Page 304 - Organic Electronics in Sensors and Biotechnology

Organic Semiconductor Lasers as Integrated Light Sources for Optical Sensors 281

FIGURE 7.13 Scheme of a transmission measurement.

1. Absorbance measurements
The simplest type of optical measurement is the determination
of the absorbance. Light of a certain frequency ω travels a
0
defined distance through the analyte. By detecting the fraction
of transmitted light the extinction coefficient κ of the analyte
for the frequency ω can be determined. The extinction depen-
0
dence of the frequency κ(ω) yields information about the
chemical composition of the analyte. Sensor schemes are
shown in Fig. 7.13.
For the extinction to be sufficiently high for detection, the light
traveling distance d through the analyte has to be long enough.
This can cause a problem, in particular if the analyte is a gas,
where d can be more than 100 m. In this case, one could incorpo-
rate a photonic cavity into the sensing region which causes the
light to be reflected back and forth multiple times, resulting in a
much longer efficient traveling distance. See Fig. 7.14.
The light incident on the cavity (shown symbolically as a
Bragg reflector here) has to be coherent, which means that a
laser source is required. Additionally, the cavity has to be
tuned according to the wavelength of the laser source.

FIGURE 7.14 Scheme of a resonator-enhanced transmission measurement.