An optoelectronic oscillator (OEO) is a microwave photonic system consisting of an amplified optoelectronic feedback loop to generate a microwave signal. To reduce the phase noise, the feedback loop must be long to ensure a high Q factor, which is usually implemented by incorporating a long optical fiber in the loop. The key limitation of using a long fiber is that a large number of closely spaced longitudinal modes exist in the feedback loop, which makes single-frequency oscillation difficult to achieve. In this article, we propose to incorporate a photonic integrated microdisk resonator (MDR) in an OEO to implement parity-time (PT) symmetry as well as frequency tuning. By employing the reciprocity of light propagation in the MDR, two mutually coupled optoelectronic loops having an identical geometry with one having a gain coefficient and the other a loss coefficient, identical in magnitude, to form a PT-symmetric OEO that support single-frequency oscillation, are implemented. The tuning of the frequency is realized by thermally tuning the MDR. Experimental results show a microwave signal with a frequency that is tunable from 2 to 12 GHz is realized. The phase noise of the generated microwave signal at a frequency of 11.5 GHz is -117.3 dBc/Hz at a 10-kHz frequency offset. The use of a photonic integrated microdisk resonator makes the entire PT symmetric system have high potential for full photonic integration.

We propose and experimentally demonstrate a parity–time (PT)-symmetric frequency-tunable optoelectronic oscillator (OEO) in which the PT symmetry is implemented based on a single dual-polarization optical loop. By employing the inherent birefringence of a z-cut lithium niobate ( L i N b O 3 ) phase modulator (PM), two mutually coupled optoelectronic loops supporting orthogonally polarized light waves with one experiencing a gain and the other a loss are implemented. By controlling the gain, loss, and the coupling coefficients between the two loops, the PT symmetry breaking condition is met, which enables the OEO to operate in single mode without using an ultranarrow passband optical or microwave filter. The frequency tunability is realized using a microwave photonic filter (MPF) implemented using the PM and a phase-shifted fiber Bragg grating (PS-FBG). The proposed PT-symmetric OEO is experimentally evaluated. A stable and frequency-tunable microwave signal from 2 to 12 GHz is generated. The phase noise of the generated signal at 11.8 GHz is measured, which is − 124 d B c / H z at a frequency offset of 10 kHz.

A finely tunable low-drift spurious-free single-loop optoelectronic oscillator (OEO) incorporating injection locking and time delay compensation is proposed and experimentally demonstrated. In the proposed OEO, one mode of a single-loop OEO is injection locked by a tunable electronic oscillator resulting in single-mode oscillation. A time delay compensation system is used to compensate the OEO’s loop length change caused by environmental changes, such as temperature and strain. Tuning of the oscillation frequency is realized by controlling the injection frequency and absolute loop length of the single-loop OEO. In the experiments, when the ambient temperature varies between 22°C and 31°C within 1000 s, an output signal at the frequency of 10.664 GHz with a frequency drift better than $-0.1\mathrm{ppb}$ and side-mode suppression ratio greater than 78 dB has been realized. Also, the OEO can be tuned with a precise frequency step of 10 Hz.

Injection locking and pulling characteristics of a long-loop optoelectronic oscillator (OEO) that has a large number of closely-spaced longitudinal modes are theoretically analyzed and experimentally evaluated. A differential phase equation that relates the phase difference between the OEO and the injected microwave signal to its instantaneous beat angular frequency is derived in the time domain. Based on the differential phase equation, both the locking and pulling characteristics of an injection-locked OEO are studied, and the phase noise performance is analyzed. It is found that the locking and pulling performance depends upon three parameters, the initial frequency difference between the frequency of the signal generated by the free-running OEO and frequency of the injected microwave signal, the voltage ratio between the signal generated by the free-running OEO and the injected microwave signal, and the Q factor of the free-running OEO. The phase noise performance depends upon the locking range, the phase noise performance of the free-running OEO as well as that of the injected microwave signal. The analysis is validated experimentally. Excellent agreement is found between the theoretical analysis and the experimental demonstration.

A widely frequency-tunable parity-time (PT) symmetric optoelectronic oscillator (OEO) implemented based on a single polarization-dependent Sagnac loop is proposed and experimentally demonstrated. Two mutually coupled feedback loops having an identical geometry with balanced gain and loss are implemented to achieve PT-symmetry, which is essential for single-mode selection without using an ultra-narrow passband microwave filter. In the implementation, a Sagnac loop consisting of a polarization beam splitter (PBS) and two polarization controllers (PCs) is employed which functions equivalently as two mutually coupled feedback loops, with the gain, loss and coupling coefficient being independently controllable. Since only a single physical loop is used, the implementation is greatly simplified and the stability is highly improved. The operation of the proposed OEO is experimentally verified. Stable single-mode oscillation with a wide frequency tunable range from 2 to 12 GHz is achieved. The phase noise is -128 dBc/Hz at an offset frequency of 10 kHz.

A high-precision and thermal-insensitive strain sensor based on two self-starting optoelectronic oscillators (OEOs) is proposed and experimentally demonstrated. Two OEOs are grouped into a cross-referencing structure by dense wavelength division multiplexing (DWDM); the two OEOs have the same characters and they are placed in the same environment. In this frequency encoded strain sensor, it converts the strain information of the single mode fiber to the frequency information, and the frequency information is acquired by measuring the intermediate frequency (IF) mixed by the two OEOs. The accumulative magnification effect at high-order resonant frequency modes makes the strain sensor achieve high sensitivity, which significantly improves the precision of the measurement strain. The cross-referencing structure of the two OEOs makes the influence of the environment, such as temperature, greatly reduced. In the experiments, measurement errors less than ± 0.3 με at a measurement range of 600 με have been realized, including a drift error due to a variation in the environment such as temperature. Furthermore, a quasi-distributed strain measurement system based on the proposed strain sensor has been designed.