Modulation Of Laser Beam Research Articles

Shrinkage of picosecond laser beam by plasma reflection in super-resolution near-field structure of SiN/Sb/SiN thin film

The transmittive and reflective Z-scan technique is used with a 10 Hz, frequency doubled, Q-switched, and mode-locked Nd:YAG laser to verify that the reflectivity of the super-resolution near-field structure of an SiN/Sb/SiN thin film increases as incident intensity decreases. This intensity-dependent reflection, called nonlinear reflection, reflects a TEM(00) mode laser beam more strongly at its periphery than at its center and so shrinks the transmitted laser beam. The observed nonlinear reflection is attributed to laser-induced change of carrier densities in Sb, to justify quantitatively the experimental results.

Intensity dependent residual amplitude modulation in electro-optic phase modulators

Residual amplitude modulation (RAM) mechanisms in electro-optic phase modulators are detrimental in applications that require high purity phase modulation of the incident laser beam. While the origins of RAM are not fully understood, measurements have revealed that it depends on the beam properties of the laser as well as the properties of the medium. Here we present experimental and theoretical results that demonstrate, for the first time, the dependence of RAM production in electro-optic phase modulators on beam intensity. The results show an order of magnitude increase in the level of RAM, around 10 dB, with a fifteenfold enhancement in the input intensity from 12 to 190 mW/mm2. We show that this intensity dependent RAM is photorefractive in origin.

NEAR AND FAR FIELD OPTICAL PATTERNS FORMATION BY ROTATIONAL SYMMETRY MASKS: APPLICATION TO OPTICAL SPATIAL SOLITON GENERATION

In this report we present the formation of necklace type optical beams from Gaussian beam by rotational symmetry masks. The detailed studies of space evolution of the optical beam formation from the near field to the far field by the use of 7-fold symmetry mask and a single mode laser beam were performed. Different types of necklace beams with different numbers of "pearls" periodically or quasi-periodically situated along the different radii rings were observed during the space evolution of the optical diffraction pattern.

Efficient second harmonic generation of a diode laser using ring cavity with a KNbO3 Crystal

The spatial mode and impedance matching of an IR diode laser beam with a ring cavity has been investigated by the example of second-harmonic generation in a KNbO3 crystal placed in the cavity. The frequency conversion efficiency into blue light is found to be as high as 20%.

Fabrication of three-dimensional electrospun microstructures using phase modulated femtosecond laser pulses

Electrospun polycaprolactone nanofibers were selectively ablated to form microstructures via the phase modulation of femtosecond laser beams. Ablation width (1–15 μm) and depth (15–110 μm) resolution were dependent upon the selection of pulse energy and microscope objective. Because phase modulation shapes light in a maskless fashion, desired templates were digitally created and physically transferred to electrospun mats within a matter of minutes. Several microarchitectures were formed in parallel by dividing pulse energy between multiple foci, substantially increasing throughput. The data presented herein demonstrates that phase-based laser ablation can be used to rapidly shape and tailor electrospun mats in three dimensions.

Autogenous laser welding investigations on modified 9Cr–1Mo (P91) steel

Modified 9Cr–1Mo steel plates of 6 mm thickness have been laser welded using CO2 laser. The effects of beam intensity and overall heat input (168-1500J/mm) on the bead characteristics, microstructure and mechanical properties of the welds have been investigated by varying the laser welding parameters such as laser beam mode, power and welding speed. The microhardness survey carried out on the welds after post-weld heat treatment did not reveal any soft zones in the intercritical heat affected zone for welds made with a heat input of up to 420 J mm−1. The tensile strengths of the welds were comparable to that of the base material. Charpy impact tests on subsize specimens revealed that the welds have good toughness. δ-Ferrite was observed in the fusion zone of the welds made at heat input of 700 J mm−1 and above, the content of which increased with the increased heat input.

High-efficiency electrically tunable phase diffraction grating based on a transparent lead magnesium niobate-lead titanite electro-optic ceramic

An electrically tunable high-efficiency phase diffraction grating using the transparent lead magnesium niobate-lead titanite (PMNT) electro-optic ceramic is proposed. A photomask technique is used for fabricating the grating with the Ti/Pt/Au interdigital electrodes. With the variation of the equivalent phase face induced by the quadratic electro-optic effect of the PMNT ceramic, the diffraction patterns of the light beam will be changed in which the diffraction efficiency may be controlled by an applied DC voltage. When the applied voltages are 41, 80, and 139 V, the zero-order, first-order, or second-order diffraction lights will disappear in turn with about 100% diffraction efficiency. The proposed grating can be used as an electrically controlled switch in the laser beam modulation and scanning, especially for the application of the high-power laser systems.

Two-dimensional angular filtering by volume Bragg gratings in photothermorefractive glass

We propose and perform two-dimensional angular filtering on the basis of volume Bragg gratings (VBGs) in a series connection for cleaning up a laser beam. The dependence between angular selectivity and VBG parameters is analyzed with the coupled wave theory. Near-field modulation, contrast ratio, and power spectral density were used to evaluate the effect of filtering in both the spatial and frequency domain. The results from our initial experiments showed that the near-field modulation and contrast ratio of laser beam were remarkably improved.

Scaling of spiking and humping in keyhole welding

Spiking, rippling and humping seriously reduce the strength of welds. The effects of beam focusing, volatile alloying element concentration and welding velocity on spiking, coarse rippling and humping in keyhole mode electron-beam welding are examined through scale analysis. Although these defects have been studied in the past, the mechanisms for their formation are not fully understood. This work relates the average amplitudes of spikes to fusion zone depth for the welding of Al 6061, SS 304 and carbon steel, and Al 5083. The scale analysis introduces welding and melting efficiencies and an appropriate power distribution to account for the focusing effects, and the energy which is reflected and escapes through the keyhole opening to the surroundings. The frequency of humping and spiking can also be predicted from the scale analysis. The analysis also reveals the interrelation between coarse rippling and humping. The data and the mechanistic findings reported in this study are useful for understanding and preventing spiking and humping during keyhole mode electron and laser beam welding.

Relativistic self-focusing and self-phase modulation of cosh-Gaussian laser beam in magnetoplasma

AbstractIn this paper, we have investigated the propagation characteristics of cosh-Gaussian laser beam in magnetoplasma using relativistic nonlinearity. The field distribution in the medium is expressed in terms of beam width parameter an and decentred parameter b. An appropriate nonlinear Schräodinger equation has been solved analytically using variational approach. The behaviour of beam width parameter with dimensionless distance of propagation ξ for various b values is examined. Self-phase modulation and self-trapping is also studied under variety of parameters. Further, the effect of magnetic field on self-focusing of the beam have been explored.

Laser beam shaping and mode conversion in optical fibers

A new class of all-fiber beam shaping devices has been realized by inverse etching the end face of single mode and multimode fibers to form a concave cone tip. Concave tip fiber can convert a Gaussian beam profile to a flat top beam profile with a uniform intensity distribution. A flat top beam with intensity variation of approx. 5% and flat top diameter to spot diameter ratio of 67% has been achieved. This device can also change the beam shape from a Gaussian to a donut by moving the observation plane. A flat top multimode fiber beam delivery is attractive for applications which require high power and uniform intensity distribution. In single mode fiber, concave tips could be used to reduce the beam spot size diameter, enabling efficient light coupling from a single mode fiber to an integrated optical waveguide.

Intensity correlation properties of high-order optical vortices passing through a rotating ground-glass plate

We study the intensity correlation properties of optical vortices passing through a rotating ground-glass (RGG) plate and compare them with those of the TEM(00) mode of an He-Ne laser beam passed through the same RGG. We have observed that the intensity correlation curves for optical vortices decrease much faster than the corresponding curve for a TEM(00) mode of the He-Ne laser. The rate of decay of the correlation increases with the increase of order of the vortices. Our experimentally observed results are supported by exact analytical results.

Single-crystal modifications of the Mach-Zehnder electro-optic modulator

Two single-crystal modifications of the Mach-Zehnder electro-optic modulator are described. Preliminary theoretical estimates show that these variants are capable of ensuring the amplitude modulation of laser beams with elevated power (no less than several watts) at frequencies up to 4 GHz. The obtained modulation characteristics show that the modulation depth in the proposed devices reaches 50–80% and a half-wave voltage of these devices amounts to 100–160 V.

Laser Welding of Steel Plates with Divided Beam

In the steel-work the rolled coils of low carbon steel plates must be unended before pickling on the continuous mill. This was solved with butt welded joint by laser beam welding at ISD DUNAFERR Dunai Vasmő ZRt. The 8 kW light power, near TEM00 mode laser beam’s final part of optical system is a divided parabolic mirror, which creates two focal spot in the welding region. In the interest of productivity rising, the rolled bands’ ends have to be welded at the highest speed as far as possible. The welded seams have to tolerate the repeated bends on the pickling mill without cracking. The optimum parameters of laser welding technology were determined by metallography and calculation of energetic efficiency.

Investigation of CO 2 laser beam modulation by rotating polygon

A beam-modulating system based on the rotating polygon has been proposed in a previous experimental work [14]. The aim was to improve the quality and efficiency of CO 2 laser surface texturing. Based on the generalized ABCD law, the focusing characteristics of a real beam through the modulating system are investigated in this paper. By introducing a precise defocusing, the velocity synchronization between the focal spot and the workpiece can be realized. Micro-dimples of nearly circular shape can be achieved on the surface. Surface texturing of mill roll has been performed. The experimental and theoretical results are in good agreement.

Transport phenomena in conduction mode laser beam welding of Fe–Al dissimilar couple with Ta diffusion barrier

Fusion welding of dissimilar metals always encounters immense difficulties due to the formation of brittle intermetallic phases that make the joint unsuitable for structural applications. Using a barrier layer that wets well with either component but does not undergo melting itself and prevents intermixing and undesirable phase formation could be a possible remedy. In order to explore this hypothesis, we have made an attempt to numerically simulate and experimentally validate the heat and mass transport phenomena in the fusion zone concerning laser assisted welding of Fe–Al dissimilar couples with 0.2 mm Ta foil as the diffusion barrier butted between Fe and Al. The results suggest that the Fe-side undergoes greater degree of melting resulting in an asymmetric thermal and velocity fields. Furthermore, partial melting of Ta seems unavoidable but proves benign as this leads to good bonding between Fe–Ta and Ta–Al while precluding formation of Fe–Al alloy/intermetallic phases.

Extending the Limits of Laser Polymer Welding using Advanced Irradiation Strategies

After enjoying a significantly increased industrial acceptance, mainly due to the development of high power diode lasers, the laser polymer welding has faced extensive investigation over the last decade in order to become nowadays a strong competitor for the other polymer joining processes. The latest developments in the field of laser materials processing such as laser sources with excellent beam quality are bringing new opportunities and also challenges for the laser welding of polymers and lead to a further extension of the process limits. This paper presents the achieved results for the polymer welding using high brilliance laser sources together with advanced irradiation strategies based on the local und temporal modulation of the laser beam such as the recently developed method called TWIST ® – Transmission Welding by an Incremental Scanning Technique. This approach leads to an optimized thermal management within the welding zone, an exact control of the temperature field generated by the absorbed laser energy, and a significant increase of process robustness, welding quality, process flexibility and efficiency. Experimental results for welding of polymers with a weld seam width from 100 µm up to 800 µm and a processing speed up to 30 m/ min are presented and discussed.

Enhanced melt pool stirring in welding with dynamic polarised laser beam

The stirring of the molten material during CO2 laser welding with a dynamic polarised laser beam (DP) was studied. The key element of the set-up is a high power interference laser beam modulator for CW power rating up to 4 kW. Metallographic cross-sections and EDX analyses of the realised welds in St37 using the contrast material nickel proved the enhancement of melt pool stirring due to DP. Moreover, roughness measurements confirmed a smoothing of the top bead surface as well as a correlation between melt pool stirring and the formation of the top bead ripple structure.

Surface Enhanced Fluorescence (SEF): Polarization Characteristics

Nanostructured platforms and specifically, array of nanoholes with subwavelength pitch, have been shown to enhance fluorescence signals. Here, we assess the effect of platform orientation with respect to the incident polarized (exciting) beam on the fluorescence signal of dye molecules, imbedded in the pores of such structures. Clear angular-dependent fluorescence signal, excited with the TM mode of the incident laser beam, has been demonstrated. The results may be explained by feedback to the scattered fluorescence mode from the array of nanoholes.

Generation of a dynamic polarized laser beam for applications in laser welding

The temporal control and optimization of laser beam parameters, e.g., the beam power, can be employed to enhance the quality and reliability of the welding process. Until now, the influence of a time-dependent beam polarization has been less investigated in welding. The publication describes a new experimental arrangement allowing the fast variation of the polarization of CO2 laser radiation [“dynamic polarization” (DP)]. The key element of the setup is a high power interference laser beam modulator for cw laser power rating of up to 4 kW. Weld seams were produced with different process parameters in the technically relevant steels St37 and Ck45 and in the precoated steel 22MnB5 (USIBOR 1500P). The welds were subsequently characterized by roughness measurements of the top bead surfaces and by the preparation of cross sections and their macroscopic analysis. The experiments on welding with DP showed a significant influence on the keyhole and melt pool dynamics, especially a reduction in the top bead roughness at certain DP frequencies up to a factor of about 3. In addition, a threshold behavior for this effect was observed. It was shown that the cross sectional shape and thus the degree of energy coupling to the material was unaffected by the DP. Finally, during welding USIBOR 1500P with DP an enhanced melt pool stirring was achieved. The results show that the temporal modulation of energy coupling to the material caused by DP is suitable to enhance the quality and reliability of the welding process.