Light-induced Self-written Waveguides for Large Core Optical Fiber Modules

Transcription

1 Speial Issue Visible Optial Fiber Communiation 11 Researh Report Light-indued Self-ritten Waveguides for Large Core Optial Fiber Modules Tatsuya Yamashita, Manabu Kagami Abstrat A fabriation tehnique for produing lightindued self-ritten (LISW) aveguides for large-ore optial fibers is proposed. The proposed tehnique employs a photopolymerizable resin onsisting of to kinds of photopolymerizable monomers that differ from eah other in terms of both refrative index and polymerization mehanism. The ore portion is formed by virtue of a self-trapping effet, in hih visible light is irradiated to the resin through an optial fiber that is inserted into the resin. Only the lo refrative index monomer an be radially polymerized to form the LISW aveguide, hih is generated from one end of the fiber. After the irradiation is stopped, the onentration gradient indued by the omsumption of the lo refrative index monomer initiates a ounter-diffusion phenomenon beteen the residual monomers. The lo refrative index radially polymerizable monomer diffuses into the ore region, hile the high refrative index ationi polymerizable monomer diffuses out of the ore region. The residual monomers are subsequently ured by exposure to UV light, and the region ith dereased onentration of high refrative index monomer beomes a ladding layer. The resultant refrative index profiles of the aveguides ere experimentally onfirmed to be "W-shaped". The measured propagation loss of the aveguide as 1.7 db/m at 680 nm avelength. Keyords Optial aveguide, Plasti optial fiber, Self-fousing, Photopolymerizable resin, Refrative index profile, Counter diffusion proess

2 12 1. Introdution Optial avelength division multiplexing (WDM) tehnology is no regarded as a promising approah for realizing high speed LANs for use in automobiles, beause optial WDM ommuniation an not only provide high-bandidth, but also simplifies the sharing of several ommuniation signals. 1) For appliations involving automobiles, reduing the system ost is the most important issue to address. Hoever, onventional fabriation tehniques for WDM modules are ostly, beause these tehniques require the use of time-onsuming proesses suh as the insertion of WDM filters. Therefore, in order to realize devies for high speed LANs for automobiles, it ould be useful to develop a simple fabriation tehnique for WDM modules. Attempts to simplify fabriation tehniques for WDM modules have lead to an inreasing interest in the use of Light-Indued Self-Written (LISW) aveguides. 2-10) When an optial fiber is adopted to irradiate a photopolymerizable resin, an optial aveguide an be produed automatially from the 2, 3) fiber by virtue of the self-trapping effet. Moreover, if WDM filters are arranged ahead of the fiber, it is also possible to form a aveguide that passes though the filters. 6) Therefore, the use of LISW aveguides ill probably serve to redue the ost of WDM modules, beause LISW aveguides enable us to eliminate the time-onsuming proess of inserting WDM filters into optial aveguide iruits. So far, to types of fabriation tehniques for LISW aveguides that also enable ladding layers to be formed by a photopolymerization proess have been demonstrated. One of these involves the exhange of residual photopolymerizable resin surrounding the ore region for another photopolymerizable resin ith a loer refrative index after the ore region has been produed by using the self-trapping effet. 13) This tehnique (the ladding substitution LISW method) has a drabak in that it requires extra proesses to exhange the resins. The other tehnique is based on a ore region that is formed using a seletive-photopolymerization proess, in hih only the higher refrative-index photopolymerizable monomer is polymerized and the loer refrative index monomer is exepted from the polymerized portion in a photopolymerizable resin mixture that inludes a higher refrative index photopolymerizable monomer and a loer refrative index photopolymerizable 6, 9) monomer. By using the latter tehnique (the ore seletive-photopolymerization LISW method), the proesses involved in the formation of the ladding layer an be greatly simplified hen ompared ith the former tehnique. Hoever, the latter tehnique is reognized as being impratiable for making large-ore aveguides, beause it takes a longer time to diffuse monomers over a longer distane. Therefore, in ases here e ish to use a largeore optial fiber (typially 500 ~ 1000 µm in diameter), another tehnique that uses a simplified proess is required. Plasti optial fiber (POF) is onsidered to be the most suitable fiber for use in automobile appliations, beause it an offer lo onnetion osts and robustness for tight bends due to its large ore size and high numerial apertures (NA). A WDM module for POF as able to be produed by the ladding substitution LISW method, and the feasibility of using a WDM module for automobile appliations has been reported. 13) In this report, a novel seletive-photopolymerization LISW method is proposed in order to redue the proess ost of produing WDM modules for POF. 2. Proposed LISW aveguide for an optial fiber ith a large ore The proposed fabriation proesses for forming LISW aveguides for large-ore optial fibers are shon shematially in Fig. 1. The photopolymerizable resin used in this tehnique inludes to kinds of photopolymerizable monomers, A m and B m. Monomer A m an be polymerized by irradiating ith light at a avelength shorter than λ a to form polymer A p, and monomer B m an be polymerized by irradiating ith light at a avelength shorter than λ b to form polymer B p. The avelength λ a must be longer than λ b, and the refrative index of polymer A p must be loer than that of polymer B p. In the first stage, a ore region is reated in the photopolymerizable resin by using the self-trapping effet of a light beam (Fig. 1). An optial fiber is

3 13 introdued into the resin and delivers a light beam at avelength λ, hih is somehere beteen λ a and λ b. This light beam an therefore only indue polymerization of monomer A m. If the optial fiber has a large ore size, then the area here photopolymerization ours is too large for monomer B m to diffuse out of the polymerized region. Thus, the ore region reated in this stage is omposed of both polymer A p and monomer B m. In the seond stage, a layer that is defiient in onentration of monomer B m is formed by the ounter-diffusion of monomers A m and B m, as follos. Monomer A m diffuses into the ore region that as reated in the first stage in order to redue the onentration gradient of monomer A m. This diffusion of monomer A m auses the unreated moleules of monomer B m that remain in the ore region to diffuse out of the ore. As a result, a layer that is defiient in onentration of monomer B m is produed around the surfae of the ore region (Fig. 1()). In the final stage, the residual monomers are polymerized by irradiating ith a light at a avelength λ, hih is shorter than λ b, as shon in Fig. 1(d). The layer that as produed in the seond stage beomes a ladding layer after polymerization in the final stage, beause the onentration of polymer B p in the layer is loer than that in the ore region. The refrative index of the resultant aveguide exhibits a W-shaped profile. of The diglyidyl ether of bisphenol A as used as monomer B m, hih has a refrative index of This is a ationi polymerizable monomer that forms polymer B p, hih has a refrative index of The eight ratio of monomer A m to monomer B m as 0.6:0.4. The refrative index of the photopolymerizable resin (n Cm ) as 1.509, and that of the polymerized resin (n Cp ) as Formation of the LISW aveguides The experimental set-up used to fabriate the LISW aveguides is shon in Fig. 2. A quasi graded-index type POF ith a 700 µm ore diameter, a 750 µm ladding diameter and an NA of 0.25 (Eska-Miu, supplied by Mitsubishi Rayon Co., Ltd., Japan) as used in this experiment. The photopolymerizable resin as introdued into a transparent ell and one end of a POF as immersed Photopolymerizable resin Core Optial fiber Core Cladding 3. Experimental methods 3. 1 Material The photopolymerizable resin used in this report as a mixture of radial and ationi polymerizable monomers, inluding photoinitiators for the monomers. The photoinitiator used for the radial polymerization as seleted to be sensitive to light at a avelength of 488 nm, hile the photoinitiator for the ationi polymerization as insensitive to light at this avelength but as sensitive to light from a high-pressure merury lamp. Ethoxylated trimethylolpropane triarylate as used for monomer A m, hih has a refrative index of This is a radial polymerizable monomer that forms polymer A p, hih has a refrative index + Ar LASER Fig. 2 Shutter Fig. 1 Objetive lens NA=0.25 () Mode srambler POF ( Eska-Miu; NA=0.25, 750 m) Photopolymerizable resin (Monomer Am + Monomer Bm) Shemati diagram of the experimental set-up used for fabriation of the LISW aveguides. Fabriation proesses to produe LISW aveguides for use ith large-ore optial fibers. (d)

4 14 into the resin. A ontinuous-ave beam from an argon-ion laser (λ = 488 nm) as foused onto the other end of the POF by means of an objetive lens ith an NA of The photopolymerizable resin as irradiated by the ontinuous-ave beam that as radiated from the immersed end of the POF, and the irradiation poer P as measured at the end. The duration of the irradiation as denoted as t 1. After the irradiation as terminated, the resin as kept in a dark plae for a set time t x to allo diffusion to take plae. The resin as finally uniformly exposed to UV light from a 200 Watt high-pressure merury lamp. All of these experiments ere arried out under atmosphere at room temperature Analysis of refrative index profiles The refrative index profiles ere analyzed using a transmitted dual-beam interferene mirosope. In order to evaluate the refrative index profiles of the fabriated LISW aveguides, slies of aveguide ere used that had been ut along the ore setion ith a diamond sa. In ases here e anted to evaluate hanges in the refrative index profiles ith time, to-dimensional samples orresponding to ross-setions of the LISW aveguides ere prepared using the set-up shon in Fig. 3, as follos. The photopolymerizable resin as sandihed beteen to transparent glass plates of 0.15 mm thikness. The thikness of the resin layer as adjusted by using a spaer plate ith a thikness of mm. One end of the optial fiber, hih had a ore diameter of 600 µm and an NA of 0.37, as then ontated onto the top of the glass plate, and light at a avelength of 488 nm as used to irradiate the photopolymerizable resin from the end of the optial fiber via the glass plate to form the ore. The refrative index profile of the todimensional sample beteen the glass plates as able to be observed, even if photopolymerizable monomers still remained in the sample. The maximum refrative index differene beteen the ore region and the ladding layer is denoted as n m (see Fig. 5). The ore diameter is defined as the distane beteen the positions that have the loest refrative indies, and is denoted as d (see Fig. 5) Measurement of optial properties An LISW optial aveguide sample that as prepared under the folloing onditions; P = 80 mw, t 1 = 150 se and t x = 5 min, as used to measure the optial properties. The propagation loss of the aveguide as measured by the utbak method, as follos. Light from a hite lamp soure as oupled into a pigtailed fiber, hih as used for forming the ore region of the LISW aveguide. Then, the outgoing light from a ertain length of the ut aveguide as oupled into a multimode fiber ith a ore diameter of 1000 µm and an NA of The optial poer from the output end of the fiber n 1.52 Transparent glass plates { { Optial fiber (600/630 m, NA=0.37) }Photopolymerizable resin Position from the ore enter [ m] Fig. 3 Experimental set-up used to investigate refrative index hanges indued by photopolymerization. Fig. 4 Refrative index profile before UV exposure. Interferogram obtained by interferene mirosopy, alulated refrative index profile.

5 15 as measured using an optial poer meter. The propagation loss as obtained from the slope of the graph of the insertion loss versus the aveguide length L. The oupling loss as estimated as the insertion loss at L = 0 mm. 4. Experimental results and disussion 4. 1 Change of refrative index profiles Figure 4 shos a typial interferogram of a to-dimensional sample fabriated under the folloing onditions; P = 30 mw, t 1 = 20 se, t x = 60 se. The estimated refrative index profile is also depited in Fig. 4. The photopolymerized region that as indued by the irradiation from the optial fiber as identified from inreases in the refrative index. At the surfae of the photopolymerized region, a derease in the refrative index as observed. Although the refrative index as inreased over the hole area of the sample by the subsequent UV exposure (~ 1.6 mj), the position here the minimum refrative index as observed remained unhanged, as shon in Fig. 5. Figure 6 shos the hanges of the maximum refrative index differene n m and the ore diameter d as a funtion of the diffusion time t x. The onditions to prepare the sample ere as follos, P = 30 mw, t 1 = 30 se. As shon in Fig. 6, the value of n m inreased rapidly as t x inreased, and exhibited a tendeny to saturate above approximately t x >10 min. The value of d dereased slightly, as shon in Fig. 6. These results support that mehanism that ounter-diffusion of monomers through the ore/ladding interfae ours after irradiation from the fiber has terminated, hereby the lo refrative index monomer A m moves into the ore region and the high refrative index monomer B m moves toard the outer portion of the ore region. Next, the uniformity of the refrative index profile along the LISW aveguide as investigated. A aveguide sample fabriated under the folloing 1.55 n n m 1.52 d Position from the ore enter [ m] Fig. 5 Refrative index profile after UV exposure. Interferogram obtained by interferene mirosopy, alulated refrative index profile. Fig. 6 Changes in the maximum refrative index differene n m and the ore diameter d against the diffusion time t x, observed after UV exposure.

6 16 onditions; P = 80 mw, t 1 = 300 se and t x = 5 min, as used in this investigation. Figure 7 shos the dependene of the maximum refrative index differene n m and the diameter d on the distane from the fiber tip. The values of n m and d ere found to vary ith aveguide position, and the minimum relative refrative index differene as estimated at 0.52 % (orresponding to an NA of 0.15). These results suggest that this aveguide ould sho a signifiant level of radiation loss Optial properties Figure 8 shos the insertion loss as a funtion of aveguide length L at a avelength of 680 nm. The oupling loss and the propagation loss at this avelength ere estimated to be 0.25 db and 1.7 db/m, respetively. A rapid inrease as observed in the propagation loss spetrum in the shorter avelength region, belo 600 nm (Fig. 8). As observed in Fig. 7, it is possible that the radiation loss arises from both an insuffiient differene in refrative index and non-uniformity of the ore diameter. In addition, some sattering loss seems to be indued beause the photopolymerized resin as slightly loudy after the UV exposure. Therefore, the propagation loss is onsidered to inlude omponents of both radiation loss and sattering loss. It is supposed that these problems ould be solved by means of both the seletion of appropriate materials and the orret fabriation onditions. 5. Conlusion A novel tehnique for forming large-ore LISW aveguides is proposed. A "W-shaped" refrative index profile as suessfully realized. The origin of this form of profile an be explained by a ounterdiffusion phenomenon of the monomers. At present, there is a remarkable sattering loss in the shorter avelength region and a propagation loss of 1.7 db/m at a avelength of 680 nm. If these optial properties ould be improved by means of both appropriate material seletion and better fabriation onditions, this tehnique ould be used to produe lo-ost WDM modules for large-ore optial fibers, and these might be expeted to enourage an expansion in the use of high speed LANs for automobiles. Insertion loss [db] Waveguide length L [mm] 5 nm d m] [ Propagation loss [db/m] L [mm] Wavelength [nm] Fig. 7 The dependene of the maximum refrative index differene n m and the ore diameter d on the distane from the fiber tip. Fig. 8 Loss properties of an LISW optial aveguide measured by a utbak method. Insertion loss as a funtion of aveguide length, Propagation loss spetrum.

7 17 Referenes 1) Kagami, M. : "Visible Optial Fiber Communiation", R&D Rev. of Toyota CRDL, 40-2(2005), 1 2) Frisken, S. J. : "Light-indued Optial Waveguide Uptapers", Opt. Lett., 18-3(1993), ) Keitsh, A. S. and Yariv, A. : "Self-fousing and Self-trapping of Optial Beams upon Photopolymerization," Opt. Lett., 21-1(1996), 24 4) Monro, T. M., de Sterke, C. M. and Poladian, L. : "Self-riting a Waveguide in Glass using Photosensitivity", Opt. Commun., 119(1996), 523 5) Maruo, S. and Kaata, S. : "Optially-indued Groth of Fiber Patterns into a Photopolymer", Appl. Phys. Lett., 75-5(1999), 737 6) Kagami, M., Yamashita, T. and Ito, H. : "Lightindued Self-ritten Waveguide Three-dimensional Optial Waveguide", Appl. Phys. Lett., 79-8(2001), ) Yoshimura, T., Ojima, M., Arai, Y. and Asama, K. : "Three-dimensional Self-organized Miroopteletroni Systems for Board-level Reonfigurable Optial Interonnets - Performane Modeling and Simulation", IEEE J. Selet. Topis Quantum Eletron., 9-2(2003), 492 8) Watanabe, N., Murata, Y., Oyama, Y., Ito, T., Ozaa, H., Mikami, O. and Shioda, T. : "Self-ritten Connetion beteen GI-MMF and VCSEL Using Green Laser", IEICE Trans. Eletron., J87-C(2004), 488 9) Yamashita, T., Kagami, M. and Ito, H. : "Waveguide Shape Control and Loss Properties of Light-indued Self-ritten (LISW) Optial Waveguides", J. Lightave Tehnol., 20-8(2002), ) Yamashita, K., Hashimoto, T., Oe, K., Mune, K., Naitou, R. and Mohizuki, A. : "Self-ritten Waveguide Struture in Photosensitive Polyimide Resin Fabriated by Exposure and Thermosetting Proess", IEEE Photo. Teh. Lett., 16-3(2004), ) Dorkenoo, K., Cregut, O., Mager, L., Gillot, F., Carre, C. and Fort, A. : "Quasi-solitoni Behavior of Self-ritten Waveguides Created by Photopolymerization", Opt. Lett., 27-20(2002), ) Sugihara, O., Tsuhie, H., Endo, H., Okamoto, N., Yamashita, T., Kagami, M. and Kaino, T. : "Lightindued Self-ritten Polymeri Optial Waveguides for Single-mode Propagation for Optial Interonnetions", IEEE Photo. Teh. Lett., 16-3(2004), ) Yonemura, M. : "250 Mbit/s Bi-diretional Single Plasti Optial Fiber Communiation Systems", R&D Rev. of Toyota CRDL, 40-2(2005), 18 (Report reeived on May 9, 2005) Tatsuya Yamashita Researh Field : Guided-ave optial devies, Polymer-based optial omponents Aademi soiety : Inst. Eletron., Inf. Commun. Eng., Jpn. So. Appl. Phys., Opt. So. Jpn., Opt. So. Am. Manabu Kagami Researh Field : Optial ommuniation devies Aademi degree : Dr. Eng. Aademi soiety : Inst. Eletron., Inf. Commun. Eng., Inst. Eletr. Eletron. Eng., Opt. So. Am., Jpn. So. Appl. Phys., Jpn. Inst. Eletron. Pakag.