Special issue—Microcavity optical frequency comb technology
2022, 51(5): 20220302.
doi: 10.3788/IRLA20220302
Optical resonators with high quality (Q) factor can restrict light in a small mode volume for a long time, greatly enhancing the interaction between light and matter, and becoming an important component with great potential in integrated optical devices. Focusing on the silicon nitride material platform, which is currently widely used in the field of integrated nonlinear optics, in order to solve the problem of large scattering loss in the large size on-chip silicon nitride microring resonator caused by the stitching error, the surface roughness and other factors, a series of fabrication process improvements were made to improve the quality factor of the large size silicon nitride microring resonator. The results show that the scattering loss of the silicon nitride waveguide can be effectively reduced by thin film redeposition process, and the intrinsic Q of the large size silicon nitride microring resonator with a radius of 560 μm is increased by 26% on average. Thanks to the improved Q of the large size microring resonator, the frequency comb with the repetition rate of 40 GHz is realized in the on-chip silicon nitride microring resonator.
2022, 51(5): 20220270.
doi: 10.3788/IRLA20220270
Chip-scale optical frequency combs based on microresonators have great potentials in spectroscopy, microwave photonics, optical atomic clocks and coherent optical communications. The non-centrosymmetric wurtzite crystal structure of aluminum nitride (AlN) and gallium nitride (GaN) allows them to exhibit both second- and third-order nonlinear optical coefficients, together with wide transparency window and large refractive index contrast against sapphire substrate, making III-nitrides an attractive platform for nonlinear photonics. The basic properties of AlN and GaN microresonators as well as recent advances in III-nitride-based microresonator frequency combs are presented, including broadband frequency comb generation and optical parametric oscillation in AlN microresonators, and soliton microcomb generation in GaN microresonators.
2022, 51(5): 20220381.
doi: 10.3788/IRLA20220381
Optical frequency comb (OFC) is the spectrum structure composed of a set of discrete and equally spaced frequency components, which has been widely used in many areas such as spectroscopy, precision measurement, optical communication and sensing as the natural scale for spectral analysis. According to its generation methods, OFC can be generated in three ways, including mode-locked laser based OFC, Kerr microresonator OFC and electro-optic frequency comb (EOFC). EOFC has been greatly developed because of its advantages including remarkable tunability of frequency spacing, high comb line power, as well as the accessible conversion from microwave to optical wave. However, there are some drawbacks in conventional EOFC generator, for instance, the bulk size and required high power, which limit its further development. As the micro/nanofabrication technology gradually grows, more and more materials are applied into integrated chip-scale optical devices, including Si, Silicon Nitride, Aluminum Nitride, Indium Phosphide, Lithium Niobate and Aluminium Gallium Arsenide. Integrated EOFC possesses the excellent characteristics, such as small volume and low power consumption, which is an important device for optoelectronic integrated chip. The research status of the integrated EOFC is reviewed in this paper. First, the classification of optical frequency comb, as well as detailed content about generation mechanism of EOFC are introduced. Next, the information comprising various material platforms, corresponding devices performance metrics and applications about EOFC is presented. Finally, the future research directions are prospected in view of the existing problems of integrated EOFC.
2022, 51(5): 20220312.
doi: 10.3788/IRLA20220312
Chalcogenide glass integrated microresonators (chalcogenide microresonators) have attracted great attention in nonlinear integrated photonics in recent years because of their high linear refraction index, high nonlinearity coefficient, ultra-wide transmittance window, low thermo-optic coefficient, and precisely regulated dispersion with conventional semiconductor micro-nanofabrication technology. Recently, the researchers from the Sun Yat-sun University developed a novel chalcogenide glass (Ge25Sb10S65) material platform and realized a series of high-quality chalcogenide integrated photonic devices. The progress of integrated soliton microcombs generation and regulation based on chalcogenide microresonators was reviewed. The integrated chalcogenide microresonators with high-quality factors(Q>106) were achieved by a modified nanofabrication process. Furthermore, mode-locked soliton microcombs with a low pump power and a widely tunable Kerr-Raman comb were achieved by precisely controlling the dispersion , respectively.
2022, 51(5): 20220271.
doi: 10.3788/IRLA20220271
Optical solitons are wavepackets that can sustain the shape via a nonlinear refractive index potential well. They exist in a wide range of optical systems spanning optical fibers, fiber lasers and parametric oscillators. Recently, a new type optical solitons have been observed in coherently pumped high-Q microcavities. The observation of microcavity optical solitons provides a well-controlled experimental platform to study soliton physics. Microcavity optical solitons also endow an array of highly stable spectral lines in the frequency domain, which advance the miniaturization of frequency comb systems. These soliton microcombs have been self-reference stabilized and could enable many chip-based applications including optical frequency synthesizers, optical atomic clocks, data transmission, spectrometer and LiDAR in the near future. Here, the fundamental of microcavity optical solitons was introduced, with a special focus on soliton interaction dynamics. The microcavity dual-comb measurement based applications in fast imaging and mid-infrared gas spectroscopy were also discussed.
Microcavity optical frequency comb (also called the microcavity comb), a subversive technology, is an integrated light source produced from a four-wave mixing process in a nonlinear optical microcavity. As a precision device with excellent properties of optical frequency, microcavity combs can be extensively applied in many fields such as molecular spectroscopy, coherent communication, LiDAR, metrology, and lightweight equipment for airborne system. Here, the fabrication of integrated silicon nitride (Si3N4) microcavity optical frequency comb devices was reported. The balance between the stress, thickness and stoichiometry of Si3N4 was well controlled. A reliable method was proposed to fabricate Si3N4 optical film with enough thickness and stoichiometry to meet the requirements of anomalous dispersion and reducing light absorption. The modified technology of Damascene process with microstructures to decline the stress of thick Si3N4 film was developed to reduce defects. Furthermore, the mask via with a 30 nm thick alumina compensation layer was optimized and a practicable etching process was used for fabricating Si3N4 microresonators with sub-15 nm roughness of lateral walls of microring and waveguide. The experimental results show a high quality of Si3N4 microcavity. Additionally, a coherent Kerr optical frequency comb spectrum can be produced with a wide spectral range from 1480 nm to 1640 nm via dual light pumping.
2022, 51(5): 20220291.
doi: 10.3788/IRLA20220291
To cope with the ever-increasing requirements on transmission capacity, spectral utilization, energy efficiency, small volume, and system simplicity, wavelength-division multiplexing (WDM) optical fiber communication systems need more advanced laser sources than those conventional laser modules used today. Kerr optical frequency comb generated in integrated on-chip micro-cavity provides a promising candidate as the next generation WDM laser source, thanks to its advantages including broadband spectrum, large number of comb lines, matched frequency interval with WDM channels, highly stable frequency, low phase noise, compatibility for chip integration, and low-cost volume production. The fundamental physics of Kerr optical frequency comb was reviewed and the fabrication methods of various Kerr optical frequency comb devices were introduced. Moreover, the unique merits of Kerr optical frequency comb were discussed, such as high spectral purity and compatibility for chip integration, which could facilitate WDM optical communication in the scenarios of long-haul coherent transmissions and data center interconnects.
2022, 51(5): 20220311.
doi: 10.3788/IRLA20220311
Self-reference Dissipative Kerr Solitons (DKSs) based on optical microring resonators have a wide range of applications, such as frequency synthesizers, coherent communication, astronomical spectrometer calibration, precision measurements, optical clocks, dual-comb spectroscopy, etc. The directly accessing octave-spanning DKS has been obtained in silicon nitride and lithium niobate microresonators. Here, a simple method that can directly access the octave-spanning DKS in an aluminum nitride (AlN) microring resonator via a single pump was proposed. The TE00 and TE10 modes act as the pump resonance and auxiliary resonance modes, respectively, which had the resonant frequencies close to each other, and the auxiliary mode on red detuning side could effectively balance the thermal drag effect during the formation of soliton. The pump wavelength was tuned slowly to access a stable soliton comb with a bandwidth of 1100-2300 nm and the maximum soliton existence range of 10.4 GHz (83 pm), which was the first time an octave-spanning Kerr soliton had been obtained on the AlN platform. The stable octave-spanning DKS with large soliton accessing window could be obtained in this scheme using a single pump, which was different from other schemes with additional complex controls means and equipments.
2022, 51(5): 20220236.
doi: 10.3788/IRLA20220236
Microwave signals with low phase noise are indispensable in wireless communication, radar, time and frequency metrology, and deep astronomy. Photonic microwave signal generation approaches are promising. They can break through the bottleneck of classical microwave signal generation methods in many aspects, such as carrier frequency, tunability, phase noise, integrability, and power consumption. The early systems of photonic microwave signal generation were complex, the lasers and related equipment were bulky, which restricted their application outside of the laboratories. In recent years, optical frequency combs based on microcavities develop rapidly. With the advantages of microcombs such as low loss, small size, and ultrahigh stability, the application of microwave photonics based on microcombs has attracted extensive attention of researchers and the systems of photonic microwave signal generation have been intensively studied. In this paper, we reviewed the topic of low phase noise photonic microwave generation with microcombs and prospected the challenges and development trend in the future application of the systems of photonic microwave signal based on microcombs at the end.
2022, 51(5): 20220294.
doi: 10.3788/IRLA20220294
Based on ultra-high quality factor(Q) and nonlinear optical microcavities, optical microcombs(microcavity optical frequency comb) have enabled a variety of important applications including high volume optical communications, optical data center, photonic neuromorphic computation and massive parallel LIDAR. Whispering gallery mode (WGM) microcavities stand for an important platform for studying the microcavity optical frequency comb technology, particularly having record ultra-high Q factors as well as the ultra-high finesse. It can realize ultra-narrow linewidth lasers and optical frequency combs, and photonic microwaves for synthesizing ultra-low noise. Here we developed high Q WGM microcavities from a silica (SiO2) rod fused and shaped with the CO2 laser. The quality factor is above 108 with a free spectrum range at the level of 10 GHz. The cavity resonances as well as the coupling ideality have been characterized, where a degradation of Q factors in a humid environment was observed and recovered with a second annealing process. Moreover, Kerr comb generation was demonstrated in such SiO2 microcavities, which at the moment is mostly in a noisy state governed by the modulation instability regime. Yet the footprint of the cavity soliton state was experimentally observed as a “soliton step” signal. The results indicate that a low-noise and fully coherent soliton microcomb is potentially accessible in home developed SiO2 microcavities, and is readily for comb-related applications.
2022, 51(5): 20220335.
doi: 10.3788/IRLA20220335
Optical frequency combs (OFCs) based on optical microcavities have the characteristics of low threshold, wide spectrum, and compact structure, and have important application prospect in the fields of precision measurement, sensing and et al. Therefore, microcavity based OFC has become an international research hotspot in recent years. At present, relevant researches focus on the generation principle and application of mode-locked OFCs in the infrared band. Although the OFCs in the visible light band have special applications in the fields of precision spectroscopy, atomic clocks and biomedicine, the realization of visible light OFCs is extremely challenging. Based on a brief description of the generation principle of OFCs, this paper introduces the main challenges of realizing OFCs in the visible light band, and the current research progress of three implementation schemes, including the use of the second-order and third-order nonlinear effects of materials, the regulation of the geometric dispersion of the microcavity, and the regulation of the dispersion via modal strong coupling effect to generate visible light frequency combs.
