Based on the ray transformation matrix formalism, we propose a simple method for the identification of the dynamic and geometric parts of the Gouy phase, acquired by an appropriate Gaussian-type beam while propagating through a first-order optical system. © 2008 Optical Society of America OCIS codes: 070.2575, 070.2580, 070.2590, 070.3185, 070.4690, 080.2468 A transversal mode with a Gaussian envelope, undergoing a cycle of transformations while propagating through a paraxial optical system, accumulates Gouy phase, usually divided into two parts: A dynamic part and a geometric part [1][2][3][4]. The identification of the Gouy phase is important in resonator theory [5], in optical trapping [6], and in possible applications of its geometric part for quantum computation [7]. The physical origin of the geometric phase, its calculation in parametric space [2,3], and its experimental measurement [4] have been discussed in a series of publications.In this Letter we propose a simple method for the determination of the Gouy phase-and in particular its dynamic and geometric parts-accumulated by an appropriate Gaussian-type mode during its propagation through a first-order optical system. It is based on the analysis of the eigenvalues of the ray transformation matrix associated with the first-order optical system.Strictly speaking, a coherent beam of light ⌿͑r͒, propagating through an optical system described by an operator R, accumulates a phase shift only if it is an eigenfunction of R with a unimodular eigenvalue exp͑i͒: R͓⌿͑r i ͔͒͑r o ͒ = exp͑i͒⌿͑r o ͒. Therefore we need to identify a system where phase accumulation is possible and determine the corresponding beams.Beam propagation through a lossless first-order optical system is described by the canonical integral transformation R T [8,9], whose kernel is parameterized by the real symplectic ray transformation matrix T, which relates the (properly normalized) position r = ͑x , y͒ t and direction p = ͑p x , p y ͒ t of an incoming ray to those of the outgoing ray. Using the modified Iwasawa decomposition [10] the (normalized) ray transformation matrix T can be written as a product of three symplectic matrices,where the first matrix T L represents a lens described by the symmetric matrix G =−͑CA t + DB t ͒͑AA t + BB t ͒ −1 , the second matrix T S corresponds to a scaler described by the positive definite symmetric matrix S = ͑AA t + BB t ͒ 1/2 , and the third matrix T O is orthogonal and can be described by the unitary matrix U = X + iY = ͑AA t + BB t ͒ −1/2 ͑A + iB͒. A simple example of Gouy phase accumulation is the propagation of the well-known HermiteGaussian (HG) modes H m,n ͑r͒ = H m ͑x͒H n ͑y͒, where H n ͑x͒ =2