Optics
Photodetector
Devices that convert light into electrical signals — eyes of digital sensors
A photodetector converts incident light (photons) into electrical signal — current or voltage. Essential for cameras, optical communications, scientific instruments, medical imaging. Types include photodiodes (most common), photomultipliers (single-photon sensitive), avalanche photodiodes, image sensors (CCD, CMOS). Based on photoelectric or photovoltaic effects.
- FunctionLight → electrical signal (current or voltage)
- Common typesPhotodiode, PMT, APD, CCD, CMOS, photoresistor
- Quantum efficiencyFraction of photons producing electrons; up to ~95% for advanced
- ResponsivityCurrent per unit optical power (A/W)
- Wavelength rangeMaterial-dependent; Si ~400-1100 nm; InGaAs ~900-1700 nm
- Used inCameras, fiber optics, lidar, spectroscopy, medical imaging
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Types of photodetectors
| Type | QE | Speed | Notes |
|---|---|---|---|
| Photoresistor | ~5-30% | Slow (ms) | Cheap; light meters, ambient sensing |
| Photodiode (Si) | ~80-90% | Fast (ns) | Most common; 400-1100 nm |
| Photodiode (InGaAs) | ~80-90% | Fast | Telecom (1310, 1550 nm) |
| Photomultiplier (PMT) | ~25-30% | Fast | Single-photon; high gain (10⁶); bulky |
| APD | ~80% | Fast | Internal gain ~10-1000 |
| SPAD (single-photon) | ~50% | Sub-ns | Photon counting |
| Superconducting nanowire | ~95% | ~ps | Best; needs cryo |
| CCD/CMOS | ~80-90% per pixel | Per-frame | Image sensors |
Responsivity
R = I_photo / P_optical (A/W)
R_max = η · q / (h · f) = η · λ / (1240 nm·A/W)where η is quantum efficiency. Theoretical max R = 0.5 A/W at 633 nm with QE = 1. Real silicon photodiodes ~0.4-0.5 A/W in visible.
Where photodetectors matter
- Cameras. CMOS/CCD sensors in phones, DSLRs, cinema cameras.
- Fiber optics. InGaAs photodiodes at receivers; foundation of internet backbone.
- Medical imaging. PMTs in PET, gamma cameras, X-ray detectors.
- Astronomy. Most modern telescopes use CCDs or CMOS.
- Spectroscopy. Detect specific wavelengths in lab instruments.
- LIDAR. APDs and SPADs detect reflected laser pulses for 3D mapping.
- Quantum technology. Single-photon detectors for QKD, quantum computing.
Common mistakes
- Confusing QE with responsivity. QE is unitless ratio. Responsivity is A/W; depends on QE AND wavelength.
- Mixing detector types. Each suits different applications. Don't use PMT for high-light; don't use slow photoresistor for fast signals.
- Forgetting wavelength range. Si photodiodes don't work at 1550 nm (out of range). Need correct material for target wavelengths.
- Treating detection as instantaneous. Real detectors have rise time, fall time, dead time. Photon counting limited by dead time after each event.
- Ignoring noise. Dark current, thermal noise (Johnson), shot noise — all limit detection. Cool detectors (e.g., -50°C for astronomy) to reduce noise.
- Confusing CCD and CMOS sensitivities. Both photodiode-based; differ in readout. Modern CMOS rivals or exceeds CCD in performance.
Frequently asked questions
How does a photodiode work?
P-N junction semiconductor. Photons absorbed in depletion region → create electron-hole pairs. Built-in electric field separates them — electrons flow one way, holes the other. External circuit measures current. Reverse-biased for fast response. Si photodiodes detect 400-1100 nm; for IR (telecom 1310, 1550 nm) use InGaAs.
What's a photomultiplier tube (PMT)?
Vacuum tube with photocathode + dynodes. One photon strikes photocathode → ejects 1 electron (photoelectric). Electron accelerated to first dynode; secondary emission → multiple electrons. Each subsequent dynode multiplies. After ~10 stages, signal multiplied 10⁶+ times. Single-photon sensitive but bulky and high-voltage (~kV). Used in: low-light astronomy, medical imaging (PET, gamma cameras), particle physics.
How does an APD differ from a regular photodiode?
Avalanche Photodiode — high reverse bias (~100s V) creates strong field. One absorbed photon creates electron-hole pair; electrons accelerated cause more ionization → avalanche multiplication. Gain ~10-1000. Faster than PMT, smaller, lower voltage. Used in: lidar, single-photon counting, telecommunications, quantum cryptography.
How do CMOS and CCD image sensors work?
Both array-based: million-pixel arrays of photodiodes. CCD — charge accumulated in pixels, shifted out row by row. CMOS — each pixel has its own readout transistor; faster, cheaper. Both convert photon → electron via photoelectric effect. Modern phone cameras have ~20-50 MP CMOS sensors with sub-µm pixels.
What's quantum efficiency?
Fraction of incident photons that produce an electrical signal. QE = electrons / photons. Modern silicon photodiodes ~80-90% QE in visible. Photomultiplier ~25-30% (limited by photocathode). Avalanche ~80%. Color cameras lose factor of 3 because RGB filters block 2/3 of photons.
How are photodetectors used in fiber optics?
Optical signal carries data through fiber. At receiver, photodetector converts back to electrical. Telecom uses InGaAs photodiodes for 1310/1550 nm. Bandwidth up to ~100 GHz. Combined with laser transmitters, modern internet runs on photodiode-laser pairs. Underseas cables, FTTH (fiber to the home), all rely on this technology.
How can I detect single photons?
Specialized photon-counting detectors. PMT (good QE in visible, bulky). Single-photon avalanche diode (SPAD) — like APD but biased above breakdown; one photon triggers avalanche, then quenched. Superconducting nanowire detectors — best QE (>95%) and timing (< ps), need cryogenic cooling. Used in: quantum optics, fluorescence imaging, quantum cryptography.