The initial stage of the optical alignment procedure was performed using multiple pinholes and a He-Ne laser. Phase five of the alignment (see section 3.13.2) required the receiver to be bore-sighted with the transmitter over the system’s range of operation. The purpose of the bore-sight is to ensure that the FoV of the detector is located approximately at the centre of the transmitted laser beam (with a tolerance of few detector pixels projected at the FoV) over a maximum range of 10 km.
148 Figure 4.17 shows an optical setup used for bore-sighting of the transmitter and the receiver. The parabolic mirror was positioned such that a collimated and expanded He- Ne laser beam travelling from the back of the optical system through the centre the pinholes placed in the system during the initial alignment phase was pointing at the centre of the mirror and was located approximately 5 m away from the aperture of the telescope. The scanning mirrors were powered up during the alignment and were kept at a constant voltage of 0 V. The tip and tilt of the parabolic mirror was adjusted to position the focused He-Ne beam at the focal point of the mirror and at the zonal radius, 23.1 cm away from the centre of the telescope. At the location of the focal spot, a laser fibre holder was mounted to which a CW fibre laser operating at a wavelength of 1550 nm was connected. The 1550 nm beam from the fibre holder was highly divergent and fully illuminated the parabolic mirror. After being reflected from the parabolic mirror, the beam propagated along the same axis (equivalent to the optical axis of the system) as the He-Ne laser, throughout the optical system.
The tip and tilt of the annular mirror were adjusted such that the CW 1550 nm laser beam propagated along the breadboard mounting holes and was levelled with the breadboard. The position of the beam was marked with two pinholes. The lens L3(b) was then placed and aligned such that the beam still travelled along the two pinholes. The single-element detector was then placed at the approximate design distance from the back of the lens L3(b) and its active region was positioned close to the location of the CW beam.
The focal plane of the parabolic mirror, where the fibre source was placed, corresponded to an object located on-axis at infinity. Aligning the detector’s active area in a focal plane that corresponds to the position of an infinite range on-axis image allowed an approximate infinity alignment of the optical system in laboratory conditions.
149 Figure 4.17. Optical setup used for the bore-sighting of the
transmitter and the receiver.
Scanning mirrors were used to scan the FoV of the detector across 17 × 17 pixels to define where the fibre source point was located in respect to the optical axis of the receiver. Figure 4.18 shows a series of intensity images of the fibre source. Figure 4.18(a) shows a “donut” shape beam located off-centre of the image. This suggested that the detector was defocused and located off-axis. Figure 4.18(b)-(l) shows images of the point source after the position of the SPAD was adjusted along x, y axis and defocused until the beam was centred and focused to one pixel.
150 Figure 4.18. Intensity images of 17 × 17 pixels of the fibre source
located in the focal point of the parabolic mirror acquired with a single-element scanning SPAD. The colour scale is linear.
Having established the position of the SPAD which is conjugate to an on-axis infinite point source, the laser transmitter was bore-sighted with the SPAD. This process involved the adjustment of two mirrors, M1 and M2 (Figure 4.17) such that the transmitted laser beam overlapped with the fibre source (FS) so that both, the transmitter and receiver were aligned to an on-axis infinite object. In order to confirm this, a Xenics camera was used. First, the fibre source was switched on and a pixel indicating the position of the fibre core was marked on an image produced by the Xenics camera placed in front of it. Then, the fibre source was switched off and the laser transmitter was switched on, the mirrors M1 and M2 were tweaked, producing an overlap of the transmitter focal spot on the pixel indicating the approximate position of the fibre core. The centre of the FoV of the CCD camera was then bore-sighted to the same point source.
151 After the laboratory-based alignment the setup was experimentally confirmed by using the corner cube retro-reflector over a range of 6.6 km. In this case, as shown in Figure 4.19, the position of the detector focus required a minor adjustment in order to focus the laser beam to approximately one pixel, where the pixel size represented the image of the detector projected onto the object plane, which is smaller than the corner cube diameter at 6.6 km. A riflescope was then bore-sighted to the position of the corner cube.
Figure 4.19. Intensity images of 9 × 9 pixels of the corner cube retro- reflector located over 6.6 km produced by the single-element SPAD configuration. Image (c) confirms that an image from infinity is focused to approximately one pixel. The colour scale is linear.