16/04/2004 Optical Tansmitters and Laser Control
Dublin Institute of Technology
School of Electronic and
Communications Engineering
Optical Communications Systems
Optical Transmitters and Laser Control
Dr. Yuliya Semenova
Unauthorised usage or reproduction strictly prohibited
Copyright 2003, Dr. Yuliya Semenova, Dublin Institute of Technology
16/04/2004
Types of Laser/LED Driver
Constant Current Drivers
•Used where modulation is not involved or
external
IC
•Connecting/Disconnecting laser can cause
voltage spikes which could destroy the laser
Series Drivers
Im
•Modulation and prebias can be set individually
•Potentially slower than Shunt drivers
•Wide variety of designs have evolved
Modulation
switch
Prebias
current
Modulation
switch
Im
Shunt Drivers
•Used to both set the prebias point and to
modulate the device
•Very high speed operation possible
•Used in integrated high speed Laser/LED drivers
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
LED Drive Circuits. Digital
Transmission
+VCC
•Series type driver.
•The circuit uses a bipolar transistor switch
in the common emitter mode.
LED
R2
•The maximum current flow through the LED
is limited by the resistor R2 whilst
independent bias to the device may be
provided by the incorporation of resistor R3.
R3
C
Vin
R1
On "1" current =
VCC − VCE ( SAT ) − V LED
Prebias current =
R2
VCC − V LED
R3
+ I prebias
•The speed of the common emitter
configuration is limited by space charge and
diffusion capacitance. This may, to a certain
extent, be compensated by overdriving (preemphasizing) the base current during the
switch-on period.
•Can modulate at speeds up to 50 MBit/sec.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
LED Driver Circuits. Low Impedance
Driving Circuit
+VCC
Increased switching speed may be
obtained from a LED by use of a low
impedance driving circuit. This may be
achieved with the emitter follower drive
circuit.
Vin
R1
R3
C
R2
LED
The use of this configuration with a
compensating
matching
network
provides fast direct modulation of LEDs
with relatively low drive power (2.5 ns,
for LED with capacitance of 180 pF,
allowing 100 Mbit s-1).
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
LED Driver Circuits. Shunt
Configuration
The switch transistor is placed in parallel with
the LED, providing a low impedance path for
switching off the LED by shunting current
around it.
+VCC
R
The switch-on performance of the circuit is
determined by the combination of resistor R ad
the LED capacitance.
Vin
LED
VEE
Stored space charge may be removed by
slightly reverse biasing the LED when the
device is switched off. This may be achieved by
placing the transistor emitter potential VEE
below ground. In this case a Schottky clamp
(dotted) may be incorporated to limit the extent
of the reverse without introducing any extra
minority carrier stored charge in the circuit.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Logic Interfacing for Digital
Transmission
Data input
Logic interface
Drive circuit
LED
Schematic
A frequent requirement for digital transmission is the interfacing of
the LED by drive circuit with a common logic family. In this case
the logic interface must be considered along with possible drive
circuits. Compatibility with TTL may be achieved by use of a
commercial IC.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Logic Interfacing for Digital
Transmission. Examples
Texas Instruments’ 74S140 line
driver provides a drive current of
around 60 mA to the LED when R1 is
50 Ω. The incorporation of suitable
speed-up capacitor (e.g. C = 47 pF)
gives optical rise times of around
5 ns when using LEDs between 150
and 200 pF capacitance.
+5 V
Shunt configuration using a standard TTL
75451 integrated circuit. The rise time of
this circuit may be improved through
maintenance of charge on the LED
capacitance by placing a resistor between
the shunt switch collector and the LED.
+5 V
LED
R1
Vin
74S140
line
driver
75451
C
R2
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Analog Transmission
+VCC
LED
Vin
•For analog transmission the drive circuit
must cause the light output from a LED
source to follow accurately a time-varying
input voltage waveform in both amplitude
and phase.
•The figure shows a driver consisting of a
common emitter transconductance amplifier
which converts an input base voltage into a
collector current. The circuit is biased for a
class A mode of operation with the quiescent
collector current about half the peak value.
Transconductance
drive circuit:
common emitter
configuration
•The circuit is unadequate in frequency
multiplexed systems where a high degree of
linearity is required.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Linearization Methods for LED drive
Circuits
Fibre
Input
Output
Linearizing
network
Drive
circuit
Receiver
Linearizing
network
Complementary distortion technique
In the complementary distortion technique additional nonlinear
devices are included in the system. It may take the form of
predistortion compensation (before the source drive circuit) or
postdistortion compensation (after the receiver).
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Negative Feedback Compensation
Technique
In the negative feedback compensation technique the LED is
included in the linearization scheme. The optical output is
detected and compared with the input waveform, the amount of
compensation being dependent on the gain of the feedback loop.
Fibre
Input
Out
Drive
circuit
Receiver
Monitor
Negative feedback
compensation technique
Feedback
control
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Selective Harmonic Compensation
Technique
Drive
circuit
Input
Phase shifter
(900 hybrid)
Fibre
Out
Receiver
Drive
circuit
Selective harmonic
compensation technique
The method employs phase shift modulation for selective harmonic
compensation using a pair of LEDs with similar characteristics.
The input signal is divided into equal parts which are phase shifted with
respect to each other. These signals then modulate the two LEDs giving
a cancellation of the second and third harmonic with 900 and 600 phase
shift respectively.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Laser Drive Circuits
Most LED drive circuits for both digital and analog transmission may be adapted for
injection laser applications with only minor changes.
Lasers, because of stimulated emission, are much faster than LEDs, this increases the need
for high speed drive circuitry, in particular above 1 GHz.
The laser, being a threshold device, has somewhat different driver current requirements
from the LED. For instance, when digital transmission is considered, the laser is usually
given a substantial prebias, in the off state.
Reasons for prebiasing the laser near but below threshold in the off state are as follows:
1.
It reduces the switch-on delay and minimizes any relaxation oscillations.
2.
It allows easy compensation for changes in ambient temperature and device
ageing.
3.
It reduces the junction heating caused by the digital drive current since the on
and off currents are not widely different for most lasers.
Although biasing near threshold causes spontaneous emission of light in the off state, this
is not normally a problem for digital transmission because the stimulated emission in the on
state is generally greater by, at least, a factor 10 (extinciton
extinciton ratio).
ratio
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Design of Drive Circuitry
Design Considerations and Possible Solutions in the Design of Laser
Drivers
Trade-offs
Possible Solutions
Design Constraint
Output Power
Increases with extinction
ratio (signal amplitude)
•Monitor output power
•Feedback and adjust bias
Dispersion
Between dispersion and
laser chirp thus affecting
whether direct or external
modulation is used
•Use of modulators can
educe or eliminate laser
chirp
•Equalization can eliminate
linear dispersive effects
Chirp
Large with direct
modulation
Use of external modulator
Extinction ratio
Decreases with increased
bias
Affects average output
POWER
Balance according to
requirements
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Design of Drive Circuitry (Cont)
Design Constraint
Trade-offs
Possible Solutions
Output impedance
matching (with
transmission line
connected to laser or
modulator)
•High output impedance
can limit bandwidth if laser
has high input capacitance
(modulators do)
•Reflections caused by
mismatch can affect
extinction ratio and jitter
Affects speed and
extinction ratio relationship
and can control power
•Low output impedance
driver
•Actively matched output
Temperature
Laser output
characteristics vary with
temperature
Feedback control of laser
bias current
Aging
Laser threshold increasing
with age because of an
increase of an internal loss
Feedback control of laser
bias current
Bias current level
•Low threshold laser
•Used as control parameter
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Simple Shunt Laser Driver Circuit
Utilizes a FET to provide high speed
operation.
+VCC
R1
Vin
D
G
R2
S
Laser
C
Sufficient voltage is maintained in series
with the laser using the resistor R2 and the
compensating capacitor C such that the
FET is biased into its active or pinch-off
region.
For particular input voltage Vin (VGS) a
specific amount of the total current flowing
through R1 is diverted around the laser
having the balance of the current to flow
through R2 and provide the off laser state.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
ECL Compatible High Speed Laser
Drive Circuit
VCD
ECL input
Iout
T3 T4
T1
Prebias
current
T5 T6
RB1
ECL return
Laser
RB2
ZD
RE1
RE2
RB
VBD
T2
IE
Drive control
Bias control
VED
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
ECL Compatible High Speed Laser
Drive Circuit
•The circuit consists of two differential amplifiers connected in parallel.
•The input stage, which is ECL compatible, exhibits a 50 Ω input
impedance by use of an emitter follower T1 and a 50 Ω resistor in parallel
with the input.
•The transistor T2 acts as a current source with the Zener diode ZD
adjusting the signal level for ECL operation.
•The two differential amplifiers provide sufficient modulation current
amplitude for the laser under the control of a d.c. control current IE
through two emitter resistors RE1 and RE2;
•IE is provided by an optical feedback control circuit.
•Prebias current is applied to the laser from a separate current source.
Contains feedback control!
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Mean Power Feedback Circuit
Vref
Set Pmean
G
•The detected signal is
integrated and
compared with a
reference by an
operational amplifier
which is used to servocontrol the d.c. bias
applied to the laser.
G
OP amp
Monitor
photodiode
Fiber
Laser
•The mean optical
power is maintained
constant by varying the
threshold current level.
•Suitable for both
analog and digital
transmission.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
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16/04/2004
Laser Feedback Control
Control of the laser modulation amplitude
•The electrical output from the monitor photodiode is fed into a low drift DC
amplifier A1 and into a wideband amplifier A2. Therefore the mean value of the
laser output power is proportional to the output from A1 whilst the AC content of
the monitoring signal is peak detected after the amplifier A2.
•The peak signals correspond to the maximum and minimum laser output powers
within a certain time interval.
•The difference signal is acquired in A3 and compared with a drive reference
voltage in order to control the current output from A4 and, consequently, the laser
drive current.
Control of the laser bias current
•Achieved from the difference between the output signal of A1and a minimum laser
power acquired in A5. The output voltage of A5 is compared to with a bias
reference voltage in A6 which supplies a current output to control the laser DC
bias.
•The circuit operates at bit rates in the GHz range.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Laser Feedback Control
Drive control
Laser drive circuit
Bias control
+V
A5
A1
A6
Laser
Fiber
Bias ref.
Pmax
Monitor
photodiode
A2
Peak
detector
A3
A4
Pmin
Drive ref.
A major disadvantage with just controlling the laser bias current is that it does not compensate
for variations in the laser slope efficiency (any slope changes with temperature and aging).
To compensate for such changes, the AC and DC components of the monitored light output
must be processed independently.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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16/04/2004
Integrated Drivers
•Integrated drivers allow higher speeds and potentially
allow lasers to be integrated with drive electronics
•Example:
MAX3850 2.7Gbps, single 3.3V, direct-coupled laser driver.
Consuming only 116mW of power, the laser driver is capable of
providing 5mA to 60mA of direct-coupled modulation current
(80mA AC-coupled) and 1mA to 100mA of bias current.
An automatic power control (APC) feedback loop is incorporated to maintain a constant
average power over temperature and lifetime. To reduce jitter, the MAX3850 has an optional
synchronizing input latch that can be implemented depending on the presence of a clock
signal. The MAX3850 provides a laser current enable control and two current monitors that are
proportional to the laser bias and modulation currents. A failure-monitor output is provided to
indicate when the APC loop is unable to compensate and maintain average optical power.
Designed to be DC-coupled to the laser diode, the MAX3850 greatly simplifies interface
requirements and reduces component count.
The MAX3850 is designed for SONET OC48/SDH STM16, metro mesh/ring architectures, metro
DWDM, and long-haul systems. Packaged in a 32-pin TQFP, the MAX3850 operates from -40°C
to +85°C.
Optical Communications Systems, Dr. Yuliya Semenova, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2003, Dr. Yulia Semenova, Dublin Institute of Technology
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