Increased tolerance polarization converters

BACKGROUND

Integration of optical functions into photonic integrated circuits (PICs or "optical chips") is a developing technology attracting increasing attention by researchers. In PICs optical functionality is created by bringing together basic optical components. One of these components is the polarization converter (or polarization rotator), which is used to control the state of polarization of light in a chip. This is important because the planar geometry typically results in polarization dependent operation, and because some applications are polarization based (e.g., polarization multiplexing in telecom applications). Polarization manipulation in PICs is also important for polarization independent operation of the integrated circuit chips, and for functionalities like polarization multiplexing and polarization switching. An ideal polarization converter would be a short low-loss passive component, which can be realized within the standard fabrication of a PIC. However, it still remains an unsolved challenge to realize such an ideal.

A number of proposals have been made for polarization converters. The most promising of these proposals seems to be the sloped sidewall devices, which operate as an integrated optical analogue of a half-wave plate. The tolerances to obtain an acceptable level of conversion, however, are relatively narrow. For example, the width deviations should be kept below 50 nm for conversion efficiency above 95%. Thus, due to their tight fabrication tolerances, polarization converters known in the art are not adequate to make them commercially attractive.

SUMMARY OF THE INVENTION

The present invention provides polarization converters that have improved fabrication tolerances. The inventor has discovered the root of the intolerances in prior devices and how fabrication errors can be compensated in novel two-section polarization converter designs. The new devices double the fabrication tolerances and wavelength range and promises conversion efficiencies above 99%.

The inventor has discovered that the main reason for the limited tolerance of prior polarization converters lies in the difficulty to control the polarization states (also referred to herein as polarized modes of the waveguide) in the device, resulting in an orientation error of these states. For example, prior waveguide polarization converters often rely on waveguides modes having polarization states that are at ±45 degrees with respect to the input and output polarization states being converted. In practice, performance of such polarization converters has been found to be dominated by deviations from this ±45 degrees condition (i.e., a tilt error). These deviations can result from non-ideal fabrication. Thus, the effect of fabrication on this angular error is the dominant source of fabrication-induced performance variation for polarization converters.

To overcome these problems, the present invention provides a novel polarization converter design that combines a conventional converter section with a mirror-imaged cross section. Consequently, an orientation error of the same magnitude, but with the opposite sign is obtained. In other words, a polarization converter including two sections, which are each other's mirror image, will compensate the orientation error. The two sections are preferably made in the same fabrication, in the same material, and operate at the same temperature, and optical wavelength. Consequently, they will have the same magnitude of the fabrication error.

The present invention provides an optical polarization converter device comprising a first polarization converter section, and a second polarization converter section; where the first polarization converter section and second polarization converter section have mirror image cross-sections of each other; and where the first polarization converter section and second polarization converter section are made of a common material and have orientation (i.e., tilt) errors equal in magnitude and opposite in sign. The invention also encompasses photonic integrated circuits comprising such optical polarization converter devices.

Desired business relationship

Patent licensing