Achromatic miniature lens system for coherent Raman scattering microscopy

Abstract

We discuss the design and performance of a miniature objective lens optimized for coherent Raman scattering microscopy. The packaged lens assembly has a numerical aperture of 0.51 in water and an outer diameter of 8 mm. The lens system exhibits minimum chromatic aberrations, and produces coherent Raman scattering images with sub-micrometer lateral resolution (0.648 μm) using near-infrared excitation pulses. We demonstrate that despite the small dimensions of the miniature objective, the performance of this lens system is comparable to standard microscope objective lenses, offering opportunities for miniaturizing coherent Raman scattering imaging probes without sacrificing the image quality.

Keywords: (180.4315) Nonlinear microscopy; (220.3630) Lenses; (350.3950) Micro-optics.

Fig. 1:

Aberration corrected miniature objective. (a) Optical ray diagram of multiple lens elements combined to form a 5mm diameter, 0.51 NA miniature objective lens. (b) Rendering of the lenses and brass holder components (8 mm total outer diameter) with rings supporting lenses on both sides. (c) Photograph of the fabricated and assembled miniature objective next to a commercial objective.

Fig. 2:

ZEMAX predicted performance of the objective design. (a) Geometric spot size diagram for two radial distances from the optical axis (blue: 817 nm, red: 1064 nm wavelength). (b) Modulation transfer function (MTF) plot.

Fig. 3:

Transmission images taken at the fundamental excitation wavelengths. (a) High-resolution USAF 1951 target visualized at 817 nm. (b) Cross section taken at the red line in (a). Scale bar is 5 μm. (c) USAF 1951 target visualized at 1064 nm. (d) Cross section taken at the red line in (c). Scale bar is 5 μm. (e) Ronchi ruling with 5 μm per line pair visualized at 817 nm. (e) Lateral deviation across the field of view for both excitation wavelengths.

Fig. 4:

Resolution measurement of the miniature objective lens. (a) Lateral and (b) axial CARS intensity profile measurement for a 0.51 NA miniature objective. FWHM values (0.648 ± 0.073 μm lateral; 4.9 ± 0.16 μm axial) were determined by averaging the results from n = 11 particles. The pump and stokes laser wavelength was 817 nm and 1064 nm, respectively, corresponding to a Raman shift of 2845 cm⁻¹. The pump and Stokes laser power at the sample are 10 mW and 20 mW respectively.

Fig. 5:

CARS images of 16 μm polystyrene beads over an extended field of view. (a) Image of beads acquired with the 0.51 NA miniature objective. (b) Image obtained with the 0.6 NA aspheric lens. (c) Image acquired using the Zeiss, 20X, 0.5NA objective lens. A fs pump source was used. All images are 512 × 512 pixels. Scale bar is 20 μm

Fig. 6:

CARS imaging of mouse ear tissue using the miniature objective lens at 2845 cm⁻¹. (a, b) Adipocytes of the subcutaneous layer at ∼80 μm from the skin surface. (c) Lipid contrast from the stratum corneum (d) Hair structures near the surface of the tissue. The pump and Stokes laser power at the sample are 20 mW and 28 mW respectively. Images were acquired in the forward detection geometry. All images are 512 × 512 pixels and acquired in 2 s. A fs pump source was used. Scale bar is 20 μm.

Fig. 7:

Multimodal imaging of tissue samples ex vivo at 2845 cm⁻¹ using the miniature objective (a) Epi-detection SHG (green) and CARS (red) image of thick rabbit skin tissue. A fs pump source was used. (b) Forward-detected SRS images of adipocytes in the mouse ear sample. A ps pump source was used. All images are 512 × 512 pixels. Scale bar is 20 μm.