We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Femtosecond pulse shaping using spatial light modulators

A large-scale silicon nanophotonic phased array with more than 4,000 antennas is demonstrated using a state-of-the-art complementary metal-oxide–semiconductor (CMOS) process, enabling arbitrary holograms with tunability, which brings phased arrays to many new technological territories. Nanophotonic approaches allow the construction of chip-scale arrays of optical nanoantennas capable of producing radiation patterns in the far field. This could be useful for a range of applications in communications, LADAR (laser detection and ranging) and three-dimensional holography. Until now this technology has been restricted to one-dimensional or small two-dimensional arrays. This paper reports the construction of a large-scale silicon nanophotonic phased array containing 4,096 optical nanoantennas balanced in power and aligned in phase. The array was used to generate a complex radiation pattern—the MIT logo—in the far field. The authors show that this type of nanophotonic phased array can be actively tuned, and in some cases the beam is steerable. Electromagnetic phased arrays at radio frequencies are well known and have enabled applications ranging from communications to radar, broadcasting and astronomy1. The ability to generate arbitrary radiation patterns with large-scale phased arrays has long been pursued. Although it is extremely expensive and cumbersome to deploy large-scale radiofrequency phased arrays2, optical phased arrays have a unique advantage in that the much shorter optical wavelength holds promise for large-scale integration3. However, the short optical wavelength also imposes stringent requirements on fabrication. As a consequence, although optical phased arrays have been studied with various platforms4,5,6,7,8 and recently with chip-scale nanophotonics9,10,11,12, all of the demonstrations so far are restricted to one-dimensional or small-scale two-dimensional arrays. Here we report the demonstration of a large-scale two-dimensional nanophotonic phased array (NPA), in which 64 × 64 (4,096) optical nanoantennas are densely integrated on a silicon chip within a footprint of 576 μm × 576 μm with all of the nanoantennas precisely balanced in power and aligned in phase to generate a designed, sophisticated radiation pattern in the far field. We also show that active phase tunability can be realized in the proposed NPA by demonstrating dynamic beam steering and shaping with an 8 × 8 array. This work demonstrates that a robust design, together with state-of-the-art complementary metal-oxide–semiconductor technology, allows large-scale NPAs to be implemented on compact and inexpensive nanophotonic chips. In turn, this enables arbitrary radiation pattern generation using NPAs and therefore extends the functionalities of phased arrays beyond conventional beam focusing and steering, opening up possibilities for large-scale deployment in applications such as communication, laser detection and ranging, three-dimensional holography and biomedical sciences, to name just a few.

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Large-scale nanophotonic phased array

Foreword. Series Editor's Foreword. Preface. 1. Liquid crystal physics.* Introduction.* Thermodynamics and statistic physics.* Orientational order.* Elastic properties of liquid crystals.* Response of liquid crystals to electro-magnetic fields.* Anchoring effects of nematic liquid crystal at surfaces. 2. Propagation of light in anisotropic optical medium.* Electromagnetic wave.* Polarization.* Propagation of light in uniform anisotropic optical media.* Propagation of light in cholesteric liquid crystals. 3. Optical modeling methods.* Jones matrix method.* Mueller matrix method.* Berreman 4x4 method. 4. Effects of Electric field on Liquid Crystals.* Dielectric interaction.* Flexoelectric Effect.* Ferroelectricity in liquid crystals. 5. Freedericksz transition.* Calculus of variation.* The Fredeericksz transition: statics.* The Freedericksz transition: dynamics. 6. Liquid Crystal Materials.* Introduction.* Refractive indices.* Dielectric constants.* Rotational Viscosity.* Elastic constant.* Figure-of-merits.* Refractive index matching between liquid crystals and polymers. 7. Modeling of liquid crystal director configuration.* Electric energy of liquid crystals.* Modeling electric field.* Simulation of liquid crystal director configuration. 8. Transmissive liquid crystal display.* Introduction.* Twisted nematic cells.* In plane switching (IPS) mode.* Vertical alignment (VA) mode.* Multi-domain Vertical Alignment (MVA) Cells.* Optically compensated bend (OCB) cell. 9. Reflective and Trasreflective display.* Introduction.* Reflective liquid crystal displays.* Transflector.* Classification of Transflective LCDs.* Dual-cell-gap Transflective LCDs.* Single-cell-gap Transflective LCDs.* Performance of transflective LCDs. 10. Liquid crystal display matrices, drive schemes and bistable displays.* Segmented displays.* Passive matrix displays and drive scheme.* Active Matrix Displays.* Bistable ferroelectric liquid crystal displays and drive scheme.* Bistable nematic displays.* Bistable cholesteric reflective display. 11. Liquid crystal/polymer composites. * Introduction.* Phase separation.* Scattering properties of liquid crystal/polymer composites.* Polymer dispersed liquid crystals.* Polymer stabilization liquid crystals.* Displays from liquid crystal/polymer composites. 12. Tunable liquid crystal photonic devices. * Introduction.* Laser beam steering.* Variable Optical Attenuators.* Tunable-Focus Lens.* Polarization-Independent LC Devices. Index.

Fundamentals of Liquid Crystal Devices

Optical phased arrays represent an enabling new technology that makes possible simple affordable, lightweight, optical sensors offering very precise stabilization, random-access pointing programmable multiple simultaneous beams, a dynamic focus/defocus capability, and moderate to excellent optical power handling capability. These new arrays steer or otherwise operate on an already formed beam. A phase profile is imposed on an optical beam as it is either transmitted through or reflected from the phase shifter array. The imposed phase profile steers, focuses, fans out, or corrects phase aberrations on the beam. The array of optical phase shifters is realized through lithographic patterning of an electrical addressing network on the superstrate of a liquid crystal waveplate. Refractive index changes sufficiently large to realize full-wave differential phase shifts can be effected using low (<10 V) voltages applied to the liquid crystal phase plate electrodes. High efficiency large-angle steering with phased arrays requires phase shifter spacing on the order of a wavelength or less; consequently addressing issues make 1-D optical arrays much more practical than 2-D arrays. Orthogonal oriented 1-D phased arrays are used to deflect a beam in both dimensions. Optical phased arrays with apertures on the order of 4 cm by 4 cm have been fabricated for steering green, red, 1.06 /spl mu/m, and 10.6 /spl mu/m radiation. System concepts that include a passive acquisition sensor as well as a laser radar are presented.

Optical phased array technology

A LiDAR-based 3-D point cloud measuring system includes a base, a housing, a plurality of photon transmitters and photon detectors contained within the housing, a rotary motor that rotates the housing about the base, and a communication component that allows transmission of signals generated by the photon detectors to external components. In several versions of the invention, the system includes a vertically oriented motherboard, thin circuit boards such as ceramic hybrids for selectively mounting emitters and detectors, a conjoined D-shaped lens array, and preferred firing sequences.

High definition lidar system

Efficient, electrically tunable, agile, inertialess, near-diffraction-limited one-dimensional optical beam steering is demonstrated at the infrared wavelength of 10.6 microm with a liquid-crystal phased array.

High-efficiency liquid-crystal optical phased-array beam steering

Phase imaging is used to compare near-field measurements with the corresponding far-field intensity distribution. A liquid-crystal device serves as a phase object that can be programmed as a variable grating. Real-time phase visualization then provides an avenue for direct optimization of complex phase gratings.

Spatially resolved phase imaging of a programmable liquid-crystal grating.

High-performance, optical phased arrays for electronic control of laser beams have recently been demonstrated. The optical aperture is lithographically fabricated to form a unity-fill-factor array of liquid-crystal-based optical phase shifters. The liquid-crystal phase shifters can be fabricated smaller than a wavelength of light, with spacings equally small, although this is not required in these devices for small angle steering applications. Computer control of the individual phase shifters establishes a phase profile across the optical aperture that steers or otherwise controls an incident optical beam. Prototypes of such optical phased arrays have been used to demonstrate electronically programmable optical beam steering, lensing, fanout, and conditioning. The devices offer high pointing accuracy and resolution in a small size, owing to the much shorter wavelength of light as compared to microwaves. This inertialess technology makes available to optical systems many of the performance capabilities and the functional versatility long afforded microwave systems by microwave phased array antennas. Operation of such optical phased arrays has been successfully demonstrated from the green (doubled Nd:YAG lasers) to the long-wave infrared (carbon dioxide lasers). The capability represents a major technological advance and is expected to enable numerous new optical systems. This paper focuses on a comparison of the new optical phased array technology with that of conventional microwave phased arrays. Operating principles are briefly reviewed from dual perspectives: namely, those of the microwave and optical regimes. A brief discussion of system applications is included.

D.P. Resler

Papers

Optical phased array technology

High-efficiency liquid-crystal optical phased-array beam steering

Spatially resolved phase imaging of a programmable liquid-crystal grating.

An optical phased array for lasers