Spectral airglow temperature imager (SATI): a ground-based instrument for the monitoring of mesosphere temperature

The spectral airglow temperature imager is a two-channel, Fabry-Perot spectrometer with an annular field of view and a cooled CCD detector. The detected fringe pattern contains spectral information in the radial direction and azimuthal spatial information from the annular field of view. The instrument measures the rotational temperature from the O2 atmospheric (0,1) nightglow emission layer at 94 km and from the Q branch of the OH Meinel (6,2) band emission layer at 87 km. The method for temperature derivation is based on the temperature dependence of the line-emission rates. This dependence allows a determination of the temperature by a least-squares fit of the measured spectrum to a set of synthetic spectra, an approach that minimizes the effect of noise from the sky background and the detector. The spectral airglow temperature imager was developed to meet a need for monitoring the role of the mesosphere in climate variability through long-term observation of the mean temperature and the gravity waves from a single station, as well as large-scale wave perturbations through the use of multiple stations.     

Improved multiple-pass Raman spectrometer

An improved Raman gain spectrometer for flame measurements of gas temperature and species concentrations is described. This instrument uses a multiple-pass optical cell to enhance the incident light intensity in the measurement volume. The Raman signal is 83 times larger than from a single pass, and the Raman signal-to-noise ratio (SNR) in room-temperature air of 153 is an improvement over that from a single-pass cell by a factor of 9.3 when the cell is operated with 100 passes and the signal is integrated over 20 laser shots. The SNR improvement with the multipass cell is even higher for flame measurements at atmospheric pressure, because detector readout noise is more significant for single-pass measurements when the gas density is lower. Raman scattering is collected and dispersed in a spectrograph with a transmission grating and recorded with a fast gated CCD array detector to help eliminate flame interferences. The instrument is used to record spontaneous Raman spectra from N(2), CO(2), O(2), and CO in a methane-air flame. Curve fits of the recorded Raman spectra to detailed simulations of nitrogen spectra are used to determine the flame temperature from the shapes of the spectral signatures and from the ratio of the total intensities of the Stokes and anti-Stokes signals. The temperatures measured are in good agreement with radiation-corrected thermocouple measurements for a range of equivalence ratios.     

光栅激光告警系统中信号光与天空光的能量分析

为了在天空背景下实时测量宽波段脉冲激光波长和方向,提出了使用闪耀光栅衍射测量脉冲激光波长和方向的方法.用傅立叶变换分析天空背景和脉冲激光的光谱分布.通过对透射率函数的傅立叶级数展开,得到光栅衍射谱线的强度,对信号光谱线的最小强度、背景光谱线的最大强度和噪声强度的比较,得到设计的可行性.光路主要由特制光栅、柱面镜以及CCD探测器构成.     

High-resolution tunable spectrograph for x-ray laser linewidth measurements with a plane varied-line-spacing grating

We describe a spectrograph for x-ray laser linewidth measurements in the range 100-220 A. The design employs a plane varied-line-spacing grating operating in the convergent light produced by imaging of the entrance slit with a concave spherical mirror. By the appropriate choice of the linear term in the grating-spacing variation, two separate wavelengths can be focused at the same image distance. As a result all wavelengths within the range of interest are focused at or near the same distance. The spectrograph can be tuned by rotation of the grating to bring any wavelength within the range to the center of the focal plane, and the spectra are dispersed on a surface that is erect or practically flat and perpendicular to the principal ray. This allows the use of a planar detector. With a streak camera used as a detector, the instrument obtained time-resolved linewidth data on x-ray lasers with a resolving power of 1 x 10(4) to 2 x 10(4). This paper presents the design methods used to optimize the varied-line-spacing grating, the design of the tunable spectrograph, and the results from the instrument in operation.     

Far-infrared sensor for cirrus (FIRSC): an aircraft-based Fourier-transform spectrometer to measure cloud radiance

We describe an aircraft-based Fourier-transform spectrometer (FTS) designed to measure the Earth outgoing radiance spectrum in the far-infrared-submillimeter spectral range. The instrument features include a rapid-scan FTS to obtain high spatial resolution from a moving aircraft platform, a sensitive two-channel detector, and a CCD camera for recording the nadir cloud scene with each scan record. Such measurements demonstrate the sensitivity of Earth radiance to high clouds and provide spectral data for improving techniques for remote sensing and retrieval of atmospheric and cloud properties.     

Properties of DMDs for holographic displays

Digital micromirror device’s (DMD) properties as being a display device for holographic displays are investigated. High speed, a large separation between reconstructed image and reconstruction beam, two symmetric diffraction patterns, and low intensity (0,0)th-order beam at a blazed grating condition are the desired properties for the displays. The blazed grating condition of a DMD can reconstruct images with higher diffraction efficiency than the line grating condition. DMD’s high speed enables to present colors and gray levels to the reconstructed image. However, reconstructed images from a gray-level computer-generated hologram (CGH) and its binary form hologram reveal no noticeable difference between them, except the background noise in the image from the CGH.     

Correction of instrument line shape in Fourier transform spectrometry using matrix inversion

A novel matrix inversion approach is proposed to correct several contributions to the instrument line shape (ILS) of a Fourier transform spectrometer. The matrix formalism for the ILS is first quickly reviewed. Formal inversion of the ILS matrix is next discussed, along with its limitations. The stability of the inversion process for large field-of view- (FOV-) limited and highly off-axis line shapes is investigated. The effect of inversion on the noise that is present in the spectrum is also presented. Use of classical iterative inversion methods, coupled with efficient synthesis algorithms, is proposed as a way to drastically speed up the inversion process. The method is applied to correct HBr spectra obtained from a laboratory spectrometer that has an adjustable field of view. ILSS from six FOVs are brought to the same spectral axis and to the same ideal sinc shape.     

The effect of AM noise on correlation phase-noise measurements

We analyze the phase-noise measurement methods in which correlation and averaging is used to reject the background noise of the instrument. All the known methods make use of a mixer, used either as a saturated-phase detector or as a linear-synchronous detector. Unifortunately, AM noise is taken in through the power-to-dc-offset conversion mechanism that results from the mixer asymmetry. The measurement of some mixers indicates that the unwanted amplitude-to-voltage gain is of the order of 5-50 mV, which is 12-35 dB lower than the phase-to-voltage gain of the mixer. In addition, the trick of setting the mixer at a sweet point--off the quadrature condition--where the sensitivity to AM nulls, works only with microwave mixers. The HF-VHF mixers do not have this sweet point. Moreover, we prove that if the AM noise comes from the oscillator under test, it cannot be rejected by correlation. At least not with the schemes currently used. An example shows that at some critical frequencies the unwanted effect of AM noise is of the same order-if not greater--than the phase noise. Thus, experimental mistakes are around the corner.     

The influence of disturbing effects on the performance of a wide field coded mask X-ray camera

The coded aperture telescope, or Dicke camera, is seen as an instrument suitable for many applications in X-ray and gamma ray imaging. In this paper the effects of a partially obscuring window mask support or collimator, a detector with limited spatial resolution, and motion of the camera during image integration are considered using a computer simulation of the performance of such a camera. Cross correlation and the Wiener filter are used to deconvolve the data. It is shown that while these effects cause a degradation in performance this is in no case catastrophic. Deterioration of the image is shown to be greatest where strong sources are present in the field of view and is quite small ( 10%) when diffuse background is the major element. A comparison between the cyclic mask camera and the single mask camera is made under various conditions and it is shown the single mask camera has a moderate advantage particularly when imaging a wide field of view.     

Improving the response of a wheel speed sensor by using frequency-domain adaptive filtering

In this paper, a frequency-domain least-mean-square adaptive filter is used to cancel noise in a wheel speed sensor embedded in a car under performance tests. In this case the relevant signal is buried in a broad-band noise background, where we have little or no prior knowledge of the signal or noise characteristics. The results of the experiments show that the signal of interest and the noise (all forms of interference, deterministic, as well as stochastic) share the same frequency band and that the filter used significantly reduced the noise corrupting the information from the sensor while it left the true signal unchanged from a practical point of view. In this paper, a signal-to-noise ratio improvement higher than 40 dB is achieved. The results of the experiment show the importance of using digital signal processing when dealing with a signal corrupted by noise.     

Signal-to-noise ratio trade-offs associated with coarsely sampled Fourier transform spectroscopy

We derive the spectral signal-to-noise ratio (SNR) trade-offs associated with coarsely sampled Fourier transform spectroscopy using a step-and-integrate measurement scheme. We show that there is no SNR penalty in the shot noise limit and a slight SNR benefit in the detector noise limit for the case of coarse sampling to achieve the same spectral resolution as a baseline Nyquist sampling scenario, where the total detector integration time is the same for both sampling cases.     

Dispersion multiplexing with broadband filtering for miniature spectrometers

We replace the traditional grating used in a dispersive spectrometer with a multiplex holographic grating to increase the spectral range sensed by the instrument. The multiplexed grating allows us to measure three different, overlapping spectral bands on a color digital focal plane. The detector's broadband color filters, along with a computational inversion algorithm, let us disambiguate measurements made from the three bands. The overlapping spectral bands allow us to measure a greater spectral bandwidth than a traditional spectrometer with the same sized detector. Additionally, our spectrometer uses a static coded aperture mask in the place of a slit. The aperture mask allows increased light throughput, offsetting the photon loss at the broadband filters. We present our proof-of-concept dispersion multiplexing spectrometer design with experimental measurements to verify its operation.     

强噪声背景下的微弱信号检测新方法

提出了新的强噪声背景下的微弱信号检测方法,设计了一种硬件与软件相结合的实现方案。采用经典的仪表放大技术和单片机控制技术对数据进行检测和处理,并通过虚拟仪器技术仿真和显示,为解决热释电红外探测器中µA级微弱信号的检测提供了十分有效的方法。该系统通过实验对模拟低频微弱信号的检测,充分显示了它在微弱信号检测方面的实用性和优越性。     

Inverse problem approach in particle digital holography: out-of-field particle detection made possible

We propose a microparticle detection scheme in digital holography. In our inverse problem approach, we estimate the optimal particles set that best models the observed hologram image. Such a method can deal with data that have missing pixels. By considering the camera as a truncated version of a wider sensor, it becomes possible to detect particles even out of the camera field of view. We tested the performance of our algorithm against simulated and experimental data for diluted particle conditions. With real data, our algorithm can detect particles far from the detector edges in a working area as large as 16 times the camera field of view. A study based on simulated data shows that, compared with classical methods, our algorithm greatly improves the precision of the estimated particle positions and radii. This precision does not depend on the particle's size or location (i.e., whether inside or outside the detector field of view).     

Throughput of tilted interferometers

The throughput of a tilted Fourier-transform spectrometer (FTS) with collimation is calculated. It is shown that the maximum off-axis angle that is acceptable in the interferometer is inversely proportional to the distance between the detector and the location where the tilt is applied to the wave fronts and is also inversely proportional to the tilt angle. This effect leads to tilt sensitivity in a scanning FTS and to the loss of the throughput advantage in a FTS with no moving part in which a tilt between two collimated beams is used to disperse the interferogram spatially. Experimental verification confirms the throughput condition with tilt angle.     

Detection and false-alarm probabilities for laser radars that use Geiger-mode detectors

For a direct-detection laser radar that uses a Geiger-mode detector, theory shows that the single-pulse detection probability is reduced by a factor exp(-K), where K is the mean number of primary electrons created by noise in the interval t between detector turn-on and arrival of laser photons reflected from the target. The corresponding false-alarm probability is at least 1 - exp(-K). For fixed-rate noise, one can improve the detection and false-alarm probabilities by reducing t. Moreover, when background-light noise is significant and dominates dark-current noise and when the laser signal is of the order of ten photoelectrons or more, the probabilities can be improved by reducing the amount of light falling on the detector, even if the laser signal is reduced by the same factor as the background light is. Additional analytical calculations show that identifying coincidences in data from as few as three pulses canreduce the false-alarm probability by orders of magnitude and, for some conditions, can also improve the detection probability.     

Characterization of a submillimeter high-angular-resolution camera with a monolithic silicon bolometer array for the Caltech Submillimeter Observatory

We constructed a 24-pixel bolometer camera operating in the 350- and 450-μm atmospheric windows for the Caltech Submillimeter Observatory (CSO). This instrument uses a monolithic silicon bolometer array that is cooled to approximately 300 mK by a single-shot (3)He refrigerator. First-stage amplification is provided by field-effect transistors at approximately 130 K. The sky is imaged onto the bolometer array by means of several mirrors outside the Dewar and a cold off-axis elliptical mirror inside the cryostat. The beam is defined by cold aperture and field stops, which eliminates the need for any condensing horns. We describe the instrument, present measurements of the physical properties of the bolometer array, describe the performance of the electronics and the data-acquisition system, and demonstrate the sensitivity of the instrument operating at the observatory. Approximate detector noise at 350 μm is 5 × 10(-15) W/√Hz, referenced to the entrance of the Dewar, and the CSO system noise-equivalent flux density is approximately 4 Jy/√Hz. These values are within a factor of 2.5 of the background limit.     

Structured light spots projected by a Dammann grating with high power efficiency and uniformity for optical sorting

We report a method for microfluidic multiple trapping and continuous sorting of microparticles using an optical potential landscape projected by a Dammann grating, enabling a high power-efficient approach to forming a composite two-dimensional spots array with high uniformity. The Dammann grating is fabricated in a photoresist by optical lithography. It is employed to create an optical lattice for multiple optical trapping and sorting in a mixture of polymer particles (n = 1.59) and silica particles (n = 1.42) with the same diameters of 3.1 μm. In addition to the exponential selectivity by the projected optical landscapes, the proposed microfluidic sorting system has advantages in terms of high power efficiency and high uniformity due to the Dammann grating.     

Kinetic Inductance Phonon Sensors for the Cryogenic Dark Matter Search Experiment

An important challenge faced by phonon-mediated detectors for the next generation of dark matter detectors (>100 kg) is to instrument large target mass at low cost, while maintaining the large background suppression offered by the combination of phonons and ionization (or scintillation) measurement. Kinetic inductance phonon sensors, operating far below the superconducting transition temperature, offer an interesting solution to this scaling problem. They do not critically depend on the uniformity of T _c and their resonant-cavity readout is easy to multiplex. We are studying a microstrip (two parallel planes) transmission line architecture that may offer the additional advantage of a separation of functions: the main detector is just covered by an unpatterned aluminum film and the number of quasi-particles created in it by athermal phonons are sensed by a second film, which has been independently patterned and is mounted a few microns away from the detector. We present current results on the responsivity and noise of large area (∼33 mm²) microstrip kinetic inductance phonon sensors.