**1. Introduction**

**Optical fiber** is a flexible, transparent *waveguide* or "*light pipe*" to transmit light; the latter, in its various forms and facets has caught the attention of humanity since prehistoric times. The ancient civilization used it as fire signals to communicate and later as a therapeutic and preventive tool for better health. In the modern era, the idea that light can be used for communication combined with the phenomenon of total internal reflection, gave rise to the concept of medium for light transmission. As a consequence, by the end of 19th century glass rods as illuminators were realized. Optical fibers were the next step as they are basically glass rods stretched very thin to become long and flexible. Gradual technological advances from 1920s when use of fiber for light transmission was first proposed, to 1980s resulted in glass fibers as the most ideal communication medium for enormous amount of data with lowest possible attenuation. Their inherent properties such as small size (have standard thickness of ~0.250 mm that can be less than that of surgical suture), biocompatibility, non-toxicity, chemical inertness and remote monitoring capability, make them quite lucrative for usage in the biomedical area. These fibers thus have diverse applications ranging from illumination to imaging, from phototherapy to precise surgery, from monitoring complex biomechanical dynamics to wearable smart sensors. In fact, after their first practical application in flexible endoscopes reported by Basil Hirschowitz in 1957 for illuminating and imaging internal organs of human body, the optical fibers have come a long way as sensors for various physiological parameters as well. This book chapter describes a special type of fiber optic tool, called fiber grating, its unique features with reference to potential applications in the field of biomedicine not only as in-fiber devices but also as sensing elements. (Mishra, 2011)

## **2. History**

Although glass fibers as endoscope were being used for medical applications since 1950s, they had very limited applications because of their very high power loss (~1000 dB/km) and non availability of a compatible light source. The solution for the second problem came with the first Laser fabrication in 1960 by Maiman that had the potential to be ideal light source

1999)

core.

**3. Fiber gratings** 

**3.1 Special features** 

rotation, etc.)

in close vicinity e.g. a catheter.

Optical Fiber Gratings in Perspective of Their Applications in Biomedicine 127

there had been explosion of research activity in this field. (Kashyap, 2009, Othonos & Kallis,

Fiber 4gratings are one of the simple intrinsic fiber devices that can reflect, filter or disperse light passing through them, suitable not only for communication applications but also finding their foothold as a fascinating sensor element in diverse areas. Figure 1 shows the schematic representation of a typical single mode fiber with fiber grating inscribed in its

**Input Spectrum Output Spectrum**

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

i. Small size and geometric versatility: with diameter in the range of 125 -500 micrometers, fiber sensors can be configured in arbitrary shapes and offer a great choice

ii. Common technology base for multiparameter sensing: a single fiber can be used to sense various physical perturbations (acoustic, magnetic, temperature, strain, pressure,

iv. Absence of crosstalk between close fibers suggests that different sensors can be housed

v. A single electro-optic unit can be utilized for all the sensors, naturally with an

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.

simultaneously because of the unique advantages they offer, such as:

in space-restricted or hard to reach environments.

iii. Compatibility with communication fiber facilitates telemetry

appropriate illumination, detection and signal-processing scheme.

Fig. 1. Schematic representation of a fiber grating

for optical fibers. After this and a revolutionary prediction in 1966 by 1C.K. Kao and George Hockham that a purer fiber with 10 or 20 decibels of light loss per kilometer is possible to produce, there was a spurt of fiber based research activities worldwide. In 1970, two important breakthroughs happened i.e. first fiber with loss less than 20 dB2/km was fabricated and room-temperature operation of semiconductor laser was demonstrated; the latter became an ideal source for optical communication. (Hecht, 1999) By 1985, optical fibers with lowest possible loss (~0.2 dB/km) were being produced routinely. In that decade, research reached its pinnacle with optical fiber as a communication medium had been standardized to perfection. To quote *Philip Russell "standard fiber had become a highly respected elder statesman with a wonderful history but nothing new to say".* (Russell, 2003). Thus subsequent advancement required exploring newer avenues like in-fiber devices and fiber optic sensors. In-fiber devices are essential for easier interconnection between fiber as communication medium and transmitter and receiver parts to complete efficient telecommunication. Other non-telecommunication applications, though started as spin-off, had been emerging simultaneously.

Concurrently, after the demonstration of fiberscopes further development in the quality of fibers and compact light sources resulted in a new offshoot of fiber optics i.e. fiber based sensors and other devices which were able to extract information about various aspects of human physiology by analyzing the reflected laser light sent and received through fibers. This method has been used in laser Doppler analysis of different cells. Study of scattered light is used to detect blood velocity to determine if sufficient blood is reaching vital organs. It can also detect the oxygen content of the blood. Miniature sensors at the end of an optical fiber were devised to measure pressures in the arteries, bladder, urethra and rectum. Some chemical analysis was also possible utilizing the phenomenon of luminescence. (Katzir 1989, Mishra et al 2009).

The discovery of grating formation in optical fibers by Hill and Coworkers in 1978 is a good example of serendipity! While studying non-linear properties of germanium doped silica by passing intense Argon ion laser radiation, they found an unexpected reflection and concluded that it was because of formation of Bragg reflection gratings inside the fiber core. This formation was attributed to interference between forward propagating wave and back reflected radiation from the far end of the fiber resulting in standing wave pattern. A refractive index distribution with the same periodicity as the interference pattern is thus created in the fiber core. This periodic perturbation of refractive index is a result of <sup>3</sup>*'photosensitivity'* phenomenon in certain kind of doped fibers. Introducing a variation of refractive index with periodicity on the scale of wavelength of light alters the light-matter interaction like a grating and results in selective reflection of light. Initially this phenomenon was just a scientific curiosity but after its first practical demonstration by Meltz in 1989,

<sup>1</sup> A part of 2000 Nobel Prize in Physics was awarded to Z.I. Alferov for his invention of semiconductor laser and that of 2009 to C.K. Kao for his "groundbreaking achievements concerning the transmission of light in fibers for optical communication"

<sup>2</sup> The dB(decibels) is related to the ratio of output optical power from an optical fiber to the input optical power; If an input power P1 results in an output power P2, the loss in decibels is given by; α dB = 10 x Log10 (P1/P2)

<sup>3</sup> photosensitivity: the change in optical properties of material on exposure to light

there had been explosion of research activity in this field. (Kashyap, 2009, Othonos & Kallis, 1999)
