Abstract:
Systems, configurations and methods of using an ultrafast, self-starting, mode-locked laser are provided. The systems, devices and methods of using stable, self-starting mode-locked lasers, can be compact, use fewer optical elements and have energies sufficient for most micro-processing and micro-structuring applications. The large spectral bandwidth of ultra-short (femtosecond) laser pulses can be used in laser sensing applications, micro-machining, time-resolved experiments, where short-lived transient species can be observed in biological or chemical reactions. Terahertz radiation can be generated using ultrashort pulses and used for imaging applications.

Description:
This invention claims the benefit of priority based on the U.S. Provisional Application Ser. No. 60/571,907 filed May 17, 2004, the contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to ultrafast mode-locked lasers generating pulses with picosecond or less in duration, and, more particularly to systems, devices and methods of using stable, self-starting mode-locked lasers, which are compact, use fewer optical elements and have energies sufficient for most micro-processing and micro-structuring applications. 
   BACKGROUND AND PRIOR ART 
   Extremely short duration optical pulses, which are also known as femtosecond pulses, are important for high speed signal processing, communications, micro-machining, imaging, sensing applications, time resolved experiments, where short-lived transient species can be observed in biological and chemical reactions. 
   The early development of laser technology involving the production of extremely short pulses is disclosed in U.S. Pat. No. 4,727,553 to Fork et al. The early version of a passively mode-locked laser is described as containing a saturable absorbing element optically coupled to a gain medium in an optical resonator. Additional variations of the saturable absorbing element positioned between reflective (mirror-like) elements are disclosed in U.S. Pat. No. 5,237,577 to Keller et al. and U.S. Pat. No. 5,278,855 to Jacobovitz-Veselka et al. The so-called Semiconductor Saturable Absorber Mirrors (SESAMs) are described in U.S. Pat. Nos. 6,538,298 B1 and 6,393,035 B1 to Weingarten et al. The saturable absorber element functions as a shutter. 
   Further development of passively mode-locked lasers includes use of astigmatic mirrors with spacing and a unique twist angle to correct the optical path in an absorption cell (U.S. Pat. No. 5,291,265 to Kebabian) and subsequently, the integration of the saturable absorber with the optical element as discussed below. 
   Current conventional, commercial, ultrafast mode-locked lasers have basic constituents, which include an active laser medium, resonator mirrors, and optical components, usually prisms that compensate for dispersion in the resonator. The mode-locking element in simpler devices is a nonlinear optical effect occurring in the laser medium itself. A typical mode-locked laser design is shown in  FIG. 1 ; the laser of this design has drawbacks in that it is not “self-starting” and is sensitive to effects of alignment, optical pumping, and the like. 
   More recently, other components have been added to the general structure shown in  FIG. 1 . Instead of using prisms as the dispersive elements, special mirrors, known as, Chirped Mirrors have been developed. Chirped Mirrors or Negative Group Velocity Dispersion (NGVD) as discussed in U.S. Pat. No. 6,055,261 to Reed et al. have been used to provide an ultrafast laser device with a significantly shortened resonant cavity. Additional prism replacements include a fold mirror (U.S. Pat. No. 5,812,308 to Kafka et al.); a dispersive dielectric mirror (U.S. Pat. No. 5,734,503 to Szipocs et al.); a self-tuning saturable reflector comprising two Bragg reflectors (U.S. Pat. No. 6,141,359 to Cunningham et al.); and heated mirrors (U.S. Pat. No. 6,188,475 to Inman et al.) in semiconductor processing. 
   The prior art includes several arrangements of gain elements, optical components, resonator mirrors, and mode-locking elements for short pulse lasers; however, all arrangements are unlike the arrangements of elements in the present invention.  FIG. 1  shows the basic architecture for a short pulse laser, a high reflector (HR) mirror  10 , Kerr-lens lasing element  20 , dispersion compensating elements, such as, a prism pair  30  and an output coupler (OC)  40 . In such lasers, the Kerr-lens effect in the gain medium is the mode-locking mechanism. This configuration is a simpler device that is not “self-starting” and is sensitive to effects of alignment, optical pumping, and the like.  FIG. 2  shows the substitution of a saturable absorber mirror  50  for the Kerr-lens mirror  20  (in  FIG. 1 ), a lasing element  60  and Chirped mirrors  70 , as the dispersion compensating element, with output coupler  80  to provide a laser that is more stable and “self-starting.” 
     FIG. 3  is another prior art arrangement of a mode-locked laser with a high reflector (HR)  90 , a multipass cell  92 , a Kerr-lens lasing element  94 , a prism pair  96  as the dispersion compensating element with output coupler  98 . The multipass cell  92  is added to slow down the repetition rate. 
   None of the prior art arrangements of gain elements, or optical elements have the addition of Chirped Mirrors (CP), multi-pass mirror system and Saturable Absorber Mirror (SAM) mode-locking elements as disclosed herein. A second embodiment of the invention includes a cavity-dumping feature in the novel arrangement of elements. The cavity-dumping feature facilitates the extraction of all energy trapped inside the cavity by dumping the beam and thereby providing a several-fold improvement in the usable pulse energy. The present invention has a unique configuration and meets the commercial need for rugged, low cost, high power, ultra-short pulse lasers useful in, but not limited to, micro-processing and micro-structuring below conventional tolerances. 
   SUMMARY OF THE INVENTION 
   It is a primary objective of the present invention to provide a family of low cost, compact, high peak power ultra-short pulse lasers. 
   A second objective of the present invention is to provide an ultrafast megahertz (MHz) mode-locked laser of increased ruggedness that is self-starting. 
   A third objective of the present invention is to provide high intensity MHz mode-locked lasers that are low cost and easy to manufacture. 
   A fourth objective of the present invention is to provide high repetition rate MHz mode-locked lasers for sub-100 μm micro-processing and micro-structuring. 
   A fifth objective of the present invention is to provide a mode-locked ultrafast laser with a cavity-dumping feature. 
   A preferred compact, high intensity megahertz (MHz) mode-locked laser is provided wherein the laser system includes, a laser source coupled to a resonator cavity having a gain medium, a saturable absorber mirror in the resonator cavity for self-starting and stable mode-locking operation, a multipass mirror in combination with the saturable absorber mirror in the resonator cavity for lowering repetition rate of the laser system to below approximately 50 megahertz (MHz), dispersion compensating element in the resonator cavity for dispersion compensation, and an output coupler to the laser for releasing pulses with energies sufficient for micro-machining and micro-structuring applications. 
   The preferred laser source includes a diode laser. The preferred multi-pass mirror slows down the repetition rate of laser pulses to between approximately 1 MHz to approximately 50 MHz. 
   The preferred gain medium includes, but is not limited to, Yb:KYW, a KY[WO 4 ] 2  (KYW) crystal doped with Ytterbium ions, Yb:YAG, a Y 3 Al 5 O 12  (YAG) crystal doped with Ytterbium ions, and Ti:Sapphire, a sapphire (Al 2 O 3 ) crystal doped with Titanium ions. 
   A cavity dumping component is used to extracting energy trapped inside of the resonant cavity. Thus, a more preferred laser system of the present invention includes a cavity dumping component having an optical gate located between the output coupler and the saturable absorber mirror. The preferred cavity dumping component is either an acousto-optically driven gate or an electro-optically driven gate. 
   A preferred method of improving the pulse energy of a compact, high intensity megahertz mode-locked laser includes, providing a resonant laser cavity having a saturable absorber mirror and a laser gain medium, positioning a cavity dumping component within the laser cavity between an output coupler and the saturable absorber, pumping the laser gain medium, and extracting energy trapped inside the cavity by dumping excess energy through an optical gate, which can be acousto-optically driven or electro-optically driven. The saturable absorber mirror is a broadband saturable absorber. 
   Another preferred method of providing low cost, simple, compact, ultrafast laser with high pulse energies for micromachining applications includes, providing a laser configuration having a laser gain medium, a resonator cavity, a saturable absorber mirror, a multipass mirror and an output coupler, pumping the laser gain medium, generating femtosecond pulses with intensities in the megawatt range from the laser source, and simultaneously lowering the repetition rate of each pulse, thereby minimizing damage associated with the thermal load accumulated pulse after pulse. The femtosecond pulses have an energy of between approximately 10 nano Joules (nJ) and approximately 150 nano Joules (nJ). The repetition rate of the femtosecond pulses is in a range between approximately 1 MegaHertz (MHz) and approximately 50 MegaHertz (MHz). 
   Another preferred high intensity megahertz (MHz) mode-locked laser system has a laser that includes, a laser source and a gain medium, a saturable absorber mirror in the system for self-starting and stable mode-locking operation, a multipass mirror in the laser system for lowering repetition rate of the laser system to below approximately 50 megahertz (MHz), a dispersion compensating element in the laser system for dispersion compensation, and an output coupler to the laser system for releasing pulses with energies sufficient for micro-machining and micro-structuring applications. The preferred gain medium is a Ti:Sapphire crystal, or a thin disk-shaped gain medium. 
   A preferred basic laser system includes a lasing element, dispersion compensating elements coupled to the lasing element, a SAM mode-locking element coupled to the dispersion compensating element, a multipass mirror system coupled to the SAM mode-locking element, and an output coupler coupled to the multipass mirror system for providing an output from the laser system. 
   A more preferred laser system includes a lasing element, dispersion compensating elements coupled to the lasing element, a SAM mode-locking element coupled to the dispersion compensating element, a multipass mirror system coupled to the SAM mode-locking element, a cavity dumper coupled to the multipass mirror system, and an output coupler coupled to the cavity dumper for providing an output from the laser system. 
   Preferred embodiments of the invention include a cavity-dumping feature to extract all the energy trapped inside the cavity by dumping the beam, using an optical gate that can be either acousto-optically or electro-optically driven. The preferred embodiments provide a several-fold improvement in the usable pulse energy; approximately one order of magnitude higher pulse energies than current mode-locked lasers. 
   Further objects and advantages of this invention will be apparent from the following detailed description of the presently preferred embodiments, which are illustrated schematically in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a conventional mode-locked laser configuration with Kerr-lens mode locking and a prism pair as the dispersion compensating element. (Prior Art) 
       FIG. 2  is a mode locked laser with the addition of Chirped mirrors, as the dispersion compensating element, and Saturable Absorber (SAM) mode-locking element. (Prior Art) 
       FIG. 3  is a mode-locked laser with Kerr-lens mode locking and a prism pair as the dispersion compensating element and the addition of Multipass mirrors. (Prior Art) 
       FIG. 4  is a mode-locked laser of the present invention with a special Saturable Absorber Mirror as the mode-locking element, Chirped mirrors as the dispersion compensating element, and Multipass mirrors. 
       FIG. 5  is a first embodiment of the present invention with a Ti:Sapphire oscillator. 
       FIG. 6  is a second embodiment of the present invention with a face-pumped Yb:YAG or Yb:KYW thin disk laser configuration. 
       FIG. 7  is a third embodiment of the present invention incorporating a cavity dumper. 
       FIG. 8  is a fourth embodiment of the present invention with a Ti:Sapphire crystal and an acousto-optic cell cavity dumping scheme. 
       FIG. 9  is a surface profile of laser micro-machined trenches in Arsenic trisulfide (As 2 S 3 ) using the mode-locked laser of the present invention. 
       FIG. 10  is an oscilloscope trace of a 2-MHz laser pulse train, having a pulse separation of approximately 500 nanoseconds (ns). 
       FIG. 11  is an oscilloscope trace of a standard 90-MHz laser pulse train, having a pulse separation of approximately 10 nanoseconds (ns). 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of further embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
   Acronyms and terminology used throughout this description are defined as follows: 
   CM—Chirped mirrors that are able to provide second- and third-order dispersion compensation using a scheme where each frequency component is reflected at different depths through the dielectric coating, which consists of multiple stacks of varying thickness. Chirped mirrors provide very robust and compact arrangements for the design of ultrafast lasers. 
   FL—focusing lens used to focus the pump beam inside the laser crystal 
   KLM—Kerr lens mode-locking 
   MHz—megahertz 
   MPC—Multipass Cell—a cell inserted inside the laser cavity that enables the repetition rate to be reduced to as low as a few MHz, maintaining a reasonable level of compactness of the laser chassis. The pulse energy, inversely proportional to the repetition rate, is therefore increased several fold, while avoiding damage problems, heating effect, recovery time artifacts associated with high (&gt;50 MHz) repetition rates. 
   OC—Output Coupler—allows parts of the light trapped inside the cavity to get out 
   HR—High Reflector—constitutes the other end of the cavity opposite the output coupler (OC) 
   SAM—Saturable Absorber Mirror is a mode-locking element that allows a stabilized pulsed operation and makes the laser self-starting without the need for mechanical adjustments. 
   SM—Spherical mirror—mirror having a reflecting surface of spherical shape as opposed to a flat mirror. Concave spherical mirrors are used to focus light at the same time it is reflected. 
   TM—Turning mirror—used to redirect the laser beam 
   Yb:KYW—gain material consisting of a KY[WO 4 ] 2  (KYW) crystal doped with Ytterbium ions 
   Yb:YAG—gain material consisting of Y 3 Al 5 O 12  (YAG) crystal doped with Ytterbium ions 
   Ti:Sapphire—gain material consisting of a sapphire (Al 2 O 3 ) crystal doped with Titanium ions 
   More recently other components have been added to this general structure previously described in reference to  FIG. 1 . Instead of using prisms, dispersive elements, special mirrors, such as, Chirped Mirrors have been developed. These make the system more stable, and reduce the overall footprint of the laser. Moreover a special nonlinear mirror  50  has been developed to replace one of the mirrors (HR) as shown in  FIG. 2 . The function of the replacement mirror is to act as a “saturable absorber” mode-locking element, replacing, in part, the nonlinear effect in the laser medium. Thus,  FIG. 2  is a mode locked laser with the addition of Chirped mirrors  70  and Saturable Absorber (SAM) mode-locking element  50  wherein the substitution of a saturable absorber mirror for the Kerr-lens mirror provides a laser that is more stable and “self-starting.” 
     FIG. 3  represents further advancement as a mode-locked laser configuration with Kerr-lens mode locking element  94  and the addition of Multipass mirrors  92  to slow down frequency.  FIG. 3  shows the use of a multi-pass mirror system  92  with the conventional non-self-starting Kerr lens mode-locking  94  in the laser medium. This arrangement slows down the laser repetition rate to approximately 4 megahertz (MHz); which is more desirable for micro-machining and microstructuring. To understand this modification, two facts important to commercial applications should be appreciated. Firstly, most of these lasers operate at frequencies around 100 MHz. Thus, individual pulses are separated by approximately 10 nanoseconds (ns). This is too short a time for nearly all processing applications. Secondly, the energy per pulse of these systems is typically a few nanoJoules (nJ), approximately 1 to approximately 3 nJ, too low for most applications. This has lead many groups, including commercial companies, to add complex and expensive amplifier systems to boost the energy. 
   The present invention recognizes that if the resonator length of the laser increases, the frequency decreases, and the pulse energy increases. The only trouble is that the cavity length must be increased by orders of magnitude, from typically 1 meter (m) overall to greater than approximately 100 meters, for many applications. To accomplish this in a convenient way, the present invention has incorporated a special multi-pass mirror system in combination with CP mirrors, in addition to, utilizing non-self starting Kerr-lens mode locking in the laser medium, as shown in  FIG. 3 . 
   According to the present invention, the above objectives are met by incorporation of special Saturable Absorber (SAM) mirrors with mode-locking elements in combination with a Multipass Mirror system to slow down the repetition rate, and create a system that is more stable, self-starting and commercially viable for micro-machining applications. 
     FIG. 4  is a mode-locked laser of the present invention with special Saturable Absorber mode-locking element  100  and Multipass mirrors  102 . Referring now to  FIG. 4 , this is a general layout of the components in the high intensity MHz mode-locked laser of the present invention. A saturable absorber mirror  100  is positioned in close proximity to the multi-pass mirror system  102  and plays the role of both the high reflector and the mode-locking element. The new laser can include a lasing element  104 , dispersion compensating elements  106  coupled to the lasing element, and an output coupler  108  to provide output from the laser system. 
   The order of the elements can vary; for example, a SAM mode-locking element  100  can be coupled to the dispersion compensating element  106 , a multipass mirror system  102  can be coupled to the SAM mode-locking element  100  and an output coupler  108  can be coupled to the multipass mirror system  102  for providing an output from the laser system. 
     FIG. 5  is a first embodiment of the present invention with a Ti:Sapphire oscillator. In  FIG. 5 , a pump beam  200  from a diode pumped solid state laser is focused, with focusing lens  12 , on a Ti:Sapphire crystal  202  positioned between two spherical mirrors  204  and  222 . The spherical mirrors  204 ,  222  have a radius of approximately 10 centimeters (cm). The light beam resonates inside the cavity formed by the Output Coupler  216  with 12% transmission, and the Saturable Absorber Mirror (SAM)  212 . For each path, the light beam goes through the gain medium, through the multipass cell formed by mirrors  210  and  214 . The multipass cell is approximately 4 inches in diameter, approximately 2 meters (m) in length with r.o.c., 6.5 mm holes. Light passes through the multipass cell and reflects on the chirped mirrors  218  and  220  each having −60 fs 2 /reflection, and to the SAM  212 . A curved mirror  208 , with a radius of approximately 50 cm, is used to focus light onto the SAM  212  in order to reach the saturation intensity. A turning mirror  206  is used to steer the beam in order to reduce the laser footprint. The order in which light passes through these elements does not matter. 
   If the same architecture were utilized with a directly-diode pumped Yb:YAG or Yb:KYW laser, for instance the configuration, and footprint would be much smaller.  FIG. 6  shows a laser schematic in a face-pumped Yb:YAG or Yb:KYW thin disk laser configuration. The thin disk laser head  300  acts as the gain medium and as a mirror at the same time. In  FIG. 6 , the beam path is very similar to that illustrated in  FIG. 5 , except that it is folded one more time on the thin disk  302 , allowing a more compact system. 
   The light beam resonates inside the cavity formed by the output coupler  304  and a Broadband Saturable Absorber Mirror  318 . Each path, the light beam goes through the multipass cell formed by mirror  312  and  314 . The multipass cell is approximately 4 inches in diameter, with a length of approximately 3 feet (ft.) with r.o.c., 6.5 mm holes. Light passes through the multipass cell and reflects on dispersion compensated mirrors  306 ,  308 , is reflected by the thin disk  302  through Brewster plate  320 , before exiting the output coupler  304 . The thin disk geometry also allows better thermal management in the crystal. As a result, the power is scaleable with pump energy, and detrimental effects, such as, thermal lensing is minimized. The invention presented here is particularly appropriate for thin disk lasers. 
     FIG. 7  shows the general layout of the novel high intensity mode-locked laser of the present invention with the addition of a cavity dumper  408  between the multi-pass mirror system  402  and the output coupler  410 . The cavity dumper  408  facilitates the extraction of all the energy trapped inside the cavity by dumping the beam, using an optical gate that can be either acousto-optically or electro-optically driven. The laser system with the cavity dumper has a several-fold improvement in the usable pulse energy. 
   Except for the addition of an acousto-optic cell  528  and the addition of spherical mirrors  504 ,  506  and  514 , the laser configuration in  FIG. 8  is the same as the diode pumped solid state laser in  FIG. 7 .  FIG. 8  shows a version of this system for a Ti:Sapphire laser using a cavity dumping scheme. 
   In  FIG. 8 , a pump laser  500  sends a beam through focusing lens  15  onto a Ti:Sapphire crystal  502  positioned between two spherical mirrors  504  and  506 . Thereafter, the light path is very much the same as that depicted in  FIG. 5 . The light beam resonates inside the cavity formed by the Output Coupler  526  with 12% transmission, and the Saturable Absorber Mirror (SAM)  516 . For each path, the light beam goes through the gain medium, to the multipass cell formed by mirrors  518  and  520 . The multipass cell is approximately 4 inches in diameter, approximately 2 meters (m) in length with r.o.c., 6.5 mm holes. Light passes through the multipass cell and reflects on the chirped mirrors  522  and  524  each having −60 fs 2 /reflection, and to the SAM  516 . A curved mirror  514 , with a radius of approximately 50 cm, is used to focus light onto the SAM  516  in order to reach the saturation intensity. A turning mirror  512  is used to steer the beam in order to reduce the laser footprint. The order in which light passes through these elements does not matter. The difference between the light path in  FIG. 8  is that every time the acousto-optic cell  528  is triggered, a transient Bragg grating is created in the cell, which deflects the beam outside the cavity. The beam then bypasses the output coupler  526 , is picked up by a mirror  530  delivering pulses of higher energy. The curved mirrors  508 ,  510  are used to focus the beam onto the grating and recollimate it. 
   Cavity dumping relies on bypassing the output coupler (OC) that has a low transmission coefficient, by dumping inside the Fabry-Perot cavity where most of the energy is located. This is achieved by inserting an acousto-optic cell, in which an acoustic wave creates a Bragg grating that diffracts light. Generation of femtosecond pulses with intensities in the MW range is essential for a number of applications including optical harmonic generation, investigation of ultrafast nonlinear optical phenomena and laser micromachining. The development of low cost, simple and compact laser sources with high pulse energies will enable a wider range of ultrafast laser applications, making this technology more available to both the research and the development communities. 
   In laser micromachining applications, minimum pulse energy of several 100 s of nano Joules (nJ) is generally required. Consequently, most research studies utilize laser systems typically composed of a laser oscillator followed by an amplification stage, employing chirped pulse amplification schemes. These systems are complex, cost-ineffective and require high pump power levels. We propose, as an alternative, to use the laser system described above for such application and demonstrate its ability to produce ultrashort light pulses, with sufficient energy for micromachining applications. 
     FIG. 9  is a surface profile of laser micro-machined trenches in Arsenic trisulfide (As 2 S 3 ) using the mode-locked laser of the present invention. The femtosecond regime minimizes heat disposition and allows the fabrication of fine features measuring less than approximately 10 microns. 
     FIG. 9  shows the surface profile of laser micromachined trenches in Arsenic trisulfide using pulse energies of approximately 20 nJ (image taken with an interferometric microscope Zygo New View 5000). 
     FIG. 10  is an oscilloscope trace of a 2-MHz laser pulse train, having a pulse separation of approximately 500 nanoseconds (ns). 
   In addition to an increase of the pulse energy, lowering the repetition rate minimizes damage problems associated with the thermal load accumulated pulse after pulse. The insertion of the multipass cell increases with time separation between each pulse, leaving more time to the material to recover from the previous pulse. Recovery time artifacts are thus avoided. This is illustrated by  FIG. 10  showing the oscilloscope trace of a 2-MHz laser pulse train having a pulse separation of approximately 500 ns. In comparison, a standard 90-MHz laser, shown in  FIG. 11 , has a pulse separation of approximately 10 ns.  FIG. 11  is an oscilloscope trace of a standard 90-MHz laser pulse train, having a pulse separation of approximately 10 nanoseconds (ns). 
   The advantages of the invention are less cost, more versatile laser equipment, greatly increased ruggedness, ease of manufacture and compatibility with both disk laser and diode pumped solid-state laser. 
   While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.