Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

The overall goal of this procedure is to generate new optical frequencies using silica microspherical whispering-gallery-mode resonators for optical and photonics applications ranging from compact laser sources to biochemical sensing. In whispering gallery-mode microresonators light can be guided with a unique combination of strong spatial confinement and long cavity lifetime. Low losses ensure high quality factors.

Such resonators are ideally suited for nonlinear optical interactions. In particular, silica microspheres show a great potential as frequency generators for continuous wave compact, narrow-line width and tunable sources. The interest of using fusion techniques for fabrication of the electric microsphere is based on low scattering losses can be achieved and in consequence very high quality factors.

We used homemade taper fiber to couple light into the microspheres. The high coupling efficiency and the high cavity buildup factor can trigger nonlinear Kerr effects with low continuous wave pump power. Demonstrating the taper procedure will be Franco Cosi, a technician from our laboratory.

To create a microsphere, first get a length of fiber to work with. Gather the equipment needed to create a microsphere using single-mode silica fiber. This includes the fiber, acetone, an optical coating stripper, and a fiber cleaver.

In addition, have ready a fusion splicer. Use the stripper to remove about 1-2 centimeters of the acrylic coating from the end of the fiber. Follow this by cleaning the stripped portion with a clean wipe and acetone.

Place the end of the fiber into the fiber cleaver and cleave to isolate the stripped portion. Collect the cleaved fiber from the cleaver. Move the fiber tip to the fusion splicer.

There, position the fiber and hold it in place according to the manufacturer's guidelines. Close the cover, check that the arc power and the arc duration are correct, then start the arc. Monitor progress on the display.

As the arc melts the glass, the fiber shape will begin to form a sphere due to surface tension. When the sphere begins to take shape, stop the arc and open the cover. Access the fiber in the holder, rotate it by 90 degrees to preserve the spherical shape, then secure the fiber in place again.

Close the cover and restart the arc. Performing at least three such 90 degree rotations over five to 10 minutes will produce a microsphere of about 160 micrometers. When removing the microsphere and its fiber stem, be prepared to store it or use it within a day.

This t-shaped holder is translation stage-mountable for use with light-coupling experiments. It has a channel down its middle for the microsphere's fiber stem. Place the residual stem end of the microsphere in the channel so the microsphere is unobstructed above the holder.

Afix the stem with clear tape. Now, begin to create a tapered fiber to couple light into the microsphere. This will require a device that can slowly pull the fiber ends apart.

Start with a length of single-mode silica fiber and identify its midpoint. Use the optical coating stripper to remove about 3-4 centimeters of the coating around this point. Once this is done, slip an alumina cylinder over the stripped portion of the fiber to serve as an oven.

Next, use a bare fiber terminator to connect one end of the fiber to a laser capable of operating at 635 nanometers. Connect the other end of the fiber to a power meter. Once the fiber is in place, arrange for an oxygen-butane flame to heat the alumina cylinder to a temperature close to the silica melting point of about 2, 100 degrees Celsius.

Start the fiber puller so that it slowly moves the two ends away from one another. With the 635 nanometer laser on, monitor the power transmitted through the fiber with the power meter. Changing values reflect the ongoing tapering.

At this point, stop pulling the fiber and stop the flame. Occasionally remove the fiber from the power meter input to project the fiber output onto a beam card to check that a homogeneous circular spot is preserved during the tapering process. Work with fiber after it has been freed from all equipment including the cylinder.

Position a glass microscope slide cut to form a U'near the fiber's taper. Next, put the fiber on the slide with taper centered on the U'Keep the fiber taut and apply glue where the fiber and slide overlap to hold the fiber in place. At this point, prepare to couple light into the microsphere with the tapered fiber.

To do this, use separate translation stages for the microsphere and the tapered fiber. On a stage with piezoelectric actuators, mount the T-shaped holder with the microsphere so that the microsphere points up. On the other translation stage, mount the glass slide with the fiber oriented perpendicular to the microsphere fiber stem.

Use a terminated fiber cable to connect one end of the tapered fiber to a tunable diode laser. Connect the other end to an indium-gallium-arsenide photodiode detector. Now, situate both of the translation stages beneath the microscope tube that will be used for observation of the gap between the microsphere and the taper.

The two should be placed so the fiber taper and microsphere are within the travel distance of the translation stages. In addition, place a mirror at 45 degrees with respect to the microscope tube to monitor the vertical position of the taper. Position the microsphere using its translation stage and feedback from the microscope tube.

The goal is to have the equator of the microsphere in contact with the taper. Once this is done, turn on the continuous wave laser and monitor the transmission spectrum on an oscilloscope. Tune the laser until Lorentzian-shaped dips due to resonances appear in the spectrum.

Measure the resonance line widths and calculate the quality factor. Continue the experiment by exploring coupling efficiency as a function of the gap between the taper and the sphere. Fiber coupling is crucial in order to find good resonance to trigger nonlinear effects.

A good resonance is narrow, it is high contrast at low power. Prepare the set-up to realize stimulated Raman scattering. Light from a tunable diode laser operating at 15, 015 nanometers enters the tapered fiber.

After the laser, there is an erbium doped fiber amplifier to boost the laser power to achieve nonlinear effect. Next, there is an attenuator and then, an inline fiber polarizer. The light then goes through the taper, which is coupled to a microsphere.

After passing the microsphere, there is a 3-decibel-milliwatt splitter. One output of the splitter goes to an optical spectrum analyzer. The other goes to a photodetector connected to an oscilloscope.

Tune the laser from high to low frequencies to achieve thermal self-locking. On the oscilloscope, observe a resonance with the thermal drift comparable to the wavelength scan speed. The resonance will broaden when thermal self-locking is achieved.

Check the output power transmitted into the optical spectrum analyzer. Manipulate the attenuator to increase the power until the Raman laser line appears. The Raman laser line is detuned from the pump wavelength at about 13.5 terahertz.

This is a measurement from 50-micrometer diameter microsphere pumped at about 1, 547 nanometers. On either side of the pump wavelength, there are two four-wave mixing lines about 13 nanometers away. Two stimulated Raman scattering lines are at about 1, 610 and 1, 710 nanometers.

Measurements of a 98-nanometer sphere produced similar results. In this case, the pump peak is at 1, 551 nanometers. The stimulated Raman scattering line is centered at 1, 646 nanometers.

The stimulated antistokes Raman scattering line is centered at about 1, 452 nanometers. The symmetric lines near the stimulated antistokes line are from degenerated four-wave mixing. A Raman cone is centered at 1, 666 nanometers.

This spectrum from a microsphere of 40-micrometer diameter provides evidence of stimulated antistokes enhancement when its frequency is resonant with the cavity mode and phased-matched with the pump and stimulated stokes signal. The pump frequency is 1, 540 nanometers, the stokes line is at 1, 630 nanometers, and the antistokes line is at 1, 460 nanometers. This spectrum from a 65-micrometer microsphere has a stimulated antistokes to stimulated stokes scattering ratio close to one.

The pump frequency is centered at 1, 572 nanometers, the stokes line at 1, 640 nanometers, the antistokes line at 1, 490 nanometers. After watching this video, you should have a good understanding of how to fabricate high-Q silica microsphere, how to draw a fiber taper, and how to efficiently couple light into microresonator to study fundamental light-matter interactions. The electrical microspheres are photonic platforms that play an extremely important role in modern optics.

They can be exploited for CW nonlinear frequency conversion due to their capability to confine light for long time periods in very small volumes. The microresonator technologies that have been demonstrated may also be effectively exploited for biosensing. Our group is achieving excellent results in this area.