An optical assembly that combines the output of mutually incoherent lasers into a multibeam source for transmission into free space was designed, assembled, and tested. This design was motivated by the need for ground-based beacon sources required for broadcasting laser beams to spaceborne, satellite-to-ground free-space optical communications systems. These beacons will provide a pointing and tracking reference for the spaceborne optical communications system and be used to uplink commands and data to the spacecraft. The assembly was field tested by broadcasting the beams through a 0.6-m-aperture telescope over a mountaintop-to-mountaintop 45-km atmospheric path, with receiving sensors located at the other end. The key advantages of the design are the following:

  • Reduction in atmospheric-turbulence-induced irradiance fluctuations, due to incoherent averaging of the overlapping beams in the far field, effectively eliminating or greatly reducing fades experienced by the spaceborne sensor.
  • The distribution of the required optical power among multiple beams, which in some cases allows greater optical throughput without exceeding the damage threshold of optical surfaces and in other cases can help maintain eye-safe irradiance levels at the transmitting aperture.
The schematic shows how Eight Laser Beams are copropagated through a single telescope aperture. The inset shows a photograph of the individual laser spots on the telescope primary mirror. The beams overlap in the far-field and noncoherent averaging causes the atmosphere-induced intensity fluctuations (scintillation) to be eliminated or greatly reduced.

The assembly (see figure) includes four multimode fiber-coupled (62.5-µm core) laser diodes emitting at ~780 nm. The output of each laser is further split into two beams, using multimode fiber-optic splitters. The resulting outputs are each free-space coupled through 11-mm focal length collimators, resulting in an optical power of ~10 mW per beam. The collimated outputs are then aimed radially toward eight right-angled prisms (see figure) arranged symmetrically about the optical axis of an optical train consisting of a positive combining lens, followed by a negative doublet lens, and finally a field lens at the Coudéfocus of the telescope. The telescope focal length used was 25 m. The entire optical assembly was chosen to provide a 123-µrad full-width (100-percent energy) beam divergence that also corresponded to ~42 mm subaperture spots arranged symmetrically on the primary mirror of the transmitting telescope.

The inset accompanying figure shows beam spots on the primary mirror. The spot sizes varied between 33 and 48 mm, resulting in a range of beam divergences.

In field tests, the expected reduction in atmospheric-turbulence-induced irradiance fluctuations was ascertained by measuring the normalized variance of the received power. Thus, an observed normalized variance range (also called scintillation index) of 0.8 to 1.3 observed for single beams was reduced to 0.13 to 0.55 upon combining all eight beams. The range of observed normalized variances was recorded over three separate campaigns conducted between June and September of 2000. The observed range of values is attributed to the variations in (1) the extent of beam overlap achieved and (2) variations in atmospheric turbulence over the period of measurements.

This work was done by Malcolm Wright, Abhijit Biswas, Norman Page, and Babak Sanii of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-21119.


This Brief includes a Technical Support Package (TSP).
Multibeam Beacon Laser Assembly

(reference NPO21119) is currently available for download from the TSP library.

Don't have an account? Sign up here.



Photonics Tech Briefs Magazine

This article first appeared in the October, 2001 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.