Reference no: EM132679741
1.1 Explain what the critical angle of an optical fibre is?
1.2 Explain what intramodal dispersion is and how it can be avoided.
1.3 Explain what modal dispersion is and how it can be avoided.
1.4 A multimode step-index fibre has a relative refractive index difference of 1%, a core diameter of 52 p.m, and a core refractive index of 1.46. The maximum optical bandwidth that may be obtained with a particular source operating at a wavelength of 1540 nm on a 4.5 km link is 3.0 MHz.
1.4.1 Determine the numerical aperture and the acceptance angle for the fibre in air.
1.4.2 Determine the number of guided modes propagating in the fibre.
1.4.3 Determine the spot size for the fibre.
1.4.4 Determine the rms pulse broadening per kilometre which results from intramodal dispersion mechanisms.
2.1 A signal of power 7 dBm is launched into a 4 km long fiber with an absorption coefficient of 5 dB/km; what is the output power in milliwatts at the output end of the fiber?
2.2 A multimode step-index fibre is fusion spliced. The core refractive index of the fibre is 1.52 and the splice exhibits an insertion loss of 0.8 dB. The insertion loss is entirely due to angular misalignment of the core axes which is 7 degrees. Determine the numerical aperture of the fibre.
2.3 An optical fibre has a core refractive index of 1.45. Two (2) length of the fibre with smooth and perpendicular end faces are butted together with a small gap between the end faces. Find the the optical loss in decibels at the joint (due to Fresnel refelction).
2.4 An 8x8 port multimode fibre reflective star coupler has -8 dBm of optical power launched into a single port. The average measured optical power at each output port is -22 dBm. Obtain the excess loss for the device and hence the total loss experienced by an optical signal in transmission through the coupler.
3.1 How can the frequency that a LED emits be changed?
3.2 For an AIGaAs heterostructure laser with a cavity length of 200 pm, an operating wavelength of 870 nm and a refractive index of 3.7, calculate the number of modes generated by the device and the separation between the modes of the cavity.
3.2.1 If the optical gain medium bandwidth is 6 nm, how many modes are 3.3 3.4
3.5 How does an LED affect the operating bandwidth of a single mode step index fibre?
3.6 Briefly describe the terms dark current and detectivity related to a photodetector.
3.7 A photodetector operating at 800 nm has a quantum efficiency of 15 %. The load resistance for the device is 100 f2 and the optical power absorbed by the load resistor is 1.7 pW. Calculate the voltage across the load.
Determine the multiplication factor for the device.
3.8 The following measurements were taken for an APD, Received optical power at 1545nm = 0.2pW Corresponding output photocurrent = 4.9 µA (after avalanche gain) Quantum efficiency at 1545 nm = 40 %
4.1 An optical fibre receiver incorporates a high impedance integrating front-end amplifier with an effective input impedance of 6 Mohm. The detector bias resistance is 10 Mohm and the total capacitance of the detector and amplifier is 2 pF (T = 290 K).
4.1.1 Calculate the bandwidth and rms thermal noise current for the amplifier.
4.2 A silicon PIN-photodiode incorporated into an optical fibre has a quantum efficiency of 60 % and operating at a wavelength of 1300 nm. The dark current in the device operating at this wavelenght is 3 nA and the load resistance is 4kf2. The incident optical power at this wavelength is 200 nW and the post-detection bandwidth of the receiver is 10 MHz. compare the shot-noise generated in the photodetector with the thermal noise in the load resistance at temperature of 20 °C.
4.3 Determine the maximum fibre length of a multimode optical fibre communications link that consists of a 1.0 mW laser diode, operating at a wavelength of 1320 nm and a pin-photodiode detector. The laser diode and detector is connected to the fibre with a connector that has a loss of 0.5 dB at each connector. The fibre attenuation at the operating frequency is 2.5 dB/km. The fibre is jointed every 5 km with a fusion splice resulting in an additional loss of 0.05 dB loss per splice. The required optical power at the detector to ensure a bit-error-rate of less than 10-° is -42 dBm.
4.4 An optical fibre system operates over a distance of 10km without repeaters. The rise-times of the components are:
LED source: 8ns
Fibre:
Intermodal: 5 ns/km Intramodal: 1 ns/km Detector: 6 ns Estimate the maximum bit rate that may be achieved on the link when using an RZ format.