Femtosecond laser-inscribed fiber interface Mach-Zehnder interferometer for temperature-insensitive refractive index measurement

High-sensitivity vector bend sensor based on a fiber directional coupler inscribed by a femtosecond laser

In this Letter, we demonstrate a high-sensitivity vector bend sensor based on a fiber directional coupler. The fiber directional coupler is composed of two parallel waveguides inscribed within a no-core fiber (NCF) by a femtosecond laser. Since the two written waveguides have closely matched refractive indices and geometries, the transmission spectrum of the fiber directional coupler possesses periodic resonant dips. Such a fiber directional coupler exhibits a good bending-dependent spectral shift response due to its asymmetric structure. Experimental results show that bending sensitivities of -97.11 nm/m^-1 and 58.22 nm/m^-1 are achieved for the 0° and 180° orientations in the curvature range of 0-0.62 m^-1, respectively. In addition, the proposed fiber directional coupler is shown to be insensitive to external humidity changes, thus improving its suitability in high-accuracy bending measurements.

In-line mode-dependent loss equalizer with femtosecond laser induced refractive index modification

We demonstrate an in-line all-fiber mode-dependent loss (MDL) equalizer with femtosecond laser induced refractive index (RI) modification. By inscribing an RI-modified structure into the core of a few-mode fiber (FMF), a differential mode attenuation (DMA) can be achieved for LP₀₁ and LP₁₁ modes. The DMA can serve as an in-line MDL equalizer for the long-haul mode-division multiplexing transmission system. Through numerical simulations, we identify that the LP₀₁ mode has a larger attenuation than that of higher-order modes, where the sign of DMA is contrary to that of the conventional FMF links and devices. Finally, a proof-of-concept experiment is implemented by inscribing an RI modified region with a width of 4 µm, a height of 13 µm, and a length of 200 µm into the FMF core. An average additional attenuation of 8.4 dB and 3 dB can be applied to the LP₀₁ and LP₁₁ modes over the C-band, respectively, leading to an MDL equalization range of 5.4 dB. Meanwhile, the average polarization dependent loss (PDL) of the LP₀₁ and LP₁₁ modes induced by the in-line MDL equalizer is approximately 0.3 dB over the C-band. Power matrix measurement indicates that the in-line MDL equalizer has a negligible mode coupling. The proposed in-line MDL equalizer with a wider range and low insertion loss is feasible by precise manipulation of femtosecond laser inscription.

Magneto-refractive properties and measurement of an erbium-doped fiber

The magneto-refractive properties of an erbium-doped fiber (EDF) are investigated by theoretically analyzing the change in mode characteristics with a magnetic field and experimentally measuring it based on a fiber-optic Mach-Zehnder interferometer (MZI). The numerical results indicate that the mode effective refractive index (RI) increases as the magnetic field increases, and the mode field intensity distribution tends to be more concentrated in the core region with an increasing magnetic field. The variation in the mode effective RI of the fundamental mode with the magnetic field is greater than that of the higher-order modes. A magneto-refractive measurement system based on a fiber-optic MZI is set up to analyze the magneto-refractive effect of the EDF. The changes in the mode effective RI measured with a direct-current (DC) magnetic field and with a 100 Hz alternating-current (AC) magnetic field are 4.838×10^-6 and 4.245×10^-6 RIU/mT, respectively. The experimental results are in reasonable agreement with the theoretical analysis. Furthermore, the error between the experimental and numerical results is discussed. The magneto-refractive properties of the EDF exhibit potential in all-fiber magnetic field or current sensing area.

Extremely sensitive multi-order mode refractive index sensor using TiO2 nanograss film and weakly bounded waveguide modes

An extremely sensitive multi-order mode refractive index (RI) sensor was fabricated by coupling titanium dioxide nanograss film coated FTO conductive glass with Kretschmann prism. Both calculation and experimental studies were carried out. Theoretical analysis by employing resonant waveguide modes indicated that the maximum sensitivity could be achieved when the mode worked at the weakly-bounded condition. The experimental results showed that for p-polarized and s-polarized light, the sensor exhibited a maximum RI sensitivity of 2938.21 nm/RI unit (RIU) and 1484.39 nm/RIU in the 1^st order mode, respectively. Its maximum figure of merit was as high as 77.77. The proposed sensor is promising to be applied in environmental monitoring, immune analysis, nucleic acid test, etc.

Optical Fiber Sensors by Direct Laser Processing: A Review

The consolidation of laser micro/nano processing technologies has led to a continuous increase in the complexity of optical fiber sensors. This new avenue offers novel possibilities for advanced sensing in a wide set of application sectors and, especially in the industrial and medical fields. In this review, the most important transducing structures carried out by laser processing in optical fiber are shown. The work covers different types of fiber Bragg gratings with an emphasis in the direct-write technique and their most interesting inscription configurations. Along with gratings, cladding waveguide structures in optical fibers have reached notable importance in the development of new optical fiber transducers. That is why a detailed study is made of the different laser inscription configurations that can be adopted, as well as their current applications. Microcavities manufactured in optical fibers can be used as both optical transducer and hybrid structure to reach advanced soft-matter optical sensing approaches based on optofluidic concepts. These in-fiber cavities manufactured by femtosecond laser irradiation followed by chemical etching are promising tools for biophotonic devices. Finally, the enhanced Rayleigh backscattering fibers by femtosecond laser dots inscription are also discussed, as a consequence of the new sensing possibilities they enable.

High-consistency fiber-optic Fabry-Perot sensor based on MEMS for simultaneous temperature and liquid refractive index measurement

We proposed a hybrid Fabry-Perot fiber-optic sensor based on the microelectromechanical system (MEMS) technique for measuring temperature and liquid refractive index simultaneously, and we verify the consistency of four sensors in the same batch. The sensor consists of a groove-array structured glass wafer and two silicon wafers, which are connected by double-sided anodic bonding. The three parts form two independent Fabry-Perot cavities for temperature and liquid refractive index sensing, respectively. We randomly selected three sensors in the same batch and conducted temperature and refractive index experiments to establish the sensing equation. The experimental results demonstrate their high consistency with temperature sensitivities of 81.6, 81.8, and 81.4 pm/°C in the range of 10°C to 80°C, and refractive index sensitivities of 1040.11, 1044.24, 1042.91 nm/RIU in the range of 1.333-1.374. The sensors have low cross-sensitivities that are less than 5.86×10^-6 RIU/°C and high precisions of 0.047°C, 2.14×10^-6RIU, respectively. To verify the validity of the sensing equation, we made another sensor in the same batch and got maximum errors of 0.36°C and 7.7×10^-5RIU, respectively.

Fiber-interface directional coupler inscribed by femtosecond laser for refractive index measurements

A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as ∼8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing.

Fiber Reshaping-Based Refractive Index Sensor Interrogated through Both Intensity and Wavelength Detection

A fiber reshaping-based refractive index (RI) sensor is proposed relying on both optical intensity variation and wavelength shift. The objective of this study is to completely reshape the core and to ultimately mimic a coreless fiber, thereby creating a highly efficient multimode interference (MMI) coupler. Thus, propagation modes are permitted to leak out into the cladding and eventually escape out of the fiber, depending on the surrounding environment. Two interrogation mechanisms based on both the intensity variation and wavelength shift are employed to investigate the performance of the RI sensor, with the assistance of leaky-mode and MMI theories. By monitoring the output intensity difference and the wavelength shift, the proposed RI sensor exhibits high average sensitivities of 185 dB/RIU and 3912 nm/RIU in a broad range from 1.339 to 1.443, respectively. The operating range and sensitivity can be adjusted by controlling the interaction length, which is appealing for a wide range of applications in industry and bioscience research.

Surface plasmon resonance refractive index sensor based on fiber-interface waveguide inscribed by femtosecond laser

A novel surface plasmon resonance (SPR) configuration based on fiber-interface waveguide was proposed and realized by combining the technology of femtosecond laser writing waveguide with SPR effect for measuring refractive index (RI) of analyte. A U-shaped waveguide is inscribed in the coreless fiber and its bottom is very close to the fiber surface, which can produce strong evanescent field being sensitive to ambient media. When the fiber surface is coated with a layer of gold film, the strong evanescent field can excite the SPR effect on the fiber surface. Most importantly, different from some types of fiber SPR sensors with a fragile physical structure, the fiber-interface waveguide SPR sensor exhibits an excellent mechanical strength. Such a SPR sensor exhibits a high sensitivity of ∼3352  nm/RIU at the RI value of ∼1.395, which may have important practical applications in medicine, environmental monitoring, and food safety.

High Sensitivity Refractometer Based on a Tapered-Single Mode-No Core-Single Mode Fiber Structure

We have proposed a novel tapered-single mode-no core-single mode (TSNS) fiber refractometer based on multimode interference. The TSNS structure exhibits a high contrast ratio (>15 dB) and a uniform interference fringe. The influence of different lengths and diameters of the TSNS on the refractive index unit (RIU) sensitivity was investigated. The experimental investigations indicated a maximum sensitivity of 1517.28 nm/RIU for a refractive index of 1.417 and low-temperature sensitivity (<10 pm/°C). The experimental and simulation results are also in good agreement.

Determination of geometry-induced positional distortion of ultrafast laser-inscribed circuits in a cylindrical optical fiber

Positional distortion is a defocusing phenomenon in ultrafast laser inscription of fiber optic circuits induced by the cylindrical geometry of an optical fiber. In this Letter, a study on the positional distortion of ultrafast laser processing assisted by tightly focusing optics has been conducted. Attention has been paid to the effect of numerical aperture (NA) of the focusing optics and location of the laser-writing plane. The occurrence of convex positional distortion that decreased with the NA was observed in an array of laser-inscribed optical tracks when scanning across the fiber. It exhibited a maximum distortion of 28.9 and 23.8 μm in the center plane of the fiber for the 0.42-NA and 0.85-NA dry objective lenses, respectively, but only a negligible positional distortion in the track array written in an off-center plane.