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The level of joint laxity, which is an indicator of accurate diagnosis for musculoskeletal conditions is manually determined by a physician. Studying joint laxity via artificial joints is an efficient and economical way to improve patient experience and joint proficiency. However, most of study focus on the joint geometry but are inadequate with regard to the tailored mechanical properties of soft tissues. On the basis of collagen fibril deformation, this study proposes bioinspired 3D fibers braided from polyethene multifilament for the reproduction of the controlled nonlinear behavior of ligaments and tendons. Four braided bands are designed, all showing biological behaviors. Two knot-based bands exhibit large toe strains of 10.98% and 5.33% but low linear modulus of 239.84 MPa and 826.05 MPa. The other two bands without knots exhibit lower toe strains of 1.61% and 1.52% but high linear modulus of 2605.27 MPa and 2050.74 MPa. Empirical formulas for braiding parameters (wales and courses) and mechanical properties are expressed to provide a theoretical basis for the mimicry of different tissues in the human body by artificial joints. All parameters have significant effects on the linear region of the load-displacement curve of a fiber due to braided structure, while changing the number of wales facilitates a major contribution to the toe region. A biofidelic human knee has been successfully reconstructed by using bioinspired 3D braided fibers. This study demonstrates that the nonlinear mechanical properties of soft tissues can be replicated by bioinspired 3D braided fibers, further yielding the design of more biomechanically realistic artificial joints.

The level of joint laxity, which is an indicator of accurate diagnosis for musculoskeletal conditions is manually determined by a physician. Studying joint laxity via artificial joints is an efficient and economical way to improve patient experience and joint proficiency. However, most of study focus on the joint geometry but are inadequate with regard to the tailored mechanical properties of soft tissues. On the basis of collagen fibril deformation, this study proposes bioinspired 3D fibers braided from polyethene multifilament for the reproduction of the controlled nonlinear behavior of ligaments and tendons. Four braided bands are designed, all showing biological behaviors. Two knot-based bands exhibit large toe strains of 10.98% and 5.33% but low linear modulus of 239.84 MPa and 826.05 MPa. The other two bands without knots exhibit lower toe strains of 1.61% and 1.52% but high linear modulus of 2605.27 MPa and 2050.74 MPa. Empirical formulas for braiding parameters (wales and courses) and mechanical properties are expressed to provide a theoretical basis for the mimicry of different tissues in the human body by artificial joints. All parameters have significant effects on the linear region of the load-displacement curve of a fiber due to braided structure, while changing the number of wales facilitates a major contribution to the toe region. A biofidelic human knee has been successfully reconstructed by using bioinspired 3D braided fibers. This study demonstrates that the nonlinear mechanical properties of soft tissues can be replicated by bioinspired 3D braided fibers, further yielding the design of more biomechanically realistic artificial joints. [ABSTRACT FROM AUTHOR]

Researches on high coherence light sources always pursue fast tuning speed, linear tunability and persistence of narrow linewidths at different tuning channels. Fortunately, acousto-optical interaction (AOI) can be employed in the fast management of the lasing wavelength. Herein, we propose a fast tunable narrow-linewidth fiber laser assisted by the acoustically-induced polarization selection in dispersion compensation fiber (DCF). The lasing wavelength is determined by the acoustically-induced band-pass grating in DCF. The large dispersion slope of the DCF suppresses the bandwidth of the TM01 peak to be less than 1 nm. The acousto-optic-controlled polarization conversion effectively restrains the acousto-optic frequency shift. With side longitudinal mode suppressions induced by the combintion of the self-injection feedback and saturable absorption filtering, the output laser successfully achieves the narrow-linewdith operation and owns a low relative intensity noise floor of −135 dB/Hz. By controlling the acoustic-wave frequency, the laser can be linearly tuned with a large wavelength range (over 20 nm, linear fitting R2 reaches 0.99781), and the laser linewidths remain about ∼2 kHz in different tuning channels. The laser tuning dynamics under the acousto-optical controlling are experimentally revealed, and the lasing tuning response time is ∼800 μs. [ABSTRACT FROM AUTHOR]

We demonstrate a hybrid solution for simultaneous transfer of ultra-stable optical frequency, RF frequency and time signal on a 100 km fiber spool with the same wavelength based on coherent demodulation and optical phase-lock-loop (OPLL). The one pulse per second (1PPS) is encoded by phase and RF frequency is encoded with an intensity modulator. The noise caused by fiber link is compensated by the round-trip method. And the bad effect on optical frequency caused by optical phase modulation (PM) can be cleaned using OPLL. When the fiber link is stabilized, the transferred time deviation (TDEV) can be 2.4E-11@1s and 1.9E-12@104s, RF frequency stability can achieve 8.0E-14@1s and 2.5E-17@104s, the modified Allan deviation (MDEV) of cleaned optical frequency can be 1.4E-16@1s and 1.6E-19@104s.

Current technological progress is driving Quantum Key Distribution towards a commercial and worldwide scale expansion. Its capability to deliver secure communication regardless of the computational power of the attackers will be a fundamental feature in the next generations of telecommunication networks. Nevertheless, demonstrations of QKD implementation in a real operating scenario and their coexistence with the classical telecom infrastructure are of fundamental importance for reliable exploitation. Here we present a Quantum Key Distribution application implemented over a classical fiber-based infrastructure. We exploit a 50 MHz source at 1550 nm, a single 13 km-long fiber cable for both the quantum and the classical channel, and a simplified receiver scheme with just one single-photon detector. In this way, we achieve an error rate of approximately 2% and a secret key rate of about 1.7 kbps, thus demonstrating the feasibility of low-cost and ready-to-use Quantum Key Distribution systems compatible with standard classical infrastructure. [ABSTRACT FROM AUTHOR]

Global data traffic is continuously growing year by year. In this regard, telecom companies faced the problem of increasing data transmission rate, which is becoming more and more challenging. One of options that could help to increase optical links capacity is an expansion of the spectral range of DWDM systems used in telecom by inclusion of the O-band and/or the E-band. An ability to engage these spectral ranges directly related to the presence of suitable effective optical amplifiers. In this work, we studied different modifications of Bi-doped fiber amplifier operating in the O- and E-bands in order to obtain the widest gain spectrum in a single device. In particular, one- and two-wavelength pumping and gain-clamped schemes, as well as usage of a Bi:P $_\text {2}$ O $_\text {5}$ -SiO $_\text {2}$ and Bi:GeO $_\text {2}$ -SiO $_\text {2}$ fibers combination, were considered. As a result, an amplifier with 150 nm –6 dB bandwidth, $>$ 25 dB peak gain operating in the range from $\sim$ 1300 to $\sim$ 1450 nm was developed. To achieve the highest spectral width, we compare a gain-clamped at 1280 nm cavity pumped at 1180 nm with dual pump scheme at 1180 nm and 1270 nm along with a combination of active media composed of Bi:P $_\text {2}$ O $_\text {5}$ -SiO $_\text {2}$ and Bi:GeO $_\text {2}$ -SiO $_\text {2}$ fibers. [ABSTRACT FROM AUTHOR]

Advanced applications in optical frequency metrology demand improved tunable lasers with high coherence. Herein, a tunable single-frequency fiber laser based on the longitude-purification induced by the distributed feedback has been demonstrated. The key device in the proposed laser structure is a distributed self-injection feedback structure (DSIFS) which function as a mode selector to have a robust frequency-domain mode suppression and wavelength adaptability. Our work is distinctly focused on the theoretical analysis of the formation process of single-longitudinal-mode (SLM) laser output and the wavelength adaptive characteristics in the DSIFS. In this experiment, the side mode suppression of the resonant longitudinal modes can be achieved in a fiber ring laser without any accurate control of the main cavity. The fiber laser obtained a compressed laser linewidth of 855 Hz, enhanced side mode suppression ratio (SMSR) of ∼67 dB, and frequency noise suppression of 40 dB. Furthermore, the fiber laser can be tuned over the entire flat gain region. The measured output power, wavelength and SMSR variations of the proposed laser over a long-term observation are less than 0.034 mW, 0.028 nm and 2.53 dB, respectively. In addition, the SLM operation of the laser can also be obtained at different pump power for every wavelength channel. Moreover, this method is applicable to any other gain-type lasers, indicating that the longitude-purification mechanism has a broad application prospect in other highly coherent tunable lasers. [ABSTRACT FROM AUTHOR]

We demonstrate a hybrid solution for simultaneous transfer of ultra-stable optical frequency, RF frequency and time signal on a 100 km fiber spool with the same wavelength based on coherent demodulation and optical phase-lock-loop (OPLL). The one pulse per second (1PPS) is encoded by phase and RF frequency is encoded with an intensity modulator. The noise caused by fiber link is compensated by the round-trip method. And the bad effect on optical frequency caused by optical phase modulation (PM) can be cleaned using OPLL. When the fiber link is stabilized, the transferred time deviation (TDEV) can be 2.4E-11@1s and 1.9E-12@104s, RF frequency stability can achieve 8.0E-14@1s and 2.5E-17@104s, the modified Allan deviation (MDEV) of cleaned optical frequency can be 1.4E-16@1s and 1.6E-19@104s. [ABSTRACT FROM AUTHOR]

The eigenmodes of an inverse-parabolic graded index fiber under torsion were investigated by using the finite element method simulation. These eigenstates correspond to orbital angular momentum modes except for the two polarized-vortex modes. The design of appropriate core parameters can lead to the difference in the effective refractive indexes between all adjacent modes being greater than 10-4 over a bandwidth of 1.5 to 1.6 μm, which is expected to be an ideal method for the generation of multiple vortex modes in few-mode fiber for optical communication. The results showed that the separation of these modes changes with twist rate. Compared to the twisted air-hole ring-core fiber, the optimal twist rate of this fiber was found to be an order of magnitude smaller, which provides a more feasible scheme for the combination of mode division multiplexing and wavelength division multiplexing. We speculate that this study may have promising prospects in various applications such as particle manipulation, imaging, and quantum communication.

In this work, we design and simulate an air-core As2S3 ring fiber for high-order orbital angular momentum (OAM) supercontinuum generation. We show that the chromatic dispersion of the ring fiber can be substantially tailored by proper optimization of the air-core radius. Two-octave supercontinuum carrying OAM17,1 mode, spanning from 1560 to 6250 nm, is obtained by pumping a 50-fs 100-kW secant hyperbolic pulse centered at the wavelength of 3800 nm into the designed fiber with 50-μm air-core radius and 1-μm ring width. We further engineer the chromatic dispersion of some other OAM modes and perform simulations of supercontinuum spectra using different kilowatt-level peak power, which indicates that the fiber we design represents a promising avenue for supercontinuum generation of all the OAMl,1 modes (|l| ≤17). The proposed fiber is suitable for the transmission of OAM beams in infrared wavelength range and it could promote the development and application of high-order OAM beams.

In this manuscript, optical fiber Fabry-Perot acoustics sensors based on corrugated PET diaphragms were proposed, fabricated and tested. Corrugations of different depths were introduced into the PET by simple hot-embossing process. The performances of the fabricated acoustics sensors were tested and analyzed. The results showed the corrugations could effectively improve the performances of the sensors. The proposed fabrication method of the corrugated diaphragm showed the advantages of simplicity and low-cost, and the acoustic sensors showed flexible control of corrugated structure, which were suitable for weak acoustic sensing. [ABSTRACT FROM AUTHOR]

We report the fabrication details and optical characterization of novel Erbium (Er) and Chromium (Cr) co-doped alumina-germania-calcia-yttria-silica fiber. Co-doping with Er and Cr is chosen as potentially promising for enhancing the fiber's fluorescent and amplifying potential in the near-infrared (NIR) region at `resonant' (viz. into Er band absorption maximum, @976 nm) and `non-resonant' (viz. off Er band while into Cr broad absorption band, @905/1064 nm) excitations. Our results reveal strong coupling, at both kinds of excitation, of Er- and Cr- subsystems in the fiber. This effect is justified by the presence of pronounced 1.5-1.6 μm emission, characteristic to Er ions, at 905/1064 nm excitations and, at 976-nm excitation, by notable spectral broadening of the fiber's NIR fluorescence and gain, comprising the lines inherent to Er3+ (1.5-1.6 μm) and Cr3+/Cr4+ (1.15-1.45 μm), as compared to purely Er-doped silicate fiber, exemplified here by commercial `L20' (Er20-4/125) and `L40' (Er40-4/125) fibers.

We propose and demonstrate a symmetric step-apodized distributed feedback (DFB) fiber laser, which has an improved efficiency and a narrow linewidth. This special DFB laser cavity is based on a unique symmetric step-apodized π-phase-shifted fiber Bragg grating (FBG) directly inscribed in a heavily erbium-doped fiber. The symmetric step-apodized phase-shifted grating is equivalent to a weak uniform phase-shifted FBG embedded in a pair of strong FBGs, and this design could increase the effective cavity length of a DFB fiber laser. Moreover, the symmetric step-apodized DFB laser cavity proposed in this work has a phase shift located in the cavity center. It could simplify the design process significantly due to the symmetry in the laser cavity. Experimental results show that the slope efficiency of a symmetric step-apodized DFB fiber laser could be increased to 1.06%, which is significantly higher than that of a uniform DFB fiber laser (i.e., 0.45%) under the same cavity length and pump conditions. In addition, the proposed DFB fiber laser exhibits a high stability. The fluctuations in lasing wavelength and output power were less than 12 pm (corresponding to a frequency shift of 1.49 GHz) and 0.13 dB within 24 h, respectively. Moreover, the full-width at half-maximum linewidth of the symmetric step-apodized DFB fiber laser was ~2.6 kHz, measured by the delayed self-heterodyne method with a 50-km fiber delayed line. As such, the proposed symmetric step-apodized DFB fiber laser could potentially be used as high-performance light source for fiber-optic sensors or coherent optical communication systems.

We fully demonstrated an ytterbium (Yb) doped triple-clad laser fiber with intense pump absorption, high efficiency, and laser stability. P5+/Al3+ molar ratio in the central region of fiber core was specially designed to be ~1.08 for compensating P2O5 evaporation, while the ratio in the outer region was set as ~0.92 for obtaining a flattened refractive index profile. Applying a two-stage deposition process, one-time collapse technique, low-temperature multi-times sintering, and suitable flow of pure POCl3 during sintering and collapsing process were instrumental in decreasing and offsetting element evaporation. Measured in an all-fiber master oscillator power amplifier configuration, the 7-m-long fiber obtained 1.39 kW near-single-mode laser output at 1079.6 nm with a slope efficiency of 85.2% and a beam quality M2 of 1.36. Output laser spectrum with no sign of nonlinear-related peaks and a narrow 3-dB-bandwidth of ~0.33 nm was obtained. The fiber-based laser setup was kept at the maximum power for 1 h with power degradation less than 0.21%. The results demonstrated Yb-doped aluminophosphosilicate triple-clad fiber is a highly competitive candidate for commercial high-power laser and high-power narrow-linewidth fiber laser.

This paper presents a fabrication method and results from optical and acoustic characterization of a miniature fiber Fabry-Perot ultrasound detector. A high Q-factor was achieved by creating a self-aligned polymer waveguide within the resonator cavity to restrict diffraction losses. As a result of the improved optical Q-factor upon introduction of the waveguide, a high acoustic sensitivity was achieved. The device presented in this paper demonstrates an optical resonator Q-factor of >2000, and a Noise Equivalent Pressure of 350 Pa over a 25MHz bandwidth on a $125~\mu \text{m}$ fiber with a $9~\mu \text{m}$ waveguide. Sensitivity can be further improved significantly with the use of dielectric mirrors. The high sensitivity of these waveguided fiber Fabry-Perot ultrasound detectors makes them suitable for detection of both ultrasound and photoacoustic signals, while their small size makes this technology suitable for adoption in applications requiring small form factor, such as in interventional cardiology. [ABSTRACT FROM AUTHOR]

We have developed a new type of piezoelectric poly-/-lactic acid (PLLA) braided cord that can be embroidered (piezoelectric PLLA embroidery) as a motion-sensing material for a new type of wearable sensor. We then examined the motion-sensing function of the piezoelectric PLLA embroidery in response to different types of movement. On the basis of the experimental results, we produced e-textiles embroidered with the piezoelectric PLLA braided cord as materials for a new wearable sensor. These e-textiles with piezoelectric PLLA embroidery, which are flexible, feel soft and are similar to high-quality fabrics, can sense complex human motion without using a complex computer system. [ABSTRACT FROM AUTHOR]

Physics phenomena of multipulse compounds have enriched the life of ultrashort pulses beyond traditional pulse singlets in passively mode-locked fiber lasers. By developing a near zero-dispersion fiber laser, we report on the generation of dispersion-managed soliton (DMS) molecules. During propagation in the laser cavity, the broadband DMSs experience a breathing process (i.e., periodic compression and stretch of the pulse width), which facilitates various molecule evolutions from pulse singlets. In particular, tightly bound DMS pairs, loosely bound DMS pairs and three-pulse to eleven-pulse molecules are respectively observed. Apart from the aforementioned DMS molecules with equal pulse separations, unequally spaced DMS molecules with different multipulse structures are also obtained, typically including the (2 + 1)-type molecule and the (2 + 3 + 1)-type molecule. The investigation of DMS molecules characterized by versatile multipulse structures is of both fundamental scientific interests in soliton dynamics and potential applications of ultrahigh-capacity optics communications based on advanced modulation formats.

By using the MCVD system and the all-gas-phase chelate precursor doping technique, we fabricated an Yb-doped aluminophosphosilicate fiber with 20 μm-core and 400 μm-clad in diameter, i.e., 20/400 Yb-APS fiber. The fiber core was doped with 1100 ppm Yb3+, 7200 ppm Al3+, and 8000 ppm P5+. With a molar ratio of Al/P close to 1:1, low refractive index difference and suitable numerical aperture was obtained for large-mode-area fiber design. Directly forward pumped by 976 nm laser diodes, 11-m-long 20/400 Yb-APS fiber presented 3.03 kW laser output at 1080 nm with a slope efficiency of 76.6% and beam quality M2 of ~1.58. Up to this power level, no signs of instable beam profile, beam quality worsening, and laser power roll-over were obtained, indirectly justified no obvious mode instability in the whole laser setup. To directly characterize its power stability and photodarkening effect, the fiber laser was kept at ~2.1 kW for over 500 min with power degradation less than 1.1%. These results indicated that the all-gas-phase chelate precursor doping technique is highly competitive for Yb-APS fiber fabrication toward high-power laser, and the fabricated fiber is very suitable for 2 kW-level or above commercial fiber laser development.

By a chelate precursor doping technique, we designed and fabricated an ytterbium/cerium codoped aluminosilicate fiber for noticeable suppression of photodarkening and demonstrated its laser performance. Cerium/ytterbium ions ratio of 0.5 was specially designed to mitigate photodarkening. Aluminum/fluorine ions ratio of 1 was devised to keep low numerical aperture of fiber core. The measurements showed the fiber core was homogeneously doped with ∼0.12 mol% Yb2O3, ∼0.05 mol% Ce2O3, ∼0.5 mol% Al2O3, and ∼0.2 mol% SiF4, respectively. Tested in a master oscillator power amplifier system, the fiber presented 1930 W laser output power with optical-to-optical efficiency of 79.2%. Stabilized at 1850 W for over 500 min, the output power presented a relatively small power degradation of

In this article, we fabricated and tested a series of extrinsic fiber Fabry–Perot interferometric acoustic sensors based on the corrugated silver diaphragms. These diaphragms were fabricated through a simple and low-cost method. The corrugations were first etched on the silicon surface permanently and then transferred to the silver diaphragm with the help of photoresist as the sacrificial layer. The sensors with different corrugation depths were fabricated. The experimental results showed the same trend with the theoretical results on the mechanical sensitivities of the fabricated corrugated diaphragms, which first increased and then decreased with the increase in the corruption’s depth. Other characteristics of the fabricated optical acoustic sensors based on the corrugated silver diaphragms were also tested in detail. The proposed fabrication method of the corrugated diaphragm showed the advantages of simplicity and low cost, and the acoustic sensors showed high sensitivity, which was suitable for weak acoustic’s sensing. [ABSTRACT FROM AUTHOR]

The eigenmodes of an inverse-parabolic graded index fiber under torsion were investigated by using the finite element method simulation. These eigenstates correspond to orbital angular momentum modes except for the two polarized-vortex modes. The design of appropriate core parameters can lead to the difference in the effective refractive indexes between all adjacent modes being greater than 10−4 over a bandwidth of 1.5 to 1.6 μm, which is expected to be an ideal method for the generation of multiple vortex modes in few-mode fiber for optical communication. The results showed that the separation of these modes changes with twist rate. Compared to the twisted air-hole ring-core fiber, the optimal twist rate of this fiber was found to be an order of magnitude smaller, which provides a more feasible scheme for the combination of mode division multiplexing and wavelength division multiplexing. We speculate that this study may have promising prospects in various applications such as particle manipulation, imaging, and quantum communication. [ABSTRACT FROM AUTHOR]

We demonstrated an Yb-doped mode-locked fiber laser system delivering both dissipative-soliton-resonance (DSR) and noise-like pulse (NLP) simultaneously. The pulse widths of DSR and NLP were 7.1 ns and 4.3 ns, respectively. Meanwhile, the spectrum widths at 1056 nm of DSR and NLP were 0.5 nm and 21.5 nm, respectively. In this dual mode-locking state operation, the intracavity laser powers (DSR, NLP) increased 24% and 41% compared to that in single mode-locking states at the pump power of 600 mW, respectively. To the best of our knowledge, it is the first time to trigger DSR and NLP simultaneously by sharing one pump in a fiber laser system. [ABSTRACT FROM AUTHOR]

We discuss technology options and challenges for scaling intra-datacenter interconnects beyond 1 Tb/s bandwidths, with focus on two possible approaches: pulse amplitude modulation (PAM)-based intensity modulation-direct detection (IM-DD) and baud-rate sampled coherent technology. In our studies, we compare the performance of various orders of PAM modulation (PAM4 to 8). In addition to these fixed PAM signaling options, a flexible PAM (FlexPAM) technique leveraging granularity in spectral efficiency (SE) is proposed to maximize link margin. For baud-rate sampled coherent technology, we propose a simplified digital signal processing (DSP) architecture to bring down power consumption of the coherent approach closer to that of IM-DD PAM. We also propose two new phase noise tolerant 2D coherent modulation formats to relax the laser linewidth requirement. In closing, a comparative study of fixed IM-DD PAM versus coherent polarization multiplexed-quadrature amplitude modulation (PM-QAM) is presented for a 1.6 Tb/s solution (200 Gb/s per dimension), with consideration of link loss/reach budget, power consumption, implementation complexity, as well as fan-out granularity. [ABSTRACT FROM AUTHOR]

We report the fabrication details and optical characterization of novel Erbium (Er) and Chromium (Cr) co-doped alumina-germania-calcia-yttria-silica fiber. Co-doping with Er and Cr is chosen as potentially promising for enhancing the fiber's fluorescent and amplifying potential in the near-infrared (NIR) region at ‘resonant’ (viz. into Er band absorption maximum, @976 nm) and ‘non-resonant’ (viz. off Er band while into Cr broad absorption band, @905/1064 nm) excitations. Our results reveal strong coupling, at both kinds of excitation, of Er- and Cr- subsystems in the fiber. This effect is justified by the presence of pronounced 1.5–1.6 μm emission, characteristic to Er ions, at 905/1064 nm excitations and, at 976-nm excitation, by notable spectral broadening of the fiber's NIR fluorescence and gain, comprising the lines inherent to Er3+ (1.5–1.6 μm) and Cr3+/Cr4+ (1.15–1.45 μm), as compared to purely Er-doped silicate fiber, exemplified here by commercial ‘L20’ (Er20-4/125) and ‘L40’ (Er40-4/125) fibers. [ABSTRACT FROM AUTHOR]

We report on an efficient wavelength-tunable gain-switched and gain-switched mode-locked Er3+-doped double-clad fiber laser with a linear cavity. Stable gain-switching and gain-switched mode-locking were achieved with slope efficiencies of 28.6% and 34.5% with respect to launched pump power, respectively. The gain-switched laser pulses were generated with a maximum average output power of around 110 mW, pulse width of 661.2 ns, and calculated peak power of ~16.5 W at a repetition rate of 10 kHz. At the same repetition, the gain-switched mode-locked laser pulses were generated with a maximum average output power of around 514 mW, subpulse repetition rate of ~30.7 MHz, and peak power of higher than 154 W at a repetition rate of 10 kHz when the wavelength was tuned to 2.78 μm. This is, to the best of our knowledge, the first demonstration of a gain-switched self-started mode-locked fiber laser near 3 μm.

We propose and demonstrate a symmetric step-apodized distributed feedback (DFB) fiber laser, which has an improved efficiency and a narrow linewidth. This special DFB laser cavity is based on a unique symmetric step-apodized π-phase-shifted fiber Bragg grating (FBG) directly inscribed in a heavily erbium-doped fiber. The symmetric step-apodized phase-shifted grating is equivalent to a weak uniform phase-shifted FBG embedded in a pair of strong FBGs, and this design could increase the effective cavity length of a DFB fiber laser. Moreover, the symmetric step-apodized DFB laser cavity proposed in this work has a phase shift located in the cavity center. It could simplify the design process significantly due to the symmetry in the laser cavity. Experimental results show that the slope efficiency of a symmetric step-apodized DFB fiber laser could be increased to 1.06%, which is significantly higher than that of a uniform DFB fiber laser (i.e., 0.45%) under the same cavity length and pump conditions. In addition, the proposed DFB fiber laser exhibits a high stability. The fluctuations in lasing wavelength and output power were less than 12 pm (corresponding to a frequency shift of 1.49 GHz) and 0.13 dB within 24 h, respectively. Moreover, the full-width at half-maximum linewidth of the symmetric step-apodized DFB fiber laser was ∼2.6 kHz, measured by the delayed self-heterodyne method with a 50-km fiber delayed line. As such, the proposed symmetric step-apodized DFB fiber laser could potentially be used as high-performance light source for fiber-optic sensors or coherent optical communication systems. [ABSTRACT FROM AUTHOR]

Conventional grating-inscription technology usually inscribes only one fiber Bragg grating (FBG) in the core of an optical fiber. A novel femtosecond laser point-by-point technology was demonstrated to parallel-inscribe multiple FBGs, i.e., so-called parallel-integrated FBGs (PI-FBGs), with either different or the same reflection wavelengths in the core of a standard single-mode fiber. Single- or multiwavelength PI-FBGs exhibited one or multiple peaks in the reflection spectrum, respectively. The length of PI-FBGs can be shortened to hundreds of micrometers to increase the spatial resolution of the FBG-based sensors. Our ultrashort PI-FBGs with a length of 500 μm could be used to realize ultrahigh temperature sensing with a high spatial resolution of less than 1 mm, a linear sensitivity of 15.0 pm/°C, and a large measurement range of up to 1100 °C, which overcomes the shortcomings of conventional FBG-based sensors with a low spatial resolution of more than 1 cm and a small measurement range of up to 300 °C. Moreover, the grating parameters, such as reflectivity, reflection wavelength, polarization-dependent loss, and full width at half maximum, can be improved to achieve desired PI-FBGs by changing the spatial distribution and grating number of the PI-FBGs. The PI-FBGs are insensitive to bending, which reduces bend-induced measurement errors. Therefore, our PI-FBGs will find potential applications in the fields of optical fiber sensors, communications, and lasers. [ABSTRACT FROM AUTHOR]

Physics phenomena of multipulse compounds have enriched the life of ultrashort pulses beyond traditional pulse singlets in passively mode-locked fiber lasers. By developing a near zero-dispersion fiber laser, we report on the generation of dispersion-managed soliton (DMS) molecules. During propagation in the laser cavity, the broadband DMSs experience a breathing process (i.e., periodic compression and stretch of the pulse width), which facilitates various molecule evolutions from pulse singlets. In particular, tightly bound DMS pairs, loosely bound DMS pairs and three-pulse to eleven-pulse molecules are respectively observed. Apart from the aforementioned DMS molecules with equal pulse separations, unequally spaced DMS molecules with different multipulse structures are also obtained, typically including the (2 + 1)-type molecule and the (2 + 3 + 1)-type molecule. The investigation of DMS molecules characterized by versatile multipulse structures is of both fundamental scientific interests in soliton dynamics and potential applications of ultrahigh-capacity optics communications based on advanced modulation formats. [ABSTRACT FROM AUTHOR]

By using the MCVD system and the all-gas-phase chelate precursor doping technique, we fabricated an Yb-doped aluminophosphosilicate fiber with 20 μm-core and 400 μm-clad in diameter, i.e., 20/400 Yb-APS fiber. The fiber core was doped with 1100 ppm Yb3+, 7200 ppm Al3+, and 8000 ppm P5+. With a molar ratio of Al/P close to 1:1, low refractive index difference and suitable numerical aperture was obtained for large-mode-area fiber design. Directly forward pumped by 976 nm laser diodes, 11-m-long 20/400 Yb-APS fiber presented 3.03 kW laser output at 1080 nm with a slope efficiency of 76.6% and beam quality M2 of ∼1.58. Up to this power level, no signs of instable beam profile, beam quality worsening, and laser power roll-over were obtained, indirectly justified no obvious mode instability in the whole laser setup. To directly characterize its power stability and photodarkening effect, the fiber laser was kept at ∼2.1 kW for over 500 min with power degradation less than 1.1%. These results indicated that the all-gas-phase chelate precursor doping technique is highly competitive for Yb-APS fiber fabrication toward high-power laser, and the fabricated fiber is very suitable for 2 kW-level or above commercial fiber laser development. [ABSTRACT FROM AUTHOR]

A corrugated silver diaphragm-based optical fiber microphone (OFM), formed by extrinsic Fabry–Perot interferometric structure, was proposed. The sensor’s sensitivity was improved by releasing diaphragm’s initial stress with the help of corrugations. A simple “gray-scale exposure” method was proposed to fabricate the corrugation. Experimental results indicated that the mechanical sensitivity of the fabricated corrugated diaphragm (CD) was 52 nm/Pa, which was twice that of the planar diaphragm under the same parameters. Other characteristics of the fabricated microphone were also tested in detail. The proposed CD-based OFM showed the advantages of high sensitivity, simplicity and low cost, which was expected to be applied in the field of weak acoustic sensing. [ABSTRACT FROM AUTHOR]

By a chelate precursor doping technique, we designed and fabricated an ytterbium/cerium codoped aluminosilicate fiber for noticeable suppression of photodarkening and demonstrated its laser performance. Cerium/ytterbium ions ratio of 0.5 was specially designed to mitigate photodarkening. Aluminum/fluorine ions ratio of 1 was devised to keep low numerical aperture of fiber core. The measurements showed the fiber core was homogeneously doped with ∼0.12 mol% Yb2O3, ∼0.05 mol% Ce2O3, ∼0.5 mol% Al2O3, and ∼0.2 mol% SiF4, respectively. Tested in a master oscillator power amplifier system, the fiber presented 1930 W laser output power with optical-to-optical efficiency of 79.2%. Stabilized at 1850 W for over 500 min, the output power presented a relatively small power degradation of <1.04%, directly justifying a strong photodarkening resistance. These results demonstrated that ytterbium/cerium codoped aluminosilicate fiber with appropriate concentration ratios of dopants could be an ideal choice for 2 kW-level commercial high power laser applications. [ABSTRACT FROM AUTHOR]

A new piezoelectric sensor was developed in the shape of a traditional Japanese braided cord known as a Kumihimo by weaving piezoelectric poly-l-lactic acid (PLLA) fibers (piezoelectric PLLA braided cord). We found that the piezoelectric PLLA braided cord tied in various decorative knots, and worn as an accessory such as a choker or necklace, can detect pulse waves, swallowing, and coughing in a systematic way in one's daily life. To demonstrate this, we developed a system for measuring pulse waves with a Wi-Fi communication system using a piezoelectric PLLA braided cord with decorative knots. [ABSTRACT FROM AUTHOR]

High-sensitivity extrinsic Fabry-Perot cantilever accelerometers were proposed and tested.Micromachined 45° angled fiber was adopted to form the cantilever to measure the acceleration. The Fabry-Perot cavity was formed by the sidewall of the fiber and the inner surface of commercial ceramic sleeves. The stepped cantilever fiber was fabricated by etching method to improve the sensitivitywithout enlarging the cantilever';s length or adding extra proof-mass. Detailed theoretical analysis, simulation, and experimental results of the proposed accelerometers were given.The measured results were in good agreement with the simulated results. The proposed accelerometers show the advantages of high flexibility, simplicity, compactness, and low cost, which are expected to be applied in the field of miniaturized vector hydrophone and seismic monitoring. [ABSTRACT FROM AUTHOR]

Zylon fibers, which are made of high-strength polymer, are used at the National High Magnetic Field Laboratory for structural reinforcement of high field pulsed magnet coils. Most polymers are subject to chain scission, which is the result of aging degradation of the fibers in the absence of chemical agent. We studied chain scission of Zylon fibers stored with and without visible light. No mechanical strength degradation was found in Zylon stored without exposure to visible light for 8 years. Prolonged exposure to lab-lighting rendered formation of surface defects, resulting in reduction in mechanical strength. [ABSTRACT FROM AUTHOR]

Z-domain model of fiber Bragg grating is developed, and its performance analysis is presented in this letter. The developed mathematical model is used for the evaluation of reflection and transmission response. Spectral characteristics in terms of bandwidth, peak transmittance, and side lobes formation is determined for a model of length 0.84 mm and is found to be in very close agreement with previously published fabrication result. A similar structure with different parameters is considered. The transmission response provides a wide full width half maximum (FWHM) bandwidth of 9.52 nm and a peak transmission of 0.067. The proposed model is extended to determine the transmission response for a very small incidence angle variation. Performance parameters, such as FWHM, stopband width, and optical density, are analyzed. Transmission response of the proposed model for different length combinations is evaluated. [ABSTRACT FROM AUTHOR]

A mode-locked all-fiber laser based on the nonlinear multimode interference (NL-MMI) effect of the graded-index multimode fiber (GIMF) is proposed and demonstrated. The saturable absorber (SA) used in the laser consists of a step-index multimode fiber (SIMF) connected by a GIMF. Compared with the theoretically proposed SMF--GIMF--SMF structure, the introduction of the SIMF eliminates the restriction on the length of the GIMF and provides a more flexible method of making an SA based on the NL-MMI. Through bending the device to a certain state, the modulation depth of the SA was measured to be 3.16%. The mode-locked laser output pulses have the pulse width of ~446 fs, the bandwidth of 4.48 nm, and the fundamental repetition rate of 11.7 3MHz. This versatility and simplicity of the SA device combined with the possibility of scaling the pulse energy in the large-mode-area double-clad fiber laser make it highly attractive in ultrafast photonics. [ABSTRACT FROM AUTHOR]

We demonstrate a 500-MHz repetition rate, self-starting Yb:fiber laser incorporated with a phase biased nonlinear amplifying loop mirror as an artificial saturable absorber. The laser has two output ports; the beam splitter port delivers pulses, which can be compressed to 84.4 fs at an average power of 51 mW and the polarization beam splitter port directly output 61-fs pulses at an average power of 197 mW. Although the fiber is non-polarization maintaining, the mode locking is environmentally stable. [ABSTRACT FROM PUBLISHER]