Air-to-ground lasercom system demonstration design overview and results summary

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Air-to-ground lasercom system demonstration

design overview and results summary

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Citation

Walther, Frederick G. et al. “Air-to-ground lasercom system

demonstration design overview and results summary.” Free-Space

Laser Communications X. Ed. Arun K. Majumdar & Christopher C.

Davis. San Diego, California, USA: SPIE, 2010. 78140Y-9. © 2010

SPIE.

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http://dx.doi.org/10.1117/12.864262

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Society of Photo-optical Instrumentation Engineers

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Final published version

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http://hdl.handle.net/1721.1/61648

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Air-to-Ground Lasercom System Demonstration

Design Overview and Results Summary

Frederick G. Walther, Steven Michael, Ronald R. Parenti, and John A. Taylor

Massachusetts Institute of Technology Lincoln Laboratory, Lexington, MA 02420-9108

ABSTRACT

We present an overview of an air-to-ground laser communications demonstration performed at MIT Lincoln Laboratory. Error-free communication at 2.5 Gb/s was demonstrated along a 25-km slant path between a 1-in transmit aperture on an aircraft at 12 kft altitude and ground terminal with 4 separate 1-cm receivers. Power ﬂuctuations from turbulence-induced scintillation are mitigated in the spatial domain by use of the multiple ground receivers and in the time domain by the use of forward error correction and interleaving. The optical terminals are monitored by multiple high-rate sensors which allow us to quantify total system performance.

Keywords: free-space optical communications, atmospheric turbulence, optical scintillation, pointing and

track-ing, forward error correction, interleavtrack-ing, diversity, ﬁeld demonstration

1. INTRODUCTION

An important future application of lasercom is support of tactical intelligence, surveillance, and reconnaissance (ISR) missions, delivering near real-time sensor data to the operational theater. Representative objective re-quirements include the ability to transport data at rates up to tens of Gb/s over a range of roughly 50 km. The air terminal should have low size, weight, and power (SWaP), and low integration impact on the aircraft. Use of standard terrestrial network protocols can simplify the implementation, particularly at the client interface. A key advantage of lasercom is its ability to provide high bandwidth without adding to RF congestion in theater. An example of such an application, currently under development at Lincoln Laboratory, is the Multiple Aperture Sparse imager Video System (MASIVS), an imaging system with 880 megapixels generating in excess of 20 Gb/s of raw data that can be near-losslessly compressed to about 2 Gb/s. The work described in this paper is directed towards providing an air-to-ground optical link that will allow the MASIVS system to relay data to users on the ground in real time.

Optical links based on single-mode fibers in the 1550-nm band are commonly used in commercial telecom and provide a readily available base of COTS components, including efficient pre-amplified receivers and components that support a wide range of modulation formats. In free space communications the fiber connectivity between transmit and receive nodes present in terrestrial telecom is replaced with a power delivery subsystem the sole purpose of which is to deliver power from the transmit fiber to the receive fiber as efficiently as possible. Communications performance depends on both the mean power in fiber and the fluctuations of power in fiber. Design approaches must maximize average power and minimize its variance in the presence of atmospheric turbulence and platform jitter.

Figure 1 diagrams the effects of the channel on communications performance. Power in fiber is a product of both power delivered to the receiver aperture and the coupling efficiency from aperture to fiber. Transmitter platform jitter and turbulence in the far field of the receiver both lead to fluctuations in power delivered to the aperture, the well-known scintillation problem. In addition, turbulence near to the receiver causes phase distortion in the aperture that reduces the efficiency of power coupling into fiber. Traditionally, phase distortion can be corrected with adaptive optics; the lowest order distortion, tilt, can be corrected with fast tracking systems. Moreover, as long as the receiver aperture is small compared to the phase distortion scale, r₀, tilt tracking is sufficient to ensure high coupling efficiency, avoiding need for higher-order phase corrections.

The design used in this work exploits 1-cm ground receiver apertures which use tilt-only tracking and which have 8-m diffraction limited imaging resolution at a 50-km slant range to the aircraft. This provides the flexibility to use multiple apertures at the aircraft, if desired, to avoid structural blockage or to allow seamless handover between multiple aircraft apertures with limited field of regard. The small apertures ensures that high-order phase aberrations are negligible; scintillation is the dominant atmospheric turbulence effect, and must be mitigated.

Proc. of SPIE Vol. 7814 78140Y-1

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A similar optical module is used in the ground terminal, as depicted in Figure 8. The beam after the FSM is directly in object space; no further transmit optics are used on the ground. One stabilization technique, reported earlier,3 is to dither the FSM in a conical scan, allowing the incoming fiber power to be detected synchronously providing a nutation tracking discriminate for boresight alignment of fiber to camera by adjusting the beam splitter during operation. As on the aircraft optical module, a flip-in retro reflector provides the means of initially boresighting the fiber to the tracking camera.

Two such modules are assembled on each side of a plate; two such plates are combined to form the 4 aperture ground module. The unit is integrated with a commercial pan-tilt unit to form a wide steerable opto-mechanical system. A broad uplink beacon and a wide ﬁeld of view acquisition camera are integrated on the pan-tilt unit. The completed assembly is mounted in a rotatable optical dome mounted on the roof of a lab building directly above the ground terminal test facility.

4. PERFORMANCE OF THE POWER DELIVERY SYSTEM

For initial signal acquisition, the pointing burden is placed on the aircraft since it has all the information required to point to the ground terminal. The ground terminal requires knowledge of the aircraft position, which can be obtained through a RF downlink if available. For this work, the aircraft was acquired optically by ﬂying through predetermined waypoints. After initial acquisition, any reacquisition can be achieved by propagating the aircraft motion. In an actual ISR mission with the aircraft in a pre-known orbit, a wide acquisition camera on the ground would suﬃce to acquire the downlink beacon.

λ / D λ / D −4 −2 0 2 4 −4 −2 0 2 4 (a) Aircraft λ / D λ / D −4 −2 0 2 4 −4 −2 0 2 4 (b) Gnd Terminal 1 λ / D λ / D −4 −2 0 2 4 −4 −2 0 2 4 (c) Gnd Terminal 2 λ / D λ / D −4 −2 0 2 4 −4 −2 0 2 4 (d) Gnd Terminal 3 λ / D λ / D −4 −2 0 2 4 −4 −2 0 2 4 (e) Gnd Terminal 4 0 1000 2000 3000 4000

Figure 9: Focal plane images of received bean at aircraft and ground. Center of white circle indicates track point; circle diameter is 2λ/D.

Sample frames from tracking camera on the aircraft and the four tracking camera on the ground are shown in Figure 9. The tilt-only tracking system is suﬃcient to deliver well-contained distributions in the focal plane. The beam spot is smaller for the ground terminal, with nearly diﬀraction limited performance. The aircraft power distribution is reasonably well contained, but is spread a bit in elevation. A known handover issue between the fast steering mirror and the gimbal contributes to this distortion on the aircraft The potential impact of airframe boundary layer turbulence is still under investigation, although it appears to be modest. Not shown in Figure 9 is the track jitter for the air and ground terminals, which we measure to be less than 0.3λ/D.

Figure 10(a) overlays probability densities of power-in-aperture and power-in-fiber. With good tracking and low higher-order phase distortion, the densities should closely match. A stiction-induced tracking disturbance in the wide area beam director caused occasional mirror jumps. Contours of the residual centroid log probability are shown in Figure 10b. The low probability jumps along the direction of the two gimbal axes are easily identified. The result is deep fades in the fiber power distribution that do not appear in the aperture distribution, resulting in the rising tail seen in Figure 10a. We can remove these jumps by ignoring points where the plane orientation (as measured by the IMU) is changing quickly. The result is well matched distributions of fiber and aperture power, indicating good tracking performance and the absence of higher-order phase disturbances.

The distributions in Figure 11 illustrate the single and combined 4-aperture distributions for two cases: 50-km range afternoon and 25-50-km range evening conditions. Overlaid on the charts are wave propagation model simulations for the respective cases, scaled with a multiplier for best ﬁt relative to the well known Hufnagel-Valley

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−15 −10 −5 0 5 10 10−6 10−4 10−2 100 Normalized Power (dB) Probability Density

Intensity on Focal Plane Power in Fiber

Power in Fiber (Gimbal Jumps Removed)

(a) Power Distributions

λ / D λ / D −4 −3 −2 −1 0 1 2 3 4 −4 −3 −2 −1 0 1 2 3 4 σ_az = 0.29 λ / D σ_el = 0.25 λ / D Elevation = −4.9 deg Azimuth = −26.2 deg

(b) Contours of Residual Centroid Log Probability

Figure 10: Coupling Eﬃciency

−70 −60 −50 −40 −30 −20 −10 0 10−5 10−4 10−3 10−2 10−1 100

Received Power in Fiber (dBm)

Probability σ₁2 = 2.44 σ₄2 = 0.65 ratio = 3.7 H−V 5/7 Multiplier = 1.1 Single Channel 4−Channel Sum Simulation

(a) Afternoon, 50-km range

−70 −60 −50 −40 −30 −20 −10 0 10−5 10−4 10−3 10−2 10−1 100

Received Power in Fiber (dBm)

Probability σ₁2 = 0.36 σ₄2 = 0.10 ratio = 3.6 H−V 5/7 Multiplier = 0.69 Single Channel 4−Channel Sum Simulation (b) Evening, 25-km range

Figure 11: Measured Scintillation with Simulation Overlay

5/7 (HV 5/7) turbulence model.4 Afternoon conditions ﬁt well to 1.1 x HV 5/7, while evening conditions fall to 0.7 x HV 5/7. Over the range of conditions measured, from 20 to 60 km and for various times of day, the HV 5/7 channel model ﬁts the data with multipliers from 0.4 to 2. In all cases, spatial diversity reduces the variance in received power.

5. COMMUNICATIONS SYSTEM PERFORMANCE

The data flow used for communications performance evaluation is shown in Figure 12. Stored 6 minute data files of MASIVS imagery were delivered in ethernet format and encapsulated in Synchronous Optical Networking (SONET) frames to match the input format of our existing high speed electronics (HSE) hardware. The the HSE system performed OTU1 data encapsulation, adding RS(255,239) FEC and interleaving over a 1.25-s span, with inter-symbol spacing of 5 ms. On-off keying was chosen as the optical modulation format. Detection and clock recovery were followed by deinterleaving, FEC decoding, delivery of SONET data and extraction of the original ethernet data stream for display processing and storage.

One metric of communications performance is the ability to deliver error free data files of significant length. Our basic unit was a 6 minute image file of 100 Gbytes. With 1 Watt of optical transmit power, the link delivered

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19:400 19:45 19:50 19:55 20:00 20:05 2 4 6 8 10x 10 5 10/22/2009 Dropped Frames Dropped Frames Distance 40 45 50 55 60 Distance (km)

(a) Moderate Turbulence

13:500 13:55 14:00 14:05 2 4 6 8x 10 5 10/19/2009 Dropped Frames Dropped Frames Distance 39 40 41 42 43 44 Distance (km) (b) Strong Turbulence

Figure 13: Frame Errors

The short 6-month development time for this experiment left little room for system optimization. Several improvements are planned as we move towards a deployable system. The HSE system will undergo a signiﬁcant SWaP reduction, even as the data rate is increased to 10 Gb/s. A more eﬃcient optical waveform and larger diameter ground apertures will allow the higher data rate without a corresponding increase in transmit power. The control system for the ground station will be reduced in size and the entire terminal will be ruggedized and packaged to be transportable. The optical bench on the aircraft will be reduced in size and the control computer reduced to a single 4-in cube stack. The beam director will be moved outside the aircraft below the fuselage to allow for hemispherical coverage. The use of passive isolation will be considered and compensation bandwidths increased as the aircraft terminal moves from the soft ride of a damped optical table within the Twin Otter to the harsh angular and linear vibration environment of a deployable aircraft. We are also considering the use of alternative commercial beam directors that provide inertial stabilization and have a heritage of aircraft integration.

REFERENCES

[1] Moores, J. D., Walther, F. G., Greco, J. A., Michael, S., William E. Wilcox, J., Volpicelli, A. M., Magliocco, R. J., and Henion, S. R., “Architecture overview and data summary of a 5.4 km free-space laser communication experiment,” Free-Space Laser Communications IX 7464(1), 746404, SPIE (2009).

[2] Greco, J. A., “Design of the high-speed framing, fec, and interleaving hardware used in a 5.4km free-space optical communication experiment,” Free-Space Laser Communications IX 7464(1), 746409, SPIE (2009). [3] Murphy, R. J., Volpicelli, A. M., William Wilcox, J., Crucioli, D. A., and Williams, T. H., “A conical scan free

space optical tracking system for fading channels,” Free-Space Laser Communications IX 7464(1), 74640P, SPIE (2009).

[4] Smith, F. G., ed., [The Infrared & Electro-Optical Systems Handbook, Volume 2: Atmospheric Propagation

of Radiation ], SPIE Optical Engineering Press (1993).

This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government