Philip M. Lubin

Beamed-energy propulsion: optical phase noise in 1064nm fiber amplifiers

Effective transfer of beamed optical energy to a spacecraft requires that the source have a sufficiently small angular divergence to contain the beam within the area of the spacecraft collector. Such a directed energy system could be achieved by coherently combining many small optical sources distributed throughout a large-scale array. The fundamental unit of such a system is a master oscillator power amplifier (MOPA) which consists of a frequency stabilized laser source distributed to amplifier trees in the array. A fundamental challenge in long baseline coherent beam combination is that each source be combined with sub-wavelength accuracy over the entire array. In addition to perturbations due to mechanical and atmospheric disturbances, phase noise introduced by the amplifiers and the seed distribution network must also be accounted for in order to achieve the necessary accuracy. This work investigates the excess phase noise introduced by the amplifier stages and fiber optic links. Locking schemes that could be used to synchronize such an array are presented. The test bed used to interrogate phase noise is based on an all polarization maintaining fiber Mach-Zehnder interferometer (MZI), controlled by an FPGA based digital quadrature detection at a sampling rate of 1 MS/s yielding direct measurement of amplitude and phase with servo control for phase locking. Results for various MOPA and fiber link configurations based on Yb-doped fiber amplifiers operating at 1064 nm and kilometer scale link lengths are presented.

Remote laser evaporative molecular absorption (R-LEMA) spectroscopy laboratory experiments

To probe the molecular composition of a remote target, a laser is directed at a spot on the target, where melting and evaporation occur. The heated spot serves as a high-temperature blackbody source, and the ejected substance creates a plume of surface materials in front of the spot. Bulk molecular composition of the surface material is investigated by using a spectrometer to view the heated spot through the ejected plume. The proposed method is distinct from current stand-off approaches to composition analysis, such as Laser-Induced Breakdown Spectroscopy (LIBS), which atomizes and ionizes target material and observes emission spectra to determine bulk atomic composition. Initial simulations of absorption profiles based on theoretical models show great promise for the proposed method. This paper compares simulated spectral profiles with results of preliminary laboratory experiments. A sample is placed in an evacuated space, which is situated within the beam line of a Fourier Transform Infrared (FTIR) spectrometer. A laser beam is directed at the sample through an optical window in the front of the vacuum space. As the sample is heated, and evaporation begins, the FTIR beam passes through the molecular plume, via IR windows in the sidewalls of the evacuated space. Sample targets, such as basalt, are tested and compared to the theoretically predicted spectra.

Experimental design for remote laser evaporative molecular absorption spectroscopy sensor system concept

Surface material on a remote target can be characterized by using a spectrometer to view a laser-heated spot on the target surface through the plume of ejected material. The concept is described as Remote Laser Evaporative Molecular Absorption (R-LEMA) spectroscopy.1,2 The proposed method is distinct from current stand-off approaches to composition analysis, such as Laser-Induced Breakdown Spectroscopy (LIBS), which atomizes and ionizes target material and observes emission spectra to determine bulk atomic composition. Initial simulations of R-LEMA absorption profiles based on theoretical models show great promise for the proposed method. This paper describes an experimental setup being developed to acquire R-LEMA spectra in the laboratory under controlled conditions that will allow comparison to theoretically predicted spectral profiles. A sample is placed in a vacuum space; a laser beam is directed at the sample, through an optical window. As the sample is heated, and evaporation begins, thermal emission from the heated spot passes through the molecular plume, then out of the vacuum space via infrared windows. The thermal emission is directed into a FT-IR spectrometer, which is equipped with a source-brightness comparator to correct for changes in source intensity during a scan. Targets of known composition are tested and laboratory measurements are compared to the theoretically predicted spectra. Laboratory spectra for composite targets are also presented, including terrestrial rocks and asteroid regolith simulant.