All-optical tunable buffering with coupled ultra-high Q whispering gallery mode microcavities

Wataru Yoshiki,Yoshihiro Honda,Takasumi Tanabe,Tomohiro Tetsumoto,Kentaro Furusawa,Norihiko Sekine

doi:10.1038/s41598-017-10035-4

Abstract

All-optical tunable buffering was recently achieved on a chip by using dynamically tuned coupled mode induced transparency, which is an optical analogue of electromagnetically induced transparency. However, the small Q s of about 105 used in those systems were limiting the maximum buffering time to a few hundred ps. Although employing an ultra-high Q whispering gallery mode (WGM) microcavity can significantly improve the maximum buffering time, the dynamic tuning of the WGM has remained challenging because thermo-optic and pressure tunings, which are widely used for WGM microcavities, have a very slow response. Here we demonstrate all-optical tunable buffering utilizing coupled ultra-high Q WGM cavities and the Kerr effect. The Kerr effect can change the refractive index instantaneously, and this allowed us to tune the WGM cavity very quickly. In addition, from among the various WGM cavities we employed a silica toroid microcavity for our experiments because it has an ultra-high Q factor (>2 × 107) and a small mode volume, and can be fabricated on a chip. Use of the Kerr effect and the silica toroid microcavity enabled us to observe an on-chip all-optical tunable buffering operation and achieve a maximum buffering time of 20 ns.

Highlights

The silica toroid microcavities employed in the experiments (C1 and C 2) were fabricated using five processes: (1) photolithography, (2) HF etching, (3) dicing, (4) XeF2 dry etching, and (5) laser reflow[28]
With the photolithography process (1), circular resist patterns 100 μm in diameter were developed on a silicon chip with a 2 μm-thick silica layer
We coupled light emitted by a TLS (Santec TSL-710, linewidth of 100 kHz) into the cavity via the tapered optical fibre and detected the output light with a PD (Thorlabs DET08CFC, bandwidth of 5 GHz)

Summary

Results

If the resonant frequencies of the two cavities (C1 and C2) are the same, the light input into a tapered optical fibre (i.e. signal light) first couples to C2, and reaches C1 (see “(1) input” in the figure). The signal light can be buffered for an arbitrary time period by changing the duration of the control pulse This is the concept of all-optical tunable buffering with coupled silica toroid microcavities. The signal light leakage is believed to be small enough to have no influence on the buffering performance This is because there was no change in the leakage when the input control power (i.e. the amount of resonance shift) was changed. It should be emphasized that the red dashed curve in the figure plots the calculated results and agrees well with the experimental results

Discussion

Summary

Methods