What is the bandwidth limit of current photodetectors for high frequency RF photonic links?

The bandwidth of photodetectors determines the maximum RF frequency that can be processed by a photonic link. Current technology limits are: (1) InGaAs PIN PD: standard 3 dB bandwidth: 40-70 GHz (for commercial models from Finisar/II-VI, Discovery Semiconductors, u2t/Thorlabs). State-of-the-art research: > 100 GHz (with specialized thin absorber designs). Responsivity: 0.5-0.8 A/W at 1550 nm. Maximum photocurrent: 10-20 mA (limited by space-charge screening). (2) Uni-Traveling-Carrier (UTC) PD: bandwidth: 50-150 GHz. Maximum photocurrent: 20-100 mA (the key advantage over PIN PDs). Higher RF output power: up to +10 dBm at 100 GHz. Used for: high-power mmWave photonic generation and high-linearity analog links. (3) Modified UTC (MUTC) PD: combines the advantages of UTC (high power) and partially depleted absorber (high responsivity). Bandwidth: 40-100 GHz. Responsivity: 0.5-0.7 A/W. Used for: mmWave signal generation in 5G and radar. (4) Traveling-wave PD: the absorber is distributed along a waveguide, and the RF output propagates along a transmission line matched to the detector impedance. Bandwidth: > 100 GHz (velocity-matched design eliminates the RC bandwidth limit). Lower responsivity per unit length (the light is absorbed gradually). Used for: ultra-wideband applications (> 100 GHz). (5) Bandwidth limitations: transit time: carriers generated in the absorber must traverse the depletion region. Thinner depletion = shorter transit time = higher bandwidth. But thinner absorber = less light absorbed = lower responsivity. Trade-off: BW × responsivity is approximately constant for a given PD design. RC time constant: the PD junction capacitance (C_j) and load resistance (R_load = 50 Ω) create a low-pass filter. Smaller PD area = lower C_j = higher RC bandwidth. But smaller area = harder to couple light efficiently. Trade-off: area vs coupling efficiency. Packaging: the PD must be packaged with a high-bandwidth output (coplanar waveguide or V-connector). The package parasitics (bond wire inductance, pad capacitance) limit the overall bandwidth. State-of-the-art packaging: > 100 GHz (using flip-chip bonding or waveguide integration).