1. 3PNT – Advanced Laser Structures and Optical Communications (SJS)
Advanced Laser Structures and Optical Communications Dr Stephen Sweeney Lecture 2 : Advanced Laser Structures
3PNT – Advanced Laser Structures and Optical Communications (SJS)

2. Next… Advanced Laser Structures
Primarily application driven:
Low cost (e.g. VCSELs)
High efficiency (e.g. Quantum Dot Lasers, Quantum Cascade Lasers)
Low maintenance (e.g. thermally stable)
Single-mode and tuneable lasing (e.g. DFB,DBR,VCSEL)
High power (e.g. tapered lasers etc.)
Many of these requirements come from optical communications (see later…)
3PNT – Advanced Laser Structures and Optical Communications (SJS)

3. Quantum Dot Lasers - Motivation
bulk - 3D 0D 1D 2D 3D E r
Q Well- 2D E r
Q Wire-1D E r
QD - 0D
Y.Arakawa, H.Sakaki, “Multidimensional quantum well laser and temperature dependence of its threshold current,” Appl.Phys.Lett., v.40(11), 1982,
3PNT – Advanced Laser Structures and Optical Communications (SJS)

4. Quantum Dot Lasers – Device Performance
Decrease in Jth due to carrier “thermalisation”
“Self-assembled” Quantum Dots
Radiative current behaves ideally and low Jth values achieved
BUT
To remains low around RT (limited by Auger processes)
Remains the subject of ongoing research…
3PNT – Advanced Laser Structures and Optical Communications (SJS)

5. Single-Mode (SM) lasers
Many applications require lasers to emit in a single mode. Examples include optical communications (WDM), sources for spectroscopy, frequency sources for timing
Standard FP lasers emit several modes so are not generally suitable
Two methods for achieving SM emission:
Make  very big (e.g. as in VCSELs, microcavity lasers)
Make the reflectivity / threshold gain a strong function of  (e.g. as in DFB lasers)
3PNT – Advanced Laser Structures and Optical Communications (SJS)

6. Single-Mode (SM) lasers
Many applications require lasers to emit in a single mode. Examples include optical communications (WDM), sources for spectroscopy, frequency sources for timing
Standard FP lasers emit several modes so are not generally suitable
Two methods for achieving SM emission:
Make  very big (e.g. as in VCSELs, microcavity lasers)
Make the reflectivity / threshold gain a strong function of  (e.g. as in DFB lasers)
3PNT – Advanced Laser Structures and Optical Communications (SJS)

7. Single-Mode (SM) lasers
The output from a standard FP laser is highly multi-moded.
From before, in a FP cavity,  is given by:
Hence, as L decreases,  increases
FWHM=22nm
Typical FP laser, Lcav=0.5mm, =3.6
Hence, at a wavelength of ~1.3 m
~0.5nm
Full Width Half Maximum (FWHM) of laser gain typically supports:
20 / 0.5 = 40 modes !
Q. How short would the cavity need to be to support only one mode?
3PNT – Advanced Laser Structures and Optical Communications (SJS)

8. Single-Mode (SM) lasers - Microdisks
Q. How short would the cavity need to be to support only one mode?
A. <10m (difficult to achieve in practice)
D = 2m
Solutions…
1. Microdisk “Thumb-tack” lasers
Q. Determine the standing wave condition for a microdisk laser (“whispering gallery” modes [c.f. St. Paul’s Cathedral…]) D
Micro “Stone Henge”
Q. For a 2m diameter microdisk with =3.6 at a wavelength of 1.3m, what is the mode spacing?
3PNT – Advanced Laser Structures and Optical Communications (SJS)

9. Single-Mode (SM) lasers - Microdisks
Pumping microdisks and getting light out…
Commonly pump using another laser (“optical pumping”
Output powers tend to be small (but very high “Q”-factor)
Light coupled in and out using a waveguide (can also be used as an all optical router)
Some recent success with electrical injection on larger microdisks
light out
light in
Source: USC
3PNT – Advanced Laser Structures and Optical Communications (SJS)
Source: DARPA
Source: ETH

10. Single-Mode (SM) lasers - VCSELs
Q. How short would the cavity need to be to support only one mode?
A. <10m (difficult to achieve in practice)
Edge-Emitting Laser (EEL)
Solutions…
2. Vertical Cavity Surface Emitting Lasers (VCSELs)
This was a revolutionary idea first realised by Kenichi Iga in 1979 and literally turned the laser on its side
Basic idea is to form cavity in the vertical (growth) direction
Light is emitted from the top surface of the device
This has numerous advantages but presents several technical challenges…
Vertical Cavity Surface-Emitting Laser (EEL)
3PNT – Advanced Laser Structures and Optical Communications (SJS)

11. Single-Mode (SM) lasers - VCSELs
EELs
VCSELs
3PNT – Advanced Laser Structures and Optical Communications (SJS)

12. Single-Mode (SM) lasers - VCSELs
This was a revolutionary idea first realised by Kenichi Iga in 1979 and literally turned the laser on its side
VCSEL Advantages
VCSEL Disadvantages
Small devices – low threshold currents
Small devices – heating is significant
On wafer testing – large cost saving in device manufacture
Requires ~100 well controlled growth steps
Easy to create several wavelength lasers from one wafer
Cavity length small, hence need v. large mirror reflectivities ~99.9% (reduces output power)
Circular beam profile – excellent for fibre coupling
Works well with GaAs, not so good with InP – a problem for telecomms wavelengths
Easy to form 2-D arrays – good for interconnects
Can be temperature stable
3PNT – Advanced Laser Structures and Optical Communications (SJS)

13. Single-Mode (SM) lasers - VCSELs
VCSEL active layer itself forms cavity in growth direction, typical thickness ~200nm
Threshold gain can therefore be very large, hence need high Reflectivities
Achieved using Distributed Bragg Reflector (DBR) mirrors either side of active region
~5m
VCSEL array
Typical VCSEL
3PNT – Advanced Laser Structures and Optical Communications (SJS)

14. 3PNT – Advanced Laser Structures and Optical Communications (SJS)
VCSEL - DBRs 1 2 t1 t2
A DBR is the simplest form of 1D Photonic Band Gap
High reflectivity mirrors achieved using multiple layer pairs
Optical thickness of layer pair equal to half wavelength:
Magnitude and sharpness of the Reflectivity spectrum depends on the number of layer pairs (p) as
s and 0 are the refractive indices of the substrate and air, respectively
NB: A typical VCSEL has
~80 DBR layer pairs…!
3PNT – Advanced Laser Structures and Optical Communications (SJS)

15. 3PNT – Advanced Laser Structures and Optical Communications (SJS)
VCSEL Design Issues
Three things need to be considered when designing a VCSEL:
Gain spectrum
Reflectivity Spectrum (DBRs)
Wavelength of Cavity Mode (usually = 1)
Cavity Mode causes apparent dip in R spectrum
Reflectivity
Light Output
Wavelength (nm)
Reflectivity
Lasing emission at Cavity Mode
Cavity Mode
Gain
3PNT – Advanced Laser Structures and Optical Communications (SJS)

16. Single-Mode (SM) lasers - VCSELs
Typical VCSEL characteristics
Stable threshold current due to “tuning”
“Roll-over” in power output due to resistive heating
3PNT – Advanced Laser Structures and Optical Communications (SJS)

17. Single-Mode (SM) lasers
Many applications require lasers to emit in a single mode. Examples include optical communications (WDM), sources for spectroscopy, frequency sources for timing
Standard FP lasers emit several modes so are not generally suitable
Two methods for achieving SM emission:
Make  very big (e.g. as in VCSELs, microcavity lasers)
Make the reflectivity / threshold gain a strong function of  (e.g. as in DFB lasers)
3PNT – Advanced Laser Structures and Optical Communications (SJS)

18. External Cavity Lasers
Emission from one laser facet is fed-back into cavity from an external grating
Angle of grating determines laser wavelength
Works reasonably well and allows wavelength tuning
Relies on moving parts and expensive to manufacture
3PNT – Advanced Laser Structures and Optical Communications (SJS)

19. Distributed Bragg Reflector (DBR) Lasers
Incorporates the grating into the edge-emitting laser structure itself
Constructive interference at Bragg Condition:
Grating pitch = 
light
Q. What grating pitch is required to produce a first order grating for a 1550nm laser with =3.4?
Semiconductor material with different refractive indices
3PNT – Advanced Laser Structures and Optical Communications (SJS)

20. Copying nature to set the colour….
This very fine structure makes the butterfly wing appear coloured. There is no pigment or colour in the wing!
1 cm
magnification
500 nm
The laser wavelength is set by the microscopic grating, fabricated in the structure. The grating ‘teeth’ are 200nm apart
3PNT – Advanced Laser Structures and Optical Communications (SJS)

21. Distributed Feedback (DFB) Lasers
Problem with DBR lasers is that the DBR regions do not produce gain and are therefore lossy
leads to high threshold currents 
Distributed Feedback Lasers (DFB) overcome this by incorporating the grating along the length of the gain region 
HR coating
AR coating
n contact
p contact
Ibias
n InP substrate
p+ InGaAs
p InP
p InGaAsP
InGaAs/InGaAsP
QW active region
light
output
3PNT – Advanced Laser Structures and Optical Communications (SJS)

22. Distributed Feedback (DFB) Lasers
Standard DFB laser gives rise to two degenerate modes
Imperfections cause one mode to dominate (in practice a /4 phase shift is introduced to select one mode)
DFB lasers are the mainstay of optical fibre telecommunication systems
Output
spectrum
Typical module performance:
> +13 dBm (20mW) on fibre
linewidth <1 MHz
Side mode suppression ratio (SMSR) ~50dB
Note log scale!
3PNT – Advanced Laser Structures and Optical Communications (SJS)

23. Manufacturing DFB/DBR gratings
DFB and DBR lasers require the fabrication of very small grating structures ~100nm scale
Achieved using either optical interference technique or electron-beam lithography
E-beam vs. Holography
Interference of UV laser beams produces pattern which can be written into semiconductor
E-beam
Holography
Feature size limited by energy of e-beam (deBroglie wavelength) therefore very flexible
Feature size limited by availability of source (UV, X-ray etc)
Slow to scan over wafer
Very fast
Subject to “match-up” problems when writing adjacent patterns
Can do complete wafer with one pattern
V. expensive equipment (>£1M)
Relatively cheap and simple
Versatile
Awkward to re-configure
semiconductor
Electron beam writes pattern directly
semiconductor
3PNT – Advanced Laser Structures and Optical Communications (SJS)

24. 3PNT – Advanced Laser Structures and Optical Communications (SJS)
Practical Systems
E-beam or Holography used to write pattern in photo-resist
E-Beam System
Photo-resist is developed leaving grating image
Holographic System
Wafer is then etched forming the grating in the semiconductor
3PNT – Advanced Laser Structures and Optical Communications (SJS)

25. 3-Section Tuneable DBR Laser
Low reflection
coating
AR coating
n contact
p contacts
light
output
Tuning Regions
QW gain region
n InP substrate
Igain
Iphase
Irear
p InP
Grating
3PNT – Advanced Laser Structures and Optical Communications (SJS)

26. DBR Reflection Response
Current injection causes a reduction in effective index (plasma/band-filling effect) and thus induces a wavelength blue-shift. The rear grating reflection response selects the longitudinal lasing mode whilst the mode wavelength can be adjusted using the phase section.
Rear (grating tuning)
Phase tuning
3PNT – Advanced Laser Structures and Optical Communications (SJS)

27. Combination of Phase and DBR Tuning
Varying phase current shifts the position of the mode boundaries. Thus the combination of DBR and phase tuning allows any lasing wavelength to be achieved whilst centering the operating point between mode boundaries.
3PNT – Advanced Laser Structures and Optical Communications (SJS)

28. Wavelength Map Longitudinal mode boundaries
Combination of DBR (horizontal) and phase (vertical) tuning plotted as a 2-D map.
3PNT – Advanced Laser Structures and Optical Communications (SJS)

29. 3PNT – Advanced Laser Structures and Optical Communications (SJS)
Digital Supermode (DS)-DBR broad-band tunable laser
Monolithic multi-section InP based DBR surface ridge laser
Minimal size, scaleable in manufacture
Industry standard epitaxy and process
Multi-element, linearly chirped and apodised front grating reflector. Multi-phase shift rear grating reflector for conventional carrier injection tuning
3PNT – Advanced Laser Structures and Optical Communications (SJS)

30. Finished Tuneable Laser Product
3PNT – Advanced Laser Structures and Optical Communications (SJS)