Feedback control of lasers in fiber optic systems

A control arrangement comprising:

• (a) a laser (10) for generating a beam of radiation along a propagation axis (12);
• (b) an optical fiber (28) oriented so as to couple a major portion (13) of said beam into said fiber;
• and
• (c) feedback means responsive to said beam for changing the excitation to said laser so as to maintain essentially constant the average power coupled into said fiber;
• characterized in that the axis of said fiber is oriented transverse to said propagation axis, whereby a displacement of said fiber in a direction transverse both to said propagation axis and to the fiber axis changes the coupling of said laser beam into said fiber, and said feedback means includes
• (i) a pair of photodetectors, each for generating a different photocurrent proportional to a respective light portion incident thereon, said photodetectors positioned on opposite sides of the plane formed by said axes so that said displacement of the fiber changes the respective said photocurrents proportional to said change in respective light portion incident thereon, and
• (ii) circuit means responsive to said photocurrents for changing the excitation to said laser whenever said displacement of said fiber causes said photocurrents to undergo said changes.