Mid-infrared Frequency Comb Generation Research Articles

Kerr nonlinearity and group velocity dispersion of InGaAs/InP and GaAsSb/InP waveguides in the mid-infrared

We report measurements of Kerr nonlinearity and group velocity dispersion in In0.53Ga0.47As/InP and GaAs0.51Sb0.49/InP ridge waveguides in the mid-infrared using four-wave mixing at λ ≈ 5 µm. Measured values of Kerr nonlinearity are significantly higher compared to those reported for any other materials systems suitable for building dielectric waveguides with low losses and low group velocity dispersion in the mid-infrared (λ ≈ 3–15 μm). Our measurements establish both In0.53Ga0.47As/InP and GaAs0.51Sb0.49/InP materials as promising platforms for the development of on-chip mid-infrared frequency comb generation and supercontinuum light sources.

High‐Power Mid‐IR Few‐Cycle Frequency Comb from Quadratic Solitons in an Optical Parametric Oscillator

AbstractPowerful and efficient optical frequency combs in the mid‐infrared (MIR) spectral region are highly desirable for a broad range of applications. Despite extensive efforts utilizing various techniques, MIR frequency comb sources are still lacking power, efficiency, or bandwidth for many applications. Here, the generation of an intrinsically locked frequency comb source centered at 4.18 µm from an optical parametric oscillator (OPO) operating in the simulton regime is reported, in which formation of purely quadratic solitons leads to enhanced performance. Advantages of operation in the simulton regime in direct experimental comparisons to the conventional regime are shown, which are also supported by simulation and theory. 565 mW of average power, 900 nm of instantaneous 3 dB bandwidth, 350% slope efficiency, and 44% conversion efficiency are achieved, a performance that is superior to previous OPO demonstrations and other sources in this wavelength range. Here, a new avenue toward MIR frequency comb generation with high power and efficiency is opened, and the great potential of soliton generation based on quadratic nonlinearity in the MIR spectral region is suggested.

Performance evaluation of silicon-chip-based mid-infrared Kerr optical frequency combs with ridge cross section

We present a complementary metal-oxide-semiconductor (CMOS) compatible platform for on-chip frequency comb generation in the mid-infrared region based on a silicon-on-insulator (SOI) microring resonator with ridge cross section. Flat dispersion tailoring is performed with dispersion variation of 4.04 × 10−6 ps/nm/km by adjusting the geometry parameter and the low-loss SOI microring resonator with total quality factor (Q) up to 106 can be realized at wavelengths from 3.3 to 3.6 µm. Furthermore, the thresholdless frequency combs consisting of 50 comb lines spanning from 3.1 to 4.0 µm (over 900 nm) can be realized using SOI microring resonator with 50 mW pump power. Besides, the study shows that the frequency interval of the comb is related to the selection of the dual-pumped wavelength. The influences of the coupling coefficient and the radius of microring on the bandwidth of mid-infrared OFC are also investigated numerically which shows that remarkable enhancement of mid-infrared OFC bandwidth can reach 449 nm when the coupling coefficient varies from 0.012 to 0.05. This research work is instructive for realizing highly integrated photonics and achieving the experimental generation of mid-infrared optical frequency combs, which could enable substantial progress in spectroscopy applications.

Versatile optical frequency combs based on multi-seeded femtosecond optical parametric generation.

This study proposes and demonstrates a versatile method for near- and mid-infrared optical frequency comb generation using multi-seeded femtosecond optical parametric generation. The method allows one to divide the repetition rate by an arbitrarily large integer factor, freely tune the offset frequency, and adjust the common phase offset of the comb modes. Since all possible degrees of freedom are adjustable, the proposed method manifests itself as versatile optical frequency synthesis.

Mid-infrared frequency comb with 25 pJ threshold via CW-seeded optical parametric generation in nonlinear waveguide.

We demonstrate efficient generation of mid-infrared frequency combs based on continuous-wave-seeded femtosecond optical parametric generation in nonlinear waveguides. Conversion of the near-infrared pump to signal and idler light takes place with very high efficiency (74%), and the threshold (25 pJ for 100 fs pulses) is over 300 times lower than in bulk analogs. Relative intensity noise of the mid-infrared comb is exceptionally low, below 5×10-5 (integrated from 10 Hz to 2 MHz). Furthermore, the mid-infrared bandwidth can be increased by driving the process with a broadband pump obtained via supercontinuum generation.

Design of all-solid fluorotellurite fibers with near-zero-flattened dispersion profiles for mid-infrared optical frequency comb generation

All-solid fluorotellurite fibers with near-zero-flattened dispersion profiles in the wavelength range of 1840–2140 nm are designed for mid-infrared optical frequency comb (OFC) generation. The fiber core materials are TeO2-BaF2-Y2O3 (TBY) glasses. The fiber cladding materials are fluoroaluminate and TBY glasses. By controlling the core diameter and the thickness of the thin annular fluoroaluminate or TBY glass, the fiber with the chromatic dispersion value between −0.3 and 0.32 ps nm km−1 in the wavelength range of 1840–2140 nm is achieved. Furthermore, we perform numerical simulations on mid-infrared OFC generation via four-wave mixing in the designed fiber. By using a 1980 nm laser with a pulse width of 1.28 ps and a peak power of 60 W as the pump source, OFC spectrum expanding from 1127 to 4942 nm is obtained in a 2 m long fiber, and the pulse width of the generated OFC is about 25 fs. By varying the repetition rate of the pump source, mid-infrared OFC with tunable mode spacing is generated in the designed fiber.

Simple method for mid-infrared optical frequency comb generation with dynamic offset frequency tuning

We present a simple method for fully stabilized mid-infrared optical frequency comb generation based on single-pass femtosecond optical parametric generation that is seeded by a continuous-wave laser. We have implemented the method in a periodically poled lithium niobate crystal that produces a frequency comb tunable across 3325 nm–4000 nm (2380 cm−1–3030 cm−1). The method generates the mid-infrared (idler) comb with known and stabilized Carrier-Envelope Offset (CEO) frequency without the need to directly detect it. The idler CEO is continuously tunable for almost half of the repetition rate and can be modulated. Together with the high output power (up to 700 mW) and low intensity noise (0.018% integrated in 10 Hz–2 MHz bandwidth), this makes the demonstrated mid-infrared frequency comb promising for many applications such as high-precision molecular spectroscopy, frequency metrology, and high harmonic generation.

Design of on-chip mid-IR frequency comb with ultra-low power pump in near-IR.

Broadband mid-infrared frequency combs are of particular interest to mid-infrared spectroscopy due to their ruler-like precise discrete comb teeth. However, the state-of-the-art mid-infrared frequency combs are usually limited to low integration level and high pump power as a result of the conventional way of mid-infrared frequency comb generation--producing a near-infrared frequency comb first and then convert it to mid-infrared regime through a nonlinear process. Here, we theoretically investigate two integrated designs for generating mid-infrared frequency combs with ultra-low power pump based on the lithium-niobate on insulator (LNOI) platform. Utilizing periodically poled lithium-niobate (PPLN) waveguides and microring electro-optic phase modulators, we switch the conventional order of comb generation and nonlinear conversion. This paradigm shift significantly improves the conversion efficiency of mid-infrared frequency comb generation and obviates the need for femtosecond lasers. Our theoretical results predict that a broadband mid-infrared frequency comb around 4.3 µm with nanowatt-power-level comb teeth can be produced from continuous-wave (CW) inputs whose power is lower than 5 mW with an ultra-high conversion efficiency above 1800 %/W. Our designs of mid-infrared frequency comb have high controllability, flexibility and integration level, enabling the miniaturization of mid-infrared spectrometers.

Interleaved difference-frequency generation for microcomb spectral densification in the mid-infrared

With their compact size and semiconductor-chip-based operation, frequency microcombs can be an invaluable light source for gas spectrcoscopy. However, the generation of mid-infrared (mid-IR) frequency combs with gigahertz line spacing as required to resolve many gas spectra represents a significant challenge for these devices. Here, a technique referred to as interleaved difference-frequency generation (iDFG) is introduced that densifies the spectral line spacing upon conversion of near-IR comb light into the mid-IR light. A soliton microcomb is used as both a comb light source and microwave oscillator in a demonstration, and the spectrum of methane is measured to illustrate how the resulting mid-IR comb avoids spectral undersampling. Beyond demonstration of the iDFG technique, this work represents an important feasibility step towards more compact and potentially chip-based mid-IR gas spectroscopy modules.

Pump condition dependent Kerr frequency comb generation in mid-infrared

We propose and simulate the mid-infrared (MIR) frequency comb generation based on the Kerr nonlinear effect in a silicon microring resonator (MRR). The MRR is formed by the suspended ridge waveguides which are designed to have the anomalous group-velocity dispersion. Taking the thermal effect into consideration, the modified Lugiato–Lefever equation (LLE) is used to simulate the frequency comb generation. The stable dissipative cavity soliton and mode-locked combs in the MIR wavelength range of 2.5 μm–5.2 μm can be obtained under different pump conditions. It is found that the ternary-pump scheme has wider bandwidth and flatter frequency combs as compared with the dual-pump scheme. The lowest total pump power is required for the ternary-pump approach to produce the same comb span in these three pump conditions.

Multi-octave mid-infrared supercontinuum and frequency comb generation in a suspended As2Se3 ridge waveguide.

In this paper, we numerically investigate the mid-infrared supercontinuum (SC) generation in a suspended ${{\rm As}_2}{{\rm Se}_3}$As2Se3 ridge waveguide, which is designed with the two zero-dispersion wavelengths. Simulation results show that when the pump pulses at wavelength 3.3 µm with width of 100 fs and peak power of 900 W are launched into the anomalous dispersion region of the designed waveguide with a length of 0.87 mm, the SC can be generated in the wavelength range from 1.76 to 14.42 µm (more than three octaves), extending deep into the "fingerprint" region. The stability of the generated SC is confirmed by the first-order coherence. Moreover, we demonstrate the performance of the SC-based frequency comb by assuming a 50 pulse pump source at a repetition rate of 100 MHz.

Numerical Simulation of Mid-Infrared Optical Frequency Comb Generation in Chalcogenide As2S3 Microbubble Resonators

Mid-infrared optical frequency comb generation in whispering gallery mode microresonators attracts significant interest. Chalcogenide glass microresonators are good candidates for operating in the mid-infrared range. We present the first theoretical analysis of optical frequency comb generation in As2S3 microbubble resonators in the 3–4 μm range. The regime of dissipative soliton plus dispersive wave generation is simulated numerically in the frame of the Lugiato–Lefever equation. Using microbubble geometry allows controlling of the zero-dispersion wavelength and the obtaining of anomalous dispersion needed for soliton generation at the pump wavelength of 3.5 μm, whereas the zero-dispersion wavelength of the analyzed As2S3 glass is ~4.8 μm. It is shown that, for the optimized characteristics of microbubble resonators, optical frequency combs with a spectral width of more than 700 nm (at the level of −30 dB) can be obtained with the low pump power of 10 mW.

Mid-infrared frequency comb via coherent dispersive wave generation in silicon nitride nanophotonic waveguides

Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Substantial advances in mid-infrared frequency comb generation have been made in recent years based on nonlinear frequency conversion, microresonator Kerr frequency combs, quantum cascade lasers and mode locking regimes. Here we demonstrate a simple, yet effective method for the direct generation of mid-infrared optical frequency combs in the region from ${2.5-4~\mu{\rm m}}$, i.e. ${2500-4000~{\rm cm}^{-1}}$ covering a large fraction of the functional group region, directly from a conventional and compact erbium-fiber-based femtosecond laser in the telecommunication band (i.e. ${1.55~\mu{\rm m}}$). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in large-cross-section and dispersion-engineered silicon nitride (${\rm Si_3N_4}$) waveguides. The long-wavelength dispersive wave, with its position lithographically determined, performs as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such a simple and versatile approach to mid-infrared frequency comb generation is suitable for spectroscopic applications in the first mid-infrared atmospheric window. Moreover, the compactness and simplicity of the approach have the potential to realize compact dual-comb spectrometers. The generated combs have a fine teeth-spacing, making them also suitable for gas phase analysis.

Mid-infrared frequency comb generation via cascaded quadratic nonlinearities in quasi-phase-matched waveguides.

We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-χ(2) nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive wave generation in the 2.5-3μm region and intrapulse difference frequency generation in the 4-5μm region. By engineering the quasi-phase-matched grating profiles, tunable, narrowband MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment-and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.

Broadband mid-infrared frequency comb generation in a Si(3)N(4) microresonator.

We demonstrate broadband frequency comb generation in the mid-infrared (MIR) from 2.3 to 3.5 μm in a Si(3)N(4) microresonator. We engineer the dispersion of the structure in the MIR using a Sellmeier equation we derive from experimental measurements performed on Si(3)N(4) films from the UV to the IR. We use deposition-anneal cycling to decrease absorption losses due to vibrational transitions in the MIR and achieve a Q-factor of 1.0×10(6). To our knowledge, this is the highest Q reported in this wavelength range for any on-chip resonator.

Generation of Kerr combs centered at 4.5 μm in crystalline microresonators pumped with quantum-cascade lasers.

We report on the generation of mid-infrared Kerr frequency combs in high-finesse CaF2 and MgF2 whispering-gallery-mode resonators pumped with continuous-wave room-temperature quantum cascade lasers. The combs were centered at 4.5 μm, the longest wavelength to date. A frequency comb wider than one half of an octave was demonstrated when approximately 20 mW of pump power was coupled to an MgF2 resonator characterized with quality factor exceeding 10(8).

Silicon-chip mid-infrared frequency comb generation.

Optical frequency combs are a revolutionary light source for high-precision spectroscopy because of their narrow linewidths and precise frequency spacing. Generation of such combs in the mid-infrared spectral region (2-20 μm) is important for molecular gas detection owing to the presence of a large number of absorption lines in this wavelength regime. Microresonator-based frequency comb sources can provide a compact and robust platform for comb generation that can operate with relatively low optical powers. However, material and dispersion engineering limitations have prevented the realization of an on-chip integrated mid-infrared microresonator comb source. Here we demonstrate a complementary metal-oxide-semiconductor compatible platform for on-chip comb generation using silicon microresonators, and realize a broadband frequency comb spanning from 2.1 to 3.5 μm. This platform is compact and robust and offers the potential to be versatile for use outside the laboratory environment for applications such as real-time monitoring of atmospheric gas conditions.

Mid-infrared frequency comb generation in coupled silicon microring resonators

We propose and simulate an on-chip mid-IR frequency comb generator based on the four-wave mixing (FWM) effect in a coupled silicon microring resonator. The resonator is formed by suspended ridge waveguides that are designed to have low group-velocity dispersion. A continuous-wave pump laser coupled into the resonator excites other longitudinal modes of the resonator through FWM-based parametric oscillations. In comparison with the device based on a single-ring resonator, we numerically demonstrate that the pump threshold can be dramatically reduced by introducing an auxiliary ring resonator to compensate the phase mismatch between the pump and the generated comb components. The proposed frequency comb generator has a low pump threshold of ~40mW and a high efficiency of ~25% at the pump power of 100mW.

Fractional-length sync-pumped degenerate optical parametric oscillator for 500-MHz 3-μm mid-infrared frequency comb generation

We demonstrate a mid-IR frequency comb centered at 3120 nm with 650-nm (20-THz) bandwidth at a comb-teeth spacing of 500 MHz. The generated comb is based on a compact ring-type synchronously pumped optical parametric oscillator (SPOPO) operating at degeneracy and pumped by a mode-locked Er-doped 1560 nm fiber laser at a repetition rate of 100 MHz. We achieve high-repetition rate by using a fractional-length cavity with a roundtrip length of 60 cm, which is one-fifth of the length dictated by conventional synchronous pumping.

Broadband degenerate OPO for mid-infrared frequency comb generation

We present a new technique suitable for generating broadband phase- and frequency-locked frequency combs in the mid-infrared. Our source is based on a degenerate optical parametric oscillator (OPO) which rigorously both down-converts and augments the spectrum of a pump frequency comb provided by a commercial mode-locked near-IR laser. Low intracavity dispersion, combined with extensive cross-mixing of comb components, results in extremely broad instantaneous mid-IR bandwidths. We achieve an output power of 60 mW and 20 dB bandwidth extending from 2500 to 3800 nm. Among other applications, such a source is well-suited for coherent Fourier-transform spectroscopy in the absorption-rich mid-IR 'molecular fingerprint' region.