A novel method for determining gain in long-cavity Fabry-Pérot diode lasers is presented. Gain is extracted from a return-loss measurement via a simple model. The experimental setup is based on the injection of a low-coherence probe signal into the laser cavity and synchronous detection of the reflected light. Using a fiber-optics setup, gain can be determined in commercial fiber-coupled laser modules. The novel technique is particularly suitable for diode lasers with very long cavities. To illustrate the method,the gain spectra of a 980-nm pump diode laser with 1800-µm cavity length are determined. Gain saturation due to probe signal injection is addressed.
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