Compact double-pass Echelle spectrometer employing a crossed diffraction grating.

This contribution presents the design and implementation of a compact and robust Echelle-inspired cross-grating spectrometer which is arranged as a double pass setup. This allows use of the employed refractive elements for collimation of the incoming light and, after diffraction at the reflective crossed diffraction grating, for imaging the diffracted light onto the detector. The crossed diffraction grating combines the two dispersive functionalities of a classical Echelle spectrometer in a single element and is therefore formed by a superposition of two blazed linear gratings which are oriented perpendicularly. The refractive elements and the plane grating are arranged in a rigid objective group which is beneficial in terms of stability and robustness. The experimental tests prove that the designed resolving power of more than 300 is achieved for the addressed spectrum ranging from 400 nm to 1100 nm by using an entrance pinhole diameter of 105 µm. The utilization of a single mode fiber increases the resolving power to more than 1000, but leads to longer acquisition times.

Realization of a compact cross-grating spectrometer and validating experimental tests.

Echelle inspired cross-grating spectrometers offer the potential to bridge the gap between classical high-end echelle spectrometers and curved-grating single-element instruments. In particular, the cross-grating approach offers the possibility to simultaneously achieve a high spectral resolution and a wide accessible spectral range in compact dimensions and without moving parts. We report on the complete realization and implementation details of an all-reflective cross-grating spectrometer based on a modified Czerny-Turner configuration including a folded beam path and a toric-convex mirror for aberration compensation. The applicability of the cross-grating spectrometer is demonstrated by test measurements including the recording of the spectra of different plant leaves. For the cross-grating spectrometer, with an accessible wavelength range between 330 and 1100 nm, a spectral resolution of 0.6 nm at 589 nm was achieved.

Concept and optical design of a compact cross-grating spectrometer.

The concept of a new compact echelle-inspired cross-grating spectrometer is introduced, and a specific optical design is presented. The new concept aims to achieve simultaneously a high spectral resolution, a wide accessible spectral range, and compact dimensions. The essential system novelty concerns the combination of different aspects: the implementation of a crossed grating comprising both the main dispersion and order separation, a folded reflective beam path, which enables a reduction of the system volume, and the introduction of a form-adjustable mirror for aberration compensation. The exemplary optical design offers a spectral bandwidth ranging from 330-1100 nm with spectral resolution better than 1.4 nm in the fourth and 0.4 nm in the 11th order. The optical setup covers a volume of 110 mm×110 mm×30 mm.

Comprehensive optical design model of the goldfish eye and quantitative simulation of the consequences on the accommodation mechanism.

To further extent our understanding of aquatic vision, we introduce a complete optical model of a goldfish eye, which comprises all important optical parameters for the first time. Especially a spherical gradient index structure for the crystalline lens was included, thus allowing a detailed analysis of image quality, regarding spot size, and wavelength dependent aberration. The simulation results show, that our realistic eye model generates a sufficient image quality, with a spot radius of 4.9 µm which is below the inter cone distance of 5.5 µm. Furthermore, we optically simulate potential mechanical processes of accommodation and compare the results with contradictory findings of previous experimental studies. The quantitative simulation of the accommodation capacity shows that the depth of field is strongly dependent on the resting position and becomes significantly smaller when shorter resting positions are assumed. That means, to enable an extended depth perception with high acuity for the goldfish an adaptive, lens shifting mechanism would be required. In addition, our model allows a clear prediction of the expected axial lens-shift, which is necessary to ensure a sufficient resolution over a large object range.

Compact echelle spectrometer employing a cross-grating.

The concept and the implementation of a compact and simplified echelle spectrometer are presented, and the working principle is demonstrated by first experimental measurements. The crucial element of the setup is a cross-grating, combining an echelle grating utilizing several higher diffraction orders (5th up to 11th) and a superposed perpendicular-oriented cross-dispersing grating. Two alternative manufacturing approaches for the cross-grating are presented and discussed. The first approach combines Talbot lithography for the deep echelle grating and interference lithography for the cross-dispersing structure. As a second approach, direct laser-beam writing was applied. The compact echelle spectrometer covers a spectral range from 380 to 700 nm and offers a spectral resolution of ∼2 nm.