Chapter 24 Photonics Question 1 Question 2 Question 3 Question 4 Question 5

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1 Chapter 24 Photonics Data throughout this chapter: e = C; h = Js (or ev s); m e = kg; c = m s 1 Question 1 Visible light has a range of photons with wavelengths 390 nm to 750 nm. An LED material with a band gap of 1.55 ev emits electromagnetic radiation. Which of the following best describes this radiation? A Infra-red radiation (invisible) B Ultra-violet radiation (invisible) C Red light (visible) D Infra-red radiation (visible) Question 2 Comparing a red light emitting LED material with a blue light emitting LED material, which of the following is true? A Their band gaps must be equal, since they both emit visible light. B The blue LED material must have a smaller band gap than the red LED material. C The red LED material must have a smaller band gap than the blue LED material. D It is not possible to conclude anything about the band gap from this information. Question 3 Gas lasers, solid-state lasers, tunable dye lasers and semiconductor laser diodes are all in use for various applications. For these types of lasers, it is true to say that: A they are all coherent and all emit light of the same wavelength. B they are all coherent and monochromatic, but may have different wavelengths from each other. C D they are all coherent and produce a range of wavelengths. not all are coherent. They produce light of one wavelength, but one of them can be tuned to different wavelengths. Question 4 Which one or more of the statements below is a correct statement about laser light? A Photons in laser light are formed by many different pairs of energy levels. B All photons that are emitted from a laser have a definite phase relationship with each other. C Population inversion of energy levels is an essential requirement for laser action. D The number of photons from a laser that are due to stimulated emission is about the same as those due to spontaneous emission. Question 5 Total internal reflection always occurs only when: A the angle of incidence is less than the critical angle. B the angle of incidence is greater than the critical angle. C the relative refractive index is less than 1. D the relative refractive index is greater than 1.

2 2 Physics The sketch below shows the end of a step-index optic fibre. The acceptance angle is labelled on the diagram as α. Question 6 Which of the following best defines the acceptance angle of an optical fibre? A The critical angle between the fibre cladding and the fibre core. B The critical angle between the fibre core and the fibre cladding. C The smallest angle a ray of light can make with the fibre axis and be propagated along the fibre. D The largest angle a ray of light can make with the fibre axis and be propagated along the fibre. Question 7 If n 1 = 1.53 and n 2 = 1.58, which of the following is closest to the value of the critical angle for a light ray travelling within the core? A 85 B 75 C 5 D 15 Question 8 Which of the following is closest to the value of the numerical aperture of the fibre? A 0.3 B 0.4 C 0.5 D 0.6 Question 9 Which of the following gives the units of numerical aperture? A degrees B m s 1 C m D it has no units Question 10 Which of the following gives the closest value for the acceptance angle for this optical fibre? A 21 B 22 C 23 D 24 Question 11 Which of the following best explains why sharp bends in an optical fibre are likely to increase the attenuation in the fibre? A Sharp bends weaken the material and hence absorb light. B Sharp bends increase the incident angle that rays make with the corecladding interface. C Sharp bends reduce the incident angle that rays make with the corecladding interface. D Sharp bends reduce the critical angle. Question 12 Which of the following best explains how rays, following different paths in a graded-index optical fibre, can remain in phase with each other? A A steadily reducing refractive index from the axis outwards. B A steadily increasing refractive index from the axis outwards. C Graded-index fibres have a much smaller diameter than other fibres. D The axial rays travel more quickly.

3 Photonics 3 Question 13 The phenomenon of material dispersion is responsible for changing the shape of pulse signals that are transmitted along optic fibres. Which of the following best explains (for the case of a single-mode, step-index fibre) how this effect can be minimised? A Use light which is as monochromatic as possible. B Use high quality materials. C Use light with a wide wavelength range. D Use high frequency light. Question 14 Which of the following best explains how modal dispersion changes the shape of pulse signals in optic fibres? A Impurities distort the paths of different modes. B Different wavelength photons travel at different speeds. C Different modes take different times to traverse the fibre. D Phase changes on internal reflections cause interference. Question 15 Which of the following best explains why graded-index, multi-mode fibres are preferred to step-index, multi-mode fibres for information transmission? A There is less pulse broadening due to material dispersion. B There is less pulse broadening due to modal dispersion. C There is less attenuation due to absorption. D There is less pulse broadening due to chromatic dispersion. Question 16 Endoscopes are instruments using bundles of optic fibres that are used to image hard-to-reach parts of the human body. Which of the following best explains why many fibres are used, carefully arranged in organised bundles? A Many fibres are more flexible than a smaller number. B Many fibres reduce chromatic dispersion. C If one fibre breaks, the others can still transmit information. D Each fibre carries small parts of the image to the outside. Question 17 Which of the following best explains what is meant by the term Rayleigh scattering? A The scattering of photons when they strike irregularities. B The scattering of photons when they internally reflect. C The scattering of photo-electrons in the fibre. D The scattering of photons due to chromatic effects. Question 18 A Bunsen burner flame has parts that are blue and parts that are yellow. Which of the options below best explains why the blue region of the flame is at a higher temperature than the yellow region of the flame? A Blue photons travel at a higher speed than yellow photons. B Blue photons are absorbed less than yellow photons. C Blue photons have higher energy than yellow photons. D Blue photons have a lower momentum than yellow photons. Question 19 Which of the following is the best explanation of the reason why light is emitted from an incandescent source (such as a candle)? A Accelerating atoms, due to collisions, emit energy as photons. B Collisions between moving atoms emit photons. C Accelerating electrons emit photons. D Photons are emitted directly from the hot atoms.

4 4 Physics A class looks at spectra from two sources. One is from an incandescent light globe; the other is from a mercury vapour lamp. Students observe that the spectra are of different types. Question 20 (VCAA, 2007 examination, adapted) Which of the following best describes the type of spectrum seen from each source? A The incandescent globe produces a continuous spectrum and the mercury lamp produces a line spectrum. B The incandescent globe produces a line spectrum and the mercury lamp produces a continuous spectrum. C The incandescent globe and the mercury lamp both produce a continuous spectrum. D The incandescent globe and the mercury lamp both produce a line spectrum. Question 21 (VCAA, 2007 examination, adapted) Which two of the following best describe the electron mechanism that produces each spectrum? A The continuous spectrum is caused by atomic energy level transitions. B The line spectrum is caused by atomic energy level transitions. C The continuous spectrum is caused by thermal electrons. D The line spectrum is caused by thermal electrons. Chris is testing LEDs (light emitting diodes). She has an LED which emits blue light and another which emits red light. Blue light has a higher frequency than red light. She uses the circuit shown below. With the blue LED in the circuit, the supply voltage is gradually increased. The LED doesn t emit light until the voltage reading is 2.64 V. At this time there is a current of 5.00 ma in the circuit. Question 22 (VCAA, 2007 examination, adapted) Which of the following best explains why the blue LED needs at least 2.64 V before it emits light? A 2.64 ev of energy is needed to raise electrons from the valence band into the conduction band. B 2.64 ev of energy is needed to raise electrons from the conduction band into the valence band. C 2.64 V is needed to overcome the resistance of the LED. D 2.64 V is the maximum voltage that can be applied to the LED. Question 23 (VCAA, 2007 examination, adapted) Which of the following is closest to the wavelength of the blue light emitted by the blue LED (assuming that the diode is ideal)? A 450 nm B 460 nm C 470 nm D 480 nm

5 Photonics 5 Question 24 (VCAA, 2007 examination) The blue LED is now replaced by the red LED. The supply voltage is left unchanged. Which one of the following sets of observation best describes the new situation? Red LED Voltage across LED Current in circuit A Light emitted Less than 2.64 V More than 5.00 ma B Light emitted More than 2.64 V Less than 5.00 ma C No light emitted Less than 2.64 V More than 5.00 ma D No light emitted More than 2.64 V Less than 5.00 ma The diagram shows a ray of light entering (from air, n AIR = 1.00) a step-index optical fibre. The fibre has n CORE = 1.46 and n CLADDING = Question 25 The best estimate of i C is: (VCAA, 2007 examination) A 12 B 43 C 47 D 78 For a different step-index fibre, the refractive index of the core is 1.46 and the refractive index of the cladding is The critical angle, i C, is 70. This is shown in the diagram below. Question 26 (VCAA, 2007 examination) The best estimate of the acceptance angle, α, is: A 20 B 30 C 47 D 70 An LED, a single-mode step-index optical fibre, and a phototransistor are used in a telecommunications system. The LED emits light in the wavelength range of nm. To test the system a pulse is transmitted as shown in the diagram below. Question 27 (VCAA, 2007 examination, adapted) The output pulse has suffered significant material dispersion. Which of the following best explains why material dispersion occurs in the system? A Different ray paths take different times of travel. B Different wavelengths travel at different speeds in the fibre. C Different wavelengths suffer different amounts of attenuation. D Pulses are broadened naturally by their passage through the fibre.

6 6 Physics Question 28 (VCAA, 2007 examination, adapted) Clare suggests that the effect of material dispersion can be reduced by using a laser in place of the LED. Which of the following best explains why this will reduce the effect of material dispersion on the transmitted pulse? A A smaller range of wavelengths is involved. B A wider range of wavelengths is involved. C A smaller divergence of light is involved. D A greater divergence of light is involved. The graph below shows the attenuation (optical loss) versus wavelength for a particular material used to make optical fibres. Question 29 (VCAA, 2007 examination) Two lasers are used: a red (visible) laser of wavelength 640 nm, and an infrared laser of wavelength 1700 nm. Which one of the following causes would produce the major attenuation for the red laser? A modal dispersion B material dispersion C Rayleigh scattering D absorption by the material Question 30 (VCAA, 2007 examination, adapted) In terms of attenuation, which of the following best explains which of the two lasers would be best for use in a fibre optic system made from this material? A Attenuation and Rayleigh scattering are both less in the infra-red wavelength range. B Attenuation and Rayleigh scattering are both less in the visible red wavelength range. C Attenuation is less in the infra-red wavelength range but Rayleigh scattering is greater. D Attenuation is less in the visible red wavelength range but Rayleigh scattering is greater.

7 Photonics 7 A single-mode, step-index optical fibre is to be used as a simple, intensity-based sensor to detect excessive bending in a concrete beam in a bridge. The fibre is embedded in the concrete in a winding pattern as shown in the diagram below, with the light path impacting on the core-cladding interface just below the critical angle on the most curved section of the fibre within the beam. Question 31 (VCAA, 2007 examination, adapted) Which of the following best describes what will be observed at the detector if the beam bends? A The intensity will reduce. B The intensity will increase. C The intensity will remain the same. D The intensity will vary from lower to greater. Question 32 (VCAA, 2007 examination, adapted) Which of the following best explains how the fibre operates as a simple intensitybased sensor? A The curvature of the bends will increase and less light will escape at these points. B The curvature of the bends will not change, but the stress in the fibre will attenuate the light. C The curvature of the bends will not change, but the stress in the fibre will intensify the light. D The curvature of the bends will increase and light will escape at these points. Question 33 Three spectra in the visible region are shown below. The three light sources are a laser, an LED and a mercury vapour lamp. Which of the following correctly matches the graph with the light source? A LED = graph 1, laser = graph 2, mercury vapour lamp = graph 3 B LED = graph 2, laser = graph 1, mercury vapour lamp = graph 3 C LED = graph 3, laser = graph 2, mercury vapour lamp = graph 1 D LED = graph 3, laser = graph 1, mercury vapour lamp = graph 2 Question 34 The band gap of an LED is 1.8 ev. Which one of the following best gives the wavelength of light emitted by this LED? A 110 nm B 690 nm C nm D nm

8 8 Physics Question 35 Comparing light from a laser and from an LED, which one of the following statements is true? A Light from an LED is coherent but light from a laser is incoherent. B Light from both an LED and a laser is coherent. C Light from a laser has a narrower range of wavelengths (more monochromatic) than light from an LED. D Light from an LED is pulsed but light from a laser is continuous. Question 36 Which one of the following statements best describes stimulated emission in a laser? A Atoms are raised to a metastable state. B A population inversion is created. C Photons interact with the atoms in a metastable state causing them to release their energy as photons. D Photons interact with the atoms in a metastable state to cause emission of electrons. The diagram shows a stepped-index fibre cable. The core has n = 1.48 and the cladding has n = A ray of light is shown inside the fibre. Question 37 Which one of the following best gives the critical angle for total internal reflection in the core of the fibre? A 18 B 67 C 72 D 78 For a different optical fibre, of cladding n = 1.30 and core n = 1.50, the critical angle is 60, as shown below. Question 38 Which one of the following best gives the acceptance angle α if the fibre is surrounded by air? A 40.5 B 48.5 C 60.0 D 86.9 Question 39 The optical fibre is now immersed in water (n = 1.33). Which one of the following choices best identifies the effect of this on the critical angle and on the acceptance angle? A Critical angle is the same; acceptance angle is greater. B Critical angle is the same; acceptance angle is smaller. C Critical angle is greater; acceptance angle is greater. D Critical angle is smaller; acceptance angle is smaller.

9 Photonics 9 An optical fibre telecommunications link uses an infrared laser of wavelength λ = 1300 nm, with a power output into the fibre of 50 mw at the transmitting end. At the receiving end, a phototransistor is used; the phototransistor requires 20 mw input to operate. The transmitter and receiver are linked by optical fibre cable. The attenuation of the cable is shown in the graph below. Question 40 Which one of the following distances gives the best estimate of the maximum length of optical fibre that could be used for the system to operate correctly? A 4 km B 6 km C 12 km D 30 km Question 41 A graded-index fibre has an advantage over a step-index fibre because a gradedindex fibre reduces the effect of: A material dispersion B Rayleigh scattering C modal dispersion D absorption Question 42 A laser source is superior to an LED source in a fibre-optic system because it causes less: A material dispersion B Rayleigh scattering C modal dispersion D absorption Question 43 Engineers wish to use fibre-optic imaging to examine corrosion on the inside of a pipe carrying water. The situation is shown in the diagram below. The mechanism for moving the device is not shown. Which one of the following is the best estimate of the number of fibres in the optical cable? A 1 B 5 C 1000 D

10 10 Physics Some physicists are discussing the design of a new system for long-distance transmission of data using light-fibre optics. They need to make a decision about two sets of options, as follows. They need to decide between red light (λ = 600 nm) and infrared radiation (λ = 1300 nm). They need to decide whether to use single-mode or multi-mode optical fibre. (You may need to refer to the graph attached to Question 40.) Question 44 Which one of the following statements gives the best choice between red light and infrared radiation, and the correct reason for this choice? A Red light, since the total attenuation is less for red light. B Red light, since absorption is less for red light. C Infrared radiation light, since the total attenuation is less for infrared radiation. D Infrared radiation, since absorption is less for red light. Question 45 Which one of the following statements gives the best choice between singlemode and multi-mode fibre, and the correct reason for this choice? A Single-mode, since it has less modal dispersion. B Single-mode, since it has less material dispersion. C Multi-mode, since it has less modal dispersion. D Multi-mode, since it has less material dispersion. Question 46 The spectra of light from three different light sources are shown below. The dashed lines indicate the range of visible wavelengths. Five possible light sources are listed below. mercury vapour lamp red laser sunlight 100 W (incandescent) globe LED (light-emitting diode) Which one of the options (A D) below correctly identifies the light source for each spectrum? Spectrum 1 Spectrum 2 Spectrum 3 A mercury vapour lamp red laser sunlight B 100 W globe mercury vapour lamp LED C 100 W globe LED mercury vapour lamp D sunlight red laser LED Question 47 Which one of the following statements best describes the production of light in an incandescent light bulb? A Stimulated emission of photons by electron in the electric current. B Transition of excited valence electrons back to lower energy states. C Acceleration of electrons in random thermal collisions. D Emission of electromagnetic radiation (light) by electrons accelerated by the applied voltage.

11 Photonics 11 Question 48 Which one of the following statements best describes the production of light in an LED (light-emitting diode)? A Electron movement from the conduction band to the valence band. B Electron movement from the valence band to the conduction band. C Electron movement from the valence band to lower energy bands. D Movement of ground state electrons to higher energy levels. A variable DC power supply is used to investigate the operation of an LED of band gap energy 2.30 ev. The voltage of the power supply is increased slowly. At a potential of 2.30 V across it, the LED emits light. A current of 4.0 ma is now flowing through the LED. The circuit used is shown below. Question 49 Assuming an ideal diode, which one of the following best gives the wavelength of light emitted by the LED? A 284 nm B 540 nm C 865 nm D nm Question 50 The power supply voltage is now increased. Which one of the following statements is true? A The wavelength of the light emitted decreases and the current increases. B The wavelength of the light emitted and the current both stay the same. C The wavelength of the light emitted stays the same and the current increases. D The wavelength of the light emitted increases and the current increases. The diagram below shows a step-index, fibre-optic waveguide, with a ray of light entering it. Question 51 The critical angle for total internal reflection between the cladding and the core of the waveguide is found to be 80. Which one of the following is the best estimate of the refractive index of the cladding? A 1.32 B 1.34 C 1.36 D 1.38 Question 52 Which one of the following best gives the value of the limiting acceptance angle, α, for the waveguide to operate correctly? A 10.0 B 13.7 C 46.4 D 80.0

12 12 Physics The waveguide is now immersed in water (refractive index 1.33). Question 53 Which one of the following best describes the effect of this immersion on the critical angle,θ? A θ will increase. B θ will remain the same. C θ will decrease. D There will be no critical angle θ. Question 54 Which one of the following best describes the effect of this immersion on the limiting acceptance angle, α? A α will increase. B α will remain the same. C α will decrease. D There will be no suitable acceptance angle α. Melanie and Max are discussing the best options for long-distance transmission using a fibre-optic waveguide. Question 55 Melanie and Max want to minimise material dispersion. Melanie argues that a laser would be a better light source than an LED to minimise material dispersion. The best reason for this would be that: A lasers are more intense than LEDs B lasers emit light with a narrower range of wavelengths than LEDs C lasers operate at higher frequencies (shorter wavelengths) than LEDs D laser light is coherent, but light from an LED is incoherent Question 56 Which one of the following would reduce modal dispersion in the waveguide? A Use of a higher refractive index cladding. B Use of a lower refractive index cladding. C Use of a larger diameter core. D Use of a smaller diameter core. Question 57 Max and Melanie decide to use a step-index fibre for the waveguide. Compared with a step-index fibre, a graded-index, optical fibre is superior because it is more likely to reduce transmission difficulties caused by: A modal dispersion B material dispersion C Rayleigh scattering D absorption Question 58 Which one of the following contains only sources that produce predominantly incoherent light? A incandescent globe, fluorescent tube, laser B laser, fluorescent tube, sun C sun, candle, incandescent globe D sun, candle, laser

13 Photonics 13 Question 59 The characteristics of a red light-emitting diode (LED) are shown in the diagram below. Which of the following best gives the average wavelength of the light from this LED? A nm B 62 nm C 621 nm D 994 nm A student sets up the following circuit to test this LED, as shown in the diagram below. The (ideal) battery has a voltage of 12 V and the resistor has a resistance of 400 Ω. Question 60 Which of the following best gives the current through the ammeter? A 5 ma B 25 ma C 30 ma D 60 ma Question 61 The red LED is replaced by a blue LED, using the same circuit. Which of the following statements best describes the effect on the circuit? A The blue LED emits light, and the current through it is greater than the current through the red LED. B The blue LED emits light, and the current through it is the same as the current through the red LED. C The blue LED emits light, and the current through it is less than the current through the red LED. D The blue LED does not emit light, and the current through it is less than the current through the red LED.

14 14 Physics The diagram below shows a step-index, fibre-optic waveguide. The outer cladding has a refractive index of 1.38, and the inner core has a refractive index of Question 62 Which of the following best gives the value of the critical angle for total internal reflection between the core and cladding? A 17 B 44 C 46 D 73 Question 63 Which of the following best gives the limiting acceptance angle, α, for the waveguide to operate correctly? A 15 B 24 C 44 D 66 Students are using a length of flexible plastic rod to investigate transmission of laser light through a fibre-optic waveguide. The plastic material has a refractive index of The refractive index of air is The situation is shown in the diagram below. Question 64 The students bend the rod more as shown in the diagram below. They find that the intensity of light emerging from the rod decreases significantly. Which of the following statements is the most likely explanation for this? A Material dispersion B The angle of incidence of the light ray on the side of the rod at some point has now become less than the critical angle for plastic to air. C The angle of incidence of the light ray on the side of the rod at some point has now become greater than the critical angle for plastic to air. D Modal dispersion

15 Photonics 15 Question 65 The bending of the plastic rod is reduced until the intensity of the light emerging from the rod increases significantly, as originally shown on the previous page. The students now immerse the plastic rod in water, as shown in the diagram below. This action results in the intensity of the light emerging from the rod once again decreasing significantly. The refractive index of water is Which of the following statements is the most likely explanation for this? A The critical angle for total internal reflection has increased. B Modal dispersion has reduced the light intensity reaching the end of the rod. C Total internal reflection cannot now occur. D Material dispersion has reduced the light intensity reaching the end of the rod. The graph below shows the attenuation (optical loss) for a particular material used in fibre-optic waveguides. Question 66 An infrared laser of wavelength 2000 nm is used. Which of the following would be the major cause of attenuation in the waveguide? A modal dispersion B material dispersion C Rayleigh scattering D absorption by material Question 67 A long-distance data link using this particular material is being planned. The designers wish to select the best wavelength to reduce signal loss in the system. Which of the following wavelengths will give the least signal loss? A 600 nm B 1000 nm C 1200 nm D 1600 nm Question 68 Graded-index fibres, even though they are more expensive, are sometimes used in place of step-index fibres in optical transmission systems. Graded-index fibres are used because they best reduce: A material dispersion. B modal dispersion. C Rayleigh scattering. D attenuation due to absorption.

16 16 Physics Question 69 Which of the following best explains the cause of Rayleigh scattering in an optical fibre? A Excessively sharp bending of the optical fibre. B Absorption at the interface of the core and cladding. C Small variations in density in the core of an optical fibre. D Refractive index of the cladding being too similar to the refractive index of the core. Question 70 The light reaching Earth from the Sun is best described as: A wide spectrum and coherent. B narrow spectrum and coherent. C wide spectrum and incoherent D narrow spectrum and incoherent Question 71 Which of the following best describes how light is produced in a light-emitting diode? A Thermal motion of valence electrons B Transition of electrons from the conduction band to the ground state C Transition of electrons from the conduction band to the valence band D Transition of electrons from the valence band to the conduction band Question 72 A light-emitting diode (LED) emits red light of wavelength 620 nm. Which of the following best gives the band-gap energy of the semiconductor material of this diode? A ev B ev C ev D 2.0 ev Question 73 As the width of the band gap in an LED increases, the colour of the light emitted changes. Which of the following gives the correct sequence of colours emitted by LEDs, in increasing order, as the band gap is increased? A blue, red, green B green, red, blue C red, green, blue D blue, green, red The characteristics of an LED are shown below. This LED is placed in the circuit shown on the next page, and operates correctly.

17 Photonics 17 Question 74 Which of the following gives the best estimate of the current through the LED? A 0 ma B 5 ma C 8 ma D 30 ma Question 75 Another identical LED, LED 2, is then placed in series with the first LED, as shown in the diagram below. Which of the following statements best describes the effect of this change? A Both LEDs will continue to operate at the same light output. B Both LEDs will continue to operate but at a lower light output. C LED 1 will operate but not LED 2. D Neither LED will operate. Question 76 Green light produced by a laser and green light produced from an LED of the same average wavelength are being compared. Which of the following statements are true? A The light from the laser is incoherent but the light from the LED is coherent. B The light from the LED is incoherent but the light from the laser is coherent. C The laser can only operate intermittently but the LED light is continuous. D The spectrum of light from the LED consists of a narrower range of wavelengths than that from the laser. Question 77 A stepped-index, fibre cable is shown in the diagram below. The cladding has a refractive index of 1.36, and the core a refractive index of Which of the following best gives the critical angle for total internal reflection at the core-cladding interface? A 17 B 45 C 47 D 73

18 18 Physics In a different optical fibre, the cladding has a refractive index of 1.32 and the core 1.48 and the critical angle is 63. This is shown in the diagram below. Question 78 Which of the following best gives the value of the acceptance angle, α? A 18 B 27 C 37 D 42 Question 79 The cable and the light source are now placed in water, as shown in the diagram below. Which of the following statements is true in this changed situation? A critical angle the same, acceptance angle greater B critical angle the same, acceptance angle less C critical angle less, acceptance angle greater D critical angle less, acceptance angle less Question 80 A graded-index optical fibre has an advantage over a step-index, optical fibre because it reduces the: A absorption. B modal dispersion C material dispersion D Rayleigh scattering Question 81 A fibre-optic transmission line has a length of 3.0 km. The material in the line causes the power of the signal to be attenuated by 50% for each kilometre travelled. The minimum power that can be detected clearly by the receiver is 2.0 µw. Which of the following best gives the minimum power required in the laser at the transmission end for the signal to be clearly received? A 6.0 µw B 8.0 µw C 16 µw D 20 µw