Structured Light Systems (SLS) give access, without contact, to a rich measurement of a cloud of points belonging to a same object surface. SLS received much interest in the past years and became a standard technique. The aim of this talk is to present the design of such a means, working in the visible spectrum, dedicated to shock physics (implying velocities up to several km/s) and to provide an example of measurements with a 3D reconstruction. A dedicated development is necessary (laser lighting, speckle smoothing, ambient light canceling, depth of field improvement), since commonly developed SLS don’t suit this field of study, mainly for three reasons: phenomena of interest (usually lasting a few microseconds) require extremely short exposure durations (few nanoseconds to few hundreds of picoseconds); the field of view ranges from millimeter for samples shocked by high power lasers to decimeter for high-explosive setups ; and finally, experimentations have single-shot acquisitions. The main domains of study are fragmentations, surface deformations and associated damages, like micro-spalling or ejected particle clouds.
Heterodyne Velocimetry (or Photonic Doppler Velocimetry) has turned out to be a major tool to study the
phenomena occurring in detonics and shock wave experiments. With accessible velocities ranging form a few m/s to
30,000 m/s, a very high sensitivity, a dynamics of more than 20 dB and a multi-velocity capability, one can understand
why this technique opens new fields of study.
This article is aimed at presenting an outlook of the setups and configurations which have been tested. We will connect
this outlook to a quick overview of different kinds of experiments that could be achieved.
In a first part we will remind how the system works. We will then detail the many setups that have already been put to
the test, with different possible hardware configurations responding to different uses and different probes aimed at
sensing specific phenomena. We also present our Matlab© based software developed to process the signals. Finally we
will go through the different applications on which PDV was implemented, both in a detonics context (free surface,
particles and Nitro Methane - or NM - characterization) and in lab experiments (measurement of laser driven shocks on
Heterodyne Velocimetry (or Photonic Doppler Velocimetry) has been used in detonics experiments for a few years
now, mainly thanks to the recent evolution of telecom components.
In its principle it is nothing else but a displacement interferometer, delivering beats versus time. A sliding Fourier
transform processing on the raw signal thus allows to derive velocity versus time. The device is made up of a 1.55 μm
Erbium laser delivering 2 W (split into 4 channels), single-mode optical fibers, fast photodetectors and digitizers (8 GHz
bandwidth, 20 GS/s sampling).
To begin with, we present a new heterodyne velocimeter setup embedding a second low-power frequency-tunable
laser (50 mW) acting as a local oscillator. Its frequency can be shifted, to make it higher than the main laser, up to the
bandwidth of the digitizer (13 GHz soon). The Doppler wave coming from the first laser and reflected by the moving
target interferes with this shifted reference, therefore doubling the overall bandwidth of the system.
On top of enhancing the measurable velocity range, the existence of beats at static gives a convenient means to tune
the power levels of the laser and match the electric signal to the dynamics of the detector.
Finally, three applications are presented: the first one deals with the classical measurement of free surface velocity
on metallic shock loaded plates, in the second part we present the velocity distribution of tin particles ejected under
shock. The third application relates to direct measurement of the velocity of detonation wave into nitromethane, by using
immersed optical fibers.
For more than 30 years now, Doppler Laser Interferometry has been used in detonics to measure velocity versus time accurately. The means is composed of: a laser source, an optical fiber bringing light to the moving target, another which collects back-reflected light, a device built around a Fabry-Perot interferometer to create the rings pattern, a streak camera. In the beginning, the source was a CW singlemode argon laser with a 7 W output power. Later, it was replaced by a wide spectrum rhodamine dye laser requiring two twin Fabry-Perot interferometers, the first one modulating the spectrum and the second analyzing the Doppler shifted light; the available power was 1 kW for a 40 μs pulse. Recently, we have improved the means by increasing the channel number with sufficient output power, replacing the dye with a solid amplifier, increasing pulse width, decreasing the velocity and time uncertainties and reducing the volume and the cost of the equipment. To achieve this quality we acquired a long-pulse singlemode Yag laser. It has 15 channels, each providing 1 kW with a 70 μs rectangular pulse at 532 nm. The analysis bench uses only 1 Fabry-Perot interferometer for 5 channels. The laser, target and analysis bench are connected through a 3 optical fibers bundle; one for lighting, one to measure velocity and a third one to record the photometric curve and determine shock breakout time accurately. To improve accuracy, we worked on two areas: (1) the equipment with the anamorphic optical device, the value of the fringe constant and the number of rings lit on the camera slit, (2) the building of a chart where we write all influent parameters in order of importance: in order to decrease the uncertainty of velocities less than 1000 m/s, we need to evaluate the influence of electronic streak camera distortion.
In our ordinary detonics experiments, the timing measurements are generally done with passive optical fiber sensors. Each sensor end is fitted with a metallic cap which contains a specially machined air chamber known as the "ionization chamber." When a projected metallic plate shocks the sensor, the air located in the ionization chamber receives a strong shock, which ionizes both the trapped air and the fiber silica core. The emitted light travels down the fiber to the slit of an electronic streak camera or an optic/electric converter coupled with a digitizer. One of our main objectives is to measure the shock breakout time very accurately. Such a measurement is practically impossible to make on a sensor of this type due to low shock pressure level and the difficulty of making contact between the sensor and the target especially in complex devices. This is why we have developed a new probe called "active fiber." This probe is located close to the surface (less than 3 mm) and is composed of two fibers; the first is used to illuminate the target with a laser source and the second collects the back-reflected light which is then analyzed with a photo-detector. It is a "no contact" measurement for shock breakout chronometry. At target impact, a light signal is produced according to the capped passive optical fiber principle. When the dynamic pressure level is low (150 kbar) we obtain a better chronometric accuracy.
Dealing with dynamic behavior of solids, detonator initiation, shock and detonation waves and other fast processes implies a number of new techniques. We are working on wave propagation at velocities of several km/s, with states existing for only a few microseconds or even nanoseconds. In this case performances of our fastest rotating mirror framing cameras are not high enough to observe states of surface or large discontinuity zones (problem of dynamic blur). We have developed a new laser technique called Instantaneous Image (I.I.). This technique consists in recording a single image in a short exposure time to minimize the dynamic blur of our fast phenomena. We use a Q-switched Nd:YAG laser made of an oscillator, a pre-amplifier, a 16 mm diameter amplifier and a KDP crystal. The available energy is in the order of 200 mJ at 532 nm for a ten nanoseconds pulse duration. A large amount of work has been done to minimize the non uniformity of the delivered light, to eliminate speckle defects and to collect the most illumination light by an optimized optic device. Under these conditions a large diameter field image (D equals 200 mm) can be achieved with a resolution better than 15 line pairs/mm. With a double proximity focused microchannel plate image intensifier (M.C.P.) it is possible to obtain faster shuttered times (a few nanoseconds) with a higher gain to observe poor reflective surfaces. But under these conditions the resolution decreases drastically to some line pairs per millimeter.