Page 95 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

88 Chapter Four

(a) Fluorescent (c)
wavelength
converter Detector
l 1
Fluorescent
l 1 l 2 waveguide l 2
l 2
Edge-mounted
(d) detector
l 1 l 1
(b)
Responsivity

l 1 l 2 l 2
Wavelength

Figure 4.8 Wavelength conversion can improve detection sensitivity in regions of poor photodiode
responsivity. Fluorescent and scintillating materials can be formed into face plates, planar wave-
guides, and fibers.

excitation wavelengths. Doped glasses researched for optical fiber amplifier use
have similar properties. Both of these may find application with detectors. A
shard of phosphor-coated glass from a broken fluorescent tube can be used for
efficient detection of 254nm UV light. When detecting rapidly modulated light
remember that phosphor wavelength conversion can be slow.
When the target wavelength becomes shorter still, fluorescence makes way
to scintillation. Thallium-doped cesium iodide is the most common scintillator
material, which generates a smooth 200nm wide visible output peak centered
on 680nm, when illuminated with 100keV x rays. Synthetic ceramic materials
are also available, emitting a series of lines at about 520nm, 68nm, and 780
nm, with similar sensitivity but reduced afterglow. Hamamatsu manufactures
a range of silicon photodiodes with these materials already bonded as faceplates.
The primary application is in x-ray tomography.
Wavelength conversion can also be a problem for photodetection. When
working with low-pressure mercury vapor lamps, which emit at 254nm, or the
UV lasers used for fiber Bragg grating manufacture, it is difficult to avoid
fluorescent wavelength conversion. At these wavelengths, almost everything
seems to fluoresce, including many glasses, polymers, dyes, aromatic chemicals,
biological materials, and fabrics. Even ordinary white bond paper makes an
effective viewing card for the UV due to additives. Fabric “brighteners” con-
tained in clothes washing powders emit intensely when excited by a violet LED
or UV lamp. If it is intended to make quantitative absorption measurements
in the UV, this extra light can lead to large errors and peculiar results. When
pulsed light sources are used and detected, the long fluorescent time constants

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.