Top: False-color scanning electron microscope (SEM) images of fabricated FPC sensor, with multipositional hinged mirror in the closed position (left), spring-body FPC pressure sensor in the half-open position (center) and flow sensor (right). Bottom left: Three reflection spectra from the top-left FPC sensor in different solutions. Bottom center: Three reflection spectra from top-center spring-body pressure sensor at different pressures. Bottom right: Reflection response of the top-right fiber tip flow sensor. [Adapted from references 2 and 3]
Two-photon lithography (TPL) has helped drive forward the advancement and miniaturization of 3D functional microsystems. Optical fibers integrated with advanced optomechanical components represent a promising approach to scale down a variety of novel sensors essential in modern engineering systems. While TPL has been used to fabricate a mechanically suspended Fabry–Pérot cavity (FPC) sensor with curved surfaces on a fiber tip, the inner optical surfaces that form the cavity are shadowed by the top structure, which prevents reflective-coating deposition. Thus the fabricated FPC yields a low quality factor.1
To solve this issue, we recently used TPL to monolithically integrate dynamic micromechanical features into an FPC sensor on a fiber tip (see accompanying figure).2 These features, we believe, signify a breakthrough in the integration and fabrication capabilities of micro-optomechanical devices and systems.
Our method leverages rotation of a movable mirror to deposit a thin reflective coating onto the inner surfaces of an FPC with curved geometry. The dynamic optical surface enables directional thin-film deposition onto obscured areas. The reflective coating, coupled with the rotatable mirror, greatly improves the FPC quality factor and enables a new class of highly integrated, multipurpose sensor systems. We used the fiber tip sensor to demonstrate liquid refractive-index sensing with a sensitivity of 2045 nm per refractive-index unit.2
In other work published this year, we further exploited TPL to create a unique spring-body FPC that is also equipped with a hinged, multipositional mirror to facilitate reflective-coating deposition onto the inner surfaces of the cavity.3 After the thin reflective coating is sputtered onto the cavity’s inner surfaces, the rotatable mirror is locked into its final position. The spring-body FPC demonstrated pressure sensing with a sensitivity of 38 pm/kPa over a range of −80 to 345 kPa.3
Finally, we used TPL to produce a flow sensor consisting of microblades that spin in response to an incident flow.3 Light exiting the optical-fiber core is reflected back into it at a flow-dependent rate as the blades pass by. The fiber-tip flow sensor operated successfully over a range of 9–25 liters per minute (LPM) using nitrogen gas, achieving a linear response of 706 reflections/LPM over a range of 10.9–12 LPM.3
Jeremiah Williams and Hengky Chandrahalim, Air Force Institute of Technology, Wright–Patterson Air Force Base, OH, USA
Joseph Suelzer and Nicholas Usechak, Air Force Research Laboratory, Wright–Patterson Air Force Base, OH, USA
1. J.W. Smith et. al. J. Micromech. Microeng. 30, 125007 (2020).
2. J.C. Williams et. al. Adv. Photonics Res. 3, 2100359 (2022).
3. J.C. Williams et. al. ACS Appl. Mater. Interfaces 14, 19988 (2022).