Single-cavity dual-comb fiber lasers and their applications

Single-cavity, dual-comb lasers are those specially designed mode-locked lasers that can emit more than one, asynchronous ultrashort pulse trains with stable repetition frequency difference between them. Unlike the long-studied, widely-used femtosecond lasers generating one stable pulse train, systematic investigation on them and their potential dual-comb applications only began, based on the fiber laser platform, around a decade ago, despite sporadic and limited reports of similar lasing phenomena since the beginning of the mode-locked laser studies. From a historic perspective, the birth of this novel technology is the lucky outcome of the timely collision of perpetual search for novel pulsing laser dynamics and concerted pursuit of open-minded solutions for out-of-lab dual-comb systems in the 2010s. In this review article, first, the current schemes to implement single-cavity dual optical frequency comb fiber lasers and their applications are summarized, based on the concept of multiplexed mode-locked lasers. The characteristics of reported single-cavity, dual-comb fiber lasers are discussed as well as their applications in spectroscopy, ranging, Terahertz (THz) spectroscopy, and asynchronous optical sampling (ASOPS). Finally, the more recent development of single-cavity, multi-comb lasers is presented

Dual terahertz comb spectroscopy with a single free-running fibre laser

Dual THz comb spectroscopy has the potential to be used as universal THz spectroscopy with high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility due to the bulky size, high complexity, and high cost. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength pulsed light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such pulsed light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. This considerably stable frequency difference enables dual THz comb spectroscopy with a single free-running fibre laser. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of not only THz spectroscopy but also THz-pulse-based measurements.Comment: 29 pages, 7 figure

Period doubling eigenstates in a fiber laser mode-locked by nonlinear polarization rotation

Due to the weak birefringence of single mode fibers, solitons generated in fiber lasers are indeed vector pulses and exhibit periodic parameter change including polarization evolution even when there is a polarizer inside the cavity. Period doubling eigenstates of solitons generated in a fiber laser mode-locked by the nonlinear polarization rotation, i.e., period doubling of polarization components of the soliton, are numerically explored in detail. We found that, apart from the synchronous evolution between the two polarization components, there exists asynchronous development depending on the detailed operation conditions. In addition, period doubling of one polarization component together with period-one of another polarization component can be achieved. When the period tripling window is obtained, much complexed dynamics on the two polarization components could be observed.Comment: 6 page

Breathing dissipative solitons in mode-locked fiber lasers

Dissipative solitons are self-localized coherent structures arising from the balance between energy supply and dissipation. Besides stationary dissipative solitons, there are dynamical ones exhibiting oscillatory behavior, known as breathing dissipative solitons. Substantial interest in breathing dissipative solitons is driven by both their fundamental importance in nonlinear science and their practical applications, such as in spectroscopy. Yet, the observation of breathers has been mainly restricted to microresonator platforms. Here, we generate breathers in a mode-locked fiber laser. They exist in the laser cavity under the pump threshold of stationary mode locking. Using fast detection, we are able to observe the temporal and spectral evolutions of the breathers in real time. Breathing soliton molecules are also observed. Breathers introduce a new regime of mode locking into ultrafast lasers. Our findings may contribute to the design of advanced laser sources and open up new possibilities of generating breathers in various dissipative systems

Collision-induced Hopf-type bifurcation reversible transitions in a dual-wavelength femtosecond fiber laser

Collision refers to a striking nonlinear interaction in dissipative systems, revealing the particle-like properties of solitons. In dual-wavelength mode-locked fiber lasers, collisions are inherent and periodic. However, how collisions influence the dynamical transitions in the dual-wavelength mode-locked state has still not been explored. In our research, dispersion management triggers the complex interactions between solitons in the cavity. We reveal the smooth or reversible Hopf-type bifurcation transitions of dual-color soliton molecules (SMs) during collision by real-time spectral measurement technique of TS-DFT. The reversible transitions from stationary SM to vibrating SM, revealing that cavity parameters pass through a bifurcation point in the collision process without active external intervention. The numerical results confirm the universality of collision-induced bifurcation behavior. These findings provide new insights into collision dynamics in dual-wavelength ultrafast fiber lasers. Furthermore, the study of intermolecular collisions is of great significance for other branches of nonlinear science.Comment: 11 pages, 4 figure