Abstract
Crystalline materials are excellent substrates for the integration of compact photonic devices benefiting from the unique optical properties of these materials. The technique of direct inscription with femtosecond lasers, as an advantage over other techniques, has opened the door to the fabrication of true three-dimensional (3D) photonic devices in almost any transparent substrate. Depressed-cladding waveguides have been demonstrated to be an excellent and versatile platform for the integration of 3D photonic circuits in crystals. Here, we present the technique that we have developed to inscribe tapered depressed-cladding waveguides with a circular section for the control of the modal behavior. As a proof of concept, we have applied the technique to fabricate structures in Nd:YAG crystal that efficiently change the modal behavior from highly multimodal to monomodal, in the visible and near infrared, with reduction factors in the waveguide radius of up to 4:1. Our results are interesting for different devices such as waveguide lasers, frequency converters or connectors between external devices with different core sizes.
Highlights
One of the most astonishing properties of femtosecond lasers is their capability to induce controlled micro-modifications in the volume of transparent dielectrics, allowing the integration of photonic devices with arbitrary three-dimensional (3D) geometry in almost any transparent substrate [1,2]
Crystalline materials, are very attractive substrates for photonic device integration due to their excellent physical and optical properties that may improve the functionality of the devices but, in many cases, the inscription of waveguides cannot be done in such a direct way provided that the modification induced by the laser consists of a refractive index decrease at the focal volume
Other strategies must be followed, with the simplest one being the double-line waveguide [4], in which two parallel tracks are written with certain separation; the material stress that is produced in the central region leads to a local increase of the refractive index, forming a waveguide between the two tracks
Summary
One of the most astonishing properties of femtosecond lasers is their capability to induce controlled micro-modifications in the volume of transparent dielectrics, allowing the integration of photonic devices with arbitrary three-dimensional (3D) geometry in almost any transparent substrate [1,2]. They consist of a tubular structure produced by multiple laser damage tracks in which the refractive index has decreased, which acts as a cladding of the waveguide, and light propagates in the central undamaged region This type of waveguide shows interesting advantages: (i) the size and shape of the cladding can be engineered on demand so as to guide any wavelength within the whole transparency range of crystals, (ii) the modal behavior can be precisely controlled for the different spectral regions and (iii) the core region keeps the optical properties (spectroscopic, non-linear, etc.) of the bulk. The tapered structures presented in this manuscript may improve the performance of these lasers, both by improving the modal behavior of the laser output, as by increasing the intensity of the pump in the tapered section
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