When the transmitted beam propagates through optics along the receiver path it causes back-reflections which are then focused onto the detector. The back-reflected signal, if not properly attenuated, can damage the detector or interfere with the signal from a target.
To eliminate back-reflections from optical components the detector was set up to operate in a gated mode where an electrical signal switched the SPAD above the avalanche breakdown voltage at pre-programmed intervals for time durations between 1 - 1000 ns. In this case the signal could only be detected if it arrived within the gate window (see Figure 5.5).
Figure 5.5. Timing diagram illustrating backscatter mitigation.
A delay between the laser trigger pulse and the SPAD gate trigger was chosen (between 1 ns – 8 µs) depending on the range to the target which ensured that the return signal arrived within the gate. The signal from internal back-reflections reaches the detector much earlier (tens of nanoseconds round-trip) compared to the return from a long-range target (thousands of nanoseconds) and therefore typically arrives at the detector during the hold-off time and is not detected.
184 When the delay between the laser pulse and the SPAD gate trigger is 0 s (no delay) then the attenuated backscatter signal from optical components can be detected. A histogram of backscatter acquired with scanning mirrors positioned on-axis is shown in Figure 5.6. There are eight peaks that can be resolved; each representing a reflection from a specific optical component in the transceiver. The time difference between the first and the last peak is ~ 20 ns which correspond to approximately 6 m round-trip distance between some of the components in the telescope and the annular mirror located closest to the detector. The last peak has the highest magnitude and probably results from the back- reflection from the corrective plate of the telescope.
Figure 5.6. Histogram showing multiple peaks resulted from the signal back-reflections from internal components.
The backscatter will appear periodically at the frequency of 8 µs (gate and laser repetition period); if an object of interest happens to be positioned at a range equivalent to a multiplication of 8 µs (1.2 km, 2.4 km, 3.6 km, etc.) then the backscatter signal will coincide with the return from the target. Temporarily changing the laser repetition and gate trigger rate can solve this issue.
5.6 Target Locations
The system was sited in a laboratory which provided access to an urban and countryside environment with a range of up to 10.5 km and varying altitude along the line sight.
185 Figure 5.7 shows locations of targets used in the alignment and experiments as well as the location of Selex ES where the transceiver was located. The map was taken from Google Maps [3] from which approximate distances to the target locations were identified. Some of the main targets and experimental sites included: a clock tower (~ 800 m), Stewart Melville College (~ 3 km), Craiglockhart Hill (~ 5.6 km), a pylon (~ 6.8 km) and Ski Slope (~ 9.4 km).
Figure 5.7. Map of Edinburgh showing locations of targets in comparison to Selex ES where the transceiver was located.
5.7
Transceiver Bore-Sighting Procedure
Initially the Xenics camera was placed in the detector plane of the lens L(b) shown in Figure 5.8 and focused on the Ski Slope at ~ 9.4 km. The system was then pointed at the parabolic mirror and the laser beam coming out of the transmitter was injected into the system through the annular mirror about 2 mm off-axis and propagated parallel to the optical axis. The beam was emitted from the telescope and was directed onto the parabolic mirror; the mechanical centre of the mirror coincided with the optical axis of the system. The tip and tilt of the mirror were adjusted such that the laser beam focused at its focal plane and the zonal radius. A fibre mount was then placed in that location, which acted as an on-axis infinite point source.
186 Figure 5.8. Optical setup used for the bore-sighting of the transmitter
and the receiver.
The position of the Xenics camera was marked with a series of lens tubes secured to the breadboard and removed from the system. The SPAD detector was then inserted into the system and secured to the tubes of a matching tube thread size – at this point its position was approximately correct to collect the signal from the fibre source. The FoV of the system was scanned in order to determine the position of the image of the fibre on the detector plane. The detector’s x, y position and defocus were adjusted until the image was focused to the size of the detector pixel and positioned at the centre of the
187 scan, i.e. the optical axis of the system. Figure 5.9 shows a series of intensity images generated during the SPAD alignment procedure with the final position shown in Figure 5.9(d) illustrating the spot focused to one pixel at the centre of the scan.
Figure 5.9. Intensity images of 16 × 16 pixels of the fibre source (FS) located in the focal point and the zonal radius of the parabolic mirror acquired with a single-element scanning SPAD. The colour scale is linear.
Once the position of the SPAD was adjusted to an on-axis infinite point source, the laser transmitter was bore-sighted with the SPAD. This involved an adjustment of the transmitter’s fold mirror until the position of the transmitter’s beam overlapped with the position of the fibre source imaged on the Xenics camera. To accomplish this the FS fibre source was switched on and an area of ~ 5 × 4 pixels, indicating the position of the fibre core on the FS plane, was marked on an image (region A in Figure 5.10) produced by the Xenics camera placed in front of it. Then, the fibre source was switched off and the transmitter was switched on, the fold mirror was adjusted, producing an overlap between the focal spot of the transmitted beam 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.
188 Figure 5.10. Xenics camera image illustrating an overlap between the
beam from the transmitter and the beam from the fibre source within an area of ~ 5 × 4 pixels.
Following the laboratory-based alignment the alignment was experimentally confirmed by using the corner cube retro-reflector over a range of 9.4 km which, according to the optical design, provides diffraction limited depth of field of approximately 5.2 km to 33 km (see section 3.10). In this case the position of the detector focal position 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 was smaller than the corner cube diameter at 9.4 km. A riflescope was then bore- sighted to the position of the corner cube.