Relative phase measurement for coherent combining of laser beams

A phase control system for controlling a relative phase of two laser beams of a laser system, the phase control system comprising:

• an optical system including:
• a beam input configured to receive measuring portions of two coherent laser beams, which are to be superimposed collinearly to form a sum laser beam, and
• a propagation section in which the measuring portions of the two coherent laser beams propagate spatially superimposed at an angle with respect to each other and form at least three measuring beam regions, wherein the at least three measuring beam regions are configured to generate different phase offsets of the associated measuring beam regions by path length differences;
• at least three photodetectors configured to output at least three photodetector signals, wherein each of the photodetectors is assigned to a respective one of the measuring beam regions, and the photodetector signals correspond to the measuring beam regions at the different phase offsets;
• an evaluation device configured to generate a control signal based on the at least three photodetector signals, wherein the control signal corresponds to a relative phase between the coherent laser beams based on an evaluation of an interference behavior of the measuring portions;
• and
• a delay device configured to be inserted into a beam path of at least one of the two laser beams to be collinearly superimposed, wherein the delay device has an optical path length adjustable in dependence on the control signal.

A laser system for providing a sum laser beam based on coherent combination, the laser system comprising:

• a laser beam source for providing a first laser beam and a second laser beam that is coherent with the first laser beam;
• an optical combiner for collinear superposition of a first beam path of the first laser beam and a second beam path of the second laser beam for generating a sum laser beam, wherein the first laser beam and the second laser beam are superposed in a first polarization state and in a second polarization state, respectively, wherein the second polarization state is different from the first polarization state;
• and
• an optical system including:
• a beam input configured to receive one or more measuring portions of the coherent first and second laser beams, which are to be superimposed collinearly to form a sum laser beam;
• and
• a beam splitter configured to generate at least three measuring beams from the one or more measuring portions of the coherent first and second laser beams, the at least three measuring beams propagating on at least three associated measuring beam paths, wherein the associated at least three measuring beam paths are configured to generate different phase offsets of the at least three measuring beams by projecting the at least three measuring beams onto at least three polarization directions, or a propagation section in which the one or more measuring portions of the coherent first and second laser beams propagate spatially superimposed at an angle with respect to each other and form at least three measuring beam regions, wherein the at least three measuring beam regions are configured to generate different phase offsets of the associated measuring beam regions by path length differences;
• at least three photodetectors coupled to the at least three measuring beam paths or the at least three measuring beam regions and configured to output at least three photodetector signals corresponding to the at least three measuring beams or to the at least three measuring beam regions having the different phase offsets;
• an evaluation device configured to generate a control signal based on the at least three photodetector signals, wherein the control signal corresponds to a relative phase between the two coherent laser beams based on an evaluation of a polarization state of the measuring portions or an evaluation of an interference behavior of the measuring portions;
• and
• a delay device configured to be inserted into a beam path of at least one of the two coherent laser beams to be collinearly superimposed, wherein the delay device has an optical path length adjustable in dependence on the control signal, wherein the delay device is provided in the first beam path of the first laser beam or in the second beam path of the second laser beam or both,
• wherein a measuring portion of the sum laser beam or measuring portions of the first laser beam and the second laser beam which propagate at an angle with respect to each other, is or are coupled into the beam input of the optical system as the one or more measuring portions of the coherent first and second laser beams.

A method of coherently combining laser beams, the method comprising:

• superimposing two coherent laser beams with different polarization states on a common propagation path to form a sum laser beam;
• determining a relative phase between the two coherent laser beams in a substantially intensity-independent homodyne measurement procedure;
• and
• adjusting an optical path length difference between the two coherent laser beams before the sum laser beam is formed to stabilize a polarization state of the sum laser beam based on the determined relative phase.

A method of coherently combining laser beams, the method comprising:

• superimposing two coherent laser beams with interfering polarization states such that at least three measuring beam regions in a beam cross-section of the superimposed coherent laser beams relate to different phase offsets, wherein the interfering polarization states cause an interference pattern relating the different phase offsets to the at least three measuring beam regions;
• determining a relative phase between the two coherent laser beams based on the at least three measuring beam regions, wherein intensity values for each of the at least three measuring beam regions are acquired and provided as input quantities for signal processing;
• and
• adjusting an optical path length difference between the two coherent laser beams prior the superposition to stabilize the interference pattern based on the determined relative phase.