Fabrication of Silica Ultra High Quality Factor Microresonators

The overall goal of this procedure is to fabricate microsphere and silica OID resonators with ultra high quality factors, which enable applications ranging from telecommunications to bio detection. Microsphere fabrication begins with a preparation of a clean, stripped optical fiber end to begin. Micro OID fabrication photolithography and etching steps are performed in the clean room to undercut the silicon beneath circular silicon pads and form silicon micro discs sitting on pillars.

The next step of both fabrication processes is to expose the optical fiber and or silica micro disc to a properly aligned CO2 laser beam. The CO2 laser beam melts the silica forming microspheres or smooth silica OIDs. The final step is to inspect the devices with a microscope to ensure proper reflow.

Ultimately, line width measurements are used to determine the quality factor of the resulting spheres and OIDs. The motivation for using CO2 laser reflow to make microsphere and micro devices is the ability to achieve quality factors above a hundred million. Visual demonstration of this method is critical because reflowing a high quality factor.

Respiratory can be difficult. Results can vary from sample to sample, so you need to watch carefully to make sure the laser beam is properly aligned and has the correct settings. To begin microsphere fabrication, select a small amount of optical fiber, strip approximately 1.5 inches of cladding from one end clean the stripped fiber end with either a methanol or ethanol, cleave the stripped end with an optical fiber cleaver.

The advantage of using an optical fiber cleaver is that it produces a very smooth uniform cut. Excessive roughness or defects from a cut may cause uneven reflow lowering the quality factor of the resulting spheres. Proper reflow is the most important step of this procedure to ensure success.

The optical fiber and micro disc samples must be properly aligned and in good condition. Proper alignment of the laser beam and use of correct intensity and time settings is crucial. Place the clean fiber into the path of the CO2 laser.

Expose the cleaned fiber end to three watts of CO2 laser power focused to a 500 micron diameter spot size for about one second. This produces spheres approximately 200 microns in diameter. However, the size can be tuned by increasing or decreasing the diameter of the optical fiber.

Slightly adjusting the laser intensity may also be necessary to reflow larger or smaller spheres. Micro OID fabrication begins with the generation of a photo mask with dark solid circles of pre-designed spacing and diameter leave at least one to two millimeters of space between each circle and at least five millimeters of space between arrays of circles and around the edges of the mask. For this procedure, a mask was made with rows of 160 micron diameter circles resulting in finished OIDs of approximately 110 microns in diameter.

Since the sample wafers must be carefully handled with tweezers, it is important to leave space for the tweezers to grip without damaging the OIDs. This space provides room for a tapered optical fiber to couple light into the finished devices and allows samples to be cut into smaller arrays more easily. Photolithography and etching steps are described in detail in the written procedure.

These steps cannot be demonstrated here as they take place in the clean room. Side view scanning electron micrographs are shown here from different points in the fabrication process. During the fabrication samples are immersed in improved buffered oxide etching, abbreviated BOE, which etches the silica not covered by the photoresist to form circular silica pads on the silicon wafer.

After removing the photoresist, a xenon di fluoride etcher is used to undercut the silicon beneath the circular silica pads to form silica micro discs. The amount etched should be approximately one third of the silica circle size, so the resulting micro discs pillar is approximately one third to one half of the total disc diameter as determined by inspection with an optical microscope. The next step is to place the samples in the path of a focused CO2 laser beam.

The center of the laser beam and the center of the micro disc must be aligned for a smooth circular micro to be formed, expose them to approximately 12 watts intensity for three seconds or until a smooth OID is formed. Depending on the exact size of the disc and the amount of xenon di fluoride undercut, a slightly higher or lower intensity and exposure time may be needed to form a micro oid. The microsphere and micro OID devices can be imaged using both optical microscopy and scanning electron microscopy.

In all images, the uniformity of the device surface is clearly evident. These are examples of incorrectly reflowed microsphere andoid resonant cavities Due to incorrect placement within the beam, the device is malformed as a result of a poor photo mask or a poor lithography. The resulting OID is moon shaped.

Shown here are representative quality factor spectra of the microsphere and micro OID resonant cavities as determined using the line width measurement method. The quality factor represents the photon storage lifetime in the cavity. These values were above 100 million.

The spectrum of the microsphere was a single resonance indicating that the light coupled into either the clockwise or counterclockwise propagating optical mode. However, the spectrum of the OID exhibited a split resonance indicating that light reflects off a small defect and circulates in both clockwise and counterclockwise directions. This phenomenon occurs when there is a slight imperfection at the coupling site.

By fitting the spectrum to a dual Lian, the Q factor of both modes can be determined. The split resonance phenomena can occur in both sphere and OID resonators, but is more frequently observed in OIDs as they're more susceptible to imperfections and have fewer optical modes compared to spheres. A comparison graph shows the Q factors of several microsphere and micro OID resonant cavities.

Ultra high quality factor devices have numerous applications ranging from fundamental physics studies, bio detection to telecommunication systems. Don't forget that working with High power CO2 lasers can be extremely dangerous. You should always wear protective eyewear and follow safe operating procedures when reflowing spheres and OIDs.