Fig. 1. Concept of the quiet-point-operated coupled optoelectronic oscillator. (a) Simplified schematic of the COEO using COA and a tunable RF bandpass filter for low-phase-noise FH signal generation. (b) Pulse synchronization in the modulator gain region through the pull-in effect. Panels I–V correspond to the modulator gain regions in advance of (I, II), simultaneous with (III), and lagging behind (IV, V) the circulating pulse. (c) Comparison of the output signal spectrum of COEO using a single EDFA and COEO using a COA along with the QP operation. (d) Schematic representation of the optimization of the phase noise when the oscillator is operating at the QP.

Fig. 2. Experimental setup of FH-COEO. SOA, semiconductor optical amplifier; EDF, erbium-doped fiber; PBS, polarization beam splitter; DSF, dispersion-shifted fiber; FRM, Faraday rotator mirror; PMF, polarization-maintaining fiber; SMF, single-mode fiber; MZM, Mach-Zehnder modulator; PD, photodetector; LNA, low-noise amplifier; VPS, voltage-controlled phase shifter; EC, electric coupler; OSA, optical spectrum analyzer; ESA, electrical spectrum analyzer; PNA, phase noise analyzer.

Fig. 3. Identification and characterization of quiet point. (a) Comparison of phase noise spectra measured under a single SOA, SOA with different gains in COA, and a single EDFA (extracted from Ref. [39]). (b) Phase noise and SMSR at 10 kHz offset frequency as a function of SOA current. (c) Measured spectrum of RF signal with filter bandwidth of 50 MHz, 500 MHz, and 1 GHz. RBW is 500 Hz. (d) Autocorrelation trace of a pulse generated under the QP operation.

Fig. 4. Experimental results of the wideband tunable oscillator. (a) Measured spectrum of 2–18 GHz with a tuning step of 1 GHz. RBW is 1 kHz. (b) Enlarged views of 6 GHz, 8 GHz, 10 GHz, and 12 GHz RF spectra. Span is 2 MHz and RBW is 500 Hz. (c) Optical spectra at different operating frequencies (the frequency combs of the 2 GHz repetition rate are not shown due to limited resolution). (d) Measured SSB phase noise of the free-running output signal with a step of 3 GHz.

Fig. 5. Measurement and characterization of COEO robustness. (a) RF power of the output when tuning the RF loop delay over 50 ps by phase shifter. The same mode is marked with the same color. (b) Amplitude-frequency response (S21) of YIG filter (top) and spectrum of the output signal (bottom) when operating at 12 GHz. (c) Measured average delays at operating frequencies from 6 to 16 GHz in 2 GHz steps. The error bars show the delay variation over a 10 MHz range near each frequency point.

Fig. 6. Characterization of FH signal. (a) Measured SMSRs of FH signals (top left) and phase noise at 10 kHz frequency offset (bottom left) and histograms of statistical results (right). (b) Output frequency at different YIG filter drive voltages. (c) Spectra measured when tuning the filter with a 0.7 mV voltage step. (d) Measured frequency hopping time with a 5.3 MHz step at 10.025 GHz.

Fig. 7. Measured phase noise and fractional frequency stability of the COEO. (a) SSB phase noise of the 10 GHz microwave signals generated from the free-running COEO, the PLL-locked COEO, and the reference source. (b) Allan deviation of the 10 GHz microwave signals generated from the free-running COEO and the PLL locked COEO.