Silica microspheres can be remarkably efficient resonators because they support whispering gallery modes that exhibit Q values of more than 1010. They are suited to applications that need ultra-narrow linewidths. Coupling light into and out of them without spoiling the Q is more difficult-and doing it in a way that could be made into an integrated device is more difficult still. Juha-Pekka Laine and others at the Massachusetts Institute of Technology (Cambridge, MA) recently demonstrated a device that overcomes both the coupling problems and potentially could be integrated onto a chip. Laine and coworkers created a channel-dropping filter using a microsphere and two unusual silica integrated waveguides.1
Most methods of coupling microspheres involve either fragile components such as thin fiber tapers, or bulk components, such as prisms. Laine says, "We are trying to get a chip-based coupling technique. Eventually we want to integrate this."
The waveguides are made of silica but they incorporate antireflection films between the waveguide and the substrate that act much like a high-reflection stack of thin films. These stripline-pedestal anti-resonant reflecting optical waveguides isolate the waveguide core from the substrate, thus decreasing loss of light that would otherwise travel from the microsphere into the waveguide and on into the substrate.
The waveguides had a silica core 2.1 µm thick; two high-reflectivity layers forming the pedestal were composed of 120 nm of amorphous silicon and 1.8 µm of silica. The microspheres had diameters ranging from 100 to 400 µm.
The input waveguide runs straight across the chip (see figure). The output waveguide on the substrate is at a diverging angle to the first, starting with a separation of 2 µm and expanding to a separation of 50 µm at the output facet. The microsphere is positioned above the two waveguides and can be moved using micropositioners. "By moving the sphere," says Laine, "one can drop different wavelengths depending on the overlap of the two waveguides with various sphere polar modes" in the microsphere.
The positioning also helps optimize coupling parameters at resonance to increase power transfer. Compared to previous methods of coupling, say the researchers, this method is robust, easy to fabricate, and provides a fixed port configuration for simple alignment and coupling-parameter control.
Transfer efficiencies of more than 50% were observed with 20- to 50-MHz resonance linewidths. Nearly 55% of the power extracted by the sphere is transferred to the drop guide at the 30-MHz linewidth. For more information, contact J.-P. Laine at firstname.lastname@example.org.
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.
1. J-P Laine et al.,Opt. Lett. 25, 22 (Nov. 15, 2000)
Planar wavelength-drop device based on stripline-pedestal antiresonant reflecting optical waveguides and a silica microsphere can transfer more than 50% of the optical power in a narrow bandwidth channel.