The static CT by Nanovision, as a new CT scanning formula, assembles a multi-source array and a ring detector array on two parallel planes with a fixed offset. The advantage of this configuration is that each source only needs to be rotated over a smaller angle range to complete a full scan than with conventional CT systems. However, the large cone angle from the source to the detector and the distribution of multiple sources lead to severe incomplete projections during the scanning process. To address this issue, this paper proposes a deep iterative network based on directional TV regularization. The network employs a tensorization module suitable for the static CT geometry in the forward and back-projection steps, and the regularization term adopts a directional TV deep learning model, which enables end-to-end reconstruction of incomplete data in the static CT. Experimental results demonstrate that the proposed method can effectively eliminate sparse artifacts, uneven artifacts and noise, and can obtain high quality images.
This paper presents our recent studies on photonic devices based on antisymmetric Bragg gratings (ASBGs). ASBGs can realize backward mode conversion between the fundamental and first order transverse electric mode, i.e., TE0 and TE1. If a π phase shift is inserted in the middle of ASBG, light resonance along with mode conversion can be set up. We call this new resonance as the “Hybrid mode resonance”. Based on this structure, several photonic devices are realized. For example, we experimentally demonstrated an on-chip light filter with dropped reflected light. It can be equivalent to the light circulator in some cases and benefits the application to photonic integrated systems. Besides, other photonic devices such as single wavelength resonator and narrow band reflector are also realized. We think the proposed grating structure may give a new way to design high-performance photonic devices.
We propose a silicon modulator in a multi-mode waveguide based on the anti-symmetric Bragg grating (AS-BG). When the incident light is in the fundamental transverse electric (TE0) mode, the reflected light of the AS-BG is in the first order transverse electric (TE1) mode, rather than the TE0 mode. Therefore, the reflected TE0 mode is radiated in a tapered waveguide (WG) that connects a multi-mode and single-mode WG. As a result, the back reflection of the Bragg grating (BG) based silicon modulator avoids the utilization of an optical circulator, especially when ingrating with a semiconductor laser, which is susceptible to the external light.
A compact cascaded tunable distributed Bragg reflection (DBR) semiconductor laser is proposed and simulated. Each laser section (LS) is formed by two passive adjacent grating sections (GSs) with slightly different Bragg wavelengths and an active section (AS) between them. A step-wise grating period profile is designed to realize wide range lasing. Since two LSs share a common GS, the total cavity length of the tunable laser is significantly reduced. As an example, a tunable laser with four GSs and three ASs was designed and analyzed, resulting in a continuous tuning range of 13.2 nm. Furthermore, an improved structure with apodized grating in each GS is proposed for good single mode property. The single mode stability and fabrication tolerance are significantly improved. Particularly, this structure based lasers has a fast switching speed of about 5ns. The proposed structure would benefit the practical applications to the low cost tunable lasers in wavelength division multiplexing (WDM) systems.
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