Abstract:
The present invention contemplates an integrated oscillator-amplifier system for deep UV generation. The system employs a long cavity oscillator to lengthen the pulse build-up time and to control the pulse spectral bandwidth. Meanwhile the system employs a short cavity amplifier to shorten the energy extraction time to produce a single short pulse with good energy extraction efficiency. The system further integrates the oscillator and the amplifier by inserting the amplifier cavity inside the oscillator cavity via a mirror of low reflectivity. As a result, the integrated system has a long build-up time to generate a seed pulse in the long cavity oscillator and has a short energy extraction time to generate a short amplified single pulse in the short cavity. Consequently, the integrated system can accommodate a relatively long pump pulse to produce a single short amplified pulse suitable for deep UV laser generation.

Description:
This application claims the benefit of U.S. provisional application No. 60/363,945, filed on Mar. 11, 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a laser system. In particular, the present invention relates to an integrated oscillator-amplifier laser system that is suitable for deep UV generation. 
     BACKGROUND OF THE INVENTION 
     Solid-state laser sources operated at deep UV wavelength around 200 nm are greatly desirable for photo-refractive surgeries. Such a deep UV laser source is expected to be more compact, more reliable, and requires less maintenance compared with excimer laser, which is currently the dominant laser source for photo-refractive surgeries. More importantly, solid-state laser sources can be operated at a much higher repetition rate and with much less energy fluctuation compared with the excimer lasers. Scanning a deep UV laser beam with high repetition rate enables a variety of ablation shapes on a cornea surface and provides a great flexibility for the refractive surgeries. The improved stability in pulse energy from a solid-state UV laser source ensures more accurate and better controllable ablation. 
     In U.S. Pat. No. 6,031,854 to Lai, a diode pumped cascade laser is proposed for deep UV generation. The second laser employs a short cavity with only a gain medium and a wavelength selection element inside the cavity. When pumped by a laser pulse of 50 ns or shorter, the second laser is gain-switched to produce a pulse of nanosecond duration. This nanosecond laser pulse is then converted to deep UV radiation by a wavelength converter. 
     In the above approach, a short pump pulse is critical for generating a single short pulse with nanosecond duration and millijole energy. The pulse build-up time is proportional to the laser cavity length and inversely proportional to the net pump pulse energy above the lasing threshold of the cavity. When the pump pulse duration is longer than the build-up time of the laser pulse, a second pulse will appear. This results in smaller energy in the first pulse and thus lowers the conversion efficiency in deep UV generation. 
     It is well known in the art that a combination of master oscillator-power amplifier system is a common approach to obtain amplified pulses of short duration, in addition to good beam profile and narrow bandwidth. In such a system, the master oscillator is usually a low gain, low power laser to produce a seed pulse of certain specifications. The power amplifier is a high gain, high power laser to amplify the seed pulse to a much higher pulse energy. A number of master oscillator-power amplifier systems are commercially available from, for example, Lambda Physics of Germany and Continuum of Santa Clara, Calif. 
     The advantage of a master oscillator-power amplifier system is that the oscillator and the amplifier laser cavities can be optimized independently. The system, however, requires two pump sources and two gain media. Also, the system requires additional optics to inject the oscillator seed pulse to the amplifier. As a result, a master oscillator-power amplifier system is usually complicated and expensive. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates an integrated oscillator-amplifier system for deep UV generation. The system employs a long cavity oscillator to lengthen the pulse build-up time and to control the pulse spectral bandwidth. Meanwhile the system employs a short cavity amplifier to shorten the energy extraction time to produce a single short pulse with good energy extraction efficiency. The system further integrates the oscillator and the amplifier by inserting the amplifier cavity inside the oscillator cavity via a mirror of low reflectivity. As a result, the integrated system has a long build up time to generate a seed pulse in the long cavity oscillator and has a short energy extraction time to generate a short amplified single pulse in the short cavity. Consequently, the integrated system can accommodate a relatively long pump pulse to produce a single short amplified pulse suitable for deep UV laser generation. 
     According to the present invention, an integrated laser oscillator-amplifier system comprises: 
     a laser oscillator having a gain medium and a long resonant cavity, said laser oscillator pre-lases to produce a seed pulse when said gain medium is excited with a pump pulse; and 
     a laser amplifier having said gain medium and a short resonant cavity, said laser amplifier is integrated into said laser oscillator via a mirror with low reflectivity and amplifies said seed pulse to generate a short amplified pulse; 
     wherein said laser oscillator-amplifier system produces a single amplified laser pulse. 
     One objective of the present invention is to provide a new and improved laser system for deep UV laser generation. 
     Another objective of the present invention is to provide a new and improved laser oscillator-amplifier system for generating single pulse of nanosecond duration and millijole energy. 
     A further objective of the present invention is to provide a new and improved laser oscillator-amplifier system accommodating a pump pulse of approximately 100 ns to generate a single pulse of nanosecond duration. 
     These and other aspects and advantages of the invention will become more apparent in the following drawings, detailed description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a first embodiment of an integrated laser oscillator-amplifier system, in accordance with the present invention. 
     FIG. 2 is a schematic diagram showing a conventional configuration of a master oscillator-power amplifier system with separated laser cavities. 
     FIG. 3 is a schematic diagram showing a second embodiment of an integrated laser oscillator-amplifier system, in accordance with the present invention. 
     FIG. 4 is a schematic diagram showing a third embodiment of an integrated laser oscillator-amplifier system, in accordance with the present invention. 
     FIG. 5 a  is a schematic diagram showing the curves of the pump pulse profile, gain, and laser pulse profile of a master oscillator. 
     FIG. 5 b  is a schematic diagram showing the curves of the pump pulse profile, gain, and laser pulse profile of a power amplifier without seeding. 
     FIG. 5 c  is a schematic diagram showing the curves of the pump pulse profile, gain, and laser pulse profile of an integrated laser oscillator-amplifier system of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram showing a first embodiment of an integrated laser oscillator-amplifier system  100 , in accordance with the present invention. The system  100  consists of a long oscillator cavity formed by mirrors  101  and  105  and a short amplifier cavity formed by mirrors  103  and  105 . The system  100  further consists of a common gain medium  104  and a wavelength selection element  102 . When excited by a pump pulse beam  107 , the system  100  produces an output pulse beam  106 . 
     In this integrated laser oscillator-amplifier system  100 , the oscillator has a much longer cavity and a much lower loss than the amplifier. The loss of the amplifier cavity is high because of the reflectivity of mirror  103  is chosen to be low. As it is shown below, when excited by a pump pulse  107 , the pulse  106  starts to build-up in the oscillator much earlier while it has a much longer build-up time. The cavity length of the oscillator is chosen such that the pulse  106  is almost fully developed when the gain produced in the gain medium  104  reaches the threshold of the amplifier cavity. The amplifier cavity is chosen such that the energy in the gain medium  104  can be extracted quickly by pulse  106  and thus the pulse  106  reaches a high peak within a short duration. This way the oscillator is there to generate a pre-lased seed pulse while the amplifier is there to amplify the seed pulse to an amplified pulse with a short duration. 
     During the build-up process of the pulse  106 , the wavelength selection element  102  is included in the oscillator cavity. As a result, the seed pulse, i.e. the early stage of pulse  106 , has its wavelength and bandwidth controlled by the oscillator. The build-up time of the seed pulse depends on the length, the gain, and the loss of the oscillator cavity. Because the gain and loss of the oscillator cavity are coupled with the amplifier cavity, a simple way to adjust the build-up time of the seed pulse is to change the cavity length of the oscillator. 
     FIG. 2 is a schematic diagram showing a conventional configuration of a master oscillator-power amplifier system  200  with separated laser cavities. The system  200  consists of a master oscillator  200   a , a power amplifier  200   b , and coupling optics  210 . The oscillator  200   a  is to generate a seed pulse  206  with predetermined parameters. The coupling optics  210  is to couple the seed pulse  206  into the amplifier  200   b . The amplifier  200   b  is to amplify the seed pulse  206  to a pulse  216  with much higher pulse energy. 
     For comparison purpose, the oscillator  200   a  and amplifier  200   b  of system  200  are simply discomposed presentations of system  100 . The oscillator  200   a  consists of a first end mirror  201 , a second end mirror  205 , a gain medium  204 , and a wavelength selection element  202 . The first end mirror  201  has a high reflectivity at the laser wavelength. The second end mirror  205  has a certain transmission at the laser wavelength and thus serves as an output coupler. The first end mirror  201  and second end mirror  205  are mirrors with multiple layer dielectric coatings to meet certain specification known to those skilled in the art. 
     The gain medium  204  is a laser crystal, such as Ti:Sapphire or Cr:LiSAlF. The gain medium  204  has a certain length and doping level such that it produces optimal gain at the laser wavelength when pumped by a pump pulse  207 . The gain medium  204  is usually cut at Brewster angle to minimize reflection loss and to define the polarization of the seed laser pulse. Cooling of the gain medium  204  is critical for obtaining constant and stable operation. 
     The wavelength selection element  202  is to select the laser wavelength and to control the spectral width of the seed laser pulse. A typical wavelength selection element  202  is a single piece or a stack of crystal quartz plates aligned at a Brewster angle of incidence. For a broadband solid state gain medium such as Ti:Sapphire or Cr:LiSAlF, a stack of multiple quartz plates is required to obtain a narrow band spectrum for the seed laser pulse. 
     The pump laser beam  207  has a shorter wavelength than the laser beam  208 . To obtain a good overlap of the pump beam  207  with the laser beam  208  inside the gain medium  204 , the pump laser beam  207  shall incident the laser crystal  204  at an angle slightly bigger than the Brewster angle for the laser beam  208 . To minimize the reflection loss of the pump beam  207  at the laser crystal  204 , the pump laser beam  207  shall have the same polarization as the laser beam  208 . 
     For comparison purpose, oscillator  200   a  should have a relatively long cavity and thus a long build-up time for seed pulse  206 . Besides, the build-up time of seed pulse  206  is also dependent on the transmission of the output coupler  205  and the pump power in the gain medium  204 . 
     On the other hand, the amplifier  200   b  has a much shorter cavity. The amplifier  200   b  consists of a first end mirror  213 , a second end mirror  215 , and a gain medium  214 . The amplifier  200   b  is seeded by seed pulse  206  through coupling optics  210 . When pumped by pump pulse  217 , the amplifier  200   b  amplifies seed pulse  206  and produces an amplified pulse  216 . 
     The first end mirror  213  and the second end mirror  215  are also dielectric mirrors. The loss of the amplifier cavity is chosen to be high such that no pulse builds up before the arrival of the seed pulse  206 . This way the output pulse  216  from the amplifier  200   b  is substantially the amplified seed pulse  206 . 
     Similar to pump laser beam  207  for the gain medium  204  in the oscillator  200   a , the pump laser beam  217  for the gain medium  214  in the amplifier  200   b  shall have a certain incident angle and polarization with respect to the cavity laser beam  218 . Besides, the first end mirror  213  shall have high transmission to the pump laser beam  217 . 
     The coupling optics  210  may include mirrors, lenses, and an optical isolator. Specifically, mirrors are used to direct the seed pulse  206  into the amplifier  200   b . Lenses are used to make the seed pulse  206  match the cavity mode of the amplifier  200   b . Optical isolator is used to prevent the radiation from the amplifier  200   b  to interfere with the operation of the oscillator  200   a . Commercially available design software may be used to assist the design of the oscillator  200   a  and amplifier  200   b  for given parameters. 
     FIG. 3 is a schematic diagram showing a second embodiment of an integrated laser oscillator-amplifier system  300 , in accordance with the present invention. In this integrated laser oscillator-amplifier system  300 , the long cavity oscillator is formed by a first end mirror  301  and a second end mirror  305 , and the amplifier short cavity is formed by a first end mirror  301  and a second end mirror  303 . The oscillator consists of the first end mirror  301 , a gain medium  304 , a wavelength selection element  302 , and the second end mirror  305 . The amplifier consists of the first end mirror  301 , the gain medium  304 , and the second end mirror  303 . When pumped by a pulsed pump laser beam  307 , the system  300  produces a pulsed output laser beam  306 . 
     Similar to the oscillator-amplifier system  100 , the oscillator in the system  300  has a low lasing threshold and starts to pre-lase early to provide a seed pulse for later amplification by the amplifier. The seed pulse thus has its wavelength and bandwidth controlled by the wavelength selection element  302  and has its build-up time controlled by the cavity length of the oscillator. 
     The amplifier in the system  300  has a high lasing threshold due to a low reflectivity of the second end mirror  303 . The amplifier has a short depletion time due to its short cavity length. This way the amplifier can boost the seed pulse to generate output pulse  306 . 
     FIG. 4 is a schematic diagram showing a third embodiment of an integrated laser oscillator-amplifier system  400  in accordance with the present invention. In this integrated laser oscillator-amplifier system  400 , the long cavity oscillator is formed by a first end mirror  401  and a second end mirror  405 , and the short cavity amplifier is formed by a first end mirror  403  and a second end mirror  405 . The oscillator consists of the first end mirror  401 , a gain medium  404 , a wavelength selection element  402 , and the second end mirror  405 . The amplifier consists of the first end mirror  403 , the gain medium  404 , and the second end mirror  405 . When pumped by a pulsed pump laser beam  407 , the system  400  produces a pulsed output laser beam  406 . 
     Similar to the oscillator-amplifier system  100 , the oscillator in the system  400  has a low lasing threshold and starts to pre-lase early to provide, a seed pulse for later amplification by the amplifier. The seed pulse thus has its wavelength and bandwidth controlled by the wavelength selection element  402  and has its build-up time controlled by the cavity length of the oscillator. 
     The amplifier in the system  400  has a high lasing threshold due to a low reflectivity of the second end mirror  403 . The amplifier has a short depletion time due to its short cavity length. This way the amplifier can boost the seed pulse to generate output pulse  406 . 
     In one embodiment, the integrated laser oscillator-amplifier system takes the configuration of FIG.  4 . The gain medium  404  is a Ti:Sapphire laser crystal, and the pump laser beam  407  is delivered from a Q-switched, frequency doubled Nd:YLF laser having pulse duration of about 100 ns and pulse energy of about 5 mJ. The oscillator cavity length is about 30 cm and the amplifier cavity length is about 10 cm. The first end mirror  403  of the amplifier has a transmission of about 60% at the laser wavelength, and the second end mirror  405  of the amplifier has a transmission of about 50%. The wavelength selection element  402  is a stack of three quartz plates. Single pulse operation is expected to produce output pulse  406  with pulse energy about 1 mJ, pulse duration shorter than 10 ns, and a spectral bandwidth narrower than 0.1 nm. 
     FIG. 5 a  is a schematic diagram showing the temporal evolution of the pump pulse intensity profile  50   la , gain profile  502   a , and laser pulse intensity profile  503   a  of a master oscillator  200   a . The pump pulse profile  501   a  shows a typical temporal profile of a pump pulse  207  delivered from a Q-switched, frequency doubled Nd:YLF laser. The gain profile  502   a  shows an estimated gain curve generated in the gain medium  204  excited by the pump pulse  207 . The laser pulse profile  503   a  shows an expected laser seed pulse  206  from the oscillator  200   a . Because of low cavity loss and long cavity length, the master oscillator  200   a  has a relatively low lasing threshold Tha and a relatively long building up time τ a  Consequently the master oscillator  200   a  has also a long energy depletion time. As a result, the master oscillator  200   a  produces a seed pulse  206  having a pulse profile of  503   a , which is a long pulse. 
     FIG. 5 b  is a schematic diagram showing the temporal evolution of the pump pulse intensity profile  501   b , gain profile  502   b , and laser pulse intensity profile  503   b  of a power amplifier  200   b . For comparison purpose, the pump pulse  217  for the power amplifier  200   b  is delivered from the same Q-switched, frequency doubled Nd:YLF laser as in FIG. 5 a . The pump pulse profile  501   b  is thus similar to that of the pump pulse profile  501   a . The gain profile  502   b  shows an estimated gain curve generated in the gain medium  214  excited by the pump pulse  217 . The laser pulse profile  503   b  shows an expected laser pulse  216  from the power amplifier  200   b . Because of high cavity loss and short cavity length, the power amplifier  200   b  has a relatively high lasing threshold Thb and a relatively short building up time τ b . Consequently the amplifier  200   b  has also a short energy depletion time. As a result, the power amplifier  200   b  produces an amplified pulse  216  having a pulse profile of  503   b , which is a short pulse. 
     As shown in FIG. 5 b , the power amplifier  200   b  is designed such that the seed pulse  206  arrives when the amplifier gain  502   b  reaches approximately to its maximum and before a pulse starts to build up inside the amplifier cavity. This way, the power amplifier  200   b  amplifies the seed pulse  206  instead of producing its own pulse. 
     FIG. 5 c  is a schematic diagram showing the temporal evolution of the pump pulse intensity profile  501   c , gain profile  502   c , and laser pulse intensity profile  503   c  of an integrated laser oscillator-amplifier system  400  of the present invention. For comparison purpose, the pump pulse  407  to the integrated system  400  is delivered from the same Q-switched, frequency doubled Nd:YLF laser as in FIG. 5 a . The pump pulse profile  501   c  is thus similar to that of the pump pulse profile  501   a . The gain profile  502   c  shows an estimated gain curve generated in the gain medium  404  excited by the pump pulse  407 . The laser pulse profile  503   c  shows an expected laser pulse  406  from the integrated system  400 . Because of low cavity loss and long cavity length for the oscillator, the integrated system  400  has a relatively low lasing threshold Thc and a relatively long building up time τ c . Meanwhile, because of high cavity loss and short cavity length for the amplifier, the integrated system  400  has a relatively short energy depletion time. As a result, the integrated system  400  produces a pulse  406  having a pulse profile of  503   c , which is a relatively short pulse. 
     As shown in FIG. 5 c , the integration system  400  is designed such that the seed pulse is generated when the gain  502   c  reaches approximately to its maximum and onsets the amplifier. This way, the integrated system  400  produces a pulse  406  having a pulse profile  503   c  similar to that of an amplified pulse  216  from a conventional master oscillator-power amplifier system  200 . 
     The integrated laser oscillator-amplifier system  100 ,  300 , or  400  significantly simplifies the structure of a master oscillator-power amplifier system  200 . By adjusting the oscillator cavity length, the amplifier cavity length, and the transmission of an end mirror  103 ,  303 , or  403  of the amplifier, the integrated system  100 ,  300 , or  400  can be optimized to produce short single pulse for relatively long pump pulse  107 ,  307 , or  407 . 
     The above figures and descriptions are intended for illustrating the present invention. It is understood that various modifications can be made without departing from the scopes of the invention as defined in the appended claims. 
     REFERENCES 
     U.S. Patent Documents 
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     U.S. Pat. No. 6,031,854 Lai Feb. 29, 2000 Diode-pumped cascade laser for deep UV generation