Comparison Between CWDM & DWDM Technology

For a better signal transmission in fiber-optic communication, different kinds of technologies are applied to the industry. Wavelength-division multiplexing (WDM) is one of the commonly used technologies which multiplexes a number of optical carrier signals onto a single optic fiber by using different wavelengths of laser light. That is to say, WDM enables two or more than two wavelength signals to transmit through different optical channels in the same optical fiber at the same time.

WDM

In the WDM system, there are two types of divisions – CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). They are both using multiple wavelengths of laser light for signal transmission on a single fiber. However, from the aspects of channel spacing, transmission reach, modulation laser and cost, CWDM and DWDM still have a lot of differences. This article will focus on these distinctions and hope you can have a general understanding about CWDM and DWDM technology.

Channel Spacing

As their names suggest, the words “coarse” and “dense” reveal the difference in channel spacing. CWDM has a wider spacing than DWDM. It is able to transport up to 16 wavelengths with a channel spacing of 20 nm in the spectrum grid from 1270 nm to 1610 nm. But DWDM can carry 40, 80 or up to 160 wavelengths with a narrower spacing of 0.8 nm, 0.4 nm or 0.2 nm from the wavelengths of 1525 nm to 1565 nm (C band) or 1570 nm to 1610 nm (L band). It is no doubt that DWDM has a higher performance for transmitting a greater number of multiple wavelengths on a single fiber.

CWDM-VS-DWDM

Transmission Reach

Since the wavelengths are highly integrated in the fiber during light transmission, DWDM is able to reach a longer distance than CWDM. The amplified wavelengths provide DWDM with the ability of suffering less interference over long-haul cables. Unlike DWDM system, CWDM is unable to travel unlimited distance. The maximum reach of CWDM is about 160 kilometers but an amplified DWDM system can go much further as the signal strength is boosted periodically throughout the run.

Modulation Laser

CWDM system uses the uncooled laser while DWDM system uses the cooling laser. Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled down to near absolute zero through the interaction with one or more laser fields. Cooling laser adopts temperature tuning which ensures better performance, higher safety and longer life span of DWDM system. But it also consumes more power than the electronic tuning uncooled laser used by CWDM system.

Cost

Because the range of temperature distribution is nonuniform in a very wide wavelength, so the temperature tuning is very difficult to realize, thus using the cooling laser technique increases the cost of DWDM system. Typically, DWDM equipment is four or five times more expensive than CWDM equipment.

Conclusion

CWDM and DWDM are both coming from the WDM technology that is capable of conveying multiple wavelengths in a single fiber. But with different characteristics, people should think twice before choosing the CWDM or DWDM system. CWDM usually costs less but its performance is far behind DWDM. Both your requirements and budget need to be taken into consideration. Moreover, the WDM products including CWDM mux/demux module, DWDM mux/demux module and optical splitter are highly welcome in the market.

Overview of Single-mode Fiber Types

According to the light transmission mode, optic fibers can be classified into single-mode and multimode. It’s easy to categorize multimode fiber into four types of OM1, OM2, OM3 and OM4. However, when it comes to single-mode, it may not be as simple as you think. The classification of single-mode fiber is much more complicated than multimode fiber. ITU-T G.65x series and IEC 60793-2-50 (published as BS EN 60793-2-50) are two primary sources for single-mode fiber specification. This article will mainly focus on the ITU-T G.65x series.

The following table introduces 19 ITU-T specifications of single-mode fiber:

Name Type
ITU-T G.652 ITU-T G.652.A, ITU-T G.652.B, ITU-T G.652.C, ITU-T G.652.D
ITU-T G.653 ITU-T G.653.A, ITU-T G.653.B
ITU-T G.654 ITU-T G.654.A, ITU-T G.654.B, ITU-T G.654.C
ITU-T G.655 TU-T G.655.A, ITU-T G.655.B, ITU-T G.655.C, ITU-T G.655.D, ITU-T G.655.E
ITU-T G.656 ITU-T G.656
ITU-T G.657 ITU-T G.657.A, ITU-T G.657.B, ITU-T G.657.C, ITU-T G.657.D

Each type has its own area of application and the evolution of these optical fiber specifications reflects the evolution of transmission system technology from the earliest installation of single-mode optical fiber to the present day. Choosing the right one for your project can be vital in terms of performance, cost, reliability and safety. Now, let’s have a look at the differences of G.65x series specifications for single-mode fiber respectively.

G.652

The ITU-T G.652 fiber is known as standard SMF (single-mode fiber) and is the most commonly deployed fiber. It comes in four variants (A, B, C, D). A and B have a water peak. C and D eliminate the water peak for full spectrum operation. The G.652.A and G.652.B fibers are designed to have a zero-dispersion wavelength near 1310 nm, therefore they are optimized for operation in the 1310nm band. They can also operate in the 1550nm band, but it is not optimized for this region due to the high dispersion. These optical fibers are usually used within LAN, MAN and access network systems. The more recent variants (G.652.C and G.652.D) feature a reduced water peak that allows them to be used in the wavelength region between 1310 nm and 1550 nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission.

G.652

G.653

G.653 fiber was developed to address this conflict between best bandwidth at one wavelength and lowest loss at another. It uses a more complex structure in the core region and a very small core area, and the wavelength of zero chromatic dispersion was shifted up to 1550 nm to coincide with the lowest losses in the fiber. Therefore, G.653 fiber is also called dispersion-shifted fiber (DSF). G.653 has a reduced core size, which is optimized for long-haul single-mode transmission systems using erbium-doped fiber amplifiers (EDFA). However, its high power concentration in the fiber core may generate nonlinear effects. One of the most troublesome, four-wave mixing (FWM), occurs in a Dense Wavelength Division Multiplexed (CWDM) system with zero chromatic dispersion, causing unacceptable crosstalk and interference between channels.

G.653

G.654

The G.654 specifications entitled “characteristics of a cut-off shifted single-mode optical fiber and cable”. It uses a larger core size made from pure silica to achieve the same long-haul performance with low attenuation in the 1550nm band. It usually also has high chromatic dispersion at 1550 nm, but is not designed to operate at 1310 nm at all. G.654 fiber can handle higher power levels between 1500 nm and 1600 nm, which is mainly designed for extended long-haul undersea applications.

G.655

G.655 is known as non-zero dispersion-shifted fiber (NZDSF). It has a small, controlled amount of chromatic dispersion in the C-band (1530-1560 nm), where amplifiers work best, and has a larger core area than G.653 fiber. NZDSF fiber overcomes problems associated with four-wave mixing and other nonlinear effects by moving the zero-dispersion wavelength outside the 1550nm operating window. There are two types of NZDSF, known as (-D)NZDSF and (+D)NZDSF. They have respectively a negative and positive slope versus wavelength. Following picture depicts the dispersion properties of the four main single-mode fiber types. The typical chromatic dispersion of a G.652 compliant fiber is 17ps/nm/km. G.655 fibers were mainly used to support long-haul systems that use DWDM transmission.

G.655

G.656

As well as fibers that work well across a range of wavelengths, some are designed to work best at specific wavelengths. This is the G.656, which is also called Medium Dispersion Fiber (MDF). It is designed for local access and long haul fiber that performs well at 1460 nm and 1625 nm. This kind of fiber was developed to support long-haul systems that use CWDM and DWDM transmission over the specified wavelength range. And at the same time, it allows the easier deployment of CWDM in metropolitan areas, and increases the capacity of fiber in DWDM systems.

G.657

G.657 optical fibers are intended to be compatible with the G.652 optical fibers but have differing bend sensitivity performance. It is designed to allow fibers to bend, without affecting performance. This is achieved through an optical trench that reflects stray light back into the core, rather than it being lost in the cladding, enabling greater bending of the fiber. As we all know, in cable TV and FTTH industries, it is hard to control bend radius in the field. G.657 is the latest standard for FTTH applications, and, along with G.652 is the most commonly used in last drop fiber networks.

Conclusion

There are different types of single-mode fiber used for different application. G.657 and G.652 are typically favored by planners and installers, and G.657 is particularly deployed for FTTH applications because of a larger bend radius. And G.655 has been taken the place of G.643 used for WDM system. In addition, G.654 is usually applied to the subsea area. To know more information about single-mode fiber, you are welcome to visit the website at FS.COM.

What Should You Know Before Choosing the Single-mode Fiber?

Fiber optical cable has single-mode and multimode type. Multimode fiber includes types of OM1, OM2, OM3, 0M4. How many kinds of single-mode fiber? There are two primary specifications of single-mode fiber. One is the ITU-T G.65x series, and the other is IEC 60793-2-50 (published as BS EN 60793-2-50). This article will introduce ITU-T G.65x series.

single-mode fiber

There are 19 types of single-mode fiber specifications defined by ITU-T (shown in the following table). Different type has different application area. From the change of single-mode fiber specifications, we can see the evolution of transmission system technology. As so many kinds of single-mode fiber, which one should you choose to get perfect performance with the fewest cost? Following will tell about each specifications in details.

ITU-T Specifications Type
ITU-T G.652 ITU-T G.652.A, ITU-T G.652.B, ITU-T G.652.C, ITU-T G.652.D
ITU-T G.653 ITU-T G.653.A, ITU-T G.653.B
ITU-T G.654 ITU-T G.654.A, ITU-T G.654.B, ITU-T G.654.C
ITU-T G.655 ITU-T G.655.A, ITU-T G.655.B, ITU-T G.655.C, ITU-T G.655.D, ITU-T G.655.E
ITU-T G.656 ITU-T G.656
ITU-T G.657 ITU-T G.657.A, ITU-T G.657.B, ITU-T G.657.C, ITU-T G.657.D

ITU-T G.652

ITU-T G.652 fiber is also known as standard SMF (single-mode fiber) and is the most commonly deployed fiber. It comes in four variants (A, B, C, D). A and B have a water peak. C and D eliminate the water peak for full spectrum operation. G.652.A and G.652.B fibers are designed with a zero-dispersion wavelength near 1310 nm, which can be optimized for the operation in 1310nm band. They can also operate in 1550nm band, but it is not optimized for this region due to the high dispersion. The two fibers are usually used within LAN, MAN and access network systems. While G.652.C and G.652.D reduce water peak and can be used in the wavelength region between 1310 nm and 1550 nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission.

ITU-T G.653

ITU-T G.653 fiber uses a more complex structure in the core region and a very small core area, and the wavelength of zero chromatic dispersion was shifted up to 1550 nm to coincide with the lowest loss in the fiber. It can address this conflict between best bandwidth at one wavelength and lowest loss at another. So G.653 fiber is also called dispersion-shifted fiber (DSF). It has a smaller core size, which is optimized for long-haul transmission system combined with erbium-doped fiber amplifiers (EDFA). However, its high power concentration in the fiber core may generate nonlinear effects. What’s more, four-wave mixing (FWM) occurs in a Dense Wavelength Division Multiplexed (CWDM) system with zero chromatic dispersion, causing unacceptable crosstalk and interference between channels.

ITU-T G.654

G.654 is called “characteristics of a cut-off shifted single-mode optical fiber and cable”. It uses a larger core size made from pure silica to achieve the same long-haul performance with low attenuation in the 1550nm band. It has high chromatic dispersion at 1550 nm but can’t operate at high chromatic dispersion of 1310 nm. G.654 fiber can handle higher power levels between 1500 nm and 1600 nm, which is mainly designed for extended long-haul undersea applications.

ITU-T G.655

G.655 is known as non-zero dispersion-shifted fiber (NZDSF). It has a small, controlled amount of chromatic dispersion in the C-band (1530-1560 nm), where amplifiers work best, and has a larger core area than G.653 fiber. NZDSF fiber can deal with four-wave mixing and other nonlinear effects by moving the zero-dispersion wavelength outside the 1550-nm operating window. There are two types of NZDSF, known as (-D)NZDSF and (+D)NZDSF. Each one has a negative and positive slope versus wavelength. G.655 fibers are mainly used to support long-haul transmission in DWDM system.

ITU-T G.656

G.656 fiber is called Medium Dispersion Fiber (MDF). It’s designed for local access and long haul fiber that performs well at 1460 nm and 1625 nm. This kind for fiber can support long-haul systems that use CWDM and DWDM transmission over the specified wavelength range. And at the same time, it allows the easier deployment of CWDM in metropolitan areas, and increase the capacity of fiber in DWDM systems.

ITU-T G.657
G.657 fiber was originally designed to be compatible with the G.652 fibers but have different bend sensitivity performance. It allows fibers to bend without affecting performance. This is achieved through an optical trench that reflects stray light back into the core and avoids the light lost in the cladding. In reality, it’s hard to control bend radius in the field, such as FTTH applications. G.657 is the latest standard for FTTH applications, and, along with G.652 is the most commonly used in last drop fiber networks.

Conclusion

From the above, different kinds of single-mode fibers have different applications. G.643 is not often used in WDM system because of some problems and is replaced by G.655. G654 is mainly for submarine use. G656 is designed for specific wavelengths. G.657 is compatible with the G.652 but has a larger bend radius than G.652, which is especially suitable for FTTH applications. Now a better understanding of these single-mode fibers will help you to choose the most suitable single-mode fiber.

Fiberstore Coarse Wavelength Division Multiplexing Devices

CWDM mux/demux is a flexible solution that enables operators make full use of available fiber bandwidth in local loop and enterprise architectures. The wavelengths used with CWDM implementations are defined by the ITU-T G.694.2, listing 18 wavelengths from 1270nm to 1610nm with 20nm increased. CWDM solution takes the most important advantage of low price which is typically 1/3rd lower than the equivalent DWDM optics. FS.COM introduces its new generation of coarse wavelength-division multiplexing (CWDM) devices boasting increased functions and improved performance to extend the reach of CWDM metropolitan networks. The following text will introduce CWDM Mux/Demux, CWDM OADM, and optical port configuration used in CWDM network.

FS.COM CWDM MUx/Demux

The CWDM Mux/Demux in FS.COM is a universal device capable of combining up to 18 optical signals into a fiber pair or 9 optical signals into a single fiber. It is designed to support a broad range of architectures, ranging from scalable point-to-point links to two fiber-protected rings.

Besides, FS.COM CWDM Mux/Demux is a passive device which allows for any protocol to be transported over the link, as long as it is at a specific wavelength (i.e. T1 over fiber at 1570nm transported alongside 10Gbps Ethernet at 1590nm). This allows for long-term future proofing of the networking infrastructure because the multiplexers simply refract light at any network speed, regardless of the protocol being deployed. The following image shows FS.COM 8 Channels 1470-1610nm Dual Fiber CWDM Mux Demux.

8 Channels 1470-1610nm Dual Fiber CWDM Mux Demux

FS.COM CWDM Mux/Demux With Different Optional Port Configurations

FS.COM also provides CWDM Mux/Demux with different optional port configurations such as, express port, monitor port, 1310nm pass band port and 1550nm port for these multiplexers according to customer choice.

  • Monitor Port: Our CWDM Mux/Demux is optional to equip with monitor port that allows our customer connect optical meter or OSA to monitor and troubleshoot the network. It is simple to operate. Add the monitor port to an existing, multiplexed link. A small sample, of each signal, is “leaked” to the outputs, then connect measurement/monitoring equipment, such as power meters or network analyzers, to the module outputs. When finished monitoring, disconnect the instruments. The network is left undisturbed. (Monitor port tap percentage is 5% as default.)
  • Expansion/Express Port: The Expansion Port (EXP) enables the cascading of two CWDM Mux/Demux modules, doubling the channel capacity on the common fiber link. For example, two 4-Channel MUX/DEMUX modules can be cascaded to create an 8-Channel fiber common link. (Express port isolation is 15dB as default.)
  • 1310 Pass Band Port: The 1310 pass band port allows a legacy 1310nm signal to pass through the CWDM MUX DEMUX module. The port can be used to combine an existing legacy 1310nm network with CWDM channels, allowing the CWDM channels to be overlaid on the same fiber pair as the existing 1310nm network. (Note: When you choose 1310nm pass band port, the CWDM 1310nm wavelength channel is NOT available on the CWDM MUX modules.) Besides, the 1310nm port can be used in this way as an optical supervisory channel (OSC) and its range is 1270nm-1350nm (1310nm±40nm). (Note: When you choose 1310nm pass band as an OSC, the available range of wavelength is 1370nm~1610nm on the CWDM MUX modules.)
  • 1550 pass band port: The 1550 pass band port allows a legacy 1550nm signal to pass through the CWDM Mux/Demux module. The 1550nm port can also be used in this way as an optical supervisory channel (OSC) and its range is 1510nm-1590nm (1550nm±40nm). When you choose 1550nm pass band as an OSC, the available range of wavelength is 1270nm~1490nm on the CWDM MUX modules.

Note: that standard (or native) 1310nm and 1550nm wavelengths are not the same as CWDM 1310nm and CWDM 1550nm wavelengths. The center wavelength tolerances for legacy 1310nm and 1550nm are much wider than the CWDM equivalents, and therefore not precise enough to run through CWDM filters. When implementing a CWDM network, a standard wavelength can be converted to CWDM wavelength, or a CWDM Mux with a pass band port can overlay the standard wavelength onto the CWDM common link. A pass band port is an additional channel port on a CWDM MUX that allows a legacy 1310nm or 1550nm signal to pass through the network within a reserved band. The legacy device is connected directly to the pass band port via fiber cabling. Standard wavelengths can be converted to CWDM wavelengths using CWDM Small Form Pluggable (SFP) transceivers, transponders, and media converters that support SFPs.

FS.COM CWDM OADM

Since adding new fiber optic cables for signal transmission of the devices would cost too much, IT managers would turn to use OADM in CWDM network, which can couple two or more wavelengths into a single fiber as well as the reverse process, saving a lot of money and installation time when they want to add or drop signal on a single fiber. FS.COM provides a wide selection of CWDM OADM which can add or drop fiber count of 1, 2 and 4. And these OADMs can be categorized into three type with different package form factors: plug-in module, pigtailed ABS box and rack mount chassis. The plug-in modules can be installed in empty rack enclosures. Three CWDM OADM types with different package form factors are shown below.

plug-in module, pigtailed ABS box and rack mount chassis

Conclusion

CWDM is a popular technology which can provide cost-effective solutions for users to upgrade their network using the least fiber strands. FS.COM provides a series of devices used in CWDM network, like CWDM Mux/Demux with different optical port configurations, CWDM OADMs, CWDM transceiver modules, etc. For any requirement, please visit FS.COM.