Abstract:
An apparatus and method for using an acousto optic scanning laser vibrometer for measuring a dynamic parameter of micro and macro components is disclosed. A coherent source of a laser beam of single wavelength and of stabilized frequency is split into two orthogonal polarized beams. One of the beams strikes the surface of investigation and gets reflected back, and the other polarized beam impinges on the reference surface and gets reflected back. The beam reflected from the surface of investigation and the beam from the reference surface are combined, thereby causing them to interfere. At least one photo detector is positioned at the point of interference. The photo detector output signals are input to a signal processor or phase meter to obtain the dynamic parameter information. Information is provided that is based on the phase shift between the beam striking on the object of investigation and the beam striking the reference surface due to the difference in the optical path. The information provided relates to the dynamic parameters of the object under investigation.

Description:
The present application is a continuation-in-part of U.S. patent application Ser. No. 09/222,731, filed on Dec. 29, 1998, entitled “Noncontact Acousto Optic Scanning Laser Vibrometer”, now U.S. Pat. No. 6,271,924. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for the dynamic measurement of micro and macro features using an acousto optic scanning means. 
     BACKGROUND OF THE INVENTION 
     Non-contact vibration measurement using laser doppler vibrometry is a well-established technique. The laser doppler vibrometry method uses an interferometer to measure the doppler frequency shift induced by the vibration of the object. In order to measure the vibration of the object, the measurement beam strikes the object to be measured and interferes with a reference beam. The resulting frequency shift induced in the interference beam is the vibration of the target surface. The measured vibration signal is the average vibration of the object over the entire beam diameter of the measurement beam striking the object. 
     U.S. Pat. No. 4,554,836 (Rudd), U.S. Pat. No. 5,394,233 (Wang), U.S. Pat. No. 5,838,439 (Zang et al.), U.S. Pat. No. 5,883,715 (Steinlechner et al.), etc., describes single point vibration measurement using laser doppler vibrometry. Some applications require scanning the beam over the object surface in order to measure the dynamic parameter of the object over an area rather than at a single point for better analysis. Moving the object stage in the X-axis and Y-axis and keeping the beam at a fixed point will allow measurement of the vibration over an area. But such a means for scanning the beam over the object is limited by the accuracy of the moving stages, and in most application it is not possible to move the object. Moreover, the motion of the stage creates vibration and increases the noise. In which case, the object of measurement should be fixed, and the beam needs to scan from one point to the next. 
     Laser doppler vibrometer can also be used to measure the flying height, i.e., the distance between the slider head and the disk surface in a hard disk. The flying height is a critical parameter and needs to be measured accurately in order to assure optimal performance. Methods such as capacitance, monochromatic interferometry, and white light interferometry are other methods which can also be applied to the measurement of flying height. All the interferometric measurement techniques involve measurement of flying height at a single point or by scanning the beam by moving the hard disk in the X-axis and the Y-axis. This process will not lead to accurate measurement of flying height due to the error induced by the mechanical movement of the hard disk. Also, the measured flying height is the average of the overall flying height of the area of the measurement spot, which is rather large in all these system. Therefore, the smaller the spot size the more accurate is the measured flying height. 
     SUMMARY OF INVENTION 
     The first preferred embodiment of the present invention is a method and apparatus for the measurement of dynamic parameters of micro and macro features. The disclosed invention includes a beam spatial filter to filter the laser beam profile. The filtered beam passes through a diaphragm or a slot to further improving the beam quality. This process of beam filtering will achieve a small and uniform profile spot. 
     The disclosed invention consists of non-mechanical scanning means using an acousto optic deflector. Acousto optic deflectors for X- and Y-axes scanning are placed in both the measurement and the reference path of the laser beam, thereby scanning the reference and the measurement beams simultaneously. The two X-axis and Y-axis acousto optic deflectors in the reference and the measurement path are driven by the common driver for X-axis and Y-axis, respectively. The acousto optic deflectors in combination with the scanning lens, scan the laser beam along the surface of the target along the X-Y plane. The position of the optical components, acousto optic deflectors and scanning lens are such that the scanning measurement and reference beam interfere automatically at all the scanning points, and they are focused on to a stationary photodetector. Further modification of the first preferred embodiment of the present invention includes using common beam splitters for producing the measurement and the reference interference signal. 
     The second embodiment of the present invention is the measurement of flying height. The reference beam is made to scan the disk surface, and the measurement beam is made to scan the slider head surface. Thus, the relative height between the disk surface and the slider head, i.e., the flying height, is measured. Further modification of the present invention includes use of common beam splitters for producing the measurement and the reference interference signals as in the previous embodiment. 
     The third embodiment of the present invention includes the modification of the previous embodiment by using two scanning lenses, one for the scanning beam on the slider surface, and the other for the scanning beam on the disk surface. This will achieve the same spot size on both the slider head and the disk surface, which is not possible by using a common scanning lens for both beams. This process will lead to more accurate results. A spherical concave reflector can be used to reflect the reference scanning beam in the same path as the input scanning beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of the overall optical system for the non-contact acousto optic scanning system for dynamic parameters measurement; 
     FIG. 2 is a drawing showing the working principle of the scanning lens along with the acousto optic deflector; 
     FIG. 3 is a drawing showing the position of the scanning lens with respect to the target surface and the acousto optic deflector; 
     FIG. 4 is a modified optical lay out for obtaining the reference interference signal from the from the beams to the acousto optic deflectors; 
     FIG. 5 is a modified optical lay out for obtaining the measurement and the reference interference signal using common optical components to minimize the error; 
     FIG. 6 is a drawing showing the optical system for flying height measurement in hard disk drive; 
     FIG. 7 is a modified optical lay out for obtaining the measurement and the reference interference signal using common optical components to minimize the error for flying height measurement in hard disk drive; 
     FIG. 8 is a drawing showing the optical layout for the measurement of flying height of hard disk drive by using two scanning lenses; 
     FIG. 9 is a drawing showing the positioning of the two scanning lenses in the third embodiment of the present invention; and 
     FIG. 10 is a modified optical lay out for obtaining the measurement and the reference interference signal using common optical components to minimize the error for flying height measurement in hard disk drive using two scanning lenses as in the first embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment of the Present Invention 
     Referring to FIG. 1, the preferred embodiment of the present invention employs a laser source  1 , which is preferably a He—Ne laser source. The laser beam  51  preferably passes through a beam spatial filtering mechanism, which is comprised of preferably a focusing lens or objective lens  2 , a pin hole  3  and a collimating or focusing lens  4 . The spatial filtering mechanism includes preferably the collimating or focusing lens  2 . The circular pin hole or slot  3  includes a hole diameter of preferably 1-1.5 times the diameter of the focal spot diameter of the laser beam focused by the lens  2 , which is a diameter of the portion of the laser beam at which the beam intensity becomes preferably 1/e 2  of its peak intensity. This arrangement is preferably to eliminate the noise component in the laser beam  51 . Thus the noise generated due to the random fluctuations from the intensity profile of the laser beam is preferably eliminated. This process of filtering may result in improving the spot size of the laser beam obtained on passing through the scanning lens  24  at the focal point of the scanning lens  24 . The pinhole or slot  3  is placed preferably at the focal point of the lens  2 . The collimating or focusing lens  4  is placed preferably at a distance equal to the focal length of the lens  4  from the slot  3 . This mechanism may also result in expanding the laser beam  51 . In order to expand the diameter of the laser beam  52  to twice the diameter of the laser beam  51 , the focal length of the lens  4  is preferably twice the focal length of the lens  2 . Therefore, the expansion ratio of the beam  52  to the beam  50  is equal to the ratio of focal length of the lens  4  to the lens  2 . In general, expansion ratio “∝” of the ultra short laser pulse is given by ∝=b/a where “b” is the focal length of the lens  4  and “a” is the focal length of the lens  2 . A portion of the total intensity of the laser beam may be lost by the filtering mechanism. 
     The filtered and expanded beam  52  may preferably pass through a slot or diaphragm  3 A which has a hole diameter of preferably 1-1.5 times the diameter of the laser beam  52  at which its intensity is preferably 1/e 2  of its peak intensity. This may further enhance the beam quality by eliminating the peripheral portion of the laser beam. The filtered beam  52 A then passes through a acousto optic modulator  5  which is preferably positioned such that the laser beam  52 A is incident on the acousto optic crystal in the acousto optic modulator  5  at the Braggs angle θ B  of the crystal. 
     The zero order beam  53  and the first order beam  54  pass through a beam splitter  6 , which is preferably a polarizing beam splitter. The zero order beam  53  then passes through a wave plate  9 , which is preferably a half wave plate to change the polarization state of the laser beam to suit the requirement of the acousto optic deflector  11 . Similarly, the first order beam  54  then passes through wave plates  8  and  10 , which are preferably half wave plates to change the polarization state of the laser beam to suit the requirement of the acousto optic deflector  13 . The beam  58  then passes through the acousto optic deflectors  11  and  12 . The first order beam  58  from the acousto optic deflectors  11  and  12  is deflected in both the X-axis and Y-axis. The beam  55  then passes through the acousto optic deflectors  13  and  14 . The first order beam  59  from the acousto optic deflectors  13  and  14  is deflected in both the X-axis and Y-axis. 
     The polarization state of the first order beam  58  is changed by the wave plate  15 , which is preferably a half wave plate so that the beam  58  passes through the beam splitters  17  and  20 . The zero order beam  57  from the acousto optic deflector  12  is deflected by the beam splitter  17  and passes through a wave plate  33 , which is preferably a quarter wave plate. The beam strikes the mirror or a reference surface  35  and is reflected back in the same path. The reflected beam  65  then interferes with the zero order beam  60  from the acousto optic deflector  14  by the beam splitter  18  and the polarizer  31 . The interference beam  66  then strikes the photo detector  32 , which acts as a reference signal. 
     The first order scanning beam from the acousto optic deflectors  11  and  12  passes through the quarter wave plate  23  and a scan lens  24 , which is preferably a F-Theta lens, telecentric lens or a confocal microscopy lens. The beam  62  is then focused on to the target surface  25  and reflected back along the same path. 
     Similarly, the first order scanning beam from the acousto optic deflectors  13  and  14  pass through the quarter wave plate  27  and collimating or focusing lens  26 . The beam  61  is focused on to the reference surface, which is preferably a super mirror  28 , which reflects the beam along the same path. The distance between the lens  26  and the center of the acousto optic deflectors  13  and  14  is equal to the focal length of the lens  26 . Also, mirror  28  is at the focal distance from the lens  26 . 
     The reflected beam from the mirror  28  is deflected by the beam splitter  21  and the polarization of the beam  63  is shifted by the half wave plate  22 . The beam interferes with the reflected beam from the object surface by the beam splitter  20  and the polarizer  29 . The interfered beam  64  strikes the photo detector  30 , and acts as the measuring signal. 
     Referring to the FIG. 1, the distance between the acousto optic deflectors  11  and  12  are positioned at a distance “a” from the acousto optic deflectors  13  and  14 . The distance “a” is same as the distance between the axis of the beams  55  and  56 . Referring to FIG.  1  and FIG. 2, the distance “b” from the center of the acoustic crystals of the acousto optic deflectors to the beam splitter  20  is equal to the distance between the beam splitter  20  and the optical window of the photo detector  30 . 
     This mechanism for positioning the acousto optic deflectors and other lenses is to make the interference beam strike the photo detector  30  at the same point at all the scan point locations as shown in FIG.  2 . Also the measuring and reference beams interfere automatically at all the scan points when the measuring and reference beams are interfered at one specific scan point. 
     The working principle of the scan lens is as shown in FIG.  2 . The scanning beam  58  comes to focus along a plane on the target surface at all the scan points and on reflection, traces the same path. 
     Referring to the FIG. 3, the positioning of scanning lens  24 , which is preferably a telecentric lens, F-theta lens, confocal microscopy lens is an important factor. The scanning lens  24  is positioned such that the forward working distance of the scanning lens  24  is preferably the distance from the lens housing of the lens and the in-between the exit face of the acousto optic crystal in acousto optic deflector  11  and the exit face of the acoustic crystal in acousto optic deflector  12 , where the lens  24  has its best performance (as shown in FIG.  3 ). The scanning lens  24  is also preferably positioned at a distance from the target or object surface called the back working distance of the scanning lens  24  so that the laser beam is focused on the work surface. The back working distance of the scanning lens  24  is preferably the distance from the target surface to the output side of the lens housing of lens  24 . 
     In order to obtain a smaller spot size at the focal point of the scanning lens  24 , the diameter of the input beam  58  is preferably larger. In other words, the larger the diameter of the input beam  58  to the scanning lens  24 , the smaller will be the focused spot size. The beam filtering mechanism using the spatial filter and the slot or diaphragm disclosed before may result in a smaller spot size (obtained by the scanning lens  24 ) by improving the quality of the beam. 
     The reference interference signal can also be obtained by altering the optical layout of the system as shown in FIG.  4 . Instead of interfering the zero order beams from the acousto optic deflectors  12  and  14 , the zero and first order beams from the acousto optic modulator  5  is made to interfere in the same manner as before. The wave plates  8  and  9 , which are preferably half wave plates, are rotated such that a fraction of the laser beams  55  and  53  are reflected by the beam splitters  17  and  18  and are made to interfere as described before. The reference photo detector  32  captures the interference signal. The zero order beams from the acousto optic deflectors  14  and  12  are blocked by a blocking means  80  and  81 . 
     Further modification of the present embodiment is the use of common beam splitter to obtain the measurement and reference interference signal. Referring to FIG. 5, common beam splitters  21  and  20  are used to produce the measuring and reference interference signal to the photo detectors  30  and  32 . This leads to a reduction in the error of the measured result. 
     Second Embodiment of the Present Invention 
     The second embodiment of the present invention is to measure the flying-height, i.e., the relative height between the slider head and the disk surface, while the disk is rotating. The optical system and the operating principle is the same as in the first embodiment but has some modification to the optical layout to suit the application as shown in FIG.  6 . 
     The reference optical path and the interference mechanism of the zero order beams  57  and  60  from the acousto optic deflectors  12  and  14  are the same as in the first embodiment. The measuring beams, i.e., the first order scanning beams  58  and  59  from the acousto optic deflectors  12  and  14 , take a different optical path as shown in FIG.  6 . 
     Referring to FIG. 6, the first order scanning beam  58  from the acousto optic deflector  12  passes through the beam splitters  17  and  20 , and then through a wave plate  23 . The beam then passes through a beam splitter  41  and is focused on to the slider head surface  25 A by the scanning lens  24 . The first order-scanning beam  59  from the acousto optic deflector  14  passes through the beam splitters  18  and  21 . The beam then passes through a wave plate  27  and is deflected by a right angle prism or a mirror  40  on to the beam splitter  41 . The beam is then deflected by the beam splitter  41  and is focused on to the disk surface  25  by the scanning lens  24 . The optical systems are aligned to make the beams  61  and  62  parallel to each other. 
     The beam  62  on reflection from the surface of the slider head traces its original path and is deflected by the beam splitter  20 . The deflected beam passes through the wave plate  22 , which is preferably a half wave plate and then through the beam splitter  21 . Similarly, the beam  61  on reflection from the surface of the disk  25  traces its original path and is deflected by the beam splitter  21 . The two beams reflected from the surface of the slider head  25 A and the disk  25  are caused to interfere by the polarizer  29 . The interference signal is captured by the photo detector  30 , which acts as the measuring signal. 
     Referring to FIG. 7 the optical system is further modified to obtain the measuring and reference interference signal using the common beam splitter  20  and  21  as in the previous embodiment. 
     Third Embodiment of the Present Invention 
     The third embodiment of the present invention is to measure the flying height using two scanning lenses in the optical layout as shown in FIG.  8 . The surface of the slider head  25 A and of the disk  25  are not at the same distance from the scanning lens and hence the laser beam cannot be focused on both the slider head and disk surface. Hence the spot of the focused beam on the slider head  25 A and the disk surface  25  will be of different size. This will lead to inaccuracy in relation to the measurement area at each scan point. In order to overcome this error, two scanning lenses are used in the present invention. Referring to FIG. 9, the scanning lens  24  is placed between the deflecting mirror  40  and the beam splitter  41 . Also scanning lens  24 A is placed before the beam splitter  41 . The scanning lens  24  is positioned such that the distance of the disk surface  25  from the scanning lens  24  is the back working distance of the scanning lens. Also the scanning lens  24 A is positioned such that the distance of the slider head surface  25 A from the scanning lens  24 A is the back working distance of the scanning lens  24 A. With this method, beams  61  and  62  will focus on the surface of the disk and of the slider head, respectively. The specification of the scanning lenses  24  and  24 A is such that the focused beam spot size at the focal point of the scanning lens is the same for both lenses. 
     Referring to FIG. 10, the optical system is further modified to obtain the measuring and reference interference signal using the common beam splitters  20  and  21  as in the first embodiment. 
     For all the embodiments of the present invention a spherical concave reflector can replace the reflecting mirror  28 , where the achromatic lens  26  is eliminated. The spherical concave reflector is placed at a distance equal to its radius of curvature or twice its focal length from the center of the acousto optic deflectors  13  and  14 . This process will lead to reflection of the scanning beam from the spherical concave reflector in the same path as the input-scanning beam. This will result in a reduction of optical components. 
     While the preferred embodiment has been disclosed, obvious modifications can be made therein without departing from the spirit and scope of the invention as defined in the following claims.