Linewidth-narrowed excimer laser cavity
An excimer laser cavity is disclosed which includes at least one grating-prism (grism) and a wavelength-selective diffraction grating arranged in sequence. The grism grating surface faces the gain medium and produces an expanded beam which is diffracted on the same side of the prism grating surface normal as the incident beam. The expanded diffracted beam is transmitted through a second surface of the grism either to another grating surface of an additional grism or to a wavelength-selective diffraction grating operating in Littrow configuration. The laser produces a laser output beam with a narrow spectral linewidth which is suitable, in particular, for lithography applications.
1. A line-narrowed excimer laser system comprising:
a laser cavity comprising a gain medium, an output coupler and a reflector assembly,
the reflector assembly comprising at least two sequentially arranged grating-prisms (grisms), each grism having an input surface and an output surface, and a wavelength-selective diffraction grating arranged in sequence, with a first of the at least two grisms having a first grating disposed on the input surface of the first grism and facing the gain medium, with the first grating operating in 2 nd order and receiving laser light from the gain medium at a first angle of incidence with respect to a first grating normal, wherein the first angle of incidence is selected so as to produce a first diffracted expanded beam which is diffracted on the same side as the incident beam in relation to the first grating normal, with the first expanded diffracted beam exiting the output surface of the first grism and impinging on a second grating disposed on the input surface of the second of the at least two grisms at a second angle of incidence with respect to a second grating normal, wherein the second angle of incidence is selected so as to produce a second expanded diffracted beam which is diffracted on the same side as the first expanded diffracted beam in relation to the second grating normal, with the second expanded diffracted beam transmitted through the output surface of the second grism to the wavelength-selective diffraction grating,
wherein the wavelength-selective diffraction grating operates in Littrow configuration and diffracts the received expanded diffracted beam back through the at least two grisms into the laser cavity, thereby narrowing the linewidth of the laser emission wavelength relative to the wavelength distribution of the laser light produced in the gain medium.
2. The laser system of claim 1 , wherein the expanded diffracted beam exits the grism substantially normal to the output surface.
3. The laser system of claim 1 , wherein at least the first angle of incidence is greater than 60° and less than 90° with respect to the first grating normal.
4. The laser system of claim 1 , wherein the grism is made of a material substantially transparent to a laser emission wavelength propagating in the gain medium.
5. The laser system of claim 1 , wherein the grism is made of a material substantially transparent to deep UV (DUV) or UV radiation, or both.
6. The laser system of claim 1 , wherein the input surface and the output surface of each of the at least two grisms subtend an angle of about 10°.
7. The laser system of claim 1 , wherein the output surfaces of the at least two grisms are AR coated at a laser emission wavelength propagating in the gain medium.