Patent Publication Number: US-2006007968-A1

Title: Long pulse laser

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. Ser. No. 08/630,829 filed Apr. 10, 1996, which application is fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to diode-pumped solid-state lasers, and in particular to diode-pumped solid-state lasers that provide long pulses at high repetition rate with high stability.  
     BACKGROUND OF THE INVENTION  
      Diode-pumped Nd:YVO 4  lasers have been used in applications that require short pulses (&lt;20 nsec) at high repetition rates (&gt;10 kHz). See for example M. S. Keirstead, T. M. Baer, S. B. Hutchison, J. Hobbs, “High repetition rate, diode-bar-pumped, Q-switched Nd:YVO 4  laser”, in Conference on Lasers and Electro-Optics, 1993, Vol. 11, OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1993), p. 642, and S. B. Hutchison, T. M. Baer, K. Cox, P. Gooding, D. Head, J. Hobbs, M. Keirstead, and G. Kintz,  Diode Pumping of Average - Power Solid State Lasers,  Proc. SPIE 1865, 61 (1993). These reports describe operation of Nd:YVO 4  lasers in a manner that provides short pulses at high repetition rate, as does W. L. Nighan, Jr., Mark S. Keirstead, Alan B. Petersen, and Jan-Willem Pieterse, “Harmonic generation at high repetition rate with Q-switched Nd:YVO 4  lasers”, in SPIE 2380-24, 1995, which discloses generation of Q-switched pulses with an end-pumped, acousto-optically Q-switched laser.  
      In Nighan et al, pulse durations of 7-20 nsec were generated for repetition rates of 10-80 kHz, at an average output power of ˜4 W in a TEM 00  mode. The pump source was a fiber-coupled diode bar, as disclosed in U.S. Pat. Nos. 5,127,068 and 5,436,990. End-pumping of Nd:YVO 4  with a pump source like this fiber-coupled bar allows generation of very high small signal gain, since this material has a stimulated emission cross-section that is much higher than that of Nd:YLF or Nd:YAG. This is useful for building a diode-pumped laser with a low laser oscillation threshold, and is also useful for building a laser that provides short pulses at high repetition rates. However, the short upper state lifetime of this material (˜100 μsec) does not allow as much energy storage as is possible with Nd:YLF (500 μsec) or Nd:YAG (200 μsec), which limits the amount of pulse energy that can be generated at repetition rates below 10 kHz. For example, an Nd:YVO 4  laser pumped at 10 W can provide 200 μJ at low repetition rates, while the YLF laser (designated “TFR” by Spectra-Physics Lasers, described by T. M. Baer, D. F. Head, P. Gooding, G. J. Kintz, S. B. Hutchison, in “Performance of Diode-Pumped Nd:YAG and Nd:YLF in a Tightly Folded Resonator Configuration”, IEEE J. Quantum Electron., vol. QE-28, pp. 1131-1138, 1992) provide ˜800 μJ.  
      While short (&lt;20 nsec, energetic pulses are typically desired for many applications, especially at high repetition rate (&gt;10 kHz), there are some applications that require long Q-switched pulses, such as pulses on the order of 50 nsec. In the prior art, the material Nd:YVO 4  has not been applied to long pulse operation at high repetition rate, since it is typically well-suited for short-pulse generation. It is well-known that a CW-pumped, repetitively Q-switched laser will provide progressively longer pulses if the repetition-rate of the laser is progressively increased. This is described in “Lasers”, by Siegman, in Chapter 26. The reason for this effect is simple. As repetition rate is increased (at rates higher than the reciprocal of the upper state lifetime), the maximum amount of energy stored in the gain medium between Q-switched pulses decreases; this stored energy is proportional to the density of ions in the upper state just before Q-switching. This means that the small-signal gain is decreased, since the small-signal gain depends upon the density of ions still in the upper state. If the small-signal gain is reduced, as it is by increasing the repetition rate, the Q-switched laser pulse will not build up as rapidly in the laser cavity as it would at lower repetition rate. Therefore, the pulse will be longer.  
      A number of diode-pumped Nd:YLF lasers, available from Spectra-Physics as the R-series, provides pulses of &lt;10 nsec duration (short) at 1 kHz (low repetition rate). If the repetition rate is increased to over 10 kHz (high repetition rate), the pulse durations on the order of 50 nsec (long) can be achieved. Although short pulses are typically desirable, long pulses (&gt;20 nsec, for example) can be useful for certain applications, especially at high repetition rate. However, the pulse-to-pulse stability of an Nd:YLF laser at high repetition rate can be poor; for example, the peak-to-peak fluctuations of an Nd:YLF laser at repetition rates over  10  kHz can easily be 50%, which can correspond to an RMS noise of ˜8%, which is too noisy for some applications. This increase in instability is common for a laser for which repetition rate has been increased; since less energy is stored, the laser oscillation is closer to threshold with each increase in repetition rate, and is therefore noisier. For applications that require greater stability at high repetition rate but still need longer pulses, there is a problem in straightforward application of a low repetition rate laser operating at higher repetition rates; stability is decreased. Some applications require high stability, long pulses, and high repetition rate. An important range that has not been provided by the prior art is repetition rate greater than 25 kHz, pulse duration greater than 35 nsec, and RMS stability less than 5%.  
      In “A new laser texturing technique for high performance magnetic disk drives”, by Baumgart et al (IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995), it is disclosed that an Nd:YLF laser with 50 nsec pulses is used to provide a highly desirable texture to a magnetic disk, such as a disk used in a computer hard drive. The references and patents that were listed in the Baumgart paper are hereby incorporated by reference; they list a variety of laser-texturing prior art. The Baumgart paper shows that a slight change in pulse energy can change the shape of the “bump” that the single laser pulse leaves on the disk. Multiple bumps are typically left on the disk, as Baumgart describes. In some cases, there is a range of variation that is acceptable, as was disclosed by Baumgart. For this reason, there is a limit on the laser pulse-to-pulse variations that are acceptable. Also, as is obvious to one skilled in the art, a high repetition rate will allow a shorter time requirement for a laser texturing job to be completed.  
      There is a need for a long pulse, Q-switched laser that provides pulses at high repetition rate with high stability. There is also a need for a laser with harmonically converted output with high stability.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide a diode-pumped solid-state laser that provides long Q-switched pulses at high repetition rate with high stability.  
      It is an object of the invention to provide a diode-pumped solid-state laser that provides long Q-switched pulses at high repetition rate with high stability, with the solid-state laser incorporating Nd:YVO 4  as the gain medium.  
      It is an object of the invention to provide a diode-pumped solid-state laser that provides Q-switched pulses longer than 35 nsec, at repetition rates higher than 25 kHz, and with RMS noise of the pulsed output at less than 5%.  
      It is an object of the invention to provide a diode-pumped solid-state laser that provides Q-switched pulses longer than 35 nsec, at repetition rates higher than 25 kHz, and with RMS noise of the pulsed output at less than 5%, with the solid-state laser incorporating Nd:YVO 4  as the gain medium.  
      It is an object of the invention to provide a diode-pumped solid-state laser that provides long Q-switched pulses at high repetition rate with high stability, with the solid-state laser incorporating Nd:YVO 4  as the gain medium, with a harmonic generator included with the laser in order to harmonically convert the output of the laser.  
      It is an object of the invention to provide a diode-pumped solid-state laser that provides long Q-switched pulses at high repetition rate with high stability, with the solid-state laser incorporating Nd:YVO 4 , as the gain medium, with this solid-state laser applied to a laser texturing application.  
      These and other objects of the invention are achieved in a diode-pumped solid-state laser, with an Nd:YVO 4  laser crystal placed in the resonator of the laser, said resonator incorporating at least two mirrors, with a Q-switch device placed in the laser resonator, with the pump power density and cavity lifetime balanced to provide long Q-switched pulses at high repetition rate with high stability.  
      In one embodiment, the laser resonator configuration is relatively symmetric, with the laser crystal placed nearly at the center of the laser resonator.  
      With this invention, Nd:YVO 4  has been incorporated for the first time in a long pulse (&gt;35 nsec), highly stable (&lt;5% RMS), high repetition rate (&gt;25 kHz) diode-pumped solid-state laser. In a preferred embodiment, it provides over 1 W in average output power. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram of a Q-switched, diode-pumped, Nd:YVO 4  solidstate laser that provides long pulses (&gt;35 nsec), while highly stable (&lt;5% RMS), at high repetition rate (&gt;25 kHz). In some embodiments it provides over 1 W of average power.  
       FIG. 2  is a plot of the output pulse duration as a function of repetition rate, and the average output power as a function of repetition rate. The pump power was 5 W. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  depicts a long pulse laser configured to provide a long pulse (&gt;15 nsec), that is highly stable (&lt;5% RMS) from pulse-to-pulse, even at high repetition rate (&gt;25 kHz). In one embodiment, the laser device of  FIG. 1  provides average output powers from about 0.25 W to about 50 W. Optionally, the system may be configured to provide output pulses of a duration of about 20 nsec to about 70 nsec at repetition rates of about 30 kHz to about 110 kHz.  
      As illustrated in  FIG. 1  the laser includes an output coupler  1  (typical reflectance is 95% at the 1.064 μm fundamental wavelength). In the illustrated embodiment, the output coupler  1  comprises a curved output coupler having a radius of curvature of about 2 m to infinity. Optionally, the output coupler  1  may comprise a flat optical device. All optics are available from Spectra-Physics Laser Components and Accessories Group in Oroville, Calif.  
      Referring again to  FIG. 1 , the laser also includes a beam path  3 , optimized in length with the output coupler  1  to provide adequate cavity lifetime and configured to provide a long pulse. For example, in one embodiment the length of the beam cavity  3  ranges from about 4 cm to about 60 c. In another embodiment the beam path is about 18 cm in length. Optionally, the beam path  3  may be any length. Examples of other embodiments of the beam path  3  which may be used in the present invention are disclosed in U.S. Pat. No. 5,412,683 and application Ser. No. 08/432,301, each of which are incorporated herein by reference.  
      As shown in  FIG. 1 , the laser may include a fold mirror  5  which is highly reflective at the 1.064, μm wavelength (R&gt;99.5%) and highly transmissive at the diode pump wavelength (T&gt;90%). The fold mirror may comprise a curved or flat reflective device. Optionally, the laser need not include the fold mirror  5 .  
      The laser also includes at least one vanadate-based laser crystal  7 . In one embodiment, the vanadate-based laser crystal  7  comprises Nd:YVO 4 , available from Litton Airtron in Charlotte, N.C., in dimension approximately 4×4×4 mm 3 , and dopant about 0.7%. The vanadate-based laser crystal may be fixtured as described in U.S. Pat. No. 5,412,683, and application Ser. Nos. 08/191,654 and 08/427,055, each of which are incorporated herein by reference. In an alternate embodiment, the vanadate-based laser crystal  7  comprise Nd:GdVO 4 , GdYVO 4 , Nd:LuVO4, Ytterbium doped Yttrium Ortho-Vanadate (Yb3+:YVO4) or a composite material comprising any combination thereof.  
      As shown in  FIG. 1 , the laser may also include at least one acousto-optic Q-switch  9 . For example in one embodiment, the Q-switch comprises SF10 glass or any other glass, like fused silica, to provide adequate loss for Q-switching. A vendor of these devices is NEOS, in Melbourne, Fla. Optionally, any variety of Q-switch devices  9  may be used within the laser system. The Q-switch device  9  may be in communication with one or more Q-switch drivers  13  configured to provide one or more control signals thereto. For example, the Q-switch driver  13  may be configured for providing a RF signal of the appropriate frequency to the Q-switch  9 , such as 80 MHz, at the appropriate power, such as 2-4 W, to provide controllable loss for Q-switching the cavity.  
      The laser also includes an end-mirror  11 , highly reflective at 1.064 nm. Like the output coupler  1 , the end mirror may comprise a curved or flat optical device. In one embodiment, the radius of curvature of the end mirror may range from about 2 m to infinity, if curved.  
      The laser may also include one or more imaging optics  21 . In one embodiment, the imaging optics  21  may be used for relaying the light from a diode pump source into the laser crystal  7 . These simple lenses are available from Melles Griot, Irvine, Calif., and many other sources. A typical pump spot size is 0.5 to 0.6 mm, in the laser crystal. In the illustrated embodiment, a fiber bundle  23  may be used for relaying diode light to the imaging optics  21 . One vendor for these bundles is Spectra-Physics Laser Components and Accessories Group in Oroville, Calif.  
      Various other optical components may be positioned within the laser resonator. For example, an aperture stop  25  with appropriate size to insure TEM 00  operation may be positioned within the beam path  3  of the laser. In another embodiment, a shutter in communication with a shutter controller may be positioned in the beam path  3  within the laser. For example, the shutter may be configured to remain closed while a lensing effect within the laser stabilizes and opened once the lensing effect has stabilized. In another embodiment, the shutter may be used to provide a pulsed laser output.  
      Referring again to  FIG. 1 , the laser may include one or more pump sources  15  configured for providing pump energy to the solid-state laser. In one embodiment, the pump source  15  comprise one or more laser diodes. For example, a common device is an OPC-13020-808-CS, available from OptoPower Corporation, Tucson, Ariz. Six to eight watts from the diode is typical, with 5 to 6 exiting the bundle  23 . In the alternate, any number and variety of pump sources  15  may be used with the laser.  
      The laser also includes power supply  17 , providing electrical power to the pump source  15  and maintaining the temperature thereof. In one embodiment, the Q-switch driver  13  may also be housed in the power supply  17 .  
      As shown in  FIG. 1 , during use the laser may be configured to emit an output beam  19 . As stated above, the output beam  19  may have a output power from about 0.25 W to about 50 W. For example, in one embodiment the output power may be greater than 1 W in average power, with highly stable, long, Q-switched pulses.  
      The combination of the laser crystal  7  in a cavity of appropriate length and cavity lifetime results in long pulses (e.g. about &gt;35 nsec) at high repetition rate (e.g. about &gt;25 kHz) at high stability (&lt;5% RMS). The high gain and short lifetime of laser crystal  7  combine with the cavity lifetime to provide this unique performance. This gain material has never been used in prior art to provide such long pulses at such high stability; this performance is required in some applications.  
       FIG. 2 . depicts the performance of the laser of  FIG. 1 . Pulses of duration approximately 70 nsec may be obtained at approximately 70 kHz, in a highly stable beam. Optionally, the laser output is TEM 00 , which enhances focusability.  
      Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.