Relative Refractive Index Difference Research Articles

Influence of Fiber Parameters on Stress-Induced Group Delay Variation in Hole-Assisted Fibers

We have studied a specific type of hole-assisted fibers (HAFs) for parallel transmission systems. A fiber ribbon using HAFs will be suitable for parallel transmission systems when used in a small space. Additional skew occurs when a fiber ribbon is used under nonuniform stress environment. We focus on stress-induced group delay variations. Influence of fiber parameters, such as a core diameter 2a and a relative refractive index difference Δ, on stress-induced group delay variations is investigated using a finite-element method. We also study the influence of hole-structure variations.

Study on Coupling Coefficient and Fabrication of a Kind of Symmetric Twin-Core Silica Fiber

The coupling coefficient of twin-core fiber is discussed with coupled-mode theory in this paper. It shows that the coupling coefficient is related with the pump wavelength, the core diameter, the distance between the centers of the two cores and the relative refractive index difference. And the parameters of the twin-core fibers can be optimized to satisfy different requirements. Using the theory, the twin-core fibers are fabricated by an improved method. The measurement results have an obvious progress than previous work.

Vertically Pluggable and Compact 10-Gb/s$\,\times\,$12-Channel Optical Modules With Anisotropic Conductive Film for Over 100-Gb/s Optical Interconnect Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have developed a vertically pluggable and compact 12-channel optical module for intercabinet and intracabinet optical interconnections. Error-free and over 10 Gb/s operation of the optical module with an electrical and optical connector have been demonstrated. The module is designed to be 10 mm in width, 10 mm in length, and 6 mm in thickness so that it can be placed around the central processing units. This compact optical module can be mounted on the inner board area. Further, this module is vertically pluggable in the <formula formulatype="inline"><tex Notation="TeX">${z}$</tex></formula>-direction, i.e., in a direction that is perpendicular to the board. Because of the vertical pluggability, the optical modules can be repaired much more easily. The optical connector used in the low-height module has 12 fine-drawn graded-index multimode fibers (MMFs). These fibers have a high relative refractive index difference (<formula formulatype="inline"><tex Notation="TeX">$\Delta$</tex></formula>) and are bent with a 2 mm radius of curvature. The electrical connector consists of an anisotropic conductive film that is sandwiched between an alumina substrate and a printed circuit board with a clamp spring. We have used the alumina substrate to mount the optical device and the driver integrated circuit, because of its high thermal conductivity. In order to reduce cost, we have not used either a microlens or a metal-coated mirror in the optical module. Furthermore, passive alignment between the MMFs and the optical device has been achieved using guide pins assembled in the alumina substrate. </para>

Small Bend Structures Using Trenches Filled With Low-Refractive Index Material for Miniaturizing Silica Planar Lightwave Circuits

We have proposed the fabrication of small bend structures using trenches along both sides of the core, filled with low-refractive index material, in order to miniaturise silica planar lightwave circuits. The minimum bending radius of a silica waveguide was reduced from 2 mm to 200 mum by filling the trenches with low-refractive index material. The local lateral relative refractive index difference (Delta) was increased to 8.64%. We fabricated cascade S-shaped waveguides to estimate a bend loss of the proposed structure. Moreover, we applied those structures to arrayed-waveguide gratings (AWG)s. Both 8-channel, 100-GHz channel-spacing and 8-channel, 12.5-GHz channel-spacing AWGs were successfully fabricated. Compared with conventional AWGs, sizes of these devices were reduced by factors of about 2 and 4, respectively.

A Short Polarization Converter Using a Triangular Waveguide

A novel polarization converter using a triangular waveguide is proposed and analyzed by the imaginary-distance beam-propagation method based on Yee's mesh and the finite-difference time-domain method. The polarization conversion length is investigated as a function of relative refractive index difference. It is found, for a silicon core embedded in a silica cladding, that the conversion length is 2 mum, while the insertion loss is 0.5 dB at a wavelength of 1.55 mum. The extinction ratio is more than 20 dB over a wide wavelength range of 1.25 to 1.65 mum. Using a geometrically expanded model, the polarization conversion behavior is verified in the experiment at a microwave frequency of 15 GHz. Finally, reasonable polarization conversion is obtained with a modified structure, in which the two corners of the triangular waveguide are cut and the cut plane is aligned with a square input (output) waveguide.

Designing a graded index depressed clad non-zero dispersion shifted optical fiber for wide band transmission system

Non-zero dispersion shifted fibers (NZ-DSFs) find extensive use in wavelength division multiplexed (WDM) system as it reduces the non-linear effects like four-wave mixing (FWM) generation. A depressed clad graded index fiber with a central dip in the refractive index profile, as well as without dip, has been modeled to perform as an NZ-DSF using the spot size optimization technique. The performance characteristics of the proposed NZ-DSF have been studied by changing different fiber parameters; such as inner core radius ( a), relative refractive index differences ( Δ +), normalized end position of depressed clad ( C), depression parameter ( ρ), etc. for a given value of Petermann-2 spot size ( W ¯ ) . By suitably adjusting the fiber parameters, the effective core areas ( A eff) as simulated here are very large (∼80 μm 2) so that the effect of non-linearities upon them can be minimized. These NZ-DSFs have also been optimized for WDM transmission system. The dispersion slopes of the proposed fibers with and without dip have been estimated which are comparable with the reported results.

THREE INTEGRATED GAINS OF WIDE BANDWIDTH MULTI-PUMPING RAMAN OPTICAL AMPLIFIER: THE MAXIMUM FLATNESS

In the present paper, the performance of wide-bandwidth multi-pumping Ramanoptical amplifier employed in high-data rate and long-haul advanced opticalcommunication systems has been investigated under different pumping and fiberconditions. Raman gain is designed as straight line-exponential decay form withsmall flat maximum gain. Three different gains are defined and investigated.Multi-pumping number of NR pumps of equal pumping powers and differentpumping wavelengths of equal spacing are investigated. The pumps interactionsare reduced under large channel spacing.A novel software program is designed to handle the amplifier bandwidth as afunction of the controlling set of parameters. This broad bandwidth of maximumflatness is deeply analyzed. A wide range of the amplifier bandwidth of goodflatness is obtained up to 278 nm. It is found that higher gains and wide flatbandwidth can obtained if either the relative refractive index difference, ∆ or NRor the shift in Raman pumping wavelength, δλR or both of them increase.Thus, the repeater spacing in long-haul transmission cables designed for ultrawidewavelength division multiplexing (UW-WDM) can be adjusted for theemployed transmission channels.

Modeling of a step index segmented core single mode optical fiber as a dispersion compensator

A segmented core central step index dispersion compensating fiber (DCF) has been modeled by using spot size optimization technique. We have designed and optimized the DCF by suitably adjusting different fiber parameters, i.e. inner core radius ( a), central relative refractive index difference ( Δ), normalized outer core position ( p), width ( b) and height ( h) at different values of given Petermann-II spot sizes ( W ¯ ) . The studies on the propagation behavior as well as bendloss and other losses of the DCF have been presented here. The proposed design of the DCF can possess improved bend performances over single core DCFs. At the same time higher negative dispersion coefficient in the single mode region as well as considerably high value of figure of merit (FOM) for optimized fiber parameters can be achieved for smaller spot sizes ( W ¯ ) of the DCF. Further, inclusion of length independent splice loss modified this conventional FOM to a slightly lesser value of modified FOM.

Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses

A microfabrication process using ultrafast laser pulses in glass was investigated. We investigated the formation of semiconductors by the irradiation of glasses with femtosecond laser pulses. ZnS- or PbS-doped SiO(2)-Al2O(3)-B(2)O(3)-CaO-ZnO-Na(2)O-K(2)O glasses were prepared by a melting method and irradiated by femtosecond laser pulses. Periodic structures in the sample glasses with a high refractive index difference were produced by femtosecond laser pulses. The maximum relative refractive index difference between the irradiated area and the nonirradiated areas was 20%. Diffraction gratings were also fabricated inside the ZnS- or PbS-doped silicate glasses. The diffraction efficiency of these gratings was approximately 90% in the infrared region.

Refractive indices of B x Al 1− x N ( x = 0–0.012) and B y Ga 1− y N ( y = 0–0.023) epitaxial layers in ultraviolet region

Refractive indices of BxAl1−xN and ByGa1−yN epitaxial layers were determined in the ultraviolet (UV) wavelength range from 230 nm to 600 nm by the light reflectance spectrum. The relative refractive index differences of (AlN/B0.01Al0.99N) and (GaN/B0.02Ga0.98N) heterostructures were 17% at 250 nm and 11% at 370 nm, respectively. These values indicate that an optical waveguide can be organized in the UV to deep-UV spectral region using the nitride semiconductors which include a small amount of boron. The relative refractive index difference of 6% was achieved with only 0.2% and 1.1% of boron for (AlN/BxAl1−xN) and (GaN/ByGa1−yN) heterostructures at 250 nm and 370 nm, respectively. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical analysis on the refractive-index distribution and bandwidth of gradient-index polymer optical fibers from a centrifugal field.

Theoretical analysis was applied to analyze the refractive-index distribution (RID) and bandwidth (BW) of gradient-index polymer optical fibers (GI POFs) prepared by a centrifugal field process. The RID of the prepared GI POF could be represented by the equation of n(r) = n1[1 - 2delta(r/alpha)g](1/2). The studied material systems were poly(hexafluoroisopropyl 2-fluoroacrylate) (PHFIP 2-FA)/dibutyl phthalate (DBP) and poly(methyl methacrylate) (PMMA)/benzyl benzoate (BEN). The RID and the BW were significantly affected by an essential parameter k, which was related to thematerial properties (density difference and molecular weight) and processing properties (rotating speed, temperature, and radius). As k increased, the characteristic constant of RID, g, decreased to a minimum and then increased sharply, owing to the separation of the polymer and the dopant. On the other hand, the relative refractive-index difference of RID, delta, increased to a steady value after k increased to a certain value. The variation of RID with k resulted in a local minimum of intermodal dispersion, and thus a maximum bandwidth was obtained. The maximum BW of the PHFIP 2-FA/DBP and PMMA/BEN systems at 1550 nm (100-m fiber length and 2-nm spectral width) for the case of k not equal to 0 were 6.7 and 3.2 Gb/s, respectively. The wavelength of light source affects the BW significantly only at k around zero because of the importance of the intramodal dispersion in this case.

Refractive indices and thicknesses of optical waveguides fabricated by silicon ion implantation into silica glass

Planar optical waveguides formed by Si ion implantation into PECVD SiO 2 have been characterized by the dark mode spectroscopy method at a wavelength of 0.6328 μm. The measured effective index values of the guided modes have been used to investigate the optical properties of the core layers of the waveguides after different pre-implantation treatments. It was found that annealing the specimens before implantation, affected both the refractive index and thickness of the core layers. In the annealed specimens a thicker core layer and a larger relative refractive index difference between the core and the buffer layer resulted.

Polarization characteristics of optical waveguides with separable symmetric refractive-index profiles

In contrast with scalar modes, the vector modes of two-dimensional waveguides with separable refractive-index profiles cannot be calculated by a simple product ansatz for the fields. It is shown that a separation of the dominant field component into two factors, with each depending on only one of the two Cartesian coordinates, is accurately possible up to first order in the small relative refractive-index difference Δ for symmetric separable profiles. This allows us to calculate the modal propagation constants by decomposing the problem into two independent TE and TM planar wave equations. Birefringence is thereby included to an accuracy of first order in Δ. The minor field component can be separated to first order in Δ, if one of the two factors constituting the major field component is the fundamental mode of the parabolic profile, but cannot be separated in general. The longitudinal field components are separable to their lowest order Δ1/2 for all separable profiles.

Measurement of depolarization ratio and ultimate limit of polarization crosstalk in silica-based waveguides by using a POLCR

A polarization optical low coherence reflectometer (POLCR) is described that enables us to excite only the transverse electric (TE) mode of a test waveguide and to measure its Rayleigh backscatter signal distributions in the TE and transverse magnetic (TM) modes at a spatial resolution of 0.3 mm. The depolarization ratios of silica-based waveguides with relative refractive index differences of /spl Delta/=0.45 and 0.75% are obtained as 0.14 and 0.10, respectively, by measuring the bias in the ratio between the distributions in the TM and TE modes of each waveguide. By using the depolarization ratios and the Rayleigh backscatter signal levels, we calculate the ultimate polarization crosstalks to be -53 and -51 dB over 1 km, respectively. The actual polarization crosstalks of previously fabricated waveguides are about 50 dB higher than their ultimate limits for the same length of fiber.

Influence of fiber parameters on skew in single-mode fiber ribbons

The influence of fiber parameters on skew has been experimentally clarified by using single-mode 16-fiber ribbons with different fiber parameters. The skews are measured for fibers in 16-fiber ribbons. It is found that not only the relative refractive index difference /spl Delta/ but also the ratio of the mode field diameter to the core diameter influence the skew. The time of flight is measured for 16-fiber ribbons under several temperature and strain values. Although the time of flight is influenced both by temperature and strain, it is confirmed experimentally that skew is independent of fiber parameters. These results indicate that the skew of a fiber ribbon under a uniform environment can be determined from fiber parameter differences. Previously proposed equations for evaluating the skew are used to design fibers in a fiber ribbon in order to realize fiber ribbons with the desired skew value.

Analysis of the effective-index method for the vector modes of rectangular-core dielectric waveguides

The approximations involved in the effective-index method for analyzing the vector modes of rectangular-core dielectric waveguides are examined in detail. It is shown that the effective-index method does not solve the full vector-wave equation that governs the modes. Instead, it solves the reduced vector-wave equation, which is accurate only for approximately linearly polarized waves. Furthermore, in solving the reduced vector-wave equation, the method of separation of variables is used, which leads to additional errors as the waveguide being analyzed is a mathematically nonseparable structure. To characterize the performance of the effective-index method, asymptotic expressions are derived for the errors in the calculation of propagation constants. Apart from separating the effects of different approximations involved, these expressions show explicitly how the accuracy of the method depends on the mode type, the normalized frequency, the mode orders, the dimensions of the waveguide, and the relative refractive-index differences between the core and the surrounding media. With the help of these expressions, it is demonstrated that more accurate results can be obtained by combining various effective-index solutions.

Transverse stress induced LP 02-LP 21 modal interference of stimulated Raman scattered light in a few-mode optical fiber

Four-photon mixing followed by stimulated Raman scattering is observed in LP 02 mode in a 7.9 μm core diameter optical fiber. A localized transverse stress efficiency couples LP 02 to the LP 21 mode with a macroscopic beat length of 1.8 mm. LP 02-LP 21 modal interference is investigated by detecting the 550–590 nm SRS through a pinhole in the far field exit plane. Quantitative explanation of wavelength dependent intensity modulation results in a precise experimental determination of ∂[β 02(λ) − β 21(λ)] ∂λ , for mode-propagation constants β 02( λ) and β 21( λ) of LP 02 and LP 21 modes respectively, as well as Δ, the relative core-cladding refractive index difference. The LP 02-LP 21 modal interference is used for sensing of temperature between 50–300°C.

Characteristics of polarization splitters constructed from a circular hollow pit outside the cores

AbstractIn this paper, a polarization splitter is proposed in which a single‐core fiber (Fiber 1) and a fiber with a circular hollow pit outside the core (Fiber 2) are coupled. Corresponding to the extraction of the two linear polarizations (x and y polarizations) from the HE11 mode excited in to Fiber 1, the polarization splitting method can be clasified into the x‐polarization type or the y polarization type. In the x‐polarization type, the x‐polarization is extracted from Fiber 1 and the y‐polarization from Fiber 2. In the y‐polarization type, the y‐polarization is extracted from Fiber 1 and the x polarization from Fiber 2. This paper discussed the structural parameters (core radius, relative refractive index difference, and core spacing), which reduce the device length of the x polarization and y polarization types and at the same time make the extinction ratio small. The main results are as follows: (1) When the relative refractive index difference of Fiber 2 was made constant, the minimum values of the device length and the extinction ratio of the x polarization type can be made smaller than those of the y polarization type. (2) When the relative refractive index difference of Fiber 2 is increased from 0.4 percent to 1.0 percent, 1 the device lengths of both the x‐ and y‐polarization types can be reduced by 0.26 times, and 2 the minimum value of the extinction ratio of the x polarization type increases +4.1 dB from ‐38.2 dB and that of the y polarization increases +4.3 dB from ‐34.1 dB.

Analysis of an intersectional optical switch by the finite difference beam propagation method

AbstractThree‐dimensional (3‐D) analysis of an intersec‐tional optical switch is performed using the finite difference beam propagation method (FD‐BPM), and design guidelines are obtained from this analysis. The operating characteristics of the switch are evaluated by extinction ratio and power attenuation in two output ports. The optimal structure parameters, such as normalized frequency (V), waveguide relative refractive index difference (Δω), and intersecting angle (θ), are obtained for 1.55 μm wavelength operation. Wideband operation in the 1.3 to 1.6 μm wavelength range is discussed taking into account the wavelength dispersion of materials. It is found that to achieve an extinction ratio that is better than 20 dB, a refractive index variation of 0.5 percent is enough for single‐wavelength operation; however, a refractive index variation larger than 0.7 percent is required at the longer‐wavelength side for wideband operation. Finally, the results of 2‐D and 3‐D analyses are compared and good agreement is obtained with an error of about 1 dB.

Integrated-optic double-ring resonators with a wide free spectral range of 100 GHz

Integrated-optic double-ring resonators with a wide free spectral range (FSR) of 100 GHz are fabricated using GeO/sub 2/-doped silica waveguides with a high relative refractive index difference (/spl Delta/) of 1.5%. The resonators are composed of two ring resonators comprising small ring waveguides with radii of 1.75 and 2.0 mm. The double-ring resonator module exhibited a wide FSR of 98.0 GHz, a finesse of higher than 138, a low crosstalk of less than -11.7 dB, and a low insertion loss of 6.1 dB. The measured FSR of 98.0 GHz is wider than any previously reported ring resonator composed of optical waveguides. >