Development of Near-IR and Beam Shaping of Mid-IR Lasers
Thesis (PhD)--Stellenbosch University, 2019.
ENGLISH ABSTRACT : This study seeks to produce short-pulsed laser output with high peak and average powers to meet the demand for industrial ranging applications. Q-switched lasers are suitable for this due to their ability to produce high energy, high average power, high peak power, short pulses and high efficiency at high pulse repetition rates. This research seeks to develop an actively Q-switched 100 kHz source that emits peak powers of ~10 kW with pulse widths in the 1 ns range. To obtain these parameters, a diode end-pumped laser was constructed and two Q-switching devices were investigated, namely Acousto-Optic Modulators (AOMs) and Electro-Optic Modulators (EOMs). An AOM uses an RF-generated acoustic grating to diffract light out of the cavity, inducing a variable loss, which Q-switches it. The advantages are that AOMs do not require high voltages, are usually polarisation insensitive and are well understood. However, the switching speed is limited by the speed of sound in the material and their restricted modulation depth often causes hold-off problems. EOMs require high voltages for their operation. However, when EOMs are not shielded, the high voltages cause electromagnetic interference (EMI) noise, as well as ringing, which result in undesired losses during Q-switching. Electro-optic Q-switching with solid state lasers mostly uses the Pockels effect to rotate beam polarisation and this, together with polarising elements, causes a varying loss within the cavity. This makes it possible to switch the cavity losses quickly since the switching time depends mainly on the high-voltage source-switching speed. EOM Q-switched are also compact, they have high extinction ratios and do eliminate the hold-off problems typically seen in AOMs. A Nd:YVO4 pulsed laser was developed using an AOM as a Q-switch element and its outputs measured. This laser produced pulses of ~2 ns widths and peak powers of over 10 kW at the pulse repetition frequency of 140 kHz. The observations on the AOM Q-switch results show that pulse widths of ~1 ns could not be reached. Double pulsing, which occurred due to slow switching speeds, was also observed at high peak powers. A comparative study was initiated to see if EOMs cannot solve these challenges that are experienced with AOMs. A new method to use these EOMs to Q-switch lasers was subsequently developed. Although the results were similar to those of the AOM, they showed significant potential for further improvement. The results of this research indicate that high peak powers and short pulse widths can be obtained using both methods. The performance and suitability of both the AOM and EOM Q-switch methods were compared in miniature, short-pulsed high-PRF lasers. The second part of this research involved beam shaping of mid-IR light in the 2 µm region. The mid-IR light has the advantages of having both high atmospheric transmission and being considered eye safe, thus making it suitable for free-space communication applications. The advancement of beam shaping, from using physical optics to using digital systems in the visible spectrum, has prompted interest in investigating the same in the mid-IR region. In order to implement a mid-IR communication link that uses spatial modes, suitable encoding and decoding techniques need to be implemented. Two of the techniques that are currently in use are detection using modal decomposition and detection using spiral phase plates. The advantage of the former is that it is dynamic and operates in real time. However, it is only optimised for low powers. The latter has the advantage of being able to operate at high power. Its disadvantage, however, is that it requires the synthesis of a number of optics so as to generate different orbital angular momenta. During this research we employed techniques based on Spatial Light Modulator (SLM) to implement modal decomposition on our structured 2 µm light to extract the modal weightings and intermodal phases. This allowed us to reconstruct the optical fields of interest as well as perform wavefront reconstruction. This work models far-field detection before the next phase of outdoor implementation. Both the detection of the optical fields and wavefront reconstruction by modal decomposition were achieved.
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