Measuring THz Radiation choose a Pyroelectric Detector or Golay Cell?

Transcription

1 Application Note 1011 Rev. A Application Note 1011 Rev. A Measuring THz Radiation choose a Pyroelectric Detector or Golay Cell? Don Dooley 5/25/ Application Note 1011 Rev. A

2 Before we start to describe the particular attributes of one detector versus the other it makes sense to spend a little time giving some general technical information about what each is and how they work. What is a Golay Cell and how does it function? It 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 input window, a small fragile gas chamber that includes a thin, partially absorbing metallic film and what is called an optical microphone section. When IR or THz radiation is absorbed by the thin film in the gas cell, the gas is heated, 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. It s output is calibrated against a source of known power output in Volts/Watt. What is a LiTaO3 Pyroelectric Detector and how does it function? It 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 Pyro is based on a thin, permanently poled, ferroelectric crystal (i.e. LiTaO3) that exhibits a pronounced thermal effect (the pyroelectric effect) where it s 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 µa/w. The Pyroelectric Detectors normally include a voltage or current mode circuit for optimum performance and are ultimately calibrated in Volts/Watt or Volts/Joule Application Note 1011 Rev. A

3 How does the performance of a Golay Cell compare to that of a LiTaO3 Pyroelectric Detector? Now lets take a look at the performance specifications that are typical for the Golay Cell versus that of a Pyroelectric Detector. Note that there is only one type of Golay Cell at present, where a Pyroelectric device can take many forms i.e. Hybrid Detector/Amp and Analog or Digital Radiometer (i.e. detector, discrete electronics and microprocessor). We ll choose the Pyroelectric Hybrid Detector for this comparison. Specification Description Golay Cell LiTaO3 Pyroelectric Comments 1. Detector Size, mm 6 2 to 9 Pyro detectors can be made larger 2. Window Material HDPE or Diamond Windowless Single crystal, wedged quartz recommended for the Pyro 3. Wavelength Range, µm 7 to to 3000 Pyro can be obtained with metallic and/or black absorbing coating 4. Maximum Power 10 µw 50 mw/cm2 5. Optimum Chopping Frequency, Hz 20 5 to NEP, W/(Hz)1/2 (typical) 1.2 x x Responsivity, V/W Hz Hz 8. Detectivity, cm(hz) 1/2 /W 7x10 9 4x10 8 Determined using a 1000 K Black Body Source 9. Power Requirement VAC Battery or VAC Golay Cell requires a Power Supply 10. Operating Temperature Range 5 to 40 C -5 to 120 C 11. Package Size, mm 126x45x87 8 diam x 19 Hybrid Pyroelectric Detectors are small 12. Response Time, sec 25 msec µsec to msec Pyro detector area inherently fast What conclusions can one draw from the comparison? The Golay Cell is slightly more sensitive, it has a larger sensing area (which may be important when measuring a point source), a fixed window (which will effect it s spectral response), it has a slow response time, is physically large and requires AC voltage for operation. The Golay Cell is one of a kind at present. The Pyroelectric Detector on the other hand is almost as sensitive, can be used windowless or with windows, may include a black absorber to enhance flat spectral response, is inherently fast, has a large operating temperature, is small and can be operated off batteries or an AC supply. Pyroelectric products are available as discrete and hybrid detectors, analog and digital Radiometer instruments.

4 Application Note 1011 Rev. A Below you can compare the typical spectral response of a LiTaO3 Pyroelectric Detector, like our SPH-62 THz or TPR-65 THz, to that of a typical Golay Cell note that the spectral response is very similar because both devices rely on a thermal effect.

5 Application Note 1011 Rev. A In conclusion let s look at the relative advantages and disadvantages of both types of sensors. Golay Cell Advantages Very sensitive (sub nw) Broad spectral response Well characterized THz spectral response Has been used as a standard in Astronomy and Extreme IR for years Disadvantages Very fragile (thin membrane) Slow in response Large, cumbersome two piece housing Very sensitive to mechanical vibration Low power only (10 µw max.) Includes HDPE or Diamond window for operation One model only Expensive (~$12K to $15K) LiTaO3 Pyroelectric Detector Advantages Sensitive (nw) Broad spectral response Small, compact housing Large operating temperature range Fast response time Can operate without a window Can handle relatively high power (50 mw) Multiple models and detector sizes Relatively inexpensive (~$450 to $2150) Quite rugged (not fragile) Readily available Disadvantages Slightly Lower Detectivity THz Spectral response is not well established microphonic response requires attention What s the final Conclusion? Though the Golay Cell has been used in the THz field for many years it has some major disadvantages compared to the LiTaO3 Pyroelectric Detector. The Pyroelectric Detector on the other hand has not been used until recently in the THz field, though the technology is mature, but has many advantages most importantly its small size, low NEP, broad spectral response and low cost. For relative measurement of THz sources the LiTaO3 Pyroelectric Detector is a great choice today and as the sensor science develops to better match the THz radiation from 30 THz to 0.1 THz the choice will only look better.