File Name: difference between step index and graded index fiber writer.zip
Depending on the material system used, this waveguide can have a step index or graded refractive index profile. The fabrication process and its effect on the waveguide performance are explained using an empirical model and afterwards experimentally verified. This approach enables easy process monitoring and optimization, effectively resulting in total insertion losses below 0. Self-written waveguides SWWs are formed by immersing an optical fiber end-face in a photosensitive polymerization mix and illuminating the mix with light emanating from the fiber, after which the exposed part of the mix polymerizes.
Mostly near- UV light is used for this process, although visible [ 1 , 2 ], and near-infrared light curing of the polymer structure has also been reported [ 3 ]. Depending on the type of fiber from which the light is launched i. Several researchers have reported this process for a variety of applications, where the SWW is typically used as an intermediate structure connecting two optical entities.
Because of the simpler fabrication process, mostly multimode SWWs have been reported, e. Within this last category, also the use of single mode SWW structures has been studied because of the popularity of single mode SMF optical fibers in telecom applications.
In order to avoid excessive insertion losses in such a fiber-to-fiber connection, the intermediate polymeric SWW structure should be precisely optically matched to the fiber, which occurs for specific RI profiles of the polymer medium between the 2 optical fibers, which will be illustrated in the current paper.
Some studies have already reported on the experimental characterization of these fiber-SWW-fiber connections [ 10 — 12 ], and on modeling the underlying chemistry and evolution of the RI during photopolymerization [ 13 — 16 ], and even on modeling the formation process of an SWW [ 17 — 19 ].
Therefore, the aim of the current paper is not to model the underlying photopolymerization principles, but to link the observed RI evolution during the SWW fabrication process with the optical performance of these structures. To this end, we have constructed a simple empirical model that helps to optimize the SWW fabrication procedure in a practical way.
Two different types of commercially available material systems were studied, i. Ormocer materials are widely used for optical interconnects because of their low absorption losses around 0.
NOA 68 is sold as an optically clear UV curable adhesive but has also been used for holographic recording. It has been reported that a permanent RI modulation can be inscribed when exposed with a spatially modulated UV beam [ 21 ].
In the case of Ormocer, a combination of 2 material variations was used, i. In the case of NOA 68 on the other hand, the single material was used to fabricate the SWW, but relying on UV exposure with a spatially varying intensity, the required RI difference between the core and cladding was achieved.
Both approaches resulted in consistent and low total insertion losses below 0. However, both material systems have their advantages; the 2-material system using Ormocer was selected because it allows an easy study of the polymerization curing kinetics since the formation of the core and cladding is decoupled, while the single material system using NOA 68 was selected because it involves a more practical technology for use in industrial applications. Analogously, the empirical model and analysis presented below describing the SWW formation is focused on the 2-material system first, and afterwards the principle is also applied for the single material system.
To create an Ormocer based SWW, first the core is fabricated and in a second step it is surrounded with a lower RI material to act as the cladding. After mixing, the material was left to rest for 1 day to let the mixing induced air bubbles escape. To achieve a 0. The empirical model consists of a cylindrical polymer waveguide core i. The fundamental TE mode at nm is launched from the leftmost fiber, propagates through the central SWW section and finally enters the rightmost fiber where a monitor is placed to record the field.
The amount of light which couples back into the fundamental fiber mode of the rightmost fiber is then obtained from the data in this monitor to determine the total transmitted power T into the rightmost fiber. The goal of this simulation is to determine the insertion losses as a function of RI difference between core and cladding of the SWW which is linked to a specific time instance during the polymerization process. In a first step, the cladding RI is kept constant and the core RI is varied while in a second step, the core RI is kept constant and the cladding RI is varied.
The output of the simulation is the total power transmitted T into the rightmost fiber. It is assumed that upon UV-illumination through the fibers, the polymerization and therefore also the RI increase of the SWW core n core occurs uniformly. Since the non-exposed, surrounding material or cladding remains unpolymerized and exhibits a lower RI n clad , a waveguide is formed between the end-faces of the SMF fibers.
This phenomenon allows inspection of the SWW polymerization process accurately by monitoring this insertion loss over time. After the formation of the SWW core, the unpolymerized surrounding material is removed and a different cladding material is applied. This material is then exposed to a uniform UV-illumination, similarly leading to insertion loss variations as the material increasingly polymerizes. To record the RI of these materials at nm before and after polymerization, a separate setup see Fig.
Cusano et al. The measuring principle is based on the RI dependent Fresnel reflections at a perpendicularly cleaved fiber end-face embedded in the polymer material under test. These are the same parameters used during the SWW fabrication. The results in Fig. A simple theoretical exponential dependence fitted to the experimental data is superimposed as the solid line on the graph.
These values were used as input data for modeling the insertion loss of the fiber-SWW-fiber structure during polymerization. In a first step, the polymerization of the SWW core was simulated by keeping the cladding RI constant at 1. In a second step, the fabrication of the SWW cladding was simulated by keeping the already fabricated core RI constant at 1. These simulations were performed for different SWW lengths and the resulting graphs depicting the fraction of transmitted optical power T are shown in Fig.
During the core fabrication, it is clear that the minimum insertion loss of about 0. In this case, a maximum overlap is achieved between the optical modal profile in the SMF fiber and in the polymer waveguide section. However, upon completion of the core formation process, the insertion loss rises again because the RI difference becomes too high to support ideal single mode propagation. Nevertheless, after substituting the unpolymerized OCore material with OrmoClad, the insertion loss finally evolves to a minimum value owing to the ideal RI difference 0.
Also note that for shorter SWWs, the maximum in the T curves as a function of the RI difference is broader, resulting in a larger processing window, meaning that shorter SWWs are more tolerant to process variations in RI, provided the pre-alignment of the fibers is perfect. Since insertion losses are easy to measure, these simulations can be linked to experimental data and used for process optimization.
The RI n t is a function of t as well as all spatial coordinates. A fiber-based setup was constructed for recording the insertion loss dynamically and in real-time during the different SWW fabrication steps, as illustrated in Fig. The current study is limited to characterizing at the popular nm telecom wavelength due to the availability of suited fiber-pigtailed components.
Figure 4 b illustrates the different steps in the SWW formation process using the 2-material system with Ormocer. During the polymerization of the core, a very similar trend is observed between the experimental and modeled data, and the main difference is a slight offset in insertion loss which can be attributed to non-perfectly aligned fibers in the experiment.
Note that for capturing this image, a longer SWW structure was prepared allowing easier visualization. Figure 7 shows a millimeters long structure, generated using the same parameters, but without the second fiber, illustrating the uniformity of the fabricated SWWs along their length.
The dashed lines represent experimental data of several identical experiments while the solid line represents the prediction from the empirical model. A longer waveguide was fabricated for easier visualization. During the polymerization of the cladding, again experiment and simulation yield similar curing kinetics see Fig.
In the previous case of the core formation, only the initial alignment of fibers resulted in slightly higher insertion loss as predicted, while in this case, also the exact RI of the already formed core is important.
Remark that in Fig. Comparing this value with the minimum achievable insertion loss in case of perfect alignment i. These experiments illustrate that monitoring the insertion loss during core and cladding formation can provide information on the instantaneous RI of the polymer and therefore also on the curing kinetics of this self-written waveguide polymerization process.
In search for a structure with the lowest possible insertion loss, tuning of RI is indeed important and in this context it is advantageous to be able to tune core and cladding separately as described above using the Ormocer 2-material system. However, a major drawback of this technology is the need to remove the uncured core material to replace it with another cladding material, which is not practical in many industrial applications. This can be overcome with the single material approach using NOA 68 material, as described in the following section.
The rationale of this approach is to write the core using UV light emanating from the fiber end-faces and performing a flood exposure to form the cladding.
Due to the specific composition of the NOA 68 material, the final RI profile after polymerization is correlated with the UV light intensity distribution during photopolyemerization. This material property has been studied extensively for application in holographic recording and it was reported that this RI modulation can be caused by a different degree of monomer conversion or by diffusion of monomers material transport between regions exposed with different UV light intensities [ 21 , 23 , 24 ].
This mechanism can therefore also be exploited for the fabrication of SWWs. Alternatively, a solution consisting of two kinds of polymers with different polymerization reaction mechanisms could be used for the fabrication of SWWs without a washing step [ 25 ], but this generally requires dedicated, non-commercially available materials.
In our approach, the UV light beam emanating from the SMF fiber combined with the UV flood exposure give rise to a non-homogeneous UV light exposure yielding a graded-index RI region between the core and cladding.
Since this graded-index region ensures the waveguiding in the polymer, no substitution step is needed for this material. We have found no evidence that the OrmoClad and OCore materials exhibit this property, therefore requiring a material substitution process to obtain a sufficient RI difference, which consequently leads to a step-index RI distribution in the waveguide region.
The SMF fiber is not single mode at the used nm exposure wavelength the cut-off wavelength of the fiber is about nm and therefore the effect of higher order modes is clearly visible. Although the beam has no ideal Gaussian profile, it was found that the higher order modes generated in the fiber reach a stable distribution and these profiles were reproducible during different experiments.
The rightmost plots show corresponding cross-sections of the near-field profiles plot along the 2 diagonals for which the highest non-uniformity in the beam can be observed. The influence of the graded index profile on the insertion losses through the polymer section was determined from simulations. An analogous model as described above was employed but now with a graded RI profile in the SWW region. This RI profile was determined by fitting a Gaussian curve through the actually measured RI data which is shown below.
Then, simulations were performed for varying amplitude of this Gaussian profile, i. The resulting graphs depicting the evolution of the transmission T as a function of varying RI difference are plotted in Fig. The maximum RI difference of 0. It can be seen that the optimum RI difference was 0. The insertion losses as a function of process time were similarly recorded using the setup shown in Fig.
Using these optimized exposure parameters for fabricating the graded index region, the core-cladding difference becomes optimum, obtaining high-quality waveguides with low insertion losses, comparable to the Ormocer material approach. Since the core and cladding are formed from the same material and therefore fabricated simultaneously in this approach, it is more difficult to predict the evolution of the core-cladding RI difference as a function of exposure time during polymerization.
Consequently, the recorded data is more difficult to compare with the optical simulations, which only take into account the insertion loss as a function of the RI difference. Therefore, the effect of both processes can be separated to a certain extent, when analyzing the experimental data, see Fig.
Step index fiber.
Plastic optical fiber was chosen for information delivery media in smart textile. Cladding layer was peeled off by chemical and mechanical methods to find optimal peeling conditions. A half-cone-shaped jig was manufactured using 3D printing to give various curvature conditions to fibers. Also POFs were embedded in plain weave textile structure to measure the light dissipation effect. The waveguide phenomenon was modeled using discrete ray tracing technique and ray-to-interface collision detection algorithm. Results from the proposed modeling technique showed linear relationship with those from experiment.
Advanced Inorganic Fibers pp Cite as. This is the number of discrete channels within a finite range of frequency. Bandwidth increases with frequency of the carrying signal. The higher the frequency used for transmission, the higher is the theoretical capacity of the system. Coaxial cables offer higher signal attenuation but require amplifiers every mile or two. Systems operation at terahertz THz frequencies  permits transmission of the contents of 30 sets of an encyclopedia in one second. Unable to display preview.
Fiber optics are used as a data transmission method whereby data is converted into modulated waves of light to be sent over optical fiber cable. Fiber optics are an alternative to traditional copper based data transmission cables over which they possess several advantages such as extremely high bandwidth , low losses even over great distances and inherent resistance to EMI. Light is transmitted through the core of a fiber optical cable by bouncing off the walls of the cladding by the principle of total internal reflection allowing the fiber to act as a light waveguide. Because the cladding does not absorb light from the core, signals can travel great distances with only slight losses occurring from impurities in the glass.
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