**3.1 Special features**

Optical fibers are long and flexible waveguides generally made of fused silica (amorphous silicon dioxide, SiO2) the most abundant mineral found in the earth crust. Though plastic optical fibers made of polymers are also quite practical and inexpensive for some applications, our focus here is on silica fibers which are the backbone of optical communication today. Their potential as efficient sensor elements was recognized simultaneously because of the unique advantages they offer, such as:


 4 "Grating" here is used in the sense of "diffraction grating" which is usually a periodic structure having very fine parallel grooves or slits and used to produce optical spectra by diffraction of light.

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 129

waveguide effect is often explained as resulting from repeated total internal reflection (TIR) of light rays at the core–cladding interface. The fiber can then guide all light impinging the input face under certain conditions. Standard fibers have core and cladding refractive indices constant along the fiber length, but if a periodic modulation of refractive index is

The periodic variation of refractive index in the fiber core is created mostly by its exposure to Ultraviolet (UV) radiation through a proper mask. These gratings are categorized as short period or fiber Bragg gratings (FBGs) and long period gratings (LPGs). In an FBG a narrow band of wavelength is reflected and there is a corresponding drop of intensity in the transmitted spectrum while LPGs work as wavelength dependant loss elements exhibiting multiple loss resonance bands in the transmitted spectrum. Though their ability to filter certain wavelengths was a major attraction in the field of telecommunication, their potential as sensing devices was recognized simultaneously. This is because the filtered wavelength is a function of its effective refractive index as well as the period of the grating and any variation in these parameters results in shift of wavelength which can be detected easily using an optical spectrum analyzer (OSA) or an interrogator. Apart from basic FBG and LPGs there are some other distinct grating types that are formed when deliberate nonuniformity in the refractive index profile is introduced. *Blazed* or *tilted* FBGs are resulted when grating planes are at an angle with the fiber axis and are used mostly for mode conversion. A tilted grating can be designed in a way that its' core mode is coupled with some of the cladding modes so that the Bragg Wavelengh becomes a function of ambient refractive index and thus can be used for refractive index sensing. Another type is *chirped* FBG in which the grating period is aperiodic having a monotonic increase/decrease or nonuniformity longitudinally. In this type of gratings each point has different Bragg wavelength and hence its spectrum can be used to monitor a parameter profile by distributed sensing

In an FBG, the guided light is scattered by each interface of different refractive index regions in the core and for a wavelength which satisfies Bragg Condition, the scattered light adds up

Where Λ is the pitch or periodicity of the grating, *n* is the effective refractive index of the core and λB is the Bragg wavelength (Kashyap 2009, Othonos & Kallis 1999). Therefore, when light from a broadband source is launched in this FBG the spectral component defined by above equation is missing from the transmitted spectrum. Bragg wavelength is shifted if the effective refractive index, the grating periodicity or both are changed due to some perturbation; in fact both these parameters are directly influenced by strain and ambient

*<sup>B</sup>*

Where *l* is change in grating length due to strain and T is change in ambient temperature. The first term on the RHS gives strain dependence while the second term

2 [ ] 2 [ ] (2)

B=2n *[Bragg Condition]* (1)

*n*

 

*<sup>T</sup> <sup>T</sup> <sup>n</sup> <sup>T</sup>*

constructively resulting in back reflection with a central wavelength (λB) given by

*<sup>l</sup> <sup>l</sup> <sup>n</sup> <sup>l</sup>*

temperature with the associated wavelength shift given as

*n*

 

created deliberately along the fiber core it results in formation of fiber gratings.

(Kashyap, 2009).

 


Along with these, fiber gratings offer some added features unique to them like selfreferencing as the information is wavelength encoded and ease of multiplexing, facilitating distributed sensing, that make them more valuable as sensors. Their potential as strain and temperature sensors array in various concrete, metal and composite structures has been established much earlier and they have been implemented successfully for *structural health monitoring (SHM*) in the arena of civil and aerospace, oil & gas exploration wells, power systems monitoring, etc. (Rao 1997, Majumdar 2008, Tiwari 2009)

### **3.2 Relevance in biomedicine**

Because the fiber gratings are made of dielectric glass, they are inherently immune to electromagnetic interference (EMI) and can be safely used without any electrical or chemical obstruction in a clinical setup. Their miniature size, flexibility, and lightness allow easy insertion in catheters/ needles making possible localized measurements inside blood and tissues. Also, multiple sensors can be accommodated on a single fiber, working independently of the other. Silica is chemically inert and fulfills the biocompatibility criterion. (Davis 1972, Yang 2003) As fibers are intrinsically safe for the patient i.e. their use produces no immunity response from human defense system, they can be used for *in- vivo* measurements and can be left in their position for repeated or continuous monitoring.

Although all these qualities are quite exciting there are some issues coming into the way of fiber grating technology to become accepted in healthcare systems that remain to be dealt with. For example, howsoever small or non-intrusive these sensors are for the patient, their read out units need to be connected through a fiber link which is not so practical in the modern world of wireless technology. With the emergence of smaller and faster interrogators one possible way out can be the development of very small wearable interrogator system with no external fiber links. Another issue is information extraction process that needs to be standardized for each application. Fiber gratings provide information in terms of wavelength shift (given by equations 2-5 in the next section) that can be due to changes in various external parameters e.g. strain, temperature, pressure, refractive index (RI) etc. when the sensor is being used for one specific measurand (strain for example) it should be unaffected by all other parameters (temperature, RI etc.) to minimize error. This can be achieved by using optimized packaging/transducer. Also, if one sensor is being used for multiparameter sensing, it will require customized interrogation system and software tool to discriminate the effect of each parameter. Practical solutions of all these issues require multidisciplinary approach with a synergy between various experts e.g. doctors, physicists and engineers.

#### **4. Working principle**

Optical fiber consists of an inner dielectric core surrounded by cladding of another dielectric material of slightly lower refractive index than that of core. In the simplest case, the

vi. Optical fibers have very low attenuation and thus long fiber links can be used to remotely monitor and control the sensing parts without disturbing the patient while

vii. Immunity to electromagnetic/ radiofrequency interference and chemically inert nature: due to their dielectric construction, they can be used in high voltage, electrically noisy,

Along with these, fiber gratings offer some added features unique to them like selfreferencing as the information is wavelength encoded and ease of multiplexing, facilitating distributed sensing, that make them more valuable as sensors. Their potential as strain and temperature sensors array in various concrete, metal and composite structures has been established much earlier and they have been implemented successfully for *structural health monitoring (SHM*) in the arena of civil and aerospace, oil & gas exploration wells, power

Because the fiber gratings are made of dielectric glass, they are inherently immune to electromagnetic interference (EMI) and can be safely used without any electrical or chemical obstruction in a clinical setup. Their miniature size, flexibility, and lightness allow easy insertion in catheters/ needles making possible localized measurements inside blood and tissues. Also, multiple sensors can be accommodated on a single fiber, working independently of the other. Silica is chemically inert and fulfills the biocompatibility criterion. (Davis 1972, Yang 2003) As fibers are intrinsically safe for the patient i.e. their use produces no immunity response from human defense system, they can be used for *in- vivo* measurements and can be left in their position for repeated or continuous monitoring.

Although all these qualities are quite exciting there are some issues coming into the way of fiber grating technology to become accepted in healthcare systems that remain to be dealt with. For example, howsoever small or non-intrusive these sensors are for the patient, their read out units need to be connected through a fiber link which is not so practical in the modern world of wireless technology. With the emergence of smaller and faster interrogators one possible way out can be the development of very small wearable interrogator system with no external fiber links. Another issue is information extraction process that needs to be standardized for each application. Fiber gratings provide information in terms of wavelength shift (given by equations 2-5 in the next section) that can be due to changes in various external parameters e.g. strain, temperature, pressure, refractive index (RI) etc. when the sensor is being used for one specific measurand (strain for example) it should be unaffected by all other parameters (temperature, RI etc.) to minimize error. This can be achieved by using optimized packaging/transducer. Also, if one sensor is being used for multiparameter sensing, it will require customized interrogation system and software tool to discriminate the effect of each parameter. Practical solutions of all these issues require multidisciplinary approach with a

Optical fiber consists of an inner dielectric core surrounded by cladding of another dielectric material of slightly lower refractive index than that of core. In the simplest case, the

high magnetic field, high temperature, corrosive, or other harsh environments

systems monitoring, etc. (Rao 1997, Majumdar 2008, Tiwari 2009)

synergy between various experts e.g. doctors, physicists and engineers.

keeping all the electronics away.

**3.2 Relevance in biomedicine** 

**4. Working principle** 

waveguide effect is often explained as resulting from repeated total internal reflection (TIR) of light rays at the core–cladding interface. The fiber can then guide all light impinging the input face under certain conditions. Standard fibers have core and cladding refractive indices constant along the fiber length, but if a periodic modulation of refractive index is created deliberately along the fiber core it results in formation of fiber gratings.

The periodic variation of refractive index in the fiber core is created mostly by its exposure to Ultraviolet (UV) radiation through a proper mask. These gratings are categorized as short period or fiber Bragg gratings (FBGs) and long period gratings (LPGs). In an FBG a narrow band of wavelength is reflected and there is a corresponding drop of intensity in the transmitted spectrum while LPGs work as wavelength dependant loss elements exhibiting multiple loss resonance bands in the transmitted spectrum. Though their ability to filter certain wavelengths was a major attraction in the field of telecommunication, their potential as sensing devices was recognized simultaneously. This is because the filtered wavelength is a function of its effective refractive index as well as the period of the grating and any variation in these parameters results in shift of wavelength which can be detected easily using an optical spectrum analyzer (OSA) or an interrogator. Apart from basic FBG and LPGs there are some other distinct grating types that are formed when deliberate nonuniformity in the refractive index profile is introduced. *Blazed* or *tilted* FBGs are resulted when grating planes are at an angle with the fiber axis and are used mostly for mode conversion. A tilted grating can be designed in a way that its' core mode is coupled with some of the cladding modes so that the Bragg Wavelengh becomes a function of ambient refractive index and thus can be used for refractive index sensing. Another type is *chirped* FBG in which the grating period is aperiodic having a monotonic increase/decrease or nonuniformity longitudinally. In this type of gratings each point has different Bragg wavelength and hence its spectrum can be used to monitor a parameter profile by distributed sensing (Kashyap, 2009).

In an FBG, the guided light is scattered by each interface of different refractive index regions in the core and for a wavelength which satisfies Bragg Condition, the scattered light adds up constructively resulting in back reflection with a central wavelength (λB) given by

$$\lambda\_{\mathsf{B}} = 2\mathsf{n}\Lambda \text{ [Bragg Condition]} \tag{1}$$

Where Λ is the pitch or periodicity of the grating, *n* is the effective refractive index of the core and λB is the Bragg wavelength (Kashyap 2009, Othonos & Kallis 1999). Therefore, when light from a broadband source is launched in this FBG the spectral component defined by above equation is missing from the transmitted spectrum. Bragg wavelength is shifted if the effective refractive index, the grating periodicity or both are changed due to some perturbation; in fact both these parameters are directly influenced by strain and ambient temperature with the associated wavelength shift given as

$$
\Delta\mathcal{X}\_{B} = \mathcal{Z} \left[ \Lambda \begin{array}{c} \hat{\mathcal{O}}n \\ \hat{\mathcal{O}}l \end{array} + \text{ } n \begin{array}{c} \hat{\mathcal{O}}\Lambda \\ \hat{\mathcal{O}}l \end{array} \right] \Lambda l + \text{ } \mathcal{Z} \left[ \Lambda \begin{array}{c} \hat{\mathcal{O}}n \\ \hat{\mathcal{O}}T \end{array} + \text{ } n \begin{array}{c} \hat{\mathcal{O}}\Lambda \\ \hat{\mathcal{O}}T \end{array} \right] \Lambda T \tag{2}
$$

Where *l* is change in grating length due to strain and T is change in ambient temperature. The first term on the RHS gives strain dependence while the second term

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 131

In our laboratory, fiber gratings are produced by exposing core of a photosensitive fiber (StockerYale Inc.) to intense UV light from a KrF (Krypton Fluoride) excimer laser at 248nm wavelength. The primary acrylate coating is first removed from the fiber section that is to be exposed to UV light, using a thermo-mechanical stripper to avoid mechanical degradation. This uncoated fiber is further loaded onto the fiber mounting stage and an appropriate tension is applied to keep it straight, using load cell. After that the fiber is first made to approach close proximity of the mask within 0. 5mm and vertical & horizontal alignments are performed by continuously monitoring on a computer screen. To inscribe FBGs the light from the KrF laser is made incident on the fiber through a phase mask of a period as per the design of the FBG being inscribed and scans on the required length of the fiber to get the FBG inscription of the designed parameters e.g. (a typical value of 1060 nm periodicity phase mask is used in our lab to achieve the peak wavelength in the range of ~1550 nm). To achieve the FBG with more than 90% reflectivity, 7-8 scans were applied. For LPG fabrication point-by point method is used to expose fiber core to laser radiation. To monitor the FBG parameter within the appropriate design, optical spectrum analyzer with broadband ASE (**Amplified Stimulated Emission)** source is used while for LPGs white light

source is employed. Figure 2 shows the experimental setup for grating fabrication.

To protect the FBG from external environment and to provide mechanical strength, the stripped fiber section is immediately re-coated with acrylate while LPGs are left uncoated so that they remain sensitive to external refractive index. Care should be taken to keep the bare LPGs well protected; they should be either fixed in a working glass cell or kept properly covered when not in use. Finally, for stabilization of grating properties over long period of time, thermal annealing of inscribed gratings at high temperature (>150 °C) is carried out.

The multiplexing, multi-parameter and minimally invasive sensing capabilities even in high electric/magnetic field environments of fiber grating sensors make them befitting for

Fig. 2. Grating fabrication set up

**6. Current research scenario** 

indicates temperature dependence of the Bragg wavelength and an FBG sensor works by monitoring Bragg wavelength shift with one or both of these parameters.

LPGs couple fundamental guided core mode to different cladding modes. The loss resonance wavelength(s) at which this coupling takes place satisfies the phase matching condition i.e.

$$\mathcal{A}\_{i} = \left[ m\_{\rm eff}^{\alpha \alpha} - m\_{\rm ieff}^{cl} \right] \mathcal{A} \tag{3}$$

Where *co neff* and *cl nieff* are the effective refractive indices of the fundamental core mode and ith cladding mode respectively and Λ is the period of the LPG. Since effective index of a cladding mode is dependent upon the refractive index of the surrounding medium, any change in the latter alters the loss resonance wavelength(s). The influence of refractive index of the surrounding medium on the LPG wavelength(s) is expressed by the following equation,

$$\frac{d\mathcal{X}\_{\circ}}{d\boldsymbol{m}\_{\circ\circ}} = \frac{d\mathcal{X}\_{\circ}}{d\boldsymbol{m}\_{\circ\circ}} \cdot \frac{d\boldsymbol{m}\_{\circ\circ}^{\circ}}{d\boldsymbol{m}\_{\circ\circ}} \tag{4}$$

Where, *sur n* is the refractive index of the surrounding medium.

Apart from the shift in loss resonance wavelengths, the variation in *n*sur is also reflected as variation in intensity of the loss resonance peak, defined by the overlap integral *I* given by equation (5),

$$\mathbf{I} = \frac{\iint \boldsymbol{\upmu\_{core}} \boldsymbol{\upmu\_{clad}^\*} r dr d\phi}{\sqrt{\iint \boldsymbol{\upmu\_{core}} \boldsymbol{\upmu\_{core}^\*} r dr d\phi} \sqrt{\iint \boldsymbol{\upmu\_{clad}} \boldsymbol{\upmu\_{clad}^\*} r dr d\phi}} \tag{5}$$

Where, s are the electromagnetic field components of the two coupling modes, r and *Φ* are radial and azimuthal co-ordinates respectively (Mishra et al 2005). Obviously, any change in cladding field distribution will affect the coupling strength. When the refractive index of the surrounding is varied, it will alter the cladding field distribution and hence the overlap integral. Since effective refractive index of a cladding mode and their coupling efficiency with the fundamental mode is dependent upon the refractive index of the surrounding medium, any change in the latter is easily detected using LPGs. This is the basic principle of an LPG based refractive index and concentration sensor. (James 2003, Patrick & Kersey 1998)

#### **5. Fabrication technology**

Direct inscription of submicron periodic pattern in optical fibers puts severe constraints in terms of stability and precision on the grating fabrication techniques. There are only a few methods, namely, the *interferometric*, the *phase mask*, and the *point-by-point* techniques that give consistently good quality gratings. (Othonos 1999)

indicates temperature dependence of the Bragg wavelength and an FBG sensor works by

LPGs couple fundamental guided core mode to different cladding modes. The loss resonance wavelength(s) at which this coupling takes place satisfies the phase matching

[ ] *co cl*

Where *co neff* and *cl nieff* are the effective refractive indices of the fundamental core mode and ith cladding mode respectively and Λ is the period of the LPG. Since effective index of a cladding mode is dependent upon the refractive index of the surrounding medium, any change in the latter alters the loss resonance wavelength(s). The influence of refractive index of the surrounding medium on the LPG wavelength(s) is expressed by the following

*dn dn*

*r*

s are the electromagnetic field components of the two coupling modes, r and *Φ* are

\* \*

*clad clad*

<sup>I</sup> (5)

*rdrd rdrd*

*rdrd*

*sur*

(4)

*cl ieff*

*dn d*

Apart from the shift in loss resonance wavelengths, the variation in *n*sur is also reflected as variation in intensity of the loss resonance peak, defined by the overlap integral *I* given by

*core clad*

 \*

radial and azimuthal co-ordinates respectively (Mishra et al 2005). Obviously, any change in cladding field distribution will affect the coupling strength. When the refractive index of the surrounding is varied, it will alter the cladding field distribution and hence the overlap integral. Since effective refractive index of a cladding mode and their coupling efficiency with the fundamental mode is dependent upon the refractive index of the surrounding medium, any change in the latter is easily detected using LPGs. This is the basic principle of an LPG based refractive index and concentration sensor. (James 2003, Patrick & Kersey

Direct inscription of submicron periodic pattern in optical fibers puts severe constraints in terms of stability and precision on the grating fabrication techniques. There are only a few methods, namely, the *interferometric*, the *phase mask*, and the *point-by-point* techniques that

*<sup>i</sup>* .

*cl ieff i*

*dn d*

Where, *sur n* is the refractive index of the surrounding medium.

give consistently good quality gratings. (Othonos 1999)

*r*

 *sur*

*core core*

 *r*

 (3)

*i eff ieff n n*

monitoring Bragg wavelength shift with one or both of these parameters.

condition i.e.

equation,

equation (5),

Where,

1998)

**5. Fabrication technology** 

In our laboratory, fiber gratings are produced by exposing core of a photosensitive fiber (StockerYale Inc.) to intense UV light from a KrF (Krypton Fluoride) excimer laser at 248nm wavelength. The primary acrylate coating is first removed from the fiber section that is to be exposed to UV light, using a thermo-mechanical stripper to avoid mechanical degradation. This uncoated fiber is further loaded onto the fiber mounting stage and an appropriate tension is applied to keep it straight, using load cell. After that the fiber is first made to approach close proximity of the mask within 0. 5mm and vertical & horizontal alignments are performed by continuously monitoring on a computer screen. To inscribe FBGs the light from the KrF laser is made incident on the fiber through a phase mask of a period as per the design of the FBG being inscribed and scans on the required length of the fiber to get the FBG inscription of the designed parameters e.g. (a typical value of 1060 nm periodicity phase mask is used in our lab to achieve the peak wavelength in the range of ~1550 nm). To achieve the FBG with more than 90% reflectivity, 7-8 scans were applied. For LPG fabrication point-by point method is used to expose fiber core to laser radiation. To monitor the FBG parameter within the appropriate design, optical spectrum analyzer with broadband ASE (**Amplified Stimulated Emission)** source is used while for LPGs white light source is employed. Figure 2 shows the experimental setup for grating fabrication.

Fig. 2. Grating fabrication set up

To protect the FBG from external environment and to provide mechanical strength, the stripped fiber section is immediately re-coated with acrylate while LPGs are left uncoated so that they remain sensitive to external refractive index. Care should be taken to keep the bare LPGs well protected; they should be either fixed in a working glass cell or kept properly covered when not in use. Finally, for stabilization of grating properties over long period of time, thermal annealing of inscribed gratings at high temperature (>150 °C) is carried out.
