- Email: [email protected]
Thermo-optic mode extinction modulation in polymeric waveguide structures
]OURNA
L OF
NON-CRYS LLINESOLIDS ELSEVIER
Journal of Non-Crystalline Solids 187 (1995) 494 497
Thermo-optic mode extinction modulation in polymeric waveguide structures F.R. Akkari*, K.H. Cazzini, W. Blau Department of Physics, Trinity College, Dublin 2, Ireland
Abstract Thermo-optic mode extinction modulation was demonstrated in polymeric waveguide structures. A strip heater was used to control mode extinction via the thermo-optic effect. Mode extinction occurs due to the counteracting effect which arises from a polymeric waveguide with a negative thermo-optic coefficient and a glass substrate with a positive coefficient. Complete mode extinction was achieved at a low driving voltage (1.6 V). Several devices were fabricated and tested using sine and square wave ac voltages up to 40 kHz. Switch ON and OFF times in the millisecond range were obtained. All devices were made of commercially available clear polyurethane varnish on BK-7 glass substrates.
1. Introduction In optoelectronics guided optical beams m a y be switched, deflected or m o d u l a t e d by electro-, acousto-, m a g n e t o - or thermo-optic effects. While the first three effects have received much attention in the past, thermo-optic effects were only considered recently on glass or polymeric thin film structures [ 1 - 4 ] . Thermally induced refractive index variation is provided by an electrically driven strip heater deposited on the interaction region of these devices. It is well k n o w n that polymers possess negative temperature coefficients while some glasses possess a positive coefficient [-5,6]. When a polymer is spun on a glass substrate a counteracting
*Corresponding author. Tel: +353-1 608 2404. Telefax: +353-1 671 1759. [email protected].
effect arises which can be utilized to design and fabricate a thermo-optic m o d e extinction m o d u l a tor ( T O M E M ) I-7]. Essentially, a thermo-optic m o d e extinction m o d u l a t o r is a weakly guiding waveguide section to which a strip heater is added to control the guidance by means of the thermooptic effect. Complete m o d e extinction occurs when the waveguide is b r o u g h t to cut-off via thermally induced refractive index changes in both the guiding polymer thin film and the glass substrate. A m o n g the c o m m o n polymers in use in integrated optics polyurethane has the highest thermo-optic coefficient ( d n / d T ) . It was chosen for this investigation to yield low driving voltages. Its thermo-optic coefficient was measured to be - 5 . 3 x 1 0 - 4 K -x. The refractive index of polyurethane was 1.522. A BK-7 glass substrate with a refractive index of 1.515 was used to reduce the change in refractive index required to produce complete mode extinction. A
0022-3093/95/$09.50 ~) 1995 ElsevierScience B.V. All rights reserved SSD! 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 2 1 3 - 8
F.R. Akkari et al. /Journal of Non-Crystalline Solids 187 (1995) 494 497
metallic stripe heater driven by a stabilized power supply was used to induce temperature changes in the weakly guiding section. Although thermo-optic devices have slow speeds of operation, they can be useful in applications where high speed is not required. In this paper, we describe the fabrication and experimental demonstration of a thermo-optic mode extinction modulator.
495
1.0 % 8 u
0.9
8 []
0.8 u []
0.7
[] []
0.6 0.5
2. Experimental details 0.4
The thermo-optic mode extinction modulator was fabricated using commercially available clear varnish polyurethane thin film. It was spin coated on a BK-7 glass substrate. It has a thickness of 3.4 ~tm and a refractive index of 1.522 determined by M-line technique. A P M M A buffer layer was spun on this layer and finally a strip heater was sputtered on top. The P M M A protective layer has a thickness of 0.3 tam and a refractive index of 1.488. The strip heater was cold sputtered through a mechanical mask, 10 mm long and 1 mm wide. The refractive index of the BK-7 glass substrate is 1.515. Several devices were fabricated and tested at different strip heater total resistances. Conventional prism coupling was used to couple light (HeNe 2 = 0.6328 ~tm) in and out of the polyurethane weakly guiding single mode and thin film waveguide. The out coupled light was focused on a photodetector connected to a storage oscilloscope. The modulator was tested while exciting the transverse electric (TE) guided mode only. Electrical control of the guided mode extinction by a dc voltage applied across the strip heater terminals. The dissipated electrical power in the strip heater was used to change the temperature of the thin film. The guided beam was coupled such that it propagated normal to the strip heater length.
3. Results A T E polarized H e - N e laser light was coupled in and out the polymeric thin film waveguide. Electrical control of the guided mode extinction by a dc voltage applied across a 15 f2 strip heater is depicted in Fig. 1, where complete mode
0.3 0.2 0.1 0.0 1.1
1.2
1.3
1.4
1.5
1.6
Applied heeter volteze(de Volts) Fig. 1. Variation of guided mode intensity with applied dc voltage with a strip heater of 15 D total resistance.
~v
t 0 . OOV'l
P,o,: SOV
t,ttO4ttq
P,¢, 50mY
'tim S,
20~,m
Fig. 2. Modulator response (upper trace) to a sine- wave 7 V peak to peak (lower trace) with a strip heater of 270 ~ total resistance.
extinction is evident at ~ 1.6 V. When the strip heater was driven with a pure sine wave the modulated amplitude had twice the frequency of the applied signal because the thermo-optic mode
496
F.R. Akkari et al. /Journal o f Non-Crystalline Solids 187 (1995) 494 497
mission state is ~-- 6,7 ms, and is determined by the strip heater geometry and heat sinking. The switching from total transmission state to cut-off state is slow and determined by the thin film thermal properties.
4. Discussion
(a)
(b) Fig. 3. Modulator square wave response: (a) upper trace switch O F F response; (b) upper trace switch O N response.
extinction effect is related to the dissipated power rather than the applied voltage. In Fig. 2 we present the modulator response to sine wave at a frequency of 10 kHz and peak to peak voltage of 7 V applied across a 270 Q strip heater. Although no attempt was made to optimize this modulator we were able to measure its response up to 40 kHz. To measure the switching response the modulator was driven with a square wave (Fig. 3) where it can be seen that the switching from extinction cut-off state to total trans-
In principle a mode extinction modulator consists of a weakly guiding waveguide section, where changes can be introduced in the guiding thin film refractive index to achieve modulation of the guided optical beam intensity. This can be achieved thermooptically in polymers using polyurethane on a BK-7 glass substrate. The refractive index changes in both the polymer thin film and the substrate can be induced by temperature changes. The temperature can be controlled by a strip heater deposited on top of the polymer thin film. The choice of a polymer having a high negative thermo-optic coefficient together with a substrate namely BK-7 having a close refractive index to that of the polymer and a positive thermo-optic coefficient resulted in a modulator requiring a low driving voltage to achieve complete (100%) mode extinction. As can be seen in Fig. 1 complete mode extinction happened at ~ 1.6 V across a 15 f~ heater. This corresponds to about 170 m W of dissipated power across the heater length, and only 40 mW are needed across the interaction region. The temperature is dependent on the dissipated power rather than the applied voltage. When a sine-wave signal is applied across the heater the guided beam is modulated at twice its frequency, as can be seen in Fig. 2. Using a square wave the modulator was tested up to 40 kHz in an on-off mode. No attempt was made to optimize the response speed in this investigation. It can be seen that the switching response from total transmission state to cut-off was very slow. It was determined by the thin film thermal properties. The response time for the switching from cut-off state to total transmission was ~ 6.7 ms. It was determined by the heater geometry and heat sinking. Thermooptic devices in general have a low speed disadvantage, but they can be very low-cost useful components in applications where high speed is not required. They can be useful for temperature, pressure and gas flow sensing applications.
F.R. Akkari et al. / Journal of Non-Crystalline Solids 187 (1995) 494-497
497
5. Conclusions
References
In conclusion, we have d e m o n s t r a t e d t h e r m o optic m o d e e x t i n c t i o n m o d u l a t i o n in p o l y m e r i c w a v e g u i d e structure using p o l y u r e t h a n e thin film on a B K - 7 glass s u b s t r a t e at low d r i v i n g voltages. T h e r e q u i r e d d r i v i n g p o w e r to achieve c o m p l e t e (100%) m o d e e x t i n c t i o n is a b o u t 40 m W . T h e speed of o p e r a t i o n is limited to milliseconds r a n g e as is the case for m o s t t h e r m o optic devices.
[1] M. Haruna and J. Koyama, Appl. Opt. 21 (1982) 3461. [2] M.B.J. Diemeer, J.J. Brons and E.S. Trommel, J. Lightwave Technol. 7 (1989) 449. [3] R.A. Mayer, K.H. Jung, W.D. Lee, D. Kwong and J.C. Campbeil, Opt. Lett. 17 (1992) 1812. [4] C.C. Lee and T.J. Su, Appl. Opt. 33 (1994) 7016. [5] J.M. Cariou, J.Dugas, L. Martin and P. Michel, Appl. Opt. 25 (1986) 334. [-6] J.M. Jewell, J. Non-Cryst Solids 146 (1992) 145. [7] P.R. Ashley and W.S.C. Chang, IEEE J. Quantum Electron. QE-22 (1986) 920.
L OF
NON-CRYS LLINESOLIDS ELSEVIER
Journal of Non-Crystalline Solids 187 (1995) 494 497
Thermo-optic mode extinction modulation in polymeric waveguide structures F.R. Akkari*, K.H. Cazzini, W. Blau Department of Physics, Trinity College, Dublin 2, Ireland
Abstract Thermo-optic mode extinction modulation was demonstrated in polymeric waveguide structures. A strip heater was used to control mode extinction via the thermo-optic effect. Mode extinction occurs due to the counteracting effect which arises from a polymeric waveguide with a negative thermo-optic coefficient and a glass substrate with a positive coefficient. Complete mode extinction was achieved at a low driving voltage (1.6 V). Several devices were fabricated and tested using sine and square wave ac voltages up to 40 kHz. Switch ON and OFF times in the millisecond range were obtained. All devices were made of commercially available clear polyurethane varnish on BK-7 glass substrates.
1. Introduction In optoelectronics guided optical beams m a y be switched, deflected or m o d u l a t e d by electro-, acousto-, m a g n e t o - or thermo-optic effects. While the first three effects have received much attention in the past, thermo-optic effects were only considered recently on glass or polymeric thin film structures [ 1 - 4 ] . Thermally induced refractive index variation is provided by an electrically driven strip heater deposited on the interaction region of these devices. It is well k n o w n that polymers possess negative temperature coefficients while some glasses possess a positive coefficient [-5,6]. When a polymer is spun on a glass substrate a counteracting
*Corresponding author. Tel: +353-1 608 2404. Telefax: +353-1 671 1759. [email protected].
effect arises which can be utilized to design and fabricate a thermo-optic m o d e extinction m o d u l a tor ( T O M E M ) I-7]. Essentially, a thermo-optic m o d e extinction m o d u l a t o r is a weakly guiding waveguide section to which a strip heater is added to control the guidance by means of the thermooptic effect. Complete m o d e extinction occurs when the waveguide is b r o u g h t to cut-off via thermally induced refractive index changes in both the guiding polymer thin film and the glass substrate. A m o n g the c o m m o n polymers in use in integrated optics polyurethane has the highest thermo-optic coefficient ( d n / d T ) . It was chosen for this investigation to yield low driving voltages. Its thermo-optic coefficient was measured to be - 5 . 3 x 1 0 - 4 K -x. The refractive index of polyurethane was 1.522. A BK-7 glass substrate with a refractive index of 1.515 was used to reduce the change in refractive index required to produce complete mode extinction. A
0022-3093/95/$09.50 ~) 1995 ElsevierScience B.V. All rights reserved SSD! 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 2 1 3 - 8
F.R. Akkari et al. /Journal of Non-Crystalline Solids 187 (1995) 494 497
metallic stripe heater driven by a stabilized power supply was used to induce temperature changes in the weakly guiding section. Although thermo-optic devices have slow speeds of operation, they can be useful in applications where high speed is not required. In this paper, we describe the fabrication and experimental demonstration of a thermo-optic mode extinction modulator.
495
1.0 % 8 u
0.9
8 []
0.8 u []
0.7
[] []
0.6 0.5
2. Experimental details 0.4
The thermo-optic mode extinction modulator was fabricated using commercially available clear varnish polyurethane thin film. It was spin coated on a BK-7 glass substrate. It has a thickness of 3.4 ~tm and a refractive index of 1.522 determined by M-line technique. A P M M A buffer layer was spun on this layer and finally a strip heater was sputtered on top. The P M M A protective layer has a thickness of 0.3 tam and a refractive index of 1.488. The strip heater was cold sputtered through a mechanical mask, 10 mm long and 1 mm wide. The refractive index of the BK-7 glass substrate is 1.515. Several devices were fabricated and tested at different strip heater total resistances. Conventional prism coupling was used to couple light (HeNe 2 = 0.6328 ~tm) in and out of the polyurethane weakly guiding single mode and thin film waveguide. The out coupled light was focused on a photodetector connected to a storage oscilloscope. The modulator was tested while exciting the transverse electric (TE) guided mode only. Electrical control of the guided mode extinction by a dc voltage applied across the strip heater terminals. The dissipated electrical power in the strip heater was used to change the temperature of the thin film. The guided beam was coupled such that it propagated normal to the strip heater length.
3. Results A T E polarized H e - N e laser light was coupled in and out the polymeric thin film waveguide. Electrical control of the guided mode extinction by a dc voltage applied across a 15 f2 strip heater is depicted in Fig. 1, where complete mode
0.3 0.2 0.1 0.0 1.1
1.2
1.3
1.4
1.5
1.6
Applied heeter volteze(de Volts) Fig. 1. Variation of guided mode intensity with applied dc voltage with a strip heater of 15 D total resistance.
~v
t 0 . OOV'l
P,o,: SOV
t,ttO4ttq
P,¢, 50mY
'tim S,
20~,m
Fig. 2. Modulator response (upper trace) to a sine- wave 7 V peak to peak (lower trace) with a strip heater of 270 ~ total resistance.
extinction is evident at ~ 1.6 V. When the strip heater was driven with a pure sine wave the modulated amplitude had twice the frequency of the applied signal because the thermo-optic mode
496
F.R. Akkari et al. /Journal o f Non-Crystalline Solids 187 (1995) 494 497
mission state is ~-- 6,7 ms, and is determined by the strip heater geometry and heat sinking. The switching from total transmission state to cut-off state is slow and determined by the thin film thermal properties.
4. Discussion
(a)
(b) Fig. 3. Modulator square wave response: (a) upper trace switch O F F response; (b) upper trace switch O N response.
extinction effect is related to the dissipated power rather than the applied voltage. In Fig. 2 we present the modulator response to sine wave at a frequency of 10 kHz and peak to peak voltage of 7 V applied across a 270 Q strip heater. Although no attempt was made to optimize this modulator we were able to measure its response up to 40 kHz. To measure the switching response the modulator was driven with a square wave (Fig. 3) where it can be seen that the switching from extinction cut-off state to total trans-
In principle a mode extinction modulator consists of a weakly guiding waveguide section, where changes can be introduced in the guiding thin film refractive index to achieve modulation of the guided optical beam intensity. This can be achieved thermooptically in polymers using polyurethane on a BK-7 glass substrate. The refractive index changes in both the polymer thin film and the substrate can be induced by temperature changes. The temperature can be controlled by a strip heater deposited on top of the polymer thin film. The choice of a polymer having a high negative thermo-optic coefficient together with a substrate namely BK-7 having a close refractive index to that of the polymer and a positive thermo-optic coefficient resulted in a modulator requiring a low driving voltage to achieve complete (100%) mode extinction. As can be seen in Fig. 1 complete mode extinction happened at ~ 1.6 V across a 15 f~ heater. This corresponds to about 170 m W of dissipated power across the heater length, and only 40 mW are needed across the interaction region. The temperature is dependent on the dissipated power rather than the applied voltage. When a sine-wave signal is applied across the heater the guided beam is modulated at twice its frequency, as can be seen in Fig. 2. Using a square wave the modulator was tested up to 40 kHz in an on-off mode. No attempt was made to optimize the response speed in this investigation. It can be seen that the switching response from total transmission state to cut-off was very slow. It was determined by the thin film thermal properties. The response time for the switching from cut-off state to total transmission was ~ 6.7 ms. It was determined by the heater geometry and heat sinking. Thermooptic devices in general have a low speed disadvantage, but they can be very low-cost useful components in applications where high speed is not required. They can be useful for temperature, pressure and gas flow sensing applications.
F.R. Akkari et al. / Journal of Non-Crystalline Solids 187 (1995) 494-497
497
5. Conclusions
References
In conclusion, we have d e m o n s t r a t e d t h e r m o optic m o d e e x t i n c t i o n m o d u l a t i o n in p o l y m e r i c w a v e g u i d e structure using p o l y u r e t h a n e thin film on a B K - 7 glass s u b s t r a t e at low d r i v i n g voltages. T h e r e q u i r e d d r i v i n g p o w e r to achieve c o m p l e t e (100%) m o d e e x t i n c t i o n is a b o u t 40 m W . T h e speed of o p e r a t i o n is limited to milliseconds r a n g e as is the case for m o s t t h e r m o optic devices.
[1] M. Haruna and J. Koyama, Appl. Opt. 21 (1982) 3461. [2] M.B.J. Diemeer, J.J. Brons and E.S. Trommel, J. Lightwave Technol. 7 (1989) 449. [3] R.A. Mayer, K.H. Jung, W.D. Lee, D. Kwong and J.C. Campbeil, Opt. Lett. 17 (1992) 1812. [4] C.C. Lee and T.J. Su, Appl. Opt. 33 (1994) 7016. [5] J.M. Cariou, J.Dugas, L. Martin and P. Michel, Appl. Opt. 25 (1986) 334. [-6] J.M. Jewell, J. Non-Cryst Solids 146 (1992) 145. [7] P.R. Ashley and W.S.C. Chang, IEEE J. Quantum Electron. QE-22 (1986) 920.