Polymeric Electro-optic Multimode Interference Devices

Transcription

1 Polymeric Electro-optic Multimode Interference Devices Abstract 執筆者 Roshan Thapliya Takashi Kikuchi Shigetoshi Nakamura Advanced Technology Research Laboratory, Corporate Research Group In order to cope with the rapid evolution of working practices, which are increasingly becoming dependent on instant data transfer of high-quality images, Fuji Xerox is researching on next-generation optical waveguide devices that offer the capability of high-speed optical information processing and network flexibility. Our technology is based on multimode interference (MMI) devices that are fabricated on a polymeric electro-optic (EO) material system. These, so-called EO-MMI devices, have been demonstrated to provide robust modulation, switching and reconfigurable power distribution characteristics, which are key functions for versatile optical networking. This report describes the design, fabrication and performance verifications of some of these devices, namely speaking, the EO-MMI modulator and reconfigurable power splitter. 60 富士ゼロックステクニカルレポート No

2 1. Introduction Optical links and interconnections are integral parts in data-intensive applications that are slowly but steadily entering our daily lives whether it is in the form of video-on-demand, teleconferencing, telemedical analysis, interconnection of office equipment, or simply browsing the Internet. The appetite for this explosive need for bandwidth has accelerated the replacement of some traditional copper based point-to-point links to optical fibers such as the last-mile of the optical core network. As optics further penetrates into shorter distances, however, at present the cost of wider bandwidth cancels out the advantage of optics, mainly due to the elevated expenditure necessary for the manufacture of the optical components when compared to traditional electrical links. This is one of the major reasons why optics has not been able to motivate a large segment of potential customers to adapt to optical solutions. Functionally, optical components based on traditional long-haul links can provide indispensable operations such as high-speed modulation and switching, however, they do not provide sufficient cost-benefits mainly due to the use of high-cost material systems and low-yield design topologies. The approach we are taking is to (a) introduce novel high-robust waveguide structures, which can reduce the overall cost of the component by maximizing yield and avoiding the use of compensation electronics and (b) harness the potential of polymeric electro-optic (EO) material systems, which are capable of high volume manufacture and are adaptable to conventional fabrication methods. With this in mind, from the device side, we base our work on multimode interference (MMI) devices, which are suitable because of its high-robustness to the change in the refractive index and tolerance to fabrication imperfections. Although having these obvious advantages, these devices have only been applied in passive applications and their potential, as active switching elements, has not been investigated before. Electro-optic polymers, on the other hand, are attractive because of its low dielectric constant and large EO coefficient, which allows us to achieve low-voltage high-speed switching. Although there are some attempts to use this material in conventional waveguides in typical Y-branch-configurations, the reliability of the resulting devices is an on-going concern, which has hampered the use of these materials in commercial devices. Our EO-MMI devices can sufficiently absorb these reliability issues and by introducing novel switching structures a cost-effective technological platform can be achieved. In this report, for the purpose of studying its potential, the feasibility of some specific functions, namely speaking, (a) 1 x 1 EO-MMI modulator and (b) 1 x 2 tunable (reconfigurable) power splitter, are presented. The paper is organized as follows. First, we explain the principle and design of the EO-MMI device; then we describe the fabrication scheme. This is followed by the experiment and discussion sections; and finally we summarize this work. 2. Principle of Multimode Interference Devices [1] The basic structure of a MMI device comprises of an input access single-mode waveguide connected to a multimode waveguide, which is, then, connected back to an output access single-mode waveguide. The field in the input single-mode waveguide excites the modes inside the multimode waveguide and generates self-images of the input optical field as the various modes propagate inside the multimode waveguide. Mathematically speaking, we can understand the principle of self-imaging as follows. The excitation input electric field ϕ ( x, y, z) of the optical signal inside the access waveguide excites the modes inside the multimode section given by Eq. (1) 富士ゼロックステクニカルレポート No

3 ϕ m 1 υ ν υ = 0 ( x, y, z) = c ϕ ( x, y) exp[ j( β β ) z] (1) where υ is the guided mode number, ϕ ν is the υth excited mode profile inside the multimode waveguide, c υ is the excitation coefficient and β υ, is the propagation constant for each excited mode ν. By adding up all the mode profiles at any given distance z, the resultant optical intensity profile at that distance can be predicted. By repeating this process for each interval of Δz, we can accurately calculate the entire multimode interference pattern generated inside the multimode waveguide. In sections 2.2 and 2.3 we will show that this method allows us to predict the positions of self-images of the input field, however, before going into the details, we will first explain some useful characteristics of the MMI device based on the following approximation. The beat length of the two lowest-order modes (ν=0,1) can be approximated by 4W L C = = β 2 π eff eff 0 β1 3λ0 (2) where β 0 and β 1 are the propagation constants of the fundamental and first-order modes, respectively, n eff is the effective index, λ 0 is the free space wavelength and W eff which is given by W 2σ λ 0 ns = + eff W eff s π neff (3) where n s and W are the surrounding effective index and geometric width of the multimode waveguide, respectively. Here, σ is equal to 0 and 1 for the TE and TM polarizations, respectively. In the case of symmetric interference, which is achieved by center feeding the multimode waveguide with a symmetric field profile, the N-fold self-images are formed at a distance L N given by Eq. (4) L 1 3L = N 4 C N = W n 2 eff Nλ ( n n ) n 0 eff ν 1 2 (4) Equation (4) shows that the length, L N, of the multimode waveguide monotonically decreases as the generation of N self-images increases. This implies that the use of large number of self-images, N, automatically allows the reduction of the length of the MMI device, which is particularly advantageous when we require compact 1 x N power splitters or N x 1 combiners. The lateral separation of these generated self-images, on the other hand, is defined by W eff /N. Hence, for the case when W eff W, we observe that by optimizing the width of the multimode waveguide, it is possible to control the two major performance parameters for waveguide devices: (a) the channel number and (b) the inter-channel cross-talk. Using these characteristics, we explain in the following sections, two types of novel devices: (i) 1 x 1 EO-MMI modulator (case for N=1) and (ii) 1 x 2 EO-MMI reconfigurable (tunable) power splitter (case for N=2). 2.1 Cross-section Design In order to design the planer configuration of the EO-MMI devices, first we must select the material system and optimize its cross-section design. A short description is given below. The cross-section of the waveguide device for the access input single-mode is shown in Fig. 1(a) and is based on the oversized rib design [2]. The clad and core materials are selected such that the refractive indices of the clad materials are lower than that of the core material, in order to achieve optical confinement in the vertical direction, while the rib structure provides the necessary condition for lateral confinement. We used a commercially available chromophore, which is nonlinear in nature and has a macroscopic EO effect when the orientation of the chromophores has a non-centrosymetric alignment obtained using a poling method. The chromophore is dispersed in a host material, an example of which is polysulfone (chemical formula of which is shown in Fig. 62 富士ゼロックステクニカルレポート No

4 1(b)) while the details of the chromophore is explained in reference 3. The clad materials, on the other hand, are commercially available acrylates, which are selected for their fabrication as well as optical compatibility to the core material. The refractive indices of each material measured using the prism coupling method for wavelength 1.55 μm are: (a) core material and for the TE and TM modes, respectively; (b) upper cladding and for the TE and TM modes, respectively; and (c) lower cladding and for the TE and TM modes, respectively. As mentioned above, the core rib dimensions are optimized based on the oversized rib design, which is useful in providing a single-mode excitation in the access waveguides of material systems that have large refractive index-differences between the core and clad materials. Using the effective index method, we find that for the ridge height of 0.65 μm and surrounding core height of 2.65 μm, the optical confinement, Δn is equal to 0.27% and 0.28% for the TE and TM modes, respectively. (a) (b) Fig. 1 : (a): Cross-section design and (b) example of polymer used as the core host-polysulfone x 1 EO-MMI Modulator [4][5] As shown in Fig. 2(a), the 1 x 1 EO-MMI consists of a silicon substrate, a bottom electrode, a lower clad layer, a core layer with a rib-structure etched on top of it, an upper clad layer and, finally, the top prism electrodes. The device operates as follows. When no voltage is applied on the top electrodes the input optical power excited in the input single-mode waveguide is self-imaged via the multimode waveguide and coupled into the output single-mode waveguide. However, when a voltage is applied on the top electrodes, a phase perturbation is generated and changes the interference patterns inside the multimode waveguide, thus, distorting or displacing the self-image depending on the shape of the top electrodes. This effect causes the optical power to couple to the radiative modes of the surrounding sections of the waveguide, thus, carrying no optical power in the output single-mode waveguide. When an electric oscillating driving voltage is applied, for example, the optical power can be modulated, and function as either a variable optical attenuator or an external modulator depending on the type of driving waveform. Based on the cross-section design of the waveguide shown in Fig. 1(a), Eq. (3) -(4) and optimization using beam-propagation method (BPM), for W=50 μm we set, L1, at which a single self-image is formed to 3175 μm and 3125 μm for TE and TM, respectively. Figure 2(b) shows the case when the length of the multimode waveguide is set to twice the length of L 1, such that two subsequent single self-images are generated. The advantage of using these so-called multiple beats is that we can reduce the driving voltage by increasing the overlap of the optical signal and the driving electric field. For example, Fig. 2(c) shows the case when a voltage of 30 V is applied, clearly illustrating our prediction that the optical power is coupled to the radiation modes. 富士ゼロックステクニカルレポート No

5 (a) (a) (b) (b) (c) (c) Fig. 2 : (a) Waveguide structure of 1 x 1 EO-MMI modulator, (b) case when no voltage is applied and (c) case when 30 V is applied x 2 EO-MMI Reconfigurable (Tunable) Power Splitter [6] The 1 x 2 EO-MMI uses the same cross - section as explained in section 2.1, but with rectangular electrode pair set on top of the multimode waveguide as shown in Fig. 3(a). Using Eq. (4) with N=1, we find for W=40 μm Fig. 3 : (a) Waveguide structure of 1 x 2 EO-MMI reconfigurable power splitter, (b) case when no voltage is applied and (c) case when 56 V is applied. that the optimal length, L1, at which a single self-image is formed is 2080 μm and 2070 μm for TE and TM, respectively. Since we require the TM mode for the EO effect, we set L1=2070 μm and, the distance L2, where a pair of self-images is formed to L1/2=1035 μm. The length of the multimode waveguide, therefore, 64 富士ゼロックステクニカルレポート No

6 is L s =3105 μm (=L 2 + L 1 ). Generally speaking, the length of multimode waveguide, L s, is selected using: L S = L 2 + ml 1 (5) where m is the frequency of the formation of the pair of self-images. The importance of using multiple beats (m>1) is discussed in section 4. As shown in Fig. 3(a), for m=1 in Eq. (5), a pair of electrodes is placed on top of the first pair of self-images at distance L 2 = 1035 μm and the output access waveguides are attached to the second pair of self images at a distance L 1 =2070 μm, which are laterally separated by 20 μm. As shown in Fig. 2(b), the 3-dB power splitter is illustrated with this design. The calculated excess loss is 0.6 db and 0.8 db for the TE and TM modes, respectively. In Fig. 2(c) we illustrate the case where a refractive index change of ± (which is equivalent to applying ± 56 V) is induced where the first pair of self-images is located. Note, in this case, that the power is shifted to channel 1 from 2. These predictions suggest that reconfigurable power splitting can be achieved by inducing phase change in the first pair of the self-images. 3. Fabrication and Experiments 3.1 Fabrication Prior to fabrication, the spin-coat solution for the core layer is prepared by dissolving the chromophore material and the host polysulfone (PSU) in cyclohexanone at room temperature followed by filtration with a 0.2 μm polypropylene membrane filter. The concentration of the loading density of the polymer was selected in order to obtain 2~4μm of film thickness. We fabricate the devices using a standard semiconductor process scheme, which is also reported in our past work [7]. Gold is sputtered on top of a two-inch diameter silicon substrate to form the bottom electrode and is set to produce a thickness of nm. The lower clad, was spin coated such that a thickness of 3.5 μm is formed. The core material is, then, spin coated, obtaining a thickness of 3.3 μm and is baked at 120 C for 60 minutes to cure and evaporate the residual solvent. After sample preparation, dry-etching using an inductively coupled plasma etching system is used to produce a 0.65 μm rib structure. The etching scheme is as follows. The etching chamber is pumped to a base pressure below 10-4 Pa. An electrostatic chuck holds the wafer, which is cooled by helium gas, maintaining the substrate temperature of 20 C. During dry etching, the antenna power and bias power of the substrate is set to 150 W and 10 W, respectively. The dry etching gas is oxygen and the flow rate is kept at 10 sccm while keeping the pressure at 0.6 Pa. The widths of the access waveguides and the multimode waveguide are 5.0 μm and 40~50 μm, respectively, depending on the 1 x 1 EO-MMI and 1 x 2 EO-MMI. Figure 4 shows the SEM picture of the joint section of the access single-mode waveguide and the multimode waveguide etched on the core layer. These films are, then, spin coated with the upper clad, and UV-cured at 6 J/cm 2. The top electrode is formed after patterning and subsequent lift-off. The wafers are, then, diced to form chips for evaluation as shown in Fig. 5. Fig. 4: SEM picture of the etched core layer at the joint of the input access single-mode and the multimode waveguide. 富士ゼロックステクニカルレポート No

7 which quantifies the degree of nonlinearity of the material. For this material system the poling details are as follows: 500 V is constantly applied for 30 minutes between the top and bottom electrodes after raising the temperature to 140 C. This is, then, cooled down to room temperature with the voltage still applied to ensure the freezing of the orientation of the chromophore to achieve the prerequisite non- centrosymetric alignment for macroscopic EO behavior [8]. Fig.5: Fabricated 1x1 and 1 x 2 EO-MMI chip array after being butt-coupling. Prior to the switching measurements, the EO-MMI devices are poled. As shown in Fig. 6, the purpose of poling is to achieve alignment of each individual chromophore thereby generating an effective dipole moment, which is the necessary condition to obtain an EO effect. The relation between the macroscopic EO coefficient, r 33, and the chromophore orientation denoted by the order parameter cos 3 θ is given by Eq. (6) r 33 M β cos (6) where M is the number of the chromophores inside the host material and β is the hyperpolarizability of the chromophores, 3 θ 3.2 Experiment The picture of the measurement system is shown in Fig. 7. The alignment of the EO-MMI with the input and output single mode fibers is achieved using a 6-axis alignment system. The polarization of the light from the DBR laser-module (λ=1.56 μm) is automatically controlled, and the TM-mode selected using a half-wave plate. The dual voltage source supplies a 10 Hz triangular waveform. The output optical power is then monitored using a phtodetector module. As shown in Fig. 8(a), the experimental result shows the switching operation of the 1 x 1 EO-MMI device, with an extinction ratio = 12 db at V b = 35.5 V between the ON and OFF states and an excess loss of < 0.5 db. The power drop at the secondary peak is due to the reduced confinement of the optical signal inside the multimode section at higher voltages. Fig. 7: Picture of measurement system. Fig. 6: Illustration of inducing an effective dipole moment by poling. 66 富士ゼロックステクニカルレポート No

8 As shown in Fig. 8(b), the switching operation of the 1 x 2 EO-MMI reconfigurable power splitter with a tuning range of ~6 db at V b = 54 V is demonstrated. The apparent V offset = 21 V between the 3-dB intersection of the two channels and V=0 V is due to the remnant pre-biased field generated during poling. When compared to the insertion loss of a straight channel, which was 18.1 db, the insertion loss of the 1 x 2 EO-MMI device was 21.8 db at the 3-dB point (i.e. at V offset =21 V), suggesting an excess loss to be 0.7 db (=21.8 db 3.0 db 18.1 db) which is fairly close to the actual measured value of 0.6 db (= db). Furthermore, the coupling loss was 3 db/side and the PDL was ~ 1 db when measured for the straight channel. Fig. 8: (a) (b) Experimental results of (a) 1 x 1 EO-MMI and (b) 1 x 2 EO-MMI devices. We also tested the photochemical robustness of the 1 x 1 EO-MMI device by continuously supplying an optical power for more than 1000 hours at 70 C. As shown in Fig. 9, the power fluctuation of the 1 x 1 EO-MMI device and that of a butt-joined fiber-to-fiber reference is very close to ~0.3 db for both TE and TM modes. This shows that the robustness of the 1 x 1 EO-MMI device is within experimental limits, hence, suggesting good thermal stability. Fig. 9 Thermal robustness of 1 x 1 EO-MMI device. 4. Discussions and On-going Works Although the driving voltages for the 1 x 1 and 1 x 2 EO-MMI devices are relatively large we can reduce this by using multiple beats [6][9] in the multimode waveguide. For example, by using m > 1, in Eq. (5) the driving voltage can be reduced by ~60% for both 1 x 2 EO-MMI and the 1 x 1 EO-MMI devices. A specific example of voltage reduction with the 1 x 2 EO-MMI is shown in Fig. 10, where V b is reduced from 54 V to 21 V by using m = 3 with tuning range of 14 db. If we are to compare it with the same tuning range of 6 db as in the case of m=1, then the required driving voltage can be further reduced to 9 V, which is already within practical limits. Furthermore, the recent development of EO-polymers has shown that, although properties related to reliability and fabrication-compatibility requires further understanding, some materials can provide r 33 of an excess of 100 pm/v. Since our material system provides an effective r 33 =15 pm/v for λ=1550 nm, by using theses materials we predict that V b ~ 1 V are obtainable. However, even without the use of these high-performance materials, we can achieve driving voltages < 3.5 V by using related novel designs and increasing the poling efficiency. 富士ゼロックステクニカルレポート No

9 Fig. 10 Reduced driving voltage for 1 x 2 EO-MMI by using multiple-beats (m=3). For our further works, we are concentrating our efforts to obtain high-speed 2 x 2 EO-MMI switches and compact MMI-based 1 x N power splitters to enhance the port number and flexibility of the optical network. For example, as shown in Fig. 11(a) and (b), a compact 1 x 10 power splitter is calculated to be ~2 cm x 0.25 cm in size, providing 10 output channels at 250 μm channel spacing with a power uniformity of < 0.4 db at excess loss of ~0.5 db. This component can be used to connect a single scanner to a multiple of printers for simultaneous printing and distribution at separated locations. By combining EO-MMI switches with these power splitters we believe that dynamically reconfigurable optical networks can be cost-effectively achieved and employed to provide customer-added value services such as network printing and distributed IIT-IOT networks. 5. Summary We have proposed novel EO-MMI devices based on FX s polymeric electro-optic waveguide technology and have tested its functionality by demonstrating 1 x 1 EO-MMI modulation and 1 x 2 EO-MMI reconfigurable power splitting. This technology offers the means for flexibility of dynamically reconfigurable optical networking which can enhance efficiency in data-intensive environments. With further development, we discussed on the possibilities of extending this technology into our core business and speculated on value-added features such as IIT-IOT optical networking. 6. Reference (a) (b) Fig. 11: (a) 1 x 10 MMI-based compact power splitter and (b) the power distribution at the 10 output channels. 1. Lucas B. Saldano and Erik C. M. Pennings, Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Applications, IEEE J. Lightwave Technol., 13, pp (1995) 2. Richard A Soref, Joachim Schidtchen, and Klaus Petermann, Large Single-Mode Rib Waveguides in GeSi-Si and Si-on-SiO 2, IEEE J. Quant. Electronics, 27, pp (1991) 3. Mingqian He, Thomas M. Leslie, John A. Sinicropi, Sean M. Garner, and Leon D. Reed, Synthesis of Chromophores with Extremely High Electrooptic Activities 2. Isophorone- and Combined Isophorone-Thiophene-based Chromophores, Chem. Mater., 14, pp (2002) 68 富士ゼロックステクニカルレポート No

10 4. Roshan Thapliya, Shigetoshi Nakamura, Takashi. Kikuchi, Electro-optic Multimode Interference Device using Organic Materials, Appl. Opt., 45, pp (2006) 5. Roshan Thapliya, Takashi Kikuchi, Shigetoshi Nakamura, Electro-optic Multimode Interference Device Based on Nonlinear Organic Materials, Proceedings of IEEE-LEOS 06, ThDD 2 (Montreal, Canada), pp (2006) 6. Roshan Thapliya, Takashi Kikuchi, Shigetoshi Nakamura, Tunable Power Splitter Based on an Electro-optic Multimode Interference Device, Appl. Opt., 46, pp (2007) 7. Roshan Thapliya, Yasunori Okano, Shigetoshi Nakamura, Electro-optic Characteristics of Thin-film PLZT Waveguide Using Ridge-type Mach-Zehnder Modulator, IEEE J. Lightwave Technol., 21, pp (2003) 8. Y. Shi, C. Zhang, H. Zhang, J. H. Bechtel, L.R. Dalton, B. H. Robinson, W. H. Steier, Low (Sub-1-Volt) Halfwave Voltage Polymeric Electrooptic Modulators Achieved by Controlling Chromophore Shape, Science 288, pp (2000) 9. Takashi Kikuchi, Shigetoshi Nakamura, Roshan Thapliya, Multiple-beat Electrooptic Multimode Interference Device Jpn. J. Appl. Phys. 46, L (2007) Author s Introductions Roshan Thapliya Corporate Research Group, Advanced Technology Research Laboratory, Theme Leader, Member of the Optical Society of America, Field: Optics (Ph.D.) Takashi Kikuchi Corporate Research Group, Advanced Technology Research Laboratory, Researcher, Field: Chemistry Shigetoshi Nakamura Corporate Research Group, Advanced Technology Research Laboratory, Researcher, Member of the Optical Society of America, Field: Applied Physics 富士ゼロックステクニカルレポート No