Creating a Precisely-tunable Single-frequency Laser Source to Excite
Introduction and Motivation
While researching methods of second harmonic generation, we learned that achieving frequency
doubling at a low power level was a novel use of whispering gallery resonators. This led to an
exploration of whispering gallery resonators, and resulted in an interest in developing a
precisely tunable laser source for the excitation of these narrow (in the MHz range) modes.
We hope to excite narrow whispering-gallery resonances in macroscopic glass spheres by
evanescent-wave coupling to a precisely-tunable single-frequency laser source. As a first step,
we have developed a 130-190 MHz tunable source for a 632.8 nm HeNe laser, using a 65-95 MHz AOM
in double pass configuration.
Whispering Gallery Modes
- Historically, whispering galleries are domed hemispherical rooms.
- The whispers from one point could be clearly heard at another point, because the resonant
sound waves constructively interfere at those points.
- A famous example of a whispering gallery is St Peter’s cathedral.
- The same resonant effect can be observed with light inside of an optical cavity.
- Optical whispering galleries are made from various transparent materials, including crystals,
amphorous materials and liquids.
Examples Of Optical Whispering Galleries
Shapes & Sizes
•Microsphere R ~ 2cm, sphericity deviation < λ0/40
•Circular disk with curved sidewall, radius from 2mm - 5mm, height ~ .5mm
Hand polishing of prefabricated crystalline structures and melting of silica into spheres.
Precisely Tunable Laser
The width of resonant modes depends on the size of the resonator and the quality of
fabrication. In our resonator, we hope to achieve mode widths in the MHz range. To observe these
modes, we have built a double pass AOM arrangement with a modified cat’s eye reflector.
- Vertically polarized light passes through a polarizing beam splitter [PBS] and into the
- After diffraction in the AOM, the first order beam, frequency shifted by f, emerges at 2x the
Bragg angle to the zero order beam.
- A plano-convex lens, a focal length away from the AOM, results in parallel rays. (modified
- The zero order beam, which is not frequency shifted, passes next to the mirror and is
directed along the return path.
- The first order beam strikes the mirror and reflects through the QWP, becoming horizontally
polarized, and retraces its path through the AOM where a second Bragg diffraction results,
creating a beam frequency shifted by 2f.
- A second PBS directs the 2f shifted light along the return path, where an aperture assists in
- A GT polarizer balances the ratio of shifted to original frequency light, incident on the
- A DET-210 photo-detector is connected to a signal amplifier and oscilloscope.
Modified Cats Eye
To demonstrate the success of our arrangement we recombined the original & shifted beams
and observed a beat.
The upper trace shows the amplified 160 MHz beat signal observed with our double-pass
arrangement. The lower trace is a sample of the 80 MHz AOM drive signal. Sweep speed of the
Tektronix 485 oscilloscope was 5ns/cm.
Souce: Schiller & Byer
The peak width of resonant frequencies is determined by the Q of the resonator. Our goal is to
measure resonant frequencies with a MHz width, using a fused silica microsphere, similar to an
experiment by Schiller & Byer  .
Current research into very high Q (~10 million) resonators requires KHz frequency tuning of
the input laser, to detect sub kilohertz resonant frequencies.
Whispering Gallery Mode Resonances
Resonant Mode Spacing
 S. Schiller, R.L. Byer, Opt. Lett. Vol 16, No. 15 (1991)
 E.A. Donley, et al. Rev. Sci. Instrum. 76, 063112 (2005)
 J.U. Fürst et al. Phys. Rev. Lett. 104, 153901 (2010)
 A.A. Savchenkov et al. Opt. Exp. Vol 15, No. 11 (2007)
 A.A. Savchenkov et al. Opt. Lett. Vol 32, No. 2 (2007)
 A.B. Matsko et al. IPN Prog. Rep. 42-162 (2005)
 V.S. Ilchenko et al. Phys. Rev. Lett. Vol 92, No. 4 (2004)
✣ Supported by NSF-REU (Summer 2011)