Gas laser amplifier, gas laser apparatus, EUV light generation apparatus, and EUV exposure apparatus

A gas laser amplifier comprising:

• a housing including an entrance window to allow entry of a first laser beam from outside and an exit window to allow exit of the first laser beam amplified;
• a discharge electrode pair to excite a laser gas supplied between discharge electrodes facing each other in the housing;
• and
• an optical resonator to cause a second laser beam to oscillate with a gain of the excited laser gas in a non-incident state where the first laser beam from outside the housing does not enter the housing through the entrance window, and suspend the oscillation of the second laser beam in an incident state where the first laser beam enters the housing through the entrance window, wherein the optical resonator includes
• a first partially reflective mirror to reflect a portion of the second laser beam and transmit a remaining portion of the second laser beam,
• a second partially reflective mirror to reflect a portion of the second laser beam and transmit a remaining portion of the second laser beam,
• a first damper to absorb the remaining portion of the second laser beam having passed through the first partially reflective mirror, and
• a second damper to absorb the remaining portion of the second laser beam having passed through the second partially reflective mirror.

A gas laser amplifier comprising:

• a housing including an entrance window to allow entry of a first laser beam from outside and an exit window to allow exit of the first laser beam amplified;
• a discharge electrode pair to excite a laser gas supplied between discharge electrodes facing each other in the housing;
• and
• an optical resonator to cause a second laser beam to oscillate with a gain of the excited laser gas in a non-incident state where the first laser beam from outside the housing does not enter the housing through the entrance window, and suspend the oscillation of the second laser beam in an incident state where the first laser beam enters the housing through the entrance window, wherein
• an optical axis of the second laser beam and an optical axis of the first laser beam that passes through the entrance window and the exit window are independent of each other,
• the optical resonator is disposed such that an optical axis of the second laser beam partly intersects an optical axis of the first laser beam, and
• the laser gas is two in number, and the gas amplifier further comprising gas flow generation units to cause the two laser gases to flow in third directions at different position, the third directions being perpendicular to first and second directions, the first direction being a direction of entry of the first laser beam, the second direction being a direction of a gap between the discharge electrode pair, the different positions being separate from each other in the first direction, wherein
• directions of the two laser gases are opposite to each other, and
• an optical axis of the second laser beam partly extends on an upstream side of an optical axis of the first laser beam, in directions of flows of the laser gases.