Refractive Index Profile Determination of Graded-index (GRIN) Waveguides from Near-Field Measurements

Knowledge of the refractive index profile (RIP) of graded-index (GRIN) waveguides can yield important characteristics necessary for photonic device design involving ion-exchanged waveguides in glass, proton-exchanged guides in LiNbO3, or other GRIN fabrication techniques. Thus, it is very important to establish an efficient, non-destructive and accurate method to determine the RIP. In the literature, many such methods have been proposed [1, 2, 3] one of which includes the well-established inverse method of RIP reconstruction from near-field measurements of the fundamental mode intensity distribution [4, 5]. Profiles for planar and channel guides have been determined by measuring the near-field intensity with infrared vidicon tubes that need to be corrected for their non-linear response. In this paper, we propose a more accurate approach using a CCD camera to image the near-field pattern and a frame grabber to capture it pixel-by-pixel. Once this pattern is measured, a simple numerical solution of the Helmholtz equation is carried out to determine the RIP from its digitized near-field image.