**3. Interferometry for OAMs detection**

#### **3.1 Mach-Zehnder interferometer**

The Mach-Zehnder interferometer is most commonly used for sensing applications and was first introduced by Ludwig Zehnder in 1891 and Ludwig Mach in 1892 independently. In this interferometer, a coherent light beam is split into two using beam splitter and then recombined on another beam splitter with the help of two mirrors to obtain interference pattern. One part of the splitted beam is called reference while other one is called sensing arm. The chiral sample is kept in the sensing arm of the interferometer and its effect on the contrast of the interference fringes is detected. The interference fringes are normally recorded by a camera and analyzed by image processing techniques. MZI has good potential for the detection of OAMs therefore, most of the time exercised in the literature for this purpose.

Calixto et al. proposed a Mach-Zehnder interferometer (MZI) based wavefront division polarimeter for the measurements of chiral solute concentrations in solutions as shown in **Figure 2**. An optically active solution was kept in a sample chamber in the sensing arm of a MZI. One beam of the polarized light was passed through a liquid sample containing dissolved OAM while the other half of the light was used as reference beam. The plane of polarization of the incident linearly polarized light was rotated when the sample beam propagated through the chiral solutions. As a result, a decrease in the visibility of the interference pattern was observed with increase in the concentration of the OAMs in the solution. Contrast of the interference fringe pattern was maximum when both the sample and reference beams presented a polarization perpendicular to the plane of incidence. However, the visibility of the fringe pattern was deteriorated fully when the polarization of the sample beam is oriented parallel to the plane of incidence. The effect of decrease in the fringes visibility was mainly due to the increase in the refractive index of the solution with increase in the concentration of the OAMs as shown in **Figure 3**. The following calibration equation was obtained for variation of the visibility of the fringe pattern and concentration of the fructose solution:

$$V(c) = 0.643 - 0.31c^2 + 0.048 \, c^4 \tag{19}$$

#### **Figure 2.**

*Schematic diagram of the Mach-Zehnder interferometer and interference fringes pattern recording system [14].*

Where, 'V' is the visibility of the interference fringe pattern and 'c' is the concentration of the chiral materials (**Figure 3**).

H.A. Razak et al. proposed an optical sensor based on fiber optic MZI for food composition detection as depicted in **Figure 4**. The MZI structure was employed as fiber optic sensor in single mode-multimode-single mode (SMS) structural configurations using fusion splicing technique. The interferometer was investigated with 4 cm and 8 cm sensing regions. The sensor response was tested for detection of water, sugar and oil from their respective refractive indices as representative major components of food. A red-shift was seen in the wavelength for increase in the refractive index of the constituent sample. The sensitivity of the sensor was found to be directly dependent on the length of the sensing region.

A miniature broad-band (BB) MZI was proposed by M. Kitsara et al. for the detection of label-free biochemical OAMs sensing as shown in **Figure 5**. A theoretical investigation was performed on Si-based MZI with BB input lights in the range 450 nm – 750 nm. They have proved that BB-MZI can be used as a miniaturized optical sensor with enhanced sensitivity, versatile biochemical sensing applications and economical fabrication and operating costs compared to its counterpart single wavelength (SW) MZI. Glucose was used as a representative biomolecular entity because of its relatively small size to demonstrate the designed BB-MZI to detect its concentration in a very diluted solutions with a higher efficiency. The phase changes of the evanescent field at Nd:YAG (532 nm) and He-Ne (633 nm) lasers were studied to evident that an optical setup could be designed where the source of light and MZI chip vary according to application. The theoretical transmission spectra of BB-MZI as a function of the refractive index of the solution were reported for 10 mM, 25 mM, 50 mM and 100 mM glucose concentrations. The recorded peak shifted with concentration of glucose. The performance of the BB-MZI was also observed with a hypothetical protein adlayer over the sensing arm of length 300 μm. An ultra-thin protein adlayer was sensed with recording of the spectral changes or by observing the variation in the integrated intensity. The potential of the designed interferometer was investigated for biomedical applications. It was observed that the performance of the BB-MZI is comparable to the SW-MZI without a requirement of a costly laser system as an input light source.

*A Review of Optical Interferometry Techniques for Quantitative Determination of Optically… DOI: http://dx.doi.org/10.5772/intechopen.104937*

**Figure 3.**

*The effect of concentration on the visibilities of fringes pattern by solutions of (a) fructose and (b) glucose [14].*

A. Psarouli et al. also investigated a monolithically integrated BB-MZI for label free detection of biomolecules with high sensitivity as shown in **Figure 6**. A transducer based on monolithic silicon microphotonic was developed for this purpose. The MZI was fabricated from monomodal silicon nitride waveguides with silicone light emitting diodes (LEDs). BB light was injected into the interferometer setup and were sinusoidal modulated by optically active biomolecules with two different frequencies of the polarization before exiting the sensor. The distinct reporting of the two polarizations and simultaneous investigation of the TE and TM signals were performed by

#### **Figure 4.**

*(a) Basic block diagram of optical fiber MZ interferometer. (b) Schematic representation of SMS structure. (c) Schematic diagram of liquid concentration detector [15].*

#### **Figure 5.**

*Schematic diagram of the prosed BB-MZI for detection of label-free biochemical chiral materials sensing [16].*

deconvolution in the Fourier transform. The quantitative determination of the binding adlyaers were made possible from their refractive indices by dual polarization analysis over the broad spectral range. The sensor was equipped with power and control electronics, a docking station, an off-chip fluidic circuit, a miniature spectrometer and an optical module. The set of ten interferometers were interrogated with a defined time delay by integrated LEDs which were operated by control electronics. The proposed interferometric sensor was found 60 and 550 times more sensitive than a two-lateral-mode spiral waveguide MZI [18] and polyimide-waveguide MZI [19], respectively.

An asymmetric Mach-Zehnder interferometer (aMZI) was introduced by M. J. Goodwin et al. for interferometric biosensing applications [11]. The device was manufactured using TriPleX technology. The interferometer was fabricated on a chip consist of Si3N4 waveguide with silica cladding which made a photonic integrated circuit (PIC). A sensing window was fabricated by locally removing the SiO2 cladding which given a provision to analyte to make a contact with the waveguide. In the

*A Review of Optical Interferometry Techniques for Quantitative Determination of Optically… DOI: http://dx.doi.org/10.5772/intechopen.104937*

#### **Figure 6.**

*(a) Schematic of the biochip showing the monolithic integration of the avalanche-type LED, the MZI, and the silicon nitride rib waveguide. (b) Layout of the 10 MZIs showing the MZI routes as well as the LED positions and metal [17].*

proposed design, the incoming light was split into two arms by the waveguide. One of the arm was exposed to the analyte and the other arm was used as reference. The interference pattern was detected on at the point of recombination of the two arms due to the deliberately induced asymmetry. Subsequent to interaction of the analyte with the evanescent field of the waveguide at the sensing window, a phase shift was introduced due to variation in the refractive index. Performance of the proposed sensor platform in terms of signal-to-noise-ratio (SNR) and absolute response is compared with the commercial quartz crystal microbalance with dissipation (QCM-D). The aMZI proved itself dominated over the QCM-D due to measurement capability streptavidin binding with no need of the added complication of hydrodynamically coupled water which allow the elucidation of absolute protein adsorption. Also the aMZI presented 200 times good SNR and therefore offered a relatively lower limit of detection.

The operation of a versatile and sensitive integrated optical MZI biosensor with three-guide coupler at the output was demonstrated by B. J. Luff et al. as shown in **Figure 7**. The interferometric devices were designed by Ag+ –Na+ ionexchange in glass substrates. The chemical modification of the waveguide surface of the interferometer made possible the detection of the biochemical species. The waveguide were designed in BGG36 glass with refractive index about 1.6 at 786 nm by Ag+ –Na<sup>+</sup> ion-exchange. Photolithographically patterned Ti film was used as the masking material with

#### **Figure 7.**

*(a) Schematic diagram of the biosensor device configuration based on MZI. (b) MZI with three-waveguide coupler [20].*

opening width of 1 μm. The fabricated device was characterized by different concentrations of sucrose solutions to vary the superstrate index. For building up multilayers of protein over the sample surface, a high affinity interaction between vitamin biotin and protein streptavidin base system was used. The refractive index and thickness of the protein multilayer system was calculated reproducibly based on waveguide model.

#### **3.2 Fabry–Pérot interferometer**

Fabry–Pérot interferometer (FPI) also called etalon is based on an optical cavity made from two parallel reflecting surfaces. The interferometer is named after Charles Fabry and Alfred Perot for their invention in 1899. The parallel reflecting surfaces of the FP cavity is separated by a distance 'd' which allows the transmission of infinite number of parallel to each other as shown in **Figure 8**. A sharp constructive interference can be recorded when these parallel beam superimposed with each other. Free spectral range (FSR) analysis can be used to calculate the separation between the reflective ends of the cavity from refractive index information of a known medium. The spacing between the two reflective surfaces can be calculated from the FSR from refractive index of a known medium, as follow;

$$\text{FSR} = \Delta \mathfrak{v} = \mathfrak{v}\_{\mathfrak{m}+1} - \mathfrak{v}\_{\mathfrak{m}} = \frac{\mathfrak{c}}{2 \text{nd} \text{cos}\mathfrak{o}} \tag{20}$$

Where, υ is the frequency at which transmission of maximum intensity occurs, m determines an integral order of transmission peaks, θ is the angle of maximum transmission, d is the separation between the end reflective cavity surfaces and n is the index of refraction of a specific medium. FP interferometers are highly sensitive for the detection of OAMs therefore, mostly applied by the researchers in the literature.

#### **Figure 8.** *Schematic diagram of the principle of the Fabry-Perot interferometer etalon. Basic structure of cavity [21].*
