Name: writing bragg gratings in multicore fibers
Uploaded: 2016-04-20T14:00:00.000+00:00
Duration: 8 min 48 s
Description: Watch this Scientific Journal Video about Writing Bragg Gratings in Multicore Fibers at JoVE.com

The SAIL labs are funded through JBH's Australian Laureate Fellowship from the Australian Research Council.
This research was supported by the Australian Research Council Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems (project number CE110001018).
Experimental work was performed in part at the OptoFab node of the Australian National Fabrication Facility, utilizing NCRIS and NSW state government funding. Fiber hydrogenation services were provided by TE Connectivity in Redfern. SLS would like to acknowledge the Optics and Electronics Laboratory, Fujikura Ltd, Japan for providing the 7-core fiber used in this experiment.

1. Preparation of Polished Capillary Tubes (ANFF OptoFab)
<ol>
	<li>Obtain glass capillary tubes with inner diameter closely matched to fiber diameter. The closer in size, the better the performance, but ensure that a &#177;10 &#181;m variation in capillary size is allowed for. Remove any protective coatings from the capillary tubes. Shave off coatings with a razor blade to remove them without damaging the tubes.</li>
	<li>Taper the capillary tubes to a smaller diameter if required. Use a computer-controlled automatic tapering machine if available.
	<ol>
		<li>Secure a length of tubing with clamps at either end.</li>
		<li>Heat the capillary to melting point evenly around its diameter using a hot filament located between the clamps.</li>
		<li>Draw the capillary through the heating element with a constant tension until the desired length has been tapered to the smaller diameter.</li>
	</ol>
	</li>
	<li>Cut the capillary tube into approximately equal lengths with a glass cutting tool. Ensure that these are at least 2 cm longer than the intended grating length but small enough to fit within the polishing equipment used. Note: For this experiment, a capillary length of 7 cm was used.</li>
	<li>Attach 8-10 of the capillary lengths to a glass puck using UV-curable glue. Install the puck onto a jig compatible with the lapping/polishing machine.</li>
	<li>Use the lapping/polishing machine to grind the exposed walls of the capillary tubes to a flat surface. CAUTION: Do not inhale loose grit. Note: The abrasive element is Al2O3 in a suspension of reverse-osmosis purified water.
	<ol>
		<li>Use 25 &#181;m grit until the remaining thickness of the capillary wall is approximately 70 &#181;m. Use a micrometer to measure the displacement of the jig during grinding and hence the amount that has been removed.</li>
		<li>Switch to 5 &#181;m grit and grind until the remaining thickness of the wall is approximately 50 &#181;m.</li>
	</ol>
	</li>
	<li>Use the lapping/polishing machine to polish the flattened surface for at least 3 hr with high-purity colloidal silica in alkaline dispersion (NaOH). Note: This restores the surface to optical quality. The silica can be prevented from solidifying by adding 1 part 0.004 M NaOH to 3 parts polishing solution.</li>
	<li>Separate the capillary tubes from the holding puck by soaking overnight in acetone.</li>
	<li>Examine the capillary tubes at both ends under a microscope with 10X&#160;magnification to check the wall thickness. Note: A good quality capillary will have a uniform thin (~ 50 &#181;m) wall along its length.</li>
</ol>
2. Creation of Gratings
<ol>
	<li>Hydrogenate the MCF to increase photosensitivity.
	<ol>
		<li>Place the fibers to be hydrogenated into a sealed airtight chamber. CAUTION: Ensure the chamber is securely bolted due to the presence of pressurized gases.</li>
		<li>Pump high purity H2 into the chamber. Use N2 gas as a booster to increase pressure.</li>
		<li>Leave the fibers inside the chamber for an extended period: 2 weeks at 300 bar and room temperature, or 3 days at 380 bar and 80 &#176;C.</li>
		<li>Vent the gases from the chamber and remove the fibers. CAUTION: Ensure the room is well ventilated. Gases may act as asphyxiants, or fire risk in the case of H2.</li>
		<li>Keep the fibers in a freezer with temperature -70 &#176;C or below until they are used. This slows the rate of hydrogen outgassing and preserves the increased photosensitivity.</li>
	</ol>
	</li>
	<li>Strip the protective coating from the MCF. Strip MCFs the same size as SMFs with a standard SMF fiber stripper; otherwise shave off the coating with a razor blade. Remove the coating from the region where the grating is to be written, all the way to the end of the fiber.</li>
	<li>Insert the stripped end of the fiber into the capillary tube, and slide the tube along the fiber so that it covers the region to be inscribed.</li>
	<li>Put on UV-protective eyewear. Mount the fiber on the moving stage which holds the phase mask, with the flat side of the capillary tube angled towards the phase mask. Ensure the fiber is positioned within the interference pattern created by the mask, but not touching the mask itself as this may cause damage.</li>
	<li>Align the 244 nm laser so that the beam is perpendicular to the flat surface of the phase mask. Ensure that the fiber receives at least 90 mW of laser power.</li>
	<li>Expose a 4 cm length of the fiber to the UV interference pattern by moving the fiber and phase mask together with respect to the incoming beam at a rate of 0.25 mm/min.</li>
	<li>Remove the capillary tube from the fiber.</li>
	<li>Anneal the grating at 110 &#176;C for 20 hr to stabilize the wavelength response. Note: This step is optional as the grating will stabilize by itself over the course of approximately three days, but annealing makes the process faster.</li>
</ol>
3. Analysis of Spectra
<ol>
	<li>Cleave both ends of the fiber. Use a fiber cleaver that allows the user to set both fiber diameter and tension to ensure a flat end surface.</li>
	<li>Illuminate one end of the fiber using a tunable laser with a central wavelength approximately matched to the Bragg wavelength.</li>
	<li>Connect a CCD camera to a PC with control software to display and record the fiber output. Image the fiber output with the CCD camera, using a microscope objective lens with 50X magnification in front of the camera to ensure that all cores cover multiple CCD pixels. Note: The following steps 3.4.1 &#8211; 3.5.5 are specific to the custom software used by the authors and represent only one method of capturing spectra.
	<ol>
		<li>Select a circular region of pixels corresponding to each core by clicking on the centers of cores as they appear in the image in the control software. Enter the diameter of the cores in units of pixels in the &#39;Length or Diameter&#39; field.</li>
		<li>Record the pixel values registered by the camera for the selected regions. Sum the values for all pixels covering a given core to quantify the total throughput at that wavelength.</li>
	</ol>
	</li>
	<li>Connect the tunable laser to the control PC so that observations and data collection can be automated.
	<ol>
		<li>Enter a wavelength approximately 5 nm below the Bragg wavelength in the &#39;Start Wavelength&#39; field.</li>
		<li>Set the wavelength increment of the laser to 0.01 nm in the &#39;Scan - Step&#39; field. Note: Set the delay between steps to at least 300 msec so that lasing is stable at each wavelength before measurements are recorded and the next wavelength step occurs.</li>
		<li>Enter a wavelength approximately 5 nm greater than the Bragg wavelength in the &#39;End Wavelength&#39; field.</li>
		<li>Click the &#39;Automatic Scan&#39; button to set the laser to the defined Start Wavelength and increase the wavelength by the chosen increment at regular time intervals.</li>
		<li>Record the intensity transmitted through each core for each wavelength step. Export the calculated values to a text file by enabling the &#39;Text File Save&#39; option.</li>
	</ol>
	</li>
	<li>Repeat the scan at least 3 times and average the data from all runs.</li>
	<li>Plot transmitted power versus wavelength for each core to generate a set of spectra.
	<ol>
		<li>Compare the spectra of all cores to confirm whether they have the same suppression characteristics. Check that the central wavelength, depth and bandwidth of each grating match.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Othonos, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fiber+Bragg+gratings.">Fiber Bragg gratings.</a> Rev. Sci. Instrum. 68 (12), 4309-4341 (1997).</li><li>Bland-Hawthorn, J., Englund, M., Edvell, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+approach+to+atmospheric+OH+suppression+using+an+aperiodic+fiber+Bragg+grating.">New approach to atmospheric OH suppression using an aperiodic fiber Bragg grating.</a> Opt. Express. 12 (24), 5902-5909 (2004).</li><li>Jovanovic, N., Guyon, O., Martinache, F., Schwab, C., Cvetojevic, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+to+inject+light+efficiently+into+single-mode+fibers.">How to inject light efficiently into single-mode fibers.</a> Proc. SPIE. 9147, 91477P (2014).</li><li>Bland-Hawthorn, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+complex+multi-notch+astronomical+filter+to+suppress+the+bright+infrared+sky.">A complex multi-notch astronomical filter to suppress the bright infrared sky.</a> Nat. Comm. 2, 581 (2011).</li><li>Leon-Saval, S. G., Argyros, A., Bland-Hawthorn, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photonic+lanterns.">Photonic lanterns.</a> Opt. Express. 18 (8), 8430-8439 (2010).</li><li>Trinh, C. Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=GNOSIS:+the+first+instrument+to+use+fiber+Bragg+gratings+for+OH+suppression.">GNOSIS: the first instrument to use fiber Bragg gratings for OH suppression.</a> Astron. J. 145 (2), 51 (2013).</li><li>Content, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=PRAXIS:+low+thermal+emission+high+efficiency+OH+suppressed+fiber+spectrograph.">PRAXIS: low thermal emission high efficiency OH suppressed fiber spectrograph.</a> Proc. SPIE. 9151, 91514W (2014).</li><li>Birks, T. A., Mangan, B. J., D&#236;ez, A., Cruz, J. L., Murphy, D. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photonic+lantern'+spectral+filters+in+multi-core+fiber.">Photonic lantern' spectral filters in multi-core fiber.</a> Opt. Express. 20 (13), 13996-14008 (2012).</li><li>Askins, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inscription+of+fiber+Bragg+gratings+in+multicore+fiber.">Inscription of fiber Bragg gratings in multicore fiber.</a> BGPP. , JWA39 (2007).</li><li>Lindley, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Core-to-core+uniformity+improvement+in+multicore+fiber+Bragg+gratings.">Core-to-core uniformity improvement in multicore fiber Bragg gratings.</a> Proc. SPIE. 9151, 91515F (2014).</li><li>Lindley, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Demonstration+of+uniform+multicore+fiber+Bragg+gratings.">Demonstration of uniform multicore fiber Bragg gratings.</a> Opt. Express. 22 (25), 31575-31581 (2014).</li><li>Tomaru, S., Yasu, M., Kawachi, M., Edahiro, T. V. A. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=VAD+single-mode+fiber+with+0.2+dB/km+loss.">VAD single-mode fiber with 0.2 dB/km loss.</a> Elec. Lett. 17 (2), 93-93 (1981).</li></ol>

The authors declare that they have no competing financial interests.

Figures 2 and 3 together show that introducing the polished capillary tube (PCT) when writing gratings is sufficient to improve the uniformity of core spectra in the MCFBG. The rest of the inscription process is largely unchanged from established methods for creating SMF gratings and can be used with most existing FBG writing systems. Hence the preparation of PCTs as outlined in section 2 of the protocol is most critical for improving MCFBG uniformity. The best results are achieved with tubes where the polished wall has a consistent, small thickness; the 50 &#181;m thickness chosen here gives a compromise between maintaining the strength of the glass and minimizing the distance between the fiber and phase mask.
However, even with the PCT there is an additional effect that causes the middle core of the MCF to have a different wavelength response to the other cores. We used a different hydrogenation regime as mentioned in Step 3.1 of the Protocol section to investigate whether the variation was caused by a lower hydrogen uptake in this core, but no improvement was observed. The variation also cannot be explained by cores casting shadows on each other during UV exposure, as this would result in the outer 6 cores also having poorly matched responses. Instead the behavior can be explained by the central core having different optical properties to the others, despite being identical when manufactured.
MCFBGs cannot be used as effective replacements for their SMF counterparts unless all cores within a single fiber have the same transmission spectrum. We intend to experiment with secondary corrections to existing MCFs made with the PCT, using the effects of thermal and mechanical strain to shift the Bragg wavelengths of the outer cores to match the center. The experiment described in this article will also be repeated for larger core numbers to determine the extent to which shadowing and radial Bragg wavelength variation effects scale with the number of &#39;rings&#39; of cores.
The technique is currently limited in effectiveness for the case of very large fibers or high core numbers. In the former scenario, the cores in a large fiber which are furthest from the incoming beam are not exposed to the interference pattern. This is because we use a Mach-Zender interferometer in these experiments which limits the maximum write depth; this effect occurs because the interference pattern extends only a few hundred microns beyond the phase mask. We intend to address this in future experiments with a redesigned Sagnac interferometer, which will have a depth of field at least twice that of the current equipment. In the second situation where the total core number is large, some cores may be positioned within shadows cast by cores closer to the phase mask. The effect of this on MCFBG quality is not yet known; we will investigate this with 19-, 37-, and 55-core fibers using the method described above.
These experiments have shown that minimal, inexpensive changes to the grating writing procedure can extend its applicability beyond SMFs. Once MCFBGs can be created with filtering capabilities equal to existing SMF technology, they can be employed in any application of photonics, allowing construction of compact and robust devices without sacrificing performance. As outlined in the introduction, the authors&#39; primary goal is to incorporate MCFBGs into new astronomical instruments; however, they can potentially be employed in any system that already makes use of single-mode optics and/or accurate wavelength filtering. Like their single-mode counterparts, MCFBGs can be used in transmission and reflection depending on the application.

Fiber Bragg gratings (FBGs) are widely used as narrowband filters due to the fact they can be customized for a large number of applications1. They are not limited to suppressing single wavelengths; complex transmission spectra can be created by the use of aperiodic refractive index variations2. One limitation is that FBGs can only be inscribed in single-mode fibers (SMFs), as the wavelength that is suppressed for a given grating period depends on the propagation constant. In a multimode fiber (MMF), where each mode has a different propagation constant, the suppressed wavelength for each mode is different and hence the grating does not give strong suppression at any single wavelength.
The impetus for this experiment comes from astronomy. Under seeing-limited conditions, direct coupling into an SMF is difficult and inefficient; extreme adaptive optics are required to do so3. Because of this, MMFs are typically used when collecting light from the telescope focal plane4. Therefore in order to keep the functionality available only to SMFs, it is necessary to have efficient conversion between SMFs and MMFs. This is made possible with the photonic lantern, a device which consists of a multimode port connected to an array of SMFs via a taper transition5. Photonic lanterns were used in the GNOSIS instrument, in which the SMFs contained FBGs to remove atmospheric emission lines (caused by OH radicals and other molecules) from near-infrared observations6. The drawbacks of using individual, single-core SMFs for this task are that they must be written one by one and spliced individually into the optical train, requiring significant time and manual effort. The technique described in this article attempts to address these shortcomings using a more complex fiber format to provide the single-mode functionality.
The next generation OH suppression instrument PRAXIS7 will make use of multi-core fibers (MCFs). These fibers contain any number of single-moded cores embedded in a single cladding. The advantage of this approach is that the MCF can be tapered into an MMF with the resulting photonic lantern being a compact and robust self-contained unit. In the completed instrument, light from the telescope will be coupled into the MMF port of the lantern; the taper transition will separate this light into the single-mode cores where it will pass through the FBGs. After the wavelength filtering the remaining light is dispersed onto a detector, the spectra collected.
Using MCFs also speeds up the process of writing gratings, as all cores can be inscribed in a single pass. However, the writing process must be modified in order to ensure that all cores have the same reflection characteristics. This is because the curved surface of the cladding acts as a lens during side-writing of the FBGs, resulting in a UV field which varies in power and direction at each core if the standard side-writing method is used. Hence each core will have a different transmission profile, and the fiber will not provide strong suppression at a single wavelength8.
A group at the Naval Research Laboratory experimented with modifying the distribution and photosensitivity of cores to cancel the effects of this variation9. The downside of using such an approach is that the fiber must be redesigned for every combination of cladding size, core size, number of cores and chemical composition. In addition, the lack of axial symmetry in the resulting designs means that the MCF cannot be effectively tapered into an MMF with a circular core. This paper details a different approach to the problem: modifying the field within the fiber by having it pass through a flat surface instead of being directly incident on the curved cladding. Using this approach results in a technique which is transferable to a variety of MCF designs and sizes, particularly the axially symmetric fibers which we wish to incorporate into photonic lanterns.
To create the necessary flat surface, the MCF is placed inside a UV-transparent capillary tube which has been ground and polished on one side to give a flat outer wall. A small gap must be left between the fiber and capillary, since the latter may contain &#177;10 &#181;m variations in diameter. See Figure 1 for a representation. This paper will describe the experimental procedure to write FBGs in this manner and provide examples of the possible improvements. For more information see previously published simulations10 and experimental results11.
<img alt="Figure 1" src="/files/ftp_upload/53326/53326fig1.jpg" /> 
Figure 1.&#160;Diagram of polished capillary tube as used in FBG production. The MCF is placed inside the capillary tube. The gap between the two should be small but allow for small variations in diameter. The UV light which has passed through the phase mask then enters the system through the flat side of the capillary tube. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53326/53326fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>Multicore fiber</td><td>Fujikura Ltd.</td><td></td><td>7 cores with diameter 5.5&amp;nbsp;&amp;micro;m, core separation 35&amp;nbsp;&amp;micro;m, hexagonally arranged within 125 &amp;micro;m cladding, NA = 0.177</td></tr><tr><td>Glass tapering machine</td><td>Vytran</td><td>GPX-3000</td><td></td></tr><tr><td>UV laser</td><td>Coherent</td><td>300 FreD Innova</td><td>Frequency doubled 244&amp;nbsp;nm, at least 150 mW output. CAUTION: eye damage; wear appropriate goggles</td></tr><tr><td>Phase mask</td><td>Lasiris</td><td>PM-244-1069.50-50.8</td><td>Custom component, 1069.50 nm grating period, 5.08 mm thickness</td></tr><tr><td>Capillary tubes</td><td>Polymicro</td><td>TSP200794</td><td>Inner diameter 200 &amp;micro;m, outer diameter 794 &amp;micro;m</td></tr><tr><td>Lapping machine</td><td>Logitech</td><td>PM5</td><td>Combination grinder/polisher</td></tr><tr><td>UV-curable glue</td><td>Norland</td><td>NOA-61</td><td>Cures rapidly, removable with acetone</td></tr><tr><td>Microgrit</td><td>Eminess</td><td></td><td>Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;: 25 &amp;micro;m and 5 &amp;micro;m particle size</td></tr><tr><td>Polishing fluid</td><td>Eminess</td><td>ULTRA-SOL 500S</td><td>SF-500S-5, ULTRA-SOL 500S N/D, 5 GAL</td></tr><tr><td>Sodium hydroxide</td><td></td><td></td><td>0.004&amp;nbsp;M</td></tr><tr><td>Fiber cleaver</td><td>Vytran</td><td>LDC-400</td><td></td></tr><tr><td>Tunable laser</td><td>JDS Uniphase</td><td>SWS15101</td><td></td></tr><tr><td>IR Camera</td><td>Xenics</td><td>XEVA-1429</td><td>320x256 pixel, 16 bit resolution</td></tr><tr><td>Oven</td><td>Thermoline Scientific</td><td>LDO-030N</td><td>For annealing at T = 110&amp;nbsp;&amp;deg;C</td></tr><tr><td>Hydrogen gas</td><td>BOC</td><td></td><td>For hydrogenating fiber. CAUTION: flammable, pressurised gas</td></tr><tr><td>Nitrogen gas</td><td>BOC</td><td></td><td>Booster for hydrogenation. CAUTION: pressurised gas</td></tr><tr><td>Acetone</td><td></td><td></td><td></td></tr><tr><td>Razor blades</td><td></td><td></td><td></td></tr></tbody></table>

writing bragg gratings in multicore fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

The effectiveness of this technique is best demonstrated by comparing the multicore fiber Bragg gratings (MCFBGs) that result from exposure with and without the capillary. Figure 2 shows the transmission characteristics of a 7-core MCF exposed using the standard method for SMFs, with individual core spectra represented by different colors. There is minimal overlap between the suppressed wavelengths, and core #5 has received weaker exposure resulting in a shallower notch. Both effects are due to variations in power within the fiber during the writing process. Note that the flat cutoff at -36 dB is due to the limited dynamic range of the camera; all transmission values are scaled relative to this minimum.
<img alt="Figure 2" src="/files/ftp_upload/53326/53326fig2.jpg" /> 
Figure 2.&#160;Performance of MCFBG cores with no compensation for lensing. This plot shows the transmission spectra of individual cores when the MCFBG is produced using the standard method for SMFs. There is minimal overlap between notches. (Inset) Diagram of core numbering. Adapted from previous publication11. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53326/53326fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In Figure 3, the same data is shown for an identical fiber which was exposed inside a capillary tube with inner diameter 140 &#181;m. (Note that the Bragg wavelengths are approximately 2 nm lower than in the previous case as this grating was annealed before measurement. The variation between cores is maintained before and after annealing.) In this MCFBG, 6 out of 7 cores have well aligned notches, with an overlap centered at 1548.25 &#177; 0.01 nm. The misaligned core, which is located at the center of the fiber, has a Bragg wavelength 100 pm shorter than the others. The effect of having this mismatched core is to limit the fiber&#39;s total suppression to -8.5 dB; in other words, 1/7th of the light at 1548.25 nm can pass freely through the MCFBG. If only the outer cores are included in the calculation (i.e., core #1 is blocked or otherwise not illuminated), a maximum suppression of &#62;36 dB is possible. These results are represented graphically in Figure 4.
<img alt="Figure 3" src="/files/ftp_upload/53326/53326fig3.jpg" /> 
Figure 3.&#160;Performance of MCFBG cores with polished capillary tube. Transmission profiles of all gratings in the 7-core fiber with the capillary tube used to compensate for lensing. The wavelengths of reflection of the outer six cores overlap centered at 1548.25 &#177; 0 0.01 nm. The grating response of Core #1, which is located in the center of the fiber, is offset towards shorter wavelengths. (Inset) Diagram of core numbering. Adapted from previous publication11. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53326/53326fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53326/53326fig4.jpg" /> 
Figure 4.&#160;Overall performance of MCFBG. Comparison of overall fiber performance with (blue) and without (green) the center core included. Adapted from previous publication11. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53326/53326fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Writing Bragg Gratings in Multicore Fibers at JoVE.com

Writing Bragg Gratings in Multicore Fibers

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

writing-bragg-gratings-in-multicore-fibers

Nanyang Technological University

Research

JoVE Journal

Engineering

8.1K Views.  University of Sydney. We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Video: Writing Bragg Gratings in Multicore Fibers

Fabrication and Characterization of Disordered Polymer Optical Fibers for Transverse Anderson Localization of Light

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed polymer optical fiber is composed of 80,000 strands of poly (methyl methacrylate) (PMMA) and polystyrene (PS) that are randomly mixed and drawn into a square cross section optical fiber with a side width of 250 &mu;m. Initially, each strand is 200 &mu;m in diameter and 8-inches long. During the mixing process of the original fiber strands, the fibers cross over each other; however, a large draw ratio guarantees that the refractive index profile is invariant along the length of the fiber for several tens of centimeters. The large refractive index difference of 0.1 between the disordered sites results in a small localized beam radius that is comparable to the beam radius of conventional optical fibers. The input light is launched from a standard single mode optical fiber using the butt-coupling method and the near-field output beam from the disordered fiber is imaged using a 40X objective and a CCD camera. The output beam diameter agrees well with the expected results from the numerical simulations. The disordered optical fiber presented in this work is the first device-level implementation of 2D Anderson localization, and can potentially be used for image transport and short-haul optical communication systems.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. This microstructured fiber can transport a small localized beam with a radius that is comparable to the beam radius of conventional optical fibers.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed ...

Implementation of a Reference Interferometer for Nanodetection

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing its free spectral range (FSR), the module is put in parallel with a fiber taper-microcavity system and then calibrated through isolating and eliminating random shifts in the laser frequency (i.e. laser jitter noise). To realize the taper-microcavity junction and to maximize the optical power that is transferred to the resonator, a single-mode optical fiber waveguide is pulled. Solutions containing polystyrene nanobeads are then prepared and flown to the microcavity in order to demonstrate the system&#8217;s ability to sense binding to the surface of the microcavity. Data is post-processed via adaptive curve fitting, which allows for high-resolution measurements of the quality factor as well as the plotting of time-dependent parameters, such as resonant wavelength and split frequency shifts. By carefully inspecting steps in the time-domain response and shifting in the frequency-domain response, this instrument can quantify discrete binding events.

A reference interferometer technique, which is designed to remove undesirable laser jitter noise for nanodetection, is utilized for probing an ultra-high quality factor microcavity. Instructions for assembly, setup, and data acquisition are provided, alongside the measurement process for specifying the cavity quality factor.

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

A Random-displacement Measurement by Combining a Magnetic Scale and Two Fiber Bragg Gratings

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed and characterized a temperature-independent fiber Bragg grating (FBG)-based random-displacement sensor that adopts a magnetic scale as a novel transferring mechanism. By detecting shifts of two FBG center wavelengths, a full-range measurement can be obtained with a magnetic scale. For identification of the clockwise and counterclockwise rotation direction of the motor (in fact, the direction of movement of the object to be tested), there is a sinusoidal relationship between the displacement and the center wavelength shift of the FBG; as the anticlockwise rotation alternates, the center wavelength shift of the second FBG detector shows a leading phase difference of around 90&#176; (+90&#176;). As the clockwise rotation alternates, the center wavelength shift of the second FBG displays a lagging phase difference of around 90&#176; (-90&#176;). At the same time, the two FBG-based sensors are temperature independent. If there is some need for a remote monitor without any electromagnetic interference, this striking approach makes them a useful tool for determining the random displacement. This methodology is appropriate for industrial production. As the structure of the whole system is relatively simple, this displacement sensor can be used in commercial production. In addition to it being a displacement sensor, it can be used to measure other parameters, such as velocity and acceleration.

A protocol to create a full-range linear displacement sensor, combining two packaged fiber Bragg grating detectors with a magnetic scale, is presented.

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed ...

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

This work reports an innovative silicon-tipped fiber-optic sensing platform (Si-FOSP) for high-resolution and fast-response measurement of a variety of physical parameters, such as temperature, flow, and radiation. Applications of this Si-FOSP span from oceanographic research, mechanical industry, to fusion energy research.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This ...

Optimized Sealing Process and Real-Time Monitoring of Glass-to-Metal Seal Structures

Residual stress is an essential factor to keeping the hermeticity and robustness of a glass-to-metal seal structure. The purpose of this report is to demonstrate a novel protocol to characterize and measure residual stress in a glass-to-metal seal structure without destroying the insulation and hermeticity of sealing materials. In this research, a femto-laser inscribed fiber Bragg grating sensor is used. The glass-to-metal seal structure that is measured consists of a metal shell, sealing glass, and Kovar conductor. To make the measurements worthwhile, the specific heat treatment of metal-to-glass seal (MTGS) structure is explored to obtain the model with best hermeticity. Then, the FBG sensor is embedded into the path of sealing glass and becomes well-fused with the glass as the temperature cools to RT. The Bragg wavelength of FBG shifts with the residual stress generated in sealing the glass. To calculate the residual stress, the relationship between Bragg wavelength shift and strain is applied, and the finite element method is also used to make the results reliable. The online monitoring experiments of residual stress in sealing glass are carried out at different loads, such as high temperature and high pressure, to broaden functions of this protocol in harsh environments.

Key procedures to optimize the sealing process and achieve real-time monitoring of the metal-to-glass seal (MTGS) structure are described in detail. The embedded fiber Bragg grating (FBG) sensor is designed to achieve online monitoring of temperature and high-level residual stress in the MTGS with simultaneous environmental pressure monitoring.

Residual stress is an essential factor to keeping the hermeticity and robustness of a glass-to-metal seal structure. The purpose of this report is to ...

Optimizing Brillouin Optical Time-Domain Analyzers Based on Gain Spectrum Engineering

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is superimposed with two symmetric Brillouin loss spectra (BLS). This leads to a complex engineered spectrum shape that is more resistant to the sensing system noise. Instead of only one pump and probe interaction as in the conventional BOTDA setup, three optical probe waves are exploited, with one probe located in the BGS and the other two symmetrically in the BLS. Due to the resistance and insensitivity of the engineered spectrum shape to the noise, the sensing performance is enhanced by 60% and the measurand resolution is doubled.

A protocol for Brillouin optical time-domain analyzers based on gain spectrum engineering is presented. Enhancements in the sensing performance, including sensing range and measurand resolution are achieved and the excess Brillouin intensity noise is studied. The protocol introduces a new way to enhance distributed Brillouin sensing performance.

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is ...

Preparation of Extracellular Matrix Protein Fibers for Brillouin Spectroscopy

Brillouin spectroscopy is an emerging technique in the biomedical field. It probes the mechanical properties of a sample through the interaction of visible light with thermally induced acoustic waves or phonons propagating at a speed of a few km/sec. Information on the elasticity and structure of the material is obtained in a nondestructive contactless manner, hence opening the way to in vivo applications and potential diagnosis of pathology. This work describes the application of Brillouin spectroscopy to the study of biomechanics in elastin and trypsin-digested type I collagen fibers of the extracellular matrix. Fibrous proteins of the extracellular matrix are the building blocks of biological tissues and investigating their mechanical and physical behavior is key to establishing structure-function relationships in normal tissues and the changes which occur in disease. The procedures of sample preparation followed by measurement of Brillouin spectra using a reflective substrate are presented together with details of the optical system and methods of spectral data analysis.

We present a protocol for the application of Brillouin light scattering spectroscopy to elastin and trypsin-purified type I collagen fibers of the extracellular matrix to extract their full elastic properties.

Brillouin spectroscopy is an emerging technique in the biomedical field. It probes the mechanical properties of a sample through the interaction of ...

Bioengineering

Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the light distortion during the propagation within the fiber. Our methodology is based on digital optical phase conjugation employing only a single spatial light modulator, where the optical wavefront is individually modulated at different regions of the modulator, one region per light signal. Digital optical phase conjugation approaches are considered to be faster than other wavefront shaping approaches, where (for example) a complete determination of the wave propagation behavior of the fiber is performed. In contrast, the presented approach is time-efficient since it only requires one calibration per light signal. The proposed method is potentially appropriate for spatial division multiplexing in communications engineering. Further application fields are endoscopic light delivery in biophotonics, especially in optogenetics, where single cells in biological tissue have to be selectively illuminated with high spatial and temporal resolution.

We demonstrate the transmission of multiple independent signals through a multimode fiber using wavefront shaping employing a single spatial light modulator. By modulating the wavefront for each signal individually, spatially separated foci are transmitted. Potential applications are multiplexed data transfer in communications engineering and endoscopic light delivery in biophotonics.

The transmission of multiple independent optical signals through a multimode fiber is accomplished using wavefront shaping in order to compensate for the ...

Writing Bragg Gratings in Multicore Fibers

Samenvatting

Meer video's verkennen

Hoofdstukken in deze video