G.L. Gruca

Ferrule-top nanoindenter: An optomechanical fiber sensor for nanoindentation

Ferrule-top probes are self-aligned all-optical devices obtained by fabricating a cantilever on the top of a ferruled optical fiber. This approach has been proven to provide a new platform for the realization of small footprint atomic force microscopes (AFMs) that adapt well to utilization outside specialized laboratories [D. Chavan et al., Rev. Sci. Instrum. 81, 123702 (2010); ibid. 82, 046107 (2011)]. In this paper we now show that ferrule-top cantilevers can be also used to develop nanoindenters. Our instrument combines the sensitivity of commercial AFM-based indentation with the ease-of-use of more macroscopic instrumented indenters available today on the market. Furthermore, the all-optical design allows smooth operations also in liquids, where other devices are much more limited and often provide data that are difficult to interpret. This study may pave the way to the implementation of a new generation user-friendly nanoindenters for the measurement of the stiffness of samples in material sciences and medical research.

Ferrule-top micromachined devices: design, fabrication, performance

We present a new all-optical micromachined device obtained by carving a rectangular mechanical beam out of the end of a ferruled optical fiber. The device is fabricated with techniques that adapt well to series production and offers performance similar to that provided by fiber-top cantilevers, with the advantage of a much lower production cost.

Top-down approach to fiber-top cantilevers

Taking inspiration from conventional top-down micromachining techniques, we have fabricated a low mass gold fiber-top cantilever via align-and-shine photolithography. The cantilever is characterized by measuring its resonance frequency and mechanical quality factor. Our results show that the device grants mass sensitivity comparable to that reported for similar standard cantilevers. This proof-of-concept paves the way to series production of highly sensitive fiber-top devices for remote detection of biochemical substances.

Ferrule-top atomic force microscope

Ferrule-top cantilevers are a new generation of all-optical miniaturized devices for utilization in liquids, harsh environments, and small volumes [G. Gruca et al., Meas. Sci. Technol. 21, 094033 (2010)]. They are obtained by carving the end of a ferruled fiber in the form of a mechanical beam. Light coupled from the opposite side of the fiber allows detection of cantilever deflections. In this paper, we demonstrate that ferrule-top cantilevers can be used to develop ultra compact AFMs for contact mode imaging in air and in liquids with sensitivity comparable to that of commercial AFMs. The probes do not require any alignment procedure and are easy to handle, favoring applications also outside research laboratories.

Measurement of the Casimir force with a ferrule-top sensor

We present a Casimir force setup based on an all-optical ferrule-top sensor. We demonstrate that the instrument can be used to measure the gradient of the Casimir force between a gold-coated sphere and a gold-coated plate with results that are comparable to those achieved by similar atomic force microscope experiments. Thanks to the monolithic design of the force sensor (which does not require any optical triangulation readout) and to the absence of electronics on the sensing head, the instrument represents a significant step forward for future studies of the Casimir effect under engineered conditions, where the intervening medium or the environmental conditions might be unsuitable for the use of more standard setups.

Demonstration of a miniature all-optical photoacoustic spectrometer based on ferrule-top technology

We present a miniaturized photoacoustic (PA) spectrometer obtained by carving a micromachined flexural pressure transducer directly at the top of a glass ferrule. The ferrule is equipped with two optical fibers, one for laser excitation of the gas and one for interferometric readout of the transducer. To demonstrate the working principle and assess the sensitivity of the device, we performed a set of measurements of C2H2 traces in an Ar buffer atmosphere. The data acquired show that our ferrule-top scheme allows one to increase the minimum detectable concentration by more than one order of magnitude with respect to the other miniaturized PA spectrometers reported in the literature, while decreasing the integration time by a factor of 10.

Ferrule-top cantilever optical fiber sensor for velocity measurements of low speed air flows

We present the application of a ferrule-top cantilever optical fiber sensor to measure the velocity of low speed air flows. The sensor is statically characterized against a Pitot tube in a small wind-tunnel and both probes are facing the flow in its laminar region. The ferrule-top sensor is equipped with a single-wavelength interferometric readout and it shows good sensitivity and short-term repeatability, despite the fact that the geometry and the read-out are not optimized.

Ferrule-top micromachined devices: a new approach to fibre-top technology

Fibre-top devices (micromachined devices fabricated on top of a single mode optical fibre) provide unprecedented opportunities for the development of user-friendly, all-optical miniaturized sensors. Unfortunately, the impact of this technology has been hampered by the high costs of fabrication. In this paper we introduce a new generation of fibre-top-like devices that can be fabricated with cost-effective processes and still maintain all the advantages of fibre-top technology.

Interrogation of multiple ferrule-top-cantilever sensors for acoustic emission sensing

A simple, yet effective, setup for the simultaneous interrogation of multiple ferrule-top-cantilever sensors for acoustic sensing is here presented and experimentally tested with two ferrule-top-cantilever sensors; results confirm the feasibility of the approach.

Analysis of fiber optic sensor application to precursory acoustic signals detection in rockfall events

Two fiber optic sensors (FOSs) for detection of precursory acoustic emissions in rockfall events are proposed and experimentally characterized. While both sensors are interferometric, the first one use a fiber coil as sensing element, whereas the second sensor exploits a micro-machined cantilever carved on the top of a ferrule. Preliminary experimental comparison with standard piezo-electric transducers shows the viability of such FOSs for acoustic emission monitoring in rock masses.