High-Performance Ultra-Thin Spectrometer Optical Design Based on Coddington's Equations

Optics miniaturization strategy for demanding Raman spectroscopy applications

Raman spectroscopy provides non-destructive, label-free quantitative studies of chemical compositions at the microscale as used on NASA's Perseverance rover on Mars. Such capabilities come at the cost of high requirements for instrumentation. Here we present a centimeter-scale miniaturization of a Raman spectrometer using cheap non-stabilized laser diodes, densely packed optics, and non-cooled small sensors. The performance is comparable with expensive bulky research-grade Raman systems. It has excellent sensitivity, low power consumption, perfect wavenumber, intensity calibration, and 7 cm-1 resolution within the 400-4000 cm-1 range using a built-in reference. High performance and versatility are demonstrated in use cases including quantification of methanol in beverages, in-vivo Raman measurements of human skin, fermentation monitoring, chemical Raman mapping at sub-micrometer resolution, quantitative SERS mapping of the anti-cancer drug methotrexate and in-vitro bacteria identification. We foresee that the miniaturization will allow realization of super-compact Raman spectrometers for integration in smartphones and medical devices, democratizing Raman technology.

Astigmatism-free Czerny-Turner spectrometer with a low f-number by a bicylinder lens

A modified optical design for a broadband astigmatism-corrected Czerny-Turner spectrometer with high resolution and large throughput is presented. The theory of astigmatism correction is analyzed with the use of a bicylinder lens with different radii of curvature in tangential and sagittal planes. The optical performances of a modified spectrometer and two traditional spectrometers are compared in detail. The results show that the modified Czerny-Turner spectrometer can obtain superior astigmatism-free performance, an f-number of 2.5, and spectral resolution of 1.5 nm in 350-750 nm. Its volume decreases by approximately 77% compared with a traditional spectrometer with the same f-number.

Optical System Design of a Planar Waveguide Spectrometer

In this paper, an optical design for a hollow planar waveguide spectrometer with Czerny-Turner is proposed. To decrease the propagation loss of the spectrometer, the design strategy of designing the tangential plane and the sagittal plane separately is proposed, corresponding to resolution and energy, respectively. The Czerny–Turner optical path is designed on the tangential plane, and the sagittal design theory and method are analyzed in detail. The ray tracing results show that the resolution of the spectrometer is better than 4 nm on the tangential plane, while on the sagittal plane, the detector receives the highest energy when the detector pixel height matches the distance between the two mirrors.

Advanced optical design of Czerny-Turner spectrometer with high flux and low aberration in broadband

An advanced optical design for a low f-number, high resolution, astigmatism-free, and broadband Czerny-Turner spectrometer is proposed. A hemispherical lens is added between the entrance slit and collimating mirror, which can correct astigmatism and increase numerical aperture at the same time. The theory and method for the aberration correction are analyzed in detail. An example of a design with the f-number as 3 working in 350-750 nm has been presented by the optimized theory. The comparison of the improved Czerny-Turner spectrometer with the conventional Czerny-Turner spectrometer is also thoroughly described in the paper. The ray-tracing results show that the RMS spot radius of the improved Czerny-Turner spectrometer decreases from over 200 µm to less than 18 µm compared with the traditional Czerny-Turner spectrometer.

Freeform gradient index generalized Coddington's equations

Coddington's equations and their generalized forms are useful for lens design and analysis of optical performance. Generalized Coddington's equations (GCE) exist in literature for analysis of decentered systems and freeform surfaces, but not for gradient index (GRIN) lenses. In this work, GCE are presented for the analysis of freeform GRIN lenses with freeform surfaces. Examples are shown where the presented theory converges on Coddington's equations and known paraxial GRIN behavior. The method also correctly shows known afocal behavior proximate to azimuthally directed rays in a cylindrical GRIN. The latter case is one of analytically validated local freeform behavior.