High-Q ring resonators in thin silicon-on-insulator

Abstract

We have fabricated high-Q microrings from thin silicon-on-insulater SOI layers and measured Q values of 45 000 in these rings, which were then improved to 57 000 by adding a PMMA cladding. The optimal waveguide designs were calculated, and the waveguide losses were analyzed. These high-Q resonators are expected to lead to interesting devices for telecommunication filters and sources as well as optical refractive index sensing. © 2004 American Institute of Physics. [DOI: 10.1063/1.1781355] Over the past several years, there has been substantial interest in the application of microring resonator structures as laser sources 1 and as optical filter elements for dense wavelength division multiplexing systems. 2 Here we describe a silicon-on-insulator (SOI) structure that is particularly advantageous. In the first place, it has low waveguide loss; from the measured Q values of 45 000 and 57 000, we can extrapolate an uncoupled Q value of 94 000 and a waveguide loss of 7.1 dB/ cm in the unclad case, and −6.6 dB/ cm in the PMMA clad case. Although higher Q values have been obtained for optical microcavities, 3 we believe that our geometry has the highest Q for a resonator based on a single mode silicon waveguide. But also noteworthy is the large amount of power contained outside the core silicon waveguide, which may be important in some applications; the modes that will be described have approximately 57% of the power outside the waveguide, as compared to 20% for a single-mode 200-nm-thick silicon waveguide, and 10% for a single-mode 300-nm-thick silicon waveguide.We chose to study wafer geometries that minimize the thickness of the SOI waveguiding layer as well as the buried oxide, but still yield low loss waveguides and bends. A number of different waveguide widths were compared by finite difference based mode solving. The geometry we chose consists of a 500-nm-wide waveguide formed in a 120-nm-thick silicon layer, atop a 1.4 m oxide layer, which rests on a silicon handle. Such a configuration supports only a single well-contained optical mode for near infrared wavelengths. The dispersion characteristics are shown in Fig. 1 for both unclad and PMMA-clad waveguides. Our interest in unclad structures stems from the ease of fabrication, as detailed in the following, as well as the flexibility an open air waveguide may provide for applications.These modes were determined by using a finite difference based 4 Hermitian eigensolver, the details of which are beyond the scope of this letter. It is possible to calculate the loss directly from the mode pattern with an analytic method valid in the low-loss limit, which the authors intend to publish in a forthcoming paper. The waveguide loss at 1.55 m calculated in such a fashion is approximately −4.5 dB. This loss figure was in agreement with the extrapolated results of FDTD simulation.It should be noted that since −4 dB/ cm loss is attributed to substrate leakage, the waveguide loss can be improved by the addition of a cladding, which tends to pull the mode upwards. This not...

A Hybrid Electrooptic Microring Resonator-Based $1 \times 4\times 1$ ROADM for Wafer Scale Optical Interconnects

“…To produce high quality silicon waveguides, we choose to use commercially available SOI wafers with a 1.4-m-thick buried oxide layer and a 120-nm silicon top layer. The high refractive index contrast between the silicon core and buried oxide and polymer cladding layers allows compact waveguides, 0.5 m in width, to be realized [22]. With these waveguide dimensions, only one optical mode, polarized in-plane with the substrate surface, is supported in the 1.4-1.6-m spectral range.…”

“…For use as low-drive voltage modulators, modest improvements, including the use of materials with higher EO activity, closer electrode spacing, and ring resonators with larger values, for example, would result in dramatically reduced (3 dB) drive voltages. Recently, organic EO materials have demonstrated over 300 pm/V EO activity and polymer-clad SOI microring devices with values of 57 k have been developed; therefore, lower targets can certainly be achieved in the near future [9], [22]. Electrode spacing can be reduced using standard procedures in high-resolution photomask and alignment technology or by using e-beam patterning.…”

“…Silicon, which has a high index of refraction and low optical loss, has allowed for the fabrication of ring resonators with diameters less than 10 m, FSRs greater than 15 nm, and values of over 1 000 000 [22], [26]. In these experiments, however, we intentionally quench to values below 5000 to allow for increased reproducibility and reasonable switching speeds.…”

Analysis of the tuning sensitivity of silicon-on-insulator optical ring resonators

“…Even a completely unclad waveguide, with simply air above the buried oxide and the waveguide, will guide; a polymethylmethacrylate (PMMA) cladding will also work. In both cases, the waveguide loss is between 6 and 7 dB/cm [1], of which about 4 dB/cm can be attributed to leakage into the silicon handle due to perturbation theory. Only one wellcontained optical mode is supported in the 1.4-1.6 µm spectral range.…”

“…H IGH-QUALITY-FACTOR ring resonators have recently been fabricated in thin silicon-on-insulator (SOI), with waveguide losses better than 7 dB/cm, and Q values in excess of 45 000 [1]. This suggests that these devices may find practical application as chemical sensors and as low-loss, very compact, add-drop optical multiplexers.…”