Spontaneous & Stimulated Emission, CO₂ Laser

Spontaneous Emission

• Emission of light photon radiation occurs without any external agency.
• Emitted photon travels in a random direction.
• Photons do not get multiplied through chain reaction.
• Less intense radiation.
• Emitted photons cannot be controlled.
• Polychromatic radiation is produced.
• Emitted radiations have no phase relationship.
• This is a key factor for ordinary light emission.

Stimulated Emission

• Emission of light photon radiation occurs with an external agency.
• Emitted photon travels in a particular direction.
• Photons get multiplied through chain reaction.
• More intense radiation.
• Emitted photons can be controlled.
• Monochromatic radiation is produced.
• Definite phase relationship exists.
• Essential for laser operation.

CO₂ Laser Characteristics

• It is a molecular gas and four-level laser.
• The active medium is a gas mixture of CO₂, N₂, and helium.
• Pumping method: Electrical discharge method is used.
• The optical resonator is formed with two concave mirrors.
• Power output: 10 kW.
• Nature of output: Continuous wave or pulsed wave.
• Wavelength of output: 9.6 µm and 10.6 µm (96000 Å and 106000 Å infra-red).

CO₂ Laser Advantages

• Simple construction.
• Continuous output.
• High efficiency.
• Very high output power.
• Output power can be increased by increasing the length of the discharge tube.

CO₂ Laser Disadvantages

• Contamination of oxygen by carbon monoxide affects laser action.
• Operating temperature significantly affects output power.
• Corrosion may occur at the surfaces of the discharge tube.
• High power laser light can damage eyes due to invisibility (infra-red region).

CO₂ Laser Applications

• High-power lasers are used in material processing (welding, drilling, cutting, soldering).
• Suitable for open-air communication due to low atmospheric attenuation (10.6 µm).
• Used in remote sensing.
• Used in treating liver and lung diseases.
• Used in neurosurgery and general surgery.
• Used to perform microsurgery and bloodless operations.

CO₂ Molecule Vibrational Modes

• Symmetric Stretching Mode: Both oxygen atoms vibrate symmetrically, carbon atom is at rest.
• Bending Mode: Both oxygen atoms and the carbon atom vibrate perpendicularly to the molecular axis.
• Asymmetric Stretching Mode: Both oxygen atoms and the carbon atom vibrate asymmetrically.

CO₂ Laser Principle

• Laser transition occurs between vibrational energy states of CO₂ molecules.

CO₂ Laser Construction

• Consists of a quartz discharge tube (5 m long, 2.5 cm diameter).
• The discharge tube is filled with a gas mixture of CO₂, nitrogen, and helium at suitable partial pressures.

Energy Transfer in CO₂ Laser

• Electrical discharge in gas mixture causes electrons to collide with nitrogen molecules, raising them to excited states.
• Excited nitrogen molecules transfer energy to CO₂ molecules, leading to population inversion.

Laser Transitions in CO₂ Laser

• Transition E₅-E₄: Produces a laser beam of wavelength 10.6 µm.
• Transition E₅-E₃: Produces a laser beam of wavelength 9.6 µm.
• Typically, 10.6 µm transition is more intense than 9.6 µm transition.

Helium's Role in CO₂ Laser

• Helium gas conducts heat generated in the central region of the discharge tube to the walls.

Military Laser Applications

• Laser range finders and LIDAR are used to determine the distance to objects.
• Ring laser gyroscopes are used for sensing and measuring very small angles of rotation.
• Lasers serve as secretive illuminators for night reconnaissance with high precision.
• Lasers can dispose of the energy of a warhead by damaging the missile.

Science and Technology Laser Applications

• Studying Brownian motion of particles.
• Proving that the velocity of light is constant in all directions using a helium-neon laser.
• Counting the number of atoms in a substance.
• Retrieving stored information from CDs.
• Storing large amounts of data in CD-ROMs.
• Measuring pollutant gases and atmospheric contaminants.
• Determining the rate of rotation of the Earth accurately.
• Use in computer printers.
• Producing three-dimensional pictures in space without lenses.
• Detecting earthquakes and underwater nuclear blasts.
• Setting up invisible fences using gallium arsenide diode lasers.

Communications Laser Applications

• Laser light facilitates optical fiber communications for long-distance, low-loss data transmission.
• Laser light is used in underwater communication networks.
• Lasers are used in space communication, radars, and satellites.

Industrial Laser Applications

• Lasers cut glass and quartz.
• Electronic industries use lasers to trim components of integrated circuits (ICs).
• Lasers are used for heat treatment in the automotive industry.
• Reading prefixed prices of products via bar codes.
• Semiconductor industries use UV lasers for photolithography (manufacturing printed circuit boards and microprocessors).
• Lasers drill aerosol nozzles with precision.

Semiconductor Lasers

• Specially fabricated PN junction devices that emit coherent light when forward biased.
• Both P and N regions are highly doped.
• PN junction formed with two heavily doped semiconductors.
• Two opposite faces polished, other two roughened.
• Polished faces act as optical resonant cavity.
• Recombination of electrons and holes at the junction is responsible for light emission (active region).
• GaAs is the typical semiconductor material used.
• Threshold current is required to achieve population inversion and laser action.
• Compact size, low operating voltages, and durable.

Laser Fundamentals

• Lasers generate intense beams of coherent monochromatic light through stimulated emission.
• Distinct properties include coherence, monochromaticity, directionality, and high intensity.

Medical Laser Applications

• Bloodless surgery.
• Destroying kidney stones.
• Cancer diagnosis and therapy.
• Eye lens curvature corrections.
• Fiber-optic endoscope to detect ulcers in the intestines.
• Treatment for liver and lung diseases.
• Studying the internal structure of microorganisms and cells.
• Produce chemical reactions.
• Create plasma.
• Remove tumors.
• Remove caries and decayed portions of teeth.
• Cosmetic treatments (acne, cellulite, hair removal).

Ruby Laser Demerits

• Large input energy is required.
• Output beam is not continuous, it is in the form of pulses called spikes.
• Extra cooling arrangement is essential.

Ruby Laser Applications

• Used as laser range finders and laser weapons.
• Used for surgery and in LIDAR (Light Detection and Ranging).
• Used for laser entertainments.
• Used for drilling, cutting, welding, and trimming of very hard materials.

Ruby Laser Mechanisms

• After a long life time in a state, atoms in that state increase, leading to population inversion.
• Once at population inversion, lasing starts from stimulated emission bringing the atoms to ground state.
• Pumping re-established to repeat the process. Flash lamp causes pulsed output.
• Photons emitted from stimulated emission travel between mirrors creating more photons and amplification.
• Photons are in phase and travel in same direction.
• Output beam is an intense laser.

Ruby Laser Merits

• The output power is of 10 kW to 40 kW range.
• Possesses long life time.
• Narrow line width.
• First successful laser.

Ruby Laser Active Medium

• Transparent pink colored ruby stone.
• Composed of aluminum oxide (Al₂O₃) with 0.05 to 0.5% triply ionized chromium atoms (Cr³⁺) as impurity.
• Pink color is due to Cr³⁺ presence.
• Cr³⁺ replaces Al in Al₂O₃ to form Cr₂O₃ (ruby), the active material.
• Chromium ions (Cr³⁺) in ruby act as active centers, Al₂O₃ as host.

Ruby Laser Resonant Cavity

• Ends of ruby rod are polished flat.
• One end is fully silvered, other partially.
• The two ends are in a resonant cavity.

Ruby Laser Optical Pumping Method

• The helical photographic flash lamp surrounds the ruby rod.
• Filled with xenon.
• Activated by power supply.
• Produces flashes of white light (optical pumping).

Ruby Laser Energy Levels

• Energy levels of Cr³⁺ ions in the crystal lattice are depicted in a figure (not detailed).
• Two wide energy levels E₃ and E₄. Closely spaced levels at E₂ (metastable state as upper lasing level).
• E₃ and E₄ act as excited levels.
• Chromium atoms in ruby absorb blue and green light from xenon.
• Atoms are raised to levels E₃ and E₄.
• Short lifetime at E₃ and E₄.
• Chromium ions go to metastable state, which then generates the laser.

Ruby Laser

• A three-level solid state laser.
• Invented by T.H. Maiman in 1960.
• Includes Ruby crystal, totally reflecting face, partially reflecting face, Xenon-flash lamp, capacitor, and power supply to get the laser beam.
• Ruby crystal of cylindrical shape, about 10cm length and 1cm in diameter, serves as the active medium

Optical Fibre Communication Systems

• Block diagram includes information source (telephone), encoder (A/D converter), optical transmitter (LED or Laser), optical fibre, optical receiver (photodiode), and decoder (D/A converter).
• Fiber is cheaper, has greater space for optical fibres, large information-carrying capacity, and immune to electromagnetic intereference

Optical Fibre Applications

• Flexible light pipes enable seeing hidden corners.
• Fiberscopes (endoscopes) for seeing inside the body without surgery.
• For transmitting digital signals in telephone networks.
• No sparks, suitable near explosives.
• Resistant to nuclear radiation, safe for signal transmission near nuclear installations.
• Scanning the interior of the heart.
• Examining gastro-intestinal systems using endoscopes.
• Transmission between submarines.
• Fibre sensors measure acoustic fields, magnetic fields, and rotation.
• Fibre optic gyroscopes detect fine rotations. Fiber temperatures measure differences.

Optical Fibre Characteristics

• High capacity to carry information.
• Immune to electromagnetic interference.
• Abundant availability of material.
• Cheaper than other methods of communication.
• Useful in defense network. Occupies little space.

Optical Fibre Types

• Three materials typically used are silica glass, multicomponent glass, and plastic.
• Total internal reflection is achieved by refractive index differences between core and cladding or diminishing steps.
• Classified: Step index fibre and graded index fibre.
• Fiber has different index types, graded medium decreases the potential that the optical signal can be lost or misinterpreted

Step Index Fibre

• Core with constant refractive index (n₁).
• Cladding with lower refractive index (n₂). Refractive index decreases core boundary.

Graded Index Fibre

• Core constructed using transparent material with gradually diminishing refractive index.
• Cladding made of material with constant refractive index.
• Bending of light toward the fiber axis makes more of the cladding to reflect than refract

Fibre Optics Essentials

• Basically, material is a thing dielectric material with transparent dielectric material made for refractive index.
• The inner cylinder is used the core, the outer is cladding.
• Propagate by internal reflection at one end and propagate material

Laser Characteristics

• Key properties include directionality, monochromaticity, coherence, and high intensity.
• Directionality: Laser light emits parallel and directional.
• Monochromaticity: Light will then produce high intensity radiation.
• Coherence: lasers the radiate in equal amounts of the beam in the same direction.
• intensity: the intensity is very high, as described in the original document provided, due to directionality of the lasers.

Optical Resonators

• In a laser, the light is directed back and forth.
• Active medium is between two mirrors.
• Photons travel releases spontaneously and triggers stimluated emission on the axis.
• the cavity selects guided radiation parralell

Metastable State in Lasers:

• Intermediate energy level between the excited level and ground level, possessing a long lifetime.
• Atoms reside in this level for an extended duration, aiding in population inversion.
• Facilitates laser transition through stimulated emission from the metastable state to a lower energy level.
• Referred to as the upper lasing level.

Basic Laser Components

• Active Medium: Substance where the lasing action takes place.
• Optical Cavity: 100% reflecting and output window, partially reflecting
• Pumping Source: Provides energy to excite the atoms in the active medium.

Single Mode Fiber:

• one mode of propagation is possible
• diameter of core is about 10micrometers
• refractive indices of core and cladding is very small
• In single mode fibers there is no dispersion, so these are more suitable for communication.
• The process of launching of light into single mode fibers is very difficult

Multi Mode Fiber:

• optical fibers many mummer of modes of propagation are possible.
• the diameter of core is 50 to 200 micrometers
• the refractive indices of core and cladding is also large compared to the single mode fibers.
• Due to multi mode transmission the dispersion is large, so these fibers are not used for communication purposes.
• The process of launching of light into single mode fibers is very easy.

Explores spontaneous and stimulated emission of photons, highlighting key differences like direction, intensity, and phase. Covers CO₂ laser characteristics, including its active medium and pumping method.