Showing papers by "Amnon Yariv published in 1996"

Self-focusing and self-trapping of optical beams upon photopolymerization.

Abstract: We demonstrate theoretically and experimentally that optical beams are self-focused and self-trapped upon initiating photopolymerization. This unique nonlinear optical phenomenon is dependent on the optical exposure and produces permanent index-of-refraction changes larger than 0.04. The resulting nonlinear wave equation is shown to be nonlocal in time and displays self-trapped solutions only for sufficiently low average optical intensities.

Wavelength selective optical couplers

Abstract: A wavelength selective optical fiber coupler having various applications in the field of optical communications is disclosed. The coupler is composed of dissimilar waveguides in close proximity. A light induced, permanent index of refraction grating is recorded in the coupler waist. The grating filters and transfers energy within a particular range of wavelengths from a first waveguide to a second waveguide. Transversely asymmetric gratings provide an efficient means of energy transfer. The coupler can be used to combine or multiplex a plurality of lasers operating at slightly different wavelengths into a single fiber. Other embodiments such as a dispersion compensator and gain flattening filter are disclosed.

Holographic storage dynamics in lithium niobate: theory and experiment

Abstract: We present a theoretical model that describes holographic ionic fixing and storage dynamics in photorefractive crystals. Holographic gratings that are based on charge redistribution inevitably decay because of ionic and electronic conduction. Relevant decay rates and transient hologram field expressions are derived. Ionic gratings are partially screened by trapped electrons on readout. The lifetimes of fixed ionic holograms are limited by the finite ionic conductivity at low (i.e., room) temperatures. Only under certain and restricted conditions can these decay times be acceptably long. A significant increase in fixed ionic hologram lifetime is realized in lithium niobate with a low hydrogen-impurity content. The residual ionic conductivity (decay-time constant) in these samples exhibits ~1.4-eV activation energy and is not due to protonic conduction. Fixed hologram lifetimes of ~2 years at room temperature in dehydrated lithium niobate crystals are projected.

Nonlinear optical properties of photoresists for projection lithography

Abstract: Optical beams are self‐focused and self‐trapped upon initiating crosslinking in photoresists This nonlinear optical phenomenon is apparent only for low average optical intensities and produces index of refraction changes as large as 004 We propose using the self‐focusing and self‐trapping phenomenon in projection photolithography to enhance the resolution and depth of focus

Self-trapping and self-focusing of optical beams in photopolymers

Abstract: A polymer material is exposed to radiation of a type that changes some aspect of the polymer's radiation passing properties. The radiation that caused the property change is then contained by the material. The property change can be self-focusing or self-trapping light can be used. In that case, the same light that causes the photopolymerization is contained by the change in index of refraction that is caused by the polymerization.

Lasers incorporating 2D photonic bandgap mirrors

Abstract: Semiconductor lasers incorporating a 2D photonic lattice as a one end mirror in a Fabry-Perot cavity are demonstrated. The photonic lattice is a 2D hexagonal close-packed array with a lattice constant of 220 nm. Pulsed threshold currents of 110 mA were observed from a 180 μm laser.

Wavelength selective optical devices

Abstract: Wavelength selective devices and subsystems having various applications in the field of optical communications are disclosed. These devices and subsystems are composed of bidirectional grating assisted mode couplers (9). The high add/drop efficiency and low loss of this coupler enable low loss wavelength selective elements such as optical switches (62), amplifiers (63), routers (5), and sources to be fabricated. The grating assisted mode coupler (23) can be wavelength tuned by modifying the optical properties of the coupler interaction region. A programmable, wavelength selective router (5) composed of multiple grating assisted mode couplers is also disclosed.

General theorem of spatial coherence: application to three-dimensional imaging

Abstract: Three-dimensional imaging of incoherent light sources by the Michelson stellar interferometer is considered. When the interferometer’s pinholes are arranged properly, its output result is equivalent to a two-dimensional Fourier hologram that stores information about the source object’s three-dimensional intensity distribution.

Electrically controlled wavelength multiplexing waveguide filter

Abstract: An optical switch for programmable spatial and spectral filtering of wavelength-multiplexed optical signals of different wavelengths. Multiple input and output waveguides are connected to a waveguide filter to allow adiabatic propagative evolution of an optical wave therebetween. A plurality of electrically-controlled diffraction gratings are formed in the waveguide filter to provide mode coupling between different optical modes. Each grating can be controlled electrically to achieve efficient power conversion between two different modes in the waveguide filter. Different modes can be directed to different output waveguides, thereby achieving programmable switching and signal modulation.

Noise characterization of a pulse train generated by actively mode-locked lasers

Abstract: We analyze the entire power spectrum of pulse trains generated by a continuously operating actively mode-locked laser in the presence of noise. We consider the effect of amplitude, pulse-shape, and timing-jitter fluctuations that are characterized by stationary processes. Effects of correlations between different parameters of these fluctuations are studied also. The nonstationary timing-jitter fluctuations of passively mode-locked lasers and their influence on the power spectrum is discussed as well.

Three-dimensional imaging of random radiation sources

Abstract: A method to image random three-dimensional source distributions is proposed. We show that, by using a Michelson stellar interferometer in a prescribed fashion, one is able to measure a special form of a three-dimensional degree of coherence. The inverse Fourier transform of this coherence function yields the three-dimensional intensity distribution of the source as seen from the paraxial far zone.

Reduction of relative intensity noise of the output field of semiconductor lasers due to propagation in dispersive optical fiber

Abstract: The effect of dispersive, linear propagation (e.g., in single‐mode optical fiber) on the intensity noise from semiconductor lasers is investigated. Relations between the frequency and amplitude noise variations of semiconductor lasers are obtained from the laser rate equations and used to calculate the change in the relative intensity noise (RIN) spectrum that occurs during dispersive propagation. Propagation in fiber with positive dispersion (D≳0) over moderate distances (several km for standard single‐mode fiber at 1.55 μm) is found to reduce the RIN over a wide range of frequencies. Measurements with a 1.56 μm distributed feedback laser confirm the main theoretical results and demonstrate reductions in RIN of up to 11 dB with 4 km of standard fiber.

Reconstruction of longitudinal distributed incoherent sources.

Abstract: We describe measurement of the degree of coherence induced by a random light source distributed along the longitudinal z axis. If this degree of coherence is measured only between all the in-plane pairs of points placed along the radial lines it is proportional to the Fourier transform of the source’s three-dimensional intensity distribution as seen from the paraxial far zone. A reconstruction of the source shape from the measured degree of coherence is also demonstrated.

Nonlinear self‐phase matching of optical second harmonic generation in lithium niobate

Abstract: We show that the nonlinear index perturbation due to light‐induced photovoltaic space‐charge field in LiNbO3 can give rise to self‐phase matching of second harmonic generation. Increase of the conversion efficiency is accompanied by formation of stationary and nonstationary patterns in the spatial structure of the generated second harmonic. The space‐charge field can be induced either by the initially non‐phase‐matched second harmonic or by an external seed.

Supermodes of high-repetition-rate passively mode-locked semiconductor lasers

Abstract: We present a steady-state analysis of high-repetition-rate passively mode-locked semiconductor lasers. The analysis includes effects of amplitude-to-phase coupling in both gain and absorber sections. A many-mode eigenvalue approach is presented to obtain supermode solutions. Using a nearest-neighbor mode coupling approximation, chirp-free pulse generation and electrically chirp-controlled operation are explained for the first time. The presence of a nonzero alpha parameter is found to change the symmetry of the supermode and significantly reduce the mode-locking range over which the lowest order supermode remains the minimum gain solution. An increase in absorber strength tends to lead to downchirped pulses. The effects of individual laser parameters are considered, and agreement with recent experimental results is discussed.

Wavelength selective grating assisted optical couplers

Abstract: A wavelength selective optical fiber coupler having various applications in the field of optical communications is disclosed. The coupler is composed of dissimilar waveguides (12, 22) in close proximity. A light induced, permanent index of refraction grating (42) is recorded in the coupler waist (52). The grating filters and transfers energy within a particular range of wavelengths from a first waveguide (12) to a second waveguide (22). Transversely asymmetric gratings provide an efficient means of energy transfer. The coupler can be used to combine or multiplex a plurality of lasers operating at slightly different wavelengths into a single fiber. Other embodiments such as a dispersion compensator and gain flattening filter are disclosed.

Average coherence approximation for partially coherent optical systems

Abstract: We introduce an approximation to the Hopkins integral for partially coherent optical systems. The average coherence approximation yields results close to the Hopkins integral for a wide range of coherent transfer functions and illumination functions and is far less computationally demanding than the full Hopkins integral.

Thermal fixing of volume holograms in potassium niobate

Abstract: Volume holograms are thermally fixed in a pure and an iron doped potassium niobate crystal through the screening of a photorefractive space‐charge field by a nonphotoactive species at elevated temperature (110 °C). The nonphotoactive species is identified as the hydrogen ion. Fixed holograms are reconstructed at different temperatures. A diffraction efficiency of 16% in a 6 mm thick KNbO3:Fe sample is measured. The time constant and activation energy associated with the thermal induced decay of the hologram are determined to be 2.9×10−7 s and 0.81 eV for the KNbO3:Fe sample, and 1.35×10−7 s and 0.79 eV for the pure KNbO3 sample.

Fixing of volume holograms in ferroelectric K(1-y)Li(y)Ta(1-x)Nb(x)O(3).

Abstract: We report on the fixing of photorefractive volume holograms in potassium lithium tantalate niobate with ionic gratings and also with ferroelectric domain-reversed gratings. A diffraction efficiency of 55% is obtained with domain reversal in a 2-mm-thick ferroelectric phase K1–yLiyTa1–xNbxO3 crystal doped with Co, V, and Ti. We measured the decay rate of the domain gratings and also of the initial electron gratings and ion gratings. The domain grating decay agrees with Vogel–Fulcher fits. The activation energies for ionic and electronic conductivity are 0.76 and 0.12 eV, respectively.

Near infrared absorption and dark conductivity of k1-yliyta1-xnbxo3 crystal

Abstract: Absorption bands in the wave‐number range of 3525–3470 cm−1 have been observed in K1−yLiyTa1−xNbxO3 (KLTN) doped crystals except in crystals doped with Cu and V. These absorption bands are attributed to the O–H vibration band. The hydrogen concentration in KLTN doped crystal is controllable either by doping or by heat treatment. The activation energy of hydrogen ion migration is between 0.6 and 0.7 eV in KLTN doped crystals. The [H +] ion is identified as responsible for fixing (screening) the holographically produced electronic grating.

Pseudo-nondiffracting beams generated by radial harmonic functions: erratum

Abstract: A pseudo-nondiffracting beam (PNDB) is characterized by an almost constant intensity along a finite propagation distance and a beamlike shape in its transverse dimensions. In other words, the intensity distribution is distributed as a sword of light. There are several methods for achieving a PNDB. From the viewpoint of geometrical optics, a sword beam has been demonstrated by axicon.1 With diffraction theory, it has been shown that the Bessel beam2 and a beam diffracted from the holographic axilens3 behave as PNDB’s. Other kinds of PNDB have been obtained by numerical iterative methods.4,5 Multiplexing a few PNDB’s has been achieved also by numerical iterative methods,6 and one-dimensional PNDB’s have been found analytically7 and numerically.8 In this paper we propose a new PNDB that is general in the sense that in the limit, when its parameters go to infinity, it converges to the Bessel beam. It consists of four independent parameters, each affecting the beam’s transverse width, sidelobe height, and longitudinal interval length. The proposed PNDB’s are created by radial harmonic functions (RHF’s) coded on Fourier holograms. The RHF can be used as a phase-only pupil filter for increasing the depth of focus of imaging systems, as demonstrated in Section 5. The relations between the hologram and the resulting beam are analyzed in Section 3. Knowing the mutual dependence between the holograms and the PNDB’s enables us to shift and rotate the beams in space, as well as change their scale, by changing a few parameters of the holograms’ distribution. These effects are also demonstrated in Section 5.

Monolithic integration of quantum well infrared photodetector and modulator

Abstract: A modulation depth of 40% (0.7 dB/μm) was obtained with an infrared (10.6 μm) modulator consisting of a stack of 50 pairs of weakly coupled asymmetric quantum wells monolithically integrated with a quantum well infrared photodetector. The monolithic integration is shown to be a promising technique for the ‘‘ac’’ coupling of infrared focal‐plane arrays as well as for the direct study of the effects of electric fields and charge density variations on intersubband transitions.

Diffusion of hydrogen in k1-xliyta1-ynbxo3 doped crystals

Abstract: Based on infrared spectra and protonic conductivity measurements, we propose a model of hydrogen ion migration in K1−yLiyTa1−xNbxO3 doped crystals and use it to obtain a theoretical estimate of the diffusion constant pre‐exponential factor D0 that is in reasonable agreement with the experimental result. This model shows that the transition energy of the second, and higher, overtone(s) of the [OH] vibration can be higher than the activation energy of hydrogen ion migration.