Page 32 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Amplified Detection Circuitry

Amplified Detection Circuitry 25

final value is achieved only after more than 2 seconds. Stated alternatively, the
photoreceiver bandwidth is approximately 1/2pR LC p = 0.22Hz. This slow per-
formance may be unacceptable for the application.

2.4.1 Reverse bias
The simplest change that can often be made to improve the bandwidth of an
existing photoreceiver design is the application of photodiode reverse bias. All
photodiodes may be reverse biased to some extent, and in some cases the gains
2
can be very worthwhile. For example, the well-known BPX65 is a 1-mm silicon
device designed for high-speed applications. With zero bias its capacitance is
about 15pF. However, this device can withstand 50-V reverse bias (Fig. 2.4), at
which point the capacitance will drop to about 3pF. This can make a critical
improvement in detection bandwidth. Most telecommunications photodiodes
are designed to be operated in this mode, some being capable of withstanding
100V or more.
Unfortunately, a bigger reverse bias will increase the reverse leakage current.
In every case it is important to calculate and measure both the DC offset and
the extra shot noise caused by the dark current. It is often convenient to make
the reverse bias potentiometer-adjustable, so that the optimum bandwidth and
noise setting can easily be found. In some cases even 1V reverse bias can make
a useful improvement in overall performance, without unduly increasing noise.

2.4.2 Photodiode choice
When reverse bias does not achieve the desired speed performance, try alter-
native devices. A very wide variety of photodiodes is available, which vary in
2
size from 1000mm tiles of silicon to tiny 25-mm diameter devices designed for

16
14
Capacitance (pF) 10
12

8
6
4
2
0
0.1 1 10 100
Reverse Voltage (V)
Figure 2.4 Photodiode capacitance is a function of reverse bias voltage.
Values for a BPX65 are shown. High voltage will increase detection band-
width at the expense of increased dark leakage current.

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