Broad Bandpass Research Articles

Reflective-SOA Fiber Cavity Laser as Directly Modulated WDM-PON Colorless Transmitter

Reflective semiconductor optical amplifier (RSOA) fiber cavity lasers are attractive colorless, self-seeded, self-tuning, and directly modulatable sources for passive optical networks (PONs). They comprise of an RSOA in the optical network unit as the active element, a distribution fiber as the laser cavity, a waveguide grating router, and a common reflective mirror with the latter two positioned at the remote node. In this paper, we introduce a model and perform simulations to elucidate the recently discovered successful operation of this new PON source. The results are in agreement with experiments; the formation of a narrow laser spectrum with a relatively constant output power is seen despite a relatively broad passband window of the waveguide grating router. We further study mode competition and mode partition noise. It is shown that proper chromatic dispersion management can overcome mode partition noise limitations. The quality of the RSOA fiber cavity laser does not degrade when being directly modulated and as a result these highly multimode lasers offer an economic way to transport Gbit/s upstream data over kilometers of fiber in a wavelength division multiplexing-PON.

A wide stopband microstrip bandpass filter with stepped coupled lines

ABSTRACTThis study develops a microstrip bandpass filter with a broader stopband. The proposed bandpass filter is constructed by cascading three stepped coupled lines and two short‐circuited stubs. Moreover, quarter‐wavelength electrical lengths and weak strong‐weak couplings are adopted for the stepped coupled line. In this way, a compact circuit size and a wide stopband can be achieved readily. In addition, the short‐circuited stubs connected to the input/output port can result in a steeper passband skirt and a broader passband. Finally, a design example is provided to show the availability of the proposed structure. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2283–2286, 2014

Tunable broadband transmission and phase modulation of light through graphene multilayers

In this work, we study the light transmission through multilayer graphene sheets that are spatially separated. By adopting the transfer matrix method modified for two-dimensional material, we find that the graphene multilayers manifest a broad passband in the transmission spectrum. The two edges of the passband are determined by the interband threshold of electron transition in graphene and the cutoff wavelength of Bloch mode in the multilayer structure. By varying the interlayer space of the graphene multilayers, the structure can be used to increase the phase velocity of normally incident light or to make the phase at rest like epsilon-near-zero material. The study shows that the graphene multilayers may find great applications in optical switches, filters, and phase modulators.

Study of a mode-locked erbium-doped frequency-shifted-feedback fiber laser incorporating a broad bandpass filter: Numerical results

We present a comprehensive numerical study on the spectral and temporal behaviors of a mode-locked erbium-doped frequency-shifted-feedback fiber laser as a function of the frequency shift and optical bandwidth of the laser cavity. For this we develop a numerical model which is based on the rate equation and nonlinear Schrödinger equation for the fiber-based active cavity. We numerically verify that if the ratio of the filter bandwidth to the frequency-shift value is higher than ~400 times, the spectral broadening of the laser output tends to break up and form a secondary spectral band (SSB) on the wavelength side of the spectrum where the spectral components of the cavity modes are constantly shifted by the intracavity frequency shifter. We also verify that the SSB forms a satellite pulse in the time domain, traveling on either the trailing edge or leading edge of the main pulse, depending on whether the SSB is formed on the shorter or longer wavelength side of the pulse spectrum. We emphasize that these numerical results are also in good agreement with the experimental results discussed in our previous report [25].

Frequency selective surfaces filters to enhance performance of Ka band applications

ABSTRACTWe present circular‐loop and fan‐shaped periodic geometries for narrow and broad passband Ka band filters using frequency selective surfaces (FSS), respectively. Two narrowband designs are obtained having less than 0.5‐dB attenuation with nearly −20‐dB out‐of‐band rejection. A three‐layer broadband (30%) FSS is also presented with high transmission and excellent flat bandpass response that covers almost the entire Ka band. Experimental results are also discussed to validate both the single‐layer and three‐layer designs. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:563–568, 2014

Narrow-Linewidth and High-Transmission Terahertz Bandpass Filtering by Metallic Gratings

Accurate spectral resolving capability is important to THz spectroscopy. So far, the THz passband filters based on metamaterial and surface plasmon suffer from the broad passband. In this paper, we propose a THz filter with calculated pass band of $(\Delta f/f) together with a peak transmission above 70% and demonstrate the experimental results with $(\Delta f/f) limited by the measurement system. The calculated value of $\Delta f/f$ is at least one order of magnitude smaller than the previous results. The unique filtering capability was attributed to the guided-mode resonance that is generated by metallic gratings and a waveguide. Compared to the dielectric gratings, the metallic gratings with a narrow air slit are proved to effectively suppress off-resonance wavelength transmission resulting in a bandpass filtering response. In addition, a weak waveguide is found to benefit a narrow-linewidth filtering. Fabrication method of metallic gratings on membrane in a size of several ${\hbox{cm}}^{2}$ was developed and the experimental results show reasonably good agreement with the simulation for a filter with $(\Delta f/f) limited by the spectral resolution and detectability of our measurement system.

Variations in EUV Irradiance: Comparison between LYRA, ESP, and SWAP Integrated Flux

The Sun Watcher Using Active Pixel System Detector and Image Processing (SWAP) telescope and Large Yield Radiometer (LYRA) are the two Sun observation instruments on-board PROBA2. SWAP extreme ultraviolet images, if presented in terms of the integrated flux over solar disk, in general, correlate well with LYRA channel 2–4 (zirconium filter) and channels QD and 18 of EVE/ESP on-board SDO between 2010 and 2013. Hence, SWAP can be considered as an additional radiometric channel. We compare in detail LYRA channel 2–4 and SWAP integrated flux in July 2010 and in particular during the solar eclipse that occurred on July 11, 2010. During this eclipse, the discrepancy between the two data channels can be explained to be related to the occultation of active region 11087 by the Moon. In the second half of July 2010, LYRA channel 2–4 and SWAP integrated flux deviate from each other, but these differences can also be explained in terms of features appearing on the solar disk such as coronal holes and active regions. By additionally comparing with timeline of EVE/ESP, we can preliminarily interpret these differences in terms of the difference between the broad bandpass of LYRA channel 2–4 and the, relatively speaking, narrower bandpass of SWAP.

Bifunctional plasmonic metamaterials enabled by subwavelength nano-notches for broadband, polarization-independent enhanced optical transmission and passive beam-steering

In this work, we present the design, numerical experiments, and analysis of a plasmonic metamaterial thin film based on subwavelength nano-notch loaded modified fishnet structures. The resulting device offers a simultaneous bandpass filtering functionality with a broad enhanced optical transmission window and a gapless negative-zero-positive index transition to enable polarization-independent passive beam-steering. This unique characteristic is made possible by the introduced subwavelength nano-notches, which provide fine tuning and hybridization of the external and internal surface plasmon polariton modes. This allows tailoring of the dispersive properties of the plasmonic metamaterial for broadband operation. Specifically, a multilayer nanostructured modified fishnet with feature sizes accessible by modern nanofabrication techniques is presented, exhibiting a broad passband at the mid-infrared wavelengths from 3.0 to 3.7 µm and stopbands elsewhere in the 2.5 ~4.5 µm window. The transmittance normalized to area is around 3 dB within the broad 20% bandwidth of the passband. Additionally, the effective index undergoes a smooth transition from negative unity through zero to positive unity with low loss within the passband. The physical mechanism and the angular dispersion of the metamaterial are analyzed in detail. Finally, full-wave simulations of a prism formed from this metamaterial are performed to demonstrate that the proposed structure achieves simultaneous polarization-insensitive passive beam-steering and filtering functionalities.

Terahertz narrow bandstop, broad bandpass filter using double-layer S-shaped metamaterials

In this study, double-layer S-shaped metamaterials (MMs) are analyzed by terahertz time-domain spectroscopy. These materials exhibit narrow bandstop and broad bandpass transmission properties at both horizontal and vertical electric-field polarizations. A 117% increase in the unloaded quality factor is experimentally observed for these materials. The center frequency is approximately 0.45 THz, with a 3-dB bandwidth of 0.52 THz from 0.20 to 0.72 THz at normal incidence. The measured average insertion loss is 0.5 dB with a ripple of 1 dB. These results show that double-layer S-shaped MMs are effective in designing tunable terahertz devices.

Compact ultra‐wideband bandpass filters with narrow notched bands based on a ring resonator

In this study, compact ultra-wideband (UWB) bandpass filters with single/dual-notched bands are proposed based on a ring resonator and transversal signal-interference concept. Broad passband and narrow notched bands with controlled even/odd-mode resonator frequencies can be adjusted conveniently by changing the characteristic impedance of the ring resonator. Transmission zeros are introduced to improve selectivity and harmonic suppression of the proposed UWB bandpass filters with notched bands. To verify the presented strategies, three prototypes (er = 2.65, h = 0.5 mm) with 3 dB fractional bandwidth >120% are designed and fabricated. Simple structure and good in/out-of-band performances can be achieved in the proposed UWB bandpass filters.

Flux calibration of broad-band far-infrared and submillimetre photometric instruments: theory and application to Herschel-SPIRE

Photometric instruments operating at far infrared to millimetre wavelengths often have broad spectral passbands (central wavelength/bandwidth ~ 3 or less), especially those operating in space. A broad passband can result in significant variation of the beam profile and aperture efficiency across the passband, effects which thus far have not generally been taken into account in the flux calibration of such instruments. With absolute calibration uncertainties associated with the brightness of primary calibration standards now in the region of 5% or less, variation of the beam properties across the passband can be a significant contributor to the overall calibration accuracy for extended emission. We present a calibration framework which takes such variations into account for both antenna-coupled and absorber-coupled focal plane architectures. The scheme covers point source and extended source cases, and also the intermediate case of a semi-extended source profile. We apply the new method to the Herschel-SPIRE space-borne photometer.

Study of a mode-locked erbium-doped frequency-shifted-feedback fiber laser incorporating a broad bandpass filter: Experimental results

We present rigorous experimental studies on the spectral and temporal behaviors of an erbium-doped frequency-shifted-feedback fiber laser (FSFL), with respect to various parameters of the laser cavity, including the direction of the frequency-shift mechanism, the quantity of frequency-shift, and the output coupling ratio (OCR) of the cavity. We show that if the filter bandwidth is much broader than the laser linewidth, the laser spectrum tends to split and form a secondary spectral band (SSB) on the shorter or longer wavelength side of the primary spectrum, depending on whether the direction of the frequency-shift mechanism is upward or downward, respectively. We found that the SSB forms a parasitic pulse with much lower peak power traveling on the leading or trailing edge of the primary pulse, which leads to a significant asymmetry in the whole pulse formation in the time domain.

A bright thermonuclear X-ray burst simultaneously observed withChandraand RXTE

The prototypical accretion-powered millisecond pulsar SAX J1808.4-3658 was observed simultaneously with Chandra-LETGS and RXTE-PCA near the peak of a transient outburst in November 2011. A single thermonuclear (type-I) burst was detected, the brightest yet observed by Chandra from any source, and the second-brightest observed by RXTE. We found no evidence for discrete spectral features during the burst; absorption edges have been predicted to be present in such bursts, but may require a greater degree of photospheric expansion than the rather moderate expansion seen in this event (a factor of a few). These observations provide a unique data set to study an X-ray burst over a broad bandpass and at high spectral resolution (lambda/delta-lambda=200-400). We find a significant excess of photons at high and low energies compared to the standard black body spectrum. This excess is well described by a 20-fold increase of the persistent flux during the burst. We speculate that this results from burst photons being scattered in the accretion disk corona. These and other recent observations of X-ray bursts point out the need for detailed theoretical modeling of the radiative and hydrodynamical interaction between thermonuclear X-ray bursts and accretion disks.

ASWIFTSURVEY OF ACCRETION ONTO STELLAR-MASS BLACK HOLES

We present a systemic analysis of all of the stellar mass black hole binaries (confirmed & candidate) observed by the Swift observatory up to June 2010. The broad Swift bandpass enables a trace of disk evolution over an unprecedented range in flux and temperature. The final data sample consists of 476 X-ray spectra containing greater than 100 counts, in the 0.6 -- 10 keV band. This is the largest sample of high quality CCD spectra of accreting black holes published to date. In addition, strictly simultaneous data at optical/UV wavelengths are available for 255 (54%) of these observations. The data are modelled with a combination of an accretion disk and a hard spectral component. For the hard component we consider both a simple power-law and a thermal Comptonization model. An accretion disk is detected at greater than the 5sigma confidence level in 61% of the observations. Lightcurves and color-color diagrams are constructed for each system. Hardness luminosity and disk fraction luminosity diagrams are constructed and are observed to be consistent with those typically observed by RXTE, noting the sensitivity below 2 keV provided by Swift. The observed spectra have an average luminosity of ~ 1% Eddington, though we are sensitive to accretion disks down to a luminosity of 10^{-3} L_Edd. Thus this is also the largest sample of such cool accretion disks studied to date. (abridged)

Tailoring Dispersion for Broadband Low-loss Optical Metamaterials Using Deep-subwavelength Inclusions

Metamaterials have the potential to create optical devices with new and diverse functionalities based on novel wave phenomena. Most practical optical systems require that the device properties be tightly controlled over a broad wavelength range. However, optical metamaterials are inherently dispersive, which limits operational bandwidths and leads to high absorption losses. Here, we show that deep-subwavelength inclusions can controllably tailor the dispersive properties of an established metamaterial structure thereby producing a broadband low-loss optical device with a desired response. We experimentally verify this by optimizing an array of nano-notch inclusions, which perturb the mode patterns and strength of the primary and secondary fishnet nanostructure resonances and give an optically thin mid-wave-infrared filter with a broad transmissive pass-band and near-constant group delay. This work outlines a powerful new strategy for realizing a wide range of broadband optical devices that exploit the unique properties of metamaterials.

Design of Wide Passband/Stopband Microstrip Bandpass Filters With the Stepped Coupled Line

Novel wide passband microstrip bandpass filters with broader stopbands are provided in this paper. The proposed bandpass filters are composed of the stepped coupled line and two short-circuited stubs. The electrical length of the stepped coupled line is a quarter of wavelength so that the proposed filter is compact size. Moreover, the stepped coupled line with weak-strong-weak coupling results in a wide stopband. Furthermore, with short-circuited stubs connected at the input/output port, there is a steeper passband skirt and broader passband. In addition, the agreement between theoretical prediction and measurement validates our proposed structure.

One-dimensional broadband phononic crystal filter with unit cells made of two non-uniform impedance-mirrored elements

A one-dimensional finite-sized phononic crystal(PC) made of a specially-configured unit cell is proposed to realize broad bandpass, high-performance filtering. The unit cell is specially-configured with two elements having mirrored impedance distributions of each other. One element has a non-uniform impedance distribution that is so engineered as to maximize wave transmission in the pass band and to minimize transmission in the adjacent stop band while the other, exactly the mirrored distribution. The mirroring approach naturally yields the overall impedance contrast within the resulting unit cell, necessary to form stop bands in a PC of the unit cells. More importantly, the good transmission performance of the orginally-engineered element can be preserved by the approach because no additional impedance mismatch is introduced along the interface of the two impedance-mirrored elements. Extraordinary performance of the PC filter made of the proposed unit cell, such as high transmission, large bandwidth and sharp roll-off, is demonstrated by using one-dimensional longitudinal elastic wave problems.

TheXMM-Newtonslew survey in the 2–10 keV band

The XMM-Newton Slew Survey (XSS) covers a significant fraction of the sky in a broad X-ray bandpass. Although shallow by contemporary standards, in the `classical' 2-10 keV band of X-ray astronomy, the XSS provides significantly better sensitivity than any currently available all-sky survey. We investigate the source content of the XSS, focussing on detections in the 2-10 keV band down to a very low threshold (> 4 counts net of background). At the faint end, the survey reaches a flux sensitivity of roughly 3e-12 erg/cm2/s (2-10 keV). Our starting point was a sample of 487 sources detected in the XMMSL1d2 XSS at high galactic latitude in the hard band. Through cross-correlation with published source catalogues from surveys spanning the electromagnetic spectrum from radio to gamma-rays, we find that 45% of the sources have likely identifications with normal/active galaxies, 18% are associated with other classes of X-ray object (nearby coronally active stars, accreting binaries, clusters of galaxies), leaving 37% of the XSS sources with no current identification. We go on to define an XSS extragalactic hard band sample comprised of 219 galaxies and active galaxies. We investigate the properties of this extragalactic sample including its X-ray logN-logS distribution. We find that in the low-count limit, the XSS is strongly affected by Eddington bias. There is also a very strong bias in the XSS against the detection of extended sources, most notably clusters of galaxies. A significant fraction of the detections at and around the low-count limit may be spurious. Nevertheless, it is possible to use the XSS to extract a reasonably robust sample of extragalactic sources, excluding galaxy clusters. The differential logN-logS relation of these extragalactic sources matches very well to the HEAO-1 A2 all-sky survey measurements at bright fluxes and to the 2XMM source counts at the faint end.

Design of a compact wide‐bandwidth band‐pass filter using a parallel‐coupled structure

AbstractIn this article, a broad bandpass filter (BPF) with a parallel‐coupled line (PCL) structure is proposed. In general, a BPF with a PCL structure has a limited bandwidth of 20%. A broader range of bandwidth than this requires a very narrow coupling‐gap that can hardly be realized. In this research, a new smaller‐sized broad‐BPF using a new type of PCL is presented. The new broad BPF shows a measured insertion loss of less than 0.31 dB and a return loss of better than 19.2 dB at the center frequency of 5.8 GHz with a 70% broad bandwidth. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2581–2584, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27157

Cosmic distances from surface brightness fluctuations

AbstractHigh spatial-resolution measurements of surface brightness fluctuations (SBFs) with the Hubble Space Telescope (HST) provide the most precise distances available to early-type galaxies beyond the Local Group. The observable SBF magnitude in a given bandpass is a basic property of any stellar system, corresponding to a ratio of the first and second moments of the stellar luminosity function. Calibration of the method has presented challenges, but we now have an excellent empirical determination of how the SBF observable varies with galaxy color in broad bandpasses at the red end of the optical spectrum, and we are working towards a similar calibration for HST's Wide-Field Camera 3 in the near-infrared wavelength range, where the SBF magnitudes are considerably brighter. From HST Advanced Camera for Surveys data, we have determined the relative distances of the Virgo and Fornax clusters to within a precision of 2%, and resolved their internal structures. More recent measurements allow us to tie the Coma cluster, the standard of comparison for distant cluster studies, to the same precise distance scale. The SBF method can be calibrated in an absolute sense either empirically using Cepheids or theoretically based on stellar population models. The agreement between model and empirical zero points provides an independent confirmation of the Cepheid distance scale.