314 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 3, MARCH 2000 Multifunctional Grating Couplers for Bidirectional Incoupling into Planar Waveguides Johan Backlund, Jörgen Bengtsson, Carl-Fredrik Carlström, and Anders Larsson, Member, IEEE Abstract—A novel type of grating coupler for coupling light from free space into a planar waveguide was designed. Compared to a conventional grating coupler which can couple light into one main direction, this new design enables simultaneous coupling into two opposite main directions. This leads to increased flexibility in integrated optics design and may also increase the overall cou- pling efficiency. We demonstrate experimentally devices for bidi- rectional incoupling with additional beam splitting and focusing capabilities. Index Terms—Couplers, holographic gratings, integrated optics, optical fiber connecting, waveguide components. I. INTRODUCTION I N MANY photonic applications the coupling of light between a waveguide and free space is critical. For instance in fiber communications research much effort is devoted to finding simpler solutions for coupling light from a fiber into a waveguide or vice versa. Solutions based on grating couplers [1]–[4] are attractive since they are compatible with standard microfabrication techniques and reduce the need for external microoptics. Recently it was shown that in addition to incou- pling, grating couplers can implement beam splitting functions [5]. Thus the need for waveguide components such as splitters and lenses can be reduced. Although these grating couplers offer high functionality their performance is limited by the fact that the incoupled light only propagates in directions confined within a narrow angular re- gion. If some of the light should be launched into the opposite direction this requires the use of, e.g., waveguide bends or wave- guide mirrors to redirect the incoupled light. Here, we report on a novel design for grating couplers that enables launching of the guided wave in two opposite directions simultaneously. The focusing and beam splitting capabilities can also be incor- porated in the design, see Fig. 1. Thus, the coupler offers in- creased flexibility in the design of complex integrated optical circuits. The added functionality of the device comes at no addi- tional cost in terms of fabrication complexity. Further, the bidi- rectional coupling can also increase the overall coupling effi- ciency, i.e., more light can be coupled into the waveguide. That is so since for monodirectional coupling all the incoupled light Manuscript received September 13, 1999; revised November 10, 1999. This work was supported by the Council of the Foundation for Strategic Research (SSF). J. Backlund, J. Bengtsson, and A. Larsson are with the Microtechnology Center at Chalmers and Photonics Laboratory, Department of Microelectronics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden. C.-F. Carlström is with the Laboratory of Semiconductor Materials, Depart- ment of Electronics, Royal Institute of Technology, SE-16440 Kista, Sweden. Publisher Item Identifier S 1041-1135(00)01985-6. Fig. 1. Multifunctional bidirectional incoupling using a grating coupler. The figure shows the experimental setup. exits the grating region, say, as a forward guided wave in Fig. 2. Thus the light incident on the left part of the coupler has propa- gated a longer distance within the grating and is thereby largely outcoupled by the grating into free space. However, for bidirec- tional coupling the light incident on the left is coupled mainly as a backward guided wave and therefore propagates a shorter dis- tance within the grating thus suffering lower outcoupling losses. In the following, we present the design of these bidirectional grating couplers and show experimental results for InP-waveg- uides for 1.55- m wavelength. II. DESIGN METHOD A bidirectional grating coupler is a grating structure etched into the surface of a planar waveguide. In the design the grating structure is divided into cells (here 256 256 cells), each cell containing a number of grating lines with a constant period, , equal for all cells. However, compared to a neighboring cell the grating lines may be dislocated by a distance as shown in Fig. 2. This dislocation imposes a phase modulation of the in- coupled light that enables the bidirectional multifunctional in- coupling. In our previous work, we studied multifunctional but monodi- rectional incoupling in what we will now refer to as the back- ward direction (negative -direction) [5]. The waveguide is as- sumed to be excited by infinitesimal emitters whose amplitude and phase are determined by the locally incident wave in the position of the emitter as depicted in Fig. 2. The phase also de- pends on the position (the -coordinate) of the emitter and the amount of grating line dislocation, , in that position. In all, an emitter in position in the waveguide transmitting in the backward direction was found to have a phase (1) 1041–1135/00$10.00 © 2000 IEEE