Fiber-Optic Basics

The range of acceptance angles for an optical fiber is generally stated as the numerical aperture (NA). For the case of the square-cut fiber shown in 

Fortunately, low-loss fibers are available over most of the visible and NIR wavelengths in common use for Raman spectroscopy. As an example, some measured attenuation values for a commercial, 200 pm fiber at argon ion laser wavelengths are listed in Table 12.1 (5). Over a 3 meter fiber length appropriate to laboratory use, light loss is trivial, while 100 meter lengths are possible with moderate light loss. Since commercial Raman probes are 

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Data Communication Wires. Electronic cables such as data communication wires employ three basic designs coaxial, twisted pair, and fiber optics (3,4) (Eig. 1). Coaxial cables are so named because the axis of curvature of its outer conductor is concentric to its inner central wire. The metal braiding wrapped around the insulated center wire acts as the return current conductor in addition to shielding the wire from various interferences. 

A number of areas in which plastics are used in electrical and electronic design have been covered there are many more. Examples include fiber optics, computer hardware and software, radomes for radar transmitters, sound transmitters, and appliances. Reviewed were the basic use and behavior for plastics as an insulator or as a dielectric material and applying design parameters. The effect of field intensity, frequency, environmental effects, temperature, and time were reviewed as part of the design process. Several special applications for plastics based on intrinsic properties of plastics materials were also reviewed. 

This chapter provides an overview of the basic principles and designs of such sensors. A chemical sensor to detect trace explosives and a broadband fiber optic electric-field sensor are presented as practical examples. The polymers used for the trace explosive sensor are unpoled and have chromophores randomly orientated in the polymer hosts. The electric field sensor uses a poled polymer with chromophores preferentially aligned through electrical poling, and the microring resonator is directly coupled to the core of optical fiber. 

Pulse Code Formats for Fiber Optical Data Communication Basic Principles and Applications, David J. Morris... 

Lasers, 9 729 14 654-706. See also Lasing atomic systems in, 14 666-669 basic mechanism of, 14 656—661 buried heterostructure, 14 701 carbon dioxide, 14 693-696 carbon monoxide application, 5 24 cavity optics and, 14 669-672 classes of, 14 666-667 cutting applications of, 14 695-696 dye, 14 702-705 effect of loss in, 14 670 excimer, 14 691-693 fast pulse production in, 14 673-678 fiber optics and, 11 129 in fine art examination/conservation, 11 412, 413... 

It is desirable to have means to measure organohalides such as carbon tetrachloride in situ in water and other environmental media. One approach to doing this has been demonstrated by the in situ analysis of chloroform-contaminated well water using remote fiber fluorimetry (RFF) and fiber optic chemical sensors (FOGS) (Milanovich 1986). With this approach, fluorescence of basic pyridine in the presence of an organohalide (Fujiwara reaction) is measured from a chemical sensor immersed in the water at the end of an optical fiber. Carbon tetrachloride undergoes a Fujiwara reaction, so its determination might be amenable to this approach. 

Sampling with fiber optic sensors can be continuous if needed otherwise they can be operated discontinuously, with a lower duty cycle. These sensors could be used for laboratory-based or in situ applications. The cost of instrumentation for fiber optic systems should be 25,000 to 50,000. Sensors would need to be replaced periodically (several weeks to many months), depending upon their design. Sensors using fiber optic probes will be available within 5 years for some applications and within 10 years for some others. Sensors for pH, C02, and 02 are in development now new sensors should be capable of measuring from high concentrations down to 1 part per million for ions and organic materials. Basic research is still required for specific applications. 

Figure 10. Basic elements for flourescent fiber optic chemical sensors a. Optode Structure Design after Lubbers and Opitz (69) b. Optode Structure Design after Hirshfeld, et al. (71).

There is no doubt that Raman spectroscopy is already a powerful tool in basic and translational medical research. However, there is still room for development of new devices to facilitate practical applications. The fiber-optic Raman... 

The innovative thermostated separation system published by de Bokx et al. [17] represents an interesting example and comprises a capillary cross intersection for sample injection and a 100 pi fluorescence detector cell based on fiber optics. This apparatus shows basically all features that are required to perform automated fast and efficient electrophoretic separations and has been used to separate a mixture of laser dyes in 35 seconds with moderate efficiency. However, in order to keep all dead volumes at the junctions sufficiently small, the connections had to be done by tedious laser-based drilling of holes through the capillary walls. A similar approach to interconnect capillaries was described for a postcolumn derivatization reactor for CE [18], and many more inventive capillary coupling devices have been designed. 

Weiner et. al. [291] determined the particle size distribution as a function of concentration for a number of colloid suspensions. The results showed the advantage of using single mode fiber optics as a practical tool. A comparison made between this and a previous design was presented and several limiting features summarized. A recent review with 54 references covers basic physics, and experimental methods [292]. Applications of the technique to the determination of mean particle diameter, polydispersity and higher order moments are discussed. 

The basic theory and discussion of results are covered in papers by Thomas [308]. who uses a Brookhaven Instrument Fiber Optics Quasi-Elastic Light Scattering System (BI-FOQELS) with dynamic light scattering obtained using the BI-DLS and diluted samples. 

The basic components of a spectrophotometer include (1) a light source (2) a means to isolate light of a desired wavelength (3) fiber optics (4) cuvets (5) a photodetector (6) a readout device (7) a recorder and (8) a microprocessor. 

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