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Applications Of Microwave Photonic Technology And Fiber Bragg Gratings In Microwave And Sensing Systems

Posted on:2015-01-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:L GaoFull Text:PDF
GTID:1228330434959351Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Microwave photonics is an interdisciplinary area that mainly studies the photonic generation, processing and transmission of microwave signals for applications of telecommunications, radar, broadband phased array beamforming, sensors and so on. It arouses great interests in the past few decades for its advantages of high frequency, broadband, light weight, and immunity to electromagnetic interference. Fiber Bragg gratings (FBGs) are widely used in microwave photonic systems. The applications of microwave photonic technology and FBGs in microwave and sensing systems are investigated in this thesis. We propose several novel microwave photonic systems in microwave generation, filtering, and fiber-optic sensing, and a method for fabrication of long FBGs. Most of the works in this thesis are based on microwave photonic technology by using FBGs as the key components.The major works are as follows:1. An optoelectronic oscillator (OEO) for generation of a microwave signal with both tunable frequency and phase is proposed and experimentally demonstrated. In the proposed OEO, a single-sideband (SSB) polarization-modulated signal is generated by a polarization modulator (PolM) and a phase-shifted FBG (PS-FBG). The SSB polarization-modulated signal is then split into two parts. One part is sent to a photodetector (PD) and then fed back to the PolM to form the OEO loop. The other part is sent to an electronically tunable polarizer (ETP) and then applied to a second PD. The frequency tunability is achieved by tuning the wavelength of the optical carrier. The tunable phase is introduced by tuning the principal axis of the ETP.2. Photonic-assisted microwave frequency multiplication with a tunable multiplication factor (MF) based on an optical comb generator (OCG) and an embedded single-passband microwave photonic filter (MPF) is proposed and demonstrated. The optical comb is generated using two cascaded modulators which are driven by a microwave reference signal. In the proposed system, the embedded single-passband MPF is formed by using a sliced broadband optical source (SBOS) and a section of dispersion compensating fiber (DCF). By applying the optical comb to a PD, a fundamental frequency corresponding to the comb spacing and its harmonics are generated. By tuning the central frequency of the passband at a frequency corresponding to that of a specific harmonic, a microwave signal at that specific frequency is generated. The proposed system is experimentally demonstrated.3. Photonic generation of a phase-coded microwave waveform with an ultra-wide frequency tunable range using two PolMs is proposed and experimentally demonstrated. The first PolM (PolM1) is used to control the polarization direction of a linearly polarized light wave to have an angle of45°or135°relative to one of the principal axis of the second PolM (PolM2). PolM2operates in conjunction with a polarization controller (PC) and a polarizer as an equivalent Mach-Zehnder modulator (MZM). Depending on the polarization direction of the incident light wave, the equivalent MZM is biased at the opposite slopes of the transfer function. Thus, by applying a binary coding signal with a switching voltage of Vπ, which is the half-wave voltage of the PolM, to PolM1, a π phase-coded microwave waveform is generated. The key significance of the technique is that a phase-coded microwave waveform with an ultra-wide frequency tunable range can be generated.4. A frequency-tunable MPF with a narrow and flat-top passband implemented using a phase modulator (PM) and a superstructured FBG (SFBG) is proposed and experimentally demonstrated. The key component in the MPF is the SFBG, which is designed and fabricated based on the equivalent phase shift (EPS) technique. Two phase shifts are introduced, and the combination of the two phase shifts would lead to a flat-bottom notch in the reflection band. By incorporating the SFBG into the MPF, a narrow and flat-top passband is achieved. 5. A dual-passband MPF implemented based on phase-modulation to intensity-modulation conversion using a PM and an equivalent phase-shifted FBG (EPS-FBG) is proposed and experimentally demonstrated. The key component in the system is the EPS-FBG, which is designed and fabricated based on the EPS technique. The unique feature of the EPS-FBG is that EPSs are introduced to both of the±1st channels, leading to a notch in each of the two channels. Thus, by implementing phase-modulation to intensity-modulation conversion in the two channels, two passbands are produced. The central frequency of each passband is determined by the wavelength different between the notch and the optical carrier. In the design and fabrication, two phase shifts are introduced to the EPS-FBG to decrease the shape factor. In addition, a stimulated Brillouin scattering (SBS) assisted filter is incorporated in the system for carrier suppression to increase the spurious free dynamic range (SFDR) and decrease the noise figure (NF) of the MPF.6. A novel multi-longitudinal mode fiber laser vibration sensor is presented. There are many longitudinal modes in the laser cavity. The longitudinal mode beat frequency (LMBF) signal is generated by any two different modes. When the vibration is applied on the laser cavity, the LMBF signal is modulated by it. The modulated LMBF signal can be demodulated by a frequency-modulation demodulator.7. A multi-longitudinal mode fiber laser sensor system for simultaneous measurement of strain and temperature is proposed and experimentally demonstrated. The LMBF of the laser shifts with both strain and temperature, while, the output power of the laser only changes with temperature because of the erbium-doped fiber (EDF) used in the laser cavity. By monitoring both of the LMBF and the output power, the strain and temperature can be achieved at the same time.8. A multimode fiber laser sensor system for simultaneous measurement of strain and temperature is proposed and demonstrated. The beat frequency demodulation technique based on the microwave photonic technology is used. Because of the long cavity and birefringence, LMBF and polarization mode beat frequency (PMBF) are achieved in the beat frequency signals of the multimode fiber laser. They have different strain and temperature response, thus, the strain and temperature can be obtained by monitoring both of them for their different strain and temperature responses.9. A method for simultaneous measurement of strain and load by monitoring both the LMBF and PMBF in a multimode fiber laser based on beat frequency demodulation technique is proposed and demonstrated. The LMBF has a linear response to strain, while, it is insensitive to load. On the other hand, the PMBF has linear responses to both strain and load. Thus, by monitoring LMBF and PMBF simultaneously the strain and load can be obtained.10. A simple fiber ring laser sensing system with beat frequency demodulation technique is proposed. Because of the absence of mode selecting device, the mode spacings between multi-longitudinal modes in the laser can be large. Therefore, the LMBFs with high frequency are generated, and the high sensitivities and accuracies will be achieved.11. We propose that long FBGs with desired properties can be fabricated with two uniform phase masks and a translation stage of submicron precision based on the reconstruction-equivalent-chirp (REC) technique. The phase mismatch between the two phase masks can be compensated with the EPS, thoroughly. The length of FBG is not restricted by the length of phase mask anymore. In the proof-of-concept experiment, it is obtained that the original phase shift (OPS) caused by phase mismatch of the two phase masks is0.85π, and it keeps unchanged throughout the experiment. The OPSs in the±1st channels can be compensated with different EPSs. Moreover, as an example, a90mm-long FBG with π phase shift at the-1st channel is written by using two50mm-long phase masks.
Keywords/Search Tags:Microwave Photonic Technology, Fiber Bragg Grating, Microwave Generation, Microwave Photonic Filter, Fiber-Optic Sensor, Multimode Fiber Laser, Beat Frequency, andReconstruction-Equivalent-Chirp Technique
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