Wavelength Division Multiplexing (WDM) optical fiber communications, a transmission technology, which utilizes a single fiber can simultaneously transmit a plurality of different wavelengths of light carrier characteristics, dividing the wavelength range of possible applications of the optical fiber into several bands, each bands used as an independent transmission channel in a predetermined wavelength of the optical signal. Different communication system design, each gap width is also a difference between the wavelengths, in accordance with the difference between the channel spacing, can be subdivided into the WDM W-WDM, M-WDM, D-WDM.

We can be an optical fiber as a multi-lane public roads, traditional TDM systems only use a lane on this road, and the use of D-WDM technology, similar to the use of lanes on public roads not yet been used, for fiber untapped tremendous transmission capacity.

WDM technology development

WDM optical fiber communication occurs beginning to appear. Since 1995, WDM development into the fast lane, Lucent first to introduce the 8 * 2.5G WDM systems, Ciena launched 16 * 2.5G system. China has completed a 4 * 2.5G field experiments, 8 * 2.5G experimental system has been signed.

Mainly due to the rapid development of WDM:
The rapid development (1) of the photovoltaic device.
(2) TDM 10Gb / s faced with the challenge of electronic components response time.
(3) fiber dispersion and polarization mode dispersion limits the 10Gb / s of transmission.
Early 1990s, EDFA (erbium doped fiber amplifier) rapid commercialization solve the problem WDM multiplexer insertion loss caused. EDFA can provide power gain of about 40dB, more importantly, EDFA amplification wavelength window, spaced apart enough to accommodate many roads with different wavelengths to obtain a power gain, which is wavelength division multiplexing (WDM) very useful system application.

WDM System

Currently, the manufacturers of WDM systems into two basic categories: integrated systems and open systems.
Integrated System Synchronous Digital (SDH) terminal has to meet the G.692 optical interface, the standard light wavelength, to meet long-distance light.

Open systems WDM front end joined wavelength transfer unit OUT, the current SDH G.957 interfaces wavelength converted to G.692 standard wavelength optical interface.

WDM technology and devices

Wavelength transfer unit OUT

SDH optical signal from the terminal sent through the light - electricity - light transfer, a specific wavelength of the signal fed to the multiplexer (OMU). In the WDM system, the need for wavelength stabilization in the following three aspects:
(1) reduce interference between adjacent passages. (2) to improve system performance. (3) reduce the wavelength division requirements to some extent.

The use of OUT presented two technical options:
- Only as a wavelength converter, no regenerative repeater OUT application functionality.
- As a regenerative repeater function OUT application.

WDM device

Optical multiplexer for transmitting end of the transmission system, a plurality of input ports and one output port of the device has. Optical receiving end filter for transmission systems, just the opposite with the optical multiplexer having a plurality of input ports and output ports, an optical amplifier as a pre-amplifier, line amplifiers, power amplifiers, optical fiber communication is the key one part. Currently developed optical amplifier into an optical fiber amplifier (OFA) and a semiconductor optical amplifier (SOA) two categories. Optical fiber amplifiers have an erbium doped fiber amplifier (EDFA), erbium-doped fiber amplifier (PDFA), niobium-doped fiber amplifier (NDFA). Wherein, EDFA superior performance, it has been widely used in WDM experimental systems, commercial systems.

EDFA consists of three parts: a pump light source, optical coupler, erbium-doped silica fiber. Its characteristics are:
(1) has a wide gain bandwidth.
(2) provides a high gain, small noise figure.
(3) pump wavelength of 980nm and 1480nm, the wavelength of the semiconductor laser to adapt.
High coupling efficiency (4) and the transmission fiber.
(5) when the input signal is too large, EDFA gain saturation output power tends to a finite fixed value.
(6) can be transparently achieve amplification, regardless of the signal rate and modulation scheme, and can amplify the signal along each path.

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