It was just a matter of time before spectroscopists (and chemists of all persuasions, too) discovered the utility of optical fibers for chemical analysis. In fact, one could say that fiber-optic-based technologies are literally allowing analyses of samples and process streams outside the spectrophotometer.
In addition to remote analyses, optical fibers-together with solid-state detectors and laser diodes optimized for use in the near-IR, IR, UV, and visible regions-are enabling the development and construction of small and truly portable instruments.
A catchall technology, fiber optics refers to a series of technologies that generate, transmit, receive, and convert light to usable signals. Applied to spectroscopy, fiber optics includes sampling devices, optical fibers, components, and other technologies in the light train between the sample and the spectrophotometer.
FIBER-OPTIC BASICS
Optical fibers consist of a silica fiber core,
cladding, buffer coating, and sheath. The core is usually fused silica,
5-600 |gmm or larger in diameter; the larger the core, the more light the
fiber can carry. Depending on the fiber and the application, plastic fibers
and fibers with different kinds of glass and core diameters are available.
These fibers include those that are optimized to carry near-IR, IR, or UV
and visible radiation from traditional spectroscopic light sources as well as
the intense high-power light from lasers.
The cladding, a thin layer of glass, plastic, or polymer coating with an index of refraction lower than the core, surrounds the core. The function of the cladding is to reflect light back into the core as it moves down the fiber. The buffer coating is a plastic or a polymer that protects the core and cladding from moisture, scratches, and other contamination while imparting additional strength to the fiber. Finally, a metallic, plastic, or polymer sheath provides additional strength, stiffness, protection, and resistance to stray light and moisture.
Three types of optical fiber are commercially available. The distinguishing characteristic of each type is the core-cladding combination and the resulting optical properties that affect the way light travels through the fiber. In step index fibers, a 100- to 1500-|gmm diameter core is surrounded by cladding that has a lower refractive index relative to the core. Light waves are reflected back into the core at the core-cladding interface, causing them to propagate down the fiber. In addition, because the speed of light at different wavelengths is a function of refractive index, the speed of transmission of different wavelengths varies. Step index fibers are typically used for spectroscopy, medical laser beam delivery and medical diagnostics systems, and industrial sensor systems.
In graded index fibers, cores with diameters from 50 |gmm to about 600 |gmm are surrounded by a layer or layers of cladding with successively lower indices of refraction. Light waves are reflected from the outer layers back to the center of the core and propagated down the fiber, compensating for the change in the refractive index-related changes in the speed of light. Graded index fibers are typically used for applications in which transmission of light pulses is important. Both step index and graded index fibers are multimode fibers because they carry polychromatic light.
Enlarge ImageSingle mode fibers, which carry monochromatic light, have the smallest diameter cores and no cladding. Because of their small diameters (5-10 |gmm), light waves propagate down the fiber virtually unchanged by refraction or reflection. Thus large amounts of data in the form of light pulses can be transmitted with low distortion. These fibers are typically used for telephone and ultrahigh-speed data communications. In fact, because they are immune to electromagnetic interference, fiber-optic cables are widely used as replacements for copper-based coaxial cables in supervisory control and data acquisition (SCADA) and other process control systems.
FIBER CHARACTERISTICS
The optical and mechanical characteristics
of the fiber are important considerations for fiber-optic spectroscopy
systems and their applications. For example, low-OH fibers (fibers with low
or ultralow levels of moisture or hydrated water-typically 2 ppm or less) are
suitable for near-IR and IR spectroscopy because the broad intense absorption
band attributable to the hydroxyl groups is absent. Fibers with high OH
levels (around 800 ppm), however, can be used for UV or visible spectroscopy.
In addition to silica fibers with low- and high-OH content, borosilicate,
quartz, zirconium fluoride, silver halide (AgCl:AgBr), chalcogenide (complex
glasses composed of tellurium, selenium, antimony, and germanium), and
plastic fibers are available, although not all are suitable for spectroscopy.
Depending on fiber composition, the length of the fiber is also limited.
For example, the low-OH fibers offered by Galileo Electro-Optics Corp.
(Sturbridge, MA) are limited to 200 m, zirconium fluoride fibers to 50 m,
and chalcogenide fibers to 4 m. Fibers offered by other original equipment
manufacturers have similar optical and mechanical limitations, depending on
the application, optical characteristics, and construction.
Claddings are usually silica, but other glasses, silicone, and other polymers may be used; however, the refractive index must be lower than the core to allow light to propagate through the fiber. Because polymer claddings can be removed with a suitable solvent, fibers with polymer-based claddings can be used for applications requiring the internal reflectance properties of the bare fiber in an application called evanescent wave spectroscopy.
Numerical aperture, another important characteristic of the fiber, is the sine of the critical angle of incidence above which the fiber will not accept light and is related to the difference in refractive indices of the core and the cladding. High numerical aperture fibers collect more "off-center" light, whereas low numerical aperture fibers require a more collimated beam. Spectral attenuation of the fiber is another design parameter that should be considered with respect to the overall performance of a fiber-optic system. Spectral attenuation or transmission losses are attributed to the purity of the glass used for the core as well as the quality of the cladding and the core-cladding interface. These quality parameters influence transmission losses caused by dispersion, absorption, reflection, and scattering. Transmission losses are determined by measuring the absorption spectrum of the fiber over the wavelength region of interest. Although vendors report attenuation per kilometer of fiber, as a practical consideration Kirk Grim, product marketing manager, Galileo Electro-Optics Corp., points out that the distance light travels to a remote sensor is typically a round trip involving both the outgoing and return light beams.
Another source of transmission loss is microbending, which results from minute curvatures in the fiber that exacerbate the losses caused by reflection, absorption, and scattering. Microbending can be minimized by stiffening or increasing the strength of the assembled cable (core, cladding, buffer, and sheath). Although vendors provide information about short- and long-term bend radii, Fiberoptics Technology, Inc. (Pomfret, CT), explains in its technical literature that, in addition to measuring the spectral attenuation of the fiber, one should characterize spectral attenuation for the entire light train, including couplings and interfaces, the sampling device, the fiber, and the spectrometer.
Just as stainless steel HPLC tubing must be properly cut and prepared to ensure leak-free connections, the ends of optical fibers must be properly cleaved and polished to maximize optical efficiency. "One has to make sure that the coupling between either the fiber and the instrument or the fiber and the probe is optically efficient," says Vince Catalano, product manager, UOP Guided Wave (El Dorado Hills, CA). "To have an optically efficient coupling, you need to make sure the fiber-optic connector is polished properly and of the correct distance to mate properly with the probe. If you have a poor coupling, you may induce fringe patterns, which are basically sinusoidal waves in the spectra."
APPLICATIONS
What can fiber optics do? As an enabling
technology, fiber optics allows in situ, real-time or near-real-time
measurements of dynamic systems in both chemical process streams and
laboratories. Much of the work by vendors of probes has focused on the
development and construction of fiber-optic probes for near-IR and IR
applications. The specifics of what people are actually doing with the
technology are harder to come by because of trade secret issues. However,
the applications reported in the literature and publicized by vendors or
users appear to be the tip of the iceberg. In addition, vendor confidence
and continued development of fiber-optic-based technologies for spectroscopy
are indications that the chemical and chemical process communities are
focusing on the technology.
For example, charge-coupled device detector technology, optical fibers, compact light sources, and inexpensive PCs enabled Ocean Optics, Inc. (Dunedin, FL), to develop a miniature fiber-optic spectrometer. According to Leeward Bean, vice president of marketing and a company partner, the idea for this instrument was born when his partner, marine biologist Michael Morris, was asked by the US Department of Energy to adapt a phenol red fiber-optic pH measuring system for use as a free-floating oceanographic pH sensor. After finding that low-cost single-strand fiber-optic spectrometers were unavailable, they developed their own. For as little as $1,999, the company now offers the PC1000 PC Plug-in Fiber Optic Spectrometer mounted on an A/D board. By selecting an appropriate grating, light source, fiber, and detector, the basic instrument can be used for analyses in the near-IR, UV, or visible regions.
In one application, the instrument is used as a field-portable spectrometer to determine qualitatively and quantitatively the type and extent of crude oil, diesel fuel, gasoline, and solvent contamination of soil and water. The field test, invented and patented by John D. Hanby, president of Hanby Environmental Laboratory Procedures, Inc. (Wimberly, TX), involves the generation of an intensely colored precipitate by a Friedel-Crafts catalyst (a stoichiometric excess of AlCl 3) in a solvent extract of soil or water and subsequent measurement of the tristimulus values of the precipitate. (Tristimulus values are related to measurement of the color space perceived by the eye at red, green, and blue wavelengths.) The method is suitable from 1 to 50,000 ppm. In addition to quantitative measurement, Hanby says, "We can differentiate between gasoline and diesel fuel because gasolines will normally have more of the BTEX-type mono-aromatics (reds and yellows), whereas diesel fuels and heavier fuel oils will have considerably more of the polynuclear aromatic compounds that will give you more of a bluish color."
In an on-line process monitoring application, N-Visions (Antioch, CA) used Ocean Optics's spectrometer on a card along with a suitable fiber-optic light guide and light sources to develop a system for monitoring the deposition of photochromic coatings on ophthalmic lenses. According to Bryan Barney, president of N-Visions, such a system costs about $7,000, whereas other fiber-optic spectrophotometer-based control systems cost about $20,000.
In addition to enabling the construction of versatile process-monitoring systems, fiber-optics technology makes it possible to do more in situ analyses of laboratory samples and process streams. For example, Spectra-Tech, Inc. (Shelton, CT), offers what the company calls Needle-Probes for reflectance and attenuated total reflectance measurements as well as a flow cell and a probe for diffuse reflectance. Connected to an FT-IR spectrometer via chalcogenide fibers and their FiberLink interface, IR spectra from 4,000 cm -1 to 900 cm 1 of a wide variety of solid, powder, and liquid samples can be measured, as is, up to 1.5 m from the spectrometer. Nicolet (Madison, WI), Galileo Electro-Optics, Oriel Corp. (Stratford, CT), and other companies offer similar probes.
Don Rasmussen, professor of chemistry and member of the Center for Advanced Materials Processing at Clarkson University (Potsdam, NY), is using home-built, fiber-optic probe-based systems to study the dynamics of polishing, precipitation, and crystallization processes. "We are able to probe the liquid with a light probe that can [determine the] size of the particles (particle size distribution) by dynamic light scattering to tell us what is happening in terms of the growth of the particles or injection of new particles into the liquid," explains Rasmussen. "One of the [key] advantages of the fiber-optic dynamic light-scattering method is the opportunity to measure solutions with high particle concentrations without having to dilute the solution during the measurement cycle."
Closer to the chemical process arena and reported in the literature, chalcogenide fibers and FT-IR spectroscopy were used to characterize the curing of a graphite fiber/polyimide prepreg (1) and polyurethane foam (i>2). In the first example, reported in 1988, a bare 1-m length of 120-|gmm-diameter fiber was used as an internal reflectance element in a detailed study of a graphite fiber/polyimide prepreg curing reaction. The researchers observed spectroscopically the loss of amine and ester groups at low temperature, the buildup of intermediate amide groups, and the conversion of intermediate amide and acid groups to imide groups at higher temperatures. In the second study, researchers using a bare 1-m length of 200-|gmm-diameter fiber as an internal reflectance element observed the changes that occur during the polyurethane-curing process. They measured the appearance and disappearance absorption bands attributed to free urea and urethane, hydrogen-bonded urethane, monodentate and bidentate hydrogen-bonded urea, isocyanate, and isocyanurate and obtained information about the reaction kinetics and morphological development of the foam.
For process analyzer chemists and process control engineers, a variety of fiber-optic probes and accessories are commercially available for near- and mid-IR analyses of liquids and gases at ambient and high-temperature and pressure conditions. These instruments include gas and liquid transmission cells and probes as well as probes for diffuse reflectance and attenuated total reflectance measurements. Although such devices are popularly called probes, Walter Doyle, president, and Norm Jennings, executive vice president, Axiom Analytical (Irvine, CA), call them "sample interface equipment for spectrometry." At least two vendors (Galileo Electro-Optics and UOP Guided Wave) market fiber-optic multiplexers that allow the use of one spectrometer to monitor many probes.
The design and construction of these probes involved maximizing optical efficiency and developing solutions to problems posed by stray light as well as the effects of high temperature, pressure, flow, and vibration on the mechanical and optical integrity of the probe itself. "The probe is simply one part of the entire optical train. You have an analyzer and fiber optics; each has to be optimized for maximizing light throughput,|P' says Catalano of UOP. "Many fiber-optic probes have been hooked up to many different types of analyzers only to fail because the analyzer itself was not suited to accept fiber optics." As a result, interfaces between commonly available spectrometers and optical fibers have been developed. One vendor cautions, however, that "there could be a great deal of difficulty in taking a fiber-optic probe and a set of fiber optics and just hooking it up to any spectrometer because of efficiency issues. You could poorly launch light into a fiber and get poor performance," he adds.
Another approach, used by Axiom, to optimize the sampling device-the probe-is to use light guides (of which optical fibers are just one type). This approach involves the use of quartz rods and nickel- and gold-coated stainless steel tubes. These light guides are optically isolated as well as thermally and mechanically stable, and the optical characteristics of the probe itself are less affected by stray light and mechanical stress. In addition, the materials, seals, and welds used to build the probe need to be matched to the process.
Process analyzer chemists at Dow (Midland, MI), 3M (St. Paul, MN), FMC (Princeton, NJ), and Exxon (Baytown, TX) are using or just starting to use probe technology to monitor adhesives, polymers, organic liquids, and solvents. Citing trade secrets, however, these users refused to be specific about their applications; similarly, vendors refused to say who's doing what and where.
UOP's Catalano, however, notes the company has installed more than 100 fiber-optic-based systems measuring more than 500 sample points for closed-loop control in chemical plants around the world. Obviously there is a great deal of activity in this area by both vendors and users. And if one has experience with a conventional spectroscopic measurement, chances are a fiber-optic-related technology is available for the measurement, too.
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