Dual-comb Laser Research Articles

Rapid-Scan Nonlinear Time-Resolved Spectroscopy over Arbitrary Delay Intervals

Femtosecond dual-comb lasers have revolutionized linear Fourier-domain spectroscopy by offering a rapid motion-free, precise, and accurate measurement mode with easy registration of the combs beat note in the radio frequency domain. Extensions of this technique already found application for nonlinear time-resolved spectroscopy within the energy limit available from sources operating at the full oscillator repetition rate. Here, we present a technique based on time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. This gives the required boost to the pulse energy and provides the flexibility to engineer pairs of arbitrarily delayed wavelength-tunable pulses for pump–probe techniques. Using a dual-channel millijoule amplifier, we demonstrate programmable generation of both extremely short, fs, and extremely long (>ns) interpulse delays. A predetermined arbitrarily chosen interpulse delay can be directly realized in each successive amplifier shot, eliminating the massive waiting time required to alter the delay setting by means of an optomechanical line or an asynchronous scan of 2 free-running oscillators. We confirm the versatility of this delay generation method by measuring χ (2) cross-correlation and χ (3) multicomponent population recovery kinetics.

Efficient pump-probe sampling with a single-cavity dual-comb laser: Application in ultrafast photoacoustics

Ultrafast pump-probe measurements are used to characterize various samples, such as biological cells, bulk, and thin-film structures. However, typical implementations of the pump-probe apparatus are either slow or complex and costly hindering wide deployment. Here we combine a single-cavity dual-comb laser with a simple experimental setup to obtain pump-probe measurements with ultra-high sensitivity, fast acquisition, and high timing precision over long optical delay scan ranges of 12.5 ns that would correspond to a mechanical delay of about 3.75 m. We employ digital signal balancing to obtain shot-noise-limited detection compatible with pump-probe microscopy deployment. Here we demonstrate ultrafast photoacoustics for thin-film sample characterization. We measured a tungsten layer thickness of (700 ± 4) Å with shot-noise-limited detection. Such single-cavity dual-comb lasers can be used for any pump-probe measurements and are especially well-suited for ultrafast photoacoustic studies such as involving ultrasonic echoes, Brillouin oscillations, surface acoustic waves and thermal dynamics.

Dynamic and precise long-distance ranging using a free-running dual-comb laser.

Long-distance ranging is a crucial tool for both industrial and scientific applications. Laser-based distance metrology offers unprecedented precision making it the ideal approach for many deployments. In particular, dual-comb ranging is favorable due to its inherently high precision and sampling rate. To make high-performance long-range dual-comb LiDAR more accessible by reducing both cost and complexity, here we demonstrate a fiber-based dual-comb LiDAR frontend combined with a free-running diode-pumped solid-state dual-comb laser that allows for sub-µm measurement precision while offering a theoretical ambiguity range of more than 200 km. Our system simultaneously measures distance with the role of each comb interchanged, thereby enabling Vernier-based determination of the number of ambiguity ranges. As a proof-of-principle experiment, we measure the distance to a moving target over more than 10 m with sub-µm precision and high update rate, corresponding to a relative precision of 10-7. For a static target at a similar distance, we achieve an instantaneous precision of 0.29 µm with an update time of 1.50 ms. With a longer averaging time of 200 ms, we reach a precision of around 33 nm, which corresponds to a relative precision of about 3·10-9 with a time-of-flight-based approach.

Relative timing jitter in a counterpropagating all-normal dispersion dual-comb fiber laser

The counterpropagating all-normal dispersion (CANDi) fiber laser is an emerging high-energy single-cavity dual-comb laser source. Its relative timing jitter (RTJ), a critical parameter for dual-comb timing precision and spectral resolution, has not been comprehensively investigated. In this paper, we enhance the state-of-the-art CANDi fiber laser pulse energy from 1 nJ to 8 nJ. We then introduce a reference-free RTJ characterization technique that provides shot-to-shot measurement capability at femtosecond precision. The measurement noise floor reaches 1.6 × 10 − 7 f s 2 / H z , and the corresponding integrated measurement precision is only 1.8 fs (1 kHz, 20 MHz). With this characterization tool, we are able to study the physical origin of the CANDi laser’s RTJ in detail. We first verify that the cavity length fluctuation does not contribute to the RTJ. Then we measure the integrated RTJ to be 39 fs (1 kHz, 20 MHz) and identify the pump relative intensity noise (RIN) to be the dominant factor responsible for it. In particular, pump RIN is coupled to the RTJ through the Gordon–Haus effect. Finally, solutions to reduce the free-running CANDi laser’s RTJ are discussed. This work provides a general guideline to improve the performance of compact single-cavity dual-comb systems such as the CANDi laser, benefitting various dual-comb applications.

Spatially multiplexed single-cavity dual-comb laser

Single-cavity dual-comb lasers are a new class of ultrafast lasers that have a wide possible application space including pump–probe sampling, optical ranging, and gas absorption spectroscopy. However, to date, laser cavity multiplexing has usually come with a trade-off in laser performance or relative timing noise suppression. We present a method for multiplexing a single laser cavity to support a pair of noise-correlated modes. These modes share all intracavity components and take a near-common path, but do not overlap on any active elements. We implement the method with an 80-MHz laser delivering more than 2.4 W of average power per comb with sub-140-fs pulses. We reach sub-cycle relative timing jitter of 2.2 fs [20 Hz, 100 kHz]. With this multiplexing technique, we can implement slow feedback on the repetition rate difference Δ f r e p , enabling this quantity to be drift-free, have low jitter, and be adjustable—a key combination for practical applications that was lacking in prior single-cavity dual-comb systems.

Asynchronous vector solitons based dual-comb in a fiber laser mode-locked by GO-COOH SA

We design a stable polarization-multiplexed dual-comb source (DCS) by deriving the effect of the introduced birefringence on the repetition frequency difference (Δfrep). There is no phase-locking device in the DCS. Carboxy-functional graphene oxide saturable absorber (GO-COOH SA) is prepared using the liquid phase exfoliation method. The polarization-maintaining fiber (PMF) introduces large birefringence, which produces asynchronous vector solitons (AVSs) with a stable Δfrep. We record and analyze the Δfrep from 79 Hz to 656 Hz. The interferograms of the dual-comb demonstrate its potential in applications. Meanwhile, we separately vary the length of the PMF and the repetition frequency of the laser to discuss the effect of these two parameters on the Δfrep. After 15 min of testing, DCS displays excellent stability, our work will facilitate the design of dual-comb lasers and enrich the study of AVSs.

Dual-comb thin-disk oscillator

Dual-comb spectroscopy (DCS) normally operates with two independent, relatively low power and actively synchronized laser sources. This hinders the wide adoption for practical implementations and frequency conversion into deep UV and VUV spectral ranges. Here, we report a fully passive, high power dual-comb laser based on thin-disk technology and its application to direct frequency comb spectroscopy. The peak power (1.2 MW) and the average power (15 W) of our Yb:YAG thin-disk dual-comb system are more than one-order-of-magnitude higher than in any previous systems. The scheme allows easy adjustment of the repetition frequency difference during operation. Both combs share all cavity components which leads to an excellent mutual stability. A time-domain signal recorded over 10 ms without any active stabilization was sufficient to resolve individual comb lines after Fourier transformation.

Timing jitter characterization of free-running dual-comb laser with sub-attosecond resolution using optical heterodyne detection.

Pulse trains emitted from dual-comb systems are designed to have low relative timing jitter, making them useful for many optical measurement techniques such as optical ranging and spectroscopy. However, the characterization of low-jitter dual-comb systems is challenging because it requires measurement techniques with high sensitivity. Motivated by this challenge, we developed a technique based on an optical heterodyne detection approach for measuring the relative timing jitter of two pulse trains. The method is suitable for dual-comb systems with essentially any repetition rate difference. Furthermore, the proposed approach allows for continuous and precise tracking of the sampling rate. To demonstrate the technique, we perform a detailed characterization of a single-mode-diode pumped Yb:CaF2 dual-comb laser from a free-running polarization-multiplexed cavity. This new laser produces 115-fs pulses at 160 MHz repetition rate, with 130 mW of average power in each comb. The detection noise floor for the relative timing jitter between the two pulse trains reaches 8.0 × 10-7 fs2/Hz (∼ 896 zs/Hz), and the relative root mean square (rms) timing jitter is 13 fs when integrating from 100Hz to 1MHz. This performance indicates that the demonstrated laser is highly compatible with practical dual-comb spectroscopy, ranging, and sampling applications. Furthermore, our results show that the relative timing noise measurement technique can characterize dual-comb systems operating in free-running mode or with finite repetition rate differences while providing a sub-attosecond resolution, which was not feasible with any other approach before.

Absolute SESAM characterization via polarization-resolved non-collinear equivalent time sampling

Semiconductor saturable absorber mirrors (SESAMs) have enabled a wide variety of modelocked laser systems, which makes measuring their nonlinear properties an important step in laser design. Here, we demonstrate complete characterization of SESAMs using an equivalent time sampling apparatus. The light source is a free-running dual-comb laser, which produces a pair of sub-150-fs modelocked laser outputs at 1051 nm from a single cavity. The average pulse repetition rate is 80.1 MHz, and the full time window is scanned at 240 Hz. Cross-correlation between the beams is used to calibrate the time axis of the measurements, and we use a non-collinear pump-probe geometry on the sample. The measurements enable fast and robust determination of all the nonlinear reflectivity and recovery time parameters of the devices from a single setup, and show good agreement with conventional nonlinear reflectivity measurements. We compare measurements to a rate equation model, showing good agreement up to high pulse fluence values and revealing that the samples tested exhibit a slightly slower recovery at higher fluence values. Lastly, we examine the polarization dependence of the reflectivity, revealing a reduced rollover if cross-polarized beams are used or if the sample is oriented optimally around the beam axis.

Ultrashort pulse duration and broadband dual-comb laser system based on a free-running passively mode-locked Er-fiber oscillator

We have demonstrated pulse duration compression and wavelength extension to a dual-comb free-running passively mode-locked Er-fiber oscillator. At first, the pulse duration of the dual-comb ultrashort pulses from the oscillator is nonlinearly compressed to ∼50 fs by fiber amplifiers. Then, a dual-comb supercontinuum laser source with broadband wavelength band from 1.0 to more than 1.75 µm is achieved when the compressed pulses are used to pump highly nonlinear fibers. At last, utilizing the dual-comb supercontinuum source as the seed for spectral-selected Yb-fiber amplifiers, sub-100 fs ultrashort pulses are obtained at 1.0 µm. This free-running dual-comb fiber laser system can provide ultrashort pulses with sub-100 fs pulse duration at 1.0 µm, 1.5 µm and broadband supercontinuum covering 1.0 to more than 1.75 µm, so it can be potentially deployed in dual-comb applications such as absolute distance measurement, coherent anti-stokes Raman spectroscopy, and mid-infrared differential frequency generation.

Two-photon dual-comb LiDAR.

The interferometric signals produced in conventional dual-comb laser ranging require femtosecond lasers with long-term carrier-envelope offset frequency stability, and are limited to an upper sampling rate by radio-frequency aliasing considerations. By using cross-polarized dual combs and two-photon detection, we demonstrate carrier-phase-insensitive cross-correlations at sampling rates of up to 12× the conventional dual-comb aliasing limit, recording these in a digitizer-based acquisition system to implement ranging with sub-100 nm precision. We then extend this concept to show how the high data burden of conventional dual-comb acquisition can be eliminated by using a simple microcontroller as a ns-precision stopwatch to record the time intervals separating the two-photon cross-correlation pulses, providing real-time and continuous LiDAR-like distance metrology capable of sub-100 nm precision and dynamic acquisition for unlimited periods.

Picosecond ultrasonics with a free-running dual-comb laser.

We present a free-running 80-MHz dual-comb polarization-multiplexed solid-state laser which delivers 1.8 W of average power with 110-fs pulse duration per comb. With a high-sensitivity pump-probe setup, we apply this free-running dual-comb laser to picosecond ultrasonic measurements. The ultrasonic signatures in a semiconductor multi-quantum-well structure originating from the quantum wells and superlattice regions are revealed and discussed. We further demonstrate ultrasonic measurements on a thin-film metalized sample and compare these measurements to ones obtained with a pair of locked femtosecond lasers. Our data show that a free-running dual-comb laser is well-suited for picosecond ultrasonic measurements and thus it offers a significant reduction in complexity and cost for this widely adopted non-destructive testing technique.

Dual‐Comb Lasers: Dual‐Comb Femtosecond Solid‐State Laser with Inherent Polarization‐Multiplexing (Laser Photonics Rev. 15(8)/2021)

In article number 2000441, Maciej Kowalczyk and co-workers developed a novel technique for simultaneous generation of two pulse trains from a single femtosecond laser. It is based on polarization-multiplexing in a birefringent gain medium and allowed to achieve the lowest relative noise ever demonstrated for any solid-state dual-comb laser. This simple technique can be utilized for free-running, but yet very precise spectroscopic measurements.

Resolving nonuniform temperature distributions with single-beam absorption spectroscopy. Part II: Implementation from broadband spectra

Several past studies have described how absorption spectroscopy can be used to determine spatial temperature variations along the optical path by measuring the unique, nonlinear response to temperature of many molecular absorption transitions and performing an inversion. New laser absorption spectroscopy techniques are well-suited to this nonuniformity measurement, yet present analysis approaches use only isolated features rather than a full broadband spectral measurement. In this work, we develop a constrained spectral fitting technique called E″-binning to fit an absorption spectrum arising from a nonuniform environment. The information extracted from E″-binning is then input to the inversion approach from the previous paper in this series (Malarich and Rieker, JQSRT 107455 [1]) to determine the temperature distribution. We demonstrate this approach by using dual frequency comb laser measurements to resolve convection cells in a tube furnace. The recovered temperature distributions at each measurement height agree with an existing natural convection model. Finally, we show that for real-world measurements with noise and absorption model error, increasing the bandwidth and the number of measured absorption transitions may improve the temperature distribution precision. We make the fitting code publicly available for use with any broadband absorption measurement.

All-fiber single-cavity dual-comb for coherent anti-Stokes Raman scattering spectroscopy based on spectral focusing

We report an all-fiber free-running bidirectional dual-comb laser system for coherent anti-Stokes Raman scattering spectroscopy based on spectral focusing. The mode-locked oscillator is a bidirectional ring-cavity erbium fiber laser running at a repetition rate of ∼114MHz. One output of the bidirectional laser is wavelength-shifted from 1560 to 1060 nm via supercontinuum generation for use as the pump source. We have been able to record the Raman spectra of various samples such as polystyrene, olive oil, polymethyl methacrylate (PMMA), and polyethylene in the C-H stretching window. We believe that this all-fiber laser design has promising potential for coherent Raman spectroscopy and also label-free imaging for a variety of practical applications.

Addition of the line list for carbon disulfide to the HITRAN database: line positions, intensities, and half-widths of the 12C32S2, 32S12C34S, 32S12C33S, and 13C32S2 isotopologues

A new ro-vibrational line list for the 12C32S2, 32S12C34S, 32S12C33S, and 13C32S2 isotopologues of carbon disulfide, including line positions, intensities, and linewidths, has been produced as part of the HITRAN2020 project. The carbon disulfide line list covers the range from about 0 to 6470 cm−1 (1.55 µm). The calculations of the line positions and intensities were performed by applying the PGOPHER program using a large set of measured line positions available in the literature, including the most recent high-precision dual-comb laser spectroscopy measurements and transition dipole moments for each measured band. The bands which did not have available intensity measurements were scaled to the experimental cross-sections from the PNNL database. Semi-empirical models for rotational dependence of air- and self-broadened half widths are developed based on the available experimental data.A total of 94992 transitions corresponding to 423 cold and hot bands of the 12C32S2, 32S12C34S, 32S12C33S, and 13C32S2 isotopologues up to a maximum rotational angular momentum J=150 have been calculated. The CS2 line list has been generated in HITRAN format and is provided as supplementary material and will be a part of the next edition of the HITRAN database distributed through https://www.hitran.org. This list will be useful in the fields of atmospheric environmental chemistry, medical diagnostics and planetary atmospheres.

Dual-comb laser spectroscopy of CS2 near 4.6 µm

High-precision absorption spectrum of carbon disulfide (CS2) has been recorded by mid-IR dual-comb laser spectroscopy between 2110 and 2200 cm−1 (4.5–4.7 µm) with sample point spacing of 5 × 10−4 cm−1 and absolute accuracy better than 10−5 cm−1. We present about 980 line positions and intensities retrieved for eleven cold and hot bands of the 12C32S2, 32S12C34S, 32S12C33S, and 13C32S2 isotopologues with the majority of the observed bands belonging to the 12C32S2 isotopologue. Direct measurements of line intensities for most of these CS2 bands are reported for the first time. We compare the measured CS2 line parameters with those obtained in the previous works near 2180 cm−1. As of today, the obtained experimental dataset is the most comprehensive and significantly improves the knowledge of the absorption spectrum of carbon disulfide near 4.6 µm dominated by the ν1 + ν3 combination band.

Dual frequency comb spectroscopy of solid fuel pyrolysis and combustion: Quantifying the influence of moisture content in Douglas fir

As the fire research community develops multi-scale models to predict the spread of wildland fires, there is a need for well-controlled, small-scale experiments which provide benchmark data and fundamental understanding of the effects of key parameters on the combustion of wildland fuels. Here, we implement a unique suite of non-intrusive, quantitative diagnostics that simultaneously measure mass loss, surface temperature, in situ gas temperature and water vapor concentration, including the first implementation of dual frequency comb spectroscopy for the study of solid fuels. As a demonstration, we examine the effect of fuel moisture content on the burning of Douglas fir wood samples that are ignited under constant radiative heat flux from a cone calorimeter to provide benchmark data for multi-scale models. Peak flame temperatures 3 mm above the sample surface measured with the dual frequency comb laser spectrometer are in excess of 1200 K. The flame temperature decreases by over 150 K as fuel moisture content increases from 0.7% to 40%. This work demonstrates a platform for simultaneous, quantitative measurements of well-controlled small-scale experiments which provide benchmark data for the development of advanced modeling and simulations capable of capturing the complex physics present in wildland fire systems.

“Periodic” soliton explosions in a dual-wavelength mode-locked Yb-doped fiber laser

We report the “periodic” soliton explosions induced by intracavity soliton collisions in a dual-wavelength mode-locked Yb-doped fiber laser. Owing to the different group velocities of the two wavelengths, the mode-locked solitons centered at different wavelengths would periodically collide with each other. By using the dispersive Fourier transformation technique, it was found that each collision would induce soliton explosions, but none of them would be identical. Therefore, this phenomenon was termed as “periodic” soliton explosions. In addition, the dissipative rogue waves were detected in the dual-wavelength mode-locked state. The experimental results would be fruitful to the communities interested in soliton dynamics and dual-comb lasers.

Computational Doppler-limited dual-comb spectroscopy with a free-running all-fiber laser

Dual-comb spectroscopy has emerged as an indispensable analytical technique in applications that require high resolution and broadband coverage within short acquisition times. Its experimental realization, however, remains hampered by intricate experimental setups with large power consumption. Here, we demonstrate an ultrasimple free-running dual-comb spectrometer realized in a single all-fiber cavity suitable for the most demanding Doppler-limited measurements. Our dual-comb laser utilizes just a few basic fiber components, allows us to tailor the repetition rate difference, and requires only 350 mW of electrical power for sustained operation over a dozen of hours. As a demonstration, we measure low-pressure hydrogen cyanide within 1.7 THz bandwidth and obtain better than 1% transmittance precision over a terahertz in 200 ms enabled by an all-computational phase retrieval and correction algorithm. The combination of the setup simplicity, comb tooth resolution, and high spectroscopic precision paves the way for proliferation of frequency comb spectroscopy on a larger scale.