Compact snapshot dual-mode interferometric system
View Patent ↗Systems, devices and methods for measuring surface roughness and surface shape of an optical element using a dual-mode interferometer are disclosed. The devices implement optical filters, with a compact form, that allows measurement of both surface characteristics without rearranging the system components. One example interferometric system includes a laser light source and a low coherence light source that alternatively provide light to a collimator, followed by a polarizer, and a polarizing beam splitter. The system further includes two optical filters, a quarter waveplate, two objectives and a reference optical component. Each light source produces a set of interferograms, where one set of interferograms is used to measure the surface shape and another set of interferograms is used to measure the surface roughness of the optical component.
1. An interferometric system for measuring surface characteristics of an object, comprising:
a collimator positioned to receive light from:
(1) a first light source having a first wavelength or first range of wavelengths, and
(2) a second light source having light with a smaller coherence length than light from the first light source, and a second wavelength or second range of wavelengths different from the first wavelength or first range of wavelengths, the interferometric system configured to selectively allow light from one of the light sources to reach the collimator at a given time;
a polarizer positioned to receive collimated light from the collimator and to produce polarized light;
a polarizing optical element positioned to receive the polarized light and to direct light having a first polarization towards a first optical filter as part of a reference path of the interferometric system, and to direct light having a second polarization towards a second optical filter as part of a test path of the interferometric system; and
a quarter wave plate positioned to receive the light having the first polarization in the reference path after reflection from the first optical filter or a reference optical element, and to receive light having the second polarization in the test path after reflection from the object, and to produce oppositely circularly polarized light beams, wherein
the first optical filter is configured to allow the light having the first wavelength or the first range of wavelengths to reflect therefrom, and the light having the second wavelength or the second range of wavelengths to transmit therethrough for illumination of a reference optical element,
the second optical filter having a thickness or a material to impart the same optical path length to light having the second wavelength or range of wavelengths as the first optical filter and configured to allow the light having the first wavelength or the first range of wavelengths, and the light having the second wavelength or the second range of wavelengths, to transmit therethrough for illumination of the object, and
the oppositely circularly polarized light beams, upon reaching a polarization sensitive camera at an image plane of the interferometric system and formation of a plurality of interferograms, enable measurements of a surface shape or a surface roughness of the object based on interferograms produced from light from the first light source or the second light source, respectively.
2. The interferometric system of claim 1 , comprising an imaging lens positioned to receive the oppositely circularly polarized light beams and to produce an image thereof at the image plane.
3. The interferometric system of claim 1 , comprising the polarization sensitive camera that includes:
a polarizer array having a plurality of polarizer elements, and
a detector array configured to produce electrical signals based on detected intensities of the one or more interferograms.
4. The interferometric system of claim 1 , comprising a first objective lens positioned in the reference path between the first optical filter and the reference optical element.
5. The interferometric system of claim 1 , comprising a second objective lens positioned in the test path between the second optical filter and a location of the object.
6. The interferometric system of claim 1 , comprising a dichroic mirror positioned to deliver light from the first and the second light sources to the collimator, wherein the dichroic mirror is configured to allow light from one of the first or the second light sources to reach the collimator upon transmission through the dichroic mirror, and the other of the first or the second light sources to reach the collimator upon reflection from the dichroic mirror.
7. The interferometric system of claim 1 , configured to:
turn on the first light source and turn off the second light source to enable measurement of the surface shape, and
turn off the first light source and turn on the second light source to enable measurement of the surface roughness.
8. The interferometric system of claim 1 , comprising:
a folding mirror positioned to fold an optical path the light that is output from the collimator; and
a beam splitter positioned to fold an optical path of the light beams that exit the polarizing beam splitter after reflection from the reference optical element or the first optical filter, and after reflection from the object.
9. The interferometric system of claim 1 , wherein the polarizer is a linear polarizer.
10. The interferometric system of claim 9 , wherein the linear polarizer is a wire grid linear polarizer.
11. The interferometric system of claim 9 , wherein the linear polarizer has a transmission axis oriented at a particular angle that is adjustable to enable a change in fringe contrast for the plurality of interferograms.
12. The interferometric system of claim 9 , wherein the linear polarizer has a transmission axis oriented at 45 degrees.
13. The interferometric system of claim 1 , wherein the reference optical element is a mirror.
14. The interferometric system of claim 1 , wherein the polarizing optical element is a polarizing beam splitter.
15. The interferometric system of claim 1 , comprising the first and the light sources.
16. The interferometric system of claim 1 , wherein the first light source is a laser light source, and the second light source is a light emitting diode.
17. The interferometric system of claim 16 , wherein the first wavelength or the first range of wavelengths is at, or around, 632.8 nm, and the second wavelength or the second range of wavelengths is at, or around, 540 nm.
18. The interferometric system of claim 1 , wherein the first optical filter is a shortpass optical filter with a transmission cutoff wavelength that is less than the first wavelength or the first range of wavelengths, and the second optical filter is a longpass optical filter.
19. The interferometric system of claim 1 , wherein the system is configured to measure the surface shape and the surface roughness by switching on or off the first and the second light sources without reconfiguration of components of the interferometric system.
20. A dual mode interferometric measurement system, comprising:
a laser light source;
a low coherence light source;
a collimator to receive light selectively from only one, but not both of, the laser light source or the low coherence light source at a given time;
a polarizer to receive light from the collimator;
a polarization beam splitter (PBS) to receive light from the polarizer;
a first optical filter to receive a first type of polarized light from the PBS, the first optical filter configured to allow light from the laser light source to reflect therefrom and light from the low coherence light source to transmit therethrough;
a second optical filter to receive a second type of polarized light from the PBS, the second optical filter configured to allow light from the laser light source and the low coherence light source to pass therethrough toward a test surface;
a reference mirror positioned to receive and reflect light corresponding to the low coherence light source after propagation through the first optical filter; and
a quarter wave plate (QWP) positioned to receive:
(a) light associated with the laser light source after reflection from the first optical filter as part of a reference path associated with the laser light source and after reflection from the test surface as part of a test path associated with the laser light source, or
(b) light associated with the low coherence light source after reflection from the reference mirror as part of a reference path associated with the low coherence light source and after reflection from the test surface as part of a test path associated with the low coherence light source, wherein:
light that is output from the QWP, upon reaching a polarization sensitive camera at an image plane of the interferometric system and formation of a plurality of interferograms, enables a measurement of both a surface shape and a surface roughness of the object based on interferograms produced from light associated with the laser light source and the low coherence light source, respectively.