Chapter 5

Laser-Fiber Connection

Content

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

Power Launching Considerations

• Numerical Aperture• Core Size • Refractive Index Profile • Core Cladding index difference of the fiber • Radiance• Angular Power Distribution of the optical

source

Coupling Efficiency

s

F

PP

sourse thefrom emittedpower fiber theinto coupledpower [5-1]

Source Optical Fiber

sP FP

Lambertian Source

Lambert's cosine law in optics says that the radiant intensity observed from a "Lambertian" surface is directly proportional to the cosine of the angle θ between the observer's line of sight and the surface normal.

The law is also known as the cosine emission law or Lambert's emission law.

It is named after Johann Heinrich Lambert, from his Photometria, published in 1760

Source Output Pattern• Radiance (or brightness) B at a given diode drive

current is the optical power radiated into a unit solid angle per unit emitting surface area and is generally specified in terms of W/cm2.sr.

• Consider Fig. 5-1, which shows a spherical coordinate system characterized by R, , and , with the normal to the emitting surface being the polar axis.

• The radiance may be a function of both and , and can also vary from point to point on the emitting surface.

• Surface-emitting LEDs are characterized by their Lambertian output pattern. The power delivered at an angle , measured relative to a normal to the emitting surface, varies as cos because the projected area of the emitting surface varies as cos with viewing direction.

• The emission pattern for a Lambertian source follows the relationship B(, ) = Bocos (5-1)

where Bo is the radiance along the normal to the radiating surface. The radiance pattern for this source is shown in Fig. 5-2.

SOURCE-TO-FIBER POWER LAUNCHING

Figure 5-1. Spherical coordinate system for characterizing the emission pattern from an optical source.

SOURCE-TO-FIBER POWER LAUNCHING

Figure 5-2. Radiance patterns for a lambertian source and the lateral output of a highly directional laser diode. Both sources have B0 normalized to unity.

Edge emitting LEDs and laser diodes radiation pattern

LT BBB coscos

cossin

),(1

0

2

0

2

The integers T and L are the transverse and lateral power distribution coefficients, respectively.

For edge emitters, L=1 (a Lambertian distribution with a 120o half-power beam width) and T is significantly larger.

For laser diodes, L can take on values over 100.

[5-3]

Power Coupled from source to the fiber

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Power coupled from LED to the Fiber

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Power coupling from LED to step-index fiber

• Total optical power from LED:

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sin),(

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[5-6]

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Equilibrium Numerical Aperture

Examples of 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:

• Low loss fiber-fiber joints are either: 1- Splice (permanent bond) 2- Connector (demountable connection)

FFL log10]dB[ [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.

2/12

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Longitudinal offset effect

Losses due to differences in the geometry and waveguide characteristics of the fibers

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for )log(10)(

[5-10]

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