Optical Filter and Laser Applications using Micro Ring Resonators

	ABOUT THE AUTHORS Dominik G. Rabus and Michael Hamacher are Ph.D. candidates at the Heinrich-Hertz-Institut fuer Nachrichtentechnik Berlin GmbH, Berlin, Germany.

Active and passive ring-resonator devices are good for wavelength filtering, routing, switching, modulation, and multiplexing/demultiplexing applications. Ring resonators do not require facets or gratings for optical feedback, making them particularly suited for monolithic integration with other components.

This article examines the characteristic responses of single and double multimode- interference- (MMI) coupled GaInAsP/InP micro-ring resonators. Specifically, the article will explore these resonators configured as racetracks with radii of 100- and 200 microns and a free spectral range (FSR) of 100- and 50-GHz, respectively (Figure 1).

Figure 1:  Double (left) and single (right) ring resonators with MMI couplers

The device's layer sequence (from bottom to top) is:

• InP substrate
• GaInAsP (_gap=1.06 µm, 0.38 µm)
• InP etch stop layer (0.020 µm)
• GaInAsP (_gap=1.06 µm, 0.84 µm)
• InP cap (0.2 µm)

The design assures a monomodal propagation of light in the waveguide and, due to a good confinement in the resonator, very low bending losses (Figure 2).

Figure 2:  The structure (left) and the mode profile (right) of the straight waveguide, calculated using BPM.

In addition, the waveguide was etched down on the outer side of the waveguide in the curvatures. The width of the waveguide is 1.8 µm (Figure 3).

Figure 3:  The structure of the waveguide in the curved region (left) and the mode profile (right) for a radius of 100 µm

MMI couplers couple light into the resonator. These couplers are widely used in photonic integrated circuits; in the ring resonators we use them as power splitters with a ratio of 50:50 (3 dB). The coupler has two input and two output ports. The splitting ratio is mainly defined by the length of the MMI (Figure 4).

Figure 4:  The intensity at the two output ports is shown as a function of the length. The dashed line corresponds to Output Port 1 and the solid line corresponds to Output Port 2. At a length of approximately 160 µm, the two curves meet and have an intensity of 0.5 each.

The ring resonators were fabricated using standard photolithography and a CH₄/H₂ reactive ion etching (RIE) technique. SiN_x was used as the etching mask, which also served as the mask for the deep etching process. A small fraction of oxygen was added to reduce the formation of polymers during dry etching and to minimize the sidewall roughness (Figure 5).

Figure 5:  The step-by-step fabrication process for the ring resonators

We added an antireflection coating to the facets of the input and output waveguides in order to avoid Fabry-Perot resonances in the straight waveguide section.

We characterized the ring resonators using a tapered fiber and an external cavity laser. The characterized ring resonators are polarization-dependent due to waveguide asymmetry. This is irrelevant if the devices comprise an active section and act as a laser. You can eliminate polarization-dependent filter characteristics with a polarization diversity architecture.

We performed the measurements reported in Figure 6 for TE polarization. As designed, an FSR of approximately 0.8 nm (100 GHz) for the 100 µm ring devices and 0.4 nm (50 GHz) for the 200 µm devices are observed near = 1.55 µm. The insertion losses of the devices are between 7-8 dB (including coupling losses of approximately 5 dB). The transmission difference between the minimum and the off-resonant values for the single micro ring resonators where R = 100 µm and R = 200 µm is more than 13 dB. The FWHM (full width at half maximum) was approximately 0.14 nm and 0.08 nm, respectively.

Figure 6:   The results for the measured and simulated ring resonators

The single-ring resonators are suitable for laser applications due to the small FWHM. You must use multiple, cascaded ring resonators for optical filters. The contrast of the throughput port and of the drop port of the double micro ring resonator with R=100 µm are around 3.5 dB and 7.5 dB, respectively. The expected broadening of the FWHM for the throughput port was 0.25 nm and 0.4 nm for the drop port. The low contrast is in agreement with theoretical calculations; using gain sections can improve the contrast further.

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