Peixuan Li

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.

High-Efficiency Photonic Microwave Downconversion With Full-Frequency-Range Coverage

Microwave frequency downconversion is optically implemented by using two cascaded polarization modulators (PolMs), characterized by full-frequency-range coverage and high conversion efficiency. In this proposed approach, the incident radio-frequency signal (RF) and the local oscillator signal (LO) are applied to the first PolM at the transmitter and the second one at the receiver, respectively. Effective optical carrier suppression can be achieved using the cascaded PolMs, whereas the optical sidebands induced by the RF and the LO signals remain unchanged. Since no additional optical filter is needed, the lower operation frequency limit in other approaches with an optical filter is eliminated, such that a full-frequency-range coverage is ensured in this approach. Meanwhile, the use of optical carrier suppression (OCS) enables high conversion efficiency for microwave downconversion. In the proof-of-concept experiments, the conversion efficiency is improved by over 20 dB within the frequency range from 2 to 15 GHz, as compared to those of cascaded modulators without OCS. Both the upper and the lower limits of such a frequency range can be greatly extended when test instruments with full frequency range are used.

Image-Free Microwave Photonic Down-Conversion Approach for Fiber-Optic Antenna Remoting

For commercial and military applications, it is highly desired to acquire broadband radio frequency (RF) signals from remotely located receiving antennas to the central office (CO) for centralized signal processing such as down-conversion. In this paper, a photonic approach for image-free microwave frequency down-conversion is proposed for antenna remoting scenarios. The RF signal is captured by the phase modulator (PM) at remote antenna unit and transmitted over a long fiber link to the CO for performing frequency down-conversion. In analog, with the Hartley architecture in the electronic domain, two in-phase (I) and quadrature (Q) intermediate frequency (IF) components are generated by using an electro-optic polarization modulator (PolM) and two polarizers. To achieve large image rejection ratio (IRR), the digital post-processing technique is introduced to accurately compensate the amplitude and phase imbalances between the I and Q IF signals. In the experiments, a 2-km single-mode fiber link is inserted between the PM and PolM. As the local oscillator signals are set as 35 and 3 GHz, two target sinusoidal signals at 35.5 and 3.1 GHz are applied with two image signals, a sinusoidal signal at 34.5 GHz and a broadband RF signal centered at 2.9 GHz, respectively. Then, both real-time analog and off-line digital processing methods are used to process the generated I and Q IF signals for image rejection, yielding an IRR over 45 and 60 dB respectively, when the sinusoidal image signal is applied. The distortions from the broadband 2.9-GHz image RF signal are also effectively suppressed by using the two processing methods. The proposed approach is capable of covering a wide frequency range from 5 to 40 GHz.

Proposal and demonstration of SIM-OFDM based radio-over-fiber system

We propose and experimentally demonstrate a radio-over-fiber (RoF) system employing subcarrier-index modulation (SIM) orthogonal-frequency-division multiplexing (OFDM) signal. Thanks to the introduction of a new freedom in terms of SIM, a robust and energy-effective transmission solution is achieved for the RoF system. In the experiments, the 8-GHz QPSK-SIM-OFDM and conventional QPSK-OFDM signals are transmitted in an RoF system with a 10-km single-mode fiber (SMF) link. Compared with conventional QPSK-OFDM, the proposed QPSK-SIM-OFDM for RoF system is verified with higher energy efficiency and a lower bit error ratio (BER) for a medium or high signal-to-noise ratio (SNR). Concerning the energy efficiency of the proposed ROF system, defined as the total amount of reliably decoded bits normalized by the energy, it is about 1.5 times higher than that of conventional QPSK-OFDM.

Simultaneous transmission of frequency-doubling vector signal and low-radiofrequency signal over RoF link free of inter-band beating interferences

A simple multi-service radio over fiber (RoF) system to simultaneously transmit frequency-doubling vector signal (FDVS) and low-radiofrequency signal (LRS) in a single wavelength is proposed. At the center office (CO), an integrated dual-parallel Mach-Zehnder modulator is utilized to implement double-sideband modulation with carrier (DSB+C) for the LRS and the carrier-suppressed double-sideband modulation (CS-DSB) for the FDVS, respectively. More importantly, a 90-degree phase shift is introduced to the DSB signal to eliminate the interband beating interferences (IBBIs) arising from the beating terms of optical subcarriers induced by these two signals. In the experiments, a 0.5-Gbaud QPSK FDVS at 16 GHz and a 4-GHz LRS carrying 1.25 Gbit/s OOK data have been simultaneously transmitted successfully, showing the effective elimination of IBBIs. In the 10-km fiber link, the power penalties for the OOK data and QPSK signal are estimated as 0.44 dB and 1 dB, respectively.

Tunable microwave photonic duplexer for full-duplex radio-over-fiber access

A tunable microwave photonic duplexer (MPD) for full-duplex radio-over-fiber access is proposed and experimentally demonstrated. The proposed MPD is implemented by two single-bandpass microwave photonic filters (MPFs) to separate the transmit (Tx) and receive (Rx) channels. More specifically, a broadband optical source (BOS) is spectrally sliced by a differential-group-delay interferometer (DGDI) and a Mach-Zehnder interferometer (MZI) to form two tunable single-bandpass MPFs with different central frequencies. The two MPFs are used for the Tx and Rx channels, respectively. By adjusting the free spectral ranges (FSRs) of the DGDI and MZI, the central frequencies of the Tx and Rx channels can be tuned independently in a wide frequency range. In the experiments, tunable ranges from 0 to 6.1 GHz for the Tx channel and from 0 to 17 GHz for the Rx channel are achieved. Meanwhile, a high isolation up to 44 dB between the Tx and Rx channels is also obtained. Based on the proposed MPD, the duplex transmission of a 10-MBaud 16-quadrature-amplitude-modulation (QAM) signal at 2.45 GHz and 2.82 GHz is demonstrated.

Tunable Photonic Radio-Frequency Filter With a Record High Out-of-Band Rejection

As radio-frequency (RF) filtering plays a vital role in electromagnetic devices and systems, recently photonic techniques have intensively been studied to implement RF filters to harness wide frequency coverage, large instantaneous bandwidth, low frequency-dependent loss, flexible tunability, and strong immunity to electromagnetic interference. However, one crucial challenge facing the photonic RF filter (PRF) is the less impressive out-of-band rejection. Here, to the best of our knowledge, we demonstrate a record out-of-band rejection of 80 dB for a tunable PRF with a high processing resolution, by incorporating the highly selective polarization control and the large narrowband amplification enabled by the stimulated Brillouin scattering effect. This record rejection is arduous to be achieved for a narrow passband (e.g., a few megahertz) and a high finesse in a PRF. Moreover, the proposed PRF is an active one capable of providing negligible insertion loss and even signal gain. A tunable central frequency ranging from 2.1 to 6.1 GHz is also demonstrated. The proposed PRF will provide an ultrahigh noise or clutter suppression for harsh electromagnetic scenarios, particularly when room-temperature implementation and remote distribution are needed.

Flexible microwave signal generation with frequency multiplication based on tunable OEO and SBS-assisted notch filter

A photonic approach to generate tunable microwave signal with frequency-multiplication is proposed. In the proposed scheme, the microwave signal with tunable fundamental frequency is generated by an optoelectronic oscillator (OEO) incorporating a tunable microwave photonic bandpass filter. Then a stimulated Brillouin scattering (SBS) assisted notch filter is used to achieve wavelength-independent optical carrier suppressing for frequency multiplication. In the experiment, the frequency-doubled microwave signals with a tunable range from 15 GHz to 25 GHz and frequency-quadrupled signals with frequencies of 8 and 12 GHz are generated..

Simplified photonic approach for high-coding-efficiency, digitalized microwave frequency measurement using multiple optical filter arrays with different FSRs

A simplified photonic approach to microwave frequency measurement with digital outputs based on multiple parallel optical filter arrays, is proposed and experimentally demonstrated. In the proposed approach, multiple optical phase-shifted filter arrays in parallel with different free spectral ranges (FSRs) are designed to obtain digitalized results in the form of binary code with high-coding efficiency and fine measurement resolution. In particular, compared with previous approaches, larger tolerances on the phase shifts of optical filter arrays are provided in the proposed approach having the same coding efficiency and resolution. Therefore, the proposed system is greatly simplified and robust to noise. A proof-of-concept experiment is performed when two optical filter arrays are used. 8-bit binary digitalized results with a 5-bit effective number and a 2-GHz measurement resolution are obtained in the range from 10 to 40GHz.

Spurious Noises Analysis of Microwave Photonic Filters Based on Phase Modulation to Intensity Modulation Conversion Using an Optical Notch Filter

The spurious noises of a narrow-bandpass microwave photonic filter (MPF) using phase modulation to intensity modulation (PM-IM) conversion and an optical notch filter are analyzed in theory and demonstrated experimentally. Due to the difference between the nonlinear filtering effect of PM-IM conversion and the linear one of finite impulse response or infinite impulse response, additional spurious noises will be generated in the former filter. Analysis models with two input microwave tones are developed for revealing the spurious noises in the MPF based on PM-IM conversion under small-signal and large-signal modulations, which are then verified by simulations and experiments, respectively. The results show that, besides the target microwave signal to be selected, the inter-modulation noise and the harmonic noises also coexist at the output of the MPF, wherein the 2nd-order inter-modulation noises and the 2nd-order harmonic noise of the target signal are the dominant spurious ones. Therefore, an excellent agreement among the results from analysis models, the simulations, and the experiments is obtained.