# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 2 Fluorinated Polymers for Passive Optical Waveguides  
Currently, several polymer materials have been developed for photonic applications. Such optical polymers have become state-of-the-art, particularly in the form of polymer optical fibers [7]. Linear polyacrylates such as poly(methyl methacrylate) (PMMA) have primarily been used, but they exhibit significant aging effects at moderate temperatures above 75 \({}^{\circ}\)C. A fluorinated polymer is used as optical cladding to enable waveguiding. Modified PMMA from Toray Industries [8] shows higher long-term thermal stability and heat resistance. Polymer optical fibers based on linear PMMA show low propagation losses of 0.0016-0.003 dB-cm\({}^{-1}\), but only in the wavelength range between 650 and 670 nm [9]. This limits their usability to lengths of less than 100 m, in contrast to optical glass fibers, which can transmit data over many kilometers (in the wavelength range around 1550 nm). In addition, PMMA can absorb up to 1.5% of water, which has a negative effect on its propagation losses [7]. The use of these materials, especially PMMA, for integrated optical components has already been described in the literature [10; 11; 12] and reviewed in 2002 [13]. However, these materials have several disadvantages, especially high losses in the wavelength range around 1550 nm, which are mainly due to the harmonics of the C-H bond [7]. For the wavelength range around 1550 nm considered here for polymer-based optical microchips, the loss values are typically worse than those of PMMA. In addition, none of these materials are cross-linked, which makes processing into integrated components difficult or even impossible (e.g., re-dissolution of the lower cladding layer by the solvent of the waveguide layer). In addition, the refractive index is too high and there is no second material available to provide a sufficient index contrast. Thermoplastic polymers also have a very high thermo-optic coefficient (TOC) [14]. In addition to the materials mentioned above, several other systems for integrated optical components have been described in the literature. Benzocyclobutene from Dow Chemicals (Midland, MI, USA) has too high optical losses for applications around 1550 nm [15]. Perfluorocyclobutane polymers, originally developed by Dow as low-k materials for microelectronics, have been further developed by the US research group led by [PERSON] and used for various optical and other applications [16; 17; 18]. They exhibit low optical losses at 1550 nm (sometimes as low as 0.2 dB-cm\({}^{-1}\) at 1550 nm), but their adhesion to silicon is poor. Fluorinated acrylates from Allied Signal [19] have low propagation optical losses at 1550 nm, but there are no material pairs with a sufficiently high index contrast. Several classes of polymers were used in our previous research [20; 21]. Functional prototypes of integrated optical components based on monomodal polymer optical waveguides were developed using different polymer classes for telecommunication applications, particularly in the wavelength range around 1550 nm. Depending on the respective integrated optical components, different polymers were developed. These included fluorinated (meth)acrylate co-polymers, polycyanurates, new triazine-containing polymers (e.g., triazine acrylates), perfluorocyclobutane (PFCB) poly mers and PFCB-polycyanurate hybrid polymers. The layers were prepared by spin-coating of prepolymer solutions, whereby the refractive index contrast \(\Delta n\) = 0.008-0.020 between the cladding and the core was achieved using specially designed co-polymers. The polymer systems developed possessed very reproducible process characteristics. With polycyanurate co-polymers, optical propagation losses below 0.3 dB-cm\({}^{-1}\) at 1550 nm could be achieved, although the processability was not sufficient. The partially fluorinated (meth)acrylate co-polymers, which can be processed on silicon and show good film formation properties, proved to be particularly promising, especially due to their good processing properties.