At present, the medical endoscope mainly uses the fixed focus optical system and relies on image processing and other means to scale the image. Due to the large space occupied by traditional mechanical zoom optical system as well as its difficulty in zooming, it has not been applied to medical endoscopes. Based on the concept of bionics, the liquid lens is a small, integrated and self-zooming lens. The electrowetting liquid lens is applied to the design of medical endoscope. Zooming can be realized by adjusting the voltage and the image quality can be guaranteed. Some scholars at home and abroad have studied the optical system of endoscope containing liquid lens, but there are many problems in the existing research contents, such as large distortion and difficulty to guarantee the image quality. In this paper, based on the traditional zoom optical design of medical laparoscope, an autonomous zoom optical system with small distortion and simple structure is designed, which is suitable for medical endoscope.
The electrowetting liquid lens, which depends on voltage to adjust the curvature of the interface, is selected as the core zoom element. The basic parameters of the optical system are determined according to the main technical indexes, the mechanical zoom optical system is designed by optimizing the initial structure of traditional zoom optics, and the initial optical structure of a zoom laparoscope is obtained. The parameters of liquid lens are determined according to the changing range of focal length and the aperture of optical system, and then the liquid lens is designed and simulated by COMSOL. The simulation diagram of liquid lens interface is given when the voltage is 0, 35 and 60 V. The corresponding relationship between voltage variation and liquid level variation is simulated. When the voltage reaches 60 V, the contact angle of the electrowetting liquid lens reaches saturation, and the liquid lens interface no longer changes. MATLAB software is used to simulate the corresponding relationship between voltage and focal length. In the voltage range of 0?30 V, the focal length is positive and shows a slow growth trend; in the voltage range of 30?35 V, the focal length increases rapidly and finally reaches infinity; when the voltage is in the 35?40 V range, the focal length is negative and rapidly decays from infinity; when the voltage is in the 40?60 V range, the focal length slowly decays, and finally the contact angle of the liquid lens reaches saturation, and the focal length of the liquid lens no longer changes. According to the data, the liquid lens is modeled in ZEMAX. The zoom lens group and compensation lens group in the initial structure are replaced with liquid lenses, and the liquid surface is controlled by voltage to achieve the purpose of zooming. ZEMAX is used to optimize the simulation.
After optimization, the liquid lens zoom optical system for laparoscopy is obtained (Fig. 7), in which the system structure diagrams under four different states are given: short focus (5 mm), medium focus (7.2 mm), medium focus (10.4 mm), and long focus (15 mm), respectively. The fourth and eleventh mirrors of the optical system are liquid lenses. In the four configurations, the focal lengths of liquid lens 1 and liquid lens 2 are (-17.89 mm, 20.88 mm), (-48.68 mm, 37.43 mm), (180.73 mm, -631.96 mm), (38.78 mm, -23.31 mm), respectively. Through 19.44?44.23 V voltage regulation, the focal length of 5?15 mm is realized, and the total length of the system is 45 mm (Table 3). Each state of the zoom system obtains the maximum root-mean-square (RMS) radius at the maximum field angle (Table 3), and all of them are smaller than the pixel size 7.4 μm. The optical transfer function curves of different configurations are all close to the diffraction limit curve (Fig. 8). At the Nyquist frequency of 68 lp/mm, the modulation transfer function (MTF) of the optical system in the zoom range is greater than 0.5, which meets the high quality requirements of the optical system of laparoscopy. The distortion value of each state of the zoom system is far less than the required value, and the maximum distortion value is 7.047% (Fig. 9). Compared with the fixed-focus laparoscopic system, the distortion value is far less than the common value of 15%, showing excellent system performance.
A zoom optical system for laparoscopy is designed with liquid lens as the core component, and the relationship between zoom range and control voltage is designed and simulated. Aberration analysis of the design results shows that the MTF of the four configurations is greater than 0.5 at the Nyquist frequency of 68 lp/mm, and the maximum distortion is 7.047%. The imaging quality can meet the requirements of laparoscopic optical system. In the process of focal length change, the position of the image plane is always unchanged, and the zooming can be realized by adjusting the voltage without mechanical structure. The results show that it is feasible to use liquid lens to realize the design of laparoscopic zoom optical system. It has a broad application prospect in reducing the overall size, realizing convenient operation and improving the image quality of laparoscopy.
