Showing papers in "Optics Letters in 2000"
TL;DR: In this article, the authors demonstrate experimentally that air-silica microstructure optical fibers can exhibit anomalous dispersion at visible wavelengths, and exploit this feature to generate an optical continuum 550 THz in width, extending from the violet to the infrared.
Abstract: We demonstrate experimentally for what is to our knowledge the first time that air–silica microstructure optical fibers can exhibit anomalous dispersion at visible wavelengths. We exploit this feature to generate an optical continuum 550 THz in width, extending from the violet to the infrared, by propagating pulses of 100-fs duration and kilowatt peak powers through a microstructure fiber near the zero-dispersion wavelength.
2,372 citations
TL;DR: An optical architecture that encodes a primary image to stationary white noise by using two statistically independent random phase codes that has an enhanced security value compared with earlier methods is proposed.
Abstract: We propose an optical architecture that encodes a primary image to stationary white noise by using two statistically independent random phase codes. The encoding is done in the fractional Fourier domain. The optical distribution in any two planes of a quadratic phase system (QPS) are related by fractional Fourier transform of the appropriately scaled distribution in the two input planes. Thus a QPS offers a continuum of planes in which encoding can be done. The six parameters that characterize the QPS in addition to the random phase codes form the key to the encrypted image. The proposed method has an enhanced security value compared with earlier methods. Experimental results in support of the proposed idea are presented.
1,066 citations
TL;DR: A new four-wave rectification method for the generation of intense, ultrafast terahertz pulses from gases that is sensitive to the relative phases of the fundamental and second-harmonic pulses.
Abstract: We describe a new four-wave rectification method for the generation of intense, ultrafast terahertz (THz) pulses from gases. The fundamental and second-harmonic output of an amplified Ti:sapphire laser is focused to a peak intensity of ~5x10(14)W/cm (2) . Under these conditions, peak THz fields estimated at 2 kV/cm have been observed; the measured power spectrum peaks near 2 THz. Phase-dependent measurements show that this is a coherent process and is sensitive to the relative phases of the fundamental and second-harmonic pulses. Comparable THz signals have been observed from nitrogen and argon as well as from air.
915 citations
TL;DR: Using the phase change between sequential scans to construct flow-velocity imaging, this technique decouples spatial resolution and velocity sensitivity in flow images and increases imaging speed by more than 2 orders of magnitude without compromising spatial resolution or velocity sensitivity.
Abstract: The invention is a fast-scanning ODT system that uses phase information derived from a Hilbert transformation to increase the sensitivity of flow velocity measurements while maintaining high spatial resolution. The significant increases in scanning speed and velocity sensitivity realized by the invention make it possible to image in vivo blood flow in human skin. The method of the invention overcomes the inherent limitations of the prior art ODT by using a phase change between sequential line scans for velocity image reconstruction. The ODT signal phase or phase shifts at each pixel can be determined from the complex function, {tilde over (Γ)}ODT(t), which is determined through analytic continuation of the measured interference fringes function, ΓODT(t), by use of a Hilbert transformation, by electronic phase demodulation, by optical means, or a fast Fourier transformation. The phase change in each pixel between axial-line scans is then used to calculate the Doppler frequency shift. Sequential measurements of a single line scan, measurements of sequential line scans or measurements of line scans in sequential frames may be used. Because the time between line scans is much longer than the pixel time window, very small Doppler shifts can be detected with this technique. In addition, spatial resolution and velocity sensitivity are decoupled. Furthermore, because two axial-line scans are compared at the same location, speckle modulations in the fringe signal cancel each other and, therefore, will not affect the phase-difference calculation.
875 citations
TL;DR: Based on spectral measurements of the polarization mode beating, it is estimated that the strongly anisotropic photonic crystal fiber has a beat length of approximately 0.4 mm at a wavelength of 1540 nm, in good agreement with the results of modeling.
Abstract: We report a strongly anisotropic photonic crystal fiber. Twofold rotational symmetry was introduced into a single-mode fiber structure by creation of a regular array of airholes of two sizes disposed about a pure-silica core. Based on spectral measurements of the polarization mode beating, we estimate that the fiber has a beat length of approximately 0.4 mm at a wavelength of 1540 nm, in good agreement with the results of modeling.
860 citations
TL;DR: In this paper, a supercontinuum light with a spectrum more than two octaves broad (370-1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser.
Abstract: Supercontinuum light with a spectrum more than two octaves broad (370–1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser. The fiber had been tapered to a diameter of ∼2 µm over a 90-mm length. The pulse energy was 3.9 nJ (average power, 300 mW). This source of high-intensity single-mode white light should find widespread applications in frequency metrology and spectroscopy, especially since no unconventional fibers are needed.
711 citations
TL;DR: Spectroscopic optical coherence tomography (OCT) as discussed by the authors is an extension of conventional OCT for performing cross-sectional tomographic and spectroscopic imaging, which allows the spectrum of backscattered light to be measured over the entire available optical bandwidth simultaneously in a single measurement.
Abstract: Spectroscopic optical coherence tomography (OCT), an extension of conventional OCT, is demonstrated for performing cross-sectional tomographic and spectroscopic imaging. Information on the spectral content of backscattered light is obtained by detection and processing of the interferometric OCT signal. This method allows the spectrum of backscattered light to be measured over the entire available optical bandwidth simultaneously in a single measurement. Specific spectral features can be extracted by use of digital signal processing without changing the measurement apparatus. An ultrabroadband femtosecond Ti:Al2O3 laser was used to achieve spectroscopic imaging over the wavelength range from 650 to 1000 nm in a simple model as well as in vivo in the Xenopus laevis (African frog) tadpole. Multidimensional spectroscopic data are displayed by use of a novel hue-saturation false-color mapping.
644 citations
TL;DR: This technique will permit scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.
Abstract: The authors report a new approach to obtain single-transverse-mode operation of a multimode fiber amplifier, in which the gain fiber is coiled to induce significant bend loss for all but the lowest-order mode. They have demonstrated this method by constructing a coiled amplifier using Yb-doped, double-clad fiber with a core diameter of 25 {micro}m and NA of {minus}0.1 (V {approx} 7.4). When operated as an ASE source, the output beam had an M{sup 2} value of 1.09 {+-} 0.09; when seeded at 1,064 nm, the slope efficiency was similar to that of an uncoiled amplifier. This technique does not require exotic fiber designs or increase system complexity and is inexpensive to implement. It will allow scaling of pulsed fiber lasers and amplifiers to significantly higher pulse energies and peak powers and cw fiber sources to higher average powers while maintaining excellent beam quality.
630 citations
TL;DR: In this paper, the authors observed photon antibunching in the fluorescent light emitted from a single nitrogen-vacancy center in diamond at room temperature, and the possibility of generating triggerable single photons with such a solid-state system is discussed.
Abstract: We observed photon antibunching in the fluorescent light emitted from a single nitrogen-vacancy center in diamond at room temperature. The possibility of generating triggerable single photons with such a solid-state system is discussed.
616 citations
TL;DR: It is demonstrated experimentally that an arbitrary phase and amplitude profile can be applied to an ultrashort pulse by use of an acousto-optic programmable dispersive filter (AOPDF) that has a large group-delay range and a 30% diffraction efficiency over 150 THz.
Abstract: We demonstrate experimentally that an arbitrary phase and amplitude profile can be applied to an ultrashort pulse by use of an acousto-optic programmable dispersive filter (AOPDF). Our filter has a large group-delay range that extends over 3 ps and a 30% diffraction efficiency over 150 THz. Experiments were conducted on a kilohertz chirped-pulse amplification laser chain capable of generating 30-fs pulses without additional pulse shaping. Compensating for gain narrowing and residual phase errors with an AOPDF in place of the stretcher results in 17-fs transform-limited pulses. Arbitrary shaping of these 17-fs pulses is also demonstrated in both the temporal and the spectral domains.
529 citations
TL;DR: A class of invariant optical fields that may have a highly localized distribution along one of the transverse directions and a sharply peaked quasi-periodic structure along the other and are described by the radial and angular Mathieu functions is presented.
Abstract: Based on the separability of the Helmholtz equation into elliptical cylindrical coordinates, we present another class of invariant optical fields that may have a highly localized distribution along one of the transverse directions and a sharply peaked quasi-periodic structure along the other. These fields are described by the radial and angular Mathieu functions. We identify the corresponding function in the McCutchen sphere that produces this kind of beam and propose an experimental setup for the realization of an invariant optical field.
TL;DR: An information security method that uses a digital holographic technique that provides secure storage and data transmission and can be electrically decrypted by use of the digital hologram of the key.
Abstract: An information security method that uses a digital holographic technique is presented. An encrypted image is stored as a digital hologram. The decryption key is also stored as a digital hologram. The encrypted image can be electrically decrypted by use of the digital hologram of the key. This security technique provides secure storage and data transmission. Experimental results are presented to demonstrate the proposed method.
TL;DR: The absorption spectra of the the gold colloid monolayers, which were immersed in liquid samples or coated with thin films, were measured and increases of both the resonance wavelength and the absorbance as the refractive indices of the sample liquids or the thickness of the coated films increased.
Abstract: A new optical sensor that uses local plasmon resonance is proposed. A peak that is due to the local plasmon resonance appears in the absorption spectrum of a gold colloid suspension in the visible region, and its height and wavelength depend on the refractive index of the suspension. These properties are used for optical sensors. We used gold colloid monolayers in which colloidal gold particles a few tens of nanometers in diameter were immobilized upon a glass slide by a functional organic coupling agent. We measured the absorption spectra of the the gold colloid monolayers, which were immersed in liquid samples or coated with thin films. We observed increases of both the resonance wavelength and the absorbance as the refractive indices of the sample liquids or the thickness of the coated films increased. The proportional constants of the resonance wavelength to the film thickness were 3.6 and 5.7 for a 13.9- and a 20.2-nm gold colloid monolayer, respectively.
TL;DR: In vivo PS OCT images of human skin are presented, showing subsurface structures that are not discernible in conventional OCT images.
Abstract: A high-speed single-mode fiber-based polarization-sensitive optical coherence tomography (PS OCT) system was developed. With a polarization modulator, Stokes parameters of reflected flight for four input polarization states are measured as a function of depth. A phase modulator in the reference arm of a Michelson interferometer permits independent control of the axial scan rate and carrier frequency. In vivo PS OCT images of human skin are presented, showing subsurface structures that are not discernible in conventional OCT images. A phase retardation image in tissue is calculated based on the reflected Stokes parameters of the four input polarization states.
TL;DR: A computer-generated hologram is used to form an optical beam with a localized intensity null at its focus that will have applications in the optical trapping of macroscopic objects or atoms; hence the term optical bottle beam.
Abstract: A computer-generated hologram is used to form an optical beam with a localized intensity null at its focus. The beam is a superposition of two Laguerre-Gaussian modes that are phased so that they interfere destructively to give a beam focus that is surrounded in all directions by regions of higher intensity. Beams of this kind will have applications in the optical trapping of macroscopic objects or atoms; hence the term optical bottle beam.
TL;DR: A technique for performing three-dimensional pattern recognition by use of in-line digital holography, where the complex amplitude distribution generated by a 3D object at an arbitrary plane located in the Fresnel diffraction region is recorded by phase-shifting interferometry.
Abstract: We present a technique for performing three-dimensional (3D) pattern recognition by use of in-line digital holography. The complex amplitude distribution generated by a 3D object at an arbitrary plane located in the Fresnel diffraction region is recorded by phase-shifting interferometry. The digital hologram contains information about the 3D object's shape, location, and orientation. This information allows us to perform 3D pattern-recognition techniques with high discrimination and to measure 3D orientation changes. Experimental results are presented.
TL;DR: A new method of measurement that essentially combines Fourier-domain optical coherence tomography with spectroscopy is introduced by use of a windowed Fourier transform to obtain spectroscopic information such as the absorption properties of materials.
Abstract: A new method of measurement that essentially combines Fourier-domain optical coherence tomography with spectroscopy is introduced. By use of a windowed Fourier transform it is possible to obtain, in addition to the object structure, spectroscopic information such as the absorption properties of materials. The feasibility of this new method for performing depth-resolved spectroscopy is demonstrated with a glass filter plate. The results are compared with theoretically calculated spectra by use of the well-known spectral characteristics of the light source and the filter plate.
TL;DR: The time-averaged total force on a subwavelength-sized particle in a time-harmonic-varying field and the addition of the radiative reaction term to the polarizability to deal correctly with the scattering force are established.
Abstract: We establish the time-averaged total force on a subwavelength-sized particle in a time-harmonic-varying field. Our analysis is not restricted to the spatial dependence of the incident field. We discuss the addition of the radiative reaction term to the polarizability to deal correctly with the scattering force. As an illustration, we assess the degree of accuracy of several previously established polarizability models.
TL;DR: A miniature optical coherence tomography (OCT) imaging needle that can be inserted into solid tissues and organs to permit interstitial imaging of their internal microstructures with micrometer scale resolution and minimal trauma is described.
Abstract: We describe a miniature optical coherence tomography (OCT) imaging needle that can be inserted into solid tissues and organs to permit interstitial imaging of their internal microstructures with micrometer scale resolution and minimal trauma. A novel rotational coupler with a glass capillary tube is also presented that couples light from a rotating single-mode fiber to a stationary one. A prototype needle with a 27-gauge ∼410‐µm outer diameter has been developed and is demonstrated for in vivo imaging. The OCT needle can be integrated with standard excisional biopsy devices and used for OCT-guided biopsy.
TL;DR: The threshold power for self-focusing collapse both in a bulk medium and in a hollow-core waveguide for various spatial profiles is determined.
Abstract: We determine the threshold power for self-focusing collapse both in a bulk medium and in a hollow-core waveguide for various spatial profiles. We find that the threshold power for collapse in the waveguide is always equal to the lower-bound prediction for a bulk medium.
TL;DR: This procedure for achieving photonic crystal fibers with nearly zero ultraflattened group-velocity dispersion permits remarkably improved suppression of third-order dispersion, particularly in the low-dispersion domain.
Abstract: We present a procedure for achieving photonic crystal fibers with nearly zero ultraflattened group-velocity dispersion. Systematic knowledge of the special guiding properties of these fibers permits the achievement of qualitatively novel dispersion curves. Unlike the behavior of conventional fibers, this new type of dispersion behavior permits remarkably improved suppression of third-order dispersion, particularly in the low-dispersion domain.
TL;DR: To the authors' knowledge this is the first clinical application of ODT to provide a fast semiquantitative evaluation of the efficacy of PWS laser therapy in situ and in real time.
Abstract: We used a novel phase-resolved optical Doppler tomographic (ODT) technique with very high flow-velocity sensitivity (10microm/s) and high spatial resolution (10microm) to image blood flow in port-wine stain (PWS) birthmarks in human skin. In addition to the regular ODT velocity and structural images, we use the variance of blood flow velocity to map the PWS vessels. Our device combines ODT and therapeutic systems such that PWS blood flow can be monitored in situ before and after laser treatment. To the authors' knowledge this is the first clinical application of ODT to provide a fast semiquantitative evaluation of the efficacy of PWS laser therapy in situ and in real time.
TL;DR: Two figures of merit based on singular value decomposition are presented, which can be used to assess the noise immunity of a complete Stokes polarimeter, used to optimize a polarimeter featuring a rotatable retarder and a fixed polarizer.
Abstract: The authors present two figures of merit based on singular value decomposition which can be used to assess the noise immunity of a complete Stokes polarimeter. These are used to optimize a polarimeter consisting of a rotatable retarder and fixed polarizer. A retardance of 132{degree} (approximately three eights wave) and retarder orientation angles of {+-}51.7{degree} and {+-}15.1{degree} are found to be optimal when four measurements are used. Use of this retardance affords a factor of 1.5 improvement in signal-to-noise ratio over systems employing a quarter wave plate. A geometric means of visualizing the optimization process is discussed, and the advantages of the use of additional measurements are investigated. No advantage of using retarder orientation angles spaced uniformly through 360{degree} is found over repeated measurements made at the four angles given previously.
TL;DR: An optical fiber that can appear single mode with propagation properties that can be achieved only in multimode waveguides is analysis of waveguide properties of microstructure optical fibers.
Abstract: We analyze the waveguide properties of microstructure optical fibers consisting of a silica core surrounded by a single ring of large air holes. Although the fibers can support numerous transverse spatial modes, coupling between these modes even in the presence of large perturbations is prevented for small core dimensions, owing to a large wave-vector mismatch between the lowest-order modes. The result is an optical fiber that can appear single mode with propagation properties that can be achieved only in multimode waveguides.
TL;DR: This filter, which is simple and inexpensive, exhibits transmission spectra and temperature stability similar to those of photoinduced LPFG's and offers the unique advantages of being tunable, erasable, and reconfigurable.
Abstract: We report the experimental characterization of mechanically induced long-period fiber gratings (LPFG's) made by pressing a plate with periodic grooves against a short length of fiber. This filter, which is simple and inexpensive, exhibits transmission spectra and temperature stability similar to those of photoinduced LPFG's. It also offers the unique advantages of being tunable, erasable, and reconfigurable. Its polarization dependence also makes it useful as a polarizer, although it can also be eliminated with simple mechanical designs.
TL;DR: A number of compounds with a Kerr nonlinearity hundreds of times larger than silica are found, making them excellent candidates for ultrafast all-optical devices.
Abstract: High-speed optical communication requires ultrafast all-optical processing and switching capabilities. The Kerr nonlinearity, an ultrafast optical nonlinearity, is often used as the basic switching mechanism. A practical, small device that can be switched with ∼1‐pJ energies requires a large Kerr effect with minimal losses (both linear and nonlinear). We have investigated theoretically and experimentally a number of Se-based chalcogenide glasses. We have found a number of compounds with a Kerr nonlinearity hundreds of times larger than silica, making them excellent candidates for ultrafast all-optical devices.
TL;DR: In this paper, a 1.5mm-wavelength fiber laser was constructed by placing glass microsphere resonators along a fiber taper, which served the dual purpose of transporting optical pump power into the spheres and extracting the resulting laser emission.
Abstract: We demonstrate a 1.5-mm-wavelength fiber laser formed by placement of glass microsphere resonators along a fiber taper. The fiber taper serves the dual purpose of transporting optical pump power into the spheres and extracting the resulting laser emission. A highly doped erbium:ytterbium phosphate glass was used to form microsphere resonant cavities with large gain at 1.5 mm. Laser threshold pump powers of 60 mW and fiber-coupled output powers as high as 3 mW with single-mode operation were obtained. A bisphere laser system consisting of two microspheres attached to a single fiber taper is also demonstrated. © 2000 Optical Society of America
TL;DR: Speckle statistics that are appropriate to the OCT measurements are developed and a simple and practical speckle-reduction technique is demonstrated.
Abstract: Studies have shown that optical coherence tomography (OCT) is useful in imaging microscopic structures through highly scattering media. Because spatially coherent light is used in OCT, speckle in the reconstructed image is unavoidable, resulting in degradation of the quality of the OCT images and impaired ability to differentiate subsurface structures. Therefore speckle reduction is an important issue in OCT imaging. We develop speckle statistics that are appropriate to the OCT measurements and demonstrate a simple and practical speckle-reduction technique.
TL;DR: Both theory and experiment of enhanced four-wave mixing in micro-ring resonators are presented that can be used for many applications and shows a 26-dB improvement in the conversion efficiency compared with that of an equivalent straight waveguide in agreement with theory.
Abstract: Tightly confined, low-loss waveguides in highly nonlinear materials permit nonlinear optical interactions to occur over much shorter distances than do fibers The nonlinear interactions are further enhanced in resonators Both theory and experiment of enhanced four-wave mixing in micro-ring resonators are presented that can be used for many applications A conversion efficiency of 14% achievable with only 10-mW peak pump power is predicted under realizable conditions The experiment, the first one to the authors’ knowledge in nonlinear optics performed in microrings, shows, even in a lossy GaAs/AlGaAs ring, a 26-dB improvement in the conversion efficiency compared with that of an equivalent straight waveguide, in agreement with theory
TL;DR: What are believed to be the first experimental observations of the existence of long-range plasmon-polariton waves, guided by a thin metal film of finite width, are presented.
Abstract: What are believed to be the first experimental observations of the existence of long-range plasmon–polariton waves, guided by a thin metal film of finite width, are presented. A waveguide composed of an 8-µm-wide, 20-nm-thick, 3.5-mm-long Au metal film embedded in SiO2 was successfully excited at a free-space wavelength of 1.55 µm in an end-fire experiment. The theoretical nature of the phenomenon is described, and experimental observations of field confinement provided by this metal waveguide are presented in detail.