Power Launching and Coupling Chapter 5 Power Launching and Coupling

Content Launching optical power into a fiber Fiber-to-Fiber coupling Fiber Splicing and connectors

Coupling Efficiency [5-1] Source Optical Fiber

Fiber-to-Fiber Joint 1) Fiber-to-Fiber coupling loss: 2) Low loss fiber-fiber joints are either: 1- Splice (permanent bond) 2- Connector (demountable connection) [5-8]

Mechanical misalignment losses Lateral (axial) misalignment loss is a dominant Mechanical loss. [5-9]

Lateral (axial) misalignment loss: Cont…

Example

Longitudinal offset effect Losses due to differences in the geometry and waveguide characteristics of the fibers [5-10] E & R subscripts refer to emitting and receiving fibers.

Losses due to differences in the geometry and waveguide characteristics of the fibers E & R subscripts refer to emitting and receiving fibers. [5-10]

Experimental comparison of Loss as a function of mechanical misalignment

Fiber end face Before connecting or splicing fiber ends must be properly faced to avoid losses and to improve the coupling efficiency. The end faces should be polished until all the scratches are removed and they become smooth. The process involves following steps: The fiber is scratched to create a stress concentration at the surface. Fiber is then bent over a curved form with applied tension to produce stress distribution. Maximum stress occurs at scratch point and crack starts propagating through fiber.

Fiber splicing Fusion Splicing Fusion splicing involves butting two cleaned fiber end faces and heating them until they melt together. Fusion splicing is normally done with a fusion splicer that controls the alignment of the two fibers to keep losses as low as 0.05 dB. Fiber ends are first prealigned and butted together under a microscope with micromanipulators. The butted joint is heated with laser pulse to melt the fiber ends so can be bonded together.

V-groove optical fiber splicing Mechanical splices join two fibers together by holding them tightly with a structure or by gluing the fibers together. Mechanical splices may have a slightly higher loss and back reflection. These can be reduced by inserting index matching gel. 3) V groove mechanical splicing provides a temporary joint i.e fibers can be disassembled if required. The fiber ends are butted together in a V – shaped groove Groove :اخدود

Optical Fiber Connectors Some of the principal requirements of a good connector design are as follows: 1- low coupling losses 2- Interchangeability 3- Ease of assembly 4- Low environmental sensitivity 5- Low-cost and reliable construction 6- Ease of connection

Connector Return Loss

Fresnel Reflection or Reflectivity

Example

Power Coupled from source to the fiber rs is the source radius

Power coupling from LED to step-index fiber Total optical power from LED:

Example

Power coupling from LED to graded index fiber Where rs <a HW: Try rs >a

Equilibrium Numerical Aperture

Laser diode to Fiber Coupling BW 30 50 degree, but area is small use lenses