Optical Biochips - Biophotonics - page 225

Fig. 12.7. Schematic illustration of an optical biosensor based on prism coupling
( left ) and typical line-sensor signals acquired with the system ( right )
12.5.2 Wavelength Tuning
The light loss or the complexity of angular adjustment of the input light can be
avoided if the wavelength of the incident light is modified instead. An e cient,
all solid-state optical biosensor results, in which the wavelength tuning of the
light source is realized by tuning the drive current of a VCSEL (vertical cavity
surface emitting laser diode) [18].
12.5.3 Grating Coupler Chirping
Another approach to avoid the mechanical adjustment of the angles of the
incident and the out-coupled light is to work with gratings whose period is
varied laterally. The use of such chirped gratings ensures that there is always
a part of the grating for which the in- or out-coupling condition (12.5) is ful-
filled. This technique has the interesting property that the lateral propagation
location of the guided wave is a direct measure of the resonance position. For
this reason, the method has also been called the “light pointer” approach to
optical biosensing [19].
A significant disadvantage of the chirped grating technique is the low e -
ciency with which the available light is used, similar to the convergent beam
approach described in Sect. 12.5.1. Only the fraction of light for which the in-
coupling condition is fulfilled can propagate in the waveguide and contributes
to the sensing signal; the largest part of the optical energy is just transmitted
through the grating.
12.6 Substrates for Optical Biochips
Although optical biochips are most often disposable devices, the optical sens-
ing techniques employed put high demands on the surface quality, the mechan-
ical stability, and the robustness of the used materials. At the same time, the
materials should be of low cost, easy to process, and of low environmental con-
cern. For these reasons, the dominant substrate materials for optical biochips