Page 94 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

Interlude: Alternative Circuits and Detection Techniques 87

40

35

30

25
Voltage (mV) 20

15

10

5
8 fine black stripes 2 broad
black stripes
0
-2500 -1500 -500 500 1500 2500
Time (μs)
Figure 4.7 Self-detecting LED output for eight 1-mm-wide black stripes on a retroreflecting disk.

trum. Emission wavelengths are usually in the 610 to 640nm range. In princi-
ple they could be coupled with a silicon detector to give improved sensitivity
beyond 1mm wavelength, but in practice it is far better to use a germanium or
InGaAs photodiode.
Below 350nm most silicon detectors also show poor and decreasing respon-
sivity, due both to use of a borosilicate glass or plastic window and to the intrin-
sic detection process. Especially at these ultraviolet wavelengths, therefore,
fluorescent wavelength conversion is an attractive option. The goal is to find a
material with high absorption at the “difficult” wavelength, which reemits in a
longer-wavelength region where the photodiode sensitivity is superior (Fig. 4.8a
and b). If the emission process is efficient, there can be a net gain in detection
sensitivity. A number of fluorescent glasses, polymers, and dyes is available for
this. For example, Sumita Optical Glass manufactures a range of fluorescent
glasses that can be mounted directly on the front of the photodetector as a wave-
length converter. Some can also be processed into planar waveguide form with
edge-mounted detectors, allowing large areas for detection of diffuse or widely
2
dispersed light (Fig. 4.8c). Large-core (mm ) optical fibers can be formed from
some fluorescent polymers. The best materials for this use show high absorp-
tion at the detected wavelength and low absorption at the shifted wavelength,
so that self-absorption of the visible light is minimized. With the great interest
in and potential markets for efficient white-light LEDs, it is likely that big
improvements will be seen in phosphor materials for use at deep blue/near-UV

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