Geoff Andersen

Large optical photon sieve.

A photon sieve with 10(7) holes has been constructed for operation at optical wavelengths. Details of the design, fabrication, and performance of this device are presented. The 1 m focal-length, 0.1 m diameter element is diffraction limited over a significant bandwidth and has a moderate field of view.

Holography-based wavefront sensing

We describe a modal wavefront sensing technique of using multiplexed holographic optical elements (HOEs). The phase pattern of a set of aberrations is angle multiplexed in a HOE, and the correlated information is obtained with a position sensing detector. The recorded aberration pattern is based on an orthogonal basis set, the Zernike polynomials, and a spherical reference wave. We show that only two recorded holographic patterns for any particular aberration type are sufficient to allow interpolated readout of aberrations to lambda/50. In this paper, we demonstrate the capability of detecting errors between +/-2lambda PV for each orthogonal set at rates limited only by the speeds of the detection electronics, which could be up to 1 MHz. We show how we take advantage of the unavoidable intermodal and intramodal cross talks in determining the type, amplitude, and orientation of the wavefront aberrations.

Holographic wavefront sensor

We have constructed a new type of modal wavefront sensor that uses a multiplexed hologram and position-sensing detectors to measure the amplitudes of a preselected set of eight Zernike modes in an input beam. The measurement is all optical, with the calculations made in encoding the holograms themselves. The result is a sensor with no computational overhead or postprocessing that has the potential to operate at megahertz speeds without the need for computer calculations. We have built and tested a prototype device, demonstrating its operation both as a stand-alone wavefront sensor and in a closed-loop adaptive optics control system.

Fast, compact, autonomous holographic adaptive optics

We present a closed-loop adaptive optics system based on a holographic sensing method. The system uses a multiplexed holographic recording of the response functions of each actuator in a deformable mirror. By comparing the output intensity measured in a pair of photodiodes, the absolute phase can be measured over each actuator location. From this a feedback correction signal is applied to the input beam without need for a computer. The sensing and correction is applied to each actuator in parallel, so the bandwidth is independent of the number of actuator. We demonstrate a breadboard system using a 32-actuator MEMS deformable mirror capable of operating at over 10 kHz without a computer in the loop.

Holographically corrected telescope for high-bandwidth optical communications

We present a design for an optical data communications receiver-transmitter pair based on the holographic correction of a large diameter, poor-quality, reflecting primary mirror. The telescope has a narrow bandwidth (<0.1 nm) with good signal frequency isolation (>60 dB) and is scalable to meter-class apertures. We demonstrate the correction of a reflector telescope with over 2000 waves of aberration to diffraction-limited operation, capable of handling data transmission rates up to 100 GHz.

Large-aperture holographically corrected membrane telescope

We have constructed a 1-m-diam holographically corrected membrane mirror telescope for optical imaging. Several thousand waves of surface error were removed using a corrective hologram, resulting in near diffraction-limited performance. A detailed discussion of the mirror, the corrective process, and the performance of the final telescope are included.

Photon sieve telescope

The creation of next generation, ultra-large space telescopes (>20m diameter) will require novel technologies. Many current concepts involve curved membrane reflectors but the problem is creating a diffraction-limited, three-dimensional surface. Here we present the idea of using a flat diffractive element which requires no out-of-plane deformation and is thus much simpler to deploy. The primary is a photon sieve--a diffractive element consisting of a large number of precisely positioned holes distributed over a flat surface. Photon sieves can be simply designed to any conic, apodization and operating bandwidth. The photon sieve is easier to fabricate than the better known Fresnel zone plate as a single substrate can be used since there are no connected regions requiring support. Presented here are results of prototypes capable of diffraction-limited imaging over wide fields and useful bandwidths.

FalconSAT-7: a membrane space solar telescope

The US Air Force Academy of Physics has built FalconSAT-7, a membrane solar telescope to be deployed from a 3U CubeSat in LEO. The primary optic is a 0.2m photon sieve – a diffractive element consisting of billions of tiny circular dimples etched into a Kapton sheet. The membrane its support structure, secondary optics, two imaging cameras and associated control, recording electronics are packaged within half the CubeSat volume. Once in space the supporting pantograph structure is deployed, extending out and pulling the membrane flat under tension. The telescope will then be directed at the Sun to gather images at H-alpha for transmission to the ground. We will present details of the optical configuration, operation and performance of the flight telescope which has been made ready for launch in early 2017.

FalconSAT-7: a membrane space telescope

The USAF Academy Department of Physics is building FalconSAT-7, a membrane solar telescope to be deployed from a 3U CubeSat in LEO. The primary optic is a 0.2m photon sieve.—a diffractive element consisting of billions of tiny holes in an otherwise opaque polymer sheet. The membrane, its support structure, secondary optics, two imaging cameras and associated control/recording electronics are all packaged within half the CubeSat volume. Once in space the supporting pantograph structure is deployed, pulling the membrane flat under tension. The telescope will then be steered towards the Sun to gather images at H-alpha for transmission to the ground. Due for launch in 2016, FalconSAT-7 will serve as a pathfinder for future surveillance missions.

Photon sieve null corrector

We present the concept of using a photon sieve as an inexpensive null-corrector. A photon sieve is a diffractive element consisting of a large number of holes precisely positioned according to an underlying Fresnel Zone Plate geometry. Using diffraction theory we can enlarge the holes significantly beyond the width of a Fresnel zone such that more light is transmitted and greater efficiency is achieved. Added to this, modification of the equations used to generate the hole locations can also permit the construction of any desired wavefront instead of a simple infinite-conjugate ratio focusing optic. This makes it an ideal optic for testing larger components in a null-corrector configuration. In this talk we will present theoretical and experimental results from tests of this idea. These include the fabrication of a 0.1m diameter intensity photon sieve null-corrector specifically for testing a parabolic primary.