Improvement of Performance for Raman Assisted BOTDR by Analyzing Brillouin Gain Spectrum.

We propose a simplified partitioned Brillouin gain spectrum (BGS) analysis method to enhance the spatial resolution and measurement accuracy of a Brillouin optical time-domain reflectometer (BOTDR) assisted by a first-order Raman pump. We theoretically derive the mathematical model of the partitioned BGS and analyze the superposition process of sub-Brillouin signals within a theoretical spatial resolution range. We unified all the unknown constant parameters of the calculation process to simplify the partitioned BGS analysis method and the value of the uniform parameter is attained through the system test data and numerical analysis. Moreover, to automate data processing, the starting point of the temperature/strain change is determined by the first occurrence of the maximum Brillouin frequency shift (BFS), then the position where the partitioned BGS analysis method calculation begins is obtained. Using a 100 ns probe pulse and partitioned BGS analysis method, we obtain a spatial resolution of 0.4 m in the 78.45-km-long Raman-assisted BOTDR system, and the measurement accuracy is significantly improved. In addition, we achieve a strain accuracy of 5.6 µÎµ and a spatial resolution of 0.4 m in the 28.5-km-long BOTDR without Raman amplification.

DNA methylation data-based molecular subtype classification and prediction in patients with gastric cancer.

BACKGROUND: Genetic and epigenetic alterations have been indicated to be closely correlated with the carcinogenesis, DNA methylation is one of most frequently occurring molecular behavior that take place early during this complicated process in gastric cancer (GC). METHODS: In this study, 398 samples were collected from the cancer genome atlas (TCGA) database and were analyzed, so as to mine the specific DNA methylation sites that affected the prognosis for GC patients. Moreover, the 23,588 selected CpGs that were markedly correlated with patient prognosis were used for consistent clustering of the samples into 6 subgroups, and samples in each subgroup varied in terms of M, Stage, Grade, and Age. In addition, the levels of methylation sites in each subgroup were calculated, and 347 methylation sites (corresponding to 271 genes) were screened as the intrasubgroup specific methylation sites. Meanwhile, genes in the corresponding promoter regions that the above specific methylation sites were located were performed signaling pathway enrichment analysis. RESULTS: The specific genes were enriched to the biological pathways that were reported to be closely correlated with GC; moreover, the subsequent transcription factor enrichment analysis discovered that, these genes were mainly enriched into the cell response to transcription factor B, regulation of MAPK signaling pathways, and regulation of cell proliferation and metastasis. Eventually, the prognosis prediction model for GC patients was constructed using the Random Forest Classifier model, and the training set and test set data were carried out independent verification and test. CONCLUSIONS: Such specific classification based on specific DNA methylation sites can well reflect the heterogeneity of GC tissues, which contributes to developing the individualized treatment and accurately predicting patient prognosis.

Silicon-based wavelength division multiplexer using asymmetric grating-assisted couplers.

A wavelength division multiplexer (WDM) based on asymmetric grating-assisted couplers is proposed, which can flexibly adjust the bandwidth by changing the corrugation width of the grating. The simulation results show that, compared with asymmetric uniform grating-assisted couplers, asymmetric unilateral amplitude apodization grating-assisted couplers and asymmetric bilateral amplitude apodization grating-assisted couplers can effectively suppress the sidelobes. The experimental results show that the insertion loss of each wavelength channel is between 0.23dB and 0.58dB, and the sidelobe suppress ratio of both unilateral amplitude apodization grating-assisted couplers and bilateral amplitude apodization grating-assisted couplers is larger than 10dB, which reduces channel crosstalk and proves the feasibility of the wavelength division multiplexers.

Analysis of germanium waveguide laser performance under external phonon injection.

We propose and design a germanium (Ge) waveguide laser under external phonon injection to reduce laser threshold. To take the phonon injection and the acousto-optic overlap into consideration, the theory of the photon-phonon laser action is further developed. The phononic crystal waveguide is introduced in the laser structure to intensify acousto-optic interaction, and characteristics of photon-phonon laser action in Ge waveguide are analyzed. With the external phonon injection, the two-quantum transition can be facilitated and the photon-phonon laser action is able to be established. The impacts of phononic crystal waveguide parameters, overlap of optical and acoustic fields, and phonon injection on the laser behavior are discussed. Optimal waveguide structural parameters are obtained to enhance acousto-optic interaction through the enlargement of the overlap of optical and acoustic fields. The results indicate that, for a Ge waveguide with the length of 200 µm, the threshold current is reduced to 0.2 µA and the slope efficiency reaches 0.7 W/A when the average phonon injection concentration is about 2.5 × 1021 cm-3. Our proposed scheme offers an effective approach to achieve laser oscillation in integrated Ge waveguide.

NOAA-20 VIIRS polarization effect and its correction.

The follow-on Visible Infrared Imaging Radiometer Suite (VIIRS) housed in the NOAA-20 satellite was launched on 18 November 2017. It has 22 spectral bands, among which 14 are reflective solar bands (RSBs) covering the wavelength range from 411 to 2258 nm. Prelaunch polarization sensitivity measurements have revealed that NOAA-20 VIIRS RSBs are much more sensitive to polarization of the incident light than its predecessor, the VIIRS on the Suomi National Polar-orbiting Partnership. For the short wavelength bands, i.e., M1-M4, the polarization sensitivities are out of specifications, especially for band M1, for which the polarization factors can be as large as ∼6%. The polarization effect induces striping in imagery along the track and radiometric bias both along the scan and along the track, resulting in much larger uncertainties in the environmental data records (EDR). In this paper, the polarization effect correction algorithms are described and applied to the NOAA-20 VIIRS RSBs for ocean scenes where the top-of-atmosphere radiance can be separated into the ocean normalized water-leaving radiance, the basis of the ocean color EDR, and the sunlight reflected by the atmosphere, which can be mostly described by the Rayleigh scattering radiance. The errors of the sensor data records (SDR or Level-1B radiance) due to the polarization effect can be as large as ∼1% for bands M1 and M2, and those in the ocean normalized water-leaving radiances are about 13% and 10% for wavelengths at 411 nm (band M1) and 445 nm (band M2), respectively. The polarization effect also induces strong striping in both NOAA-20 VIIRS RSB SDR and normalized water-leaving radiances. It is demonstrated that with the polarization correction applied, the aforementioned errors and artifacts are successfully removed.

Experimental demonstration of DFB lasers with active distributed reflector.

We experimentally demonstrated DFB lasers containing an active distributed reflector that has the same waveguide core as the active section. Although without current injection, the distributed reflector will be optically pumped to near transparency by the laser itself, and therefore can provide relatively high reflection to the laser. The laser, fabricated with processing steps similar to standard DFB lasers, has achieved 10-mA threshold current, 0.38-mW/mA slope efficiency, above 55-dB side mode suppression ratio, and 24-GHz modulation bandwidth at 60-mA current injection. 28-Gb/s transmission over 10-km single-mode fibers with a power penalty of less-than 0.5 dB has been demonstrated as well.

Visible Infrared Imaging Radiometer Suite reflective solar bands on-orbit calibration using solar diffuser illuminated by scattered light through the nadir port.

A new variant to the standard on-orbit calibration of the reflective solar bands (RSBs) using a solar diffuser (SD) is formulated. Instead of direct solar exposure through the SD port in the front of the instrument as originally designed, the variant method uses light reflecting off Earth's surface coming through the nadir port as the light source to illuminate the built-in onboard SD. The methodology is applied to the Visible Infrared Imaging Radiometer Suite on board the Suomi National Polar-orbiting Partnership satellite, and is shown to be viable and useful. This approach effectively preserves the standard calibration pipeline other than using a different set of illumination data, corresponding to a different illumination source, for computing the radiance emanating from the SD. It has the added advantages of not dealing with operational needs for the standard calibration activities and completely bypassing the characterization of the transmission function of the attenuation screen in the front of the SD port. The RSB calibration coefficients are computed from the data of scattered light from the SD sector per each orbit, and a 16-day average is taken. The variant calibration coefficients are shown to well match the standard solar-based RSB calibration coefficients for Bands M5 to M8, but diverging results emerge for Bands M1 to M4, highlighting the known non-ideal behavior in the degradation of SD that contributes to the worsening error in RSB calibration. The result also shows a consistent 2% variation mission-long for all RSBs, showing the overall consistency of this first analysis of the new method but also the level of the uncertainty. The result and the implications of this study are discussed.

On-orbit characterization of the VIIRS solar diffuser and attenuation screens for NOAA-20 using yaw measurements.

The follow-up visible infrared imaging radiometer suite (VIIRS) housed in the NOAA-20 satellite and the first of four in the NOAA Joint Polar Satellite System satellite series, was launched on 18 November 2017. The on-orbit satellite yaw maneuver operation was carried out on 25-26 January 2018 over 15 scheduled orbits to obtain responses of the reflective solar bands (RSBs) and the solar diffuser stability monitor (SDSM) over a specified angular range. This paper presents a comprehensive analysis of the yaw measurements that characterizes the three required input functions for the standard on-orbit RSB calibration pipeline. The characterization functions of the product of the bidirectional reflectance factors (BRFs) of the solar diffuser (SD) with the vignetting function (VF) of the SD screen (SDS), dubbed the BRF-VF-products (BVPs), are derived for the two required outgoing directions from the SD, one set for the RSB BVPs from the SD to the rotation telescope assembly that directs light to the RSBs, and another set for the SDSM BVPs for the outgoing direction from the SD to the SDSM. The VFs for the attenuation screen placed in front of the Sun-view port, the Sun-view screen (SVS), are analyzed as a set of standalone functions to characterize the direct solar illumination reaching the SDSM through the SVS, but the complexity of their non-smooth two-dimensional dependence requires an additional direct treatment in the derivation of the degradation of the SD, the H-factors. The results for the RSB BVPs, SDSM BVPs, and the SVS VFs are presented and discussed, and further applied to derive the early-mission performance of H-factors and the RSB calibration coefficients, or F-factors. The overall results of the H-factors and F-factors showing smooth trends with negligible residuals indicate that the derived BVP functions and the SVS VFs have been accurately characterized or treated, and are ready for use for the standard on-orbit RSB calibration of NOAA-20 VIIRS.

Theoretical analysis of electro-refractive index variation in asymmetric Ge/SiGe coupled quantum wells.

We propose and analyze theoretically an asymmetric Ge/SiGe coupled quantum well (CQW) for silicon based optical phase modulator. An 8-band k⋅p model is used to calculate the eigenstates and absorption spectra of the CQWs. The simulation results exhibit unique physical characteristics owing to the coupling between the two wave functions through the thin barriers. We can achieve an electro-refractive index variation as high as 9 × 10-3 at the wavelength of about 1461 nm under the electric field of 30 kV/cm. The product VπLπ of half-wave voltage and length of phase shift region is estimated to be 0.01 V cm. The proposed asymmetric Ge/SiGe CQW scheme provides a promising candidate for high speed, low voltage, low power consumption and compact optical phase modulators in silicon-based integrated optoelectronic devices.

Design and analysis of electro-absorption modulators with uniaxially stressed Ge/SiGe multiple quantum wells.

We propose and analyze theoretically electro-absorption modulators with uniaxially tensile strained Ge/Si0.19Ge0.81 multiple quantum wells (MQWs). The effects of uniaxial strain on band structures including Γ-valley and L-valley are discussed. The simulation results indicate that the absorption contrast of TE mode is improved by 3.1 dB while the TM mode absorption is reduced by two-thirds under 1.6% uniaxial tensile strain. Zero-biased electro-absorption modulators covering 1380-1550 nm wavelength can be achieved by introducing 0.18%-1.6% uniaxial tensile strain. Taking into account the TE-polarized mode excited usually in integrated waveguides, the proposed scheme provides a promising approach to design highly efficient Ge/SiGe MQWs electro-absorption modulators for on-chip optical transmission and cross-connect applications.

Analysis of threshold current of uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers.

We propose and design uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers with the stress along <100> direction. The micro-bridge structure is adapted for introducing uniaxial stress in Ge/SiGe quantum well. To enhance the fabrication tolerance, full-etched circular gratings with high reflectivity bandwidths of ~500 nm are deployed in laser cavities. We compare and analyze the density of state, the number of states between Γ- and L-points, the carrier injection efficiency, and the threshold current density for the uniaxially tensile stressed bulk Ge and Ge/SiGe quantum well lasers. Simulation results show that the threshold current density of the Ge/SiGe quantum well laser is much higher than that of the bulk Ge laser, even combined with high uniaxial tensile stress owing to the larger number of states between Γ- and L- points and extremely low carrier injection efficiency. Electrical transport simulation reveals that the reduced effective mass of the hole and the small conduction band offset cause the low carrier injection efficiency of the Ge/SiGe quantum well laser. Our theoretical results imply that unlike III-V material, uniaxially tensile stressed bulk Ge outperforms a Ge/SiGe quantum well with the same strain level and is a promising approach for Si-compatible light sources.

Wide-band achromatic flat focusing lens based on all-dielectric subwavelength metasurface.

A new method for realizing achromatic flat focusing based on all-dielectric silicon subwavelength metasurface is presented. The designed subwavelength silicon-air slits waveguide array with varied widths can provide desired phase shift of beam focusing and has the non-dispersive characteristic when the period of each unit cell is far less than the wavelength of incident electromagnetic wave (about λ/10) in mid-infrared and far-infrared spectral range. Numerical simulation of an achromatic flat focusing lens in wide spectral range from 8µm to 12µm is performed by the finite difference time domain method and the results show agreement with theory analysis results. This work indicates an effective solution for wide-band achromatic flat optical elements and potential application in integrated achromatic infrared optical systems.

Design and analysis of a CMOS-compatible distributed Bragg reflector laser based on highly uniaxial tensile stressed germanium.

We design a CMOS-compatible Distributed Bragg Reflector (DBR) laser based on highly uniaxial tensile stressed germanium. Our design first incorporates three critical elements including high uniaxial tensile stress, low loss optical resonator and heterojunction for electrical injection. A threshold current density of 80 kA/cm2 and an internal quantum efficiency of 8.5% are estimated when the Shockley-Reed-Hall (SRH) lifetime is chosen to be 3 ns. Furthermore, the performance of the DBR laser can be enhanced by improving the crystal quality and carefully designing the p-n junction. The simulation results also indicate that the limitation of the improvement of threshold current density and internal quantum efficiency are 29 kA/cm2 and 19.6%, resulting from the Auger recombination. The influences of strain and n-type doping on the threshold current density and the internal quantum efficiency are discussed. The proposed DBR laser offers a new approach to realize on-chip light source for silicon photonics.

Optimized calibration methodology of VIIRS day-night band low-gain stage using a solar diffuser.

An enhanced methodology of the standard on-orbit calibration of the day-night band (DNB) of the visible infrared imaging radiometer suite on board the Suomi National Polar-orbiting Partnership satellite is presented, specifically for the low-gain stage. The range of the "sweet spot" for the fully illuminated scans in the calibration events is expanded from 4° to 7.8° in order to increase the number of fully illuminated scans to 72, which then permits a complete calibration of the DNB to be carried out within one orbit for all 36 aggregated modes and the two mirror sides. This improves over the current operational methodology, which is with a more restricted range for the sweet spot that requires multiple orbits to complete a calibration. The expansion necessarily requires the expansion of a key calibration input, the product of the bidirectional reflectance factor of the solar diffuser (SD) with the vignetting function describing the transmission of the attenuation screen in front of the SD port, or BVP for short. The expanded BVP is derived by fitting the function in three separate intervals and joining them smoothly to form a single function. Additional enhancements include the adaptation of the previously improved SD degradation and all other improvements developed for reflective solar bands from the previous work. The time-dependent relative spectral response is also characterized and implemented, and its impact and improvements are studied and discussed. The result shows that the improved DNB calibration coefficients are more stable and smooth, and less noisy.

Theoretical analysis of optical gain in uniaxial tensile strained and n+-doped Ge/GeSi quantum well.

The direct gap optical gain of [100] uniaxial tensile strained and n+-doped Ge/GeSi quantum well (QW) is calculated. The theoretical models for strained band structures near the Γ- and L-point, optical gain and free carrier absorption are provided. Simulation results show that the optical gain can be dramatically enhanced with the help of uniaxial tensile strain and n-type doping. Furthermore, to consider the competition between gain and loss and get insight into the effects of strain and doping, the net peak gain and transparency carrier density at various strain value and doping concentration are evaluated. A net peak gain up to 2061 cm-1 for TE-polarized light is predicted at a strain value of 4%, a doping concentration of 1x1019 cm-3and an injected carrier density of 4x1019 cm-3.

Simultaneous measurement of strain and temperature based on hybrid EDF/Brillouin laser.

Simultaneous temperature and strain sensing is experimentally demonstrated based on erbium-doped fiber laser (EDFL) and Brillouin erbium fiber laser (BEFL) incorporated in a single ring laser cavity. The EDFL can be switched to BEFL by injecting the Brillouin pump into the laser cavity. Longitudinal modes beat frequency and Brillouin frequency shift are monitored to discriminate strain and temperature. The longitudinal modes beat frequency is measured by observing the self-beating signals of the EDFL, while the Brillouin frequency shift is measured by monitoring the heterodyning signal of the BEFL. The simultaneous measurement errors of strain and temperature are within ± 25.8µÎµ and ± 0.8°C. The sensor is of simple structure and compact size.

Analysis of acousto-optic interaction based on forward stimulated Brillouin scattering in hybrid phononic-photonic waveguides.

We present the generation of forward stimulated Brillouin scattering (FSBS) in hybrid phononic-photonic waveguides. To confine the optical and acoustic waves simultaneously, a hybrid waveguide is designed by embedding the silicon line defect in the silicon nitride phononic crystal slab. By taking into account three kinds hybrid waveguide, the appropriate structural parameters are obtained to enhance the acousto-optic interaction. We fabricate the honeycomb hybrid waveguide with a CMOS compatible technology. The forward Brillouin frequency shift is measured up to 2.425 GHz and the acoustic Q-factor of the corresponding acoustic mode is 1100. The influences of pump power, acoustic loss, nonlinear optical loss and lattice constant on the acousto-optic interaction in FSBS are analyzed and discussed. The proposed approach has important potential applications in on-chip all-optical signal processing.

Design of compact and efficient polarization-insensitive taper coupler for SiGe photonic integration.

We have proposed a polarization-insensitive laterally tapered coupler for the integration of the active Ge/SiGe multi-quantum-well device with a passive SiGe waveguide. A 45-µm-long taper is designed to achieve more than 90% coupling efficiency for both TE- and TM-polarized modes. The mode interference is utilized to obtain a compact taper coupler with high coupling efficiency. Fabrication tolerances are analyzed in terms of taper width, thickness, material refractive and operation wavelength. The results indicate that a width variation of ± 200 nm and a thickness variation of ± 100 nm are allowed. The designed taper coupler can provide efficient coupling under normal operating conditions in the wavelength range of 1460-1625nm.

Design and simulation of two-section DFB lasers with short active-section lengths.

Distributed feedback lasers comprised of a reflection section and an active section have been proposed for high direct-modulation bandwidth. The reflection section has the same core layer as the active section so butt-joint re-growth is avoided. Without current injection the reflection section will be pumped to near transparency by the emission from the laser itself so high reflection (> 0.75) can still be achieved as confirmed by the simulation. Therefore a short (150 µm) active section can be used, which enables a low threshold current (~5 mA) and a high direct modulation bandwidth (>30 GHz) as demonstrated by the simulation.

Optical single sideband modulation based on a high-order birefringent filter using cascaded Solc-Sagnac and Lyot-Sagnac loops.

We propose and experimentally demonstrate a simple and flexible photonic approach to implementing single sideband (SSB) modulation based on optical spectral filtering. The high-order birefringent filter is realized through the cascaded Solc-Sagnac and Lyot-Sagnac loops. By adjusting the rotation angle of the polarization controller (PC), the notch position to remove undesired sidebands changes. The frequency for SSB modulation varies accordingly. The periodical response of the filter spectrum allows both the carrier wavelength and the optical carrier to sideband ratio (OCSR) to be tunable. SSB modulation over a frequency range from 5 to 40 GHz and tunable OCSR ranging from -9.174 to 34.408 dB are obtained. The significant merits of the proposed approach are the simple structure, easy operation, large frequency range, tunable OCSR, and wavelength independence. The approach has potential applications in optimizing the transmission performance of photonic microwave signal processing systems.