Patent Application
20090169149
Embodiments of the present invention are directed to optical ring resonators and, more particularly is directed to a ring resonator modulator with improved stability.
Ring resonators are wavelength selective devices which may be used for various optical filter and modulation applications. Optical Ring Resonators (RRs) are useful components for wavelength filtering, multiplexing, switching, and modulation. The key performance characteristics of the RR includes the Free-Spectral Range (FSR), the finesse (or Q-factor), the resonance transmission, and the extinction ratio. These quantities depend not only on the device design but also on the fabrication tolerance. Although state-of-the-art lithography may not be required for most conventional waveguide designs, Ring Resonator designs involve critical dimension (CD) values at or below 100 nm.
For such designs, resolution and CD control are both important to the success of the devices. In the case of Si based ring resonators, one of the important parameters to control is the coupling efficiency between the RR and the input/output waveguide. As a compact waveguide (for example, 220 nm×500 nm strip waveguide) is usually used in the RR to obtain a large FSR, the gap between the ring and bus waveguide may only be 100-200 nm. Since the device operates through evanescent coupling, the coupling is exponentially dependent on the size of the separating gap. Thus, in order to reliably process high-Q RR devices, control of a few nm demands CD control readily achieved by modern 0.18 μm or 0.13 μm lithography.
Since the ring resonators are by their very nature very sensitive devices, there are many things that could require re-adjustment. Some things which could cause a “de-tuning” of the resonance include but are not limited to temperature variations, process variations, materials degradation, voltage droop, strain, wavelength drift of the laser, etc.
The foregoing and a better understanding of the present invention may become apparent from the following detailed description of arrangements and example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing arrangements and example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto.
In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing FIG. drawings. Well-known power/ground connections to integrated circuits (ICs) and other components may not be shown within the figures for simplicity of illustration and discussion. Where specific details are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without these specific details.
An example of a micro-ring resonator is shown in
Thus, a ring resonator is a device which works by having a very narrow band where light of a particular wavelength is in resonance with the ring and that light gets coupled into the ring 100. Here, the resonant wavelength λR is the wavelength that is coupled into the ring 100 since it satisfies the condition λR=LNeff/m, were L is the length of the ring 100, Neff is the effective index of the ring 100 and m is an integer value. With this device, multiple wavelengths go into the ring resonator device, and all may be filtered out but the wavelength of interest, or resonant wavelength, λR.
Referring now to
Different modulation methods may be employed by changing the refractive index of the waveguide or the cladding of the ring 200, thus changing the resonance frequency. For example this may be accomplished by thermal tuning or using an electro optic material such as a chromophore doped polymer or semiconductor whose index can be changed by injecting (or removing) free carriers. Other electro optic material options are also available, as well as other tuning options.
As shown in the example of
According to an embodiment, an integrated monitor photodetector, or photodiode, 220 may be placed to capture the light from the throughput port 208. The photodiode 220 at the throughput port 208 essentially sees the inverse intensity of light at the output port 210. The photodiode 220 translates the signal intensity from the optical domain to the electrical domain. A feed back circuit including a transimpedance amplifier (TIA) 222 then translates the electrical current received from the photodiode 220 to an electrical voltage which may be applied to the terminals to modulate light in the ring 214.
As illustrated in
The photodiode 220 at the end of the throughput port 208, reading this light, outputs a signal 240 that may be connected to CMOS circuits 242 to amplify the signal through the transimpedance amplifier (TIA) 222 or other amplifier. A feedback circuit 244 may read the difference between the on and off state and then apply a voltage to the control electrodes 214 of the ring modulator to maximize this difference. The real time feedback circuit thus aids in maintaining stability and maximize performance of ring modulators, which are by nature very sensitive to small changes in refractive index caused for example by processing variations and thermal drift.
The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
