Richard J. Tansey

Practical issues in wave-front sensing by use of phase diversity

We present the results of the phase-diversity algorithm applied to simulated and laboratory data. We show that the exact amount of defocus distance does not need to be known exactly for the phase-diversity algorithm on extended scene imaging. We determine, through computer simulation, the optimum diversity distance for various scene types. Using laboratory data, we compare the aberrations recovered with the phase-diversity algorithm and those measured with a Fizeau interferometer that uses a He-Ne laser. The two aberration sets agree with a Strehl ratio of over 0.9. The contrast of the recovered object is found to be ten times that of the raw image.

On chip Fourier processing to enhance SNR in the presence of background noise

Lockheed Martins Advanced Technology Center in Palo Alto has developed a prototype 8x8 NIR focal plane capable of simultaneous Fourier processing per pixel. Experiments will be described in which 10 kHz Fourier processed frame rates are used to drive a closed loop tracking servo loop. The pointing direction of one laser with wavelength λ1, and amplitude modulation f1 is adjusted with a fast stirring mirror to track the motion of two targets illuminated by two other lasers with wavelengths λ2 and λ3, amplitude modulated at two other distinct frequencies. Closed loop tracking control at 1kHz is demonstrated using only the single 8x8 focal plane to sense position of the three lasers simultaneously at all pixels. Random noise generated by heat sources and a fan applied to the track laser beam path, and a white light source with 100x larger signal then the received laser signal shown directly on the focal plane, has no effect on the track loop. A final discussion will show the capability of the sensor to simultaneously measure range, as well as position.

The black fringe wavefront sensor: white light real time analog phase control

White light interferometry techniques to obtain 3D surface profiles or rms surface roughness measurements for microscopic instruments are used to develop an adaptive optics wavefront sensor for long range correction of lasers and images. Combining these techniques with a high power incoherent or multiline laser and a radial shear interferometer, a black fringe wavefront sensor (bfwfs) has been developed over the last two years at Lockheed Martins Advanced technology Center. The bfwfs will be described, and results of recent tests shown using a 16 channel device. The 16 ch system is used to obtain measurements of open loop influence functions, and closed loop Bode plots using a Mems mirror. The bfwfs device can be used for adaptive optics at long ranges on weight or volume limited platforms because it allows high power incoherent lasers or other broadband sources to be combined with a parallel architecture and inherently analog servo system. Results are reported in which a superluminescent laser diode (SLD) and a multiline cw Argon laser are combined with a radial shear interferometer to measure phase at 200 Hz with 1/20 pv accuracy.

Practical concerns for phase diversity implementation in wavefront sensing and image recovery

In this paper, we present the results of the phase diversity algorithm applied to simulated and laboratory data. We show that the exact amount of defocus distance does not need to be known exactly for phase diversity algorithm. We determine, through simulation, the optimum diversity distance. We compare the aberrations recovered with the phase diversity algorithm and those measured with a Fizeau interferometer using a HeNe laser. The two aberration sets agree with a Strehl of over 0.9. The contrast of the recovered object is found to be 10 times that of the raw image.

Strehl ratio meter

The on axis intensity of a focused beam is a critical measurement of laser wavefront quality. This paper describes a simple method and device for measuring the Strehl ratio. The device requires a collimated beam of known input size and wavelength. The experimental Strehl ratio is determined by the power within the central 1/5 of the Airy disc diameter. Error sources are discussed, as well as experimental measurements.

Astigmatism effects in a CO2 unstable ring resonator

This paper details experimental and numerical studies of the effect of astigmatism on the performance of an unstable ring resonator with eight mirrors, causing a round trip 180 deg beam rotation, or seven mirrors, producing a beam flip. The resonators were studied with integer and integer plus one half equivalent Fresnel numbers and with and without intracavity spatial filtering. The experiment was performed on a CW electric discharge, fast flow CO2 unstable ring resonator with two removable and orthogonal focal line apertures (FLAs). The astigmatism was produced by rotating one cylindrical focusing mirror with respect to the other, producing astigmatism oriented at 45 deg to the focal line axes. Intracavity power, near field intensity distribution, and far field power and beam quality measurements were taken. The far field beam quality behavior with astigmatism was not very sensitive to the Fresnel number, but was dramatically dependent on the number of mirrors.

Experiments on an unstable ring resonator with an intracavity spatial filter: some anomalies

During a series of experiments to test the concept of using an intracavity spatial filter to improve far field resonator performance, some interesting anomalies with established theory was discovered. A brief description of the gain medium and resonator geometries will be followed by a detailed discussion of the behavior.