Infrared photodetectors benefit greatly from cooling to cryogenic temperatures. For this reason, manufacturers mount the majority of these instruments in evacuated Dewars that use cryogenic coolants such as liquid nitrogen or helium (Figure 1). Figure 1. Cryogenically cooled infrared detectors display a lower drift in responsivity when mounted in liquid-nitrogen-cooled Dewars with metal O-rings and cemented windows. Dewars with rubber O-rings suffer more from moisture leakage.Laboratories around the world employ these detectors. Until recently, however, nobody reported that their output (spectral responsivity) drifts significantly with time after cooling. Drift of many percent per minute occurs, making the validity of the data acquired with such detectors suspect (Figure 2). Figure 2. The normalized response of a cooled HgCdTe detector at 3.1 μm drops by 60 percent over seven hours. The sharp decline is the result of the exhaustion of the coolant. Users of these detectors will have noticed that the cryo-coolant hold time of a Dewar deteriorates with time. For example, it will remain cold for more than 16 hours when new, but that hold time can drop to half that after a couple of years.Work at the National Physical Laboratory in Teddington, UK, also has shown that moisture can seep past the O-ring seals for the Dewar window and evacuation port. After the Dewar is cooled, the cold finger — which includes the infrared detector — acts like a cryopump, depositing the moisture as a thin film of ice on all cold parts of the Dewar, including the surface of the infrared detector.Effects of icingThe film preferentially absorbs wavelengths coinciding with a number of strong ice absorption bands. This reduces the IR detector’s responsivity at those wavelengths over time, causing drift of many percent per minute at wavelengths around 3.1 μm.1 Significant drifts also have been observed around 12 μm, another strong ice absorption band. Evacuating the Dewar and baking it at approximately 60 °C temporarily eliminates the drift. However, the drifts slowly reappear and grow in magnitude. Dewars with rubber O-rings appear the least effective in preventing the drifts from reappearing; those with metal O-rings and/or cemented windows appear to be the most effective. Moisture affects the detectors in other ways as well. For example, ice has strong dielectric properties. When it forms on the active area of a photodetector, it interacts with the multilayer dielectric antireflection coatings, causing their transmission to change and, thus, the detector responsivities to drift with time. This occurs in InSb detectors,2 HgCdTe detectors3 and silicon detectors (in the visible) cooled to –40 °C.The introduction of cold filters that restrict the range of wavelengths reaching a cryogenically cooled infrared photodetector and that block some of the thermal background radiation reduces the total noise power at the output of the detector.4 This is a well-established technique for improving a detector’s noise equivalent power and the specific detectivity of an infrared detector over specific wavelength ranges. Figure 3. An InSb detector may have a bandpass filter mounted on the Dewar cold shield. The cold bandpass filters are mounted on the cold shield of the detector, so any moisture that is inside the Dewar vacuum is deposited on the bandpass filter rather than on the detector (Figure 3). The filter comprises a stack of dielectric layers designed to have a specified transmission profile. A film of ice only a few nanometers thick interacts with the dielectric structure of the filter, changing its transmission characteristics and thus the response of the filter/detector combination.5 The response decreases at some wavelengths and increases at others. In the absence of alternatives, InSb detectors with low-pass filters on their cold shields are used extensively in the 1.6- to 2.6-μm range. The filters are glass plates that transmit wavelengths shorter than roughly 3 μm and that block longer ones. When moisture is present in the Dewar vacuum, it is deposited on the cold glass plate, where it behaves like a thin film of oil on the surface of water. As the thickness of the ice film grows, the transmission goes through maxima and minima.6 The response of the filter/detector combination exhibits an identical behavior (Figure 4). Figure 4. The normalized response of an InSb detector monitoring radiation at 1.3 μm shows oscillations in its responsivity for the first seven hours after cooling to 77 K. The “oscillations” are fast at first because the moisture in the Dewar vacuum is relatively high. As ice forms, the moisture gradually depletes, dropping the deposition rate and the frequency of the oscillations.Icing in spaceUsers of these detectors should evacuate them frequently, depending on the type of Dewar and on the required uncertainty of the final measurement. Even after evacuation, the detector should be maintained at cryogenic temperatures for a day or so to stabilize the rate of drift of the spectral responsivity.Some space-based Earth-observation instruments that employ cooled detectors exhibit similar behavior.7 The oscillations in the output of these instruments also are caused by a thin film of ice that forms as moisture is deposited on cold windows and on the detectors. Space is, by definition, vacuum. Satellites, however, can exhibit severe outgassing. The thermal insulation used in some missions, for instance, contains moisture that is liberated slowly into space. Some of it is deposited on the cold parts of the satellite, including on the cooled detectors and filters. Figure 5. The National Physical Laboratory’s Absolute Measurement of Blackbody Emitted Radiance facility uses a cooled, filtered InSb detector whose spectral responsivity in the 4.5- to 4.7-μm wavelength range must be stable to ±0.1 percent over the period of a month. The filter radiometer is housed in a Dewar with metal O-rings and with a cemented sapphire window. The Dewar is evacuated and baked every six months. The absolute spectral irradiance responsivity of the filter radiometer is checked against the lab’s standards every month when it is in use to ensure that the responsivity has not drifted by more than the specified amount. The problem can be so severe that the cooling must be switched off periodically to allow the detectors and filters to warm up and the ice to melt and escape. Examples of instruments that receive this treatment include the Along Track Scanning Radiometer 2 aboard the European Space Agency’s ERS-2, the payloads on NASA’s Landsat-4 and -5, and the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (Sciamachy) on the European Space Agency’s Envisat.Specifically, the outputs of Sciamachy’s channels seven and eight decrease by 10 percent per week after cooldown. The instrument must be decontaminated every few months by warming it to just above 0 °C to reset the channels.Meet the authorEvangelos Theocharous is principal research scientist in the Quality of Life Div. at the National Physical Laboratory in Teddington, UK; e-mail: e.theo@npl.co.uk.References1. E. Theocharous and N.P. Fox (Feb. 7, 2003). Reversible and apparent ageing effects in infrared detectors. METROLOGIA, pp. S136-S140.2. E. Theocharous (October 2005). Stability of the spectral responsivity of cryogenically cooled InSb infrared detectors. APPL OPTICS, pp. 6087-6092.3. E. Theocharous (online Aug. 10, 2005). On the stability of the spectral responsivity of cryogenically cooled photoconductive HgCdTe infrared detectors. INFRARED PHYS TECH, doi:10.1016/j.infrared.2005.06.002.4. E. Theocharous and J. Birch (February 2002). Handbook of Vibrational Spectroscopy, Vol. 1. J.M Chalmers and P.R. Griffiths, eds. John Wiley & Sons, pp. 349-367.5. E. Theocharous, G. Hawkins and N.P. Fox (April 2005). Reversible ageing effects in cryogenically cooled infrared filter radiometers. INFRARED PHYS TECH, pp. 339-349.6. E. Theocharous (July 2005). Drifts exhibited by cryogenically cooled InSb infrared filtered detectors and their importance to the ATSR-2 and Landsat-5 Earth observation missions. APPL OPTICS, pp. 4181-4185.7. D.L. Smith, P.D. Read and C.T. Mutlow (December 1997). Calibration of the visible/near-infrared channels of the along-track scanning radiometer-2 (ATSR-2), Proc. SPIE, Vol. 3221, Sensors, Systems and Next-Generation Satellites, pp. 53-62.