Polished Optical Fiber Research Articles

Ultrafast laser direct writing of in-line polarizers based on nano-gratings.

In-line polarizers play an important role in the emerging quantum computing, integrated photonics, and ultrafast science. However, to our knowledge, no one has actually used in-line polarizers based on nano-gratings (NGs) in those research fields. Here, we present a novel approach to write a fiber polarizer that utilizes nano-gratings directly within the fiber core for the first time to our knowledge. An optical fiber polarizer measuring 1 mm in length, 2.5 dB in polarization-dependent loss (PDL), and 2 dB in insertion loss was developed and initially utilized for the artificially saturable absorber nonlinear polarization evolution (NPE) experiment. In the experiment, several nonlinear transmission curves are measured. In-line polarizers based on nano-gratings are expected to be used in future integrated optical chips because of their small size and easy direct writing.

Design of Magnetic Fluid-Enhanced Optical Fiber Polarization Filter.

In this paper, we demonstrated a method of filling the air holes of a photonic crystal fiber (PCF), coated with gold film, with magnetic fluid (MF) to enhance the Surface Plasmon Resonance (SPR). The simulation results show that at the wavelength of 1260-1675 nm, the minimum loss coefficient of the y-polarization mode is 4.7 times that before filling with MF, and the x-polarization mode is 0.45 times greater. Then, based on this method, we designed a polarizing filter with a core diameter of 9 µm. The numerical simulation results indicate that it not only maintains the same core diameter as the single-mode fiber, but also has a larger bandwidth and a higher extinction ratio (ER). Additionally, we can optimize its ER at a specific wavelength by adjusting the magnetic field.

Broadband and tunable fiber polarizer based on a graphene photonic crystal fiber.

The recent flourishing development of two-dimensional (2D) graphene has sparked considerable interest and extensive research on graphene-based optical fiber polarizers. However, studies on graphene-optical fiber polarizers focused on the structure with graphene films attached to side-polished fibers, which face challenges such as low birefringence of 10-6, low polarization extinction ratio (PER), and narrow polarizing window of tens of nanometers. Here, a fiber polarizer based on a graphene-photonic crystal fiber (Gr-PCF) is proposed firstly, which exhibits high birefringence of ∼2.5 × 10-3, high PER of ∼111 dB/mm, broad polarizing window of >400 nm, and tunable polarization states. Graphene or graphene/hBN/graphene (Gr/hBN/Gr) heterojunctions are attached to the surface of two square holes in the PCF to make one of the polarizing modes attenuate significantly. The tunability of the Fermi level (EF) in Gr/hBN/Gr enables the proposed device to function as a polarizer or a polarization-maintaining fiber. The combination of PCF's endless single-mode feature and graphene's broadband optical response feature enables the fiber polarizer to exhibit a wide spectrum range with single-mode transmission characteristics.

Effects of thickness and roughness on plasmonic characteristics of gold thin films deposited on polished optical fiber

The thickness and roughness of metal layers substantially affect the performance of surface plasmon resonance (SPR)-based sensors. The deposition methods, control parameters, and substrate characteristics influence the layer thickness and roughness. This study investigates the SPR characteristics of a polished optical fiber surface coated with gold (Au) metal of different thicknesses. The Au layer is deposited via the thermal evaporation method, and its thickness is varied by controlling the deposition time (3–6 min). A proportionality relationship between thickness and deposition time is observed. Island-shaped structures in gold (Au) morphology are formed due to low adhesion to the substrate. The shape of this island creates gaps in the layer, causing scattering. In addition, the roughness on the gold surface triggers the Localized Surface Plasmon Resonance (LSPR) phenomenon. As a result, the measured dielectric characteristics differ from the reference. The SPR curve calculation simulation was carried out based on reference optical parameters and measurement results by an ellipsometer, which were then compared with experiments. The obtained results show that the substrate roughness, morphology, and thickness of the Au layer play an essential role in determining the characteristics of the SPR curve in a fiber optic plasmonic sensor. As a result, in basic experiments, the sample with an Au thickness of 27.37 nm (deposition time = 3 min) shows better characteristics (half-maximum full width, minimum transmittance, and resonance wavelength) compared with the sample with an Au thickness of 53.97 nm (deposition time = 4 min), Although 53.97 nm is the optimal thickness from the simulation using reference optical parameters (smooth substrate surface and smooth gold layer).

High resolution and accuracy model fitting interrogation method for optical fiber Polarization interferometric sensor

Interrogation methods largely determine the performance of the entire sensing system. However, interrogation methods alone are unlikely to provide very good results. An accurate model for the optical fiber polarization interferometric sensor (PIS) has been meticulously formulated within the frequency domain, incorporating the mode birefringence dispersion term of polarization maintaining fibers (PMFs). A novel model-fitting interrogation approach for the PIS is initially proposed and validated through experimental demonstration, which is bifurcated into parameter initialisation and model fitting interrogation process during the operation of the PIS. Subsequently, the parameters are determined at varying temperatures using this method. The interrogation capabilities of the proposed method are then experimentally contrasted with the valley wavelength tracking method. Results corroborate that the measurement range of model-fitting approach is no longer restricted by the spectrum bandwidth. The resolution and accuracy of our approach are 89% and 8 times higher than the valley wavelength tracking method, respectively.

Long Distance Military Fiber-Optic Polarization Sensor Improved by an Optical Amplifier

The ever-increasing demands for the use of fiber-optic sensors powered by long optical fibers is forcing developers to solve problems associated with powering these remote sensors. Due to their non-electric character, these sensors are suitable for many uses, including military applications. The Army of the Czech Republic is very interested in this type of optical fiber sensor as it fulfils the significant prerequisites for use in military areas. However, the army’s requirements are challenging because they require long supply cables in which there is significant attenuation of optical power. At the same time, there is a need for high sensitivity. The subject of our research team’s work was to use amplifiers to power these sensors. The army already uses this type of sensor for short distances as it cannot ignite a gas mixture with an explosive concentration and thus meet the strict requirements of the explosion-poof standard. The novelty of our research lies in the discovered measurement technique that allows the sensors to be powered remotely and in the saving of optical fibers by utilizing duplex communication with a circulator. Furthermore, the research presents an innovative approach to the optimization of the entire sensor by using a bidirectional, sensory, polarization-maintaining optical fiber. The proposed sensor was first verified in laboratory conditions at the Optoelectronics Laboratory of the University of Defense in Brno, and further tests were carried out in the military training areas of Boletice and Březina in the Czech Republic, which is a member of North Atlantic Treaty Organization.

Tilted fiber grating polarizer in a 40-µm polarization-maintaining fiber.

The optical polarizer is a crucial component widely used in many optical systems and applications. Fiber-optic polarizers have the merits of excellent compatibility and ease of integration with other fiber components. We report an in-line polarizer enabled by a 45° tilted fiber grating inscribed into a specialty fiber for the next generation fiber-optic gyroscope, i.e.,a 40-µm ultra-fine-diameter tiger-type polarization-maintaining fiber with which the size of fiber-optic sensors can be miniaturized. The results show that a 40-mm-long polarizer operates at a center wavelength of around 830nm with high-performance characteristics, such as a polarization extinction ratio exceeding 30dB, a low insertion loss of less than 1.5dB, and a large 3-dB optical bandwidth more than 60nm. This kind of fiber-optic polarizer may have a broad scope across applications and systems such as fiber lasers and sensors, especially high-precision fiber-optic gyroscopes.

Simulation study of a temperature-calibrated double-sided polished optical fiber SPR refractive index sensor.

The temperature of the environment directly affects the accuracy of refractive index (RI) measurement. Therefore, we propose a double-sided polished surface plasmon resonance (SPR) RI fiber sensor, which is available for simultaneous measurement of the RI and temperature in real time. The proposed sensor uses single-mode fiber as a special double-sided polishing structure. The double-sided polishing regions are coated with a gold-silver hybrid film; one side is additionally coated with graphene layers to increase detection sensitivity, and the other side is coated with polydimethylsiloxane on the metal layer for temperature sensing. The simulation result shows that in the range from 1.33 to 1.35, RI sensitivity reaches as high as 2600nm/RIU. In the range from 15°C to 85°C, temperature sensitivity reaches as high as -3.5n m/∘ C. The full width at half maximum is 65nm. Compared with previous studies, the sensitivity is slightly improved, and an excellent temperature compensation effect can be achieved. It is suitable for high-precision measurement of the environment and biochemical aspects.

MXene supported surface plasmons on telecommunications optical fibers

MXenes, an emerging class of two-dimensional materials, exhibit characteristics that promise significant potential for their use in next generation optoelectronic sensors. An interplay between interband transitions and boundary effects offer the potential to tune the plasma frequencies over a large spectral range from the near-infrared to the mid-infrared. This tuneability along with the ‘layered’ nature of the material not only offer the flexibility to produce plasmon resonances across a wide range of wavelengths, but also add a degree of freedom to the sensing mechanism by allowing the plasma frequency to be modulated. Here we show, numerically, that MXenes can support plasmons in the telecommunications frequency range and that surface plasmon resonances can be excited on a standard MXene coated side polished optical fiber. Thus, presenting the tantalising prospect of highly selective distributed optical fiber sensor networks.

Enhancement of Spr Effect and Sensing Characteristics in D-Shaped Polished Grapefruit Microstructured Optical Fiber with Silver Film

Enhancement of Spr Effect and Sensing Characteristics in D-Shaped Polished Grapefruit Microstructured Optical Fiber with Silver Film

Utilization of telecommunication optical routes to power fiber-optic polarization sensors

A single-mode telecommunication optical route can be used for reliable power supplies of a remote non-electric temperature fiber-optic polarization sensor, but the optical route, due to many physical factors, affects an immediate state of polarization during the transmission. This negative phenomenon changes the sensitivity of the sensor itself. The thesis proposes two main approaches to solving that problem. The first approach is based on the suitable connection of a depolarizer and linear polarizers. The second approach is based on signal interference, which takes place in a polarization-maintaining fiber coupler. This article also evaluates the advantages and disadvantages of the two approaches and graphically demonstrates the functionality of the fiber-optic sensor, which was tested by applying a container with water of different temperatures. A big advantage of this type of sensor is that it is not necessary to have components, that are dependent on electricity, near the monitored place, where there may be no access to electricity, or the place may be sensitive to an electric charge. Paper demonstrates the possibility of successfully powering the non-electric sensor via a classical optical route.

Novel Technique for Meniscus Height Measurement of Liquids Using Optical Fibers

A novel technique was designed to measure the meniscus height (MH) of liquids on a standard single mode fiber (SMF) using fiber optics. Once the MH is known, the contact angle could be calculated, and the surface tension of liquids can be determined. In this paper, the MH is measured using a cleaved and side polished optical fiber attached to a SMF with a known separation distance between their fiber tips. When the liquid reaches the SMF, the liquid meniscus rises on the vertical wall of the SMF, which also causes a variation in mass (measured by a μg scale). At this point, the liquid moves upwards until the liquid meniscus touches the side polished fiber, which registers a variation of the reflected optical power. The MH seems to increase as the surface tension decreases, and the variation of the mass measured by the scale increase with the increase of the surface tension. This sensor was also used to measure the interfacial surface tension of the air-liquid and liquid-liquid interface since the contact angle could be measured. In addition, the small dimension of the sensor is adequate for applications where using small amounts of liquid is critical, i.e., biomedical applications.

Optimization of Fiber Bragg Gratings Inscribed in Thin Films Deposited on D-Shaped Optical Fibers.

A fiber Bragg grating patterned on a SnO2 thin film deposited on the flat surface of a D-shaped polished optical fiber is studied in this work. The fabrication parameters of this structure were optimized to achieve a trade-off among reflected power, full width half maximum (FWHM), sensitivity to the surrounding refractive index (SRI), and figure of merit (FOM). In the first place, the influence of the thin film thickness, the cladding thickness between the core and the flat surface of the D-shaped fiber (neck), and the length of the D-shaped zone over the reflected power and the FWHM were assessed. Reflected peak powers in the range from −2 dB to −10 dB can be easily achieved with FWHM below 100 pm. In the second place, the sensitivity to the SRI, the FWHM, and the FOM were analyzed for variations of the SRI in the 1.33–1.4 range, the neck, and the thin-film thickness. The best sensitivities theoretically achieved for this device are next to 40 nm/RIU, while the best FOM has a value of 114 RIU−1.

Study of Pulse Formation in an EDFL Under a Large Dispersion Variation Hybridly Mode-Locked by Graphene and Nonlinear Polarization Rotation

In this work, we present a detailed experimental study of pulse formation mechanism in an EDFL, hybridly mode-locked by use of a CVD monolayer graphene onto a D-shaped optical fiber and nonlinear polarization rotation (NPR), under a large intracavity dispersion influence (up to 100 times) ranging from 12.3 to 1382 m cavity lengths. Analyzing the spectral evolution, temporal and soliton behaviors of the pulse, our results revealed the presence of four laser operating regimes with non-soliton and soliton formation from short to ultralong cavity lengths, all related to graphene/NPR nonlinearities, dispersion and soliton propagation influences.

Monitoring of solenoid parameters based on neural networks and optical fiber squeezer for solenoid valves diagnosis

As crucial parts of various engineering systems, solenoid valves (SVs) operated by electromagnetic solenoid (EMS) are of great importance and their failure may lead to cause unexpected casualties. This failure, characterized by a degradation of the performances of the SVs, could be due to a fluctuations in the EMS parameters. These fluctuations are essentially attributed to the changes in the spring constant, coefficient of friction, inductance, and the resistance of the coil. Preventive maintenance by controlling and monitoring these parameters is necessary to avoid eventual failure of these actuators. The authors propose a new methodology for the functional diagnosis of electromagnetic solenoids (EMS) used in hydraulic systems. The proposed method monitors online the electrical and mechanical parameters varying over time by using artificial neural networks algorithm coupled with an optical fiber polarization squeezer based on EMS for polarization scrambling. First, the MATLAB/Simulink model is proposed to analyze the effect of the parameters on the dynamic EMS model. The result of this simulation is used for training the neural network, then a simulation is proposed using the neural net fitting toolbox to determine the solenoid parameters (Resistance of the coil R, stiffness K and coefficient of friction B of the spring) from the coefficients of the transfer function, established from the model step response. Future work will include not only diagnosing failure modes, but also predicting the remaining life based on the results of monitoring.

High sensitivity refractive index sensor in long-range surface plasmon resonance based on side polished optical fiber

A side-polished single mode optical fiber based on conventional long-range surface plasmon resonance (cLRSPR) sensor and symmetrical long-range surface plasmon resonance (sLRSPR) sensor was proposed and experimentally demonstrated. The proposed structure of cLRSPR is composed of optical fiber/dielectric buffer layer (DBL)/Ag film while for sLRSPR is consisted of optical fiber/DBL/Ag film/DBL. Magnesium fluoride, MgF2 with 50 nm thickness is used as DBL layer that lies beneath of Ag film in cLRSPR and covering both sides of Ag film in sLRSPR. 40 nm thickness of Ag layer is used with 5 mm length of sensing region. The result suggested that the sensing performance in terms of sensitivity of the cLRSPR sensor was better compared to sLRSPR and previous works of conventional surface plasmon resonance (cSPR) as it achieves as high as 2812.5 nm/RIU at 0.01 interval of refractive index with refractive index (RI) range of 1.33–1.34. The larger the sensitivity value indicates the better the performance of the SPR sensor. This design is expected to play an important role on chemical sensing and biological sensing as it displays higher penetration depth.

Distributed Polarization Measurement for Fiber Sensing Coils: A Review

Distributed polarization measurement is a significant way to evaluate polarization-maintaining fiber. It can assess the fiber's polarization preserving ability and locate internal defects and external perturbations of a long-distance PM fiber. Distributed polarization crosstalk of a fiber-optic polarization device accurately describes its time-dependent optical performance. Therefore, distributed polarization measurement technology can be used to predict and prevent the performance degradation and failure. However, the remaining shortcomings of existing measurement methods are low dynamic range, short measurement length, inadequate spatial resolution, and low diagnostic ability. This paper reviews recent advances in solving the above issues and presents ways to obtain high performances: a dynamic range > 90 dB, a measurement length > 10 km, a full range spatial resolution <; 10 cm, and developments in time domain, frequency domain, and time-frequency domain diagnosis methods. All these high-performance techniques have led to more effective diagnosis of fiber optic sensing coils. This can be demonstrated by a series of comparative measurements and analysis of three PM fiber optic sensing coils in various configurations such as a randomly-wound spool of fiber and a free-standing coil quadrupole-pattern-wound from it, a second coil with and without a flange at various temperatures, and a third coil spliced to a Y waveguide. Each is one of the key configurations during the production processes or the application of the fiber sensing coil. The technical progress reviewed here will greatly promote the development, production, and application of fiber sensing coils.

Wavelength Switchable Multi-Wavelength Erbium-Doped Fiber Laser Based on Polarization-Dependent Loss Modulation

We propose and demonstrate a novel and simple wavelength switchable multi-wavelength erbium-doped fiber (EDF) laser based on polarization-dependent loss (PDL) modulation with cascaded fiber Bragg gratings (FBGs) as the wavelength comb filter. PDL is generated and controlled by different polarization angles between the polarization state of each lasing line and the optical fiber polarizer in the laser cavity. The PDL modulation balances the difference between the total loss and the wavelength-dependent lasing gain. The relations among gain, loss, and laser output power under different PDL conditions are numerically analyzed on the basis of a simplified Giles model in an EDF laser system. Results show that multi-wavelength lasing lines can be built up by properly tuning the PDLs in different wavelengths. The amount of lasing lines can be switched by changing the PDL intensity in a certain wavelength. In an experiment, the PDL in that certain wavelength is modulated by utilizing four polarization controllable elements, each of which includes an FBG and a corresponding polarization controller (PC). One PC is used to control the polarization state of the reflected lasing line on the center wavelength of the FBG. A four-wavelength individually switchable EDF laser system is demonstrated by utilizing this wavelength switchable method, in which the switching operation for the single-, dual-, triple-, and four-wavelength laser output is verified. The experimental results are consistent with the theoretical results. All wavelength switching operations are conveniently controlled by merely adjusting the PCs. This simple structure and flexible wavelength switchable operations show that this multi-wavelength EDF laser has great potential in practical applications.

A Novel Ultra-Broadband Polarization Filter Based on a Microstructured Optical Fiber with a Gold-Coated Air Hole.

In this paper, a pentagonal microstructured optical fiber polarization filter by utilizing a surface plasmon resonance effect is proposed. The characteristics of the mode-coupling and filtering of the filter are studied by making use of the full-vector finite element method. The performance of the filter is greatly affected by the structure parameters. The losses of Y and X polarization of the fiber core are 665.97 and 0.17 dB/cm at 1.55 μm, respectively, and the loss ratio is 3917.47. This shows that the filter has a greater loss ratio. Moreover, both the extinction ratio and tolerance are also researched, which shows that the proposed filter has a wider filtering bandwidth and better fabrication tolerance. The designed filter has an important role in wavelength-division multiplexing (WDM) and coherent optical fiber communication systems.

Polarization Selectivity of the Thin-Metal-Film Plasmon-Assisted Fiber-Optic Polarizer.

The interaction between light and metallic nanostructures leads to many impressive achievements and has a wide range of applications. The thin-metal-film plasmon-assisted fiber-optic polarizer is one of the essential applications. However, the polarization mechanism and the transmitted polarization of the plasmon-assisted polarizer have given rise to controversy over the past decade. Which of the polarizations is preferentially transmitted through the polarizer? The transverse electric polarization or the transverse magnetic polarization? Here, special emphasis is placed upon the polarization mechanism and the transmitted polarization of thin-metal-film plasmon-assisted fiber polarizers. We first investigate the polarization mechanism of the polarizers theoretically and numerically. Furthermore, a novel approach is proposed to demonstrate the transmitted polarization in the plasmon-assisted fiber polarizers experimentally. We demonstrate that the polarization mechanism is based on the polarization selective absorption of the metallic material, and the transverse electric polarization is the only transmitted polarization of the metallic plasmon-assisted polarizer. Moreover, the plasmon-assisted polarizer can offer a high polarization extinction ratio (33.1 dB) and a low insertion loss (1.1 dB) at room temperature and have excellent temperature stability in the range of -48 to 82 °C. Experimental results agree well with our theoretical and numerical analyses. The findings clarify the confusion about the polarization mechanism and the transmitted polarization of metallic plasmon-assisted fiber polarizers over the past decade, providing new ground for the exploration of polarization-sensitive optical systems, with good potential applications in the fields of optical sensors, plasmonic lasers, coherent optical communications, and biosensor systems.