| 摘要 |
PURPOSE:To stabilize performance for a long period of time by sealing a gas which absorbs light of the wavelength of an incident luminous flux into a chamber means disposed at the mid-point of an optical path and varying the absorption quantity of the light at the time of passing through the chamber means by a control means. CONSTITUTION:An attenuation chamber 10 is made of a stainless steel and is installed with an incident window 12 and exit window 14 formed of quartz at both ends. A laser beam LB passes the inside of the chamber 10. For example, the beam of 249nm oscillation line of KrF which is an excimer laser is used for the beam LB. Freon FC11 is sealed into the chamber 10. The pressure in the chamber 10 is measured by a pressure gage 16. The internal pressure of the chamber 10 is controlled by the respective opening and closing actions of gas feed and discharge valves 18, 20 by utilizing the phenomenon that the transmittance of the beam LB changes with the internal pressure of the chamber 10, by which the attenuation rate of the beam LB passing the chamber 10 in the final is controlled. Since the attenuation of the light is executed by the absorption of the gas in such a manner, the cleaned gas is usable at all times and the performance of the light attenuation is stabilized for a long period of time.
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