Gaussian-shaped Beam Research Articles

Spatial optical soliton in (1 + 2)-dimensional synthetic nonlocal nonlinear media

This paper studies the propagation of Gaussian-shaped optical beams in (1+2)-dimensional synthetic nonlocal media by variational approach. The evolution equations for the parameters of the optical beam are obtained and the critical power of forming a soliton is found. The optical soliton can be formed in “focusing–focusing” synthetic nonlocal media with arbitrary degree of nonlocality. However, we find that σ2 must be grater than a certain value which was depended on the other material parameters of the “focusing–defocusing” synthetic nonlocal media. The analytical results were confirmed by the numerical results.

One-step fabrication of annular microstructures based on improved femtosecond laser Bessel–Gaussian beam shaping

In this work, a favorable approach is proposed for the high-efficiency fabrication of annular microstructures by shaping femtosecond Gaussian laser into Bessel–Gaussian beam. An adjustable annular beam is generated by a combination of an axicon, a lens, and an objective. Uniform annular microstructures are fabricated on the surface of fused silica in a single illumination step with only one pulse and without stage translation, the parameters of which are coincident with theoretical design. By adjusting the relative distance between the lens and objective, the diameters of the annular microstructures can be flexibly controlled in a wide range. In addition, it has been discussed that the effects of laser pulse energy and off-focus on the depth and line-width of the fabricated annuli.

A high-power narrow-linewidth 1018 nm fiber laser based on a single-mode–few-mode–single-mode structure

We demonstrate an all-fiber high-power Yb-doped 1018 nm fiber laser with a Gaussian-shaped output beam profile based on a mismatched structure, which consists of a pair of single-mode fiber Bragg gratings and a section of few-mode double-cladding gain fiber. The output power is up to 107.5 W with an optical-to-optical efficiency of 63%, and the 3 dB band is 0.26 nm at this power level. Such a structure of single-mode–few-mode–single-mode fiber oscillator can be used to generate high-power narrow-linewidth lasing with excellent beam quality in other spectral ranges.

Laser Welding of Transparent Polymers by Using Quasi-simultaneous Beam Off-setting Scanning Technique

Traditionally laser polymer welding has been done by using lasers with wavelength of c. 1μm wavelength. With this wavelength one of the pair of polymers to be joined needs to be transparent and one absorbing, usually clear and black, respectively. Joining of two transparent polymers with laser has been possible only by using absorbing additive between the samples or by doping the polymer with absorbing additive. During the last years lasers with new wavelengths between 1.4 to 2μm have been developed. With this wavelength range welding of the transparent polymer to another transparent one without any absorbing additives is now possible. In this paper joining of transparent to transparent polymers with quasi-simultaneous laser welding (QSLW) technique and fibre lase with wavelength of 1.94μm is studied. Also a new scanning technique for QSLW is introduced. In this technique laser beam path is off-setting to a different place in every scanning round. This way the Gaussian beam shape of fibre laser can be smoothened to be more suitable for polymer welding and more flexibility on laser welding of polymers is achieved without losing the benefits of quasi-simultaneous laser welding.

Gaussian beam shaping based on aspheric cylindrical lenses

We propose a specific aspheric cylindrical optical system to transform Gaussian beam to flat-top and rectangular beam. The Gaussian beam shaping system is composed of dual orthogonal aspheric cylindrical lenses. The principle of shaping Gaussian beam is studied theoretically. The mapping function of arbitrary rays in an incident plane and an image plane is deduced based on the law of energy conservation, and the real ray tracing method is adopted to design the shaping system. Finally, the lens system is processed by single point diamond turning techniques. Testing results indicate that the system achieves the theoretical expectation, and the uniformity of flat-top and rectangular beam is 88.2%. The method is not only simple but also practical.

Reflection of a TE-polarised Gaussian beam from a layered structure under conditions of resonance excitation of waveguide modes

The problem of reflection of a TE-polarised Gaussian light beam from a layered structure under conditions of resonance excitation of waveguide modes using a total internal reflection prism is considered. Using the spectral approach we have derived the analytic expressions for the mode propagation lengths, widths and depths of -lines (sharp and narrow dips in the angular dependence of the specular reflection coefficient), depending on the structure parameters. It is shown that in the case of weak coupling, when the propagation lengths of the waveguide modes are mainly determined by the extinction coefficient in the film, the depth of -lines grows with the mode number . In the case of strong coupling, when is determined mainly by the radiation of modes into the prism, the depth of -lines decreases with increasing . The change in the TE-polarised Gaussian beam shape after its reflection from the layered structure is studied, which is determined by the energy transfer from the incident beam into waveguide modes that propagate along the structure by the distance , are radiated in the direction of specular reflection and interfere with a part of the beam reflected from the working face of the prism. It is shown that this interference can lead to the field intensity oscillations near -lines. The analysis of different methods for determining the parameters of thin-film structures is presented, including the measurement of mode angles and the reflected beam shape. The methods are based on simultaneous excitation of a few waveguide modes in the film with a strongly focused monochromatic Gaussian beam, the waist width of which is much smaller than the propagation length of the modes. As an example of using these methods, the refractive index and the thickness of silicon monoxide film on silica substrate at the wavelength are determined.

Beam shaping characteristics of an unstable-waveguide hybrid resonator.

The unstable-waveguide hybrid resonator emits a rectangular, simple astigmatic beam with a large number of high-spatial-frequency oscillations in the unstable direction. To equalize the beam quality, in this paper, a beam shaping system with a spatial filter for the hybrid resonator was investigated by numerical simulation and experimental method. The high-frequency components and fundamental mode of the output beam of the hybrid resonator in the unstable direction are separated by a focus lens. The high-frequency components of the beam are eliminated by the following spatial filter. A nearly Gaussian-shaped beam with approximately equal beam propagation factor M² in the two orthogonal directions was obtained. The effects of the width of the spatial filter on the beam quality, power loss, and intensity distribution of the shaped beam were investigated. The M² factor in the unstable direction is changed from 1.6 to 1.1 by optimum design. The power loss is only 9.5%. The simulation results are in good agreement with the experimental results.

Optical spectral imaging of a single layer of a quantum gas with an ultranarrow optical transition

We demonstrated high-sensitivity optical spectral imaging of a single layer of a quantum gas of ytterbium atoms in a two-dimensional optical lattice using the ultranarrow ${}^{1}{S}_{0}$-${}^{3}{P}_{2}$ transition. We successfully obtained a set of excitation-frequency-dependent fluorescence images with an excitation laser of the linewidth of 1 kHz (FWHM), and the overall features were well explained by considering the inhomogeneous light shift originating from the Gaussian beam shape of the optical lattice potential which provided the steepest potential gradient of 3.6 kHz/$\ensuremath{\mu}$m. This result is also the successful demonstration of the tunable local atom addressing along the equipotential contour depending on the excitation laser frequency with the frequency resolution of 8 kHz and the spatial resolution of approximately 2 $\ensuremath{\mu}$m. The demonstrated technique will be useful for many purposes including the measurement of interaction shift in the study of a quantum gas and quantum information processing.

Generation and patterning of Si nanoparticles by femtosecond laser pulses

The unique optical properties of nanoparticles are highly sensitive in respect to particle shapes, sizes, and localization on a sample. This demands for a fully controlled fabrication process. The use of femtosecond laser pulses to generate and transfer nanoparticles from a bulk target towards a collector substrate is a promising approach. This process allows a controlled fabrication of spherical nanoparticles with a very smooth surface. Several process parameters can be varied to achieve the desired nanoparticle characteristics. In this paper, the influence of two of these parameters, i.e. the applied pulse energy and the laser beam shape, on the generation of Si nanoparticles from a bulk Si target are studied in detail. By changing the laser intensity distribution on the target surface one can influence the dynamics of molten material inducing its flow to the edges or to the center of the focal spot. Due to this dynamics of molten material, a single femtosecond laser pulse with a Gaussian beam shape generates multiple spherical nanoparticles from a bulk Si target. The statistical properties of this process, with respect to number of generated nanoparticles and laser pulse energy are investigated. We demonstrate for the first time that a ring-shaped intensity distribution on the target surface results in the generation of a single silicon nanoparticle with a controllable size. Furthermore, the generated silicon nanoparticles presented in this paper show strong electric and magnetic dipole resonances in the visible and near-infrared spectral range. Theoretical simulations as well as optical scattering measurements of single silicon nanoparticles are discussed and compared.

Reply to “Comment on ‘Control of the Goos-Hänchen shift using a duplicated two-level atomic medium’ ”

The concept behind using a Gaussian-shaped probe beam to control the Goos-H\anchen shift using a duplicated two-level atomic medium inside a cavity [Phys. Rev. A 85, 055804 (2012)] is presented. It is obvious from the analysis that by using certain approximations, the required conditions can be imposed on the probe field corresponding to the Gaussian-shaped incident beam and control field driving the intracavity atomic medium.

A high-field 3He metastability exchange optical pumping polarizer operating in a 1.5 T medical scanner for lung magnetic resonance imaging

After being hyperpolarized using the technique of Metastability Exchange Optical Pumping (MEOP), 3He can be used as a contrast agent for lung magnetic resonance imaging (MRI). MEOP is usually performed at low magnetic field (∼1 mT) and low pressure (∼1 mbar), which results in a low magnetization production rate. Polarization preserving compression with a compression ratio of order 1000 is also required. It was demonstrated in sealed cells that high nuclear polarization values can be obtained at higher pressures with MEOP, if performed at high magnetic field (non-standard conditions). In this work, the feasibility of building a high-field polarizer that operates within a commercial 1.5 T scanner was evaluated. Preliminary measurements of nuclear polarization with sealed cells filled at different 3He gas pressures (1.33 to 267 mbar) were performed. The use of an annular shape for the laser beam increased by 25% the achievable nuclear polarization equilibrium value (Meq) at 32 and 67 mbar as compared to a Gaussian beam shape. Meq values of 66.4% and 31% were obtained at 32 and 267 mbar, respectively, and the magnetization production rate was increased by a factor of 10 compared to the best results obtained under standard conditions. To study the reproducibility of the method in a polarizing system, the same experiments were performed with small cells connected to a gas handling system. Despite careful cleaning procedure, the purity of the 3He gas could not be matched to that of the sealed cells. Consequently, the polarization build-up times were approximately 3 times longer in the 20–30 mbar range of pressure than those obtained for the 32 mbar sealed cell. However, reasonable Meq values of 40%–60% were achieved in a 90 ml open cell. Based on these findings, a novel compact polarizing system was designed and built. Its typical output is a 3He gas flow rate of 15 sccm with a polarization of 33%. In-vivo lung MRI ventilation images (Signal to Noise Ratio (SNR) of approximately 55 for a voxel size of 50 mm × 3 mm × 3 mm) were acquired to demonstrate the polarizer's application.

A 200 W INNOSLAB Tm:YLF laser

We report obtaining 200 W of continuous wave laser emission at 1.9 μm from an INNOSLAB Tm:YLF (YLiF4) laser. The slope efficiency is 52.2% with respect to the absorbed pump power. The wavelength of the laser output and the polarization direction can be switched between 1890 and 1908 nm. Due to the stable resonator design in both axes of the slab crystal, the output is strongly elliptical with a Gaussian distribution beam shape along the semi-minor axis and a top-hat-like distribution beam shape along the semi-major axis. This laser source is ideal for pumping a holmium-doped slab crystal in a pulsed INNOSLAB amplifier.

Angular spectrum and localized model of Davis-type beam

The angular spectrum of the Davis fifth-order linearly polarized, dual, and symmetrized fields of a focused Gaussian laser beam is obtained. Since the original Davis fields are not an exact solution of the vector wave equation and Maxwell's equations, a beam remodeling procedure within the angular spectrum is described that produces an exact solution. The spherical multipole beam shape coefficients of the remodeled beam are then obtained, and it is shown that in the weak focusing limit they simplify to the localized model Gaussian beam shape coefficients for both on-axis and off-axis beams. The angular spectrum method is then applied to a transversely confined electromagnetic beam with arbitrary profile in the focal plane, and to a general zero-order Bessel beam.

A novel hybrid algorithm for the design of the phase diffractive optical elements for beam shaping

In this paper, a novel hybrid algorithm for the design of a phase diffractive optical elements (PDOE) is proposed. It combines the genetic algorithm (GA) with the transformable scale BFGS (Broyden, Fletcher, Goldfarb, Shanno) algorithm, the penalty function was used in the cost function definition. The novel hybrid algorithm has the global merits of the genetic algorithm as well as the local improvement capabilities of the transformable scale BFGS algorithm. We designed the PDOE using the conventional simulated annealing algorithm and the novel hybrid algorithm. To compare the performance of two algorithms, three indexes of the diffractive efficiency, uniformity error and the signal-to-noise ratio are considered in numerical simulation. The results show that the novel hybrid algorithm has good convergence property and good stability. As an application example, the PDOE was used for the Gaussian beam shaping; high diffractive efficiency, low uniformity error and high signal-to-noise were obtained. The PDOE can be used for high quality beam shaping such as inertial confinement fusion (ICF), excimer laser lithography, fiber coupling laser diode array, laser welding, etc. It shows wide application value.

Terahertz wavefront measurement with a Hartmann sensor

The measurement of the wavefront of a terahertz (THz) beam is essential for the development of any optical instrument operating at THz frequencies. We have realized a Hartmann wavefront sensor for the THz frequency range. The sensor is based on an aperture plate consisting of a regular square pattern of holes and a microbolometer camera. The performance of the sensor is demonstrated by characterizing the wavefront of a THz beam emitted by a quantum-cascade laser. The wavefront determined by the sensor agrees well with that expected from a Gaussian-shaped beam. The spatial resolution is 1 mm, and a single-wavefront measurement takes less than 1 s.

Goos–Hänchen shift of the reflected wave through an anisotropic metamaterial containing metal/dielectric nanocomposites

Goos-Hänchen (GH) shift of a transverse-magnetic (TM) wave reflected from a semi-infinite anisotropic metamaterial consisting of aligned metallic nanowires in a dielectric matrix is investigated. Based on Bruggeman effective medium theory, we obtain the conditions for realizing the negative refraction, which are dependent on both the incident wavelength and the volume fraction of metallic inclusions. Then, we investigate the GH shifts from the composite metamaterial with positive and negative refractions with the stationary-phase method. Numerical results show that the enhancement of GH shift can be achieved near the pseudo-Brewster angle for small volume fractions and at the close-to-grazing incidence for large volume fractions. We further find that for positively refractive metamaterials with weak absorption, one can realize the transition from negative GH shift to the positive one by adjusting the incident wavelength. However, for negatively refractive composite metamaterials, the reversal of the GH shifts may take place by the adjustment of the volume fraction instead of the incident wavelength. In order to demonstrate the validity of the stationary-phase approach, numerical simulations are performed for a Gaussian-shaped beam. In the end, by using COMSOL simulation, a comprehensive understanding is given and the above analysis is confirmed.

Ion beam based composition and texture control of titanium nitride

Titanium nitride was formed by electron beam evaporation of titanium in an atmosphere of backfilled nitrogen. The growing film was simultaneously irradiated with argon ions. From of a single-aperture ion source, a Gaussian-shaped ion beam was extracted. Each position on the sample was correlated with a particular ion beam current density. With this method, on a single substrate a large variation in ion irradiation intensity could be obtained. Since the titanium evaporation rate was uniform over this area, the parameter varied over the sample surface was only the ion-to-atom arrival ratio. It turns out that with a constant Ti condensation rate and nitrogen impact rate, the Ti:N ratio in the film is a strong function of the argon ion impact rate. Ion-beam induced nitrogen sorption saturates at a certain ion irradiation intensity. While under all conditions the TiN (mono)phase is obtained, it shows different preferred crystal orientations in dependence on ion bombardment intensity. At low level, the film grows in (111)-orientation, at high levels in (100)-orientation. In a small transition zone, (110)-orientation prevails. Thus, the orientation can be selected by the ion beam intensity, which is relevant for application.

Tailoring Gaussian Laser Beam Shape Through Controlled Etching of Single-Mode and Multimode Fibers: Simulation and Experimental Studies

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 approximately 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.

Gaussian laser beam tailoring using acoustooptic cell

Profile shaping of a Gaussian laser beam by an acoustic wave is well described using Collins integral and ABCD matrix formalism. It is shown by a numerical simulation that the relative width of the laser beam to the ultrasonic wavelength and the acoustic pressure inside the acoustooptic cell act on the light intensity diffraction pattern. Obtained results show that the output intensity profile differs from the incident Gaussian beam shape, and it is more broadened with an increase in the acoustic pressure. The intensity of a focused laser beam is transformed in a flat form in the central region if the acoustic pressure is proprely controlled. On the other hand the intensity longitudinal range (ILR) of the flat shape is discussed along the propagation axes, we have found the ILR is about 2 mm for a focal length distance f=100 mm.

Automatic optimization design of Gaussian beam shaping system by using ZEMAX software

This paper studies the method of automatic optimization design of Gaussian beam shaping. Firstly, the principle of Gaussian beam shaping was studied theoretically and the flattened Lorentzian function (FL) is chosen as the distribution of flattened beam. The mapping function of arbitrary ray in incident plane and image plane was deduced based on the law of conservation of energy. And then, according to the characteristics of this system, Zemax programming language (ZPL) was used to compile ZPL macro orders to optimize the Gaussian beam shaping automatically. Finally, optical analyzing software was used to test the aspheric lens system. The method is not only simple but also practical with high efficient energy conversion and significant engineering application value.