Spectrally Combinable, Instability-Mitigating Lasers

Current high-energy laser (HEL) demonstrations through the OSD-funded HELSI program proved the viability of 250kW-class HELs.  Both spectral beam combination (SBS) and coherent beam combination (CBC) were validated, although the power-on-target metric (power-in-the-bucket, PIB) for the CBC system became degraded with increasing powers.  Even if this problem is solved, SBC holds more promise since the beams are perfectly overlapped, incurring no PIB penalty for the spatial tiling that is incurred in CBC systems.  For this reason, SBC systems having been demonstrated at counter-ICBM powers are viewed as the HEL weapon of choice. Interestingly, the requirements for the laser sources that drive both SBC and CBC systems have been determined to be nearly identical.  The most critical of these is the spectral linewidth, which should be = 15GHz.  Because of the need for absolute phase control in CBC systems and absolute wavelength control in SBC systems, it has always been held that the same laser sources would be used for both CBC and SBC systems, namely Master-Oscillator/Power-Amplifier (MOPA) systems.  In a MOPA system, the wavelength and phase are controlled by a low power oscillator, and the power is attained through the use of a chain of amplifiers to reach the requisite kW power levels.  However, MOPA systems are unnecessarily complex for SBC systems due to the strict phase control requirements.  This creates not only unnecessary failure modes but also unnecessary increases in SWaP.  The latter is particularly important in attaining HELs for airborne platforms. Both CBC and SBC require wavelength locking to = 1 GHz and a total spectral envelope of = 15 GHz.  However, within the spectral envelope, CBC requires full coherence whereas SBC does not.  Moreover, CBC required strict polarization control in order to maintain coherent interference conditions, whereas SBC has no such requirement.  It is recognition of these difference where the opportunity lies to differentiate sources for SBC vs. CBC to enable significant reduction of both SWaP and failure modes. In the conceptualization of a new type of laser source, two nonlinear instabilities must be addressed: stimulated Brillouin scattering (SBS) and thermal mode instability (TMI).  While TMI reduced the power on target through a dynamic beam instability, SBS can catastrophically destroy the laser system. In order to address both SWaP and the instabilities described above, RAM has developed a concept for a new laser source that is specifically designed for use in SBC systems.  The new laser source is significantly smaller and simpler than current MOPA systems, the latter leading to a reduced set of failure modes, and inherently mitigates both SBS and TMI passively.  Moreover, the failure modes are reduced by a factor or 4, and the catastrophic failure modes are reduced by a factor of 10.