Abstract:
Objective Transversely Excited Atmospheric (TEA) CO2 laser can achieve tunable output at 9-11 μm which covers several atmospheric windows. It has improved a lot towards higher peak power and repetition frequency in the applications such as laser radar and laser manufacturing since the beginning of the 21st century. Heat effect as a product of gas discharging may bring unwelcomed effects on laser components and parts. Previous researches mainly focused on thermal deformation of laser windows under the situation of multi-kilowatt output. With the finite element theory, specific deformation data features were studied and corresponding compensation methods were proposed as well. However, thermal deformation of laser resonant cavities was commonly neglected as the huge bulk of cavities had large thermal capacity and good heat dissipation. With the recent development of lidar applications, opposite demand of TEA CO2 lasers for small size and high reliability was put forward. Consequently, thermal deformation of resonant cavity instead of laser window is more likely to take place due to local heat effect. By far rare research works were carried out on this topic. For this purpose, thermal stability and compensation of compact dual-path TEA CO2 laser resonant cavities were studied.
Methods A compact dual-path TEA CO2 laser was developed. Deformation of resonant cavity in the action of local heat effect was measured with an indication beam (Fig.2, Eq.(14)). It was also simulated in both azimuth and pitch directions with Ansys Workbench (Fig.4-5). Hypothesis of the deformation counteract by balancing the local heat effect with thermal expansion was calculated (Fig.7) and experimentally realized by an axial flow DC fan assisted heat extraction structure (Fig.6, Fig.8). Moreover, angular compensation measures were taken after restraining the local heat effect. The azimuth angular compensation was achieved by adjusting servo motor steps under different ambient temperatures. The pitch angular compensation was realized by changing the bottom displacement of the cantilever tuning structure with a PZT actuator.
Results and Discussions The angular deviation of resonant cavity mirrors changed linearly regardless the presence of local heat effect. While local heat effect caused unequal angular deviation between resonant cavity mirrors. By adjusting heat exchanging efficiency, local heat and overall heat expansion reached a balanced counteraction in cavity deformation, thus the deviation of resonant cavities mirrors was limited to a low level within 13-22 ℃. Output pulse energies decreased more gently in the condition of enhanced heat exchange. Angular compensation slopes were confirmed to realize the stability of resonant cavities within a wider temperature range, they were 0.28 step/℃ for servo motor in the azimuth direction and 0.79 μm/℃ for PZT actuator in the pitch direction respectively.
Conclusions Thermal stability and compensation of compact dual-path TEA CO2 laser resonant cavities was studied. The heat effect of distributed inner heat sources was studied both in experimental and simulated method. Active heat exchange plan by adopting an axial flow DC fan was adopted to constrain the local heat effect. Hypothesis of the deformation counteract between local heat effect and thermal expansion was proposed and demonstrated. Moreover, the adaptability in wider temperature range was realized by precise angular compensation. Investigation results provided comprehensive view about thermal deformation in compact volume and angular compensation idea would be a reference for similar laser devices.
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