A torsion sensor based on the near-helical (NH) long period fiber grating (LPFG) is fabricated by using a high frequency pulsed CO₂ laser. Each groove of the NH-LPFG is spirally written in the four sides of a single-mode fiber. The NH-LPFG has a helical periodic vertical index modulation. This is different from the screw-type index modulation of the common helical LPFGs (H-LPFGs) fabricated in a twisted fiber. The torsion and temperature characteristics of the NH-LPFG are experimentally investigated. The temperature sensitivity is about 0.0668 nm/°C. The torsion sensitivity is 0.103 nm/(rad/m) and independent of the polarization state of incident light.

A stable, passively Q-switched thulium fluoride fiber laser (TFFL) using a multi-walled carbon nanotube (MWCNT)-based saturable absorber (SA) for operation in the S-band region is proposed and demonstrated. The proposed TFFL has a central lasing wavelength of 1486.4 nm and an input power range of 87.1–126.6 mW. The output pulses have a repetition rate and pulse width range of 30.1–40.0 kHz and 9.0–3.2 μs, respectively, with a maximum pulse energy of 28.9 nJ. This is the first time, to the author’s knowledge, of the successful demonstration of a passively Q-switched S-band TFFL using an MWCNT-based SA.

In this Letter, an alternative solution is proposed and demonstrated for simultaneous measurement of axial strain and temperature. This sensor consists of two twisted points on a commercial single mode fiber introduced by flame-heated and rotation treatment. The fabrication process modifies the geometrical configuration and refractive index of the fiber. Different cladding modes are excited at the first twisted point, and part of them are coupled back to the fiber core at the second twisted point. Experimental results show distinct sensitivities of 34.9 pm/με with 49.23 pm/°C and 36.19 pm/με with 62.99 pm/°C for the two selected destructive interference wavelengths.

We present a single-mode multilayer-core fiber with a large mode area (LMA) and a low bending loss in this Letter. A low equivalent core-cladding refractive index difference is achieved by exploiting the multilayer structure. The multilayer structure has a better bending performance than a traditional step-index core and this structure also contributes to realizing different curved refractive index profiles that have a better bending performance. An index trench is also introduced to dramatically reduce the bending loss. The experimental results show that, at a wavelength of 1550 nm, the mode area of the fabricated fiber is about 215.5 μm2 and the bending loss is 0.58 dB/turn at a 10 mm bending radius. The LMA and excellent bending performance can be obtained simultaneously with the proposed fiber.

We report on the high-power amplification of a 1064 nm linearly polarized laser in an all-fiber polarization-maintained master oscillator power amplifier, which can operate at an output power level of 1.3 kW. The beam quality (M2) was measured to be <1.2 at full power operation. The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities (MIs) set in, which reduced to about 90% after the onset of MI. The power scaling capability of strategies for suppressing MI is analyzed based on a semianalytical model, the theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber is an effective and practical method in standard double-cladding large mode area fiber, and, by tight coiling of the gain fiber to the radius of 5.5 cm, the MI threshold can be increased to three times higher than that without coiling or loose coiling. Experimental studies have been carried out to verify the idea, which has proved that MI was suppressed successfully in the amplifier by proper coiling.

We present a compact displacement measurement system possessing the capability of nanometer-scale precision. On basis of integrating single grating with 3×3 coupler for phase shift in interference signal, the present scheme features advantages of simple structure, convenient alignment, and insensitivity to air turbulence. Linear comparisons between our system and HP5530 show a residual error less than 81 nm during step motions along a 10 mm round-trip, and a discrepancy less than 15 nm in the case of 200 μm movement. We also demonstrate a measurement stability test in a duration of 300 s, which shows the proposed scheme potentially performs better than HP5530 in terms of long-term stability.