TECHNOLOGY AREA(S): Electronics,
OBJECTIVE: To create a monolithic, chip-scale mid-wave infrared photonic circuit that emits an order of magnitude more single mode average power than single emitters. Lasers should be directly coupled to the beam combining components either on the same chip or by effectively creating one “super-chip” containing the lasers and the beam combiner
DESCRIPTION: The coherent beam combining of semiconductor lasers has been pursued thru a number of methods. Major efforts have been pursued at near infrared diode laser wavelengths near 1 micron, with some success. Examples include the evanescent super-mode concept recently attempted for mid-wave infrared quantum cascade lasers which built upon prior work with shorter wavelength lasers. Although this approach may still be viable to a certain level, it is complex. More straight forward approaches are seen that leverage advances in low loss integrated photonics whereby lasers can be combined coherently from individual lasers spaced at any given degree as dictated most likely by thermal management consideration. Thus, the beam combining can leverage designs and processes developed over the past decade for the very best high power single mode mid infrared lasers to create a combined single mode output of ten times or more continuous wave power (from continuous wave input lasers). Research is progressing on these methods in the near infrared, but should now be investigated for U. S. Army needs at longer wavelengths. Much improved SWaP (Size, Weight, and Power) systems can be envisioned for a number of relevant applications. Other approaches to direct diode near-IR beam combined lasers rely upon the spatial multiplexing of broad-area diode lasers through beam shaping optics. These approaches cannot provide the desired high brightness (or high coherence) since at best they can achieve the beam quality of a single high-power broad-area diode laser, which has a M2 value of ~10 or higher. In addition, these systems are usually realized by use of free space and/or fiber optical components, not suitable for chip-scale integration. However, it is well known that photonic integrated circuits (PICs) can significantly reduce the SWaP of many optical and laser systems and are under large scale research and development for use in telecommunications and data centers. Thus, the aim is to hereby use PICs to replace the bulk optics approaches in the current beam combining systems by encompassing recent advances in coupling from lasers to integrated waveguides and by use of low-loss silicon nitride or other very low loss integrated photonics materials that could significantly reduce the system SWaP and improve the beam quality. The potential research topic includes creating a PIC-based beam combining architecture, improving the coupling between semiconductor lasers and PICs, and increasing the power handling capability of PICs. Thermal management of closely spaced mid-infrared lasers including some kind of cooling may be a concern for studying the ultimate limits of such chip-scale approaches but would probably not be too challenging for a significant order of magnitude improvement over a single laser emitter. Another consideration for one of the key applications would be achieving high modulation speed. Therefore, once the beam combining has been established one would also like to pursue the high speed modulation of such an array. Other considerations such as distributed feedback cavities for improved linewidth and modulation performance and coherence lengths may also be pursued.
PHASE I: Conduct research, theoretical analysis, and numerical studies on PIC based beam combining systems for high power single mode mid-wave infrared semiconductor lasers (3-5 micron wavelengths), develop measures of expected performance, and document results in a final report. The phase I effort should investigate specific PIC based laser beam combining system architectures and include modeling and simulation results supporting performance claims. The proposed beam combining system should use coherent beam combining and leverage state-of-the-art semiconductor lasers (average power and wall-plug efficiency of at least 1 W and 15% or more) at the chosen wavelengths. Simulations should show capability to scale the coherent combining of at least 10 lasers on chip of about 1 cm2 with close to 90% combining efficiency.
PHASE II: 1) Demonstrate PIC based beam combining of multiple (10 or more) 3-5 micron single mode semiconductor lasers with high beam quality (near diffraction limited) and experimental combining efficiency >85%; 2) Demonstrate high coupling efficiency (>90%) between the lasers and the beam combining PIC; 3) Explore the power scaling limits and power handling capability of the integrated beam combining systems. The data, reports, and tested hardware will be delivered to the government upon the completion of the phase II effort. 4) Begin studies of high-speed modulation of such beam combined arrays. Modulation speeds of at least 1 Gb/s are requested as the phase II goal at the 10 W power level.
PHASE III: Further scaling of power level and modulation speed can be pursued in the phase III. Applications of such mid-wave infrared lasers can be pursued for various military and civilian applications. Free-space laser communications systems can be developed and tested, and narrow linewidth distributed feedback or external cavity systems may be pursued for coherent lidar or other uses. Directed energy countermeasures type of applications would also be of interest. Industrial applications for material processing and fabrication may be desirable depending upon the power scaling potential. Scaling to much high power levels by using 100s of lasers (possibly with multiple PIC stacks) can be investigated experimentally.
REFERENCES: 1: S. Li and D. Botez, "Analysis of 2-D Surface-Emitting ROW-DFB Semiconductor Lasers for High-Power Single-Mode Operation", IEEE Journal of Quantum Electronics, Vol. 43, No. 8, August 2007.KEYWORDS: Coherent Beam Combining, Mid-infrared Lasers, Photonic Integrated Circuits, Free-space Laser Communications
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