Applications of optical polymer waveguide devices on future optical communication and signal processing N.Keil, B.Strebel, H.Yao, J.Krauser* Heinrich-Hertz-Institut für Nachrichtentechnik Berlin GmbH Einsteinufer 37, D-1000 Berlin 10, FRG * Fachhochschule der Deutschen Bundespost Telekom Berlin Ringbahnstr. 130, D-1000 Berlin 42, FRG ABSTRACT Optical technology is now established as the basis of a future integrated broadband communication network (IBCN). The capacity of an optical system can be increased by the development of transmission and switching facilities of the coherent multi carrier (CMC) technique. This contribution concentrates on the structure and technology of a CMC crossconnect, which may be realized as an integrated optical polymer circuit. The manufacture of passive polymer waveguide devices is presented. Further electro optical polymer devices and erbium doped PMMA applications are discussed. 1. INTRODUCTION In a future integrated broadband communication network (IBCN) distribution and bidirectional services may be expected'. The purpose of the development will be the transparent transmission path for point to point and point to multipoint applications. Transparency is characterized by the independence of signals from the network within specified qualifications. It is expressed by the terms bit transparency, modulation transparency and service transparency. The general transmission scheme for a transparent communication is shown in Fig. 1. Transmitters and receivers are connected to the network. It contains multiplexers and crossconnects controlled by the network management. Fig. 1: Communication scheme. mult Iplexers & crossconnects. I 278 / SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, andapplications 11(1991) O-8194-0687-2/91/$4.OO
2. OPTICAL CARRIER FREQUENCY CHANNEL One promising candidate for an optical transparent medium is given by the optical carrier frequency channel2. It is defined by a limited window on the optical frequency axis. Signals with arbitrary modulation and arbitrary bit rate can be transmitted on coherent optical carriers. Signals are independent from the network and can be processed in the space/frequency domain by electro optic or optic optical switching devices. th 'z 3 opt S11 21 th S12 S22 S32 'i Z 3 'opt crossconnect th s33 31ll I f'2 3 1'opt th f'1 z '3 1opt 3221 11 Fig. 2: CMC crossconnect fi, f2 optical carrier frequencies S, broadband signals Fig. 2 shows the principle of a transparent crossconnect for coherent optical carriers. It is important to note, that this crossconnect can switch signals S,, from arbitrary carrier frequencies at arbitrary waveguide input ports to arbitrary carrier frequencies at arbitrary waveguide output ports. Therefore two operations have to be managed, space switching and frequency conversion. Structures of crossconnects for coherent optical carriers were recently published3'4. One example is given by Fig. 3. Fig. 3: Structure of a CMC crossconnect (example) SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, and Applications 11(1991) / 279
All structures of these crossconnects contain the following signal transport operations: - broadband power splitting, - optical channel separation and insertion, - selective and broadband space routing, - routing of signals from one optical carrier to another and - tuned optical filtering. Additionally some monitoring and stabilization procedures are needed. These circuits are used for controlling the spectral distribution of channels and for tracking filters and optical carrier frequencies5. Basic devices for the broadband signal transport circuit can now be specified: - filters and power splitters, - amplifiers for compensating the splitting losses, - isolators for a unidirectional data flow, - frequency converters, - space switching devices and - bistable optical gates. The operation wavelength should be within the third window of optical communication near )¼ = 1.55 jim, in which the fiber exhibits its minimum loss. 3. OPTICAL POLYMERS IN CROSSCONNECTS Technologies for optical crossconnects are: - the combination of fiber devices, semiconductor amplifiers, mixers and compact isolators, - optical integrated circuits on InP and - optical polymer integrated circuits. The functionality of the fiber/laser technology for crossconnects has been successfully demonstrated6. Now further research should reveal, which integration technology is able to dispose a comparable high functionality. As a special feature a high number of identical switching circuits has to be integrated. 280 / SPIE Vol. 1559 Photopo/ymer Device Physics, Chemistry, and Applications II (1991)
Optical polymer circuits give the impression, that they can be the ideal technology for optical crossconnects, because it seems realistic to expect the following components: - passive monomode structures like directional couplers, passive stars and wavelength multiplexers, - electro optical waveguide switches, - parametric frequency converters and - parametric amplifiers using second order nonlinear waveguides, - amplifiers with rare earth doped polymer waveguides, - isolators using waveguides doped with magneto optic dipoles. If the polymer technology can be realized, it shows the following structural advantages for the transparent optical channel: - broadband and nonreciprocal parametric frequency conversion, - low noise and nonreciprocal parametric amplification, - no detection circuits in the signal transport path, - integration of a high number of identical circuits at low cost technology. 4. FABRICATION PROCESS FOR POLYMER WAVEGUIDE DEVICES The fabrication process7 used in the Heinrich-Hertz-Institut starts with a mixture of PMMA and photoinitiator deposited on a substrate by spin coating. The film on the substrate is exposured with UV-light followed by an annealing process (Fig. 4). Different substrate materials were used like optical glasses and Si wafers with SiO2 buffer layers. UV-l ight L_L I I I exposure annealing h.v polymer film (PMMA photo initiator) substrate L -/ n2 Fig. 4: Fabrication process SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, and Applications II (1991) / 281
A refractive index step of approximately 0.02 can be produced (Fig. 5). The refractive index is matched for a butt coupling to the optical fiber. 1. 54 a) C a) > -4- L. C.- a) 1.53 1.5Z 1.51 1.50 - illuminated area unilluminated area I 1 1 1 10 Z0 30 40 50 concentration of' photo initiator E%I Fig. 5: Refractive index Photoinitiator: Benzildimethylketal Fig. 6 shows the picture of a part of a Mach-Zehnder interferometer. The material attenuation of PMMA is roughly 1 db/cm measured on a 1 m multimode fiber specimen at 1.55,um wavelength. Fig. 6: Part of a Mach Zehnder interferometer 282 / SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, and Applications 11(1991)
Passive components like strip waveguides and 3 db directional couplers exhibit near field patterns (Fig. 7) with monomode behaviour. Fig. 7: Near field pattern of polymer waveguides at A = 1.55 m. left : strip waveguide right: 3 db coupler Subsequently fibers are connected to the waveguide endfaces (Fig. 8), which are manufactured by cleaving the Si02/Si wafers on a knife edge. Fig. 8: Fiber/polymer waveguide transition The technology to fabricate dynamic passive structures is in principle the same, but there are some additional processing steps. First a doping by nonlinear molecules is necessary. After fabrication of waveguides the nonlinear molecules are poled by corona discharge. Electrodes were produced by evaporating Cr, Cu and Au. The final component is a multi-layer-structure shown in Fig. 9. The devices of interest are electro optical switches and tunable optical channel filters, optic optical frequency converters and parametric amplifiers. SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, and Applications 11(1991) / 283
electrodes buffer layer -I ------. poled wavegu ides buffer layer electrode substrate Fig. 9: Multi layer structure substrate: glass, Si,... buffer layer: Si02, epoxy,... electrode: Cr, Cu, Au,... poled waveguides: PMMA DR1,... For future active devices Er3 and Nd3 doped PMMA has been synthesized. Absorption spectra were measured in the optical short wave region8. Typical absorption resonances could be observed. The 530 nm wavelength may be used for pumping an Er3doped polymer waveguide to amplify a signal at 1550 nm. Monomode waveguides made of Er3doped PMMA were successfully fabricated. 5. SPECIFICATIONS OF A FUTURE OPTICAL POLYMER CROSSCONNECT A typical section of a crossconnect circuit is shown in Fig. 10. The input 2 x 1 switch should be an electrooptic one in a structure, known from LiNbO3-technology like directional coupler type. The input carrier is selected by a tunable channel filter. Reflection free filter types are the Mach- Zehnder or the ring type structure cascaded according the number of optical input carriers. The 10 db directional coupler is needed for adding the pump wave to the signal wave in the parametric frequency converter. At the converter output a tunable optical filter is used to clean the converted optical carrier from parasitic mixing products and to protect the following circuits against the pump wave P1. Last stage of the crossconnect is an amplifier realized either as a parametric or an Er3-doped one. Both types must be pumped by P2. If a parametric amplifier could be inserted the technology advantage of a unique doping is obvious. For the complete section a nonlinear doping of approximately 50-100 pm/v should be sufficient, if pump powers of 100-200 mw can be supplied. For a long interaction length a phase matching by periodic poling is preferred. 284 / SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, and Applications 11(1991)
input selected selected mixture signal on signal on amplified signals input input C, P1 Cii Cp signal on I waveguide carrier C,, parasitics I I s! i Ci..- I pump wave P1 Fig. 10: CMC crossconnect element pumping light P2 6. HYBRID INTEGRATION CONCEPT At this time neither semiconductor technology nor polymer technology is able to dispose a totally integrated CMC crossconnect element. However the polymer technology is conceptional hybrid because polymer circuits can be fabricated on all kinds of substrate materials (Fig. 11). That means, that for example the optic optical frequency converter or the parametric amplifier can be firstly realized as a semiconductor element, exchanged later by polymer elements if the material parameters (attenuation, nonlinear coefficients etc.) are sufficient. electrode photodetector buffer layer (pol optical input signals Fig. 11: Hybrid integration concept laser diode substrate SPIE Vol. 1559 Photopo/ymer Device Physics, Chemistry, and Applications II (1991) / 285
Fig. 12 shows a comparison of most important material systems containing physical, technological and functional arguments. It is obvious, that most elements can be realized as well in semiconductor as in polymer technology. Besides the above mentioned functional advantages of polymer components like unidirectional frequency converters, parametric amplifiers and potential integrated optical isolators, these devices can be realized on a high number of substrate materials by a cost effective technology. argument semiconductor LINbO3 glass polymer passive electro optic parametric active nonreciprocal substrate fabrication process attenuation fiberbchip coupling remarks? inp epitaxy <0. 5 dbbcm difficult monolithic integration erbium? LINbO3 diffusion <0, 1 dbbcm difficult eiectro opt. elements 1- erbium? glass diffusion <0, 1 dbbcm easy passive elements erbium (mb-dipole) glass. Si. inp I ithography <0, 5 dbbcm easy hybrld integration Fig. 12: Comparison of material systems In addition there will be many other applications of integrated polymer circuits for optical communication and signal processing, in which a complete integration of polymer components is not possible. In this case the proposed hybrid integration scheme is a practicable solution. Consequently this hybrid concept opens up for the first time a possibility for a step by step transition to a small scale integration of single components to functional devices. 7. ACKNOWLEDGEMENT The authors would like to thank the Deutsche Bundespost TELEKOM for funding this work. 286 / SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, and Applications II (1991)
8. REFERENCES 1 RACE 91: 2. The RACE Programme p.3 2 B.Strebel, E.-J.Bachus, J.Vathke: Switching in Coherent Multi-Carrier Systems, Globecom'89, Dallas, Conference Record Vol. 1 pp.32-36 3 M.Nishio, S.Suzuki: Photonik Wavelength-Division Switching Network using Parallel A-Switch, 1990 International Topical Meeting on Photonic Switching, Post-Deadline Paper 14B-9 4 E.-J.Bachus, R.P.Braun, C.Caspar, H.-M.Foisel, N.Keil, H.H.Yao, B.Strebel: Broadband Exchange by Coherent Multi-Carrier Switching, Proc.SPIE, Coherent Lightwave Communications, Sept. 1989, Boston, Vol.1175, pp.198-207 5 E.J.Bachus, R.P.Braun, C.Caspar, H.-M.Foisel, E.Grol3mann, B.Strebel, F.-J.Westphal: Coherent Optical Multicarrier Systems, J.of Lightw.Teclm., Vol.7, No.2, February 1989, pp. 375-383 6 E.-J.Bachus, R.P.Braun, C.Caspar, H.-M.Foisel, K.Heimes, N.Keil, B.Strebel, J.Vathke, M.Weickhmann: Coherent optical multicarrier switching node, OFC'89, Houston, Post-Deadline Paper, PD-13 7, 8 N.Keil, B.Strebel, H.H.Yao: Polymer Waveguides in Integrated Optics, Micro System Technologies 90, 1.Intern.Conf. on Micro Electro, Opto, Mechanic Systems and Components, Berlin, 10.- 13.Sept.1990, Springer Verlag, pp. 289-294 SPIE Vol. 1559 Photopolymer Device Physics, Chemistry, arid Applications II (1991) / 287