Response Uniformity Measurements

The response uniformity has been measured using a recently available facility. This facility consists of a three axis translator mounted on a reflective objective and housed in a light tight box. Figure 5 shows the opened black box on the left and a detail of the reflective objective on the right.

Figure 5. (left) Black box containing the reflective objective and (right) a detail of it.

The motorized translators are able to move the reflective objective in the X (150 mm) and Y (30 mm) directions with resolution and repeatability of 1 ^m and in the Z (12 mm) direction with a better resolution to allow the beam focusing. The reflective objective is illuminated through a 10 ^m pinhole by a fiber optic fed by monochromatic radiation produced by the characterization system. In this way we can obtain a monochromatic spot with a size smaller than 10 ^m FWHM at the focal plane of the reflective objective to evaluate the pixel-to-pixel non-uniformity as a function of wavelength. Figure 6 shows a sketch of the system.

Figure 6. Schematic sketch of the motorized reflective objective.

To find the best condition in terms of focus and position we developed an automated procedure that operates a scanning procedure. The procedure, shown in Fig. 7, begins with the objective not illuminating the SPAD (a), and moves along the X direction of a given AX (b). Having reached the end X position, the procedure moves the objective at the initial X position and moves along the Y direction of a given AY (c) starting a new X direction scan.

SPAD.

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1 " step: X movement b

2" step: Y movement c

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Figure 7. Schematic sketch of the scanning procedure.

Figure 7. Schematic sketch of the scanning procedure.

With each movement the corresponding count rate is acquired and stored in a file. Simultaneously, an IDL procedure reads the file and produces a 3D plot of the currently acquired data. Examples of 3D plots are showed in Figure 8. The left plot was obtained by setting AX and AY to 150 ^m and the right plot by setting AX and AY to 20 ^m.

Figure 9 shows a 3D plot of 50^50 ^m scan with 5 ^m resolution. It is evident that the 5x5 ^m central area presents a flat response corresponding to ~5000 counts/s. Thus we can confirm SPAD uniformity in that area. The shape of the 3D plot can be explained by considering the convolution of the SPAD sensitive area (20 ^m) and the luminous spot (FWHM 10 ^m). To better study the SPAD uniformity we will use a smaller spot and a scan resolution of 2 ^m.

Figure 8. 3D plots obtained by scanning the SPAD at different resolutions: (left) 150 ^m and (right) 20 ^m. It is evident in the left plot that the beam is unfocused while in the right the beam is near the focus position.

Figure 8. 3D plots obtained by scanning the SPAD at different resolutions: (left) 150 ^m and (right) 20 ^m. It is evident in the left plot that the beam is unfocused while in the right the beam is near the focus position.

Figure 9. 3D plot of 50x50 ^m scan with 5 ^m resolution.

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