Plastic Fiber Components (PFC)
Application Note 5342
Introduction
Optical-Mechanical Design
Optical communications offer important advantages
over electrical transmission links. The following characteristics make the technology particularly attractive for a
wide range of applications:·
The product range builds upon proven fabrication technology 5 mm LEDs. The task of coupling the device to the
fiber is given over to the housing, the design is shown in
Figure 8a.
• Insensitivity to electromagnetic interference·
A particular advantage of the Avago PFCs is the housing
aperture into which a standard plastic fiber (external
diameter of 2.2 mm) may be introduced without having
to remove the cladding. This has the additional benefit of
automatically aligning the fiber on the chip.
• Voltage decoupling between emitter and detector·
• Security against tapping·
• No sparking at fiber ends or breaks·
• No ground loops
Yet despite the many potential application areas arising
from these advantages, the use of optical glass fiber is
restricted due to its relatively high cost. Where demands
are for medium bit rates and distances, by far the more
cost effective solution today is offered by Plastic Optical
Fiber (POF) in combination with Avago Plastic Fiber Components (PFC) emitters and detectors. These low cost
components permit the use of plastic fibers even in the
most cost-sensitive applications, such as:
• Industrial and medical networks
• Motor controls, links between power and control units
• Replacement of connections with copper wire and
optocoupler (within cabinets)
• High voltage optocouplers
• Automotive bus applications
• Building information and control systems
The Avago PFC product range consists of three different
emitter diodes (SFH450, SFH750, and SFH756) and three
optodetectors (SFH250, SFH350 and SFH551/1). Whatever
the system requirements, a combination of Avago
PFCs may be selected to provide the optimal solution.
detectors are shown in Figure 1.
Figure 1. Bit Rate versus Transmission Span
Sticking the component and the fiber together results
in a permanent connection, which not only saves on
space but also is very cost effective. Should a temporary
connection be required, the PFCs are also available in
a housing with a mounting screw (see Figure 8b). This
option has numerous advantages:
• The cladding need not be removed
• A connector on the fiber is not necessary
• The plastic fiber is connected by a simple turn of the
screw cap
• The screw cap cannot be removed (loss-proof )
• The component is suitable for automatic board
assembly
• The fiber itself does not turn when the cap is screwed
tight
• Every plastic fiber with an external diameter of 2.2 mm
and an internal diameter of 1 mm can be used
• Small housing dimensions
• The housing protects photodetectors from external
light sources
The emitters are mounted in grey housing and the
detectors in a black one. Two mounting pins are provided
on the housing for firm attachment to boards.
Electrical and Optical Characteristics of PFCs
Table 2. Parameters of PFC Photodetectors
Both the emitters and detectors display the same electrical characteristics of standard optoelectronic devices.
They may be operated in a temperature range of –40° C
to +85° C.
Characteristics of the Emitter Diodes
Different technologies employed in chip fabrication
lead to significant variation in parameters for the various
emitter diodes. All the emitters distinguish themselves in
offering high output power coupled into the plastic fiber,
low fiber attenuation and long lifetimes. Table 1 gives
a summary of the most important device parameters,
where Pin is the output power coupled into the fiber,
and tr and tf are the rise and fall switching times for the
optical signal.
Table 1. Parameters of the PFC Emitters
Sensitivity 660 nm
(950 nm)
SFH350
Phototrasistor
0.25 (0.3)
80 (120)
Switching Level
SFH551/1
Integrated
Photodetector
Unit
A/W
6
µW
Phototransistor SFH350
The phototransistor SFH350 is a very cost effective photodetector. In operation it yields a high output current
even at low optical input power. Its performance is
limited by low switching speeds. The external base
connector on the SFH350 may be used to divert current
using a base-emitter resistor, with the following advantages:
Part
SFH450
SFH750
SFH756
Unit
• Reduction of the collector-emitter cut-off current
Wavelength
950
660
660
nm
• Reduction of the switch off time
Typ. Pin(IF = 10 mA)
90
9
200
µW
tr 10%/90%
1000
120
80
• Suppression of noise signals and signals with low
power
tf 90%/10%
1000
50
80
ns
As the temperature coefficient is positive for phototransistors, this in some measure compensates for the
negative coefficient of the emitter diodes, assuming that
the ambient temperature of the emitter and receiver are
the same.
The SFH450 emits in the infrared range, whereas the
SFH750 and SFH756 emit visible light in the red range,
namely at 660 nm, which is optimal for plastic fiber. The
choice of emitter for a particular application is dealt with
in Systems with PFC Diodes (page 5), where system issues are
discussed.
The output power coupled into the fiber is measured
using a standard plastic fiber of numerical aperture 0.47
and length approximately 30 cm, for a forward current of
10 mA. It should be noted that this power is not transmitted over long distances. As a consequence the first few
meters of a long length of fiber appear to have higher attenuation (see Connecting the PFCs to the Fiber Emitter, page 6).
The optical power does not rise linearly over the entire
operating range. At low currents the optical power rises
more than proportionately with the current; at higher
currents, saturation sets in.
With respect to the effect of device temperature, all
emitter diodes have negative temperature coefficients.
As a result the output power coupled into the fiber
decreases at higher temperatures.
Characteristics of the Photodetectors
Within the PFC range, three photodetectors are available:
a fast PIN-Photodiode (SFH250), a phototransistor
(SFH350) and an integrated photodetector with TTL-output (SFH551/1).
The main parameters of the photodetectors are summarized in Table 2.
2
Part
SFH250
PINPhotodiode
Photodiode SFH250
The photodiode SFH250 has a switching time of 10 ns
(with 50 Ω for Popt = 50 µW), which makes it the fastest
available detector. When driving a load of greater than
200 Ω, the capacitance of the diode also determines the
switching time.
Temperature dependence of IP is less than that for a phototransistor. The photodiode SFH250 has lower, negative
temperature coefficients for wavelengths of 565 or 500
nm. At 950 nm wavelength, the coefficient becomes
positive.
Digital Receiver SFH551/1
A simple design of the electronic circuitry is an essential
step for the implementation of low cost optical links. In
many cases optical emitters can be directly driven by
logic gates through a resistor for adjusting the current.
On the receiver side, however, a special low-noise
amplifier is required due to the small photocurrent. Thus,
receivers with integrated preamplifiers have succeeded
to realize simple and low cost transmission links.
For this purpose Avago offered the SFH551 (not for
new design), which stood the test in many applications.
Now this receiver is replaced by the upgrade version
(SFH551/1) which is compatible in pinning and identical
in most of its properties. It was the goal of the SFH551/1
development to improve its performance with respect
to noise and dynamics. The features of the SFH551/1 are
now discussed in detail.
Principle of the Digital Receiver
An integrated optical receiver in an optical transmission
system (Figure 2) comprises the following units:
Using a built-in lens, the light leaving the plastic fiber is
focused onto the photodiode of the IC. The photodiode
integrated in the device converts the received light into
photocurrent.
The preamplifier converts the photocurrent into voltage.
Usually, a transimpedance amplifier is applied. A resistor
in the amplifiers feed-back loop determines the currentvoltage conversion. To avoid pulse disturbance, the
amplifier has to offer a linear performance over the entire
power -range of the received light.
The comparator following the signal path converts the
signal to a logic signal. Here, the preamplifiers output
voltage is compared to a reference voltage and the
decision to set the output high or low is made. As the
signal may be noisy, a (small) undefined voltage range
occurs around the reference level. Typical input signals
go through this range very fast and no degradation
of the output signal can be observed. However, if the
signal slowly increases towards the decision threshold,
the output can run into undefined states. Thus, noise
peaks may occur with amplitudes reaching the switching
threshold of the following stages.
To remove this effect, the SFH551/1 is equipped with
a Schmitt trigger which works with different thresholds. If the output is ’off’ a high signal level is required
for switching to the on-state, whereas a lower level has
to be reached to switch back to the off-state. Therefore,
the probability is greatly reduced that small noise signals
cause unwanted switching errors.
Vref
Vcc
Data
Vcc
GND
Figure 2. Optical Transmission Line with Integrated Optical Receiver
Vref
Vcc
Schmitt-Trigger
Data
Vref
Vcc
Differential
amplifier
Cref
Figure 3. Circuitry of the SFH551/1 with Built in Schmitt Trigger
3
TTL-Output
Inverting ooopopr
GND
How the DC-Coupled Receiver with a Fixed Threshold
Works
The form of the signal that is internally delivered from
the transimpedance amplifier to the decision circuit considerably determines the performance of the receiver.
This form itself is determined by the optical receiver
signal and its processing in the preamplifier.
The received optical signal is determined by the following
items:
• Bias light caused by scattering or residual emission of
the transmitter in the switched off state
• Rise and fall times depending on the transmitters
control signal, the transmitter’s speed and possibly
effects from the transmission link
• Pulse amplitude depending on emission power and
the loss of the optical transmission link.
The preamplifier integrated in the SFH551/1 is designed
for wide bandwidth. Consequently no considerable disturbance occurs within its linear working range.
Depending Where the Fixed Decision Threshold Crosses...
If the input signal is too small and does not reach the
decision threshold, the decision circuit does not change
its output state. A short peak exceeding the threshold
results in a correspondingly short output pulse.
If the level of the input signal is very high this results in
early transition to the on-state and a delayed transition
to the off-state. Consequently, the delay of the electrical
signal caused by the falling edge of the optical signal is
longer than the delay of the rising edge (optical signal).
If the transmitter signal is DC-biased (at logical zero) or
additional light hits the receiver, the noise signal is shifted
to the decision threshold. In the case of the SFH551
(without Schmitt trigger) unwanted pulses may occur
at the output. Using a SFH551/1 with its built-in Schmitt
trigger eliminates this problem.
Highly biased signals (at logical zero) cause permanent
switching to the on-state and therefore prohibit data
transmission for both the SFH551 (not for new designs)
and the SFH551/1.
4
The effects described above demonstrate, that the performance of the optical receiver can only fully be described
if the level and form of the optical input signal are taken
into account.
Limits of the Receiver Operation
Dynamic range:
An important figure for the application of an optical
receiver is the dynamic range, defining the range
between smallest and highest signal level in which
perfect operation is assured.
As described above noise pulses can occur at the receiver
output:
• If no light is received (negative peaks on high level)
• If the input power is close to or below the specified
limit of 4 µW (positive peaks on low level)
As these noise pulses occur randomly, they are hard to
detect, requiring a modern storage oscilloscope.
The measurement has shown, that the receiver switches
at an input level as low as 2 µW. In order to avoid noise
as described above, a switching level of 4 µW for nearly
error free operation has been specified in the data sheet.
SFH551/1: minimal light power
As described above, the SFH551/1 contains a Schmitt
trigger. To ensure safe operation, the limit of the switching-threshold was shifted to 5 μW. Compared to the
SFH551 this appears as a lower sensitivity, but taking
the very low bit error rate into account the transmission system can be reliably operated with less system
margin. Due to possible coupling losses, a value of 6 μW
is specified in the datasheet.
On request a low cost option (SFH551/1 -2) with
switching level < 10 μW is available.
SFH551/1: maximal light power
The SFH551/1 offers a considerably increased dynamic
range. Using conventional plastic fiber transmitter diodes
overdriving is practically impossible. Depending on the
system application a limit is given by pulse disturbances
in time depending on the pulse form and increasing with
the input level at high input power. It is recommended
not to exceed a signal level of 400 μW. For higher levels
the pulse form has to be checked in the circuit individually.
Hints for the Application in an Electronic Circuit
Systems with PFC Diodes
Power supply
Plastic Optical Fiber (POF)
For printed circuit board mounting it has to be taken
into account that the SFH551 and SFH551/1 are fast electronic circuits. A blocking capacitor (100 nF) is obligatory
for the SFH551 (not for new designs) and recommended
for the SFH551/1. As both circuits can be operated at 5
V voltage, a noisy power supply can be connected via
a resistor building a low pass filter together with the
capacitor. Despite this means, the full output level can be
reached, if the pull up resistor is connected to Vcc.
The most common type of fiber consists of a polymethylmethacrylate (PMMA) core approximately 970 µm thick
with 30 µm thick cladding made of fluoride-containing carbon polymer. Given the refractive indices of the
core and cladding are 1.492 and 1.417 respectively, the
numerical aperture is 0.47 and the fiber acceptance angle
56°. With a PVC or PE protecting sheath the POF has a
total diameter of 2.2 mm.
How to select the pull-up resistor
To keep the current consumption low, a high value
within the possible range is usually chosen. For very
fast systems it might be important to work with the
minimal value. The calculation of the minimal value,
however, should not be performed with the maximal
output current. For the SFH551 (not for new designs) the
minimal value is 330 Ω (VCC = 5 V). The SFH551/1 can also
be operated with this value, although 390 Ω is recommended here.
Signal Delay
The redesign of the receiver has resulted in a small
increase of the delay times. This becomes clear if the
pulse form of the SFH551/1 is taken into account. The
rising edge is fast and; the delay time increases only for a
very small light power.
Figure 5 shows that the delay of the electrical signal
caused by the falling edge of the optical signal is longer
than the delay of the rising edge (optical signal).
These fibers are obtainable from many manufacturers. The CUPOFLEX fibers, data for which is given in the
appendix, are typical of the POFs available.
The appendix also gives the typical attenuation as a
function of wavelength. Of the two attenuation minima
in the visible spectrum, the one in the red region at = 650
nm is suitable for distances up to 100 m. Due to the low
quantum efficiency of the green emitter, the attenuation
minimum at 570 nm is unsuitable for communications
applications. Moreover, the switching times of the red
emitters are significantly lower, and the degradation is
less than for green emitters. In spite of the high attenuation of POF in the IR-spectrum (4 dB/m), infrared emitters
(SFH450) can still produce sufficient power levels at the
fiber end over a few meters to be of practical use.
Plastic optical fiber made of PMMA can be used in
ambient temperatures of –20° up to +85° C. For lower
temperatures (down to –50° C) the constraint of mechanical flexibility has to be taken into account. Fibers
for higher temperatures, up to 100° C, such as those
necessary for automotive applications, are in development.
When using POF, the bend radius should not be less than
20 mm, as otherwise the fiber attenuation increases.
Smaller radii are possible using fibers with higher
numerical aperture.
250
Light off [ns]
Light on [ns]
tphl, tplh [ns]
200
LED Driving Current
tpLH
Figure 4. Definition of the Delay Times
5
100
50
Output
tpLH
150
0
6 45 84 123
161
200
239 277 316 355
Popt (µW)
394
432
471
Figure 5. Measured Signal Delay vs the Level of Received Light. SFH756 on
the Transmitter Side
Treatment of Fiber Ends
Detector
Due to the thick diameter of POF it is easier to handle
than glass fiber. Thus for very short distances where
plenty of allowance has been made for attenuation, it is
possible to cleave the fiber using a sharp edged blade.
For longer stretches wet polishing the fiber end with
600 grain sandpaper yields greatly improved results.
To achieve very flat surfaces at the fiber ends, the fiber
should be cut with a blade heated to 160 - 180° C or the
clean cut fiber end may be pressed for 2 - 4 seconds on to
a plate heated to 100 - 140°C. Avago recommends that
the manufacturer recommendations be followed.
Due to the lens of the detector’s housing, virtually all the
input radiation is focused on to the chip.
Connecting the PFCs to the Fiber Emitter
When current flows in the forward direction the emitter
diode emits optical radiation. The task of the housing
is to bundle the output radiation so that the greater
portion of it is coupled into the fiber. The components
are intended for use with plastic fibers and thus the data
regarding optical coupling into and out of the fiber refers
to standard plastic fiber with a core diameter of 1 mm
and a numerical aperture (NA) of 0.47. So that as much
as possible of the radiation emitted within this angle falls
upon the fiber, the radiation is concentrated on the fiber
using a built-in reflector and a lens.
The chip in the SFH756 is a large area radiating chip with the
bonding ball in the centre. Therefore not just one position
between chip and fiber core (indicated by the lens) is
possible for optimum coupling, as shown in Figure 6.
Dimensioning the Emitter Driver and the Detector
The Emitter Driver
The emitter is stimulated into emission by current
flowing in the forward direction. There are several possibilities for the design of the driver circuit, which has
the task of adjusting and stabilizing the current flow. In
Figure 7 the basic types of driver circuits are given.
In the simplest example (Figure 7a) the emitter diode
is connected in series with a resistance Rs to the supply
voltage Vs. The current If is dependent of the forward
voltage Vf of the diode:
If = (Vs – Vf )/Rs
The diode may also be driven using the output transistor
of a TTL gate or a separate driver transistor (Figure 7b), in
which case the collector-emitter voltage should be taken
into account. In this configuration the current passed
through by the transistor is given as:
If = (Vs – Vf – Vce)/Rs
In order to keep the current and thus the optical power
constant, it is preferable to control the current flow
(Figure 7c). For this example, as with the others, it is
necessary to ensure than sufficient voltage is provided.
One of the diagrams in the emitter data sheet shows
the dependency of the forward voltage Vf on the
current. Since Vf may reach 3 V for currents of 300 mA,
the maximum duty cycle may be limited by the supply
voltage.
Lens
Light Emitting Area
Figure 6. Lens Position in the Transmitter
In Characteristics of the Emitter Diodes (page 1) it was already
pointed out that the optical power which can be coupled
into the fiber is measured using a standard plastic
fiber with a numerical aperture of 0.47 and of length
approximately 30 cm. The power defined in this way
is not transmitted along large lengths (> 3 m) of fiber.
For long stretches of fiber the first meters therefore
apparently exhibit a higher level of attenuation. This can
be taken into account by adding 2 dB to the attenuation
incurred due to the length of the fiber itself.
Another important parameter when dimensioning the
driver is the permissible pulse load. It is clear from the
relevant diagram in the data sheet, that in regard to
power loss, peak currents of up to 1 A are possible for
short duty cycles. Whether this peak current can be used,
depends on the available supply voltage.
Figure 7. Types of Emitter Diode Driver Circuits
6
Detector
Recommendations for Mounting
When dimensioning the detector it is necessary to
establish the dynamic range, ie. the relation between the
minimum current required and the maximum permissible current.
When soldering it should be ensured that the components is not overheated. Given a distance of 2 mm
between the component and the soldering point, when
using a soldering iron the maximum permissible temperature is 300° C for at most 3 s. In the case of flow and dip
soldering the maximum temperature is 260° C for up to a
5 s period.
System Planning
In order to demonstrate the process of system planning,
an example of the design of an optical transmission
system over a long distance using the SFH551/1 detector
will be discussed.
According to the data sheet, the SFH551/1 detector has
a dynamic range of around 18 dB. This range must be attributed to the various levels of attenuation along the
fiber and the tolerance of the emitter power.
Attenuation in plastic fiber over long distances for light of
wavelength 660 nm is of the order 0.3 dB/m. However the
first section of fiber exhibits higher attenuation (around
2 dB additional to the regular losses in the first meters).
Since the emitter power is measured and specified using
a short length of fiber, the effect needs to be taken into
account (see Connecting the PFCs to the Fiber Emitter, page 6 ).
The following example demonstrates how a PFC system
with a length of 50 m should be driven:
Table 3 Planning of a Transmission Line
Detector power
22.0
dBm
50 m distance at 0.3 dB/m
15.0
dB
Additional attenuation of first few meters
2.0
Needed emitter power without tolerances
5.0
dBm
Further effects which contribute to the reduction of the
operating range:
Temperature (–20 to 45°C)
1.0
Aging
2.0
Range of emitter power distribution
1.5
Emitter power
0.5
dB
dBm
Thus the configuration in this example requires at
minimum an emitter 890 µW output power achievable
with SFH756 driven by 50 mA peak current.
For easy planning SFH551/1 with SFH756 the following
table for recommended drive current in SFH756 may be
used:
Table 4 Easy Planning of a Transmission Line SFH551/1 with
SFH756
7
Maximum Length
10m
20m
30m
40m
50m
Needed Peak Current
5 mA
10 mA
20 mA
30 mA
40 mA
When soldering particular care should be taken so that
the component is not subject to any mechanical stress.
When bending the connection pins, the packaging
should not be loaded in any way.
The soldered components may be cleaned using organic
solvents, with an alcohol base, a base of certain fluorohydrocarbons or a mix of the two. Under no circumstance
should solvents or solvent mixtures be used which
contain chorohydrocarbons or ketones, since these can
attack or dissolve the housing.
Reliability
Degradation
For optoelectronic emitters, the emitted radiation power
reduces over the component’s lifetime. This effect is
known as ageing. It is dependent on the technology of
the diode system, the load current If and the ambient
temperature TA.
If one defines the failure criteria as a 50% (–3 dB) fall in
output power relative to the start value, the lifetime of
the PFC emitter is more than 105 khours.
Emitter Diode Permissible Current Load
When the emitter diode is in operation most of the
electrical input power (Uf x If ) is converted into heat.
The temperature of the semiconductor chip and the
packaging rises as a consequence. On grounds of reliability, the chip should not be heated above the
maximum permitted for the depletion layer. This in
turn yields a maximum power loss, dependent on the
ambient temperature, which cannot be exceeded (refer
to the relevant data sheets). These values are valid for DC
operation.
Significantly better operating conditions are possible
by using the emitter diodes in pulse operation, as the
average power decreases inversely proportional to the
duty cycle. However this means that the maximum
forward current If can be increased for shorter pulse
widths and/or longer duty cycles, as shown in the Permissible Pulse Load plot in the data sheets. These plots give
the maximum permissible forward current dependent on
the pulse width for an ambient temperature TA of 25° C.
The duty cycle parameter D (ratio of pulse width to cycle
period) given as D = ‘n/T. For very short pulse widths (10
s) and small duty factor (1:200) a current up to 20 times
greater than the maximum direct current is permissible
for PFC emitter diodes. Even in the range of 100 ms and
a duty cycle of 1:5 a doubling of the forward current may
be reached, thereby raising the power coupled into the
fiber by a factor two with the SFH450 and a factor 2.5
with the SFH750 in comparison with DC operation.
Permissible Mechanical Load
The fiber optic components SFH250, SFH350, SFH450,
SFH750, SFH756 and SFH551/1 are designed in such a
way, that they can cope with a heavy mechanical load, as
required in automotive applications. The following tests
have been successfully conducted on all the above components:
• Oscillation test (according to DIN IEC Teil 2 - 6, Test Fc)
• Schock test (according to IEC 68-2-27, Test Ea)
In these tests diodes were employed with a fiber of approximately 5 cm in length. They were soldered to a
board and subjected to oscillations and schocks (single
impulses) in the three orthogonal axes. The oscillation
frequency was varied between 10 and 500 Hz with an acceleration of 5 g’s.
Package Outlines
.093
(2.35) .087
Surface not flat
Anode
.100 (2.54)
Cathode
.181
(4.6 )
.071
(1.8) .047
.187
(4.75) .177
.307 (7.8)
.201 (5.1)
.295 (7.5)
1.142 (29)
.354 (9.0)
1.063 (27)
.323 (8.2)
.217 (5.5)
a.SFH250, SFH750, SFH450
.362(9.20)
346 (8.80)
.953 (24.20)
.937 (23.80)
(9.00)
(8.60)
.138 ±.020
(3.5 ±.5)
.024 (0.60)
.024
(0.60) .016
.300
(7.62)
Screw-on
connector
b. SFH350V
Figure 8. Design of the PFC Housing
8
.200
(5.08)
.100
(2.54)
Dimensions in inches (mm)
Disclaimer
The information herein is given to describe certain components and shall not be considered as a guarantee of
characteristics.
Terms of delivery and rights to technical change reserved.We hereby disclaim any and all warranties, including but
not limited to warranties of non-infringement, regarding circuits, descriptions and charts stated herein.
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Due to technical requirements components may contain dangerous substances. For information on the types inquestion please contact your nearest Avago Technologies Office.
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of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support
devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustainand/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons
maybe endangered.
Information
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Data subject to change. Copyright © 2005-2010 Avago Technologies. All rights reserved. Obsoletes AV01-0746EN
AV02-2647EN - September 16, 2010