Enhanced evanescent field via integration of a graphene oxide/poly(methyl methacrylate) hybrid film on coreless D-shaped fibers.

This study presents the implementation of an evanescent field (EF)-based sensing platform employing a hybrid film composed of graphene oxide (GO) and poly(methyl methacrylate) (PMMA), integrated onto coreless D-shaped fibers (cDsFs). The operational framework of the hybrid film-coated cDsFs (GoP-cDsFs) was comprehensively elucidated through theoretical and experimental analyses. To establish a baseline for comparison, the performance of the cDsFs with the sole inclusion of the PMMA film was investigated. Our investigations underscore the substantive role of graphene oxide in augmenting the evanescent field, thereby generating a synergistic effect that contributes to the overall enhancement of the evanescent field in the device. Consequently, the fabricated GoP-cDsF sensor manifests an outstanding sensitivity of -4.936 nm/°C, rendering it particularly well-suited for applications demanding high-sensitivity temperature sensing. Moreover, the unique attributes of the GoP-cDsF position it as a promising candidate for the measurement of both magnetic and electric fields, presenting an effective strategy for multifunctional sensing applications.

Physical-layer security of optical communication based on chaotic optical encryption without an additional driving signal.

We propose and numerically demonstrate a scheme for physical-layer security based on chaotic phase encryption, where the transmitted carrier signal is used as the common injection for chaos synchronization, so there is no need for additional common driving. To ensure privacy, two identical optical scramblers consisting of a semiconductor laser and a dispersion component are used to observe the carrier signal. The results show that the responses of the optical scramblers are highly synchronized but are not synchronized with the injection. By properly setting the phase encryption index, the original message can be well encrypted and decrypted. Moreover, the legal decryption performance is sensitive to the parameter mismatch, since it can degrade the synchronization quality. A slight drop in synchronization induces an evident deterioration in decryption performance. Therefore, without perfectly reconstructing the optical scrambler, the original message cannot be decoded by an eavesdropper.

Multiple core modes conversion using helical long-period fiber gratings.

We propose and demonstrate the fabrication of an all-fiber mode converter enabling simultaneous generation of multiple high-order core modes, which is realized by inscribing a helical long-period grating (HLPG) in a few-mode fiber (FMF) using a femtosecond laser. Helical refractive index modulation is introduced by continuously irradiating the core region with a highly focused femtosecond laser, while the fiber moves in a spiral path through a three-dimensional translation stage. Mode conversion from the LP01 mode to high-order core modes, including LP11, LP21, LP31, LP02, LP12, and LP41 modes, is achieved by controlling the inscription pitch of the grating. Moreover, first-, second-, third-, and fourth-order orbital angular momentum (OAM) modes can be directly generated using the HLPGs, and multiple OAM modes of different topological charges can be simultaneously excited using a single high diffraction order HLPG. This approach offers a new option for implementing with high-integration high-order mode converters or OAM mode generators.

Soliton and bound-state soliton mode-locked fiber laser based on polarization-dependent grating.

We demonstrate the generation of solitons and bound-state solitons in a passively mode-locked fiber laser based on the nonlinear polarization rotation effect by polarization-dependent helical grating. The CO2-laser-inscribed grating has a high polarization-dependent loss of 24.4 dB at 1558.4 nm, which has facilitated the achievement of stable mode locking. The soliton laser could generate 548.9 fs pulses at 1560.59 nm with a spectrum bandwidth of 5.45 nm and a signal-to-noise ratio of 75.2 dB. Through adjustment of the polarization controller and pump power, a bound-state soliton mode-locked pulse with a spectral modulation period of 3.11 nm was achieved and the temporal interval between the two solitons was 2.19 ps. Furthermore, its repetition rate can be easily manipulated by varying the pump power. The results indicated that the polarization-dependent helical grating is an excellent polarizer that could be applied in an ultrafast fiber laser.

Broadband and tunable bandgap guidance based on ring-pattern liquid-filled photonic crystal fibers.

A ring-pattern liquid-filled photonic crystal fiber (R-LPCF) scheme, in which the first-ring holes (the six holes adjacent to the core) are filled with high-index inclusions, has been experimentally demonstrated to extend over a wide-guided spectral range. In such new fiber, the bandgap-like core mode is investigated, among which the telecommunication bandgap exhibits confinement losses five orders of magnitude smaller than those of the corresponding fully liquid-filled photonic bandgap fibers. Besides, the R-LPCF serving the thermal tunability when filled with index-matching liquid enables guided bandwidth switching from the 1.5-µm-band to the 1.3-µm-band communication window. Moreover, the structural parameters for two commercial photonic crystal fiber are quantified to confirm the feasibility of the proposed method.

Numerical investigation for the mode transmission characteristics of a large mode area optical fiber with heterogeneous helical claddings designed for 2.0 µm.

Large mode area (LMA) fibers are widely used in high-power fiber lasers to solve the nonlinear problems. In this Letter, a novel LMA fiber with heterogeneous helical claddings (HHCs) is proposed. A coordinate transformation simulation technique is adopted to analyze the fiber mode transmission characteristics. The effects of the fiber parameters such as θ,Λ,r,n0,n1,n2,n3, and λ on the mode transmission characteristics include loss coefficient L and effective mode area Aeff. When θ=14∘, Λ=22mm, n0=n2=1.439, and n1=n3=1.438 are adopted. The loss coefficients are L01=0.093dB/m, L11=9.47dB/m, and L21=23.36dB/m; the single-mode fiber diameter 2r is at least 60 µm; and the corresponding effective mode field area Aeff is 2025µm2. As the helix pitch is a centimeter order of magnitude and the fiber is all solid state, the HHC fiber is relatively easy to fabricate and convenient for cutting, splicing, etc., and will be well applied in the field of high-power fiber lasers.

Multi-wavelength random fiber laser with switchable wavelength interval.

A random fiber laser with flexible wavelength interval switching is proposed and demonstrated through two switching methods. One is to change the effective structure of the laser cavity by controlling the switches of 980 nm pump laser diodes (LDs) for erbium-doped fibers (EDFs), which can achieve the switching of the wavelength interval from a single Brillouin frequency shift (BFS) of 0.088 nm to a double BFS of 0.176 nm. Another method is to manipulate the gain provided by the two EDF amplifiers by controlling the power of the three 980 nm LDs, thereby realizing the optical switching of the wavelength interval. This kind of wavelength interval switchable random fiber laser increases the flexibility and functionality of multi-wavelength light sources, and further expands the application range of the random fiber lasers. Furthermore, the alternative wavelength interval switching mechanisms with simple structure enable it to meet the application requirements of various occasions.

Complex pulsating dynamics of counter-propagating solitons in a bidirectional ultrafast fiber laser.

Bidirectional ultrafast fiber lasers capable of generating counter-propagating (CP) coherent solitons are promising to be served as a dual-comb light source for the applications in spectroscopy and gyroscope. In the absence of efficient numerical model, the understanding of the operation of bidirectional fiber lasers is very limited. In this paper, we experimentally explore the pulsating dynamics of CP solitons in a bidirectional mode locked fiber laser and present a set of rich complex dynamics of CP solitons, revealing for the first time, to the best of our knowledge, the periodic breathing and acyclic pulsating dynamics of CP solitons. With a bi-directional pumping configuration, the impacts of gain distribution along the fiber on the dynamics of CP solitons have been investigated and discussed. These results provide further evidence of the universality of breathing dynamics of solitons. More importantly, the abundant dynamical behavior of CP solitons demonstrated in this paper, collaborating with a handful of previous reports on the buildup dynamics of CP solitons in bidirectional fiber lasers, underline further the independent evolving dynamics of CP solitons. These findings contribute to the understanding of how bidirectional lasers work and, consequently, will accelerate the development of bidirectional lasers in the applications such as gyroscope.

Sensitivity enhanced temperature sensor with serial tapered two-mode fibers based on the Vernier effect.

A sensitivity enhanced temperature sensor with cascaded tapered two-mode fibers (TTMFs) based on the Vernier effect is proposed and experimentally demonstrated. It is confirmed that series connection exhibits higher extinction ratio than parallel one both by theory and experiments, which provides guidance for related experiments. In experiments, two TTMFs have the same single-mode fiber-TTMF-single-mode fiber configuration, while the free spectral ranges (FSRs) are chosen with slightly difference by modifying the parameters in the tapering process. Experimental results show that the proposed temperature sensor possesses sensitivity of -3.348 nm/°C in temperature measurement range from 25 °C to 60°C, 11.3 times sensitivity enhancement in comparison with single TTMF. Benefiting from advantages of high temperature sensitivity, simplicity of manufacture and long distance sensing, this novel sensitivity enhanced temperature sensor can be applied to various particular fields, such as oil wells, coal mines and so on.

Broadband tunable single-longitudinal-mode erbium-doped fiber ring laser based on a microfiber knot resonator.

We propose and demonstrate a broadband tunable single-longitudinal-mode (SLM) erbium-doped fiber laser based on a microfiber knot resonator (MKR). The MKR is made from a double-ended fiber taper and employed for SLM filtering based on the Vernier effect. An unpumped erbium-doped fiber is used in the fiber laser cavity to suppress mode-hopping for a stabilized SLM laser operation. When combined with an optical fiber filter, widely tunable SLM laser generation is achieved. The proposed SLM laser can be tuned from 1545 to 1565 nm with a high side-mode suppression ratio of about 55 dB and a high stability.

Two-dimensional hexagonal boron nitride as saturable absorber for a 1.5 µm passively Q-switched erbium-doped fiber laser.

We report on two-dimensional (2D) hexagonal boron nitride (hBN) as saturable absorber (SA) material in a passively Q-switched erbium-doped fiber laser (EDFL) operating at 1.5 µm. The 2D hBN film as an SA is fabricated and transferred onto the optical fiber tip by natural deposition technology. In the Q-switched operation, we obtain stable Q-switched laser operation with a maximum average 10% output power of 2.25 mW, corresponding to a repetition frequency of 55.5 kHz, shortest pulse width of 6.77 µs, and single pulse energy of 40.49 nJ. The achieved PQS at 1.5 µm EDFL with 2D hBN as an SA may have potential applications in many novel 2D materials and all-fiber lasers.

Novel temperature-independent microfiber sensor fabricated with the tapering-twisting-tapering technique.

In this paper, a novel twist and refractive index microfiber sensor fabricated with the tapering-twisting-tapering technique has been proposed and demonstrated, for the first time to the best of our knowledge. Experimental results show that the interference intensity of the microfiber increases with the increment of pre-tapered length and the intervening twist number. The sensitivity of the microfiber sensor with respect to twist is found to be 2.817 dB/(rad/m) and the refractive index sensitivity of this microfiber sensor can reach to ∼809 nm/refractive index unit in the refractive index ranging from 1.30 to 1.33. Moreover, considering that its temperature sensitivity is 0.005 dB/°C, it will not suffer from the cross sensitivity to temperature.

Ultrasensitive sensing in air based on graphene-coated hollow core fibers.

The mismatching between permittivities of guided mode and air limits the operation of accurately monitoring the change in the refractive index of the surrounding air. To solve it, we propose a platform using a hollow core fiber with the integration of graphene coating. Experimental results demonstrate that the anti-resonant reflecting guidance has been enhanced while it induces sharply and periodically lossy dips in the transmission spectrum. We conclude a sensitivity of -365.9 dB/RIU and a high detection limit of 2.73 × 10-6 RIU by means of interrogating the intensity of the lossy dips. We believe that this configuration opens a direction for highly sensitive sensing in researches of chemistry, medicine, and biology.

High-sensitivity and low-temperature magnetic field sensor based on tapered two-mode fiber interference.

A high-sensitivity and low-temperature fiber-optic magnetic field sensor based on a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers has been proposed and demonstrated. The section of TTMF has a specifically designed transition region as an efficient tool to filter higher-order modes, where the uniform modal interferometer just involved with LP01 and LP11 modes is achieved. The transmission spectral characteristics and the magnetic response of the proposed sensors have been investigated. The experimental results show that a maximum sensitivity of 98.2 pm/Oe within a linear magnetic field intensity ranging from 0 to 140 Oe can be achieved. Significantly, the temperature cross-sensitivity problem can be resolved owing to the lower thermal expansion coefficient of the TTMF. Finally, with its low insertion loss, compactness, and ease of fabrication, the proposed sensor would find potential applications in the measurement of a magnetic field.

High efficiency mode-locked, cylindrical vector beam fiber laser based on a mode selective coupler.

We propose and demonstrate an all-fiber passively mode-locked laser with a figure-8 cavity, which generates pulsed cylindrical vector beam output based on a mode selective coupler (MSC). The MSC made of a two mode fiber and a standard single mode fiber is used as both the intracavity transverse mode converter and mode splitter with a low insertion loss of about 0.65 dB. The slope efficiency of the fiber laser is > 3%. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity which are measured to be > 94%. The laser operates at 1556.3 nm with a spectral bandwidth of 3.2 nm. The mode-locked pulses have duration of 17 ns and a repetition rate of 0.66 MHz.

Switchable dual-wavelength Q-switched and mode-locked fiber lasers using a large-angle tilted fiber grating.

We proposed and demonstrated pulsed fiber lasers Q-switched and mode-locked by using a large-angle tilted fiber grating, for the first time to our best knowledge. Owing to the unique polarization properties of the large-angle tilted fiber grating (LA-TFG), i.e. polarization-dependent loss and polarization-mode splitting, switchable dual-wavelength Q-switched and mode-locked pulses have been achieved with short and long cavities, respectively. For the mode-locking case, the laser was under the operation of nanosecond rectangular pulses, due to the peak-power clamping effect. With the increasing pump power, the durations of both single- and dual-wavelength rectangular pulses increase. It was also found that each filtered wavelength of the dual-wavelength rectangular pulse corresponds to an individual nanosecond rectangular pulse by employing a tunable bandpass filter.

All-fiber passively mode-locked Tm-doped NOLM-based oscillator operating at 2-µm in both soliton and noisy-pulse regimes.

A self-starting all-fiber passively mode-locked Tm(3+)-doped fiber laser based on nonlinear loop mirror (NOLM) is demonstrated. Stable soliton pulses centered at 2017.33 nm with 1.56 nm FWHM were produced at a repetition rate of 1.514 MHz with pulse duration of 2.8 ps and pulse energy of 83.8 pJ. As increased pump power, the oscillator can also operate at noise-like (NL) regime. Stable NL pulses with coherence spike width of 341 fs and pulse energy of up to 249.32 nJ was achieved at a center wavelength of 2017.24 nm with 21.33 nm FWHM. To the best of our knowledge, this is the first 2 µm region NOLM-based mode-locked fiber laser operating at two regimes with the highest single pulse energy for NL pulses.

Sub-100 fs mode-locked erbium-doped fiber laser using a 45°-tilted fiber grating.

We demonstrate generation of sub-100 fs pulses at 1.5 µm in a mode-locked erbium-doped fiber laser using a 45°-tilted fiber grating element. The laser features a genuine all-fiber configuration. Based on the unique polarization properties of the 45°-tilted fiber grating, we managed to produce sub-100 fs laser pulses through proper dispersion management. To the best of our knowledge, this is the shortest pulse generated from mode-locked lasers with fiber gratings. The output pulse has an average power of 8 mW, with a repetition rate of 47.8 MHz and pulse energy of 1.68 nJ. The performance of laser also matches well the theoretical simulations.

Sub-50 fs Yb-doped laser with anomalous-dispersion photonic crystal fiber.

We report on the generation of 42 fs pulses at 1 µm in a completely fiber-integrated format, which are, to the best of our knowledge, the shortest from all-fiber-integrated Yb-doped fiber lasers to date. The ring fiber cavity incorporates anomalous-dispersion, solid-core photonic crystal fiber with low birefringence, which acts as a broadband, in-fiber Lyot filter to facilitate mode locking. The oscillator operates in the stretched-pulse regime under slight normal net cavity dispersion. The cavity generates 4.7 ps long pulses with a spectral bandwidth of 58.2 nm, which are dechirped to 42 fs via a grating pair compressor outside of the cavity. Relative intensity noise (RIN) of the laser is characterized, with the integrated RIN found to be 0.026% in the 3 Hz-250 kHz frequency range.

Photoelectrochemical sensing of glutathione using bismuth vanadate (BiVO4) decorated with polyaniline (PANI) and cadmium sulfide (CdS).

A ternary nanocomposite photoelectrode composed of cadmium sulfide (CdS), polyaniline (PANI), and bismuth vanadate (BiVO4) was successfully designed by combining cyclic voltammetry (CV) with electrochemical deposition and high-temperature calcination. The first synthesized CdS/PANI/BiVO4 composite was used as a photoelectrochemical (PEC) monitoring platform for glutathione (GSH). The ternary CdS/PANI/BiVO4 nanocomposites exhibited higher PEC activity, which was attributed to the accelerated electron transfer by the loading of CdS and PANI, which enables the material surface to better adsorb the electrons separated by GSH, thereby oxidizing it into GSSH. The photoanodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy, and photoelectrochemical experiments. Under the optimal experimental conditions, the BiVO4 electrode modified with CdS and PANI exhibited a linear response in the concentration range of 0.1-20 µM with a sensitivity of 0.669 µA mM-1 cm-2 and a detection limit of 40 nM. Moreover, the PEC sensor exhibits good reproducibility and long-term stability. In summary, the designed materials have excellent electrochemical properties, which make them ideal candidates for PEC detection of GSH.