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We demonstrate a novel hollow waveguide optical switch composed of an multi-mode interference (MMI) coupler with a variable air core. The numerical simulation and experiment of the proposed optical switch is carried out for investigating the operation of the switch. Switching operation can be obtained by the mechanical displacement of the air core of an MMI hollow waveguide. A hollow waveguide consists of Au mirrors deposited on two GaAs substrates for optical confinement. The measured result shows a possibility of switching of about 85% optical power fraction with a switch length of 1.1 mm and small displacement (ΔDcore=3 µm) of an air core thickness. The measured insertion losses of a 1.1 mm long hollow waveguide with 12 and 9 µm air core are 5.4 dB and 5.7 dB respectively.
©2005 Optical Society of America
1. Introduction
The rapid growth of broadband internet traffic requires high-speed and high-capacity photonics networks. A wavelength division multiplex (WDM) photonic network, which can provide an efficient increase in transmission capacity and functionalities in routing, has been attracting much attention. In addition, the rapid growth of short reach links, metro-area networks, and FTTH, etc. is going on. Also, photonic networks will be developed to form either large-scale rings or mesh networks. In order to build reliable and flexible photonic networks, compact and large-scale photonic integrated circuits are becoming important. An optical switch is one of key optical devices. Important issues for optical switches are compact size, cross-talk, scalability and so on. In waveguide optical switches, either an electro-optic effect or a thermo-optic effect has been used for switching operations [1–8]. However, both effects except excitonic electrorefraction [1] give us small change in refractive index, resulting in a large device length of several mm [3–8].
We proposed tunable hollow waveguides for a new class of photonic Integrated circuits, enabling a large change in propagation constants of light with a variable core [9–10]. We suggested a possibility of large tuning in propagation constant where the mode field changes in a vertical direction [10]. The relative change in propagation constant is as large as a few % or larger with a small air core, which is very difficult to be realized in conventional dielectric waveguides. Such a large change in propagation constant may allow us to reduce the size of optical switches. Also, a hollow waveguide MMI coupler was demonstrated [11–12]. Recently, we proposed a hollow waveguide optical switch composed of a MMI coupler and a variable air core [13].
In this paper, we present the first demonstration of a hollow waveguide optical switch composed of MMI coupler with a variable air core. Switching operation can be obtained in a 1.1 mm long hollow waveguide MMI with a small displacement in an air core.
2. Operating principle and fabrication of hollow waveguide optical switch
Fig. 1. Schematic structure of proposed hollow waveguide optical switch composed of a MMI coupler with a variable air core.
We proposed an optical switch consisting of an MMI coupler as shown in Figs. 1(a) and 1(b). The proposed MMI coupler type optical switch is based on the interference between the hollow waveguide modes with a self-imaging effect. The beat length Lπ of the two lowest-order modes is expressed by the difference in propagation constants of two guided modes as shown in the following equation [14]:
where nr is the effective refractive index, We is the effective width of an MMI waveguide and λ0 is the wavelength of light. Moreover, the beat length Lπ is dependent on the effective width and effective refractive index of the hollow waveguide. The lateral spot size of a hollow waveguide, which is the effective width, can be changed with a variable air core. Thus, the beat length Lπ can be variable in a tunable hollow waveguide. As a result, switching of a mode field at the end of an MMI coupler is obtained by changing the thickness of an air core gap.
The mechanical displacement can be obtained by using an electrostatic force with applying a voltage in the core gap. Figures 2(a) and 2(b) show the cross-sectional view of an input waveguide and MMI waveguide of the fabricated switch, respectively. In this experiment, we used an PZT actuator for the mechanical displacement, which will be replaced by a monolithic electrostatic actuator in future. Guided modes can be confined in an Au-coated 3D hollow waveguide. First, we made a trench structure by lithography followed by dry etching, and then we coated both the etched GaAs substrate and planar cover GaAs substrate with a thin Au film. The parameters of the fabricated switch are a step difference in an air core of 5 µm, an input waveguide width of 10 µm and an MMI waveguide width of 20 µm, which is twice that of the input waveguide. The air core thickness was changed by moving the upper substrate with a PZT actuator as shown in Fig. 2.
3. Results
Fig. 3. Calculated field distribution of an MMI coupler. Top view and cross-sectional view for (a-1) 12 µm and for (a-2) 10 µm, respectively. Calculated optical intensity distribution at a distance of 1.2mm.
We carried out the numerical simulation of the proposed hollow waveguide optical switch with an MMI coupler using the full-vectorial simulator of FIMM-WAVE/FIMMPROP (Pohoton Design Co.). At first, a core thickness Dcore of an input waveguide was changed and we found a fundamental guided mode confined in the air core. We assumed that the input light is a TE mode with an electric filed parallel to a substrate and the wavelength is 1.55 µm. The wavelength dependence of switching characteristics is under study. We expect small wavelength dependence based on Eq. (1). The top view and cross-sectional view of the intensity distribution in the hollow waveguide are shown in Figs. 3(a-1) and 3(a-2) for different air core thicknesses of 12 µm and 10 µm, respectively. When the core thickness changes from 12 µm to 10 µm, the lateral intensity peak moves from the left hand side to the right hand side at a distance of 1.2 mm. Figure 3(b) show the lateral intensity distribution at a distance of 1.2 mm from the input port for the core thickness of 12 and 10 µm, respectively. The numerical simulation shows a possibility of optical switching operation with a small displacement of air core ΔDcore=2 µm, when we connect an output waveguide with the MMI coupler. The displacement ΔDcore used in the numerical simulation is different from the experiment. It is because we are currently able to find a solution for discrete values ΔDcore due to the high contrast in refractive indices in the air core. The structure dependence of switching characteristics is under study and will be reported elsewhere.
Fig. 5. Measured optical intensity distribution at a distance of 1.1 mm for 9 µm and 12 µm air core.
We carried out the measurement by exciting guided modes with a single-mode fiber at an input for the fabricated device. The air core thickness was precisely controlled by using the PZT actuator. Figures 4(a) and 4(b) show the measured field distribution on the output edge of the fabricated hollow waveguide optical switch with a 1.1 mm long MMI coupler. Figure 5 shows the measured lateral intensity distribution at a distance of 1.1 mm from the input port for the core thickness of 12 µm and 9 µm, respectively. The lateral intensity peak moves from the bar side to the cross side. We estimate the switching of optical power fraction of about 85% with a switching length of 1.1 mm and a small displacement of air core ΔDcore=3 µm, when we assume that a 10µm wide output waveguide is integrated with the MMI coupler. The measured insertion losses of a 1.1 mm long hollow waveguide with 12 and 9 µm air cores are 5.4 dB and 5.7 dB, respectively. The fabricated MMI switch is polarization dependent since metal-coated mirrors are used. The insertion loss for TM mode decreases to be 2–3 dB.
Further reduction in the size can be expected with reducing the gap of the air core. If we assume the device with an MMI core width of Wcore=3 µm, an initial core thickness of Dcore=3 µm and displacement of ΔD=2 µm, an effective refractive-index change and spot size change in Eq. (1) will increase to be about 10 times or more. Thus, we expect a compact optical switch with a switching length of 100µm.
4. Conclusion
We demonstrated a novel MMI waveguide switch based on a tunable hollow waveguide technology. We carried out the full-vectorial numerical simulation and measurement for an MMI hollow waveguide switch. The result shows a possibility of compact-size waveguide switches with a switching length of 1.1 mm. Further reduction in the size can be expected with reducing the gap of the air core. The monolithic integration of the actuator is also a remaining work for realizing compact multi-channel waveguide switches.
Acknowledgments
The authors acknowledge Professor Emeritus Kenichi Iga of Tokyo Institute of Technology for his encouragement. This work was supported by the Grant-in-Aid for Creative Scientific Research from the Ministry of Education, Science, Sport and Culture (#14GS0212”). The authors acknowledge Dr. A. Matsutani for his support in dry etching process of experiments.
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