What is DWDM? An easy way to understand DWDM and CWDM solution.

DWDM stands for Dense Wavelength Division Multiplexing, which it the top solution to make the optical fiber channel been utilized adequately in long distance transmitting.

Why do we need DWDM in the telecom systems?

The optical fiber cabling (e.g. submarine optical cable), interrelated equipment (e.g. the core layer switch) and maintenance cost make the each optical fiber channel very expensive.

Why we choose Wavelength O-Band, C-Band and L-Band to Transmit?

For long distance optical communication, the attenuator is very important.

1. We need the loss in fiber channel as lower as it can be. On transmitting side (Tx), the optic power should be low to save power consumption and reduce heat dissipation. The cooling system also a big part of telecommunication infrastructure. For example, datacenter have the special cooling system, fans and cooling pipe, oil cooling, even water cooling by water in the river.
2. It require less on the sensitivity on receiving components(Rx).

Please find the suitable wavelength in the picture below.

What is the initial alphabet “Density” of DWDM?

DWDM wavelength gap less than or equal to 0.8nm, CWDM wavelength gap 20nm.

Different frequencies or modes in the optical pulse have different group velocities in the fiber, so that these frequency components and modes arrive at the fiber end before the optical pulse is broadened, which is the dispersion of the fiber.

The dispersion in the fiber can be divided into modal dispersion, chromatic dispersion, polarization mode dispersion:

1. Mode dispersion is also called inter-mode dispersion, and modal dispersion mainly exists in multimode fiber;

2. Chromatic dispersion (CD), also known as in-mold dispersion, can be divided into material dispersion and waveguide dispersion;

3. Polarization Mode Dispersion (PMD) is caused by the fact that two orthogonal polarization states of signal light have different propagation velocities in the fiber. Polarization mode dispersion is generated by random factors, so it is a random amount and difficult to compensate. ;

The chromatic dispersion coefficient is the delay difference produced by the optical wavelength signal passing through the unit length fiber in the unit wavelength interval, expressed by D, and the unit is ps/nm.km. The polarization mode dispersion coefficient is expressed in PMDQ, and measured by ps/kmn (n is 1/2).

From the perspective of TDM, dispersion will lead to intersymbol interference.

The light source is non-zero spectrally wide, and the optical signal output by the light source is intensity modulated by an electrical pulse having a modulated wavelength component of each wavelength.

Since the time of arrival of each wavelength component is inconsistent, the optical pulse is lengthened (T + ΔT), which is called pulse broadening. The farther the light pulse is transmitted, the more severe the pulse broadening. Pulse broadening will cause the front and rear light pulses to overlap, called intersymbol interference. Inter-code interference will cause errors, thus limiting the code rate and transmission distance of the transmission.

However, from the perspective of WDM, chromatic dispersion is beneficial to overcome inter-channel interference caused by nonlinearity of optical fibers, such as FWM and XPM.

We need to dialectically look upon the effects of chromatic dispersion.

The main type of WDM

1. The coupling type

2. Dielectric layer type

3.  AWG type