Fibre Switch

The fibre switch sequentially couples the laser light into one of 32 source fibres. It is a fast and very compact DiCon (Berkeley, CA) VX500 low-profile 1x32 switch with an inte­ grated controller. The switch interfaces with the PC via a number of TTL pins. Its specifications are summarised in Table 6-3. A photograph of the switch, mounted in a specially designed 19” housing, is shown in Figure 6-9.

Insertion Loss Repeatability

Switching Time

(trade off with repeatability) Back-Refiection Cross-talk Controller Interface 0.6 dB typ., 1.2 dB max. ± 0.01 dB max. sequential ± 0.03 dB max. random < 425 ms 4- 12 ms per channel - 20 dB typ. -80 dB max. 12 TTL pins Table 6-3 DiCon fibre switch specifications.

Figure 6-9 Photograph of the custom-made 19” rack mounted Fibre Switch hous­ ing. The optical input and output connectors are located on the left-hand side of the front panel (with the input and only one output fibre connected). The actual DiCon

1 x32 switch module is hidden underneath the PCB on the right-hand side.

6.2.1.3 Source Fibres

All fibres on the source side of the system are standard Coming (Coming, NY) graded index 62.5/125 p.m fibres with a numerical aperture (NA) of 0.275. They are protected by a 900 pm outer tube jacket. The laser light is fibre coupled via a low back-reflection fibre optic delivery system supplied by Point Source (Winchester, UK), and sequentially switched into one of 32 source fibres, each 2.5 m in length. These are terminated with small 1.22 mm outer diameter stainless steel fermles (supplied by Coopers Needle Works, Birmingham, UK) in order to occupy a minimum of space on the patient’s head. All other terminations (going in and out of the fibre switch) are angle polished Huber-t-Suhner (Herisau, Switzerland) PUSH (E2000) connectors with integrated shutters^. These mechani­ cal shutters automatically close when two mated fibres are pulled apart, and thus provide an extra safety mechanism - a useful feature in a clinical environment. The dispersion of the source fibres is very low because of the short length and the graded index profile. Streak camera measurements indicate a contribution of <20 ps in pulse dispersion.

It w as found to be important to keep the connector ends very clean, because at high laser pow ers (> 100 m W ) a particle o f dust at the interface can cau se permanent thermal dam age to the fibres due to e x c e s siv e heating.

6.2.1.4 Detector Fibre Bundles

The 2 m long detector fibre bundles have been constructed from 45/50 |im step index Hoya- Schott (Tokyo, Japan) glass fibres (type LB21E, HC02 coated). The bundles are 2.5 mm in diameter with an NA of 0.21 (24° full acceptance angle) and brass ferrules mounted on both ends. Streak camera measurements indicate that the dispersion is an acceptable <30 ps. Graded index fibres could improve this but are significantly more expensive, because fibres with a small core to cladding ratio (essential for a high packing fraction) are non-standard.

mmmm

O lO lO lO lO lO lO IO lO lO .

m mm m mm m

©X©X©X©

Figure 6-10 Illustration of fibre bundle geometry (left). The packing frac­ tion equals the shaded area divided by the area of the triangle (right).

Figure 6-10 illustrates the geometry of a closely packed fibre bundle. The small core/cladding ratio (45/50 |im) of the individual fibres ensures a small packing fraction (PF) loss. The packing fraction is the ratio of the total fibre core area to the fibre bundle cross- sectional area, and after a simple geometric analysis (see also Figure 6-10) can be expressed as

PF =

2 S ^ b u ffe r

xlOO% (6.2)

For 0core = 45 qm and puffer = 50 |im, this gives an excellent PF = 73%, corresponding to a packing fraction loss of 27%. Section 7.1.3 discusses various other loss factors in this fibre bundle, and in other optical and electronic components.

6.2.1.5 Fibre Holder

A circular fibre holder assembly with an inner diameter of 70 mm has been constructed for experiments on tissue equivalent cylindrical phantoms and the human forearm (see chapter 8). The ring has been designed to hold up to 32 source fibres and 32 detector fibre bundles, and is shown in Figure 6-11. It is made of black Delrin and the fibre ferrules are held in position by grub screws. While this arrangement is adequate for performing planar scans.

the fibres may be distributed among several similar rings for 3D acquisitions of cylindrical objects (see Figure 9-2). Features of a future ‘helmet’ design suitable for performing neonatal brain scans are outlined in the discussion in section 9.1.1.

I

Figure 6-11 Photograph of a fibre holder ring, which is used for imaging cylindri­ cal phantoms. Thirty-two smal 1-diameter source fibres, and 32 large-diameter detector fibre bundles are arranged uniformly around its circumference.

6.2.1.6 Variable Optical Attenuators

In a typical imaging experiment there is a very large dynamic range of detected intensities between different detectors for any given source position. This is illustrated in Figure 6-12, which shows the intensity variation obtained from a model calculation for a typical planar acquisition geometry. The following issues therefore need to be addressed.

• The MCP-PMT detectors and pulse processing electronics may saturate at very high count rates. Section 7.1.2 discusses this effect in more detail.

• The pulse processing electronics has a maximum count rate (see also section 7.1.2). • Excessive illumination levels may damage the MCP-PMT.

• The nature of photon counting detection dictates that no more than one photon must be detected per duty-cycle. Otherwise only the first photon would be registered, thus pro­ ducing a distortion of the resulting histogram. It is therefore of fundamental importance to ensure that the intensity of light collected by each fibre in the bundle is sufficiently weak so that the relevant detector operates in the single-photon counting regime. A rule of thumb is to adjust the light level so that each channel can only detect a photon for

one per cent or less of incident laser pulses. Hence the probability of detecting two photons per duty cycle is negligible at <1/100^ (0.01%). For a laser operating at about 80 MHz this gives a single photon counting limit of 800,000 counts per second (cps) per channel.

• Cross-talk between physically adjacent channels of the same 8-anode MCP-PMT (see section 7.4) may become significant if the corresponding count rates are very different. For instance, detector channels 1 and 8 in the geometry illustrated in Figure 6-12 have incident intensities that are different by several orders of magnitude.

Therefore Variable Optical Attenuators (VOAs) are required to cope with the large dynamic range of signal intensities. If the dynamic range of the VO As is not sufficient to approximately equalise intensities between the different detector channels, a mechanical shutter, incorporated into the VO A, can be used to completely shut off channels that are located close to the source input optode.

M l l j } " .

K/7tV\v”'

2* i 24 23 (a) (b) 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 19 2021 2 2 2 3 2 4 25 2 6 2 7 2 8 29 3 0 3 1 3 2 D e tec to r N u m b e r

F igure 6-12 Schematic diagram illustrating the variation in detected intensity (right) for 32 detectors optodes arranged around a circle (left). The large arrow on the left-hand side indicates the source, which in this example is located between detectors 1 and 32. The intensity values are approximations based on a calculation using infinite space Greens functions for all 32 source-detector optode separations (see also section 4.2.2.1). The diameter of the circle in this model is 80 mm, with = 1.0 mm'^ and |Xa = 0.01 m m '\

VO A R equirem ents

The VOAs must meet certain specifications. In particular, they must be capable of rapid and reproducible attenuation over a wide dynamic range for large diameter fibres or fibre bundles. They must operate at any NIR wavelength of transmitted light, should introduce

minimal temporal dispersion into the system, produce minimal spurious reflections, and should not introduce a significant residual light loss when set to maximum transmission.

There are currently no suitable commercially available attenuators that meet our re­ quirements. For example, some attenuators are based on variable neutral density filters, which can cause very strong stray reflections that may affect the quality of the temporal measurements^. VOAs that are based on liquid-crystal technology generally have too low a dynamic range. But most importantly, no suitable VGA unit was found that is able to hold large diameter fibre bundles. Instead commercially available attenuators tend to be designed for regular single- and multi-mode single fibres, because they are aimed at the telecommu­ nications market*. Therefore a new design has been specifically developed as part of this project, and is described in some detail below.

Design Features

The principle of operation of these VOAs is based on a very thin mask that can be positioned to obscure a varying fraction of the detector fibre bundle aperture. Figure 6-13 shows the circular mask.

illu m in a ted a re a slit fo r ‘c o n tin u o u s ’ O D v ariatio n hole fo r z e ro sw itch p in h o le m o u nts h oles of d ifferent diameters Figure 6-13 VGA mask layout.

The disk has been etched from 100 p.m thick stainless steel by Photofabrication Services (St. Neots, UK). The 52.5 mm diameter mask is rotated to the desired position by a stepper motor whose shaft is attached at the centre. Its slit and arrangement of small holes allow for

^ Apart from reflective, thin-film coated, variable neutral density filters, there are also absorptive filters. However, these have a rather limited dynamic range, and the variation in absorption is achieved by varying the thickness of the filter. This is a disadvantageous feature because it introduces a (small) variable temporal delay and demands a large input to output fibre separation, the latter resulting in larger residual losses when the attenuator is set to maximum transmission.

continuous and discrete attenuation, respectively. The detector fibre bundle (coming from the patient) and the polymer fibre (going to the MCP-PMT detector) are positioned adjacent to either side of the mask as indicated in Figure 6-7.

While the elongated slit continuously^ obscures different quantities of light de­ pending on its angular position relative to the optical axis, the holes on the bottom right (diameters 0.2, 0.3, 0.5, 0.7, 1.0, 1.4, 1.9 and 3.5 mm) allow the attenuation to be adjusted in discrete steps. If very high attenuations should be required in the future, small pinholes can be glued onto the four 1 mm holes on the left-hand side of the mask. The mask can also be positioned to act as a mechanical shutter, capable of completely cutting off any light^°. The hole to the far right, together with a photo-switch (Omron SXl 101, Tokyo, Japan)” , is used as a non-contact position-sensing switch for the stepper motor controller (see also section 6.2.3.2) that allows a zero-position to be set when the VOAs are switched on. Currently only the 8 etched holes are being used for performing imaging studies. The largest hole in the mask (3.5 mm diameter) is used to set the VGA to full transmission. Any coupling losses incurring in the VGA are minimised since the input fibre bundle has a small NA of 0.21 and a diameter of approximately 2.5 mm, while the output (light-collecting) polymer fibre has a larger NA of 0.5 and diameter of 3.0 nun.

The mask has been chemically blackened (using the so-called ‘PX3 with oil’ finish, provided by Somerville Laboratories, London, UK) to reduce stray reflections. Tests (see Figure 6-14) using an integrating sphere suggest that the diffuse reflectance is ~6%” . However, it can be assumed that the actual amount of reflected light re-entering the fibre bundle is significantly less because of the finite acceptance angle.

Examples of commercially available attenuators suitable for single fibres include the DiCon FA500, or the E- TEK (San Jose, CA) PITA.

^ Although the aperture width is varying continuously, there is of course discretisation due to the stepper motor. This is not only useful to block off detector channels that are too close to the source fibre, but it also represents a very usefiol safety feature that helps to protect the MCP-PMT between successive scans and during storage.

” Although the photo-switch has a peak emission wavelength at 940 nm, tests indicated that the emission spectrum is broad enough to overlap with the detection window of the MCP-PMT detector, potentially causing stray light be recorded as background noise. Therefore the photo-switch LEDs are switched on by the system control computer only when needed while the VOAs are reset to their zero-position.

” Tests with various different samples suggest that a ‘Copper Black’ finish produces a more matt surface with only -3% reflectance. However, the surface is very soft, and can be scratched very easily. Should a lower reflectance prove to be essential in the future, a new set of masks made of beryllium copper will be made. This material is slightly weaker, but gives a more matt and uniform black finish. Another potential coating is the considerably more expensive ‘Ni-P black’ provided by the National Physical Laboratory (NPL, Teddington, UK). It has an extremely low diffuse reflectance of -0.1% in the NIR (manufacturer’s .specification).

700 800 900

X [nm]

1000

Figure 6-14 VOA mask spectral reflectance.

The VOA mask is glued to a small holder disk that can be screwed onto the stepper motor shaft. A housing, that is attached to the stepper motor, has been designed for holding the fibre bundle and polymer fibre, both of which are equipped with ferrules at their ends. This design ensures accurate co-alignment of the flbres and guiding of the mask, as well as optical shielding from external sources. The zero-position sensing photo switch is also incorporated into the housing. The design of the housings, which were CNC-machined from black PVC’^, is illustrated in Figure 6-15, and a detailed technical drawing can be found in the appendix in Figure B-1.

hole for detector fibre bundle ferrule housing m ask holder m ask zero switch-

hole for polymer fibre ferrule (hidden) stepper

motor

F igure 6-15 Illustration of the VOA housing with top plate mounted (left) and removed (right). (Drawing prepared by Yoko Schmidt).

Figure 6-16 shows eight VOAs mounted in a diecast box. There are four such boxes, one for each 8-channel MCP-PMT detector (c.f. Figure 6-6). The attenuators are stacked on two

1 3

Black PVC was found to be a suitable material because it is strongly absorbing in the NIR, and has a relatively matt surface.

levels in order to save space. Apart from being convenient for mounting, the box also provides shielding from optical and RF radiation.

(a) (b) (c)

Figure 6-16 Photograph of a VOA box, housing eight attenuators, at three stages of its assembly: (a) the stepper motors are mounted onto the back-plane of the VOA box, (b) the actual VOAs are mounted on the stepper motors (note that the top plates of most VOAs in this picture have been removed, thus revealing the circular VOA masks), and (c) the front panel is mounted on the VOA box, with the 8 detector fibre bundles attached to the individual VOA units. The polymer fibres connecting the VOAs to the MCP-PMT photocathode are mounted at the rear of the VOA box (hidden).

Performance

The measured and estimated attenuation values in terms of optical density (OD) are depicted in Figure 6-17. An attenuation of approximately 2 OD can be achieved using the 8 etched holes. The measured values are slightly smaller than the estimates, which is probably due to the fact that some light emerging from the obscured parts of the fibre bundle aperture manages to pass through the VOA hole at an angle. Should higher attenuations be required in the future, small pinholes may provide up to ~4 OD. The slit provides a similar range of attenuation values, but is not included here because the holes were found to be adequate. More details on the VOAs performance, in particular relating to the effect they have on the temporal characteristics of the Instrument Response Function (IRF), are included in section 7.6.2, which discusses various system performance issues. Table 6-4 below summarises the advantages and disadvantages of this particular attenuator design.

4.5 -- 4.0 -- ^ 3.5 -- Q 0 3.0 ■I 2.5 -I- (0 1 2.0 + | l . B f 1.0 - - 0.5 0.0 ■ measured o calculated * j pinholes ^ 3500 1900 1400 1000 700 500 300 200 100 50 25 12.5

VOA Hole D iam eter [pm]

Figure 6-17 Chart of the estimated and measured VOA attenuation values. The measured values for the 8 etched holes (3500 down to 200 tun) represent the aver­ ages of all 32 VOAs, and include the standard deviations. Note that differences between the individual channels can be calibrated for should absolute intensity measurements be required. The calculated values for the pinholes (100 down to 12.5 |im), which are not currently incorporated, are included for completeness.

Advantages Disadvantages

simple and rugged mechanical device (requires moving parts)

low-cost requires a diffuser at output for uniform

detector illumination (we use a Polymer fibre anyway)

no collimation required selective (non-uniform) transmission of light

emerging from fibre bundle aperture

no temporal dispersion some effect on the IRF characteristics

(see section 7.6.2) compact (< 60x60 mm)

vibration insensitive

sufficiently high OD dynamic range (in particular if pinholes are used) low losses (high maximum throughput) wavelength insensitive

relatively low, but non-negligible, back- reflection

relatively fast change in OD (see also section 7.1.4)

suitable for large fibres/fibre bundles Table 6-4 Summary of VGA features.

In document Development of a Time-Resolved Optical Tomography System for Neonatal Brain Imaging (Page 99-109)