Showing papers in "Optics Letters in 2013"
TL;DR: This study reveals that Kerr combs are associated with temporal cavity solitons and dispersive waves, and opens up new avenues for the understanding of Kerr-comb formation.
Abstract: A generalized Lugiato-Lefever equation is numerically solved with a Newton-Raphson method to model Kerr frequency combs. We obtain excellent agreement with past experiments, even for an octave-spanning comb. Simulations are much faster than with any other technique despite including more modes than ever before. Our study reveals that Kerr combs are associated with temporal cavity solitons and dispersive waves, and opens up new avenues for the understanding of Kerr-comb formation.
541 citations
TL;DR: It is shown analytically and experimentally that such a vortex can be approximately generated in the Fourier transforming optical system with a computer-controlled liquid-crystal spatial light modulator.
Abstract: We introduce the concept of the perfect optical vortex whose dark hollow radius does not depend on the topological charge. It is shown analytically and experimentally that such a vortex can be approximately generated in the Fourier transforming optical system with a computer-controlled liquid-crystal spatial light modulator.
426 citations
TL;DR: The results demonstrate that the microfiber-based TI photonic device can operate as both the high nonlinear optical component and the SA in fiber lasers, and could also find other applications in the related fields of photonics.
Abstract: We report on the generation of passive harmonic mode locking of a fiber laser using a microfiber-based topological insulator (TI) Bi2Te3 saturable absorber (SA) The optical deposition method was employed to fabricate the microfiber-based TISA By virtue of the excellent nonlinear optical property of the proposed TISA, the fiber laser could operate at the pulse repetition rate of 204 GHz under a pump power of 126 mW, corresponding to the 418th harmonic of fundamental repetition frequency The results demonstrate that the microfiber-based TI photonic device can operate as both the high nonlinear optical component and the SA in fiber lasers, and could also find other applications in the related fields of photonics
379 citations
TL;DR: The formation of topologically protected localized midgap states in systems with spatially distributed gain and loss can be selectively amplified, which finds applications in the beam dynamics along a photonic lattice and in the lasing of quasi-one-dimensional photonic crystals.
Abstract: One of the principal goals in the design of photonic crystals is the engineering of band gaps and defect states. Here I describe the formation of topologically protected localized midgap states in systems with spatially distributed gain and loss. These states can be selectively amplified, which finds applications in the beam dynamics along a photonic lattice and in the lasing of quasi-one-dimensional photonic crystals.
362 citations
TL;DR: A fabricated mode (de)multiplexer with cascaded asymmetrical directional couplers with low excess loss as well as low crosstalk over a broad wavelength range (~20 nm) is demonstrated experimentally.
Abstract: A small silicon mode (de)multiplexer with cascaded asymmetrical directional couplers is demonstrated experimentally. As an example, a four channel mode (de)multiplexer is designed and realized for TM polarization. The fabricated mode (de)multiplexer has a low excess loss (<1 dB) as well as low crosstalk (≤23 dB) over a broad wavelength range (~20 nm). More channels can be achieved with two sets of orthogonal-polarization modes (e.g., 2N=8) multiplexed when desired.
349 citations
TL;DR: White light diffraction phase microscopy is presented as a quantitative phase imaging method that combines the single shot measurement benefit associated with off-axis methods, high temporal phase stability associated with common path geometries, and high spatial phase sensitivity due to the white light illumination.
Abstract: A microscope and methods for obtaining a phase image of a substantially transparent specimen. Light collected from a specimen illuminated by a temporally incoherent source is diffracted into a first order and either the zeroth or first order is low-pass filtered in a Fourier transform plane before the orders are recombined at a focal plane detector. By low pass filtering the first order diffracted beam into a plurality of wavelengths, a spectrally- and spatially-resolved quantitative phase image of the specimen is obtained.
288 citations
TL;DR: This Letter compares FPM data to theoretical prediction and phase-shifting digital holography measurement to show that its acquired phase maps are quantitative and artifact-free, and explores the relationship between the achievable spatial and optical thickness resolution offered by a reconstructed FPM phase image.
Abstract: Fourier ptychographic microscopy (FPM) is a recently developed imaging modality that uses angularly varying illumination to extend a system’s performance beyond the limit defined by its optical components. The FPM technique applies a novel phase-retrieval procedure to achieve resolution enhancement and complex image recovery. In this Letter, we compare FPM data to theoretical prediction and phase-shifting digital holography measurement to show that its acquired phase maps are quantitative and artifact-free. We additionally explore the relationship between the achievable spatial and optical thickness resolution offered by a reconstructed FPM phase image. We conclude by demonstrating enhanced visualization and the collection of otherwise unobservable sample information using FPM’s quantitative phase.
280 citations
TL;DR: Using the known solutions of the Lugiato-Lefever equation, normalized properties of temporal cavity soliton solutions lead to a simple analytic estimate of the maximum attainable bandwidth for given pump resonator parameters.
Abstract: Using the known solutions of the Lugiato–Lefever equation, we derive universal trends of Kerr frequency combs. In particular, normalized properties of temporal cavity soliton solutions lead us to a simple analytic estimate of the maximum attainable bandwidth for given pump resonator parameters. The result is validated via comparison with past experiments encompassing a diverse range of resonator configurations and parameters.
264 citations
TL;DR: This Letter proposes and demonstrates a high-speed and power-efficient thermo-optic switch using an adiabatic bend with a directly integrated silicon heater to minimize the heat capacity and therein maximize the performance of the thermosensitive switch.
Abstract: In this Letter, we propose and demonstrate a high-speed and power-efficient thermo-optic switch using an adiabatic bend with a directly integrated silicon heater to minimize the heat capacity and therein maximize the performance of the thermo-optic switch. A rapid, τ=2.4 μs thermal time constant and a low electrical power consumption of P(π)=12.7 mW/π-phase shift were demonstrated representing a P(π)τ product of only 30.5 mW·μs in a compact device with a phase shifter of only ~10 μm long.
263 citations
TL;DR: A method to encode the amplitude and the phase of an optical field into a phase-only hologram, which allows the exact control of spatial transverse modes and experimentally verify the accuracy of this method by applying it to the generation and detection of transverse spatial modes in mutually unbiased bases of dimension two and three.
Abstract: A phase-only hologram applies a modal transformation to an optical transverse spatial mode via phase encoding and intensity masking. Accurate control of the optical field crucially depends on the method employed to encode the hologram. In this Letter, we present a method to encode the amplitude and the phase of an optical field into a phase-only hologram, which allows the exact control of spatial transverse modes. Any intensity masking method modulates the amplitude and alters the phase of the optical field. Our method consists in correcting for this unwanted phase alteration by modifying the phase encryption accordingly. We experimentally verify the accuracy of our method by applying it to the generation and detection of transverse spatial modes in mutually unbiased bases of dimension two and three.
251 citations
TL;DR: A linear relationship is observed between the rotation speed and orbital angular momentum content of the beam and a perfect vortex beam with integer or fractional topological charges is observed.
Abstract: We analyze microparticle dynamics within a "perfect" vortex beam. In contrast to other vortex fields, for any given integer value of the topological charge, a "perfect" vortex beam has the same annular intensity profile with fixed radius of peak intensity. For a given topological charge, the field possesses a well-defined orbital angular momentum density at each point in space, invariant with respect to azimuthal position. We experimentally create a perfect vortex and correct the field in situ, to trap and set in motion trapped microscopic particles. For a given topological charge, a single trapped particle exhibits the same local angular velocity moving in such a field independent of its azimuthal position. We also investigate particle dynamics in "perfect" vortex beams of fractional topological charge. This light field may be applied for novel studies in optical trapping of particles, atoms, and quantum gases.
TL;DR: Silicon waveguide asymmetric Y junction mode multiplexers and demultiplexers are demonstrated for applications in on-chip mode-division multiplexing (MDM) and interference effects are shown to be advantageous for low-crosstalk MDM, even while using compact Y junctions designed to be outside the mode-sorting regime.
Abstract: Silicon waveguide asymmetric Y junction mode multiplexers and demultiplexers are demonstrated for applications in on-chip mode-division multiplexing (MDM). We measure demultiplexed crosstalk as low as -30 dB, <-9 dB over the C band, and insertion loss <1.5 dB for multimode links up to 1.2 mm in length. The frequency response of these devices is shown to depend upon Y junction angle and multimode interconnect length. Interference effects are shown to be advantageous for low-crosstalk MDM, even while using compact Y junctions designed to be outside the mode-sorting regime.
TL;DR: The experimental results show that the power of the transmitted OAM mode will tend to spread uniformly onto the neighboring mode in medium-to-strong turbulence, resulting in severe crosstalk at the receiver, and the power penalty is found to exceed 10 dB in a weak- to-medium turbulence condition.
Abstract: We experimentally investigate the performance of an orbital angular momentum (OAM) multiplexed free space optical (FSO) communication link through emulated atmospheric turbulence. The turbulence effects on the crosstalk and system power penalty of the FSO link are characterized. The experimental results show that the power of the transmitted OAM mode will tend to spread uniformly onto the neighboring mode in medium-to-strong turbulence, resulting in severe crosstalk at the receiver. The power penalty is found to exceed 10 dB in a weak-to-medium turbulence condition due to the turbulence-induced crosstalk and power fluctuation of the received signal.
TL;DR: Optical frequency comb generation from high-quality-factor AlN microring resonators integrated on silicon substrates is reported by engineering the waveguide structure to achieve near-zero dispersion at telecommunication wavelengths and optimizing the phase matching for four-wave mixing.
Abstract: Aluminum nitride (AlN) is an appealing nonlinear optical material for on-chip wavelength conversion. Here we report optical frequency comb generation from high-quality-factor AlN microring resonators integrated on silicon substrates. By engineering the waveguide structure to achieve near-zero dispersion at telecommunication wavelengths and optimizing the phase matching for four-wave mixing, frequency combs are generated with a single-wavelength continuous-wave pump laser. Further, the Kerr coefficient (n2) of AlN is extracted from our experimental results.
TL;DR: The results confirm the advantage of using cryogenic cooling of the lithium niobate crystal that significantly reduces the THz absorption, enabling the scaling of THz pulse energies to the millijoule level via OR.
Abstract: We demonstrate highly efficient terahertz (THz) generation by optical rectification (OR) of near-optimum pump pulses centered at 1.03 μm in cryogenically cooled lithium niobate. Using a close to optimal pulse duration of 680 fs and a pump energy of 1.2 mJ, we report conversion efficiencies above 3.8±0.4%, which is more than an order of magnitude higher than previously reported. The results confirm the advantage of using cryogenic cooling of the lithium niobate crystal that significantly reduces the THz absorption, enabling the scaling of THz pulse energies to the millijoule level via OR.
TL;DR: An optical fiber magnetic field sensor based on the single-mode-multimode-single-mode (SMS) structure and magnetic fluid (MF) is proposed and demonstrated and investigated.
Abstract: An optical fiber magnetic field sensor based on the single-mode-multimode-single-mode (SMS) structure and magnetic fluid (MF) is proposed and demonstrated. By using a piece of no-core fiber as the multimode waveguide in the SMS structure and MF sealed in a capillary tube as the magnetic sensitive media, which totally immersing the no-core fiber, an all-fiber magnetic sensor was fabricated. Interrogation of the magnetic field strength can be achieved either by measuring the dip wavelength shift of the transmission spectrum or by detecting the transmission loss at a specific wavelength. A demonstration sensor with sensitivities up to 905 pm/mT and 0.748 dB/mT was fabricated and investigated. A theoretical model for the design of the proposed device was developed and numerical simulations were performed.
TL;DR: A method for determining the core and cladding refractive indices of a microring resonator from its measured quasi-transverse electric and magnetic resonant modes and uses the singular value decomposition method to find the best fit parameters for the measured data.
Abstract: We present a method for determining the core and cladding refractive indices of a microring resonator from its measured quasi-transverse electric and magnetic resonant modes. We use single wavelength reflective microrings to resolve the azimuthal order ambiguity of the measured resonances. We perform accurate electromagnetic simulations to model the dependence of the resonances on geometrical and material parameters. We linearize the model and use the singular value decomposition method to find the best fit parameters for the measured data. At 1550 nm, we determine n(Si(3)N(4))=1.977±0.003 for stoichiometric silicon nitride deposited using low-pressure chemical vapor deposition (LPCVD) technique and n(SiO(x))=1.428±0.011 for plasma-enhanced chemical vapor deposition (PECVD) oxide. By measuring the temperature sensitivities of microring resonant modes with different polarizations, we find the thermo-optic coefficient of the stoichiometric silicon nitride to be dn(Si(3)N(4))/dT=(2.45±0.09)×10(-5) (RIU/°C) and the PECVD oxide to be dn(SiO(x))/dT=(0.95±0.10)×10(-5) (RIU/°C).
TL;DR: The first theoretical modeling of the full spectral-temporal dynamics of octave-spanning parametric microresonator comb generation through use of the Lugiato-Lefever model extended to include higher-order dispersion and self-steepening is performed.
Abstract: We perform the first theoretical modeling of the full spectral-temporal dynamics of octave-spanning parametric microresonator comb generation through use of the Lugiato-Lefever model extended to include higher-order dispersion and self-steepening. We show that three distinct stages are necessary to achieve single-pulse modelocking and discuss the dispersion characteristics required for ultrabroadband, stabilized comb generation. Our simulations agree well with previous experimental demonstrations and predict many of the observed features, including multipulse generation, dispersive wave generation, modelocking, and comb stabilization.
TL;DR: The proposed nanoantenna metasurface can find applications in many optical devices, such as birefringent modulators, waveplates, polarizers, and splitters.
Abstract: An infrared reflectarray metasurface with engineered birefringent behavior is demonstrated. The array reradiates incoming light into two orthogonal, linearly polarized reflections. The reflectarray is composed of rectangular metallic patch nanoantennas placed on top of a grounded dielectric stand-off layer. The patches are designed to locally manipulate the phase front of the incoming wave. They tailor the reflection phase to transform the phase front on the surface to the one desired for both orthogonal polarizations at the same time. The proposed nanoantenna metasurface can find applications in many optical devices, such as birefringent modulators, waveplates, polarizers, and splitters.
TL;DR: Two classes of scalar, stochastic sources are introduced, each capable of producing far fields with intensities forming rings, and the new beams are of importance for optical methods of particle manipulation.
Abstract: Two classes of scalar, stochastic sources are introduced, each capable of producing far fields with intensities forming rings. Although the Bessel–Gaussian and the Laguerre–Gaussian Schell-model sources are described by two different math models, the behavior of their degrees of coherence and, hence, the shapes of their far fields are qualitatively similar. The new beams are of importance for optical methods of particle manipulation.
TL;DR: It is demonstrated that the resonances of infrared plasmonic antennas can be tuned or switched on/off by taking advantage of the thermally driven insulator-to-metal phase transition in vanadium dioxide (VO(2).
Abstract: We demonstrate that the resonances of infrared plasmonic antennas can be tuned or switched on/off by taking advantage of the thermally driven insulator-to-metal phase transition in vanadium dioxide (VO2). Y-shaped antennas were fabricated on a 180 nm film of VO2 deposited on a sapphire substrate, and their resonances were shown to depend on the temperature of the VO2 film in proximity of its phase transition, in good agreement with full-wave simulations. We achieved tunability of the resonance wavelength of approximately 10% (>1 μm at λ∼10 μm).
TL;DR: The effect of detector array size on resolution and signal collection efficiency of image scanning microscopy based on pixel reassignment is studied and it is shown how the method can be employed if there is a Stokes shift in fluorescence emission wavelength.
Abstract: The effect of detector array size on resolution and signal collection efficiency of image scanning microscopy based on pixel reassignment is studied. It is shown how the method can also be employed if there is a Stokes shift in fluorescence emission wavelength. With no Stokes shift, the width of the point spread function can be sharpened by a factor of 1.53, and its peak intensity increased by a factor of 1.84.
TL;DR: In the authors' experiments, the structure corroded for ~1620 s is found to have maximum sensing performance and the figure of merit of the sensing system is employed to evaluate the sensing performance comprehensively.
Abstract: Magnetic field sensing based on magnetic fluid (MF) and a singlemode-multimode-singlemode (SMS) fiber structure is proposed. The sensitivity of the proposed sensing system can be enhanced by corroding the cladding of the multimode fiber of the SMS fiber structure. The achieved maximum magnetic field sensitivity of our experimental structures is -16.86 pm/Oe as the fiber is corroded for 1680 s. The visibility of the interference dip for the MF-clad SMS fiber structure decreases with corrosion time. Considering the trade-off between sensitivity and visibility, the figure of merit of the sensing system is employed to evaluate the sensing performance comprehensively. In our experiments, the structure corroded for ~1620 s is found to have maximum sensing performance.
TL;DR: A simple approach to broadband generation of orbital angular momentum (OAM) carrying vector beams based on compact metamaterials that consists of two concentric rings in a gold film, where each ring is composed of subwavelength rectangular apertures with gradually varied orientation.
Abstract: We propose a simple approach to broadband generation of orbital angular momentum (OAM) carrying vector beams based on compact metamaterials. It consists of two concentric rings in a gold film, where each ring is composed of subwavelength rectangular apertures with gradually varied orientation. The subwavelength rectangular aperture serves as a localized spatial polarizer. We show the generation of different OAM-carrying vector beams with OAM charge number and polarization order varying from -3 to +3 using a 11.2×11.2 μm device. The extinction ratio can exceed 20 dB, and the operation bandwidth (1500 nm) can cover from 1000 to 2500 nm (from near-infrared to mid-infrared). The device provides three degrees of freedom (polarization order l, polarization of input beam σ, and initial orientation angle α(0)) to flexibly generate different OAM-carrying vector beams. We can use a single device to generate two OAM-carrying vector beams with opposite charge sign of OAM by simply controlling the polarization of the input beam. We further study the performance dependence of the designed metamaterials on the offset of the initial orientation angle, length, and width of the rectangular apertures. The obtained results indicate favorable fabrication tolerance.
TL;DR: The lack of phase coherence across the seemingly regular array of pulses is proved by measuring the degree of first-order coherence using a Young's-type interference experiment and proving the existence of spectral fluctuations that remain unresolved in conventional time-averaged ensemble measurements.
Abstract: We report on experimental studies of coherence and fluctuations in noise-like pulse trains generated by ultrafast fiber oscillators. By measuring the degree of first-order coherence using a Young's-type interference experiment, we prove the lack of phase coherence across the seemingly regular array of pulses. We further quantify the pulse-to-pulse fluctuations by recording the single-shot spectra of the megahertz pulse train, and experimentally demonstrate the existence of spectral fluctuations that remain unresolved in conventional time-averaged ensemble measurements. Phase incoherence and spectral fluctuations are contrasted with quantified coherence and spectral stability when the laser is soliton mode-locked.
TL;DR: A method for measurement of a magnetic field by combining photonic crystal fibers (PCFs) and magnetic fluid is presented and experimentally demonstrated and possesses high sensitivity and low cost.
Abstract: A method for measurement of a magnetic field by combining photonic crystal fibers (PCFs) and magnetic fluid is presented and experimentally demonstrated. The magnetic fluid is filled into the air holes of the cladding layer in the PCF. Due to the tunable refractive index property of the magnetic fluid, the refractive index difference between the fiber core and cladding layer is changed with the external magnetic field. The magnetic field can be directly detected by measuring the intensity of the transmission light. A series of magnetic fields with different strengths have been measured with the sensor. The experimental results show that a resolution of up to 0.09 Oe is achieved, and a good repetition is demonstrated experimentally. Compared with other expensive methods, the proposed method possesses high sensitivity and low cost.
TL;DR: Ptychography is combined with optical image encryption for the first time and their complex-amplitude functions can serve as a kind of secret keys that lead to the enlarged key space and the enhanced system security.
Abstract: Ptychography is combined with optical image encryption for the first time. Due to the nature of ptychography, not only is the interferometric optical setup that is usually adopted not required any more, but also the encryption for a complex-valued image is achievable. Considering that the probes overlapping with each other is the crucial factor in ptychography, their complex-amplitude functions can serve as a kind of secret keys that lead to the enlarged key space and the enhanced system security. Further, since only introducing the probes into the input of common system is required, it is convenient to combine ptychography with many existing optical image encryption systems for varied security applications.
TL;DR: It is demonstrated, both numerically and experimentally, that metal-insulator-metal configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus supporting gap-surface plasmon resonances, can be designed to function as reflective broadband half-wave plates.
Abstract: We demonstrate, both numerically and experimentally, that metal–insulator–metal configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus supporting gap-surface plasmon resonances, can be designed to function as reflective broadband half-wave plates. Using gold as the metal, the constructed wave plates in the near-infrared regime show scalability, bandwidth of ∼20% of the design wavelength, and theoretical reflectivity above 85%, while a reflectivity of ∼50% is experimentally measured.
TL;DR: An efficient method to design apodized grating couplers with Gaussian output profiles for efficient coupling between standard single mode fibers and silicon chips is presented.
Abstract: We present an efficient method to design apodized grating couplers with Gaussian output profiles for efficient coupling between standard single mode fibers and silicon chips. An apodized grating coupler using fully etched photonic crystal holes on the silicon-on-insulator platform is designed, and fabricated in a single step of lithography and etching. An ultralow coupling loss of -1.74 dB (67% coupling efficiency) with a 3 dB bandwidth of 60 nm is experimentally measured.
TL;DR: This is the longest laser wavelength at which graphene-based passive mode-locking has been demonstrated to date and it is reported, for the first time, femtosecond pulse generation from a graphene mode-locked Cr:ZnSe laser at 2500 nm.
Abstract: We report, for the first time to our knowledge, femtosecond pulse generation from a graphene mode-locked Cr:ZnSe laser at 2500 nm. To minimize the insertion losses at the lasing wavelength, high-quality monolayer graphene transferred on a CaF2 substrate was used in the experiments. Once mode-locking was initiated, the laser generated a stable train of 226 fs pulses with a time–bandwidth product of 0.39. The mode-locked laser operated at a pulse repetition rate of 77 MHz and produced 80 mW output power with an incident pump power of 1.6 W. To our knowledge, this is the longest laser wavelength at which graphene-based passive mode-locking has been demonstrated to date.