Thursday, July 18, 2013

We're Back at BU!
Maureen and Michelle sporting laser safety glasses

I have been lucky enough to return to Boston for another summer in Dr. Bifano's lab in the Photonics Center at BU.   I feel especially fortunate that Michelle has returned too and we are able to work together again.  This year there are five RET teachers, all returning for a second year.  The other three teachers are one each from 2010, 2011, and 2012.  We are working on new projects in labs and helping to formulate what the RET experience will include in future years. 

Last year Michelle and I used a deformable mirror in an adaptive optics control loop that reduced the aberrations in an image.   The application is the improvement of image quality of telescope systems, in which the aberrations are caused by the atmosphere and of systems that image the back of the eye, in which the aberrations are caused by the tissue and fluid in the eye.  See my blog posts from last year for more details.  

This year we are using a deformable mirror in a different application: fiber optics.  My experience with fiber optics was limited.  I knew that fiber uses light to transmit large amounts of data, and I knew that total internal reflection is the phenomenon that keeps light traveling along the fiber.  I have learned that there are two main types of fibers:  single mode and multimode.  The difference is in the diameter of the fiber.  A single mode fiber is very narrow and only passes one mode of light. The light leaving the fiber is a single coherent beam. A multi mode fiber has a larger diameter (ours is 50 microns) which makes it easier to couple light to it.  The larger diameter results in light bouncing off of the walls inside the fiber at different angles.  The resulting light that comes out of the fiber is the sum of all of these rays.  The sum creates an interference pattern that looks like speckles.  The number of speckles corresponds to the number of modes which can be in the thousands.  
Single Mode (yellow) and Multimode (orange) Fibers

Our goal is to control the light going through the fiber with a deformable mirror (DM) such that one speckle will be brighter than the others.  Instead of a wide speckle pattern we would see one coherent bright spot.