Superconducting submillimeter and millimeter wave detectors Page: 13 of 135
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thermometer, which in turn changes the temperature of the bolometer through the change in
bias heating. The net effect can be incorporated as a modification of the thermal
conductance from G to Gerf.=G-I2dR/dT. When Geft0 (and for dR/dT>O), the resistance
of the thermometer and the temperature of the bolometer increase without bound (until a
stable configuration is reached), a condition known as thermal runaway. Throughout the
remainder of tdis di:sertation, I will assume that the bias current is sufficiently low so that
the effects of bias current heating and thermal feedback can be ignored.
2.3 Measurements of Bolometer Responsivity
It is often necessary to measure the responsivity of the bolometer. The simplest
approach is to dissipate a known amount of power in the bolometer and to measure the
resultant voltage rise. This information is contained in the dc I-V characteristic of the
bolometer. From the previous discussion, the voltage across the bolometer is
V=IRO+I(AT)dR/dT. The temperature rise due bias heating is just AT=I2R,/G, so that the
I-V relation is
V = IRO + IdR/dT I2Ro . (2.4)
The slope of the voltage V versus the bias power I2Ro is just the voltage responsivity S.
The efficiency of the optical system can then be measured by using a calibrated source of
radiation (such as a blackbody source) and comparing the measured signal with the
expected magnitude.
One concern with this type of measurement is due to the presence of non-thermal non-
linearities in the thermometer. Thus, in addition to the temperature dependence of the
thermometer resistance, it is necessary to account for the possibility that the resistance of
the thermometer also depends on the bias current. This effect has been observed in doped
germanium16,17,17a eAd transitin ,cdg. thermomets. 5 The Ffec Of non-,hermal non-
ge..=a"UJIa u au*5~i1LL'.JAg u5'. morneU11ersA. 1 I 11 L 01O UIUI-1W1l 11011-7
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Nahum, M. Superconducting submillimeter and millimeter wave detectors, thesis or dissertation, October 20, 1992; California. (https://digital.library.unt.edu/ark:/67531/metadc1311365/m1/13/?rotate=90: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.