After decades of development, mode-locked fiber lasers can provide laser pulses with high coherence, high pulse energy, and controllable pulse width and repetition rate. Mode-locked pulsed lasers can play a key role in some specific research areas. For instance, in biomedicine, lasers are used as light sources to perform coherent tomography imaging and the information of the samples under test can be collected and recorded at the same time. However, in the process, the signals of some substances with similar excitation wavelengths can interfere with each other, thus affecting the measurement results. Therefore, the development of wavelength tunable mode-locked lasers to improve spectral resolution is of great significance to the research in this field. We study the rapid tuning of the center wavelength of a narrow-spectrum passive mode-locked ytterbium fiber laser based on fast acousto-optic filtering technology. Combining fast acousto-optic filtering technology, we obtain a stable mode-locked pulse with a center wavelength tuning function. To investigate the reconstruction process of laser pulses during intracavity filtering and confirm the reliability of this technology, we record the real-time reconstruction process of laser pulses during the tuning of the center wavelength. We hope that our research can provide a reliable solution for applications requiring high spectral resolution.

The laser consists of a laser cavity and a two-stage amplifier. The fiber cavity consists of a semiconductor saturable absorption mirror (SESAM), a wavelength division multiplexer (WDM), a 40 cm long ytterbium-doped fiber (CorActive Yb406, YDF), a 90∶10 fiber coupler (90∶10 OC), a collimator, and a λ/2 waveplate (HWP). It is composed of acousto-optic tunable filter, reflect mirror, and piezoelectric ceramic transducer (PZT). The piezoelectric ceramic is combined with a mirror to lock and stabilize the output laser repetition rate by adjusting the length of the phase-locked loop feedback. The phase-locked loop is composed of a photodetector (PD), an RF amplifier, a bandpass filter, a mixer, a signal source, a low pass filter, and a proportional integral derivative (PID). The voltage intensity of the externally modulated signal can alter the intracavity pumping energy. The rising edge of the modulated signal can be recognized by the acousto-optic tunable filter driver and used to switch the filter wavelength. The arbitrary waveform generator drives the acousto-optic tunable filter and laser semiconductor with the edited modulation signal, such that the center wavelength of the laser can be tuned at high speed while maintaining the mode-locked state. To explore the pulse conversion process in the cavity during wavelength switching, a part of the laser after the first stage amplification is fed into the dispersion compensation fiber, and the stretched optical signal is converted into an electrical signal through a photodetector and transmitted to a high-speed oscilloscope. The real-time observation of the laser pulse reconstruction process can be realized by generating signals through the external arbitrary signal generator, and simultaneously modulating the pump working current of the cavity and the wavelength switching of the acousto-optic tunable filter.

The parameters of the laser are tested (Figs. 2 and 3), and the wavelength tuning ability and frequency stability of the laser are verified (Fig. 4). The phase noise and time jitter of the locked pulse are significantly improved. The time jitter of the locked laser is 9.58 ps, and the phase noise at 10 Hz is -72 dBc/Hz. The information on the pulse reconstruction process of the laser in the state of high pump power and the operation of the single pulse after adjusting the external modulation signal is recorded (Figs. 5 and 6). The information shows the pulse reconstruction time and spectrum of the wavelength tuning process. The spectral stability and the highest wavelength tuning speed can be defined. Also, the result of the dispersive Fourier transform test proves that by editing the external modulation signal to change the internal pump energy of the laser cavity and the filtering band of the acousto-optic tunable filter, a reliable mode-locked fiber laser with high-speed tuning of the center wavelength can be obtained.

We study the rapid tuning of the center wavelength of a narrow-spectrum passive mode-locked ytterbium fiber laser based on fast acousto-optic filtering technology. The narrow-spectrum mode-locked fiber laser system has an output power of 200 mW, a pulse width of 5.87 ps, a repetition rate of 40.874 MHz, and a spectral bandwidth of 0.15 nm. By programming the RF signal to drive the acousto-optic tunable filter, a stable mode-locked pulse with a center wavelength tunable in the range of 1016-1042 nm can be obtained. To investigate the reconstruction process of laser pulses during intracavity filtering, we employ the dispersive Fourier transform technology to visualize the real-time reconstruction process of laser pulses during the tuning of the center wavelength, and the results confirm that the highest central wavelength tuning frequency of the laser is about 5 kHz.