FIGS. 8 and 9 illustrate several spherical tiling embodiments that can be used to cover wider areas or even the full four pi solid angle, using multiple LPASS modules. In embodiment 340 of FIG. 8, 4 90-degree×90-degree LPASS transmit modules 350, 360, 370, and 380, are shown tiled to provide a 360-degree lateral coverage. Two more modules could be used to cover the top and bottom regions, providing full spherical coverage. System 440 of FIG. 9 shows that when three 120-degree×120-degree FOV LPASS modules 450, 460, and 470, are tiled together for 360-degree lateral coverage, there is only a small top and bottom cap region uncovered. Again, these could be covered by two additional modules. Other tiling embodiments of the present teaching follow from these embodiments. While the solid angles shown in these examples are symmetric, any shape or geometry Solid Angles Can Be Filled Using the LPASS Modules.

Referring to FIG. 8, two more modules, one for the top cap and one for the bottom cap, will provide 4-Pi steradian coverage. Alternatively wider field ASSOCLPASS modules can be used such as the 120-degree field case shown in FIG. 9.

Referring to FIG. 9, in this case only a small field is left uncovered in two directions, and additional modules may not be required depending on the application.

A corner cube retroreflector 500 is illustrated in FIG. 10. The corner cube retroreflector and similar devices retro-reflect light back in the direction of the source and can be used in conjunction with the LPASS technology as described above for vehicle location and tracking.

This and similar devices retro-reflect light back in the direction of the source and can be used in conjunction with the above description for tracking. When a modulator is incorporated in the device 500, it becomes a retromodulator and can transmit data back to the source or first vehicle with an extremely low power requirement in the second vehicle since the second vehicle only modulates the incident light from the first vehicle and does not need to source any optical power. The modulator can be formed One embodiment of a Wavelength Division Multiplexed (WDM) retromodulator of these teachings is described below, and a variation on it to provide very broad angular modulations at one or several wavelengths is particularly useful in conjunction with the LPASS technology, particularly when power is extremely limited on the second vehicle, Or when rapid communication must be established between two vehicles.

In the wavelength division multiplexed (WDM) retromodulator, multichannel capability and allocable bandwidth is obtained by cascading a single or typically up to 5 to 10 or more relatively narrow band reflectance modulators to form independently modulated parallel optical channels which can be simultaneously read at the same time. The retromodulator of these teachings is illustrated in FIG. 11.

In the high data rate retromodulator 590 shown in FIG. 11, one to ten or more narrow-band reflectance modulators (switchable volume holographic mirrors) are cascaded in the aperture of a corner cube retroreflector. A multi-wavelength probe beam is used to simultaneously extract the multichannel data. As shown in the inset, each switchable holographic mirror modulates an isolated narrow spectral band and leaves other wavelengths unaffected-they transmit through nonresonant switchable mirrors with little or no interaction. Separate vehicles or satellites can communicate on different channels, or the channels can be combined in various ways (real time bandwidth allocation) to provide multiples of single-channel bandwidth to needy links.

In the high data rate retromodulator 590, a cascade 570 of narrow-band reflectance modulators 510, 515, 520, . . . , 540, 545, 550 (switchable volume holographic mirrors) are cascaded in the aperture of a corner cube retroreflector 500. These switchable mirrors are driven by multichannel communications electronics 574. A multi-wavelength probe beam 580 is used to simultaneously extract the multichannel data. This probe beam can emanate from vehicle or ground station 585. As shown in the inset, each switchable holographic mirror modulates an isolated narrow spectral band 511, 516, 521, . . . , 541, 546, 551, respectively, and leaves other wavelengths unaffected. Separate satellites can communicate on different channels, or the channels can be combined in various ways (real time bandwidth allocation) to provide multiples of single-channel bandwidth to needy links.

The spectral width of the reflectance bands for each of the cascaded holographic electro-optic mirror modulators can be narrow, and yet still provide for the wavelength tuning over a relatively wide field of view. For the wide angle communication scenarios described earlier where wide FOVs may be required, the WDM modulator described above can be modified to an ultra wide angle retromodulator by spacing the neighboring spectral channels closely and sending the same data to the neighboring channels. In this mode, as one switchable mirror angularly detunes and looses contrast, the neighboring channel spectrally picks up and recovers the contrast.

FIG. 12 shows how a rugged monolithic non-mechanical rad-hard switch technology can be used to steer a high power diode or fiber laser output to any element in a dense packed switch output plane. This plane can replace the VCSEL array with a much higher power array. When this is cascaded behind the LPASS imager, the communication range of the LPASS system can be greatly extended.

Referring to FIG. 12, the switch technology shown there in can route the light from a high power diode laser to one of 32×32 positions in a volume not much larger than 1″×1″×1.5″. This can replace the VCSEL array with a dense source that contains several watts or more of power per beam. Optical switch 600 of FIG. 12 uses cascaded switchable transmission gratings 610, 620, 630, 640, and 650 to route and input beam from channel 660 to any of the 16 output channels and column 686. The switchable gratings are separated by spacers 642, 644, and 646. The first switchable rating 610 switches the input signal from channel 660 between transmitted channel 662 and diffracted channel 664. Second switchable gratings 620 routes light incident on channel 662 among transmitted channel 666 and 668. If the signal has been routed in channel 664 by the first switchable rating the second six switchable grading routes that light between channels 672 and 670, and so on. There are 15 more input channels directly behind in parallel to channel 660 that work in the same fashion but are independently controlled. An array of 16 pixels or high power sources can be used to drive each of the 16 channels which can therefore be steered to any upper channel in each of the 16 columns about the channels. Alternatively a 1×16 switch using the same technology can be used with a single high-powered laser or source to steer it among any of the 16 inputs to switch 600 which thereby allows routing to any of the 16×16 output channels. The 16×16 output channels can be used to replace the VCSEL arrays in the earlier described embodiments.

Referring to FIG. 13, as described previously the compact optical switch routes one or more high power diode laser output to the dense input array of the infinite conjugate imager 60, effectively replacing the VCSEL array with a much higher power source 720. This can extend the LPASS communication range over mid-to long distances. The output plane of the switch is roughly an inch square.

Other embodiments of these teachings include the retromodulator disclosed above.

The embodiments of the retro-modulator of these teachings shown herein above obtain multichannel capability and allocable bandwidth by cascading 5 to 10 relatively narrow band reflectance modulators to form independently modulated parallel optical channels which can be simultaneously read at the same time.

In one instance, a retromodulator device includes switchable volume holographic mirrors (SVHMs). Switchable volume holographic gratings that exhibit a 10-20 kHz bandwidth and a 20 dB have been demonstrated and can be used in place of the switchable mirrors. These are the same switchable transmission gratings described in system 600. In the grading off for transmitting state the transmission grating is transparent and light propagates into and out of the retro-modulator. To modulate the beam of the corresponding wavelength of the grading off the volume transmission grating is turned on Indians and lightest diffracted at a large angle outside the acceptance of the corner cube prism. The thickness, refractive index modulation, and spatial. Of each of the cascaded volume transmission gratings are selected to interact substantially only with the beam of the corresponding wavelength for that grating modulator.

Optical Properties of Bragg Reflectors

In one instance, the retromodulator is based on switchable volume holographic mirrors. The relevant optical properties of static Bragg reflectors (BR) are presented here as a background to understanding the switchable holographic mirrors referenced above. In detail, a volume holographic mirror has sinusoidally or otherwise varying refractive index regions (rather than step-index profiles), however, we expect that the major features of the two structures are very similar. Here the simpler dielectric stack model is used for first order properties. A schematic diagram of a Bragg reflector, or switchable holographic mirror BR, is shown in FIG. 14. The BR 800 is comprised of a periodic stack of dielectric film pairs 864 with layer thicknesses 860 and 862 and optical indices n₁and n₂, respectively. The spatial period 864 of the structure is Λ=a+b and in the present model, the indices of refraction of the media surrounding the BR is taken to be the spatially averaged index of the mirror.

The BR shown in FIG. 14 is comprised of a multilayer stack of dielectric media. The spatial period is Λ=a+b, where a and b are the thicknesses of layers with indices of refraction n₁and n₂, respectively. Such structures act as mirrors over limited wavelength and angular ranges for both TE and TM polarized light, and otherwise transmit light with little loss.

The optical properties of Bragg structures have been described in terms of the physical quantities presented above using coupled mode theory. Such analysis shows that the center wavelength, λ_C, and spectral bandwidth, Δλ, of the periodic structure for light incident at normal incidence are given by

λ_C=2nΛ, (1a)

and

Δ
⁢
λ

=

λ
C

·

(

2
π

⁢

Δ
⁢
n

n
¯

)

,

(

1
⁢
b

)

where

n
¯

=

(

n
1
2

+

n
2
2

)

/
2

is the spatially averaged mirror index and Δn=|n₁−n₂| is the layer index mismatch. Equation (1a) is a statement of Bragg's law in the optical regime, hence the nomenclature, Bragg reflector.

The analysis shows that the first-order reflectance feature of the structure shown in FIG. 14 is given by

R
=

❘
"[LeftBracketingBar]"

κ

❘
"[RightBracketingBar]"

2

s
2

⁢

coth
2

(

s
⁢
L

)

+

(

Δβ
/
2

)

2

,

(
2
)

where s²=|κ|²−(Δβ/2)², the phase mismatch factor is Δβ=2 k cos θ−2π/Λ, k=nω/c is the optical wavenumber, the angle of incidence is θ, the total film thickness is L, and the coupling constants for TE and TM light are given by