Overcoming oxygen inhibition in UV photolithography for the fabrication of low-loss polymer waveguides.

Perfluorinated acrylate polymer materials exhibit low absorption loss at 1310 and 1550 nm, but molecular oxygen inhibits their photocuring. We propose a novel, to our knowledge, UV photolithography method incorporating a pre-exposure process for fabricating low-loss perfluorinated acrylate polymer waveguides. During the pre-exposure process, a partially cured thin layer forms on the core layer, effectively overcoming oxygen inhibition in subsequent lithography. Furthermore, the functional group contents of the polymerized materials were characterized by a Raman spectrometer to analyze the development reaction under the pre-exposure layer. Utilizing this improved method, we fabricated a straight waveguide with a length of 21 cm. The experiments showed that the propagation losses are 0.14 dB/cm at 1310 nm and 0.51 dB/cm at 1550 nm. The inter-channel cross talk for a core pitch of 250 µm was measured as low as -49 dB at 1310 nm. Error-free NRZ data transmission over this waveguide at 25 Gb/s was achieved, showcasing the potential in optical interconnect and communication applications.

Vibration-insensitive polarimetric fiber optic current sensor based on orbital angular momentum modes in an air-core optical fiber.

Current or magnetic field sensing is usually achieved by exploiting the Faraday effect of an optical material combined with an interferometric probe that provides the sensitivity. Being interferometric in nature, such sensors are typically sensitive to several other environmental parameters such as vibrations and mechanical disturbances, which, however, inevitably impose the inaccuracy and instability of the detection. Here we demonstrate a polarimetric fiber optic current sensor based on orbital angular momentum modes of an air-core optical fiber. In the fiber, spin-orbit interactions imply that the circular birefringence, which is sensitive to applied currents or resultant magnetic fields, is naturally resilient to mechanical vibrations. The sensor, which effectively measures polarization rotation at the output of a fiber in a magnetic field, exhibits high linearity in the measured signal versus the applied current that induces the magnetic field, with a sensitivity of 0.00128 rad/A and a noise limit of 1×10-5/H z. The measured polarization varies within only ±0.1% under mechanical vibrations with the frequency of up to 800 Hz, validating the robust environmental performance of the sensor.

A 2 µm Wavelength Band Low-Loss Spot Size Converter Based on Trident Structure on the SOI Platform.

A 2 µm wavelength band spot size converter (SSC) based on a trident structure is proposed, which is coupled to a lensed fiber with a mode field diameter of 5 µm. The cross-section of the first segment of the tapered waveguide structure in the trident structure is designed as a right-angled trapezoidal shape, which can further improve the performance of the SSC. The coupling loss of the SSC is less than 0.9 dB in the wavelength range of 1.95~2.05 µm simulated by FDTD. According to the experimental results, the lowest coupling loss of the SSC is 1.425 dB/facet at 2 µm, which is close to the simulation result. The device is compatible with the CMOS process and can provide a good reference for the development of 2 µm wavelength band integrated photonics.

Distributed optical fiber magnetic field sensor based on polarization-sensitive OFDR.

A distributed optical fiber magnetic field sensor based on a polarization-sensitive optical frequency domain reflectometer (POFDR) is proposed. It extracts the accumulated Faraday rotation by combining the Stokes vectors and the backward Mueller matrices from the measured states of polarization (SOPs) and obtains the magnetic field component. This method avoids adjusting the input polarization during the magnetic field sensing process. It overcomes the drawback of the conventional POFDR scheme, which requires at least two sets of different input SOPs for each sensing. Finally, the aforementioned effectiveness has been experimentally verified by using a single-mode sensing fiber. The results show that the sensor has good repeatability and linearity. The measurement error of the magnetic field sensor is 19.4 mT. The measured magnetic field variations agree with the applied ones with similarities higher than 0.98.

Formation of E-band luminescence-active centers in bismuth-doped silica fiber via atomic layer deposition.

In this study, a Si defect structure was added into the silica network in order to activate the bismuth and silica structure active center. TD-DFT theoretical simulations show that the Bi and Si ODC(I) models can excite the active center of the E-band at 1408 nm. Additionally, the Bi-doped silica fiber (BDSF) with improved fluorescence was fabricated using atomic layer deposition (ALD) combined with the modified chemical vapor deposition (MCVD) technique. Some tests were used to investigate the structural and optical properties of BDSF. The UV-VIS spectral peak of the BDSF preform is 424 cm-1, and the binding energy of XPS is 439.3 eV, indicating the presence of Bi° atom in BDSF. The Raman peak near 811 cm-1 corresponds to the Bi-O bond. The Si POL defect lacks a Bi-O structure, and the reason for the absence of simulated active center from the E-band is explained. A fluorescence spectrometer was used to analyze the emission peak of a BDSF at 1420 nm. The gain of the BDSF based optical amplifier was measured 28.8 dB at 1420 nm and confirmed the effective stimulation of the bismuth active center in the E-band.

Bandwidth extension to 1627†nm of over 20†dB gain in an erbium-doped silica fiber via two-photon absorption.

In this study, PbS/Er co-doped fibers (PEDFs) were fabricated by atomic layer deposition (ALD) combined with modified chemical vapor deposition (MCVD). A pumping scheme based on two-photon absorption at 1310 nm of PEDF is proposed for L + band amplification. Through the theoretical analysis, the local environment of Er3+ is changed due to the co-doping of PbS, which improves the two-photon absorption efficiency near 1300 nm. Compared with the 980 nm pump, the PEDFs excited by the 1310 nm pump show better amplification performance in the L + band. And in a bi-directional pumping system, PEDF achieves over 22 dB of gain in the whole L band. In particular, the bandwidth of over 20 dB gain was extended to 1627 nm with a noise figure as low as 4.9 dB. To the best of our knowledge, this is the first time that a high-gain bandwidth of L band amplification has been extended to 1627 nm. The results of unsaturated loss also show that PbS co-doping improves the two-photon absorption efficiency of PEDF to broaden the amplification bandwidth of L + band. These results demonstrate that an effective L + band amplification method is practically provided for future ultra-wideband optical communications.

Heterogeneous integrated optical fiber with side nickel core for distributed magnetic field sensing.

A design of a heterogeneous integrated optical fiber with side nickel core (SNCF) has been proposed and demonstrated for distributed fiber-optic magnetic field sensing. Experimental results show that magnetic properties of nickel can be preserved well after the high temperature drawing process. The functionality of the SNCF has been well verified, with the sensitivity for DC magnetic field being up to -2.42 µÎµ/mT (below 8 mT). Besides, the SNCF finally presents magnetostriction saturation under a certain magnetic field, which agrees with the simulation. The proposed direct thermal drawing method to produce metal-heterogeneous integrated optical fiber paves the way for a simple and scalable means of incorporating metallic materials into fibers, as well as providing a promising candidate for long-distance distributed magnetic field sensing.

Towards high-sensitivity and high-accuracy forward Brillouin scattering-based optomechanical temperature sensing in thin-diameter fibers.

We numerically and experimentally demonstrated a high-sensitivity and high-accuracy temperature sensor based on guided acoustic radial modes of forward stimulated Brillouin scattering (FSBS)-based optomechanics in thin-diameter fibers (TDF). The dependence of the FSBS-involved electrostrictive force on the fiber diameter is systematically investigated. As the diameters of the fiber core and cladding decrease, the intrinsic frequency of each activated acoustic mode and corresponding FSBS gain are expected to be accordingly increased, which benefits the significant enhancement of its temperature sensitivity as well as the optimization of the measurement accuracy. In validations, by utilizing TDFs with fiber diameters of 80 µm and 60 µm, the proof-of-concept experiments proved that sensitivities of the TDF-based FSBS temperature sensor with radial modes from R0,4 to R0,15 increased from 35.23 kHz/°C to 130.38 kHz/°C with an interval of 8.74 kHz/°C. The minimum measurement error (i.e., 0.15 °C) of the temperature sensor with the 60 µm-TDF is 2.5 times lower than that of the 125 µm-SSMF (i.e., 0.39 °C). The experimental and simulated results are consistent with theoretical predictions. It is believed that the proposed approach with high sensitivity and accuracy could find potential in a wide range of applications such as environmental monitoring, chemical engineering, and cancer detection in human beings.

Two-photon 3D printing diaphragm-integrated ring waveguide coupler for ultrasound detection.

We demonstrate a diaphragm-integrated ring waveguide coupler fabricated by the two-photon direct laser wring technique as an ultrasonic sensor, which is integrated on an optical fiber tip. The device consists of a micro-ring waveguide with a diameter of 5 µm functionalized as an optical fiber tip light reflection mirror and a straight waveguide connecting a diaphragm. The evanescent field coupling can be realized between the two waveguides, and the coupling efficiency can be changed due to the variation of the coupling gap induced by ultrasound. Accordingly, the light reflection can be changed. Based on the plate vibration theory, the vibration frequency can be changed through optimizing the diaphragm size. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 1.07 mPa/Hz1/2 at 100 kHz, which has great potential in various acoustic wave sensing applications.

Low loss side-polished pumping coupler for high order OAM modes amplification.

An all-fiber fiber coupler was demonstrated for pumping orbital angular momentum (OAM) modes amplification, which was fabricated by side-polishing and bonding a ring-core erbium-doped fiber (RC-EDF) and a pre-tapered side-polished single-mode fiber (SMF). With the selected phase-matching condition at 976 nm, the pumping laser was coupled into the RC-EDF from the SMF with optimized high efficiency, whereas the 1st to 3rd-order OAM mode signals were transmitted with the low insertion loss in the RC-EDF over a broadband wavelength range from 1530 to 1565 nm. This all-fiber wavelength division multiplexing coupler was optimized by the polished length and depth of the two coupled fibers. The insertion loss for the OAM signal modes was obtained lower than 0.58 dB with the pump power coupling ratio of above 90%. The proposed side-polished pumping coupler technique can ensure high-order OAM modes amplification, paving the way for the all-fiber optical amplifier in high-capacity modal-division multiplexing fiber communication systems.

Modeling and calibration of micro/nano FBG temperature probe for scanning probe microscopy.

To accurately measure the local temperatures of the micro-nano area, we propose an optical method using a tapered fiber Bragg grating (FBG) probe with a nano tip for scanning probe microscopy (SPM). When the tapered FBG probe senses local temperature through near-field heat transfer, the intensity of the reflected spectrum decreases, along with a broadening bandwidth and a shift in the central peak position. Modeling the heat transfer between the probe and the sample shows that the tapered FBG probe is in a non-uniform temperature field when approaching the sample surface. Simulation of the probe's reflection spectrum reveals that the central peak position shifts nonlinearly with increasing local temperature. In addition, the near-field temperature calibration experiments show that the temperature sensitivity of the FBG probe increases nonlinearly from 6.2 pm/°C to 9.4 pm/°C as the sample surface temperature increases from 25.3°C to 160.4°C. The agreement of the experimental results with the theory and the reproducibility demonstrate that this method offers a promising approach for exploring micro-nano temperature.

Sub-kHz high-order mode Brillouin random fiber laser based on long-period fiber grating and distributed Rayleigh scattering in a half-open linear cavity.

We demonstrate a narrow-linewidth high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on a long-period fiber grating (LPFG) and distributed Rayleigh random feedback in a half-open linear cavity. The single-mode operation of the laser radiation with sub-kilohertz linewidth is achieved thanks to distributed Brillouin amplification and Rayleigh scattering along kilometer-long single mode fibers whilst a few mode fiber-based LPFGs enable the transverse mode conversion among a broadband wavelength range. Meanwhile, a dynamic fiber grating (DFG) is embedded and incorporated to manipulate and purify the random modes, which hence suppresses the frequency drift resulting from random mode hopping. Consequently, the random laser emission with either high-order scalar or vector modes can be generated with a high laser efficiency of 25.5% and an ultra-narrow 3-dB linewidth of 230 Hz. Furthermore, the dependence of the laser efficiency and frequency stability on the gain fiber length are also experimentally investigated. It is believed that our approach could provide a promising platform for a wide range of applications such as coherent optical communication, high-resolution imaging, highly sensitive sensing, etc.

Sub-kHz-linewidth continuous-wave single-frequency ring-cavity fiber laser based on high-gain Er: YAG crystal-derived silica fiber.

Throughout the development of single frequency fiber lasers (SFFLs), gain fiber is one of the most important components, which can greatly affect the quality of SFFLs. Here, we fabricated an Er: YAG crystal-derived silica fiber (EYDSF) using a CO2 laser-heating drawing technique, with a high gain coefficient of 1.74 dB/cm. Employing the EYDSF of only 10 cm as a gain medium, we constructed a continuous-wave ring-cavity SFFL with an all-fiber system. An ultra-narrow linewidth <660 Hz was achieved harnessing a homemade low-concentration Er-doped silica fiber as a saturable absorber. Importantly, the SFFL output power was up to 32.7 mW at 1560 nm. What's more, no multi longitudinal mode or mode hopping were found in 2 hours, and the fluctuation of power was <0.63% in 8 hours. Furthermore, the relative intensity noise was lower to -145 dB/Hz at frequencies over 1 MHz. The results indicate that the ring-cavity SFFL has desirable performance in output power, linewidth, stability and noise, which serves a prospective candidate applied to coherent optical communications, high-precision sensors, laser radars and other advanced fields.

Concise method for high-precision vibration recovery of Φ-OTDR based on the GF-FastICA algorithm.

To efficiently restore the vibration signals of a phase-sensitive optical time domain reflectometer (Φ-OTDR), the GF-FastICA joint algorithm is proposed, which combines guided filtering with fast independent component analysis (FastICA). The marked region of vibration is precisely located by guided filtering. FastICA deals with the optimal phase mixing matrix of the marked region to separate the vibration signals from the noise-containing phase signals. The experimental results show that the GF-FastICA achieves a correlation coefficient of 0.998 for 5-Hz vibration signal recovery from a 14.3-km sensing fiber, verifying the potency of the algorithm. Compared with the traditional method and FastICA only, GF-FastICA improves the root mean square error (RMSE) metric by an order of magnitude, which is approaching an experience value of 10-3.

Disordered mullite grains in a sapphire-derived fiber for high-temperature sensing.

In this study, a sapphire-derived fiber (SDF)-based Fabry-Pérot interferometer (FPI) is proposed and experimentally demonstrated as a high-temperature sensor using the arc discharge crystallization process, forming a region with disordered mullite grains. This shows that the disordered mullite grains are related to the gradual temperature distribution during the arc discharge process, which results in a larger refractive index (RI) modulation of the SDF near the fusing area, forming a reflection mirror. An FPI was obtained by combining the optical fiber end facet. Considering the high-temperature resistance of the fiber, the fabricated FPI was used for high-temperature sensing. This shows that the device can operate at temperatures of up to 1200 °C with a sensitivity of 15.47 pm/°C, demonstrating that the proposed devices have potential applications in high-temperature environments.

Broadband high-gain Yb: YAG crystal-derived silica fiber for low noise tunable single-frequency fiber laser.

An over 75 nm broadband spectrum with a gain per unit length of >2 dB/cm was obtained from a homemade Yb: YAG crystal-derived silica fiber (YCDSF) with Yb-doping concertation of 6.57 wt.%. Using a 13-cm-long YCDSF, a low-noise wavelength-tunable single-frequency fiber laser has been constructed, enabling a single longitudinal mode oscillation from 1009 to 1070 nm. In particular, in the 1023-1056 nm waveband, the laser operating at any wavelength exhibited a maximum output power over 37 mW with power fluctuations below 0.38%, a slope efficiency >8%, and an optical signal-to-noise ratio higher than 60 dB. A linewidth of less than 2.8 kHz was also observed at the maximum pump powers, and relative intensity noise was as low as -155 dB/Hz at frequencies above 1.0 MHz. These results indicate that the YCDSFs with broadband high-gain characteristics are promising for wavelength-tunable fiber lasers in applications such as optical coherence tomography, precision metrology, nonlinear frequency conversion, and so on.

Towards optimal conversion efficiency of Brillouin random fiber lasers in a half-open linear cavity.

We proposed and demonstrated an unprecedented high-efficiency Brillouin random fiber laser (BRFL) by fiber length optimization in a half-open linear cavity. In terms of the trade-off between Brillouin gain saturation and weak distributed Rayleigh feedback strength, optimal laser efficiency associated to proper fiber length in a BRFL was theoretically predicted. As a proof-of-concept, a unidirectional-pumped BRFL with a half-open linear cavity was experimentally conducted, in which a fiber Bragg grating at one end of gain fiber served as a high-reflection mirror while Rayleigh scattering enabled distributed feedback for random lasing resonance. Results show that the optimal fiber length of ∼3.4 km in the BRFL offers sufficient Rayleigh scattered random feedback whilst alleviating the Brillouin gain saturation to a large extent. Consequently, an optimal laser efficiency of 77.0% in the BRFL was experimentally demonstrated, which reaches the state-of-the-art high record. Laser characteristics, including the linewidth, statistics and frequency jitter were also systematically investigated. It is believed that such efficient BRFL could provide a promising platform for inspiring new explorations of laser physics as well as potentials in long-haul coherent communication and fiber-optic sensing.

Microbubble-based optical fiber Fabry-Perot sensor for simultaneous high-pressure and high-temperature sensing.

An all-silica Fabry-Perot interferometer (FPI) based on a microbubble for high-pressure and high-temperature measurements is proposed and demonstrated. The microbubble-based air cavity is fabricated using a hollow silica tube and a single-mode optical fiber for pressure sensing. The suitable thickness between the two end faces of the microbubble enables the silica cavity to be used for temperature sensing. The wavelength shift of the reflection spectrum versus pressure is linear, and the sensitivity reaches -5.083 nm/MPa at room temperature (20 °C) within the range of 0 - 4 MPa. The temperature sensitivity reaches 12.715 pm/°C within the range of 20 - 700 °C. The very low temperature-pressure cross-sensitivity of the two cavities indicates that the proposed FPI sensor offers great potential for simultaneous high-pressure and high-temperature measurements in harsh environments.

High-efficient disturbance event recognition method of ϕ-OTDR utilizing region-segmentation differential phase signals.

We demonstrate a disturbance event recognition method based on region segmentation, which utilizes differential phase signals of a phase-sensitive optical time-domain reflectometer (ϕ-OTDR) to recognize disturbance events efficiently. The long-haul sensing fiber is divided into subsensing regions; whereas the phase signals at the two end points of the subsensing regions are subtracted, unwrapped, and differenced to represent the disturbance information. Feature extraction and classification are performed separately on the subsensing regions datasets. The experimental results indicate that the average recognition accuracy of the region-segmentation-based event recognition method is up to 92.9%. Compared to the method without region segmentation, this proposed method improves the average recognition accuracy by 8%; whereas the recognition time of three disturbance events on a 14.8-km sensing system is only 0.39 s. The proposed method provides significant support for the development of disturbance event recognition of the ϕ-OTDR sensor system.

Improved Fluorescence and Gain Characteristics of Er-Doped Optical Fiber with PbS Nanomaterials Co-Doping.

Er-doped optical fiber (EDF) with ultra-broad gain bandwidth is urgently needed given the rapid advancement of optical communication. However, the weak crystal field of the host silica glass severely restricts the bandwidth of traditional EDF at 1.5 µm. In this study, we theoretically explored the introduction of PbS nanomaterials in the silica network assisted with the non-bridging oxygen. This can significantly increase the crystal field strength of Er3+ ions in the local structure, leading to their energy level splitting and expanding the fluorescence bandwidth. Additionally, the PbS/Er co-doped optical fiber (PEDF) with improved fluorescence and gain characteristics was fabricated using modified chemical vapor deposition combined with the atomic layer deposition technique. The presence of PbS nanomaterials in the fiber core region, which had an average size of 4 nm, causes the 4I13/2 energy level of Er3+ ions to divide, increasing the fluorescence bandwidth from 32 to 39 nm. Notably, the gain bandwidth of PEDF greater than 20 dB increased by approximately 12 nm compared to that of EDF. The obtained PEDF would play an important role in the optical fiber amplifier and laser applications.