Multiphysics analysis of chip and resonator designs for increased damage threshold of external cavity high-power laser diodes.

We present a detailed analysis of multiphysics simulation results to evaluate the threshold for catastrophic optical damage (COD) of high-power laser diodes under misaligned external optical feedback. Three different chip designs are investigated: the non-injecting mirror concept, the non-absorbing mirror concept and the introduction of an additional energy barrier within the waveguide near the front facet. Furthermore, a modification of the external resonator that promises a lower sensitivity towards misalignments is considered. The dependence of the COD threshold on the additional design parameters (bandgap change, modification length, focal length) and the impact of the different approaches on electro-optical efficiency as well as beam quality are analyzed. Compared to the initial design, the different chip design concepts promise an increase of the achievable output power by 8%, 27% and 27% respectively, whereas the modified resonator fully prevents feedback-induced failure.

High fidelity laser beam shaping using liquid crystal on silicon spatial light modulators as diffractive neural networks.

Spatial light modulators (SLMs) based on liquid crystal on silicon (LCoS) are powerful tools for laser beam shaping as they can be used to dynamically create almost arbitrary intensity distributions. However, laser beam shaping with LCoS-SLMs often suffers from beam shaping artifacts in part caused by unconsidered properties of the LCoS devices: astigmatism that stems from the non-normal incidence of the laser beam on the SLM and the effect commonly referred to as the '0-th diffraction order' that is caused by both the crosstalk between neighboring pixels and the direct reflection at the cover glass of the SLM. We here present a method to consider and compensate for these inherent properties of LCoS devices by treating the SLM as a diffractive neural network.

Advanced beam shaping for laser materials processing based on diffractive neural networks.

We propose a method based on neural network training algorithms for the design of diffractive neural networks - with the aim to perform advanced laser beam shaping in the NIR/VIS spectrum for laser materials processing. The method enables the efficient design of systems including multiple cascaded diffractive optical elements (DOEs) and allows the simultaneous optimization for complex (intensity and phase) target field distributions in multiple target planes. The multi-target boundary condition in the optimization method offers great potential for advanced laser beam shaping.

Design of an efficient illuminator for partially coherent sources in the extreme ultraviolet.

In this paper, the design of an efficient illuminator for extreme ultraviolet (EUV) applications such as photolithography, metrology, and microscopy is investigated. Illuminators are arrangements of optical components that allow us to tailor optical parameters to a targeted application. For the EUV spectral range, illuminators are commonly realized by an arrangement of several multilayer mirrors. Within this publication, design methods are developed to tailor optical parameters such as the intensity distribution, the spatial coherence, and the spectral bandwidth by using only one multilayer mirror. For the demonstration of the methods, an illuminator is designed for a compact in-lab EUV interference lithography system that is suited for industrial EUV resist qualification and large-area nanopatterning. The designed illuminator increases the wafer-throughput and improves the imaging quality.

Rigorous approach for unifying wave optics and state of the art rendering techniques.

With the capabilities of diffractive optics there is a rising demand for determining the light interaction of diffractive elements with arbitrary illumination and scenery. Since the structured surfaces' scale lies within the visible wavelengths and below, the light's interaction cannot be simulated with state of the art geometric optic rendering approaches. This paper presents a new model for the inclusion of wave-optical effects into Monte Carlo path rendering concepts. The derived method allows the coupling of a rigorous full-field approach with the concept of backward ray propagation through complex scenes. Therefore, the rendering of arbitrarily structured periodic optical components is now possible for complex sceneries for design, verification and testing purposes. The method's performance is demonstrated by comparing rendering results of complex sceneries including CDs with corresponding photographs.

Automated lens design for optical systems consisting of stock lenses.

The design of lens systems requires advanced knowledge and the mastery of highly specialized software tools. Furthermore, for the realization of the designed lens systems often custom-made lenses are needed, which are expensive and have lead times of several weeks compared to stock lenses with several days. To shorten realization time, a new approach for the automated design of lens systems consisting of stock lenses is developed. In this work, a multi-step process is described which identifies the most robust stock lens combination fulfilling prior defined requirements. The approach is realized with a computer program that can be used by a non-expert to find the most suited selection of stock lenses for a three-lens system for a set of requirements.

GPU-accelerated wave-optical model for external-cavity diode lasers.

The spatial distribution of the optical feedback field in an external-cavity diode laser (ECDL) is an important parameter influencing its lifetime and filamentation. From a modeling point of view, the evaluation of the full diffraction integral is required to calculate the propagation of light through thick lenses such as fast axis collimation lenses or beam transformation systems used for high-power diode lasers. In this Letter, we illustrate an algorithm for the accelerated computation of the Rayleigh-Sommerfeld diffraction integral on a graphics processing unit. We further show a validation of the results by measurements on an ECDL.

Simulation of spectral stabilization of high-power broad-area edge emitting semiconductor lasers.

The simulation of spectral stabilization of broad-area edge-emitting semiconductor diode lasers is presented in this paper. In the reported model light-, temperature- and charge carrier-distributions are solved iteratively in frequency domain for transverse slices along the semiconductor heterostructure using wide-angle finite-difference beam propagation. Depending on the operating current the laser characteristics are evaluated numerically, including near- and far-field patterns of the astigmatic laser beam, optical output power and the emission spectra, with central wavelength and spectral width. The focus of the model lies on the prediction of influences on the spectrum and power characteristics by frequency selective feedback from external optical resonators. Results for the free running and the spectrally stabilized diode are presented.

High-power dense wavelength division multiplexing of multimode diode laser radiation based on volume Bragg gratings.

We present a dense wavelength division multiplexer based on volume Bragg gratings (VBGs) with a channel spacing of Δλ = 1.5 nm. Multiplexing efficiencies of ηSM = 97% have been demonstrated with single-mode, frequency-stabilized diode laser radiation. By use of VBGs in an external-cavity laser we constrict the spectral bandwidth of passively cooled multimode diode laser bars with 19 broad-area emitters to δλ95% = 120 pm. When the multimode high-power diode laser radiation with a beam propagation factor of M(2) ≈ 45 is overlaid, the multiplexing efficiency decreases to ηMM = 85%. Temperature control of the VBGs expands the high-efficiency operation range.