N. G. Pavlov

Soliton dual frequency combs in crystalline microresonators

We present a novel compact dual-comb source based on a monolithic optical crystalline MgF2 multi-resonator stack. The coherent soliton combs generated in the two microresonators of the stack with the repetition rate of 12.1 GHz and difference of 1.62 MHz provided after heterodyning a 300 MHz wide radio frequency comb. An analogous system can be used for dual-comb spectroscopy, coherent LIDAR applications, and massively parallel optical communications.

Harmonization of chaos into a soliton in Kerr frequency combs

Dissipative Kerr solitons have paved the way to broadband and fully coherent optical frequency combs in microresonators. Here, we demonstrate numerically that slow frequency tuning of the pump laser in conjunction with phase or amplitude modulation corresponding to the free spectral range of the microresonator, provides reliable convergence of an initially excited chaotic comb state to a single dissipative Kerr soliton (DKS) state. The efficiency of this approach depends on both frequency tuning speed and modulation depth. The relevance of the proposed method is confirmed experimentally in a MgF2 microresonator.

Spatial multiplexing of soliton microcombs

Dual-comb interferometry utilizes two optical frequency combs to map the optical field’s spectrum to a radio-frequency signal without using moving parts, allowing improved speed and accuracy. However, the method is compounded by the complexity and demanding stability associated with operating multiple laser frequency combs. To overcome these challenges, we demonstrate simultaneous generation of multiple frequency combs from a single optical microresonator and a single continuous-wave laser. Similar to space-division multiplexing, we generate several dissipative Kerr soliton states—circulating solitonic pulses driven by a continuous-wave laser—in different spatial (or polarization) modes of a MgF2 microresonator. Up to three distinct combs are produced simultaneously, featuring excellent mutual coherence and substantial repetition rate differences, useful for fast acquisition and efficient rejection of soliton intermodulation products. Dual-comb spectroscopy with amplitude and phase retrieval, as well as optical sampling of a breathing soliton, is realized with the free-running system. Compatibility with photonic-integrated resonators could enable the deployment of dual- and triple-comb-based methods to applications where they remained impractical with current technology.Up to three distinct frequency combs are simultaneously generated from an optical microresonator and a continuous-wave laser, enabling the deployment of dual- and triple-comb-based methods to applications unachievable by current technologies.

Self-injection locking of a laser diode to a high-Q WGM microresonator

We present the analysis of the self-injection locking of a single-frequency laser diode to a high-Q whispering gallery mode (WGM) microresonator with Rayleigh backscattering. Simple analytical formulas for the width of the locking band and resulting laser linewidth are derived.

Modeling the whispering gallery microresonator-based optical modulator

We present a theoretical analysis and numerical simulations of an electro-optic double resonant modulator based on interaction of fundamental whispering gallery modes with a radio frequency field in a dielectric microdisk made from electro-optical material. Models of the modulator in two dimensions and three dimensions are developed and compared. Both optical and RF fields are simulated using the finite element method. The magnitude of the effect in such a system may be maximized with an optimum configuration of a microstrip resonator used for radio-frequency coupling.

Highly efficient coupling of crystalline microresonators to integrated photonic waveguides

Crystalline optical whispering gallery mode resonators made from alkaline earth fluorides can achieve exceptionally large optical finesse, and are used in a variety of applications, from frequency stabilization and narrow linewidth lasers, to low-noise microwave generation or soliton Kerr frequency combs. Here we demonstrate an efficient coupling method to resonators of these materials, which employs photonic integrated waveguides on a chip based on silicon nitride. By converting a mode from silicon nitride to a free-hanging silica waveguide on a silicon chip, coupling to a crystalline resonator is achieved with a high extinction, while preserving a quality factor exceeding 200xa0million. This compact, heterogeneous integration of ultra-high Q-factor crystalline resonators with photonic waveguides provides a proof of concept for wafer scale integration and robust, compact packaging for a wide range of applications.

Narrow-linewidth lasing and soliton Kerr microcombs with ordinary laser diodes

Narrow-linewidth lasers and optical frequency combs generated with mode-locked lasers have revolutionized optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators has opened new horizons in academic research and industrial applications. These combs, as was naturally assumed, however, require narrow-linewidth, single-frequency pump lasers. We demonstrate that an ordinary cost-effective broadband Fabry–Pérot laser diode at the hundreds of milliwatts level, self-injection-locked to a microresonator, can be efficiently transformed to a powerful single-frequency, ultra-narrow-linewidth light source with further transformation to a coherent soliton comb oscillator. Our findings pave the way to the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production.A broadband multi-frequency Fabry–Pérot laser diode, when coupled to a high-Q microresonator, can be efficiently transformed to an ~100u2009mW narrow-linewidth single-frequency light source, and subsequently, to a coherent soliton Kerr comb oscillator.

Kerr soliton combs in crystalline microresonators pumped by regular multifrequency diode lasers

Kerr frequency comb generated in ultra-high Q whispering gallery mode (WGM) microresonators is a promising light source for ultra-compact photonic devices due to its potential advantages of low power consumption and possibility of chip integration. We introduce a technique to stabilize and control effective generation of dissipative Kerr solitons (DKS) in nonlinear crystalline microresonators using regular commercial broad spectrum multi-frequency CW laser diodes (1550 nm; P ∼ 200 mW), self-injection locked to magnesium fluoride microresonators (Q > 109).

Kerr soliton combs with regular multifrequency diode lasers

Kerr optical frequency combs in high-Q microresonators [1] are attracting growing interest [23], especially after mode-locking via dissipative Kerr solitons (DKS) has been demonstrated on a variety of platforms [3, 4]. Such combs are a promising source for compact applications due to its potential advantages of low power consumption and possibility of chip integration. A traditional approach to obtaining DKS in microresonators relies on narrow-linewidth tunable lasers for pumping. Independently the same type microresonators could be used for significant line narrowing of diode lasers exploiting resonant Rayleigh backscattering [5] for self-injection locking [6]. Kerr soliton frequency combs have also been demonstrated with self-injection locked diode lasers [7]. Previously for self-injection locking only single frequency stabilized diode lasers were used with either Bragg-grating [6] or distributed feedback configuration [7], having narrow linewidth comparable to the resonance linewidth of the high-Q microresonator. Surprisingly, we found that the initial stabilization is not required for soliton comb generation, and simpler but more powerful diode lasers may be used, and demonstrate a technique to stabilize, generate and control coherent low-noise soliton Kerr combs using commercial broad spectrum multi-frequency CW laser diodes, self-injection-locked to an ultra-high-Q crystalline whispering-gallery-mode microresonator. In this configuration the role of the microresonator is twofold: 1) it selects and narrows the linewidth of the laser via self-injection locking, and 2) soliton Kerr comb is generated in the microresonator. We manufactured a MgF2 resonator, 5 mm in diameter with computer controlled single-point diamond turning machine and polished it with diamond slurries, achieving Q > 109. For pumping, we used free-space laser diodes (Seminex, λ∼1535, 1550 and 1650 nm, spectrum width ∼10 nm, P∼200 mW) coupled to the resonator with a total internal reflection prism. Generation of self-injection locking soliton combs stable for hours (beat note linewidth <1kHz) was observed when the laser current was adjusted [Fig. 1]. By changing current it was possible to select the pumped mode of the resonator thus gradually changing the central frequency of the soliton by 10 nm. In several cases, we observed simultaneous excitation of two solitons with different central frequencies. In this case beat note spectrum demonstrated two narrow lines separated by ∼ 10 MHz distance, corresponding to FSR difference at central frequencies. The diode multimode spectrum (10 nm) was narrowed to single mode line with FWHM of only 5 kHz, comparable to the results achieved with self-injection-locked DFB lasers.

Kerr combs in microresonators: from chaos to solitons and from theory to experiment (Conference Presentation)

Kerr frequency combs in optical passive microresonators promise new breakthrough in photonics. Such combs result from multiple hyper-parametric four-wave mixing processes. The report presents the results of recent theoretical and experimental studies, leading to the development of compact and integrated coherent frequency comb sources.