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

The FPI is highly sensitive to any perturbation causing a variation in the optical path length between its two reflective mirrors as presented in **Figure 9**. Due to its compact size, high sensitivity and fast response, the FPI is applied for different physical parameters sensing, biosensing, gas sensing, current and magnetic field detection etc. [23]. FP etalons based optical sensors provide an efficient label-free biosensing capability with enhanced sensitivity. The biosensing of etalon is measured in terms of absorption or phase shift subsequent to interference between the reflected light beams from the two reflecting surfaces in its cavity [24–29].

G. Allison et al. investigated an efficient FP cavity coupled surface plasmon photodiode for electrical label-free biomolecular sensing [30]. The surface plasmonic sensor was developed inside a photovoltaic cell. The information of solutions containing biomolecules was extracted from its refractive indices in the form of electronic signal generated subsequent to incident light. The resultant photocurrent was enhanced due to surface plasmon mode coupling with the FP modes inside the photovoltaic cell due to its absorbing layer. An optically transparent substrate with special ability for surface plasmon resonance (SPR) was replaced by a silicone layer of semi-transparent optical nature. With the help of this mechanism, an absorbing layers was sandwiched between a metallic layer and an optically transparent conducting electrode. Photocurrent was caused as a result of incident light due to built-in

#### **Figure 9.**

*(a) FP interferometer having reflective surfaces with reflectivity of R1 and R2, respectively. Examples of intrinsic and extrinsic FP interferometers. (b) Schematic representation of the FP based experimental setup employed for detection of gaseous biomolecules [22].*

potential of fabricated device in a similar nature to that of photovoltaic cell. The surface plasmon was excited in the metal layer and generated the photocurrent simultaneously by illuminating a thin silicon layer by a visible light with single wavelength at a resonant angle. The photocurrent was reduced drastically by surface plasmon due to disruption of the distribution of electric field in the silicone layer. The mechanism were further enhanced by the silicone layer as an optical FP cavity to produce the FP modes which were coupled with the plasmon mode. The mechanism was confirmed by the simulation of the distribution of electric field which was further confirmed experimentally by electric detection of mode and resultantly the variations in the refractive index and the protein – protein interactions were measured.

A microfluidic optical sensor integrated with FP etalon geometry was investigated for detection of concentrations of different biochemical species in solution by K. E. You et al.. The concentration information were extracted in terms of the refractive index variation with concentration with high accuracy and sensitivity. The FP cavity was fabricated from a liquid channel with two partially reflected Ag/SiO2 reflective surfaces. The refractive index dependent interference peaks were achieved in the transmission spectra subsequent to transmission of light through the fluid channel. Concentrations of different biomolecules, i.e., glucose, potassium chloride and sodium chloride were calculated from their refractive indices in terms of a shift in the position of maxima of wavelength of the interference peaks in the transmission spectra. The devised optical sensor shown a linear response with good accuracy. Sensitivities of 10<sup>3</sup> , 1.4x10<sup>3</sup> and 1.8x10<sup>3</sup> refractive indices per percent of glucose, KCl and NaCl, respectively were obtained. Schematic diagram of the investigated FP cavity based optical sensor and its response to the optically active glucose sample was shown in **Figure 10**.

#### **Figure 10.**

*Schematic diagram of the devised modified microfluidic Fabry-Perot etalon (a) 2D view, (b) simulation (c) refractive index vs. glucose concentration (%) [21].*

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

**Figure 11.**

*(a) Schematic diagram of the experimental setup for MFPI based sugar concentration sensing, (b) schematic view of the fabricated MFPI, (c) reflected energy and the refractive index of the exit medium at λ = 1538.27 nm as a function of the sugar concentration (d) measured reflected power spectra for different sugar concentrations and (e) enlarged view of the spectra [31].*

A micro Fabri-Pérot interferometer (MFPI) was designed and developed for quantitative determination of sugar in a transparent solutions by G. Chavez et al. as shown in **Figure 11(a)**. The MFPI was developed in the form of a micro bubble made of a hollow core photonic crystal fiber (PCF) as shown in **Figure 11(b)**. The cavity was fabricated by splicing of a segment of PCF to a single mode fiber (SMF) by a conventional arc fusion splicer which form an air MFPI. The fabricated MFPI then acted like an optical cavity having two reflecting surface of different refractive indices separated by a distance d. The erbium doped fiber (EDF) was illuminated by 200 mW laser diode with 980 nm wavelength with the help of wavelength division multiplexer (WDM). Output light from EDF was incident on the MFPI subsequent to passing through a three-port circulator. The reflection from MPI was guided towards optical spectrum analyzer (OSA) by port three of the circulator. The MFPI was immersed inside a sugar solution filled cuvette and the reflected interference patterns were recorded at different concentrations of sugar solution in the range 0–30.88 g/100 ml. The contrast of the interference fringes decreases with increased in the sugar concentration in solution as illustrated in **Figure 11(d)**. The predicted results from simulation were also experimentally confirmed with good agreement. The reflected optical power was directly decreased with increase in the concentration and resultant refractive index of solution with sensitivity of 0.0123 dBm/(g/100 ml) at 1538.27 nm wavelength.

J. Martini et al. also proposed a glucose concentration sensor in interstitial fluids based on a small size double-chamber FP etalon. One of the FP chamber of the proposed sensor was used as reference to overcome the effect of ambient temperature variations. The 400 μm etalon cavity was filled with water – glucose solution which had FSR of 680 pm in response to the normal incident light of wavelength 850 nm. A wavelength shift of 1 pm was produces per 1 mg/dl of the optically active analyte (glucose). The light beam from an SM vertical cavity surface emitting laser (VCSEL,

#### **Figure 12.**

*(a) Schematic diagram of the experimental setup of the double-chamber FP etalon (b) spectral position of FP modes for two different refractive indices (c) calibration curve of the refractive index with varied concentration of glucose [32].*

850 nm) was guided towards 50/50 beam splitter and was incident on one of the FP chamber subsequent to proper collimation. The perpendicular half of the beam was redirected into the other FP chamber with the help of a prism. The transmitted light signal from FP chamber were recorded by PIN photodiodes with two active segments as shown in **Figure 12**. Difference in the refractive indices of the two etalon chambers produced a phases in the transmitted optical signals. The proposed optical sensor was studied in the range 0–700 mg/dl of glucose concentration with precision of 2.5 mg/ dl. The temperature compensation was confirmed in the range 32–42°C.

#### **3.3 Sagnac interferometer**

In the Sagnac interferometer, the light beam is split to follow the same optical path but in opposite directions in the form of a closed loop. The beams get interference upon returning back to the point of entry. In the case of optical fiber Sagnac interferometer, a section of birefringent fiber is splices to the loop which causes interference between the counter propagating light beams.

A Sagnac interferometer based optical sensor system was designed, developed and demonstrated by T. Kumagai et al. for quantitative analysis of glucose in a solution. The optical rotation proportion to concentration of glucose concentration was measured in a Sagnac loop from a phase difference introduced between the clockwise

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

(CW) and counter clockwise circularly polarized light. The proposed optical sensor was composed of a Sagnac interferometer made from a polarization maintaining fiber (PMF) and other optical components to avoid the unwanted sources of noise due to different reasons. A coherent light of wavelength 840 nm from a super luminescent diode was transmitted in the sensor system. Two orthogonal mode of polarizations with a minor difference in their propagations constants were transmitted in the interferometer. The ambient sources of noise those were temperature variations and external vibrations may vary the zero level of the output signal which were controlled by fabricating the interferometer from PMF. A quarter-wave retarder was used to convert the linearly polarized light into circularly polarized light which was subsequently passed through a low birefringence span SMF or in free space. The interference signal achieved from recombination of CCW and CW lights was confirmed from the output of a preamplifier and was physically observed on an oscilloscope. The polarization measurement system (polarimeter) was checked by measuring the phase difference with the help of Faraday effect optical rotation measurement setup. In this regard, current was applied to a 1300-turn copper coil and a span fiber was a wounded around the coil. Concentration of glucose was measured from the degree of optical rotation using Biot's law as shown in Eq. (1). The phase difference analysis was performed in a I dm long measurement cell. The specific rotation of +51.6 and 91.2 were measured for glucose and fructose respectively which were close to their physical properties. To make the sensor suitable for practical applications, the active length of the measurement cell was reduced to about 2 mm and the resolution of the sensor was monitored. A trial based noninvasive measurement with human body was performed by skin webbing between fingers. The resultant interference between CCW and CW lights was investigated. The skin webbing caused a bias in the interference signal which was observed in the form of noise due to a phase difference between CCW and CW light. A resolution of 1 mg/dl was achieved for glucose concentration and 0.5 mdeg resolution of optical rotation was detected for the devised sensor (**Figure 13**).

An optical polarimetry based Sagnac interferometry technique was investigated for noninvasive glucose sensing by A. M. Winkler et al. [34]. A phase sift in the interference fringes of the Sagnac interferometer was detected in a glucose solution as a representative OAM. The proposed method was linked with the sugar detection from the aqueous humor of human eye. The interferometer was simulated such that the counter propagating beams in which one of them passed through an optically active sample caused a difference in the optical path traversing. The effect was due to a difference in the refractive indices of the left and right circularly polarized beams.

A compact PCF Sagnac interferometer based glucose sensor was introduced by G. Ann et al. [35]. A light signal from a broadband light source was launched in a 3 dB coupler and split into two beams.. A Saganac loop was mainly comprised of a polarization controller and PCF spliced with an SMF. The splitted beams counter propagated and interfered with an accumulated phase difference when passed through the PCF. The interference signal was effected greatly by the phase difference between the two orthogonal guided modes of PCF. A similar trend of phase birefringence was observed when the wavelength was varied in the range 1000–2000 nm with a gradual decrease in the maxima of the curve with increase in the glucose concentration. The nature of the devised sensor was analyzed between 15 cm to 40 cm. It was observed that the response of the sensor was highly sensitive to the PCF length. A prominent interference peaks were observed between 1050 nm to 2000 nm. The interference

#### **Figure 13.**

*(a) Configuration of optical rotation measurement system with a Sagnac interferometer. (b) Concentration dependent optical rotation of sugar samples [33].*

signatures becomes highly sensitive with good SNR for 20 cm PCF length. An average sensitivity of 0.76 mg/dL of glucose solution in water was recorded which is lower than 70 mg/dl of hypoglycemia episodes. The sensor was designed for effective sensing of glucose level in the patients with hypoglycemia.

#### **3.4 Michelson interferometer**

In this type of interferometer, a coherent light beam is split into two by a beam splitter. Each of the two beams are reflected back and recombines at the same beam splitter to get interference pattern. Although, the Michelson interferometer has good potential for the detection of OAMs but rarely applied due to its relatively complex optical arranges. L. K. Chin reported a droplet Michelson interferometer for biochemical and protein detection. The interferometer was fabricated in the form of on-chip liquid grating as schematically shown in **Figure 14(a)**. A branch of the interferometer was spared for injection of two immiscible liquids. A T-junction was developed for the formation of the liquid grating. The other branch with microchannel was filled with immersion oil which caused a phase shift due to optical path difference produced in the paths of light transmitted through the core and the cladding. A buffer solution was injected in the third branch to measure its refractive index. An optical fiber was

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

**Figure 14.**

*Schematic illustrations of (a) the droplet Michelson interferometer and (b) the physics model of Michelson interferometer (c) reflection spectra of liquid grating Michelson interferometer [36].*

aligned with one end of the microchannel for input/output light detection and the other end was coated with a gold film to use it as end mirror. The light was coupled from core to the cladding with the help of liquid grating. In the interferometer, the second and third branches of the microchannel was used to propagate the light which was reflected back by the gold layer which caused an optical path difference. An interference pattern was observed with attenuation band when both the light signals recombines in the core subsequent to passing through the liquid grating as shown in **Figure 14(b)**. The microchip was fabricated in polydimethylsiloxane (PDMS) using lithography. Sputtering technique was applied to coat the sidewall of the PDMS edge. The PDMS slab with the pattern was attached with the unpatented PDMS slab using plasma bonding. The broadband light was coupled by an optical circulator with the optofluidic chip. The reflection from the optofluidic interferometer was detected by an OSA. The immersion oil and glycerol with refractive indices 1.462 and 1.420, respectively were used as carrier and dispersed flow, respectively. Reflection from the devised interferometer for different buffers with distinct refractive indices was recorded by the spectrum analyzer as shown in **Figure 14(c)**. Two distinct attenuation peaks were observed in the overall attenuation band at slightly different positions and intensities.
