Optimization of sensitivity in SMS structures

As has been mentioned, there are two main parameters that can be modified in SMS structures, in order to obtain a better sensitivity: The MMF segment length and diameter. However, in view of the potential of the materials deposited as thin-films presented along this thesis, it is possible that their intrinsic characteristics may also influence the behavior of the final device. Therefore, the main goal of this section will be trying to improve the SMS sensitivity by varying these parameters and extract some conclusions on the evidence.

5.2.1. Influence of the MMF segment length

By simply depositing the same [PAH/PAA] thin-film previously used for the 58 mm-length MMF segment to an MMF segment of 20 mm it can be observed that there are no differences when the evolution of the spectrum as a function of the thin-film thickness is compared to a 58 mm MMF segment length SMS. Fig. 5.7 shows several attenuation and transmission bands (brighter and cooler colors, respectively) corresponding to fourth and last spectra plotted in Fig. 5.2. It is easy to observe that the wavelength shift rate is similar with independence on the length of the structure [24].

Therefore, the main conclusion is that the MMF segment length is not a key factor on the sensitivity of the device, although it is crucial to obtain a self-image band and, therefore, a maximum in transmission. In view of this, as long as there are attenuation bands that can be used to track a further wavelength detection, it seems logical to use shorter length devices. This will lead to more miniaturized structures without sensitivity losses.

5.2.2. Influence of the deposited material

To make a study on this, several theoretical and experimental results are provided. They are based on the deposition of high refractive index thin-films of polymeric [PAH/PAA] and [TiO2/PSS] metal oxide/polymer matrixes onto 20 mm-

length MMF segments on SMS structures. The results are shown in Fig. 5.8. The selection of a material with a refractive index that approximates that of the substrate increases the wavelength shift. As an example, the self-image band moves from 1521 to 1613.7 nm with [PAH/PAA] and from 1521 to 1568.6 nm with [TiO2/PSS]. However, the scale of the x-axis is different for [PAH/PAA] and

as the wavelength shift divided by nanocoating thickness range, this parameter is lower with [PAH/PAA] (92.7/504 nm = 0.184) than with the higher refractive index matrix [TiO2/PSS] (47.6/132 nm = 0.361). However, the wavelength shift is

nonlinear. Consequently, a better knowledge of the sensitivity can be obtained with the derivative of the wavelength as a function of thickness. The maximum derivative is obtained in the proximity of the fading region and it is 0.383 and 0.690, respectively, for both [PAH/PAA] and [TiO2/PSS] thin-films.

The reason why the deposition of a thin-film with a higher refractive index leads to a higher sensitivity can be explained by the phenomenon of modal transition [10,25]. As it has been previously indicated, this basically consists of the guidance of a mode in the thin-film, which induces a reorganization of the effective indices of the modes in the MMF section. With a higher refractive index in the thin- film the transition region is achieved for a lower thickness and it is more abrupt, which causes and increase in the wavelength shift induced by variation in the thin- film refractive index.

Fig. 5.7. Evolution of the spectrum as a function of thickness for an SMS structure of (a and b) 58 mm

and (c and d) 20 mm of MMF segment length. On the bottom, the experimental results. Above, the theoretical results assuming thin-film RI = 1.5+0.0025i [24].

Fig. 5.8. Evolution as a function of thickness of the central wavelength of transmission bands

obtained with an SMS structure of 20 mm. Simulation data, continuous line; experimental data, squares. (a) Deposition of [PAH/PAA] assuming RI = 1.5+0.0025i; (b) Deposition of [TiO2 /PSS]

assuming RI from 1.749+0.01i to 1.737+0.007i [23].

5.2.3. Influence of the MMF segment diameter

Once it has been observed that the depositing a material with a higher refractive index than that of the substrate is a better option in terms of sensitivity, the diameter of the MMF segment is analyzed from now on. The expectations, as mentioned in section 5.1 are that the sensitivity is increased if the MMF segment diameter is reduced.

Following this idea, an SMS structure with an MMF segment diameter of 61.5 microns was deposited and coated with [PAH/PAA]. The length of the device was 15 mm, which permits to visualize the self-image band in the optical spectrum. As expected, theoretical and experimental results in Figs. 5.9 prove that the total wavelength shift is increased by a factor that approximates the ratio between the diameters of the optical fibers analyzed: 125 /61.5 μm. By comparing in Figs.

5.9a and b, the central wavelength of the transmission bands of the 58 mm-length SMS with diameter 125 μm and the 15 mm-length SMS with diameter 61.5 μm as a function of the nanocoating thickness. For example, in Fig. 5.9a the band located at 1574nm for a nanocoating thickness of 420 nm experiments a wavelength shift of 89.6 nm if a nanocoating of 468 nm is deposited, whereas in Fig. 5.9b, the band located at 1494.5 nm for a nanocoating of thickness 420 nm experiments a wavelength shift of 172.3 nm if a nanocoating of 420 nm is deposited. This behavior can be explained by considering the results obtained in [10,25], where it is proved that the resonance wavelength shift is caused by changes in the difference between the effective indices of modes HE1,1 and HE1,3 in the MMF section. Since the

separation between the effective indices of HE1,1 and HE1,3 modes in the MMF

section of an uncoated SMS structure with a lower diameter is increased, the wavelength shift is higher as the nanocoating is progressively deposited on the structure.

Fig. 5.9. Evolution as a function of thickness of the central wavelength of transmission bands

obtained with an SMS structure where a [PAH/PAA] thin-film is deposited. Simulation data, continuous line; experimental data, squares. (a) MMF segment diameter 125 μm and length 58 mm