By Moez Haque and Peter R. Herman
The manipulation of femtosecond laser light inside transparent media can be directed on varying interaction pathways of micro-explosions, photochemistry and self-focusing filamentation to open new directions in creating dense memory storage, three-dimensional (3D) optical circuits, 3D microfluidic networks, and high-speed scribing tracks[1-3]. Our group has been following these fundamental and nonlinear interactions to control femtosecond laser processes in transparent coreless and single-mode optical fibers (SMFs) and thereby form highly functional and compact fiber devices that may seamlessly integrate with microelectronic chips. Such optical fibers are currently deployed over a billion kilometers of worldwide networks and can also reach into challenging environments such as advanced aircraft structures or cardiovascular systems.
The concept of developing ubiquitous sensing networks relies on the development of novel miniaturized and integrated in-fiber microsystems. Following the miniaturization and integration of chemical and biological devices with optical components for multifunctional lab-on-chip (LOC) microsystems, femtosecond laser processing has enabled us to create a new optofluidic lab-in-fiber platform for environmental, mechanical and analytical sensing that may be widely distributed into fiber networks or inside flexible biomedical probes that is otherwise not possible with more traditional LOC-based technologies.
An essential component for the lab-in-fiber is the laser-formed optical tap that predictably couples light into and out of the light-guiding core of SMFs for connecting with optical probing sensors that have been written in the surrounding fiber cladding. To enable such a novel concept of “fiber cladding photonics”[5,6], Figure 1 shows three traditional approaches developed to partially redirect light from the core waveguide into the laser-written cladding waveguide: (1) A “X-coupler” (Figure 1a) that crosses the center waveguide at a discrete angle, (2) an “S-bend” coupler (Figure 1b) that forms an “S” shaped waveguide to emerge from the SMF core, and (3) a “directional coupler” (Figure 1c) that runs offset and parallel with the SMF core. Our group has demonstrated an unprecedented flexibility in tuning the coupling ratio to values as high as 99 percent while also controlling the light polarization and spectral bandwidth[5,6].
A wide variety of photonic cladding sensing circuits is now available within general types of glass fibers. For example, the formation of helical waveguides to define an in-fiber Mach-Zehnder interferometer has offered unambiguous sensing of fiber torsion. Alternatively, we showcase the distributed fiber-optic 3D shape and temperature sensor shown in Figure 2 that was written in a single laser exposure step. Oil-immersion focusing into the buffer-stripped optical fiber offers a continuous and distortion-free inscription, where nine different Bragg grating waveguides (BGWs) were distributed along three parallel waveguide tracks and interconnected via a 1×3 directional coupler. The instantaneous 3D fiber shape is computed from shifts in the nine BGW wavelengths when probed by a spectrometer as shown in Video 1, where the center waveguide was designed for minimal sensitivity to bend-induced strain to permit the simultaneous measurement of the temperature profile along the fiber as shown in Video 2. Such a freestanding, flexible and lightweight 3D shape sensor is attractive in wide ranging applications, including the guidance of drug delivery, biomedical catheters and other instruments used in minimally invasive surgeries.
We also exploit the selective hydrofluoric acid etching of laser-modification tracks to enable precise 3D structuring of through and blind holes, reservoirs, microfluidic networks and near-optical quality (12 nm rms) hollow resonators anywhere inside an optical fiber. In this way, existing fiber-optic technology can be elevated from “cladding photonics” into new types of “fiber optofluidics” or MEMS sensors, for example, permitting refractive index and pressure sensing with a fiber-embedded wavefront splitting interferometer. Figure 3 showcases a higher level of integration, combining cladding photonics, microfluidics and optical resonators that efficiently connect with the probing SMF core waveguide. This multiplexed lab-in-fiber offers simultaneous probing of an inline BGW and a cladding Fabry Perot resonator either in-core with a laser-formed X-coupler or externally with a total internal reflection mirror. This highly compact lab-in-fiber was spliced to a SMF for real-time sensing of temperature, axial strain, bending strain, gas pressure, fluid or gas refractive index, or analyte fluorescence.
The overall approach of femtosecond laser structuring in optical fiber extends much further to enable selective formation of through and blind holes, evanescent and plasmonic sensing elements, MEMS, 3D microfluidic networks, reservoirs, micro-optics, inline BGW filters, polarization elements, interferometers, spectrometers and bioprobes. New research tools, commercial products and biomedical devices may now be manufactured to, for example, (1) develop analyte-specific sensors into optical fibers for monitoring oil and gas exploration systems and water supplies, (2) exploit the 3D fiber shape sensing capability for catheter guidance, (3) enable optical coherence tomography probing devices in the human body, and (4) construct micro- to nano-holes for differentiating cells, bacteria, viruses and DNA. University of Toronto spin out company, Incise Photonics Inc. (www.incisephotonics.com) is now targeting such commercial applications.
Dr. Moez Haque is a postdoctoral researcher developing novel lab-in-fiber sensors for commercial applications. Prof. Peter R. Herman is full professor in the Department of Electrical and Computer Engineering at the University of Toronto.
 S. A. Hosseini, P. R. Herman, “Method of material processing by laser filamentation”, U.S. Patent 20130126573 A1, filed July 24, 2011. http://www.google.com/patents/US20130126573
 R. Osellame, G. Cerullo, R. Ramponi, Femtosecond laser micromachining: photonic and microfluidic devices in transparent materials, Springer-Verlag Berlin Heidelberg, 2012. http://dx.doi.org/10.1007/978-3-642-23366-1
 K. Sugioka, Y. Cheng, Ultrafast laser processing: from micro- to nanoscale, Pan Stanford, Boca Raton, 2013. http://dx.doi.org/10.1201/b15030
 M. Haque, K. K. C. Lee, S. Ho, L. A. Fernandes, P. R. Herman, “Chemical-assisted femtosecond laser writing of lab-in-fibers”, Lab Chip 14, 3817-3829 (2014). http://dx.doi.org/10.1039/C4LC00648H
 J. R. Grenier, M. Haque, L. A. Fernandes, K. K. C. Lee, P. R. Herman, “Femtosecond laser inscription of photonic and optofluidic devices in fiber cladding”, in G. Marowsky (ed.), Planar waveguides and other confined geometries, p. 67, Springer Series in Optical Sciences vol. 189, New York (2015). http://dx.doi.org/10.1007/978-1-4939-1179-0_4
 J. R. Grenier, L. A. Fernandes, P. R. Herman, “Femtosecond laser inscription of asymmetric directional couplers for in-fiber optical taps and fiber cladding photonics”, Opt. Express 23(13), 16760-16771 (2015). http://dx.doi.org/10.1364/OE.23.016760
 L. A. Fernandes, J. R. Grenier, J. S. Aitchison, P. R. Herman, “Fiber optic stress-independent helical torsion sensor”, Opt. Lett. 40(4), 657-660 (2015). http://dx.doi.org/10.1364/OL.40.000657
 K. K. C. Lee, A. Mariampillai, M. Haque, B. A. Standish, V. X. D. Yang, P. R. Herman, “Temperature-compensated fiber-optic 3D shape sensor based on femtosecond laser direct-written Bragg grating waveguides”, Opt. Express 21, 24076-24086 (2013). http://dx.doi.org/10.1364/OE.21.024076
 V. Maselli, P. R. Herman, “Integrated optical circuits in fiber cladding by tightly focused femtosecond laser writing”, Proc. SPIE 7585, 75850F-1-75850F-11 (2010). http://dx.doi.org/10.1117/12.845431
 Video 1: 3D fiber shape sensing by laser-written cladding photonics, from , available at http://www.opticsinfobase.org/oe/viewmedia.cfm?uri=oe-21-20-24076-1
 Video 2: 3D fiber shape and distributed temperature sensing, from , available at http://www.opticsinfobase.org/oe/viewmedia.cfm?uri=oe-21-20-24076-2
 M. Haque, Y. Shen, A. A. Gawad, and P. R. Herman, ‘’Chemical-assisted femtosecond laser structuring of waveguide-embedded wavefront-splitting interferometers”, J. Lightwave Technol. 33(21), 4478-4487 (2015). http://dx.doi.org/10.1109/JLT.2015.2473795