The Optical Heterodyne Microwave (millimeter Wave) Signals

The demand of communication rate is going up quickly, but the existent microwaves are too congested to provide enough bandwidth to support the services. So we need wider wavelength bands, such as millimeter waves. Because of the restriction millimeter waves cannot transmit at a long distance, and a new technique called Radio Over Fiber (ROF) is proposed. This technique combines optical fiber communication and wireless communication, and microwave is sub-carrier of the communication system. One of the most important techniques of the system is optical generation of microwave signals. This thesis mainly studied microwaves (millimeter waves) generation by optical heterodyne.First, this paper has introduced the principle of microwaves (millimeter waves) generation by optical heterodyne. This technique uses the differential frequency between two light-waves to generate microwaves. We can take one light source or two light sources, each of which uses different lasers. So there are some theoretic differences between them. When using one light source, we make optical filter to choose these two frequencies we need, and to subtract them. And then microwaves can be obtained. When using two light sources, we use the frequency deviation of two lasers, and the frequency offset is the frequency of microwaves. When the number of light source is different, we choose different lasers, so we compared two common lasers.Because of the limitation of optical devices, some optical parameters can affect the generated microwaves. This thesis conducted a deeply theoretical analysis and derivation of the polarization, initial phase offset, intensity offset, and the relative intensity noise of two lasers. And the results of the derivation are simulated. At the same time, this thesis calculated the formula of the phase noise and the line width of two lasers. The dispersion factor of fiber should be considered if we use this system in ROF communication.In order to compress the noises of the system, this thesis made some improvements. It used balanced detection for"optical generation of microwaves", and used optical injection locking. Balanced detection can reduce the noise of the system, and optical injection can reduce the frequency offset. This thesis had established the noise mode of the system, analyzed and calculated the formula of the system's signal to noise ratio (SNR). The result verified balanced detection can improve SNR of communication system. Optical injection locking will cause the salver laser to lock onto the master's sideband frequency and to resonate there. This thesis had analyzed the theory of OIL, and used software to simulate it. The result of simulation established that the characteristic of microwaves (millimeter waves) generated by optical injection locking is better. At last, we combined optical injection locking and balanced detection together, and simulated the system.