Fiber laser sources with narrow linewidths in the short-wave infrared and mid-infrared spectral regions have many defense, commercial, and sensing related applications. To generate wavelengths not produced by commercially available lasers, we introduce a compact design of a hollow core fiber-based optical parametric amplifier (OPA) with flexible phase-matching schemes. An electric field applied to fiber core liquids or gases can induce the required effective second-order nonlinearity. Quasi-phase-matching for efficient frequency conversion can be realized by applying spatially periodic electric fields along the length of the fiber. We investigate fiber-based OPA designs with xenon gas and liquids such as carbon disulfide to determine the viability of these architectures for efficient wavelength conversion and flexibility.
We demonstrate a single pulse LiDAR polarimeter that is optimized to measure the diagonal elements of a target Mueller matrix and thereby dramatically reduce CSWAP of the system. Previous work showed that 12 out 16 elements of a Mueller Matrix can be resolved using three polarization state analyzers (PSA); for example, a linear horizontal/vertical PSA, a linear +45º/135º PSA, and a circular PSA. Here we employ a single elliptical PSA to measure the diagonal matrix elements. The system is composed of a transmitter beam produced by directing the laser pulse through a Pockels cell wherein a high-voltage ramp is synchronously applied thereby creating a time varying birefringence. The resulting pulse is characterized by a time varying polarization across the temporal envelope. The receiver PSA is composed of an elliptical PSA (quarter wave plate at angle Ɵ and linear polarizer) followed by a high bandwidth detector capable of measuring the polarization modulation of the return signal. In this particular work, we use analytical models to optimize the Pockels cell angle and the elliptical PSA quarter-wave plate angle for maximum matrix element estimation accuracy. A system demonstrator employing a 1.06 μm, 10 ns pulse laser is used to demonstrate target diagonal Mueller matrix measurement. We measure diagonal Mueller matrix elements of air, Spectralon, and paint samples.
In a recently developed compact LiDAR polarimeter, the transmit beam cycles through multiple polarization states within each laser pulse and the receiver splits the received signal into multiple polarization state analyzers (PSAs), with the PSA outputs temporally multiplexed into a single detector. This enables measurement of up to 12 of the 16 Mueller matrix elements of a target using limited hardware. However, due to numerical issues, one entire column of the matrix is not accessible, including the M22 element (counting from zero) on the matrix diagonal. Experimental data show that for most surfaces of interest in a defense/security setting, the off-diagonal elements tend to be negligibly small and the diagonal elements vary between targets, so access to the diagonal elements is of high interest. In this paper, we show that if an elliptical PSA is used at the receiver and if most of the off-diagonal Mueller matrix elements are assumed to be zero a priori, then the otherwise inaccessible M22 element can be estimated. We explore the linear algebra of the problem to determine the full list of which subsets of Mueller matrix elements can be estimated with this hardware configuration. We also theoretically and empirically investigate the effects of the rotations of the electro-optic transmitter plate and receiver elliptical PSA on the overall system performance.
We present, mathematically and experimentally, a novel temporally multiplexed polarimetric LADAR (TMP-LADAR) architecture which is capable of characterizing the polarimetric properties (Mueller matrix elements) of a target using a single 10 ns laser pulse. By exploiting the Kerr nonlinear optical effect, birefringence within an optical fiber can be modulated based on the instantaneous intensity of the input laser pulse, which results in temporally varying polarization states within the laser pulse exiting the fiber. We introduce a model that describes the varying polarization of a laser pulse through an optical fiber and experimentally verify the operation of this novel polarization state generator (PSG).
We numerically and experimentally explore a new scanning LADAR architecture that enables Mueller matrix measurement for each point in a scene using a single ~10 ns illumination laser pulse. For the transmitter, we direct the laser pulse through an electro-optic crystal wherein a high-voltage ramp is synchronously applied. As the laser pulse propagates through the crystal the high-voltage ramp induces a time varying birefringence which results in a time varying polarization across the temporal envelope of the laser pulse. The receiver channel can be designed to measure the full Mueller matrix of the target by employing three polarization state analyzers or a subset of the Mueller matrix by employing fewer analyzers. This transmitter produces a well-defined temporal polarization variation which is used to illuminate the target. Knowing the temporal distribution of the transmitted polarization signal and using polarization analyzers in the receiver chain to measure the temporal distribution of the return signal’s polarization one can measure the entire Mueller matrix. We will introduce a model describing the transmitted and received signal polarization temporal distribution and show how the Mueller matrix can be extracted from this information. We will demonstrate the concept using a 10 ns, 1.06 μm laser pulse and a lithium tantalate electro-optic phase retarder in the transmitter and using two polarization analyzers in the receiver chain to measure a subset of the Mueller matrix in a point and shoot configuration. Measurements will be compared to theoretical reference to assess accuracy.
It is known that LADAR imaging can characterize reflective properties of a scene and provide high resolution threedimensional spatial information useful for target classification; however, scanning and processing high resolution LADAR data is extremely time and computational resource consuming. In remote sensing applications, polarization sensitive imagery can improve target-clutter discrimination of man-made objects in a natural background and anomaly detection algorithms have been shown to accurately identify areas of interest in low resolution imagery. In this paper, we investigate the possibility of enabling passively augmented LADAR for target detection by utilizing polarimetric thermal imagery to cue high resolution LADAR scans of anomalous regions of a scene. A statistical outlier detection algorithm is explored with features extracted from passive polarimetric LWIR imagery collected on an outdoor range under various conditions. The data collection process and products are discussed as well as the performance of anomaly detection algorithms for LADAR cueing. In both data collection and image processing, foliage penetration of partially hidden targets is considered. Data analysis shows polarization information of paired systems improves true positive rate and target detection rate with an acceptable false positive rate while greatly reducing LADAR scan time. As a result, a spatial clustering and anomaly ranking system is introduced to prioritize the most likely anomaly among multiple detections; minimizing time consumed performing LADAR scanning and processing.
In order to develop LADAR-based sensors that satisfy the cost, size, weight, and power constraints imposed by increasingly demanding systems, new LADAR architectures need to be developed to support requirements in areas such as intelligence, surveillance, and reconnaissance. Knowledge of the spectral reflectivity of objects in a complex scene may prove useful to distinguish object from background, or even to identify partially occluded objects where a full set of identifying pixels may be impossible to measure. We present a novel LADAR architecture to enable spectral reflectivity measurement with a single pulse of a multispectral laser and a single receiver detector, eliminating spectrally dispersive elements which spatially multiplex the return signal to multiple detectors. This is accomplished by exploiting the wavelength-dependent temporal waveforms that arise from stimulated Raman scattering based multispectral laser sources to multiplex multispectral signals inside a single pulse envelope. With knowledge of these effects in a transmitted laser pulse, a measured pulse envelope at the receiver can be modeled as a sum of reflectivity-scaled spectral components, and the individual object reflectivities estimated. The system performance of this architecture is evaluated using measured pulses of a Raman-based multispectral fiber laser to simulate the measurement of objects of interest, including the influence of detector noise. System performance is quantified by calculating the target reflectivity estimation error as a function of signal-to-noise ratio, receiver bandwidth, and receiver sample rate, demonstrating the feasibility of a temporally multiplexed architecture.
The 13 nm emission that results from laser plasmas created from tin targets, results from a milliard of transitions occurring in many ions of tin (Sn6+-Sn13+). Understanding the energy manifolds within these multiple states will further our ability to manipulate energy into the narrow emission band demanded by EUV Lithography. A combined experimental theoretical program is underway to measure and interpret the detailed EUV emission spectra from laser plasmas suitable for EUVL, particularly mass-limited droplet laser plasmas. We employ high resolution spectroscopy in the 2 - 60 nm wavelength regions to characterize the emission from the plasma. This is interpreted with the aid of combined hydrodynamic/ radiation transport computer models. The results of this study will have impact on the in-band EUV conversion efficiency, estimation of the out-of-band short-wavelength emission, and in the development of electron temperature plasma diagnostics.
Detailed understanding of the complex UTA emission from Xe and Sn laser plasmas is imperative to the development of efficient 13.5 nm sources for EUVL. We are developing a comprehensive theoretical modeling approach to these sources, utilizing state-of-the-art hydrodynamic and radiation transport plasma codes. These models are specifically applied to Xe and Sn-doped microscopic droplet targets laser-plasmas irradiated with nanosecond laser pulses. The plasma expansion models are compared to experimental determinations of the plasma electron density distributions. The output of the radiation transport code is used to interpret details of the spectral emission measured from these plasmas over a broad range of parameters.
The most pressing technical issue for the success of EUV lithography is the provision of a high repetition-rate source having sufficient brightness, lifetime, and with sufficiently low off-band heating and particulate emissions characteristics to be technically and economically viable. We review current laser plasma approaches and achievements, with the objective of projecting future progress and identifying possible limitations and issues requiring further investigation.
We have previously proposed the use of mass-limited, tin-containing laser plasma sources for EUV lithography applications. Here we report advances in measurements of the spectral output, conversion efficiency, and debris emission from these sources. We also report progress in the use of repeller field debris inhibition techniques for this source.
One of the key leverage factors in determining the viability of laser-plasma sources for EUVL is the conversion efficiency of laser light to EUV emission in the 13-nm region. We describe experiments and theoretical calculations on a mass-limited laser target design using tin that offers high conversion efficiency.
As a mass limited target the water droplet laser plasma source has been shown to have many attractive features as a continuous, almost debris-free source for extreme ultraviolet (EUV) and X-ray applications. Through a dual experimental and theoretical study, we analyze the interaction physics between the laser light and the target. The hydrodynamic laser plasma simulation code, Medusa103 is used to model the electron density distribution for comparison to electron density distributions obtained through Abel inversion of plasma interferograms. In addition, flat field EUV spectra are compared to synthetic spectra calculated with the atomic physics code RATION.
The droplet laser plasma source has previously been shown to have many attractive features as a continuous, almost debris-free source for EUV and x-ray applications. In a combined experimental and theoretical study, we analyze the interaction physics between the laser light and the target over a range of conditions.