Tunable High-order Subharmonic Injection-locked Optoelectronic Oscillator in K-band Based on a Dual-polarization Mach-Zehnder Modulator

The advancement of modern communication， radar， and electronic warfare systems imposes stringent requirements on microwave signal sources， including exceptional spectral purity， low phase noise， broad frequency tunability， and operation at high-frequency bands. Conventional electronic oscillators， such as quartz crystal oscillators， typically require multiple frequency-multiplication stages to generate high-frequency signals. This process inherently degrades phase noise by 20log N （where N is the multiplication factor）， thereby limiting their applicability in high-frequency systems. In contrast， the optoelectronic oscillator employs an optoelectronic feedback loop to generate microwave signals， offering the distinctive advantage of low phase noise that is independent of oscillation frequency. This makes the optoelectronic oscillator a pivotal solution for high-frequency， high-stability microwave generation. Since its inception by YAO X S in 1996， research on high-frequency optoelectronic oscillators has advanced considerably. The K-band （18~27 GHz）， with its broad available bandwidth， compact antenna size， and promising applications in satellite communications， high-resolution radar， and future fifth-generation and sixth-generation mobile systems， is regarded as an ideal frequency range for such investigations.However， conventional optoelectronic oscillators operating at high frequencies are susceptible to multi-mode oscillation and frequency hopping due to the limited quality factor of available radio frequency filters， resulting in degraded side-mode suppression ratio and frequency instability. Injection locking is a well-established technique to suppress side modes and stabilize the output. Nevertheless， it requires an injection signal with a frequency close to the free-running oscillation frequency of the optoelectronic oscillator. At high frequencies， this typically depends on expensive and complex high-frequency signal sources， which hinders the adoption of injection locking in high-frequency optoelectronic oscillators.This paper proposes and experimentally demonstrates a novel scheme for a fifth-order subharmonic injection-locked optoelectronic oscillator in the K-band， utilizing a dual-polarization Mach-Zehnder modulator. The system exploits the polarization independence of the two optical paths within the dual-polarization Mach-Zehnder modulator. An external， relatively low-frequency radio frequency signal is injected into the modulator that drives the X-polarization state， generating high-order optical sidebands. Following photodetection， the electrical spectrum contains harmonics of the injected signal. A subsequent bandpass filter then selects the fifth harmonic component. This filtered high-frequency signal， precisely at the target K-band frequency （five times the injection frequency）， is injected into a conventional optoelectronic oscillator loop constructed using the Y-polarization state of the same dual-polarization Mach-Zehnder modulator. This architecture effectively integrates harmonic frequency multiplication with injection locking within a single， compact photonic configuration. Crucially， it eliminates the need for a dedicated high-frequency signal source for injection， as the high-frequency locking signal is internally generated from a low-frequency input.An experimental system was implemented to validate the proposed concept. A 1 550 nm laser diode served as the optical source. The dual-polarization Mach-Zehnder modulator provided independent modulation paths for the X-and Y-polarization states. A low-frequency radio frequency signal from a signal generator was injected into the X-polarization modulator. A photodetector converted the modulated optical signal into an electrical signal. A broadband bandpass filter （24~26 GHz） selected the fifth harmonic， which was then amplified and fed into the optoelectronic oscillator loop formed by the Y-polarization path； this loop included an additional modulator bias point， a fiber delay line， an amplifier， and a filter. When an injection signal with a power of 17.5 dBm and a tunable frequency range of 4.8~5.2 GHz was applied， the system successfully achieved fifth-order subharmonic injection locking， producing stable single-mode oscillation outputs at 24.0 GHz， 24.5 GHz， 25.0 GHz， 25.5 GHz， and 26.0 GHz， respectively.The performance of the injection-locked optoelectronic oscillator was thoroughly characterized. Output spectra at all five frequencies exhibited excellent spectral purity， with side-mode suppression ratio values consistently exceeding 70 dB—a significant improvement over free-running optoelectronic oscillators， which are prone to multi-mode oscillation. The output power variation across the 24-26 GHz tuning range was less than 1 dB， indicating good gain flatness. Phase noise measurements revealed outstanding performance： below -70 dBc/Hz at 10 Hz offset and below -129 dBc/Hz at 10 kHz offset for all output frequencies. The near-in phase noise is primarily determined by the quality of the low-frequency injection source， whereas the far-out phase noise retains the inherently low noise floor of the optoelectronic oscillator's long delay line， thereby effectively circumventing the 20log N degradation associated with pure electronic frequency multiplication. Frequency stability， assessed via Allan deviation measurements over averaging times （τ） from 1 μs to 10 s， demonstrated consistent behavior across the five output frequencies. For short averaging times （ττ， stability was influenced by environmental perturbations on the fiber delay.In conclusion， this work successfully demonstrates a tunable， high-performance K-band optoelectronic oscillator based on a dual-polarization Mach-Zehnder modulator-enabled subharmonic injection-locking scheme. The system efficiently generates high-frequency （24~26 GHz）， high-spectral-purity microwave signals using only a low-frequency radio frequency source， thereby overcoming key limitations of traditional approaches. The demonstrated performance in terms of side-mode suppression ratio （>70 dB） and phase noise （<-70 dBc/Hz at 10 Hz offset， <-129 dBc/Hz at 10 kHz offset） meets the requirements for advanced microwave systems. Although the current tuning range is constrained by the components employed （filter bandwidth， modulator response）， the underlying principle is not inherently frequency limited. Utilizing devices with higher bandwidths or adopting cascaded modulation structures could potentially extend operation to even higher frequencies， including millimeter-wave and terahertz bands. This approach offers a promising and practical photonic pathway toward realizing compact， low-noise， and tunable high-frequency oscillators