In this apparatus, both single mode and multi-mode fibers are used for the experiments. The apparatus makes use of rail and carriage system for mounting and adjusting the optical components required for experiments. Diode laser is used as light source. Laser fiber coupler is used to couple light from laser to fiber input end efficiently. There are mounts to hold input and output ends of the fiber firmly. Detector is placed on an XYZ stage. Distance between each component can be adjusted using the rail and carriage mechanism.
The experiment helps students to understand concepts of numerical aperture, bending loss, splice loss etc. The laser light is coupled to optical fiber with the use of an objective lens for maximum coupling efficiency. Numerical aperture is found out by scanning the far field of the optical fiber using a photo detector mounted on a translation stage. Corrosion resistant materials like stainless steel and aluminium alloys are used for the construction of all components used in this apparatus.
The Numerical Aperture is given by,
NA = Sin θa
Where θa is the Acceptance angleThe bend of a fiber causes loss in emittance and increase in attenuation as the angle of incidence decreases at the points where curveted radius is too small and the condition of total internal reflection is not fulfilled. In this experiment an apparatus of varying radii is used to study the bending losses involved. When a fiber is bend specified number of turns on various diameters, loss occurs in accordance with the diameter and it can be seen that, the loss will increase with respect to the decrease in diameter.
Splice loss is caused by a number of factors. Loss is minimized when the two fiber cores are identical and perfectly aligned, splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fiber's core will propagate, so all the rest of the light becomes splice loss
The Numerical Aperture is given by,
NA = Sin θa
Where θa is the Acceptance angleIn an optical fiber, the normalized frequency, the V number is given by
V = ( 2πa / λ ) √ ( n1 - n2 )
V = ( 2πa / λ ) NA
Where a is the core radius, λ is the wavelength in vacuum, n1 is the maximum refractive index of the core, n2 is the refractive index of the homogeneous cladding, and applying the usual definition of the numerical aperture NA.
For a Gaussian power distribution (lasers used in communications have Gaussian power distribution) in a single mode optical fiber, the mode field diameter (MFD) is defined as the point at which the electric and magnetic field strengths are reduced to 1 / e2 of their maximum values, i.e., the diameter at which power is reduced to 1 / e2 (0.135) of the peak power (because the power is proportional to the square of the field strength). For single mode fibers, the peak power is at the center of the core.
NA of Multimode fiber
Intensity Vs Micrometer Reading
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