Study On Several Key Technologies Of Microwave Photonic Signal Processing

Microwave photonics is an emerging interdisciplinary subject, the research areas of which include photonic generation, processing and measurement of microwave signals; radio-over-fiber systems; opto-electronic devices processing signals at microwave rates and optical control of microwave devices. Compared with the conventional microwave systems, microwave photonic systems could not only effectively solve the "electronic bottleneck" restrictions, but also lead to some unique advantages, such as large bandwidth, low loss, light weight, and immunity to electromagnetic interference. Microwave photonic technologies could even achieve many functions that are difficult for conventional microwave systems. Therefore, microwave photonics are widely used in many fields, such as military and defense systems, medical treatments, communication systems, and aeronautics & astronautics in recent years.In this thesis, we first give a brief introduction to the historical development, the main application areas and the main devices of microwave photonics. Then, three topics in microwave photonic signal processing area are discussed, which are microwave arbitrary waveform generation, photonic measurement of microwave frequency, optical single-sideband modulation and microwave photonic fractional Hilbert transformers. The major innovations and contributions are as follows:1. Two novel photonic approaches to realizing phase-coded microwave/mm-wave signal generation with large frequency tunability are proposed and demonstrated based on electro-optic modulators and fiber Bragg gratings (FBG). The generation of frequency tunable phase-coded microwave/mm-wave signals with the tuning range of 22-27 GHz and 40-50 GHz are achieved. A pseudo-random binary sequence is used as a phase-coding signal to verify the robustness of the pulse compression technique to noise. A pulse compression ratio of 67.5 or 128 is achieved, respectively. A theoretical frequency tuning range of 12.5-100 GHz or 40-110 GHz is obtained. Such a large tunable frequency range is difficult to achieve using conventional electronic techniques. 2. A thorough analysis on pulse distortions due to the third-order dispersion (TOD) and dispersion mismatches in a phase-modulator-based temporal pulse shaping (TPS) system for the generation of a repetition-rate-multiplied pulse burst is performed. We demonstrate that the profile of a repetition-rate-multiplied pulse burst and the shape of the individual pulses in the pulse burst are distorted due to the TOD and the dispersion mismatches of the dispersive elements. The tolerance of the system to the TOD and the dispersion mismatches when employing an input optical pulse with different pulse width is studied. A technique to use predistortion of the RF modulation signal to tackle the pulse distortions is discussed.3. Three novel photonic approaches to realizing instantaneous microwave frequency measurement based on frequency-to-power mapping are proposed and demonstrated. The first two approaches are based on microwave frequency to optical power mapping, in which a measurement range of 1-10 GHz is realized. The first approach is based on the calibration-look-up-table method, which sets no strict requirements on the shape of the FBG Thus, the measurement error caused by the shape error of the FBG in the second approach could be eliminated, which makes the measurement error of the first approach within 0.08 GHz. In the second approach, a linear power ratio function is constructed, leading to a constant resolution in the maximum measurement range. However, as this approach requires a high accuracy of the FBG spectral shape, the measurement error is about 0.2 GHz. The third approach is based on frequency to microwave power mapping, in which the measurement range is 0.5-40 GHz. However, as the linear segment of the sinusoidal transfer function is used, and there is an inherent error, which makes the impact of noise relatively large, the measurement error of the third approach is about 0.5 GHz.4. A photonic approach to implementing a microwave channelized receiver based on wavelength division multiplexing using an optical comb is proposed. In the approach, a flat optical comb with 11 comb lines is generated using two cascaded Mach-Zehnder modulators. Frequency analysis of a microwave signal with multiple-frequency components is realized by using the optical comb together with an optical etalon with a periodic transfer function, a wavelength division multiplexer (WDM) and a photodetector array. The system is investigated numerically. The reconfigurability of the system realized by tuning the comb-line spacing and the peak positions of the etalon is also evaluated. The improvement of the dynamic range of the system using an optimized periodic filter is also discussed.5. An all-optical approach to realizing optical single sideband (OSSB) modulation based on optical Hilbert transform implemented using an FBG is proposed and demonstrated. In the experiment, an OSSB signal with a frequency from 6 to 15 GHz and a sideband suppression ratio as large as 20 dB is generated. The transmission of the OSSB signal over a single mode fiber of 45.6 km is also studied, and the power fading effect resulted from the chromatic dispersion is eliminated successfully.6. Two approaches to realizing continuously tunable microwave fractional Hilbert transformer (FHT) based on a uniformly-spaced microwave photonic delay-line filter (MPF) and a nonuniformly-spaced MPF are proposed and demonstrated. In the first approach, the MPF with true negative coefficients is realized based on a polarization modulator and polarization-modulation to intensity-modulation conversion in an optical polarizer. The tunability of the fractional order is achieved by tuning the coefficient of the 0th tap. An FHT with a tunable order from 0.3 to 1 is demonstrated. The accuracy of the FHT is evaluated; a phase deviation less than 5°within the passband is achieved. In the second approach, nonuniformly-spaced MPF is used. The advantage of using nonuniform spacing is that an equivalent negative coefficient can be achieved by introducing an additional time delay leading to aπphase shift, corresponding to a negative coefficient. An FHT with a tunable order between 0.24 and 1 is implemented. The accuracy of the FHT is also evaluated and the phase deviation within the passband is also less than 5°.