Microchannels are fabricated into conventional single-mode fibers by femtosecond laser processing and chemical etching. Fabrication limitations imposed by the fiber geometry are highlighted and resolved through a simple technique without compromising fabrication flexibility. A microfluidic fiber device consisting of a 4 μm wide microchannel that intersects the fiber core for refractive index sensing is further demonstrated. © 2006 Optical Society of America

Bennion, I.

Lai, Yicheng

Zhang, L.

Zhou, K.

English

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September 1, 2006 / Vol. 31, No. 17 / OPTICS LETTERS 2559Microchannels in conventional single-mode fibersY. Lai, K. Zhou, L. Zhang, and I. BennionPhotonics Research Group, School of Engineering and Applied Science, Aston University, Birmingham B4 7ET, UKReceived May 17, 2006; accepted June 8, 2006;posted June 14, 2006 (Doc. ID 71077); published August 9, 2006Microchannels are fabricated into conventional single-mode fibers by femtosecond laser processing andchemical etching. Fabrication limitations imposed by the fiber geometry are highlighted and resolvedthrough a simple technique without compromising fabrication flexibility. A microfluidic fiber device consist-ing of a 4 m wide microchannel that intersects the fiber core for refractive index sensing is furtherdemonstrated. © 2006 Optical Society of AmericaOCIS codes: 220.4000, 060.2310, 290.3030.The advancement toward microphotonics compo-nents for modern biomedical, chemistry, and sensingapplications has generated increasing interest in mi-crofluidic optical devices. Within the broad-rangingdevelopment of optical microfluidics, there has beeninterest to integrate such advances into fiber opticsto incorporate the operational merits of optical fibers.To date, such developments have adopted specialphotonic crystal fibers where the fluids are accessedwithin the cladding air holes.1 Alternative reportedmethods include immersing cladding-etched fibers orfiber gratings into planar bulk-machined fluidchannels.2 In general, the operations of these pro-posed device formats are based on the interactionsbetween the evanescent field and the surroundingfluids, which modify the optical waveguiding proper-ties of the fiber.The foundation of microfluidic fiber devices an-chors on an effective overlap between the optical fieldand the channel fluids. For a guided light wave, itsinteraction with the fluid can be maximized whenboth the light and the fluid are confined simulta-neously in a waveguide core. Such assembly is desir-able for achieving a large dynamic response rangeand short interaction length. To date, practical imple-mentations to realize such microfluidic configura-tions have relied on abutting two fiber surfaces to themicrofluidic channel,3 while other proposed tech-niques involve coupling fibers into an optical hollow-core planar waveguide.4Toward improved channel integration, reliability,and ease of production, an alternative concept is con-sidered here where the microchannels are fabricateddirectly into an optical fiber. It is advantageous to doso since the desired alignment, and hence the inter-actions between the microfluidic channels and the op-tical waveguide, can be accurately defined and guar-anteed. The fiber waveguide constitutes the backboneconduit for input/output light delivery, and effectivelight-fluid interaction is achieved when the microflu-idic channel intersects the optical pathway. To thebest of our knowledge, there has been no descriptionto date of microfluidic fiber devices based on such en-gineered integrated microchannels.In this Letter we report, for the first time to ourknowledge, the fabrication of microchannels withinsingle-mode fibers based on femtosecond laser expo-sure combined with chemical etching. The fabrication0146-9592/06/172559-3/$15.00 ©process takes advantage of the high spatial resolu-tion inherent to femtosecond laser inscription, andthe preferential chemical etching of the modified ma-terial to realize the designed microchannels withinthe fiber.5 We highlight the fabrication limitationsimposed by the surface geometry of the fiber anddemonstrate a simple solution involving an additionof a glass slip attachment. We further illustrate a mi-crofluidic fiber device consisting of a microchannelthat intersects the fiber core for refractive indexsensing applications. The index measurements areachieved based on straightforward optical power de-tections, and the effective light-fluid interactionlength of the microfluidic fiber device is only of the or-der of 4 m.The fabrication process involves two main steps:(1) inscription of the desired structure into the fiberby using a tightly focused femtosecond laser beam,and (2) etching of the fiber in a solution of 5%hydrofluoric acid (HF) for selective removal of thelaser-modified regions. In the laser inscription pro-cess, the femtosecond laser pulses (centerwavelength=800 nm) were focused into the silica fi-ber by using a 100objective lens with a NA of 0.55and a working distance of 13 mm. The laser pulsewidth was measured to be 150 fs, and the repetitionrate was at 1 kHz. The focused spot size was evalu-ated to be 1.5 m, and the average pulse energywas measured to be 550 nJ. The fiber was mountedon a dual-axes air-bearing translation stage, so thatthe desired structure could be written by moving thefiber with respect to the propagation direction of thelaser beam.The fiber was first translated at 10 m/s along thedirection of propagation of the laser beam so as to in-scribe a straight-line track transversely through thefiber core. A single-pass exposure over a distancemuch larger than the fiber diameter was carried outto ensure the inscription process covered the entirefiber. A high-resolution optical microscope was thenused to visually inspect the inscribed feature withinthe optical fiber.As shown in Fig. 1a, the femtosecond laser-inducedindex modification in the fiber is clearly visible. Moreimportant, a distorted feature is obtained instead ofthe expected straight-line track. The inscription pro-cess was repeated whereby the fiber was translatedin the reverse direction and a similar distorted laser-2006 Optical Society of America2560 OPTICS LETTERS / Vol. 31, No. 17 / September 1, 2006induced structure was obtained. It is clear that theinscribed feature expands as it approaches the fibercore and subsequently fades away as the laser beammoves further into the fiber. This is attributed to thecurved surface of the fiber, which defocuses the inci-dent femtosecond laser beam, leading to a decrease influence. This effect increases with increasing process-ing depth into the fiber, and the fluence subsequentlydrops below the necessary threshold to cause any sig-nificant index modification.To alleviate the defocusing-induced distortions, asimple approach was attempted whereby the fibersurface geometry, presented to the path of the inci-dent femtosecond laser beam, is rectified by using aclosely attached flat glass slip. Conceptually similarto the use of an oil-immersion lens, this approachhowever does not restrict fabrication flexibility and iseasily implemented within the current fabricationsetup. The inscription process was repeated underthe same conditions except a conventional microscopeglass slip thickness=125 m was attached to the fi-ber on the near side of the objective lens as shownschematically in Fig. 2. Index-matching oil was usedto remove any air gap and to form a continuum be-tween the fiber and the glass slip. The resultant in-scribed feature in the fiber based on this technique isas shown in Fig. 1b, and an undistorted straight-linetrack through the fiber core was obtained expectedly.After the laser inscription process, the fiber waschemically etched in the HF solution assisted by anultrasonic bath at ambient conditions for 12 min.The optimum etching time was determined throughreal-time monitoring of the optical loss through thefiber. The fiber was removed from the HF solutionwhen the rate of transmission loss slowed down dras-tically; further etching only serves to widen theetched channel. The processed fiber was visually in-spected under the microscope, and images of theetched feature within the fiber are as shown in Figs.1c and 1d. Evidently, a microchannel that intersectsthe fiber core has been achieved.Even though the detail morphology of the micro-Fig. 1. a, Microscope image of the femtosecond laser-induced index modification in the fiber after direct laser ex-posure. b, Image of the inscribed straight-line track withinthe fiber when a glass slip attachment is adopted duringthe laser exposure. c and d, Images of the etched channel inthe fiber viewed at orthogonal angles, indicating that themicrochannel intersects the fiber core.channel cannot be accurately determined throughthe microscope images, one can observe a direct spa-tial correlation between the laser-modified track andthe region that is preferentially etched that led to thecreation of the microchannel. However, the etchedzone extends over a distance that is larger than thelaser-modified track, highlighting the presence oflaser-induced densification/stress zones around theinscribed region.5,6 Taking note that the etching rateof an identical, unexposed fiber under the same con-ditions was measured to be 0.06 m/min, the laser-modified feature here is therefore 200 times moresusceptible to the etching process. This etching selec-tivity is useful for creating narrow channels as wellas preserving the structural integrity around the in-scribed feature. The physical dimensions of the mi-crochannel were determined under a 40oil-immersion microscopic lens as shown in Fig. 3. Thewidth of the channel tapers very slightly toward thefiber core because the inside of the fiber experiences ashorter exposure time to the HF solution comparedwith regions nearer to the surfaces.5 The channelwidth, measured to be 4.0 m, near to the fiber coreis smooth and uniform but deteriorates noticeablynear to the fiber surfaces. We believe this is due tothe clipping effect of the laser beam whose power isperturbed as the beam transits the surface edge. Theinduced stress fields within the laser-modifiedregions are consequently disturbed, leading to anonhomogeneous etched pattern.6 Such features canbe further reduced if optimally matched index oil isused between the glass slip and the fiber.Fig. 2. Schematic setup of using a glass slip to alleviatethe effect of the fiber curved surface. The diagram is notdrawn to scale.Fig. 3. Microscope images of the microchannel within thefiber viewed under a 40oil-immersion microscopic lens.September 1, 2006 / Vol. 31, No. 17 / OPTICS LETTERS 2561The presence of the microchannel within the fibercore introduces scattering loss to the propagatingcore mode. The refractive index of the channel on theother hand affects the amount of scattering, hencethe transmission loss, through the fiber device. Tak-ing into account the dimensions of the microchannel,the required fluid volume to fill the channel for opti-cal probing is minimal. The effective light-fluid inter-action length is essentially the diameter of the micro-channel and is only on the order of 4 m. A series ofmixtures consisting of different volume concentra-tions of glycerine in water were prepared to create so-lutions of different indices ranging from 1.333 to1.475 for the microchannel fill. The transmissionpower through the microfluidic fiber device was mea-sured at different channel indices by using a C-bandbroadband source and an optical spectrum analyzeras the detection unit. The normalized transmittedpower at 1538 nm through the microfluidic fiber de-vice at different channel indices is plotted as shownin Fig. 4.Between index values of 1.333 to 1.407, the trans-mitted power through the microfluidic fiber deviceexhibits a linear increment with increasing micro-channel index at a rate of 0.56 dB/10−2. Further in-dex increments up to 1.475 saw a much smaller im-provement to the transmission loss, which eventuallyFig. 4. Normalized transmitted optical power through themicrofluidic fiber device at different microchannel refrac-tive indices.levels at 0.07 dB. For an optical power detectionresolution of 0.01 dB, the resolution in the refractiveindex measurement with this simple device configu-ration is therefore on the order of 1.7910−4 betweenindex values of 1.333 to 1.407. Evidently, the dy-namic power response range, hence sensitivity, canbe increased by cascading a larger number of micro-channels. The spatial resolution of the femtosecondlaser inscription process can allow high-density pre-cisely positioned microfluidic channels within the fi-ber. It is also important to note that because of theminiature cross-sectional feature size of the micro-channel, the polarization dependent loss of the fiberdevice is relatively insignificant and measured to be0.08 dB. These attributes highlight the potential ofsuch microfluidic fiber devices as ultrasmall index-sensing components or in-fiber attenuators for manyphotonics applications. Furthermore, with the inclu-sion of fiber grating structures, we believe a range ofdevices with enhanced functionalities and perfor-mances can be made possible.In conclusion, the fabrication of microchannelswithin conventional single-mode fibers is demon-strated. Fabrication limitations imposed by the fibergeometry are highlighted and resolved by using asimple glass slip attachment. A microfluidic fiber de-vice consisting of a 4 m wide microchannel for re-fractive index sensing applications is further demon-strated. The results obtained illustrate the potentialattributes of engineered integrated microchannels infibers for tunable photonics and sensing applications.Y. Lai’s e-mail address is laiy@aston.ac.uk.References1. J. B. Jensen, L. H. Pedersen, P. E. Hoiby, L. B. Nielsen,T. P. Hansen, J. R. Folkenberg, J. Riishede, D.Noordegraaf, K. Nielsen, A. Carlsen, and A. Bjarklev,Opt. Lett. 29, 1974 (2004).2. J. Hsieh, P. Mach, F. Cattaneo, S. Yang, T. Krupenkine,K. Baldwin, and J. A. Rogers, IEEE Photon. Technol.Lett. 15, 81 (2003).3. C. Grillet, P. Domachuk, V. Ta’eed, E. Mägi, J. A.Bolger, B. J. Eggleton, L. E. Rodd, and J. Cooper-White, Opt. Express 12, 5440 (2004).4. S. Campopiano, R. Bernini, L. Zeni, and P. M. Sarro,Opt. Lett. 29, 1894 (2004).5. A. Marcinkevicius, S. Juodkazis, M. Watanabe, M.Miwa, S. Matsuo, H. Misawa, and J. Nishii, Opt. Lett.26, 277 (2001).6. Y. Bellouard, A. Said, M. Dugan, and P. Bado, Opt.Express 12, 2120 (2004).

Microchannels in conventional single-mode fibers

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Microchannels in conventional single-mode fibers

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