Submitted on: 18 July 2001.
Revised on: 14 November 2001.
Accepted on: 12 December 2001.
___________________Journal of Physics-Photonics---IOP PUBLISHING LTD
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Submitted on: 15 August 2011.
Revised on: 22 October 2011.
Accepted on: 20 December 2011.
___________________Journal of Physics-Photonics---IOP PUBLISHING LTD
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Submitted on: 06 September 1998.
Revised on: 08 December 1998.
Accepted on: 07 January 1999.
___________________Journal of Physics-Photonics---IOP PUBLISHING LTD
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