Measuring with light is a source of unprecedented applications. This is why TNO is putting so much energy into developing photonic sensors and corresponding read-out systems. These read-out systems – interrogators – are becoming increasingly smaller, cheaper and better. And so more interesting from a commercial perspective.

Around 2007 TNO conceived an interrogator that was some ten times better than the versions then available. The concept was used in the fibre sensor systems (Fibre Bragg Gratings or FBGs) TNO was developing at the time. In 2011 this interrogator concept appeared in the market as the Technobis Fibre Technologies Deminsys system. This generation of interrogators, however, still has a shoe-box format and is expensive at between ten and fifty thousand euros. Although one interrogator can simultaneously read thirty to a hundred sensors spread over a glass fibre, photonic sensors are less used than possible due to the high price. Also, the maximum precision is a factor ten too low for the very latest generation of photonic sensors: sensors on an optical chip.

Yet another square centimetre

Many potential applications require interrogators that are not only cheaper and better but that are also smaller – for instance for use in a windmill sail or car or aircraft. A shoebox really is too big for this. TNO already has a concept of a fibre-based interrogator on the shelf that is potentially as small as a USB-stick, costs less than five thousand euros and is ten times as precise. Based on the same technological knowledge, work is focusing on an interrogator on an optical chip whose size and cost can fall much further: just under a square centimeter for less than a hundred euros. manufacturing will use CMOS production, with the same degree of flexibility in optimisation and modification as for electronic ICs.

Medical implants

All interrogators developed by TNO are, in fact, spectrometers. Sensors resubmit information in the form of light frequency about the sensor value and location. What is special about the fibre-based version and the interrogator on an optical chip is that only three or four detectors are needed and it is not dependent on the signal strength of the source and sensor. This makes them much more robust than most other solutions. For fibre-based sensing the interrogators can be used to monitor the structural health of windmill sails and turbines, for example, or to safely measure oil and gas installations. Since the measuring head contains no electronics, no sparks can occur and thus there is no danger of explosion. The most promising market for integrated photonics is medical implants. In the future when there is a need for internal examination, patients will simply swallow a pill that will collect the information required as it proceeds through the body …