Three-dimensional (3D) remote imaging may improve target characterization and help improve tracking and aim-point maintenance. Digital holography (DH) has been shown to be very effective in generating 3D images of objects using frequency diversity. The digital holographic nature of the process can eliminate the need for imaging lenses when the pupil-plane is used for the detection plane. In fact, this approach is the optical analog of the widely used frequencymodulation (FM) radar imaging. The current implementation of the holographic FM 3D imaging involves forming a set of holographic 2D images each at a different temporal frequency and then inverse Fourier transform to recover the actual 3D image. In this paper we present results from both laboratory and simulations of a coherent 3D holographic imaging method based on continuous frequency chirping of a laser. Laboratory 3D imaging results were obtained in both the pupilplane using a spatial heterodyne method, and in the image-plane using a temporal heterodyne method.
A significant enhancement in the rate of material removal is demonstrated using a nanosecond-pulsed UV fiber laser in multi-pulsing burst mode, as compared to the case without bursting. Percussion drilling and scribing of thin-film and bulk material tests show that, in general, laser bursts with increased pulse count and reduced pulse spacing show higher rates of material removal. A considerable improvement in removal rate is demonstrated, when bursting is applied to scribing of mono-crystalline silicon (m-Si) and up to 30% in percussion drilling speed. Likewise, improved material removal is demonstrated for scribing of thin film of indium tin oxide (ITO) on glass or metal film on sapphire. Examples of material processing are given with and without bursting at similar experimental conditions of average power, scan speed, and burst/pulse energies. Experimental results included are for m-Si, ITO thin films on glass, and metal films on sapphire.
We propose a simple physical mechanism to explain observed instabilities in the dynamics of passively phased fiber amplifier arrays that arises from two properties: First that a weak phase disturbance of the output field of the array is converted into a strong intensity disturbance through the mode-selective feedback mechanism. Second, that this intensity fluctuation regenerates a phase fluctuation due to the nonlinear properties of the amplifying media. At sufficiently high operating power levels this cyclic disturbance continues to grow upon each cavity round trip, creating instability. This simple picture is supported by the results of a linear stability analysis of the set of propagation and population rate equations, which are in good agreement with observed critical power levels. A third level of quantitative confirmation was obtained by comparison to the results of numerical integration of the original set of nonlinear equations. This predicted instability is entirely a property of passively phased arrays of more than one element.
An optimized 1064nm MOPA fiber laser configuration is used for generating pulse widths from 10 ns to 250 ns, with
pulse repetition frequencies that range from single-shot to 500 kHz, and peak powers up to 10kW. These parameters are
independently controlled and used to investigate the effect of peak power and pulse energy on material ablation. Test
results are demonstrated for processed silicon and ceramic materials using pulse energies up to 0.5 mJ and peak powers
up to 10kW. We demonstrate that pulses with high peak powers have shallow penetration depths, as compared to
longer pulses. These experimental results are well correlated with those from a theoretical thermal model for silicon
ablation that is based on silicon temperature rise as the incident pulse energy is absorbed.
Passive phasing of fiber amplifier arrays are promising for the power scaling of high power fiber
laser systems. The broadband operation of passively phased systems mitigates nonlinear effects such
as Stimulated Brillouin Scattering. This leads to the possibility of scaling the individual fiber
amplifiers in the passively phased arrays to multi-kilowatt power levels. In effect, a smaller number
of fiber amplifiers can be used compared to other methods of fiber amplifiers combining. We report
the passive phasing of 16 Yb-doped fiber amplifiers at 5W each for a total of 80W.
System temporal response in passively phased fiber amplifier arrays dictates how fast a passively phased system can correct for phase fluctuations due to thermal and mechanical effects. The system response time was measured by employing a variable-speed mechanical chopper in the feedback loop of a passively phased system then measuring the on-axis output intensity of the system as a function of time. Observed relaxation oscillations are compared to theory. The system response time was measured to be about 20 μsec. We also find that passive phasing improved the system's beam stability and extraction efficiency.
Efficiency and mode control in coupled grating-surface-emitting diodes are important issues in the proper design of these devices. In these devices, radiation emission properties depend on the injection current of the different sections, and phase effects due to length variations, grating phase effects, and the boundary conditions of the particular design. A realistic model incorporating power saturation, saturable losses, and saturable dispersion effects is established. Optimization of operation by injection current control and phase modulation are studied.
The performance of a system of unstable resonators is addressed. The resonators are coupled to one another through a portion of each output that is mutually shared through reverse (adjoint) waves. The ideal performance of the coupled resonator system is dependent upon operation in a single supermode, insuring that the individual resonator outputs are mutually coherent. A wave optics analysis of two coupled resonators is described. The model considers diffraction effects in Cartesian coordinates and in a single transverse dimension (strip resonator). The sensitivity of coupled resonator performance to the length control of the individual resonators and the strength of coupling between them are evaluated.
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