A Robust and Tunable Add–Drop Filter Using Whispering Gallery Mode Microtoroid Resonator

Abstract

We fabricated and theoretically investigated an add-drop filter using an on-chip whispering gallery mode (WGM) microtoroid resonator with ultra-high quality factor (Q) side coupled to two taper fibers, forming the bus and drop waveguides. The new device design incorporates silica side walls close to the microresonators which not only enable placing the coupling fibers on the same plane with respect to the microtoroid resonator but also provides mechanical stability, leading to an add-drop filter with high drop efficiency and improved robustness to environmental perturbations. We show that this new device can be thermally tuned to drop desired wavelengths from the bus without significantly affecting the drop efficiency, which is around 57%. Index Terms-Whispering gallery mode (WGM), microresonator, add-drop filter, microtoroid, thermal effects I. INTRODUCTION PTICAL add-drop filters (ADFs) that add or remove narrow band wavelengths of light from a broader optical signal being carried along a bus waveguide are fundamental building blocks of optical transmission and communication systems. They are key elements in multiplexers, modulators and optical switches. Thus, great efforts have been put in designing ADF architectures with improved efficiency, spectral selectivity (i.e., quality factor), and spectral tunability. The past two decades have seen significant progress in ADF designs ranging from all-fiber architectures [1]-[5] to photonic crystal (PhC) structures [6]-[8] and waveguide coupled whispering gallery mode (WGM) microresonators (e.g., microsphere, microring, microdisk, microtoroid, etc) [9]-[15].All-fiber systems relying on Bragg gratings accomplish high drop efficiency but have quality factors of only about ∼ 10 3 , which yields a wavelength selectivity of about 0.7nm (or equivalently a frequency selectivity of ∼87.7GHz) [2]- [5]. Another all-fiber scheme using a fiber taper coupled microfiber knot, which works as a resonator, has been reported to have a quality factor of Q ∼ 1.3 × 10 4 [1]. In a two-dimensional PhC with a complete photonic band-gap, nearly 100% drop efficiency and a Q close to 700 have been numerically demonstrated, however, with a very small overall transmission (∼ 15% of the incident power) [6]. A proposal using PhC ring resonator has promised more than 96% drop efficiency with 160 < Q < 10 3 . These PhC based ADFs are fabricated with very high

An active add-drop filter using an ytterbium and erbium co-doped silica microsphere

“…Although Er 3+ ions alone can be used for the purpose, we preferred to co-dope Yb 3+ and Er 3+ ions, because absorption cross-section of Yb 3+ is larger than that of Er 3+ in the 980nm band, providing a more efficient pumping route for the gain medium. The waveguides for the ADF was a pair of fiber-tapers, which were fabricated together by heating and pulling two fibers simultaneously under hydrogen flame [5,6]. Figure 2 shows an image of the fabricated ADF and the illustration of the experimental setup.…”

Reconfigurable Optical Directed-Logic Circuits

“…As the intrinsic loss of the ring cavity is negligible compared with the coupling efficiency, the add/drop ring is still under the critical coupling condition with an extinction ratio larger than 20 dB. This high extinction ratio promises high power--drop efficiency at the resonant state [12]. The large coupling efficiency can also broaden the coupling bandwidth and reduce the thermal instability in the ring cavity.…”

Universal hybrid three-qubit quantum gates assisted by a nitrogen-vacancy center coupled with a whispering-gallery-mode microresonator

“…Also, the σ + -polarized photon can only induce the transition |−1 ↔ |A 2 and the frequency of the photon is different from ω 0 owing to the the external magnetic field. The WGM microtoroid resonator interfaced with two tapered fibers is defined as an add-drop structure [36,37], with the four ports marked with a 1 , a 2 , b 1 , and b 2 , respectively. Photons being sent into port a 1 (a 2 ) would be redirected to port b 2 (b 1 ) under the resonance condition, while the path of the photons remains unchanged under the nonresonance case.…”