The document discusses laser diode structures, focusing on heterojunctions and semiconductor materials for enhancing laser efficiency and characteristics. Key points include the use of double-heterojunction structures to reduce threshold currents and the role of various semiconductor materials in achieving desired wavelengths. Additionally, it covers different types of lasers like gain guided and index guided lasers, along with their operational principles and challenges.

1. 1
Laser Diode Structures
MEC

2. 2
Contents
• Heterojunctions.
• Semiconductor Materials.
• Semiconductor Injection Lasers.
• Gain guided Lasers.
• Index Guided Lasers.
• Buried Heterostructure Lasers.
• Injection Laser Characteristics.

3. 3
Heterojunctions
• Dielectric step due to different refractive
indices at either side of the junction.
• Radiation confinement to active region (walls
of an optical waveguide).
• Efficiency of containment depends on the
magnitude of the step, dictated by difference
in bandgap energies and wavelength of
radiation.
• Double-heterojunction (DH) structure - carrier
and optical confinement reduce threshold
currents (50 to 200 mA) for lasing by around
100.

4. 4
Heterojunctions
• Homojunction - single p-n junction from a
single-crystal semiconductor material.
• Radiative properties improved using
heterojunctions.
• Heterojunction - interface between two
adjoining single crystal semiconductors
with different bandgap energies.
• Heterostructure - devices fabricated with
heterojunctions.

5. 5
Heterojunctions
• Isotype (n–n or p–p) or anisotype (p–n).
• Isotype heterojunction - potential barrier
within the structure for confinement of
minority carriers to a small active region
(carrier confinement).
• Reduces carrier diffusion length and volume
within the structure where radiative
recombination take place.
• Fabrication of injection lasers and high-
radiance LEDs.

6. 6
Heterojunctions
• Isotype heterojunctions used in LEDs to
provide a transparent layer close to active
region - reduces absorption of light emitted
from structure.
• Anisotype heterojunctions - large bandgap
differences improve injection efficiency of
either electrons or holes.

7. 7
Heterojunctions

8. 8
Heterojunctions
• Applied voltage for bandgap energy of the active
layer - large number of electrons(or holes)
injected into active layer, laser oscillation
commences.
• Carriers confined to active layer by energy
barriers provided by heterojunctions placed
within diffusion length of injected carriers.
• Refractive index step at heterojunctions provides
radiation containment to active layer.

9. 9
Heterojunctions
• Active layer forms center of dielectric
waveguide, confines electroluminescence
within the region.
• Refractive index step desirable to prevent
losses due to lack of waveguiding.
• Reduce defects at interfaces - misfit
dislocations or inclusions cause nonradiative
recombination, reduce internal quantum
efficiency.
• Lattice matching needed.

10. 10
Semiconductor Materials
• Materials to lend themselves to the formation
of p–n junctions.
• Efficient electroluminescence - high
probability of radiative transitions, high
internal quantum efficiency.
• Useful emission wavelength - materials to
emit light at wavelength to be utilized with
current optical fibers and detectors.
• Direct bandgap semiconductors/indirect
bandgap semiconductors with appropriate
impurity centers.

11. 11
Semiconductor Materials

12. 12
Semiconductor Materials
• GaAs/AlGaAs DH system used for fabricating
lasers and LEDs for shorter wavelengths.
• InGaAsP/InP material system for both long
wavelength light sources and detectors.
• Ternary-nitride-based alloys have advantage
over GaAs or InP-based alloys - better index
match with silica optical fibers.
• Introducing nitrogen reduces bandgap
sufficiently, gallium nitride (GaN) for blue
lasers.

13. 13
Semiconductor Injection Lasers
• Also called Injection Laser Diodes (ILD) or
Injection Lasers.
• Stimulated emission by recombination of
injected carriers.
• Optical cavity in the crystal structure to
provide feedback of photons.

14. 14
Advantages of Semiconductor
Injection Lasers
• High radiance due to amplifying effect of
stimulated emission - milliwatts of optical
output power.
• Narrow linewidth of the order of 1 nm.
• Modulation capabilities to extend up to GHz.
• Relative temporal coherence to allow
heterodyne (coherent) detection.
• Good spatial coherence - output focused by
lens into a spot - efficient coupling.

15. 15
GaAs Homojunction Injection
Lasers

16. 16
GaAs Homojunction Injection
Lasers
• High threshold current density.
• Lack of carrier containment.
• Inefficient light sources.
• Improved carrier containment, lower
threshold current densities (around 103 A
cm−2) with heterojunction (DH) structures.
• DH injection laser fabricated from lattice-
matched III–V alloys - carrier and optical
confinement on both sides.

17. 17
GaAs/AlGaAs DH Injection Lasers
Lasing across the whole width
stripe geometry for optical containment in horizontal plane.

18. 18
Stripe Geometry DH Injection Lasers
Single transverse mode operation

19. 19
Gain Guided Lasers

20. 20
Gain Guided Lasers
Kink
Filamentary behavior within active region due to crystal defects
Lasing changes from fundamental lateral mode to higher order lateral mode.
spike

21. 21
Index Guided Lasers
• Active region
waveguide thickness
varied by growing it
over a channel or
ridge in the substrate.
• Edges of the ridge
reflect light, guides it
within active layer,
forms a waveguide.

22. 22
Optical Confinement in Index-
Guided Lasers
• Positive Index Guided – central region has
higher refractive index.
• Negative Index Guided - central region
has lower refractive index.
+-Gain guided

23. 23
Index Guided Lasers
Kink

24. 24
Buried Heterostructure Lasers
Strong index guiding along junction plane
Active volume completely buried in a material of wider band gap and lower refractive index
Optical field confined in transverse and lateral directions

25. 25
Distributed Feedback Lasers
• Approach to single-frequency operation.
• Use of distributed resonators, fabricated into
laser structure to give integrated wavelength
selectivity.
• Distributed Bragg diffraction grating provides
periodic variation in refractive index in the laser
heterostructure waveguide along the direction of
wave propagation.
• Feedback of optical energy obtained through
Bragg reflection.

26. 26
Distributed Feedback Lasers
• Distributed feedback (DFB) laser and
Distributed Bragg reflector (DBR) laser
employs distributed feedback mechanism.

27. 27
Distributed Feedback Lasers

28. 28
DFB Buried Heterostructure Laser
with a window structure

29. 29
Quarter-wavelength-shifted
Double-channel Planar DFB BH
Laser
stable, single-frequency operation, π/2 phase shift

30. 30
Injection Laser Characteristics
• Threshold current temperature dependence
T0 - threshold temperature coefficient
Jth - threshold current density

31. 31
Injection Laser Dynamic Response

32. 32
Injection Laser Noise
• Phase or frequency
noise.
• Instabilities - kinks in
light output against
current characteristic
and self-pulsation.
• Reflection of light
back into the device.
• Mode partition noise.
• Dynamic shift of peak
wavelength emitted -
frequency chirping.

33. 33
Mode Hopping
Responsible for the kinks

34. 34
Reliability
• Catastrophic degradation – result of mechanical
damage of mirror facets, leads to partial or
complete laser failure.
• Gradual degradation:
(a) defect formation in the active region.
(b) degradation of the current-confining
junctions.
- Characterized by increase in threshold current
for the laser, decrease in external quantum
efficiency.

35. 35
Fiber Coupling
butt coupling
tapered hemispherical fiber coupling
confocal lens system

36. 36
Thank You