Mid-infrared and far-infrared lasers
Infrared spectroscopy has been used for this purpose for many years, although industrial systems are typically not sensitive enough to detect trace amounts. A properly designed infrared laser system could be used as a decoy, or, preferably, to confuse or damage an incoming missile attack.
Features of the mid infrared and far infrared lasers:
- For practical communication systems in the mid-infrared wavelength region the requirement is for semiconductor or fiber lasers which are capable of operating at, or close to, room temperature
- Semiconductor materials with direct band gaps which encompass both the mid-infrared and far-infrared (8 to 12 μm) wavelength range include many of the III–V, II–VI and IV–VI alloys.
- Injection lasers operating in this longer wavelength region are to increased carrier losses over devices emitting at wavelengths up to 1.6 μm which result from nonradiative recombination via the Auger interaction
- The recombination energy of the injected carriers is dissipated as thermal energy to the remaining free carriers by this process.
- Moreover, the probability of the occurrence of such a process increases as the bandgap of the semiconductor is reduced.
- In addition, optical losses due to free carrier absorption are also greater because of their dependence on the square of the wavelength.
- Both of these effects present more problems in the mid-infrared wavelength range and they exhibit increased importance at higher temperatures as a result of the higher concentration of free carriers.
- They therefore play a major role in the determination of the injection laser threshold current and efficiency, as well as providing a limit to the maximum operating temperature of the device.
- The total current required to provide the injection laser threshold is greater than the amount attributable only to radiative recombination by the addition of an Auger current.
- The Auger current depends upon the precise electronic band structure of the material, and often consists of contributions from a number of different Auger transitions; it is generally large for materials with band gaps which provide longer wavelength emission.
- In this context the results of calculations for threshold current and internal quantum efficiency for several long-wavelength semiconductor alloys are displayed in Figure 1
Quantum cascade lasers:
- A fiber laser produced for operation around the 3 μm wavelength point is the diode cladding- pumped erbium praseodymium–doped fluoride device
- This laser is capable of producing very high output power of more than 1 W, it comprises expensive double-clad fluoride fiber which is difficult to cleave with consistently high optical quality over the entire cross-section of the pump fiber cladding.
- a new technique based on inter sub-band transitions has resulted in a device known as a quantum cascade (QC) laser which has been successfully demonstrated for mid-infrared emission.
- In principle this technique provides for emission of an optical signal across the full wavelength range of the mid- and far-infrared regions (i.e. 2 to 12 μm) since the emitted wavelength is determined by quantum mechanical band structure engineering.
- The QC laser is a layered semiconductor device comprising a series of coupled quantum wells grown on GaAs or InP substrates