[1] P.Pace and M.Lacombe,Optics Comm.,1978,Vol.26,P.405.
[2] D.S.Stark,P.H.Cross and H.Fsiter,IEEE J,Quant.Elect1975, Vol.QE-11,P.774.
[3] P.Pace and M.Lacombe,IEEE J.Quant.Elect.,1978,Vol.QE-14,P.263.
[4] M.J.Taylor,P.H.Davies,D.W.Brown,W.F.Woods,I.D.Bell and C.J.kennedy,Applied Opties,1978,Vol.17,P.885.
[5] H.M.Lamberton and P.R. Pearson,Electron Lett.,1971,Vol.7,P.141.
[6] P.R.Pearson and H.M.Lamberton,1972,Vol.26,P.405.QE-8., P.145.
[7] M.C. Richardson, K.Leopold and A.J.Alcock, IEEE J,Quant.Elect1973, Vol,QE-9,p.934.
[8] D.S.Stark, P.H.Cross and M.R.Harris, J.Phys.E.,1978,Vol.11,P.311.
[9] A.H,Bushnell,M.Gundersen and R, T.Burkes, IEEE J,Quant, Elect.,1976, Vol.QE-12, P.447.
[10] G. S. Dzakowic and S.A, Wutzke,J.App.Phys.,1973, Vol.11,P.5061.
[11] D. C.Hamilton, D.J.James and S.A,Ram sden,J,Phys,E.,1977, Vol.10,P.208.
[12] P. W. Pace and M.Lacombe., IEEF J,Quant.Elect.,1978,Vol.QE-14,P. 263.
[13] R. J. Sanderson, Proceedings SPIE, to be published.