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]

Different modal distribution of the optical beam emerging from a fiber lead to different degrees of coupling loss. a) when all modes are equally excited, the output beam fills the entire output NA. b) for a steady state modal distribution, only the equilibrium NA is filled by the output beam.

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

Lateral (axial) misalignment loss: Cont…

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 Fiber end defects

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 putted 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

Radiance (Brightness) of the source B= Optical power radiated from a unit area of the source into a unit solid angle [watts/(square centimeter per stradian)]

Surface emitting LEDs have a Lambertian pattern: [5-2]

Edge emitting LEDs and laser diodes radiation pattern [5-3] For edge emitting LEDs, L=1

Power Coupled from source to the fiber [5-4]

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

Power coupled from LED to the Fiber rs source radius [5-5]

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

Examples of possible lensing schemes used to improve optical source-to-fiber coupling efficiency

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