A new method, called barrier-coupling method, for coupling light into ion-implanted waveguides is .... is placed on a motorized precision rotation stage. The.
A novel nondestructive method for the characterization of ion-implanted waveguides Andrea Guarino and Peter G¨ unter Nonlinear Optics Laboratory, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH H¨ onggerberg, CH-8093 Z¨ urich, Switzerland Compiled June 24, 2005 A new method, called barrier-coupling method, for coupling light into ion-implanted waveguides is presented in analogy to the prism-coupling method. Light is coupled by frustrated total reflection at the barrier region of decreased refractive index by properly varying the incident angle. Effective indices of guided modes are determined by the minima of the not incoupled reflected light. The method is used for the determination of the effective indices of an ion-implanted waveguide in KNbO3 . It is simpler than most other techniques, more c 2005 Optical Society of America accurate and non destructive. OCIS codes: 120.4290, 130.2790, 230.7390
Ion implantation has proven to be an easy and powerful method to fabricate optical waveguides in optical materials.1 In many important nonlinear and electrooptic crystals such as KNbO3 ,2 BiB3 O6 3 and other borate crystals,4 lasing materials (Nd:YAG5 ) this is the only available technique and it is widely used since it mostly preserves their linear and nonlinear properties. The waveguiding is ensured by the implanted light ions perturbing a small region at the end of their track inside the crystal and usually decreasing the refractive index of the material forming an optical barrier. The fundamental characterization of planar waveguides1 implies the determination of propagation constants, i.e. the modes effective indices. The most common procedure to accomplish this task is the well known prism-coupling method.6 Even though it has been widely used since its introduction, this method has a fundamental drawback: surface imperfections require a considerable pressure to be applied to the sample to achieve significant light coupling through the air gap between the prism and the waveguide. If the investigated material is fragile or has ferroelastic properties, the applied mechanical stress can lead to irreversible damages or even break the sample. We present a novel method which is also based on frustrated total internal reflection in a layer adjacent to waveguide core, but does not require any pressure to be applied, since it does not imply any additional optical element to couple the probe light into the waveguide. Therefore our method is also suitable when the prism-coupling method cannot be used due to the lack of prisms, as with materials having refractive index exceeding the one of rutile. Our method applies to ion implanted waveguides which exhibit a decrease of refractive index, as the vast majority of investigated materials of practical interest. This type of waveguides are leaky, since the optical barrier can always be partially tunneled by the guided light, even though with a proper choice of implantation param-
eters the leakage losses can be made negligible for most applications. Conversely, a beam propagating in the underlying bulk crystal will experience frustrated total internal reflection when encountering the optical barrier. We want to exploit this property to efficiently transfer light from the bulk crystal region below the implanted layer and therewith get information about the propagation constants of the guided modes. A beam entering into the crystal (of unperturbed index nc ) from the lateral surfaces with an external angle θ (see Fig 1b) will have a propagation constant along the waveguide propagation axis z (and a corresponding effective index N ) given by Snell’s law: p (1) β := kz = k0 N = k0 n2c − cos2 θ. where k0 = 2π/λ is the wavevector in vacuum and isotropy of the material was assumed. For sufficiently high values of θ, β ≥ k0 nb , i.e. the electrical field becomes evanescent in the optical barrier whose minimal refractive index is nb . Nevertheless, because of the finite size of the barrier (typically 0.25 µm for a single implantation energy), light will not be totally reflected and a fraction of it will tunnel inside the core region. The incoupled light will excite a mode when the propagation constant along the axis z of the incoming beam matches the real part of the effective index Nm of the m-th mode of the structure: β = k0 Nm .
(2)
Conversely, by using general properties of transmission and reflection coefficients, the same condition implies that every ray tunneled out of the core will be exactly phase-shifted by π with respect to the directly reflected beam. This means that the reflected light will exhibit a minimum when the incoming angle satisfies the coupling condition, allowing us to determine the effective indices of the propagating modes using (1). The simple experimental set-up needed to measure the effective indices of propagating modes using the barriercoupling method is illustrated in Fig. 1a. The polarized 1
He-Ne (633 nm)
(b)
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z
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Refractive Index n(x)
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Optical Barrier nb< n(x)
