In the subproject theoretical fundaments and concepts for the development of miniaturized endomicroscopic systems are to be developed.
This includes concrete system designs, simulations of function and performance, tolerances and sensitivities respectively the selected technologies, adjustment concern and recommendations for the construction and testing of the systems. The necessary models, simulation methods and tools are also to be developed.

AP 1.1: Definition of the necessary specifications of the demonstrator "Endomicroscope"

The aim of this work package is to determine realistic achievable performance data, taking physical-technological boundary conditions into account.
For the applications of microendoscopy outlined in the compound description, the necessary specifications for transverse resolution, the size of the image field to be achieved, color correction, system-related distortion errors, etc. must be considered. As a prerequisite for the successful use of the system are also to be defined the wavelengths and sensible ranges of temperature and refractive index of the tissue medium. For focusing, the necessary dynamic ranges of the adjustment are to be estimated.
Results: According to the planning and taking into account realistic possibilities for the correction of the optical systems with OCT and microscope imaging modality, the following target values were worked out:
OCT-modality: numerical aperture NA = 0.1, FoV object size 2 mm, wavelength center 1.2 my, free working distance 1.5 mm, intended quality: diffraction-limited.
High-resolution microscopic imaging: numerical aperture NA = 0.5 ... 0.6, FoV object size 0.4 mm, wavelength in NIR at 900 nm, free working distance 0.2 mm, targeted quality: diffraction-limited.

AP 1.2: Definition of the necessary specifications of the demonstrator "OCT-Endoscope"

Same in AP 1.1, transversal resolution, image field size, color correction, refractive index fluctuations of the surrounding tissue, range of post-focusing must also be defined. The scheduled spectral bandwidth of the light source will determine the maximum achievable axial resolution. In addition, the size of the provided dynamic built-in zoom function is important, which is intended to allow adaptation to different sizes of the image fields.
The solution parameters of the OCT approach are to be explored with a view to a reasonable compromise between numerical aperture / lateral spatial resolution, working distance and the image field size.
Results: According to the planning and taking into account realistic possibilities for the correction of the optical systems with OCT and wide field imaging modality, the following target values were worked out:
OCT modality: numerical aperture NA = 0.1, FoV object size 1.5-2 mm, wavelength center 1.2 megapixel, free working distance 2 mm, targeted quality: diffraction-limited.
Figure with large field: numerical aperture NA = 0.03-0.05, FoV object size 12-15 mm, wavelength in the visible range from 490 nm to 633 nm, free working distance approx. 10 mm, targeted quality: diffraction-limited.

P 2.1: Concepts, design studies and simulations for demonstrators "Endomicroscope"

The novel combination of endoscopic small lateral dimensions with the desired high-resolution imaging with numerical apertures in the range NA = 0.3 ... 0.8 first requires a fundamental investigation with which conceptual approaches such a system can be realized. This involves the use and combination of novel system components with a gradient index profile, diffractive effect and aspherical forms. It is to be expected that this can only be achieved by a combination of such technologies.
In the present work package, a literature and patent investigation is intended to provide information on which approaches are promising. In a design study it is then necessary to investigate the problems and limitations of the system structure and the consequences of miniaturization on the classical structures. Since diffraction effects are to be expected as a result of the small transverse dimensions, a physical simulation, which goes beyond the traditional raytrace, is necessary to guarantee the system concepts. Miniaturization also requires critical testing of the technological constraints on tolerances, manufacturing errors, achievable accuracy, etc.
A particular challenge is the scanning process via a rotating fiber. Especially, it is necessary to investigate if known approaches from classical microscopy can be directly transferred to endomicroscopy. The approach of diffractive elements for the chromatic corrections requires here most likely an integrated simulation of macro- and micro-effects with real structures in order to be able to determine tolerances for clarifying the feasibility. Furthermore, it is important to simulate the calculation of point image functions in confocal, 2-photon and CARS imaging modalities. For this purpose, a tool development is necessary - corresponding calculation possibilities are not yet available.
As a result of this work package, viable system concepts and the technological feasibility will be developed, discussed and compared, taking into account the specifications laid down in AP 1. Regarding the technological feasibility, the concepts are closely aligned with the technological evaluations in the subprojects of the partner GrinTech.
Results: In the demonstrator 1, a multimodal imaging between high-resolution microscopic imaging and a weakly resolving OCT image is to be realized. For this purpose, switchable or variable components are required, but no stepless shifting has to be realized. Extensive studies, design studies on Alvarez plate systems have been carried out. The desired data are conceivable with this kind of solution, but the necessary small diameter of the system (2mm) is problematic.

AP 2.2: Concepts, design studies and simulations for demonstrator "OCT-Endoscope"

Here, too, an initial literature and patent investigation is to be made whether there are already indications for system constructions of this type. In particular, the combination of the focusing and zooming functionality under the boundary conditions of the limited diameters is to be investigated. Here, no comparable system concepts are available in literature. From the present point of view, it will always be necessary to simulate the single-mode fiber coupling in OCT mode and to examine the influence of aberrations on the functionality.
As a result of this work package, various conceivable concepts should be proposed, evaluated and compared where possible, which meet the requirements under the technological possibilities.
Results: In demonstrator 2, between a wide field and an OCT mode should be switched. According to the examinations and design variants, a classic zoom lens with a sliding group is favored. In order to achieve the required quality, it will be necessary to use aspherical and / or diffractive elements. Components with Alvarez principle are also conceivable, but in a somewhat more elaborate variant.

AP 3.1: Optical and system design of the demonstrator "Endomicroscope"

The result of this AP is a full production documentation for the implementation of the demonstrator.
To achieve this, the system parameters must be optimized, data sheets must be created, the system performance to be expected simulated as well as information on tolerances for system components and assembly. The main part of the work are concrete dimensioning with the fulfillment of the system requirements while at the same time considering the technological limitations as well as cost-related criteria for an application - which can generally only be carried out iteratively.

AP 3.2: Optical and system design of the demonstrator "OCT-Endoscope"

The system parameters will be optimized as in AP 3.1. In particular, the dynamic positioning elements for post-focusing and zoom adjustment must also be dimensioned in this case.
As a result of this AP, complete data sheets will be available for the realization of the demonstrator.

AP 4.1: Integration and evaluation of the demonstrator "Endomicroscope"

The construction and the test of the demonstrator are supported from the point of optics design. The work includes comparisons with the simulation results and, where appropriate, problem analyzes, proposals and quantitative determination of test optics and criteria. In the case of a not achieved system performance, the individual critical components are characterized for the purpose of error analysis and returned to the simulation under real conditions.

AP 4.2: Integration and evaluation of the demonstrator "OCT-Endoscope"

Similar to AP 4.1 the construction and test of the demonstrator are supported from the point of view of the optics design and appropriate work will carried out.
The variable setting options of this demonstrator make the analysis particularly difficult. Therefore, the development of a concrete test scenario that operates on a restricted parameter space may also be necessary.

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