1. A.M. Prokhorov, Molecular amplifier and generator for submillimeter waves, Sov. Phys. JETP, 7: 1140–1141 (1958).
2. A.L. Shawlow and C.H. Townes, Infrared and optical masers, Phys. Rev., 112: 1940–1949 (1958).
3. R.V. Ambartsumyan, N.G. Basov, P.G. Kryukov, and V.S. Letokhov, Laser with nonresonant feedback [Pis’ma Zh. Eksp. i Teor. Fiz., 3: 261–264 (1966) Russian] JETP Lett., 3: 167–169 (1966).
4. R.V. Ambartsumyan, N.G. Basov, P.G. Kryukov, and V.S. Letokhov, A laser with nonresonant feedback [Zh. Eksp. i Teor. Fiz., 51: 724–729 (1966) Russian] Sov. Phys. JETP, 24: 481–485 (1967).
5. R.V. Ambartsumyan, N.G. Basov, P.G. Kryukov, and V.S. Letokhov, A laser with a non-resonant feedback, IEEE J. Quantum Electron., QE-2: 442–446 (1966).