5.3 Optical Components
5.3.2 Light detector
As NIR photons leave the irradiated tissue they can be retrieved from the scalp surface using sensitive detectors, sometimes requiring coupling fiber optic cables. The following section deals with the design considerations for selection of an appropriate detection device for a CWNIRS system.
5.3.2.1 Light detector - design considerations
There are various detectors used in NIRS which are chosen depending on the application, but the choice is primarily dependent on the NIRS modality used, i.e. continuous wave, time domain, or frequency domain NIRS. A NIRS detector should satisfy the following constraints:
• It must be sensitive to photons in the NIR wavelength range - typically 650nm
4 Parallel configurations are possible if the manufacturer chooses die from the same wafer with matched potential drops.
to 950nm, but specifically for LED sources used in this project (LED specifications in Table 5.1) - taking into account the broadband nature of the LEDs' wavelength - sensitivity should cater for 745nm-775nm for the shorter wavelength LEDs (30nm FWHM) and from 840nm-920nm for the longer wavelength LEDs (80nm FWHM);
• It should have a high sensitivity for this wavelength range - with consideration for both (a) the 7-9 orders of magnitude loss from NIR photons traversing the head, and (b) the capabilities of the light source primarily optical output power, i.e. how much light remains after exiting the head will depend on the tissue's absorption and scattering effects and on the amount of photons emerging from the light source in the first instance. Thus, the detector should be sensitive enough to detect the remaining photons;
• It have an appropriate speed (rise-time and fall-time, i.e. time constant) - depending on the mode of NIRS used the detector may have to respond to very fast pulses, e.g. during time-of-flight measurement for time-domain systems. For CWNIRS, detectors should be able to detect signals in the DC-to-several kHz range. The modulation strategy used (described later in Section 5.4.1 and Section 5.4.2) operates with sinusoidal carrier signals up to 16kHz, so a detector with response rates capable of detecting these signals is needed. Furthermore the detector should only have a bandwidth sufficient for the measurement made since anymore lets in more noise (Johnson noise). However, there is a trade-off in the design in terms of complexity, cost, portability, and other issues such as e.g. photomultiplier tubes are more fragile;
• Coupling - the detector should be able to retrieve light from the head. Nevertheless many systems use coupling fiber optics to carry the light to remote detectors, with varied distances in different designs. However, there are losses involved in this coupling technique. Thus, losses should not be more than the detector can afford for the photonic flux exiting the head.
Four types of detectors are generally used (Rolfe, 2000; Strangman, et al., 2002). Charge-coupled devices (CCDs); photomultiplier tubes (PMTs); silicon photodiodes (SiPDs); and avalanche photodiodes (APDs). PMTs may require cooling and need a high bias potential, as mentioned above, and are not very portable. They also require high voltages and protection from high currents to prevent them being damaged. These have been used in a single-channel photon counting system at Maynooth (Lebid, et al., 2004) with the aim to detect the fast signal (event related optical signal), since they can be configured to be highly-sensitive fast detectors. CCD devices could be used when there are many wavelengths
of light to collect, such as with a tungsten lamp. The wavelengths can be dispersed using a prism, spreading light of different wavelengths across the CCD surface. SiPDs can be used and placed in direct contact with the scalp, making them useful for effective portable probes. Nevertheless they are less sensitive than APDs, since APDs achieve electrical gain via the amplification from the high electric field applied to the depletion layer of this semiconductor detector. This yields higher quantum efficiency for the device. In addition, onboard temperature modules can be used to keep the APD gain constant. APDs on the other hand cannot be placed in direct contact with the scalp (due to the high voltage electronics for these modules) but they can be highly effective when used with coupling fiber optics, since their sensitivity is appropriate for low light levels emerging from the head. Thus, APDs were chosen as the light detector. Furthermore these devices are also used by others in the NIRS community, especially in CWNIRS devices (Koizumi, et al., 2003; Coyle, et al., 2004; Joseph, et al., 2006).
5.3.2.2 Avalanche photodiode (APD) detection for CWNIRS
The major specifications of the APD detector used in this project (full list at Hamamatsu_Photonics_K.K., 2009) are outlined in Table 5.3. Seven APDs were used for the complete multichannel CWNIRS instrument, as illustrated earlier in Figure 5.1.
Specification Description
Manufacturer Hamamatsu Photonics K.K., Japan
Device Name C5460-01 APD
Wavelength Range 400nm to 1000nm (peak sensitivity @ 800nm)
Sensitivity -1.5*108V/W
Active Area
φ
3.0mm (diameter of circular photodiode)Bandwidth DC to 100kHz
Noise Equivalent Power (NEP)
0.02pW/Hz
12Minimum Detection Level 0.005nW (RMS)
Maximum Input Level (light) 0.06
μ
WTable 5.3: Avalanche photodiode (C5460-01) detector used in the CWNIRS instrument.
It can be seen from the specifications above that this device satisfies many of the requirements of a detector for CWNIRS with the LED light sources used. Firstly, the wavelength range is adequate with a central peak at 800nm which is at the centre of the LED wavelengths and the isosbestic point for the haemoglobin species. The bandwidth of the
device is also suitable up to 100kHz. An analysis of the light detection level limits (minimum and maximum) can be seen as suitable for cerebral interrogation. The 760nm component (four die) of each LED package outputs approximately 25mW and the 880nm die 13mW. With the high absorption of cranial tissue (~107) it can be crudely assessed that 2.5nW and 1.3nW reach the detector, a combined total of 3.8nW which lies within the detectors range (0.005nW - 60nW). The active area of the detector is also an important factor - the larger it is the more light can be detected. However, for increased spatial specificity a smaller active area may be required. Given the difficulties with hair follicles and hair absorbing light it is a trade-off between size and what is practical for applications on a subject's scalp, principally signal-to-noise ratio. Hair can be brushed aside and clear up to 10mm of 'free' scalp typically, and so an area less than say 8mm would suffice - here an active area of 3mm is employed in this APD detector. Furthermore, a fiber optic bundle (see later in Figure 5.13) of 6.35mm diameter (
φ
) was used to increase the SNR of the instrument compared to the first prototype by Coyle et al at Maynooth (Coyle, 2005). It was anticipated that perhaps software-based demodulation techniques would not be as good (in terms of SNR of the demodulated signals) as hardware-based phase-sensitive lock-in detection techniques (examined later in Section 5.6.2) used in that first prototype previous to this project - where Dr. Coyle used aφ
3.175mm light guide (Coyle, 2005). (These light guides are described in the next section - Section 5.3.2.3). Apart from these specifications the APD module also has a temperature-compensation bias circuit (to keep APD gain constant at ±2.5% at an ambient temperature of 25±10˚C) and a high-speed current-to- voltage converter (linear) in order to have the light levels recordable from the BNC output (as a potential difference). A block diagram of the APD board is shown in Figure 5.12 below. The high-voltage generator (+12V to +200V) is needed for the amplification process.Figure 5.12: C5460-01 APD block diagram showing the main components for transducing light levels linearly to recordable potentials (~ 0V to -10V) from the BNC output.
5.3.2.3 APD coupling optics and signal conditioning
The APD module from Hamamatsu comes as shown in the previous block diagram, with a connection for the DC power supply. A design issue then is how to couple the light from the head to the APD's
φ
3mm active area. Fiber optics bundles can be used as light guides although there is a trade-off between losses and fiber cable length (evaluated later). In addition, if the diameter of the light guide is larger than the active area then a focusing lens of some sort is required to couple as much light as possible to the APD. Again, there are some other design considerations in selecting a light guide: 1) it should have good transmission for the NIR wavelengths used; 2) to maximise photons collected, it should have a large acceptance angle; 3) the length of the light guide should be as short as is practical to minimise losses, or incur acceptable ones; 4) the bend radius of the fiber bundle should allow for practical application of multiple light guides onto a subject's head for a multichannel CWNIRS instrument; and 5) it should be possible to fix the fiber cable to the subject's head and to the APD at the other end securely, to minimise artefacts arising from motion of the subject\light guides. This would also have the affect of changing the optical pathlength in an experiment if there is some artefact introduced (subject motion). The modified Beer-Lambert Law is not well equiped to handle these motions due to it being an ill-posed inverse problem.10.72mm
Figure 5.13: Fiber optic bundles (glass light guides) used to couple light from the scalp to the active area of the APD detector (not to scale).
The optic fiber bundles used for this project have an acceptance angle of 68° (with a numerical aperture of 0.55) with approximately 12,800 fibers per bundle for the 6.35mm diameter light guide used (Edmund Optics - NT42-346), with individual fibers 50 microns in diameter. The fiber bundle ends are polished and sealed with stainless steel ferrules. For this light guide, ~70% of light enters with losses of 6% per 300mm. Thus, for this light guide, ~60% of the light makes it through the fiber to the detector. They also have a minimum bend radius of 38mm.
Two other optically related accessories were added to the detection system. First, the light guide used has approximately twice the diameter of the active area of the APD, calling for some additional lensing apparatus. However, due to the spatially extended light coming
12.7mm
609.6mm
from multiple fibers in the fiber bundle, an aluminium bucket was used to collect the light. The aluminium bucket was also designed to allow for better, more secure coupling to the APD. This is illustrated in Figure 5.15. The aluminium bucket design also catered for rigid application of a second accessory - a NIR glass bandpass filter (NT46-082:Edmund Optics, UK; RT830; 12mm diameter with a central wavelength at 830nm and a FWHM of 260nm; 2.5mm thickness). The transmission spectrum for this filter is shown in Figure 5.14. This filter was put in place so as to protect the detector from saturation (ambient light) and reduce noise in the system from stray light arising from lack of secure fitting of the APD to the light guide, i.e. the photodiode is fastened with screws to the aluminium bucket and the light guide is flush with the opposite side of the glass BPF (the assembly is shown in Figure 5.15). Since the detector is sensitive in the wavelength range of 400nm-1000nm, the main effect of this filter is to reject visible light, i.e. <~700nm and, also those infrared sources above ~960nm.
Figure 5.14: NIR glass bandpass filter with central wavelength at 830nm (Adapted from Edmund_Optics, 2009)
Figure 5. 15: Detection sy stem configuration. optic bundle fi ts into t he aluminiu m bucket held in pla ce with a gru b screw. Fiber bun dle fits f lush
against the glass BPF, whi
ch is slotted in
a tight groove in the aluminium
buck
et. APD
is held tight against the alum
ini um buck et on the right han d
side via two 3mm diameter
screws which
secure the APD
m
odule, al
um
iniu
m
bucket, and the
alu m inium enclosure case ( shown in "fro nt face" inset) . Fiber