In recent years, optoelectronic oscillators (OEOs) have undergone rapid development due to their remarkable advantages such as high frequency, large bandwidth, and magnetic interference immunity. They are widely used to generate microwave photonic signals with ultralow phase noise level, which are mainly used in important fields such as communications, radar systems, and measurements. In the past decades, double-loop, coupled, injection-locked, and cosymmetric-time symmetry (parity-time symmetry) OEOs have been developed. These different structures of OEOs can be used to generate high-frequency microwave signals as well as complex microwave signals, such as chirped microwave signals via applied modulation, phase-locked microwave signals, and chaotic signals. However, the abovementioned OEOs rely on their own fiber cavity lengths to produce a single oscillation frequency with a limited frequency tuning range. In 2020, Prof. Ming Li's group at the Institute of Semiconductors, Chinese Academy of Sciences, proposed to combine backward Rayleigh scattering (RS) in an optical fiber with optoelectronic oscillation to realize a broadband random OEO (ROEO), and an ultra-broadband (DC-40 GHz) random microwave signal is obtained in an open cavity through backward RS. The generated signal possesses random characteristics, and its oscillation frequency was not limited by the fixed length of the resonant cavity. Random signals have potential in many applications such as random bit generation, radar systems, and electronic jamming and countermeasures. However, their open-loop noise results in a large substrate noise because of the active amplifier devices, and the random signal power is only 20 dB greater than the noise power. Schemes for improvement of the signal-to-noise ratio (SNR) of random microwave signals have not been reported. In this study, we propose an ROEO for the generation of high-SNR random microwave signals. The ROEO consists of a random fiber laser and an optoelectronic oscillation loop, which improves the SNR of random microwave signals by introducing a wavelength division multiplexer (WDM) in the random cavity and eliminating the remaining Raman pump power. The regulated polarization controller (PC) controls the polarization state of the signal light to suppress the stimulated Brillouin scattering (SBS) effect in the dispersion-compensating fiber (DCF).

The ROEO consists of two devices a random fiber laser and an OEO. Broadband random microwave signals are generated by the ROEO through random distribution feedback provided by RS in the random fiber laser. The SNR of the random microwave signal generated by the ROEO is improved by adding another WDM (WDM2) to the random fiber laser to eliminate the residual Raman pump light from the random cavity. The polarization state of the signal light is controlled by the PC to reduce the Raman gain and thus suppress the SBS effect in the OEO loop. The signal light from port 2 of the optical circulator (OC) is combined with 1455-nm Raman pump light via WDM1 and excited in the DCF at 7.2 km for achieving RS to produce a random distributed feedback. RS can occur at any position in the DCF; therefore, the loop length of the OEO is not determined, and all eligible frequencies can oscillate in the OEO. WDM2 is added to the system following the DCF to filter out residual Raman-pumped optical power. The backscattered Rayleigh light propagates from port 2 to port 3 of the OC, and the optical amplifier amplifies the weak, backscattered optical signal by 10 dB. 10% port of OC2 allows for the measurement of the amplified spectrum and the optical power in the photodetector (PD) (DC-18 GHz), with 90% power coupling into the loop. The PD converts the optical signal into an electrical one, which is fed through electronic amplifier 1 (EA1) (DC-15 GHz). The microwave signal with 50% power is analyzed using a spectrum analyzer, and the remaining 50% microwave signal is fed back to a phase modulator (PM) through secondary amplification by EA2 with 27-dB gain, which constitutes the complete ROEO. When the loop gain exceeds the loop loss, the microwaves with different frequencies oscillate simultaneously.

In Fig. 3(a), DC-30 GHz bandwidth is shown for a random signal for which the open-loop noise rejection may reach ~40 dB. The power of the spectrum for 18-30 GHz frequencies rapidly decays because the bandwidth of the PD used is DC-18 GHz. The spectrum obtained for DC-18 GHz has uneven distribution because the amplification bandwidth of EA1 and EA2 is 15 and 10 GHz, respectively. The resolution bandwidth of the spectrum analyzer (RMS FSV30) is 500 kHz, the video bandwidth is 500 kHz, and the cutoff bandwidth of ESA is 30 GHz. The Raman gain is dependent on polarization, and the SBS effect in the DCF is suppressed by the adjustment of the polarization state of the signal light by the PC to attenuate the Raman gain. Compared with systems presented in the literature, the power contrast suppression of the Stokes optical signal and the random microwave signal is improved by ~15 dB by the present system. The RF signal and open-loop noise plots for DC-10 GHz, shown in Fig. 5(a), which exhibit a significant power difference of ~40 dB, show the excellent noise rejection of the present system, which is due to WDM2 that eliminates the excess Raman pump power and attenuates the substrate noise. Compared with the 20-dB power difference achieved in previous studies, the gain of the present OEO system is more prominent and the RF power is more uniform for DC-10 GHz. In addition, the system can generate random microwave signals with a large bandwidth if the two-stage EA has a wider amplification bandwidth. Upon removing WDM2, as shown in Fig. 5(b), the open-loop substrate noise level increases and the OEO power at DC-10 GHz is between 20 dB and 30 dB from the noise power, when the feedback loop is closed to form OEO, the power observed at OSA1 increases. Compared with the literature, our study uses a PM with a lower insertion loss (1.7 dB); the OEO microwave power is more effectively fed back into the cavity, and the difference between the OEO oscillation power and the open-loop power is higher.

This study demonstrates a high-SNR ROEO that generates random microwave signals with good flatness when considering DC-10 GHz and a noise rejection of 40 dB. The SNR of the ROEO is enhanced by stripping the excess Raman-pumped light from the random cavity using WDM2. The polarization state of the signal light is controlled using a PC to reduce the Raman gain and thus suppress the SBS effect in the OEO loop. The open cavity of the ROEO eliminates the limitation of conventional OEOs of generating single-frequency microwave signals, and the generated broadband random microwave signals exhibit considerable application potential in noisy radar systems, electromagnetic interference, secure communications, and random coding. Narrowband random signals can be generated using narrowband filters for channel encryption applications.