III.3.The detector’s time behaviour

A detector’s time resolution is limited by its response to an instantaneous change of the input signal. Due to electrical capacities of the light sensitive element and the electronics, the output signal does not change instantaneously as well but gradually increases or decreases until it reaches its final value. The detector’s rise time is defined by the time span required for the output signal to rise from a certain low percentage (usually 10%) to a certain high percentage (usually 90%) level of the maximum value when a steady input is instantaneously applied. Accordingly, fall time is defined by the time span required for the output signal to drop from a certain high percentage (usually 90%) to a certain low percentage (usually 10%) of the maximum value when a steady input is instantaneously removed.

Typically, the detector’s response to an instantaneous change of the input signal exponentially approaches the final value. In that case, the detector’s time behaviour is best described by the time constant τ, which is the time span required for the output signal to vary from its initial value by 63% of its final change (the value of 63% is derived from 1 – 1/e, which equals 0.63). The temporal change of the output signal Y(t) from its initial value Y0 to its final value Yf is then given by



Gigahertz Optik’s integral detectors use photodiodes, which are typically characterised by time constants of us. As most variable light sources change their intensity levels in significantly longer time scales, the detector’s time constant is not really an issue for most applications. However, especially lasers are often pulsed with a frequency in the order of 109 Hz (for example in telecommunication), which corresponds to signals periods in the order of 1 ns. In that case, the relatively slow response of normal photodiodes prevents the accurate characterisation of the laser signal’s time characteristics. For this application, Gigahertz Optik offers the LPPA-9901 detector, which uses a photodiode with an especially small capacity. This allows to reduce the LPPA-9901’s time constant to a value of 1 ns.