Control Laser Beam Research Articles

Tunable Infrared Semiconductor Lasers Based on Electromagnetically Induced Optical Defects

We propose tunable midinfrared laser systems based on dynamic formation of electromagnetically induced optical defect sites. Such defects occur in a waveguide structure having a uniformly corrugated quantum well structure in the absence of any structural defect or phase slip. In the absence of a control laser field, such a corrugated structure causes a uniform perturbation of refractive index along the waveguide. However, when a relatively small region of such a waveguide structure is illuminated from the side by the control laser field, electromagnetically induced transparency occurs in this region while its refractive index corrugation is removed. We also show that such a coherently induced defect can be dynamically moved along the waveguide structure by just steering the control laser beam to illuminate different locations. We utilize the fact that the phase associated with this defect site can be adjusted via changing the length of the illuminated region to present a tunable distributed feedback laser where its lasing wavelength can be continuously varied within the stop-band. We study the case where two coherently induced defect sites happen along the waveguide structure and discuss the impacts of illumination of the whole waveguide structure with the control laser field. We show that the latter can either make the waveguide coherently transparent by destroying its refractive index perturbation and stop-band, or generate a gain-without-inversion grating. Formation of such a grating allows the waveguide to act as a tunable partly gain-coupled distributed feedback laser.

Evaluation of Polarization Angle Rotation of Propagating Light in a Partially Ionized Atmosphere under Discharge Conditions

The rotation angle of polarization plane of propagating beam was experimentally measured using a polarization controlled laser beam, repeating square mirror, and discharge apparatus. A rotation angle of 0.29 degrees was obtained in the case of discharge gap of 4 cm and charge voltage of 50 kV. The result was in good agreement with the simulation of the experimental model. The waveform of the rotation angle showed a similar time dependence as the discharge current, with a correlation coefficient of>0.94.

Expansion of the relative time delay by switching between slow and fast light using coherent population oscillation with semiconductors

The slow-light effect based on free-carrier population oscillation in semiconductors is analyzed using generalized macroscopic Bloch equations. Using this model, we are able to investigate the relation between the fractional time delay and the intensity of control laser beams for semiconductors. It is found that although increasing the control-beam intensity can enlarge the fractional time delay, the maximum delay that can be achieved is limited and determined by a critical value of the separation between the quasi-Fermi levels of electrons and holes. We also show that there is an intensity threshold for the control beam above which the probe signal becomes gain-assisted fast light rather than slow light. We therefore suggest a method to expand the relative time delay by switching between the fast-light region and the slow-light region through changing the intensity of the control laser beams.

Efficient laser polishing of silica micro-optic components

We report a study of the basic characteristics of laser polishing of fused silica with a protocol that is particularly suitable for surface smoothing of micro-optic elements fabricated by a laser ablation process. We describe a new, to our knowledge, approach based on scanning a highly controlled small size laser beam and melting areas of tens to hundreds of micrometers of glass using a computer-controlled raster scan process, which does not require beam shaping, substrate preheating, or special atmospheres. Special test samples of silica substrates with prescribed spatial frequency content were polished using a range of irradiation conditions with the beam from a well-controlled CO2 laser operating at a wavelength of 10.59 microm. An analysis is presented of the laser-generated reduction in surface roughness in terms of measurements of the spatial frequency characteristics, and the results are compared with the predictions of a simple model of surface-tension-driven mass flow within the laser-melted layer. This technique is shown to be capable of smoothing silica surfaces with approximately 1 microm scale roughness down to levels < 1 nm with no net effect on the as-machined net surface shape, at realistic production rates without a preheating stage, and with noncritical residual stresses.

A novel method to improve spatial resolution of acousto-optic deflector

In this paper, a novel method of laser beam deflecting is introduced to improve the spatial resolution of an acoustic-optic deflector (AOD). We use double AOD (DAOD) to control laser beam. Compared with single AOD, the double AOD can improve speed and precision of target tracking and pointing significantly. Finally, a method of calculating the static and dynamic number of resolvable poins (NRP) is and it is supported by the experimental data excellently.

Enhancement in the number of trapped atoms in a cesium magneto-optical trap by a near-resonant control laser

We demonstrate enhancement in the number of trapped cesium atoms in a magneto-optical trap (MOT) using a control laser that illuminates only a small faction of the capture region of the trap without interacting with the cold cloud of atoms. The enhancement is maximized when the laser is slightly blue detuned with respect to the cooling transition. Trap loading curves point to approximately a twofold increase in the capture rate, which as a consequence results in the increase in the steady state number of trapped atoms. Enhanced loading is confirmed by MOT loading and decay curves taken under the modulation of the control laser beam. Optical pumping of the inaccessible Zeeman states into the stretched states is suggested as a possible mechanism.

Laser polishing of silica slotted rods

Lasers have been widely used in surface modification. In the present research, a CO2 continuous wave laser has been used to polish slot surfaces of silica rods. Strong absorption of 10.6 μm CO2 radiation by the silica surface promotes softening of a very thin layer of material that flows under the action of surface tension forces. A smooth surface forms, which decreases the surface roughness without substantial change in surface geometry. The effect of laser-surface inclination angle on the requisite power requirement has been assessed experimentally and theoretically. Using a computer controlled laser beam scanning system, reliability and reproducibility have been greatly improved compared with an alternative surface finishing method, manual fire polishing.

All-Optical Triode Using Dual-Stage Wavelength Converter in Erbium-Doped Fiber Amplifiers

An all-optical triode was demonstrated using a dual-stage wavelength converter in two erbium-doped fiber amplifiers. The first stage of the converter was based on cross-gain modulation in the amplifier whereby conversion from an input wavelength of 1534 nm to a bias wavelength of 1555 nm was accomplished. The 1555 nm wavelength was converted back to 1534 nm in the second stage. Moreover, a laser-induced signal enhancement phenomenon consists of the optical signal of spontaneous emission being amplified while maintaining an almost constant modulation degree when a continuous wave laser beam is injected into the amplifier. The output signal power was amplified by a factor of approximately 20, maintaining an almost constant input modulation degree, when the control laser beam was injected into the device.

Projects on coal characterization

Danish power stations use coals, imported from various countries around the world. Since the coals may differ considerably in burning properties, the purchasers have a marked interest in characterization of the coals prior to use. In a Danish project existing methods were compared, using ten selected coal samples and equipment and techniques for particulate coal in the USA and Sweden were also used. TGA measurements and chemical and petrographic characterizations were made in Denmark. A multivariate analysis of six selected characterizations, using the Sandia equipment (CA, USA) results as target parameters, was made to create a statistical model which would allow characterization of an “unknown” coal with a minimum of analytical effort and cost. In a separate investigation, involving a flat-flame burner, an entrained-flow reactor (both in USA) and a TGA, oxidative coal pyrolysis was studied. A distributed activation energy concept was applied to produce kinetic parameters for a large-scale combustor model to evaluate the conditions in a flame. A parallel project (named IKI) presents an innovation with pyrolysis by a controlled laser beam of a very thin sample plate in a specific chamber, followed by isothermic gasification. The characterizations obtained with the IKI method on coal samples of the DP agree well with the Sandia results. Compared with the concept of coal ranks, for which two coals of same rank may behave differently, coal characterization methods have taken big steps forward, but we are still far from seeing the new tools employed in practice.

Ceramic Composites Fabricated by Selective Laser Sintering

Abstract Selective laser sintering (SLS) uses a computer controlled laser beam to create objects directly from the CAD data without part-specific tooling. In this paper, the effect of material and processing parameters on the microstructure and mechanical properties of an alumina/aluminum phosphate composite fabricated by SLS is described. The precursor material is a blend of alumina and ammonium phosphate powder. This paper deals primarily with the effect of alumina particle size and material composition on the strength of the composite. A constitutive equation is also proposed to relate the mechanical properties of the composite to its microstructural characteristics.

A system for Coulomb explosion imaging of small molecules at the Weizmann Institute

A description of a system for molecular structure imaging at the Weizmann Institute is presented. A novel method of controlled laser beam photo detachment inside the high voltage terminal of a tandem accelerator, enabling the study of neutral fast molecules by the Coulomb Explosion Imaging technique, is described. Also, a new type of three dimension multiparticle detector is presented.

An introduction to medical lasers.

This review deals with the following topics: why use lasers?; what is a laser?; containing and controlling laser beams; types of medical lasers; laser beam delivery systems; evaluation and safety of medical lasers; buying a laser; laser tubes; general maintenance; staff training and safety codes; future outlook.

Carbon Dioxide Laser Beam Control for Corneal Surgery

Precise control of output power and beam divergence of a CO2 laser resulted in focused beam diameters down to 25 microns and essentially continuous control of energy deposition on target. Application of controlled laser beams on cow, hog, and human eye-bank reject corneal tissue give evidence of well-defined, narrow cuts. The controllable penetration depth and width of the CO2 laser incisions seem to make the laser an ideal "knife" for such corneal modifications as radial keratotomy and epikeratophakia. Our results indicate that the CO2 laser, when successfully integrated with the standard slit lamp, may be a safer and useful tool in laser surgery of the cornea.

Endoscopic laser coagulation of bleeding gastrointestinal lesions with report of the first therapeutic application in man

In acute and chronic experiments in animals we have shown that it is possible to effect photocoagulation with the argon-ion laser beam in any part of the gastrointestinal tract. The degree of coagulation depends upon the duration of application of the laser, power density, and the properties of the tissue treated. Hemostasis was achieved with the laser in 50 artificially induced bleeding lesions in the mucosa and 80 incised arteries and veins having a diameter of up to 1 mm and 2mm, respectively. Endoscopically controlled laser beam coagulation has since been applied successfully in the treatment of hemangiomas, angiodysplasia, intestinal angiomatosis, bleeding gastric erosions, bleeding gastric and duodenal ulcers, and iatrogenic hemorrhage in the stomach following big particle biopsy in 94 lesions.

Investigation of Laser Modulation by Modifying the Internal Reflection Barrier

The well-known problem of penetration through an internally reflecting barrier is treated as it relates to the extraction of a laser beam from an internally reflecting cavity. A quantity d logT/d logn which relates the fractional change of transmittance to the fractional change of refractive index is defined and is shown to be large for large barrier spacing. It is suggested that this strong dependence of transmittance on the value of n may be used in studying and controlling laser beams.