Pyroelectric detector has advantages for terahertz sensing

May 28, 2008--The Golay cell is a device traditionally used to measure the intensity of terahertz radiation, but an alternative device--the lithium tantalate (LiTaO3) pyroelectric detector--has some major advantages, according to Don Dooley, president of Spectrum Detector (Lake Oswego, OR). An application note written by Dooley describes and compares the two types of detectors.1

May 28th, 2008

May 28, 2008--The Golay cell is a device traditionally used to measure the intensity of terahertz radiation, but an alternative device--the lithium tantalate (LiTaO3) pyroelectric detector--has some major advantages, according to Don Dooley, president of Spectrum Detector (Lake Oswego, OR). An application note written by Dooley describes and compares the two types of detectors.1

Dooley describes the two devices as follows.

The Golay cell is a "photo-acoustic" device that is sensitive, works at ambient temperatures and has broad spectral response. The basic elements that make up a Golay cell are the 6 mm HDPE (high-density polyethylene) input window, and a small fragile gas chamber that includes a thin, partially absorbing film and what is called an "optical microphone section." When infrared or terahertz radiation is absorbed by the thin film in the gas cell, the gas is heated, and expands and distorts the mirrored back wall of the cell. This distortion (or movement) is monitored and measured by the combination of an LED, optics, grating, and photodiode. The output of the photodiode is proportional to the displacement of the mirrored wall of the gas cell. Its output is calibrated against a source of known power output in volts/watt.

The LiTaO3 pyroelectric detector is an AC thermal detector that is sensitive, typically used at room temperature, and has broad, flat spectral response across most of the electromagnetic spectrum. The detector is based on a thin permanently poled, ferroelectric crystal (here, LiTaO3) that exhibits a pronounced thermal effect (the pyroelectric effect) where its instantaneous polarization is a function of the rate of temperature change of the crystal. By applying conductive electrodes to the top and bottom surfaces of the crystal, the resulting charge can be coupled out of the device and calibrated in terms of microamps per watt. The pyroelectric detectors normally include a voltage- or current-mode circuit for optimum performance and are ultimately calibrated in volts/watt or volts/joule.

Dooley notes that the Golay cell can take only one form at present, whereas the pyroelectric detector can take many forms--for example, the hybrid detector/amp, and the analog or digital radiometer (that is, a detector, discrete electronics, and microprocessor).

Next, Dooley compares the performance specifications for the two devices, choosing the pyroelectric hybrid detector/amp for the comparison. While the Golay cell has a 6 x 6 mm detector size and requires a HDPE window, the pyroelectric detector can range from 2 x 2 to 20 x 20 mm in size, requires no window, and includes a black absorbing coating. The Golay cell has a responsivity (in volts/watt) of 150 K at 15 Hz, a noise-equivalent power (NEP, in W/(Hz)0.5) of 1.2 x 10-10, a detectivity (in cm(Hz)0.5/W) of 7 x 109, and an optimum chopping frequency of 20 Hz. The pyroelectric detector has a responsivity of 150 K at 5 Hz, a NEP of 4.0 x 10-10, a detectivity of 4 x 108, and an optimum chopping frequency of 5 to 10 Hz.

Further, the Golay cell handles a maximum power of 10 microwatts, a wavelength range of 7 to 2000 microns, and an operating temperature of 5 to 40 ºC, while the pyroelectric detector handles a max power of 50 mW per square centimeter of detector area, a wavelength range of 0.1 to 3000 microns, and an operating temperature of -5 to 120 ºC. The Golay cell is bulky (126x45x87 mm) and slow (response time of 25 msec), while the pyroelectric detector is only 8 mm in diameter by 19 mm long and responds in microseconds to milliseconds.

From the comparison, Dooley draws the conclusions that the Golay cell is slightly more sensitive, has a larger sensing area (which may be important when measuring a point source), a fixed window (which will effect its spectral response), a slow response time, and is physically large; he notes that a Golay cell also requires AC voltage for operation.

The pyroelectric detector, on the other hand, is almost as sensitive, can be used windowless or with windows, includes a black absorber for flat spectral response, is inherently fast, has a large operating temperature, is small, and can be operated off either batteries or an AC supply.

Most-important advantages and disadvantages

The Golay cell is very sensitive (subnanowatt), has a broad and well-characterized spectral response, and has been used as a standard in astronomy and infrared sensing for years. However, it is very fragile (thin membrane) and quite slow in response, has a large two-piece housing, is very sensitive to mechanical vibration, has a transient response (varies with room pressure), handles low power only (10 microwatts max), requires a HDPE window for operation, comes in one model only, has a long lead time, and is expensive (~$12K to $15K).

The LiTaO3 pyroelectric detector is sensitive (nanowatts), has a broad, flat spectral response, a small housing, a large operating-temperature range, and a fast response time, can operate without a window, can handle relatively high power (50 mW), comes in multiple models and detector sizes, is relatively inexpensive (~$450 to $1950), and is quite rugged and readily available. However, it also has a lower detectivity, its spectral response is not well established, and it can have a microphonic response.

Dooley notes that, while the pyroelectric detector has not been used until recently in the terahertz field (though the technology itself is mature), its small size, low NEP, broad spectral response, and low cost make it a good choice. As the sensor science develops to better match the radiation range from 20 to 0.1 THz, Dooley believes that the pyroelectric-detector choice will only look better.

--John Wallace

REFERENCE:

1. Application Note 1011: THz intensity measurement ...choose a Golay Cell or a LiTaO3 Pyroelectric Detector? 4/28/08, Tom Dooley, Spectrum Detector.

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