Method and apparatus for locking the wavelength of beacon beam in a free-space laser communications system

A method for setting the wavelength of a laser beacon in a free-space laser communication system, comprising the steps of:

• (a) generating an output beam from a wavelength-tunable laser beacon;
• (b) tuning the output beam over a selected range of wavelengths;
• (c) directing at least part of the output beam through an absorption cell having at least one absorption peak;
• (d) detecting wavelengths output from the absorption cell corresponding to the selected range of wavelengths;
• (e) determining laser beacon tuning parameters corresponding to at least one absorption peak from the detected wavelengths;
• and
• (f) setting a wavelength for the output beam corresponding to the determined tuning parameters.

An apparatus for setting the wavelength of a laser beacon in a free-space laser communication system, comprising:

• (a) a wavelength-tunable laser beacon having an output beam;
• (b) an optical module disposed relative to the laser beacon to direct at least part of the output beam through an absorption cell having at least two closely-spaced absorption peaks;
• (c) a photodetector positioned relative to the absorption cell for detecting wavelengths output from the absorption cell corresponding to a selected range of wavelengths for the output beam;
• (d) a circuit, coupled to the wavelength-tunable laser beacon and the photodetector and configured to determine laser beacon tuning parameters corresponding to at least two absorption peaks from the detected wavelengths, for generating a laser beacon tuning parameter between the determined tuning parameters, and for setting a wavelength for the output beam corresponding to the generated tuning parameter.

A method for setting the wavelength of a laser beacon in a free-space laser communication system, comprising the steps of:

• (a) generating an output beam from a wavelength-tunable laser beacon;
• (b) tuning the output beam over a selected range of wavelengths;
• (c) directing at least part of the output beam through an absorption cell having at least two closely-spaced absorption peaks;
• (d) detecting wavelengths output from the absorption cell corresponding to the selected range of wavelengths;
• (e) determining laser beacon tuning parameters corresponding to at least two absorption peaks from the detected wavelengths;
• (f) generating a laser beacon tuning parameter between the determined tuning parameters;
• (g) setting a wavelength for the output beam corresponding to the generated tuning parameter.

A system for locking the wavelength of a laser beacon of a transmitting transceiver in a free-space laser communication system to the passband of an atomic line filter of a receiving transceiver, comprising:

• (a) a wavelength-tunable laser beacon having an output beam;
• (b) optical means for directing at least part of the output beam through an absorption cell having at least two closely-spaced absorption peaks corresponding to light filtering characteristics of the atomic line filter of the receiving transceiver;
• (c) a photodetector positioned relative to the absorption cell for detecting wavelengths output from the absorption cell corresponding to a selected range of wavelengths for the output beam;
• (d) a circuit, coupled to the wavelength-tunable laser beacon and the photodetector, for determining laser beacon tuning parameters corresponding to at least two absorption peaks from the detected wavelengths, for generating a laser beacon tuning parameter between the determined tuning parameters, and for setting a wavelength for the output beam corresponding to the generated tuning parameter.

A method for locking the wavelength of a laser beacon of a transmitting transceiver in a free-space laser communication system to the passband of an atomic line filter of a receiving transceiver, comprising the steps of:

• (a) generating an output beam from a wavelength-tunable laser beacon;
• (b) tuning the output beam over a selected range of wavelengths;
• (c) directing at least part of the output beam through an absorption cell having at least two closely-spaced absorption peaks corresponding to light filtering characteristics of the atomic line filter of the receiving transceiver;
• (d) detecting wavelengths output from the absorption cell corresponding to the selected range of wavelengths;
• (e) determining laser beacon tuning parameters corresponding to at least two absorption peaks from the detected wavelengths;
• (f) generating a laser beacon tuning parameter between the determined tuning parameters;
• (g) setting a wavelength for the output beam corresponding to the generated tuning parameter.

A free-space laser communication system, comprising:

• (a) a laser transmitter, including:
• (1) a wavelength-tunable laser diode operable to produce an output beam having a laser wavelength determined by a driving current, wherein the laser wavelength is tunable within a first wavelength range in response to a current tuning range of the driving current in which the laser wavelength is monotonically dependent on the driving current;
• (2) a laser control circuit coupled to the wavelength-tunable laser diode to control the driving current and operable to maintain the driving current at a predetermined value to control the laser wavelength at a predetermined laser wavelength within the first wavelength range, wherein the laser control circuit is calibrated with respect to at least one known frequency standard so that each current value with the current tuning range corresponds to a single wavelength within said first wavelength range;
• and
• (b) a laser receiver having an atomic line filter with a transmission band defined by a second wavelength range that includes the predetermined laser wavelength of the wavelength-tunable laser diode and operating to selectively receive an optical signal at a desired wavelength with the second wavelength range.

A method for locking the wavelength of a laser diode of a transmitting transceiver in a free-space laser communication system to the passband of an atomic line filter of a receiving transceiver, comprising:

• (a) generating an output laser beam from the laser diode by providing a driving current;
• (b) determining absolute wavelength values of the laser diode in a tunable wavelength range corresponding to a selected range of the driving current by using at least one wavelength standard, wherein the tunable wavelength range includes the passband of the atomic line filter;
• (c) determining a selected driving current value at which the wavelength of the output laser beam from the laser diode is at or near the center of the passband of the atomic line filter according to the step (b);
• and
• (d) setting and maintaining the driving current at the selected driving current to control the wavelength of the laser diode at or near the center of the passband of the atomic line filter.