- A highly efficient intracavity coherent addition of nine individual laser distributions is presented. It is achieved with two passive interferometric combiners that are introduced into the combined laser cavity. The results reveal that the combined output power is greater by almost a factor of 9 compared to that of the single laser distributions, while the beam quality is the same. © 2006 American Institute of Physics. The potential for high output power concomitantly with good beam quality has led to the development of many meth-ods for phase locking and the coherent addition of lasers over the past several decades. These include evanescent waves coupling, 1,2 Talbot and Fourier transform resonators, 3,4 active feedback control of both cw and ultrafast lasers, 5–7 the introduction of diffractive components and phase elements into the laser resonators, 8–11 Vernier-Michelson resonators, 12,13 and the incorporation of fiber couplers. 14,15 In general two major difficulties must be over-come when performing coherent addition. The first results from the need for proper phase locking which requires very accurate relative alignment between the lasers. The second, and somewhat related difficulty, results from the need to accurately control the relative phase so as to obtain construc-tive interference between the laser distributions. In the past, we demonstrated an approach for efficient coherent addition of two Gaussian laser distributions, using relatively simple intracavity interferometric combiners. 16 The ability to scale this approach in a compact manner to coherently add multiple beam distributions is of great prac-tical importance. Here we extend this approach, to coher-ently add larger arrays of laser distributions, using somewhat more complicated interferometric combiners. Specifically, we first extend the approach in order to obtain efficient co-herent addition using a single sequential interferometric combiner that is designed to deal simultaneously with three laser distributions. This is followed by the coherent addition of nine laser distributions, using two such interferometric combiners that are orthogonally oriented inside the combined laser cavity. Essentially, similar combiners as those intro-duced here can be used to coherently add larger arrays 4 4, 5 5, etc. using only two interferometric combiners, thus enabling compact laser configurations with high output power. The single-substrate interferometric combiner and how it coherently adds three parallel laser beams along a line, is presented in Fig. 1. The interferometric combiner is formed of a high precision plane parallel plate, with specially de-signed coatings. Specifically, a third of the front surface is coated with an antireflection layer, a third with a 50% beam splitter layer, and the remaining third with a 66% reflec-tance beam splitter layer. Two thirds of the rear surface are coated with a highly reflecting layer and the remaining third is coated with an antireflection layer. As shown in Fig. 1, beam 1 enters the interferometric combiner, reflected from the backsurface and intercepts beam 2 at the 50% beam split-ter layer. The reflected part of beam 1 interferes construc-tively with the transmitted part of beam 2, while the remain-ing parts of the two beams interfere destructively towards the direction of loss channel 1a. The combined beam, composed of reflected beam 1 and transmitted beam 2, propagates to-wards the backsurface, where it is reflected and intercepts beam 3 at the 66% beam splitter layer. The reflected part of the combined beam interferes constructively with the trans-mitted part of beam 3, while the remaining parts of the two beams interfere destructively towards the direction of loss channel 2a. The overall combined beam, hence, composed of all three beams, emerges from the output interface of the interferometric combiner. With two such interferometric combiners, oriented orthogonally with respect to each other, a two-dimensional array of nine laser distributions can be coherently added. As is well known, phase locking and coherent addition self-occurs due to the inherent ability of the combined laser resonator to select the mode of operation with minimal losses, but only if all the individual laser distributions have some common longitudinal modes. In general, as the number of laser distributions that must be coherently added in-creases, the probability for obtaining common longitudinal modes may rapidly decrease. 12 Such a decrease is alleviated with our interferometric combiners, whose constant thick-ness ensures that the path difference between sequential laser distributions is always the same.