Showing papers on "Radiation mode published in 1984"

Bending effects in optical fibers

Abstract: Mode coupling at bends in optical fibers supporting one or only a few guided modes is analyzed by considering the local normal modes for the corresponding straight waveguide. Matrix elements giving the strength of coupling between guided modes at a corner bend, and for coupling between guided modes and radiation modes, are calculated as a function of guiding strength for this "geometrical" effect. The correction to these matrix elements due to the longitudinal strain in a bent fiber is also determined. The increase in propagation constant for the fundamental mode of a fiber wrapped in a coil of constant radius is calculated from information on the coupling strengths and mode propagation constants. The phase shift and attenuation of the fundamental mode caused by a spatially periodic microbending of the fiber axis are also considered. Finally, potential applications of these effects in fiber-optic devices such as mode converters, phase shifters, switches, and sensors are discussed.

High-power millimetre-wave mode converters in overmoded circular waveguides using periodic wall perturbations

Abstract: This work reports on measurements and calculations (coupled mode equations) on the conversion of circular electric TE0n gyrotron mode compositions (TE01 to TE04) at 28 and 70 GHz to the linearly polarized TE11 mode by means of mode converter systems using periodic waveguide wall perturbations. Mode transducers with axisymmetric radius perturbations transform the TE0n gyrotron mode mixture to the more convenient TE01 mode for long-distance transmission through overmoded waveguides. Proper matching of the phase differences between the TE0n modes and of lengths and perturbation amplitudes of the several converter sections is required. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TE01 mode into the TE11 mode which produces an almost linearly polarized millimetre-wave beam needed for efficient electron cyclotron resonance heating (ECRH) of plasmas in thermonuclear fusion devices. The experimentally determined TE0n -to-TE01 conversion efficiency...

Two-mode fiber modal coupler.

Abstract: When a single-mode fiber is used at a wavelength below the cutoff wavelength, the fiber guides second-order modes, which travel at different phase velocities from the fundamental mode. Periodically stressing this two-mode fiber once per beat length can cause coherent coupling between the modes. Such a modal coupler has been developed and is described here. Coupling to one of the second-order modes has been achieved, leaving less than -40-dB residual power in the fundamental mode. Two couplers have been mounted on a single strand of fiber to construct a Mach-Zehnder interferometer with better than a 30-dB on/off ratio. The coupler is polarization sensitive and can be used as an in-line polarizer. A 36-dB extinction ratio between polarizations has been observed.

Measurements of polarization mode couplings along polarization-maintaining single-mode optical fibers

Abstract: A new technique for measuring the polarization mode coupling along a polarization-maintaining optical fiber is demonstrated Additional analyses of optical time-domain reflectometry signals are used Using a 134-μm Nd3+:YAG laser as a light source and an acousto-optical light switch to reduce the Fresnel reflection at the input end of the fiber, we have examined characteristics of the polarization mode couplings for four fibers with different extinction ratios The extinction ratios evaluated by the present method are in good agreement with those obtained by a conventional technique within ±05 dB

Apparatus for generating and amplifying new wavelengths of optical radiation

Abstract: Method and apparatus for the tunable generation of new wavelengths of optical radiation from input sources of predetermined wavelength. Optical radiation from a source (20), or sources, is launched into both the fast and slow axis of a birefringent optical waveguide (12) as pump waves. The pump waves generate new Stokes and anti-Stokes waves through enhanced four photon mixing processes. The Stokes wave is wavelength or frequency shifted from the pump wave by a small amount and exits the end of the optical waveguide (12) where it can be interfaced to various apparatus for utilization. The apparatus preferably employs polarization control means for controlling the polarization of optical radiation launched into the birefringent optical waveguide. The waveguide can also be an optical fiber or planar waveguide structure. The wavelength or radiation generated and output can be tuned by altering the birefringence of the waveguide.

Length dependence of effective cutoff wavelength in single-mode fibers

Abstract: Transmission characteristics of single-mode fiber operated in the two-mode (LP 01 and LP 11 ) propagation region is investigated experimentally and theoretically. The effective cutoff wavelength of the LP 11 mode λ ce is commonly used to assure practically the single-mode operation of the LP 01 mode above λ ce up to certain fiber length. The experimental λ ce decreases linearly in a logarithmic scale of fiber length as increasing fiber length, and eventually becomes constant after a few kilometers' propagation. Theoretical λ ce is obtained from a coupled power equation by taking into account the mode coupling as well as the losses of the two modes. To calculate λ ce , the losses of the two modes are individually measured by a new measurement technique. It is clarified from the theory that the nonlinear property of the length dependence of λ ce in the logarithmic scale is due to a relatively small mode coupling between the LP 01 and LP 11 modes.

1.55‐μm InGaAsP ridge waveguide distributed feedback laser

Abstract: Optical and modulation properties of ridge waveguide distributed feedback lasers grown by liquid phase epitaxy and operating in the spectral region of 1.51–1.58 μm have been investigated. These lasers were characterized by room temperature cw threshold currents as low as 40 mA, external quantum efficiency of up to 40%, and lowest order transverse mode operation up to 10 mW. Stable, single longitudinal mode operation under direct modulation of up to 4 GHz was obtained using second order gratings fabricated by electron beam writing as well as optical holography. The absence of partition mode events has allowed routine error free transmission over 60 km of single mode fiber at data rates of 2 Gb/s. The relationship between grating parameters and the Bragg length has been studied experimentally and compared with the theoretical results.

Single longitudinal mode semiconductor laser

Abstract: The semiconductor laser diode comprises a distributed Bragg reflector formed on a substrate. The reflector includes an optical waveguide sandwiched between first and second cladding regions formed over the substrate. The optical waveguide has a corrugated region extending within the optical waveguide in a direction parallel to the surface of the substrate. The thickness of the corrugated region varies in a prescribed period and the refractive index of the corrugated region differs from that of the optical waveguide. An optically active layer formed over the substrate is butt-jointed to the optical waveguide, and emits light beams when a current is injected into it. This single longitudinal mode semiconductor laser has high performance features. Its equivalent reflecting power is increased by so structuring the grating section that it is highly efficient in coupling the periodic structure and light.

LP(11) mode loss measurements in the two-mode-propagation region of optical fibers.

Abstract: We propose a new method of separately measuring the LP(01) and LP(11) mode losses in the two-mode-propagation region of optical fibers. It is found that LP(11) mode loss measured by this technique rapidly increases as the wavelength approaches the effective cutoff wavelength, while the LP(01) mode loss shows the expected Rayleigh scattering loss versus wavelength.

Equivalent optical waveguides

Abstract: We develop a comparison method which identifies a waveguide with properties that are well known and are very similar to the one under investigation. It has a very simple mathematical basis which allows a trivial derivation of the moment method for circular cross-sectional fibers including W -fibers, and an extension to noncircular optical waveguides. We first use the scalar approximation, and then extend to the full vector theory which accounts for polarization. Our concern is with fundamental mode propagation constants and higher mode cutoffs.

Optical study of the EH11 mode in a hollow circular oversized waveguide and Gaussian approximation of the far-field pattern.

Abstract: This paper gives the main characteristics and an optical description of the fundamental EH11 mode propagating in hollow circular oversized waveguides. The quasi-Gaussian beam radiated by this mode in the far field is derived from a recently developed optical model to calculate the Gaussian beam transmission through hollow circular oversized waveguides. A simple relation is found between optical characteristics of the EH11 mode in the waveguide and the quasi-Gaussian beam in the far field. The results agree well with previous theoretical and experimental works.

Single mode operation with non-single mode optical fiber transmission cable

Abstract: An optical fiber transmission path for interconnecting a transmitter and receiver for single mode operation at a predetermined wavelength. Most of the path is formed by a transmission cable made up of several optical fibers, spliced end-to-end, that are single mode at a wavelength longer than the predetermined wavelength, but possibly not single mode at the predetermined wavelength. The splice between the fibers is fabricated so as to exhibit minimized splice loss, as measured for only the fundamental mode at the predetermined wavelength. The transmission path of the invention also includes an optical fiber which is very short relative to the transmission cable and is in the path between the transmitter and the cable. The short fiber is selected to be single mode at the predetermined wavelength. A mode stripper is in the path, near the end of the transmission cable that is nearest the receiver. The mode stripper transmits only the fundamental mode at the predetermined wavelength.

Fluid level and condition detector system

Abstract: A fluid level and condition detector system including a multimode optical waveguide for insertion into a medium including at least a first fluid whose level is to be monitored, a source of radiation for introducing into one end of the waveguide radiation in a plurality of modes, and means for detecting the difference in radiation flux transmitted by the waveguide with the waveguide covered by the first fluid over its equilibration length, with the waveguide covered by a second fluid of lower index of refraction over its equilibration length, and with the waveguide covered partially by each fluid over its equilibration length and for detecting the difference in radiation flux transmitted by the waveguide due to the combined effects of the change in index of refraction and change in optical absorption coefficient of the first fluid covering at least a portion of the waveguide.

Frequency-correlation properties of optical waveguide intensity patterns

Abstract: A modal analysis has been used to explain frequency-correlation characteristics in waveguide cross-sectional intensity patterns. It is also shown that the power spectral density of the waveguide impulse response can be obtained from continuous-wave frequency-correlation measurement.

Cutoff-wavelength determination in single-mode fibers by mode interference.

Abstract: An effective cutoff wavelength lambda(ceff) in single-mode fibers is defined, and a simple interferometric method for its determination is demonstrated. This method is essentially independent of launching conditions and differential mode attenuation. At lambda(ceff) the power fraction of the LP(11) mode in the core is less than 1/e.

Mode conversion analysis in a single-mode planar taper optical waveguide

Abstract: We have investigated mode conversion in a taper waveguide used in a planar multi-branch waveguide optical power divider designed for fundamental mode operation and fabricated by Ti-diffusion into a z-cut LiNbO3 substrate. With an appropriate aspect ratio and index distribution in the taper, mode conversion loss into a higher-order mode can be maintained under 5%.

Overmoded waveguide components for the ECH system on PDX

Abstract: Waveguide components designed specifically for transmitting power into PDX for electron cyclotron heating (ECH) at 60 GHz are described. These include mode converters from the circular electric TE01 mode to the polarized HE11 mode, compact corrugated waveguide bends with a hyperbolic secant curvature variation, compact corrugated waveguide diameter tapers with a parabolic profile, and a high voltage dc break incorporating a section of dielectric waveguide, all designed for low-loss HE11 propagation. Optimization of the corrugation depth and curvature for bends propagating TE01 is also discussed.

Light intensity modulation based on guided-to-radiation mode coupling in heterostructure waveguides.

Abstract: The performance of light intensity modulation using guided-to-radiation mode coupling through the electrooptic effect is investigated theoretically and experimentally. We show that high modulation efficiency is obtainable if a heterostructure waveguide is employed that has appropriate refractive indices to bring forth large field overlap of the two coupled modes. Waveguides of Nb2O5 or Li(Nb0.1Ta0.9)O3 film on a structure could have modulation efficiency of more than 100 dB/cm. The experiment was LiTaO3 substrate carried out in a Nb2O5–LiTaO3 structure fabricated by rf reactive sputtering, and we obtained the measured modulation depth as high as 45 dB/cm at an applied voltage of 400 V across 55-μm gap electrodes.

Optimum beam thickness for the Raman‐type free‐electron laser using a two‐dimensional hollow beam

Abstract: The effect of an arbitrary beam thickness is investigated for a two‐dimensional Raman‐type free‐electron laser composed of a hollow relativistic electron beam and a parallel plate waveguide containing it. On the basis of the fluid theory for electron beams, analytical expressions are given for the dispersion relation and the spatial growth rate of a scattered wave (even TE mode) and an electron plasma wave (even TM mode) coupled under the influence of a pump wave (even TE mode). Then, from the numerical analysis for an important case where both the pump and scattered waves are the lowest‐order even TE mode, the following interesting results are obtained. First, the hollow beam can propagate two distinct modes of the electron plasma wave with even symmetry characterized by different field distributions. One mode (mode 1) corresponds to the normal mode of the solid beam while the other mode is inherent to the hollow beam. In addition, the growth rate for the mode 1 is considerably larger than that for the m...

Enhancement of the efficiency of the process of mode diffraction on corrugated sections of optical waveguides

Abstract: An analysis is made of the process of propagation of radiation in unperturbed and perturbed multilayer optical waveguides. A dispersion relationship convenient for the calculation of the characteristics of complex waveguide structures is derived. A determination is made of the parameter representing the degree of localization of the optical flux of a mode in the vicinity of a given interface between media forming a waveguide. A study is made of the maximalization of the light flux of a mode in the region of a perturbed boundary of a waveguide. Relationships are obtained for calculations dealing with the processes of emission, reflection, and transformation of modes in sections of multilayer waveguides perturbed by corrugations. The influence of an asymmetry of corrugations on the process of mode diffraction is determined. Conditions are found for the use of corrugated sections of optical waveguides in the construction of nonreciprocal integrated- optics components.

Nonlinear scattering at 10.6 μ in a fiber waveguide made of KRS-5

Abstract: Nonlinear backscattering of pulsed CO2 laser radiation of 10.6 μ wavelength was observed for the first time in a KRS-5 fiber waveguide when the intensity of the exciting radiation was ~ 5 MW/cm2. A comparison of the results obtained with the stimulated Brillouin mechanism was observed in these experiments.

Instability of an optical beam in a bent single-mode fiber waveguide

Abstract: Spatial inhomogeneities of an optical beam, critically sensitive to the bent radius, are considered for a bent single-mode fiber waveguide.

Optical levitation using single mode fibers and its application to self‐centering of microlenses

Abstract: Optical levitation using single mode fibers has been experimentally demonstrated. The use of fibers in levitation experiments presents obvious advantages, in particular, concerning the case with which complicated optical paths can be realized without additional optical components. In addition the microsphere can be accurately deposited on the fiber end by simply controlling the optical guided power. This technique has been used to position microlenses self‐centered on the guided mode to improve the coupling efficiency between a semiconductor laser and a single mode fiber.

Integrated semiconductor laser device

Abstract: PURPOSE:To enable rear section specular coating and cleavage, and to improve performance by extracting beams from a side surface in the rectangular direction to a resonant optical axis of an active layer or a wave guide layer in a semiconductor laser element and supplying a photodetector with said beams. CONSTITUTION:With a semiconductor laser element 1, a resonator is formed by resonator specular surfaces 4a, 4b, it is oscillated by currents flowing between a power supply terminal 7 for drive and a common electrode 9, and oscillation beams are wave-guided by an active layer 6 and an optical guide layer 3 optically coupling with the active layer 6. One part of beams is converted into a radiation mode by a projecting section 10 formed to the optical guide layer 3 at that time, and emitted toward a photodetector 2.

Optical waveguide analysis using the fourier transform and its application to intersecting waveguides.

Abstract: In analyzing inhomogeneous waveguides, this paper derives an analytical method that is effective even for the radiation mode light. Also, the propagation characteristic of intersecting waveguides is studied by using this method. In this analysis, considering virtual waveguides that include the real waveguide completely, the propagating light is expressed by the Fourier transform with virtual modes set up in the virtual waveguides. Moreover, using the concept of the power flow equation, the Helmholtz equation is transformed into a difference equation of Fourier coefficients, where weak coupling and paraxial rays are assumed. Applying this method to intersecting waveguides having inhomogeneous index gradients, we study the dependency of the propagation characteristics of the intersecting angle and the index gradient difference. From these results, light propagation in branching waveguides is found for low intersecting angles. This phenomenon is explained successfully by a model which combines TM0 and TM1 modes. Furthermore, the propagation characteristics of inhomogeneous waveguides are simulated and their dependency on the index gradient is studied.

Mode birefringence of a three-layer elliptic single-mode fiber wavequide

Abstract: A vector solution is obtained for the fields of the lowest-order modes in a three-layer elliptic fiber waveguide which is valid in the case of a small ellipticity of the boundaries between the layers, but is not restricted in respect of the differences between the refractive indices of the core and claddings. This solution is used to study mode birefringence as a function of the fiber waveguide parameters. It is shown that for a given core and an outer cladding, a waveguide with the W profile ensuring a high concentration of the energy of the lowest-order modes exhibits a stronger mode birefringence than a waveguide with a two-step profile. Near the limit of single-mode behavior an increase in the thickness of the inner cladding enhances the mode birefringence of a W-type waveguide, but reduces the corresponding birefringence of a two-step waveguide. The sensitivity of a three-layer waveguide to elliptic distortions of the boundaries increases on increase in the difference between the refractive indices of the three media forming the waveguide.

Waveguide type electrooptic optical modulator

Abstract: PURPOSE:To obtain an inexpensive modulator superior in temperature characteristic, by providing one channel-type optical waveguide on the surface of an electrooptic crystal, and providing an ion exchange layer, whose thickness is changed cyclically in the light propagation direction, and a voltage applying means around this waveguide. CONSTITUTION:The modulator consists of a Z-cut lithium niobate crystal plate 1, and a titanium diffusion channel waveguide 2 and an ion exchange layer 3 are formed. An electrode 4 for applying an electric field into the waveguide 2 is provided on the upper face of the ion exchange layer. The depth of the ion exchange layer 3 is changed cyclically in the light propagation direction, and its cycle is changed to LAMBDA1-LAMBDA2 (LAMBDA2>LAMBDA1) monotonously. An average thickness (d) of the layer 3 is so determined that an equivalent refractive index nr of the radiation mode is lower than an equivalent refractive index ng of the guided mode. Thus, the difference between ng and nr, namely, the change of the radiation angle due to the temperature change of double refraction is reduced, and the mode conversion is performed with a low voltage.

Characterization of single-mode fibers from wavelength dependence of modal field and far field

Abstract: We present direct methods for determination of equivalent-step-index (ESI) parameters and file modal field by measurement of the wavelength dependence of the far-field intensity in single-mode fibers. A comparison is made with the spot-size method commonly used for characterization of such fibers. A new technique for measuring the LP 11 mode cutoff wavelength is also presented.