Modelling of Self Phase Modulation for Broadband DIRCM Lasers

摘要

Efficient laser emission in the medium wave infrared (MWIR) is a long established requirement for directed infrared countermeasures (DIRCM). However, until the last decade, there has not been a viable technology for the direct generation of wavelengths in the 3-5μm region and instead indirect methods using optical parametric conversion have been the subject of intense development. Several indirect methods have been developed using different pump wavelengths which represent mature laser technology. For example, 1.54μm from Er:YAG or non-linear conversion from Nd:YAG (1.064μm); 2.1μrn from Ho derived from Tm at 1.97μm, or co-doped Ho:Tm in a fibre. These approaches produce the required pump wavelength for efficient 3-5μm generation using either ZGP or OPGaAs [1], however, they are less efficient than direct generation by quantum cascade lasers (QCL). The direct conversion from electrical to optical energy in a QCL is very efficient; wall-plug efficiencies of >10%, depending on wavelength and operating temperature, are typical. High efficiency, together with the high average powers that are now commercially available suggests that the QCL is an attractive laser for DIRCM.rnHowever, 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/λ_p>1 for a pump wavelength λ_p 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.rnIn this paper, a theoretical calculation is presented which shows that a continuous spectrum spanning a few hundred nanometres in the mid infrared (δλ/λ_p～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 CO_2 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 n_2. The prospects for developing a suitable pump laser are also discussed, in particular, the possibility of using a mode-locked QCL.