which, in addition to choosing material with large n and G, implies a low temperature T and a large equivalent resistance R. The detector is then said to be photon-noise limited.

If the power P originates mainly in the background rather than the source then the detector is said to be operating in the 'BLIP' (background limited intrinsic photoconductor) condition. Figure 9.7 shows how most detectors approach this condition at their wavelength of peak performance.

9.4.2 Photovoltaic detectors

Photovoltaic mode detectors are fast and sensitive, and operate without the need for any bias voltage. The commonest examples are semiconductor p-n diodes made of silicon, indium antimonide, or indium arsenide. The principle of operation is that pair production by photons induces a measurable voltage across the junction, which is related to the incoming photon flux. For wavelengths longer than about 3 to 4 pm, they have to be cooled to prevent the output from being swamped by spontaneous electron-hole pairs produced as a result of thermal excitation.

For a diode in the photoconductive mode, with an applied bias voltage Va, the current i is given by the normal expression for a diode, plus a photocurrent in the opposite direction, i.e.

In photovotaic mode the device is open circuit, with a voltage

appearing across the terminals. Note that this type of detector is inherently nonlinear; for this reason, it is generally arranged in a suitable circuit where the diode acts as a current source, where there is no voltage across the detector.

Photon noise in these devices is known as shot noise, which has its origin in the finite nature of the charge on the electron. Suppose that the detection system is characterized by a time t0 during which time N photon events occur. The mean current is eN

is given by

If N is assumed to have a Poisson distribution about the mean, a standard identity gives

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