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Figure 4. Left: Thickness of the deposited layer of PMMA versus the diameter of the fiber. Square: Experimental data. Comparison with the prediction of the model with one layer and 6 successive layers. Right: SEM image of an ONF coated with six layers of PMMA.
a refractive index after polymerization of 1.39, well below the refractive index of silica( 1.45). This encapsulation would enhance the mechanical resistance of the ONF while maintaining light guidance. In this process the ONF is immersed in the silicone precursor. The liquid is then outgassed under vacuum and cured at ambient temperature for 48 h. For this demonstration of principle, we pulled two fibers with a diameter of 10 lm. The fibers were fusion-spliced on both ends to two pigtails and their optical transmission of the components monitored with a tunable laser diode. The first fiber was encapsulated in silicone without PMMA coating. The average optical transmission from 1456 to 1548 nm increased from 48 % in air to 57 % in silicone, firstly validating the possibility to encapsulate tapered fibers in silicone without breaking them and secondly showing that this process does not induce important losses around 1.5 lm( the transmission even increased, due to the refractive index of silicone higher than the one of air). The second fiber was coated with PMMA and then encapsulated in silicone. After both processes, we measured an optical transmission of 38 % for the fiber at 1.5 lm, which remains relatively high. We believe that the losses are mainly induced by the Plateau-Rayleigh effect in PMMA coating. Even if further experiments and measurements need to be performed to optimize the coatings, this promising result validates the feasibility of this two-step treatment.
5 Conclusion and perspectives
We have successfully developed two processes to coat ONF with nonlinear materials. The first technique is the deposition of thin layers of TiO 2 by ALD. The precise control of the deposited thickness, the homogeneity of the deposition and the low induced losses obtained on the treated fibers indicate that ALD is a very efficient technique for the functionalization of cm long silica ONF. The second technique involves depositing thin layers of PMMA. We have realized a multiple layer deposition process to reach thicknesses of about 100 nm on ONF diameters as small as 1 lm. The encapsulation of a coated fiber in silicone has also been performed with low additional losses, demonstrating the great promises of such a treatment. Further investigations are now needed to improve these proofs of concept, including a more precise understanding of the origin of losses. Thanks to these techniques applied to ONF, we believe new experiments in nonlinear optics can be considered as the realization of second or third order nonlinear effects in the evanescent field of the optical propagating mode and for tailoring the chromatic dispersion to control phase matching conditions.
Funding
The work was supported by the French National Research Agency( FUNFILM-ANR-16-CE24-0010-03).
Conflicts of interest
Theauthorsdeclare thattherearenoconflicts of interest related to this article.
Data availability statement This article has no associated data generated.
Author contribution statement
Laurent Divay and Sylvie Lebrun wrote the article. Laurent Divay supervised the coatings. Sylvie Lebrun supervised the project. Etienne Eustache realized the coatings. Maha Bouhadida fabricated the ONF. Abderrahim Azzoune, Christian Larat and Jean-Charles Beugnot participated to all the discussions related to this work and reviewed the manuscript.
References
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