Steven J. M. Habraken

Geometric phases in astigmatic optical modes of arbitrary order

The transverse spatial structure of a paraxial beam of light is fully characterized by a set of parameters that vary only slowly under free propagation. They specify bosonic ladder operators that connect modes of different orders, in analogy to the ladder operators connecting harmonic-oscillator wave functions. The parameter spaces underlying sets of higher-order modes are isomorphic to the parameter space of the ladder operators. We study the geometry of this space and the geometric phase that arises from it. This phase constitutes the ultimate generalization of the Gouy phase in paraxial wave optics. It reduces to the ordinary Gouy phase and the geometric phase of nonastigmatic optical modes with orbital angular momentum in limiting cases. We briefly discuss the well-known analogy between geometric phases and the Aharonov–Bohm effect, which provides some complementary insights into the geometric nature and origin of the generalized Gouy phase shift. Our method also applies to the quantum-mechanical desc...

Rotational stabilization and destabilization of an optical cavity

We investigate the effects of rotation about the axis of an astigmatic two-mirror cavity on its optical properties. This simple geometry constitutes an optical system that can be destabilized and, more surprisingly, stabilized by rotation. As such, it has some similarity with both the Paul trap and the gyroscope. We illustrate the effects of rotational (de)stabilization of a cavity in terms of the spatial structure and orbital angular momentum of its modes.

Orbital angular momentum in twisted and rotating cavity modes

We use an algebraic method to derive explicit expressions for the structure of paraxial modes in a cavity consisting of astigmatic mirrors. The algebra is based upon the use of ladder operators that raise or lower the mode indices, when acting on a mode function. We show that the method is also applicable when the mirrors perform a uniform rotation about their axes. We also find expressions for the orbital angular momentum in these modes.

Universal description of geometric phases in higher-order optical modes bearing orbital angular momentum.

We study geometric phases that arise from (cyclic) transformations of the transverse spatial structure of paraxial optical modes. Our approach involves bosonic ladder operators that, in the spirit of the quantum-mechanical harmonic oscillator, generate sets of transverse optical modes. It applies to modes of all orders in a very natural way and provides a universal geometric interpretation of the phase shifts acquired by nonastigmatic modes under typical experimental conditions.

Rotationally induced vortices in optical cavity modes

We show that vortices appear in the modes of an astigmatic optical cavity when it is put into rotation about its optical axis. We study the properties of these vortices and discuss numerical results for a specific realization of such a set-up. Our method is exact up to first order in the time-dependent paraxial approximation and involves bosonic ladder operators in the spirit of the quantum-mechanical harmonic oscillator.

Structure of cavity modes with general astigmatism

The modes in an optical cavity between two astigmatic mirrors have a twisted structure when the mirror axes are not aligned. We use operator techniques to obtain a full characterization of these modes. The method is exact in the paraxial limit. The structure of the modes is completely determined by the geometry of the resonator. This geometry is given by the separation between the mirrors, their radii of curvature, and the relative orientation of their symmetry axes. The fundamental mode has elliptical Gaussian intensity profiles and the intersections of a nodal plane with a transverse plane normal to the axis can be ellipses or hyperbolae. The symmetry axes of the intensity curves and the nodal curves are not aligned. At the mirrors, the higher-order modes have a Hermite-Gaussian structure. Their analytical form can be generated from the fundamental mode by using raising operators that generalize the operators that are known in the description of the quantum harmonic oscillator. In the interior region of the resonator, admixture of Laguerre-Gaussian structures can arise, resulting in vortices.

Geometric phases in higher-order transverse optical modes

We study the geometric origin of generalized Gouy phases in paraxial optical modes of arbitrary order. We focus on the specific case of cyclic beam transformations of non-astigmatic vortex beams, thereby, generalizing the well-known geometric phase shift for first-order beams with orbital angular momentum to modes of arbitrary order. Our method involves two pairs of bosonic ladder operators, which, analogous to the algebraic description of the quantum-mechanical harmonic oscillator in two dimensions, connect transverse modes of different order. Rather than studying the geometry of the infinite-dimensional space of higher-order modes, we focus on the space underlying the ladder operators. We identify overall phases of the ladder operators, thereby obtaining the phases of all higher-order modes, and show that the variation of these phases under optical elements and transformations has a geometric interpretation in terms of the other parameters involved.

Stability properties of a rotating astigmatic optical cavity

We study the effects of rotation on the stability properties of an astigmatic two-mirror cavity. We show that rotation can both stabilize and destabilize a cavity and investigate the effects of such a rotationally-induced transition on the spatial structure and the orbital angular momentum of the cavity modes. Our method relies on the connection between ray and wave optics and is exact within the time-dependent paraxial approximation.

Orbital angular momentum of twisted cavity modes

Paraxial modes of a two mirror cavity can have a very rich spatial structure. We study this structure and its physical properties by using purely algebraic techniques. This approach allows for general astigmatism, which arises if the two mirrors are astigmatic and non-aligned.