Narrowband Sources Research Articles

Radiochromic film thickness correction with convergent cone- beam optical CT scanner

A cone-beam optical computed tomography (CT) scanner was modified by replacing the diffuse planar yellow light emitting diode (LED) source with violet and red LEDs and a large Fresnel lens. The narrow band sources provided transmission images of radiochromic EBT2 film at 420 and 633 nm, with air as a reference. The dose image was not detectable with the violet source. This demonstrated spectral independence of the two images. Assuming attenuation at 420 nm was dominated by absorption from yellow dye in the active film layer allowed a relative thickness image to be calculated. By scaling the 633 nm optical density image for relative thickness, non-uniformities in the recorded dose distribution due to film thickness variations, were removed.

Evaluation and Application of Dual-Reactor Chamber for Studying Atmospheric Oxidation Processes and Mechanisms

A new smog chamber with dual reactors was designed and constructed to study atmospheric oxidation processes that may form ozone or secondary organic aerosols (SOAs). The chamber consists of two 5 m(3) fluorinated ethylene propylene (FEP) Teflon-film reactors housed in a thermally isolated enclosure, in which the temperature can be well controlled in the range of -10 to 40 degrees C. The influence of the light source on the gas-phase oxidation mechanism of propene was investigated. The results showed that multiple ultraviolet (UV) light sources were better than traditional narrow-band black-lamp light sources. Preliminary experiments on propene and m-xylene photo-oxidation processes were performed. The results showed that the dual-reactor chamber can simulate the gas-phase oxidation processes that form ozone or SOAs, and can be used to determine the effects of various species by comparing experiments performed using different initial concentrations. The SOA yield data from m-xylene photo-oxidation under different NOx conditions were in good agreement with those from previous studies. This proves that the chamber can simulate gas-to-particle conversion processes. The dual reactors have the advantage of enabling experiments to be performed with only one key parameter being changed. This will help us to further understand the role of key factors in complex atmospheric pollution processes.

DOA Estimation for Mixed Uncorrelated and Coherent Sources in Multipath Environment

A novel direction-of-arrival (DOA) estimation method is proposed to cope with the scenario where a number of uncorrelated and coherent narrowband sources simultaneously impinge on the far-field of a uniform linear array (ULA). In the proposed method, the DOAs of uncorrelated sources are firstly estimated by utilizing the property of the moduli of eigenvalues of the DOA matrix. Afterwards, the contributions of uncorrelated sources and the interference of noise are eliminated completely by exploiting the improved spatial differencing technique and only the coherent components remain in the spatial differencing matrix. Finally, the remaining coherent sources can be resolved by performing the improved spatial smoothing scheme on the spatial differencing matrix. The presented method can resolve more number of sources than that of the array elements and distinguish the uncorrelated and coherent sources that come from the same direction as well as improving the estimation performance. Simulation results demonstrate the effectiveness and efficiency of the proposed method.

Generation of quasiequally spaced ultrashort microbunches in a photocathode rf gun

A photocathode rf gun can generate trains of THz subpicosecond electron bunches by illuminating the cathode with trains of laser pulses, but it suffers from the increasing charge in the beam. The THz structure blurs and tends to disappear when the longitudinal space charge forces begin to play a significant role in the beam evolution. In this paper, we propose a scheme to restrain the space charge forces by expanding the transverse size of the laser pulses to reduce the charge density and adopting a multicell gun to increase the beam energy. Thus, quasiequally spaced ultrashort microbunches with relatively high charges can be generated according to our studies. Postacceleration can be used to freeze the longitudinal phase space dynamics. The proposed scheme is in principle able to generate intense multi-color narrow-band THz radiation and offers a promising way towards the tunable intense narrow-band THz sources.

True color scanning laser ophthalmoscopy and optical coherence tomography handheld probe.

Scanning laser ophthalmoscopes (SLOs) are able to achieve superior contrast and axial sectioning capability compared to fundus photography. However, SLOs typically use monochromatic illumination and are thus unable to extract color information of the retina. Previous color SLO imaging techniques utilized multiple lasers or narrow band sources for illumination, which allowed for multiple color but not "true color" imaging as done in fundus photography. We describe the first "true color" SLO, handheld color SLO, and combined color SLO integrated with a spectral domain optical coherence tomography (OCT) system. To achieve accurate color imaging, the SLO was calibrated with a color test target and utilized an achromatizing lens when imaging the retina to correct for the eye's longitudinal chromatic aberration. Color SLO and OCT images from volunteers were then acquired simultaneously with a combined power under the ANSI limit. Images from this system were then compared with those from commercially available SLOs featuring multiple narrow-band color imaging.

Broad-band sources of single-photon pulses, based on spontaneous parametric scattering in nonlinear impurity crystals

This paper discusses the possibilities of using spontaneous parametric scattering (SPS) in impurity crystals with a periodic domain structure for the efficient generation of narrow-band single-photon pulses. Using a periodically poled potassium titanyl phosphate crystal doped with trivalent erbium ions as an example, it is shown that, in the case of a nonlinear crystal with a periodic domain structure, allowing the generation of photons on the opposite sides, the width of the SPS spectrum can be less than the free spectral zone of a single-cavity parametric generator based on this crystal. Such an SPS regime can be useful when creating narrow-band sources of single-photon states that can be recorded and reproduced in optical quantum-memory devices, as well as for combining SPS processes and quantum memory in a single medium—a nonlinear impurity crystal.

Time-stretch microscopy based on time-wavelength sequence reconstruction from wideband incoherent source

Time-stretch microscopy has emerged as an ultrafast optical imaging concept offering the unprecedented combination of the imaging speed and sensitivity. However, dedicated wideband and coherence optical pulse source with high shot-to-shot stability has been mandated for time-wavelength mapping—the enabling process for ultrahigh speed wavelength-encoded image retrieval. From the practical point of view, exploiting methods to relax the stringent requirements (e.g., temporal stability and coherence) for the source of time-stretch microscopy is thus of great value. In this paper, we demonstrated time-stretch microscopy by reconstructing the time-wavelength mapping sequence from a wideband incoherent source. Utilizing the time-lens focusing mechanism mediated by a narrow-band pulse source, this approach allows generation of a wideband incoherent source, with the spectral efficiency enhanced by a factor of 18. As a proof-of-principle demonstration, time-stretch imaging with the scan rate as high as MHz and diffraction-limited resolution is achieved based on the wideband incoherent source. We note that the concept of time-wavelength sequence reconstruction from wideband incoherent source can also be generalized to any high-speed optical real-time measurements, where wavelength is acted as the information carrier.

33 W quasi-continuous-wave narrow-band sodium D2a laser by sum-frequency generation in LBO

We demonstrate an all-solid quasi-continuous-wave (QCW) narrow-band source tunable to sodium D2a line at 589.159 nm. The source is based on sum-frequency mixing between lasers at 1064 nm and 1319 nm in a LBO crystal. The 1064 nm and 1319 nm lasers are produced from two diode side-pumped Nd:YAG master oscillator power amplifier (MOPA) laser systems, respectively. A 33 W output of 589 nm laser is obtained with beam quality factor M2 = 1.25, frequency stability better than ±0.2 GHz and linewidth less than 0.44 GHz. A prototype 589 nm laser system is assembled, and a sodium laser guided star has been successfully observed in the field test.

Color difference amplification between gold nanoparticles in colorimetric analysis with actively controlled multiband illumination.

Spectral chemical sensing with digital color analysis by using consumer imaging devices could potentially revolutionize personalized healthcare. However, samples with small spectral variations often cannot be differentiated in color due to the nonlinearity of color appearance. In this study, we address this problem by exploiting the color image formation mechanism in digital photography. A close examination of the color image processing pipeline emphasizes that although the color can be represented digitally, it is still a reproducible subjective perception rather than a measurable physical property. That makes it possible to physically manage the color appearance of a nonradiative specimen through engineered illumination. By using scattering light imaging of gold nanoparticles (GNPs) as a model system, we demonstrated via simulation that enlarged color difference between spectrally close samples could be achieved with actively controlled illumination of multiple narrow-band light sources. Experimentally, darkfield imaging results indicate that color separation of single GNPs with various sizes can be significantly improved and the detection limit of GNP aggregation-based colorimetric assays can be much reduced when the conventional spectrally continuous white light was replaced with three independently intensity-controlled laser beams, even though the laser lines were uncorrelated with the LSPR maxima of the GNPs. With low-cost narrow-band light sources widely available today, this actively controlled illumination strategy could be utilized to replace the spectrometer in many spectral sensing applications.

A Compact and Low-Cost Optical Pickup Head-Based Optical Microscope

An optical pickup head (OPH) is a compact and low-cost but of high-precision optical system. In the past, it has been used for much scientific research besides optical disc storage, in particular, for the length-related measurements. In this paper, we propose a compact and low-cost OPH-based optical microscope (OM). A CMOS sensor disassembled from the web camera is fixed below the fold mirror of the OPH. The revised structure is simple and low cost but without permanently altering the original structure of the OPH. The user can choose operation modes of either the OPH or OM by a simple switching. We estimated that the optical magnification of the OM is 15.5X. The experimentally measured resolution of the OM is ~2 μm for the visible light, and it achieved diffraction limit for narrow-band light source. From the results of optical simulation, the main factor deteriorating the resolution is chromatic aberration. Astigmatic aberration is not important. Some plant cells investigation results using the microscope are also demonstrated in this paper.

Out-of-Band Measurement of LED-based Solar Blind UV Filters

Due to the difficulty in measuring very low out-of-band cutoff depths of solar blind UV filters, we propose a cutoff depth adjustable measurement system (CDAM) to test deep cutoff filters with a large dynamic range. The CDAM utilizing the substitution method is elaborately composed of several parts, including narrow-band LED light sources, standard reflective neutral attenuators with known attenuation coefficients, and a photomultiplier (PMT). This paper also presents an attenuator combination method ensuring that the PMT works within its linear response range. In addition, numerical simulation testifies to the method, and experiment shows that the CDAM system can achieve an extension of dynamic range from 0-6 OD to 0-10 OD, which is sufficient for the measurement of out-of-band cutoff depths of solar blind UV filters. Above all, the CDAM system, being easily implemented, of wide dynamic range, and highly precise, could be widely used in the measurement of filter cutoff depth.

A Grid-Less Approach to Underdetermined Direction of Arrival Estimation Via Low Rank Matrix Denoising

The problem of direction of arrival (DOA) estimation of narrowband sources using an antenna array is considered where the number of sources can potentially exceed the number of sensors. In earlier works, the authors showed that using a suitable antenna geometry, such as the nested and coprime arrays, it is possible to localize O(M2) sources using M sensors. To this end, two different approaches have been proposed. One is based on an extension of subspace based methods such as MUSIC to these sparse arrays, and the other employs l1 norm minimization based sparse estimation techniques by assuming an underlying grid. While the former requires the knowledge of number of sources, the latter suffers from basis mismatch effects. In this letter, a new approach is proposed which overcomes both these weaknesses. The method is hybrid in nature, using a low rank matrix denoising approach followed by a MUSIC-like subspace method to estimate the DOAs. The number of sources is revealed as a by-product of the low rank denoising stage. Moreover, it does not assume any underlying grid and thereby does not suffer from basis mismatch. Numerical examples validate the effectiveness of the proposed method when compared against existing techniques.

CRB-Based Design of Linear Antenna Arrays for Near-Field Source Localization

This paper is devoted to the Cramer Rao bound (CRB) on the angle and range of a narrow-band near-field source localized by means of an arbitrary linear array using the exact expression of the time delay parameter. First, we prove that the conditional and unconditional CRBs are proportional for an arbitrary parametrization of the steering vector (that may include, but is not limited to, the source DOA, range or polarization). Then, a Taylor expansion of the CRB is conducted to obtain accurate nonmatrix closed-form expressions of the CRB on angle and range. In contrast to the existing expressions, our expressions are simple, interpretable and more general because the sensors are only constrained to be placed along some axis. Our analysis leads to the characterization and design of a class of centro-symmetric linear arrays with improved near-field angle and range estimation capabilities.

Energy-Efficient Time-of-Flight Estimation in the Presence of Outliers: A Machine Learning Approach

The time-of-flight (ToF) estimation problem is common in sonar, ultrasound, radar, and other remote sensing applications. The conventional ToF maximum-likelihood estimator (MLE) exhibits a rapid deterioration in the accuracy when the signal-to-noise ratio (SNR) falls below a certain threshold. This threshold effect emerges mostly due to appearance of outliers associated with the side lobes in the autocorrelation function of a narrowband source signal. In our previous work, we have introduced a bank of unmatched filters and biased ToF estimators derived using these filters. These biased estimators form a feature vector for training a classifier which, subsequently, is used for reducing the bias and the variance parts induced by outliers in the mean-square error (MSE) of the MLE. In this paper, we extend the above method by introducing an adaptive scheme for controlling the number of measurements (pulses) required to achieve a desired accuracy. We show that using the information provided by a classifier, it is possible to achieve the estimation error of the MLE but by using significantly less number of pulses and thus energy on average.

Ultraviolet continuous-wave laser source at 205 nm for hydrogen spectroscopy

This paper reports on the generation of 15mW of continuous narrow-band laser source at 205nm. The infra-red light source provided by a Titanium-Sapphire (TiSa) laser is mixed with the fourth harmonic of a Nd:YVO4 laser by the use of a β-barium borate (BBO) non-linear crystal. This highly reliable and powerful ultraviolet (UV) source is an ideal tool for the 1S–3S hydrogen spectroscopy. Moreover, the wide tunability of the TiSa laser combined with this experimental set up makes the generation of bright deep ultra-violet (D-UV) sources possible. In particular, we plan to produce a 194nm continuous light beam which is necessary to perform the 1S–4S transition in hydrogen.

Carboxylic Group and Its Tetrazolyl Isostere in One Molecule. Matrix Isolation FTIR and DFT Studies on Thermal Decomposition and Photochemistry of (Tetrazol-5-yl)acetic Acid

The title compound (tetrazol-5-yl)acetic acid (TA) is an interesting molecule that contains both a carboxylic group and its isostere tetrazolyl group. Out of nine theoretically predicted stable structures of TA, three appeared to be present in solid argon. Thermal decomposition of the species aided by water molecules was studied in detail both experimentally using FTIR matrix isolation technique and theoretically at the B3LYP/6-311++G(2d,2p) level. Experimentally, it was found that the decarboxylation process appeared at the presence of water traces in the system. Theoretically, it was shown that the energy barrier of the water assisted process was lower by ca. 30 kJ mol(-1) comparing to the process without water participation. The UV photolysis of the TA/Ar system was studied using both broad-band and narrow-band sources. The main photoproducts appeared to be carbodiimidylacetic acid and (1H-diaziren-3-yl)acetic acid. The progress of the reactions induced was followed by FTIR spectroscopy, whereas interpretation of the results was supported by quantum chemical calculations (DFT, TD-DFT).

Effects of incomplete modal sampling on the underwater target detetion and performance improvement method

The generalized likelihood ratio test (GLRT) detector and its theoretical detection performance for an underwater narrowband source with an unknown position are both given in this paper. Via the eigenvalue decomposition of the mode correlation matrix (MCM), the GLRT detector is decomposed into different spectrum components corresponding to the eigenvalues of the MCM. Based on the derived statistical property of each component, the spatial processing gain of each spectrum component with respect to the input signal is obtained, which is proportional to the corresponding eigenvalue. As there are several eigenvalues of the MCM approaching to zero when the modal information is incompletely sampled, the components corresponding to these small eigenvalues contribute much less spatial processing gain to the input signal than other components. By discarding the components corresponding to these small eigenvalues, the effective spectrum detector (ESD) is proposed, of which the target signal component in the output is approximately identical to that of the GLRT detector, and the noise in the ESD output is much less. Therefore, a much more robust detection performance is obtained by ESD than by the GLRT detector. Numerical simulations in a typical shallow water environment demonstrate that 1) the theoretical analyses, derivations and the effectiveness of the proposed ESD are verified; 2) the more incomplete the modal information sampling is, the more significant performance improvement of ESD over the GLRT detector can be acquired; 3) the numerical stability of the ESD is better than that of the GLRT detector.

Dispersive element based on grating and tunable Fabry–Perot filter in miniature spectrometer

We present a new design for the integration of a tunable Fabry-Perot (FP) filter and the grating etched on top of the cavity (IGFP) in the miniature spectrometer. It is based on the predispersion of the grating with the capacity of spatial separation of the spectral component and filter effect of the tunable FP filter. The free spectral range (FSR) of the IGFP is determined by the FSR of the grating, and its resolution depends on the filtering capacity of the FP filter. In the experiment, the high-resolution and wavelength scanning process of the IGFP were demonstrated with a narrowband and broadband light source, respectively. The results of the sub-nanometer resolution agree well with those from a commercial optical spectrum analyzer. Further, the IGFP provides an effective approach to solve the problem of the decrease of spectral resolution in the miniaturization process.

Spectrally shaped broadband study of up-conversion in Y2O3:Er3+

We present a novel scheme for studying up-conversion through excited state absorption (ESA) by using a broadband excitation source with spectral shaping capabilities. Up-conversion processes have typically been investigated using a single, narrowband excitation source, when the two steps of the process are coincident in frequency, which is often made possible by broadening mechanisms of the intermediate excited state manifolds. Thus, narrowband sources are limited in the systems they can excite and what material information they can provide. With broadband light, we are able to drive up-conversion with non-coincident frequencies as well. Finally, by windowing the spectrum, we determine the optimal excitation bandwidth for low-concentration (1%) Y2O3:Er3+ nanocrystals.

Scaling laws of the cavity enhancement for nitrogen-vacancy centers in diamond

We employ a fiber-based optical microcavity with high finesse to study the enhancement of phonon sideband fluorescence of nitrogen-vacancy centers in nanodiamonds. Harnessing the full tunability and open access of the resonator, we explicitly demonstrate the scaling laws of the Purcell enhancement by varying both the mode volume and the quality factor over a large range. While changes in the emission lifetime remain small in the regime of a broadband emitter, we observe an increase of the emission spectral density by up to a factor of 300. This gives a direct measure of the Purcell factor that could be achieved with this resonator and an emitter whose linewidth is narrower than the cavity linewidth. Our results show a method for the realization of wavelength-tunable narrow-band single-photon sources and demonstrate a system that has the potential to reach the strong-coupling regime.