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A Review on Pedestal Waveguides for Low Loss Optical Guiding, Optical Amplifiers and Nonlinear Optics Applications T ⁎ Daniel O

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A Review on Pedestal Waveguides for Low Loss Optical Guiding, Optical Amplifiers and Nonlinear Optics Applications T ⁎ Daniel O Journal of Luminescence 203 (2018) 135–144 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin A review on pedestal waveguides for low loss optical guiding, optical amplifiers and nonlinear optics applications T ⁎ Daniel O. Carvalhoa, Luciana R.P. Kassabb, , Vanessa D. Del Cachob, Davinson M. da Silvab, Marco I. Alayoc a Univ. Estadual Paulista Júlio de Mesquita Filho (UNESP), São João da Boa Vista, SP 13876-750, Brazil b Faculdade de Tecnologia de São Paulo, CEETEPS/UNESP, 01124-060 São Paulo, Brazil c Escola Politécnica, Universidade de São Paulo (USP), 05508-010 São Paulo, Brazil ARTICLE INFO ABSTRACT Keywords: In this work we review the recent advances and demonstrate possible future perspectives regarding pedestal Pedestal waveguides waveguide fabrication technology. The optical waveguides produced using this technology represent a new Rare earth ions luminescence enhancement platform to study novel optical materials that could not be explored differently as this fabrication process does fi Optical ampli ers not require etching of the core material itself. Thus, novel materials that are highly inert to chemical etchants, Gain enhancement due to gold nanoparticles such as those composed of heavy metal elements, can be used with this new architecture that allows the fab- Integrated nonlinear optics rication of integrated optical devices. Lower propagation losses with pedestal waveguides when compared to rib waveguides are reviewed. The possibility to construct Mach-Zehnder interferometer structures using the pedestal waveguide technology is shown. The optical amplifiers with high gain are demonstrated using this pedestal process. Furthermore, using the pedestal architecture, it is presented the possibility to produce rare earth ions doped waveguides with gold nanoparticles, which demonstrated gain enhancement attributed to the efficient luminescence due to gold nanoparticles, thus opening possibilities for potential applications in integrated photonics. Finally, another research field that could benefit greatly in the near future from this new architecture is the integrated nonlinear optics as the pedestal waveguide fabrication technology opens up the possibility of using novel materials with high optical nonlinear susceptibilities. 1. Introduction hosts for rare-earth ions. Consequently, several studies on heavy metal oxide glasses [10–14] and also on heavy metal oxyfluoride glasses Recently the optical community has shown a very high demand for [15,16] were conducted motivated by the desire to identify low phonon novel material platforms for the development of different areas of energy hosts that have large mechanical strength, low manufacturing photonics. This is especially true for the fields of optical amplification temperatures and high refractive index. Recently, considerable em- and nonlinear optics. Doped fiber amplifiers such as Erbium Doped phasis has been given to the discovery of new heavy metal oxide glass Fiber Amplifiers (EDFAs) and Ytterbium Doped Fiber Amplifiers compositions to be used as rare-earth ions doped luminescent hosts. (YDFAs) have become key components of optical communication net- Important features that make them suitable for devices fabrication and works. A natural step is to integrate these devices in photonic chips for attracted several researches are their relatively high refractive index (≥ miniaturization and to compensate for optical losses in integrated 1.8), high dispersion, low melting temperatures (normally in the range photonic devices. New host materials based on heavy metals are of 700–1200 °C), wide transmission window (visible to mid - infrared), − much interest in this area [1] since they may present high nonlinear and low cutoff phonon energy (500–800 cm 1) among oxide glass optical susceptibilities, which is important for applications like para- formers. The reduced phonon energy increases the efficiency of lumi- metric amplification, wavelength conversion, all-optical switching, nescence and provides the possibility to develop more efficient visible among others [2–7]. Heavy metal oxide glasses [8,9] exhibit adequate optical lasers; normally frequency upconversion photoluminescence is thermal properties, mechanical strength and chemical durability, can more efficient in these hosts. Heavy metal oxide glasses are also suitable be melted in ambient atmosphere requiring no sophisticated apparatus, for white light generation and can replace the conventional in- present low phonon energy and high refractive index, and are suitable candescent and fluorescent lamps [17,18]. Particularly because of their ⁎ Corresponding author. E-mail address: [email protected] (L.R.P. Kassab). https://doi.org/10.1016/j.jlumin.2018.06.037 Received 1 April 2018; Received in revised form 14 June 2018; Accepted 14 June 2018 Available online 18 June 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved. D.O. Carvalho et al. Journal of Luminescence 203 (2018) 135–144 potential for laser hosts, tellurite and germanate glasses have been the 1050 nm and demonstrated possible applications as integrated optical subject of several investigations [19–22]. sensors [34]. Also, fabrication of Yb3+/Er3+ codoped On the other hand, it is well known that materials prepared with TeO2−WO3−Bi2O3 pedestal waveguides showed applications as op- metallic elements are prone to problems during the etching process tical amplifiers at 1530 nm [35] and advances on the fabrication pro- 3+ 3+ used to define the waveguide sidewalls. This is essentially due to micro- cess of Yb /Er codoped GeO2-PbO pedestal waveguides reported masking caused by atoms of metallic elements which lead to: rough the possibility to enhance the gain at about 50%, at 1530 nm [1]. Fi- etched surfaces and defects, commonly known as “grass”, (because of nally, the influence of gold nanoparticles on Yb3+/Tm3+ codoped its resemblance to actual grass), formed on the top of the etched regions GeO2-PbO pedestal waveguides was reported more recently. In this case [23]. This artifact is detrimental to light propagation inside the wave- the high luminescence and gain enhancement, at 805 nm, due to the guides, as they lead to scattering and radiation losses. Properly de- presence of gold nanoparticles showed relevant results for the devel- signed pedestal waveguides have been shown to mitigate significantly opment of optical amplifiers to be used in integrated optics, especially these effects [24]. Other configurations used to fabricate waveguides on optical network at short distances [36]. have been proposed in which hollow core arrows are fabricated over We remark that Begou et. al. [37] performed a theoretical study of self-aligned pedestals (SAP) [25], resulting in a minimization of the pedestal waveguides. In this study an extension of Marcatili's [38] waveguides propagation losses. Nonetheless, as stated, this concept has method was developed in order to adapt it to pedestal structures. Si- not been applied to solid core waveguides and this process is slightly mulations were performed for two different T-pedestal waveguide (T- different from the pedestal process. PW) configurations corresponding respectively to a high and a lower With the pedestal waveguide fabrication technology, the definition core to pedestal widths ratio. Then the extended Marcatili method was of the waveguide sidewalls is performed on the lower cladding by the compared with a semi-vectorial finite difference method (SVFD) [39] construction of a pedestal using conventional photolithography and and a spectral method developed by Galerkin [40–42]. By using all this Reactive Ion Etching (RIE) techniques. The core is deposited after the approach the authors found a good agreement between Marcatili's definition of the pedestal, no longer being necessary the etching of this method and the numerical methods. Finally these three approaches layer and thus resulting in rounded sidewalls. Since the light propa- were used to study the modal birefringence minimization for pedestal gating in the core does not interact with etched sidewalls, lower pro- structures. The study showed that when the core height or core width, pagation losses can be achieved when compared to conventional rib or are high enough the pedestal does not play a significant role on the strip waveguides fabricated with similar processing steps. modal birefringence. Furthermore, devices with applications on integrated nonlinear optics have attracted much attention over the last decade. By allowing a 2. Pedestal waveguides fabrication process larger refractive index contrast between core and cladding materials, when compared to the typical optical fibers, integrated devices tend to The first step of the pedestal waveguides fabrication process using have far smaller transverse mode dimensions. Consequently, larger silicon technology is the thermal growth on a silicon substrate of a nonlinear parameters can be achieved, which means that shorter de- relatively thick silicon dioxide film (1–2 μm-thick) with refractive index vices could be used with similar efficiencies. Silicon On Insulator (SOI) of 1.46 [1,24,34–36]. It is worth mentioning that there is no restriction was the first material platform used for integrated nonlinear optics. with respect to the use of other material platforms than silicon since the Devices for wide-band optical amplification, wavelength conversion as same concept can be applied to other materials. However, when silicon well as multiple wavelength coherent optical sources have been
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