We report the fabrication of multimode polymeric waveguides using spin coating, photolithography, and reactive ion etching. Different layer structures have been used, e.g., a UV curable resin is used as a core layer and PMMA as a lower and upper cladding. The layer systems can be spun with good uniformity with thicknesses matching multimode fiber dimensions, and smooth waveguide side-walls have been obtained after etching. The measured performance of these waveguides demonstrates that low-loss waveguides can be fabricated cheaply. The waveguides are used in the realization of novel compact multimode power splitters and star couplers

Driessen, A.

Lagali, N.

Musa, S.

Sengo, G.

Sulur,

English

Lagali, N.S.

Sulur, N.

University of Twente Research Information

 	 !!"#$%'&()* ,+.-0/)123547688894;:<* =>2,4?%/)A@BC2C/)D* 1)	Fabrication of Polymeric Multimode Waveguides forApplication in the Local Area Network and OpticalInterconnectsS. Musa, N. Lagali, Sulur, G. Sengo, and A. DriessenLightwave Devices Group, MESA Research Institute, University of Twente, P.O. Box 217,7500 AE Enschede, The Netherlands, e-mail: s.musa@el.utwente.nlAbstractWe report the fabrication of multimode polymeric waveguides using spin coating,photolithography, and reactive ion etching. Different layer structures have been used, e.g.,a UV curable resin is used as a core layer and PMMA as a lower and upper cladding. Thelayer systems can be spun with good uniformity with thicknesses matching multimode fiberdimensions, and smooth waveguide side-walls have been obtained after etching. Themeasured performance of these waveguides demonstrates that low-loss waveguides can befabricated cheaply. The waveguides are used in the realization of novel compactmultimode power splitters and star couplers.I IntroductionRecently, there has been a revived strong interest in multimode fiber for applications inshort distance communication systems, such as optical interconnects and local areanetworks (LAN). Multimode fiber has the advantage of having high bit rate capacity incomparison with copper wire, which is used in conventional systems, and has the merit oflow-cost and ease of splicing and coupling to light sources when compared with singlemode fiber. Moreover, the availability of the flexible multimode plastic optical fiber(POF) [1] adds higher bandwidth capacity to multimode fiber systems and make it moreattractive for application in short distance communication. However, efficient use ofmultimode fiber requires a vast number of optical components, such as power splitters,star couplers, and wavelength division multiplexing components, which for low-costapplications should be realized in planar waveguide technology. Realization of thesemultimode integrated optical components still represents a considerable challenge,because of the complexity of the design and the immaturity of the technology that issuitable to fabricate large waveguides required in these applications.Polymer optical waveguide technology has appeared as the most suitable means forrealization of optical multimode thick planar devices. This technology is very attractivebecause; 1) it makes possible the fabrication of very thick waveguides, e.g., ~80x80 µm2,with very low optical losses using simple equipment [2], 2) it has the benefit of havingpotentially very low costs, and 3) it is suitable for mass production, especially in the caseof molding [3]. There have been several reports in literature about fabrication andcharacterization of polymeric multimode waveguides with different physical, optical, andmechanical properties [2,3]. However, no optimum waveguide structure that satisfies mostof the application demands including good optical, physical, and mechanical propertieshas yet been obtained.In this paper, we report on the fabrication of novel large core structures with lowand high ∆n, where ∆n is relative index contrast between the core and the cladding, tomatch the 50-µm-diameter multimode graded-index fiber and the plastic optical fiberhaving a core diameter > 100 µm, respectively. In section II, we will describe theEF  1  12  9 =  9*     * 2   	 1 ,9 	, =>	%*  12  9 2/)A# 1*	% 1@BC20 1)	 $ 2  1* 23 ,9)C2 051015202530350 1000 2000 3000 4000 5000 6000speed (rpm)thickness (microns)Fig. 1 Film thickness of UV15 vs spinning speed .fabrication process of these structures including layer deposition method and channelwaveguides definition. Optical properties will be shown and discussed in section III. Theconclusions will give in section IV.II Fabrication of polymeric optical waveguidesWe have fabricated two types of polymeric optical waveguides, one with low ∆n andanother with a high one, using spin coating, photolithography, and oxygen reactive ionetching (RIE). In the following we will describe the fabrication of these two waveguides.a) Fabrication of low   n polymeric waveguides The low ∆n polymeric optical waveguide consists of PMMA as a lower and an uppercladding and a UV-curable resin (UV15, commercially available from Master Bond [4]) asa core layer.  The core and the cladding having refractive indices 1.51 and 1.488 at λ=0.633 µm, respectively.  A buffer PMMA layer (~10 µm) was spun on a thermallyoxidized Si-substrate on top of which a 40 µm of UV15 used as a guiding layer wasdeposited in the same way. After definition of the channel waveguide a thick overcladding of PMMA was deposited.The PMMA solution is prepared by dissolving 20 w% of PMMA in Cyclo-pentanone. The solution is then placed in ultrasonic bath for ~ 5 hours to completelydissolve PMMA. The 10 µm-thick layer was obtained by spin deposition of three layers ata rotation speed of 1000 rpm during 30 seconds. The layer thickness can be varied bychanging the concentration and / or the spinning speed. Each layer was cured by heating itto 100 °C for 10 min. Using the spin coating process a uniform and a well controlled layerthickness with a smooth film surface could been obtained. The thick PMMA overcladdingwas prepared by spinning twice at low speed followed by flooding the wafer with PMMAsolution.The UV15 has a viscosity of 1000cps which makes it suitable for spincoating of thick multimode layers.   Fig. 1shows the layer thickness as a function ofspinning speed. It is shown that the filmthickness can be varied between ~ 34 to 3µm in spinning speed range from 400 to6000 rpm. Thinner film can be depositedby decreasing the viscosity of UV15,which can be obtained by adding acetone.Thickness uniformity is estimated to be2.5 %. The UV15 is cured by exposingthe deposited layer to a uniformlydistribution of the 360-nm UV line of themask aligner. A constant N2 flow waskept during exposure to avoid oxidationduring the curing process.For the channel definition, a 150 nm thick Al layer was used as a resistant etchmask. To avoid evaporating Al directly on the UV15, which might diffuse into the guidinglayer and hence increases the optical losses, a 3-µm PMMA film was spun on UV15 to actas a protection layer. On top of the Al mask a 1.5 µm photoresist was deposited andE 	 !!"#$%'&()* ,+.-0/)123547688894;:<* =>2,4?%/)A@BC2C/)D* 1)	developed in the normal way [2]. The exposed Al is then etched away by the standard Aletchant before the structure is etched down by O2 RIE.The RIE is carried out at a constant O2 flow rate of 20 standard cubic centimetersper minute (SCCM) with a total pressure of 10 mtorr. The cathode was maintained cooledat 10 °C during the etching process. In this investigations the applied power was variedbetween 50 to 100 Watts and a moderate increase in the corresponding etch rate from 0.3to 0.5 µm/min has been observed. Also, it is has been observed that by varying the cathodetemperature between 10 to 20 °C no significant change in the etch rate has taken place.We have found that in etching of thick layers (> 40 µm) one has to flush the chamber withN2 after every 30 min of etching in order to maintain a constant etch rate.   (a) (b)Fig. 2 shows a scanning electron micrograph of multimode channel waveguidesetched 40 µm deep into UV15 with different channel widths from 20 to 50 µm. It can beseen that the channels are free from cracks. Also, the side-walls are smooth without theneed for extra treatment [2]. The waveguides shown in Fig. 2 are etched at an power of 75W. We have noticed that the side wall roughness increases and the waveguides are slightlydamaged if the power is increased to 100 W.  In Fig. 2 it can be seen that there is aremaining thin layer of Al in spite of the etching of the Al mask after the RIE process.b) Fabrication of high    n polymeric waveguidesThe structure of high ∆n comprises a polyurethane (PU) core layer with refractive index of1.565 at λ = 0.633 µm spun on a thermally oxidized Si-wafer. As an upper cladding arelatively low refractive index (n = 1.43 at λ = 0.633 µm) commercially available resinMastersil (from Master Bond [4]) is used.The PU is commercially available as a solution with low viscosity. In order toobtain thick layers the viscosity has to be increased. This can be achieved by warming thesolution at 45°C for ~ 5 hours. A thickness of 10 µm is obtained spin speed of 500 rpm.To obtain layers thicker than 50 µm the spinning process has to be repeated several times.The films are cured by heating at 90 °C for ~1 hour. The resulted layers are smooth andwithout cracks. In addition there is strong adhesion between the PU film and the SiO 2.The channel waveguides are fabricated using a method similar to that used toproduce low ∆n channel waveguides. After the etching process a thick overcladding ofMastersil was overlaid on the structure. Mastersil was then cured by heating at 90 °C for 3hours.Fig. 2: (a) SEM picture of UV15 multimode channel waveguides. (b) A top view of the etched waveguidesby an optical microscope. The gray islands on the channels are due to residuals of the thin Al layerE  1  12  9 =  9*     * 2   	 1 ,9 	, =>	%*  12  9 2/)A# 1*	% 1@BC20 1)	 $ 2  1* 23 ,9)C2 Fig. 3: Absorption spectrum of a UV15 waveguide.III Optical characterizationsIn the loss measurement setup a 50/125 µm graded multimode fiber was used to couplelight in and out of the waveguides. An index matching gel was placed between thewaveguide and the multimode fiber to reduce the coupling losses. The loss spectra weremeasured using a halogen lamb and a spectrometer.Fig. 3 shows the absorption spectrum of a 40 x 50 µm2 PU channel waveguide.The spectrum shows that forwavelengths < 1000 nm that there isno considerable intrinsic absorptionby the PU polymer indicating that thewaveguide is quite suitable forapplication in short distancecommunication with light sources ofwavelength range between 600 and900 nm. Also around the secondtelecom window centered at 1.3 µm,there low absorption. For the thirdtelecom window centered around λ =1.5 µm the material absorption isquite high, meaning that thiswaveguide can not be used in thistelecom window. Optical lossmeasurements were done using cut-back. The optical losses for UV15, at λ = 0.633 µm, were determined to be 1.9 dB/cm.These relatively high losses might be due to the influence of the thin remaining Al layer,which in the case of PU has been completely striped off.IV ConclusionsWe have shown that large core polymeric optical waveguides can be fabricated cheaplyusing spin coating, photolithography, and RIE. Spin coating gives layer with uniform andcontrollable thickness and smooth surfaces. The obtained channel waveguides havesmooth side-walls and are free from cracks. Very low optical losses were determined forthese waveguides. The fabricated waveguides will be used for realization of novelcompact power splitters and star couplers.References[1] G. D. Khoe et al. “status of GIPOF systems and related technologies,” Proc. ECOC’99, September1999.[2] M. Kagami, H. Ito, T. Ichikawa, S. Kato, M. Matsuda, and N. Takahashi, “Fabrication of large-corehigh-∆ optical waveguides in polymers,” Appl. Optics, Vol. 34 No. 6, pp. 1041, 1995.[3] R. T. Chen, S. Tang, T. Janson, and J. Janson, “45-cm long compression-molded polymer-based opticalbus,” Appl. Phys. Lett., 63(8), pp. 1032, 1993.[4] Master Bond Inc., http://www.masterbond.com.10123456789600 800 1000 1200 1400 1600wavelength(nm)excess losses(dB/cm)E 

Fabrication of Polymeric Multimode Waveguides for Application in the Local Area Network and Optical Interconnects

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Fabrication of Polymeric Multimode Waveguides for Application in the Local Area Network and Optical Interconnects.

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