Teach Yourself Electricity and Electronics, 5th edition |

Stan Gibilisco |

Explanations for Quiz Answers in Chapter 33 |

1. In a pulsed laser, the ratio of the peak output power to the average output power depends on the duty cycle (the proportion of time that the laser actually produces output), assuming all other factors remain constant. The correct choice is (d). |

2. A helium-neon (He-Ne) laser produces output at a wavelength of 633 nm. The correct choice is (c). Some He-Ne lasers also produce output at other wavelengths, but not at 1155 nm (a) or 793 nm (b), and not in the UV range (d). |

3. If we reverse-bias a laser diode and keep the voltage below the avalanche point, we get no current through the device, and we observe no radiation from it. In fact, under these conditions, the laser diode behaves exactly as an ordinary diode does. The correct choice is (d). |

4. In a pulsed cavity laser (or any other type of cavity laser), the output wavelength depends on the length of the cavity. The correct choice is (a). The wavelength doesn't depend on the duty cycle (b), the peak energy output (c), or the characteristic shape of the output pulses (d). |

5. In a gallium-arsenide (GaAs) laser, the high carrier mobility (that is, the speed with which the charge carriers move) makes the task of modulation easy. The correct choice is (b). |

6. When an electron moves from a certain shell to a smaller shell within an atom, that electron loses energy. In the process, it radiates a photon having a specific wavelength corresponding to the amount of energy that it loses. The correct choice is (b). |

7. If know the peak output power (P_{pk-out}) and the average output
power (P_{avg-out}) from a pulsed laser with rectangular pulses, then we
can calculate the duty cycle as
We're told that the peak output power from a particular laser having rectangular pulses
is 660 GW, or 6.60 x 10
The correct choice is (d). |

8. We can determine the efficiency eff of any laser device by dividing the
average output power P_{avg-out} by the average input power P_{avg-in}
to get
The statement of Question 7 (the previous problem) tells us that P
The correct choice is (b). |

9. By definition, coherent radiation occurs in such a way that all the waves have the same frequency, and they all follow each other in phase coincidence. The correct choice is (c). |

10. A typical vertical-cavity surface-emitting laser (VCSEL) has a narrower beam, at great distances, than a typical injection laser operating a similar wavelength. In other words, the VCSEL has less beam divergence. The correct choice is (a). |

11. We want to find the average output power, P_{avg-out}. We know that
the peak pulse power output equals 1200 W and the device works at a duty cycle of 0.020.
Let's set P_{pk-out} = 1200 and D = 0.020, and then plug these
numbers into the formula
to get 0.020 = We can rewrite this equation as
which solves to
The correct choice is (d). |

12. As we did in the solution to Question 8, we can determine the efficiency eff
with the formula
In the solution to Question 11, we determined that P
The correct choice is (a). |

13. To calculate the power (in watts) that a gas-discharge tube dissipates, we
multiply the applied voltage (in volts) by the current that flows through the gas (in
amperes). In this case, we apply 5.0 kV DC, which equals 5000 V DC, and get a current of
2.0 mA, which equals 0.0020 A. The dissipated power P is therefore
The correct choice is (c). |

14. This problem constitutes a "repeat" of Question 7 with different
numbers. If know the peak output power (P_{pk-out}) and the average output
power (P_{avg-out}) from a pulsed laser with rectangular pulses, then the
duty cycle is
We're told that the peak output power from a particular laser having rectangular pulses
is 500 GW, or 5.00 x 10
The correct choice is (b). |

15. Let's turn the formula for efficiency "inside-out." Recall that we can
calculate the efficiency eff by dividing the average output power P_{avg-out}
by the average input power P_{avg-in} to obtain
Using algebra, we can rewrite this formula as
Now we can plug in the values
The correct choice is (b). |

16. If we reverse-bias a laser diode, the laser diode will behave essentially as an ordinary diode does. Beyond the avalanche voltage, the device will conduct current, but it will not radiate. The correct choice is (c). |

17. Energy radiates from ionized hydrogen gas in the form of photons having multiple
discrete wavelengths w according to the Rydberg-Ritz formula
where we express
In this case, we're told that an electron falls from a shell with
When we work out this quotient and round off to three significant figures, we get
The correct choice is (a). |

18. Look back at the solution to Question 11 for reference. This time, we want to find
the peak output power, P_{pk-out}. We know that the average pulse power
output equals 24 W (it hasn't changed) and the device works at a duty cycle of 0.010 (half
of what it was before). Now we can set P_{avg-out} = 24 and D =
0.010, and then plug these numbers into the formula
to get 0.010 = 24 / We can rewrite this equation as
which solves to
The peak output power has doubled from 1200 W, its value in the scenario described by Question 11. The correct choice is (c). |

19. Look back at Question 11 for reference. Now we want to find the efficiency, eff.
The average pulse power output, P_{avg-out}, was 24 W in that situation,
and it hasn't changed. The average input power, P_{avg-in}, was 30 W back
then, and it's still the same now. Although the duty cycle has changed, it has no effect
on the calculation of efficiency in this particular situation. We have simply
which is the same as it was in the scenario of Question 11. The correct choice is (b). |

20. At first, the P-N junction does not conduct, and the device doesn't radiate at all. When forward breakover occurs, current begins to flow, and the diode emits incoherent infrared (IR). When the voltage rises to a certain critical point, the IR emission becomes coherent and we observe lasing. The correct choice is (d). |