Page 155 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

148 Chapter Seven

defined, and stable to about 1nm in even the least stable solid state laser diode.
Hence we can use filters centered on the signal source wavelength and block-
ing at wavelengths only a few nanometers away. These bandpass filters can be
very effective. Thin-film interference filters formed from multiple layers of
deposited alternating high- and low-index transparent material are widely avail-
able with center wavelengths from about 220nm in the UV to 2mm in the near
IR, with bandwidths of 10 to 100nm. Small pieces of these filters, diamond sawn
from larger units, are big enough to cover many photodetectors. However, they
do have limitations which affect system performance. The transmission in the
passband is typically 50 percent, so half of the desired signal is also thrown
away. They are often designed for normal incidence and their transmission
changes markedly off axis. They always have other passbands separated from
the main transmission region, which require additional filters such as colored-
glass types to block them. They can be expensive, and last their suppression is
-3
-4
not infinite (typically 10 to 10 ), especially as the light is reflected by the mul-
tilayer stack, not absorbed. Nevertheless their performance with simple band-
pass configurations with collimated light can be stunningly effective in isolating
well-characterized sources from bright ambient light.
Another useful class of multilayer interference filter is the dichroic filter.
These are produced in huge volumes as “hot” or “cold” mirrors for halogen
lightbulbs and projector mirrors, reflecting most visible light but allowing
infrared to be transmitted and removed from the instrument. They offer an
edge response, reflecting either to longer or shorter wavelengths (Fig. 7.8).
Devices can be purchased for normal incidence and for 45° incidence, and some
types are very economical given the high performance delivered.
Recently another class of interference filter has become popular, the rugate
filter. This uses a single film of deposited material with a refractive index profile
normal to the filter surface which is continuously graded. This extra degree of
freedom compared with a high/low refractive index stack allows the fabrication
of filters with almost arbitrary transmission spectra. Filter synthesis software
tools have also been developed to make this flexibility practical. One applica-
tion is to make narrow notch filters for eyesight protection which reflect a
design laser wavelength, but otherwise transmit to give a nonfatiguing low-color
view of the scene (Fig. 7.9). Multiple notches can be made in the same filter.
Even color-correction filters with a complex intensity reflection coefficient spec-
trum can be fabricated in such a manner.
Although they offer much more limited wavelength filtration functions,
colored-glass filters, made from absorbing materials in a glass host, can often
provide much higher ultimate suppression than interference filters. They
are available primarily with long-wavelength pass responses, although a few
bandpass filters exist (Fig. 7.10). There is a particularly useful range of
long-wavelength passing glass filters from Schott with designations such
as WG with 280nm to 345nm wavelength cutoffs, and the GG, OG and
RG series with wavelength ranging from the 375nm (deep blue) to 1000nm

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.