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Showing papers in "Optics Letters in 2004"


Journal ArticleDOI
TL;DR: It is shown that by use of a novel waveguide geometry the field can be confined in a 50-nm-wide low-index region with a normalized intensity of 20 microm(-2), approximately 20 times higher than what can be achieved in SiO2 with conventional rectangular waveguides.
Abstract: We present a novel waveguide geometry for enhancing and confining light in a nanometer-wide low-index material. Light enhancement and confinement is caused by large discontinuity of the electric field at highindex-contrast interfaces. We show that by use of such a structure the field can be confined in a 50-nm-wide low-index region with a normalized intensity of 20 mm 22 . This intensity is approximately 20 times higher than what can be achieved in SiO2 with conventional rectangular waveguides. © 2004 Optical Society of America OCIS codes: 030.4070, 130.0130, 130.2790, 230.7370, 230.7380, 230.7390, 230.7400. Recent results in integrated optics have shown the ability to guide, bend, split, and f ilter light on chips by use of optical devices based on high-index-contrast waveguides. 1–5 In all these devices the guiding mechanism is based on total internal ref lection (TIR) in a highindex material (core) surrounded by a low-indexmaterial (cladding); the TIR mechanism can strongly confine light in the high-index material. In recent years a number of structures have been proposed to guide or enhance light in low-index materials, 6–1 1 relying on external ref lections provided by interference effects. Unlike TIR, the external ref lection cannot be perfectly unity; therefore the modes in these structures are inherently leaky modes. In addition, since interference is involved, these structures are strongly wavelength dependent. Here we show that the optical field can be enhanced and conf ined in the low-index material even when light is guided by TIR. For a high-index-contrast interface, Maxwell’s equations state that, to satisfy the continuity of the normal component of electric f lux density D, the corresponding electric field (E-field) must undergo a large discontinuity with much higher amplitude in the low-index side. We show that this discontinuity can be used to strongly enhance and confine light in a nanometer-wide region of low-index material. The proposed structure presents an eigenmode, and it is compatible with highly integrated photonics technology. The principle of operation of the novel structure can be illustrated by analysis of the slab-based structure shown in Fig. 1(a), where a low-index slot is embedded between two high-index slabs (shaded regions). The novel structure is hereafter referred to as a slot waveguide. The slot waveguide eigenmode can be seen as being formed by the interaction between the fundamental eigenmodes of the individual slab waveguides. Rigorously, the analytical solution for the transverse E-field profile Ex of the fundamental TM eigenmode of the slab-based slot waveguide is

1,716 citations


Journal ArticleDOI
TL;DR: A lensless optical security system based on double random-phase encoding in the Fresnel domain is proposed, which can encrypt a primary image to random noise by use of two statistically independent random- phase masks in the input and transform planes, respectively.
Abstract: A lensless optical security system based on double random-phase encoding in the Fresnel domain is proposed. This technique can encrypt a primary image to random noise by use of two statistically independent random-phase masks in the input and transform planes, respectively. In this system the positions of the significant planes and the operation wavelength, as well as the phase codes, are used as keys to encrypt and recover the primary image. Therefore higher security is achieved. The sensitivity of the decrypted image to shifting along the propagation direction and to the wavelength are also investigated.

859 citations


Journal ArticleDOI
TL;DR: A novel type of Fourier-transform infrared spectrometer based on two Ti:sapphire lasers emitting femtosecond pulse trains with slightly different repetition frequencies that superimposed upon a detector to produce purely time-domain interferograms that encode the infrared spectrum is demonstrated.
Abstract: A novel type of Fourier-transform infrared spectrometer (FTIR) is demonstrated. It is based on two Ti:sapphire lasers emitting femtosecond pulse trains with slightly different repetition frequencies. Two mid-infrared beams-derived from those lasers by rectification in GaSe-are superimposed upon a detector to produce purely time-domain interferograms that encode the infrared spectrum. The advantages of this spectrometer compared with the common FTIR include ease of operation (no moving parts), speed of acquisition (100 micros demonstrated), and not-yet-shown collimated long-distance propagation, diffraction-limited microscopic probing, and electronically controllable chemometric factoring. Extending time-domain frequency-comb spectroscopy to lower (terahertz) or higher (visible, ultraviolet) frequencies should be feasible.

697 citations


Journal ArticleDOI
TL;DR: A novel silicon waveguide structure for guiding and confining light in nanometer-wide low-refractive-index material is experimentally demonstrated and it is shown that the structure can be implemented in highly integrated photonics.
Abstract: We experimentally demonstrate a novel silicon waveguide structure for guiding and confining light in nanometer-wide low-refractive-index material. The optical field in the low-index material is enhanced because of the discontinuity of the electric field at high-index-contrast interfaces. We measure a 30% reduction of the effective index of light propagating in the novel structure due to the presence of the nanometer-wide low-index region, evidencing the guiding and confinement of light in the low-index material. We fabricate ring resonators based on the structure and show that the structure can be implemented in highly integrated photonics.

678 citations


Journal ArticleDOI
TL;DR: A high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers and the size of the grooves is optimized numerically.
Abstract: We have designed a high-efficiency broadband grating coupler for coupling between silicon-on-insulator (SOI) waveguides and optical fibers. The grating is only 13 µm long and 12 µm wide, and the size of the grooves is optimized numerically. For TE polarization the coupling loss to single-mode fiber is below 1 dB over a 35-nm wavelength range when using SOI with a two-pair bottom reflector. The tolerances to fabrication errors are also calculated.

676 citations


Journal ArticleDOI
TL;DR: An ultrahigh-speed spectral domain optical coherence tomography (SD-OCT) system is presented, demonstrating the ability to acquire high-quality structural images with an axial resolution of 6 microm at ultrahigh speed and with an ocular exposure level of less than 600 microW.
Abstract: An ultrahigh-speed spectral domain optical coherence tomography (SD-OCT) system is presented that achieves acquisition rates of 29,300 depth profiles/s. The sensitivity of SD-OCT and time domain OCT (TD-OCT) are experimentally compared, demonstrating a 21.7-dB improvement of SD-OCT over TD-OCT. In vivo images of the human retina are presented, demonstrating the ability to acquire high-quality structural images with an axial resolution of 6 microm at ultrahigh speed and with an ocular exposure level of less than 600 microW.

643 citations


Journal ArticleDOI
TL;DR: By use of the Fourier decomposition of a low-coherence optical image field into two spatial components that can be controllably shifted in phase with respect to each other, a new high-transverse-resolution quantitative-phase microscope has been developed.
Abstract: By use of the Fourier decomposition of a low-coherence optical image field into two spatial components that can be controllably shifted in phase with respect to each other, a new high-transverse-resolution quantitative-phase microscope has been developed. The technique transforms a typical optical microscope into a quantitative-phase microscope, with high accuracy and a path-length sensitivity of lambda/5500, which is stable over several hours. The results obtained on epithelial and red blood cells demonstrate the potential of this instrument for quantitative investigation of the structure and dynamics associated with biological systems without sample preparation.

466 citations


Journal ArticleDOI
TL;DR: By measuring the resonant wavelength of a two-dimensional photonic crystal microcavity, a time-resolved sensing capability is demonstrated that can detect the change in refractive index of 0.002.
Abstract: We report an experimental demonstration of an ultracompact biochemical sensor based on a two-dimensional photonic crystal microcavity. The microcavity, fabricated on a silicon-on-insulator substrate, is designed to have a resonant wavelength (λ) near 1.5 µm. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from n=1.0 to n=1.5. From observation of the shift in resonant wavelength, a change in ambient refractive index of Δn=0.002 is readily apparent. The correspondence between absolute refractive index and resonant wavelength agrees with numerical calculation to within 4% accuracy. The evaporation of water in a 5% glycerol mixture is also used to demonstrate the capability for in situ time-resolved sensing.

454 citations


Journal ArticleDOI
TL;DR: An advanced random phase-shifting algorithm to extract phase distributions from randomly phase- shifted interferograms is proposed, based on a least-squares iterative procedure, but it copes with the limitation of the existing iterative algorithms by separating a frame-to-frame iteration from a pixel- to-pixel iteration.
Abstract: An advanced random phase-shifting algorithm to extract phase distributions from randomly phase-shifted interferograms is proposed. The algorithm is based on a least-squares iterative procedure, but it copes with the limitation of the existing iterative algorithms by separating a frame-to-frame iteration from a pixel-to-pixel iteration. The algorithm provides stable convergence and accurate phase extraction with as few as three interferograms, even when the phase shifts are completely random. The algorithm is simple, fast, and fully automatic. A computer simulation is conducted to prove the concept.

428 citations


Journal ArticleDOI
TL;DR: A noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately and is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.
Abstract: We introduce a noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately. The method uses a pulse shaper that scans calibrated phase functions to determine the unknown spectral phase of a pulse. The pulse shaper can then be used to synthesize arbitrary phase femtosecond pulses or it can introduce a compensating spectral phase to obtain transform-limited pulses. This method is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.

425 citations


Journal ArticleDOI
TL;DR: High-quality single-walled carbon nanotubes were directly synthesized on quartz substrates and fiber ends and successfully applied the SWNTs to mode lock a fiber laser producing subpicosecond pulses at a 50-MHz repetition rate.
Abstract: We present novel carbon-nanotube-based saturable absorbers. Using the low-temperature alcohol catalytic chemical-vapor deposition method, high-quality single-walled carbon nanotubes (SWNTs) were directly synthesized on quartz substrates and fiber ends. We successfully applied the SWNTs to mode lock a fiber laser producing subpicosecond pulses at a 50-MHz repetition rate.

Journal ArticleDOI
TL;DR: It is shown that the optical bistability allows all-optical functionalities, such as switching and memory with microsecond time response and a modulation depth of 10 dB, driven by pump power as low as 45 microW.
Abstract: We demonstrate, for the first time to our knowledge, optical bistability on a highly integrated silicon device, using a 5-microm-radius ring resonator. The strong light-confinement nature of the resonator induces nonlinear optical response with low pump power. We show that the optical bistability allows all-optical functionalities, such as switching and memory with microsecond time response and a modulation depth of 10 dB, driven by pump power as low as 45 microW. Silicon optical bistability relies on a fast thermal nonlinear optical effect presenting a 500-kHz modulation bandwidth.

Journal ArticleDOI
TL;DR: A threshold for THz generation is identified that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain measurements, and the plasma emitter is competitive with other state-of-the-art THz emitters.
Abstract: Intense radiation in the terahertz (THz) frequency range can be generated by focusing of an ultrashort laser pulse composed of both a fundamental wave and its second-harmonic field into air, as reported previously by Cook [Opt. Lett.25, 1210 (2000)]. We identify a threshold for THz generation that proves that generation of a plasma is required and that the nonlinearity of air is insufficient to explain our measurements. An additional THz field component generated in the type I β-barium borate crystal used for second-harmonic generation has to be considered if one is to avoid misinterpretation of this kind of experiment. We conclude with a comparison that shows that the plasma emitter is competitive with other state-of-the-art THz emitters.

Journal ArticleDOI
TL;DR: A phase-locked frequency comb in the near infrared is demonstrated with a mode-locked, erbium-doped, fiber laser whose output is amplified and spectrally broadened in dispersion-flattened, highly nonlinear optical fiber to span from 1100 to >2200 nm.
Abstract: A phase-locked frequency comb in the near infrared is demonstrated with a mode-locked, erbium-doped, fiber laser whose output is amplified and spectrally broadened in dispersion-flattened, highly nonlinear optical fiber to span from 1100 to >2200 nm. The supercontinuum output comprises a frequency comb with a spacing set by the laser repetition rate and an offset by the carrier-envelope offset frequency, which is detected with the standard f-to-2f heterodyne technique. The comb spacing and offset frequency are phase locked to a stable rf signal with a fiber stretcher in the laser cavity and by control of the pump laser power, respectively. This infrared comb permits frequency metrology experiments in the near infrared in a compact, fiber-laser-based system.

Journal ArticleDOI
TL;DR: The feasibility for noninvasive optical measurement of blood flow through the skull of an adult brain is demonstrated, and the clinical potential of this hybrid, all-optical nonin invasive, methodology can now be explored.
Abstract: We combine diffuse optical and correlation spectroscopies to simultaneously measure the oxyhemoglobin and deoxyhemoglobin concentration and blood flow in an adult human brain during sensorimotor stimulation. The observations permit calculation of the relative cerebral metabolic rate of oxygen in the human brain, for the first time to our knowledge, by use of all-optical methods. The feasibility for noninvasive optical measurement of blood flow through the skull of an adult brain is thus demonstrated, and the clinical potential of this hybrid, all-optical noninvasive, methodology can now be explored.

Journal ArticleDOI
TL;DR: In this paper, a record peak intensity of 0.7×1022 W/cm2 was achieved by focusing a 45-TW laser beam with an f/0.6 off-axis paraboloid.
Abstract: We generated a record peak intensity of 0.7×1022 W/cm2 by focusing a 45-TW laser beam with an f/0.6 off-axis paraboloid. The aberrations of the paraboloid and the low-energy reference laser beam were measured and corrected, and a focal spot size of 0.8 µm was achieved. It is shown that the peak intensity can be increased to 1.0×1022 W/cm2 by correction of the wave front of a 45-TW beam relative to the reference beam. The phase and amplitude measurement provides for an efficient full characterization of the focal field.

Journal ArticleDOI
TL;DR: In this article, a closed-loop correction of both ocular and system aberrations results in a residual uncorrected wavefront rms of 0.1 µm for a 3.68 mm pupil diameter.
Abstract: Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 µm) and improved transverse resolution (5–10 µm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm×300 mm) closed-loop AO system, based on a real-time Hartmann–Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 µm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.

Journal ArticleDOI
TL;DR: The existence of the Ince-Gaussian beams is demonstrated that constitute the third complete family of exact and orthogonal solutions of the paraxial wave equation and has an inherent elliptical symmetry.
Abstract: We demonstrate the existence of the Ince-Gaussian beams that constitute the third complete family of exact and orthogonal solutions of the paraxial wave equation. Their transverse structure is described by the Ince polynomials and has an inherent elliptical symmetry. Ince-Gaussian beams constitute the exact and continuous transition modes between Laguerre and Hermite-Gaussian beams. The propagating characteristics are discussed as well.

Journal ArticleDOI
TL;DR: It is demonstrated that the localization of the predicted plasmons in acute grooves may be substantially stronger than what is allowed by the diffraction limit.
Abstract: One-dimensional localized plasmons (channel polaritons) guided by a triangular groove on a metal substrate are investigated numerically by means of a finite-difference time-domain algorithm. Dispersion, existence conditions, and dissipation of these waves are analyzed. In particular, it is demonstrated that the localization of the predicted plasmons in acute grooves may be substantially stronger than what is allowed by the diffraction limit. As a result, the predicted waves may be significant for the development of new subwavelength waveguides and interconnectors for nano-optics and photonics.

Journal ArticleDOI
TL;DR: A scheme is proposed for high-precision, absolute length measurement for an arbitrary optical distance of a few meters to beyond 10(6) m using a phase-stabilized femtosecond laser to provide both incoherent, time-of-flight information and coherent, fringe-resolved interferometry.
Abstract: A scheme is proposed for high-precision, absolute length measurement for an arbitrary optical distance of a few meters to beyond 106 m. The approach utilizes a phase-stabilized femtosecond laser to provide both incoherent, time-of-flight information and coherent, fringe-resolved interferometry. Such a combined measurement capability allows an optical wavelength resolution to be achieved for absolute length measurement over a large dynamic range.

Journal ArticleDOI
TL;DR: It is demonstrated that highly efficient evanescent-wave detection of fluorophore-labeled biomolecules in aqueous solutions positioned in the air holes of the microstructured part of a photonic crystal fiber even at wavelengths in the visible range is demonstrated.
Abstract: We demonstrate highly efficient evanescent-wave detection of fluorophore-labeled biomolecules in aqueous solutions positioned in the air holes of the microstructured part of a photonic crystal fiber. The air-suspended silica structures located between three neighboring air holes in the cladding crystal guide light with a large fraction of the optical field penetrating into the sample even at wavelengths in the visible range. An effective interaction length of several centimeters is obtained when a sample volume of less than 1 µL is used.

Journal ArticleDOI
TL;DR: The existence of parabolic beams that constitute the last member of the family of fundamental nondiffracting wave fields and their associated angular spectrum is demonstrated and their eigenvalue spectrum is continuous.
Abstract: We demonstrate the existence of parabolic beams that constitute the last member of the family of fundamental nondiffracting wave fields and determine their associated angular spectrum. Their transverse structure is described by parabolic cylinder functions, and contrary to Bessel or Mathieu beams their eigenvalue spectrum is continuous. Any nondiffracting beam can be constructed as a superposition of parabolic beams, since they form a complete orthogonal set of solutions of the Helmholtz equation. A novel class of traveling parabolic waves is also introduced for the first time.

Journal ArticleDOI
TL;DR: The development of ultrahigh-quality-factor (Q) silicon-on-insulator (SOI) microring resonators based on silicon wire waveguides is presented, illustrating that in addition to low propagation losses the critical coupling condition is essential for optimizing device characteristics.
Abstract: The development of ultrahigh-quality-factor Q silicon-on-insulator (SOI) microring resonators based on silicon wire waveguides is presented. An analytical description is derived, illustrating that in addition to low propagation losses the critical coupling condition is essential for optimizing device characteristics. Propagation losses as low as 1.9±0.1 dB/cm in a curved waveguide with a bending radius of 20 µm and a Q factor as high as 139.000±6.000 are demonstrated. These are believed to be the highest values reported for a curved SOI waveguide device and for any directly structured semiconductor microring fabricated without additional melting-induced surface smoothing.

Journal ArticleDOI
TL;DR: Efficient single-photon detection at 1.55 microm by means of sum-frequency mixing with a strong pump with high background counts at strong circulating pump powers due to efficient upconversion of pump-induced fluorescence photons is demonstrated.
Abstract: We demonstrate efficient single-photon detection at 1.55 microm by means of sum-frequency mixing with a strong pump at 1.064 microm in periodically poled lithium niobate followed by photon counting in the visible region. This scheme offers significant advantages over existing InGaAs photon counters: continuous-wave operation, higher detection efficiency, higher counting rates, and no afterpulsing. We achieved single-photon upconversion efficiency of 90% at 21.6 W of circulating power in a resonant pump cavity with a 400-mW Nd:YAG laser. We observed high background counts at strong circulating pump powers due to efficient upconversion of pump-induced fluorescence photons.

Journal ArticleDOI
TL;DR: A truly broadband CARS imaging instrument is demonstrated that is used to acquire hyperspectral images with vibrational spectra over a bandwidth of 2500 cm (-1) with a resolution of 13 cm(-1).
Abstract: Coherent anti-Stokes Raman scattering (CARS) microscopy is emerging as a powerful method for imaging materials and biological systems, partly because of its noninvasiveness and selective chemical sensitivity. However, its full potential for species-selective imaging is limited by a restricted spectral bandwidth. Recent increases in bandwidth are promising but still are not sufficient for the level of robust component discrimination that would be needed in a chemically complex milieu found, for example, in intracellular and extracellular environments. We demonstrate a truly broadband CARS imaging instrument that we use to acquire hyperspectral images with vibrational spectra over a bandwidth of 2500 cm(-1) with a resolution of 13 cm(-1).

Journal ArticleDOI
TL;DR: A spatially adaptive two-dimensional wavelet filter is used to reduce speckle noise in time-domain and Fourier-domain optical coherence tomography (OCT) images.
Abstract: A spatially adaptive two-dimensional wavelet filter is used to reduce speckle noise in time-domain and Fourier-domain optical coherence tomography (OCT) images. Edges can be separated from discontinuities that are due to noise, and noise power can be attenuated in the wavelet domain without significantly compromising image sharpness. A single parameter controls the degree of noise reduction. When this filter is applied to ophthalmic OCT images, signal-to-noise ratio improvements of >7 dB are attained, with a sharpness reduction of <3%.

Journal ArticleDOI
TL;DR: A miniature, flexible two-photon microscope consisting of a fused coherent optical fiber bundle with 30,000 cores and a gradient-index lens objective is presented and fluorescent light was detected through the fiber bundle.
Abstract: We present a miniature, flexible two-photon microscope consisting of a fused coherent optical fiber bundle with 30,000 cores and a gradient-index lens objective. The laser focus of a standard two-photon laser-scanning microscope was scanned over the entrance surface of the fiber bundle, resulting in sequential coupling into individual cores. Fluorescent light was detected through the fiber bundle. Micrometer-sized fluorescent beads and pollen grains were readily resolved. In addition, fluorescently labeled blood vessels were imaged through the fiber bundle in rat brain in vivo.

Journal ArticleDOI
TL;DR: An analysis for polarization-sensitive optical coherence tomography that facilitates the unrestricted use of fiber and fiber-optic components throughout an interferometer and yields sample birefringence, diattenuation, and relative optic axis orientation is presented.
Abstract: We present an analysis for polarization-sensitive optical coherence tomography that facilitates the unrestricted use of fiber and fiber-optic components throughout an interferometer and yields sample birefringence, diattenuation, and relative optic axis orientation. We use a novel Jones matrix approach that compares the polarization states of light reflected from the sample surface with those reflected from within a biological sample for pairs of depth scans. The incident polarization alternated between two states that are perpendicular in a Poincare sphere representation to ensure proper detection of tissue birefringence regardless of optical fiber contributions. The method was validated by comparing the calculated diattenuation of a polarizing sheet, chicken tendon, and muscle with that obtained by independent measurement. The relative importance of diattenuation versus birefringence to angular displacement of Stokes vectors on a Poincare sphere was quantified.

Journal ArticleDOI
TL;DR: A high-sensitivity surface plasmon resonance (SPR) biosensor based on the Mach-Zehnder interferometer design is presented and a significant improvement over previously obtained results should allow SPR biosensors to become a possible replacement for conventional biosensing techniques based on fluorescence.
Abstract: A high-sensitivity surface plasmon resonance (SPR) biosensor based on the Mach-Zehnder interferometer design is presented. The novel feature of the new design is the use of a Wollaston prism through which the phase quantities of the p and s polarizations are interrogated simultaneously. Since SPR affects only the p polarization, the signal due to the s polarization can be used as the reference. Consequently, the differential phase between the two polarizations allows us to eliminate all common-path phase noise while keeping the phase change caused by the SPR effect. Experimental results obtained from glycerin-water mixtures indicate that the sensitivity limit of our scheme is 5.5 x 10(-8) refractive-index units per 0.01 degrees phase change. To our knowledge, this is a significant improvement over previously obtained results when gold was used as the sensor surface. Such an improvement in the sensitivity limit should allow SPR biosensors to become a possible replacement for conventional biosensing techniques based on fluorescence. Monitoring of the bovine serum albumin (BSA) binding reaction with BSA antibodies is also demonstrated.

Journal ArticleDOI
TL;DR: Phase noise characterization of a 10-W Yb fiber amplifier is presented, and phase locking of two fiber amplifiers with near-perfect fringe contrast at power levels of as much as 10 W per fiber are demonstrated.
Abstract: Phase noise characterization of a 10-W Yb fiber amplifier is presented, and we demonstrate phase locking of two fiber amplifiers with near-perfect fringe contrast at power levels of as much as 10 W per fiber. Coherent beam combining is maintained during the turn-on transient as well as in thermal steady-state operation.