Abstract

We propose a novel method to design a tapered waveguide structure for low loss mode conversion As the mode mismatch causes coupling loss and the loss depends on the tapered shape, it is important to design a tapering structure for various applications, such as low loss hetero-structure waveguide coupling [1] or the spot-size-converter integrated laser diode.[2] It has been very common to design a tapered structure with a simple shape, as shown in Fig. 1 conceptually, of a parabolic, linear, or exponential tapers in order to get adiabatic changes in structure, and thus, in the optical mode profile. Recently, a novel concept for mode confinement index of a waveguide is proposed by using the overlap integration of two adjacent modes in tapered waveguide structure as a single step loss (SSL) in the unit of dB. It provides valuable information on the mode profile and on the maximum optical confinement as shown in Fig.2 for the differential increment. δw (≡s• w), of a given portion s of a width w, because the waveguide width of minimum SSL. w_m is coincident with the width of best optical confinement. Note that the values of w_m are 0.291 λ and 0.505 λ for values of the index difference, $\text{Δ}\equiv \left[n_1{}^2-n_2{}^2\right]/\left[2n_1{}^2\right]$ of 0.0647 and 0 0236, respectively. As the variation of w_m are systematic in that Δ multiplied by w_in is nearly constant, the normalized frequeney V generalizes SSL curves in one as shown in Fig 3 Note that the value of minimum SSL, V_m is 1 12 always even for other values of s. Now, it is possible to design optimized taper by choosing s at each step of taper width with constant SSL. Comparison of several taper structures with the optimized taper are shown in Fig.4 as an example for the case of w_in < w_m < w_out. The calculated coupling loss of the optimized structure is improved to be about half of other taper's. Proposed designing method is expected to play an important role in general optimization process of coupling or junction of hetero-structure waveguides in near future.

© 2000 IEEE