Silica-Based Fiber Boosts Broad-Spectrum Spectroscopy

Teodor Tichindelean, Polymicro Technologies

Fibers with a broad optical spectrum are useful in spectroscopic applications because they enable the collection and analysis of more information over a greater wavelength range.

A variety of spectroscopic applications require light transmission over a broad spectrum. Optical fiber with a broad optical spectrum can transmit a wide range of wavelengths with relative uniformity across the wavelength range. This is particularly advantageous in spectroscopic applications because it expands the measurement range and sensitivity of the device. In many cases, it allows the spectrometer to be remotely located, connected to the analysis areas through broad-spectrum optical fibers. As a result, more spectroscopic information over a greater wavelength range can be collected and analyzed.

Figure 1. The setup for the FBPI fiber’s spectral attenuation test.

In the field of fiber sensing, there are many industrial, chemical and biomedical applications that could benefit from the capability to measure optical signals across a very wide spectral range. But optical fibers have been historically limited in their transmission spectral range: Fibers with high -OH content perform better at UV wavelengths; however, the -OH content creates very large absorption regions in the near-IR wavelengths, particularly at 980, 1250 and 1383 nm. Conversely, fibers with low -OH content can perform very well in the near-IR region of the spectrum, but tend to exhibit poor UV performance. While both types of traditional fibers transmit well in most of the visible spectrum, they are both limited in spectrum range.

A new broad-spectrum optical fiber with a low -OH pure silica core demonstrates significantly reduced content of UV defects and other UV absorption centers. This new fiber effectively combines the benefits of standard fibers while mitigating the drawbacks. Exhibiting improved transmission properties over a much wider spectral range, the silica-based, broadband FBPI fiber can be produced in core diameters from 50 to 600 µm.

In extensive spectral performance testing using different light sources, including a deuterium lamp and tungsten-halogen lamp, the attenuation of the new fiber in the near-IR wavelength region to beyond 2100 nm is equivalent to standard near-IR fibers having a low -OH silica core and F-doped cladding.

Figure 2. The spectral attenuation performance comparison between the FBPI fiber and three other Polymicro fibers.

Another important characteristic of this new optical fiber is the degree of resistance to solarization damage caused by exposure to high levels of UV illumination. It has excellent UV transmission down to 200 nm, and significantly reduced UV- defect concentrations, so that its solarization degradation properties are comparable to standard UV-optimized high -OH fibers with high radiation resistance.

Spectral attenuation performance

The fiber’s attenuation has been rigorously tested in three different wavelength bands: UV (200 to 400 nm), visible (400 to 900 nm) and near-IR (900 to 2100 nm), using three Ocean Optics fiber optic spectrometers, each designed for the specific wavelength range. Because the attenuation varies at the different wavelength ranges, different lengths of fiber are used for optimum test sensitivity. Specifically, 15-m lengths are tested in the UV range, while approximately 200-m lengths are used for visible and near-IR testing. The source used for UV testing is a deuterium lamp. For visible and near-IR testing, the source is a tungsten-halogen lamp.

Figure 3. The setup for the four-hour UV-exposure test.

Attenuation was tested using a cutback method (Figure 1). Light is launched through the full length of the test sample, and then the spectrum received by the spectrometer is recorded. Then the sample is cut back to a short 2-m length, and this spectrum is also recorded. Comparing the two spectra wavelength by wavelength, the attenuation is calculated as a function of wavelength.

Figure 2 shows the charted spectral attenuation performance comparing the new broad-spectrum fiber, optimized for near-IR attenuation and UV solarization resistance, against three other Polymicro fiber types:

• Low -OH: A standard FIP fiber for near-IR.

• High -OH: FVP (a standard high -OH fiber for UV/visible), UVM (UV-optimized high -OH preform), UVMI (hydrogen-loaded UVM fiber) and FDP (deep-UV-optimized fiber with high UV-radiation resistance).

• FBP: An existing broad-spectrum fiber, not optimized for near-IR attenuation or UV solarization resistance.

Figure 4. Results of the four-hour UV-exposure test for the new FBPI fiber.

As shown in Figure 2, the new FBPI fiber demonstrates the optimum performance of high -OH fibers in the UV and low -OH fibers in the near-IR.

UV-solarization resistance

Silica optical fibers can be susceptible to UV-induced attenuation. Transmission loss is caused by damage induced by solarization. Most of the attenuation occurs in wavelengths less than 250 nm, with the peak damage occurring at 214 nm. The degree of damage varies greatly with the type of fiber. Figure 2 shows a comparison of the new fiber’s UV exposure performance to that of other fibers used in the UV.

Figure 5. Results of the four-hour UV-exposure test for standard high -OH FVP fiber.

Solarization resistance is evaluated using the four-hour UV-exposure test (Figure 3) using a 2-m segment of fiber. Light from a high-intensity deuterium lamp is launched into the fiber using a focusing lens to maximize intensity at 214 nm (generally the most sensitive wavelength for UV solarization). The output of the test sample is monitored using a UV spectrometer (this test used one from Ocean Optics), and data is collected for four hours.

Six important wavelengths are traced throughout the test process: 214, 229, 245, 255, 266 and 330 nm. Also, the entire spectrum is measured and compared at the beginning and end of the test. The degradation rate of each wavelength decreases as the test progresses, ideally reaching a saturation point before the four-hour test has ended. Quick saturation is a desirable quality in a UV fiber, along with minimum degradation. The level of degradation at saturation is mostly independent of light intensity; increasing the intensity only tends to change the speed with which the saturation is reached.

Figure 6. Results of the four-hour UV-exposure test for UV-optimized UVM fiber.

Results of the four-hour UV-exposure test are shown in Figure 4. For comparison, data for other fibers commonly used in the UV region are also shown. Standard high -OH FVP fiber, often used in visible/UV applications, is shown in Figure 5. The data for UVM fiber, drawn from a special preform optimized for UV performance, is shown in Figure 6. And finally, the data for FDP fiber, a fiber optimized for deep-UV operation with high radiation resistance, is shown in Figure 7.

The test results show that the new broadband fiber has solarization properties significantly improved over the standard high -OH FVP fiber, while reasonably comparable to the UV-optimized UVM fiber.

Figure 7. Results of the four-hour UV-exposure test for deep-UV FDP fiber.

Potential applications

Combining the UV performance of a high -OH fiber and the near-IR performance of a low -OH fiber, the FBPI broadband optical fiber has potential for use in applications that require sensing over a wide range of wavelengths. Prototypes are already in development using the new fiber: long-life broadband light sources with fiber-coupled output for a simpler optical system. Designed to eliminate requirements for multiple lamps (deuterium/tungsten/xenon arc), the compact light sources deliver very high brightness across a complete UV-VIS-NIR spectrum (170 to 2100 nm).

Figure 8. The new FBPI combines the UV performance of a high -OH fiber and the near-IR performance of a low -OH fiber.

The capabilities of the FBPI fiber have the potential to propel spectrograph development in a range of existing and emerging applications that would benefit from broader-spectrum chemical analysis. As full-spectrum readers are developed to keep pace with optical fiber technology, this fiber has the potential to replace traditional fiber in a range of applications, including broadband spectroscopy, advanced imaging, optical fiber testing, environmental monitoring, gas phase measurements, precision surgical and industrial lasers, and gas or liquid chromatography.

Meet the author

Teodor Tichindelean is global product manager at Polymicro Technologies in Phoenix; email: teo.tichindelean@molex.com.