Plastic optical fibers: properties and practical applications

Transcription

1 Invited Paper Plastic optical fibers: properties and practical applications M. Loch, University of Applied Sciences, Haardtring 100, D Darmstadt, Germany ABSTRACT Driven by the increasing data traffic and the increasing demand for bandwidth optical fiber technologies play a greater role in todays and future data-communication networks. Although the well-known silica fiber have the potential of achieving very large bandwidth, this fiber is not the ideal medium for high bit rate data-communication for office and home applications because its small dimension requires well sophisticated components as well as installation technologies. This increases the total system cost, inevitably. However, the technologies of plastic (polymer) optical fibers (POF) and the devices for POF nowadays show rapid process /2/. So, we could benefit from the special advantages of these fibers over a wide field of applications, from decoration to local networks, including lighting, image guides and sensor technique. Today, inexpensive and robust POF transmission systems are available on the market with high bit rate capacity. Bus-systems, e.g. MOST and Byteflight, are applied in the rough automotive environment. Keywords: polymer optical fibers, automotive environment, data communication, micro-structured optical fibers, bandwidth, attenuation, wavelength division multiplexing 1. INTRODUCTION In the last 10 to 15 years, completely new markets for digital transmission systems are being developed for short-range applications. Polymer optical fibers have the power to fulfill many of these requirements given by the new demands in a wide range. Therefore polymer optical fibers are a transmission medium of high interest. These fibers show high resistance to damage, potential high rate of information transfer and they can operate in the visible range, to mention only some characteristics, here. Generally, different types of optical fibers are distinguished by either their internal structure, including the profile characteristics, e.g. step-index or graded index profile, and the core dimensions, e.g. single-mode or multimode behavior, or by the materials the fibers are made of. The internal structure influences in a very strong way the pulse propagation and the bandwidth characteristics of the fiber /1/. In single-mode fibers the chromatic dispersion, summarizing the material and wave-guide dispersion, can be tuned in a very fine way to minimize the resulting pulse spreading for the operating wavelength by optimal fiber parameters. But also in multi-mode fibers an accurate selection and precise realization of profile structure is necessary for minimal mode dispersion. On the other hand, the material is also a very important factor, not only regarding to the spectral attenuation properties of the fibers. Today, particular attention being paid to polymer (plastic) optical fibers (POF), polymer clad fibers (PCF) and glass optical fibers (GOF). In this paper, we will outline the special properties of polymer optical fibers combined with the advantages compared to their glass equivalents. Furthermore, considering special fields of application we want to give an overview over the actual state of the art. 2. PROPERTIES OF POLYMER OPTICAL FIBERS Beside many general advantages optical fibers have compared to copper based transmission lines, e.g. electromagnetic immunity, high transmission capacity, low weight and others, there are some exceptional advantages of polymer optical fibers. In comparison to glass fibers the handling simplicity is very important. Due to the large fiber dimension (see also Fig. 1) the positioning of the polymer optical fiber with respect to the transmitter and the coupling of polymer fibers is very easy and can be done without any special expensive instruments and extensive experience. The large fiber core diameter is also a very important advantage looking to the application of the fiber in industrial environment, Optical Transmission Systems and Equipment for WDM Networking III, edited by B. B. Dingel, W. Weiershausen, A. K. Dutta, K.-I. Sato, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2004) X/04/$15 doi: /

2 particular in the field, where dust is inevitable. So, minor contamination could be accepted without any risk of failure of total system functionality. Fig. 1: Comparison of numerical aperture and core diameter of optical fibers: glass fibers and POF /1/ Given by the material the handling of the polymer optical fibers is very simple in comparison to glass fibers. At first, glass tends to break applying it to a small bending radius in contrast to polymer. Additionally, the fiber end face preparation is very easy. Especially for Polymethylmethacrylate fibers (PMMA-POF) only little time and simple components are required for processing the end face to achieve a clean and mirror similar surface. Finally, also the costs should be mentioned: Due to large core dimension, end face preparation and connectivity simplicity systems based on polymer optical fibers could be performed extremely cost-efficiently. Due to these advantages there will be a wide field of application for polymer optical fibers from automotive engineering, lightning, to in-house network structure and more. 3. HISTORICAL CONSIDERATION OF POLYMER OPTICAL FIBERS 3.1. Bandwidth considerations The first POF, reported in the year 1968 /3/, was based on PMMA. At first Du Pont realized the pure step-index POF (SI-POF) with an attenuation of about 500 db/km at the wavelength 650 nm and the typical disadvantage of very high mode dispersion. Assuming typical values for core diameter of 980 µm and numerical aperture (NA) of 0.5 (corresponding to an angle of acceptance of 30 ) the number of modes able to propagate is about influencing a pulse spreading of 30ns/km, theoretically. The bandwidth of such a fiber is typically 40 MHz over about 100 m transmission length. To replace the well established copper cable for ATM applications with data rates of 155 Mbit/s an improved fiber structure was necessary: the low NA-POF was realized by Mitsubishi /4/. This POF with the reduced numerical aperture of about Fig. 2: Photograph of a DSI-MC-POF (Univ. of Appl. Sciences, Telekom, Leipzig) 300 Proc. of SPIE Vol. 5596

3 Fig. 3: Comparison of different polymer fibers 0.3 allows a bandwidth of about 100 MHz over 100 m, but was critical to additional bending loss. The problems could be overcome by adding a second cladding around the core with decreased refractive index in the outer cladding (double step index profile fiber, DSI-POF, Fig. 3). Generally, it is very difficult to fulfill the demands for high bandwidth and low sensitivity to bending /15/. For this demand fibers with small core diameter are suitable, but this is in contrast to the requirement of easy handling. So, the multi-core step-index polymer optical fiber (MC-POF) was born. There, many separate fibers (up to 200 fibers with a core radius of single fiber between 30 µm and 100 µm) are put together (Fig. 2). Graded-index polymer optical fibers (GI-POF) were reported by the Keio University in 1982 for the first time. By the profile structure following the power law g 2 2 r n () r = n 1 2 r < a, core region 1 a 2 () r n 2 n = r > a, cladding region 2 - where g is the profile exponent, and describes the relative refractive index difference - Data rate [Gbit/s] 10 5,0 120 µm PF-GI 2,0 1,0 0,5 1 mm MC/MSI DSI MC MSI PF-GI 1 mm Standard POF 0,2 Fiber length [m] 0, Fig. 4: Possible data rates and application fields depending on the fiber structure /2/ Double Step-Index POF Multi-core POF Multi Step-Index POF Perfluorinated Graded-Index POF Proc. of SPIE Vol

4 the bandwidth could be even greater. Due to the reduced mode dispersion by the continuously changing refractive index data transmission in the Gbit/s-range is possible. Fig. 3 shows the different polymer fibers; Fig.4 summarizes the bandwidth considerations Spectral attenuation Besides to the bandwidth the spectral attenuation of the POF is a very important and critical parameter /13/. To win importance especially in the field of in-house networks the fiber needs have the potential for minimum transmission line length of about 100 m. In the last years the attenuation of POF was continuously decreasing according Fig. 5. The first report of POF in the year 1968 /3/ mentioned an attenuation of about 500 db/km at a transmission wavelength of 650 nm (manufactured by Du Pont). 200 loss [db/km] year Fig. 5: Decrease of fiber loss of POF in the last 10 years In earlier times the most frequently used material for POF was PMMA. Polymethylmethacrylate (PMMA) is characterized by a refractive index of n = and a glass transition temperature between +95 C and +125 C. Generally, the attenuation of POF is mainly given by the Rayleigh-scattering following the relation 4 1 α R λ and the absorption caused by molecular vibrations. The vibrations of the C-H bonds of the PMMA and its harmonic waves cause the high loss of the material in the near infrared wavelength region. So, according to /14/, a PMMA based POF has a theoretical minimum attenuation of 106 db/km at the wavelength 650 nm, which is due to the scattering and absorption of the C-H bonds. But for shorter wavelength the minimum loss is about 35 db/km. Fig. 6: Spectral attenuation of PMMA step-index fiber /2/ 302 Proc. of SPIE Vol. 5596

5 The spectral attenuation of a PMMA based step-index POF is shown in Fig. 6. There are three preferred windows for data transmission: at 520 nm, 570 nm and 650 nm. At these wavelength simple LED and laser diodes as transmitters are available and many different systems are realized in the past. For the use in the area of automotive technology special POFs with high temperature resistance are necessary. Therefore, special fibers with polycarbonate (PC) for the core region (n = 1.51) and Teflon-AF serving as cladding are developed, which are characterized by a temperature resistance up to about 145 C. But these fibers suffer form an increased spectral loss compared to the PMMA fibers. The best attenuation values for PC-core POF reported are less then 1 db/m, which is quite acceptable for automotive applications. Due to their high numerical aperture these fibers are also very interesting for lighting applications. To reduce the H-C bonds vibration based absorption loss it is necessary to substitute the hydrogen by heavy atoms. This results in a lower vibration frequency moving the absorption bands to higher wavelengths. Using deuterium (heavy water) attenuation values of 20 db/km at 650 nm could be realized /3/. In the last years, this value could be improved by the Lucina TM -POF. However, deuterinated polymers tends to react with the atmospheric water, so that the absorption based attenuation will increase if no further protection of the material is given. By the same idea the fluorinated POF were born: Caused by the high atomic mass of fluorine in comparison to hydrogen, the absorption bands are moved significantly to longer wavelength, so that the theoretical attenuation of fluorinated polymers is in the range as the standard quartz glass fiber (Fig. 7) /5/. For practical realization there are some technical problems. In the last time remarkable success in the field of low loss POF has been achieved by Asahi Glass, Japan, by the material CYTOP, which no longer contains hydrogen. a) b) Fig.7: Theoretical comparison of PF-polymer and silica glass materials (a) and measured attenuation spectrum of real PF Graded- Index POF, OFS 2002 /6/ attenuation [db/km] wavelength [nm] In Table 1 the refractive indices of different most important polymer materials are listed. These material are used for optimal realization of core and cladding region or graded index structure, respectively, with different numerical apertures (NA) between 0.30 and Concerning the angle of acceptance we have to distinguish between low-na and high NA fiber. A larger numerical aperture, so say a larger angle of acceptance of the fiber, simplifies / increases the launching of light from the transmitter, but amplifies the problem of modal dispersion. Also other parameters are influences by the choice of numerical aperture: Increasing the numerical aperture reduces the bending sensitivity and the connection loss based on fiber axis angle differences. In contrary, reducing the angle of acceptance for better bandwidth characteristics implicates a higher connecting loss based on fiber gap or fiber axis displacement. Finally, it should be mentioned, that often there are large discrepancies between measured and expected attenuations, which are mainly caused by the measuring technique /13/. One typical problem consists in the assumption of equilibrium of mode distribution. Additionally, we have to take into consideration losses resulting from wave-guide structure, especially arising from inhomogenities in the cladding region. Proc. of SPIE Vol

6 Polymer material Refractive Index Polymethylmethacrylate (PMMA) Polycarbonate (PC) 1.51 Polystyrene (PS) 1.59 Fluorinated polymer (PF) Table 1: Refractive indices of different polymer materials 4. DATACOM APPLICATIONS AND TECHNOLOGY: WAVELENGTH DIVION MULTIPLEXING Compared to glass fibers the attenuation of POF is still high, so that POF based systems are limited to a transmission length of about 0.5 km to 2 km. However, comparing the PF-GI-POF with a standard graded-index multimode fiber made of glass there is the remarkable advantage of higher bandwidth over a wide wavelength region of the polymer medium. The reason for this is given by a smaller dispersion of polymer material /7/, which is defined by the first derivative of the propagation constant with respect to the wavelength according to dτ g dλ d n c λ dn 2 λ = : = M( λ) with τ 2 g = τ φ 1 = n λ dλ n dλ c dλ 1 dn In Fig. 8 the dispersion coefficient of fluorinated polymer material is compared with other standard fiber materials; the increase of bandwidth resulting from the flat dispersion curve is shown in Fig. 9. According to theoretical considerations /8/ the bit rate achieved by PF-GI-POF is twice as high as that by quartz glass based multimode fibers combined with a D [ps/nm km] PF-GI-POF SiO 2 SiO 2 + GeO 2 wavelength [nm] Fig. 8: Spectral behavior of dispersion coefficient of different materials 10 bandwidth [GHz 100m] PF-GI-POF 1 MM-GOF wavelength [nm] 0, Fig. 9: Comparison of bandwidth: fluorinated graded-index POF and glass graded-index fiber /8/ 304 Proc. of SPIE Vol. 5596

7 small wavelength dependence in the range between 850 nm and 1300 nm. Due to this fact the polymer fiber seems to be an ideal transmission medium utilized for wavelength division multiplexing systems of short length, e.g. future inhouse networks. Up to now this advantage of GI-POF is not used because of sub-optimal fiber manufacturing procedures, but there are some activities at this range. Fig. 10 shows an example of WDM system at 790 nm and 860 nm operating with simple VCSEL /9/. A data rate of 400 Mbit/s was transmitted over 50 m GI-POF. Because of the large core diameter of the POF both transmitters, which are spaced at a distance of 75µm, could be coupled directly to the fiber by a simple lens configuration. At the receiver side a dichroid mirror was used as demultiplexer. Fig.11 shows another example working at three wavelength for video applications /10/. Selective mirror 790 nm/860 nm VCSEL 50 m PF-GI-POF 120/200 µm CYTOP GaAs-PD GaAs-PD Fig. 10: Wavelength division multiplexing systems using a PF-GI-POF λ 2 λ 1 λ 3 PF-GI-POF 150/250 µm A N = 0,20 camera MUX/DEMUX e/o transmitter o/e receiver monitor Fig. 11: WDM-POF system in the wavelength range between 1.2 µm and 1.6 µm Thinking at WDM systems based on POF as discussed in the last figure additional new elements are necessary: multiplexer (MUX) and demultiplexer (DEMUX). In /11/ about the development of low-cost multiplexer and demultiplexer is reported. The Fraunhofer Institute in Nuernberg, Germany, demonstrates a multimedia system applying these elements for transmission of video, audio and data signal. Using a multiplexer three optical channels at 460 nm, 520 nm and 650 nm are transmitted over 100 m POF. At the receiver side these signals are demultiplexed before they are converted opto-electrically. The demultiplexer device, which is based on thin-filter technology, is characterized by insertion losses between 3 db to 5 db and cross talk suppression greater than 20 db. 5. AUTOMOTIVE APPLICATION The increasing number of digital equipment in the car, e.g. digital radio, CD player, navigation system and more, arises the necessity to install a network structure. There are at least three main reasons for automotive applications of optical fibers: the demand to transmit higher date rate in the future, the demand to decrease the weight of the transmission media, and finally, the demand to avoid the EMI problems. Combined with the requirement for extremely low costs the POF seems to be an ideal medium for this range. The MOST (Media Oriented Systems Transport) specification gives Recommendations for a multimedia fiber-optic network optimized for automotive applications. Proc. of SPIE Vol

8 MOST is a synchronous network characterized by ease of use, wide application range and low implementation costs. For example, in /12/ a high speed optical system up to 250 Mbit/s, using 650 nm band RC-LED and SI-POF with NA = 0.5 is described. The receiver consists of a standard pin photodiode (Si) having an active area of 800 µm diameter. The power of the system was 17.8 db including life degradation of 3 db and an extra margin of 1.3 db. The fiber loss was 4.6 db. Especially the temperature dependence was analyzed within the range of 40 C to 85 C. In this range the optical output power changed 4.1 db and the minimal power of the receiver changed 1.2 db. 6. MICRO-STRUCTURES POLYMER OPTICAL FIBERS A new development on the field of POF research is the micro-structured polymer optical fibers. Micro-structured fibers, which were fabricated at first time in 1995 /18/, open in general totally new applications. The guiding of light is achieved by the introduction of a pattern of microscopic air holes running down the entire fiber length. The advantage of micro-structured fibers in comparison to standard doped fibers could be explained by the fact that only one basic material is necessary, e.g. PMMA. The fine-tuning of refractive index to realize the required profile structure could be reached by the correct arrangement of holes. Beside to the refractive index of PMMA (n = 1.492), there is only the index of air. So, depending on the accurate arrangement of the holes the effective index varies in a wide range. Up to the last few years, especially micro-structured single-mode glass fibers were analyzed because of their optimized transmission behavior. However, micro-structured polymer fibers offer additional advantages: Due to the simplified manufacturing process non-rotation structures and holes of different diameter are realizable. In POF the microstructure is not restricted to simple close-packed circular arrangements of holes. Also, due to the much lower processing temperature of polymer materials (about 175 C in contrast to about 2000 C for glass fibers) more material modifications are possible. A large range of different fibers can be fabricated with new functionalities for specific applications. Here, we want to give some examples. Single-mode fibers were presented with a wide spectral behavior. In /16/ such fibers where shown to be single moded at the wavelength 633 nm. Special arrangement of holes allows special tuning of chromatic dispersion, e.g. a negative slope of dispersion curve with zero-dispersion at 1.35 µm, so the MPOF could be used as dispersion control fiber. Introducing an asymmetric distribution of holes and / or hole sizes, there is the possibility to realize highly birefringent characteristics. In this way polarization fibers with beat lengths below 1mm were demonstrated /17/, which are very interesting for sensor application. Special experiment relates also to optical sensing or to switching: Implementation of internal electrodes, which create an electro-optical effect, results in poled MPOF and gives characteristics controlled by an external electrical field. The advantage of MPOF over silica-based fibers that the hole structure is not restricted to special geometry could also be used very effectively for the realization of graded-index MPOF. By varying the size of the holes we can replicate a parabolic index structure of optimal light guiding characteristics designed to reduce mode dispersion. Due to the missing of doping material these GI-MPOF will show a much better temperature and aging resistance as the corresponding multi-mode fibers (GI-POF). Fig. 12 shows clearly the structure of a GI-MPOF /19/. Fig. 12: End face of a graded-index micro-structured polymer optical fiber (GI- MPOF) 306 Proc. of SPIE Vol. 5596

9 7. CONCLUSIONS We studied the most important characteristics of polymer optical fibers with different structures under consideration of different materials. Today, the polymer optical fiber has established in totally different fields of applications. In the field of data communication the POF has some advantages in comparison to the well-known glass competitor: Not only the ease of use and the low costs but also technical aspects as high bandwidth will increase the implementation of PF- GI-POF. For future wavelength division multiplexing systems based on GI-POF the further development of new components as e.g. multiplexer and demultiplexer is necessary. In the automotive technique the standard POF has already utilized to overcome problems with increasing data rate within an environment with electro-magnetic contamination. Also in the industrial environment the POF can fulfill the hard demands given in the field of complex system control. Because of the wide range of application it was not possible to discuss all different practical aspects, here. So, e.g. there are many other applications of the polymer optical fiber in the fields of sensoring technique and medical technology. Also the area of lighting should be mentioned. In future, micro-structured fibers based on polymer material will be of special interest, because this structure allows totally other possibilities to influence the feature of the wave-guide. Some laboratories - including our institute - are working on the optimization of this fiber type. REFERENCES /1/ W. Daum, J. Krauser, P.E. Zamzow, 0. Ziemann. POF, Polymer Optical Fiber for Data Communication ; Springer Verlag, Berlin, Heidelberg, New York, 2002 /2/ H. Poisel, K.F. Klein, M. Loch: POFMIX, Multiplextechniken für Polymere Optischen Fasern ; Project desription, 2003 /3/ Y. Koike: Status of POF in Japan, 5 th POF conference, Oct. 1996, Paris, pp. 1-8 /4/ J. Vobian, G. Herschenröder, M. Loch, K.-F. Klein, H. Poisel; Impulse Response of Polymer Optical Fibers ; 7 th POF conference, Oct. 1998, Berlin, pp /5/ M. Murofushi: Low loss perfluorinated POF ; 5 th POF conference, Oct. 1996, Paris, pp /6/ L.L. Blyer, W.R.White, R. Ratnagin: Materials Technology for Perfluorinated Graded-Index Polymer Optical Fibers ; 11 th POF conference, Sept. 2002, Tokyo, pp /7/ T. Ishigure: Test & Measurement For High Bandwidth POF, Tutorial POF 2003, Sept. 2003, Seattle /8 Y.Koike: POF Technology for the 21 th Century, 10 th POF conference, Sept. 2001, Amsterdam, pp. 5-8 /9/ T. Kaneko, S. Kitamura, T. Ide, T. Kawase, S. Shimoda, Y. Watanabe, R. Yoshida, Y. Takano: VCSEL module for optical data links using perfluorinated POF, 7 th POF conference, Berlin, October 5-8, 1998, pp /10/ K. Uehara, J. Mizusawa: Evaluation of POF WDM Video Transmission System of Long Wavelength Band Region ; 8 th POF conference, 1999, Chiba, pp /11/ S. Junger, W. Tschekalinskiy, N. Weber: POF EDM Transmission for Multimedia Data; 11 th POF conference, Sept. 2002, Tokyo, pp /12/ K. Numata, S. Furusawa, S. Morikura: High Speed POF Transmission for Automotive System ; 10 th POF conference, Sept. 2001, Amsterdam, pp /13/ K.-F. Klein, S. Doris, D. Zevoglis, H. Poisel, O. Zeimann, M. Loch: Attenuation Measurement of Graded- Index POF ; 10 th POF Conference, Sept. 2001, Amsterdam, Proc. of SPIE Vol

10 /14/ W. Kaiser, R. Ruf: Untersuchungen von Kunststofflichtwellenleitern für den Einsatz im Breitband- Teilnehmerinstallationsnetz; Final report on research projects at German Federal Post Office, Dec /15/ S. Gies, M. Odenwald, K.-F. Klein, M. Loch, H. Poisel, O. Zeimann: Characterization of polymer fibers by using different farfield methods; 7 th POF conference, Oct. 1998, Berlin, pp /16/ M. Eijkelenborg, A. Argyros, G. Barton, F. Cox: New Possiblities with Microstructured Polymer Optical Fibers; 11 th POF conference, Sept. 2002, Tokyo, pp /17/ A. Ortigosy-Blanch et al.: Highly birefringent photonic cryital fibers; Optics letters 25, No. 18, pp (2000) /18/ P. Russell: Photonic Crystal Fibers; Science 299, 358 (2003) /19/ K.-F. Klein, C. Bunge, A. Bachmann, S. Feistner, G. Barton: Mikrostrukturierte Multimode-Polymerfasern; 10 th conference on communication cables, Köln 2003, pp Proc. of SPIE Vol. 5596