Biophotonics_Concepts_to_Applications

Measuring the difference in transmissivity through the two directions of optical
signal rotation is an alternative use of a Sagnac interferometer. In either case, the
Sagnac interferometer has a linear relationship between the output changes and the
parameter being measured. Thus, a Sagnac interferometer can be calibrated by
plotting the phase difference or transmissivity variations as a function of these
measured parameters.

7.6 Photonic Crystal Fiber Biosensors

Photonic crystalfiber-based (PCF) devices are being applied increasingly in various
types of biosensors [ 45 – 51 ]. One attractive feature is that the air holes in a PCF can
be used as natural miniature chambers for holding a liquid sample [ 3 ]. This feature
greatly reduces the required sample volume that generally is much larger in other
biosensors. The mechanism for evaluating the analyte solution inside the PCF can
be based on the use of a FBG, interferometry, mode coupling, evanescentfield
monitoring, or observation of a bandgap shift.

7.6.1 Interferometry Sensing Methods.

Onefiber sensor type that uses a PCF as the sensing element is based on intermodal
interference between forward-propagating core and cladding modes in a PCF [ 38 ,
45 ]. The basic configuration of a two-beam interference method uses the core mode
as the reference beam and the cladding modes as the sensing beam [ 3 ]. An example
of such a PCF-based interferometer is shown in Fig.7.24, which is similar to the

Laser

input

signal

2 2 optical

3-dB coupler

Splice points

Sensing

environment

CCW

To optical

signal analyzer

CW

Bidirectional

optical fiber loop

Sensing fiber

of length L

Polarization

controller

Fig. 7.23 Operation of a basic Sagnac interferometer

7.5 Interferometric Sensors 221