Morozov Mikhail Yu. RussiaSaratovSaratov Branch of Kotel’nikov Institute of Radio Engineering and Electronics of the Russian Academy of Sciences, 38, Zelenaya Str., Saratov 410019, Russiamikhail.yu.morozov@gmail.com Author's articles Linear Model of Surface Terahertz Plasmons Amplification in Two Parallel Graphene Sheets Difference-Frequency Generator and Optical Parametric Oscillator Pumped by a Semiconductor Disk Laser: Comparative Study with a Time Delay Model