Patent Publication Number: US-2013250383-A1

Title: Apparatus for multi-wavelength holographic imaging

Description:
FIELD OF THE INVENTION 
     The field of the invention is the field of measuring surface topography of an object. 
     BACKGROUND OF THE INVENTION 
     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  FIG. 1 , has been difficult to implement for objects with surfaces with steps or slopes greater than a half wavelength of light per resolution element of the imaging system, because the phase count is lost, and the height of the surface is known only modulo λ/2, where λ is the wavelength of light used for the interferometer. 
     If a series of interferograms are recorded with different wavelengths λ 1 , 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. 
     RELATED PATENTS AND APPLICATIONS 
     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. 
     OBJECTS OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sketch of an interferometric imaging system. 
         FIG. 2  is a sketch of the tunable light source for the interferometric imaging system of the invention. 
         FIG. 3  is a sketch of the most preferred frequency converter of the invention. 
         FIG. 4  is graph of the power output the most preferred frequency converter of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A number of n measurements for synthetic holography at each of a number m of wavelengths λ m  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.  FIG. 1  shows a prior art interferometric imaging diagram. A light source  10  produces coherent light output which is converted into a parallel light beam  12  by a lens  11 . The beam  12  is split by a beamsplitter  13  into two parts, one part which illuminates an object  15 , the other part which illuminates a reference surface  14 . The reference surface  14  may be a specularly reflecting surface, a diffusely scattering surface, or any combination of the two. Light scattered from the reference surface  14  and the object  15  is combined at the beamsplitter  13  and propagates to the lens  16 , which images both the surface of the object  15  and the surface of the reference surface  14  on to an image detector  17 . Preferably, an image is exposed, and the reference surface  14  in the reference beam is moved to change the relative phase of the reference beam with respect to the object beam measured at the image receiver. Each image recorded for each relative phase difference is called a phase image. Preferably, a number n phase images are exposed. The wavelength of the light source  10  is then changed, and the process repeated for m wavelengths. 
     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  FIG. 1 . Optical fibers are used to carry light from light source  10  to points where the object and the image receiver are illuminated. Optical fiber techniques speed up the changes in relative phase of the object and reference beams. 
       FIG. 2  shows the tunable coherent light source  10  of the invention. Light  20  from a tunable diode laser  22  operating in the IR spectral region is directed into a non linear frequency converting device  24  which converts the light  20  to a shorter wavelength light  26  in the visible or near IR spectral region which can be detected by a silicon CDD or CMOS image receiver. These devices are not sensitive to light having wavelength longer than about 1100 nm. Image receivers which can detect light wavelengths longer than 1100 nm are expensive and have much less resolution than the silicon devices. Tunable diode IR lasers are used in the 1300 and 1550 nm bands for communications through optical fibers. These lasers are commercially available, reliable, and much cheaper than tunable lasers in the visible and near IR spectral region. 
     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. 
       FIG. 3  shows a preferred embodiment of the invention. A first region  32  of the crystal is poled by one of several techniques know in the art so that efficient conversion for one spectral region is obtained. Then, the poling period is changed in a second region  34 . Second harmonic light generated in the first region is little affected by the second, as there is no phase matching. However, if the frequency of the input light  20  is changed, little second harmonic light will be generated in the first region  32 , but the second region  34  will be phase matched. Thus, the non-linear frequency doubler of  FIG. 3  will be able to convert light in two spectral bands. 
     More than two different poling regions are most preferred for the invention. 
       FIG. 4  shows the measured output second harmonic power as a function of C band laser channel generated by a poled KTP crystal having 4 different periodic poling regions. Note that there are 17 output wavelengths with over 5 mw power distributed over a 15 nm band near 775 nm. The frequency difference between neighboring C band channels is 50 GHz, which would give a depth of field of =/−0.75 mm for resolving height differences on the object surface. The inventors anticipate that this depth of field may be expanded by using subband tuning of the communication lasers to give a set of frequencies spaced closer than 50 GHz apart. 
     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&amp;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.