Micromachining with ultrashort pulsed lasers
Generation of high-resolution optics in the glass volume
Laser-based writing of DOEs using a scannable microscope optics
We have developed a fast and high-resolution process with which any diffractive optical elements (DOEs) can be written directly into a glass volume. To do this, we combine a galvanometric scanner with microscope optics (NA = 0.4) and a 1-ps laser source, which enables fast and precise structuring. The strong focussing and non-linear absorption results in very small and localized refractive index changes (∆n ≈ 0.5 × 10-2), making it possible to generate a pixel sizes of 1.2 µm and using single pulse writing. The resulting phase is stacked layer by layer in the z-direction in order to be able to realize multi-level phase masks (e.g. 4-level 250×250 px at 2 µm and 10-level 416×416 px at 1.2 µm). For comparison with the calculated templates, the corresponding phase structures are visualised in the phase contrast microscope. Optical tests at 532 nm show good reproduction quality of the target images (overlap > 80 % vs. the discretised mask) with short production times (≈ 8-9 min for the high-resolution mask, ≈ 60 s for the 4-level variant) - making the method attractive for prototyping and small series.
The results are published at:
C. Ingenhag et al, Direct laser writing of in-volume diffractive optical elements with high speed and high resolution, Appl. Phys. B 132, 1 (2026)
Phase masks from refractive index modifications
How to write an optical element with light?
In this project, we are investigating the effect of non-linearly absorbed laser light on the optical properties of glass. Just above the absorption threshold, we generate customised refractive index modifications that can be used as building blocks to generate arbitrary phase masks. The required phase distribution is calculated using an iterative Fourier transform method (Gerchberg-Saxton) and the desired phase shift per pixel is realised by axially superimposing several modifications (z-stacking). Experimentally, we used a ps laser system with µJ energies, a galvanometric scanner and BK7 glass samples; typical writing parameters were several 100-1000 pulses per location (e.g. 1000 × 15-20 µJ at 100 kHz), whereby we were able to determine Δn ≈ 1.5 × 10-3. Discretisation to Nz = 8 planes and pixel sizes close to the lateral modification size (Δx ≈ 9-12 µm) resulted in masks (e.g. 64×64 px) that could be produced in <10 min and showed well-recognisable intensity distributions in optical tests. As limiting effects, we identified the remaining diffraction at zero order and scattering due to the inhomogeneous cross-sectional shape of the modifications; possible improvements include smaller focal volumes, larger illumination apertures, adapted scanning strategies and algorithmic pre-compensation.
The results are published at: A. Schüller-Ruhl et al, Direct fabrication of arbitrary phase masks in optical glass via ultra-short pulsed laser writing of refractive index modifications, Appl. Phys. B 128, 208 (2022)
