A method for spanning the 100−1000−μ portion of the spectrum with continuously tunable coherent radiation is described. The approach is based upon laser light scattering from the long−wavelength side of the A1−symmetry soft mode in LiNbO3. In contrast with other techniques, this method uses a single fixed−frequency pump source, requires no magnetic field, provides continuous rather than discrete tuning, can cover most of the 100−1000−μ range, operates at room temperature, and is simple to tune. The experimental data show that tuning was obtained from approximately 150 to 700 μ.

A free electron laser operated with a dual electron beam A Nd : Y AG laser has been used to modulate the energy of a free electron beam. With a single pass of the 2-MW laser light through the lOO-MeV electrons we have measured a 37-keV increase in the width of the energy spectrum for the particles. The interaction occurred in helium at standard temperature and pressure over a distance corresponding to 10' optical wavelengths. and a phase synchronism condition was maintained by means of the inverse Cerenkov effect. That is. along the direction of motion of the electrons the phase velocity of the electromagnetic wave equaled the electron velocity. Therefore, the electrons remained in an electric field of constant phase. resulting in significant energy exchange.= c/ n. n is the index of refraction of the medium, u the velocity of the electrons, c is the velocity of light in a vacuum, and 0 is the Cerenkov angle.

We have operated a high-gain angle-tuned singly resonant LiNbO3 parametric oscillator pumped directly at 1.06 μm by a Q -switched Nd:YAG laser. The oscillator angle tunes from degeneracy and operates over the entire 1.4–4.4-μm range. Output energies of greater than 1 mJ/pulse at 5 pps have been observed with a 15% energy conversion efficiency. The key to this device is the large LiNbO3 crystals fabricated from new [01.4] grown boules. Crystals up to 15 mm in diameter and 5 cm in length have been cut at the nominal 47° orientation. These crystals will potentially handle over 2 J of optical energy at 1.06 μm.

We have generated continuously tunable infrared output between 3.5 and 13 μm by coherent Raman mixing in H2 gas at 20 atm using a Nd : YAG laser-pumped LiNbO3 parametric oscillator as the input source. The coherent Raman mixing method offers significant advantages over stimulated Raman scattering for frequency conversion.

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