We propose and demonstrate a high-frequency interferometric optical fiber hydrophone based on acoustic resonance isolation. Novel air chambers are introduced into common type of mandrel sensitization structure to prevent acoustic resonance which occurs inside the inner cylindrical cavity of hydrophone frame at specific high frequency acoustic filed. 1kHz-30kHz frequency response of this new kind of hydrophone is measured in a lake. Experiment shows that, by this acoustic structure optimization, the working frequency bandwidth of optical fiber hydrophone, whose acoustic pressure sensitivity is-147dB, is expanded from 10kHz to above 30kHz.
We report in this paper a miniature adjustable-focus camera module integrated with a solid tunable lens driven by MEMS-thermal actuators. Thanks to the compact structures of the MEMS actuators and the optical-power-variation capability of such solid tunable lenses, such a camera module shows great potential in various applications, especially in tiny underwater imaging systems where an optical zoom or focus-tuning function is required. The camera module presented in this paper consists of a solid tunable lens for optical power tuning, two identical MEMS-thermal actuators for lens component driving and one commercial CMOS chip for image recording, assisted with necessary mechanical parts for supporting and housing. We first introduce the optimal design of the solid tunable lens which consists of two freeform components moving in the direction perpendicular to the optical axis, and subsequently, the MEMS thermal actuators which consist of V-shaped micro beams. The lens components are fabricated by the technique of single-point diamond turning followed by the PMDS modelling process while the MEMS-thermal actuators are materialized by a commercial MUMPS process. The static performance of the designed system, including the temperature distribution, displacement-versus-voltage curve and the optical tuning capability, is characterized in detail, together with the dynamic responding speed, hysteresis and imaging performance stability during tuning. Results show that a maximum output displacement of 135 μm is achieved by the optimized MEMS-thermal actuator with a driving voltage of 10V, and consequently a focal length tuning range from 9.2 to 7.9 mm with a responding speed of about 90 ms is realized by the solid tunable lens. The highest temperate on the actuator is about 690 K during the operation while the temperate increase on the lens components is found to be about 30 K, which guarantees the optical performance stability. Targets placed at different object distances are clearly focused by the assembled miniature camera module with various driving voltages, which demonstrate its adjustable focus capability. Such miniature adjustable-focus camera modules show promising future in underwater applications due to their compact structures and optical-power-variation capability.