We demonstrate a convenient approach to precisely tune the polarization state of a nonlinear-polarization-rotation mode-locked Yb-doped fiber laser by using an electronic polarization controller. It is shown to benefit self-starting of mode-locking states, with precise tuning of the spectral profile, pulse width, and carrier-envelope offset frequency. The pulse width changed linearly by 0.78 ps in the time domain, and the carrier-envelope offset frequency shifted ~77.5 MHz in the frequency domain with a slight change of the driving voltage of 30.7 mV applied on the controller, corresponding to a polarization rotation of 0.0135π. This facilitated precise and automatic regeneration of a particular mode-locking state by setting an accurate voltage at the polarization controller with a programmed microprocessor control unit.

With two asynchronous frequency combs beating on a single detector, dual‐comb spectroscopy (DCS) enables rapid, broadband, and precise molecular fingerprinting on a comb tooth‐by‐tooth basis and thus holds much promise in molecular sensing. Sensitivity, however, has been an Achilles’ heel of this technique. Here, an approach for ultrasensitive nonlinear DCS based on plasmon‐enhanced coherent Raman spectroscopy (CRS) with ultrathin (100 nm) nanoporous gold films is presented. A high‐resolution (<8 cm−1) Raman spectrum is obtained within 32.8 µs for characterizing low‐density chemicals. The achieved sensitivity enhancement reaches ≈106–8 relative to nonenhanced dual‐comb CRS of liquid samples. The demonstrated approach may open up a new avenue for fast identification of trace amounts of chemicals.

We demonstrated a polarization-maintained dissipative soliton fiber laser. It was self-started and capable of outputting stable nanosecond mode-locked pulses through the nonlinear rotation inside fibers connected with a Faraday reflective mirror. The setup of the fiber laser ensured the output pulses with good polarization extinction, high pulse contrast, and quite good long-term stability against environmental disturbance. The mode-locked pulses were centered at 1030 nm with a narrow spectral bandwidth and had a pulse duration of 7 ns, while the pulse repetition rate could be varied from 1 to 8 MHz by setting the fiber cavity length. The output pulse energy reached 20 nJ at the repetition rate of 1 MHz.

Dual-comb spectroscopy (DCS), without moving mirrors, enables fast optical sampling of molecular vibrations and results in high-resolution and high-accuracy Fouriertransform spectra. This motionless technique holds much promise in gas sensing and environmental monitoring. However, in many cases, these applications require a mature device continuously operating for days or even months, thus posing a challenge to long-term stability of this delicate technique. In this paper, we demonstrate the feasibility of DCS for long-term routine spectral monitoring. A compact dual-comb spectrometer is built based on adaptive sampling and simple frequency stabilization schemes. The spectrometer is ceaselessly recording and displaying, in real time, transmission spectra of a phase-shifted fiber Bragg grating and of gas-phase samples for over 1200 h (50 days). We spectroscopically validate the system by measuring absorption lines of ν 1 + ν 3 band of C 2 H 2 and of 2ν 3 band of CH 4 and comparing the experimental data with HITRAN database.

We experimentally explored the influence of passive harmonic mode locking on the temporal and spectral features of a ytterbium-doped fiber laser. Similar dependences of free-running linewidth of the laser carrier-envelope offset frequency (f0) on the intracavity net dispersion were observed for the fundamental-, second-, and third-order mode-locking states. Due to the reduction of nonlinear effects and supermode phase locking that balanced the third-order dispersion in the fiber cavity, the third-order harmonic mode-locking exhibited the narrowest free-running f0 linewidth of ~120 kHz in the near-zero net dispersion regime.