Detectors and System Mar. 28, 2018

+ Photodetection: Photoconductive effect is fast and used in communication.
Photovoltaic effect is slow and used in solar cells.
Exchange photons for electrons.

+ Photo-current: [equation] where [equation] is responsivity ( A/W) and [equation] is input optical power. Note - current [equation] where [equation] is the dark current.

+ Responsivity: [equation].
Current is generated by excitation of electrons to upper level, i.e. [equation] [equation] cut-off wavelength [equation].

+ Photodiode: Reverse biased to photoconductive regime of a diode. Electrons need to travel across the depletion layer to be excited. Output voltage [equation] where [equation] is the load resistor.

+ Modulation response: two factors control it.
Transit time - [equation] where [equation] is the rise time or the time to travel across depletion layer.
Circuit time - [equation] and [equation] where [equation] is the junction capacitance.
Example on calculation with detector response time

+ Mean square value of noise current: Shot noise [equation]
Note: noise is proportional to signal
Thermal noise [equation].
Note: noise increases as [equation] decreases.
Example for SNR calculation with DC signal
Comment on SNR calculation with AC signal

+ Overall bandwidth budget: total system delay [equation] and [equation].
[equation] -- delay of detector, [equation] -- delay of source, [equation] -- delay of fiber.

Last Modified: March 27, 2018
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