Optiwave Semiconductor LASER Modulation Response

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

Content

1. Optiwave Software

2. Laser

3. Modulation types

4. Project Description

5. Components used in Project

6. Observations

7. Conclusion

8. References

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 :-

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

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.

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

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

http://en.wikipedia.org/wiki/Random_number_generation

http://en.wikipedia.org/wiki/Random

http://en.wikipedia.org/wiki/Random_seed

http://en.wikipedia.org/wiki/Hardware_random_number_generator

http://en.wikipedia.org/wiki/Hardware_random_number_generator

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


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


3. 75mA


4. 110mA


5. 145mA


From Figure (3)


Minimum BER: 0


From Figure (4)


Minimum BER: 0


From Figure (5)


Minimum BER: 0


B). Bit Rate = 1.1GHz

1. 10mA


2. 40 mA


3. 75mA


From Figure (6)


Minimum BER: 0.0151708

Eye Height: -0.0111118

From Figure (7)


Minimum BER: 0


From Figure (8)


Minimum BER: 0

Eye Height: .00632661

4. 110mA


5. 145mA


C). Bit Rate = 1.2GHZ

1. 10mA


From Figure (9)


Minimum BER: 0


From Figure (10)


Minimum BER: 0


From Figure (11)


Minimum BER: 0.0136359

Eye Height: -0.00104701

2. 40mA


3. 75mA


4. 100mA


From Figure (12)

Maximum Q –Factor: 197.09

Minimum BER: 0


From Figure (13)


Minimum BER: 0


From Figure (14)


Minimum BER: 0


5. 145mA


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-F

acto

r

Bias Current

Q-Factor vs Bias Current

1 Gbps

1.1 Gbps

1.2 Gbps

From Figure (15)


Minimum BER: 0


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

Min

imu

m B

ER

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

Eye

Hei

ght

Bias Current

Eye Height

1 Gbps

1.1 Gbps

1.2 Gbps

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.

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

http://www.ciscopress.com/articles/article.asp?p=30886&seqNum=9

http://www.rp-photonics.com/relaxation_oscillations.html

http://www.rp-photonics.com/chirp.html

Engineering

Optiwave Semiconductor LASER Modulation Response