MEMS optical switch research

　　Optical switches are important functional components of optical communication networks, and MEMS optical switches are one of the most promising optical switches. Based on the introduction of the principles and characteristics of different types of optical switches, the classification, structure, process and performance characteristics of the current main MEMS optical switches are analyzed in detail, and the research and development status of this field is given.

　　I. Introduction

　　The advent and development of optical fiber communication technology has brought revolutionary changes to the communication industry. Currently, about 85% of the world's communication services are transmitted through optical fiber, and optical fiber has also been widely used in long-distance trunk networks and local relay networks. At the same time, the development and maturity of dense wavelength division multiplexing (DWDM) technology has opened up a vast space for full utilization of the bandwidth and capacity of optical fiber transmission. DWDM optical communication networks with obvious advantages of high speed and large bandwidth have become the basis for the development of current communication networks. trend. Especially in recent years, IP-based Internet services have shown explosive growth. This growth trend has not only changed the relationship between the IP network layer and the underlying transmission network, but also put forward the requirements for the networking method, node design, management and control of the entire network. new requirements. An intelligent network architecture - automatically switched optical networks (ASON: automatic switched optical networks) has become a hot topic in system research today. Its core node is composed of optical cross connect (OXC: optical cross connect) equipment. Through OXC, it can be realized Dynamic wavelength routing and flexible and effective management of optical networks. Optical cross-connect (OXC) technology is one of the key technologies in the increasingly complex DWDM network, and optical switches, as functional devices for switching optical paths, are a key part of OXC. The optical switch matrix is ​​the core part of OXC. It can realize dynamic optical path management, optical network fault protection, dynamic wavelength allocation and other functions. It can solve the wavelength contention in the current complex network, improve the wavelength reuse rate, and achieve flexible network configuration and balance. Is of great significance.

　　Optical switches are not only the core devices in OXC, they are also widely used in the following fields.

　　(1) Optical network protection switching system. There are spare optical fibers in actual optical cable transmission systems. When the working channel transmission is interrupted or the performance deteriorates to a certain extent, the optical switch automatically transfers the main signal to the backup optical fiber system for transmission, thereby allowing the receiving end to It can receive normal signals without feeling that the network has failed, and it will connect network nodes into a ring to further improve the survivability of the network.

　　(2) Real-time monitoring system for network performance. At the remote optical fiber test point, multiple optical fibers are connected to the optical time domain reflectometer through a 1×N multi-channel optical switch for real-time network monitoring, and the optical switch switching sequence is controlled by a computer. and time to detect all optical fibers and transmit the detection results back to the network control center. Once a problem is found on a certain path, it can be dealt with directly in the network management center.

　　(3) Optical switches are also used in optical fiber communication device testing systems and differential/demultiplexing and switching equipment in metropolitan area networks and access networks. The introduction of optical switches makes future all-optical networks more flexible, intelligent, and survivable. Optical switching technology has become a key technology for future optical networking and optical switching, playing an increasingly important role in communications, automatic control and other fields.

　　Among the many types of optical switches, micromachined (MEMS) optical switches are considered to be the most likely to become the mainstream device of optical switches. Based on an overview of the principles and characteristics of various optical switches, this article focuses on analyzing several major MEMS optical switches and explains their respective structures and performance characteristics.

　　2. Principles and types of optical switches

　　Optical switches have various performance parameters, such as fast switching speed, high isolation, small insertion loss, insensitivity to polarization and reliability. Different fields have different requirements for it. Its types include traditional optomechanical switches commonly used in protection and switching systems, as well as new optical switches that have developed rapidly in recent years, such as thermo-optical switches, liquid crystal switches, electro-optical switches, acousto-optical switches, and micro-optical electromechanical system optical switches (MOEMS). , micro? optic? electro mechanical systems), bubble switches, etc. In the field of ultra-high-speed optical communications, there are also optical switches such as Mach-Zehnder interference optical switches and nonlinear optical fiber loop mirror (NOLM) optical switches.

　　1. Mechanical optical switch

　　The working principle of traditional mechanical optical switches: through heat, static electricity and other power, the micro-reflector is rotated to directly send or reflect light to the output end. It is characterized by relatively slow switching speed and good cost performance, and has market prospects in many fields. However, its large size and difficulty in large-scale integration limit its application in the field of optical communications in the future. On this basis, the MOEMS optical switch has developed rapidly in recent years. It is a new type of switch that combines micro-electromechanical systems and traditional optical technology. In particular, the data format of the optical signal is transparent, independent of polarization, and has small loss. It has the advantages of good reliability, fast speed and easy integration.

　　2. Electro-optical effect switch

　　Electro-optical effect optical switches are mostly made of photoelectric crystal materials (such as LiNbO3 or other semiconductor materials) waveguide materials. Two waveguide paths are connected to form an MZ interference structure. The applied voltage can change the refractive index of the waveguide material, thereby controlling the phase difference between the two arms. The interference effect is used to achieve the switching of light. It is characterized by fast speed, but is related to polarization and has higher cost. The working principle is shown in Figure 1.

　　　Figure 1 Electro-optical effect optical switch based on Mach-Zehnder structure

　　For the 3dB coupler, the two light waves satisfy the mode coupling equation. Let the propagation constants of the two optical waveguides be equal, B0=0, and get at the output end of the 3dB coupler 2:

　　|A3|2=|A0|2sin2(Ф/2)

　　|B32=|A0|2cos2(Ф/2)

　　In the formula: A0, B0 - input light wave amplitude; A3, B3 - output light wave amplitude; Ф - light wave phase.

　　From the above formula, it can be seen that Ф is related to the applied voltage. If the voltage is changed, Ф will change, so that the light intensity is tuned. The switching speed depends on the time when the phase difference occurs between the two lights, that is, the time when the refractive index changes in the optical waveguide.

　　In the stage when modern communication systems are developing toward high speed and intelligence, in order to solve the problem of slow response time of electronic switches and inability to match ultra-high-speed data transmission, and to achieve faster switching speed and lower insertion loss, quartz can also be used. The method of changing the refractive index by the self-phase modulation or cross-phase modulation effect of optical fibers and semiconductor optical amplifiers is called optically controlled optical switching technology.

　　3. Light control switch

　　The more mature models now include all-optical switches based on the NOLM principle and SOA nonlinear effects (such as XPM: cross phase modulation). They are not only used for ultra-fast switching, but also for all-optical signal regeneration and ultra-fast wavelength conversion. They are currently a promising all-optical switching technology. In general, various ultrafast all-optical switches are ultimately inseparable from the nonlinear effect of light. Here, SOA-XPM is used as an example to illustrate. The experimental principle is shown in Figure 2.

　　　　Figure 2 Experimental device for realizing light switching using SOA-XPM

　　The SOA is placed on the two arms of the MZ interferometer respectively, and the switch controls the pulse injection into one arm. The change of the pulse will cause the change of the refractive index of the SOA, thereby causing the change of the phase difference △Ф of the two arms, that is:

　　△Ф=-(2π/ l)(dn/dN)(τe/[1+(wτe)2]1/2L×Vg×g×△S×cos(wτ-q)

　　Among them, l - signal wavelength; dn/dN - the change of refractive index with carrier density; L - cavity length of SOA; τe - carrier lifetime; Vg - group velocity; g - gain coefficient; △S - Amplitude of carrier density change; q—phase delay between carrier density change and modulation signal.

　　When △Ф=0, π, the output ends of the two arms are switched on and off. Since the switching speed of SOA can reach the picosecond level, it can be used in ultra-high-speed optical fiber communication systems. In addition to SOA, if the two branches of the MZ interferometer are composed of nonlinear optical waveguide materials such as GaAs/AlGaAs, the purpose of switching can also be achieved.