Generation of Optical Beat Frequencies from Megahertz to Terahertz with Daul-Wavelength Mode-Locked Lasers

• Main Authors: Sung Cheng-Lin, 宋政霖
• Other Authors: Chen Yung-Fu
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/u6b46y

博士 === 國立交通大學 === 電子物理系所 === 106 === The aim of the thesis is focused on exploring various methods to realize the dual-wavelength lasers at mode-locked operation for generating the optical beat frequencies from megahertz to gigahertz. To begin with, the criterion for the employment of a single gain medium to operate at dual-wavelength oscillation is constructed with a simple formula as a function of the reflectivity of the output coupler, roundtrip cavity losses, and the emission cross section for each desirable lasing wavelength. In the experiments, we design the reflectivity of the output coupler to realize the synchronously self-mode-locked operation at 1064 and 1123 nm in a diode-end-pumped Nd:YAG laser for generation of 14.7-THz optical beat waves and an experiment of the synchronously self-mode-locked operation at 1061 and 1064 nm for achieving 0.67-THz optical beating by exploiting a monolithic Nd:YAG laser under cryogenic process. The two experiments offer a simple way to fulfill the dual-wavelength lasing for the 4F3/2 → 4I11/2 transition in a single Nd:YAG crystal. Next, we demonstrate the dual-gain-media laser systems for emitting the dual-wavelength emissions with the advantage of controllable output intensities for each lasing line. We experimentally utilize a diffusion-bonded Nd:YVO4/Nd:GdVO4 composite crystal and a semiconductor saturable absorber to achieve a dual-wavelength mode-locked laser with full modulation in the 0.31-THz optical beating. For achieving the higher order beat frequency, the other experiment is conducted by coupling the self-mode-locked monolithic Nd:YAG laser with 946-nm emission to an self-mode-locked Nd:YVO4 resonator with 1064-nm emission. With the synchronization of the 1064- and 946-nm mode-locked pulses, the optical beating is up to 35.2 THz. Finally, the simultaneous self-mode-locking of two orthogonally polarized states in Nd:YAG lasers are generated to produce the tunable optical beating. The mechanism of the orthogonal polarized Nd:YAG laser is theoretically and experimentally explained by thermally-induced birefringence of the Nd:YAG crystal. The order of optical beat frequencies induced by pump power is found to be in the order of tens of megahertz. In order to extend the optical beat frequencies between orthogonally polarized components to gigahertz regimes, the mechanical stresses are additionally applied to a monolithic Nd:YAG laser. The experimental results reveal that the beat frequencies can be tuned from 0.14 GHz to 1.48 GHz by controlling the external forces from 0.056 N to 0.620 N.