LASER Fundamentals and Applications

Comprehensive practice flashcards covering the characteristics, physics, pumping schemes, specific types (He-Ne, Semiconductor), and applications (LIDAR, printing, bridge deflection) of LASER technology.

What does the acronym LASER stand for?

Light Amplification by Stimulated Emission of Radiation.

What are the four characteristic properties of Laser light?

1. Unidirectionality, 2. Monochromaticity, 3. High Intensity, 4. Coherence.

Compare the line width of ordinary light sources to that of laser light.

For ordinary light sources it is of the order of $1000\,Å$, whereas for laser light it is of the order of $10\,Å$ to $10^{-4}\,Å$.

How much brighter is a $1\,mW$ power laser compared to light from the sun at the earth’s surface?

$10,000$ times brighter.

Define the process of induced absorption.

The process of absorption of incident photon by an atom and the excitation of the atom to the higher energy state, where the energy of the incident photon is exactly equal to the difference in energies of two states: $A + h\nu = A^*$.

How long does an atom typically stay in the excited state $E_2$ before spontaneous emission occurs?

$10^{-8}$ seconds.

Why is spontaneous emission considered incoherent?

Because the atom can emit a photon in any direction and two photons emitted by different atoms may or may not be in phase.

What is stimulated emission?

The process where an incident photon stimulates an atom in an excited state to emit an identical photon and transition to a lower energy state ($A^* + h\nu \rightarrow A + 2h\nu$).

What is the Boltzmann relation for the ratio of population of two atomic states in thermal equilibrium?

$$\frac{N_2}{N_1} = e^{-\frac{E_2 - E_1}{kT}}$$

What is population inversion?

A condition where the population of any higher energy state is more than the population of any specified lower energy state ($N_2 > N_1$).

What are Einstein's coefficients for induced absorption, spontaneous emission, and stimulated emission?

$B_{12}$ is the coefficient for induced absorption, $A_{21}$ for spontaneous emission, and $B_{21}$ for stimulated emission.

What are the two primary conditions for LASER action?

1. The rate of stimulated emission must be much higher than spontaneous emission (high $U_{\nu}$); 2. The rate of stimulated emission must be much higher than induced absorption (achieved via population inversion $N_2 > N_1$).

What is the resonance condition for the distance between mirrors ($L$) in an optical resonant cavity?

$L = n(\frac{\lambda}{2})$, where $n = 1, 2, 3, \dots$

Why is a metastable state required in an active medium for laser action?

To achieve population inversion by allowing atoms to stay in an excited state for a long duration ($10^{-2}$ seconds) to increase stimulated emissions.

How do 3-level and 4-level pumping schemes differ regarding their power requirements and output mode?

3-level schemes require very high pumping power and produce light in pulses; 4-level schemes require less power and operate in continuous wave mode.

What is the ratio of Helium to Neon and the pressure maintained in a He-Ne laser?

The ratio is 10:1 and the pressure is maintained at $1:0.1\,torr$..

What is the output wavelength of a Helium-Neon laser?

$$6328\,Å$$

What principle is the semiconductor laser based on?

Electron hole recombination in a direct band gap semiconductor.

Calculate the wavelength of an emitted photon from a Gallium Arsenide (GaAs) semiconductor with an energy gap of $1.4\,eV$.

$\lambda = 8400\,Å$ (based on $E_g = \frac{hc}{\lambda}$).

In a LIDAR system, what are the two types of lasers used for mapping land and water?

Topographic lidar uses a near-infrared laser; bathymetric lidar uses water-penetrating green light.

How is bridge deflection calculated using the laser image method?

By calculating the variation of the center of a laser spot on a screen, using the relationship $\Delta = l \cos(\phi)$, where $l$ is the length of the deformation relative to the datum plane.