Method for alignment of a laser beam emitted from an optical communication transmitter with a receiving station

Method for transmitting information from an optical communication transmitter (100) to a receiving station (110) via a laser beam (18) and for alignment of said laser beam emitted from said optical communication transmitter with said receiving station, characterised in that:

• said optical communication transmitter is displaced relative to said receiving station and comprises a laser (16), a radio receiver (32), a microprocessor (14) and a liquid crystal on silicon spatial light modulator (24) comprising an element for diffracting and reflecting said laser beam, whereby said laser beam is emitted from said laser and is projected over an area by diffraction and reflection using said liquid crystal on silicon spatial light modulator, wherein said laser and said element for diffracting and reflecting said laser beam are controlled by said microprocessor, wherein said laser beam has a longitudinal axis parallel to the propagation path of said laser beam, whereby the displacement of the receiving station relative to said optical communication transmitter is a component of the vector representing the movement of the optical communication transmitter relative to said receiving station, wherein said component is in the plane perpendicular to a line between said receiving station and said optical communication transmitter,
• said receiving station comprises a photodiode receiver (52) for detecting said transmitted laser beam and a radio transmitter (60), and
• said method comprises the following steps:
• (i) projecting said laser beam in consecutive intervals over an area in which said receiving station is located by diffraction and reflection using said spatial light modulator;
• (ii) dividing said projected laser beam into quadrants intersecting at the longitudinal axis of said projected laser beam;
• (iii) interrupting the projection of each quadrant of said laser beam during each interval of a set of consecutive intervals by pulsation of said laser beam using amplitude modulation and by distortion of the wavefront of said laser beam using a pointing diffraction mask generated in said element for diffracting and reflecting said laser beam, for each interval of said set, wherein any given quadrant of said projected laser beam is projected over a subset of said set of intervals which is different from the subset of said set of intervals over which other quadrants of said laser beam are projected and wherein said quadrant is exclusively projected over at least one interval of said subset,
• wherein the frequency of the pulses of said amplitude-modulated laser beam that are emitted during any given interval are the same for all quadrants over which said laser beam is projected, and wherein the frequency of the pulses of said amplitude-modulated laser beam that are emitted during at least one interval of said set of intervals are different from the frequency of the pulses of said amplitude-modulated laser beam that are emitted during at least one other interval of said set of intervals;
• (iv) identifying the quadrant of said projected laser beam which is detected in said photodiode receiver in the receiving station by determining the frequency of the pulses of the subset of the set of intervals over which the quadrant of said laser beam is projected, and communicating this information to said optical communication transmitter via said radio transmitter and said radio receiver;
• (v) projecting said laser beam in consecutive intervals over the area which the quadrant of the laser beam identified in step (iv) was projected over by distorting the wavefront of the laser beam emitted from the laser using the pointing diffraction mask generated in said element for diffracting and reflecting said laser beam, which exclusively projected said quadrant of the laser beam in step (iii);
• and
• (vi) repeating steps (ii) to (v) at least a further three times or until the angle between the longitudinal axis of said laser beam projected in step (v) and said longitudinal axis of said projected laser beam in step (ii) is less than π/9500 radians,
• wherein the direction in which the longitudinal axis of said projected laser beam is pointed in steps (i) to (v) is changed every t seconds as a function of the displacement of the optical communication transmitter with respect to the receiving station by distorting the wavefront of the laser beam emitted from the laser using a tracking diffraction mask which is generated in said element for diffracting and reflecting said laser beam,
• wherein the intervals have a frequency of value r of between 10 and 500 Hz, the pulses have a frequency of value f of between 0.1 kHz and 100 MHz, and t is between 0.001 and 10 seconds, wherein each tracking diffraction mask is used:
• (a) alone in step (i) and in the first iteration of step (ii);
• and
• (b) in combination with a pointing diffraction mask in steps (iii) to (v) and in subsequent iterations of step (ii),
• and wherein:
• (c) each tracking diffraction mask is comprised in a holographic greyscale pattern which, when generated in said element for diffracting and reflecting said laser beam, diffracts and reflects the laser beam which is emitted from said laser, and projects the projected laser beam over the area which it was projected using the tracking diffraction mask used immediately prior thereto,
• (d) each combination of tracking diffraction mask and pointing diffraction mask is comprised in a holographic greyscale pattern which, when generated in said element for diffracting and reflecting said laser beam, diffracts and reflects the laser beam which is emitted from said laser, and projects the projected laser beam either:
• over the area which it was projected using the combination of tracking diffraction mask and pointing diffraction mask used immediately prior thereto, when the pointing diffraction masks used in each combination are the same;
• or
• over a subset of the area over which it was projected using:
• the tracking diffraction mask used prior thereto when said combination is the first combination used after using a tracking mask;
• or
• the combination of tracking diffraction mask and pointing diffraction mask used prior thereto when the pointing diffraction masks used in each combination are different.