The MorphOptic Rapid Additive Mirror Process
In order to address these challenges we have developed a Rapid Additive Mirror Process (RAMP). The RAMP techniques MorphOptic (MO) has developed are unlike abrasive or printable deposition-based nanoscale additive surface technologies.
RAMP uses relatively inexpensive 3D printer technologies in combination with new physics-based techniques for shaping mm-scale thickness glass without roughening the smooth fire-polished front surface of our mirror starting point – commercially available thin borofloat glass plates.
Current Methods of Optics Polishing are Difficult, Labor Intensive and Expensive
The energy concentrator mirrors that are used for low-power lasers and high resolution imaging are often a bottleneck in the development and deployment of optical communication constellations (on the ground or in space) and for remote sensing and direct imaging.
High quality paraboloidal optics (on- or off-axis) typically require many cycles of abrasive polishing and 10-nanometer-scale metrology. Meter-scale paraboloids can cost $0.4M/m2 and are often the pacing component of imaging (or communication) systems.
Through experiment, precise metrology, and physics-based models, MorphOptics will advance our RAMP curvature-polishing as an important tool for the low-cost fabrication of low-mass high-quality imaging and energy concentrator optics in support of space domain awareness, robust satellite constellations for ISR and optical communication networks.
The MorphOptic RAMP Mirrors are:
Mirrors are an order of magnitude lighter than abrasively polished mirrors
The front surface of our mirrors are smooth, with non-specular scattered light an order of magnitude less than typical abrasively polished or deposition-based mirrors
Our mirrors are easily fabricated as off-axis energy concentrators or mild free-form optical surfaces,
Our glass-shaping techniques are deterministic and accurate at the 10-nanometer scale
Our technology is inexpensive using COTS fabrication equipment and can decrease optical mirror costs by an order of magnitude or more.
Our technology has several advantages that enable:
Smaller and less massive satellites
Curvature polished mirrors are typically 10x lighter with savings that propagate into the overall satellite system mass and costs. For example, a 1kg 10cm conventional mirror may be only 50g
Detection of faint objects
Faint targets that are closely spaced to bright sources can be more readily detected: MorphOptic mirrors are smooth on the smallest spatial scales and so have low scattered light to see spatially complex high contrast space targets.
Optical systems cost reduction
Satellite optical subsystem costs can be reduced by 10x: MorphOptic mirror fabrication is deterministic and requires only a few iterations in contrast to the 10-50 cycles that current abrasive techniques require.
Complex reflective shapes
More light efficient and compact optical system designs are possible using complex reflective shapes: The MorphOptic polishing algorithms can readily create aspheric and even mild free-form optical mirrors.
Less expensive small quantity optics
Overall costs of even small quantity optics can be 10x less than conventional polished optics: MorphOptic fabrication technology uses slightly modified off the shelf hardware with small entry and continuing costs.
High bandwidth optical communication
Our process promotes high bandwidth optical communication and dense satellite networks. The cost of a typical meter-scale precision mirror can cost millions. This cost is often the overall pacing components in imaging and communication systems.
Figure 1: Spatial temperature distribution during 100ms laser pulse and the corresponding central surface temperature versus time. The purple region on the left graphic is the glass and the surface region illuminated by CO2 laser is shown by the central circle. The color scale gives the instantaneous temperature in this result from the end of the laser pulse.
Since the glass is opaque to 10μm infrared light the laser energy is deposited within just a few microns of the surface in a roughly 300micron diameter spot. Note that the vertical scale in the figure below left graphic is the glass thickness of 2mm. The laser energy is rapidly conducted into the glass but the local temperature still greatly exceeds the melting temperature for 10’s of millisec while it is a relatively inviscid liquid for about 100ms. The mechanical response of glass in the interior plate to this rapid thermal expansion and radiative and conductive cooling produces a change in the material stress distribution. The Figure 2 illustrates the stress magnitude (VonMises) and material deformation long after (30s) the laser pulse. The smooth deformation of the glass plate lower surface in this simulation extends far beyond the energy deposition region because it is the local stress, not ablation that shapes the mirror. The stress, like surface tension, locally changes the glass curvature on the optical surface. This is the curvature polishing effect we can apply to an arbitrarily large area of the glass plate in order to shape a resulting thin glass mirror. The shape change due to this effect in the figure below extends far beyond the laser spot – as we validate with direct measurements.
Figure 2: Induced stress amplitude and material deformation 30s after laser pulse (Left). The bottom (optical) surface deformation is shown on the right)
Figure 3 illustrates the white-light flash generated by the glass from the laser pulse when the energy density exceeds a certain threshold. We have measured optical spectra of the integrated flash in the figure below from 4 different laser spot sizes. These illustrate several features. Narrow resonant atomic emission lines for sodium near 589nm and from potassium near 770nm are clearly visible. The broader emission peaks near 490nm are likely due to glass fluorescence and the broadest peak near 750nm, superposed on a redward-increasing IR background, appears to be the peak temperature component of the integrated blackbody radiation from the glass. These curves suggest that the highest glass temperatures we observe can approach 4000K — larger than the 2000K in the Figure 1 simulation. This is not surprising since some of the viscous and thermal material properties of the COMSOL simulation of glass are not well known. It is interesting that the visible luminosity seems to be more directly related to the peak energy density deposited on the glass rather than the total luminosity in the lased-glass spot.
Figure 3: Glass lasing image captures the visible light thermal pulse from glass (left) due to the CO2 laser. Integrating this light in time and space over a single pulse and over the laser spot profile reveals the light spectrum (right). These were obtained from 4 different focus positions – 0 defocus yielded the highest spatial energy density deposited in the glass and the largest visible power, while the other focus settings absorbed the same total energy, but successively lower peak spatial energy density.
Interestingly, the numerical models show that the maximum stress in the glass does not occur at the central spot. Figure 4 illustrates the final von Mises stress in the glass and shows that the stress reaches a peak in a circular arc at a radius of about 1mm from the lased spot in these models. This effect is qualitatively validated in our measurements.
Figure 4:Final laser-induced stress distribution within the glass (left). The final stress distribution is also plotted along a radial cut on the glass top surface (right).
Figure 5: Micrograph images of laser spots on glass. The 10ms pulsed laser spot image (left) has 300 micron diameter. Laser spot (right) shows circular crack formation due to high stress.
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