Showing papers in "Optics Letters in 1999"
TL;DR: The relations for the dispersion and the group velocity of the photonic band of the CROW's are obtained and it is found that they are solely characterized by coupling factor k(1) .
Abstract: We propose a new type of optical waveguide that consists of a sequence of coupled high- Q resonators. Unlike other types of optical waveguide, waveguiding in the coupled-resonator optical waveguide (CROW) is achieved through weak coupling between otherwise localized high- Q optical cavities. Employing a formalism similar to the tight-binding method in solid-state physics, we obtain the relations for the dispersion and the group velocity of the photonic band of the CROW's and find that they are solely characterized by coupling factor k 1 . We also demonstrate the possibility of highly efficient nonlinear optical frequency conversion and perfect transmission through bends in CROW's.
1,671 citations
TL;DR: A new application of digital holography for phase-contrast imaging and optical metrology and an application to surface profilometry shows excellent agreement with contact-stylus probe measurements.
Abstract: We present a new application of digital holography for phase-contrast imaging and optical metrology. This holographic imaging technique uses a CCD camera for recording of a digital Fresnel off-axis hologram and a numerical method for hologram reconstruction. The method simultaneously provides an amplitude-contrast image and a quantitative phase-contrast image. An application to surface profilometry is presented and shows excellent agreement with contact-stylus probe measurements.
1,202 citations
TL;DR: To overcome depth-of-field limitations the authors perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth, which is to their knowledge the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.
Abstract: Ultrahigh-resolution optical coherence tomography (OCT) by use of state of the art broad-bandwidth femtosecond laser technology is demonstrated and applied to in vivo subcellular imaging. Imaging is performed with a Kerr-lens mode-locked Ti:sapphire laser with double-chirped mirrors that emits sub-two-cycle pulses with bandwidths of up to 350 nm, centered at 800 nm. Longitudinal resolutions of ~1mum and transverse resolution of 3mum, with a 110-dB dynamic range, are achieved in biological tissue. To overcome depth-of-field limitations we perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth. To our knowledge this is the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.
1,134 citations
TL;DR: The resolution limit of scanning far-field fluorescence microscopy is overcame by disabling the fluorescence from the outer part of the focal spot by a spatially offset pulse.
Abstract: We overcame the resolution limit of scanning far-field fluorescence microscopy by disabling the fluorescence from the outer part of the focal spot. Whereas a near-UV pulse generates a diffraction-limited distribution of excited molecules, a spatially offset pulse quenches the excited molecules from the outer part of the focus through stimulated emission. This results in a subdiffraction-sized effective point-spread function. For a 1.4 aperture and a 388-nm excitation wavelength spatial resolution is increased from 150±8 nm to 106±8 nm with a single offset beam. Superior lateral resolution is demonstrated by separation of adjacent Pyridine 2 nanocrystals that are otherwise indiscernible.
789 citations
TL;DR: A hybrid Ti:sapphire-Nd:glass laser system that produces more than 1500 TW (1.5 PW) of peak power and focuses to an irradiance of >7x10(20) W/cm (2) is achieved by use of a Cassegrainian focusing system employing a plasma mirror.
Abstract: We have developed a hybrid Ti:sapphire–Nd:glass laser system that produces more than 1500??TW (1.5??PW) of peak power. The system produces 660??J of power in a compressed 440±20 fs pulse by use of 94-cm master diffraction gratings. Focusing to an irradiance of >7×1020 W/cm2 is achieved by use of a Cassegrainian focusing system employing a plasma mirror.
479 citations
TL;DR: Encryption and decryption of optical memory in a LiNbO(3) :Fe photorefractive crystal by use of angular multiplexing is demonstrated.
Abstract: An encrypted optical memory system using double random phase codes in the Fresnel domain is proposed. In this system, two random phase codes and their positions form three-dimensional keys for encryption of images and are used as keys to recover the original data. The third dimension is the positions of the codes, which can have as many as three degrees of freedom. Original images encrypted by use of the two phase codes located in the Fresnel domain are stored holographically in a photorefractive material. We demonstrate in preliminary experiments encryption and decryption of optical memory in a LiNbO3:Fe photorefractive crystal by use of angular multiplexing.
476 citations
TL;DR: These pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW.
Abstract: Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.
472 citations
TL;DR: Computer-generated holograms written on a liquid-crystal display can be used to generate dynamic light fields of arbitrary shape and was used to simultaneously trap polystyrene particles laterally and to displace them independently of one another.
Abstract: Computer-generated holograms written on a liquid-crystal display can be used to generate dynamic light fields of arbitrary shape. This method was used to simultaneously trap polystyrene particles laterally and to displace them independently of one another.
456 citations
TL;DR: The use of infrared femtosecond laser pulses to induce highly localized refractive-index changes in fused-silica glasses is investigated and the potential of this technique for writing three-dimensional photonic structures in bulk glasses is demonstrated by the fabrication of a Y coupler within a sample of pure fused silica.
Abstract: We investigate the use of infrared femtosecond laser pulses to induce highly localized refractive-index changes in fused-silica glasses. We characterize the magnitude of the change as a function of exposure and measure index changes as large as 3x10(-3) and 5x10(-3) in pure fused silica and boron-doped silica, respectively. The potential of this technique for writing three-dimensional photonic structures in bulk glasses is demonstrated by the fabrication of a Y coupler within a sample of pure fused silica.
448 citations
TL;DR: The fiber gratings that were fabricated by this technique have a high resistance to thermal decay and the values of the loss peak wavelength and the transmittance after heat treatment below 500 degrees C are the same as initial values before heat treatment.
Abstract: We have fabricated long-period fiber gratings by use of a novel technique using focused irradiation of infrared femtosecond laser pulses. We investigate the thermal stability of the fabricated fiber gratings. The values of the loss peak wavelength and the transmittance of the fiber gratings after heat treatment below 500°C are the same as initial values before heat treatment. The fiber gratings that were fabricated by this technique have a high resistance to thermal decay. We propose that this technique mill be useful for fabrication of fiber gratings with a superior aging characteristic.
439 citations
TL;DR: A two-dimensional subwavelength structured (SWS) surface upon a crystal silicon substrate patterned by electron beam lithography and etched by an SF(6) fast atom beam was fabricated and the reflectivity was examined.
Abstract: We fabricated a two-dimensional subwavelength structured (SWS) surface upon a crystal silicon substrate. The SWS surface was patterned by electron beam lithography and etched by an SF(6) fast atom beam. The SWS grating had a conical profile, the period was 150 nm, and the groove was approximately 350 nm deep. The reflectivity was examined at 2002500-nm wavelengths. At 400 nm the reflectivity decreased to 0.5% from the 54.7% of the silicon substrate. We also used HeNe laser light to examine the reflectivity as a function of the incident angle.
TL;DR: This work has used the comb of optical frequencies emitted by a mode-locked laser as a ruler to measure differences of as much as 20 THz between laser frequencies, to their knowledge the largest gap measured with a frequency comb.
Abstract: We have used the comb of optical frequencies emitted by a mode-locked laser as a ruler to measure differences of as much as 20 THz between laser frequencies. This is to our knowledge the largest gap measured with a frequency comb, with high potential for further improvements. To check the accuracy of this approach we show that the modes are distributed uniformly in frequency space within the experimental limit of 3.0 parts in 1017. By comparison with an optical frequency comb generator we have verified that the mode separation equals the pulse repetition rate within the experimental limit of 6.0 parts in 1016.
TL;DR: Polarization-sensitive optical coherence tomography (PS-OCT) was used to characterize completely the polarization state of light backscattered from turbid media as discussed by the authors, which has potentially useful applications in biomedical optics by imaging simultaneously the structural properties of turbid biological materials.
Abstract: Polarization-sensitive optical coherence tomography (PS-OCT) was used to characterize completely the polarization state of light backscattered from turbid media. Using a low-coherence light source, one can determine the Stokes parameters of backscattered light as a function of optical path in turbid media. To demonstrate the application of this technique we determined the birefringence and the optical axis in fibrous tissue (rodent muscle) and in vivo rodent skin. PS-OCT has potentially useful applications in biomedical optics by imaging simultaneously the structural properties of turbid biological materials and their effects on the polarization state of backscattered light. This method may also find applications in material science for investigation of polarization properties (e.g., birefringence) in opaque media such as ceramics and crystals.
TL;DR: An improvement in bandwidth has been made possible by carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.
Abstract: Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.
TL;DR: A pair of thick birefringent retarders is incorporated into the spectroscopic polarimeter, so the generated channeled spectrum is composed of three quasi-cosinusoidal components carrying the information about the SOP of the light that is being measured.
Abstract: We describe a novel method for the spectroscopic measurement of the state of polarization (SOP) of light. A pair of thick birefringent retarders is incorporated into the spectroscopic polarimeter, so the generated channeled spectrum is composed of three quasi-cosinusoidal components carrying the information about the SOP of the light that is being measured. Fourier inversion of the channeled spectrum provides significant parameters for determination of the spectrally resolved Stokes parameters of light. No mechanically movable components for polarization control or active devices for polarization modulation are used, and all the Stokes parameters can be determined at once from only the single spectrum. The effectiveness of this method is demonstrated by the generation of elliptically polarized light whose SOP varies with wave number.
TL;DR: The rotational frequency shift for light beams with orbital angular momentum is interpreted as a dynamically evolving geometric phase using the Poincaré-sphere equivalent.
Abstract: The polarization state of a light beam is related to its spin angular momentum and can be represented on the Poincare sphere. We propose a sphere for light beams in analogous orbital angular momentum states. Using the Poincare-sphere equivalent, we interpret the rotational frequency shift for light beams with orbital angular momentum [Phys.??Rev.??Lett.??80, 3217 (1998)] as a dynamically evolving geometric phase.
TL;DR: A polarization-sensitive optical coherence tomographic system is built and the Mueller matrix measurements, made by varying the polarization states of the light source and the detector, yielded a complete characterization of the polarization property of the tissue sample.
Abstract: We built a polarization-sensitive optical coherence tomographic system and measured the two-dimensional depth-resolved full 4×4 Mueller matrix of biological tissue for what is believed to be the first time. The Mueller matrix measurements, which we made by varying the polarization states of the light source and the detector, yielded a complete characterization of the polarization property of the tissue sample. The initial experimental results indicated that this new approach reveals some tissue structures that are not perceptible in standard optical coherence tomography.
TL;DR: In initial clinical studies using the EOCT system, the esophagus, stomach, duodenum, ileum, colon, and rectum of patients with normal endoscopic findings were examined; microscopic structures such as glands, blood vessels, pits, villi, and crypts were also observed.
Abstract: We report on the design and initial clinical experience with a real-time endoscopic optical coherence tomography (EOCT) imaging system. The EOCT unit includes a high-speed optical coherence tomography interferometer, endoscope-compatible catheter probes, and real-time data capture and display hardware and software. Several technological innovations are introduced that improve EOCT efficiency and performance. In initial clinical studies using the EOCT system, the esophagus, stomach, duodenum, ileum, colon, and rectum of patients with normal endoscopic findings were examined. In these initial investigations, EOCT imaging clearly delineated the substructure of the mucosa and submucosa in several gastrointestinal organs; microscopic structures such as glands, blood vessels, pits, villi, and crypts were also observed.
TL;DR: This work demonstrates control of the transmission through variation of these parameters and shows that perforated metal films may form a novel basis for electro-optic devices such as flat-panel displays, spatial light modulators, and tunable optical filters.
Abstract: The transmission spectrum of a metal that is perforated with a periodic array of subwavelength holes exhibits well-defined maxima and minima resulting from, respectively, a transmission enhancement by surface plasmons and Wood's anomaly, a diffraction effect. These features occur at wavelengths determined by the geometry of the hole arrays, the refractive index of the adjacent medium, and the angle of incidence. We demonstrate control of the transmission through variation of these parameters and show that perforated metal films may form a novel basis for electro-optic devices such as flat-panel displays, spatial light modulators, and tunable optical filters.
TL;DR: The small effective area that is possible in this type of fiber significantly enhances its effective nonlinearity relative to standard fiber.
Abstract: Holey fibers combine two-dimensional microstructuring with one-dimensional longitudinal propagation, resulting in fibers with tailorable dispersive and nonlinear properties. We measure the effective nonlinearity of a typical holey fiber. The small effective area that is possible in this type of fiber significantly enhances its effective nonlinearity relative to standard fiber.
TL;DR: In this article, a family of power-conserving fiber-optic interferometer designs for low-coherence reflectometry that use optical circulators, unbalanced couplers, and balanced heterodyne detection is introduced.
Abstract: We introduce a family of power-conserving fiber-optic interferometer designs for low-coherence reflectometry that use optical circulators, unbalanced couplers, and (or) balanced heterodyne detection. Simple design equations for optimization of the signal-to-noise ratio of the interferometers are expressed in terms of relevant signal and noise sources and measurable system parameters. We use the equations to evaluate the expected performance of the new configurations compared with that of the standard Michelson interferometer that is commonly used in optical coherence tomography (OCT) systems. The analysis indicates that improved sensitivity is expected for all the new interferometer designs, compared with the sensitivity of the standard OCT interferometer, under high-speed imaging conditions.
TL;DR: A simple pigtailing method for microspheres permits their wider use in fiber optics and photonics devices and in the experimental setup, the total fiber-to-fiber transmission at resonance reached 23%.
Abstract: We demonstrate a simple method for efficient coupling of standard single-mode optical fibers to a high- Q optical microsphere cavity. Phase-matched excitation of whispering-gallery modes is provided by an angle-polished fiber tip in which the core-guided wave undergoes total internal reflection. In the experimental setup, which included a microsphere with both an input and an output coupler, the total fiber-to-fiber transmission at resonance reached 23% (total insertion loss, 6.3 dB), with loaded quality factor Q> or =3 x 10(7) and unloaded Q approximately 1.2 x 10(8) at 1550 nm. A simple pigtailing method for microspheres permits their wider use in fiber optics and photonics devices.
TL;DR: It is shown that, for a generic polarization state of the incident field, such a grating gives rise to first-order diffracted beams with counterrotating circular polarizations and can be used for measuring the Stokes parameters of a light beam in an achromatic manner.
Abstract: Ordinary gratings act on the amplitude and (or) the phase of a wave front. Polarization gratings produce instead a periodic modulation of the state of polarization. A simple grating of the latter type is constituted by a linear polarizer whose orientation varies periodically along a line. It is shown that, for a generic polarization state of the incident field, such a grating gives rise to first-order diffracted beams with counterrotating circular polarizations. It is also shown that such a grating can be used for measuring the Stokes parameters of a light beam in an achromatic manner. Several extensions are briefly discussed.
TL;DR: A new linear-scanning fiber-optic catheter is demonstrated that avoids the rotary optical junction that is required in circumferential scanning systems, and these advancements have permitted the clinical implementation of OCT imaging in the human gastrointestinal tract.
Abstract: A nonreciprocal fiber-optic interferometer is demonstrated in an optical coherence tomography (OCT) system. The increased power efficiency of this system provides a 4.1-dB advantage over standard Michelson implementations. In addition, a new linear-scanning fiber-optic catheter is demonstrated that avoids the rotary optical junction that is required in circumferential scanning systems. These advancements have permitted the clinical implementation of OCT imaging in the human gastrointestinal tract.
TL;DR: An electrostatically deformable, gold-coated, silicon nitride membrane mirror was used as a phase modulator to compress pulses from 92 to 15 fs to test the range of the deformable-mirror-based compressor.
Abstract: An electrostatically deformable, gold-coated, silicon nitride membrane mirror was used as a phase modulator to compress pulses from 92 to 15 fs. Both an iterative genetic algorithm and single-step dispersion compensation based on frequency-resolved optical gating calibration of the mirror were used to compress pulses to within 10% of the transform limit. Frequency-resolved optical gating was used to characterize the pulses and to test the range of the deformable-mirror-based compressor.
TL;DR: A combination of advanced silicon-processing techniques was used to create three-dimensional (3D) photonic crystals with a 180-nm minimum feature size that displayed a strong stop band at optical wavelengths.
Abstract: A combination of advanced silicon-processing techniques was used to create three-dimensional (3D) photonic crystals with a 180-nm minimum feature size. The resulting 3D crystal displayed a strong stop band at optical wavelengths from lambda=1.35 microm to lambda=1.95 microm . This is believed to be the smallest 3D crystal with a complete 3D photonic bandgap ever created.
TL;DR: In this article, the influence of the pulse length of ultrashort laser pulses at 780 and 920nm on cell vitality and cellular reproduction has been studied and it was found that cell destruction is likely based on a two-photon excitation process rather than a one- or a threephoton event, indicating that femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe twophoton fluorescence microscopy.
Abstract: The influence of the pulse length, ?, of ultrashort laser pulses at 780 and 920??nm on cell vitality and cellular reproduction has been studied. A total of 2400 nonlabeled cells were exposed to a highly focused scanning beam from a mode-locked 80-MHz Ti:sapphire laser with 60??s pixel dwell time. For the same pulse energy, destructive effects were more pronounced for shorter pulses. The damage behavior was found to follow approximately a P2/? dependence (P, mean power), indicating that cell destruction is likely based on a two-photon excitation process rather than a one- or a three-photon event. Therefore, femtosecond as well as picosecond pulses provide approximately the same relative optical window for safe two-photon fluorescence microscopy.
TL;DR: This work predicts dramatically reduced switching thresholds for nonlinear optical devices incorporating fiber ring resonators, allowing for photonic switching devices that operate at milliwatt power levels in ordinary optical fibers.
Abstract: We predict dramatically reduced switching thresholds for nonlinear optical devices incorporating fiber ring resonators. The circulating power in such a resonator is much larger than the incident power; also, the phase of the transmitted light varies rapidly with the single-pass phase shift. The combined action of these effects leads to a finesse-squared reduction in the switching threshold, allowing for photonic switching devices that operate at milliwatt power levels in ordinary optical fibers.
TL;DR: It is shown that the spatial distribution and the effective index of these modes are determined largely by the design of the lattice and that the grating spectra are unaffected by the refractive index surrounding the fiber.
Abstract: We report what is believed to be the first demonstration of optical fiber gratings written in photonic crystal fibers. The fiber consists of a germanium-doped photosensitive core surrounded by a hexagonal periodic air-hole lattice in a silica matrix. The spectra of these gratings allow for a detailed characterization of the fiber. In particular, the gratings facilitate coupling to higher-order leaky modes. We show that the spatial distribution and the effective index of these modes are determined largely by the design of the lattice and that the grating spectra are unaffected by the refractive index surrounding the fiber. We describe these measurements and corresponding simulations and discuss their implications for the understanding of such air-hole structures.
TL;DR: The well-known duality between paraxial diffraction in space and dispersion in time is used to propose a time-domain analog to spatial Fraunhofer diffraction that permits the design of real-time optical Fourier-transformer systems.
Abstract: We use the well-known duality between paraxial diffraction in space and dispersion in time to propose a time-domain analog to spatial Fraunhofer diffraction. This analog permits the design of real-time optical Fourier-transformer systems. These systems are shown to be realizable by use of linearly chirped fiber gratings as dispersive media.