High Capacity Hybrid Fiber-Optic/Wireless Communications System.

摘要

The microchip laser is a good candidate for producing a low noise optical beam. As the microchip laser works with locked modes, it is suitable for the optical generation of microwave or millimeter wave signals. However, the frequency difference between the two modes is not stable enough and therefore the generated signal will be unstable as well. We developed a new approach for the stabilization of the relative frequency difference between the modes. For that purpose a high stability millimeter wave oscillator was designed and constructed utilizing dielectric disc resonators. The signal of the stable oscillator was injected into the laser resonator providing this way a stable difference frequency for the optical generation of microwave signals. However, there is another problem because the microchip laser has a noise peak in the transmission band. We developed a new method to suppress that noise peak. The new method was verified by both simulations and measurements. An experimental set-up was also built. It contained a high-power diode laser pumping a mode locked crystal laser. The frequency difference between the modes was around 20 GHz. After fiber transmission a 20 GHz signal was generated at the reception side by detecting the optical signal having two modes. About 15 dB noise suppression was achieved in the experiments. New modulation methods were theoretically investigated to reduce the bandwidth requirement in combined optical-wireless systems. First of all, the 16 QAM modulation of a subcarrier proved to be a proper new approach.