Lockheed Martin's bold project to shrink the telescope
PICs offer a radically different approach to space exploration, as experts from Lockheed Martin's Advanced Technology Centre explain in an exclusive article prepared for PIC International Magazine.
The future of space exploration isn't a race for Mars, it's a race for size. As space agencies reconcile smaller budgets with the call to study the cosmos, many are looking to shrink the size of the optical systems that dominate spacecraft, like the Hubble Space Telescope.
"If the instruments can be smaller, they could go on smaller spacecraft, opening the door for multi-platform space probes and lower launch costs," said Alan Duncan, an optical systems expert and Lockheed Martin Fellow at the corporation's Advanced Technology Centre in Palo Alto, California. "That's a big quandary, how to shrink down massive structures to something you can hold in your hand. But that's the type of challenge we like at Lockheed Martin."
Figure 1. Lockheed Martin's compact lenslets are tiny in scale compared to this penny. With small lenslet groupings, the shape of a larger sensor can take many forms, even conformal architectures. Credit: Lockheed Martin
From space, the need for higher-resolution imaging requires bigger and bigger telescopes to the point where the size, weight and power of the telescope can completely dominate a system. Duncan's solution, an imaging technology called Segmented Planar Imaging Detector for Electro-optical Reconnaissance (or SPIDER), could reduce the size, weight and power needs for telescopes by 10 to 100 times.
Duncan's team at Lockheed Martin is taking a new look at how to process imagery by using interferometry. Funded by NASA and the Defense Advanced Projects Research Agency, they are developing this capability in the heart of Silicon Valley at the Advanced Technology Center. This is also the home of Lockheed Martin's Optical Payload Center of Excellence, which brings together the collective expertise of the corporation's space observation professionals, including a set of renowned interferometry experts.
Flipping the concept
Large-scale interferometer arrays, located in observatories around the world, are used to collect data over large periods of time to form ultra-high-resolution images of objects up in space. However, SPIDER flips that concept, staring instead from space, and trading person-sized telescopes and complex combining optics for hundreds or thousands of tiny lenses using PICs to combine the light to form interference fringes.
Figure 2. SPIDER holds significant potential for space applications. A communication satellite like this one could serve a dual purpose by hosting a flat, optical payload made possible with SPIDER technology. Previously, due to size and weight, optical payloads often needed their own satellite structure and power to carry out their missions. Credit: Lockheed Martin
"What's new is the ability to build interferometer arrays that have the same number of channels as a digital camera," Duncan said. "The conventional approach for imaging interferometers requires complex mechanical delay lines to form the interference fringes resulting in designs that are not traceable to more than a few simultaneous spatial frequency measurements. SPIDER achieves simultaneous measurements on many baselines by employing micron-scale optical waveguides and nanophotonic structures fabricated on a PIC with micro-scale packing density to form the necessary interferometers."
Lockheed Martin's unique use of PICs helps create sharp images without increasing size.
"Our SPIDER concept consists of thousands of direct detection white-light interferometers densely packed onto PICs to measure the amplitude and phase at frequencies that span the full synthetic aperture," Duncan said. "This provides an increase in resolution while maintaining a thin disk."
PIC technology was critical for the design. Duncan partnered with the University of California, Davis to leverage their research using PICs for compact electronic systems. Duncan thought the way PICs manage light could be useful in orbit.
For the newest version of the SPIDER concept, PICs perform several forms of optical processing: 1) collect light into wave-guides, 2) apply phase shifts to the light, 3) coherently combine baseline pairs (lenslets with different separation), 4) demultiplex the light into spectral bins, and 5) photo detection of the complex fringe pattern.
Figure 3. This detailed look at how the SPIDER system works includes rows of lenslets (top) connected to PICs (middle) that feed data to processors that generate an image. Each lens-PIC grouping works in teams to take in light from various points to create an image. Credit: Lockheed Martin
On the system level, the SPIDER demonstration design uses an array of 19 high resolution PIC cards arranged in a radial pattern where each card contains collection optics, photonic circuits, detectors and readout electronics. Along the top of each card is a linear array of 16 lenslet assemblies. Each lenslet assembly is a small, compact telescope with a K-mirror for image rotation and an off-axis parabolic mirror to couple light into the waveguides on the PIC.
Risk reduction
SPIDER's PICs do not require complex, precision alignment of large lenses and mirrors. That means less risk on orbit. And its many eyes can be rearranged into various configurations, which could offer flexible placement options on its host. Telescopes have always been cylindrical, but SPIDER could begin a new era of different thin-disk shapes staring in the sky, from squares to hexagons and even conformal concepts.
Figure 4. SPIDER can be configured in different arrays, from radial like the early demonstration models, to this rectangular concept. Despite the larger array - taking in more light - its thickness remains the same. Credit: Lockheed Martin
"When you think of what mission customers such as NASA want to accomplish, like going to Jupiter's moon Europa, they need affordable alternatives to sending the mass of a optics-laden spacecraft into deep space," Duncan said. "Our creative use of PICs in SPIDER helps to ensure that scientists can continue exploring our solar system, just with an incredibly different kind of optical system based on incredibly different kind of thinking."
