162
J. Eur. Opt. Society-Rapid Publ. 21, 15( 2025)
Figure 2. Left: Thickness of the deposited layer of TiO 2 versus the diameter of the ONF. Right: Example of a SEM image of the cleaved fiber’ s cross section of an ONF coated with TiO 2.
thin layer can be deposited on the whole surface of this cylindrical component. During the process, a silicon substrate is placed in the chamber with the sample. The study of the substrate enables us to determine precisely the refractive index of the TiO 2 layer and its thickness by ellipsometry. On silicon test-plate, the measured refractive index at 1.5 lm is 2.238, which is relatively low. This is attributed to a consequence of the low deposition temperature( 70 ° C). Indeed, we believe TiO 2 is then in an amorphous phase, less dense than the crystalline ones. In Figure 2, ONFs with diameters ranging from 1.3 lm to 6 lm have been coated, cleaved and the cross-sections have been observed by Scanning Electron Microscopy( SEM). We measured an average thickness of the deposited layer of 48 nm, very close to the target thickness and probably within the accuracy of the measuring procedure.
To measure the losses induced by the TiO 2 deposition, we have drawn an ONF with a diameter of 1 lm anda length of 2 cm. Then we have fusion-spliced its two untapered ends to two fibre pigtails. A laser beam at 1.5 lm has been injected in the component. The losses of the bare ONF with the two pigtails were 2 dB. After having coated the ONF with TiO 2 the losses increased to 2.5 dB, leading to additional losses of no more than 0.5 dB.
These preliminary results show that ALD is a powerful and reproducible technique for the deposition of controlled thin layers of TiO 2 on ONF. The additional losses are very low, even for diameter as small as 1 lm and relatively long ONF length. Further investigations are needed to optimize the coatings to get lower losses. At first sight, the surface rugosity is very low, so we believe this is not the main origin of the extra losses. One point that should be investigated is the process subsequent tapers’ coating that may change their adiabaticity condition and thus their transmission.
3 PMMA coating
The second selected material is PMMA. Its refractive index is 1.49 at 1.5 lm. This polymer is also an interesting material for nonlinear optics. As an example, PMMA optical fibers have been studied for the realization of Raman amplifiers [ 13 ]. Moreover, PMMA can also be doped with nonlinear dyes, which could enlarge the field of applications. As detailed in [ 7 ], our target is to obtain deposition thicknesses of a few hundreds of nm on ONF having a diameter around 1 lm.
Here, the coating process requires the use of liquid PMMA solution. Acetophenone is selected as solvent for its low toxicity and low vapor pressure for ease of handling and to avoid solvent evaporation during coating. The concentration of PMMA in the liquid solution is 264 g / L. The technique we have developed has been adapted from the classical dip-coating technique and is inspired by Velázquez-Benítez et al. [ 8 ]. A drop of the solution is placed in a U-shape profile which is translated gently at a constant speed along the fiber in the horizontal direction, making the deposition. The fiber is then heated under reduced pressure in a vacuum oven to evaporate the solvent. By using this technique, the success rate of deposition without breaking the ONF is close to 100 %.
It is possible to estimate the thickness of the deposited layer of PMMA solution since the studied case corresponds to conditions in which the deposited solution thickness is small compared to the fiber’ s diameter. Indeed, it can be described by the model of Landau, Levitch and Derjaguin [ 14, 15 ] applied to the coating of a fiber [ 16 ]. The PMMA film thickness e p remaining after the solvent evaporation canbedeterminedusingthePMMAvolumefraction.
For a constant deposition speed v thereisalinearrelation between the thickness e p and the local diameter of the fiber d f:
2 = 3 lv e p ¼ 0:17 k d f; ð1Þ c
k is the PMMA volume fraction in the solution, l is the dynamic viscosity and c is the surface tension. c was measured to be ~ 48 mN m �1 using the drop weighing technique, and l was determined to be 0.71 Pa s for the deposition conditions’ estimated shear rate.