Types of Lasers and CO2 Vibrational Modes

Types of Lasers

• Carbon dioxide (CO2) lasers are molecular gas lasers
• Electrons in molecules can be excited to higher energy levels, defining the electronic state of the molecule
• Molecules have other energy levels besides electronic levels
• Carbon dioxide (CO2) is a symmetric molecule (O=C=O) with three modes of vibration

Vibrational Modes of Carbon Dioxide

• Symmetric vibration: Carbon atom stationary, oxygen atoms move simultaneously along the molecular axis (0.172 eV)
• Bending vibration: Oxygen and carbon atoms vibrate perpendicular to the molecular axis (0.083 eV)
• Asymmetric vibration: Oxygen and carbon atoms oscillate asymmetrically, with oxygen moving in one direction while carbon moves in the opposite (0.291 eV)

Active Medium

• Mixture of CO2, N2, and He or water vapor

Optical Resonators

• Pair of concave mirrors on either side of the discharge tube
• One mirror is completely polished, the other partially polished

Population Inversion

• Achieved through applying an electric discharge to the gas mixture
• Electron collisions excite CO2 molecules to higher electronic, vibrational, and rotational energy levels
• Radiationless transitions populate higher levels
• Presence of other molecules (like N2) facilitates resonant energy transfer, enhancing pumping efficiency
• Nitrogen acts like helium in a He-Ne laser

Carbon Dioxide Laser Characteristics

• Generates continuous laser beam with high power output (kilowatts) and high spectral purity and spatial coherence
• Energy level structure more complex compared to atoms/ions (electronic, vibrational, rotational motions)

Energy-Level Diagram

• Illustrates vibrational-rotational states and physical processes in the laser
• Electric discharge excites nitrogen molecules, transferring enough energy to excite them to their first vibrational-rotational energy level (4(001))
• Majority of electrical discharge energy absorbed by nitrogen molecules; small fraction absorbed by CO2 molecules to excite them from (000) to (001)
• Energy transfer occurs between N2 and CO2 molecules, further populating excited vibrational states in CO2

Energy Release and Laser Emission

• Excited CO2 molecules release energy and transition to lower vibrational states (e.g., (100), (020))
• Energy release corresponds to laser light emission at frequencies of 10.6 μm or 9.6 μm
• Residual decay transitions (from (100) to (010), (020) to (010), (010) to (000)) primarily dissipate as heat, not light

Additional Characteristics

• Population of the fourth energy level in CO2 is increased by the presence of He molecules, which helps depopulate lower laser levels
• Output power of a CO2 laser is directly proportional to the length of the active medium
• Continuous-wave (CW) CO2 lasers with low power outputs (up to 50 W) are often sealed tube configurations

Advantages of CO2 Laser

• Simple construction
• Continuous wave (CW) output for applications requiring steady laser beams
• High efficiency (significant electrical input converted to laser output)
• High output power beneficial for industrial and medical applications
• Scalable power output (length of the discharge tube adjusted for greater control over power levels)

Disadvantages of CO2 Laser

• Contamination effects (presence of oxygen can form CO, affecting efficiency)
• Temperature sensitivity (operating temperature significantly influences output power; optimal temperature critical for consistent performance)
• Corrosion of reflecting plates (mirrors, particularly high-reflectivity ones, can corrode over time).
• Risk of eye damage (infrared radiation, invisible to the naked eye, can cause serious eye damage without warning)

Vibrational Energy Modes (CO2)

• V1: Symmetric stretching
• V2: Bending
• V3: Asymmetric stretching
• Each mode corresponds to a vibrational energy level within the CO2 molecule

Explore the fascinating world of lasers, specifically focusing on carbon dioxide (CO2) lasers. This quiz covers the intricacies of vibrational modes in CO2, the active medium used, and the design of optical resonators. Test your knowledge and understand the key principles behind these powerful devices.