Channel spacing

Introduction

The telecommunications industry constantly strives to maximize the use of fiber optic capacity. Despite the broad spectral width of the conventional C-band, which offers over 40 THz, the limited use of optical channels at 10 or 40 Gbit/s results in substantial under utilization. The solution lies in Wavelength Division Multiplexing (WDM), a technique that can significantly increase the capacity of optical fibers.

Understanding Spectral Grids

WDM employs multiple optical carriers, each on a different wavelength, to transmit data simultaneously over a single fiber. This method vastly improves the efficiency of data transmission, as outlined in ITU-T Recommendations that define the spectral grids for WDM applications.

The Evolution of Channel Spacing

Historically, WDM systems have evolved to support an array of channel spacings. Initially, a 100 GHz grid was established, which was then subdivided by factors of two to create a variety of frequency grids, including:

12.5 GHz spacing
25 GHz spacing
50 GHz spacing
100 GHz spacing

All four frequency grids incorporate 193.1 THz and are not limited by frequency boundaries. Additionally, wider spacing grids can be achieved by using multiples of 100 GHz, such as 200 GHz, 300 GHz, and so on.

ITU-T Recommendations for DWDM

ITU-T Recommendations such as ITU-T G.692 and G.698 series outline applications utilizing these DWDM frequency grids. The recent addition of a flexible DWDM grid, as per Recommendation ITU-T G.694.1, allows for variable bit rates and modulation formats, optimizing the allocation of frequency slots to match specific bandwidth requirements.

Flexible DWDM Grid in Practice

The flexible grid is particularly innovative, with nominal central frequencies at intervals of 6.25 GHz from 193.1 THz and slot widths based on 12.5 GHz increments. This flexibility ensures that the grid can adapt to a variety of transmission needs without overlap, as depicted in Figure above.

CWDM Wavelength Grid and Applications

Recommendation ITU-T G.694.2 defines the CWDM wavelength grid to support applications requiring simultaneous transmission of several wavelengths. The 20 nm channel spacing is a result of manufacturing tolerances, temperature variations, and the need for a guardband to use cost-effective filter technologies. These CWDM grids are further detailed in ITU-T G.695.

Conclusion

The strategic use of DWDM and CWDM grids, as defined by ITU-T Recommendations, is key to maximizing the capacity of fiber optic transmissions. With the introduction of flexible grids and ongoing advancements, we are witnessing a transformative period in fiber optic technology.

In a non-coherent WDM system, each optical channel on the line side uses only one binary channel to carry service information. The service transmission rate on each optical channel is called bit rate while the binary channel rate is called baud rate. In this sense, the baud rate was equal to the bit rate. The spectral width of an optical signal is determined by the baud rate. Specifically, the spectral width is linearly proportional to the baud rate, which means a higher baud rate generates a larger spectral width.

Baud (pronounced as /bɔ:d/ and abbreviated as “Bd”) is the unit for representing the data communication speed. It indicates the signal changes occurring in every second on a device, for example, a modulator-demodulator (modem). During encoding, one baud (namely, the signal change) actually represents two or more bits. In the current high-speed modulation techniques, each change in a carrier can transmit multiple bits, which makes the baud rate different from the transmission speed.

In practice, the spectral width of the optical signal cannot be larger than the frequency spacing between WDM channels; otherwise, the optical spectrums of the neighboring WDM channels will overlap, causing interference among data streams on different WDM channels and thus generating bit errors and a system penalty.

For example, the spectral width of a 100G BPSK/DPSK signal is about 50 GHz, which means a common 40G BPSK/DPSK modulator is not suitable for a 50 GHz channel spaced 100G system because it will cause a high crosstalk penalty. When the baud rate reaches 100 Gbaud/s, the spectral width of the BPSK/DPSK signal is greater than 50 GHz. Thus, it is impossible to achieve 50 GHz channel spacing in a 100G BPSK/DPSK transmission system.

(This is one reason that BPSK cannot be used in a 100G coherent system. The other reason is that high-speed ADC devices are costly.)

A 100G coherent system must employ new technology. The system must employ more advanced multiplexing technologies so that an optical channel contains multiple binary channels. This reduces the baud rate while keeping the line bit rate unchanged, ensuring that the spectral width is less than 50 GHz even after the line rate is increased to 100 Gbit/s. These multiplexing technologies include quadrature phase shift keying (QPSK) modulation and polarization division multiplexing (PDM).