Biophotonics_Concepts_to_Applications

[Note: 1 dyne = 10−^5 N; 1 Pa (Pa) = 1 N/m^2 =10^5 dyne/10^4 cm^2 =10

dyne/cm^2 ].

For comparison purposes, atmospheric pressure at sea level is

101.325 kPa.

7.5 Interferometric Sensors

Interferometric sensors are based on measuring the phase difference between two
superimposed light beams that have the same frequency. As is shown by the generic
interferometer diagram in Fig.7.18, typically an incident light beam is split into at
least two parts by a 3-dB coupler or beamsplitter in the interferometer. These parts
follow different paths through the interferometer system and then another coupler
recombines the parts to create an interference pattern. If the optical path lengths
differ by an integer number of wavelengths then a constructive interference pattern
will be displayed. If the optical path lengths differ by an odd number of half
wavelengths then a destructive interference pattern will appear.
The optical paths could be in the same opticalfiber if two or more distin-
guishable opticalfiber modes are used. Here each mode defines one optical path.
An example is the Sagnac interferometer where the different optical paths are
defined by clockwise and counter clockwise modes. Another option is to have the
optical paths pass through separate opticalfibers. For example, this is the case in the
commonly deployed Mach-Zehnder optical fiber interferometer. This section
describes three interferometer architectures commonly used in biophotonics. These
are the Mach-Zehnder, Michelson, and Sagnac interferometers [ 32 ].

Input

optical

signal

3-dB optical

coupler or

beamsplitter

3-dB optical

coupler or

beam combiner

Output

interference

Light path 1 pattern

To optical

signal

analyzer

Light path 2

Fig. 7.18 Operational concept of an interferometer sensor

216 7 Optical Probes and Biosensors