3.1. RI Sensing with LPG or µMZI Structure
The spectral responses of the LPG and Al2
-nanocoated LPG (Al2
-LPG) to RI before μIMZI micromachining are shown in Figure 3
A,B, respectively. An increase of the spectral distance between the resonances is observed when the RI increases, which is very characteristic for this type of sensing structure working at the DTP of higher-order cladding modes [14
]. The RI sensitivity for each resonance is close to 2000 and 9000 nm/RIU for LPG and Al2
-LPG, respectively. Due to the higher RI sensitivity of the Al2
-LPG, the resonances shift significantly more with RI.
In Figure 4
, in turn, the spectral response of a reference µIMZI is shown. This structure was micromachined separately for comparison. The microcavity has the same diameter d
= 60 µm as the µIMZIs fabricated later in the LPGs. In general, we observe here a shift of the transmission minima towards shorter wavelengths with increasing RI values in the cavity. The substantiation of the spectrum evolution was presented in our previous work [27
]. It is worth mentioning that due to the micromachined interferometric structure, the overall transmission dropped by ca. 7–10 dB. This effect is induced by the formation of the µIMZI cavity, which makes direct interaction between the fiber core and the external medium possible. The RI sensitivity of the presented µIMZI reaches 15,000 nm/RIU.
3.2. RI Sensing with LPG-µMZI Structures
A,B shows the spectral response to RI in its different ranges for the LPG after µIMZI micromachining, i.e., with the microcavity in the middle of the LPG.
It is interesting that the LPG still ordinarily responds to the RI despite a significant discontinuity in the fiber cladding. Given that the LPG’s working principle relies on the cladding modes, it is perhaps counter-intuitive that the microcavity had no direct effect on the LPG response. The micromachining process led to an increase in the overall insertion loss (transmission dropped by ca. 10–14 dB). The presence of the cavity also manifests itself in the oscillatory character of the spectrum, which in this case is caused by scattered waves at the cavity interferences. Finally, two specific ranges in the spectrum are evident, above and below 1400 nm. The higher part resembles the spectrum of the LPG before micromachining (Figure 3
A), while the lower one is dominated by the response characteristic to the µIMZI (Figure 4
). Based on the curves shown in Figure 5
and Figure 6
, we can state that the LPG-µIMZI response is a composite of the LPG and µIMZI spectra. Figure 5
shows that with an increase of the RI, the spectral response of the combined device is similar to the responses of the LPG and µIMZI working independently, which is more evident for smaller values of RI. Specifically, we observe an increase of the resonance wavelength separation in the range of 1400–1700 nm, which corresponds to the profile of the LPG. There is also a blue shift of the minima in the 1200–1400 nm range which, in turn, typically corresponds to responses of the µIMZI (Figure 5
A). For higher values of RI, the second minima, characteristic for the µIMZI, appear at the higher wavelengths and mingle with the response induced by the LPG. This causes distortions in the spectrum and makes the two effects interfere with one another (Figure 5
B). The dominant regions for each effect are still distinguishable, but very much disturbed, especially in the case of the minima characteristic for the LPGs. The interfering effects in some RI ranges are critical and this fact needs to be taken into consideration while designing the device.
Next, the RI-induced spectral evolution for the Al2
-LPG with microcavity was investigated (Figure 6
As in the case of the LPG, the overall transmitted power after micromachining is significantly reduced. The transmission dropped by ca. 7–11 dB. Thanks to the higher sensitivity of the Al2O3-LPG, the effect is even more noticeable at higher RI values. On the other hand, the more sensitive the LPG structure is, the more distortion and undulations appear in the spectrum after the micromachining. Although it was always possible to determine one of the LPG’s minima, as well as the minimum for the µIMZI, the obtained spectrum was not that smooth and well pronounced as in the case of the sensors working separately.
A few possible causes for the origins of these distortions are, e.g., exciting additional modes, or introducing a phase shift to the already existing one. It is also worth reminding that both the sensing structures were made in a standard single-mode fiber where the cutoff wavelength is λ = 1260 nm. Below this value, the fiber works in a multimode regime and the high-frequency oscillations in the spectrum are observed mainly in the short wavelength region. As shown in Figure 6
A, the minimum at around 1200 nm corresponding to the µIMZI diminishes with RI, i.e., moves towards the shorter wavelengths and cannot be further observed due to the cut-off wavelength of the fiber. Nevertheless, the second minimum can be seen above RI of 1.3416 RIU at about 1600–1700 nm (Figure 6
It can be concluded that the results obtained for the two types of LPGs with µIMZIs differ only quantitatively. The similarities are explained by the fact that both have the same effect and the same general principle of operation, while the differences have several reasons. The spectra shown in Figure 6
indicate that the minimum corresponding to the µIMZI is in the short wavelength region. The second minimum, in contrast to the LPG, emerges for relatively lower RI values. Moreover, due to the considerably higher sensitivity of the Al2
-LPG, its resonances shift significantly more with the RI and the µIMZI effect is more pronounced and less distorted than for higher RI in case of the LPG. It might be expected that a RI range exists for which the distortion makes discrimination of the effects difficult. However, in the investigated cases during the whole experiment, i.e., for the entire RI range, it was always possible to determine one of the LPG’s minima, as well as the minimum of the µIMZI. It is important to note that in this case, i.e., Al2
-LPG, the exact definition of the location of the minima requires additional signal processing, e.g., curve fitting or pattern recognition. Furthermore, along with some additional signal processing, the full-width half minimum (FWHM) parameter could also be improved, especially for the µIMZI part of the spectrum. Even though its reduction requires additional fine-tuning of the sensor, such as reactive ion etching as reported in our previous work [28
]. In this discussion, we must acknowledge that also the placement of the cavity might influence the transmission spectrum of the device. In the case of both structures, despite the significant discontinuity in the fiber cladding, which sustains the cladding modes, the LPG-related effect remains valid and the cavity does not affect the output of the sensor significantly.
From the above discussion, we can conclude that the two effects, the first stemming from the LPG and the second from the µIMZI, seem to be independent. Furthermore, we have seen that in lower RI ranges they do not affect each other and can be well separated. This separation of effects may be used to reduce the cross-sensitivity of the proposed device. Since the lower RI range of operation is essential in many applications, including bio-sensing, the combined structure could well serve for taking measurements where two different RI sensitivity effects are expected. Experiments such as those reported here have been carried out previously for both platforms separately [14
], but never for a combination of the platforms. The prospect of being able to simultaneously detect and distinguish the earlier mentioned different sensitivities (surface and volume) effects using a two-in-one platform serves as a continuous incentive for ongoing studies and motivates future research.
3.3. Monitoring of RI at the Surface with the LPG-µMZI
In the previous sections, we discussed the high SV
of the sensor. However, the primary motive behind the combination of the LPG and µIMZI was rather a determination of the difference between SV
. Based on the working principles of each platform it can be stated that the LPG is best suited for detecting surface RI changes, while the µIMZI excels in volume RI measurements, where the surface effect can be ignored to some extent. To prove our hypothesis and simulate the biological film formation, we deposited a high-RI thin overlay of Al2
using the ALD method on the entire LPG-µIMZI. ALD provided a highly controllable and uniform 30 nm layer all over the LPG-µIMZI sensor. The Al2
can be dissolved in both highly concentrated acids and alkalis, which enables studying the response of the sensor to different thicknesses of the thin film [26
]. In Figure 7
the influence of the Al2
etching process on the spectral response of the LPG-µIMZI is shown. The arrows indicate the change of the spectral response with the progression of the etching process.
The most apparent change concerns the LPG part of the spectrum. LPGs are highly sensitive towards external RI including even the thinnest overlay formed on their surface. Any change of the thickness of the nanocoating manifests itself as a shift of the resonance wavelength, and it was observed after the deposition process. Both resonances shifted away from each other for about 50 nm from the initial stage. Moreover, the LPG operates in the proximity of the DTP. Thus, the deposition process splits both resonances and makes them apparent and defined. Slow etching in 10 mM NaOH induced the shift of the spectrum (indicated by black arrows) and with time the resonances were getting close to each other, and finally, they got back to the initial working point. In contrast to the response from the LPG, the µIMZI part of the spectrum only slightly reacted by the deposited Al2O3 overlay. We can observe the changes mainly in terms of the transmission amplitude. The properties of the deposited layer—its RI and thickness—did not change the working conditions of the µIMZI. In conclusion, if we are targeting detection of surface changes (ca. tens of nm overlays), it will be too small to affect the volume sensitivity of the µIMZI part of the sensor and in effect, it will stay unrecognizable to the µIMZI, or it will induce changes just in terms of transmission amplitude. However, combined with surface-sensitive LPG, the sensor can stand as a perfect tool to measure the growth of an overlay and, for example, simultaneous fluctuations of the external RI. This, in turn, could find the application in the detection of biological targets such as DNA aptamers, which are a key target in medical diagnostic tests.
presents a schematic comparison of three cases when the use of combined LPG-µIMZI would allow to deliver enriched information about the analyzed thin film, including biofilm, and the surrounding liquid. Figure 8
A shows a situation when both the LPG and µIMZI are influenced by a high-RI liquid (n1
). Figure 8
B, in turn, presents a case where a very thin layer (h2
), such as a biological film is deposited, and the external RI is lower than in the case shown in Figure 8
). The last example (Figure 8
C) concerns a thicker layer (h3
) and the lowest RI (n3
). For the LPG, the measurement will be a superposition of effect coming from RI, the thickness of the nanolayer, and to some extent RI of the surrounding medium. Thus, on the LPG part of the spectrum for specific values of n1
, and n3
, as well as h2
there will be no or slight difference between thin and thicker film surrounded by higher and lower external RI. Only the contribution from µIMZI would allow to discriminate the cases and identify changes in RI and, in consequence, changes in film growth, too. During the biological experiments, the discrimination of such differences is crucial, e.g., the thickness of the film may correspond to the concentration of biological targets, while external RI indicates proper removal of excess of the unbound targets. Thus, the proposed sensing combination enables clear identification of the changes at the surface and in its proximity, as well as further interpretation of the results.
3.4. Temperature Sensitivity of LPG-µIMZI
In the preceding Section 3.2
and Section 3.3
, it was shown that the combined structures work independently and do not affect each other’s operation in any significant way. This can be even more interesting when one realizes that in addition to a slight reaction to the thin overlay deposition, in contrast to the single LPG, the µIMZI is almost T-insensitive [18
]. To demonstrate the T-insensitive RI sensing with the µIMZI, the reference µIMZI was placed in a T-controlled cell and immersed in water. The transmission spectrum was monitored while the T was gradually increased from 10 to 45 °C with a 5 °C step. The spectra obtained during these measurements show that with an increase of T, the minimum shifted towards longer wavelengths (Figure 9
The RI of the fiber core, as well as diameter of the microcavity, are treated as constants over the range of the T applied during the described experiment (10–45 °C) considering the thermo-optic coefficient (6.3 × 10−6
/°C) and thermal expansion coefficient (0.55 × 10−6
/°C) of the fiber materials [29
]. The obtained T sensitivity of the µIMZI in water is 1.2 nm/°C at ca. 1250 nm and is induced mainly by RI sensitivity of the structure which is ≈15,000 nm/RIU. Since the thermo-optic coefficient of water reaches −1 × 10−4
RIU/°C and is two orders of magnitude higher than that of fused silica (6.3 × 10−6
/°C), we can conclude that the observed shift of the minimum is induced almost exclusively by the change in the RI of water caused by T variation.
In Figure 10
evolution of Al2
-LPG-µIMZI transmission spectra with T is presented. Here, we see that with the increase of T, the spectral distance between the resonances of the LPG decreases, while the part dominated by the microcavity response barely changes. Assuming that the two effects are independent and that the change of the spectrum caused by the µIMZI is relatively small when compared to that caused by the LPG, we can infer that the sensor is suited for highly accurate RI measurements in cases where the T of the investigated medium varies.
Multiparameter sensing is often unavoidable to acquire desired information on the performance of a chosen system. Thus, in recent years many configurations have been presented, especially for T and RI sensing. However, just two other sensors are incorporating a grating and in-fiber cavities. The first sensor was created by combining the micromachined cavities in-between two FBGs [31
]. The second was made by a combination of FBG with the µIMZI [6
]. In both cases, the FBGs provided very high sensitivity. However, because of the combination, as indicated in the introduction, differences in the values of the traced wavelength shifts limit the practical application of the sensors. The design and manufacturing method of the first-mentioned sensor also highly bounds its performance. Two microcavities made in one fiber significantly weakened the sensing structure. Besides, the reproducibility of the sensor is questionable due to the Excimer laser processing combined with highly uncontrollable hydrofluoric acid etching. Regardless of the combination, both sensors provided the information about T and RI. Therefore, they cannot be compared in terms of surface and volume sensitivities. This is the first sensor to date which has been considered for such application.