Optical fiber temperature sensor based on the upconversion fluorescence intensity ratio of NaYF4:Er3+ excited by a 1525-nm laser.

Remote and accurate temperature measurements in severe environments are of great importance. A 1525-nm wavelength located in the C band of optical fiber communication is used as a pumping light source for NaYF4:Er3+ phosphor possessing high upconversion efficiency. The upconversion luminescence characteristics were demonstrated in the temperature range of 160-400 K. Based on the thermal coupling energy level theory, the temperature measurement principle of the fluorescence intensity ratio is analyzed. The energy gap between the 2H11/2 and 4S3/2 energy levels of the Er3+ ions is approximately 787cm-1, which is appropriate for a temperature sensor. The experimental results indicated that its maximum temperature sensitivity was 0.00335K-1. The proposed optical fiber temperature sensor indicates good hysteresis and repeatability and has potential applications in resisting electromagnetic interference and remote temperature sensing.

A polarization-based image restoration method for both haze and underwater scattering environment.

Existing polarization-based defogging algorithms rely on the polarization degree or polarization angle and are not effective enough in scenes with little polarized light. In this article, a method of image restoration for both haze and underwater scattering environment is proposed. It bases on the general assumption that gray variance and average gradient of a clear image are larger than those of an image in a scattering medium. Firstly, based on the assumption, polarimetric images with the maximum variance (Ibest) and minimum variance (Iworst) are calculated from the captured four polarization images. Secondly, the transmittance is estimated and used to remove the scattering light from background medium of Ibest and Iworst. Thirdly, two images are fused to form a clear image and the color is also restored. Experimental results show that the proposed method obtains clear restored images both in haze and underwater scattering media. Because it does not rely on the polarization degree or polarization angle, it is more universal and suitable for scenes with little polarized light.

Theoretical and experimental investigation of broadband dispersion tailoring of high-order mode in the hybrid microsphere cavity.

The large normal dispersion of the fundamental mode (TEn=1 mode) in the whispering gallery modes (WGM) microsphere is detrimental to the visible comb generation. Herein, we demonstrate that this fundamental limitation can be removed by considering the high-order radial modes (TEn=2 mode) of the hybrid microsphere cavity (HMC). The studied HMC consists of a high-refractive-index coating (TiO2 or HfO2) and silica microsphere. The simulated electric field energy distribution and measured Q value in our experiment show that optical confinement of the coating effectively excites the TEn=2 mode and reduces the free spectral range (FSR) and modal dispersion. In addition, the observed redshift of WGM and decreased trend of FSR are in accordance with simulations. The zero-dispersion wavelength can be linearly shifted to a shorter wavelength or even into the visible region with the reduction of coating thickness or refractive index and larger microcavity, which advances the visible comb generation.

Full-Stokes polarization imaging method based on the self-organized grating array in fused silica.

A full-Stokes polarization imaging method based on the self-organized grating array was presented. By focusing the ultra-fast laser with moderate fluence into fused silica, the self-organized grating array was fabricated, featuring the optical properties similar to wave plates. A set of four independent polarization measurements were simultaneously acquired with designed grating array mounted in the focal plane of an imaging detector. Experimental results including the device fabrication, calibration and optimization were presented. Finally, a principle verification experiment was implemented for our polarization imaging method.

Numerical study of a DoFP polarimeter based on the self-organized nanograting array.

The self-organized nanograting manufactured by irradiating the transparent materials with the femtosecond laser has aroused wide interests in photonic applications in recent years. Although the mechanism of nanograting formatting has not yet been fully understood, the essential property of the optical birefringence can be precisely acquired by controlling the energy fluence of the femtosecond laser. In this paper, we proposed a novel application of the self-organized nanograting in a division-of-focal-plane polarimeter. Based on the rigid-coupled-wave algorithm, the optical characteristics of the nanograting and the polarimeter were comprehensively analyzed and discussed.

Realization of an O-waveband laser based on cascaded stimulated Raman scattering of microspheres.

We report an experimental realization of five-order Stokes stimulated Raman scattering lasing in silica microspheres pumped by a 1030 nm continuous-wave laser. The wavelength of the Stokes Raman laser is extended to 1348.55 nm, which is located in the second low loss window of the optical fiber. It has potential applications in the wavelength converter and Raman amplifier in O-waveband optical communication. The minimum pump power is about 50 µW when the first-order Stokes Raman laser can be observed.

2 µm laser oscillation of Ho3+:Tm3+-codoped silica microspheres.

2 µm laser oscillation with a low threshold has been achieved in Ho3+:Tm3+-codoped silica microspheres (HTCSMs). Ho3+:Tm3+-codoped solgel functionalization film is applied to the surface of a silica microsphere, and an optical tapered fiber is adopted to couple an 808 nm continuous-wave laser to serve as the pump light source. Multimode and single-mode laser oscillations around 2 µm within the eye-safe wave band are observed due to the I75→I85 transitions of Ho3+ ions sensitized by Tm3+. The morphology characteristics of microspheres determine the multimode laser oscillation spectrum. The free spectral range is in good accordance with the calculated value based on Mie scattering theory. The HTCSM laser oscillation shows characteristics of good capability, simple process, high flexibility, and low cost.

Low-threshold stimulated Brillouin scattering in high-Q whispering gallery mode tellurite microspheres.

We demonstrate the first observation of stimulated Brillouin scattering (SBS) in a high-Q whispering gallery mode tellurite microsphere. Tellurite glass with composition of 70TeO2-20ZnO-5Na2O-5La2O3 (molar ratio) was prepared in-house using a melt-quenching technique. Moreover, tellurite microspheres with Q in excess of 13 millions at 1550 nm were fabricated by melting tellurite microwires using a CO2 laser. By pumping the tellurite microspheres with a tunable single frequency laser, SBS is further realized with a threshold as low as 0.58 mW. At last, the beat notes between the pump and the Stokes signals were measured, which indicated the Brillouin frequency shift is at the 8.2 GHz band for our tellurite glass. Our results could propel significant applications utilizing SBS by employing tellurite microspheres.

Laser oscillation of Yb³âº:Er³âº co-doped phosphosilicate microsphere [invited].

A fiber-taper-microsphere-coupled system was used to research the characteristics of laser oscillation and upconversion luminescence of Yb3+:Er3+ co-doped phosphosilicate (YECP) microspheres. The YECP microspheres were fabricated by melting the end of phosphosilicate filaments. Single- and multimode laser oscillation at 1535-1565 nm within the C-band were obtained. In addition, the output power of the single-mode laser at 1545.5 nm can be as high as 48.98 µW, which was achieved under pump power of 9.63 mW, and the side-mode suppression ratio was 51.49 dB. Upconversion fluorescence of Er3+ at 521, 532, and 544 nm also were measured, and the pump power dependence was studied. The fluorescence intensity was lower than that of Yb3+:Er3+ co-doped silica and oxyfluoride glass ceramic microspheres. Moreover, the physical mechanism of upconversion suppression and laser oscillation enhancement observed in our experiment was presented, which is beneficial to the preparation of rare-earth-doped microcavity lasers.