Page 93 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

86 Chapter Four

100
Noise Voltage (nV/ Hz) 10 200 Ohm 200

100
50
20
10
1

0.1 0 Ohm 0
1 10 100 1000 10000
Emiter Current (mA)
(a)
10 20 Ohm
Noise Current (pA/ Hz) 0.1 1000 Ohm 20

1
100
200
500
1000

0.01
1 10 100 1000 10000
Emiter Current (mA)
(b)
Figure 4.6 (a) Bipolar transistor voltage noise generator for various values of r bb.
(b) Bipolar transistor current noise generator for various values of emitter resistance r e.

still restricted to AC coupling. Figure 4.7 shows a time-trace of a 660nm (red)
LED operated in close proximity to a spinning disk of retroreflective adhesive
tape masked by opaque black stripes. Eight black stripes of different widths are
visible. Most of the visible noise is due to true variations in surface reflectivity.
The signal-to-noise for detection is more than 35dB in 50kHz bandwidth.

4.5 Wavelength Shifted Detection
Some regions of the wavelength spectrum are difficult to detect. For instance,
above about 700nm wavelength the eye becomes less and less sensitive, which
makes working with the near-infrared light (IR) of current optical communi-
cation systems difficult. The well-known IR viewing cards are used to make
visible the energy in this region, from about 800 to 1600nm. They function by
storing visible energy from room lights and sunlight in a layer of phosphor
material, which is then stimulated by the IR to be released in the visible spec-

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