What is the role of a photodetector in an analog photonic link and how does its responsivity affect link gain?

The photodetector (PD) is the optical-to-electrical converter at the receive end of an analog photonic link, converting the modulated optical signal back into an RF electrical signal. Its responsivity directly determines the link gain: (1) Function: the PD absorbs photons from the optical fiber and generates a proportional electrical current (photocurrent). The photocurrent is: I_PD = R_PD × P_opt. Where R_PD = responsivity (A/W) and P_opt = received optical power (W). The RF component of the photocurrent reproduces the original RF modulation signal. (2) Responsivity: InGaAs PDs (for 1310-1550 nm): R = 0.6-0.9 A/W. Silicon PDs (for 850 nm): R = 0.4-0.6 A/W. Ge PDs (for 1550 nm, silicon photonics): R = 0.5-0.8 A/W. The maximum theoretical responsivity: R_max = qλ/(hc) = quantum efficiency × λ(nm)/1240. At 1550 nm with 100% quantum efficiency: R_max = 1.25 A/W. (3) Effect on link gain: the RF link gain is proportional to R_PD²: G_link ∝ R_PD². A 50% increase in responsivity (0.6 → 0.9 A/W): increases link gain by 20 log(0.9/0.6) = 3.5 dB. (4) Types of photodetectors: PIN photodiode: the standard PD for analog links. Bandwidth: DC to 40+ GHz (depending on the PD design). Responsivity: 0.6-0.9 A/W. Linearity: excellent (the photocurrent is linear with optical power over a wide range). Used for: all standard RFoF applications. Avalanche photodiode (APD): internally amplifies the photocurrent (gain M = 5-20). Effective responsivity: R_eff = M × R_PD = 5-15 A/W. Advantage: higher effective responsivity increases link gain. Disadvantage: the avalanche process adds noise (excess noise factor F(M) = M^x, where x = 0.2-0.7 depending on the material). The noise increase may offset the gain advantage. Used for: long-distance, low-optical-power links where the link is thermal-noise limited. Uni-Traveling-Carrier (UTC) PD: specialized high-power, high-linearity PD. Can handle very high photocurrent (50-100 mA) without saturation. Used for: high-power RFoF links and photonic mmWave generation.