Dual-comb Laser Research Articles

High-sensitivity pump-probe spectroscopy with a dual-comb laser and a PM-Andi supercontinuum.

Amplifier-based pump-probe systems, while versatile, often suffer from complexity and low measurement speeds, especially when probing samples require low excitation fluences. To address these limitations, we introduce a pump-probe system that leverages a 60-MHz single-cavity dual-comb oscillator and an ultra-low noise supercontinuum. The setup can operate in equivalent time sampling or in programmable optical delay generation modes. We employ this system to study the wavelength-dependent excited-state dynamics of the non-fullerene electron acceptor Y6, a compound of interest in solar cell development, with excitation fluences as low as 1 nJ/cm2, well below the onset of nonlinear exciton annihilation effects. Our measurements reach a shot-noise limited sensitivity in differential transmission of 3.4·10-7. The results demonstrate the system's potential to advance the field of ultrafast spectroscopy.

Detection of carbon monoxide using a polarization-multiplexed erbium dual-comb fiber laser

Abstract We present a simple method to develop a compact, reliable, and robust free-running erbium single-cavity dual-comb (DC) laser via polarization multiplexing. The key features of our design include dynamic tuning in the difference in repetition rates of the laser, long-term stability, and the use of off-the-shelf components. Polarization multiplexing exploits the fast and slow axes of the fiber, while modelocking is achieved through a nonlinear amplifying loop mirror scheme using readily available components. The laser operates at a repetition rate of around 74.74 MHz with a tuning capability in the difference in repetition rates from 500 Hz to 200 kHz. This tunability makes the system more flexible for DC spectroscopy experiments. Consequently, using this laser, we demonstrated a proof-of-principle DC spectroscopy of carbon monoxide, operating without any active stabilization.

Dynamic counterpropagating all-normal dispersion (DCANDi) fiber laser

The fiber single-cavity dual-comb laser (SCDCL) is an emerging light-source architecture that opens up the possibility for low-complexity dual-comb pump-probe measurements. However, the fundamental trade-off between measurement speed and time resolution remains a hurdle for the widespread use of fiber SCDCLs in dual-comb pump-probe measurements. In this paper, we break this fundamental trade-off by devising an all-optical dynamic repetition rate difference (Δf rep ) modulation technique. We demonstrate the dynamic Δf rep modulation in a modified version of the recently developed counterpropagating all-normal dispersion (CANDi) fiber laser. We verify that our all-optical dynamic Δf rep modulation technique does not introduce excessive relative timing jitter. In addition, the dynamic modulation mechanism is studied and validated both theoretically and experimentally. As a proof-of-principle experiment, we apply this so-called dynamic CANDi (DCANDi) fiber laser to measure the relaxation time of a semiconductor saturable absorber mirror, achieving a measurement speed and duty cycle enhancement factor of 143. DCANDi fiber laser is a promising light source for low-complexity, high-speed, high-sensitivity ultrafast dual-comb pump-probe measurements.

Water-vapor absorption database using Dual Comb Spectroscopy from 300 to 1300 K Part II: Air-Broadened H2O, 6600 to 7650 cm−1

We present broadband dual frequency comb laser absorption measurements of 2 % H2O (natural isotopic abundance of 99.7 % H216O) in air from 6600 to 7650 cm−1 (1307–1515 nm) with a spectral point spacing of 0.0068 cm−1. Twenty-nine datasets were collected at temperatures between 300 and 1300 K (±0.82 % average uncertainty) and pressures ranging from 20 to 600 Torr (±0.25 %) with an average residual absorbance noise of 8.0E-4 across the spectrum for all measurements. We fit measurements using a quadratic speed-dependent Voigt profile to determine 7088 absorption parameters for 3366 individual transitions found in HITRAN2020. These measurements build on the line strength, line center, self-broadening, and self-shift parameters determined in the Part I companion of this work. Here we measure air-broadened width (with temperature- and speed-dependence) and air pressure shift (with temperature-dependence) parameters. Various trends are explored for extrapolation to weak transitions that were not covered in this work. Improvements made in this work are predominantly due to the inclusion of air pressure shift temperature dependence values. In aggregate, these updates improved RMS absorbance error by a factor of 4.2 on average, and the remaining residual is predominantly spectral noise. This updated database improves high-temperature spectroscopic knowledge across the 6600-7650 cm−1 region of H2O absorption.

Study of Time-Resolved Dynamics in Turbid Medium Using a Single-Cavity Dual-Comb Laser.

In measuring cerebral blood flow (CBF) noninvasively using optical techniques, diffusing-wave spectroscopy is often combined with near-infrared spectroscopy to obtain a reliable blood flow index. Measuring the blood flow index at a determined depth remains the ultimate goal. In this study, we present a simple approach using dual-comb lasers where we simultaneously measure the absorption coefficient (μa), the reduced scattering coefficient (μs '), and dynamic properties. This system can also effectively differentiate dynamics from various depths, which is crucial for analyzing multilayer dynamics. For CBF measurements, this capability is particularly valuable as it helps mitigate the influence of the scalp and skull, thereby enhancing the specificity of deep tissue.

THz-TDS with gigahertz Yb-based dual-comb lasers: noise analysis and mitigation strategies

We investigate terahertz time-domain spectroscopy using a low-noise dual-frequency-comb laser based on a single spatially multiplexed laser cavity. The laser cavity includes a reflective biprism, which enables generation of a pair of modelocked output pulse trains with slightly different repetition rates and highly correlated noise characteristics. These two pulse trains are used to generate the THz waves and detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominal repetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We perform THz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1 µm lasers for the first time, to our knowledge, and obtain THz signal currents approximately as strong as those from reference measurements at 1.55 µm and 80 MHz. We investigate the influence of the laser’s timing noise properties on THz measurements, showing that the laser’s timing jitter is quantitatively explained by power-dependent shifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power and show up to 20 dB suppression in noise by the combination of shared pumping and shared cavity architecture. The laser’s ultra-low-noise properties enable averaging of the THz waveform for repetition rate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55 dB when operating at 1 kHz and averaging for 2 s. We show that the obtained dynamic range is competitive and can be well explained by accounting for the measured optical delay range, integration time, as well as the measurement bandwidth dependence of the noise from transimpedance amplification. These results will help enable a new approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.

Long-distance and high-precision ranging with dual-comb nonlinear asynchronous optical sampling

Precise distance metrology and measurements play an important role in many fields of scientific research and industrial manufacture. Dual-comb laser ranging combines sub-wavelength ranging precision, large non-ambiguity range, and high update rate, making it the most promising candidate in precise distance metrology and measurements. However, previous demonstrations of dual-comb ranging suffer from short working distances, limited by the decoherence of lasers in interferometric schemes or by the low sensitivity of the photodetectors in response to the sparse echo photons. Here, we propose and demonstrate time-of-flight laser ranging with dual-comb nonlinear asynchronous optical sampling and photon counting by a fractal superconducting nanowire single-photon detector, achieving ranging precision of 6.2 micrometers with an acquisition time of 100 ms and 0.9 micrometers with an acquisition time of 1 s in measuring the distance of an outdoor target approximately 298 m away.

Scan-less 3D microscopy based on spatiotemporal encoding on a single-cavity dual-comb laser.

Dual-comb microscopy enables high-speed and high-precision optical sampling by simultaneously extracting both amplitude and phase information from the interference signals with frequency division multiplexing. In this Letter, we introduce a spatiotemporal encoding approach for dual-comb microscopy that overcomes previous limitations such as mechanical scanning, low sampling efficiency, and system complexity. By employing free-space angular-chirp-enhanced delay (FACED) and a low-noise single-cavity dual-comb laser, we achieve scan-less 3D imaging with nanometer precision and a 3D distance-imaging rate of 330 Hz, restricted only by the repetition rate difference of the dual-comb laser. Specifically, the FACED unit linearly arranges the laser beam into an array. A grating subsequently disperses this array transversely into lines, facilitating ultrafast spectroscopic applications that are 1-2 orders of magnitude quicker than traditional dual-comb methods. This spatiotemporal encoding also eases the stringent conditions on various dual-comb laser parameters, such as repetition rates, coherence, and stability. Through carefully designed experiments, we demonstrate that our scan-less system can measure 3D profiles of microfabricated structures at a rate of 7 million pixels per second. Our method significantly enhances measurement speed while maintaining high precision, using a compact light source. This advancement has the potential for broad applications, including phase imaging, surface topography, distance ranging, and spectroscopy.

Water-vapor absorption database using dual comb spectroscopy from 300 to 1300 K part I: Pure H2O, 6600 to 7650 cm-1

We present broadband dual frequency comb laser measurements of pure H2O absorption (natural isotopic abundance of 99.7 % H216O) from 6600 to 7650 cm−1 (1307–1515 nm) with a spectral point spacing of 0.0068 cm−1. Twenty-nine absorption spectra were collected at temperatures between 300 and 1300 K (±0.81 % average uncertainty) and pressures ranging from 0.5 to 16 Torr (±0.27 %) with an average residual absorbance noise of 7.4E-4 across the spectrum for all measurements. We fit measurements using a quadratic speed-dependent Voigt profile to determine 17,030 absorption parameters for 5986 transitions found in HITRAN2020. The measured parameters include line strength, line center, self-broadening, and self-shift, along with their power-law temperature-dependence exponents, and speed-dependent self-width parameters. A second version of the retrieved parameters with fits that do not include speed-dependence is also presented. We explore various trends to enable parameter extrapolation for weak transitions that were not covered in this work. We measured line parameters for an additional 614 features not presently catalogued in the HITRAN2020 database. In aggregate, these updates improved RMS absorbance error by a factor of 2.6 on average, and the remaining residual is predominantly spectral noise. This updated database improves high-temperature spectroscopic knowledge across the 6600–7650 cm−1 region of H2O absorption. An accompanying paper (Part II) describes similar measurements and a new database for air-broadened H2O absorption.

Controlling intracavity dual-comb soliton motion in a single-fiber laser.

Ultrafast science builds on dynamic compositions of precisely timed light pulses, and evolving groups of pulses are observed in almost every mode-locked laser. However, the underlying physics has rarely been controlled or used until now. Here, we demonstrate a general approach to control soliton motion inside a dual-comb laser and the programmable synthesis of ultrashort pulse patterns. Introducing single-pulse modulation inside an Er:fiber laser, we rapidly shift the timing between two temporally separated soliton combs. Their superposition outside the cavity yields ultrashort soliton sequences. On the basis of real-time spectral interferometry, we observe the deterministic switching of intersoliton separation arising from the interplay of attracting and repulsing forces via ultrafast nonlinearity and laser gain dynamics. Harnessing these insights, we demonstrate the high-speed all-optical synthesis of nano- to picosecond pump-probe delays and programmable free-form soliton trajectories. This concept may pave the way to a new class of all-optical delay generators for ultrafast measurements at unprecedented high tuning, cycling, and acquisition speeds.

Gigahertz semiconductor laser at a center wavelength of 2 µm in single and dual-comb operation.

Dual-comb lasers are a new class of ultrafast lasers that enable fast, accurate and sensitive measurements without any mechanical delay lines. Here, we demonstrate a 2-µm laser called MIXSEL (Modelocked Integrated eXternal-cavity Surface Emitting Laser), based on an optically pumped passively modelocked semiconductor thin disk laser. Using III-V semiconductor molecular beam epitaxy, we achieve a center wavelength in the shortwave infrared (SWIR) range by integrating InGaSb quantum well gain and saturable absorber layers onto a highly reflective mirror. The cavity setup consists of a linear straight configuration with the semiconductor MIXSEL chip at one end and an output coupler a few centimeters away, resulting in an optical comb spacing between 1 and 10 GHz. This gigahertz pulse repetition rate is ideal for ambient pressure gas spectroscopy and dual-comb measurements without requiring additional stabilization. In single-comb operation, we generate 1.5-ps pulses with an average output power of 28 mW, a pulse repetition rate of 4 GHz at a center wavelength of 2.035 µm. For dual-comb operation, we spatially multiplex the cavity using an inverted bisprism operated in transmission, achieving an adjustable pulse repetition rate difference estimated up to 4.4 MHz. The resulting heterodyne beat reveals a low-noise down-converted microwave frequency comb, facilitating coherent averaging.

High capacity NP-Complete problems solver based on dual-comb asynchronous optical sampling

Photonic schemes to solve the Nondeterministic Polynomial-Complete (NP–C) problems could offer an interesting alternative to such computationally intensive tasks. Previous studies had shown that they can be solved by measuring the propagation delays through an optical oracle that corresponds to the NP-C problem. However, to tackle a problem with a large instance, the previous schemes face significant challenges to achieve a large measurement range, fast speed and high resolution, simultaneously. In this work, we propose an optical solver based on asynchronous optical sampling driven by a simple fiber dual-comb laser. It is experimentally demonstrated that it can realize picosecond resolution and tens of nanosecond range with a fiber oracle network consisting of 15 couplers and sections of fibers that represents a typical Traveling Salesman problem. With a width of the pulse interferogram of less than 1.5 ps, i.e. an optical path resolution of ∼0.3 mm in fiber, the output from an oracle with 58 likely paths is measured accurately, from which the Hamilton path is easily identified in ∼0.7 ms. It is expected that further optimizing the parameters of the laser as well as the oracle would enable the solving larger scale NP-C problems.

Single-short-cavity dual-comb fiber laser with over 120 kHz repetition rate difference based on polarization multiplexing.

An all-fiber single-short-cavity dual-comb laser with a high repetition rate of up to 500 MHz and a high repetition rate difference of over 120 kHz was demonstrated. The laser setup exploits high birefringence of a polarization-maintaining gain fiber to generate asynchronous combs based on the polarization-multiplexing method. By adopting short-linear-cavity and all-birefringent configuration, a repetition rate difference several orders of magnitude larger than that of a previous work was achieved. The soliton dual-comb showed good mutual coherence and stability, which reveals the potential to enhance the acquisition rate and accuracy of dual-comb measurement systems.

Multi-dimensional multiplexed, tri-comb and quad-comb generation from a bidirectional mode-locked fiber laser.

Tri-comb and multi-comb techniques could enable many advanced measurement applications beyond the reach of traditional dual-comb schemes. However, the sophisticated and bulky control systems of the conventional schemes based on three comb lasers render them impractical for many potential applications. Like their dual-comb counterparts, tri-comb and multi-comb lasers are being investigated as attractive alternatives. In contrast to previous dual-comb lasers using only one multiplexing dimension of optical pulses, this work simultaneously leverages multiplexing methods in three physical dimensions, i.e. wavelength, polarization, and direction, to generate triple to quadruple asynchronous pulse trains in a bidirectional mode-locked fiber laser. Because of the unique cavity structure studied here, both wavelength-multiplexed and polarization-multiplexed dual-comb generation from a completely shared-cavity and wavelength/polarization-multiplexed multi-comb generation from a bidirectional partially shared-cavity are achieved. Good relative stability among the generated combs of the fiber laser is demonstrated, as well as proof-of-concept dual-comb spectroscopy measurements, which validates the mutual coherence between the combs. The analysis of the experimental results further reveals interesting performance comparisons between combs from different multiplexing schemes, thanks to the special laser design used here that allows a side-by-side dual-comb demonstrations from different combinations of outputs from the same laser. Our investigation could facilitate multi-comb generation based on one light source for field-deployable multi-comb applications.

Noise characteristics of a polarization-duplex dual-comb fiber laser based on a single gain fiber

We demonstrate gain-sharing polarization-duplex dual-comb laser. Here, a nonlinear amplifying loop mirror (NALM) with a single gain fiber is used to achieve simultaneous mode-locking for the generation of the two frequency combs with slightly different repetition rates. Our system provides stable mode-locking and a good overlap of frequency comb spectra. The repetition rate difference is about 100 kHz and has high stability: we determined a standard deviation of 0.75 Hz, and the Allan deviation is 0.073 Hz at an integration time of 1 s for this free-running system. In the case of simultaneous mode-locking, our system shows a relative intensity noise (RIN) characteristic that has, to the best our knowledge, not yet been reported for dual-comb lasers. On the other hand, the phase noise characteristic of our mode-locked dual-comb laser is similar to the characteristic obtained when one beam path is blocked and the laser is used for single frequency-comb generation.

Coherently averaged dual-comb spectroscopy with a low-noise and high-power free-running gigahertz dual-comb laser: erratum.

This erratum corrects a typographical error in equation (8) of our published paper [Opt. Express31, 7103 (2023)10.1364/OE.479356]. All the calculations used the correct equation, so all the results and conclusions remain unchanged.

Dual-comb fiber laser for stable frequency distribution.

A passive dual-comb laser can generate two optical frequency combs with different repetition frequencies. These repetition differences have high relative stability and mutual coherence through passive common-mode noise suppression without complex tight phase locking from a single-laser cavity. The comb-based frequency distribution requires the dual-comb laser to have a high repetition frequency difference. This paper presents a high repetition frequency difference bidirectional dual-comb fiber laser based on an all-polarization-maintaining cavity configuration and a semiconductor saturable absorption mirror with single polarization output. The proposed comb laser has a standard deviation of 69 Hz and an Allan deviation of 1.17 × 10-7 at τ = 1 s under different repetition frequencies of 12.815 MHz. Moreover, a transmission experiment has been conducted. Owing to the passive common-mode noise rejection capability of dual-comb laser, after passing an 84 km fiber link, the frequency stability of the repetition frequency difference signal is improved by two orders of magnitude than the repetition frequency signal at the receiver side.

Recent Advances and Outlook in Single-Cavity Dual Comb Lasers

Dual-comb spectroscopy as an emerging tool for spectral analysis has been investigated in a wide range of applications, including absorption spectroscopy, light detection and ranging, and nonlinear spectral imaging. Two mutually coherent combs facilitate high-precision, high-resolution, and broadband spectroscopy. Recently, dual combs generated from a single cavity have become compelling options for dual-comb spectroscopy, enabling huge simplification to measuring systems. Here, we review the progress of single-cavity dual comb lasers in recent years and summarize the distinctive advantages of single-cavity dual combs. First, the principles of optical frequency comb and dual-comb spectroscopy are introduced in time and frequency domains. Then, the implementation techniques and typical applications of single-cavity dual comb lasers are discussed, including directional multiplexing, wavelength multiplexing, polarization multiplexing, and space multiplexing. Finally, an outlook on the development of single-cavity dual combs is presented.

Coherently averaged dual-comb spectroscopy with a low-noise and high-power free-running gigahertz dual-comb laser.

We present a new type of dual optical frequency comb source capable of scaling applications to high measurement speeds while combining high average power, ultra-low noise operation, and a compact setup. Our approach is based on a diode-pumped solid-state laser cavity which includes an intracavity biprism operated at Brewster angle to generate two spatially-separated modes with highly correlated properties. The 15-cm-long cavity uses an Yb:CALGO crystal and a semiconductor saturable absorber mirror as an end mirror to generate more than 3 W average power per comb, below 80 fs pulse duration, a repetition rate of 1.03 GHz, and a continuously tunable repetition rate difference up to 27 kHz. We carefully investigate the coherence properties of the dual-comb by a series of heterodyne measurements, revealing several important features: (1) ultra-low jitter on the uncorrelated part of the timing noise; (2) the radio frequency comb lines of the interferograms are fully resolved in free-running operation; (3) we validate that through a simple measurement of the interferograms we can determine the fluctuations of the phase of all the radio frequency comb lines; (4) this phase information is used in a post-processing routine to perform coherently averaged dual-comb spectroscopy of acetylene (C2H2) over long timescales. Our results represent a powerful and general approach to dual-comb applications by combining low noise and high power operation directly from a highly compact laser oscillator.

Temperature-dependent CO2 line mixing models using dual frequency comb absorption and phase spectroscopy up to 25 bar and 1000 K

We perform dual frequency comb laser absorption spectroscopy of CO2 at high pressure and temperature, and use the spectra to test and improve CO2 line mixing models and their temperature dependence. The dual-comb spectrometer spans 6800–7000 cm−1 with 0.0066 cm−1 point spacing, and is coupled to a specialized, high-pressure, high-temperature gas cell providing conditions up to 977 K and 25 bar. We compare our measurements to spectra calculated using an advanced line mixing model for pure CO2 based on the energy-corrected sudden (ECS) approximation. We determine a new set of temperature-dependent ECS model parameters, and show that the new parameters significantly improve the accuracy of the ECS line mixing model over a temperature range spanning 298–977 K. We also compare the measured spectra to a simpler line mixing model developed using the modified exponential gap (MEG) scaling law, and report the temperature and pressure dependence of the MEG model parameters required to scale the model across wide ranges of conditions. Finally, we report high-pressure, ambient-temperature measurements of CO2 spectra in both amplitude (absorption) and phase using the dual-comb spectrometer. We use these measured absorption and phase spectra to assess the performance of the ECS and MEG line mixing models at high densities near room temperature. To the best of our knowledge, these results represent the first study of line mixing using phase spectroscopy. Overall, the results of this study significantly expand line mixing models for CO2 at high pressure and temperature and improve the accuracy and availability of absorption models for harsh conditions encountered in laser-based sensing and planetary science.