Critical Coupling Point Research Articles

Demonstration of critical coupling in microfiber loops wrapped around a copper rod

The authors demonstrate critical coupling condition in microfiber loops wrapped around a copper rod. The critical coupling condition is achieved by tuning the coupling coefficient to balance the circulation loss. A maximum extinction of 30dB has been obtained around the critical coupling point with a quality factor of about 4000. The resonance wavelength can be tuned by applying an electric current through the copper rod. The copper rod, usually deemed as a high-loss medium for handling light at optical frequency, serves as a low-index robust support for achieving critical coupling in the microfiber loops with acceptable loss.

Ultralow-threshold microcavity Raman laser on a microelectronic chip.

Using ultrahigh-Q toroid microcavities on a chip, we demonstrate a monolithic microcavity Raman laser. Cavity photon lifetimes in excess of 100 ns combined with mode volumes typically of less than 1000 (microm)3 significantly reduce the threshold for stimulated Raman scattering. In conjunction with the high ideality of a tapered optical fiber coupling junction, stimulated Raman lasing is observed at an ultralow threshold (as low as 74 microW of fiber-launched power at 1550 nm) with high efficiency (up to 45% at the critical coupling point) in good agreement with theoretical modeling. Equally important, the wafer-scale nature of these devices should permit integration with other photonic, mechanical, or electrical functionality on a chip.

Observation of Critical Coupling in a Fiber Taper to a Silica-Microsphere Whispering-Gallery Mode System

We present the observation of critical coupling in a high- Q fused-silica microsphere whispering-gallery mode resonator coupled to a fiber taper. Extremely efficient and controlled power transfer to high- Q ( approximately 10(7)) resonators has been demonstrated. Off-resonance scattering loss was measured to be less than 0.3%. On-resonance extinction in transmitted optical power through the fiber coupler was measured as high as 26 dB at the critical coupling point. This result opens up a range of new applications in fields as diverse as near-field sensing and quantum optics.