Abstract
We report on the theory and design of adaptive objective lens for ultra broadband near infrared light imaging with large dynamic optical depth scanning range by using an embedded tunable lens, which can find wide applications in deep tissue biomedical imaging systems, such as confocal microscope, optical coherence tomography (OCT), two-photon microscopy, etc., both in vivo and ex vivo. This design is based on, but not limited to, a home-made prototype of liquid-filled membrane lens with a clear aperture of 8mm and the thickness of 2.55mm ~3.18mm. It is beneficial to have an adaptive objective lens which allows an extended depth scanning range larger than the focal length zoom range, since this will keep the magnification of the whole system, numerical aperture (NA), field of view (FOV), and resolution more consistent. To achieve this goal, a systematic theory is presented, for the first time to our acknowledgment, by inserting the varifocal lens in between a front and a back solid lens group. The designed objective has a compact size (10mm-diameter and 15mm-length), ultrabroad working bandwidth (760nm - 920nm), a large depth scanning range (7.36mm in air) - 1.533 times of focal length zoom range (4.8mm in air), and a FOV around 1mm × 1mm. Diffraction-limited performance can be achieved within this ultrabroad bandwidth through all the scanning depth (the resolution is 2.22 μm - 2.81 μm, calculated at the wavelength of 800nm with the NA of 0.214 - 0.171). The chromatic focal shift value is within the depth of focus (field). The chromatic difference in distortion is nearly zero and the maximum distortion is less than 0.05%.
Highlights
Large depth scan is very important for both in vivo and ex vivo three dimensional (3D) volume imaging of bio-tissue
Light needs to be focused onto the specific layer of the tissue to which the pinhole is set conjugated for filtering out the scatter light from different out-of-focus layers
It is beneficial to have an adaptive objective lens which allows an extended depth scanning range that is larger than the focal length zoom range, since this will keep the magnification of the whole system, numerical aperture (NA), field of view (FOV), and resolution more consistent
Summary
Large depth scan is very important for both in vivo and ex vivo three dimensional (3D) volume imaging of bio-tissue. Light needs to be focused onto the specific layer of the tissue to which the pinhole is set conjugated for filtering out the scatter light from different out-of-focus layers. The depth scan is usually achieved mechanically by moving either the sample or the optics for refocusing. In the OCT system, a range of depth scan can be obtained either by moving the reference mirror (time domain), or by selecting the depth information spectrally (Fourier domain). The refocusing is needed for selecting a zero delay line, where the sensitivity is the highest, and for imaging different volumes of interest
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