Enhancing the spectral sensitivity of interferometers using slow-light media

We demonstrate experimentally that the spectral sensitivity of an interferometer can be greatly enhanced by introducing a slow-light medium into it. The experimental results agree very well with theoretical predictions that the enhancement factor of the spectral sensitivity is equal to the group index n(g) of the slow-light medium.     

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).     

Micro-scale Fabry–Perot interferometer with high spectral resolution and tunable transmission frequency via chiral waveguide-emitter coupling

A micro-scale Fabry–Perot interferometer with high spectral resolution and tunable transmission frequency is proposed. In this scheme, two partially reflecting mirrors with a separation of several wavelengths is fabricated in a waveguide, and a two-level emitter is located between the mirrors and coupled to the waveguide with chiral interaction. We analytically show that the single emitter plays the role of a strongly dispersive medium and the full width at half maximum (FWHM) of the transmission fringes around the resonance frequency of the emitter can be narrowed by 5 orders of magnitude. The proposed micro-scale interferometer can have the same spectral resolution as meter-scale traditional interferometers. We also show that the central frequency of the narrowed transmission fringe can be tuned by adjusting the asymmetry of the emitter-waveguide coupling. Our scheme has potential applications in the fields of integrated optical circuit and quantum information processing.     

Sagnac棱镜角公差与干涉光谱仪光谱分辨率的关系分析

根据干涉成像光谱仪光谱分辨率对角向差的要求,通过对实体Sagnac干涉仪结构和光路进行分析,从三个相互垂直的方向出发,研究了光谱分辨率和棱镜角公差之间存在的关系;并推导了满足光谱仪光谱分辨率要求的实体Sagnac干涉棱镜的角公差公式;用实例说明了关系式的应用方法,如果不考虑棱镜变形引起的色散及棱镜的面型误差和付氏镜的残余像差的影响,而只考虑棱镜的角误差对光谱分辨率的影响,则通常情况下干涉棱镜的角公差要求较严,约20″以内.     

Sagnac interferometer with adaptive nonlinear detection

We present a Sagnac interferometer that uses a nonlinear adaptive medium for detection. The nonlinearity of the medium being characterized by a finite response time, the detection has a finite frequency bandwidth determining two operational regimes: at low variation rate of the rotation speed the interferometer operates in the slow-light regime, providing a direct measurement of the angular acceleration. For high variation rate of the rotation speed the amplitude of the alternating component of the Sagnac phase shift is instead detected, with the continuous and low-frequency components filtered out by the adaptive nonlinear process in the medium.     

A tuneable 600 GHz bandpass-filter with large free-spectral-range

Considering recent ozon problems in the upper atmosphere the measuring technique in the spectral range of about some hundred GHz gains in importance increasingly. Ozon concentration is measured by high sensitive radiometers consisting of several quasioptical components. A tunable bandpass filter represents a versatile component in such a test assembly. The bandpass filter is realized as interferometer and can be adapted to its respective application by individual tuning of its coupling links. Metallic meshes of different aperture size are used for injection and feedback of the electromagnetic wave. A stepper motor control serves for the integration in an automatic measurement setup. The construction principles of the interferometer are summarized. An analysis of the component by means of plane wave theory is extended to a theory of diffraction. Finally the theoretical approach is verified on an application as single-sideband filter comparing the measured results with the computed values of the simulation.     

Guided subwavelength slow-light mode supported by a plasmonic waveguide system

We introduce a plasmonic waveguide system, which supports a subwavelength broadband slow-light guided mode with a tunable slowdown factor at a given wavelength. The system consists of a metal-dielectric-metal (MDM) waveguide side-coupled to a periodic array of MDM stub resonators. The slowdown factor at a given wavelength can be tuned by adjusting the geometrical parameters of the system. In addition, there is a trade-off between the slowdown factor and the propagation length of the supported optical mode. Finally, we show that light can be coupled efficiently from a conventional MDM waveguide to the plasmonic waveguide system.     

干涉型计算层析成像光谱仪的图像重建

针对干涉型计算层析成像光谱仪(CTII)提出了一种光谱图像数据立方体的重建方法。干涉型计算层析成像光谱仪是一种将空间调制傅里叶变换成像光谱仪(FTIS)的原理与计算层析成像光谱仪(CTIS)的原理相结合的一种新型成像光谱仪,具有高通量、高光谱分辨力以及高空间分辨力的特点。分析和讨论了干涉型计算层析成像光谱仪的工作原理以及获取图像的特征,介绍了光谱图像数据立方体的重建方法。根据多角度投影数据的特点提出采用卷积反投影计算层析成像图像重建算法,给出了图像重建步骤以及相应的数学表达式。对D65光源照明条件下的396×396像素目标进行了仿真实验,投影角度为0~180°,步长为0.5°,列出了仿真实验部分结果。实验结果验证了干涉型计算层析成像光谱仪及其图像重建算法的可行性。     

大孔径空间外差干涉光谱成像技术多谱段成像仿真

在大孔径空间外差干涉光谱成像技术（LASHIS）的基础上提出了一种多谱段成像方案.其采用LASHIS的外差探测原理,一方面,可通过较少的采样点数实现很高的光谱分辨率,保留了LASHIS的高光谱分辨率、高稳定性和高探测灵敏度的特点;另一方面,利用光栅的多级衍射性质,实现同一系统的多谱段同时探测,拓宽了光谱探测范围.首先,阐述了LASHIS多谱段成像方案的基本原理;然后,分析了多谱段探测与谱段解混方式;最后,对该方案进行了计算机仿真模拟,通过ZEMAX光线追迹的干涉图结果与理论计算结果相符合,验证了方案的正确性.基于LASHIS的多谱段成像方案所具有的高光谱分辨率、高探测灵敏度以及可实现同一系统的多谱段同时探测特点,尤其适合温室气体等高稳定性、高探测灵敏度的多谱段高光谱探测应用.     

Ultra-compact TDLAS humidity measurement cell with advanced signal processing

In this paper, tunable diode laser absorption spectroscopy humidity measurements with an ultra-compact measurement cell are presented. The optical path length is 2 cm. The system uses a vertical cavity surface emitting laser at 1.854 μm. The main limiting factor of the humidity resolution is not the noise but interference fringes produced by reflecting surfaces. Next to the system setup, a novel rejection method to eliminate these fringes, based on Fourier domain analysis of the absorption line, is described. In contrast to other fringe rejection methods, the presented method is able to handle fringes, whose free spectral range is in the range of the half width of the absorption line. The achievable humidity resolution for the presented cell is below 0.25 % relative humidity at room temperature.     

Balancing interferometers with slow-light elements

In this Letter we show that interferometers with unbalanced arm lengths can be balanced using optical elements with appropriate group delays. For matched group delays of the arms, the balanced interferometer becomes insensitive to the frequency noise of the source. For experimental illustration, a ring resonator is employed as a slow-light element to compensate the arm-length mismatch of a Mach-Zehnder interferometer. An arm-length mismatch of 9.4?m is compensated by a ring resonator with a finesse of 70 and a perimeter of 42?cm.     

Precision measurement of the refractive index of air with frequency combs

We report an experimental method for high-precision refractive index measurement using a Michelson interferometer setup with a femtosecond optical frequency comb as the light source. We demonstrate this technique by measuring the refractive index of air, under different pressures, inside a 29 m multipass cell. Both spectral and temporal interferograms are recorded. The method has a sensitivity of 6.2 x 10(-9) and a standard error of 7.4 x 10(-8) at fixed parameters (pressure and temperature). The possible variation of the density factor from the Lorentz-Lorenz equation is also discussed.     

48-波长线形腔多波长掺铒光纤激光器

采用两个光纤环镜作为腔镜构成线形腔可调谐多波长掺铒光纤激光器.将铒光纤浸入77 K的液氮中,选择可调谐光纤环镜作为输出腔镜,利用Mach-Zehnder干涉仪的滤波特性和输出端光纤环镜反射率的宽带可调特性,获得了波长间隔～0.4 nm、最大波长数目48个的多波长激光的稳定输出,同时实现了在1525～1555 nm范围内,多波长激光运转区域的灵活调节.     

马赫-曾德光纤干涉光谱测量系统的设计

光纤干涉光谱仪由于其体积小、分辨率高、抗电磁干扰和耐高温的特点,广泛应用于各种领域的物质检测中。设计了一种使用电动光纤延迟线实现光程差调制的基于Mach-Zehnder光纤干涉仪的光谱测量系统。介绍了系统的构成,推导了基于该系统的光谱计算公式以及分辨率计算公式。搭建了实验平台,对一宽谱光源进行测试,并采集其干涉信号,解调出了相应的光谱图。对结果进行分析,当最大光程差变为0.479 5 cm时,光谱分辨率为2.085 4 cm-1。电动光纤延迟线的性能在很大程度上决定了最终解调出的光谱的分辨率、精度,尤其是速度和延时时间的精度和线性度。     

Broad tunability of erbium-doped fiber ring laser based on few-mode polarization maintaining fiber Sagnac interferometer

A highly stable erbium-doped fiber (EDF) ring laser with broad tunability is proposed and experimentally demonstrated. It is accomplished by using a few-mode polarization maintaining fiber (FM-PMF) Sagnac interferometer, in which simultaneous Sagnac and modal interferences occur. Continuously tunable output is obtained by exerting the longitudinal strain on the FM-PMF in combination with adjusting a polarization controller (PC) next to the FM-PMF in the Sagnac interferometer. In addition, the adverse impact of uneven EDF gain distribution on the laser output power uniformity may be offset by the PC-dependent transmission loss of the FM-PMF Sagnac interferometer and reflected backward C-band amplifier spontaneous emission from the pumped EDF. During the whole tuning process, the tunable span covers more than 40 nm with the output power flatness better than 1 dB, 3 dB spectral linewidth narrower than 0.1 nm and optical signal-to-noise extinction ratio higher than 45 dB.     

Wavelength-dispersive device based on a Fourier-transform Michelson-type arrayed waveguide grating

We propose a new type of arrayed waveguide grating (AWG) device that operates as a Fourier-transform (FT) spectrometer without the need of scanning elements. The large input aperture size typical of a FT spectrometer eliminates the requirement for a narrow single-mode input waveguide while still achieving high spectral resolution with a markedly increased light-gathering capability (etendue). An example of the device with a resolution of 0.07 nm (approximately 10 GHz) and designed for a silicon-on-insulator platform is presented. The calculated spectra show no noticeable deterioration for aperture widths as large as 40 microm, yielding more than a 50-fold increase in aperture size compared with conventional AWG or echelle grating based devices at the equivalent resolution.     

Tunable light source for coherent anti-Stokes Raman scattering microspectroscopy based on the soliton self-frequency shift

We present a photonic crystal fiber (PCF)-based light source for generating tunable excitation pulses (pump and Stokes) that are applicable to coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The laser employed is an unamplified Ti:sapphire femtosecond laser oscillator. The CARS pump pulse is generated by spectral compression of a laser pulse in a PCF. The Stokes pulse is generated by redshifting a laser pulse in a PCF through the soliton self-frequency shift. This setup allows for probing up to 4000 cm(-1) with a spectral resolution of approximately 25 cm(-1). We characterize the stability and robustness of CARS microspectroscopy employing this light source.     

法布里—珀罗干涉仪研究光谱线超精细结构的理论分析

在前一篇论文^[1]中,我们提出了利用法布—珀罗干涉仪(以下简称F—P干涉仪)研究光谱线超精细结构的一种新方法。在本文中我们将就表征F—P干涉仪特性的二个参量——自由光谱范围与分辨本领,把它与经典方法^[2,3],作一理论上的分析与比较。其结论是:二者都必须遵守自由光谱范围。但在相同条件下,我们方法所能达到的分辨本领要比经典方法提高一倍。     

高灵敏度色散型慢光干涉仪的仿真研究

设计了一种基于慢光介质色散特性的新型干涉装置,研究其物理过程,证明利用慢光介质可以有效提高干涉仪的光谱灵敏度,并且光谱灵敏度与慢光介质的群折射率成正比。同时,进一步具体分析慢光介质GaAs的色散特性,给出研究色散型慢光干涉仪的一般性方法,并证明使用慢光介质GaAs在近红外区域可以提高光谱灵敏度2倍左右。     

高灵敏度色散型慢光干涉仪的仿真研究

设计了一种基于慢光介质色散特性的新型干涉装置,研究其物理过程,证明利用慢光介质可以有效提高干涉仪的光谱灵敏度,并且光谱灵敏度与慢光介质的群折射率成正比。同时,进一步具体分析慢光介质GaAs的色散特性,给出研究色散型慢光干涉仪的一般性方法,并证明使用慢光介质GaAs在近红外区域可以提高光谱灵敏度2倍左右。