Understanding Lasers: Chapter 9 Quiz

Source: Jeff Hecht, Understanding Lasers: An Entry-Level Guide, fourth edition (2019), Chapter 9 quiz, printed pages 337–339. The questions are paraphrased.

Quick answers

Question

Answer

1

d

2

a

3

b

4

e, ytterbium

5

b, fibre Bragg gratings

6

b, Nd:YAG

7

a, \(7.14\%\)

8

d, \(2.5\ \mathrm{kW}\) saved

9

c

10

a

11

b, erbium

12

e, no added element

Worked reasoning

  1. Not a fibre-laser advantage: d. The long, small core gives excellent cooling and beam quality but nonlinearities and optical damage limit very high pulse energy.

  2. Inner cladding: a. It accepts pump light from a comparatively large area and guides it along the doped core, where repeated overlap allows absorption.

  3. Location of active species: b. Rare-earth ions are doped into the light-guiding core so pump and signal fields overlap them along the fibre.

  4. Highest-power rare-earth fibre laser: e. Ytterbium combines efficient diode pumping, a small quantum defect, and a useful emission band near one micrometre.

  5. Cavity reflectors: b. Fibre Bragg gratings written into the fibre provide wavelength-selective reflection without free-space alignment.

  6. Same wavelength replacement: b. Ytterbium fibre gain includes \(1064\ \mathrm{nm}\), the main Nd:YAG wavelength.

  7. Thulium-to-holmium quantum defect: a.

    \[q=1-\frac{E_l}{E_p}=1-\frac{\lambda_p}{\lambda_l} =1-\frac{1950}{2100}=0.07143=7.14\%.\]

  8. Input-power saving: d.

    \[P_{20\%}=\frac{1\ \mathrm{kW}}{0.20}=5\ \mathrm{kW},\qquad P_{40\%}=\frac{1\ \mathrm{kW}}{0.40}=2.5\ \mathrm{kW},\]

    so the more efficient laser saves \(2.5\ \mathrm{kW}\) of input.

  9. Why 50-fs pulses are possible: c. A short transform-limited pulse requires many phase-locked frequencies, hence broad emission bandwidth.

  10. Long-wavelength silica limit: a. Multiphonon absorption in silica rises strongly beyond roughly two micrometres, motivating fluoride and other non-silica hosts.

  11. Telecommunications amplifier dopant: b. Erbium gain around \(1550\ \mathrm{nm}\) overlaps the low-loss window of silica fibre.

  12. Raman amplifier dopant: e. Raman gain comes from the host glass’s vibrational response and a strong pump; no rare-earth active species is required.