Abstract:
Apparatus and method for inspecting the interior surfaces of devices such as vessels having a single entry port. Laser energy is launched into the vessel, and the light reflected from the interior surfaces is interfered with reference laser energy to produce an interference pattern. This interference pattern is analyzed to reveal information about the condition of the interior surfaces of the device inspected.

Description:
The present invention generally relates to procedures for inspecting the condition of manufactured articles, and, more particularly to the inspection of the interiors of manufactured articles such as tanks, medical implants, turbines, and other closed applications in which only a limited access is provided. This invention was made with Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Inspection of manufactured articles is of great importance, particularly to manufacturers of critical equipment whose failure can produce catastrophic results. However, some of the articles in this category defy close inspection because the geometry of the article makes inspection extremely difficult for conventional inspection techniques. 
     Fiber optical devices are extensively used to view objects that normally would be considered inaccessible. The optical fiber elements for this purpose are typically smaller than 50 μm in diameter, including protective layers. Optical fibers such as these can be bent into radii as short as 3 cm, allowing their infiltration into areas that normally preclude direct imaging techniques. 
     The medical community has made extensive us of such optical fibers for endoscopic applications, such as the real-time imaging of internal organs to provide guidance for microsurgical techniques. Fiber-optic technology also has been applied in many other areas, such as opto-mechanical applications for numerous industrial and medical applications. 
     The primary problem with prior art, fiber optic imaging is that it is difficult to achieve acceptable measurement of surface tolerances or roughness through a single port or single optical fiber. Conventional fiber optic imaging also does not provide any quantitative measurement of the interior region being imaged. This is because prior art systems generally must rely on interferometry concepts requiring multiple fibers. 
     It is therefore an object of the present invention to provide profilometer apparatus for inspecting the interior surfaces of tanks and other device under tests, including medical implants and any enclosure having a single access port. 
     It is therefore an object of the present invention to provide apparatus for inspecting interiors of tanks and other device under tests that is capable of accurate quantitative measurement of interior surface imperfections with a high degree of resolution. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects, apparatus for inspecting the interior of a device under test through a single port in the device under test comprising laser means for producing laser energy, with beam splitter means for directing the laser energy to a first direction and to a second direction. Delay means receive the laser energy from the second direction for introducing a predetermined delay to the laser energy from the second direction and outputting the delayed laser energy from the second direction. Optical routing means receive the laser energy from the first direction of at a first input for directing the laser energy from the first direction to ones of a first at least one optical fibers that enter the device under test through the port, and for transmitting laser energy reflected from interior surfaces of the device under test to ones of a second at least one optical fibers. Interferometer means receive the laser energy reflected from the interior surfaces of the device under test and the delayed laser energy from the second direction for interfering said laser energy reflected from the interior surfaces of the device under test with the delayed laser energy from the second direction and outputting interference patterns, wherein the interference patterns are representative of the interior surfaces of the device under test. 
     In a further aspect of the present invention, and in accordance with its objects and principles, a method for inspecting the interior surfaces of a device under test having a single entry port comprises the steps of launching laser energy into the device under test; receiving light reflected from the interior surfaces of the device under test; interfering the light reflected from the interior surfaces of the device under test with reference laser energy, creating an interference pattern; analyzing the interference pattern to discern information about the interior surfaces of the device under test; and outputting the information about the interior surfaces of the device under test. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the components of the present invention. 
     FIG. 2 is a schematical illustration of one of the interferomet units for output from the present invention. 
     FIG. 3 is a side view detail of the end of three optical fibers inserted into a device under test to be inspected so that upon rotation and translation of the device under test virtually all of the interior suffice can be inspected. 
     FIG. 4 is a block diagram of the control portion of the present invention and its interconnection to other control devices. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides apparatus for the high resolution inspection of the interior of device under tests, having the capability of such inspection even should the device under test only have a single port of access. The invention can be understood most easily through reference to the drawings. 
     In FIG. 1, a block diagram of the components of the present invention is illustrated. As seen, laser system  11 , which may be a Titanium doped Sapphire laser pumped by an Argon ion laser outputting approximately 30-100 femtosecond (10 −13  seconds) pulses to beamsplitter  12 , which divides the pulse output of laser system  11  between optical path  12   a  and optical path  12   b . However, as will be discussed below, many forms of laser energy with different modulation techniques can be used with the present invention. Optical path  12   a  carries a portion of the laser energy of laser system  11  to optical routing module  13 . Optical fiber  12   b  carries the remaining portion of the laser energy output of laser system  11  to mirror  14  by which the correlated output of laser system  11  is directed into correlation module  15 . 
     As will hereinafter be more particularly described, optical routing module  13  splits the pulses from beamsplitter  12  and directs them into inspection fibers  13   a ,  13   b , and  13   c  which are inserted into device under test  16 . Device under test  16  is mounted onto fixture  17 , which is capable of translating and rotating device under test  16 , and which is itself controlled by fixture and laser control unit  18   a . Fixture and laser control unit  18   a  indexes the rotational movement of fixture  17  and inserts inspection fibers  13   a ,  13   b , and  13   c  into device under test  16  by an amount commensurate with the desired cross-sectional resolution of the profile measurement. Fixture and laser control unit  18   a  also controls the output of laser system  11 , controlling such functions as pulse width and rate, or frequency (wavelength). The Fixture and laser control unit  18   a , is in turn under the control of main controller  18   d.    
     Optical routing module  13  serves several optical functions. Initially, optical routing module  13  directs the incident pulses output from laser system  11  into optical fibers  13   a ,  13   b , and  13   c  for transmission to device under test  16 . Optical routing module  13  also directs pulses returning from device under test  16  on optical fibers  13   a ,  13   b , and  13   c  into optical fibers  13   d ,  13   e , and  13   f  respectively. Optical fibers  13   d ,  13   e , and  13   f  are routed to interferometer detectors  15  and to individual nonlinear crystals  15   a ,  15   b , and  15   c , respectively. 
     As shown in FIG. 2, for an individual nonlinear crystal  15   a ,  15   b , or  15   c  of interferometer detectors  15 , receive the signal reflected from the interior of device under test  16  (FIG. 1) which are on optical fibers  13   d ,  13   e , and  13   f . Also input to nonlinear crystals  15   a ,  15   b , or  15   c  is the transmitted signal on optical fiber  12   b . The interference of the transmitted signal with the received signal produces interference pattern  22  that is magnified by magnification lens  23 . The magnified interference pattern is input to CCD camera  18   b  for conversion into an electrical signal representative of high resolution inspection of the interior surface of device under test  16 . 
     In operation, non-linear optical crystals  15   a ,  15   b  and  15   c  serve to convert the optical energy in the pulses arriving from the transmit reference fiber,  12   b  and from the receive signals on optical fibers  13   a ,  13   b , or  13   c  into energy detectable by the Charge Coupled Device (CCD)  18   b . The pattern of optical energy observed in the non-linear crystals,  15   a ,  15   b and  15   c  is a direct measure of the difference in total path difference between the transmit paththrough optical fibers  12   a , and  13   a ,  13   b , and  13   c  respectively, and the transmit reference path optical fiber  12   b.    
     Non-linear optical crystals  15   a ,  15   b , and  15   c  may be any appropriate non-linear optical crystal. Examples of suitable crystals of use in the present invention are Beta Barium Borate (BBO) and Potassium Dihydrogen Phosphate (KDP) crystals. 
     Turning now to FIG. 3, there can be seen optical fibers  13   a ,  13   b , and  13   c  entering device under test  16  through a single port  16   a . As shown, optical fibers  13   a  and  13   c  have their ends finished to form a 45° reflector, with a high reflective coating applied to surfaces  31  and  32 , and an anti-reflecting coating applied to surfaces  33  and  34 . These coatings will direct light traveling along optical fibers  13   a  and  13   c  at a right angle to the longitudinal axes of optical fibers  13   a  and  13   c . The end of optical fiber  13   b  is finished at a 90° angle, and has an anti-reflective coating applied to its surface  35 , to direct light along its longitudinal axis. With optical fibers  13   a ,  13   b , and  13   c  so configured, virtually complete coverage of the interior surfaces of device under test  16  is possible when device under test  16  is rotated and laterally moved by fixture  17  (FIG.  1 ). 
     Returning now to FIG. 1, it should be noted that the output of laser system  11  is modulated in order to facilitate analyzation of the signals returning from the interior surface of device under test  16 . This modulation, which is applied prior to output from laser system  11 , can be chosen from the group consisting of short pulse, frequency modulated continuous wave (FMCW) or chirp, and stepped frequency phase measurement. Each type of modulation has its own advantages, with the particular type dependent on the particular application. However, any one of these modulation techniques should provide satisfactory results with the present invention. 
     As stated, each of these modulation techniques has its own advantages. Short pulse modulation offers excellent resolution and direct measurement, but is somewhat complex and the pulse resolution is inversely proportional to pulse width. FMCW or chirp modulation, a linear chirp over the same bandwidth as short pulse modulation, has its resolution limited only by the total bandwidth, and accomplishes range measurement in the frequency domain, but is disadvantaged by the possibility its wide bandwidth can produce RF signals, and by its complexity in developing a high linearity sweep. Finally, stepped frequency phase measurement provides discrete wavelength, and accurate phase measurement. Its advantages are that the resolution is limited only by the total wavelength, and that range measurement is in the phase domain. Its disadvantages include that its resolution is limited by phase stability, and its high complexity in accurately measuring phase at optical wavelengths. 
     It is to be understood that the particular type of modulation employed in practicing this invention will require analysis of the requirements so that the correct modulation for the application can be utilized. It is a question of the resolution required and the amount of complexity allowed in the application. In the invention, the modulation of laser system  11  is provided through fixture and laser control  18   a.    
     As illustrated for one interferometer in FIG. 2, interference pattern  22  is imaged onto CCD camera  18   b  for each optical fiber  13   a ,  13   b , and  13   c  shown in FIG.  1 . CCD camera  18   b  provides its output to frame grabber and DSP (Digital Signal Processor)  18   c  for output. 
     Main controller  18   d  provides control to fixture and laser control  18   a  and to frame grabber and DSP  18   c . The controller can be any general-purpose personal computer. Control bus  18   c  provides the connection between frame grabber and DSP  18   c , main controller  18   d  and fixture and laser control  18  to provide the necessary communication between these devices. 
     FIG. 4 is a block diagram of one embodiment of the computer control system for the present invention. As shown, fixture and laser control  18   a  and frame grabber and DSP  18   c  are connected to I/O bus  18   e . Central processing unit  24  also is connected to I/O bus  18   e  as well as to removable hard drive  25 , fixed hard drive  26 , and display  27 . Central processing unit  24  is programmed with the software necessary to provide the appropriate control signals to fixture and laser control  18   a  and frame grabber and DSP  18   c , and to analyze and display the signals returned to it from the interior surfaces of device under test  16  (FIG.  1 ). 
     The present invention can find application in numerous important areas. The inspection of interior surfaces of important vessel assemblies can further reduce the danger of vessel rupture and the concomitant dangers and expense. Devices intended for implantation into human bodies likewise must be examined so that any possible leakage is discovered and corrected prior to implantation. Thus, the present invention may prove to be invaluable in the discovery of surface problems in any vessel that has limited entry ports. 
     The foregoing description of the embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.