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An Introduction to
Fiber Optic Imaging
by SCHOTT North America
An Introduction to
Fiber Optic Imaging©
SCHOTT Fiber Optics offers customized high-tech solutions in markets such as
automotive, lighting, medical, industrial, and defense. By mastering glass, fibers,
LEDs, and processes for the production of fiber optic and LED components, we
develop outstanding, market-oriented products. With our leading technological
know-how and innovative ideas, we ensure the success of our customers— around the world, around the clock.
An Introduction to Fiber Optic Imaging© SCHOTT North America, Inc.—Southbridge, MA, USA Published by SCHOTT North America, Inc.
122 Charlton Street Southbridge, MA 01550-1960, USA www.us.schott.com/fiberoptics Revised February 2007 SCHOTT North America would like to recognize the following employees who
contributed their time and expertise to this publication:
Patricia Alter, Colleen Bayrouty, Jennifer Benoit, Kapil Bhura, Mike Dargie, Gary DiGregorio, Brigitte Esposito, Ann Kutsch, Rick Miller, Katie Pepler, Kevin Tabor, Jim Triba, Evans Waldron, Michael Weisser, and Roberta Zacek t h ig r y p o C © © Copyright SCHOTT North America 2007 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means — electronic, mechanical, photocopying, recording, or otherwise — without prior written permission of the publishers.
What You Need to Know
Chapters 1 Mono Fiber Drawing
2 Multi Fiber Drawing
3 Multi-Multi Fiber Drawing
4 Boule Fusing
5 Material Quality
7 Image Inverters
9 Wound Image Bundles
10 Leached Image Bundles
Fiber optic components for image transfer have found extensive use in military, commercial, and scientific applications. As new ideas develop, questions arise regarding the manufacture and design of these imaging components. This book has been written to answer many of these questions, as well as provide a solid overview of the manufacturing processes and products offered by SCHOTT North America.
The details in this text have been obtained from extensive research of articles and documents available from the scientific community.
Information on the manufacture of fiber optics, as well as specific products offered by SCHOTT, was obtained through discussions with various members of the SCHOTT North America manufacturing/engineering team.
We hope this textbook provides you with a basic understanding of the fundamentals of fiber optic technology and imaging components. We also hope that the contents will ignite new ideas for the use of fiber optic components in new imaging applications.
When one mentions the subject of fiber optics today, telecommunications fiber optic applications come immediately to mind.
However, these represent only one use of fiber optic technology.
Fiber optic technology was initially developed for use in light and image transfer applications. During the past several decades, fiber optic light guides have been applied to such varied applications as optical inspection, architectural illumination, medical and dental lighting, retail and museum display lighting, and traffic and changeable message signs.
Fiber optic technology has also found widespread use in image transfer applications. Imaging fiber optics have been used in night vision, medical, dental, commercial, industrial, and scientific applications. These technologies continue to find homes in many new fields, such as X-ray crystallography, machine vision, and projection displays.
The following text explains the fundamentals of fiber optics, and describes the design and manufacture of imaging fiber optics to encourage their use in new applications. In addition to a general glossary of terms, there is a detailed discussion of the manufacturing process, design considerations, quality characteristics, and present applications—as well as a summary of SCHOTT’s fiber optics products.
Although the earliest observations of light conduction in transparent rods must predate scientific literature, the first recorded demonstration of this principle appears credited to John Tyndall in 1854, before a meeting of the Royal Institution of London. Tyndall showed that light was conducted by internal reflections in a curving stream of water. However, it was not until 1927 that J.L. Baird1 of England and C.W. Hansell2 of the United States defined a similar principle, and independently filed patents covering the basic idea of conducting images through bundles of fibers. The first attempt to produce an image-transmitting fiber bundle appears to have been made by H. Lamm3, of Germany, in 1930. His results were disappointing, however, and fiber optic development was not widely pursued. A number of patents relating to this technology were issued in the intervening period, but it was not until the early 1950s — when C.S. van Heel4 of Holland and Dr. Brian O’Brien5 of the United States began investigating the problem of optical insulation — that the principle of fiber optics was placed on a practical foundation.
In 1953, an article appearing in the Dutch technical journal De Ingenieur described a new method of transmitting optical images without the use of lenses. It described a new form of image encoding device that used delicately scrambled bundles of fibers. It reawakened the optical concept of “fiber optic technology,” in which images could be relayed through precisely aligned bundles of glass fibers.
The De Ingenieur article attracted the interest of the American government. At that time, the only American scientist who had worked on this idea was Dr. Brian O’Brien, V.P. of Research for American Optical (AO) Corporation in Southbridge, Massachusetts. These entities merged and formed a contract to develop a fiber optic image encoding device. Work began in 1954 in AO’s research laboratory under a cloak of confidentiality. First, it was necessary to produce glass fibers to efficiently transmit light.
XIAn Introduction to Fiber Optic Imaging
For two years, progress was disappointingly slow. In 1957, using several optical glasses from SCHOTT Glas in Mainz, Germany, there were a series of technical breakthroughs that formed the basis for much of today’s technology. The glass-coated, glass-core fiber concept was implemented, providing the useful transmission efficiency previously lacking. The second breakthrough was the invention of the optical multi fiber. This made possible large assemblies of either flexible or rigid fibers of very small diameters.
Taken together, the all-glass fiber made in the form of multi fibers led to a wholly new category of fiber optic components for use as windows in electronic image intensifiers — namely, fused faceplates. The third major breakthrough was the invention of the “hoop winding and assembly process” for making precisely aligned, flexible fiber image bundles in greater lengths.
Also during that time, the American Optical researchers did, in fact, develop an image encoder based on the early Dutch journal concept. However, it proved to be difficult to make, awkward to use, and not as secure as originally assumed.
Since its development, fiber optic technology has grown from a laboratory curiosity to a critical enterprise. Today, it continues to be used worldwide to produce many thousands of components, flexible bundles, products, and systems for fiber optic imaging applications in endoscopy, medicine, science, industry, night vision, law enforcement, telecommunications, and computer imaging.
Walter P. Siegmund, Ph.D.
See Appendix for “The History of SCHOTT North America.”
1. Baird, J.L., British Patent Specification No. 20,969/27 (1927)
2. Hansell, C.W., United States Patent 1,751,584 (1930)
3. Lamm, H., “Biegsame Optische Geräte,” Z. Instrumentenk, Vol. 50, pp. 579-581, 1930
4. Van Heel, C.S., “Optische Afbeelding Zonder Lenzes of Afbeeldingsspiegels Aanoulzing,” De Ingenieur, Vol. 65, p. 25, 1953
5. O’Brien, B., United States Patent 2,285,260 (1958)
Before we begin a discussion of the design and manufacture of fiber optic components for imaging applications, we must give an overview of some of the fundamental fiber optic terms that we will use throughout our discussion.
I Numerical Aperture Numerical aperture is the key concept that illustrates how fiber optics work as an optical wave guide. Snell’s Law gives us the relationship between the angle at which light enters the fiber α and the angle at which the light travels down the fiber θ1. Simple geometry gives us the relationship between θ1 and θ2. The rule of total internal reflection shows that the angle θ2 must have a minimum value in order for reflection to occur. When total internal reflection occurs, it is 100% reflection.
If θ2 is smaller than this minimum value, the light will not be totally reflected; it will be lost out the side of the wave guide.
From the three relationships, we can relate the requirements for total internal reflection back to the angle α. There is a maximum angle for which light can be transmitted down the fiber. Light will be lost out of the sides of the fiber for the angles larger than α. The limiting angle α is called the “acceptance angle” of the fiber.
Standard core glasses are available with indexes of refraction near 1.8 and cladding glasses near 1.5. With the index of refraction of air being 1.0, the equation for numerical aperture results in an acceptance angle of nearly 90°.
With the fundamental terms of fiber optics defined, we are now prepared to discuss the manufacturing of coherent fiber optic bundles. The following assembly, drawing, and fusing processes are designed to ensure that multiple fibers are assembled and fused in a precise stack that will allow for coherent image transfer. The first stage of this process is mono fiber drawing; it consists of the assembly of the core and cladding, as well as the drawing of these materials into a single clad fiber. The process begins with the selection of a core and cladding glass.
Core Glass Core glasses vary, based on the specific needs of the finished product. The standard core glass used in tapers and faceplates has an index of about 1.8. When combined with the standard cladding glass, which has an index of about 1.48, the resulting mono fiber has a numerical aperture, or NA, of 1.0 (the standard NA for most coherent fiber applications).
Some of the key properties that help to differentiate core glasses include:
I Index of Refraction Discussed earlier; see page XVIII.
I Transmission Internal spectral transmittance may need to be considered in the design of a fiber optic product. Most SCHOTT core glasses have excellent transmission from 500 to 800 nm. However, because there is some variability in the transmission performance from 380 to 500 nm, the spectral transmission of each glass should be considered.
I Coefficient of Thermal Expansion The coefficient of thermal expansion (CTE) of a glass is defined as the measure of the rate at which the glass will change dimensions when exposed to temperature changes. The core glass expansion is important because, when combined with the cladding, it will produce a certain expansion for the fiber optic product. The CTE of a fiber optic product is an important consideration when the product is going to be bonded or coupled with another product.
An Introduction to Fiber Optic Imaging
Core Glass Materials Below is a summary of the lanthanum borate and lead-based core glasses manufactured by SCHOTT, as well as the properties that differentiate them. Other core glasses, with other numerical apertures, are available from SCHOTT upon request.
I CG-1 SCHOTT’s standard taper core glass. This glass has thermal properties that make it more forgiving during the additional heat cycle required for tapering (see diagram showing the difference in working temperatures for faceplates and tapers). CG-1 is also a phosphor compatible glass. nd = 1.81.
I RWY47 SCHOTT’s standard faceplate core glass. Does not contain cadmium oxide or lead oxide, making it both phosphor and photocathode compatible. This glass is used primarily in input plates for night vision devices. RWY47 also has excellent X-ray attenuation properties, making it an excellent choice for X-ray imaging applications. nd = 1.81.
An Introduction to Fiber Optic Imaging Glass Melting
Core Bar Configurations SCHOTT manufactures core glasses of various configurations. Most SCHOTT core glasses are proprietary glasses manufactured at our facility in Duryea, Pennsylvania. The five-sided core bar allows for easy insertion of the black extramural absorption, or EMA, fibers into the multi fiber structure, which will be discussed in more detail later. Round and square core configurations are also manufactured for some applications.
Cladding Glass The standard cladding glass used in SCHOTT’s fiber optic materials is a borosilicate tubing glass with an index of refraction of about nd = 1.48. The cladding glass is purchased in the form of a tube, into which the core is inserted. Most SCHOTT tubes are manufactured at SCHOTT’s facility in Germany.