This document summarizes a student project on laser modulation response using Optiwave simulation software. The project involves: 1. Generating a pseudorandom bit sequence to drive a laser, producing an optical output. 2. Using a photodiode to convert the optical signal back to electrical, introducing harmonics. 3. Applying a Bessel filter to reduce harmonics and obtain the desired output signal quality in terms of eye height, Q-factor and bit error rate. The document discusses laser modulation types, components used like the pseudorandom sequence generator, photodiode and Bessel filter, and observations on the laser's optical output response to modulation.

1. Report on
Advanced LAB
Semiconductor Laser Modulation Response
Name : Rajneesh kumar Salgotra
En. Rolment : 163/2010
Roll no. : 29
Semester : 8th
National Institute of Technology,
Hazartbal, Srinagar

2. Content
1. Optiwave Software
2. Laser
3. Modulation types
4. Project Description
5. Components used in Project
6. Observations
7. Conclusion
8. References

3. 1. Optiwave Introduction[2]
Optiwave Software is very useful one . It helps in Simulation of the optic-circuits , which can then to be
used by design engineers to evaluate the effect of different parameters and the efficiency of the
designed system. It also helps the marketing teams of component and system vendors to powerfully
and cost-effectively demonstrate their products .It reduces the investment cost incurred for the
research as the software offers conditions close to the real one. This software is widely used because
of the features offered by it, following are few of them
FEATURES:
A) Graphical user interface :
A comprehensive Graphical User Interface (GUI) controls the optical component layout, component
models, and presentation graphics. The Performer Control window provides simple, easy-to-use button
controls for simulation control.
B) Mixed signal Representation :
OptiSystem handles mixed signal formats for optical and electrical signals in the Component Library.
OptiSystem calculates the signals using the appropriate algorithms related to the required simulation
accuracy and efficiency.
C) Quality and performance algorithms :
In order to predict the system performance, OptiSystem calculates parameters such as BER and Q-
Factor using numerical analysis or semi-analytical techniques for systems limited by inter symbol
interference and noise.
D) Advanced visualization tools :
Advanced visualization tools produce Oscilloscope, and Eye diagrams. Also included are WDM analysis
tools listing signal power, gain, noise figure.
2. LASER :
Laser is a device which emits amplified light signal, amplified through the process of stimulated
emission, that is why LASER is named after LIGHT AMPLIFICATION STIMULATED EMISSION RADIATION
.The light emitted by the laser is monochromatic , directional and coherent in nature.
As we know optical links for data flow offered much more advantages
over the conventional metal links and are being used as transmission medium. As only light can flow
through these links we require an electro-optic converter at sending end and optic-electro converter at
receiving end for signal conversion. We require a source which can convert high frequency electrical
signal to optical domain. Laser is the best known Modulation source of optical light , which can be
coupled to the optical link with lower losses .Generally two techniques in for Modulation are defined :-

4. 1. External Modulation : A continuous Wave Laser is used to emit light , and external modulator
Switches the Light “ON” or “OFF”
2. Direct Modulation : The Driving Signal is fed to Laser and Light is emitted when
“MARK” is to sent and no light when “SPACE” it be sent .
Figure 1. (a) : The Driving current to Laser is the Modulating signal, so it turns Laser On and Off (b) : Driving
current to Laser is constant and Light thus emitted is modulated by external modulator
Important parameters consideration about the laser to be used:
1. Speed : The speed of operation is required to be high as input data frequency is high ,so the laser
should be able to switch between states immediately
2. Extinction Ratio: It is the ratio of the power level corresponding to “MARK” to the power level
corresponding to “SPACE”
ER=P1/P0
P1 – Power Level for MARK
P0 – Power Level for SPACE
The value of ER should be high for easy recognition of MARK and SPACE , otherwise if high power is
transmitted but ER is less , the power transmitted is useless as probability of error at receiver is much
more. Power Penality at Output is δER = 10 log10 [(ER+1)/(ER-1)]
3. Frequency Chirp[3,5]: When an external field is applied as driving signal which forces changes in the
charge density in the laser medium resulting in the changes in the refractive index and hence the phase
of the output changes with time

5. ω(t) = ωo + dφ/dt
dφ/dt – equivalent to the change in signal intensity. The instantaneous frequency depends upon phase
change, this undesired frequency modulation is known as frequency chirping.
In optical fiber different frequency components travel in optical fiber link with
different velocities resulting in pulse broadening at output
3. Types of Modulation
External Modulation:
Two techniques are used for External Modulation
A) Electro-Absorption Modulation:
In this technique we use Semiconductor material whose energy gap can be changed depending upon the
input electric field, so when electric field is applied the Energy Gap reduces and the Light is absorbed,
hence no light output from modulator. So when there is no electric field the Energy Gap remains the
same, hence Modulator acts as transparent to the Laser light and it passes through. Hence it become
possible to achieve Optical modulation controlled by External signal.
B) Electro-optic Modulator:
In this Technique the refractive index is changed based on the external electric field.
φ = (2π/λ)nL experienced by light wave
φ- phase shift
λ- wavelength of that light wave
L- length of medium
n- refractive index of medium
The applied voltage will modulate the refractive index of the waveguide material. However, optical
fiber communication is based on intensity modulation, so for converting Phase modulated signal to
intensity modulated interferometric structure is used
Direct Current Modulation of Semiconductor Laser :
As stated before output power of laser depends upon the current injected. when Laser is turned ON
there is no output for some time , as it takes time to build up population inversion in Laser , as current
increases the population inversion is achieved this current is known as threshold current at which
population inversion achieved. The Laser output then increases almost linearly with the input current.
Varying current depending on data we can convert electrical signal in to the optical signal which is to
be transmitted.

6. Some Important aspects of DML :
1. Turn on delay[6] :
If we try to achieve Space in below the threshold current and mark above threshold , so when ever
there is change in state like from Space to mark so sufficient current needs to be injected to fulfil the
condition for the population inversion and it incorporates some delay, which is given by equation
Td = τo ln [(I1-I0)/(I1-Ith) ]
I1 – Driving Current for mark
I0 - Driving Current for space
Ith - Threshold Current
2. Extinction Ratio:
There is trade-off between speed of operation and Extinction Ratio possible, as more the Extinction
Ratio lesser is the speed of operation possible and lesser is Extinction ratio, more is the speed of
operation possible, but in case of Lower Extinction ratio power penalty is more.
3. Bandwidth
The Bandwidth denotes the Data rate and delay in a DML limits the data rate, more is the Bias current
more carriers injected, hence lesser the delay more the data rate more the bandwidth
Δf = [3Gn (Ib-Ith) / 4π2 e ]1/2
Gn - constant related to the rate of stimulated emission on the carrier number
Ib - Bias current
Ith - Threshold current
e – elementary charge
4. Relaxation Oscillation
When Laser is subjected to change state between Mark and Space, oscillations occur at the output
power known as Relaxation Oscillations. But , these oscillations are under-damped and steady state is
attained , the damping depends upon the input current higher the current higher the damping , the
origin if these oscillations is interplay between injected charge and the emitted photons.
4. PROJECT:
This project is based on laser emission the nature of the optical power output from the Laser
depending upon various parameters of the Laser and the Input signal is observed. So in Circuit we use
a pseudo-random sequence generator, which produces sequence which is only known to sender and
receiver and purely random to others, Non-return-to-zero pulse generator is used which feeds NRZ

7. pulses to the Laser, which is the driving signal to the Laser , the laser thus producing optical output
depending upon the driving current (NRZ signal) , As we know Laser is a non- linear device producing
harmonics at the output also along with the desired signal. The light output at Laser’s output is fed to
Fig. Layout of Semiconductor Laser Modulation Response
the Photo diode for conversion back to the Electric domain which now also includes Harmonics , so a
Low Pass Bessel filter is connected after Photo diode to reduce harmonics and get output at some
desired range of frequencies, which produces fiddle Bits at the Output with more Q-factor , more Eye
Height and Lesser Bit error rate possible.
5. Description of components used in project
1. PSEUDO-RANDOM BIT SEQUENCE GENERATOR: A pseudorandom Bit Sequence generator is used
for generating a sequence of numbers that approximates the properties of random numbers. In terms
of random number generation, the sequence is not truly random in that it is completely determined by
a relatively small set of initial values, called the PRNG's state, which includes a truly random seed.
Although sequences that are closer to truly random can be generated using hardware random number
generators, pseudorandom numbers are important in practice for their speed in number generation
and their reproducibility.
2. Photo Diode: Photo Diode is used to convert optical signal back to the electric signal, the current is
generated when photons are absorbed and appreciable amount of electrons and holes produced. A small

8. amount of current also flows even when there is no light known as dark current. The response time of
the Photo Diode depends on the area exposed to light.
3. Bessel Filter: This filter has a flat group delay in the pass band that is all the frequencies that are
allowed to pass experience a constant delay and hence the fidelity of the signal at the output is
maintained.
6. Observations:-
Default parameters Ith = 33.45mA, Tsp = 1ns, Tph = 3ps, Frequency=193.099 THz
Observation is taken at three different data rates , sweeping the Bias current from 10mA to 145mA
A). Bit rate = 1 Gbps
1. Bias Current =10mA
Figure(1) Eye Diagram Output Screen
2. Bias Current =40 mA
Figure(2) Eye Diagram Output Screen
From Figure (1)
Maximum Q-Factor: 5.59134
Minimum BER: 6.04e-009
Eye Height: 0.00103595
From Figure (2)
Maximum Q-factor: 191.391
Minimum BER: 0
Eye Height: 0.00626599

9. 3. 75mA
Figure(3) Eye Diagram Output Screen
4. 110mA
Figure(4) Eye Diagram Output Screen
5. 145mA
Figure(5) Eye Diagram Output Screen
From Figure (3)
Maximum Q-Factor: 619.593
Minimum BER: 0
Eye Height: 0.0063296
From Figure (4)
Maximum Q-Factor: 631.319
Minimum BER: 0
Eye Height: 0.00632961
From Figure (5)
Maximum Q-Factor: 597.883
Minimum BER: 0
Eye Height: 0.00632769

10. B). Bit Rate = 1.1GHz
1. 10mA
Figure(6) Eye Diagram Output Screen
2. 40 mA
Figure(7) Eye Diagram Output Screen
3. 75mA
Figure(8) Eye Diagram Output Screen
From Figure (6)
Maximum Q-Factor: 1.88261
Minimum BER: 0.0151708
Eye Height: -0.0111118
From Figure (7)
Maximum Q-Factor: 181.448
Minimum BER: 0
Eye Height: 0.00626553
From Figure (8)
Maximum Q-Factor: 561.383
Minimum BER: 0
Eye Height: .00632661

11. 4. 110mA
Figure(9) Eye Diagram Output Screen
5. 145mA
Figure(10) Eye Diagram Output Screen
C). Bit Rate = 1.2GHZ
1. 10mA
Figure(11) Eye Diagram Output Screen
From Figure (9)
Maximum Q-Factor: 575.383
Minimum BER: 0
Eye Height: 0.00632769
From Figure (10)
Maximum Q-Factor: 547.359
Minimum BER: 0
Eye Height: 0.00632632
From Figure (11)
Maximum Q-Factor: 1.90759
Minimum BER: 0.0136359
Eye Height: -0.00104701

12. 2. 40mA
Figure(12) Eye Diagram Output Screen
3. 75mA
Figure(13) Eye Diagram Output Screen
4. 100mA
Figure(14) Eye Diagram Output Screen
From Figure (12)
Maximum Q –Factor: 197.09
Minimum BER: 0
Eye Height: 0.0062785
From Figure (13)
Maximum Q-Factor: 522.951
Minimum BER: 0
Eye Height: 0.00632517
From Figure (14)
Maximum Q-Factor: 540.863
Minimum BER: 0
Eye Height: 0.00632627

13. 5. 145mA
Figure(15) Eye Diagram Output Screen
7. Conclusion :
Graph(1)
In Graph(1) Q-Factor vs Bias current, for a particular data rate Q-Factor below kink current is very less
and increasing very slowly up to kink current, because in region below threshold current the
spontaneous emission in more pronounced, so the Q-Factor is less. Then, after the threshold current
the Extinction Ratio concept comes into play, when we move upwards on optical Power vs Bias Current
curve, the ER ratio decreases and power penalty increases and Q-Factor starts decreasing. Similar
concept is followed in each bit rate curve, but the relation between various bit rate is dictated by
dispersion. At lower data rate, Inter symbol interference effect is low hence Q-Factor more with
respect to a curve corresponding to higher bit rate .
0
100
200
300
400
500
600
700
10 40 75 110 145 180
Q-Factor
Bias Current
Q-Factor vs Bias Current
1 Gbps
1.1 Gbps
1.2 Gbps
From Figure (15)
Maximum Q-Factor: 519.949
Minimum BER: 0
Eye Height: 0.00632481

14. Graph(2)
In Graph(2) Minimum BER vs Bias current, in region below threshold only there is appreciable existence
of Minimum Bit Error rate. As we know below threshold current value there is Spontaneous Emission
only So as the current is increasing more spontaneous emissions possible hence the BER decreases
accordingly, but as the threshold value is crossed only Stimulated emission takes place, all the photons
emitted are coherent and BER is very low, but as the data rate is increased Dispersion increases and
the BER increases too in spontaneous region.
Graph(3)
0.00E+00
2.00E-03
4.00E-03
6.00E-03
8.00E-03
1.00E-02
1.20E-02
1.40E-02
1.60E-02
10 40 75 110 145 180
MinimumBER
Bias Current
Min. BER vs Bias Current
1 Gbps
1.1 Gbps
1.2 Gbps
-0.002
0
0.002
0.004
0.006
0.008
10 40 75 110 145 180
EyeHeight
Bias Current
Eye Height
1 Gbps
1.1 Gbps
1.2 Gbps

15. First of all we need to understand what does the Eye Height represent , the Eye Height represents the
difference between the levels of MARK and SPACE , so more is the eye height more easily the
two(MARK and SPACE) are distinguishable. As clear from Graph(3) In the Lasing Region, Bias current
increases the optical output power of both mark as well as space, so doesn’t matter how much Bias
current you increase the Eye Height will remain the same.

16. 8.References:
1. G. Keiser , Optical Fiber Communications , McGraw Hill
2. optiwave.com/pdf/OptiPerformer_User_Reference.pdf
3. http://www.ciscopress.com/articles/article.asp?p=30886&seqNum=9
4. www.rp-photonics.com/relaxation_oscillations.html
5. www.rp-photonics.com/chirp.html
6. http://physics.stackexchange.com/questions/21510/turn-on-delay-time-for-laser-diode
7. en.wikipedia.org/wiki/Pseudorandom_number_generator
8. en.wikipedia.org/wiki/Photodiode
9. en.wikipedia.org/wiki/Bessel_filter