The purpose of this thesis was to design, implement and analyze power sub-nanosecond light source that is able to generate powerful optical pulses with lengths shorter than 100 ps on one side and lengths up to 2 ns on other side of the range. Their peak powers had to have values between 500 mW and 1 W and the light source also had to have option for programmable tuning of pulse duration, amplitude and repetition frequency in the range between 100 kHz and 50 MHz. Developed device presents more practical and economical solution of seed source for pulse fiber laser systems, that are build for applications where mentioned pulse lengths are used.
The background and motivation for development of such light source are presented in the beginning of the introduction chapter. The difference between removal of material with melting (evaporation) and cold ablation, where material is removed without thermal deformation of a workpiece, is explained. It turns out, that this is correlated with duration and power of optical pulse. The cold ablation process is interesting for micromachining of materials, where master oscillator power amplifier (MOPA) type fiber lasers systems are used. Mentioned system consists from two main sections. They are, seed source (oscillator) and optical amplifier. What a typical fiber amplifier and chain of them is made of and description of typical seed sources like mode-locked laser, microchip laser and laser diode is presented in the second part of introduction chapter.
Because mode locked laser and microchip laser, as they are, do not allow simple tuning of pulse duration and repetition frequency, we decided to try and achieve this with direct modulation of a laser diode. In order to extract very short starting oscillation from a diode's gain switch response and for generation of longer optical pulses, we need suitable electric driving. Since we need electrical pulses with current amplitude round 1 A and durations from 500 ps to 2 ns at repetition frequencies from 100 kHz to 50 MHz, only candidate devices like photoconductive semiconductor switch, avalanche transistor, step recovery diode (SRD) and gallium nitride (GaN) high electron mobility (HEMT) transistor have shown potential for achieving needed parameters of electrical pulses. How each of the selected devices work and also how pulse generators built with them perform, is explained in second chapter. The analysis of photoconductive semiconductor switch was made analytically, where other three elements were tested empirically. At analyzing avalanche transistor, besides basic working of the element, two circuits for voltage magnification were also tested. The circuits were serial connection of transistors and Marx type circuit. SRD diodes were in our case used for pulse shaping of a pre-generated longer pulse, to which we sharpened rise time, fall time and by controlling bias currents through diodes, we were also able to tune its duration. GaN HEMT transistor was used in amplification chain where baseband
amplification was used for attaining powerful electrical pulses. Chain was consisted of pulse generator followed by amplifiers.
As it turned out during the experiments, only with GaN HEMT transistor were we able to achieve all the parameters for electrical pulses. It was used as key element in main amplification stage in the baseband amplification concept. There are two more segments that are part of mentioned concept. Before main amplification stage there is preamplification stage consisting of two wideband (DC - 6 GHz) monolithic microwave integrated circuit (MMIC) type amplifiers and before this stage there is low power pulse generator. The latter is built from emitter coupled logic (ECL) components and also includes a delay line for tuning pulse duration. How this baseband amplification chain is assembled and what its response is, is presented in the third chapter.
In our case the light source is assembled from two parts. One is electrical pulse generator and the second is laser diode. The properties of optical pulses from this source are dependent on combination of both parts. In the fourth chapter it is explained how we combined pulse generator with three different seed laser diodes in order to attain the best possible optical pulses. Two of tested laser diodes were distributed feedback (DFB) type and one was Fabry-Perot (FP) type laser diode. First we tested how to obtain the shortest and most powerful optical pulse via the extraction of the first oscillation of gain switch response. We were aiming to get this pulses with as less as possible secondary oscillations or tails that usually follow first oscillation. Second test was the optical response of diodes when they are driven with electrical pulses up to 2 ns. Finally we measured optical spectrums of selected laser diodes and also analyzed how their temperature influences on optical pulse stability.
For final experiment we used light source for seeding fiber amplification chain. It generated pulses with duration of 60 ps, amplitude of 500 mW and repetition frequency of 40 MHz. These pulses were then divided into bursts containing 20 pulses at burst repetition frequencies from 1 kHz to 20 kHz. Because of the high average power of the pulses from source and with conjunction with between stage spectral filtering and pulse pumping of last amplifier stage, we were able to obtain reduction of amplified spontaneous emission and improvement in signal to noise ratio compared to continuous pumping. This experiment is explained in the fifth chapter.
|
