Frequency-spreading coupler

A frequency spreading coupler for coupling electromagnetic energy from a source to a propagation mode in a tapered, cylindral interaction waveguide of a travelling wave amplifier, where said interaction waveguide has a small first end and a large second end and said interaction waveguide is of the type with a plurality of first cutoff planes corresponding to frequencies in the frequency band of the amplifier, said frequency spreading coupler comprising:

• a tapered input waveguide disposed coaxially about said interaction waveguide, said input waveguide with a small third end and a large fourth end, wherein said input waveguide is positioned with its small third end adjacent to the small first end of said interaction waveguide and its large fourth end adjacent to the large second end of said interaction waveguide, where said input waveguide is a transversely ridged low pass fiter, said input waveguide with a plurality of second cutoff planes corresponding to frequencies in the frequency band of the amplifier, where said input waveguide is positioned relative to said interaction waveguide so that each of said second cutoff planes in said input waveguide corresponding to .omega..sub.c is displaced in the direction of increasing taper of said interction waveguide from each of said first cutoff planes in said interaction waveguide corresponding to .omega..sub.c where .omega..sub.c is any frequency in the frequency band of the travelling wave amplifier, and wherein electromagnetic energy is coupled from said input waveguide into said interaction waveguide by a plurality of slots disposed between the cutoff planes in said interaction waveguide and said input waveguide corresponding to .omega..sub.c.

A travelling wave amplifier comprising:

• a tapered interaction waveguide with a circular cross-section wherein the cross-section thereof gradually increases from a small first end to a large second end for propagating electromagnetic energy in a broad frequency band therein, said interaction waveguide having a plurality of first cutoff planes corresponding to frequencies in the frequency band of the amplifier;
• means for injecting an electron beam into the small end of said interaction waveguide;
• an input waveguide for receiving electromagnetic waves to be amplified therein, wherein said input waveguide is a tapered low pass filter having a small third end and large fourth end, said input waveguide being coaxially disposed around said interaction waveguide with said small third end adjacent to said small first end and said large fourth end adjacent to said large second end, said input waveguide with a plurality of second cutoff planes corresponding to frequencies in the frequency band of the amplifier, wherein said input waveguide is positioned so that each of said second cutoff planes in said input waveguide corresponding to .omega..sub.c is displaced in the direction of increasing cross-section of said interaction waveguide from each of said first cutoff planes in said intereaction waveguide corresponding to .omega..sub.c, where .omega..sub.c is any frequency in the frequency band of the amplifier;
• and
• coupling means, disposed between the cutoff planes corresponding to .omega..sub.c in said interaction waveguide and said input waveguide, for coupling the electromagnetic waves in said input waveguide into said interaction waveguide.