ITU-T G.692 (Optical interfaces for multi-channel systems with optical amplifiers)

Recommendation ITU-T G.692 (Optical interfaces for multi-channel systems with optical amplifiers) has defined a comb of wavelengths allowed in the only window of 1530-1565 nm transmission. It normalizes the spacing in nanometer (nm) or Gigahertz (GHz) between two wave lengths permitted the window: 200 nm or 1.6 GHz and 100 GHz or 0.8 nm.

The technology is called dense WDM (DWDM) used when the spacing is equal to or less than 100 GHz. Systems at 50 GHz (0.4 nm) and 25 GHz (0.2 nm) to obtain respectively 80 and 160 optical channels.

For even lower densities, we refer to U-DWDM: Ultra - Dense Wavelength Division Multiplexing. Thus, systems
10 GHz (0.08) to obtain 400 optical channels.

Systems WDM / DWDM most marketed today include 8, 16, 32, 80 optical channels, which can reach capacities of 80, 160, 320, 800 Gb / s through a flow capacity of 10 Gb / s. You can reach a capacity of 4 000 Gb / s (4 Tera b / s) with 400 optical channels at 10 Gb / s, U-DWDM technology.

One of the key components of WDM / DWDM amplifier is the erbium-doped fiber (EDFA) which can compensate for losses due to insertion multiplexing / demultiplexing wavelengths.

However DWDM introduces non-linear phenomena which have the effect of limiting in practice the distance between amplifiers between 50 and 100 Km:

- Crosstalk between channels (XPM: Cross Phase Modulation),
- The blend quatre ondes (FWM Four Wave Mixing) which created the inter-modulation between the different optical channels,
- The Raman effect (SRS: Stimulated Raman Scattering), which increases the received power differences between channels and therefore produces a too large dispersion of the signal to noise ratio.

Over singlemode optical fiber 652 G non-linear effects do not appear in the 1550 nm window as the number of channels remains less than or equal to 32 channels and the power per channel remains below 1 mw.

Different techniques used to correct these phenomena: the case of the DCF (Dispersion Compensating Fiber) which is to introduce in the binding section of fiber producing a negative dispersion (around -100 ps / nm.km) compensation.

However, the technologies and DWDM DWDM-U have not yet reached their limits.

New techniques under development will further increase the capacity of optical systems:
- Soliton transmission allowing the transport of very narrow pulses over thousands of km without distortion, while maintaining a broad bandwidth;
- Pulse modulation, or duo-binary transmission, allowing the multiplication by two or three times the flow-mail, using pulse 2 or 3 levels binaries;
- Amplification and multiplexing in the 1300 nm window to improve returns on conventional optical fibers G652 experiencing limitations in the use of DWDM systems at 1550 nm.


In addition, devices such as multiplexers to insertion / extrat optics (Optical Add Drop Multiplexing: OADM) Reconfigurable:

brewers and optical (Optical Cross-Connect: OXC)

give operators flexibility to optimize their networks, primarily long-haul networks. Indeed, the price of these technologies can not be used on networks of local loops, where it should be more equipment. They particularly need to use cooled lasers. Therefore, it has grown quite recently another less expensive technology called CWDM (Coarse WDM) which saves about 30% over a DWDM system.