Fourier-domain low-coherence interferometry for differential mode delay analysis of an optical fiber

We propose a novel mode analysis and differential mode delay measurement method for an optical fiber using Fourier-domain low-coherence interferometry. A spectral interferometer based on a Mach-Zehnder interferometer setup was used with a broadband source and an optical spectrum analyzer to detect relative temporal delays between the guided modes of a few-mode optical fiber by analyzing spectral interference signals. We have shown that experimental results of the proposed method agree well with those results obtained by using a conventional time-domain measurement method. We have demonstrated that this new mode analysis technique has high sensitivity (<60 dB) and very good resolution (<1 ps/m).     

光谱位相相干电场法的参数分析及测量

从光谱位相相干电场重构法（ＳＰＩＤＥＲ）的原理出发,实现了算法、讨论时间延迟、光谱剪切量、滤波窗口宽度、色散量等几个方面的优化选取。当展宽器色散和脉冲宽度一定时,脉冲对的时间延迟存在一个最佳取值范围。相对光谱剪切量在５％～１５％间,滤波窗口宽度为／３,重构出的位相误差最小。对干涉图取平均来减小噪声的影响。用ＳＰＩＤＥＲ算法还原了脉冲的电场和位相信息,由ＳＰＩＤＥＲ测量的脉冲宽度为１７.７ fs。同时为了比较,用自相关法测量了同一脉冲,由自相关曲线可估算出脉冲宽度为１６.８ fs,与ＳＰＩＤＥＲ 的误差比为５．１％,说明了实验的有效性。     

Direct measurement of group dispersion of optical components using white-light spectral interferometry

We present a simple white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the group dispersion of optical components over a wide wavelength range. The technique utilizes an unbalanced Mach-Zehnder interferometer with a component under test inserted in one arm and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference. We measure the absolute group refractive index as a function of wavelength for a quartz crystal of known thickness and the relative one for optical fiber. In the latter case we use a microscope objective in front and a lens behind the fiber and subtract their group dispersion, which is measured by a technique of tandem interferometry including also a Michelson interferometer.     

High-precision index measurement in anisotropic crystals using white-light spectral interferometry

White-light spectral interferometry appears an excellent tool for precise determination of indices, and has already been successfully applied to different isotropic or weakly dispersive materials such as glass or rhodamine films. In this paper, we extend the spectral method to the measurement of anisotropic media with strong dispersion. The method is discussed below and allows an accuracy of the order of 10^-5 on the principal indices of a birefringent silver thiogallate (AgGaS₂) crystal.     

Linear and quadratic dispersion characterization of millimeter-length fibers and waveguides using common-path interferometry

We measured linear and quadratic dispersion on millimeter-length fibers, waveguides, and nanowires based on common-path spectral interferometry. We obtained the linear dispersion parameter, beta', with a relative precision of 1.45 x 10(-4), and extracted the quadratic dispersion parameter, beta', from the Taylor expansion of beta' x beta' values show a discrepancy of < 1% when compared with simulation as well as with measurement results obtained by a conventional Michelson interferometer. Using this method, we experimentally confirmed the sign inversion of the group velocity dispersion of AlGaAs nanowires for what is believed to be the first time.     

Rapid and accurate chromatic dispersion measurement of fiber using asymmetric Sagnac interferometer

We propose and experimentally demonstrate an advanced method for chromatic dispersion measurement of fiber. This technique is based on spectral interferometry by using an asymmetric Sagnac loop and broadband optical source. The chromatic dispersion can be directly obtained from the spectral interferogram seen from an optical spectral analyzer. This method is rapid (<1?s), accurate, simple, low cost, and can provide a large dispersion measurement range.     

Broadband near-infrared fibers dispersion measurement using white-light spectral interferometry

A new technique for an experimental determination of the effective refractive index, group refractive index and dispersion of fibers in a broad near-infrared spectral range is presented. The method is based on a white-light spectral interference which utilizes an unbalanced Michelson interferometer. The effective refractive index is obtained by a direct fitting the cosine function to the spectral interference pattern recorded by a low resolution spectrometer. The method has been tested in the spectral range of 1000-1700 nm both with standard telecommunication fibers and a sample of a photonic fiber. The accuracy of dispersion measurement () exceeds those from the previously reported near-infrared white-light spectral interference methods.     

Measurement of fiber chromatic dispersion using spectral interferometry with modulation of dispersed laser pulses

We propose and experimentally demonstrate a method for fiber dispersion measurement based on the modulation of laser pulses stretched by the fiber under test. The measured spectrum of the modulated pulses is the result of the interference between the stretched pulse spectra shifted by the modulation harmonics. The interference pattern is processed as in Fourier transform spectral interferometry. Unlike to conventional spectral interferometry, environmental conditions do not affect the interferogram due to the lack of any interferometer; additionally, large dispersions can be characterized by the method proposed. Its high accuracy is demonstrated in experimental comparison with the widely used phase shift technique.     

Harmonic interferometry in the visible and UV, based on second- and third-harmonic generation of a 25 ps mode-locked Nd:YAG laser

A second-harmonic interferometer using 25 ps pluses is presented, achieving a sensitivity of 9x10(-10) m in measuring line-integrated dispersion between 1064 and 532 nm. Using the same laser source, a third-harmonic interferometer is demonstrated for the first time to our knowledge, achieving a sensitivity of 7x10(-9) m in measuring line-integrated dispersion between 1064 and 355 nm. The prospect for the development and use of high-harmonic interferometry is discussed.     

Measurements of Key Parameters for Birefringent Single-Mode Optical Fibers by Optical Frequency-Domain Interferometry

This Letter describes measurements of modal birefringence (MB) and polarization mode dispersion (PMD) of a birefringent single-mode optical fiber for use at 1.5 #x03BC;m in optical wavelength. Optical frequency-domain interferometry based on a fixed analyzer method is applied to the measurement of orthogonal polarization components of light guided in the fiber. An amplified spontaneous emission source having an optical wavelength range between 1520 and 1560 nm is used. A channeled interference spectrum is obtained to measure MB and PMD in turn. These two parameters are not frequency-dependent in the above optical wavelength range and MB = 4.0 #x00D7; 10^#x2212;4 and PMD = 1.33 ps/m are obtained.     

双目视觉DIC测量系统的离面位移测量精度

采用离面位移测量精度达到10 nm~20 nm的电子散斑干涉测量系统验证了双目视觉数字散斑相关测量系统的离面位移测量精度。分别用电子散斑干涉测量系统和双目视觉数字散斑相关测量系统同时测量了平板离面位移,并对所测量的位移最大值进行了分析处理及比较。结果表明,双目视觉数字散斑相关测量系统的物体离面位移分布云图与电子散斑干涉测量系统的结果基本相同,且两者位移均方根相差为2.76 m~3.56 m,相对误差为4.59%~7.60%。因此,当被测量物体的离面位移大于4 m时,双目视觉Q400测量系统精度可达到电子散斑干涉测量系统的精度。     

A Criterion for Using Spectral Interferometry in Far Infrared Dispersion Measurements

High precision index measurements (n/n 10^-5) have been recently achieved on an AgGaS₂ crystal using a spectral interferometry technique with a polarized white-light continuum. Up to now, these measurements are limited to the visible and near-infrared range, the most dispersive domain. We investigate the possibility to extend the technique to far infrared index measurements. We demonstrate a criterion depending only on group velocity dispersion and crystal thickness for planning future experiments.     

Crystal-dithering method applied to spectral phase interferometry for direct electric-field reconstruction (SPIDER) for sensitivity enhancement of the pulse phase measurement

The application of a crystal-dithering technique to the spectral phase interferometry for direct electric-field reconstruction (SPIDER) was experimentally demonstrated. Applying a crystal-dithering technique to SPIDER resulted in the improvement of the measurement sensitivity. In our approach, a 20 μm-thick nonlinear crystal for a thin-crystal SPIDER method was replaced by a 100 μm-thick crystal, and the crystal was dithered. The improvement was confirmed by comparing the statistical error of the crystal-dithering SPIDER to the statistical error of the thin-crystal SPIDER method. The dispersion of a BK7 plate was investigated to evaluate the reliability of the proposed approach.     

Single-shot multiple-delay crossed-beam spectral interferometry for measuring extremely complex pulses

We demonstrate a single-shot measurement technique based on spectral interferometry (SI) for measuring the complete intensity and phase vs. time of extremely complex ultrashort laser pulses. Ordinarily, such a method would require an extremely-high-resolution spectrometer, but, by temporally interleaving many SI measurements, each using a different reference-pulse delay, our method overcomes this need. It involves introducing a transverse time delay into the reference pulse by tilting its pulse front transversely to the spectrometer dispersion plane. The tilted reference pulse then gates the unknown pulse by interfering with it at the image plane of a low-resolution imaging spectrometer, yielding an effective increase in the delay range and spectral resolution—by a factor of 30 in our proof-of-principle implementation. Our device achieved a temporal resolution of ~ 130 fs and a temporal range of 120 ps. This simple device has the potential to measure even longer and more complex pulses.     

Many-wavelength interferometry with thousands of lasers for absolute distance measurement

We demonstrate a new technique for absolute distance measurement with a femtosecond frequency comb laser, based on unraveling the output of an interferometer to distinct comb modes with 1?GHz spacing. From the fringe patterns that are captured with a camera, a distance is derived by combining spectral and homodyne interferometry, exploiting about 9000 continuous wave lasers. This results in a measurement accuracy far within an optical fringe (λ/30), combined with a large range of nonambiguity (15?cm). Our technique merges multiwavelength interferometry and spectral interferometry, within a single scheme.     

Distributed measurement of birefringence dispersion in polarization-maintaining fibers

A new method to measure the birefringence dispersion in high-birefringence polarization-maintaining fibers is presented using white-light interferometry. By analyzing broadening of low-coherence interferograms obtained in a scanning Michelson interferometer, the birefringence dispersion and its variation along different fiber sections are acquired with high sensitivity and accuracy. Birefringence dispersions of two PANDA fibers at their operation wavelength are measured to be 0.011 ps/(km nm) and 0.018 ps/(km nm), respectively. Distributed measurement capability of the method is also verified experimentally.     

Spectrally resolved phase-shifting interferometry for accurate group-velocity dispersion measurements

We report an accurate method to measure the group-velocity dispersion (GVD) of transparent materials by use of spectrally resolved phase-shifting interferometry. The GVD of silica glass slide measured using an eight-step phase-shifting algorithm agrees well with that calculated using the Sellmeier dispersion equation over the entire visible wavelength region, with a rms error of < or =0.0036 microm(-2), better than that of other measurement methods reported so far.     

Amplified spontaneous emission enhanced forward stimulated Raman scattering in dye solutions

We study forward stimulated Raman emission from the weakly fluorescent dye 4^′-diethylamino-N-methyl-4-stilbazolium tosylate (DEST) in 1,2 dichloroethane solution excited by a 28-ps, 532-nm Nd:YAG laser. Neat 1,2 dichloroethane emits the first Stokes line at 631 nm with a spectral width of 1.6 nm, corresponding to a Raman shift of 2956 cm^-1. We observe a reduction of spectral width with the addition of DEST in 1,2 dichloroethane solution. The single-pass conversion efficiency for forward Raman emission is as high as 10% in a 1-cm-path-length sample. The pulse duration of forward stimulated Raman emission, measured by a third-order autocorrelation technique, is 10 ps in neat 1,2 dichloroethane, whereas it is ～3 ps for 4×10^-5 mol/l of DEST solution.     

Spectral variation of the birefringence,group birefringence and retardance of a gypsum plate measured using the interference of polarized light

Polarized white light interferometry is used to characterize the wavelength dependence of the birefringence, group birefringence and retardance of a gypsum crystal. Two different calculation schemes are used to extract values of the birefringence across the whole visible spectrum. In the spectral range 435 nm–642 nm, the variation of the gypsum birefringence is fitted to the two terms Cauchy formula and to a fourth order dispersion function. The gypsum birefringence is found to be inversely proportional with wavelength. The experimental method used gives a relative error in finding the gypsum birefringence of an order of 6×10^?4. The wavelength dependence of gypsum group birefringence is also calculated with a relative error of order 5×10^?4. In the same spectral range, the retardance changes by 28π and the gypsum plate introduced halfwave retardance 15 times.