CHAPTER
13
DECODING
OPTICAL INFORMATION
13.1
INTRODUCTION
Optical decoding entails detecting an optical signal and retrieving (or demodulating)
binary coded information from the received modulated lightwave based on one of
the three coding techniques discussed earlier:
• Detecting optical amplitude level if amplitude-shift keying (ASK) is used.
• Detecting phase change (from 0 to 180°) if phase-shift keying (PSK) is used.
• Detecting frequency change (from w - aw to co
keying (FSK) is used.
+
aw) if frequency-shift
Because each technique is different from the others, clearly the receiver device and
the overall design differ as well.
13.2
ASK DEMODULATORS
ASK demodulators use photodetectors that directly detect incident photonic amplitude. Based on a threshold level, incident amplitude above it is interpreted as logic
1 and below it as logic o. When the incident amplitude is ambiguous due to photonic
noise sources, an erroneous 1 or 0 may be produced. To remove noise from the signal, the retrieved signal is low-pass-filtered and sampled by a local phase-locked
loop (PLL) at the expected incoming bit rate.
It should be pointed out that for long fiber lengths signal attenuation weakens
the incident light at the receiver considerably, and it is counted in single-digit photons per bit (logic 1); the amount is known as the quantum limit of the receiver.
173
174
Part III
Cod ing Optical Information
Consequently, the incoming signal should be coupled with the receiver with no
losses, the signal should be dispersion-compensated, and the sensitivity of the receiver should be very high.
13.3
PSK AND FSK DEMODULATORS
PSK and FSK demodulators are based on coherent (homodyne and heterodyne) principles . This means that in addition to the received optical signal from the fiber, the
receiver must have one or two local oscillators (optical sources) to interact with the
received optical signal and to extract binary information. The basic principle of a homodyne PSK demodulator (without noise) is as follows (Figure 13.1).
Optica ,Igoal , "'s(
~
WLO
•
*
~
Photodetector
Filter
_ __ _
r----.
Recovered
electrical
binary signal
Figure 13.1 Basic principle of a homodyne PSK demodulator.
The received optical modulated signal Ws is mixed coherently with a locally
generated frequency WLO' These frequencies are the same, and they interact interferometri cally. Clearly, when both frequencie s are in phase, there is maximum optical contribution; otherwise, they cancel each other. Because this arrangement is
based on optical interference, phase stability is important. The mixed optical signal
is detected by a photodetector that provides electrical output of logic 1 or O. Clearly,
the demodulator becomes more complex when noise is included.
The basic principle of a homodyne FSK demodulator (without noise) is as follows (Figure 13.2). The received optical modulated signal Ws is split into two equal
parts. Then, each part interacts interferometrically with a locally generated frequency, one part with WI = Ws - Aw and the other with Wz = Ws + Aw. Because
each local frequency coincides with the frequency of a logic 0 or a logic 1, it is obvious that if two optical filters, OF) and OF 2 , are used, each centered at one of
Optical signal, Ws
Recovered
electrical
binary signal
Splitter
Figure 13.2 Basic principle of a homodyne FSK demodulator.
Chapter 13
175
Decoding Optical Information
the two respective frequencies, the temporal bits 1 and 0 are recognized. Upon recombining the two halves in the time domain, the complete data bit stream will be
recovered. Because this arrangement is based on optical interference, optical phase
alignment and stability are important. Again, the demodulator becomes more complex when noise is included.
A simplified FSK demodulator can be constructed by passing the received optical modulated signal Ws through a tunable filter tuned to pass the frequency Wz =
w + ~w. Whenever this frequency is detected, logic I is generated; otherwise, logic
o is detected (Figure 13.3). The tunable filter provides the flexibility of selecting a
given frequency and acts like an FSK-to-ASK converter. As usual, the demodulator
becomes more complex when noise is included.
Photodetector
~
Filter
- - _ .
Recovered
electrical
binary signal
Figure 13.3 A simplified FSK demodulator.
EXERCISES
1. Could a receiver designed to detect a PSK signal detect an FSK signal ?
2. A PSK modulator shifts the phase by 180° thus decoding a 1 or a 0, or two states, depending on the received phase . How many states are possible in a PSK modulator that
shifts by 30°?
3. A signal has traveled about 200 km of fiber without amplification . Comment on the receiver precautions for reliable signal detection.
4. Consider a PSK receiver. How do the received signal and the local oscillator interact to decode the signal?
5. Consider an FSK signal. What kind of optical device is required to decode the signal?
REFERENCES
[1] J.C. Palais, Fiber Optic Communications, 3rd ed., Prentice-Hall , Englewood Cliffs, NJ,
1992.
[2] I.P. Kaminow, ed., and TL Koch, ed., Optical Fiber Communications IlIA and Optical
Fiber Communications IlIB, Academic Press, San Diego, CA, 1997.
[3] R. Ramaswami and K.N. Sivarajan, Optical Networks, Morgan Kaufmann , San
Francisco, 1998.
[4] B. Mukhergie, Optical Communications Networks, McGraw-Hill, New York, 1997.
[5] R.A. Linke, "Optical Heterodyne Communications Systems," IEEE Communications
Magazine , October 1989, pp. 36-41.
[6] R.A. Linke and A.H . Gnauck, "High-Capacity Coherent Lightwave Systems," Journal of
Lightwave Technology, vol. 6, no. 11, 1988, pp. 1750-1769.
[7] R.E. Slusher and B. Yurke, "Squeezed Light for Coherent Communications," Journal of
Lightwave Technology, vol. 8, no. 3, 1990, pp. 466-477.
[8] J.A.C. Bingham, "Multicarrier Modulation for Data Transmission: An Idea Whose Time
Has Come," IEEE Communications Magazine, vol. 28, no. 5, May 1990, pp. 5-14.
176
Part III
Coding Optical Information
[9] R.D. Gitlin, IF. Hayes, and S.B. Weinstein, Data Communications Principles, Plenum,
New York, 1992.
[10] T. Wildi, Units and Conversion Charts, IEEE Press, Piscataway, NJ, 1991.
[11] J. Nellist, Understanding Telecommunications and Lightwave Systems, IEEE Press,
1996.
[12] A. Borella, G. Cancellieri, and F. Chiaraluce, Wavelength Division Multiple Access
Optical Networks, Artec House, Boston, 1998.
[13] P. Black and T. Meng, "A I-Gb/s Four-State Sliding Block Viterbi Decoder," IEEE
Journal on Solid-State Communications, vol. 32, no. 6, June 1997, pp. 797-805.
[14] D. Edforrs et aI., "An Introduction to Orthogonal Frequency-Division Multiplexing,"
http://www.tde.lth.se/home/oes/publications.html.
[15] R.E. Matick, Transmission Lines for Digital and Communication Networks, IEEE Press,
Piscataway, NJ, 1995.
[16] Members of the Technical Staff, Transmission Systems for Communications, Bell
Telephone Laboratories, Murray Hill, NJ, 1982.
[17] S.U.H. Qureshi, "Adaptive Equalization," Proceedings of IEEE, vol. 73, no. 9,
September 1985, pp. 1349-1386.
STANDARDS
[1] ANSIIIEEE 812-1984, "Definition of Terms Relating to Fiber Optics."
[2] Bellcore, TR-NWT-499, "Transport Systems Generic Requirements [TSGR): Common
Requirements," issue 5, December 1993.
[3] IEEE 802.3ab, 1000BaseT.
[4] IEEE 802.1 to 802.6, Local Area Networks.
[5] ITU-T Draft Recommendation G.692 [ex Gsaf), "Optical Interfaces for Multichannel
Systems with Optical Amplifiers," October 1998.
[6] ITU-T Recommendation G.702, "Digital Hierarchy Bit Rates," 1988.
[7] ITU-T Recommendation G.741, "General Considerations on Second Order Multiplex
Equipment," 1988.
[8] ITU-T Recommendation 1.432, "B-ISDN User-Network Interface-Physical Layer
Specification."