As an unprecedented opportunity to dramatically increase the bandwidth capacity, WDM (Wavelength Division Multiplexing) technology is an ideal solution to get more bandwidth and lower cost in nowaday telecommunications networks. By virtue of fame, WDM becomes a household word now. Yet, most of the time, we only know what is “WDM” but do not really know WDM technology. Actually, there are various of terminologies used in WDM that are always a headache for us. Now, let’s see what are they.
First of all, there are three basic terminologies you should clearly know.
A technology that multiplexes a number of optical carrier signals onto a single optical fiber by using different optical wavelengths (i.e., colors) of laser light. It breaks white light passing through fiber optic cable into all the colors of the spectrum, much like light passed through a prism creates a rainbow. Every wavelength carries an individual signal that does not interfere with the other wavelengths.
CWDM is a specific WDM technology defined by the ITU (International Telecommunication Union) in ITU-T G.694.2 spectral grids, using the wavelengths from 1270 nm to 1610 nm within a 20nm channel spacing. It is a technology of choice for cost efficiently transporting large amounts of data traffic in telecoms or enterprise networks.
DWDM is a specific WDM technology also defined by the ITU but in ITU-T G.694.1 spectral grids. The grid is specified as frequency in THz, anchored at 193.1 THz, with a variety of specified channel spacing from 12.5 GHz to 200 GHz, among which 100 GHz is common. In practice, DWDM frequency is usually converted to wavelength. DWDM typically has the capability to transport up to 80 channels (wavelengths) in what is known as the Conventional band (C-band) spectrum, with all 80 channels in the 1550 nm region.
When referring to fiber optic transmission in WDM system, you should know these:
Single fiber, namely bi-directional communication on one single fiber. This system utilizes two identical sets of wavelengths for both directions over a single fiber. Individual channels residing on the single fiber system may propagate in either direction.
Dual fiber, namely comprised of two single fibers, one fiber is used for the transmit direction and the other is used for the receive direction. In dual fiber transmission system, the same wavelength is normally used in both the transmit and receive directions. The second fiber may serve as a backup fiber as in a redundant system, or it may provide an optical path in the opposite direction.
The direction of a communication signal can be refered using these two terminologies. The downstream direction is defined as communication originating at a service provider and sent to the service user. Upstream is in the opposite direction.
The topology in which WDM systems are used plays a key role in determining the extent to which the WDM network is utilized. The related terminologies are:
WDM products bring higher efficiency to fiber networks through multiple channel usage of fiber. Networks are identified by their fiber layout or topology. Network topologies such as Mesh, Ring, P2P (Point-to-Point), and P2MP (Point-to-Multipoint) will sometimes use WDM products particularly designed for the network. So, it is important to understand the intended network use when selecting WDM products. Entire networks are often comprised of several kinds of sub-network topologies.
In metropolitan area networks, infrastructures are generally organized over a ring topology. Ring topology is a type of network topology consisting of a closed loop. Fiber ring networks are comprised of a series of fiber spans that terminate at network nodes spread throughout the loop. Each node in the ring will connect to two, and only two, adjacent nodes. Ring networks are often dual fiber systems. Contrast ring topology with an unclosed, end-to-end or point-to-point fiber span.
In network topology, a node is a termination of a single branch or multiple branches of the network. A WDM network consists of a set of nodes, physically interconnected by optical fiber (the physical topology), upon which a logical topology is overlaid by establishing lightpath interconnections between the nodes. The use of WDM on the fiber side allows the node to be segmented or divided into additional serving areas thus expanding the customer base and available bandwidth.
AWG, including Athermal AWG (AAWG) and Thermal AWG (TAWG), is commonly used as optical MUX/DeMUX in WDM systems. AAWG have equivalent performance to standard TAWG but require no electrical power, software or temperature.
FBGs are versatile wavelength filters for multiplexing and demultiplexing WDM signals. They also can compensate for chromatic dispersion that can degrade the quality of the WDM signal in an optical fiber.
Thin film filters were adopted very early on and have been widely deployed since because they have the unique attributes that meet the stringent requirements of optical communication systems. The main advantage of thin film filters is its ability to achieve high accuracy in processing in small device sizes when compared it to competing technologies.
To build a WDM system, these WDM equipment are required:
WDM multiplexer is a device that multiplexes or combines optical signals of different wavelengths (colors) together on one single fiber.
In contrast to multiplexer, DeMux is a device that de-multiplexes or splits optical transmission comprised of multiplexed wavelengths onto individual fibers assigned to each wavelength.
Note: In today’s market, there are CWDM Mux/DeMux products and DWDM Mux/DeMux products. These products have the Mux and DeMux inside and comes in a package like 1RU 19″ rackmont, LGX box and ABS module etc.
OADM is a device used in WDM systems for multiplexing and routing different channels of light into or out of a single fiber.
Filter-based Wavelength Division Multiplexer (FWDM) is a kind of WDM multiplexer based on the Thin Film Filter (TFF) technology. FWDM combines or separates light at different wavelengths in a wide wavelength range and is extensively used in EDFA, Raman amplifiers, and WDM optical networks.
As the name suggests, these are multi-channel WDM products that have relatively small footprints so that they can provide more channels with a device footprint small enough to fit within a FOSC (Fiber Optic Splice Closure), splice tray or splice holder. These products utilize a free-space multiple bounce technology in which light reflects from each filter element directly onto the next filter element instead of being collimated and launched into a fiber as in individual, discrete TFF components. In addition, bend insensitive fiber permits the use of smaller housings for joining (concatenating) individual TFFs into a multi-channel product.
Banded skip filters are used to build BWDM (Band WDM) products. These filters are TFFs that have wide pass bands, which contain multiple channels. For example, DWDM Red/Blue C-band Filter is used to separate or combine Red and Blue band wavelength signals in C-band DWDM systems and high-power amplification systems. It is just like a regular FWDM, with the only difference that the wavelengths are split in Red/Blue filter while bonded in WDM.
Ports on WDM equipment, do you really know their functions?
The connection point of a WDM product where combined channels appear. For a MUX product, combined channels are transmitted from the common port. For a DEMUX, the combined channels are received at the common port.
For CWDM products, there will normally be either an upgrade or an express port, but not both. The upgrade or express port on a CWDM Mux or DeMux is used to add, drop, or pass through additional channels which enables the cascading of two CWDM Mux/DeMux modules, doubling the channel capacity on the common fiber link.
For DWDM products, the purpose of an upgrade port is to be able to add, drop, or pass through C-band DWDM channels not already in use, namely only channels that reside in the band 1530 – 1565 nm. If the DWDM product also has an express port, then that port is normally used for additional channels residing outside the C-band, such as most of the CWDM channels.
The 1310nm port is a wide band optic port added to other specific CWDM wavelengths in a module. For example if an 8 channel CWDM is called out it may use wavelengths 1470 nm to 1610 nm and request the 1310nm port. The 1310nm port is used in some legacy networks and sometimes as a return path. If an existing legacy network is using 1310nm port and they have exhausted all fibers and are looking for ways to increase their network capacity they can add in other CWDM wavelengths on to the same fiber while still allowing the use of the 1310nm port. Meanwhile, it can carry LR optics, LX optics etc.
Similar to 1310nm port, allows a legacy 1550nm signal to pass and can carry ER optics, ZR optics, LX optics, ZX optics etc.
This port is used to monitor or test the power signal coming out of a Muxed CWDM or before it gets demuxed from the signal coming through the fiber network usually at a 5% or less power level. Generally, it can be connected with measurement or monitoring equipment, such as power meters or network analyzers. Network administrators will use this to test of monitor if a signal has failed or changed without having to interrupt the existing network.
You should also know these parameters when operating a WDM system:
Wavelength is the distance, measured in the direction of propagation, between two points of the same phase in consecutive cycles of a wave. The wavelength λm of monochromatic light travelling in a optical fiber is expressed:
Note: In WDM practice, wavelengths such as the wavelength of a communications laser, the wavelength specifications for optical filters, and the wavelengths of optical transmission channels over fiber are all given as λ, the wavelength in nanometers as would occur in a vacuum.
In WDM systems, each input channel is assigned a unique wavelength (i.e. color of light), thus the channels can traverse the fiber “in parallel”.
A pass band is the range of frequencies or wavelengths that can pass through a filter. It is one of the parameters of WDM filters. In practice, it is the tolerance of the filter for laser drift away from the center wavelength. For example, a typical pass band for CWDM filters is ± 6.5 nm about the center wavelength. So a 1551nm laser could operate within a range of 1544.5 nm to 1557.5 nm without encountering extra channel loss.
Insertion loss is the attenuation caused by the insertion of WDM filter in an optical transmission system. It is normally specified as the maximum insertion loss occurring across the filter pass band. The insertion loss of a WDM product is given as the maximum insertion loss occurring at the channel port with the highest loss. In WDM networks, insertion loss is one of several contributors to the total loss of the communication link. Thin film filters exhibit fairly wide manufacturing variance in their insertion loss values and are screened prior to use in WDM products.
The loss exhibited by a WDM filter is dependent on the optical polarization of the light. PDL is the largest difference in maximum insertion loss occurring at all states of optical polarization. PDL for a WDM product is specified as the largest allowed PDL for any channel.
PMD is an important linear phenomenon occurring inside optical fibers, which can cause the optical receiver to be unable to interpret the signal correctly, and results in high bit error rates. It is another polarization effects that lead to impairments in the long-haul optical fiber transmission systems.
Return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fiber of WDM systems. A large value of return loss is desirable for preventing problems with source lasers and reducing transmitted loss. Return loss for a WDM product is the smallest, measured return loss at all ports.
Passband ripple is defined as the maximum peak-to-peak loss variation within the passband of one channel.
Isolation is a measure of light at an undesired wavelength at any given point. Expressed in dB, it is the difference of the maximum insertion loss within the filter pass band and the minimum loss occurring within other filtering pass bands. Isolation is measured by applying a swept optical power source to the filter’s common port and measuring the loss within the filter’s pass band and the pass bands of other filters. When other filters are those with pass bands nearest to the filter’s pass band, it is called the Adjacent Channel Isolation. For the remaining ports, it is called the Non-Adjacent Channel Isolation.
Operating Temperature (°C) is the ambient temperature range over which the device’s performance spec can be met.
Storage Temperature (°C) is the ambient temperature range over which the device can be stored without affecting its intended application afterwards.
At 1385 nm, overtones of the OH- contamination-related molecular absorption manifest themselves in the well known “water peak”. Attenuation at and near the water peak can exceed 2 dB/km compared with attenuation values of <0.2 dB/km as can occur in the C-band (1530 – 1565 nm).
On the basic of WDM, some new network technologies have emerged, such as:
WDM-PON is an innovative concept for access and backhaul networks. It uses WDM over a physical P2MP fiber infrastructure that contains no active components (i.e., PON). WDM-PON allows operators to deliver high bandwidth to multiple endpoints over long distances.
OTN was designed to provide support for optical networking using wavelength-division multiplexing (WDM) unlike its predecessor SONET/SDH. It is able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals.
The knowledge is endless, thus, the word you see on this page as only the tip of an iceberg. But once you know all of them, you may better know about WDM systems.
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