Optoelectronic Semiconductor Devices Principals and Characteristics

 
Figure 17.: Methods for controlling longitudinal modes of ILDs 
[1]
 

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The discrimination of longitudinal modes is much more difficult. In order to restrict or select longitudinal 
modes, three basic schemes have been devised, (Figure 
17.
). 
• 
[1]Geometry Control - since the longitudinal mode separation is inversely proportional to the 
cavity length d (Formula (3)) only one longitudinal mode can fit within the gain bandwidth if d is 
sufficiently small. 
o 
short cavity laser. 
• 
[2]Frequency-Selective Feedback. In the usual Fabry-Perot cavities, broadband reflectors are used 
and the frequency selectivity is mainly due to the large mirror separation. Alternative - use of the 
gratings with a large number of grooves. For such gratings, the reflectivity is frequency 
dependent. 
o 
external grating laser; 
o 
distributed Bragg reflector (DBR) laser; 
o 
distributed feedback laser. 
• 
[3]Coupled Cavities. Instead of having a single optical cavity, a laser diode can have two coupled 
cavities with slightly different cavity lengths. If one set is tuned such that the two wavelength sets 
have one wavelength in common, the laser diode will lase at that particular wavelength. 
o 
external mirror cavity; 
o 
grooved coupled cavity; 
o 
cleaved coupled cavity; 
o 
integrated etalon interference structure. 
Table 5.: Evolution of laser diode structure and geometries. 
[1]
I. Layer structures
 
1.Homojunctions [by diffusion or liquid phase epitaxial (LPE) 
techniques]
Example: 
n–GaAs/p–GaAs/GaAs
2. Single heterojunctions (by LPE techniques)
Example: 
n–GaAs/p–GaAs/Al
x
Ga
1-x
As 
3. Double heterostructions (by various epitaxial methods)
Example: 
n–Al
x
Ga
1-x
As/GaAs/p–Al
x
Ga
1-x
As 
4.Quantum wells