Source: De Gruyter

Understanding Injection Locking in Laser Technology

Injection locking is a crucial technique in laser technology, particularly for continuous-wave single-frequency laser sources. This method is primarily used when high output power needs to be combined with minimal intensity noise and phase noise. Achieving low-noise performance is challenging in high-power lasers due to their susceptibility to mechanical vibrations, thermal influences, and the inability to use very low-noise pump sources.

The Challenges of High-Power Lasers

High-power lasers often face difficulties in maintaining low-noise performance. Mechanical vibrations and thermal influences can significantly impact the laser’s stability. Additionally, using optical filters within the laser resonator is not desirable as they can degrade power efficiency and may not withstand high power levels.

Traditional Amplification Techniques

A conventional approach involves creating a low-noise, low-power laser and amplifying its output using a high-power fiber amplifier, known as the master oscillator power amplifier (MOPA). However, this method has inherent problems. Amplifier noise can raise the noise level, preventing the achievement of the standard quantum noise level. Moreover, multiple amplifier stages may be needed for large amplification factors, and fiber nonlinearities can cause issues, especially for single-frequency operation.

Injection Locking: A Different Approach

Injection locking offers a solution to these challenges. In this technique, a high-power laser, referred to as the slave laser, generates the output power. The noise level is reduced by injecting the output of a low-noise, low-power master laser (or seed laser) through a partially transparent resonator mirror. The close frequency relationship between the master laser and the free-running slave laser ensures that the slave laser operates precisely on the injected frequency with minimal noise.

Injection locking allows for operation near the quantum-limited intensity and phase noise. The resonator mode typically used is a Gaussian mode, although higher-order modes can be enforced if needed. It is important to note that injection locking is distinct from injection seeding, which does not involve frequency locking.

Self-Injection Locking

Self-injection locking is a related technique where the seed signal originates from the same laser instead of an external source. This involves providing optical feedback to the laser resonator, often with spectral filtering. For instance, a narrowband fiber Bragg grating can provide filtered optical feedback to a laser diode. This setup can significantly reduce the laser linewidth compared to situations without an external reflector.

Conclusion

Injection locking and self-injection locking are essential techniques in laser technology, offering solutions to the challenges faced by high-power lasers. By understanding and implementing these methods, laser systems can achieve high output power with minimal noise, enhancing their performance in various applications.

Source: ResearchGate
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