David Cavalieri

Airborne Aero-Optics Laboratory

Abstract. We provide a background into aero-optics, which is the effect that turbulent flow over and around an aircraft has on a laser projected or received by an optical system. We also discuss the magnitude of detrimental effects which aero-optics has on optical system performance, and the need to measure these effects in flight. The Airborne Aero-Optics Laboratory (AAOL), fulfills this need by providing an airborne laboratory that can capture wavefronts imposed on a laser beam from a morphable optical turret; the aircraft has a Mach number range up to low transonic speeds. We present the AAOL concept as well as a description of its optical components and sensing capabilities and uses.

The Airborne Aero-Optics Laboratory, AAOL

This paper gives a background into aero-optics which is the effect that turbulent flow over and around an aircraft has on a laser projected or received by an optical system. The background also discusses the magnitude of the detrimental effects that aero-optics has on optical system performance and the need to measure these effects in flight. The Airborne Aero-Optics Laboratory, AAOL, fulfills this need by providing an airborne laboratory that can capture wavefronts imposed on a laser beam from a morphable optical turret; the aircraft has a Mach number range up to low transonic speeds. This paper presents the AAOL concept as well as a description of its optical components and sensing capabilities and uses.

iLocater: a diffraction-limited Doppler spectrometer for the Large Binocular Telescope

We are developing a stable and precise spectrograph for the Large Binocular Telescope (LBT) named “iLocater.” The instrument comprises three principal components: a cross-dispersed echelle spectrograph that operates in the YJ-bands (0.97-1.30 μm), a fiber-injection acquisition camera system, and a wavelength calibration unit. iLocater will deliver high spectral resolution (R~150,000-240,000) measurements that permit novel studies of stellar and substellar objects in the solar neighborhood including extrasolar planets. Unlike previous planet-finding instruments, which are seeing-limited, iLocater operates at the diffraction limit and uses single mode fibers to eliminate the effects of modal noise entirely. By receiving starlight from two 8.4m diameter telescopes that each use “extreme” adaptive optics (AO), iLocater shows promise to overcome the limitations that prevent existing instruments from generating sub-meter-per-second radial velocity (RV) precision. Although optimized for the characterization of low-mass planets using the Doppler technique, iLocater will also advance areas of research that involve crowded fields, line-blanketing, and weak absorption lines.

Optical Measurement of a Compressible Shear Layer Using a Laser-Induced Air Breakdown Beacon

The aero-optic aberrations due to a compressible shear-layer flow with high- and lowspeed Mach numbers of 0.75 and 0.12 were measured using the return light from an artificial guide star. The guide star was created by focusing a pulsed, frequency-tripled Nd:YAG laser emitting in the ultraviolet to create a laser-induced air breakdown spark. The experiments showed that accurate wavefront data could be obtained, including accurate measurements of the wavefront tip/tilt, when the shear layer was forced and the measurements were phase-locked to the forcing signal. Issues involved in integrating the beacon system into a feedforward adaptive-optic correction approach are discussed.

Flight-Test Measurement of the Aero-Optical Environment of a Helicopter in Hover

Methods and results are described for flight-test measurements of the aero-optical effect of the near-field flow surrounding a medium-sized helicopter in hover. The data were acquired using a novel, passive measurement approach in which optical wave-front aberrations were computed from the displacements of light-emitting diodes attached to a target array mounted under the helicopter fuselage. The resulting aero-optical data are shown through the rotor downwash and through the engine exhaust of the helicopter, and they are compared to estimates computed using parametric models for the helicopter wake structure and rotor–vortex diffusion. The experimental data provide additional information on the magnitude and frequency content of the aero-optical aberrations, as well as vibration data, that will be valuable for future beam-control and modeling studies.

Aero-Optic Measurements Using a Laser-Induced Air Breakdown Beacon

Abstract : An experimental investigation into the optical behavior of a laser-induced air breakdown spark is described. The investigation concentrates on qualities of the air-breakdown spark, particularly the non-point-source character of the spark, that have a critical influence on the accuracy with which aero-optic aberration can be measured using the return light from the spark. Data are presented that show that the spark dimensions conform to established physical models, and baseline spark wavefront noise figures are presented as a function of the optical system parameters. Wavefront measurements are shown that indicate that a well-designed beacon system should be capable of accurately measuring aero-optic aberration created by realistic compressible shear-layer flows.

Evaluation of Laser Beacon for Adaptive-Optic Correction of a Compressible Shear Layer

S PHERICAL turrets are effective platforms for integrating optical systems into aircraft because they are able to aim the optical system over a large field of regard without altering their basic aerodynamic shape. The spherical turret shape is susceptible, however, to flow separation from its leeward surface. At compressible flow speeds, the shear layer associatedwith the separated-flow region becomes optically active, producing index-of-refraction variations that arise primarily from the reduced pressure and density inside vortical structures in the shear layer [1,2]. The resulting optical aberrations can break up the irradiance profile of a beam that traverses the shear layer, thereby impairing the ability to focus the optical system on far-field targets [3]. One approach to mitigating the effect of the optically active shear layer is to employ an adaptive-optic (AO) system [4] that uses a deformable mirror (DM) to place the conjugate waveform of the aberration onto the wave front of the beam before its transmission through the flow. Current AO technologies may, however, lack the frequency response necessary for the compensation of high-speed shear-layer flows where the associated aero-optic aberrations can have frequency content of hundreds of hertz up to several kilohertz [5,6]. As such, modified AO correction approaches have been developed in which the optical aberration of the shear layer is first made predictable using flow-control techniques to control the frequency and phase of the vortex shedding [5,6]. As described in studies such as [7], the passing frequency of vortical structures in the unforced shear layer, which are the dominant source of optical aberrations [2], decreases as the shear layer thickens with downstream distance; however, application of forcing induces the shear layer to growmore rapidly than in the unforced case, so that the passing frequency of the vortical structures reaches the value dictated by the applied forcing at a location that is upstream of where it would occur in the unforced case.Downstreamof this rapid-growth region is a “regularized” region in which the vortical structures pass through the shear layer with a frequency that is locked in to the applied forcing; more importantly, the turbulent energy and optical aberrations of the shear layer in the regularized region are contained primarily in the coherent vortical structures rather than in random, broadband fluctuations. As such, shear-layer regularization greatly alleviates the demands on the AO system because it eliminates the need for the AO system to measure and react to the optical aberration presented by the flow at each instant in time. Instead, with the shearlayer regularized, the DM can be preprogrammed to compensate for the largely repeatable and predictable aberration of the regularized region of the shear layer, which occurs at the frequency dictated by the forcing actuator. This kind of “feedforward” adaptive-optic correction of a compressible shear layer is described in [6], where the Strehl ratio of a collimated beam that was passed through a compressible shear layer was improved from approximately 0.14 (shear layer unforced with no correction) to 0.66 (shear layer forced and corrected). In the feedforward correction described previously, the frequency of the dominant shear-layer aberrations corresponds to the forcing frequency; however, there can still be a phase difference between the forcing signal and the shear-layer aberrations at the location of the traversing beam. For the feedforward correction described in [6], the phase of the AO system was synchronized with the phase of the forced and regularized shear layer manually in a trial-and-error fashion. As such, feedforward AO control would still benefit from an optical measurement of the shear-layer that would provide information on the current phase of the regularized shear layer, thereby enabling automation of this phase synchronization. This measurement could bemade using amanmade guide star or “beacon” that is generated by focusing a high-energy pulsed laser to create a laser-induced breakdown (LIB) spark with sufficient brightness for optical measurements [8]. The operational advantages offered by this kind ofmanmade guide star are significant because it can be placed at any point in space outside of the transporting aircraft. Our previous investigations into the use of the LIB spark as a guide star for aero-optic measurements are detailed in [9,10]. In [9], it was shown that small-amplitude fluctuations in the wave front of the emitted light from an LIB spark arise due to variations in the spark length between ignitions; however, these wave-front fluctuations are an order of magnitude smaller than typical shear-layer aberrations. In [10], it was shown that anisoplanatism effects produced by the diverging light from the LIB spark areminimized due to the localized nature of shear-layer flows and, in any case, can be compensated. In this note, we present experimental measurements of the optical aberration produced by a forced compressible shear layer using the light emitted from a LIB spark and estimate the effect of error in the measured phase of the forced shear-layer aberration on the performance of a feedforward AO correction.

Feedforward Adaptive-Optic Correction of a Compressible Shear Layer Using a Laser Beacon

A feedforward adaptive-optic correction approach was investigated in which the return light from an artificial guide star is used to synchronize the deformable mirror with the regularized aberrations of a forced compressible shear layer with high- and low-speed Mach numbers of 0.75 and 0.12. The guide star was created by focusing a pulsed, frequency-tripled Nd:YAG laser emitting in the ultraviolet to create a laser-induced air breakdown spark. The experiments showed that the beacon system could successfully synchronize the feedforward correction to within a few hundredths of a millisecond, and that this temporal error would introduce a residual OPDrms on the corrected wavefront of around 0.06 μm.

An Optical Propagation Improvement System and the Importance of Aeroacoustics

Use of an airborne platform for a directed energy system is currently severely limited by aero-optic aberrations arising from density variations in air flowing over the aircraft; the primary limitation is for aft pointing applications. Innovative Technology Applications Company (ITAC), in collaboration with the University of Notre Dame (ND), is working to develop, design, construct and test a turret/adaptive fairing that provides a large field of regard for propagation of a lethal beam from an airborne platform at up to transonic speed. The conceptual design incorporates a fairing that includes a tuned cavity between the aperture and the aft-fairing that excites a resonance mode that robustly regularizes optical aberrations imposed by the shear layer over the entire Mach number range. The cavity will be exposed to the flow only when using the beam in an aft pointing direction. Optical-aberration regularization is the exact requirement for robust feed-forward adaptive-optic correction of a laser propagated through the controlled shear layer. This paper will describe the importance of understanding aeroacoustic behavior to effectively develop aero-optic capability that is based on cavity resonances.

The iLocater cryostat: design and thermal control strategy for precision radial velocity measurements

The current generation of precision radial velocity (RV) spectrographs are seeing-limited instruments. In order to achieve high spectral resolution on 8m class telescopes, these spectrographs require large optics and in turn, large instrument volumes. Achieving milli-Kelvin thermal stability for these systems is challenging but is vital in order to obtain a single measurement RV precision of better than 1m/s. This precision is crucial to study Earth-like exoplanets within the habitable zone. iLocater is a next generation RV instrument being developed for the Large Binocular Telescope (LBT). Unlike seeinglimited RV instruments, iLocater uses adaptive optics (AO) to inject a diffraction-limited beam into single-mode fibers. These fibers illuminate the instrument spectrograph, facilitating a diffraction-limited design and a small instrument volume compared to present-day instruments. This enables intrinsic instrument stability and facilitates precision thermal control. We present the current design of the iLocater cryostat which houses the instrument spectrograph and the strategy for its thermal control. The spectrograph is situated within a pair of radiation shields mounted inside an MLI lined vacuum chamber. The outer radiation shield is actively controlled to maintain instrument stability at the sub-mK level and minimize effects of thermal changes from the external environment. An inner shield passively dampens any residual temperature fluctuations and is radiatively coupled to the optical board. To provide intrinsic stability, the optical board and optic mounts will be made from Invar and cooled to 58K to benefit from a zero coefficient of thermal expansion (CTE) value at this temperature. Combined, the small footprint of the instrument spectrograph, the use of Invar, and precision thermal control will allow long-term sub-milliKelvin stability to facilitate precision RV measurements.