The noise introduced by the APD in the optical receiver is

The noise in an APD optical receiver arises from shot noise, thermal noise, dark current noise, and excess noise due to the stochastic avalanche multiplication process.

Key Noise Mechanisms in APDs

1. Shot Noise Shot noise results from the discrete nature of charge carriers (electrons and holes) in the APD. It occurs because the photocurrent and dark current consist of randomly generated charges, leading to current fluctuations. Shot noise increases with the average photocurrent and is amplified by the APD's avalanche gain, making it a dominant noise source in low-light conditions . 2. Dark Current Noise Dark current is the leakage current present even without incident light. It includes bulk leakage (thermal generation of electron-hole pairs) and surface leakage (current across the device surface). Both components undergo avalanche multiplication, contributing to noise. Dark current noise is temperature-dependent and limits APD sensitivity, especially in low-light scenarios . 3. Excess Noise (Multiplication Noise) Avalanche multiplication is inherently random. The generation of secondary carriers during the avalanche process fluctuates, producing excess noise. This noise is characterized by the excess noise factor, which depends on the multiplication gain and the ionization coefficients of electrons and holes. Higher avalanche gain generally increases excess noise, limiting the useful gain and degrading the signal-to-noise ratio (SNR), . 4. Thermal Noise (Johnson-Nyquist Noise) Thermal noise arises from the random motion of charge carriers due to thermal energy in resistive elements of the APD circuit. It is independent of the applied bias voltage and proportional to temperature and resistance. While typically smaller than shot or excess noise, it contributes to the total noise floor . 5. Other Noise Components Additional noise types include 1/f noise and generation-recombination noise, which can affect APD performance at low frequencies. Multiplication shot noise, a combination of shot and avalanche noise, also contributes to fluctuations in the output current .

Impact on Optical Receiver Performance

The total noise in an APD affects the signal-to-noise ratio (SNR) and the sensitivity of the optical receiver. While the APD's internal gain amplifies the signal, it also amplifies inherent noise. Careful design, including material selection and avalanche control, can reduce noise and improve detection of weak optical signals . In summary, the noise introduced by an APD in an optical receiver is a combination of shot noise, dark current noise, thermal noise, and excess noise from avalanche multiplication, all of which influence the receiver's sensitivity and SNR.