Tech Briefs

Optical Design of an Optical Communications Terminal

This airborne system would keep itself aimed at a ground station.

An optical communications terminal (OCT) is being developed to enable transmission of data at a rate as high as 2.5 Gb/s, from an aircraft or spacecraft to a ground station. In addition to transmitting high data rates, OCT will also be capable of bidirectional communications. The OCT is meant to incorporate all of the design features of a prior apparatus denoted the Optical Communications Demonstrator (OCD), plus some improvements.

Like the OCD, the OCT would utilize a single telescope aperture for both transmitting and receiving. Also as in the OCD, a fine-steering mirror (FSM) would be included in the transmitting optical train.

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The Optical Assembly of the OCT would be compact, yet would accommodate six optical channels, each playing a different role in transmission or reception.
The OCT design utilizes a 1,550-nm fiber-optic amplifier transmitter like that used in the telecommunications industry. Such an amplifier includes a single mode oscillator, to which one can apply modulation such that the laser light emanating from the fiber can convey data at a rate in the gigabit-per-second range. The laser beam from each such amplifier would be coupled, via a collimating interface module, to a transceiver optical assembly, major optical components of which are shown in the figure.

The OCT shall include large-field-of-view focal planes for receiving optical communications and for sensing remote beacon lasers for controlled acquisition, tracking, and pointing (in other words, beacons toward which the OCT would be aimed for transmitting or receiving). The OCT could be connected to a gimbal assembly that could be used for coarse aiming.

The OCT would utilize six optical channels — three for transmitting, three for receiving. The transmitting channels would be the following:

  • A channel for a 1,550-nm-wavelength laser beam, which would be the main data-modulated beam to be transmitted via the telescope;
  • A channel for part of a split 980-nm laser beam used as a reference beam for fine-pointing servo control; and
  • A channel for the other part of the split 980-nm beam used for calibration of a coarse-acquisition charge-coupled device (CCD). The receiving channels would be the following:
  • A channel for a portion of an 852-nm wavelength beacon-and-data-communication signal from a ground station for use in the coarse-acquisition control system;
  • A channel for another portion of the 852-nm signal for use in the fine-acquisition- and-tracking system; and
  • A channel for yet another portion of the 852-nm signal, used for reception of data from the ground station.

The telescope in the OCT would have an aperture 100 mm wide and would be afocal: all beams would be collimated at the points where they would be split. The design would minimize vignetting and would include field stops, Lyot stops, and baffles to block stray light. To make the optical system compact, the primary mirror would have a focal-length/diameter ratio (“f” number) of 1.2.

In the first-mentioned transmitting channel, the 1,550-nm laser light coming from a single-mode optical fiber would be collimated and directed to a spot on the FSM coincident with a pupil image plane, then reflected from the FSM to a dichroic beam splitter (DBS), then reflected by four more mirrors, the last two of which would be the secondary and primary telescope mirrors. The divergence of the outgoing 1,550- nm laser beam could be tailored by altering the design of the collimating interface module: one would choose the amount of divergence according to range of the free-space optical link and the degree of mechanical stability of the aircraft to carry the OCT. The FSM could steer the 1,550-nm laser beam over an angular range about 10 milliradians wide.