Abstract:
A compact high average power mid infrared range laser for ultrasound inspection. The laser comprises one of a Nd:YAG or Yb:YAG laser pumped by a diode at 808 nm to produce a 1 micron output beam. The 1 micron output beam is directed to an optical parametric oscillator where the beam wavelength is converted to 1.94 microns and conveyed to a mid infrared emission head. The emission head comprises one of a Ho:YAG or Ho:YLG laser optically coupled with a second optical parametric oscillator. The second optical parametric oscillator forms a generation output beam for creating ultrasonic displacements on a target. The generation output beam wavelength ranges from about 3 to about 4 microns, and can be 3.2 microns.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention relates generally to the field of non-destructive testing. More specifically, the present invention relates to a system to create a mid-range infrared generation laser beam. 
         [0003]    2. Description of Prior Art 
         [0004]    Recent developments in creating composite materials have expanded the use of composite materials into a wide variety of applications. Because of its high strength and durability combined with its low weight, composites are replacing metals and metal alloys as the base material for certain load bearing components. For example, composites are now commonly used as a material for body parts and structure in vehicles such as automobiles, watercraft, and aircraft. However, to ensure composite mechanical integrity, strict inspections are required. The inspections are typically required upon fabrication of a component made from a composite and periodically during the life of the component. 
         [0005]    Laser ultrasound is one example of a method of inspecting objects made from composite materials. The method involves producing ultrasonic vibrations on a composite surface by radiating a portion of the composite with a pulsed generation laser. A detection laser beam is directed at the vibrating surface and scattered, reflected, and phase modulated by the surface vibrations to produce phase modulated light. Collection optics receives the phase modulated laser light and directs it for processing. Processing is typically performed by an interferometer coupled to the collection optics. Information concerning the composite can be ascertained from the phase modulated light processing, the information includes the detection of cracks, delaminations, porosity, foreign materials (inclusions), disbonds, and fiber information. 
         [0006]      FIG. 1  provides one prior art example of a laser system  10  for producing a pulsed generation laser beam. The laser system  10  is configured to produce laser light in the mid infrared range and comprises a mid infrared laser head  11  optically coupled to a mid infrared emission head  30 . The mid IR laser head  11  includes a holmium yttrium lithium fluoride (Th:YLF) end pumped by a pair of diode pumps ( 16 ,  18 ). The output  19  of diode pump  18  pumps one end of the Th:YLF laser  20 , the other end of the laser  20  is pumped by pump diode  16 . Pump diode  16  output beam (not shown) passes through the transmissible side of a dichroic mixer  24  and into the lower end of the laser  20 . The thulium laser output  20  is directed towards the reflective side of the dichroic mixture  24  and is reflected towards the emission head  30 . Optional input and output couplers ( 21 ,  25 ) are provided on the respective input and output of the thulium laser  20 . In this prior art embodiment, the pump diodes ( 16 ,  18 ) pump the thulium laser  20  at a wavelength of 794 nanometers. The thulium output beam  22  operates at approximately 1.94 microns. 
         [0007]    The emission head  30  has a holmium yttrium aluminum garnet (Ho:YAG) laser  34  operatively coupled with a frequency converter  38 . The frequency converter  38  is depicted as an optical parametric oscillator (OPO). The Ho:YAG laser  34  receives the reflected laser output  26  at a wavelength of approximately 1.94 microns and emits its corresponding output beam  36  at a wavelength of about 2.05 microns. The OPO converts the output beam  36  to a signal beam and an input beam, where the signal beam has a wavelength of about 3.2 microns and the idler beam has a wavelength of about 5.7 microns. 
         [0008]    The laser system  10  emits from about five to about ten watts of 3 to 4 micron light, but requires about 1 kilowatt in power of pump diodes. Accordingly, the mid IR laser head  11  is equipped with an associated cooling circuit  14  and power supply  12  that requires a substantial capacity to support laser system  10  operation. The increased power in cooling capacity for the system  10  results in a large volume and a large mass laser head. Additionally, the 794 nanometer pump diodes are not common readily available items. Typically the Ho:YAG laser output is at about 2.05 microns where it is converted within the OPO to a mid IR laser output of about 3.2 microns. A Q-switching device (not shown) is typically included within the emission head  30 . Q-switching provides pulsing to the output laser beam  40  for creating the thermo-elastic displacements on a target surface that then forms ultrasonic displacements on the target surface. The Ho:YAG laser  34  is shown with an input coupler  32  at its input and an output coupler  33  at its output. The OPO  38  is illustrated having both an input and output coupler ( 35 ,  37 ). 
       SUMMARY OF INVENTION 
       [0009]    Disclosed herein is a mid infrared range laser system for ultrasonic testing comprising a yttrium aluminum garnet (YAG) laser having an output beam, a pump diode operatively coupled to the YAG laser, an optical frequency converter operatively coupled to the YAG laser output beam and having an output beam, where the optical frequency converter output beam wavelength is about 2 microns, and a laser system output beam directed at an ultrasonic testing target. The YAG laser may be a neodymium yttrium aluminum garnet (Nb:YAG) laser or a ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser. The optical frequency converter may be an optical parametric oscillator and the pump diode beam output may have a wavelength of about 808 nanometers. The optical frequency converter output beam can be about 1.94 microns. The mid infrared range laser system may also include an emission head coupled to the optical frequency converter output beam, where the emission head includes an output beam having a wavelength in the mid infrared spectral range, and where the emission head output beam forms the laser system output beam. The emission head output beam wavelength can be about 3.2 microns and may include a laser device, the laser device coupled to receive the optical frequency converter output beam, and a second optical frequency converter, the laser device having an output directed to the second optical frequency converter. The laser device of the emission head may be one of a holmium yttrium lithium fluoride laser or a holmium yttrium aluminum garnet laser. The second optical frequency converter may be an optical parametric oscillator. 
         [0010]    Also disclosed herein is a system for ultrasonic analysis of a test object comprising, a laser head pumped with a laser beam having a wavelength of about 808 nanometers and having a laser head output beam having a wavelength of about 2 microns, and a mid infrared range emission head configured to receive the laser head output beam and emit a generation output beam in the mid infrared wavelength range, wherein the generation output beam is directed at the test object to create thermo-elastic expansion thereon and form ultrasonic displacements. The generation output beam wavelength may be about 3.2 microns. The laser head can be an yttrium aluminum garnet laser having an output beam having a wavelength of about 1 micron and may be one of a neodymium yttrium aluminum garnet laser or a ytterbium-doped yttrium aluminum garnet laser. The laser head may include an optical parametric oscillator configured to receive the yttrium aluminum garnet laser output beam and emit a converted beam that forms the laser head output beam. The mid infrared range emission head has a laser device having a laser output beam with a wavelength of about 2.05 microns, and an optical parametric oscillator configured to receive the laser output beam and emit a converted beam, wherein the converted beam forms the generation output beam. The laser device can be a holmium yttrium lithium fluoride laser or a holmium yttrium aluminum garnet laser. An optical fiber may be used to couple the pump head and emission head. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic view of a prior art ultrasonic laser source. 
           [0013]      FIG. 2  is a schematic view of a mid range infrared ultrasonic laser source in accordance with the present disclosure. 
           [0014]      FIG. 3  is a schematic representation of a laser ultrasonic system. 
       
    
    
       [0015]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0016]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as “upper”, “lower”, “above”, “below”, and the like are being used to illustrate a relational location. 
         [0017]    It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims. 
         [0018]    With reference now to  FIG. 2  depicts a schematic view of an embodiment of a mid IR laser system  48 . The mid IR laser system  48  comprises a mid IR laser head  50  optically coupled to an emission head  74  via an optical fiber  72 . The mid IR laser head  50  includes a yttrium aluminum garnet (YAG) laser. The YAG laser may be one of a neodymium yttrium aluminum garnet (Nb:YAG) laser or a ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser. The YAG laser  62  is end pumped by a pump outlet beam  60  from a diode pump  58  on one end; on its opposite end the YAG laser  62  receives a pump outlet beam (not shown) from a pump diode  56  through the transmissive side of a dichroic mixer  66 . The YAG laser  62  output  64  is optionally directed to a reflective side of the dichroic mixer  66  to form a reflected output beam  68 . The reflected output beam  68  forms the optical frequency converter input  68  that is directed to the optical frequency converter  70 . In one embodiment, the optical frequency converter  70  comprises an OPO. The YAG laser  62  is shown with an optional input coupler  61  and an optional output coupler  63  disposed in the path between the YAG laser  62  and the optical frequency converter  70 . An input coupler  65  is shown provided at the input of the optical frequency converter  70  and a corresponding output coupler  67  is disposed at the output. 
         [0019]    The optical frequency converter  70  receives the YAG laser  62  output and emits a converted beam that forms the emission head input beam  72 . As noted above an optical fiber may provide the conduit path between the optical frequency converter  70  and the emission head  74 . The emission head  74  comprises a holmium laser  76  configured to receive the emission head input beam  72 . An output beam  77  from the holmium laser  76  is directed to a second optical frequency converter  80 . The second optical frequency converter  80  receives the laser output beam  77  and emits a converted beam forming the mid IR laser output beam  82 . The holmium laser  76  may be one of a holmium yttrium lithium fluoride (Ho:YLF) laser or a holmium yttrium aluminum garnet (Ho:YAG) laser. Optionally, the second optical frequency converter  80  may also comprise an OPO. The holmium laser  76  is illustrated in a cavity formed between an input coupler  78  and an output coupler  79 . Similarly, the the second optical frequency converter  80  is shown residing in a cavity between an input coupler  81  and an output coupler  83 . 
         [0020]    In one embodiment of the mid IR laser system  48  of  FIG. 2 , the YAG laser is pumped by the pump diodes ( 56 ,  58 ) where the pump laser wavelength is 808 nanometers. The YAG laser  62 , in this embodiment, emits an output laser beam  64  of about 1 micron. The optical frequency converter  70  is configured to convert the approximately 1 micron output beam  64  to a converted beam having a wavelength of about 1.94 microns. In one embodiment, the holmium laser  76  further converts the beam wavelength to about 2.05 microns and the second optical frequency converter  80  emits a beam in the mid infrared range of about 3 microns to about 4 microns. The beam emitted from the second optical frequency converter forms the mid IR laser output beam  82 . Optionally, the mid infrared laser output beam  82  is about 3.2 microns. More specifically, in yet another embodiment, the mid IR laser output beam  82  comprises an idler beam having a wavelength of about 5.7 microns and a signal wavelength of about 3.2 microns. 
         [0021]    One of the many advantages of the system of  FIG. 2  is the availability of pump diodes operating in the absorption band usable for the YAG laser  62 . Pump diodes having an output wavelength of about 808 nanometers are more plentiful than pump diodes whose output is about 794 nanometers. Additionally, the power requirements of the mid IR laser head  50  is reduced by use of the YAG laser  62  over the mid IR laser head  11  of the thulium laser. Accordingly, the associated cooling circuit  54  size used for cooling the mid IR laser head  50  can be smaller due to the lower cooling demands. This further reduces the power requirements required from the power supply  52  to the pump diodes ( 56 ,  58 ). In one alternative embodiment, the YAG laser  62  may be side pumped with pump diodes thereby potentially enhancing the power efficiency of the system. Additionally, the YAG laser  62  may be powered by a single pump diode instead of the dual end pump diodes provided in  FIG. 2 . 
         [0022]      FIG. 3  provides a side perspective view of one embodiment of a laser ultrasonic detection system  85 . The detection system  85  comprises a laser ultrasonic unit  87  that may optionally include the mid infrared laser system  48  as described herein. The detection system  85  emits a generation beam  86  directed to an inspection target  91 , where the generation beam  86  comprises the mid IR laser output beam  82  formed by the mid infrared laser system  48 . The generation beam  86  contacts the inspection target  91  on an inspection surface  92 . The generation beam  86  thermo-elastically expands the inspection surface  92  to produce corresponding displacements  93  on the inspection surface  92 . In one embodiment, the generation beam  86  is a pulsed laser configured to produce the displacements  93  on the inspection surface  92 . A detection beam  88  is also illustrated emanating from the laser ultrasonic unit  87  and is shown coaxial around the generation beam  86 . Although emanating from the same laser ultrasonic unit  87 , the detection and generation beams ( 86 ,  88 ) are generated by different sources. However, the detection beam  88  may optionally originate from a different unit as well as a different location. As is known, the detection beam  88  comprises a detection beam that is scattered, reflected, and phase modulated upon contact with the displacements  93  to form phase modulated light  90 . The phase modulated light  90  from the detection beam  88  is then received by collection optics  89  and processed to determine information about the inspection target  91 . The generation and detection beams ( 86 ,  88 ) may be scanned across the target  91  to obtain information regarding the entire surface  92 . A mechanism (not shown) used to scan the beams ( 86 ,  88 ) may be housed within the laser ultrasonic unit  87 . A processor (not shown) for controlling the mechanism and optionally for processing the data recorded by the collection optics, may also be housed in the laser ultrasonic unit  87 . The collection optics  89  are shown separate from the laser ultrasonic unit  87  and in communication with the laser ultrasonic unit  87  through the arrow A, however the collection optics may be included with or within the laser ultrasonic unit  87 . 
         [0023]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.