Application Cases

Experiment Name: System Testing and Analysis of OEO Based on High-Q Micro-Ring Resonator

Testing Equipment: High-voltage amplifier, oscilloscope, tunable laser, spectrum analyzer, optical coupler, photodetector, etc.

Experimental Process:

Figure 1: Schematic diagram of the fiber ring cavity OEO with a frequency-locking circuit

A frequency-locking circuit for a high-Q fiber ring cavity optoelectronic oscillator (OEO) was designed, as shown in Figure 1. The principle of the locking circuit is as follows: A laser generates a 1550nm wavelength optical wave, which then enters a phase modulator for phase modulation, passes through the high-Q fiber ring cavity, and is injected into a photodetector for photoelectric conversion. The output signal is then sent to a frequency-locking module for signal demodulation. Finally, it is fed back to the laser's frequency control terminal via a high-voltage amplifier. By controlling a piezoelectric ceramic to change the laser's output frequency, it can always follow the resonant frequency changes of the high-Q micro-ring cavity. This feedback regulation locks the laser's output optical frequency, keeping photons in resonance as they pass through the fiber ring cavity. This acts as the OEO's delay energy storage unit, forming a feedback loop.

After adding the frequency-locking system, the OEO system works as follows: The laser's light, after being modulated by an intensity modulator, enters a phase modulator. The modulated optical signal is amplified by an erbium-doped fiber amplifier, then enters the fiber ring resonator cavity. One path goes to the frequency-locking module, where the signal is fed back to the laser via a high-voltage amplifier to lock the output optical frequency at the resonant point of the fiber ring cavity. The other path goes to a photodetector, then through a radio frequency amplifier and an optical coupler, back to the intensity modulator's modulation port, forming the OEO oscillator loop.

Figure 2: Spectrum of the frequency-locked OEO system

After powering on the optoelectronic devices in the OEO loop, such as microwave amplifiers and photodetectors, and the frequency-locking circuit module, the laser's light intensity (optical power) is adjusted to start the OEO system. The optical microcavity is locked by the frequency-locking circuit. When the system reaches a stable oscillation state, the oscillation peak is at 10.4GHz, as shown in Figure 2.

Experimental Results:

Figure 3: Left: Phase noise without frequency-locking circuit; Right: Phase noise with frequency-locking circuit

With the spectrum analyzer's frequency range set from 9.6GHz to 11.3GHz and the center frequency at 10.4GHz, single-mode oscillation was achieved. The phase noise of the OEO system was measured before and after the frequency-locking circuit was applied, as shown in Figure 3. At a frequency offset of 10kHz, the single-sideband phase noise was -71.34dBc/Hz and -91.35dBc/Hz, respectively. The introduction of the frequency-locking circuit reduced the phase noise by about 20dBc.

Recommended High-Voltage Amplifier: ATA-2161

Figure: ATA-2161 High-Voltage Amplifier Specifications

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