Name: writing bragg gratings in multicore fibers
Uploaded: 2016-04-20T14:00:00
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 ...

<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>

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 ...

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/files/ftp_upload/53326/53326fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table border="0" cellpadding="0" cellspacing="0" width="1171">
	<tbody>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Multicore fiber</td>
			<td style="width:148px;">Fujikura Ltd.</td>
			<td style="width:163px;"></td>
			<td style="width:645px;">7 cores with diameter 5.5 &#181;m, core separation 35 &#181;m, hexagonally arranged within 125 &#181;m cladding, NA = 0.177</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Glass tapering machine</td>
			<td style="width:148px;">Vytran</td>
			<td style="width:163px;">GPX-3000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">UV laser</td>
			<td style="width:148px;">Coherent</td>
			<td style="width:163px;">300 FreD Innova</td>
			<td>Frequency doubled 244 nm, at least 150 mW output. CAUTION: eye damage; wear appropriate goggles</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Phase mask</td>
			<td style="width:148px;">Lasiris</td>
			<td style="width:163px;">PM-244-1069.50-50.8</td>
			<td>Custom component, 1069.50 nm grating period, 5.08 mm thickness</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Capillary tubes</td>
			<td style="width:148px;">Polymicro</td>
			<td>TSP200794</td>
			<td>Inner diameter 200 &#181;m, outer diameter 794 &#181;m</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Lapping machine</td>
			<td style="width:148px;">Logitech</td>
			<td style="width:163px;">PM5</td>
			<td>Combination grinder/polisher</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">UV-curable glue</td>
			<td style="width:148px;">Norland</td>
			<td style="width:163px;">NOA-61</td>
			<td>Cures rapidly, removable with acetone</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Microgrit</td>
			<td style="width:148px;">Eminess</td>
			<td style="width:163px;"></td>
			<td>Al2O3: 25 &#181;m and 5 &#181;m particle size</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Polishing fluid</td>
			<td style="width:148px;">Eminess</td>
			<td style="width:163px;">ULTRA-SOL 500S</td>
			<td>SF-500S-5, ULTRA-SOL 500S N/D, 5 GAL</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium hydroxide</td>
			<td style="width:148px;"></td>
			<td style="width:163px;"></td>
			<td>0.004M</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Fiber cleaver</td>
			<td style="width:148px;">Vytran</td>
			<td style="width:163px;">LDC-400</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Tunable laser</td>
			<td style="width:148px;">JDS Uniphase</td>
			<td style="width:163px;">SWS15101</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">IR Camera</td>
			<td style="width:148px;">Xenics</td>
			<td style="width:163px;">XEVA-1429</td>
			<td>320x256 pixel, 16 bit resolution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Oven</td>
			<td style="width:148px;">Thermoline Scientific</td>
			<td style="width:163px;">LDO-030N</td>
			<td>For annealing at T=110&#176;C</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Hydrogen gas</td>
			<td style="width:148px;">BOC</td>
			<td style="width:163px;"></td>
			<td>For hydrogenating fiber. CAUTION: flammable, pressurised gas</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Nitrogen gas</td>
			<td style="width:148px;">BOC</td>
			<td style="width:163px;"></td>
			<td>Booster for hydrogenation. CAUTION: pressurised gas</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Acetone</td>
			<td style="width:148px;"></td>
			<td style="width:163px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Razor blades</td>
			<td style="width:148px;"></td>
			<td style="width:163px;"></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. ...

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 ...

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