Why laser beam shape matters

Optical lithography with its 193nm technology is pushed to reach and shift its limits even further. There is strong demand on innovations in illumination part of exposure tools. Current illumination systems consisting of diffractive and refractive optical elements offer numerous benefits such as optimized laser beam shape with high homogeneity and high numerical aperture enabling high efficiency. LIMO's unique production technology is capable to manufacture free form surfaces on monolithic arrays larger than 250mm with high precision and reproducibility. Different kinds of intensity distributions with best uniformities or customized profiles have been achieved by using LIMO's refractive optical elements. Recently LIMO pushed the limits of this lens production technology and was able to manufacture first diffractive optical elements (DOE) based on continuous relief's profile. Beside for the illumination devices in lithography, DOEs find wide use in optical devices for other technological applications, such as optical communications and data processing. Up to now DOE designs follow the principle of phase diffraction gratings. Its diffraction structure with a periodic phase profile performs a superposition of beams with predefined energy ratios. Due to the application for high precise laser-beam shaping and beam splitting in optical technologies and optical fiber networks, number of grating orders is increased up to some tens or even hundreds. Classic lithographic technologies lead to quantized (step-like) profiles of diffractive micro-reliefs, which causes a decrease of beam splitter's diffractive efficiency. The newest development of LIMO's microlens fabrication technology allows us to make a step from free programmable microlens profiles to diffractive optical elements with high efficiency. Our first results of this approach are demonstrated in this paper. Diffractive beam splitters are presented. A special mathematical method is used to design diffractive optical elements with continuous surface profiles. Comparison between theoretical simulations and experimental results shows very good correlation.

Researchers from China have proposed a hollow top-hat beam (HTB) diffractive optical element (DOE) shaping method, for use in space uplink chaotic communications systems (SUCCS). Simulation results show that by shaping the beam into a HTB, the bit error-rate (BER) improvement is substantial when compared to the traditional hollow Gaussian beam (HGB). The results could be significant for the optimisation of designs relating to SUCCS. Laser communication is a type of communication technique that, unsurprisingly, uses laser beams as a signal carrier, allowing the transmission of images, sound and other data in free space. It has been conceptualised since the development of the first lasers in the 1960s. When compared to more traditional microwave communication, laser communication has the advantages of allowing faster transmission due to higher data-rate (in principle anywhere from 1,000 to 100,000 times faster), a larger communication capacity, smaller terminal size and lower power consumption. It is anticipated that, due to its higher data-rate, laser communication will be the preferred candidate for the problem of bottlenecking when it comes to space uplink communications. Prof Mi Li (left of centre) and his research group. (a) original Gaussian distribution of laser beam (b) hollow Gaussian beam (HGB) shaped by DOE (c) hollow top-hat beam (HTB) shaped by DOE. Space chaotic laser communication system. Reducing the complexity of designing laser communications systems is a hot topic in the research field of free-space communication. Optical antennas play an important role, where they function as beam collators, expanders and convergers – all of which can improve the performance of free-space communication. One of the most commonly used antennas is the Cassegrain reflective optical antenna, featuring a short mirror body, high magnification and elimination of spherical aberrations (which result in blurry images due to numerous undesired focal points). Cassegrain antennas do come with a drawback, however, in that the secondary mirror can block part of the transmission beam, affecting the transmission efficiency of the optical system. The work presented in this issue of Electronics Letters is applicable to space chaotic laser communications systems, which are more complex than ground optical fiber chaotic communication systems. The authors use a DOE to shape a carrier beam into a hollow, top-hat shape (see inset images). DOEs have been widely used in the field of beam-shaping due to the advantages of high diffraction efficiency, flexible design and simple structure. Intelligent shaping of the beam can eliminate the problem of secondary mirrors in a Cassegrain antenna blocking part of the transmission beam. It can also be used to improve the transmission efficiency of the optical sub-system, meaning that the design pressure on other sub-system parameters is eased. DOEs can be used to shape and split laser beams in an energy-efficient manner and are often used in production facilities for laser material processing, or in medical laser treatments and diagnostics. They also see use in lighting, printing and lithography. In short, DOEs are frequently used in areas where shaping and splitting with minimal light loss is necessary. In order to effectively demonstrate how their suggested HTB improves the quality of the communication system, the authors use BER to evaluate their work. They also compare the HTB to a HGB, which they used in previous research works, to see if there is any difference in quality when considering the two shapes for SUCCS. The research considers strong and weak atmospheric conditions, as atmospheric turbulence also plays a role in the BER of signals moving between Earth and orbit. The work shows that, at higher beam power, BER is reduced by around 10−0.9 when using the HTB shaping method at a low chaotic mismatch and 10−1.5 at a high chaotic mismatch. In both scenarios the HTB shaping outperforms the HGB shaping, showing that the HTB is more suitable for SUCCS. In recent years there have been a number of exciting achievements in the free-space laser communication field. Laser communication has moved from concept to practice, Mbps rates to Gbps rates and we are now seeing the next iteration of communication technology. Research scientists are carrying out research work on small-size, high-speed laser communication stations. The work presented here not only improves the efficiency of the beam, but also makes steps towards reducing the design difficulty of other sub-systems within the communication terminal. The authors hope that this work will also allow a meshing of 5G ground wireless communications and space laser communications networks, and even pave the way for easier realisation of 6G in the future.

A major use of metal additive welding is the repair of damaged or worn turbine blades. Problems seen in the repair of these components are porosity and cracking. These problems are caused by the intense temperature gradients induced by circular beam shapes. When a laser beam is scanned across a workpiece, the shape of the beam as well as the flux distribution will cause a temperature gradient to form across the weld pool. High thermal gradients lead to the growth of weak microstructures, along with high thermally induced stresses during and after the processing. In Inconel 625 there is a region of 700˚C between a molten state and boiling of the alloy constituents. The temperatures present in traditional beam profiles may lead to boiling of the material at the centre of the weld. These extremes of temperature will typically cause porosity and alloy constituent migration. This paper looks at a modified beam intensity profile made using a novel beam shaping method. Computer generated diffractive optic elements (DOE) developed at Loughborough University have been shown to reconstruct a laser beam intensity profile into any user defined structure with typically 90% efficiency. Through the use of thermal modelling, the temperature field required in the weld pool was determined and back propagated to the surface to find the beam flux distribution required. The complexity of the resulting beam, is not intuitive and can only be achievable using a DOE. The reconstructed beam profile is used to reduce the temperature gradient across the molten pool, removing overheating of the pool centre. This has shown to have had a dramatic positive effect on the deposition microstructure, by halving the average grain size, without degradation of crystal growth directions. Electron Back-Scatter Diffraction (EBSD) has been used to characterize the grain characteristics of the resulting depositions. Tailored beam profiles show improvements in the mechanical and corrosion properties of the depositions, when compared to welds using circular beams. Using these methods, depositions of crack sensitive, high nickel alloys have been made with an absence of cracking or porosity.

Diode lasers are playing an increasingly important role in various applications, including pump sources for solid-state and fiber lasers, optical communications, printing systems, optical data storage, and electro-optical sensors. Compact package size, lower cost, and emission spectra from UV to IR, combined with conversion efficiencies in excess of 50% makes diode lasers a preferred choice over several other laser types. High-power diode laser beams are multimode, and can be described as an incoherent superposition of the limited number of individual lateral modes contained in the beam. The ability to shape and control the output beam characteristics of diode lasers requires establishing an accurate model of the multimode laser beam. We propose a general multimode laser beam model based on incoherent superposition of apodized Hermite-Gaussian modes to describe the spatial intensity distribution and propagation characteristics of the high-power diode laser beams. Free-space propagation characteristics of high power diode laser beams are compared with propagation characteristics of propagation invariant Hermite-Gaussian beams. Diffraction effects caused by micro-optics components will be presented, which show a significant impact on the intensity distributions of the individual lateral modes composing the beam, as well as on the multimode laser beam spatial distribution as a whole.

Optical microsensors for fluid flow diagnostics

Applications may vary with time, and so may the technical realization of these applications. Lasers change and consequently application wavelengths, due to requirements imposed by material changes, power constraints, scaling of lateral features, or other parameters and specifications. In applications where diffractive optical elements (DOEs) are employed a change in laser or wavelength usually demands not only a new fabrication batch but a redesign of the diffractive structure. This is almost always the case in the special application of laser beam shaping where a DOE is designed to redistribute the energy of a specific laser beam profile, generally into a flat-top intensity profile (either round or square). In this paper we will describe how a mono-mode beam shaping DOE design for 355 nm was adapted to compensate a wavelength change to 532 nm. A preliminary telescope is necessary to ensure that the input beam retains the initial properties of beam waist and beam diameter. In this case, the original DOE design can be maintained with some minor compensation in phase depth for the wavelength change and in positioning of the elements to compensate the scaled diffraction angles. Further data will be shared showing a side by side comparison of testing results of the 355 nm and 532 nm designs.

The development of DOEs fabrication techniques is continuously growing due to the wide range of industrial applications, such as beam manipulation or optical position encoders. In this work, we use Femtosecond laser direct writing to manufacture DOEs, which uses a simpler and more efficient way to fabricate amplitude binary masks. Also, we have analyzed the performance of the DOEs. The fabrication technique is validated since the experimental results are in accordance to numerical simulations.

Four-petal Gaussian beam is a special type of Gaussian beam, and its propagation properties are widely used in micro optics, optical communication and splitting technology. Recently, the generations and the properties of different types of hollow beams have become a hot research topic, such as research on hollow optical vortex beams. The Gyrator transform can be used to fulfill the mode conversion of laser beam. In this paper, based on the Gyrator transform, the analytical expression of four-petal Gaussian beam passing through such a transform system is derived, and the intensity distribution and the corresponding phase distribution associated with the transforming four-petal Gaussian beam are analyzed by numerical simulations. It is found that the four-petal Gaussian beam can be transformed into rectangular hollow beam by Gyrator transform, under the appropriate conditions of the beam order, the beam parameter, the transform angle of Gyrator transform, and the waist width. For the beam order n=m=3, the transform angle of Gyrator transform = 0.4133, the beam parameter K=30, and the waist width = 0.9, the rectangular hollow optical vortex beams can be obtained. Under such conditions, the maximum intensities appear in the four corners, and they are almost uniform on the four sides. The effects of the beam parameters, the transform angle, and the beam order on the distributions of intensity and phase of the rectangular hollow beam are analyzed in detail. The numerical results show that for the beam parameter K10, the rectangular hollow beam always is obtained, and for a lager beam parameter, the intensity distribution of the rectangular hollow beam is more uniform. Different beam order generates different type of hollow beam. For example, for n=m = 2, = 1.2, K = 30, and = 0.5409, a new strange circular hollow beam with solid circular nucleus can be obtained. The transform angle of Gyrator transform has a significant effect on the energy distribution of the hollow beam. When the transform angle changes in a small range, the uniformity of the intensity distribution of the rectangular hollow beam is lost. The bigger the transform angle change, the more serious the loss of uniformity of the hollow beam intensity is. The size of the hollow beam bright ring is determined by the waist width of the four-petal Gaussian beam: the larger the waist width, the smaller the bright ring is. The results further enriches the applications of Gyrator transform system and the four-petal Gaussian beam in the beam shaping.

In high-power laser system such as Petawatt lasers, the laser beam can be intense enough to result in saturation of nonlinear refraction index of medium. We present an analytical and simulative investigation of hot image formation in an intense laser beam through a saturable nonlinear medium slab based on Fresnel-Kirchhoff diffraction integral and the standard split-step Fourier method. The analytical results are found in agreement with the simulative ones. It is shown that, hot images can still form in an intense laser beam through a saturable nonlinear medium slab, additionally, the saturable nonlinearity does not change the location of hot images, while may decrease the intensity of hot images, i.e., the intensity of hot images decreases with the saturation light intensity lowering, and can stop to increase with the intensity of the incident laser beam heightening due to saturation of nonlinearity. Moreover, variations of intensity of hot images with the obscuration type and the slab thickness are discussed.

A unique design method of two cascaded diffractive optical elements (DOEs) with different sizes of effective phase region to modulate broadband beam is presented with consideration of single production material and low relief height on DOE. The iterative algorithm to calculate the relief heights on these DOEs is introduced at first. Where after, a broadband beam at wavelength from 500nm to 600nm propagates through the designed DOEs and is focused on the target plane in the simulation part. The shaping results demonstrate the excellent shaping ability of this unique design method. The shaping system proposed in this paper is significant for nonmonochromatic light modulation and has many applications such as graphic encryption, three-dimensional color display and multi wavelength division multiplexing.

Diffractive optical elements are being used in increasing numbers of applications such as beam shaping, optical interconnects and filtering. Their main advantages over traditional refractive optical elements are their reduced size and weight and the possibility of being mass fabricated. In order to reduce the design complexity and increase the performance, a novel, full complex-amplitude modulation diffractive optical element was designed, which allows full control over both phase and amplitude modulation of whatever optical wave front. In this element the wavefront phase modulation is controlled by the variations in the thickness of an optical film and the magnitude modulation is controlled by variations in the area of the apertures in a reflective layer deposited over this film. The use of a reflective layer in this new diffractive element avoids the risk of laser-induced damage when a powerful laser is used with the element since no light absorption is involved in the process.

Measuring far-field laser beam profile is an effective way to study the atmospheric transmission effects of high power laser and analyze the laser transmission efficiency in a long distance. A laser beam profiler based on detector array is developed to measure the far-field beam absolute intensity and temporal-spatial distribution of the high repetition rate pulsed laser beam. In the experiment, the pulse width of laser is about 100ns, the repetition rate is adjustable between 1 kHz and 20 kHz, and the diameter of the far-field beam is about 20cm. The detector array is composed of 297 Si-PIN photodiode detectors with the spatial resolution of 1cm in the central region. The image of the far-field beam profile is reconstructed with the bilinear interpolation method. The beam profiler is capable for measuring far-field beam profile of high repetition rate pulsed laser, with response wavelength between 400nm and 1100nm, minimal detectable pulse width of about 10ns, and dynamic range of energy density of 0.02–100µJ/cm2.

Near-field shaping with two binary diffractive optical elements in tandem

A lot of source shapes have been proposed for resolution enhancement in semiconductor exposure field, and so-called OAI (OAI: Off Axis Illumination) in them improves not only the resolution but also defocus behavior. Such kind of small window illumination was realized aperture filter at first stage but there were issues of efficiency of light source and complex OPC due to higher coherency. The advantages of use DOE (DOE : Diffractive Optical Element ) are not only the flexible illumination shape available but also the controllability of intensity profile in addition to the higher efficiency of light source. However DOE design and fabrication to obtain enough resolution are difficult due to the huge design load and leading-edge fabrication. Not enough design and fabrication error lead unintended intensity distribution, and the distribution degrades the resolution and makes OPC less effective so that photomask specification shall be tighter. In this paper, as one example, dipole source shape named "Soft-Dipole" is optimized considering intensity distribution targeting 90nm with simulator and is estimated the impact to resolution and OPC. Then actual DOE is fabricated for the intended distribution and evaluated the behavior with the simulator of the DOE using captured intensity distribution. The result showed Soft-Dipole illumination had possibilities to reduce OPC load with enough resolution. Then the DOE design, the fabrication and the evaluation are discussed in this paper.

Recent advances in compression molding of glass optical elements for mass production offer the potential of extending this technology to elements with micro and nano scale features. In this research, glass diffractive optical elements (DOEs) with lateral features in the order of 10 µm and vertical height of 330 nm were fabricated using a fused silica glass mold and a special low Tg (glass transition temperature) glass material K-PG325. Molded DOEs were studied using an atomic force microscope (AFM) and scanning electron microscope (SEM) to evaluate the glass molding process capability. Optical testing of the molded DOEs was a further demonstration of the effectiveness of the molding process for high volume micro and diffractive optical component fabrication. The combination of two high-precision, high-volume processes, i.e., semiconductor batch process for optical mold making and glass molding for DOE replication, is an effective alternative manufacturing method for high-quality, low-cost optical components. The reported experiment is a detailed illustration of the glass molding process capability. With further process optimization a robust manufacturing process can be developed for mass production of diffractive and micro glass optical elements.