An optical phased array (OPA) uses phase technology to produce a wide
range of optical images and emissions.. A phased array is a set of
electrical oscillators hooked up to antennas, arranged on a surface and
with a spacing of a moderate fraction of the wavelength of the
radiation they emit. By controlling each of the oscillators
independently, the amplitude, phase, and polarization of the
electromagnetic field can be precisely controlled at the resonant
oscillator frequency and its wavefronts can be shaped as required. A
planar wave front propagates straight (neglecting diffraction), but the
beam can have its direction shifted nearly instantly, and can be
rapidly scanned back and forth if desired. Alternately, the beam can be
made converging, focused to a point in front of the array, or made to
diverge away from the array's surface. This technology is often used in
microwave frequencies as a powerful and rapid form of radar.
An oscillator that resonates at the frequency of visible light requires
very small circuits only nanometers across distributed across a surface
with appropriate spacing. Processing elements on a similar scale
connect these emitters together and control their output. Using such an
array it is possible to create complex visual displays which can mimic
three dimensional objects. By emitting a diverging
beam of light, it will act the same as a beam that came from a point
source behind the OPA's surface.
Someone seeing the light from this beam will see a point of light that
appears to float behind the OPA. If the beam is converging, anyone
seeing the light from the beam past the focal point will see a point of
light hovering at the focal point. By displaying a continuous
distribution of points that describe a surface it is possible to
display an apparently solid object, either in front of, or apparently
behind the OPA.
An OPA can recreate
wavefronts as if they originate from in front or behind the array
Such a display can project a 3 dimensional image at any location, even
a distant object far behind the array. If oscillators of not just one
frequency, but many, are used, it is possible to give the illusions a
variety of colors and make them look lifelike. The OPA can now be used
as a general purpose "holographic" display, replacing televisions,
movies, and computer terminals.
There is an important limitation to this display that may not be
immediately obvious. As long as it is only projecting images that
appear to originate behind the display's surface, there is no problem.
The OPA looks just like a window onto its displayed world.
However, if it projects an image in front of it, an observer can
only
see the image if the light from the beam can enter your eye. This
can only happen if the image is along the line of sight from the
observer to the OPA's surface. Any part of the image that moves so
that it is no longer directly in front of the OPA will cease to be
visible. If the observer moves between the OPA and the image, you can
see the light, but it will make no sense since the observer cannot
focus it onto er retina.
To produce such complex waveforms and images sophisticated computer
programs and ample processing power is required. Such images can be
generated from scratch, or the OPA itself can be used to record the
scene for retransmission. The light which impinges on the OPA is
absorbed by the nanoscale antennas and triggers currents in the
oscillators, currents which can be measured and recorded. A complete
record of the light coming into the OPA can be obtained, which can be
played back by that OPA, or another, to produce a perfect fidelity
recreation of the scene.
An OPA can make objects invisible. As an example, consider two
flat OPA surfaces 30 cm apart facing away from each other. Each
OPA is recording, the signal is processed to account for that extra 30
cm, and then played by the other to produce an image on its surface as
if that extra 30 cm was simply empty space. An object placed between
the two surfaces will disappear. Now, take the concept of the two
OPA slabs, and wrap them around to form a closed surface, for instance
a suit of clothing that covered the entire body. Anyone wearing that
suit would become invisible. Although the eyes would need to be covered
for complete invisibility, the wearer need not be blind - just have an
OPA head's up display on the inside of the suit play back what is
recorded from the outside.
Buildings with supports coated in OPA technology will appear to be
floating in air- however they can be accessed by an (entirely
invisible) elevator or even stairs. A mass of Utility Fog may have an
external layer of OPA which will make it appear transparent. However
this illusion of invisibility is not perfect. One minor limitation to
this technique is that there must be a processing delay between when
the signal is recorded and when it is played back. Even if this is so
small that no one would ever notice it, it means that there is a phase
difference between the light emitted from the edge of the OPA and that
which travels through empty space. To a sensor with good enough
resolution, this appears as a very faint discontinuity, a fringe around
the wearer's silhouette. This effect is very hard for most people to
detect without visual enhancements. While OPAs can work well in the
near and mid infrared, the thermal emission of a room or body
temperature OPA interferes with its ability to emit controlled light in
the far infrared, making thermographic invisibility impossible.
This assumes the OPA itself is at room temperature - if the OPA were
cryogenically cooled, it would work in the far infrared just as well as
at higher frequencies.
OPAs can project beams of light. They can be highly collimated,
coherent, monochromatic beams of light that can be focused at any
desired distance. They can thus be used like lasers. They can be used
for lidar sensors, range finders, bar code scanners, communication,
welding, cutting, drilling, and, if intense enough, as a weapon.
An OPA capable of emitting light at the same intensity as sunlight
would produce approximately a kilowatt per square metre.. A
kilowatt focused to a diffraction-limited point is quite powerful and
can be used for industrial welding, cutting, and drilling, but it is
too weak for most weapon purposes. However a 30 m x 30 m array at 1
kW/m^2 can emit about a megawatt, which is quite sufficient to cause
lethal flash burns while igniting clothing, or to vaporize holes
through thin skinned vehicles given a dwell time of a few seconds. As
the available intensity and/or surface area increases, so does the
potential to use the OPA as a weapon.
Ultimately a phase array can be used as a long distance interplanetary
or interstellar weapon. If an entire planet, moon, habitat or ISO is
covered in phased array optics, the entire surface can produce a beam
which can be focused on a distant point. The larger the object emitting
the light, the longer the range, due to diffraction limitations.
Covering
an entire dyson sphere or Matrioshka brain creates a weapon (known as
a
Nicoll-Dyson Beam) which can
be used to send a powerful beam an immense distance.
