WDM : Wavelength division multiplexing is a technology that simultaneously transmits two or more optical signals of different wavelengths in the same optical fiber.

CWDM : Coarse wavelength division multiplexing, generally covering wavelengths of 1270~1610nm , DFB lasers will be used , the channel spacing is relatively wide, usually 20nm , the main application, a total of 18 waves, relatively low cost, mainly used in Aggregation layer and access layer. So it can be used for our fronthaul, CWDM6 application.

DWDM : dense wavelength division multiplexing, the wavelength interval will be relatively small, 0.2nm~1.2nm . The main working wavelengths are 1525nm~1565nm in C -band and 1570nm~1610nm in L- band . It focuses on long-distance transmission and is applied to the transmission network. Therefore, the low-loss C -band is selected, and EML lasers are usually used. To ensure the wavelength, they are all equipped with TEC , so the cost of the device will be relatively high.

Classification of optical modules

 There are many types of optical modules, let’s talk about them from the application point of view today. Modules are usually divided into three types from the perspective of application:

1.Wireless

Wireless is mobile communication, and the transmission of signals is carried out between mobile phones and devices through wireless electromagnetic waves. Wireless communication is divided into forward transmission, medium transmission and back transmission. Different types of optical modules have different transmission distances. Currently, 4G and 5G wireless transmissions are commonly used .  

The optical modules used for transmission are used in various base stations outdoors, so the requirements of industrial regulations are required, and there are strict requirements on reliability. The reliability life is evaluated according to 20 years . The requirement is hermetic packaging. Especially for front-haul applications, the power consumption and cost required are very low because of the large amount of usage. 4G transmission uses 10G optical modules, 5G transmission uses 25G optical modules, and next-generation fronthaul applies 50G optical modules. The current mainstream is the rate of 25G . The fronthaul mainly uses a 10 km optical module and a 25G DFB laser+Pin. Because of the maturity, price advantage and reliability of the 10G laser, an overclocking solution is usually used, that is, 10G DFB + 25 G header or 1 0G DFB + 10G header.

2. Access

      For the access network, we usually use the PON technology of all-fiber access, which is the mainstream technology the wired access network at present. It is divided into OLT (local end) and ONU (terminal), but currently ONU no longer uses optical modules, but uses BOB (BOSA on board), which is our home optical modem. There are GPON, XGPON, and XGSPON. Its applications include FTTB (fiber to building ), FTTO (Fiber to office), FTTH (Fiber to home), FTTR (Fiber to room), FTTD (Fiber to desktop ). At present, the domestic mainstream is XGPON (1 0 G asymmetric) , Overseas is XGSPON (1 0 G symmetric). XGPON OLT mainly uses 10 GEML lasers and APD. What XGPON ONU uses is 1 0 G DFB laser and APD. Access to optical modules requires relatively long transmission distances, so lasers and APDs with high output power are required. Because of the one-to-many feature of the access network, although the rate is not high, high-power output, high sensitivity and transmission timing are the places that need to be paid attention to. Of course, cost and power consumption are also important considerations. Because it is usually indoors, the temperature requirements are not as stringent as those for wireless.

3. Data center

     The optical module used in the data center is mainly the connection of various parts of the data center server. With the development of digital communication and the application of various cloud services, data centers are playing an increasingly important role. We need to process all kinds of data and then send them to where they are needed. Because the temperature of the computer room in the data center is relatively stable, the operating temperature does not need to be so strict. The update iteration of the data center is very fast, and it will be changed every 2 to 3 years, and the redundancy design is also different, so compared with wireless applications, the reliability requirements are not so strict. However , the data center updates and iterates quickly, so it develops rapidly. Single-wavelength 50G and 100G are currently commercially available.

According to its application scenarios, the requirements of optical modules are divided into SR (<300 m ), DR (500 m ), FR (2km) , LR (10Km) , ER (40km) , ZR (80km) and so on. At present, the most used modules are the TOR guide from the server， SR， 25G SR (AOC)，100G SR4 ( AOC )， 200G SR4 ( AOC ) , 400G SR8 (AOC) for multimode applications.