Page 149 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

142 Chapter Seven

Ambient light
C f
R
~mW f
I p
1nW -
C p +
LED
source
~mW
Figure 7.1 Many laboratory experiments are bathed in interfer-
ing light, which can sometimes be much more intense than the
desired signal. The first defence weapon is modulation.

V Time
b I p
+ Time
C - A
p L
R L R 1 V o = I R
1M 100M
20dB/dec
(a) Voltage Follower V o

f c
Time
R 1M 120Hz Frequency
L
V o = I R
p L
I p + A
-
(b) Transimpedance
Time
Figure 7.2 AC coupling is not much help to reduce the DC response, as
the front end can still overload. Choose f c high enough to get adequate
suppression at the interfering frequency.

7.2 Frequency Domain
7.2.1 AC Coupling: voltage follower
The first characteristic difference between our weak modulated signal and
the strong ambient light is of course its modulation frequency; ambient light
intensity generally doesn’t change rapidly. To help the synchronous detection
process we could therefore try to suppress the receiver’s DC and low-
frequency response, for example by AC coupling the first stage. Where a bias
box and voltage follower are used, this is straightforward to arrange (Fig. 7.2).
Photocurrent due to the incident light, containing both our modulated signal

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.