Integrated Flat‐Top Beam Shaper Using Subwavelength Engineered Critical Guiding

Flat‐top optical beams with uniform intensity profiles have potential for applications in microscopy, laser machining, and laser welding. Previously reported methods for flat‐top beam shaping rely on free‐space optics or fiber‐based systems, which are bulky and sensitive to vibrations or misalignment. In contrast, the use of optical waveguides can offer a more compact and robust approach to flat‐top beam shaping. In this work, a novel design of an integrated flat‐top beam shaper (FTBS) is proposed and experimentally demonstrated, wherein subwavelength gratings (SWGs) are utilized to realize the critical guiding of light. The SWGs are employed to engineer the index profile of a triple‐segment waveguide structure to access the critical point. The fabricated FTBSs can generate flat‐top beams with small intensity variations of 0.066 and 0.084 over 10 and 20 µm beam dimensions, within compact device lengths of 100 and 200 µm, respectively. A 1 × 6 power splitter is then realized using the proposed FTBS, demonstrating a loss of 0.7 dB and power uniformity of 0.6 dB at the operating wavelength.