Optical frequency combs(1-3) provide equidistant frequency markers in the infrared, visible and ultraviolet(4,5), and can be used to link an unknown optical frequency to a radio or microwave frequency reference(6,7). Since their inception, frequency combs have triggered substantial advances in optical frequency metrology and precision measurements(6,7) and in applications such as broadband laser- based gas sensing(8) and molecular fingerprinting(9). Early work generated frequency combs by intra- cavity phase modulation(10,11); subsequently, frequency combs have been generated using the comb- like mode structure of mode- locked lasers, whose repetition rate and carrier envelope phase can be stabilized(12). Here we report a substantially different approach to comb generation, in which equally spaced frequency markers are produced by the interaction between a continuous- wave pump laser of a known frequency with the modes of a monolithic ultra- high- Q microresonator(13) via the Kerr nonlinearity(14,15). The intrinsically broadband nature of parametric gain makes it possible to generate discrete comb modes over a 500- nm- wide span (similar to 70 THz) around 1,550 nm without relying on any external spectral broadening. Optical- heterodyne- based measurements reveal that cascaded parametric interactions give rise to an optical frequency comb, overcoming passive cavity dispersion. The uniformity of the mode spacing has been verified to within a relative experimental precision of 7.3 x 10(-18). In contrast to femtosecond mode- locked lasers(16), this work represents a step towards a monolithic optical frequency comb generator, allowing considerable reduction in size, complexity and power consumption. Moreover, the approach can operate at previously unattainable repetition rates(17), exceeding 100 GHz, which are useful in applications where access to individual comb modes is required, such as optical waveform synthesis(18), high capacity telecommunications or astrophysical spectrometer calibration(19).
Optical frequency comb generation from a monolithic microresonator
P. Del'Haye,
A. Schließer,
O. Arcizet,
T. Wilken,
R. Holzwarth,
T. J. Kippenberg
Quantitative measurements of the vibrational eigenmodes in ultrahigh-Q silica microspheres are reported. The modes are excited via radiation-pressure-induced dynamical backaction of light confined in the optical whispering-gallery modes of the microspheres (i.e., via the parametric oscillation instability). Two families of modes are studied and their frequency dependence on sphere size investigated. The measured frequencies are in good agreement both with Lamb's theory and numerical finite-element simulation and are found to be proportional to the sphere's inverse diameter. In addition, the quality factors of the vibrational modes are studied. (C) 2007 Optical Society of America.
Radiation-pressure-driven vibrational modes in ultrahigh-Q silica microspheres
R. Ma,
Albert Schließer,
Pascal Del'Haye,
A. Dabirian,
Georg Anetsberger,
Tobias J. Kippenberg
