Optics on silicon lose polarization dependence
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2007
MIT researchers recently announced that they may have found a solution to polarization-dependence problems in ICs. Light traveling in fiber-optic cables experiences randomized polarization, which makes steering light in silicon difficult. By eliminating this dependence, an IC can process optical signals without needing to convert them to electrical signals.
The MIT researchers overcame the problem by manipulating the light polarization, thus making the light’s polarization parallel to the substrate’s surface. The device best manipulates light that has a polarization state that is parallel to the substrate’s surface.
The process involves separating the signal’s vertical transverse-magnetic (TM) and horizontal transverse-electric (TE) polarization components, rotating one polarization by 90º, sending both through identical structures, rotating the other component by 90º, and combining the polarization components. The process makes the IC immune to polarization errors.
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Researchers at MIT used this setup to measure the optical output power of a polarization-independent IC. |
To find out how the researchers measured the technique’s effectiveness, I met with researcher Peter Rakich at his office on the MIT campus. Rakich explained that amplified spontaneous emission from the tunable laser limits the purity of the polarization state, making some extra steps necessary to obtain high-fidelity measurements.
The signal is first modulated at 5 kHz before going to the polarization controller. A Glan-Thompson polarizer in the first polarization controller cleans the polarization of the light. It also acts as a course polarization tuner, enabling the synthesis of any polarization state through computer control. The second controller maps the polarization into TM and TE states relative to the IC’s surface.
To verify that the device under test (DUT) is polarization independent, Rakich used a computerized polarization controller that generates 100 known polarization states. A spectrometer and InGaAs PIN diode follow the device, which lets researchers filter the laser light and convert it to an electrical signal.
The researchers proved that the DUT was polarization independent because its outgoing optical power was virtually the same for all 100 known polarization states. If the device had not been polarization independent, power measurements would have shown a wide variation.
Press release, “MIT 'optics on a chip' may revolutionize telecom, computing,” Massachusetts Institute of Technology, Cambridge, MA, Feb 6, 2007, web.mit.edu/newsoffice/2007/optics.html
Tymon Barwicz, Michael R. Watts, Milos A. Popovic, Peter T. Rakich1, Luciano Socci, Franz X. Kärtner, Erich P. Ippen, and Henry I. Smith, “Polarization-transparent microphotonic devices in the strong confinement limit,” Nature Photonics, January 2007. www.nature.com/nphoton/journal/v1/n1/full/nphoton.2006.41.html
