Achromatic waveguide couplers

70-nm-bandwidth achromatic waveguide coupler

“…Different configurations to increase the spectral bandwidth of planar waveguide couplers have been proposed. Spaulding and Morris 11,12 suggested two different approaches to the problem: 1a2 a surfacerelief transmission grating on top of a prism coupler and 1b2 a grating coupler combined with a transmission grating tilted at a specific angle. The reported experimental full-widths at half-maximum 1FWHM's2 were 41 nm and 13 nm for prism-grating and doublegrating couplers, respectively.…”

“…In this paper we introduce a different approach to the design of achromatic couplers. Instead of working with coupling angles, [11][12][13][14] we describe the achromatization problem as an effective-index-matching process between waveguide and coupler, which is directly related to coupling efficiency. 16 The effective-index mismatch function is defined and expanded into a Taylor series in which the coefficients are defined as chromatic orders.…”

“…1, which combines two grat-ings and a prism, has simplicity and sufficient degrees of freedom. The two gratings are parallel to the waveguide, avoiding the critical alignment required in previous designs, [11][12][13][14] and can be fabricated on a single slide. As we show below, the gratings are designed with slightly different spatial frequencies to function as an equivalent long-period grating, and their linear wavelength dependence is used to correct the first chromatic order.…”

“…From an improved design we were able to extend the input-coupling spectral bandwidth much beyond that of previously reported experiments. [12][13][14][15]…”

“…In Table 1 the expressions for each limiting case are given, where we have used Db1l2 ; 12p@l2F1l2. Unlike previous studies [11][12][13][14] that used the incident coupling angle, here there exists a straightforward connection between the design function F1l2 and the variable h1l2 that is experimentally determined.…”