The overall goal of this procedure is to fabricate and characterize a disordered optical fiber for transverse Anderson localization of light. This is accomplished by first mixing strands of polymethyl, methacrylate, and polystyrene, and making a preform for the draw process. The second step is to image the refractive index profile of the drawn fiber.
Once the PMMA fibers have been dissolved using scanning electron microscopy. Next, the samples of a drawn fiber are cleaved and polished for the optical characterization of Anderson localization. The final step is to test the samples in the optical setup by illuminating the disordered fiber with helium neon laser light.
Ultimately, the output intensity profile of the localized beam is imaged on a CCD camera beam profiler. To show the transverse Anderson localization of light Begin by spreading about 200 PMMA strands on a table, and then spreading the same number of polystyrene strands on top of the PMMA. Mix the strands together and then place them to one side.
Repeat this procedure until 40, 000 strands of PMMA are randomly mixed with 40, 000 strands of polystyrene. Then assemble the randomly mixed strands into a square, preform with sides of about 2.5 inches. Next, have the preform drawn into an optical fiber with a diameter of 250 microns by a company that specializes in this type of procedure.
This represents roughly a 64, 000 times reduction in cross-sectional area of the original fibers. Then submerge the polymer optical fiber in liquid nitrogen for about 10 minutes, so a clean fracture can be made. The liquid nitrogen quickly cools the fiber and makes it very brittle as the fibers cool.
Prepare a bath of ethyl alcohol and warm it to 65 degrees Celsius. After 10 minutes, remove the fibers from the liquid nitrogen and quickly break them in half. Break at least 20 fibers for imaging as a success rate for obtaining a smooth surface is still only about 15%Using this method then submerge the broken tips of the samples in 70%ethyl alcohol for about three minutes.
During this time, the ethyl alcohol dissolves the PMMA sites in the fiber leaving behind only the polystyrene fibers. Next, place the fibers onto a sample holder and coat each sample with a 10 nanometer thick layer of gold palladium. Then place the coated samples in the chamber of the scanning electron microscope for imaging image, the refractive index profile of the samples using a five kilo electron volt electron beam at a magnification that allows for the best resolution of the 0.9 micron diameter fibers.
Prepare about 25 centimeter long fiber samples for optical characterization by pre warming the fibers to 37 degrees Celsius and a curved blade used for cutting the fibers to 65 degrees Celsius. These temperatures prevent deformation of the fiber tip that can occur during the cleaving process. Next, a line of fiber on a cutting surface so that a clean perpendicular cut can be made across the tip.
Then place the blade on the side of the fiber and quickly roll the blade across it. Keep the razor blade at a right angle to the fiber at all times. Inspect the fiber tip using an optical microscope to make sure the fiber tip is cleaved perpendicular to the fiber sides.
This allows for improved coupling later on in the procedure. Next, polish each fiber one at a time by gripping them 1.5 millimeters away from one end and drawing the fiber in one inch long. Figure eight shaped paths on 0.3 micron aluminum oxide polishing paper.
Make approximately eight loops to ensure that the whole tip gets polished. Then couple of helium neon laser emitting a 633 nanometer wavelength of light to a four micron diameter SMF 630 HP fiber. To accomplish this first place, a 20 x objective onto a stage with three degrees of freedom.
Also put in place two flat mirrors with two degrees of freedom. Initially place the SMF fiber eight millimeters away from the objective tip, then reposition the mirrors and the objective to direct the laser light to the tip of the fiber. Once aligned, connect the other side of the SMF fiber to a power meter.
Continue to adjust the position of the mirrors and objective until a coupled power of one milliwatt is achieved. One milliwatt is sufficient power for the fiber characterization measurements. Next couple, the SMF 630 HP fiber to the polymer optical fiber.
Using a motorized stage, the motorized stage should be able to be moved in all three Cartesian directions. First, center the two fibers together using transversal control. Then use longitudinal displacement to move the SMF fiber as closely as possible to the polymer fiber.
A smaller air gap between the SMF fiber and polymer fiber will reduce the expansion of the beam. Next place, the entire setup on a second motorized stage that moves in the longitudinal direction. The second motorized stage will be used for aligning the beam with the CCD camera.
Then observe the position of the fibers using an optical microscope and a right angled mirror. A small tilt or deformation in the polymer fiber tip because of the cleaving or polishing processes, can limit the minimum air gap between the SMF and polymer fiber. Next, use a CCD camera beam profiler to measure the output of the fiber using a 40 x objective.
First, saturate the CCD camera to monitor the boundaries of the polymer fiber using the knobs on the objective holder. Make sure that the polymer fiber boundaries can be observed on the CCD. Then use the motorized stage to focus the image on the CCD camera by moving the setup away or towards the 40 x objective while the CCD and objective are fixed.
Finally, transversely sweep the incident beam that comes out of the SMF across the tip of the polymer fiber to observe localization in different regions of the polymer fiber. Measure the output beam intensity for different incident beam positions with the lights off. Collect data at five different positions of the incident beam and carry out the measurements for 20 fiber samples for a total of 100 different measurements.
100 different measurements of the beam profiles are then averaged to show the transverse Anderson localization of the fiber. Shown here is the end product of a polished polymer fiber tip. The electron microscope image of the polymer fiber tip shows that most regions end up being well polished following this protocol.
When the polymethyl methacrylate fibers are dissolved away and ethanol only the polystyrene fibers remain. This is what's known as the refractive index profile, which easily displays the arrangement of polystyrene and polymethylmethacrylate fibers. An example intensity profile as measured by CCD camera is shown here after five centimeters of propagation.
The red indicates the localized profile and the yellow is the tail of the intensity. This image is of a fiber that is localized in the transverse direction of the disordered fiber.
