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The negative thermo-optic coefficient of water has been used to fabricate an athermal Bragg grating device. Glucose was used to modify the refractive index of water, allowing this response to be tuned to the desired operating temperature range. Over a 108C temperature range the Bragg response was found to vary by 3.7 pm, an improvement of nearly two orders of magnitude over the unetched Bragg grating. The low cost of fabrication compared to traditional athermal Bragg devices makes them ideal for incorporation within integrated photonic networks.Introduction: Complex, narrow band optical components, such as filters, can be an important part of multiple-wavelength telecommunications systems. There are already many forms of optical filters in use, often utilising the Bragg effect for effective wavelength filtering. Such devices can be fabricated through direct UV writing of Bragg gratings into photosensitised silica [1]. In this way Bragg gratings can be written directly into both optical fibre and planar waveguide systems. One difficulty in the use of these gratings is the inherent sensitivity of the Bragg wavelength (l B ) to variations in temperature (and strain). This is observed as the shift of the central Bragg wavelength of the grating with changes in the local temperature. The thermo-optic response (dl B /dT) for a UV written planar Bragg grating is measured to be 11.1 pm/8C at a wavelength of 1550 nm [2].A method of making an athermal Bragg grating is to control the temperature of the grating with an active stabilisation system; however, this is costly to implement and requires electrical power. A second approach used in fibre involves creating a negative expansion via a hybrid mounting arrangement in which materials with dissimilar positive expansion coefficients create a reduction in the length of the Bragg grating as temperature increases [3]. Such devices have several undesirable properties, these include the fabrication of a reliable union with the fibre and the fabrication costs associated with the mechanical assembly and adjustment of such devices. Such systems have also been found to show hysteresis [4], degrading the performance under repeated thermal cycling. Furthermore, the requirement that the grating be suspended within such a device can make it incompatible to applications requiring tolerance to mechanical shock and vibration. Instead of creating a hybrid mounting, it is possible to use composite materials to achieve an athermal response [5]. A fourth method of incorporating negative expansion is to provide a substrate for mounting the Bragg grating that is fabricated from a material with an intrinsic negative coefficient of expansion. An example of such a material is the beta-eucryptite solid solution of cerammed lithium aluminosilicate [6]. However, such an approach is less well suited to integrated photonics owing to difficulties in overcoming the substrate expansion.In this Letter we demonstrate an alternate method of achieving an athermal response by using an evanescent interaction into ...
In this work, the generation of optical vortices in an optical integrated circuit is numerically demonstrated. The optical vortices with topological charge m = ±1 are obtained by the coherent superposition of the first order modes present in a waveguide with a rectangular cross section, where the phase delay between these two propagating modes is Δφ = ±π/2. The optical integrated circuit consists of an input waveguide continued with a y-splitter. The left and the right arms of the splitter form two coupling regions K1 and K2 with a multimode output waveguide. In each coupling region, the fundamental modes present in the arms of the splitter are selectively coupled into the output waveguide horizontal and vertical first order modes, respectively. We showed by employing the beam propagation method simulations that the fine tuning of the geometrical parameters of the optical circuit makes possible the generation of optical vortices in both transverse electric (TE) and transverse magnetic (TM) modes. Also, we demonstrated that by placing a thermo-optical element on one of the y-splitter arms, it is possible to switch the topological charge of the generated vortex from m = 1 to m = −1.
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