Ultrafast high speed photonics are shown to provide the necessary temporal and spectral information required for understanding FBG response under impulsive loading from either high explosive detonation or an inert shock wave interaction. Demonstration of both, chirped and uniform, silica based FBGs are presented for sensing under harsh conditions that vary from thermal ignition in high explosives to inert tracking of high pressure shock waves. Ultrafast laser based chirped pulse methods are used to time-stretch and streak the spectral response of the FBG sensor to provide information about material response under loading. Coherent broadband pulses from a femtosecond modelocked fiber laser at 1560 nm are used to illuminate and interrogate the FBG at a repetition rate of 100 MHz. After reflecting off the FBG, chromatic dispersion is applied to time stretch the pulse and separate spectral channels for detection with a 35 GHz photoreceiver and recording with a 25 GHz digitizing oscilloscope. Results include pressure wave tracking in weak inert shocks and pressure measurements in thermal ignition of high explosives detonation. The focus of the presentation is present the method and tools used for this approach to high speed FBG sensing.
In 2006 an approach was developed that used chirped fiber gratings in combination with a high speed read out configuration to measure the velocity and position of shock waves after detonation of energetic materials. The first demonstrations were conducted in 2007. Extensions of this technology were made to measure pressure and temperature as well as velocity and position during burn, deflagration and detonation. This paper reviews a series of improvements that have been made by Columbia Gorge Research, LLC, Lawrence Livermore National Lab and Los Alamos National Lab in developing and improving this technology.
George Rodriguez, Marcelo Jaime, Chuck Mielke, Fedor Balakirev, Abul Azad, Richard Sandberg, Bruce Marshall, Brandon La Lone, Bryan Henson, Laura Smilowitz, Mark Marr-Lyon, Tom Sandoval
A 100 MHz fiber Bragg grating (FBG) interrogation system is described and applied to strain, pressure, and shock position sensing. The approach relies on coherent pulse illumination of the FBG sensor with a broadband short pulse from a femtosecond modelocked erbium fiber laser. After interrogation of the FBG sensor, a long multi-kilometer run of single mode fiber was used for chromatic dispersion to temporally stretch the spectral components of the reflected pulse from the FBG sensor. Dynamic strain or pressure induced spectral shifts in the FBG sensor were detected as a pulsed time domain waveform shift after encoding by the chromatic dispersive line. Signals were recorded using a single 35 GHz photodetector and a 25 GHz bandwidth digitizing oscilloscope. Application of this approach to high-speed strain sensing of magnetic materials in pulsed magnetic fields to ~150 T is demonstrated. The FBG wavelength shifts were used to study magnetic field driven magnetostriction effects in LaCoO3. A sub-microsecond temporal shift in the FBG sensor wavelength attached to the sample under first order phase change appears as a fractional length change (strain: ΔL/L<10-4) in the material. A second application to FBG sensing of pressure dynamics to nearly 2 GPa in the thermal ignition of the high explosive PBX-9501 is also demonstrated. Then, as final demonstration, we use a chirped FBG (CFBG) to resolve shock propagation dynamics in 1-D from an explosive detonation that produces fragmentation in an inert confinement vessel. These applications demonstrate the use of this FBG interrogation system in dynamical extreme conditions that would otherwise not be possible using traditional FBG interrogation approaches that are deemed too slow to resolve such events.
Fiber grating pressure sensors have been used to support pressure measurements
associated with burn, deflagration and detonation of energetic materials. This paper provides an
overview of this technology and serves as a companion paper to the application of this
technology to measuring pressure during high speed impacts.
Fiber Bragg gratings (FBGs) are developing into useful sensing tools for measuring high pressure dynamics in extreme environments under shock loading conditions. Approaches using traditional diode array coupled FBG interrogation systems are often limited to readout speeds in the sub-MHz range. For shock wave physics, required detection speeds approaching 100 MHz are desired. We explore the use of two types of FBG sensing systems that are aimed at applying this technology as embedded high pressure probes for transient shock events. Both approaches measure time resolved spectral shifts in the return light from short (few mm long) uniform FBGs at 1550 nm. In the first approach, we use a fiber coupled spectrometer to demultiplex spectral channels into an array (up to 12) of single element InGaAs photoreceivers. By monitoring the detectors during a shock impact event with high speed recording, we are able to track the pressure induced spectral shifting in FBG down to a time resolution of 20 ns. In the second approach, developed at the Special Technologies Lab, a coherent mode-locked fiber laser is used to illuminate the FBG sensor. After the sensor, wavelength-to-time mapping is accomplished with a chromatic dispersive element, and entire spectra are sampled using a single detector at the modelocked laser repetition rate of 50 MHz. By sampling with a 12 GHz InGaAs detector, direct wavelength mapping in time is recorded, and the pressure induced FBG spectral shift is sampled at 50 MHz. Here, the sensing systems are used to monitor the spectral shifts of FBGs that are immersed into liquid water and shock compressed using explosives. In this configuration, the gratings survive to pressures approaching 50 kbar. We describe both approaches and present the measured spectral shifts from the shock experiments.
An all optical-fiber-based approach to measuring high explosive detonation front position and velocity is described. By measuring total light return using an incoherent light source reflected from a fiber Bragg grating sensor in contact with the explosive, dynamic mapping of the detonation front position and velocity versus time is obtained. We demonstrate two calibration procedures and provide several examples of detonation front measurements: PBX 9502 cylindrical rate stick, radial detonation front in PBX 9501, and PBX 9501 detonation along a curved meridian line. In the cylindrical rate stick measurement, excellent agreement with complementary diagnostics (electrical pins and streak camera imaging) is achieved, demonstrating accuracy in the detonation front velocity to below the 0.3% level when compared to the results from the pin data. In a similar approach, we use embedded fiber grating sensors for dynamic pressure measurements to test the feasibility of these sensors for high pressure shock wave research in gas gun driven flyer plate impact experiments. By applying well-controlled steady shock wave pressure profiles to soft materials such as PMMA, we study the dynamic pressure response of embedded fiber Bragg gratings to extract pressure amplitude of the shock wave. Comparison of the fiber sensor results is then made with traditional methods (velocimetry and electro-magnetic particle velocity gauges) to gauge the accuracy of the approach.
We have developed a suite of optical diagnostics and analyses for
probing the velocity and spatial distribution of ablatively launched
metal with nano-scale precision. We utilize a nanosecond laser
pulse to launch a thin layer of metal and then use optical and
opto-electronic devices to diagnose the velocity and topography.
Our Photonic Doppler Velocimeter (PDV) utilizes the heterodyne
principle that allows us to track multiple velocity components. We
have investigated a number of different methods for analyzing this
data to provide increased velocity and temporal resolution. We also
discuss the possibilities to extend the sensitivity of the PDV
system to provide a compact diagnostic with a broad range of
capabilities. Our topographer is based on the Shack-Hartmann
interferometer that can resolve the changing shape of the ablated
metal surface as it is launched. We compare the experimental data
to hydrodynamic simulations to provide a feedback loop to improve
our theoretical models. The ultimate goal is to develop a
well-understood laser-based firing set for direct optical initiation
(DOI) of explosives.
We report on the development of novel high-speed techniques to measure the surface topography and instantaneous velocity of ablatively launched thin metal layers with sub-nanosecond temporal resolution. Applications for laser detonator technology require the understanding of laser fiber optical energy deposition and ablative launch of a thin metal layer into an explosive. Characterization of the ablation process requires a time-resolved diagnosis of the ejected material state (topography, velocity, density, pressure, etc.). A pulsed Nd:YAG fibercoupled laser is used to ablate a 250 nm layer of titanium deposited on a 500 μm thick fused silica substrate at fluences below 10 J/cm2. Time-resolved imaging of the free expansion of the metal surface is accomplished with Fourier plane imaging using a Shack-Hartmann lenticular array coupled to a fast framing camera. The imager performs topographical surface measurements by detecting changes in the optical wavefront of a reflected picosecond probe laser beam off the expanding surface. Consequently, single-event sub-nanosecond time-resolved "movies" of surface motion dynamics are captured. Crosscheck of the Shack-Hartmann imager is done using advanced velocimetry. A 1550 nm heterodyne laser-based Photonic Doppler Velocimeter is used to measure surface velocity. Using a 1550 nm single mode fiber laser, 10 GHz InGaAs detectors and telecom hardware, we directly record the resulting beat signal produced by the accelerated surface onto a fast digitizer. Free surface velocities as high as 6.5 μm/ns are recorded. Comparisons between the dynamic topography, surface velocimetry and laser hydrocode simulations are presented.
We investigate the phase transitions of intense ultrashort laser-heated solids, from the cold solid to the hot dense plasma state, by measuring the complex electrical conductivity (or refractive index) transients at terahertz (1 THz = 1012 Hz) frequencies. Using optical-pump, terahertz-probe spectroscopy, we measured the phase shifts and absorption of terahertz probe pulses that were reflected from the warm dense plasma. To characterize the THz field, we developed and used a single-shot, high-temporal-resolution THz diagnostic capable of measuring free-space electromagnetic pulse fields in time and space. Due to relatively large focal spot sizes of the THz probe (~mm), mainly limited by the diffraction properties of THz radiation, the optical pump pulse was weakly focused onto the target in order to overfill the THz probe spot size with a peak intensity of ~1013 W/cm2. In contrast to the previous measurements of conductivities at optical frequencies, our THz non-contact probe method can directly measure quasi-DC electrical conductivities, providing insight into the transport nature of warm dense matter and any present discrepancies with the Drude model. In case of warm dense aluminum, we observe a noticeable deviation from the Drude model even in the ~1013 W/cm2 laser intensity regime. In addition, we observe strong coherent THz emission produced by a current surge in the laser-produced plasma.
We report on the development of a suite of novel techniques to measure important characteristics in intense ultrashort laser solid target experiments such as critical surface displacement, ablation depth, and plasma characteristics. Measurement of these important characteristics on an ultrafast (~50 fs) time scale is important in understanding the primary event mechanisms in laser ablation of metal targets. Unlike traditional methods that infer these characteristics from spectral power shifts, phase shifts in frequency domain interferometry (FDI) or laser breakthrough studies of multiple shots on bulk materials, these techniques directly measure these characteristics from a single ultrafast heating pulse. These techniques are based on absolute displacement interferometry and nanotopographic applications of wavefront sensors. By applying all these femtosecond time-resolved techniques to a range of materials (Al, Au, and Au on plastic) over a range of pulse energies (1011 to 1016 W/cm2) and pulse durations (50 to 700 fs), greater insight into the ablation mechanism and its pulse parameter dependencies can be determined. Comparison of these results with hydrocode software programs also reveals the applicability of hydrocode models.
Experimental and theoretical studies at Los Alamos of nonlinear optical phenomena important to the design of the National Ignition Facility are summarized. These include measurements of nonlinear index coefficients, Raman scattering in atmospheric oxygen, and theoretical studies of harmonic conversion.
The change in phase of ultrashort optical pulses induced by travelling through nonlinear media is used to determine the nonlinear index of refraction and to study the dynamics of femtosecond optical-field-ionization of noble gases.
Frequency conversion of an ultrafast 810 nm Ti:sapphire laser has produced 15 mJ, 165-fs pulses at 405 nm with an energy conversion efficiency of 35%. Upon tripling the fundamental to 270 nm, an energy of 0.8 mJ is obtained. Unconverted 405-nm light is used to generate an ultrafast supercontinuum in water that extends from 200 nm to 600 nm. The system operates at a 5 Hz repetition rate, and is useful for applications in deep ultraviolet pump-probe ultrafast spectroscopy.
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