LASERS

Lasers

• Lasers are important tools in science, medicine, and industry. They are a concentrated beam of coherent, monochromatic light, traveling in a specific direction. Laser light waves have aligned peaks (in phase).
• This allows lasers to be highly focused and travel long distances.

Coherence

• Spatial coherence ensures a laser beam stays focused, enabling uses like cutting and lithography as well as pointing.
• High temporal coherence means lasers emit a single color of light, which allows for very short pulses (like femtoseconds).

Laser Physics

• Laser physics is a branch of optics. It's concerned with quantum electronics, laser construction, optical cavity design, producing population inversion in laser media, and the temporal evolution of the light field within the laser.

Historical Overview

• Albert Einstein developed the theory of single-frequency light (1917)
• Gordon Gould invented the first laser concept (1957).
• Theodore Maiman created the first working laser system(1960).
• Laser Technology Inc. patented the first police traffic laser (1989)
• The Marksman 20-20 police LIDAR was marketed (1991)

Basic Theory of a Laser (Einstein 1917)

• Atoms have electron clouds and nuclei
• Absorbing high-energy photons raises atoms to excited states.
• Excited atoms can return to lower states via spontaneous or stimulated emission.

Spontaneous Emission

• Electrons spontaneously transition from excited to ground states, emitting photons.
• Emitted photons are incoherent (random phases) and vary spatially and temporally.
• Photons emitted from a tungsten lamp are a good example.

Laser vs. Ordinary Light

• Ordinary light atoms consist of nuclei and electron clouds in different energy levels. Electrons absorb energy to move to higher levels. Then release energy through spontaneous emission to return to the lower level.
• Laser light's photons are emitted in a specific direction and energy. The emitted photons are in phase.

Stimulated Emission

• Electromagnetic radiation triggers electrons to emit.
• Emitted photons are coherent (in phase).
• Laser emission is an example.

Laser vs. Ordinary Light (Production)

• If a photon with appropriate energy hits an atom, it will be absorbed, and the atom becomes excited to a higher energy state.
• The excited atom can release the energy through spontaneous emission by emitting a photon in a random direction. Instead, if the appropriate amount of energy is input into the excited atom, the atom can transition to a lower energy state and emit another photon that travels in the same direction and phase as the initial photon.
• The mechanism is called stimulated emission.

Laser vs. Ordinary Light

• The emitted photon stimulates the atom to emit another photon.
• The emitted photon has the same energy, wavelength, and color.
• The emitted photons are in the same direction and phase. Laser light is coherent.
• Lasers thus have a pure color (single wavelength), and the beam of light travels in a narrow, parallel beam.

Spontaneous vs. Stimulated Emission

• Spontaneous emission occurs when electrons transition to lower energy levels.
• Stimulated emission occurs when incoming photons induce electrons to transition to lower energy levels, and emit a new photon with the same properties (energy, frequency, phase and direction) as the incoming photon..

Production of Laser Light

• The likelihood of a stimulated emission equals that of absorption.
• Increasing excited atoms relative to ground state atoms increases the chance of stimulated emission. Pumping is used to achieve this population inversion (more excited atoms).

Absorption

• Incident photons with the correct energy cause an atom to transition to a higher energy level, absorbing energy. This is the absorption process.

Population Inversion

• Under normal operating conditions atoms tend to occupy lower energy levels more often than higher levels.
• Population inversion is when an upper energy level is occupied more often by atoms than a lower level. This typically requires external excitation of the atoms, which is accomplished through processes called pumping.

Population Inversion & Laser Production

• An upper-state system, which has a population inversion, will release photons of the correct wavelength returning to the ground state.
• This process stimulates further release of additional photons.
• This amplification of photons through stimulated emission leads to the production of laser light.

Population Inversion

• For population inversion, atoms are moved from ground state to the excited state and then to a metastable state from which they transition slower to the ground state.
• The transition to the ground state occurs through stimulated emission.

Spontaneous vs. Stimulated Emission

• Spontaneous emission is a natural process. It doesn’t need external stimuli. Less intense light. Unpolarized (random directions). Incoherent (different frequencies).
• Stimulated emission requires external stimuli. More intense light. Polarized (same direction). Coherent (same frequencies).

Mirrors

• Two mirrors (one partially reflective), form an optical cavity.
• Reflect light back and forth, increasing the chance of stimulated emission.
• The emitted photon travel in the same direction as the initial photon, and in phase. This produces a coherent narrow laser beam.

Conditions for Laser Light

• Stimulated emission
• Population inversion
• Mirrors

Properties of Ordinary Light vs. Laser

• Ordinary light is multi-directional, incoherent, and has low intensity.
• Laser light, on the other hand, is unidirectional (parallel), coherent (same phase), and high intensity. It is monochromatic.

Ordinary Light: High Divergence

• Ordinary light sources emit light photons equally in all directions.
• Intensity of the light decreases according to the inverse square law (intensity is spread out).

Ordinary Light: Incoherence

• Excited atoms in ordinary light bulbs act independently.
• Photons have random phases and frequencies, leading to an incoherent light beam.

Ordinary Light: Low Power and Intensity

• Ordinary light photons are emitted equally in all directions.
• Spreading of energy results in decreasing intensity with distance.
• Incoherent photons at different phases lead to reduced amplitude of combined waves and reduced intensity.

Properties of a Laser Light

• Narrowness: Light travels in a very tight beam.
• Monochromaticity: Light is of the same frequency/wavelength/color.
• Coherence: Light photons are in phase and travel in the same direction.
• High Power and Intensity: Photons emitted in the same direction and in phase lead to higher intensity.

Types of Lasers

• Pulsed Lasers: Atoms are energized periodically. Photons multiply. Excitation is repeated.
• Continuous Lasers: Atoms are energized continuously. Excitation occurs simultaneously. Photons multiply and produce continuous laser output.

Types of Lasers (Lasing Media)

• Gas Lasers (e.g., He-Ne, CO2)
• Dye Lasers (e.g., Rhodamine 6G)
• Metal-Vapor Lasers (e.g., copper vapor)
• Solid-State Lasers (e.g., ruby)
• Semiconductor Lasers (e.g., diode lasers).

Interaction of Laser with Tissue

• Reflection: Light bounces off tissue.
• Scattering: Light disperses within tissue.
• Transmission: Light passes through tissue, traveling in a different direction.
• Heating: Light absorption in tissue releases heat energy.
• Photo-dissociation: Break molecular bonds within cells.
• Shock wave: Breaks mineralized deposits.
• Fluorescence: Light emitted for diagnostic purposes.
• Photo-chemistry: Destroy target material.

Interaction of Laser with Tissue (Additional)

• Absorption: Tissues absorb light and release energy as heat.
• Photocoagulation: Tissue is heated beyond the normal body temperature, which destabilizes proteins. This is used in surgery.
• Photovaporization: Tissues are heated above the boiling point, which turns the water into gas. Application : removal of tissues.

Heat by Laser

• Destructive effects can be precise and selective depending on applications.
• Heating tissues at different temperatures elicits different physiological responses.

Laser Penetration Depth

• Penetration depth is the depth at which light intensity declines to 1/e (37%) of the original value.
• Penetration depth varies according to wavelength. Different wavelengths have varying penetration depths into tissues.

Applications of Lasers in Medicine

• General: Destroying cancerous tissues, sealing blood vessels, and energy delivery.
• Dermatology: Hair removal, cosmetic surgeries
• Ophthalmology: Treating retinal issues.
• Dentistry: Treating soft tissue in the mouth.

Treatment Laser Systems

• Tissue heating,
• Coagulation
• Vaporization
• Tattoo removal
• Cold cutting
• Photoacoustic (lithotripsy)
• Photodissociation.

Lasers in Surgery

• Laser surgery uses lasers to cut tissues. Examples include LASIK and phorefractive keratectomy.
• Laser resurfacing dissolves molecular bonds using lasers. Laser surgery is often used on the eye.

Lasers in Surgery (Additional Examples)

• Lymphoangioma treatment
• Hemangioma treatment
• Photocoagulation of the retina
• Laser angioplasty

Photocoagulation of the Retina

• Laser beam heats blood vessels.
• Blood coagulates, stopping blood flow.
• Typically used in ophthalmology, treating retinal issues.

Treatment of the Retina

• The dark brown melanin pigment of the retina absorbs the green light from an argon laser without damaging other parts of the eye. This is useful for treating retina disorders.
• Selective targeting of tissue regions. This is a type of treatment for eye issues.

Laser Angioplasty

• Used to remove plaque from obstructed blood vessels.
• Performed through fiber optics.
• Can include fluorescence characterization of vessel walls.

Photodynamic Therapy (PDT)

• Dyes selectively concentrate in cancerous tissue after injection.
• Blue violet light (Krypton laser light) is administered through optical fiber, causing dye fluorescence for diagnosis.
• Laser light of a different wavelength destroys the tumor.

Laser in Dermatology

• Techniques rejuvenate and resurface skin by targeting water absorption in the mid-infrared spectrum.
• Treats conditions like wrinkles, sun damage, and acne.

Advantages and Disadvantages of Lasers

• Advantages: Increased precision, less bleeding.
• Disadvantages: Heat can spread uncontrollably, possible damage to surrounding tissues if not controlled.

Revision Questions

• What is a laser?
• Two mechanisms of an atom returning to its lowest energy state.
• What is spontaneous emission?
• What is stimulated emission?
• Difference between spontaneous and stimulated emission.
• What is population inversion?
• Purpose of mirrors in lasers
• Three conditions for production of laser light.
• What is the difference between ordinary and laser light?
• Types of lasers based on excitation processes.
• Principles of laser interaction with tissue.
• Applications of lasers in medical treatment.
• Applications of lasers in medical diagnostics
• Diagnostic Laser Systems
• Laser Doppler Imaging (LDI)
• Laser spectrum

This quiz explores the concepts of photon emission, particularly focusing on spontaneous and stimulated emission processes. Questions address key characteristics of emitted photons, the mechanisms involved, and the properties of laser light compared to ordinary light sources. Perfect for students studying optics and photonics.