Research On Photonics-based Wideband Signal Acquisition

Wideband signal acquisition has become an essential technique in modern radar, radio astronomy, electronic warfare (EW) and other advanced communication systems. Conventional electronic solutions are limited with narrow bandwidth, high power consumption and susceptible to electromagnetic interference (EMI). To solve these problems, photonic techniques are preferable candidates due to its various advantages, such as high bandwidth, light weight, low loss, and immunity to EMI. Currently, photonic analog-to-digital conversion (ADC) and photonic compressive sensing (CS) are two major approaches of photonics-based wideband signal acquisition. This thesis is targeted at improving the performance of signal acquisition, including higher resolution and wider bandwidth. Several schemes of wideband signal acquisition based on the technique of photonic ADC and photonic CS are proposed and demonstrated.In this dissertation, the research background, major applications and important devices of microwave photonics are firstly introduced. Then, photonic techniques in the application of wideband signal acquisition is studied. The advantages, of photonic ADC and photonic CS based wideband signal acquisition compared to their electronic counterparts are analyzed. At last, some innovative structures and techniques are proposed and demonstrated to solve some existed problems. The major innovations and contributions are as follows:1. For overcoming the major limitation of classical model of photonic ADC, which uses electro-optic modulators (EOMs) with geometrically scaled half wave voltages, differentically encoded photonic ADC is proposed and demonstrated. In the scheme, a phase modulator (PM) and an interferometer are applied, which improves the feasibility of the ADC system. In addition, the realization of differential endcoding can increases the equivalent quantization level of the system. In order to improve the performance of the system, we replace the PM and interformeter with a dual-drive unbalanced modulator, which can reduce the size and loss of the system and is also benefical to the integration of the system.2. Based on the study of symmetrical number system (SNS) based encoding property, a novel photonic ADC scheme based on SNS is proposed and demonstrated. In the scheme, interferometer array with identical half wave voltages and several electronic comparators are applied. By setting the phase shift of adjacent interferometers, SNS with Gray code property are realized, which enhance the error correction capability of the system. To eliminate the amplitude jitter of the optical source, the technique of balanced detection is applied to improve the stability of the system.3. Wideband signal acquisition based on photonic CS is studied. The feasibility and existed limitations of spacial light modulator (SLM) and EOM based photonic CS is demonstrated and analyzed. Optical random mixing with balanced Mach-Zehnder modulator (MZM) structure for photonic CS is proposed, which simplify the mathematical model and the realized zero-mean measurement matrix can improve the performance of the system.4. One disadvantage of CS based signal acquistion is the system bandwidth is limited as applied random sequence should not less than twice of the signal bandwidth. To overcome this problem, photonic CS with the technique of photonic time stretch is proposed and demonstrated. To eliminate the dc component introduced by intensity modulating and direct detection based optical link, photonic time stretch with balanced structure is further proposed. System performance under different parameters is analyzed though numerical simulations.5. Three schemes of photonic compressive with microwave photonic filter to realize the process of low pass filtering or integration of CS are proposed and demonstrated. In the first scheme, incoherent multi-wavelength optical source and dispersive medium are applied and the integration of random mixed signal is realized with effect of group velocity dispersion. In the second scheme, a continuous-wave and dispersive medium is used to simplify the structure and the random mixed signal is low pass filtered with the function of dispersion induced RF power fading. In the third scheme, a multi-tap microwave photonic filter is realized by utilization of an optical frequency comb and a dispersive medium. The optical frequency comb is achieved by spectrally shaping of an amplified spontaneous emission (ASE) source with a silicon micro-ring resonator. The realized filter function has higher side-lobe suppression and better frequency response due to the controllable comb number. In addition, the use of compact on-chip component is beneficial to the integration of the system.