Showing papers on "Radiation mode published in 1975"

Electrooptic coupling between TE and TM modes in anisotropic slabs

Abstract: In an electrooptic crystal TE and TM modes can be coupled by application of a modulation voltage. Two types of modulators using anisotropic crystals are discussed. In the first type a voltage that is constant along the direction of wave propagation couples a guided mode to a radiation mode of the orthogonal polarization so that the radiation loss of the guided mode can be controlled by the applied voltage. The modulator of the second type uses voltage-controlled coupling between guided TE and TM modes. The computation of the coupling coefficients is outlined and numerical examples are plotted for LiNbO 3 and LiTaO 3 crystals.

Surface-wave radiation loss from curved dielectric slabs and fibers

Abstract: A newly developed systematic procedure to calculate the real and imaginary parts of the change in propagation constant of a surface-wave mode on a curved open waveguide of general cross section is used here to determine these quantifies for the TE modes of an asymmetric slab waveguide and for all the modes of an optical fiber. Comparison of these results with the existing literature points up the care which must be taken in making approximations in this problem, since errors of significant factors in the radiation loss have been made in previous work.

Optical waveguide interferometer modulator-switch

Abstract: An optical waveguide arrangement operates in the manner of an interferometer to provide modulator and/or switching functions. Two optical waveguide branches having a common connection diverge along a substantially coextensive distance and reconverge to provide first and second light paths of identical optical length. Conductive electrodes are disposed contiguous to at least one of the optical waveguide branches and are connectable through a controlling switch to a source of electrical energy for producing an electric field across the energized electrodes. In the absence of such electric field, light energy entering the two optical waveguide branches will propagate along identical optical path lengths and recombine constructively at the reconvergence of the waveguide branches. However, a selectively applied electrical energy changes the optical property of at least one of the optical waveguide branches causing phase differences in the optical energy transmitted by the branches and producing destructive interference at the reconvergence of the optical waveguide branches. The addition of two optical waveguide sections having different dimensions provides a switch function when one of such additional waveguides is dimensioned to be conducive to the propagation of first order mode light energy while the other waveguide is dimensioned to be conducive to the propagation of second order mode light energy.

Mode study, beam characteristics and output power of a CW 377μm HCN waveguide laser

Abstract: Describes the first continuous submillimetre waveguide laser operating on the 337 mu m HCN line, and presents the results of the first comprehensive experimental study of its low-loss modes. The laser cavity is a 5 cm diameter, 2.4 m long Pyrex tube with plane reflectors (one of them is a metal mesh) against its ends. Eight waveguide modes were identified. Intensity distributions in the beam cross section and mode polarization were studied. The results agree with theoretical predictions for dielectric waveguide modes. The propagation of the EH11 mode in free space was also studied. The beam propagates as a gaussian beam at moderate and large distances from the mesh coupler. The maximum output power of the laser is of the order of 100 mW, representing the highest output power per unit volume so far reported for a continuous HCN laser.

Chasi-Guided Modes and Related Radiation Losses in Optical Dielectric Waveguides with External Higher Index Surroundings

Abstract: Mode filter actions are found theoretically in an optical dielectric waveguide consisting of a core and a thin cladding layer which is further surrounded by an external higher index region. The propagating waves, which are usually considered to be cutoff modes, can be guided with a small amount of loss under a certain condition. These waves are defined here as quasi-guided modes. These modes tend to the guided modes of the guide when the cladding thickness increases infinitely. A method is given to estimate the losses. As an example, the radiation losses are formulated for a symmetric slab waveguide, and are found to be approximately proportional to the cube of the mode number of the quasi-guided mode. Therefore, losses of the quasi-guided modes depend strongly on the mode number. It is suggested that fibers with large core diameters can be used as quasi-single mode fibers by covering the clad-type multimode fibers with external higher index surroundings and choosing the parameters properly.

Directional optical waveguide coupler and power tap arrangement

Abstract: A directional optical waveguide coupler and power tap is disclosed for use with optical fibers or planar-form optical waveguides. Abutted to the end of a cladded optical transmission waveguide is a section of a second optical waveguide comprising a core surrounded by a cladding layer, the core having a smaller cross-sectional area than the core of the transmission waveguide. In the present arrangement, light energy propagating in the core of the second waveguide towards the transmission waveguide is coupled into the core of the transmission waveguide at the point of abuttment, whereas light energy propagating in the core of the transmission waveguide towards the second waveguide is coupled into both the core and cladding layer of the second waveguide at the point of abuttment. Means disposed along the second waveguide adjacent the point of abuttment couples out of the cladding layer the cladding modes transferred from the core-guided modes of the transmission waveguide for subsequent conversion into a representative electrical signal. A stripper-absorber means is also disposed along the second waveguide for removing spurious cladding modes propagating therebeneath towards both the coupling out means and the point of abuttment between the two waveguides.

Excitation of parabolic-index fibers with incoherent sources

Abstract: We study the excitation of a parabolic-index fiber by an incoherent source. The theory is based on approximating the fiber modes by Laguerregaussian functions. The dependence of the total light power injected into the fiber core on the separation between source and fiber, and on the transverse displacement of the source, is shown in graphic form. Also shown are far-field radiation patterns, which indicate the distribution of power versus mode number, for several launching conditions and plots of power versus azimuthal mode number for given values of the compound mode number. The study of launching efficiency versus source radius leads to prescriptions for optimizing the ratio of source to fiber core radius without a matching lens. Use of a lens for matching the image of a small source to the size of the fiber core increases the launching efficiency relative to the power consumption of the light-emitting-source diode.

Image waveguide transmission line and mode launchers utilizing same

Abstract: A receiver module including a mode launcher, a band pass filter, a local oscillator and a balanced mixer for use in the frequency range 1 GHz to 1,000 GHz, wherein the components are all connected by an insular waveguide transmission line having a conductive image plane, a dielectric waveguide attached to the image plane by a thin film of plastic, the thin film being low loss in character and having a low permittivity compared with that of the dielectric waveguide, the ratio between the thickness of the thin film and the square root of the cross-sectional area of the dielectric waveguide being in the range from about 0.02 to about 1.0.; there also are disclosed signal processing components made of insular waveguide transmission lines including a directional coupler, a symmetrical quadrature hybrid coupler, and a ring resonator filter that can be used as a band rejection or band pass filter either with a single pole or multiple pole, as well as mode launchers for connecting the insular waveguide transmission line to a metal waveguide and a coaxial line.

Radiation Fields of a Tapered Film and a Novel Film-to-Fiber Coupler

Abstract: Light in a tapered thin-film optical waveguide radiates into the substrate because the waveguide mode in the taper becomes cut off. Our measurement of the radiation pattern shows that the light emerges from the taper as a narrow beam with an angular width of only 2/spl deg/-4/spl deg/. We have studied the problem based on ray optics and based on a wave theory of radiation modes. We also have demonstrated a film-to-fiber coupler in which an optical fiber collects all the light emerging from the taper.

Optical waveguide coupler employing deformed shape fiber-optic core coupling portion

Abstract: More efficient optical coupling between light wave energy traveling in a fiber-optic, which may be a multimode fiber-optic, and single or few mode light wave energy traveling in a planar optical waveguide is accomplished by deforming the initially cylindrical shape of the coupled portion of the fiber-optic core, such as by flattening the coupled portion of the fiber-core in a direction substantially perpendicular to the plane of the planar optical waveguide and fanning out this portion of the fiber-optic core in a direction substantially parallel to the plane of the planar optical waveguide.

Selection of modes perpendicular to the junction plane in GaAs large-cavity double-heterostructure lasers

Abstract: The gains and reflectivities of the transverse-electric (TE) modes of GaAs large-cavity double-heterostructure (DH) (P-p-n-N) lasers havebeen calculated in order to study mode selection perpendicular to the junction plane. The GaAs laser is modeled as a passive dielectric-slab waveguide composed of four layers whose optical constants are taken from recent literature. In lasers having a cavity thickness of a few microns, a small difference in refractive index between the p- and n-region leads to the fundamental mode being confined to the p-region. Discrimination in favor of this mode without significant penalty in threshold gain may be achieved by utilizing the mode-dependent loss in the n-layer. It is found that optimum mode discrimination in favor of the fundamental mode is governed by a suitable combination of the p-layer thickness, loss in the n-region, and the difference of refractive indices of the p- and n-layers. This analysis contrasts with an earlier analysis of the case of uniform refractive index in which it was found that the p-layer should be twice as thick as the n-layer for optimum mode discrimination. The calculated mode shapes in P-p-n-N lasers are found to be in qualitative agreement with those observed recently by Hakki, and also with earlier observations that catastrophic mirror damage in P-p-n-N lasers is confined to the p-region. Contrary to experimental observation, the model predicts that lasers having cavity thicknesses as large as 3 μm should operate stably in the fundamental mode. Possible reasons for this discrepancy are advanced. A simplification of the P-p-n-N laser, which consists of omitting the N-layer thus yielding an asymmetric double-heterostructure laser, is suggested, and the merits and drawbacks of such a design are discussed.

Acoustic waveguide with a cladded core geometry

Abstract: A new type of acoustic waveguide analogous to the clad optical fiber waveguide is described and experimental results are given. Modes exist in this structure which confine the acoustic energy to approximately the core due to the lower acoustical velocity in this region. The waveguide is excited with specially electroded bulk shear transducers bonded to the end surfaces. In the ∼100−MHz region both radial and torsional type modes may be attractive for long−delay (near 1 msec) low−loss serial memory applications.

Diffraction coupling of radiation into a thin-film waveguide

Abstract: A thin-film optical waveguide with a sinusoidally corrugated surface is considered. This waveguide acts as a phase diffraction grating. The relationships between the waveguide parameters and the parameters of the exciting beam, ensuring the most effective coupling of the radiation into a specified waveguide mode, are determined for an arbitrary ratio of the grating period to the wavelength of light and for a relatively weak depth of modulation of the film thickness (i.e., for relatively weak coupling conditions).

Radiation losses of the HE 11 mode of a fiber with sinusoidally perturbed core boundary

Abstract: The radiation losses of the HE11 mode of a fiber with sinusoidal radius fluctuations are calculated in the weak guidance approximation. The loss formula is derived by means of the volume current method and also by using coupled mode theory. The result presented here extends earlier results to the case of radiation escaping nearly perpendicular to the fiber axis.

Coupled mode analysis of light modulation in dielectric waveguides

Abstract: A detailed analysis of electrooptic light modulation in optical waveguides is presented. Several important problems of a waveguide modulator, such as the difference of waveguide axes from crystalline electrooptic ones, the distribution of transverse and longitudinal field components of light modes, and the traveling-wave property of the modulating field, are discussed. The analysis is based on the coupled mode theory, regarding the modulation as the coupling among sidebands of unperturbed waveguide modes. The coupled mode equation is derived for the modulation in optical waveguides. It can be solved if the normal modes of the waveguide are given. Actually the equation is solved for the modulation in dielectric slab waveguides and the mechanism of modulation is discussed. The results of the analysis are applied to designing two types of waveguide modulators. In an example (10.6 μm modulator with a GaAs slab waveguide) a new efficient crystal orientation is found. The calculated phase retardation with this orientation is 0.13 rad/(V·cm) with a 1 μm thick slab. Another example of a 0.633 μm modulator using a LiTaO3 crystal as a substrate is also described.

Coupling to graded index waveguide

Abstract: A specific propagation mode is excited in a multi-moded, graded index, planar waveguide by coupling an optical fiber to the waveguide a specific distance below the surface of the guide.

Mode coupling and radiation loss of clad-type optical waveguides due to the index inhomogeneities of the core material

Abstract: Closed form approximate expressions are derived for the mode-coupling, radiation loss and losses of each guided mode in multimode clad-type dielectric slab waveguides due to the index inhomogeneities of the core material. The analysis considers the inhomogeneities of the core-index in the axial and transverse directions and includes correlation lengths of any magnitude. It allows us to conclude that there will be considerable mode-dependency for the mode-coupling and radiation loss if the axial and/or transverse correlation lengths are of the order of or greater than the wavelength. It is also found that the mode-coupling and radiation loss peak at some value of the axial correlation length, and the radiation loss can be reduced by increasing the refractive-index difference of the core and cladding materials. Finally, it is shown that if the correlation lengths are shorter than, or of the order of the wavelength, the losses of each guided mode will mainly be due to the radiation process. But, in the case where the axial correlation length is much greater than the wavelength, the scattering into other guided modes is the dominant mechanism.

Advances in Communication through Light Fibers

Abstract: Publisher Summary This chapter focuses on the advances in communication through light fibers. Recent breakthrough in the purification of glass has made it possible to manufacture very low-loss optical fibers. The fibers provide almost all the attractive features of an ideal optical transmission line, such as, low cost, small size, lightweight, flexible, larger bandwidth, high tensile strength, no short circuiting issues, resistant to nuclear radiation, and heat resistance. These optical fibers are used as the communication media for transmitting optical signals. The bandwidth of a single-mode fiber is limited by the dispersion of the normal mode and by the material dispersion effects. One of the unique characteristics of a dielectric fiber waveguide is that below a certain frequency called the cut-off frequency of a particular mode, that mode simply no longer exists. A fiber of a given cross section will support a finite number of guided modes and a continuous spectrum of radiation modes. Any irregularity in the fiber because of diameter variation, bending, interface irregularities, or the presence of scattering centers will produce coupling of the energy of one mode to the others. Mode coupling within an optical fiber may produce the desirable effect of reducing the delay distortion that result from uncoupled multimode operation or may produce the undesirable effect of signal contamination.

Communication system having low dispersion glass optical waveguide

Abstract: An optical waveguide for use in an optical communication system employing a broad spectral source. Mode coupling is present in the propagation of optical signals in the waveguide, and the parameters of the waveguide are such that pulse dispersion produced by the finite spectral width of the source is minimized by utilizing the mode coupling to substantially cause cancellation of dispersion which is linear with respect to waveguide length and which is caused by the mode independent material dispersion and by the mode dependent waveguide dispersion.

Fiber acoustic waveguide and system

Abstract: A fiber acoustic waveguide comprising a single crystal fiber and a single ystal film deposited on the outer elongated surface of the single crystal fiber, wherein the single crystal film has a higher velocity of propagation than the single crystal fiber. The fiber acoustic waveguide has a delay that is directly proportional to its length, whereupon fixed lengths of the waveguide provided fixed delay. A fiber acoustic waveguide system is provided by connecting one end of the waveguide to driving transducer means for converting an electrical signal to an acoustic signal, and the other end of the waveguide to pickup transducing means for converting the transported acoustic signal from the waveguide back into an electrical signal.

Waveguide section for connecting rectangular waveguide with elliptical waveguide

Abstract: A connecting piece for connecting a rectangular waveguide with an elliptical waveguide is composed of a waveguide section having a rectangular cross section identical to that of the rectangular waveguide to be connected, the waveguide section being provided, in the center of at least one of its wide sides, with a cylindrical recess at a distance of 1/8 of the waveguide wavelength with respect to the center of its longitudinal dimension from the plane of connection with the elliptical waveguide, the length of the recess in the longitudinal direction of the waveguide section being less than 1/4 such wavelength and the depth of the recess being selected in dependence on its length to match the two connected waveguides over a broad frequency band.

Low order mode propagation photo fiber

Abstract: PURPOSE:Holding group delay time difference between modes small, allowing wide band characteristic, and making core diameter larger than that of single mode fiber.

Launching efficiencies of the HE(1m) modes in a selffocusing optical fiber waveguide.

Abstract: The launching efficiencies of the HE1m modes and the over-all launching efficiency are calculated for the cases in which a self-focusing optical fiber is excited axially by a TEM0n beam mode or by a beam consisting of several beam modes. It is shown that, when the fiber is excited by a TEM00 mode whose spot size s is very large compared with the characteristic spot size so of the fiber, the HE1m modes are almost equally excited and that, if s is matched to so, the over-all launching efficiency is nearly 100%. The over-all launching efficiency as well as the launching efficiencies of the HE1m modes decrease considerably when the fiber is excited by a beam containing the TEM01 mode that is out of phase with the TEM00 mode. The effect of the off-axis excitation by a TEM00 mode on the over-all launching efficiency is also examined.

Transverse mode control of junction lasers optically coupled to single-mode fibers

Abstract: Preliminary experimental results are reported to demonstrate that fundamental transverse mode operation can be obtained from a multimode double-heterostructure (DH) junction laser while simultaneously coupling its output into a single-mode optical fiber. Mode selection has been achieved by three different methods: 1) incorporating the fiber into the resonant cavity; 2) using the reflections off the diode-fiber interface; and 3) by a combination of the aforementioned methods. In all approaches the diode-fiber interface effectively becomes a mirror with a reflectivity which is higher for the fundamental mode than for all other modes of the DH waveguide. By using the reflections off the diode-fiber interface, a peak pulsed power of 80 mW has been obtained from the uncoated end of a single-mode fiber butted against a P-p-n-N junction laser. This laser radiated a peak power of 165 mW in a first-order mode when operated without coupling to the fiber.

Two wavelength measurements of optical waveguide parameters.

Abstract: Optical waveguide parameters, guide index of refraction and thickness, can be measured from the knowledge of mode propagation constants at two different wavelengths. This method is as accurate as the two-mode method and can be used for both single-mode guides and multimode guides. The index of refraction of various optical components must be known at both the wavelengths; in cases where these indices are not directly available it is possible to interpolate this information from the known properties of other glasses. This paper describes the experimental and analytical aspects of the two-wavelength measurement technique.

Calculation of the matching of an injection laser to a dielectric waveguide

Abstract: Calculations are reported of the parameters of a rectangular-core fiber waveguide needed to achieve an efficient matching to an injection laser which is in direct optical contact with the waveguide. Simple expressions are obtained for the relationships between the permittivity discontinuity at the core-cladding interface and the optimal thickness of the core on the one hand, and the laser parameters on the other. The requirements which the passband of the waveguide must satisfy impose restrictions on the laser-waveguide coupling coefficient. If the permissible distortion of a laser pulse is 30 nsec/km, about 65% of the injection laser radiation can be coupled into the waveguide.

The experimental determination of higher-order mode conversion in a multimode waveguide

Abstract: An overmoded rectangular waveguide system which operates in the vicinity of 30 GHz is used to investigate multimode propagation problems. By careful design of the input waveguide either a relatively pure TE10 or a combination of TE10 and higher modes may be launched into a 3 m long overmoded test section of the system. The overmoded section is terminated in a matched load. The waveguide will support TEn0 modes where n is any odd integer between 1 and 13. Even-order modes are normally suppressed, if they arise at all, by a longitudinal slot along the centre of one of the broad faces of the waveguide. Electric field amplitude and phase profiles within the overmoded waveguide are measured continuously, either transversely or longitudinally. Experimental measurements of the field within the waveguide enable mode content to be determined for comparison with theoretical data. In this way the amplitudes and phases of the higher-order propagating modes are determined.

Circular teon umode filter

Abstract: A circular arc polygonal type waveguide provides an excellent TEon mode filter. In a circular waveguide and/or a helix waveguide for transmission of millimeter-wave energy, the electromagnetic field of the fundamental TE01 mode converts easily to that of undesirable higher order TEon modes. This conversion is undesirable for the transmission of millimeter-wave energy. According to the invention, the undesirable TEon mode is converted purposely to a higher order TEpq mode by the present mode filter, the profile of which is slightly deformed from as compared to the perfectly accurate circular one, and said converted TEpq mode is absorbed in a conventional helix waveguide. Thus, the fundamental TE01 mode is transmitted in a waveguide without distortion. BACKGROUND OF THE INVENTION The present invention relates to a mode filter or a suppressor for an undesired mode for a circular waveguide for transmission of millimeter-wave energy, which involves very small loss of the TE01 mode and very large loss for higher modes. The TEO1 mode is generally utilized for the transmission of millimeter-wave energy with millimeter wavelengths (for instance 40 - 100 GHz) since the transmission loss of the TE01 mode is very small in that frequency band. In a circular waveguide with a diameter several times larger than the wavelength of the energy to be transmitted many higher modes other than the TE01 mode appear, since the TE01 mode is not a dominant or principle mode for a waveguide of the above size. A slight deformity of the circular waveguide, a corner waveguide at bend portions and/or an elastic or expansion waveguide are triggers for generation of higher TEon modes. These undesirable higher TEon modes should be absorbed for the TE01 mode transmission. It is very difficult to eliminate unwanted TEon modes since the electromagnetic field of the higher TEon modes are closely similar to that of the fundamental TE01 mode. Various types of TE mode filters which absorb these TEon modes have been proposed. Some of them are a distribution coupling type filter, a long slit type filter, a resonant slit type filter and a phase reverse type filter. But these types of prior mode filters have the following disadvantages: (i) Their structures are very complex, requiring high degree of manufacturing accuracy and thus are very expensive: (ii) It is difficult to obtain large inner diameters such as 51 mm and so, tapered waveguides are needed to mate and these tapers may generate other TEon modes. (iii) TE01 mode loss is relatively large. (iv) The structures of the filters are very complicated and different from that of the waveguide line. SUMMARY OF THE INVENTION The object of the present invention is to provide a TEon mode filter which overcomes the above-mentioned drawbacks. Another object of the present invention is to provide a mode filter of simple structure which is similar to the structure of a circular or helix waveguide. Still another object of the present invention is to provide a mode filter of short length, with a very wide frequency band. The above and other objects are attained by a mode filter comprising a circular waveguide of a predetermined length, the cross-section of said waveguide being almost circular but being slightly deformed by the existence of a plurality of ridges on the periphery of the waveguide, whereby the mode conversion loss in said waveguide from TEon mode (n ≥ 2) to TEpm mode is much greater than that from TE01 mode to another modes.