Lecture 7: Laser and its Applications PDF

Types of Lasers Carbon dioxide laser Introduction CO2 lasers belong to the class of molecular gas lasers. Electrons in molecules can be excited to higher energy levels, and the distributions of electrons in the levels define the electronic state of the molecule. Besides, these electronic lev...

Types of Lasers Carbon dioxide laser Introduction CO2 lasers belong to the class of molecular gas lasers. Electrons in molecules can be excited to higher energy levels, and the distributions of electrons in the levels define the electronic state of the molecule. Besides, these electronic levels, the molecules have other energy levels. Energy states of CO2 molecules Carbon dioxide (CO2) is a symmetric molecule (O=C=O) and it has three modes of vibration: The Vibrational Modes of Carbon Dioxide Symmetric stretching mode In this vibrational mode, the carbon atom remains stationary while both oxygen atoms move simultaneously along the molecular axis, either approaching or moving away from the fixed carbon atom. Bending mode In this vibration mode, oxygen atoms and carbon atoms vibrate perpendicular to molecular axis. Asymmetric stretching mode In this vibration mode, the oxygen and carbon atoms oscillate asymmetrically, with the oxygen atoms moving in one direction while the carbon atoms move in the opposite direction. Active medium It consist of a mixture of Co2, N2 and He or water vapor. Optical resonators A pair of Concave mirrors placed on either side of the discharge tube, one completely polished and the other partially polished.  Population inversion is achieved through the application of an electric discharge to the gas mixture. When an electric discharge is introduced into a tube containing CO₂, electron collisions excite the molecules to higher electronic, vibrational, and rotational energy levels. Additionally, these levels are populated via radiationless transitions from higher excited states. The presence of other molecules, such as N₂, facilitates resonant energy transfer, thereby enhancing the pumping efficiency. In this system, nitrogen serves a role analogous to that of helium in a He-Ne laser.  A carbon dioxide (CO₂) laser is capable of generating a continuous laser beam with a power output of several kilowatts while maintaining a high degree of spectral purity and spatial coherence. Compared to atoms and ions, the energy level structure of molecules is more complex, arising from three distinct sources: electronic motions, vibrational motions, and rotational motions.  The energy-level diagram of vibrational-rotational states illustrates the main physical processes occurring within this laser. When an electric discharge is applied to the tube containing a mixture of carbon dioxide, nitrogen, and helium gases, electrons collide with nitrogen molecules, transferring enough energy to excite them to their first vibrational-rotational energy level. This excited state corresponds to a vibrational- rotational energy level in CO₂ molecules, commonly referred to as level 4 (001). Initially, the majority of the electrical discharge energy is absorbed by nitrogen gas molecules, while only a small fraction of the energy is directly absorbed by CO₂ molecules. This direct absorption promotes CO₂ molecules from their ground state (000) to the excited vibrational state (001). Subsequently, energy transfer occurs through collisions between N2 and CO₂ molecules, where N2 transfers its excitation energy to CO₂, further populating the excited vibrational states of the CO₂ molecules. Once excitation occurs, the CO₂ molecules in the vibrational (001) state will release energy and transition to lower vibrational energy levels, such as the (100) or (020) states. This energy release corresponds to the emission of laser light at frequencies of 10.6 μm or 9.6 μm, respectively. The residual decay transitions from the (100) state to the (010) state, from the (020) state to the (010) state, or from the (010) state to the ground state (000) will result in the dissipation of energy primarily as heat rather than as emitted light.  The population of the fourth energy level in the CO₂ laser is increased by the presence of He molecules, which also contribute to depopulating the lower laser levels. The output power of a CO₂ laser exhibits a linear dependence on the length of the active medium. Continuous-wave CO₂ lasers with low power outputs (up to 50 W) are commonly available in sealed tube configurations. Advantages of Carbon Dioxide Laser Simple Construction. Continuous Output: CO2 lasers are capable of producing a continuous wave (CW) output, making them suitable for a range of applications requiring steady laser beams. High Efficiency: CO2 lasers exhibit high efficiency, with a significant portion of the electrical input converted into laser output, making them energy-effective. High Output Power: CO2 lasers can generate high output power, which is beneficial for industrial and medical applications requiring powerful beams. Scalable Power Output: The output power of a CO2 laser can be increased by adjusting the length of the gas discharge tube, which allows for greater control over the power levels.. Disadvantages of Carbon Dioxide Laser Contamination Effects: The presence of oxygen in the CO2 laser mixture can lead to the formation of carbon monoxide (CO), which can affect the efficiency and performance of the laser. Temperature Sensitivity: The operating temperature significantly influences the output power of the laser. Maintaining optimal temperature conditions is critical for achieving consistent performance. Corrosion of Reflecting Plates: The mirrors used in CO2 lasers, particularly the high-reflectivity mirrors, may suffer from corrosion over time. Risk of Eye Damage: CO2 lasers emit radiation in the infrared spectrum, which is invisible to the human eye. Accidental exposure to the laser beam can cause serious eye damage without any visual warning.  In the context of a CO₂ laser, v₁, v₂, and v₃ refer to the vibrational energy modes of the CO₂ molecule. CO₂ is a linear molecule with three vibrational modes, each corresponding to a specific set of atomic movements: 1.v₁: This is the symmetric stretching vibration mode, where the carbon (C) atom moves along the axis of the molecule, and the two oxygen (O) atoms move symmetrically in and out. It involves the stretching of the C=O bonds in a symmetric manner. 2.v₂: This represents the bending vibration mode, where the two oxygen atoms move in opposite directions, causing a bending motion of the C-O bonds. This is often referred to as the "degenerate bending mode" because both oxygen atoms move in the same plane but in opposite directions. 3.v₃: This is the antisymmetric stretching vibration mode, where the carbon (C) atom moves in the opposite direction to the two oxygen atoms. The O atoms move asymmetrically, stretching the C=O bonds in an antisymmetric manner. Each of these modes corresponds to a particular vibrational energy level within the CO₂ molecule. When CO₂ molecules are excited, they move to higher energy states in these vibrational modes. The energy states are labeled v₁ = 0, v₂ = 0, v₃ = 0 for the ground state (no vibration) and higher values (e.g., v₁ = 1, v₂ = 1, v₃ = 1) for excited states where the molecule is in an excited vibrational mode.