- Email: [email protected]
1550 nm wavelength division multiplexer with output facet-tilted MMI waveguide
Optics Communications 232 (2004) 371–379 www.elsevier.com/locate/optcom
A novel 1 2 single-mode 1300/1550 nm wavelength division multiplexer with output facet-tilted MMI waveguide Shyh-Lin Tsao *, Huang-Chen Guo, Chun-Wei Tsai Optical Fiber System Laboratory, Institute of Electro-Optical Science and Technology, National Taiwan Normal University, No. 88, Sec. 4, Ting-Chou Rd., Taipei 116, Taiwan, ROC Received 8 April 2003; received in revised form 4 November 2003; accepted 25 December 2003
Abstract In this paper, we design a novel 1 2 1300/1550 nm wavelength division multiplexer based on silicon-on-insulator (SOI) substrate. The wavelength division multiplexer has a tilted facet of our designed multimode interference (MMI) region. Tuning the angle of the tilted MMI facet, one can adjust the output wavelength, power ratio and polarization deviations. A design example shows that the insertion loss for 1300 and I550 nm wavelengths can be reduced to )0.227 and )0.31 dB, respectively. According to the beam-propagation method (BPM) simulation, the crosstalk can be improved to below )22 dB. The polarization-dependent (PDL) loss is 0.033 and 0.01 dB for 1300 and 1550 nm wavelengths without attaching single-mode fiber (SMF), respectively. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Wave optics; Multiplexers; Optical beam splitters; Integrated optics; Interference
1. Introduction Intensive research has recently been conducted on semiconductor optical devices for wavelength division multiplexer as reported in literatures [1–5]. Wavelength division multiplexing (WDM) devices for subscriber networks have become an important issue [6]. Moreover, the integration of optical element devices with wavelength selective functions has also attracted a lot of attention. Several
*
Corresponding author. Tel.: +886-2-29350349; fax: +886-229350382. E-mail address: [email protected] (S.-L. Tsao).
wavelength division devices have been demonstrated using Bragg gratings [7–9], liquid crystal filters [10,11], fiber ring filters [12,13], MEMS systems [14,15] and other methods [16–18]. One promising technique is using silicon-on-insulator [19–22] waveguide devices in WDM optical communication systems [23–25]. CMOS (complementary metal-oxide semiconductor) electronics on SOI wafers have shown promise as the future technology for low-power and high-speed applications [26–28]. Although Bragg gratings, liquid crystals and MEMS are candidates, the multimode interference (MMI) WDM device has a smaller size and is compatible with the CMOS fabrication process. Some good SOI waveguide structures
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.12.082
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have also been proved for reducing the optical insertion loss and optical crosstalk in 1300 and 1550 nm optical communication windows [28,29]. For the above reasons, we choose the SOI structure for producing wavelength division devices. Recently, some good WDM applications using MMI waveguide structure were reported [30,31]. Therefore, we apply the MMI technique to realize a 1 2 compact MMI SOI 1300/1550 nm wavelength division multiplexer that is smaller than the traditional fiber-optic 1300/1550 nm wavelength division multiplexer. Meanwhile, integration of hybrid photonic devices is becoming an important issue for applications in optical communication networks [32]. We think that a compact MMI wavelength division multiplexer is very useful in hybrid photonic device integration. Couplers play an important role in optical communication systems and can be used to combine or split the power of different optical channels. Single-mode fiber couplers are also used to make optical fiber WDM components [33–35], such as 1300/1550 nm WDM fiber-optic directional couplers. Although these fiber-optic directional couplers typically exhibit losses less than 0.5 dB and channel crosstalk lower than )16 dB [36], fiber-optical directional couplers have a large coupling length (about several centimeters). SOI MMI devices can reduce the coupling length to the millimeter range. Compared with WDM fiberoptic directional couplers, our SOI WDM coupler can have a much smaller size (ffi1 mm long) which is useful in realizing the OE-VLSI devices. Recently, fiber and integrated-optical couplers have been combined by various methods for WDM systems [37]. Therefore, such a device can also be applied to fiber-optic transmission systems. In this paper, we design and simulate the wavelength response of MMI wavelength division multiplexers based on an SOI wafer. The BPM_CAD software is used to simulate a lot of conditions for device design parameter sorting. The simulation method in this software package uses the beampropagation method (BPM) [38], which can simulate the light propagation in waveguides by performing a step-by-step numerical calculation along the propagation direction. The wave equations can be solved in three dimensions.
2. Tilted facet MMI SOI wavelength division multiplexer In this section, we design a 1 2 MMI SOI WDM device for two wavelength bands (1300 and 1550 nm) multiplexing. Fig. 1 shows the crosssection of the 1 2 MMI SOI waveguide we designed at the input plane. For standard UNIBOND SOI substrates, the silicon film thickness is usually 3 lm. The buried silicon oxide layer performed as a cladding layer is 1 lm thick. The design for SOI waveguide single-mode operation should follow the two equations [39]: h ¼ r P 0:5 H and W r 6 0:3 þ pffiffiffiffiffiffiffiffiffiffiffiffi : H 1 r2
ð1Þ
ð2Þ
Following the above equations, we design the rib waveguide with width Wi ¼ 3 lm, rib height H ¼ 3 lm and slab height h ¼ 1:9 lm for providing single-mode operation. Fig. 2 shows the top view of the output facet-tilted 1 2 MMI SOI WDM rib waveguide region. The width and length of the input port are denoted as Wi and L1 , respectively. The width and length of the output
Fig. 1. Cross-section of our 1 2 MMI SOI waveguide at the input plane.
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Fig. 2. Top view of the 1 2 output facet-tilted MMI SOI wavelength division multiplexer.
port1 are denoted as WO1 and L2 , respectively. The width and length of the output port2 are denoted as WO2 and (L2 þ DL), respectively. The length of the MMI waveguide is defined as Lm and width as Wm . The slanted distance between output1 and output2 is denoted as DL. The wavelength of input light is kin . The nr is the refractive index of the MMI region. We follow the design formula for designing the MMI waveguide [40], L ¼ kk
nr W 2 ; kin
ð3Þ
where kk is the corresponding wavelength coefficient constant. Using the above equations, we did many simulations after calculating the parameters. The BPM simulations are described in the following section.
and 1550 nm by using BPM. After many simulations, we found a compact MMI size for WDM usage. Based on Eqs. (1) and (2), we calculate the single-mode waveguide width for SOI wafer. Based on the simulations, we choose the geometrical parameters Wi ¼ 3 lm and L1 ¼ 50 lm for 1300 and 1500 nm wavelength demultiplexing. Then, we adjust the length of the lower edge of the MMI region for tilting the output facet. Figs. 3 and 4 show the BPM simulation results for the transmittance with varying DL between tilted length 0 and 90 lm for 1300 and 1550 nm wavelength, respectively. For 1300 nm wavelength, the
3. BPM simulation results The 1300 and 1550 nm wavelength bands are the major communication windows for optical fiber communication systems. Single-mode SOI waveguide devices with low propagation loss have also been fabricated at k ¼ 1300 and 1550 nm [29]. To characterize our tilted-facet MMI SOI wavelength division multiplexer, the analysis of propagation modes (TE and TM modes), waveguide losses, near field output intensity mode pattern and crosstalk are investigated at wavelength 1300
Fig. 3. BPM simulated tilting effect at output1 with 1300 nm wavelength.
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Fig. 4. BPM simulated tilting effect at output2 with 1550 nm wavelength.
output1 maximum transmittance is 0.969 and occurs at the tilted length DL ¼ 52 lm as shown in Fig. 3. For 1550 nm wavelength, the output2 maximum transmittance value occurs at the tilted length DL ¼ 47 lm in the output2 and the transmittance is 0.937 as shown in the Fig. 4. From our simulation results, we can tune the tilted MMI structure to change the self-imaging conditions. Due to the self-imaging effect in a multimode waveguide, the input spot profile can be repro-
duced in single and multiple spot images at periodic distances along the propagation direction of the waveguide. We find that tilting the MMI geometrical design can vary the self-image pattern. Hence, we use these results to design the self-image original position and let the light propagate along the direction of our designed tilted MMI waveguide. Using the tilted MMI structure not only preserves wavelength division performance, but also serves to substantially reduce the PDL and the crosstalk of the device. The best parameters for 1300 and 1550 nm wavelength division multiplexers are: WO1 ¼ WO2 ¼ 2:5 lm, d ¼ 1 lm and L2 ¼ 20 lm. The optimal values of the tilted length are DL ¼ 52 lm for 1300 nm wavelength and DL ¼ 47 lm for 1550 nm wavelength. The length of MMI region Lm in the tilted region is from 890 to 980 lm, and the width of MMI region Wm is 6 lm. So, the propagation loss of the single-mode SOI rib waveguides can be controlled to below 0.1 dB/cm. In our analysis, we use Eq. (3) to adjust the width and length of the MMI waveguide. The k1300 and K1550 coefficients are set to 9.667 and 12.06, respectively. Fig. 5 shows the lightwave propagation diagram of the 1 2 1300/1550 nm MMI SOI wavelength division multiplexer with S-bend waveguides. The lx and lz are the lengths of the x and z coordinates, O1 , O2 are outputs at the MMI
Fig. 5. Lightwave propagation diagram of 1 2 1300/1550 nm MMI SOI wavelength division multiplexer with S-bending waveguides.
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outputs. O01 and O02 are the output ports at S-bend output. The designed single-mode SOI S-bending waveguides are used for connecting optical fibers. The optical fibers can be connected to the output ends of the rib waveguides, the separation between the two waveguides must be larger than the fibercladding diameter. However, the bending loss will increase as the separation between the two output waveguides increases. Hence, using the following equation for calculating the separation 2lx between the two output waveguides of the S-bending rib waveguides [41], 2lx ¼
4pxlz : 2pz þ lz sin 2pðz=lz Þ
ð4Þ
We chose 160 lm as the separation. We also designed the S-bending rib waveguides with lz ¼ 1 cm to decrease the bending loss as much as possible. Finally, the insertion loss of the SOI bending output waveguide is below 0.6 dB. Two light wavelengths 1300 and 1550 nm with fundamental mode are fed from the input of a single-mode rib waveguide of our designed 1 2 MMI SOI wavelength division multiplexer. The simulated output mode patterns of light wavelengths at 1300 and 1550 nm coming from O1 and O2 are shown in Figs. 6 and 7, respectively. The insertion loss and crosstalk are )0.227 and )25.36
375
dB for the 1300 nm wavelength, respectively. And for the 1550 nm wavelength, the insertion loss and crosstalk are )0.31 and )22.73 dB, respectively. We applied a tilted MMI region to design the 1 2 MMI SOI WDM because of its low polarization sensitivity. Fig. 8 shows the BPM simulation results of the MMI output at 1300 nm for TE mode and TM mode polarized lights. The dash lines and solid lines show the TE and TM mode output light fields, respectively. We can find that the output peak position shift between the TE and TM modes is about 0.4 lm deviation. The TE and TM mode outputs of wavelength 1550 nm are shown in Fig. 9. For the S-bending waveguides, Fig. 10 shows the TE and TM mode outputs from O01 and O02 corresponding to wavelengths 1300 and 1550 nm, respectively. The output peak position shift between the TE and TM modes is only 0.15 lm, which is smaller than the deviation at wavelength 1300 nm. In Fig. 10, the difference in transmitted power for the two modes at the same wavelength is defined as the polarization-dependent loss (PDL). We calculate the PDL with and without attaching a standard step-index single-mode fiber (SMF). Without attaching the single-mode fiber, the PDL of the device of the output port1 is 0.033 dB for 1300 nm wavelength. The PDL of the device of the output port2 is 0.01 dB for 1550 nm wavelength.
Fig. 6. Simulated output mode pattern of light wavelength 1300 nm at output O1 .
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Fig. 7. Simulated output mode pattern of light wavelength 1550 nm at output O2 .
Fig. 8. BPM simulation results of the MMI output at 1300 nm for TE-mode and TM-mode lightwave.
By attaching the single-mode fiber without taping SOI waveguide, the PDL value of the output port1 may reach 5 and 7 dB for wavelength 1300 and 1550 nm, respectively. However, some papers [42] report that the mode mismatching loss can be re-
duced to as low as 0.17 dB between a rectangular waveguide and a circular waveguide, such as fibers, by using mode matching methods [43]. We believe that by adding the SOI mode matching taper, the PDL can be significantly improved.
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Fig. 9. BPM simulation results of the MMI output at 1550 nm for TE-mode and TM-mode lightwave.
Fig. 10. Wavelength response of the 1 2 SOI output facet-tilted MMI SOI wavelength division multiplexer for TE-mode and TMmode at outputs O1 and O2 , respectively.
Fig. 11 shows the wavelength response of the 1 2 MMI SOI wavelength division multiplexer output facet tilted for TE-mode and TM-mode at output O1 and O2 , respectively. The wavelength shifts between the TE and TM modes for the 1300 and
1550 nm wavelength bands are defined as S1 and S2 , respectively. The optimal values of the tilted length are DL ¼ 52 lm for 1300 nm wavelength and DL ¼ 47 lm for 1550 nm wavelength. For S1 ¼ 0:3 lm and S2 ¼ 0:5 lm, the crosstalk at O1 ,
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Fig. 11. Wavelength response of the 1 2 SOI output facet-tilted MMI SOI wavelength division multiplexer for TE-mode and TMmode at outputs O01 and O02 , respectively.
of our designed 1 2 MMI SOI WDM is )25 dB for both of the TE and TM modes at 1300 nm wavelength.
4. Conclusion The design and analysis of a 1 2 single-mode 1300/1550 nm output facet-tilted MMI SOI wavelength division multiplexer has been presented in this paper. Using BPM_CAD simulations, we show the wavelength response of this device. According to results of simulation, the transmitted power (insertion loss) of output port1 is )0.227 dB for 1300 nm wavelength, and the transmitted power of output port2 is )0.31 dB for 1550 nm wavelength. The crosstalk for 1300 nm wavelength is )25.36 dB and the crosstalk for 1550 nm wavelength is )22.73 dB. Without attaching the single-mode fiber, the device PDL value of the output port1 is 0.033 dB for 1300 nm wavelength, and the device PDL value of the output port2 is 0.01 dB for 1550 nm wavelength. We also can find the optimal values of the tilted length DL ¼ 52 lm for 1300 nm wavelength and DL ¼ 47 lm for 1550 nm wavelength. These values achieve the minimal
TE and TM mode wavelength response deviation. Using such a tilted structure to minimize the polarization effect for both outputs can be optimized with proper geometrical MMI SOI structure design by tuning the slant distance DL. From this research, we numerically demonstrated a feasible compact WDM device with low crosstalk, low polarization sensitivity and good wavelength response. No complex fabrication technology should be required for our 1 2 MMI SOI WDM device. Such a device can be mass produced for future optical WDM networks. We believe very large-scale photonic integrated circuits (VLSPIC) will include many such devices at I/O ports for independent processing of 1300 and 1550 nm signals inside the VLSPIC.
Acknowledgements This work is supported in part by the National Science Council under contract NSC 92-2213-E003-013, NDL 92S-C-025, NSC 92 2219-E-003-001 and NSC 92-2622-E-003-001-CC3. This work also partially supported by National Taiwan Normal University under contract 91091016 (research pro-
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ject for new educator) and ‘‘Application and development of novel magneto-optical materials’’ under contract ORD 93-1. The authors thank Mr. Yi-Jr Jou and Mr. Yi Jr Chen for their typing assistance. The authors also appreciate the editor and reviewers valuable comments. Our gratitude also goes to the Academic Paper Editing Clinic, NTNU.
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A novel 1 2 single-mode 1300/1550 nm wavelength division multiplexer with output facet-tilted MMI waveguide Shyh-Lin Tsao *, Huang-Chen Guo, Chun-Wei Tsai Optical Fiber System Laboratory, Institute of Electro-Optical Science and Technology, National Taiwan Normal University, No. 88, Sec. 4, Ting-Chou Rd., Taipei 116, Taiwan, ROC Received 8 April 2003; received in revised form 4 November 2003; accepted 25 December 2003
Abstract In this paper, we design a novel 1 2 1300/1550 nm wavelength division multiplexer based on silicon-on-insulator (SOI) substrate. The wavelength division multiplexer has a tilted facet of our designed multimode interference (MMI) region. Tuning the angle of the tilted MMI facet, one can adjust the output wavelength, power ratio and polarization deviations. A design example shows that the insertion loss for 1300 and I550 nm wavelengths can be reduced to )0.227 and )0.31 dB, respectively. According to the beam-propagation method (BPM) simulation, the crosstalk can be improved to below )22 dB. The polarization-dependent (PDL) loss is 0.033 and 0.01 dB for 1300 and 1550 nm wavelengths without attaching single-mode fiber (SMF), respectively. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Wave optics; Multiplexers; Optical beam splitters; Integrated optics; Interference
1. Introduction Intensive research has recently been conducted on semiconductor optical devices for wavelength division multiplexer as reported in literatures [1–5]. Wavelength division multiplexing (WDM) devices for subscriber networks have become an important issue [6]. Moreover, the integration of optical element devices with wavelength selective functions has also attracted a lot of attention. Several
*
Corresponding author. Tel.: +886-2-29350349; fax: +886-229350382. E-mail address: [email protected] (S.-L. Tsao).
wavelength division devices have been demonstrated using Bragg gratings [7–9], liquid crystal filters [10,11], fiber ring filters [12,13], MEMS systems [14,15] and other methods [16–18]. One promising technique is using silicon-on-insulator [19–22] waveguide devices in WDM optical communication systems [23–25]. CMOS (complementary metal-oxide semiconductor) electronics on SOI wafers have shown promise as the future technology for low-power and high-speed applications [26–28]. Although Bragg gratings, liquid crystals and MEMS are candidates, the multimode interference (MMI) WDM device has a smaller size and is compatible with the CMOS fabrication process. Some good SOI waveguide structures
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.12.082
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have also been proved for reducing the optical insertion loss and optical crosstalk in 1300 and 1550 nm optical communication windows [28,29]. For the above reasons, we choose the SOI structure for producing wavelength division devices. Recently, some good WDM applications using MMI waveguide structure were reported [30,31]. Therefore, we apply the MMI technique to realize a 1 2 compact MMI SOI 1300/1550 nm wavelength division multiplexer that is smaller than the traditional fiber-optic 1300/1550 nm wavelength division multiplexer. Meanwhile, integration of hybrid photonic devices is becoming an important issue for applications in optical communication networks [32]. We think that a compact MMI wavelength division multiplexer is very useful in hybrid photonic device integration. Couplers play an important role in optical communication systems and can be used to combine or split the power of different optical channels. Single-mode fiber couplers are also used to make optical fiber WDM components [33–35], such as 1300/1550 nm WDM fiber-optic directional couplers. Although these fiber-optic directional couplers typically exhibit losses less than 0.5 dB and channel crosstalk lower than )16 dB [36], fiber-optical directional couplers have a large coupling length (about several centimeters). SOI MMI devices can reduce the coupling length to the millimeter range. Compared with WDM fiberoptic directional couplers, our SOI WDM coupler can have a much smaller size (ffi1 mm long) which is useful in realizing the OE-VLSI devices. Recently, fiber and integrated-optical couplers have been combined by various methods for WDM systems [37]. Therefore, such a device can also be applied to fiber-optic transmission systems. In this paper, we design and simulate the wavelength response of MMI wavelength division multiplexers based on an SOI wafer. The BPM_CAD software is used to simulate a lot of conditions for device design parameter sorting. The simulation method in this software package uses the beampropagation method (BPM) [38], which can simulate the light propagation in waveguides by performing a step-by-step numerical calculation along the propagation direction. The wave equations can be solved in three dimensions.
2. Tilted facet MMI SOI wavelength division multiplexer In this section, we design a 1 2 MMI SOI WDM device for two wavelength bands (1300 and 1550 nm) multiplexing. Fig. 1 shows the crosssection of the 1 2 MMI SOI waveguide we designed at the input plane. For standard UNIBOND SOI substrates, the silicon film thickness is usually 3 lm. The buried silicon oxide layer performed as a cladding layer is 1 lm thick. The design for SOI waveguide single-mode operation should follow the two equations [39]: h ¼ r P 0:5 H and W r 6 0:3 þ pffiffiffiffiffiffiffiffiffiffiffiffi : H 1 r2
ð1Þ
ð2Þ
Following the above equations, we design the rib waveguide with width Wi ¼ 3 lm, rib height H ¼ 3 lm and slab height h ¼ 1:9 lm for providing single-mode operation. Fig. 2 shows the top view of the output facet-tilted 1 2 MMI SOI WDM rib waveguide region. The width and length of the input port are denoted as Wi and L1 , respectively. The width and length of the output
Fig. 1. Cross-section of our 1 2 MMI SOI waveguide at the input plane.
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373
Fig. 2. Top view of the 1 2 output facet-tilted MMI SOI wavelength division multiplexer.
port1 are denoted as WO1 and L2 , respectively. The width and length of the output port2 are denoted as WO2 and (L2 þ DL), respectively. The length of the MMI waveguide is defined as Lm and width as Wm . The slanted distance between output1 and output2 is denoted as DL. The wavelength of input light is kin . The nr is the refractive index of the MMI region. We follow the design formula for designing the MMI waveguide [40], L ¼ kk
nr W 2 ; kin
ð3Þ
where kk is the corresponding wavelength coefficient constant. Using the above equations, we did many simulations after calculating the parameters. The BPM simulations are described in the following section.
and 1550 nm by using BPM. After many simulations, we found a compact MMI size for WDM usage. Based on Eqs. (1) and (2), we calculate the single-mode waveguide width for SOI wafer. Based on the simulations, we choose the geometrical parameters Wi ¼ 3 lm and L1 ¼ 50 lm for 1300 and 1500 nm wavelength demultiplexing. Then, we adjust the length of the lower edge of the MMI region for tilting the output facet. Figs. 3 and 4 show the BPM simulation results for the transmittance with varying DL between tilted length 0 and 90 lm for 1300 and 1550 nm wavelength, respectively. For 1300 nm wavelength, the
3. BPM simulation results The 1300 and 1550 nm wavelength bands are the major communication windows for optical fiber communication systems. Single-mode SOI waveguide devices with low propagation loss have also been fabricated at k ¼ 1300 and 1550 nm [29]. To characterize our tilted-facet MMI SOI wavelength division multiplexer, the analysis of propagation modes (TE and TM modes), waveguide losses, near field output intensity mode pattern and crosstalk are investigated at wavelength 1300
Fig. 3. BPM simulated tilting effect at output1 with 1300 nm wavelength.
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Fig. 4. BPM simulated tilting effect at output2 with 1550 nm wavelength.
output1 maximum transmittance is 0.969 and occurs at the tilted length DL ¼ 52 lm as shown in Fig. 3. For 1550 nm wavelength, the output2 maximum transmittance value occurs at the tilted length DL ¼ 47 lm in the output2 and the transmittance is 0.937 as shown in the Fig. 4. From our simulation results, we can tune the tilted MMI structure to change the self-imaging conditions. Due to the self-imaging effect in a multimode waveguide, the input spot profile can be repro-
duced in single and multiple spot images at periodic distances along the propagation direction of the waveguide. We find that tilting the MMI geometrical design can vary the self-image pattern. Hence, we use these results to design the self-image original position and let the light propagate along the direction of our designed tilted MMI waveguide. Using the tilted MMI structure not only preserves wavelength division performance, but also serves to substantially reduce the PDL and the crosstalk of the device. The best parameters for 1300 and 1550 nm wavelength division multiplexers are: WO1 ¼ WO2 ¼ 2:5 lm, d ¼ 1 lm and L2 ¼ 20 lm. The optimal values of the tilted length are DL ¼ 52 lm for 1300 nm wavelength and DL ¼ 47 lm for 1550 nm wavelength. The length of MMI region Lm in the tilted region is from 890 to 980 lm, and the width of MMI region Wm is 6 lm. So, the propagation loss of the single-mode SOI rib waveguides can be controlled to below 0.1 dB/cm. In our analysis, we use Eq. (3) to adjust the width and length of the MMI waveguide. The k1300 and K1550 coefficients are set to 9.667 and 12.06, respectively. Fig. 5 shows the lightwave propagation diagram of the 1 2 1300/1550 nm MMI SOI wavelength division multiplexer with S-bend waveguides. The lx and lz are the lengths of the x and z coordinates, O1 , O2 are outputs at the MMI
Fig. 5. Lightwave propagation diagram of 1 2 1300/1550 nm MMI SOI wavelength division multiplexer with S-bending waveguides.
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outputs. O01 and O02 are the output ports at S-bend output. The designed single-mode SOI S-bending waveguides are used for connecting optical fibers. The optical fibers can be connected to the output ends of the rib waveguides, the separation between the two waveguides must be larger than the fibercladding diameter. However, the bending loss will increase as the separation between the two output waveguides increases. Hence, using the following equation for calculating the separation 2lx between the two output waveguides of the S-bending rib waveguides [41], 2lx ¼
4pxlz : 2pz þ lz sin 2pðz=lz Þ
ð4Þ
We chose 160 lm as the separation. We also designed the S-bending rib waveguides with lz ¼ 1 cm to decrease the bending loss as much as possible. Finally, the insertion loss of the SOI bending output waveguide is below 0.6 dB. Two light wavelengths 1300 and 1550 nm with fundamental mode are fed from the input of a single-mode rib waveguide of our designed 1 2 MMI SOI wavelength division multiplexer. The simulated output mode patterns of light wavelengths at 1300 and 1550 nm coming from O1 and O2 are shown in Figs. 6 and 7, respectively. The insertion loss and crosstalk are )0.227 and )25.36
375
dB for the 1300 nm wavelength, respectively. And for the 1550 nm wavelength, the insertion loss and crosstalk are )0.31 and )22.73 dB, respectively. We applied a tilted MMI region to design the 1 2 MMI SOI WDM because of its low polarization sensitivity. Fig. 8 shows the BPM simulation results of the MMI output at 1300 nm for TE mode and TM mode polarized lights. The dash lines and solid lines show the TE and TM mode output light fields, respectively. We can find that the output peak position shift between the TE and TM modes is about 0.4 lm deviation. The TE and TM mode outputs of wavelength 1550 nm are shown in Fig. 9. For the S-bending waveguides, Fig. 10 shows the TE and TM mode outputs from O01 and O02 corresponding to wavelengths 1300 and 1550 nm, respectively. The output peak position shift between the TE and TM modes is only 0.15 lm, which is smaller than the deviation at wavelength 1300 nm. In Fig. 10, the difference in transmitted power for the two modes at the same wavelength is defined as the polarization-dependent loss (PDL). We calculate the PDL with and without attaching a standard step-index single-mode fiber (SMF). Without attaching the single-mode fiber, the PDL of the device of the output port1 is 0.033 dB for 1300 nm wavelength. The PDL of the device of the output port2 is 0.01 dB for 1550 nm wavelength.
Fig. 6. Simulated output mode pattern of light wavelength 1300 nm at output O1 .
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Fig. 7. Simulated output mode pattern of light wavelength 1550 nm at output O2 .
Fig. 8. BPM simulation results of the MMI output at 1300 nm for TE-mode and TM-mode lightwave.
By attaching the single-mode fiber without taping SOI waveguide, the PDL value of the output port1 may reach 5 and 7 dB for wavelength 1300 and 1550 nm, respectively. However, some papers [42] report that the mode mismatching loss can be re-
duced to as low as 0.17 dB between a rectangular waveguide and a circular waveguide, such as fibers, by using mode matching methods [43]. We believe that by adding the SOI mode matching taper, the PDL can be significantly improved.
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Fig. 9. BPM simulation results of the MMI output at 1550 nm for TE-mode and TM-mode lightwave.
Fig. 10. Wavelength response of the 1 2 SOI output facet-tilted MMI SOI wavelength division multiplexer for TE-mode and TMmode at outputs O1 and O2 , respectively.
Fig. 11 shows the wavelength response of the 1 2 MMI SOI wavelength division multiplexer output facet tilted for TE-mode and TM-mode at output O1 and O2 , respectively. The wavelength shifts between the TE and TM modes for the 1300 and
1550 nm wavelength bands are defined as S1 and S2 , respectively. The optimal values of the tilted length are DL ¼ 52 lm for 1300 nm wavelength and DL ¼ 47 lm for 1550 nm wavelength. For S1 ¼ 0:3 lm and S2 ¼ 0:5 lm, the crosstalk at O1 ,
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Fig. 11. Wavelength response of the 1 2 SOI output facet-tilted MMI SOI wavelength division multiplexer for TE-mode and TMmode at outputs O01 and O02 , respectively.
of our designed 1 2 MMI SOI WDM is )25 dB for both of the TE and TM modes at 1300 nm wavelength.
4. Conclusion The design and analysis of a 1 2 single-mode 1300/1550 nm output facet-tilted MMI SOI wavelength division multiplexer has been presented in this paper. Using BPM_CAD simulations, we show the wavelength response of this device. According to results of simulation, the transmitted power (insertion loss) of output port1 is )0.227 dB for 1300 nm wavelength, and the transmitted power of output port2 is )0.31 dB for 1550 nm wavelength. The crosstalk for 1300 nm wavelength is )25.36 dB and the crosstalk for 1550 nm wavelength is )22.73 dB. Without attaching the single-mode fiber, the device PDL value of the output port1 is 0.033 dB for 1300 nm wavelength, and the device PDL value of the output port2 is 0.01 dB for 1550 nm wavelength. We also can find the optimal values of the tilted length DL ¼ 52 lm for 1300 nm wavelength and DL ¼ 47 lm for 1550 nm wavelength. These values achieve the minimal
TE and TM mode wavelength response deviation. Using such a tilted structure to minimize the polarization effect for both outputs can be optimized with proper geometrical MMI SOI structure design by tuning the slant distance DL. From this research, we numerically demonstrated a feasible compact WDM device with low crosstalk, low polarization sensitivity and good wavelength response. No complex fabrication technology should be required for our 1 2 MMI SOI WDM device. Such a device can be mass produced for future optical WDM networks. We believe very large-scale photonic integrated circuits (VLSPIC) will include many such devices at I/O ports for independent processing of 1300 and 1550 nm signals inside the VLSPIC.
Acknowledgements This work is supported in part by the National Science Council under contract NSC 92-2213-E003-013, NDL 92S-C-025, NSC 92 2219-E-003-001 and NSC 92-2622-E-003-001-CC3. This work also partially supported by National Taiwan Normal University under contract 91091016 (research pro-
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ject for new educator) and ‘‘Application and development of novel magneto-optical materials’’ under contract ORD 93-1. The authors thank Mr. Yi-Jr Jou and Mr. Yi Jr Chen for their typing assistance. The authors also appreciate the editor and reviewers valuable comments. Our gratitude also goes to the Academic Paper Editing Clinic, NTNU.
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