Applications Of Microwave Photonic Technology Based On External Modulators In Microwave And Sensing Systems

Microwave photonics (MWP) has been defined as an interdisciplinary area that mainly studies the generation, processing and transmission of microwave signals in the optical domain. The applications of MWP include radio-over-fiber systems, radar systems, optical signal processing, sensors and so on. MWP is regarded as a technology which can overcome the electronic bottleneck due to the advantages provided by the photonic devices such as large time-bandwidth product, high tunability and reconfigurability, and electronicmagnetic interference immunity. Based on different external modulators, this thesis studies some microwave photonic systems in microwave transmission, generation, processing. This thsis also applies the MWP technology to multi-longitudinal fiber laser sensors.The major works are as follows:1. A broadband chromatic dispersion (CD) compensation scheme using optical phase conjugation (OPC) based on a DFB semiconductor laser is proposed and experimentally demonstrated in microwave photonic links (MPLs). The DFB laser acts as a nonlinear medium to induce four-wave mixing (FWM) and a pump laser simultaneously. Compared with the conventional OPC based on a semiconductor optical amplifier (SOA) or a dispersion-shifted fiber (DSF), an extra pump laser is no longer required. Experimental results show the power fading caused by the CD in a 50.4 km MPL can be compensated with a 3 dB bandwidth up to 33 GHz. Meanwhile, the spur-free dynamic range (SFDR) is enhanced with 12.6 dB·Hz2/3; the transmission performance is also improved.2. A novel scheme for the generation of full-duty-cycle triangular pulses based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The fundamental tone corresponding to the repetition rate of the pulses is generated by the OEO. Biasing the Mach-Zehnder modulator at the quadratic point, the small-signal gain of the OEO is maximized while even-order harmonics in the optical intensity are suppressed. By properly setting the gain of the OEO loop, the amplitude of the first-order harmonic is 9 times of that of the third-order harmonic. After a 90° electrical phase shifter adjusts the phase of the electrical signal at the output of the photodetector, triangular pulses are obtained in the time domain. The oscillating frequency is tuned from 2 GHz to 10 GHz by the yttrium-iron-garnet filter and the phase noise of generated signal is also investigated. Triangular pulses with repetition rates of 3 GHz and 6 GHz are successfully generated. The root-mean-square errors between the generated and the ideal triangular waveforms are 6.4927e-4 and 9.0932e-4 at 3 and 6 GHz, respectively.3. A novel approach to generating a microwave frequency shift keying (FSK) signal using a polarization modulator (PolM) and a dual-polarization modulator is proposed and experimentally demonstrated. In the proposed system, the PolM is employed to modulate the polarization state of an input linearly polarized lightwave and switch it between two orthogonal directions, leading to the generation of a polarization shift keying (PolSK) signal. Then the PolSK signal is sent to a dual-polarization modulator which is made up of two polarization multiplexed sub-MZMs. Two microwave signals with different frequencies are applied to the sub-MZMs, respectively. After aligning the orthogonal directions of the PolSK signal with the axes of the dual-polarization modulator by a polarization controller, the PolSK signal can be converted to a microwave FSK signal. A proof-of-concept experiment is carried out to verify the proposed system. When the two sub-MZMs in the dual-polarization modulator are biased at quadrature point, a double sideband (DSB) microwave FSK signal at 3/6.5 GHz with a bit rate of 1.25 Gb/s is generated and transmitted over 10 km of single-mode fiber (SMF). When the two sub-MZMs in the dual-polarization modulator are biased at minimum point, an optical carrier suppression (OCS) microwave FSK signal which is immune to the power fading effect in SMF is implemented at frequency-doubled 6/14 GHz with a bit rate of 2.5 Gb/s.4. A novel microwave photonic filter (MPF) with multiple independently tunable passbands is proposed. A broadband optical source (BOS) is employed and split by a 1:N coupler into several branches. One branch is directed to a phase modulator which is modulated by a radio frequency signal and the other branches are delayed by optical delay lines (ODLs), respectively. All of these branches are combined by another 1:N coupler and sent to a dispersion compensation fiber which is used to introduce group delay dispersion to the optical signal. At a photodetector (PD), each time-delayed broadband lightwave beating with the sidebands produced by the phase modulator forms a passband of the MPF. By tuning the delay of each broadband lightwave, the center frequency of the passband can be independently tuned. An MPF with two independently tunable passbands is experimentally demonstrated. The two passbands can be tuned from DC to 30 GHz with a 3-dB bandwidth of about 250 MHz. The stability and dynamic range of the MPF are also evaluated. By employing more branches delayed by ODLs, more passbands can be generated.5. A tunable single bandpass microwave photonic filter (MPF) with an improved spurious free dynamic range (SFDR) based on a phase-shifted fiber Bragg grating (PS-FBG) is proposed and experimentally demonstrated. A phase-modulated double-sideband (DSB) signal is sent to a PS-FBG; when one of the sidebands falls in the notch of the reflection band of the PS-FBG, the reflected signal is converted to an intensity-modulated single-sideband (SSB) signal which leads to a single passband of the MPF. By tuning the wavelength of the optical carrier, the center frequency of the passband can be tuned correspondingly. The SFDR is improved by employing polarization mixing technique in a LiNbO3 phase modulator and a polarization diversity receiver. Experimental results show that the passband width and the frequency-tunable range of the single bandpass MPF are 80 MHz and 5.5 GHz, respectively. An SFDR improvement of 13.1 dB is successfully achieved.6. A photonic microwave downconverter with improved conversion efficiency and spurious-free dynamic range (SFDR) is proposed and experimentally demonstrated based on a dual-parallel Mach-Zehnder modulator (DPMZM) and a digital signal post-processing algorithm. The RF signal and LO signal are fed to the two sub-MZMs of the DPMZM which leads to an infinite isolation between the RF and LO ports. By biasing the two sub-MZMs and the parent MZM of the DPMZM at the minimum transmission point, the optical carrier can be greatly suppressed. As a result, the conversion efficiency is improved for the same power impinged on the PD. A preliminary experiment shows that a conversion efficiency of-12.7 dB can be achieved. On the other hand, without emulating the inverse link transfer function where exact parameters of the photonic link should be known, a simple post-processing algorithm which just needs the modulation index of the LO signal is employed to suppress the intermodulation distortion products. The SFDR of the downconverter is improved from 101.5 dB·Hz2/3 to 114.5 dB·Hz4/5by using digital linearization.7. A multiplexed multi-longitudinal mode fiber laser sensor system is proposed and demonstrated. By incorporating two matched wavelength division multiplexers (WDMs) and a semiconductor optical amplifier (SOA) into a fiber laser cavity, multiwavelength oscillation is established. Each wavelength corresponding to one channel of WDMs contains multi-longitudinal modes. The multiwavelength output of the laser is directed to another WDM which functions as a demultiplexer. By monitoring the longitudinal mode beat frequency generated at photodetectors following the WDM, the sensing information can be demodulated. Preliminary results for multiplexing two sensors measuring strain and temperature are presented to verify the principle of the system.