Meelis Sildoja

Predictable quantum efficient detector: II. Characterization and confirmed responsivity

The predictable quantum efficient detector (PQED) is intended to become a new primary standard for radiant power measurements in the wavelength range from 400 nm to 800 nm. Characterization results of custom-made single induced junction photodiodes as they are used in the PQED and of assembled PQEDs are presented. The single photodiodes were tested in terms of linearity and spatial uniformity of the spectral responsivity. The highly uniform photodiodes were proved to be linear over seven orders of magnitude, i.e. in the radiant power range from 100 pW to 400 µW. The assembled PQED has been compared with a cryogenic electrical substitution radiometer with a very low uncertainty of the order of 30 ppm. Experimental results show good agreement with the modelled response of the PQED to optical radiation and prove a near unity external quantum efficiency.

Simulations of a predictable quantum efficient detector with PC1D

The spectral responsivity of a predictable quantum efficient detector (PQED) is calculated based on the responsivity of an ideal quantum detector and taking into account reflection losses from the surface of the photodiode and internal charge-carrier gains/losses inside the diode. The internal quantum deficiency (IQD) is obtained from simulations with the PC1D software using the material data of the produced PQED photodiodes. The results indicate that at room temperature the predicted IQD of the PQED is close to zero with an uncertainty of about 100 ppm over the visible range. It is further concluded that a primary standard of visible optical power with an uncertainty of approximately 1 ppm is achievable using the PQED at low temperatures.

Reflectance calculations for a predictable quantum efficient detector

Calculations of reflectance required for designing a predictable quantum efficient detector (PQED) are presented. The PQED will be based on natural inversion layer photodiodes and it is expected to measure optical power with an uncertainty of 1?ppm. To achieve such a low level of uncertainty, the associated reflectance of the PQED must be of the same order of magnitude. The wavelength range from 450?nm to 900?nm and the oxide layer thickness from 10?nm to 500?nm were used in the calculations of specular reflectance. The results show that a simple two-photodiode design is suitable for the PQED if scattering from the surface of the photodiode is less than 1?ppm. For higher scattering up to 100?ppm, a more advanced detector structure may allow control of this uncertainty component at the level of 1?ppm.

Reducing photodiode reflectance by Brewster-angle operation

Methods for reducing reflectance losses at multiple wavelengths with a silicon photodiode detector are described. Using thick oxide layer and Brewster-angle operation it is shown that specular reflectance losses can be theoretically decreased below 1 ppm (part per million) in a simple measurement arrangement. Additionally, a detector structure is presented which can be used to reduce the measurement uncertainties due to diffuse reflectance below 1 ppm, even if the total diffuse reflectance losses from a plain photodiode would be two orders of magnitude larger.

A primary standard of optical power based on induced-junction silicon photodiodes operated at room temperature

We present the design and construction of a new compact room temperature predictable quantum efficient detector (PQED). It consists of two custom-made induced-junction photodiodes mounted in a wedge trap configuration and a window aligned at Brewsters angle for high transmission of p polarized light. The window can also be removed, in which case a dry nitrogen flow system is utilized to prevent dust contamination of the photodiodes. Measurements of individual detectors at the wavelength of 488 nm indicate that reflectance and spectral responsivity are consistent within 4 ppm and 13 ppm peak-to-peak variation, respectively, and agree with the predicted values. The spatial non-uniformity of the responsivity of the PQED is an order of magnitude lower than that of single photodiodes. The internal quantum efficiency of the photodiodes is concluded to be spatially uniform within 50 ppm. These measurement results—together with the responsivity predictable by fundamental laws of physics—provide evidence that the room temperature PQED may replace the cryogenic radiometer as a primary standard of optical power in the visible wavelength range of 380 nm to 780 nm.

Liquid nitrogen cryostat for predictable quantum efficient detectors

We present the design and testing of a cryostat to be used with induced junction photodiodes of the Predictable Quantum Efficient Detector (PQED). Long-term reflectance measurements indicate that possible ice growth on the photodiodes at the temperature of liquid nitrogen (LN) is significantly reduced from earlier PQED cryostat designs.