04-04-2012, 12:58 PM
Fiber Optic Transmitter Design using Intersil Digitally Controlled Potentiometer
[attachment=19440]
Abstract
Laser Diode (LD) controller/driver IC's at gigabit data-rates
typically use specially designed chipsets. In many cases
numerous control parameters are required to be set, using
resistors which define reference or control currents used by
the LD driver circuit. Precise and dependable settings are
essential in order to achieve a maximized extinction ratio
and minimize jitter.
Intersil Digitally Controlled Potentiometers (XDCPs) allow
automated setting of these resistor values in a manner which
is rapid, stable, repeatable and precise.
In addition, Intersil XDCPs offer many other integrated
features such as EEPROM and trip alarms.
Introduction
Optical fiber communication systems often use
semiconductor light sources such as a laser diode (LD) or
light emitting diode (LED). Both have the advantage that the
digital modulation at logic circuit voltages can be directly
applied to the operating bias current of the diode, which in
turn varies the output power. Laser diodes have advantages
for communication systems, since they provide high coupled
output power and routinely have high cutoff frequencies
which allow for data rates in excess of 10Gb/s. Such high
rates, call for purpose-designed circuits to control laser
diode currents. A challenge for control circuits is the knee or
threshold effect, below which optical output is minimal as
shown in Figure 1.
Many such chipsets for laser diode control exist, most
sharing a similar architecture. This comprises a transistor
which sets a bias current, another transistor providing extra
current to modulate the laser diode into the ‘1’ (maximum
power) condition, and a common-emitter differential
transistor pair switch. The function of this switch is to divert
the modulation current away from the laser diode when a ‘0’
is required. The differential pair can be directly controlled
from an ECL or PECL input (Figure 2).
The ‘1’ level is often set near the maximum safe level of
laser diode power output, while the ‘0’ is set close to the
threshold current (ITH), since this allows higher data rates.
In practice a small amount light is expected at the ‘0’ level
since the “knee” at the threshold of the Light versus Current
(L vs. I) characteristic curve is somewhat rounded.
Laser diodes exhibit a very wide range of tolerances of their
various parameters, and this requires a number of reference
values to be adapted to suit the individual component. In
addition these values vary significantly with temperature.
The bias current (IB) is controlled by a feedback loop from
the laser diode’s monitor diode (Refer to Figure 3).
FIGURE 1. IDEALIZED LIGHT OUTPUT vs. CONTROL
CURRENT
CURRENT
LIGHT
OUTPUT
MODULATION
CURRENT
DESIGN VALUE OF
LIGHT OUTPUT
FOR A “1”
DESIGN VALUE OF
LIGHT OUTPUT
THRESHOLD (ITH) FOR A “0”
I (mA)
L (dBm)
FIGURE 2. LASER DIODE DRIVER BLOCK DIAGRAM
IMOD IBIAS
LASER
DIODE
COMPLEMENTARY
ECL COMPATIBLE
LOGIC LINES
DATA
SOURCE
OPTICAL FIBER
Application Note
Authors: Joe Ciancio, Product Development Engineer,
Rex Niven, Design Engineer, Forty Trout Electronics Pty. Ltd.
June 28, 2005
2 AN140.0
June 28, 2005
.
The monitor photo-diode and feedback loop cannot “track”
the optical signal at full data rate. Therefore, the bias
feedback loop is designed such that it maintains an average
optical power, which is influenced by the relative number of
data ‘1’s and ‘0’s. The number of data ‘1’s and ‘0’s may be
made consistent by employing a line code such as simple
Manchester Coding, or more efficient, OMIT “8B/10B”
Coding - as used in GBIC (fiber channel) transceiver
modules.
As previously mentioned, at high data rates the modulation
current (IMOD) cannot have feedback control, and must be
set at a constant level. This therefore imposes the
requirement that the modulation current must be
desensitized OMIT, to the effects of temperature fluctuations
and device ageing. In the schematic shown in Figure 3, the
modulation current is set by choosing the appropriate value
of RMODSET.
An example of the effect of temperature variation, is to
reduce the gradient of the Light versus Current (L vs. I)
characteristic curve of the laser diode. This is referred to as
reduced slope efficiency. The lower slope efficiency has the
potential effect of making the laser diode in ‘0’ condition
transmit significant light power, while also reducing the
power of the optical ‘1’ level (Figure 4).
This approaching of the two levels reduces the extinction
ratio (the ‘0’ optical power level as a fraction of the ‘1’ optical
power level), which has a detrimental effect on the
theoretical bit error rate (BER) by reducing the size of the
“eye diagram” opening (Figure 5) [1].
To achieve a BER smaller than 1 x 10-10, requires the ‘1’
signal level to be at least 12.5σ (standard deviations) greater
than the received noise. Should the ‘0’ level rise to approach
one standard noise deviation, the BER will deteriorate
markedly. Therefore, the extinction ratio must be maximized
for optimum results, although this must be balanced against
the need for the zero level to be as far above threshold as
possible to avoid bit-pattern dependent jitter [2]. This is
achieved by reliable, and precise setting of the various
control circuit parameters. These control parameters are
usually currents, which are set/programmed by adjustable
resistors.
FIGURE 3. SY88922 LASER DIODE DRIVER AND SY88905 LASER DIODE CONTROLLER BLOCK DIAGRAM
IMOD IBIAS
LASER
DIODE
COMPLEMENTARY
LOGIC LINES
DATA
SOURCE
OPTICAL FIBER
RMODSET IBIASSET
RBIASSET
MONITOR
PHOTO-DIODE
RPINSET EXTERNAL
VOLTAGE
REFERENCE
IMD
IPINSET
Current
Comparator
ERROR AMPLIFIER
(CURRENT ERROR X10)
IBIASFB
Vcc
GND
MODULATION
CURRENT MIRROR
BIAS
CURRENT MIRROR
MODULATION
CURRENT SWITCH
AMPLIFICATION FACTOR
ß = x40
AMPLIFICATION FACTOR
ß = x40
EXTERNAL
VOLTAGE
REFERENCE
EXTERNAL
VOLTAGE
REFERENCE
FIGURE 4. EFFECT OF REDUCED SLOPE EFFICIENCY DUE TO LASER DIODE TEMPERATURE VARIATION
CURRENT
MODULATION
CURRENT
MEAN VALUE
OF MONITOR
PHOTO-DIODE
OUTPUT.
AT 0 °C
CURRENT
INCREASED
MODULATION
CURRENT
AT 65 °C
LOWER SLOPE EFFICIENCY
RESULTS IN SMALLER EXTINCTION
RATION (“0” AND “1”
LEVELS TO APPROACH EACH
OTHER - EXAGGERATED).
LIGHT
OUTPUT
I (mA)
L (dBm) L (dBm)
I (mA)
LIGHT
OUTPUT
Application Note 140
3 AN140.0
June 28, 2005
Intersil Digitally Controlled
Potentiometers in Laser Diode Driver
Control
Ideally the resistors used to program/set the laser diode
driver circuit parameters, should have the properties of:
• Have high resolution
• Allow adjustment over a wide range of resistance
• Be ready immediately after power-up
• Have low temperature coefficient
• Be immune to vibration and shocks
• Introduce minimal noise, both internally generated and
coupled from the exterior
• Have low “wiper” resistance
• Be very stable with age and other environmental
conditions
• Be tamper proof
• Allow readjustment many times (at circuit set up time, for
maintenance, or regular checkups)
• Permit a given setting to be reproduced with high
accuracy
• Have a small footprint for use in small form-factor
transceiver modules
• Avoid out-of-range values which may overdrive (and
possibly damage) critical components such as the laser
diode
• Readily allow automatic adjustment under digital/computer
control
• Allow multiple units in one package
• Be integrated with other ancillary functions
• Require no additional power supplies other than those
available for other system functions
• Be non-volatile (does not require power to retain resistor
settings)
• Allow a link to documentation of settings, with time/date
stamp, serial number and operator data
The Intersil Digitally Controlled Potentiometer (XDCP)
products offer all these features, and most will be shown in
the example design below.
Laser Diode Driver Circuit Design
In this circuit design example, the laser diode is controlled by
the Micrel-Synergy chipset SY88922 and SY88905. This
chipset uses PECL compatible data signals, and reference
voltages (less control circuit input bias) of 1.0V. The laser
diode used is a Vertical Cavity Surface Emitting Laser
(VCSEL) with monitor diode, type Honeywell HFE4380-321.
The complete schematic for this laser diode driver circuit is
shown in the Appendix.
The VCSEL has some favorable characteristics compared to
the more traditional plane cavity (edge-emitter) type laser
diodes. For example, a VCSEL has the properties of
• Lower threshold current
• Less influence of temperature on threshold current, with a
minimum near room temperature [3]
• Critically-damped relaxation oscillation
• Only one longitudinal mode, giving narrow spectral width
[4]
Some important characteristics of the HFE4380-321 are
summarized in Table 1 and Table 2.
FIGURE 5. BIT ERROR RATE AS A FUNCTION OF SIGNAL
TO NOISE RATIO
10-5
10-6
10-7
10-8
10-9
10-10
2 3 4 5 6 7 8
n
N = RATIO OF (MEAN ‘1’ – MEAN
‘0’) SIGNAL TO (ONE STANDARD
DEVIATION OF NOISE ‘0’ + ONE
STANDARD OF DEVIATION OF
NOISE ‘1’).
BER 0.5 1 – erf(n)
1.414
= -------------------------
BER
TABLE 1. KEY PARAMETERS OF VCSEL HFE4380-321
MIN TYP MAX UNITS
Optical Power
Output
0.2 0.35 0.8 mW
Threshold - 3.5 6 mA
Slope efficiency 0.02 0.04 0.1 mW/mA
Monitor current
(at 0.35mW optical)
0.07 0.275 mA
Series resistance 15 25 50 Ω
Forward voltage 1.6 1.8 2.2 V
TABLE 2. TEMP. DEPENDENCY OF VCSEL HFE4380-321
MIN TYP MAX UNITS
Threshold (1)
-1 +1
mA
(0 to 70°C)
Slope efficiency -0.4 %/°C
Monitor current +0.2 %/°C
1. The threshold varies parabolically with temperature having a
minimum at mid range of temperature (around +30°C).
Application Note 140
4 AN140.0
June 28, 2005
Some precautions to be observed with VCSELs are:
• The zero point should be as far as possible above
threshold to prevent very long tails of the falling edges and
bit-pattern dependent jitter [5].
• The fiber coupling and fiber should not favour one
polarization or transverse mode as this may result in pulse
distortion such as large overshoot or undershoot at the
rising edge and zero bounce after the falling edge [6].
• Optical reflections from the fiber to the laser should be
minimized (as for edge emitters)
Selecting Resistor Values
In setting the various control circuit parameters, allowance
must be made for maximum and minimum values, as
shown in Table 3. The calculated resistor values are shown
in Table 4.
An initial design decision is that the laser diode will be
operated at the nominal value of 0.35mW. Also, operation is
assumed to be at 25°C.
To calculate the range of resistor values required, some
basic design equations are required:
(we assume zero error current in nominal conditions with
normal data signal being transmitted)
With the laser diode giving the optical power output “P1” at a
‘1’ level, and with an equal number of ‘1’s and ‘0’s, the
monitor current will be 50% of the full-power value.
VINM
= input voltage of current mirror
ßBIAS = bias current mirror gain
Each resistor (R6, R7, R8) depends upon three or more
parameters, some of which can have tolerances as high as
(+100% / -50%). For example, a simple calculation of
minimum and maximum values for RMODSET reveals a
potential range of values of 10:1.
However, common-sense engineering would dictate that
absolute minimum or maximum possible values represent
the combination of the “three-sigma” (3σ) limit of every
component - an extremely unlikely scenario, or gross
overdesign! An approach which acknowledges the statistical
nature of product parameter distributions would use a rootmean-
square method, where the square root of the sum of
the squares of the possible variation (in relative terms) is
calculated. Even the three sigma limit could be considered
overkill, since a two-sigma threshold still gives 95% of
products within the design range.
This approach assumes a normal or gaussian distribution for
each parameter. For components which are selected from a
population with a much wider distribution, this may not be
valid. For parameters which vary over a 4:1 range, this
method is obviously only approximate.
For the laser diode selected however, the distribution of
threshold is indeed close to Gaussian form [7].
[attachment=19440]
Abstract
Laser Diode (LD) controller/driver IC's at gigabit data-rates
typically use specially designed chipsets. In many cases
numerous control parameters are required to be set, using
resistors which define reference or control currents used by
the LD driver circuit. Precise and dependable settings are
essential in order to achieve a maximized extinction ratio
and minimize jitter.
Intersil Digitally Controlled Potentiometers (XDCPs) allow
automated setting of these resistor values in a manner which
is rapid, stable, repeatable and precise.
In addition, Intersil XDCPs offer many other integrated
features such as EEPROM and trip alarms.
Introduction
Optical fiber communication systems often use
semiconductor light sources such as a laser diode (LD) or
light emitting diode (LED). Both have the advantage that the
digital modulation at logic circuit voltages can be directly
applied to the operating bias current of the diode, which in
turn varies the output power. Laser diodes have advantages
for communication systems, since they provide high coupled
output power and routinely have high cutoff frequencies
which allow for data rates in excess of 10Gb/s. Such high
rates, call for purpose-designed circuits to control laser
diode currents. A challenge for control circuits is the knee or
threshold effect, below which optical output is minimal as
shown in Figure 1.
Many such chipsets for laser diode control exist, most
sharing a similar architecture. This comprises a transistor
which sets a bias current, another transistor providing extra
current to modulate the laser diode into the ‘1’ (maximum
power) condition, and a common-emitter differential
transistor pair switch. The function of this switch is to divert
the modulation current away from the laser diode when a ‘0’
is required. The differential pair can be directly controlled
from an ECL or PECL input (Figure 2).
The ‘1’ level is often set near the maximum safe level of
laser diode power output, while the ‘0’ is set close to the
threshold current (ITH), since this allows higher data rates.
In practice a small amount light is expected at the ‘0’ level
since the “knee” at the threshold of the Light versus Current
(L vs. I) characteristic curve is somewhat rounded.
Laser diodes exhibit a very wide range of tolerances of their
various parameters, and this requires a number of reference
values to be adapted to suit the individual component. In
addition these values vary significantly with temperature.
The bias current (IB) is controlled by a feedback loop from
the laser diode’s monitor diode (Refer to Figure 3).
FIGURE 1. IDEALIZED LIGHT OUTPUT vs. CONTROL
CURRENT
CURRENT
LIGHT
OUTPUT
MODULATION
CURRENT
DESIGN VALUE OF
LIGHT OUTPUT
FOR A “1”
DESIGN VALUE OF
LIGHT OUTPUT
THRESHOLD (ITH) FOR A “0”
I (mA)
L (dBm)
FIGURE 2. LASER DIODE DRIVER BLOCK DIAGRAM
IMOD IBIAS
LASER
DIODE
COMPLEMENTARY
ECL COMPATIBLE
LOGIC LINES
DATA
SOURCE
OPTICAL FIBER
Application Note
Authors: Joe Ciancio, Product Development Engineer,
Rex Niven, Design Engineer, Forty Trout Electronics Pty. Ltd.
June 28, 2005
2 AN140.0
June 28, 2005
.
The monitor photo-diode and feedback loop cannot “track”
the optical signal at full data rate. Therefore, the bias
feedback loop is designed such that it maintains an average
optical power, which is influenced by the relative number of
data ‘1’s and ‘0’s. The number of data ‘1’s and ‘0’s may be
made consistent by employing a line code such as simple
Manchester Coding, or more efficient, OMIT “8B/10B”
Coding - as used in GBIC (fiber channel) transceiver
modules.
As previously mentioned, at high data rates the modulation
current (IMOD) cannot have feedback control, and must be
set at a constant level. This therefore imposes the
requirement that the modulation current must be
desensitized OMIT, to the effects of temperature fluctuations
and device ageing. In the schematic shown in Figure 3, the
modulation current is set by choosing the appropriate value
of RMODSET.
An example of the effect of temperature variation, is to
reduce the gradient of the Light versus Current (L vs. I)
characteristic curve of the laser diode. This is referred to as
reduced slope efficiency. The lower slope efficiency has the
potential effect of making the laser diode in ‘0’ condition
transmit significant light power, while also reducing the
power of the optical ‘1’ level (Figure 4).
This approaching of the two levels reduces the extinction
ratio (the ‘0’ optical power level as a fraction of the ‘1’ optical
power level), which has a detrimental effect on the
theoretical bit error rate (BER) by reducing the size of the
“eye diagram” opening (Figure 5) [1].
To achieve a BER smaller than 1 x 10-10, requires the ‘1’
signal level to be at least 12.5σ (standard deviations) greater
than the received noise. Should the ‘0’ level rise to approach
one standard noise deviation, the BER will deteriorate
markedly. Therefore, the extinction ratio must be maximized
for optimum results, although this must be balanced against
the need for the zero level to be as far above threshold as
possible to avoid bit-pattern dependent jitter [2]. This is
achieved by reliable, and precise setting of the various
control circuit parameters. These control parameters are
usually currents, which are set/programmed by adjustable
resistors.
FIGURE 3. SY88922 LASER DIODE DRIVER AND SY88905 LASER DIODE CONTROLLER BLOCK DIAGRAM
IMOD IBIAS
LASER
DIODE
COMPLEMENTARY
LOGIC LINES
DATA
SOURCE
OPTICAL FIBER
RMODSET IBIASSET
RBIASSET
MONITOR
PHOTO-DIODE
RPINSET EXTERNAL
VOLTAGE
REFERENCE
IMD
IPINSET
Current
Comparator
ERROR AMPLIFIER
(CURRENT ERROR X10)
IBIASFB
Vcc
GND
MODULATION
CURRENT MIRROR
BIAS
CURRENT MIRROR
MODULATION
CURRENT SWITCH
AMPLIFICATION FACTOR
ß = x40
AMPLIFICATION FACTOR
ß = x40
EXTERNAL
VOLTAGE
REFERENCE
EXTERNAL
VOLTAGE
REFERENCE
FIGURE 4. EFFECT OF REDUCED SLOPE EFFICIENCY DUE TO LASER DIODE TEMPERATURE VARIATION
CURRENT
MODULATION
CURRENT
MEAN VALUE
OF MONITOR
PHOTO-DIODE
OUTPUT.
AT 0 °C
CURRENT
INCREASED
MODULATION
CURRENT
AT 65 °C
LOWER SLOPE EFFICIENCY
RESULTS IN SMALLER EXTINCTION
RATION (“0” AND “1”
LEVELS TO APPROACH EACH
OTHER - EXAGGERATED).
LIGHT
OUTPUT
I (mA)
L (dBm) L (dBm)
I (mA)
LIGHT
OUTPUT
Application Note 140
3 AN140.0
June 28, 2005
Intersil Digitally Controlled
Potentiometers in Laser Diode Driver
Control
Ideally the resistors used to program/set the laser diode
driver circuit parameters, should have the properties of:
• Have high resolution
• Allow adjustment over a wide range of resistance
• Be ready immediately after power-up
• Have low temperature coefficient
• Be immune to vibration and shocks
• Introduce minimal noise, both internally generated and
coupled from the exterior
• Have low “wiper” resistance
• Be very stable with age and other environmental
conditions
• Be tamper proof
• Allow readjustment many times (at circuit set up time, for
maintenance, or regular checkups)
• Permit a given setting to be reproduced with high
accuracy
• Have a small footprint for use in small form-factor
transceiver modules
• Avoid out-of-range values which may overdrive (and
possibly damage) critical components such as the laser
diode
• Readily allow automatic adjustment under digital/computer
control
• Allow multiple units in one package
• Be integrated with other ancillary functions
• Require no additional power supplies other than those
available for other system functions
• Be non-volatile (does not require power to retain resistor
settings)
• Allow a link to documentation of settings, with time/date
stamp, serial number and operator data
The Intersil Digitally Controlled Potentiometer (XDCP)
products offer all these features, and most will be shown in
the example design below.
Laser Diode Driver Circuit Design
In this circuit design example, the laser diode is controlled by
the Micrel-Synergy chipset SY88922 and SY88905. This
chipset uses PECL compatible data signals, and reference
voltages (less control circuit input bias) of 1.0V. The laser
diode used is a Vertical Cavity Surface Emitting Laser
(VCSEL) with monitor diode, type Honeywell HFE4380-321.
The complete schematic for this laser diode driver circuit is
shown in the Appendix.
The VCSEL has some favorable characteristics compared to
the more traditional plane cavity (edge-emitter) type laser
diodes. For example, a VCSEL has the properties of
• Lower threshold current
• Less influence of temperature on threshold current, with a
minimum near room temperature [3]
• Critically-damped relaxation oscillation
• Only one longitudinal mode, giving narrow spectral width
[4]
Some important characteristics of the HFE4380-321 are
summarized in Table 1 and Table 2.
FIGURE 5. BIT ERROR RATE AS A FUNCTION OF SIGNAL
TO NOISE RATIO
10-5
10-6
10-7
10-8
10-9
10-10
2 3 4 5 6 7 8
n
N = RATIO OF (MEAN ‘1’ – MEAN
‘0’) SIGNAL TO (ONE STANDARD
DEVIATION OF NOISE ‘0’ + ONE
STANDARD OF DEVIATION OF
NOISE ‘1’).
BER 0.5 1 – erf(n)
1.414
= -------------------------
BER
TABLE 1. KEY PARAMETERS OF VCSEL HFE4380-321
MIN TYP MAX UNITS
Optical Power
Output
0.2 0.35 0.8 mW
Threshold - 3.5 6 mA
Slope efficiency 0.02 0.04 0.1 mW/mA
Monitor current
(at 0.35mW optical)
0.07 0.275 mA
Series resistance 15 25 50 Ω
Forward voltage 1.6 1.8 2.2 V
TABLE 2. TEMP. DEPENDENCY OF VCSEL HFE4380-321
MIN TYP MAX UNITS
Threshold (1)
-1 +1
mA
(0 to 70°C)
Slope efficiency -0.4 %/°C
Monitor current +0.2 %/°C
1. The threshold varies parabolically with temperature having a
minimum at mid range of temperature (around +30°C).
Application Note 140
4 AN140.0
June 28, 2005
Some precautions to be observed with VCSELs are:
• The zero point should be as far as possible above
threshold to prevent very long tails of the falling edges and
bit-pattern dependent jitter [5].
• The fiber coupling and fiber should not favour one
polarization or transverse mode as this may result in pulse
distortion such as large overshoot or undershoot at the
rising edge and zero bounce after the falling edge [6].
• Optical reflections from the fiber to the laser should be
minimized (as for edge emitters)
Selecting Resistor Values
In setting the various control circuit parameters, allowance
must be made for maximum and minimum values, as
shown in Table 3. The calculated resistor values are shown
in Table 4.
An initial design decision is that the laser diode will be
operated at the nominal value of 0.35mW. Also, operation is
assumed to be at 25°C.
To calculate the range of resistor values required, some
basic design equations are required:
(we assume zero error current in nominal conditions with
normal data signal being transmitted)
With the laser diode giving the optical power output “P1” at a
‘1’ level, and with an equal number of ‘1’s and ‘0’s, the
monitor current will be 50% of the full-power value.
VINM
= input voltage of current mirror
ßBIAS = bias current mirror gain
Each resistor (R6, R7, R8) depends upon three or more
parameters, some of which can have tolerances as high as
(+100% / -50%). For example, a simple calculation of
minimum and maximum values for RMODSET reveals a
potential range of values of 10:1.
However, common-sense engineering would dictate that
absolute minimum or maximum possible values represent
the combination of the “three-sigma” (3σ) limit of every
component - an extremely unlikely scenario, or gross
overdesign! An approach which acknowledges the statistical
nature of product parameter distributions would use a rootmean-
square method, where the square root of the sum of
the squares of the possible variation (in relative terms) is
calculated. Even the three sigma limit could be considered
overkill, since a two-sigma threshold still gives 95% of
products within the design range.
This approach assumes a normal or gaussian distribution for
each parameter. For components which are selected from a
population with a much wider distribution, this may not be
valid. For parameters which vary over a 4:1 range, this
method is obviously only approximate.
For the laser diode selected however, the distribution of
threshold is indeed close to Gaussian form [7].