The transfer of TEM00 and TEM01 beams through a hard-aperture

Abstract

We determine the far-field divergence angle θ ₀₀ (θ ₀₁) resulting from the diffraction of a TEM₀₀ (TEM₀₁) beam incident on a circular aperture. It is demonstrated that, with a suitable choice of the aperture radius, the ratio θ ₀₁/θ ₀₀ is enhanced to about 2, while its value is 1.64 without the aperturing effect. It is shown that this effect allows high discrimination between TEM₀₀ and TEM₀₁ modes either in the case of a spatial filter or in the case of an apertured resonator.     

Diffractive optical elements for intra-cavity beam-shaping of laser modes

Abstract

Diffractive optical elements (DOE) are applied as intra-cavity mode selection devices for customizing the fundamental mode of laser resonators for high power laser systems. Using a phase-conjugating mode selecting element (MSE) in a laser oscillator, we are able to produce a good approximation to a super-Gaussian mode with a near flat intensity profile. This offers higher energy extraction from any following laser amplifiers compared to an unmodified Gaussian TEM₀₀ mode. Two different designs for operation in a 1 m cavity length Nd:YAG master oscillator are presented. Both designs are surface relief phase elements fabricated in fused silica using photolithography with reactive-ion etching to produce 16 level elements for use in transmission. One element is designed to replace the cavity end mirror, while the other stands off an arbitrary distance from the end mirror. A novel iterated design for these transmissive elements is introduced. Numerical results and experimental measurements are presented and discussed.     

A Diode-laser-bar Pumped Stable CW Nd: YAG Slab Laser and its Frequency Doubling

Abstract

A high power Nd: YAG mini-slab laser transversely pumped by two diode laser arrays has been investigated. The simplicity and compactness of this pumping scheme provides intrinsic stability in both the output power and the beam pointing. A linearly polarized TEM₀₀ mode output power of 3·6 W at 1064 nm was achieved with a linear cavity, and 1 W of single frequency green light at 532 nm was generated by intracavity doubling in a ring cavity.     

Stable 4 W CW solid-state green source pumped by fibre-coupled diode-laser arrays

Abstract

A high-power Nd:YAG laser end-pumped by a fibre-coupled diode-laser array system is described. When operated at 1 μm, the device produced 10 W of output power in the TEM₀₀ transverse mode with a slope efficiency of 43%. The system was operated using both 600 μm and 400 μm core, 0·37 N.A. multimode fibres for delivery of the pump radiation, and also with Nd:YVO₄ as the gain material. By intracavity doubling with KTP, 4 W of non-chaotic two-mode 0·5 μm output was obtained.     

Thermal Effects in Nd:YAG Slabs

Abstract

In the design of a diode-pumped slab solid-state laser, the knowledge of the temperature distribution and the thermal lensing in the medium is very important when attempting to achieve high output power in the TEM₀₀ mode. A general 3D analytical calculation is given of the temperature distribution in a slab of finite size, side-pumped by many laser diodes; the spatial distribution of the pump power is taken into account. An approximate thermal focal length is calculated from the temperature distribution. Three different ways of cooling the slab are treated and included in the model as boundary conditions. We derive general solutions of the heat distribution for each case of cooling. The test of the model includes numerical calculations applied to the case of our slab prototype and a comparison with experimental measurements is made.     

Flat-topped light beams with non-circular cross-sections

Abstract

A set of flat-topped light beams with non-circular cross-sections is introduced. This set includes beams with rectangular, elliptical, pyramidal and one-dimensional Gaussian cross-sections. The one-dimensional Gaussian denotes a beam with Gaussian distribution in one direction and a long flattened distribution in the perpendicular direction. Field distributions of this set of beams can be expressed as sums of the lowest order Gaussian modes (TEM₀₀) having different parameters. The main features of the beam are investigated throughout the space without using the orthogonal polynomials.     

Hybrid “antenna-amplifier” relativistic traveling-wave tube using an azimuthally asymmetric mode of a rod slow-wave system

A linear theory is developed for a Cherenkov microwave amplifier with a slow-wave system in the form of a circular waveguide with a dielectric rod. In this system the rod antenna can couple out the radiation into free space without additional channels and mode converters. An analysis is made of the general case with no azimuthal symmetry and conditions are determined where the dominant TEM₁₁ asymmetric mode predominates. Pis’ma Zh. Tekh. Fiz. 25, 43–50 (April 12, 1999)     

Theoretical modeling on resonating the second harmonic for ultraviolet laser generation

The resonance of the second harmonic was proposed to improve conversion efficiency in ultraviolet laser. A schematic to generate the fourth harmonic laser with a compound cavity was modeled based on plane-wave approximation. The simulated results show that the mode-matching condition of the multi-mode lasers depends strongly on both frequency-mode spacing and resonator loss. In the case of the same cavity lengths and losses, the tolerance on the optical cavity length should be maintained on tens or hundreds of nanometer scales. Meanwhile, the TEM₀₀ beam can be achieved due to the best mode-matching at resonance and the beam size increases arising from the cavity detune.     

Cross-focusing of Mixed Mode Operation in a Collisional Plasma

This paper presents a study of the nonlinear propagation of a mixture of two degenerate modes (TEM₀₀ and TEM₁₀) of a high power laser beam in a collisional plasma. The non-linear dependence of the dielectric constant inside the collisional plasma is due to non-uniform redistribution of carriers on account of their inhomogeneous heating. The focusing effects have been studied for different power ratios of the two modes and the absorption effects have also been considered. In certain power regions considered the beam either converges or defocuses in both the directions, while in some other defined regions of the power spectrum one dimension of the beam focuses and the other defocuses.     

Mode-volume enhancement in an apertured laser

In a laser TEM(00)-mode selection by the use of a hard intracavity aperture gives rise to losses that are experimentally observable and to an increase in the fundamental-mode volume that is more difficult to observe. We use the parasitic resonances that are characteristic of multicavity lasers to demonstrate this volume enhancement in a He-Ne laser operating at 3.39 μm. The increase in mode volume in the apertured laser can increase the output power by a factor of 3 with respect to that of the nonapertured laser.     

A heterodyne laser system to study frequency stabilized Zeeman 633 nm He-Ne lasers deficient in temperature steadiness

A heterodyne laser system is constructed to study the effect of external transverse magnetic field on the dynamic instability of internal mirror TEM_00q He-Ne lasers having temperature instability. The system could provide precise detailed knowledge about the roles of both the temperature and the applied magnetic field separately. Two different internal mirror 633 nm TEM_00q He-Ne lasers with frequency stability of 10⁻⁶ are studied. The applied transverse magnetic field on He-Ne lasers increased the frequency stability to be in the order of ∼10⁻¹⁰, in spite of temperature instability. In the same time, a single-mode operation with an enhanced laser output power is obtained. The sensitivity of the method showed that the laser exhibits a short-term frequency stability of 4.7×10⁻¹⁰, which after one minute, when mode collapse starts, decreased to 2.6×10⁻⁸, over the next four minutes. This increase in the frequency difference of the inter-mode beat signal that affect the stability is attributed to the uncontrolled temperature of the laser tube. The results revealed that the magnetic field plays the dominant role to achieve the maximum frequency stability, while the unsteadiness of temperature of the laser tubes limits the long term single-mode laser operation.     

Technique of calculating the divergence at the exit of afocal optical systems, taking aberrations into account

A flexible system of approximations by means of which it is possible to precisely compute the Kirchhoff diffraction integral is analyzed. A beam of the null component mode (TEM₀₀) at the exit from a flat hollow resonator transformed by an afocal optical system that reduces aberration is considered. The possibility of reducing aberrations by a computation of the structural parameters of the lens using a newly developed procedure is investigated. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 45–48, October, 2006.     

Analytical non-paraxial TM polarized Gaussian beam in the source region

The non-paraxial TM polarized Gaussian beam, which approximately reduces to be a Gaussian TEM₀₀ mode at the paraxial case, is the rigorous solution of Maxwell's equations for a confocal resonator. By means of mathematical techniques, the analytical expression in the source region has been derived without any approximation and expressed as a sum of the propagating part and the evanescent part. With some certain given values of the initial Gaussian half width w ₀, the distributions of the squared modulus of the propagating part, the evanescent part, and the whole beam are depicted in the reference plane close to the source. When w ₀ is small enough, the squared modulus of the non-paraxial TM polarized Gaussian beam is composed of two lobes, which results from the contribution of the evanescent wave. This research further reveals the difference of a non-paraxial TM polarized Gaussian beam from a non-paraxial Gaussian beam.     

Fast mode decision based on systematic deployment of predictive resources for H.264/AVC

Abstract

This paper presents a well organised fast mode decision scheme for H.264/AVC that systematically integrates and deploys applicable predictive resources using constructive compromise. This scheme is constructed by primary SKIP mode checking, spatial prior temporal hard decision (STHD), and succinct mode decision (SMD). The STHD is connected with the predictive macroblocks searched. In STHD, only when spatial hard decision fails, temporal hard decision is performed using the potential partition characteristic vector (PPCV) addressed. If it fails, too, then SMD takes care of mode checking also using PPCV. The SMD consists of succinct P16?×?16 mode decision and succinct geometric/statistical extension examination, which are formed as an effective two-stage judgment. Extensive experimental results demonstrate that the proposed method by systematically deploying the predictive resources can achieve 58–74% runtime savings in encoding QCIF, CIF and high definition videos with low performance losses. In addition, most processes of the proposed scheme could be implemented on logical operations, only.     

A few-mode optical fiber retaining optical vortices

The ability of an optical fiber with axial losses to selectively suppress the fundamental HE ₁₁ mode, as well as the TE and TM waveguide modes, and, simultaneously, to transmit optical vortices with almost zero energy losses is considered. The attenuation coefficients for the corresponding eigenmodes and vortices are determined. It is shown that such a fiber operates as a mode filter for the feeding beam.     

Propagation of a Non-Gaussian Laser Beam in SELFOC Fibres

In this communication we have presented an analysis of the propagation of a laser beam operating in a TEM₀₁ mode, in a medium of varying dielectric constant. The cylindrical and conical optical fibre guides have been considered. The results have been put in a form suitable for numerical computation.     

Beam propagation factor of truncated laguerre—gauss beams

Abstract

Beam propagation factor (usually referred to as M ² factor) and brightness of high-order symmetrical Laguerre–Gauss (LG) beams that diffract upon a hard-aperture are investigated. We demonstrate that a circular aperture is able to decrease the M ² factor of a high-order symmetrical LG beam and also to enhance its brightness although losses are induced.     

Absorption Efficiency of Traveling Wave Photodetectors in Superconducting Fiber Plasmon–Polariton Optical Waveguides

By using an analytical model and a finite element method, we investigate a new, very sensitive, superconducting traveling wave photodetector made by an optical fiber, which includes a high index layer (LaAlO₃) with small losses, a metallic (gold) layer, and two active superconducting layers (YBCO). While the fundamental guided degenerate mode HE₁₁ is highly confined in the interior YBCO layer, the plasmon non-degenerate mode TM₀₁ and plasmon degenerate mode HE₁₂ are tightly confined in the exterior superconducting layer. In our structure of the fiber with five layers (without a buffer layer), the imaginary parts of the HE₁₁, TM₀₁, HE₁₂ modes and the power absorption efficiency in superconducting layers are larger in comparison with that of a fundamental mode from a LaAlO₃?CYBCO?CAu planar waveguide. The confinement regimes of the light and the power absorption efficiency in superconducting layers can be optimized by only acting on the fiber geometry.     

Asymmetric Mode of an Off-axis Diaphragmed Laser

Abstract

Diaphragmed resonators have been widely studied in the axially symmetric case. Here we present some experimental and theoretical results concerning the field distribution of an off-axis apertured laser. Measurements have been made with a commercial Argon laser based on a plano-concave cavity and operating at 488 nm. Calculations have been carried out using and extending a method presented previously for a symmetric resonator. Numerical results are given for the field distributions inside and outside the laser, the round-trip loss for the amplitude and the round-trip phase shift for various transverse misadjustments of the aperture. We find the asymmetric field distribution of the fundamental mode in both experimental and theoretical aspects.     

Oscillation characteristics analysis of transversely and coaxially pumped Ti-diffused Nd:MgO:LiNbO3 waveguide lasers

Abstract

A theoretical analysis of oscillation characteristics is developed for Ti-diffused Nd:MgO:LiNbO₃ waveguide lasers by using the formalism of mode overlap. Simple and explicit expressions in terms of mode sizes are derived for effective mode volume and coupling efficiency between pump mode and laser mode, which are directly proportional to the threshold pump power and slope efficiency, respectively, both for transverse and coaxial pumping considering the spatial distribution of the pump beam and the laser beam. A comparison for the two pumping configuration has shown that the transverse pumping requires much higher input pump power and has much lower slope efficiency than the coaxial pumping. The fundamental transverse mode (TM ₀) oscillation conditions relative to the cavity loss are also discussed for the two pumping configurations.

Indexing terms: Ti-diffused Nd:MgO:LiNbO ₃ waveguide laser, effective mode volume and coupling efficiency, threshold pump power and slope efficiency, transverse and coaxial pumping, single mode oscillation condition.