Kelly Moran, CeramOptec Industries Inc.
The intersection of spectroscopy and fiber optics continues to give rise to some very interesting results. Spectroscopy, with its ability to determine chemical and mineralogical composition, temperature, emission and molecular activity, is still growing as a primary agent of discovery and measurement in science and engineering. Meanwhile, optical fibers carry visible light in a massive array of applications — from tiny medical devices to huge animated billboards — as well as energy across the entire ultraviolet to infrared spectrum. Where they intersect, fibers increase spectroscopy’s range, timeliness and sensitivity, while at the same time lowering costs.
The basic strengths of fiber optics dovetail neatly with the needs of spectroscopy. A spectroscopic investigation often requires precise delivery of electromagnetic energy to a target, and it always requires the precise detection of transmitted or reflected energy from a target. Fibers provide a highly customizable means of energy transmission both to and from targets. Additionally, they can convey energy across or through tortuous paths — literally around corners and through walls — while also isolating spectrographic instrument components from any hostile ambient conditions at the target. Similarly, optical fibers offer the freedom of movement to follow targets that are in motion.
Another benefit is that they can decrease the time required for analysis, enabling highly accurate on-site analysis with portable spectrographs, such as in point-of-sample environmental water-quality monitoring. Formerly, when samples had to be collected and transported to a lab, time, temperature and even en route vibration could play havoc with the target sampling. And modern on-site equipment sometimes costs a mere fraction of earlier, lab-based units.
Where are fiber-based spectroscopic applications being used? A full listing would take pages, but some examples include scientific research, medicine, forensics, pharmaceuticals, astronomy (including the Mars Exploration Rover Mission that recently went into full swing), paper manufacturing, polymer science, mineralogy and geology, environmental monitoring and food processing.
A few representative uses include the measurement of histochemical levels of medication and oxygenation in patients, determination of the composition of objects in space from suns to planets to plasma, imaging of molecules in DNA and other research, hydrocarbon exploration and refining, and in situ monitoring of hazardous waste and environmental phenomena ranging from river water composition to atmospheric haze to waste digester operation.
But there are, of course, critical differences among types of optical fibers. Standard silica-based, commodity fibers (such as those used in telecommunications or computer networking) operate more quantitatively than qualitatively. At a cost of a few cents per meter, they can transmit electromagnetic energy pulses over kilometers, but precise qualitative information about spectral strengths at specific wavelengths is neither required nor targeted for preservation during transmission.
In contrast, fibers for spectroscopy must provide highly qualitative transmission, and a variety of specialized fibers drawn from a wide range of formulations has sprung up in the past 10 years. Laser delivery fibers, for example, can be made from calcogenide, heavy metal fluoride, polycrystalline silver halide, thallium halide, single-crystal sapphire and silica. Each offers specific strengths in wavelength energy preservation, fingerprint, resolution and sensitivity, as well as application-specific needs such as biocompatibility for medical uses.
The variety of optical fiber applications in spectroscopy continues to increase. Fueling this growth, in part, is a general advance in the science and engineering of spectroscopy, including burgeoning libraries, ever-expanding ways to differentiate increasingly subtle spectrographic profiles, development of increasingly powerful detectors that can be manufactured at relatively low cost and advances in scientific computing.
But contributing equally are developments in the fibers themselves. Purity of materials has gone ever higher, minimizing transmission losses or distortion traceable to such contaminants as water, inclusions or unwanted compounds. Cladding materials, critical in maintaining internal reflectivity to minimize attenuation, have grown in sophistication. Choice of fiber diameters, bundling options and a myriad of other product variations permit configuration of optical systems that are highly tuned to their applications in spectrum width, transmissivity, conductivity and freedom from distortion.
Three important recent advances have strengthened the relationship of fiber to spectroscopy. The first is an increase in the numerical aperture rating of high-sensitivity optical fibers for spectroscopy. The second is the availability of fiber capable of withstanding elevated ultraviolet energy — nonsolarizing fibers. The third is the development of optical fibers that can extend use at high heat as well as rapid transition from elevated temperatures to low ones.
Numerical apertures of pure silica fibers for the ultraviolet, visible and near-infrared spectral ranges have increased from 0.22 to 0.30, 0.44 and even 0.53, allowing instruments both to gather more energy and to capture a wider field of view. For example, high-numerical-aperture fibers have proved useful in milk monitoring and other “in process” applications.
Advances in UV optical fibers permit long exposure to high levels of laser and other UV energy without damage. Although UV spectroscopy goes back decades, it is only in the past 18 months that nonsolarizing fibers have been commercially available. Applications that are especially aided by nonsolarizing fibers include the characterization and analysis of materials using excimer lasers and high-power UV lamp sources. These UV-resistant fibers permit long-term exploitation of a rich spectroscopic region.
An assortment of UV nonsolarizing, high-numerical-aperture (0.54) silica/silica fused-end fiber bundles is available for a variety of spectroscopic applications.
Optical fibers can now reach upward of 1500° C, while previous limits were around 400° C. Probes made of the ultrapure material will not melt or outgas. In addition, the material can be quickly chilled, making it ideal for uses that require relatively quick probe application to and withdrawal from heat faces or rapid chilling over a short distance. Heat resistance brings spectroscopy to areas that formerly would have destroyed equipment, from food preparation at the relatively mild end to industrial furnaces and jet and rocket propulsion at the other extreme.
As with much activity surrounding scientific instrumentation, there is often a significant market lag around innovations. New entries into the market must be fingerprinted, validated and calibrated. More fundamentally, instrument manufacturers themselves need time to incorporate advances. Optical fibers have had UV capabilities for more than 10 years, yet adoption of them for UV spectroscopy has been only relatively recent, particularly since the introduction of nonsolarizing UV fiber.
If nothing else, this inevitable lag is a primary reason for users of spectroscopy to work closely with producers of specialized fibers, so that those who must forever wring increasing productivity from equipment can take advantage of technical and economic advances as quickly as possible.
Contact: Kelly Moran, chief operating officer, CeramOptec Industries Inc., East Longmeadow, Mass.; e-mail: [email protected].