Gentec-eo THZ-D Manuel d'utilisateur télécharger pdf (Page 19)

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7 Appendix B: THZ detectors calibration

Now for the first time, Gentec-EO has developed a rigorous calibration method and new thermal THz

detector that allow absolute power and energy measurement based on a validated reference curve from

10 µm to 440 µm (30 THz to 0.70 THz). The purpose of this application note is to explain the new method

of calibration and validation for Gentec-EO THz detectors and to characterize its new spectrally flat THz

Thermal Probe.

CALIBRATING IN THE THz REGION - A CHALLENGE

Typical traceable detector calibration methods involve the calibration at one particular wavelength with

traceable Gold Standard previously obtained from a recognized international institution such as NIST in

USA and PTB in Germany (most typical wavelength are 1.064 or 10.6 μm). In addition, the relative

spectral absorption of the sensor is determined, using a Near IR (0.25 to 2.5 μm) spectrometer and a

traceable spectral standard. A wavelength correction factor is then applied to provide the best possible

calibration uncertainty over the detector’s calibrated spectral range. Both calibration systems need to be

traceable to an international calibration laboratory. Calibration can then be called traceable to NIST

and/or other recognized international standards laboratory. Thus, with quantitative traceability, the total

uncertainty of the calibration can be calculated and specified. It is important to know that, however, the

calibrated range using this method is somewhat limited and doesn’t begin to cover the broad THz

wavelength range.

The rapidly expanding development of THz sources, both CW and Pulsed, has posed numerous

challenges to our industry, including how to make accurate measurements of power and energy. One of

the biggest difficulties is that there has been no recognized international calibration standard or service

available that covered THz spectrum. This has forced us to offer THz products that are not calibrated in

the THz range and can therefore only be used for relative measurements. However, just recently, the staff

at PTB in Germany announced that they now provide traceable, low uncertainty calibration of THz

detectors at the single wavelength 119 μm (or 2.52 THz).

We are working very closely with PTB Germany and NIST USA and other well-known international

laboratories in order to take advantage of these new standards and to continue to develop better

calibration methods for the Gentec-EO THz product line.

Because of the lack of calibrated spectral reference in the THz range, it has been critical that Gentec EO

develop a new spectrally flat absorber for a THz sensor. It has been demonstrated that our organic black

and metallic coatings display significant changes in sensitivity in the THz range and thus cannot be used

as a wavelength reference there. In order to be a valid THz detector reference, the optical absorption

must be measured with high accuracy. This requires measuring the total reflectance, both specular and

diffuse. In addition, the transmission of the material must be negligible.

Currently, only specular reflectance can be measured in the spectral range of interest; 10 μm to 440 μm.

Thus, it was an essential requirement that our new absorber have only specular reflectance and

negligible diffuse reflectance. In addition, a very high and constant absorption throughout the THz range

is also necessary because of the lack of multi-wavelength THz standards in the World.

THE BREAKTHROUGH

Gentec-EO has intensified its THz development program in recent months, which has led to a technical

breakthrough. We have discovered a spectrally flat and very high absorption material for the THz range,

from 10 μm to 440 μm (and up to 600 μm) that will be used as the THz absorber for our new thermal

probe. This probe, model THZ12D-3S-VP, is believed to be the first low uncertainty spectral reference in

this portion of broad THz spectral range.

Figure 1 below shows the spectral absorption of two Gentec-EO THz detectors. The red curve represents

the low uncertainty “reference absorption spectrum” for our new THZ12D-3S-VP probe. The blue curve

gives the typical relative absorption of our “BL” coated Pyroelectric THz probes THZ9D-20mS-BL. The

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