Optical frequency combs generated by multiple four-wave mixing of two stabilized single-frequency lasers in optical fibers are proposed for use as high precision frequency markers, calibration of astrophysical spectrometers and metrology. Use of highly nonlinear and photonic crystal fibers with very short lengths and small group velocity dispersion, combined with energy and momentum conservation required by the parametric generation, assures negligible phase mismatch between comb frequencies. In contrast to combs from mode-locked lasers or microcavities, the absence of a resonator allows large tuning of the frequency spacing from tens of gigahertz to beyond teraHertz.
We demonstrate a robust and simple method for measurement, stabilization and tuning of the frequency of cw mid-infrared (MIR) lasers, in particular of quantum cascade lasers. The proof of principle is performed with a quantum cascade laser at 5.4 μm, which is upconverted to 1.2 μm by sum-frequency generation in orientation-patterned GaAs with the output of a standard high-power cw 1.5 μm fiber laser. Both the 1.2 μm and the 1.5 μm waves are measured by a standard Er:fiber frequency comb. Frequency measurement at the 100 kHz-level, stabilization to sub-10 kHz level, controlled frequency tuning …
Highly sensitive terahertz (THz) sensors for a myriad of applications are rapidly evolving. A widespread sensor concept is based on the detection of minute resonance frequency shifts due to a targeted specimen in the sensors environment. Therefore, cutting-edge high resolution continuous wave (CW) THz spectrometers provide very powerful tools to investigate the sensors' performances. However, unpredictable yet non negligible frequency drifts common to state-of-the-art CW THz spectrometers limit the sensors' accuracy for ultra-high precision sensing and metrology. Here, we overcome this deficie…
Frequency combs have triggered an impressive evolution of optical metrology across diverse regions of the electromagnetic spectrum, from the ultraviolet to the terahertz frequencies. An unexplored territory, however, remains in the region of vibrational bending modes, mostly due to the lack of single-mode lasers in the long-wavelength (LW) part of the mid-infrared (MIR) spectrum. We fill this gap through a purely MIR-based nonlinear laser source with tunability from 12.1 to 14.8 $μ$m, optical power up to 110 $μ$W, MHz-level linewidth and comb calibration. This enables the first example of bend…
Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. We retrieve the group delay dispersion of the cavity mirror coatings and pure N${_2}$ with 0.1 fs${^2}$ precision and 1 fs${^2}$ accuracy, as well as t…
The operation of a frequency comb at extreme ultraviolet (XUV) wavelengths based on pair-wise amplification and nonlinear upconversion to the 15th harmonic of pulses from a frequency comb laser in the near-infrared range is reported. Following a first account of the experiment [Kandula et al., Phys. Rev. Lett. 105, 063001 (2010)], an extensive review is given of the demonstration that the resulting spectrum at 51 nm is fully phase coherent and can be applied to precision metrology. The pulses are used in a scheme of direct-frequency-comb excitation of helium atoms from the ground state to the …
We combine a tunable continuous-wave optical parametric oscillator and a femtosecond Ti:Sapphire laser frequency comb to provide a phase-coherent bridge between the visible and mid-infrared spectral ranges. As a first demonstration of this new technique we perform a direct frequency comparison between an iodine stabilized Nd:YAG laser at 1064 nm and an infrared methane optical frequency standard at $3.39 μ$m.
Dual comb spectroscopy (DCS) is a broadband technique offering high resolution and fast data acquisition. We describe a hybrid dual comb spectrometer comprising a broadband commercial fiber laser system offering a wide range of sample interrogation, and an actively mode locked semiconductor laser (MLL) having a widely tunable, relatively narrow spectrum. The mutual coherence over 100 seconds has been realized between the two combs. We employed the DCS system to characterize the absorption spectrum of rubidium atoms at 313 K with a high signal to noise ratio. The broadband laser is directly loc…
We propose a novel measurement technique that exploits the high coherent controllability of multi-comb systems, which corresponds to a generalization of the frequency control between the comb sources. In this paper, we particularly focus on the arbitrary relative carrier envelope phase (CEP) control through the relative offset frequency of two combs. We successfully demonstrate polarization-modulated comb generation and its coherent detection in the developed system. The proof-of-principle experiment indicates the potential of multi-comb systems, enabling a rapid, precise, and arbitrary cohere…
We have developed a technique to measure the absolute frequencies of optical transitions by using an evacuated Rb-stabilized ring-cavity resonator as a transfer cavity. We study possible wavelength-dependent errors due to dispersion at the cavity mirrors by measuring the frequency of the same transition in the $D_2$ line of Cs at three cavity lengths. We find no discernable change in values within our error of 30 kHz. Our values are consistent with measurements using the frequency-comb technique and have similar accuracy.
Dual comb spectroscopy (DCS) is a powerful technique for broadband spectroscopy with high precision and fast data acquisition. High-frequency resolution requires long data acquisition times, limiting the temporal resolution in time-resolved measurements. Here we overcome this limitation by engineering the DCS pulse train that interacts with the sample. The measurement is performed in steps where the number of pulses interacting with the sample varies in each step. The DCS spectrum is recorded in each stage, and a multi-dimensional spectrum is generated from which the system time evolution is d…
A laser frequency combs is a broad spectrum composed of equidistant narrow lines. Initially invented for frequency metrology, such combs enable new approaches to spectroscopy over broad spectral bandwidths, of particular relevance to molecules. With optical frequency combs, the performance of existing spectrometers, such as Michelson-based Fourier transform interferometers or crossed dispersers, involving e.g. virtual imaging phase array (VIPA) étalons, is dramatically enhanced. Novel types of instruments, such as dual-comb spectrometers, lead to a new class of devices without moving parts for…
We demonstrate a self-referenced fiber frequency comb that can operate outside the well-controlled optical laboratory. The frequency comb has residual optical linewidths of < 1 Hz, sub-radian residual optical phase noise, and residual pulse-to-pulse timing jitter of 2.4 - 5 fs, when locked to an optical reference. This fully phase-locked frequency comb has been successfully operated in a moving vehicle with 0.5 g peak accelerations and on a shaker table with a sustained 0.5 g rms integrated acceleration, while retaining its optical coherence and 5-fs-level timing jitter. This frequency comb sh…
We demonstrate a high-accuracy dual-comb spectrometer centered at 3.4 μm. The amplitude and phase spectra of the P, Q, and partial R-branch of the methane ν3 band are measured at 25 MHz to 100 MHz point spacing with ~kHz resolution and a signal-to-noise ratio of up to 3500. A fit of the absorbance and phase spectra yield the center frequency of 132 rovibrational lines. The systematic uncertainty is estimated to be 300 kHz, which is 10-3 of the Doppler width and a tenfold improvement over Fourier transform spectroscopy. These data are the first high- accuracy molecular spectra obtained with a d…
Optical frequency combs are an essential tool for precision metrology experiments ranging in application from remote spectroscopic sensing of trace gases to the characterization and comparison of optical atomic clocks for precision time-keeping and searches for physics beyond the standard model. Here we describe the architecture and fully characterize a telecom-band, self-modelocking frequency comb based on a free-space laser with an Er/Yb co-doped glass gain medium. The laser provides a robust and cost-effective alternative to Er:fiber laser based frequency combs, while offering stability and…
We present a directly generated on-chip dual-comb source at THz frequencies. The multi-heterodyne beating signal of two free-running THz quantum cascade laser frequency combs is measured electrically using one of the combs as a detector, fully exploiting the unique characteristics of quantum cascade active regions. Up to 30 modes can be detected corresponding to a spectral bandwidth of 630 GHz, being the available bandwidth of the dual comb configuration. The multi-heterodyne signal is used to investigate the equidistance of the comb modes showing an accuracy of $10^{-12}$ at the carrier frequ…
We present a dual-comb spectrometer based on two passively mode-locked waveguide lasers integrated in a single Er-doped ZBLAN chip. This original design yields two free-running frequency combs having a high level of mutual stability. We developed in parallel a self-correction algorithm that compensates residual relative fluctuations and yields mode-resolved spectra without the help of any reference laser or control system. Fluctuations are extracted directly from the interferograms using the concept of ambiguity function, which leads to a significant simplification of the instrument that will …
We demonstrate the use of dual-comb spectroscopy for isotope ratio measurements. We show that the analysis spectral range of a free-running near-infrared dual-comb spectrometer can be extended to the mid-infrared by difference frequency generation to target specific spectral regions suitable for such measurements, and especially the relative isotopic ratio $δ{}^{13}$C. The measurements performed present a very good repeatability over several days with a standard deviation below 2$\unicode{x2030}$ for a recording time of a few tens of seconds, and the results are compatible with measurements ob…
Nanometer-precision surface metrology of millimeter-size stepped objects using full-cascade-linked synthetic-wavelength digital holography using a line-by-line full-mode-extracted optical frequency comb
Supercontinuum generation using chip-integrated photonic waveguides is a powerful approach for spectrally broadening pulsed laser sources with very low pulse energies and compact form factors. When pumped with a mode-locked laser frequency comb, these waveguides can coherently expand the comb spectrum to more than an octave in bandwidth to enable self-referenced stabilization. However, for applications in frequency metrology and precision spectroscopy, it is desirable to not only support self-referencing, but also to generate low-noise combs with customizable broadband spectra. In this work, w…
Fourier transform spectroscopy based on incoherent light sources is a well-established tool in research fields from molecular spectroscopy and atmospheric monitoring to material science and biophysics. It provides broadband molecular spectra and information about the molecular structure and composition of absorptive media. However, the spectral resolution is fundamentally limited by the maximum delay range (Δ$_{max}$) of the interferometer, so acquisition of high-resolution spectra implies long measurement times and large instrument size. We overcome this limit by combining the Fourier transfo…
Two frequency combs can be used to measure the full complex response of a sample in a configuration which can be alternatively viewed as the equivalent of a dispersive Fourier transform spectrometer, infrared time domain spectrometer, or a multiheterodyne laser spectrometer. This dual comb spectrometer retains the frequency accuracy and resolution inherent to the comb sources. We discuss, in detail, the specific design of our coherent dual-comb spectrometer and demonstrate the potential of this technique by measuring the first overtone vibration of hydrogen cyanide, centered at 194 THz (1545 n…
Optical frequency-comb-based-high-resolution spectrometers offer enormous potential for spectroscopic applications. Although various implementations have been demonstrated, the lack of suitable mid-infrared comb sources has impeded explorations of molecular fingerprinting. Here we present for the first time a frequency-comb Fourier transform spectrometer operating in the 2100-to-3700-cm-1 spectral region that allows fast and simultaneous acquisitions of broadband absorption spectra with up to 0.0056 cm-1 resolution. We demonstrate part-per-billion detection limits in 30 seconds of integration …
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…
As a potent greenhouse gas and ozone depleting agent, nitrous oxide (N$_2$O) plays a critical role in the global climate. Effective mitigation relies on understanding global sources and sinks which can be supported through isotopic analysis. We present a cross-dispersed spectrometer, coupled with a mid-infrared frequency comb, capable of simultaneously monitoring all singly substituted, stable isotopic variants of N$_2$O. Rigorous evaluation of the instrument lineshape function and data treatment are discussed. Laboratory characterization of the spectrometer demonstrates sub-GHz spectral resol…
We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and frequency metrology with quantum cascade lasers (QCLs). We have implemented frequency stabilization of a QCL to an ultra-low expansion (ULE) reference cavity, via upconversion to the near-IR spectral range, at a level of $1\times10^{-13}$. The absolute frequency of the QCL is measured relative to a hydrogen maser, with instability $<1\times10^{-13}$ and inaccuracy $5\times10^{-13}$, using a frequency comb phase-stabilized to an independent ultrastable laser. The QCL linewidth is determined to be 60 Hz, dominated by fibe…
Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optic…
Optical frequency combs have revolutionized the field of frequency metrology within the last decade and have become enabling tools for atomic clocks, gas sensing and astrophysical spectrometer calibration. The rapidly increasing number of applications has heightened interest in more compact comb generators. Optical microresonator based comb generators bear promise in this regard. Critical to their future use as 'frequency markers', is however the absolute frequency stabilization of the optical comb spectrum. A powerful technique for this stabilization is self-referencing, which requires a spec…
The calibration and traceability of high resolution frequency modulated continuous wave (FMCW) ladar sources is a requirement for their use in length and volume metrology. We report the calibration of a FMCW ladar length measurement system by use of spectroscopy of molecular frequency references HCN (C-band) or CO (L-band) to calibrate the chirp rate of the FMCW source. Propagating the stated uncertainties from the molecular calibrations provided by NIST and measurement errors provides an estimated uncertainty of a few ppm for the FMCW system. As a test of this calibration, a displacement meas…
Dual-frequency comb spectroscopy has emerged as a disruptive technique for measuring wide-spanning spectra with high resolution, yielding a particularly powerful technique for sensitive multi-component gas analysis. We present a spectrometer system based on dual electro-optical combs with subsequent conversion to the mid-infrared via tunable difference frequency generation, operating in the range from 3 to 4.7 $μ$m. The simultaneously recorded bandwidth is up to 454(1) GHz and a signal-to-noise ratio of 7.3(2) x 10$^2$ Hz$^{-1/2}$ can be reached. The conversion preserves the coherence of the d…
Non-perturbative and phase-sensitive light-matter interactions have led to the generation of attosecond pulses of light and the control electrical currents on the same timescale. Traditionally, probing these effects via high harmonic generation has involved complicated lasers and apparatuses to generate the few-cycle and high peak power pulses needed to obtain and measure spectra that are sensitive to the phase of the light wave. Instead, we show that nonlinear effects dependent on the carrier-envelope phase can be accessed in solid state crystals with simple low-energy frequency combs that we…
We have developed a dispersive spectrometer using a compact immersion grating for direct frequency comb spectroscopy in the long-wave infrared region of 8-10 μm. A frequency resolution of 463 MHz is achieved, which is the highest reported in this wavelength region with a dispersive direct frequency comb spectrometer. We also demonstrate individual mode-resolved imaging of the frequency comb spectrum by Vernier cavity filtering and apply this to obtain both simple and complex molecular spectra. These results indicate that the immersion grating spectrometer offers the next advancement for sensit…
Dual Comb Spectroscopy proved its versatile capabilities in molecular fingerprinting in different spectral regions, but not yet in the ultraviolet (UV). Unlocking this spectral window would expand fingerprinting to the electronic energy structure of matter.This will access the prime triggers of photo-chemical reactions with unprecedented spectral resolution. In this research article, we discuss the milestones marking the way to the first UV dual comb spectrometer. We present experimental and simulated studies towards UV dual comb spectroscopy, directly applied to planned absorption measurement…
We demonstrate the performance of a dual frequency comb QCL spectrometer for the application of vibrational Stark spectroscopy. Measurements performed on fluorobenzene with the dual-comb spectrometer (DCS) were compared to results obtained using a conventional Fourier transform infrared (FTIR) instrument in terms of spectral response, parameter estimation, and signal-to-noise ratio. The dual-comb spectrometer provided similar qualitative and quantitative data as the FTIR setup in 250 times shorter acquisition time. For fluorobenzene, the DCS measurement resulted in a more precise estimation of…
Acquisition of laser frequency with high resolution under continuous and abrupt tuning conditions is important for sensing, spectroscopy and communications. Here, a single microresonator provides rapid and broad-band measurement of frequencies across the optical C-band with a relative frequency precision comparable to conventional dual frequency comb systems. Dual-locked counter-propagating solitons having slightly different repetition rates are used to implement a Vernier spectrometer. Laser tuning rates as high as 10 THz/s, broadly step-tuned lasers, multi-line laser spectra and also molecul…
In the current study, a quantum-cascade-laser-based dual-comb spectrometer (DCS) was used to paint a detailed picture of a 1.0 ms high-temperature reaction between propyne and oxygen. The DCS interfaced with a shock tube to provide pre-ignition conditions of 1225 K, 2.8 atm, and 2% p-C3H4/18% O2/Ar. The spectrometer consisted of two free-running, non-stabilized frequency combs each emitting at 179 wavelengths between 1174 and 1233 cm-1. A free spectral range, f_r, of 9.86 GHz and a difference in comb spacing, Δf_r, of 5 MHz, enabled a theoretical time resolution of 0.2 us but the data was time…
Fast (sub-second) spectroscopy with high spectral resolution is of vital importance for revealing quantum chemistry kinetics of complex chemical and biological reactions. Fourier transform (FT) spectrometers can achieve high spectral resolution and operate at hundreds of ms time scales in rapid-scan mode. However, the linear translation of a scanning mirror imposes stringent time-resolution limitations to these systems, which makes simultaneous high spectral and temporal resolution impossible. Here, we demonstrate an FT spectrometer whose operational principle is based on continuous rotational…
We demonstrated that an iodine stabilized Distributed Bragg Reflector (DBR) diode based laser system lasing at a wavelength in close proximity to $λ= 633\,$nm could be used as an alternative laser source to the He-Ne lasers in both scientific and industrial metrology. This yields additional advantages besides the optical frequency stability and coherence: inherent traceability, wider optical frequency tuning range, higher output power and high frequency modulation capability. We experimentally investigated the characteristics of the laser source in two major steps: first using a wavelength met…
With the advent of laser frequency combs, coherent light sources that offer equally-spaced sharp lines over a broad spectral bandwidth have become available. One decade after revolutionizing optical frequency metrology, frequency combs hold much promise for significant advances in a growing number of applications including molecular spectroscopy. Despite its intriguing potential for the measurement of molecular spectra spanning tens of nanometers within tens of microseconds at Doppler-limited resolution, the development of dual-comb spectroscopy is hindered by the extremely demanding high-band…
