Dynamic enhancement of semiconductor laser using external optical feedback

Directly modulated fiberoptic links, because of their small size and low power consumption, are a clear choice for the distribution of signals to the active MMIC modules in antenna remoting applications. Previous attempts by many researchers have shown that the simultaneous improvement of laser bandwidth, linearity and noise characteristics is extremely challenging using the existing laser structure. To address this limitation, this thesis introduces a new approach for modifying the laser diode structure to attain a high modulation frequency, suppress intensity noise, and enhance dynamic range for baseband information. The new approach is based on the use of an external optical cavity to provide feedback of coherent light to the laser cavity. First, new rate equations were developed from Maxwell-Bloch equations, resulting in a model that overcomes the difficulties associated with phenomenologically modified standard rate equations. Phase and amplitude information on the feedback light and modulation frequency of the microwave signal were incorporated into the new equations. The model was then used for small-signal analysis and noise modeling. Experiments were conducted to verify the theoretical predictions. In these experiments, the measured threshold current reduction of the external cavity laser indicated the amount of feedback light in the active region of laser. A low frequency uncoated InGaAsP laser diode was used with an external cavity to attain higher microwave frequencies. In addition, laser nonlinearity was modified through a harmonic conversion process. Other experiments showed that the relative intensity noise of a laser with external optical feedback is significantly colored noise, due to the external optical feedback. More specifically, the relative intensity noise increases at the resonant frequency corresponding to the round-trip time delay in the external cavity. However, relative intensity noise is suppressed by about 20 dB to 40 dB at all non-resonant frequencies. Through the combination effective bandwidth increase and intensity noise reduction, the spurious free dynamic range improved at least by 8 dB over a broad bandwidth. Through this new approach, high quality data signals and high frequency reference signals can be simultaneously transmitted. This capability is highly attractive in applications such as the phased array antenna used in satellite communication.