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Characterization of Thermopile for Laser Power Measurements
A. K. Türkoğlu
Ulusal Metroloji Enstitüsü, Kocaeli, Turkey
Abstract. Due to their high wavelength and power range,
thermal detectors were employed in our measurement
set-up for optical power measurements. A link between
optical power calibrations at medium power levels and our
primary standard cryogenic radiometer was established by
means of transfer standards. Analysis of the set-up and
characterization of our working standard thermal detectors
to yield uncertainty budget is presented in this study.
Detector Based Measurements
As the needs and applications of optical measurements
increase, we frequently face with new radiation detectors
and lasers with different structure, speed, range or beam
properties. However in the calibration and measurements,
commercially, there is no single suitable working standard
detector available to cover all the power and optical
wavelength range of interest.
In our laboratory, treaceability from our Cryogenic
Radiometer (Oxford Instr., Radiox) is provided by using a
set of home-built 3-element reflection type trap detectors
as the transfer standards. Due to the type of silicon
detectors used, those trap detectors can be used safely in
300 to 1100 nm wavelength range but only up to 3 mW
incident power due to their linearity limits. The
measurements at wider wavelength ranges have become
possible by employing our Electrically Calibrated
Pyroelectric Radiometer. But for higher power ranges, it
was found out that it is not feasible to use filtered
photodetectors due to the heating effects, nonhomogeneity
of the filter surface and strong depen- dence of the
alignment and on incident angle. Therefore, we have
chosen and employed thermopiles in our laser power
measurement and detector calibration set-up.
Laser Sources
Our monochromatic line source facility contains an 11
mW HeNe of 632.8 nm and a 20 mW Tunable Argon
lasing at 488 and 514 nm. For medium power ranges, we
added an intensity stabilized 2W Nd-YAG laser to our
set-up, which has a main beam at 1064 nm wavelength and
an additional green line at 532 nm.
Detectors
For internal checks and dissemination, we have used
filtered UV and Ge detectors which are capable up to
optical powers of 30 mW. We have added three
thermopiles with 3, 30 and 250 W maximum optical power
thresholds to our measurement facility. Circular active
areas of the thermopiles were defined with the diameters of
12, 18 and 50 mm respectively. Selected detectors had
response times of 0.8 to 2.5 s. These detectors were chosen
especially for their good repeatibility, spatial uniformity
and rather flat responsivity over a wide range which was
stated as 0.2 to 20 µm.
Figure 1. Reflectance Curve of Thermopile Coating
Characterization
For the flatness of the spectral responsivity, the
diffuse reflectance of the thermopile was measured by
mono- chromator and found to be as 95,2 ± 1,2 % between
200 and 800 nm. Reflectances at single laser lines were
also measured seperately as shown in Fig.1.
Nonlinearity behaviour was studied by means of
detector sensitivity measurements taken at eight different
levels and found to be increasing up to 0.4 % level as
optical power goes to 1.6 W. Homogeneity of sensitivity
was analyzed 7 mm around the center and detected as
below 0.25 % except for the largest area detector. The
stray light and divergence effects was also measured to be
very low, around 0.05 % for nominal 60 cm measurement
distance on the set-up. Over 100 mW level, random power
fluctuations by the source were seen to be more effective
than the any other factor, and therefore were tried to be
minimized by employing a monitor detector.
Results
A thermopile based laser power measurement set-up
working at medium power levels from visible to near
infrared range has been established. Responsivity, linearity
and uniformity focused analysis resulted that
measurements at an uncertainty level (k=2) of 1.4 % for
the visible range (up to 20 mW) and of 1.9 % in the
infrared (for 1.6 W at 1064 nm), have become possible by
this power and wavelength range extension study.
References
Li, X., T. R. Scott, C. L. Cromer, D. Keenan, F. Brandt, K.Möstl,
Power Measurement Standards for High-Power Lasers:
Comparison between the NIST and the PTB, Metrologia, 37,
pp.445-447, 2000.
Li, X., T. Scott, S. Yang, C. Cromer, M. Dowell, Nonlinearity
Measurements of High-Power Laser detectors at NIST,
Journal of Research of the NIST, 109, No.4, pp429-434, 2004.
Möstl, K., Laser Power and Energy Radiometry, Institute of
Physics, Conference Series, No.92, pp.11-18, London, 1988.
Turkoglu, K., F. Samadov, M. Durak, U. Kucuk, Construction of
a Reference Photometer Head for the Realization of Candela,
Proceedings of CIE Congress, Volume II, pp. 379-386,
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 329