| Microwave Photonics is an interdisciplinary field that combines radio-frequency engineering and photonic technologies. It involves the study of photonic devices and technologies used to generate, transmit and process microwave and millimeter signals. Thus it benefits many advantages that are difficult or even impossible to be realized in the radio-frequency domain, such as low loss, wide bandwidth, light and immunity to electromagnetic interference, etc. Hence it has various applications in radar systems, wireless communications, cable television systems, optical signal processing and high-speed optical packet switched networks.As an effective means of changing the characteristics of a DFB semiconductor laser, optical injection introduces dynamic properties that can be widely used in Microwave Photonics. Through optical injection, the modulation response and output properties of the slave laser can be obviously improved, such as the broadened modulation bandwidth, the enhanced resonance frequency and the reduced chirp and noises. Besides, the injected signals can be selectively amplified by the slave laser under optical injection.In this dissertation, the dynamic properties of the DFB semiconductor laser under optical injection are investigated in theory and experiments. By applying these effects in Microwave Photonics, we have done some research in optoelectronic oscillators (OEOs) and radio-over-fiber (RoF) systems. The main content provided in this dissertation are concluded as follows:1. The background and applications of Microwave Photonics are briefly introduced. And the study history of optical injection and its applications are reviewed.2. A novel scheme to realize a frequency widely tunable OEO, in which neither a high-speed external modulator nor an electrical filter is required to enable the loop oscillation, is proposed and demonstrated by experiment. Through optical injection, the relaxation oscillation frequency of the DFB laser is enhanced and its high modulation efficiency can enable the loop oscillation with a RF threshold gain of less than 20 dB. Using a commercial DFB laser with the bandwidth of 10 GHz, microwave signals with a frequency tuning range from 5.98 to 15.22 GHz are generated by adjusting the injection ratio and frequency detuning between the master and slave lasers. The phase noise of the generated 9.75 GHz microwave signal is measured to be-104.8 dBc/Hz@ 10 kHz frequency offset. Using a photo-detector with higher bandwidth and an electrical amplifier with flatter gain response, the maximum tunable frequency can be further increased.3. Photonic microwave upconversion using an optically injected distributed feedback (DFB) semiconductor laser based optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. Due to the wavelength-selective amplification characteristic of the DFB laser under optical injection, a stable oscillation can be established in the OEO loop and the slave laser is injection-locked by the+1st sideband. When the DFB laser is modulated by a baseband signal, only the injection-locked+1st sideband is strongly modulated while the others are almost not affected, thus a near single sideband modulation (SSB) is realized. Moreover, the optical carrier to sideband ratio of 0 dB can be achieved by adjusting the injection ratio and frequency detuning, which is optimum for the transmission performance of a radio-over-fiber (RoF) system. An electrical bandpass filter is incorporated in the OEO loop to prevent affection from the modulated signal to the oscillation. The OEO produces a high-quality local oscillator (LO) signal and simultaneously serves as a broadband mixer. By photo-detecting the optical signal after the circulator, an upconverted signal is obtained. In this experiment, baseband signals with data rates of 622 Mb/s,1.25 Gb/s and 2.5 Gb/s are successfully upconverted by the OEO operating at 10.66 GHz and transmitted over 25.2 km single mode fiber without dispersion compensation.4. Cross gain modulation (XGM) and four wave mixing (FWM) effects in optical injection process are investigated. Due to the XGM effect in DFB semiconductor under optical injection, when modulating the injected light with a LO signal, the emitted light of the slave laser will carry the phase-reversed LO signal. Directly modulating the DFB laser with an IF signal, then the upconversion process is realized.There is an optical phase conjugation effect in FWM process which can be used to compensate the chromatic dispersion in analog photonic links. In this section, the DFB laser simultaneously acts as a nonlinear medium to induce FWM and a pump laser. Compared with the conventional OPC based on a semiconductor optical amplifier or a dispersion shifted fiber, an extra pump laser is no longer required. Experimental results show that the power fading caused by the CD in a 50.4 km fiber span can be compensated with a 3-dB bandwidth up to 33 GHz. Meanwhile, the spur-free dynamic range is enhanced by 12.6 dB·Hz2/3.5. This chapter is the conclusion of all the research work in the dissertation. |
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