We present a compact and fast wavelength monitor capable of resolving pm wavelength changes. A photosensor array or
position detector element is coated with a linear variable filter, which converts the wavelength information of the
incident light into a spatial intensity distribution on the detector. Differential read-out of two adjacent elements of the
photosensor array or the position detector is used to determine the centroid of this distribution. A wavelength change of
the incident light is detected as a shift of the centroid of the distribution. The performance of this wavelength detector
was tested with a wavelength tunable light source. We have demonstrated that our device is capable of detecting
wavelength changes as small as ~0.1 pm. The wavelength monitor can be used as read-out unit for any optical sensor
that produces a wavelength shift in response to a stimulus. In particular, changes in the reflection properties of one and
two-dimensional photonic crystals can been detected. The performance of this interrogation method has been tested for
the case of temperature and strain sensors based on Fiber Bragg Gratings (FBG).
In optical biosensors waveguides are a good choice to deliver light to the area used for sensing. In traditional optical waveguides the light is confined by total internal reflection inside of a high index layer surrounded by regions of low refractive index. Since many sensing applications are based on liquids, it is necessary to guide the light within the liquid. Liquids usually have a lower refractive index than their surroundings. Hence, conventional waveguides provide only a weak interaction between light and target molecules.
In order to improve the interaction we are using a novel anti-resonant waveguide concept, in which the core region has a lower refractive index than the cladding layers. With this concept the light can be guided within the target-containing medium, thereby enabling an extended interaction length. An anti-resonant waveguide is especially compatible with a fluidic biosensor because the fluidic channel itself can be used as the core of the anti-resonant waveguide.
The light propagation and coupling mechanism of an anti-resonant waveguide is reviewed and is demonstrated with large area fluorescence excitation. By coupling the excitation light into a liquid film between two glass slides we are able to excite fluorescence within a 5 cm long channel. The measured fluorescence intensity per unit area is equal to that obtained by focusing the total excitation power onto a small spot. From analyzing the angular intensity distribution at the end facet of the waveguide we gain a better understanding of the guiding mechanism.