Customizing the output beam shape from a laser invariably involves specialized optical elements in the form of apertures, diffractive optics and free-form mirrors. Such optics require considerable design and fabrication effort and suffer from the further disadvantage of being immutably connected to the selection of a particular spatial mode. Here we overcome these limitations with the first digital laser comprising an electrically addressed reflective phase-only spatial light modulator as an intra-cavity digitally addressed holographic mirror. The phase and amplitude of the holographic mirror may be controlled simply by writing a computer-generated hologram in the form of a grey-scale image to the device, for on-demand laser modes. We show that we can digitally control the laser modes with ease, and demonstrate real-time switching between spatial modes in an otherwise standard solid-state laser resonator. Our work opens new possibilities for the customizing of laser modes at source.

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A digital laser for on-demand laser modes

In this article, we review the phenomenon of dispersion, paying particular attention to its impact in the optics of ultrashort pulses, as well as its measurement and management. At present, lasers generating coherent bandwidths of several hundred nanometers have been demonstrated and correspondingly short pulses of 10 fs or so are quite usual. The limits to the breadth of optical spectra and brevity of pulse durations that may be achieved are often set by the dispersive properties of the linear optical elements of which the source is constructed. Progress in ultrafast optics to date has therefore relied extensively on the development of ways to characterize and manipulate dispersion. The means by which this can be accomplished are significantly different for laser oscillators and laser amplifiers, as well as for nonlinear interactions that are used to extend the range of frequencies at which short optical pulses are available, but in all cases it is this phenomenon that determines the output of current optical sources.

The role of dispersion in ultrafast optics

Abstract This article reviews recent progress leading to the generation of optical vortex beams. After introducing the basics of optical vortex beams and their promising applications, we summarized different approaches for optical vortex generation by discrete components and laser cavities. We place particular emphasis on the recent development of vortex generation by the planar phase plates, which are able to engineer a spiral phasefront via dynamic or geometric phase in nanoscale, and highlight the independent operation of these two different phases which leads to a multifunctional optical vortex beam generation and independent spin-orbit interaction. We also introduced the recent progress on vortex lasing, including vortex beam generation from the output of bulk lasers by modification of conventional laser cavities with phase elements and from integrated on-chip microlasers. Similar approaches are also applied to generate fractional vortex beams carrying fractional topological charge. The advanced technology and approaches on design and nanofabrications enable multiple vortex beams generation from a single device via multiplexing, multicasting, and vortex array, open up opportunities for applications on data processing, information encoding/decoding, communication and parallel data processing, and micromanipulations.

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Recent advances on optical vortex generation

The polarization properties of diffraction from volume phase gratings in photorefractive sillenite crystals such as bismuth silicon oxide (Bi12SiO20), bismuth germanium oxide (Bi12GeO20), and bismuth titanium oxide (Bi12TiO20) are strongly modified by the presence of concomitant natural optical activity and electric-field-induced linear birefringence. A set of coupled-wave equations that characterize the Bragg regime has been derived for the 〈110〉 and the 〈001〉 crystallographic orientations typically employed in volume holographic storage and multiwave-mixing applications. The predicted anisotropic behavior of the grating diffraction is experimentally confirmed, and a significant efficiency improvement is shown to occur for proper choice of the operating mode and the probe beam polarization in a given configuration.

Polarization properties of photorefractive diffraction in electrooptic and optically active sillenite crystals (Bragg regime)

Structured Light from Lasers

We present a new beam-shaping technique with an intracavity optically addressed liquid-crystal spatial light modulator. The Nd:YAG resonator is able to deliver beams with various spatial profiles such as flat-topped super-Gaussian and square-shaped beams.

Spatial mode control of a diode-pumped Nd:YAG laser by an intracavity liquid-crystal light valve.

We present the principle of a new photodetector based on the synchronous drift in a semiconductor of photogenerated carriers with a moving interference pattern. It provides a large detection volume which could be suitable for the optical generation of microwave signals. A theoretical analysis is detailed and followed by an experimental demonstration in a MBE grown GaAs layer illuminated with a moving interference pattern. We study the evolution of the detected signal at frequency f=1.6 GHz as a function of applied electric field, carrier lifetime, and fringe period.

Photodetector for microwave signals based on the synchronous drift of photogenerated carriers with a moving interference pattern

We report the characterization of a new pulse stretcher that provides a linear and positive variation of the group delay as a function of the optical frequency. It consists of a perpendicular chirped-grating pair introduced by Tournois [Opt. Commun. 106, 253 (1994)] that allows 20-fs pulses to be stretched to 100 ps. The system is tested by short-pulse spectral interferometry. We designed and realized the chirped gratings by phase volume holographic recording in a highly efficient photopolymer.

Characterization of perpendicular chirped phase volume grating pairs for laser pulse stretching.

Projection systems based on liquid-crystal displays (LCD's) offer new opportunities to display high-definition and large-size TV images. There are two types of LCD projector architectures: the 3-LCD architecture uses one LCD for each primary color, red, green, and blue, whereas a single-LCD configuration employs only one LCD paved with color filters. The single-LCD projector is simple and compact but suffers from a poor luminous efficiency because of losses in the color filters: each filter transmits only ~1/3 of the flux emitted by the lamp. To increase this optical efficiency, we propose to introduce volume holographic elements in the architecture of a single-LCD projector. Innovative systems are presented in which volume holographic elements realize the spatiochromatic illumination of the LCD. This illumination consists of selectively directing all the light that corresponds to a primary color, red, green, or blue, in the pixel addressed with the corresponding video composite signal and exploits the spectral selectivity and dispersion properties of volume holographic gratings and lenses. The two main advantages of such illumination are the suppression of the color filters and the recovery of the light lost in a classical architecture by absorption of the color filters. A complete luminous efficiency analysis of spatiochromatic illumination with volume holographic elements is presented. The achieved performances are compared with classical single-LCD projectors.

Phase volume holographic optical components for high-brightness single-LCD projectors.

We present and experimentally demonstrate a new method for enhancing the signal-to-background ratio of two-wave mixing in photorefractive crystals. The method uses a mutually incoherent third beam to suppress the fanning in a dark ring-shaped region in which the amplified signal is located. A 20-fold improvement of the signal-to-background ratio is measured in BaTiO3 at λ = 514 nm. The extension of this principle to wide-field-of-view heterodyne detection is discussed.

J.-P. Huignard

Papers

Spatial mode control of a diode-pumped Nd:YAG laser by an intracavity liquid-crystal light valve.

Photodetector for microwave signals based on the synchronous drift of photogenerated carriers with a moving interference pattern

Characterization of perpendicular chirped phase volume grating pairs for laser pulse stretching.

Phase volume holographic optical components for high-brightness single-LCD projectors.

Enhancement of the signal-to-background ratio in photorefractive two-wave mixing by mutually incoherent two-beam coupling