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Application Note 
April 2001 

RF and Microwave Fiber-Optic Design Guide 

Introduction 

Agere Systems Inc., through its predecessors, began 
developing and producing lasers and detectors for linear 
fiber-optic links nearly two decades ago. Over time, 
these optoelectronic components have been continually 
refined for integration into a variety of systems that 
require high fidelity, high frequency, or long-distance 
transportation of analog and digital signals. As a result 
of this widespread use and development, by the late 
1980s, these link products were routinely being treated 
as standard RF and microwave components in many 
different applications. 

There are several notable advantages of fiber optics 
that have led to its increasing use. The most immediate 
benefit of fiber optics is its low loss. With less than 

0.4 dB/km of optical attenuation, fiber-optic links send 
signals tens of kilometers and still maintain nearly the 
original quality of the input. 
The low fiber loss is also independent of frequency for 
most practical systems. With laser and detector speeds 
up to 18 GHz, links can send high-frequency signals in 
their original form without the need to downconvert or 
digitize them for the transmission portion of a system. 
As a result, signal conversion equipment can be placed 
in convenient locations or even eliminated altogether, 
which often leads to significant cost and maintenance 
savings. 

Savings are also realized due to the mechanical flexibility 
and lightweight fiber-optic cable, approximately 1/25 
the weight of waveguide and 1/10 that of coax. Many 
transmission lines can be fed through small conduits, 

allowing for high signal rates without investing in 
expensive architectural supports. The placement of 
fiber cable is further simplified by the natural immunity 
of optical fiber to electromagnetic interference 
(EMI). Not only can large numbers of fibers be tightly 
bundled with power cables, they also provide a 
uniquely secure and electrically isolated transmission 
path. 

The general advantages of fiber-optics first led to 
their widespread use in long-haul digital telecommunications. 
In the most basic form of fiber-optic communications, 
light from a semiconductor laser or 
LED is switched on and off to send digitally coded 
information through a fiber to a photodiode receiver. 

By comparison, in linear fiber-optic systems developed 
by Lucent, the light sent through the fiber has 
an intensity directly related to the input electrical current. 
While this places extra requirements on the 
quality of the lasers and photodiodes, it has been 
essential in many applications to transmit arbitrary 
RF and microwave signals. As a result, tens of thousands 
of Agere Systems’ transmitters are currently 
in use. 

The information offered here examines the basic link 
components, provides an overview of design calculations 
related to gain, bandwidth, noise, and 
dynamic range and distortion. A section on fiber-
optic components discusses a number of key 
parameters, among them wavelength and loss, dispersion, 
reflections, and polarization and attenuation. 
Additional information evaluates optical 
isolators, distributed-feedback lasers and Fabry-
Perot lasers, predistortion, and short- vs. long-wavelength 
transmission. 


Application Note 
RF and Microwave Fiber-Optic Design Guide April 2001 

Table of Contents 

Contents Page 

Introduction............................................................................1
Basic Link Applications and Components ............................ 3
Typical Linear Link Applications........................................ 3
Typical Linear Link Components....................................... 3
Optical Transmitter ............................................................4
Optical Receiver ...............................................................5
Fiber-Optic Cable .............................................................5
Link Design Calculations...................................................... 5
Gain ..................................................................................6
Doubling the Optical Loss Term.................................... 7
Resistively Matched Components................................. 7
Reactively Matched Link............................................... 8
Bandwidth ......................................................................... 8
Noise .................................................................................9
Laser Noise ...................................................................9
Shot Noise ..................................................................10
Receiver Noise............................................................ 10
Total Link Noise ..........................................................11
Cascading Noise Figures............................................ 12
Unconverted Noise and SNR...................................... 12
Noise-Equivalent Bandwidth .......................................13
Dynamic Range and Distortion .......................................13
1 dB Compression Point .................................................13
Third-Order Intercept and Spur-Free Dynamic Range 13
Large Number of Carriers ........................................... 15
Placement of Amplifiers .............................................. 15
Example.......................................................................... 15
Transmitter and Receiver Choice ...............................15
Gain ............................................................................ 16
Noise ...........................................................................16
Dynamic Range .......................................................... 17
Distortion .....................................................................17
Selection of Optical Fiber Components.............................. 19
Wavelength and Loss ..................................................... 19
Dispersion....................................................................... 20
Ruggedization................................................................. 21
Connecting Fibers........................................................... 21
Fusion Splice .............................................................. 21
Mechanical Splices ..................................................... 21
Connectors ................................................................. 22
Reflections and Laser Noise ...........................................23
Reflections and Interferometric Noise............................. 24
Polarization Mode Dispersion ......................................... 25
Optical Attenuators ......................................................... 25
Additional Transmitter Considerations................................ 25
Optical Isolators .............................................................. 25
Distributed-Feedback vs. Fabry-Perot Lasers................ 25
Predistortion.................................................................... 26
1310 nm vs. 1550 nm Wavelengths................................ 27
Summary ............................................................................27
Appendix .............................................................................28
Performance Characteristics ...........................................28
Gain.................................................................................... 28
Total Link Noise ..................................................................29
Glossary ............................................................................33
References .........................................................................43


Figure Page 

 Figure 1 . Block Diagram Depicting a Basic Fiber-Optic Link 
of Transmitter, Receiver, and Optical Fiber ...... 3 
Figure 2. Input Current vs. Output Power ........................... 4 
Figure 3. A1611A DFB Laser Module ................................. 4 
Figure 4. Transmitter Package Styles: Flange-Mount (left) 
and Plug-In (right)............................................. 4 
Figure 5. Photodiode Linear Responsivity Curve................ 5 
Figure 6. Effects of Optical Loss and Transmitter RF 
Efficiency .......................................................... 6 
Figure 7. Resistively Matched Link ..................................... 7 
Figure 8. Laser Frequency Response as a Function 
of Increasing Bias Current ................................ 8 
Figure 9. Cumulative Loss Effects of Laser Noise, Photodiode 
Shot Noise, and Receiver Thermal 
Noise on Total Link Performance .................... 11 
Figure 10. Reactive Matching at the Transmitter and 
Receiver Imparts Improved Noise 
Performance ................................................... 11 
Figure 11. Effects of Unconverted Low-Frequency 
Noise on SNR ................................................. 12 
Figure 12. Third-Order Intermodulation Distortion 
Spectrum ........................................................ 13 
Figure 13. Third-Order Intercept and Spur-Free Dynamic 
Range ............................................................. 14 
Figure 14. Precise Indices of Refractions Enables Total 
Internal Reflection in Optical Fiber ................. 19 
Figure 15. Scattering and Absorption Losses vs. 
Wavelength ..................................................... 19 
Figure 16. Wavelength Dependency of Dispersion for Standard 
and Dispersion-Shifted FIber.................. 20 
Figure 17. Cross Section of Typical Ruggedized, Simplex 
Cable ............................................................... 21 
Figure 18. Periodic Spikes Can Degrade Performance 
in Analog Fiber-Optic Links............................. 23 
Figure 19. Low-Reflection Components (Terminators 
and Optical Noise Splitters) Help To Avoid 
Interferometric Noise ...................................... 24 
Figure 20. Comparative Noise Performance for DFB 
and F-P Lasers ............................................... 26 
Figure 21. Gain Curve 1, Resistively Matched Photodiode 
for 50 Ω ........................................................... 28 
Figure 22. Gain Curve 2, Unmatched Photodiode ............ 28 
Figure 23. Gain Curve 3, Unmatched Photodiode ............ 29 
Figure 24. Noise Curve 1, Equivalent Input Noise vs. 
Optical Losses ................................................ 29 
Figure 25. Noise Curve 2, Equivalent Input Noise vs. 
Optical Losses ............................................... 30 
Figure 26. Noise Curve 3, Equivalent Input Noise vs. 
Optical Losses ................................................ 30 
Figure 27. Noise Curve 4, Equivalent Input Noise vs. 
Optical Losses ................................................ 31 
Figure 28. Noise Curve 5, Equivalent Input Noise vs. 
Optical Losses ................................................ 31 
Figure 29. Noise Curve 6, Equivalent Input Noise vs. 
Optical Losses ................................................ 32 
Figure 30. Noise Curve 7, Equivalent Input Noise vs. 
Optical Losses ................................................ 32 
Agere Systems Inc. 


Application NoteApril 2001 RF and Microwave Fiber-Optic Design Guide 

Basic Link Applications and Components 

Typical Linear Link Applications 

One of the primary uses of linear fiber-optic links is 
sending RF and microwave signals between transmit or 
receive electronics and remotely located antennas. 

Due to the flexibility of fiber-optic links, the antennas 
may be designed for analog or digital signals from any 
of a number of sources, including military and commercial 
communication satellites, global positioning satellites, 
telemetry/tracking beacons, or wireless cellular 
networks. 

Another type of link is the fiber-optic delay line, which 
combines a transmitter, a receiver, and a long length of 
fiber in a single package to provide a unique combination 
of long delay times, high bandwidths, and low 
weight. These higher-frequency RF and microwave 
products have benefitted indirectly from another application, 
the overwhelming use of linear fiber optics in 
cable television. Here, fiber extends the transmission 
distance of TV signals, improves their quality and system 
reliability, and even reduces costs when compared 
with systems employing only coax cables. 

Typical Linear Link Components 

In each of these applications, as well as many others, 
the Agere Systems’ transmitters and receivers comprising 
the links are similar and can be treated as standard 
microwave components. Focusing on these 
common elements, this design guide describes the 
general technical considerations and equations necessary 
for engineers to choose the most appropriate 
Agere Systems’ components for their systems. These 
equations also have been incorporated into various 
programs, which an Agere Systems’ applications engineer 
can use to provide an analysis for a specific link 
application. 

Figure 1 shows the three primary components in a 
fiber-optic link: an optical transmitter, a fiber-optic 
cable, and an optical receiver. In the transmitter, the 
input signal modulates the light output from a semiconductor 
laser diode, which is then focussed into a fiber-
optic cable. This fiber carries the modulated optical signal 
to the receiver, which then reconverts the optical 
signal back to the original electrical RF signal. 

OPTICAL 
RF 
IN 
Z 
MATCH 
TRANSMITTER 
AMP 
ISOLATOR 
LASER 
DIODE 
dc BIAS 
POWER 
MONITOR 
TEMP. 
CONTROL 
MODULE 
OPTICAL 
RF 
OUT 
Z 
MATCH 
RECEIVER 
AMP 
PHOTODIODE 
dc BIAS 
MONITORS 
AND ALARMS 
MODULE 
LIGHT 
OPTICAL 
FIBER 
1-1215F 
Figure 1. Block Diagram Depicting a Basic Fiber-Optic Link of Transmitter, Receiver, and Optical Fiber 

Agere Systems Inc. 


Application Note 
RF and Microwave Fiber-Optic Design Guide April 2001 

Basic Link Applications and Components (continued) 

Optical Transmitter 

For RF systems, distributed feedback (DFB) lasers are 
used for low-noise, high-dynamic range applications, 
and Fabry-Perot lasers for less demanding applications. 
The wavelength of these lasers is either 1310 nm 
or 1550 nm. 

The intensity of the laser light is described by the simplified 
light-current (L-I) curve in Figure 2. When the 
laser diode is biased with a current larger than the 
threshold current, ITH, the optical output power 
increases linearly with increasing input current. Analog 
links take advantage of this behavior by setting the dc 
operating point of the laser in the middle of this linear 
region. Typically, this bias current for Agere Systems’ 
transmitters is set somewhere between 40 mA to 
90 mA. The threshold current ranges from 10 mA to 30 
mA. 

INPUT CURRENT ITh IO 
OUTPUT LIGHT POWERP1 
SLOPE = MODULATION 
MODULATION 
GAIN (W/A) 
LIGHT OUTPUT 
1-1216F 
Figure 2. Input Current vs. Output Power 

The efficiency with which the laser converts current to 
usable light is given by the slope of the L-I curve and is 
called the modulation gain. For typical Agere Systems’ 
lasers, this dc modulation gain ranges from 0.02 W/A 
to 

0.3 W/A, depending on the model chosen. The wide 
variation is largely due to differing methods of coupling 
the light into the optical fiber. The modulation gain also 
varies somewhat with frequency, so it must be specified 
whether a particular value is a dc or higher-frequency 
gain. 
In addition to the laser diode, transmitters also contain 
a variety of other components, depending on the specific 
application or level of integration desired. The 
most basic laser module package contains the laser 
chip, optical fiber, and impedance-matched electrical 

connections in a hermetically sealed container such as 
the one shown in Figure 3. Modules also may contain a 
photodiode for monitoring the laser power, a thermistor 
and a thermoelectric (TE) cooler for monitoring and 
controlling the laser temperature, and an optical isolator 
for reducing the amount of light reflected back to the 
laser from the fiber. 


Figure 3. A1611A DFB Laser Module 

The basic laser module, although available as a subcomponent, 
is usually integrated into a complete transmitter 
housing such as the flange-mount and plug-in 
packages shown in Figure 4. These transmitters also 
may include dc electronics to control the laser temperature 
and bias current, amplifiers and other circuitry to 
precondition the RF signal, and various indicators for 
monitoring the overall transmitter performance. Because 
analog fiber-optic transmitters are used in a variety 
of applications, the exact implementation of these 
product features varies as well. 

Figure 4. Transmitter Package Styles: Flange-
Mount (left) and Plug-In (right) 
Agere Systems Inc. 


Application NoteApril 2001 RF and Microwave Fiber-Optic Design Guide 

Basic Link Applications and Components (continued) 

Optical Receiver 

At the other end of the fiber-optic link, the light is 
detected by the receiver PIN photodiode, which converts 
the light back into an electrical current. The 
behavior of the photodiode is given by the responsivity 
curve shown in Figure 5. Once again, note that the 
response is very linear. The slope of this curve is the 
responsivity, which typically is greater than 

0.75 mA/mW for a photodiode chip without any impedance 
matching. 
Similar to Lucent’s laser diodes, photodiodes are packaged 
in a hermetic module containing an impedance 
matching network and electrical lines to provide dc bias 
and RF output. However, unlike the laser, the photodiode 
is relatively insensitive to temperature so a thermoelectric 
cooler (TEC) is not required. Special 
precautions also are made to minimize optical reflections 
from returning back through the fiber, which otherwise 
could degrade a link’s performance. 

OUTPUT CURRENT

SLOPE = RESPONSIVITY 
(A/W) 


INPUT LIGHT POWER 


1-1218F 

Figure 5. Photodiode Linear Responsivity Curve