Methods and apparatuses for spatially filtering optical pulses
An optical filter apparatus including an optical divergence device, operable to receive optical pulses and spatially distribute the optical pulses over an optical plane in dependence with a pulse energy of each of the optical pulses; and a spatial filter, located at the optical plane, operable to apply spatial filtering to the optical pulses based on a location of each of the optical pulses at the optical plane resulting from the spatial distributing.
1 . An optical filter apparatus, comprising:
an optical divergence device, configured to receive optical pulses and spatially distribute the optical pulses over an optical plane in dependence with a pulse energy of each of the optical pulses; and
a spatial filter, located at the optical plane, configured to apply spatial filtering to the optical pulses based on a location of each of the optical pulses at the optical plane resulting from the spatial distributing.
2 . The optical filter apparatus as claimed in claim 1 , wherein the optical divergence device comprises a deflectable mirror configured to receive and subsequently reflect the optical pulses, the deflectable mirror further configured to deflect upon every reflection of each of the optical pulses, the magnitude of the deflection being dependent on a pulse energy of each of the optical pulses and the deflection resulting in spatially distributing the optical pulses over the optical plane.
3 . The optical filter apparatus as claimed in claim 2 , wherein the deflectable mirror comprises a cantilever having a fixed end which is fixed in position and a free end which is moveable in the plane of reflection.
4 . The optical filter apparatus as claimed in claim 3 , wherein the deflection of the deflectable mirror comprises a radial movement of the free end with respect to the fixed end due to each impact by the optical pulses.
5 . The optical filter apparatus as claimed in claim 2 , wherein at least one selected from:
the deflectable mirror comprises a width in the range between 1 μm and 1000 μm,
the deflectable mirror comprises a length in the range between 1 μm and 1000 μm, and/or
the deflectable mirror comprises a thickness in the range between 0.1 μm and 100 μm.
6 . The optical filter apparatus as claimed in claim 2 , wherein the deflectable mirror is configured such that upon a second reflection, each of the optical pulses is reflected into a propagation direction according to its pulse energy.
7 . The optical filter apparatus as claimed in claim 2 , wherein the deflectable mirror comprises at least one reflection coating configured for reflecting the optical pulses.
8 . The optical filter apparatus as claimed in claim 7 , wherein the reflection coating provides a reflectivity of at least 80% in a spectral range defined by the optical pulses and the spectral range covered by the at least one reflection coating is between 100 nm and 4000 nm.
9 . The optical filter apparatus as claimed in claim 2 , wherein the deflectable mirror is configured to provide sufficient deflection to spatially distribute the optical pulses when the pulse energy of the optical pulses is in the range between 0.1 μJ and 100 μJ.
10 . The optical filter apparatus as claimed in any of claim 2 , wherein the optical divergence device further comprises an optical delay arrangement configured to apply a delay time to the optical pulses subsequent to them having undergone a first reflection from the deflectable mirror and direct them back to the deflectable mirror to undergo a second reflection by the deflectable mirror.
11 . The optical filter apparatus as claimed in claim 10 , wherein the optical delay arrangement comprises a configurable delay time.
12 . A broadband radiation source device, configured for generating broadband output radiation comprising the optical pulses upon receiving pump radiation, the broadband radiation source comprising the optical filter apparatus as claimed in claim 2 and, configured such that a pulse separation time of the optical pulses is configured to be substantially equal to or longer than a period of time required for the deflection of the deflectable mirror to decrease to zero or substantially near zero.
13 . A broadband radiation source device configured for generating broadband output radiation comprising the optical pulses upon receiving pump radiation, the broadband radiation source comprising the optical filter apparatus as claimed in claim 1 .
14 . A metrology device comprising the optical filter apparatus according to claim 1 .
15 . A method of spatial filtering optical pulses, the method comprising:
spatially distributing a plurality of optical pulses over an optical plane in dependence with a pulse energy of each of the optical pulses; and
spatially filtering the optical pulses based on a location of each of the optical pulses at the optical plane resulting from the spatial distributing.
16 . The method as claimed in claim 15 , wherein the spatial distributing of the optical pulses further comprises using a deflectable mirror to reflect optical pulses, wherein the deflectable mirror is deflected upon every reflection of each of the optical pulses, the deflection of the deflectable mirror being dependent on a pulse energy of each of the optical pulses.
17 . The method as claimed in claim 16 , wherein the spatial distributing of the optical pulses further comprises:
applying a delay time to the optical pulses first reflected from the deflectable mirror and subsequently directing them back to the deflectable mirror; and
using the deflectable mirror to second reflect the delayed optical pulses to spatially distribute the optical pulses.
18 . The method as claimed in claim 17 , wherein the delay time applied is such that each of the optical pulses arrives at the deflectable mirror for the second reflection at a time when the deflection of the deflectable mirror is substantially at a maximum or wherein at least one pulse separation time of the optical pulses is substantially equal to or longer than a period of time required for the deflection of the deflectable mirror to return to zero or substantially near zero.
19 . The method as claimed in claim 15 , wherein the spatial filtering comprises at least partially blocking some of the optical pulses that are spatially distributed at the optical plane.
20 . The method as claimed in claim 15 , further comprising controlling a beam diameter of the optical pulses prior to their spatial distribution over the optical plane.