Hollow-core optical fiber based radiation source
A radiation source including: a hollow core optical fiber having a body having a hollow core for confining a working medium, the hollow core optical fiber being operable to receive pulsed pump radiation such that the received pulsed pump radiation propagates through the hollow core from an input end to an output end of the hollow core optical fiber, wherein one or more source parameters of the radiation source are configured such that the pulsed pump radiation undergoes a soliton self-compression process so as to change a spectrum of the pulsed pump radiation to form output radiation; and at least one dispersion control mechanism being operable to change dispersion characteristics in a first portion of the optical fiber so as to spectrally shift a dispersive wave generated in the soliton self-compression process.
1 . A radiation source comprising:
a hollow core optical fiber comprising a body having a hollow core for confining a working medium, the hollow core optical fiber being operable to receive pulsed pump radiation such that the input pulsed pump radiation propagates through the hollow core from an input end to an output end of the hollow core optical fiber, wherein one or more source parameters of the radiation source are configured such that the pulsed pump radiation undergoes a soliton self-compression process so as to change a spectrum of the pulsed pump radiation to form output radiation; and
at least one dispersion control mechanism configured to change dispersion characteristics starting at a location, in a portion of the optical fiber, after a point of maximum compression so as to spectrally shift a dispersive wave generated in the soliton self-compression process,
wherein the location is selected so as to attenuate a peak of the dispersive wave to reduce sensitivity of the peak to pulse energy variations.
2 . The radiation source as claimed in claim 1 , wherein the one or more source parameters are also such that the spectrum of the pulsed pump radiation has reached its maximum breadth before exiting the optical fiber via the output end.
3 . The radiation source as claimed in claim 2 , wherein a first end of the portion of the optical fiber is located at or near a position along the length of the optical fiber where the spectrum of the pump radiation reaches the maximum breadth and a second end of the portion of the optical fiber is located at or near the output end of the optical fiber.
4 . The radiation source as claimed in claim 1 , wherein the at least one dispersion control mechanism comprises a tapered portion of the optical fiber which overlaps with the portion of the optical fiber.
5 . The radiation source as claimed in claim 4 , wherein the hollow core has a diameter and wherein the tapered portion is configured such that the diameter of the hollow core continuously increases along the tapered portion.
6 . The radiation source as claimed in claim 1 , wherein the at least one dispersion control mechanism comprises a pressure gradient arrangement configured to apply a pressure gradient to the working medium within the portion of the optical fiber.
7 . The radiation source as claimed in claim 1 , wherein the at least one dispersion control mechanism comprises a temperature gradient arrangement configured to apply a temperature gradient to the working medium within the portion of the optical fiber.
8 . The radiation source as claimed in claim 1 , wherein the portion of the optical fiber is the full length of the optical fiber.
9 . The radiation source as claimed in claim 1 , further comprising:
the working medium disposed within the hollow core; and
a pulsed pump radiation source arranged to produce the pulsed pump radiation,
wherein the one or more source parameters comprise parameters of the pulsed pump radiation, the optical fiber and the working medium.
10 . The radiation source as claimed in claim 9 , configured such that a soliton order of the input pulsed pump radiation is less than 20.
11 . The radiation source as claimed in claim 1 , configured such that the dispersive wave comprises wavelengths between 450 nm and 550 nm.
12 . The radiation source as claimed in claim 1 , configured such that the dispersion characteristics comprise a wavelength of zero group velocity dispersion of the optical fiber.
13 . The radiation source as claimed in claim 1 , wherein the at least one dispersion control mechanism applied in the portion of the optical fiber is arranged such that a ratio of a maximum power spectral density to a minimum power spectral density of a spectrum of the output radiation is no more than 5.
14 . A metrology device comprising the radiation source as claimed in claim 1 .
15 . A method of generating output radiation, the method comprising:
selecting one or more parameters of one or more selected from pulsed pump radiation, a hollow core optical fiber comprising a body having a hollow core and a working medium comprised within the hollow core such that the pulsed pump radiation undergoes a soliton self-compression process so as to change a spectrum of the pulsed pump radiation so as to form output radiation,
generating the pulsed pump radiation for reception by, and propagation through, the hollow core from an input end to an output end; and
applying at least one dispersion control mechanism in a portion of the optical fiber so as to change dispersion characteristics starting at a location, in the portion of the optical fiber, after a point of maximum compression to spectrally shift a dispersive wave generated in the soliton self-compression process, wherein the location is selected so as to attenuate a peak of the dispersive wave to reduce sensitivity of the peak to pulse energy variations.
16 . The method as claimed in claim 15 , wherein the spectrum of the pulsed pump radiation has reached its maximum breadth before exiting the optical fiber via the output end.
17 . The method as claimed in claim 15 , wherein the at least one dispersion control mechanism comprises one or more selected from:
a continuously increasing or decreasing core diameter of the optical fiber along the portion of the optical fiber;
a continuously increasing or decreasing pressure to which the working medium is subject along the portion of the optical fiber; or
a continuously increasing or decreasing temperature to which the working medium is subject along the portion of the optical fiber.
18 . The method as claimed in claim 15 , wherein the portion of the optical fiber is the full length of the optical fiber.
19 . The method as claimed in claim 15 , wherein the portion of the optical fiber begins at or near to a position where the spectrum of the pump radiation reaches the maximum breadth and ends at the output end of the optical fiber.
20 . The method as claimed in claim 15 , wherein the dispersion characteristics comprise a wavelength of zero group velocity dispersion of the optical fiber.