Rd Rl Rd ARd RlRd

assuming a log-linear dependence, with coefficient a, between the temperature of the detector element, T, and its resistance RD, and RD ^ RL. In general, VS is accompanied by VN and V0, the noise and offset components of the output voltage as before. The advantages of thermistor bolometer detectors include sensitivity to a wide range of wavelengths, and reasonable performance (D* « few x 108 cm Hz1/2) without cooling. The main drawback is the need to provide a high (noise-free) bias voltage.

Superconducting versions have been developed that use a detector element held at its normal-to-superconducting transition temperature. Enormous coefficients of resistivity are thereby obtained, but maintaining a very precise operating temperature without noise-inducing fluctuations presents practical difficulties, especially for deployment on satellites.

Current source i

Detector element

Detector element

Current source i

FlG. 9.3. Pyroelectric detector equivalent circuit. The components inside the box represent the detector element; RL is the load resistor connected across the element, and CL the stray capacitance of the external circuit. The output voltages V are r.m.s. values of an a.c. signal.

Pyroelectric detectors offer good performance requiring neither bias voltage nor cooling by exploiting the properties of ferroelectric materials, which exhibit a permanent electric dipole moment. Pyroelectric crystals are a subclass of these characterized by a high coefficient of the temperature dependence of polarizabil-ity, p. A common example, used for instance as motion detectors in domestic burglar alarms, is lithium tantalate LiTO3, but the material most used in practice is deuterated triglycine sulphate, known as DTGS, which has both a large value of p (about 10~8 C cm~2 Ku1) and a small dielectric constant, which corresponds to small internal losses.

In pyroelectric detectors used in the a.c. mode the chopped incoming radiation induces fluctuations in the polarization of the crystal that appears as a varying charge separation across the detector element. The detector behaves like a capacitor with a charge that varies at the modulation frequency, and can be modelled as a capacitance of a few pF in parallel with a current source (Fig. 9.3). Externally, the element is connected across a large load resistor RL through which the induced charge can flow, and the signal is then the voltage across this load. A practical model of the detector will include the internal resistance of the element Ro and the stray capacitance CL of the external circuit. If the modulation (chopping) frequency is w = 2nf so that the incident power is P(t) = Po exp(iwt), the corresponding detector temperature T as a function of time t will be the solution of

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