Understanding Lasers: Chapter 1 Quiz
Source
Jeff Hecht, Understanding Lasers: An Entry-Level Guide, fourth edition
(Wiley/IEEE Press, 2019), Quiz for Chapter 1, printed pages 18–20.
The questions below are paraphrased rather than reproduced. The goal is to explain why each answer follows from the chapter, including the calculation steps for the efficiency questions.
Quick answers
Question |
Answer |
Main idea |
|---|---|---|
1 |
c |
|
2 |
b |
Photon behaviour is not unique to laser light. |
3 |
a |
Semiconductor lasers dominate by unit count. |
4 |
c |
Laser amplification is based on stimulated emission. |
5 |
d |
Chromium ions are the active emitters in ruby. |
6 |
b (intended) |
A semiconductor laser is an electrically driven diode device. |
7 |
e, \(99\ \mathrm{W}\) |
A 1%-efficient device needs \(100\ \mathrm{W}\) input. |
8 |
b, \(3\ \mathrm{W}\) |
A 25%-efficient device needs \(4\ \mathrm{W}\) input. |
9 |
b |
Waves with a stable phase relationship are coherent. |
10 |
Personal result |
Count lasers hidden in devices as well as standalone lasers. |
1. Where the word laser came from
Answer: c. The name was formed from the initial letters of Light Amplification by Stimulated Emission of Radiation. It is therefore an acronym, not a trade name or a translation of a foreign word.
The phrase is also a compact description of the process: stimulated emission adds photons to the optical field, thereby amplifying light.
2. The incorrect statement about light
Answer: b. All light can display wave-like and photon-like behaviour. Laser light is special because of properties such as high coherence, directionality, and brightness, not because its energy alone comes in photons.
The other statements are consistent with the electromagnetic spectrum. Radio, visible, and ultraviolet radiation are all electromagnetic waves; their wavelength and frequency ranges differ.
3. The most common lasers
Answer: a. Most lasers counted as individual devices are semiconductor laser diodes embedded in electronic and communications equipment. Familiar examples include optical-disc drives and fibre-optic transmitters.
This question concerns the number of devices, not which laser type has the highest power or occupies the most space. Large scientific and industrial lasers are much less numerous.
4. The emission process used by a laser
Answer: c. A photon whose energy matches an excited-state transition can stimulate the excited atom, molecule, or semiconductor carrier to emit another photon. This is stimulated emission.
Spontaneous emission can help start the process, and mirrors can provide optical feedback, but neither one is the defining amplification mechanism.
5. The active emitter in ruby
Answer: d. Ruby is sapphire, \(\mathrm{Al_2O_3}\), containing a small concentration of chromium ions. The sapphire forms the host crystal, while the chromium ions supply the energy levels responsible for the red laser transition.
It is useful to keep these two roles separate:
6. Why the name diode laser is used
Answer: b is the intended choice. A semiconductor laser is based on a diode junction with two electrical terminals. An applied current injects electrons and holes; their recombination can produce stimulated emission.
The wording of choice b in the book says that the device conducts light between terminals. That is imprecise: electrical current passes between the terminals, while light is generated in and guided out of the active region. Nevertheless, b is the only choice describing the diode structure and is also the choice in the book’s answer key.
7. Heat from a 1%-efficient laser
Answer: e, \(99\ \mathrm{W}\).
We are given optical output power \(P_{\mathrm{out}}=1\ \mathrm{W}\) and efficiency \(\eta=1\%=0.01\). Efficiency is
Rearrange Equation (1) for input power:
If the non-optical output is treated as heat, conservation of energy gives
Check: one percent of \(100\ \mathrm{W}\) is the stated \(1\ \mathrm{W}\) optical output.
8. Heat from a 25%-efficient laser
Answer: b, \(3\ \mathrm{W}\).
Now \(P_{\mathrm{out}}=1\ \mathrm{W}\) and \(\eta=25\%=0.25\). Apply the same equation:
Therefore,
Check: \(1\ \mathrm{W}\) of light plus \(3\ \mathrm{W}\) of heat accounts for the full \(4\ \mathrm{W}\) input.
9. The result of a stable phase relationship
Answer: b, coherent. Waves are coherent when their relative phase is stable. Stimulated emission produces a new photon matched to the stimulating field, which supports the coherent field in a laser cavity.
Important
Answer-key discrepancy
The answer key on printed page 543 gives c for question 9, which would
mean pulsed. That conflicts with the question and with the definition
of coherence: being in phase does not require a laser to be pulsed. Choice
b is the physically correct answer, so the printed key appears to
contain an error.
10. Personal laser inventory
There is no single correct answer. Count only devices you actually own and distinguish lasers from ordinary light-emitting diodes. Places worth checking include:
CD, DVD, and Blu-ray drives or players, which may contain more than one wavelength of laser;
laser printers;
fibre-optic networking equipment;
laser distance meters, levels, barcode scanners, and pointers;
computer mice specifically labelled as laser mice.
For a repeatable answer, make a table with columns for the device, number of
lasers, purpose, and confidence. Devices whose documentation is unclear can
be marked unknown rather than guessed.
What this quiz established
The chapter’s central chain of ideas is:
Practical lasers differ in their active material and efficiency, but the stimulated-emission principle is common to them.