Light line system: road to 100Tbps

Advances in transmission, light amplification and ROADM switching technology have allowed more wavelengths to be transmitted through fiber optic systems, according to a new study from Ovum. The increased transmission wavelength makes the total transmission volume per pair of optical fibers greater, thus improving the economic benefits of the network. Today’s industry transport standard is to transmit 96 wavelengths in the extended C-band.

Currently, the industry is evaluating the value and benefits of the Super C-band and C-L-band solutions. Higher-wavelength systems paved the way for the development of 100 Tbps transmissions. Network operators are always looking for the lowest cost for each solution. Super C and C-L bands need to be supported by the development of the technology ecosystem. Once the Super C-band and C-L are mass-produced, telecom operators will be able to benefit from lower per-bit system costs.

From 80 wavelengths in history to 96 wavelength standards today

Since the mid-2000s, 80 wavelengths per pair of fiber transmissions have long been the historical standard. Since then, the industry has increased the wavelength of each pair of fiber transmissions to 96 using the extended C-band.

Light line system: road to 100Tbps

Figure 1: Increase system capacity by leveraging existing feasible bands.

Source: https://www.itu.int/rec/T-REC-G.694.1-201202-I/en

Extended C-band provides low-cost system transfer capability

The extended C-band supports up to 96 wavelengths compared to the traditional C-band transmission of 80 wavelengths, resulting in a 20% increase in bandwidth per pair of fibers. For example, at a wavelength of 100Gbps, the transmission bandwidth increases from 8Tbps to 9.6 Tbps. The extended C-band operation utilizes the traditional C-band optical line system, which achieves bandwidth increase with minimal incremental cost of the line system.

Super C and C-L bands further expand capacity

The next step in the evolution of light transmission will be the use of Super C and L bands. An extra 120 wavelengths can be transmitted on the extra-wide C-band, while an additional 96 wavelengths can be added to the L-band with C-L. Both Super C and C-L require the development of a technology ecosystem to support an expanded band range. Switching elements in transmission lasers, amplifiers, and ROADM require upgrades to support extended bands.

Super C will require tunable lasers and modulators to operate over a larger Super C band range. The laser output power will need to be kept flat within the extended wavelength range. The Super C-band amplifier also needs to operate in the Super C range with minimal gain change across the entire band range. The industry has considered using new doping elements to run the amplifier gain in the Super C range. Silicon-on-silicon liquid crystal (LCOS) is the wavelength selection switch (WSS) technology used in ROADM today. In order to achieve Super C wavelength exchange, it is necessary to increase the wavelength resolution to precisely adjust and control more wavelengths.

The Super L band is an ultra-wide version of the regular L-band and can provide more than 96 channels based on 50GHz standard channel intervals. So far, the L-band has been very small and has not yet reached the generally adopted position of practical technology. Both Super C and Super C-L are typical technologies – the development of the economic ecosystem market is in trouble. Large-scale production is required to make the system cost-effective, but telecom operators will only make purchases if they see attractive economic benefits.

Ian Redpath, head of transport and routing component research at Ovum, says the catalyst for this market is likely to be a business where telecommunications operators, enterprises or other fibre-optic resources are very limited. Businesses and organizations that lack fiber will be very motivated to maximize their scarce fiber to increase transmission bandwidth. These businesses and organizations will be able to afford the early market equipment costs of low shipments, which will generally be cheaper than purchasing additional fiber pairs. The momentum of market development will be established, so that the Super C and C-L ecosystem is expected to develop into a bulk shipping market.

The industry’s focus is on higher speeds per wavelength. By leveraging all the economically viable spectrum bands, network operators can deploy more wavelengths per pair of fibers, reducing the overall cost per bit of the system.

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