Cavity length control system for a multiline HEL

An apparatus for controlling the cavity length of an injection locked laser comprising:

• (a) means for generating a master oscillator beam;
• (b) means for generating a frequency-shifted beam therefrom, having at least two lines shifted in frequency from corresponding lines in said master oscillator beam by first and second predetermined frequencies;
• (c) means for transmitting a portion of said master oscillator beam as a reference beam, along a predetermined reference path length;
• (d) a resonant optical cavity comprising a gain medium, means for transporting a portion of said master oscillator beam about a cavity path length passing through said gain medium, and cavity length means for varying said cavity path length;
• (e) means for coupling said portion of said master oscillator beam into said optical cavity, and for coupling an amplified beam out of said cavity along an amplified beam path, including said cavity path, having a variable length substantially equal to said predetermined reference path length and path length means for varying said variable length;
• (f) path length control means, responsive to said frequency-shifted beam, said reference beam and said amplified beam, for forming a signal for equalizing said predetermined reference path length and said variable length, coupled to said path length means;
• (g) cavity length control means, responsive to said frequency-shifted beam, said reference beam and said amplified beam, for generating a signal for controlling said cavity path length, further comprising:
• (i) means for generating detected signals, at said first and second frequencies, from a reference detector responsive to said frequency-shifted beam and said reference beam and from an output detector responsive to said frequency-shifted beam to said amplified beam;
• (ii) means for mixing said detected signals at said first frequency from both said detectors to form a first fine error signal, means for further mixing said signals at said second frequency from both said detectors to form a second fine error signal;
• (iii) means for homodyning the detected signal from said reference detector to form a reference beat signal, having a beat frequency equal to the difference of said first and second frequencies, means for homodyning the signal from said output detector, to form an output beat signal at said beat frequency;
• (iv) means for mixing said reference beat signal and said output beat signal to form a coarse error signal;
• and
• (v) means for combining said first and second fine error signals with said coarse error signal in a predetermined manner to form a cavity length error signal for controlling said cavity length means.