Research On The High Dynamic Range Microwave Photonic Links

Microwave Photonic Link (MPL) refers to the microwave signal is modulated on an optical carrier and is transferred through the optical fiber. Compared to an all electronic link, MPL can provide many advantages, such as low loss, large bandwidth, small size, lightweight and immunity to electromagnetic interference. So the MPL has been widely used in radio over fiber, radio astronomy, radar system and other applications. Currently, the radar and electronic warfare system require the high dynamic range MPL. For instance, the anti-jamming radar requires the MPL with the Spurious-Free Dynamic Range (SFDR) of 120-130dB Hz2/3. However, the external modulation MPL based on Mach-Zehnder Modulator (MZM) with the SFDR of 110dB·Hz2/3 is one of the most popular links. Therefore, the high dynamic range MPL is the key technique of the microwave photonic technologies, and is one of the research hotspots in microwave photonics. In addition, the long-distance transmission of local oscillator requires the MPL with low residual phase noise. This paper focuses on the dynamic range and the residual phase noise of the MPL. A series of research work has been done and listed as follows:(1) The development process of the microwave photonics, the applications of MPL and the research status of MPL are reviewed. The structure and the main devices of external modulation MPL based on MZM are introduced. The theoretical model of this link is studied, and the relationship between the performance metrics of link and the parameters of the devices is derived. The impacts on the performance of link by the loss, dispersion and stimulated Brillouin scattering of optical fiber are analyzed.(2) A dual-wavelength dual-parallel modulation linearization method is proposed to improve the dynamic range of the external modulation MPL. The theoretical and experimental study of this linearization method is performed. The measurement results show that the SFDR of the linearized link is up to 122.5dB·Hz4/5. Using the MZM low-biasing technology to improve the performance of linearized link, the SFDR of linearized link can be reached 127.6dB·Hz4/5.(3) A two-tone cross-correlation method is proposed to measure the residual phase noise of long-haul MPL. This method can eliminate the phase noise of measurement system introduced by the referencing source, which enable the measurement of residual phase noise of long haul MPL. The theoretical model of this method is deplored and the experimental research on this method is also performed. The residual phase noise of 1m,2km and 6km MPL are measured based on this method. The measurement results show that the residual phase noise of 6km MPL deteriorates 10dB than 1m MPL.(4) The experimental research on the residual phase noise of photodiode is performed based on the two-tone cross-correlation method. The measurements show that:a) the residual phase noise of photodiode increase with the reverse-biased voltage, b) When the photodiode works at linear region, its residual phase noise does not change with the input optical power. When the photodiode works at the saturated nonlinear region, its residual phase noise increases with the input optical power, c) When the linearity of photodiode gets worsen, its residual phase noise will get larger.(5) The current driver and constant temperature controller of laser and the biasing controller of MZM are developed. Further more, the microwave photonic transmitter and receiver are designed. The experimental measurement shows that:a) In 12 hours, the output optical power of laser drifts less than 0.2%. The variation of the MPL gain caused by optical power drifting is no more than 0.02dB. b) When the environmental temperature varies between 15℃ and 35℃ repeatedly, the output optical power of MZM drifts 1.4% and the biasing angle of MZM drifts less than 0.8°. The variation of the MPL gain caused by the biasing angle of MZM drifting is no more than 0.001dB. c) The performance of the microwave photonic transmitter and receiver without electric amplifier is measured. Its gain equals -18.4dB. The noise figure is equal to 35.6dB and the SFDR is 109.3dB·Hz2/3. d) The low-biased microwave photonic transmitter with electric amplifier is also designed. Its gain equals 2.5dB and the noise figure is equal to 6.7dB. The SFDR is 112.3dB·Hz2/3.(6) A novel measurement approach for the half-wave voltage of phase modulator is presented. The half-wave voltage can be directly obtained by measuring the input 1dB gain compression point of phase modulation Mach-Zehnder Interferometer (MZI) MPL and the differential delay time of MZI. Compared to the traditional method, this method does not require large driving power, eliminates the manual adjustment of microwave power at every frequency point, enables the measurement at the low frequency range, prevents the calibration of optical insertion loss and responsivity of photodiode. The half-wave voltage of Covega LN53 phase modulator is measured by this method. The measurement error is smaller than 0.2V and the measurement results agree well with the data provided by the manufacturer.