Fluorescent Optical Fibers for Data Transmission - ACS Symposium

Chapter 15 Fluorescent Optical Fibers for Data Transmission 1

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Downloaded by OHIO STATE UNIV LIBRARIES on September 1, 2012 | http://pubs.acs.org Publication Date: September 13, 2001 | doi: 10.1021/bk-2001-0795.ch015

Hans Poisel , Karl F. Klein , and Vladimir M. Levin 1

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FHN, University of Applied Sciences, Wassertorstrasse 10, 90489 Nuernberg, Germany FH-GF, University of Applied Sciences, W. Leuschnerstrasse 16, 61169 Friedberg, Germany RPC, Moskovskoe SH, 157, Tver 170032, Russia 3

Fluorescent optical fibers offer new possibilities for building optical bus systems or rotary joints. They allow for a lateral coupling of information to a dye doped polymerfiber.Selecting therightparameters concerning lateral launch, absorption and emission wavelengths, data transfer rates beyond 500 Mbits/s are feasible.

Introduction Usuallyfluorescentpolymer optical fibers and data transmission belong to dif ferent worlds; until recentlyfluorescentfibershave been used for sensor applications or for decorative purpose whereas data transmission is usually done with clear undoped fibers. In the following it will be demonstrated that a combination of both worlds opens the door to new applications in data transmission systems.

Terminal 1

Terminal 3

Terminal 2

^

_ D a t a stream

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Figure 1. Data bus system: Linear topology © 2001 American Chemical Society

In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Background and Motivation In data transmission systems there are a lot of applications where lateral cou pling to optical fiber is needed. Optical bus systems need a coupler or tap for each terminal and this can only be realized at discrete positions. These terminals could be lined up more easily and the system design were more flexible if lateral access were possible at any position along the fiber. It should be possible to install a terminal and to remove it without the need to install and to re move a coupler or tap. Thefiberbetween the access points should behave as ideally as possible, i.e. no additional loss. Another application where lateral access is desirable is an optical rotary joint or slipring. This device allows for transmission of data from a rotating device such as radar antennas, a computer tomography apparatus or from robot arms to a stationary device, e.g. a transmitter or a computer. The first description of fiber optical rotary joint with lateral access was published in 1994 (1).

Data stream

to detector (stationary)

•O if common axis

Orbit of data source

'Fiber ring

Figure 2. Rotary optical joint

Lateral Coupling to Optical Fibers Apart from solutions using coupling prisms or surface gratings which are suit able preferably in the laboratory, ordinary fibers do not allow for lateral access, at least not with a considerable coupling coefficient (2). Measuring the radiation at the end of a laterally illuminated fiber yielded the results shown in Fig. 3. In addition this light is coupled mainly to high order helically or skew modes with attenuation in the range of several hundred dB/m. A closer look to the origin of light scattering by scanning the fiber cross section with a narrow laser beam showed, that the main contribution is due to scattering at imperfections from the core - cladding interface (cf. Fig. 4). In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


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Figure 3: Ratio of radiation coupled in to incident radiation as a function distance betweenfiberend and point of illumination.

Figure 4: Power (normalized to axis value) coupled to thefiberby scannin thefiberdiameter with a spot of < 0.1 mm diameter In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


214 Therefore it was necessary to look for other solutions. One very successful solu tion was using dye doped polymer optical fibers, so called fluorescent optical fibers (FOF). Thus it is possible to circumvent the problem to couple light radially to the fiber by fulfilling the original task which is to couple the information to the fiber. This can be done by exciting fluorescent radiation inside the FOF by an external light source which can be modulated with the information to be transmitted. Since now the fluorescent light is generated inside of the waveguide, part of it (i.e. that part which is emitted inside the acceptance cone of the fiber giving the piping effi ciency) can be trapped and guided along the fiber until to its ends. The capabilities depend on parameters such as • • • • • •

Coupling efficiency from the external light source to the FOF Absorption of exciting radiation Fluorescence yield Piping efficiency FOF attenuation for fluorescent light Bandwidth of the system determined essentially by fluorescent lifetime of the dopant

For the first prove of principle standard FOFs made of polystyrene had been used and data rates up to 100 Mbits/s (1) had been achieved. Based on experiences with that first demonstrator a development program for optimizing the FOF could be started.

Boundary Conditions Because of optical and mechanical properties are the best, PMMA was chosen as a substrate material. The dyes were selected according to following criteria: • • •

• • • •

Spectral region such that there are high bandwidth and powerful light sources (LEDs or laser diodes) available or to be expected in the near future. Spectral region such that the spectral attenuation of the host material is suffi ciently low. Short fluorescence lifetime. In this case these values were only published for so lution in organic solvents such as ethanol, values which may not hold for solution in polymers. Good solubility in selected host polymers Stable under normal environment e.g. daylight irradiation Non-toxic Thermally stable up to fiber extrusion temperatures


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Experimental


Fibers with different dyes, concentrations and processing during extrusion have been fabricated and tested. For measuring absorption and emission spectra a set up shown in Fig. 5 was used:

Figure 5. Set up for measuring absorption and emission spectra offluorescen cal fibers A 450 W XBO lamp produces sufficient white light which is coupled side-on to the FOF under test via a first monochromator controlling the absorption wavelength. The fluorescent light thus generated is analyzed by a second monochromator and detected by a thermoelectrically cooled photon counter. A PC controls the measure ment sequence, processes and stores the data. Fluorescence lifetime is measured by a laser spectrometer shown in Fig.6. A ni trogen laser pumps a dye laser producing short pulses of about 0.3 ns half width. These pulses are focussed to the FOF under test with a repetition rate of about 10/s. The fluorescence light is captured optically and coupled to a monochromator where the spectral regime to be detected by a following photomultiplier tube can be tuned. The signals of this PMT are fed to a boxcar card inside a PC where the repetitive signals are averaged and analyzed. For enhancing the time resolution, the output signal is deconvoluted with the known input signal thus giving a resolution in the range better than 0.1 ns.



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Figure 6. Set up for measuringfluorescentlifetime of dye doped optical fibers

Results and Discussion Experimental results are shown for one of the most promising dyes: Nile blue. This dye showed a very good solubility in M M A and was stable under polymeriza tion and extrusion conditions.

As expected, the absorption maximum is around 630 nm, the wavelength, where laser diodes as excitation sources are commercially available. Emission maximum is around 700 nm, unfortunately close to an absorption peak of PMMA. For testing the reproducibility the sample was rotated along its axis giving a varia tion of less than 10% due to imperfection of end face preparation of the fiber sample. Emission and absorption curves overlap in the range between 600 and 700 nm giv ing reasonable self-absorption of fluorescent light. This self-absorption is expected to prolong thefluorescencelifetime.



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Figure 7. Absorption and emission spectrum of Nile blue doped PMMA fib ple AC-K). Different traces show dependence on sample orientation.

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Figure 8. Excitation pulse and response function of Mile-blue doped sample In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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The dynamic behavior for this sample can be seen in Fig.8. After deconvolution with the system function a fluorescence lifetime of 3.5 ± 0.2 ns was calculated which is veiy fast for a red emitting dye (3) The concentration was varied by a factor of 10 but within the measurement ac curacy only a slight increase of fluorescence lifetime could be detected, indicating only a small concentration dependence if any (Fig.9). To investigate the influence of self absorption, the wavelength of the output monochromator was varied, but again no influence was noted.

Figure 9: Fluorescence lifetime as function of dopant concentration Several different dyes have been tested, most of them with different concentra tions or different treatment during the polymerization and extrusion process. Some of the originally selected dyes did not solve in the monomer, some reacted chemi cally with the monomer and some were not stable under the conditions of polymeri zation. Those dyes which gave a reasonable fluorescence signal are listed in Table I. The shortest lifetime measured with a good signal amplitude was 1.9 ± 0.2 ns. This allows data transmission bandwidth in the order of 500 Mbits/s and beyond, depending on the modulation scheme.


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Table L Fluorescent Lifetimes of selected Dyes* Dye Nile blue Oxazine 1 Styril 9M** Styrilo LD700 LD688 Oxazine 750**

Lifetime (ns) λ Absorption max (nm) λ Emission max. (nm) 3.5 672 633 4.2 670 646 1.3 840 585 1.9 720 615 700 3.8 643 2.8 610 492 2.9 750 667

* all dyesfromLambdachrome, Goettingen, Germany ** very weak signal

Conclusion Fluorescent optical fibers have been investigated for data transmission applica tions because of their very attractive property, to couple information laterally to the waveguide. Different fluorescent dopants have been tested showing that data trans mission systems with data rates beyond 500 Mbit/s are feasible. More investigations have to be done to enhance the concentration of the most promising dyes in order to achieve data rates in the Gbit/s regime.

Acknowledgment Parts of this work have been supported by the German Ministry BMB+F con tract # 1704897. The authors would like to thank Oliver Stefani and Markus Beck for most valuable contributions.

References th

(1) Poisel, H., Dandin, E., Klein, K.F., Proceedings of 4 International Conference on Polymer Optical Fibers POF'94, Yokohama, Japan, 1994, 82. (2) Poisel, H., Hager, Α., Levin, V., Klein, K.F., Proceedings of 7 International Conference on Polymer Optical Fibers POF'98, Berlin, Germany, 1998, 114. (3) P.A. Cahill, Radiat. Phys. Chem. Vol.41, 1993, pp. 351-363 th


Fluorescent Optical Fibers for Data Transmission - ACS Symposium

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