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Photodetectors

Definitions:

Responsivity (A/W): The ratio of photocurrent to a corresponding level of incident light. Responsivity varies with wavelength.

Spectral Responsivity: Responsivity plotted as a function of wavelength.

Risetime (Tr): The time required for the photodetector output level to change from 10% to 90% of the peak output level.

Falltime (Tf): The time required for the photodetector output level to change from 90% to 10% of the peak output level.

Frequency Response: The electrical output response to a sinewave modulated light input. This is typically measured in dB vs. Hertz.

Cut Off Frequency: The frequency at which the detector output power decreases by 3dB from the output at 100kHz.

Bandwidth: The difference between the high and low cutoff frequencies, measured in Hertz. The bandwidth of the photodetector is approximately related to the rise time (Tr) by:

Bandwidth (Hz) ≈ 0.35/Tr

Dark Current: The small current which flows when a reverse voltage is applied to a photodiode and no optical input is present.

Junction Capacitance: An effective capacitor is formed at the P-N junction of a photodiode. The junction capacitance is the major factor in determining the speed of a photodiode.

Reverse Breakdown Voltage: The level of reverse voltage which can cause breakdown and deterioration of the detector.

Noise Equivalent Power (NEP): The amount of incident photon energy equivalent to the intrinsic noise level of the device, providing a signal-to-noise ratio of 1.

NEP = Noise Current (A/√Hz)
Responsivity a λp (A/W)

λp = wavelength of detector's peak responsivity