New Broad Spectrum Fiber for Fiber Sensor and Spectroscopy Applications
DOWNLOAD PDF

John Shannon, Valery Khalilov and Rick Timmerman
Polymicro Technologies, a subsidiary of Molex Incorporated
18019 N. 25th Avenue
Phoenix, AZ, USA 85023
Tel: (602) 375-4100

Introduction

In the fields of fiber sensing and spectroscopy, many applications require the capability to measure optical signals across a very wide spectral range. Historically, optical fibers have been 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 Infrared (NIR) wavelengths, particularly at 980, 1250 and 1383nm. Conversely, fibers with low -OH content can perform very well in the NIR region of the spectrum, but have very poor UV performance. Both types of traditional fibers transmit well in most of the visible spectrum.

A new silica-based fiber design has been developed at Polymicro Technologies which exhibits improved transmission properties over a very wide spectral range. This fiber is known as FBPI fiber and can be produced in core diameters from 50μm to 600μm. In the NIR wavelength region to beyond 2100nm, the attenuation of the new fiber is similar to standard low -OH fibers. Simultaneously, the fiber has excellent UV transmission down to 200nm comparable to standard high -OH fibers.

Another important characteristic of optical fibers is its resistance to solarization damage caused by exposure to high levels of UV illumination. In the case of the new FBPI fiber, the UV-defect concentrations have been reduced significantly, such that the solarization degradation properties are close to UV optimized high -OH fibers with high radiation resistance.

Spectral Attenuation Performance

Attenuation is tested in three different wavelength bands: 1) UV (200-400nm), 2) Visible (400-750nm) and 3) NIR (750-2100nm). Each test uses an Ocean Optics fiber-optic spectrometer designed for the specific wavelength range. Due to the varying attenuation at the different wavelength ranges, different lengths of fiber are used for optimum test sensitivity. 15m lengths are tested in the UV range, while approximately 200m lengths are used for visible and NIR testing. The source used for UV testing is a Deuterium lamp. For visible and NIR testing, the source is a Tungsten-halogen lamp.

Attenuation is tested using a cutback method as shown in Figure 1 below. Light is launched through the full length of the test sample. The spectrum received by the spectrometer is then recorded. Then the sample is cut back to a short length. This spectrum is then also recorded. Comparing the two spectra wavelength by wavelength, the attenuation is calculated as a function of wavelength.

Figure 1. Spectral Attenuation Test Set-up

Below in Figure 2 is a chart of the spectral attenuation performance for four separate fiber types for comparison.
The fiber types are:

• Low -OH: Standard FIP fiber for NIR
  
• High -OH: FVP (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: Existing broad spectrum fiber, not optimized for NIR attenuation or UV solarization resistance
  
• FBPI: New broad spectrum fiber, optimized for NIR attenuation and UV solarization resistance

As can be seen in the chart, the new FBPI demonstrates the optimum performance of high -OH fibers in the UV and low -OH fibers in the NIR.

Figure 2. Spectral Attenuation Comparison

UV Solarization Resistance

Silica optical fibers are susceptible to UV induced attenuation. This increase in transmission loss is caused by damage induced by exposure to UV radiation. It is commonly known as solarization. Most of the attenuation occurs in wavelengths less than 250nm, with the peak damage occurring at 214nm. The degree of damage varies greatly with the type of fiber. In the section below, the new FBPI fiber UV exposure performance is compared to other fibers used in the UV.

The solarization resistance is evaluated using a test known as a “Four Hour UV Exposure Test” as shown in Figure 3 below. For this test, a 2m segment of fiber is used. Light from a high intensity Deuterium lamp is launched into the fiber using a focusing lens to maximize the intensity. The focus is aligned to maximize the intensity at 214nm (generally the most sensitive wavelength for UV solarization). The output of the test sample is monitored using an Ocean Optics UV spectrometer, and data is collected for four hours. Six characteristic wavelengths are traced throughout the test process (214nm, 229nm, 245nm, 255nm, 266nm, and 330nm). Also, the entire spectrum is measured and compared at the beginning and end of the test. The rate of degradation of each wavelength decreases as the test progresses, ideally reaching a saturation point before the end of the 4 hour test. 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 tends to only change the speed with which the saturation is reached.

Figure 3. Four Hour UV Exposure Test Set-up

The results of the test are given below for the new fiber 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, a 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.

Figure 4. Four Hour UV Exposure Test for new FBPI Fiber

Figure 5. Four Hour UV Exposure Test for standard high -OH FVP Fiber

Figure 6. Four Hour UV Exposure Test for UV optimized UVM Fiber

Figure 7. Four Hour UV Exposure Test for Deep UV FDP Fiber

Reviewing the test results shows 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. However, the deep UV FDP fiber shows significantly better solarization results compared to the new FBPI fiber.

Applications

The new broadband fiber has potential for use in applications which require sensing over a wide range of wavelengths. Historically, if an application required sensing of UV and NIR light simultaneously then two separate fiber types would be required. The use of a single fiber has the potential to simplify system complexity and reduce cost.

Various potential applications could benefit from this fiber, including:

• Bio-chemical analysis
• Industrial chemical sensing
• Broadband spectroscopy
• Gas sensing
• Materials analysis

Summary

Polymicro Technologies has developed a new broadband optical fiber which exhibits the UV performance of a high -OH fiber and the NIR performance of a low -OH fiber. This fiber is optimized for sensing applications where -1.00.01.02.03.04.05.06.07.08.09.010.00:000:301:001:302:002:303:003:304:00Attenuation ( dB/2m)TitleRelative Attenuation vs Time200μm Core FDP Fiber 214nm229nm244nm254nm266nm330nm extremely wide spectral performance is required. In addition, the UV solarization resistance has been demonstrated to be similar to UV optimized UVM fibers.

# # #