Patent Application
20130250383
The field of the invention is the field of measuring surface topography of an object.
Interferometry has been used for over a century to measure the surface topography of objects, typically optical components, and distances and small changes in such distances. With the advent of lasers having long coherence lengths and high brightness, the field has expanded greatly. Interferometric imaging, as depicted by
If a series of interferograms are recorded with different wavelengths λ1, the ambiguity in the phase may be resolved, and the heights on the object surface relative to a particular location on the particle surface may be calculated, as is shown in the patents cited below.
U.S. Pat. Nos. 5,907,404 and 5,926,277, assigned to the assignee of the present invention, show that a number of such interferograms taken with various phase delays in the reference beam and various wavelengths of the light source 10 may be recorded and computer analyzed to construct a “synthetic interferogram”, which is an interferogram that one would measure if one had a light source of much different wavelength from the wavelengths from the light source 10. Thus, the “lines” on the interferogram could show height differences of, say, 100 microns instead of 0.4 micron height differences, so the lines would be much further apart and much easier to keep track of. Lasers of 200 micron wavelength are hard to find, and electronic imaging equipment for such wavelengths is even harder to find, and spatial resolution of such a detector, if available, could not possibly match the resolution of detectors for visible and near infra-red light.
U.S. Pat. No. 5,907,404 by Marron, et al. entitled “Multiple wavelength image plane interferometry” issued May 25, 1999.
U.S. Pat. No. 5,926,277 by Marron, et al. Method and apparatus for three-dimensional imaging using laser illumination interferometry” issued Jul. 20, 1999.
U.S. Pat. No. 7,317,541 by Mater entitled “Interferometry method based on the wavelength drift of an illumination source” issued Jan. 8, 2008.
U.S. Pat. No. 7,359,065 by Nisper, et al. entitled “Method of combining holograms” issued Apr. 15, 2008.
U.S. Pat. No. 7,440,114 by Klooster , et al. entitled “Off-axis paraboloid interferometric mirror with off focus illumination” issued Oct. 21, 2008.
U.S. Pat. No. 7,456,976 by Mater entitled “Statistical method of generating a synthetic hologram from measured data” issued Nov. 25, 2008.
The above identified patents and patent applications are assigned to the assignee of the present invention and are incorporated herein by reference in their entirety including incorporated material.
It is an object of the invention to introduce a novel multiwavelength coherent interferometric imaging system using relatively inexpensive lasers which are commercially available and which can switch wavelengths in a very short time.
Commercially available diode lasers used for communication are relatively inexpensive, reliable, tunable over a relative large spectral region, and can switch frequencies rapidly. The lasers which typically are in the wavelength regions of 1300 and 1550 nanometers (nm) can, unfortunately, not be imaged using high quality silicon CCD and CMOS image receivers. In addition, light in the infra-red (IR) spectral region can give as high resolution images as light in the visible and near IR region. The present invention uses a frequency converter to convert the light from such communication lasers to visible or near IR light in the wavelength regions around 650 and 775 nm which can be used in a multiwavelength interferometric imaging system to measure surface topography of objects.
A number of n measurements for synthetic holography at each of a number m of wavelengths λm of light are made to determine the phase of light scattered from an object and received at an image receiver such as film, or an electronic CMOS or CCD array detector.
A problem with the prior art is that phase changes in the reference arm of the interferometer are not set accurately enough due to time lags in moving mechanical parts and hysteresis in the piezo drivers for moving the reference phase surface. If the wavelength of the laser used to expose the interferograms is changed, it will not be set accurately enough for the same reason. For the number of images required for accurate surface measurement, the sum of time lags in setting phase and frequency of the light source 10 can be much greater than the exposure times or the time needed to process the image information to make a surface map of the object.
U.S. Pat. No. 7,440,114 by Klooster , et al. entitled “Off-axis paraboloid interferometric mirror with off focus illumination” issued Oct. 21, 2008 describes a multiwavelength interferometric imaging system with a number of improvements over the basic system shown in
Preferable non linear frequency conversion devices are frequency doublers, triplers, frequency subtraction devices, and other parametric frequency conversion devices. In the art of frequency conversion, a non linear conversion device is a device whose output converted power is a non linear function of the input power in a particular power region. Such devices are preferably crystals lacking a center of symmetry for frequency doubling. One type of crystal commonly used has a different index of refraction for different polarizations of the light. The second harmonic must be in phase with the first harmonic over the length of the crystal for efficient conversion, and the orientation of the crystal is chosen so that the first and second harmonic have opposite polarizations and are phased matched over the length of the crystal. For a tunable input laser beam, such crystals are generally phase matched by changing temperature and/or angle of the crystal to the incoming light beam. When changing the input frequency, the crystal has a relatively narrow wavelength output band before the temperature or angle must be changed. Such changes are too slow to allow for the rapid acquisition of the number of images needed for multiwavelength interferometric imaging. If a frequency doubling crystal is not phase matched, the first harmonic will convert to the second harmonic for a length called a coherence length, and then the generated second harmonic will convert back to the first harmonic. The second harmonic power in the crystal will be a sinusoidal function of the distance traveled.
Another method of frequency doubling is the use of poled crystals, where the symmetry of the crystal is changed periodically by changing the domain structure. Then, when the doubled frequency power starts to convert back to the first harmonic, the changed crystal symmetry allows the second harmonic power to build up once again. The crystal has many such regions and the second harmonic can build up. The conversion efficiency is determined by the number of such poled regions. The bandwidth of conversion is relatively narrow. Preferred crystals are ferroelectric crystals where the poling is controlled by electric fields in the crystal.
More than two different poling regions are most preferred for the invention.
The wavelength region accessible to this technique may be extended using even more poling regions and by concatenating two or more lasers 22. An article describing such a combination of communication band lasers by Brandon George and Dennis Derickson may be found at Proc. of SPIE Vol. 7554 75542O-pp 1-8 (2010) and on the web at
http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1176&context=eeng_fac
The above articles reports usable output spanning the C and L communication bands from 1523 to 1610 nm, which when frequency doubled would give near IR light from 760 to 805 nm.
The above identified publications and reports are hereby incorporated herein by reference in their entirety including incorporated material.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
