Optical Communications

1. The Bragg wavelength λᴮ of a fiber Bragg grating depends primarily on. A. Core diameter. B. Grating period and effective index. C. Input optical power. D. Fiber dispersion.

2. Which device typically uses a built-in Bragg grating inside the active region?. A. DBR laser. B. DFB laser. C. VCSEL with external grating. D. Long-period grating.

3. Chirped fiber Bragg gratings are mainly used for: A. ASE noise suppression. B. Polarization filtering. C. Dispersion compensation. D. Pump recycling.

4. The coupled-mode theory for FBGs describes: A. Thermal expansion of fiber. B. Mode interaction caused by periodic index modulation. C. Laser cavity dynamics. D. Fiber bending loss.

5. An apodized grating is designed to: A. Increase refractive-index contrast. B. Eliminate core–cladding coupling. C. Reduce sidelobes in the reflection spectrum. D. Increase grating length.

6. Which of the following is NOT a typical application of FBGs in sensing?. A. Strain measurement. B. Temperature monitoring. C. Chemical detection. D. RF signal amplification.

7. Long-period gratings differ from standard FBGs because they: A. Reflect only the Bragg wavelength. B. Couple forward-propagating modes. C. Use metal gratings. D. Only operate below 1000 nm.

8. VCSEL devices use Bragg gratings mainly as: A. Heating elements. B. Optical isolators. C. Cavity mirrors. D. Output couplers only.

9. A DBR fiber laser uses FBGs as: A. Pump combiners. B. End mirrors of the resonator. C. Noise suppressors only. D. Dispersion compensators.

10. The Bragg wavelength of an FBG shifts due to temperature because of: A. Core dopant diffusion. B. Thermo-optic effect and thermal expansion. C. Changes in fiber cladding thickness. D. Variation of splice loss.

11. A photon generates an electron–hole pair in a photodetector only if: A. Photon energy is below the bandgap. B. Photon energy equals bandgap. C. Photon energy exceeds the bandgap. D. Frequency is lower than cutoff.

12. Responsivity ℜ is defined as: A. Ratio of voltage to optical power. B. Ratio of photocurrent to optical power. C. Ratio of quantum efficiency to bandwidth. D. Ratio of incident photons to electrons.

13. The dominant noise sources in photodiodes are: A. Acoustic noise and scattering. B. Thermal noise and shot noise. C. Quantization noise only. D. Dark current only.

14. Increasing reverse bias in a PN photodiode mainly: A. Reduces depletion width. B. Increases capacitance. C. Speeds up carrier collection. D. Blocks photon absorption.

15. The main purpose of the intrinsic layer in a PIN photodiode is to: A. Reduce absorption. B. Increase depletion width. C. Limit photocurrent. D. Increase series resistance.

16. Detector bandwidth depends strongly on: A. Core refractive index only. B. Junction capacitance and carrier transit time. C. Fiber bend radius. D. Ambient temperature only.

17. Quantum efficiency describes: A. Probability of photon reflection. B. Number of electron–hole pairs per incident photon. C. Noise level of the photodiode. D. Detector response speed.

18. Which material allows very high-speed PIN detectors?. A. Silicon. B. Germanium. C. InGaAs. D. Sapphire.

19. Dark current in a photodiode: A. Does not contribute to noise. B. Is always zero. C. Produces shot noise. D. Eliminates the need for reverse bias.

20. The rise time of a photodetector is defined as the time needed for the output to rise from: A. 0 → 100%. B. 20% → 80%. C. 10% → 90%. D. 5% → 95%.

21. Which modulation format is commonly used in IM/DD systems?. A. FSK. B. OOK. C. QAM. D. DPSK only.

22. Direct modulation of a semiconductor laser mainly suffers from: A. Slow carrier injection. B. Chirp. C. Low optical power. D. Excessive heating only.

23. External modulation is preferred for high-speed links because it: A. Increases chirp. B. Reduces laser lifetime. C. Minimizes chirp and extends bandwidth. D. Requires no biasing.

24. Electro-optic modulators operate based on: A. Raman scattering. B. Pockels effect. C. Fresnel reflection. D. Thermal diffusion.

25. An electro-absorption modulator works by controlling: A. Carrier recombination rate. B. Material absorption with voltage. C. Refractive index only. D. Polarization state.

26. The half-wave voltage Vπ of an EO modulator is the voltage required to: A. Reduce output intensity by half. B. Switch polarization. C. Induce a π phase shift. D. Change wavelength by 1 nm.

27. A Mach–Zehnder modulator uses: A. Two gratings in series. B. Two interferometric paths. C. A pair of lasers. D. Two optical isolators.

28. X-cut LiNbO₃ MZ modulators are often preferred because they: A. Provide chirp-free operation. B. Have zero insertion loss. C. Require no bias control. D. Support only phase modulation.

29. The extinction ratio of a modulator is defined as: A. Loss in the ON state. B. Bandwidth at 3 dB. C. Ratio of OFF to ON output power. D. Phase shift between arms.

30. A dual-drive Mach–Zehnder modulator is used to: A. Generate polarization modulation. B. Achieve chirp-free intensity modulation. C. Reduce laser linewidth. D. Lower optical power consumption.

31. The primary physical mechanism enabling reflection in a Bragg grating is: A. Rayleigh scattering. B. Constructive interference of weak reflections. C. Mode-locked cavity action. D. Total internal reflection.

32. A phase-shifted FBG is typically used to create: A. A wide reflection bandwidth. B. A narrow transmission peak. C. Broadband absorption. D. High-gain amplification.

33. Increasing the index modulation amplitude Δn in an FBG will primarily: A. Reduce reflectivity. B. Increase reflectivity. C. Decrease Bragg wavelength. D. Increase fiber loss.

34. The Fourier transform of the grating index profile gives an approximation of: A. Fiber nonlinear coefficient. B. Mode-field diameter. C. Grating reflection spectrum. D. Fiber attenuation curve.

35. A sampled grating is designed to: A. Filter out all wavelengths. B. Create multiple reflection peaks. C. Increase thermal stability. D. Prevent bending losses.

36. In a distributed Bragg reflector (DBR) laser diode, the Bragg gratings are located: A. Inside the gain region. B. Outside the gain region. C. Only at the output facet. D. At mid-cavity only.

37. A volume holographic grating (VHG) stabilizes a laser by: A. Creating high polarization extinction. B. Reflecting only the Bragg-matched wavelength. C. Suppressing thermal noise in the diode. D. Reducing spontaneous emission.

38. In a fiber laser using FBGs, which parameter is most strongly determined by the gratings?. A. Output polarization. B. Laser wavelength. C. Core NA. D. Pump absorption coefficient.

39. A uniform FBG produces a reflection spectrum whose central wavelength is determined by: A. Average index and fiber diameter. B. Average index and grating period. C. Cladding thickness and pitch. D. Laser drive current.

40. A long-period grating (LPG) is best described as a device that: A. Reflects light backward. B. Couples core mode to cladding modes. C. Operates only at 850 nm. D. Acts as an isolator.

41. In photodiodes, the absorption coefficient α determines: A. The dark current noise. B. The fraction of photons absorbed per unit length. C. The frequency response. D. The reverse-bias voltage.

42. In a PIN photodiode, the intrinsic region is useful because it: A. Lowers quantum efficiency. B. Provides a large depletion region. C. Prevents any dark current. D. Eliminates transit-time limitations.

43. A detector with high junction capacitance will generally have: A. Higher bandwidth. B. Lower bandwidth. C. Higher quantum efficiency. D. Lower dark current.

44. Which current exists in a photodiode even in complete darkness?. A. Photocurrent. B. Avalanche current. C. Dark current. D. Transit current.

45. The RC-limited bandwidth of a detector improves when: A. Capacitance increases. B. Resistance increases. C. Capacitance decreases. D. Depletion width decreases.

46. Shot noise arises because: A. Carriers arrive as a random process. B. Photons reflect at the interface. C. Temperature gradients vary. D. Optical power fluctuates at DC.

47. Avalanche photodiodes (APDs) provide increased sensitivity mainly because they: A. Have zero dark current. B. Multiply carriers through impact ionization. C. Use thicker intrinsic layers. D. Require no bias voltage.

48. 18. The 3 dB bandwidth of a first-order photodiode system corresponds to: A. 10% of maximum response. B. 50% of maximum response. C. 70.7% of maximum response. D. 90% of maximum response.

49. A photodiode with a very wide depletion region typically has: A. Higher transit time. B. Higher capacitance. C. No dark current. D. Very low quantum efficiency.

50. Matching the size of a photodiode to the optical fiber core is important to: A. Reduce avalanche gain. B. Maximize coupling efficiency. C. Increase reflectivity. D. Eliminate thermal noise.

51. The primary purpose of using an external modulator with a CW laser is to: A. Increase spontaneous emission. B. Perform high-speed modulation with low chirp. C. Achieve higher optical absorption. D. Reduce the optical carrier frequency.

52. Electro-optic modulators based on LiNbO₃ rely on which crystal property?. A. High thermal conductivity. B. Anisotropic refractive index variation. C. Strong Raman gain. D. Linear absorption.

53. In a Mach–Zehnder modulator, constructive or destructive interference depends on: A. Fiber dispersion. B. Phase difference between the two arms. C. Laser bias current. D. Refractive index of cladding only.

54. The drive voltage Vπ of a modulator depends strongly on: A. Waveguide length and electrode spacing. B. Laser wavelength only. C. Fiber coupling. D. Optical output power.

55. A dual-drive MZM typically uses: A. A single voltage applied to both arms. B. Complementary electrical signals. C. A mechanical tuning mechanism. D. No electrodes.

56. Z-cut LiNbO₃ in modulators typically offers: A. Lower Vπ but more chirp. B. Higher Vπ and zero chirp. C. No phase modulation capability. D. No EO effect.

57. The electro-absorption effect alters: A. The resonant frequency of the laser. B. The material’s absorption spectrum. C. Laser linewidth. D. Fiber attenuation coefficient.

58. A polarization-based EO modulator uses: A. Crossed polarizers around a Pockels cell. B. A pump laser and nonlinear fiber. C. A Bragg mirror inside the waveguide. D. Dual laser cavities.

59. In EO modulators, the Pockels effect is characterized by: A. Quadratic dependence on electric field. B. Linear dependence on electric field. C. Zero dependence on electric field. D. Thermal expansion.

60. A typical LiNbO₃ MZ modulator can achieve bandwidths up to: A. 1 GHz. B. 5 GHz. C. 40 GHz. D. 200 GHz.

61. Which part of a telecommunication system performs modulation?. A. Receiver. B. Transmitter. C. Medium. D. Photodetector.

62. Why is wavelength preferred over frequency in optical communications?. A. Frequency is unstable. B. Frequency values are too small. C. Optical frequencies are extremely large. D. Wavelength does not depend on the medium.

63. Which wavelength window has the lowest fiber attenuation?. A. 850 nm. B. 1310 nm. C. 1550 nm. D. 980 nm.

64. Free-space optical communication uses which medium?. A. Water. B. Copper. C. Air. D. Optical fiber.

65. In FTTH, the return (uplink) channel typically uses: A. 1310 nm. B. 1490 nm. C. 1550 nm. D. 1625 nm.

66. Which device is used for optical detection in receivers?. A. Laser diode. B. PIN or APD photodetector. C. EDFA. D. Mach-Zehnder modulator.

67. ASK or OOK modulation in fiber systems mainly modifies: A. Phase. B. Polarization. C. Amplitude (intensity). D. Frequency.

68. The power budget determines: A. Maximum dispersion allowed. B. Minimum received power required. C. Modulation format. D. System’s rise time.

69. In the logarithmic power budget: A. Losses are added. B. Gains are multiplied. C. Power is linear only. D. Losses are ignored.

70. The rise-time budget ensures: A. The optical fiber is long enough. B. The system supports the target bit rate. C. Output power is constant. D. No reflections occur.

71. Standard Single Mode Fiber (SSMF) is defined under: A. ITU-T G.655. B. ITU-T G.657. C. ITU-T G.652. D. ITU-T G.653.

72. DSF (Dispersion Shifted Fiber) has minimum dispersion at: A. 850 nm. B. 1310 nm. C. 1550 nm. D. 1625 nm.

73. NZ-DSF fibers are mainly used in: A. Short-haul copper networks. B. DWDM systems. C. Microwave radio links. D. CATV coaxial networks.

74. A 1×8 optical splitter typically uses which technology?. A. FBT for best performance. B. PLC for best performance. C. Copper cabling. D. Thin-film filters.

75. Excess loss in a splitter refers to: A. Power lost due to reflection only. B. Power lost besides ideal splitting. C. Power returned to the input port. D. Loss due to polarization changes.

76. Insertion loss measures: A. Total system loss. B. Loss at the wavelength center. C. Loss from input to output for a given path. D. Loss caused by modulation.

77. Fused Biconic Taper (FBT) splitters are mainly limited by: A. Cost. B. Limited wavelength range. C. High uniformity. D. Very large package size.

18. PLC splitters are preferred for: A. 1×2 only. B. High port counts (1×16, 1×32…). C. Very narrowband filtering. D. Polarization rotation.

79. A Fabry–Perot filter behaves as: A. A broadband amplifier. B. A wavelength-selective resonator. C. A polarization scrambler. D. A circulator.

80. Free Spectral Range (FSR) in FP filters depends on: A. Fiber dispersion. B. Laser linewidth. C. Cavity length and refractive index. D. Input power.

81. Thin-film filters (TFF) operate based on: A. Nonlinear scattering. B. Multi-cavity interference. C. Doppler shift. D. Raman effect.

82. Fiber Bragg Gratings reflect wavelengths that satisfy: A. Maxwell’s equations. B. Bragg condition. C. Brewster’s angle. D. Rayleigh scattering.

83. AWGs (Arrayed Waveguide Gratings) perform multiplexing using: A. Polarization rotation. B. Mach-Zehnder interferometers. C. Path-length differences. D. Chromatic dispersion.

84. Polarizers are rated mainly using: A. Isolation ratio. B. Extinction ratio. C. Finesse. D. Return loss.

85. A quarter-wave retarder transforms: A. Linear to circular polarization. B. Circular to linear only. C. TE to TM modes. D. High power to low power.

86. A half-wave retarder rotates linear polarization by: A. 45°. B. 90°. C. 180°. D. 360°.

27. Optical isolators are based on: A. Faraday rotation. B. Rayleigh scattering. C. Brewster’s law. D. Piezo-electric actuation.

88. Circulators are typically used for: A. Amplifying optical signals. B. Temperature sensing. C. Add-drop filtering with FBGs. D. Preventing SBS nonlinearities.

89. The main purpose of optical attenuators is to: A. Increase optical power. B. Stabilize temperature. C. Reduce optical power to safe/optimal levels. D. Split signals evenly.

90. WDM couplers allow: A. Modulating multiple lasers. B. Combining or separating wavelengths. C. Reducing dispersion. D. Amplifying signals in C-band.