Silica optical fiber is well recognized as a decent nonlinear optical medium to carry out nonlinear optics due to the long-interaction length and high-power density in the fiber core. However, the centrosymmetric nature of silica fiber precludes the excitation and utilization of the second-order nonlinear optical process. To solve this problem, researchers from the Northwestern Polytechnical University, China, reported an in-fiber frequency-converter by using second harmonic generation (SHG) in an optical hollow-core fiber (HCF). The HCF filled with a dispersion of gallium selenide (GaSe) nanosheets in the ultraviolet-cured optical adhesive supports a well-propagating mode in the fiber core, which enables effective light interaction with dispersed GaSe nanosheets and a strong SHG process. Due to the filled GaSe nanosheets and the optimized HCF length, SHG can be produced under low pump power (milliwatt-level), which is reduced by three orders of magnitude compared with the poled optical fiber scheme.
The SHG could be excited under slightly relaxed phase-matching condition, and therefore broadband tuning of SHG signal can be implemented. In a wide wavelength range from 1460 nm to 1600 nm, the SHG have been excited effectively, which is difficult to be realized under prefect phase match condition or in resonance-based nonlinear devices.
A self-enhanced SHG in the HCF device has been identified as well, which originates from the moderately poled multi-mode fiber after the HCF device.
Last but not least, the all-fiber structure proposed in this paper effectively avoids the complex processes existing in conventional fiber poling and doping techniques. The second-order nonlinear optical effect indicated by the strong SHG in the all-fiber structure can not only expand the operation wavelength range of fiber laser, but also promise the realization of the electro-optic modulation and entangled photon pair, which can greatly promote the applications of mature optical fiber communication and sensing technology in information processing. Therefore, it can be widely concerned by peers in the fields of fiber-based laser, sensor and nonlinear optics.
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Second harmonic generation in a hollow-core fiber filled with GaSe nanosheets
Science China Information Sciences