Xinkai Liu

Photonic Generation of Triangular-Shaped Microwave Pulses Using SBS-Based Optical Carrier Processing

A photonic approach to generate triangular-shaped microwave pulses using stimulated Brillouin scattering (SBS)-based optical carrier processing is proposed and experimentally demonstrated. In the proposed approach, only odd-order optical sidebands are obtained by externally modulating the CW light wave using a Mach-Zehnder modulator (MZM) biased at the minimum-transmission-point (MITP). Then the suppressed optical carrier is recovered by the amplification effect from the SBS gain. After that, all negative-order optical sidebands are removed by using an optical filter. Through opto-electronic conversion in a photodetector (PD), the amplitude of the 1st-order electrical harmonic can be tuned just to be nine times of that of the 3rd-order one when the modulation index is specified as 1.51. Therefore the target pulse is generated and its repetition rate can be flexibly tuned by adjusting the frequency of the radio frequency (RF) signal applied to the MZM. According to the principle above, triangular-shaped microwave pulses with repetition rates of 5, 8, and 10 GHz are generated in our experiments, respectively, showing desired waveforms and excellent tunability.

Optoelectronic Oscillators (OEOs) to Sensing, Measurement, and Detection

Besides distinct features on RF/optical signal generation, optoelectronic oscillators (OEOs) have also been rapidly developed as emerging techniques towards sensing, measurement, and detection. In this paper, we start with the conceptual architecture and the analytical model of OEOs. Then, three operation principles behind sensing, measurement, and detection applications are categorized, including the variation on the time delay of loop, the passband reconfiguration of microwave photonic filter in loop, and the oscillation gain from injection locking, which clearly clarify the X-to-frequency mapping (X denotes target parameter or signal) for supporting practical solutions and approaches. Next, a comprehensive review to advances in OEO-based sensing, measurement, and detection applications is presented, including length change and distance measurement, refractive index estimation, load and strain sensing, temperature and acoustic sensing, optical clock recovery, and low-power RF signal detection. As a new application example, a novel approach for in-line position finding is proposed. When a long fiber Bragg grating inserted into OEO is locally heated to slightly broaden its reflection spectrum, the target position heated is mapped into the oscillating frequency shift, according to the first operation principle. A sensitivity of 254.66 kHz/cm is obtained for position finding in the experiment. Afterward, solutions for calibration and stabilization are briefly introduced, which enable us to improve the accuracy and reliability. Finally, features and future prospects on the sensing, measurement, and detection applications are discussed, such as compact and integrated OEOs.

A reconfigurable optoelectronic oscillator based on cascaded coherence-controllable recirculating delay lines.

A novel optoelectronic oscillator (OEO) using cascaded recirculating delay lines (RDLs) is proposed and experimentally demonstrated. In the proposed OEO, instead of the use of an electronic microwave ðlter, two infinite impulse response (IIR) photonic microwave ðlters (PMFs) formed by two RDLs are employed to select oscillation frequencies. Specifically, an amplified spontaneous emission (ASE) source is adopted to avoid self-interference of each RDL, and two approximately equal gain RDLs are employed to reduce the influence of mutual interference between the two RDLs. Therefore, a stable microwave signal can be generated from the OEO loop. In the experiment, by tuning the lengths of RDLs, microwave signals at different frequencies, such as 194.1MHz, 648.5MHz and 2.99GHz, have been generated. The phase noise performance of the generated microwave signal is also investigated. The proposed approach has the potential for the generation of microwave signals up to tens of GHz with the use of integrated micro-ring devices.

Photonic Frequency Measurement and Signal Separation for Pulsed/CW Microwave Signals

A novel photonic approach to simultaneously implement frequency measurement and signal separation is proposed and experimentally demonstrated for pulsed and continuous-wave (CW) microwave signals. In this approach, a light wave is externally modulated with carrier suppressed by a pulsed or CW microwave signal to be measured, and then sent to two optical complementary filters to perform frequency-to-amplitude mapping. The outputs of the two filters are detected by low-speed photodetectors, low-frequency alternating currency (AC) electronic component and direct current (DC) one being generated. The carrier frequency of the pulsed microwave signal can be estimated by monitoring the power of the AC component, while the frequency of the CW microwave signal is discriminated from the power of the DC component, with the frequency measurement and the signal separation simultaneously realized. In proof-of-concept experiments, within the frequency range from 5 to 20 GHz, measurement errors less than ±0.1, ±0.11 , or ±0.13 GHz are achieved for a pulsed signal with pulse repetition frequency of 0.25, 0.5, or 1 MHz, whereas the errors less than ±0.08 GHz are derived for a CW signal.

Frequency-Doubling Optoelectronic Oscillator Using DSB-SC Modulation and Carrier Recovery Based on Stimulated Brillouin Scattering

A tunable frequency-doubling optoelectronic oscillator (OEO) is proposed and experimentally demonstrated, of which the novelty lies in the conjunction of the double-sideband suppressed carrier (DSB-SC) modulation and the carrier recovery based on stimulated Brillouin scattering (SBS) effect. Frequency-doubled signals are generated via the DSB-SC modulation, which is realized by using a polarization modulator (PolM) in combination with an optical polarizer. Then, the gain provided by the SBS effect is used to recover the suppressed optical carrier, such that a fundamental-frequency oscillating required in the OEO loop is maintained. In the experiment, frequency-doubled microwave signals at 6.1 and 20 GHz are generated and analyzed. Meanwhile, the stability of the generated signals is also investigated.

High-Spectral-Efficiency Photonic Frequency Down-Conversion Using Optical Frequency Comb and SSB Modulation

A high-spectral-efficiency photonic frequency down-conversion approach for wavelength-division-multiplexing radio-over-fiber (WDM-ROF) uplinks is proposed and experimentally investigated. In the approach, an optical frequency comb serves as the carriers of multiple channels, and each comb line is modulated by the upstream radio-frequency (RF) signal via single-sideband modulation. At the central station, the beating between the sideband with upstream signal and its adjacent optical carrier leads to photonic frequency down-conversion without additional optical frequency shifts. Due to the band overlapping or the reduction of guard band between adjacent channels, a high spectral efficiency is achieved for WDM-ROF uplinks. A two-channel experimental uplink is established, where a 15-GHz RF signal carrying quadrature phase shift keying (QPSK) data is successfully down-converted to a 2-GHz intermediate frequency (IF) signal. For the 10-Mb/s QPSK data, the measured eye diagrams and the constellation diagrams clearly show the effectiveness of the frequency down-conversion and the influence of the transmission distance. In fact, the proposed approach is applied to the down-conversion at a high data rate up to several gigabits per second, such as a simulation demonstration at 2.5 Gb/s.

Wideband Microwave Doppler Frequency Shift Measurement and Direction Discrimination Using Photonic I/Q Detection

An enhanced approach to realizing wideband microwave Doppler frequency shift (DFS) measurement and direction discrimination based on photonic in-phase and quadrature coherent detection is proposed and demonstrated experimentally. In the proposed approach, the DFS between the transmitted microwave signal and the received echo signal is converted into two quadrature low-frequency electrical signals through the coherent detection by using an optical hybrid and two balanced photodetectors. The microwave DFS of interest can be estimated with an unambiguous direction, and in particular with a greatly improved resolution. Meanwhile, photonic coherent and balanced detection effectively eliminates the optical signal to signal beating interferences. In the proof-of-concept experiment, the DFSs from -90 to +90 kHz are successfully estimated for microwave signals at 10, 14, 18, and 38 GHz. The measurement errors are estimated to be less than ±5.8 Hz which are an order of magnitude lower than those (i.e., ±60 Hz) released before. Such results provide a high resolution for radial velocity measurement as well. In addition, the performance of the proposed approach in term of the signal-to-noise ratio and stability is discussed.

Investigation on Tunable Modulation Index in the Polarization-Modulator-Based Optoelectronic Oscillator

Tunable characteristic of the modulation index in the polarization-modulator (PolM)-based optoelectronic oscillator (OEO) is investigated and demonstrated experimentally. The PolM-based OEO is theoretically analyzed and a simplified analytical expression to reveal the variation of the modulation index is derived. Numerical results indicate that a tunable modulation index from 0 to 2.405 can be achieved for stable oscillation inside the OEO. In addition, a proof-of-concept experiment is carried out and the measured modulation indexes exactly follow the simplified analytical expression. Such a tunable modulation index enriches the applications of the OEO in the field of microwave/optical signal generation and processing, such as the generation of the return-to-zero (RZ) and the carrier-suppressed RZ optical pulse trains in our experiment.

Photonic-assisted chirped microwave pulses generation with a flexible and fine parameter manipulation.

A photonic approach for generating chirped microwave pulses with a flexible and fine parameter manipulation is proposed and experimentally demonstrated. In the proposed system, an intensity modulator (IM) biased at the minimum transmission point is used to generate two ± 1st-order optical sidebands which are then sent to a phase modulator (PM) for implementing large-signal phase modulations. A de-interleaver combined with an optical variable delay line (OVDL) is utilized to introduce a time delay between two phase-modulated optical signals. A second IM that acts as a time domain intensity switch (TDIS) is used to select different phase modulation ranges of the two phase-modulated optical signals. After the optical-electrical conversion in a photodetector (PD), chirped microwave pulses are generated. The key feature of this approach is that the parameters of the generated chirped microwave pulses including central frequency, pulse repetition frequency, and chirp rate can be flexibly and precisely manipulated by the radio frequency (RF) signals applied to modulators. A proof-of-principle experiment is carried out to verify the proposed approach. Consequently, positive or negative chirped microwave pulses with different central frequencies at 20, 22, 24 or 26 GHz and different pulse repetition frequencies at 1.5 or 2 GHz are generated, respectively.

Enhanced Doppler frequency shift measurement and direction discrimination using photonic i/Q detection

We propose an enhanced approach to implement wideband Doppler frequency shift (DFS) measurement and direction discrimination based on photonic in-phase and quadrature (I/Q) coherent detection. Through the I/Q coherent detection by using an optical hybrid and two balanced photodetectors (BPDs), the microwave/millimeter-wave DFS of interest is estimated with clear direction discrimination, and in particular with a greatly improved resolution. In the proof-of-concept experiment, the DFSs from -90 to 90 kHz are successfully estimated for microwave signals at 14 and 18 GHz. The measurement errors are estimated to be less than ±5.8 Hz which is an order of magnitude lower than those (i.e., ±60 Hz) released before. Such results correspond to a higher resolution for radial velocity measurement as well.