UNIT - IIILASER-FIBER CONNECTION AGENDA • Launching optical power into a fiber • Fiber-to-Fiber coupling • Fiber Splicing and connectors Coupling Efficiency PF power coupled into the fiber η= = power emitted from the sourse Ps Ps Source PF Optical Fiber 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: B (θ , φ ) = B0 cos θ Edge emitting LEDs and Laser Diodes radiation pattern 1 sin ϕ cos ϕ = + T L B(θ , ϕ ) B0 cos θ B0 cos θ 2 For edge emitting LEDs, L=1 2 Power Coupled from source to the fiber As and Ω s : area and solid emission angle of the source PF = ∫ ∫ B ( As , Ω s )dΩ s dAs = A f and Ω f : area and Af Ω f solid acceptance angle of fiber rm 2π 2π θ 0 max = ∫ ∫ ∫ ∫ B (θ , ϕ ) sin θdθdϕ dθ s rdr 0 0 0 0 Power coupled from LED to the Fiber θ 0 max 2πB0 cosθ sin θdθ dθ s rdr ∫0 ∫0 2π rs P=∫ 0 rs 2π 0 0 = πB0 ∫ 2 sin ∫ θ 0 max dθ s rdr rs 2π 0 0 = πB0 ∫ ∫ NA 2 dθ s rdr PLED,step = π rs B0 ( NA) ≈ 2π rs B0 n1 ∆ 2 2 2 2 2 2 Power coupling from LED to step-index fiber • Total optical power from LED: 2π π / 2 Ps = As ∫∫ 0 B (θ , ϕ ) sin θdθdϕ 0 Ps = πrs 2πB0 2 π /2 2 d rs B0 cos θ sin θ θ = π ∫ 2 0 PLED,step Ps ( NA) 2 = a 2 2 P ( NA ) s rs if rs ≤ a if rs ≥ a Equilibrium Numerical Aperture Possible Lensing Schemes used to improve optical source-to-fiber coupling efficiency Laser diode to Fiber Coupling Fiber-to-Fiber Joint • Fiber-to-Fiber coupling loss: LF [dB] = −10 log η F • Low loss fiber-fiber joints are either: 1- Splice (permanent bond) 2- Connector (demountable connection) 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. η F ,step Acomm 2 d d d = = arccos − 1 − 2 π 2a πa 2a πa 2 1/ 2 Longitudinal offset effect Losses due to differences in the geometry and waveguide characteristics of the fibers aR LF (a ) = −10 log( ) aE for a R ≤ a E NA R LF (a ) = −20 log( ) NA E for NA R ≤ NA E E & R subscripts refer to emitting and receiving fibers. Experimental comparison of Loss as a function of mechanical misalignment Fiber end face Fiber end defects Fiber splicing Fusion Splicing V-groove optical fiber splicing 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