Bragg gratings have been written in exposed-core microstructured optical fibers for the first time using a femtosecond laser. Second and third order gratings have been written and both show strong reflectivity at 1550 nm, with bandwidths as narrow as 60 pm. Due to the penetration of the guided field outside the fiber the Bragg reflections are sensitive to the external refractive index. As different modes have different sensitivities to refractive index but the same temperature sensitivity the sensor can provide temperature-compensated refractive index measurements. Since these Bragg gratings have been formed by physical ablation, these devices can also be used for high temperature sensing, demonstrated here up to 800°C. The fibers have been spliced to single mode fiber for improved handling and integration with commercial interrogation units.
© 2014 Optical Society of America
There is increasing interest in utilizing optical fiber Bragg gratings (FBG) as refractive index sensors for applications such as biosensing . While other techniques such as surface plasmon resonance (SPR)  or combined SPR and FBG sensors [3, 4] can provide very high sensitivity, advantages of Bragg gratings are that they can be readily wavelength division multiplexed due to their narrow bandwidth  and have intrinsic reflected signals making them ideal for use as a dip-sensor. Gratings can also be designed to operate in the telecommunications wavelength range, giving access to a wide variety of sources and detectors.
For use as a refractive index sensor a portion of the guided field must be made accessible to the external environment. The greater the fraction of the guided field that propagates outside the fiber the greater the shift in Bragg wavelength relative to the external refractive index, that is, the sensitivity . A propagating external field can be accessed by using a variation of conventional step-index fiber known as a D-fiber [6, 7]. However, only limited sensitivity to external refractive index can be achieved. This is because the index contrast on the open sensing side of the fiber is typically large (glass-air or glass-liquid boundary) compared to the core-cladding boundary (germanium doped glass core to pure silica cladding) and thus the propagating mode is shifted away from the sensing region. This results in D-fibers having an external field percentage the order of only 0.1-0.2% .
Micro/nano-fibers are an attractive alternative as the high numerical aperture formed by the core (glass) to cladding (air or liquid) boundary on all sides of the core allows the core diameter to be as small as the wavelength of light. This can produce a significant portion of the guided light being located outside the fiber material, where it is available to interact with its surroundings .
Numerous methods have been demonstrated for creating Bragg gratings in microfibers. By sufficiently reducing the core diameter, sensitivities as large as 660 nm/RIU (1.8 µm diameter) have been demonstrated . One method of fabrication is to take a conventionally written single-mode fiber Bragg grating and then wet-etch until the core is exposed [11–13]. Alternatively, the microfiber can be fabricated first, such as by tapering standard silica fiber, and then the grating can be written into the microfiber. For traditional ultra-violet grating fabrication the starting fiber core needs to be large and highly photosensitive if a small diameter taper is to be used . Alternatively, techniques that do not require a photosensitive glass can be used such as focused ion beam milling , femtosecond writing , and femtosecond laser ablation . As direct write techniques such as focused ion beam milling and femtosecond laser ablation create physical holes or lines the index contrast that forms the grating can be very large. For example, a focused ion beam milled grating as short as 500 µm with 7 dB reflectivity has been reported .
A challenge associated with the use of microfibers is their fragility and handling. An alternative to a pure microfiber is the microstructured optical fiber (MOF). In particular, the suspended-core microstructured optical fiber is essentially a microfiber suspended within a protective jacket that has similar dimensions to conventional fiber [17–22]. There have been a number of demonstrations of writing Bragg gratings in MOFs . For example, an ultra-violet (UV) written Bragg grating in a suspended-core microstructured optical fiber made entirely from photosensitive glass has been demonstrated and could detect a refractive index variation of 3x10−5 refractive index units (RIU) . However, conventional microstructured optical fibers need to be filled from a distal end in order to access the evanescent field. Therefore, if the sensor is to be functionalized as a biosensor, such as for an immunoassay, there must be several filling and emptying steps. While this has been achieved, for example, for fluorescence based assays [25–29], this is typically a time consuming process with significant risk of blocking the fiber holes  and also negates the possibility of real time measurements.
For such reasons, a class of MOF known as exposed-core microstructured optical fibers (EC-MOFs) has been developed. EC-MOFs have a core that is exposed to the external environment on one side along the length of the fiber, while still being held within an outer jacket with traditional fiber dimensions [30–33]. While similar in concept to a D-fiber [6–8, 34–36], the EC-MOF has the advantage that the index contrast on the sensing side of the fiber core can actually be less than the supported (structured) side of the core if the fiber is immersed in a liquid . For liquid immersed EC-MOFs the mode will shift in the direction of the sensing region, allowing for external field percentages well above 10% . Techniques to fabricate EC-MOF preforms include: extrusion of soft glass , drilling and cutting polymer , the molding method for chalcogenide glass , and ultrasonic drilling and cutting of silica glass .
In this paper, we fabricate gratings from silica glass EC-MOFs for the first time using femtosecond laser ablation to create a linear array of nanoholes. We demonstrate refractive index and temperature sensing and outline methods for improving the sensitivity, namely, reducing the core diameter. The fiber has been spliced to single mode fiber to improve handling and to facilitate connection to commercial interrogation units.
2. Sensor fabrication
2.1. Exposed-core microstructured optical fiber fabrication
Fabrication of the exposed-core microstructured optical fibers (EC-MOFs) used in this paper followed the same procedure as detailed previously  and is briefly outlined here. The preform was fabricated by first sonic drilling three holes into the center of a 12 mm F300HQ (Heraeus) silica rod in an equilateral triangle pattern and then cutting a 1.0 mm slot along the length to create the open structure. The preforms were then drawn into fiber using a 6 m tall drawing tower with a graphite resistance furnace and positive internal pressure. Scanning electron microscope (SEM) images of the final fiber geometry are shown in Fig. 1.The fiber has a maximum outer diameter of 200 µm and a core diameter of 12.5 µm, where the core diameter is defined as the diameter of a circle that has the same area as a triangle that fits wholly within the core region.
2.2. Grating fabrication
Bragg gratings were written using an 800 nm femtosecond Ti:sapphire laser (Hurricane, Spectra Physics) system that has previously achieved directly written Bragg gratings in bulk fused silica  and single mode optical fiber . In these cases the femtosecond beam was focused sufficiently below the glass surface to allow index modulation without ablating the glass surface, such as 450 µm deep for the bulk fused silica . For the EC-MOF used here the core diameter is only 12.5 µm and thus the Bragg gratings were instead written by focusing the beam at the glass surface and ablating holes. The laser was pulsed at 200 Hz with a pulse energy of 200 nJ and focused with a long working distance 50X microscope objective. The fiber was translated at a speed calculated to yield either a second or third order Bragg grating at approximately 1550 nm. The pitches of these gratings were 1080 and 1620 nm, respectively. The length of the fiber was approximately 20 cm while the length of the grating was 22 mm, that is, approximately 20,300 points were inscribed for the second order grating and 13,600 points for the third order grating. Particular care was taken to ensure that the fiber was mounted straight as a deviation of more than approximately 2 µm away from the focal plane of the femtosecond laser resulted in insufficient intensity to create the nanoholes. In addition, a drift of this magnitude across the exposed region would shift the grating away from the center of the core. SEM images of the second order grating are shown in Fig. 2.The images show that the hole diameters at the surface of the fiber are approximately 600 nm.
2.3. Splicing to single-mode fiber
To improve the ability to handle the sensor for sensing experiments the EC-MOF with the third order grating was spliced to standard FC/APC connectorized single mode fiber (SMF). This is the first report of splicing to EC-MOF. The splicing was achieved using an Ericson arc splicer (FSU 975). The standard SMF program was used with the exception of a 100 µm offset towards the SMF and manual alignment. Figure 3 shows an SEM image of the EC-MOF spliced to SMF. A splice loss of 3 dB was measured for this fiber at a wavelength of 1550 nm. While Fig. 3 shows that there is some deformation of the EC-MOF at the splice, as is generally expected for splicing to microstructured optical fiber [40, 41], the large air holes help to ensure that guidance in the core is still maintained.
To obtain an upper theoretical limit we evaluated the splice loss for an analytically evaluated mode of the single mode fiber  and its coupling into the numerically evaluated fundamental mode of the exposed-core fiber (Sec. 5) using the coupling efficiency expression in . This assumes vectorial solutions to propagating modes and reflections into the guided modes only. The theoretical splice loss was evaluated to be 0.51 dB, indicating that there is some scope for improving the splice loss.
Splicing the EC-MOF to connectorized SMF means that the fiber sensor can be readily connected to commercial fiber interrogation systems. It also blocks the holes of the fiber at one end while still allowing an optical signal to propagate. The distal end of the fiber was then sealed with ultra-violet (UV) curable glue so that the entire EC-MOF could be immersed in a liquid without the liquid penetrating the internal MOF holes.
3. Sensor characterization
To characterize the Bragg gratings an Optical Sensor Interrogator (OSI, National Instruments PXIe-4844) was used, which sweeps a 1 pm linewidth tunable laser from 1510 to 1590 nm. Note that in these experiments a resolution of 4 pm was used to reduce data volume. For the second order grating the output of the OSI was collimated in free space using a 15 mm lens and then coupled into the EC-MOF using a 7.5 mm lens, the fiber was subsequently used for SEM imaging. The spectra obtained for the second order grating is shown in Figs. 4(a) and 4(b). The full width half maximum (FWHM) bandwidth of the longest wavelength reflection, which results from the forward propagating fundamental mode coupling to the backward propagating fundamental mode, was measured to be 60 pm. Figure 4(a) also shows that there are several additional reflections, which result from various combinations of forward and backward propagating higher order modes. These will be discussed in more detail in Sec. 5.
The third order grating was connected to the OSI via splicing to SMF (Sec. 2.3) and the recorded reflection is shown in Figs. 4(c) and 4(d). Note that the broadband reflection for the third order grating was measured to be substantially smaller than for the second order grating. This is because the spliced connection reflects significantly less than the free space coupling into the EC-MOF. Reducing the reflection for the free space coupled measurement could be improved by angle cleaving the EC-MOF, though this is difficult to achieve reproducibly in practice for this class of fiber. Thus, it can be seen that there is an additional advantage to splicing when considering management of unwanted reflections. Note that there are also more higher order mode Bragg grating reflections visible at shorter wavelengths for the third order grating as they are not hidden by the broadband reflection background.
4. Refractive index and temperature sensing
To measure the refractive index response the spliced third-order grating was held vertically in a Pasteur pipette. This helped to keep the fiber straight; otherwise bending of the fiber was observed to sufficiently shift the grating reflection as to interfere with the refractive index measurements. This occurs due to the asymmetric geometry of the EC-MOF and thus asymmetric compression and expansion of the core upon bending . The pipette was then replaced with pipettes filled with isopropanol solutions whose refractive index had been calculated by interpolating the data in . The reflection was measured using the same system as detailed in Sec. 3. As noted in Sec. 2.3, the holes at the distal end of the fiber were sealed using glue to prevent the ingress of liquid so that the shift in grating reflection results only from the evanescent field interaction at the exposed region of the fiber core. Between different samples the pipette was removed and the fiber was allowed to dry for two minutes. During this time the peaks were observed to return to their original position once the isopropanol solution had evaporated. The shift in the peak positions of the two longest wavelength peaks from Fig. 4(c) are shown in Fig. 5(a).It is seen that the reflected peaks shift to longer wavelengths as the external refractive index increases. The two peaks shift by a different amount due to a different percentage of externally propagating optical field experienced by different propagating modes. The shorter wavelength peak shifts further as this correlates to a smaller effective index, thus a higher order mode and greater evanescent field. Even greater sensitivities can be expected for the shorter wavelength peaks; however, analysis of the data is difficult because the reflections from the different higher order modes overlap.
When considering the potential of this sensor to be used as a refractive index based sensor, such as a biosensor, an important parameter is sensitivity (shift in wavelength per refractive index units). For this particular sensor a sensitivity of 1.1 nm/RIU was measured for the longest wavelength peak. While this value is lower than previously reported smaller-diameter microfiber values, such as 660 nm/RIU , we note that a more important parameter is the bandwidth divided by the sensitivity, as this indicates the minimum difference in refractive index that can be measured. For this sensor, the bandwidth over sensitivity is 0.055 RIU. If it is assumed that it is possible to measure a shift of 10% of the bandwidth of the peak (still greater than the OSI resolution) then the minimum refractive index difference that can be measured is approximately 5.5 x10−3 RIU. As will be discussed in Sec. 5, this sensitivity can be greatly improved by reducing the core diameter of the fiber.
To measure the temperature sensitivity the fiber was placed into a tube furnace and the temperature increased up to 800°C. The positions of the two longest wavelength reflections are shown in Fig. 5(b) and match very closely. It should be noted that the shorter wavelength peaks were also observed to shift at the same rate but are not included here for clarity in the graph.
Figure 5 shows that the different propagating modes have different external refractive index sensitivities but the same temperature sensitivity, thus comparing modes can provide temperature compensated refractive index sensing. However, this has a trade off in terms of sensitivity as error in the position of the two peaks will be combined. The difference in sensitivity between the longest and second longest wavelength reflections is also relatively small, thus improving the sensitivity by reducing the core diameter is important to create a sensitive sensor that can also be temperature compensated.
The position of the peak reflection wavelength, λB, for an m-order Bragg grating with pitch Λ, considering only the sensitivity to the external refractive index, ns, is given by the phase matching condition represented by Eq. (1) .Eq. (1) to yield Eq. (2).
Thus, by determining the effective index of the modes that propagate in the fiber the peak position of the Bragg reflection can be determined. Furthermore, by evaluating the slope of the effective index relative to the external refractive index the sensitivity can be predicted in order to guide fiber and sensor design.
To evaluate the effective indices of the propagating modes the SEM image in Fig. 1 was imported into a commercial finite element method solver (COMSOL 3.4). For simplicity, all results were evaluated at 1557 nm, the position of the longest wavelength reflection. Sample results showed negligible impact on the following results if the wavelength was changed within the range of wavelengths experimentally measured. The fiber was solved firstly for a series of external refractive indices and the results for the first three mode combinations are shown in Fig. 6(a) along with the experimental results from Fig. 5(a). Here a pitch of 1621.52 nm was found to give the best agreement with experimental results, which differs only 0.1% from the target pitch. Figure 6(a) shows that the 1557 nm peak experimentally measured shows good agreement with the theoretical prediction for the forward propagating fundamental mode (HE11) coupling to the backward propagating fundamental mode. Likewise, the 1555 nm measured peak agrees closely with the fundamental mode coupling to both the first order TE and TM modes. Only modes with their polarization parallel to the surface of the fiber core have been included in Fig. 6(a) as these are expected to have a higher reflection strength . Note that as the fiber is highly asymmetric hybrid modes will be non-degenerate and thus lead to splitting of the reflection peaks. An example calculation for an air-clad fiber (i.e. ns = 1) gives a splitting of 12 pm and likely accounts for the slightly asymmetric shape seen in Figs. 4(b) and 4(d). Indeed, highly asymmetric waveguides can be used for temperature compensation if the two polarizations are sufficiently separated .
Matching the theoretical results with the experimental results gives confidence to the model. The model can then be extended to optimize the sensitivity of the fiber, which can be achieved primarily by reducing the core diameter and thus increasing the percentage of optical field propagating externally to the fiber. This has been previously reported for a single-mode fiber , where it was shown that the sensitivity of a Bragg grating microfiber can provide a sensitivity up to a theoretical maximum of approximately 1000 nm/RIU. To evaluate this for the EC-MOF structure for parameters relevant to biological sensing the external refractive index was set to 1.333 and the sensitivity (Eq. (2) was evaluated versus core diameter by scaling the SEM image in Fig. 1(b), noting that the core diameter of the actual fiber used was 12.5 μm. The results are shown in Fig. 6(b) and show that reducing the core diameter increases the sensitivity. For example, a core diameter of 7.1 μm can increase the sensitivity from the current 1.1 nm/RIU to 10 nm/RIU (assuming the HE11 to TE01 reflection) and a core size of 3.3 μm can increase the sensitivity to 100 nm/RIU. Thus, by reducing the core diameter the EC-MOF Bragg grating refractive index sensor can readily achieve a refractive index detection limit as low as 5 x10−5 RIU, making it comparable to other optical biosensors but with the added advantages of multiplexing and dip-sensing capability.
6. Discussion and conclusions
Bragg gratings have been written into an exposed-core microstructured optical fiber by femtosecond laser ablation. A portion of the guided field of the EC-MOF is available to interact with the external environment allowing the grating to be used as a refractive index sensor with a measured sensitivity of 1.1 nm/RIU. The narrow bandwidth of the grating allows for a refractive index detection limit of 5 x10−3 RIU. In order to increase the sensitivity the core diameter can be reduced and modeling shows an expected sensitivity of 100 nm/RIU for a 3.3 µm core diameter fiber. Recently, EC-MOFs with core diameters as small as 1.7 µm have been fabricated . The challenge now is to write gratings in such structures without damaging the fine structure and inducing large losses. Splicing of these fibers is also of interest as it allows easy integration into commercial interrogation systems. The relatively large 12.5 µm core of the fiber used in this work allows the fiber to be spliced directly onto standard single mode fiber with reasonable insertion loss. The use of smaller core fibers will require advances in the splicing technique, such as the use of intermediate fibers to reduce splice loss. This new class of fiber sensor will be a powerful platform for biosensing, enabling immunoassays without many of the practical challenges that come with using the voids within MOFs. The multiplexing capability of Bragg gratings, compared to techniques such as surface plasmon resonance, is particularly useful as it will allow for the simultaneous detection of multiple biomarkers in a dip sensor configuration.
This work was performed in part at the OptoFab node of the Australian National Fabrication Facility utilizing Commonwealth and South Australian State Government funding. The authors acknowledge Ben Johnston from Macquarie University for writing the Bragg gratings and Peter Henry, Roman Kostecki, Heike Ebendorff-Heidepriem, Linh Nguyen, and Erik Schartner from the University of Adelaide for their contribution to the silica fiber fabrication. Stephen Warren-Smith acknowledges the support of an Australian Research Council Super Science Fellowship and Tanya Monro acknowledges the support of an Australian Research Council Georgina Sweet Laureate Fellowship. This work is supported via the Sensing Technologies for Advanced Reproductive Research (STARR) laboratory, supported by the South Australian State Government via the Premier's Science & Research Fund (PSRF) scheme.
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