A novel liquid crystal on silicon (LCOS)-based wavelength selective switch (WSS) is proposed, fabricated, and demonstrated. It employs a multilayered arrayed waveguide grating (AWG) as a wavelength multiplex/demultiplexer. The LCOS deflects spectrally decomposed beams channel by channel and switches them to desired waveguide layers of the multilayered AWG. In order to obtain the multilayered AWG with high yield, phase errors of the AWG is externally compensated for by an additional phase modulation with the LCOS. This additional phase modulation is applied to the equivalent image of the facet of the AWG, which is projected by a relay lens. In our previously-reported WSS configuration, somewhat large footprint and increased cost were the drawbacks, since two LCOSs were required: one LCOS was driven for the inter-port switching operation, and the other was for the phase-error compensation. In the newly proposed configuration, on the other hand, both switching and compensation operations are performed using a single LCOS. This reduction of the component count is realized by introducing the folded configuration with a reflector. The volume of the WSS optics is 80 × 100 × 60 mm3, which is approximately 40% smaller than the previous configuration. The polarization-dependent loss and inter-channel crosstalk are less than 1.5 dB and -21.0 dB, respectively. An error-free transmission of 40-Gbit/s NRZ-OOK signal through the WSS is successfully demonstrated.
A wavelength cross-connect switch (WXC) is proposed and demonstrated. The cross-connect optics have orthogonal imaging systems that operate differently in the switching and spectral planes. The switching plane has 2f Fourier optics with a Rayleigh length. On the other hand, the spectral plane has 4f imaging optics. Two types of switching engines, microelectromechanical system (MEMS) mirrors and liquid crystal on silicon (LCOS), are applied for the same cross-connect optics. The 5×5 WXC with MEMS mirrors has a 100 GHz channel spacing, which is compatible with the International Telecommunication Union (ITU) grid. On the other hand, the WXC with LCOS has a variable channel spacing. The characteristics of two types of WXC are compared. In addition, the port count, which is one of the important parameters, is discussed.
Abstract:We propose a novel method to compensate for the phase errors of a multilayered arrayed waveguide grating (AWG) used in a liquid-crystal-on-silicon (LCOS)-based wavelength selective switch (WSS). In this scheme, an additional LCOS is employed to externally compensate for the phase errors of the AWG in a layer-by-layer manner for both planes of polarization. This compensation scheme enables us to improve the yield of the WSS: specifically, we demonstrate that the additional LCOS drastically reduces the loss and polarizationdependent loss (PDL) of the WSS.
Abstract:We propose a novel method to efficiently correct the aberration in a free-space optical switch, where a spatial light modulator (SLM) is used to control wavefronts. A particle swarm optimization (PSO) method is applied to finding the optimum set of Zernike modes to compensate the aberration. We find out that the obtained coefficients of the lower-order modes and those of the higher-order ones exhibit a linear relationship when the optimization flow is interrupted by the local optima. We can drastically reduce the time for the calibration by using this relationship among the Zernike coefficients.
Abstract:We propose a microelectromechanical system (MEMS) mirror design, which is suitable for use in a flexible-grid wavelength selective switch (WSS). In the flexible grid operation with a conventional MEMS-based WSS, the diffraction loss at the inter-mirror gap regions caused a large ripple in the transmission spectra. On the other hand, the newly introduced slot structures in each mirror can reduce the ripples by intentionally attenuating the light on the mirrors to equalize the reflectance of the entire mirrors. We optimize the slot structure aiming for the flexible-grid WSS operation with a bandwidth-granularity of 12.5 GHz. The ripples are successfully reduced to below 0.004 dB with a loss penalty of only 1.05 dB.
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