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UV-laser assisted Fabrication of integrated-optical Waveguides
UV-laser assisted Fabrication of integrated-optical Waveguides F. Vollertsen (2), C. Wochnowski BIAS - Bremer lnstitut fur Angewandte Strahltechnik Klagenfurter Str. 2, 28359 Bremen, Germany [email protected]
Abstract By UV-laser-irradiation a controllable change of the refractive index of some polymers can be attained in the irradiated zone. The detailed UV-photon-induced process mechanisms have been investigated on the basis of PMMA employed as an UV-modifiable model polymer. By mask lithographic methods the UV-laserassisted technology for the modification of the polymer optical properties enables the local increase of the refractive index and thus the fabrication of integrated-optical waveguiding elements in the surface of a planar polymer chip. These elements are relevant for the manufacturing of complex integrated-optical components. The optical and functional properties of the integrated-optical waveguides have been characterized. Keywords: polymer, optical chip, excimer laser
1 INTRODUCTION Integrated optics plays more and more a relevant role in optical microsystems due to the miniaturization in the optical information and sensor technology [I]. However, the classical anorganic basic materials for integratedoptical components like silica-on-silicon, LiNb03 and binary semiconductors like GaAs or InP are very costly and difficult in processing [I]. Organic materials like polymers are an alternative for those materials combining both cost-effectiveness and simple handling [2]. Unfortunately the most manufacturing process of integrated-optical polymer components are still too complex for wide industrial application [2].
In this paper a simple but effective process for the fabrication of integrated-optical polymer devices is presented which is based on the UV-laser-assisted photochemical modification of the optical properties of the polymer surface. The UV-photochemical processes permit the local and controllable increase of the refractive index at the surface of the polymer. Consequently in this way it is possible to generate integrated-optical micro-structures on the surface of a polymer chip [3][4]. 2 EXPERIMENTAL As a model polymer polymethylmethacrylate (PMMA) was chosen because of the low content of additives, its good optical surface and bulk quality. As an UV-source an excimer laser was used which can emit different wavelengths (193 nm and 248 nm) depending on the gas mixture.
In order to modify the polymer refractive index locally by the UV-laser beam a mask-lithographic alignment was built up. In case of contact mode a chrome mask is positioned directly on the polymer substrate. Consequently the incident UV-laser beam only irradiates the uncovered regions of the polymer surface. In the illuminated areas of the polymer surface the refractive index is increased locally in a controllable way thus generating the required integrated-optical structure.
In this paper the work is confined to contact mode as shown in Fig. 1. As an alternative lithographic method the projection mode can be used as described in [5].
Figure 1: fabrication of integrated-optical waveguide by contact mask laser lithography
2.1
Fabrication and characterization of integratedoptical waveguides The optical-functional properties of the UV-laser generated integrated-optical waveguides strongly depend on the irradiation conditions. Especially the irradiation parameters affect the refractive index profile of the waveguide and so the mode distribution and the loss rate of the transmitted light. By choosing the right process parameters the functional properties of the integrated-optical waveguide can be optimized. Refractive index Fig. 2 shows the refractive index of the UV-illuminated polymer substrate surface in dependence on the irradiation dose for the two wavelengths 193 nm and 248 nm (fluence = 17 mJ/cm2, repetition rate R = 5 Hz). The refractive index was measured by a white-light Abberefractometer. For the two wavelengths the refractive index curves are quite similar as in both cases two
I
Figure 2: Refractive index of UV-laser treated PMMA substrate surface measured by white-light Abbe refractometry maxima exist: a first small maximum at a relatively low laser pulse number and a second big one at quite a high pulse number. Generally the refractive index changes at 248 nm are superior to those ones at 193 nm. Experimental measurements of the refractive index depth profile inside the integ rated-optical waveguide structure are done by a free-space optical two-beam-interferometer (Mach-Zehnder-Interferometer) as shown in Fig. 3 [6]. The experiments yield a gradient refractive index profile shown in Fig. 4. One has to distinguish between two cases: in the first case the waveguide is generated by a low irradiation dose (wavelength h = 248 nm, fluence f = 17 mJ/cm2, repetition rate R = 5 Hz, total number of pulses N = 1000). According to Lambert-Beefs law the refractive index depth profile of the waveguide has a flattened shape as shown in Fig. 4a. In the second case the waveguide is generated by a high irradiation dose ( h = 248 nm, f = 17 mJ/cm2, R = 5 Hz, N = 5000). In this case the refractive index reaches its maximum shortly beneath the surface as shown in Fig. 4b. With increasing depth the refractive index decreases quite sharply, so the refractive index profile is almost step-like. In the deeper region of the UVirradiated polymer substrate the refractive index rises again in a moderate way until it reaches a second small maximum at approx. 55 pm. The controllable and local increase of the refractive index permits the generation of integrated-optical waveguiding structures in the surface of a planar polymer chip as shown in Fig. 5.
.
I
Figure 3: free-space Mach-Zehnder interferometer for refractive index measurements generated at a low irradiation dose (irradiation parameters: h = 248 nm, f = 20mJ/cm2, R = 1 Hz and N = IOOO), so the refractive index profile of the waveguide is similar to the one shown in Fig. 4a. Due to the low irradation dose the refractive index rise inside the waveguide is quite small and its lateral diameter and depth are 30 pm, respectively, so only one mode is propagating inside the planar waveguide. In Fig. 6b one can see the mode distribution of red light (632.8 nm) inside the integrated-optical polymer waveguide with the same width, but generated at a high irradiation dose (irradiation parameters: h = 248 nm, f = 20mJ/cm2, R = 1 Hz and N = 5000). Thus the refractive index profile of the waveguide is similar to the one shown
Figure 4a: Refractive index depth profile of the waveguide generated by a low irradiation dose [6]
Mode characterization For measurement of the mode propagation of light inside the integrated-optical waveguide a laser test beam was coupled into the waveguide by fiber-chip-coupling. A HeNe laser ( h = 632.8 nm) is used as a light source. Then the mode propagation inside the waveguide was characterized by the following method: after propagating through the integrated-optical waveguide the light exits from the rear side of the polymer chip to a free-space standing objective lense magnifying and projecting the image onto the chip of a CCD camera. The camera digitalizes and transmits the data to a computer in order to record and evaluate them. In Fig. 6a one can see the mode distribution of red light (632.8 nm) at the exit of an integrated-optical polymer waveguide with a width of 30 pm. The waveguide is
Figure. 4b: Refractive index depth profile of the waveguide generated by a high irradiation dose [6]
Figure 5: Integrated-optical waveguides
in Fig. 4b. Due to the relatively high irradiation dose the rise of the refractive index is also high, thus several lateral modes are formed passing through the planar waveguide. A comparison between waveguides generated by different irradiaton doses shows the influence of the irradiation conditions on the optical properties of the waveguide, especially in the deeper region. This feature can be effectively used for a controllable modification of the refractive index in order to obtain the predetermined optimum functional properties.
Loss rate measurements The loss rate of the waveguided light inside the integratedoptical polymer waveguides was measured by the transmission method with a fiber-chip-fiber alignment. On the front side of the waveguide light was coupled in to propagate through the waveguiding structure. On the back side of the waveguide the light was coupled out and its intensity was measured. Comparing with a zero-point measurement, which was done for calibration purposes, one obtains the total attenuation of the waveguide including all kinds of losses (coupling and transmission loss). The experimental alignment for loss rate measurement is similar to the experimental set-up for mode propagation examination described above. In order to measure the loss rate the test laser beam is coupled into the integrated-optical waveguide by fiber-chipcoupling. As a light source a HeNe laser is used. On the exit side light is also coupled out by a fiber and is guided to a photo diode for intensity measurements. For visible light (wavelength 632.8 nm) the loss rate for a monomode waveguide is about 3 dBkm and for a multimode waveguide the loss rate is about 1 dBlcm. Thus it was shown that the waveguides generated by a high irradiation dose have a lower attenuation than waveguides generated by a low irradiation dose.
3 DISCUSSION The UV-photon-induced modification of the refractive index in Fig. 2 is based on several independently and subsequently occurring photochemical processes.
Figure. 6a: Mode propagation in a waveguide generated by low irradiation dose
According to the literature at low UV-irradiation dose a direct UV-photopolymerization occurs [7][8][9]: in every PMMA sample a non-negligible rest monomer content exists which can be activated by the incident UV-photons ( h = 250 nm) as shown in Fig. 7. The partial direct breaking of the double bonding C=C of the acrylate monomer yields two radical electrons initiating a photoninduced polymerization process. By the growth of the polymer molecular chain the refractive index starts to increase moderately at low pulse numbers as seen in Fig. 2. After a certain dose the polymer chain growth stops due to direct photon-induced main chain scission. Therefore a depolymerization process starts [ I O][l I ] decreasing the refractive index. Both the direct UV-photopolymerization as well as the depolymerization processes explain the existence of the first small refractive index maximum in Fig. 2. By ongoing UV-laser irradiation a photochemical modification process of the physical-chemical polymer structure takes place at a high irradiation dose: the incident UVphotons trigger a cleavage of the side chains from the main polymer chain as shown in Fig. 8. UV-laser-assisted modification mechanism of the PMMA-polymer is described in details elsewhere [3]. As this photo-induced reaction is a photochemical effect one has to distinguish between two cases with different wavelengths: by a 248 nm laser irradiation a complete side chain scission occurs while by a 193 nm laser irradiation only an incomplete side chain scission arises. The side chain cleavage results in a rise of the mechanical density due to Van-der-Waals interactions between the polymer molecules and thus yielding an increase of the optical refractive index [12]. Hence the different photochemical behaviour of the side chain of the polymer towards the two wavelengths 193 nm and 248 nm is reflected by the different dependences of the refractive index on the irradiation conditions as shown in Fig. 2: for both wavelengths the refractive index was measured in dependence on the irradiation dose. For 248 nm irradiation the curve is rising more strongly and the refractive index attains higher values than for 193 nm irradiation because a complete side chain scission yields a higher volume densification than an incomplete side chain scission. After reaching a maximum a decrease of the refractive index occurs in both cases. This can be explained by the destabilization and destruction of the polymer main chain by the side chain cleavage [13]. Hence by ongoing UV-illumination too many polymer main chains scissions occurs leading to the total defragmentation of the entire polymer structure at very high irradiation dose [14]. Both photochemical processes the direct UVphoto(de)polymerization and the UV-photochemical modification process (side chain cleavage from the polymer main chain) have an influence on the formation of the refractive index depth profile as shown in Fig. 4. At a low irradiation dose the moderate refractive index increase is due to the direct UV-photopolymerization (see Fig. 4a).
Figure. 6b: Mode propagation in a waveguide generated by high irradiation dose
At a high irradiation dose a double waveguide structure is generated (s. Fig. 4b): the upper waveguide is formed by the UV-photochemical modification process (side chain cleavage from the polymer main chain with subsequent densification of the polymeric material), while the lower
4
CONCLUSION It has been shown that by UV-laser assisted photochemical modification of the polymer surface it is possible to fabricate integrated-optical waveguides on the surface of a polymer chip. The characterization of the optical and functional properties of the waveguides shows that they are suited for the operation in the optical information technology. It is expected that their properties can be further improved by optimization of the process parameters.
5
ACKNOWLEDGEMENT he authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work (project ME 991110 - 1). I
I
04w \ Figure. 7: direct UV-photopolymerization of Methylmethacrylate-monomers waveguide is mainly generated by the direct UV-photo(de)polymerization process. Thus by the right choice of irradiation parameters the refractive index profile of the polymer can be modified in a controllable way having an effect on the functional properties of the waveguiding structure like the mode field distribution as well as attenuation. The mode propagation in the waveguide is mainly determined by its geometric shape and the refractive index
/c
'0
I
\O-CH,
+ + -0-CH,
partial side chain scission
/c
O/
\O-CH,
complete side chain scission
Figure 8: UV-photon induced photochemical modification mechanism of PMMA profile. At a low irradiation dose the increase of the refractive index is quite moderate so the integrated-optical waveguide is a single-mode one as shown in Fig. 6a. At a high irradiation dose the refractive index increases strongly permitting the formation of more than one mode as shown in Fig. 6b. Also it was shown that the waveguides generated by a high irradiation dose have a lower attenuation than waveguides generated by a low irradiation dose due to the better confinement of the propagating modes to the waveguide core and hence to the lower power leakage in the cladding region.
LITERATURE Alferness, R.C., 1997, Integrated Optics: Technology and System Applications Converge, Optics 8 Photonics News, 9: 16-22 Theis, J., Brockmeyer, A,, Groth, W. et al., 1992, Polymer Optical Fibers in Data Communications and Sensor Applications", Polym. f. Lightwave a. Integr. Optics: Techn. a. Appl.: 39-70 Wochnowski, C., Metev, S., Sepold, G., 2000, UVlaser-assisted Modification of the optical Properties of Polymethylmethacrylate, Appl. Surf. Sc., 154-155: 706-71 1 Schmidt, M., Dirscherl, M., Kaufmann S., Stark, M., 2003, Laser-assisted Fabrication of micro-optical Components, Proc. ICALEO -03 (USA) LIA Pub #: 595 (CD-version) Vol. 95 (2003) Paper-ID MI07 Kley E.-B., Tunnermann, A,, Zeitner, U. D., Karthe W., 2001, Mikrooptische Elemente, Funktionen und Herstellungstechnologien, lithographische LaserOpto, 3313: 78-84 Shams-el-Din, M., Wochnowski C., Metev, S., Hamza A,, Juptner W., 2004, Determination of the refractive index depth profile of an UV-laser generated waveguide in a planar polymer chip Appl. Surf. Sc. to be submitted Morita, H., Sadakiyo T., 1995, Laser-induced polymeric film formation from gaseous methyl acrylate, J. Photochem. Photobiol. A: Chemistry, 87: 163-167 Lewis, F.D., Nepras, M.J., Hampsch H.L., 1987, Tetrahedron, 4317: 1635-1642 Tsao, J.Y., Ehrlich D.J., 1983, UV laser photopolymerization of volatile surface-absorbed methyl methacrylate, Appl. Phys. Lett. 42/12: 997999 Kopietz M., Lechner M.D., Steinmeier D. G., 1984, Light-Induced Refractive Index Changes in Polymethylmethacrylate (PMMA) blocks, Polym. Photochem., 5: 109-119 Garnett J.L., 1995, Radiation Curing - Twenty Five Years On, Radiat. Phys. Chem., 4614-6: 925-930 Schmoldt A,, 1994, Veranderung der optischen Eigenschaften von PMMA durch ionisierende Strahlung, dissertation work, TU Berlin Dole M., 1973, Radiation Chemistry of Macromol., Vol II, Acad. Press NY, London: 97-117 Kuper, S., Stuke M., 1989, UV-Excimer-Laser Ablation of Polymethylmethacrylate at 248 nm: Characterization of Incubation Sites with Fourier Transformation IR-and UV-Spectroscopy, Appl. Phys. A, 49: 211-215
Abstract By UV-laser-irradiation a controllable change of the refractive index of some polymers can be attained in the irradiated zone. The detailed UV-photon-induced process mechanisms have been investigated on the basis of PMMA employed as an UV-modifiable model polymer. By mask lithographic methods the UV-laserassisted technology for the modification of the polymer optical properties enables the local increase of the refractive index and thus the fabrication of integrated-optical waveguiding elements in the surface of a planar polymer chip. These elements are relevant for the manufacturing of complex integrated-optical components. The optical and functional properties of the integrated-optical waveguides have been characterized. Keywords: polymer, optical chip, excimer laser
1 INTRODUCTION Integrated optics plays more and more a relevant role in optical microsystems due to the miniaturization in the optical information and sensor technology [I]. However, the classical anorganic basic materials for integratedoptical components like silica-on-silicon, LiNb03 and binary semiconductors like GaAs or InP are very costly and difficult in processing [I]. Organic materials like polymers are an alternative for those materials combining both cost-effectiveness and simple handling [2]. Unfortunately the most manufacturing process of integrated-optical polymer components are still too complex for wide industrial application [2].
In this paper a simple but effective process for the fabrication of integrated-optical polymer devices is presented which is based on the UV-laser-assisted photochemical modification of the optical properties of the polymer surface. The UV-photochemical processes permit the local and controllable increase of the refractive index at the surface of the polymer. Consequently in this way it is possible to generate integrated-optical micro-structures on the surface of a polymer chip [3][4]. 2 EXPERIMENTAL As a model polymer polymethylmethacrylate (PMMA) was chosen because of the low content of additives, its good optical surface and bulk quality. As an UV-source an excimer laser was used which can emit different wavelengths (193 nm and 248 nm) depending on the gas mixture.
In order to modify the polymer refractive index locally by the UV-laser beam a mask-lithographic alignment was built up. In case of contact mode a chrome mask is positioned directly on the polymer substrate. Consequently the incident UV-laser beam only irradiates the uncovered regions of the polymer surface. In the illuminated areas of the polymer surface the refractive index is increased locally in a controllable way thus generating the required integrated-optical structure.
In this paper the work is confined to contact mode as shown in Fig. 1. As an alternative lithographic method the projection mode can be used as described in [5].
Figure 1: fabrication of integrated-optical waveguide by contact mask laser lithography
2.1
Fabrication and characterization of integratedoptical waveguides The optical-functional properties of the UV-laser generated integrated-optical waveguides strongly depend on the irradiation conditions. Especially the irradiation parameters affect the refractive index profile of the waveguide and so the mode distribution and the loss rate of the transmitted light. By choosing the right process parameters the functional properties of the integrated-optical waveguide can be optimized. Refractive index Fig. 2 shows the refractive index of the UV-illuminated polymer substrate surface in dependence on the irradiation dose for the two wavelengths 193 nm and 248 nm (fluence = 17 mJ/cm2, repetition rate R = 5 Hz). The refractive index was measured by a white-light Abberefractometer. For the two wavelengths the refractive index curves are quite similar as in both cases two
I
Figure 2: Refractive index of UV-laser treated PMMA substrate surface measured by white-light Abbe refractometry maxima exist: a first small maximum at a relatively low laser pulse number and a second big one at quite a high pulse number. Generally the refractive index changes at 248 nm are superior to those ones at 193 nm. Experimental measurements of the refractive index depth profile inside the integ rated-optical waveguide structure are done by a free-space optical two-beam-interferometer (Mach-Zehnder-Interferometer) as shown in Fig. 3 [6]. The experiments yield a gradient refractive index profile shown in Fig. 4. One has to distinguish between two cases: in the first case the waveguide is generated by a low irradiation dose (wavelength h = 248 nm, fluence f = 17 mJ/cm2, repetition rate R = 5 Hz, total number of pulses N = 1000). According to Lambert-Beefs law the refractive index depth profile of the waveguide has a flattened shape as shown in Fig. 4a. In the second case the waveguide is generated by a high irradiation dose ( h = 248 nm, f = 17 mJ/cm2, R = 5 Hz, N = 5000). In this case the refractive index reaches its maximum shortly beneath the surface as shown in Fig. 4b. With increasing depth the refractive index decreases quite sharply, so the refractive index profile is almost step-like. In the deeper region of the UVirradiated polymer substrate the refractive index rises again in a moderate way until it reaches a second small maximum at approx. 55 pm. The controllable and local increase of the refractive index permits the generation of integrated-optical waveguiding structures in the surface of a planar polymer chip as shown in Fig. 5.
.
I
Figure 3: free-space Mach-Zehnder interferometer for refractive index measurements generated at a low irradiation dose (irradiation parameters: h = 248 nm, f = 20mJ/cm2, R = 1 Hz and N = IOOO), so the refractive index profile of the waveguide is similar to the one shown in Fig. 4a. Due to the low irradation dose the refractive index rise inside the waveguide is quite small and its lateral diameter and depth are 30 pm, respectively, so only one mode is propagating inside the planar waveguide. In Fig. 6b one can see the mode distribution of red light (632.8 nm) inside the integrated-optical polymer waveguide with the same width, but generated at a high irradiation dose (irradiation parameters: h = 248 nm, f = 20mJ/cm2, R = 1 Hz and N = 5000). Thus the refractive index profile of the waveguide is similar to the one shown
Figure 4a: Refractive index depth profile of the waveguide generated by a low irradiation dose [6]
Mode characterization For measurement of the mode propagation of light inside the integrated-optical waveguide a laser test beam was coupled into the waveguide by fiber-chip-coupling. A HeNe laser ( h = 632.8 nm) is used as a light source. Then the mode propagation inside the waveguide was characterized by the following method: after propagating through the integrated-optical waveguide the light exits from the rear side of the polymer chip to a free-space standing objective lense magnifying and projecting the image onto the chip of a CCD camera. The camera digitalizes and transmits the data to a computer in order to record and evaluate them. In Fig. 6a one can see the mode distribution of red light (632.8 nm) at the exit of an integrated-optical polymer waveguide with a width of 30 pm. The waveguide is
Figure. 4b: Refractive index depth profile of the waveguide generated by a high irradiation dose [6]
Figure 5: Integrated-optical waveguides
in Fig. 4b. Due to the relatively high irradiation dose the rise of the refractive index is also high, thus several lateral modes are formed passing through the planar waveguide. A comparison between waveguides generated by different irradiaton doses shows the influence of the irradiation conditions on the optical properties of the waveguide, especially in the deeper region. This feature can be effectively used for a controllable modification of the refractive index in order to obtain the predetermined optimum functional properties.
Loss rate measurements The loss rate of the waveguided light inside the integratedoptical polymer waveguides was measured by the transmission method with a fiber-chip-fiber alignment. On the front side of the waveguide light was coupled in to propagate through the waveguiding structure. On the back side of the waveguide the light was coupled out and its intensity was measured. Comparing with a zero-point measurement, which was done for calibration purposes, one obtains the total attenuation of the waveguide including all kinds of losses (coupling and transmission loss). The experimental alignment for loss rate measurement is similar to the experimental set-up for mode propagation examination described above. In order to measure the loss rate the test laser beam is coupled into the integrated-optical waveguide by fiber-chipcoupling. As a light source a HeNe laser is used. On the exit side light is also coupled out by a fiber and is guided to a photo diode for intensity measurements. For visible light (wavelength 632.8 nm) the loss rate for a monomode waveguide is about 3 dBkm and for a multimode waveguide the loss rate is about 1 dBlcm. Thus it was shown that the waveguides generated by a high irradiation dose have a lower attenuation than waveguides generated by a low irradiation dose.
3 DISCUSSION The UV-photon-induced modification of the refractive index in Fig. 2 is based on several independently and subsequently occurring photochemical processes.
Figure. 6a: Mode propagation in a waveguide generated by low irradiation dose
According to the literature at low UV-irradiation dose a direct UV-photopolymerization occurs [7][8][9]: in every PMMA sample a non-negligible rest monomer content exists which can be activated by the incident UV-photons ( h = 250 nm) as shown in Fig. 7. The partial direct breaking of the double bonding C=C of the acrylate monomer yields two radical electrons initiating a photoninduced polymerization process. By the growth of the polymer molecular chain the refractive index starts to increase moderately at low pulse numbers as seen in Fig. 2. After a certain dose the polymer chain growth stops due to direct photon-induced main chain scission. Therefore a depolymerization process starts [ I O][l I ] decreasing the refractive index. Both the direct UV-photopolymerization as well as the depolymerization processes explain the existence of the first small refractive index maximum in Fig. 2. By ongoing UV-laser irradiation a photochemical modification process of the physical-chemical polymer structure takes place at a high irradiation dose: the incident UVphotons trigger a cleavage of the side chains from the main polymer chain as shown in Fig. 8. UV-laser-assisted modification mechanism of the PMMA-polymer is described in details elsewhere [3]. As this photo-induced reaction is a photochemical effect one has to distinguish between two cases with different wavelengths: by a 248 nm laser irradiation a complete side chain scission occurs while by a 193 nm laser irradiation only an incomplete side chain scission arises. The side chain cleavage results in a rise of the mechanical density due to Van-der-Waals interactions between the polymer molecules and thus yielding an increase of the optical refractive index [12]. Hence the different photochemical behaviour of the side chain of the polymer towards the two wavelengths 193 nm and 248 nm is reflected by the different dependences of the refractive index on the irradiation conditions as shown in Fig. 2: for both wavelengths the refractive index was measured in dependence on the irradiation dose. For 248 nm irradiation the curve is rising more strongly and the refractive index attains higher values than for 193 nm irradiation because a complete side chain scission yields a higher volume densification than an incomplete side chain scission. After reaching a maximum a decrease of the refractive index occurs in both cases. This can be explained by the destabilization and destruction of the polymer main chain by the side chain cleavage [13]. Hence by ongoing UV-illumination too many polymer main chains scissions occurs leading to the total defragmentation of the entire polymer structure at very high irradiation dose [14]. Both photochemical processes the direct UVphoto(de)polymerization and the UV-photochemical modification process (side chain cleavage from the polymer main chain) have an influence on the formation of the refractive index depth profile as shown in Fig. 4. At a low irradiation dose the moderate refractive index increase is due to the direct UV-photopolymerization (see Fig. 4a).
Figure. 6b: Mode propagation in a waveguide generated by high irradiation dose
At a high irradiation dose a double waveguide structure is generated (s. Fig. 4b): the upper waveguide is formed by the UV-photochemical modification process (side chain cleavage from the polymer main chain with subsequent densification of the polymeric material), while the lower
4
CONCLUSION It has been shown that by UV-laser assisted photochemical modification of the polymer surface it is possible to fabricate integrated-optical waveguides on the surface of a polymer chip. The characterization of the optical and functional properties of the waveguides shows that they are suited for the operation in the optical information technology. It is expected that their properties can be further improved by optimization of the process parameters.
5
ACKNOWLEDGEMENT he authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work (project ME 991110 - 1). I
I
04w \ Figure. 7: direct UV-photopolymerization of Methylmethacrylate-monomers waveguide is mainly generated by the direct UV-photo(de)polymerization process. Thus by the right choice of irradiation parameters the refractive index profile of the polymer can be modified in a controllable way having an effect on the functional properties of the waveguiding structure like the mode field distribution as well as attenuation. The mode propagation in the waveguide is mainly determined by its geometric shape and the refractive index
/c
'0
I
\O-CH,
+ + -0-CH,
partial side chain scission
/c
O/
\O-CH,
complete side chain scission
Figure 8: UV-photon induced photochemical modification mechanism of PMMA profile. At a low irradiation dose the increase of the refractive index is quite moderate so the integrated-optical waveguide is a single-mode one as shown in Fig. 6a. At a high irradiation dose the refractive index increases strongly permitting the formation of more than one mode as shown in Fig. 6b. Also it was shown that the waveguides generated by a high irradiation dose have a lower attenuation than waveguides generated by a low irradiation dose due to the better confinement of the propagating modes to the waveguide core and hence to the lower power leakage in the cladding region.
LITERATURE Alferness, R.C., 1997, Integrated Optics: Technology and System Applications Converge, Optics 8 Photonics News, 9: 16-22 Theis, J., Brockmeyer, A,, Groth, W. et al., 1992, Polymer Optical Fibers in Data Communications and Sensor Applications", Polym. f. Lightwave a. Integr. Optics: Techn. a. Appl.: 39-70 Wochnowski, C., Metev, S., Sepold, G., 2000, UVlaser-assisted Modification of the optical Properties of Polymethylmethacrylate, Appl. Surf. Sc., 154-155: 706-71 1 Schmidt, M., Dirscherl, M., Kaufmann S., Stark, M., 2003, Laser-assisted Fabrication of micro-optical Components, Proc. ICALEO -03 (USA) LIA Pub #: 595 (CD-version) Vol. 95 (2003) Paper-ID MI07 Kley E.-B., Tunnermann, A,, Zeitner, U. D., Karthe W., 2001, Mikrooptische Elemente, Funktionen und Herstellungstechnologien, lithographische LaserOpto, 3313: 78-84 Shams-el-Din, M., Wochnowski C., Metev, S., Hamza A,, Juptner W., 2004, Determination of the refractive index depth profile of an UV-laser generated waveguide in a planar polymer chip Appl. Surf. Sc. to be submitted Morita, H., Sadakiyo T., 1995, Laser-induced polymeric film formation from gaseous methyl acrylate, J. Photochem. Photobiol. A: Chemistry, 87: 163-167 Lewis, F.D., Nepras, M.J., Hampsch H.L., 1987, Tetrahedron, 4317: 1635-1642 Tsao, J.Y., Ehrlich D.J., 1983, UV laser photopolymerization of volatile surface-absorbed methyl methacrylate, Appl. Phys. Lett. 42/12: 997999 Kopietz M., Lechner M.D., Steinmeier D. G., 1984, Light-Induced Refractive Index Changes in Polymethylmethacrylate (PMMA) blocks, Polym. Photochem., 5: 109-119 Garnett J.L., 1995, Radiation Curing - Twenty Five Years On, Radiat. Phys. Chem., 4614-6: 925-930 Schmoldt A,, 1994, Veranderung der optischen Eigenschaften von PMMA durch ionisierende Strahlung, dissertation work, TU Berlin Dole M., 1973, Radiation Chemistry of Macromol., Vol II, Acad. Press NY, London: 97-117 Kuper, S., Stuke M., 1989, UV-Excimer-Laser Ablation of Polymethylmethacrylate at 248 nm: Characterization of Incubation Sites with Fourier Transformation IR-and UV-Spectroscopy, Appl. Phys. A, 49: 211-215