With the rapid development of science and technology, people's life changes with each passing day. The rapid development of 5g, big data, blockchain, cloud computing, Internet of things, artificial intelligence and other application markets has brought explosive growth to data traffic. Data center interconnection has gradually developed into a research hotspot of optical communication. At present, the application scenarios of high-speed optical module are mainly divided into Internet data center network, metropolitan area network optical transmission network and telecommunications network represented by 5g bearer network.

Application of optical module in data center

  Now the data center is no longer just one or several computer rooms, but a group of data center clusters. In order to realize the normal work of various Internet services and application markets, it is required that the data centers work together. The massive real-time interaction of information between data centers leads to the demand of data center interconnection network, and optical fiber communication has become a necessary means to realize interconnection.

  Different from traditional telecommunication access network transmission equipment, data center interconnection requires switching equipment to have higher rate, lower power consumption and more miniaturization in order to realize larger and more intensive transmission of information. The core factor that determines whether these performances can be realized is the optical module. The information network mainly uses optical fiber as the transmission medium, but at present, the calculation and analysis must also be based on electrical signals, and the optical module is the core device to realize photoelectric conversion.

The data center communication optical module can be divided into three categories according to the connection type:

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

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

(3) Inside the data center, it is mainly used for information storage, generation and mining. According to the prediction, the internal communication of the data center accounts for more than 70% of the communication of the data center. The great development of the construction of the data center has given birth to the development of high-speed optical modules.

The data flow continues to grow, and the trend of large-scale and flat data center promotes the development of optical modules in two aspects:

·Increased transmission rate requirements

·Quantity demand growth

      The trend of large-scale data center leads to the increase of transmission distance demand. The transmission distance of multi-mode fiber is limited by the increase of signal rate, and it is expected to be gradually replaced by single-mode fiber. The cost of optical fiber link is composed of optical module and optical fiber. There are different applicable schemes for different distances. In terms of medium and long-distance interconnection required for data center communication, there are two revolutionary schemes born from MSA:

· PSM4（Parallel Single Mode 4 lanes）

· CWDM4（Coarse Wavelength Division MulTIplexer 4 lanes）

Among them, the use of psm4 optical fiber is four times that of cwdm4. When the link distance is long, the cost of cwdm4 scheme is relatively low.

There are differences in the application of optical modules in data centers at home and abroad

The interconnection of internal switches in American data centers is mainly based on single-mode optical fiber. Cwdm4 / psm4 optical modules are widely used in 100g era, and DR4 is mainly used in 400g era at present; The interconnection between server and switch mostly adopts cable DAC. With the passage of time and the increase of module rate, the proportion of multimode fiber and direct cable DAC in the internal interconnection scheme of U.S. data centers will be lower and lower.

The interconnection of switches in China's data center is mainly multi-mode optical fiber, and the proportion of single-mode optical fiber is gradually increasing. At present, there is little demand for 400g in China. In the 100g era, Sr4 / cwdm4 module is adopted, and the interconnection between servers and switches mostly adopts active optical cable AOC.

The leader in optical fiber transmission network: CWDM optical module

CWDM optical module adopts CWDM Technology, which can combine optical signals of different wavelengths through external wavelength division multiplexer and transmit them through one optical fiber, so as to save optical fiber resources. At the same time, the receiver needs to use a wavelength decomposition multiplexer to decompose the complex optical signal.

CWDM optical module is usually used in CWDM system, which has lower cost than DWDM optical module and is widely used. In a CWDM system, the CWDM optical module is inserted into the switch, and the CWDM optical module is connected with the CWDM demultiplexer or OADM optical add drop multiplexing by jumpers.

CWDM optical module plays a great role in CWDM system and successfully solves the problems in optical fiber transmission network. CWDM optical module has eight advantages, which are summarized as follows:

1. "Transparent" transmission of data;

2. Large capacity, making full use of the huge bandwidth resources of optical fiber;

3. The optical fiber resources are greatly saved and the construction cost is reduced;

4. High networking flexibility, economy and reliability;

5. It can be switched by all-optical network to realize long-distance non electric relay transmission;

6. The simplified laser module reduces the volume of the equipment and saves the room space;

7. Optical layer recovery is independent of service and rate, and can effectively protect data;

8. Without semiconductor cooler and temperature control function, it can significantly reduce the power consumption, which is only 12.5% of DWDM.

Application scenario of 5g bearing optical module

The 5g era is coming, which brings unlimited business opportunities to the field of optical communication. The optical module based on 5g base station has become a research hotspot in recent two years. 5g bearer network is generally divided into metro access layer, Metro convergence layer, metro core layer / provincial trunk line to realize the forward and middle return functions of 5g services. Among them, the devices at each layer mainly rely on optical modules to realize interconnection.

Under the condition of 5g forward transmission network, the 25gb / S (ecpri / CPRI) rate module is mainly sfp28. The schemes of dual fiber bidirectional, single fiber bidirectional and 25g WDM (including tunable wavelength) module can reduce the use of optical fiber and greatly reduce the construction cost.

The existing mature 25g optical device can be used by the intermediate transmission, and the bandwidth of the optical device can be doubled by using pam4 technology; The transmission distance of 10km and 40km will cover more than 90% of the application scenarios, and the transmission distance of more than 80km will adopt coherent technology.

Typical application scenarios of 5g forward transmission: including optical fiber direct connection, passive WDM and active WDM / optical transmission network (OTN) / sliced packet network (SPN), etc. 25gb / s gray light module is generally used in the scene of direct optical fiber connection, which supports two types of dual fiber bidirectional and single fiber bidirectional, mainly including two transmission distances of 300m and 10km. Passive WDM scenarios mainly include point-to-point passive WDM and WDM-PON. One pair or one optical fiber is used to realize the connection between multiple AAU and Du. Typically, 10Gb / s or 25gb / s color light module is required. In active WDM / OTN scenario, 10Gb / s or 25gb / s short-range gray light module is generally required between AAU / Du and WDM / OTN / SPN equipment, and N is required between WDM / OTN / SPN equipment × 10 / 25 / 50 / 100GB / s dual fiber bidirectional or single fiber bidirectional color light module.

Typical requirements for optical module in 5g front transmission application scenario are as follows:

(1) Meet the industrial temperature range and high reliability requirements: considering the full outdoor application environment of AAU, the front light transmission module needs to meet the industrial temperature range of - 40 ℃ ~ + 85 ℃ and dust-proof requirements.

(2) Low cost: the total demand for 5g optical modules is expected to exceed 4G. In particular, there may be tens of millions of demands for front transmission modules. Low cost is one of the main demands of the industry for optical modules. 5g medium return covers the metropolitan area access layer, convergence layer and core layer. The required optical module is not different from the optical module technology used in the existing transmission network and data center. The access layer will mainly use gray light or color light modules with rates of 25gb / s, 50GB / s and 100GB / S, and the convergence layer and above will mostly use DWDM color light modules with rates of 100GB / s, 200GB / s and 400gb / s.

Development status of 5g bearing optical module

At present, standardization organizations at home and abroad, such as ITU-T, Institute of electrical and Electronic Engineers (IEEE), optical interconnection Forum (OIF), 4wdm and other multi-source protocols (MSA), and China Communication Standardization Association (CCSA), are developing specifications for optical modules related to 5g bearer, involving different types and interface characteristics. The front light transmission module mainly includes two rate types: 25gb / s and 100GB / s, and supports a typical transmission distance of hundreds of m to 20km. The specific technical status is shown in Table 2.

The return optical module in 5g mainly includes 25gb / s, 50GB / s, 100GB / s, 200GB / s, 400gb / s and other rates. The typical transmission distance ranges from a few kilometers to hundreds of kilometers. It supports CPRI, ecpri, Ethernet, OTN and other interface protocols, as well as NRZ, pam4, DMT and other modulation formats. The specific technical status is shown in Table 3.

 With the development of optical device chip technology, standards and application requirements, the types of optical modules may increase in the future. Too many product types and specifications will lead to the fragmentation of the overall industrial market of optical modules, resulting in a waste of resources in the upstream and downstream R & D, manufacturing, operation and maintenance and many other links of the industrial chain.

                                                       Statement: This article comes from the Internet