Laser Notes

Laser

Introduction

• Laser impacts various fields. LASER: Light Amplification by Stimulated Emission of Radiation.

• Maiman created the first laser in 1960 (chromium-doped Ruby).

Spontaneous Emission and Stimulated Emission

• Atoms transition between energy levels by absorbing or emitting photons ($h\nu = E<em>2 - E</em>1$).

• Spontaneous Emission: Atom releases energy as a photon after a short lifetime ($\approx 10^{-8}$ s); common in ordinary light.

• Stimulated Emission: Incident photon causes excited atom to emit another photon of same frequency, phase, and direction, creating coherent light.

Spectrum Emission

Spontaneous Emission

• Atom spontaneously emits a photon when relaxing to lower state.

• Short lifetime ($10^{-8}$ s), arbitrary phase/direction, wide range of wavelengths, intensity decreases (inverse square law).

• Dominant in ordinary light sources.

Stimulated Emission

• External photon stimulates excited atom to emit energy as another photon.

• Requires metastable state (long lifetime, $10^{-3}$ s).

• Two photons (original + stimulated) with same properties, forming parallel beam.

• Single wavelength, constant intensity, no spreading.

• Dominant in laser sources.

Conditions of Stimulated Emission

1. External photon matches energy needed to excite the atom.

2. Excited atom in metastable state (long lifetime).

3. Stimulation occurs before lifetime ends.

Properties of Laser Beam

1. Monochromaticity (Spectral Purity)

• Laser emits single spectral line with limited bandwidth.

2. Collimation (Parallel Rays)

• Laser beam diameter remains constant over distance.

3. Coherence (Same Phase and Direction)

• Photons emitted coherently, maintaining same phase.

4. Intensity (High Concentration)

• Laser maintains constant intensity, not following inverse square law.

Laser Applications

• Lasers span electromagnetic spectrum (visible, UV, IR).

a) Holography

• Uses reference beam to create interference fringes (hologram) for 3D imaging.

b) Medicine

• Treats retinal detachment, nearsightedness, farsightedness with thermal energy.

• Used in medical endoscopes.

c) Communications

• Used with optical fibers as alternative to cables.

d) Industry

e) Military Fields

• Precision guidance (LADAR).

f) Recording on Compact Discs (CDs)

g) Laser Printer

• Transfers info to drum using laser beam.

h) Arts and Light Shows

i) Surveying Work

• Determines dimensions.

j) Space Research

Laser Applications by Property

1. Intensity (Heat Effect)

• Medicine, Industry, Laser Printing, CD Recording.

2. Collimation (Long Distance)

• Surveying, Military Applications, Space Research.

3. Coherence (In Phase)

• Arts, Laser Shows, Holography, Communications.

Holography

• Phase difference: $\text{Phase difference} = \frac{2 \pi}{\lambda} \times \text{Path difference}$

Plane (2D) Images

• Records intensity ($I \propto A^2$).

Hologram (3D)

• Records intensity and phase.

Holography Operation

1. Laser split: one beam illuminates object, other acts as reference.

2. Beams meet at plate, interfering.

3. Plate shows fringes, creating hologram.

4. Illuminating hologram reveals 3D image.

Laser Applications and Scientific Basis

• Intensity: Medicine, Laser printing, Industry, CD recording.

• Collimation: Communication, Space research, Military applications, Surveying.

• Coherence: Holography, Arts and laser shows.

Main Components of Laser

• Active Medium, Energy Sources, Resonant Cavity.

Active Medium

• Gas, liquid, or solid with metastable state.

Sources of Energy (Pumping)

• Electrical, optical, thermal, chemical excitation.

Resonant Cavity

• External (gas lasers) or internal (ruby laser) mirrors for amplification.

Theory of Laser Action

1. Population Inversion: More atoms in excited than lower state.

2. Stimulated Emission: Emission triggered by external photon.

3. Amplification: Photons amplified in the medium.

Helium-Neon (He-Ne) Laser

Construction

• Quartz tube with He-Ne mixture (10:1) at low pressure.

Source of Energy

• Electric discharge excites He atoms which excite Ne atoms.

Resonant Cavity

• Parallel/concave mirrors; one 99.5% reflective, other semi-transparent (98%).

Operation

1. Voltage excites He atoms.

2. He collides with Ne, exciting them (population inversion).

3. Ne relaxes, emitting photons.

4. Photons reflected, stimulating more emission.

5. Amplification occurs; laser beam released.

6. Cycle repeats with He regaining energy.

7. Emission at 632.8 nm (visible light).

He-Ne Laser Operation

1. Voltage excites He, which excites Ne (population inversion).

2. Ne relaxes, emitting photons.

3. Photons reflected, stimulating Ne atoms.

4. Amplification, portion emitted.

5. He collides with Ne, cycle repeats.

6. Emission: 632.8 nm.

He-Ne Laser energy

• He excited from $E<em>0$ to $E</em>e$, Ne from $E<em>1$ to $E</em>2$

Energy Conversion

• Electrical -> Thermal -> Electromagnetic (Laser from Ne).

Points of comparison