Modelling spectra of hot alkali vapour in the saturation regime

Daniel Häupl, Clare R Higgins, Danielle Pizzey, Jack D Briscoe, Steven A Wrathmall, Ifan G Hughes, Robert Löw, Nicolas Y. Joly

Laser spectroscopy of hot atomic vapours has been studied extensively. Theoretical models that predict the absolute value of the electric susceptibility are crucial for optimising the design of photonic devices that use hot vapours, and for extracting parameters, such as external fields, when these devices are used as sensors. To date, most of the models developed have been restricted to the weak-probe regime. However, fulfilling the weak-probe power constraint may not always be easy, desired or necessary. Here we present a model for simulating the spectra of alkali-metal vapours for a variety of experimental parameters, most distinctly at intensities beyond weak laser fields. The model incorporates optical pumping effects and transit-time broadening. We test the performance of the model by performing spectroscopy of Rb-87 in a magnetic field of 0.6 T, where isolated atomic resonances can be addressed. We find very good agreement between the model and data for three <br>different beam diameters and a variation of intensity of over five orders of magnitude. The non-overlapping absorption lines allow us to differentiate the saturation behaviour of open and closed transitions. While our model was only experimentally verified for the D2 line of rubidium, the software is also capable of simulating spectra of rubidium, sodium, potassium and caesium over both D lines.<br>

Exploring the role of polarization in fiber-based quantum sources

Carla M. Brunner, Nicolas Y. Joly

Optical fibers constitute an attractive platform for the realization of nonlinear and quantum optics processes. Here we show, through theoretical considerations, how polarization effects of both third-order parametric down-conversion and four-wave-mixing in optical fibers may be exploited to enhance detection schemes. We apply our general framework specifically to the case of tapered fibers for photon triplet generation, a long-standing goal within quantum optics, and obtain explicit expectation values for its efficiency. A quantitative investigation of four-wave-mixing in a microstructured solid-core fiber provides significant consequences for the role of polarization in experimental design.

Flying Particle Thermosensor in Hollow-Core Fiber Based on Fluorescence Lifetime Measurements

Jasper Freitag, Max Koeppel, Maria N. Romodina, Nicolas Joly, Bernhard Schmauß

Thermosensitive fluorescence lifetime measurements enable accurate thermometry independent of intensity fluctuations along the optical path. Here, we report lifetime-based temperature measurements of a single europium-doped particle optically trapped in an air-filled hollow-core fiber. A frequency-domain fluorescence lifetime measurement setup was integrated into a dual-beam optical trap. The measured apparent lifetime shows a linear temperature dependence of −1.8 µs/K for excitation at 400Hz . The results were repeatable over multiple cooling and heating cycles. In addition to temperature sensing, the influence of the high-power trapping laser on the measured apparent lifetime and fluorescence intensity was investigated. The observed laser-induced particle heating can be exploited to increase the fluorophore's sensitivity and operating range for low-temperature sensing. Fluorescence lifetime measurements of optically trapped particles inside a hollow-core fiber are promising for temperature sensing with micrometer spatial resolution over meter-scale distances.

Performance analysis of tabletop single-pulse terahertz detection at rates up to 1.1 MHz

Nicolas Couture, Markus Lippl, Wei Cui, Angela Gamouras, Nicolas Joly, Jean-Michel Ménard

Standard terahertz time-domain spectroscopy uses a relatively slow multidata acquisition process that has hindered the technique’s ability to resolve “fast” dynamics occurring on the microsecond timescale. This timescale, inaccessible to most ultrafast pump-probe techniques, hosts a range of phenomena that has been left unexplored due to a lack of proper real-time monitoring techniques. In this work, chirped-pulse spectral encoding, a photonic time-stretch technique, and high-speed electronics are used to demonstrate time-resolved terahertz detection at a rate up to 1.1 MHz. This configuration relies on a tabletop optical source and a setup able to resolve every terahertz transient generated by the same source. We investigate the performance of this single-pulse terahertz detection system at different acquisition rates in terms of experimental noise, dynamic range, and signal-to-noise ratio. Our results pave the way towards single-pulse terahertz time-domain spectroscopy at arbitrarily fast rates to monitor complex dynamics in real time.

Flying Particle Thermosensor in Hollow-Core Fiber Based on Fluorescence Lifetime Measurements

Jasper Freitag, Max Koeppel, Maria N. Romodina, Nicolas Joly, Bernhard Schmauß

Thermosensitive fluorescence lifetime measurements enable accurate thermometry independent of intensity fluctuations along the optical path. Here, we report lifetime-based temperature measurements of a single europium-doped particle optically trapped in an air-filled hollow-core fiber. A frequency-domain fluorescence lifetime measurement setup was integrated into a dual-beam optical trap. The measured apparent lifetime shows a linear temperature dependence of −1.8 µs/K for excitation at 400Hz . The results were repeatable over multiple cooling and heating cycles. In addition to temperature sensing, the influence of the high-power trapping laser on the measured apparent lifetime and fluorescence intensity was investigated. The observed laser-induced particle heating can be exploited to increase the fluorophore's sensitivity and operating range for low-temperature sensing. Fluorescence lifetime measurements of optically trapped particles inside a hollow-core fiber are promising for temperature sensing with micrometer spatial resolution over meter-scale distances.

Experimental Optical Simulator of Reconfigurable and Complex Quantum Environment

P. Renault, J. Nokkala, G. Roeland, Nicolas Y. Joly, R. Zambrini, S. Maniscalco, J. Piilo, N. Treps, V. Parigi

No quantum system can be considered totally isolated from its environment. In most cases the interaction between the system of interest and the external degrees of freedom deeply changes its dynamics, as described by open quantum system theory. Nevertheless engineered environment can be turned into beneficial effects for some quantum information tasks. Here we demonstrate an optical simulator of a quantum system coupled to an arbitrary and reconfigurable environment built as a complex network of quantum interacting systems. We experimentally retrieve typical features of open quantum system dynamics like the spectral density and quantum non-Markovianity, by exploiting squeezing and entanglement correlation of a continuous-variable optical platform. This opens the way to the experimental tests of open quantum systems in reconfigurable environments that are relevant in, among others, quantum information, quantum thermodynamics, quantum transport, and quantum synchronization.

Low-noise supercontinuum generation in chiral all-normal dispersion photonic crystal fibers

Markus Lippl, Michael H. Frosz, Nicolas Y. Joly

We present the advantages of supercontinuum generation in chiral, therefore circularly birefringent, all-normal dispersion fibers. Due to the absence of nonlinear power transfer between the polarization eigenstates of the fiber, chiral all-normal dispersion fibers do not exhibit any polarization instabilities and thus are an ideal platform for a low-noise supercontinuum generation. By pumping a chiral all-normal dispersion fiber at 802 nm, we obtained an octave-spanning, robustly circularly polarized supercontinuum with a low noise.

Tunable fiber source of entangled UV-C and infrared photons

Santiago López-Huidrobro, Noureddin Mohammad, Maria V. Chekhova, Nicolas Y. Joly

Pairs of entangled photons—biphotons—are indispensable in quantum applications. However, some important spectral ranges, like the ultraviolet, have been inaccessible to them so far. Here, we use four-wave mixing in a xenon-filled single-ring photonic crystal fiber to generate biphotons with one of the photons in the ultraviolet and its entangled partner in the infrared spectral range. We tune the biphotons in frequency by varying the gas pressure inside the fiber and thus tailoring the fiber dispersion landscape. The ultraviolet photons are tunable from 271 nm to 231 nm and their entangled partners, from 764 nm to 1500 nm, respectively. Tunability up to 192 THz is achieved by adjusting the gas pressure by only 0.68 bar. At 1.43 bar, the photons of a pair are separated by more than 2 octaves. The access to ultraviolet wavelengths opens the possibility for spectroscopy and sensing with undetected photons in this spectral range.

Single-pulse terahertz spectroscopy monitoring sub-millisecond time dynamics at a rate of 50 kHz

Nicolas Couture, Wei Cui, Markus Lippl, Rachel Ostic, Defi Junior Jubgang Fandio, Eeswar Kumar Yalavarthi, Aswin Vishnuradhan, Angela Gamouras, Nicolas Y. Joly, et al.

Slow motion movies allow us to see intricate details of the mechanical dynamics of complex phenomena. If the images in each frame are replaced by terahertz (THz) waves, such movies can monitor low-energy resonances and reveal fast structural or chemical transitions. Here, we combine THz spectroscopy as a non-invasive optical probe with a real-time monitoring technique to demonstrate the ability to resolve non-reproducible phenomena at 50k frames per second, extracting each of the generated THz waveforms every 20 μs. The concept, based on a photonic time-stretch technique to achieve unprecedented data acquisition speeds, is demonstrated by monitoring sub-millisecond dynamics of hot carriers injected in silicon by successive resonant pulses as a saturation density is established. Our experimental configuration will play a crucial role in revealing fast irreversible physical and chemical processes at THz frequencies with microsecond resolution to enable new applications in fundamental research as well as in industry.<br><br>

Measurement of Minute Liquid Volumes of Chiral Molecules Using In-Fiber Polarimetry

Florian Schorn, Arabella Essert, Yu Zhong, Sahib Abdullayev, Kathrin Castiglione, Marco Haumann, Nicolas Y. Joly

We report an optofluidic method that enables to efficiently measure the enantiomeric excess of chiral molecules at low concentrations. The approach is to monitor the optical activity induced by a Kagome-lattice hollow core photonic crystal fiber filled with a sub-mu L volume of chiral compounds. The technique also allows monitoring the enzymatic racemization of Rmandelic acid.

Editorial: Future directions in novel laser source development: Dynamical properties, and beam manipulation

Xing Fu, Tijmen Euser, Shu-Wei Huang, Nicolas Y. Joly, Shangran Xie

Editorial on the Research Topic: Future Directions in Novel Laser Source Development: Dynamical Properties, and Beam Manipulation

Spatially resolved spectroscopy of alkali metal vapour diffusing inside hollow-core photonic crystal fibres

Daniel Häupl, Daniel Weller, Robert Loew, Nicolas Y. Joly

We present a new type of compact and all-glass based vapour cell integrating hollow-core photonic crystal fibres. The absence of metals, as in a traditional vacuum chamber and the much more compact geometry allows for fast and homogeneous heating. As a consequence we can fill the fibres on much faster timescales, ranging from minutes to hours. Additionally the all-glass design ensures optical access along the fibre. This allows live monitoring of the diffusion of rubidium atoms inside the hollow-core by measuring the frequency-dependent fluorescence from the atoms. The atomic density is numerically retrieved using a five-level system of Bloch-equations.

Tunable and state-preserving frequency conversion of single photons in hydrogen

Rinat Tyumenev, Jonas Hammer, Nicolas Joly, Philip St.J. Russell, David Novoa

In modern quantum technologies, preservation of the photon statistics of quantum optical states upon frequency conversion holds the key to the viable implementation of quantum networks, which often require interfacing of several subsystems operating in widely different spectral regions. Most current approaches offer only very small frequency shifts and limited tunability, while suffering from high insertion loss and Raman noise originating in the materials used. We introduce a route to quantum-correlation–preserving frequency conversion using hydrogen-filled antiresonant-reflecting photonic crystal fibers. Transient optical phonons generated by stimulated Raman scattering enable selective frequency up-conversion by 125 terahertz of the idler photon of an entangled pair, with efficiencies up to 70%. This threshold-less molecular modulation process preserves quantum correlations, making it ideal for applications in quantum information.

Twist and strain tuning of third harmonic generation in glass nanostrand with two sub-wavelength hollow channels

Yang Chen, Jonas Hammer, Nicolas Joly, Philip Russell

A major challenge in third harmonic generation and its converse, parametric down-conversion, is how to arrange phase matching between signals at omega and 3 omega while maintaining a high nonlinear overlap. In this Letter, we present a design consisting of a nanostrand of glass with two hollow channels. The fundamental and third harmonic modal fields, enhanced in the region between the channels, have high nonlinear overlap, while the phase-matching wavelength can be coarse-tuned by gas pressure and fine-tuned by axial strain and mechanical twist, which, remarkably, have opposite effects. The ability to adjust the phase-matching condition may facilitate efficient generation of entangled photon triplets. (C) 2021 Optical Society of America.

Deep-UV-enhanced supercontinuum generated in a tapered gas-filled photonic crystal fiber

Mallika Irene Suresh, Jonas Hammer, Nicolas Y. Joly, Philip Russell, Francesco Tani

We present the use of a linearly down-tapered gas-filled hollow-core photonic crystal fiber in a single stage, pumped with pulses froma compact infrared (IR) laser source, to generate a supercontinuum (SC) carrying significant spectral power in the deep ultraviolet (UV) [200-300 nm]. The generated SC extends from the near IR down to similar to 213 nm with 0.58 mW/nm and down to similar to 220 nm with 0.83 mW/nm in the deepUV. (C) 2021 Optical Society of America

Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle

Vanessa Wachter, Victor A. S. V. Bittencourt, Shangran Xie, Sanchar Sharma, Nicolas Joly, Philip Russell, Florian Marquardt, Silvia Viola-Kusminskiy

We propose a platform that combines the fields of cavity optomagnonics and levitated optome-<br>chanics in order to control and probe the coupled spin-mechanics of magnetic dielectric particles. We theoretically study the dynamics of a levitated Faraday-active dielectric microsphere serving as an optomagnonic cavity, placed in an external magnetic field and driven by an external laser. We find that the optically driven magnetization dynamics induces angular oscillations of the particle with low associated damping. Further, we show that the magnetization and angular motion dynamics<br>can be probed via the power spectrum of the outgoing light. Namely, the characteristic frequencies attributed to the angular oscillations and the spin dynamics are imprinted in the light spectrum by two main resonance peaks. Additionally, we demonstrate that a ferromagnetic resonance setup with an oscillatory perpendicular magnetic field can enhance the resonance peak corresponding to<br>the spin oscillations and induce fast rotations of the particle around its anisotropy axis.

Fiber-based biphoton source with ultrabroad frequency tunability

Santiago López-Huidrobro, Markus Lippl, Nicolas Joly, Maria Chekhova

Tunable biphotons are highly important for a wide range of quantum applications. For some applications, especially interesting are cases where two photons of a pair are far apart in frequency. Here, we report a tunable biphoton source based on a xenon-filled hollow-core photonic crystal fiber. Tunability is achieved by adjusting the pressure of the gas inside the fiber. This allows us to tailor the dispersion landscape of the fiber, overcoming the principal limitations of solid-core fiber-based biphoton sources. We report a maximum tunability of 120 THz for a pressure range of 4 bar with a continuous shift of 30 THz/bar. At 21 bar, the photons of a pair are separated by more than one octave. Despite the large separation, both photons have large bandwidths. At 17 bar, they form a very broad (110 THz) band around the frequency of the pump.

Tumbling and anomalous alignment of optically levitated anisotropic microparticles in chiral hollow-core photonic crystal fiber

Shangran Xie, Abhinav Sharma, Maria N. Romodina, Nicolas Y. Joly, Philip Russell

The complex tumbling motion of spinning nonspherical objects is a topic of enduring interest, both in popular culture and in advanced scientific research. Here, we report all-optical control of the spin, precession, and nutation of vaterite microparticles levitated by counterpropagating circularly polarized laser beams guided in chiral hollow-core fiber. The circularly polarized light causes the anisotropic particles to spin about the fiber axis, while, regulated by minimization of free energy, dipole forces tend to align the extraordinary optical axis of positive uniaxial particles into the plane of rotating electric field. The end result is that, accompanied by oscillatory nutation, the optical axis reaches a stable tilt angle with respect to the plane of the electric field. The results reveal new possibilities for manipulating optical alignment through rotational degrees of freedom, with applications in the control of micromotors and microgyroscopes, laser alignment of polyatomic molecules, and study of rotational cell mechanics.

Specialty Photonic Crystal Fibers and Their Applications

David Novoa, Nicolas Joly

This year not only commemorates the 60th anniversary of nonlinear optics with the seminal experiment of second harmonic generation, but it is also the 30th anniversary of the invention of the photonic crystal fiber (PCF). Following their first practical demonstration in 1996, PCFs have rapidly evolved into an established platform for applications in both academic and industrial environments. Their unique ability to confine light in a far more versatile way than possible with conventional optical fibers facilitated the expansion of the multifaceted world of PCF to cover not only nonlinear optics, but also many other disparate fields such as interferometry, beam delivery, laser science, telecommunications, quantum optics, sensing, microscopy, and many others.

Online Monitoring of Microscale Liquid-Phase Catalysis Using in-Fiber Raman Spectroscopy

Florian Schorn, Manfred Aubermann, Richard Zeltner, Marco Haumann, Nicolas Y. Joly

We report on the use of hollow-core photonic crystal fibers to monitor the evolution of chemical reactions. The combination of tight confinement and long interaction length allows single-pass spectroscopic measurements using less than a microliter volume of chemicals with good accuracy. As a proof of principle, we used here nonlinear Raman spectroscopy for a reaction screening of the acidic catalyzed esterification of methanol and acetic acid.

Doppler optical frequency domain reflectometry for remote fiber sensing

Max Koeppel, Abhinav Sharma, Jasper Podschus, Sanju Sundaramahalingam, Nicolas Y. Joly, Shangran Xie, Philip Russell, Bernhard Schmauss

Coherent optical frequency domain reflectometry has been widely used to locate static reflectors with high spatial resolution. Here, we present a new type of Doppler optical frequency domain reflectometry that offers simultaneous measurement of the position and speed of moving objects. The system is exploited to track optically levitated "flying" particles inside a hollow-core photonic crystal fiber. As an example, we demonstrate distributed temperature sensing with sub-mm-scale spatial resolution and a standard deviation of similar to 10 degrees C up to 200 degrees C. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

All normal dispersion nonlinear fibre supercontinuum source characterization and application in hyperspectral stimulated Raman scattering microscopy

Pedram Abdolghader, Adrian F. Pegoraro, Nicolas Joly, Andrew Ridsdale, Rune Lausten, Francois Legare, Albert Stolow

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful label-free, chemical-specific technique for biomedical and mineralogical imaging. Usually, broad and rapid spectral scanning across Raman bands is required for species identification. In many implementations, however, the Raman spectral scan speed is limited by the need to tune source laser wavelengths. Alternatively, a broadband supercontinuum source can be considered. In SRS microscopy, however, source noise is critically important, precluding many spectral broadening schemes. Here we show that a supercontinuum light source based on all normal dispersion (ANDi) fibres provides a stable broadband output with very low incremental source noise. We characterized the noise power spectral density of the ANDi fibre output and demonstrated its use in hyperspectral SRS microscopy applications. This confirms the viability and ease of implementation of ANDi fibre sources tier broadband SRS imaging. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Progress toward third-order parametric down-conversion in optical fibers

A. Cavanna, J. Hammer, C. Okoth, E. Ortiz-Ricardo, H. Cruz-Ramirez, K. Garay-Palmett, A. B. U’Ren, M. Frosz, X. Jiang, et al.

Optical fibers have been considered an optimal platform for third-order parametric down-conversion since they can potentially overcome the weak third-order nonlinearity by their long interaction length. Here we present, in the first part, a theoretical derivation for the conversion rate both in the case of spontaneous generation and in the presence of a seed beam. Then we review three types of optical fibers and we examine their properties in terms of conversion efficiency and practical feasibility.

Broadly tunable photon-pair generation in a suspended-core fiber

Jonas Hammer, Maria V. Chekhova, Daniel Häupl, Riccardo Pennetta, Nicolas Y. Joly

Nowadays fiber biphoton sources are nearly as popular as crystal-based ones. They offer a single spatial mode and easy integrability into optical networks. However, fiber sources lack the broad tunability of crystals, which do not require a tunable pump. Here, we report a broadly tunable biphoton source based on a suspended core fiber. This is achieved by introducing pressurized gas into the fibers hollow channels, changing the step index. The mechanism circumvents the need for a tunable pump laser, making this a broadly tunable fiber biphoton source with a convenient tuning mechanism, comparable to crystals. We report a continuous shift of 0.30 THz/bar of the sidebands, using up to 25 bar of argon.

Laser refrigeration of gas filled hollow-core fibres

Christian Sommer, Nicolas Y. Joly, Helmut Ritsch, Claudiu Genes

We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered<br>refrigeration device is based on exciplex-mediated frequency up-conversion inside hollow-core fibers pressurized with a dopant - buffer<br>gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the<br>ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during<br>atomic collisions inducing light assisted exciplex formation followed by blue-shifted spontaneous emission on the atomic line of the bare<br>dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large<br>fraction of collision energy to be extracted in each cycle. The hollow-core fiber plays a crucial role as it allows for strong light-matter<br>interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while<br>limiting re-absorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations<br>and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding<br>solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the<br>exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification<br>of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific<br>applications.<br>

Generation of broadband circularly polarized supercontinuum light in twisted photonic crystal fibers

Rafal Sopalla, Gordon Wong, Nicolas Joly, Michael Frosz, Xin Jiang, Goran Ahmed, Philip Russell

We compare the properties of the broadband supercontinuum (SC) generated in twisted and untwisted solid-core photonic crystal fibers when pumped by circularly polarized<br>40 picosecond laser pulses at 1064 nm. In the helically twisted fiber, fabricated by spinning the preform during the draw, the SC is robustly circularly polarized across its entire<br>spectrum whereas, in the straight fiber, axial fluctuations in linear birefringence and polarization-dependent nonlinear effects cause the polarization state to vary randomly with the wavelength. Theoretical modelling confirms the experimental results. Helically twisted photonic crystal fibers permit the generation of pure circularly polarized SC light with excellent polarization stability against fluctuations in input power and environmental perturbations.

Seeded and unseeded high-order parametric down-conversion

Cameron Okoth, Andrea Cavanna, Nicolas Joly, Maria Chekhova

Spontaneous parametric down-conversion (SPDC) has been one of the foremost tools in quantum optics for over five decades. Over that time, it has been used to demonstrate some of the curious features that arise from quantum mechanics. Despite the success of SPDC, its higher-order analogs have never been observed, even though it has been suggested that they generate far more unique and exotic states than SPDC. An example of this is the emergence of non-Gaussian states without the need for postselection. Here we calculate the expected rate of emission for nth-order SPDC with and without external stimulation (seeding). Focusing primarily on third-order parametric down-conversion, we estimate the photon detection rates in a rutile crystal for both the unseeded and seeded regimes.

High-resolution multimodal flexible coherent Raman endoscope

Alberto Lombardini, Vasyl Mytskaniuk, Siddharth Sivankutty, Esben Ravn Andresen, Xueqin Chen, Jérôme Wenger, Marc Fabert, Nicolas Y. Joly, Frédéric Louradour, et al.

Coherent Raman scattering microscopy is a fast, label-free, and chemically specific imaging technique that shows high potential for future in vivo optical histology. However, the imaging depth in tissues is limited to the sub-millimeter range because of absorption and scattering. Realization of coherent Raman imaging using a fiber endoscope system is a crucial step towards imaging deep inside living tissues and providing information that is inaccessible with current microscopy tools. Until now, the development of coherent Raman endoscopy has been hampered by several issues, mainly related to the fiber delivery of the excitation pulses and signal collection. Here, we present a flexible, compact, coherent Raman, and multimodal nonlinear endoscope (4.2 mm outer diameter, 71 mm rigid length) based on a resonantly scanned hollow-core Kagomé lattice double-clad fiber. The fiber design enables distortion-less, background-free delivery of femtosecond excitation pulses and back-collection of nonlinear signals through the same fiber. Sub-micrometer spatial resolution over a large field of view is obtained by combination of a miniature objective lens with a silica microsphere lens inserted into the fiber core. We demonstrate high-resolution, high-contrast coherent anti-Stokes Raman scattering, and second harmonic generation endoscopic imaging of biological tissues over a field of view of 320 µm at a rate of 0.8 frames per second. These results pave the way for intraoperative label-free imaging applied to real-time histopathology diagnosis and surgery guidance.

Dispersion tuning in sub-micron tapers for third-harmonic and photon triplet generation

Jonas Hammer, Andrea Cavanna, Riccardo Pennetta, Maria Chekhova, Philip St. J. Russell, Nicolas Joly

Precise control of the dispersion landscape is of crucial importance if optical fibers are to be successfully used for the generation of three-photon states of light—the inverse of third-harmonic generation (THG). Here we report gas-tuning of intermodal phase-matched THG in sub-micron-diameter tapered optical fiber. By adjusting the pressure of the surrounding argon gas up to 50 bars, intermodally phase-matched third-harmonic light can be generated for pump wavelengths within a 15 nm range around 1.38 μm. We also measure the infrared fluorescence generated in the fiber when pumped in the visible and estimate that the accidental coincidence rate in this signal is lower than the predicted detection rate of photon triplets

Low-loss single-mode negatively curved square-core hollow fibers

Yang Chen, Mohammed F. Saleh, Nicolas Y. Joly, Fabio Biancalana

We introduce a novel design of anti-resonant fibers with negative-curvature square cores to be employed in 1.55 and 2.94 mu m transmission bands. The fibers have low losses and single-mode operation via optimizing the negative curvature of the guiding walls. The first proposed fiber shows a broadband transmission window spanning 0.9-1.7 mu m, with losses of 0.025 and 0.056 dB/m at 1.064 and 1.55 mu m, respectively. The second proposed fiber has approximately a 0.023 dB/m guiding loss at 2.94 mu m with a small cross-sectional area, useful for laser micromachining applications. (C) 2017 Optical Society of America

Effect of stray fields on Rydberg states in hollow-core PCF probed by higher-order modes

G. Epple, N. Y. Joly, T. G. Euser, P. St. J. Russell, R. Loew

The spectroscopy of atomic gases confined in hollow-core photonic crystal fiber (HC-PCF) provides optimal atom-light coupling beyond the diffraction limit, which is desirable for various applications such as sensing, referencing, and nonlinear optics. Recently, coherent spectroscopy was carried out on highly excited Rydberg states at room temperature in a gas-filled HC-PCF. The large polarizability of the Rydberg states made it possible to detect weak electric fields inside the fiber. In this Letter, we show that by combining highly excited Rydberg states with higher-order optical modes, we can gain insight into the distribution and underlying effects of these electric fields. Comparisons between experimental findings and simulations indicate that the fields are caused by the dipole moments of atoms adsorbed on the hollow-core wall. Knowing the origin of the electric fields is an important step towards suppressing them in future HC-PCF experiments. Furthermore, a better understanding of the influence of adatoms will be advantageous for optimizing electric-fieldsensitive experiments carried out in the vicinity of nearby surfaces. (C) 2017 Optical Society of America

Characterization and shaping of the time-frequency Schmidt mode spectrum of bright twin beams generated in gas-filled hollow-core photonic crystal fibers

M. A. Finger, N. Y. Joly, P. St. J. Russell, M. V. Chekhova

We vary the time-frequency mode structure of ultrafast pulse-pumped modulational instability (MI) twin beams in an argon-filled hollow-core kagome-style photonic crystal fiber by adjusting the pressure, pump pulse chirp, fiber length, and parametric gain. Compared to solid-core systems, the pressure-dependent dispersion landscape brings increased flexibility to the tailoring of frequency correlations, and we demonstrate that the pump pulse chirp can be used to tune the joint spectrum of femtosecond-pumped.(3) sources. We also characterize the resulting mode content, not only by measuring the multimode second-order correlation function g((2)), but also by directly reconstructing the shapes and weights of time-frequency Schmidt (TFS) modes. We show that the number of modes directly influences the shot-to-shot pulse-energy and spectral-shape fluctuations in MI. Using this approach we control and monitor the number of TFS modes within the range from 1.3 to 4 using only a single fiber.

Single-shot reconstruction of spectral amplitude and phase in a fiber ring cavity at a 80 MHz repetition rate

Jonas Hammer, Pooria Hosseini, Curtis R. Menyuk, Philip St. J. Russell, Nicolas Y. Joly

Femtosecond pulses circulating in a synchronously driven fiber ring cavity have complex amplitude and phase profiles that can change completely from one round-trip to the next. We use a recently developed technique, combining dispersive Fourier transformation) with spectral interferometry, to reconstruct the spectral amplitude and phase at each round-trip and, thereby, follow in detail the pulse reorganization that occurs. We focus on two different regimes: a period-two regime in which the pulse alternates between two distinct states and a highly complex regime. We characterize the spectral amplitude and phase of the pulses in both regimes at a repetition rate of 75.6 MHz and find good agreement with modeling of the system based on numerical solutions of the generalized nonlinear Schrodinger equation with feedback. (C) 2016 Optical Society of America

Hybrid photonic-crystal fiber for single-mode phase matched generation of third harmonic and photon triplets

Andrea Cavanna, Felix Just, Xin Jiang, Gerd Leuchs, Maria V. Chekhova, Philip St. J. Russell, Nicolas Y. Joly

All-fiber systems for third harmonic generation are of great interest because they can be used for the inverse process, namely, the generation of entangled photon triplets. Usually, chromatic dispersion prevents phase matching between the incident and generated radiation when they are both guided in an LP01-like mode. Here, we present a hybrid photonic crystal fiber that has been designed for phase matched third harmonic generation from 1596 to 532 nm in single-lobed modes. The third harmonic radiation is guided by an all-solid bandgap microstructure, while the pump frequency is confined by conventional total internal reflection. The fiber is also suitable for the generation of photon triplet states.

Reducing losses in solid-core photonic crystal fibers using chlorine dehydration

Michael H. Frosz, Goran Ahmed, Nadezda Lapshina, Ralf Keding, Fehim Babic, Nicolas Y. Joly, Philip St. J. Russell

Solid-core and hollow-core photonic crystal fiber for generation of bright ultraviolet light (Conference Presentation)

Nicolas Y. Joly, Xin Jiang, John C. Travers, Alexey Ermolov, Philip St. J. Russell

Guiding 2.94 mu m using low-loss microstructured antiresonant triangular-core fibers

Yang Chen, Mohammed F. Saleh, Nicolas Y. Joly, Fabio Biancalana

We introduce a new simple design of hollow-core microstructured fiber targeted to guide mid-infrared light at a wavelength of 2.94 mu m. The fiber has a triangular-core supported via silica-glass webs enclosed by a large hollow capillary tube. The fiber specific dimensions are determined by the anti-resonant guiding mechanism. For a triangular-core with side length 100 mu m, the fiber has a minimum transmission loss 0.08 +/- 0.005 dB/m and dispersion 2.3 ps/km/nm at the operational wavelength of 2.94 mu m. (C) 2016 AIP Publishing LLC.

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores

Xin Jiang, Nicolas Y. Joly, Martin A. Finger, Fehim Babic, Meng Pang, Rafal Sopalla, Michael H. Frosz, Samuel Poulain, Marcel Poulain, et al.

Stable subpicosecond soliton fiber laser passively mode-locked by gigahertz acoustic resonance in photonic crystal fiber core

M. Pang, X. Jiang, W. He, G. K. L. Wong, G. Onishchukov, N. Y. Joly, G. Ahmed, C. R. Menyuk, P. St J. Russell

Ultrafast lasers with high repetition rates are of considerable interest in applications such as optical fiber telecommunications, frequency metrology, high-speed optical sampling, and arbitrary waveform generation. For fiber lasers mode-locked at the cavity round-trip frequency, the pulse repetition rate is limited to tens or hundreds of megahertz by the meter-order cavity lengths. Here we report a soliton fiber laser passively mode-locked at a high harmonic (similar to 2 GHz) of its fundamental frequency by means of optoacoustic interactions in the small solid glass core of a short length ( 60 cm) of photonic crystal fiber. Due to tight confinement of both light and vibrations, the optomechanical interaction is strongly enhanced. The long-lived acoustic vibration provides strong modulation of the refractive index in the photonic crystal fiber core, fixing the soliton spacing in the laser cavity and allowing stable mode-locking, with low pulse timing jitter, at gigahertz repetition rates. (C) 2015 Optical Society of America

Deep-ultraviolet to mid-infrared supercontinuum generated in solid-core ZBLAN photonic crystal fibre

Xin Jiang, Nicolas Y. Joly, Martin A. Finger, Fehim Babic, Gordon K. L. Wong, John C. Travers, Philip St J. Russell

Silica-based photonic crystal fibre has proven highly successful for supercontinuum generation, with smooth and flat spectral power densities. However, fused silica glass suffers from strong material absorption in the mid-infrared (>2,500 nm), as well as ultraviolet-related optical damage (solarization), which limits performance and lifetime in the ultraviolet (<380 nm). Supercontinuum generation in silica photonic crystal fibre is therefore only possible between these limits. A number of alternative glasses have been used to extend the mid-infrared performance, including chalcogenides, fluorides and heavy-metal oxides, but none has extended the ultraviolet performance. Here, we describe the successful fabrication (using the stack-and-draw technique) of a ZBLAN photonic crystal fibre with a high air-filling fraction, a small solid core, nanoscale features and near-perfect structure. We also report its use in the generation of ultrabroadband, long-term stable, supercontinua spanning more than three octaves in the spectral range 200-2,500 nm.

Raman-Free, Noble-Gas-Filled Photonic-Crystal Fiber Source for Ultrafast, Very Bright Twin-Beam Squeezed Vacuum

Martin A. Finger, Timur Sh. Iskhakov, Nicolas Y. Joly, Maria V. Chekhova, Philip St. J. Russell

We report a novel source of twin beams based on modulational instability in high-pressure argon-filled hollow-core kagome-style photonic-crystal fiber. The source is Raman-free and manifests strong photonnumber correlations for femtosecond pulses of squeezed vacuum with a record brightness of similar to 2500 photons per mode. The ultra-broadband (similar to 50 THz) twin beams are frequency tunable and contain one spatial and less than 5 frequency modes. The presented source outperforms all previously reported squeezed-vacuum twin-beam sources in terms of brightness and low mode content.

Accuracy of the capillary approximation for gas-filled kagome-style photonic crystal fibers

M. A. Finger, N. Y. Joly, T. Weiss, P. St. J. Russell

Precise knowledge of the group velocity dispersion in gas-filled hollow-core photonic crystal fiber is essential for accurate modeling of ultrafast nonlinear dynamics. Here we study the validity of the capillary approximation commonly used to calculate the modal refractive index in kagome-style photonic crystal fibers. For area-preserving core radius alpha(AP) and core wall thickness t, measurements and finite element simulations show that the approximation has an error greater than 15% for wavelengths longer than 0.56 root(alpha(AP)t), independently of the gas-filling pressure. By introducing an empirical wavelength-dependent core radius, the range of validity of the capillary approximation is extended out to a wavelength of at least 0.98 root(alpha(AP)t). (C) 2014 Optical Society of America

Rydberg atoms in hollow-core photonic crystal fibres

G. Epple, K. S. Kleinbach, T. G. Euser, N. Y. Joly, T. Pfau, P. St J. Russell, R. Loew

The exceptionally large polarizability of highly excited Rydberg atoms-six orders of magnitude higher than ground-state atoms-makes them of great interest in fields such as quantum optics, quantum computing, quantum simulation and metrology. However, if they are to be used routinely in applications, a major requirement is their integration into technically feasible, miniaturized devices. Here we show that a Rydberg medium based on room temperature caesium vapour can be confined in broadband-guiding kagome-style hollow-core photonic crystal fibres. Three-photon spectroscopy performed on a caesium-filled fibre detects Rydberg states up to a principal quantum number of n = 40. Besides small energy-level shifts we observe narrow lines confirming the coherence of the Rydberg excitation. Using different Rydberg states and core diameters we study the influence of confinement within the fibre core after different exposure times. Understanding these effects is essential for the successful future development of novel applications based on integrated room temperature Rydberg systems.

Atomic mercury vapor inside a hollow-core photonic crystal fiber

Ulrich Vogl, Christian Peuntinger, Nicolas Y. Joly, Philip St. J. Russell, Christoph Marquardt, Gerd Leuchs

We demonstrate high atomic mercury vapor pressure in a kagome-style hollow-core photonic crystal fiber at room temperature. After a few days of exposure to mercury vapor the fiber is homogeneously filled and the optical depth achieved remains constant. With incoherent optical pumping from the ground state we achieve an optical depth of 114 at the 6(3)P(2) - 6(3)D(3) transition, corresponding to an atomic mercury number density of 6 x 10(10) cm(-3). The use of mercury vapor in quasi one-dimensional confinement may be advantageous compared to chemically more active alkali vapor, while offering strong optical nonlinearities in the ultraviolet region of the optical spectrum. (C) 2014 Optical Society of America

Multistability and spontaneous breaking in pulse-shape symmetry in fiber ring cavities

M. J. Schmidberger, D. Novoa, F. Biancalana, P. St J. Russell, N. Y. Joly

We describe the spatio-temporal evolution of ultrashort pulses propagating in a fiber ring cavity using an extension of the Lugiato-Lefever model. The model predicts the appearance of multistability and spontaneous symmetry breaking in temporal pulse shape. We also use a hydrodynamical approach to explain the stability of the observed regimes of asymmetry. (C) 2014 Optical Society of America

Damage-free single-mode transmission of deep-UV light in hollow-core PCF

F. Gebert, M. H. Frosz, T. Weiss, Y. Wan, A. Ermolov, N. Y. Joly, P. O. Schmidt, P. St. J. Russell

Transmission of UV light with high beam quality and pointing stability is desirable for many experiments in atomic, molecular and optical physics. In particular, laser cooling and coherent manipulation of trapped ions with transitions in the UV require stable, single-mode light delivery. Transmitting even similar to 2 mW CW light at 280 nm through silica solid-core fibers has previously been found to cause transmission degradation after just a few hours due to optical damage. We show that photonic crystal fiber of the kagome type can be used for effectively single-mode transmission with acceptable loss and bending sensitivity. No transmission degradation was observed even after >100 hours of operation with 15 mW CW input power. In addition it is shown that implementation of the fiber in a trapped ion experiment increases the coherence time of the internal state transfer due to an increase in beam pointing stability. (C) 2014 Optical Society of America

Nonlinear optics in Xe-filled hollow-core PCF in high pressure and supercritical regimes

M. Azhar, N. Y. Joly, J. C. Travers, P. St J. Russell

Supercritical Xe at 293 K offers a Kerr nonlinearity that can exceed that of fused silica while being free of Raman scattering. It also has a much higher optical damage threshold and a transparency window that extends from the UV to the infrared. We report the observation of nonlinear phenomena, such as self-phase modulation, in hollow-core photonic crystal fiber filled with supercritical Xe. In the subcritical regime, intermodal four-wave mixing resulted in the generation of UV light in the HE12 mode. The normal dispersion of the fiber at high pressures means that spectral broadening can be clearly obtained without influence from soliton effects or material damage.

Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF

Ka Fai Mak, John C. Travers, Philipp Hoelzer, Nicolas Y. Joly, Philip St. J. Russell

An efficient and tunable 176-550 nm source based on the emission of resonant dispersive radiation from ultrafast solitons at 800 nm is demonstrated in a gas-filled hollow-core photonic crystal fiber (PCF). By careful optimization and appropriate choice of gas, informed by detailed numerical simulations, we show that bright, high quality, localized bands of UV light (relative widths of a few percent) can be generated at all wavelengths across this range. Pulse energies of more than 75 nJ in the deep-UV, with relative bandwidths of similar to 3%, are generated from pump pulses of a few mu J. Excellent agreement is obtained between numerical and experimental results. The effects of positive and negative axial pressure gradients are also experimentally studied, and the coherence of the deep-UV dispersive wave radiation numerically investigated. (C) 2013 Optical Society of America

Raman-free nonlinear optical effects in high pressure gas-filled hollow core PCF

M. Azhar, G. K. L. Wong, W. Chang, N. Y. Joly, P. St J. Russell

The effective Kerr nonlinearity of hollow-core kagome-style photonic crystal fiber (PCF) filled with argon gas increases to similar to 15% of that of bulk silica glass when the pressure is increased from 1 to 150 bar, while the zero dispersion wavelength shifts from 300 to 900 nm. The group velocity dispersion of the system is uniquely pressure-tunable over a wide range while avoiding Raman scattering-absent in noble gases-and having an extremely high optical damage threshold. As a result, detailed and well-controlled studies of nonlinear effects can be performed, in both normal and anomalous dispersion regimes, using only a fixed-frequency pump laser. For example, the absence of Raman scattering permits clean observation, at high powers, of the interaction between a modulational instability side-band and a soliton-created dispersive wave. Excellent agreement is obtained between numerical simulations and experimental results. The system has great potential for the realization of reconfigurable supercontinuum sources, wavelength convertors and short-pulse laser systems. (C)2013 Optical Society of America

Passive mode-locking of fiber ring laser at the 337th harmonic using gigahertz acoustic core resonances

M. S. Kang, N. Y. Joly, P. St. J. Russell

We report the experimental demonstration of a passively mode-locked Er-doped fiber ring laser operating at the 337th harmonic (1.80 GHz) of the cavity. The laser makes use of highly efficient Raman-like optoacoustic interactions between the guided light and gigahertz acoustic resonances trapped in the micron-sized solid glass core of a photonic crystal fiber. At sufficient pump power levels the laser output locks to a repetition rate corresponding to the acoustic frequency. A stable optical pulse train with a side-mode suppression ratio higher than 45 dB was obtained at low pump powers (similar to 60 mW). (C) 2013 Optical Society of America

Two techniques for temporal pulse compression in gas-filled hollow-core kagome photonic crystal fiber

K. F. Mak, J. C. Travers, N. Y. Joly, A. Abdolvand, P. St. J. Russell

We demonstrate temporal pulse compression in gas-filled kagome hollow-core photonic crystal fiber (PCF) using two different approaches: fiber-mirror compression based on self-phase modulation under normal dispersion, and soliton effect self-compression under anomalous dispersion with a decreasing pressure gradient. In the first, efficient compression to near-transform-limited pulses from 103 to 10.6 fs was achieved at output energies of 10.3 mu J. In the second, compression from 24 to 6.8 fs was achieved at output energies of 6.6 mu J, also with near-transform-limited pulse shapes. The results illustrate the potential of kagome-PCF for postprocessing the output of fiber lasers. We also show that, using a negative pressure gradient, ultrashort pulses can be delivered directly into vacuum. (C) 2013 Optical Society of America

Influence of timing jitter on nonlinear dynamics of a photonic crystal fiber ring cavity

M. Schmidberger, W. Chang, P. St. J. Russell, N. Y. Joly

We demonstrate that timing jitter has a strong influence on supercontinua generated in a photonic crystal fiber ring cavity synchronously pumped by 140 fs pulses. The global dynamics with respect to cavity detuning is analyzed both numerically and experimentally by tracking the cavity pulse energy. The results show that low-frequency timing jitter, induced by both the pump oscillator and the external cavity, masks the fine underlying bifurcation structure of the system. Numerical simulations in the absence of timing jitter reveal that the system dynamics fall into four qualitatively different regimes. The existence of these regimes is experimentally observed in first-return diagrams. (c) 2012 Optical Society of America

Kagome hollow-core photonic crystal fiber probe for Raman spectroscopy

Petru Ghenuche, Silke Rammler, Nicolas Y. Joly, Michael Scharrer, Michael Frosz, Jerome Wenger, Philip St J. Russell, Herve Rigneault

We demonstrate the use of a large-pitch Kagome-lattice hollow-core photonic crystal fiber probe for Raman spectroscopy. The large transmission bandwidth of the fiber enables both the excitation and Raman beams to be transmitted through the same fiber. As the excitation beam is mainly transmitted through air inside the hollow core, the silica luminescence background is reduced by over 2 orders of magnitude as compared to standard silica fiber probes, removing the need for fiber background subtraction. (C) 2012 Optical Society of America

Optofluidic refractive-index sensor in step-index fiber with parallel hollow micro-channel

H. W. Lee, M. A. Schmidt, P. Uebel, H. Tyagi, N. Y. Joly, M. Scharrer, P. St. J. Russell

We present a simple refractive index sensor based on a step-index fiber with a hollow micro-channel running parallel to its core. This channel becomes waveguiding when filled with a liquid of index greater than silica, causing sharp dips to appear in the transmission spectrum at wavelengths where the glass-core mode phase-matches to a mode of the liquid-core. The sensitivity of the dip-wavelengths to changes in liquid refractive index is quantified and the results used to study the dynamic flow characteristics of fluids in narrow channels. Potential applications of this fiber microstructure include measuring the optical properties of liquids, refractive index sensing, biophotonics and studies of fluid dynamics on the nanoscale. (C) 2011 Optical Society of America

Bright Spatially Coherent Wavelength-Tunable Deep-UV Laser Source Using an Ar-Filled Photonic Crystal Fiber

N.Y. Joly, J. Nold, W. Chang, P. Hoelzer, A. Nazarkin, G. K. L. Wong, F. Biancalana, P. St. J. Russell

We report on the spectral broadening of similar to 1 mu J 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving <1 m of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8 %. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.

Influence of ionization on ultrafast gas-based nonlinear fiber optics

W. Chang, A. Nazarkin, J. C. Travers, J. Nold, P. Hoelzer, N. Y. Joly, P. St. J. Russell

We numerically investigate the effect of ionization on ultrashort high-energy pulses propagating in gas-filled kagome-lattice hollow-core photonic crystal fibers by solving an established uni-directional field equation. We consider the dynamics of two distinct regimes: ionization induced blue-shift and resonant dispersive wave emission in the deep-UV. We illustrate how the system evolves between these regimes and the changing influence of ionization. Finally, we consider the effect of higher ionization stages. (C) 2011 Optical Society of America

Entangling Different Degrees of Freedom by Quadrature Squeezing Cylindrically Polarized Modes

C. Gabriel, A. Aiello, W. Zhong, T. G. Euser, N. Y. Joly, P. Banzer, M. Foertsch, D. Elser, U. L. Andersen, et al.

Quantum systems such as, for example, photons, atoms, or Bose-Einstein condensates, prepared in complex states where entanglement between distinct degrees of freedom is present, may display several intriguing features. In this Letter we introduce the concept of such complex quantum states for intense beams of light by exploiting the properties of cylindrically polarized modes. We show that already in a classical picture the spatial and polarization field variables of these modes cannot be factorized. Theoretically it is proven that by quadrature squeezing cylindrically polarized modes one generates entanglement between these two different degrees of freedom. Experimentally we demonstrate amplitude squeezing of an azimuthally polarized mode by exploiting the nonlinear Kerr effect in a specially tailored photonic crystal fiber. These results display that such novel continuous-variable entangled systems can, in principle, be realized.

Birefringence and dispersion of cylindrically polarized modes in nanobore photonic crystal fiber

T. G. Euser, M. A. Schmidt, N. Y. Joly, C. Gabriel, C. Marquardt, L. Y. Zang, M. Foertsch, P. Banzer, A. Brenn, et al.

We demonstrate experimentally and theoretically that a nanoscale hollow channel placed centrally in the solid-glass core of a photonic crystal fiber strongly enhances the cylindrical birefringence (the modal index difference between radially and azimuthally polarized modes). Furthermore, it causes a large split in group velocity and group velocity dispersion. We show analytically that all three parameters can be varied over a wide range by tuning the diameters of the nanobore and the core. (C) 2010 Optical Society of America

Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers [Invited]

John C. Travers, Wonkeun Chang, Johannes Nold, Nicolas Y. Joly, Philip St. J. Russell

We review the use of hollow-core photonic crystal fibers (PCFs) in the field of ultrafast gas-based nonlinear optics, including recent experiments, numerical modeling, and a discussion of future prospects. Concentrating on broadband guiding kagome-style hollow-core PCF, we describe its potential for moving conventional nonlinear fiber optics both into extreme regimes-such as few-cycle pulse compression and efficient deep ultraviolet wavelength generation-and into regimes hitherto inaccessible, such as single-mode guidance in a photoionized plasma and high-harmonic generation in fiber. (C) 2011 Optical Society of America

Theory of Photoionization-Induced Blueshift of Ultrashort Solitons in Gas-Filled Hollow-Core Photonic Crystal Fibers

Mohammed F. Saleh, Wonkeun Chang, Philipp Hoelzer, Alexander Nazarkin, John C. Travers, Nicolas Y. Joly, Philip St. J. Russell, Fabio Biancalana

We show theoretically that the photoionization process in a hollow-core photonic crystal fiber filled with a Raman-inactive noble gas leads to a constant acceleration of solitons in the time domain with a continuous shift to higher frequencies, limited only by ionization loss. This phenomenon is opposite to the well-known Raman self-frequency redshift of solitons in solid-core glass fibers. We also predict the existence of unconventional long-range nonlocal soliton interactions leading to spectral and temporal soliton clustering. Furthermore, if the core is filled with a Raman-active molecular gas, spectral transformations between redshifted, blueshifted, and stabilized solitons can take place in the same fiber.

Pressure-assisted melt-filling and optical characterization of Au nano-wires in microstructured fibers

H. W. Lee, M. A. Schmidt, R. F. Russell, N. Y. Joly, H. K. Tyagi, P. Uebel, P. St. J. Russell

We report a novel splicing-based pressure-assisted melt-filling technique for creating metallic nanowires in hollow channels in microstructured silica fibers. Wires with diameters as small as 120 nm (typical aspect ration 50:1) could be realized at a filling pressure of 300 bar. As an example we investigate a conventional single-mode step-index fiber with a parallel gold nanowire (wire diameter 510 nm) running next to the core. Optical transmission spectra show dips at wavelengths where guided surface plasmon modes on the nanowire phase match to the glass core mode. By monitoring the side-scattered light at narrow breaks in the nanowire, the loss could be estimated. Values as low as 0.7 dB/mm were measured at resonance, corresponding to those of an ultra-long-range eigenmode of the glass-core/nanowire system. By thermal treatment the hollow channel could be collapsed controllably, permitting creation of a conical gold nanowire, the optical properties of which could be monitored by side-scattering. The reproducibility of the technique and the high optical quality of the wires suggest applications in fields such as nonlinear plasmonics, near-field scanning optical microscope tips, cylindrical polarizers, optical sensing and telecommunications. (C) 2011 Optical Society of America

Femtosecond Nonlinear Fiber Optics in the Ionization Regime

P. Hoelzer, W. Chang, J. C. Travers, A. Nazarkin, J. Nold, N. Y. Joly, M. F. Saleh, F. Biancalana, P. St. J. Russell

By using a gas-filled kagome-style photonic crystal fiber, nonlinear fiber optics is studied in the regime of optically induced ionization. The fiber offers low anomalous dispersion over a broad bandwidth and low loss. Sequences of blueshifted pulses are emitted when 65 fs, few-microjoule pulses, corresponding to high-order solitons, are launched into the fiber and undergo self-compression. The experimental results are confirmed by numerical simulations which suggest that free-electron densities of similar to 10(17) cm(-3) are achieved at peak intensities of 10(14) W/cm(2) over length scales of several centimeters.

Single-mode hollow-core photonic crystal fiber made from soft glass

X. Jiang, T. G. Euser, A. Abdolvand, F. Babic, F. Tani, N. Y. Joly, J. C. Travers, P. St J. Russell

We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagome-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter similar to 30 mu m) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices. (C) 2011 Optical Society of America

Ultraviolet-enhanced supercontinuum generation in tapered photonic crystal fiber

S. P. Stark, A. Podlipensky, N. Y. Joly, P. St. J. Russell

We investigate numerically and experimentally the propagation of visible sub-50 fs pulses in a tapered small core photonic crystal fiber. The fiber has anomalous dispersion between two closely spaced zero dispersion wavelengths at 509 and 640 nm, and the excitation wavelength was varied within this range. We find that the spectral evolution in the low power regime is dominated by higher-order soliton fission, soliton self-frequency shift, and dispersive wave generation. At higher powers, extremely wide spectral broadening of the input pulse occurs within the first few millimeters of fiber. The wavelength conversion into the blue and red spectral ranges is studied as a function of the input power and excitation wavelength. Conversions into the spectral range 300-470 nm at efficiencies as high as 40% are observed when pumping at 523 nm. (C) 2010 Optical Society of America

Bridging visible and telecom wavelengths with a single-mode broadband photon pair source

C. Soeller, B. Brecht, P. J. Mosley, L. Y. Zang, A. Podlipensky, N. Y. Joly, P. St. J. Russell, C. Silberhorn

We present a spectrally decorrelated photon pair source bridging the visible and telecom wavelength regions. Tailored design and fabrication of a solid-core photonic crystal fiber (PCF) lead to the emission of signal and idler photons into only a single spectral and spatial mode. Thus no narrowband filtering is necessary and the heralded generation of pure photon number states in ultrafast wave packets at telecom wavelengths becomes possible.

Plasmon resonances on gold nanowires directly drawn in a step-index fiber

H. K. Tyagi, H. W. Lee, P. Uebel, M. A. Schmidt, N. Joly, M. Scharrer, P. St. J. Russell

We report the successful production of high-quality gold wires, with diameters down to 260 nm, by direct fiber drawing from a gold-filled fused-silica cane. The stack-and-draw technique makes it straightforward to incorporate a conventional step-index core, adjacent to the gold wire, in the cane. In the drawn fiber, strong coupling of light from the glass core to SPP resonances on the gold wire is observed at specific well-defined wavelengths. Such embedded wires have many potential applications, for example, as nanoscale electrodes, in nonlinear optical plasmonics, and as near-field scanning optical microscope tips. (C) 2010 Optical Society of America

Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber

J. Nold, P. Hoelzer, N. Y. Joly, G. K. L. Wong, A. Nazarkin, A. Podlipensky, M. Scharrer, P. St J. Russell

We report tunable third-harmonic generation (THG) in an Ar-filled hollow-core photonic crystal fiber, pumped by broadband < 2 mu J, 30 fs pulses from an amplified Ti:sapphire laser system. The overall dispersion is precisely controlled by balancing the negative dielectric susceptibility of the waveguide against the positive susceptibility of the gas. We demonstrate THG to a higher-order guided mode and show that the phase-matched UV wavelength is tunable by adjusting the gas pressure. (C) 2010 Optical Society of America

Pulse Splitting in Short Wavelength Seeded Free Electron Lasers

M. Labat, N. Joly, S. Bielawski, C. Szwaj, C. Bruni, M. E. Couprie

We investigate a fundamental limitation occurring in vacuum ultraviolet and extreme ultraviolet seeded free electron lasers (FELs). For a given electron beam and undulator configuration, an increase of the FEL output energy at saturation can be obtained via an increase of the seed pulse duration. We put in evidence a complex spatiotemporal deformation of the amplified pulse, leading ultimately to a pulse splitting effect. Numerical studies of the Colson–Bonifacio FEL equations reveal that slippage length and seed laser pulse wings are core ingredients of the dynamics.

Influence of air-filling fraction on forward Raman-like scattering by transversely trapped acoustic resonances in photonic crystal fibers

A. Brenn, G. S. Wiederhecker, M. S. Kang, H. Hundertmark, N. Joly, P. St. J. Russell

Raman-like forward scattering by acoustic phonons transversely trapped in birefringent silica-air photonic crystal fibers is studied. As the air-filling fraction increases, core-confined acoustic resonances become increasingly apparent at higher frequencies (> 1.1 GHz), while the number of cladding-confined acoustic modes involved in scattering falls. Two main types of scattering are observed: intramodal (scattering to new frequencies within the same optical mode) and intermodal (frequency-shifted scattering to a different optical mode). It is shown that the twofold symmetric microstructure in a birefringent fiber causes strongly polarization-dependent intramodal scattering. Good agreement is obtained between the experimental measurements and numerical solutions of both the acoustic and electromagnetic wave equations by using a full-vectorial finite-element approach. Phononic bandgaps are found to play a significant role at higher air-filling fractions, leading to the appearance of additional bands in the scattering spectrum. (C) 2009 Optical Society of America

Self-oscillations in a cascaded Raman laser made with a highly nonlinear photonic crystal fiber

P. Suret, N. Y. Joly, G. Mélin, S. Randoux

We report the observation of self-oscillations of the output power in a cascaded Raman fiber laser delivering two Stokes components. Our cascaded Raman fiber laser is made with a highly nonlinear photonic crystal fiber and it oscillates within a Perot-Fabry cavity formed by weak Fresnel reflections from the fiber ends. From our experimental and theoretical study, we identify stimulated Raman scattering as being the physical effect dominant in the emergence of unstable behaviors inside the Perot-Fabry cavity. Mechanisms of laser destabilization are thus found to be very different from polarization mechanisms previously identified as being responsible for unstable behaviors in conventional one-stage Raman fiber lasers [14, 15].

Anomalous pulse breakup in small-core photonic crystal fibers

A. Podlipensky, P. Szarniak, N. Y. Joly, P. St. J. Russell

Detailed numerical and experimental studies of propagation of 110 fs laser pulses at 800 am in small-core photonic crystal fibers (gamma = 100 W (1) km(-1)) reveal that pulse breakup occurs in two distinct regimes defined by the input power. At low peak power (soliton order N <= g7) higher-order soliton fission occurs: individual solitons being ejected from the input pulse one after the other and are at-ranged in wavelength and in time by peak power. At higher levels of peak power (N>8), pulse breakup results in ejection of bound soliton pairs and the formation of single solitons that collide during propagation. (C) 2008 Optical Society of America

Grating-free Raman laser using highly nonlinear photonic crystal fiber

S. Randoux, N. Y. Joly, G. Mélin, A. Fleureau, L. Galkovsky, S. Lempereur, P. Suret

We demonstrate a Raman laser made from a grating-free highly-nonlinear photonic crystal fiber. The laser threshold power is lower than 600 mW and laser power characteristics recorded in experiments are accurately described from the usual simplest model dealing only with stationary evolutions of total optical powers [J. Opt. Soc. Am. 69, 803–807 (1979)]. In our theoretical treatment, reflectivity coefficients are fixed parameters, in strong contrast with procedures usually implemented to describe Raman fiber lasers made with fiber Bragg gratings. Experimental investigations of the spectral properties of our grating-free Raman fiber laser evidence that the shape of the Stokes power spectrum remains remarkably Gaussian whatever the incident pump power. Increasing the incident pump power induces a drift of the Stokes wavelength together with a broadening of the Stokes optical spectrum. Investigations on the role of light polarization on laser characteristics show that our grating-free Raman fiber laser behaves as a Raman laser made with a standard polarization maintaining fiber.

Bound soliton pairs in photonic crystal fiber

A. Podlipensky, P. Szarniak, N. Y. Joly, C. G. Poulton, P. St. J. Russell

We demonstrate experimentally the formation and stable propagation of bound soliton pairs in a highly nonlinear photonic crystal fiber. The bound pairs occur at a particular power as the consequence of high-order soliton fission. They propagate over long distances with constant inter-soliton frequency and time separation. During propagation, the soliton self-frequency shift causes the central frequency of the pairs to move towards longer wavelength. The formation and characteristics of the bound soliton pairs are confirmed numerically. We believe this to be the first experimental observation of such bound soliton pairs. (c) 2007 Optical Society of America.

The Q-switching instability in passively mode-locked lasers

Theodore Kolokolnikov, Michel Nizette, Thomas Erneux, Nicolas Joly, Serge Bielawski

A new asymptotic analysis of the equations for a mode-locked laser exhibiting a Q-switching instability is proposed. We determine the Hopf bifurcation point responsible for this instability and discuss its behavior in terms of the laser parameters. Our analysis is valid for all class B lasers exhibiting slowly damped relaxation oscillations which include solid state and semiconductor lasers. In the limit of weak nonlinearities, we recover Haus stability conditions. In the case of strongly nonlinear losses, as for the usual picosecond and femtosecond laser devices, we determine the stability conditions.

Polarization modulation instability in photonic crystal fibers

R. J. Kruhlak, G. K. Wong, J. S. Chen, S. G. Murdoch, R. Leonhardt, J. D. Harvey, N. Y. Joly, J. C. Knight

Polarization modulation instability (PMI) in birefringent photonic crystal fibers has been observed in the normal dispersion regime with a frequency shift of 64 THz between the generated frequencies and the pump frequency. The generated sidebands are orthogonally polarized to the pump. From the observed PMI frequency shift and the measured dispersion, we determined the phase birefringence to be 5.3×10−5 at a pump wavelength of 647.1 nm. This birefringence was used to estimate the PMI gain as a function of pump wavelength. Four-wave mixing experiments in both the normal and the anomalous dispersion regimes generated PMI frequency shifts that show good agreement with the predicted values over a 70 THz range. These results could lead to amplifiers and oscillators based on PMI.

Cross-phase modulation instability in photonic crystal fibers

J. S. Chen, G. K. Wong, S. G. Murdoch, R. J. Kruhlak, R. Leonhardt, J. D. Harvey, N. Y. Joly, J. C. Knight

We report on the observation of cross-phase modulation instability in a highly nonlinear photonic crystal fiber. In such fibers the presence of higher orders of dispersion results in a complex phase-matching curve. We are able to observe this behavior experimentally and obtain excellent agreement between the measured and predicted shifts.

Raman-like light scattering from acoustic phonons in photonic crystal fiber

P Dainese, PSJ Russell, GS Wiederhecker, N Joly, HL Fragnito, V Laude, A Khelif

Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core. (c) 2006 Optical Society of America

Spectrally smooth supercontinuum from 350 nm to 3 mu m in sub-centimeter lengths of soft-glass photonic crystal fibers.

FG Omenetto, NA Wolchover, MR Wehner, M Ross, A Efimov, AJ Taylor, VVRK Kumar, AK George, JC Knight, et al.

The conversion of light fields in photonic crystal fibers ( PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced. (c) 2006 Optical Society of America.

Stimulated Brillouin scattering from multi-GHz-guided acoustic phonons in nanostructured photonic crystal fibres

P. Dainese, P. St. J. Russell, N. Joly, J. C. Knight, G. S. Wiederhecker, H. L. Fragnito, V. Laude, A. Khelif

Wavelength-scale periodic microstructuring dramatically alters the optical properties of materials. An example is glass photonic crystal fibre(1) ( PCF), which guides light by means of a lattice of hollow micro/nanochannels running axially along its length. In this letter, we explore stimulated Brillouin scattering in PCFs with subwavelength-scale solid silica glass cores. The large refractive-index difference between air and glass allows much tighter confinement of light than is possible in all-solid single-mode glass optical fibres made using conventional techniques. When the silica-air PCF has a core diameter of around 70% of the vacuum wavelength of the launched laser light, we find that the spontaneous Brillouin signal develops a highly unusual multi-peaked spectrum with Stokes frequency shifts in the 10-GHz range. We attribute these peaks to several families of guided acoustic modes each with different proportions of longitudinal and shear strain, strongly localized to the core(2,3). At the same time, the threshold power for stimulated Brillouin scattering(4) increases fivefold. The results show that Brillouin scattering is strongly affected by nanoscale microstructuring, opening new opportunities for controlling light-sound interactions in optical fibres.

Interaction of an Optical Soliton with a Dispersive Wave

A. Efimov, A. V. Yulin, D. V. Skryabin, J. C. Knight, N. Joly, F. G. Omenetto, A. J. Taylor, P. Russell

Scattering of a dispersive wave by optical solitons is studied experimentally in photonic crystal fibers in cases when the soliton and the dispersive wave have either identical or orthogonal polarization states. Observations of new resonant frequencies are reported. The experimental results are compared to numerical simulations and predictions from the recently derived wave vector matching conditions.

Continuous-wave tunable optical parametric generation in a photonic-crystal fiber

G. K. L. Wong, A. Y. H. Chen, S. G. Murdoch, R. Leonhardt, J. D. Harvey, N. Y. Joly, J. C. Knight, W. J. Wadsworth, P. St. J. Russell

Continuous-wave and quasi-cw operation of tunable optical parametric generation has been demonstrated in a photonic-crystal fiber. The frequency shift of the generated sidebands, which arise from modulation instability, depends strongly on the detuning of the pump from the fiber’s zero-dispersion wavelength. Over 30 nm of sideband tunability has been demonstrated using a 300 mW cw pump, and over 185 nm of tunability using a 1.6 W quasi-cw pump. Continuous wave and quasi-cw pumps eliminate the detrimental effects of pump–sideband walk-off. In the absence of walk-off it is the fluctuations in the index profile of the photonic-crystal fiber along its length that limit the tunable sideband range.

Characterization of chromatic dispersion in photonic crystal fibers using scalar modulation instability

G. K. L. Wong, A. Y. H. Chen, S. W. Ha, R. J. Kruhlak, S. G. Murdoch, R. Leonhardt, J. D. Harvey, N. Y. Joly

A simple and accurate method is proposed for characterizing the chromatic dispersion of high air-filling fraction photonic crystal fibers. The method is based upon scalar modulation instability generated by a strong pump wave propagating near the zero-dispersion wavelength. Measuring the modulation instability sideband frequency shifts as a function of wavelength gives a direct measurement of the fiber’s chromatic dispersion over a wide wavelength range. To simplify the dispersion calculation we introduce a simple analytical model of the fiber’s dispersion, and verify its accuracy via a full numerical simulation. Measurements of the chromatic dispersion of two different types of high air-filling fraction photonic crystal fibers are presented.

Linear and nonlinear guidance in an ultralow loss planar glass membrane

N. Y. Joly, T. A. Birks, A. Yulin, J. C. Knight, P. St.J. Russell

We describe the fabrication and characterization of a free-standing silica glass membrane waveguide formed using fiber fabrication processes. The membrane has a thickness of 0.6 mm and a width of 60 mm and is many meters long. The optical attenuation is measured as 0.4 dB⁄m. Such attenuation outperforms that of conventional planar waveguides by several orders of magnitude.

Splice-free interfacing of photonic crystal fibers

S. G. Leon-Saval, T. A. Birks, N. Y. Joly, A. K. George, W. J. Wadsworth, G. Kakarantzas, P. St.J. Russell

We report a new method for making low-loss interfaces between conventional single-mode fibers and photonic crystal fibers (PCFs). Adapted from the fabrication of PCF preforms from stacked tubes and rods, this method avoids the need for splicing and is versatile enough to interface to virtually any type of index-guiding silica PCF. We illustrate the method by forming interfaces to two problematic types of PCF, highly nonlinear and multicore. In particular, we believe this to be the first method capable of individually coupling light into and out of all the cores of a fiber with multiple closely spaced cores, without input or output cross talk.

Intermediate asymptotic evolution and photonic bandgap fiber compression of optical similaritons around 1550 nm

C. Billet, J. M. Dudley, N. Joly, J. C. Knight

We report the complete characterization of the self-similar scaling of parabolic pulse similaritons in an optical fiber amplifier. High dynamic range frequency resolved optical gating allows the direct observation of the evolution of a hyperbolic secant-like input pulse to an asymptotic amplifier similariton, and reveals the presence of intermediate asymptotic wings about the parabolic pulse core. These results are used to optimize additional self-similar propagation in highly-nonlinear fiber and subsequent compression in hollow-core photonic bandgap fiber.

Time-spectrally-resolved ultrafast nonlinear dynamics in small-core photonic crystal fibers: Experiment and modelling

Anatoly Efimov, A. J. Taylor, F. G. Omenetto, A. V. Yulin, N. Y. Joly, F. Biancalana, D. V. Skryabin, J. C. Knight, P. St.J. Russell

Nonlinear dynamics of ultrashort optical pulses in the vicinity of the second zero-dispersion point of a small-core photonic crystal fiber is visualized and studied using cross-correlation frequency-resolved optical gating. New spectral features observed in the experiments match well with recent theoretical predictions of the generation of new frequencies via mixing of solitons and dispersive waves. Power- as well as length-dependent dynamics is obtained showing strong interaction between solitons and dispersive waves, soliton-soliton interaction, soliton stabilization against Raman self-frequency shift and Cherenkov continuum generation.

Competition between spectral splitting and Raman frequency shift in negative-dispersion slope photonic crystal fiber

Nicolas Y. Joly, Fiorenzo G. Omenetto, Anatoly Efimov, Antoinette J. Taylor, Jonathan C. Knight, Philip St.J. Russell

We report on the nonlinear behavior of high air-filling fraction solid-core photonic crystal fibers pumped with ultrashort pulses in the vicinity of a negative-slope zero-crossing of the group velocity dispersion. We observed dramatically different behavior when the pump wavelength lies in the normal or the anomalous dispersion range. When pumping at the zero-dispersion wavelength the combined effects of spectral splitting, self-phase modulation and soliton self-frequency shift result a “comma”-shape of the power-dependant spectra. This spectral feature is explained using a simple model.

Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibres

W. J. Wadsworth, N. Joly, J. C. Knight, T. A. Birks, F. Biancalana, P. St. J. Russell

Photonic crystal fibres exhibiting endlessly single-mode operation and dispersion zero in the range 1040 to 1100 nm are demonstrated. A sub-ns pump source at 1064 nm generates a parametric output at 732 nm with an efficiency of 35%, or parametric gain of 55 dB at 1315 nm. A broad, flat supercontinuum extending from 500 nm to beyond 1750 nm is also demonstrated using the same pump source.

Optical time division multiplexing: a powerful technique for experimental investigations of spatiotemporal dynamics

Nicolas Joly, Dominique Derozier, Igor Razdobreev, Serge Bielawski

We show that fast and complex spatio-temporal dynamics of pulsed lasers or OPOs can be recorded using a simple and low-cost technique. The system is based on the well-known principle of time-division multiplexing currently used in telecommunications, and is composed of a set of fibers, conventional photodiodes, and an oscilloscope. The resolution is typically limited by the ratio between the oscilloscope sampling rate and the pulse repetition rate. We have tested this technique on a femtosecond titanium:sapphire laser. This allowed us to observe in real time the irregular displacement of one-dimensional cuts of the output beam (every 12 ns), a process that has been studied up to now only by indirect means. This device can be potentially extended to non-mode-locked sources.

Self-starting of feedback control in lasers with a tendency to Q-switch

Nicolas Joly, Serge Bielawski

We show that control of Q-switch instabilities using derivative feedback [Phys. Rev. A 47 (1993) 3276] can fail in a range of conditions. This problem appears in particular when the allowed perturbations on the pump power are limited in size. From a numerical bifurcation analysis, it appears that this problem arises from the coexistence of a spurious stable periodic solution together with the desired stabilized state. From this bifurcation analysis, we deduce a protocol that solves the problem, and permits to bring the system to steady-state behavior. It consists in applying the feedback before slowly increasing the pump power. This technique is expected to find applications in the suppression of Q-switch in mode-locked lasers.

Suppression of Q-switch instabilities by feedback control in passively mode-locked lasers

Nicolas Joly, Serge Bielawski