**3. Fiber gratings**

126 Biomedicine

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,

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,

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,

1 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

2 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;

3 photosensitivity: the change in optical properties of material on exposure to light

had been emerging simultaneously.

light in fibers for optical communication"

α dB = 10 x Log10 (P1/P2)

Mishra et al 2009).

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 core.

Fig. 1. Schematic representation of a fiber grating
