Analysis of the Current Coherent Optical Module Market

Researchers have made great progress in optical devices. The output power of laser, linewidth, stability and noise, as well as the bandwidth of photodetectors, power capacity and common mode rejection ratio have been greatly improved. Microwave electronic devices have also been greatly improved. Then, the coherent optical communication technology has gradually become an important capacity-lifting solution for the current 100G line-side.

Market Demand for Coherent Optical Communication

One of the biggest drivers of growth in the current communications market is the transition from 10G to 100G in the metro, core and Data Center Interconnect (DCI) sectors.

With the explosive growth of information generated by the use of communication technologies such as video conferencing and the spread of the Internet, the market has proposed higher transmission performance requirements for the physical layer that is the basis of the entire communication system.

OSNR: 16QAM vs. 64QAM

In terms of digital communication, how to expand the capacity of C-band amplifiers, overcome the deterioration of fiber dispersion effects, and increase the capacity and range of free-space transmission have become important considerations for researchers; in analog communication, sensitivity and dynamic range are key parameters of systems.

Driven by strong demand, large-scale DWDM systems are gradually depleting their wavelength resources, and the efficiency of Time-Division Multiplexing (TDM) systems through compressed optical pulses also has a large technical bottleneck. People began to consider replacing the original Wavelength Division Multiplexing (WDM) system with a coherent optical communication system.

Advantages of Coherent Optical Modules

The coherent optical communication system modulates the signal to the optical carrier by adjusting the amplitude, phase and frequency by means of external light modulation (such as DP-QPSK) at the transmitting end.

Compared with the traditional direct detection system, coherent detection can obtain more signal information through the signal light and the beat frequency of the local oscillator; after the signal reaches the receiving end, it uses high-speed Digital Signal Processing (DSP) technology to perform front-end processing such as equalization. The optical mixer and the optical signal generated by the local oscillator are coherently mixed to realize signal reconstruction and distortion compensation.

Baud rates: QPSK vs. DP-QPSK

Coherent optics can be used in both 100G and 400G applications, primarily because it enables service providers to send more data over existing fiber, reducing the cost and complexity of network upgrades for bandwidth expansion.

  1. Coherent detection combined with DSP technology:
    • Cleared barriers to traditional coherent reception
    • Compensate for various transmission impairments in the electrical domain, simplifying transmission links
    • Make high-order modulation formats and polarization states possible
  2. At the same time, the application of high-order modulation formats enables coherent optical communication to have higher single-wavelength channel spectrum utilization compared to traditional system systems.
    Coherent receivers have no special requirements for fiber channel, so coherent optical communication can use already laid fiber lines. With the aid of digital signal processing algorithms, coherent receivers compensate for signal distortion caused by fiber dispersion, polarization mode dispersion, and carrier phase noise at a very small cost.
  3. A coherent receiver is about 20 dB more sensitive than a normal receiver, so the distance that is not relayed in the transmission system becomes longer, which reduces the number of amplifications in the transmitted light path.

Based on the above reasons, coherent optical communication can reduce the cost of optical fiber erection for long-distance transmission, simplify optical path amplification and compensation design, and become the main application technology of current long-distance transmission network.

Application Scenarios of Coherent Optical Modules

At present, the coherent optical communication is mainly used on the line side of the backbone network and the metropolitan area network, and belongs to the technical research field of DWDM long-distance transmission. In the application scenarios of the metropolitan area network and the core network with distance more than 80km, the coherent optical communication features good performance of Optical Signal-to-Noise Ratio (OSNR), sensitivity, dispersion tolerance and so on.

Coherent Applications: DWDM Long-Haul Transmission

WDM System

The operating wavelength range is C-band (1530nm to 1565nm), and the fiber type is G.652D (prefered) or G.655. The key performance index is OSNR.

Error correction coding technology can jump out of the limitations of the physical layer of transmission, and compensate for all physical transmission impairments at the logic layer, especially the effects of nonlinear effects.

Coherent Applications

5G Middlehaul/Backhaul Network

In the 5G middlehaul scenario, 100G/200G DWDM system will be deployed, and the 100G CFP-DCO and 200G CFP2-DCO optical module can be used to implement the 80km scene application; the 400G DCO product is applied in the 5G backhaul scenario with distance less than 200km.

DCI

Whether the coherent communication will be used in the DCI field of 40km to 80km depends mainly depends on the commercial cost performance and whether the market capacity is large enough.

At the current 100G rate, products such as 100G ER with EML modulation are sufficient for the use; the 100G CFP-DCO ZR series will appear in the future.

Coherent Applications: Bandwidth by Speed

The OIF organization is developing a 400ZR specification that uses a combination of DWDM and coherent technology.

Andrew Schmitt, principal analyst at Cignal AI, said: “Coherent 400G will limit the development of existing 200G and 100G technologies by 2020, and new devices will maximize optical capacity without relying on coverage.” Foreseeable Yes, more and more 400ZR products will enter the market.

Summary

The coherent optical communication system is a more advanced and complex optical transmission system suitable for longer distance and larger capacity information transmission.

At present, coherent modules with the CFP form-factor are bulky and consume large power. Compact coherent modules will replace existing coherent products. The innovation of semiconductor technology and the improvement of chip technology will greatly promote the replacement of 400G coherent products.

In recent years, Gigalight, a global optical interconnect innovator, has increased its research and development of coherent modules and has achieved a series of achievements. In the next few years, it will strengthen cooperation with the industry and jointly promote the progress of related industries.

Source: Analysis of the Current Coherent Optical Module Market

Which Is Better for 80km Links? PAM4 or Coherent Technology

A significant portion of Data Center Interconnections (DCIs) and telecom router-to-router interconnections rely on simple ZR or 80km transceivers. The former is mostly based on 100Gbps per 100GHz ITU-T window C-band DWDM transceivers, while the latter is mostly 10G or 100G grey wavelength transceivers. In DWDM links, the laser wavelength is fixed to a specified grid, so that with DWDM Mux and Demux 80 or more wavelength channels can be transported through a single fiber. Grey wavelengths are not fixed to a grid and can be anywhere in the C-Band, limiting capacity to one channel per fiber. DCI links tend to use DWDM because they have to utilize the optical fiber bandwidth as much as possible due to the extremely high-volume traffic between data centers.

Another emerging 80km market is the Multi-System Operator (MSO) or the CATV optical access networks. This need emerges because MSOs are running out of their access optical fibers and they need a transmission technology which would allow them to grow to a very large capacity by using the remaining fibers. For this reason they need to use DWDM wavelengths to pack more channels in a single fiber.

The majority of the 10G transceivers on 80km links will be replaced by 100G or 400G transceivers in the coming years. For that to happen, there are two modulation techniques to enable 80km 100G transceivers.

  • 50G PAM4 with two wavelengths in a 100G transceiver
  • Coherent 100G dual-polarization Quadrature Phase Shifted Keying (DP-QPSK)

Generally speaking, PAM4 is a low-cost solution but require active optical dispersion compensation (which could be a big headache as well as extra expense to data center operators) and extra optical amplification to compensate for the dispersion compensators. By contrast, Coherent approaches do not need any dispersion compensation and the price is coming down rapidly, especially when the same hardware can be configured to upgrade the transmission data rate per wavelength from 100G to 200G (by using DP-16QAM modulation).

When 400G per wavelength is needed in a DCI network within a 100GHz ITU-T window, coherent technology is the only cost-effective solution, because it will not be feasible for PAM4 to achieve the same high spectral efficiency of 4 bit/sec/Hz.

On the standards front, many standards organizations are adopting coherent technology for 80km transmission. The Optical Inter-networking Forum (OIF) will adopt coherent DP-16QAM modulation at up to 60Gbaud (400G per wavelength) in an implementation agreement on 400G ZR. This is initially for DCI applications with a transmission distance of more than 80km, and vendors may come up with various derivatives for longer transmission distances. Separately, CableLabs has published a specification document for 100G DP-QPSK coherent transmission over a distance of 80km aimed at MSO applications. In addition, IEEE802.3ct is in the process of adopting coherent technologies for 100G and 400G per wavelength transmissions over 80km.

As data rates increase from 100G to 400G and capacity requirements per fiber are driven by DCI needs, and assisted by volume driven cost reductions in coherent optics and in coherent DSPs, we expect coherent transmission to be the technology of choice for 80km links.