A 1.5 μm pulse fiber amplifier in the eye-safe band is one of the prominent areas of research in pulse fiber laser technology. Depending on the pulse width, fiber amplifiers are used on different occasions; nanosecond fiber amplifiers are used in lidar, and microsecond fiber amplifiers are used in special material processing, biomedicine, and other fields. However, compared with nanosecond fiber amplifiers, the study of high-power microsecond pulsed fiber amplifiers is more challenging because of the steepening of the pulse waveform during amplification and the significantly increase in amplified spontaneous emission (ASE) at low repetition frequencies. In this study, a two-stage master oscillator power amplifier (MOPA) all-fiber structure was used to design and realize a 1550 nm microsecond rectangular pulse fiber amplifier with an adjustable repetition frequency of 10 Hz to 10 kHz. Compared with the fiber amplifier mentioned by M. Yu. Koptev and Svitlana Pavlova, the present one reduced the system complexity and cost and realized a wider range of adjustable repetition frequencies.
The entire fiber amplifier consists of four parts: signal optical pulse modulation, preamplifier, power amplifier, and pulse signal driving and control (Fig. 1). The optical signal pulse is modulated by a continuous seed source using an acousto-optic modulator (AOM). The preamplifier consisted of a double-clad erbium-ytterbium co-doped fiber (Nufern, PM-EYDF-12/130, length of 2.4 m), pump laser diode (pump wavelength is 940 nm, maximum optical power of 10 W), multimode pump beam combiner (MPC), and cladding pump stripper (CPS). The power amplifier included a double-clad erbium ytterbium co-doped fiber (Nufern, PM-EYDF-12/130, length 3.8 m), pump laser diode (940 nm, 80 W), MPC, and CPS. Both the preamplifier and power amplifier use the pulse-pump mode. The RF driver of the AOM, preamplifier pump driver, and power amplifier pump driver can be accurately synchronized and controlled, and the pulse width can be adjusted. The AOM operation is used to solve the waveform distortion caused by the narrowing of the pulse width in two different modes. By controlling the timing of the pulse signal and pre-shaping the waveform, the challenges of rapid ASE growth and transient effect of the pulse fiber amplifier are addressed.
By setting the pulse timing and pulse width of the AOM RF driver, the preamplifier pump driver, and power amplifier pump driver of the pulse fiber amplifier (Table 2), and optimizing the parameter settings of the AOM to pre-shape the pulse waveform of the input signal light, the output signal optical pulse waveform of the fiber amplifier is made rectangular (Fig. 4). The peak power reached 30 W at different pulse repetition frequencies and widths (Table 3). When the AOM was operated in mode 1, the optical-to-optical conversion efficiency was approximately 34%. Because the timing of the signal light pulse and pump pulse has been optimized to suppress the growth of the ASE, the spectral side-mode suppression ratio is close to 55 dB (Fig. 5), and there is no ASE generation. When the AOM operates in mode 2, the optical-to-optical conversion efficiency gradually increases from 19.50% to 24.19%. As the pulse repetition frequency continues to increase, the interval between two adjacent pulses becomes shorter, and the ASE accumulation time decreases, which is conducive to an increase in the signal optical power. However, keeping the pulse width constant and increasing the pulse repetition frequency leads to a continuous increase in the pulse duty cycle and number of pulses per unit time, resulting in an increase in the absorbed pump optical energy per unit time. The spectral side-mode suppression ratio is 25 dB. This is because when the pulse width is narrow, and the excess energy of a single pump pulse continues to convert energy between ions. The rapid accumulation of the ASE leads to a decrease in the spectral side-mode suppression ratio.
A 1550 nm microsecond pulse fiber amplifier with an adjustable repetition frequency of 10 Hz-10 kHz was designed and realized. The fiber amplifier adopts a pulse-pumped all-fiber dual-stage MOPA structure. By setting two working modes of the acousto-optic modulator (AOM), it realizes a working wavelength of 1550 nm, peak output power of 30 W, pulse width of 10 μs to 1 ms, and wide range adjustable rectangular pulse repetition frequency of 10 Hz to 10 kHz. In this system, the rapid growth of the ASE is suppressed by optimizing the timing and pulse width of the signal and pump beams. The waveform distortion problem caused by the transient gain effect in microsecond pulse fiber amplification is overcome by pre-shaping the signal optical pulse waveform, and a better rectangular pulse laser output is obtained. A microsecond pulse fiber amplifier has high peak power. The pulse width and repetition frequency of the amplifier can theoretically be adjusted to a wider range. The microsecond pulse fiber amplifier has broad application prospects in laser drilling, laser coloring, and biomedicine.
