Optical frequency comb generation from a monolithic microresonator

Abstract

Optical frequency combs [1,2,3] provide equidistant frequency markers in the infrared, visible and ultra-violet [4,5] and can link an unknown optical frequency to a radio or microwave frequency reference [6,7]. Since their inception frequency combs have triggered major 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], while to date frequency combs are generated utilizing the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized [12]. Here, we report an entirely novel approach in which equally spaced frequency markers are generated from a continuous wave (CW) pump laser of a known frequency interacting with the modes of a monolithic high-Q microresonator [13] via the Kerr nonlinearity [14,15]. The intrinsically broadband nature of parametric gain enables the generation of discrete comb modes over a 500 nm wide span (≈ 70 THz) around 1550 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 7.3×10 −18 . In contrast to femtosecond mode-locked lasers[16] the present work represents an enabling step towards a monolithic optical frequency comb generator allowing significant reduction in size, cost and power consumption. Moreover, the present approach can operate at previously unattainable repetition rates [17] exceeding 100 GHz which are useful in applications where the access to individual comb modes is required, such as optical waveform synthesis [18], high capacity telecommunications or astrophysical spectrometer calibration [19].Optical microcavities [20] are owing to their long temporal and small spatial light confinement ideally suited for nonlinear frequency conversion, which has led to a dramatic improvement in the threshold of nonlinear optical light conversion [21]. In contrast to stimulated gain, parametric frequency conversion [22] does not involve coupling to a dissipative reservoir, is broadband as it does not rely on atomic or molecular resonances and constitutes a phase sensitive amplification process, making it uniquely suited for tunable frequency conversion. In the case of a material with inversion symmetry -such as silica -the non linear optical effects are dominated by the third order non linearity. The process is based on four-wave mixing among two pump photons (frequency ν P ) with a signal (ν S ) and idler photon (ν I ) and results in the emergence of (phase coherent) signal and idler sidebands from the vacuum fluctuations at the expense of the pump field (cf. Fig.1). The observation of parametric interactions requires two conditions to be satisfied. First momentum conservation...

Frequency comb assisted diode laser spectroscopy for measurement of microcavity dispersion

“…This precision measurement allows to determine the dispersion of optical microcavity modes by exploiting all advantages of the spectroscopy scheme discussed before. Recently, it has been shown that ultra-high-Q microcavities allow to generate frequency combs with a bandwidth of more than 500 nm via four-wave mixing [13,14,15]. The total bandwidth of these combs is limited by the dispersion of the microcavity, which render the resonator modes nonequidistant.…”

“…Taking only the leading terms in (3) we can deduce a rather crude analytical approximation for ℓ which ignores additional dispersion in very flattened spheroids with large a: A more intuitive way to specify the dispersion of a microresonator pertaining to optical frequency comb generation via four-wave mixing [13] is the mismatch between the generated comb modes ν comb and the respective microcavity modes ν ℓ (with ℓ being the angular mode number). Assuming that the central comb mode correponds to a certain microresonator mode at frequency ν ℓ0 , the frequency mismatch is given by…”

Microresonator-based solitons for massively parallel coherent optical communications

“…Microresonator-based Kerr comb sources [19,20] intrinsically offer unique advantages such as small footprint, large number of narrow-linewidth optical carriers, and line spacings of tens of GHz, which can be designed to fit established WDM frequency grids. However, while these advantages were recognized, previous transmission experiments [15] were limited to aggregate line rates of 1.44 Tbit/s due to strong irregularities of the optical spectrum associated with the specific Kerr comb states.…”

Frequency comb generation in a continuously pumped optical parametric oscillator

“…In order to generate optical frequency combs (OFCs), the use of passive nonlinear cavities has been shown to represent an attractive alternative to traditional techniques based on femtosecond mode-locked lasers. Indeed, comb generation has been demonstrated to occur in continuosly pumped resonators with third-or second-order nonlinearities [1,2]. In cavities dominated by the second-order nonlinerity, OFCs can be generated when the nonlinear crystal is phase-matched for second harmonic generation and the cavity is pumped above the threshold of the so-called internally-pumped optical parametric oscillation [2,3].…”