Laser Properties and Types

Outline of the Presentation

• Topics covered include wave optics, laser properties, basic laser concepts, three and four level systems, laser types, and applications.
• Specific lasers discussed include Ruby, Nd-YAG, CO2, and He-Ne lasers.

Introduction to Wave Optics

• Wave optics studies phenomena like wavelength, crests, and troughs of light waves.
• Electromagnetic spectrum ranges from gamma rays to radio waves, with visible light in between ultraviolet and infrared.
• Shorter wavelengths have higher frequency and energy, and longer wavelengths have lower frequency and energy.
• Constructive interference occurs when waves are in phase, resulting in a combined wave with larger amplitude.
• Destructive interference happens when waves are 180 degrees out of phase, resulting in wave cancellation.

What is LASER?

• LASER stands for Light Amplification by Stimulated Emission of Radiation.
• A laser device produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher energy levels.
• When these excited particles return to lower energy levels through stimulated emission, they emit energy in the form of highly coherent photons.

Properties and Characteristics of Lasers

• Monochromatic light has a narrow range of wavelengths, essentially a single wavelength and color.
• Unidirectional light travels in a single direction with less divergence, made possible by using a resonant cavity.
• Variation of divergence with distance measures directionality.
• Coherent light consists of photons with a constant phase relationship in time and space.
• High Intensity is due to stimulated emission.

Basic Concepts Involved in Laser Operation

• Absorption: Atoms absorb energy and electrons get excited to a higher energy level
• Spontaneous Emission: Electrons in an excited state return to the ground state by emitting photons, creating incoherent and polychromatic light; electrons stay in the excited state for ~10-8 seconds.
• Stimulated Emission: An incident photon interacts with an excited electron, forcing it to return to the ground state and emit two photons.
• The emitted light is coherent and monochromatic.
• Population Inversion (N1N2): Achieving more atoms in an excited state than in the ground state.
• Active systems achieve population inversion.
• Pumping: The method of raising atoms to a higher energy state.
• Metastable State: An excited state with a relatively long lifetime (~10^-3 seconds).
• Active Medium: Also known as a gain medium, it's where stimulated emission and lasing action occur.
• Lifetime: The time an electron stays in a particular energy level.
• Excited state is 10^-8 second.
• Metastable state - 10^-3 second.
• Resonant Cavity: Provides unidirectional laser beams.
• Enhances light amplification inside the laser.
• Pumping : Atoms raised from a lower energy level to a higher energy level.

Three & Four Level Systems

• Diagrams are shown illustrating energy level transitions in three-level and four-level laser systems.

Types of Lasers

• Solid State Laser types include Ruby and Nd-YAG lasers.
• Gas Laser types include CO2 and He-Ne lasers.

Solid State Laser: Ruby Laser

• A cylindrical ruby rod with aluminum oxide doped with 0.05% chromium oxide makes the laser.
• One rod end is fully silvered, and the other is partially silvered to act as a resonant cavity.
• A glass tube surrounds the rod, which is surrounded by a helical flash lamp filled with xenon gas.
• The flash lamp light excites chromium ions to higher energy states.
• Following excitation, after 10^-8 seconds, ions get transmitted to the metastable state.
• Laser radiation of wavelength 6943 Aº (694.3 nm) with laser emission in pulsed mode.
• A xenon lamp is used for optical pumping.
• Operates based on three energy levels,
• Ruby lasers used in holography, tattoos, and hair removal.

Solid State Laser: Nd-YAG Laser

• Laser radiation with a wavelength of 1064 nm and laser emission produced by Nd-YAG lasers.
• It is made from neodymium-doped yttrium aluminum garnet.
• Nd is the active medium in Nd:YAG lasers.
• Used in cosmetic surgery, dermatology, optical tweezers, and cancer tumor treatment.
• Optical pumping is used to achieve population inversion.
• Xenon flash lamps is used for optical pumping.
• Nd-YAG lasers are four level laser systems.

Gas Laser: CO2 Laser

• The CO2 gas laser is one the most powerful and efficient laser available (up to 30%).
• The laser has produced continuous wave (CW) powers over 100 kW and pulsed energies up to 10kJ.
• Three normel modes of vibration like Asymmetric stretch, Bending mode, and Symmetric stretch mode.
• Has applications in materials processing, cutting, drilling, and etching.
• The laser consists of a discharge tube with one end having a complete reflector and the other a partial reflector.
• The system is filled with a mixture of CO2, N2, and He gases at a pressure of a few mm of mercury.
• High DC voltage provides electric discharge, breaking CO2 molecules into CO & O (O2 corrodes electrodes)
• Uses electrical pumping.
• The vibrational and rotational modes of the CO2 cannot be excited by photons so when voltage is applied across gas, electrons colloids with the N2 molecules and excite them to their vibrational levels.
• Laser emission is continuous mode.
• It is a four level laser system.
• The excited N2 molecules populate the CO2 vibrational state via collision.
• LASER wavelengths produced include 10.6 micrometer and 9.3 micrometer.

Gas Laser: He-Ne Laser

• Helium and Neon fills a narrow discharge tube in a ratio of 85%/15%.
• The tube is enclosed between fully and partially reflective mirrors which serve as an optical cavity.
• Population inversion is achieved via electrical discharge (electrical pumping).
• Emits a beam with continuous mode.
• It is a four level laser system.
• Requires a high-voltage source.

Applications of Lasers

• In optical fiber communication, lasers provide very high bandwidth compared to radio and microwave.
• Large bandwidth, more data sent.
• More channels can be simultaneously transmitted.
• Dennis Gabor developed holography in 1947 and won the Nobel Prize.
• Holography records and reconstructs three-dimensional images of an object.
• Stored on every point of a hologram.
• Uses the interference of two laser beams: one from the object and one reference beam.
• 2-D recording vs 3D recording.
• Ordinary light can be used vs. laser light can be used only.
• Lens system vs Lensless stem
• Lasers are used in industry to blast holes in diamonds and hard steel.
• Lasers cut, drill, weld, and remove metal, even in inaccessible areas.
• Laser range finders measure surveyor distances.
• Lasers are used in eye treatment for detached retinas, and blood vessel removal from the retina.
• Lasers in military systems allow target ranging to determine speed and distance.
• Lasers are used as weapons
• High-energy lasers can destroy enemy aircraft and missiles.

Advantages of Lasers

• Lasers are used to cut glass, drill holes in ceramics, perform bloodless surgery, kidney and gallstone destruction, and study microorganisms.
• Also used in air pollution analysis.