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WDM Links

WDM technology is based on the ability to transmit several light signals on a single fiber using different wavelengths. It turns out that such different light wavelengths do not interfere with each other, and thus can be split apart at the other end of the fiber to form separate channels (Fig.4 is a schematic drawing of such a system). In this figure only half-duplex channels are depicted. To realize the other direction, a duplicate system is typically used in that direction The WDM link essentially comprises the following elements (scanning the figure from left to right):



Figure 4 - A WDM link.


        Different interfaces per port to enable different protocols to communicate over the link.
        An electro-optical converter which includes a laser per channel at different wavelengths.
        An optical multiplexer, typically a piece of glass called a grating.
        Due to attenuation, amplifiers may be needed along the fiber or at the endpoints.
        When the signal gets to the other end of the fiber it is split by an optical demultiplexer, which acts like a prism, to separate wavelength-specific optical signals (the same grating could be used again).
        A wavelength-insensitive receiver converts the signal into electrical form.
        The signal is output via the specific interface of the channel's port.


The pace of improvement for this technology is spectacular: while in 1994 the only commercial product for the telco market multiplexed four OC-48 (2.5 Gb/s) channels to a distance of 550 km, the high-end multiplexer in 1995 has multiplexed 8 such channels, 16 channels to 600 km in 1996, and a 32-channel system to 1200 km in 1997, and much more (100s) has been announced currently. As for the enterprise market, a multi-protocol lower-speed and shorter-distance system has existed since 1994. As mentioned earlier, such systems are already deployed and are rapidly gaining popularity due to their maturity, the large gain they offer, and the simplicity of integration with legacy equipment.

Several other demonstrations of WDM that have been reported recently are:

        34 wavelengths x10 Gb/s signal transmitted over 8 514 km fiber using an equalization technique via Fourier synthesis of the gain characteristic of the system;
        50 GHz spaced, 32 wavelengths x10 Gb/s signal trans-mitted over 640 km dispersion-shifted fiber with multi-wavelength distributed Raman amplification;
        25 wavelengths x40 Gb/s, 1 Tb/s throughput experiment demonstrated over 342 km in nonzero dispersion fiber.

Early deployment of WDM has mainly been point-to-point to increase link bandwidth, supports little in terms of networking functionality, and does not yet perform traffic protection or restoration. In cases of fiber cuts or network failures, synchronous optical network/synchronous digital hierarchy (SONET/SDH) equipment would usually provide these functions, with WDM used strictly for fiber capacity expansion.


WDM Link Enabling Technologies

The optical WDM revolution in transmission and emerging applications in optical networking are enabled by a range of key optical technologies. At the foundation is the low loss, 0.25 dB/km, single-mode optical fiber, which allows long-distance transmission with a bandwidth window of about 25 THz.


Source category

Tuning range

Tuning time


Mechanically tuned lasers

Full range of laser (1020 nm)

100500 ms

Mechanical tuning of the facets of an FP external cavity

Acousto-optically and electro-optically tuned lasers

Full range of laser (1020 nm)

Tens of s

Find application in packet switching

Injection current tuned lasers

4 nm

0.510 ns

Limited tuning range

Switched sources


< 65 ps

Integration improves functionality and speed

Array sources (using AWG)

Typically16 channels spaced by 200 GHz

100200 ms

Locked wavelengths and temperature tuning of whole comb by 275 GHz demonstrated

Array sources (DFB)

Limited by number of elements in array

110 ns

DFB lasers act independently, and can drift and cause crosstalk

Table 1. Properties of tunable sources


Erbium-doped fiber amplifiers (EDFAs) provide optical amplification to compensate power loss in optical signal transmission and processing (splitting, multiplexing etc.). Conventional C-band EDFAs cover the 15301565 nm wavelength range and extended L-band. EDFAs cover the 15651605 nm range for a total available gain bandwidth of 9 THz. Commercial systems have already capitalized on this bandwidth by using 100 GHz and 50 GHz spaced channels at channel bit rates of up to 10 Gb/s and a total fiber capacity of greater than 1 Tb/s. Future trends are toward smaller channel spacing, higher bit rate per channel, and higher spectral efficiency (bits per second per hertz).

WDM filters are another key technology; they allow splitting and combining of the available wavelength band into more than 100 individual wavelength channels. A variety of WDM filter types are available, such as fiber Bragg grating filters, dielectric thin-film filters, and Mach-Zehnder and waveguide-grating-router devices.

Semiconductor lasers are used in transmitters for the WDM multi-channel systems. Fixed-wavelength distributed feedback (DFB) semiconductor lasers with wavelengths on an internationally standardized wavelength grid are now widely available. Semiconductor lasers tunable over as much as 40 nm (4.6 THz) are also becoming commercially available.



Filter category

Tuning range

Tuning time



500 nm

110 ms

Fiber implementation available


250 nm

10 s

Can be used as a router as well

AWG tunable filter

40 nm

10 ms

Thermo-optic tuning

Liquid crystal Fabry-Perot

30 nm

0.510 s

Low power consumption (< 1mW)


16 nm

110 ns

Resolvable channels about 10

Fiber Bragg grating

10 nm

110 ms

Temperature or mechanical stretching tuning

Cascaded Mach-Zehnder interferometer filters

4 nm

50 ns

Insertion loss can be an issue

Tunable filters based on semiconduct or laser structures

5 nm

0.11 ns

Small number of channels

Table 2. Characteristics of tunable filters.


Electro-mechanical, and in the very near future micro-electromechanical system (MEMS), optical fiber switches switch full-band optical signals between multiple fibers; switching times are now typically between a few and tens of milliseconds.

Signal splitters, combiners, and variable attenuators allow further manipulation of optical signals.

High-speed, greater than 10 Gb/s, single-channel transmission is enabled by a variety of optical and electronic components. Among the key devices here are low-wavelength-chirp external lithium-niobate modulators, as well as integrated or copackaged semiconductor electroabsorption and Mach-Zehnder modulators, with modulation bandwidths in excess of 10 GHz.

Commercial PIN photodetectors and avalanche photodetectors (APDs) provide receiver bit rates of 10 Gb/s and higher; EDFA-based optical preamplifiers enable high sensitivity of these optical receivers.

Dispersion-compensating fiber allows compensation of the fiber link dispersion for high-speed long-distance signal propagation, as well as tailoring of the dispersion profile in the dispersion-managed optical signal path.

These and other optical technologies are being further developed to provide richer functionality for WDM applications while improving their performance and reducing component-induced signal impairments.


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