A wavemeter for a high repetition rate gas discharge laser having a laser output beam comprising a pulsed output of greater than or equal to 15 mJ per pulse, sub-nanometer bandwidth tuning range pulses having a femtometer bandwidth precision and tens of femtometer bandwidth accuracy range, for measuring bandwidth on a pulse to pulse basis at pulse repetition rates of 4000Hz and above, and comprises: a beam splitter for extracting a portion of the laser output beam from a power amplifier and for passing a seed beam from a master oscillator to the power amplifier wherein the beam splitter comprises a primary beam splitter mirror, wherein the primary beam splitter mirror accommodates the seed beam from the master oscillator and the laser output beam from the power amplifier, the seed beam from the master oscillator and the laser output beam from the power amplifier propagating in opposite directions, and wherein the primary beam splitter mirror is oriented at 70 degrees or greater to the laser output beam portion from the power amplifier and to the seed beam from the master oscillator; a first diffuser that receives the laser output beam portion; a focusing lens having a focal length; a second diffuser, wherein the focusing lens is placed between the first diffuser and the second diffuser; an etalon creating an interference fringe pattern from the laser output beam portion that has passed through the first diffuser, the focusing lens, and the second diffuser, the etalon having a slit function; an optical detector positioned at the focal length from the focusing lens; and a bandwidth calculator calculating bandwidth of the laser output beam from the position of interference fringes in the interference fringe pattern incident on the optical detector, wherein the slit function and the focal length are selected to deliver to the optical detector only the two innermost fringes of the optical interference fringe pattern. a beam splitter for extracting a portion of the laser output beam from a power amplifier and for passing a seed beam from a master oscillator to the power amplifier wherein the beam splitter comprises a primary beam splitter mirror, wherein the primary beam splitter mirror accommodates the seed beam from the master oscillator and the laser output beam from the power amplifier, the seed beam from the master oscillator and the laser output beam from the power amplifier propagating in opposite directions, and wherein the primary beam splitter mirror is oriented at 70 degrees or greater to the laser output beam portion from the power amplifier and to the seed beam from the master oscillator; a first diffuser that receives the laser output beam portion; a focusing lens having a focal length; a second diffuser, wherein the focusing lens is placed between the first diffuser and the second diffuser; an etalon creating an interference fringe pattern from the laser output beam portion that has passed through the first diffuser, the focusing lens, and the second diffuser, the etalon having a slit function; an optical detector positioned at the focal length from the focusing lens; and a bandwidth calculator calculating bandwidth of the laser output beam from the position of interference fringes in the interference fringe pattern incident on the optical detector, wherein the slit function and the focal length are selected to deliver to the optical detector only the two innermost fringes of the optical interference fringe pattern.