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High-precision measurements of reflectance, transmittance, and scattering at 632.8 nm

Humbat Nasibov*a, Izmir Mamedbeili a, Dadash Riza a, Ertan Balaban a, Fikret Hacizade a

aTUBITAK-BILGEM, Scientific and Technical Research Council of Turkey, Optoelectronics

Department, Gebze, Kocaeli 41470, Turkey

ABSTRACT

Progresses in the optical coatings and optical material fields require an increase in the sensitivity and accuracy of the optical parameters’ measurement methods and systems. In this work we describe a flexible and high-accuracy system for measuring the main optical characteristics at 632.8 nm wavelength. The system comprises two methods: a laser ratio-metric measurement method for absolute measurement of the transmittance and the specular reflectance, and an integrating-sphere method for assessment of the total integrated scattering. The system utilizes an intensity stabilized He-Ne laser as a light source. Two four-element trap detectors are used: the first for monitoring of laser power, the second (fixed on a motorized stage) for the measurement of reflectance and transmittance, one after another. A PMT mounted to the exit port of a 40 cm diameter integrating hemisphere, is used for measuring the total integrated scattering. A series of measurements with several reference mirrors showed that the system is able to measure the specular reflectance with a reproducibility of <0.005%, transmittance of 0.005% with a reproducibility <0.005%, and total integrated scattering about 10 ppm, with a reproducibility of < 5 ppm at 2 sigma. The system allows characterizing of optical components with diameters between 5 mm and 50 mm.

Keywords: Specular reflectance, transmittance, total integrated scattering, optical coatings, laser ratio-metric method, trap detector.

1. INTRODUCTION High precision instrumentation for the measurements of reflectance (R), transmittance (T) and scattering (RTS measurements) play a key role in many fields of optics; ranging from the optical materials science to the development of ultra-high reflective optical coatings. The maturation of densified film deposition processes has enabled the routine manufacturing of extremely complex optical designs, which a decade earlier were only of theoretical interest but not practical. Nowadays, the introduction of ion beam and magnetron sputtering techniques and advanced plasma source processes for the deposition of high-reflective dielectric coatings on super-polished fused silica substrates allow manufacturing of high reflective mirrors with the reflectivity approaching the unity. The evolution of the field has accompanied new achievements in high precision measurement techniques and methods for optical surface characterizations. The current status of measurement methods’ potentials and drawbacks for assessments of optical coatings can easily be reviewed from the reports of the Optical Interference Coatings Topical Meetings. During the last decade, in the frame of the Measurement Problems (MP) topic, information about inter-comparisons on the measurements of optogeometrical parameters of coated surfaces between various laboratories around the globe is provided1,2,3. For example, in 2004 MP frame, fourteen institutions were asked to investigate optogeometrical parameters of a low-index single-layer HfO2 coated fused-silica substrates at quasi-0° angle of incidence. According to the inter-comparison report1, a good agreement between the participants’ results were found for T spectra, refractive index and geometric thickness, while large deviations occurred for R spectra and extinction coefficient. In 2007, the MP task was extended and the participants were invited to measure T and R in the spectral range from 300 to 1000 nm at a 45° angle

* [email protected]; phone +9 0262 6481905; +9 0262 6481947;

Laser Sources and Applications, edited by Thomas Graf, Jacob I. Mackenzie, Helena Jelínková, John Powell, Proc. of SPIE Vol. 8433, 843313 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.921651

Proc. of SPIE Vol. 8433 843313-1

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of incidence for both states of polarization (s and p)2. The sample under investigation was a high-index single-layer Ta2O5 dielectric coated mirror. Analysis of the results from fourteen participants showed that, the deviations for the R were higher than for the T. Moreover, evaluation of the participants’ results revealed larger deviations in all 45° spectra compared with zero incidence problems in 2004. Finally, in 2010 MP two samples of high reflective dielectric laser mirrors (designed for 1064 nm, with angle of incidence near 0°) were the subjects of the investigations. The mirrors were 25 mm diameter fused silica substrates coated with Ta2O5 /SiO2. The reflectance of these low-loss mirrors was measured at 1064 nm at 0° angle of incidence by means of prescreening the cavity ring-down method, and it was found to be of 99.996% and 99.878%. An evaluation of the sixteen participants’ results revealed large deviations in R for both sample types, although they were most pronounced for 99.996% mirror. The critical impact of surface contamination on the reflectance level emphasized the importance of sophisticated sample handling3.

Nowadays most widely used methods for measurements of the base optical properties of mirrors are the spectrophotometric method 4,5, the integrating sphere method6,7, the laser ratio-metric method8 , the optical cavity ring-down technique9 (OCRD) and the modified versions of OCRD, the phase-shift OCRD10 , and a recently introduced method called the optical feed-back OCRD11. In spectrophotometric methods the measurements are performed in a wide interval of spectrum. Generally, due to the instability of light sources (commonly lamps) the accuracy of spectrophotometric results is lower then that in laser-based measurement methods. Literature depicts the latest ultra-high precision reflectivity measurement results in11, where using a cw Fabry-Perot diode laser the reflectivity of ultra-high reflective cavity mirrors were measured to be 99.99606 with a reproducibility of 0.00003% at 1063.1 nm wavelength.

High reflective mirrors are widely used in manufacturing of high intensity and/or high frequency stabilized lasers and laser applications. Generally, in these applications the scattering and absorption of mirrors are usually ignored12. However in some applications, such as gravitational-wave detection interferometry, cavity ring-down spectroscopy, ultra-short pulse length coatings, and in the areas of nanostructures and micro-structured coating materials, the measurement of losses due to the scattering and absorption are becoming an important issue.

In this work we describe a system for measuring the main optical characteristics of optical interference coated surfaces at 632.8 nm, by utilizing both laser ratio-metric and integrating hemisphere methods. We have made efforts to construct a very precise, but at the same time flexible and easy to use instrument for measurements of R, T and total integrating scattering (TIS) for the in situ thin film characterization diagnostics. Moreover, the system design allows performing RTS measurements at 45°, quasi-0°, and -90° angles of incidence.

2. INSTRUMENT DESIGN AND CHARACTERISTICS Figure 1 illustrates the schematic representations of the system designed for the 45° RTS measurements. The system is remarkable for using the same detector for measurements of an incident laser beam power, R and T. This allows us to minimize a total measurement uncertainty by avoiding uncertainty components originated from an absolute calibration of two detectors. The optical table (OT) (Figure 1.a) is put on a marble plate to reduce possible mechanical vibrations, and placed in a blackened inside ligneous enclosure to eliminate stray light. To achieve better temperature stabilization, the laser tube (L), its power supply and all other electronics circuits were separated from the optical measurement facility.

(a) (b) (c)

Figure 1. Schematic representations of the system.



The laser beam, after passing the shutter (St) (Figure 1.b), is divided into two beams by means of a cubic beam splitter (BS). The first beam – the reference beam, is used to monitor the laser power during the measurements. The second beam – the measurement beam, is sent to a sample (S) surface for subsequent measurement purposes. The intensity of the reference beam is recorded by the reference detector RD, and of the measurement beam - by the measurement detector MD. The signal of the RD is to correct the MD signal for possible laser output fluctuations.

The MD is fixed on a computer controlled motorized stage. The stage is able to move the MD on the quarter-round stage around an integrating hemisphere (IHS) with highly precise steps. It allows performing simultaneously T and incident laser beam intensity measurements at the MD’s top position, and R – at the MD’s bottom position.

A series of iris (I) diaphragms are used on the beam way to reduce stray light and back reflections.

The IHS (40 cm diameter) and a photomultiplier (PMT) (mounted to the wall of the IHS) are used in TIS measurements. The inner walls of the hemisphere are coated with diffuse reflective barium sulfate paint. Scattered from the sample light, after multiple diffuse scatterings inside the IHS, is recorded with the PMT.

For further data analysis, dc signals from all detectors are simultaneously measured by means of three 6½ digit digital multimeters (DMMs) by a homemade software. Dark signals of all detectors are measured before and after each RTS measurement. For this purpose, a software controlled mechanism is used to close and open the shutter automatically.

The measurement procedure consists of three steps:

1) Each RTS measurement begins with the empty sample holder. At the closed position of the shutter the dark signals of all detectors are recorded by the software. Then the MD is moved to the top position, and the incident beam intensity at the initial time is recorded. After collecting a sufficient number of data points, and performing the averaging of the detecots’ dark signal and incident beam intensity value, the software allows performing of the next step.

2) At the second step, a top door of the enclosure is opened and a sample (mirror) is placed to the sample holder SH. After closing the door a few seconds of delay is needed for temperature and vibration stabilizations. Then the MD measures the transmittance of the mirror. The beam reflected from the mirror passes through the hemisphere’s bottom exit port and falls into the light trap (LT). While an amount of the back-reflected light form the LT is negligibly smaller than that from the MD, the TIS measurements are performed simultaneously with the transmittance measurements. After collecting a sufficient number of data, the MD is moved to the bottom position to measure the reflectance R of the sample.

3) At the third step, the operator removes the mirror from the holder, and again the MD is sent to the top position for the recording of the incident laser beam intensity (for a detection of possible drifts during the measurements). Then the system closes the shutter and measures the dark signals from all detectors. Finally, the software calculates all parameters and performs the validity of the measurements. A number of experiments are shown that about 3 minutes is enough to perform one measurement.

A color firewire CCD camera (1360x1024 pixels) is used to determine the relative position of the laser beam and the mirror surface center. For this purpose, using any diffuse reflective surface (for example, a piece of white cardboard paper) the weighted center coordinates of the laser beam inside the SH is determined (Figure 2. a)). This position is fixed on the image box of the software and is used in positioning the mirror onto the sample holder (SH).

00 (a) (b) (c)

Figure2. Alignment of the laser beam and the center of the surface under the test.



2.1 Laser stability

The performance of a laser ratio-metric system, when used for assessments of the optical properties is directly governed by the intensity stability of the used laser system. In our system we employed a continuous wave HeNe laser (Research Electro-Optics, Inc.) as a light source. Usually, HeNe lasers have excellent beam characteristics (TEM00 mode) and a long lifetime. The laser has an output power of about 8 mW at 633 nm wavelength. The laser operates in two modes: the frequency stabilized mode and the intensity stabilized mode. In the intensity stabilized mode, according to the manufacturer’s specification, it has an output intensity stability of about 0.01%. Figure 3. a) depicts an example of the output power variation of the laser within the 60 seconds. Generally, variations at the output power of HeNe lasers consist of: a) periodic component (which is mainly originated from the ripple of the discharge current caused by an insufficiently filtered power supply), b) random fluctuations components (mainly due to spikes of the power supply, instabilities of the gas discharges, and vibrations of the resonator) and c) long-term drifts (caused by slow temperature or pressure changes in the gas discharge, or by detuning of the resonator)13. Further stabilization of the laser output is a difficult task: passive methods, such as creating and controlling highly stable environmental conditions to eliminate the cavity expansion’s contraction and vibration and using power supplies with low-ripple levels and spikes is a very complicated issue. Active methods, such as using fast response electro-optic active crystals (Pockels cell or Faraday rotator) are very reliable to compensate for the intensity fluctuations at the cost of the loss of output intensity. Another way to stabilize a HeNe laser output intensity is to use a digital feedback method to regulate the discharge current14. In 14, by employing this method an intensity stabilization of 0.014% over 12 h was reached.

The other most practical and widely used method is to split the laser beam into two beams by means of a beam splitter, and use one of them as a reference beam, to control the stability of the laser, and the other – the working beam – in optical measurements (Figure 1. c)). In Figure 3. a) an example of the simultaneously measured intensity signals of the reference beam (pink open symbol curve, right Y axis, RD) and the working beam (blue solid symbol curve, left Y axis, MD) is shown. For clarity, (to show the synchronization between detectors) only a short time (60 s) stability data are shown. Figure 3. b) illustrates the normalized ratio of these data (the standard deviation is about of 9.2x10-6). Finally, Figure 3. c) depicts the long term stability (during the 5000 sec) of the system, where the standard deviation of the normalized ratio of the working and the reference signals is about 2.04 x10-5.

(a) (b) (c)

Figure 3. (a) Laser intensity stability, where pink open symbol curve denotes the reference beam stability (right Y axis) –and blue solid symbol curve, left Y axis - the measurement beam; (b) short term stability of the normalized ratio of RD

and MD signals; (c) Long term stability of the system.

2.2 Detectors

The MD and RD detectors are four-element transmittance-type trap detectors15,16,17 comprising of four Hamamatsu S1337-BQ series silicon photodiodes. The diodes have an active area of 10x10 mm2 and a responsivity of about 0.4 A/W at 628.3 nm wavelength. According to the manufacturer’s specification all eight diodes were taken from the same batch.

Both traps were used with an applied bias voltage of 16 V using a precise and low temperature coefficient (0.5 ppm) resistor. As it was shown in18 , in the biased regime a linearity of S1337 series silicon detectors is increased. To supply the bias voltage a rechargeable battery was used. The use of batteries allows one to elude the grounding problems and to decrease the noise. When the measurements start the system automatically disconnects the batteries from a charging drive. If the charge of the battery is less then the predefined limit the system stops measurements until the battery



charges up to the required value. During the measuring process the discharging of the battery i.e. drop of the bias voltage has a negligibly small influence on the measurement results.

The scattering measurements are carried out using a Hamamtsu R5900U-20A photomultiplier. It is driven by a homemade electronic driver. The PMT is mounted to the exit port of the integrating hemisphere in such a way that it registers only scattered light. While after multiple diffuse scattering inside the IHS the light becomes completely depolarized, the PMT output signal doesn’t depend on the polarization of the incident beam. However, a polarization dependence of the trap detectors was examined19,20. The HeNe laser used as light source is vertically linearly polarized and its output intensity was controlled by means of a reference detector. The traps were mounted on a precision rotation table. Great care was taken to ensure all internal angles of incidence were kept as the same as in the case of the RTS measurements (in both positions). A pinhole aperture was placed in front of the entrance pupil of the trap to ensure that the beam always hit the same area on the surface of the first diode in the trap. The measurement results of both traps showed no significant systematic dependence on the polarization direction. The MD detector has a smaller relative standard deviation of the measured signal (less then 2x10-5 in absolute units) than that of the RD detector.

To increase the dynamic range (more then 6 decades) of RTS measurements, special attention was paid to the linearity measurements21,22 of the trap detectors and the PMT. The non-linearities of all three detectors were studied very carefully. The non-linearity of the trap detectors were measured by the flux addition method, enhanced by means of a set of neutral density filters23. The nonlinearity of both traps was measured at a laser beam diameter of about 1.2 mm. The linearity measurements were repeated at least five times for each traps detectors, and the detector with a good linearity was used in reflectance and transmittance measurements. The nonlinearity of the PMT was assessed using a set of neutral filters with various optical densities in the range of 1 to 7, and then was corrected by the nonlinearity function of the MD. Although we have performed surface scanning to determine the non-uniformity of spatial responsivity of both trap detectors, in all calibrations and measurement procedures special attention was paid to keep the beam to hit the same point on the surface of the first diode.

3. MEASUREMENTS 3.1 Signal recording and analyzing

Each detector was connected to a dedicated DMM (Keithley 2000). The DMMs communicate with the PC through the GPIB interface. The homemade software triggers all DMMs for the simultaneous measurement of the output voltage from the detectors. At the starting of the measurements, the software sets the DMMs to the same internal measurement configuration (such as, an averaging filter, nplc, a triggering mode, etc.), except that which is connected to the MD. During the incident beam intensity and the dark signal measurements, the measurement range of the DMM corresponding to the MD is set to the “10 V range” and the “100 mV range”, respectively, while during the reflectance R and transmittance T measurements, depending on the value of the signal it is adjusted to the corresponding measurement range. The homemade software was developed to order the apparatus. In addition to driving the beam shutter, motorized stage for positioning of the MD, controlling the battery charge process, measuring the temperature inside the facility and communicating with the DMMs, the software performs data collection, storage and calculations.

3.2 Reflectance R and transmittance T measurements

In measurements, transmittance T and reflectance R were determined from the ratio of the corrected and normalized signals of incident, transmitted and reflected beam intensities, as it is shown in the following formulas:

( ) , ,, ,

S T S S SMD dark IBI MD darkTransmittanceS S S SRD RD dark RD RD dark

⎛ ⎞ ⎛ ⎞−< > −< >= < > < >⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟−< > −< >⎝ ⎠ ⎝ ⎠

(1)

( ) , ,, ,

S R S S SMD dark IBI MD darkReflectanceS S S SRD RD dark RD RD dark

⎛ ⎞ ⎛ ⎞−< > −< >= < > < >⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟−< > −< >⎝ ⎠ ⎝ ⎠

(2)



where, S(T) and S(R) are the transmittance and reflectance signals of a sample, measured by the MD at the top and at the bottom positions, respectively; IBIS is the incident beam intensity signal, RDS is the signal of reference beam,

,MD darkS and ,RD darkS are the dark signals of the MD and the RD detectors, respectively; < , > angle brackets denote an average value.

3.3 Scattering measurements

Generally, the scattering in optical thin film coated mirrors comprises two components: the surface scattering and the volume scattering. The surface scattering contains components originated from the roughness of the first surface, from the interface of each layer or from the roughness of the substrate. The volume scattering originates from the microstructure of optical coating associated with the columnar growth or defect in the thin film12. In this work the measured scattering is a total integrated scattering, which is registered by the PMT, whose output signal S(PMT) is proportional to the amount of total integrated scattered light. While the sphere-wall coating is never a perfect Lambertian diffuser, baffles and ports perturb the light distribution within the sphere, all detectors exhibit some angular dependence, etc., we performed calibration of the PMT signal versus the incident beam intensity. For this purpose, several high diffuse scattering metallic (with a high surface roughness) and diffuse paint coated surfaces were used to scatter completely the incident beam inside the IHS. Then, by varying the incident beam intensity (by means of a set of neutral density filters) and measuring its intensity by the MD, a calibration curve of the PMT signal versus the incident beam intensity was plotted (Figure 4. (a)). Only the linear part (from 10-4 to 10-1) of this curve was used in a linear fit for determination of the TIS:

73.1745996 S 6.9925 10sigScattering −= + , where

( ) , ,

, ,

S PMT S S SPMT dark IBI MD darkSsig S S S SRD RD dark RD RD dark

⎛ ⎞ ⎛ ⎞−< > −< >= < > < >⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟−< > −< >⎝ ⎠ ⎝ ⎠

(3)

Here S(PMT) is a PMT signal, and ,SPMT dark is the dark signal of the PMT.

(a) (b)

Figure 4. (a) PMT signal versus MD signal (red line depicts the linear fit to the experimental data, corresponding to linear part of the dependence, i.e. in the MD signal’s range from 10-4 to 10-1 V). (b) Specular reflectance of five

measurements of the same mirror.



3.4 Practical measurements

A number of optical components, including windows, filters, mirrors (dielectric and metallic coated) have been characterized by the system. Antireflection and high-reflection coatings on various substrates were accurately characterized and measured.

A number of commercially available high reflectivity, low-loss laser-mirrors were used to confirm the accuracy of the system. The standard uncertainty associated with the measurement repeatability was calculated using the results of several measurements of the same mirror. For example, Figure 4. (b) shows five measurements of the same dielectric mirror during a day. An average value of reflectivity is about 99.9654 with an associated standard deviation of ±0.0021. The reproducibility of the experimental setup was calculated using the results of the various sets of measurements with high reflective mirrors (R>99.9%), performed by two operators independently during three weeks. From these results, the average reproducibility of <0.005% for the reflectance R, a reproducibility <0.005% for transmittance T, and < 5 ppm – for the total integrated scattering of about 10 ppm (at k=2) were obtained.

For low loss mirrors with the reflectivity of more then 99.99% the measurement sensitivity should be in the 1x10-5 level or better. At the same time, the reflectivity measurements should always be performed under vacuum conditions, where the scattering and absorption losses are minimized11. We had only one dielectric coated high reflective mirror with the reflectivity more then 99.99%. At initial measurements the values of reflectance and transmittance of the mirror were in a good agreement with the values that had with the mirror; however the measured scattering value was found higher (about 40 ppm) than that in the specification. After cleaning the surface under microscope, and repeating the measurements the scattering value was found to be about 25 ppm, which is in a good consistency for this type of mirrors.

4. SUMMARY A system, comprising a HeNe laser, two trap detectors, a PMT and an integrated hemisphere were built to characterize optical coated surfaces at 632.8 nm wavelength. In the facility, part of the laser beam is directed to the reference trap detector by a beam splitter. To reduce the effects of laser output variations, the signals of reference and measurement trap detectors are measured simultaneously, and then correlated. By this way, during the overall time of one measurement cycle (less then 3 minutes) a stability better then 1x10-5 was maintained. To increase the overall measurement accuracy, the same measurement detector, fixed on a motorized stage, is used to perform transmittance and specular reflectance measurements, while a total integrated scattering is measured by means of the PMT. We have reached measurement reproducibility of <0.005% for transmittance and reflectance measurements of high reflectivity mirrors (R>99.9%). A number of measurements with an ultra-high reflective mirror (reflectivity more then 99.99%) shows that the sensitivity of the system in total integrated scattering measurements is about 10 ppm.

REFERENCES

[1] Duparré, A., and Ristau, D., “2004 Topical Meeting on Optical Interference Coatings: Measurement Problem,” in Optical Interference Coatings, OSA Technical Digest Series (Optical Society of America, 2004), paper WD1.

[2] Duparré, A. and Ristau, D., “Optical interference coatings 2007 measurement problem”, Appl. Opt., 47, C179-C184 (2008).

[3] Duparré, A. and Ristau, D., “Optical interference coatings 2010 measurement problem”, Appl. Opt., 50, C172-C177 (2011).

[4] Voarino, P., Piombini, H., Sabary, F., Marteau, D., Dubard, J., Hameury, J. and Filtz, J.R., “High-accuracy measurements of the normal specular reflectance,” Appl. Opt., 47, C303-C309 (2008).

[5] Piombini, H., and Voarino, P., “Apparatus designed for very accurate measurement of the optical reflection,” Appl. Opt., 46 (36), 8609-8618 (2007).

[6] Bastin, J.A., Mitchell, E.W.J., and Whitehouse, J., “Use of an integrating sphere to distinguish between absorption and scattering in solids,” British Journal of Applied Physics, 10, 308, 412-416 (1959).

[7] Hanssen, L., “Integrating-sphere system and method for absolute measurement of transmittance, reflectance, and absorptance of specular samples,” Appl. Opt., 40 (19), 3196-3204 (2001).

[8] Zhang, Z.M., Gentile, T.R., Migdall, A.L., and Datla, R.U., “Transmittance measurements for filters of optical density between one and ten,” Appl. Opt., 36 (34), 8889-8895 (1997).



[9] Herbelin, J.M., and McKay, J.A., “Development of laser mirrors of very high reflectivity using the cavity-attenuated phase-shift method,” Appl. Opt., 20 (19), 3341-3344 (1981).

[10] Gong, Y., and Li, B., “High reflectivity measurement with a broadband diode laser based cavity ring-down technique,” Appl. Phys. B, 88(3), 477-482 (2007).

[11] Gong, Y., Li, B., and Han, Y., “Optical feedback cavity ring-down technique for accurate measurement of ultra-high reflectivity,” Appl. Phys. B-Lasers and Optics, 93, 355-360 (2008).

[12] Cho, H.-J., Shin M.-J., and Lee, J.-C., “Effects of substrate and deposition method onto the mirror scattering,” Appl. Opt., 45, 1440-1446 (2006).

[13] Anishenko, M.L., Ermachenko, V.M., Petrovskiy, V.N., and Protsenko, E.D., “Natural intensity fluctuations in two-mode HeNe and HeNe/Ch lasers,” Opt. Commun., 71, 359–362 (1989).

[14] Wei, Z., Lu, G., Yang, K., Long, X., and Huang, Y., “A digital intensity stabilization system for HeNe laser,” Opt. Laser. Technology, 44, 63-66 (2012).

[15] Fox, N.P., “Trap Detectors and their Properties,” Metrologia, 28, 197-202 (1991). [16] Gardner, J.L., “Transmission trap detectors,” Appl. Opt., 33 (25), 5914-5918 (1994). [17] Balling, P., and Kren, P., “Linearity limits of biased S1337 trap detectors,” Metrologia, 38, 249-251 (2001). [18] Toivanen, P., Manoochehri, F., Kärhä, P., Ikonen, E., and Lassila, A., “Method for characterization of filter

radiometers,” Appl. Opt., 38, 1709-1713 (1999). [19] Goebel, R., Yilmaz, S., and Pello, R., “Polarization dependence of trap detectors,” Metrologia, 33, 207-213 (1996). [20] Kübarsepp, T., Kärhä, P., and Ikonen, E., “Characterization of a polarization-independent transmission trap

detector,” Appl. Opt., 36 (13), 2807-2812 (1997). [21] Fischer, J., and Fu, L., “Photodiode nonlinearity measurement with an intensity stabilized laser as a radiation

source,” Appl. Opt., 32(22), 4187-4190 (1993). [22] Korde, R., Prince, C., Cunningham, D., Vest, R. E., and Gullikson, E., “Present status of radiometric quality silicon

photodiodes,” Metrologia, 40, S145-S150 (2003). [23] Corredera, P., Hernanz, M.L., Gonzalez-Herraez, M., and Campos, J., “Anomalous non-linear behaviour of InGaAs

phodiodes with overfilled illumination,” Metrologia, 40, S150-S153 (2003).



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