After the invention of lasers, in the past 50 years progress made in laser-based display technology has been very promising, with commercial products awaiting release to the mass market. Compact laser systems, such as edge-emitting diodes, vertical-cavity surface-emitting lasers, and optically pumped semiconductor lasers, are suitable candidates for laser-based displays. Laser speckle is an important concern, as it degrades image quality. Typically, one or multiple speckle reduction techniques are employed in laser displays to reduce speckle contrast. Likewise, laser safety issues need to be carefully evaluated in designing laser displays under different usage scenarios. Laser beam shaping using refractive and diffractive components is an integral part of laser displays, and the requirements depend on the source specifications, modulation technique, and the scanning method being employed in the display. A variety of laser-based displays have been reported, and many products such as pico projectors and laser televisions are commercially available already.

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Laser-based displays: a review

In general, in one aspect, the invention features an objective arranged to image radiation from an object plane to an image plane, including a plurality of elements arranged to direct the radiation from the object plane to the image plane, wherein the objective has an image side numerical aperture of more than 0.55 and a maximum image side field dimension of more than 1 mm, and the objective is a catoptric objective.

Catoptric objectives and systems using catoptric objectives

In optical apparatus for illuminating a mask-plane with a line of light, a light source includes four fast-axis stacks of laser diode bars. One optical arrangement collects light beams from all of the diode-laser bar stacks, homogenizes and expands the beams in the fast-axis direction of the diode-laser bar stacks and partially overlaps the homogenized and expanded beams in the fast-axis direction. Another optical arrangement homogenizes the sum of the partially overlapped beams and images the sum of the beams as a line of light having a length in the fast-axis-direction and a width in the slow-axis direction.

Line-projection apparatus for arrays of diode-laser bar stacks

We propose a method of designing a freeform lens array for off-axis illumination (OAI) in optical lithography to produce desired OAI patterns and improve efficiency. Based on the Snell law and the conservation law of energy, a set of first-order partial differential equations are derived and the coordinate relations for each OAI pattern are established. The contours of the freeform lens unit are calculated numerically by solving the partial differential equations, and the freeform lens array is obtained by arraying the lens units. Moreover, the optical performance for each OAI pattern is simulated and analyzed by software. Simulation results show that the irradiance distribution of each OAI pattern can be well controlled with a maximum uniformity of 92.45% and a maximum efficiency of 99.35%. Also, analysis indicates that this method has the advantages of reducing the complexity of the exposure system and having good tolerance to the input intensity variations of the laser beam.

Freeform lens arrays for off-axis illumination in an optical lithography system.

Optical characterisation and modelling for oblique near-infrared laser heating of carbon fibre reinforced thermoplastic composites

A device for homogenizing light has a first lens array provided with a number of convex lenses, and at least one second lens array disposed at a distance from the first lens array in the beam scattering direction and through which the light refracted by the first lens array can pass. The second lens array is formed with a number of first lenses that are respectively arranged at a distance from each other. At least one of the first lenses of the second lens array is associated with each convex lens of the first lens array. The convex lenses of the first lens array have a smaller curvature than the first lenses of the second lens array associated with said convex lenses.

Device for homogenizing light

Micro-lens arrays are widely used for beam shaping, especially beam homogenization of various laser sources.
Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages in laser
applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical
micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and
application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMO's unique production
technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an
even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of
aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis.
Additionally, the opportunities in simultaneous illumination from numerous light sources to one target are extended by
just geometrical arrangement without the need for collinear beam alignment. First application results of such micro-lens
arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100
W laser with tilted illumination at an angle of 35° is shown. A multi-kW line generator based on the superposition of
over 50 diode laser bars under different illumination angles is demonstrated as well. Thus, laser material processing like
plastics welding, soldering or annealing becomes much more convenient and less demanding regarding beam steering.

Beam shaping of high power diode lasers benefits from asymmetrical refractive micro-lens arrays

Beam shaping improvements of line generators based on high power diode lasers in combination with newly designed and produced high precision micro-optics lead to new applications such as hardening, metallization and annealing of different materials.
Two aspects are mainly needed to be focused on for getting the best results and throughput in these applications. The first one is the overall power content along the narrow axis of the line, namely the peak intensity in combination with the beam shape. The second one is the intensity homogeneity along the long axis of the line.
Herewith, a beam shaping concept that fulfils the desired requirements in a variable modality is presented. The concept
consists of macro-lenses and newly designed micro-optics and results in a passively cooled high power diode laser emitting at 808nm. The laser has an output power of 1000W. The generated line has a length of 13mm and a width of 80kW/cm 2 and uniformity >97%. To achieve such an extraordinary homogeneity level, several approaches based on cylindrical lens arrays were designed and tested. Methods to reduce inhomogeneities caused by diffraction effects and effects based on geometric optics are presented as well as their results. Additionally, the potential of this concept with regard to modularity, expandability and variability is reviewed.
Finally, an application example - crystallisation of a thin film of a-Si on a glass substrate - is presented.

Beam shaping of line generators based on high power diode lasers to achieve high intensity and uniformity levels

Uniform illumination of the mask is a key factor for the lithography process. The requirements of Immersion Lithography make illumination systems even more complex e.g. by adding additional parameters like polarization and improved throughput. Arrays of refractive microoptics are the ideal solution for high transmission homogenizing elements since several tool generations. These arrays can provide very steep intensity profiles (top hat and other profiles), enable lossless polarization control and do not suffer from zero order losses like diffractive elements.
Usually refractive microlens arrays are used with macrooptical field lenses in order to illuminate a field very uniformly or with a customized intensity distribution. High numerical apertures create the necessity for aspherical surfaces which leads to significantly higher lens cost especially for the macrooptics. In this paper we present novel microoptical homogenizers which create extremely uniform intensity distributions for high numerical apertures without any field lens or at least only with spherical field lenses. Especially multi-pole off-axis illumination can be improved with less optical components. An important prerequisite for these special types of homogenizers is that LIMO can produce free form surfaces on monolithic arrays larger than 200 mm with high precision and reproducibility. Every lens can be designed individually and can also be shaped asymmetrically.
We will present surface test methods and the final UV tests, guaranteeing the performance for the applications. Example data gained with these tests will be shown with regard to: meeting the design parameters, reproducibility over one wafer and reproducibility in large lots. 
Monolithic elements based on crossed cylindrical lenses provide a fill factor close to 100%. Simulations and measurements prove that microoptic arrays can be produced which provide a uniformity of the homogenized laser light of significantly better than 1% P-V at numerical apertures above 0.35.
Refractive microoptic arrays do not change the polarization state of the transmitted light which is an important prerequisite in immersion exposure tools. LIMO homogenizer sets are manufactured from fused silica and Calcium Fluoride thus they are suitable for all DUV wavelengths at highest laser fluxes.

Novel high-throughput micro-optical beam shapers reduce the complexity of macro-optics in hyper-NA illumination systems

LIMO's unique production technology based on computer-aided design enables the manufacture of high precision
asphere single lenses and arrays, where every single lens can be individually shaped. These free form micro-optical
cylindrical lens and lens arrays find their application in various types of metrology systems. Due to the high precise
manufacturing of specially designed surface, single lenses can be bond directly onto sensor or sensor arrays, performing
efficient projection of signal onto detector. Optical modules based on micro-lenses arrays enable special intensity
distribution, as well as highly homogeneous illumination with inhomogeneity less then 1% (peak to valley) used in
illumination parts of inspection tools. Due to the special free form profile, a special case of asymmetric lens arrays can
offer extreme uniformity illumination at the target non orthogonal to the illumination path. The feature under inspection
can be uniformly illuminated even if it lies at a specific angle to the illumination. This allows better conditions for
measurement devices arranged orthogonal to the mask or wafer. Furthermore the use of micro-optics enables more
sufficient inspection of laser beam parameters for excimer or 
CO 2 lasers. Additionally very accurate metal patterns can
be applied on the optics and used as alignment marks, apertures or bonding features.

Thomas Mitra

Papers

Device for homogenizing light

Beam shaping of high power diode lasers benefits from asymmetrical refractive micro-lens arrays

Beam shaping of line generators based on high power diode lasers to achieve high intensity and uniformity levels

Novel high-throughput micro-optical beam shapers reduce the complexity of macro-optics in hyper-NA illumination systems

Inspection and metrology tools benefit from free-form refractive micro-lens and micro-lens arrays