A wavemeter and method for measuring bandwidth for a high repetition rate gas
discharge
laser having an output laser bean comprising a pulsed output of greater than or
equal to 15 mJ per pulse, sub-nanometer bandwidth tuning range pulses having a
femptometer bandwidth precision and tens of femptometers bandwidth accuracy range,
for measuring bandwidth on a pulse to pulse basis at pulse repetition rates of
4000Hz and above, is disclosed which may comprise a focusing lens having a focal
length; an optical interferometer creating an interference fringe pattern; an optical
detection means positioned at the focal length from the focusing lens; and a bandwidth
calculator calculating bandwidth from the position of interference fringes in the
interference fringe pattern incident on the optical detection means, defining a
DID and a DOOD, the respective distances between a pair of
first fringe borders and between a pair of second fringe borders in the interference
pattern on an axis of the interference pattern, and according to the formula =0
[DOD2-;DID2]/[8f2-;D02],
where 0 is an assumed constant wavelength and D0=(DOD-;DID)/2,
and f is the focal length. The optical detector may be a photodiode array.
The wavemember may have an optical interferometer having a slit function; the slit
function and the focal length being selected to deliver to the optical detector
the two innermost fringes of the optical interference ring pattern. The optical
detector may comprise an array of pixels each having a height and width and the
array having a total width; and an aperture at the optical input to the optical
interferometer may selectively input to the optical interferometer a portion of
a beam of light sufficient for the output of the etalon to illuminate the optical
detector over the height of each respective pixel height and the total width. The
optical interferometer may comprise an etalon having a slit function of 3 pm or
less and a finesses of 25 or greater; and the focal length may be 1.5 meters. A
second stage diffuser may be placed between the first stage diffuser and the etalon
delivering a narrow cone of light to the etalon, and an aperture between the second
stage diffuser and the etalon may deliver to the etalon a thin strip of the narrow
cone of light.
