Optoelectronic Oscillator (OEO)
Definition: An OEO generates a radio frequency (RF) signal by converting optical signals into electrical ones, where a feedback loop is employed to sustain oscillations.
How does OEO Works?
A laser generates a light signal, which passes through a modulator, modulating the light with an RF signal. The modulated light travels through an optical delay line or resonator. The photodetector converts the modulated light back into an electrical signal, which is amplified and fed back into the modulator, sustaining oscillation.
Key Parameters in OEO Design
Phase Noise: It refers to the noise in the phase of the signal, manifesting as small random fluctuations in the frequency.
Importance: Low phase noise is essential for maintaining signal integrity, particularly in high-frequency systems where even slight noise can degrade performance.
Mitigation: In OEOs, long optical delay lines or high-Q (quality factor) resonators are used to lower phase noise. Photonic integration can also reduce noise by minimizing electrical-to-optical conversion losses and component misalignment.
Quality factor (Q-Factor):
The Q-factor measures the sharpness of the resonance of a system, reflecting its ability to store energy. A high-Q resonator (like an optical cavity) is critical for maintaining low phase noise and stable oscillation at high frequencies. PIC-based OEOs often aim for high Q-factors through resonators like micro-ring resonators or Bragg gratings.
Optical Delay Line:
The optical delay line introduces a delay in the feedback loop, effectively controlling the frequency of oscillation.It has impact on Frequency and Phase Noise for example if we have longer delay, the phase noise will be lower, but it is more challenging to integrate such system.
PICs aim to achieve compact delay lines through waveguide structures.
Laser Linewidth: The linewidth of a laser is the range of frequencies over which it operates. Narrow linewidth lasers are essential in reducing noise.
Impact: A broad linewidth adds more noise to the OEO system, so using a laser with narrow linewidth is essential for low phase noise operation.
Photodetector Responsivity: Responsivity is the efficiency with which the photodetector converts optical signals to electrical signals.
Importance: High-responsivity photodetectors can improve the efficiency of signal conversion and reduce the overall loss in the OEO loop.
Amplification and Gain: Electronic amplifiers are required to maintain the loop gain and ensure oscillation. However, amplification stages can introduce noise, so optimizing the gain while keeping noise low is essential.
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