This PDF file contains the front matter associated with SPIE Proceedings Volume 9950, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.
Focusing of laser radiation is most often used approach in various industrial micromachining applications like scribing, PCB drilling, and is important in scientific researches like laser heating in geophysics experiments with diamond anvil cells (DAC). Control of intensity distribution in focal spot is important task since optimum intensity profiles are rather flat-top, doughnut or “inverse-Gauss” than typical for lasers Gaussian profile. Because of high intensity of modern CW and pulsed lasers it is advisable to use refractive beam shaping optics with smooth optical surfaces providing high radiation resistance. Workable optical solutions can be built on the base of diffraction theory conclusion that flat-top intensity profile in focal plane of a lens is created when input beam has Airy-disk intensity distribution. It is suggested to apply refractive beam shapers converting, with minimum wavefront deformation, Gaussian profile of TEM00 beam to a beam with Airy disk intensity distribution, thereby optimizing conditions of interference near the focal plane of a lens after the beam shaper and providing flat-top, doughnut, “inverse-Gauss” profiles. This approach allows operation with CW and ultra-short pulse lasers, using F-theta lenses and objectives, mirror scanners, provides extended depth of field similar to Rayleigh length of comparable TEM00 beam, easy integration in industrial equipment, simple adjustment procedure and switching between profiles, telescope and collimator implementations. There will be considered design basics of beam shapers, analysis of profile behaviour near focal plane, examples of implementations in micromachining systems and experimental DAC setups, results of profile measurements and material processing.
Orbital angular momentum (OAM) states of light have been recently considered in new mode-division multiplexing techniques in order to increase the bandwidth of today’s optical networks. Many optical architectures have been presented and exploited in order to sort the different OAM channels. Here we present a diffractive version of the sorting technique based on log-pol optical transformation and we further improve the miniaturization level by integrating the two components into a single diffractive optical element. Samples have been fabricated with high-resolution electronbeam lithography and characterized in the optical range. The presented design is promising for integration into nextgeneration optical platforms performing optical processing of OAM modes, for applications both in free-space and optical fibers.
Laser beam shaping enables the simultaneous redistribution of the irradiance and phase of a laser beam. The desired shape of the laser beam is thereby determined by the respective application. One possible way to achieve the desired irradiance and phase at the same time is from double freeform surfaces. We investigate the numerical design of double freeform surfaces for collimated beam shaping with arbitrary irradiances by using ray-mapping techniques, where at first a proper ray mapping between the source and target irradiance is calculated, and in a subsequent step the freeform surfaces are constructed. The difficulty thereby is to find an integrable ray mapping which leads to two continuous surfaces. Combining the law of refraction and an integrability condition, we derive a condition for a ray mapping and show that it can be fulfilled in a small-angle approximation by a mapping derived with optimal mass transport. As a consequence the design process decouples into the separate calculation of the ray mapping as well as both freeform surfaces. A quantitative estimate for the approximate integrability of the optimal mass transport mapping is derived. The decoupling of the design process offers an efficient way of constructing both freeform surfaces by solving linear advection equations. The efficiency of the design algorithm is demonstrated by applying it to a challenging design example, furthermore the limitations of the numerical approach are investigated.
The demand for a uniform intensity distribution of working laser beams (top-hat beam) is growing steadily, especially in the field of laser material processing. Therefore, a refractive beam-shaping system that transforms a collimated Gaussian beam into a collimated top-hat beam was developed. As a result, a compact optical design was derived, which has a very high optical performance and is suitable for series production. Additionally, it can be combined with existing monolithic beam expanders from asphericon GmbH to achieve modular top-hat generation. A characterization of the system performance was carried out. The results of the design and the characterization will be presented.
Laser scanners are critical components in material processing systems, such as welding, cutting, and drilling. To achieve high-accuracy processing, the laser spot size should be small and uniform in the entire objective flat field. However, traditional static focusing method using F-theta objective lens is limited by the narrow flat field. To overcome these limitations, a dynamic focusing unit consisting of two lenses is presented in this paper. The dual-lens system has a movable plano-concave lens and a fixed convex lens. As the location of the movable optical elements is changed, the focal length is shifted to keep a small focus spot in a broad flat processing filed. The optical parameters of the two elements are theoretical analyzed. The spot size is calculated to obtain the relationship between the moving length of first lens and the shift focus length of the system. Also, the Zemax model of the optical system is built up to verify the theoretical design and optimize the optical parameter. The proposed lenses are manufactured and a test system is built up to investigate their performances. The experimental results show the spot size is smaller than 450um in all the 500*500mm~2 filed with CO2 laser. Compared with the other dynamic focusing units, this design has fewer lenses and no focusing spot in the optical path. In addition, the focal length minimal changes with the shit of incident laser beam.
In this work we will review some of the novel applications, recently proposed, where the use of structured light has played a crucial role. First, in the field of laser remote sensing, we discuss about a technique that allows to measure, in a direct way, the component of velocity perpendicular to the line of sight. This technique has found applications in the field of fluid dynamics, where an effective and simple optical technique capable to provide accurate measurements of ow vorticity, the tendency of a ow to rotate, was recently demonstrated. We then move to the field of profilometry to revise the key ideas behind a highly sensitive interferometric technique for thickness measurement, which is based on mode projection. We finally enter the field of optical activity to explore a novel proposal where an enhanced interaction between the handedness of structured light and chiral molecules was predicted.
Current optical communication technologies are predicted to face a bandwidth capacity limit in the near future. The nature of the limitation is fundamental rather than technological and is set by nonlinearities in optical fibers. One solution, suggested over 30 years ago, comprises the use of spatial modes of light as information carriers. Along this direction, light beams endowed with orbital angular momentum (OAM) have been demonstrated as potential information carriers in both, free space and fibres. However, recent studies suggest that purely OAM modes does not increase the bandwidth of optical communication systems. In fact, in all work to date, only the azimuthal component of transverse spatial modes has been used. Crucially, all transverse spatial modes require two degrees of freedom to be described; in the context of Laguerre-Gaussian (LGp`) beams these are azimuthal (l) and radial (p), the former responsible for OAM. Here, we demonstrate a technique where both degrees of freedom of LG modes are used as information carrier over free space. We transfer images encoded using 100 spatial modes in three wavelengths as our basis, and employ a spatial demultiplexing scheme that detects all 100 modes simultaneously. Our scheme is a hybrid of MIMO and SMM, and serves as a proof-of-principle demonstration. The cross-talk between the modes is small and independent of whether OAM modes are used or not.
We present the experimental conversion of a spatially-Gaussian optical mode into a self-healing, approximate Bessel-Gauss mode by a non-collinear, spatially-multimode four-wave mixing process in warm atomic vapor. In addition to the mode conversion, a second, spatially-separate conjugate beam is created in a non-Gaussian mode that mimics that of the resulting converted probe beam. Additionally, we show that these resulting beams exhibit the ability to partially self-heal their mode profiles after encountering an obstacle in their paths. This multi-spatial-mode nonlinear gain platform may thus be used as a new method for all-optically generating pairs of self-healing beams.
The laser beam M2 quality parameter is based on the second moments’ theory, as defined by ISO standards, and provides a common approach for defining the propagation characteristics of laser beams as a whole. At the same time, the M2 parameter fails to quantitatively distinguish the quality of laser beams with different spatial characteristics. For example, several laser beams with very different spatial profiles may have the same M2 value. To overcome this ambiguity, a different beam quality criterion is introduced, allowing for a quantitative definition of both the structured laser beam shape and its propagation characteristics. This criterion, called the encircled power M2 (EPM2), bridges the gap between the M2 quality parameter and the structured laser beam shape. Based on several examples we demonstrate the utility of EPM2 as applied to characterization of several structured laser beam types.
Vector beams are spatial modes of light with spatially variant polarization states in the transverse profile. Over the years, vector beams have found their way into plenty of applications ranging from material processing and lithography to electron acceleration and particle trapping. Though qualitative measurements are routinely used to analyse vector beams, there is currently no quantitative measure for vector beam purity. Here, we introduce a new measure, the vector quality factor (VQF), that maps the purity of vector beams to a scale ranging from 0 to 1. We demonstrate a simple optical setup to generate and detect vector beams using a birefringent phase plate known as a q-plate. Tomographic measurements are performed by decomposing the vector beam into its circular basis states, and measuring the expectation values of the Pauli matrices as intensity measurements which, are used to evaluate the VQF of vector beams.
Conventionally, it is a tedious work to measure the beam quality factor for a laser beam because one needs to move a camera-based beam profiler from one location to another for many times to record intensity profiles at different positions around the beam waist. We present a simple method for determining the laser beam quality factor from only two laser intensity profiles at different cross sections around the waist. We first used an iterative phase-retrieval algorithm, based on the Huygens-Fresnel principle, to reconstruct the phase profiles at the two cross sections where the intensity profiles had been measured. Once the optical field amplitude (the square root of intensity) and phase distribution functions at certain cross section of a laser beam had been determined, we can propagate the light wave at this cross section by using the Fresnel diffraction formula to obtain the intensity profiles at different positions, from which the beam quality factor can be determined. Using a HeNe laser for test, we had experimentally demonstrated the feasibility of our idea by showing that the result from our proposed method is in good agreement with that obtained from the conventional method. Our setup is capable of executing a real-time measurement of the beam quality factor because the two intensity profiles can be simultaneously recorded by using a beam splitter and two beam-profilers controlled by the same computer.
Based on a few-mode fiber Bragg grating as polarization-selective output coupler and topological insulators Bi2Te3 as the saturable absorber, we propose a passively Q-switched fiber laser with cylindrical vector beam output. Both radially and azimuthally polarized beams can be readily generated, and the output polarization can be switchable through tuning the polarization controllers inside the laser cavity. The laser operates at the wavelength of 1557.5 nm with a 3 dB linewidth of less than 0.04 nm. The repetition rate of the Q-switched laser can be tuned from 31.54 kHz to 49.40 kHz when the pump power increases from 103.5 mW to 139.5 mW.
Slab geometry is a promising architecture for power scaling of solid-state lasers. By propagating the laser beams along zigzag path in the gain medium, the thermal effects can be well compensated. However, in the non-zigzag direction, the thermal effects are not compensated. Among the overall aberrations in the slab lasers, the major contributors are two low-order aberrations: astigmatism and defocus, which can range up to over 100 microns (peak to valley), leading to detracted beam quality. Another problem with slab lasers is that the output beams are generally in a rectangular aperture with high aspect ratio (normally 1:10), where square beams are favorable for many applications. In order to solve these problems, we propose an automatic low-order aberration compensation system. This system is composed of three lenses fixed on a motorized rail, one is a spherical lens and the others are cylindrical lenses. Astigmatism and defocus can be compensated by merely adjusting the distances between the lenses. Two wave-front sensors are employed in this compensation system, one is used for detecting the initial parameters of the beams, and the other one is used for detecting the remaining aberrations after correction. The adjustments of the three lenses are directly calculated based on beam parameters using ray tracing method. The initial size of the beam is 3.2mm by 26mm, and peak to valley(PV) value of the wave-front is 33.07λ(λ=1064nm). After correction, the dimension becomes 40mm by 40mm, and peak to valley (PV) value of the wave-front is less than 2 microns.
Using Liquid Crystal Spatial Light Modulator (LC-SLM) as a beam shaping device to improve beam quality in high-gain amplification system is reported. 1.6 nJ injected small-size signal Gaussian beam can be amplified to 5 J by 4 stages amplification, and finally output beam is a 50mm×50mm square spot with flat-top intensity distribution. In the amplification system we designed, LC-SLM is placed after the second level of amplifier, where the signal laser energy is about 20mJ, and beam size is 10mm×10mm. The structure of Fourier image transfer is also implemented in this amplifications system to be capable of maintaining high-quality image transmission in the amplification process. The LC-SLM as an object, is imaged by beam expand lenses and spatial filters lenses in the amplifications system to get good quality of imaging. By catching output spot and making a feed-back, transmission efficiency of each pixel on LC-SLM is modulated, high energy density area can be decreased to realize flat-top intensity distribution. A spot modulation function is defined as, using the maximum grey value on spot area divided by the average grey value of the image after background correction. By this, amplified laser obtains the spot modulation of 1.24 on central 90% area of the spot. Furthermore, un-uniform distribution on the full spot, soften effects of spot edge, and output beam shape can also be optimized by the LC-SLM shaping scheme in the amplification system.
In this work we revisit Young's experiment and show how it can be done with digital holography. We study different properties of light and show that depending on how light interferes, fringe patterns in other observables arise. We explain this conceptually and demonstrate how this can be implemented experimentally. We aid the reader with a tutorial-like approach and provide the necessary tools to easily perform the experiments.
In this work we report on our achievements in generating switchable and arbitrary vector beams by means of q-plates. Two kind of q-plates are considered: i) a physical prototype from Citizen Co. and ii) a virtual device that is encoded onto a spatial light modulator (SLM). In both cases experimental and analytical results within the Jones formalism are shown. The performance of a segmented and tunable liquid crystal q-plate prototype is characterized at visible and telecommunications wavelengths, and the generation of first-order vector beams is probed. By using a reflective geometry and tuning the q-plate at half-wave or at quarter-wave retardance, it is shown how the device can operate either as a q-plate with double order. Finally, we show the generation of arbitrary programmable integer and fractional vector beams by encoding a q-plate onto a SLM based system. The system is based on a double-pass configuration that consecutively modulates the vertical and the horizontal polarization components of light using a transmissive LCoS display. Therefore, new and exotic q-plate designs can be analyzed prior to their fabrication.
Based on our constructed robust π/2 mode converter, we report a concise yet high-efficient experiment to realize the detection of high-order orbital angular momentum (OAM). The π/2 mode converter that consists of a pair of cylindrical lens is actually not new. However, our experiment show clearly its excellent robustness, as we have detected the high-order OAM numbers up to ℓ = 150 carried by standard Laguerre-Gaussian (LG) modes. The observed patterns of two-dimensional optical lattices indicate that the radial index p of LG beams can be straightforwardly inferred as well. Our demonstration has potential in both classical and quantum information applications where high OAM modes are needed.
We demonstrate a new method to detect the vortex beams carrying orbital angular momentum (OAM) by a sectorial screen. When the sectorial screen is illuminated with vortex beams, the far-field diffraction pattern can be used to define the modulus and sign of topological charges. The number of the petals denotes the number of topological charge. The direction of intensity pattern flip by 180° for a change in the sign of topological charge. The experimental results agree well with the simulated results.
We demonstrate a thulium-doped all fiber actively mode-locked laser by synchronously pumping without electronic modulator. A mode-locked fiber laser operating at 1550 nm based on nonlinear polarization rotation (NPR) is innovatively utilized as the pulsed pump. Through cavity length matching, stable mode-locking that operate at 1891.25 nm is achieved with a spectral width of 0.52 nm at 3 dB. The repetition rate is 11.59 MHz with an estimated pulse duration less than 125 ps.
The commutation between the Helmholtz equation and the derivative operator allows us to generate novel nondiffracting beams. We apply a general differential operator to Bessel beams and study the resulting phase structure and orbital angular momentum (OAM). We find the parameters that preserve the OAM of the seed beam and show how to produce and control shape preserving vortex arrays. In analogy to the Poincaré sphere, our approach is used to develop an operator sphere connecting higher-order Bessel beams.
We study the energy ow pattern in the superposition of two off-axis optical vortices with orthogonal polarization states. This system presents a rich structure of polarization singularities, which allows us to study the transverse spin and orbital angular momentum of different polarization morphologies, which includes C points (stars, lemons and monstars) and L lines. We perform numerical simulations of the optical forces acting on submicron particles and show interesting configurations. We provide the set of control parameters to unambiguously distinguish between the spin and orbital ow contributions.
In this report the new black-glass fiber-preform fabricated by the vapor-phase axial deposition (VAD) method to realize high-resolution optical bundle fibers is discussed with the Energy Dispersive X-ray (EDX) analysis and the transmittance spectrum measurement. The black glass consists of SiO2, GeO2, Bi2O3 and Al2O3. Firstly, the rod-shaped soot of SiO2 and GeO2 is prepared by blowing SiCl4 and GeCl4 into the oxyhydrogen burner. Then the soot is dipped into the solution of the Bi and Al compounds. After drying the soot with Bi and Al penetrated, the soot is consolidated into the glass preform by heating with the carbon heater at 1650 degrees Celsius. The diameter of the obtained preform is 10.5 mm and the black glass layer thickness is 2.6 mm located at the periphery. The Bi concentration distribution shows the content of several wt% in the black glass layer. The black glass preform is drawn into the black optical fiber being expected to make a clear image because of no light leaking from the neighboring optical fibers as compared to the conventional fiber endoscope.
In this work we present a numerical analysis of the mode coupling between the pump-beam and the laser-beam in a Ti:Sapphire crystal used as a gain medium of a femtosecond laser. Using the Matrix ABCD and propagation gaussian beam models, we obtained an optimal configuration for compensate the astigmatism in the output beam laser. Also we analysed pump-beam propagation and got the settings to fix the astigmatism in the crystal. Furthermore we apply this configuration to a homemade femtosecond laser, accomplishing an overall efficiency of laser to 20% in continuum wave (CW) and 16% in mode looking (ML) operation. The femtosecond laser have 30 nm bandwidth to FWHM at 810 nm corresponding 30fs.
A means to digitally generate a partially coherent beam with orbital angular momentum is presented. Our approach is based on encoding the randomness of broadband light passing through a spiral phase plate in a spatial light modulator. We illustrate the technique by generating partially coherent beams with orbital angular momentum content and different coherence lengths, with no moving optical elements. We study the cross correlation spectra which yields to good agreement with theory.
We study the realization of quantum algorithms using classical optical elements and a coherent laser source. The encoded qubits are present in form of path qubits, polarization and orbital angular momentum. In particular, we propose an implementation for the Deutsch Algorithm in a Sagnac interferometer and the Deutsch-Jozsa Algorithm in a ring cavity.
Laser beams have long been applied across many disciplines, extending degrees of freedom for purely spatial control to polarization spatial control. Adaptive beam shaping in Na Laser Guide Star approaches will be assessed for progress and lessons learned. Laser Guide Stars based on Rayleigh Scattering at 530 nm is straightforward: simply frequency double a Nd:YAG. For Na Laser Guide Stars, there is no easy way to get 589 nm and is more cotp:plicated. Significate Laser Guide Star Systems include the Starfire Optical Range (SOR), The Lick Laser Gude Star (UC), Caltech/Mt. Palomar, the Keck Laser Guide Star, ESQ VLT, and Gemini South. These will be compared for progress and future developments.