A crossed-strip detector initially developed at UC Berkeley’s Space Sciences Laboratory has been demonstrated
as a laboratory benchtop instrument and is now in the process of being integrated by Los Alamos National
Laboratory into a portable, real-time, single-photon-counting camera system. The crossed-strip detector consists
of 32 anode strips along each of two axes sealed inside a vacuum tube behind a photocathode and a microchannelplate
stack. A photon incident on the photocathode produces a cloud of charge from the microchannel plates
that falls onto a portion of the 64 anode strips, producing a signal on a subset of channels along each axis and
requiring that all anode channels continually be analyzed simultaneously. To maximize the photon flux that
can be accepted by the sensor with minimal deadtime, the crossed-strip sensor has been combined with shortershaping-
time amplifiers and higher-rate digitizers than previously used. With the ultimate goal of reaching 100
million events per second, FPGA-implementable algorithms have been developed for the identification of pulses
on each anode channel and the determination of the pulses’ time and amplitude. From the pulse times and
amplitudes, a charge cloud can be reconstructed and the centroid determined to produce the time and position
of each incident photon. The data-analysis procedure will be discussed, measurements detailing the performance
of the camera system as it exists at this point will be presented, and the planned layout of the embedded camera
system hardware will be detailed.
We have implemented cross strip readout microchannel plate detectors in 18 mm active area format including open
face (UV/particle) and sealed tube (optical) configurations. These have been tested with a field programmable gate array
based parallel channel electronics for event encoding which can process high input event rates (> 5 MHz) with high
spatial resolution. Using small pore MCPs (6 μm) operated in a pair, we achieve gains of >5 x 10<sup>5</sup> which is sufficient to
provide spatial resolution of <35 μm FHWM, with self triggered event timing accuracy of ~2 ns for sealed tube optical
sensors. A peak quantum efficiency of ~19% at 500 nm has been achieved with SuperGenII photocathodes that have
response over the 400 nm to 900 nm range. Local area counting rates of up to >200 events/mcp pore sec<sup>-1</sup> have been
attained, along with image linearity and stability to better than 50 μm.
There exists a wealth of information in the scientific literature on the physical properties and device characterization
procedures for complementary metal oxide semiconductor (CMOS), charge coupled device (CCD) and avalanche
photodiode (APD) format detectors. Numerous papers and books have also treated photocathode operation in the
context of photomultiplier tube (PMT) operation for either non imaging applications or limited night vision capability.
However, much less information has been reported in the literature about the characterization procedures and properties
of photocathode detectors with novel cross delay line (XDL) anode structures. These allow one to detect single photons
and create images by recording space and time coordinate (X, Y & T) information. In this paper, we report on the
physical characteristics and performance of a cross delay line anode sensor with an enhanced near infrared wavelength
response photocathode and high dynamic range micro channel plate (MCP) gain (> 10<sup>6</sup> ) multiplier stage. Measurement
procedures and results including the device dark event rate (DER), pulse height distribution, quantum and electronic
device efficiency (QE & DQE) and spatial resolution per effective pixel region in a 25 mm sensor array are presented.
The overall knowledge and information obtained from XDL sensor characterization allow us to optimize device
performance and assess capability. These device performance properties and capabilities make XDL detectors ideal for
remote sensing field applications that require single photon detection, imaging, sub nano-second timing response, high
spatial resolution (10's of microns) and large effective image format.ÿ
Remote Ultra-Low Light Imaging detectors are photon limited detectors developed at Los Alamos National
Laboratories. RULLI detectors provide a very high degree of temporal resolution for the arrival times of detected photoevents,
but saturate at a photo-detection rate of about 10<sup>6</sup> photo-events per second. Rather than recording a conventional
image, such as output by a charged coupled device (CCD) camera, the RULLI detector outputs a data stream consisting
of the two-dimensional location, and time of arrival of each detected photo-electron. Hence, there is no need to select a
specific exposure time to accumulate photo-events prior to the data collection with a RULLI detector - this quantity can
be optimized in post processing. RULLI detectors have lower peak quantum efficiency (from as low as 5% to perhaps as
much as 40% with modern photocathode technology) than back-illuminated CCD's (80% or higher). As a result of these
factors, and the associated analyses of signal and noise, we have found that RULLI detectors can play two key new roles
in SSA: passive imaging of exceedingly dim objects, and three-dimensional imaging of objects illuminated with an
appropriate pulsed laser. In this paper we describe the RULLI detection model, compare it to a conventional CCD
detection model, and present analytic and simulation results to show the limits of performance of RULLI detectors used
for SSA applications at AMOS field site.
We have developed rapidly tuned RF-pumped CO<SUB>2</SUB> waveguide laser transmitters for remote sensing in the 9 - 11 micrometers spectral range. The small size, high power and efficiency, and tunability of these lasers offer significant advantages over other laser sources in this spectral region. Employing acousto-optic modulators to achieve random-access tuning at pulse rates up to 100 kHz permits rapid gathering of data on time scales short compared to times for change in atmospheric turbulence and absorption effects, thereby improving the signal-to-noise ratios that can be achieved. Laser system design and performance characteristics of present systems are described, along with proposed concepts to increase optical bandwidths and extend the tuning range to cover the full long-wave atmospheric transmission window from 8 - 12 micrometers .
As a complement to our work developing rapidly-tunable (approximately 10 - 100 kHz) CO<SUB>2</SUB> lasers for differential absorption lidar (DIAL) applications, we have developed a rapidly-tunable spectrometer. A rapid spectral diagnostic is critical for a high speed DIAL system, since analysis of the return signals depend on knowing the spectral purity of the transmitted beam. The spectrometer developed for our lidar system is based on a double-passed large- (75 mm) aperture acousto-optic deflector, a grating, and a fast single-element room temperature mercury-cadmium-telluride detector. The spectrometer has a resolution of approximately 0.5 cm<SUP>-1</SUP>, a tuning range of 9.0 - 11.4 micrometers , a random-access tuning speed of greater than 80 kHz and a S/N ratio of greater than 100:1. We describe the design and performance of this device, as well as of future devices featuring improved resolution, higher speed and easier and more robust alignment. We will also briefly discuss the applications and limitations of the technique in a space environment.
A high sensitivity, CO<SUB>2</SUB> lidar detector, based on recent advances in ultra-low noise, readout integrated circuits (ROIC), is being developed. This detector will combine a high speed, low noise focal plane array with a dispersive grating spectrometer. The spectrometer will filter the large background flux, thereby reducing the limiting background photon shot noise. In order to achieve the desired low noise levels, the HgCdTe FPA will be cooled to approximately 50 K. High speed, short pulse operation of the lidar system should enable the detector to operate with the order of a few noise electrons in the combined detector/ROIC output. Current receiver design concepts will be presented, along with their expected noise performance.
We are developing 2-100 kHz repetition rate CO<SUB>2</SUB> lasers with milliJoule pulse energies, rapid acousto-optic tuning and isotopic gas mixes, for differential absorption LIDAR applications. We explain the tuning method, which uses a pair of acousto-optic modulators and is capable of random access to CO<SUB>2</SUB> laser lines at rates of 100 kHz or more. The laser system is also described, and we report on performance with both normal and isotopic gas mixes.
Reflection of laser light from a diffuse surface exhibits a complex interference pattern known as laser speckle. Measurement of the reflected intensity from remote targets, common to `hard-target' differential absorption lidar, requires consideration of the statistical properties of the reflected light. We have explored the effects of laser speckle on the noise statistics for CO<SUB>2</SUB> DIAL. For an ensemble of independent speckle patterns it is predicted that the variance for the measured intensity is inversely proportional to the number of speckle measured. We have used a rotating drum target to obtain a large number of independent speckle and have measured the predicted decrease in the variance after correlations due to system drifts were removed. Measurements have been made using both circular and linear polarized light. These measurements show a slight improvement in return signal statistics when circular polarization is used. We have conducted experiments at close range to isolate speckle phenomena from other phenomena, such as atmospheric turbulence and platform motion thus allowing us to gain a greater understanding of speckle issues. We have also studied how to remove correlation in the data caused by albedo inhomogenuties producing a more statistically independent ensemble of speckle patterns. We find that some types of correlation are difficult to remove from the data.