FIBER
OPTICS
John K. Crum , ANALYTICAL CHEMISTRY, Washington, D. C.
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THE FASCINATING DEVELOPMENTS in science within t h e past dozen or so years, much p u b licity has been given to breakthroughs such as t h e transistor and the laser. Yet, a development t h a t has gone relatively unheralded within the chemical community, and t h a t is of potential use in a variety of applications t h a t boggles t h e imagination, is t h e science of fiber optics. Fiber optics makes possible the twisting, bending, and turning of light; it makes possible t h e illumination and observation of r e mote and inaccessible areas, t h e measurement of depth, and examination of the interior of the h u m a n body with heretofore unapproachable clarity. Such applications, and a brief review of the physics related to fiber optics, is t h e subject of this Report. W e hasten to add t h a t nothing new is presented here. However, constant contact with analytical chemists leads us to t h e conclusion t h a t insight into the possible uses of fiber optics might bring a new dimension to our capabilities in dealing with difficult design problems in instrumentation, in automation of analytical procedures, and in more efficient handling of light problems in the laboratory.
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I ! Figure 1. Fiber optic bundles, which can transmit light over certain lengths, are shown here wound on large spools 26 A ·
ANALYTICAL CHEMISTRY
Fiber optics bundles (see Figure 1) a r e composed of numerous strands of light-transmitting glass or plastic fused at the ends, and are of sufficient length t h a t they can be easily bent while maintaining their light-transmitting capability (within practical limits). A single fiber (available in diameters ranging from about 2 microns to V 4 inch and in lengths of about 150 feet in glass, and in diameters of 75 to 2500 microns in plastic) will t r a n s mit light, but a bundle of fibers will transmit both light and images,
REPORT FOR ANALYTICAL CHEMISTS
and the analytical chemist which is important in m a n y of t h e applications t h a t will be discussed later. Individual fibers are clad with a material t h a t eliminates light leakage a n d t h a t traps light within t h e fiber for "total internal reflection;" or, more simply, light t h a t enters one end of a fiber a t the proper angle will be reflected from t h e walls as it traverses t h e length of the fiber and will emerge from t h e other end a t t h e same angle. As interest in the range of appli cations has increased, t h e technol ogy of fiber fabrication has tried t o keep pace. For example, individual glass fibers of 0.0025-inch diameter in bundles of 0.045-inch diameter can be clad in a 0.020-inch P V C coating in lengths of u p t o 10,000 feet (1). Costs have been rather high in the past, b u t as production technology a n d demand have in creased, t h e costs have come down and will probably continue t o do so. N e w companies are rapidly coming into the fiber optics fabrica tion field in hope of enjoying p a r t of a m a r k e t t h a t has been estimated by one source t o be 20—50 million dollars annually within the next five years.
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Bend, Twist, Turn Light and Images
2
Theory and Principles
I t has mainly been the ability t o work with new materials, and j u d i cious application of well-known physics principles, t h a t has brought about a new depth t o t h e under standing of light management. As can be seen from the following, it is the approach to total internal reflec tion t h a t permits wondrous things t o be accomplished. Figure 2 is a sim ple schematic t h a t illustrates this principle in a fiber strand with index of refraction Nlt coated with a ma terial of index of refraction JV2.
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where Ns is the refractive index of the medium through which light ap proaches the end of the fiber. For most applications discussed here, the sin of the half angle of ac ceptance of t h e incoming light a t the fiber end is defined as t h e numercial aperture, and t h e angle is the maximum a t which a light r a y can be transmitted through t h e fi ber. Of course, various factors af fect the actual transmission of light through a given fiber or bundle, such as absorption by the fiber m a terial, reflection losses, and the con dition of t h e ends of t h e fiber or bundle. Most of the literature indi cates t h a t there are m a n y pitfalls in design of fiber optics systems when practiced b y beginners, so we rec ommend a careful study of all fac tors involved and consultation with experts available a t t h e manufac turing companies before a commit ment to use fiber optics is made. A good brief description of design con siderations is given in Ref. (2). T h e commercial glasses normally used for preparing fiber optics bun dles t r a n s m i t light in the 4000-9000 Â range; for infrared transmission arsenic trisulfide fibers are available and silver chloride is being studied ; for ultraviolet, fused quartz appears to work quite well (3). Light Pipes ami tmagescepes
Tapered Fibers for Magnification Figure 3 . optics
S o m e basic f o r m s o f f i b e r
Some of the basic uses of fiber opties are shown schematically in Figure 3. I n its simplest form the single fiber or fiber bundle or a r r a y can be used to transmit light from one point t o another, perhaps b y a tortuous path. This capability is VOL. 4 1 , NO. 2, FEBRUARY 1969
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Repart for Analytical Chemists
Infrared Detector
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Fifeer Optics
COURTESY OF IRON AQC
Figure 4. Fiber epfcics utsed in device fer determining the quality of a microweld
significant, however. For example, "cold light" can be obtained over a surgeon's operating table, where heat is not desired, by piping it from a source in another room. As another example, consider the illumination of a microscope stage with cold light, so as not to bother a biological specimen being studied (4) • Another useful application in this vein is the piping of light through flexible fibers to areas t h a t are difficult to illuminate conventionally ; therefore, the dentist finds a fiber optic bundle useful for examining the d a r k reeesses of the mouth (S). When NASA needed a highly reliable (100% !) method for determining if every weld in the electronic systems of the Apollo spacecraft was good, t h e y turned to R a y theon for an on-line quality eontrol system t h a t used fiber optics for transmission of infrared d a t a (6). Three parameters of a micreweM can be examined by infrared—temperature, heat rise time, and heat decay time. Because the weld quality is a function of time for a thermal body to reach equilibrium, heat decay time is the most important of these parameters. A silicon infrared detector t h a t can detect frequencies in the 1-micron region was used in conjunction with fiber optics to observe these parameters in the melting point regions of the m a t e rials used for microwelding the electronic circuits (Figure 4 shows the basics of this a r r a n g e m e n t ) . T h e next step from simple piping WA
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ANALYTICAL CHEMISTRY
of light is the illumination of several objects from a single source. A control panel, for instance, can be lighted a t several points by one bulb (some automobiles now have fiber optic light guides for illumination of interior panel, m a p readers, etc., and for failure indicators for outside head- and tail-lamp assemblies) . This "split lighting" is accomplished by dividing the strands of a bundle of fibers. Fiber optics really won its spurs by an adaptation of the above for reading computer tapes and eards. I n fact, this application has given great impetus to fiber optics development in Great Britain, where R a n k Taylor Hobson is the leading manufacturer (this one company commands about 3 0 % of the world's fiber optics business). D a t a t r a n s formation from punched tapes or eards is accomplished by passing light through punched holes and sensing the transmitted light with a phototransistor or photodiode (7). T h e different sensors which arc activated cause generation of code signals t h a t cause characters to print a message. Traditionally, several individual light bulbs were placed alongside the tape or card, but fiber optics illumination from a single bulb was found to obviate replacement and maintenance of the several small bulbs, thus providing for a more trouble-free operation. As mentioned before, each fiber within a bundle is capable of t r a n s mitting light independently. A bundle is noncoherent if the fibers are not aligned within the bundle, and is therefore capable of t r a n s mitting only light; a bundle is coherent if the individual fibers are aligned within the bundle and at the ends—such a bundle is capable of transmitting both light and images. T h e image can be t r a n s mitted erect or, if desired, the ends of the bundle can be twisted in relation t o each other to provide an inverted image. T h e image-transmitting capability of fiber optic bundles gives rise to a whole new spectrum of applications. Fiber sigmoidoscopes for viewing the h u m a n alimentary canal and bronchial tubes with amazing clarity open u p new vistas in medical diagnosis. Glass fibers are quite
flexible, but they sometimes break when bent in the 1.5-inch radius often required for traversing certain areas of the alimentary canal, so much research is being done on development of plastic fibers for this purpose (8). I n the devices under development, full consideration is given to the physician's need for manipulation of the instrument and provision of air, water, suction, and illumination to the area under scrutiny. Illumination is provided by an outer ring of noncoherent fibers t h a t transmit light from a source controlled by the physician, and image viewing is accomplished by an inner core of coherent fibers. Air and water are used to clean the lens attached to the image-transmitting bundle, and suction is provided for removal of foreign m a t t e r such as mucous. P e r m a n e n t records of the finding can be made by a t t a c h ing a camera to the image-transmitting section of the fiberscope. The Illinois Institute of Technology Research Institute at Chicago has developed several of these probes for inspection of the colon, bronchial tubes, esophagus, larynx, and stomach. Elsewhere, a t Children's Hospital in Boston, a fiberscope has been used for viewing (and taking movies of) the interior of the h e a r t ; at W a y n e State University School of Medicine an a m nioscope has been developed for viewing the fetus during intra-uterine transfusions (5). Using devices similar to those just described, it is possible to observe the inner workings of machines for wear or damage to parts without resorting to such time-honored techniques as borescopes or X - r a y s (9). For example, the condition of a fuellevel indicator in the gas t a n k of an automobile can be studied without removing the p a r t from the vehicle. Doesn't this suggest possible applications in viewing hazardous reactions in the laboratory? Fiber optics has been useful in the solution of m a n y unique optical problems. An interesting example is its use in a device for monitoring the thickness of very thin films during vacuum deposition. Optical interference methods, in which reflectivity or transmission of monochromatic light for a'monitor glass
Report for Analytical Chemists
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COURTESY OF CHEMICAL AND ENGINEERING NEWS
Figure 5. optics
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is measured, are in general use. B e cause placement of the light source and detector within the vacuum chamber is contrary to good vacuum practice, and because of design considerations mounting the source outside the vacuum chamber requires an amplifier for the detector which causes serious electrical interference, a more efficient light source is desired. Such a light source was provided by fiber optic bundles with remote light source t h a t gave a 25° half-angle beam into the vacuum chamber of the instrument, thus solving a complex control problem (10).
Another example of t h e use of fiber optic light guides is in a system using a F a b r y - P é r o t interferometer and high-gain photomultipliers for spectral analysis of scattered laser light from plasmas. I n such a photoelectric multichannel spectrometer, the F a b r y - P é r o t fringe is imaged onto a set of concentric an-
nular light guides, each of which leads to a separate photomultiplier ; the annuli each correspond to the same increment of wavelength when they are of equal area. Fiber optics was used for matching the concentric annuli light guides to their respective photomultipliers (11). Now, for some applications more a t the heart of analytical chemistry. Beroza, Hill, and Norris (12) have recently described an instrument for rapid quantitative scanning of thinlayer chromatograms by measuring diffuse reflectance—the most interesting component of their system is the fiber optic scanning head t h a t conducts light to the T L C plate and conducts the reflected light back to a photocell, where response is registered on a recorder. A second fiber bundle is used to monitor the blank space adjacent to the spots and to correct for background variations, thus providing "double-beam" operation. This innovative procedure
and possible extrapolation of t h e principles to other techniques such as fluorometry and electrophoresis certainly merits our attention. Another application of immediate interest to analytical chemists is the transmission of colorimetric end points by fiber optics (13). For automatic titrations this is accomplished by elimination of monochromators or optical cells used in manual photometric titrators, and replacement with a specially designed photometer and two fiber optic bundles, in accompaniment with a variable-interference wedge filter t h a t makes possible wavelength selection over the entire visible spectrum. T h e ends of the two fiber optic light pipes are located facing each other across the buret tip in the titrating a p p a r a t u s ; one pipe conducts monochromatic light to the sample, and the other pipe carries the light t h a t has passed through solution to a photocell (see Figure 5 ) . T h e device was developed at Fisher Scientific Co. Faceplates are prepared by fusing together an a r r a y of small-diameter, clad fibers as shown in Figures 2 and 6. After suitable surface t r e a t ment such as polishing, grinding, or coating, these faceplates become excellent image transporters and can be used as special image orthicons, cathode ray tubes, vidicons, and intensifiers. The thermal and mechanical properties of these plates are nearly identical with solid glass, and they are thus suitable for transporting an image in or out of a vacuum enclosure. A new range of applications of potential interest to laboratory scientists is thus apparent. Such a faceplate has been used, for example, in image recording in field ion microscopy (14)· Direct contact photography has thus been made possible, some intermediate optics have been eliminated from the conventional apparatus, and shorter exposure times and increased quality of the field ion micrograph have been achieved. As another possible advantage, coupling to an external image intensifier t h a t also has a fiber optic faceplate is feasible. Photographic prints cannot be made directly from a cathode ray VOL. 4 1 , NO. 2, FEBRUARY 1969
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29 A
Report for Analytical Chemists
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coatings in the appropriate places (see diagram) discharge electrons when light strikes them. The elec trons pass through three 15,000-volt electric fields (from a battery-fed oscillation voltage multiplier sys tem) , and the electron acceleration builds a progressively brighter im age. Such night vision devices have been used by the Army in Vietnam for quite some time. Tapered Fibers
COURTESY OF CORNING GLASS WORKS
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Image enlargement or reduction can be obtained by fiber optic face plates as a result of varying the cross-sectional area of the bundle at the input and output (see Figure 8). For example, a bundle with an in put cross-sectional dimension of 1 in. χ 1 in. and an output cross sec tion of 2 in. χ 2 in. will achieve 2 χ magnification at the larger end, or a 2 χ reduction at the smaller end [16). At present these magni fiers are available with a fiber size minimum of about 5 microns, and an output size of 3 in. χ 3 in. Pos sible applications include film enlargers and other areas where ex cellent resolution and low distor tion are required. Other Applications
COURTESY OF OPTICAL SPECTRA
Figure 7. Schematic of night vision device
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