4 laser dio destruct

8/16/2019 4 Laser Dio Destruct

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Dublin Institute of Technology

Dr. Gerald Farrell

Optical Communications Systems

School of Electronic andCommunications Engineering

Unauthorised usage or reproduction strictly prohibited

Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology

Semiconductor Laser Diodes

27/02/02 2.4 Semicdr Laser diode structures and characteristics.prz


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Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering

Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology

Laser Structures

Source: Master 4_3



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Semiconductor Laser Structures

A wide variety of laser structures have evolved, with the aim of reducedthresholds, improved efficiency and narrow spectral output:

Basic broad area laser

Stripe geometry laser

Gain guided laser

Index guided laser

Single frequency laser

Multi-section laser

Source: Master 4_3



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Double Heterostructure

The double heterostructure is one of the most basic Laser structures.Typical 5 layer structure is shown below.

Bandgap energy is higher in the confinement regions, resulting in a concentration of radiativerecombination in the lower bandgap energy active region, improving efficiency.

Refractive index in the confinement region is lower, resulting in optical confinement within theactive region.

Contact region

Contact region

p-GaAs

p-AlGaAs

Active Layern-GaAs

n-AlGaAs

n-GaAs

Electrode

Heterojunctions

Light output normal to

page

Confinementregions

Electrode

Refractiveindex profile

Source: Master 4_3



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Broad Area DH Injection Lasers

Roughened sides

n-AlGaAs

Light Output

Cleaved Mirror

n+ -GaAs

p -AlGaAs

n+ -GaAs

Confinement Layers

Contact metallization

p -GaAsActive Layer

In this simple early laser structure the DH structure confines the light to the active regionin the vertical direction.

Lasing still takes place across the whole width of the device, hence it is called a broadarea laser.

Low quantum efficiency, by comparison with more advanced designs, resulting in highthreshold current values.

Output light geometry is unsuitable for coupling to fibre.

Source: Master 4_3



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Gain Guided Lasers

Laser structures are designed to keep the threshold as low as possible, with a high efficiencyand a narrow output beam.

Two basic design approaches are gain guiding and index guiding.

In a gain guided laser the current flow is restricted to a narrow stripe by placing high resistivityregions within the contact regions.

Gain guiding is not very successful, thresholds are high, >100mA, with low differential quantum

efficiencies and non-linear kinks in the output characteristic.

p-GaAs

p-AlGaAs

Active Layern-GaAs

n-AlGaAs

n-GaAs

Electrode

Heterojunctions

Confinementregions

Electrode

High resistivityregion

Source: Master 4_3



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DH Stripe Geometry Lasers

Stripe formed by inclusion of insulation layers, thus most of the current enters the activeregion in a narrow stripe that runs the length of the device.

Result is a narrow emission region, with a lower lasing threshold and a narrower outputbeam.

Source: Master 4_3



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Index Guided Lasers

Index guiding overcomes most of the disadvantages of gain guided designs.In an index guided structure the active region is surrounded by a region of lowerrefractive index, confining the photons to a narrow stripe, in both the transverse andvertical directions.

Several designs have emerged including the ridge waveguide (weakly index guided)and buried heterostructure (BH) (fully index guided) designs.

Typically the threshold currents lie in the region of 10-20 mA for BH lasers, with activeregions a couple of microns wide.

Buried heterostructure laser

diode

Source: Master 4_3



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Twin Section Lasers

Gain Section

Absorber Section

Active Region

Two distinct sections, based on split anode contacts.Forward biased section is so-called gain section.

Other section is left unconnected or reversed biased, called the absorber.

Produces hysteresis in the light-current characteristic and repetitive self-pulsation.

Numerous optical signal processing applications, including all optical frequency changing.

Basic Fabry-Perot twin section laser

Source: Master 4_3



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Twin section LaserCharacteristics

0 10 20 30 40 50 60 700

2

4

6

8

10

Light

Intensity

(a.u..)

Gain section current (mA)

Twin section laser light-current curve,displays hysteresis

Results in two distinct states, potentiallyuseful for optical memory and logic

5 mV/div

1 ns/div

O/P

I/P

Twin section lasers can also exhibitrepetitive on-off behaviour, calledself-pulsation.

Proposed applications include all-opticalsynchronisation for frequencymultiplication / division and clockextraction.

Trace shows all-optical frequencymultiplication by 2:1

Source: Master 4_3



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LaserCharacteristics

Source: Master 4_3



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Laser Efficiency

Basic internal laser quantum efficiency ηη i is defined as:

ηη i =number of photons produced in the laser cavity

number of injected electrons

Defined in a number of ways:

Laser differential efficiency ηηd is defined as the ratio of the increase in thephoton output for a given increase in the number of injected electrons:

ηηd =Approximateexpression

dPe

dI.(Eg)

where dPe is the change in the optical power emitted

from the device, dI is the change in input current and Eg

is the bandgap energy.

Total laser efficiency ηηt is defined as (with approximate expression):

ηηt =total number of output photons

total number of injected electrons

Pe

I.(Eg)≈≈

Source: Master 4_3



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Laser Characteristics: Threshold

Spontaneousemission regime

Stimulatedemission regime

Light output

Injection current

Laser threshold current

Saturation

All Semiconductor laser diodes have alight current characteristic, with a definedthreshold current.

Below the threshold spontaneousemission dominates

Beyond the threshold, where stimulatedemission dominates, the differentialquantum efficiency increase dramatically.

The threshold current by convention isthe intercept on the current axis of a linedrawn along the characteristic, as shown

Source: Master 4_3



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Temperature Dependence (II)

In general the threshold current density Jth temperature dependence is :

Light output versus input current characteristic atvarious temperatures for an InGaAsP laser

0 10 20 30 40 50 60 70 80

10 mW

7.5 mW

5 mW

2.5 mW

0 mW

DC current(mA)

10 20 30 40 50 60

Laser temperature indegrees C

LightOutput

Jth is proportional to expT

To

To is about 120 to 190 degrees K for AlGaAs devices,

InGaAsP devices have a stronger dependence with To values of 40 to 75 degrees K

Source: Master 4_3



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Temperature DependenceProblem

1.The threshold for an InGaAsP laser diode is measured and is found tobe 31 mA and 34 mA for a device temperatures of 20 °C and 25 °Crespectively.

2.Show clearly how the above information can be used to derive anapproximate value for the characteristic temperature of the laser.

3. If this laser diode is used in system which drives the laser with aconstant current of 50 mA, what is the maximum device temperaturepermissible if the laser is to operate above threshold?

Source: Master 4_3



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Temperature DependenceSolution (I)

Source: Master 4_3

Solution: In general the threshold current density J th temperature dependence is given by:

Jth = A exp (T/To)

where A is a constant. Assuming that the distribution of current within the laser is not stronglytemperature dependent then the laser threshold (I

th) temperature dependence can be approximated by:

Ith = B exp (T/To)

where B is some constant. Assuming that at two temperatures T1 and T2 the laser threshold currents are

I1 and I 2 respectively then:

[ ]ln

I

I

T T

T o

1

2

1 2

=

−



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Temperature DependenceSolution (II)

Source: Master 4_3

Based on the measurements provided the value of To, the characteristic temperature is 54.1 °K. If thedevice is to lase at 50 mA, then the threshold must be less than 50 mA. If the maximum temperature at

which lasing will occur is Tx then (temperatures in °K) :

[ ]o

x

T

T

mA

mA 298

34

50ln

−=

Substituting for To and solving for Tx gives Tx = 318.8 °K or 45.8 °C.



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Laser Diode Optical Spectrum

Laser diodes generally display multiple longitudinal modes (multimode)Gain guided lasers are multimode at all drive currents levels

With index guided lasers several modes exist near threshold, but as current increasesone or two modes dominate.

True singlemode lasers have only one mode

Index guidedlaser diode

Sharp LT022

Gain guidedlaser diode

Sharp LT023

Source: Master 4_3



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Single Frequency Lasers

Demand for ultra-narrow, so called single frequency, laser diodes is increasing for anumber of reasons, including low dispersion and frequency division multiplexing.

One of the most popular types is the Distributed Feedback Laser (DFB).

Instead of feedback from the cleaved ends of the laser, an internal diffraction gratingis fabricated within the laser, the period of which sets the operatingfrequency/wavelength. Linewidths of 10-50 MHz have been demonstrated.

Multisection lasers have been developed which are tunable by electrical bias.

Distributed FeedbackLaser diode

Bias Tuningof a

MultisectionDFB

Source: Master 4_3



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Laser Modulation Bandwidth

All semiconductor laser diodes exhibit a so-called relaxation oscillation

Current pulse injected into the laser produces an optical output pulse exhibitingrelaxation oscillation

Relaxation oscillation can be seen as a resonance frequency for the interchange ofenergy between photons and carriers

Relaxation oscillation normally sets the limit on the modulation frequency of the laser

(0.5 to 10 GHz)

time

Current pulse input to laser

time

Optical pulse, with relaxation oscillation

Source: Master 4_3


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