Showing papers in "Optics Letters in 2002"
TL;DR: A theoretical analysis and experimentally demonstrate a Pancharatnam-Berry phase-based diffraction grating for laser radiation at a wavelength of 10.6microm.
Abstract: Space-variant Pancharatnam-Berry phase optical elements based on computer-generated subwavelength gratings are presented. By continuously controlling the local orientation and period of the grating we can achieve any desired phase element. We present a theoretical analysis and experimentally demonstrate a Pancharatnam-Berry phase-based diffraction grating for laser radiation at a wavelength of 10.6microm.
664 citations
TL;DR: Submicrometer-resolution OCT is demonstrated in vitro on human colorectal adenocarcinoma cells HT-29, a novel light source that has great potential for development of spectroscopic OCT because its spectrum covers the absorption bands of several biological chromophores.
Abstract: Optical coherence tomography (OCT) with unprecedented submicrometer axial resolution achieved by use of a photonic crystal fiber in combination with a compact sub-10-fs Ti:sapphire laser (Femtolasers Produktions) is demonstrated for what the authors believe is the first time The emission spectrum ranges from 550 to 950 nm (?c=725 nm , Pout=27 mW) , resulting in a free-space axial OCT resolution of ~075 ?m , corresponding to ~05 ?m in biological tissue Submicrometer-resolution OCT is demonstrated in vitro on human colorectal adenocarcinoma cells HT-29 This novel light source has great potential for development of spectroscopic OCT because its spectrum covers the absorption bands of several biological chromophores
596 citations
TL;DR: A new regime of guidance is identified in which the spectral properties of these structures are largely determined by the thickness of the high-index layers and the refractive-index contrast and are not particularly sensitive to the period of the cladding layers.
Abstract: We propose a simple analytical theory for low-index core photonic bandgap optical waveguides based on an antiresonant reflecting guidance mechanism. We identify a new regime of guidance in which the spectral properties of these structures are largely determined by the thickness of the high-index layers and the refractive-index contrast and are not particularly sensitive to the period of the cladding layers. The attenuation properties are controlled by the number of high/low-index cladding layers. Numerical simulations with the beam propagation method confirm the predictions of the analytical model. We discuss the implications of the results for photonic bandgap fibers.
576 citations
TL;DR: By reconstruction of both amplitude and phase, a new implementation of complex spectral optical coherence tomography (OCT) in biomedical imaging is demonstrated, able to use the negative and positive optical path differences to get images of objects of considerable thickness.
Abstract: We demonstrate a new implementation of complex spectral optical coherence tomography (OCT) in biomedical imaging. By reconstruction of both amplitude and phase we are able to use the negative and positive optical path differences to get images of objects of considerable thickness. An accompanying reduction of coherent noise improves the quality of the images. The property of the complex spectral OCT that permits the measurement range to be increased and permits the simultaneous use of phase and amplitude in spectral systems was not described previously. To show the potential of this technique we measured an anterior chamber of a porcine eye in vitro.
533 citations
TL;DR: The coherence of the supercontinuum is shown to depend strongly on the input pulse's duration and wavelength, and optimal conditions for the generation of coherent supercontinua are discussed.
Abstract: Numerical simulations have been used in studies of the temporal and spectral features of supercontinuum generation in photonic crystal and tapered optical fibers. In particular, an ensemble average over multiple simulations performed with random quantum noise on the input pulse allows the coherence of the supercontinuum to be quantified in terms of the dependence of the degree of first-order coherence on the wavelength. The coherence is shown to depend strongly on the input pulse’s duration and wavelength, and optimal conditions for the generation of coherent supercontinua are discussed.
488 citations
TL;DR: A novel method for forming radially and azimuthally polarized beams by using computer-generated subwavelength dielectric gratings is presented and it is shown that such beams have certain vortexlike properties and that they carry angular momentum.
Abstract: We present a novel method for forming radially and azimuthally polarized beams by using computer-generated subwavelength dielectric gratings. The elements were deposited upon GaAs substrates and produced beams with a polarization purity of 99.2% at a wavelength of 10.6 µm. We have verified the polarization properties with full space-variant polarization analysis and measurement, and we show that such beams have certain vortexlike properties and that they carry angular momentum.
473 citations
TL;DR: A series of chalcogenide glasses from the As-S-Se system that is designed to have strong nonlinearities are synthesized and measurements reveal that many of these glasses offer optical Kerr non linearities greater than 400 times that of fused silica.
Abstract: We have synthesized a series of chalcogenide glasses from the As–S–Se system that is designed to have strong nonlinearities. Measurements reveal that many of these glasses offer optical Kerr nonlinearities greater than 400 times that of fused silica at 1.25 and 1.55 µm and figures of merit for all-optical switching greater than 5 at 1.55 µm.
411 citations
TL;DR: This system, based on a Linnik-type interference microscope, illuminated by a white-light thermal lamp, has the highest resolution demonstrated to date for OCT imaging and realistic volume rendering of structures inside biological tissues is possible.
Abstract: We have built a high-resolution optical coherence tomography (OCT) system, based on a Linnik-type interference microscope, illuminated by a white-light thermal lamp. The extremely short coherence length of the illumination source and the large aperture of the objectives permit resolution close to 1 µm in three dimensions. A parallel detection scheme with a CCD camera provides cross-section x–y image acquisition without scanning at a rate of up to 50 Hz. To our knowledge, our system has the highest resolution demonstrated to date for OCT imaging. With identical resolution in three dimensions, realistic volume rendering of structures inside biological tissues is possible.
409 citations
TL;DR: This work reports on the incorporation of an axicon lens into the sample arm of an interferometer to overcome the trade-off between lateral resolution and focusing depth when conventional optical elements are used.
Abstract: In optical coherence tomography, axial and lateral resolutions are determined by the source coherence length and the numerical aperture of the sampling lens, respectively. Whereas axial resolution can be improved by use of a broadband light source, there is a trade-off between lateral resolution and focusing depth when conventional optical elements are used. We report on the incorporation of an axicon lens into the sample arm of an interferometer to overcome this limitation. Using an axicon lens with a top angle of 160 degrees , we maintained 10-microm or better lateral resolution over a focusing depth of at least 6 mm. In addition to having high lateral resolution, the focusing spot has an intensity that is approximately constant over a greater depth range than when a conventional lens is used.
406 citations
TL;DR: Continuously tunable and coherent radiation in the wide range 56-1618 mum (0.18-5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal.
Abstract: Continuously tunable and coherent radiation in the wide range 56.8–1618 µm (0.18–5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal. This source has the advantages of high coherence, simplicity for tuning, simple alignment, and stable output. The peak output power for the terahertz radiation reaches 69.4 W at a wavelength of 196 µm (1.53 THz), which corresponds to a photon conversion efficiency of 3.3%. A simple optimization of the design can yield a compact terahertz source.
401 citations
TL;DR: The essence of the method is the use of two combs of slightly different mode spacing to achieve spectral resolution, which has advantages of speed, frequency resolution, sensitivity, absence of dispersive components, and high spatial resolution.
Abstract: A method is proposed for performing accurate spectral characterization of samples with frequency combs, e.g., from mode-locked lasers, over the spectral range covered by the combs. The essence of the method is the use of two combs of slightly different mode spacing to achieve spectral resolution. The advantages of the method are speed, frequency resolution, sensitivity, absence of dispersive components, and high spatial resolution.
TL;DR: High-Q traveling-wave-resonators can enter a regime in which even minute scattering amplitudes associated with either bulk or surface imperfections can drive the system into the so-called strong modal coupling regime, and resonance properties radically altered and can mimic a narrowband reflector.
Abstract: High-Q traveling-wave-resonators can enter a regime in which even minute scattering amplitudes associated with either bulk or surface imperfections can drive the system into the so-called strong modal coupling regime. Resonators that enter this regime have their coupling properties radically altered and can mimic a narrowband reflector. We experimentally confirm recently predicted deviations from criticality in such strongly coupled systems. Observations of resonators that had Q>10^8 and modal coupling parameters as large as 30 were shown to reflect more than 94% of an incoming optical signal within a narrow bandwidth of 40 MHz.
TL;DR: Angular dispersion of pump frequencies is shown to be an efficient mechanism for bandwidth enhancement in a noncollinear optical parametric amplifier and feedback for an iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by second-harmonic generation frequency-resolved optical gating.
Abstract: Angular dispersion of pump frequencies is shown to be an efficient mechanism for bandwidth enhancement in a noncollinear optical parametric amplifier. We demonstrate the generation of a continuous, simultaneously phase-matched 250-THz parametrically amplified spectrum. The resultant visible-near-IR signal-wave pulses were compressed to a 4-fs duration by a micromachined flexible mirror. Feedback for an iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by second-harmonic generation frequency-resolved optical gating.
TL;DR: With the proposed technique, it is shown experimentally that the viewing resolution can be improved without reducing the three-dimensional viewing aspect of the reconstructed image.
Abstract: We propose the use of synchronously moving micro-optics (lenslet arrays) for image pickup and display in three-dimensional integral imaging to overcome the upper resolution limit imposed by the Nyquist sampling theorem. With the proposed technique, we show experimentally that the viewing resolution can be improved without reducing the three-dimensional viewing aspect of the reconstructed image.
TL;DR: Spiral phase elements with topological charges based on space-variant Pancharatnam-Berry phase optical elements are presented and spiral geometrical phases for infrared radiation at a wavelength of 10.6microm are demonstrated.
Abstract: Spiral phase elements with topological charges based on space-variant Pancharatnam-Berry phase optical elements are presented Such elements can be achieved by use of continuous computer-generated space-variant subwavelength dielectric gratings We present a theoretical analysis and experimentally demonstrate spiral geometrical phases for infrared radiation at a wavelength of 106microm
TL;DR: The interaction between spatial optical solitons in undoped nematic liquid crystals is governed by reorientational nonlinearity with a significant nonlocality, and a scalar model accounts for the main features that were experimentally observed.
Abstract: The interaction between spatial optical solitons in undoped nematic liquid crystals is governed by reorientational nonlinearity with a significant nonlocality. We demonstrate several cases of interactions, including crossing, interlacing, and merging of spatial solitons in voltage-biased liquid-crystal cells. A scalar model accounts for the main features that were experimentally observed.
TL;DR: In this article, a computer-based three-dimensional image reconstruction method and system is presented, where an elemental image array of a 3D object is formed by a micro-lens array, and recorded by a CCD camera.
Abstract: A computer-based three-dimensional image reconstruction method and system are presented. An elemental image array of a three-dimensional object is formed by a micro-lens array, and recorded by a CCD camera. Three-dimensional images are reconstructed by extracting pixels periodically from the elemental image array using a computer. Images viewed from an arbitrary angle can be retrieved by shifting which pixels are to be extracted. Image processing methods can be used to enhance the reconstructed image. Further, the digitally reconstructed images can be sent via a network. A system for imaging a three-dimensional object includes a micro-lens array that generates an elemental image array. The elemental image array is detected by a CCD camera to generate digitized image information. A computer processes the digitized image information to reconstruct an image of the three-dimensional object. A two-dimensional display device may be connected directly or indirectly to the computer to display the image of the three-dimensional object. The computer may also be used to generate virtual image information of a virtual three-dimensional object. This can then be combined with the digitized image information to provide combined image information. The two-dimensional display device may be used to display a virtual image or a combined image.
TL;DR: A tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional structures in the far-infrared, analogous to conventional CT techniques such as x-ray CT.
Abstract: We demonstrate a tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional (3D) structures in the far-infrared. This THz-wave computed tomography (T-ray CT) system provides sectional images of objects in a manner analogous to conventional CT techniques such as x-ray CT. The transmitted amplitude and phase of broadband pulses of THz radiation are measured at multiple projection angles. The filtered backprojection algorithm is then used to reconstruct the target object, including both its 3D structure and its frequency-dependent far-infrared optical properties.
TL;DR: With buried waveguides formed in periodically poled lithium niobate by annealed and reverse proton exchange, what is believed to be the highest normalized conversion efficiency for SHG in the 1550-nm communications band reported to date is demonstrated.
Abstract: Efficient three-wave mixing devices have numerous applications, including wavelength conversion, dispersion compensation, and all-optical switching. Second-harmonic generation (SHG) is a useful diagnostic for near-degenerate operation of these devices. With buried waveguides formed in periodically poled lithium niobate by annealed and reverse proton exchange, we demonstrate what is believed to be the highest normalized conversion efficiency (150%/W cm(2)) for SHG in the 1550-nm communications band reported to date.
TL;DR: The LTRS system combines optical trapping and near-infrared Raman spectroscopy for manipulation and identification of single biological cells in solution and may provide a valuable tool for the study of fundamental cellular processes and the diagnosis of cellular disorders.
Abstract: We report on the development and testing of a compact laser tweezers Raman spectroscopy (LTRS) system. The system combines optical trapping and near-infrared Raman spectroscopy for manipulation and identification of single biological cells in solution. A low-power diode laser at 785 nm was used for both trapping and excitation for Raman spectroscopy of the suspended microscopic particles. The design of the LTRS system provides high sensitivity and permits real-time spectroscopic measurements of the biological sample. The system was calibrated by use of polystyrene microbeads and tested on living blood cells and on both living and dead yeast cells. As expected, different images and Raman spectra were observed for the different cells. The LTRS system may provide a valuable tool for the study of fundamental cellular processes and the diagnosis of cellular disorders.
TL;DR: A double-beam polarization-sensitive system based on optical coherence tomography was built to measure the Mueller matrix of scattering biological tissue with high spatial resolution and the polarization parameters of a porcine tendon were extracted by decomposition of the measured Mueller matrix.
Abstract: A double-beam polarization-sensitive system based on optical coherence tomography was built to measure the Mueller matrix of scattering biological tissue with high spatial resolution. The Jones matrix of a sample can be determined with a single scan and subsequently converted into an equivalent nondepolarizing Mueller matrix. As a result, the system can be used to measure the Mueller matrix of an unstable sample, such as soft tissue. The polarization parameters of a porcine tendon, including magnitude and orientation of birefringence and diattenuation, were extracted by decomposition of the measured Mueller matrix.
TL;DR: The OCT system successfully reveals the birefringent nature of human skin tissue through the measurement of the distribution of each element of the Müller matrix of a measured object from 16 OCT images.
Abstract: We have developed a spectral interferometric optical coherence tomography (OCT) system with polarization sensitivity that is able to measure a two-dimensional tomographic image by means of one-dimensional mechanical scanning. Our system, which has an axial resolution of 32 mum , calculates the distribution of each element of the Muller matrix of a measured object from 16 OCT images. The OCT system successfully reveals the birefringent nature of human skin tissue.
TL;DR: This work built a fiber-based PS-OCT setup that produces quasi-real-time images of the human retina in vivo and preliminary measurements showed that the double-pass phase retardation per unit depth of the RNFL near the optic nerve head is 39±6°/100 µm.
Abstract: To our knowledge, this is the first demonstration of in vivo depth-resolved birefringence measurements of the human retinal nerve fiber layer (RNFL) by use of polarization-sensitive optical coherence tomography (PS-OCT). Because glaucoma causes nerve fiber layer damage, which may cause loss of retinal birefringence, PS-OCT is a potentially useful technique for the early detection of glaucoma. We built a fiber-based PS-OCT setup that produces quasi-real-time images of the human retina in vivo. Preliminary measurements of a healthy volunteer showed that the double-pass phase retardation per unit depth of the RNFL near the optic nerve head is 39±6°/100 µm.
TL;DR: By using tailored pulse sequences from a novel, 1.5-microm direct space-to-time pulse shaper driving a high-speed photodetector, the first time to the authors' knowledge, millimeter-wave arbitrary waveform generation at center frequencies approaching 50 GHz is achieved.
Abstract: By using tailored pulse sequences from a novel, 1.5-microm direct space-to-time pulse shaper driving a high-speed photodetector, we have achieved, for the first time to our knowledge, millimeter-wave arbitrary waveform generation at center frequencies approaching 50 GHz. By appropriately designing the driving optical pulse sequences, we demonstrate the ability to synthesize strongly phase- and frequency-modulated millimeter-wave electrical signals on a cycle-by-cycle basis.
TL;DR: Compared the images reconstructed by two algorithms using a single Fourier transformation and a convolution with each other by both experiments and numerical simulations have proved not to cause serious deterioration of the reconstructed images.
Abstract: Digital holography with a three-wavelength laser and a color CCD has been demonstrated. With the phase shifting of the reference beam, in-line holograms for three wavelengths are recorded simultaneously for derivation of the complex amplitude at each wavelength, and then the three monochromatic images are reconstructed and combined into full-color images in the computer. Laser power variation for wavelengths can be compensated for in the reconstruction process. We have compared the images reconstructed by two algorithms using a single Fourier transformation and a convolution with each other by both experiments and numerical simulations. Phase-shifting errors arising at two of the three wavelengths have proved not to cause serious deterioration of the reconstructed images.
TL;DR: An overlay material was deposited by the Langmuir-Blodgett technique onto a single-mode optical fiber containing a long-period grating that exhibits characteristic attenuation bands in its transmission spectrum whose central wavelengths were observed to depend on the optical thickness of the overlay material.
Abstract: An overlay material was deposited by the Langmuir–Blodgett technique onto a single-mode optical fiber containing a long-period grating. The long-period grating exhibits characteristic attenuation bands in its transmission spectrum whose central wavelengths were observed to depend on the optical thickness of the overlay material, even for materials that have a refractive index higher than that of silica.
TL;DR: A novel integrated optical sensor based on a cylindrical microcavity (MC) that sustains so-called whispering-gallery modes (WGMs), in which the energy of the optical field can be efficiently stored is proposed.
Abstract: A novel integrated optical sensor based on a cylindrical microcavity (MC) is proposed. A MC sustains so-called whispering-gallery modes (WGMs), in which the energy of the optical field can be efficiently stored. By monitoring the scattering intensity from the MC, one can detect minute changes in the refractive index of the WGM, for instance, as a result of analyte adsorption. Measurement of a change in refractive index of as little as 1024 is demonstrated experimentally. The MC-based integrated optical sensor may have a size of approximately 8 mm, and it is rugged and inexpensive.
TL;DR: The ability to trigger and guide high-voltage discharges with ionized filaments generated by femtosecond terawatt laser pulses with plasma filaments extended over the whole gap, providing a direct ohmic connection between the electrodes.
Abstract: We have demonstrated the ability to trigger and guide high-voltage discharges with ionized filaments generated by femtosecond terawatt laser pulses. The plasma filaments extended over the whole gap, providing a direct ohmic connection between the electrodes. Laser-guided straight discharges have been observed for gaps of as much as 3.8 m at a high voltage reduced to 68% of the natural breakdown voltage. The triggering efficiency was found to depend critically on the spatial connection of the laser filaments to the electrode as well as on the temporal coincidence of the laser with the peak of the high voltage.
TL;DR: A digital spectral shaping technique to reduce the sidelobes (ringing) of the axial point-spread function in optical coherence tomography for non-Gaussian-shaped source spectra is presented.
Abstract: We present a digital spectral shaping technique to reduce the sidelobes (ringing) of the axial point-spread function in optical coherence tomography for non-Gaussian-shaped source spectra. The spectra of two superluminescent diodes were combined to generate a spectrum with significant modulation. Images of onion cells demonstrate the improved image quality in a turbid biological sample. A quantitative analysis of the accompanying penalty in signal-to-noise ratio is given.
TL;DR: Tests in a controlled environment indicate that the novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings has a linear, reversible, and accurate response behavior.
Abstract: A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.