1. Photo Detectors for Fiber Optic Communication
Eric Gallo
Fiber Optics ECE-E641
Winter 2003

2. Photo-detectors
Convert light energy (photons) back into electrical signals in communication
Broad range of devices with varying range of light absorption
Speed, efficiency and cost vary widely from device to device

3. Operation of Detectors

4. Types of Detectors Photo Conductors PIN photodiodes
Avalanche photodiodes
Phototransistors
MSM Photodetectors
Heterostructure MSM Photodetectors

5. Quality Factors Responsivity Quantum Efficiency Capacitance
Ratio of photons incident to current produced
Quantum Efficiency
Ratio of photons incident to EHP produced
Capacitance
Gain-Bandwidth Product
FWHM, rise time, fall time
Noise
Signal-to-noise ratio

6. PIN Photodiodes Large absorption area Simple fabrication Unity gain
Speed dependent on width
Thermally generated carriers create noise within region

7. PIN Photo diodes

8. Avalanche Effect

9. Avalanche Photodiodes
High gain due to avalanche multiplication effect
Increased noise
Silicon has high gain but low noise
Si-InGaAs APD often used(diagram on right)

10. Metal-Semiconductor-Metal Photodiodes
Similar operation to PIN diodes
Low noise due to Schottky barriers
Speeds dependent on carrier transit time (350GHz measured)

11. Heterostructure MSM Photodiodes
Faster Response Time
Lower Dark Current due to enhanced Schottky barriers
Compatible with HEMT technology

12. HMSM with Distributed Bragg Reflector
FWHM 11ps
Internal quantum efficiency approximately 1
Dark Current 15pA
High wavelength selectivity