Shot noise of optical receiver

Shot noise is a fundamental quantum noise in optical receivers, arising from the discrete nature of photons and electrons, and it sets a lower limit on the achievable signal-to-noise ratio.

Origin of Shot Noise

Shot noise occurs because photocurrents are generated by discrete electrons in response to incident photons, which arrive randomly over time. Even when the optical signal has constant power, the current fluctuates due to the probabilistic nature of photon absorption events in the photodiode. These fluctuations follow Poisson statistics, often approximated as Gaussian for large photon numbers, and are independent of frequency, making shot noise a type of white noise (constant spectral density) in the receiver output.

Mathematical Description

For a photodiode with average photocurrent Ip, the shot noise current is(t) can be expressed as: I(t)=Ip+is(t) The power spectral density of shot noise is given by Si(f)=2eIp for a one-sided spectrum, where e is the elementary charge. This shows that shot noise increases with the average photocurrent, meaning higher optical power leads to higher absolute shot noise, though the relative noise decreases. Impact on Optical Receiver PerformanceShot noise limits the signal-to-shot-noise ratio (SSNR) in optical receivers.

In intensity modulation direct detection (IM/DD) systems, the SSNR can be optimized using receiver filters such as integrators or matched filters. For coherent detection systems, the SSNR improves significantly when the local oscillator power exceeds the received signal power, approaching the quantum limit of detection. Shot noise is particularly important in low-light or high-speed optical communication, where it often dominates over thermal noise.

Avalanche photodiodes (APDs) introduce additional multiplication noise, which combines with shot noise to affect the overall SNR.

Mitigation and Measurement

To accurately measure shot noise, high quantum efficiency photodetectors and low-noise electronics are required. Balanced detection schemes are often used to subtract common-mode noise and isolate the shot noise contribution. While shot noise is intrinsic and cannot be eliminated, receiver design and filtering can maximize the SSNR and approach the quantum-limited performance.

In summary, shot noise is an unavoidable quantum effect in optical receivers, arising from the discrete nature of photons and electrons, and it fundamentally limits the sensitivity and performance of optical communication systems. Proper receiver design can optimize performance relative to this noise floor.