We report on the generation of a supercontinuum (SC) by nonlinear polarization-rotating (NPR) mode-locked fiber laser, which includes a pulsed switchable NPR mode-locked fiber laser, an Er-doped fiber amplifier, and high nonlinear fiber (HNLF). In the experiment, by adjusting the polarization controller (PC) angle and pump power, we obtained three different pulses. They are traditional soliton bunch pulses, the coexistence of soliton bunch pulses and square pulses, and multi-longitudinal mode noise like square pulses. All three pulses can generate SC with spectrum width be about of 1000 nm. We fixed the pump power in the laser and observed the influence of amplifier power on the spectrum width and output power of SC. The SC width of soliton bunch pulse reaches 950.9 nm, the SC of soliton bunch pulse and square pulse coexisting pulse is 1009.7 nm, and the SC spectrum width of multi-longitudinal mode noise like square pulse is 887.1 nm. Our results demonstrate the generation of SC with three types of switchable pulses, opening up possibilities for various application requirements.

Surface plasmon resonance (SPR) sensing methods enable highly sensitive, fast response, and label-free analysis of biomolecular interactions. For SPR sensors, sensitivity and full width at half maximum (FWHM) are two incompatible performance parameters. We propose a refractive index (RI) sensor using a dual-core photonic crystal fiber (PCF) and SPR effects to achieve high sensitivity and a narrow FWHM simultaneously. The air holes of the sensor appear in a hexagonal arrangement, and polishing technology introduces two polishing planes into the cladding. A gold film is deposited on one side of the polished plane to form a highly sensitive RI sensing channel. Five gold nanowires are deposited on the other side of the polished plane to form a RI sensing channel with a narrow FWHM. We analyzed and optimized its structural parameters using the finite element method and determined the optimal structural parameters. The numerical results demonstrate that the maximum sensitivity of the sensor is 21,000 nm/RIU with the narrowest FWHM of 31 nm. Therefore, measuring the refractive index simultaneously with two sensing channels increases the detection accuracy of the measurement. In addition, the findings further indicate that variations in structural parameters do not significantly impact the sensing performance of the sensor, which makes the production of the sensor relatively simple. In conclusion, our work provides a new research method for realizing high sensitivity and a narrow FWHM simultaneously.

Objective As one of the important properties of the light field, polarization plays an important role in the interaction between light and matter. The modulation of polarization plays an indispensable role in optical communication systems, fiber sensors, fiber lasers, and other fields. However, in view of the twist, defects, environment perturbations, and other factors in the process of optical fiber manufacturing, the manufactured optical fiber is not completely uniform, which introduces random birefringence and leads to unpredictable polarization states. Therefore, it is of great practical value to study optical fibers with excellent polarization states. Although the existing single-polarization single- mode negative-curvature hollow-core fiber has the advantages of simple structure, easy preparation, endless single- mode transmission, and low loss, due to the limitation of research habits and optical materials, the current research mainly focuses on common communication bands. But obviously, the mid-infrared band will become the next hot band of the negative-curvature hollow-core fiber. Research shows that a wavelength of 3-5 mu m plays an important role in national defense, medical care, communications, and other fields, especially near the wavelength of 4 mu m, which is an ideal band for quantum cascade detectors to detect low-level light. Single-mode single-polarization light helps to provide a more pure light source for quantum cascade detectors. Therefore, it is of great practical significance to study the single-mode single- polarization negative-curvature hollow-core fiber with a wavelength of 4 mu m. Methods A hollow-core anti-resonant fiber composed of six nested tubes working near 4 mu m is designed, which can transmit single-mode single-polarization with low loss. The influence of structural parameters on fiber performance is calculated by using the control variable method. The capillary wall thickness will lead to an obvious change in the fiber loss with the working band, which is the key factor affecting the characteristics of the negative-curvature hollow-core anti-resonant fiber. Therefore, the capillary wall thickness is analyzed and optimized. Through the scanning study of the capillary wall thickness, the local optimal parameter values of the minimum fundamental mode loss and the maximum high-order mode extinction ratio in the 4 mu m band are determined, and the design goal of the single- mode performance of the fiber is successfully realized. The second step is to optimize the capillary radius. This parameter mainly affects the polarization state of the fiber, and different parameter combinations of the six inner tube radii correspond to different implementation effects. The optimization of capillary radius successfully achieves single-polarization operation in a single-mode state. In the third step, the core diameter of the fiber is optimized. Although the study does not reflect the further optimization effect of the parameters that have been optimized and determined in the previous steps, the parameter design still retains the effective mode area and the maximum transmission power tolerance value of the fiber. The fourth step is to study and characterize the bending resistance of optical fiber. Research shows that this design fully meets the preset requirements for bending resistance and verifies that the natural advantages of negative-curvature hollow-core anti-resonant fibers, such as large effective mode field area and less substrate material coverage, can contribute to the bending resistance of the fiber. Results and Discussions A negative-curvature hollow-core fiber with low-loss single-mode single-polarization transmission is proposed and analyzed by the finite element method. By calculating the influence of fiber parameters on the fiber structure, the high-order mode extinction ratio reaches 163 (Fig. 3), and the fiber successfully realizes single-mode transmission. However, in order to further ensure the single polarization performance of the fiber, the size of the capillary radius is optimized, and the single polarization function is realized based on single-mode transmission (Fig. 4). In order to ensure that the fiber has good bending resistance, the critical bending radius of the fiber is defined, and it is found that the bending loss of the x-polarization fundamental mode of the fiber is always less than 10(-3) dB/m (Fig. 7). In addition, the fiber structure also has a large effective mode field area (Fig. 8), which meets the transmission requirements of high power lasers. The results show that the designed structure achieves both single-polarization performance and single-mode transmission. Conclusions In this paper, a single-mode, single-polarization, low-loss, negative-curvature, hollow-core, and anti-resonant fiber is proposed. The substrate material of the fiber is As40S60, which is specially studied and experimentally prepared by Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences. Its refractive index is 2. 395 at 4 mu m. It has low intrinsic loss and great chemical stability in the mid-infrared band, which is beneficial to realize the low loss performance of the fiber. The fiber structure adopts a six-nested, capillary-type, negative-curvature, hollow-core, and anti-resonant structure with relatively mature preparation technologies and a simple structure. After optimizing the parameters of the fiber, the single-mode single-polarization effect can be achieved from 3. 99 mu m to 4. 00 mu m. Especially at the wavelength of 4 mu m, the polarization extinction ratio (PER) and high order mode extinction ratio (HOMER) reach 491 and 694, respectively, which meet the conditions of single-polarization single-mode transmission, and the loss is as low as 1. 8x10(-4) dB/m. The fiber also has excellent bending resistance. At the wavelength of 4 mu m, single-mode single-polarization transmission of the fiber can be achieved by selecting the appropriate bending radius at any bending angle. When the bending angle is equal to 0 degrees, and the bending radius is from 1 cm to 10 cm, the confinement loss of the fiber is less than 5. 3x10(-3) dB/m. The negative-curvature, hollow-core, and anti-resonant fiber proposed in this paper has the advantages of simple structure, single-mode single-polarization operation, low loss, and excellent bending resistance. It can not only be applied to the communication industry and medical system but also is expected to provide a more pure light source for quantum cascade detectors operating in the band of 4 mu m.

A negative curvature hollow core fiber (NCHCF) refractive index (RI) sensor based on localized surface plasmon resonance (LSPR) is proposed. The gold nanowires are deposited in four cladding tubes in the y -direction, with the core region serving as the analyte channel. The full vector finite element method (FEM) is used to analyze the influence of various structural parameters on this sensor, and the structural parameters are optimized and selected. Ultimately, an average sensitivity of 9356.59 nm/RIU is accomplished within a spacious refractive index detection scope of RI = 1.28–1.43. The sensor attained a maximum sensitivity of 10,220 nm/RIU at RI = 1.36. In the bargain, there is an excellent linear correlation between the resonance wavelength and the refractive index of the analyte, with a value of 0.99901 and a factor of merit (FOM) range of 119.9563–155.9432 RIU −1 , achieving a resolution of 10 −6 RIU. The sensor has potential applications in various fields such as environmental protection, food safety, and medical diagnostics due to its high sensitivity, spacious detection scope, and positive linear response.

Passively mode locked fiber lasers (PML-FLs) can produce a variety of phenomena due to the nonlinear effects. With the introduction of the nonlinear polarization rotation (NPR) and a multimode fiber polarization controller (MMF-PC), we observed nonlinear effects in our erbium-doped mode-locked fiber laser, such as the evolution of the soliton bunch to dissipates soliton resonance (DSR) pulses. In the evolution of the soliton bunch to DSR pulses, the soliton square bunch and the multi-longitudinal mode oscillation also appeared in our experiment with the increasing the pump power. To illustrate the importance of MMF-PC, a control experiment is conducted by replacing the MMF-PC with a single mode fiber PC (SMF-PC). The experimental results indicate that the MMF-PC are crucial for the evolution of soliton pairs to DSR pulses and the generation of soliton pairs. The NPR mode-locked structure is used to achieve dual-wavelength filtering, its strength-dependent non-uniform loss can effectively suppress the mode competition in the gain fibers. The MMF-PC in the mode-locked structure adds a polarization burned hole effect inside the resonator. We achieved dual-wavelength mode locking by adjusting the PC to control the birefringence and loss in the resonator.

A passive mode-locked erbium-doped fiber laser based on graphene operating in the anomalous dispersion state is designed in this paper. In this experiment, the complete convert process from traditional soliton pulses to soliton rains and bound-state (BS) soliton pulses, and ultimately to traditional soliton pulses, is achieved by adjusting the pump power and polarization controller (PC) to control factors such as nonlinearity, dispersion, and birefringence in the cavity. In the BS soliton state, switching between the 0 phase and π phase BS solitons is achieved. We also replaced the 20/80 coupler with the 30/70 coupler to achieve the soliton rains by varying the energy alteration of the intracavity pulse. When the pump power is increased to 350 mW, the laser generates multi-pulses and eventually forms a new BS. In addition, we also studied the soliton vector characteristics during this conversion process and found the polarization-locked vector soliton (PLVS). The research results enrich nonlinear vector soliton dynamics and provide valuable data for further theoretical studies.

A chalcogenide hollow-core anti-resonance fiber (HC-ARF) filter based on surface plasmon resonance (SPR) at 3 μm band is designed. The substrate materials are As40S60 and GeAsS, which are used to create the conditions for the formation of SPR, so as to realize the directional energy coupling in the specific polarization direction of the fiber. The effects of core diameter, capillary radius, and capillary wall thickness on the polarization performance of HC-ARF filter are analyzed by full-vector finite element method (FV-FEM). The numerical results show that the confinement loss of x-polarized fundamental mode (FM) reaches 1917.24 dB/m at the wavelength of 3.02 μm, while that of y-polarized FM is 23.11 dB/m. When the fiber length is 8 mm, the bandwidth with extinction ratio (ER) better than 20 dB covers the wavelength range of 320 nm. In addition, the resonance wavelength can be effectively adjusted by changing the capillary wall thickness. The proposed HC-ARF filter has potential applications in biomedicine, scientific research, and atmospheric detection.