1. 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

2. Introduction to the Semiconductor Laser
LASER — Light Amplification by Stimulated Emission of Radiation
The Laser is a source of highly directional, monochromatic, coherent light.
The Laser operates under a “stimulated emission” process.
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 the He-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.

3. From LED to LD: Improvement by an Optical Cavity

4. 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

5. 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 to dominate over absorption of photons from the radiation field

6. Optical Resonant Cavity
parallel reflecting mirrors to reflect the photons back and forth, allowing the photon 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-Perot faces must larger than the losses (such as the transmission at the ends, scattering from impurities absorption, and others)
In the semiconductor laser, optical resonant cavity is made by cleaving.
Cleave the oriented sample (GaAs) along a crystal plane (110), letting the crystal structure itself provide the parallel faces.

7. 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 in “kinks” 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 mode is necessary to improve the performance of lasers.
Single-mode laser: the laser operates in the fundamental transverse and lateral modes but with several longitudinal modes.
Single-frequency laser: the laser operates in only one longitudinal mode.

8. Longitudinal modes of a laser cavity
For stimulated emission, the length L of the cavity must satisfy the condition (for resonant): m [ 0 / 2n ] = L or m 0 = 2 n L
m is an integral number and  is the refraction index in the semiconductor corresponding to the wavelength 0 (n is generally a function of  0)
The separation 0 between the allowed modes in the longitudinal direction is
Since dn/d0 is very small,  0  02 / 2Ln (for m = 1)
For typical GaAs laser of  0 = 0.94 nm, n = 3.6 and L = 300 m,   0 = 4 Å.

9. Population Inversion (1)
Forward biasing a p-n junction formed between degenerate semiconductors under high-injection condition.
 Population inversion appears about the transition region
The condition necessary for population inversion is (EFC - EFV) > Eg where EFC, and EFV are the quasi-Fermi levels
In the figure shows then energy diagrams of a degenerate p-n junction
(a) at thermal equilibrium
(b) under forward bias
(c) under high-injection condition

10. Population Inversion (2)
(a) incoherent (spontaneous) emission
 EFC - EFV > h > Eg
(b) laser modes at threshold
 There modes correspond to successive numbers of integral half-wavelengths fitted within the cavity
(c) dominant laser mode above threshold
 h = Eg

11. Carrier and Optical Confinement
Carrier and Optical Confinement can be obtained by using the heterostructure design in the LD
Carrier Confinement
reduce the threshold current density
laser can operate continuously at room temperature
Optical Confinement
confinement factor  : the ratio of the light intensity within the active layer to the sum of light intensity both and outside the active 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

12. 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

13. Double-Heterojunction (DH) Laser

14. 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 the cavity without attenuation
 is the confinement factor,  is the loss per unit length, L is the length of the cavity, R is the reflectance of the ends of the cavity
Threshold Current Density (Jth)
the minimum current density required for lasing to occur
To reduce Jth, we can increase , , L, R and reduce d, 

15. Characteristics of the DH laser
Threshold current density vs. active layer thickness
The threshold current density decreases with decreasing d, reaches a minimum, and then increases. The increase of Jth at very narrow active thickness is caused by poor optical confinement.
Output power vs. diode current
The light-current characteristics is quite linear above threshold.
Temperature dependence
The threshold current increases exponentially with temperature  Jth ~ exp [ T/T0 ]

16. Emission Spectra of the typical DH laser
Emission spectra of a perfect laser
above the threshold, the laser may approach near-perfect monochromatic emission with a spectra width in the order of 1 to 10 Å.
High-resolution emission spectra (of a typical stripe-geometry DH laser)
Sub-peaks, which are evenly spaced with a separation of  = 7.5 Å, appear in the spectra. belong to the longitudinal modes.
Because of these longitudinal modes, the stripe geometry laser is not a spectrally pure light source for optical communication.

17. Design considerations for laser diode performance
Low threshold current
low threshold can be generated by electronic devices which can be modulated at high speed to provide a high speed modulation in the output
(1) reducing the active layer thickness (d)
↣ Quantum-Well (~ Å), 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

18. Stripe Geometry Laser
Using the “gain-guided cavity” to carry out the lateral 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 (because of the smaller active volume and the greater protection offered by isolating the active region from an open surface along two sides)
Fundamental mode operation is valid for all stripe widths below µm.
Different types of stripe-geometry structure:
oxide stripe
implantation
selective diffusion
Mesa stripe
buried heterostructures
ridge structures

19. Single Frequency Laser
Single frequency lasers is desirable in the optical fiber communication system to increase the bandwidth of an optical signal.
This is because light pulses of different frequencies travel through optical fiber at different 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 using long wavelength sources of narrow spectral width (a single frequency laser) in conjunction with 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

20. Distributed Feedback (DFB) Laser
In periodic structures, special effects occur when the wavelength of the wave approaches the wavelength of the periodic structure. In semiconductor crystals, this leads to bandgaps and Bragg reflections.
The wavelength selective periodic grating with a corrugated structure, made by E-beam lithography and RIE, is incorporated into to the laser.
The period of the grating is d = 2qB /2n where 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 suppress the Fabry-Perot modes.
The DFB laser’ main advantage is its very small temperature dependence.

21. Distributed Bragg Reflector (DBR) Laser
In the DBR laser, the period reflecting mirror stack is placed outside the active lasing region.
The advantages of the DBR lasers:
high coupling efficiency between the active lasing region and the passive waveduide structures.
the wavelength of the output light is tunable.
The reflective index of the stack is alterable by current injection.
The wavelengths that get the highest feedback must satisfy B = 2 q (nr1 d1 + nr2 d2) where is a positive integer
The values of nr1 d1 and nr2 d2 can be altered electronically, therefore can have a certain degree of wavelength tunability

22. 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 a two-cavity resonator.
Because the laser light has to travel through an additional cavity (modulator), the only radiation that is reinforced is at a wavelength that resonates both in the laser’s cavity and also in the modulator.
The two cavities can have their currents controlled independently and this is the main advantage of the C3 laser.

23. Quantum Well Laser
If the thickness of the active region Ly is made small enough (Ly ~ the “de Broglie wavelength”  = h/p < 500Å, depending on the materials for GaAs, Ly ~ 20 nm), the carriers are confined in a finite potential well in which the energy band splitting into a “staircase” of discrete levels (the quantization effect)
E-h recombination can only occur with “n = 0 transition” in the quantum well.
In a quantum well (QW), a large number electrons all of the same energy can recombine with a similar block of holes.
Hence, a QW laser should gives a much narrower output wavelength, unlike the other lasers with the bulk effect, where recombining carriers are distributed in energy over a parabolically varying density of states

24. Multiple Quantum Well (MQW) Laser
Several single quantum wells are coupled into a “multiple quantum well (MQW)” structure.
The significantly reduced temperature sensitivity of MQW lasers has been related to the staircase density of states distribution and the distributed electron and photon distributions of the active region.
This optical confinement helps to contain the otherwise large losses from a narrow active region, leading to low threshold currents.
An MQW is the active region of a laser that can emit a single frequency at several different wavelengths, known as a multiple array grating integrated cavity (MAGIC) laser.

25. Graded Index Separate Confinement Heterostructure (GRINSCH) Laser
A narrower carrier confinement region (d) of high recombination is separated from a wider optical waveguide region
Optical confinement can be optimized without affecting the carrier confinement
GRINSCH-SQW and 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.

26. GRINSCH Laser

27. Vertical Cavity Surface Emitting Laser (VCSEL)
The structure of an VCSEL is very much like a standard 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 on incorporating high reflectivity mirrors in the structures
The incorporations of DBR and MQW structures highly improve the performance of the VCSEL.
Various DBRs in the VCSEL:
crystalline BRD
amorphous DBR stacks
MgF/ZnSe DBR