Phillip Sprangle

The effect of laser noise on the atmospheric propagation of high-power CW lasers and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. High-power CW laser systems have relatively large fractional levels of intensity noise and frequency noise. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. A paraxial wave equation, containing dispersion and nonlinear effects, is expanded in terms of fluctuations in the intensity and phase. Longitudinal and transverse intensity noise and frequency noise are considered. The laser propagation model includes group velocity dispersion, Kerr, delayed Raman response, and optical self-steepening effects. A set of coupled linearized equations are derived for the evolution of the laser intensity and frequency fluctuations. In certain limits these equations can be solved analytically. We find, for example, that in a dispersive medium, frequency noise can couple to and induce intensity noise, and vice versa. At high intensities the Kerr effect can reduce this intensity noise. In addition, significant spectral modification can occur if the initial intensity noise level is sufficiently high. Finally, our model is used to study the transverse and longitudinal modulational instabilities. We also present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm , 1 μm , and 10.6 μm .

Raman amplified seed splitting was demonstrated in the backscattering scheme in a plasma, when a significant chirp was introduced into the pump. Particle-in-cell simulations have shown that a single seed laser pulse gradually splits into two, self-compressed pulses in the nonlinear amplification regime. This is in difference to previous studies of the pump chirp, which mainly have focused on its compensation for the plasma density gradient, in order to maximize the resonant amplification. The splitting, as revealed by the spectral evolution of the amplified seed, is attributed to resonance slipping, which is the result of the spatio-temporal distribution of the gain of the chirped pump.

The comment by Vorontsov and Weyrauch [Appl. Opt. 55, 9950 (2016) [CrossRef] ] is aimed at rebutting the critiques in Sprangle et al. [Appl. Opt. 54, F201 (2015) [CrossRef] ] and Nelson et al. [Appl. Opt. 55, 1757 (2016) [CrossRef] ]. In the comment, Vorontsov and colleagues describe their experiments aimed at demonstrating the feasibility of coherent combining of lasers on a distant target, using relatively low-power lasers and a cooperative retro-reflective target. The Naval Research Laboratory has demonstrated the capability to project high power on a distant target by making use of an incoherent combining architecture. The proof-of-concept experiments were performed in a realistic environment without employing cooperative targets and without sophisticated adaptive optics instrumentation.

We do not believe that there is any inconsistency between the findings of our paper [Appl. Opt. 55, 1757 (2016) [CrossRef] ] and the observations of Goodno and Rothenberg [Appl. Opt. 55, 8335 (2016)]. [CrossRef]

The free electron laser (FEL) has lived up to its promise as given in (Madey 1971 J. Appl. Phys. 42 1906) to wit: 'As shall be seen, finite gain is available ...from the far-infrared through the visible region ...with the further possibility of partially coherent radiation sources in the x-ray region'. In the present paper we review the history of the FEL drawing liberally (and where possible literally) from the original sources. Coauthors, Madey, Scully and Sprangle were involved in the early days of the subject and give a first hand account of the subject with an eye to the future.

This paper analyzes a mechanism for the remote optical measurements of magnetic field variations above the surface of seawater. This magnetometry mechanism is based on the polarization rotation of reflected polarized laser light, in the presence of the earth's magnetic field. Here the laser light is reflected off the surface of the water and off an underwater object. Two mechanisms responsible for the polarization rotation are the surface magneto-optical kerr effect (SMOKE) and the Faraday effect. In both the mechanisms, the degree of polarization rotation is proportional to the earth's local magnetic field. Variations in the earth's magnetic field due to an underwater object will result in variations in the polarization rotation of the laser light reflected off the water's surface (SMOKE) and off the underwater object (Faraday effect). An analytical expression is obtained for the polarization-rotated field when the incident plane wave is at arbitrary angle and polarization with respect to the water's surface. We find that the polarization rotated field due to SMOKE is small compared with that due to the Faraday effect.

We investigate the feasibility of using optically generated channels of reduced air density to provide trajectory correction (guiding) for a supersonic projectile. It is shown that the projectile experiences a force perpendicular to its direction of motion as one side of the projectile passes through a channel of reduced air density. A single channel of reduced air density can be generated by the energy deposited from filamentation of an intense laser pulse. We propose changing the laser pulse energy from shot-to-shot to build longer effective channels. Current femtosecond laser systems with multi-millijoule pulses could provide trajectory correction of several meters on 5 km trajectories for sub-kilogram projectiles traveling at Mach 3.