Mid-IR laser for generation of ultrasound

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.

BACKGROUND

1. Field of Invention

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.

2. Description of Prior Art

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.

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.

FIG. 1provides one prior art example of a laser system10for producing a pulsed generation laser beam. The laser system10is configured to produce laser light in the mid infrared range and comprises a mid infrared laser head11optically coupled to a mid infrared emission head30. The mid IR laser head11includes a thulium yttrium lithium fluoride (Th:YLF) end pumped by a pair of diode pumps (16,18). The output19of diode pump18pumps one end of the Th:YLF laser20, the other end of the laser20is pumped by pump diode16. Pump diode16output beam (not shown) passes through the transmissible side of a dichroic mixer24and into the lower end of the laser20. The thulium laser output20is directed towards the reflective side of the dichroic mixer24and is reflected towards the emission head30. Optional input and output couplers (21,25) are provided on the respective input and output of the thulium laser20. In this prior art embodiment, the pump diodes (16,18) pump the thulium laser20at a wavelength of 794 nanometers. The thulium output beam22operates at approximately 1.94 microns.

The emission head30has a holmium yttrium aluminum garnet (Ho:YAG) laser34operatively coupled with a frequency converter38. The frequency converter38is depicted as an optical parametric oscillator (OPO). The Ho:YAG laser34receives the reflected laser output26at a wavelength of approximately 1.94 microns and emits its corresponding output beam36at a wavelength of about 2.05 microns. The OPO converts the output beam36to 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.

The laser system10emits 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 head11is equipped with an associated cooling circuit14and power supply12that requires a substantial capacity to support laser system10operation. The increased power in cooling capacity for the system10results 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 head30. Q-switching provides pulsing to the output laser beam40for creating the thermo-elastic displacements on a target surface that then forms ultrasonic displacements on the target surface. The Ho:YAG laser34is shown with an input coupler32at its input and an output coupler33at its output. The OPO38is illustrated having both an input and output coupler (35,37).

SUMMARY OF INVENTION

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 (Nd: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.

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.

DETAILED DESCRIPTION OF INVENTION

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.

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.

With reference now toFIG. 2depicts a schematic view of an embodiment of a mid IR laser system48. The mid IR laser system48comprises a mid IR laser head50optically coupled to an emission head74via an optical fiber72. The mid IR laser head50includes a yttrium aluminum garnet (YAG) laser. The YAG laser may be one of a neodymium yttrium aluminum garnet (Nd:YAG) laser or a ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser. The YAG laser62is end pumped by a pump outlet beam60from a diode pump58on one end; on its opposite end the YAG laser62receives a pump outlet beam (not shown) from a pump diode56through the transmissive side of a dichroic mixer66. The YAG laser62output64is optionally directed to a reflective side of the dichroic mixer66to form a reflected output beam68. The reflected output beam68forms the optical frequency converter input68that is directed to the optical frequency converter70. In one embodiment, the optical frequency converter70comprises an OPO. The YAG laser62is shown with an optional input coupler61and an optional output coupler63disposed in the path between the YAG laser62and the optical frequency converter70. An input coupler65is shown provided at the input of the optical frequency converter70and a corresponding output coupler67is disposed at the output.

The optical frequency converter70receives the YAG laser62output and emits a converted beam that forms the emission head input beam72. As noted above an optical fiber may provide the conduit path between the optical frequency converter70and the emission head74. The emission head74comprises a holmium laser76configured to receive the emission head input beam72. An output beam77from the holmium laser76is directed to a second optical frequency converter80. The second optical frequency converter80receives the laser output beam77and emits a converted beam forming the mid IR laser output beam82. The holmium laser76may 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 converter80may also comprise an OPO. The holmium laser76is illustrated in a cavity formed between an input coupler78and an output coupler79. Similarly, the second optical frequency converter80is shown residing in a cavity between an input coupler81and an output coupler83.

In one embodiment of the mid IR laser system48ofFIG. 2, the YAG laser is pumped by the pump diodes (56,58) where the pump laser wavelength is 808 nanometers. The YAG laser62, in this embodiment, emits an output laser beam64of about 1 micron. The optical frequency converter70is configured to convert the approximately 1 micron output beam64to a converted beam having a wavelength of about 1.94 microns. In one embodiment, the holmium laser76further converts the beam wavelength to about 2.05 microns and the second optical frequency converter80emits 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 beam82. Optionally, the mid infrared laser output beam82is about 3.2 microns. More specifically, in yet another embodiment, the mid IR laser output beam82comprises an idler beam having a wavelength of about 5.7 microns and a signal wavelength of about 3.2 microns.

One of the many advantages of the system ofFIG. 2is the availability of pump diodes operating in the absorption band usable for the YAG laser62. 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 head50is reduced by use of the YAG laser62over the mid IR laser head11of the thulium laser. Accordingly, the associated cooling circuit54size used for cooling the mid IR laser head50can be smaller due to the lower cooling demands. This further reduces the power requirements required from the power supply52to the pump diodes (56,58). In one alternative embodiment, the YAG laser62may be side pumped with pump diodes thereby potentially enhancing the power efficiency of the system. Additionally, the YAG laser62may be powered by a single pump diode instead of the dual end pump diodes provided inFIG. 2.

FIG. 3provides a side perspective view of one embodiment of a laser ultrasonic detection system85. The detection system85comprises a laser ultrasonic unit87that may optionally include the mid infrared laser system48as described herein. The detection system85emits a generation beam86directed to an inspection target91, where the generation beam86comprises the mid IR laser output beam82formed by the mid infrared laser system48. The generation beam86contacts the inspection target91on an inspection surface92. The generation beam86thermo-elastically expands the inspection surface92to produce corresponding displacements93on the inspection surface92. In one embodiment, the generation beam86is a pulsed laser configured to produce the displacements93on the inspection surface92. A detection beam88is also illustrated emanating from the laser ultrasonic unit87and is shown coaxial around the generation beam86. Although emanating from the same laser ultrasonic unit87, the detection and generation beams (86,88) are generated by different sources. However, the detection beam88may optionally originate from a different unit as well as a different location. As is known, the detection beam88comprises a detection beam that is scattered, reflected, and phase modulated upon contact with the displacements93to form phase modulated light90. The phase modulated light90from the detection beam88is then received by collection optics89and processed to determine information about the inspection target91. The generation and detection beams (86,88) may be scanned across the target91to obtain information regarding the entire surface92. A mechanism (not shown) used to scan the beams (86,88) may be housed within the laser ultrasonic unit87. 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 unit87. The collection optics89are shown separate from the laser ultrasonic unit87and in communication with the laser ultrasonic unit87through the arrow A, however the collection optics may be included with or within the laser ultrasonic unit87.