Optoelectronic Semiconductor Devices Principals and Characteristics

3.5 DIFFERENCES OF ILD FROM GAS, LIQUID DYE AND 
SOLID STATE LASERS 
• 
[1]Lasing transition in ILDs are between the valence and conduction bands of semiconductors 
rather then between the discrete energy levels of isolated atoms or ions in solid-state host 
materials. 
• 
[2]The energy diagram of semiconductors depends on the orientation with respect to the crystal 
axis. The energy levels in gases or liquids are independent of direction or orientation. 
• 
[3]Direct, high-speed amplitude modulation of ILDs is possible by controlling the current injected 
into the semiconductor junction. For other lasers, direct amplitude modulation is extremely 
difficult. 
• 
[4]The overall size of ILDs, including the active medium and the optical cavity, is very small in 
comparison with other lasers. Because ILDs are small, the output is not as well confined spatially 
or spectrally, as in other lasers that are larger. 
Diode lasers are distinguished from these other types primarily by their ability to be pumped directly by 
an electrical current. This results in much more efficient operation. Overall power conversion efficiencies 
of ~50% are not uncommon for a diode laser. But for gas and solid state lasers, which generally pumped 
by plasma excitations or an incoherent optical flash lamp source respectively, efficiencies on the order of 
1% are common. 
Because of their longer cavities and more narrow gain bandwidth, gas and solid state lasers also tend to 
have more coherent outputs than simple semiconductor lasers. However, more advanced and 
sophisticated single frequency diode lasers can have comparable line widths in the low megahertz range. 
Another difference between semiconductor and other lasers is the net size. Gas and solid state lasers are 
typically tens of centimeters in length, diode laser chips generally about the size of the grain of salt and 
increased to the order of a cubic centimeter mostly for the purpose of handling (mounting and packaging). 
Diode lasers, due to their high reliability or useful lifetime, find a widespread use in such an applications 
as fiber-optic communication systems. 
The useful life of gas or solid state lasers is typically measured in thousands of hours, diode lasers - 
hundreds of years. 
The characteristics of semiconductor injection lasers may be described by six parameters: 
- radiation 
wavelength; 
- threshold current density; 
- external quantum efficiency; 
- power 
efficiency; 
- far-field radiation pattern; 
- spectral 
width. 

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