The overall goal of this procedure is to generate optical frequency combs using Whispering Gallery mode, disc resonators for microwave photonics applications. The first step is to grind and polish the rim of a crystalline disc until the scale of its surface roughness is sub nanometer. The second step is to measure this surface roughness using AAU interferometer microscope.
Next, a, a single mode optical fiber is drawn while heated with the blowtorch in order to obtain a tapered fiber with micrometer scale diameter. The final step is to use the taper to couple laser light into the disc resonator and to excite a whispering gallery mode whose round trip light path corresponds to an integer number of total internal reflections. Ultimately, the pump power is increased and other whispering gallery modes are excited via the care non-linearity.
This leads to short pulses in the temporal domain and a frequency comb in the frequency domain. The test in using mechanically polished crystalized disc ator is that it's possible to achieve very high surface fast quality with a small internal absorption. The surface ness can be accurately using a customized optical profilometer, and the related C factor is using the cavi methods To launch the light into the disc originators.
We use type fibers fabricated in our lab. Unlike other methods. Type fibers provide the very high coupling efficiency needed to trigger nonlinear effects.
At low pump power, The care nonlinearity induces new frequencies as sufficiently high laser power leading to an optical frequency column. For phase correlated frequencies, a trustable microwaves can be generated, which we use for photonix applications. The first step in this protocol is to produce a resonator.
Start with optical window crystalline magnesium fluoride, or calcium fluoride. Here, a six millimeter diameter disc of calcium fluoride is used. The crystal will be shaped and polished on a polishing tower like the one custom made for this experiment.
Samples in this polishing tower are attached to a rod that is held by the spindle motor glue, the end of the rod to the center of the crystalline optical window, mount the rod in the spindle motor. Next, cover a V-shaped metallic guide with a polishing support pad appropriate to the crystal used. Take a prepared mixture of an abrasive powder, such as diamond or silicon carbide with a particle size of 10 microns and water and pour it over the pad in the guide.
Start the rotation of the crystal at about 5, 000 RPM and move the guide toward the spinning crystal to begin grinding it at 20 grams. Pressure Continue grinding until the crystal has a by convex shape as in this computer generated image. This can take between two and four hours depending on the material.
After shaping the crystal, the next step is further grinding and polishing. Using progressively smaller abrasive particles mixed with water in the guide, the complete process takes four to eight hours. As the grinding and polishing proceeds, it is important to obtain feedback on the state of the surface.
Initially, when the crystal is opaque, use an optical microscope for visual inspection. The surface should remain defect free with roughness on the scale of the abrasive grain size. Later after successful polishing with one micron particles, the crystal becomes transparent and optical polish has been reached at later stages with smaller abrasives, interferometric measurements are required to characterize the surface.
Use a microscope equipped with the morale interferometer objective to study surface height variations qualitatively via the observed interference patterns in the morale objective. Half the incident light is directed to the surface of the disc while the other half is reflected by a mirror. The light reflected by the disc and the light from the mirror produce an interference pattern that is used to characterize the disc surface.
This interferometric image is from a surface that has been polished with one micron powder. It shows significant height variations. This image is from a surface that has been polished with 100 nanometer powder.
It indicates a much smoother surface. For more detailed characterization, use a computer controlled measurement of the height variations of the surface in a small area. This image was completed in about one hour and covers approximately 0.25 square millimeters.
Grinding and polishing should continue until the interferometric data suggests the surface is as smooth as possible. In order to couple light to the disc. A taper is drawn in an optical fiber.
When the taper is drawn, the fiber diameter in the taper is reduced from 125 microns to one micron. Higher order modes are progressively excited and they interfere with each other. A critical point is reached.
When these higher order modes are ejected, then only one mode remains guided and its evanescent field can be used to couple light into the disc to create the optical fiber for coupling to the resonator. Begin with about one meter of single mode silica fiber. Splice the bare fiber to FC connectors on each side.
Strip about five centimeters of the plastic and polymer coating from the center section of a single mode silica fiber. Then connect one end of the fiber to a continuous wave near infrared laser source, and the other to a photo diode. This allows for monitoring of the fiber.
Next, fix a coated portion on either side of the stripped region of the fiber to a computer controlled high resolution motor. The motors should be computer controlled and configured to pull the ends in opposite directions at a constant acceleration. Now, heat the uncoated portion of the fiber with a blowtorch for about one minute.
The flames should be gentle to avoid damaging the evolving taper. After one minute while still applying a flame, use a computer interface of the motors to start each of them moving with a constant acceleration of about five microns per square. Second, once the drawing has started, monitor the transmission of the taper using the laser source and photo diode.
IOD interference patterns will appear during the process, increase in frequency, and then disappear for a waste diameter near one micron. At this point, stop both the motor motion and the flame immediately to couple light to the resonator and measure the agan modes of the cavity. The resonator and fiber taper must be brought close to one another under a microscope.
Fix the resonator on a three axis PAO controlled translation stage in the vicinity of the fiber taper while using the microscope to monitor the relative position of the resonator and the fiber taper. Move the resonator to a distance of less than one micron from the taper. Employ a mirror to help with vertical positioning and tilt angle to test the coupling of the fiber and the resonator.
Connect the tapered fiber to a visible laser diode. When the coupling is efficient, the resonator should be illuminated. Once efficient coupling has been achieved, replace the visible laser diode with a mode hop free laser that has aligned with narrower than that of the resonance.
The other end of the taper should be input to a photo diode that is connected to an oscilloscope. When light is into the cavity, its intensity decreases exponentially with time. The characteristic decay time is called the photon lifetime.
An important measure of the energy storage capacity of the resonator is the quality factor defined as the product of the photon lifetime and the photon angular frequency. An accurate measurement of the quality factor can be made with the cavity ring down experiment to perform it. Sweep the input laser through wavelengths fast enough to cause interference between the resonating light decaying in the chamber and de-tuned light from a subsequent time.
Then fine tune the position of the resonator and taper to maximize the coupling quality factor. Using the oscilloscope trace as a guide, the quality factor is found by fitting the measured curve. Nonlinear effects can be seen by inserting an optical amplifier between the tunable laser and the resonator.
Using the photo diode oscilloscope trace for feedback, tune the laser to an input wavelength close to resonance. At this point, connect the output fiber to a high resolution optical spectrum analyzer. Increase the laser input power while slightly detuning the pump wavelength.
New frequencies will appear on each side of the pump peak. This is a care optical frequency comb. In this plot of transmission versus time, the trace in blue was found using the cavity ring down technique on a whispering gallery mode resonator fabricated from calcium fluoride.
With the methods of this protocol, the red curve is the fitted curve from which an intrinsic quality factor of 1.5 times 10 to the ninth can be determined. This data shows the care optical frequency comb obtained for a whispering gallery mode resonator. The central frequency is that of the input laser, about 193.3 terahertz.
The other spectral lines are created by the non-linear interactions in the resonator. The frequency between each spectral line is six gigahertz corresponding to the free spectral range of the resonator. After watching this video, you should have a good understanding about how to fabricate ultra HighQ whispering gallery mode, resonators, how to couple them to laser pump using a tapered fiber and how to generate an optical frequency comp.
These methods enable investigation of the fundamental light matter interactions induced by strongly confined laser fields in nonlinear media, an exploration of a wide variety of technological applications. Don't forget to take precautions such as masks, glues, and certified laser safety glasses while performing this procedure.
