Refractive Index Profile Of Optical Fibers Research Articles

The optical fiber refractive index profile measurement based on phase-correct quantitative phase microscopy

The optical fiber refractive index profile measurement based on phase-correct quantitative phase microscopy

Development of a Method for Measuring the Optical Fiber Refractive Index Profile

Development of a Method for Measuring the Optical Fiber Refractive Index Profile

Modelling and record technologies of address fibre Bragg structures based on two identical ultra-narrow gratings with different central wavelengths

Address fiber Bragg structures (AFBS) make it possible to effectively solve the problem of sensors interrogation and multiplexing in multi-sensor networks with microwave photonic processing of information. Based on the method of inverse Fourier transform, a mathematical model of the optical fiber refractive index profile was constructed to form 2λ-FBG AFBS with two FBG with identical spectral responses at separated wavelengths. As the initial parameters for the construction of the mathematical model, the desired spectral profile of 2λ-FBG AFBS was specified, including the reflection coefficient and the width of the transmission band of its two identical ultra-narrow-band gratings and the separation between them. On the basis of the study of the mathematical model, the possibility of selecting the necessary values of the refractive index and the laws of its modulation is shown, allowing the spectral profile of 2λ-FBG AFBS to be formed so that they can be used as a sensitive element, transforming information from optical range to radiofrequency one. The analysis of the formation and recording methods for 2λ-FBG AFBS was carried out. To implement given structures, the technology, using of an ultraviolet argon laser, the classic phase masks with sequential recording of several arrays with precise movement and strain of the fiber were chosen. Further paper deals with issues of interrogation of the developed structures and principles of measurement system construction on their basis.

Optical Fiber Refractive Index Profiling by Iterative Optical Diffraction Tomography

A non-destructive iterative interferometric tomographic technique is proposed for imaging fibers with high refractive index contrast, complex structures, and/or large optical path differences. Both simulation and experimental results demonstrate that this technique enables the reconstruction of fiber refractive index profiles accurately and robustly with sub-wavelength resolution.

Adaptive simultaneous algebraic reconstruction technique for retrieving refractive index profiles of optical fiber

An efficient adaptive simultaneous algebraic reconstruction technique (ASART) to calculate optical fiber refractive index profiles is proposed based on phase difference curves obtained by digital holography technique. We develop adaptive relaxation parameter (ARP) on simultaneous algebraic reconstruction technique (SART) to increase the convergence speed and improve the reconstruction accuracy. A formula of ARP is derived mathematically and multilevel scheme (MLS) is used to increase convergence speed in the first iteration. Experimental results show the proposed ASART convergences over 40% faster than SART and achieve significantly higher reconstruction accuracy. Experimental verification shows that ASART is more efficient than SART and filtered back projection in image reconstruction, especially with few projection views. The running time of ASART is much shorter than that of SART, and ASART needs fewer iteration numbers to obtain the same reconstruction effects. In addition, it can be used to measure optical fibers with various diameters that cannot be measured with S14 refractive index profiler (S14).

Measurement of the focusing constant of gradient-index fiber lens and its application in developing GRIN fiber probes

The curve-fitting algorithm is adopted to measure the focusing constant of a gradient-index (GRIN) fiber lens, which is used to further design ultra-small GRIN fiber probes. Firstly, the refractive index profile of a GRIN fiber lens is acquired by using the Optical Fiber Refractive Index Profile tester. And the focusing constant of the GRIN fiber lens is obtained by means of the curve-fitting algorithm of quadraticpolynomial. Secondly, the measured focusing constant is used to design ultra-small GRIN fiber probes with different focusing properties. Thirdly, according to the designed optical probe models, GRIN fiber probe samples have been fabricated and tested. Finally, comparative analysis is conducted between the theoretical focusing performance parameters and the experimentally measured values. The results show that, in the given condition, the fitted value of the focusing constant of GRIN fiber lens and the theoretical focusing performance parameters of GRIN fiber probes, respectively agree well with the nominal value provided by the manufacturer and the experimentally measured data. Therefore, the proposed methods of measuring the focusing constant of GRIN fiber lens and its application in designing GRIN fiber probes are validated and feasible to further developing ultra-small GRIN fiber probes requiring specific optical focusing performance.

Gain‐guided optical fibre refractive index synthesis starting from arbitrary modal electric field

Novel results on how to reconstruct gain-guided optical fibres starting from arbitrary modal electric fields are presented. It is known that gain-guided optical fibre refractive index profiles are described in terms of complex numbers. Therefore, to achieve the design of such fibres from a knowledge of modal electric fields one is required to use complex refractive index profile waveguides, where one can determine the required refractive index profile on both real and imaginary parts of the refractive index. The novelty here is that the reconstruction starts by using complex-valued desired electric fields. This inverse problem is solved via modelling the waveguide transversely as a transmission line and working inversely from the electric field. Example reconstructions of refractive index profiles are offered which are useful for high-power lasers, in designing optical fibres for sensors, where one may require specifically shaped electric fields from a waveguide, and also in refractive index profiling instrumentation.

Two-dimensional scanning focused refractive-index microscopy and applications to refractive-index profiling of optical fibers.

The refractive-index profile (RIP) of optical fibers is of fundamental significance in determining critical fiber properties. Here, we present the application of a two-dimensional (2-D) scanning focused refractive-index microscopy (SFRIM) to accurately obtain the 2-D RIP of a graded-index optical fiber. Some modifications are made to SFRIM for better 2-D measurement. Quantitative RIP of the fiber is obtained with derivative total reflection method. The refractive-index accuracy is 0.002. The measured result is in good agreement with theoretical expectation. This method is straightforward, simple, repeatable, and free from signal distortion. This technique is suitable for symmetric and asymmetric optical fibers. The results indicate that this technique can be applied to obtain the RIPs of a wide range of materials and has broad application prospect in many fields.

Multi-Wavelength Optical Fiber Refractive Index Profiling by Spatially Resolved Fourier Transform Spectroscopy

A non-destructive technique to measure an optical fiber's refractive index profile with sub-¿m spatial resolution over a wavelength range spanning more than one octave (from 480 to 1040 nm) in a single measurement is described. Data showing the variation of refractive index with wavelength for several fiber types is presented.

Algorithm performance in the determination of the refractive-index profile of optical fibers

Three algorithms for computing the refractive-index profile of azimuthally symmetric optical fibers via the inverse Abel transform are compared to determine their relative accuracies. Appropriate values of algorithm parameters are also determined. The direct differentiation algorithm, the iterative algorithm, and the Fourier algorithm are used to calculate the refractive-index profile from simulated measurements of the phase shift of light transmitted transversely through the fiber. The rms error in the calculated index profile is used to quantify the accuracy of each algorithm. The Fourier algorithm is typically the most accurate of the three.

Mode dispersion and delay characteristics of optical waveguides using equivalent TL circuits

A new analysis leading to an exact and efficient algorithm is presented for calculating directly and without numerical differentiation the mode dispersion characteristics of cylindrical dielectric waveguides of arbitrary refractive-index profile. The new algorithm is based on the equivalent transmission-line (T-L) technique. From Maxwell's equations, we derive an equivalent T-L circuit for a cylindrical dielectric waveguide. Based on the TL-circuit model we derive exact analytic formulas for a recursive algorithm which allows direct calculation of mode delay and dispersion. We demonstrate this technique by calculating the fundamental mode dispersion for step, triangular, and linear chirp optical fiber refractive index profiles. The accuracy of the numerical results is also examined. The proposed algorithm computes dispersion directly from the propagation constant without the need for curve fitting and subsequent successive numerical differentiation. It is exact, rapidly convergent, and it results in savings for both storage memory and computing time.

Propagation characteristics of single-mode optical fibers with arbitrary complex index profiles

A rapidly converging numerical technique for the evaluation mode characteristics of circularly symmetric optical fibers with an arbitrary complex refractive index profile is presented. This method is based on transmission-line principles. From Maxwell's equations, we derive a transmission-line equivalent circuit for the optical fiber refractive index profile and we demonstrate how it can be used to determine the mode effective index (normalized propagation constant) of cylindrical dielectric waveguides. To illustrate the effectiveness of the procedure, we have applied it to circularly symmetric fibers with complex step, parabolic, and segmented optical refractive index profiles. We have used this method to evaluate and manipulate the gain in a typical 980-nm pumped erbium-doped fiber as well as for calculating attenuation of optical fibers when radial loss factors are present.

Refractive-index profiling of optical fibers with axial symmetry by use of quantitative phase microscopy

The application of quantitative phase microscopy to refractive-index profiling of optical fibers is demonstrated. Phase images of axially symmetric optical fibers immersed in index-matching fluid are obtained, and the inverse Abel transform is used to obtain the radial refractive-index profile. This technique is straightforward, nondestructive, repeatable, and accurate. Excellent agreement, to within approximately 0.0005, between this method and the index profile obtained with a commercial profiler is obtained.

Refracted near-field measurements of refractive index and geometry of silica-on-silicon integrated optical waveguides

The standard refracted near-field technique for measuring the refractive-index profile of optical fibers cannot be directly used for silica-on-silicon integrated optical waveguides because of the opacity of silicon. A modified method is thus presented to characterize this kind of waveguide. The resolution it gives, both spatially and in the refracted index, is practically as good as that obtained with the standard technique for measuring optical fibers.

Modified two-dimensional fast Fourier transform beam propagation method for media with random variations of refractive index

Exact knowledge of the refractive-index profile of optical fibers and ion-exchange glass waveguides is needed for accurate calculation of their electromagnetic properties. Since there are always uncertainties in the manufacturing process, various methods have been proposed for the calculation of the refractive-index profile. All of these methods however, give only approximate results, and the electromagnetic properties of these devices are not always calculated with the required accuracy. A method to compensate for this inaccuracy is presented, with the addition of a small random term to the approximate estimation of the refractive-index profile. The contribution of this random term to the electromagnetic properties of the device under study is calculated with the aid of an expansion to the well-known fast Fourier transform beam propagation method.

Dispersion compensation based on dual-mode optical fiber with inhomogeneous profile core

It is well known that the first higher order mode of optical fibers exhibits large negative waveguide dispersion by operating close to its cutoff wavelength. By using this dispersion, the positive dispersion in conventional 1.3 /spl mu/m zero dispersion optical fibers for 1.55 /spl mu/m signal light can be compensated. In this paper, we focus on the dispersion compensation technique using the first higher order mode in a dual-mode optical fiber, which supports both the fundamental LP/sub 01/ and first higher order LP/sub 11/ modes. Numerical calculations show that the waveguide dispersion of the first higher order mode is very sensitive to the refractive-index profile of optical fibers. In addition, it is demonstrated that a directional coupler composed of a single-mode fiber and a dual-mode fiber operates as a mode converter, which converts the signal light from the LP/sub 01/ mode to the LP/sub 11/ mode.

Retrieval algorithm for refractive-index profile of fibers from transverse interferograms

A novel retrieval algorithm for the refractive-index profile of optical fibers from transverse interferograms is presented. This algorithm allows a quick and simple index-profile reconstruction even in the presence of measurement noise and near the fiber axis. The response to noise of different retrieval algorithms is discussed and computer simulations are presented.

Measuring the refractive-index profile of optical fibers by the cladding-mode near-field technique

A new measurement system has been developed that simply determines the refractive-index profile of any optical fiber by analyzing the propagation of cladding modes. Measurement accuracy was verified by a comparison with the refractive near-field method, and a sufficient resolution level was achieved to produce the fine structural details in single-mode fibers.

Measurement of axially nonsymmetrical refractive-index distribution of-a single-mode fiber by a multidirectional scattering-pattern method

A high spatial resolution, typically 0.2 \sim 0.3 \mu m, has previously been achieved by the proposed by one of the authors for measuring refractive-index profiles of optical fibers. This method, therefore, is especially suitable for measuring the index profiles of single-mode fibers. Described in this paper is the modification of this technique for measuring nonsymmetrical index distributions in single-mode fibers. In the modified method, a two-dimensional index distribution in an axially nonsymmetrical fiber is computed from a set of its forward scattering patterns for different directions of incident light. The distribution to be measured is somewhat restricted, i.e., it must have point symmetry. However, the point symmetry includes double symmetry which holds in most of polarization-maintaining fibers. The spatial resolution is as high as the scattering-pattern method for the axially symmetrical case; in the present experiment the resolution limit is 0.28 μm. Results of computer simulation and measurement are shown.

Immersionless single-mode preform index profiling

Refractive-index profiles of optical fibers and their preforms can be measured by the focusing method which, in its original form, required that the preform (or fiber) be immersed in index-matching fluid to ensure that incident, collimated light reaches its core undistorted. Since immersion of the preform may be undesirable, we propose to keep the preform dry and obtain the required collimated light by means of a cylindrical matching lens. For simplicity, we are using homogeneous cylindrical glass rods as matching lenses and have achieved excellent agreement between index profiles obtained by the new technique and by the conventional focusing method employing index-matching fluid. The new technique works well with single-mode preforms, but it may not be applicable to index profiling of multimode preforms. This limitation arises for two reasons: (1) The requirement that the light beam be collimated inside the preform limits the region that is accessible to a light beam coming from the outside. (2) Aberrations of the matching lens (correctable to some extent) restrict the region inside the preform over which light collimation can be achieved by optical means. In practice, only half of the central preform cross section is accessible to the focusing method if a matching lens replaces the index-matching fluid. Where applicable, the new technique eliminates the possibility of damaging the surfaces of the preforms during the cleaning operation. Damaged preform surfaces could result in fibers with reduced tensile strength. In addition, the new technique can provide information on special preforms with high refractive index or long operating wavelengths for which matching fluids may not be available.