A pulse multiplier comprising: A first ring cavity including: a first beam splitter for receiving a laser pulse; and A first set of one or more mirrors (106, 107); A first prism; A second ring cavity including: A second beam splitter; A second set of one or more mirrors (116, 117); A second prism; Wherein the first beam splitter is configured such that it directs a first fraction of the input laser pulse to the second beam splitter, and directs a second fraction of each input laser pulse into the first ring cavity; and Wherein the second beam splitter is configured such that it directs a third fraction of each laser pulse incident on it to the output of the pulse multiplier, and directs a fourth fraction of the energy of each pulse incident on it into the second ring cavity; wherein the first ring cavity has an optical path length of about half the distance between the successive incoming laser pulses, wherein the distance between the successive incoming laser pulses is equal to the velocity of light multiplied by the time interval between the successive incoming laser pulses, wherein an optical path length of the second ring cavity is approximately an odd integer times half the optical path length of the first ring cavity; wherein the first and second prisms (108, 118) each have an input face which is cut such that light which is incident thereon is incident at an angle substantially or approximately equal to Brewster's angle; and wherein the first and second prisms (108, 118) are each rotatable such that light no longer strikes them at Brewster's angle. A first ring cavity including: a first beam splitter for receiving a laser pulse; and A first set of one or more mirrors (106, 107); A first prism; a first beam splitter for receiving a laser pulse; and A first set of one or more mirrors (106, 107); A first prism; A second ring cavity including: A second beam splitter; A second set of one or more mirrors (116, 117); A second prism; A second beam splitter; A second set of one or more mirrors (116, 117); A second prism; Wherein the first beam splitter is configured such that it directs a first fraction of the input laser pulse to the second beam splitter, and directs a second fraction of each input laser pulse into the first ring cavity; and Wherein the second beam splitter is configured such that it directs a third fraction of each laser pulse incident on it to the output of the pulse multiplier, and directs a fourth fraction of the energy of each pulse incident on it into the second ring cavity; wherein the first ring cavity has an optical path length of about half the distance between the successive incoming laser pulses, wherein the distance between the successive incoming laser pulses is equal to the velocity of light multiplied by the time interval between the successive incoming laser pulses, wherein an optical path length of the second ring cavity is approximately an odd integer times half the optical path length of the first ring cavity; wherein the first and second prisms (108, 118) each have an input face which is cut such that light which is incident thereon is incident at an angle substantially or approximately equal to Brewster's angle; and wherein the first and second prisms (108, 118) are each rotatable such that light no longer strikes them at Brewster's angle.