Control Electronics, Data Acquisition and Analysis

A schematic diagram showing the timing electronics used to acquire experimental data is shown in Figure 5.2. A commercially sourced digital delay and pulse generator was used to synchronise and gate the SPAD detector and clock the laser pulse at a frequency of 125 kHz. The output power of the laser was adjusted via a current driver controlled by a PC.

Figure 5.2. Schematic diagram showing a timing scheme used to acquire data with a single-element SPAD operating in a scanning mono-static configuration.

The laser trigger signal was split into two by a power splitter and used to initiate the scanner FPGA, sending TTL pulses to the servo drivers of the scanning mirrors set by a control software. The event timing analyser – GuideTech – was used to continuously

178 time-stamp two events: laser trigger pulses and photon events recorded by the detector. Similarly to the bi-static arrangement described in Chapter 4, the FPGA was set up to trigger scanning mirrors after clocking a certain number of laser trigger pulses and released a TTL pulse after a scanning mirror moved. This pulse initiated an arm input of the GuideTech which started acquiring time tags from channel A and channel B. During the time-tag acquisition the GuideTech was synchronised to an internal 50 MHz time base. After each scan point, the time counting process was re-set to a common starting reference at the time-interval analyser.

The delay generator was used to adjust the delay between the SPAD gate trigger pulse with respect to the laser/scanner trigger pulse such that, depending on the range at which the target was located in respect to the optical system, photon returns from the target were collected approximately in the centre of the SPAD gate.

The acquisition time at the GuideTech was adjusted by setting a number of laser trigger time tags acquired by channel A. The maximum number of tags that could be collected by the GuideTech was one million, which, at a laser operation of 125 kHz limited the acquisition time to approximately 8 s per pixel. The time tags were collected with 80 ps temporal resolution which was set by the intrinsic limitations of the time interval analyser.

The data acquisition process was controlled by software in the following sequence: - Laser trigger is enabled, pulses are emitted and travel towards a target, the return

photons are collected by the telescope and recorded by the SPAD detector;

- The FPGA clocks in “copy of laser trigger” pulses (see Figure 5.2) and uses them as a base for setting the acquisition time;

- GuideTech software starts, this includes configuring and initialising the electronics board;

- Scanner control software starts which moves scanning mirrors according to a user- defined and specified in a script voltage;

- After clocking a certain number of laser trigger pulses, approximately equal to the acquisition time, the FPGA releases a TTL pulse which initialises the GuideTech arm input;

179 - GuideTech initialises channel read-out operations, arrays for storing time-tags and starts time-tag acquisition for channel A (laser trigger pulses) and channel B (photon events recorded by the SPAD) and stops when the initialised array is full; - The acquisition time set on the scanner control software is required to be about

25 % longer than the time-tag acquisition time to allow enough time for the data to be stored and saved;

- After the data from the first pixel is stored the GuideTech resets and “waits” for the next pulse from the FPGA to arrive (which signifies that the scanning mirrors have moved to the next position) and the process repeats until the scanning mirrors cover the FoVScanner.

The scanner control applies voltage to the mirror x and y in a sequence which follows a user defined script. After a mirror moves, a pulse is released which activates the arm input of the GuideTech collecting time-tags from channel A and channel B. Time tags from channel A are marked with a marker “0” (laser trigger time stamps) while tags from channel B are marked with a marker “1” (photon event time stamps). The time tags from both channels are then sorted with respect to the time and combined into one row with the corresponding markers in another row; data representative of each scan point is separated by a marker “2” and appended to the previous data in a text file. Table 5.1 shows an example of a text file for two scan points (the data has been altered for the purpose of illustrating the principle); the first column shows a series of time stamps and the second column shows a marker for each of the time stamp. The first marker “2” indicates that a mirror moved into the correct location and that the data acquisition has started. Each subsequent “2” marker separates a set of data collected for different scan points.

A three-dimensional map of a target is generated in the following steps:

- LabVIEW software extracts a set of data corresponding to each scan point and calculates the ToF for each detected photon by subtracting the photon arrival time from the time of previous laser pulse trigger;

- A histogram of ToF for each detected photon is generated for every scan point; - Each histogram is analysed using the peak finding method. The peak finder finds a

peak in a histogram based on the least-squares fitting which minimises the square of the error between the experimental data points and the values calculated from the

180 fitting function (see section 4.13.1.1). Typically, a quadratic polynomial is fitted into a set of data allowing locations of peaks to be identified [5].

- The software calculates a timing location for each peak from which the range information is calculated;

- The data is collected in a text file where the x and y coordinate of the scan point for each identified peak is listed with the maximum number of photon counts present in a peak and a range to the target corresponding to each peak;

- This data is then used as an input file for a MatLab code which produces a three- dimensional scatterplot [1].

Table 5.1. A set of example data acquired by the system with time stamps in the first column and markers in the second row. Marker “2” indicates the movement of a scanning mirror to the position defined by a script, marker “0” indicates laser pulse trigger and marker “1” indicates a photon event.

181

5.4 Transmitter

The transmitter was designed to operate in a mono-static mode where both, the transmitter and the receiver have a common optical axis. As illustrated in Figure 5.3, when a single-element detector is used in the mono-static configuration, the FoVScanner is

sequentially illuminated by the transmitter of divergence ~ 2.5 times larger than the FoVPixel which is 104 µrad (1/e) and 40 µrad respectively. The backscatter from internal

components is mitigated using electrical gating of the SPAD detector (see section 5.5 for details).

Figure 5.3. Illustration of transmitter and receiver FoV when the system operates in a scanned mono-static configuration. Scanning mirrors were programmed to scan the FoV in a stop-and stare mode using a bi-lateral pattern

As illustrated in Figure 5.3, the stop-and-stare approach, described in section 4.4 is applied.

5.4.1

Transmitter Design

A schematic diagram of the mono-static transmitter setup is shown in Figure 5.4. A laser fibre output was mechanically combined with a collimating lens via a SMA connector. A 3× beam expander used in reverse was then mounted along the path of the beam and the distance between the two lenses of the expander was adjusted to provide the beam divergence of 104 µrad.

182 Figure 5.4. Schematic diagram of the mono-static transmitter

designed to operate in a scanning system that incorporated a single- element SPAD detector.

In this experiment, an off-the-shelf collimation package with a lens of focal length f = 15.58 mm was combined with an SMA-28 fibre of mode field diameter D = 10.5 µm at 1550 nm providing the 1/edivergence angle, θ° ≈ 0.022° (3.9 × 10-4 rad) with the output beam diameter of 1.8 mm [2]. The 3× beam expander used in reverse de- magnified the beam to a 0.6 mm diameter (which fitted in between the edge of the annular mirror and the telescope obstruction) and increased the beam divergence to θ° ≈ 0.066 ° (1.7 × 10-3

rad). After travelling through the receiver optics the beam diameter increased by the system magnification, M = 11.28. Thus, at the output of the telescope the beam divergence was θ°Output ≈ 0.006 ° (104 µrad) 1/e.

Ideally, the beam divergence at the intensity of 1/e is required to be slightly larger than the FoVPixel (40 µrad). The divergence of a collimator is a trade-off between its

divergence and the beam diameter. Achieving low divergence with a small beam diameter that could fit between the edge of the annular mirror and the telescope central obstruction with a fixed magnification of the telescope-eyepiece pair meant that the lowest possible divergence with off-the-shelf components is 104 µrad.