High-Power Tapered Direct-Diode MOPA Arrary with MPLC Coherent Beam Combining

We propose a master oscillator power amplifier concept that employs a single master oscillator distributed to multiple power amplifiers with intermediate phase and amplitude modulators to produce a 2-D array of laser beams with controlled phase and intensity relationships. This allows for the coherent beam combining using a free space multiplane light conversion approach to achieve a multi-kW laser beam with high beam quality (M2 < 1.5). We plan to fabricate an array of tapered semiconductor optical amplifiers integrated with input intensity and phase modulators using a 1xN waveguide star coupler. These will be implemented on a strained InGaAs/GaAs MQW laser structure that is optimized for highest optical gain at 980 nm. We will employ controlled MQW disordering to achieve a slightly larger bandgap of the MQW in the modulator sections and either employ more extensive disordering of the MQW layer or hybrid silicon nitride layer for the passive waveguide sections and the 1xN integrated coupler. The semiconductor chips will be stacked and a single DFB laser diode will be distributed to the individual integrated SOA bar using a fiber 1xN coupler. The 2-D array of coherent output beams will be then combined using a free-space multiplane light conversion technique. Because we have the freedom to control the phase and amplitude of every single beam in the matrix of laser beams, it will be possible to achieve a very high combination efficiency while minimizing optical beam distortions to produce a high brightness single mode laser beam. This method of coherent beam combining will effectively mitigate any beam array spacing irregularity caused by “smile”, as long as the irregularity is static or predictable.       We will analyze the heat flow dynamics of the SOA arrays and use both conventional microchannel liquid cooling of the heatsinks as well as an integrated aerosol spray cooling delivered through an array of micro-jets positioned directly above the heat dissipating surface. This will dictate the fill factor of the SOA arrays. Traditionally most commercial tapered SOAs employ gain guiding in the tapered amplifying section. While this make fabrication much simpler and the exact taper angle is less critical, there a penalty in the attainable best wall-plug efficiency.  As long as the electrical contact metal on top of the active region is wide enough to cover the whole width of the Gaussian–like mode, the whole beam is amplified. However, the very nature of a Gaussian beam is that it does not really have a finite width, as will be discussed in more details in the proposal. In contrast, the index guided tapered SOAs potentially yield a significant improvement on the wall-plug efficiency.   We have assembled a strong team with outstanding expertise in semiconductor optoelectronic device integration, optical system integration, as well as technology transfer and commercialization.