Showing papers on "Mode volume published in 1985"

Theoretical analysis of optical fiber laser amplifiers and oscillators

Abstract: Using the formalism of mode overlap, a theoretical analysis of optically pumped fiber laser amplifiers and oscillators is developed. The concept of normalized overlap coefficients is introduced to account for the effects of the transverse structure of the interacting signal and pump modes on the device characteristics. Simple and accurate closed-form expressions are derived for the gain of fiber amplifiers and the threshold and energy conversion efficiency of fiber laser oscillators in terms of the fiber and laser material parameters and the pump and signal modes. When applied to step-index Nd:YAG fiber lasers, this study predicts optimum fundamental mode oscillation in fibers with a V number of 5-25 with submilliwatt thresholds and nearly quantum-limited conversion efficiencies.

Device for tapping radiation from, or injecting radiation into, single made optical fiber, and communication system comprising same

Abstract: Optical radiation can be efficiently removed from, or injected into, single mode optical fiber at an intermediate point along the fiber, by causing quasi-resonant coupling of the guided mode LP 01 to an appropriate tunneling leaky (TL) mode, e.g., LP 11 . Such coupling is caused by means of a "grating" in the fiber, with the grating being formed by impressing a periodic (or pseudo-periodic) modulation on the fiber, or by causing a periodic (or pseudo-periodic) variation of the refractive index of the fiber by means of the photoelastic or the photorefractive effect. The nominal grating spacing Λ(z) is chosen such that Ω o =(2π/Λ)>β 01 -2πn cl /λ o , where Λ is the average grating spacing, β 01 is the propagation constant of the LP 01 mode, n cl is the refractive index of the fiber cladding, and λ o is the wavelength of the radiation to be coupled from or into the fiber. Furthermore, Λ(z) is to be chosen such that Ω o ˜β 01 -β rs , where β rs is the propagation constant of the selected TL mode. In order for the coupling to be quasi-resonant, it is necessary that α rs , the attenuation constant of the selected TL mode, be relatively small, typically <1 dB/cm. By appropriately choosing Λ(z) and/or the amplitude function of the grating, it is possible to increase the coupling efficiency above what is possible with a constant spacing, constant amplitude grating. Devices according to the invention can be advantageously used not only as radiation couplers but also as filters and, if they are of the photorefractive type, as amplitude modulators.

Transmission properties of a multimode optical-fiber taper

Abstract: A simple and useful formula governing the skew rays propagating through a multimode fiber taper has been derived by using geometrical optics. Using this formula, the transmission properties of the fiber taper are studied. The total light transmitted and the effective numerical aperture for both the meridional rays and the skew rays are compared with those of a uniform fiber.

Similarities and Differences Between Fiber Acoustics and Fiber Optics

Abstract: An introductory comparison of guided wave properties and sensor applications between weakly guiding acoustic and optical fibers is given. The mathematical expressions of dispersion relations, cutoff c onditions, etc for the flexural, t orsional and radial-axial acoustic modes in a weakly guiding acoustic fiber a re shown to be identical to those for the hybrid, transverse electric and transverse magnetic optic modes in a weakly guiding optical fiber. But no analogy is found between the longitudinal acoustic mode and any optical mode. Cladded glass f iber waveguides using pure and doped fused silica as core or cladding materials can be used for e ither f iber acoustic or fiber optical waveguides or both simultaneously . For both types of waveguides, dispersion properties, loss mechanisms, coupling considerations and industrial applications are discussed.

Modal-insensitive biconical taper couplers

Abstract: The modal-insensitive coupler consists of a number N of optical multimode fiber lengths, each fiber length having a core of initial predetermined cross-sectional area. The fiber lengths are fused together in a coupling section such that the cores of the fiber lengths have a reduced cross-sectional area in the coupling section which supports a minimum number of modes of optical energy at a predetermined wavelength, and such that the fiber lengths have different core cross-sectional areas to produce preselected coupling ratios.

Force measuring apparatus and method using stress-induced birefringence in a single-mode optical fiber

Abstract: The force measuring apparatus includes a single-mode optical fiber having a defined anisotropy, whereby coherent light suppled to one end of the optical fiber is guided as two waves having linear orthogonal polarizations, respectively. The optical fiber is arranged between a pair of parallel planar pressure plates, the orientation of the optical fiber being such that the aforementioned linear polarizations are parallel with and normal to the remote surfaces of the pressure plates, respectively. Upon the application of a force to be measured in the compressive direction normal to one remote surface of a plate, a phase shift is produced between the waves which is a function of the magnitude of the load to be measured, as determined by interferometer means arranged at the other end of the optical fiber. The optical fiber may be of a helical configuration having turns that are contained between successive pairs of pressure plates, respectively, arranged in a stack, thereby achieving a multiplication of the measuring effect with increased accuracy of the corresponding measurement.

Modal cutoffs in single- and few-mode fiber couplers

Abstract: Optical coupling, or crosstalk, between two parallel, identical single- or few-mode fibers is describable as a superposition of the modes of the composite waveguide. Like the modes of a single-fiber waveguide, the modes of the two-fiber waveguide are cut off. Exact and simple accurate asymptotic formulae are derived giving the cutoff value as a function of separation. For single-mode couplers, perturbation theory gives the fundamental and second modes \Psi+ and \Psi- as the sum and difference of the fundamental modes \Psi_{1} and \Psi_{2} of each fiber. In reality \Psi+ is the fundamental mode of the composite waveguide, whereas \Psi- is the second mode and has a finite cutoff. This limits the smallest V -value for which low-loss coupling is possible. The limitation is least severe for step-profile fibers.

Combination lens method for coupling a laser diode to a single-mode fiber.

Abstract: A new combination lens method using a relatively large spherical lens and a virtual fiber is proposed for coupling an InGaAsP laser diode to a single-mode fiber. The virtual fiber is formed by attaching a GRIN rod lens to the input endface of the single-mode fiber. Coupling efficiency of from −3 to −4 dB is easily obtained. Alignment sensitivities of the proposed optical coupling circuit are investigated both theoretically and experimentally. The lateral and axial misalignment tolerances for the virtual fiber are improved three and nine times, respectively, in comparison with those for the single-mode fiber. The measured dependence of the coupling efficiency on the misalignments for the optical components agrees relatively well with the theoretical results through the ray matrix approach.

Accurate analysis of single-mode graded-index fiber directional couplers

Abstract: We present an accurate coupled-mode analysis to evaluate the coupling characteristics of identical single-mode graded-index fiber directional couplers. We also give an empirical relation for calculating the coupling length of a fiber coupler formed by fibers having profile parameter q , normalized frequency V , and normalized separation d/a ; d is the distance between the fiber centers and a , the radius of the fiber core.

Polarimetric fibre sensor

Abstract: A polarmetric fiber sensor comprises a lead fiber (1) and a shorter length of sensing fiber (2). Both fibers comprise higly birefringent optical fiber having orthogonal eigen modes and are spliced together with their eigen modes mutually rotated by 45°. The sensing fiber (2) has a reflective distal end (6). Linearly polarized light from a laser source (3) is launched into one mode of the lead fiber (1) and light reflected from the distal end of the sensing fiber is recovered from the other mode of the lead fiber via a beam splitter (7) and photodiode (8). The resulting electrical signal produced by the photodiode (8) corresponds to the modal birefringence of the sensing fiber (2) and changes in this birefringence are monitored by processing means (9) to detect changes in a physical parameter sensed by the sensing fiber.

Vibrating optical fiber accelerometer and gyroscope

Abstract: There is provided by this invention a fiber optic transducer for accelerometers and gyroscopes that utilize an optical fiber suspended between two points and supported on a vibrating platform such that the optical fiber is deflected to cause light passing through the fiber to be phase modulated. When the device is accelerated, the deflection of the vibrating fiber is assymetrical, and the light passing through the fiber is phase modulated at frequency `f` and odd harmonics of `f`. The first in odd harmonic phase modulation has an amplitude proportional to the acceleration and a phase relative to the driving signal that depends on the direction of acceleration.

Evanescent-field coupling between a monomode fiber and a high-index medium of limited thickness

Abstract: By applying a coupled-mode theory to the angle degenerate modes of a planar waveguide and to the single mode of a neighboring monomode fiber, we derive an integrodifferential system describing the transfer of light intensity from the fiber to the planar waveguide. By numerically solving this system, the transmission loss of the fiber is computed as a function of various parameters such as the planar-guide thickness, the planar index, and the remaining fiber-cladding thickness.

Radiation losses from couplers

Abstract: The antisymmetric mode of couplers, composed of two parallel fibers, becomes cut off when V is sufficiently small. We show that the familiar perturbation modes of this coupler give an excellent approximation to the cutoff value of V previously obtained by an exact analysis. More importantly, by using the perturbation modes, we establish that the rate of leakage below cutoff sets a genuine practical limitation to operation. To do this, we develop a scalar theory for radiation losses.

A new fundamental mode field-radius definition usable for non-Gaussian and noncircular field distributions

Abstract: A new definition for the mode field radius of fundamental mode fibers is proposed which can be applied to circular as well as to noncircular and non-Gaussian field distributions. The mode field radius according to this definition can be strictly determined by the well-established transverse offset technique. Moreover, an alternative test method is presented offering the advantage of using one standard launching fiber for each measurement.

Acousto-optic frequency shifter

Abstract: A fiber optic frequency shifter comprises an optical fiber (54) having two propagation modes and an acoustic transducer (50) for generating an acoustic wave. The transducer (50) is positioned relative to the fiber (54) to cause the wave fronts of the acoustic wave to acoustically contact the fiber (54) at an angle of incidence (0) which is less than 90° greater than 0°. Preferably, the wavelength of the acoustic wave is substantially equal to the beat length of the fiber (54) times the sine of the angle of incidence (θ.

Fiber Optics In High Dose Radiation Fields

Abstract: A review of the behavior of state-of-the-art optical fiber waveguides in high dose (≥ 105 rad), steady state radiation fields is presented. The influence on radiation-induced transmission loss due to experimental parameters such as dose rate, total dose, irradiation history, temperature, wavelength, and light intensity, and material parameters such as dopant type, concentration, and OH content is described. Recommendations for future work in high dose environments are given.© (1985) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Desensitization of the ultrasonic response of single-mode fibers

Abstract: The ultrasonic sensitivity of single-mode fibers has been studied by taking into account the exact composition and geometry of multilayer fibers in the isotropic regime (where the acoustic wavelength is much larger than the fiber diameter) and in the anisotropic or high frequency regime. Good agreement was found between analysis and experiments, and coating compositions desensitizing the ultrasonic response have been identified.

Correction to "Transmission properties of graded-index triple-clad single-mode fiber for 1.55 &#181;m system operation"

Abstract: Graded-index triple clad (GITC)single-mode fiber exhibits low loss and low dispersion (< 2 ps/km-nm) over a wide wavelength range. The fundamental mode field radius of this fiber was measured as a function of wavelength using the transverse offset technique. This showed that the fundamental mode radial power (or field) distribution can be well represented by a Gaussian approximation. The basket weave loss test was made to obtain the fiber bend sensitivity. The bend induced additional losses of GITC single-mode fiber at 1.55 μm is much smaller than that of single-clad single-mode fibers.

Analysis of mode partition noise in optical fiber line systems

Abstract: A theoretical model of the effects of Laser mode partition noise on system performance is presented. The model distinguishes between the noise in binary zero and one and takes into account the influence of interfering symbols, which has not been the case in previously published analyses. Both types of mode partition noise, caused by either chromatic dispersion or wavelength-dependent attenuation, are treated. Calculations show that a conventional FP Laser could probably be used for a 1300 nm 2.4 Gbit/s system. However, problems can arise even at low bit rates when using a fiber with strong OHabsorption peaks.

Boundary element analysis of polarization holding fibers

Abstract: The boundary element method has drawn wide attention as a novel numerical method of the boundary integration equation, and it has already been applied to the electromagnetic initial value problems. In this paper, we apply the boundary element method to the electromagnetic wave eigenvalue problems and establish a numerical method which is applicable to analyze the guided modes of waveguides with arbitrary structure. Then as an application example of the present method, we make numerical calculation of the elliptical core optical fiber, which is the typical polarization holding optical fiber, and clarify the cutoff frequencies of the first- and second-order higher modes of the fiber. Also, the propagation constant and the geometrical anisotropy of the fundamental mode of the fiber are obtained. Again, the maximum anisotropy of the elliptical core fiber is shown to be obtained for the normalized frequency of 1.85 and ellipse factor of the core equal to 0.8. From these results we conclude that the boundary element method has high accuracy of calculation, is easy to apply, and has highly stable solution.

The bandwidth of a multimode fiber chain

Abstract: We propose a new method for evaluating the baseband transmission in a multimode fiber chain. Carnevale and Paek (Bell Syst. Tech. J., vol. 62, pp. 1415-1431, 1983) stated that errors in the fiber manufacturing process will randomly distort a desired index profile, presumably of power-law type. We extend their discussion to the bandwidths of concatenated fibers, by considering Gaussian approximations to actual transfer functions. The bandwidth can thus be separated into two parts, one of which is due to the over- and undercompensation of individual, idealized power-law profiles and the other of which refers to random profile distortions as well as possible mode coupling within mode groups. The former part should normally dominate the length dependence of longer chains. The latter part may be replaced by an expectation value, typical for the actual manufacturing process. A remarkably good agreement is achieved between experimental and predicted bandwidths for various chain configurations.

Theoretical analysis of optical fiber mode exciters constructed with alternate concatenation of step-index and graded-index fibers

Abstract: Mode exciters which consist of alternate concatenation of step-index (S) and graded-index (G) fibers are used widely for the measurement of transmission characteristics (optical loss and baseband frequency response) of multimode fibers. This paper presents a theoretical analysis for the mode exciters based on geometrical ray optics approximation. The mode power distribution controlling effect of the exciters, excited mode power distribution in the test fiber, and accuracy and reproducibility of optical loss measurements are clarified theoretically. It is shown that optical loss measurements under steady-state distribution can be done by using either SGS or GSG mode exciters which consist of either small core or low NA fibers.

Measuring cutoff wavelength of HE21, TE01, and TM01 modes: a novel method using single-polarization launching.

Abstract: A new method for measuring the cutoff wavelength of HE21, TE01, and TM01 modes in single-mode fibers is reported. The method is based on the difference in polarization between the HE11 and second-order modes launched in the offset condition. A single-polarization fiber several meters in length is used as a launching fiber to eliminate cladding mode disturbance and realize linear polarization launching into a test fiber. Good agreement between theoretical and measured cutoff wavelengths for fibers with various refractive-index profiles confirms the high accuracy of the method. A resolution of ±0.005 μm is attained with the method.

Evaluating method of loss of optical connector

Abstract: PURPOSE:To evaluate the loss of a connector by using a master plug fitted to an optical fiber which is different in mode volume, and making light made incident on the optical fiber on the side of a plug to be measured and detecting the light leaking from the bent part of the fiber. CONSTITUTION:The master plug 2c fitted to the optical fiber 8 having the same mode volume that an optical fiber 1 has and a master plug 2d fitted to the optical fiber having smaller mode volume than the optical fiber 1 are provided. Light from a light source 5 is made incident on the optical fibers 8 and 9, and the plugs 2c and 2a are coupled with each other to make the light incident on the optical fiber 1; and the fiber 1 is bent near the plug 2a to leak the light, and to detect light power P1. The plugs 2d and 2a are coupled with each other and the light is made incident on the fiber 1; and the light power P2 leaking from the bent part formed in the pretreatment of the fiber 1 is detected, thereby evaluating the loss characteristics of the plug 2a on the basis of P1 and P2. Thus, the light leaking from the bent part of the fiber is detected to evaluate the loss of the connector easily.

Decentralization compensation method for optical communication system

Abstract: PURPOSE:To increase remarkably the transmission capacity by inserting a single mode optical fiber for compensation decentralized with a prescribed length to control the decentralization in the entire optical fiber transmission line between a light source and a transmission long optical fiber or between the transmission long optical fiber and a light receiver. CONSTITUTION:One end of a decentralization compensation single mode optical fiber 3 is coupled directly with an electrooptic converter 2 and the other end is connected to an optical fiber cable 5 by a connection section 4. As the decentralization compensation single mode optical fiber, a high NA single mode optical fiber, a W type single mode optical fiber or a multi-clad single mode optical fiber or a triangle core single mode optical fiber, etc., are used and a polarized plane preserving optical fiber is used as well. In inserting the decentralization compensation single mode optical fiber 3, even when a conventional multi-mode laser is used, since the transmission capacity is increased nearly as semi-infinitely practically, the system is very effective to a wide band.

Method of and apparatus for measuring the cut-off wavelength of the first higher order mode in optical fibers

Abstract: The cut-off wavelength of the first higher order mode in optical fibers for telecommunication is measured based on the fact that the introduction of perturbations in an optical fiber can generate greater losses on higher order modes than on the lower order modes A spectral scanning of the fiber output power is effected under a certain number of different perturbations conditions and the values obtained are combined so as to determine the fraction of power guided in the fundamental mode The cut-off wavelength is that where this fraction exceeds a certain value

Study of vectorial properties of optical-fiber modes using a fiber resonator

Abstract: Each eigenmode in optical fibers is experimentally analyzed individually by changing the resonance condition of the eigenmode in optical-fiber resonators. When a stress is applied to fibers, it is found that eigenmodes in a single-mode fiber and a two-mode fiber are LP modes, and eigenmodes in a fiber Faraday rotator are elliptically polarized modes. A significant result for the two-mode fiber is that the directions of polarization and orientation of the LP 11 modes are those of a transverse pressure and principal axes of the elliptical core, respectively. A coupled-mode theory is also presented to explain these experimental results.

Mode designations of laser beams

Abstract: Conventionally, the resonant modes of open resonators are designated as TEMpl. This kind of mode designation is inappropriate because of the fact that all the beam modes have at least an axial field component, the relative strength of which, in comparison with the principal transverse field components, increases with the higher-order mode numbers p and l. It is shown in this paper that the higher-order beam modes can be separated into three types of modes; namely, transverse-circular-electric modes, transverse-circular-magnetic modes and hybrid modes. Based on this result, a new scheme of mode designations is proposed. This scheme, in fact, follows the mode designations for cylindrical dielectric (fibre and hollow) waveguide modes. Consequently, a good analogy can be obtained between the Laguerre-Gaussian beam modes and the dielectric waveguide modes, along with their common applications in optical transmission.

Fiber type wavelength filter

Abstract: PURPOSE:To provide a titled filter which is small in size, is highly flexible and is easily connectable to optical fibers by fully constituting the wavelength filter of optical fibers. CONSTITUTION:The fiber type wavelength filter is formed by successively connecting the dual-core fibers 15 each of which has the 1st core 16 at the center of the clad and the 2nd core 17 in the offcentered position of the clad and satisfies cl1=mpi (c; coupling constant at the wavelength lambda0 of the dual-core fiber, l1; length of the shortest dual-core fiber, m; positive integer). The length ln of the n-th dual-core fiber satisfies ln=2 l1 (n; 1-N integer, N; the number of the dual-core fibers). The direction where the 1st core and the 2nd core of the n-th dual-core fiber are connected is designated as Xn axis and the respec tive 1st cores of the n-th and (n+1)th dual-core fibers are so connected to each other by rotating the Xn axis and Xn+1 axis as not to induce the optical cou pling. The terminal of the N-th dual-core fiber is a single mode fiber having only the 1st core.

A method for reducing optical fiber diameter fluctuations

Abstract: An optical fiber diameter monitor, Type GY-A, was developed especially for fiber drawing. A new method, the pulling force-temperature method, is presented. The fiber drawing stability is improved and the fluctuation of the fiber diameter is 0.3 μm (rms).