Femtosecond laser direct written fiber Bragg gratings with narrow bandwidth and high sideband suppression

Aiming for suppressing side-mode and spectrum broadening,a slit beam-shaping method and super-Gaussian apodization processing for femtosecond laser point-by-point(PbP)inscription technology of fiber Bragg gratings(FBGs)are reported here.High-quality FBGs,featuring narrow bandwidth of less than 0.3 nm,high reflectivity above 85%,low insertion loss(0.21 dB),and low cladding loss(0.82 dB),were obtained successfully.By a semi-automatic PbP inscription process,an array consisting of six FBGs,exhibiting almost no side-mode peaks with high suppression ability and narrow bandwidth,was fabricated along three independently developed single-mode fibers with an interval of 20 mm.

Underwater ghost imaging with pseudo-Bessel-ring modulation pattern

In this study,we propose an underwater ghost-imaging scheme using a modulation pattern combining offset-position pseudo-Bessel-ring(OPBR)and random binary(RB)speckle pattern illumination.We design the experiments based on modulation rules to order the OPBR speckle patterns.We retrieve ghost images by OPBR beam with different modulation speckle sizes.The obtained ghost images have a better contrast-to-noise rate compared to RB beam ghost imaging under the same conditions.We verify the results both in the experiment and simulation.In addition,we also check the image quality at different turbidities.Furthermore,we demonstrate that the OPBR speckle pattern also provides better image quality in other objects.The proposed method promises wide applications in highly scattering media,atmosphere,turbid water,etc.

Ultrashort Bessel beam photoinscription of Bragg grating waveguides and their application as temperature sensors

Ultrashort pulsed Bessel beams with intrinsic nondiffractive character and potential strong excitation confinement down to 100nm can show a series of advantages over Gaussian beams in fabricating efficient Bragg grating waveguides(BGWs).In this work, we focus on parameter management for the inscription of efficient BGWs using the point-by-point method employing Bessel beams.Due to their high aspect ratio, the resulting one-dimensional void-like structures can section the waveguides and interact efficiently with the optical modes. Effective first-order BGWs with low birefringence can then be fabricated in bulk fused silica. By controlling the size and the relative location of grating voids via the Bessel pulse energy and scan velocities, the resonant behaviors of BGWs can be well regulated.A high value of 34 dB for 8 mm length is achieved.A simple predictive model for BGWs is proposed for analyzing the influences of processing parameters on the performance of BGWs. The technique permits multiplexing several gratings in the same waveguide.Up to eight grating traces were straightforwarcliy inscribed into the waveguide in a parallel-serial combined mode, forming the multiplex BGWs.As an application, the multiplex BGW sensor with two resonant peaks is proposed and fabricated for improving the reliability of temperature detection.