From Photonics and Lasers

Photoconductive Mode

Saturation behavior in the photoconductive mode can be understood by referring to the load-line analysis of Fig. 14-3. The load line has a slope -1/RL, with an intercept on the voltage axis of Vd = -VB. As the incident optical power increases, the operating point moves downward and to the right along the load line, decreasing the magnitude of reverse-bias voltage and increasing the magnitude of the current. Both the voltage and current change linearly with increasing optical power, until the operating point reaches the


Figure 14-7 Diode voltage versus incident optical power for the photovoltaic mode. Smaller load resistance RLgives a larger dynamic range but lower sensitivity. For RL , the effective resistance reaches the upper limit of Rsh.


current axis (Vd = 0). At that point, the detector circuit saturates, and the output is no longer linear with the incident optical power.

In the linear regime, we can obtain a simple analytical expression for the detector signal as follows. It is clear from Fig. 14-3 that Vd < 0 in the linear regime. Eq. (14-3) can then be written as


Unless the...

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Topics of Interest

14-3. RESPONSE TIME   An important characteristic of any photodetector is its response time the time it takes for the detector output to change in response to changes in the input light...

14.1. BIASING THE PHOTODIODE   Although there are several types of circuits used to measure the photodiode signal current (see Section 14-5), the way that the photodiode is biased falls into one...

14-5. SIGNAL-TO-NOISE RATIO   The detectability of a small signal depends on how large it is compared to the noise. This is usually expressed by the signal-to-noise ratio (SNR), defined as the...

14-6. DETECTOR CIRCUITS   So far, we have considered only the simple detector circuits shown in Fig. 14-2, in which the output is taken as the voltage across the series load resistor RL. Here, we...

PROBLEMS   14.1    A photodiode with responsivity 0.3 A/W and dark current 2 nA is biased in the photoconductive mode, with a 9 V battery and 500 k resistor. Make a sketch like that of...