Variable, two-color acquisition beam for free-space laser communication terminals

This paper reports on a two-color acquisition beam with continuously variable divergence for free-space laser communication links. This approach is useful for terminals using wavelength-separated beacon and communi- cation signals amplified by a common high-power optical amplifier (HPOA). The acquisition beam features a controllable power-split ratio between the two waveforms that varies during the acquisition sequence. With this scheme, the area of the acquisition beam can be expanded by a factor of 100, while maintaining power levels of the two colors within a specified range on an acquisition sensor in a partner terminal. An optical transmitter produces beacon and communication waveforms from distinct master oscillators, and balances these signal levels during a multi-step transition sequence to achieve the desired relative power levels at each wavelength. An HPOA then boosts this transmitter signal to power levels necessary to overcome link losses. The HPOA and transmitter adjust power levels during the sequence to maintain beacon and communication irradiances within specified ranges. A divergence setting assembly (DSA) simultaneously adjusts the beam width from a 10x broadened beacon to a narrow, diffraction-limited beam. We demonstrate control of the on-axis beacon power to within a 10-dB dynamic range while the beacon area varies by a factor of approximately 20 dB. This paper describes the hardware and software control of the various units used to perform acquisition.