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Understanding Small-Signal Gain in Photonics

What is Small-Signal Gain?

The small-signal gain of a gain medium, such as a laser medium in a laser or amplifier, refers to the gain achieved for a very weak input signal that does not cause gain saturation. This implies that the signal intensity throughout the gain medium is significantly below the saturation intensity, especially in continuous-wave operation.

Factors Affecting Small-Signal Gain

In a four-level laser gain medium with minimal amplified spontaneous emission (ASE) and no parasitic lasing, the small-signal gain in a steady state is typically proportional to the absorbed pump power and the stored energy.

Applications of Small-Signal Gain

The threshold of a laser or an optical parametric oscillator is reached when the small-signal gain equals the total resonator losses. A high small-signal gain in a Q-switched laser helps achieve a shorter pulse duration. However, in high-gain amplifiers like fiber amplifiers, the achievable small-signal gain is often restricted by ASE or parasitic lasing.

Measurement of Small-Signal Gain

For measuring the small-signal gain of a fiber amplifier (based on a rare-earth-doped fiber), one must inject a weak test signal at the relevant wavelength that does not significantly alter the amplifier gain. Subsequently, both the launched signal power and the resulting amplified signal power need to be measured.

In cases like bulk lasers, where the injected input signal may be affected by the resonator mirrors, an indirect approach may be necessary. For instance, calculating the required small-signal gain to reach the laser threshold based on the known reflectivities of the resonator mirrors and determining at what pump power that threshold is achieved. The gain at other pump power levels can then be estimated, often assuming a linear dependence on the pump power.

Conclusion

Understanding small-signal gain in photonics is crucial for optimizing the performance of lasers, amplifiers, and other optical devices. By measuring and controlling small-signal gain, researchers and engineers can enhance the efficiency and effectiveness of various photonics applications.

Source: Electronics Stack Exchange
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