Presented By :-
ME (REGULAR- 2011)
January 30, 2015 1Presented by :- Diwaker Pant
To achieve the reliable and secure communication using optical fiber it is
essential that all the components within the transmission system are
compatible so that their individual performances, as far as possible enhance
the overall system performance.
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Prior to the discussion of the transmitter circuits , we will discuss the
differences and limitations between LED and ILD.
There are certain parameters on the basis of which we can discuss
the limitations of Optical Sources.
1. Power Considerations 2.Linearty
3.Thermal Consideration 4.Response
January 30, 2015 Presented by :- Diwaker Pant 3
1. Power Considerations
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Fig. 2 Light Output Vs. Input Current
Optical power coupled into a fiber
from a LED can be 10 to 20 dB
below that obtained a with a typical
Power advantage gained with
the ILD is a major factor in the
choice of the source.
2. Linearity Considerations
The linearity of the optical output power against current
characteristics is an important consideration with both ILD and LED.
Source non-linearity can cause severe distortion of the transmitted
An alternative approach to obtaining a linear source characteristic is
to operate an injection laser in the light generating region above its
January 30, 2015 Presented by :- Diwaker Pant 5
3. Thermal Considerations
The variation of injection laser threshold current with the device
junction temperature can cause a major operating problem.
Hence any signiﬁcant increase in the junction temperature may
cause loss of lasing and a subsequent dramatic reduction in the
optical output power.
In general we can say that LEDs are less temperature dependent
as compare to ILDs.
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4. Response Considerations
The speed of response of the two types of optical source is largely
dictated by their respective radiative emission mechanisms.
Spontaneous emission for LEDs and Stimulated Emission for Laser
diode are responsible for their response.
In general we can say that ILDs have fast response as compare to
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5. Spectral Width
Incoherent emission from an LED usually displays a spectral line-
width of between 20 and 50 nm when operating in the 0.8 to 0.9 μm
LEDs have wide spectral line-width as compare to ILDs.
Wide line width of the LEDs causes material dispersion within the
Use of the injection laser greatly reduces the effect of material
dispersion within the fiber.
January 30, 2015 Presented by :- Diwaker Pant 8
As we know that ILDs are more powerful and having high speed
over LEDs but LEDs are also used because LEDs are very easy to
operate as compare to ILDs.
Drive circuits are circuit configurations that may be used to convert
the information voltage signal at the transmitter into a modulation
current suitable for an LED source.
We divided the drive circuitry into two parts namely
1. Analog Transmission drive circuitry
2.Digital Transmission drive circuitry
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Reason for drive circuitry..
For analog transmission the drive circuit must cause the light output
from an LED source to follow accurately a time varying input voltage
waveform in both amplitude and phase.
It is important that the LED output power responds linearly to the
input voltage or current.
Unfortunately, this is not always the case because of inherent
nonlinearities within LEDs which create distortion in the signal.
So suitable compensation techniques are used.
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Two possible high speed drive circuitry are given below-:
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CE Configuration Drive circuit Darlington transistor pair Drive Circuit
Common emitter ampliﬁer which converts an input base
voltage into a collector current.
Darlington transistor pair is used to reduce the
impedance of the source or kind of current amplification.
Darlington transistor pair is used to drive the high
radiance LEDs at freq. of 70 MHz.
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Another simple drive circuit configuration is differential amplifier
operated over its linear region which directlly modulates the LED.
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Differential amplifier drive circuit continued…
The LED operating point is controlled by a reference voltage Vref.
The transistor T3 is used as current generator and T1, T2 limit the
maximum current through the device.
R1 and R2 provides a current feedback and is used for reducing
transimpedance of driver.
Normally the value of R1 and R2 are equal.
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Linearization Methods for LED drive circuits
1.Complementary Distortion Technique
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• Non-linear devices are used for distortion compensation.
•Pre-distortion compensation & Post- distortion Compensation.
Linearization Methods for LED drive circuits…
2.Negative Feedback Compensation Technique
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• The optical output is detected and compared with the input
waveform, amount of compensation depends on gain of
Linearization Methods for LED drive circuits…
3.Selective Harmonic Compensation Technique
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•The input signal is divided into equal parts which are phase shifted with
respect to each other.
The operation of the LED for binary digital transmission requires the
switching ON and OFF of a current in the range of several tens to
several hundreds of milli-amperes.
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This single-stage circuit provides current gain.
Small Voltage drop across the switch when
transistor is in saturation.
The maximum current ﬂow through the LED is
limited by the resistor R2.
Biasing is provided by the resistor R3
A simple drive circuit for binary digital transmission consisting of a CE
Low- impedance drive circuit consisting of an emitter follower with
compensating matching network.
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The capacitor used here is known as speed up
Pre-emphasis is accomplished by use of speed
Resistor R3 Capacitor C together forms a
Compensating Matching Network.
The current amplification is produced by
Emitter Follower circuit.
Now days drive circuitry with common logic interface are also available.
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Fig.(a) Block schematic showing
the interfacing of the LED drive circuit with logic input
Fig.(b) A simple TTL-
compatible LED drive circuit employing a Texas
Instruments’ 74S140 line driver.
Fig.(b) A TTL shunt drive circuit using a commercially
available integrated circuit.
A number of conﬁgurations which we have used for LED can be
used for ILD with certain modifications.
As we know that ILD is a threshold device so its drive current
requirements are different than LED.
For example when digital transmission is considered, the laser is
usually given a substantial applied bias in the off state and this
biasing is called Pre-biasing.
Benefits of Pre-biasing are as follows-:
(1).Reduction in switch on delay. (2).Reduction in junction heating.
(3).Easy compensation for change in ambient temperature and device
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A shunt drive circuit for ILD
Here we have used FET for high speed laser operation.
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
FET works as an inverter here.
Using a stable gallium arsenide MESFETs, a rate of
1Gbit per second can be achieved.
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An ECL-compatible high-speed laser drive circuit
Two differential amplifiers are connected in parallel for providing
modulation current amplitude for laser.
Input stage is ECL compatible , by the help of T1
(Emitter Follower) , T2 as a current source with the Zener diode for
adjusting the signal levels for ECL operation.
IE is provided by optical feedback control.
Optical feedback control is required to ensure continuous LASER
operation as threshold level of LASER is sensitive to temperature
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We conclude that drive circuits are very important part of an optical
fiber communication system.
By the proper use of drive circuits the output power of sources can
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Optical Fiber Communications by John M. Senior 3rd
edition, Pearson Publication
Optical Fiber Communication by Gerd Keiser 3rd
edition, Mc.Graw Hill.
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