I. Introduction
The advent and development of optical fiber communication technology has brought revolutionary changes to the communications industry. At present, about 85% of the world's communication services are transmitted through optical fibers, and long-distance trunk networks and local relay networks have also widely used optical fibers. An intelligent network architecture-automatic switched optical network (ASON: automaTIc switched opTIcal networks) has become a hotspot in today's system research. Its core node is composed of optical cross-connect (OXC: opTIcal cross connect) equipment. Dynamic wavelength routing and flexible and effective management of optical networks. Optical cross interconnect (OXC) technology is one of the key technologies in the increasingly complex DWDM network, and the optical switch as a functional device that switches the optical path is a key part of OXC. The optical switch matrix is ​​the core part of OXC. It can realize functions such as dynamic optical path management, optical network fault protection, and wavelength dynamic allocation. Is of great significance.
Optical switch is not only the core device in OXC, it is also widely used in the following fields.
(1) Optical network protection switching system, the actual optical cable transmission system has spare optical fibers. When the transmission of the working channel is interrupted or the performance is degraded to a certain degree, the optical switch automatically transfers the main signal to the backup optical fiber system for transmission, so that the receiving end It can receive normal signals without feeling that the network has failed, and it will connect the network nodes into a ring to further improve the survivability of the network.
(2) A real-time monitoring system for network performance. At the remote fiber test point, multiple optical fibers are connected to the optical time domain reflectometer through a 1 & TImes; N multiplex optical switch to perform real-time network monitoring, and the optical switch switching sequence is controlled by a computer Time and time to realize the detection of all optical fibers, and transmit the detection results to the network control center. Once a problem is found on a certain path, it can be directly handled in the network management center.
(3) Optical switches are also used in optical fiber communication device test systems and differential / multiplexing 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 switch technology has become the key technology of optical networking and optical switching in the future, playing an increasingly important role in the fields of communication and automatic control.
Among many types of optical switches, micromechanical (MEMS) optical switches are considered to be the most likely to become the mainstream device of optical switches. Based on an overview of the characteristics of various optical switches, this paper focuses on the analysis of several major MEMS optical switches, and describes their structure and performance characteristics.
Second, the principle and type of optical switch
There are a variety of optical switch performance parameters, such as: fast switching speed, high isolation, small insertion loss, polarization insensitivity and reliability, and its requirements in different fields are also different. Its types include traditional optomechanical switches commonly used in protection and switching systems, as well as new types of optical switches that have developed rapidly in recent years, such as: thermo-optic switches, liquid crystal switches, electro-optic switches, acousto-optic switches, micro-optical electromechanical system optical switches (MOEMS , Micro optic electro mechanical systems), bubble switch, etc. In the field of ultra-high-speed optical communication, there are also optical control switches such as Maeh-Zehnder interference type optical switches and nonlinear optical fiber loop mirror (NOLM) optical switches.
1. Mechanical optical switch
The working principle of the traditional mechanical optical switch: through heat, static electricity and other power, the micro-mirror is rotated to send or reflect the light directly to the output end. The characteristic is that the switching speed is relatively slow, the cost performance is good, and it has market prospects in many fields, but the disadvantages of large size and difficulty in scale integration limit its application in the future optical communication field. 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 technologies. Especially the data format with optical signals is transparent, independent of polarization, and has small loss. The advantages of good reliability, fast speed and easy integration.
2. Electro-optic effect switch
Electro-optic effect optical switches are mostly made of photoelectric crystal materials (such as LiNbO3 or other semiconductor materials) waveguide materials. The two waveguide paths are connected into an MZ interference structure. The applied voltage can change the refractive index of the waveguide material, thereby controlling the phase difference of the two arms The interference effect is used to realize the on-off of light. It is characterized by high speed, but it is related to polarization and has a relatively high cost. The working principle is shown in Figure 1.
Figure 1 Electro-optical effect optical switch based on Mach-Zehnder structure
For a 3dB coupler, the two light waves satisfy the mode coupling equation, making the propagation constants of the two optical waveguides equal, B0 = 0, obtained at the output 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.
It can be seen from the above formula that Ф is related to the applied voltage. If the voltage is changed, Ф changes, so that the light intensity is tuned. The switching speed depends on the time when the phase difference occurs between the two paths, that is, the refractive index change time in the optical waveguide.
In the stage of modern communication system developing towards high speed and intelligence, in order to solve the contradiction of slow response time of electronic switches and inability to match ultra-high-speed transmission data, to achieve faster switching speed and lower insertion loss, quartz can also be used The method of self-phase modulation or cross-phase modulation effect of optical fiber and semiconductor optical amplifier to change the refractive index, that is, optical control optical switch technology.
3. Light control switch
Now more mature models are: all-optical switches based on 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. It is a promising all-optical switching technology. In general, all kinds of ultra-fast all-optical switches are ultimately inseparable from the nonlinear effects of light. Here we take SOA-XPM as an example to illustrate, the experimental principle is shown in Figure 2.
Fig. 2 Experimental device for realizing light opening using SOA-XPM
Put the SOA on the two arms of the MZ interferometer, and the switch control pulse is injected into one arm. The change of the pulse will cause the refractive index of the SOA to change, which will cause the phase difference of the two arms to change.
△ Ф =-(2π / l) (dn / dN) (τe / [1+ (wτe) 2] 1 / 2L × Vg × g × △ S × cos (wτ-q)
Among them, l——signal wavelength; dn / dN—refractive index changes with carrier density; L—SOA cavity length; τe—carrier lifetime; Vg—group velocity; g—gain factor; △ S— Carrier density change amplitude; q-phase delay between carrier density change and modulation signal.
When △ Ф = 0, π, the output ends of the two arms are turned on and off. Because the switching speed of SOA can reach picoseconds, 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.
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