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Optical Switch
Published in Kenichi Iga, Yasuo Kokubun, Encyclopedic Handbook of Integrated Optics, 2018
Grating Light Valve (GLV) that uses a diffraction grating composed of six silicon nitride movable ribbons is used for switches as well as projectors [4]. As shown in Figure 2(a), six ribbons of 3 μm width, 100 μm long, and 150 nm thick correspond to one DMD micromirror described later. The ribbons are arranged at 4 μm intervals and displaced up and down alternately to change the light direction (same Figure 2[b]). When all ribbons are placed on the same plane, the reflected light returns along the incident light path. When the movable ribbons in every other position are pulled down with electrostatic force, the reflected light intensity decreases gradually and the diffracted light intensity increases. When the movable ribbons lower by λ/4, the diffracted light is the most intense and the reflected light is the least intense. In addition, the MARS switch [3,5] moves the silicon nitride membrane up and down to vary the gap in the Fabry–Perot etalon and to vary the intensity of reflected and transmitted light accordingly (Figure 1)[6]. This switch is suitable for use as an on/off switch, and the switching time is approximately 40 nsec. Also, this switch is used as a variable attenuator by converting the displacement to an analog value. An example where a DMD mirror [7] is inserted between two input/output fibers for application as a switch is shown in Figure 3.
MEMS gratings and their applications
Published in International Journal of Optomechatronics, 2021
Guangcan Zhou, Zi Heng Lim, Yi Qi, Fook Siong Chau, Guangya Zhou
The surface profile of a grating within one period can be changed using MEMS actuators, thereby varying the grating’s diffraction efficiency and redistributing light energy among different diffraction orders. These MEMS grating with tunable grating profiles can be used in various light modulation and sensing applications. In the field of MEMS gratings with variable profile, vertical tuning is often used, such as the grating light valve (GLV) device.[94,95] The electrostatic actuation is the most commonly adopted actuation method to construct a GLV device where each reflective beam, also called “ribbon”, is suspended over its underlying electrode with a small air gap, as depicted in Figure 11(a). All movable ribbons reflect the incident light at the initial state, while alternating ribbons can be pulled down through electrostatic force with a moving distance of approximately one-quarter wavelength to generate diffraction effect on the incident light. Thus, GLV devices achieve two different modulation states to commonly serve as the light modulator, and the performance of such devices is primarily dependent on the design of ribbon structures, such as its length, width and shape.[97,98] The movable ribbon may not maintain its flat profile due to the bending of reflective ribbons during the modulation process, hence significantly affecting the grating efficiency. Then some research works have been conducted to optimize the beam parameters to reduce this effect induced by the non-flat deflection,[99,100] and the shape of the beam could also be modified to achieve better tuning behavior.[101]Figure 11(b) indicates a novel device design where the micromirrors are suspended with soft flexures that can be pulled towards the substrate when a voltage is supplied to the flexures and the underlying electrode.[96] With the linkage arm, the mechanical coupling between the mirror and the flexures could change the tuning behavior of micromirrors in traditional GLV devices to avoid the micromirror bending during the actuation process. The reported device was designed with 700 µm length and 50 µm width, and the results showed negligible crosstalk and bowing to be 0.14 µm over the total length. The GLV device has also been fabricated with other materials, such as poly-SiGe, by taking advantage of CMOS-integrated MEMS technology.[102] Such GLV device could achieve good tuning behavior under a 16 V driving voltage, and the resonant frequency was reported to be 90 kHz for the ribbon with 50 µm width. In Ref.[103,104], the GLV device was constructed with dielectric film with high permittivity and a field-controlled air gap to reduce the required driving voltage.