Precise optical fiber drilling for sensing application

March 9, 2011—The latest optical-fiber developments have impacted more than just telecommunications technology, they have also had a big effect on measuring and sensing technologies.

The interferometry principle embodied in fibers promotes extremely high accuracy when they are used for measuring rotation, translation, acceleration, electric and magnetic field, temperature, pressure, acoustics, vibration, linear and angular position, strain, humidity, viscosity and chemical composition. And, fibers are relatively simple, cost-effective solution when used in these applications.

Figures 1 a and b: Holes in 125µm diameter single mode fibers.

Small size, lightweight and passively operating sensors reveal new measuring capabilities and often replace other traditional sensor technologies. Advance in laser diodes and significant drop of their prices heavily influenced prevalence of optical fiber sensors. In overall, it seems that optical fibers are becoming the most valuable technological platform for design and manufacture of sensors for precise measurement operations.

Optical fiber drilling using femtosecond laser radiation is an unprecedented technique, which shows what’s impossible with other conventional laser processing or mechanical drilling techniques and is much faster and more cost-effective than electron or ion-beam lithography. With tested <300 fs duration pulses, holes as small as 10 μm have been drilled, while keeping fairly good rigidity of the fiber in the region. Femtosecond laser processing eliminates occurrence of micro-cracks and melt and the flatness of the inner wall is of hundreds of nanometers.

Figure 2a: Micro lens on the single-mode optical fiber tip.

Figure 2b: "Workshop of Photonics" logo formed inside of optical fiber.

However several challenges are still to be dealt with. The most considerable ones are:

• Difficulties in control of the laser radiation inside the fiber due to non-linear effects, such as self-focusing and cylindrical nature of the fiber itself;
• Debris removal (especially evident at drilling <10 μm diameter holes);
• Right angle corner fabrication when making holes of rectangular cross-section.

Cases are not so trivial since applying high viscosity immersion oils tending to compensate the change of refractive index around the fiber, impedes debris removal properties, thus increasing the smallest possible size of a hole to be drilled.

All experiments have been carried out using originally developed femtosecond laser micromachining system FemtoFAB having implemented SCA series software. Drilling duration varies from less than 10 seconds for a 13 μm circular hole to 1.5 minute for a rectangular shaped hole in 125 μm diameter single-mode and multi-mode fibers.

‘Workshop of Photonics’ is planning to develop the drilling process to an industrial level for mass-production of optical fiber sensors used in Fabry-Perot interferometers, bio- and chemical sensing, and micro-fluidics. Some extraordinary ideas, like optical fiber based microphone or integrated micro-optic assemblies on the fiber tip are also in potential application list. WOP also develops a 3D direct laser writing technique, called multi-photon polymerization, which supplements the planned activities in fiber fabrication with flexible capabilities of forming desired shape 3D nanostructures from hybrid SOLGEL materials having matched refractive index to the core of a fiber. If we add the capability to locally change the refractive index inside the fiber material, by using a femtosecond laser radiation, the whole precision toolkit is available for making what’s desired in optical fibers.

References: http://www.wophotonics.com/get.php?f.319 http://www.bluerr.com/images/Overview_of_FOS2.pdf