Optimization of high-order harmonic generation by adaptive control of a sub-10-fs pulse wave front

Automated fast computational adaptive optics for optical coherence tomography based on a stochastic parallel gradient descent algorithm

The transverse resolution of optical coherence tomography is decreased by aberrations introduced from optical components and the tested samples. In this paper, an automated fast computational aberration correction method based on a stochastic parallel gradient descent (SPGD) algorithm is proposed for aberration-corrected imaging without adopting extra adaptive optics hardware components. A virtual phase filter constructed through combination of Zernike polynomials is adopted to eliminate the wavefront aberration, and their coefficients are stochastically estimated in parallel through the optimization of the image metrics. The feasibility of the proposed method is validated by a simulated resolution target image, in which the introduced aberration wavefront is estimated accurately and with fast convergence. The computation time for the aberration correction of a 512 × 512 pixel image from 7 terms to 12 terms requires little change, from 2.13 s to 2.35 s. The proposed method is then applied for samples with different scattering properties including a particle-based phantom, ex-vivo rabbit adipose tissue, and in-vivo human retina photoreceptors, respectively. Results indicate that diffraction-limited optical performance is recovered, and the maximum intensity increased nearly 3-fold for out-of-focus plane in particle-based tissue phantom. The SPGD algorithm shows great potential for aberration correction and improved run-time performance compared to our previous Resilient backpropagation (Rprop) algorithm when correcting for complex wavefront distortions. The fast computational aberration correction suggests that after further optimization our method can be integrated for future applications in real-time clinical imaging.

Fast stabilization of a high-energy ultrafast OPA with adaptive lenses

The use of fast closed-loop adaptive optics has improved the performance of optical systems since its first application. Here we demonstrate the amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier devoted to Attosecond Science exploiting two high speed adaptive optical systems for the correction of static and dynamic instabilities. The exploitation of multi actuator adaptive lenses allowed for a minimal impact on the optical setup.

Handheld Adaptive Optics Scanning Laser Ophthalmoscope

Adaptive optics scanning laser ophthalmoscopy (AOSLO) has enabled in vivo visualization and enhanced understanding of retinal structure and function. Current generation AOSLOs have a large footprint and are mainly limited to imaging cooperative adult subjects. To extend the application of AOSLO to new patient populations, we have designed the first portable handheld AOSLO (HAOSLO) system. By incorporating a novel computational wavefront sensorless AO algorithm and custom optics, we have miniaturized our HAOSLO to weigh less than 200 grams. HAOSLO imaged the cones closest to the fovea with a handheld probe in adults and captured the first AO-enhanced image of cones in infants.

Optimizing single-mode collection from pointlike sources of single photons with adaptive optics

The collection efficiency of light from a point-like emitter may be extremely poor due to aberrations induced by collection optics and the emission distribution of the source. Analyzing the aberrant wavefront (e.g., with a Shack-Hartmann sensor) and correcting accordingly can be infeasible on the single-photon level. We present a technique that uses a genetic algorithm to control a deformable mirror for correcting wavefront aberrations in single-photon signals from point emitters. We apply our technique to both a simulated point source and a real InAs quantum dot, achieving coupling increases of up to 50% and automatic reduction of system drift.

High-throughput beamline for attosecond pulses based on toroidal mirrors with microfocusing capabilities

We have developed a novel attosecond beamline designed for attosecond-pump/attosecond probe experiments. Microfocusing of the Extreme-ultraviolet (XUV) radiation is obtained by using a coma-compensated optical configuration based on the use of three toroidal mirrors controlled by a genetic algorithm. Trains of attosecond pulses are generated with a measured peak intensity of about 3 × 10(11) W/cm(2).

Wavefront sensorless adaptive optics optical coherence tomography for in vivo retinal imaging in mice

We present wavefront sensorless adaptive optics (WSAO) Fourier domain optical coherence tomography (FD-OCT) for in vivo small animal retinal imaging. WSAO is attractive especially for mouse retinal imaging because it simplifies optical design and eliminates the need for wavefront sensing, which is difficult in the small animal eye. GPU accelerated processing of the OCT data permitted real-time extraction of image quality metrics (intensity) for arbitrarily selected retinal layers to be optimized. Modal control of a commercially available segmented deformable mirror (IrisAO Inc.) provided rapid convergence using a sequential search algorithm. Image quality improvements with WSAO OCT are presented for both pigmented and albino mouse retinal data, acquired in vivo.

Wavefront sensorless modal deformable mirror correction in adaptive optics: optical coherence tomography

We present a method for optimization of optical coherence tomography images using wavefront sensorless adaptive optics. The method consists of systematic adjustment of the coefficients of a subset of the orthogonal Zernike bases and application of the resulting shapes to a deformable mirror, while optimizing using image sharpness as a merit function. We demonstrate that this technique can compensate for aberrations induced by trial lenses. Measurements of the point spread function before and after compensation demonstrate near diffraction limit imaging.

Optimization of two-photon wave function in parametric down conversion by adaptive optics control of the pump radiation

We present an efficient method for optimizing the spatial profile of entangled-photon wave function produced in a spontaneous parametric down conversion process. A deformable mirror that modifies a wavefront of a 404 nm CW diode laser pump interacting with a nonlinear β-barium borate type-I crystal effectively controls the profile of the joint biphoton function. The use of a feedback signal extracted from the biphoton coincidence rate is used to achieve the optimal wavefront shape. The optimization of the two-photon coupling into two, single spatial modes for correlated detection is used for a practical demonstration of this physical principle.

Active diffraction gratings: development and tests

We present the realization and characterization of an active spherical diffraction grating with variable radius of curvature to be used in grazing-incidence monochromators. The device consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The experimental results show that the active grating can optimize the beam focalization of visible wavelengths through its rotation and focus accommodation.

Invited review article: technology for attosecond science

We describe a complete technological system at Imperial College London for Attosecond Science studies. The system comprises a few-cycle, carrier envelope phase stabilized laser source which delivers sub 4 fs pulses to a vibration-isolated attosecond vacuum beamline. The beamline is used for the generation of isolated attosecond pulses in the extreme ultraviolet (XUV) at kilohertz repetition rates through laser-driven high harmonic generation in gas targets. The beamline incorporates: interferometers for producing pulse sequences for pump-probe studies; the facility to spectrally and spatially filter the harmonic radiation; an in-line spatially resolving XUV spectrometer; and a photoelectron spectroscopy chamber in which attosecond streaking is used to characterize the attosecond pulses. We discuss the technology and techniques behind the development of our complete system and summarize its performance. This versatile apparatus has enabled a number of new experimental investigations which we briefly describe.

Improving high-order harmonic yield using wavefront-controlled ultrashort laser pulses

In this work we show that it is possible to increase the high-order harmonic yield when using wavefront-shaped laser beams. The investigation of the beam profile near the interaction region shows that the optimized beam is asymmetric and has a larger diameter. Thus, the optimized beam leads to a higher yield even if the peak intensity is lower compared to an unoptimized beam. This indicates that the wavefront of the fundamental laser beam and, accordingly, the focal profile play an important role in the efficient generation of high-order harmonic radiation.

High resolution wavefront correction with photocontrolled deformable mirror

We demonstrate that a novel actuation scheme, employed in an optical control deformable mirror, can be more convenient than the conventional discrete fixed actuators approach. The Photo-Controlled Deformable Mirror (PCDM) mirror leverages consumer LCD display technology in the wavefront forming control, enabling flexible programmable configuration of the actuation geometry. This new approach simplifies the driving electronics, relaxing the per channel cost of high spatial control of the wavefront forming surface. In our experiment we tested the PCDM by applying the equivalent of 36, 76 and 201 actuators, this by just changing the light driving pattern. We demonstrated the effectiveness of this technique in a closed loop setup, which showed performances superior to the state of the art for similar DM, while providing a significant reduction in the hardware complexity.

Shack-Hartmann wavefront-sensor-based adaptive optics system for multiphoton microscopy

The imaging depth of two-photon excitation fluorescence microscopy is partly limited by the inhomogeneity of the refractive index in biological specimens. This inhomogeneity results in a distortion of the wavefront of the excitation light. This wavefront distortion results in image resolution degradation and lower signal level. Using an adaptive optics system consisting of a Shack-Hartmann wavefront sensor and a deformable mirror, wavefront distortion can be measured and corrected. With adaptive optics compensation, we demonstrate that the resolution and signal level can be better preserved at greater imaging depth in a variety of ex-vivo tissue specimens including mouse tongue muscle, heart muscle, and brain. However, for these highly scattering tissues, we find signal degradation due to scattering to be a more dominant factor than aberration.

Efficient selection of single high-order harmonic caused by atomic autoionizing state influence

It is shown that the influence of atomic autoionizing states on the phase matching of high harmonic generation results in efficient selection of the single harmonic generated in the plateau region. The change of relative concentration of the atomic medium components and variation of the fundamental laser frequency can be used for tuning of the selected harmonic frequency. The possibility of achievement of contrast for the selected single harmonic of more than 10(4) at intensity conversion efficiency of approximately 10(-3) has been shown.

No wavefront sensor adaptive optics system for compensation of primary aberrations by software analysis of a point source image. 2. Tests

The description of an adaptive optics (AO) system with no wavefront sensor to correct primary aberrations is presented. This system is based on closed loop software that iteratively analyzes a point source target image on the instrument focal plane and suitably modifies the AO device. The performed tests with a pull-only deformable mirror (DM) have shown that the system works very well, reaching an optimal focusing condition in a few seconds using standard components. Such a system can be conveniently applied in all the fields in which a not very fast optical adaptation is acceptable.

Time-delay compensated monochromator in the off-plane mount for extreme-ultraviolet ultrashort pulses

The design of ultrafast monochromators using grazing-incidence gratings in the off-plane mount for the spectral selection of extreme-ultraviolet femtosecond pulses in a broad spectral region is presented. Their application in the selection of high-order laser harmonics is analyzed in detail. The main advantage of the off-plane mount is a much higher efficiency than that of the classical mount. It is shown that two-grating configurations preserve the length of the optical paths of different diffracted rays, maintaining the extremely short time duration of the pulse. Configurations with plane or toroidal gratings are discussed. As a test case, the design of a monochromator for the 17-61 nm region with a time compensation better than 1 fs is presented.

Adaptive correction of a tightly focused, high-intensity laser beam by use of a third-harmonic signal generated at an interface

By using the third-harmonic signal generated at an air-dielectric interface, we demonstrate a novel way of correcting wavefront aberrations induced by high-numerical-aperture optics. The third harmonic is used as the input physical parameter of a genetic algorithm working in closed loop with a 37-actuator deformable mirror. This method is simple and reliable and can be used to correct aberrations of tightly focused beams, a regime where other methods have limitations. Improvement of the third-harmonic signal generated with an f/1.2 parabolic mirror by 1 order of magnitude is demonstrated.

Gratings in a conical diffraction mounting for an extreme-ultraviolet time-delay-compensated monochromator

The conical diffraction mounting in which the direction of incident light belongs to a plane parallel to the direction of the grooves has the unique property of maintaining high diffraction efficiency, even in the extreme-ultraviolet (EUV) region. This property is useful for designing high-throughput time-delay-compensated monochromators for the spectral selection of ultrashort EUV pulses as the high-order harmonics generated by the interaction between an ultrashort laser pulse and a gas jet. The time compensation allows one to exploit the femtosecond scale duration of the harmonics both to have high intensity and to reach an unprecedented temporal resolution for pump and probe experiments. Because two gratings have to be used for time compensation, the high diffraction efficiency becomes an essential requirement, which can be fulfilled by the conical diffraction mounting. Measurements recently accomplished at the Bending Magnet for Emission Absorption and Reflectivity (BEAR) beam line (ELETTRA Synchrotron, Trieste, Italy) for three gratings in the 10-90 nm region are reported here that show a peak efficiency of as much as 0.7 in the first order. A model computing the electromagnetic propagation and the grating efficiency, implemented and tested with the experimental data, permits the study and design of rather complex systems operating in the conical mounting. Basic physical principles and mathematical aspects of the model are discussed here.

Adaptive spatial control of fiber modes and their excitation for high-harmonic generation

We present the control of high-harmonic generation (HHG) in hollow fibers using adaptive pulse shaping techniques. The shaping capabilities of our spatial light modulator (SLM) are demonstrated by the excitation of specific fiber modes inside a hollow fiber with a helium-neon laser. Afterwards spatially shaped ultrashort pulses are used to generate phase-matched high-harmonic radiation in a fiber. We show that by controlling the mode structure, we can manipulate the spatial and spectral properties of the generated harmonics.

Spatial control of high-harmonic generation in hollow fibers

We demonstrate the control of high-harmonic generation in a hollow fiber by shaping the spatial structure of the generating laser pulse. We use a liquid-crystal-based two-dimensional spatial light modulator to control the spatial phase of the driver pulse. An evolutionary algorithm finds the spatial laser phase distribution that is optimal for reaching maximum total harmonic yield and for selectively enhancing the cutoff region of the spectrum. We show that enhacement of harmonic generation is related to coupling into a single fiber mode. Our results directly show that spatial properties of the laser are important parameters in fully controlling the high-harmonic spectrum. It is thus not possible to derive the controllability of the high-harmonic generation from the single-atom response only.

Validity of wave-front reconstruction and propagation of ultrabroadband pulses measured with a Hartmann-Shack sensor

Wave-front reconstruction for ultrabroadband laser pulses is verified by use of a Hartmann-Shack sensor. We estimate the accuracy of numerical wave-front propagation by comparing numerical with experimental results and verify that wave fronts of ultrabroadband laser pulses from a hollow fiber can be propagated correctly by a single polychromatic wave-front measurement to a place where detection is not practicable, e.g., inside a vacuum chamber or laser focus.