Continuous-wave pumped frequency upconversions in an InSe-integrated microfiber.

We report the achievement of continuous-wave (CW)-pumped second-harmonic generation (SHG) and sum frequency generation (SFG) in a layered indium selenide (InSe)-integrated microfiber. As a result of the strong interaction between the InSe nanosheets and the evanescent field, the second-order nonlinear processes are greatly enhanced in the InSe-integrated microfiber pumped by a few milliwatt CW lasers. The experimental results reveal that the intensities of SHG and SFG are quadratic and linear dependencies with the incident pump power, respectively, which is consistent with theoretical predictions. Additionally, the SHG intensity is strongly polarization-dependent on the nonaxisymmetrical distribution of the InSe nanosheets around the microfiber, providing the possibility of the SHG-polarized manipulation. The proposed device has the potential to be integrable into all-fiber systems for nonlinear applications.

Co-located angularly offset fiber Bragg grating pair for temperature-compensated unambiguous 3D shape sensing.

A 10 mm-long three-dimensional shape sensor in a single-mode fiber is described and demonstrated experimentally. The sensor is based on a pair of fiber Bragg gratings inscribed at the same location along the fiber axis but offset along different radial directions away from the fiber center. Each offset grating generates cladding mode resonances over a ${\sim}{20}\;{\rm{nm}}$-wide spectral bandwidth, and the two gratings are also offset in period so that their transmission spectra are separated by 40 nm, and thus non-overlapping and fully distinguishable. Directional bending sensitivity results from the differential amplitude response of the cladding mode resonances from the two gratings, depending on the relative orientation of the bend with the azimuthal direction of the grating offsets. It is further demonstrated that both axial deformation and temperature have no influence on the shape measurement as they both only cause a global wavelength shift of the spectra without amplitude change. The experimental results demonstrate that the shape orientation of an object can be unambiguously determined for bend directions covering the full 360° range around the fiber axis with sensitivities of the order of ${{1}}\;{\rm{dB/}}{{\rm{m}}^{- 1}}$ and small curvatures between 0 and ${{1}}\;{{\rm{m}}^{- 1}}$.

High-efficiency second-order nonlinear processes in an optical microfibre assisted by few-layer GaSe.

The centrosymmetric nature of silica fibre precludes the realisation of second-order nonlinear processes in optical fibre systems. Recently, the integration of 2D materials with optical fibres has opened up a great opportunity to develop all-fibre active devices. Here, we demonstrate high-efficiency second-order nonlinear frequency conversions in an optical microfibre assisted with few-layer gallium selenide (GaSe) nanoflakes. Attributed to the strong evanescent field of the microfibre and ultrahigh second-order nonlinearity of the GaSe nanoflakes, second harmonic generation (SHG) and sum-frequency generation (SFG) are effectively achieved with only sub-milliwatt continuous-wave (CW) lasers in the wavelength range of 1500-1620 nm, covering the C and L telecom bands. The SHG intensity from the microfibre is enhanced by more than four orders of magnitude with the assistance of the GaSe nanoflakes on fibre nonlinear processes. Moreover, in the SFG process, the intensity transfer between different frequencies can be effectively manipulated by changing the wavelengths and powers of two pump lasers. The realised strong second-order nonlinearity in the GaSe-integrated microfibre might expand the applications of all-fibre devices in all-optical signal processing and new light source generation at awkward wavelengths.