WL1 (Invited) 1o:m - 11 :oo

Active Pointing for Terrestrial Free Space Optics

G. Stephen Mecherle Chief Scientist,

fSONA Communications smecherle@ fsona.com

www.fsona.com

Abstract Fixed-pointed Free Space Optics (FSO) systems must use beamwidths that are broadened sufficiently to account for building motion, wind loading, and other pointing instabilities. Some general tradeoffs with regard to active pointing are discussed, and the specific approach selected for fSONA's newly developed commercial actively pointed FSO product is described.

Introduction Fixed-pointed Free Space Optics (FSO) systems for communication through the atmosphere must use transmitting and receiving beamwidths that are broadened sufficiently to account for building motion, wind loading, and other pointing instabilities. The addition of an active pointing system - which includes an optical tracker and a gimbal or fast-steering mirror - can enable beamwidths more than an order of magnitude narrower, with potentially more than 20 dB improved link margin. Actively pointed systems can theoretically support beamwidths that are limited by broadening due to atmospheric turbulence and scattering. Some general tradeoffs with regard to active pointing are discussed, and the specific approach selected for fSONA's newly developed commercial actively pointed FSO product is described. Test results in real world atmospheric conditions are presented.

Active Pointing Tradeoffs System design tradeoffs for an active pointing system for terrestrial free space optics include:

Gimbaled transceiver vs. gimbaled mirror vs. fast steering mirror (FSM) Separate tracking beacon vs. track on communication beam Quadrant vs. array tracker

fSONA initially explored a gimbaled transceiver design [l], considering a gimbaled mirror approach as too unwieldy for pointing six separate apertures: four one inch transmitters, one eight inch receiver, and a three inch tracker. The gimbal capitalized on the limited angular travel requirements of the FSO application to incorporate frictionless flex-pivot and voice coil actuator technology for carrier class reliability, and provided a capability for a control bandwidth approaching 10 Hz and f 1" angular travel. More recently, fSONA has prototyped an active pointing design that uses a single FSM to deflect both the transmitting and receiving beams simultaneously within the FOV of eight inch transmitting and receiving telescopes. The FSM uses similar frictionless pivots and actuators and also provides carrier class reliability. Since a much smaller mass is moved with the FSM, the control loop bandwidth can approach 100 Hz, with somewhat reduced angular travel of iO.25". In the FSM design, the tracking optic is shared

0-7803-7500-9/02/$17.~002 IEEE 451

with the eight-inch communication receiver, allowing for more aperture averaging of scintillation than with the three inch tracker for the gimbal.

While there are benefits from array tracking in using spatial processing and correlation tracking, for an unresolved spot tracker the benefit is typically small. Because BONA is committed to high power semiconductor laser technology at 1550 nm, the use of inexpensive CCD and MOS arrays was not feasible, and 1550 nm array technology is still expensive. Therefore we chose ano InGaAs quadrant tracker, which is proven technology. There was an option to add a beacon laser at 830 nm to facilitate a CCD array, but this would provide a single point of failure to the system, and introduces retinal hazard issues which don’t exist at 1550 nm. Also, by tracking directly on the communication lasers, we get the benefit of four (to eight) multiple transmitters to further reduce scintillation effects on tracking.

fSONA Active Pointing System and Test Results The fSONA baseline system approach uses dual eight inch transmit and receive apertures with a single fast steering mirror which directs both incoming and outgoing beams. (See figure at right.) The quadrant tracker shares the receive aperture with the communication receiver using a 20% beamsplit. A multi-fiber combiner can spatially combine up to eight fiber-coupled lasers by synthesizing virtual one inch transmitting apertures which surround the central telescope obscuration.

As of September 2002 prototype testing was still in progress, using BONA’S 5 km test range in Vancouver, BC to ensure a saturated scintillation condition. Preliminary testing shows that scintillation effects on the tracker, rather than transmit beam broadening effects, may set the limitation on how narrow a transmitting beam can be used. The latest test results and f i a l design

. will be discussed at the Conference.

Conclusion BONA has developed a commercial active pointing system for FSO which uses a single FSM to steer both transmit and receive beams, and which has the potential for 20 dB or more additional link margin relative to f i e d pointed solutions. The system uses totally frictionless mechanical technology, has nearly 100 Hz bandwidth, i 0.25” field-of-regard, and uses fSONA standard transceiver technology (52 Mbps to 1.25 Gbps).

References [l] D.J. Hiley, G.S. Mecherle, J. Decanini, “Novel active pointing system for point-to-point optical communication,” hoc. SPIE Vol. 4272, pp. 181-189, San Jose, January 2001

452