Series-coupled pairs of whispering-gallery-mode optical microresonators have been demonstrated as prototypes of stable, narrow-band-pass photonic filters. Characteristics that are generally considered desirable in a photonic or other narrow-band-pass filter include response as nearly flat as possible across the pass band, sharp roll-off, and high rejection of signals outside the pass band. A single microresonator exhibits a Lorentzian filter function: its peak response cannot be made flatter and its roll-off cannot be made sharper. However, as a matter of basic principle applicable to resonators in general, it is possible to (1) use multiple resonators, operating in series or parallel, to obtain a roll-off sharper, and out-of-band rejection greater, relative to those of a Lorentzian filter function and (2) to make the peak response (the response within the pass band) flatter by tuning the resonators to slightly different resonance frequencies that span the pass band.
The first of the two microresonators in each series-coupled pair was a microtorus made of germania-doped silica (containing about 19 mole percent germania), which is a material used for the cores of some optical fibers. The reasons for choosing this material is that exposing it to ultraviolet light causes it to undergo a chemical change that changes its index of refraction and thereby changes the resonance frequency. Hence, this material affords the means to effect the desired slight relative detuning of the two resonators. The second microresonator in each pair was a microsphere of pure silica. The advantage of making one of the resonators a torus instead of a sphere is that its spectrum of whispering-gallery-mode resonances is sparser, as needed to obtain a frequency separation of at least 100 GHz between resonances of the filter as a whole.
The two microresonators in each pair were mounted in proximity to each other so that the two were optically coupled. Half of the amplified light from a laser diode at a nominal wavelength of 1.55 μm was coupled into the first microresonator by means of an angle-polished optical fiber. The other half of the amplified laser light was passed through a Fabry-Perot cavity having a free spectral range of 20 GHz; this cavity served as both a reference to correct for laser frequency drift and a scale for measuring the difference between resonance frequencies. By use of a second angle-polished optical fiber, light was coupled out of the second microresonator to a photodiode.
An argon-ion laser operating at a wavelength of 351 nm (the wavelength most efficient for producing the desired photochemical reaction) was focused into the germania-doped microresonator. The current applied to the photodiode was modulated with a sawtooth waveform in order to sweep the laser wavelength repeatedly through a frequency range that included the pass band and surrounding frequencies. Using knowledge of the laser frequency vs. time, along with the measurements of photocurrent vs. time, it was possible to determine the magnitude of the filter spectrum. From time to time, the argonion laser was turned on to tune the germania-doped microresonator, and then the spectrum determined. Care was taken to discriminate against the transient contribution of laser-induced thermal expansion to the change in the spectrum. The process was repeated until the desired separation between the two resonance frequencies was obtained (for example, see figure).
This work was done by Anatoliy Savchenkov, Vladimir Iltchenko, Lute Maleki, and Tim Handley of Caltech for NASA’s Jet Propulsion Laboratory.
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