The Applications of MEMS Optical Switches

 MEMS optical switch is based on micro-electro-mechanical system, using optical micromirror or optical micromirror array to change the propagation direction of light beam to realize the switching of optical path. What scenarios can MEMS optical switches be applied to?

MCS（Multicast Optical Switch）
The multicast optical switch (MCS) based on PLC technology and MEMS technology is a key component of the next-generation reconfigurable optical add-drop multiplexing system (ROADM); Every functional unit consists of 1xM Splitters and 1xN MEMS optical switches; provide connections from N add (or drop) ports to M directions.

iODF (Intelligent Optical Distribution Frame)
Through the cascade integration of optical switches, it can be used in iODF to replace the traditional distribution frame in the industry private network.

OXC (Optical Cross Connect)
Through the cascade integration of optical switches, it can be used in small-scale OXCs to meet the needs of industry private networks and key lines in data centers.

Optical Performance Monitoring
Integrated with TOF or OPM, combined with monitoring software, through Time Division Multiplexing TDM, to monitor the signal performance of DWDM channel in the multi-core optical fiber in the optical cable, widely used in optical transmission network optical cable monitoring, ROADM network, DCI, etc.

Optical Cable Monitoring
Integrated with OTDR, combined with monitoring software, through time-division multiplexing OTDR, to monitor the quality status of multi-core optical fibers in optical cables, widely used in the optical cable monitoring of PON network, optical transmission network, enterprise private network, etc.

Fiber Optic Sensing
The main products are 1x4 and 1x8 for fiber optic sensing.

Test Instrumentation and Factory Automation
The test instrument and factory automation markets are relatively small, but they have high added value and have high requirements on the optical performance of optical switches, such as insertion loss, return loss, and repeatability.

DWDM System
Channel power equalization, link node power attenuation, optical receiver protection, and fast control of optical line on/off.

Basic Knowledge of Optical Transceiver Modules

Definition
Optical module is an optical transceiver integrated module.

Structure
The optical transceiver integrated module is composed of optoelectronic devices, functional circuits and optical interfaces, etc. The optoelectronic devices include the two parts of transmitting and receiving.

The transmitting part is the electrical signal of a certain code rate is input and processed by the internal driver chip to drive the semiconductor laser (LD) or light-emitting diode (LED) to emit a modulated optical signal at a corresponding rate. It has an optical power automatic control circuit inside, so that The output optical signal power remains stable.

The receiving part is an optical signal with a certain code rate is input into the module and then converted into an electrical signal by a light detection diode. The electrical signal of corresponding code rate is output after passing through the preamplifier, and the output signal is generally at the PECL level. At the same time, an alarm signal will be sent when the input optical power is less than a certain value.

Parameters
There are many important photoelectric technical parameters for optical transceiver module, but for a hot plug optical module, the following three parameters are the most concerned when selecting:

Center Wavelength
850nm（MM, multi-mode, low cost but short transmission distance, generally only 500M）；
1310nm (SM, single mode, large loss but small dispersion during transmission, generally used for transmission within 40KM);
1550nm (SM, single mode, small loss but large dispersion during transmission, generally used for long-distance transmission above 40KM, the farthest can be up to 120KM without any repeater)
In addition to the above conventional wavelengths, CWDM wavelength (SM, single-mode, color optical module) and DWDM wavelength (SM, single-mode, color optical module) are also used in multiplex transmission.

Data Rates
The number of bits of data transmitted per second (bit), in bps.
There are 7 types in common use currently: 155Mbps, 1.25Gbps, 2.5Gbps, 10Gbps, 25Gbps, 40Gbps, 100Gbps, etc. The transmission rate is generally downward compatible, so 155M optical modules are also called FE (100M) optical modules, 1.25G optical modules are also called GE (Gigabit) optical modules, and 10G optical modules are also called 10GE (10 Gigabit) optical modules. 10G optical modules are the most widely used module in optical transmission equipment currently. In addition, in the optical storage system (SAN), its transmission rate is 2Gbps, 4Gbps and 8Gbps.

Transmission Distance
The distance in kilometers (km) that optical signals can be transmitted directly without relay amplification. The transmission distance of the optical module is generally 550m for multi-mode, 20km, 40km, 80km and 120km for single-mode, etc.

Laser Types
The laser is a key device in the optical transceiver module. It injects current into the semiconductor material, and emits laser light through the photon oscillation and gain of the resonator. At present, the most commonly used lasers are FP and DFB lasers. The difference between them is that the different semiconductor materials and different resonant cavity structures. The price of DFB lasers is much more expensive than that of FP lasers. Optical modules with transmission distances within 40KM generally use FP lasers; optical modules with transmission distances ≥ 40KM generally use DFB lasers.

Loss and Dispersion
Loss refers to the loss of light energy due to the absorption, scattering and leakage of the medium when light is transmitted in the optical fiber. This part of the energy is dissipated at a certain rate as the transmission distance increases. Dispersion is mainly due to the fact that electromagnetic waves of different wavelengths propagate at different speeds in the same medium, resulting in different wavelength components of the optical signal arriving at the receiving end at different times due to the accumulation of transmission distances, resulting in pulse broadening, which makes it impossible to distinguish signals value. These two parameters mainly affect the transmission distance of the optical module. In the actual application process, the link loss of the 1310nm optical module is generally calculated at 0.35dBm/km, the link loss of the 1550nm optical module is generally calculated at 0.20dBm/km. But the calculation of the dispersion value is very complicated and generally only for reference.

Transmitting Optical Power and Receiving Sensitivity
Transmitting optical power refers to the output optical power of the light source at the transmitting end of the optical module, and receiving sensitivity refers to the minimum receiving optical power of the optical module under a certain rate and bit error rate. Both are in dBm and are important parameters affecting transmission distance. The transmission distance of optical modules is mainly limited by loss and dispersion. The loss limit can be estimated according to the formula: loss limited distance = (transmitted optical power - receiving sensitivity) / fiber attenuation. The fiber attenuation is related to the actual selected fiber. Generally, the G.652 optical fiber can achieve 0.5dB/km in the 1310nm band, and 0.3dB/km in the 1550nm band or even better. The 50um multimode fiber is 4dB/km in the 850nm band and 2dB/km in the 1310nm band. For 100M and 1000M optical modules, the dispersion limitation is far greater than the loss limitation, so it can be ignored. The 10GE optical module complies with the 802.3ae standard, and the transmission distance is related to the type of optical fiber and the optical performance of the optical transceiver module.

Service Life
International unified standard, 7x24 hours of uninterrupted work for 50,000 hours (equivalent to 5 years).

Interface Types
LC, SC, MPO, RJ45

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.

SFP-10G-SR vs. SFP-10G-LR

 With the development of technology, from the original GBIC, XENPAK, X2 to the current SFP28, QSFP2 optical modules are used in different application scenarios. However, based on the market analysis, the 10G SFP+ optical modules are still the most in demand, especially SFP-10G-SR and SFP-10G-LR.

SFP-10G-SR and SFP-10G-LR have different wavelengths, transmission distances, fiber types and laser types：

Module	SFP-10G-SR	SFP-10G-LR
Wavelength	850nm	1310nm
Transmission Distance	300m	10km
Fiber Type	MMF	SMF
Laser Type	VCSEL	DFB
DOM	Support	Support

SFP-10G-SR optical module, where SR means short range (Short Range), the center wavelength is 850nm, it can be used with 62.5/125um multimode fiber and 50/125um multimode fiber, and its transmission distance will be affected by the fiber type, as follows (refer to IEEE Standard 802.3)

Data Rate	Fiber Type	Model Bandwidth @850nm (MHz-km)	Distance Range(m)
9.83-11.3Gb/s	62.5/125um MMF	160	26
9.83-11.3Gb/s	62.5/125um MMF	200	33
9.83-11.3Gb/s	50/125um MMF	400	66
9.83-11.3Gb/s	50/125um MMF	500	82
9.83-11.3Gb/s	50/125um MMF	2000	100

SFP-10G-LR optical module, where LR refers to the long range (Long Range), the center wavelength is 1310nm, it is used with 9/125um single-mode fiber, and the effective transmission is 2km to 10km.

Applications of SFP-10G-SR & SFP-10G-LR
Comparison of the characteristics of the two modules shows that the advantages and applications are as follows: SFP-10G-SR optical module has the characteristics of miniaturization and low power consumption. The TOSA laser used is a VCSEL laser, and the cost is low. The short transmission distance is suitable for short-distance application scenarios, such as enterprises, hospitals and data centers. SFP-10G-LR optical module has the characteristics of miniaturization, low power consumption and long transmission distance. It is suitable for medium and long-distance application scenarios, such as campus, industrial districts, and operator customers, big data centers, etc.

When using these two types of optical modules with optical fibers, should be paid attention to the fact that single-mode optical fibers cannot be inserted into multi-mode optical modules, because the divergence angle of the optical signal sent by the multi-mode optical module is large, while the aperture of single-mode optical fibers is small. Too little light enters the fiber to travel long distances. Meanwhile, it is not recommended to multi-mode optical fiber into the single-mode optical module, because although the light emitted by the single-mode optical module can all enter the optical fiber, it is transmitted in multi-mode mode in the optical fiber, and the dispersion is large, so it can only be transmitted over a short distance. And in this case, the optical power of the receiving end will increase, which will cause the optical power over"" of the receiving end. Therefore, it is not recommended to use multi-mode optical fibers with single-mode optical modules.

What's Optical Protection?

 Optical protection means that once the optical fiber cable is broken, but the communication network won’t be interrupted.

 

OLP optical line protection is a functional device used in optical fiber communication to automatically switch between the main and backup optical paths. It can monitor the optical signal status of the main and backup links in real time. When the optical cable of the main link fails, the system automatically switches to the backup link. transmission, protect the system from running normally, and improve system stability.

Optical line protection can realize functions such as optical power monitoring, automatic switching of optical paths, and network management. In the optical communication network, OLP monitors the optical power on the working fiber and the standby fiber in real time. When the optical power value on the current working fiber is detected to be lower than the set switching threshold, it will issue a warning prompt and automatically switch to the standby fiber, thereby Realize the protection of the optical transmission system. OLP can simply and economically form protection schemes for various channels and trunk lines, and can also protect various networks that require optical path switching, so as to form a non-blocking, high-reliability, safe, flexible, and disaster-resistant optical communications network.

Features
⦁ Automatic Instantaneous Switching
⦁ Real-time Monitoring of Luminous Power
⦁ Reduce Various Losses Caused by Network Node Failures
⦁ Increase the Reliability of Transmission Network
⦁ Improve the Service Quality of Operation
⦁ Main Route and Backup Route can be Scheduled Arbitrarily

Applications
⦁ Backbone Network
⦁ 5G Base Station Fronthaul
⦁ Government and Enterprise Backbone Network
⦁ Banking and Finance Network, etc

 

OBP Optical bypass protection is an intelligent system that is applied in the field of optical fiber communication and can automatically bypass faulty network nodes. The system automatically recognizes the power supply status and optical signal output status of network nodes. The faulty network nodes can be avoided, so as to avoid all-blocking obstacles in the network nodes and maintain the normal connection of the system.

 

Features
⦁ High Security
⦁ Transmitting Signals Transparently
⦁ Automatic Switching without Blocking
⦁ Effectively Respond to Power-off and Light-Emitting Faults

 

Applications
⦁ Local Network Base Station
⦁ Local Network Transmission Station
⦁ Network Room
⦁ Others

What Are the Key Components of Optical Transceiver Module?

 The function of optical transceiver module is to perform photoelectric conversion, and its internal TOSA, ROSA and BOSA are the key components to realize the photoelectric conversion function. The optical device is composed of transmitter and receiver to complete the optical-electrical or electrical-optical conversion of optical signals.

The interior is composed of optical devices, functional circuits and optical interfaces. The optical device is the main component of the optical transceiver module.

The optical devices used for optical signal conversion are called TOSA and ROSA.

TOSA (Transmitting Optical Sub-Assembly) mainly completes the conversion of electrical signals into optical signals. With the light source (semiconductor light-emitting diode or laser diode) as the core, LD chip, monitor photodiode (MD) and other components are packaged in a TO coaxial package or butterfly package, which constitutes TOSA.

In TOSA, laser diode is the most commonly used semiconductor emitting device for optical transceiver modules. Threshold current (Ith) and slope efficiency (S) are the two main parameters. In order to make the LD work quickly, a DC bias current slightly greater than the threshold current must be provided to the LD, that is, the laser is emitted only when the forward current exceeds the threshold current.

ROSA (Resceiving Optical Sub-Assembly) optical receiving assembly, in the high data rate optical fiber module, PIN or APD photodiode and TIA are usually assembled in a sealed metal casing to form an optical receiving assembly.

The figure below is the schematic diagram of the optical module ROSA, which is composed of a photodetector (PIN/APD), a TIA pre-amplifier, and a limiting amplifier.

Photodetector, the main device of ROSA, is mainly used to convert optical signals into electronic signals through the photoelectric effect. The common photodetectors in optical communications are PIN photodiodes and avalanche photodiodes (APDs). APDs are high-sensitivity photodetectors that use the avalanche multiplication effect to double the photocurrent. Compared with PIN photodiode, the receiving sensitivity of APD can be improved by 6~10dB.

The weak signal current generated by the photodetector is converted into a signal voltage of sufficient magnitude by the preamplifier TIA, and then output. TIA is actually a voltage converter, which converts electro-optic current into voltage.

At this time, the voltage signal output by the TIA is still an analog signal, which needs to be converted into a digital signal before the signal processing circuit can recognize it. The function of the Poster Amplifier behind the TIA is to convert signals of different amplitudes into digital signals with the same amplitude.

After introducing TOSA and ROSA, let's take a look at what is BOSA?
With the development of process level technology, the modules can be made smaller. TOSA and ROSA integrate the transmission and reception of light (LD and PIN/APD) through the coaxial coupling process, plus splitters, optical fibers and other components, called BOSA (Bi-Directional Optical Sub-Assembly).

Nowadays, the high-speed optical transceiver module integrates high-performance DSP at the receiving end, and its performance in terms of dispersion and noise processing is really good.

Application Scenarios of CFP Transceiver Modules

 CFP optical transceiver modules are mainly used in switches, routers, converters, OTN optical transmission platforms and other equipment for line-side optical transmission of 100G wavelength division systems. Compared with other forms of line-side optical modules, It has good OSNR performance, sensitivity, dispersion tolerance, and DGD tolerance. At the same time, CFP coherent optical modules using wavelength division multiplexing technology are widely used in 100G metropolitan area networks to meet their high-capacity and long-distance requirements.

100G Multi-channel DWDM Transmission Network
Since 100G is more susceptible to dispersion, it is necessary to dispersion and improve optical power. First, a 100GHz DWDM multiplexer is used to combine all 100Gs, and then the dispersion compensation and amplification of the whole combination are performed. This architecture supports "pay-as-you-grow" service providers. Existing legacy 10G channels can be seamlessly swapped with 100G services when bandwidth is exhausted. The rest of the same components can even be reused to extend the data rate to 2.4Tb/s.

This solution requires 24 color CFP optical transceiver modules and a 48-channel 100GHz DWDM multiplexer. First, all 100G lines are multiplexed together, and then only one dispersion compensation and amplification is required. Obviously, such a network structure can provide higher density at a reasonable cost, and has the ability to flexibly reuse the existing infrastructure.

100G Distance Extension Solution
SFP + OEO Optical amplification repeaters can be used in conjunction with switches to achieve distance extension. After the 100G output signal of the switch is converted into a DWDM signal, it can realize longer distance transmission. This solution eliminates the distance limitation problem by using CFP coherent optical transceiver modules to connect the output signal to the line fiber to achieve longer distance signal transmission.

The above architecture achieves longer-distance signal transmission by adding MTP branch cables and WDM SFP + OEO converters, 6-channel dual-fiber DWDM dense wavelength division multiplexers and 8-channel dual-fiber CWDM coarse wavelength division multiplexers. The role of OEO is to convert conventional SR wavelengths to DWDM wavelengths. Therefore, the 100G CFP coherent optical module is an effective solution for building a 2500km long-distance DWDM network.