Laser Physics: Population Inversion Mechanism

Questions and Answers

What is the primary material used in constructing a simple semiconductor laser?

AlGaAs

GaAs (correct)

InP

SiO2

What is the purpose of roughening the sides of the diode in a semiconductor laser?

To enhance the aesthetic appearance

To allow better heat dissipation

To eliminate lasing action in that direction (correct)

To facilitate easier electrical connections

In a semiconductor laser, the conduction band serves as what level in the laser action?

Ground level

Inactive level

Excited level (correct)

Valence level

What must be achieved to create population inversion in a semiconductor laser?

Higher concentration of electrons in conduction band Signup and view all the answers

What effect does low forward current have on the operation of a semiconductor diode laser?

Causes spontaneous emission of photons Signup and view all the answers

Which of the following describes the dimensions of a typical diode chip used in semiconductor lasers?

500 µm long and 100 µm wide Signup and view all the answers

What is primarily contained in the valence band of a semiconductor for laser operation?

Holes Signup and view all the answers

What role does the Fabry Perot resonator play in a semiconductor laser?

It enhances optical feedback Signup and view all the answers

What occurs when the forward current through the junction exceeds a certain threshold value?

A significant population inversion is achieved. Signup and view all the answers

Which of the following best describes the term 'inversion region' in the context of the junction?

A narrow zone with a high density of electrons and vacant states. Signup and view all the answers

Why do spontaneous photons get emitted in the junction region?

Because of chance recombination of electron-hole pairs. Signup and view all the answers

What is the wavelength of light emitted by a GaAs laser in the infrared region?

9000 Å Signup and view all the answers

In which of the following applications are lasers NOT commonly used?

Generating electrical power Signup and view all the answers

What process involves using a laser beam to create holes in materials?

Laser Drilling Signup and view all the answers

Which of the following statements best describes laser communication?

It requires fiber optics for signal modulation. Signup and view all the answers

How do lasers contribute to the surface texturing process?

They create fine features that traditional methods cannot achieve. Signup and view all the answers

What characterizes the light produced by a LASER?

It is highly intense, monochromatic, and unidirectional. Signup and view all the answers

Why is population inversion not possible in a two-level system?

The probabilities for stimulated absorption and emission are equal. Signup and view all the answers

What role does a metastable state play in a laser system?

It allows electrons to remain excited for a longer duration. Signup and view all the answers

In a three-level laser system, which state do atoms decay to before achieving population inversion?

The metastable state. Signup and view all the answers

What is a characteristic of the excited state in a three-level laser system?

It is often short-lived. Signup and view all the answers

Which of the following accurately describes the energy transition in a three-level system?

Transition from the upper level to the metastable level may be weakly radiative. Signup and view all the answers

What happens to atoms when they are pumped to the excited state in a three-level system?

They decay to a metastable state where population inversion occurs. Signup and view all the answers

Which statement is true regarding the energy levels in a two-level system?

Both levels will always have the same population. Signup and view all the answers

What is the primary purpose of cladding in optical fibres?

To prevent physical damage to the fibre Signup and view all the answers

What does the principle of Total Internal Reflection (TIR) depend on?

The refractive indices of the two media Signup and view all the answers

Which of the following statements is true about the behaviour of light in an optical fibre?

A small fraction of light may escape through the sidewalls. Signup and view all the answers

What happens when the angle of incidence in the denser medium exceeds the critical angle ϕC?

The light reflects back into the denser medium. Signup and view all the answers

Why can light travel through a bent optical fibre?

Total Internal Reflection allows it to propagate. Signup and view all the answers

In Snell’s law, what do the symbols n1 and n2 represent?

The refractive indices of the two media Signup and view all the answers

What effect does cladding have on the transmission of data in optical fibres?

It increases the speed at which data can be transmitted. Signup and view all the answers

What can be inferred about light when it refracts at the boundary between a denser and a rarer medium?

It bends away from the normal if the angle of incidence is less than the critical angle. Signup and view all the answers

What is the primary advantage of laser welding?

It is especially effective for complex geometries or dissimilar materials. Signup and view all the answers

Which laser is commonly used for the treatment of liver cancer?

Pulsed neodymium laser Signup and view all the answers

For a laser beam with a wavelength of 6328 Å, what is the calculated energy of a photon?

1.96 eV Signup and view all the answers

What equation relates the energy and momentum of a photon?

p = E/c Signup and view all the answers

Which two lasers are used for treatments involving the lungs?

Argon and carbon dioxide lasers Signup and view all the answers

What is the significance of the constant k = 1.38 × 10^{-23} J/K in the population ratio equation?

It is Boltzmann's constant. Signup and view all the answers

Which of the following is NOT a medical application of lasers?

Welding of surgical instruments Signup and view all the answers

What does the symbol λ represent in laser physics?

The wavelength of the laser beam Signup and view all the answers

Study Notes

Laser Fundamentals

Multiplication of photons leads to light amplification, resulting in intense, monochromatic, unidirectional laser light.
Photons produced by stimulated emission share the same energy, direction, and phase as incident photons.

Population Inversion

Two-Level System:
- Consists of two energy levels (E1 and E2).
- Einstein coefficients for upward (B12) and downward (B21) transitions are equal.
- Population inversion is not achievable; upper and lower levels remain equally populated.
Metastable State:
- A higher energy state allowing electrons to remain longer, bypassing limitations of two-level systems.
Three-Level System:
- Atoms are pumped to an excited state (E3) and then transition to a metastable state (E2).
- Short-lived E3 allows for spontaneous decay to E2, resulting in population inversion between E2 and the ground state.

Semiconductor Laser Construction

Built using semiconductor materials, typically GaAs.
Composed of heavily doped n-type material with a p-region on top, creating a junction.
Typical size: 500 µm long, 100 µm wide and thick.
Features polished faces acting as a Fabry-Perot resonator, roughened sides terminate lasing in non-target directions.

Semiconductor Laser Working Mechanism

Achieves population inversion through carrier injection in a PN-junction diode.
Electrons occupy the conduction band (excited level) while holes in the valence band (ground level) lead to recombination.
As forward current rises, spontaneous emission occurs (initially LED action).
Threshold current achieves high electron concentration in the conduction band and holes in the valence band, establishing the inversion region.
Electron-hole recombination releases photons, resulting in coherent radiation at wavelengths, e.g., 9000 Å in the IR region.

Applications of Lasers

Computers:
- Facilitate data transfer over LANs and are integral in CD-ROM technology.
Communication:
- Modulated light signals transmitted via optical fibers.
Manufacturing:
- Capable of precise surface texturing, laser cutting, drilling, and welding, especially with complex geometries or dissimilar materials.
Medical Uses:
- Employed in laser surgeries, retinal treatments, and various laser therapies targeting conditions like liver cancer using neodymium, argon, and carbon dioxide lasers.

Optical Fiber Basics

Optical fibers utilize Total Internal Reflection (TIR) for light propagation.
Light can zigzag along the fiber length and can bend while maintaining transmission.

Total Internal Reflection

Occurs at certain angles when light moves from a dense medium to a rarer medium, bending away from the normal.
Critical angle (ϕC) is the angle at which refraction ceases, leading to reflection back into the denser medium.
Snell’s Law governs the relationship between angles of incidence and refraction in these media transitions.

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Description

Explore the principles of laser light amplification and the concept of population inversion in a two-level system. This quiz delves into the behavior of photons, their energy levels, and the characteristics that lead to the generation of monochromatic laser light.