Historically, there has been a trade-off in spectroscopic measurements between high spectral resolution, broadband coverage, and acquisition time. Optical frequency combs, initially envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet region, are now commonly used for probing molecular rotational-vibrational transitions throughout broad spectral bands in the mid-infrared with superior accuracy, resolution and speed. Here we demonstrate acquisition of 2.5 million spectral points over the continuous wavelength range of 3.2 to 5.1 microns (frequency span 1200 cm^{-1}, resolution <100 kHz) via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequency-comb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying spectra with gradually shifted comb lines we fully resolved amplitude and phase spectra of molecules with narrow Doppler-broadened lines, such as ν1+ν3 band of carbon disulfide (CS2) and its isotopologues. Also, we were able to observe the chemical oxidation reaction of CS2 by detecting traces of carbonyl sulfide (OCS) via its asymmetric ν3 stretch band.
A chirped-pulse interleaving method is reported for generation of dual optical frequency combs based on electro-optic phase modulators (EOM) in a free-running all-fiber based system. Methods are discussed to easily modify the linear chirp rate and comb resolution by more than three orders of magnitude and to significantly increase the spectral bandwidth coverage. The agility of the technique is shown to both capture complex line shapes and to magnify rapid passage effects in spectroscopic and molecular dynamics studies of CO2. These methods are well-suited for applications in the areas of remo…
We propose a new type of broadband and low repetition rate frequency comb generator which has the shape of an elongated and nanoscale-shallow optical bottle microresonator created at the surface of an optical fiber. The free spectral range (FSR) of the broadband azimuthal eigenfrequency series of this resonator is the exact multiple of the FSR of the dense and narrowband axial series. The effective radius variation of the microresonator is close to a parabola with a nanoscale height which is greater or equal to lambda/2pi*n0 (here lambda is the characteristic radiation wavelength and n0 is the…
We present a monolithic solution to extract efficiently light from terahertz quantum cascade lasers with metal-metal waveguides suitable for broadband frequency comb applications. The design is optimized for a bandwidth of 500 GHz around a center frequency of 2.6 THz. A five-fold increase in total output power is observed compared to standard metal-metal waveguides. The extractor features a single-lobed far-field pattern and increases the frequency comb dynamical range to cover more than 50 % of the laser dynamic range. Frequency comb operation up to a spectral bandwidth of 670 GHz is achieved…
Frequency comb assisted diode laser spectroscopy, employing both the accuracy of an optical frequency comb and the broad wavelength tuning range of a tunable diode laser, has been widely used in many applications. In this letter we present a novel method using cascaded frequency agile diode lasers, which allows extending the measurement bandwidth to 37.4 THz (1355 to 1630 nm) at MHz resolution with scanning speeds above 1 THz/s. It is demonstrated as a useful tool to characterize a broadband spectrum for molecular spectroscopy and in particular it enables to characterize the dispersion of inte…
Development of chip-scale optical frequency comb with the coverage from ultra-violet (UV) to mid-infrared (MIR) wavelength is of great significance. To expand the comb spectrum into the challenging UV region, a material platform with high UV transparency is crucial. In this paper, crystalline aluminum nitride (AlN)-onsapphire film is demonstrated for efficient Kerr frequency comb generation. Near-infrared (NIR) comb with nearly octave-spanning coverage and low parametric threshold is achieved in continuous-wave pumped high-quality-factor AlN microring resonators. The competition between stimul…
We suggest a technique for using off-resonance spectral comb generation to produce broadband frequency modulated, and therefore, amplitude quieted light. Results include closed-form formulae for the amplitudes and phases of all of the spectral components.
We have recently introduced the Vernier-based Direct Frequency Comb Cavity-Enhanced Spectroscopy technique and we present the corresponding formalism for quantitative broadband spectroscopy. We achieve high sensitivity and broadband performance by acquiring spectra covering more than 2000 cm$^{-1}$ around 12600 cm$^{-1}$ (800 nm), resolving the 3$ν$+$δ$ band of water vapor and the entire A-band of oxygen in ambient air. 31 300 independent spectral elements are acquired at the second time scale with an absorption baseline noise of 2$\times$10$^{-8}$ cm$^{-1}$ providing a merit figure of 1,1.10$…
We report on a heterogeneous active region design for terahertz quantum cascade laser based frequency combs. Dynamic range, spectral bandwidth as well as output power have been significantly improved with respect to previous designs. When operating individually the lasers act as a frequency comb up to a spectral bandwidth of 1.1 THz, while in a dispersed regime a bandwidth up to 1.94 THz at a center frequency of 3 THz can be reached. A self-detected dual-comb setup has been used to verify the frequency comb nature of the lasers.
A high-Q silica whispering-gallery mode microresonator is an attractive platform on which to demonstrate a broad and phase-locked Raman comb in various wavelength regimes. Raman combs can be used for applications such as compact pulse laser sources, sensors, optical clocks, and coherence tomography. However, the formation dynamics of a Raman comb has not been well exploited. Here we study the dynamics of the Raman comb formation in silica rod microresonators, which have cavity free spectral ranges in microwave rates. We generated a Raman comb with a smooth spectral envelope and also observed t…
We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 μm using two fiber-based frequency combs phase-locked to each other. This ultra-broad wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6 Hz using the sub-Hz-linewidth fiber combs. The recorded spectrum contains five vibration-rotation bands of C${_2}$H${_2}$, CH${_4}$, and H${_2}$O at different wavelengths across the whole spectrum. The determined transition frequencies of C${_2}$H${_2}$ agree with those from …
We report and characterize low-temperature, plasma-deposited deuterated silicon nitride thin films for nonlinear integrated photonics. With a peak processing temperature less than 300$^\circ$C, it is back-end compatible with pre-processed CMOS substrates. We achieve microresonators with a quality factor of up to $1.6\times 10^6 $ at 1552 nm, and $>1.2\times 10^6$ throughout $λ$ = 1510 -- 1600 nm, without annealing or stress management. We then demonstrate the immediate utility of this platform in nonlinear photonics by generating a 1 THz free spectral range, 900-nm-bandwidth modulation-instabi…
Spectroscopy is the basic tool for studying molecular physics and realizing bio-chemical sensing. However, it is challenging to realize sub-femtometer resolution spectroscopy over broad bandwidth. In this paper, broadband and high-resolution spectroscopy with calibrated optical frequency is demonstrated by bridging the fields of speckle patterns and electro-optic frequency comb (EOFC). A novel wavemeter based on whispering-gallery-mode (WGM) speckles (or WGM barcodes) is proposed to link the frequency of a tunable continuous-wave (CW) laser to an optical reference provided by an ultra-stable l…
Nanophotonic waveguides with sub-wavelength mode confinement and engineered dispersion profiles are an excellent platform for application-tailored nonlinear optical interactions at low pulse energies. Here, we present fully air clad suspended-silicon waveguides for infrared frequency comb generation with optical bandwidth limited only by the silicon transparency. The achieved spectra are lithographically tailored to span 2.1 octaves in the mid-infrared (2.0-8.5 um or 1170--5000 cm-1) when pumped at 3.10 um with 100 pJ pulses. Novel fork-shaped couplers provide efficient input coupling with onl…
We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23°x19°) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser's emission spectrum and current-voltage characteristics are not affected, as well as frequency co…
The quest for extrasolar planets and their characterisation as well as studies of fundamental physics on cosmological scales rely on capabilities of high-resolution astronomical spectroscopy. A central requirement is a precise wavelength calibration of astronomical spectrographs allowing for extraction of subtle wavelength shifts from the spectra of stars and quasars. Here, we present an all-fibre, 400 nm wide near-infrared frequency comb based on electro-optic modulation with 14.5 GHz comb line spacing. Tests on the high-resolution, near-infrared spectrometer GIANO-B show a photon-noise limit…
We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83 THz ($Δf/f=50\%$) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low …
We characterize the frequency response of channel-interleaved photonic analog-to-digital converters (CI-PADCs) theoretically and experimentally. The CI-PADC is composed of a photonic frontend for photonic sampling and an electronic backend for quantization. The photonic frontend includes a photonic sampling pulse generator for directly high-speed sampling and an optical time-division demultiplexer (OTDM) for channel demultiplexing. It is found that the frequency response of the CI-PADC is influenced by both the photonic sampling pulses and the OTDM, of which the combined impact can be characte…
We demonstrate fiber mode-locked dual frequency comb spectroscopy for broadband, high resolution measurements in a rapid compression machine (RCM). We apply an apodization technique to improve the short-term signal-to-noise-ratio (SNR), which enables broadband spectroscopy at combustion-relevant timescales. We measure the absorption on 24345 individual wavelength elements (comb teeth) between 5967 and 6133 cm-1 at 704 microsecond time resolution during a 12-ms compression of a CH4-N2 mixture. We discuss the effect of the apodization technique on the absorption spectra, and apply an identical e…
Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multi-heterodyne spectroscopy using two terahertz quantum cascade laser combs. With just 100 $μ$s of integration time, we achieve peak signal-to-noise ratios exceeding 60 dB and a spectral coverage greater than 250 GHz centered at 2.8 THz. Even with room-temperature detectors we are able to achieve peak signal-to-noise ratios of 50 dB, and as a proof-of-principle we use these…
We report a 92 channel RF channelizer based on a 48.9 GHz integrated micro-comb that operates via soliton crystals, together with a passive high-Q ring resonator that acts as a periodic filter with an optical 3dB bandwidth of 121.4 MHz. We obtain an instant RF bandwidth of 8.08 GHz and 17.55 GHz achieved through temperature tuning. These results represent a major advance to achieving fully integrated photonic RF spectrum channelizers with reduced low complexity, size, and high performance for digital-compatible signal detection and broadband analog signal processing.
Time-resolved spectroscopic temperature measurements of a sealed carbon dioxide sample cell were realized with an optical frequency comb combined with a two-dimensional dispersive spectrometer. A supercontinuum laser source based on an erbium fiber mode-locked laser was employed to generate coherent light around 2000 nm (5000 cm-1). The laser was passed through a 12-cm long cell containing CO2, and the transmitted light was analyzed in a virtually imaged phased array- (VIPA-) based spectrometer. Broadband spectra spanning more than 100 cm-1 with a spectral resolution of roughly 0.075 cm-1 (2.2…
Mid-infrared (mid-IR) spectroscopy provides a way to study structures and dynamics of complicated molecules in condensed phases. Therefore, developing compact and broadband mid-IR spectrometer has been a long-standing challenge. Here, we show that a highly coherent and broadband mid-IR frequency comb can be generated by using an intra-pulse difference-frequency-generation with a train of pulses from a few-cycle Ti:Sapphire oscillator. By tightly focusing the oscillator output beam into a single-pass fan-out-type periodically-poled lithium niobate crystal and tilting the orientation of the crys…
Programmable ultra-broadband photonic chaos platform enabled by microwave-chaos-driven electro-optic frequency combs
State Key Laboratory of Optical Field Manipulation Science and Technology, Institute of Optics and Electronics, Chinese Academy of Sciences（中国科学院光电技术研究所光场操控科学与技术重点实验室）；Research Center on Vector Optical Fields, Institute of Optics and Electronics, Chinese Academy of Sciences（中国科学院光电技术研究所矢量光学场研究中心）；Tianfu Xinglong Lake Laboratory（天府兴隆湖实验室）；College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences（中国科学院大学材料科学与光电技术学院）
In recent years, the need for high-power laser is of great interest for different applications ranging from direct-laser processing, light detection, medicine, and lighting. However, high-power lasers with high intensities give rise to fundamental problems for optical detectors and imaging systems with low threshold damage, which still need reliable solutions. Here, we report and numerically demonstrate a hybrid system that synergically combines a broadband optical power limiter with a transmittance difference between on-state (70^o C) and off-state (25^o C) about 62.5%, and a diffraction-limi…
Program gain and loss for broadband soliton microcombs
State Key Laboratory for Artificial Microstructure and Mesoscopic Physics and Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University（北京大学物理学院人工微结构和介观物理国家重点实验室及纳米光电前沿科学中心）；Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所北京凝聚态物理国家研究中心）；Peking University Yangtze Delta Institute of Optoelectronics（北京大学长三角光学光电研究院）；Collaborative Innovation Center of Extreme Optics, Shanxi University（山西大学极端光学协同创新中心）；Hefei National Laboratory（合肥国家实验室）
We present a chip-scale scanning dual-comb spectroscopy (SDCS) approach for broadband ultrahigh-resolution spectral acquisition. SDCS uses Si3N4 microring resonators that generate two single soliton micro-combs spanning 37 THz (300 nm) on the same chip from a single 1550-nm laser, forming a high-mutual-coherence dual-comb. We realize continuous tuning of the dual-comb system over the entire optical span of 37.5 THz with high precision using integrated microheater-based wavelength trackers. This continuous wavelength tuning is enabled by simultaneous tuning of the laser frequency and the two si…
In a uniform microresonator, the generation of a broadband Kerr frequency comb is triggered by Turing patterns. Here, we study a distinctly different route to initiate the Kerr frequency comb by Faraday ripples. Momentum conservation is ensured by azimuthal modulation of the cavity dispersion. With a good agreement with the theoretical analysis, we demonstrate a multispectral Kerr frequency comb covering telecommunication O, C, L, and 2 μm bands. Comb coherence and absence of a subcomb offset are confirmed by cw heterodyne beat note and amplitude noise spectra measurements. The device can be u…
Suspended thin-film lithium niobate modulator for broadband mid-infrared light modulation and frequency comb generation
University of California, Berkeley（加州大学伯克利分校）；Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California（南加州大学明海电气与计算机工程学院）；PHY research lab, Intel lab（英特尔实验室PHY研究组）；Materials Sciences Division, Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室材料科学部）
Broadband dual-comb spectroscopy has attracted increasing interests due to its unique advantages in high spectral resolution, fast detection, and so on. Although the dual-comb technique is relatively mature in the infrared wavelengths, it is, currently, not commercially capable of practical applications in the terahertz regime due to the lack of high performance broadband terahertz dual-comb sources. In the terahertz frequency range, the electrically pumped quantum cascade laser (QCL) is a suitable candidate for the dual-comb operation. However, free running terahertz QCL dual-comb sources nor…
Uncertainties of refractive and group index in dispersion measurement by spectrally resolved white light interferometry are deeply analyzed. First, the contribution to uncertainty of the different parameters affecting both indices is identified. Afterwards, results are presented for a 1.5 mm thick fused silica sample over a broad spectral range, from 400 to 1000 nm, and the effects that mostly deteriorate the measurement accuracy are established. Finally, the different contributions are quadratically combined to determine the total uncertainty of the two indices.
We report molecular detection using dual frequency-comb spectroscopy with highly coherent broadband (6.6-11.4 {\m}m) long-wavelength infrared (LWIR) combs. The combs were produced via intra-pulse difference frequency generation (IDFG) in ZGP crystals using sub-three-cycle (20 fs) driving pulses from mode-locked Cr:ZnS lasers at the central wavelength of 2.4 {\m}m). Real-time and up to video rate (0.1-12 s per spectrum) acquisition of molecular spectra with some 240,000 comb-mode-resolved data points spaced by 80 MHz and referenced to a Rb clock with a signal-to-noise ratio (SNR) >300 has been …
The realization of geometric phase optical device operating over a broad spectral range is usually confronted with intrinsic limitations depending of the physical process at play. Here we propose to use chiral nematic liquid crystal slabs with helical ordering that varies in three dimensions. Namely, gradient-pitch cholesterics endowed with in-plane space-variant angular positioning of the supramolecular helix. By doing so, we show that the recently introduced Bragg-Berry mirrors [Opt. Lett. {\bf 41}, 3972-3975 (2016)] can be endowed with ultra-broadband spectral range. Experimental demonstrat…
Noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) is a recently developed technique that utilizes phase modulation to obtain immunity to frequency-to-amplitude noise conversion by the cavity modes and yields high absorption sensitivity over a broad spectral range. We describe the principles of the technique and discuss possible comb-cavity matching solutions. We present a theoretical description of NICE-OFCS signals detected with a Fourier transform spectrometer (FTS), and validate the model by comparing it to experimental CO2 spectra around 1575 nm. Our system is ba…
