Michael P. Bristow

Use of water Raman emission to correct airborne laser fluorosensor data for effects of water optical attenuation.

Airborne laser fluorosensor measurements of fluorophore concentrations in surface waters are highly sensitive to interference from changes in optical attenuation. This interference can be eliminated by normalizing the fluorescence signal with the concurrent water Raman signal. In our application to remotely monitor chlorophyll a concentrations in fresh waters, the Raman-corrected chlorophyll a fluorescence measurements were found to be highly correlated with chlorophyll a ground truth data. Also, the reciprocal of the water Raman signal was seen to vary directly as the beam attenuation coefficient. It is suggested that this latter relationship be exploited as a means for remotely sensing changes in the optical attenuation of surface waters.

Airborne laser fluorosensor survey of the Columbia and Snake rivers: simultaneous measurements of chlorophyll, dissolved organics and optical attenuation

Abstract An airborne laser fluorosensor has been used to make concurrent, near-continuous (microscale) profiles of chlorophyll-a, dissolved organic carbon (DOC) and the beam attenuation coefficient along the Columbia and Snake rivers. Cross-reference between profiles is used to expose anomalies and explain ambiguities encountered in their interpretation. The unique potential for remote sensing of DOC concentrations is based on a strong correlation with the fluorescence of dissolved organic matter (DOM). Similarly, the reciprocal of the remotely sensed water Raman emission is shown to be a reliable predictor of the beam attenuation coefficient. Profiles of a 734 km segment of the lower Snake and Columbia rivers obtained during peak discharge revealed a number of interesting features. In particular, the chlorophyll-a fluorescence data suggested the existence of subsurface chlorophyll-a maxima in the reservoirs formed by the eight dams of the Columbia-Snake Inland Waterway, and the absence of variability in ...

Signal linearity, gain stability, and gating in photomultipliers: application to differential absorption lidars.

We present the results of a study that identifies a photomultiplier tube (PMT), divider networks, and gating circuitry for use in the current detection mode, in which the specific objectives were to hold variations in both signal gain over a 25-µs gate period and signal linearity up to 20 mA to less than ±0.1%. The study, aimed at optimizing the performance in a nadir-looking airborne UV differential absorption lidar, is sufficiently general to apply to other critical gated or pulsed PMT applications in which performance at the 0.1% level is required. Signal-induced gain increases peculiar to pulsed or gated signals from PMTs with BeCu dynodes that can have values between 1 and 10% over 25 µs were reduced to less than 0.1% by the use of a 2-in. (5.08-cm)diameter PMT (EMI 9214) with CsSb dynodes. Compliance with the linearity requirement was achieved for gated signals up to 8 mA at a current gain of ~10(7) with the EMI 9214 PMT controlled by a resistive divider network with an inverted taper, in which the linearity data showed no tendency toward overlinearity caused by either space charge effects or induced divider-network voltage changes.

Suppression of afterpulsing in photomultipliers by gating the photocathode

A number of gating schemes to minimize the long-term afterpulse signal in photomultipliers have been evaluated. Blocking the excitation pulse by gating the photocathode was found to reduce the gate-on afterpulse background by a factor of 230 over that for nongated operation. This afterpulse or signal-induced background (SIB), which is particularly troublesome in stratospheric lidar measurements, appears as a weak exponentially decaying signal extending into the millisecond region after the photomultiplier tube (PMT) is exposed to an intense submicrosecond optical pulse. Photocathode gating is not feasible in PMTs with semitransparent bialkali photocathodes because of their slow gate response time, but is easily implemented in PMTs with opaque bialkali or semitransparent multialkali (S-20) photocathodes that can be gated with nanosecond response. In those PMTs with semitransparent bialkali photocathodes, a gated (adjacent) focus grid (if available) also produces a significant reduction in the SIB.

Stokes shifted laser lines in KrF-pumped hydrogen : reduction of beam divergence by addition of helium

The beam divergences of the recollimated Stokes shifted lines of H(2) excited by a focused KrF laser are reduced by addition of helium.

Fluorescence of short wavelength cutoff filters

Although the light-induced fluorescence of colored filter glass is a well-known phenomenon, reliable spectral intensity data pertaining to the fluorescence of commercially available colored filters are scarce. The Corning Optical Glass Company and the Schott Optical Glass Company provide only qualitative information on the nature of the fluorescence of some of their glass filter products.Wyszecki and Stiles have compiled a list of colored glasses made by Chance Pilkington (U.K.), Corning (U.S.A.), and Schott (U.S.A. and W. Germany) which exhibit strong fluorescence in the visible part of the spectrum when excited at 365 nm. French and Pfeiffer and Porter have provided some data on the fluorescence of available filter glass. A study of colored filter glass fluorescence by Turner provides more detailed information on the uv, visible, and near-ir fluorescence for a selection of Corning glass filters. The purpose of this Letter is to provide data complementary to those already published, based on specific applications experienced by the author which might be useful to those involved in similar projects. The applications involve the detection of a weak Raman signal at 515 nm and a weak fluorescence signal at 685 nm in the presence of intense laser excitation at 440 nm. Because of the limited effectiveness of employing temporal, spatial, and polarization discrimination as means for rejecting the laser radiation in our application, a spectral discrimination approach was adopted using a combination of interference and absorption filters. In one

Lidar-signal compression by photomultiplier gain modulation: influence of detector nonlinearity

The application of photomultiplier gain modulation to the compression of wide-dynamic-range lidar signals is investigated in relation to the effect of the gain level on anode-signal linearity. Gain reduction is achieved by the coupling of modulation signals through either multidynode or focus-grid gating networks. This technique facilitates signal recovery and prevents detector nonlinearity and dynode damage caused by high near-field lidar signals. The measurements were performed in the current mode primarily on a 50-mm-diameter, 12-stage photomultiplier (EMI 9214) with a bialkali photocathode. With 3- or 4-dynode-based modulation made at a photomultiplier voltage of 1300 V and a gain of 1 x 10(7), signals of ~6 mA can be maintained at the 1% linearity limit from 100% to 0.2% modulation, corresponding to a 500-fold reduction in the lidar-signal dynamic range. A significant advantage to dynode modulation is that it preserves the shot-signal-to-noise ratio of the incoming signal, which is not true for focus-grid modulation or external predetection schemes such as controlled obscuration or Pockels-cell modulation that attenuate the as-yet unamplified signal.

Compensational three-wavelength differential-absorption lidar technique for reducing the influence of differential scattering on ozone-concentration measurements

A three-wavelength differential-absorption lidar (DIAL) technique for the ultraviolet spectral region is presented that reduces the influence of aerosol differential scattering on measured ozone-concentration profiles. The principal advantage of this approach is that, to a good first approximation, no correction for aerosol differential-extinction and backscattering effects are needed. Therefore, one avoids having to obtain an aerosol extinction-coefficient profile at a reference wavelength; nor does one have to invoke questionable assumptions regarding the spectral dependence of the aerosol total and backscatter coefficients.

Short- and long-term memory effects in intensified array detectors: influence on airborne laser fluorosensor measurements

Phosphorescence and thermoluminescence memory effects in the phosphors of image intensifiers have been examined as they relate to the performance of intensified optical multichannel analyzers. Although most users of these devices may not need to be concerned with or aware of these secondary effects, their influence on an airborne laser fluorosensor application is shown to be significant. The magnitude of these strongly coupled effects, which are very sensitive to experimental conditions, has been modeled for a specific application, and algorithms are presented that can be used to remove these effects from airborne measurements of laser-induced fluorescence spectra of aquatic and terrestrial targets. It is a straightforward matter to adopt these procedures to other situations involving different gating routines, repetition rates, and diode group sizes.

Nonlinear-approximation technique for determining vertical ozone-concentration profiles with a differential-absorption lidar

A new technique is presented for the retrieval of ozone-concentration profiles (O(3)) from backscattered signals obtained by a multiwavelength differential-absorption lidar (DIAL). The technique makes it possible to reduce erroneous local fluctuations induced in the ozone-concentration profiles by signal noise and other phenomena such as aerosol inhomogeneity. Before the O(3) profiles are derived, the dominant measurement errors are estimated and uncertainty boundaries for the measured profiles are established. The off- to on-line signal ratio is transformed into an intermediate function, and analytical approximations of the function are then determined. The separation of low- and high-frequency constituents of the measured ozone profile is made by the application of different approximation fits to appropriate intermediate functions. The low-frequency constituents are approximated with a low-order polynomial fit, whereas the high-frequency constituents are approximated with a trigonometric fit. The latter fit makes it possible to correct the measured O(3) profiles in zones of large ozone-concentration gradients where the low-order polynomial fit is found to be insufficient. Application of this technique to experimental data obtained in the lower troposphere shows that erroneous fluctuations induced in the ozone-concentration profile by signal noise and aerosol inhomogeneity undergo a significant reduction in comparison with the results from the conventional technique based on straightforward numerical differentiation.