Measuring polarisation, spectrum, temporal dynamics, and spatial complex amplitude of optical beams is essential to studying phenomena in laser dynamics, telecommunications and nonlinear optics. Here, we harness principles of spatial state tomography to measure a complete description of an unknown beam as a set of spectrally, temporally, and polarisation resolved spatial state density matrices. Each density matrix slice resolves the spatial complex amplitude of multiple mutually incoherent fields, which over several slices reveals the spectral or temporal evolution of these fields even in scenarios when they spectrally or temporally overlap. We demonstrate these features by characterising the rich spatiotemporal and spatiospectral output of a vertical-cavity surface-emitting laser.
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.
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.
We describe an approach capable of generating structured light beams from a compact laser source based on the coherent combination of multiple tailored Gaussian beams emitted from a multicore fiber (MCF) amplifier. We report a proof-of-concept structured light generation experiment, using a cladding-pumped 7-core MCF amplifier as an integrated parallel amplifier array and a spatial light modulator to actively control the amplitude, polarization and phase of the signal light input to each fiber core. We demonstrate the generation of various structured light beams including high-order linearly polarized spatial fiber modes, cylindrical vector beams and helical phase front optical vortex beams.
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.
Computer-generated holography is a computationally intensive process particularly well suited to the architecture of graphics processing units (GPUs). This work investigates the performance improvements achievable through utilization of a GPU for optimization of holograms via simulated annealing. Two examples are given; accelerated training of an optical correlator to accept or reject inputs over sets of varying sizes, followed by an investigation into optimization of a hologram to produce a desired complex distribution in a portion of the far field with varying resolutions. Specifically, results comparing a Quad-core 3.0-GHz CPU and an nVidia GTX260 GPU are presented, demonstrating performance improvements of up to 2400%. This work offers details on what steps have been taken to optimize the algorithm for both the CPU and GPU platforms, and may be of interest to those looking to utilize GPU hardware for scientific computation.
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