The Applications of SFP Optical Transceivers

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Today’s data centers are no longer just one or a few computer rooms, but a group of data center clusters. In order to realize the normal operation of various Internet services and application markets, data centers are required to operate in coordination. The real-time mass exchange of information between data centers has created the demand for data center interconnection networks, and optical fiber communication has become a necessary means to achieve interconnection.

Different from the traditional telecommunication access network transmission equipment, the data center interconnection requires higher speed, lower power consumption, and smaller size of the switching equipment in order to realize larger and denser transmission of information. The SFP optical transceiver is a core factor that determines whether these performances can be achieved. The information network mainly uses optical fiber as the transmission medium, but the current calculation and analysis must be based on electrical signals, and the SFP optical transceiver is the core device for realizing photoelectric conversion.

The Three Applications of SFP Optical Transceivers

(1) From the data center to the user, it is generated by end-user behaviors such as accessing the cloud to browse web pages, send and receive emails, and stream video;

(2) Data center interconnection, mainly used for data replication, software and system upgrade;

(3) Inside the data center, it is mainly used for information storage, generation and mining.  

What is CWDM SFP optical Transceiver?

The CWDM SFP optical transceiver adopts CWDM technology, which can combine optical signals of different wavelengths through an external wavelength division multiplexer, and transmit them through a single fiber, thereby saving fiber resources. At the same time, the receiving end needs to use a wavelength demultiplexer to decompose the complex optical signal.

CWDM transceiver modules are usually used in CWDM systems. In a WDM system, the CWDM SFP module is inserted into the switch, and the CWDM SFP module and the CWDM demultiplexer or OADM optical add-drop multiplexer are connected to work with fiber optic jumpers.

The SPF optical Transceiver Module Application in 5G Network

The 5G era is coming, bringing unlimited business opportunities to the field of optical communication. SFP modules based on 5G base stations have become a research hotspot in the past two years. The 5G network is generally divided into metro access layer, metro aggregation layer, metro core layer/provincial trunk line, and realizes the fronthaul and mid-backhaul functions of 5G services. The devices at each layer mainly rely on SFP modules to achieve interconnection.

The typical requirements for optical modules in 5G fronthaul application scenarios are as follows:

(1) Meet the industrial temperature range and high reliability requirements: Considering the full outdoor application environment of AAU, the fronthaul optical module must meet the industrial temperature range of -40°C to +85°C, as well as dustproof requirements.

(2) Low cost: The total demand for 5G SFP modules is expected to exceed 4G. In particular, there may be tens of millions of demand for front-haul optical modules. Low cost is one of the main demands of the industry for SFP optical modules. In 5G, the backhaul covers the access layer, aggregation layer and core layer of the metro area. The required SFP optical modules are not much different from those used in the existing transmission network and data center. The access layer will mainly use 25Gb/s, 50Gb/s s, 100Gb/s and other gray light or color light modules, the convergence layer and above will mostly use 100Gb/s, 200Gb/s, 400Gb/s and other rates of DWDM color light modules.

How to Realize the Precise Connection of Optical Fiber by Optical Fiber Adapter?

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When two optical fibers are connected, due to the difference in position, shape and structure of the two optical fibers, the energy cannot be 100% from one optical fiber to the other. That is, there will be connection loss. In order to minimize connection loss, the two fibers must be precisely aligned. The main function of the optical fiber adapter is to quickly connect two optical fibers, so that the optical signal can be continuous to form an optical path. And how does the optical fiber adapter realize the accurate connection of the optical fiber?

There are many types of optical fiber adatpers, but the precise alignment between optical fibers depends on two factors. One is a ceramic ferrule with a precise inner diameter, outer diameter, and concentricity, and the other is a ceramic sleeve with a slit. The ceramic sleeve is a very smart design.

You can see how the two optical ferrules are precisely aligned through a ceramic sleeve. The inner diameter of the ceramic sleeve is slightly smaller than the outer diameter of the ferrule. Because the sleeve has a slit, the fiber can be inserted. The expanded sleeve tightens the two ferrules to achieve precise alignment.

The core diameter of the single-mode fiber SMF is only about 8 ~ 10μm, in order to ensure low connection loss, the two fibers must be precisely aligned. For single-mode fiber adapter, the lateral misalignment between the two fibers should be less than 0.5um.

However, mere precision alignment is far from enough for fiber connection. We know that light will reflect back at the interface between two different media. The refractive index of quartz fiber at 1.55um is about 1.455, so the reflected echo BR at the end face of the fiber is 3.4%. Back-reflected light will affect the stability of the communication system, and at the same time, each quartz glass-air interface will introduce an insertion loss of about 0.15dB. Therefore, each fiber connector will increase the loss of 0.3dB.

People usually apply antireflection coatings on the end faces to reduce reflected echoes. However, coating problems are not considered in fiber optic adapters. First, the AR coating will increase the cost of the adapter. Second, the fiber connection is not fixed, repeated insertion and removal will damage the AR coating. So, can we apply AR coating on the fiber end face and keep the fiber end face out of contact?

When the two optical fibers are buttted, the longitudinal distance as small as 50um will introduce nearly 1dB loss, which is intolerable in the optical fiber communication system. Therefore, we have reached a consensus that the two fibers must be in contact and the end surface of the fiber cannot be coated. The reflected echo occurs at the interface between two different media, and the air between the fiber end faces must be exhausted, so that the the two fiber ends are in physical contact (PC), as if the medium is fused. Since the optical fiber is fixed in the middle of the ceramic ferrule, any roughness on the ceramic surface will affect the physical contact between the optical fibers. In order to ensure the physical contact between the optical fibers, the ferrule surface is usually ground into a spherical surface. The end surface of the optical fiber is located at the vertex of the spherical surface. This is the second smart design in the optical fiber adapter.

As shown in Figure 1, the ferrule is inserted into the sleeve, and under pressure, the end surface of the ferrule deforms under the pressure, and the deformation of the end surface can ensure physical contact between the optical fibers. Because the physical contact depends on the deformation of the end face, and the ceramic is both wear-resistant and has a certain elasticity, which is why it is selected as the ferrule material rather than glass.

The physical contact between the optical fibers can ensure low loss of the fiber connection point, but the return loss RL can only reach 55dB. For some applications that require higher RL, the end face of the optical fiber connector is ground to a certain angle, which is called a bevel physical contact APC. The fiber end face is usually polished to an 8° slope, and the RL can be increased by an additional 36dB, so the total RL of the APC connector is usually greater than 65dB.

The optical fiber adapter is the most basic optical passive device in the optical fiber communication system. The system’s basic technical requirements for the optical fiber connector include low insertion loss IL and high return loss RL. That is, the lowest possible reflected echo BR. However, as the most widely used optical passive device, its cost and connection convenience are as important as the technical indicators.

What Is A Low-Speed Optical Transceiver Module

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With the rapid development of data centers to ultra-high speed and large capacity, the market demand for high-speed optical transceiver modules is also increasing. Against this background, is there still market demand for low-speed optical transceiver modules? What does low-speed optical  transceiver module specifically refer to? Let us take a look below.

Low-speed optical module concept

According to the transmission rate, optical transceiver modules can be divided into 100M optical modules, Gigabit optical modules, 10G optical modules, 40G optical modules and 100G optical modules. We generally refer to optical transceiver modules with transmission rates of 1000M and below as low speed optical Module.

Classification of low-speed optical modules

Low-speed optical transceiver modules currently mainly include GBIC and SFP package types, they have certain differences in size and shape and their actual application.

GBIC optical transceiver module: The GBIC optical transceiver module is an optical module used in the early Gigabit Ethernet. It is mostly connected by sc interface. At present, only some of the earlier switch models of older models still have application requirements.

SFP optical transceiver module: The SFP optical transceiver module is an optimized version of the GBIC optical module. Its function is basically the same as that of the GBIC optical module, but its volume is much smaller. This type of optical transceiver module uses LC interface connection, which has the characteristics of compact structure and more Suitable for high-density wiring, it can greatly simplify the deployment of Gigabit Ethernet.

The working principle of low speed optical transceiver module

The low-speed optical transceiver module is composed of a transmitting part and a receiving part.

The function of the transmitting part is to modulate the electrical signal to the laser, so as to realize the electro-optical conversion. Specifically, the electrical signal with a certain code rate is processed by the internal driving chip to drive the semiconductor laser (LD) or the light emitting diode (LED) to emit the corresponding rate The modulated optical signal has an optical power automatic control circuit inside, which keeps the output optical signal power stable.

The function of the receiving part is that the detector receives the optical signal and realizes the photoelectric conversion. Specifically, an optical signal with a certain code rate is converted into an electrical signal by a light detection diode after being input into the module, and then an electrical signal with a corresponding code rate is output after passing through a preamplifier.

Application of low speed optical transceiver module

The low-speed optical transceiver supports many interface devices, including some other network devices such as switches and routers. In practical applications, SFP low-speed optical transceiver modules are more widely used than GBIC low-speed optical transceiver modules in the field of telecommunications and data communications.

However, there are still some network devices equipped with GBIC optical module interfaces, so the appropriate type of low-speed optical transceiver should be selected according to the interface type of the specific device.

The Latest Methods of Aerial Fiber Cable Construction

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Many people are confused about the hanging of aerial optical cables. In fact, there are two methods for aerial optical cables laying: one is “fixed-pulley traction method”, including “manual traction method” and “mechanical traction method”; the other is “cable tray moving and releasing method”. Next, we will take a detailed look at the concrete operation of the latest methods of aerial cable construction technology.

(1) Fixed-pulley traction method. When hanging the optical cable, the cable tray should be set first, and the big pulley for laying the optical cable shall be fixed on the pole at the up and down positions. A small pulley (guide pulley) shall be hung every 10-20 m on the suspension line in each pole, and the traction rope shall be put into the small pulley. Then, make the traction head, connect the traction rope with the optical cable, and prepare for laying.

① When applying manual traction method, please manually tighten the optical cable tray to gradually release the optical cable, and then one person shall be arranged on every two poles for auxiliary traction (if necessary, install guide pulley on the angle pole). Gradually tighten the traction rope at the traction end to slowly release the optical cable. In order to fully ensure the safety of optical fiber, three contact telephones shall be set at the cable laying end, cable receiving end and cable traction head of optical cable line, and always keep the contact unblocked. When the single tray optical cable is too long, it is generally placed in two times: after the first half of the tray is placed, put it on the ground with an “8” shape; and then the second half of the tray is placed in another direction.

② When mechanical traction is adopted, the main machine of mechanical traction shall be placed at the retrieving cable end, and the auxiliary traction machine shall be placed at the appropriate position of the cable laying end. Put the cable tray at the cable laying end, manually tighten the cable tray to gradually release the cable from the top, and apply machinery instead of manual traction to achieve the purpose of cable laying. When manufacturing the traction end, make sure that the traction force mainly acts on the reinforcement of the optical cable. With allowable traction force limit of optical cable, the length of optical cable laid in one time shall be determined according to the terrain conditions. In addition, the traction speed of optical cable traction machine shall be determined in advance. When the traction tension exceeds the standard figure, there shall be automatic cut-off or alarm device.

③ After laying optical cable, the length of optical cable for the joint and for each pole shall be reserved. Finally, hang the remaining cable with the wrapped end on the pole.

When laying the aerial optical cable of hanging type, the expansion bend can be properly made on the pole in the middle load area, heavy load area, overweight load area. It is generally required to be reserved on each pole. In the light load area, one reservation can be made every 3-5 poles; in the ice-free area, it is ok to make no reservation (but the optical cable cannot be pulled too tight, and pay attention to the natural sag).

The central part of the pole shall be protected by polyethylene bellows with a reserved length of 2m (generally not less than 1.5m). Both sides and binding parts shall be reserved, and shall not be tied up, so that it could expand freely when the temperature changes and the optical cable could thus be protected. When the optical cable passes through the cross-shaped suspension line or T-shaped suspension line, the protective pipe shall also be installed.

④ When the aerial optical cable is led upward, the aerial fiber optic cable installation method and requirements can refer to the following figure. The steel pipe (led upward to protect the steel pipe) is used to protect the lower part of the pole to prevent human injury; the upper suspension part shall be bound at the place 30cm away from the pole, and there should be an expansion bend (pay attention to the bending radius of the expansion bend and ensure the safety of optical cable with the condition of severe temperature change).

⑤ Generally, hooks are used for fixing and hanging aerial optical cables. The distance between optical cable hooks shall be 50cm, and the allowable deviation shall not exceed ± 3cm.

The buckle direction of the hook on the suspension line shall be the same, and the hook plate shall be complete without mechanical damage. The distance between the first hook on both sides of the pole and the clamp plate of suspension line shall be 25cm, and its allowable deviation shall not exceed ± 2cm.

(2) Cable tray moving and releasing method. Such hanging method is to set the optical cable tray on the truck (fix one end of the optical cable at the suspension line of the electric pole), manually push the optical cable tray, so that the optical cable is laid along the overhead line, while the optical cable is hung on the suspension line. After the laying work with one pole spacing, the optical cable can be fixed on the suspension line with next pole spacing. This method is simple, but the operation of truck is limited by conditions. In general, the following requirements shall be met.

It should be noted that the distance between the overhead line and the roadside is not more than 3m, and the suspension line should be located at the lowest layer of other lines on the overhead line. Unitek focuses on the R & D and sales of optical fiber and cable products. Optical fiber jumper, optical fiber adapter, MPO / MTP data center and other optical fiber products are all in line with international and domestic standards such as ICE, ROSH, and YD/T. We hope that you can firmly grasp the latest hanging methods above and apply them to actual wiring operation.

The Introduction of Popular Fiber Optical Cables

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Central tube optical cable possesses loose tube structure of the central tube. Because its optical fiber is in the center of the cable, which has good bending characteristics and compact structure, it has become the main type after being introduced. Wuhan Institute of Post and Telecommunications Science, based on the characteristics of China’s vast territory and small optical fiber communication capacity at that time, has developed the first optical cable patent product with Chinese characteristics, that is, the central tube optical cable.

There are several forms of central tube structure. The first one has no steel belt armor, which is strengthened by parallel galvanized steel wire and has the problem of poor water penetration and bad low-temperature characteristics. Later, steel belt armor is added, and it is the first forming technology of optical cable steel strip longitudinal wrapping in China, adopting classification molding, steel belt flat belt butt joint, hot melt adhesive bonding process, which not only solves the problem of water penetration and high/low temperature, but also determines the rudimentary technical standard of steel strip. However, the optical cable is very difficult to peel. Finally, the technology of water blocking tape or oil paste filling is developed to further rationalize the structure. However, from the perspective of structural characteristics, if there are more than 12 cores are used, complex bundling technology is needed to distinguish optical fiber. Later, spraying ring technology is developed for OPGW. When multiple groups of optical fiber are placed in one tube, multiple fiber shelves are needed, so the utilization ratio of the equipment is limited, which is not conducive to large-scale production and not easy to be divided in project.

In 1991, Wuhan institute of post and telecommunications introduced NOKIA production line of loose tube, and introduced S-stranding cable from France. Then layer-stranding optical cable has been widely used. For S-stranding structure, the technology of synchronous back-twist of receiving and releasing line has been adopted. The cable pitch is stable, and the oil paste is filled and wrapped with polyester tape. The advantages are that the cable performance is stable, and the theoretical basis of cable surplus design today has been established. The disadvantages are complex equipment, low efficiency and high energy consumption. In 1995, SZ-stranding cabling equipment was developed, which greatly simplified the cabling equipment and improved the production efficiency. And the double-core shift winding, SZ reciprocating stranding process, ointment filling, longitudinal wrapping or wrapping tape were used to produce 12-unit tubes at most. The filling rope was developed as the filling unit, and two distinguishing methods of pilot color indication and full chromatography were designed.

The difference of cuff technique is that multiple optical fibers are arranged side by side. This technology was first used in skeleton ribbon cable in Japan, and the first one introduced into China was central tube ribbon cable. The key technology of optical fiber ribbon is paralleling fibers. The main index is the flatness control, and the colored optical fiber used in paralleling fibers is also different from the coloring process control of ordinary colored fiber. The other key is that the tidiness and tension of the winding line should match the cuff technique.

In cuff technique of ribbon fiber optic cable, ribbon fiber optic cable is usually stranded into tube with S or Z stranding structure, so the concept of the remaining length is different from that of the ordinary cuff. Because the cuff is thick, the wheel type traction is generally not used, while the track traction is mostly used. The key parameters to control the remaining length are: tension of ribbon releasing, pitch of pipe inlet, molds (multiple), water temperature and line receiving tension. Its theoretical calculation should specially include strain of each sideband and side fiber, and be connected with the actual test. In the initial stage, the duty ratio and pitch designed by each plant are relatively large. With the increase cost pressure, the technology began to be pushed towards the direction of small structure, which can best reflect the technology control ability of each plant in the ribbon fiber optic cable structure.

Skeleton ribbon fiber optic cable has its own characteristics in application because it has no ointment. In this period, only Chang Fei company introduced this technology, but it was not widely used. The main problems are: fail to produce 12-core ribbon; reduces the number of cores in one-time fusion; adopt S-stranded structure, which is not convenient for branching; complex equipment; high cost and low efficiency.

The emergence of butterfly optical cable is different from the previous design ideas. Its bare fiber is directly sheathed, which is designed based on the requirements of the last 100m of access network. There are pipeline and overhead, pipeline introduction and other laying forms. The process route is colored, sheathed or protected by aluminum tape sometimes. The tensile and temperature test of optical cable are different from those of ordinary optical cable. Burning characteristics and environmental safety factors are also considered.

The difficulty of this structure lies in the control of optical fiber loss, the control of optical cable structure size (need to cooperate with quick connector), the latest requirements of small surface friction coefficient for the construction of multiple optical cables in one pipe.

The traditional indoor optical cable is mainly jumper optical cable and short-distance indoor connection optical cable. With the development of FTTX, the types of optical cables in the building began to increase, including branch optical cables, wiring optical cables, etc.

These optical cables are all based on the tight cuff technology, and their materials are PE, AT, PU, PVC, LSZH, etc. They are quite different from the previous loose tube technology, and have various requirements for stripping.

The Key Technology and Application of CWDM

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The emergence of CWDM (coarse wavelength divisio

n multiplexing) technology allows operators to find a low-cost, high-performance transmission solution. Because of its low cost, low power consumption, small size, and other advantages, CWDM has been widely used in metro transport networks. Many domestic and foreign manufacturers have also begun to develop and launch products, and ITU is also accelerating its standardization process. CWDM technology improves fiber utilization and gives operators and users greater flexibility. This article will discuss the key technologies such as CWDM characteristics, wavelength selection, and fiber type, and make a detailed comparison between CWDM and DWDM. Finally, the application and development of CWDM are prospected.

Advantages of CWDM system metro transport networks

The biggest advantage of the CWDM system is its low cost, which is mainly manifested in several aspects of device, power consumption, and integration.

CWDM technology will greatly reduce the cost of construction and operation and maintenance, especially the cost of lasers and multiplexers/demultiplexers. For DWDM systems with awavelength interval of less than 50GHz, the laser needs to use a precise temperature control circuit to control the wavelength. And sometimes a wavelength locker is required to ensure the accuracy and stability of the wavelength. An optical multiplexer (filter type) requires hundreds of layers of multi-layer dielectric film devices. To prevent crosstalk between the same frequency and different frequencies, multiple filtering must be used. CWDM does not require complex technologies such as laser cooling, wavelength locking and precise coating, which greatly reduces equipment costs.

The DWDM system laser integrates a Peltier cooler. The temperature detection and control circuit uses a large amount of power and consumes about 4W per wavelength. The CWDMnon-refrigerated laser and its control circuit only require about 0.5W per wavelength. For multi-wavelength and high-speed DWDM systems, single-disk power consumption control is a difficult problem in system design. The low power consumption of the CWDM system using a non-refrigerated laser reduces power backup batteries and reduces costs.

The physical size of CWDM laser is much smaller than that of DFB laser. The size of DWDM optical transmitter is about 5 times that of CWDM optical transmitter. Due to the CWDM laser structure and simple control circuit, a single module can realize multi-channel optical transceiver. At present, commercial devices have achieved 4-channel transceiver integrated into a module with a size of only 16cm´9cm´1.65cm, which is equivalent to a DWDM system optical forwarding Size. The CWDM system does not use optical amplifiers, so it may be designed as a compact desktop or box-type device, which is very convenient for installation and maintenance.

The technology of CWDM

G.694.2 defines 18 nominal wavelengths of CWDM from 1270nm to 1610nm, and the wavelength interval is 20nm. This interval allows the simultaneous transmission of various wavelengths under the condition of using a non-refrigerated light source. The CWDM wavelength covers the O, E, S, C, L and other five bands of the single-mode fiber system.

The operating temperature (tube temperature) of non-refrigerating lasers usually ranges from 0°C to 70°C, and its thermal drift coefficient is about 0.08nm/°C. The nominal center wavelength value refers to the output wavelength of the laser at normal temperature, that is, 23°C. The filtering characteristics of passive devices (such as multiplexers) hardly change with temperature. It is generally believed that the nominal center wavelength of passive devices should be aligned with the output signal wavelength of the laser at 35°C, because 35°C is in the middle of the entire operating temperature range. In other words, the nominal center wavelength of the passive device should be l0 plus the wavelength drift value of the laser output from 23°C to 35°C, that is, l0 + 0.08nm/°C *(35°C -23°C) = l0+1nm. In order to solve the problem of wavelength difference caused by the difference between the nominal temperature of the laser wavelength and the actual operating temperature. The ITU shifts the G.694.2 wavelength up by 1 nm (1271 nm/1291 nm/…/1611 nm), so that the laser wavelength will just work at (1270 nm/1290 nm/…/1610 nm) in the actual environment.

In actual application, CWDM products mainly have two forms, 8-wavelength system and 16-wavelength system. The 8-wavelength system is a system with many applications. In theory,any one of the 18 wavelength choices given by ITU can be used as the operating wavelength. However, considering the type and loss characteristics of the optical fiber that has been laid, the 8 wavelength is generally selected at (1460-1620) nm, that is, the S+C+L band, avoiding the fiber water peak E band and the O band with greater loss. Additional requirements are required for optical fibers. The 16-wavelength system will impose requirements on the type of fiber, that is, a “full-wave” fiber with a flat loss must be used, and currently such fiber is rarely used. CWDM is mainly used in the access area lacking in optical fiber, and the capacity of 8 waves can mostly meet the system requirements.

According to the current laser manufacturing technology, the wavelength change of the non-refrigerating laser should be within +/- 6~7nm under working conditions and throughout its lifetime. Taking into account sufficient adjacent channel isolation and a certain guard band (generally the minimum channel spacing is about one-third), G.694.2 selects 20nm as the channel spacing of the CWDM system. In the CWDM system, the output wavelength of the uncooled laser at 0°C~70°C changes by about 6nm, plus the wavelength deviation of the laser manufacturing process is about±3nm, and the total wavelength change will not exceed±6nm. The passband of the optical filter and the wavelength spacing between adjacent channels must be wide enough to meet the requirement of no wavelength shift of the cooling laser.

The channel spacing of the CWDM system is usually 20 NM, and the filter passband width is about 13 NM. The laser center wavelength shift must be considered consistent with the filter passband width, and the laser output wavelength must be within the filter passband range.

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