Introductory Chapter: Optical Interferometry in Interdisciplinary Applications

1. Introduction

Optical interferometry (OI) involves the superposition of electromagnetic waves to form interference fringes where tiny changes in optical path difference cause significant variation in the intensity pattern of the detected light. The precision and flexibility associated with modern-day interferometric techniques have enabled researchers to apply them to a wide range of applications for solving problems related to science and engineering [1, 2, 3, 4, 5, 6, 7]. In the recent years, applications have become more sophisticated. For example, optical interferometric methods are now useful for detecting early chemistry, to analyze 3D surface topography, in biosensing, and in several topics related to investigate the matter under extreme conditions [6, 7, 8, 9, 10]. The use of OI in astronomy should be mentioned separately due to the volume of research that has been reported over the last few decades [11]. In this chapter, a brief overview on interferometric applications will be presented. The focus will be on the most recent advances from several fronts of science and engineering to note the width of applications where OI techniques have been relevant. The examples are by no means exhaustive due to the limited scope of this chapter. The mentions here serve only pointers for future discussions or an extended review on the state-of-the-art of OI techniques.

In most simple terms, in OI, a coherent light beam is split into two using a beam splitter. The two beams cover different trajectories, commonly known as “paths” in optics before superposing to create interference fringes. The fringes or intensity patterns depend on phase differences between the two beams, resulting from the different lengths of the paths taken by them. Classification of interferometers could be done in many ways, depending on the focus and purpose. If the carrier frequency is of importance, two major groups named homodyne and heterodyne could be mentioned based on if the frequency has changed in the process.

Likewise, interferometers could be classified into major groups. Most OI devices that are being discussed here are double-path interferometers, where the signal and reference beams travel along distinct paths before recombining. A common path interferometer is, as their name suggests, example where the two beams travel the same paths.

2. Typical applications of OI in science and engineering

Interferometry resulting from electromagnetic waves has been used in many routine applications. One of the most popular methods in OI involves the superposition of Doppler-shifted laser beams. For simplicity and convenience of discussion, applications of interferometry could be divided into three major areas of science and engineering research. Historically physics and astronomy have used interferometry in many ways and could be taken as the first major application area. The second group of applications could be grouped under the engineering and applied science header. Finally, applications focused on research problems related to biology makes the third major area. Of course, this oversimplified classification is insensitive to some nuances of interdisciplinary works and serves only an ad hoc purpose.

These techniques found extensive relevance in investigations where probing small displacements, refractive index changes, surface irregularities, and remote analysis of optical signals are needed [11]. The progress of technology has played a critical role in shaping the advancement of these techniques. Even the “successful” demonstration of the famous negative result in physics from the 19th century, named after Michelson and Morley, found extraordinary level of precision with the help of modern technologies [12]. Interferometers have typically been used in the scientific industry for manufacturing and characterization of surfaces, integrated circuits, multiplexing methods needed for applications in telecommunications, in navigation systems, and to aid spectroscopy for materials characterization [13].

Extremely active and rapidly progressing, the typical applications provide the foundations for many interdisciplinary applications of OI techniques. These examples, noted in Section 3, are the most awe-inspiring results of modern-day scientific research.

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 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, 15, 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, 19, 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.

4. Summary

OI has been a strong application tool in almost every discipline of science and engineering. As devices based fundamentally on the superposition of light and all-optical, noninvasive signal processing, these methods have remarkable flexibility and relevance. In this chapter, an overview of the most recent applications has been presented. While typical applications in science and engineering research have seen remarkable updates in the past few years, some novel applications have also emerged as a result of collaborative efforts in cross-disciplinary projects.

Acknowledgments

The author is thankful for the resources provided by the Miami University Regionals for conducting research.

Conflict of interest

The author declares no conflict of interest.

Notes/thanks/other declarations

None.