J. Basic. Appl. Sci. Res., 1(11)2324-2328, 2011

© 2011, TextRoad Publication

ISSN 2090-4304 Journal of Basic and Applied

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*Corresponding Author: Sameh M. Reda, Photometry and Radiometry Division, National Institute for Standards (NIS), EGYPT. E-mail: egyreda@hotmail.com, sameh.reda@nis.sci.eg.

Improving Uncertainty of Determining the Uniformity of Optical Detectors

Sameh M. Reda

Photometry and Radiometry Division, National Institute for Standards (NIS), EGYPT.

ABSTRACT

Using optical detectors in measuring and monitoring the optical power in daily applications in our life requires the complete knowledge and accurate determination of its optical characteristics. In this work we use the most conmen optical detector made of silicon, which have wide responsivity region covering all the visible and extending to the near infrared. One of the most important characters is the spatial uniformity of a detector. The present work discusses by simulation, the parameters affecting the determination of the spatial uniformity. The scanning simulations depend on applying different parameters, which ultimately lead to improving the ways for precise spatial uniformity determination. Also this work shows that, the parameters such as: small cross section of a light source, narrow scans steps, and using stabilized-power light sources not only improve the determination of the spatial uniformity but also lead directly to a plausible reduction in uncertainty. The best combination of applicable and reasonable parameters has been chosen carefully for a better determination of the spatial uniformity of an optical detector. KEYWORDS: Radiometry, Optical detector, characterization of optical detector,

uniformity, uncertainty.

1. INTRODUCTION

The applications of using light power with its wide range from ultraviolet to infrared are increasing rapidly. In parallel, the need for measuring this power using different types of optical detectors is increasing. Silicon photo diodes are becoming the preferred kind of detectors to perform this task, and in some cases, they present the transfer standard for light measurements. These detectors are produced by different methods and have different internal structures [1]. Accurate determination of the responsivity of silicon photodiodes is highly desired in photometry and radiometry. The change of responsivity over the surface, the so-called spatial non-uniformity, affects power measurements especially in photodiodes with large active areas. The determination of the spectral responsivity characteristics of the silicon photodiode is the most important factor for photodetector. In a standard spectral responsivity measurement, the detector is under-filled, i.e. the incoming light is adjusted so that all the light falls inside the active area of the photodiode. An ideal photodiode should have a spatially-uniform responsivity which has the same value over the active area regardless of the position of the incoming beam. In the case of uniform responsivity, for the radiation sensed by a photodiode at different locations of its sensitive surface, the same electrical output signal is generated. But due to various effects, such as crystal structure of silicon, fabrication quality, radiation, heat conduction and convection losses, the detector suffers from spatial non-uniformities. Most inhomogeneities of the photodiode material itself cause non-uniformity. Therefore, for the same optical beam with the same radiant power coming to different parts of a photodetector, generally, different output signals are generated. This effect causes the need to have different signal readings and extra uncertainties that must be evaluated for a successful calibration of these devices [2]. The establishment of the absolute spectral responsivity scale traceable to the cryogenic radiometer depends on well-characterized optical detectors as a radiometric scale transfer standards. The photodiodes used for the construction of such transfer standards need to be characterized in terms of spatial uniformity, spectral responsivity, shunt resistance and linearity [3] .There are many ways to determine the spatial uniformity of a photodetector. Antti Lamminpää [4], measured the spatial uniformity of single photodiodes. His measurements were carried out at two helium–cadmium laser wavelengths, 325 nm and 442 nm. The 1/e2 diameter of the laser beam was approximately 2 mm. Also, Mart Noorma [5], measured the spatial uniformity of the responsivity of the trap detector, and the

corresponding uncertainty component at 325 nm was calculated to be 4×10-4

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2. Measurement The uniformity of responses across the sensitive area of a detector can be determined using the set-up in Figure (1). It scans a small circular patch of radiation of diameter in the range 25 μm to 2 mm across the surface of the detector and compares the response at selected positions with the maximum response value. A standard measurement provides an equally spaced eleven by eleven matrix of response variations across the surface Figure (2) [6 ].

Figure 1: Setup for uniformity determination [6].

Figure 2: 3D plot of the output scan of a detector [6].

In previous work a simple optical setup was assembled to quantify the spatial power response of the optical detector as depicted in Figure (3). The lasers were aligned perpendicular to the plane of entrance of the detector aperture. Via an X-Y micrometer translational stage, a scanning procedure in equal steps for both directions was used to determine the spatial power response of the detector. It is found that determining the spatial uniformity of an optical detector depending on different parameters

1- Stability of the source 2- The cross section of the used source. 3- The area of the detector. 4- Number of scanned points.[7]

In this work a theoretical simulation based on these parameters was carried out to determine their effect on the spatial uniformity of optical detectors.

3. SIMULATION AND DISCUSSION

3.1 The first simulation studies the effect of power stability of the input light source. Assuming a stable source emits 100 units varying with ±1% of its nominal value and another source emits 100 units with a variation of ±10 around its nominal value.

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Figure 3: A schematic diagram of the experimental set up [7].

Figures (4A and B) clarify the difference between the two different stability of the input light sources. This maybe used in determining the spatial uniformity of an optical detector. The two plots show that with stable ±1% light source, we get high uniformity i.e. small differences on the scanned areas outputs. The obtained values of uniformity uncertainty are better by the stabilized source.

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Figure 4: "A" 3D Plot of the output of a scanned detector with a variation of ±1% in the input light power. "B" 3D Plot

of the output of a scanned detector with a variation of ±10% in the input light power.

3.2 The second simulation examines to what extent the spatial uniformity of optical detectors is affected by the change in the cross sectional area of the input light source. Some times laser sources are used, and alternatively, spot or collimated monochromatic sources obtained through bandpass filters or a monochromator can also be used. For this simulation, assume to have the same light source with different output aperture areas or cross sections.

The small cross section lays on separate areas of the detector as shown in Figure (5D), while in Figure (5C), the wide cross section produces overlapped exposed areas on the detector when we use the same scan step as that used in scanning by the small cross section aperture.

Figures (5 A and B) show the comparison of the two cross section areas. The obtained spatial uniformity using the small cross section is better than that obtained using wide cross section. The simulation runs assuming one light source with two different cross sections. Then the detector surface is scanned twice using both cross sections. We obtain a large variation in the quantitative value for the spatial uniformity (standard deviation) of outputs of the detector. 3.3 The third simulation studies the affect due to the detector area.

The result of this simulation can be noticed by looking at Figure (6) and Figure (4A) which show the spatial uniformity of two scans for the two similar detectors but differ only in their areas. There is no clear difference in the shape even by quantitative calculation of the spatial uniformity for both detectors, however, the difference is too small and not that much significant.

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Figure 5: Comparison between two cross sections of light sources, "A" and "C" are the spatial uniformity and the sketch diagram of a light source with a wide cross section. "B" and "D" are the spatial uniformity and the sketch

diagram of a light source with a small cross section input.

The smaller detector area coerce us to use small cross section for the incident light, otherwise the scan of the detector will not be accurate.

3.4 The fourth simulation studies the effect of the number of scanned points when obtaining the spatial uniformity. For this simulation, we assume the same detector is exposed to the high stabilized source of light with a fixed cross section. Which scanned twice one step scan and double steps scan.

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Figure6: Spatial uniformity of a detector with small area scanned using small cross section light source.

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Figure (7) compares between the two scans, clarifying that double steps scan means missing data from the detector surface. Also the obtained spatial uniformity is not reflecting the characteristics of the whole detector and resulting in a high source of uncertainty.

4. Conclusion The simulations of a scanned detector with different source stability, source cross section, detector sensitive area, and scanning step width, lead to the improvement the ways determining one of the most important characteristics of silicon optical detectors. The parameters affecting the spatial uniformity of an optical detector must be taken in consideration when evaluating its spatial uniformity. A small cross-sectional light source, narrow scan steps, and using stabilized-power light sources improve the determination of the spatial uniformity and also lead directly to better uncertainty. This work recommends to fixing in choosing the best combination of applicable and reasonable parameters to determine the spatial uniformity of a silicon optical detector.

REFERENCES

1- White M.G. and Bittr A, 1993. Uniformity of Quantum Efficiency of Single and Trap-configured Silicon Photodiodes, Metrologia, 30,361-364.

2- Murat DURAK, Farhad SAMADOV, A. Kamuran TÜRKOĞLU, 2002. Spatial Non-uniformity Measurements of Large Area Silicon Photodiodes, Turk J Phys 26, 375 – 379.

3- Alves L. C., Reis F., Torres M. C., Almeida G. B. and Couceiro I. B., 2009. Spatial uniformity of the Silicon photodiodes for establishment of spectral responsivity scale, XIX IMEKO World Congress, Fundamental and Applied Metrology.

4- Antti Lamminpää, 2006.High-accuracy characterization of optical components: detectors, coatings and fibers, Doctoral dissertation, Helsinki University of Technology.

5- Mart Noorma , 2005. Development of Detectors and Calibration Methods for Spectral Irradiance and Radiometric Temperature Measurements , Helsinki University of Technology ,Department of Electrical and Communications Engineering, Metrology Research Institute Report 25.

6- Optical Radiation Measurement Group, NPL, 2003. Spectral Characterization of Detectors and Meters, Measurements and Calibration service.

7- Sameh M. Reda and Hatem Hussein, 2010, Spatial Power Response for Optical Detectors using Multiple Laser Wavelengths, International Metrology conference,CAFMET, April (2010).

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Figure 7: A and B Spatial uniformity of the same detector using the same light sources with one and two steps scans.

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