Mid-infrared (MIR) spectroscopy is instrumental in addressing gas molecule-related environmental and ecological challenges. Especially, massively parallel sensing capability is critical to multi-species molecules analysis, enabling the demands for various MIR gas characterizations. However, real-time, high-accuracy parallel sensing for multiple gases remains a significant challenge due to the limitations in laser bandwidth and sampling speed. Here, we present a broadband MIR dual-comb spectrometer for the simultaneous detection of multiple greenhouse gases. This MIR spectrometer employs a scheme of difference frequency generation (DFG), directly producing a wide spectrum spanning 3.2–4.7 μm with over 300,000 comb-tooth-resolved frequency lines at a 100 MHz resolution. In addition, we demonstrated the parallel detection of four mixed gas molecules (CH4, C2H2, CO, and N2O), in which the absorptions were in excellent agreement with HITRAN database. This broadband MIR dual-comb spectrometer is promising to be integrated with only fiber devices and periodically poled lithium niobate waveguides, providing a high-precision, high-efficiency approach for massively parallel sensing in atmospheric or industrial monitoring.

Optical frequency combs (OFCs) have enabled significant opportunities for high-precision frequency metrology and high-resolution broadband spectroscopy. Although nonlinear photonics chips have the capacity of frequency expansion for OFCs, most of them can only access the limited bandwidths in the partial infrared region, and it is still hard to satisfy many measurement applications in the ultraviolet-to-visible region. Here, we demonstrate a compact broadband OFC scheme via the combination of three χ(2) nonlinearities in a three-stage periodically poled lithium niobate (PPLN) chain. With a supercontinuum spectrum OFC delivered into the PPLN chain, the intra-pulse diffidence frequency generation, optical parametric amplification, and high-order harmonic generation were carried out in sequence. It is crucial that the harmonics of the 1st–10th orders are simultaneously obtained with an offset-free OFC spectrum from 0.35 to 4.0 μm. In view of the great potential for integration and spectral expansion, this wideband frequency comb source will open a new insight for the valuable applications of two-dimensional material analysis, biofluorescence microscopy, and nonlinear amplitude-phase metrology.

We demonstrate an all-polarization-maintaining (APM) fiber mode-locked laser based on nonlinear polarization evolution (NPE). A well-designed Sagnac fiber loop is employed to establish the nonlinear polarization evolution process in a polarization beam splitter (PBS) figure-8 fiber laser. Nonlinear loss curves are calculated to verify the saturable absorption characteristic of this NPE-based APM oscillator. Then, we simulate the pulse propagation process in the cavity to demonstrate the pulse mode-locked formation. Finally, we also design a realizable compact scheme to further reduce noise disturbances, achieving a 101-fs mode-locked pulse train with a 0.3-mrad integrated phase noise and a 0.006% integrated relative intensity noise (RIN). This figure-8 fiber laser provides a new scheme for compacting low-noise compact APM fiber lasers based on the NPE mode-locked mechanism.

The mid-infrared optical frequency comb is a powerful tool for gas sensing. In this study, we demonstrate a simple mid-infrared dual-comb spectrometer covering 3–4 μm in LiNbO₃ waveguides. Based on a low-power fiber laser system, the mid-infrared comb is achieved via intra-pulse difference frequency generation in the LiNbO₃ waveguide. We construct pre-chirp management before supercontinuum generation to control spatiotemporal alignment for pump and signal pulses. The supercontinuum is directly coupled into a chirped periodically poled LiNbO₃ waveguide for the 3–4 μm idler generation. A mid-infrared dual-comb spectrometer based on this approach provides a 100 MHz resolution over 25 THz coverage. To evaluate the applicability for spectroscopy, we measure the methane spectrum using the dual-comb spectrometer. The measured results are consistent with the HITRAN database, in which the root mean square of the residual is 3.2%. This proposed method is expected to develop integrated and robust mid-infrared dual-comb spectrometers on chip for sensing.

Dual-comb spectroscopy (DCS) has revolutionized numerous spectroscopic applications due to its high spectral resolution and fast measurement speed. Substantial efforts have been made to obtain a coherent dual-comb source at various spectral regions through nonlinear frequency conversion, where the preservation of coherence has become a problem of great importance. In this study, we report the generation of coherent dual-comb sources covering from the ultraviolet to mid-infrared region based on high-order harmonic generation. Driven by high-repetition-rate femtosecond mid-infrared dual-comb pump pulses, up to ninth-order harmonic was generated from the ultraviolet to mid-infrared region using an aperiodically poled lithium niobate waveguide. To investigate the coherence property of the high-order harmonic generation, DCS was performed at every generated spectral region from 450 to 3600 nm. The measured dual-comb spectra with distinctive tooth-resolved structures show the well-preserved coherence without apparent degradation after the cascaded quadratic nonlinear processes. The subsequent methane absorption spectroscopy at multiple spectral regions of different harmonics was carried out to characterize the spectroscopic capability of the system. These results demonstrate the potential of our scheme to generate compact and coherent broadband optical frequency combs for simultaneous multi-target detections.

We report a compact, tunable, self-starting, all-fiber laser-based asynchronous optical sampling (ASOPS) system. Two Er-doped fiber oscillators were used as the pulsed-laser source, whose repetition rate could be set at 100 MHz with a tuning range of 1.25 MHz through a fiber delay line. By employing phase-locked and temperature control loops, the repetition rate offset of the two lasers was stabilized with 7.13 × 10^-11 fractional instability at an average time of 1 s. Its capabilities in the terahertz regime were demonstrated by terahertz time-domain spectroscopy, achieving a spectral bandwidth of 3 THz with a dynamic range of 30 dB. The large range of repetition rate adjustment in our ASOPS system has the potential to be a powerful tool in the terahertz regime.

Gain-parameter-dependent transfer functions and phase-noise performances in a mode-locked Yb-doped fiber laser are measured in this study. It is discovered that the corner frequency in the amplitude and phase domains is determined by the absorption coefficient of the gain fiber, when the total absorption and other cavity parameters are fixed. This shows that an oscillator using gain fiber with higher dopant concentration accumulates more phase noise. Furthermore, we present net cavity dispersion-dependent transfer functions to verify the effect of dispersion management on the frequency response. We derive a guideline for optimizing mode-locked fiber laser design to achieve low phase noise and timing jitter.

Mid-infrared dual-comb spectroscopy is of great interest owing to the strong spectroscopic features of trace gases, biological molecules, and solid matter with higher resolution, accuracy, and acquisition speed. However, the prerequisite of achieving high coherence of optical sources with the use of bulk sophisticated control systems prevents their widespread use in field applications. Here we generate a highly mutually coherent dual mid-infrared comb spectrometer based on the optical–optical modulation of a continuous-wave (CW) interband or quantum cascade laser. Mutual coherence was passively achieved without post-data processes or active carrier envelope phase-locking processes. The center wavelength of the generated mid-infrared frequency combs can be flexibly tuned by adjusting the wavelength of the CW seeds. The parallel detection of multiple molecular species, including C2H2,CH4,H2CO,H2S, COS, and H2O, was achieved. This technique provides a stable and robust dual-comb spectrometer that will find nonlaboratory applications including open-path atmospheric gas sensing, industrial process monitoring, and combustion.