Fiber-optic communication works on the principle of ‘Total Internal Reflection’ which is summarized below

 

Total Internal Reflection

           When a light ray is incident on a boundary separating two different media, part of the ray is reflected back into the first medium and while the other part is bent and enters into the second medium. The bending in the second medium known as refraction and depends on the refractive index of the two media.

           By Snell’s Law

n₁sinφ₁ = n₂sinφ₂

sinφ₁/sinφ ₂ = n₂/n₁

where n₁ and n₂ are the refractive indices of the medium 1 and 2 respectively and φ₁ and φ₂ are the angle of incidence and angle of refraction respectively.

         For n₁>n₂, if sinφ₁=n₂/n₁  then φ₂=90°.(Here, φ₁ is known as the ‘Critical Angle’). If we further increase φ₁ from this angle there will be no refracted wave and the light ray will be completely reflected into the first medium. This phenomenon is known as the Total Internal Reflection.

 

        A fiber-optic cable consists of a core surrounded by cladding materials which respectively serve as the denser and the rarer medium. It may be noted that fiber used in this experiment has a core refractive index is 1.492 and the cladding refractive index is 1.406.

 

 Acceptance Angle in Fiber-Optic Cable

           Since the optical fiber works on the principle of ‘Total Internal Reflection’ only the rays with an angle greater than the ‘Critical Angle’ at core-cladding interface will be transmitted. The particular angles of incidence at which it can transmit a light ray is called the Acceptance Angle.In our experiment, the Acceptance Angle is nearly 60°.

Meridional ray at critical angle φ within the fiber at core cladding interface.

 

Numerical Aperture

            This gives the relation between the acceptance angle and the refractive indices of the three media involved viz. the core, the cladding and the air. This is given as

Numerical Aperture = n₀sinθ_a = (n₁²-n₂²)^1/2

where n₀ is the refractive index of air, n₁ is the refractive index of the core and n₂ is the refractive index of the cladding.In our experiment, Numerical Aperture is nearly 0.5.

Transmitter

          An optical transmitter is needed to convert an electrical signal to a light pulse for transmission in the optical medium i.e. the fiber-optic cable. The most commonly used optical transmitters are semiconductor devices such as Light-Emitting Diodes (LEDs) and Laser Diodes. In our experiment, we have used an LED transmitter.

Fiber-optic Cable

         An optical fiber is a dielectric waveguide that operates at optical frequencies. This is normally in a cylindrical wave. It confines the electromagnetic energy in the form of light to within its surface and guides the light in the direction parallel to its axis.

         The simplest fiber optic cable consists of two concentric layers of transparent materials. The inner portion (the core) transports the light; the outer covering (the cladding) must have a lower refractive index than the core so the two of them are made of different materials. To provide mechanical protection to the cladding, an additional plastic layer, the Primary Buffer is added. 

The way the material composition of the core is varied gives rise to the two commonly used fiber types

1.      Step index fiber

2.      Graded index fiber

 

1.Step-index Fiber

      In a step-index fiber, the core of refractive index of n₁ is surrounded by a cladding of refractive index n₂ where

n₂=n₁(1-∆)

The parameter ∆ is called the core-cladding index difference. Values of n2 are chosen such that ∆ is nominally 0.01.Typical values of this range from 1 to 3 percent for multimode fibers and 0.2 to 1 percent for single-mode fibers

 

2.Graded-index fiber

    In graded-index fiber, the core refractive index decreases continuously with increasing radial distance r from the centre of fiber, but the refractive index is generally constant in the cladding.

n(r)=n₁[1-2∆(r/a)^α ]^1/2  for 0 ≤ r ≤ a

n₁(i-2∆)^1/2 ≈ n₁(1-∆) = n₂ for r ≥ a

    Here, a is the core radius, n₁ is the refractive index at the core axis, n₂ is the refractive index of the cladding, and the dimensionless parameter α defines the shape of the index profile. The index difference ∆ for the graded-index fiber is given by

∆=(n₁²-n₂²)/2n₁² = (n₁-n₂)/n₁

We have used a step-index fiber in this experiment.

 

Receiver

        An optical receiver is needed to convert light pulses to electrical signals. The main component of an optical receiver is a photodetector, which converts light into electricity using thephotoelectric effect.In our experiment the photodetector is a semiconductor based photodiode.

 

Types of Transmission

         There are two types of transmission schemes may be present in fiber-optic transmission system.

 

Single Mode

        As the name implies, single mode optical fiber is designed to propagate onlyone light ray. It is used in high speed long distance communication.

Multi Mode

      A multi mode optical fiber is designed to propagate more than one light wave at a time. A larger diameter of the core is required to accommodate more light rays facilitate the transmission. It is typically used in short distance communication.In this particular experiment, we have used amulti mode fiber.