There is currently considerable interest from the military and civil authorities in defences against man portable air defence systems. One such approach utilizes directing in-band modulated mid-infrared lasers at missile seekers to disrupt the missile tracking. The work presented here discusses some of the engineering issues associated with the development of such a laser system. The laser system is based on the MURLIN (Multi-band Research Laser INfrared). A prototype system has been developed based on a highly efficient diode laser pumped Nd: YAG co-planar folded slab laser that is frequency converted by cascaded optical parametric oscillators and an optical parametric amplifier. The water-cooled system produces up to 1.4 W of modulated power which is distributed across 3-lines in the 2-5 mm spectral region. The potential countermeasure utility of this system is enhanced by the single beam multi-line output and capability to directly modulate the pump diodes with a range of waveforms. The follow-on air-cooled version currently under development has recently achieved 8 W of modulated average power at the 1.064 mm pump wavelength. Measured beam quality is twice the diffraction limit.