Showing papers on "Equilibrium mode distribution published in 1990"

Bi-domain two-mode single crystal fiber devices

Abstract: A mode converter comprises an a-axis LiNbO3 optical fiber exhibiting a ferroelectric bi-domain structure. The fiber is subject to an electrical field that induces a +π/2 phase retardation in one domain of the fiber and a -π/2 phase retardation in the other domain. A light signal launched in the fundamental mode of the fiber is converted into a light signal propagating in the second order mode. When the electrical field is selected so that the phase retardations are not multiples of π/2, the mode conversion is partial and the LiNbO3 fiber can operate as an optical switch or as an amplitude modulator. The mode converter can also be operated as a second harmonic generator. The fiber is heated to a phase matching temperature so that a signal launched in the fundamental mode of the fiber and at a frequency ω is converted to the second order mode at a frequency 2ω. The LiNbO3 fiber can also simultaneously operate as an optical switch and as a second harmonic generator. Other non-linear interactions are possible such as sum or difference frequency generation or parametric generation. The various embodiments of the present invention are reciprocal.

1/f phonon-number fluctuations in quartz observed by laser light scattering.

Abstract: The energy of each phonon mode is fluctuating around its equilibrium value, and its power spectral density was derived with a quartz specimen which was illuminated by a laser light. Fluctuations of the Brillouin scattered light intensity were measured by means of the photon-counting technique. The power spectral density has been derived to be 8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$/f per phonon mode provided that phonon-number fluctuations are independent from mode to mode.

The mechanism of mode selection for an external cavity laser

Abstract: The process of mode hopping between external cavity modes of a laser diode with optical feedback is described by analogy with noise driven transitions of a particle between potential valleys. The model explains why the dominant mode is the mode with the lowest linewidth and not the mode with the lowest carrier density. It also gives a simple analytic expression for the rate of mode hopping. The expression is shown to predict the results of computer simulations to a good approximation. >

Mode structure of hollow dielectric waveguide lasers

Abstract: The mode structure of hollow dielectric waveguide lasers with free space sections and flat mirrors is studied theoretically and experimentally. The study covers the fundamental mode and the three most important higher order modes, and graphs are given which identify regions of high mode discrimination in the parameter space. Calculated coupling losses are verified experimentally by detailed studies of the output power of CO2 lasers as a function of resonator geometry. The intensity profile inside and outside the resonator is calculated, and the profile outside the resonator is compared with experiments for the fundamental mode as well as for the higher order modes. It is shown that in general the fundamental mode is non-Gaussian, and that drastically different output characteristics are obtained for different choice of output plane. The paper identifies design criteria for obtaining single line and single mode oscillation over a wide tuning range, even in the densest region of the CO2 laser line spectrum, and this is exemplified by a spectroscopic application.

Dependence of the linewidth of a semiconductor laser on the mode distribution

Abstract: A theory and measurements that show the dependence of the frequency noise spectrum and, therefore, of the semiconductor laser linewidth on the mode distribution are presented. The theory is based on rate equations where the linewidth is strongly influenced by the gain saturation coefficients (nonlinear gain). Analytical results are restricted to two modes. Measurements show that the linewidth change of the main mode depends on the wavelength side of the side mode (related to the wavelength mode of the main mode) and the effect decreases with increasing mode separation. Furthermore, the theoretical and measured frequency noise spectra of a three-mode laser are depicted, showing a characteristic resonance peak for certain mode distributions. This property of the frequency noise spectrum is also described by the theory. >

Fiber optic mode conditioner

Abstract: A fiber optic mode conditioner for obtaining an equilibrium light distribution in a multimode fiber optic link under test. Light is supplied to the link through a gap between two successive fiber optic segments, followed by a high-curvature region created by wrapping the second fiber optic segment around a mandrel. The gap functions as a mode filter, stripping off all but the lowest modes, while the curved region acts as a mode scrambler redistributing the light energy into the intermediate modes.

Analysis of the cross-coupled lateral mode in a twin-stripe four-contact laser with diagonal current injection

Abstract: Cross-coupled resonant lateral modes in a twin-stripe four-contact laser are analyzed by coupled-mode theory for conditions in which the current injection into each diagonal pair of stripes is equal. The dependence of the optical power distribution on the laser length and on the carrier-density or the current-density distribution is investigated. The most interesting result is that the highest optical power interesting result is that the highest optical power appears in the waveguide with the lowest carrier density when the carrier-density difference between the adjacent waveguides is small and the laser is shorter than the coupling length. This makes it possible to show that a cross-coupled mode can be supported even in a two-contact twin-stripe laser and that bistable switching between the mode and its mirror image is possible. >

Analysis of leaky modes supported by a slab waveguide

Abstract: The electromagnetic waves propagating along an open waveguide can be classified in several ways. For instance: (1) guided modes and leaky modes; (2) real modes and complex modes; and (3) proper modes and improper modes. However, their mutual relationships are not clear. In addition, the physical meaning of the leaky mode is not well understood in comparison with the guided mode. In this paper, the relationship of the modes propagating in a dielectric slab waveguide is investigated. First, from the dispersion equation, the guided mode cutoff frequency and the leaky mode cutoff frequency are obtained so that the regions of the guided mode and the leaky mode are found. It is shown also that there exists an improper mode region with real propagation constants between the guide and leaky mode regions. This improper mode does not decay in the direction of propagation and the amplitude increases away from the waveguide along a straight line with an arbitrary angle from the propagation direction. In this region, the group velocity may exceed the speed of light in free space or become infinite. Thus, in this paper, this region is called the anomalous group velocity region. The width of this region is inversely proportional to the mode number and the waveguide width.

Mode conversion losses in highly overmoded waveguide cavities

Abstract: A technique to quantify the effects of mode conversion losses in highly overmoded waveguide cavity resonators using the WKB method is presented. The WKB theory is accurate for weakly irregular waveguide cavities with small amounts of mode conversion. This approach to the problem has the advantage that the mode conversion losses in waveguide cavities can be quantified quickly and cheaply. The objective is to compute the amplitude and phase of a resonant mode in the cavity to zeroth order (i.e. without regard to mode conversion effects), and then use amplitude and phase of the computed zeroth-order fields to compute the amplitude of the converted mode. This approach is valid to the extent that the converted modes are not a major perturbation to the resonant mode. The results of the theory are presented in the context of the trade-offs associated with the design of an 85 GHz TE/sub 1.3/ phase-locked gyroklystron oscillator experiment. Mode conversion losses in the input cavity are a particular problem, and place limits on the amount of usable beam current (and therefore the output power capabilities of the gyroklystron). The isolation of the input and output cavities due to mode conversion must be carefully considered in any design. >

Y-branched light circuit

Abstract: PURPOSE:To branch the optical power of an incident waveguide to two outgoing waveguide in stable and uniform manner by irradiating the high order mode and leaky mode for the propagation of the incident waveguide in a Y-branched light circuit into a substrate in a low equivalent refractive index waveguide part and propagating only the basic ground mode into a branched part. CONSTITUTION:An incident waveguide 1 and an outgoing waveguides 2 and 3 are the straight line waveguides, and these waveguides are connected so as to cross in Y-figure form in a branched part 4. Since the high order mode and leaky mode for the propagation in the incident waveguide 1 are irradiated in a substrate 5 by a low equivalent refractive index waveguide passage part 6, and since only the basic ground mode s propagated to the branched part 4, the optical power of the incident waveguide passage 1 can be branched uniformly into two outgoing waveguides. Accordingly, the branched ratio variation for the axis deflection incidence, preparation error of the waveguides 1-3 and the preparation dispersion is hardly generated, and the branching ratio of 50:50 or close to this value can be obtained within the wide wave length range.

Second Harmonic Generation as a Tool for Measuring Mode Coupling, Pulse Length and Mode Size in Short Laser Pulses

Abstract: We report on a detailed analysis of second harmonic generation with a multi-modal beam for a non-collinear geometry. As a result some improvements for a picosecond pulse width analyser are suggested. Experimental results relative to both a free electron laser and to a mode locked Nd: YAG laser are discussed. This modified version of the pulse analyser is able to monitor, in real time, the relative pulse-to-pulse change of mode size, pulse length and mode mixing effect independently of the pulse length from femtoseconds to milliseconds.

Waveguide arrangement for low-attenuation transmission of microwaves of the dominant mode

Abstract: It is intended to specify a waveguide arrangement which transmits microwaves of the dominant mode with as little attenuation as possible, using means of little complexity. A transition waveguide (HL2), which consists of a plurality of sections (A1...A7) of graduated cross-section, is connected to at least one end of a multi-mode transmission waveguide (HL1). Arranged at the side of the transition waveguide (HL2) is at least one chamber (K11, K12, K21, K22) which is at least partially filled with an absorbent material (A3) and is coupled to the transition waveguide (HL2) at a point where it is possible to output only at least one higher mode and not the dominant mode. Those sections (A1) of the transition waveguide in which other higher modes are capable of existence, for which no output is provided, and those sections (A2) which are adjacent to these sections and cause periodic attenuation of the said modes are dimensioned in terms of their length such that forward-running and backward-running wave elements which occur in these sections (A1, A2) of each mode are superimposed in antiphase. This waveguide arrangement is particularly suitable for the transmission of the dominant mode at frequencies which are greater than twice its cut-off frequency.

Waveguide-type optical isolator with in-plane magnetization structure

Abstract: A novel unidirectional mode converter, which has the advantage of a simple magnetization structure, is proposed for an application to a waveguide-type optical isolator. The proposed mode converter is constructed with in-plane magnetization in the whole device, where the magnetization component along a light propagation direction produces a mode conversion between TE0 and TM0 modes, while the magnetization perpendicular to the propagation direction provides a nonreciprocal phase shift to the TM0 mode. The unidirectional mode converter, in which, in the backward direction, 100% TE-TM mode conversion is produced but, in the forward direction, an input TE mode emerges as a TE mode, is realizable by virtue of the nonreciprocal phase shift of the TM0 mode. Design examples are shown by using the (LuBi)3Fe5O12 guiding layer / (YBi)3Fe5O12 cladding layer / Gd3Ga5O12 substrate structure. A typical length of the mode converter is 15.5 mm at the wavelength of 1.152 .tm for the guiding layer thickness of O.575jim and with the magnetization direction tilted by 87 degree from the light propagation direction in the film plane. A relatively large device length is reduced by using a magnetooptic material having a large first-order magnetooptic coefficient.

Waveguide type optical circuit element

Abstract: A waveguide type optical circuit element having a directional coupler for converting light intensity between two waveguides, which is used as a polarity-independent optical switch or a variable optical divider for optical fiber communications and the like. The two waveguides (2), (3) have a coupling section length which is in a TE mode about three times a complete coupling length of the TE mode, and in a TM mode about one time a complete coupling length of the TM mode. Electrodes (4), (5) comprise three-part electrodes of different lengths which act as reverse Δβ electrodes for alternately reversing a direction of an electric field. This construction promotes the light-confinement effect of the waveguides, reduces the drive voltage necessary for carrying out an optical switching operation, and realizes optimal element characteristics in accordance with a ratio of variations in the propagation coefficient in the TE mode and TM mode of the directional coupler manufactured.

Measurement Of Mode Field Distribution And Evaluation Of Mode Field Radius In Single Mode Optical Waveguides

Abstract: There are many techniques currently used for measuring the fundamental Mode Field Distribution (MFD) of a single-mode optical waveguide. There are also many definitions of field width which can be used for characterising the measured MFD. These definitions are key parameters for describing the propagation, and other properties of any optical waveguide. A theoretical study of the evaluation of the various field width definitions from measurement data (containing noise) is described. From this study, the relationship between both accuracy and repeatability of evaluation of the definitions of Mode Field Radius (MFR) and the measurement Signal to Noise Ratio (SNR) for the various measurement techniques has been found. Tables and graphs of data are presented which indicate the necessary measurement SNR for a required accuracy of evaluation of a desired definition of MFR.