The spectral output of cw DH GaAs/GaAlAs laser is analyzed by using Czerny-Turner spectrometer at room temperature. Below the threshold current (85mA), the effect of the spontaneous emission is observed and it decreases with the increasing current, the spectral width at FWHM being about 50 - 240Å. Above the threshold current, the stimulated emission is abruptly narrowed, and the single mode operation is obtained.
Each longitudinal mode is composed of four transverse modes and the mode spacing between the 0th and 1st transverse mode is increased from 0.61 Å at 64mA to 1.2 Å at 96mA. By using the theory of Hermite-Gaussian mode, FWHM is calculated for the transverse optical field distribution inside the resonator and it is found that FWHM is 7.2 ㎛ at 64 mA and 5.3 μm at 96 mA of injection current.
Below the threshold current, the net gain coefficient is obtained and based on the measurement of the ratio of the maximum to minimum of the Fabry-Perot resonance of the spontaneous emission, at the wavelength of the peak intensity (λ_p=8350Å), the net gain coefficient is found as a linear function of the injection current and its slope is about 3.2 $cm^{-1}$/mA.
The shift of the spectral mode is measured as a function of the injection current, the total shift of the spectral mode at the threshold current is about -13.4Å and the equivalent reduction of the refractive index is 0.006. This result agrees with the previously reported result by Thompson.(17)
