Gas Lasers

Gas lasers use a gaseous medium for lasing action and are classified into:

1. Neutral atom lasers – e.g., Helium-Neon (He-Ne) laser

2. Ion lasers – e.g., Argon ion laser
3. Molecular lasers – e.g., Carbon dioxide (CO₂) laser

Helium-Neon (He-Ne) Laser

Introduction:

• It is a neutral atom gas laser.

• Common wavelength: 632.8 nm (red light).
• It is tunable from infrared to various visible wavelengths.
• Typical output power: less than 50 mW.
• It is the most widely used visible light laser.

Working Principle:

1. Gas Mixture:
o Mixture of Helium (He) and Neon (Ne) gases.
o Pressure: ~1 Torr He and ~0.1 Torr Ne.
2. Pumping Mechanism:
o A DC electric discharge is applied to the gas mixture inside a discharge tube.
o Helium atoms are excited first by the electrical energy.
3. Energy Transfer:
 o Helium atoms transfer energy to Neon atoms through collision.
o Neon atoms are excited to a metastable energy level close to that of excited
helium.
4. Laser Emission:
o Excited Neon atoms drop to lower energy levels and emit laser light at 632.8
nm.
o It follows a 3-level energy system.

Construction of He-Ne Laser:

• Discharge Tube: Contains He and Ne gases.

• DC Source: Provides pumping energy.
• Ballast Resistor: Controls current and prevents damage.
• Optical Resonator: Consists of two mirrors at both ends:
o One fully reflective
o One partially transmissive (output coupler)
• Brewster Windows: Placed at both ends to minimize reflection losses and ensure
polarized output.

Energy Level Diagram (Three-level System):

(A labelled diagram of energy levels should be drawn here)

 • Shows excitation of He atoms, energy transfer to Ne, and the laser transition in Neon.
• Key levels:
o He: Excited state
o Ne: Upper laser level → Lower laser level → Ground state

Transverse Modes (TEM):

• The laser supports different Transverse Electromagnetic Modes (TEM).

• By adjusting cavity reflectors, various TEM modes (like TEM₀₀, TEM₀₁) can be
observed.
• These modes define the beam shape and intensity distribution.

(Add a diagram showing different TEM modes if possible)

Applications:

• Holography
• Laser scanning and printing
• Metrology (precision measurement)
• Optical fiber communication
• Scientific demonstrations and alignment tools
Conclusion:

The Helium-Neon laser is an essential type of gas laser known for its stability,
coherence, and visible output. Its simple working principle and reliable design make it
ideal for a wide range of scientific and industrial applications.

Carbon Dioxide (CO₂) Laser

Introduction:

• CO₂ laser is a molecular gas laser.

• It operates in the far infrared region at a wavelength of 10.6 μm.
• It was first developed by Prof. C.K.N. Pillai (Indian-born American scientist).
• It is a four-level laser system with high efficiency and high output power (up to 10
kW).

Vibrational Modes of CO₂ Molecule:

A carbon dioxide molecule consists of one carbon atom between two oxygen atoms. It
exhibits three major vibrational modes:

1. Symmetric Stretching Mode:

o Oxygen atoms move in and out simultaneously while the carbon atom remains
stationary.

2. Bending Mode:
o All atoms vibrate perpendicular to the molecular axis.
 3. Asymmetric Stretching Mode:
o Oxygen atoms move in one direction and the carbon atom in the opposite
direction.

Principle of CO₂ Laser:

• Laser transition occurs between vibrational energy levels of the CO₂ molecules.
• A mixture of CO₂ (active medium), N₂ (excitation medium), and He (cooling gas)
is used.
• Energy is transferred from excited N₂ molecules to CO₂ molecules, resulting in laser
emission.

Construction:

• Discharge Tube: Quartz tube of about 5 m length and 2.5 cm diameter filled with a
gas mixture of CO₂, N₂, and He.
• Power Supply: DC power supply for electrical discharge.
• Brewster Windows: NaCl windows ensure polarized output.
• Optical Resonator: Two concave mirrors, one fully reflecting and one partially
reflecting.
Working:

1. Electrical discharge excites nitrogen molecules:

N₂+e∗→N₂∗+e

2. Excited nitrogen molecules transfer energy to CO₂ molecules:

N₂∗+CO₂→CO₂∗+N₂

3. Excited CO₂ molecules undergo transitions between vibrational states, producing

laser radiation.

Laser Transitions:

• E₅ → E₄:
o Produces a laser beam at 10.6 μm (most intense and common).
• E₅ → E₃:
o Produces a laser beam at 9.6 μm.

Energy Level Diagram:

Characteristics of CO₂ Laser:

 Feature Description
Type Molecular gas laser
Active Medium CO₂, N₂, He mixture
Pumping Method Electrical discharge
Optical Resonator Two concave mirrors
Output Power Up to 10 kW
Output Nature Continuous or pulsed
Output Wavelength 10.6 μm (main), 9.6 μm

Advantages:

1. Simple construction
2. High continuous output power
3. High efficiency
4. Power can be increased by lengthening the discharge tube

Disadvantages:

1. Oxygen contamination by CO affects performance

2. Output is invisible, so accidental exposure can harm eyes
3. Reflector corrosion may occur
4. Output is temperature-dependent

Applications:

1. Material processing: Cutting, welding, drilling, soldering

2. Open-air communication: Low atmospheric attenuation at 10.6 μm
3. Remote sensing
4. Medical field:
o Liver and lung treatment
o Neurosurgery and general surgery
o Microsurgery and bloodless operations

Conclusion:

CO₂ lasers are among the most powerful and efficient lasers developed. Their continuous
and high-power output in the infrared region makes them suitable for industrial, medical,
and communication applications.
What is a Laser Resonator?

A laser resonator (or laser cavity) is a structure made of mirrors that allows light to bounce back and
forth, amplifying it to create a laser beam. It determines:

• Laser frequency (color)

• Beam shape
• Stability

🔸 Types of Resonator Configurations

1. Plane Parallel Resonator

• Structure: Two flat (plane) mirrors placed parallel to each other.

• Working: Light reflects straight back and forth.
• Resonant Frequency:

ν=k⋅c/2L

where:
k = integer,
c = speed of light,
L = distance between mirrors

• Use: Simple design, mostly for small lasers.

2. Concentric Resonator
• Structure: Two spherical mirrors with radius R, separated by L = 2R.
• Working: Light focuses to a point in the center.
• Use: Highly stable, used in powerful lasers.

3. Confocal Resonator

• Structure: Two spherical mirrors with radius R, placed at a distance L = R.

• Working: Focus points (foci) of both mirrors coincide.
• Use: Very stable, good mode control.

4. Generalized Spherical Resonator

• Structure: Two spherical mirrors with same radius R, and R < L < 2R.
 • Working: In-between concentric and confocal types.
• Use: Commonly used for flexibility and good performance.

5. Ring Resonator
• Structure: Mirrors arranged in a closed loop (ring or triangle).
• Working: Light travels in a circular path.
• Resonant Frequency:
o ν=kc/Lp

Lp = total loop path length

• Use: No standing wave interference, preferred in advanced systems.

🔸 Stable vs Unstable Resonators

• Stable Resonator: Keeps beam inside the cavity.

✔ Best for lasers ≤ 2kW
✔ Output from center
✔ Good beam quality
• Unstable Resonator: Allows beam to spread out.
✔ Used for high-power lasers (>2kW)
✔ Output from edge (ring-shaped beam)
✔ Lower risk of mirror damage
Cavity Dumping – Explained (10 Marks)

✅ Definition:

Cavity Dumping is a technique used in lasers to extract all the stored energy in the resonator (cavity)
at once, instead of continuously allowing light to escape. It gives a very short and powerful output
pulse.

✅ Why Cavity Dumping is Needed:

• Normal lasers release energy slowly over time.

• Some applications (like fast communication, high-speed imaging) need short, high-intensity
pulses.
• Cavity dumping helps to achieve this by storing energy first, then releasing it suddenly.

✅ Working Principle:

1. Laser Setup:
The laser resonator is made with mirrors that reflect light back and forth. In cavity dumping,
the output mirror is usually 100% reflective (no output normally).
2. Pulse Build-up:
A pulse builds up inside the cavity and becomes stronger and stronger because the light is
not allowed to escape.
3. Dumping the Cavity:
A fast switch (like an acousto-optic or electro-optic modulator) is used to suddenly change
the path of the light and dump all the energy at once to an output.
4. Result:
You get a very intense and short laser pulse (in nanoseconds or even femtoseconds).
✅ Key Components:

Component Function

100% Reflective Mirrors Store light energy inside the cavity

Fast Optical Switch Releases stored light as a pulse (very fast)

Q-switch (optional) Can be used to control when the dumping starts

✅ Applications:

• High-speed laser communication

• Time-resolved spectroscopy
• Laser micromachining
• Scientific experiments needing ultrashort laser pulses

✅ Advantages:

• Very short and high-power laser pulses

• Better control over pulse timing
• Can generate pulses faster than normal Q-switching
🌟 Levels of Lasers – 20 Marks

Lasers work based on the concept of stimulated emission and population inversion. The number of
energy levels involved in achieving laser action defines the type of laser system. There are four main
types:

🔹 1. Two-Level Laser

✅ Definition:

A two-level laser system involves only two energy levels:

• Ground state (Level 1)

• Excited state (Level 2)

✅ Working:

• Electrons are pumped from level 1 to level 2 using external energy (light or electricity).
• For lasing, electrons must fall from level 2 to level 1 and emit photons.
• To get population inversion (more electrons in level 2 than level 1), a strong light source at
frequency hν is used.

❌ Limitation:

It is very difficult to achieve population inversion in this system because:

• As soon as electrons are pumped to level 2, they quickly fall back to level 1.
• The same transition is used for pumping and lasing, causing efficiency issues.

✅ Diagram:
🔹 2. Three-Level Laser

✅ Definition:

In a three-level laser, there are three energy levels:

• Level 1: Ground state

• Level 2: Metastable state (electrons stay longer)
• Level 3: High-energy pump level

✅ Working:

1. Electrons are pumped from level 1 to level 3 using an external source (like a flash lamp).
2. They quickly fall to level 2 without emitting light (radiationless transition).
3. From level 2 to level 1, stimulated emission occurs, producing laser light.

✅ Example:

Ruby Laser is a common example of a three-level laser.

✅ Key Point:

Population inversion is between level 2 and level 1.

✅ Diagram:
🔹 3. Quasi-Three-Level Laser

✅ Definition:

A quasi-three-level laser works similarly to a three-level laser, but:

• The lower laser level is slightly above the ground state.

• The population inversion is easier to achieve than a pure 3-level system.

✅ Working:

• Excitation occurs by absorption of electromagnetic radiation or collisions.

• Electrons absorb energy equal to E2 - E1 = hν and jump to the upper state.
• Stimulated emission occurs between upper and lower laser levels.

✅ Key Point:

Though it involves three energy levels, it behaves slightly like a four-level system since the lower
level is quickly emptied after emission.

✅ Example:

Nd:YAG laser is a common quasi-3-level system when operated at 0.94 μm wavelength.

🔹 4. Four-Level Laser

✅ Definition:

A four-level laser system uses four energy levels:

 • Level 1: Ground state
• Level 2: Lower laser level
• Level 3: Upper laser level
• Level 4: Pump level

✅ Working:

1. Pumping excites electrons from level 1 to level 4.

2. Electrons quickly fall to level 3 (non-radiative).
3. Stimulated emission happens between level 3 and level 2.
4. Electrons fall quickly from level 2 to level 1, completing the cycle.

✅ Advantages:

• Easier to maintain population inversion.

• Lower threshold pump power needed.
• Level 2 is above the ground state, so it is quickly depopulated.
• No re-absorption of laser light.

✅ Example:

• Nd:YAG Laser (when operated at 1.06 μm)

• This is the most common four-level solid-state laser.

✅ Diagram:

🔸 Comparison Table

Feature Two-Level Three-Level Quasi-Three-Level Four-Level

Number of levels 2 3 3 (effective) 4

 Feature Two-Level Three-Level Quasi-Three-Level Four-Level

Population inversion Very difficult Difficult Easier Very easy

Lower laser level Ground state Ground state Near ground state Above ground

Efficiency Low Moderate Better High

Example Theoretical Ruby Laser Nd:YAG (0.94 µm) Nd:YAG (1.06 µm)

✅ Conclusion:

Different types of laser systems use different numbers of energy levels to achieve population
inversion. Among them, four-level lasers are the most efficient and widely used.
Understanding these level systems helps in designing lasers suitable for different scientific,
industrial, and medical applications.

Semiconductor Laser (PN-Junction Laser)

Definition

A semiconductor laser is a highly doped p-n junction diode made of materials like Gallium
Arsenide (GaAs). It emits coherent light when forward biased.

Construction
• Made from GaAs with heavily doped p and n regions.
• p-region: Doped with zinc.
• Size: ~500 µm long, 100 µm wide.
• Metal contacts on top and bottom for current.
• Front and rear faces polished to act as mirrors (resonator).
Working

• Forward bias causes electrons and holes to recombine at junction.

• At threshold voltage, population inversion occurs.
• Stimulated emission starts: photons stimulate more emissions.
• Laser light is amplified inside the cavity and exits from one side.

Output

• GaAs emits light at 900 nm (IR region).

Features & Uses

• Small, efficient, low power
• Used in CD players, barcode scanners, optical fibers, printers, etc.