Optical Detectors

Optical Detectors

The detector is a device in which incident radiation produces a measurable physical effect like temp, pressure, electrical signals…..

The primary goal of Photodetection is the conversion of an optical signal to an electrical signal. So here we are going to study the performance parameters of the detector that must satisfy for its performance and compatibility.  

1. High sensitivity at the operating wavelength
2. High fidelity, linearity is very important
3. Short response time to obtain a suitable wavelength
4. A minimum noise produced by the detector
5. Stability of performance characteristics
6. Small size
7. High reliability

The explanation is as follow…..

• Spectral responsitivity: A detector is used to measure the radiometric or photometric quantities like radiant flux, irradiance, luminous flux, illuminance or value of such a quantity integrated over a given period of time.
• In radiometry, the detector has a flat spectral response. In photometry, the detector should closely match V(λ) response.

Spectral response: all parameter varies with wavelength. The peak emission wavelength of the source with a peak response of the detector.

Spectral responsitivity(S) : is the detector output/ detector input .

S(λ)= detector output/ detector input

If a detector has more spectral responsitivity the better is the detector.

• Detector output: the physical quantity yielded by a detector in response to a detector input like current, voltage or change in resistance.

Quantum Efficiency: a measure of how many photoelectrons are produced for every photon incident on the photosensitive surface.

η= output in form of electrons/ number of the photon as input

Larger the efficiency better is the detector.

• Response time: A measure of how long it takes a detector to respond to a change in light power falling on it. Less the response time better the detector is.
• Signal to noise ratio: is the measure of noise characteristics of a detector. Lesser the noise better the detector is.
• NEP (Noise Equivalent Power): is defined as the radiant flux which produces an output signal equal in magnitude to that produced by the noise signal. Its units are W/√hz.

Lower the value of NEP the better is the characteristic of the detector for detecting a small signal in presence of noise.

• Specific Detectivity (D): It is reciprocal of NEP.
• Linearity: It is the property that the output quantity of the detector is proportional to input quantity.

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