Writing Bragg Gratings in Multicore Fibers

The overall goal of this procedure is to simultaneously inscribe fiber Bragg gratings with matching characteristics into each core of a multi-core fiber. Well this technique allows efficient inscription of multi-core fiber Bragg gratings, and the main advantage of using this method is that it can be performed with the same inscription equipment as for a single mode fiber. Generally individuals new to this method will struggle because it requires a range of skills to fabricate and use the capillary tubes.

The idea behind this method came from trying to avoid the lensing effect that happens when the light gets into the fiber at right angles. Demonstrating the procedure will be myself, a PHD student, and Doctor Seong-sik Min, a research fellow in the fiber Bragg grating laboratory. In preparation, taper glass capillary tubes, as described in the text protocol.

Then begin by cutting the tapered tubes into approximately equal lengths that are at least two centimeters longer than the intended grating length, but small enough to fit within the polishing equipment. Using a UV curable glue, attach eight to ten of the capillary lengths to a slide, and then attach the slide to a glass puck. Next, install the puck onto a jig compatible with the lapping polishing machine.

Now, put on protective gear, and load the lapping polishing machine with 25 micron grit made of aluminum oxide in reverse osmosis purified water. Set the puck into the machine, and make certain it is absolutely parallel to the grinding surface. Grind the capillary tubes until the thickness of the capillary wall is about 70 microns.

Use a very accurate micrometer to measure the displacement of the jig during the grinding, and hence the amount of wall that has been removed. Once the wall thickness drops to 70 microns, switch to a five micron grit, and grind the wall down to about 50 microns. Now, swap the grit for a high purity colloidal sillica in alkaline dispersion.

Then polish the flattened surface for at least three hours to restore the surface's optical quality. To separate the capillary tubes from the holding puck, soak them overnight in acetone. The next day, examine the capillary tubes at both ends under a microscope with ten times magnification to check the wall thickness.

A good quality capillary will have a uniformly thin wall along its length. Prior to beginning, hydrogenate the MCFs to increase their photo sensitivity. Next, strip the protective coating from the MCFs using a standard SMF fiber stripper.

Remove the coating from where the grating will be written to the end of the fiber. Then, 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. Now put on protective goggles and gently mount the fiber on the moving stage, which holds the phase mask.

Angle the flat side of the capillary tube towards the phase mask. To protect the phase mask, use small pieces of foil to separate it from the capillary. Once positioned, the fiber should be within the masked interference pattern.

Now align a 244 nanometer laser so that the beam is perpendicular to the flat surface of the phase mask. Before writing, make sure that the fiber gets at least 90 milliwatts of laser power. Use a beam splitter to separate out about one percent of the power and measure it.

Now grate the fiber. Expose four centimeters to the UV interference pattern by moving the fiber and phase mask together at 0.25 millimeters per minute with respect to the incoming beam. Once completed, remove the capillary tube from the fiber.

Then anneal the grating at 110 degrees Celcius for 20 hours to stabilize the wavelength response. Using a fiber cleaver, cleave both ends of the fiber. Set the fiber diameter intention to ensure a flat surface.

Then illuminate one end of the fiber using a tunable laser with a central wavelength that is approximately matched to the Bragg wavelength. 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 50 times magnification in front of the camera to ensure that all cores cover multiple CCD pixels.

Using software, select a circular region of pixels corresponding to each core. Click on the centers of cores and enter their diameters in units of pixels in the length or diameter field. Record the pixel values for the selected regions and sum all the pixels covering a given core to quantify the total throughput at that wavelength.

Next connect the tunable laser to a computer to automate the data collection. In the software, enter a wavelength five nanometers below the Bragg wavelength in the Start Wavelength field. Then set the wavelength increment to 0.01 nanometers in the Scan Step field, and set the delay between the steps to at least 300 milliseconds so that the laser is stable at each wavelength.

Now enter a wavelength five naometers above the Bragg wavelength in the End Wavelength field. Then scan three times and take an average value. Click the Automatic Scan button to set the laser to the defined parameters and perform the scan.

Plot the transmitted power versus wavelength for each core to generate a set of spectra. Then 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.

Without a capillary, transmission characteristics were assessed for a seven core fiber exposed using the standard methods. The individual core spectra are represented by different colors. There was minimal overlap between the suppressed wavelengths.

The same characteristics were measured using an identical fiber inside a capillary tube with an inner diameter of 140 microns. In this multi-core fiber, Bragg ratings are much better overlapped. The single misaligned core was located at the center of the fiber, which had a Bragg wavelength 100 picometers shorter than the others.

If only the outer cores are included in the calculation, a suppression greater than 36 decibels is possible. After watching this video, you should have a good understanding of how to modify the procedure for writing single mode fiber Bragg gratings for multi-core fibers. Once the capillaries are fabricated, they can be reused for each new grating.

Following the method, we can produce larger size capillaries to much larger size fibers with several number cores. This will help us to understand why the inner cores behave differently, and indeed, to compensate or correct for this effect.