Microdisk Resonance Tuned with Optical Signal

Breck Hitz

All-optical switches — those in which one group of photons alters the flow of other photons — are an essential component of many next-generation systems, from optical computers to transparent optical networks.

Recently, researchers at Université des Sciences et Technologies de Lille in Villeneuve d’Ascq, France, demonstrated what they believe is the first all-optical switch based on an InP/InGaAsP microdisk resonator. The device could serve not only as an optical switch, but also as a tunable optical multiplexer/demultiplexer or as a tunable filter.

The switch comprised a microdisk resonator and two adjacent waveguides separated from the disk by an ∼100-nm gap (Figure 1). The waveguide structure was essentially a sandwich with a 300-nm-thick layer of InGaAsP between two 1.2-μm-thick layers of InP cladding. The researchers fabricated the raised structure in Figure 1 in the InGaAsP layer using electron-beam photolithography and inductively coupled plasma etching. The waveguides were 0.5 μm wide, tapered to 2.5 μm at the ends to facilitate matching with external optics, and the microdisk was 15 μm in diameter.

Figure 1. The 15-μm-diameter microdisk couples resonant wavelengths (i.e., waves that fit exactly an integral number of times around the disk’s circumference) from the straight waveguide on the left to the z-output waveguide on the right. (All other wavelengths emerge from the y output.) Free carriers are photoinduced in the disk when it is illuminated with 980-nm light from a diode laser. The free carriers change the disk’s refractive index and, hence, its resonant wavelengths, so a different set of wavelengths is coupled from the straight waveguide to the z output. Reprinted with permission of Optics Letters.

Before experimenting with the switching capability of their device, the investigators characterized its passive optical properties. The microdisk’s measured free spectral range, 14.8 nm, was consistent with the calculated value based on its circumference and effective refractive index. Its finesse was 14.8 and 5.8 for the TM and TE polarizations, respectively, indicating a significantly better confinement for the TM polarization.

When they illuminated the microdisk with 70 mW at 980 nm from a diode laser, the transmission peak that emerged from the y-output port blueshifted by ~3 nm, or 375 GHz (Figure 2). Primarily as a result of the high Fresnel reflection at the air-InP interface, only about 20 of the incident 70 mW was absorbed in the disk. But because the experimental arrangement prevented them from accurately measuring the dimensions of the illuminated area, the researchers did not attempt to calculate the wavelength shift as a function of absorbed power.

Figure 2. When the researchers illuminated the microdisk with 70 mW from the diode laser, the transmission peak emerging from the y-output port was blueshifted by ~3 nm, or ~375 GHz. These data are for the TM polarization, although similar results were obtained for the TE polarization.

They did, however, calculate that a refractive-index change of –6 x 10^–3, induced by the free carriers, would cause the observed 3-nm wavelength shift. That result led them to calculate the density of free carriers induced by the 980-nm light to be 3.5 × 10¹⁷ cm^–3.

The researchers considered the possibility that a temperature change induced by the 980-nm light might be responsible for the index change, rather than photoinduced free carriers. However, they noted that previous investigators had found that thermally induced refractive-index change is positive with temperature, while the effect they observed had the opposite sign. Moreover, when they modulated the 980-nm light at 1 kHz, a rate too fast for thermal fluctuations to follow, the optical signal exhibited identical modulation.

Optics Letters, Jan. 1, 2007, pp, 35-37