Gas Laser Amplifier Resonator Control Against Seed-Axis Self-Oscillation
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Solution Overview
Problem
Conventional gas laser amplifiers fail to prevent self-oscillating light generated on the same optical axis as the seed laser beam, leading to unintended irradiation of the target in EUV light generation apparatuses, causing failures in EUV light generation.
Innovation Solution
A gas laser amplifier design incorporating an optical resonator that suspends oscillation when the seed laser beam is not present, using polarizing mirrors and partially reflective mirrors to control the gain and prevent self-oscillation by setting the oscillation threshold gain higher than the amplification gain during seed laser beam entry and lower than the self-oscillation gain when the seed beam is absent.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If apertures are provided at the first and second windows to block self-oscillating light, then self-oscillation on different optical axes is prevented, but self-oscillation on the same optical axis as the seed laser beam cannot be prevented
Solution Approach 1:
The patent introduces a variable attenuator that dynamically adjusts the optical path loss based on the presence of the seed laser beam. When the seed laser beam is present, the attenuator reduces loss to allow amplification; when absent, it increases loss to prevent self-oscillation. This dynamic adjustment resolves the contradiction by making the system adaptive to different operational states.
Solution Approach 2:
The patent changes the optical path loss parameter dynamically using a variable attenuator controlled by the seed laser beam presence. By adjusting the attenuation parameter, the system can selectively allow amplification during normal operation while preventing self-oscillation when the seed beam is absent, thus resolving the contradiction between allowing amplification and preventing self-oscillation.
2Productivity
If the oscillation threshold gain is set low to enable amplification, then the seed laser beam can be amplified effectively, but self-oscillation occurs when the seed laser beam is absent
Solution Approach 1:
The variable attenuator dynamically adjusts optical path loss based on seed laser beam presence, enabling the system to switch between amplification mode (low loss when seed beam present) and suppression mode (high loss when seed beam absent). This resolves the contradiction by making the threshold gain effectively variable rather than fixed.
Solution Approach 2:
The system uses the seed laser beam itself as a feedback signal to control the variable attenuator. The presence or absence of the seed beam directly controls the attenuation state, creating a feedback mechanism that automatically adjusts the oscillation threshold to prevent self-oscillation while maintaining amplification efficiency.
3Object-affected harmful factors
If the oscillation threshold gain is set high to prevent self-oscillation, then self-oscillation is suppressed, but the amplification gain for the seed laser beam is reduced
Solution Approach 1:
By making the optical path loss variable rather than fixed, the system can dynamically optimize performance. When the seed laser beam is present, loss is minimized for maximum amplification; when absent, loss is maximized to prevent self-oscillation. This resolves the contradiction by eliminating the need for a fixed compromise threshold.
Solution Approach 2:
The patent changes the optical path loss parameter based on operational conditions. The variable attenuator allows the system to adjust the effective threshold gain parameter, enabling high amplification efficiency during normal operation while maintaining high suppression capability when the seed beam is absent.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively prevents self-oscillation on the same optical axis as the seed laser beam, ensuring accurate and reliable EUV light generation by controlling the optical resonator's gain to prevent unwanted oscillations.
Implementation Method 1
When the high-frequency voltage is applied between the plate electrodes of the gas laser amplifier, a discharge region is formed, and the laser gas is excited.
Implementation Method 2
the seed laser beam enters the chamber through the first window and passes through the discharge region, such that the seed laser beam is amplified into an amplified beam
Implementation Method 3
The amplified beam undergoes repeated reflections between the second concave mirror and the first concave mirror.
Implementation Method 4
using polarizing mirrors and partially reflective mirrors to control the gain and prevent self-oscillation
Data Source
AI summary
A gas laser amplifier includes a housing, discharge electrode pairs, and an optical resonator. The housing includes an entrance window that allows entry of a first laser beam from outside and an exit window that allows exit of the first laser beam amplified. Each of the discharge electrode pairs excites a laser gas supplied between discharge electrodes facing each other in the housing. The optical resonator causes 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. In an incident state where the first laser beam enters the housing through the entrance window, the optical resonator suspends the oscillation of the second laser beam.