Laser beam shaping at focus or focal beam shaping is essential for many applications. The most common approach makes use of the Fourier transforming properties of lenses to generate at their focal planes the desired irradiance patterns, e.g., the flattop. There are two inherent limitations for this approach. First, the shaping quality depends strongly on the dimensionless parameter β. In the case of a long focal length or small beam sizes giving a small β value, additional beam expanders are needed to achieve a satisfying irradiance pattern at the focus. Second, without considering the phase, the irradiance patterns beyond the focal plane are not controlled. We propose a different approach with two plano-aspheric lenses that allow control of both irradiance and phase at focus. The design method comprises an extended ray mapping procedure combined with backward wave propagation from focus. With this design approach, the shaping quality is guaranteed without the possible need for extra beam expanders, offering the potential for a more compact system with fewer elements. Through the additional phase control, the depth of focus is enlarged to a large extent and the designed system becomes more tolerant.
Focal beam shaping (FBS), or laser beam shaping at focus, is required in many laser applications. The most common approach is to use a phase element and a Fourier transform lens to generate at the focal plane of the lens the desired irradiance pattern, usually a at-top. The shaping quality depends strongly on a dimensionless parameter β. In case of long focal length and/or small focal spot, the input laser beam should be sufficiently large in order to get a large β value for a satisfying shaping quality. Therefore additional beam expansions might be needed. In this work, we propose a different approach with two plano-aspheric lenses that allows to control both irradiance and phase at focus. The two lenses are designed by an extended ray mapping technique combined with a rigorous backward wave propagation method, so that diffraction effects around laser focus can be implemented in a reliable way. With the developed approach, the shaping quality is guaranteed without the possible need for extra beam expanders, which makes the system more compact. The advantage of our design approach is demonstrated in direct comparison with the conventional Fourier approach for the same design example to transform a Gaussian beam to have a circular flat-top irradiance pattern.
A refractive laser beam shaper typically consists of either two plano-aspheric lenses or one thick lens with two aspherical surfaces. Ray mapping is a general optical design technique for irradiance reshaping based on geometric optics. Although ray mapping, in principle, allows generating any rotational-symmetric irradiance profile, in the literature this technique is mainly used to transform a Gaussian irradiance profile to a uniform rotational-symmetric profile. For more complex profiles, especially with low intensity in the inner region (such as annular profiles), a high sampling rate is required to ensure an accurate calculation of the surfaces. In practice, the high sampling rate increases the numerical effort to calculate the aspherical surfaces and the simulation time to verify the design considerably. In this work, we evaluate different sampling approaches and surface construction methods. This allows us to propose and demonstrate a comprehensive numerical approach to efficiently design refractive laser beam shapers to generate rotational-symmetric collimated beams with annular irradiance profiles. Ray tracing analysis for several annular irradiance profiles demonstrates the excellent performance of the designed lenses and the versatility of our design procedure.
A Gaussian laser beam is reshaped to have specific irradiance distributions in many applications in order to ensure optimal system performance. Refractive optics are commonly used for laser beam shaping. A refractive laser beam shaper is typically formed by either two plano-aspheric lenses or by one thick lens with two aspherical surfaces. Ray mapping is a general optical design technique to design refractive beam shapers based on geometric optics. This design technique in principle allows to generate any rotational-symmetric irradiance profile, yet in literature ray mapping is mainly developed to transform a Gaussian irradiance profile to a uniform profile. For more complex profiles especially with low intensity in the inner region, like a Dark Hollow Gaussian (DHG) irradiance profile, ray mapping technique is not directly applicable in practice. In order to these complex profiles, the numerical effort of calculating the aspherical surface points and fitting a surface with sufficient accuracy increases considerably. In this work we evaluate different sampling approaches and surface fitting methods. This allows us to propose and demonstrate a comprehensive numerical approach to efficiently design refractive laser beam shapers to generate rotational-symmetric collimated beams with a complex irradiance profile. Ray tracing analysis for several complex irradiance profiles demonstrates excellent performance of the designed lenses and the versatility of our design procedure.
A novel method is presented to inject the light of millimeter-sized high-brightness blue LEDs into light guides of submillimeter
thickness. Use is made of an interference filter that is designed to pass only those modes that will propagate in
the light guide by total internal reflection. Other modes are reflected back to the LED cavity and recycled, leading to an
With this method a collimator has been designed and made that is only 1mm thick, with a diameter of 6.5mm. It creates a
beam of 26deg Full Width at Half Maximum. Presently, collimators with these characteristics have a thickness of 10-20mm and a diameter of 20-30mm and require careful mounting and alignment. The new collimator contains a
4.5micron thick interference filter made of 54 layers of Nb2O5 and SiO2 layers. The filter is optically coupled to the LED
with Silicone adhesive which makes the configuration very robust. A cylindrical lightguide, tapered from 6.5mm to
2.5mm diameter and 1mm thick captures the light that passes the filter, folds the light path and redirects the beam.
Measurements on collimator prototypes show good agreement with the designed characteristics. This promising
approach enables much more compact collimators optics that offer material cost savings and design freedom.