Industry pushes limits of WDM transmission

Scientists at advanced optical research organizations are racing to improve WDM transmission capacity far beyond the capabilities of today’s systems. Increasing capacity can be achieved by manipulating technology in three separate domains: rate, frequency, and space.
In preparation for March’s OSA OFC/NFOEC conference, Alcatel-Lucent’s New Jersey–based Bell Labs announced several successful lab experiments to increase wavelength capacity and spectral efficiency using multiple subcarrier super-channels, PDM-16QAM modulation formats, spatial division multiplexing (SDM), and MIMO (multiple-input multiple-output) technology.
An hour down the road in Princeton, New Jersey, scientists from NEC America announced they have completed a successful trial of real-time 1Tbps super-channel transmission over a trans-oceanic distance of 7,200km.
Huawei just completed a 2Tbps transmission trial over 3,325km. Though laboratory and field experiments are a far stretch from viable products, they offer an interesting glimpse into possible solutions for overcoming the physical limitations of transmission over optical fiber.
Increase the line rate and the number of bits transmitted
The most common method to increase capacity on WDM systems is to up the line rate on each wavelength, allowing transmission of more bits in the same channel space. This is accomplished by changing the modulation format by which optical channels are encoded and transmitted. Most 100G WDM systems deployed today use polarization-division multiplexing (PDM) quadrature phase-shift keying (QPSK) modulation coupled with coherent digital signal processing (DSP).
Advanced modulation formats such as 16-QAM (quadrature amplitude modulation) that increase the number of bits per symbol transmitted have been in development for several years. These formats will soon make their way into commercial WDM systems, while even more advanced modulation schemes such as 32-QAM, 64-QAM, and 256-IPM (iterative polar modulation) are being investigated by the scientific community.
While upping the channel rate is the most common method to increase capacity, it comes with a requirement for significantly higher optical signal-to-noise ratio (OSNR), which reduces the tolerance to fiber nonlinearity and noise.
Divide and conquer: subcarrier super-channel transmission
Another way to increase capacity on optical systems is to increase the number of frequencies that are transmitted on a fiber. Early DWDM transmission systems used 40 channels spaced 100GHz apart.
Reducing the space between the frequencies to 50GHz allowed 80 channels to be transmitted in the same spectrum. Super-channel transmission breaks apart the existing (50GHz and 100GHz) channel grids to combine wavelengths and build wider channels. Multiple subcarriers, each carrying a higher-rate signal (e.g. 100G) are combined to create a higher rate channel.
Recent lab trials at Alcatel-Lucent and NEC show transmission of 1Tbps super-channels at distances greater than 5,000km can be achieved. NEC researchers were able to extend the reach of its recently completed 1Tbps trial to 7,200km by using digital Nyquist-shaped subcarriers. In October 2012 Huawei completed a 1Tb 2,913km field trial and then followed that up in November with a 2Tb field trial with Vodafone.
Huawei transmitted the 2Tb signal 3,325km using 20 subcarriers, 660GHz bandwidth, and PDM QPSK modulation. The company also completed a 1,440km trial using 10 subcarriers (375GHz bandwidth) and 16-QAM modulation. Infinera’s commercially available DTN-X platform supports 500G super-channel transmission with five 100G subcarriers transmitted in each super-channel.
Space, the final frontier for WDM
Given recent advances in manipulating signal rates and frequencies, it’s easy to see how 1Tb long-distance transmission is possible. Still, there is a physical limit to how much capacity can be supported over a nonlinear optical system, as discovered by Shannon. In that regard, research in spatial division multiplexing (SDM) may represent the “final frontier” allowing development of systems that scale to petabits per second and more.
Spatial division multiplexing uses new types of multi-core or “few-mode” fiber that provides multiple paths for light to travel. This is akin to lighting up additional fibers, essentially replicating capacity for each instance.
Practical limitations of today’s single-mode fiber (SMF) place the upper limit of capacity transmitted at roughly 50Tbps, but by adding additional paths using multi-core or multimode fiber, capacity can be pushed into the petabit range. Alcatel-Lucent is currently investigating SDM coupled with photonic integration and MIMO (multiple-input, multiple-output) processing for future-generation petabit WDM systems.
Ron Kline is a principal analyst for network infrastructure at Ovum. For more information, visit