This paper presents a highly-sensitive, miniature, all-silica, dual parameter fiber-optic Fabry-Perot sensor, which is suitable for independent measurement of the refractive index and the temperature of the fluid surrounding the sensor. The experimental sensor was produced by a micromachining process based on the selective etching of doped silica glass and a simple assembly procedure that included fiber cleaving, splicing and etching of optical fibers. The presented sensor also allows for direct compensation of the temperature’s effect on the fluid’s refractive index change and consequently provides opportunities for the detection of very small changes in the surrounding fluid’s composition. A measurement resolution of 2x10−7 RIU was demonstrated experimentally for a component of the refractive index that is related purely to the fluid’s composition. This resolution was achieved under non-stabilized temperature conditions. The temperature resolution of the sensor proved to be about 10−3 °C. These high resolution measurements were obtained by phase-tracking of characteristic components in a Fourier transform of sensor’s optical spectrum.
© 2014 Optical Society of America
Fiber-optic refractive index (RI) sensors have recently been intensively investigated [1–31]. Their small size, high measurement resolution, ability to operate sensors remotely through optical fiber, bio-compatibility, compatibility with harsh environments, wide temperature range, and electromagnetic interference immunity, are properties that make these sensors interesting for chemical, bio-chemical, bio-medical, industrial, environmental and a variety of other sensing applications. Considerably attention has been devoted to those fiber optic RI sensors with high RI measurement sensitivity [6–17]. High measurement sensitivity is required to achieve high RI measurement resolution, which is gaining importance in many applications like for example within bio-chemical and bio-medical sensing. High sensitivity fiber optic RI sensors were built in the past by utilizing various sensing principles including Fabry-Perot interferometers [6–10], Mach-Zehnder interferometers [11, 12], surface plasmon resonance [13, 14], long-period gratings , twisted highly-birefringent microfibers , tapered rectangular silica microfibers , and others.
Sensor’s sensitivity is however not the only parameter that determines the resolving capability of the RI measuring system. In most applications, RI measurements are used to determine small changes in fluid structure or composition. The great majority of fluids however possess high dependence of RI on the temperature, which can severely limit a sensing system’s capability of detecting a fluid’s composition change through observation of its RI variation. For example, typical fluids (water, organic solvents, etc.) have their dn/dT’s within a range of −10−4/°C, which further change (nonlinearly) according to the temperature. Thus, to reliably detect very small changes in a fluid’s composition through RI observation either a highly temperature stable environment is required or high precision temperature compensation must be introduced into the measurement process.
In order to overcome this limitation combined (dual-parameter) RI-temperature sensors were proposed and studied [18–31]. These approaches included the uses of long-period and fiber Bragg gratings [18–21], Mach-Zehnder, Michelson and Fabry-Perot interferometers [25–28], multimode interference [22, 23], photonic crystal fibers , double-clad fibers , tapered bend-resistant fiber interferometers , and tilted Bragg reflector fiber lasers .
Unfortunately many of these dual parameter sensing approaches require compromises in RI and temperature sensor designs, which leads to limited performances of such combined dual parameter sensors. From all above-listed references, the highest predicted (not actually demonstrated) resolution was 2.25 x 10−5 RIU (as reported in ).
RI sensing within true 10−6 or even 10−7 RIU resolution within thermally unstable environments thus remains challenging as it requires sensor with both highly sensitive RI and temperature sensing capabilities. RI sensing with such a resolution requires the acquisition of measured sample temperatures with resolutions within the 1-10 mK range. This additionally demands sensor’s compact size and close proximity of the temperature and RI sensing parts of the sensor in order to prevent occurrence of temperature discrepancy between temperature and RI sensing parts of the sensor even at mK scale.
In this paper we present a fiber optic sensor and corresponding signal integration scheme that can fulfill the above requirements and perform temperature independent RI measurements of the fluid while achieving a resolution 2x10-7 RIU.
2. Sensor design and manufacturing
The proposed sensor is built on the tip of a silica optical fiber and is composed of two Fabry-Perot interferometers (FPI). The first interferometer (1st FPI) is an open-path microcell FPI with a length of around 100μm, and is intended to perform surrounding fluid’s RI measurements. It is defined by an in-fiber mirror located close to the fiber-end and a reflective cap that is suspended at the front of the fiber-end, as shown in Fig. 1. The cap is attached to the fiber by a cap supporter that has a “waxing crescent moon” cross-section shape. The cap supporter provides suitable mechanical strength and dimensional stability required for the structure’s practical use. The surrounding fluid can thus freely enter/exit the region that defines the optical path of the 1st FPI. The second, temperature measuring FPI (2nd FPI), is defined by the same in-fiber mirror as in the case of the 1st FPI and an additional in-fiber mirror located about 1.3 mm further down the lead-in fiber, as depicted in Fig. 1. Temperature change of a silica core in this section causes variations of the refractive index, which further causes variation of the optical path-length in this 2nd FPI. The distance between the first fiber mirror and the fiber end-face was reduced to a minimum allowed by production process, in our particular case to about 5 μm. The proposed structure forms a three-beam interferometer (three semi-reflective mirrors). Three beam fiber interferometers were successfully used in the past for dual parameter sensing [32, 33] and can be interrogated in various ways as for example described in signal processing section below.
The production process is started by the preparation of a short section of coreless silica fiber (CF). The CF is flat cleaved and then fusion-spliced to a section of specially-designed structure-forming fiber (SFF)-Fig. 2(a). The cross-section of SFF is shown in Fig. 3 and consists of a large P2O5 doped core (8.5 mol%), that is offset from the fiber center. At one side of the SFF cross-section the P2O5 doped core approaches the outer boundary of the fiber circumference to about 27 μm, while at the opposite side this distance corresponds to 5 μm. The SFF preform was produced by using the conventional modified chemical vapor deposition process (MCVD). The produced preform was then mechanically machined to offset the doped region out of the preform center. Fiber was drawn by the conventional fiber drawing process.
The SFF is then cleaved away at a distance corresponding to about 100 μm measured from the CF to SFF splice– Fig. 2(b). The obtained structure is further spliced to another section of CF, which is again cleaved at a distance of about 100 μm away from the last splice- Fig. 2(d). This concludes the preparation of the first part of the sensor’s assembly. The second part of the sensor’s assembly is prepared by flat cleaving the end of the lead-in single-mode fiber (SMF). This lead-in fiber is then etched in 40% HF for about 45 s to create a shallow cavity in place of the SMF’s germanium doped core - Fig. 2(e). Further splicing of such an etched fiber tip with another section of SMF creates an in-fiber mirror – Fig. 2(f). This mirror reflectance is then set to about 2% through a controlled fusion-splicing process, as described in detail in . This spliced and cleaved section of SMF is additionally cleaved at about 1.3 mm distance from the in-fiber mirror, as shown in Fig. 2(g) and again etched in HF to create another shallow cavity in place SMFs fiber core – Fig. 2(h). This completes the preparation of the second fiber assembly. The first and the second fiber assemblies were then spliced together in a controlled way (as described in ) to create in-fiber mirror at the splice between both assemblies – Fig. 2(i), this mirror reflectance was set at 2.3%. The coreless fiber constituting the first assembly is then cleaved in such a way as to provide about a 10 μm long section of CF at the tip of the SFF – Fig. 2(j). Such an assembly is then immersed in HF for 6 min– Fig. 2(k). After the initial uniform removal of silica around the assembly circumference, HF gets in contact with the P2O5 doped region of SFF that etches away about 30 times faster than the pure silica , which creates a void in place of the large eccentric P2O5 doped core. Finally, this etched structure is placed into a standard connector ferrule (to protect the inner surfaces), while being exposed to the sputtering process in order to deposit an about 200 nm thick layer of TiO2 onto the structure’s cap – Fig. 2(l). High RI of TiO2 provides high reflection of the cap, even when the sensor is immersed into liquids with various RIs (in case of measurement of liquids with very high RI, TiO2 coating can be replaced either by multilayer or metal coating). The scanning-electron microscope (SEM) image of the produced all-silica sensor structure is shown in Fig. 4.
3. Signal processing
Both sensor interferometers have significantly different lengths with characteristic free spectral ranges (FSRs) and thus generate distinctive frequency components in a back-reflected optical spectrum (BROS). Proper signal processing of BROS provides the opportunity to measure temperature and RI with high resolution in the absence of crosstalk. In our experimental characterization we used a National Instrument NI PXIe-4844 spectral interrogator to acquire sensor spectral characteristics (we performed 80 nm wide sweeps with 10 Hz repetition rate). These spectral characteristic were then transferred to a personal computer (PC) for processing using a custom-developed code in LabView that extensively relied on Furrier transform (FT) analysis. A typical recorded sensor’s BROS and its amplitude FT is shown in Fig. 5 (sensor was immersed in water).
Three distinct peaks in the FT of BROS can be recognized. The first peak appears at 1ps, the second at 13ps and the third at 14.1 ps. These peaks correspond to individual interferometers round trip times of flight (i.e. interferometers optical lengths, when multiplied by one half of the vacuum speed of light). The first peak thus corresponds to the shortest, RI measurement cavity, the middle peak corresponds to the temperature measurement FPI, and the third peak corresponds to the FPI which is formed by the second in-fiber mirror and the sensor’s cap (sum of lengths of RI and temperature measurements of FPIs). In order to observe the small path-length changes of all three FPIs with high resolution we further calculated/observed those phases of FT components corresponding to all three FPIs round-trip times of flights (lengths), i.e. we observed those phases of the FT components where peaks in the amplitude of the FT of BROS occurred. These phase changes corresponded to the interferometers’ optical path length variations (ΔOPL), i.e.:Fig. 1. Thus the entire length of the interferometer, which also defines position of the first peak in FT of BROSS, corresponds to LRI FPI + LM + LC, however, in the presented design, LM + LC accounts for about 10% or less of LRI FPI.
The phase change of the component in FT of BROS, which corresponds to the round-trip-time of flight of the long (temperature measuring) FPI, can be correlated to the temperature as:Eq. (5) by using information from the temperature measuring FPI and Eq. (3):
Since dnf/dT in the upper equation is often temperature-dependent, it might be more convenient to re-write the upper equation asEq. (7) to zero, while varying the temperature and recording f(T).
In the second instance, compensation can be performed directly at the physical level using phase-tracking of the FT component which corresponds to the sum of the round-trip-times of the flights of both FPIs (i.e. to the third peak in FT of BROS). The Eq. (4) can be thus rewritten as:Eq. (9) is satisfied, the phase of the third peak in FT of BROS becomes temperature-independent and proportional to the RI component that is directly related to the surrounding fluid composition change:
4. Experimental results
The experimental setup is shown in Fig. 6. It consisted of a sensor, the already described spectrally resolved interrogation system (spectral interrogator connected to a PC), container with a test fluid, stirrer with magnetic stirring bar, two burets and heater. A heater was used to change the fluid’s temperature while burets were used to precisely dose fluids with different RIs into the container.
In the first series of experiments we filled the container with demineralized water and then we linearly added/increased the mass concentration of glycerin within the solution, while observing the phase change of the 1st FPI. This experiment was performed in a temperature stabilized water bath (glycerin that was added to the water was also temperature stabilized). The experiment lasted approximately 20 minutes. During this experiment no temperature compensation was carried out. The results are shown in Fig. 7. The refractive index sensing interferometer showed linear change of phase with the RI change having a slope of 48.7 deg/RIU, which also corresponds to spectral sensitivity of 1067 nm/RIU. Estimated non-linearity proved to be better than 0.3% (with correlation coefficient R2 = 0.99993).
In the second experiment we filled the container with demineralized water and we varied the temperature within the range from 25 to 65 °C. The responses of both interferometers are shown in Fig. 8. The temperature-sensing interferometer showed a linear change of phase regarding the temperature with a slope of −5.78 deg/°C, which corresponds to a dn/dT = 9.87 pm/°C (this value is consistent with data for fused silica). The RI sensing interferometer showed nonlinear n(T) dependence, which is characteristic of water (dn/dT is not constant over the test temperature range). This nonlinear curve is however well in agreement with known water RI versus temperature variation .
In the third experiment we attempted to demonstrate the produced sensors temperature resolution. The test fluid was stabilized within a laboratory environment and then we switched on the heater for a short period of time (30 s) to deliver a small amount of heat to the fluid. The response is shown in Fig. 9. The insert in Fig. 9 also shows a magnified section of the curve that indicates measurement system output noise of less than 1 mK. The total time of observation was 42 min at a sample rate 0.025 Hz (400 samples were averaged).
In the fourth experiment we introduced temperature compensation into the measurement algorithm to demonstrate the measurement of the RI component that depends only on the fluid’s composition. The compensation was performed by using Eq. (7). The temperature compensation function f(ΔT) in Eq. (7) was approximated by second-order polynomial and was obtained for water in a separate calibration procedure (cycle) where we varied water temperature over a 24-31°C range, while calculating and recording f(ΔT). The calibration curve for the demineralized water and its polynomial approximations are shown in Fig. 10(b). All subsequent temperature measurements using water as a base fluid thus utilized the curve in Fig. 10. A typical measurement result is shown in Fig. 10(a) where we immersed the sensor within the demineralized water and then performed RI measurements while varying the temperature between 25 and 30°C. Figure 10(a) shows the relative RI change, as measured directly by the first FPI, and the component of the RI that was temperature-compensated and thus depended only on the fluid’s composition. The latter was stable, with drift/fluctuations well below 10−6 RIU over a relatively broad temperature range.
Figure 10 also demonstrates those RI measurements obtained from the third peak phase tracking within the FT of BROS spectrum (peak that represents the combined temperatures-RI FPI) when we immersed a particular sensor in demineralized water. The latter represents the partially compensated response as predicted by Eqs. (8) and (9). The length of the experiential temperature measurement FPI was 1345 μm while the length of the RI measurement FP was 100 μm. Thus to achieve full compensation in water without using numerical corrections the length of the temperature measurement FPI should be around 832 μm. The same sensor was then submersed in 20%w water-glycerol solution and the temperature of the solution was varied over 26.4-36.7 °C range. The phase change of the third peak in FT of BROS and corresponding RI response are shown in Fig. 11.
The glycerin-water solution has considerably higher dn/dT than pure water and thus requires a longer section of temperature measuring FPI to achieve temperature compensation. As shown by Fig. 11, a good intrinsically compensated RI measurement was obtained within the 30.7 to 31.7 °C range for particular combinations of sensor dimensions and water-glycerin solution. This temperature range can be shifted by changing the length of the temperature-sensing section (i.e. second FPI length) as already discussed in the signal processing section of the manuscript. The useful width of the compensated temperature range in such passive compensation using the phase-tracking of the third peak in BROS primary depends on the nonlinearly of fluid’s dn/dT and acceptable RI measurement error.
In the fourth and last experiment we demonstrated the ability of the proposed system to reliably resolve very small changes in the RIs of the measured fluid that are caused by the fluid’s composition change. In order to allow for changing fluid’s RI in both directions, e.g. to be able to increase and decrease the RI of the fluid in the container, we prepared a test solution using demineralized water and glycerol in a mass ratio of 99:1 (the RI of such a solution was 1.31913@1550 nm). Before proceeding with further tests we also performed calibration of the temperature compensation function (f(ΔΤ)) for this mixture. Then we interchangeably added small and precisely dosed quantities of pure water and glycerol-water (98:2) solution through separate burets into the test container. This caused up/down variations of RI over time for preset amounts. Furthermore, we initially heated the solution for a short period of time and then let it cool during the test to demonstrate the practical operation of temperature compensation under varying temperature conditions. The results are shown in Fig. 12(a) and 12(b). The Figures show sensing system output versus time together with the RI of the measured fluid (calculated from the injected amounts of test fluids into the main container).
Figure 12(a) demonstrates the stable response and sensor system resolution of about 2x10−7 RIU under varying temperature conditions. It should be stressed that measurement stability and resolution within the 2x10−7 range require temperature compensation with mK range precision (i.e. dn/dT of water at 25 °C is about 0.8x10−4 K−1), which was also archived by phase-tracking of the temperature measurement FPI phase using a FFT algorithm.
This paper presented a miniature, all-silica, dual-parameter sensor for the simultaneous measurement of surrounding fluid’s RI and temperature. This sensor permits a full temperature-compensated RI measurement that can be used to determine very small changes in fluid structure or composition. Temperature-independent measurements of RI with resolution and stability within the range of 10−7 RIU were demonstrated experimentally under varying (non-constant) temperature conditions (measuring with this resolution normally requires stabilization of the measured fluid’s temperature with an mK accuracy). Sensor design allows for different temperature-compensating schemes including directed compensation that uses the combination of a temperature measuring section with positive dn/dT to compensate for the measured fluid’s negative dn/dT.
The proposed sensor is miniature in size (total length is below 1.5 mm), created on the tip of a standard optical fiber and fabricated by the micromachining process based on selective etching and conventional splicing, cleaving, and etching steps. All-silica design provides high chemical and thermal inertness, while the miniature size provides opportunities for measuring very small (nL) fluid volumes.
We would like to thank the Slovenian Research Agency for its support (Grant No. P2-0368 and L2-5494), and the Optacore d.o.o. team for producing samples of SFF. The paper was also produced within the framework of the operation entitled “Centre of Open innovation and ResEarch UM” that is co-funded by the European Regional Development Fund and conducted within the framework of the Operational Programme for Strengthening Regional Development Potentials for the period 2007-2013.
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