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.
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.
Spatial and temporal properties of an ultrashort pulse of light are naturally scrambled upon propagation in thick scattering media. Significant progresses have been realized over the last decade to manipulate light propagation in scattering media, mostly using monochromatic light. However, applications that require a broadband ultrashort pulse of light remain limited, as the pulse gets temporally broadened because of scattering effects. A monochromatic optical transmission matrix does not allow temporal control of broadband light. Although measuring multiple transmission matrices with spectral resolution allows fine temporal control, it requires lengthy measurements, as well as stability of the medium.
In this work, we show that a single linear operator that we named Broadband Transmission Matrix, can be straightforwardly measured for a broadband pulse with a co-propagating reference. We exploit this operator for focusing purposes, and we analyze its phase conjugation properties. While the operator naturally allows for spatial focusing, unexpectedly, the focus duration is on average shorter than the natural temporal broadening due to the medium. More precisely, we observe a two-fold temporal recompression at the focus that we fully explain theoretically. We also explore the spectral content at the focus, and demonstrate a narrowing of the spectrum.
These results are particularly relevant for non-linear imaging techniques in biological tissues, at depth where an ultrashort excitation pulse is broadened.
When an ultrashort pulse of light propagates in a scattering medium, its spatial and temporal properties get mixed and distorted because of the scattering process. Spatially, the output pattern is the result of the multiple interference between the scattered photons. Temporally, light gets stretched within the medium due to its characteristic confinement time, thus the output pulse is broadened in the time domain. Nonetheless, as the scattering process is linear and deterministic, the spatio-temporal profile of light at the output can be controlled by shaping the input light using a single spatial light modulator (SLM).
We report the first experimental measurement of the Time-Resolved Transmission Matrix of a multiple scattering medium using a coherent time-gated detection system. This operator contains the relationship between the input field, controllable with a SLM, and the output field accessible with a CCD camera for a given arrival time of photons at the output of medium. The delay line of the time-gated detection system sets the arrival time at will within the time of flight distribution of photons of the output pulse.
We exploit this time-resolved matrix to achieve spatio-temporal focusing of the output pulse at any arbitrary space and time position. The pulse is recompressed in time to its original Fourier-limited temporal width and spatially to the diffraction-limited size defined by the speckle grain size. We also generate more sophisticated spatio-temporal profiles such as pump-probe like pulse, thus opening interesting perspectives in coherent control, light-matter interaction and imaging in disordered media.
We have recently reported on a method to design at will the spatial profile of transmitted coherent light after propagation through a strongly scattering sample, exploiting wavefront shaping in combination with a transmission matrix approach. In this paper, we explore experimentally and theoretically the ability of this approach to generate foci whose full width at half maximum are smaller than the diffraction-limited speckle grain size, using (Bessels) beam variations implemented with virtual annular filters.
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