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In this paper, a ratiometric approach to sensing temperature variations is shown using specialty fiber optic devices. We analyzed the transmission response of cascaded segments of multicore fibers (MCFs), and dissimilar lengths were found to generate an adequate scheme for ratiometric operation. The perturbation of optical parameters in the MCFs translates to a rich spectral behavior in which some peaks increase their intensity while others decrease their intensity. Thus, by selecting opposite-behavior peaks, highly sensitive ratiometric measurements that provide robustness against spurious fluctuations can be performed. We implemented this approach using seven-core fiber (SCF) segments of 5.8 cm and 9.9 cm. To test the system’s response under controlled perturbations, we heated one of the segments from ambient temperature up to 150 °C. We observed defined peaks with opposite behavior as a function of temperature. Two pairs of peaks within the interrogation window were selected to perform ratiometric calculations. Ratiometric measurements exhibited sensitivities 6–14 times higher than single-wavelength measurements. A similar trend with enhanced sensitivity in both peak pairs was obtained. In contrast to conventional interferometric schemes, the proposed approach does not require expensive facilities or micrometric-resolution equipment. Moreover, our approach has the potential to be realized using commercial splicers, detectors, and filters.

We demonstrate the use of thermally insensitive fiber optic multimode interference (MMI) devices to measure the thermo-optic coefficient (TOC) of liquids. Such athermal MMI devices are all-fiber, easy to fabricate and, in general, consist of a cascade of several multimode sections. Removing the inherent thermo-optical response of the MMI device is achieved passively in a material-based fashion. Starting from the general theory, we derive simple analytical expressions for the design of two-element MMI cascaded devices with arbitrary materials. Moreover, we verify that those expressions are valid irrespectively of the self-imaging operation. We designed and fabricated an athermal MMI device with a simple architecture consisting of a so-called no-core optical fiber (NCF), which is partly coated by PDMS, and we used it to measure the TOC of several standard liquids. Our results are in good agreement with the values of the TOC reported in the literature. By removing the thermal cross-sensitivity in our device, only one calibration measurement is necessary, which allows for straightforward information retrieval.

We propose an experimental setup to measure the refractive index (RI) of two independent liquid samples by analyzing a single optical signal. We spliced a section of a seven-core fiber (SCF) into each arm of a 1 × 2 fiber coupler and spliced additional single mode fiber (SMF) to use as fiber probe tips. As the spatial frequency of the characteristic sinusoidal response of an SCF is a function of the fiber length, we used two different lengths (3 and 10 cm) in our realization. A superluminescent diode (SLD) is used as the light source, and the reflected light by both fiber tips is acquired through an optical circulator. An optical spectrum analyzer (OSA) is used to analyze the spectral response of the combined signals from both fiber tips. We characterized this sensing system by immersing the fiber tips in liquid samples with a RI between 1.3175 and 1.4040. We show that the two independent RI measurements can be retrieved by performing a Fourier transform on the recorded optical spectrum. In particular, we demonstrate that with adequate calibration, the Fresnel coefficients can be calculated independently. The presented approach represents a straightforward and cost-effective fiber implementation for multipoint sensing.

We present an ultra-high sensitivity fiber loop mirror temperature sensor based on an asymmetric two-hole fiber (ATHF) filled with distilled water. The design of the ATHF allows for larger interaction between the optical mode and the region with large induced birefringence. This enhanced interaction provides large changes in the sensing signal when the temperature is varied. The sensing signal can easily be tracked by following the change in the frequency of the interference signal as a function of the temperature, thus allowing for absolute temperature measurements. Experimental results show that the ATHF birefringence is modified from ${1.18} \times {10}^{{{-{5}}}}$ to $3.33\times 10^{{{-{4}}}}$ when the temperature changes from 40 to 80 °C; generating, in terms of frequency, a sensitivity of $6\times 10^{{{-{3}}}}$ nm−1/°C. In the case of very small temperature changes, where the wavelength shift is also small, simulations were performed using experimental parameters of the ATHF and an ultra-high sensitivity of 240 nm/°C was calculated. The sensitivity of the presented sensor can also be tuned by changing the liquid infiltrated into the fiber holes.

A fiber optic strain sensor, based on the Vernier effect, was constructed using two parallel MZIs. The strain sensitivity of one MZI was 0.84 pm/µε, using this configuration the sensitivity increased up to 13.00 pm/µε.

In this work, we present a wavelength-tunable erbium-doped fiber ring laser based on a Mach- Zehnder interferometer composed of a small piece of capillary hollow-core fiber (CHCF), and two small sections of multimode fibers (MMF).

We demonstrate optical fiber sensors based on highly coupled multicore fibers operating with the optical Vernier effect. The sensors are constructed using a simple device incorporating single-mode fibers (SMFs) and a segment of a multicore fiber. In particular, we evaluated the performance of a sensor based on a seven-core fiber (SCF) spliced at both ends to conventional SMFs, yielding a versatile arrangement for realizing Vernier-based fiber sensors. The SMF–SCF–SMF device can be fabricated using standard splicing procedures and serve as a “building block” for both, reflection and transmission sensing configurations. As demonstrated with our experimental results, the Vernier arrangements can yield a ten-fold increase in sensitivity for temperature measurements compared to a conventional single SMF–SCF–SMF device, thereby confirming the enhanced sensitivity that can be attained with this optical effect. Furthermore, through theoretical analysis, we obtain the relevant parameters that must be optimized in order to achieve an optimal sensitivity for a specific application. Our findings thus provide the necessary guidelines for constructing Vernier-based sensors with all-fiber devices based on highly coupled multicore optical fibers, which constitutes an ideal framework to develop highly sensitive fiber sensors for different applications.

In this paper, we propose a highly sensitive temperature sensor based on two cascaded Mach–Zehnder interferometers (MZIs) that work using the Vernier effect. The all-fiber MZIs were assembled by splicing a segment of capillary hollow-core fiber (CHCF) between two sections of multimode fibers (MMFs). This cascaded configuration exhibits a temperature sensitivity of 1.964 nm/°C in a range from 10 to 70 °C, which is ~67.03 times higher than the sensitivity of the single MZI. Moreover, this device exhibits a high-temperature resolution of 0.0153 °C. A numerical analysis was carried out to estimate the devices’ temperature sensitivity and calculate the magnification of the sensitivity produced by the Vernier effect. The numerical results have an excellent agreement with the experimental results and provide a better insight into the working principle of the MZI devices. The sensor’s performance, small size, and easy fabrication make us believe that it is an attractive candidate for temperature measurement in biological applications.

The applications of serology tests to the virus SARS-CoV-2 are diverse, ranging from diagnosing COVID-19, understanding the humoral response to this disease, and estimating its prevalence in a population, to modeling the course of the pandemic. COVID-19 serology assays will significantly benefit from sensitive and reliable technologies that can process dozens of samples in parallel, thus reducing costs and time; however, they will also benefit from biosensors that can assess antibody reactivities to multiple SARS-CoV-2 antigens. Here, we report a high-throughput microfluidic device that can assess antibody reactivities against four SARS-CoV-2 antigens from up to 50 serum samples in parallel. This semi-automatic platform measures IgG and IgM levels against four SARS-CoV-2 proteins: the spike protein (S), the S1 subunit (S1), the receptor-binding domain (RBD), and the nucleocapsid (N). After assay optimization, we evaluated sera from infected individuals with COVID-19 and a cohort of archival samples from 2018. The assay achieved a sensitivity of 95% and a specificity of 91%. Nonetheless, both parameters increased to 100% when evaluating sera from individuals in the third week after symptom onset. To further assess our platform's utility, we monitored the antibody titers from 5 COVID-19 patients over a time course of several weeks. Our platform can aid in global efforts to control and understand COVID-19.

Light-induced deposits of carbon nanotubes (CNTs) are obtained in multimodal interference (MMI) fiber tips. Differences between deposits obtained with solutions with and without surfactant were observed.

The output signal of Mach-Zehnder curvature sensor was Fourier transformed to increase its curvature range to high value of 1.69 m-1, relative to the traditional fringe contrast measurement 1.09 m-1.

We demonstrate a passive, material-based athermalization of all-fiber architectures consisting of cascaded multimode-interference (MMI) devices. Specifically, the athermal MMI filter used for in-line thermo-optic compensation consists of a no-core multimode fiber, which is spliced between two single-mode fibers, and is partly covered with a polymer coating. The design of the temperature-compensation unit is straightforward and its fabrication is simple. The applicability of our approach is experimentally verified by fabricating wavelength-locked MMI lasers with a temperature sensitivity lower than 1.0 pm/°C, which is several times smaller than that achieved with other fiber-optic devices and is comparable to that targeted in silicon photonics.

In this paper, a narrow spectral linewidth and wavelength-tunable erbium-doped fiber laser based on a Mach-Zehnder interferometer (MZI) as a filter is proposed and experimentally demonstrated. The filter is fabricated by splicing a short length of capillary hollow-core fiber (CHCF) between two small sections of multimode fibers (MMF). The interference pattern generated by the filter has a free spectral range of 10.5 nm and a maximum extinction ratio of 36 dB at 1560 nm. Then, by incorporating the filter inside a ring cavity laser, a single laser emission centered at 1566.46 nm with a spectral width around 0.03 nm and a high SNR of 66 dB (also has a SMSR of 53 dB) is obtained. By changing the temperature of the filter, the wavelength emission of the laser can be tuned from 1566.46 to 1570.09 nm. Considering that the fabrication process of the filter is relatively simple and can be easily implemented in an EDFL, we strongly believe that it can have a great deal of interest in telecom applications.

In this paper, we demonstrate highly sensitive refractive index (RI) sensing based on multicore coupled structures. Specifically, we use a seven-core fiber (SCF) as the sensing element. The interaction of the SCF and its surroundings is induced by controllably etching material from the cladding in order to expose its external cores. Thus, the sensor sensitivity can be tuned by simply controlling the etching depth. In our case, the cladding of a 10-mm-long piece of SCF, spliced between two single-mode fibers, was slowly removed until the remaining cladding around the external cores is only 2.5 μm. Sensitivity on the order of 1 × 104 nm/RIU is experimentally demonstrated. These sensing architectures are compact, all-fiber, simple to fabricate, and highly sensitive. Based on our results RI changes of 10–4–10–5 can be resolved with standard laboratory-graded equipment, which opens the possibility of using this type of sensing architectures in biological applications.

We study the effect of linear coupling on nonlinear phase noise in two-core fibers (TCFs). Considering the elementary TCF switching process, we demonstrate reduction of nonlinear phase noise in the propagation of optically amplified pulses through TCFs. Our results show that the reduction occurs just when the first maximum transfer of optical power is carried out between the TCF cores.

A compact, simple-to-fabricate, low-cost, and highly sensitive optical fiber temperature sensor based on surface plasmon resonance (SPR) is reported. The sensor consists of a core mismatch fiber structure fabricated by splicing a small piece of single-mode fiber (SMF) between two multimode fibers (MMF). SPR is generated when evanescent field interacts with the gold layer deposited over the SMF cladding. Then, the sensor was embedded in polydimethylsiloxane (PDMS), which acts as a temperature to refractive index transducer. Due to PDMS high thermooptic coefficient, the SPR dip underwent a noticeable wavelength shift when a variation of temperature occurred. The device was tested in the 20–60 °C range showing a linear response and a sensitivity of 2.60 nm/°C. This sensor is appealing for temperature monitoring in microfluidic devices made of PDMS due to its high performance and simply fabrication process.

The integration of carbon nanotubes (CNTs) into optical fibers allows the application of their unique properties in robust and versatile devices. Here, we present a laser-induced technique to obtain the deposition of CNTs onto the fiber optics tips of multimode interference (MMI) devices. An MMI device is constructed by splicing a section of no-core fiber (NCF) to a single-mode fiber (SMF). The tip of the MMI device is immersed into a liquid solution of CNTs and laser light is launched into the MMI device. CNTs solutions using water and methanol as solvents were tested. In addition, the use of a polymer dispersant polyvinylpyrrolidone (PVP) in the CNTs solutions was also studied. We found that the laser-induced deposition of CNTs performed in water-based solutions generates non-uniform deposits. On the other hand, the laser-induced deposition performed with methanol solutions generates uniform deposits over the fiber tip when no PVP is used and deposition at the center of the fiber when PVP is present in the CNTs solution. The results show the crucial role of the solvent on the spatial features of the laser-induced deposition process. Finally, we register and study the reflection spectra of the as-fabricated CNTs deposited MMI devices.

A multimode interference (MMI) sensor was designed and experimentally demonstrated for simultaneous measurement of curvature and temperature. A typical fiber structure of single-mode fiber – multimode fiber – single-mode fiber (SMS), mounted in a long and thin carbon steel sheet, and then coated with polydimethylsiloxane (PDMS) was manufactured and tested in curvature and temperature. Bending laboratory results showed that the proffered sensor has a curvature sensitivity of -0.9835 dB/m-1 over a range from 0 m-1 to 1.3652 m-1 , measurements were taken by keeping a constant temperature of 30°C. The laboratory temperature response was -119 pm/°C at a temperature range from 30°C to 60°C, showing an improvement in temperature response, temperature measurements were taken by keeping a constant bending of 0 m-1 . The results show that PDMS coatings are a good way to improve multimode interferometer sensitivity during temperature measurement while keeping a good curvature measuring response, moreover the device shows a linear response within the curvature and temperature ranges. Another advantage of the PDMS coating is that it makes the sensor insensitive to refractive index changes, it gives the sensor robustness and protection against dust.

We present a resonant magnetic field sensor based on microelectromechanical systems technology with optical detection. The sensor has single resonator composed of two orthogonal silicon beams (600 µm × 26 µm × 2 µm) with an integrated mirror (50 µm × 34 µm × 0.11 µm) and gold tracks (16 µm × 0.11 µm). The resonator is fabricated using silicon-on-insulator wafer in a simple bulk micromachining process. The sensor has easy performance that allows its oscillation in the first bending vibration mode through the Lorentz force for monitoring in-plane magnetic field. Analytical models are developed to predict first bending resonant frequency, quality factor, and displacements of the resonator. In addition, finite element method models are obtained to estimate the resonator performance. The results of the proposed analytical models agree well with those of the finite element method models. For alternating electrical current of 30 mA, the sensor has a theoretical linear response, a first bending resonant frequency of 43.8 kHz, a sensitivity of 46.1 µm T−1, and a power consumption close to 54 mW. The experimental resonant frequency of the sensor is 53 kHz. The proposed sensor could be used for monitoring in-plane magnetic field without a complex signal conditioning system.

In this work, an analysis of the luminescent properties of zirconium oxide (ZrO 2 ) doped with Er 3+ in three different concentrations was performed to investigate which sample could be useful for optoelectronic applications. To achieve it, a set of four samples of ZrO2:Er 3+ with the variation in Er 3+ concentration about 0%, 0.1%, 0.2% y 0.3% were synthesized by the sol-gel method. The luminescence measurements for all samples were performed using an excitation wavelength of 380 nm, by this way the upconversion spectrum for the sample containing the 0.2% of Er 3+ was measured at 980 nm. The luminescence spectra showed three emission bands attributed to the electronic transitions 4 F 7/2 → 4 I 15/2 , 2 H 11/2 → 4 I 15/2, and 2 S 3/2 → 4 I 15/2 , all this band belonged to Er 3+ radiative transitions, we found that the sample with 0.2% presented a higher emission intensity at 550 nm compared with the others samples, also the average lifetime was calculated, we found that this value decrease with the increment of Er 3+ concentration, finally, the upconversion spectrum were measured for the sample with higher luminescent intensity at 380 nm, the result revealed that the most probable emission after the photon absorption is that around 550 nm.

In this paper, we experimentally analyze the intensity and phase dependence of the electric current in an active long-range surface plasmon polariton (LRSPP) device. It consists of a metallic waveguide embedded in a dielectric material where we have included external electric contacts. Experimental results show that the intensity beam decays almost 15 dB when we apply 150 mW, as we can obtain a phase change of 15p for the same value of electric power.