SYSTEM AND METHOD FOR HIGH-SPEED LASER DETECTION OF ULTRASOUND

Abstract: A system and method for laser light amplification provides amplification of a laser light emitted from a laser light source as low-amplification seed laser light signal. The low-amplification seed laser light signal is transmitted to an amplification component. The amplification component amplifies the low-amplification seed laser light signal by stimulating emissions of the population inversion provided by a pumping diode to generate an amplified laser light signal. The system and method further directs the amplified laser light signal to an output destination. The result of the present invention is a system and method of operation providing higher pulse rates, improved pointing stability, and optionally variable pulse rates for a variety of uses, including for non-destructive laser ultrasonic testing of materials.

Description: SYSTEM AND METHOD FOR HIGH-SPEED LASER DETECTION OF ULTRASOUND TECHNICAL FIELD The present invention relates generally to a system and method for laser light amplification and, more particularly, to a system and method for providing amplification of a laser beam emitted from a solid state laser that provides higher pulse rates, improved pointmg stability, and optionally variable pulse rates for a variety of uses, including for non-destructive laser ultrasonic testing of materials BACKGROUND ART Amplification of laser light is required for a variety of applications. Long haul telecommunication applications, such as those employing single mode optical fiber, often require optical repeater/amplifiers to boost sagging signal levels Material processing applications may require very high power laser light to perform functions such as cutting of various materials and preparation of mateπal surfaces. Applications requiring intense energy pulses of laser light employ some configuration for providing either time-varying optical amplification or intensity modulation of laser light. One method for amplifying a laser beam is to employ a laser medium whose optical gam may be controlled by optical pumping. Optical pumping of a solid state laser medium is a common and conventional method used to create a population inversion of energy states for laser applications requirmg high-gam. The laser medium providing high-gam, when optically pumped, may comprise a mateπal such as neodymium yttπum- aluminum garnet (Nd 3+ :YAG), neodymium glass (Nd 3+ -glass), Erbium doped optical fiber (Er 3+- sιlιca), or Ruby rods (Cr 3+ .A1 2 0 3 ) These materials are merely exemplary candidates for high-gain laser media, and those skilled in the art will appreciate that any suitable mateπal capable of maintammg an inverted population of energy states when optically-pumped may serve as an optical amplifier Those laser media utilizing Nd 3+ .YAG are common, given the substantial optical gam near desired wavelengths near the 1 064 μm range. Additionally, Nd 3+ YAG laser media provide linearity of pumping rate with respect to inverted population given its four-level transition system that is also well known to those skilled in the art To saturate an entire laser medium with an inverted population through optical pumpmg, a conventional method is to distπbute a large array of laser diodes across the surface of the laser medium to form a pumping array. The light emitted from the individual laser diodes of the pumpmg array excites the laser medium and provide a very high optical gam for the energy transition level of the optically-pumped, inverted population within the high-gam laser medium, e g., near the 1 064 μm range for Nd 3+ :YAG, near the 1.06 μm range for Nd 3+ :glass, near the 0 6943 μm range for Cr 3+ .A1 2 0 3 , near the 1 55 μm range for Er 3+ .sιhca, etc An integrated approach to performing laser light amplification and generating optical pulses utilizes gain switchmg of a laser medium. In this method of providmg a high energy pulsed laser beam, the optical pumping of a high-gam laser medium itself is pulsed to generate a time varying gam of the high-gam laser medium through which a laser beam is propagating This results in a pulsed output laser beam after an original laser light source has traveled through the high-gain laser medium that is being optically pumped m a time varying manner Each optical pumpmg cycle takes the high-gam laser medium through a transition which consists essentially of generating a sufficient energy state population through optical pumping to reach threshold for amplification Before the optical pumping begins, the population of energy states is initially below threshold and optical amplification does not occur After the high-gain laser medium has operated in an amplification mode for some time, then optical pumpmg is switched off, and the energy state population is subsequently depleted By turning off the optical pumpmg, the population falls below threshold and the optical amplification is interrupted until the optical pumping again resumes and the population of energy states again reaches threshold Such a method provides for a pulsing of the conditions m which laser light amplification may occur Such a method is preferable to a method which merely blocks a highly amplified laser beam in that design considerations need not include the potentially loss energy due to the dumping of electromagnetic energy into a shutter assembly Many other advantages are inherent to the fact that the solution is electronic, not incorporating any mechanical components for a mechanical shutter system Another method for providing an electronic solution is to generate a high energy pulsed laser beam to maintain continuous optical pumpmg of the high-gain laser medium and to modulate the high-gam laser medium's loss coefficient One method to perform such loss switchmg is to electronically modulate an optical absorber that is placed within the optical resonator cavity next to the high-gam laser medium Such a configuration will permit the user to control the loss of the laser light traveling through the high-gain laser medium as opposed to controlling the rate at which optical pumpmg occurs Those skilled in the art will recognize a variety of methods for performing loss switchmg of laser light contained within a the high-gain laser medium including electrical modulation of an electro-optic crystal to perform intensity modulation of the laser beam Such a method is an extension of the gain switching method as an optical resonator's threshold energy state population difference is proportional to the resonator's loss coefficient In this method, the loss coefficient is modulated to provide intermittent periods when the optical loss of the high-gain laser medium is prohibitively high to maintain oscillation This results is creating an increased energy state threshold population to sustain oscillation, given the increased loss of the high-gam laser medium Even though the energy state population would be sufficiently high for oscillation were the loss coefficient of the high-gam laser medium not increased, no optical amplification can occur during the period when the loss coefficient is elevated When the loss in suddenly decreased during the transition of a pulsing cycle the loss coefficient, the energy state population begins to deplete resulting from the decreased optical losses The high-gam laser medium will amplify the laser light during the period when the energy state population exceeds the threshold condition for oscillation during the period that the loss coefficient is minimum However, as the population continues to decrease, the population will eventually fall below the newly established energy state threshold for oscillation corresponding to the period of time when the loss coefficient is at its minimum during a modulation cycle These methods of performing electronic switchmg of either the gain or loss coefficients of the high-gam laser medium often employ flashlamp optical pumpmg The use of such a light source for performing the optical pumpmg presents some undesirable effects which significantly limit performance of the high-gam laser medium m providing pulses of laser light including the maximum pulse rate of the laser beam and the intensity with which the optical pumpmg must be performed Such inherent problems may present significant problems for applications which require high pulse rates and suffer from limited power budgets Another problem that is introduced by the utilization of flashlamps to provide optical pumping is the broad spectral width of flashlamp produced light may prove very inefficient m that a large proportion of the light produced by the flashlamp does not serve to generate the inverted population of energy states Flashlamp light outside of the spectral density range required for generating the inverted population is simply lost into the high- gain laser medium in the form of thermal heating This heating of the high-gam laser medium may itself produce undesirable effects including beam pointmg errors and self- focusing The heating of the high-gam laser medium may increase to such levels that fracture of the solid-state crystals will limit the maximum peak or average power The pulse rate at which the laser amplifier may be switched is also limited by the physical properties of the flashlamps which provide the optical pumpmg The electrical switchmg of the flashlamps is often associated with the thermal heating problems associated with the flashlamps themselves This upper limit of pulse rate may also be determined m part by the intensity level at which the flashlamps must operate to generate an inverted energy state population above threshold For example, if the energy transition of interest is near the periphery of the spectral density of the flashlamp, the flashlamp may necessitate operation at a very high power level to generate the inverted population Such a situation may at the very least limit the duty cycle of the pulse rate to avoid overheatmg of the flashlamps themselves Additionally, the flashlamps lntnnsically possess a start up time constant before they begin optical pumpmg They do not respond instantaneously with the vertical transition of the electric signal which drives them Consequently, the maximum pulse rate of the optical amplifier may be limited by the time constant correspondmg to the start up of the flashlamps Another consequence of the intrinsic response limitations of the flashlamps is a lower limit on the width of the pulse which may be generated using such a laser amplification system Such a problem stems from the similar characteπstic of the flashlamps m that they are limited in the speed with which they may switch on and off The minimum pulse width which may be generated is often dictated by the minimum time in which the flashlamps may mm on and then m off, including considering of the start up time constant of the flashlamps and evanescent decay of radiation from the flashlamps when turned off The present invention overcomes or eliminates the problems and limitations of known systems and methods for detecting high-speed laser- induced ultrasound to provide a system and method for laser beam amplification from a solid state laser that yields high pulse rates, improved pointing stability, and optionally variable pulse rates for non-destructive laser ultrasonic testing of materials, as well as a variety of other uses DISCLOSURE OF INVENTION Accordmg to one aspect of the present invention, there is provided a method for generating an amplified laser beam at a high pulse rate that mcludes generatmg a low-amplification seed laser light signal The method further includes transmitting the low-amplification seed laser light signal to an amplification component The low-amplification seed laser light signal is amplified in the amplification component by stimulating emissions of the population inversion that a pumping diode provides The result of this amplifying step is to yield an amplified laser light signal The amplified laser light signal is then directed to an output destination The present invention provides a system and method for providmg amplification of laser light from a solid state laser while maintaining the physical properties of the laser light by minimizing amplification induced distortion A seed laser possessing desired physical properties includmg a single longitudmal mode with a desired hnewidth is passed through a high-gam laser medium The high-gain laser medium is optically pumped using a pumpmg array of laser diodes distributed across the high-gam laser medium The electric current which drives the pumping array may be a time-varying signal which consequently provides time-varymg optical gain and lasmg conditions withm the laser medium The amplified laser beam may then be pulsed at a pulse rate corresponding to the frequency of the time- varying signal comprising the electric current which drives the pumping array of laser diodes The present invention may be employed in applications which require a particularly narrow or pure spectral density such as applications involving optical lnterferometry which often require a single longitudinal mode having a very stable center frequency and hnewidth For such applications in which the purity of the initial seed laser is amplified to generate a pulse stream of laser light having a desired intensity level, duty cycle and pulse rate, an optical isolation assembly may be used to minimize back reflection of the laser light into the seed laser light source Undesirable parasitic feedback may corrupt the seed laser source resulting in deleterious performance of the seed laser includmg amplitude noise and multimode operation Such effects may be disastrous for those applications requiring a spectrally pure laser source One conventional method, well known to those skilled in the art, for providing optical isolation of a laser beam which minimizes back reflections into the original light source compπses a Faraday rotator and two polarizers which provide for light propagation in only one direction through the optical isolation assembly by using the non-reciprocal rotation of a polarized lightwave provided by a Faraday rotator Additional optical isolation assemblies may be included m the invention for providing beam splitting of the original laser beam for directing the amplified, pulsed output laser beam or directing of the original laser beam through the high-gam laser medium multiple times for even greater amplification than a single pass through the high-gain laser medium The present invention generates an amplified and pulsed laser beam which possesses similar physical properties of the original seed laser including a desired center frequency and hnewidth for use within an optical interferometer to perform ultrasonic detection Additionally, the output intensity of the pulsed, amplified laser beam may be modulated to extend the dynamic range of detection within an optical mterferometπc system Electro-optic modulators compnsing Pockels or Kerr effect crystals are well known to those skilled in the art for providmg intensity modulation of a laser beam The present invention provides an important technical advantage by performing optical pumpmg of a high-gam laser medium usmg a pump g array of laser diodes This method permits optical pumping withm a very narrow wavelength regime selected by the use of appropriate laser diodes to minimize optically-induced thermal heating of the laser medium This advantage is provided primarily by the fact that the appropriate choice of laser diodes which comprise the pumping array may be chosen to optically pump withm a specific wavelength regime thereby not incurring significant thermal heating by radiation bombardment of the high-gain laser medium with optical pumpmg outside of the energy transition level of interest The present invention provides another technical advantage by providing for maximizing the pulse rate and controlling the pulse width of the resulting amplified pulsed laser beam by performing the optical pumpmg using a pumping array comprising a multiplicity of laser diodes The maximum pulse rate for the present invention will be determined largely by the speed at which the laser diodes may be turned on and turned off and the allowed duty cycle The present invention provides another technical advantage by minimizing amplification induced distortion of a seed laser beam Many applications, including optical lnterferometry, require a highly amplified beam with a uniform wavefront and good pointing stability The present invention provides another technical advantage by providing for intensity modulation of the output laser beam thereby expandmg the detection dynamic range within lnterferometπc systems BRIEF DESCRIPTION OF DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numerals indicate like features and wherem FIGURE 1 A shows a polarization selective assembly comprising an optical isolator, FIGURE IB shows a polaπzation selective assembly comprising a four port optical device providing polarization selective directing of a laser beam, FIGURE 2 shows a configuration of a laser light source capable of preventing optical feedback and varying the intensity variation of the laser light by rotating a half waveplate, FIGURE 3 shows another configuration of a laser light source capable of preventing optical feedback and varying the intensity variation of the laser light using an electro-optic modulator, FIGURE 4 shows one possible embodiment of the invention comprising a four pass, dual rod laser media pulsed laser light source, FIGURE 5 shows another possible embodiment of the invention compnsing a dual pass, dual rod laser media pulsed laser light source, FIGURE 6 shows another possible embodiment of the mvention compnsing a four pass, single slab laser medium pulsed laser light source, FIGURE 7 shows another possible embodiment of the invention compπsmg a four pass, dual slab laser media pulsed laser light source, FIGURE 8 shows another possible embodiment of the invention comprising an eight pass, single slab laser medium pulsed laser light source, FIGURE 9A shows one alternative embodiment of the present invention; and FIGURE 9B shows still a further alternative embodiment of the present invention MODES FOR CARRYING OUT THE INVENTION Embodiments of the present invention are illustrated m the FIGUREs, like numerals being used to refer to like and conespondmg parts of the various drawmgs The present invention provides a system and method for providing amplification of laser light from a solid state laser while maintaining the physical properties of the laser light while minimizing amplification induced distortion. A seed laser possessing desired physical properties including a single longitudinal mode with a desired hnewidth is passed through a high-gam laser medium. The center frequency of the seed laser source may be chosen appropriately as to perform withm specific applications such as optical lnterferometry which require very coherent light The high-gam laser medium is optically pumped usmg a pumpmg array of laser diodes distributed across the high-gam laser medium The electric cuπent which drives the pumping array may be a time-varymg signal which consequently provides time-varying optical gam and lasmg conditions within the laser medium. The amplified laser beam may then be pulsed at a pulse rate corresponding to the frequency of the time-varying signal compnsmg the electric cunent which drives the pumpmg anay of laser diodes. FIGURE 1 A shows a polarization selective assembly compnsmg an optical isolator 10. A typical aπangement of an optical isolator passes a laser beam 18 through a first polarizer 12. The polarized light then passes through a Faraday rotator 14. A Faraday rotator provides optical rotation of a polarized light way in a non- reciprocal fashion. That is to say, polarized light passes through the Faraday rotator will rotate in one and only one direction regardless of the direction of propagation of the laser beam through the material For example, a laser beam 18 traveling through Faraday rotator 14 rotates clockwise as it propagates in the direction of the arrows of laser beam 18, then a laser beam 18 traveling in the opposite direction of the arrows of laser beam 18 will also rotate clockwise. Faraday rotators 14 are well known to those skilled in the art, and may comprise a number of materials including yttrium-iron-garnet (YIG) or terbium-galhum-garnet (TGG). The first polarizer 12 transmits only P-state light from the input 18. After traveling through Faraday rotator 14, laser beam 18 is rotated 45°. Following the Faraday rotator is a half waveplate 16 which provides an additional 45° of polaπzation rotation m the opposite direction of Faraday rotator 14. Light polarized in a P-state traveling m the direction of the anows has a net rotation of zero. A half waveplate and its operation are well known to those skilled in the art. Light polarized in a p-state traveling in the direction of the anows of laser beam 18 through the optical isolator 10 travels unencumbered, yet light (possibly unpolaπzed, such as from random scattering) traveling in the opposite direction of the anows of laser beam 18 through the optical isolator 10 is blocked. This stems from the fact that the Faraday rotator exhibits non-reciprocal angular rotation of a polarized light beam. Light traveling m the opposite direction of the anows of laser beam 18 is polarized by the a polarizer 12, then pass through a half waveplate 16 and then through the Faraday rotator 14 for a net rotation of 90°. The Faraday rotator 14 will rotate the light one half of the angular difference between the two polarizers 12 so that when light traveling in the opposite direction of the anows of laser beam 18 meets the originally first polarizer 12, at an angle of incidence that is perpendicular to the polarizer's pass axis thereby completely blocking back reflections through the optical isolator 10 In effect, an optical isolator 10 serves as an optical diode or check valve FIGURE IB shows a polarization selective circulator assembly comprising a four port optical device 11 providing polarization selective directing of a laser beam A laser beam 18 enters a polarizing beam splitter 17 in which polarized light aligned to one axis of the polarizing beam splitter 17 passes through, and polarized light not aligned to that one axis of the polarizing beam splitter 17 is rejected For the portion of laser beam 18 which passes through polarizing beam splitter 17, it then passes through a Faraday rotator 14 followed by a half waveplate 16 for directmg the laser beam 18 Another polarizing beam splitter 17 may be used to direct the remaining portion of laser beam 18 again into two sub-components comprising gain path laser beam 13 and aperture path laser beam 15 Four port optical device 11 may be used to provide steering of components of laser beam 18 m various directions within an optical circuit while minimizing back reflections in the opposite direction of the anows of laser beam 18 FIGURE 2 shows a first configuration of a laser light source 20 capable of preventing optical feedback and varying the intensity variation of the laser light by rotatmg a half waveplate Laser light source 22 may comprise a smgle longitudinal light source operated at contmuous wave operation The emitted laser beam 18 from the laser light source 22 inherently possesses a significant polarization and a half waveplate 16 permits the aligning of that polarization along a predetermined axis of polaπzer 12 contained within optical isolator 10 Such a configuration is used to minimize any parasitic, undesirable back reflection of laser light into the laser light source 22 which may result in deleterious effects such as wavelength drift and hnewidth broadening of the laser light source 22 The half waveplate 16 may be angularly aligned to various angles of incidence of the polarizer 12 to vary the intensity of the laser beam 18 which exits laser light source 22 and travels through the optical system Those skilled m the art will recognize that a number of optical isolators 10 may be used to decrease even further the possibility of back reflected light into the laser light source 22 by cascading several optical isolators 10 FIGURE 3 shows another configuration of a laser light source 30 capable of preventing optical feedback and varying the intensity vanation of the laser light using an electro-optic modulator An optical isolator 10 may also be used to prevent reflected light from enteπng into the laser light source 22 and causing undesired effects as described above Additionally, an intensity modulator 32 may be placed either in front of the optical isolator 10 or after for modulating the intensity of the laser beam 18 as it then travels through the remainder of an optical circuit Those skilled m the art will recognize a wide variety of electro-optic modulators which will serve the function of intensity modulator 32 includmg Pockels effect elements utilizing the linear electro-optic effect and Ken effect elements utilizing the quadratic electro-optic effect A very common material candidate for a Pockels cells is Lithium mobate (LιNb0 3 ) FIGURE 4 shows one possible embodiment of the mvention compnsmg a four pass, dual [could be a smgle or "n" rods, two is just an example] rod laser media pulsed laser light source 40 Note, however, that source 40 may be formed of one or many rods, as other considerations may dictate This embodiment shows laser light source 20 which emits laser beam 18 which passes through an isolator 10 and is directed using a first minor 42 through a beam expander 46 for broadening the beam waist of laser beam 18 to minimize the divergence of laser beam 18 as it propagates through free space given its inherently Gaussian nature Laser beam 18 then passes through four port optical device 11 m which laser beam 18 is directed to the gain path laser beam 13 The gain path laser beam 13 passes through two high-gain laser media 48 and then through a phase conection plate and a Faraday rotator 49 where the linear polarization of laser beam 18 is rotated 45° Typically, high-gain laser media 48 comprising glass materials such as Nd 3+ glass are inherently amorphous and non-birefπngent whereas semiconductor materials such as Nd 3+ YAG might degrade the polarization state of laser beam 18 as it passes through them In the case of optically birefπngent high-gam laser media, the use of an adjustable waveplate for phase compensation can improve the performance of the system The phase conection plate compensates for any rod birefringence, which may be thermally induced The high- gain laser media 48 may comprise any material that will sustain an inverted population of energy states when optically pumped It then reflects off of an end minor 44 which then passes through the Faraday rotator 49 again where it is rotated an additional 45°, orthogonal to the polarization of the original laser beam 18 The reflected laser beam then passes a second time through high-gam laser media 48 and into four port optical device 11 where it is directed mto the direction of aperture path laser beam 15 and then reflects off a minor 42 where it passes through an aperture 43 which helps to minimize self-oscillations caused by amplified spontaneous emissions from the gam medium Beam 15 reflects at end mi or 44 retracing its path back into four port optical device 11 where it is again directed to pass through high-gain laser media 48 for a third pass It then travels through to Faraday rotator 49 and to end minor 44 where it is again reflected and retraces its path in passing through Faraday rotator 49, being converted into the same linear polarization as the original gam path laser beam 13 in its first pass through high-gam laser media 48 The gam path laser beam 13 then passes a fourth tune through high-gam laser media 48 and into the four port optical device 11 where it is directed to pass out in the direction of output laser beam 19 Each high-gam optical medium 48 is optically pumped to generate an inverted population of energy states using a pumpmg anay 51 of laser diodes driven by a diode driver 52 which delivers electric cunent to operate the laser diodes of the pumping anay 51 To generate pulses of amplified laser light, a trigger signal 54 is used to drive the diode driver 52 which operates the pumpmg anay 51 of laser diodes The pulse rate at which the output laser beam 19 may be pulsed is governed most directly by the frequency of the trigger signal 54 which is used to pulse the diode driver 52 The switchmg of the optical pumpmg results in gam switching of the high-gam laser medium 48 which serves to provide for a pulsing of the conditions in which laser light amplification may occur The result is an amplified, pulsed output laser beam 19 FIGURE 5 shows another possible embodiment of the mvention comprising, for example, a dual pass, dual rod laser media pulsed laser light source 50 This embodiment is strikingly similar to four pass, dual rod laser media pulsed laser light source 40 The mam difference is that there exists no four port optical device 11 is replaced by a smgle polarizing beam splitter 17 FIGURE 6 shows another possible embodiment of the invention compnsmg, for example, a four pass, smgle slab laser medium pulsed laser light source 60 This embodiment shows laser light source 20 which emits laser beam 18 which is isolated from feedback with isolator 10 and is directed through beam expander 46 for broadening the beam waist of laser beam 18 to minimize the divergence of laser beam 18 as it propagates through free space given its inherently Gaussian nature. Laser beam 18 then passes through polarizing beam splitter 17 m which polarized light aligned to one axis of the polarizing beam splitter 17 passes through, and polarized light not aligned to that one axis of the polarizing beam splitter 17 is redirected in the direction of an output laser beam 19 For the portion of laser beam 18 which passes through polarizing beam splitter 17, it then passes through a Faraday rotator 14 followed by a half waveplate 16 and a second polarizer 17 for directing the laser beam 18 through high-gam laser media 48 Slab designs pass P-state with high efficiency because the face is almost at Brewsters Angle. The high-gam laser media 48 may comprise any material that will sustain an inverted population of energy states when optically pumped It then reflects off of turning minor 42. The reflected laser beam then reflects off of a second mmmg minor 42, passes a second time through high-gam laser media 48 and is directed into the direction of laser beam 15 and then reflects off end minor 44 before retracing its path back into the high-gain laser media 48 for a third pass It then reflects off minor 44 to minor 44 where it is again reflected and retraces its path in passing through high-gain laser media 48 for a fourth time and into the polarizer 17 and half waveplate for aligning the laser beam along a predetermined incidence angle. The laser beam 18 then passes a second time through Farady rotator 14. After traveling through Faraday rotator 14, the laser beam 18 has been rotated to be orthogonal to the original laser beam. Laser beam 18 enters polaπzmg beam splitter 17 where it is directed to pass out in the direction of output laser beam 19. In the present example, each high-gam optical medium 48 is optically pumped to generate an inverted population of energy states using pumping anays 51 of laser diodes driven by a diode driver 52 which delivers electric cunent to operate the laser diodes of the two pumping anays 51 To generate pulses of amplified laser light, a trigger signal 54 is used to drive the diode driver 52 which operates the pumpmg anays 51 of laser diodes The pulse rate at which the output laser beam 19 may be pulsed is governed most directly by the frequency of the trigger signal 54 which is used to pulse the diode driver 52. The switching of the optical pumping results in gain switching of the high-gam laser medium 48 which serves to provide for a pulsing of the conditions in which laser light amplification may occur The result is an amplified, pulsed output laser beam 19. FIGURE 7 shows another possible embodiment of the invention compnsmg a four pass, dual slab laser medium pulsed laser light source 70. This embodiment is stπkmgly similar to four pass, smgle slab laser medium pulsed laser light source 60, m that it uses the same number of diodes as in the FIGURE 6 example. The main difference is that there exists a second slab laser medium 48 and minor assemblies 72 (either minors or coatings) are placed on both of the high-gain laser media 48 This may result in a system that is more efficient (per diode) than the system of FIGURE 6, but with only a small added cost of the second slab. FIGURE 8 shows another possible embodiment of the mvention compnsmg an eight pass, single slab laser medium pulsed laser light source 80. This embodiment is strikingly similar to four pass, single slab laser medium pulsed laser light source 60. The mam difference is that there exists a plurality of turning minors 42 to direct the laser beam through the high-gam laser media 48 eight times and minor assembly 72 (either minors or coatings) is placed on the high-gam laser media 48 FIGURE 9A shows another possible embodiment of the present invention FIGURE 9A includes remote seeding of amplifier 54 with a fiber optic link Laser light source 20 emits laser beam 18 which is isolated from feedback with optical isolator 10 and directed into input coupling optics and polarization preserving single-mode fiber optics Polarization preserving single-mode fiber optic is coupled to amplifier 54, which includes output coupling optics Amplifier 54 (not including output coupling optics) is equivalent to the amplifier section shown in FIGURE 6 Amplifier methods illustrated in FIGUREs 4, 5, 7 or 8 can be used, as well Long-term stability of amplifier 54 is improved by de-coupling of laser beam 18 If laser beam 18 laser "walks" but part of the light still couples into the polarization preserving single-mode fiber optic, then output of amplifier 54 may only drop a small amount FIGURE 9B shows another possible embodiment of the present invention FIGURE 9B represents remote seeding of amplifier with mtemal modulators Laser light source 20 emits laser beam 18 which is isolated from feedback with optical isolator 10 Laser beam 18 is input in amplitude modulator, phase modulator and other beam/laser conditioning components Prior to being input to polarization preserving single-mode fiber optics, laser beam 18 is input to input coupling optics Laser beam 18 is output from polarization preserving single-mode fiber optics to output coupling optics Laser beam 18 is amplified in a manner similar to that shown m FIGURE 6 Amplifying methods illustrated in FIGUREs 4, 5, 7 or 8 can be used, as well The present invention provides several benefits including minimizing thermal heating of the high-gam laser medium by using laser diodes to perform the optical pumping Using laser diodes which operate within a very nanow wavelength regime minimize optically-induced thermal heating of the laser medium in that little electromagnetic radiation outside of the desired spectrum bombards the high-gam laser medium as with conventional methods Using laser diodes for optically pumping the high-gam laser medium provides additional benefits including permitting a faster pulse rate, variable pulse rate By performing optical amplification and pulsing in the manner described above, the present invention also minimizes amplification induced distortion of a seed laser beam Consequently, the physical properties of the original seed laser beam are maintained in the resultant amplified, output beam Many applications includmg optical lnterferometry require a highly amplified, spectrally pure output laser beam which the present invention will provide To broaden the dynamic detection range of an optical interferometer employmg the present invention, the intensity of the output laser beam may also be modulated Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as described by the appended claims

Claims: WE CLAIM 1 A method for generating an amplified laser beam at a high pulse rate, comprising the steps of generatmg a low amplification seed laser light signal from a low-amplification seed laser light source, transmitting said low-amplification seed laser light signal to an amplification component, amplifying said low-amplification seed laser light signal in said amplification component by stimulating emissions of the population mversion provided by a pumping diode to generate an amplified laser light signal, and directing said amplified laser light signal to an output destination 2 The method of Claim 1, wherein said amplifying step further comprises the steps of amplifying said low-amplification seed laser light signal in the amplifying component comprising the pumpmg diode and amplifying said low-amplification seed laser light signal by repeatedly transmitting said low-amplification see laser light signal through said amplification medium 3 The method of Claim 1, wherem said directmg step further comprises the step of extracting said low-amplification seed laser light signal from an isolator associated with said amplifier 4 The method of Claim 1, further compnsmg the step of isolating said amplification component from the source of said low-amplification seed laser light signal 5 The method of Claim 1, wherein said amplifying step further comprises the step of amplifying said low-amplification seed laser light signal using a diode pump rod as said amplification medium 6 The method of Claim 1 , wherein said amplifying step further compπses the step of amplifying said low-amplification seed laser light signal usmg a diode pump slab as said amplification medium 7 The method of Claim 1, wherem said generatmg step further compπses the step of generating a continuous wave low-amplification seed laser light signal, and further wherein said amplifying step further comprises the step of amplifying said contmuous wave low-amplification seed laser light signal to generate said amplified laser light signal havmg a maximum gain of a predetermined gain coefficient times the amplification of said continuous wave low-amplification seed laser light signal 8 The method of Claim 1, further compnsmg the step of isolating said low-amplification seed laser light source from said amplification component for preventmg feedback into said low-amplification seed laser light source 9 The method of Claim 1 , further compnsmg the step of isolating said amplification component from said output destination for extracting said amplified laser light signal from said amplification source 10 The method of Claim 1 , further compnsmg the steps of transmitting said low-amplification seed laser light via an optical fiber to an isolator that interfaces with said amplification component, and transmitting said amplified laser light signal through an optical fiber from said amplification component to said output destmation 11 The method of Claim 1, wherein said generating step further comprises the step of generating microsecond pulses of said low-amplification seed laser light signal 12 The method of Claim 1, wherem said generatmg step further compπses the step of generating variable pulse rate pulses of said low-amplification seed laser light signal 13 A system for amplifying a laser beam and generatmg an output pulse of laser light, compnsmg a seed laser light source for providing a laser beam with a desired hnewidth, at least one optical isolation assembly placed in the path of propagation of the laser beam for preventing reflected laser light feedback mto the seed laser light source, a polarization selective assembly aligned in the path of propagation of the laser beam for directing a first polarization state of the laser beam in a first propagation direction and at least one additional polarization state of the laser beam in at least one additional propagation direction and for minimizing back reflection of one of either the first polarization state or the at least one additional polarization state of the laser beam, and at least one pumpmg anay comprising a multiplicity of laser diodes distributed across at least one high- gam laser medium aligned m the path of propagation of either the first polaπzation state or the at least one additional polarization state of the laser beam for optically pumpmg the at least one high-gain laser medium for generating a population mversion of energy states withm the at least one high-gain laser medium for amplifying the laser beam and generatmg an output pulse of laser light 14 The system of Claim 13, further compnsmg a diode driver for delivering an electric cunent to the multiplicity of laser diodes for optically pumping the at least one high-gam laser medium, and a trigger signal for pulsmg the optical pumpmg of the high-gam laser medium by delivering the trigger signal to the diode dπver for switching the electric cunent delivered to the at least one pumping anay 15 The system of Claim 13, wherein the at least one optical isolation assembly compπses a first polaπzer for polarizing the laser beam, a Faraday rotator for providmg non-reciprocal angular rotation of the laser beam, and a half waveplate for aligning the laser beam along a predetermined incidence angle of a second polarizer 16 The system of Claim 13, further compnsmg an end minor for reflecting the laser beam back through the at least one high-gain laser medium and mto the polarization selective assembly wherein the polarization selective assembly comprises. a polarizing beam splitter in the propagation path of the laser beam; and a Faraday rotator located between the polarizing beam splitter and the end reflecting minor along the propagation path of the laser beam. 17 The system of Claim 13, further compnsmg an end minor for reflecting the laser beam back through the at least one high-gain laser medium and mto the polarization selective assembly wherein the polarization selective assembly comprises a polarizing beam splitter m the propagation path of the laser beam, and a quarter waveplate located between the polarizing beam splitter and the end reflecting minor along the propagation path of the laser beam. 18. The system of Claim 13, further comprising at least one end minor for reflecting the laser beam back through the at least one high-gain laser medium and mto the polarization selective assembly wherem the polarization selective assembly compnses- a first polarizing beam splitter m the propagation path of the laser beam, a Faraday rotator for providing non-reciprocal angular rotation of the laser beam in the propagation path of the laser following the first polarizing beam splitter; a half waveplate in the propagation path of the laser beam for aligning the laser beam along a predetermined incidence angle of a second polarizing beam splitter; and a quarter waveplate located between the second polarizing beam splitter and the at least one end reflecting minor along the propagation path of the laser beam 19 The system of Claim 13, further compnsmg at least one end minor for reflecting the laser beam back through the at least one high-gam laser medium and into the polarization selective assembly wherem the polarization selective assembly compπses- a first polarizing beam splitter m the propagation path of the laser beam; a Faraday rotator for providing non-reciprocal angular rotation of the laser beam in the propagation path of the laser following the first polaπzing beam splitter, a half waveplate in the propagation path of the laser beam for aligning the laser beam along a predetermined incidence angle of a second polarizing beam splitter; and a least one additional Faraday rotator located between the second polaπzmg beam splitter and the at least one end reflecting minor along the propagation path of the laser beam 20. The system of Claim 13, further compnsmg a beam expander for minimizing the divergence of the laser beam 21 The system of Claim 13, further comprising a plurality of minors for directing and reflecting the laser beam throughout the laser light amplification system 22 The system of Claim 13, wherem the at least one high-gam laser medium compπses a solid state laser medium having a reflecting assembly for maximizing the optical pumping 23 The system of Claim 13, wherem the at least one high-gam laser medium comprises a solid state laser medium havmg an entrance surface and an exit surface for providing at least one predetermmed incidence angle and at least one predetermined exit angle of the laser beam for providing multiple passes of the laser beam through the high-gam laser medium while maintaining a smgle longitudinal mode of the laser beam 24 The system of Claim 13, wherem the at least one high-gain laser medium compπses a glass rod laser medium capable of generatmg an inverted population of energy states through optical pumpmg 25. The system of Claim 13, further comprising an electro-optic crystal driven by a signal processor for modulating the intensity of the laser beam 26 The system of Claim 13, further compnsmg a half waveplate for aligning the laser beam along at least one predetermmed incidence angle of a polarizer for varying the intensity of the laser beam 27 The system of Claim 13, further compnsmg a temperature sensor for monitoring the temperature of the at least one high-gain laser medium and for providmg a temperamre signal for maximizing the efficiency of the optical pumpmg and for maintammg at least one desired optical property of the laser beam generated by the seed laser light source 28 The system of Claim 13, wherem the at least one high-gain laser medium compπses an optically lsotropic laser medium. 29 A system for amplifying a laser beam and generating an output pulse of laser light, comprising a seed laser light source for providing a laser beam with a desired hnewidth; at least one optical isolation assembly aligned m the path of propagation of the laser beam for preventing reflected laser light feedback into the seed laser light source, compnsmg: a first polaπzer for polarizing the laser beam; a Faraday rotator for providmg non-reciprocal angular rotation of the laser beam, a second half waveplate for aligning the laser beam along a predetermmed incidence angle of a second polarizer, and the second polarizer for maintaining the polarization of the laser beam, a first half waveplate aligned m the path of propagation of the laser beam for aligning the laser beam along at least one predetermmed angle of incidence of the first polaπzer, a beam expander aligned in the path of propagation of the laser beam for minimizing the divergence of the laser beam, a plurality of minors for directing and reflecting the laser beam throughout the laser light amplification system, a polarizing beam splitter aligned in the path of propagation of the laser beam for directing a first polarization state of the laser beam in a first direction and at least one additional polarization state of the laser beam in at least one additional direction, at least one of either a quarter waveplate or a second Faraday rotator aligned in the path of propagation of the laser beam for minimizing back reflection of at least one polarization state of the laser beam into the polarizing beam splitter, at least one high-gain diode-pumped laser amplifier aligned in the path of propagation of the laser beam for generating a population inversion of energy states thereby amplifying the laser beam, comprising a high-gam laser medium, at least one pumpmg anay comprising a multiplicity of laser diodes distributed across the high- gam laser medium, a diode dπver for providing electric cunent to the multiplicity of laser diodes for optically pumping the high-gain laser medium, and a trigger signal for pulsing the optical pumpmg of the high-gam laser medium by delivering the trigger signal to the diode driver for switching the electric cunent delivered to the at least one pumpmg anay thereby switchmg the optical gam of the high-gam laser medium, and an end minor aligned in the path of propagation of the laser beam for reflecting the laser beam back through the at least one high-gain diode-pumped laser amplifier 30 A method for amplifying a laser beam and generating an output pulse of laser light, comprising preventing reflected laser light feedback mto a seed laser light source, wherem the seed laser light source provides a laser beam with a desired hnewidth, by using at least one optical isolation assembly placed in the path of propagation of the laser beam, directmg a first polarization state of the laser beam m a first propagation direction and at least one additional polaπzation state of the laser beam in at least one additional propagation direction usmg a polarization selective assembly aligned in the path of propagation of the laser beam, wherem the polaπzation selective assembly minimizes back reflection of one of either the first polarization state or the at least one additional polarization state of the laser beam, and optically pumpmg at least one high-gam laser medium aligned m the path of propagation of either the first polarization state or the at least one additional polarization state of the laser beam usmg at least one pumpmg anay comprising a multiplicity of laser diodes distributed across the at least one high-gain laser medium for generatmg a population inversion of energy states withm the at least one high-gain laser medium for amplifying the laser beam and generating an output pulse of laser light 31 The method of Claim 30, further compnsing delivering an electric cunent to the multiplicity of laser diodes using a diode driver for optically pumping the at least one high-gam laser medium, and pulsing the optical pumpmg of the high-gain laser medium by delivering a trigger signal to the diode driver for switching the electric cunent delivered to the at least one pumpmg anay 32 The method of Claim 30, further comprising minimizing the divergence of the laser beam using a beam expander 33 The method of Claim 30, further comprising directing and reflecting the laser beam throughout the laser light amplification system using a plurality of minors 34 The method of Claim 30, wherem the at least one high-gam laser medium comprises a solid state laser medium having a reflecting assembly for maximizing the optical pumpmg 35 The method of Claim 30, wherein the at least one high-gain laser medium compπses a solid state laser medium having an entrance surface and an exit surface for providing at least one predetermined mcidence angle and at least one predetermined exit angle of the laser beam for providing multiple passes of the laser beam through the high-gam laser medium while maintaining a single longitudinal mode of the laser beam 36 The method of Claim 30, wherem the at least one high-gam laser medium compπses a glass rod laser medium capable of generating an inverted population of energy states through optical pumpmg

Inventors: DRAKE THOMAS E JR, DRAKE, THOMAS, E., JR.
Applicants: LOCKHEED CORP, LOCKHEED MARTIN CORPORATION
Application year: 2000
Main classification: H01S3
IPC codes: G02B27, H01S3