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Introduction to WDM Theory

WDM is the abbreviation for Wavelength Division Multiplexing. It is a new technology of transmitting signals with different wavelengths (colors of light) over a single fiber. Thus in the given information transmission capacity, the fewer fibers are required, which can result in the popularity of WDM technology among telecommunications companies since they do not need to lay more fibers in order to enlarge the capacity.
wavelength division multiplexing

As a system concept, the technology of WDM is generally divided into coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM).
CWDM  grid

CWDM: Coarse Wavelength Division Multiplexing
CWDM mainly differs DWDM from the channel spacing (CWDM has almost 100 times wider channel spacing) and the required frequency stability. In 2002 the ITU (International Telecommunications Union) standardized a channel spacing grid for use with CWDM, using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm. Due to the wilder channel spacing, the technical index of the laser is relatively low. And since the channel spacing reaches 20 nm, thus the longest wavelength shift is 6.5℃~+6.5℃. Besides, with the -5℃~70℃ operating temperature range, the wavelength shift result from temperature variations still within the acceptable limits, temperature controlling mechanism is not required by a laser, thus the structure of a laser has been greatly simplified, and the yield gets improved.

  • Merits of CWDM
    Reduction of the operating cost of network
    Small in size, low in dissipation
  • Demerits of CWDM
    Comparing with DWDM, the advantage of low-cost is not that obvious. Optical transceiver modules and optical devices are the keys to reduce the cost. But due to the small scale and the shipping from suppliers, the device cost is not apparent. Simplifying the device functions is another way to reduce the cost, but this could damage the reliability and manageability of the system. Furthermore, the continuous price-decreasing of DWMD products is also exerting pressure on CWDM technology.

DWDM: Dense Wavelength Division Multiplexing
DWDM refers originally to optical signals multiplexed within the 1550 nm band so as to leverage the capabilities (and cost) of erbium doped fiber amplifiers (EDFAs), which are effective for wavelengths between approximately 1525–1565 nm (C band), or 1570–1610 nm (L band). DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers. DWDM-based networks can transmit data in IP, ATM, SONET /SDH, and Ethernet, and handle bit rates between 100 Mb/s and 2.5 Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel.

  • Merits of DWDM
    Protocol- and bit-rate-independent.
    Transmission distance and network capacity improvements
    Electronic networking to optical networking evolution
  • Demerits of DWDM
    Comparing with CWDM, DWDM requires higher energy. Let’s take an example. DWDM laser temperature stabilized through coolers integrated modules encapsulation, these devices together with the associated PIN and the control circuit consumes approximately 4w of power per wavelength monitor. However, an uncooled CWDM laser transmitter only consumes about 0.5w power.

WDM has been applied in optical transceiver module, and used widely. The following is the brief introduction to CWDM and DWDM SFP transceiver.

SFP short for Small Form-factor Pluggable, a upgraded version of GBIC(Gigabit interface converter) and is a type of optical transceiver modules. CWDM SFP transceiver is widely used in optical communications for both telecommunication and data communication, designed for operations in Metro Access Rings and Point-to-Point networks using Synchronous Optical Network (SONET, 2.5 Gb/s), SDH (Synchronous Digital Hierarchy), Gigabit Ethernet and Fiber Channel (4Gb/s) networking equipment.

  • CWDM SFP transceiver is a convenient and cost-effective solution for the adoption of Gigabit Ethernet and Fiber Channel (FC) in campus, data-center, and metropolitan-area access networks, with maximum optical budget 41dB. From 1270 nm to 1610 nm there are 18 center wavelengths available in CWDM SFP, with each step 20 nm.
  • DWDM SFP transceiver provides a high-speed serial link at signaling rates from 100 Mbps to 2.5Gbps. And DWDM is typically used in large optical networks over longer distance.The DWDM SFP modules are suitable for interconnections in Gigabit Ethernet and Fiber Channel environments. There are 32 fixed-wavelength DWDM SFPs that support the International Telecommunications Union (ITU) 100-GHz wavelength grid.

I hope I have made myself clear through the above content, and enlighten you a little in the field of WDM. Moreover, WDM theory applied in many other optical network devices, and we will talk it about in the days to come.

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