System implementation

Figure 3-5 shows the computer aided design (CAD) drawing and a photograph of the transceiver unit. The slotted baseplate and the enclosure of the unit were made from black anodised aluminium to minimise stray reflections. The free-space optical

components are arranged on the top of the baseplate while the control electronic modules for the scanning optics are mounted on the back surface of the baseplate. A lid that also made from anodised aluminium is usually placed over the assembly, to reduce background light during measurements.

Figure 3-5 A computer aided design (CAD) drawing and photograph of the transceiver unit. PBS: polarising beam splitter; SM1 and SM2: galvanometer scan mirrors in X and Y; RL1, RL2, and RL3: relay lenses.

In the transceiver unit, two independent mirror configurations can be achieved, either with or without the use of intermediate optics. Without a relay optical system between the two flat scan mirrors on perpendicular axes, the two mirrors must be positioned as close as possible and share a telecentric plane located at the mid-point between them. If two mirrors are close-coupled without intervening relay optics, it can only implement an approximate telecentric configuration. By contrast, for the case with longer spacing between two flat scan mirrors on perpendicular axes, a relay optical system containing three telecentric lenses (RL1, RL2 and RL3) shown in Figure 3-2 (not limited to this arrangement) is required to optimise the performance of the scanning optics. As this approach can meet the telecentric requirement perfectly it is used in the transceiver (see Figure 3-5) for all the measurements in this Thesis. More specifically, an image of the Y scan mirror (SM2) can be formed on the rotation axis of the X scan mirror (SM1) by using an achromatic pair of RL1 and RL2. Consequently, the X scan mirror is also within a conjugate telecentric plane when the Y scan mirror is placed in a telecentric plane [3.13]. In addition, once RL3 is employed, an image of scan mirrors in X and Y can be formed in the fixed primary image plane of the system’s objective lens. In fact, RL3 plays the role of a scan lens and hence this limits the effective acceptance angle of

the scanning transceiver by the effective numerical aperture of RL3 in the image space.

In the current system described in this Thesis, this numerical aperture corresponds to a minimum number of f/4. In this case, even if the system objective lens had a low f-number (e.g. Canon EF 200 mm f/2.8L II USM), the f-f-number of the transceiver system for imaging is constrained to f/4.

To optimise the transceiver for imaging at different wavelengths and stand-off working distances, the free-space optical components in the transceiver were carefully selected and aligned. As our transceiver is based on the monostatic configuration, it is inevitable to have optical back-reflections from the component surfaces along the common transmit and receive optical path. These optical back-reflections have the potential to form “ghost signals”, which can confuse the signal of interest. In addition, in a time-correlated single-photon counting set-up, these back-reflections have the potential to cause pulse pile-up or cause a saturation of the detector acquisition chain, rendering the entire system ineffective. However, there are several ways to reduce the optical back-reflection effect, such as using optical components with high performance anti-reflective coating. In addition, the performance of components PBS, RL1, RL2 and RL3 is wavelength-dependent. For imaging at specific wavelengths, components PBS, RL1, RL2 and RL3 each with an appropriate coating at the wavelengths of operation were chosen. On the other hand, the employed components RL1, RL2 and RL3 were all achromatic lenses. The spacing between SM2, RL1, RL2, SM1 and RL3 was optimised to form a good-quality scanning beam. In contrast, for broadband imaging, the components with good performance at a weighted wavelength for the broad wavelength range were selected. It is worth noting that as the effective aperture of both two galvanometer scan mirrors (i.e. SM1 and SM2) with silver coating is 10 mm, they have broadband operation with high reflection for transmit and receive light beams [3.11].

More details about the software and hardware approaches to remove these unwanted back-reflections are discussed in the Chapter 4 - 7.

In terms of the alignment of the scanning optics, the spacing between the SM1 and SM2 was set to 4 , based on the pair of relay lenses (i.e. RL1 and RL2 shown in Figure 3-2).

The off-the-shelf achromatic lenses were required to have suitable AR-coatings, clear apertures and focal lengths to optimise the relay optics between two scan mirrors. The separation of + associated with different pairs of relay lenses was simulated in Zemax. This can ensure that the combination of the relay lens pair with optimum parameters has a good match to the spacing between the SM1 and SM2. The relevant

commercial lenses can then be appropriately chosen.

In order to complete the high performance relay and scanning optics for the scanners, the selected optical components were required to appropriately align in the transceiver unit. Once the placement of SM1 and SM2 is fixed, it is necessary to align the relay optical system to the scan mirrors. As shown in Figure 3-6(a), a temporary lens with a proper focal length is placed between two scan mirrors and it is employed to focus the collimation laser beam on the moving SM1. Adjusting the spacing between RL3 and SM1 until the point source (P1) on the moving SM1 has a relatively stationary image in sharp focus on the CCD camera, ensures that RL3 is a distance equal to a focal length, , from SM1. The next step is to align a pair of achromatic lenses, RL1 and RL2, to SM2. Note that the achromatic lenses, RL1 and RL2, are pre-aligned to be separated by + at the operating wavelengths. As shown in Figure 3-6(b), the other temporary lens with a proper focal length is used to focus the collimation laser beam on the moving SM2. By adjusting the spacing between the achromatic pair and SM2 until the point source (P2) on the moving SM2 has a relatively stationary image in sharp focus on the CCD camera, this can ensure that RL1 is a distance equal to a focal length, , from SM2.

Figure 3-6 Alignment between the relay lenses (RL1, RL2, &RL3) and the scan mirrors (SM1 and SM2) within the transceiver unit. (a) shows the schematic of positioning RL3 with a distance equal to a focal length, , from SM1. The focussed spot on SM1 is considered as a point source (P1) for the image in the CCD camera. (b) shows the schematic of positioning an achromatic pair of RL1 and RL2 and ensures that RL1 a distance equal to a focal length, , from SM2. The focussed spot on SM2 is considered as a point source (P2) for the image in the CCD camera.

The optical path of the transmit and receive channels within the transceiver unit for alignment is highlighted in Figure 3-7. The figure also shows the pair of mirrors included in the transmit channel, and the pair in the receive channel, that are adjusted to align each channel to ensure that both channels are coaxial. It is imperative that the coaxial alignment of the system is correct for use in long range imaging applications.

Figure 3-7 The optical path within the transceiver unit of the coaxial optical alignment scheme shown in Figure 3-8 between the transmit (Tx) and receive (Rx) channels.

Figure 3-8 illustrates a coaxial optical alignment scheme to optimise the coaxial overlapping between the transmission and collection aperture of the monostatic transceiver and thus offer high performance detection. Figure 3-8(a) shows the schematic of a coaxial optical alignment setup for the transmit and receive channels of the transceiver unit using a concave mirror with a focal length of 505 mm and a CCD camera. Given that there are fibre collimators in both the transmit channel front-end and receive channel back-end, laser beams can be delivered from a fibre and collimated in free space for both the transmit and receive channels. The fibre-to-free-space coupled laser beams pass through the transceiver unit and are focused into the CCD camera by the concave mirror. As illustrated in Figure 3-8(b), the centroid overlapping of the focussed fibre core images does not guarantee that the transmit and receive

channels are perfectly coaxial. This is because there is the potential for orientation difference with respect to the optical axis between the two focussed beams from the transmit and receive channels. To achieve coaxial alignment of the transmit and receive channels, the centroid overlapping between the images of the in-focus and out-of-focus fibre cores needs to be inspected. This was done by shifting the CCD camera forward and backward along the optical axis. Inspection of the images of the fibre cores can provide alignment hints and visual tracing in order to perform coarse adjustments for coaxial optimisation between the transmit and receive channels. By combining a microscope objective lens with a CCD camera, the magnified images of the fibre core can be used to implement fine adjustments for further coaxial optimisation.

Figure 3-8 The coaxial optical alignment scheme between the transmit and receive channels. (a) The schematic of an optical alignment setup that is used to align the transmit and receive channels of the transceiver unit using a concave mirror and a CCD camera. (b) Top: Illustrations of the focusing beam ray traces on the CCD camera in the Y – Z plane of the transmit and receive channels. Bottom: Illustrations of the images of the fibre cores in focus (centre) and out of focus (left and right) associated with the transmit and receive channels. The light beam and the images of the fibre cores in the transmit receive channel are shown in blue while the ones shown in red are for the receive channel.

An electrical driver system was used to simultaneously control the two galvanometer scan mirrors and thus achieve a raster scan. The schematic of the driver system for the XY scanning galvo mirrors with support devices can be seen in Figure 3-9. Note that the implemented driver system operates in a non-closed-loop. A digital-to-analogue converter (DAC) generates control voltages to drive the X-axis and Y-axis galvo motors. Note that the control voltages are preset values assembled in XY from a control computer in order to achieve a certain scan pattern for the X-axis and Y-axis servo controllers. In the meantime, a trigger signal associated with the generation of control

voltages can be generated from the DAC module to an external system, for example to be as a marker for the TCSPC module. By contrast, this trigger signal can also be provided from the external system to trigger the DAC module. For example, the silicon single photon avalanche diode (SPAD) array camera described in Chapter 5 generates a trigger as a marker to control the DAC module. In addition, a power supply unit can power on two servo controllers simultaneously.

Figure 3-9 Schematic of a driver system for XY scanning galvo mirrors. A digital-to-analogue converter generates control voltages which are preset values sent to the X and axis servo controllers to achieve a certain scan pattern for the X-axis and Y-axis galvo motors. In the meantime, a trigger signal associated with the generation of control voltages is provided to/from the data acquisition module. In addition, a power supply unit can power the two servo controllers simultaneously.