# Design of Zoom Optical System from Visible to NIR-II for Vivo Fluorescence Imaging Device
## 1 Introduction  
In vivo imaging techniques play a crucial role in the exploration and development of biology and medicine. Currently, in vivo imaging techniques include computed tomography CT, MRI, ultrasound US, fluorescence imaging FI, etc. In the past decade, short-wave infrared (SWIR) fluorescence imaging has been gradually and widely used in biomedical fields because of its excellent imaging effects [1; 2; 3; 4]. In most biological tissues, light scattering within the tissue is the main factor limiting the depth of imaging penetration. Since scattering becomes weaker at longer wavelengths [5], the use of longer-wavelength light sources should significantly improve the penetration depth. To date, silicon-based CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) cameras have been used mainly as two-dimensional detector arrays, mainly because reliable silicon (Si) arrays with high-speed imaging capabilities are readily available. However, beyond 1.0 um, the sensitivity of silicon-based detectors is significantly reduced, making it difficult to use these devices at longer wavelengths. In blood, skin, and fat tissue, the absorption in the 600-1800 nm band is at a low range, which is much better than visible light; the scattering of short-wave infrared by biological tissues is also much smaller than visible light; in the short-wave infrared band, the excited fluorescence of the organism itself is also weaker, making the interfering light less, which is also the advantage of short-wave infrared fluorescence in vivo imaging. Before being limited by the cut-off wavelength of 1100 nm of CCD and CMOS, the mainstream form of an imaging system takes the form of multiple optical systems with multiple detectors. However, the scheme of two cameras in concert makes the whole system bulky and inconvenient for clinical or laboratory use. The use of multiple wavelengths for fluorescence surgical navigation has been demonstrated, as shown in Figure 1[2], but the current devices are bulky; three sets of imagingequipment were used to achieve wide-band imaging-visible NIR-I and NIR-II, and their bottlenecks are in the optical system and detectors as mentioned above. The pre-shot pictures of our previous equipment also point out the necessity of the zoom optical system, such as the mouse in Figure 2a, whose fluorescence luminescence area is only a small part marked by a red rectangle because it is limited by our 16 mm fixed-focus lens and fluorescence timeliness, we can only take experimental pictures with an unsuitable field of view. Figure 2b shows the dilemma of not being able to perform wide-band imaging, and the left and right sides can only be taken at different angles with different equipment-visible and NIR-II cameras. All the above shows the application scenario of a wide-band zoom system. With the gradual maturation of InGaAs Vis-SWIR detectors with 0.4-1.7 \(\upmu\)m wide band detection capability [6], the realization of visible and IR light confocal plane imaging systems has become possible. The question of how to realize a compact multi-mode system is receiving more and more attention. The large variation in dispersion properties of different optical materials in such a band from 400 nm to 1700 nm makes the design of wide-band optical systems difficult [7, 8]. With the improvement of the design theory of zoom optical systems and the mechanical processing capability of optical elements, the imaging quality of good zoom optical lenses in the signal spectral band can even be comparable to that of fixed-focus lenses. This enables the implementation of a simple and easy-to-use system of in vivo imaging. In this paper, we design a wide-band zoom optical system adapted to InGaAs detectors, which makes full use of the system space and avoids the problems of matching multiple cameras and switching the field-of-view. Figure 1: The illustration of multiple wavelengths for fluorescence surgical navigation [2]. Figure 2: Pre-shot pictures of our previous equipment.