Questions: Stimulated Emission and Lasers

Question 1

Why must a laser medium have a population inversion?

• A. So that spontaneous emission stops
• B. So that stimulated emission outpaces absorption and the light is amplified
• C. So that the atoms stay in their ground state
• D. So that the cavity mirrors reflect more light

Answer: (B)

A photon passing through the medium can be absorbed by a lower-level atom or trigger emission from an upper-level atom. Net gain needs more atoms in the upper level.

Question 2

Steady optical pumping of a simple two-level system can produce more atoms in the upper level than in the lower level.

• T. True
• F. False

Answer: False

In a two-level system the pump light both excites and de-excites atoms. The populations can approach equality but cannot invert. Practical lasers use three or four levels.

Question 3

A laser cavity is 30 cm long. What is the frequency spacing between neighbouring longitudinal modes, c/2L?

• A. 50 MHz
• B. 500 MHz
• C. 1 GHz
• D. 5 GHz

Answer: (B)

c/2L = (3.0 × 10⁸ m/s)/(0.60 m) = 5.0 × 10⁸ Hz = 500 MHz.

Question 4

At room temperature (kT ≈ 0.026 eV), estimate the ratio of atoms in an excited level 2.0 eV above the ground state to atoms in the ground state, assuming equal degeneracies.

Model answer: About 10⁻³⁴, essentially zero.

N₂/N₁ = exp(−ΔE/kT) = exp(−2.0/0.026) ≈ exp(−77) ≈ 10⁻³⁴. Thermal excitation cannot produce an inversion; it must be created by pumping.