In spectrometry, the more light gathered the better the results. Yet in many applications, both military and commercial, the need to protect instruments or operators from harsh environments, as well as installation constraints, make conventional line-of-sight optics impractical. Specialty fibers can offer the robustness and light-handling characteristics that such applications require.
In the past decade, small and remarkably inexpensive spectrometers have appeared that can determine the “chemical fingerprint” of a wide variety of liquids and gasses. These devices have seen use in a host of diagnostic and field analysis situations involving explosions, flames and combustion, and airborne contaminants. They have been particularly important in military applications for rapid detection and analysis of potential chemical threats in battlefield environments.
For these spectrometers to be effective they need to gather enough light to perform a rapid and accurate spectral analysis. They may also require the use of a stimulus light source to excite a response from a target in order to make an analysis. In many applications, however, the harshness of the operating environment or complexities in installation and mounting make conventional optics impractical for these purposes. Optical fibers have the potential to overcome such restrictions.
Conventional, mass-produced optical fiber, however, may not be adequate for many of these challenging situations. Most of the widely-available fibers were developed for telecommunications system use and have several operating limitations. One is that they come in fixed sizes – typically 8 um single-mode and 62.5 um multimode core diameters – with relatively low numerical apertures. These characteristics are useful in communications, where they serve to minimize the capture of stray light when a laser diode is the optical signal generator. In spectrometry, especially where the goal is to capture light over a wide area, these characteristics are sub-optimal.
Because telecommunications fibers are intended for installation in buildings, conduits, and other relatively benign environments, they do not offer much resistance to the effects of harsh environments. The polymer cladding and plastic coatings typically used on telecom fibers cannot tolerate temperatures above 190°C. In the presence of radiation, as well as UV wavelengths, telecommunications fiber core materials can suffer darkening and the formation of color centers that reduce light-gathering effectiveness.
Where telecommunications fibers fall short, specialty fibers can fill the gap. Specialty fibers can use a variety of pre-form materials such as silver halides and fluorine-doped silicas that allow them to handle light ranging from UV well into the mid-IR range, and they can be broad-spectrum or optimized for specialized wavelengths (Figure 1). Control of the entire process, from preform manufacture to finished assembly, also allows the creation of fibers that are heat resistant to temperatures above 375°C, fibers that are resistant to radiation darkening, and low and high NA (numerical aperture) fibers for broad light-acceptance angles (NA = sin(acceptance angle)).
Control of the fiber manufacture is not only important for maximizing transmission at the desired wavelengths, it is essential for controlling other optical properties. Some fiber materials, for instance, can fluoresce in response to certain wavelengths. This fluorescence generates stray lightwaves that contaminate signals coming from a sample to the spectrometer and can reduce the accuracy of results. Specialty fibers drawn from preforms fabricated to reduce fluorescence, especially in the blue and green wavelengths, have thus been essential to the use of fibers in medical and biological spectroscopy applications.