Abstract:
An autocorrelator apparatus and method for economically measuring physical properties of an object where the measurement path is at least semi-translucent to light, such as thicknesses in multilayered optical structures, group index of refraction, and distance to a surface. The apparatus includes a non-coherent light fiber interferometer and an optional coherent light fiber interferometer in association so as to share PZT fiber modulators. Thickness and boundary extent measurements can be made, for example, of solids, liquids, liquids moving along a horizontal plane, or liquids flowing down a plane.

Description:
FIELD OF INVENTION 
   The invention relates generally to autocorrelators constructed from optical fiber instead of bulk optics useful for measuring reflection areas in materials and physical properties thereof that are at least semi-transparent to light. 
   BACKGROUND OF THE INVENTION 
   In web and coating manufacturing operations, expensive bulk optic interferometric apparatuses are used for accurate, on-line measurements of web and coating layer thickness. An apparatus, such as shown in U.S. Pat. No. 5,633,712, by Venkatesh, et al., U.S. Pat. No. 5,659,392, by Marcus, et al. which issued Aug. 19, 1997 and the associated method taught in U.S. Pat. No. 5,596,409 by Marcus, et al. which issued Jan. 21, 1997 have a high degree of lateral resolution, are light weight, compact, easy to set up, and are robust in high and low temperature environments, in the presence of solvents, high air flow, and various levels of relative humidity. Such apparatuses are self-calibrating or able to remain in calibration for extended periods of time so that the apparatus can be installed on a production machine without the need for re-calibration. Unfortunately, the expensive bulk optics and mechanics required in such devices reduce their usefulness except in high value production applications. In addition, the mechanical nature of the scanning optics of such devices reduce the possible scan rate and service life. 
   Therefore, there has been a need for an economical, long lifetime measuring device, which can produce accurate measurements at high scan rates that require little periodic maintenance and can be packaged with minimal size and mass. 
   SUMMARY OF THE INVENTION 
   In the present invention, the disadvantages of the prior art interferometric devices are overcome by eliminating bulk optic and mechanical components, and providing an all fiber device. The mechanical components and open optics are eliminated and instead, scanning is accomplished by means of piezo electric fiber stretchers usually configured in tandem to provide a rapid, accurate measurement with a relatively large dynamic range. Replaceable sensing probes may be any length and do not require length matching to other components of the measuring instrument. 
   In the present invention, broadband light (of a first wavelength) is guided by a probe fiber of arbitrary length to the sample material. Multiple reflections of the broadband light from the sample are oppositely guided back through the probe fiber, and subsequently guided to an all-fiber, optical path matched autocorrelator assembly where this light is split into two beams which each pass through fiber stretchers, which are driven in opposite directions. Both beams are reflected back upon themselves so as to double pass their respective fiber stretchers to be recombined with the original splitter. The recombined signal is guided to a photodetection device creating a first electronic signal representative of the scanned reflections from the sample material. 
   The present invention also allows for a wavelength stable coherent optical source of a second wavelength different than that of the broadband source to be co-injected to the autocorrelator assembly with the broadband light. This second wavelength co-propagates with the first wavelength through the autocorrelator taking the same paths of the first wavelength and is also recombined with the original splitter causing an interferometric fringe rate proportional to the optical path variations caused by the fiber stretchers. This second wavelength is then guided to a separate photodetection device causing a second electronic signal which represents a highly accurate measure of the autocorrelator scan. This second electronic signal may be used to assist in the interpretations of the first electronic signal such that a highly accurate displacement measure of the multiple reflections from the sample material may be made. These interpretations in turn may be used to accurately define physical properties of the sample material. 
   Therefore, it is an object of the present invention to provide an improved white light interferometric reflective measuring device at a fraction of the cost of similar measuring devices available in the prior art. 
   Another object is to provide a white light interferometric reflective measuring device that implements its scanning mechanism by means of fiber stretching. 
   Another object is to provide a white light interferometric reflective measuring device that incorporates an optical path matched autocorrelator section which is independent of the probe fiber, hence removing the requirement of path matching for the probe fiber. 
   Another object is to provide a white light interferometric reflective measuring device with high resolution and fast scan rates. 
   Another object is to provide a white light interferometric reflective measuring device that provides for a first broad band wavelength to probe the sample material and a second coherent wavelength to measure the scan distance variations of the autocorrelator. 
   Another object is to provide a white light interferometric reflective measuring device that utilizes low cost single mode fiber and associated low-cost single mode fiber components and implements orthoconjugate mirrors (known as Faraday rotator mirrors (FRM)) in the autocorrelator section which assures both high interferometric visibility and minimal birefringence modulation of the broadband light. This in turn provides the highest possible resolution capability of the instrument. 
   These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detailed specification, together with the accompanying drawings wherein: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a partial fiber configured interferometric reflective measuring device (Michelson white light interferometer) constructed in accordance with the prior art and having a fiber probe; 
       FIG. 2  is a schematic diagram of a modified version of the device of  FIG. 1  with an all fiber scanning assembly; 
       FIG. 3  is a schematic diagram of a all fiber autocorrelator constructed in accordance with the present invention; 
       FIG. 3A  is a graph of output level vs displacement for the autocorrelator of  FIG. 3  when its probe is pointed at a rear surface mirror; and 
       FIG. 4  is a schematic diagram of an enhanced version of the autocorrelator of  FIG. 3  including a reference laser. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings more particularly by reference numbers, number  10  in  FIG. 1  refers to a prior art Michelson scanning interferometer in which a broadband light source  12  provides white light  13  through a fiber  14  to a polarizer  16 . The polarizer  16  and the polarization maintaining fibers  20 ,  22 ,  24 , and  26  downstream therefrom as well as a polarization maintaining fiber coupler  28  are included to eliminate polarization fading. 
   The polarized white light  30  passes through the fiber  20  and is split into two light beams  32  and  34  by the polarization maintaining fiber coupler  28 . The beam  32  is projected out of a non-reflective fiber termination  36 , through a focusing lens  38  and onto a mirror device  40 . The mirror device  40  may be a corner cube or retro-reflector to assure that the reflected reference beam  44  returns to the termination  36  without much attenuation. The referenced beam  44  is formed by translating the device  40  typically by means of either a motorized linear slide, rocker assembly or a beam deformer  45 , whose motion is shown symbolically by the arrow  46 . Such techniques have proven to be effective in providing appropriate scan ranges, but have a number of undesirable features when being considered for instrument production. These include: high cost of the launch and collection optics, and specialized motor controllers; low speed scan rates because inertia limits scan rate, and rocker assemblies limit range through angle changes; the requirement for periodic maintenance such as optical alignment and cleaning; the limited lifetimes inherent in mechanical systems with moving parts; and package limitations because compact packages are delicate, so size and mass must be increased for robustness. 
   The sensing beam  34  in the sensing leg of fiber  24  is projected out of a fiber termination  50 , through a focusing lens  52  and onto a sample  54  whose spacing of reflective interfaces are to be determined. The fiber termination  50  may be a partial reflective termination to enable a reference distance to the sample by producing a reference reflection or the sensing leg fiber termination  50  may be non-reflective and a reference reflection be incorporated into the sample  54 . 
   The beams  44  and  56  are combined in the polarization maintaining fiber coupler  28  as an optical signal  60  whose intensity varies with time in reference to the motion of the mirror device  40  where there will be zones of incoherent intensity summation and zones of coherent recombination (of the two beams  44  and  56 ). The beam  60  passes through fiber  26  to be projected onto an optical receiver  62  by a non-reflective fiber termination  64 . The optical receiver  62  converts the optical intensity levels into electrical signals, which are digitized for subsequent signal processing. Also digitized is the electrical monitor or pick-off signal along line  66  from the translating device  45  by the data acquisition demodulator  68 . 
   The concept of an all-fiber Michelson white light interferometer  70 , as shown in  FIG. 2 , is highly appealing in that the light is self contained and eliminates the packaging complexities associated with integration of bulk optics with fiber waveguides. A polarization maintaining fiber arrangement is shown. The scanner apparatus  70  includes a broadband light source  72  that provides white light  73  through a fiber  74  to a polarizer  76 . The polarizer  76  and the polarization maintaining fibers  80 ,  82 ,  84 , and  86  downstream therefrom as well as a polarization maintaining 50/50 fiber coupler  88  are included to eliminate polarization fading. 
   The polarized white light  90  passes through the fiber  80  and is split into two light beams  92  and  104  by the fiber coupler  88 . The beam  92  is passed through a piezoelectric fiber stretcher  95  and is reversed in direction by mirror  100  as reference beam  102 . 
   In piezoelectric fiber stretchers  95 ,  105 , typically a length of fiber is wound around the circumference of a ceramic piezo cylinder element with a sufficient tension that assures that the fiber never goes limp. Using the white light interferometer configuration  70 , with available fibers of reasonable length and appropriate piezo ceramic material for the modulators, 10 mm of scan range can be obtained for low frequency scan rates and at 1mm scans, an order of magnitude faster scan rate can be produced. It is typical that when using 2.3 inch diameter cylinders, each with 40 meters of fiber applied that 10 mm scans at 50 Hz rates and 1 mm scans at 500 Hz may be achieved. 
   The sensing beam  104  in the sensing leg of fiber  84  is passed through a piezoelectric fiber stretcher  105 , driven opposite to the stretcher  95 , and is projected out of a fiber termination  110 , through a optional focusing lens  112  which forms the probe  113 , and onto a sample  114  whose spacing of reflective interfaces are to be determined. Like before, the fiber termination  110  may be a partial reflective termination to enable a reference distance to the sample  114  by producing a reference reflection in the return reference beam  116  or the sensing leg fiber termination  110  may be non-reflective and a reference reflection be incorporated into the sample  114 . 
   The beams  102  and  116  are combined in the fiber coupler  88  as an interference beam  120  whose intensity varies with time in reference to the stretching of the fibers  82  and  84 . To assure that interference between the two beams  102  and  116  occurs, the pathlengths out and back to the fiber coupler  88  must be very close, since any mismatch reduces the dynamic measuring range of the instrument  70 . 
   The beam  120  passes through fiber  86  to be projected onto an optical receiver  122  by a non-reflective fiber termination  124 . The optical receiver  122  converts the intensity changes in the beam  120  into electrical signals, which are demodulated in a demodulator  124  in accordance to the stretching by the piezoelectric fiber stretchers  95  and  105 . 
   The PM fiber arrangement in  FIG. 2  is the design of choice. The use of lower cost single mode fiber scanners with PZT modulators produces birefringence modulation caused by the modulation process, which broadens the coherence which reduces measurement resolution and also causes polarization fading although such are available for very low cost applications. 
   A variation of the All-Fiber Michelson white light interferometer  70  is realized when the probe  113  is located external to the interferometer  70 . In this case, the light returning from a sample is sent to a scanning interferometer and then processed. This all-fiber autocorrelator  150  as shown in  FIG. 3 , has the advantage of using a probe of arbitrary length without having to match the length of a probe fiber to the length of a reference fiber. Another advantage involves the use of lower cost single mode fiber to replace the polarization maintaining fiber where no birefringence modulation degradation is experienced when Faraday rotator mirrors are used. 
   The all-fiber autocorrelator  150  is shown in  FIG. 3  in its most generic form. Here the same type of fiber modulator scanning mechanism as in instrument  70  is used. A disadvantage to this approach when compared to the Michelson approach is that it has a larger optical loss resulting from the extra coupler. This coupler could however be replaced (at an additional expense) with a circulator to improve the throughput power such that is roughly equivalent to that of the Michelson approach. Use of the circulator also provides immunity of the broadband source from back reflected light from the sample. 
   Unlike before, a single mode fiber arrangement is shown. The autocorrelator  150  includes a broadband light source  152  that provides white light  153  through a single mode fiber  154 . The single mode fiber  154  and the single mode fibers  161 ,  162 ,  163 ,  164  and  165  downstream therefrom as well as 50/50 single mode fiber couplers  166  and  168  form the primary light paths for the autocorrelator  150 . 
   The white light  153  passes through the fiber  154 , fiber coupler (or three port circulator)  166 , fiber  161  and out of a probe  171  for reflection off the sample  172  under test. The reflected beam  174  is re-acquired by the probe  171  is conducted by the fiber coupler  166  and fiber  162  to the second 50/50 single mode coupler  168  where it is split into two light beams  175  and  176 . The probe  171  could include a separate optical fiber for re-acquiring the reflected beam  174 , in which case, fiber coupler  166  can be eliminated. The beam  175  is passed through a single mode fiber wound piezoelectric fiber stretcher  177  and is reversed in direction by Faraday rotator mirror  180  providing an orthoconjugate reflection causing a  908  polarization rotation as first reference/signal beam  182 . The beam  176  is passed through a second piezoelectric fiber stretcher  184  driven opposite from stretcher  177  and is reversed in direction by Faraday rotator mirror  185  where its polarization is also rotated by 90° as second reference/signal beam  186 . The second fiber stretcher does not need to be present when a reduced measurement range is all that is required so long as the light beam  176  travels a similar light path distance to that of light beam  175 . Passage through the stretchers  177  and  184  can cause birefringence variations so shifting the return beams state of polarization by 90° causes any birefringence variations of the light going through in one direction to be corrected during the reverse passage. The piezoelectric fiber stretchers  177  and  184  are constructed as described for stretchers  95  and  105 , with the fibers  163  and  164  forming the light guides thereof being optical path matched. 
   The first and second reference/signal beams  182  and  186  are combined into an interference beam  190  by the single mode coupler  168  and conducted by single mode fiber  165  to receiver and processing electronics  192 . The response of the autocorrelator  150  shown as a rectified envelope of the inteferogram from a single autocorrelator scan of a rear surface mirror having a partial reflecting front surface separated by a distance X to the rear reflector as the sample is shown in  FIG. 3A  with the probe through a piezoelectric fiber stretcher  278  and is reversed in direction by Faraday rotator mirror  280  where its state of polarization is rotated  90 ° as first reference/signal beam  282 . The beam  277  is passed through a second piezolectric fiber stretcher  284  driven opposite from stretcher  278  and is reversed in direction by Faraday rotator mirror  285  where its state of polarization is rotated  90 ° as second reference/signal beam  286 . The piezoelectric fiber stretchers  278  and  284  are constructed as described for stretchers  95  and  105 , with the fibers  263  and  264  forming the light guides therof being optical path matched. 
   The autocorrelator  250  shown in  FIG. 4  uses an additional coherent optical source which co-propagates with the broadband light inside the scanning interferometer. Wavelength division multiplexer&#39;s (WDM) or other appropriate combining/splitting and filter elements are used to inject and separate out the returns from the broadband and coherent sources. The detected fringe crossings from the coherent source are used to determine the exact displacement of the scan at all points in the sweep. 
   The modified autocorrelator  250  is shown in  FIG. 4  is essentially identical to the autocorrelator  150  except for modifications to allow injection and separation of returns from a coherent source so that detected fringe crossings from the coherent source can be used to determine the exact displacement of the scan at all points in the sweep. The modified autocorrelator  250  includes a broadband light source  252  that provides white light  253  at a center frequency λ 1  through a fiber  254  to a 50/50 coupler  256 , or in applications that require low light loss, a three port circulator. The coupler  256  is shown with a termination  257  on its unused leg  258 . The coupler  256  and the single mode fibers  260 ,  261 ,  262 ,  263 ,  264 ,  265 ,  312 , and  266  downstream therefrom as well as 50/50 fiber coupler  268  form the main sensing light path of the autocorrelator  250 . 
   The white light  253  passes through the fiber  260  and out of a probe  270  for reflection off the sample  271  under test. Like before, the probe  270  includes a fiber termination  272  and an optional focusing lens  273 . The reflected beam  274  is re-acquired by the probe  270 , and is conducted by the coupler  256 , fiber  261 , and a wavelength division multiplexer  275  (or coupler used to combine the two beams) to the second 50/50 coupler  268  where it is split into two light beams  276  and  277 . The beam  276  is passed through a piezoelectric fiber stretcher  278  and is reversed in direction by Faraday rotator mirror  280  where its state of polarization is rotated 90° as first reference/signal beam  282 . The beam  277  is passed through a second piezoelectric fiber stretcher  284  driven opposite from stretcher  278  and is reversed in direction by Faraday rotator mirror  285  where its state of polarization is rotated 90° as second reference/signal beam  286 . The piezo-electric fiber stretchers  278  and  284  are constructed as described for stretchers  95  and  105 , with the fibers  263  and  264  forming the light guides thereof being optical path matched. 
   The first and second reference/signal beams  282  and  286  are combined into an interference beam  290  on fiber  265  by the coupler  268  and directed through a fiber termination  291  to an optical receiver  292  by a WDM  293  or other appropriate splitter and filter. 
   The autocorrelator  250  shown in  FIG. 4  uses a coherent light source  296  and injects a coherent beam  298  into fiber  299  at a frequency λ 2 , that is not common with any of the frequencies centered at λ 1 , by means of the WDM  275  positioned between the couplers  256  and  268 . It is important that λ 2  be selected such that it can propagate single mode through the same fiber and couplers used for λ 1  and also it be close enough in wavelength to λ 1  (25% is sufficient) so that the couplers and Faraday rotator mirrors (which are typically adjusted to λ 1 ) are able to (but with small errors) function correctly for this second wavelength. For example, if standard telecommunications single mode fiber is used, selections of λ 1  and λ 2  of 1300 nm and 1550 nm satisfies the criteria. Commercial devices (wide band or dual window couplers, single mode fiber, circulators, Faraday rotator mirrors, and WDM&#39;s) are abundant at these two wavelengths. The coherent beam  298  co-propagates with the broadband light inside the scanning interferometer  250 . The coherent beam  298  is split by the coupler  268  into coherent beams  300  and  302 . The coherent beams  300  and  302  are passed through the stretchers  278  and  284 , reflected off the Faraday rotator mirrors  280  and  285 , and passed again though the stretchers  278  and  284  for combination on the coupler  268  into coherent fiber modulator sensing beam  306 . The beam  306  is conducted along fiber  265  and is separated by the WDM  293  onto a second optical receiver  310  by means of fiber  312  and termination  314 . WDM&#39;s or other appropriate splitter/filters are used to inject and separate out the returns from the broadband and coherent sources. The detected fringe variations from the coherent light source  296  are used to determine the exact displacement of the scan at all points in the sweep. 
   Thus, there has been shown novel all-fiber autocorrelators which fulfill all of the objects and advantages sought therefor. Many changes, alterations, modifications and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification together with the accompanying drawings. All such changes, alterations and modifications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims that follow.