Wavelength and bandwidth-tunable silicon comb filter based on Sagnac loop mirrors with Mach-Zehnder interferometer couplers.

We propose and experimentally demonstrate a wavelength and bandwidth-tunable comb filter based on silicon Sagnac loop mirrors (SLMs) with Mach-Zehnder interferometer (MZI) couplers. By thermally tuning the MZI couplers in common and differential modes, the phase shift and reflectivity of the SLMs can be changed, respectively, leading to tunable wavelength and bandwidth of the comb filter. The fabricated comb filter has 93 comb lines in the wavelength range from 1535 nm to 1565 nm spaced by ~0.322 nm. The central wavelength can be red-shifted by ~0.462 nm with a tuning efficiency of ~0.019 nm/mW. A continuously tunable bandwidth from 5.88 GHz to 24.89 GHz is also achieved with a differential heating power ranging from 0.00 mW to 0.53 mW.

High-capacity and low-cost long-reach OFDMA PON based on distance-adaptive bandwidth allocation.

We propose and experimentally demonstrate a distance-adaptive bandwidth allocation scheme to realize high-capacity long-reach orthogonal frequency division multiple access passive optical network (OFDMA PON) with cost-effective electro-absorption modulator (EAM). In our scheme, the subcarriers in downstream OFDM signal are properly allocated to the optical network units (ONUs) with different fiber transmission lengths. By this means, the detrimental influence of power fading induced by dispersion and chirp can be avoided, thus all OFDM subcarriers can be modulated with high-order quadrature amplitude modulation (QAM) format, leading to a high transmission capacity. A proof-of-concept experiment is performed, in which three ONUs with transmission distances of 25, 50, and 100 km are assigned with different subcarriers, respectively. By using distance-adaptive bandwidth allocation technique, an OFDM signal of 34.5 Gb/s is successfully delivered to the ONUs with a bit error ratio (BER) lower than 2 × 10(-3).

Broadband photodetection in a microfiber-graphene device.

We propose and experimentally demonstrate a microfiber-graphene device. Owing to the interaction between the graphene film and the evanescent field leaked from the microfiber, the hybrid photoconductive device exhibits a high photoresponse. A maximum photocurrent responsivity of ~2.81 mA/W is achieved in the telecommunication band. A nearly flat photoresponse spectrum within broad operational band ranging from 1500 nm to 1600 nm is also obtained as a consequence of the dispersionless and flat absorption of graphene. These results show that the proposed photocurrent generation device could provide an effective solution for broadband photodetection.

An Intron of Invertebrate Microphthalmia Transcription Factor Gene Is Evolved from a Longer Ancestral Sequence.

Introns are highly variable in number and size. Sequence simulation is an effective method to elucidate intron evolution patterns. Previously, we have reported that introns are more likely to evolve through mutation-and-deletion (MD) rather than through mutation-and-insertion (MI). In the present study, we further studied evolution models by allowing insertion in the MD model and by allowing deletion in the MI model at various frequencies. It was found that all deletion-biased models with proper parameter settings could generate sequences with attributes matchable to 16 invertebrate introns from the microphthalmia transcription factor gene, whereas all insertion-biased models with any parameter settings failed to generate such sequences. We conclude that the examined invertebrate introns may have evolved from a longer ancestral sequence in a deletion-biased pattern. The constructed models are useful for studying the evolution of introns from other genes and/or from other taxonomic groups. (C++ scripts of all deletion- and insertion-biased models are available upon request.).

Human SARS-CoV-2 has evolved to reduce CG dinucleotide in its open reading frames.

The outbreak of COVID-19 has brought great threat to human health. Its causative agent is a severe acute respiratory syndrome-related coronavirus which has been officially named SARS-CoV-2. Here we report the discovery of extremely low CG abundance in its open reading frames. We found that CG reduction in SARS-CoV-2 is achieved mainly through mutating C/G into A/T, and CG is the best target for mutation. Meanwhile, 5'-untranslated region of SARS-CoV-2 has high CG content and is capable of forming an internal ribosome entry site (IRES) to recruit host ribosome for translating its RNA. These features allow SARS-CoV-2 to reproduce efficiently in host cells, because less energy is consumed in disrupting the stem-loops formed by its genomic RNA. Notably, genomes of cellular organisms also have very low CG abundance, suggesting that mutating C/G into A/T occurs universally in all life forms. Moreover, CG is the dinucleotide related to CpG island, mutational hotspot and single nucleotide polymorphism in cellular organisms. The relationship between these features is worthy of further investigations.

Simulation of Chordate Intron Evolution Using Randomly Generated and Mutated Base Sequences.

Introns are well known for their high variation not only in length but also in base sequence. The evolution of intron sequences has aroused broad interest in the past decades. However, very little is known about the evolutionary pattern of introns due to the lack of efficient analytical method. In this study, we designed 2 evolutionary models, that is, mutation-and-deletion (MD) and mutation-and-insertion (MI), to simulate intron evolution using randomly generated and mutated bases by referencing to the phylogenetic tree constructed using 14 chordate introns from TF4 (transcription factor-like protein 4) gene. A comparison of attributes between model-generated sequences and chordate introns showed that the MD model with proper parameter settings could generate sequences that have attributes matchable to chordate introns, whereas the MI model with any parameter settings failed in doing so. These data suggest that the surveyed chordate introns have evolved from a long ancestral sequence through gradual reduction in length. The established methodology provides an effective measure to study the evolutionary pattern of intron sequences from organisms of various taxonomic groups. (C++ scripts of MD and MI models are available upon request.).