While the gold standard for infection diagnostics remains the PCR test, the pandemic has shown the importance of Point-of-Care devices that carry the test in the field and yield results in an hour or less. Lab-on-Chip systems have been a game changer in this respect, but PCR based devices are still rather expensive. Among the main cost drivers are the optical readout modules that excite and detect fluorescence signals in multiple spectral channels. Depending on the device concept, the bulk of this cost is either shifted towards component cost, precise alignment, or complex mechanics, but there is a lack of compact, simple irradiation and detection modules that scale well in mass production. In this contribution we focus on the excitation optics, which must provide visible radiation in well-defined spectral bands that illuminate the sample. While the state of the art teaches either a precise assembly of dichroic mirrors or a filter changing mechanism, we have devised and demonstrated a concept that involves a holographic optical element (HOE). This HOE does not only act as a beam combiner, directing light from the individual sources towards the sample, but also exploits the intrinsic spectral selectivity for bandpass filtering, rendering any dielectric multilayer components obsolete. We present a 4-channel excitation optics with a single HOE at its core that unites light from four different LEDs towards a sample, demonstrating quantitative fluorescence readouts from fluorophore concentrations in the sub-micromolar range.
Lab-on-chip analysis for molecular diagnostics use an analyzer unit and cartridges. Since the cartridges are disposables, their development cycle can be much shorter than the one of the analyzer unit. Hence, it is important to have a high degree of exibility in the analyzer unit to react to new cartridge developments. Our research tries to overcome limitations due to fixed interfaces by replacing a static illumination system for uorescence analysis by an adjustable module. The benefit of adjustability goes along with harder requirements. In the case of the illumination system, that means a decrease in etendue of the optical system. Purpose of the presented research is to define the spectral radiance requirements on the light source as a bottleneck in an etendue limited optical system. As light source a phosphor converted light source should be used due to the benefits of a broad spectrum. First, the concept of etendue is used to specify a theoretical spectral radiance requirement on the light source. In the second step, the best available light source is used in a prototype to measure the performance of the adjustable illumination system and to derive a practical spectral radiance requirement. With the currently used light source in the lab prototype, requirements are fulfilled for one out of four spectral wavelength bands. The comparison between theory and experiment shows that the theoretical requirement must be corrected by a factor of two to a practical spectral radiance requirement of about 14mW=(sr • mm2 • nm). It can be concluded that especially in the blue and red wavelength range below 500nm and over 600nm bright phosphor converted light sources are required. Since the time the light source was selected, a new light source candidate with stronger emission for shorter wavelengths was found.
We present highly transparent, wave front printed volume holographic optical elements (vHOEs), realized with a new recording method based on the pre-illumination of incoherent light patterns. The introduced amplitudemodulated pattern illuminates a distinct area on the unexposed, photopolymer-based holographic recording material prior to the hologram recording sequence. The incoherent pre-illumination scheme enables a precise tuning of the material’s local photosensitivity without the formation of a holographic volume diffraction grating. As a consequence, the pre-illumination exposure significantly suppresses the formation of transparency diminishing structures in the material that are formed concurrently with the volume diffraction grating during the hologram recording sequence. The pre-illumination component is integrated in an extended immersion-based wave front printing setup, which realizes vHOEs by sequentially recording single holographic elements in an array-like structure. A wide range of different recording configurations is enabled by our recording setup due to independent modulation of both wave fronts and the possibility to realize large off-axis recording angles. We introduce two hologram characterization methods, based on a diffraction efficiency and a slanted-edge method analysis, which are used to evaluate the implemented pre-illumination method and demonstrate significant improvements to the see-through quality of the presented wave front recorded vHOEs.
Recently, holographic optics such as volume holographic optical elements (vHOEs) receive increasing attention as optical combiners in augmented reality (AR) applications. Especially vHOEs fabricated by means of wave front printing have the potential to realize complex optical functions with high diffraction efficiency while maintaining excellent transmittance. We present the recording of a holographic combiner for AR applications fabricated by means of individually modulated recording wave fronts in our extended immersion-based holographic wave front printer setup. Holographic elements from our setup are made up of individual sub-holograms, so called Hogels. The implementation of two phase-only reflective spatial light modulators (SLMs) allows for the recording of Hogels and consequently vHOEs in a wide range of different configurations. Large-area vHOEs are achieved by adjacent recording of multiple Hogels in a step-wise fashion. Our immersion-based printer setup ensures a high numerical aperture for the recording configuration, which is directly linked to a wide angular range of possible replay configurations for wave front propagation in air. We present a reflective vHOE realizing a large off-axis to on-axis wave front transformation suitable as holographic combiner for retinal scanning displays. The vHOE is characterized by evaluating the diffractive properties of the hologram’s volume gratings, as well as investigating the vHOE’s combiner characteristics by means of field of view (FoV) and eye box size evaluation.
The recording of computer-generated holographic optical elements (HOEs) via the concept of holographic wave front printing has been a topic of rising interest in many research groups over the last years. Especially for applications in augmented reality (AR), holographic wave front printing has the potential to realize HOEs with complex optical transformations and high diffraction efficiencies while maintaining excellent transmittance. Here, we present a novel immersion-based holographic wave front printer setup, which allows the recording of reflection volume holographic optical elements (vHOEs) in both on-axis and off-axis configurations. HOEs fabricated via our wave front printing process are made up of individual sub-holograms, so called Hogels. Each sub-hologram is recorded via two phase-only reflective spatial light modulators (SLMs). Large-area vHOEs are achieved by adjacent recording of multiple Hogels in a step-wise fashion. Our immersion-based holographic printer setup ensures a high numerical aperture for the recording configuration, that is directly linked to a wide angular range in which recorded wave fronts can be replayed in air configuration. As a possible AR application, we demonstrate the recording of a holographic combiner for retinal projection. A single eye box is projected in the user's field of view (FOV) by means of a scanned laser projector source. Each Hogel of the holographic combiner performs an individual wave front transformation of large off-axis to on-axis angles, which contributes to the global holographic transfer function of the vHOE. Haze and clarity analysis of the recorded vHOE confirm high transmittance, which is crucial for AR applications.
In the context of highly automated and autonomous driving growing market interest arises with respect to technologies enabling new use cases relying on the front, side and back windows of the car to serve as projection screens for display systems. Holographic diffusers represent a promising approach for the realization of transparent projection screens. We pursue the development of holographic diffusers composed of individual holographic micro-mirrors based on recording in our holographic wave front printer. This recording method allows for the realization of large-area holographic structures with non-conventional diffraction characteristics. We recorded a Holographic Convex Micro-Mirror Array (HCMMA) for red and green target wavelengths via spatial frequency multiplexing. We report on the fundamental properties of HCMMAs and depict the relevant recording procedure. We demonstrate the fitness of the HCMMA to serve as a transparent diffuser screen in conjunction with a LED projector and discuss potential for further improvement of our structures.
We previously demonstrated the recording of transmission volume holographic optical elements (TVHOE) in our holographic wave front printer setup. The setup is based on two phase-only reflective spatial light modulators (SLMs), which provide the individual recording wave fronts in an adaptive fashion. Recently, we extended the optical system of our holographic wave front printer, so that reflection volume holographic optical elements (RVHOE) can be fabricated. To this end, the optical system, which guides the recording wave fronts from the SLMs to the holographic film, has been duplicated on either side of the film. The SLMs are located in opposite half-spaces relative to the holographic film. The film is placed in the confocal plane of the two optical systems for wave front relay, which face in opposite directions. Consequently, arbitrary recording wave fronts generated at the SLMs are superimposed in the holographic film and produce a reflection hologram structure. Adjacently recording many sub-holograms allows for fabrication of large RVHOEs. In this publication, we report on the working principle of our holographic wave front printing approach. We present our wave front printer laboratory setup along with insights on setup alignment strategies. We demonstrate the functionality of our setup; we successfully produced holographic micro-mirror arrays with short focal length in an off-axis configuration of excellent optical quality.
We introduce a technique for recording volume holographic optical elements (vHOEs) designed for operation at a wavelength different from the recording wavelength. This technique relies on our holographic wave front printer, which employs two spatial light modulators for recording wave front generation. We present the procedure for computing the required recording wave fronts accurately and noniteratively from the desired vHOE optical transformation function. We exploit this technique for the fabrication of a converging lens vHOE with an operation wavelength of 915 nm at a recording wavelength of 515 nm. Finally, we demonstrate the holographic lens functionality directly in the infrared spectral region.
Holographic wave front printing is an emerging technique for the fabrication of computer-generated image holo- grams and holographic optical elements based on wave front synthesis via adaptive optical elements. Progress is fueled by the availability of high quality spatial light modulators and industrial grade holographic film materials. Striving for increased relevance for applications, a flexible and precise way of fabricating large-area, highly efficient holographic optical elements providing complex wave front transformations is sought after. In this pa- per, we report on a novel holographic printer setup for the recording of volume holographic optical elements (vHOEs) in a step-wise fashion by adjacently recording many sub-holograms. The setup is centered around two phase-only reflective spatial light modulators, which are used to shape the recording wave fronts. The recording wave fronts propagate through the setup on a common path and interfere in the focal plane of a reversely illuminated microscope objective, where the holographic film is located. The setup is additionally equipped with two characterization beam paths based on CMOS cameras allowing for accurate control of printing parameters and in-situ characterization of diffraction properties of the printed holograms. We demonstrate the recording of high-efficiency transmission vHOEs in development grade Bayfol HX TP* photopolymer in our setup. In order to exemplify the setup’s characterization capability, we present results of test series concerning vHOE diffraction efficiency as a function of exposure dose and the influence of photopolymer development dynamics on multiplexed exposure. Finally, we showcase a printed high-contrast holographic diffuser screen for see-through display applications.
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