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Fiber alignment and joint loss

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Ali Baig

This document discusses optical losses associated with fiber optic joints. It describes losses from Fresnel reflection at the interface between fibers due to differences in refractive index. It also discusses losses from various types of geometric misalignments between fibers, including longitudinal offset, lateral offset, and angular misalignment. Finally, it examines losses from variations in optical parameters between fibers, such as core diameter, numerical aperture, and refractive index profile mismatches. Formulas are provided for calculating losses from many of these sources.

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Fiber alignment and joint loss
CONTENTS ï‚—Introduction ï‚—Losses due to Fresnel Reflection. ï‚—Losses due to Geometric and Optical Parameter variations. ï‚—Multimode Fiber Joints. ï‚—Single Mode Fiber Joints.
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.
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.
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.
Losses due to Geometric Variations ï‚—Misalignment may occur in three dimensions: ï‚—the separation between the fibers (longitudinal misalignment) ï‚— the offset perpendicular to the fiber core axes (lateral misalignment) ï‚—angle between the core axes (angular misalignment).
Losses due to Longitudinal & Lateral misalignments
Losses due to Angular Misalignment
Losses due to Optical Parameter Variations ï‚—Losses at joints may occur due to the following reasons: ï‚— variations in core and/or cladding diameters. ï‚— variations in numerical apertures and/or relative refractive index differences. ï‚— variations in refractive index profiles. ï‚— fiber faults (core ellipticity, core concentricity, etc.).
Losses due to Optical Parameter Variations
Multimode Fiber Joints ï‚—Lateral misalignment reduces the overlap region between the two fiber cores. ï‚—The lateral coupling efficiency for two similar step index fibers (in a multimode step index fiber) is given by equation 1. ï‚—The lateral misalignment loss in dB is given by equation 2. ï‚—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.
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.
Losses due to Geometric Variations at a Multimode Fiber Joint ï‚—loss resulting from a mismatch of core diameters is given by equation 1. ï‚—loss caused by a mismatch of numerical apertures is given by equation 2.
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:

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Fiber alignment and joint loss

  • 2. CONTENTS ï‚—Introduction ï‚—Losses due to Fresnel Reflection. ï‚—Losses due to Geometric and Optical Parameter variations. ï‚—Multimode Fiber Joints. ï‚—Single Mode Fiber Joints.
  • 3. INTRODUCTION ï‚— Fiber splices: These are semipermanent or permanent joints which find major use in most optical fiber telecommunication systems (analogous to electrical soldered joints). ï‚— 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). ï‚— A crucial aspect of fiber jointing concerns the optical loss associated with the connection. ï‚— A major consideration with all types of fiber–fiber connection is the optical loss encountered at the interface.
  • 4. Losses due to Fresnel Reflection ï‚—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 ï‚—Associated with the step changes in refractive index at the jointed interface (i.e. glass–air–glass). ï‚—The magnitude of this partial reflection of the light transmitted through the interface is given by equation 1.
  • 5. Losses due to Fresnel Reflection ï‚—The loss in decibels due to Fresnel reflection at a single interface is given by equation 2. ï‚—Where n1 is the refractive index of the fiber core and n is the refractive index of the medium between the two jointed fibers. ï‚—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 ï‚—Misalignment may occur in three dimensions: ï‚—the separation between the fibers (longitudinal misalignment) ï‚— the offset perpendicular to the fiber core axes (lateral misalignment) ï‚—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 ï‚—Losses at joints may occur due to the following reasons: ï‚— variations in core and/or cladding diameters. ï‚— variations in numerical apertures and/or relative refractive index differences. ï‚— variations in refractive index profiles. ï‚— fiber faults (core ellipticity, core concentricity, etc.).
  • 10. Losses due to Optical Parameter Variations
  • 11. Multimode Fiber Joints ï‚—Lateral misalignment reduces the overlap region between the two fiber cores. ï‚—The lateral coupling efficiency for two similar step index fibers (in a multimode step index fiber) is given by equation 1. ï‚—The lateral misalignment loss in dB is given by equation 2. ï‚—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 ï‚—Lateral misalignment loss in multimode graded index fibers is given by equation 1. ï‚—Angular misalignment losses at joints in multimode step index fibers is given by equation 2. ï‚—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 ï‚—loss resulting from a mismatch of core diameters is given by equation 1. ï‚—loss caused by a mismatch of numerical apertures is given by equation 2.
  • 14. Losses due to Geometric Variations at a Multimode Fiber Joint ï‚—A mismatch in refractive index profiles results in a loss given by: ï‚—The total intrinsic losses obtained at multimode fiber–fiber joints provided by the following equation: