306
8 Digital Optical Fiber Links
optical transmitter output and the minimum receiver sensitivity needed to establish a
specified BER. This margin can then be allocated to connector, splice, and fiber losses,
plus any additional margins required for other components, possible component
degradations, transmission-line impairments, or temperature effects. If the choice
of components did not allow the desired transmission distance to be achieved, the
components might have to be changed or amplifiers might have to be incorporated
into the link. Once the link power budget has been established, the designer can
perform a system rise-time analysis to ensure that the desired overall operational
performance has been met.
8.1.1 Signal Formats for Transporting Information
In designing a communication link for transporting digitized information, a significant consideration is the format of the transmitted digital signal [1–5]. One important
factor concerning the signal format that is sent out from the transmitter is that the
receiver must be able to extract precise timing information from the incoming signal
[6]. The following are the three main purposes of timing:
• To allow the signal to be sampled by the receiver at the time the signal-to-noise
ratio is a maximum
• To maintain proper spacing between digital pulses
• To indicate the start and end of each timing interval
In addition, it may be desirable for the signal to have an inherent error-detecting
capability, as well as an error-correction mechanism, if it is needed or is practical.
These timing and error-minimizing features can be incorporated into the data stream
by restructuring or encoding the digital signal [7–9]. This process is called channel
coding or line coding. This section examines the basic binary line codes that are used
in optical fiber communication systems.
One of the principal functions of a line code is to minimize errors in the bit stream
that might arise from noise or other interference effects. Generally one does this by
introducing extra redundant bits into the raw data stream at the transmitter, arranging
them in a specific pattern, and extracting the redundant bits at the receiver to recover
the original signal. Depending on the amount of redundancy that is introduced into the
data stream, various degrees of error reduction in the data can be achieved, provided
that the data rate is less than the channel capacity.
NRZ and RZ Signal Formats The simplest method for encoding data is the
unipolar nonreturn-to-zero (NRZ) code. Unipolar means that a logic 1 is represented
by a voltage or light pulse that fills an entire bit period, whereas for a logic 0 no pulse
is transmitted, as shown in Fig. 8.2 for the data sequence 1010110. Because this
process turns the light signal on and off, it is known as amplitude shift keying (ASK)
or on–off keying (OOK). If 1 and 0 pulses occur with equal probability, and if the
amplitude of the voltage pulse is A, then the average transmitted power for this code
is A
2 /2. In optical systems one typically describes a pulse in terms of its optical power
8 Digital Optical Fiber Links
optical transmitter output and the minimum receiver sensitivity needed to establish a
specified BER. This margin can then be allocated to connector, splice, and fiber losses,
plus any additional margins required for other components, possible component
degradations, transmission-line impairments, or temperature effects. If the choice
of components did not allow the desired transmission distance to be achieved, the
components might have to be changed or amplifiers might have to be incorporated
into the link. Once the link power budget has been established, the designer can
perform a system rise-time analysis to ensure that the desired overall operational
performance has been met.
8.1.1 Signal Formats for Transporting Information
In designing a communication link for transporting digitized information, a significant consideration is the format of the transmitted digital signal [1–5]. One important
factor concerning the signal format that is sent out from the transmitter is that the
receiver must be able to extract precise timing information from the incoming signal
[6]. The following are the three main purposes of timing:
• To allow the signal to be sampled by the receiver at the time the signal-to-noise
ratio is a maximum
• To maintain proper spacing between digital pulses
• To indicate the start and end of each timing interval
In addition, it may be desirable for the signal to have an inherent error-detecting
capability, as well as an error-correction mechanism, if it is needed or is practical.
These timing and error-minimizing features can be incorporated into the data stream
by restructuring or encoding the digital signal [7–9]. This process is called channel
coding or line coding. This section examines the basic binary line codes that are used
in optical fiber communication systems.
One of the principal functions of a line code is to minimize errors in the bit stream
that might arise from noise or other interference effects. Generally one does this by
introducing extra redundant bits into the raw data stream at the transmitter, arranging
them in a specific pattern, and extracting the redundant bits at the receiver to recover
the original signal. Depending on the amount of redundancy that is introduced into the
data stream, various degrees of error reduction in the data can be achieved, provided
that the data rate is less than the channel capacity.
NRZ and RZ Signal Formats The simplest method for encoding data is the
unipolar nonreturn-to-zero (NRZ) code. Unipolar means that a logic 1 is represented
by a voltage or light pulse that fills an entire bit period, whereas for a logic 0 no pulse
is transmitted, as shown in Fig. 8.2 for the data sequence 1010110. Because this
process turns the light signal on and off, it is known as amplitude shift keying (ASK)
or on–off keying (OOK). If 1 and 0 pulses occur with equal probability, and if the
amplitude of the voltage pulse is A, then the average transmitted power for this code
is A
2 /2. In optical systems one typically describes a pulse in terms of its optical power