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IV. Laser Diode (LD) or Semiconductor Laser

Operation Mechanism

Characteristics of LD

LD Design (1): control of electronic properties

LD Design (2): control of optical properties

Advanced LD Structures

Applications of LD

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Introduction to the Semiconductor Laser

LASERLightAmplification byStimulatedEmission ofRadiation

The Laser is a source of highly directional, monochromatic, coherent light.

The Laser operates under a stimulated emissionprocess.

The semiconductor laser differs from other lasers (solid, gas, and liquid lasers):

small size (typical on the order of 0.1 0.1 0.3 mm3) high efficiency

the laser output is easily modulated at high frequency by controlling the junction current

low or medium power (as compared with ruby or CO2 laser, but is comparable to theHe-Ne laser)

particularly suitable for fiber optic communication

Important applications of the semiconductor lasers: optical-fiber communication, video recording, optical reading, high-speed laser printing.

high-resolution gas spectroscopy, atmospheric pollution monitoring.

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From LED to LD: Improvement by an Optical Cavity

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Comparison between an LD and LED

Laser Diode Stimulated radiation

narrow linewidth

coherent higher output power

a threshold device

strong temperature dependence

higher coupling efficiency to a fiber

LED Spontaneous radiation

broad spectral

incoherent lower output power

no threshold current

weak temperature dependence

lower coupling efficiency

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Stimulated Emission

Stimulation emission

The two basic requirements for a stimulated emission process to occur:

(1) providing an optical resonant cavity to build up a large enough photon field

a very large photon field energy density (12) will enhance the stimulated emission

over spontaneous emission

(2) obtaining population inversion condition

under the population inversion condition (n2 > n1) the stimulated emission is todominate over absorption of photons from the radiation field

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Optical Resonant Cavity

Optical resonant cavity

parallel reflecting mirrors to reflect thephotons back and forth, allowing thephoton energy density to build up.

The Fabry-Perot faces (cavity)

The reflecting ends of the laser cavity The gain in photons per pass between

the Fabry-Perotfaces must larger thanthe losses (such as the transmissionat the ends, scattering from impuritiesabsorption, and others)

In the semiconductor laser, optical resonant

cavity is made by cleaving. Cleave the oriented sample (GaAs)

along a crystal plane (110), letting thecrystal structure itself provide theparallel faces.

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Resonant modes of a laser cavity

Longitudinal modes

determine the output-light wavelength

Lateral modes leading to subpeaks on the sides of the

fundamental modes, and resulting inkinks in the output-current curve.

suppressed by the stripe-geometry

structure

Transverse modes generating hot spots

suppressed by thin active layer design

Suppressing lateral and transverse modeis necessary to improve the performance

of lasers. Single-mode laser: the laser operates in the

fundamental transverse and lateral modes butwith several longitudinal modes.

Single-frequency laser: the laser operates inonly one longitudinal mode.

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Longitudinal modes of a laser cavity

For stimulated emission, the length L ofthe cavity must satisfy the condition (forresonant):

m [ 0/ 2n] = L or m 0 = 2 n L m is an integral number and is the

refraction index in the semiconductorcorresponding to the wavelength 0 (n isgenerally a function of0)

The separation 0 between the allowedmodes in the longitudinal direction is

Since dn/d0 is very small,002 / 2Ln (form = 1)

For typical GaAs laser of0 = 0.94 nm,n = 3.6 and L = 300 m, 0 = 4 .

md

nd

nLno

1

0

0

2

01

2

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Population Inversion (1)

Forward biasing a p-n junction formedbetween degenerate semiconductorsunderhigh-injection condition.

Population inversion appears about thetransition region

The condition necessary for populationinversion is (EFC - EFV) > Egwhere EFC, andEFV are the quasi-Fermilevels

In the figure shows then energy diagramsof a degenerate p-n junction

(a) at thermal equilibrium(b) under forward bias

(c) under high-injection condition

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Population Inversion (2)

(a) incoherent (spontaneous) emission

EFC - EFV > h > Eg(b) laser modes at threshold

There modes correspond tosuccessive numbers

of integral half-wavelengths fitted withinthe cavity

(c) dominant laser mode above threshold

h= Eg

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Carrier and Optical Confinement

Carrier and Optical Confinement can beobtained by using the heterostructuredesign in the LD

Carrier Confinement reduce the threshold current density laser can operate continuously at room

temperature Optical Confinement

confinement factor : the ratio of thelight intensity within the active layer to thesum of light intensity both and outside theactive layer

=1 - exp ( - C n d )n :the difference in the reflective index

d:the thickness of the active layer the larger the n and d are, the higher

the will be Optical confinement provides effective

wave-guide for optical communication

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Homojunction and Heterojunction Laser

Homojunction Laser

pulse mode output

large threshold current density

operated at low temperature

broad spectral width of output light

Improvement Heterojunction Laser

Heterojunction Laser

(1) Single-Heterojunction Laser (SH Laser)

(2) Double-Heterojunction Laser (DH Laser)

(3) Stripe-geometry DH Laser

(4) Single quantum well (SQW) Laser(5) Multiple quantum well (MQW) Laser

(6) Strained layer superlattice (SLS) structure

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Double-Heterojunction (DH) Laser

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Threshold Current Density

Gain (g) the incremental optical energy flux per unit

length

Threshold Gain the gain satifies the condition that a light

wave makes a complete traveral of thecavity without attenuation

is the confinement factor, is the loss perunit length, L is the length of the cavity, R isthe reflectance of the ends of the cavity

Threshold Current Density(J th) the minimum current density required for

lasing to occur

To reduce Jth, we can increase , , L, Rand reduce d,

RLg

dJ

dJJth

1ln

1

0

0

0

Rn

Lg

11

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Characteristics of the DH laser

Threshold current density vs. activelayer thickness

The threshold current density decreaseswith decreasing d, reaches a minimum,and then increases. The increase ofJth atvery narrow active thickness is causedby poor optical confinement.

Output power vs. diode current

The light-current characteristics is quitelinear above threshold.

Temperature dependence

The threshold current increases exponentiallywith temperature Jth ~ exp [ T/T0]

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Emission Spectra of the typical DH laser

Emission spectra of a perfect laser

above the threshold, the laser mayapproach near-perfect monochromaticemission with a spectra width in theorder of 1 to 10 .

High-resolution emission spectra(of a typical stripe-geometry DH laser)

Sub-peaks, which are evenly spacedwith a separation of= 7.5 , appearin the spectra. belong to the longitudinalmodes.

Because of these longitudinal modes,the stripe geometry laser is not aspectrally pure light source for opticalcommunication.

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Design considerations for laser diode performance

Low threshold current

low threshold can be generated by electronic devices which can be modulated at highspeed to provide a high speed modulation in the output

(1) reducing the active layer thickness (d)

Quantum-Well (~ 50 - 100 ), Strain Quantum-Well(2) N-doped active region

(3) Stripe geometry

Lateral confinement

to avoid the kink effect, which produces noise in the optical transmitter

reduce the lateral dimension of the Fabry-Perot cavity

(1) Stripe geometry (Gain-guided cavity)

(2) Buried heterostructures

Selective Optical Cavity to reduce the laser linewidth

(1) Distributed Feedback (DFB) structures

(2) Buried heterostructures

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Stripe Geometry Laser

Using the gain-guided cavityto carry out thelateral confinement

Advantages of a stripe geometry structure Removing side peaks from the main modes by

suppression of the lateral mode.

Reducing the threshold current

less stringent demands on fabrication (becauseof the smaller active volume and the greaterprotection offered by isolating the active regionfrom an open surface along two sides)

Fundamental mode operation is valid for all stripewidths below 10 - 15 m.

Different types of stripe-geometry structure: oxide stripe

implantation

selective diffusion

Mesa stripe

buried heterostructures

ridge structures

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Single Frequency Laser

Single frequency lasers is desirable in the opticalfiber communication system to increase thebandwidth of an optical signal.

This is because light pulses of differentfrequencies travel through optical fiber atdifferent speeds thus causing pulse spread.

Dispersion mechanisms for a step-index fiber:(1) intermodal dispersion(2) waveguide dispersion(3) material dispersion

Dispersion effects can be minimized by usinglong wavelength sources of narrow spectralwidth (a single frequency laser) in conjunctionwith single mode fibers.

Methods to achieve the single frequency lasers:

(1) Frequency Selective Feedback

External Grating, Distributed-Feedback

(DFB), Distributed Bragg Reflector (DBR)

(2) Coupled Cavity

Cleaved Coupled Cavity (C3) laser

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Distributed Feedback (DFB) Laser

In periodic structures, special effects occur whenthe wavelength of the wave approaches thewavelength of the periodic structure. Insemiconductor crystals, this leads to bandgapsand Bragg reflections.

The wavelength selectiveperiodic gratingwith acorrugated structure, made by E-beam lithographyand RIE, is incorporated into to the laser.

The period of the grating is d= 2qB /2nwhere B is the Bragg wavelength give by

where 0 is the oscillating wavelength DFB lasers have been made with sawed end

facets or with antireflection coating to suppressthe Fabry-Perot modes.

The DFB laser main advantage is its very smalltemperature dependence.

nL

mB

B

2

21 2

0

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Distributed Bragg Reflector (DBR) Laser

In the DBR laser, the period reflecting mirrorstack is placed outside the active lasing region.

The advantages of the DBR lasers:

high coupling efficiency between theactive lasing region and the passive

waveduide structures. the wavelength of the output light istunable.

The reflective index of the stack is alterable bycurrent injection.

The wavelengths that get the highest feedbackmust satisfy

B = 2 q (nr1 d1 + nr2 d2)where is a positive integer

The values ofnr1 d1 and nr2 d2 can be alteredelectronically, therefore can have a certaindegree of wavelength tunability

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Cleaved-Coupled-Cavity (C3) Laser

The C3 laser consists of two standard Fabry-Perot cavity laser diodes which are self-aligned and very closely coupled to form atwo-cavity resonator.

Because the laser light has to travel throughan additional cavity (modulator), the onlyradiation that is reinforced is at a wavelengththat resonates both in the lasers cavity and

also in the modulator.

The two cavities can have their currentscontrolled independently and this is the mainadvantage of the C3 laser.

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Quantum Well Laser

If the thickness of the active region Ly ismade small enough (Ly~ the de Brogliewavelength= h/p < 500, depending onthe materials for GaAs, Ly ~ 20 nm), thecarriers are confined in a finite potential wellin which the energy band splitting into a

staircase of discrete levels (the quantizationeffect)

E-h recombination can only occur withn = 0 transition in the quantum well.

In a quantum well (QW), a large numberelectrons all of the same energy canrecombine with a similar block of holes.

Hence, a QW laser should gives a muchnarrower output wavelength, unlike the otherlasers with the bulk effect, whererecombining carriers are distributed in energyover a parabolically varying density of states

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Multiple Quantum Well (MQW) Laser

Several single quantum wells are coupled into amultiple quantum well (MQW) structure.

The significantly reduced temperature sensitivityof MQW lasers has been related to the staircasedensity of states distribution and the distributedelectron and photon distributions of the activeregion.

This optical confinement helps to contain theotherwise large losses from a narrow activeregion, leading to low threshold currents.

An MQW is the active region of a laser that canemit a single frequency at several differentwavelengths, known as a multiple array gratingintegrated cavity (MAGIC) laser.

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Graded Index Separate Confinement Heterostructure(GRINSCH) Laser

GRadedINdexSeparate ConfinementHeterostructure (GRINSCH) Laser

A narrower carrier confinement region (d) ofhigh recombination is separated from a wideroptical waveguide region

Optical confinement can be optimizedwithout affecting the carrier confinement

GRINSCH-SQWand GRINSCH-MQW

The threshold current for a GRINSCH is

much lower than that of a DH laser

For a standard DH laser, both mirror and

absorption losses increase rapid for thin active

region, leading to very high threshold current.

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GRINSCH Laser

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Vertical Cavity Surface Emitting Laser (VCSEL)

The structure of an VCSEL is very much like astandard heterojunction LED.

Advantages of the VCSEL: the possibility of single frequency operation due

to the short cavity

the removal of the fragile cleavage process that

creates the end mirrors in a standard laser.

The success of the VCSEL depends onincorporating high reflectivity mirrors in thestructures

The incorporations of DBR and MQWstructures highly improve the performance ofthe VCSEL.

Various DBRs in the VCSEL: crystalline BRD

amorphous DBR stacks

MgF/ZnSe DBR