System and method for protecting a seed laser in an EUV light source with a Bragg AOM
View Patent ↗A method and apparatus for protecting the seed laser in a laser produced plasma (LPP) extreme ultraviolet (EUV) light system are disclosed in one embodiment, a Bragg AOM is used as a switch on the beam path from the seed laser to other optical components and ultimately to an irradiation site. Power is applied to the Bragg AOM and pulses from the seed laser are thus deflected onto the desired beam path rather than passing straight through the Bragg AOM. Once the pulses have passed through the Bragg AOM, power to the Bragg AOM ceases, so that any reflections from the irradiation site will pass straight through the Bragg AOM and will not be deflected back to the seed laser. Use of the Bragg AOM rather than components previously used results in lower power consumption and better protection for the seed laser.
1. A system comprising:
a laser source producing laser light on a first path;
a Bragg acousto-optic modulation device switchable between a first state in which the laser light is deflected from the first path onto a second path toward an irradiation site at which a target material may he irradiated by the laser light and a second state in which the laser light is transmitted in a direction other than onto the second path;
a delay device on the second path between the Bragg device and the irradiation site for delaying the laser light; and
means for switching the Brag device from the first state to the second state after the laser light has been deflected onto the second path such that at least a portion of any reflections of the laser light from the irradiation site are transmitted in a direction other than along the first path, and thus prevented from reaching the laser source.
2. The system of claim 1 wherein, the laser source is a seed laser.
3. The system of claim 1 wherein the delay device comprises a folding optical arrangement.
4. The system of claim 3 wherein the delay device creates a delay in the transmission of the laser light to the irradiation site and any reflection of the laser light from the irradiation site such that the time it takes light on the second path to reach the irradiation site from the Bragg device and return is greater than the time in which the Bragg device is able to switch from the first state to the second state.
5. The system of claim 1 wherein the time in which the Bragg device is able to switch from the first state to the second state is in the range of 300 to 500nanoseconds.
6. The system of claim 1 further comprising an optical amplifier positioned on the second path between the Bragg device and the irradiation site.
7. The system of claim 1 further comprising an optical isolator on the second path between the Bragg device and the irradiation site.
8. The system of claim 7 wherein the optical isolator comprises a polarization discriminating optical component and a phase retarding optical component.
9. A system comprising:
a pre-pulse seed laser that produces laser light;
a first Bragg acousto-optic modulation device switchable between a first state in which the laser light from the pre-pulse seed laser is deflected onto a first beam path and a second state in which the laser light is transmitted in a direction other than onto the first beam path;
a main pulse seed laser that produces laser light;
a second Bragg acousto-optic modulation device switchable between a first state in which the laser light from the main pulse seed laser is deflected onto a second beam path and a second state in which the laser light is transmitted in a direction other than onto the second beam path;
a combiner for directing the laser light from the pre-pulse seed laser on the first beam path and the laser light from the main pulse laser on the second beam path onto a common beam path toward an irradiation site at which a target material may be irradiated by the laser light;
means for switching the first Bragg device from the first state to the second state after the laser light from the pre-pulse seed laser has been deflected onto the first beam path such that at least a portion of any reflections of the laser light from the irradiation site are prevented from reaching the pre-pulse seed laser; and
means for switching the second Bragg device from the first state to the second state after the laser fight from the main pulse seed laser has been deflected onto the second beam path such that at least a portion of any reflections of the laser light from the irradiation site are prevented from reaching the main pulse seed laser.
10. The system of claim 9 further comprising an optical amplifier positioned on the common beam path between the combiner and the irradiation site.
11. The system of claim 9 further comprising, a delay device on the common beam path between the combiner and the irradiation site for delaying the laser pulses.
12. The system of claim 11 wherein the delay device comprises a beam folding optical arrangement.
13. The system of claim 12 wherein the delay device creates a delay in the transmission of the laser light to the irradiation site and any reflection of the laser light from the irradiation site such that the time it takes light on the beam path to reach the irradiation site from either the first Bragg device or the second Bragg device and return is greater than the time in which the first Bragg device and the second Bragg device are able to switch from the first state to the second state.
14. The system of claim 9 wherein the time in which the first Bragg device and the second Bragg device are able to switch from the open state to the closed state is in the range of 300 to 500 nanoseconds.
15. The system of claim 9 further comprising an optical isolator on the common beam path between the combiner and the irradiation site.
16. The system of claim 9 wherein the optical isolator comprises a polarization discriminating optical component and a phase retarding optical component.
17. A method of protecting a laser pulse source from pulse reflections comprising:
generating a laser pulse on a first path from a laser source;
passing the laser pulse through a Bragg acousto-optic modulation device, the Bragg device switchable between a first state in which the pulse is deflected onto a second path toward an irradiation site at which a target material may be irradiated by the laser pulse and a second state in which the laser pulse is transmitted in a direction other than onto the second path, while the Bragg device is in the first state; and
switching the Bragg device from the first state to the second state after the laser pulse has been deflected onto the second path such that at least a portion of any reflections of the laser pulse from the irradiation site are transmitted in a direction other than along the first path, and thus prevented from reaching the laser source.
18. The method of claim 17 , wherein the laser source is a seed laser.
19. The method of claim 17 , further comprising passing the laser pulse through a delay device after the laser pulse has passed through the Bragg device while the Bragg device is in the first state.
20. The method of claim 19 wherein the delay device creates a delay in the transmission of the laser light to the irradiation site and any reflection of the laser light from the irradiation site such that the time it takes light on the second path to reach the irradiation site from the Bragg device and return is greater than the time in which the Bragg device is able to switch from the first state to the second state.
21. A system comprising:
a laser source producing laser light on a first path;
a Bragg acousto-optic modulation device switchable between a first state in which the laser light is deflected from the first path onto a second path toward an irradiation site at which a target material may be irradiated by the laser light and a second state in which the laser light is transmitted in a direction other than onto the second path:
a delay device comprising a folding arrangement of optical components, located on the second path between the Bragg device and the irradiation site for delaying the laser light; and
a controller for switching the Bragg device from the first state to the second state after the laser light has been deflected onto the second path such that at least a portion of any reflections of the laser light from the irradiation site are transmitted in a direction other than along the first path, and thus prevented from reaching the laser source.