Understanding Lasers: Chapter 7 Quiz

Source: Jeff Hecht, Understanding Lasers: An Entry-Level Guide, fourth edition (2019), Chapter 7 quiz, printed pages 249–251. The questions are paraphrased.

Quick answers

Question

Answer

1

b in the printed key; c is also false

2

b, helium–cadmium

3

d, argon fluoride

4

a, approximately \(150\)–\(1500\ \mathrm{nm}\)

5

c

6

b, \(488.0\) and \(514.5\ \mathrm{nm}\)

7

d

8

e

9

e, all listed discharge geometries

10

a, carbon dioxide

11

a

12

b, HF

Worked reasoning

  1. General gas-laser statement: b in the key, but the item is ambiguous. Metal-vapour lasers use material that must be heated because it is not gaseous at room temperature, so b is false. However, c is also false: mixtures such as helium–neon deliberately contain a second species that transfers excitation energy. The item therefore has two defensible answers.

  2. Shortest continuous-wave output: b. A helium–cadmium laser can emit continuously in the ultraviolet. The shorter-wavelength nitrogen and ArF choices normally operate as pulsed lasers.

  3. Shortest wavelength overall: d. ArF excimer emission near \(193\ \mathrm{nm}\) is shorter than KrF, nitrogen, He–Cd, or argon-ion output in the choices.

  4. Electronic-transition span: a. Atomic and ionic gas-laser electronic transitions extend approximately from the deep ultraviolet near \(150\ \mathrm{nm}\) to the near infrared near \(1500\ \mathrm{nm}\). Molecular vibrational transitions extend much farther into the infrared but are not electronic transitions.

  5. Why retain a red He–Ne laser: c. It offers excellent spatial mode and beam quality. Diode lasers are generally smaller, cheaper, and more efficient.

  6. Strong argon-ion lines: b. The familiar blue and green lines are \(488.0\ \mathrm{nm}\) and \(514.5\ \mathrm{nm}\).

  7. After ignition of an argon discharge: d. High voltage initiates ionization; after breakdown, the voltage drops while a large current sustains the plasma and laser emission.

  8. Helium–cadmium distinction: e. Cadmium must be heated to supply metal vapour. Neon is already gaseous under the tube’s operating conditions.

  9. CO2 excitation geometries: e. CO2 lasers have been built with longitudinal, transverse, high-pressure, and flowing-gas discharges.

  10. Highest industrial gas-laser power: a. Carbon-dioxide lasers scale to high continuous power and have long been major cutting and welding tools.

  11. Excimer lower level: a. The excited dimer is bound, but its ground state is repulsive. After photon emission it promptly dissociates, helping keep the lower laser level empty.

  12. Not an excimer molecule: b. ArF, KrF, XeCl, and XeF are excimer or exciplex laser species. HF is the molecule in a chemical laser.