Showing papers on "Equilibrium mode distribution published in 2004"

Fundamental mode operation of a 19-core phase-locked Yb-doped fiber amplifier.

Abstract: We demonstrate, theoretically and experimentally, a high power/energy 19-core Yb-doped fiber amplifier that operates in its fundamental in-phase mode. The calculated result using an improved coupled mode theory with gain shows that, with a Gaussian beam as seed, the in-phase supermode dominates. Experimentally, we use a Q-switched single-core fiber laser with single transverse mode as seed, and amplify it with a 5.8 m 19-core fiber. The measured near and far fields are close to the in-phase supermode. The measured M2 factor of the amplified beam is 1.5, which is close to the theoretical value. A pulse energy gain of 20 dB is obtained with the amplified pulse energy up to 0.65 mJ at a repetition frequency of 5 kHz. No appreciable stimulated Brillouin scattering is observed at this power level.

X-band TM01-TE11 mode converter with short length for high power

Abstract: A mode converter for X-band located between a generator and an antenna to convert TM01 to TE11 mode with high efficiency for a high power transmitting system is presented. The proposed mode converter has extended radius and excessive curvature with symmetric structure in order to convert between two modes efficiently with short length. The measured and simulated results of the return loss, mode patterns and efficiency of the proposed mode converter with 200 mm length are provided.

Numerical solution of the power flow equation in step-index plastic optical fibers

Abstract: The numerical solution of the complete power flow equation is reported and employed to investigate the state of mode coupling along a step-index plastic optical fiber. This solution is based on the explicit finite-difference method and, in contrast to earlier solutions, does not neglect absorption and scattering loss. It is the only solution that can accommodate any input condition throughout the entire range of feasible input angles without the need for restriction to those angles that are sufficiently far away from critical. Our results for the field patterns at different locations along one type of fiber are in agreement with reported measurements earlier. Furthermore, the length of fiber required for achieving a steady-state mode distribution matches the analytical solution that is available for such distribution as a special case. Mode coupling in plastic fibers is known to affect fiber-optic power delivery, data transmission, and sensing systems.

Influence of numerical aperture on mode coupling in step-index plastic optical fibers.

Abstract: Using the power-flow equation, we have examined the state of mode coupling in step-index plastic optical fibers with different numerical apertures. Our results confirm that the coupling rates vary with the coupling coefficient of the fibers as the dominant parameter, especially in the early stage of coupling near the input fiber end. However, we show that the fiber’s numerical aperture has a significant influence on later stages of this process. Consequently, equilibrium mode distribution and steady-state distribution are achieved at overall fiber lengths that depend on both of these factors. As one of our examples demonstrates, it is possible for the coupling length of a high-aperture fiber to be similar to that of a low-aperture fiber despite the three-times-larger coupling coefficient of the former.

Measurement of optical dispersion in multimode polymer waveguides

Abstract: We have determined the dispersion in multimode polymer waveguides at 850 nm and 990 nm. Studies using short pulse propagation through differing lengths of waveguides show sub-linear temporal pulse broadening and short equilibrium mode distribution lengths.

Distributed optical structures exhibiting reduced optical loss

Abstract: An optical waveguide includes a set of diffractive elements. The diffractive element set routes within the waveguide a diffracted portion of an input optical signal between input and output optical ports. The input optical signal is successively incident on the diffractive elements. The optical signal propagates in the waveguide in a corresponding signal optical transverse mode substantially confined in at least one transverse dimension. A modal index of the signal optical mode or a modal index of a loss optical mode spatially varies along a signal propagation direction within the optical waveguide, or the loss optical mode is optically damped as it propagates along the optical waveguide. Said signal modal index variation, said loss modal index variation, or said loss mode damping yields a level of optical coupling between the signal optical mode and the loss optical mode at or below an operationally acceptable level.

Expanded single-mode fiber using graded index multimode fiber

Abstract: A compact large Gaussian mode field diameter single-mode fiber is developed This expanded single-mode fiber consists of splicing fibers with various lengths and characteristics The principle of this optical arrangement is to excite the first mode of propagation of the multimode graded index to preserve a Gaussian field The coupling properties are numerically analyzed, considering the effects of gap, radial offset, and tilt The fiber connections loss between the same kinds of expanded single-mode fibers with a Gaussian mode field diameter of 18 µm is measured with an average of 055 dB This large mode field diameter is preserved on a long length of fiber >20 cm

Intermode conversion in a near-field optical fiber probe

Abstract: The propagation of light in metal-coated fiber probes used in scanning near-field optical microscopy is studied computationally by means of the cross-section method. In contrast to previous work focusing on the behavior of the HE11 mode in the vicinity of the end aperture of the probe, we trace the propagation of the incident fundamental mode of the optical fiber through the tapered probe. The transformation of the fiber mode into the probe modes, intermode conversion, and modes cutoff are consistently calculated. In particular, it is shown that in the conventional gradual tapered probe only a few percent of the input power are transformed into the HE11 mode which has the smallest cutoff radius. In addition, an easily manufacturable modification of the shape of the probe’s tip that can provide a tenfold enhancement in optical near-field intensity has been proposed.

Initial-value problem for three-dimensional disturbances in a hypersonic boundary layer

Abstract: An initial-value problem is formulated for a threedimensional wave packet in a hypersonic boundary layer flow. The problem is solved using a Laplace transform with respect to time and Fourier transforms with respect to the streamwise and spanwise coordinates. The solution can be presented as a sum of modes consisting of continuous and discrete spectra of temporal stability theory. Two discrete modes, known as Mode S and Mode F, are of interest since they may be involved in a laminar -turbulent transition scenario. The continuous and discrete spectrum are analyzed numerically, and the following features are revealed: (1) the synchronism of Mode S with acoustic waves at low wave number is primarily twodimensional; (2) at high angles of dis turbance propagation, Mode F is no longer synchronized with entropy and vorticity waves; (3) at high angles of di sturbance propagation, the synchronism between Mode S and Mode F no longer leads to a Mode S instability, and at even higher angles of disturbance propagation, Mode S and Mode F are not synchronized.

Suppression of undesirable signal propagation mode(s) downstream of a mode converter

Abstract: An optical device (D) is dedicated to the transformation of the propagation mode of optical signals. This device comprises at least a first mode converter (3) that is supplied with signals that are propagated in a first guided mode and that delivers the signals in a multimode fibre (4) partly in the first guided mode and partly in a second guided more of a higher order that the first. The multimode fibre (4) comprises at least first passive filtering means (R) which have the task of converting the first guided mode into at least one dissipative cladding mode in order to prevent or limit the propagation of the signals in this first guided mode while at the same time authorising the propagation of the signals having the second guided mode in the multimode fibre (4).

Mode converter device using omnidirectional reflectors

Abstract: Provided is a mode converter for use in optical communication. The mode converter includes a first waveguide, a second waveguide optically coupled into the first waveguide, and omnidirectional reflectors that are disposed on either side of the first waveguide and have forward reflectivity with respect to a mode propagating in the first waveguide. The mode converter provides minimum loss coupling between either optical fiber or low index difference waveguide and a high index contrast waveguide. Furthermore, the mode converter achieves high, bi-directional optical coupling over a wide wavelength range with a simple manufacturing process.

Tearing-mode natural frequency behaviour under mode-locking in T-10 tokamak

Abstract: The results of the m = 2, n = 1 tearing mode natural frequency measurements in the course of the mode-locking by a controlled halo-current are presented. The halo-current in the rail limiter?plasma?vacuum vessel?external circuit with a controlled EMF source?rail limiter loop was used. The halo-current represented a superposition of a direct-current pulse for the mode-locking and an oscillating-current component for the natural frequency measurements by the resonant response of the mode. In the experiment, the mode signal oscillations at the frequency of the halo-current oscillating component were observed even if the mode rotation stopped under the pulse of the halo-current. The maximum amplitude of these mode oscillations was achieved when the frequency of the halo-current oscillating component was equal to the frequency of the mode before switching on the halo-current. It means that under the conditions of the T-10 experiment, there was no variation of the mode natural frequency upon termination of the mode rotation by the halo-current. This mode behaviour can be attributed to the well-known effect of frequency locking in an externally driven non-linear oscillator. The experiment was simulated with a model of a rotating non-linear tearing mode in the presence of the halo-current.

Robust slab-coupled buried-rib semiconductor laser with high fibre coupling efficiency

Abstract: A robust, single transverse mode slab-coupled buried-rib waveguide laser by combining the concept of slab-coupled waveguide and the technique of diluted waveguide is demonstrated. The large mode size and near-circular mode profile result in high fibre coupling efficiency and improved alignment tolerance.

A new type of coaxial TEM-circular waveguide TE_(11) mode converter

Abstract: Many high-power microwave (HPM) sources utilize an azimuthally symmetric output mode, like the TM_(01) circular waveguide or the coaxial TEM modes. If radiated directly these modes produce a doughnut-shaped pattern, with a bore-sight null. A novel mode-conversion technique for transforming the azimuthally symmetric mode to one with a more desirable radiated pattern is described and analyzed. The novel approach is as follows: first, the coaxial TEM mode is converted to TE_(11) fan-shaped waveguide mode by inserting fins into the coaxial waveguide, then the coaxial TE_(11) mode is got by varying the length of the fins, finally the coaxial TE_(11) mode is transformed to TE_(11) circular waveguide mode. A mode converter for 3.8GHz based on this principle is designed and optimized. The calculated results show that it has high conversion efficiency about 98.5% at 3.8GHz with an over 10% bandwidth where the efficiency exceeds 90%. The return loss is below 0.3dB in the range of 3.5～4.1GHz. The converter has the input and output co-aligned on the same axis and it’s easy to fabricate.

Study on the transmission loss of single mode of birefringence winding optical fiber

Abstract: Using the instrument for the transmission loss of single mode of winding birefringence optical fiber, the spectrogram of loss and the graph of mode density are obtained. The experimental results show that at the fixed incident angle the distribution of mode is independent for different curvature radius brings for the different loss.

Investigation of tm01 mode inversion in circular waveguide filled with dielectric layers

Abstract: The cutoff characteristic equation of the modes in circular waveguide is derived in this paper The TM01 mode inversion has been investigated Two kinds of technique of dielectric loading have been discussed When the dielectric tube is filled in the waveguide next to the waveguide wall, there is no window of TM01 mode inversion observed in the cutoff characteristics Contrarily, when the dielectric is filled centrally in the circular waveguide, there always exists one "window", within which the TM01 mode can propagate in the waveguide in the sense of single mode

Observation of noise-like solitons

Abstract: Noise-like ultra-short soliton pulses train of 72fs without CW components are observed from Figure-8 passively mode locked fiber laser; noise-like bound states of asymmetrical solitons train with pulse width of 103fs and separation of 585.5fs are also observed. The bound soliton separation and pulsewidth keep unchanged even after 1.2Km Single Mode Fiber transmission.

Optical Limiting in a Single Mode Waveguide System

Abstract: : The objective of this Trident project is to study the optical properties of single mode waveguide systems that exhibit an absorption that increases with the intensity of the light incident upon them. In this way these devices limit the transmission of optical energy and are referred to as optical limiters . Such systems are of great value to both the military and the telecommunications industry because of their ability to protect sensitive equipment from exposure to high intensity light. Experiments were performed using very small glass capillaries filled with materials that exhibit a nonlinear absorption. In this case, the materials absorb a greater amount of light as the intensity increases. These systems act as waveguides, confining the light to a small core region where the nonlinear material resides. Furthermore, only one intensity distribution (or mode ) is allowed if the index of refraction of the core is very close to that of the surrounding glass. In this case, the waveguide is called single mode. Single mode behavior was achieved by controlling the temperature of the waveguide in order to tune the index of refraction of the core material. Pulses of visible light (wavelength = 532 nm, pulse width = 4ns) were coupled into single mode waveguides and the input and output intensities were measured. These results were compared to optical limiting observed in multi-mode waveguides. It was observed that multi-mode waveguides exhibit better optical limiting than single-mode waveguides. This is because the entire mode structure is confined to the core in a multi-mode waveguide. However, a portion of the mode extends into the cladding in a single-mode waveguide. In addition, single-mode waveguides show a decreased ability to limit at higher temperatures where the difference in index of refraction is smallest.

POSSIBILITY OF PLASTIC OPTICAL FIBER AMPLIFIERS DOPED WITH Er(C8H11F8O2)3

Abstract: Plastic optical fiber amplifiers (POFAs), doped with Er(C8H11F8O2)3, having an energy system showing 3-level features have been investigated theoretically focusing on the condition for amplifying mainly the lowest signal mode (LP01 mode). We have studied the dependence of signal gain on fiber structure design for circular and ring doping conditions and have shown that the gain for the fundamental mode (LP01) and the second order mode (LP11 mode) is a strong function of the outer radius (ro) of the doping area. The gain for the LP01 mode becomes larger than that of LP11 mode when ro is less than 110 μm with doping concentration (N0) of 3 × 1020cm-3. A signal gain of 18.2 dB is predicted for LP01 mode for 9 cm long POFA with circular doping and ro of 80 μm when N0, the pump power and the core radius are 2.66 × 1020cm-3, 30 W and 250 μm, respectively. The gain is 12 dB larger than that for LP11 mode under the same condition.

Device and method of transformation of signal propagation modes by interference

Abstract: The device (D) has two mode converters (3,5) coupled to each other by a portion of a multimode fiber (4). The converter (3) receives signals propagating in a fundamental mode, and delivers two signals according to the fundamental mode and a higher order mode respectively, to the fiber (4). The converter (5) is arranged to interfere the signals according to the two modes to provide a chosen spectral spacing at output. A mode transformation means (3-5) transforms modes associated to a multimode fiber (2). Independent claims are also included for the following: (a) a process for transformation of a propagation mode of optical signals (b) a use of an optical device and a propagation mode transformation process in a group having interlacing or multiplexing of modes, modes filleting, and mode changing.

Selective coupling to higher order modes dual mode step index fiber

Abstract: The use of a vortex phase optic as a coupling element for a dual mode step index fiber system is presented. We introduce a novel but rather simple coupling scheme for selectively exciting the LP11 mode in a step index fiber that is a single mode at a longer wavelength. This scheme can be utilized to optically pump fiber lasers and amplifiers.