Widely tunable second-harmonic generation in a chalcogenide-tellurite hybrid optical fiber

Multi-wavelength second-harmonic generation in a double-clad high nonlinear fiber induced by multiple phase-matching

In this study, multi-wavelength second-harmonic generation (SHG) based on self-phase modulation (SPM) broadband supercontinuum (SC) was observed by employing a double-clad high nonlinear optical fiber (HNLF) in conjunction with a femtosecond laser. At a wavelength of 1050 nm and an average pump power of 320 mW, multiple phase-matching conditions were achieved, and SH signals of central wavelengths ∼530.7 nm, ∼525.1 nm, ∼503.5 nm, and ∼478.7 nm were observed, with SHG efficiency reaching ∼1.34 × 10-4. The SHG in this experiment can be attributed to the utilization of a doped optical fiber, where dopants create defect states, facilitating optical-chemical transformation and enhancing second-order polarization susceptibility. Additionally, theoretical simulations were conducted, aligning closely with the experimental findings. To the best of our knowledge, this work marks the first demonstration of multi-wavelength SHG in optical fibers. It offers a distinctive avenue for customizing multi-wavelength ultrafast light sources, exhibiting great application potential in the fields of medical diagnostics and optical sensing.

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.

Highly efficient second-harmonic generation in a tellurite optical fiber

A step-index tellurite optical fiber with loss ∼0.02  dB/m at ∼1545  nm was fabricated based on TeO₂-Bi₂O₃-ZnO-Na₂O glass. With a nanosecond laser operated at ∼1545  nmas the pump source, second-harmonic generation (SHG) was observed in the 2.5 m tellurite fiber with conversion efficiency up to ∼1.15% at 20 mW, which, to the best of our knowledge, is the highest for non-silica optical fibers. The experimental phenomenon can be explained by the χ⁽²⁾-induced model of nonlinear polarization at the core-cladding interface. Furthermore, it can also be explained by a χ⁽³⁾-induced model via the four-wave-mixing effect, where a quasi-SH signal and a millimeter wave were generated simultaneously. This new model may help shed light on the physical mechanism of SHG in optical fibers.

Deep-ultraviolet second-harmonic generation by combined degenerate four-wave mixing and surface nonlinearity polarization in photonic crystal fiber

Deep-ultraviolet (UV) second-harmonics (SHs) have important applications in basic physics and applied sciences. However, it still remains challenging to generate deep-UV SHs especially in optical fibers. Here, for the first time, we experimentally demonstrate the deep-UV SH generations (SHGs) by combined degenerate four-wave mixing (FWM) and surface nonlinearity polarization in an in-house designed and fabricated air-silica photonic crystal fiber (PCF). When femtosecond pump pulses with average input power P_av of 650 mW and center wavelength λ_p of 810, 820, 830, and 840 nm are coupled into the normal dispersion region close to the zero-dispersion wavelength of the fundamental mode of the PCF, the anti-Stokes waves induced by degenerate FWM process are tunable from 669 to 612 nm. Then, they serve as the secondary pump, and deep-UV SHs are generated within the wavelength range of 334.5 to 306 nm as a result of surface nonlinearity polarization at the core-cladding interface of the PCF. The physical mechanism of the SHGs is confirmed by studying the dependences of the output power P_SH of the SHs on the PCF length and time. Finally, we also establish a theoretical model to analyze the SHGs.