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Methods to enable electro mag netic waves to propagate t hroug h t h e sheath of pla s ma created by a high s peed ve hicle ree n Le rin g a planeta ry at mos phere are reviewed and compa red. Attention is p a id to the possibi lit ies of e mployin g ve ry h igh frequency waves, low frequ ency waves, .of creatin g magneto-ioui c wave modes by imposin g static magn c tic fi elds, a nd of modlflll1g t he plasma by chemICal scedJI1~ an d ae rody nam ic effects. No ne of t he tec hniques are w It ho u t malor ltml tatIO ns and cons ldc rable cxperimen tal st udi es are st ill r equ ired to es tablish thcir feas ibi lity.

An all-fiber 1.48-mum generator based on a laser-diode-pumped Yb-doped double-clad laser and a cascaded Raman wavelength converter has been developed. Second-order Raman Stokes radiation was generated in a phosphosilicate-fiber resonator formed by two pairs of Bragg gratings. A slope efficiency of the Raman converter of 48% with respect to the power emitted by the double-clad Yb laser has been achieved. We obtained an output power of 1 W at a slope efficiency of 34% with respect to the laser-diode array power, with a total optical-to-optical efficiency of 23%.

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