Q-switched laser is one of the important techniques to achieve high-energy laser pulses and greatly promotes the development and application of the field of optical engineering. With the modulation of cavity loss by optical switch in the laser cavity, the energy output by the laser is compressed into a pulse with a very short duration. For passively Q-switched lasers, saturable absorbers (SAs) with nonlinear saturable absorption are commonly used as optical switches. In recent years, 2D materials are used widely as SAs, such as topological insulators, black phosphorous, transition metal dichalcogenides and transition metal oxides. As one of the TMDs, the antimonous sulfide (Sb2S3) with smaller band gap as well as excellent nonlinear optical properties is a promising SA material. In this paper, we demonstrate a passively Q-switched erbium-doped fiber laser based on Sb2S3 SA, which could generate stable short pulses. By utilizing the Sb2S3 SA fabricated with optical deposition method, an all-fiber cavity is built. In the experiment, a stable Q-switched pulse trains with a repetition frequency of 30 kHz, a central wavelength of 1558.4 nm and a pulse width of 6.4 μs are obtained at a 980 nm pump power of 35 mW. The signal-to-noise ratio of radio-frequency spectrum at the fundamental frequency is about 50 dB, indicating the high stability of the Q-switched pulse. To the best of our knowledge, this is the first demonstration of Q-switched fiber laser based on Sb2S3 material, which could contribute as a new potential SA material for pulsed fiber lasers.
Programmable and fast wavelength-switchable ultrashort pulses have important applications in optical communication, optical sensing, micro-wave photonics, and other fields. Multi-wavelength mode-locked lasers are commonly developed to achieve wavelength tunability by using tunable optical filters, fiber Bragg gratings, Lyot filters providing the direct spectral manipulation of the pulses. Actually, manual control is essential in process of wavelength switching, which is very time-consuming and sometimes it is difficult to find the appropriate state of polarization for a certain mode-locking state when utilizing polarization adjustment. So a programmable mode-locked laser which can automatically switch to different mode-locking states is desired. Here, we demonstrate a fast electrically-controlled wavelength-switching scheme in ultrafast nonlinear polarization rotation mode-locked lasers. It is achieved by simply introducing an inline polarization beam splitter (PBS) with two output ports and followed by a 1×2 electrically-controlled optical switch in a fiber ring laser. By utilizing the polarization-maintaining-fiber-pigtailed inline PBS and a polarization controller, a nonlinear polarization evolution (NPE) mode-locking effect as well as a tunable Lyot filter are enabled in the fiber cavity. The optical switch could select the optical path with different polarization state for mode-locking at different wavelength.
Developing ultrafast lasers with controllable flexible pulses, such as wavelength and pulsewidth tunable lasers are desired for various applications like fiber telecommunication and optical sensing. To achieve wavelength or/and pulsewidth tunability in saturable absorber (SA) based mode-locked fiber lasers, some technologies including tunable band pass filter, fiber Bragg grating, diffraction grating mirror, 45°tilted fiber grating, and tunable Lyot filter are proposed to provide the direct spectral manipulation of the pulses. In this paper, we demonstrate a simple bandwidth-tunable ultrashort pulse generation scheme for a nonlinear polarization rotation mode-locked laser. By utilizing a polarization-maintaining-fiber-pigtailed inline polarizer and a polarization controller, a nonlinear polarization evolution (NPE) mode-locking effect as well as a bandwidth-tunable Lyot filter are enabled in the laser cavity when the intracavity polarization settings adjusted. The Lyot filter formed by the inline polarizer and its birefringence of the two fiber pigtails with lengths of 0.3 m could introduce a spectral filtering effect. By only tuning the intracavity polarization controller, the spectral bandwidth is continuously tuned in the range of 7.8 to 3.5 nm. We attribute the lower limit of the spectral bandwidth to the nonlinear self-phase modulation requiring narrow pulses in nonlinear polarization rotation mode-locked lasers. These results provide a simple way for generating subpicosecond pulse with variable spectral bandwith or pulse duration without using a saturable absorber.
We demonstrate a passively mode-locked and a passively Q-switched erbium-doped fiber laser respectively by utilizing a same saturable absorber fabricated with tungsten trioxide (WO3). When the WO3 saturable absorber is employed to provide the pulse narrowing effect, Q-switched pulses were observed with a repetition rate of 44.11 kHz and a pulse width of 3.42 μs. Moreover, the Q-switched laser could realize hybrid mode-locking after an in-line polarizer was inserted, which could introduce an additional pulse narrowing effect of nonlinear polarization rotation under a certain polarization state. The 3- dB spectral bandwidth and the repetition rate of mode-locked pulses are about 7.5 nm and 22.51 MHz respectively. The pulse train is stable with a signal to noise ratio of 70 dB.
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