- Tiny Ring Resonator in Silicon Is a Wavelength Converter
Wavelength converters perform the important function of changing an optical signal from one wavelength to another. Such a capability is vital, for example, when a channel to be added to a wavelength-multiplexed fiber has the same wavelength as an existing channel. Many approaches to wavelength conversion have been proposed and demonstrated during the past several years, but they all have been dependent on large and/or expensive devices whose incorporation into low-cost, highly integrated systems would be problematic at best. Now researchers at Cornell University in Ithaca, N.Y., have demonstrated an efficient, tiny wavelength converter in silicon.
Figure 1. Information placed on the control light by the modulator is transferred to the signal light in the silicon chip. Images ©OSA.
The device is based on a 10-µm-diameter ring waveguide fabricated on a silicon-on-insulator substrate. Light passing through a straight waveguide adjacent to the ring can be coupled into the ring if its wavelength matches one of the ring’s resonances. Two low-power lasers are connected to the straight waveguide (Figure 1). One laser is modulated and serves as the control; the other is a continuous-wave probe beam and serves as the signal. The concept is that information entering the device on the control light is transferred to the signal light.
The signal laser can be tuned either to a resonance of the ring or to a slightly shorter wavelength. If it is tuned to a resonance, signal light passing through the silicon chip is strongly attenuated because most of it is coupled into the ring, where it circulates until it eventually converts to heat. On the other hand, if the signal is tuned to a wavelength that is slightly below a ring resonance, it is not coupled into the ring, and it passes through the silicon chip with minimal loss.
Figure 2. Information on the control light (top) can be directly transferred to the signal light (middle) or transferred in inverted form to the signal (bottom).
Free carriers generated by two-photon absorption of the control light can change the refractive index of the silicon and lower the ring’s resonant wavelengths. If the signal wavelength initially is resonant with the ring — that is, being strongly attenuated in the chip — turning on the control destroys the resonance so that the signal light passes through the chip with minimal attenuation. In other words, the information on the control light is passed directly to the signal light.
Alternatively, if the signal initially is tuned to a slightly lower wavelength than a ring resonance and, therefore, is not strongly absorbed, turning on the control will shift the ring resonance to the wavelength of the signal, and the signal will be attenuated as it passes through the chip. In other words, the information on the control light is inverted and passed to the signal light.
The scientists observed both inverted and noninverted wavelength conversion with a control power level of 4.5 mW (Figure 2). The power in the signal beam was approximately 0.5 mW. The bit rate achieved in the current device was 0.9 Gb/s, limited by the carrier lifetime in the waveguide.
The carrier lifetime could be shortened by perhaps an order of magnitude by reducing the waveguide dimensions and fabricating a reverse-biased PIN junction around the waveguide to sweep the carriers out. The smaller waveguide, however, would necessitate a larger control power unless the resonator’s Q were significantly increased. The scientists recently demonstrated ring resonators with the required Q, however, so they believe that bit rates of 10 Gb/s are feasible.
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