Fiber-comb filters demonstrate significant application potentials in ultrafast optics, optical information processing, fiber sensing, laser processing, and laser guidance. As a class of fiber-laser control devices, filters play an irreplaceable role in laser generation, soliton formation, and pulse-waveform control, which are essential components of tunable, narrow-bandwidth, and multi-wavelength fiber lasers. Therefore, studying the effect of filtering on pulse generation and transmission and manufacturing new filter devices are valuable.
In the experiments, first, a Mach-Zehnder interferometer (MZI) is developed by connecting two arms with two 3 dB couplers through fiber splicing. The fabricated MZI is then incorporated into a laser cavity. The laser cavity adopts a hybrid cavity mode-locking method that combined nonlinear polarization rotation (NPR) and few-layer black phosphorus (BP). Second, the ultrafine fiber is extracted from the standard single-mode fiber (SMF) via the flame brush technique, the tapered fiber is knotted, and the fabricated micro-knot resonator (MKR) is obtained. The fabricated MKR is then incorporated into the laser cavity. The laser cavity is mode-locked using a real saturable absorber.
Based on the MZI multi-wavelength mode-locked fiber laser, when the pump power reaches 270 mW, a multi-wavelength continuous laser output is generated (Fig. 4). To obtain different pulse states, we increase the pump power to 327 mW and adjust the polarization controller (PC) to realize single-wavelength locking (the central wavelength is adjustable) (Fig. 5). The NPR suppresses mode competition and introduces the filtering effect. The modulation depth of the sample MZI is large; thus, multi-wavelength mode-locked laser sequences are realized at room temperature. A dual-wavelength mode-locked laser is obtained by increasing the power to 400 mW (Fig. 6). Three-wavelength mode-locked lasers with different distance intervals are obtained by further increasing the power to 420 mW (Fig. 7). The interval between two adjacent wavelengths is an integer multiple of the free spectral range. By further increasing the power to 440 mW, four-wavelength mode-locked laser is obtained (Fig. 7).
In the passive harmonic mode-locked fiber laser based on MKR, by increasing the pump power to 60 mW, we observe bound-state mode-locking, accompanied by a gradual accumulation of nonlinearity in the cavity (Fig. 9). Under the same pump power, in the absence of MKR, the pulse shape output by the laser cavity is observed to be a typical soliton pulse (Fig. 10). In a numerical simulation, researchers found that the addition of a narrow-band filter to the resonator limits the pulse bandwidth, the filtering effect leads to pulse splitting, and the splitting threshold of the soliton can be further reduced to generate more pulses. Under the combined action of dispersion, nonlinear effects, filter effects, and other factors in the passive mode-locked fiber laser cavity, multiple pulses are obtained and transmitted back and forth in the resonator simultaneously. Each pulse is evenly distributed at equal intervals. The phases between the pulses are matched, thus achieving passive harmonic mode-locking. The fundamental repetition frequency of the laser cavity is 8.6 MHz. When the pump power is increased to 100 mW, the 8th-order harmonic mode-locking is generated. By increasing the power and adjusting the PC, the 24th-order harmonic mode-locking is obtained. The fundamental repetition frequency in this case is 206.4 MHz (Fig. 11).
A theoretical model to further study the effects of the filter on the laser is established. The propagation path of the pulse in the resonator cavity is simulated, and the influence of various devices in the cavity on the pulse is considered. The pulse circulated in the cavity for one circle is used as the input signal for the next cycle until the pulsed light field reaches a stable self-consistent state. An evident modulation phenomenon is observed in the spectrum (Fig. 12), and stable double pulses can be observed in the time domain. In this case, the formation of multiple pulses is mainly a result of the spectral filtering effect.
Filtering effects have a significant impact on pulse formation, waveform regulation, and dynamic transmission. In this study, a multifunctional thulium-doped fiber laser with an all-fiber MZI as a compact comb filter is demonstrated to achieve a multi-wavelength continuous wave (CW) laser output and multi-wavelength mode-locked laser pulse output. A single-wavelength mode-locked laser in the spectral range can be used to tune the output and simultaneously realize four-wavelength mode-locked pulses. An all-fiber MKR is fabricated using the tapering technique. Bound solitons are obtained in the cavity of an erbium-doped fiber laser, and the mode-locked state is realized by switching from the fundamental frequency state to harmonic mode-locking. By adjusting the power and polarization state in the resonator, the 24th-order harmonic mode-locking is achieved. The pulse shape is still bound-state soliton. The experimental results and numerical analysis reveal that the filtering effect has a significant influence on the pulse shape.
