Dynamic Brillouin cooling for continuous optomechanical systems

Changlong Zhu, Birgit Stiller

Up until now, ground state cooling using optomechanical interaction is realized in the regime where optical dissipation is higher than mechanical dissipation. Here, we demonstrate that optomechanical ground state cooling in a continuous optomechanical system is possible by using backward Brillouin scattering while mechanical dissipation exceeds optical dissipation which is the common case in optical waveguides. The cooling is achieved in an anti-Stokes backward Brillouin process by modulating the intensity of the optomechanical coupling via a pulsed pump to suppress heating processes in the strong coupling regime. With such dynamic modulation, a significant cooling factor can be achieved, which can be several orders of magnitude lower than for the steady-state case. This modulation scheme can also be applied to Brillouin cooling generated by forward intermodal Brillouin scattering.

Optical Vortex Brillouin Laser

Xinglin Zeng, Philip Russell, Yang Chen, Zheqi Wang, Gordon Wong, Paul Roth, Michael Frosz, Birgit Stiller

Optical vortices, which have been extensively studied over the last decades, offer an additional degree of freedom useful in many applications, such as optical tweezers and quantum control. Stimulated Brillouin scattering (SBS), providing a narrow linewidth and a strong nonlinear response, has been used to realize quasi-continuous wave lasers. Here, stable oscillation of optical vortices and acoustic modes in a Brillouin laser based on chiral photonic crystal fiber (PCF) is reported, which robustly supports helical Bloch modes (HBMs) that carry circularly polarized optical vortex and display circular birefringence. A narrow-linewidth Brillouin fiber laser that stably emits 1st- and 2nd-order vortex-carrying HBMs is implemented. Angular momentum conservation selection rules dictate that pump and backward Brillouin signals have opposite topological charge and spin. Additionally, it is shown that when the chiral PCF is placed within a laser ring cavity, the linewidth-narrowing associated with lasing permits the peak of the Brillouin gain that corresponds to acoustic mode to be measured with resolution of 10 kHz and accuracy of 520 kHz. The results pave the way to a new generation of vortex-carrying SBS systems with applications in optical tweezers, quantum information processing, and vortex-carrying nonreciprocal systems.

Quantum coherent control in pulsed waveguide optomechanics

Junyin Zhang, Changlong Zhu, Christian Wolff, Birgit Stiller

Coherent control of traveling acoustic excitations in a waveguide system is an interesting way to manipulate and transduce classical and quantum information. So far, these interactions, often based on optomechanical resonators or Brillouin scattering, have been studied in the steady-state regime using continuous waves. However, waveguide experiments are often based on optical pump pulses, which require treatment in a dynamic framework. In this paper, we present an effective Hamiltonian formalism in the dynamic regime using optical pulses that links waveguide optomechanics and cavity optomechanics, which can be used in the classical and quantum regime including quantum noise. Based on our formalism, a closed solution for coupled-mode equation under the undepleted assumption is provided and we found that the strong coupling regime is already accessible in current Brillouin waveguides by using pulses. We further investigate several possible experiments within waveguide optomechanics, including Brillouin-based coherent transfer, Brillouin cooling, and optoacoustic entanglement.

Nonreciprocal vortex isolator via topology-selective stimulated Brillouin scattering

Xinglin Zeng, Philip Russell, Christian Wolff , Michael Frosz, Gordon Wong, Birgit Stiller

Optical nonreciprocity, which breaks the symmetry between forward and backward propagating optical waves, has become vital in photonic systems and enables many key applications. So far, all the existing nonreciprocal systems are implemented for linearly or randomly polarized fundamental modes. Optical vortex modes, with wavefronts that spiral around the central axis of propagation, have been extensively studied over the past decades and offer an additional degree of freedom useful in many applications. Here, we report a light-driven nonreciprocal isolation system for optical vortex modes based on topology-selective stimulated Brillouin scattering (SBS) in chiral photonic crystal fiber. The device can be reconfigured as an amplifier or an isolator by adjusting the frequency of the control signal. The experimental results show vortex isolation of 22 decibels (dB), which is at the state of the art in fundamental mode isolators using SBS. This device may find applications in optical communications, fiber lasers, quantum information processing, and optical tweezers.

Stimulated Brillouin scattering in chiral photonic crystal fiber

Xinglin Zeng, Wenbin He, Michael Frosz, Andreas Geilen, Paul Roth, Gordon Wong, Philip Russell, Birgit Stiller

Stimulated Brillouin scattering (SBS) has many applications; for example, in sensing, microwave photonics, and signal processing. Here, we report the first experimental study of SBS in chiral photonic crystal fiber (PCF), which displays optical activity and robustly maintains circular polarization states against external perturbations. As a result, circularly polarized pump light is cleanly backscattered into a Stokes signal with the orthogonal circular polarization state, as is required by angular momentum conservation. By comparison, untwisted PCF generates a Stokes signal with an unpredictable polarization state, owing to its high sensitivity to external perturbations. We use chiral PCF to realize a circularly polarized continuous-wave Brillouin laser. The results pave the way for a new generation of stable circularly polarized SBS systems with applications in quantum manipulation, optical tweezers, optical gyroscopes, and fiber sensors.

Picosecond acoustic dynamics in stimulated Brillouin scattering

Johannes Piotrowski, Mikołaj K Schmidt, Birgit Stiller, Christopher G. Poulton, Michael Steel

Recent experiments demonstrating storage of optical pulses in acoustic phonons via stimulated Brillouin scattering raise questions about the spectral and temporal capacities of such protocols and the limitations of the theoretical frameworks routinely used to describe them. We consider the dynamics of photon-phonon scattering induced by optical pulses with temporal widths comparable to the period of acoustic oscillations. We revisit the widely adopted classical formalism of coupled modes and demonstrate its breakdown. We use a simple extension to the formulation and find potentially measurable consequences in the dynamics of Brillouin experiments involving ultrashort pulses. (C) 2021 Optical Society of America

Brillouin scattering - theory and experiment: tutorial

C. Wolff, M.J.A. Smith, Birgit Stiller, C. G. Poulton

Brillouin scattering is an important and interesting nonlinear effect involving the interaction between optical and acoustic fields in optical waveguides. It is increasingly useful in the field of photonics, where it supplies a tunable ultra-narrow linewidth response that can be used for applications including sensing, filtering, and lasing, as well as the acoustic storage of optical pulses. This tutorial gives an overview of the fundamentals of Brillouin scattering aimed at newcomers to the field, and covers the physics underlying the interaction, the mathematical theory, and setup details of foundational Brillouin experiments. (C) 2021 Optical Society of America

Agile and versatile quantum communication: Signatures and secrets

Stefan Richter, Matthew Thornton, Imran Khan, Hamish Scott, Kevin Jaksch, Ulrich Vogl, Birgit Stiller, Gerd Leuchs, Christoph Marquardt, et al.

Agile cryptography allows for a resource-efficient swap of a cryptographic core in case the security of an underlying classical cryptographic algorithm becomes compromised. Conversely, versatile cryptography allows the user to switch the cryptographic task without requiring any knowledge of its inner workings.<br>In this paper, we suggest how these related principles can be applied to the field of quantum cryptography by explicitly demonstrating two quantum cryptographic protocols, quantum digital signatures (QDS) and quantum secret sharing (QSS), on the same hardware sender and receiver platform. Crucially, the protocols differ only in their classical postprocessing. The system is also suitable for quantum key distribution (QKD) and is highly compatible with deployed telecommunication infrastructures, since it uses standard quadrature phase-shift keying encoding and heterodyne detection. For the first time, QDS protocols are modified to allow for postselection at the receiver, enhancing protocol performance. The cryptographic primitives QDS and QSS are inherently multipartite, and we prove that they are secure not only when a player internal to the task is dishonest, but also when (external) eavesdropping on the quantum channel is allowed. In our first proof-of-principle demonstration of an agile and versatile quantum communication system, the quantum states are distributed at GHz rates. A 1-bit message may be securely signed using our QDS protocols in less than 0.05 ms over a 2-km fiber link and in less than 0.2 s over a 20-km fiber link. To our knowledge, this also marks the first demonstration of a continuous-variable direct QSS protocol.

On-chip broadband nonreciprocal light storage

Moritz Merklein, Birgit Stiller, Khu Vu, Pan Ma, Stephen J. Madden, Benjamin J. Eggleton

Breaking the symmetry between forward- and<br>backward-propagating optical modes is of fundamental scientific interest and enables crucial functionalities, such as isolators, circulators, and duplex communication<br>systems. Although there has been progress in achieving optical isolation on-chip, integrated broadband nonreciprocal signal processing functionalities that enable transmitting<br>and receiving via the same low-loss planar<br>waveguide, without altering the frequency or mode of the signal, remain elusive. Here, we demonstrate a nonreciprocal delay scheme based on the unidirectional transfer of<br>optical data pulses to acoustic waves in a chip-based integration platform. We experimentally demonstrate that this scheme is not impacted by simultaneously counterpropagating optical signals. Furthermore, we achieve a bandwidth more than an order of magnitude broader than<br>the intrinsic optoacoustic linewidth, linear operation for a wide range of signal powers, and importantly, show that this scheme is wavelength preserving and avoids complicated multimode structures.

Coherently refreshed acoustic phonons for extended light storage

Birgit Stiller, Moritz Merklein, Christian Wolff, Khu Vu, Pan Ma, Stephen J. Madden, Benjamin J. Eggleton

Acoustic waves can serve as memory for optical information; however, propagating acoustic phonons in the gigahertz (GHz) regime decay on the nanosecond time scale. Usually this is dominated by intrinsic acoustic loss due to inelastic scattering of the acoustic waves and thermal phonons. Here we show a way to counteract the intrinsic acoustic decay<br>of the phonons in a waveguide by resonantly reinforcing the acoustic wave via synchronized optical pulses. We experimentally demonstrate coherent on-chip storage in amplitude and phase up to 40 ns, 4 times the intrinsic acoustic lifetime in the waveguide. Through theoretical considerations, we anticipate that this concept allows for storage times up to microseconds within realistic experimental limitations while maintaining a GHz bandwidth of the optical signal.

Coherently refreshed acoustic phonons for extended light storage

Birgit Stiller, Moritz Merklein, Christian Wolff, Khu Vu, Pan Ma, Stephen J. Madden, Benjamin J. Eggleton

Acoustic waves can serve as memory for optical information, however, acoustic phonons in the GHz regime decay on the nanosecond timescale. Usually this is dominated by intrinsic acoustic loss due to inelastic scattering of the acoustic waves and thermal phonons. Here we show a way to counteract the intrinsic acoustic decay of the phonons in a waveguide by resonantly reinforcing the acoustic wave via synchronized optical pulses. This scheme overcomes the previous constraints of phonon-based optical signal processing for light storage and memory. We experimentally demonstrate on-chip storage up to 40 ns, four times the intrinsic acoustic lifetime in the waveguide. We confirm the coherence of the scheme by detecting the phase of the delayed optical signal after 40 ns using homodyne detection. Through theoretical considerations we anticipate that this concept allows for storage times up to microseconds within realistic experimental limitations while maintaining a GHz bandwidth of the optical signal. The refreshed phonon-based light storage removes the usual bandwidth-delay product limitations of e.g. slow-light schemes.

Cross talk-free coherent multi-wavelength Brillouin interaction

Birgit Stiller, Moritz Merklein, Khu Vu, Pan Ma, Stephen J. Madden, Christopher G. Poulton, Benjamin J. Eggleton

Stimulated Brillouin scattering drives a coherent interaction between<br> optical signals and acoustic phonons and can be used for storing optical<br> information in acoustic waves. An important consideration arises when<br> multiple optical frequencies are simultaneously employed in the<br> Brillouin process: in this case, the acoustic phonons that are addressed<br> by each optical wavelength can be separated by frequencies far smaller<br> than the acoustic phonon linewidth, potentially leading to cross talk<br> between the optical modes. Here we extend the concept of Brillouin-based<br> light storage to multiple wavelength channels. We experimentally and<br> theoretically show that the accumulated phase mismatch over the length<br> of the spatially extended phonons allows each optical wavelength channel<br> to address a distinct phonon mode, ensuring negligible cross talk and<br> preserving the coherence, even if the phonons overlap in frequency. This<br> phase-mismatch for broad-bandwidth pulses has far-reaching implications<br> allowing dense wavelength multiplexing in Brillouin-based light storage,<br> multifrequency Brillouin sensing and lasing, parallel microwave<br> processing, and quantum photon-phonon interactions. (C) 2019 Author(s).

On-chip correlation-based Brillouin sensing: design, experiment, and simulation

Atiyeh Zarifi, Birgit Stiller, Moritz Merklein, Yang Liu, Blair Morrison, Alvaro Casas-Bedoya, Guanghui Ren, Thach G. Nguyen, Khu Vu, et al.

Wavelength-scale stimulated Brillouin scattering (SBS) waveguides are<br> enabling novel on-chip functionalities. The microscale and nanoscale SBS<br> structures and the complexity of the SBS waveguides require a<br> characterization technique to monitor the local geometry-dependent SBS<br> responses along the waveguide. In this work, we demonstrate an<br> experimental spatial resolution of 500 mu m, which can detect feature<br> sizes down to 200 mu m on a silicon-chalcogenide photonic waveguide<br> using the Brillouin optical correlation domain analysis technique. We<br> provide extensive simulation and analysis of how multiple acoustic and<br> optical modes associated with geometrical variations influence the<br> Brillouin spectrum. (C) 2018 Optical Society of America

High Resolution Brillouin Sensing of Micro-Scale Structures

Atiyeh Zarifi, Birgit Stiller, Moritz Merklein, Benjamin J. Eggleton

Brillouin distributed measurement techniques have been extensively<br> developed for structural health monitoring using fibre optic nerve<br> systems. The recent advancement in the spatial resolution capabilities<br> of correlation-based Brillouin distributed technique have reached the<br> sub-mm regime, making this approach a suitable candidate for monitoring<br> and characterizing integrated photonic devices. The small dimension<br> associated with the short length of these devices-on the order of the<br> cm- and mm-scale-requires high sensitivity detection techniques and<br> sub-mm spatial resolution. In this paper, we provide an overview of the<br> different Brillouin sensing techniques in various micro-scale structures<br> such as photonic crystal fibres, microfibres, and on-chip waveguides. We<br> show how Brillouin sensing is capable of detecting fine transverse<br> geometrical features with the sensitivity of a few nm and also extremely<br> small longitudinal features on the order of a few hundreds of mu m. We<br> focus on the technique of Brillouin optical correlation domain analysis<br> (BOCDA), which enables such high spatial resolution for mapping the<br> opto-acoustic responses of micro-scale waveguides.

On-chip multi-stage optical delay based on cascaded Brillouin light storage

Birgit Stiller, Moritz Merklein, Christian Wolff, Khu Vu, Pan Ma, Christopher G. Poulton, Stephen J. Madden, Benjamin J. Eggleton

Storing and delaying optical signals plays a crucial role in data<br> centers, phased array antennas, communication, and future computing<br> architectures. Here, we show a delay scheme based on cascaded Brillouin<br> light storage that achieves multi-stage delay at arbitrary positions<br> within a photonic integrated circuit. Importantly these multiple<br> resonant transfers between the optical and acoustic domain are<br> controlled solely via external optical control pulses, allowing<br> cascading of the delay without the need of aligning multiple structural<br> resonances along the optical circuit. (c) 2018 Optical Society of<br> America.

Brillouin spectroscopy of a hybrid silicon-chalcogenide waveguide with geometrical variations

Atiyeh Zarifi, Birgit Stiller, Moritz Merklein, Yang Liu, Blair Morrison, Alvaro Casas-Bedoya, Guanghui Ren, Thach G. Nguyen, Khu Vu, et al.

Recent advances in design and fabrication of photonic-phononic<br> waveguides have enabled stimulated Brillouin scattering in silicon-based<br> platforms such as underetched silicon waveguides and hybrid waveguides.<br> Due to the sophisticated design and, more importantly, high sensitivity<br> of the Brillouin resonances to geometrical variations in micro- and<br> nano-scale structures, it is necessary to have access to the localized<br> opto-acoustic response along those waveguides to monitor their<br> uniformity and maximize their interaction strength. In this Letter, we<br> design and fabricate photonic-phononic waveguides with a deliberate<br> width variation on a hybrid silicon-dialcogenide photonic chip and<br> confirm the effect of the geometrical variation on the localized<br> Brillouin response using a distributed Brillouin measurement. (C) 2018<br> Optical Society of America

Brillouin-based light storage and delay techniques

Moritz Merklein, Birgit Stiller, Benjamin J. Eggleton

Storing or delaying optical signals is of key interest for fundamental research and applications in radar, microwave signal processing, quantum information as well as optical communication systems. Delay systems based on stimulated Brillouin scattering (SBS), a coherent interaction between optical and acoustic waves can be implemented in optical fiber or chip-scale platforms, operate at room temperature and can be fully controlled via the optical pump wave. In this article, we review delay and storage techniques based on SBS. We present SBS slow-light, fiber delay lines based on Brillouin dynamic gratings, Brillouin quasi-light storage and Brillouin-based light storage via photon-phonon conversion and compare their performance. In this process, we also include an overview of different platforms used for Brillouin light storage and delay schemes, such as optical fiber, planar soft-glass waveguides, and silica fiber-tip resonators.

Highly localized distributed Brillouin scattering response in a photonic integrated circuit

Atiyeh Zarifi, Birgit Stiller, Moritz Merklein, Neuton Li, Khu Vu, Duk-Yong Choi, Pan Ma, Stephen J. Madden, Benjamin J. Eggleton

The interaction of optical and acoustic waves via stimulated Brillouin<br> scattering (SBS) has recently reached on-chip platforms, which has<br> opened new fields of applications ranging from integrated microwave<br> photonics and on-chip narrow-linewidth lasers, to phonon-based optical<br> delay and signal processing schemes. Since SBS is an effect that scales<br> exponentially with interaction length, on-chip implementation on a short<br> length scale is challenging, requiring carefully designed waveguides<br> with optimized opto-acoustic overlap. In this work, we use the principle<br> of Brillouin optical correlation domain analysis to locally measure the<br> SBS spectrum with high spatial resolution of 800 mu m and perform a<br> distributed measurement of the Brillouin spectrum along a spiral<br> waveguide in a photonic integrated circuit. This approach gives access<br> to local opto-acoustic properties of the waveguides, including the<br> Brillouin frequency shift and linewidth, essential information for the<br> further development of high quality photonic-phononic waveguides for SBS<br> applications. (C) 2018 Author(s).

A chip-integrated coherent photonic-phononic memory

Moritz Merklein, Birgit Stiller, Khu Vu, Stephen J. Madden, Benjamin J. Eggleton

Controlling and manipulating quanta of coherent acoustic<br> vibrations-phonons-in integrated circuits has recently drawn a lot of<br> attention, since phonons can function as unique links between<br> radiofrequency and optical signals, allow access to quantum regimes and<br> offer advanced signal processing capabilities. Recent approaches based<br> on optomechanical resonators have achieved impressive quality factors<br> allowing for storage of optical signals. However, so far these<br> techniques have been limited in bandwidth and are incompatible with<br> multi-wavelength operation. In this work, we experimentally demonstrate<br> a coherent buffer in an integrated planar optical waveguide by<br> transferring the optical information coherently to an acoustic<br> hypersound wave. Optical information is extracted using the reverse<br> process. These hypersound phonons have similar wavelengths as the<br> optical photons but travel at five orders of magnitude lower velocity.<br> We demonstrate the storage of phase and amplitude of optical information<br> with gigahertz bandwidth and show operation at separate wavelengths with<br> negligible cross-talk.

Cascaded forward Brillouin scattering to all Stokes orders

Christian Wolff, Birgit Stiller, Benjamin J. Eggleton, M. J. Steel, C. G. Poulton

Inelastic scattering processes such as Brillouin scattering can often function in cascaded regimes and this is likely to occur in certain integrated opto-acoustic devices. We develop a Hamiltonian formalism for cascaded Brillouin scattering valid for both quantum and classical regimes. By regarding Brillouin scattering as the interaction of a single acoustic envelope and a single optical envelope that covers all Stokes and anti-Stokes orders, we obtain a compact model that is well suited for numerical implementation, extension to include other optical nonlinearities or short pulses, and application in the quantum-optics domain. We then theoretically analyze intra-mode forward Brillouin scattering (FBS) for arbitrary waveguides with and without optical dispersion. In the absence of optical dispersion, we find an exact analytical solution. With a perturbative approach, we furthermore solve the case of weak optical dispersion. Our work leads to several key results on intra-mode FBS. For negligible dispersion, we show that cascaded intra-mode FBS results in a pure phase modulation and discuss how this necessitates specific experimental methods for the observation of fiber-based and integrated FBS. Further, we discuss how the descriptions that have been established in these two classes of waveguides connect to each other and to the broader context of cavity opto-mechanics and Raman scattering. Finally, we draw an unexpected striking similarity between FBS and discrete diffraction phenomena in waveguide arrays, which makes FBS an interesting candidate for future research in quantum-optics.

Quantum-limited measurements of optical signals from a geostationary satellite

Kevin Guenthner, Imran Khan, Dominique Elser, Birgit Stiller, Oemer Bayraktar, Christian R. Mueller, Karen Saucke, Daniel Troendle, Frank Heine, et al.

The measurement of quantum signals that travel through long distances is fundamentally and technologically interesting. We present quantum-limited coherent measurements of optical signals that are sent from a satellite in geostationary Earth orbit to an optical ground station. We bound the excess noise that the quantum states could have acquired after having propagated 38,600 km through Earth's gravitational potential, as well as its turbulent atmosphere. Our results indicate that quantum communication is feasible, in principle, in such a scenario, highlighting the possibility of a global quantum key distribution network for secure communication. (C) 2017 Optical Society of America

Temporal and spectral properties of quantum light

B. Stiller, U. Seyfarth, G. Leuchs, C. Fabre, V. Sandoghdar, N. Treps, L.F. Cugliandolo

Widely tunable, low phase noise microwave source based on a photonic chip

Moritz Merklein, Birgit Stiller, Irina V. Kabakova, Udara S. Mutugala, Khu Vu, Stephen J. Madden, Benjamin J. Eggleton, Radan Slavik

Spectrally pure microwave sources are highly desired for several<br> applications, ranging from wireless communication to next generation<br> radar technology and metrology. Additionally, to generate very pure<br> signals at even higher frequencies, these advanced microwave sources<br> have to be compact, low in weight, and low energy consumption to comply<br> with in-field applications. A hybrid optical and electronic cavity,<br> known as an optoelectronic oscillator (OEO), has the potential to<br> leverage the high bandwidth of optics to generate ultrapure<br> high-frequency microwave signals. Here we present a widely tunable, low<br> phase noise microwave source based on a photonic chip. Using on-chip<br> stimulated Brillouin scattering as a narrow-band active filter allows<br> single-mode OEO operation and ultrawide frequency tunability with no<br> signal degeneration. Furthermore, we show very low close-to-carrier<br> phase noise. This Letter paves the way to a compact, fully integrated<br> pure microwave source. (C) 2016 Optical Society of America

Efficient microwave to optical photon conversion: an electro-optical realization

Alfredo Rueda, Florian Sedlmeir, Michele C. Collodo, Ulrich Vogl, Birgit Stiller, Gerhard Schunk, Dmitry V. Strekalov, Christoph Marquardt, Johannes M. Fink, et al.

Linking classical microwave electrical circuits to the optical telecommunication band is at the core of modern communication. Future quantum information networks will require coherent microwave-to-optical conversion to link electronic quantum processors and memories via low-loss optical telecommunication networks. Efficient conversion can be achieved with electro-optical modulators operating at the single microwave photon level. In the standard electro-optic modulation scheme, this is impossible because both up-and down-converted sidebands are necessarily present. Here, we demonstrate true single-sideband up-or down-conversion in a triply resonant whispering gallery mode resonator by explicitly addressing modes with asymmetric free spectral range. Compared to previous experiments, we show a 3 orders of magnitude improvement of the electro-optical conversion efficiency, reaching 0.1% photon number conversion for a 10 GHz microwave tone at 0.42 mW of optical pump power. The presented scheme is fully compatible with existing superconducting 3D circuit quantum electrodynamics technology and can be used for nonclassical state conversion and communication. Our conversion bandwidth is larger than 1 MHz and is not fundamentally limited. (C) 2016 Optical Society of America

Attacks on practical quantum key distribution systems (and how to prevent them)

Nitin Jain, Birgit Stiller, Imran Khan, Dominique Elser, Christoph Marquardt, Gerd Leuchs

With the emergence of an information society, the idea of protecting sensitive data is steadily gaining importance. Conventional encryption methods may not be sufficient to guarantee data protection in the future. Quantum key distribution (QKD) is an emerging technology that exploits fundamental physical properties to guarantee perfect security in theory. However, it is not easy to ensure in practice that the implementations of QKD systems are exactly in line with the theoretical specifications. Such theory-practice deviations can open loopholes and compromise security. Several such loopholes have been discovered and investigated in the last decade. These activities have motivated the proposal and implementation of appropriate countermeasures, thereby preventing future attacks and enhancing the practical security of QKD. This article introduces the so-called field of quantum hacking by summarising a variety of attacks and their prevention mechanisms.

Classically entangled optical beams for high-speed kinematic sensing

Stefan Berg-Johansen, Falk Toeppel, Birgit Stiller, Peter Banzer, Marco Ornigotti, Elisabeth Giacobino, Gerd Leuchs, Andrea Aiello, Christoph Marquardt

Tracking the kinematics of fast-moving objects is an important diagnostic tool for science and engineering. Here, we demonstrate an approach to positional and directional sensing based on the concept of classical entanglement in vector beams of light [Found. Phys. 28, 361 -374 (1998)]. The measurement principle relies on the intrinsic correlations existing in such beams between transverse spatial modes and polarization. The latter can be determined from intensity measurements with only a few fast photodiodes, greatly outperforming the bandwidth of current CCD/CMOS devices. In this way, our setup enables two-dimensional real-time sensing with temporal resolution in the GHz range. We expect the concept to open up new directions in metrology and sensing. (C) 2015 Optical Society of America

Risk Analysis of Trojan-Horse Attacks on Practical Quantum Key Distribution Systems

Nitin Jain, Birgit Stiller, Imran Khan, Vadim Makarov, Christoph Marquardt, Gerd Leuchs

An eavesdropper Eve may probe a quantum key distribution (QKD) system by sending a bright pulse from the quantum channel into the system and analyzing the back-reflected pulses. Such Trojan-horse attacks can breach the security of the QKD system, if appropriate safeguards are not installed or if they can be fooled by the Eve. We present a risk analysis of such attacks based on extensive spectral measurements, such as transmittance, reflectivity, and detection sensitivity of some critical components used in a typical QKD systems. Our results indicate the existence of wavelength regimes, where the attacker gains considerable advantage as compared to launching an attack at 1550 nm. We also propose countermeasures to reduce the risk of such attacks.

Depolarized guided acoustic wave Brillouin scattering in hollow-core photonic crystal fibers

Wenjia Elser Nee Zhong, Birgit Stiller, Dominique Elser, Bettina Heim, Christoph Marquardt, Gerd Leuchs

By performing quantum-noise-limited optical heterodyne detection, we observe polarization noise in light after propagation through a hollow-core photonic crystal fiber (PCF). We compare the noise spectrum to the one of a standard fiber and find an increase of noise even though the light is mainly transmitted in air in a hollow-core PCF. Combined with our simulation of the acoustic vibrational modes in the hollow-core PCF, we are offering an explanation for the polarization noise with a variation of guided acoustic wave Brillouin scattering (GAWBS). Here, instead of modulating the strain in the fiber core as in a solid core fiber, the acoustic vibrations in hollow-core PCF influence the effective refractive index by modulating the geometry of the photonic crystal structure. This induces polarization noise in the light guided by the photonic crystal structure. (C) 2015 Optical Society of America

Phase regeneration of a star-8QAM signal in a phase-sensitive amplifier with conjugated pumps

B. Stiller, G. Onishchukov, B. Schmauss, G. Leuchs

We demonstrate numerically phase regeneration of a star8QAM signal with two amplitude and four phase states in a phase-sensitive amplifier. In a dual-stage setup, two phase-conjugated idlers are generated in a first stage consisting of two fiber-optic parametric phase-insensitive amplifiers operated in highly nonlinear gain regime. These are used as pumps in the second, phase-sensitive amplification stage which enables efficient phase regeneration via a degenerate four-wave-mixing process. The latter can be operated in two different operation modes: without format conversion or with phase-shifted amplitude levels. In both regimes, we observe high phase-regeneration efficiency for all amplitude levels: the initial phase noise with 0.2 rad standard deviation is reduced by a factor of 5. (C) 2014 Optical Society of America

Observation of acoustically induced modulation instability in a Brillouin photonic crystal fiber laser

Birgit Stiller, Thibaut Sylvestre

We report the experimental observation of self-induced modulation instability (MI) in a Brillouin fiber laser made with a solid-core photonic crystal fiber (PCF) with strong anomalous dispersion. We identify this MI as the result of parametric amplification of optical sidebands generated by guided acoustic modes within the core of the PCF. It is further shown that MI leads to passive harmonic mode locking and to the generation of a picosecond pulse train at a repetition rate of 1.15 GHz which matches the acoustic frequency of the fundamental acoustic mode of the PCF. (C) 2013 Optical Society of America

Long-range and high-resolution distributed Brillouin fiber sensors

T. Sylvestre, M. W. Lee, D. M. Nguyen, Birgit Stiller, J.-C. Beugnot, H. Maillotte, J. Hauden, L. Thévenaz

Brillouin-based distributed optical fiber sensors have been the subject of intense research in recent years because they offer a unique solution for continuous, real-time monitoring in civil engineering and petroleum industry. These sensors provide strain or temperature measurements with meter spatial resolution over several tens of kilometers. In this work we demonstrate two new Brillouin fiber sensors with enhanced performances based on advanced modulation formats from high-speed lightwave communications systems. We first report a Brillouin distributed sensor with enhanced centimeter resolution using a digital phase-shift keying technique. The second one uses a quadrature phase-shift keying modulator as a single-sideband modulator to balance the pump depletion and the fiber loss by the Brillouin gain. Combined with a specially-designed in-line bidirectional Erbium-doped fiber amplifier, we demonstrate that this technique allows for the achievement of long-range distributed sensing over 100 km.

Distributed Brillouin Fiber Sensor With Enhanced Sensitivity Based on Anti-Stokes Single-Sideband Suppressed-Carrier Modulation

Minh Duc Nguyen, Birgit Stiller, Min Won Lee, Jean-Charles Beugnot, Herve Maillotte, Alexandre Mottet, Jerome Hauden, Thibaut Sylvestre

We demonstrate an alternative technique to reduce pump depletion and to<br> improve sensitivity in the long-range Brillouin optical time-domain<br> analysis (BOTDA) sensor. Our BOTDA system uses an anti-Stokes<br> single-sideband suppressed-carrier modulation technique that balances<br> the pump depletion due to fiber loss. With this technique, we show both<br> numerically and experimentally a great improvement compared to the dual<br> sideband standard technique.

Demonstration of polarization pulling using a fiber-optic parametric amplifier

Birgit Stiller, P. Morin, D. M. Nguyen, J. Fatome, S. Pitois, E. Lantz, H. Maillotte, C. R. Menyuk, T. Sylvestre

We report the observation of all-optical polarization pulling of an initially polarization-scrambled signal using parametric amplification in a highly nonlinear optical fiber. Broadband polarization pulling has been achieved both for the signal and idler waves with up to 25 dB gain using the strong polarization sensitivity of parametric amplifiers. We further derive the probability distribution function for the final polarization state, assuming a randomly polarized initial state, and we show that it agrees well with the experiments.

SBS Mitigation in a Microstructured Optical Fiber by Periodically Varying the Core Diameter

Birgit Stiller, Alexandre Kudlinski, Min Won Lee, Geraud Bouwmans, Michael Delque, Jean-Charles Beugnot, Herve Maillotte, Thibaut Sylvestre

In this letter, we experimentally demonstrate a 4-dB increase of the<br> stimulated Brillouin scattering threshold in a microstructured optical<br> fiber. This result is obtained by periodically varying the size of the<br> air-hole structure by only 7% amplitude, while keeping a low attenuation<br> coefficient. The efficiency of this passive technique is verified by use<br> of the Brillouin echoes-distributed sensing technique where the<br> Brillouin frequency-shift oscillation is clearly observed.

Black-light continuum generation in a silica-core photonic crystal fiber

T. Sylvestre, A. R. Ragueh, M. W. Lee, Birgit Stiller, G. Fanjoux, B. Barviau, A. Mussot, A. Kudlinski

We report the observation of a broadband continuum spanning from 350 to 470 nm in the black-light region of the electromagnetic spectrum as a result of picosecond pumping a solid-core silica photonic crystal fiber at 355 nm. This was achieved despite strong absorption and a large normal dispersion of silica glass in the UV. Further investigations reveal that the continuum generation results from the interplay of intermodally phase-matched four-wave mixing and cascaded Raman scattering. We also discuss the main limitations in terms of bandwidth and power due to temporal walk-off, fiber absorption, and the photo darkening effect, and we suggest simple solutions.

Differential Phase-Shift-Keying Technique-Based Brillouin Echo-Distributed Sensing

Min Won Lee, Birgit Stiller, Jerome Hauden, Herve Maillotte, Carole Roch, Luc Thevenaz, Thibaut Sylvestre

In this letter, we experimentally demonstrate Brillouin echoes-based<br> distributed optical fiber sensing with centimeter spatial resolution. It<br> is based on a differential phase-shift-keying technique using a single<br> Mach-Zehnder modulator to generate a pump pulse and a pi-phase-shifted<br> pulse with an easy and accurate adjustment of delay. The results are<br> compared to those obtained in standard Brillouin echo-distributed<br> sensing system with two optical modulators and clearly show a resolution<br> of 5 cm in a spliced segment between two fibers by applying a<br> pi-phase-shifted pulse of 500 ps.

Temperature coefficient of the high-frequency guided acoustic mode in a photonic crystal fiber

Emile Carry, Jean-Charles Beugnot, Birgit Stiller, Min W. Lee, Herve Maillotte, Thibaut Sylvestre

High-frequency guided acoustic Brillouin modes have recently been<br> observed in small-core silica photonic crystal fibers. In this paper, we<br> investigate the temperature dependence of the optical sideband frequency<br> generated by one of these guided acoustic waves. The experimental<br> results show a temperature coefficient of 100 kHz/degrees C at an<br> acoustic resonance frequency of 1.15GHz and are in very good agreement<br> with the theoretical predictions. This coefficient demonstrates a<br> temperature sensitivity 10 times larger than that previously reported in<br> conventional single-mode fibers, which is promising in view of potential<br> applications to optical fiber sensors. (C) 2011 Optical Society of<br> America

Frequency-selective excitation of guided acoustic modes in a photonic crystal fiber

Birgit Stiller, M. Delqué, J.-C. Beugnot, M. W. Lee, G. Mélin, H. Maillotte, V. Laude, T. Sylvestre

We present experimental and numerical results demonstrating the simultaneous frequency-selective excitation of several guided acoustic Brillouin modes in a photonic crystal fiber with a multi-scale structure design. These guided acoustic modes are identified by using a full vector finite-element model to result from elastic radial vibrations confined by the wavelength-scale air-silica microstructure. We further show the strong impact of structural irregularities of the fiber on the frequency and modal shape of these acoustic resonances.

Son et lumière dans les fibres optiques microstructurées

Birgit Stiller

Photonic crystal fiber mapping using Brillouin echoes distributed sensing

Birgit Stiller, S. Foaleng-Mafang, J.-C. Beugnot, M. W. Lee, M. Delqué, A. Kudlinski, L. Thévenaz, H. Maillotte, T. Sylvestre

In this paper we investigate the effect of microstructure irregularities and applied strain on backward Brillouin scattering by comparing two photonic crystal fibers drawn with different parameters in order to minimize diameter and microstructure fluctuations. We fully characterize their Brillouin properties including the gain spectrum and the critical power. Using Brillouin echoes distributed sensing with a high spatial resolution of 30 cm we are able to map the Brillouin frequency shift along the fiber and get an accurate estimation of the microstructure longitudinal fluctuations. Our results reveal a clear-cut difference of longitudinal homogeneity between the two fibers.