Page 205 - Fiber Optic Communications Fund

186 Fiber Optic Communications

4.7 An electro-optic modulator operating at 1550 nm has the following parameters:
Thickness d = 8μm
Index n = 2.2
0
Pockel coefficient r 33 = 30 pm/V
It is desired that the half-wave voltage V is less than 2 V. Find the lower limit on the length L.

(Ans: 1.94 cm.)
4.8 Explain how an electroabsorption modulator can be used as an amplitude modulator.

2
2
4.9 The d.c. extinction ratio of a MZM is 13 dB. Calculate the power-splitting ratio ∶ . Assume that
1 2
2
2
+ = 1.
1 2
(Ans: 0.71 : 0.29.)
4.10 The input power to a dual-drive MZM is 0 dBm. The MZM is used to generate a NRZ-OOK signal.
Find the drive voltage V and the bias voltage V bias . If the d.c. extinction ratio is 10 dB, calculate the
optical power levels corresponding to bit ‘1’ and ‘0’. Assume V = 4 V.

(Ans: V = 1 V, V bias = 2 V, power of bit ‘1’ = 1mW, power of bit ‘0’ = 0.1 mW.)
4.11 Consider the input data sequence
′
{b }=[110111011].
n
This data passes through the differential encoder and add-and-delay filter shown in Fig. 4.28. Deter-
mine the duobinary data voltage sequence m at instants nT (=m(nT )). To proceed with differential
n b b
encoding, add an extra bit to the encoder output. State the decision rule.
(Ans: {m }= [0 0 −2000200] V.)
n
4.12 Repeat Exercise 4.11 for the case of AMI generation as shown in Fig. 4.38. State the decision rule.
(Ans: {m }=[2 −202 −220 −22].)
n
4.13 Explain how the correlative coding combined with differential coding simplifies the decision rule at
the receiver.

4.14 The 4-ASK signal is transmitted over a fiber channel with a mean power of 0 dBm. Rectangular pulses
with 50% duty cycle are used in each symbol slot. Sketch the waveform of the 4-ASK signal for a
sequence {3, 1, 3, −1, −1, 1} showing the peak powers of each symbol. Assume that the optical field
amplitudes are equally spaced.
4.15 Repeat Exercise 4.14 for a square 16-QAM signal.

Further Reading
G.P. Agrawal, Lightwave Technology. John Wiley & Sons, New York, 2005.
S. Haykin and M. Moher, Communication Systems, 5th edn. John Wiley & Sons, New York, 2009.