## Article

Published Published online: 01 November 2021 , doi: 10.37188/lam.2021.029
In this paper, we propose a dual-wavelength digital holographic interferometry method based on a compact dual vertical-cavity surface-emitting laser (VCSEL) source. The source simultaneously emits light from two highly stabilized coherent light sources with slightly different wavelengths. A highly stabilized and adjustable current source enables the application of digital holographic dual-wavelength techniques to measure the shape of an object with height steps of a few millimeters. The wavelength drift over 12 h over the entire measurement range, which was evaluated using a wavemeter, was smaller than 1 pm. In addition to the low measurement uncertainty at large height jumps, the dual-wavelength digital holographic system distinguishes itself by its robustness to environmental disturbances such as air turbulence, heat load, and/or mechanical vibrations. This is enabled via a fiber-based almost common-path single-shot digital holographic acquisition of the information of the two different wavelengths using angular multiplexing. The experimental setup and data evaluation are discussed, and we present measurements of non-cooperative objects with specular reflective and/or diffuse reflective surfaces having different colors.
Published Published online: 01 December 2021 , doi: 10.37188/lam.2021.033
Structural coloration stemming from microstructure-induced light interference has been recognized as a promising surface colorizing technology, based on its potential in a wide array of applications, including high-definition displays, anti-counterfeiting, refractive index sensing, and photonic gas and vapor sensing. Vibration-assisted ultraprecision texturing using diamond tools has emerged as a high-efficiency and cost-effective machining method for colorizing metallic and ductile surfaces by creating near-wavelength microstructures. Although theoretically possible, it is extremely challenging to apply the vibration-assisted texturing technique directly to colorize non-metallic and brittle materials (e.g., silicon and acrylic polymers) with high-quality, crack-free microstructures owing to the intrinsic brittleness of these materials. This study demonstrates the feasibility of direct texturing near-wavelength-scale gratings on brittle surfaces in the ductile regime to fabricate crack-free micro/nanostructures. The effects of tool vibration trajectories on the ductile-to-brittle transition phenomena were investigated to reveal the cutting mechanism of ductile-regime texturing and optimize the processing windows. Structural coloration on silicon and acrylic surfaces was successfully demonstrated by creating programmable and pixelated diffraction gratings with spacing values ranging from 0.75 to 4 μm.
Published Published online: 01 December 2021 , doi: 10.37188/lam.2021.032
In this study we present a novel and flexibly applicable method to measure absolute and relative vibrations accurately in a field of 148 mm × 110 mm at multiple positions simultaneously. The method is based on imaging in combination with holographic image replication of single light sources onto an image sensor, and requires no calibration for small amplitudes. We experimentally show that oscillation amplitudes of 100 nm and oscillation frequencies up to 1000 Hz can be detected clearly using standard image sensors. The presented experiments include oscillations of variable amplitude and a chirp signal generated with an inertial shaker. All experiments were verified using state-of-the-art vibrometers. In contrast to conventional vibration measurement approaches, the proposed method offers the possibility of measuring relative movements between several light sources simultaneously. We show that classical band-pass filtering can be omitted, and the relative oscillations between several object points can be monitored.
Published Published online: 01 October 2021 , doi: 10.37188/lam.2021.026
Materials are, in general, either transparent or not. In principle, it is impossible to switch a material from a reflective optical state to a fully transparent one. Transparency has received relatively less research attention compared to other optical properties such as absorption, research on which have successfully produced perfect blacks, that is, highly absorbing materials. The ability to change optical transparency, especially locally and on demand, can enable several applications. Here, we present an absorbing three-layer system whose transparency can be altered by pulsed laser processing to realize different states, ranging from full transparency to mirror, black, and combinations thereof. An initially black surface can be made highly reflective or transparent by changing the laser pulse energy. The corresponding process window, including the influence of the substrate material, was investigated in detail.
Published Published online: 01 November 2021 , doi: 10.37188/lam.2021.030
Minimally invasive endoscopes are indispensable in biomedicine. Coherent fiber bundles (CFBs) enable ultrathin lensless endoscopes. However, the propagation of light through a CFB suffers from phase distortions and aberrations that can cause images to be scrambled. The correction of such aberrations has been demonstrated using various techniques for wavefront control, especially using spatial light modulators (SLMs). This study investigates a novel aberration correction without SLM for the creation of an efficient and compact system. The memory effect of CFBs enables a paradigm shift in the use of static diffractive optical elements (DOEs) instead of dynamic modulation with SLM. We introduce DOEs produced by 2-photon polymerization lithography for phase conjugation on a CFB for focusing, raster scanning, and imaging. Furthermore, a DOE with random patterns is used to encode the three-dimensional (3D) object information in a 2D speckle pattern that propagates along the ultra-thin CFB. Neural networks decode the speckles to retrieve the 3D object information using single-shot imaging. Both DOE methods have compact low-cost concepts in common, and both pave the way for minimally invasive 3D endomicroscopy with benefits for optical imaging in biomedicine.
Published Published online: 01 October 2021 , doi: 10.37188/lam.2021.027
The thin-film optical inverse problem has attracted a great deal of attention in science and industry, and is widely applied to optical coatings. However, as the number of layers increases, the time it takes to extract the parameters of thin films drastically increases. Here, we introduce the idea of exploiting the structural similarity of all-optical neural networks and applied it to the optical inverse problem. We propose thin-film neural networks (TFNNs) to efficiently adjust all the parameters of multilayer thin films. To test the performance of TFNNs, we implemented a TFNN algorithm, and a reflectometer at normal incidence was built. Operating on multilayer thin films with 232 layers, it is shown that TFNNs can reduce the time consumed by parameter extraction, which barely increased with the number of layers compared with the conventional method. TFNNs were also used to design multilayer thin films to mimic the optical response of three types of cone cells in the human retina. The light passing through these multilayer thin films was then recorded as a colored photo.
Published Published online: 01 October 2021 , doi: 10.37188/lam.2021.025
Precise control and measurement of the optical fiber diameter are vital for a range of fields, such as ultra-high sensitivity sensing and high-speed optical communication. Nowadays, the measurement of fiber diameter relies on point measurement schemes such as microscopes, which suffer from a tradeoff between the resolution and field of view. Handling the fiber can irreversibly damage the fiber samples, especially when multi-point measurements are required. To overcome these problems, we have explored a novel technique in which the mechanical properties of fibers are reflected by forward stimulated Brillouin scattering (FSBS), from which the diameters can be demodulated via the acoustic dispersion relation. The distributed FSBS spectra with narrow linewidths were recorded via the optimized optomechanical time-domain analysis system using coherent FSBS, thereby achieving a spatial resolution of 1 m over a fiber length of tens of meters. We successfully obtained the diameter distribution of unjacketed test fibers with diameters of 125 μm and 80 μm. The diameter accuracy was verified by high-quality scanning electron microscope images. We achieved a diameter resolution of 3.9 nm, virtually independent of the diameter range. To the best of our knowledge, this is the first demonstration of non-destructive and distributed fiber diameter monitoring with nanometer resolution.
Published Published online: 30 June 2021 , doi: 10.37188/lam.2021.006
Future quantum technology relies crucially on building quantum networks with high fidelity. To achieve this challenging goal, it is of utmost importance to connect individual quantum systems such that their emitted single photons overlap with the highest possible degree of coherence. This requires perfect mode overlap of the emitted light from different emitters, which necessitates the use of single-mode fibres. Here, we present an advanced manufacturing approach to accomplish this task. We combined 3D printed complex micro-optics, such as hemispherical and Weierstrass solid immersion lenses, as well as total internal reflection solid immersion lenses, on top of individual indium arsenide quantum dots with 3D printed optics on single-mode fibres and compared their key features. We observed a systematic increase in the collection efficiency under variations of the lens geometry from roughly 2 for hemispheric solid immersion lenses up to a maximum of greater than 9 for the total internal reflection geometry. Furthermore, the temperature-induced stress was estimated for these particular lens dimensions and results to be approximately 5 meV. Interestingly, the use of solid immersion lenses further increased the localisation accuracy of the emitters to less than 1 nm when acquiring micro-photoluminescence maps. Furthermore, we show that the single-photon character of the source is preserved after device fabrication, reaching a \begin{document}$g^{(2)} (0)$\end{document} value of approximately 0.19 under pulsed optical excitation. The printed lens device can be further joined with an optical fibre and permanently fixed.This integrated system can be cooled by dipping into liquid helium using a Stirling cryocooler or by a closed-cycle helium cryostat without the necessity for optical windows, as all access is through the integrated single-mode fibre. We identify the ideal optical designs and present experiments that demonstrate excellent high-rate single-photon emission.