Average Spectral Width Research Articles

Multi-wavelength fiber laser via evanescent field confinement with Al2O3 nanolaminate-coated thinned fiber

We present the fabrication and analysis of an Erbium-doped optical fiber laser emitting at multiple wavelengths. This is accomplished via a filter constructed from a fiber taper coated with alumina (Al2O3). The taper is created using the flame brushing induction technique with a combination of Butane (25 %) and Oxygen (75 %). The diameter is reduced to a 20 µm waist and 2 mm burner displacement. The thin film of Al2O3 was deposited via atomic layer deposition (ALD), resulting in a 15 nm layer. The coated fiber taper was then inserted into a ring fiber laser cavity functioning as a selector filter. This fiber laser produced an output comprising six simultaneous wavelengths: 1531.14, 1531.79, 1532.45, 1541.85, 1544.45, and 1562.22 nm. Laser signal stability tests were performed for one hour at 25 °C at the six emission wavelengths, resulting in a maximum optical power fluctuation of 0.15 to 0.33 dB, a wavelength shift variation of 0.02 nm, and a signal-to-noise ratio of 31.79 dB in average. The laser lines emitted have an average spectral width of 0.06 nm. Our results show that the typical performance of the fiber laser configuration based on an optical fiber taper was improved by the addition of the Al2O3 layer and can be further improved. Our findings demonstrate that the standard operation of the fiber laser setup, which utilizes an optical fiber taper, was enhanced through the incorporation of an Al2O3 layer and has the potential for further development.

Stratiform and Convective Rain Classification Using Machine Learning Models and Micro Rain Radar

Rain type classification into convective and stratiform is an essential step required to improve quantitative precipitation estimations by remote sensing instruments. Previous studies with Micro Rain Radar (MRR) measurements and subjective rules have been performed to classify rain events. However, automating this process by using machine learning (ML) models provides the advantages of fast and reliable classification with the possibility to classify rain minute by minute. A total of 20,979 min of rain data measured by an MRR at Das in northeast Spain were used to build seven types of ML models for stratiform and convective rain type classification. The proposed classification models use a set of 22 parameters that summarize the reflectivity, the Doppler velocity, and the spectral width (SW) above and below the so-called separation level (SL). This level is defined as the level with the highest increase in Doppler velocity and corresponds with the bright band in stratiform rain. A pre-classification of the rain type for each minute based on the rain microstructure provided by the collocated disdrometer was performed. Our results indicate that complex ML models, particularly tree-based ensembles such as xgboost and random forest which capture the interactions of different features, perform better than simpler models. Applying methods from the field of interpretable ML, we identified reflectivity at the lowest layer and the average spectral width in the layers below SL as the most important features. High reflectivity and low SW values indicate a higher probability of convective rain.

Investigation on Ultra-Compact 2DPC Coarse Wavelength Division Demultiplexer

A two dimensional Photonic Crystal (2DPC) based eight-channel wavelength division de-multiplexer is proposed and designed for Coarse Wavelength Division Multiplexing (CWDM) applications. The circular ring resonator, channel selector, circulator rod, L bend waveguide and linear bus waveguide are essential parts of the proposed system. The system’s functional parameters such as Transmission efficiency, resonant wavelength, spectral width, channel spacing, Quality factor and crosstalk investigated this paper. The eight different wavelengths of channels are filtered out by altering the size of channel selector rod, setting the radius of the circle shaped cavity and relative refractive index of circulator rod. Initially the Photonic Band gap (PBG) is manipulated by applying Plane Wave Expansion (PWE) method of the 2DPC structure. The functional parameters are analysed by Finite Difference Time Domain (FDTD) method in periodic and non-periodic structure of the proposed system to arrive normalized transmission spectrum. The resonant wavelengths of designed eight paths of the device are varying from 1420nm to 1460nm with average spectral width and channel spacing are 5.8nm, 5.6nm respectively. The footprint of the device is 286.84μm2. Hence, this small device can be implemented for CWDM systems in Photonic Integrated Circuits (PIC)

All-polarisation-maintaining modified figure-of-8 fibre laser as a source of soliton molecules

The conditions are identified, under which a figure-of-eight all-polarisation-maintaining fibre laser with two independently pumped active media, each in one of the two cavity loops, generates soliton molecules with adjustable number of solitons in the molecule. The soliton count in the molecule may be reproducibly set between 2 and 6 by adjustment of the ratio of the pump powers of the active media. The average duration of the autocorrelation function of a single soliton is 11.6 ps, the repetition rate of the pulse train is 14.19 MHz, and its average spectral width is 10 nm. An analysis of the map of the mode-locked regimes set by different ratios of the pump source powers has shown that there exist two separate areas, one corresponding to generation of noise-like pulses and the other, to generation of soliton molecules. We believe that our findings will extend the application area of soliton molecules and will further bring a new insight into understanding of soliton molecule formation inside fibre cavities.

Optical quantization based on soliton self-frequency shift and a flexible spectrum compression scheme utilizing time-dependent filtering.

We propose and experimentally demonstrate a novel optical spectrum compression scheme utilizing time-dependent filtering effect, which can be used to improve the resolution of the optical quantization based on soliton self-frequency shift (SSFS). The spectrum compression is realized through power-dependent filtering utilizing polarizers and nonlinear polarization rotation effect in a spool of high-nonlinear fiber (HNLF) after mapping the soliton spectrum to the time domain in a section of dispersive fiber. Theoretical analysis and numerical simulation indicate that, compared with the conventional spectrum compression scheme based on chirp compensation, the proposed scheme does not have a restrictive length match requirement of the dispersive fiber and the HNLF. In addition, the output spectral width variation is small for various pulse peak power, which is favorable for the subsequent optical coding. In the experiment, the spectra after SSFS are compressed to an average spectral width of 1.65 nm, which is 45% of that using comb-like fibers. The quantization resolution using the proposed scheme is 5.95 bits for a maximum wavelength shift of 100 nm, which is 1.13 bits higher than that using comb-like fibers.

Phase Coexistence in Ternary Lipid Mixtures Containing POPC and Phytosterols, Ergosterol or 7-Dehydrocholesterol

2H-NMR spectroscopy was used to investigate the occurrence of phase coexistence in multilamellar vesicles of DPPC and POPC (33mol%: 33mol%) with either stigmasterol, brassicasterol, ergosterol or 7-DHC (each 33mol%). In all cases, the sn-1 chains of DPPC and POPC were deuterated in turn, and 2H-NMR spectra were measured for both lipid components as a function of temperature between 5 oC and 48 oC. The chain order of DPPC was found to be greater than that of POPC in all the above mixtures. An equimolar DPPC/POPC membrane exhibits a clear solid-ordered (so)-liquid disordered (ld) transition between 5 oC and about 33 oC as observed for each lipid by a fairly steep reduction in the average spectral width. Adding sterol generally results in a more gradual change in average spectral width over this temperature range. Depending on the particular sterol, NMR spectra taken in this temperature range either appeared poorly resolved (“blurry”) or displayed coexisting liquid ordered (lo) and ld spectra. Liquid-liquid phase coexistence is observed in DPPC/POPC/ergosterol mixtures. The spectra for DPPC/POPC/ brassicasterol only exhibited liquid-liquid coexistence when a small concentration (0.02 mol%) of the fluorescent lipid dye TR-DHPE was added to the system. Only one liquid phase was observed for either DPPC/POPC/7DHC or DPPC/POPC/stigmasterol. In summary, our interest was to study how liquid phase coexistence in DPPC/POPC/sterol is modified by small changes in the sterol structure. These studies show that ergosterol is the most effective sterol in fostering phase coexistence. Note that in comparison to cholesterol, which does not cause micron scale phase coexistence in DPPC/POPC, ergosterol has an extra double in the fused ring, as well as a double bond at C22. Possible connections between the detailed sterol structures and these observations will be discussed.

Majority of PMSE spectral widths at UHF and VHF are compatible with a single scattering mechanism

Motivated by a recent controversy in the literature about the agreement or disagreement of spectral widths (after conversion to Doppler-velocities) of PMSE observed simultaneously with the EISCAT VHF and UHF radars in Tromsø, Norway, this issue was reconsidered taking into account a much larger data set than in any of the previous studies. While these previous studies only considered case studies of a few minutes in length, we considered a total of 380min of simultaneous and common volume observations with these two radars at a time resolution of 30s, i.e., a total of 760 samples. This study is important to discern whether PMSE at these two frequencies originate from the same coherent scattering mechanism or if different physical mechanisms are at work. We considered both shape and width of the considered spectra and found that – within the available spectral resolution – the VHF spectra are on average well described by a Gauss shape (with spectral parameter n=2—which is derived from a generalized formulation of the autocorrelation function), whereas the UHF-spectra show a small deviation from this shape with an average n of 1.6. Spectral widths do largely agree but show a small systematic difference, i.e., the UHF spectra are on average 0.1m/s narrower than the VHF spectra at an average spectral width of 3.5m/s. This small systematic effect is largely explained considering the overall effect of beam-, shear-, and wave-broadening. This means that the slightly different beam widths of the VHF and UHF-radars and the hence slightly different observing volumes account for the small systematic difference in mean spectral width. Finally, we demonstrated that the small deviation of UHF-spectra from a perfect Gauss shape can be either due to relatively small signal-to-noise ratios of the UHF-data and/or a superposition of incoherent and coherent scatter in cases where both contributions add equally to the total observed power. Since VHF PMSE are much more stronger than the incoherent scatter background this effect cannot be recognized in VHF observations. This superposition effect may occasionally also lead to larger deviations between VHF and UHF spectra in that it can potentially narrow the UHF-spectra significantly. However, excluding these rather rare and exotic cases, we conclude that the majority of our observations are compatible with a single coherent scattering at both VHF and UHF.

The exchange across the tropical tropopause in overshooting convective cores

Abstract. Coordinated measurements of Indian MST radar and radiosonde during the passage of overshooting convective cores in SAFAR pilot campaign (May–November 2008) are utilized to quantify the mass flux across the tropopause and strength of the turbulence in up- and down-draft cores. The distribution of retrieved mass flux is found to be wide, ranging from −0.6 (downward flux) to 0.8 kg m−2 s−1 (upward flux). The net mass flux, is, however, from the troposphere to stratosphere, in spite of the existence of significant amount of downward flux. For the first time, the turbulence strength in the vicinity of the tropopause in convective cores is quantified. Large spectral widths with magnitudes >4 m s−1 are observed during convection. However, such large spectral widths are rare and are observed only for 1.6% of total observations. The average spectral width (and also the variation or standard deviation) in draft cores is found to be ~4.5 times larger than that in fair-weather. The turbulence strength in updraft cores is much higher than that of in downdraft cores. The mean (median) spectral width in updraft cores is larger by ~0.8 m s−1 (a factor of ~2) than that of in downdraft cores. The turbulence strength does not show any systematic variation with the intensity of convection in both up- and down-draft cores. The distributions and mean values of mass flux and turbulence strength obtained in the present study will be useful to quantify the STE due to direct intrusion of mass by overshooting convection and the exchange of constituents (in particular water vapor) due to turbulence in a better way.

The Physical Properties of Sphingomyelin/Cholesterol Membranes: a Deuterium NMR Study to Map a Partial Phase Diagram

We have used deuterium nuclear magnetic resonance (2H NMR) to study the effect of cholesterol on N-palmitoyl(D31)-D-erythro-sphingosylphosphorylcholine (PSM) membranes. NMR spectra were taken as a function of temperature (from 25 to 70°C) and cholesterol concentration (0 - 40%). The constructed phase diagram exhibits both solid-ordered (so) + liquid-ordered (lo) and liquid-disordered (ld) + lo phase coexistence regions with a clear three-phase line at 37.5°C between 8 and approx. 18 mol% cholesterol. The ld + lo region was characterized by examining the cholesterol dependence of the width of resolved peaks in the depaked spectra, as well as the average spectral width (M1), at a given temperature. The so + lo region was defined using spectral subtraction. Analogous experiments were done using 1-palmitoyl,2-palmitoyl(D31)-sn-glycero-3-phosphocholine (DPPC)/cholesterol membranes in order to carefully compare the data obtained using palmitoyl chains which have similar “kinked” conformations. The three-phase line in sn-2 perdeuterated DPPC/cholesterol is at 39.5°C and extends from approx. 8 to 20 mol% sterol. The PSM/cholesterol membrane is significantly more ordered than the DPPC/cholesterol membrane in the liquid crystalline phase. For example, at 60°C and 20 mol% cholesterol, the average order parameter of PSM is 34% higher than for DPPC. This should be compared to the difference between the cholesterol-free membranes at 60°C, where the average order of PSM is 23% more than that of DPPC. However the PSM/chol interaction is more effective than that of DPPC/chol at removing conformational fluctuations in the palmitoyl chain.

A Statistical study of the Doppler spectral width of high-latitude ionospheric F-region echoes recorded with SuperDARN coherent HF radars

Abstract. The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide measurements of the E × B drift of ionospheric plasma over extended regions of the high-latitude ionosphere. We have conducted a statistical study of the associated Doppler spectral width of ionospheric F-region echoes. The study has been conducted with all available radars from the Northern Hemisphere for 2 specific periods of time. Period 1 corresponds to the winter months of 1994, while period 2 covers October 1996 to March 1997. The distributions of data points and average spectral width are presented as a function of Magnetic Latitude and Magnetic Local Time. The databases are very consistent and exhibit the same features. The most stringent features are: a region of very high spectral width, collocated with the ionospheric LLBL/cusp/mantle region; an oval shaped region of high spectral width, whose equator-ward boundary matches the poleward limit of the Holzworth and Meng auroral oval. A simulation has been conducted to evaluate the geometrical and instrumental effects on the spectral width. It shows that these effects cannot account for the observed spectral features. It is then concluded that these specific spectral width characteristics are the signature of ionospheric/magnetospheric coupling phenomena.Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; ionospheric irregularities)

Spectral characteristics of a free electron laser with time-dependent beam energy

The spectral characteristics of a continuous beam free electron laser are studied both numerically and analytically. The average spectral width and the probability distribution of mode amplitudes determined from nonlinear numerical simulations are found to agree with a simple theory based on the linear growth of each mode in the presence of a white noise source. Included in the numerical simulation model is the time dependence of the beam energy and its effect on the spontaneous emission rate. Based on this model predictions of the frequency of observing a single dominant mode in the recent University of California at Santa Barbara experiments [FEL ’89, 11th International Conference on Free Electron Lasers, Conference Digest (IEEE), New York, 1989), Paper p. 2.19] are made. The predicted frequency is somewhat lower than observed. However, other features of the simulations do agree with experimental results.