Design of an All-Fiber MOPA High-Power Pulsed Laser

Introduction

The main structural types of fiber lasers include single resonant cavity, beam combination, and Master Oscillator Power Amplifier (MOPA) configuration. Among them, the MOPA structure has become a research hotspot due to its ability to realize high-performance pulsed laser output with tunable pulse width and repetition frequency.

MOPA Laser Design

The proposed MOPA laser consists of an electronic control system and an optical path system. Its working principle is as follows: the master oscillator (MO) is a high-performance semiconductor seed laser, which generates a seed signal with adjustable parameters through direct pulse modulation. The field-programmable gate array (FPGA) main controller outputs pulse current signals with adjustable parameters. These signals drive the seed source through the driver circuit, completing the initial modulation of the seed light. The pump source driver circuit receives control instructions from the FPGA board, starts the pump sources, and generates pump light. The seed light and pump light are coupled by a combiner and injected into a two-stage amplifier module with Yb³⁺-doped double-cladding fiber (YDDCF). During amplification, Yb³⁺ ions absorb the pump energy to form population inversion, and through traveling-wave amplification and stimulated emission, the seed light is amplified to generate high-power nanosecond pulsed laser output. Due to the increased peak power, gain-clamping effects may lead to pulse compression. In practical applications, multi-stage amplification is often used to further enhance output power and efficiency.

Electronic Control System

The MOPA laser circuit system consists of an FPGA controller, pump sources, seed source, driver boards, and amplifiers. The FPGA generates pulse electrical signals with tunable pulse width (5–200 ns) and repetition frequency (30–900 kHz), driving the seed source to produce mW-level seed pulses. This seed light enters the two-stage amplifier module, composed of pre-amplifier and main amplifier, and is finally output through a collimated isolator as high-energy short pulses. The seed source includes a photodetector for real-time output monitoring, feeding signals back to the FPGA. The FPGA controls pump driver circuits to operate pumps 1, 2, and 3. If no seed signal is detected, the pump shuts down automatically to prevent fiber and device damage.

Optical System

The MOPA optical system adopts an all-fiber design with an oscillator and two-stage amplifier. The oscillator uses a semiconductor laser diode (LD) with a center wavelength of 1064 nm, linewidth of 3 nm, and maximum CW power of 400 mW as the seed source, combined with a fiber Bragg grating (FBG) reflector of 99% reflectivity at 1063.94 nm and 3.5 nm linewidth.

The two-stage amplifier module uses reverse pumping with YDDCFs of 8 μm and 30 μm core diameters, with cladding absorption coefficients of 1.0 and 2.1 dB/m at 915 nm. Key components include:

1. Pump combiners with coupling efficiency of 95% using (1+1)×1 and (2+1)×1 FCs.

2. Mode field adaptors (MFA) to match different YDDCF cores and minimize loss.

3. Cladding mode strippers (CMS) to remove residual pump and reflected light.

4. Bandpass isolators (BPF-ISO) to suppress ASE and ensure unidirectional signal propagation.

For thermal management, the YDDCF is wound onto aluminum grooves coated with thermal grease and fixed with high-temperature tape. The optical components are connected by splicing, ensuring optical performance and system stability.

Experimental Analysis and Discussion

Seed Source and First-Stage Amplifier

Oscilloscopes, optical spectrum analyzers, and power meters were used at multiple test points. By varying pulse width and repetition rate, seed source output power and waveforms were recorded. Results show that reducing pulse width decreases output power but increases amplification gain (22.8–56.8×). Spectrum shifts were minimal. At 200 ns/50 kHz, first-stage output power increases linearly with pump power until distortion occurs at high pump levels. The optimal operating point was chosen at 1 V driving voltage, corresponding to 2.2 W pump power.

Laser Output Characteristics

Output characteristics tested include power, waveform, spectrum, spot, and beam quality. At 200 ns/50 kHz, output power increased linearly with pump power, reaching 20.85 W at 34.38 W pump input (32.22 dB gain, 60.65% efficiency). Stability exceeded 95% over 48 hours continuous operation.

Pulse Waveform and Peak Power

After amplification, pulse compression occurs. Shorter seed pulses yield output more similar to the original waveform, while longer pulses distort more. Peak power analysis showed a maximum of 12.9 kW (200 ns/30 kHz) and minimum of 6.0 kW (20 ns/180 kHz).

Spectrum and Beam Quality

Spectral measurements at different pulse parameters confirmed stable central wavelength and absence of ASE or SRS nonlinearities. Beam quality (M²) initially measured ~1.79. By coiling YDDCF into a cloverleaf shape, beam quality improved significantly to M² = 1.345.

High-Reflection Tolerance and Stability

Beam quality measurements confirmed stable operation during high-speed and low-speed marking on stainless steel. Spectral monitoring indicated improved resistance to back-reflection interference, especially in the longer wavelength components.

Conclusion

A MOPA pulsed fiber laser based on direct modulation of a semiconductor seed source was designed. Experimental results show that tunable pulse width and repetition frequency seed signals can be effectively amplified, producing high-power pulses without significant ASE or SRS effects. By optimizing fiber coiling and optical layout, beam quality and back-reflection tolerance were greatly improved, meeting the requirements of industrial material processing applications.