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Register a new userSecond-harmonic assisted four-wave mixing in chip-based microresonator frequency comb generation
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Simultaneous Kerr comb formation and second-harmonic generation with on-chip microresonators can greatly facilitate comb self-referencing for optical clocks and frequency metrology. Moreover, the presence of both second- and third-order nonlinearities results in complex cavity dynamics that is of high scientific interest but is still far from well understood. Here, we demonstrate that the interaction between the fundamental and the second-harmonic waves can provide an entirely new way of phase-matching for four-wave mixing in optical microresonators, enabling the generation of optical frequency combs in the normal dispersion regime, under conditions where comb creation is ordinarily prohibited. We derive new coupled time-domain mean-field equations and obtain simulation results showing good qualitative agreement with our experimental observations. Our findings provide a novel way of overcoming the dispersion limit for simultaneous Kerr comb formation and second-harmonic generation, which might prove especially important in the near-visible to visible range where several atomic transitions commonly used for stabilization of optical clocks are located and where the large normal material dispersion is likely to dominate.
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We theoretically study the generation of optical frequency combs and corresponding pulse trains in doubly resonant intracavity second-harmonic generation (SHG). We find that, despite the large temporal walk-off characteristic of realistic cavity systems, the nonlinear dynamics can be accurately and efficiently modelled using a pair of coupled mean-field equations. Through rigorous stability analysis of the systems steady-state continuous wave solutions, we demonstrate that walk-off can give rise to a new, previously unexplored regime of temporal modulation instability (MI). Numerical simulations performed in this regime reveal rich dynamical behaviours, including the emergence of temporal patterns that correspond to coherent optical frequency combs. We also demonstrate that the two coupled equations that govern the doubly resonant cavity behaviour can, under typical conditions, be reduced to a single mean-field equation akin to that describing the dynamics of singly resonant cavity SHG [F. Leo et al., Phys. Rev. Lett. 116, 033901 (2016)]. This reduced approach allows us to derive a simple expression for the MI gain, thus permitting to acquire significant insight into the underlying physics. We anticipate that our work will have wide impact on the study of frequency combs in emerging doubly resonant cavity SHG platforms, including quadratically nonlinear microresonators.
Optical frequency combs provide equidistant frequency markers in the infrared, visible and ultra-violet and can link an unknown optical frequency to a radio or microwave frequency reference. Since their inception frequency combs have triggered major advances in optical frequency metrology and precision measurements and in applications such as broadband laser-based gas sensing8 and molecular fingerprinting. Early work generated frequency combs by intra-cavity phase modulation 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. 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 microresonator13 via the Kerr nonlinearity. The intrinsically broadband nature of parametric gain enables the generation of discrete comb modes over a 500 nm wide span (ca. 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).
Frequency comb generation in microresonators at visible wavelengths has found applications in a variety of areas such as metrology, sensing, and imaging. To achieve Kerr combs based on four-wave mixing in a microresonator, dispersion must be in the anomalous regime. In this work, we demonstrate dispersion engineering in a microbubble resonator (MBR) fabricated by a two-CO$_2$ laser beam technique. By decreasing the wall thickness of the MBR down to 1.4 $mu$m, the zero dispersion wavelength shifts to values shorter than 764 nm, making phase matching possible around 765 nm. With the optical textit{Q}-factor of the MBR modes being greater than $10^7$, four-wave mixing is observed at 765 nm for a pump power of 3 mW. By increasing the pump power, parametric oscillation is achieved, and a frequency comb with 14 comb lines is generated at visible wavelengths.
We demonstrate control and stabilization of an optical frequency comb generated by four-wave mixing in a monolithic microresonator with a mode spacing in the microwave regime (86 GHz). The comb parameters (mode spacing and offset frequency) are controlled via the power and the frequency of the pump laser, which constitutes one of the comb modes. Furthermore, generation of a microwave beat note at the combs mode spacing frequency is presented, enabling direct stabilization to a microwave frequency standard.
Optical-frequency combs enable measurement precision at the 20th digit, and accuracy entirely commensurate with their reference oscillator. A new direction in experiments is the creation of ultracompact frequency combs by way of nonlinear parametric optics in microresonators. We refer to these as microcombs, and here we report a silicon-chip-based microcomb optical clock that phase-coherently converts an optical-frequency reference to a microwave signal. A low-noise comb spectrum with 25 THz span is generated with a 2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium frequency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clocks output is the electronically countable 33 GHz microcomb line spacing, which features an absolute stability better than the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of tools needed for interfacing microcombs to state-of-the-art optical clocks.
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