Optical fibers alignment loss

2. CONTENTS
Introduction
Losses due to Fresnel Reflection.
Losses due to Geometric and Optical Parameter
variations.
Multimode Fiber Joints.
Single Mode Fiber Joints.

3. INTRODUCTION
 Fiber splices: These are semipermanent or permanent joints which find
major use in most optical fiber telecommunication systems (analogous to
electrical soldered joints).
 Demountable fiber connectors or simple connectors: These are
removable
joints which allow easy, fast, manual coupling and uncoupling of fibers
(analogous to electrical plugs and sockets).
 A crucial aspect of fiber jointing concerns the optical loss associated with the
connection.
 A major consideration with all types of fiber–fiber connection is the optical
loss encountered at the interface.

4. Losses due to Fresnel
Reflection
A small proportion of the light may be reflected back into the
transmitting fiber causing attenuation at the joint. This phenomenon,
known as Fresnel reflection
Associated with the step changes in refractive index at the jointed
interface (i.e. glass–air–glass).
The magnitude of this partial reflection of the light transmitted
through the interface is given by equation 1.

5. Losses due to Fresnel
Reflection
The loss in decibels due to Fresnel reflection at a single interface is
given by equation 2.
Where n1 is the refractive index of the fiber core and n is the refractive
index of the medium between the two jointed fibers.
The effect of Fresnel reflection at a fiber–fiber connection can be
reduced by the use of an index-matching fluid between the gap.

6. Losses due to Geometric Variations
Misalignment may occur in three dimensions:
the separation between the fibers (longitudinal misalignment)
 the offset perpendicular to the fiber core axes (lateral
misalignment)
angle between the core axes (angular misalignment).

7. Losses due to Longitudinal & Lateral
misalignments

8. Losses due to Angular Misalignment

9. Losses due to Optical Parameter Variations
Losses at joints may occur due to the following reasons:
 variations in core and/or cladding diameters.
 variations in numerical apertures and/or relative refractive index
differences.
 variations in refractive index profiles.
 fiber faults (core ellipticity, core concentricity, etc.).

10. Losses due to Optical Parameter Variations

11. Multimode Fiber Joints
Lateral misalignment reduces the overlap region between the two
fiber cores.
The lateral coupling efficiency for two similar step index fibers (in a
multimode step index fiber) is given by equation 1.
The lateral misalignment loss in dB is given by equation 2.
where n1 is the core refractive index, n is the refractive index of the
medium between the fibers, y is the lateral offset of the fiber core
axes, and a is the fiber core radius.

12. Multimode Fiber Joints
Lateral misalignment loss in multimode graded index fibers is given
by equation 1.
Angular misalignment losses at joints in multimode step index fibers
is given by equation 2.
where θ is the angular displacement in radians and Δ is the relative
refractive index difference for the fiber.

13. Losses due to Geometric Variations
at a Multimode Fiber Joint
loss resulting from a mismatch of core diameters is given by equation
1.
loss caused by a mismatch of numerical apertures is given by equation
2.

14. Losses due to Geometric Variations
at a Multimode Fiber Joint
A mismatch in refractive index profiles results in a loss given by:
The total intrinsic losses obtained at multimode fiber–fiber joints
provided by the following equation: