Do You Know These about CWDM?


With the emerging of CWDM(Coarse Wavelength Division Multiplex) tech in the optical communication industry, various CWDM optical transceiver products appear in the markets, such as CWDM SFP+, CWDM SFP, CWDM XFP, 100G QSFP CWDM4 and so on. Well, today, Gigalight will have an introduction to CWDM from these viewpoints that you should have control of it.

CWDM Definition and CWDM System Principle

1. The Definition of CWDM

CWDM(Coarse Wavelength Division Multiplex), is a low-cost WDM transmission technology for MAN(Metropolitan Area Network) access layer. Speaking in principle, CWDM uses the optical multiplexer to multiplex the optical signals with different wavelengths onto the single fiber for transmission. On the receiving end of the link, the mixed signals on the optical fiber are demultiplexed into signals with different wavelengths via optical demultiplexer and are connected to the corresponding receiving devices.

2. The Principle of CWDM System

On account that the wavelength interval of the CWDM system is wide, the requirement for technology index of the laser is low. Besides, due to that the wavelength interval reach 20nm, the maximum wavelength shift of the system can reach 6.5℃ to +6.5 ℃; the emission wavelength accuracy of laser can be extended to ±3nm; in the range of working temperature (-5℃~70℃), the wavelength shift caused by temperature change is still within the allowable range, the laser is not required to control the system without temperature. After that, the structure of the laser is greatly simplified and the yield rate is increased.

In addition, the larger wavelength spacing means that the structure of the multiplexer/demultiplexer is greatly simplified. For example, the coating layer number of the filter in CWDM system can be reduced to about 50 layers, while that of the 100GHz filter in the DWDM system is about 150 layers, which leads to higher yield rate and lower cost. The cost of CWDM filter is less over 50% of than that of DWDM filter and will be further reduced with the increase of automatic production technology and batch size.

Advantages and Features of CWDM Tech

1. Taking Full Advantage of the Low-Loss Waveband of Optic Fiber

Making full use of the low-loss waveband of the optic fiber and increasing the transmission capacity of optical fiber, the physical limit of transmitting information by one fiber can be doubled or be added by several times. Currently, a fraction of low-loss spectrum (1310nm-1550nm) of the optical fiber is applied. The wavelength division multiplexing(WDM) can make full use of the huge bandwidth(25THz)of the single-mode fiber, and the transmission bandwidth is sufficient.

2. Able to Transmit Multiple Signals On the Same Optical Fiber

With the ability to transmit 2 or several non-synchronous signals on the same fiber, it is beneficial for digital signals and analog signals, independent of the data rate and modulation mode. Simultaneously, can be flexibly removed out of or added to the channels in the middle of the line.

3. With High Flexibility

For the established optic fiber system, especially for early-laid optical cable with few fibers, as long as the original system is with redundant power rate, the capacity can be expanded, multiple unidirectional signals or bidirectional able to be transmitted without great changes to the original system. It is with high flexibility.

4. Quick and Convenient to Restore

Due to the reduction to the application of optic fiber to a large extent, the cost is greatly decreased. Besides, because it is less in the number of optic fiber, it is quick and convenient to restore when failure appears.

5. Reducing the Cost

The shareability of active optical devices reduces the cost of transmitting multiple signals or adding new services.

6. Improving the Reliability of the System

The active equipment in the system has been greatly reduced so that the reliability of the system is improved. Currently, because multichannel carrier optical wavelength division multiplexing (WDM) has high requirements for optical transmitter and an optical receiver, it is difficult to implement in tech. Simultaneously, the application of optical cables with multi fibers does not bring about the critical shortage of transmission businesses of traditional broadcast TV.

Therefore, WDM is not widely applied. However, with the development of the CATV integrated services, the increasing demand for network bandwidth,the implementation of various selective services, the consideration on cost for the network upgrade, etc, the features and advantages of WDM are gradually emerging in the CATV transmission system, showing a broad application prospect and even affecting the development pattern of the CATV network.

Applications of CWDM

1. Network Expansion and Upgrading: able to convert any input optical wavelength into fixed ITU-CWDM output optical wavelength, simultaneously to transmit up to dozen channels of optical signals, which greatly expands the transmission capacity and utilization of optical fiber, saves the time and cost of laying optical fiber cables, and starts new businesses without affecting the original businesses.

2. Hybrid Transmission of Various Signals: suitable for the upgrading of SDH, ATM, Ethernet, Fiber Channel devices, long distance line relay, analog signal transmission, and hybrid transmission of digital and analog signals in one fiber, within 10Mb/s to 2.5 Gb/s rate.

3. Mode Conversion: able to convert the single-mode optical wave into any single-mode/multi-mode optical wave, which is suitable for various complex network conditions.

4. Wavelength Conversion: able to convert any wavelength of single-mode/multi-mode optical wave into CWDM wavelength, or to convert CWDM wavelength into another arbitrary wavelength, the transmission distance able to reach several hundreds of kilometers.

5. Optical Relay: multiple CWDM can be connected in series to increase transmission distance (up to several hundred kilometers).

6. Security Networking: multiple separate virtual optical networks on the physical channels can be constituted on a pair of optical fibers, to protect the network from attacks of all software viruses and hackers. Its security is far higher than that of the general VPN, especially suitable in the fields of government, public security, and banking, etc.


Above all is an overview of coarse wavelength division multiplex. Apart from meeting the requirements for cost-saving, it is also with high reliability and flexibility, which are also key factors in the application of optical transceiver products and in the process of data transmission. For CWDM products, except for CWDM SFP+, CWDM SFP, CWDM XFP optical transceiver module, the mainly-recommended one of Gigalight is QSFP28 100G CWDM4 optical transceiver at present.

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