Design conditions in the middle range for implementation of integrated ring resonator in LiNbO3 by direct laser writing

P.PAGANO; D. PRESTI; R.PEYTON; F.VIDELA; G.A. TORCHIA

Cardiff

Conferencia; The Semiconductor and Integrated Opto-Electronics (SIOE); 2019

Universidad de Cardiff

Fiber ring resonators have demonstrated to be useful for a variety of applications especially in the field of inertial  navigation.  As  a  typical  requirement,  these  devices  have  a  free  spectral  range  (FSR)  of  3pm  and resolution of few °/h being with a radius ranging in the centimeter scale [1][2]. Unfortunately, they are too bulky and power consuming. For integrated ring design, due to the resolution, radius dimensions must be as larger as possible around 1cm. Their main features resolution about 10°/h,  FSR of 10 pm and a quality factor about 106  [3]. Simulation software for integrated optical circuits and implementation techniques are indeed challenging. The first, are memory and time constrained, normally based on Finite Differential Time Domain (FDTD) or Beam Propagation Method (BPM). FDTD allows back-propagation, but BeamProp tool (part of RSOFT suite, BPM based) does not, however enable to simulate larger rings than FDTD (r < 200 m). As implementation technique to fabricate large rings, we  choose Femtosecond Laser Direct Writing [5] on Lithium Niobate  crystals (LiNbO3) a typical used electro optical material.  An important issue of this method is to optimize the tracing of multiple straight lines to represent waveguides with some bending degree, so in this sense it is relevant to define how the curves will be plotted by this approach. In  this  paper,  we  tried  BPM  as  a  convenient  tool  to  simulate  rings  and  analyze  preliminary  results  of propagation on waveguides fabricated whose geometry corresponds to inscribed polygons where each double track [5] segment is a side. Because of bending losses, it is important to select a suitable splice angle between tracks [6][7]. Additionally, based on simplified coherent coupling [8],[4] and in the hope of some relaxation of the splice angle condition, we believe it is possible to reduce the number of  sides. Several strategies to compute large ring resonator (over 5 mm radii) extracted from the literature will be presented in this work and then they will be compared with our design and calculations approach used in this paper.