Femtosecond laser direct writing of non-reciprocal structures in magneto-optical glasses
thesisposted on 28.03.2022, 20:48 by Qiang Liu
Femtosecond laser direct writing is a fast developing technique for photonic integration. As this technique has matured, various optical devices have been incorporated into a range of photonic materials. However, an optical isolator, which is the indispensable component to mitigate unwanted back reflection in any platform, has not been fabricated by using the femtosecond laser direct write (FLDW) technique so far. As a step towards an integrated FLDW isolator, this thesis presents a systematic investigation for construction of non-reciprocal structures based on the Faraday effect by using the FLDW technique. Two femtosecond laser systems, operating at kHz and MHz repetition rates respectively, were used to fabricate functional waveguides into two types of magneto-optical glasses. In the MHz regime, we obtained relatively low-loss waveguides but the waveguide modes were relatively large and elliptical and likely to lead to coupling issues and birefringence. The confinement of waveguide modes was improved by using superposition ring structures. In the kHz regime, the created waveguides had more symmetric modes but their propagation loss was high due to associated photodarkening. Different amounts of photodarkening were induced in the two types of magneto-optical glasses tested by laser exposure, and we attributed the difference to distinct concentrations of La and Ga, which likely played a major role in suppressing photodarkening in the glass. The induced photodarkening could be reversed via photo- or thermal annealing, and we reduced the propagation loss of waveguides fabricated in the kHz regime by thermal annealing. One annealed waveguide created in the kHz regime was chosen as the candidate for examination of the Faraday effect. About 9° Faraday rotation was observed in the 11 mm long waveguide. The competition between Faraday effect and laser induced birefringence was theoretically analyzed and the calculation was confimed by experimental results. The estimated waveguide birefringence was on a scale of 10-6. The results included in this thesis expand the capability of integrated optical isolator for the FLDW platform. Meanwhile, investigations of glass properties and photodarkening developed the understanding of the response of magneto-optical materials to laser exposure and the relative inuential elements. The outcomes may benefit manufacture or applications of magneto-optical glasses. The study of competition between Faraday rotation and birefringence is a subtle topic and the results provide a reference and a possible approach to analyze the birefringence induced by femtosecond laser inscription.