Light enables extremely sensitive measurements of, for example temperature, pressure and radiation using extreme small sensors in extreme conditions. TNO co-develops with users optical sensors in fibreglass and on integrated circuits (ICs or chips) along with better and cheaper corresponding read-out units.
A Fibre Bragg Grating (FBG) comprises a grate in fibreglass that reflects a particular wavelength. If the grate is stretched, the wavelength of the reflected light shifts. This shift is a measure of the strain exerted on the FBG. TNO has developed a detection system that can measure up to 1/1,000,000,000th. In combination with specially designed mechanistic constructions, sensors can be produced for parameters like pressure and velocity. We have recently presented the smallest FBG series pressure sensor to measure blood pressure in arteries, the brain and the heart. However, there are many applications that require an even higher sensitivity. So we have begun development of the fibre-laser sensor, whereby a special laser in fibre brings sensors within reach of a sensitivity some thousand times higher than the FBG sensor. This enables picostrain measurement, equivalent to the change in length of about 1/1000th of an atom.
TNO has used fibre-laser technology in the development of a hydrophone, a sensor for detecting acoustic pressure under water. In the echo-free water basin of TNO the hydrophone proved able to easily measure the weakest natural ambient noise of a calm sea. Whether a group of orcas swimming by or a submarine far away, the sensor perceives the object(s). The fibre-laser hydrophone is not only very sensitive but it is also smaller than the conventional electrical hydrophone. An advantage of its size is that a series of sensors can be produced one behind the other in a thin hose. This can be easily placed on submarines or unmanned craft to improve the detection of other submarines. The new fibre-laser technology can also be used for the development of sensors for other physical parameters like strain, temperature, velocity and magnetic fields.
Building on a long tradition in fibre sensor systems, TNO has developed sensors on an optical chip. The most striking feature of 'silicon photonics' is that the wave conductors and corresponding components are extremely small, so we use silicon-on-insulator (SOI) technology that fixes the light in the silicon layer. We chose a wavelength of 1550 nanometres, the value used by telecom applications on a massive scale. Several physical parameters can be read at one spot at the same time. Chemical fluids and gases can also be detected by adding coatings on the top of the optical sensor. Proven examples are relative humidity, H2S and CO2. We can also develop or utilise sensitive materials that can detect other chemical substances. Silicon photonics and fibre may perhaps be used anywhere that a change has to be monitored.