Three-dimensional imaging using spectral encoding heterodyne interferometry

We present a novel heterodyne approach for performing fast, three-dimensional spectrally encoded imaging. Volumetric data of a volunteer's finger and of coin surfaces were acquired at a rate of 5 volume sets per second with a depth resolution of 145 microm.     

Quadratic grating apodized photon sieves for simultaneous multiplane microscopy

We present a new type of imaging device, named quadratic grating apodized photon sieve (QGPS), used as the objective for simultaneous multiplane imaging in X-rays. The proposed QGPS is structured based on the combination of two concepts: photon sieves and quadratic gratings. Its design principles are also expounded in detail. Analysis of imaging properties of QGPS in terms of point-spread function shows that QGPS can image multiple layers within an object field onto a single image plane. Simulated and experimental results in visible light both demonstrate the feasibility of QGPS for simultaneous multiplane imaging, which is extremely promising to detect dynamic specimens by X-ray microscopy in the physical and life sciences.     

Multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser

We demonstrate a novel multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser at 1230 nm. By acquiring the whole nonlinear spectrum in the visible and near-NIR region, this novel technique allows a combination of different imaging modalities, including second-harmonic generation, third-harmonic generation, and multiple-photon fluorescence. Combined with the selected excitation wavelength, which is located in the IR transparency window, this microscopic technique can provide high penetration depth with reduced damage and is ideal for studying living cells.     

Simultaneous multichannel nonlinear imaging: combined two-photon excited fluorescence and second-harmonic generation microscopy

Simultaneous two-photon excited fluorescence (TPF) and second-harmonic generation (SHG) imaging is demonstrated using a single femtosecond laser and a scanning microscope. This composite nonlinear microscopic technique was applied to imaging DNA and chromosomes, and it was shown that the two different interaction mechanisms provide complementary information on the structure and nonlinear properties of these biological materials, beyond that achievable using either TPF or SHG imaging alone. The use of separate modes of detection, in reflection and transmission respectively, and the simultaneous nature of the acquisition of the two images allows pure TPF and SHG images in precise registration to be obtained.     

Simultaneous fs pulse spectral broadening and third harmonic generation in highly nonlinear fibre: experiments and simulations

Experiments and numerical simulations are used to study non-phasematched single-mode third harmonic generation occurring simultaneously with fs pulse spectral broadening in highly nonlinear fibre. Pump pulses around 100 fs at 1560 nm injected into sub-5 cm lengths of commercially-available highly nonlinear fibre are observed to undergo spectral broadening spanning over 700 nm at the -30 dB level, and to simultaneously generate third harmonic radiation around 520 nm. Simulations based on a generalized nonlinear envelope equation are shown to well reproduce the spectral structure of the broadened pump pulses and the generated third harmonic signal. PACS 42.65.-k; 42.81.Dp     

Proposal for nonlinear refractive index measurement using spectral ratio in modulated OFRR dynamics

A novel method for measuring the nonlinear refractive index of an optical fiber using a spectral ratio between the modulation frequency and a harmonic component in a modulated optical fiber ring resonator (OFRR) is proposed. The spectral ratio between the modulation frequency and the 2nd-harmonics generated by phase-modulation through the OFRR is increased with increasing the input light power and has peaks above 5 W input power, however, the peaks was shifted to the lower input power below 1 W by averaging taken into account of the phase distribution. A experimental setup consisted of an OFRR system and an Ar-laser as a pump light source was used to determine the nonlinear refractive index of an optical fiber. In the experimental results, the peaks of the spectral ratio as a function of the input power was found out at 0.8 W and 0.45 W of the input power corresponding to the input source line at 488.0 nm and 514.5 nm, respectively. The profile was similar to that obtained by the simulation and the nonlinear refractive index of a optical fiber was determined as 1.0 × 10⁻²² m²/V² by a relationship between the input power giving the peak and the nonlinear refractive index.     

Multimodal nonlinear optical microscopy

Because each nonlinear optical (NLO) imaging modality is sensitive to specific molecules or structures, multimodal NLO imaging capitalizes the potential of NLO microscopy for studies of complex biological tissues. The coupling of multiphoton fluorescence, second‐harmonic generation, and coherent anti‐Stokes Raman scattering (CARS) has allowed investigation of a broad range of biological questions concerning lipid metabolism, cancer development, cardiovascular disease, and skin biology. Moreover, recent research shows the great potential of using a CARS microscope as a platform to develop more advanced NLO modalities such as electronic‐resonance‐enhanced four‐wave mixing, stimulated Raman scattering, and pump‐probe microscopy. This article reviews the various approaches developed for realization of multimodal NLO imaging as well as developments of new NLO modalities on a CARS microscope. Applications to various aspects of biological and biomedical research are discussed.     

Simultaneous spectral recovery of long-period grating sensor array using optical time-division multiplexing

A novel sensor network based on optical time-division multiplexing (OTDM) is proposed and demonstrated for simultaneous sensing of long-period grating (LPG) sensor array with overlapping spectra. A star network is configured by pairing a circulator with each of the LPG sensors. Experiments are conducted by measuring the changes of surrounding refractive index at different sensors, which show excellent agreement between the demultiplexed and pristine transmission spectrum of the LPG sensors.     

Accuracy of spectrum estimate in fluorescence spectral microscopy with spectral filters

We establish the accuracy of the spectrum that is estimated with an inexpensive fluorescence spectral microscope utilizing a small set of spectral filters [Soriano et al, Opt. Exp. 10, 1458–1464 (2002)]. The spectrum at an arbitrary image location of the fluorescent sample is estimated as a linear superposition of basis spectra that are derived by singular value decomposition (SVD) or principal component analysis (PCA) from a spectral library of fluorescence spectra. Estimation performance is analyzed as a function of library statistics, filter selection and sequencing, minimum negativity constraint and signal to noise ratio (SNR) of fluorescence image. We consider image SNR degradations that arise from weakening of image intensity, additive Gaussian noise, intensity-dependent Poisson noise and quantization errors. The recovery of specific spectral features like spectral resolution, general similarity and peak alignments, is assessed using Linfoot’s criteria of fidelity, structural content, and correlation. We found that estimation with SVD basis spectra is more robust against image noise than that with PCA basis spectra. However for high SNR images, accurate estimation is achieved more quickly with PCA basis spectra and with better response to the application of minimum negativity constraint.     

Engineering the spectral profile of photon pairs by using multi-stage nonlinear interferometers

Using the quantum interference of photon pairs in N-stage nonlinear interferometers(NLIs), the contour of the joint spectral function can be modified into an islands pattern. We perform two series of experiments. One is that all of the nonlinear fibers in pulse pumped NLIs are identical; the other is that the lengths of N pieces of nonlinear fibers are different. We not only demonstrate how the pattern of spectral function changes with the stage number N, but also characterize how the relative intensity of island peaks varies with N. The results well agree with theoretical predictions, revealing that the NLI with lengths of N pieces of nonlinear fibers following binomial distribution can provide a better active filtering function. Our investigation shows that the active filtering effect of multi-stage NLI is a useful tool for efficiently engineering the factorable two-photon state—a desirable resource for quantum information processing.     

联合谱域与深度域光谱相位显微方法

提出了一种联合谱域与深度域光谱相位显微方法, 该方法利用谱域相位信息克服2π歧义, 并结合深度域相位信息, 以实现高动态范围、高灵敏度的相位检测. 首先通过理论推导和信号模拟, 进行了深度域相位和谱域相位的灵敏度比较, 证明了深度域相位在灵敏度上要高于谱域相位. 进而详细介绍了联合谱域与深度域光谱相位显微方法. 最后通过盖玻片和光学分辨率板实验验证了所提出的联合谱域 与深度域光谱相位显微方法能够在实现高动态测量范围的同时保持高相位灵敏度. 关键词： 光谱相位显微方法 动态范围 灵敏度     

Low-intensity nonlinear spectral effects in compton scattering

Nonlinear effects are known to occur in Compton scattering light sources, when the laser normalized potential A approaches unity. In this Letter, it is shown that nonlinear spectral features can appear at arbitrarily low values of A, if the fractional bandwidth of the laser pulse Δ??1 is sufficiently small to satisfy A2Δ??1. A three-dimensional analysis, based on a local plane wave, slow-varying envelope approximation, enables the study of these effects for realistic interactions between an electron beam and a laser pulse, and their influence on high-precision Compton scattering light sources.     

Simultaneous spectral editing for gamma-aminobutyric acid and taurine using double quantum coherence transfer

Conventional double quantum (DQ) editing techniques recover resonances of one metabolite at a time and are thus inefficient for monitoring metabolic changes involving several metabolites. A DQ coherence transfer double editing sequence using a dual-band DQ coherence read pulse is described here. The sequence permits simultaneous spectral editing for two metabolites with similar J coupling constants in a single scan. Simultaneous editing for taurine and gamma-aminobutyric acid (GABA) is demonstrated using solution phantoms and rat brain tissue. Selectivity of the double editing sequence for the target metabolites is as good as that achieved using conventional DQ editing which selects each metabolite individually. With experimental parameters of the double editing sequence chosen to optimize GABA editing, the sensitivity for GABA detection is the same as that with GABA editing only, while the sensitivity for taurine detection is decreased slightly compared to that with taurine editing only.     

Multifocal optical-resolution photoacoustic microscopy in vivo

Although ultrasound arrays have been exploited in photoacoustic imaging to improve imaging speed, ultrasound-array-based optical-resolution photoacoustic microscopy (OR-PAM) has never been achieved previously to our knowledge. Here we present our development of multifocal OR-PAM using a microlens array for optical illumination and an ultrasound array for photoacoustic detection. Our system is capable of imaging hemoglobin concentration and oxygenation in individual microvessels in vivo at high speed. Compared with a single focus, multiple foci reduce the scanning load and increase the imaging speed significantly. The current multifocal system can acquire 1000 × 500 × 200 voxels at ~10 μm lateral resolution within 4 min.     

Estimation of spectral transmittance curves from RGB images in color digital holographic microscopy using speckle illuminations

Optical Review - We investigate the estimation of spectral transmittance curves in color digital holographic microscopy using speckle illuminations. In color digital holography, it has the...     

Solitons and spectral renormalization methods in nonlinear optics

Localized wave solutions, often referred to as solitary waves or solitons, are important classes of solutions in nonlinear optics. In optical communications, weakly nonlinear, quasi-monochromatic waves satisfy the “classical” and the “dispersion-managed” nonlocal nonlinear Schrödinger equations, both of which have localized pulses as special solutions. Recent research has shown that mode-locked lasers are also described by similar equations. These systems are variants of the classical nonlinear Schrödinger equation, appropriately modified to include terms which model gain, loss and spectral filtering that are present in the laser cavity. To study their remarkable properties, a computational method is introduced to find localized waves in nonlinear optical systems governed by these equations.     

Simultaneous measurement of anomalous group-velocity dispersion and nonlinear coefficient in optical fibers using soliton-effect compression

We present a novel and simple method to measure both the value of the second-order dispersion coefficient and the nonlinear coefficient in optical fibers. This method is based on the higher-order soliton-effect pulse compression phenomenon and is valid for dispersion values greater than 0.5 ps/km/nm. A non-zero dispersion-shifted fiber, a standard single-mode fiber and a highly-dispersive highly-nonlinear fiber have been measured using this method.     

Stochastic nonlinear aeroelastic analysis of a supersonic lifting surface using an adaptive spectral method

An adaptive stochastic spectral projection method is deployed for the uncertainty quantification in limit-cycle oscillations of an elastically mounted two-dimensional lifting surface in a supersonic flow field. Variabilities in the structural parameters are propagated in the aeroelastic system which accounts for nonlinear restoring force and moment by means of hardening cubic springs. The physical nonlinearities promote sharp and sudden flutter onset for small change of the reduced velocity. In a stochastic context, this behavior translates to steep solution gradients developing in the parametric space. A remedy is to expand the stochastic response of the airfoil on a piecewise generalized polynomial chaos basis. Accurate approximation andaffordable computational costs are obtained using sensitivity-based adaptivity for various types of supersonic stochastic responses depending on the selected values of the Mach number on the bifurcation map. Sensitivity analysis via Sobol' indices shows how the probability density function of the peak pitch amplitude responds to combined uncertainties: e.g. the elastic axis location, torsional stiffness and flap angle. We believe that this work demonstrates the capability and flexibility of the approach for more reliable predictions of realistic aeroelastic systems subject to a moderate number of uncertainties.     

Simultaneous spectral calibration of two phase plates

Based on a new model for the spectral calibration of phase plates in pairs, experimental results are presented for three mica plates between 430 and 684 nm. One of the plates is 36.4 μm thick and the remaining two are identical, each being 28.6 μm thick. In the studied spectral range, the retardances varied from 123.5° to 86.8° for the first plate and from 96.9° to 68° for each of the two other plates. Different procedures for performing the calibration of the plates and the special case of two identical plates are discussed.