|
However, as protection measures and signal processing techniques advance, one can anticipate that the requirement for sophisticated laser emission in the MWIR becomes more refined. In particular, broadband emission covering a wider, continuous, spectral region will prove harder to counter than that from a few discrete wavelengths. A supercontinuum has been suggested as a possible mechanism for broadband emission. In most investigations into supercontinuum generation, the emphasis has been on producing a wide, flat spectrum covering several hundred nanometres in the visible, near and short wave infrared for stand-off spectroscopic sensing of chemical agents, atmospheric sensing or hyperspectral sensing. These supercontinua are characterised by a spectral bandwidth to pump wavelength ratio of, δλ/λp<1 for a pump wavelength λp in the visible or near infrared. In most applications, the simultaneous generation of a wide spectrum is not required; instead a tuned output suffices. This has the added benefit of improving the efficiency of the laser sensor system since wavelengths which are not required, are not generated. The problem is to understand how a limited continuum might be generated. In the context of DIRCM, the spectral requirement is to produce a controlled spectral emission which matches the 3-5μm atmospheric transmission window. In this paper, a theoretical calculation is presented which shows that a continuous spectrum spanning a few hundred nanometres in the mid infrared (δλ/λp~0.2) can be generated in a simple pump geometry from a mode-locked, ultra-short pulse train using self phase modulation (SPM). Spectral broadening centered on the CO2 absorption band at 4.26μm can be excited to produce all wavelengths for emission in band IV DIRCM. The parameters which affect the spectral output such as pulse power, interaction length, pulse duration and pulse shape are considered for the case where the pump geometry is a collimated beam propagating through a mid infrared glass characterised by a non-linear refractive index n2. The prospects for developing a suitable pump laser are also discussed, in particular, the possibility of using a modelocked QCL. |
ACCESS THE FULL ARTICLE
No SPIE Account? Create one
CITATIONS
Cited by 1 scholarly publication.
Scanning probe microscopy
Mid-IR
Directed infrared countermeasures
Quantum cascade lasers
Phase modulation
Atmospheric propagation
Fourier transforms
