**3. Some selected interdisciplinary examples**

In this section, some selected applications/techniques are highlighted as examples. Once again, this selection is not exhaustive, but representative of interesting emerging techniques.

Spatial and temporal resolutions are at the heart of modern-day research questions. A relatively recent technique based on the phase of fluorescence signals, named as fluorescence interferometry, has emerged over the last few years where meso and nanoscale resolutions were achieved in biological specimens [9]. The method is fully optical and simple in terms of instrumentation. Typically, the setup consists of excitation, lens pairs for scanning, and detection units. The self-referencing interferometer collects fluorescence from the emission from both sides of the specimen, when the specimen is being translated along the optical axis. An interference pattern is created when the coherence conditions are met from the depth of the sample, thus generating a spatial map. Considering the challenges of noninvasive clinical procedures, including but not limited to precision and cost of potential automation in medical fields, fluorescence interferometry could open up new horizons in imaging/sending applications.

High time and space resolution at real time in ambient conditions is challenging. Achieving the same under extreme conditions is understandably more challenging, since the detector of light gets destroyed in the process [7]. OI using a laser beam and Doppler-shifted light forms a branch known as "laser interferometry". Examples include popular and established methods such as VISAR, ORVIS, and stress gauges [6]. One of the novel applications of such systems is in shock compression studies, where finding velocities of free surface and embedded layers have been extremely important [7]. During a shock compression process, with a powerful

#### *Introductory Chapter: Optical Interferometry in Interdisciplinary Applications DOI: http://dx.doi.org/10.5772/intechopen.108687*

shockwave progressing along the depth of a sample at ~ km/s velocities, it is critical to know these parameters inside the specimen. There are several aspects to it. First, as described above, the specimen cannot host the detector "device", since it would be destroyed after one single-stage shockwave experiment. Second, the geometry, opacity, and nature of shock response vary widely from one sample system to another, making it very difficult to design and implement the technique. A solution to those challenges was to keep the detecting device outside of the specimen, and to vary the laser wavelength according to the opacity of the material, as governed by the transmission spectra. In many materials (solids and liquids) this works due to their transparency at telecommunication wavelengths such as 1550 nm. Moreover, the already-established technology at those wavelengths makes it easy to overcome engineering challenges, especially related to signal-to-noise ratios. This technique, known as photon Doppler velocimetry (PDV), uses a single-mode laser beam, which gets split into two. One of them is then focused on the surface whose velocity is being measured. For example, in case of a projectile moving in free space, the beam would be focused on the nearest surface of it, incident normally. This beam can be reflected easily if the surface is polished or coated with metal films. The second beam from the beam splitter is the reference, with which the reflected beam is superposed for creating the interference pattern. By counting the beat frequency of the pattern once can generate a velocity history of the moving surface. The same technique can be applied to any surface inside the specimen if one is interested to know the evolution of shockwave amplitude. In this case, a thin layer (~ 30 nm) of the highly reflective surface is created at the surface from where the shockwave would be detected. By repeating the experiment with samples of different thicknesses, a complete picture of the shockwave propagation could be created. Shockwave propagation has many applications if the spatial and temporal resolution is high, for example, when it is approximately in the ns range. Early chemistry and photo-physics, mechano-optical manifestations of stress-related events, and impedance changes are only a few example applications [14–16].

Another notable application area of OI is interferometer-based sensors. Highly versatile, compact and sensitive, these sensors use to extract a wide variety of information such as temperature and refractive index changes in materials. In most common setups, Fabry-Perot systems, cascaded structures, parallel interferometric structures, fiber Bragg grating incorporated interferometer, and a Mach-Zehnder system with fiber-optic components could be found [17]. This class of interferometers is particularly effective in complementary studies to static and dynamic pressure experiments due to their sensitivity to strain-related effects in the matter. It is possible to extract information on temperature and strain (or effects) simultaneously from such measurements, and often more complex applications are possible with creatively hybridizing this technique with other related OI techniques (such as combining a Fabry-Perot with a Mach-Zehnder system).

Small sample detection, specifically related to biosensing, is another major area based on optical sensing applications where OI has contributed significantly. Traces of chemicals as small as micro or picolitres could be sensed using a technique where microfluidic devices are used in combination with OI [18–20]. This technique has enabled real-time monitoring of reaction systems and product formations, useful for novel biochemical applications in the microscopic scale. A related more application has used a ring cavity ultrafast laser to achieve small sample detection. This system employed a fiber optic Michelson interferometer in tandem with optofluidic devices to achieve label-free hybridization and sensing of DNAs with high precision [21].

The above examples are only representative applications where OI was used in overlapping areas of scientific research. A vast pool of examples could be easily found in literature where uses of OI in physics and astronomy, telecommunications, holography, navigation systems, atomic clocks, and other time/frequency domain applications could be found. Apart from the biosensing applications mentioned above, techniques such as phase contrast imaging based on differential interferometry have been developed.
