Temperature-insensitive fiber-optic tip sensors array based on OCMR for multipoint refractive index measurement

Xiu He; Zengling Ran; Tingting Yang; Yaqin Xiao; Yaxin Wang; Yunjiang Rao

doi:10.1364/OE.27.009665

1. Introduction

Refractive index (RI) sensors could find important applications in many fields, such as quality control of foods during processing and packaging, adulteration detection (edible oils, gasoline or automotive lubricants) [1], monitoring of environmental pollution [2], biomedical applications, and process monitoring of composite materials [3]. Among different RI sensors, the fiber-optic ones have attractive extensive attentions due to advantages of good electromagnetic resistance, small size, high sensitivity, etc. Different types of fiber-optic RI sensors have been reported, such as fiber Bragg gratings (FBGs) [4,5], long period fiber gratings (LPFGs) [6], Fabry-Perot interferometers (FPs) [7–10], in-fiber Mach-Zehnder interferometers (MZIs) [11–13], Sagnac interferometers [14], Michelson interferometers [15,16], multimode interferometers [17–21], and optical fiber surface plasmon resonance (SPR) [22,23]. Most of them are sensitive to temperature and then bring high cross-talk between RI and temperature, as well as involving relatively complex fabrication process. Meanwhile, a single point RI sensor is difficult to implement effective functions when the materials under test have complex chemical ingredients and structures. Generally, multipoint integrated RI sensors are highly desired especially for these applications, such as identification of complex structure in genetics, analysis of complex substances in proteomics, and drug screening.

Several configurations for multipoint fiber-optic refractometers have been reported, based on methods such as spatial frequency multiplexing (SFM) [24–28], wavelength division multiplexing (WDM) [29], short pulse optical time domain reflectometer (OTDR) [30,31], or multi-wavelength OTDR with an arrayed waveguide grating used as the wavelength division multiplexer [32], and cascaded fiber-optic interferometers [33]. The SFM method in [24–28] requires optical spectrum analyzer (OSA) with high resolution and a large spectrum range. The WDM method in [29,32] is suffered from adjacent channels cross-talk, consequently degrading measurement accuracy. The OTDR method in [30,31] is difficult to multiplex sensors with high spatial resolution, since its spatial resolution depends on the pulse width. Y. Xiang [33] proposed a cascaded and wavelength multiplexing FBG based FP RI sensing system, but the sensor size is too large to be well integrated.

In this paper, a simple, temperature-insensitive and highly integrated fiber-optic tip sensors array is proposed for multipoint RI measurement, which are highly multiplexed utilizing optical carrier based on microwave reflective (OCMR) technology. The approach involves sending a microwave-modulated optical signal through the fiber-optic tip sensors array. The microwave signal is used to obtain reflective amplitudes of the sensing tips which depend on their reflectivity. And the reflectivity of the sensing tips is affected by the RI measured by them thereby achieving RI sensing. The tip sensors array is only made of a series of cleaved fiber end-faces. It can offer many advantages of low cost, robustness, small size, and easy integration. Based on OCMR technology, sensors could be highly multiplexed with a large number, since the OCMR technology can easily and accurately obtain the reflectivity of mirrors in the fiber with millimeters to centimeters high spatial resolution, due to the fact that it is a frequency domain analysis technology. Therefore, the system is suitable for biochemical sensing application, such as identification of complex structure in genetics, analysis of complex substances in proteomics.

2. Principle of operation

2.1. System construction

The proposed approach is schematically shown in Fig. 1. A broadband continuous light modulated by a modulator becomes microwave-modulated light, where the optical light is the carrier and the microwave is the envelope. Among that, the modulator is driven by a microwave signal whose frequency can be scanned via computer control. The modulated light inputs to a planar waveguide optical divider (PLC) with sensing tips via a circulator, and then the reflected sensing signal is transmitted through the circulator to a photodetector (PD). Finally, the PD outputs to the microwave system for signal processing. Additionally, the optical detection is synchronized with microwave frequency by a phase lock loop so that the amplitude and phase of the reflected signal can be obtained. After the microwave frequency scanning covering the entire available range, the reflection microwave spectrum is gotten, and then the inverse complex Fourier transform of the microwave spectrum provides the time-resolved discrete reflection spectrum.

Fig. 1 Schematic of OCMR system for multipoint RI measurements.

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2.2. Theoretical analysis

A simplified mathematical expression of the optical wave of all the reflections received by the PD in the OCMR system can be written by [34]

{| \sum_{i = 1}^{N} E_{i} |}^{2} = DC term + Microwave term + Optical term

I_{D C} = \sum_{i = 1}^{N} R_{i} A^{2}

I_{m} = \sum_{i = 1}^{N} R_{i} A^{2} B \cos [f (t + \frac{W}{c} + \frac{2 z_{i} n}{c})]

I_{o} = \frac{2}{Δ ω} \sum_{i = 1, j = 1 j \neq i}^{N} \sqrt{(1+ B \cos [f (t + \frac{W + 2 z_{i} n}{c})) (1+ B \cos [f (t + \frac{W + 2 z_{j} n}{c})]}) \sqrt{R_{i}} \sqrt{R_{j}} A^{2} \int_{ω_{\min}}^{ω_{\max}} {\cos [\frac{2 ω}{c} (z_{1} - z_{2}) n]} d ω

where I_DC, I_m, I_o are the DC term, the microwave term, the optical term of the total reflections intensity of optical wave, respectively; E_i is the reflective electric field of the i-th microwave-modulated wave; t is the time; N is the total number of sensing tips; i is the i-th sensing tip; A and B are amplitudes of the optical carrier and the microwave envelope, respectively; ω and f are the optical angular frequency and the microwave angular frequency, respectively; W is the electrical length of the common microwave path and this delay is the same for all the path; c is the speed of light in vacuum; z_i represents the location of the i-th sensing tip; ω_max and ω_min are the maximum and minimum optical angular frequency of light source; R_i is the reflectivity of the i-th sensor, which can be expressed by

R_{i} = {(\frac{n_{f i b e r} - n_{i}}{n_{f i b e r} + n_{i}})}^{2}

where n_fiber is the effective refractive index of fiber, n_i is the liquid RI measured by the i-th sensing tip.

In the OCMR system, a broadband light source is chosen for eliminating systematic instability induced by polarization and coherence, due to its coherence length of micrometers. Under the condition, the optical path difference (OPD) of arbitrary two sensing tips is sufficiently larger than the coherence length of the optical source, i.e., (z₁-z₂) n>>c/Δω. Therefore, the optical term (I_o) approaches zero.

The OCMR system uses synchronized detection to eliminate the DC term and records the amplitude and phase of the microwave term. By a complex and inverse Fourier-transform to the microwave term, a series of cardinal sinc functions are obtained at discrete sensors in time domain, given by:

X (t) = \sum_{i = 1}^{N} X_{i} (t) = \sum_{i = 1}^{N} R_{i} A^{2} B | \sin c [(f_{\max} - f_{\min}) (t + τ_{i})] |

where f_max and f_min are the maximum and the minimum microwave angular frequency, respectively; X_i is the reflective amplitude of the i-th sensing tip in time domain;

τ_{i} = (W + 2 z_{i} n) / c

is the propagation delay of the signal corresponding to the i-th sensing tips. Equation (6) provides the location information of each sensing tip in the array. The peaks of the sinc functions are at the specific reflectors locations (z_i) that can be found when the delays (τ) are determined. Further, the reflective amplitude of each sensing tip in time domain can be expressed by:

X_{i} = k R_{i} t = τ_{i} a n d i = 1, 2, \dots, N at t = t_{i} and i = 1, 2, ..., N .

where k is related to the amplitudes of light and microwave, that is, k = A²B. As described in Eq. (7), the discrete time domain reflective amplitudes (X_i) are proportional to the reflectivity (R_i) of the sensing tips, which is affected by the liquid with different RI. And, the RI of the liquid measured by the i-th sensing tip is derived from Eq. (5) and Eq. (7), as follows:

\begin{matrix} \begin{array}{l} n_{i} = \frac{1 - \sqrt{R_{i}}}{1 + {\sqrt{R}}_{i}} n_{f i b e r} = \frac{1 - \sqrt{X_{i} / k}}{1 + \sqrt{X_{i} / k}} n_{f i b e r}, n_{i} < n_{f i b e r} \\ n_{i} = \frac{1 + \sqrt{R_{i}}}{1 - \sqrt{R_{i}}} n_{f i b e r} = \frac{1 + \sqrt{X_{i} / k}}{1 - \sqrt{X_{i} / k}} n_{f i b e r}, n_{i} > n_{f i b e r} \end{array} & i = 1, 2, \dots, N \end{matrix}

According to Eq. (8), the measured RI (n_i) depends on the reflectivity (R_i) of the i-th sensing tip. Or it depends on the amplitudes of the optical carrier and the microwave envelope.

Then, the RI sensitivity of the i-th sensing tip can be defined by:

S_{R I, i} = \frac{Δ X_{i}}{Δ n} = k \frac{Δ R_{i}}{Δ n} = - \frac{4 k n_{f i b e r} (n_{f i b e r} - n_{i})}{{(n_{f i b e r} + n_{i})}^{3}}

where ΔX_i and ΔR_i are the variation of the reflective amplitude of the i-th sensing tip in time domain and the change of its reflectivity, when the measured RI changes by Δn, respectively.

Since the n_fiber has a temperature dependence [35], the reflectivity (R) of the sensing tip will change when the surrounding temperature changes, eventually influences the measured liquid RI value, that is, temperature change will degrade the accuracy of the RI measured by the sensing tips. To evaluate the influence of the temperature on the accuracy of the RI measurement, the temperature sensitivity of the i-th sensing tip is defined under the condition where n_i is temperature independent, as follows:

S_{T, i} = \frac{Δ X_{i}}{Δ T} = k \frac{Δ R_{i}}{Δ T} = \frac{4 k n_{i} (n_{f i b e r} (T) - n_{i})}{{(n_{f i b e r} (T) + n_{i})}^{3}} \frac{\partial n_{f i b e r} (T)}{\partial T}

where ΔX_i and ΔR_i are the variation of the reflective amplitude of the i-th sensing tip in time domain and the reflectivity change of the i-th sensing tip, when its temperature changes by ΔT, respectively. n_fiber(T) represents the effective RI of the sensing tip, which is related to temperature. Temperature sensitivity (S_T,i) of the sensing tip represents the change in the reflectivity (R_i) of the sensing tip caused by changes in the effective RI of the fiber as a function of temperature. Next, the temperature-RI cross sensitivity of the i-th sensing tip is defined below:

C_{i} = \frac{S_{T, i}}{S_{R I, i}}

The temperature-RI cross sensitivity (C_i) represents the influence on measurement accuracy of the RI when temperature changes by 1 °C.

Owing to the peak amplitude influenced by fluctuation in laser light source power, microwave source, PD response, and environmental factors such as temperature, humidity, it directly affects the accuracy and stability of the RI measurement in this system. Hence, a self-calibration method is used. Since peak 1 as a reference is not affected by the measured liquid, we can compensate the fluctuation of other peaks by simply taking ratio with peak 1 as

X_{i}^{'} = \frac{X_{m e a s u r e d}}{X_{1}} = \frac{X_{i}}{X_{1}}, i = 2, 3, ..., N

where X_i^’ is the calibrated reflective amplitude of the i-th sensing tip. Therefore, the i-th liquid refractive index (n_i) measured by the i-th sensing tip by the calibration method can be expressed below:

\begin{matrix} \begin{array}{l} n_{i} = \frac{1 - \sqrt{X_{i} / X_{1}}}{1 + \sqrt{X_{i} / X_{1}}} n_{f i b e r}, n_{i} < n_{f i b e r} \\ n_{i} = \frac{1 + \sqrt{X_{i} / X_{1}}}{1 - \sqrt{X_{i} / X_{1}}} n_{f i b e r}, n_{i} > n_{f i b e r} \end{array} & i = 1, 2, \dots, N . \end{matrix}

Compared to n_i in Eq. (8), the measured RI (n_i) is not affected by the fluctuation of the light and microwave source.

In addition, the spatial resolution based on the OCMR can be described as

D = \frac{c}{2 n_{g r} (f_{\max} - f_{\min})}

where n_gr is the group RI of fiber. Generally, a microwave system like a vector network analyzer can obtain the reflectivities of mirrors in the fiber with 1 centimeter high spatial resolution which is much shorter than those reported in [27–29], when its scanning microwave frequency range is 10 GHz.

3. Experiments and discussions

In order to verify the feasibility of the system, a RI measurement test is performed. The experimental set up is illustrated in Fig. 2. The broadband light source used is an amplified spontaneous emission (ASE) source with wavelength ranging from 1520 to 1620 nm. The intensity modulation of the light is achieved through an electro-optic modulation (EOM, JDS Uniphase X5). To make sure the electro-optic modulator (EOM) with high modulation efficiency, a polarization controller (PC) is placed before the EOM. The microwave source and signal detection are realized through a vector network analyzer (VNA, TRANSCOM INSTRUMENTS T5280A). The EOM is driven by Port 1 of VNA. The microwave-modulated light is coupled into a 1 × 8 planar light wave circuit (PLC) by a fiber optical circulator.

Fig. 2 Experimental setup for multipoint refractive index measurement based on OCMR.

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The reflected signal is received by a PD with bandwidth of 300MHz. The PD outputs to the Port 2 of the VNA. By sweeping the VNA frequency ranging from 300 KHz to 295 MHz with the sampling point set to 1601 and intermediate frequency bandwidth (IFBW) set to 3 KHz, the amplitude and phase information of the reflected signal is obtained. The obtained signal (S21 of the VNA) is the microwave frequency spectrum of the tip sensors array. After applying a complex inverse Fourier transform to the microwave frequency spectrum, the time-domain spectrum of the array is gotten.

There are 8 sensing elements integrated into an array probe, which is convenient to operate and realize multipoint sensing. The transmission fiber length among sensing tips is different, and the delay fiber between adjacent sensing tips is chosen larger 3 or 4 times than the calculated value by Eq. (14) for purpose of decreasing crosstalk between them.

In the test, the first sensing tip is put in the water as reference, whose reflective amplitude is independent to the measured RI of the liquid, and others are immersed in a tank simultaneously in saline solutions with different RIs ranging from 1.3382 to 1.3727. Figure 3(a) shows reflective amplitude changes of all sensing tips in the time domain at various RIs. From the spectrum, 8 discrete peaks corresponding to 8 sensing tips show different reflective amplitudes resulting from the different end-faces flatness, cutting angles of sensing tip, and different losses in the fiber connectors. The reflective amplitude of the first sensing head has a little decrease due to the instability of the optical light source and the interference of the surrounding factors, and others decreased sharply with RI increasing. Figure 3(b) is the zoom graph of the fourth sensing tip, and clearly shows the reflective amplitude is dramatically decreased with RI increasing.

Fig. 3 (a) Reflective amplitudes of the 8 sensing tips in time domain at various RIs. (b) Zoom graph of the fourth sensing tip.

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Then a quantitative analysis about RI response of the sensing tips is performed with two processing methods: without self-calibration (in Fig. 4(a)) and with self-calibration (in Fig. 4(b)). Among that, Fig. 4(a) shows the RI sensitivities are from 8.585 х 10³ μv/RIU to 1.645 х 10⁴ μv/RIU. There are different sensitivities among 7 sensing tips, and the same phenomenon occurs in Fig. 4(b). From Fig. 4, the higher reflective amplitude is, the larger RI sensitivity is. This phenomenon may be induced by the different cutting angles of fiber end-face. Moreover, a comparative analysis about linearity of each sensing tip between two calculation methods is shown in Table 1. It shows the linearity using self-calibration method is better, indicating that the influence induced by fluctuations of optical light source and other external surroundings can be effectively decreased.

Fig. 4 RI responses of the 7 sensors (a) without self-calibration, (b) with self-calibration.

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Table 1. Comparison of the linearity of each sensing tip with two calculation methods

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From Fig. 4, the difference of the absolute values of RI sensitivities among 7 sensing tips is too large to ensure sensors’ manufacture consistency. One way to solve it is that fiber end-faces are polished to degrade difference of the cutting angle among the sensing tips. The other effective way is to use a normalized method, that is, normalize the reflective amplitude of sensing tips as followed:

X_{i, j}^{*} = \frac{X_{i, j}^{'} (t)}{X_{i, 1}^{'} (t)} = \frac{\frac{X_{i, j}}{X_{1, j}}}{\frac{X_{i, 1}}{X_{1, 1}}}, i = 2, 3, ..., N, j = 2, 3, ..., M .

where i and j represent for the i-th sensing element and the j-th RI test, respectively. M is the total number of RI test. Therefore,

X_{i, j}^{*}

is the normalized reflective amplitude of the i-th sensing element in the j-th RI test.

X_{i, j}^{'}

is the calibrated reflective amplitude of the i-th sensing element in the j-th RI test. X_i,j is the reflective amplitude of the i-th sensing element in the j-th RI test in time domain. The normalized reflective amplitude (

X_{i, j}^{*}

) as a function of the reflective index (n_i) measured by 7 sensing tips is shown in Fig. 5 (blue line). It shows the absolute values of RI sensitivities of the 7 sensing tips have a pretty consistency. A comparative analysis about the RI sensitivity and linearity of each sensing tip between the normalized method (in Fig. 5) and self-calibration method (in Fig. 4(b)) is shown in Table 2. It shows almost 50 percent difference of RI sensitivities with self-calibration method and only 4 percent difference with normalized method. Besides, the fitting linearity degree of RI responses among the 7 sensors is respectively consistent using these two methods. This normalized method can effectively reduce the demand for consistency in sensor fabrication.

Fig. 5 Normalized reflective amplitudes as a function of refractive index (blue line) and temperature (red line).

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Table 2. Comparison of RI sensitivities and fitting linearity degree of sensors with the two calculation methods

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Due to the small variation of the RI of air over the temperature ranging from room temperature to 400 °C, almost 10⁻⁴ RIU [36], which is much smaller than that of liquid in this temperature range, the temperature dependence of the tip RI sensors array is measured in air. The temperature of the first sensing tip is kept at room temperature during the whole temperature test, and the other sensors are placed to a high temperature furnace with a temperature accuracy of ± 1°C (Micro-X Furnace, MX 1180043). The temperature is gradually rising from 23.5 °C to 400 °C, at each temperature point, it is kept for 20 min to ensure uniform distribution of the temperature. The normalized reflective amplitude as a function of the temperature is shown in Fig. 5 (red line). It shows each sensing tip has a small variation from 23.5 °C to 400 °C. From Fig. 5, the temperature sensitivity and RI sensitivity of one of the sensing tips is 4.94 × 10⁻⁶ a.u/°C and –13.202 a.u./RIU, respectively, and its temperature-RI cross-sensitivity is only 3.74 × 10⁻⁷ RIU/°C, which is negligible and the sensing tips can be considered temperature-insensitive.

To get the resolution of the RI sensor, we also experiment the stability of the RI sensors in air. Figure 6 shows the fluctuation of reflective amplitudes among the 7 sensing tips as a function of time, and obtains the maximum short-term fluctuation of 0.06 μv. According to Fig. 4(a) and Fig. 6, the minimum RI resolution among the sensing tips is calculated to reach 3.60 × 10^-6 RIU. The comparison of sensing performances of multipoint RI measurement methods is shown in Table 3.

Fig. 6 Stability of the sensing tips, and the inset is the corresponding spectrum in time domain.

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Table 3. The comparison of sensing performances of multipoint RI measurement methods

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4. Conclusions

A simple, temperature-insensitive and highly integrated fiber-optic tip sensors array is proposed and demonstrated for multipoint RI measurement based on OCMR, for the first time to our knowledge. The tip sensors array is only made of a series of cleaved fiber end-faces, and it has great potential that can be integrated into a tinier chip when using microfibers. Using a self-calibration method can effectively reduce the fluctuation of optical light source and external surroundings, and a normalized method is proposed and demonstrated to ensure RI sensitivity consistency of sensing tips. Experimental results indicate that the tip sensors have a high resolution of 3.60 × 10⁻⁶ RIU and a low refractive index-temperature cross-talk of 3.74 × 10⁻⁷ RIU/°C. Moreover, the OCMR system can easily and accurately obtain the reflectivity of mirrors in the fiber with centimeters high spatial resolution. It is expected that the sensing system could be used in a wide range of biochemical and environmental applications, such as identification of complex structure in genetics, analysis of complex substances in proteomics, monitoring of environmental pollution.

Funding

National Natural Science Foundation of China (NCSF) (51205049, 51875091, 51327806); State 111 Project (B14039).

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Sensor	S₂	S₃	S₄	S₅	S₆	S₇	S₈
Linearity without self-calibration	0.990	0.993	0.995	0.990	0.993	0.994	0.991
Linearity with self-calibration	0.998	0.999	0.998	0.999	0.999	0.999	0.999

Character	Value	S₂	S₃	S₄	S₅	S₆	S₇	S₈
Sensitivity (a.u./RIU)	K_{self-calibration}	8.981	14.544	16.844	13.333	14.242	11.120	11.590
Sensitivity (a.u./RIU)	K_normalized	13.202	12.906	13.519	12.963	13.159	13.475	12.937
Fitting Linearity degree	$R_{s e l f - c a l i b r a t i o n}^{2}$	0.998	0.999	0.998	0.999	0.999	0.999	0.999
Fitting Linearity degree	$R_{n o r m a l i z e d}^{2}$	0.998	0.999	0.998	0.999	0.999	0.999	0.999

Method	Ref	Sensing performance	comment
SFM method	[24]	Resolution: 3 × 10⁻⁴ Repeatability: 5 × 10⁻⁴	-Requires OSA with high resolution and a large spectrum range
	[26]	Temperature-refractive index cross sensitivity: 6.0 × 10⁻⁵ RIU/°C	-Complex fabrication of the sensors -The sensing performances of the multiplexed sensors are different
	[27]	Resolution: 5 × 10⁻⁴ The standard deviations of the measured RI: ~10⁻³	-Complex fabrication of the sensors -Requires OSA with high resolution and a large spectrum range
WDM method	[29]	Average sensitivity: −101.9 dB/RIU in the range of 1.33~1.42	+ Simple fabrication of the sensors -The number of the sensing points is limited by the spectral range of the source -Adjacent sensing channels exist cross-talk
WDM method	[32]	Temperature-reflective index cross sensitivity: 10⁻⁴ RIU/°C Precision: 1.7 × 10⁻⁴	+ The distance between sensor points are not limited + Simple fabrication of the sensors -The number of the multiplexed sensors is limited by the spectral range of light source
OTDR method	[30]	RI sensitivity: 38.785~305.430 dB/RIU	-The distance between sensor points is limited by the spatial resolution of the OTDR + Simple fabrication of the sensors
OTDR method	[31]	The standard deviations of the measured RI: 2.8 × 10⁻⁶ for distilled water, 2.99 × 10⁻⁵ for methanol	-The distance between sensor points is limited by the spatial resolution of the OTDR + Simple fabrication of the sensors
OCMR method	This work	Resolution: 3.60 × 10⁻⁶ Temperature-RI cross sensitivity: 3.74 × 10⁻⁷ RIU/°C	+ Simple fabrication of the sensors + The sensing performances of the multiplexed sensors are similar + Highly multiplexed with a large number

Sensor	S₂	S₃	S₄	S₅	S₆	S₇	S₈
Linearity without self-calibration	0.990	0.993	0.995	0.990	0.993	0.994	0.991
Linearity with self-calibration	0.998	0.999	0.998	0.999	0.999	0.999	0.999

Character	Value	S₂	S₃	S₄	S₅	S₆	S₇	S₈
Sensitivity (a.u./RIU)	K_{self-calibration}	8.981	14.544	16.844	13.333	14.242	11.120	11.590
Sensitivity (a.u./RIU)	K_normalized	13.202	12.906	13.519	12.963	13.159	13.475	12.937
Fitting Linearity degree	$R_{s e l f - c a l i b r a t i o n}^{2}$	0.998	0.999	0.998	0.999	0.999	0.999	0.999
Fitting Linearity degree	$R_{n o r m a l i z e d}^{2}$	0.998	0.999	0.998	0.999	0.999	0.999	0.999

Temperature-insensitive fiber-optic tip sensors array based on OCMR for multipoint refractive index measurement

Abstract

1. Introduction

2. Principle of operation

2.1. System construction

2.2. Theoretical analysis

3. Experiments and discussions

4. Conclusions

Funding

References

Cited By

Figures (6)

Tables (3)

Equations (15)

Optics Express