# Experiment No. 7 Optical Fiber Receiver

```Experiment No. 7
Experiment Aim
To design and study the optical fiber receiver.
List of Equipment
• Oscilloscope
• Optical Fiber Communication Experiment Kit
• Signal generator
• AVO meter
• Wires
• Optical fiber: 3m multi-mode
Theory
It can detect optical signals by using a reversed biased diode and a
resistor as shown in Fig(1-a). The voltage across the resistor is just given by
the product of the photocurrent and the resistance. The small-signal
equivalent circuit for sinusoidal optical signals incident on the detector, is
shown in Fig.(2-b). Note that the voltage at low frequencies is still given by
the V=IR relation, but the diode capacitance rolls off high frequencies with a
3dB point at f=1/(2πRC).
(a)
(b)
Fig(1): a) Circuit to measure optical power incident on a reverse-biased detection.
b) Small signal AC equivalent circuit
But with low received optical power, the photodiode can't delivered
enough current. Therefore, the trans-impedance amplifier used to amplify
the photodiode current. The trans-impedance amplifier in this experiment
is constructed from a resistor and an op-amp, shown schematically in Fig.(2a). It converts current into voltage. An explanation of this is given in Figure
(2-b) using the simplified circuit model for the op-amp. An important point
is that the input impedance of the trans-impedance amplifier is Rf/A, where
A is the frequency dependent open loop gain. This lower effective
impedance affects the bandwidth and the dynamic range of the amplifier.
If we attach the reverse-biased photodiode as shown in Fig.(3-a), the
amplifier now gives a voltage V=I×R, except that now the resistance "seen"
by the photodiode is Rf/A. Therefore, the operation amplifier gives us A
times the bandwidth of amplifier.
Z
in
=
Va
I
=
1 Rf
Rf
× =
≈
1+ A I 1+ A A
IRf
V = -IRf
(a)
(b)
Fig.(2): a) Trans-impedance Amplifier
b) Equivalent circuit of trans-impedance amplifier.
receiver, which converts the distorted detected pulses into digital logic
signals again. For this, we will be using a LM311 comparator with ground as
our decision level. Connect the output of the trans-impedance amplifier to
the input of the comparator. Then by using the DC offset of the op-amp,
position the amplifier waveform about the decision level (halfway between
the high and low voltages) so that the comparator can make its decision. The
comparator circuit is shown in Fig. (4). Connect one scope input to point A
and the other to point B, and both scope inputs should be on DC since we are
interested in some absolute DC threshold.
Procedure
1. Connect the amplifier circuit shown in Fig.(3) on the Optical Fiber
Trainer "OPF". The circuit consists of the trans-impedance amplifier,
a voltage reference, and a reverse biased photodiode as a current
source. A current limit is imposing on the photodiode without
changing the RC time constant of the circuit at high frequencies. Be
very careful that the photodiode is connected in the correct polarity in
the circuit.
2. Compare the two waveforms "the detector output at point C of Fig.(3)
and LED drive current as before".
3. Measure the rise and fall times (Remember that this is an inverting
amplifier, so a rising edge will look like a falling edge on the scope).
4. Look at the detected waveform rising and falling edge when one of
the 2.2pF feedback capacitors is removed.
5. Increase the frequency, examine simultaneously on the scope the
electrical pulse of the LD drive current, and compare to the detected
pulse as you increase the frequency.
6. Examine in detail the frequency "roll-off" of the detected signal by
measuring the peak-to-peak amplitude of the received signal versus
frequency.
7. As shown in Figure (3), a voltage adjust on the positive op amp input
working.
Discussion
1. Explain the circuit operation in your report.
2. From results, determine the bandwidth of trans-impedance amplifier
and the rise times you have measured.
3. Comment on the relationship between the output voltage and
frequency. Explain why these changes might occur.
4. Explain the relation between the input impedance "Zin" and photo
current amplifier gain.
5. What difference between bipolar transistor operational amplifier and
FET operational amplifier. What type of OP-AMP used in the optical
12V
22pF
1k
22pF
A
3.2k
560k
D1
12V
B
U1
C
12V
+
1k
-12V
+
330
10uF
D3
+
+
R6
10uF
1uF
D2
330
Fig.(3): Practical circuit
-12V
```