Scanning system

The scanning transceiver used to obtain the results in this Thesis was designed and built by the Single-Photon group at Heriot-Watt University for single-photon depth imaging over long distances in free-space [1]. With appropriate reconfiguration, the transceiver unit has been used with a range of different detectors and laser sources. The system has been used over a range of wavelengths from 500nm to 1550nm, and has also been used with several wavelengths operating simultaneously [2-4]. In the transceiver assembly, the optical components are arranged on a slotted base-plate, which allows the optical alignment to be optimised for the experiment [5], and maintain long-term stability in field trial situations. The transceiver assembly was made of black anodised aluminium, in order to reduce the stray light inside the transceiver. The unit was then covered with a black anodised aluminium lid to further minimise the background light during the experiments. Figure 5.1 is a schematic representing the optical configuration, while Figure 5.2 shows a photograph of the transceiver unit.

The transmit and receive channels in the transceiver system were coaxial and therefore most of the optical components were common to both channels. The light from the laser was collimated using an optical fibre collimation package in the transmit channel

polarising beam splitter (PBS). The light was reflected by the first scanning mirror (y-axis), passed through two telecentric relay lenses and reflected by the second scanning mirror (x-axis). The light was then focused by a third relay lens before passing through the objective lens. The objective lens was used to focus the transmitted light to the target and collect the back-scattered light from the target. The return photons followed the same path up to the polarising beam splitter, where the receive and transmit channels are separated. Then, the return signal is coupled into an optical fibre with a collimation package (FCP-Rx), and then onto the detector.

Figure 5.1. Schematic of the transceiver unit. The optical components shown in the transceiver unit included two optical fibre collimation packages, for the transmit channel Tx) and receive channel (FCP-Rx). A polarising beam splitter (PBS) was used to overlap and separate the transmit and receive channels. Three relay lenses (RL1, RL2, RL3) were used in conjunction with two galvanometer mirrors (SM1, SM2) to perform the raster scan of the target. A camera objective lens (OBJ) was used to focus the transmitted laser light onto the target surface and collect the scattered return signal.

Figure 5.2. Photograph of the transceiver unit. The input light was coupled into the transmit channel, shown in red, via a fibre-collimation package (FCP-Tx). The path in yellow is the receive channel, which was coupled to the detector via the fibre-collimation package FCP-Rx. The two channels were overlapped at a polarising beam splitter (PBS), and the common optical path is highlighted in blue. The common channel comprised three relay lenses (RL1 - RL3), which relayed the image operating at infinite conjugates.

Two galvanometer scanning mirrors (SM1 and SM2) were used to perform a raster scan of the target. A camera objective lens (OBJ) was used to focus the light on the target and collect the light scattered by the target.

The telecentric configuration was needed to guarantee that the beam was always on-axis, independent of the deflection angle. At the same time, the relay lens RL3 was used as a scan lens, forming an image on the image plane of the objective lens. This limited the field of view of the transceiver, imposing a lower limit on the f-number of the entire system of approximately f/4. The transceiver unit was optimised and aligned before every experiment, and the optical components were selected on the basis of their performance at the wavelength range to be used in the experiment. In general, the optical elements were chosen with a high performance anti-reflective coating in order to

this monostatic configuration. If the wavelength during the experiment was varied over a wide range, the alignment was performed at a weighted intermediate wavelength to take into account the chromatic characteristics of the system. The same alignment procedure was followed for all the optical configurations reported in this Thesis.

The two scanning mirrors were placed at conjugate planes of the system, and the relay lenses were chosen so that the distance between the mirrors was twice the sum of their focal lengths. In order to perform the alignment, the light was delivered from the laser source (described in section 5.3) to the transmit channel via a polarisation maintaining optical fibre. The PBS was placed in the transceiver unit to overlap the transmit and receive channels, and the light transmitted by the PBS was maximised rotating the optical fibre collimation package FCP-Tx. A more precise overlap of the two channels was performed after all the optical components were aligned. Two temporary lenses were used to place the relay lenses in the right position. As shown in Figure 5.3a), a temporary lens with appropriate focal length was used to focus the light on the SM2 galvo-mirror, while this was moving. An image was formed on a CCD camera using a concave mirror with focal length 505 mm to allow the relay lens RL3 to be placed at a distance equal to its focal length from SM2. The relay lenses RL1 and RL2 were pre-aligned in a lens tube, on a separate setup, to set them at a distance equal to the sum of their focal lengths. Then, the block with the two lenses was placed between SM1 and SM2, as well with a temporary lens with appropriate focal length before SM1 (Figure 5.3b). Through this configuration a spot was imaged on the CCD camera, and the focus was adjusted varying the distance of the pair from SM1.

Figure 5.3. Setup for the alignment of the relay lenses RL1, RL2, and RL3 with the two galvanometer mirrors SM1 and SM2. The focus was adjusted using two temporary lenses, in (a) to adjust the position of RL3, and in (b) to adjust the position of the pair RL1 and RL2.

The last step was the optimisation of the coaxial overlapping of the transmit and receive channels. This was achieved by delivering the laser light from both channels through optical fibres, and collimated in free space. Then, light was focused on the CCD camera by the concave mirror, showing the light spots from the two channels. To perform an efficient coaxial alignment, the overlapping of the transmit and receive spots needs to be verified in two different positions. Hence, the camera was moved forward and backward to inspect the images formed in focus and de-focus positions, as shown schematically in Figure 5.4. For a more precise adjustment of the overlap between transmit and receive channels, a microscope objective lens was mounted on the camera to magnify the image, and better highlight small mis-alignments.

Figure 5.4. Coaxial overlapping of transmit and receive channels. In a) the two channels are not overlapped, hence moving the CCD camera away from the focus position two separate spots will be visible (de-focus position in the figure). In b) the two channels are coaxial, and the spots imaged are overlapped for different positions of the CCD camera.

The two galvanometer mirrors were controlled by an electrical driver system, schematically shown in Figure 5.5. The voltage ranges required to scan the target area in x and y-axes were manually set through custom software, as well with the overall acquisition time of the scan and the pixel format. These parameters were used by a digital-to-analogue converter (DAC), which provided the control voltage to drive the galvanometer mirrors, and the trigger signal to mark a new pixel. The servo controllers and the motors for the mirrors were located beneath the slotted base-plate.

Figure 5.5. Schematic of galvanometer mirrors scanning system. A digital-to-analogue converter provided control voltages (preset by computer) to the servo controllers to move the X and Y mirrors,