Showing papers on "Equilibrium mode distribution published in 1997"

Mode coupling and equilibrium mode distribution conditions in plastic optical fibers

Abstract: The equilibrium mode distribution (EMD) condition is critical for optical fiber measurements. In this letter, we examine the achievement of EMD condition inside plastic optical fibers (POF's) under different launch conditions. Our pulse broadening measurements of step-index (SI) POF's as a function of fiber length show that the EMD condition can readily be achieved in an SI POF much shorter than in glass optical fibers. The observation of the EMD condition was also independently verified using the far-field pattern technique.

Reflective-mode conversion with UV-induced phase gratings in two-mode fiber

Abstract: In future fiber communications systems, efficient mode conversion has the potential to enable improved devices for dispersion compensation and wavelength routing. Previous reports of symmetric-asymmetric mode conversion in circular, multimode optical fibers have shown nonideal spectral characteristics due to polarization dependence and/or mode splitting of nearly degenerate components of the higher order LP/sub 11/ mode. This work avoids these problems through use of nominally polarization-independent UV-induced phase gratings while exploiting a reflective phase-matching condition that is much less sensitive to the mode splitting of the LP/sub 11/ mode than prior work.

Multimode optical waveguides, waveguide components and sensors

Abstract: The invention describes two types of multimode optical waveguides having special dip in the refractive index profile of their core providing conditions for propagation of a higher order mode with sharp central peak which field carries considerable part of the mode energy, while the fields of all other modes in the waveguide are mostly concentrated outside of this central peak region. The waveguide of the first type guides the mode only with one central peak while the mode in the waveguide of the second type has also an additional peak at the interface between the waveguide core and cladding providing a possibility to detect any influence on the mode field in its outer region by measuring the signal in the central peak of the mode. The useful properties of these two kinds of modes can be employed for designing new wavelength selective waveguide components for optical communications and sensors applications: an optical waveguide modulator, amplitude and interferometric sensors for different applications, bistable nonlinear components for logic switching and optical memory, wavelength selective chemical sensors of both amplitude and interferometric type, etc. Employing a material exhibiting electro-optical properties provides a possibility of tuning of the components proposed as well as fabrication of the sensors of electric fields.

Intermodal phase difference controller for beam angle modulation in index guided semiconductor devices

Abstract: A novel device and method of beam steering for semiconductor lasers or optical amplifiers is disclosed. The method of the present invention achieves high signal extinction ratios, high speed, low chirp modulation by biasing a multi-lateral mode beam steering section. The device of the present invention comprises an active single vertical and lateral mode optical waveguide, a multi-lateral mode waveguide, and a mode converter. The mode converter efficiently couples output from an active single mode waveguide to two or more modes of a multi-lateral mode waveguide. Two guided modes arrive at a device facet with a particular intermodal phase difference based on initial mode phasing, multi-lateral mode waveguide length and modal dispersion properties, and facet angle. Beam steering is achieved through carrier antiguiding effect by injecting current into the multi-lateral mode waveguide from the mode converter thus changing the intermodal dispersion. Changing the intermodal phase difference changes the direction of beam propagation relative to the device facet, providing enhanced beam steering.

Integrated polymer waveguide mode size transformer with a vertical taper for improved fiber coupling

Abstract: We describe a novel vertical taper structure fabricated at the ends of polymer optical waveguide devices to improve the coupling between channel waveguides and single-m,ode fibers. The taper smoothly converts a highly elliptical waveguide mode into a bigger and more circular mode for low loss coupling and relaxed fiber alignment tolerances. A vertical taper 0.5-2 mm in length is made in the low index upper cladding to reduce its thickness from several micrometers to zero, followed by the coating of a second upper cladding with index higher than that of the previous upper cladding but slightly lower than that of waveguide core. In the taper, the channel waveguide mode gradually loses confinement by the upper cladding so that the mode size grows bigger a light propagates, whereas the confinement by the lower cladding and lateral confinement are hardly affected. The waveguide mode grows in the vertical direction away from the lossy ground electrode and substrate; therefore no compromise between mode size and propagation loss is involved. Two special but simple reactive ion etching techniques, shadow masked etching and tapered photoresist etching mask, are develop for making this vertical taper. Mode expansion and a 1.8 dB reduction in coupling los, which is not sensitive to waveguide width and polarization, is obtained in our preliminary experiment.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optically pumped optical waveguide amplifier

Abstract: An optically amplifying waveguide guides both the LP 01 and the LP 02 mode. Its erbium distribution profile is preferentially matched with the modal field distribution of the LP 02 mode. The waveguide is provided with long period perturbations of a pitch and strength that resonantly couples signal power of different wavelengths from propagation in the LP 01 mode to propagation in the LP 02 mode and back again to propagation in the LP 01 mode whereby the longitudinal distribution of gain along the waveguide at one wavelength is longitudinally displaced with respect to that at least one other wavelength.

Optical component wherein either an optical field distribution of received light or an optical field distribution of a propagation mode of a receiving waveguide has a double-hump shape

Abstract: An optical waveguide having a light receiving end and a propagation mode with a double-hump shaped optical field distribution at the light receiving end to receive light into the optical waveguide. The light incident on the light receiving end typically has a single-hump shaped optical field distribution. Overlap in the optical field distribution of the incident light and the optical field distribution of the propagation mode of the optical waveguide causes the light to be received by the optical waveguide. Alternatively, the optical waveguide can have a propagation mode with a single-hump shaped optical field distribution at the light receiving end, and the incident light can have a double-hump shaped optical field distribution at the light receiving end. These configuration provide a much greater range of transmission as compared to a typical case where the incident light has a single-hump shaped optical field distribution and the waveguide has a propagation mode with a single-hump shaped optical field distribution at the light receiving end. The present invention is especially useful for use in an optical multiplexer or an optical demultiplexer which uses an angular dispersion element, such as a diffraction grating.

Rational harmonic mode-locking fiber laser

Abstract: Optical pulse sources with repetition rate approaching terahertz are very important for many photonics applications including ultra-high speed optical communication and generation of sub-mm waves. Both active and passive mode locked fiber lasers are the appropriate choice for this purpose because of the availability of erbium doped fiber amplifier. In general, the mode locking occurs with a repetition rate of nf0, where n is an integer and f0 is the longitudinal mode frequency spacing. This is called harmonic mode locking. In the case of rational harmonic mode locking, the repetition rate is (np plus 1) f0 where p is also another integer. For the case of active mode locking, this is obtained when the modulation frequency to the amplitude or phase modulator used for mode locking is given by (n plus 1/p) f0. For the case of passive mode-locking, the rational harmonic mode-locking occurs when the saturable absorber in a ring laser is offset by a fraction p/L: from the center where L is the length of the cavity. We have developed a theory of the rational mode locked fiber laser. The results of the theory are compared with experimental results obtained from a 1.5 (mu) fiber laser actively mode-locked with a LiNbO3 electro-optic phase modulator.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Wavelength selective fiber to fiber optical tap

Abstract: Light is coupled out of a first mono-mode optical waveguide such as an optical fiber an into another single mode waveguide through a substantially planar slab waveguide. The first mono-mode optical waveguide has a grating impressed within which has refractive index perturbations that reflect predetermined wavelengths of incident light out of a portion of a side of the optical waveguide adjacent the grating; The slab planar waveguide has an end optically coupled to the portion of the first mono-mode optical waveguide, for guiding light coupled out of the side of the waveguide, and for maintaining a uni-phase wavefront of said light by having a response that confines the light to a single mode in one dimension, and multi-mode in another dimension and allows at least a portion of the light to converge.

Basic studies of fluctuations, transport and their control

Abstract: The first experimental study of a mixed slab - toroidal ITG (ion temperature gradient) driven mode in the Columbia linear machine (CLM) is reported. The effects of the modest levels of magnetic curvature are seen to be further destabilization and reduction of the real frequency of the mode. However, with a further increase in the mirror ratio, the mode amplitude decreased. The mode has the appearance of a hybrid ITG-dissipative trapped electron mode. We have measured the reduction of anomalous fluctuation driven particle transport due to feedback via cross-correlation of Langmuir probe and capacitive probe data for a centrifugal mode driven by E×B rotation of the plasma column. The particle flux is obtained from cross power spectra of density and potential fluctuation. The resulting diffusion coefficient clearly indicates that feedback can reduce the transport by up to a factor of three. To investigate the question of non-linear saturation of the mode we have performed a bispectral analysis of fluctuation data. The results clearly indicate strong mode coupling in the spectral width of the mode. Furthermore, we resolve the spectral width to reveal that it is composed of radial harmonics. These observations suggest that the nonlinear mode coupling between these radial harmonics is the saturation mechanism of the instability, as well as the underlying cause of the consequent anomalous transport.

Behavior of HE1m mode wavenumbers and power flow near the cutoff frequency in absorbing fiber tapers

Abstract: The known term cutoff denoting an abrupt transition from the guided mode to the leaky one becomes undefined when absorption losses in fiber material are taken into consideration.In order to study continuous transformation of the HE1m guided mode into the leaky one in the fiber taper with absorbing cladding we have analyzed behavior of eigenvalue equation solutions in the complex plane of transverse wavenumber of the mode in cladding. Radial profiles of cylindrical components of power flow density showing HE12 mode transformation in the fiber taper are plotted. Size of the region of cutoff is found depending on the losses in the fiber material. Caustical spatial structures are shown forming by the rays which has different directions in the radial points of transverse cross-sections of the fiber. Radial power flow is shown to form multiple cones of the power distribution in the plane parallel to the fiber axis, the number of the cones depending on the absorption losses.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Displacement sensing by spatial mode conversion between single mode to multi-mode fiber

Abstract: Summary form only given. There have been many displacement sensing methods based on optical fibers such as using mode scrambling of multi-mode fibers or intensity change of single mode fibers due to coupling mismatch. In this work, we used as displacement sensing the mode excitation change caused by spatial variation of input beam. A single mode fiber is used as a point-like input beam source for a multi-mode fiber. Since the core size of the multi-mode fiber is about 10 times larger than that of the single mode fiber, if two fibers are closely located and the multi-mode fiber is sufficiently long, the input beam from the single mode fiber excites many modes of the multi-mode fiber without power loss.