Diffraction gratings have been widely used in wavelength-controlled non-mechanical laser beam steering for high data-rate indoor optical wireless communications (OWC). Existing free-space diffraction gratings suffer from inherent difficulties of limited diffraction efficiency, bulky configuration, high cost and significant coupling loss with optical fiber links. In this work, a new optical approach for highly efficient, compact and fiber compatible laser beam steering using an in-fiber diffraction grating is proposed and experimentally demonstrated for the first time to our best knowledge. In-fiber diffraction is made possible based on a 45° tilted fiber grating (TFG), where wavelength dependent lateral scattering is obtained due to the strongly tilted grating structure. Improved diffraction efficiency of 93.5% has been achieved. In addition, the 45° TFG works perfectly for both light emission and reception, enabling full-duplex optical wireless transmission. Utility of the 45° TFG in all-fiber laser beam steering for multi-user full duplex optical wireless communications has been verified in experiments. 1.4 m free-space full-duplex wireless transmission has been demonstrated with data rate up to 12 Gb/s per beam using 2.4 GHz bandwidth OFDM signals.

In the field of interferometric fiber-optic sensing, the differential-cross-multiplication type of phase-generated carrier (PGC) demodulation technique has a low stability defect and the arc tangent PGC demodulation technique has a high harmonic distortion defect. Thus, we present an improved PGC demodulation technique based on arc cotangent function and differential-self-multiplying. Both theoretical and experimental results show that the interferometric fiber-optic sensing system that has adopted the improved PGC demodulation technique has the merits of high stability of resisting the rapid change of light intensity and low harmonic distortion; experimental results show that the demodulation system has a resolution of 2 × 10^-5 rad/√Hz, a total harmonic distortion less than 0.1%, a linearity of better than 0.9999, and a dynamic range of 112 dB at 50 Hz.

A novel highly efficient, fiber-compatible spectrally encoded imaging (SEI) system using a 45° tilted fiber grating (TFG) is proposed and experimentally demonstrated for the first time, to the best of our knowledge. The TFG serves as an in-fiber lateral diffraction element, eliminating the need for bulky and lossy free-space diffraction gratings in conventional SEI systems. Under proper polarization control, due to the strong tilted reflection, the 45° TFG offers a diffraction efficiency as high as 93.5%. Our new design significantly reduces the volume of the SEI system and improves energy efficiency and system stability. As a proof-of-principle experiment, spectrally encoded imaging of a customer-designed sample ($9.6\mathrm{mm}\times 3.0\mathrm{mm}$) using the TFG-based system is demonstrated. The lateral resolution of the SEI system is measured to be 42 μm in our experiment.

Photonic time stretch enables real-time high-throughput optical coherence tomography (OCT), but with massive data volume being a real challenge. In this paper, data compression in high-throughput optical time-stretch OCT has been explored and experimentally demonstrated. This is made possible by exploiting the spectral sparsity of an encoded optical pulse spectrum using a compressive sensing approach. Both randomization and integration have been implemented in the optical domain avoiding electronic bottleneck. A data compression ratio of 66% has been achieved in high-throughput OCT measurements with 1.51-MHz axial scan rate using greatly reduced data sampling rate of 50 MS/s. Potential to improve compression ratio has been exploited. In addition, using a dual pulse integration method, capability of improving frequency measurement resolution in the proposed system has been demonstrated. A number of optimization algorithms for the reconstruction of the frequency-domain OCT signals have been compared in terms of reconstruction accuracy and efficiency. Our results show that the l1 magic implementation of the primal-dual interior point method offers the best compromise between accuracy and reconstruction time of the time-stretch OCT signal tested.

Most overlooked challenges in ultrafast optical time stretch imaging (OTSI) are sacrificed spatial resolution and higher optical loss. These challenges are originated from optical diffraction devices used in OTSI, which encode image into spectra of ultrashort optical pulses. Conventional free-space diffraction gratings, as widely used in existing OTSI systems, suffer from several inherent drawbacks: limited diffraction efficiency in a non-Littrow configuration due to inherent zeroth-order reflection, high coupling loss between free-space gratings and optical fibers, bulky footprint, and more importantly, sacrificed imaging resolution due to non-full-aperture illumination for individual wavelengths. Here we report resolution-improved and diffraction-efficient OTSI using in-fiber diffraction for the first time to our knowledge. The key to overcome the existing challenges is a 45° tilted fiber grating (TFG), which serves as a compact in-fiber diffraction device offering improved diffraction efficiency (up to 97%), inherent compatibility with optical fibers, and improved imaging resolution owning to almost full-aperture illumination for all illumination wavelengths. 50 million frames per second imaging of fast moving object at 46 m/s with improved imaging resolution has been demonstrated. This conceptually new in-fiber diffraction design opens the way towards cost-effective, compact and high-resolution OTSI systems for image-based high-throughput detection and measurement.

In this work, a detailed study on volume and surface refractive index (RI) sensitivity of cladding modes for a fiber Bragg grating (FBG) based sensor is presented. Surface RI sensitivity of the cladding mode of FBGs has been illustrated and quantified with the concept of add-layer sensitivity for the first time to the best of our knowledge. A detailed investigation of mode transition of higher-order cladding modes has been revisited and important characteristics of the cladding modes are observed which could open a new designing path of fabrication and innovative way of use of this family of optical fiber grating-based sensors. The effect of “mode transition” of higher-order cladding modes, higher operating wavelength for respective cladding mode and “mode stretching” effects are combined together to achieve higher volume and surface RI sensitivity of cladding mode of FBG. It has been shown numerically that with proper designing, sub-nanometer (~0.04 nm) attachment of target analyte could be recognized by cladding mode of FBG which is quite promising for application in optical fiber grating bio-sensors. This critical designing method of FBG based surface refractometer would be very helpful in case of fabrication of highly sensitive sensors for distinct bio-chemical applications.