Characteristics and applications of frequency modulation of gallium arsenide external cavitiy lasers

The chirp-to-power ratio (CPR) spectrum of two types of external cavity lasers, the quantum well laser and the channel-substrate planar (CSP) laser, were measured by interferometric techniques. The CPR spectrum was derived including the effects of intrinsic material gain compression, carrier spatial hole burning in the lateral dimension of the gain region, and the carrier transport and capture in the quantum well laser. The theoretical model fits very well with the experimental data and gives a good explanation for the phase discrepancy between the CSP laser and the quantum well laser. By fitting the data with the model, some important parameters like photon lifetime and the equivalent gain compression coefficients due to carrier spatial hole burning and carrier transport and capture effects in quantum well lasers can be obtained. The model shows that two major contributions to the CPR spectrum at low modulation frequencies, carrier spatial hole burning and carrier transport and capture effects, are structure dependent. Therefore, with proper design of the laser structure, one can either enhance or reduce the CPR at low modulation frequencies.; A fiber grating external cavity laser which is very robust, compact, and free of alignment problems was constructed. The linewidth of the fiber grating external cavity laser is measured to be about 10 kHz, which is equivalent 6 km of coherence length in the optical fiber. An optical frequency domain reflectometry (OFDR) system containing this fiber grating external cavity laser is demonstrated to have 62 dB of sensitivity when detecting Fresnel backreflection and 2 meters of resolution at a 115 meter range in optical fiber. This system was able to detect Rayleigh backscattering in optical fiber with 20 dB signal-to-noise ratio (SNR). The phase noise limitation on the distance range for the OFDR was investigated, and the measured SNR data agreed with theoretical simulations for various distances. The OFDR technique has the potential to be applied to 1550 nm wavelength with a very high dynamic range by using an erbium doped fiber laser.