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.
Radiation resistance is another optical fiber property that can be augmented when the vendor has full control of the fiber manufacture. Both UV and ionizing radiation can alter the electronic properties of the fiber matrix through a variety of mechanisms including pushing the atoms of dopant materials into excited states. The end result is reduction of the fiber’s ability to propagate light, darkening the fiber for key wavelengths. Through the use of such techniques as proprietary silica formulations or the control of the matrix structure during preform fabrication, vendors can create fibers that resist or recover from radiation-induced damage.
In addition to offering control over fiber manufacture, specialty fiber manufacturers have the ability to fully control the fiber’s critical core and cladding dimensions. This allows fabrication of fibers with numerical apertures ranging from 0.06 to greater than 0.5, with core sizes selectable from 50 um to 2000 um. Yet these fibers can be made flexible with low micro bending losses for routing the light to a protected location without compromising signal strength.
The availability of specialty fibers allows developers to successfully address a wide variety of application challenges in spectroscopy. In an airborne battlefield threat analysis system, for instance, specialty fibers solved an installation challenge. The system needed to be helicopter mounted, using a high-powered laser to excite atmospheric target zones and imaging instruments to gather the resulting emissions for analysis. Unfortunately, the system required the stimulus to be emitted from the nose of the helicopter platform and the laser was too heavy and bulky to be safely mounted in the preferred location. A special fiber capable of handling high optical power levels allowed mounting of the laser farther back in the craft with the fiber transferring the beam to its preferred location.
High temperature operation was the critical factor in a diagnostic tool for spectroscopic monitoring of engine performance on boats and planes. A fiber bundle that could withstand temperatures as high as 700°C allowed the instrument to look inside the engine’s combustion chamber. This allowed direct, real-time analysis of fuel combustion rather than inferring the chemistry from the exhaust vapor.
The wide acceptance angles and large core diameters available in specialty fibers help in a variety of ways. A wide acceptance angle (high NA) makes gathering light from a sample easier and more efficient, for instance, because it eases restrictions on alignment and simplifies lensing (Figure 2). The same is true in reverse, when the fiber carries an illumination source such as the helicopter-carried laser. The large core diameters also help ease lens requirements by allowing larger spot sizes. If fibers are used in a bundle, large cores yield greater active areas at the bundle’s end, resulting in greater light gathering efficiency.
Choosing a Fiber
The wide range of options that specialty fibers make available requires that developers fully consider their application needs before placing an order. Some of the key questions to resolve are:
What wavelengths must the fiber carry efficiently?
What is the illumination source for this application? This helps identify the numerical aperture and power-handling ability the fiber must offer.
What is the operating environment – temperature, moisture, radiation, etc. – that the fiber must operate within?
How is the fiber handled during the application? Possibilities include reeling off a spool, being paid out and rewound repeatedly, suffering sharp bending during installation or repetitive bending during use (as with a fiber on a flexible robotic arm), and the like.
Are there bundling requirements? Many spectroscopy applications, for instance, use a bundle of six fibers to capture the signal wound around one fiber carrying the illumination.
What are the fiber’s lifecycle factors? Is it to be a disposable one-time use or must it last for years in its installed environment?
With the answers to these questions in hand, developers will be able to work with specialty fiber vendors to identify materials and dimensions for the fiber. The vendors may also be able to provide assistance in bundling the fibers, adding end-ferrules, or developing customized mounting or alignment hardware.
Specialty fibers thus dramatically expand the range of applications for spectrometry as well as anywhere else light must travel some way other than via line-of-sight. With the right materials for core, cladding, and coating, these fibers can handle whatever wavelengths are needed and can tolerate temperature, radiation, and other harsh environmental conditions with ease. By extending spectrometers’ views of the world, both instruments and operators can reside in safe and secure locations while the fibers tackle the environments where the going gets tough.