High Effective Area Research Articles

Design optimization of a dual-core dispersion-compensating fiber with a high figure of merit and a large effective area for dense wavelength-division multiplexed transmission through standard G.655 fibers.

We report design optimization in terms of index-profile parameters of a dual-core dispersion-slope-compensating fiber suitable for broadband dispersion compensation in standard G.655 and G.655b single-mode fibers over the C and L bands of fiber amplifiers and additionally over the S band for the G.655b fibers. It takes into account profiles that can be achieved with state-of-the-art fabrication techniques such as modified chemical-vapor deposition. Relatively high mode effective areas ensure the reduced sensitivity of the fiber to detrimental nonlinear effects when the fiber is integrated into a dense wavelength-division-multiplexed network. The theoretical figures of merit of these DCFs were found to be > or = 700 (ps/dB)/nm; furthermore, the estimated bend losses were also quite low, even for bend radii as small as 16 mm.

Is it better on top? The triumph of supra-annular valves

V alve selection in the patient with a small aortic root remains a difficult decision for cardiac surgeons. Use of smaller stented porcine tissue valves in these patients may result in high gradients and low effective orifice areas.1,2 In some cases, there may be prosthesis-patient mismatch,3 which can result in persistent left ventricular hypertrophy after valve replacement and has been reported in up to 52% of patients.4 Jin and colleagues5 found no significant regression of left ventricular hypertrophy in patients after implantation of either stented tissue valves or mechanical valves, although patients receiving unstented tissue valves and homografts did demonstrate significant regression. Recognition of the potential importance of prosthesis-patient mismatch has led to interest in developing valve substitutes with improved hemodynamics. Although much of the recent literature has focused on stentless tissue valves, more traditional valve designs have also evolved to meet this challenge. The St Jude Medical HP (High Performance) valve was designed to allow supra-annular placement of the valve through modification of the sewing ring on a standard St Jude Medical valve. This allows a valve approximately one size larger to be inserted in the same size aortic root. Intuitively, one would expect this valve to have lower pressure gradients possibly resulting in better remodeling of the ventricle and more complete regression of hypertrophy after valve replacement than the standard cuff (SC) St Jude Medical valve. However, evidence for an improvement in hemodynamics with the HP valve has been lacking. In this issue of the Journal, Vitale and coworkers6 report a randomized, multicenter trial comparing the postoperative and 6-month hemodynamics between the St Jude Medical HP and SC valves. The results are interesting on several levels. When HP and SC valves were compared at 6 months, both peak and mean pressure gradients were lower in the 21-mm and 23-mm HP valves than in the SC valves of the same size. However, effective orifice area did not differ significantly between SC and HP valves. A possible explanation for this finding lies in the greater cardiac output in the patients with SC valves in this series, which makes it difficult to draw definitive conclusions about the hemodynamic superiority of HP over SC valves. At 6 months, both the 21-mm and 23-mm HP valves demonstrated similarly low gradients (mean gradients of 18.8 and 14.9 mm Hg, respectively) and good effective orifice areas (1.56 cm and 1.60 cm, respectively). These observations suggest that aortic root enlargement procedures may not be needed to upgrade from a 21-mm HP valve to a 23-mm HP valve since little additional hemodynamic benefit accrues. Vitale and coworkers also examined the prevalence of postoperative patient-prosthesis mismatch. In contrast to the high prevalence in prior reports noted above, no patient in this series had patient-prosthesis mismatch according to an effective area index less than 0.9 cm/m, slightly stricter criteria than recommended by Pibarot and Dumesnil.4 Interestingly, HP valves showed a progressive reduction in gradient and effective orifice area in serial measurements after implantation. Since the HP valve is mechanical and fixed in size, no changes in shape or geometry of the valve itself are possible. This suggests that there may be remodeling either of the outflow tract or of the distal aorta allowing a closer matching of valve size with either outflow tract or aortic size. This late reduction in gradient has been reported in several previous studies with stentless valves.7,8 Vitale’s article demonstrates that this late reduction From the Division of Cardiothoracic Surgery, Cedars-Sinai Medical Center, Los Angeles, Calif.

All-fiber grating-based higher order mode dispersion compensator for broad-band compensation and 1000-km transmission at 40 Gb/s

We use a novel fiber-grating device to demonstrate the first polarization-insensitive all-fiber higher order mode dispersion compensator for broad-band dispersion compensation. Its low loss and high effective area have enabled transmission through 1000 km (10/spl times/100 km) of nonzero dispersion-shifted fiber (NZDSF) at 40 Gb/s.

Simulations and Mitigation of Pile-Up in XMM CCD Instruments

Simulations of the XMM EPIC instruments are presented. They suggest the correct operating mode must be chosen to ensure that spectral analysis of the data is not compromised by ‘pile-up’ effects. The performanceis contrasted with the CHANDRA AXAF CCD Imaging Spectrometer, showing how the higher effective area but better over-sampling of its mirror response function in XMM, impacts this pile-up criteria. Targetsexceeding a flux of a few 10-12 ergs cm-2 s-1 will becompromised for spectral analysis in these CCD instruments, unlesscare is taken in defining correct instrument configurations. Somesimple methods of mitigating the effects are described, but this featureof X-ray pile-up warrants careful attention in calibration.

A low noise HTS SQUID magnetometer with an on-chip pickup loop coupled via an intermediate flux transformer

For many SQUID applications robust, compact magnetometers are required with low flux noise and high effective area at 77 K. Low flux noise is achieved by using a low inductance SQUID. A low inductance SQUID will also have a high transfer function, allowing simplified direct readout schemes to be used. For maximum field sensitivity the SQUID needs to be coupled to a pickup loop of large area and therefore large inductance. We have modelled, designed and fabricated a novel HTS magnetometer consisting of a low inductance (<16 pH) dc SQUID coupled to a 28 nH pickup loop flux transformer fabricated on the same 1 cm2 substrate. Efficient coupling was achieved by using a flip-chipped intermediate flux transformer. This configuration produces a magnetometer with a high sensitivity per unit area. The magnetometers incorporate PrBa2Cu3O7 isolation layers and two 2 μm diameter CAM variant junctions. Thick YBa2Cu3O7 layers were used to improve coupling and decrease the demagnetisation factor wherever possible. Our device had a transfer function of 913 μV/Φ0 which allows direct readout without any matching or additional positive feedback arrangements. The measured effective area at 77 K was 0.58 mm2 (3.6 nT/Φ0). The magnetometer white noise was 18 fT/ and at 1 Hz was 380 fT/. This was reduced at 1 Hz to 120 fT/ using bias reversal. The measured white noise was higher than the design value and is largely due to Johnson noise from the PrBa2Cu3O7 isolation layer used in our devices.

The ecology of the climbing fern Dicranopteris linearis on windward Mauna Loa, Hawaii

Summary Dicranopteris linearis (Gleicheniaceae), a native fern common throughout the Old World tropics and Polynesia, forms dense thickets &gt; 3m deep over large areas of open‐canopy, oligotrophic, wet Hawaiian rainforests. Our objectives were to identify leaf‐ and whole plant‐level traits that are key to its success and to determine its community‐and ecosystem‐level consequences in primary successional sites. Along an elevational gradient from 90 to 1660m, mean maximum net assimilation rates of Dicranopteris ranged from 2.9 to 5.0 μmol m−2 s−1, compared with 3.6–9.5 μmol m−1 s−1 in the codominant tree Metrosideros polymorpha. Gas‐exchange characteristics did not explain Dicranopteris' success, nor its trends in production. However, indeterminate, clonal growth form, shallow rhizomes, marcescent leaves with low decomposability, and a mat‐forming capacity enabled Dicranopteris to colonize sites and to maintain dominance via high effective leaf area, despte its low biomass. Phosphorus use efficiency, which reached 24 kg g−1, was exceptionally high, allowing colonization of phosphorus‐poor sites. Dicranopteris contributed up to 74% of above‐ground net primary productivity in a site where it contained only 14% of live biomass. It accounted for up to 57% and 47% of total nitrogen and phosphorus uptake by plants, respectively, where it contained only 24% and 30% of plant nitrogen and phosphorus. Dicranopteris leaves are short‐lived and slow to decompose; thus, fixed carbon is transferred quickly to soil detrital pools where it contributes to aggrading soil organic matter pools and may exacerbate oligotrophic conditions, thereby strongly influencing soil genesis and ecosystem development. The fern therefore influences forest‐floor light regimes and directs later community development. An exclusion experiment demonstrated that Dicranopteris competed with Metrosideros, but lack of revegetation in 40% of the exclusion area after 39 months showed that Dicranopteris also colonized microenvironments unavailable to its endemic codominants. Dicranopteris may play an important role in resisting invasions of exotic species into Hawaiian rainforests.

Development of high-efficiency a-Si solar cell submodule with a size of 30 cm × 40 cm

We have achieved a stabilized conversion efficiency of 8.9% in a single-junction a-Si solar cell and 10.6% in a double-junction a-Si a-SiGe solar cell for a size of 1 cm 2, which are the world's highest values achieved so far for this size and structure. We have been investigating the improvement of stability in a-SiGe film with regard to the bottom cell i-layer, and the control of E opt in a-SiGe film in order to confirm the tandem cell design. On the other hand, uniformity of ± 1% has been obtained in conversion efficiency for many small cells fabricated in a size of 30 cm × 40 cm, evaluated by using a-Si single-junction structure. As a result, we have achieved the stabilized high-effective area conversion efficiency of 8.64% in a 30 cm × 40 cm a-Si/a-Si tandem submodule. The combination of the above techniques and further optimization can be expected to achieve a stabilized conversion efficiency of more than 10% for a 30 cm × 40 cm double-junction a-Si a-SiGe submodule.

The Medtronic-Hall valve: a design in 1977 to improve the results of valve replacement

The prototype Medtronic-Hall valve was developed from improvements to the Lillehei-Kaster and Björk-Shiley valves. The housing was constructed of a single piece of titanium and the disc was composed of pyrolytic carbon; both materials had shown no structural breakdown in previous clinical use. The valve was tested with pulse simulator and electromagnetic flow meter studies and yielded a high effective orifice area. Results in 110 patients were significantly better with the Medtronic-Hall valve than with either of the previous valves. In the 21-mm aortic valve the peak-peak gradient was 12 mmHg; it was 9.2 mmHg in the 23-mm, 3.8 mmHg in the 25-mm, and 2.5 mmHg in the 27-mm. Studies found 5% of forward flow regurgitation in the aortic position and more than desired regurgitation through the mitral valve. After the opening angle of the mitral valve was reduced to 70 degrees, regurgitation reached an acceptable level and the improved hemodynamics resulted in lower transvalvular gradients than in previous mechanical valves. Some patients are naturally more prone to thromboembolism regardless of valve type or anticoagulant therapy; thus the human factor plays a large role in a valve's success. More randomized studies are needed for accurate valve comparison, in addition to investigations of sewing ring factors. Nevertheless, the Medtronic-Hall valve has been used successfully for 15 years and has exhibited no housing fractures.

Peeking through the picket fence: What astrophysical surprises may be present in the 100–1200 Å region?

In anticipation of more sensitive EUV and FUV spectroscopic instruments, spectra are simulated including interstellar absorption of solarlike, RS CVn, and flare stars as folded through the instrument parameters of the EUVE, Lyman/FUSE Phase A, and a desirable next-generation spectrometer. Even the relatively insensitive EUVE spectrometer is able to detect sufficient spectral lines from many active binary and dMe stars to determine their coronal-emission-measure distributions. The Lyman/FUSE or next-generation spectrometers are needed to study solar-type stars or flaring stars with high time resolution. The high throughput and effective area of a nex-generation spectrometer is needed for Doppler imaging studies, stellar wind and downflow measurements, and high time and spectral resolution of stellar flares.