Chapter 6 Optical System Design and Performance. 6.1 Point-to-Point Transmission Systems 6.1.1 Traditional Single-channel Systems 6.1.2 Amplified Single-Channel Systems 6.1.3 WDM Systems Overview 6.2 Modulation (Making the Light Carry a Signal) 6.2.1 On-Off Keying (OOK) NRZ Coding
6.1 Point-to-Point Transmission Systems
Figure 6.1 Conventional Long Distance Fiber Transmission System.
Thus it becomes necessary to boost the signal.
it amplifies a distorted signal.
Repeater Function compared
Figure 6.3 Amplified Single-Channel Transmission System.
The systems use optical amplifiers (EDFAs) with span lengths
from 110 to 150 km.
This is done for two reasons:
1. To exploit the low attenuation window of fiber in the 1500 nm "window“.
2. To allow the use of Erbium Doped Fiber Amplifiers (EDFAs).
1.In older systems, the fiber didn't disperse the signal by very much because we were using the 1310 nm band.
However, by moving to the 1550 nm band, we have brought on a dispersion problem.
2. The link may be upgraded to use higher speeds and the modulation format may be changed without changing equipment in the field. You only have to change the equipment at each end!
3. Provided the link has been planned properly it can now be upgraded to use WDM technology again without change to the outside plant.
1. Amplifiers cost less than repeaters and require less
2. The use of an amplifier enables future upgrades and
changes to take place with minimal impact (read cost)
on the installed link.
3. The use of the amplifier allows for future use of WDM
technology with minimal change to the outside plant.
Figure 6.4 WDM Long Distance Fiber Transmission System.
It may run at its own rate (speed) and use its own encodings and protocols without any dependence
on the other channels at all.
1). to take advantage of the "low loss" transmission
window in optical fiber;
2). to enable the use of erbium-dopped fiber amplifiers.
6.2.1 On-Off Keying (OOK)
A one bit is represented as the presence of light and a zero bit is represented as the absence of light.
NRZI Encoding Example
Figure 6.7 Return-to-Zero (RZ) Coding
1. The incoming optical signal is converted to an electronic one using either a PIN-diode or an APD.
2. The signal is then pre-amplified and passed through a band-pass filter. There are a number of very low frequencies that get into the signal and there will be very high frequency harmonics that we don't need.
Figure 6.8 Digital Receiver Functions.
3. Further amplification with feedback control of the gain is used to provide stable signal levels for the rest of the process. This control circuit usually controls the bias current and thus the sensitivity of the photodiode as well.
4. A phase-locked loop is then used to recover a bit stream and (optionally) the timing information.
6. At this stage the stream of bits needs to be decoded from the coding used on the line into its data format coding. This process varies depending on the encoding and is occasionally integrated with the PLL depending on the code in use.
The important issues for the receiver are:
and cost they suffer from three major deficiencies:
1. Even at quite slow speeds they cannot recover a good enough quality clocking signal for most applications
where timing recovery is important.
2. As link speed is increased, they become less and less effective. Because circuit speeds have not increased in
the same ratio as have communication speeds.
3. As digital signals increase in speed, they start behaving more like waveforms and less like "square waves" and the simplistic DPLL technique becomes less appropriate.
What is needed is a continuous-time, analogue PLL that is illustrated in Figure 6.9.
Figure 6.9 Operating Principle of a Continuous
to the operation.
1. The VCO is designed to produce a clock frequency
close to the frequency being received.
2. Output of the VCO is fed to a comparison device
(a phase detector) which matches the input signal to
the VCO output.
3. The phase detector produces a voltage output which
represents the difference between the input signal
and the output signal.
4. The voltage output is then used to control (change)
the frequency of the VCO.
1. Recovering the bit stream (that is, providing the
necessary timing to determine where one bit starts
and another one ends).
2. Recovering the (average) timing (that is, providing
a stable timing source at exactly the same rate as
the timing of the input bit stream).
of standard Fabry-Perot lasers.
with an analogue waveform.
The characteristics of various transmission systems are summarized in Table 6.1.
Table 6.2 shows the attenuation characteristics of various transmission media and the maximum spacing of repeaters available on that medium.
6.4.1 System Power Budgeting
1. 10 connectors at 0.3 dB per connector = 3 dB
2. 2 km of cable at 2 dB/km (Multimode Graded index
fiber at 1300 nm) = 4 dB
3. Contingency of (say) 2 dB for deterioration due to
ageing over the life of the system.
Figure 6.10 shows the characteristics of some typical devices versus the transmission speed (in bits/second).
1. Every laser has a limit to the maximum speed at which it can be modulated but up to that limit power output is relatively constant.
2. LEDs produce less and less output as the modulation rate is increased. The difference in fiber types only relates to the amount of power you can couple from an LED into the different types of fiber.
3. All receivers require higher power as the speed is increased. To reliably detect a bit a receiver needs a certain number of photons. Therefore every time we double the modulation speed we need to also double the required power for a constant signal-to-noise ratio.
the mean (average).
2. If the manufacturers of the two connectors are different, then you can expect an average loss of 0.35 dB with a standard deviation of 0.25 dB.
3. One type of single-mode connector may have an "average loss" of 0.2 dB but in practical situations this loss might vary from perhaps 0.1 dB to 0.8 dB.
1. The average (mean) of the total is just the average loss of a single connector multiplied by the number of connectors.
2. The standard deviation (s) of the total is just the standard deviation of a single connector multiplied by the square root of the number of connectors involved.
1. System noise
2. Effect of dispersion and
3. Extinction ratio
Signal-to-Noise Ratio (SNR)
Inter-Symbol Interference (ISI)
1. Disruption of laser operation
2. Return Loss
3. Amplifier operation
1.Disruption of laser operation :
Reflections entering a laser disturb its stable operation adding noise and shifting the wavelength.
2. Return Loss :
Reflections can vary with the signal and produce a random loss of signal power. This is termed “ return loss ”.
3. Amplifier operation :
Reflections returning into an optical amplifier can have two main effects:
- In the extreme case of reflections at both ends the amplifier
becomes a laser and produces significant power of its own.
- In lesser cases reflections can cause the amplifier to saturate
(by taking away power) and again introduce noise to the signal.
1. Joins between high RI material and fiber (such as at
the junction between a laser or LED and a fiber or
between any planar optical component and a fiber).
2. Joins between fibers of different characteristics.
For example where a Pr-doped amplifier employing
ZBLAN host glass is coupled to standard fiber for
input and output.
3. Any bad connector produces significant reflections.
4. Some optical devices such as Fabry-Perot filters
reflect unwanted light as part of their design.
1. Taking care with fiber connectors and joins to ensure that
they are made correctly and produce minimum reflections.
This can be checked using an OTDR.
2. The inclusion of isolators in the packaging of particularly
sensitive optical components (such as DFB lasers and
amplifiers). The isolators attenuate the signal and are
polarization sensitive. Their use should be carefully
planned and minimized.
3. In critical situations a diagonal splice in the fiber or a
connector using a diagonal fiber interface can be employed.
The diagonal join ensures that any unwanted reflections
are directed out of the fiber core.
Table 6.4 Optical Network Technologies
and Major Applications
FTTx : Fiber-to-the-x
Any network which extends fiber optic capacity
directly to the user; x can be replaced with “home”,
“business”, “curb”, “cabinet”, depending on how
close the fiber connection actually is to the end user.
Fig. 6.11 PON network Diagram.
Table 6.5 A PON network elements description
1. sharing the cost of optical terminal equipment
(the electrical-to-optical conversion) in the
2. reducing the cost of servicing active devices
located in the field, and
3. sharing the fiber with multiple subscribers.
Time Domain Multiplexed Access (TDMA).
the data networking standard. As such, EPON
can handle traffic better which is more often
data- than is voice-related.