3.1.8-Electrical to Optical conversion
In the optical communication systems, the desired signal travels in an optical form. But the
source produces an electrical signal.
So, there is a need to convert this electrical signal to the optical form so the optical
communication system can handle with it.
There are some modulation techniques to make this conversation, and in our project we used
the direct intensity modulation.
22.214.171.124-Intensity Modulation (IM):
In optical communications, intensity modulation (IM) is a form of modulation in which the
optical power output of a source is varied in accordance with some characteristic of the
Figure-126.96.36.199.1 Current & Optical Power
In intensity modulation, there are no discrete upper and lower sidebands in the usually
understood sense of these terms, because present optical sources lack sufficient coherence to
188.8.131.52-Electrical to Optical circuit:
The circuit which is responsible to do the intensity modulation is actually composed of two
parts (buffer and driver) coupled to each other in series.
184.108.40.206.1-Common Collector (Buffer Amplifier):
Figure(220.127.116.11.1) is the schematic diagram of a buffer amplifier. This circuit is a common-
collector amplifier. A common-collector amplifier has high input impedance and low output
impedance. Since the input of summing circuit is connected to the high impedance of the
common-collector amplifier, the buffer has little effect on the operation of the summing
circuit. The output of the common-collector buffer is then connected to an external load;
therefore, the changes in the output load cannot reflect back to the oscillator circuit. Thus,
the buffer amplifier reduces interaction between the load and the summing circuit.
Figure-18.104.22.168.1 Buffer Schematic
22.214.171.124.2-Common Emitter (Driver Circuit):
In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-
junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. In this
circuit the base terminal of the transistor serves as the input, the collector is the output, and
the emitter is common to both (for example, it may be tied to ground reference or a power
supply rail), hence its name. The analogous field-effect transistor circuit is the common-
Basic NPN Adding an emitter
common-emitter circuit resistor decreases
(neglecting biasing details). gain, but increases
In our project we used the common-emitter-with a resistance amplifier (Figure-126.96.36.199.2.3) to
drive the optical source and convert the signal from the electrical form into optical form
using the intensity modulation.
Figure-188.8.131.52.2.3 Driver Circuit Schematic
As shown in figure-(184.108.40.206.2.3), there is a coupling capacitors (C3,C4) in series between the
Buffer and the Driver circuits,
Optical-Tx (HFBR1414) 1
By coupling the two circuits (Buffer & Driver) to each other in series we’ve got the electrical
to optical circuit as shown in figure-(220.127.116.11.2.4).
Electrical to Optical circuit schematic
From the values in table-(18.104.22.168.2),
R6=47Ω, R4=5kΩ, R5=4.5kΩ, Vcc=9volts.
Thevenin's resistor equivalent (Rth) and voltage equivalent (Vth) have to be found first.
Rth = , Rth = = 2.368kΩ
Vth = = = 4.264 volts
Once these two values have been found, the base current (IB) can be found.
IB = ( )
= ( )
= 5 × 10-4 Ampers
IC = β.IB , IE = IC + IB,
IC = 50 mA, IE = 50.5 mA,
22.214.171.124.4-Capacitor Coupling (CR-coupling):
Why using coupling capacitors?
Sections of electronic circuits may be linked with a capacitor because capacitors pass AC
(changing) signals but block DC (steady) signals. This is called capacitor coupling or CR-
For successful capacitor coupling, the signals must pass through with little or no distortion.
This is achieved if the time constant (RC) is larger than the time period (T) of the lowest
frequency audio signals required (typically 20Hz, T = 50ms).