Since a decade, wavefront shaping techniques has allowed to coherently manipulate speckle patterns. It opens the possibility to focus light through complex media and ultimately to image in them, provided that the medium can be considered as stationary during the process. However, scattering by tissues evolves over millisecond timescales, creating a fast decorrelation of the speckle pattern, thus limiting the use of this technique for in vivo microscopy. Therefore, focusing through biological tissues requires fast wavefront shaping devices, sensors and algorithms.
It has been demonstrated by Akemann et al that an Acousto-Optic Deflector (AOD) time locked on the output laser pulses of a regenerative amplifier can be used as an arbitrary 1D beam shaper: the locally modulated acousto-optic phase grating allows the spatial control of the laser pulse wavefront, with refresh rate of several tens up to several hundreds of kHz, limited by the size of the AOD aperture.
We have investigated through simulations and experiments, the use of two crossed AODs to implement 2D spatial wavefront shaping, and perform focusing by optimization through a scattering media. We have used different algorithms adapted to this grating modulator and analyzed in each case the AOD bandwidth used, the speed of convergence and the maximum intensity enhancement. In particular, we have shown that two crossed 1D modulators provide larger enhancement than a single 2D wavefront shaper with the same number of pixels. We will present our latest results towards achieving the ultimate optimization, limited by the AOD speed of 40 kHz.
The propagation of light in biological tissues is rapidly dominated by multiple scattering: ballistic light is exponentially attenuated, which limits the penetration depth of conventional microscopy techniques. For coherent light, the recombination of the different scattered paths creates a complex interference: speckle. Recently, different wavefront shaping techniques have been developed to coherently manipulate the speckle. It opens the possibility to focus light through complex media and ultimately to image in them, provided however that the medium can be considered as stationary.
We have studied the possibility to focus in and through time-varying biological tissues. Their intrinsic temporal dynamics creates a fast decorrelation of the speckle pattern. Therefore, focusing through biological tissues requires fast wavefront shaping devices, sensors and algorithms. We have investigated the use of a MEMS-based spatial light modulator (SLM) and a fast photodetector, combined with FPGA electronics to implement a closed-loop optimization. Our optimization process is just limited by the temporal dynamics of the SLM (200µs) and the computation time (45µs), thus corresponding to a rate of 4 kHz. To our knowledge, it’s the fastest closed loop optimization using phase modulators.
We have studied the focusing through colloidal solutions of TiO2 particles in glycerol, allowing tunable temporal stability, and scattering properties similar to biological tissues. We have shown that our set-up fulfills the required characteristics (speed, enhancement) to focus through biological tissues. We are currently investigating the focusing through acute rat brain slices and the memory effect in dynamic scattering media.
Decoding of information in the brain requires the imaging of large neuronal networks using e.g. two-photon microscopy (TPM). Fast control of the focus in 3D can be achieved with phase shaping of the light beam using acoustooptic deflectors (AODs). However, beam shaping using AODs is not straightforward because of non-stationary of acousto-optic diffraction. Here, we demonstrated a new stable AOD-based phase modulator, which operates at a rate of up to about hundred kHz. It provides opportunity for 3D scanning in TPM with the possibility to correct aberrations independently for every focus position or to achieve refocusing of scattered photons in rapidly decorrelating tissues.
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.