Showing papers on "Equilibrium mode distribution published in 2001"

Mode transformer between low index difference waveguide and high index difference waveguide

Abstract: A mode transformer (100) that enables low-loss coupling between optical modes of two waveguides with different index difference. The mode size and the effective index are gradually changed between two waveguides to gradually transform the mode shape, size, and speed with minimum power loss. The mode transformer is useful for coupling the mode of an optical fiber waveguide with low index difference to the mode of a planar high index difference waveguide, and vice versa.

Numerical analysis of core-mode to radiation-mode coupling in long-period fiber gratings

Abstract: This letter presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in long-period fiber gratings (LPGs) that are surrounded with dielectric material whose refractive index is higher than that of the cladding. We calculated core-mode transmission spectra through LPGs by integrating numerically the coupled-mode equations between the core mode and a continuum of radiation modes. We show calculated core-mode transmission spectra that exhibit such loss band continuums as have already been observed experimentally but not yet theoretically estimated.

Measurement of birefringence correlation length in long, single-mode fibers

Abstract: Measurements of birefringence correlation length performed on long single-mode telecommunication fibers are reported. The proposed technique relies on the statistical properties of the backscattered-field polarization, which was measured by means of a polarization-sensitive optical time-domain reflectometer. Experimental results are reported that show good agreement with the underlying theory based on stochastic differential equations.

Optical waveguide multimode to single mode transformer

Abstract: An optical waveguide mode transformer (2) has a high refractive index core layer (102a) surrouded by lower refractive index cladding (102b). The core layer includes a wide input waveguide section (110) to accept a multimode, including a fundamental mode, light input. The input waveguide section (110) is coupled to a narrow output waveguide section (140) by a tapered region (120) having a taper length enabling adiabatic transfer of the fundamental mode of the multimode light for the wide input waveguide section to the output waveguide section while suppressing (stripping) other modes of the multimode light input. The narrow output waveguide section (140) supports a single mode light output comprising said fundamental mode. In one embodiment, the core layer is contoured, preferably defined by a ridge intermediate the sides of the core layer, to provide an effective index step portion to control lateral waveguiding of light propagated along the waveguide structure, and the output waveguide section (140) includes a portion having a real index step between the core layer and cladding layers, and is functional to output a light beam having similar vertical and horizontal divergences.

Low loss mode converter based on adiabatically tapered hollow optical fibre

Abstract: A new mode converter based on tapered hollow optical fibre is proposed and experimentally demonstrated. Two segments of a singlemode fibre and a tapered hollow ring core fibre were concatenated serially to convert a fundamental mode to a ring-shaped mode. The fabrication, far field pattern and insertion loss of the proposed mode converter were measured and discussed.

Novel design concept of waveguide mode adapter for low-loss mode conversion

Abstract: We proposed a novel design concept of a mode adapter using a new parameter. The overlap integration of two adjacent mode profiles is defined as a single step loss (SSL) parameter for the characterization of optical mode confinement as well as the quantification of mode mismatch. The variation of SSL is described with two parameters. These parameters are V, a normalized frequency, and s, a differential width variation ratio. The contour for constant SSL in the V, s plane provides the optimized s curve as a function of V. The design concept with this constant SSL is shown to provide optimized adapters of minimized conversion loss.

Polarization mode dispersion probability distribution for arbitrary distances.

Abstract: The probability distribution of the differential group delay (DGD) at any fiber length is determined by use of a physically reasonable model of the fiber birefringence. We show that if the fiber correlation length is of the same order as or larger than the beat length, the DGD distribution approaches a Maxwellian in roughly 30 fiber correlation lengths, corresponding to a couple of kilometers in realistic cases. We also find that the probability distribution function of the polarization dispersion vector at the output of the fiber depends on the angle between it and the local birefringence vector on the Poincare sphere, showing that the DGD remains correlated with the orientation of the local birefringence axes over arbitrarily long distances.

Single mode fibre

Abstract: An optical fibre (20) is disclosed, the fibre is adapted in a manner such that it guides an optical signal substantially only in one non-degenerate mode, wherein an electro-magnetic field carrying the optical signal is symmetric with respect to rotation about the fibre axis. Preferably, the non-degenerate mode is the TE01 mode. In one embodiment, the optical fibre (20) includes a central hole region (22) surrounded by a concentric guiding region (24), which is in turn surrounded by a concentric cladding region 826). The guiding region (24) may comprise a Bragg reflector region. Through appropriate selection of design parameters of the fibre (20), the effective refractive index for the TM01 mode can be reduced relative to the TE01 mode to make it below a refractive index of the cladding region (269, resulting in leaking of the TM01 mode into the cladding region (26).

Fiber Modal Index Measurements Based on Fiber Gratings

Abstract: A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil immersion method is proposed. By analyzing the interaction between the cladding mode and the oil applied on the cladding surface, the mode order and the effective index of the involved cladding mode might be calculated. Experimental results about the interference fringe shifts induced by the oil index and the oil-applied length are also presented.

Temperature optical sensor based on a silicon bimodal Y branch

Abstract: A novel temperature optical sensor configuration, based on mode mixing principle, is theoretically discussed. The sensing element consists of a single-mode all-silicon waveguide, followed by a two-modes section, which acts as sensing region, and an output Y branch to separate the two output channels. The fundamental mode coming from input waveguide excites both the fundamental mode and the first higher order mode in the sensing region. The interference between the two propagating modes in the sensing region produces a periodically repeated optical intensity distribution along the propagation axis. It is possible to observe a light steering from one output channel to the other caused by the temperature change of the structure itself. This fact is related to the variation of the material refractive index with the temperature, that is the thermo-optic effect, which implies a variation of the mode effective refractive indices, and, consequently, a phase shift between the modes themselves. In this way, a continuous variation of the power distribution on the two output waveguides as a function of the temperature can be observed. A simultaneous acquisition of both output signals, followed by a simple elaboration, allows obtaining a temperature evaluation independent on light source instability. Moreover, it is possible to design the device in order to obtain the desired initial output power distribution on two channels. This permits to design sensors characterized by greater accuracy, if we use the linear part of the optical transfer function, or larger measure range, if we utilize the whole output excursion.

Semiconductor laser device with optical waveguide exhibiting high kink output

Abstract: A semiconductor laser device has at least an active layer and an optical waveguide region, which includes at least a part of the active layer, wherein Δ gain /Γ is at least 85 [cm −1 /%], where Δ gain [cm −1 ] is a difference in gain between a zero-order fundamental mode and a one-order high-order mode in a lateral transverse mode of the optical wvaveguide, and Γ [%] is a total optical confinement rate of the at least part of the active layer in the zero-order fundamental mode.

Integrated planar composite coupling structures transform between a small mode size and a large mode size waveguide

Abstract: Composite optical waveguide structures or mode transformers(1300) and their methods of fabrication and integration are disclosed, wherein the structures or mode transformers are capable of bi-directional light beam transformation between a small mode size waveguide and a large mode size waveguide. One aspect of the present invention is directed to an optical mode transformer (1300) comprising a waveguide core (1345) having a high refractive index contrast between the waveguide core (1345) and the cladding (1350 or 1310), the optical mode transformer (1300) being configured such that the waveguide core (1345) has a taper wherein a thickness of the waveguide core tapers down (1348) to a critical thickness value, the critical thickness value being defined as a thickness value below which a significant portion of the energy of a light beam (1341) penetrates into the cladding layers (1350 or 1310) surrounding the taper structure thereby enlar in the small mode size.

Ultrashort-pulse investigation of the propagation properties of the LP(11) mode in 1.55-mum communication fibers.

Abstract: The coupling of an ultrashort laser pulse into a single-mode optical communication fiber gives rise to two propagating pulses as a result of the excitation of two guided modes, the fundamental, LP01, and the leaky, LP11. Such a phenomenon provides a new approach to the study of the propagation properties of the LP11 mode. An experiment with tunable 110-fs pulses at a wavelength near 1550 nm is described. Information about the group velocity, the polarization-rotation length, the attenuation coefficient, and the cutoff wavelength of the LP11 mode is obtained in a simple and direct way for various fibers.

Novel methods of intracavity beam shaping

Abstract: Methods of shaping laser beams within the laser resonator are studied. The simplest form of shaping is the spatial modegeneration inherent in the laser cavity due to the geometry of the resonator in conjunction with gain to compensate forroundtrip losses, such as diffraction and output coupling. Typically the fundamental mode or multimode behavior isexhibited from such configurations. Passive mode shaping can be accomplished by introducing static amplitude or phasemasks. An example of an amplitude mask is provided in order to generate a higher-order spatial mode. Active mode shapingcan be accomplished by altering the optical pump distribution. This case is studied experimentally with a diode-end-pumpedNd:YVO 4 laser and compared to modeling of expected Hermite-Gaussian mode generation. Active mode shaping allows thepreferred mode distribution to be altered in real time. Additional shaping can be done following the resonator to modify aHermite-Gaussian mode into a psuedo-Laguerre-Gaussian mode. This work also shows that using the coherent propagationmethod of Gaussian beam decomposition is capable of modeling and describing intracavity beam shaping.Keywords: laser beam shaping, transverse modes, solid-state lasers, laser resonator, coherent propagation, Hermite-Gaussianmode, Laguerre-Gaussian mode

Polarization Dependence Analysis of Optical Loss in Small-Aperture Metal Waveguides for Near-Field Optics

Abstract: Optical intensity profile and optical loss in small-aperture metal waveguides were analyzed by a one-dimensional waveguide model and the beam propagation method. Guided mode profile and optical loss have a great dependence on the polarization direction of the guided lightwave. The loss for the TM0 mode is two orders of magnitude smaller than that for the TE0 mode for a slit width of less than half-wavelength. A very small spot is available at high efficiency by using a metal waveguide and an incident light with TM polarization.

Investigation of coupling between a fiber and a planar or a rectangular waveguide: Effective index coupled mode approach

Abstract: We present here an effective index-coupled mode model describing the action of a fiber-to-planar (rectangular) waveguide evanescent field coupler, which takes into account the real two-dimensional feature of the waveguidecross-sections. In addition to the increased speed of calculation compared to the earlier model of Marcuse, the effective index method-coupled mode theory model describes the coupling from a fiber to a real rectangular waveguide with a finite width.

Ring-Shaped Field Pattern: The Fundamental Mode of a Multimode Optical Fiber

Abstract: The observation of a ring-shaped single-mode field pattern at Stokes wavelengths of backward stimulated Raman scattering in a multimode fiber is reported for the first time. This phenomenon and some other novel experimental and numerical results are presented to explain the mechanism. It has been proved that self-focusing plays little role in the formation of the ring-shaped structure. The ring-shaped pattern is attributed to the refractive index profile of the fiber used in the experiment. It corresponds to the fundamental mode of the fiber and mode competition causes the Stokes waves to propagate in the fundamental mode.

Symmetric and asymmetric mode interaction in high-power traveling wave amplifier

Abstract: High power microwave TWT amplifier operation has been studied for use in electron accelerators. The performance of the amplifiers has been marred, in some cases, by pulse shortening of the microwave signal, possibly due to hybrid HEM11 mode interaction with the beam. In this paper we describe experiments which investigate high power operation and the effects of HEM modes on the amplifier performance. We report the high output powers (>50 MW) with efficient (>54%) amplification of microwaves in an X-band traveling wave amplifier, and present preliminary data showing operation at a few megawatt output levels in Ka band. In some experiments peak power levels in X-band exceeding 120 MW were measured at an efficiency of 47%. The excitation of the asymmetric hybrid electro-magnetic mode was monitored carefully, but does not seem to have a critical impact on the main interaction process in spite of the fact that its dispersion curve almost overlaps that of the symmetric interacting mode. Theoretical analysis of ...

Optical coupler has first waveguide with matt end facing light source on input side or scattering centers, resulting in completely homogeneous light distribution over fiber cross-section

Abstract: The device has a first optical waveguide (2) as its input and at least two waveguides (3,4) as its outputs. The end (5) of the first waveguide facing the light source on the input side is matt, resulting in a completely homogeneous light distribution over the fiber cross-section, or scattering centers can be uniformly distributed in the volume of the first waveguide to achieve a completely homogeneous light distribution over the cross-section.

A Theoretical Analysis of Transverse Mode Control in Waveguide Gas Lasers

Abstract: Theoretical of multiple transverse mode laser oscillation involving spatially varying gain and loss are investigated. The effect of gain and loss distribution on mode competition is analyzed. A theoretical analysis of laser transverse mode competition is investigated from the perspective of the spetial overlap of modes with a transverse gain-loss distribution. The dominant mode of laser oscillation is the mode that is stable under small perturbations.

Failure of the scalar approximation near cutoff frequency of a step-index fiber mode

Abstract: The feasibility of the scalar approximation used in the theory of weakly-guiding optical fibers is considered near the cutoff frequency of the HE/sub 1n/(LP/sub 0n/) mode. The asymptotic behavior of polarisation corrections for transverse and longitudinal wavenumbers of a lossy fiber mode is investigated both analytically and numerically. These results as well as the transmittance calculated for an irregular fiber consisting of single-mode and bimodal segments demonstrate the failure of scalar approximation near the cutoff frequency.

A method of distinguishing combined mode coupling regimes when polarisation mode dispersion is measured by the interference method

Abstract: This paper has suggested that the standard method of measuring PMD by interferometry is inadequate. In particular it fails to address the presence of both random and deterministic PMD in a fibre. Such a situation can lead to erroneous results. This paper has shown that one method of detecting this situation is by use of the FT properties of convolution.

Analysis of cascaded dispersion compensation system

Abstract: In this paper, a pulse propagation analysis of cascaded compensation scheme using higher order mode propagation is carried out. The output of the cascaded system consists of compensated and uncompensated components. A generalized expression for the compensated and uncompensated components has been derived. Results show that for a broadband efficient mode converter only previous one or two sections contributes to the pulse broadening of the uncompensated components. For 50 sections (2000 km), the ratio of uncompensated to the compensated power is 24 dB for a mode converter efficiency of 100%.

The propagation properties analysis of asymmetric fiber couplers

Abstract: The finite element method mode analysis of optical waveguides in asymmetric fiber couplers is presented. An adaptive-size triangular mesh is employed to obtain the propagation constants of the first four modes in different constructions. The numerical result shows that the radius ratio has obvious effects on the propagation constant of even and odd modes at the same degree of fiber fusion. Compared with the odd mode, the propagation constant of the even mode is insensitive to the changes of the degree of fiber fusion.