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Optical waveguides of polymethyl methacrylate doped with benzophenone and coumarin
Volume 66, number
2,3
OPTICS
COMMUNICATIONS
OPTICAL WAVEGUIDES OF POLYMETHYL BENZOPHENONE AND COUMARIN Naomichi
OKAMOTO
and Shintaro
5 October
1987; revised manuscript
DOPED
WITH
TASHIRO
Shizuoka University, Department ofElectronics Engineering, Received
METHACRYLATE
15 April 1988
received
3-5-I Johoku, Hamamarsu, Japan
4 January
1988
To determine its feasibility as a polymer optical waveguide, the system polymethyl methacrylate-benzophenone-coumarin is studied. The refractive-index change increases linearly up to - 1.8%with coumarin quantity as a result of photodimerization of coumarin, while the propagation loss is restrained. A loss of 0.7 dB/cm and an index change of 1.4% are obtained for slab waveguides having 0.3 and 0. I weight ratios of coumarin and benzophenone to PMMA, respectively.
1. Introduction Several methods of forming plastic optical waveguides using photochemical reactions have been reported to date. It was found that the refractive index of polymethyl methacrylate (PMMA) increases slightly after irradiation with uv light [ 1,2]. Chandross et al. reported a photolocking method [ 3,4 ] in which a photochemical reaction locks a dopant in a polymer film with a lower index. The unreacted dopant is then removed by heating. The polymer used is a 1: 1 copolymer of methyl methacrylate (MMA) and glycidyl methacrylate, and the dopant is naphthalenethiol. Kato et al. [ 51 showed similar results using cinnamic acid as the dopant. A selective photopolymerization method was reported by Kurokawa et al. [6,7], in which the monomer methylacrylate is doped in polycarbonate films containing benzoinethylether as a photosensitizer, and then photopolymerized by uv light irradiation. It was shown by Franke [ 81 that a refractive-index increase can be obtained in PMMA films doped with benzildimethylketal provided MMA or styrene is used as a solvent of PMMA. However, for high ketal content (high index )? the film color changes from transparent to yellow. In this paper it is first shown that the refractive’ Present address: Mitsubishi Kawasaki,
Rayon
Co. Ltd., 3816 Noborito,
Japan.
0 030-4018/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
index change of PMMA films doped with benzophenone (BP) increases linearly with BP quantity, as a result of photochemical reactions of BP. However, the propagation loss of the slab waveguides also increases, since the reactions yield yellow products. Accordingly, the system PMMA-BP-coumarin is next examined in an effort to reduce the BP concentration and thus the loss. A larger quantity of coumarin is used as a photodimerized substance. In this system, the index change increases linearly with coumarin quantity, while the propagation loss is restrained.
2. Experiments
and results
2.1. Slab waveguides of PUMA
doped with BP
Initially, MMA was distilled and its polymerization inhibitor was removed. By adding a small quantity ( -0.2 g/l) of polymerization initiator, azobis isobutyronitrile, MMA was bulk-polymerized at - 70°C. The mixture of the polymer and monomer was dissolved in benzene and added to methanol which acted as a precipitated for the polymer PMMA. The precipitated PMMA was rinsed in methanol and dried. In order to fabricate optical waveguides on Pyrex glass plates (n = 1.472), PMMA was dissolved by methyl isobutylketone whose quantity was six times heavy. Then, the dopant benzophenone (BP), B.V.
93
Volume 66. number
OPTICS
2.3
C6H5COC6HS was added. The doped polymer was coated on Pyrex plates using a spinner. It was then exposed using a standard photoresist illuminator with a 250 W ultrahigh pressure Hg lamp. The photochemical reactions of BP yielded some products which were difficult to evaporate and had higher refractive index than that of PMMA (n= 1.494). After exposure, the unreacted BP was removed by heating at - 95°C in vacuum. The refractive index and film thickness were determined by measuring the synchronous angles [ 91 for the TEO and TE, guided modes using a prism coupler. In the following figures, the film thickness of about 1.5 Km was so selected that only two modes could be supported. The propagation loss was found to be approximately equal to the case of monomode films. All experiments in this study were done at the i=633 nm wavelength of a HeNe laser. The indices n,, n, of the exposed and unexposed regions decrease with increasing baking time Tb, and become constant for T, greater than 20 hours. In fig. 1(a) the defined as d=(nzrelative index change, n: ) / 2n:, is plotted as a function of BP weight ratio, wbp to PMMA, for Th= 24 h and exposure time T,= 2 h. Fig. 1 (b) also shows the propagation loss L,, of the TE,, mode versus M’~~.The loss was measured by observing the change in the scattered light intensity along the direction of propagation. It is seen from the figures that the index change d increases linearly with u+,~, but the loss L,, also increases. This loss increase is quite undesirable for optical-waveguide operation. The reason for the loss increase is due to the fact that the photochemical reactions of BP yield yellow products which absorb light. Accordingly, the new system PMMA-BP-coumarin is next examined as a means of reducing the BP concentration. 2.2 Slab waveguides coumarin
of PMMA
doped with BP and
Coumarin (m.p. 64°C) is photodimerized by uv light exposure, provided there exists a photosensitizer [ 100 ] as shown in fig. 2. The dimer has a higher melting point 176.5’(3, and thus may remain in PMMA films after the baking process. BP is a effective photosensitizer. Therefore, we use a smaller quantity ( wbp= 0.1) of BP to suppress the optical loss, and a larger quantity of coumarin to obtain a 94
I 5 April I988
COMMUNICATIONS
2
1.5-
2 Q 0)
l.O-
0
0.3
0.2
0. I
Weight
ratio
WbP
(a)
Weight
ratio
wbP
(bl Fig. I. (a) Relative rcfractlve-index change d and (b) propagation loss I_,,, of PMMA films doped with benzophcnone arc shown as a function of its weight ratio, I+, to PMMA. where T,,= 24 h and 7,=2 h.
larger index change. In developed films, the exposed areas have thickness 3-10% greater than the unexposed areas. Figs. 3(a) and (b) are plots of the index change A and the loss L,, of the TEO mode as a function of coumarin weight ratio, w,, to PMMA, respectively. The index change increases linearly to greater than 2% with increasing \v,,. Compared to fig. 1 (b), the loss in fig. 3(b) is approximately 1 dB/cm for MJ,,, in the range O-0.4. This yields an index change 4=0.6-1.8%. When u’,,,, exceeds 0.4. coumarin tends
UV light 0
sensitizer 0
Coumarin Fig. 2. Coumarln is photodimerized vided photoscnsitzer exists.
0
Dimer by uv light exposure
pro-
Volume 66, number
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0’ 0
2,3
0. I
OPTICS
0.2
Weight
0.3
0.4
ratio
COMMUNICATIONS
0.5
Wcm
(a)
15April 1988
to crystallize and, hence, the loss increases due to scattering of the guided wave. Figs. 4(a) and (b ) show the index change d and the loss L,, versus the exposure time T, for wbp= 0.1, W =,,,= 0.3 and T,=24 h. The refractive index increases steeply for T, in the range O-l h, and starts to saturate for Tea 2 h. Up to T,= 2 h, the loss L,, is less than 1 dB/cm. Therefore, in the other experiments T, = 2 h was selected, yielding A= 1.4% and L,,=O.7 dB/cm. It is expected that because of rather large index change the guided light is well confined in bent channel waveguides so that the total bending loss is small.
3. Conclusions
m 30
I
0.1
0
I
I
0.2
Weight
0.3
ratio
0.5
0.4
wcm
(b)
Fig. 3. (a) Relative refractive-index change d and (b) propagation loss L,, of PMMA films doped with both benzophenone and coumarin are shown as a function of coumarin weight ratio, w to PMMA, where !I+,”= 0.1, Tb= 24 h, and T, = 2 h.
The waveguiding properties of doped PMMA films were investigated. For the dopant BP, the refractiveindex change increased with BP quantity as a result of photochemical reactions of BP, however, the propagation loss also increased. Accordingly, the new system PMMA-BP-coumarin was examined as a means of reducing the BP concentration. In this system, the index change increased linearly with coumarin quantity due to photodimerization of coumarin, while the propagation loss was restrained. The slab waveguides had a loss of 0.7 dB/cm, and an index change of 1.4%.
Acknowledgement I
I
0
2
I
Exposure
3
time
4 Te Ihour)
5
The authors are much indebted to Mr. H. Fujimura for his technical support of the experiments.
(a)
References g----yj
0
[ 1] W.J. Tomlinson,
I
2 Exposure
time
T.
4 (hour)
5
(b) Fig. 4. (a) Relative refractive-index change d and (b) propagation loss L,, of PMMA films doped with both benzophenone and coumarin are plotted as a function of exposure time, T,, where w.,=0.3, w,_=O.l, and T,=24 h.
I.P. Kaminow, E.A. Chandross, R.L. Fork and W.T. Silfvast, Appl. Phys. Lett. 16 (1970) 486. [2] M.J. Bowden, E.A. Chandoss and I.P. Kaminow, Appl. Optics 13 (1974) 112. [ 3 ] E.A. Chandross, C.A. Pryde, W.J. Tomlinson and H.P. Weber, Appl. Phys. Lett. 24 (1974) 72. [4] W.J. Tomlinson, H.P. Weber, C.A. Pryde and E.A. Chandross, Appl. Phys. Lett. 26, ( 1975 ) 303.
9.5
Volume 66. number
2.3
OPTICS
COMMUNICATIONS
[5] I. Kato, M. Komatsu, S. Kawamoto and Y. Matsumoto. Trans. IECE Japan J65-C (1982) 860. [6] T. Kurokawa, N. Takato. S. Oikawa and T. Okada. Appl. Optics 17 ( 1978) 646. [ 71 T. Kurokawa. N. Takato and Y. Katayama. Appl. Optics 19 (1980) 3124.
96
I 5 April 1988
[ 81H. Franke, Appl. Optics 23 (1984) 2729. [9] R. Ulrich and R. Torge, Appl. Optics 12 (1973) 2901. Preparative organic photochemistry (Springer-Verlag. Berlin, 1968 ).
[ 101 A. Schonberg,
2,3
OPTICS
COMMUNICATIONS
OPTICAL WAVEGUIDES OF POLYMETHYL BENZOPHENONE AND COUMARIN Naomichi
OKAMOTO
and Shintaro
5 October
1987; revised manuscript
DOPED
WITH
TASHIRO
Shizuoka University, Department ofElectronics Engineering, Received
METHACRYLATE
15 April 1988
received
3-5-I Johoku, Hamamarsu, Japan
4 January
1988
To determine its feasibility as a polymer optical waveguide, the system polymethyl methacrylate-benzophenone-coumarin is studied. The refractive-index change increases linearly up to - 1.8%with coumarin quantity as a result of photodimerization of coumarin, while the propagation loss is restrained. A loss of 0.7 dB/cm and an index change of 1.4% are obtained for slab waveguides having 0.3 and 0. I weight ratios of coumarin and benzophenone to PMMA, respectively.
1. Introduction Several methods of forming plastic optical waveguides using photochemical reactions have been reported to date. It was found that the refractive index of polymethyl methacrylate (PMMA) increases slightly after irradiation with uv light [ 1,2]. Chandross et al. reported a photolocking method [ 3,4 ] in which a photochemical reaction locks a dopant in a polymer film with a lower index. The unreacted dopant is then removed by heating. The polymer used is a 1: 1 copolymer of methyl methacrylate (MMA) and glycidyl methacrylate, and the dopant is naphthalenethiol. Kato et al. [ 51 showed similar results using cinnamic acid as the dopant. A selective photopolymerization method was reported by Kurokawa et al. [6,7], in which the monomer methylacrylate is doped in polycarbonate films containing benzoinethylether as a photosensitizer, and then photopolymerized by uv light irradiation. It was shown by Franke [ 81 that a refractive-index increase can be obtained in PMMA films doped with benzildimethylketal provided MMA or styrene is used as a solvent of PMMA. However, for high ketal content (high index )? the film color changes from transparent to yellow. In this paper it is first shown that the refractive’ Present address: Mitsubishi Kawasaki,
Rayon
Co. Ltd., 3816 Noborito,
Japan.
0 030-4018/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )
index change of PMMA films doped with benzophenone (BP) increases linearly with BP quantity, as a result of photochemical reactions of BP. However, the propagation loss of the slab waveguides also increases, since the reactions yield yellow products. Accordingly, the system PMMA-BP-coumarin is next examined in an effort to reduce the BP concentration and thus the loss. A larger quantity of coumarin is used as a photodimerized substance. In this system, the index change increases linearly with coumarin quantity, while the propagation loss is restrained.
2. Experiments
and results
2.1. Slab waveguides of PUMA
doped with BP
Initially, MMA was distilled and its polymerization inhibitor was removed. By adding a small quantity ( -0.2 g/l) of polymerization initiator, azobis isobutyronitrile, MMA was bulk-polymerized at - 70°C. The mixture of the polymer and monomer was dissolved in benzene and added to methanol which acted as a precipitated for the polymer PMMA. The precipitated PMMA was rinsed in methanol and dried. In order to fabricate optical waveguides on Pyrex glass plates (n = 1.472), PMMA was dissolved by methyl isobutylketone whose quantity was six times heavy. Then, the dopant benzophenone (BP), B.V.
93
Volume 66. number
OPTICS
2.3
C6H5COC6HS was added. The doped polymer was coated on Pyrex plates using a spinner. It was then exposed using a standard photoresist illuminator with a 250 W ultrahigh pressure Hg lamp. The photochemical reactions of BP yielded some products which were difficult to evaporate and had higher refractive index than that of PMMA (n= 1.494). After exposure, the unreacted BP was removed by heating at - 95°C in vacuum. The refractive index and film thickness were determined by measuring the synchronous angles [ 91 for the TEO and TE, guided modes using a prism coupler. In the following figures, the film thickness of about 1.5 Km was so selected that only two modes could be supported. The propagation loss was found to be approximately equal to the case of monomode films. All experiments in this study were done at the i=633 nm wavelength of a HeNe laser. The indices n,, n, of the exposed and unexposed regions decrease with increasing baking time Tb, and become constant for T, greater than 20 hours. In fig. 1(a) the defined as d=(nzrelative index change, n: ) / 2n:, is plotted as a function of BP weight ratio, wbp to PMMA, for Th= 24 h and exposure time T,= 2 h. Fig. 1 (b) also shows the propagation loss L,, of the TE,, mode versus M’~~.The loss was measured by observing the change in the scattered light intensity along the direction of propagation. It is seen from the figures that the index change d increases linearly with u+,~, but the loss L,, also increases. This loss increase is quite undesirable for optical-waveguide operation. The reason for the loss increase is due to the fact that the photochemical reactions of BP yield yellow products which absorb light. Accordingly, the new system PMMA-BP-coumarin is next examined as a means of reducing the BP concentration. 2.2 Slab waveguides coumarin
of PMMA
doped with BP and
Coumarin (m.p. 64°C) is photodimerized by uv light exposure, provided there exists a photosensitizer [ 100 ] as shown in fig. 2. The dimer has a higher melting point 176.5’(3, and thus may remain in PMMA films after the baking process. BP is a effective photosensitizer. Therefore, we use a smaller quantity ( wbp= 0.1) of BP to suppress the optical loss, and a larger quantity of coumarin to obtain a 94
I 5 April I988
COMMUNICATIONS
2
1.5-
2 Q 0)
l.O-
0
0.3
0.2
0. I
Weight
ratio
WbP
(a)
Weight
ratio
wbP
(bl Fig. I. (a) Relative rcfractlve-index change d and (b) propagation loss I_,,, of PMMA films doped with benzophcnone arc shown as a function of its weight ratio, I+, to PMMA. where T,,= 24 h and 7,=2 h.
larger index change. In developed films, the exposed areas have thickness 3-10% greater than the unexposed areas. Figs. 3(a) and (b) are plots of the index change A and the loss L,, of the TEO mode as a function of coumarin weight ratio, w,, to PMMA, respectively. The index change increases linearly to greater than 2% with increasing \v,,. Compared to fig. 1 (b), the loss in fig. 3(b) is approximately 1 dB/cm for MJ,,, in the range O-0.4. This yields an index change 4=0.6-1.8%. When u’,,,, exceeds 0.4. coumarin tends
UV light 0
sensitizer 0
Coumarin Fig. 2. Coumarln is photodimerized vided photoscnsitzer exists.
0
Dimer by uv light exposure
pro-
Volume 66, number
-
0’ 0
2,3
0. I
OPTICS
0.2
Weight
0.3
0.4
ratio
COMMUNICATIONS
0.5
Wcm
(a)
15April 1988
to crystallize and, hence, the loss increases due to scattering of the guided wave. Figs. 4(a) and (b ) show the index change d and the loss L,, versus the exposure time T, for wbp= 0.1, W =,,,= 0.3 and T,=24 h. The refractive index increases steeply for T, in the range O-l h, and starts to saturate for Tea 2 h. Up to T,= 2 h, the loss L,, is less than 1 dB/cm. Therefore, in the other experiments T, = 2 h was selected, yielding A= 1.4% and L,,=O.7 dB/cm. It is expected that because of rather large index change the guided light is well confined in bent channel waveguides so that the total bending loss is small.
3. Conclusions
m 30
I
0.1
0
I
I
0.2
Weight
0.3
ratio
0.5
0.4
wcm
(b)
Fig. 3. (a) Relative refractive-index change d and (b) propagation loss L,, of PMMA films doped with both benzophenone and coumarin are shown as a function of coumarin weight ratio, w to PMMA, where !I+,”= 0.1, Tb= 24 h, and T, = 2 h.
The waveguiding properties of doped PMMA films were investigated. For the dopant BP, the refractiveindex change increased with BP quantity as a result of photochemical reactions of BP, however, the propagation loss also increased. Accordingly, the new system PMMA-BP-coumarin was examined as a means of reducing the BP concentration. In this system, the index change increased linearly with coumarin quantity due to photodimerization of coumarin, while the propagation loss was restrained. The slab waveguides had a loss of 0.7 dB/cm, and an index change of 1.4%.
Acknowledgement I
I
0
2
I
Exposure
3
time
4 Te Ihour)
5
The authors are much indebted to Mr. H. Fujimura for his technical support of the experiments.
(a)
References g----yj
0
[ 1] W.J. Tomlinson,
I
2 Exposure
time
T.
4 (hour)
5
(b) Fig. 4. (a) Relative refractive-index change d and (b) propagation loss L,, of PMMA films doped with both benzophenone and coumarin are plotted as a function of exposure time, T,, where w.,=0.3, w,_=O.l, and T,=24 h.
I.P. Kaminow, E.A. Chandross, R.L. Fork and W.T. Silfvast, Appl. Phys. Lett. 16 (1970) 486. [2] M.J. Bowden, E.A. Chandoss and I.P. Kaminow, Appl. Optics 13 (1974) 112. [ 3 ] E.A. Chandross, C.A. Pryde, W.J. Tomlinson and H.P. Weber, Appl. Phys. Lett. 24 (1974) 72. [4] W.J. Tomlinson, H.P. Weber, C.A. Pryde and E.A. Chandross, Appl. Phys. Lett. 26, ( 1975 ) 303.
9.5
Volume 66. number
2.3
OPTICS
COMMUNICATIONS
[5] I. Kato, M. Komatsu, S. Kawamoto and Y. Matsumoto. Trans. IECE Japan J65-C (1982) 860. [6] T. Kurokawa, N. Takato. S. Oikawa and T. Okada. Appl. Optics 17 ( 1978) 646. [ 71 T. Kurokawa. N. Takato and Y. Katayama. Appl. Optics 19 (1980) 3124.
96
I 5 April 1988
[ 81H. Franke, Appl. Optics 23 (1984) 2729. [9] R. Ulrich and R. Torge, Appl. Optics 12 (1973) 2901. Preparative organic photochemistry (Springer-Verlag. Berlin, 1968 ).
[ 101 A. Schonberg,