RF Over Fiber and Photonic Links Microwave Photonics Applications Informational

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).

Category: RF Over Fiber and Photonic Links

Updated: April 2026

Product Tie-In: Photonic Components, Oscillators, Modulators

Photodetector Bandwidth Limits

Photodetector bandwidth is the current bottleneck for extending photonic link technology beyond 100 GHz into the sub-THz and THz regimes.

Parameter	Option A	Option B	Option C
Performance	High	Medium	Low
Cost	High	Low	Medium
Complexity	High	Low	Medium
Bandwidth	Narrow	Wide	Moderate
Typical Use	Lab/military	Consumer	Industrial

Margin Allocation

(1) Photomixers for THz: at frequencies > 300 GHz: photoconductive switches (GaAs/LT-GaAs) are used instead of PIN/UTC PDs. The photoconductor is illuminated by two CW lasers (or a pulsed laser), and the beat frequency generates THz radiation. Bandwidth: DC to 3+ THz (but output power decreases rapidly above 1 THz: -20 to -40 dBm). (2) Plasmonic PDs: emerging technology using plasmonic antenna structures to concentrate light onto a nanoscale absorber. Sub-femtofarad capacitance = THz RC bandwidth. Research demonstrations: > 500 GHz bandwidth. Not yet commercially available. (3) Ballistic PDs: carriers traverse the depletion region ballistically (without scattering). Transit time < 100 fs for InGaAs with 100 nm depletion width. Theoretical bandwidth: > 1 THz. Practical challenges: very low responsivity (thin absorber) and difficulty in coupling light to a 100 nm layer.

Propagation Modeling

When evaluating the bandwidth limit of current photodetectors for high frequency rf photonic links?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.

Performance verification: confirm specifications against the application requirements before finalizing the design
Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades

Fade Mitigation

When evaluating the bandwidth limit of current photodetectors for high frequency rf photonic links?, engineers must account for the specific requirements of their target application. The optimal choice depends on the frequency range, power level, environmental conditions, and cost constraints of the overall system design.

Common Questions

Frequently Asked Questions

Can I buy a 100 GHz photodetector?

Yes. Commercial sources: Discovery Semiconductors (DSC series): 50-70 GHz 3 dB bandwidth. Finisar/II-VI (XPDV series): 50-70 GHz. u2t Photonics (now Thorlabs): XPDV series up to 75+ GHz. NTT Electronics: UTC-PDs with 100+ GHz bandwidth (limited availability). Fraunhofer HHI: custom UTC-PDs up to 150 GHz for research. Cost: $500-5000 per device (depending on bandwidth and packaging). Packaging: coaxial (V-connector, 1.0mm), waveguide (WR-10 for W-band), or chip-on-carrier (for custom integration).

What about silicon germanium PDs?

SiGe PDs (for silicon photonic platforms): bandwidth: 40-50 GHz (current state-of-the-art on GlobalFoundries/IMEC platforms). Responsivity: 0.5-0.8 A/W at 1550 nm (comparable to InGaAs). Advantage: monolithic integration with silicon photonic waveguides and CMOS electronics. Disadvantage: the Ge absorber is strained (lattice mismatch with Si), limiting the thickness and responsivity. Dark current: higher than InGaAs PDs (Ge has smaller bandgap). SiGe PDs are the standard for silicon photonic transceivers used in data center interconnects (> 100 billion units/year market).

How does PD saturation affect linearity?

At high photocurrent: the space-charge buildup in the depletion region distorts the electric field, causing: compression (the photocurrent no longer increases linearly with optical power), increased distortion (OIP3 degrades), and bandwidth reduction (the modified field changes carrier transit times). The saturation photocurrent (I_sat) depends on: PD area (larger area = higher I_sat), bias voltage (higher reverse bias extends the linear range), and PD design (UTC PDs have much higher I_sat than PIN PDs). For high-linearity analog links: operate at I_PD < I_sat/2 (the linear regime). Use UTC PDs for high-power applications.

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