Sensors are all around us: they enable the interaction between ourselves and the environment or they deliver us detailed information about our surroundings. Classes of sensors which are becoming increasingly important are chemical and bio-sensors. These sensors are used for ensuring food quality and safety or in the medical sector for detecting so-called “biomarkers”. These are molecules containing information about possible diseases or the “health-status” of our body in general. Recent advances in photonics have led to a breakthrough in this field and an important type of optical sensor allows measuring the refractive index of a liquid (bulk sensing) or a thin layer (surface sensing).
The underlying detection mechanism in both cases relies on refractive index measurements. Therefore, if we can develop a sensor to measure small refractive index changes, it can be used for a variety of applications.
The goal of this master thesis is graphically illustrated in the figure below: developing a device for detection of liquids using polymer waveguide Bragg grating sensors. These sensors act as wavelength selective mirrors, reflecting a specific wavelength λB, defined by K. λB = 2.neff.Λ (K is an integer; usually 1). The reflected wavelength λB depends on the period of the grating (Λ) and the effective index (neff) of the mode propagating in the waveguide grating region. The value of this effective index depends on the waveguide properties (dimensions and refractive indexes) and therefore will vary when the refractive index of the liquid on top of the grating changes. Since the sensitivity of such a sensor is usually very high, it allows us to accurately measure the refractive index of the liquid which is for example a direct indication of its composition (e.g. the concentration of sugar in wine).
Currently, several technologies are available at Cmst for fabricating waveguides and grating structures in various polymer materials and the basic concept has been proven. These technologies can be used as a starting point for this thesis to make sensitive refractive index sensors.
In general, different possibilities for the implementation of sensing structures in optical waveguides will be studied. The first step for the development of these optical sensors is the optical simulation of the sensing principle. Secondly, the sensing system itself will be designed and fabricated using the CMST cleanroom capabilities. Later in the development cycle, these newly developed sensors can be integrated with an optical sensing/interrogation system and tested for measuring refractive index of various liquids.
The work is divided in the following parts:
Together with the master student, the focus of the master thesis can be slightly shifted towards one of the specified parts.