We spent every day in the lab perfecting using lasers and flame to produce high quality sensors for our research. Each sensor consists of an optical resonator which in our situation was an optical microsphere and a waveguide or tapered fiber to let light enter the microsphere. Both components affect the overall quality of the sensor system. I will first describe the process of creating the microsphere through a reflow technique.
The base of each microsphere is created by removing the protective coating of an optical fiber, cleaning the newly exposed fiber, and cleaving the end so that the end of the fiber is flat and unshattered. This prevents any outside material from melting into the sphere and unshattered glass will melt into a smoother surface during the reflow process. With this base finished, we have to align the center of the laser so that it is a few hundred micrometers above the bottom of the fiber where we exposed the end. This is to introduce more material into the sphere when the reflow begins. The reflow begins when we turn on the laser. By shooting a beam of about five watts at the sphere for 20 seconds we are able to create a sphere of about 150 micrometers. The sphere develops because the glass is instantaneously melted when it is hit by the focused laser. Surface tension then causes the glass to pull upwards creating a droplet like structure. As heating is allowed continue, the glass has more time to set into a stable smoother form; however, if the glass is melted for too long, the sphere will become more ovalic as glass is vaporized by the laser. We found that about 20 seconds, the sphere has had enough time to set while also not taking an oval shape.
The next process is fiber tapering. It’s a process that sounds easy, but is actually very difficult requiring more skill than you would expect. It involves cleaning the center of a strand of fiber by removing the protective coating, and then placing the fiber over a flame while having it pulled my two motors so that the center begins to shrink in diameter as shown above. The motors move the fiber in increments of micrometers at a time pulling the fiber thinner. As the fiber thins, the light of a laser that is shown through the fiber can be seen to start blinking. This effect is caused as the fiber changing from multimode to single mode. The fiber will stop blinking once it is a single mode. A this time the diameter of the fiber at the taper is less than 5 micrometers in diameter. The fiber is able to transmit light into optical resonators at close distances(less than a micrometer) at this size, but the fiber is also very fragile at this size. Too much vibration could tear the fiber and the process would have to begin again. The flame during this process can also affect the results. If the flame is too high or too low the taper will be incorrect or nonexistent. If you taper for too long the fiber will break on its own as well. All of these factors make producing and implementing the waveguides difficult throughout the tapering process.
With both pieces ready, we secure them into place under a microscope and attach the fiber to a sensor and a laser allowing for the sensor to be tested. Spheres need to be replaced with each experiment, but fibers last about a day while being used.