Optical waveguides are far more than mere connecting elements in integrated optical systems and circuits. Benefiting from their high optical confinement and miniaturized footprints, waveguide structures established based on crystalline materials, particularly, are opening exciting possibilities and opportunities in photonic chips by facilitating their on-chip integration with different functionalities and highly compact photonic circuits. Femtosecond-laser-direct writing (FsLDW), as a true three-dimensional (3D) micromachining and microfabrication technology, allows rapid prototyping of on-demand waveguide geometries inside transparent materials via localized material modification. The success of FsLDW lies not only in its unsurpassed aptitude for realizing 3D devices but also in its remarkable material-independence that enables cross-platform solutions. This review emphasizes FsLDW fabrication of waveguide structures with 3D layouts in dielectric crystals. Their functionalities as passive and active photonic devices are also demonstrated and discussed.

Femtosecond laser direct writing of flexibly configured waveguide geometries in optical crystals: fabrication and application

We propose a TE/TM-pass polarizer based on a shallow X-cut lithium niobate-on-insulator (LNOI) ridge waveguide. In shallow X-cut LNOI ridge waveguides, the leaky mode is interchanged between TE and TM polarization waves depending on the ridge waveguide height. The phenomenon does not occur in silicon-on-insulator ridge waveguides. We use this special loss mechanism for TE/TM-pass polarizers. We evaluate structural and wavelength dependencies of polarizers by a vectorial finite-element method. The extinction ratios of 108 dB/mm and 27 dB/mm are obtained for TE-pass and TM-pass polarizers.

C-3-24 Proposal of TE/TM-pass Polarizer Based on Lithium Niobate on Insulator Ridge Waveguide

Rare‐earth (RE) ion doped crystalline materials have a wide spectrum of applications in lasers, amplifiers, sensors, as well as classical and quantum information processing. The incorporation of RE ions into integrated photonics holds great promise for enriching the designer's toolbox with a view to addressing key performance features not available in existing photonic integration platforms. RE‐ion‐doped thin‐film LiNbO3 (also called RE‐ion‐doped lithium‐niobate‐on‐insulator, RE:LNOI), which inherits nearly all the material advantages as well as nanophotonic integration from the LNOI technology, meets the urgent demands for chip‐integrated laser sources, optical amplifiers, and quantum memories based on LNOI photonics. In this article, a timely review is provided on the development of RE:LNOI photonics in terms of ion‐doping techniques, chip‐integrated lasers, and amplifiers, as well as low‐temperature optical characterizations for quantum photonics. To conclude, some well‐noted topics that may shape the future directions in lithium–niobate integrated photonics are discussed.

Integrated Photonics Based on Rare‐Earth Ion‐Doped Thin‐Film Lithium Niobate

We report a novel mode modulator based on a KTN crystal waveguide produced by femtosecond laser (fs-laser) writing. The KTN waveguide is a typical dual-line structure that depends on the decrease of refractive index at the focus area. The propagation loss of the waveguide along the TE polarization is as low as ~0.9 dB/cm at 632.8 nm. The investigation of confocal micro-Raman spectra reveals that the microstructure of the waveguide region has no obvious change during the femtosecond laser direct writing (FsLDW) procedure. Under the applied direct current (DC) electric field, the waveguide modes in the guiding structure can be modulated, resulting in mode conversion from TE00 to TE02. Both the experimental results and the theoretical simulation indicate that the mode modulation phenomenon originates from the space-charge-field induced refractive-index increment between the waveguide core and the bulk substrate. The simplicity and remarkable performance of the demonstrated device paves the way for integrating low-cost and high-efficient electric-optical modulator on photonics circuits.

A waveguide mode modulator based on femtosecond laser direct writing in KTN crystals

Numerical study of an ultra-broadband, wide-angle, polarization-insensitive absorber in visible and infrared region

Enhanced Raman intensity in ZnS planar and channel waveguide structures via carbon ion implantation

Here we report the fabrication of a magneto-optic waveguide based on TGG crystal via 15 MeV C3+ ion irradiation. The ion irradiation process leads to the optical anisotropy in the as-irradiated TGG waveguide, which hinders the magneto-optical rotation in the waveguide. To remove the irradiation-induced optical anisotropy, we annealed the as-irradiated TGG waveguide under different conditions. After annealing at 400 °C for one hour, the magneto-optical rotation of 14° per centimeter is observed in the waveguide at the wavelength of 632.8 nm, under the magnetic field of 0.24 T, which is comparable to that observed in the TGG crystal under the same magnetic field. This work paves the way for applications of TGG waveguides as integrated optical rotators and isolators.

Ion irradiated magneto-optic waveguide based on TGG crystal

We report on the fabrication of evanescently-coupled one-dimensional waveguide laser arrays in Nd:YAG crystals by employing femtosecond-laser direct writing (FsLDW). The far-field discrete diffraction patterns of fabricated waveguide arrays under a passive regime can be observed at the active regime. By adjusting the incoupling condition, diffraction patterns with different laser intensity distribution can be realized, which is also supported by our simulation. Results in this study suggest promising applications of such an active arrayed device for complex integrated photonic circuits.

Yicun Yao

Papers

A waveguide mode modulator based on femtosecond laser direct writing in KTN crystals

Enhanced Raman intensity in ZnS planar and channel waveguide structures via carbon ion implantation

Ion irradiated magneto-optic waveguide based on TGG crystal

Femtosecond laser direct writing of evanescently-coupled planar waveguide laser arrays

Lattice structures and optical properties of Ce:LYSO crystal waveguides with implanted C, Si and Cu ions