Module 9 : Photo Detectors
Lecture : Principle of Photo Detection
 

Noise Equivalent Power :

Source of noise in a detector is thermal fluctuation. Charged particles are always in a state of motion. Even when no radiation is incident on a device, a background current, whose magnitude could be in nano-amperes or pico-amperes, is generated. This is known as dark current . In order that a detector may be able to differentiate between such random noise and an incoming signal, the power of the signal must be greater than the noise signal. In a detector design, one defines signal to noise ratio (SNR) as

$\displaystyle {\rm SNR}\ = \frac{{\rm signal\ \ power}}{{\rm noise\ \ power}}$

Noise equivalent power (NEP) is an important figure of merit for a detector. NEP is defined as the rms incident power which gives rise to a current (or voltage) whose rms value is equal to the rms value of the current (voltage) due to noise effects.
For a detector, the NEP is usually specified at particular wavelength and temperature. The bandwidth for the incident radiation for the measurement of NEP is generally taken as 1 Hz. Noise power within a bandwidth of $ \Delta f$is expected to be proportional to $ \Delta f$itself. Since the current (voltage) is proportional to the square root of the power, the noise current (voltage) is proportional to $ \sqrt{\Delta f}$. The unit of NEP is, therefore, watts/ $ \sqrt{{\rm Hz}}$.(Several texts give the unit of NEP as watt. However, it is more common to use NEP as a misnomer as given here)
Detectivity and Dee Star (D $ ^\ast$)
Both these terms are frequently used interchangibly, though some definitions make a difference between the two. D $ ^\ast$ is essentially the inverse of NEP normalized to unit area of the detector.

$\displaystyle D^\ast = \frac{\sqrt A}{{\rm NEP}}$

The unit of D $ ^\ast$ is m-(Hz) $ ^{1/2}$/w. (Detectivity is often defined as the inverse of NEP.)

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