Keita Yamaguchi

Proposal and experimental demonstration of SDM node enabling path assignment to arbitrary wavelengths, cores, and directions

We propose a novel simple space division multiplexing (SDM) node which is rearrangeble nonblocking, and effectively utilizes enhanced network resources through SDM. The proposed node can reduce a number of ports of wavelength selective switches (WSSs) and a number of WSS modules by modifying conventional multi-stage switches and employing integrated multiple arrayed WSSs. We experimentally actualized the newly proposed node, and demonstrate wavelength, core, and direction switching functions based on 127-Gbps Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) signals. We also confirm the feasibility of the proposed SDM node through SDM transmission experiments using a 40-km multicore fiber and a multicore amplifier.

Wavelength selective switches for SDM networks

Space division multiplexing (SDM) technology is becoming more attractive because it offers large transmission capacity of a multiple of the spatial channel count. For applications to core and metro networks, switching devices are needed that can handle both wavelength and space channels. An SDM network with uncoupled SDM media such as multi-core fiber (MCF) provides a degree of freedom of spatial switching in addition to wavelength switching. Here we demonstrate wavelength selective devices for an MCF-based SDM network. A wavelength selective switch with core-by-core switching, a wavelength cross-connect for SDM channel interexchange, and a gain equalizer for a multi-core erbium doped fiber amplifier are demonstrated by using the spatial and planar optical circuit platform, which utilizes the advantages of free-space and waveguide optics.

Analysis and suppression of high-order diffractions in liquid-crystal-based spatial light modulator for photonic switch application

Spatial light modulators based on liquid crystal on silicon (LCOS) are widely used for large-scale photonic switches in optical telecom network. For this application, high-order diffractions in LCOS is a critical issue because it causes signal crosstalk. In this paper, we analyze the impact of phase inaccuracy due to the fringing electric field in LCOS on the signal crosstalk in optical switches. We also propose a crosstalk reduction method that is analogous to frequency modulation in signal processing. The method is simple and optimized by only using a few parameters of the applied phase pattern without the need to modify the optics or electronics in use. With the proposed method, the worst crosstalk of a photonic switch was decreased from −16.2 to −31.6 dB.

Elimination of Curvature in Microelectromechanical-System Membrane

The SOI (silicon-on-insulator)-based MEMS (micro electro-mechanical systems) are attracting a great deal of interest from the viewpoints of increasing fabrication accuracy, process simplicity, and device performance [1]. The recent rapid performance advances of MEMS devices, which frequently require nanometer-scale control of movable-membrane shape, have focused attention on the problem of the membrane deformation in microfabrication [2]. In optical MEMS devices, which frequently employ a free-standing membrane structure to reflect or diffract light, the out-of-plane deformation sensitively causes critical degradation of optical characteristics, such as coupling loss and crosstalk. Therefore, the deformation must be small in comparison to the optical wavelength of interest to avoid compromising device performance. This paper presents a process in which the curvature in the membrane structure can be eliminated by annealing and describes the mechanism involved. In MEMS device fabrication, oxygen plasma exposure is often used to ash off an organic sacrificial layer and clean the surface. However, the process often cause the oxidization of the silicon membrane surface, which results in unexpected movement of the membrane due to the charge accumulated in the oxidized surface. Therefore, the oxidized surface has to be removed to prevent from such charge drift. Argon plasma exposure is usually used for this purpose, as shown in Fig. 1. In this process, we measured the shape of membrane with 4.5-uf06dm thickness with white-light microscope-based interferometer (NewView TM 200; Zygo). The obtained shape is out-ofplane deformation, which is derived from the compressive stress in the membrane, and the curvature, uf072, was -3.4 m -1 , a value larger than before exposure, -1.0 m -1 . To suppress the curvature enhanced by argon plasma exposure, we examined an annealing treatment. Figure 2 shows the relationship between the curvature and annealing time at 500 °C in nitrogen ambient. The curvature decreased with annealing time, and after a two-hour annealing it saturated,uf020uf072∞ = -0.6 m -1 , which is slightly smaller than that of before argon plasma exposure. This difference might be related to the compressive stress due to the oxide layer. To investigate the difference in the surface state before and after annealing, we carried out a total-reflection X-ray fluorescence (TXRF) analysis. Figure 3 shows that there is argon on silicon surface before annealing and that there is no argon after it. This indicates that argon is implanted into the crystal lattice of silicon by argon plasma exposure and that there is a correlation between the curvature and implanted argon. Figure 4 shows the relationship between implanted argon concentration, obtained from the peak intensity in TXRF spectra, and the curvature change with the saturated one (uf044uf072 = uf072∞ -uf020uf072). It is found that there is a linear relationship between them. These results mean that curvature change of membrane increases in response of the argon concentration, and that the desorption of implanted argon by annealing causes the decrease of membrane curvature. In summary, we clarified the relationship between curvature change in membrane and implanted argon concentration and demonstrated the elimination of curvature by annealing.