All-Fiber Passive Q-Switched Laser Based on Stimulated Brillouin Scattering

Introduction

Pulsed fiber lasers have attracted widespread attention due to their compact structure, high efficiency, and excellent beam quality. The high peak power pulses generated by these lasers play an important role in material processing, medical surgery, defense applications, and scientific research. Among the common approaches for generating pulsed output, Q-switching technology is one of the most effective methods.

Q-switching can be categorized into active Q-switching and passive Q-switching. Active Q-switching typically relies on electro-optic or acousto-optic modulators to control the intracavity loss, while passive Q-switching often uses saturable absorbers to achieve pulse modulation. However, conventional Q-switching devices usually have a limited damage threshold and are not well suited for full integration into fiber-based systems.

In this context, all-fiber pulsed lasers have clear advantages such as high damage threshold, low insertion loss, and excellent integration capability. Therefore, realizing passive Q-switching entirely within an all-fiber configuration has become an important research direction.

One promising approach is the all-fiber passive Q-switched laser based on Stimulated Brillouin Scattering (SBS), which utilizes Rayleigh scattering and SBS effects inside optical fibers to achieve passive Q-switching. The entire laser cavity is composed solely of fiber components, eliminating fragile bulk devices and enabling compact, stable operation.

Principle of SBS-Based Passive Q-Switched Fiber Laser

The Q-switching process is illustrated conceptually as follows:

1. Pump light is coupled into the gain fiber through a fiber combiner.

2. The gain fiber absorbs pump energy, achieving population inversion and generating amplified spontaneous emission (ASE).

3. During backward transmission, ASE undergoes Rayleigh scattering in single-mode fiber, with part of the scattered light redirected back into the cavity.

4. Together with a low-reflectivity fiber Bragg grating, this feedback forms a resonant cavity, narrowing the linewidth and building up laser oscillation.

5. As intracavity intensity increases, the SBS effect is triggered, dramatically increasing the cavity Q factor. Once the gain surpasses losses, a high-power pulse is emitted.

6. After the SBS effect subsides, the cavity returns to its low-Q state. Under continuous pumping, this cycle repeats, producing a stable sequence of Q-switched pulses.

SBS Threshold in Optical Fibers

SBS is a nonlinear optical effect that occurs when high-intensity pump light excites acoustic phonons in the fiber. These phonons modulate the refractive index, generating backward-scattered light that couples energy from the pump into a lower-frequency Stokes wave. This positive feedback mechanism enhances scattering until the SBS threshold is reached.

The SBS threshold can be expressed as a function of effective mode area, SBS gain, and effective fiber length. In general:

• Smaller core diameters reduce the mode area, lowering the SBS threshold.

• Longer fiber lengths increase interaction, further lowering the threshold.

For example, standard single-mode fibers with small core diameters can significantly reduce the SBS threshold, making it easier to trigger passive Q-switching. Experimental results show that when the fiber length exceeds ~20 m, the SBS threshold drops below 0.5 W, after which further length increases yield diminishing effects.

Experimental Results and Analysis

By carefully selecting the fiber length to optimize Rayleigh scattering and SBS, a fully fiber-integrated passive Q-switched laser output was achieved. The system exhibited three regimes as pump power increased:

1. ASE emission,

2. random laser action,

3. Q-switched pulsed laser output.

At higher pump powers (around 48 W), the laser produced:

• An average output power of ~12 W,

• A tunable repetition rate in the range of 3.7–100 kHz,

• Pulse duration of ~7 ns,

• Broadband near-infrared supercontinuum output covering 1030–1650 nm,

• Peak power up to ~17 kW.

These results confirm the feasibility of achieving high-power passive Q-switching entirely within an all-fiber configuration.

Engineering Applications

The demonstrated all-fiber passive Q-switched laser offers several key advantages compared with conventional Q-switched lasers:

• Simple and compact structure – no bulk optical modulators are required.

• High reliability – no easily damaged intracavity devices.

• Scalability – capable of producing higher average power and peak power.

• Cost-effectiveness – the all-fiber design reduces system complexity and maintenance.

Such systems are particularly promising for industrial and scientific applications, including:

• Laser marking and engraving, where high peak power pulses enable precise and clean ablation with minimal thermal effects.

• Micromachining of metals, ceramics, and polymers, benefiting from short pulse widths and high repetition rates.

• Medical laser systems, where compact and stable pulsed lasers can be applied in surgical tools.

• Defense and sensing technologies, where robust all-fiber architectures are desirable for field deployment.

Conclusion

The development of all-fiber passive Q-switched lasers based on SBS represents a significant step toward compact, reliable, and cost-effective pulsed laser sources. By leveraging the intrinsic nonlinear properties of optical fibers, such systems avoid the limitations of traditional Q-switching components, enabling high-power, short-pulse generation with broad application potential in materials processing, medical treatment, laser marking, and advanced research.