Objective Dual-comb mode-locked lasers are emerging laser sources that have attracted significant attention in the fields of dual-comb spectroscopy, optical sensing, and absolute-distance measurements. In recent years, the use of dual-comb spectroscopy to detect fast dynamic processes has become a new development trend but has also presented corresponding technical challenges. The detection of fast processes requires the shortest possible detection time and high update speed while maintaining a certain spectral bandwidth, signal-to-noise ratio, and accuracy. A high repetition rate and large repetition rate difference are essential for improving the detection speed of dual-comb spectroscopy, which can reach a time resolution on the order of nanoseconds. However, this may reduce the spectral bandwidth. Another approach is to use a tunable wavelength in a dual-comb light source. In this study, we present a dual-comb laser with tunable wavelength and flat-topped spectrum generated from an all-normal dispersion linear-cavity fiber laser. This novel dual-comb light source with flat-top spectral characteristics and tunable wavelength provides a scheme for fast segmented scanning dual-comb spectroscopy that has good reference value for rapid dual-comb spectroscopic applications. Methods The hybrid cavity investigated in our study is linear and consists of a fiber and free space, as shown in Fig. 1. The cavity is 3.97 m in length. In the fibered section, a single-mode ytterbium-doped fiber (YDF) with a length of 0.4 m is pumped by a 976-nm diode through a single-mode fiber wavelength division multiplexer. The group velocity dispersion introduced by the YDF is 23 ps(2)/km. The output of the fiber output coupler is 40% of the intra-cavity power. At one end of the cavity, a semiconductor saturable absorber mirror (SESAM) is attached to the fiber jumper, which has a polarization-maintaining fiber (PMF) tail of 0.75 m in length. The SESAM, which has a modulation depth of 30% and relaxation time of 500 fs, is used to achieve self-started mode locking. The PMF provides a certain amount of birefringence to the cavity, producing two pulsed lasers with different repetition rates. The light is coupled from the fiber to free space by a collimator. In the free space, a quarter-wave plate (QWP), half-wave plate (HWP), and blazed grating are used. The blazed grating has the engraved line density of 600 line/mm and is arranged in a Littrow configuration, which enables first-order diffraction light to be reflected into the cavity to form a filtering mirror. The filtering bandwidth can be controlled by adjusting the distance between the grating and fiber collimator. The polarization states of the intra-cavity light are adjusted using the HWP, QWP, and polarization controller 1. Polarization controller 2 and a fiber polarization beam splitter are used to isolate the two pulse trains that are orthogonally polarized. Results and Discussions In the experiments, when one pulse train is triggered, the other pulse train moves periodically due to the different repetition rates. Figure 2(d) shows the spectrum when the two pulse trains are not isolated. The central wavelength is 1037.5 nm and the 3 dB bandwidth is 4.05 nm. The flat-top shape of the spectrum indicates that the pulses are dissipative solitons derived from the all-normal dispersion. Notably, the use of dissipative soliton mode-locking under all-normal dispersion not only prevents the problem of intra-cavity dispersion compensation and simplifies the laser structure, it also allows for higher single-pulse energy without pulse splitting and produces a flat-top shape spectrum, which are attractive features for dual-comb light sources. To verify whether common-mode noise can be effectively eliminated, we simultaneously measure the change in the repetition rate of each comb and derive the repetition rate difference (RRD), as shown in Fig. 4. This measurement lasts 35 min and no shield is used to protect the laser from environmental disturbances. Results show that due to the common noise cancelation in the shared cavity, the peak-to-peak value and standard deviation of RRD fluctuation is suppressed to 7.1 Hz and 2.5 Hz, respectively. An interferogram is observed, as shown in Fig. 5, which benefits from the excellent mutual coherence. When the grating is rotated, the center wavelength of the spectrum is tuned from 1028.3 nm to 1041.1 nm, representing a tuning range of 12.8 nm, as shown in Fig. 6(b). During wavelength tuning, the spectral bandwidth and RRD of the dual-comb laser remain stable. A larger wavelength tuning range is possible, but the pump power and optical polarization state must be readjusted to reconstruct the dual-comb mode-locking state. Conclusions We present a dual-comb laser with tunable wavelength and flat-topped spectrum generated from an all-normal dispersion linear-cavity fiber laser. Results show that the central wavelength is tunable to a range larger than 10 nm, and the spectra are flat-topped with a bandwidth of approximately 4 nm. The fluctuation in the repetition rate is cancelled out by 80%, and the amount of drift in the repetition rate difference in the long-term drift is compressed to the Hz-order. In the experiment, the entire device is not encapsulated and therefore the coherence of the dual-comb laser can be further improved by isolating the device from the external environment. We believe that with appropriate optimization, this innovative dual-comb light source featuring flat-top spectral characteristics and an adjustable wavelength offers a promising approach for rapid segmented scanning dual-comb spectroscopy and thus has significant reference value for applications in the field of dual-comb spectroscopy.
