Dual-comb Laser Research Articles

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.

Methane spectroscopy using a free-running chip-based dual-comb laser.

Absorption lines of methane in the 2ν3 band centered at 1650nm were measured with a free-running mode-locked dual-comb laser based on a single erbium-doped glass chip. The laser's spectra were broadened up to 1670nm using amplifiers and highly nonlinear fiber. A comb was used to interrogate the complex transmission spectrum of a methane-filled gas cell with an optical point spacing of 968MHz and an interferogram (IGM) rate of 27kHz to yield absorption lines of the R and Q branches. A 1.28s sequence of IGMs was measured and phase-corrected using a self-sufficient correction algorithm seeded only by the IGMs. The associated transmission spectrum was then compared to HITRAN yielding residuals limited by photodetector nonlinearity.

Tunable dual-color operation of Yb:fiber laser via mechanical spectral subdivision.

We present a versatile method to generate a dual-color laser from a single fiber laser cavity by spectral subdivision using a tunable mechanical filter. As a proof-of-principle, we implement the concept in a nonlinear polarization evolution (NPE)-mode-locked ytterbium (Yb)-fiber laser. The division into two independent spectral regions is achieved by inserting a narrow blade-shaped beam block into the free-space grating compressor section of the cavity, where the spectrum is spatially dispersed. By mode-locking both spectral regions, two pulse trains, with different repetition rates around 23 MHz, are generated. Each pulse train has a FWHM of ~10 nm. The method presented here enables tuning of the difference in repetition rate as well as the spectral separation of the two independent pulse trains. The difference in repetition rates originates from intracavity dispersion and can be tuned over a large range (650 Hz - 3 kHz in this setup) by changing the length of the grating compressor. By changing the effective width of the beam block the spectral separation can be dynamically adjusted. This approach's simplicity holds great promises for the development of single-cavity dual-comb lasers featuring tunable sampling rates.

Single-Blind Quantification of Natural Gas Leaks from 1 km Distance Using Frequency Combs.

A new method is tested in a single-blind study for detection, attribution, and quantification of methane emissions from the natural gas supply chain, which contribute substantially to annual U.S. emissions. The monitoring approach couples atmospheric methane concentration measurements from an open-path dual frequency comb laser spectrometer with meteorological data in an inversion to characterize emissions. During single-blind testing, the spectrometer is placed >1 km from decommissioned natural gas equipment configured with intentional leaks of controllablerate. Single, steady emissions ranging from 0 to 10.7 g min-1 (0-34.7 scfh) are detected, located, and quantified at three gas pads of varying size and complexity. The system detects 100% of leaks, including leaks as small as 0.96 g min-1 (3.1 scfh). It attributes leaks to the correct pad or equipment group (tank battery, separator battery, wellhead battery) 100% of the time and to the correct equipment (specific separator, tank, or wellhead) 67% of the time. All leaks are quantified to within 3.7 g min-1 (12 scfh); 94% are quantified to within 2.8 g min-1 (9 scfh). These tests are an important initial demonstration of the methodology's viability for continuousmonitoring oflarge regions, with extension to other trace gases and industries.

Timing Jitter of the Dual-Comb Mode-Locked Laser: A Quantum Origin and the Ultimate Effect on High-Speed Time- and Frequency-Domain Metrology

We study on the timing jitter characteristics of a passively mode-locked dual-wavelength dual-comb Er-fiber laser that generates two femtosecond optical frequency combs with an offset repetition rate in a shared cavity. The relative timing jitter is characterized directly in the time domain with tens of attoseconds precision by utilizing the intrinsic asynchronous optical sampling process between the two laser beams. This sensitive measurement reveals an amplified spontaneous emission (ASE) noise dominated random walk of the relative pulse timing, which has not been observed before in a dual-comb laser. The quantum-limited relative timing jitter behavior shows that the common-mode noise suppression based on a shared cavity does not apply to quantum noise. We further conduct a time-of-flight ranging experiment and a Fourier-transform spectroscopy simulation based on this dual-comb laser, showing that the quantum-limited relative timing jitter sets an ultimate limit on the reachable performance in time- and frequency-domain dual-comb applications particularly when the high acquisition rate is desired.

Dual-Comb Dynamic Interrogation of Fiber Bragg Grating With One Mode-Locked Fiber Laser

Dual-comb spectroscopy schemes have been demonstrated as an attractive solution for advanced optical spectroscopic analysis. The widely used fiber Bragg grating (FBG) sensing could benefit from the high spectral resolution and broad spectral coverage they could offer. However, the complexity, cost, and size of traditional dual-comb systems are prohibitively high for FBG applications. Dual-wavelength mode-locked fiber laser can generate two asynchronous ultrashort pulse trains and could enable the development of compact and low-complexity dual-comb application systems. In this paper, we propose and demonstrate a high-resolution, dynamic FBG interrogation scheme based on dual-comb spectroscopy with one free-running fiber laser. Accurate spectroscopic measurement of the FBG’s reflective spectrum is realized with each acquired dual-comb interferogram without further averaging. In our experiments, the resolution of such single-shot FBG wavelength measurement results can reach 1.7 pm, while the spectral non-aliasing measurement range is over 10 nm. Enabled by this capability of making fast dual-comb spectroscopy measurement of the FBG sensor response, the sampling speed of the interrogation system can reach the repetition rate difference of the dual combs, e.g., 1 kHz in this demonstration. Dynamic measurements of FBG responses driven by frequency up to 400 Hz are experimentally demonstrated. Our results further illustrate the potential and advantages of such single-cavity dual-comb laser sources in realizing low-complexity dual-comb systems that could appeal to a wider variety of applications.

Polarization-multiplexed, dual-comb all-fiber mode-locked laser

Mode-locked fiber lasers that can simultaneously generate two asynchronous ultrashort pulse trains could play an attractive role as the alternative light sources for low-complexity dual-comb metrology applications. Although a few multiplexing schemes to realize such lasers have been proposed and demonstrated, here we investigate the lasing characteristics of a passively mode-locked fiber laser with a finite amount of intracavity birefringence. By introducing a section of polarization-maintaining (PM) fiber into the otherwise-non-PM-single-mode cavity, dual asynchronous pulses with nearly orthogonal states of polarization are generated. With a repetition rate difference of hundreds of hertz, the pulses have well-overlapped spectra and show typical features of polarization-locked vector solitons. It is demonstrated that under an anomalous or net normal dispersion regime, either dual vector solitons or dual dissipative vector solitons can be generated, respectively. Such polarization-multiplexed single single-cavity dual-comb lasers could find further uses in various applications in need of simple dual-comb system solutions.

Regional trace-gas source attribution using a field-deployed dual frequency comb spectrometer

Identification and quantification of trace-gas sources is a major challenge for understanding and regulating air quality and greenhouse gas emissions. Current approaches provide either continuous but localized monitoring, or quasi-instantaneous “snapshot-in-time” regional monitoring. There is a need for emissions detection that provides both continuous and regional coverage, because sources and sinks can be episodic and spatially variable. We field deploy a dual frequency comb laser spectrometer for the first time, enabling an observing system that provides continuous detection of trace-gas sources over multiple-square-kilometer regions. Field tests simulating methane emissions from oil and gas production demonstrate detection and quantification of a 1.6 g min−1 source (less than the average emissions from a small pneumatic controller) from a distance of 1 km, and the ability to discern two leaks among a field of many potential sources. The technology achieves the goal of detecting, quantifying, and attributing emissions sources continuously through time, over large areas, and at emissions rates ∼1000× lower than current regional approaches. It therefore provides a useful tool for monitoring and mitigating undesirable sources and closes a major information gap in the atmospheric sciences.

Singular spectrum analysis for precision improvement in dual-comb laser ranging

Optical methods in distance measurement, which are categorized by interferometry and time-of-flight (TOF) detection, have received widespread attention in recent years. However, interferometry cannot provide absolute distance and traditional TOF measurement cannot obtain a high precision measurement result either. The TOF ranging by femtosecond lasers, a novel precise measurement approach, enabling a sub-micrometer precision for long distance absolute ranging, can solve the problems above and has a wide application prospect in aerospace, remote sensing and surface profilometry. Particularly, a dual-comb ranging approach has attracted great attention due to high update rate (~kHz) and a simple system structure (i.e., working with free running mode-locked laser system). However, the quantum limited timing jitter of mode-locked lasers will inevitably introduce uncertainty into TOF estimation due to the equivalent sampling nature of a dual-comb scheme. As a result, the distance measurement precision is significantly degraded. Even though a simple multiple averaging can be used to alleviate this problem, the measurement speed is limited to a very low level, which is unacceptable to many applications. Moreover, multiple averaging fails in the presence of more complex noise sources. Singular spectrum analysis (SSA), known as a non-parametric spectral estimation technique, has been widely used in dynamic systems to distinguish complex patterns in signals without a priori knowledge of the dynamical model. In this paper, for the first time, we apply SSA to extract distance information from a noisy time series generated by a high update rate dual-comb ranging system. Numerical simulation shows that the SSA is a powerful tool for separating distance series into signal and random noise regardless its color. Specifically, we extract a one-dimensional step profile with high precision in the presence of violet noise (density proportional to f2). In experiment, a dual-comb ranging system is built based on two home-built polarization maintaining mode-locked fiber lasers by using carbon nanotube as saturable absorber. Their repetition rates are both about 74 MHz, their difference being about 2 kHz. We measure the distance of a moving target placed at ~0.5 m away from the range finder and use the SSA for signal extraction. The direct measurement precision is 1.9968 m rms at 200 Hz update rate. The SSA successfully separates the quantum noise from the ranging time series, resulting in 0.1522 m rms ranging precision, corresponding to about 13 times ranging precision improvement. This method can be further extended to high dimension, enabling high precision and high speed profilometry for complex surfaces based on femtosecond laser ranging.

Asynchronous and synchronous dual-wavelength pulse generation in a passively mode-locked fiber laser with a mode-locker.

The evolution from asynchronous to synchronous dual-wavelength pulse generation in a passively mode-locked fiber laser is experimentally investigated by tailoring the intracavity dispersion. Through tuning the intracavity-loss-dependent gain profile and the birefringence-induced filter effect, asynchronous dual-wavelength soliton pulses can be generated until the intracavity anomalous dispersion is reduced to ∼8 fs/nm. The transition from asynchronous to synchronous pulse generation is then observed at an elevated pump power in the presence of residual anomalous dispersion, and it is shown that pulses are temporally synchronized at the mode-locker in the cavity. Spectral sidelobes are observed and could be attributed to the four-wave-mixing effect between dual-wavelength pulses at the carbon nanotube mode-locker. These results could provide further insight into the design and realization of such dual-wavelength ultrafast lasers for different applications such as dual-comb metrology as well as better understanding of the inter-pulse interactions in such dual-comb lasers.

Unidirectional, dual-comb lasing under multiple pulse formation mechanisms in a passively mode-locked fiber ring laser.

Dual-comb lasers simultaneously generating asynchronous ultrashort pulses could be an intriguing alternative to the current dual-laser comb source. When generated through a common light path, the low common-mode noises and good coherence between the pulse trains could be realized. Here we demonstrate the completely common-path, unidirectional dual-comb lasing using a carbon nanotube saturable absorber with additional pulse narrowing and broadening mechanisms. The interactions between multiple soliton formation mechanisms result in bifurcation into unusual two-pulse states with pulses of four-fold bandwidth difference and tens-of-Hz repetition rate difference. Coherence between the pulses is verified by the asynchronous cross-sampling and dual-comb spectroscopy measurements.

Dual frequency comb laser absorption spectroscopy in a 16 MW gas turbine exhaust

We demonstrate the first frequency comb laser absorption spectroscopy in an industrial environment. Recent advancements in robust frequency comb design enabled installation of the sensor in an operating power plant, where we simultaneously measured temperature, H2O and CO2 concentration in the exhaust of a 16 MW stationary gas turbine. The frequency comb laser spectrometer probed 16,000 individual wavelengths of light spaced by 0.007 cm−1 (0.0014 nm) near 1440 nm, spanning 279 absorption features of H2O and 43 features of CO2. Fits to the measured absorption spectra yield simultaneous temperature, H2O and CO2 concentrations with between 10 and 60 second time resolution. Measurements over a 5 hour period tracked variations in the exhaust consistent with various changes to the gas turbine operation. Much larger wavelength ranges (200+ nm) and different time resolutions are possible depending on the desired precision by changing various settings on the same spectrometer. Overall, this work demonstrates the potential for frequency comb laser absorption spectroscopy in industrial combustion environments.

Dual-comb modelocked lasers generated from a single source

Dual-comb modelocked lasers generated from a single source

Dual-comb modelocked lasers: semiconductor saturable absorber mirror decouples noise stabilization.

In this paper we present the stabilization of the pulse repetition rate of dual-comb lasers using an intracavity semiconductor saturable absorber mirror (SESAM) for passive modelocking and an intracavity birefringent crystal for polarization-duplexing to obtain simultaneous emission of two modelocked beams from the same linear cavity sharing all components. Initially surprising was the observation that the cavity length adjustments to stabilize one polarization did not significantly affect the pulse repetition rate of the other. We gained insight in the underlying physics using both a semiconductor and Nd:YAG laser gain material with the conclusion that the pulse arrival timing jitter of the two beams is decoupled by the uncorrelated time delay from the saturated SESAM and becomes locked with sufficient but not too much pulse overlap. Noise stabilization is in all cases still possible for both combs. The dual-comb modelocked laser is particularly interesting for the semiconductor laser enabling the integration of gain and absorber layers within one wafer (referred to as the modelocked integrated external-cavity surface emitting laser--MIXSEL).

Dual Ti:sapphire comb lasers by a fiber laser pumping scheme and a hand-sized optical frequency reference

A dual-comb laser system containing two femtosecond Ti:sapphire (Ti:S) lasers is reported, in which the ultrashort pulses are pumped by one common fiber laser and the influences of the different pumping noise are presented both in time-domain and frequency-domain. In addition, the dimensions of dual Ti:S comb system are reduced due to the implementation of one hand-sized optical frequency reference, from which the needed space and cost for two “self-reference” optics are saved.