Dual-comb Spectroscopy Research Articles

Multi-octave two-color soliton frequency comb in integrated chalcogenide microresonators.

Mid-infrared (MIR) Kerr microcombs are of significant interest for portable dual-comb spectroscopy and precision molecular sensing due to strong molecular vibrational absorption in the MIR band. However, achieving a compact, octave-spanning MIR Kerr microcomb remains a challenge due to the lack of suitable MIR photonic materials for the core and cladding of integrated devices and appropriate MIR continuous-wave (CW) pump lasers. Here, we propose a novel slot concentric dual-ring (SCDR) microresonator based on an integrated chalcogenide glass chip, which offers excellent transmission performance and flexible dispersion engineering in the MIR band. This device achieves both phase-matching and group velocity matching in two separated anomalous dispersion regions, enabling phase-locked, two-color solitons in the MIR region with a commercial 2-μm CW laser as the pump source. Moreover, the spectral locking of the two-color soliton enhances pump wavelength selectivity, providing precise control over soliton dynamics. By leveraging the dispersion characteristics of the SCDR microresonator, we have demonstrated a multi-octave-spanning, two-color soliton microcomb, covering a spectral range from 1156.07 to 5054.95nm (200 THz) at a -40dB level, highlighting the versatility and broad applicability of our approach. And the proposed multi-octave MIR frequency comb is relevant for applications such as dual-comb spectroscopy and trace-gas sensing.

Calibration experiments for dual-comb IPDA XCO2 measurements using a variable pressure absorption cell

Measuring XCO2, the column-averaged dry-air mixing ratio of CO2 in the atmosphere, is essential for monitoring climate change and guiding mitigation efforts. Integrated Path Differential Absorption (IPDA) is a highly effective XCO2 measurement solution for space-borne lidar. However, the limited number of wavelengths limits its capabilities. IPDA, enhanced with Electro-Optical Frequency Comb (EOFC) technology, offers a robust solution. This study validates IPDA measurements using both asymmetric and symmetric Electro-Optic Dual-Comb Spectroscopy (EO-DCS) systems at 1572 nm. This indoor experimental system does not operate in a true lidar mode, but focuses on IPDA measurements of XCO2 through a transmission-style cell. We achieved atmospheric spectral transmittance by a variable pressure CO2 absorption cell, correlating pressures from 425 to 470 mb to XCO2 of 386.81–447.08 ppm. The XCO2 measurements of asymmetric/symmetric systems both exhibit excellent linearity. Their linear fittings yielded the Root Mean Square Error (RMSE) as low as 0.198/0.064 ppm, and the Mean Absolute Percentage Errors (MAPE) was as low as 0.0386%/0.0140%. This level of measurement performance has never been demonstrated in other works. Accuracy was assessed by comparing IPDA results with a pressure gauge, and precision was confirmed through repetitive measurements (1000 times over 10 min) and Allan variance analysis. Notably, our system operates effectively without frequency stabilization, showing tolerance to laser frequency drift—a significant advantage for IPDA applications. The entire experiment was conducted in a comparison between asymmetric and symmetric systems, especially in the analysis of absorption line residuals. We discuss the error transfer from phase to spectral transmittance and its implications and explained why the residuals in the asymmetric system were larger. These results underscore the superior performance of EO-DCS IPDA in measuring XCO2 and its promising potential for applications. To our knowledge, this is the only DCS IPDA calibration demonstration to date.

Monolithic dispersion engineered mid-infrared quantum cascade laser frequency comb

The high-power quantum cascade laser (QCL) frequency comb capable of room temperature operation is of great interest to high-precision measurement and low-noise molecular spectroscopy. While a significant amount of research is devoted to the longwave spectral range, shortwave 3–5 μm QCL combs are still relatively underdeveloped due to the excessive material dispersion. In this work, we propose a monolithic integrated multimode waveguide scheme for effective dispersion engineering and high-power-efficiency operation. Over watt-level output power at room temperature with a wall plug efficiency of 7% and robust dispersion reduction is achieved from a quantum cascade laser frequency comb at a wavelength approximately 4.6 μm. Narrow beatnote linewidth less than 1 kHz and clear dual-comb multiheterodyne comb lines manifest the coherent phase relation among the comb modes which is crucial to fast molecular spectroscopy. This monolithic dispersion engineered waveguide design is also compatible to an efficient active–passive optical coupling scheme and would open up a new research playground for ring comb and on-chip dual-comb spectroscopy.

Using open-path dual-comb spectroscopy to monitor methane emissions from simulated grazing cattle

Abstract. Accurate whole-farm or herd-level measurements of livestock methane emissions are necessary for anthropogenic greenhouse gas inventories and to evaluate mitigation strategies. A controlled methane (CH4) release experiment was performed to determine if dual-comb spectroscopy (DCS) can detect CH4 concentration enhancements produced by a typical herd of beef cattle in an extensive grazing system. Open-path DCS was used to measure downwind and upwind CH4 concentrations from 10 point sources of methane simulating cattle emissions. The CH4 mole fractions and wind velocity data were used to calculate CH4 flux using an inverse dispersion model, and the simulated fluxes were then compared to the actual CH4 release rate. For a source located 60 m from the downwind path, the DCS system detected 10 nmol mol−1 CH4 horizontal concentration gradient above the atmospheric background concentration with a precision of 6 nmol mol−1 in 15 min interval. A CH4 release of 3970 g d−1 was performed, resulting in an average concentration enhancement of 24 nmol mol−1 of CH4. The calculated CH4 flux was 4002 g d−1, showing good agreement with the actual CH4 release rate. Periodically altering the downwind path, which may be needed to track moving cattle, did not adversely affect the ability of the instruments to determine the CH4 flux. These results give us confidence that CH4 flux can be determined by grazing cattle with low disturbance and direct field-scale measurements.

Digitally generated high-resolution mid-infrared dual-comb spectroscopy system based on electro-optic modulation.

In this Letter, we propose a high-resolution dual-comb spectroscopy (DCS) in the mid-infrared (MIR) region. A broadband electro-optic frequency comb (EOFC) with a line spacing of 13 GHz is generated in the near-infrared region. The injection locking technique is employed to lock the distributed feedback (DFB) laser to each comb line of the 34 comb lines as the seed laser for the subsequent electro-optic modulation. A dual radio frequency (RF) comb source with a 50 MHz line spacing and a 13 GHz bandwidth drives a single IQ Mach-Zehnder modulator (IQ-MZM), functioning as a single-sideband (SSB) generator and producing a DCS with high spectrum flatness and resolution flexibility. The generated DCS is converted to the MIR region via a nonlinear difference frequency generation (DFG) system. A DCS with a bandwidth of 442 GHz and a resolution of 50 MHz is achieved in the 3.3 µm region, and the figure of merit reaches 2.94×106 H z 12 in a 183.6 ms measurement time.

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.

Brillouin laser pumped tunable low-threshold mid-IR Kerr comb at 2 μm

Optical frequency combs in the 2 μm wavelength region are important for applications ranging from sensing of gases such as CO2 and CO to optical communications, LIDAR, and gravitational wave detection. The development of low-loss waveguides and high-Q microresonators with anomalous dispersion and the availability of tunable narrow linewidth lasers around 1.55 μm have enabled the realization of small footprint soliton combs and low-threshold Kerr combs in this wavelength region; demonstrations of microresonator frequency combs in the 2 μm wavelength region have been limited. Here, we harness an intracavity pumping scheme to demonstrate a low-threshold (<100 mW) microresonator Kerr comb at 2 μm. We exploit Brillouin lasing in a silica microsphere (∼310 μm diameter) to create an intracavity pump, which then generates a ∼140 nm wide Kerr comb in the backscattered Stokes direction. We demonstrate the tolerance of the comb generation scheme to microsphere dimensions and the input pump wavelength by achieving Kerr comb generation in microspheres of diameters ranging from 295 to 318 μm and also at different input pump wavelengths for a particular microsphere diameter. Intracavity pumping opens up opportunities for the development of soliton combs and Kerr combs in the mid-IR wavelength region for applications such as dual-comb spectroscopy, LIDAR, and optical communications.

Free-form dual-comb spectroscopy for compressive sensing and imaging

Free-form dual-comb spectroscopy for compressive sensing and imaging

Cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using free-running quantum cascade lasers

Cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using free-running quantum cascade lasers

Dual-comb spectroscopy over a 100 km open-air path

Dual-comb spectroscopy over a 100 km open-air path

Reference-free dual-comb spectroscopy with inbuilt coherence: publisher's note.

This publisher's note contains a correction to Opt. Lett.49, 2473 (2024)10.1364/OL.521866.

High-sensitivity dual-comb and cross-comb spectroscopy across the infrared using a widely tunable and free-running optical parametric oscillator

Dual-comb spectroscopy (DCS) enables high-resolution measurements at high speeds without the trade-off between resolution and update rate inherent to mechanical delay scanning. However, high complexity and limited sensitivity remain significant challenges for DCS systems. We address these via a wavelength-tunable dual-comb optical parametric oscillator (OPO) combined with an up-conversion detection method. The OPO is tunable from 1300-1670 nm (signal) and 2700-5000 nm (idler). Spatial multiplexing in both the laser and OPO cavities creates a near-common path arrangement, enabling comb-line-resolved measurements in free-running operation. The narrow instantaneous bandwidth results in high power per comb-line up to 160 μW in the mid-infrared. Through intra-cavity up-conversion based on cross-comb spectroscopy, we leverage these power levels while overcoming the sensitivity limitations of direct mid-infrared detection. This approach yields a high signal-to-noise ratio (50.2 dB Hz1/2) and high dual-comb figure of merit (3.5 × 108 Hz1/2). This scheme enabled detecting ambient methane over a 3-meter path length in millisecond time scale.

Single-modulator, dual comb serrodyne spectroscopy.

Dual optical frequency comb spectroscopy allows for high speed, broadband measurements without any moving parts. Here, we combine differential chirp downconversion to probe large spectral bandwidths and serrodyne modulation to separate the positive and negative sidebands in a single modulator. As an initial demonstration, we apply this approach to measure a sharp cavity resonance to illustrate the system performance. We then measure methane transitions in the near-infrared and compare the resulting spectra to models based upon the current spectroscopic databases. The serrodyne method has lower hardware requirements compared to many existing approaches, and its simplicity enables a high degree of mutual coherence between the two combs. Further, this method is readily amenable to chip-scale photonic integration.

InP-based integrated optical frequency shifter for heterodyne detection applications

We present the design and characterization of an integrated optical frequency shifter based on an IQ modulator featuring standard building blocks from a generic photonic integration Indium Phosphide (InP) foundry. The device is conceived to be used in applications with low bandwidth requirements such as dual-comb spectroscopy or integrated low-cost LiDARs. An initial analytical study of the system was carried out and subsequently validated by simulations. Then, we obtained experimentally the most relevant parameters of the system and evaluated its performance under frequency shifter operation. Good performance up to 250 MHz is achieved, thus proving its suitability for the intended applications.

Solitons, bifurcation and chaotics in a bidirectional mamyshev oscillator

Bidirectional mode-locked fiber lasers are able to deliver two outputs of pulses in the clockwise (CW) and counterclockwise (CCW) directions, having important applications in dual-comb spectroscopy, asynchronous optical sampling and terahertz time-domain spectroscopy. In this paper, we have experimentally demonstrated an all-fiber Er/Yb-doped bidirectional passively mode-locked Mamyshev oscillator (MO) without an external seed. Two stable mode-locked states with remarkable different characteristics are observed by adjusting the pump power. The transition process between the two mode-locked states shows chaotic behaviors, which are further investigated with numerical simulations based on the Ginzburg-Landau equation. The bidirectional MO proposed in this work is not only a good laser source, but also a good platform to study soliton dynamics, system bifurcation, and chaos in ultrafast fiber laser systems.

Dual-Comb Spectroscopy With Enhanced Resolution by Pseudorandom Phase Modulation of Gain-Switched Optical Pulses

Dual-Comb Spectroscopy With Enhanced Resolution by Pseudorandom Phase Modulation of Gain-Switched Optical Pulses

Pulse interaction induced systematic errors in dual comb spectroscopy

Systematic errors are observed in dual comb spectroscopy when pulses from the two sources travel in a common fiber before interrogating the sample of interest. When sounding a molecular gas, these errors distort both the line shapes and retrieved concentrations. Simulations of dual comb interferograms based on a generalized nonlinear Schrodinger equation highlight two processes for these systematic errors. Self-phase modulation changes the spectral content of the field interrogating the molecular response but affects the recorded spectral baseline and absorption features differently, leading to line intensity errors. Cross-phase modulation modifies the relative inter-pulse delay, thus introducing interferogram sampling errors and creating a characteristic asymmetric distortion on spectral lines. Simulations capture the shape and amplitude of experimental errors which are around 0.1% on spectral transmittance residuals for 10 mW of total average power in 10 meters of common fiber, scaling up to above 0.6% for 20 mW and 60 m.

Dual-comb spectroscopy of ammonia formation in non-thermal plasmas

Plasma-activated chemical transformations promise the efficient synthesis of salient chemical products. However, the reaction pathways that lead to desirable products are often unknown, and key quantum-state-resolved information regarding the involved molecular species is lacking. Here we use quantum cascade laser dual-comb spectroscopy (QCL-DCS) to probe plasma-activated NH3 generation with rotational and vibrational state resolution, quantifying state-specific number densities via broadband spectral analysis. The measurements reveal unique translational, rotational and vibrational temperatures for NH3 products, indicative of a highly reactive, non-thermal environment. Ultimately, we postulate on the energy transfer mechanisms that explain trends in temperatures and number densities observed for NH3 generated in low-pressure nitrogen-hydrogen (N2–H2) plasmas.

Reference-free dual-comb spectroscopy with inbuilt coherence.

We demonstrate a simple system for dual-comb spectroscopy based on two inherently coherent optical frequency combs generated via seeded parametric downconversion. The inbuilt coherence is established by making the two combs share a common comb line. We show that the inbuilt coherence makes it possible to use a simple numerical post-processing procedure to compensate for small drifts of the dual-comb interferogram arrival time and phase. This enables long-time coherent averaging of the interferograms.

Cavity-enhanced dual-comb spectroscopy with wide spectral band tuning

Cavity-enhanced dual-comb spectroscopy holds significant research value by combining the high sensitivity of cavity enhancement and dual-comb high resolution and rapid measurement. However, due to the presence of intracavity dispersion, the insufficient coupling between the enhancement cavity and the optical frequency comb restricts the measurement bandwidth of the system, making it unable to provide abundant spectral information of the sample. To overcome this limitation, we propose a cavity-enhanced dual-comb spectroscopy with wide spectral band tuning by a single-point locking tuning scheme, which achieves a tuning capability of over 9 THz at the central wave number of 6400 cm−1. This represents a significant improvement compared to the untuned cavity-enhanced spectrum, increasing the measurable bandwidth by 2.5 times. We verified the capability of the scheme by measuring the rovibrational spectrum of CO2 in various frequency bands within the 6250–6550 cm−1 range. In each tuned frequency band, we achieved an enhancement factor of ∼950, signal-to-noise ratio (SNR > 600) in 30 s, and resolution of 250 MHz. The cavity-enhanced dual-comb spectroscopy with wide spectral band tuning holds promise for potential applications in fields such as trace gas analysis and respiratory diagnostics.