- Email: [email protected]
Crosstalk improvement of a thermo-optic polymer waveguide MZI–MMI switch
Optics Communications 281 (2008) 5764–5767
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Crosstalk improvement of a thermo-optic polymer waveguide MZI–MMI switch Abdulaziz M. Al-Hetar *, Abu Sahmah M. Supa’at, A.B. Mohammad, I. Yulianti Photonics Technology Centre, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudia, Johor, Malaysia
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 16 August 2008 Accepted 16 August 2008
Keywords: Multimode interference Mach–Zehndar Thermo-optic switch Crosstalk
a b s t r a c t A 2 2 MZI–MMI switch based on thermo-optic effect with a ridge in the silicon substrate was proposed and the performance of switch was simulated. The main purpose behind this change in substrate layer is to localize the heating at a heated arm single mode waveguide and limit the increasing temperature at a second one. The switch performance of the device should be improved, compare to the usual one. Using finite difference beam propagation method (FD-BPM) and thermal computing simulation based on finite element method (FEM), the results clearly indicate that the MZI–MMI switch can satisfy 31 dB crosstalk at two states. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing need for optical switch matrices for routing, switching, protection switching, and cross connection. It is desirable that such switches have large optical bandwidth, small physical dimensions, large fabrication tolerances, and good performances. These properties are sought in order to reduce optical network system costs and improve efficiency. The conventional integrated optical switches, such as directional coupler switch [1] which is highly sensitive to dimension variation during fabrication and the polarization of optical field, or Y-branch switch [2,3] which requires high electrical power to achieve switch function and has very limited fabrication tolerances. In this regard Mach–Zehndar interferometer (MZI) switches based on multimode interference (MMI) couplers, have gained considerable popularity in recent years. MMI couplers have compactness [4], relaxed fabrication tolerance, and large optical bandwidths [5], as well as polarization insensitivity [6,7] and suitability for device integration [8] which are the subjects of interest in the high capacity WDM network. All these features make MZI based on MMI an ideal candidate over directional couplers or Ybranches. The MZI–MMI switches based on thermo-optic effect are very attractive due to their simplicity and flexibility. The thermo-optic effect refers to the variation of the refractive index of a heated dielectric material [9–11]. The thin-film heater is utilized
* Corresponding author. Tel.: +60 75535302; fax: +60 75566272. E-mail address: [email protected] (A.M. Al-Hetar). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.08.041
to change the refractive index and propagation characteristics of the waveguide. The heat generated by a thin-film heater causes the temperature of nearby arm waveguide to increase. According to this reason, the crosstalk in all MZI–MMI switches based on thermo-optic effect that have been done is unequal at two states. In this work, we propose a new structure layers for 2 2 thermo-optic MZI–MMI switch in which only one heater electrode is used with a silicon ridge at substrate layer. The silicon ridge is expanded from silicon substrate layer to the lower cladding layer and between two single mode waveguides, which are located at intermediate MMI couplers. As a result, the crosstalk in this new structure switch and conventional one is 31 dB at the cross state, while at the bar state is 31 dB and 24 dB, respectively. In Section 2, we have introduced the configuration of this device. A thermal simulation is presented in Section 3. The FD-BPM numerical simulation is shown and discussed in Section 4. 2. Configuration The configuration of 2 2 thermo-optic MZI–MMI is shown in Fig. 1. It consists of an MZI with two 3-dB MMI couplers and two straight parallel waveguide arms between them. The light is lunched using 4 lm wide inputs/outputs waveguides. The dimensions of the MMI couplers were calculated using the well known relation for general interference [6,7]. The length of each MMI coupler is set 3Lp/2 (where Lp corresponds to the beat length) [7] and the width is chosen 10 lm. The proposed device adopted a ridge type waveguide as shown in Fig. 1b with the assumed materials as ZPU series from Chemop-
A.M. Al-Hetar et al. / Optics Communications 281 (2008) 5764–5767
5765
Fig. 1. (a) Schematic diagram of MZI–MMI optical switch, and (b) a–a0 cross section.
Table 1 Thermal parameters and refractive indices used in the simulation Material
Thermal conductivity (W m1 °C1)
Thermal coefficient (°C1)
Refractive index
Position
Cr ZPU12480 ZPU13430 Si
94 0.2
– 1.7 104
– 1.48
Heater Core
0.2
1.8 104
1.43
Lower and upper cladding Substrate
163
1.8 10
4
3.5
tics Co. Ltd. It consists of a core layer of ZPU12-480 with the refractive index of 1.48 and thickness = 1.5 lm surrounded by upper and lower cladding regions of ZPU13-430 with the refractive index of 1.43. The structure is considered as a strong guiding ridge waveguide, with high lateral index contrast due to the lateral air interface and high transverse index contrast ðDn ¼ 0:05Þ between the core and cladding regions. The MZI–MMI configuration allows one to change the light intensities in the output channel by introducing a phase difference in the two arms. The phase difference Du in Eq. (1) [12] is achieved
Fig. 2. Simulation results based on FEM of the temperature distribution of (a) 2D layer structure without silicon ridge, (b) 2D layer structure with silicon ridge, (c) 1D at center of heated guiding layer and (d) change of refractive index and temperature distribution of 1D at center of non-heated guiding layer.
5766
A.M. Al-Hetar et al. / Optics Communications 281 (2008) 5764–5767
in thermo-optic effect by heating one arm resulting in a temperature difference DT between them.
Du ¼
2p v DT Lheater k
ð1Þ
where DT is the temperature difference between the two waveguide cores of the arms in the MZI section, v is the thermo-optic coefficient of the core material. k is the operation wavelength, and Lheater is the length of the heated section. Table 1 tabulated the thermal parameters and refractive indices used in the simulation. For our case with Lheater = 300 lm, k = 1.55 lm, and v = 1.7 104 °C1, a phase difference of p radian is achieved with a temperature difference of DT = 14.35 °C for ZPU12-480.
3. Thermal coupling In this paper, Femlab simulation software from COMSOL is used, which is based on finite element method (FEM) to evaluate the heat transfer from the thin-film heater to structure’s layers as shown in Fig. 2a and b. Fig. 2a shows a layer structure without Si ridge (conventional one). When the power is applied on the heated arm, the change of refractive index in the non-heated arm is high and not constant along its cross section as shown in Fig. 2d due to rise in temperature in this region. Therefore, the different value of crosstalk from the cross state to the bar state is result of thermal crosstalk. To reduce thermal crosstalk, the silicon ridge is expanded from silicon substrate layer to the lower cladding layer and between two single mode waveguides as shown in Fig 2b. When the power is applied on the heated arm, the change of refractive index in the nonheated arm is small and constant (homogeneous) along its cross section as shown in Fig. 2d. The thermal conductivity of silicon is much higher than that of polymer cladding layer; consequently, the Si ridge will act like a thermal bypass path to the substrate
and limit the heat to diffuse into the second arm, thus the crosstalk will be maintained when the switch change from one state to the other. The thermal coupling coefficient representing the magnitude of the temperature field interference on a nearby waveguide, is defined as
g¼
DT 2 DT 2
ð2Þ
where DT2, and DT1 are the temperature rises in the arm 2 and 1, respectively. According to the Eq. (2), the thermal coupling efficiency of the conventional MZI–MMI switch and MZI–MMI switch with Si ridge as shown in Fig. 2c and d are 0.13 and 0.014, respectively. Steadystate simulation confirms good heat confinement in the second structure. The heat confinement effects of the MZI–MMI switch performance will be observed in the next section.
4. Results and discussion The optical and thermal behaviors of proposed MZI–MMI switch were verified by BeamPROP, it was based on Finite difference Beam Propagation Method (FD-BPM), and includes the effects of heater. In this simulation, an induced refractive index change of Dn = 2.583 103 in the guiding layer is sufficient to change the phase between the two arms by p. The changing phase is done by certain value of applied power as shown in Fig. 3. The simulation result shows that the crosstalk of new structure and conventional one is 33 dB at the cross state for both structures, while at the bar state is 25 dB and 31 dB, respectively. The thermal coupling at the new structure is lower than at the conventional structure. Therefore, the crosstalk at the new MZI– MMI switch is almost constant, while in the conventional one is
Fig. 3. Simulation results based on FD-BPM of channel output versus driving power of MZI–MMI switch with and without Si ridge.
A.M. Al-Hetar et al. / Optics Communications 281 (2008) 5764–5767
deteriorated when a power was applied to change the state of the switch. 5. Conclusion A 2 2 MZI–MMI switch based on thermo-optic effect with a ridge in the silicon substrate was proposed and investigated. The simulation results based on FEM and FD-BPM are presented. It shows that the crosstalk of conventional MZI–MMI and MZI– MMI with Si ridge is 33 dB at the cross state, while at the bar state is 25 dB and 31 dB, respectively. Acknowledgement The authors would like to thank the Ministry of Science, Technology and Innovation (MOSTI) for sponsoring this work under Project No. 01-01-06-SF0488.
5767
References [1] P.J. Duthie, N. Shaw, M. Wale, I. Bennion, Electron. Lett. 27 (1991) 1747. [2] A. Sneh, J.E. Zucker, B.I. Miller, IEEE Photon. Technol. Lett. 8 (1996) 1644. [3] Y. Silberberg, P. Perlmutter, J.E. Baron, Appl. Phys. Lett. 51 (1987) 1230. [4] Shuichi Nagai, Goh Morishima, Hirokazu Inayoshi, Katsuyuki Utaka, J. Lightwave Technol. 20 (4) (2002) 675. [5] J.M. Heaton, R.M. Jenkins, D.R. Wight, J.T. Parker, J.C.H. Birheck, K.P. Hilton, Appl. Phys. Lett. 61 (15) (1992) 1754. [6] P.A. Besse, M. Bachmann, H. Melchior, L.B. Soldano, M.K. Smit, J. Lightwave Technol. 12 (1994) 1004. [7] L.B. Soldano, Erik C.M. Pennings, J. Lightwave Technol. 13 (4) (1995) 615. [8] R.M. Jenkins, J.M. Heaton, D.R. Wight, J.T. Parker, J.C.H. Birbeck, G.W. Smith, K.P. Hilton, Appl. Phys. Lett. 64 (6) (1994) 686. [9] Mart B.J. Diemeer, Opt. Mater. 9 (1998) 192. [10] M.H. Ibrahim, Mee Koy, N.M. Kassim, A.B. Mohammad, Chin. Phys. Lett. 23 (10) (2006) 2796. [11] Abdulaziz M. Al-hetar, I. Yulianti, Abu Sahmah M. Supa’at, A.B. Mohammad, Opt. Commun. 281 (2008) 4653. [12] Juerg Leuthold, Charles H. Joyner, J. Lightwave Technol. (195) (2001) 700.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Crosstalk improvement of a thermo-optic polymer waveguide MZI–MMI switch Abdulaziz M. Al-Hetar *, Abu Sahmah M. Supa’at, A.B. Mohammad, I. Yulianti Photonics Technology Centre, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudia, Johor, Malaysia
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 16 August 2008 Accepted 16 August 2008
Keywords: Multimode interference Mach–Zehndar Thermo-optic switch Crosstalk
a b s t r a c t A 2 2 MZI–MMI switch based on thermo-optic effect with a ridge in the silicon substrate was proposed and the performance of switch was simulated. The main purpose behind this change in substrate layer is to localize the heating at a heated arm single mode waveguide and limit the increasing temperature at a second one. The switch performance of the device should be improved, compare to the usual one. Using finite difference beam propagation method (FD-BPM) and thermal computing simulation based on finite element method (FEM), the results clearly indicate that the MZI–MMI switch can satisfy 31 dB crosstalk at two states. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing need for optical switch matrices for routing, switching, protection switching, and cross connection. It is desirable that such switches have large optical bandwidth, small physical dimensions, large fabrication tolerances, and good performances. These properties are sought in order to reduce optical network system costs and improve efficiency. The conventional integrated optical switches, such as directional coupler switch [1] which is highly sensitive to dimension variation during fabrication and the polarization of optical field, or Y-branch switch [2,3] which requires high electrical power to achieve switch function and has very limited fabrication tolerances. In this regard Mach–Zehndar interferometer (MZI) switches based on multimode interference (MMI) couplers, have gained considerable popularity in recent years. MMI couplers have compactness [4], relaxed fabrication tolerance, and large optical bandwidths [5], as well as polarization insensitivity [6,7] and suitability for device integration [8] which are the subjects of interest in the high capacity WDM network. All these features make MZI based on MMI an ideal candidate over directional couplers or Ybranches. The MZI–MMI switches based on thermo-optic effect are very attractive due to their simplicity and flexibility. The thermo-optic effect refers to the variation of the refractive index of a heated dielectric material [9–11]. The thin-film heater is utilized
* Corresponding author. Tel.: +60 75535302; fax: +60 75566272. E-mail address: [email protected] (A.M. Al-Hetar). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.08.041
to change the refractive index and propagation characteristics of the waveguide. The heat generated by a thin-film heater causes the temperature of nearby arm waveguide to increase. According to this reason, the crosstalk in all MZI–MMI switches based on thermo-optic effect that have been done is unequal at two states. In this work, we propose a new structure layers for 2 2 thermo-optic MZI–MMI switch in which only one heater electrode is used with a silicon ridge at substrate layer. The silicon ridge is expanded from silicon substrate layer to the lower cladding layer and between two single mode waveguides, which are located at intermediate MMI couplers. As a result, the crosstalk in this new structure switch and conventional one is 31 dB at the cross state, while at the bar state is 31 dB and 24 dB, respectively. In Section 2, we have introduced the configuration of this device. A thermal simulation is presented in Section 3. The FD-BPM numerical simulation is shown and discussed in Section 4. 2. Configuration The configuration of 2 2 thermo-optic MZI–MMI is shown in Fig. 1. It consists of an MZI with two 3-dB MMI couplers and two straight parallel waveguide arms between them. The light is lunched using 4 lm wide inputs/outputs waveguides. The dimensions of the MMI couplers were calculated using the well known relation for general interference [6,7]. The length of each MMI coupler is set 3Lp/2 (where Lp corresponds to the beat length) [7] and the width is chosen 10 lm. The proposed device adopted a ridge type waveguide as shown in Fig. 1b with the assumed materials as ZPU series from Chemop-
A.M. Al-Hetar et al. / Optics Communications 281 (2008) 5764–5767
5765
Fig. 1. (a) Schematic diagram of MZI–MMI optical switch, and (b) a–a0 cross section.
Table 1 Thermal parameters and refractive indices used in the simulation Material
Thermal conductivity (W m1 °C1)
Thermal coefficient (°C1)
Refractive index
Position
Cr ZPU12480 ZPU13430 Si
94 0.2
– 1.7 104
– 1.48
Heater Core
0.2
1.8 104
1.43
Lower and upper cladding Substrate
163
1.8 10
4
3.5
tics Co. Ltd. It consists of a core layer of ZPU12-480 with the refractive index of 1.48 and thickness = 1.5 lm surrounded by upper and lower cladding regions of ZPU13-430 with the refractive index of 1.43. The structure is considered as a strong guiding ridge waveguide, with high lateral index contrast due to the lateral air interface and high transverse index contrast ðDn ¼ 0:05Þ between the core and cladding regions. The MZI–MMI configuration allows one to change the light intensities in the output channel by introducing a phase difference in the two arms. The phase difference Du in Eq. (1) [12] is achieved
Fig. 2. Simulation results based on FEM of the temperature distribution of (a) 2D layer structure without silicon ridge, (b) 2D layer structure with silicon ridge, (c) 1D at center of heated guiding layer and (d) change of refractive index and temperature distribution of 1D at center of non-heated guiding layer.
5766
A.M. Al-Hetar et al. / Optics Communications 281 (2008) 5764–5767
in thermo-optic effect by heating one arm resulting in a temperature difference DT between them.
Du ¼
2p v DT Lheater k
ð1Þ
where DT is the temperature difference between the two waveguide cores of the arms in the MZI section, v is the thermo-optic coefficient of the core material. k is the operation wavelength, and Lheater is the length of the heated section. Table 1 tabulated the thermal parameters and refractive indices used in the simulation. For our case with Lheater = 300 lm, k = 1.55 lm, and v = 1.7 104 °C1, a phase difference of p radian is achieved with a temperature difference of DT = 14.35 °C for ZPU12-480.
3. Thermal coupling In this paper, Femlab simulation software from COMSOL is used, which is based on finite element method (FEM) to evaluate the heat transfer from the thin-film heater to structure’s layers as shown in Fig. 2a and b. Fig. 2a shows a layer structure without Si ridge (conventional one). When the power is applied on the heated arm, the change of refractive index in the non-heated arm is high and not constant along its cross section as shown in Fig. 2d due to rise in temperature in this region. Therefore, the different value of crosstalk from the cross state to the bar state is result of thermal crosstalk. To reduce thermal crosstalk, the silicon ridge is expanded from silicon substrate layer to the lower cladding layer and between two single mode waveguides as shown in Fig 2b. When the power is applied on the heated arm, the change of refractive index in the nonheated arm is small and constant (homogeneous) along its cross section as shown in Fig. 2d. The thermal conductivity of silicon is much higher than that of polymer cladding layer; consequently, the Si ridge will act like a thermal bypass path to the substrate
and limit the heat to diffuse into the second arm, thus the crosstalk will be maintained when the switch change from one state to the other. The thermal coupling coefficient representing the magnitude of the temperature field interference on a nearby waveguide, is defined as
g¼
DT 2 DT 2
ð2Þ
where DT2, and DT1 are the temperature rises in the arm 2 and 1, respectively. According to the Eq. (2), the thermal coupling efficiency of the conventional MZI–MMI switch and MZI–MMI switch with Si ridge as shown in Fig. 2c and d are 0.13 and 0.014, respectively. Steadystate simulation confirms good heat confinement in the second structure. The heat confinement effects of the MZI–MMI switch performance will be observed in the next section.
4. Results and discussion The optical and thermal behaviors of proposed MZI–MMI switch were verified by BeamPROP, it was based on Finite difference Beam Propagation Method (FD-BPM), and includes the effects of heater. In this simulation, an induced refractive index change of Dn = 2.583 103 in the guiding layer is sufficient to change the phase between the two arms by p. The changing phase is done by certain value of applied power as shown in Fig. 3. The simulation result shows that the crosstalk of new structure and conventional one is 33 dB at the cross state for both structures, while at the bar state is 25 dB and 31 dB, respectively. The thermal coupling at the new structure is lower than at the conventional structure. Therefore, the crosstalk at the new MZI– MMI switch is almost constant, while in the conventional one is
Fig. 3. Simulation results based on FD-BPM of channel output versus driving power of MZI–MMI switch with and without Si ridge.
A.M. Al-Hetar et al. / Optics Communications 281 (2008) 5764–5767
deteriorated when a power was applied to change the state of the switch. 5. Conclusion A 2 2 MZI–MMI switch based on thermo-optic effect with a ridge in the silicon substrate was proposed and investigated. The simulation results based on FEM and FD-BPM are presented. It shows that the crosstalk of conventional MZI–MMI and MZI– MMI with Si ridge is 33 dB at the cross state, while at the bar state is 25 dB and 31 dB, respectively. Acknowledgement The authors would like to thank the Ministry of Science, Technology and Innovation (MOSTI) for sponsoring this work under Project No. 01-01-06-SF0488.
5767
References [1] P.J. Duthie, N. Shaw, M. Wale, I. Bennion, Electron. Lett. 27 (1991) 1747. [2] A. Sneh, J.E. Zucker, B.I. Miller, IEEE Photon. Technol. Lett. 8 (1996) 1644. [3] Y. Silberberg, P. Perlmutter, J.E. Baron, Appl. Phys. Lett. 51 (1987) 1230. [4] Shuichi Nagai, Goh Morishima, Hirokazu Inayoshi, Katsuyuki Utaka, J. Lightwave Technol. 20 (4) (2002) 675. [5] J.M. Heaton, R.M. Jenkins, D.R. Wight, J.T. Parker, J.C.H. Birheck, K.P. Hilton, Appl. Phys. Lett. 61 (15) (1992) 1754. [6] P.A. Besse, M. Bachmann, H. Melchior, L.B. Soldano, M.K. Smit, J. Lightwave Technol. 12 (1994) 1004. [7] L.B. Soldano, Erik C.M. Pennings, J. Lightwave Technol. 13 (4) (1995) 615. [8] R.M. Jenkins, J.M. Heaton, D.R. Wight, J.T. Parker, J.C.H. Birbeck, G.W. Smith, K.P. Hilton, Appl. Phys. Lett. 64 (6) (1994) 686. [9] Mart B.J. Diemeer, Opt. Mater. 9 (1998) 192. [10] M.H. Ibrahim, Mee Koy, N.M. Kassim, A.B. Mohammad, Chin. Phys. Lett. 23 (10) (2006) 2796. [11] Abdulaziz M. Al-hetar, I. Yulianti, Abu Sahmah M. Supa’at, A.B. Mohammad, Opt. Commun. 281 (2008) 4653. [12] Juerg Leuthold, Charles H. Joyner, J. Lightwave Technol. (195) (2001) 700.