4.6 Spectral Response
Spectral response, Sl, of a device refers to its responsivity as a function of wavelength. In this section the experimental details and data interpretation methods are described while detailed discussions on the underlying theory is presented in Chapter 6.
Spectral response measurements were carried out using a tungsten filament lamp, chopper and
a Bentham monochromator (M300) over the 450 to 1830nm wavelength range. Light from
the monochromator was incident on a device biased beyond the turn-on point in series with a
suitable resistor. The voltage drop across the resistor was monitored through a lock-in
amplifier and the spectral response was recorded on a synchronized plotter.
Figure 4.13: Schematic diagram of the experimental set-up for spectral response
measurements on detector devices
There are sources of non-linearities in a complex optical system such as the one described
earlier. Two such sources are the tungsten filament lamp and the grating. These need to be
taken into account in order to obtain a true spectral response of the device.
As the white light source is a tungsten filament lamp, the amount of energy radiated through
the emission spectrum is not uniform; in practice it behaves like a blackbody source at a
temperature of the filament which in this case is 3100K. The spectral radiancy, Wl, of a
blackbody at temperature T is given by Planck's formula
[135,136]:
where all the symbols have their usual meaning and the units for
Wl here is in
Wm-2mm-1.
The spectral radiancy of the Sun and a tungsten filament lamp are plotted using the above
equation in Figure 4.14; the temperature of the Sun and the lamp are
assumed to be 4500K and 3100K respectively.
Figure 4.14: Spectral Radiancy of the Sun and a tungsten filament lamp
For accurate interpretation of the data, it is important to remove the effect of this non-linearity
present at the light source from the measured spectral response of the device under test. This
is known as deconvolution. The measured spectral response is divided by the blackbody
radiancy corresponding to each wavelength. The non-linearity due to the grating, on the other
hand, was removed by using the actual spectral efficiency of the grating supplied by the
manufacturers.
In practice, the absolute responsitivites for most of the devices studied were measured at two
or more wavelengths using separate lasers where appropriate (630nm, 780nm and 1300nm
respectively). The measured spectral response, Sl, was then normalised to one of these
wavelengths while the second absolute responsivity was used to cross-check the validity of the
Sl.
4.6.1 Experimental Set-up
4.6.2 Deconvoluting Optical Non-linearities
Wl =
(2p.hc2/l5).(1/{exp[hc/lkT] - 1})
(eqn. 4.36)
© 1998: Shabbir A. Bashar (in accordance with paragraph 8.2d, University of London
Regulations for the Degrees of M.Phil. and Ph.D., October 1997). The Copyright of
this thesis rests with the author, and no quotation from it or information derived
from it may be published without the prior written consent of the author.
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Abstract
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