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.
Conventional temporal pulse shaping (TPS) for radio frequency (RF) arbitrary waveform generation (RF AWG) based on the Fourier transform relation between the input–output waveform pair requires the electronic AWG to generate RF input signal, which greatly limits the output waveform diversity due to the relative low sampling rate and bit resolution of electronic AWG, i.e., high-fidelity square waveforms are hard to be achieved since high-resolution broadband Sinc input signals are difficult to be generated by current commercial electronic AWGs. The approaches based on TPS with phase modulation incorporating with iterative algorithms can relatively improve the waveform diversity by applying the optimal phase information. However, time-consuming iterative algorithms significantly restrict the waveform reconfigurability, i.e., desired RF waveforms cannot be generated in real-time. We propose a novel high-stable and reconfigurable RF AWG scheme with multi-tone inputs, which aims to improve the output waveform diversity with simple manipulation and high stability. In our design, any desired RF waveform can be achieved in real-time by simply adjusting the power values of multi-tone inputs. A proof-of-concept experiment was implemented, which fully verified the feasibility of the approach. The system performance in terms of output waveform stability was investigated in detail. As no electronic AWG is employed and no iterative algorithms are required, the proposed design provides a promising solution for high-performance reconfigurable photonic-based RF arbitrary waveforms generation.
A unique high-performance digital to analog conversion (DAC) approach based on dual-electrode Mach–Zehnder modulators (DEMZMs) is proposed and experimentally demonstrated. According to the modulation transfer characteristics of DEMZM, the output waveform can be edited in-line by properly adjusting the input digital signals. In our design, a 2-bit photonic DAC module can be realized by using only one DEMZM, which is biased at the minimum transmission point. The scheme is extendable, i.e., a 4-bit DAC system can be achieved by using two parallel DEMZMs, and an 8-bit system can be obtained with four DEMZMs. As few modulators are employed and the system configuration is simplified, the proposed approach can greatly improve the bit resolution with less complexity. A proof-of-concept experiment of a 4-bit DAC system is successfully carried out, which fully verifies the feasibility of the proposed approach. Moreover, the system performance in terms of integral nonlinearity and differential nonlinearity is also discussed. The proposed scheme provides a potential solution for high-bandwidth and high-resolution photonic DAC.
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
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