Athermal and flat-topped silicon MachZehnder filters

Transcription

1 Vol. 4, No. 6 6 Dec 06 OPTICS EXPRESS 9577 Athermal and flat-topped silicon MachZehnder filters QINGZHONG DENG, LU LIU, RUI ZHANG, XINBAI LI, JURGEN MICHEL,,* AND ZHIPING ZHOU State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Electronics Engineering and Computer Science, Peking University, Beijing 0087, China MIT Microphotonics Center, Massachusetts Institute of Technology, Cambridge, MA 039, USA * zjzhou@pku.edu.cn Abstract: Athermal and flat-topped transmissions are the two main requirements for a silicon WDM filter. A Mach-Zehnder (MZ) filter which simultaneously satisfies these two requirements has been experimentally demonstrated in this paper. A combination of strip waveguide and hybrid strip-slot waveguide is introduced for athermalization, and two-stage interference is utilized for flat-topped transmission. The temperature dependent wavelength shift is measured to be ~-5 pm/k while the best db bandwidth is 5.5 nm with 4.7 nm free spectral range (FSR). The measured minimum insertion loss is only 0.4 db with a device dimension of 70 μm 580 μm. Moreover, Such a MZ filter is compatible with the state-ofart CMOS-fabrication process and its minimum feature size is as large as 00 nm. 06 Optical Society of America OCIS codes: (30.30) Integrated optics devices; (30.790) Guided waves; ( ) Wavelength filtering devices. References and links A. Rickman, The commercialization of silicon photonics, Nat. Photonics 8(8), (04). Z. Zhou, B. Yin, Q. Deng, X. Li, and J. Cui, Lowering the energy consumption in silicon photonic devices and systems, Photonics Res. 3(5), B8 B46 (05). S. Song, X. Yi, S. X. Chew, L. Li, L. Nguyen, and P. Bian, Integrated SOI stagger-tuned optical filter with flattop response, J. Lightwave Technol. 34(9), (06). H. Deng, Y. Yan, and Y. Xu, Tunable flat-top bandpass filter based on coupled resonators on a graphene sheet, IEEE Photonics Technol. Lett. 7(), 6 64 (05). P. Chen, S. Chen, X. Guan, Y. Shi, and D. Dai, High-order microring resonators with bent couplers for a boxlike filter response, Opt. Lett. 39(), (04). S. H. Jeong, D. Shimura, T. Simoyama, M. Seki, N. Yokoyama, M. Ohtsuka, K. Koshino, T. Horikawa, Y. Tanaka, and K. Morito, Low-loss, flat-topped and spectrally uniform silicon-nanowire-based 5th-order CROW fabricated by ArF-immersion lithography process on a 300-mm SOI wafer, Opt. Express (5), (03). S. Pathak, M. Vanslembrouck, P. Dumon, D. Van Thourhout, and W. Bogaerts, Optimized silicon AWG with flattened spectral response using an MMI aperture, J. Lightwave Technol. 3(), (03). D. Dai, L. Liu, and S. He, Three-dimensional hybrid modeling based on a beam propagation method and a diffraction formula for an AWG demultiplexer, Opt. Commun. 70(), 95 0 (007). S. N. Khan, D. Dai, L. Liu, L. Wosinski, and S. He, Optimal design for a flat-top AWG demultiplexer by using a fast calculation method based on a Gaussian beam approximation, Opt. Commun. 6(), (006). D. Dai, W. Mei, and S. He, Using a tapered MMI to flatten the passband of an AWG, Opt. Commun. 9(-6), (003). F. Horst, W. M. J. Green, S. Assefa, S. M. Shank, Y. A. Vlasov, and B. J. Offrein, Cascaded Mach-Zehnder wavelength filters in silicon photonics for low loss and flat pass-band WDM (de-)multiplexing, Opt. Express (0), (03). C. G. H. Roeloffzen, F. Horst, B. J. Offrein, R. Germann, G. L. Bona, H. W. M. Salemink, and R. M. de Ridder, Tunable passband flattened -from-6 binary-tree structured add-after-drop multiplexer using SiON waveguide technology, IEEE Photonics Technol. Lett. (9), 0 03 (000). S. H. Jeong, D. Shimura, T. Simoyama, T. Horikawa, Y. Tanaka, and K. Morito, Si-nanowire-based multistage delayed Mach-Zehnder interferometer optical MUX/DeMUX fabricated by an ArF-immersion lithography process on a 300 mm SOI wafer, Opt. Lett. 39(3), (04). S. Assefa, S. Shank, W. Green, M. Khater, E. Kiewra, C. Reinholm, S. Kamlapurkar, A. Rylyakov, C. Schow, F. Horst, H. Pan, T. Topuria, P. Rice, D. M. Gill, J. Rosenberg, T. Barwicz, M. Yang, J. Proesel, J. Hofrichter, B. Offrein, X. Gu, W. Haensch, J. Ellis-Monaghan, and Y. Vlasov, A 90nm CMOS integrated Nano-Photonics #7876 Journal 06 Received 3 Oct 06; revised 5 Nov 06; accepted 8 Nov 06; published 3 Dec 06

2 Vol. 4, No. 6 6 Dec 06 OPTICS EXPRESS 9578 technology for 5Gbps WDM optical communications applications, in 0 IEEE International Electron Devices Meeting (IEDM)(IEEE, 0), pp B. Guha, J. Cardenas, and M. Lipson, Athermal silicon microring resonators with titanium oxide cladding, Opt. Express (), (03). 6. W. N. Ye, J. Michel, and L. C. Kimerling, Athermal high-index-contrast waveguide design, IEEE Photonics Technol. Lett. 0(), (008). 7. J. Teng, P. Dumon, W. Bogaerts, H. Zhang, X. Jian, X. Han, M. Zhao, G. Morthier, and R. Baets, Athermal Silicon-on-insulator ring resonators by overlaying a polymer cladding on narrowed waveguides, Opt. Express 7(7), (009). 8. B. Guha, B. B. C. Kyotoku, and M. Lipson, CMOS-compatible athermal silicon microring resonators, Opt. Express 8(4), (00). 9. Q. Deng, X. Li, Z. Zhou, and H. Yi, Athermal scheme based on resonance splitting for silicon-on-insulator microring resonators, Photonics Res. (), 7 74 (04). 0. K. Hassan, C. Sciancalepore, J. Harduin, T. Ferrotti, S. Menezo, and B. Ben Bakir, Toward athermal silicon-oninsulator (de)multiplexers in the O-band, Opt. Lett. 40(), (05).. S. Dwivedi, H. D Heer, and W. Bogaerts, A compact all-silicon temperature insensitive filter for WDM and bio-sensing applications, IEEE Photonics Technol. Lett. 5(), (03).. M. Uenuma and T. Moooka, Temperature-independent silicon waveguide optical filter, Opt. Lett. 34(5), (009). 3. P. Xing and J. Viegas, Subwavelength grating waveguide-integrated athermal Mach-Zehnder interferometer with enhanced fabrication error tolerance and wide stable spectral range, Proc. SPIE 975, 9750U (06). 4. P. Xing and J. Viegas, Broadband CMOS-compatible SOI temperature insensitive Mach-Zehnder interferometer, Opt. Express 3(9), (05). 5. B. Guha, A. Gondarenko, and M. Lipson, Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers, Opt. Express 8(3), (00). 6. Q. Deng, R. Zhang, L. Liu, X. Li, J. Michel, and Z. Zhou, Athermal and CMOS-compatible flat-topped silicon Mach-Zehnder filters, in 3th International Conference on Group IV Photonics (IEEE Photonics Society, Shanghai, China, 06), p. FC5. 7. Q. Deng, Q. Yan, L. Liu, X. Li, J. Michel, and Z. Zhou, Robust polarization-insensitive strip-slot waveguide mode converter based on symmetric multimode interference, Opt. Express 4(7), (06). 8. Q. Deng, L. Liu, X. Li, and Z. Zhou, Strip-slot waveguide mode converter based on symmetric multimode interference, Opt. Lett. 39(9), (04). 9. Q. Deng, L. Liu, X. Li, J. Michel, and Z. Zhou, Linear-regression-based approach for loss extraction from ring resonators, Opt. Lett. 4(0), (06).. Introduction Silicon photonic wavelength division multiplexing (WDM) is widely anticipated to solve the bandwidth bottleneck problem in nowadays high-performance computing systems [,]. One main requirement of a WDM filter is the flat-topped transmission pass-bands and almost all kinds of silicon WDM filters have been modified to satisfy this requirement, such as cascade for ring resonators [3 6], multimode interference (MMI) assisted aperture for Arrayed Waveguide Gratings (AWGs) [7 0], multi-stage interference for Mach-Zehnder (MZ) filters [ 4]. All of these methods are efficient in flatting the pass-bands. For a silicon WDM filter, another main requirement is the athermal transmission spectrum since the large positive thermo-optic coefficient of silicon material (TOC K ) will cause considerable temperature dependent wavelength shift (~80 pm/k). Aiming at athermal filters, several methods have been reported, such as cladding with negative TOC materials [5 7], coupling with multiple structures [8, 9], and combination of multiple types of waveguide [0 5]. Constructing the arms of a MZ filter with the combination of multiple types of waveguide is the most promising athermal scheme, as summarized in [], due to the merits of CMOScompatible fabrication process and zero extra energy consumption. Therefore, MZ filters are the leading candidate to simultaneously satisfy the two main requirements. However, an athermal and flat-topped silicon MZ filter has not been experimentally demonstrated since the flat-top scheme is proposed only for the MZ with the same loss in the two arms while the athermal scheme makes the two arms hold significantly different losses. In this paper, we expand the flat-top theory to the MZ with different losses in the two arms firstly. Then, an athermal and flat-topped MZ filter is experimentally demonstrated. We have presented some preliminary results in the 3th International Conference on Group IV Photonics [6], while the complete simulation and measurement results are analyzed here.

3 Vol. 4, No. 6 6 Dec 06 OPTICS EXPRESS Flat-topped Mach-Zehnder filter Two-stage interference [Fig. (a)] is utilized to flatten the pass-bands of a MZ filter. It consists of 3 directional couplers (DCs) and phase shifters (PSs). κ i denotes the crosscoupling coefficient of DC i (i,, 3), and the self-coupling coefficient (r i ) can be expressed as ri κi if the coupling loss is negligible. Δ OLeff denotes the optical path length difference of the two arms in PS while the value is Δ for PS. a denotes the optical OLeff field attenuation factor in the shorter arm of PS, which means incident light with electric field amplitude of E will attenuate to a E after transmission through this waveguide, and a corresponds to the longer arm. We define the relative attenuation factor of PS as a a /a, and the value of PS will be a. Then, the transmission spectrum of Ch. (Tr ) and Ch. (Tr ) can be expressed as truncated Fourier series. 6 jφ jφ 3 j3φ Tr a κκ r3 rr r3a e + κrκ3a e + rκκ 3a e 6 jφ jφ 3 j3φ Tr a κκκ 3 rr κ3a e κrr 3a e rκr 3a e () π ; φ Δ OLeff ri κi ( i,, 3) λ In the form of Eq. (), a contributes to the insertion loss only. Therefore, we fix a in the following theoretical analysis for the flat-top performance. If the loss difference in the arms are neglected (a ), designing the coupling coefficients to satisfy κ 0.50, κ 0.9, and κ will simultaneously make Ch. and Ch. flat-topped [Fig. (b)] []. We mark the value of κ i under this condition as Κ i (i,, 3) for the convenience of statement. Fig.. (a) Schematic and (b) theoretically calculated transmission spectra of the two-stage MZ filter. According to Eq. (), we find that the flat-top feature of Ch. can be maintained by modifying the coupling coefficients as Eq. () for the case of a. After such modifications, the whole spectrum will scale down with the decrease of a while the shape of the spectrum are kept unchanged. As shown in Fig. (a), the insertion loss increases with the decrease of a while the db bandwidth is independent with a. Similarly, Eq. (3) can keep the flat-top feature for Ch. as shown in Fig. (b). For Ch.: κ ; κ ; κ 4 3 Κ 3 4 ( a ) Κ ( a ) Κ3 For Ch.: κ ; κ ; κ 4 3 Κ 3 4 ( a ) Κ ( a ) Κ3 a Κ () (3)

4 Vol. 4, No. 6 6 Dec 06 OPTICS EXPRESS 9580 Comparing between Eqs. () and (3), one can find that the required coupling coefficients of Ch. and Ch. are different only in κ [Fig. (c)]. Moreover, the required κ of the two 3 output ports differ slightly when a close to. Therefore, Eq. (4) is utilized when the two ports need to be considered simultaneously [Fig. (d)]. Even though the spectra are not strictly scaled down, the db bandwidths are not significantly degenerated. 3 For Ch.and Ch.: κ ; κ ; κ Κ 3 Κ3 ( a ) Κ (4) Fig.. The flat-top transmission spectra of (a) Ch. and (b) Ch.; (c) the required coupling coefficients; (d) transmission spectra with compromised coupling coefficients as Eq. (4). Solid lines, Ch.; dashed lines, Ch.; Gray dots, wavelength boundaries of db bandwidths. 3. Athermalization with hybrid strip-slot waveguide To achieve athermal performance, two types of waveguides are utilized to construct the arms of the phase shifters [Fig. 3(a)]. The effective thermo-optic coefficient ( neff T ) and the waveguide length ( Δ L ) of the two waveguide types are designed to satisfy the athermal condition, I II neff n I eff II Δ L ΔL. (5) T T Under athermal condition, the free spectral range (FSR) of this filter can be expressed as λ FSR ΔOLg I i TO I I g n i eff T Δ OLg ng Δ L ; TOg ( i I, II) II i TOg ng (6) where n n λ n / λ is the group refractive index. Equation (6) indicates that larger difference in g eff eff TO g of the two waveguide types will produce shorter phase shifters for a certain

5 Vol. 4, No. 6 6 Dec 06 OPTICS EXPRESS 958 FSR, which means more compact device. Due to the large difference with the strip waveguide in TO g, shown in Fig. 3(b), the hybrid strip-slot waveguide is introduced to combine with normal single-mode waveguide (strip waveguide with W 450 nm) for athermal performance. To ease the fabrication process, the total width of the hybrid strip-slot waveguide is chosen to be W 600 nm so that the minimum feature size of the filter can be kept as large as 00 nm. The optical mode profiles of the two waveguides are displayed in Figs. 3(c) and 3(d) where mode mismatch can be observed. To overcome this mode mismatch, a multimode interference (MMI) based waveguide mode converter [Fig. 3(e)] is utilized to connect the waveguides [7, 8]. The device fabricated by Singapore A*STAR Institute of Microelectronics (IME) Multi-Project Wafers (MPW) is shown in Fig. 3(f) where ΔL I 400 μm, ΔL II 553 μm and the coupling length of the three DCs are 0.5 μm, 7.0 μm and.4 μm respectively. Fig. 3. (a) The schematic of the proposed athermal flat-topped MZ filter; (b) TO g of the fundamental TE mode in the two waveguide types; The simulated transverse electric field profile of fundamental TE mode in (c) the strip waveguide with W 450 nm, (d) the hybrid strip-slot waveguide with W 600 nm and (e) the waveguide mode converter with W m.5 μm, L m.40 μm, L 5 μm; (f) Micrograph of the fabricated filter. All devices presented in this paper are based on a material platform of silicon-on-insulator (SOI) with SiO cladding, and H 0 nm, Slab 90 nm, Slot 00 nm; The slot is located at the center of the waveguide; The simulations are performed with 3D full vector finite element method (FEM) at 550 nm wavelength while the refractive index of Si and SiO are set to 3.48 and.45 respectively. Designing the filter in Fig. 3(a) to have flat-top pass-bands, characterization for the relative attenuation factor (a) is essential according to the analysis in the previous section. The mode converter is cascaded for measurement as described in [8] while the hybrid stripslot waveguide and strip waveguide are constructed into rings to measure the attenuation factor with the method in [9]. The measured optical field attenuation factors are listed in Table. Based on the measured a, the required coupling coefficients for top-flat performance can be calculated with Eq. (4), shown in Table. Moreover, the coupling coefficients of the DCs in the fabricated filter are characterized and listed in Table also. Comparing the two sets of values, one can find that the fabricated one satisfy the requirements roughly.

6 Vol. 4, No. 6 6 Dec 06 OPTICS EXPRESS 958 Therefore, this filter is expected to have athermal and flat-topped transmission spectra. The measured transmission spectra under different temperature are plotted in Fig. 4: (i) the temperature-dependent wavelength shift is only ~-5 pm/k around 550 nm wavelength which has been prominently improved when compared with traditional MZ filters (~80 pm/k); (ii) the device has flat pass-bands with db-bandwidths of 5.5 nm for Ch. and 5.0 nm for Ch. while the FSR is 4.7 nm; (iii) the cross talk between the two channels is ~-3.6 db; and (iv) the insertion loss is 0.4 db (Ch.) and.4 db (Ch.). It is worth to mention that even though the athermal performance is characterized in the temperature range of 5~45 C, the proposed filter is expected to hold the athermal performance in a wider temperature range since the athermal condition [Eq. (5)] is not sensitive to temperature fluctuations. Table. The measured optical field attenuation factors. Mode converter 553 μm hybrid strip-slot 400 μm strip Relative attenuation factor (a (a c ) waveguide (a SL ) waveguide (a ST ) a c a SL /a ST ) 0.97 ± ± ± ± 0.03 Table. The required and the fabricated DC coupling coefficients. κ Required 0.4 ± Fabricated 0.38 ± ± ± 0.0 κ κ 3 4. Conclusion Fig. 4. The measured transmission spectra of the fabricated athermal flat-topped MZ filter. In this work, a MZ filter which simultaneously satisfies the two main requirements for a silicon WDM filter, athermal and flat-topped transmission, has been experimentally demonstrated. Combination of strip waveguide and hybrid strip-slot waveguide is introduced for athermalization, and two-stage interference is utilized for flat-topped transmission. The designing theory for flat-topped filter is expanded to the MZ with different losses in the two arms so that it can be applied to an athermal MZ filter. The athermal performance is measured to be ~-5 pm/k while the db bandwidth is 5.5 nm for Ch. and 5.0 nm for Ch. (@ 4.7 nm FSR). The measured minimum insertion loss is 0.4 db with a device dimension of 70 μm 580 μm. Such a MZ filter can be easily fabricated with the state-of-art CMOS-fabrication process since its minimum feature size is as large as 00 nm. Furthermore, the device can be scaled up to multi-channel WDM filters by taking the proposed filter as the fundamental building block and cascading like a binary tree []. Funding National Natural Science Foundation of China (NSFC) (60060).