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Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 285

Quality factor
Q

V
V
M l

FIGURE 7.19 Overview of different microresonator types.

should be high enough (i.e., ≥ 1000). Additionally, the modal
volume of the cavity mode should be small to tap the full
potential of a lab-on-a-chip (LOC) using very little amounts
of analyte. For an overview, the different types of resonators
68
are listed in Fig. 7.19 together with the corresponding qual-
ity factors and modal volumes:

Waveguide-Based Sensors
The use of waveguide-based sensor systems has numerous advan-
tages over traditional concepts. 69–71 In a waveguide excitation
scheme, light needed for the analysis can be guided efficiently to the
detection zone. At the detection zone a well-defined radiation char-
acteristic without the need for sophisticated alignment procedures
is possible. Also, the integration of further functional components
such as splitters or couplers requires no additional fabrication pro-
cesses. Waveguide applications span from glass fibers to integrated
stripe or rib waveguides. Waveguide-based sensors are operated
nearly solely with a laser as light source. Main reasons are the typi-
cally high spectral intensity densities, efficient coupling, and mono-
chromatic emission spectrum. Especially, the measuring method
laser-induced fluorescence (LIF) depends on a laser light source.
This procedure uses a laser to excite a sample substance and relies
on the analysis of the resulting fluorescence signal (see Sec. 7.4.1).
Normally specific marker dyes with a characteristic emission spec-
72
trum are used to distinguish different substances or DNA sequences.
Despite the expensive and often patented marker dyes, the LIF
method is quite popular because it provides a high sensitivity with
a very low limit of detection (LOD).