Beam Quality Parameter Research Articles

Dynamic filamentation and beam quality of quantum-dot lasers

We present a comparative study of numerical simulations and experiments on the spatiotemporal dynamics and emission characteristics of quantum-well and quantum-dot lasers of identical structure. They show that, in the quantum-dot laser, the strong localization of carrier inversion and the small amplitude–phase coupling enable a significant improvement of beam quality compared to quantum-well lasers of identical geometry. Near-field profiles and beam quality (M2) parameters calculated on the basis of time-dependent effective Maxwell–Bloch equations into which the physical properties of the active media are included via space-dependent material parameters, effective time constants, and matrix elements are fully confirmed by experimental measurements. Together they indicate that, in the quantum-dot laser, the strong localization of carrier inversion and the small amplitude–phase coupling enable a significant improvement of beam quality compared with quantum-well lasers of identical geometry.

Parametric characterization of the spatial structure of partially coherent and partially polarized beams

On the basis of the second-order intensity moments formalism, the relationship between the spatial structure and the overall polarization characteristics of partially polarized Gaussian Schell-model beams of a certain kind has been investigated. More specifically, attention has been focused on a type of source that cannot be distinguished from ordinary Gaussian Schell-model fields when polarization measurements are disregarded. For this class of beams several general properties have been obtained that enable us to link the beam coherence polarization matrix and the beam quality parameter with certain polarization degrees recently introduced in the literature.

The influence of thermal effects in a beam control system and spherical aberration on the laser beam quality

With the strehl ratio, power in the bucket, β parameter and beam widths taken as the characteristic parameters of laser beam quality in the far field, the influence of thermal effects in a beam control system and spherical aberration on the laser beam quality in the far field is studied. Detailed calculations are performed using a four-dimensional simulation code, and numerical results are given and analyzed physically. It is shown that the thermal effects in the propagation channel and positive spherical aberration give rise to an expansion of intensity distributions and reduce the focus ability in the far field, and degrade the beam quality. It is interesting that a suitable choice of the negative spherical aberration would result in a higher peak intensity and a better focus abilty at the geometrical focal plane than that of the spherical-aberration-free case, as the emissive power reaches a certain value; whereas a similar result is achieved at the real focal plane due to the focal shift as in the previous work. The physical interpretation is that there exists the nonlinear thermal blooming, which is illustrated with numerical examples.

Computation of beam quality parameters for Mo/Mo, Mo/Rh, Rh/Rh, and W/Al target/filter combinations in mammography.

A computer program was implemented to predict mammography x-ray beam parameters in the range 20-40 kV for Mo/Mo, Mo/Rh, Rh/Rh, and W/Al target/filter combinations. The computation method used to simulate mammography x-ray spectra is based on the Boone et al. model. The beam quality parameters such as the half-value layer (HVL), the homogeneity coefficient (HC), and the average photon energy were computed by simulating the interaction of the spectrum photons with matter. The checking of this computation was done using a comparison of the results with published data and measured values obtained at the Netherlands Metrology Institute Van Swinden Laboratorium, National Institute of Standards and Technology, and International Atomic Energy Agency. The predicted values with a mean deviation of 3.3% of HVL, 3.7% of HC, and 1.5% of average photon energy show acceptable agreement with published data and measurements for all target/filter combinations in the 23-40 kV range. The accuracy of this computation can be considered clinically acceptable and can allow an appreciable estimation for the beam quality parameters.

A diode-bar side-pumped waveguide laser with an extended stable cavity for spatial mode control

We describe the use of an extended stable cavity to control the output spatial mode of a diode-bar side-pumped slab waveguide laser. The active medium consists of a Nd:YAG core with a one-dimensional double-clad guiding structure. The device gives >10 W output power with a 56% slope efficiency for multi-mode operation, which is reduced to 33% for the best output beam quality parameters ( M 2 values) of 2.8×1.1. The prospects for obtaining higher efficiencies and scaling to larger average powers are discussed.

Monte Carlo calculations for assessment of radiation dose to patients with congenital heart defects and to staff during cardiac catheterizations.

Effective dose is an important quantity in relation to assessment of radiation risk. Organ and effective doses to paediatric patients undergoing diagnostic and therapeutic heart catheterization procedures can be assessed by combining relatively simple measurements, e.g. of dose-area product (DAP), and calculated dose conversion factors (DCF). This also holds for the radiation dose to the hospital staff, e.g. the cardiologist. Monte Carlo (MC) simulation of radiation transport in mathematical anthropomorphic phantoms is used to obtain the DCFs, which strongly depend on beam quality and geometrical parameters. The performance of a dedicated fast MC code (PCXMC) for patient dosimetry is compared with that of a more elaborate general purpose MC code (MCNP). Resulting organ doses sometimes may differ considerably, partly due to phantom differences. While MCNP uses separate male and female mathematical phantoms, PCXMC uses a hermaphrodite. However, both codes yield effective doses that agree rather well, so PCXMC can be used for convenience. The MCNP code is used to calculate the effective dose to the cardiologist exposed to radiation scattered from the patient. Without protective clothing, effective dose per procedure to the cardiologist is at least two orders of magnitude lower than that to the patient. The effectiveness of various types and thickness of protective clothing has been evaluated for one view of one cardiac catheterization. The results of the calculations do not contradict experimental studies from the literature. MC simulation may serve as a useful tool to improve the accuracy of estimating occupational effective dose from personal dose monitors.

1 W high brightness index guided tapered laser at 980 nm using Al-free active region materials

Index guided tapered laser diodes at 0.98 /spl mu/m have been optimised for high brightness. We demonstrate an index guided tapered laser using an Al-free active region with an optical power of 1 W at 20/spl deg/C under continuous-wave operation. The parallel beam quality parameter M/sup 2/ of these devices has been measured to be less than 3 at 1 W at 980 nm.

Improvement of brightness and output power of high-power laser diodes in the visible spectral region

This paper reports on a novel design for a high-brightness quantum-well AlGaInP laser-diode system operating at 635 nm. The spatial coherence of two broad area laser diodes is improved using spatial filtering and optical feedback from a narrow mirror stripe on each laser. To increase power, the outputs from the two systems are passively combined using a polarizing beam splitter. The result is a compact system providing a high-brightness beam. The beam-quality parameter is improved in one direction from M 2=12 and M 2=16 for the two lasers, to M 2=2.1 for the combined beam. The other direction of the beam is left unchanged due to its natural high quality with an M 2 value around 1.5. A procedure for efficient coupling of the two lasers into a small core-diameter optical fiber is given. This capability facilitates a wide range of applications including interstitial medical therapies, where delivery of the treatment light through thin optical fibers is essential.

Anisotropic pure-phase plates for quality improvement of partially coherent, partially polarized beams.

From a theoretical point of view, the use of anisotropic pure-phase plates (APP) is considered in order to improve the quality parameter of certain partially coherent, partially polarized beams. It is shown that, to optimize the beam-quality parameter, the phases of the two Cartesian components of the field at the output of the APP plate should be identical and should exhibit a quadratic dependence on the radial polar coordinate.

Design of a Large Orbit Gyrotron with a Permanent Magnet System

The paper presents results of numerical analysis and outlines the computer-aided design of a novel high-harmonic gyrotron with a beam of electrons gyrating along axis-encircling trajectories. The electron beam is formed by a novel electron-optical system (EOS) based on an electron gun of diode type with thermionic cathode and gradual reversal of the magnetic field. The results of numerical simulations predict satisfactory performance of the EOS and appropriate beam quality parameters. The tube design allows one to install different cavities optimized for excitation of TE4,1 mode at the fourth harmonic of the cyclotron frequency or TE3,1 mode at the third one. The target parameters of the device are: frequency about 112 GHz; output power near 1 kW and efficiency of several percent.

Single Mode Laser Oscillation in an Nd-Doped Large Core Double Clad Fiber Cavity with Concatenated Adiabatic Tapers

ABSTRACT¾ We created a new design for an Nd-doped clad-pumped silica fiber laser to enhance the pump absor ption and lasing efficiency for a butt-coupled, end-pumped scheme. Two concatenated adiabatic tapers formed within the laser cavity simultaneously removed higher order modes and were spliced to conventional single mode fibers. We theoretically analyzed mode propagation along the composite cavity and experimentally achieved continuous wave oscillation in the LP 01 mode at 1.06 mm and a laser output power of over 820 mW with a slope e fficiency of 27%. I. INTRODUCTION The rapid development of arrayed semiconductor laser diodes (LDs) with high output power has influenced many applic ations in optical communications, especially high power fiber amplifiers and fiber lasers. The beam quality parameters of the arrayed LDs, however, are not compatible with those of conventional single mode fiber (SMF). To overcome this mode mismatch and increase power delivery, some researchers have developed laser cavities based on double clad fiber (DCF), in which the pump and signal beams have different optical paths interacting with each other over the length of the cavities [1], [2]. The pump is launched in the silica cladding waveguide with a large area and a high numerical aperture supported by a low refractive index polymer jacket.

Quality improvement of partially polarized beams

Improvement of the beam-quality parameter of partially polarized beams is investigated. We focus on the use of a Mach-Zehnder-type interferometric arrangement with crossed polarizers. The analysis has been carried out within the framework of the intensity moment formalism. Conditions are given under which the beam-quality parameter is optimized.

Beam propagation through uniaxial anisotropic media: global changes in the spatial profile

The propagation of electromagnetic beams through uniaxial anisotropic media is investigated. The Maxwell equations are solved in the paraxial limit in terms of the plane-wave spectrum associated with each Cartesian field component. Attention is focused on the global changes in the spatial structure of the beam, which are described by means of the second-order intensity moment formalism. In particular, the propagation law for the intensity moments through this kind of media is obtained. As a consequence it is inferred that it is possible to improve the beam-quality parameter by using these media.

Effect of holes and vortices on beam quality.

The effect of a vortex on the invariant quality factors of a light beam is studied. It is shown that a vortex degrades beam quality. The beam intensity at the eye of the vortex necessarily vanishes, creating a hole in the intensity distribution. The degradation in the beam quality is shown to be due partly to the vortex phase and partly to the hole. The results are illustrated graphically. An important inequality to be obeyed by the beam-quality parameters is exhibited.

High beam quality of ultraviolet radiation generated through resonant enhanced frequency doubling of a diode laser

The infrared emission from an external-cavity-tapered diode laser is frequency doubled in a 5-mm β-BaB2O4 crystal that is placed in a resonant-enhancement cavity. For a coupled diode power of 535 mW, 100 mW of continuous-wave, second-harmonic power is generated at λ=392 nm. The ultraviolet (UV) beam is measured to have a nearly ideal beam-quality parameter, M2≃1, in both meridians even though substantial double refraction is present in the nonlinear crystal. Printing of an array of submicron dots on a phase-change film is demonstrated with this UV beam. Also, by printing on photochemical film we demonstrate that improved film sensitivity is possible when a UV source is used.

Beam quality of partially polarized beams propagating through lenslike birefringent elements

Changes generated by lenslike birefringent elements on the beam-quality parameter of partially polarized beams are investigated. Analytical expressions for the beam-quality gain at the output of such systems are given in terms of the second-order intensity moments of the field. Explicit conditions to improve and optimize the beam-quality parameter after propagation through these birefringent transmittances are also shown.

Concept for Measuring Laser Beam-Quality Parameters.

This paper provides brief descriptions of recent developments in the basic formulas and measurement methods for determining laser beam-quality. The M2 factor concept has been studied in order to describe the laser beam quality of real laser beams, including potentially multimode and arbitrarily distorted laser beams. The study shows that the M2 value can be determined by measuring the beam spot sizes around the beam-waist. In addition, various methods for measuring beam spot sizes are discussed. Finally, the beam quality of fiberbundled diode lasers is evaluated by the moving-slit method. A good agreement is obtained between the theoretical and experimental beam propagation results.

Pulsed single-mode dye laser for coherent control experiments

We describe a pulsed single-mode dye laser whose output characteristics satisfy the stringent requirements of coherent control experiments. This simple, low cost laser exhibits 1.6× transform-limited frequency performance with a single-shot linewidth of 58 MHz, and a time averaged linewidth of 240 MHz. The spatial mode is nearly Gaussian and has a beam quality parameter M2=1.2. We modified the pulse shape using saturable absorption in the dye amplifier. The resultant pulses are temporally smooth and have a duration of approximately 20 ns.

How many times is a laser beam diffraction-limited?

We present an approach to the definition and estimation of the “times diffraction limit” (TDL) of highly diffracted laser beams that takes into account the near- and far-field energy distribution of the real beam instead of an ideal Gaussian beam. We have used it to find the TDL factor of two hard-edge-unstable-resonator laser beams having different quality and spatial energy distributions. The results are compared with the widely used beam quality parameter M 2. The discrepancy between our TDL and the M 2 coefficient becomes significant when the near-field energy distribution of a spatially coherent beam deviates from the pure Gaussian shape.

Beam quality changes produced by quartic phase transmittances

The effects that quartic phase distortions generate in tridimensional beam quality parameters of laser fields are investigated. Two cases of special interest are considered: a rotationally symmetric spherically aberrated lens, and a pairof identical but orthogonal cylindrical spherically aberrated lenses in contact with one another. Analytical expressions for the quality parameters at the output plane of these quarticphase optical systems are derived. A number of conditions enabling determination in a definite way whether the quality improves or deteriorates are given for several significant kinds of fields. In particular, some general conclusions are found for perfect Gaussian beams that discriminate whether the two-lens system provides better output beam quality than the single-lens system.