7 Optical Communication Systems

1

Photonics and Optical Communication, Spring 2007, Dr. D. Knipp

Optical Communication Systems

Photonics and Optical Communication(Course Number 300352)

Spring 2007


Dr. Dietmar Knipp

Assistant Professor of Electrical Engineering

http://www.faculty.iu-bremen.de/dknipp/

2



Photonics and Optical Communication

7 Optical Communication Systems

7.1 Introduction 7.2 Point to Point Transmission in a single channel system7.3 Repeaters in optical communication systems7.4 Optical Amplifiers in Single Channel Systems7.5 Multiplexing Strategies7.6 Coding Techniques

7.6.1 NRZ Coding7.6.2 Non-Return to Zero Inverted (NRZI) Coding7.6.3 RZ Coding

References

3



7.1 IntroductionSo far we discussed the operation principle of individual optical and optoelectronic components. In the following the individual components will be combined to form a state of the art optical communication network.

In this course we will concentrate on digital optical communication systems. Analog modulation schemes are only of minor interest in optical communications. As a consequence the signal coupled in an optical fiber can always be regenerated at the end of the transmission channel due to the digital nature of the signal. Hence, the signal is transmitted without a loss of information. In the following we will start with the discussion of point-to-point connection before moving on to multi-channel systems.

4



7.2 Point to Point Transmission in a single channel system

A traditional single channel transmission system is shown on this slide. Traditional single channel systems were in use during the 1980’s. Time Division Multiplex was used to combine a number of electrical channels, before feeding the channels in an optical fiber via an optical transmitter module. The signal is optically transmitted, but the regeneration of the signals is carried out by electro-optical repeaters. The repeater spacing is 30-50km and the transmission speed ranges from 155Mbit per second (Mbps) to 1.2Gbit per second (Gbps).

Traditional long distance single channel fiber transmission system.

Ref.: H. J.R. Dutton, Understanding optical communications

5



7.2 Point to Point Transmission in a single channel systemWhen the signal is received at a repeater station the signal is first converted in an electrical signal. In the second step the signal is amplified. Furthermore, distortions are removed from the signal, which are caused by noise or signal dispersion.

The major bottleneck of such traditional optical communication systems were the repeaters. When the system had to be upgraded - meaning the transmission rate was increased – all repeaters had to be replaced by new repeaters.

6



7.3 Repeaters in optical communication systems

The propagation of light through a fiber leads to an attenuation, which is usually proportional to the length of the optical fiber. The main sources of attenuation in an optical fiber are absorption and scattering. We already discussed the properties of fibers as part of the lecture on optical waveguides/fibers. If the performance of an optical communication link would be limited only by the attenuation of the signal a simple amplification of the signal would be sufficient to regenerate the original signal. However, it is obvious that the regeneration of a signal due to a repeater is not only an amplification. In general, as a signal travels along a wire, a cable or an optical fiber (it does not matter whether it is transmitted electrically or optically) the signal is distorted.

Comparison of amplification and signal regeneration.


7



7.3 Repeaters in optical communication systems

At this point we will focus our discussion on the influence of distortion. This problem is quite severe for electronic communication systems. Distortion of signals due to transmission is less of a problem for optical communication systems, because these effects can easier be removed or compensated.

The fact that we are dealing with a digital communication systems simplifies the regeneration of the signal drastically. Repeaters in optical communication systems receive the signal, convert the signal in an electrical signal, re-clock and re-shape it, amplify it, converted it back to an optical signal before coupling the signal back in the optical fiber. It is important to mention that this process is code and timing sensitive. The repeaters have to be designed to handle the transmission code and the timing scheme.

8



7.4 Optical Amplifiers in Single Channel SystemsDuring the 1980’s most of the optical communication systems operated at a wavelength of 1310nm (2rd Generation of Optical Communication Systems). That changed at the beginning of 1990’s, when operating wavelengths were increased to 1550nm (3rd Generation). In the mid 1990’s Erbium Doped Fiber Amplifiers (EDFAs) where introduced. Therefore, the electro-optical repeaters were replace by all-optical repeaters. This major breakthrough leads to a further increase of the bandwidth-distance product of optical communication systems.

Single Channel optical transmission system using optical amplifiers (4rd Generation of Optical Communication Systems).


9



7.4 Optical Amplifiers in Single Channel SystemsAs a consequence the spacing of the repeater stations could be increased to 110km-150km, which leads to a significant cost reduction. The transmission speed was increased to 1.2 Gbps or 2.4 Gbps.

The transition from the 2rd generation to the 4th generation of optical communication systems leads to the following consequences:

1. The chromatic (material) dispersion in the medium wave band (1300nm band) is close to be zero, whereas the dispersion at 1550nm is significantly increased. The increased dispersion has to be compensated. This is clearly a disadvantages of optical communication systems operating in the long wave band.

2. Due to the introduction of optical amplifiers the system is “modulation transparent” and “speed transparent”. This means that the modulation scheme can be changed from without replacing the optical amplifiers. This simply requires that the amplifier is linear. The same applies if thetransmission speed of the transmitter modules is increased. It is not necessary to exchange the optical amplifiers.

10



7.4 Optical Amplifiers in Single Channel Systems

3. In order to increase the overall transmission rate the modulation scheme can be changed, the transmission speed can be increased or/and the number of channels can be increased. So far we discussed the first and the second option. Now we will speak about the third alternative. In general, fiber based optical amplifiers amplify an entire wavelength range, for example from 1530nm to 1560nm. Therefore, it should not be of importance whether the optical amplifier amplifies a single or several channels. However, an increase of the number of channels requires an increase of the optical output power of the optical amplifier. In order to avoid an exchange of the optical amplifiers the optical output power of the amplifier can be chosen in such a way that the amplifiers still work with an increased number of channels.

Such design constraints have be considered while designing an optical communication system. If the output power of the optical amplifier is sufficiently high the amplifiers does not have to be replaced if the transmission rate is increased as a consequence of the change of the modulation scheme, the change of the transmission speed and the increase of the transmitted channels. Only the transmitter module on the input and the receiver module on the output side has to be replaced.

11



7.5 Multiplexing StrategiesThe operating speed of electronic components reaches a practical limit at frequencies of 10GHz-100GHz. However, in order to exploit more of the available bandwidth of optical fibers alternative multiplexing schemes have be employed. In the following we will discuss the two most important multiplexing schemes which make better use of the available optical fiber bandwidth.

• Time Division Multiplexing (TDM)

• Wavelength Division Multiplexing (WDM)

Comparison of Optical Time Division Multiplexing and Wavelength Division multiplexing

Digital voice channel: 64kbit/s

TDM: Time division multiplexing, WDM: Dense Wavelength Division Multiplexing

12



Total capacity (bit rate) per fiber.

Ref.: Bell Labs Technology, Corporate Journal

7.5 Multiplexing StrategiesThe transmission rate per channel is kept constant nearly for a decade (1990 to 1998), but the overall capacity of an optical fiber increased drastically due to the introduction of wavelength division multiplexing. State of the art optical communication systems are based on wavelength division multiplexing.

13



7.5 Multiplexing Strategies Wavelength division multiplexing involves the transmission of several channels at the same time (parallel) through a single optical fiber. Each channels has its own optical wavelength. Wavelength Division Multiplexing is quite different from electrical frequency based modulation schemes. Each channel in an Wavelength division multiplex system has access to the entire bandwidth of the optical fiber.

Basic concept of point-to-point connection in an DWDM system. Ref.: S.V. Kartalopoulos, Introduction to DWDM technology

14



7.5 Multiplexing StrategiesA complete schematic sketch of a Wavelength Division Multiplex point-to-point transmission system is shown on this slide. Each optical transmitter module transmits at a different wavelength so that the channels are completely independent of each other.

The optical multiplexer can be realized by a passive optical power combiner, because the different optical signals have to be simply feed into a single fiber.

Wavelength Division Multiplex System. All optical transmitters operate at a different wavelengths.


λ1

λn λn

λ1

15



7.5 Multiplexing StrategiesIt is even possible to transmit channels that operate at different transmission speeds or different modulation schemes.

Systems typically use a range of wavelengths from 1540 nm to 1560 nm. We already discussed the advantages of this wavelength range (long wave range) several times. Optical fibers exhibit their overall minimum of attenuation at 1550nm and optical amplifiers operate only at this wavelength. Transmission speeds for such (D)WDM systems of several Tbps have been demonstrated.

16



7.6 Coding TechniquesIn order to transmit data through an optical channel the optical signal has to be varied in a systematic fashion (coded) to represent the signal. In general different coding scheme can be used to transmit optical signals like ASK, PSK, or FSK.

Frequency shift keying (FSK) is a technique that modulates the frequency of the light beam. However, it is difficult to directly modulate the frequency of a laser. An external modulator (electro acoustic Bragg modulator) can be used, but so far this method is not yet in general use.

The phase of a laser beam can’t be directly modulated, which would be required in the case of Phase Shift Keying (PSK) and therefore this coding technique is not used in optical communications.

What is left is Amplitude Shift Keying (ASK), which is the most applied coding technique in optical communications. Amplitude shift keying (ASK) is a technique which uses a bit stream to modulate the intensity of the light beam directly. Maximum intensity is usually considered to be “1” and minimum or zero intensity is considered to be “0”.

17



7.6 Coding TechniquesOn-off Keying (OOK) can be considered as a special case of amplitude shift keying (ASK) in which a number of discrete signal amplitude levels are used to carry a digital signal. In the following we will discuss different On-Off Keying schemes.

7.6.1 NRZ CodingThe simplest coding scheme is NRZ coding (no return to zero coding) where a “1” is represented by the presence of light (maximum light) and a “0” is represented by the absence of light (no light or minimum level of light). Even thought the coding scheme is very simple it is only used for slow speed optical links. The major disadvantage of NRZ Coding is the recreation of the signal levels. There is no timing information present in the received signal. The receiver has to define where a sample start. Therefore re-clocking is difficult.

No return to zero (NRZ) coding.


18



7.6.2 Non-Return to Zero Inverted (NRZI) Coding

Instead of using a NRZ Coding scheme a Non-Return to Zero Inverted (NRZI) scheme is often used. Most digital communication systems are applying the NRZI scheme. An “0” is coded if a transition from light to no light or from no light to light is observed. When there are two successive pulses of light or two successive periods of minimum (zero) intensity a “1” is detected. Therefore, a “0” is represented as a change of a state whereas a “1” is detected as the absence of a change.

No return to zero inverted (NRZI) coding.


19



7.6.3 RZ CodingIn Return to Zero (RZ) coding the signal returns to the “0” throughout every bit. An “1” is detected if the optical intensity is maximized during half of the bit time. During the subsequent second half of the bit time the optical intensity has to be minimized. In electrical communication system this coding scheme is not an option because the required bandwidth is increased. In optical communication the overall performance of the system is not limited by the bandwidth of the optical fiber itself. Therefore such coding schemes can be used.

RZ coding is or was used as a standard coding scheme for Optical Time Division Multiplexing (OTDM) communication systems.

Return to zero (RZ) coding.


20



References:

John M. Senior, Optical Fiber Communications, Prentice Hall Series in Optoelectonics, 2nd edition, 1992.

Bahaa E.A. Saleh, Malvin Carl Teich, Fundamentals of Photonics,Wiley-Interscience (1991)

Harry J. R. Dutton, Understanding Optical Communications, Prentice Hall Series in Networking, 1998. (Formerly freely available as a red book on the IBM red book server.

Joseph C. Palais, Fiber Optic Communications, Prentice Hall Series, 1998. 4th edition.

Documents

7 Optical Communication Systems