Tag Archives: CWDM

Differences between CWDM System and DWDM System

As more bandwidth and faster data transmission rates over long distances are needed, users have to rely on more fiber optics. But laying more fibers will cost much. So WDM (Wave Division Multiplexing) technology appeared and solved this problem.

WDM is a technology which transmits multiple optical signals on a single fiber by using different wavelengths, or colors, of laser light to carry the different signals. By using bidirectional communications over a single fiber, network users can realize a multiplication effect in their available fiber’s capacity. The effect of expanding the capacity of the network without laying more fibers makes WDM systems so popular. The concept was first published in 1978. At the very beginning, only two signals could be combined by using WDM systems. But now more signals could be handled and transmission rates get faster.

Currently WDM systems are divided into two different types: CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). CWDM systems typically have 8 channels with 8 wavelengths separated by 20 nm from 1470 nm to 1610 nm. But the channels could also be increased to 16. Typical DWDM systems use 40 channels and provide wavelengths up to 96 with 0.4nm spacing. The channel spacing decides the call of “coarse” and “dense”. There are many differences between these two systems.


Transmission Distance

CWDM systems can’t transmit data over distances as long as DWDM system because the wavelengths are not amplified. Usually CWDM can travel somewhere about 100 miles. Therefore CWDM is limited in its functionality over longer distances. While DWDM systems are set for long haul transmission by keeping the wavelengths tightly packed. They can transmit more data over longer distances with less interference than a CWDM system. So it’s suitable for data transmission over long distances.

Power Requirements

The power requirements for DWDM are significantly higher. For example, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4 W per wavelength. But CWDM uses uncooled laser transmitter and it consumes about 0.5 W of power.


Two main factors like operating and hardware cost cause difference between CWDM and DWDM. The first example has been referred before. CWDM modulation laser is uncooled, but DWDM laser is cooling. Cooling laser using temperature tuning, uncooled laser adopts electronic tuning. The range of temperature distribution is non-uniform in a very wide wavelength, so the temperature tuning is very difficult to realize, which results in high cost. CWDM doesn’t have this problem. Second, for instance, DWDM transceivers are typically four or five times more expensive than CWDM counterparts. That’s mainly because of the lasers. Typical wavelength tolerances for DWDM lasers die are on the order of ±0.1 nm; whereas tolerances for CWDM lasers die are ±2-3 nm. Lower die yields increase the costs of DWDM lasers relative to CWDM lasers. From these two sides, CWDM system is cheaper than DWDM system.

Each kind of WDM system has its advantages and disadvantages. CWDM is very flexible and is good for campus LAN expansion. DWDM has large capacity and is ideal for long distance data transmission. CWDM and DWDM technology continue be to improved. The two have different functions and will complement not replace the other.

Optical Amplifiers – a Way to Improve the Signal Quality

An optical amplifier is a fiber optic device that can amplify optical signals directly without the need to convert it to an electrical signal. Optical amplifiers can increase the strength of the optical signal and allow the signals transmit through the cable over long distances. The emergence of optical amplifiers brings great improvement to networking and communication system.

Why Do We Need Optical Amplifiers?

There are three reasons. One reason is that the optical to electrical and electrical to optical conversions requires high-speed electronics and costs too much. The other reason is that signals carried by a fiber will be attenuated because no fiber material is absolutely transparent. Then repeaters must be used in the optical fibers which maybe longer than 100 kilometers. And the last one is that the signal-to-noise ratio of detected signals is too low and bit error rate becomes too high. So the most efficient and best optical method is an optical amplifier. Optical amplifiers could avoid the need for E-O and O-E conversions. It’s beneficial to compensate for attenuation losses during signals transmitting over long distances.

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Types of Optical Amplifiers

There are three types of optical amplifiers: rare earth doped fiber amplifiers (the most popular one- Erbium Doped Fiber Amplifiers), semiconductor optical amplifiers, Raman amplifiers. But the question is which one is suitable for your specific transmission system. By the following comparison you may find the answer.

Erbium doped fiber amplifiers (EDFAs): they have high pump power utilization. Amplifiers can directly and simutaneously amplify a wide wavelength band (>80nm) in the 1550nm region with a relatively flat gain. And EDFAs have low noise, which is suitable for long haul applications. However, compared with other two kinds of amplifiers, EDFAs can’t be integrated with other semiconductor devices and can obtain small signal gain only in a specific frequency band.

optical amplifiers

Semiconductor optical amplifiers: they are of small size and electrically pumped. Its high optical nonlinearity is good for all optical signal processing like optical switching and wavelength conversion. SOA provides the possibility for gain in different wavelength regions. The gain reacts rapidly to changes of pump or signal power and the changes of gain cause phase changes which can distort the signals. Compared with EDFA, SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time. However, it’s less expensive than EDFA and can be integrated with semiconductor lasers, modulators, etc.

Raman amplifiers: they gain exists in every fiber, which provides a cost-effective means of upgrading from the terminal ends. Furthermore, the gain is nonresonant so that it’s available over a wide transparent region of fiber. And the gain spectrum can be tailored by adjusting the pump wavelength. Compared with EDFA, Raman amplifiers have relatively poor pumping efficiency at lower signal powers and require a longer gain fiber.

The main purpose of this article is to give you suggestions to obtain a high transmission capacity over long distances. Optical amplifiers are quite important elements in wavelength division multiplexing (WDM) system. You may apply DWDM (dense wavelength division multiplexing) or CWDM (coarse wavelength division multiplexing). And these optical amplifiers could be used in both DWDM and CWDM transmission system. So you don’t need to take this as a problem for imrpoving your signal quality.