International Lighting in Controlled Environments Workshop

  T.W.Tibbitts (editor)  1994   NASA-CP-95-3309               home | contents


PRINCIPLES AND CHARACTERISTICS OF OPTICAL FIBERS

Atikem Haile-Mariam

DEFINITIONS

Core, Cladding, Coating

An optical fiber is made of three sections:

The core carries the light signals

The cladding keeps the light in the core

The coating protects the cladding

Fig Hai 1.jpg (27453 bytes)

How an Optical Fiber Works

An Optical Fiber works on the principle of Total Internal Reflection

Light rays are reflected and guided down the length of an optical fiber.

The acceptance angle of the fiber determines which light rays will be guided down the fiber.

CORE CHARACTERISTICS

1. The diameter of the light carrying region of the fiber is the "core diameter."

2. The larger the core, the more rays of light that travel in the core.

3. The larger the core, the more optical power that can be transmitted.

4. The core has a higher index of refraction than the cladding.

5. The difference in the refractive index of the core and the cladding is known as delta.

STANDARD OPTICAL FIBER SIZES

Fig Hai 2.jpg (65736 bytes)

SPECIALTY ILLUMINATION FIBER

Fig Hai 3.jpg (76651 bytes)

Total Internal Reflection

Total Internal Reflection occurs when any ray traveling from a medium with a high refractive index is incident on a boundary of a lower refractive index at an angle greater than or equal to the critical angle.

Fig Hai 4.jpg (44266 bytes)

(From: Michael Brininstool, 1993, Fiber Optic Design Principles Tutorial, ROV93 Conference San Diego, CA.)

Numerical Aperture (NA)

1. Measure of the acceptance angle of light that a fiber can support through total internal reflection.

2. Designed into the fiber by the difference in indices of refraction between the core and the cladding material.

Ray Tracing in Optical Fiber

Fig Hai 5.jpg (62482 bytes)

(From: Michael Brininstool, 1993, Fiber Optic Design PrinciplesTutorial, ROV93 Conference San Diego, CA.)

FIBER PERFORMANCE

The efficiency of light transmission of optical fibers depends on fiber design and physical environment.

FIBER MATERIAL COMPOSITION

1. Corning optical fiber is an amorphous noncrystaline material made of pure fused silica and germania dopant.

2. Plastic optical fiber is generally made of a polymethyl methacrylate (PMMA).

3. Experimental fibers are made of other materials such as sapphire.

4. Coatings are usually proprietary to the manufacturer but are usually acrylate or polyimide based.

5. The primary function of coating is to protect the glass fiber from flaws.

EXAMPLES OF SPECTRAL ATTENUATION IN OPTICAL FIBER

Fig Hai 6.jpg (103454 bytes)

COMPARISON OF GLASS AND PLASTIC OPTICAL FIBER

 

 

Characteristics

Glass

Plastic

Fiber core diameter, microns

clad diameter

Attenuation at 650 nm, dB/km

Maximum transmission distance

for 75% power loss, meters

Usable spectral range

Numerical aperature

Acceptance angle (cone)

50-200

125-500

4.0

1,500

 

UV,VIS,IR

.1-.4

35 degrees

250-5000

450-6000

150*

53

 

VIS

.3-.65

60-75 degrees

*Current commercial limits, not theoretical limits

PHYSICAL ENVIRONMENT

Bend Induced Attenuation

1. Macrobending

2. Large bends in an Optical Fiber will shed rays of light. Power is lost at the bend.

1. Microbending

Small axial bends/bumps along the fiber axis that cause mixing or loss of power. This can be induced by fiber jacketing, cabling or environment.

Cable Design

        1. Performance of fibers in cables depends on the following components:

                    strength members (kevlar, steel)

                    fill compounds

                    tight buffer vs loose tube

        2. Temperature/Humidity

                    The performance of fiber/cable depends on the extent to which temperature and humidity produce microbending.

Specifications

         3. Temperature (Celsius)

                    Standard Glass Optical Fiber - -60 to +85 degrees

                    Specialty Glass Optical Fiber - -60 to +200 degrees

                    Plastic Optical Fiber -40 to 85 degrees

        4. Temperature/Humidity

                    Standard Glass Optical Fiber - -10 to +85 degrees and 4% to 98% RH

                    Specialty Glass Optical Fiber - -10 to +85 degrees and 4% to 98% RH

                    Plastic Optical Fiber - max 85% humidity for 2000 hours

TECHNICAL ISSUES THAT MERIT FURTHER INVESTIGATION

1. Cost effective diffusers and concentrators

2. Cost effective coupling techniques between light sources and fibers

3. "Multi-use' fibers

SUMMARY

1. Optical fibers works on the principle of total internal reflection.

2. Optical fibers can be used at various wavelengths including illumination applications.

3. Factors affecting the performance of fiber include material composition, geometry, and the physical environment.

4. Fiber/cabling can be optimized for the specific application and environment.

5. Manufacturing processes are available for producing glass fibers of differing refractive indices and diameters.

 


Haile-Mariam, A.1994. Principles and characteristics of optical fibers, p 319-324. In: T.W.Tibbitts (ed.). International Lighting in Controlled Environments Workshop, NASA-CP-95-3309. 


top of page | home | contents

 

Copyright March 1994 NASA [National Aeronautics and Space Administration].

All rights reserved.