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

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

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

SPECIALTY ILLUMINATION FIBER

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.

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

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.

(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

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

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.

        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. 

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