Mode-division multiplexing (MDM) technology enables high-bandwidth data transmission using orthogonal waveguide modes to construct parallel data streams. However, few demonstrations have been realized for generating and supporting high-order modes, mainly due to the intrinsic large material group-velocity dispersion (GVD), which make it challenging to selectively couple different-order spatial modes. We show the feasibility of on-chip GVD engineering by introducing a gradient-index metamaterial structure, which enables a robust and fully scalable MDM process. We demonstrate a record-high-order MDM device that supports TE0–TE15 modes simultaneously. 40-GBaud 16-ary quadrature amplitude modulation signals encoded on 16 mode channels contribute to a 2.162 Tbit / s net data rate, which is the highest data rate ever reported for an on-chip single-wavelength transmission. Our method can effectively expand the number of channels provided by MDM technology and promote the emerging research fields with great demand for parallelism, such as high-capacity optical interconnects, high-dimensional quantum communications, and large-scale neural networks.
In this talk, a new type of beam shaper will be discussed, capable of generating arbitrary vector spatiotemporal beams, where the user can define the amplitude, phase, and polarization independently for each point in space and time. This beam shaper was recently used to demonstrate time reversed optical waves. Such waves propagate through complex media, as if watching a traditional scattering process in reverse - starting as a complicated ‘pre-scattered’ wave, which then becomes a desired target field at the distal end of the complex media.
Free-space propagating optical modes are acutely affected by aberrations such as those caused by atmospheric turbulence. These optical distortions are due to variations in optical density that arise from changes in temperature and pressure along an optical path. These distortions don’t only change in time, but along the entire optical path. Such aberrations results in bulk optical turbulence that cannot be readily corrected using single plane optical correction. We present an approach, using a 7 plane multiple light converter that can correct optical aberrations arising from 1km optical paths with up to a D/r0=20 and with less than -9dB combined crosstalk for 11 spatial modes.
We demonstrate a device capable of controlling simultaneously all the degrees of freedom of a light beam (spatial/polarisation and spectral/temporal, 38,000 spatiotemporal modes are fully controlled through the C-band), after propagation through a multimode optical fiber that adds extra mode coupling. For this, we have combined a polarisation-resolved multi-port spectral pulse shaper (control of 1D spatial/polarisation and spectral modes) and a multi plane light conversion device (conversion 1D to 2D spatial/polarisation modes). The ability to deliver accurate volumetric light fields could be applied to control both linear and non-linear optical processes.
Multi-core fibers, few-mode fibers and their hybrid combination, few-mode-multi-core fibers are promising transmission media for future high-capacity, space-division multiplexed optical fiber transmission systems. In this paper, we report on our latest short and long-haul transmission demonstrations, including record breaking 10.66 Pb/s transmission in a 38-core, three-mode fiber as well as 172 Tb/s over more than 2000 km coupled-core three core fiber, using more than 75 nm bandwidth in C- and L-bands. We further discuss key transmission channel parameters, such as the impulse response time spread and mode-dependent loss and their consequences on the transmission performance.
We describe the process of analysing a beam using spatial state tomography, a generalization of Stokes polarimetry to higher-dimensions. The process consists of measuring the intensity of spatial components in various spatial bases to construct a generalized Stokes vector and its corresponding density matrix. Just as for polarization, this matrix can describe coherent, partially coherent and completely incoherent states of light incoming from a telescope. As applied to the spatial properties of a beam, this density matrix quantifies the spatial amplitude and phase of each spatial state of which the beam is composed, giving the correlated wavefront phase information.
We propose the use of Laguerre-Gaussian (LG) mode sorters to spatially filter and analyse light from a telescope. An LG mode sorter spatially decomposes an incoming beam into a Cartesian grid of identical Gaussian diffraction-limited spots. Each spot contains a particular LG spatial component of the original beam. These individual LG components, which are now independently accessible as spatially separated Gaussian spots could be analysed and processed in various ways. For example, circularly symmetric components could be removed, in a function similar to a vortex coronagraph. Individual LG spatial components could also be selectively interfered and spectrally decomposed.
We discuss the extension of Laguerre-Gaussian (LG) mode sorters to higher spatial mode counts. LG mode sorters based on multi-plane light conversion were recently demonstrated. The device consist of a cascade of phase planes separated by free-space propagation which performs a spatial decomposition in the Laguerre-Gaussian basis. Whereby an incoming beam, described by a basis of N LG modes is mapped onto a Cartesian array of N Gaussian spots in the output plane. Each spot in the array contains a particular LG spatial component of the original beam. Previously, LG mode sorters have been demonstrated supporting as many as 325 modes using 7 planes. In this paper we present a design for a device that supports 1035 modes corresponding with the first 45 degenerate mode groups using 14 planes. At the centre wavelength, the device has a theoretical insertion loss of 2.10dB. The lowest loss LG mode is -1.65dB and the highest loss LG mode is -3.22dB. The average crosstalk over all modes is 12.75dB. The worst-case mode has a crosstalk of 9.20dB.
Wave propagation is a linear process in the time domain in the absence of loss. This property has been exploited over the past 20 years for wave control through highly disordered media. Let’s consider a short pulse propagating through a disordered system. If the field associated to the pulse is recorded and played backwards, the wave is focused back to the source at a single delay. This time reversal control has been evidenced for low frequency waves such as acoustics, water waves and microwaves. Over the last decade, partial spatiotemporal control of optical waves has been demonstrated by means of spatial light modulators. However full optical time reversal remains elusive. In this paper, we demonstrate time reversal of optical waves with a device that can manipulate independently amplitude and phase of 90 spatial and polarization modes, over 4 THz of bandwidth and 20 ps of delay. For the first time we demonstrate arbitrary control of all the degrees of freedom: spatial (amplitude and phase), polarization, spectral and temporal after propagation through a multimode fiber. This new ability to control and manipulate at will optical waves opens promising opportunities for linear and nonlinear optical phenomena, such as imaging and optical communications.
Multi-plane light conversion is a method of performing spatial basis transformations using cascaded phase plates separated by Fourier transforms or free-space propagation. In general, the number of phase plates required scales with the dimensionality (total number of modes) in the transformation. This is a practical limitation of the technique as it relates to scaling to large mode counts. Firstly, requiring many planes increases the complexity of the optical system itself making it difficult to implement, but also because even a very small loss per plane will grow exponentially as more and more planes are added, causing a theoretically lossless optical system, to be far from lossless in practice. Spatial basis transformations of particular interest are those which take a set of spatial modes which exist in the same or similar space, and transform them into an array of spatially separated spots. Analogous to the operation performed by a diffraction grating in the wavelength domain, or a polarizing beamsplitting in the polarization domain. Decomposing the Laguerre-Gaussian, Hermite-Gaussian or related bases to an array of spots are examples of this and are relevant to many areas of light propagation in free-space and optical fibre. In this paper we present our work on designing multi-plane light conversion devices capable or operating on large numbers of spatial modes in a scalable fashion.
Multi-plane light conversion is a method of performing spatial basis transformations using cascaded phase plates separated by Fourier transforms or free-space propagation. In general, the number of phase plates required scales with the dimensionality (total number of modes) in the transformation. This is a practical limitation of the technique as it relates to scaling to large mode counts. Firstly, requiring many planes increases the complexity of the optical system itself making it difficult to implement, but also because even a very small loss per plane will grow exponentially as more and more planes are added, causing a theoretically lossless optical system, to be far from lossless in practice. Spatial basis transformations of particular interest are those which take a set of spatial modes which exist in the same or similar space, and transform them into an array of spatially separated spots. Analogous to the operation performed by a diffraction grating in the wavelength domain, or a polarizing beamsplitting in the polarization domain. Decomposing the Laguerre-Gaussian, Hermite-Gaussian or related bases to an array of spots are examples of this and are relevant to many areas of light propagation in free-space and optical fibre. In this paper we present our work on designing multi-plane light conversion devices capable or operating on large numbers of spatial modes in a scalable fashion.
As the nonlinear capacity limit of single mode fiber (SMF) transmission systems is being approached, space-division multiplexing (SDM) in multicore fibers (MCFs) or few-mode fibers (FMFs) is currently under intense investigations to achieve ultrahigh spectral efficiency per fiber. Meanwhile, a key advantage of SDM over simply increasing the number of SMFs, is its inherent device integration and resource sharing capability. This can potentially provide significant benefits in terms of the cost per bit in future optical networks. In order to efficiently address capacity scaling in a single optical fiber, few-mode and multicore erbium-doped fiber amplifiers are being developed. Critical for the implementation of SDM amplifiers is to achieve almost the same amount of gain for all spatial channels. In this respect, we have recently demonstrated multimode fiber amplifiers, supporting >15 modes, with a maximum differential modal gain of 2 dB and negligible mode mixing.
Free-space optical communications links have the perpetual challenge of coupling light from free-space to a detector or fiber for subsequent detection. It is especially challenging to couple light from free-space into single-mode fiber (SMF) in the presence of atmospheric tilt due to its small acceptance angle; however, SMF coupling is desirable because of the availability of extremely sensitive digital coherent receivers developed by the fiber-telecom industry. In this work, we experimentally compare three-mode and single-mode coupling after propagating through 1.6 km of free-space with and without the use of a fast-steering mirror (FSM) control loop to mitigate atmospherically induced tilt. Here, the 3-mode fiber is a 3-mode photonic lantern multiplexer (PLM) that passively couples light into three SMF outputs. With the FSM control loop active, coupling into the PLM and the SMF yielded nearly identical coupling efficiencies, as expected. Experimental results with the FSM control loop off show that coupling from free-space to PLM increases the average power received, and mitigates the negative impacts of tilt-induced fading relative to coupling directly to SMF.
Integrated space-division multiplexed (SDM) erbium-doped fiber amplifiers (EDFAs) are not only inevitable for SDM systems, but can be an alternative solution to nowadays EDFA array for parallel amplification. SDM EDFAs are expected to provide substantial complexity and cost savings through spatial-integration compared to duplicating single-mode fiber amplifiers. High output power and low noise figure can be achieved by cladding-pumped SDM EDFAs. In this paper, different cladding pumping solutions, cladding-pumped single-mode and multimode multi-core EDFAs will be discussed.
Space-division multiplexing (SDM) systems transmit over multiple spatial-paths to increase capacity or photon efficiency. In these links, coupling between the spatial paths scramble each input signal across all the output channels. This scrambling is described by a N × M frequency dependent transfer matrix which can be undone using multiple-input multiple-output processing. Using a swept-wavelength interferometer with spatial diversity, we can completely characterize the amplitude and phase transmission between each input and output across the entire C-band in a single 100-ms scan. Matrix eigenanalysis of the transfer matrix enables extraction of the system's insertion loss, mode-dependent loss, and the principle-states of polarization. Results will include transfer matrix measurements of few-mode fibers, coupled-core fibers, photonic lantern spatial-multiplexers, and SDM compatible wavelength routing components such as a wavelength selective switch.
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