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What’s the Difference Between WDM and DWDM?

What is the difference between WDM and DWDM?

Wavelength Division Multiplexing (WDM) is an optical fiber transmission technology enabling the simultaneous transmission of multiple optical wavelengths (or colors) through a uniform medium. Multiple colors of light can propagate on the same fiber, carrying multiple signals at different wavelengths or frequencies within the optical waveguide.

In the early days of optical fiber transmission, information was conveyed using light pulses on glass fibers. A lit bulb indicated a digital one or zero, with the light spanning a broad range of wavelengths from around 670nm to 1550nm. WDM is an optical fiber transmission technology that employs multiple wavelengths to transmit data through a uniform medium.

However, this capability rapidly matured. With advancements in optoelectronic components, system designs could transmit multiple light wavelengths simultaneously on a single fiber, greatly enhancing its capacity. This gave rise to WDM, facilitating the multiplexing of high-bit-rate data streams such as multiple 10 Gb/s, 40 Gb/s, 100 Gb/s, 200 Gb/s, and more recently, 400 Gb/s and 800 Gb/s. Each data stream carries distinct payloads on a single fiber.

WDM光耦合器電纜圖

WDM Coupler Cable Diagram

There are currently two types of WDM:

  • Coarse WDM (CWDM): CWDM is a WDM system with fewer than eight effective wavelengths per fiber, primarily used for short-distance communication. It utilizes a broad range of frequencies with a wider wavelength spread. The standardized channel spacing allows for wavelength drift due to laser heating and cooling. CWDM is a compact and cost-effective choice when spectrum efficiency is not a crucial factor.
  • Dense WDM (DWDM):DWDM is frequency-based and employs tighter wavelength spacing, supporting more channels on a single fiber. However, this comes with higher implementation and operational costs. DWDM is ideal for systems needing over eight active wavelengths per fiber. It finely divides the spectrum and accommodates over 40 channels within the C-band frequency range.

Vendors utilize DWDM to populate C-band fiber spectrum with fixed-spaced 40, 88, or 96 wavelengths. Conventional DWDM line systems use wavelength-selective switches (WSS) with fixed 50GHz or 100GHz filters. These systems support channels from older coherent transponders requiring spectra below 50GHz or 100GHz (based on filters). Networks nearing capacity limits for high-bandwidth applications shift to C+L-band solutions, doubling fiber capacity by leveraging L-band spectrum.

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Electromagnetic Spectrum Chart

To meet growing bandwidth demands, optical networks rely more on programmable coherent technology to maximize fiber capacity and reduce cost per bit. To fully leverage these benefits, a flexible grid line system is necessary to accommodate higher-rate channels, including 800G wavelengths with spectra beyond 100GHz.

WDM is a technology in optical fiber transmission that uses multiple optical wavelengths to transmit data through the same medium.

Next-gen coherent modems are intelligent and programmable, offering a broader range of constellations and bit rate options for fine-tuning. Flexible channel planning allows for various channel configurations, from 64 x 75GHz channels to 40-45 channels at higher 800G line rates. This is possible through a flexible grid (or gridless) architecture supporting channels as small as 37.5GHz with 6.25GHz increments, ensuring compatibility with current and future channels.

With the advancements of EDFAs and Raman amplification, DWDM systems now have extended coverage ranges spanning thousands of kilometers. To ensure stable operation in systems with dense channels, high-precision optical filters are needed to remove specific wavelengths without affecting neighboring ones. Precision lasers operating at a constant temperature are also crucial for maintaining channel alignment targets in DWDM systems.

Deploying DWDM on a flexible grid photonic line system offers the advantage of signal independence, supporting multi-generational transponders irrespective of formats, etc. This allows network designs initially designed for 10Gb and 40Gb/s to now carry 200Gb/s channels, while channels with flexible grid capabilities can now transmit 400Gb/s or even 800Gb/s signals!

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