Power Launching And Coupling 2z5a2n

UNIT - III LASER-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 t • Fiber-to-Fiber coupling loss:

LF [dB] = −10 log η F • Low loss fiber-fiber ts 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