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.

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

Time-delayed optoelectronic oscillators are at the center of a large body of scientific literature. The complex behavior of these nonlinear oscillators has been thoroughly explored both theoretically and experimentally, leading to a better understanding of their dynamical properties. Beyond fundamental research, these systems have also inspired a wide and diverse set of applications, such as optical chaos communications, pseudorandom number generation, optoelectronic machine learning based on reservoir computing, ultrapure microwave generation, optical pulse-train synthesis, and sensing. The aim of this review is to provide a comprehensive survey of this field, to outline the latest achievements, and discuss the main challenges ahead.

Optoelectronic oscillators with time-delayed feedback

In this Letter, we propose and experimentally demonstrate a novel broadband and wide-range feedback tuning scheme for phase-locked loop (PLL) stabilization of tunable optoelectronic oscillators (OEO). The proposed scheme was realized in a simple and feasible way based on a dual-parallel Mach-Zehnder modulator (DPMZM) and optical bandpass filter (OBPF). The wide RF phase-shift range and broadband performance of the proposed scheme were demonstrated theoretically and experimentally, which ensured OEO's wide operating temperature range and tunability in PLL stabilization. As a result, PLL stabilization for OEO was achieved at different oscillating frequencies and the long-term stability was greatly improved without any thermal control. The measured overlapping Allan deviation of PLL-locked OEO reached lower than 10⁻¹⁰ after 1000-s averaging time, which was four orders of magnitude better than the free-running OEO. The phase noise performance was also improved at low-offset frequencies and remained the same at high-offset frequencies.

Broadband and wide-range feedback tuning scheme for phase-locked loop stabilization of tunable optoelectronic oscillators.

Microwave photonic approaches for the generation of microwave signals have attracted substantial attention in recent years, thanks to the significant advantages brought by photonics technology, such as high frequency, large bandwidth, and immunity to electromagnetic interference. An optoelectronic oscillator (OEO) is a paradigmatic microwave photonic oscillator that produces microwave signals with ultra-low phase noise, thanks to the high-quality-factor of the OEO cavity that is achieved with the help of optical energy storage elements, such as low-loss optical fiber or a high-quality-factor optical resonator. Different OEO architectures have been proposed to generate spectrally pure single-frequency microwave signals with ultra-low phase noise. Multiple oscillation mode control methods have been proposed in recent years to obtain different kinds of microwave signals. With the rapid development of photonic integration technologies, prototypes of integrated OEOs have been demonstrated with compact size and low power consumption. Moreover, OEOs have also been used for sensing, computing, and signal processing. This Tutorial aims to provide a comprehensive introduction to the developments of OEOs. We first discuss the basic principle and the key phase noise property of OEOs and then focus on its developments in spectrally pure low phase noise signal generation and mode control methods, its chip-scale integration, and its applications in various fields.

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Tutorial on optoelectronic oscillators

Injection-locking characteristics of a single-loop optoelectronic oscillator (OEO) under an independent sinusoidal RF signal injection are investigated. The mathematical model of the unlocked-driven system is developed to represent phase perturbation of the generated output RF signal caused by the RF injection signal. The closed-form expressions of the output RF spectra distorted due to RF signal injection are derived to formulate the behavior of the injection-pulled OEO. Using the phase dynamics equation, a phase noise model is developed to quantify the influence of RF injection signal on the phase noise performance of the oscillator. It is shown that our analytical model is capable of predicting the injection locking and estimating the injection pulling behavior of the single-loop OEO under RF signal injection. The theoretical results are validated by the experimental results.

Analysis of Injection Locking and Pulling in Single-Loop Optoelectronic Oscillator

This paper describes a novel phase-locked loop (PLL) architecture utilizing an optoelectronic oscillator (OEO) as a voltage-controlled oscillator (VCO). The OEO demonstrates excellent far-out phase-noise performance while the PLL reduces the close-in phase noise. The nonmonotonic VCO characteristics of the OEO placed stringent demands on the loop filter electronics and startup conditions. The crystal reference, prescalar, frequency synthesizer, and loop filter were all implemented with discrete high-performance components. The resulting frequency synthesizer yields a $-$ 10-dBm output at 20 GHz with phase noise of $-$ 80 dBc/Hz at 100-Hz offset, and $-$ 134 dBc/Hz at 10-kHz offset. These results are far superior to PLL synthesizers utilizing only an electronic VCO and illustrate the power of optoelectronic integration.

An Ultra-Low Phase-Noise 20-GHz PLL Utilizing an Optoelectronic Voltage-Controlled Oscillator

A microwave-frequency generator comprising silicon-photonic devices and a resonant cavity integrated on a single substrate is disclosed. Integration of silicon-photonic devices and waveguides on a single monolithic substrate enables lower phase noise in the microwave-frequency output signal. Further, the integration of low-loss waveguides that define the resonant cavity, low-noise operation can be achieved without introducing significant optical loss, thereby enabling high Q. In some embodiments, RF electronics devices are also integrated on the same substrate.

Low-noise microwave-frequency generator

We demonstrate phase noise improvement in a high frequency 20 GHz optoelectronic oscillator through dual delay lines and phase locking to a low frequency oven controlled crystal oscillator. Introduction Modern wireless communication systems, such as analog to digital converters and radar systems, require low phase noise local oscillators at frequencies greater than 10 GHz. At such high frequencies, electrical oscillators suffer from parasitic losses that decrease the quality factor and increase close to carrier phase noise which degrades the signal-to-noise ratio at the receiver. Direct up-conversion of stable oscillators at low frequencies suffer from a 20×log10(N) increase in phase noise for a multiplication factor of N. Optoelectronic oscillators (OEO) utilize the low loss of optical fiber to achieve very high microwave quality factors at high frequencies that are much higher than those achievable in the electrical domain [1]. However, further research into proper operation, tunability, and stabilization for long time scales is required. We present a dual loop OEO operating at 20 GHz that has been stabilized with multiple techniques in the optical domain as well as electronically stabilized as a voltage controlled oscillator (VCO) with a phase lock loop (PLL) to a low frequency reference for long term stability, a region in which OEOs usually suffer due to fiber drift. OEO System Design The OEO generates spectrally pure microwave tones through the use of an analog optical link with electronic feedback. The output of a high power fiber laser passes through an intensity modulator, fiber delay, and is detected on a high speed photodetector (PD). The frequency of oscillation is selected with a microwave filter, and any required amplifiers and phase shifter are added before closing the microwave loop to the intensity modulator. The main advantages of the OEO architecture are the tunability of microwave frequency with a microwave phase shift, and the long delay times that can be achieved because of very low optical losses of 0.2 dB/km. The longest delay of 10 km used in this study can produce effective microwave quality factors of 10 billion, though this is degraded by any component’s flicker noise. With such long lengths, the free spectral range (FSR) of the loop becomes very narrow (20 kHz) and requires additional filtering for mode selection and spurious tone suppression. Since 20 GHz microwave filters are not commercially available with sub-MHz linewidths, we implement a dual loop OEO that utilizes the Vernier effect of a second 100 m loop to select a single oscillating mode, as shown in the left side of Fig. 1. Two microwave amplifiers are placed directly after the photodetectors to keep the open loop noise figure low. Fig. 1. Schematic of the dual loop OEO (left) utilized as a VCO for PLL feedback (right). The fiber laser output is controlled with a polarization controller (PC) before being sent to a Mach-Zender Modulator (MZM). Tunable optical delay lines (TODL) control the Vernier effect, and the photodetectors (PD) generate the microwave signal sent through the band pass filter (BPF). After frequency division, the phase frequency detector (PFD) compares the dual loop OEO noise to an oven controlled crystal oscillator (OCXO), and sends the feedback signal to the OEO. Measurements are performed on an electrical spectrum analyzer (ESA), frequency counter, and optical frequency discriminator. The OEO is also known to be highly sensitive to reflections reaching the laser, and the intracavity optical power can become limited by fiber scattering [2]. To circumvent these problems, we added a fiber circulator after the high power fiber laser and broaden the fiber linewidth by dithering the built in piezo frequency tuner, respectively. The resulting short term linewidth of the microwave signal is sub-Hz level, but a major issue with fiber delays is the dn dT ~1.2×10/°C associated 412 WC1.2 (Contributed) 9:00 AM 9:15 AM 978-1-4577-1504-4/14/$26.00 ©2014 IEEE with thermal fluctuations [3]. A change in the optical path length from temperature drift will cause the 20.05 GHz oscillating frequency to shift, which we observed to be ~1 kHz/min. We reduced this to ~200 Hz/min by placing the system in a sealed anechoic enclosure with a stabilization time of 5 hours. To further reduce the long term linewidth (or close in phase noise), we have used a PLL circuit board which utilizes the dual loop OEO as a VCO. As shown in the right side of Fig. 1, the PLL circuit requires the 20.05 GHz signal to be frequency divided to a digital 12.5 MHz signal. The on board mixer then compares the OEO noise to a 100 MHz OXCO, and sends the correction signal to the voltage controlled microwave phase shifter with a bandwidth of 1 kHz. Tunable optical delay lines were placed in each arm to precisely tune the oscillating frequency to a multiple of the reference frequency at the PFD. Results Three measurement methods were employed to characterize the performance of the OEO. The ESA (Rhode & Schwarz FSU50) provides the most straightforward method of measuring phase noise, as it has an internal synthesizer, reference oscillator, and tunable filters for a large measurement of offset frequencies from 1 Hz to the Nyquist rate. The second method used an optical delay line based frequency discriminator and an Agilent E5505 phase noise test set, to provide a lower phase noise floor versus the ESA for certain frequency offsets. Both results are shown in Fig. 2a. Using the ESA, the white phase noise floor is -138 dBc/Hz, limited by the thermal noise of our microwave amplifiers. The rise in phase noise at 1 MHz in is due to the ESA’s internal noise floor. Using the frequency discriminator, we show a phase noise of -134 dBc/Hz at 10 kHz with a 1/f 3 slope, limited by the residual noise of our discriminator. Fig. 2. (a) Phase noise spectrum of the dual loop OEO as measured by the ESA and 4.2 km optical frequency discriminator. (b) Overlapping Allan deviation of the dual loop OEO, measured at 2.5 GHz. We also used a frequency counter with an OCXO reference to characterize the difference between the locked and unlocked OEO. Fig. 2b plots the frequency deviation, f f  , for different averaging times to obtain the Allan deviation. Both the locked and unlocked oscillators show white frequency noise (τ -1/2 ), but the locked oscillator converges to a stability of less than 1 ppb at 50 seconds averaging time before reaching frequency drift (τ 2 ). We believe the current results to be limited by the quantization error of our frequency counter.

http://ieeexplore.ieee.org/iel7/6973525/6994958/06995423.pdf

Danielle Guerra

Papers

An Ultra-Low Phase-Noise 20-GHz PLL Utilizing an Optoelectronic Voltage-Controlled Oscillator

Low-noise microwave-frequency generator

A low phase noise dual loop optoelectronic oscillator as a voltage controlled oscillator with phase locked loop