Single-mode Fiber Research Articles

An asymmetric STNS-MZI structure and its applications in temperature and Cd2+ monitoring

In this paper, an asymmetric fiber Mach-Zehnder interferometer (MZI) is presented and investigated. The proposed asymmetric MZI structure is mainly constructed with thin core fiber (TCF) and no core fiber (NCF), sandwiched between single mode fibers (SMFs). Note that the TCF is spliced with a slight offset such that higher order cladding modes could be effectively exited. The SMF-TCF-NCF-SMF (STNS) structure is adjusted by a finite-difference beam propagation method simulation to achieve an optimal interference spectrum. Temperature monitoring performance is addressed and the calculated sensing resolution is about 0.28℃ with high precision of ± 0.3 °C. Moreover, as for the Cd2+ monitoring application, the TCF is further etched and then coated with 1-allyl-2-thiourea (ATU) forming cross-linked “-S-Cd-S-” structure. The results show that the resolution of Cd2+ could reach 2.37 × 10−11mol /L, which shows a four order of magnitude improvement compared with our previous work. Therefore, the proposed asymmetric STNS-MZI interference structure has great potential in future applications.

Research on the Fabrication and Parameters of a Flexible Fiber Optic Pressure Sensor with High Sensitivity

In recent years, flexible pressure sensors have garnered significant attention. However, the development of large-area, low-cost, and easily fabricated flexible pressure sensors remains challenging. We designed a flexible fiber optic pressure sensor for contact force detection based on the principle of backward Rayleigh scattering using a single-mode optical fiber as the sensing element and polymer PDMS as the encapsulation material. To enhance the sensor’s sensitivity and stability, we optimized its structural design, parameters, and fabrication process and measured the fiber strain using an optical frequency domain reflectometer (OFDR). The results showed that the sensor achieved a high sensitivity of 6.93247 με/kPa with a PDMS concentration ratio of 10:1, a curing time of 2 h, and a substrate thickness of 5 mm. The sensor demonstrated excellent linearity and repeatability in static performance tests and was successfully used to monitor the plantar pressure distribution in real time. This flexible fiber optic pressure sensor can be developed via a simple fabrication process, has a low cost, and has high sensitivity, highlighting its potential applications in smart wearables and medical diagnostics.

Sensitivity-enhanced optical fiber sensor based on the Vernier effect for detection of ammonia in water

A sensitivity-enhanced optical fiber sensor for the detection of dissolved ammonia in water based on the Vernier effect is proposed and demonstrated. The sensor comprises a sensing Fabry-Perot interferometer (S-FPI) and a reference Fabry-Perot interferometer (R-FPI) in parallel. The S-FPI is an open cavity fabricated by the dislocation welding of single-mode fiber(SMF), which is filled with a mixture of ultraviolet-curable resin NOA 170 and Oxazine 170 perchlorate (O17). The R-FPI is a sealed cavity, which is constructed by fusion splicing of SMF and hollow core fiber (HCF). The proposed sensor is based on the chemical reaction between the dissolved ammonia and O17. This chemical reaction alters the refractive index (RI) of the S-FPI, resulting in a wavelength shift of the reflected spectrum. The two interferometers exhibit a nearly identical free spectral range (FSR), thereby enabling the generation of the Vernier effect, which markedly enhances the ammonia sensitivity of the sensor. Experimental results indicate that the sensor sensitivity is 0.34 nm/ppm in the dissolved ammonia range of 5-40 ppm, which is approximately 9.1 times that of the single S-FPI. The proposed sensor exhibits both good repeatability and a short response time, in addition to selectivity for the detection of ammonia.

Improving SMF coupling efficiency in aberrated optical system through optical system aberration correction method based on MPLC

In spatial laser communication terminals, the distortion of wavefront caused by the aberrations due to optical system processing and installation errors reduces the coupling efficiency of single-mode fiber (SMF). A method based on Multi-Plane Light Conversion (MPLC) is proposed to correct optical system aberrations and improve the coupling efficiency by converting the distorted light into Gaussian beam which matches SMF through a succession of phase planes. The Gaussian beam to SMF coupling efficiency model is firstly established and demonstrates that the SMF coupling efficiency can be effectively improved to 100% by converting the incident light into Gaussian beam which matches the SMF mode. The optical system aberration correction comparison simulation and experiment is conducted, and the results show after correcting aberrations using MPLC, the coupling efficiency increases from less than 0.1%–58.61% compared to distorted light directly coupling into SMF. Results show that the optical system aberrations are effectively corrected by this method, which has a certain reference value for the study of improving the coupling efficiency.

Fiber optic magnetic field sensor based on a magnetic-fluid-induced phase-shift FLRD

A magnetic field sensing system based on a phase-shift fiber loop ring-down (FLRD) technique and multi-mode interferometer (MI) coated with magnetic fluid (MF) is proposed and demonstrated. The MI is constructed by splicing a segment of no-core fiber between two sections of single-mode fibers, which is then immersed in MF to serve as a sensing head with the advantages of simple fabrication and specific magnetic sensitivity. Due to the magnetic refractive index tunable properties of the MF, the magnetic-field-dependent loss will be introduced in the fiber loop by the sensing head. Such magnetic-induced loss would be accumulated during the round trip of the optical carrier and reflected on the phase information of the modulated signal. The phase-shift changes with the applied magnetic field strength, enabling magnetic field sensing through phase-shift measurements. The sensing system is experimentally demonstrated and a sensitivity of 0.704×10−3deg/Gs in the linear region is achieved. Moreover, the stability and repeatability of the system are verified, leading to a promising method for magnetic field measurements.

1.3 μm higher order LP11 mode pulses in praseodymium-doped fluoride fiber laser

Abstract This work demonstrated an LP11 mode pulse laser at O-band using praseodymium-doped fluoride fiber as a gain medium. The Q-switched laser was initially generated using antinomy-telluride/polyvinyl-alcohol (Sb2Te3/PVA) thin film, and the LP11 modes were analyzed using a beam profiler. The pulse laser was achieved at a pumped power of 273–303 mW. With the increase in pump power, the pulse repetition rate increased to a maximum of 60.2 kHz, and the pulse width decreased to a minimum of 2.86 μs. The operating wavelength of the Q-switched output was 1303.2 nm. The corresponding pulse energy and peak power also increased with the increase in pump powers to the maximum of 12.12 nJ and 4.24 mW. The higher-order modes were obtained after the fiber was offset between the single-mode fiber and the two-mode fiber (TMF). The TMF output was then connected to the multimode fiber, a collimator, and the scanning-slit optical beam profiler to observe the higher-order modes. A uniform two-lobe structure of higher-order LP11 modes was observed.

Detection of riboflavin concentration by sol-gel silica coated dual-peak long period fiber grating sensor

A dual-peak long period fiber grating (DP-LPFG)-based sensor was designed and fabricated for the sensitive detection of riboflavin concentration. LPFGs with a period of 163 μm and a cladding mode of LP0, 12 were inscribed on a standard single-mode silica fiber using a femtosecond laser direct writing technique. Coatings of silica-based materials doped with β-cyclodextrin (β-CD), prepared via the sol-gel method, were applied to the surface of the LPFGs for the detection of riboflavin. To investigate the impact of coating thickness on the sensing sensitivity, sol-gel coatings of different thicknesses were applied to the surface of the LPFG sensor. Experimental findings revealed that the sensor with a thickness of approximately 540 nm sol-gel coatings exhibited superior sensing performance. The detection limit of the prepared DP-LPFG sensor was tested to be 0.08 nM, and it exhibits a response time of less than 3 minutes, along with high sensitivity, excellent repeatability, and selectivity. Thus, this sensor holds significant promise for applications in disease treatment and diagnosis, as well as in ensuring food quality.

All-fiber tunable mode converter for a mini-two-arm Mach-Zehnder interferometer.

In this Letter, an all-fiber tunable mode converter for a mini-two-arm Mach-Zehnder interferometer (MTA-MZI) is proposed and realized for the first time to our knowledge. Employing an electric arc discharge technology, we couple a multi-mode fiber (MMF) and a single-mode fiber (SMF), resulting in an MZI characterized by centimeter-scale arms. The varied sensitivity of fiber modes to curvature makes the high-order modes of the MMF more prone to leakage when subjected to bending, leading to alterations in the output interference fringe pattern of the MZI. Through continuous monitoring of the interference fringes of the MZI, we effectively detect the mode properties within the MZI in real time. In addition, a verification experiment is conducted by introducing a peanut structure to excite further higher-order modes of light to enter the MZI in advance. Upon modifying the curvature of the MZI, these excited higher-order modes leak, causing the interference fringe pattern to revert to its initial state without a peanut structure. This experimental validation highlights the potential employment of the MTA-MZI as an all-fiber mode converter and paves the way for optical field mode conversion within an all-fiber framework.

Welding electrical connections between battery sheet metal materials and additively manufactured (AM) aluminum components: The effect of surface roughness

The integration of additive manufacturing's design versatility with a push for electrification can lead to unique innovations. In this context, when discussing welded joints, their quality and reliability play a significant role. The restricting factor for the use of laser powder bed fusion of metal (PBF-LB/M) additive manufactured (AM) parts is often their surface quality in the as-built state. In addition, the surface quality changes when part plane orientation changes. In this study, AlSi10Mg aluminum test samples were additive manufactured in seven different build angles. The AM samples were glass bead blasted after manufacturing, and the surface roughness of the joint surface area was optically measured. After the measurement, the AM samples were welded to nickel-plated copper and aluminum sheet metals. A continuous wave single-mode fiber laser with scanner optics was used to carry out the welding experiments in lap joint configuration. The structural and mechanical properties of the welds were evaluated by the macroscopic images of weld cross sections and tensile testing of the samples. The electric resistivity of the samples was also tested. Based on the measurement results of the samples, the impact of surface quality was discussed. The results indicated that there was no clear effect of building angle on the final weld quality. The worst surface quality in the joint area was on the test pieces in which the joint surface was facing down toward the build plate.

Distributed salinity sensor based on optical frequency domain reflectometry with polyimide coated single mode fiber

A distributed optical fiber sensor for salinity sensing is proposed and analyzed. The sensor uses a single mode fiber coated with polyimide and is based on optical frequency domain reflectometry. By applying the polyimide solution to the surface of the fiber, a layer of polyimide film is formed by heating. As the polyimide coating is very sensitive to changes in the salinity of the external solution, the polyimide film will expand or shrink as the external concentration changes. This converts the value of salt concentration into the frequency shift in the spectrum through cross-correlation. To characterize the sensor, four polyimide-coated sensors with different average coating thicknesses of 126 µm, 170 µm, 192 µm and 249 µm were fabricated. The sensitivities of -21.71 GHz/(mol/L), -28.55 GHz/(mol/L), -29.97 GHz/(mol/L) and -42.78 GHz/(mol/L) were achieved when the salinity was measured from 0 mol/L to 3.09 mol/L, respectively. The sensitivity and response time of polyimide fibers with different diameters were measured. The minimum salinity measurement uncertainty of the sensor is 0.05 mol/L. The sensor has high sensitivity and has promising applications in observing ocean parameters and ion chemical sensing.

High sensitivity gas pressure and temperature optical sensors based on tapered no-core fiber coated with polydimethylsiloxane (PDMS)

This article introduces a novel high-sensitivity pressure and temperature sensor utilizing a tapered no-core fiber (NCF) structure created by splicing the NCF with a single-mode fiber (SMF) to form a “SMF-NCF-SMF” structure, tapering the NCF, and applying a polydimethylsiloxane (PDMS) film on the tapered NCF surface. The experimental results show that the device with a diameter of 10 μm has a pressure sensitivity of 86.63 nm/MPa within one to two atmospheres. Furthermore, the temperature sensitivity is −1.97 nm/°C from 30 °C to 60 °C. The sensor exhibits excellent stability and repeatability, and the PDMS film enhances the mechanical strength of the device, indicating promising potential for high-sensitivity pressure and temperature applications.

Real-time demonstration of 64 × 200 Gbps UDWDM-PON downstream transmission based on silicon photonic integrated transceiver

Coherent detection, high-order modulation, and dense wavelength division multiplexing (DWDM) technologies provide solutions for increasing bandwidth demand of future access networks. We experimentally demonstrate a real-time 64 × 200-Gb/s coherent ultra-dense wavelength-division (UDWDM) coherent passive optical networks (PONs) at 75-GHz channel spacing. The demonstrated downlink transmission is realized with the dual-polarization QPSK (DP-QPSK) modulation scheme. The cost is reduced by using silicon photonic integrated coherent transceiver modules and wider grid AWGs. The power budget is evaluated when all 200 channels are launched at C band. The power budget is 26 dB after 20-km G.652D standard single mode fiber (SSMF) transmission without amplifier at the receiver side under soft-decision FEC (SD-FEC) threshold of 1 × 10−2, and can achieve 32.5 dB with pre-amplified semiconductor optical amplifier (SOA) used at the receiver side.

Photonic generation and transmission of dual-band double-TBWP chirped microwave waveforms with frequency multiplication and immunity to dispersion-induced power fading

A novel photonic approach for generating and transmitting dual-band double-time bandwidth product (TBWP) chirped microwave waveforms with frequency multiplication and immunity to dispersion-induced power fading is proposed. Utilizing an integrated dual-drive dual-parallel Mach-Zehnder modulator (DD-DPMZM), single-sideband modulation of the RF input and suppression of odd-order optical sidebands are achieved. By adjusting the RF input's centre frequency and the DD-DPMZM's bias voltages, dual-band waveforms with frequency multiplication are realized, enabling transmission through optical fibres without dispersion-induced power fading. Experimental validation demonstrates successful transmission over dispersion compensation modules equivalent to 60 km or 20 km of single-mode fibre. The proposed system features a simplified architecture, reconfigurability, and robust anti-dispersion capabilities, making it highly suitable for distributed multi-band radar networks over optical fibre links.

Realization of triple dispersion turning points in tapered seven-core fiber interferometer for sensitivity enhancement

Seven-core fibers have been widely applied in optical network, astronomy, and sensing applications. In this work, we report and demonstrate the existence of triple dispersion turning points of the super-mode interferences in a seven-core fiber spliced between two single-mode fibers by modifying the diameter of the multi-core fiber. The sensitivities of the interferometer on external variables, i.e., temperature, axial strain, and refractive index (RI), are measured, showing significant enhancement around the dispersion turning points. Compared to the regular multi-core fiber device without dimension modification, the sensitivities of temperature and axial strain are improved by about 60 times and 73 times around the dispersion turning points, respectively. While the regular device has no response to the external refractive index, the magnitude of RI sensitivity can reach as high as 33 459.0 nm/RIU in air, when the fiber diameter is modified to approach the dispersion turning point, enabling the discrimination of the refractive indices of N2 and CO2. The modification method of locating the modes at the dispersion turning points significantly enhances device sensitivity, providing an avenue for integrating ultra-sensitive sensors into commercial fiber networks.

DC current sensor based upon a fused fiber optic Fabry-Perot interferometer (FPI) and copper wire

In order to meet the demand for direct current (DC) current measurements in industrial production, a new type of fiber optic DC current sensor, combining thin copper wire and fiber optic Fabry-Perot interferometer (FPI), is reported. FPI is a sandwich structure formed by splicing a single-mode fiber (SMF) and a quartz capillary, with a cavity length of approximately 200 μm. The copper wire, with a length of 3.2 cm and a diameter of 1 mm, is attached to the FPI to form the sensor. Due to the excellent thermal conductivity of copper, the results showed that the sensor increased the sensitivity from 4.1 pm/°C to 11 pm/°C. After applying direct current to the sensor, a large quantity of heat generated by the copper wire was absorbed by the FPI, resulting in a significant shift in the resonant wavelength of FPI, which can indirectly measure the current. Three current experiments were performed, and parabolic curves have been used in fitting of the Dip wavelength and the square of the current. The results provide high correlation coefficients with values exceeding 0.99. Therefore, this curve may be used to measure the square of the current, further obtaining the current. In summary, the sensor is compact and durable, easy to manufacture, and provides high correlation coefficients, providing an alternative for measuring DC current.

The Impact of 1030 nm fs-Pulsed Laser on Enhanced Rayleigh Scattering in Optical Fibers.

This article presents a comprehensive study on the impact of irradiation optical fiber cores with a femtosecond-pulsed laser, operating at a wavelength of 1030 nm, on the signal amplitude in Rayleigh scattering-based optical frequency domain reflectometry (OFDR). The experimental study involves two fibers with significantly different levels of germanium doping: the standard single-mode fiber (SMF-28) and the ultra-high numerical aperture fiber (UHNA7). The research findings reveal distinct characteristics of reflected and scattered light amplitudes as a function of pulse energy. Although different amplitude changes are observed for the examined fibers, both can yield an enhancement of amplitude. The paper further investigates the effect of fiber Bragg grating inscription on the overall amplitude of reflected light. The insights gained from this study could be beneficial for controlling the enhancement of light scattering amplitude in fibers with low or high levels of germanium doping.

Demonstration of a low-complexity real-time FBMC transmitter for a spectral-efficiency IMDD system.

Filter bank multi-carriers (FBMC) have higher spectral efficiency, lower out-of-band spectrum leakage, and stronger ability to resist synchronization errors compared to orthogonal frequency-division multiplexing (OFDM), which has been widely considered as a promising solution for short-reach applications, such as passive optical networks. However, the traditional fast Fourier transform (FFT)-based FBMC scheme has quite high implementation complexity for high-speed optical communications. In this work, we proposed a low-complexity real-time FBMC transmitter scheme enabled by a single fully parallel pruned inverse FFT and simplified short prototype filter-based polyphase networks for a low-cost intensity-modulation and direct-detection (IMDD) system. Compared to the OFDM transmitter, the proposed FBMC one occupies a similar number of look-up tables and registers but saves 21% of real multipliers, reduces latency by 16.2%, and increases the data rate by 12.5%. The real-time FBMC/OFDM signals are generated with a field programmable gate array chip and 6-bit 30-GSa/s digital-to-analog converter and are experimentally demonstrated in an IMDD system. After a 20-km standard single-mode fiber transmission, the negligible power penalty can be achieved with the proposed FBMC transmitter scheme at the bit error rate of 3.8e-3 compared to the OFDM counterpart.

Enhancements to quantum communication performance utilizing a prototype photonic lantern and multiplexed single-photon detection.

The optical interfacing between a free-space channel and single-photon detectors (SPDs) can greatly impact the inherent performance of a free-space quantum key distribution receiver. Direct coupling to detectors creates engineering challenges, and a single-mode fiber requires adaptive optics. Using a multimode fiber (MMF) is common; however, larger core diameters limit the achievable bandwidth. We demonstrate a prototype multimode fiber-based photonic lantern that allows us to retain the benefits of the large multimode coupling while transitioning to multiple, less multimodal fibers, reducing bandwidth limitation.

Study of Dependence of the Energy of the Field in Weakly Conductive Fiberlight-Guide with Gradient Refractive Index Profile Without Regard to Polarization

A round-in-cross-section weakly guiding fiber optic light guide is considered. The system of Maxwellian equations for electrically conductive transparent media without due regard to polarization with an index of refraction in the case under consideration is reduced to the homogeneous Helmholtz equation, a particular solution of which is found using the Green's function. For a unimodalmode a common decision was obtained for the field and energy inside the fiber in the general case of a gradient refractive index profile depending on the radial coordinate. The concept of normalized energy is introduced as the ratio of the energy inside a fiber with gradient refractive index profile to the energy inside a fiber with a step-index profile. Since in a single-mode regime the zero-order Bessel function, which describes the field inside a step-index profile fiber, can be replaced by a Gaussoid, then, when extending this replacement to the case of a single-mode gradient fiber, as a first approximation we shall obtain a finite expression for the normalized energy for a general gradient profile. The dependences of the normalized energy on the waveguide number were obtained for each of the degrees from the first to the fourth inclusive. It is shown that in the considered approximation, the energy increases up to a certain parameter values (for the first degree – a triangle profile) or slowly decreases (other profiles), after this value the energy increases for all profiles. The results obtained will help to select a suitable single-mode fiber for practical application.

All-fiber spatial and wavelength gain-flattening of few-mode EDFA via mode selective coupler

The mode division multiplexing (MDM) technique together with wavelength division multiplexing (WDM) shows the great prospect to solve the capacity crunch of current standard single mode fiber (SSMF) transmission system. However, the modal and wavelength gain fluctuation fundamentally limit both the capacity and the reach of hybrid MDM-WDM transmission system. Here, we propose an all-fiber few-mode gain-flattening filter (FM-GFF) based on mode selective coupler (MSC), to effectively mitigate the gain fluctuation of few-mode erbium doped fiber amplifier (FM-EDFA) in both spatial and wavelength dimensions. With the help of genetic algorithm (GA), variable mode dependent loss spectra of cascaded MSCs can be efficiently obtained, according to the gain profile of the used FM-EDFA. Consequently, gain-flattening among all guided linearly polarized (LP) mode groups over the C-band can be realized. Simulation results reveal that, as for the spatial and wavelength gain flattening among 4/6-LP mode groups, only 5/7 cascaded MSCs are good enough to mitigate the modal and wavelength gain fluctuation from 9.89 dB and 11.79-dB to less than 0.38 dB and 0.46 dB over the C-band. Finally, we carry out experimental verification based on 2-LP mode groups gain flattening over C-band. When 3 cascaded MSCs are involved, the gain fluctuation of FM-EDFA over the C-band can be flattened from 2.46 dB to less than 0.96 dB.