We correct an error in one of the equations in our original manuscript.
© 2013 Optical Society of America
The second equation in our original manuscript  contains a typo in which appears as . Thus, the correct form of the equation should be
The remainder of the manuscript is unaffected by the change.
Our thanks to Andrey Alenin for pointing out the error.
1. N. Hagen, K. Oka, and E. L. Dereniak, Opt. Lett. 32, 2100 (2007). [CrossRef]
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