We present an experimental investigation of the optical gain properties of 1.3 mu m GaInNAs double quantum well ridge waveguide laser diodes. High-resolution gain spectra versus injection current and temperature were obtained by measuring the modulation depth introduced into the spontaneous emission spectrum-by the Fabry-Perot resonances. As the injection current increases, the modal gain spectral peak experiences a small blueshift over the photon energy, and the magnitude increases asymptotically, saturating at the lasing threshold level of 24.4 cm(-1). The peak of the modal gain spectra exhibits a redshift with an average rate of 0.58 nm/degrees C as the temperature increases from 30 to 50 degrees C. For wavelengths corresponding to photon energy below the band gap, the modal gain spectra converge to the internal loss of 7 cm(-1). The full width at half maximum of the gain spectrum is 41.1 meV at 30 degrees C, 40 mA and increases with injection current at a rate of 0.42 meV/mA. The high optical gain and low internal loss indicate that GaInNAs is a promising active material for long wavelength laser diodes. (c) 2006 American Vacuum Society.