Radio frequency modulation characteristics and application of short coherent semiconductor laser

  Objective  Short coherent laser light source can eliminate the stray light formed by the reflection of the front and rear surfaces of the optical element to be measured in high-precision interferometry, which is an ideal light source for low-coherence interferometers. There are important applications in optical coherence tomography, refractive index and thickness measurement of organic materials, surface profile detection of optical elements, etc. The imaging quality of the interferometer will be affected by the light source, and the appropriate parameters are very important for the semiconductor laser to obtain high-quality short coherent light source through RF modulation. However, the spectral linewidth of semiconductor lasers is narrow. It is of great significance to reduce coherence length through the coherence control technology.

  Methods  A short coherent light source was obtained by radio frequency modulation using a Fabry-Perot laser diode with central wavelength of 637 nm. The spectral properties of short coherent semiconductor lasers under RF modulation are theoretically studied based on laser rate equations and modulation characteristics. A short coherent light source system (Fig.2) was built to study the effects of laser slope efficiency, bias current, RF signal frequency and amplitude on the coherence length of semiconductor lasers. Compared with the existing short coherent light source with RF modulation under the same conditions, its improvement effect on the interference image quality was verified.

  Results and Discussions  The spectral linewidth of laser was measured by spectrometer. The short coherence characteristics of two Fabry-Perot lasers with different slope efficiency were studied and the results show that the semiconductor laser with high slope efficiency has greater output power variation under the same modulation signal, which is helpful to achieve good modulation effect. The coherence length of a semiconductor laser is the smallest when the bias current is slightly larger than the threshold current (Fig.4). When the bias current is small, the linewidth of the laser is narrow. This is because some RF signals work below the threshold current, which leads to the abnormally low output power of the laser and affects the RF modulation performance. When the bias current is too large, the coherence of the light source is enhanced, and the increased injection current intensifies the mode competition, the number of longitudinal modes output by the semiconductor laser is reduced and the spectral line width is narrowed. The coherence length of the semiconductor laser is negatively correlated with the frequency (Fig.5) and amplitude (Fig.6) of the RF modulation signal. With the increase of the frequency and amplitude of the modulation signal, the emission spectrum of the semiconductor laser shows multi-longitudinal mode output, the spectral line width is broadened, and the coherence length decreases. An experimental setup for measuring the surface profile of transparent parallel plate glass was built (Fig.8). The short coherent light source obtained by using the parameters in this paper makes the interference image have higher image quality. Compared with the existing short coherent light source, the contrast can reach 0.9318, which is increased by about 51.1%. While avoiding background noise, the interference fringes with surface information are displayed more clearly (Fig.9).

  Conclusions  Under the condition of bias current I_\rmb = 1.3I_\rmth, a semiconductor laser with higher slope efficiency is selected. With the increase of modulation signal frequency and amplitude, the coherence length of the laser decreases, and the shortest coherence length can reach 90 \textμm at RF signal frequency f_m = \text950 MHz and amplitude A_m = \text19 dBm . It can be used to measure transparent parallel plate optical elements as thin as 0.09 mm, and the interference image contrast is 0.9318, which is higher than the existing short coherent light source. The research improves the performance of short coherent light source and has broad application prospects in the field of low coherent interferometry.