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December 1992

Nonlinear Phase Shifts Using Second Order Nonlinearities

M. Sheik-Bahae, R. DeSalvo, D.J. Hagan, G. Assanto, G. Stegeman, and E.W. Van Stryland

A continuing problem in nonlinear optics has been to find third order materials with large, fast nonlinearities in spectral regions of low linear and nonlinear absorption. Typical applications of such materials require large nonlinear phase shifts. It has been known, but not widely appreciated, that second order nonlinear interactions lead to effective third order nonlinearities and nonlinear phase shifts in the fundamental beam. While such phenomena are implicitly included in the standard equations governing, for example, second-harmonic generation (SHG) and have been theoretically predicted previously, only recently has the phase distortion been studied in detail experimentally and theoretically.

Excited State Enhancement of Nonlinear Optical Properties

In the search for efficient materials for all-optical photonic devices based on the third order optical susceptibility X(3), organic and polymeric materials are being intensely investigated because nonresonant virtual excitation of the delocalized π-electron systems of these materials can result in large ultrafast nonlinear optical responses with minimal background absorption. We have recently shown through theoretical and experimental studies that x(3)(-ω4;ω1,ω2,ω3) can be increased by orders of magnitude over the normally observed ground state values by population of electronic excited states.

200 fsec Optical Pulse Generation from a Hybrid Mode-locked Semiconductor Diode Laser-Amplifier System

Compact sources of ultrashort, high repetition rate, high peak power optical pulses are desirable for photonic switching, optical computing, optical clocking, and synchronization, nonlinear optical methods of volumetric optical data storage, and other areas of applied photonics networks. Monolithic hybrid modelocked semiconductor diode lasers provide ultrashort optical pulses at ultrahigh bit rates; however, the peak output power may not be sufficient for certain applications. In this report, a hybrid modelocked external cavity semiconductor laser system is described that produces pulses of 200 fsec in duration with 165 watts of peak power. These optical pulses represent both the shortest and most intense ever generated from an all semiconductor laser system.

Optical Dephasing and Acoustic Plasmon Undamping in Highly Excited Semiconductors

Optical dephasing, i.e., the decay of the polarization field in a semiconductor, is a direct consequence of electron and hole scattering. Under high excitation conditions or in a semiconductor amplifier/laser, carrier-carrier scattering is often the dominant relaxation mechanism, which also leads to energy-level broadening and dynamical screening of the Coulomb interaction potential. The theoretical analysis of carrier-carrier scattering is based on the quantum Boltzmann equation. Even though this equation is well known in the many-body literature, its solution for high excitation conditions and nonequilibrium carrier distributions is a substantial challenge.

Propagation-Induced Escape from Adiabatic Following in a Semiconductor

Coherent pulse propagation, including self-induced transparency and pulse breakup, is well-known in atomic media. Pulse propagation and optical soliton formation have been studied extensively in optical fibers. However, investigations of coherent pulse propagation effects in semiconductors are very rare. We have reported a combined experimental and theoretical study showing coherent pulse breakup in a semiconductor waveguide for photon energies below the lowest (ls) exciton resonance.

Guiding Light by Light Using Dark Spatial Solitons

The guiding of light by light is one long term aim of nonlinear photonics since it offers the possibility of developing all-optical circuitry where a controlling light beam can rapidly switch or modulate a second information-carrying beam. In this context, there has been considerable recent interest in spatial solitons and their interaction.

Kink Solitons in Nonlinear Optics

In the last few years the word "soliton" has become a common word in optics and photonics, thanks to the current worldwide study of potential applications of solitons in optical communications and photonic switching. Even though the phenomenon of self-induced transparency in a resonant absorbing medium provided the first example of solitons in optics, it was the discovery of solitons in optical fibers that transformed solitons from a mathematical curiosity into a practical and useful entity. Their existence is attributed to the simultaneous presence of anomalous dispersion and nonlinear refraction (responsible for self-phase modulation) in the 1.5 μm wavelength region of silica fibers.

Coherent Optical Wavelet Transforms

After a successful application to the analysis of seismic data in 1984, geophysicist J. Morlet1 formalized the concept of wavelets by generalizing similar mathematical and physical decomposition concepts from the previous works of Harr (1910), Gabor (1946), Calderon (1964), etc.. Closely related to the concept of pyramids, the key component of the wavelet theory is a set of look-alike functions called wavelets that can form an efficient and powerful decomposition basis. Any square-integrable signal can be selected as the so-called mother wavelet that, in turn, generates a family of daughter wavelets by dilations.

Femtosecond Waveform Processing via Spectral Holography

Although the technology of generating femtosecond optical pulses is now rather advanced, our ability to manipulate or process ultrashort pulses is still limited. Techniques for accomplishing linear filtering of ultrashort pulses have been developed and have been used to precisely synthesize femtosecond pulse waveforms. Nevertheless, certain basic signal processing operations, such as time reversal, correlation, and convolution, cannot be achieved by linear filtering. In this summary, we describe the use of holographic processing techniques to perform nonlinear filtering of femtosecond waveforms.

Quantum Optics of a Single Molecule in a Solid

Trapping of single atomic ions in a radio-frequency trap by photon-recoil cooling and detection of the emitted fluorescence has proven to be a successful way to investigate the fundamental interaction of light and matter and to test our understanding of quantum physics. The extension of these studies to molecules has failed so far, because all molecules— even diatomic species—possess several internal degrees of freedom that prevent laser cooling.

New Questions in the Multiphoton Ionization of Atoms

Multiphoton ionization (MPI) of atoms by intense laser fields has now received over a decade of research. Despite great progress in understanding this seemingly simple process, new observations continue to be made that challenge the accepted picture. The most recent set of experiments highlight the important role of excited states in the MPI process.

The ac Stark Shifts of High-Lying Rydberg Levels in Intense Electromagnetic Fields

When an atom is subject to an intense, time-varying electric field (such as the electromagnetic field produced by intense laser radiation), the atomic energy levels are shifted by the external field, an effect called the light shift or ac Stark shift. A thorough understanding of these shifts is necessary to interpret results from experiments that use intense lasers to probe atomic structure. For a highly monochromatic laser not tuned to any atomic transition and for a Rydberg level with binding energy much less than the laser photon energy, theory predicts that the energy shift in the level will approach the value given by the so-called ponderomotive potential. In a Rydberg level, a single electron is in a large orbit loosely bound to the nucleus (which is embedded in the core of the remaining electrons), and this electron exhibits near-classical behavior in certain circumstances. The ponderomotive potential is simply the classical average kinetic energy a free electron gains when driven into oscillation by an external electromagnetic field.

Near-Field Magneto-Optics and High Density Storage

We have demonstrated a new read/write data storage technique capable of recording up to 45 billion bits of information per square inch of media, or 100 times more than in current compact disks and 300 times more than in state-of-the-art magnetic hard drives. At this density, a palm sized disk could record up to 17 hours of compressed HDTV programming. Alternatively, two copies of War and Peace could be stored in an area about the size of the head of a pin.

Visible Subwavelength Point Source of Light

Micro-dimension light sources that can span a wide variety of wavelengths in the visible have been a major goal in optoelectronics for more than a decade. The applications for such light sources span numerous areas of advanced technology, not the least of which is the important problem of storing and retrieving the deluge of information that is engulfing mankind. For example, if a blue emitting source could be produced without any reduction in the size or the optical methodologies employed, factors of 4 improvement in data capacity could be achieved and, thus, a full length movie could be stored on one of today's compact discs. If, on the other hand, geometrical optics is bypassed and an optical device could be created that would be able to interface with the exciting developments of near-field optics, in which a subwavelength point of light is brought within the near-field of a surface to be read or written on, then pixel sizes in the nanometer regime can be achieved and orders of magnitude improvement in storage densities could be attained.

Multi-Mode Interference Optical Devices Based on Self-Imaging Effects

As opto-electronic integrated circuits (OEICs) become a reality, there is an increasing need for optical signal routing and signal processing devices with smaller dimensions, improved fabrication tolerances, and polarization-independent operation. Recently, several papers reported on multi-mode interference (MMI) 2X2 directional couplers that, in contrast with conventional two-mode interference (TMI) couplers, can fulfill all of the above requirements. Insertion losses as low as 0.5 dB and extinction ratios better than 30 dB have been obtained2,3 with very compact, polarization-independent fabrication tolerant devices. The key feature of the high performance of these couplers is the self-imaging effect by multi-mode interference. Self-imaging is a property of multimoded waveguides by which the exciting field at the entrance will be reproduced (either replicated or mirrored, single or n-folded) at periodic intervals along the propagation direction of the guide.

Sidelobe-Suppressed Acousto-Optic Filter

The integrated acousto-optic filter (AOF) has found many applications in optical signal processing, from fast scanning optical spectroscopy to routing in wavelength-division-multiplexed (WDM) systems, to femtosecond pulse shaping. Devices fabricated by integrated optical and acoustic technology on x-cut LiNbO3 have the ability to filter, or to switch between output ports, one or many independent nanometer-wide wavelength channels.

Monolithically Integrated 21-Wavelength DFB Laser Array with a Star Coupler and Optical Amplifiers

We have reported previously multi-wavelength distributive feedback DFB laser arrays with as many as 20 wavelengths on a single chip fabricated by the use of strained-layer InGaAs/InGaAsP multi-quantum wells. A wavelength span as large as 131 ran in the 1.5 μm wavelength region, a 3 dB modulation bandwidth as high as 16 GHz, and a linewidth as small as 180 kHz have been obtained. More recently, we have demonstrated monolithic integration of the laser array with a star coupler and optical amplifiers on the same chip to simplify fiber pigtailing. These devices have potential application for high density wavelength divisive multiplexing WDM systems, as well as for optical networks using wavelength routing.

Monolithic InP Grating-Based Wavelength Division Multiplexing Components

The past year saw breakthroughs in a new component technology that promises to provide wavelength-specific devices for future wavelength division multiplexed (WDM) networks that have a high degree of wavelength control and a low manufacturing cost. To date, one of the greatest challenges facing implementation of proposed WDM networks is the practical realization of low-cost wavelength-specific opto-electronic components (lasers, detectors, etc.) that possess and maintain the wavelength accuracy needed for effective network operation. While many components are available or under development, there is no technology that provides these devices with the required wavelength accuracy, tolerance, and stability at low enough cost to make many of the networks commercially viable.

Wavelength Selective Optical Logic

Increasing interest in digital opto-electronic circuits for optical communication systems and potential optical computing applications has created a demand for new device structures capable of optical input/output. Novel designs for photonic switching1 enabling multiple functions in a single device are desirable for reducing the complexity of optical receivers and transmitters, as well as for avoiding additional time delays due to interconnects. Wavelength-division-multiplexing (WDM) technology, by which multiple optical channels can be transmitted without any interference, is one of the major advantages of the optical signal transmission.

Optical Wavelength Shifter for Wavelength-Division-Multiplexed Networks

High capacity optical communications systems can be constructed by wavelength division multiplexing (WDM), in which signals carried at different wavelengths are combined through a single optical fiber transmission network. A WDM network with many users would be based on frequency routing, in which the address of a narrow-bandpass receiver is determined by its assigned wavelength. The capacity of such a wavelength-routed WDM network is limited by the tuning range of the transmitter laser. Frequency reuse within the available optical bandwidth can substantially increase the capacity of such networks.

Broadband Dispersion Compensation for High-Bit-Rate Fiber Transmission

The application of erbium-doped fiber optical amplifiers in lightwave communication systems has made it possible to transmit lightwaves over thousands of kilometers of single-mode fiber without electronic regeneration. With fiber loss no longer a factor, the primary limitation to high-bit-rate transmission over presently installed fiber networks is group-velocity dispersion, a property of single-mode fiber that causes different spectral components of an optical pulse to propagate with different group velocities. As a result, there has been considerable interest in finding an optical device that can compensate group velocity dispersion by inducing a dispersion equal in magnitude but opposite in sign to the dispersion of single-mode fiber.

Anti-Reflection Structured Surfaces

Anti-reflection structured (ARS) surfaces can be regarded as a subset of surface-relief gratings. Unlike typical gratings, ARS surfaces have subwavelength periods that enable only the reflected and transmitted zeroth order to propagate (higher diffraction orders are evanescent). ARS surfaces have been shown both theoretically and experimentally to exhibit extremely low reflectivities over broad spectral bandwidths and wide field-of-views (see, for example, Refs. 1 and 2). Unlike thin-film coatings, ARS surfaces are created by etching a surface-relief pattern directly into a substrate. Consequently, ARS surfaces do not experience cohesion problems or problems with thermal expansion mismatches, as do thin-film coatings. Analysis of ARS surfaces are performed using both vector diffraction theory and effective medium theories (EMTs).

Grazing Incidence Anti-Reflection Coatings

High power dye lasers are a formidable source of tunable and coherent radiation in many scientific, industrial, and medical applications. In pulsed dye oscillators, multiple prism grating combination is generally used to get narrow linewidths and to reduce the possibility of damage to the grating surface. Since the magnification provided by the prism increases rapidly as the incidence angle approached 90°, these are used at grazing incidence angles. A serious disadvantage of these beam expanders is the reflection loss, which can be as high as 27% for a glass-air interface at 80° incidence for p-polarized radiation. Moreover, to use these beam expanders in widely tunable lasers, it is important that the reflection loss be minimized over a broad range of wavelengths. This makes the use of broadband anti-reflection coatings essential for these prism beam expanders.

Laser Diode Delivers Atomically-Resolved Images

Optical microscopy has a diffraction-limited resolution determined by the wavelength of the laser and the numerical apertures of the objective and condenser. Even for the best optical microscope, this resolution is limited to several hundreds of nanometers.

Photonic-Crystal Planar Antennas

Periodic dielectric structures have recently generated great interest because they can exhibit a forbidden range of frequencies, or photonic band gap, in the electromagnetic dispersion relation. In view of this property, these structures are called photonic crystals. They are well suited to electromagnetic-propagation and radiation-control applications, analogous to the current-transport and charge-control applications of semiconductors. An application that has recently been demonstrated at Lincoln Laboratory is the photonic-crystal planar antenna.

Observation of Intense Far Infrared Picosecond Pulses of Coherent Transition Radiation

Transition radiation is produced by the passage of a charged particle through the interface between media with different dielectric constants. It is caused by a collective response of the matter surrounding the particle trajectory to readjust to the electromagnetic field of the charged particle. Part (a) of the figure is a schematic corresponding to the geometry used in our experiment. A negatively charged particle is incident at 45° with respect to a metallic mirror. In the low-frequency region where the metal acts like a perfect conductor, its electric shielding effects can be represented in the nonrelativistic limit by an image charge placed at an equal distance behind the metal surface. As the charge approaches the boundary, so does this image charge. The emitted radiation pattern with a frequency spectrum extending from the microwave to the x-ray region [see (b)] is emitted when the charged particle enters the metal and the charge and its image cancel.

A New Family of Self-Modelocked Chromium Doped Solid State Lasers

Since the first observation of self-modelocking in a Ti:sapphire laser in 1990, remarkable progress has been made in developing these lasers to the point where they are rapidly replacing modelocked dye lasers as the preferred source for ultrafast laser spectroscopy. The large spectral bandwidth and relative simplicity of the modelocking mechanism provides stable, high power, sub-100 fsec duration pulses with wide tunability in the near infrared.

CW Room-Temperature Blue Upconversion Laser

There has recently been tremendous interest in the development of efficient, compact, longlived, solid state blue laser sources. Whereas most diode-based blue sources use frequency doubling or mixing techniques, the process of upconversion has attracted a great deal of interest due to the simplicity of this approach used to convert infrared laser diodes to visible light. Recently, the advantages of the fiber laser geometry for upconversion lasers have become apparent and have led to the demonstration of 11 different cw, room temperature visible upconversion laser/pumping combinations.

Concentric-Circle-Grating, Surface-Emitting Semiconductor Lasers

It is possible to make use of a concentric-circle grating (CCG) to define a novel Bragg resonator for a semiconductor laser. This fundamentally unique, two-dimensional optical resonator confines the propagation of circular waves through a distributed feedback (DFB) process. Such a laser, when operated in the second Bragg order, offers the prospect of surface emission from a broad area, circularly symmetric aperture. As a result, this laser can emit a low divergence, spectrally narrow beam that is circular in cross section.

Optical Signal Processing Functions in Laser Diodes: Frequency Division and Multiplication

Because laser diodes are attractive optical sources in many communications and information processing systems, there is now great interest in optical signal processing functions with potential to replace those currently performed electronically, especially if they can be performed by laser diodes. One such function is optical clock extraction, which involves obtaining an optical clock from an optical data signal. Recently, the self-pulsation frequency of a diode laser has been shown to synchronize to the bit-rate of an injected optical signal to extract an optical clock.

Frequency-Stabilized Diode-Laser-Pumped Solid State Lasers: Optical Clocks of the Future

Monolithic diode-laser pumped solid state lasers1 are compact, reliable, and inherently-stable optical oscillators, providing nearly diffraction-limited beams with linewidths narrower than 10 kHz. While this linewidth is much narrower than that of most other lasers, the quantum-limited value is less than 1 Hz for an output power of 1 mW. However, the dependence of the cavity length and refractive index on the environment (temperature, acoustic vibrations, etc.), causes the lasing frequency to drift at a rate of several MHz per minute. External frequency stabilization can therefore improve the oscillator performance.

Reflections in Christmas Tree Balls

Many optical phenomena can be seen and many beautiful effects can be produced with Christmas tree ornaments. The kind to use is the plain silver balls, the bigger the better. The main optical property involved here is that a sphere reflects light from all directions into any given direction or to any point. Thus, an eye looking at a sphere sees in all directions, as shown in the figure. (We can also think of the sphere as forming an image of all that surrounds it. The image is contained within the sphere, and is seen when the eye lens re-images it onto the retina.)

Standards Can Play Role in Scientific Progress

Standards deals not only with codifying existing science and technology, but with the development of new test methods and procedures to deal with the demands of new technology. Adolf Giesen of the Institute für Strahlwerkzeuge at the University of Stuttgart is the convener for ISO/ TC172/SC9/WG6 on standards for laser optical components such as lenses and mirrors. I had the pleasure of meeting with Giesen again in Paris and learning more about his work.

Nonlinear Polarization Beats Spectroscopy

It is difficult to find third order nonlinear optical materials with ultrafast response and low loss that satisfy material figures of merit defined for all-optical switching. The key problem is to be able to achieve a 2πn2ILeff/λ > π nonlinear Phase shift where n2 I is the nonlinear index change and Leff is one absorption α-1 > Leffwith the absorption α containing linear α0 and nonlinear β2I + β3I2 + .. contributions where β2 and β3 are the two and three photon absorption coefficients, respectively. To date, these figures of merit have been satis- Fraction of the power output into the bar and cross channels of a one -half beat length nonlinear directional coupler when the input intensity in one channel (bar) is varied. fied by glasses and a few organic materials. As a result, it has proven very difficult to make efficient all-optical switching and demultiplexing devices in integrated optics formats. Below one half the bandgap of a semiconductor, the two photon absorption approaches zero and the linear absorption can also be very small. For example, for Al0.18GaO.82As, which has a bandgap at 750 nm, operation at 1550 ran satisfies these conditions very well and we find that total nonlinear phase shifts >5π are achievable with low loss and sub-picosecond response. This wavelength is also very attractive for communications.

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