Tuning of external-cavity semiconductor lasers with chirped diffraction gratings

We present a novel scheme of tunable semiconductor laser based on the use of a chirped grating in an external cavity. The chirped grating is fabricated using a simple holographic technique: two Gaussian beams having wavefronts with different radii of curvature are brought to interfere on a photoresist layer. The tuning properties of chirped gratings have been investigated with semiconductor lasers operated with an external cavity. With this type of grating positioned in Littrow configuration, the wavelength selection can be done by translating the grating without any need to rotate it. This cavity configuration provides a tunable output beam with an angle of propagation that is independent of the wavelength. The translation of chirped gratings was shown to tune a visible diode laser and an infrared diode laser over the same spectral band as the conventional tuning scheme where an unchirped grating is rotated.     

Synthesis of fiber Bragg grating parameters from experimental reflectivity: a simplex approach and its application to the determination of temperature-dependent properties

A simple, accurate, and fast method to synthesize the physical parameters of a fiber Bragg grating numerically from its reflectivity is proposed and demonstrated. Our program uses the transfer matrix method and is based on a Nelder-Mead simplex optimization algorithm. It can be applied to both uniform and nonuniform (apodized and chirped) fiber Bragg gratings. The method is then used to synthesize a uniform Bragg grating from its reflectivity taken at different temperatures. It gives a good estimate of the thermal expansion coefficient and the thermo-optic coefficient of the fiber.     

Analysis of fiber bragg gratings by a side-diffraction interference technique

A new nondestructive, noncontact, and sensitive technique for fiber Bragg grating geometry and index-fault location measurements is presented. Two plane-wave probe laser beams are incident upon the grating from the side at angles that satisfy the Bragg-reflection condition. An interference pattern is formed behind the fiber between the first-order diffracted beam (from one probe beam) and the zero-order transmitted beam (from the second probe beam). The axial grating index modulation and the grating period are functions of the fringe visibility and the fringe period, respectively. The method is sensitive and is applicable even in the case of relatively weak gratings. Unchirped and chirped Bragg gratings have been studied with the proposed technique. We demonstrate accuracies of 1 x 10(-4) for measurement of the index modulation and 0.01 nm for measurement of the period. As well as for the analysis of most already-fabricated gratings, this technique is useful for in situ analysis of a long fiber Bragg grating as such a grating is translated along its axis during the fabrication process.     

Characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors

The characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors are discussed. This sensor is able to detect macro bending with the transmitted power variation of the first side mode in the blazed fiber Bragg grating. Since an incident ray experiences different variations of tilt angles with respect to bending direction, the blazed fiber Bragg grating has different coupling efficiencies of the first side mode, which can be reduced considerably in the case of twisted blazed fiber Bragg gratings.     

Dispersion control with use of long-period fiber gratings

Chirped long-period fiber gratings are analyzed for management of dispersion in optical fiber communications systems. A ray model is used to derive simple analytic expressions that describe the transmission, chromatic delay, and dispersion properties of chirped long-period fiber gratings. A numerical model based on coupled-mode theory is used to verify the accuracy of the analytic expressions and explore design issues of the chirped long-period grating. With certain reasonable restrictions, chirped long-period gratings are found to be a viable and desirable alternative to existing dispersion compensation techniques.     

Reconstruction of fiber grating refractive-index profiles from complex bragg reflection spectra

Reconstruction of the refractive-index profiles of fiber gratings from their complex Bragg reflection spectra is experimentally demonstrated. The amplitude and phase of the complex reflection spectrum were measured with a balanced Michelson interferometer. By integrating the coupled-mode equations, we built the relationship between the complex coupling coefficient and the complex reflection spectrum as an iterative algorithm for reconstructing the index profile. This method is expected to be useful for reconstructing the index profiles of fiber gratings with any apodization, chirp, or dc structures. An apodized chirped grating and a uniform grating with a depression of index modulation were used to demonstrate the technique.     

线性啁啾相位掩模的设计

啁啾相位掩模法是啁啾光纤光栅的一种非常重要的制作方法。本文研究讨论了用全息干涉方法制作线性啁啾位相掩模的设计方法,提出用两球面波干涉产生条纹密度随空间距离线性变化的干涉条纹记录啁啾位相光栅,按设定光栅端点空频和端点空频差两种优化设计方法设计了长度100mm、啁啾量1nm/mm的线性啁啾光栅,理论和实验结果给出两种设计方案的非线性系数分别为2.5%和1.6%,理论设计和实验结果相符。     

Distributed measurement of static strain in an optical fiber with multiple bragg gratings at nominally equal wavelengths

The Fourier transform relationship between the reflected light froma Bragg grating and the complex spatial modulation of the Bragg grating is used to produce a distributed strain sensing system. A tunable external cavity diode laser along with a reference reflector in anoptical fiber are used to produce a measurement of the phase and amplitude of the reflected light from the modulated Bragg grating as a function of wavelength. The system is demonstrated with 22 Bragg gratings in a single fiber on a cantilever beam and compared with foil strain gauge readings.     

Analysis of chirped-sampled and sampled-chirped fiber Bragg gratings

We analyze sampled fiber Bragg gratings that have chirp in the grating period, in the sampling function, or in both. In the last-named case the sampling period can be chirped, the sample length can be chirped, or both. We explain the features in the spectral and group-delay (dispersion) responses for gratings that have a single chirp parameter. We then show how the response of sampled gratings that have multiple chirp parameters can be explained in terms of the single-chirp parameter cases.     

Spatio-temporal operator formalism for holographic recording and diffraction in a photorefractive-based true-time-delay phased-array processor

We present a spatio-temporal operator formalism and beam propagation simulations that describe the broadband efficient adaptive method for a true-time-delay array processing (BEAMTAP) algorithm for an optical beamformer by use of a photorefractive crystal. The optical system consists of a tapped-delay line implemented with an acoustooptic Bragg cell, an accumulating scrolling time-delay detector achieved with a traveling-fringes detector, and a photorefractive crystal to store the adaptive spatio-temporal weights as volume holographic gratings. In this analysis, linear shift-invariant integral operators are used to describe the propagation, interference, grating accumulation, and volume holographic diffraction of the spatio-temporally modulated optical fields in the system to compute the adaptive array processing operation. In addition, it is shown that the random fluctuation in time and phase delays of the optically modulated and transmitted array signals produced by fiber perturbations (temperature fluctuations, vibrations, or bending) are dynamically compensated for through the process of holographic wavefront reconstruction as a byproduct of the adaptive beam-forming and jammer-excision operation. The complexity of the cascaded spatial-temporal integrals describing the holographic formation, and subsequent readout processes, is shown to collapse to a simple imaging condition through standard operator manipulation. We also present spatio-temporal beam propagation simulation results as an illustrative demonstration of our analysis and the operation of a BEAMTAP beamformer.     

Simultaneous measurement of stress and temperature with a fiber Bragg grating based on a loop thin-wall section beam

We designed and demonstrated what we believe to be a novel sensor for simultaneous measurement of stress and temperature. A fiber Bragg grating is flatly adhered to the surface of a loop thin-wall section beam. The theoretical analyses and the experimental results show that both the central wavelength shift and the chirped bandwidth of the grating reflection spectrum have a linear relationship with the stress and the temperature, respectively, and the slopes of them are different. Therefore, the temperature and stress can be discriminated by interrogating the chirped fiber grating. Moreover, we also investigated the strain of the loop thin-wall section beam, and the results show that the strain is cosine proportional to the double positional angle.     

Echelle gratings acting as one

A new device for producing order-sortable echelle spectra by use of consecutive diffactions in several echelle grating surfaces is described. This echelle emulating device (EED) comprises a prism placed in the path between a pair of echelle grating surfaces. The refractive angle of the prism is fixed through a simple relation. The device reproduces all the main properties of a single virtual echelle, obtained from a simple grating equation describing the combined action of the gratings and the prism. The spectral order of the EED is the sum of the spectral orders of the individual gratings. A spectrograph that utilizes the device is described, and several applications are discussed.     

Spectra of dual-overwritten chirped fiber Bragg gratings

The coupled-mode equations for dual-overwritten chirped fiber Bragg gratings (CFBGs) are obtained and numerically calculated. By simulating the reflection spectra of dual-overwritten CFBGs, we have investigated the effects of chirp coefficient, period difference, grating length and index modulating coefficient, on the reflection spectrum and its intensity and full-width-at-half-maximum (FWHM).     

光纤Bragg光栅的特性及应用研究

介绍了光纤光栅研制及其应用研究的情况。研究包括：光纤光栅的基本特性,光纤光栅外腔半导体单频和锁模激光器、光纤光栅稳频光纤环形激光器和主动锁模光纤激光器、基于光纤光栅的波长转换器和基于啾啁光纤光栅的可调谐激光器等。     

Improving the performance of fiber gratings with sinusoidal chirps

Sinusoidal chirps are introduced in fiber gratings to improve their performance as dispersion compensators and multichannel filters. The sinusoidally chirped fiber gratings exhibit a flattop spectrum with steep edges and high reflectivity. The bandwidth utilization defined as the ratio of -1:-30-dB bandwidth could be very high (>0.85). This structure can be applied to the sampled fiber gratings to enhance channel uniformity. We demonstrate 264 uniform channels with a 25-GHz-spacing for high-density-wavelength multiplexing applications. Multichannel dispersion compensations with seven uniform channels of 50-GHz-spacing in short fiber gratings are also demonstrated. The impact of possible fabrication errors on the spectra of the gratings is discussed.     

Profile measurement taken with liquid-crystal gratings

Profile measurement taken with liquid-crystal gratings and a phase-shifting technique is proposed, and its effectiveness is verified by experiment. The surface profile is obtained by measurement of the phase distributions of the sinusoidal gratings deformed by an object's surface. The liquid-crystal grating gives an accurate phase shift, an arbitrary projection pitch, and a constant surface brightness compared with conventional gratings such as a laser interference fringe grating and a Ronchi grating. Therefore a flexible measuring system may be developed with it. Two gratings with different pitches are used to measure an object with large steps. A two-color projection system can be used to produce such gratings simultaneously. Locally varying reflectivity on a surface can also be compensated by adjustment of the color component of the projected grating with a liquid-crystal grating. Thus the contrast in the projected grating can be made uniform, and a good profile measurement can be accomplished.     

Fabrication of chirped gratings on GaAs optical waveguides

After a brief review of the applications and fabrication techniques of gratings in integrated and guided wave optics, a new method of fabricating chirped gratings is presented. The method is based on the interference of two spherical laser beams after spatial filtering by pinholes. The experimental conditions required for fabricating antisymmetric linearly chirped gratings, symmetric quadratically chirped gratings and uniform gratings are presented by way of example. By this method chirped gratings with small variations in the period can be fabricated on GaAs thin film optical waveguides. Profiles and patterns of chemically etched corrugations are quite uniform, mainly because of the direct interference of beams immediately after spatial filtering.     

Wideband two-port beam splitter of a binary fused-silica phase grating

The usual beam splitter of multilayer-coated film with a wideband spectrum is not easy to achieve. We describe the realization of a wideband transmission two-port beam splitter based on a binary fused-silica phase grating. To achieve high efficiency and equality in the diffracted 0th and -1st orders, the grating profile parameters are optimized using rigorous coupled-wave analysis at a wavelength of 1550 nm. Holographic recording and the inductively coupled plasma dry etching technique are used to fabricate the fused-silica beam splitter grating. The measured efficiency of (45% x 2) = 90% diffracted into the both orders can be obtained with the fabricated grating under Littrow mounting. The physical mechanism of such a wideband two-port beam splitter grating can be well explained by the modal method based on two-beam interference of the modes excited by the incident wave. With the high damage threshold, low coefficient of thermal expansion, and wideband high efficiency, the presented beam splitter etched in fused silica should be a useful optical element for a variety of practical applications.     

Directional dependence of spectra of fiber Bragg gratings due to excess loss

The reflectance spectra of chirped fiber Bragg gratings can depend substantially on the direction from which the measurement is taken. The measured difference between forward and backward reflectance spectra measured in a linearly chirped grating was shown to be due to the measured excess loss. Simulation using the popular transfer-matrix model demonstrated that the observed asymmetric behavior could be obtained only when excess loss has an asymmetric spectral shape about the local Bragg wavelengths. Application of cladding mode excess losses to the result of a transfer-matrix model accounted for the experimental observation.     

A comparison of wavelength dependent polarization dependent lossmeasurements in fiber gratings

We have measured wavelength dependent Polarization Dependent Loss (PDL) in chirped fiber Bragg gratings for dispersion compensation, grating filters for wavelength add/drop multiplexing and long period gratings for EDFA gain flattening. The PDL is measured in devices used in reflection and in transmission by applying the Jones matrix method, the Mueller matrix method and the polarization scanning method. A comparison of the experimental results and an analysis of the sources of errors are presented