We have developed a technique of achieving continuous-wave far-IR lasing in NH$-3$/ using a line-tunable mid-IR NH$-3$/ laser which is optically pumped by a CO$-2$/ laser. In the sequential pumping, a number of far-IR laser lines is obtainable from a single CO$-2$/ laser pump line. Continuous-wave laser action on 10 far-IR lines of $+15$/NH$-3$/, including 4 new ones, has been observed in the wavelength range from 60 to 400 $mu@m.
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