A simple photonic approach to generating and anti-dispersion transmitting the quaternary phase-coded microwave waveforms is proposed and experimentally demonstrated. In this approach, an integrated dual-polarization binary phase shift keying (DP-BPSK) modulator is adopted and driven by two parallel pre-encoding signals as well as two RF inputs with 90-deg phase shift. By simply presetting the bias voltages on the DP-BPSK modulator, the phase-coded microwave signals with desired quadrature phase shifts and the immunity to dispersion-induced power fading can be obtained. A complete theoretical analysis on operation principle is presented. Experiments on the generation and anti-dispersion transmission of the 1 Gb/s quaternary phase-coded waveforms with the center frequencies of 6 or 7 GHz over 10 km SMF are successfully carried out. The proposed system can achieve the waveforms with multiple phase coded formats and transmit them over long-distance optical fiber without dispersion-induced power fading. Besides, it also features simple architecture, excellent reconfigurability, better Doppler tolerance and sensitivity of objective recognition, which is highly desirable in modern radar networks based on optical fiber transmission.
KEYWORDS: Ultrafast phenomena, Pulse signals, Signal attenuation, Dispersion, Single mode fibers, Analog electronics, Education and training, Signal detection, Optical engineering, Signal intensity
An all-optical digital-to-analog conversion (DAC) scheme based on time-domain pulse spectrum encoding is proposed and experimentally demonstrated. In this approach, the ultrafast optical pulses are first time-broadened and frequency-chirped based on wavelength-to-time mapping and then segmented and power weighted in both time and spectrum domains to produce the multi-band optical carrier. The optical carrier is intensity-modulated by the serial digital inputs to realize the time-domain pulse spectrum encoding. Each spectrum-encoded pulse is then time-compressed to achieve the incoherent weighted intensity summation of digital bits within each word. This approach generates the multi-band optical carrier and achieves the corresponding incoherent intensity summation using all-fiber structure; the system configuration is greatly simplified. Moreover, the time-domain pulse spectrum encoding could efficiently exploit the superwide spectrum resource offered by ultrafast optical pulses and potentially improve the system conversion resolution. A proof-of-concept experiment of a 4-bit DAC system based on time-domain pulse spectrum encoding is carried out, and the obtained results validate the feasibility of the proposed approach. In addition, the system performance in terms of the effective number of bits is investigated.
Dual-chirp waveform, as a type of wideband radar signal that can improve the range-Doppler resolution in radar system has been studied widely. In this paper, a photonic scheme for generation of frequency- and bandwidth-doubling dualchirp waveform is proposed and experimental demonstrated. An integrated dual-parallel MZM is applied, where a microwave signal and a single-chirp signal are modulated, biasing at null and full points, respectively. An electrical rather than optical band-pass filter that used in the system can reduce the complexity. Experimental results perform the generated dual-chirp waveforms with frequency of 6 GHz and bandwidth of 2 GHz. The time-bandwidth product is about 154.2. Potentially, this approach can be a method for high frequency, large bandwidth and long duration dual-chirp waveforms generation
A scheme for compensation of dispersion-induced power fading is proposed and experimentally demonstrated. The system transmits two modulated laser lights via a wavelength division multiplexer and a wavelength division demultiplexer. By optimizing the power ratio and the phase difference between the two modulated signals, the dispersion-induced power fading can be adequately compensated. A key significance of our approach is that no adjustments are required when the input radio frequency signal scans over a broad bandwidth of 1 to 12 GHz. We successfully demonstrate the compensation of the power fading for both 20- and 30-km fiber links. An improvement of spurious-free dynamic range is also achieved.
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