Distributed refractive index sensing based on bending-induced multimodal interference and Rayleigh backscattering spectrum.

A distributed refractive index (RI) sensor based on high-performance optical frequency domain reflectometry was developed by bending a piece of standard single-mode fiber to excite sets of higher-order modes that penetrate the surrounding medium. External variations in RI modifies the profiles of the sets of excited higher-order modes, which are then partially coupled back into the fiber core and interfere with the fundamental mode. Accordingly, the fundamental mode carries the outer varied RI information, and RI sensing can be achieved by monitoring the wavelength shift of the local Rayleigh backscattered spectra. In the experiment, an RI sensitivity of 39.08 nm/RIU was achieved by bending a single-mode fiber to a radius of 4 mm. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.

Imaging enhancement based on stimulated Brillouin amplification in optical fiber.

The detection of weak optical signals embedded in strong background illumination has broad application prospects. We propose an imaging enhancement method based on stimulated Brillouin scattering (SBS) in a single-mode fiber, which is capable of amplifying the weak optical signal while neglecting the broadband background noise because of its narrow gain bandwidth. In experiment, a high gain of 60 dB was achieved. An imaging enhancement experiment was carried out, where a target which cannot be seen because of transmission loss could be clearly captured with the amplification of SBS in the fiber. Because of the employment of continuous pump rather than a pulsed pump, this system has wide application in the monitoring of non-cooperative targets.

Low-noise and high-gain of stimulated Brillouin amplification via orbital angular momentum mode division filtering.

We demonstrate a Brillouin amplifier scheme by using orbital angular momentum mode division filtering, which is able to amplify the weak optical signals with low noise and high gain. The system retains the advantages of a conventional collinear Brillouin amplifier structure, and employs a liquid-crystal spatial light modulator to generate distinguishable degree-of-freedom of beams. As we all know, the noise mainly derives from the unwanted coupling in stimulated Brillouin scattering (SBS) processes, which severely limits the amplifier's performances. The efficient SBS noise-filtering method is discussed, which could improve the output beam quality effectively. The experimental results show the proposed amplifier scheme can overcome this obstacle, providing the magnification (or called signal gain) of 71 dB for an input signal of 4.7×10-12 J. In addition, the amplified signal is recognized from the amplifier spontaneous-emission (ASE)-like noise with a signal-to-noise ratio of 6.7 dB.

Orbital angular momentum mode division filtering for photon-phonon coupling.

Stimulated Brillouin scattering (SBS), a fundamental nonlinear interaction between light and acoustic waves occurring in any transparency material, has been broadly studied for several decades and gained rapid progress in integrated photonics recently. However, the SBS noise arising from the unwanted coupling between photons and spontaneous non-coherent phonons in media is inevitable. Here, we propose and experimentally demonstrate this obstacle can be overcome via a method called orbital angular momentum mode division filtering. Owing to the introduction of a new distinguishable degree-of-freedom, even extremely weak signals can be discriminated and separated from a strong noise produced in SBS processes. The mechanism demonstrated in this proof-of-principle work provides a practical way for quasi-noise-free photonic-phononic operation, which is still valid in waveguides supporting multi-orthogonal spatial modes, permits more flexibility and robustness for future SBS devices.