Optoelectronic Semiconductor Devices Principals and Characteristics

3.2.2 
INDEX-GUIDED STRIPE GEOMETRY LASERS 
 
As we can see on the Figure 
13.
, there is a large index change in the lateral dimension. Because of this, 
lateral optical confinement is built into the laser structure. 
Example: the regions outside the stripe in the mesa isolated stripe are removed completely and the index 
for that region outside the stripe is reduced to that of air (n=1). 
The disadvantage of this particular structure is that the index difference between the stripe region and the 
surrounding region is too large and many higher-order transverse modes are exited unless the active stripe 
is very narrow. 
An alternative techniques help to get read of this disadvantage. 
 
Figure 14.: Schematic cross section of five index-guided stripe geometry ILDs. 
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TJS (transverse junction stripe) laser is quite different from other diode structures. Impurity diffusion is 
performed twice. Two heterojunctions are formed. Lateral confinement in the x direction is provided by 
the two heterojunctions and in the y direction it is provided by the two homojunctions. Current is injected 
laterally into the active region (horizontally), when in other stripe geometry laser diodes current flows 
across the active region. 
3.3 SINGLE-FREQUENCY SINGLE-MODE INJECTION 
LASERS: FABRY-PEROT LASERS 
The basic structure of the Fabry-Perot type of DH laser diodes is illustrated in Figure 
15.
 
 
Figure 15.: Basic structure of a Fabry-Perot laser diode. 
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It has two mirror facets - which are usually (110) cleaved facets - one at each end of the active layer. The 
mirror facets are parallel to each other and perpendicular to the active layer. This structure forms a Fabry-
Perot (FP) type of optical cavity for lasing, and this type of laser is called the Fabry-Perot type laser. 
The coherent light from the stimulated emission process in the active layer is emitted from the facets in 
opposite directions.