Miniature passively Q-switched lasers and their application to nonlinear frequency conversion in microstructured optical fiber

摘要

Laser devices are an integral part of the technological background of our modern society. Laser light transmits data over the fiber-optical cabling of internet, reads DVD disks and bar codes, cuts metals and plastics, etc. Many applications, such as refractive eye surgery, breakdown spectroscopy, and various time-resolved methods, rely on optical power that is delivered in short pulses. The bite of laser light on matter is also greatly enhanced by pulsing. Q-switching is a method of producing pulsed laser light by periodically suppressing the optical feedback of a laser resonator. In passive Q-switching the cycle is run without any external control by the process of saturable absorption. Miniature passively Q-switched lasers are small solid-state devices that produce nanosecond long, kilowatt peak power pulses of coherent light at near-infrared wavelengths. They are simple to fabricate and have found applications, e.g., in range finding, micro machining and spectroscopy. In this thesis the operation of miniature passively Q-switched lasers is studied both theoretically and experimentally. Also, nonlinear frequency conversion of the laser output is applied in microstructured optical fiber. The conversion allows for the laser output to be extended into wavelengths at which no laser sources exist. In miniature lasers the cavity decay time and the thermalization time of the laser multiplets are at close to equal. In order to take this into account, the normalized geometric rate-equation model of passively Q-switched lasers is refined to include the thermalization process. As an experimental case, a high peak power 1123 nm Nd:YAG laser is demonstrated, and the nonsaturable loss level of the Cr:YAG saturable absorber crystal is measured at the laser's wavelength. The output spectrum of a passively Q-switched and frequency-doubled Nd:YAG laser is converted to narrowband visible light ranging from blue to red wavelengths by means of nondegenerate four-wave mixing, and to a broad spectrum of blue light by means of cascaded cross-phase modulation. The output spectra are adjusted by the group-delay profile of the microstructured optical fiber. A broadband source continuously covering the wavelength range of 420-1300 nm is realized by pumping a microstructured optical fiber with a miniature gain-switched Ti:Sapphire laser.