Fibre Optic Time-Resolved Spectroscopy Using CMOS-SPAD Arrays

.K. Ehrlich, A. Kufcsák, N. Krstajić, R. K. Henderson, R. R. Thomson and M. G. Tanner
Heriot-Watt University/University of Edinburgh

The title of this paper means using an optical fibre to collect the light emitted from specific chemical substances called fluorophores, and transport it to an instrument. The instrument is equipped with a special sensor to collect the spectral information of the emitted light, as well as the arrival time of the light at the sensor.


At the moment, the diagnosis of infections in the lung lacks an accurate bed-side tool to do so. Currently, one path that is investigated within the Proteus project is sensing of pH, because a higher pH level in the lung could be an indicator for an infection. pH is a scale for acidity: the lower the pH level is, the more acidic the environment. Together with the detection of bacteria that is also done in our group, this can lead to a better diagnosis. The measurement of pH is done through measuring changes in the emitted light intensity from a fluorophore in response to the environment. There are several distortions which affect the emitted light, and hence will distort the measurement and its accuracy.


A fluorophore is a chemical substance that absorbs light of one wavelength, and emits in another. The light emitted by the fluorophore changes in response to changes in its environment. This light is captured by an optical fibre and transported to our instrument. An optical fibre is a glass wire in which light can propagate without almost any distortion. The instrument breaks up, or disperses, the light into its wavelength, before the light hits a sensor. The sensor is a device that measures photons and converts them into a signal, which can be read by an observer. By observing changes in the wavelengths, the intensity, and the arrival time of the light emitted from the fluorophore, we can draw conclusions about the environment that the fluorophore is in.


We are using a sensor which is a line of 256 pixels and made by our group. This sensor is special as it has the capability to count photons on all 256 pixels at the same time, as well as recording the arrival time of the photons. This means that we know the wavelength and intensity of the light and the time when the light is emitted. The wavelength and intensity of the light are affected by distortions, such additional light from tissue and the optical fibre, or by a destruction process of the fluorophore itself, which causes the intensity to decrease with time. However, the arrival time of the light is not affected by these distortions.


As a proof-of-principle experiment, the arrival times of the light in a changing environment (in this case, changing pH level) was measured. It was seen that the arrival time increases with higher pH level.


This paper was published in the SPIE conference proceedings:

Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XVII