E&CE 477: Photonic Communication Systems & Devices

E&CE 477: Photonic Communication Systems & Devices

Sample of Final exam from W05

Instructor : A. Hamed Majedi

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Name: Student ID#:

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Part I: Questions

Q1- [6 points]

a)Define the sensitivity of a photodetector?

b)Name three principal noise sources associated with photodetectors.

Q2- [4 points]

a) What type of noise dominates at low optical power level in semiconductor photodetector?

b) Mention another type of noise that is dominant at high optical power level?

Q3- [6 points]

a)Explain the trade-off between the width of the photodetector area and photodetectors’ quantum efficiency?

b)What would be the optimal value for the width of the photodetector area in terms of the absorption coefficient of the semiconductor that has been used for photodetection area?

Part II: Problems

P1- [10 points]

A photodiode is operating at 850 nm wavelength with a semiconductor material whose exhibits an absorption coefficient and reflectivity 0.1 at the operating wavelength. If the width of detection area is 1 m and the input optical power entering the detection area is -10 dBm,

a) Calculate the primary photocurrent.

b) Calculate the internal quantum efficiency.

c) Calculate the responsivity of the photodiode.

P2- [15 points]

The pin photodiode receiver shown in Fig P2 has a 1 k load resistor. The voltage across the resistor is 0.20 mV when the incident optical power is 800 nW at a wavelength 850 nm.

a)Calculate the photodetector’s responsivity.

b)If the pin photodiode in Fig P2 has a depletion width of 30 micron, a carrier drift velocity of m/s, and a junction capacitance of 16 pF compute the transit-time-limited bandwidth.

c)What is the largest load resistance that will not affect the bandwidth calculated in part b)?

d)Suppose a sinusoidally modulated 850 nm optical signal having a unity modulation index falls on this pin photodiode. Consider the following parameters at room temperature (T=300K): 5 nA, =1 nA, =1 k, and the surface leakage current is negligible. If the incident optical power is -35 dBm and the receiver bandwidth is 65 MHz calculate the signal-to-noise ratio.

Fig P2

P3- [25 points]

Consider a photonic DS1 link with NRZ digital transmission system at 1550 nm optical wavelength. In this system, the signal voltage for binary 1 is a pulse with amplitude V, and the threshold voltage is V/2, and no pulse is transmitted for binary 0. Assume that variance noise for each binary is the same.

a)If the probabilities of 0 and 1 signals are equal, what signal-to-noise ratio in dB is required for achieving a bit error rate .

b)If the bit rate of this system is 50 Mb/s, how often an error will be generated?

P4- [15 points]

Consider two different surface emitting LEDs with circular emitting areas each with radius . One has emission pattern and the other one has an emission pattern given by .

a)Calculate the power coupled via air into a step index fiber, with core radius and a given numerical aperture, NA, from each source.

b)Which LED couples more optical power into the fiber?

Hint:

1) where n is the refractive index of the external medium.

2)

Physical constants:

, , ,

,,,

1