Study On MOEMS Optical Passive Devices For Optical Communication Applications

As a key component of optical communication systems, optical passive devices play an important role in optic networks. With the rapid development of the fiber optical communication technologies, optical networks based on WDM technologies have become a trend of the new-generation high-speed broadband communication network. To satisfy the great need for high performance optical passive devices,many technologies have been adopted. Among of them, Micro-Opto-Electro-Mech- anical-System (MOEMS) technology has been proven to be a promising approach due to the advantages of batch fabrication and low cost. In recent years, MOEMS technology has been becoming one of key technologies to develop optical passive devices with small size, low cost and high performance.In recent years, various novel optical passive devices using MOEMS technology have been demonstrated, such as optical switch, variable optical attenuator, optical filter, light modulator and so on. Among of these devices,related researches of MOEMS optical switches and MOEMS VOAs are representative. In this thesis, MOEMS optical switches and MOEMS VOAs are investigated, and the details of design, fabrication and testing are also presented.1. MOEMS optical switchesIn this paper, 1×2 and 2×2 MOEMS optical switches are fabricated, which change input light transmission direction by moving reflective micromirror. The optical switches include the micromirror, actuator, fiber alignment grooves and single mode fibers (SMFs).A novel bulk-silicon micromachining technology,called the twice boron- diffusion/etching (TBDE) process,was developed to fabricate the device. The TBDE process is simple, and all of the structures are formed on an n-type (100) single-crystal-silicon (SCS) wafer by once mask patterning. Furthermore, In order to reduce the IL caused by misalignments between the input fiber and the output fiber, V-grooves used for precise alignment of fibers are integrated and formed simultaneously on the SCS wafer by the TBDE process.The insertion loss of optical switches has been analyzed by principle of SMF connecting loss, the main reasons which cause insertion loss in the cross state are the lateral misalignment, the angular misalignment and the longitudinal separation between two connected optical fibers. The reasons which cause insertion loss on reflective state include the loss of two optical fibers connect, surface roughness, area and thickness of reflective mirror. When the insertion loss is less than 2.5dB, the coupling between two common single mode optical fibers is quite difficult, so doing some special treatment to the endfaces of SMFs are necessary. For 1×2 and 2×2 MEMS optical switches developed here, fiber ends are tapered into conical forms which allow fibers to come closer. Additionally, the insertion loss of optical switches, which use GRIN lenses as the input and output, has been analyzed also.The relationship between driven voltage and structure dimension of the electrostatic comb-type linear actuator is analyzed. The design of a high speed, large displacement and low voltage driven comb-type linear actuator is proposed. The static characteristics and the dynamic characteristics of the actuator are analyzed by using the finite element modeling (FEM) method. The analysis indicates the switching time of the proposed actuator is less than 1ms.Fabricated MOEMS optical switches were tested to determine their optical and mechanical characteristics. The insertion loss of across state and reflective state are <0.8dB and <1.2dB, respectively, the crosstalk is less than -50dB. The driven voltage is 22V, the switching time is less than 1ms, and the lifetime is more than 108 cycles.Based on fabrication of 1×2 and 2×2 MOEMS optical switch, the design scheme of fabricating 1×8,4×4 and 8×8 optical switching matrix are proposed. 1×8 optical switching matrix architecture is a three-stage interconnection network that requires 6 1×2 optical switches. To achieve a rearrangeable non-blocking optical switching matrix, N-stage planar architecture is used to form 4×4 and 8×8 optical switching matrix. 6 2×2 optical switches and 28 2×2 optical switches are used in 4×4 and 8×8 optical switching matrix respectively..2.MOEMS VOAThe fabricated device is a shutter-type MOEMS VOA, in which the attenuation is induced by blocking a portion of light using a shutter. The MOEMS VOA includes shutter, fiber alignment grooves and actuator and SMFs. The device chip is fabricated on an n-type (100) SCS wafer by the TBDE process.The return loss (RL), the wavelength dependent loss (WDL) and the polarization dependent loss (PDL) of the shutter-type M0EMS VOA are analyzed respectively. To achieve high RL and low WDL, angled polishing is used, and the endfaces of SMFs are cleaved and polished to form an oblique angle of 8°. The attenuation model of the shutter-type MOEMS VOA is proposed. Based on this model, the relationship between the attenuation and the position of the shutter is analyzed.The relationship between driven voltage and structure dimension of the electrostatic comb-type linear actuator and the relationship between the attenuation accuracy and dimension of the moveable structure are analyzed. Based on these analyses, the structure of the electrostatic comb-type linear actuator is optimized by using the FEM method.Performance measurements showed that the IL, the attenuation range, the RL, the WDL and the PDL were <0.5dB, >50dB, >43dB, <0.1dB and <0.25dB respectively. The operating voltage was less than 30V to obtain a large attenuation range of >50dB, and the response time was less than 1ms.In this research,a number of key technologies have been broken through. Most of them are common technologies such as fiber coupling,chip processing, device packaging and parameter testing, which could be used to develop other MOEMS optical passive devices. Based on these breakthrough, many devices have been fabricated, including 1×2 MOEMS optical switch, 2×2 MOEMS optical switch, 1×8 MOEMS optical switching matrix, 4×4 MOEMS optical switching matrix, 8×8 MOEMS optical switching matrix and MOEMS VOA. 1×2, 2×2MOEMS optical switches and MOEMS VOAs have reached international advanced level. Compared with other MEMS micromachining technologies such as the surface micromachining technology, SOI technique and LIGA technology, the TBDE process developed here is simple, low cost and high precision. It can not only make process simple, increase fabrication precision, and decrease light loss, but also increase finished product ratio, and decrease fabrication cost.