3D Laser Writing of Low‐Loss Cross‐Section‐Variable Type‐I Optical Waveguide Passive/Active Integrated Devices in Single Crystals

Abstract Optical waveguides fabricated in single crystals offer crucial passive/active optical components for photonic integrated circuits. Single crystals possess inherent advantages over their amorphous counterpart, such as lower optical losses in visible‐to‐mid‐infrared band, larger peak emission cross‐section, higher doping concentration. However, the writing of Type‐I positive refractive index modified waveguides in single crystals using femtosecond laser technology presents significant challenges. Herein, this work introduces a novel femtosecond laser direct writing technique that combines slit‐shaping with an immersion oil objective to fabricate low‐loss Type‐I waveguides in single crystals. This approach allows for precise control of waveguide shape, size, mode‐field, and refractive index distribution, with a spatial resolution as high as 700 nm and a high positive refractive index variation on the order of 10 −2 , introducing new degrees of freedom to design and fabricate passive/active optical waveguide devices. As a proof‐of‐concept, this work successfully produces a 7 mm‐long circular‐shaped gain waveguide (≈10 µm in diameter) in an Er 3+ ‐doped YAG single crystal, exhibiting a propagation loss as low as 0.23 dB cm −1 , a net gain of ≈3 dB and a polarization‐insensitive character. The newly‐developed technique is theoretically applicable to arbitrary single crystals, holding promising potential for various applications in integrated optics, optical communication, and photonic quantum circuits.