Due to their advances in achieving large focal-length tuning ranges with compact structures, solid tunable lenses based on the Alvarez-Lohmann principle show a promising potential in various applications, especially modern miniature imaging systems. In this paper, we report miniature endoscopic systems integrated with solid tunable lenses for optical adjustable focusing or zooming. The solid tunable lenses are designed according to the improved Alvarez-Lohmann principle, where two independent extended polynomials are employed to govern the two freeform surfaces, respectively. Slim piezo benders aligned along the optical axis are utilized to drive the solid tunable lenses to move laterally. An image fiber bundle is used to transmit the images captured by the optical system to the external cameras. Results show that the endoscopic system is equipped with a capability of optical power tuning from about 135 diopters to about 205 diopters when there is a single solid tunable lens integrated in the system, which enables it to achieve adjustable focus for objects located at different positions. The integration of two solid tunable lenses and two fixed lenses further enables the endoscopic system to have the optical-zooming capability. A zoom ratio of 3x and a maximum full field of view as high as about 80 degrees are realized experimentally. The cross sectional diameter of the endoscopic probe is controlled below 4 mm. The captured images are clear and sharp. Such adjustable-focus or zoom endoscopic systems would be useful in future medical or industrial applications.
Photonic crystal split-beam nanocavities allow for ultra-sensitive optomechanical transductions but are degraded due to their relatively low optical quality factors. We report our recent work in designing a new type of one-dimensional photonic crystal split-beam nanocavity optimized for an ultra-high optical quality factor. The design is based on the combination of the deterministic method and hill-climbing algorithm. The latter is the simplest and most straightforward method of the local search algorithm, which provides the local maximum of the chosen quality factors. This split-beam nanocavity is made up of two mechanical uncoupled cantilever beams with Bragg mirrors patterned onto it and separated by a 75 nm air gap. Experimental results emphasize that the quality factor of the second order TE mode can be as high as 19,900. Additionally, one beam of the device is actuated in the lateral direction with the aid of a NEMS actuator and the quality factor maintains quite well even there’s a lateral offset up to 64 nm. We also apply Fano resonance to further increase the Q-factor by constructing two interfering channels. Before tuning, the original Q-factor is 60,000; it’s noteworthy that the topmost Q-factor reaches 67,000 throughout out-of-plane electrostatic force tuning. The dynamic mechanical modes of two devices is analyzed as well. Potentially promising applications, such as ultra-sensitive optomechanical torque sensor, local tuning of fano resonance, all-optical-reconfigurable filters etc, are foreseen.
The emerging dual-focus lenses are drawing increasing attention recently due to their wide applications in both academia and industries, including laser cutting systems, microscopy systems, and interferometer-based surface profilers. In this paper, a miniature electrically tunable rotary dual-focus lens is developed. Such a lens consists of two optical elements, each having an optical flat surface and one freeform surface. The two freeform surfaces are initialized with the governing equation Ar2θ (A is the constant to be determined, r and θ denote the radii and angles in the polar coordinate system) and then optimized by ray tracing technique with additional Zernike polynomial terms for aberration correction. The freeform surfaces are achieved by a single-point diamond turning technique and then a PDMS-based replication process is utilized to materialize the final lens elements. To drive the two coaxial elements to rotate independently, two MEMS thermal rotary actuators are developed and fabricated by a standard MUMPs process. The experimental results show that the MEMS thermal actuator provides a maximum rotation angle of about 8.2 degrees with an input DC voltage of 6.5 V, leading to a wide tuning range for both the two focal lengths of the lens. Specifically, one focal length can be tuned from about 30 mm to 20 mm while the other one can be adjusted from about 30 mm to 60 mm.