3D Lightwave Circuits for Data Transfer
New applications in space-division multiplexing.
Optical fiber is the backbone of all global internet communication, and due to the basic human desire to communicate, data transfer over the internet continues to increase exponentially. The technology services giant Cisco has forecast that web traffic will nearly triple from 2015 to 2020, from 2.4 Exabytes per day to 6.4 Exabytes.
However, there is a fundamental limit to how much data can be carried across currently deployed single-mode optical fibers. This nonlinear Shannon capacity limit is caused by optical nonlinearities intrinsic to the fiber design and material.
As a result, new technologies are required to further scale the transmission capacity of optical fibers to avoid the impending capacity crunch.
Many photonics researchers believe the most promising technology to transmit internet data faster is space-division multiplexing (SDM).
The basic idea of space-division multiplexing is to increase the capacity of an optical fiber by either having multiple cores within a common cladding (multicore fibers) or by using few-mode fibers which have larger cores. In the latter case, the larger core allows for a reduced energy density, which lessens nonlinear impairments and allows for each mode to be used as a different data channel.
With coherent SDM, digital signal processing can compensate for any mode intercore crosstalk, and the transmission capacity can be increased linearly with the number of modes or cores. Furthermore, SDM could bring significant improvement in the cost per bit as well as the energy efficiency compared to simply having several parallel single-mode fibers. This is due to the optical communication system requiring fewer discrete components.
For the case of few-mode optical fibers, a major challenge lies in the ability to selectively excite and detect the individual modes of the fiber. Mode multiplexing is usually achieved using free-space optical setups. However, these tend to be bulky and have high loss. Such free-space optical setups are used instead of planar integrated technologies because they are more suited to multiplexing of the rotationally asymmetric linearly polarized (LP) modes of optical fibers.
The multiplexing of rotationally asymmetric spatial modes in few-mode optical fibers can be more naturally realized using three-dimensional waveguide architectures.
In recent years, this has motivated the use of 3D photonic fabrication techniques such as ultrafast laser inscription (ULI) for fabricating monolithic mode multiplexers. ULI relies on a high-pulse-rate laser beam tightly focused into a glass sample that is placed on computer-controlled stages, which translate the sample in three dimensions with respect to the focal spot of the beam as shown at left.
The high peak intensity at the focal point causes nonlinear optical breakdown of the material. This results in energy deposition, triggering a highly localized refractive index modification of the glass substrate, which forms the waveguides. This refractive index modification is very stable, and we have earlier reported on waveguides withstanding temperatures in excess of 700°C.
ULI is a maskless process in contrast to common lithographic photonic fabrication techniques. This enables rapid prototyping of photonic circuits. Moreover, simple components such as splitters can be inscribed within seconds. In recent years ULI has allowed for the realization of practical mode multiplexers that are compact, passive, low-loss, broadband in performance, and also compatible with standard fiber.
ULI has been used to successfully fabricate photonic lanterns, mode-selective couplers, tapered-mode couplers, and multicore and few-mode multicore multiplexers.
For the case of photonic lanterns, often used in astrophotonic devices, multiple individual cores are adiabatically tapered down to merge into a single multimode waveguide — and these devices can be used to simultaneously multiplex a large number of modes. Several research groups and companies have demonstrated laser-written photonic lanterns in recent years.
Photonic lanterns, however, typically do not exhibit direct mode selectivity. This means that light injected into one of the single-mode input ports will couple into multiple modes at the few- or multimode output. This mode crosstalk can be unraveled using multiple-input multiple-output (MIMO) digital signal processing in coherent networks, albeit at the expense of high computational complexity.
Direct mode selectivity is important because it allows for the compensation of differential mode delay (DMD) and mode-dependent loss (MDL) in coherent SDM networks. This reduces the complexity of the MIMO digital signal processing and improves the system capacity, respectively.
Alternatively, direct mode selectivity allows for SDM transmission in basic time-division multiplexed (TDM) passive optical networks (PON) without the need for any such sophisticated digital signal processing.
In order to achieve mode selectivity in a photonic lantern, asymmetry has to be introduced to break the degeneracy at the single-mode ports. Researchers at the University of California, Davis ULI facility, for instance, reported using single-mode cores of different propagation constants to demonstrate this in 2015.
Asymmetry can be introduced when using ULI by simply writing the waveguides with different size or index contrast by adjusting the laser power. Researchers at Heriot-Watt University (UK) have also successfully used photonic lanterns to interface with multicore fiber.
As mentioned, other mode-selective devices that have been realized using ULI include mode-selective couplers and tapered-mode couplers.
A mode-selective directional coupler is essentially an asymmetric coupler consisting of a few-mode waveguide and an adjacent single-mode waveguide, where the propagation constant of the single-mode waveguide matches the propagation constant of a particular higher-order mode in the few-mode waveguide. With the appropriate choice of interaction region length, full power transfer between the modes can be achieved.
In order to multiplex or demultiplex both orientation states of rotationally asymmetric, higher-order modes, three-core arrangements are required. A linear cascade of two- and three-core mode-selective couplers could therefore be used to multiplex a large number of modes in future telecommunications networks.
The requirement of precise phase matching means that these devices have low fabrication tolerances and are also wavelength-dependent. In order to overcome these limitations, we proposed and modeled tapered variants of the couplers.
Tapered-mode couplers are similar in geometry to mode-selective directional couplers, only the cores are counter-tapered within the interaction region. This ensures the adiabatic evolution of the fundamental mode in one core to a given higher-order mode in the adjacent core.
Since no phase matching is required over a prolonged distance, the devices are insensitive to fabrication variations and are also broadband in performance, albeit at the expense of a slightly larger device footprint.
We demonstrated in 2014 a 3D laser-inscribed, tapered-mode coupler operating over a 400 nm wavelength range with 20 dB (i.e., 99%) coupling into the LP11 modes. The device also exhibited high mode purity in excess of 20 dB and low crosstalk. Similar devices are now also commercially available for operation across the short, conventional, and long (S+C+L) telecommunications bands. In either case, tapering of the waveguides is achieved by altering the laser power during the sample translation.
The versatility of the ULI fabrication technique has also allowed for an array of such tapered-mode couplers to be integrated with a fan-in device to create a few-mode multicore fiber (FM-MCF) multiplexer. This array allows for the multiplexing of three modes in each core of a three-mode, four-core fiber across the S+C+L bands with insertion losses of around 1.5 dB and mode extinction ratios of greater than 17 dB. Devices such as these could allow for order of magnitude increases in capacity in future SDM optical fiber networks.
We expect that more optical components like these will be taken to market in the next few years, as space-division multiplexing emerges in high-bandwidth optical networks.
–Nicolas Riesen, Simon Gross, and SPIE Senior Member Michael Withford are photonics researchers and cofounders of Modular Photonics, an Australian startup that specializes in 3D-femtosecond-laser-written mode multiplexers for telecommunications applications. Riesen, a postdoctoral fellow at University of Adelaide, is chief science officer at Modular Photonics. Gross, a postdoctoral Fellow at Macquarie University, is CTO, and Withford, node director for the Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS) at Macquarie, is CEO.
–John Love, emeritus professor of guided-wave photonics at Australian National University (ANU) and a cofounder of Modular Photonics, died 19 June 2016 in Canberra. See spie.org/jlove
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