Application of Fiber Optics in Bio-Sensing

*Lokendra Singh, Niteshkumar Agarwal, Himnashu Barthwal, Bhupal Arya and Taresh Singh*

### **Abstract**

The unique properties of optical fibers such as small size, immunity to electromagnetic radiation, high sensitivity with simpler sensing systems have found their applications from structural monitoring to biomedical sensing. The inclusion of optical transducers, integrated electronics and new immobilization methods, the optical fibers have been used in industrial process, environmental monitoring, food processing and clinical applications. Further, the optical fiber sensing research has also been extended to the area of detection of micro-organisms such as bacteria, viruses, fungi and protozoa. The validation of optical fibers in bio-sensing applications can be observed from the growing number of publications. This chapter provides a brief picture of optical fiber biosensors, their geometries including the necessary procedure for their development. This chapter could be a milestone for the young researchers to establish their laboratory.

**Keywords:** optical fiber, biosensors, biomedical sensing, environmental monitoring, micro-organisms detection

#### **1. Introduction**

The inclusion of optical fibers in bio-sensing applications was started by two different, but interrelated discoveries, such as the laser light and optical fibers. The theoretical work of C. H. Townes and A. L. Schawlow was used by T. H. Maiman to develop the first laser. A optical signal obtained through laser is highly collimated, inherently coherent, and quasi monochromatic with the data transfer capability. The optical signal propagates in optical fiber by obeying the principle of total internal reflection (TIR) with very low losses and the first working model of optical fiber was proposed in 1965 [1]. The working model of optical fiber was put forwarded 100 years after the demonstration of concept of light. Since, then the main focus was to improve the transmission of optical signal through fibers. Nowadays, the key focus is on long distance high speed communication with low transmission losses such as 2 dB/km [2]. The unique properties of optical fibers such as immunity to electromagnetic (EM) interference and miniature footprints, the optical fiber has found niche application in sensing [3].

A schematic of conventional single mode fiber (SMF) used in the field of telecommunication is shown in **Figure 1**, consisting of three layers such as a silica core having diameter of in order of several microns ( 2–9 μm) and doped with germanium to boost up its refractive index (RI), a silica cladding of diameter of 125 μm

**Figure 1.** *Schematic of single mode optical fiber [1].*

and a coating of plastic jacket. Although, the plastic coating does not play any role in light propagation but provides the mechanical strength to the fiber. The optical fibers can be fabricated by using some other materials such as chalcogenide [4], plastic [5], and composites, with different composite materials in core and cladding. Based on the core size, operating wavelength, and RI difference of core and cladding, an optical fiber can work in the regime of single or multimode. In single mode fibers, the distribution of optical signal profile in core is Gaussian, while in multimode signal profile is more complex [2].

The optical sensors detect the variation in optical properties of propagating signal, that occurs due to the physiochemical change in targeted environment. The optical fiber based sensors classified into two categories on the basis of sensing region such as extrinsic or intrinsic sensors. The sensors directing or collecting optical signal to and from external environment are termed as extrinsic sensors [6]. The sensors in which the properties of optical signal vary within the fiber are known as intrinsic sensors [7]. In general, extrinsic sensors being used for the detection of external stimuli such as physical or biochemical parameters. The optical fiber based measurement techniques have received a great attention especially in the field of structural monitoring, railway and aerospace, chemical and biological sensing, medical diagnosis and environmental monitoring.

Since, the key application of SMF were in the field of telecommunication, and hence, fabricated in such a way that the influence of external field can be minimized on propagating signal. However, for the efficient operation of optical fiber sensors, the interaction of optical signal with external environment should need to be maximized. This can be attained by adopting different optical fiber processing schemes which frequently utilizes the interaction of leaking fields with external environment. The commonly used geometry of optical fiber in sensing applications are discussed in following subsections.

#### **1.1 Cladding less evanescent based optical fiber sensors**

The easiest way to increase the interaction of evanescent waves (EW) with external medium is removal of cladding, and a schematic of cladding less optical fiber sensor is illustrated in **Figure 2**. The changes in propagation of optical signals due to variation in external environment facilitates the EW spectroscopy [8]. The facilitation of EW spectroscopy is highly sensitive and powerful technique to quantitatively and qualitatively investigate the environment present in the vicinity of sensing region of sensor. The EW leaks from core to cladding and the distance *Application of Fiber Optics in Bio-Sensing DOI: http://dx.doi.org/10.5772/intechopen.99866*

#### **Figure 2.**

*Schematic of cladding less optical fiber sensor structure [3].*

is termed as penetration depth. The penetration depth of EW can be evaluated as [9]:

$$d\_p = \frac{\lambda}{2\pi \left(n\_{\rm eff}^2 - n\_s^2\right)^{1/2}} \tag{1}$$

where, *λ* is the propagating wavelength, ns is the RI of surrounding environment and neff is the RI of guided mode propagating in the core.

The absorption spectrum of surrounding medium attenuates the EW which hindered the propagating mode. This can be understood from Lambert–Beer Law which is given as:

$$\frac{I}{I\_0} = c^\* a^\* L \tag{2}$$

where, c is the concentration of absorption substance, *α* is the attenuation constant of EW, and L is the path length in which optical signal interacts with the surrounding medium. I0 and I are the intensities of the optical signal before or after the interaction to the external environment, respectively. The optical fiber sensor structure presented in **Figure 2** can be attained by removing the cladding part by using conventional approach such as treating the fiber with hydrofluoric (HF) acid [10]. To remove the cladding, fiber structure should need to be immersed in HF acid at constant stirring at 50 rpm. In cladding less optical fiber sensors the interaction of optical signal with surrounding can be enhanced by bending it in U-shape [11]. The U-shape bend is also useful for monitoring because source and detector will be on same side. Although, the cladding less fiber can also be attained by using other techniques such as plasma etching, but it will turn into expensive systems.

#### **1.2 Tapered optical fiber sensor**

An access to EW can also be obtained by tapering the optical fiber structure. The tapering of optical fiber usually done within the dimensions varying from

**Figure 3.** *Schematic of tapered optical fiber structure [12].*

submillimeter to several millimeters. The tapered region of the optical fiber maintains the uniform diameter with conical ends to merge it with unaltered part of optical fiber as illustrated in **Figure 3**. The tapering of fiber is done by heating the fiber structure by using flame or CO2 laser beam. The properties of tapered optical fiber sensor is based on the diameter of conical ends, diameter of tapered region, and RI of surroundings. The proportion of EW power in tapered fiber structure, increases with decrease in diameter of tapered region and with decreasing RI difference of external environment and of fiber [13]. The tapered optical fiber provides numerous advantages to the sensors such as compactness, higher sensitivity and flexibility. The tapered optical fiber classified into categories such as adiabatic and non-adiabatic. When the tapered transition region is small in such a way that maximum optical power confines within the core, then such structure are termed as adiabatic tapered fibers [13]. However, in non-adiabatic one the diameter of tapered region is less than 10 μm and the propagating modes couples into higher order modes [14]. The tapered optical fibers have been utilized in various sensing applications [15–17]. In case of tapered fiber structures, the interaction of EW with surrounding medium can be analyzed by two different.

approaches. In first approach, the attenuation of signal is to be measured which is propagating through tapered region and depends on the RI of surrounding medium [18]. In second one, the variation in surrounding medium affects the RI of modes propagating in the tapered section of fiber and works interferometrically, by using mode theory [19].

#### **1.3 Interferometers**

The optical fiber interferometers provide very high sensitivity because of their unique operational mechanism and usually known as modal interferometers (MI). In MI basically, the propagating modes splits into two modes at sensing region which are traveled in different RI regime that causes a difference in their phase and wavelength. The different properties of propagating modes lead to the interference in fundamental and higher order modes and results into a transmission spectrum with fringes. The phase of the fringes ca be given as:

$$
\rho = \frac{2\pi}{\lambda} (\text{\(\text{\(\)}}\_{\text{eff}}) L
\tag{3}
$$

where, L is the center to center distance between two modes and *λ* is the operating wavelength [20]. A SMF and thin core fiber (TCF) based Mach-Zehnder interferometer (MZI) is presented in **Figure 4**. The first strand of SMF carries a single mode which splits into two parts at TCF due to variation in core diameter. In second strand of SMF the modes from TCF gets recombined at SMF. The difference in phase of recombined modes leads to the addition or cancelation of phase at output of MZI [20].

The optical fiber based Michelson interferometers were also proposed and a schematic is illustrated in **Figure 5**. In Michelson interferometer, the core modes

**Figure 4.** *Schematic of SMF and TCF fiber based Mach-Zehnder interferometer [21].*

*Application of Fiber Optics in Bio-Sensing DOI: http://dx.doi.org/10.5772/intechopen.99866*

#### **Figure 5.**

*Schematic of optical fiber based Michelson interferometer [3].*

#### **Figure 6.**

*Schematic of optical fiber based FPI sensor [22].*

distributed into higher order modes at tapered section and after striking to gold film reflects back and recombined at the tapered section. Therefore, an interference between the modes occurs at the tapered region that causes the generation of fringes. The presence of external medium in the region separating the taper and gold films introduces the interfering features in the received signal. In similar physical length, Michelson interferometer provides higher sensitivity because the twice interaction of optical signal with sensing region. These interferometers works on the basis of measurement of wavelength or amplitude of the spectrum.

received at the output. An another type of optical fiber based interferometer is Fabry-Perot interferometer (FPI). The FPI is consisting of a cavity between two reflectors and illustrated in **Figure 6**. Alternatively, a FPI can be developed by coating a thin metallic layer at the tip of the fiber which acts as a mirror and the distance between metallic layer and surrounding medium as an another mirror. A change in the RI of cavity or its length can modulate the signal. The modulated signal will be further used to measure the targeted measurand that modulates the signal.

#### **1.4 Grating based optical fiber sensor**

An optical fiber grating is consisting of slots placed periodically with an equal proportion. The slots in optical fiber structure leads to the modulation of the propagating optical signal. The grating can be incorporated by exposing the fiber structure to the ultra-violent or femtosecond laser with desired geometry [23]. The optical fiber based grating structure were also found to be a good candidate for the sensing applications [23]. A schematic of FBG sensor with its measurement setup is illustrated in **Figure 7**. The grating structure couples the forward and backward propagating modes of the core at the particular wavelength that satisfies the Bragg condition. A Bragg grating is considered as reflector which reflects a specific wavelength band

**Figure 7.** *Schematic of measurement setup of FBG sensor [24].*

along the optical fiber and transmitted all others. The reflected Bragg wavelength is governed by a mathematical expression which can be given as [23]:

$$
\lambda\_{\text{Bragg}} = \mathcal{Q}\eta\_{\text{eff}} \tag{4}
$$

In Bragg grating based sensors, the interaction of EW with surroundings can be maintained or enhanced by modifying the fiber geometry such as tapering, etching of cladding of sensing region. Therefore, to overcome this limitation, tilted Bragg grating can be utilized in which the gratings are designed at a specific angle with respect to the axis of the core. The interaction of cladding modes with EW changes the wavelength of propagating cladding modes [25]. The interaction of EW with surrounding medium leads to the induction of inherent sensitivity to the external RI and to the nano-coatings placed over the cladding layers. While considering the fact long periodic (LPG) grating structures were come into origin. The LPG are generally created with in the length of 100 microns to 1 mm as illustrated in **Figure 8**. LPG usually couples the light form the core modes to the co-propagating modes of the structure [27]. The cladding mode suffers higher attenuation, therefore, the transmission spectrum of LPG can be analyzed by using the series of resonance bands.

From the above discussion, it can be concluded that optical fiber based sensors have wide applications in bio-sensing applications. A short summary of above discussed different geometries of optical fiber sensor structures is tabulated in **Table 1**. The tabulated form is easy enough to get a brief introduction to the required geometry of sensor.

**Figure 8.** *Schematic of NP coated LPG sensor structure [26].*


**Table 1.**

*Summary of measurand and light parameters of different sensor structures.*
