MEMS Optical Cross Connect Switch (OXC)

Optical cross-connect switch (OXC) is a matrix optical switch with NxN ports. It can be used to build a CDC ROADM system (colorless, directionless, contentionless reconfigurable optical add/drop multiplexer).

OXC can be built with a 1xN port optical switch. In order to build NxN port OXC module, it needs 2N 1xN port optical switches. The size and cost of the OXC module increase dramatically when the number of port increases, so the port number of this OXC is usually limited to 32x32 ports.

4x4 OXC module built with 8 1x4 switches

2D MEMS optical switch

The second technical solution to realize OXC is a Cross-Bar optical switch based on a MEMS mirror array. It has 2 input ports and 2 output ports. The optical path switching is achieved through 4 MEMS mirrors. Every mirror has two states, lying horizontally on the substrate to allow the beam to pass through (OFF) or standing upright on the substrate to reflect the beam (ON).

SEM photos of the MEMS chip and a single mirror, as well as MEMS torsion mirror. The mirror is supported by a polysilicon beam. When there is no voltage applied to the electrode, the mirror remains horizontally. When powered on, the mirror stands upright on the substrate driven by electrostatic force.

Matrix optical switches based on 2D MEMS technology require a NxN scale mirror array to make a NxN optical switch. All optical paths of the device are in one plane, that is why it is called a 2D MEMS optical switch.

First Generation 2D MEMS matrix switch

The switching of the optical path is achieved through a mirror. The mirror is hinged on the base. One end of the two pull rods is connected to the mirror, and the other end is connected to a translation stage. The translation stage is driven by a scraper-type actuator to pull the mirror forward. The mirrors rotates as they are pulled.

 

2D MEMS Mirror Array

Matrix optical switches based on 2D MEMS mirror arrays have the advantages of simple structure and easy packaging, but their scalability is limited. For different port link relationships, the optical path lengths vary greatly, which will introduce coupling losses and loss uniformity. The tolerance for the optical path difference depends on the beam size in the free-space optical structure. According to equation (1), the smaller the spot ω0 is, the more divergent it is. According to equation (2), the shorter the collimation distance is.

 

The coupling situation between two single-mode fiber SMFs increases sharply as the spacing between the fiber end faces increases. The spacing between two single-mode fibers is usually limited to <20μm. To increase fiber spacing to allow the placement of various free space optical components, thermal expanded core (TEC) fibers or lens fibers are usually used. Both TEC fiber and lens fiber can expand the spot size to be suitable for free space light transmission. The distance between two TEC fibers can be up to 10mm, while the distance between two lens fibers can be up to 50mm. For applications that require longer free space optical paths, collimating lenses are often required.

Applying TEC fiber or lens fiber to 2D MEMS optical switches helps to increase the free space optical path length to accommodate more MEMS mirrors and expand the optical switch port. However, the maximum allowed spot size is limited by the size of the mirror, which depends on the MEMS design and process. Mirror diameterФ>3ω0 is usually required to reflect more than 99% of the optical power. Therefore, the maximum port number of 2D MEMS optical switches is usually limited to 32×32.

OXC based on 3D MEMS technology

In order to further develop OXC system, 3D MEMS optical switches were developed. The basic structure of 3D MEMS OXC includes two MEMS mirror arrays and two two-dimensional fiber collimator arrays. Each input fiber collimator corresponds to a mirror in the first MEMS chip, and each output fiber collimator The straightener corresponds to a mirror in the second MEMS chip. All mirrors on the MEMS chip can deflect in two axes.

The beam from each input port is independently controlled by a mirror on the first MEMS chip and directed through biaxial deflection to another mirror on the second MEMS chip (corresponds to the output port). A mirror adjusts the direction of the reflected beam to point to the output through biaxial deflection. Therefore, through the control of two MEMS chips, optical signals can be exchanged from any input port to any output.

The OXC developed by Bell Labs includes two MEMS mirror arrays, two 2D fiber arrays and a Fourier lens. Each input-output link is built through a mirror on the first MEMS chip and another mirror in the second MEMS chip.

The new generation 3D MEMS OXC from NTT Lab replaces Fourier lenses with a circular concave reflector. The use of a circular concave mirror can reduce the off axis aberration of the edge port to reduce insertion loss.

What’s a Fiber Optic Switch?

Fiber Optic Switch is a device with one or more selected transmission windows that can perform mutual conversion or logical operation on optical signals in optical transmission lines or integrated optical circuits. The basic form of optical switch is 2x2, that is, every input port and output port have two optical fibers, which can complete two connection states, parallel connection and cross connection. The large space optical switch unit can be composed of the combination of a basic 2x2 and 1x2 fiber optic switch.

 

Optical switches play an important role in optical networks. In Wavelength Division Multiplexing (WDM) transmission systems, optical switches can be used for wavelength driving, regeneration and clock extraction. In Optical Time Division Multiplex (OTDM) system, optical switches can be used for demultiplexing; in all-optical switching systems, optical switches are key components of Optical Cross-connect (OXC), and are also important components for wavelength conversion. The number of input and output ports of the switch can be divided into 1×1, 1×2, 1×N, 2×2, 2×N, M×N, etc. They have different uses in different occasions. They can be widely used in protection switching system of optical network, light source control in optical fiber testing, real-time monitoring system of network performance, testing of optical devices, construction of switching core of OXC equipment, optical add/drop multiplexing, optical testing, optical Sensing systems, etc.

 

Main Types of Fiber Optic Switches

At present, the most widely used ones are still 1×2 and 2×2 mechanical optical switches. Traditional opto-mechanical optic switches can directly couple light to the output end through moving optical fibers, use prisms and reflectors to switch light paths, and send or reflect light directly to the output end.

There are three main types of mechanical optical switches: one uses prism switching light path technology, the other uses mirror switching technology, and the third uses moving optical fiber to switch the light path. The optical fiber is connected to the lens (collimator) that plays a collimating role and is fixed. The optical path between the input and port output is changed by moving the prism. When the reflector does not enter the light path, the optical switch is in a straight-through state. The light entering from fiber 1 enters fiber 4, and the light entering from fiber 2 enters fiber 3. When the reflector is at the intersection of the two light rays, the optical switch is in the intersection state. , the light entering fiber 1 enters fiber 3, and the light entering fiber 2 enters fiber 4 to achieve optical path switching. The mobile optical fiber optical switch is an optical fiber with fixed ends. The device at the other end of the mobile device is connected to different ports of the fixed device to realize switching of optical paths. This type of optical switch has low return loss and is greatly affected by ambient temperature. There is no real switching product.

The advantages of mechanical optical switches are low insertion loss (<1dB), high isolation (>45dB), independent of wavelength and detour, and mature production technology. Faced with the total switching action time (ms), the size is relatively large, and it is not suitable for large-scale foreign optical switch matrices, and sometimes there are problems of rebound and poor repeatability. Mechanical optical switches have been widely used in recent years. However, as the scale of optical networks continues to expand, this type of switch is difficult to adapt to the future development needs of high-speed and large-capacity optical transmission networks.

 

Micro-electro-mechanical System (MEMS) Optical Switches

Microelectronic mechanical optical switches have developed rapidly in recent years. They are a new type of micro-electro-optical integrated switch produced by combining semiconductor micro-processing technology with micro-optical and micro-mechanical technologies. It is a new type of switch for large-capacity switching optical networks. The mainstream direction of switch development.

 

MEMS(Micro Electro-Mechanical System) optical switches are carved into a number of tiny lenses on a silicon crystal. Through the action of electrostatic force or electromagnetic force, the movable mirrors can be raised, lowered, rotated or moved, thereby changing the propagation direction of the input light to realize the function of optical path on/off. MEMS optical switches have obvious advantages over other optical switches. The switching time is measured in microseconds. MEMS fiber optic switch adopts IC manufacturing technology, is small in size and highly integrated. The working method has nothing with the format, protocol, wavelength, transmission direction, matrix direction, and modulation of the optical signal. It can process optical signals of any wavelength. Besides, it has the advantages of low insertion loss, low crosstalk, low polarization sensitivity, high extinction ratio, high switching speed, small size, and easy large-scale integration.

 

According to functions, MEMS optical switches can be divided into optical path bias type, moving fiber contact type and mirror reflection type. Mirror reflection type MEMS optical switches are easy to integrate and control, and can easily form an optical switch array. They are the focus of MEMS optical switch research. They can be divided into 2D MEMS optical switches and 3D MEMS optical switches. The concept of 1D MEMS fiber optic switches is also proposed. The so-called 2D means that the movable mirror and fiber are located on the same plane, and the mirror is either on or off at any specified moment. In this mode, the mirror array is connected to N input fibers and M output fibers. The number of mirrors required for an N×N matrix optical switch is N². Therefore this method is also called N² structure scheme.

Optical Transceiver Module Industry Chain

Optical transceiver module consists of transmitting unit, transmission unit and receiving unit. The transmitting unit inputs an electrical signal of a certain power. After being processed by the internal driver chip, the driving laser or light-emitting diode emits a modulated optical signal of corresponding power and transmits it through the optical fiber. The receiving end converts the optical signal into an electrical signal by a light detection diode, and output an electrical signal of corresponding power after passing through the pre-amplifier.


The upstream of the optical transceiver module industry includes optical chips, optical devices, electrical chips, etc. The downstream of the optical transceiver module industry mainly includes telecom operators, Internet and cloud computing companies, etc. The application fields of optical transceiver module products cover industries such as Internet services and the telecom market.


The optical transceiver module follows the packaging sequence of chip-assembly (OSA)-module. The laser chip and detector chip are formed into TOSA and ROSA through traditional TO packaging. At the same time, the supporting electrical chip is mounted on the PCB, and then the optical channel and optical fiber are connected through precision coupling, and finally packaged into a complete optical transceiver module. The emerging COB, which is mainly used in short-distance multi-mode, uses a hybrid integration method. The chip is mounted on the PCB through a special bonding and welding process and uses non-hermetic packaging. The upstream of optical transceiver modules is mainly optical chips and passive optical devices, and the downstream customers are mainly telecom main equipment manufacturers, operators and Internet & cloud computing companies. Therefore, optical transceiver modules have a broad and growing market space.


Optical transceiver modules can be divided into QSFP, QSFP28, CFP, CFP2, CFP4, CXP, SFP, CSFP, SFP+, GBIC, XFP, XENPAK, X2, SFF and other types according to the packaging method. Among them, SFP and QSFP have the advantages of high performance and low power consumption, and have become products currently used on a large scale.


Optical transceiver module rates can be divided into 1Gbps, 2.5Gbps, 10Gbps, 25Gbps, 40Gbps, 50Gbps, 100Gbps, 200Gbps, 400Gbps, 800Gbps, etc. According to market stratification by speed, low-speed optical transceiver modules have the greatest demand and can be used for broadband users, servers, and enterprise network access; higher-speed optical transceiver modules have smaller demand and are mainly used for long-distance communications by operators and data centers. The current growing demand for 100G/400G high data rate optical transceiver modules will synergistically drive the growth of the QSFP, QSFP-DD, and OSFP optical transceiver module markets.


According to the fiber access type, optical transceiver modules can be divided into single-mode optical transceiver modules and multi-mode optical transceiver modules. The transmission distance of multi-mode optical transceiver modules is shorter, usually between 500 meters and 2,000 meters, while the transmission distance of single-mode optical transceiver modules is longer, usually between 10 kilometers and 160 kilometers.


With the rapid development of the information technology industry and the rapid growth of data traffic, the performance index requirements for optical transceiver modules are getting higher and higher. Data rate, transmission distance, power consumption, and volume have become important considerations. At present, optoelectronic components are constantly improving in the direction of high speed, wide spectrum, small loss, low power consumption, high sensitivity, short delay, weak nonlinearity, high integration, small size, low price, and silicon photonics.


The current main application markets of optical transceiver modules include the data communication market, telecommunications market and emerging markets. The data communication market is the fastest growing market for optical transceiver modules. It has surpassed the telecommunications market to become the largest market and is the mainstream growth of the optical transceiver module industry in the future. The telecommunications market is the first market for optical transceiver modules. 5G construction will significantly boost the demand for optical transceiver modules for telecommunications. While emerging markets include consumer electronics, autonomous driving, industrial automation and other markets, which are the markets with the greatest potential for future development.


The downstream optical transceiver modules are mainly used in three major scenarios: telecommunications bearer network, access network, data center and Ethernet. Telecom bearer networks and access networks both belong to the telecom operator market. Wavelength division multiplexing (xWDM) optical transceiver modules are mainly used in medium and long-distance telecom bearer networks, and optical interconnects are mainly used in long-distance and long-distance backbone core networks. Capacity transmission, and the access network market is the "last mile" from operators to users, including fiber-to-the-home passive optical networks (FTTH PON), wireless fronthaul (Wireless) and other application scenarios. The data center and Ethernet market mainly includes data center internal interconnection, data center interconnection (DCI), enterprise Ethernet and other scenarios.


Telecommunications Market:Mainly used in base stations/PON/WDM/OTN/switches/routers and other equipment.


Data center market (data communications market):Main application servers/top-of-rack switches/core switches and other equipment.

OBP 光バイパス保護システムのアプリケーション

光ケーブル リソースによって制限される基地局の大規模かつ迅速な展開に伴い、基地局ネットワークがチェーン構造または大規模なリング構造を持つことが非常に一般的です。 このようなネットワーク構造では、ひとたびリンク切断や多点オープンループ事故が発生すると、広範囲にわたるサイト停止につながります。 このため、停電、伝送設備の故障、局アクセス困難等による大規模な局停止をいかに回避するかが喫緊の課題となっている。

 

光バイパス保護システムOBPは、基地局の送信装置の電力状態と発光状態をリアルタイムで監視することができ、停電や送信装置の故障による大規模な局停止を効果的に回避でき、信頼性が高いという利点があります。 、スイッチング速度が速く、コストが低いため、シンプルで効果的なソリューションです。

 

光バイパス保護技術の紹介

 

上図に示すように、光バイパス保護システムは、光ファイバ通信の分野で適用される光スイッチング システムの一種で、障害のあるネットワーク ノードを自動的にバイパスすることができ、ネットワーク ノードの電源状態と障害を自動的に識別できます。 光信号の出力状態を監視し、ローカルの光機器に障害(停電、ハードウェアまたはソフトウェアの障害など)が発生した場合には、即座にバイパス光パスに切り替え、通信回線がローカル機器をバイパスします(つまり、ローカル光機器の障害)。 、障害のあるノード)を使用して、障害のあるノードによって引き起こされる完全なブロッキング障害を回避し、システムの接続を確保します。

 

OBP は、保護対象機器の電源状態と保護対象機器の光パワーの両方をリアルタイムで検出できる内蔵の双安定光スイッチによってメインおよびバックアップ ルートの切り替えを完了します。動作原理は次のとおりです。
(1) 保護対象機器に電力が供給され、正常に放射されている場合、光路は正常な状態にあります。
(2) 保護された機器の電源がオフで、光パワーが事前に設定されたしきい値より低いことが検出されると、OBP は自動的にファイバーパススルー状態に切り替わります。
(3) OBPバイスのみの電源がオフで、保護されるデバイスが正常に動作している場合、OBP は内部の高安定コンデンサの助けを借りて、保護されるデバイスの光パワーを即座に検出します。 光パワーが正常であれば、切り替え動作は行われません。

 

結論

OBP 装置の光パワー検出は正確で、挿入損失は小さいです。 基地局の停電時には即座にバイパス状態に切り替わり、ローカル伝送装置をバイパスすることができます。 基地局の電源が回復すると、伝送装置が完全に起動した後、即座にメイン モードにカットバックでき、バイパスおよびカットバックでネットワーク上の他のネットワーク要素が正常に動作することを保証します。 OBP 装置の保護機能を備えているため、リンクが多すぎる場合や障害が頻繁に発生する基地局に大量に適用できるため、リンクが多すぎる場合や障害が頻繁に発生する多数の基地局で使用することができます。 サービスの正常な運用を適切に保護するために、メンテナンスのために基地局に入ることが困難な場所。

What's the Differences Between Single Mode and Multimode Fiber?

Technical Difference
Core Diameter
Single-mode fiber has a small core diameter (8.3 to 10 microns), allowing only one mode of light to propagate. Multimode fiber optic cables have large diameter cores (50 to 100 microns) that allow multiple modes of light to propagate.


Light Source
Multimode devices typically use LEDs or lasers as the light source, while singlemode devices use lasers or laser diodes to generate the light injected into the cable.


Main Differences
Distance
Light travels longer in single-mode cables than in multimode cables, so multimode fiber is suitable for short-distance applications, up to about 550m at 10Git/s. When the distance exceeds 550m, single-mode fiber is preferred.


Price
Multimode fiber usually costs less than singlemode fiber.


Bandwidth
Singlemode has higher bandwidth than multimode, up to 100,000 GHz.


Multimode Fiber Connector Types
The types of multimode fiber optic connectors in circulation include ST, SC, FC, LC, MU, E2000, MTRJ, SMA, DIN, and MTP&MPO, etc. The most commonly used types of fiber optic connectors include ST, SC, FC, and LC.

MMF Connector

Ferrule Size

Typical Insertion Loss (dB)

Application Features

SC

φ2.5mm ceramic

0.25-0.5

Mainstream, reliable, fast deployment, filed fit

LC

φ1.25mm ceramic

0.25-0.5

High density, cost-effective,filed fit

FC

φ2.5mm ceramic

0.25-0.5

High precision, vibration environment, field fit

ST

φ2.5mm ceramic

0.25-0.5

Military, filed fit

What are the advantages of multimode fiber?
While single-mode fiber patch cables offer advantages in terms of bandwidth and transmission distance, multimode fiber can easily support most of the distances required by enterprise and data center networks at a much lower cost than single-mode fiber. In addition, multimode fiber optic cables have many significant advantages.


Multi-user framework without lossy interference
The characteristic of multimode fiber is that it can carry multiple signals simultaneously in the same line, and most importantly, there is almost no loss of total power inside the signal.
Thus, a network user can send multiple data packets down the cable at the same time, and all information will be delivered to its destination without any interference and remain unchanged.


Support Multiple Protocols
Multimode fiber can support a variety of data transmission protocols, including Ethernet, Infiniband, and Internet Protocol. As a result, one can use the cable as a backbone for a range of high-value applications.


Cost-effective
With larger cores and good alignment tolerances, multimode fibers and components are less expensive, easier to use with other optical components such as fiber optic connectors and fiber optic adapters, and the operation, installation and maintenance of multimode patch cords Costs less than single-mode fiber optic cables.


Conclusion
Due to its high capacity and reliability, multimode fiber is commonly used in backbone applications in buildings, and in general, MMF cable remains the most cost-effective option for enterprise and data center applications up to a range of 500-600 meters.


But this is not to say that we can replace single-mode optical fiber with multi-mode optical cable. As for choosing single-mode optical fiber jumper or multi-mode jumper, it all depends on the application, transmission distance and coverage you need. Total budget allowed.

Application Scenarios of Optical Switches

An optical switch is a device with one or more selectable transmission windows, which can perform mutual conversion or logical operation on optical signals in optical transmission lines or integrated optical circuits. The basic form of the optical switch is 2×2, that is, there are two optical fibers at the input end and the output end, and two connection states can be completed, parallel connection and cross connection. Larger space-splitting optical switching units can be composed of basic 2×2 optical switches and corresponding 1×2 optical switches cascaded and combined.


Optical switches play a very important role in optical networks. In Wavelength Division Multiplexing (WDM) transmission systems, optical switches can be used for wavelength adaptation, regeneration and clock extraction. In Optical Time Division Multiplexing (OTDM) system, an optical switch can be used for demultiplexing. In an all-optical switching system, an optical switch is a key device for an Optical Cross-connect (OXC) and an important device for wavelength conversion. According to the number of input and output ports of the optical switch, it can be divided into 1×1, 1×2, 1×N, 2×2, 2×N, M×N, etc. They have different uses in different occasions.


1×1 optical switch has the function of making the optical path on and off, and is usually used to block the light transmission in the optical path.


1×2 optical switch has a protection switching function and is usually used for network fault recovery. When the fiber breaks or other transmission failures occur, the optical switch is used to realize the detour route of the signal, and switch from the main route to the backup route.


2×2 optical switch is the most commonly used one in the optical switch series, widely used in FDDI, optical node bypass, loop test sensing system, etc., It also can be used in combination with other types of optical switches to make the switching system better and more flexible.


Network Monitoring
When it is necessary to monitor the network, it just needs to connect the multi-fiber optical switch to the network monitoring instrument (such as OTDR) at the remote monitoring point. When the optical path needs to be monitored, use the optical switch to switch each optical fiber in a cycle. Every fiber will be tested by the light source to realize online monitoring. The optical switch mainly plays the role of testing in the optical cable monitoring system. Using a 1×N optical switch can connect multiple fibers.


Optical Device Testing
Connect multiple optical devices to be tested through optical fibers and 1×N optical switches, the devices can be tested by monitoring the signal of each channel of the optical switch.


Build the Switching Core of OXC
OXC is mainly used in the backbone network to aggregate and switch services of different subnets. Therefore, it is necessary to switch the services of different ports. At the same time, the use of optical switches enables OXC to dynamically configure and switch services and support protection switching functions, and supports wavelength routing configuration and dynamic routing at the optical layer. Because OXC is mainly used in high-speed and large-capacity dense wavelength division multiplexing optical backbone network, it is required that the optical switch has the characteristics of transparency, high speed, large capacity and multi-granularity switching.

Applications of Optical Switches

The optical switch plays a very important role in the optical network. It not only constitutes the switching core of the key equipment in the wavelength division multiplexing network, but also is a key device in the optical network.


Optical switches have become the research focus of major communication companies and research institutes due to the advantages of high speed, high stability, and low crosstalk. Optical switches have broad market prospects and are one of the most promising optical passive devices. Its application mainly includes:

Switching Function: Optical switches are usually used for network fault recovery. When a fiber break or other transmission failure occurs, the optical switch is used to realize the signal transfer route, switching from the main route to the backup route. This protection usually only requires the simplest 1x2 optical switch.


Network Monitoring Function: at the remote fiber test point, connect multiple fibers to an optical time domain reflectometer through a 1xN optical switch, and monitor all fibers through optical switch switching. In addition, the optical switch can also be used to insert a network analyzer into the optical fiber line to realize online network analysis. This optical switch can also be used for fiber optic device testing.


Optical Device Testing: multiple optical devices to be tested can be connected through optical fibers, and through 1xN optical switches, the devices can be tested by monitoring the signal of each channel of the optical switch.


OADM and Optical Cross-connection: optical add/drop multiplexers are mainly used in ring-shaped metropolitan area networks, to realize single wavelength and multiple wavelengths to be added and dropped freely from the optical path without electrolytic multiplexing or multiplexing process. The OADM realized by the optical switch can be dynamically controlled by software to add and drop arbitrary wavelengths, which greatly increases the flexibility of network configuration. OXC is composed of an optical switch matrix, which is mainly used for cross-connection of the core optical network, realizing fault protection of the optical network, dynamic optical path management, and flexible addition of new services.


Due to the wide range of applications of optical switches and many functions, the technology of optical switches is already very advanced. The use of optical switches can protect the optical layer of the network to a certain extent, making the speed of the network very fast. The development of the switch has promoted the survival and development of the network, and greatly improved the efficiency of using the network, so the optical switch has become an indispensable key device in the network.