R. Lawrence

Active correction of thermal lensing through external radiative thermal actuation

Absorption of laser beam power in optical elements induces thermal gradients that may cause unwanted phase aberrations. In precision measurement applications, such as laser interferometric gravitational-wave detection, corrective measures that require mechanical contact with or attachments to the optics are precluded by noise considerations. We describe a radiative thermal corrector that can counteract thermal lensing and (or) thermoelastic deformation induced by coating and substrate absorption of collimated Gaussian beams. This radiative system can correct anticipated distortions to a high accuracy, at the cost of an increase in the average temperature of the optic. A quantitative analysis and parameter optimization is supported by results from a simplified proof-of-principle experiment, demonstrating the methods feasibility for our intended application.

Beam control of a 2D polarization maintaining fiber optic phased array with high-fiber count

We demonstrate, for the first time to our knowledge, successful beam control of a fiber optic phased array containing a large number of polarization maintaining fibers. As many as forty-eight fibers have been coherently combined via individual all-fiber phase modulators. The residual phase error is less than 1/30th of a wave. Results with both near-field interferometric control and target-in-the-loop control have been obtained. Experimental results are compared with numerical simulations and excellent agreement has been achieved. We investigated propagation of this phased array output through a turbulent atmosphere, and used the all-fiber phase modulators for the compensation of turbulence effects on the array output. This work paves the way towards scaling such fiber optic phased arrays to very high fiber count. Eventually thousand of fibers can be controlled via such a scheme.

Adaptive thermal compensation of test masses in advanced LIGO

As the first generation of laser interferometric gravitational wave detectors nears operation, research and development has begun on increasing the sensitivity of the instrument while utilizing the existing infrastructure. In the laser interferometer gravitational wave observatory (LIGO), significant improvements are being planned for installation around 2007, increasing strain sensitivity through improved suspensions and test mass substrates, active seismic isolation and higher input laser power. Even with the highest quality optics available today, however, finite absorption of laser power within transmissive optics, coupled with the tremendous amount of optical power circulating in various parts of the interferometer, results in critical wavefront deformations which would cripple the performance of the instrument. A method of active wavefront correction via direct thermal actuation on optical elements of the interferometer is discussed. A simple nichrome heating element suspended off the face of an affected optic will, through radiative heating, remove the gross axisymmetric part of the original thermal distortion. A scanning heating laser will then be used to remove any remaining non-axisymmetric wavefront distortion, generated by inhomogeneities in absorption of the substrate, thermal conductivity, etc. A proof-of-principle experiment has been constructed at MIT, selected data of which are presented.

Characterization of Diffraction Gratings for Use in Wavelength Beam Combining at High Average Power

Wavelength beam combined architectures typically depend on the use of diffraction gratings. We explore the high average power limitations of commercial gratings using experiments and theoretical modeling.

Thermal-blooming compensation using target-in-the-loop techniques

We report experimental results that demonstrate compensation of extended turbulence and thermal-blooming of high-energy lasers using target-in-the-loop techniques in a scaled laboratory environment. For these experiments the deformable mirror figure was controlled by an algorithm designed to maximize the target-plane intensity as measured by a camera at the transmitter. Results using this TIL configuration were compared under identical conditions to results obtained under control of a Hartmann wavefront sensor and least-squares reconstructor. Experiments were performed for a variety of propagation scenarios anticipated for tactical HEL applications and in all cases the TIL system was seen to outperform the conventional Hartmann-driven adaptive-optics system. We will discuss the details of the the target-in-the-loop algorithm, the laboratory configuration, and the experimental results.

Comparison of cooperative and non-cooperative adaptive optics reference performance for propagation with thermal blooming effects

Atmospheric turbulence and laser-induced thermal blooming effects can degrade the beam quality of a high-energy laser (HEL) weapon, and ultimately limit the amount of energy deliverable to a target. Lincoln Laboratory has built a thermal blooming laboratory capable of emulating atmospheric thermal blooming and turbulence effects for tactical HEL systems. The HEL weapon emulation hardware includes an adaptive optics beam delivery system, which utilizes a Shack-Hartman wavefront sensor and a 349 actuator deformable mirror. For this experiment, the laboratory was configured to emulate an engagement scenario consisting of sea skimming target approaching directly toward the HEL weapon at a range of 10km. The weapon utilizes a 1.5m aperture and radiates at a 1.62 micron wavelength. An adaptive optics reference beam was provided as either a point source located at the target (cooperative) or a projected point source reflected from the target (uncooperative). Performance of the adaptive optics system was then compared between reference sources. Results show that, for operating conditions with a thermal blooming distortion number of 75 and weak turbulence (Rytov of 0.02 and D/ro of 3), cooperative beacon AO correction experiences Phase Compensation Instability, resulting in lower performance than a simple, open-loop condition. The uncooperative beacon resulted in slightly better performance than the open-loop condition.