UV Laser Optical Isolator Layout for Return Light Suppression
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Solution Overview
Problem
Chromatic aberrations in projection lenses due to wide spectral linewidth of KrF and ArF excimer laser apparatuses, leading to decreased resolution, necessitate the use of line narrowing modules which can cause deterioration in laser performance from return light.
Innovation Solution
Incorporation of an optical isolator with first and second Faraday rotators and polarizers to rotate polarization direction, suppressing return light without using a half-wave plate, thereby maintaining polarization stability and reducing heat load on the laser apparatus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a line narrowing module is provided in the laser resonator to narrow the spectral linewidth, then chromatic aberrations are reduced and resolution is improved, but the laser performance deteriorates due to return light
Solution Approach 1:
The patent extracts the isolator function from the laser resonator by placing it in the amplifier instead. The optical isolator is positioned in the amplifier to prevent return light from reaching the laser resonator, while the line narrowing module remains in the resonator to maintain resolution. This separation resolves the contradiction by allowing both functions to coexist without interference.
Solution Approach 2:
The patent introduces an optical isolator as an intermediary component between the laser resonator and amplifier. This isolator acts as a mediator that blocks return light from the amplifier from reaching the resonator, protecting the line narrowing module while allowing the forward propagation of laser light. The isolator includes Faraday rotators and polarizers to achieve non-reciprocal light isolation.
2Object-affected harmful factors
If a half-wave plate is used in the optical isolator to rotate polarization direction, then return light is suppressed, but heat load increases and polarization stability deteriorates
Solution Approach 1:
The patent replaces the mechanical half-wave plate with a magnetic field-based Faraday rotator system. Instead of using a mechanically rotating half-wave plate that generates heat, the invention uses Faraday rotators that rotate polarization direction through magnetic field interaction with magneto-optic materials. This substitution eliminates the heat generation associated with mechanical rotation while achieving the same polarization rotation function.
Solution Approach 2:
The patent changes the operating parameters of the optical isolator by using two Faraday rotators with opposite magnetic field directions instead of a single half-wave plate. The first Faraday rotator rotates polarization by +45 degrees and the second by -45 degrees, achieving net zero rotation for forward light while providing isolation for return light. This parameter change eliminates the need for high-power mechanical rotation and reduces heat load.
3Manufacturing precision
If the spectral linewidth is narrowed to reduce chromatic aberrations, then projection lens resolution is improved, but the device complexity increases due to additional components
Solution Approach 1:
The patent merges the isolator function with the existing amplifier structure by integrating the optical isolator into the amplifier's optical path. Rather than adding a separate isolator unit to the laser resonator system, the invention incorporates the isolator components (Faraday rotators and polarizers) directly into the amplifier, combining multiple functions into a single integrated system and reducing overall device complexity.
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
Enhances laser performance by reducing return light, improving energy and linewidth stability, and maintaining polarization direction without altering dependent modules, thus enhancing the resolution of the ultraviolet laser apparatus.
Implementation Method 1
a first Faraday rotator configured to rotate a polarization direction of the pulse laser light output from the oscillation-stage laser by a first angle in a first rotation direction with aid of a magnetic field in a first direction
Implementation Method 2
a second Faraday rotator configured to rotate the polarization direction of the pulse laser light passing through the first polarizer by a second angle in a second rotation direction that is an opposite direction to the first rotation direction with aid of a magnetic field in a second direction that is an opposite direction to the first direction
Data Source
AI summary
An ultraviolet laser apparatus includes an oscillation-stage laser, an amplifier that amplifies the pulse laser light, and an optical isolator. The optical isolator includes a first Faraday rotator that rotates the polarization direction of the pulse laser light output from the oscillation-stage laser by a first angle in a first rotation direction, a first polarizer so disposed to transmit the pulse laser light that exits out of the first Faraday rotator at normalized transmittance greater than or equal to 0.9, a second Faraday rotator that rotates the polarization direction of the pulse laser light passing through the first polarizer by a second angle in the opposite direction to the first rotation direction, and a second polarizer so disposed to transmit the pulse laser light that exits out of the second Faraday rotator at the normalized transmittance greater than or equal to 0.9.