Title : Theoretical investigation of spectral characteristics of CW CO lasers
Abstract:
In free running mode of operation, Continuous wave (CW) CO (carbon monoxide) lasers emit a continuous beam of coherent light over a range of wavelengths, in the mid-infrared region. This wavelength range is particularly useful for studying molecular vibrations and rotations, making CO lasers invaluable tools for spectroscopic analysis of gases and solids.
Free-running CO lasers typically utilize a simple optical cavity configuration consisting of mirrors without dispersive elements like gratings. This simplifies the laser setup and maintenance, while still providing the necessary coherence and power for scientific investigations. The output spectrum of a CO laser typically consists of few lines over a range of wavelengths. These lines result from transitions between different rotational and vibrational energy levels of the CO molecule within the laser cavity. This spectrum depends on many factors such as the specific CO isotopologue used, the operating conditions of the laser (such as gas pressure, mixture and temperature), and the reflectivity of the output coupler mirror.
We have created a predictive model that accurately forecasts the output spectrum of a laser when its operational parameters are supplied. This model has been successfully tested and validated with three distinct laser systems, each uniquely designed. The rate equation model and the methodology of carrying out the calculations, will be discussed in our presentation. The model provides some insight in the working of the CO laser, which is mentioned below.
In an electrically excited laser, the Vibration Distribution Function (VDF) describes the distribution of population density across CO vibration levels. This density is further divided among various rotational lines. At a given gas temperature, the Boltzmann distribution dictates that one rotational line will exhibit the highest population density. The small signal gain coefficient for these lines is then influenced by the type of broadening—Doppler or pressure broadening—which varies based on the gas mixture and pressure. Initially multiple spectral lines from various vibrational levels are observed due to their high gain coefficients. As the process progresses, a phenomenon known as cascade lasing emerges, altering the initial output spectrum significantly. Cascade lasing occurs when a specific P transition links a rotational level from an upper vibrational level to one in a lower vibrational level. This linkage modifies the initial distribution of population density across the rotational levels in the lower vibrational level. The alteration then extends to even lower vibrational levels. Consequently, a rotational line within a vibrational level that initially had a lower gain coefficient may eventually become the most prominent one. In the model, it is presumed that the cascade lasing mechanism occurs at a significantly quicker pace than the rotational relaxation process. This rapidity, coupled with the continuous wave (CW) operation, dictates the ultimate distribution of population density and hence gain among rotational lines. Additionally, the gas’s temperature sets a threshold vibrational band; below which lasing cannot occur. Consequently, these two elements—the speed of the cascade lasing relative to rotational relaxation and the gas temperature—define the laser’s output spectrum. This model will help a designer of CW CO laser to choose the laser parameters judiciously, for use of the laser in a specific spectroscopic application.
Audience Take Away Notes:
- CO lasers emit radiation in large number of lines in the mid infrared region efficiently. Many gases have absorption lines in this region. Thus, it is a good candidate for gas analysis and chemistry experiments. CO lasers are built with varying operating parameters and they differ in their emission spectrum.
- Our model is capable of
- Predicting the output spectrum if the operating parameters of the laser is known.
- Helping in the design of a CW CO laser for a specific application.
- Optimising the performance of a CW CO laser.
- For a scientist involved in the field of gas lasers and specially CO gas laser he can use our work as an additional tool to broaden his research work.
