Optical filtering at the receiver

The inherent time-gated filtering mechanism can distinguish the time-resolved signal

from the background noise with a constant power spectral density using TCSPC measurements. In order to perform detection with high signal-to-noise ratio, spectral and spatial filtering approaches can be also used in the receive channel to restrict the background noise originating predominantly from the solar background. The spectral filtering approach can be based on the dielectric optical filters or diffraction gratings.

When a single, fixed operating wavelength is used, dielectric optical filters were chosen, which offered a combination of out-of-band rejection and high in-band transmittance.

Once the objective lens, relay lenses, and the fibre collimators are fixed within the transceiver unit, the size of the received beam spot is determined at a specific target stand-off distance. In this case, the core of the fibre that is employed in the receive channel plays a role of a spatial filter. The core diameter of the receive fibre corresponds to an effective received aperture of ϕ on the target under test. As shown in Figure 3-21, three different sizes of ϕ against the same effective illumination aperture of ϕ on the target under test are illustrated.

Figure 3-21 Effective illumination aperture (ϕ ) for the transmit channel versus effective collection aperture ( ϕ ) for the receive channel. Left: ϕ < ϕ ; Centre:

ϕ = ϕ ; Right: ϕ > ϕ .

The collection area of scattered return photons in the line-of-sight of the transceiver is located within the active illumination region (i.e. ϕ ≤ ϕ ), offering highly efficient collection of the scattered return photons. It can restrict background noise when ϕ = ϕ . By contrast, when ϕ < ϕ there is a non-overlapping area. This area corresponds to the passive illumination region where the background noise is dominated. Thus, in this case, higher background noise is coupled by the transceiver. However, the advantage in this scenario is the higher tolerance for the beam wander effect, which commonly occurs in free space systems due to atmospheric turbulence. Note that the beam wander effect can be described as the real-time tracing of the centroid of the instantaneous beam spot. Each instantaneous beam spot is with a pattern of random wandering in the plane of a receiver aperture (refer to Pages 29-30 of [3.26]). The beam wander effect contributes to photon counts scintillation at the receiver (see Chapter 7).

In terms of ϕ > ϕ , the scattered photon collection efficiency is lower, but it can offer higher coaxial tolerance for background noise reduction and also better spatial resolution for 2D scan mapping. Thus, this was implemented in most of the imaging systems described in this Thesis. The scintillation, signal-to-background ratio and spatial resolution of the scanning system were characterised in Chapter 7 using a number of combinations of objective lenses with different focal lengths and receive fibres with different core diameters. alignment techniques described in section 3.2.4 for the transceiver assembly ensured that the scanning system was stable and worked over a very large dynamic range. Note that field trials carried out using the scanning system at stand-off working distances of 40 , 325 , 910 , and 4500 metres are reported later in the Thesis. In additon, the optical components and optomechanics of the transceiver assembly of the scanning system have shown good long-term mechanical stability with the assembly maintaining its optical alignment over the course of month-long field trials. Typically, these trials were conducted from our roof laboratory facility where the temperature can vary by about 10 °C over the course of a day.

The author, with assistance from Dr. Aongus McCarthy, devised the alignment techniques (described in section 3.2.4) for the relay and scan optics in the transceiver system. The author solely characterised the optical power spectra of the laser sources and carried out the power monitoring measurement of the supercontinuum laser source (described in section 3.3). The author made key contributions to the determination of appropriate optical components (by assessing their specifications and modelling in Zemax) and the developments of the transceiver systems with several different configurations and functions, which are more fully described in Chapter4 – 7.

3.8 References

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Chapter 4

Kilometre-range depth imaging using single-pixel single-photon detectors at λ ~ 1550nm