Abstract:
The present invention contemplates a simplified laser oscillator-amplifier system for deep UV generation. The simplified system employs same type of gain media in both the oscillator and the amplifier and utilizes a single pump pulse split to pump both the oscillator and the amplifier. A short cavity oscillator is operated near lasing threshold to produce a seed pulse with a narrow spectral bandwidth and long pulse duration. A short cavity amplifier is Q-switched to amplify the seed pulse to produce a single short pulse with good energy extraction efficiency. The amplifier is simply a short cavity, Q-switched laser. Short pulse is obtained with short cavity length and high gain of the amplifier. Consequently, the simplified laser oscillator-amplifier system can accommodate a long pump pulse to produce a nanosecond pulse suitable for deep UV laser generation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a laser system. In particular, the present invention relates to a simplified 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 require less maintenance in comparison with the excimer lasers, 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 accurate and 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 master oscillator—power amplifier system is a common approach to obtain amplified pulses of short duration, 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 up to 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 seed pulse from the oscillator to the amplifier and to isolate the amplified pulse from feeding back to the oscillator. As a result, a master oscillator—power amplifier system is usually complicated and expensive. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a simplified laser oscillator-amplifier system for deep UV generation. The simplified system employs the same type of gain media in both the oscillator and the amplifier and utilizes a single pump pulse split to pump both the oscillator and the amplifier. A short cavity oscillator is operated near lasing threshold to produce a seed pulse with a narrow spectral bandwidth. A short cavity amplifier is Q-switched to amplify the seed pulse to produce a single short pulse with good energy extraction efficiency. The amplifier is simply a short cavity, Q-switched laser. Short pulse is obtained with short cavity length and high gain of the amplifier. Consequently, the simplified laser oscillator-amplifier system can accommodate a long pump pules to produce a nanosecond pulse suitable for deep UV laser generation. In addition, the complication of pulse synchronization and optical isolation between oscillator and amplifier are substantially eliminated in this simplified laser oscillator-amplifier system. 
     According to a preferred embodiment of the present invention, a simplified laser oscillator-amplifier system comprises:
         a laser oscillator having a first gain medium and a spectrum control mechanism, said laser oscillator produces a seed pulse when said first gain medium is excited with a pump pulse;   a laser amplifier having a second gain medium and a Q-switch, said laser amplifier amplifies said seed pulse to generate a short amplified pulse; and   a pump pulse of radiation split to pump both said laser oscillator and said laser amplifier;   wherein said laser oscillator-amplifier system produces a single amplified laser pulse with predetermined pulse energy and spectrum bandwidth.       

     Accordingly, 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 approximately 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 long pump pulse 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 simplified laser oscillator-amplifier system, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram showing a simplified laser oscillator-amplifier system  100 , in accordance with the present invention. The system  100  consists of primarily a laser oscillator  10 , a laser amplifier  20 , and a pump pulse  30 . The system  100  further consists of coupling optics  17 ,  27 ,  41 ,  42 , and  43 . When excited by a pump pulse  30 , the system  100  produces an output pulse  29 . 
     The laser oscillator  10  consists of a first end mirror  11 , a second end mirror  13 , a gain medium  14 , and a wavelength control element  12 . The laser oscillator  10  is designed to operated at low pump threshold and to obtain a seed pulse  18  of narrow bandwidth and long pulse duration. A general guideline for the design of the laser oscillator  10  is low gain, low loss, and short cavity length. 
     The first end mirror  11  has a high reflectivity at the laser wavelength and high transmission at the pump pulse wavelength. The second end mirror  13  has a certain transmission at the laser wavelength and thus serves as an output coupler. The first end mirror  11  and second end mirror  13  are mirrors with multiple layer dielectric coatings to meet certain specifications known to those skilled in the art. Typically, the first end mirror  11  has a reflectivity higher than 99% at the laser wavelength, while the second end mirror  13  has a reflectivity of about 90 to 95% at the laser wavelength. 
     The gain medium  14  is a laser crystal, such as Ti:sapphire or Cr:LiSAIF. The gain medium  14  has a certain length and doping level such that it produces optimal gain at the laser wavelength when pumped by a pump pulse  16 . The gain medium  14  is usually cut at Brewster angle to minimize reflection loss and to define the polarization of the seed laser pulse. Typically, the laser crystal  14  shall absorb about 90 to 95% of the pump energy in a single pass. Cooling to the gain medium  14  is critical for obtaining constant and stable operation. 
     The wavelength selection element  12  is to select the laser wavelength and to control the spectrum bandwidth of the seed laser pulse  18 . A typical wavelength selection element  12  is a birefringent filter, which can be a single piece or a stack of crystal quartz plates aligned at a Brewster angle of incidence. For broad band solid state gain medium such as Ti:sapphire or Cr:LiSAIF, a stack of multiple quartz plates is required to obtain a narrow bandwidth spectrum in seed pulse generation. Typically, a quartz plate for this wavelength selection purpose has a thickness of 0.5 to 10 mm. When it is pumped with the pump pulse beam  16 , the laser oscillator  10  produces a seed pulse beam  18 . 
     The pump laser pulse  16  has a shorter wavelength than that of the seed pulse  18 . To obtain a good beam overlap of the pump pulse beam  16  with the seed pulse beam  18  inside the gain medium  14 , the pump pulse beam  16  is incident on the laser crystal  14  at an angle slightly bigger than the Brewster angle for the seed laser beam  18 . To minimize the reflection loss of the pump pulse beam  16  at the laser crystal  14 , the pump pule beam  16  shall have the same polarization as the seed pulse beam  18 . 
     The laser amplifier  20  consists of a first end mirror  21 , a second end mirror  23 , a gain medium  24 , and a Q-switch  25 . The amplifier  20  is seeded by seed pulse  28  through coupling optics  41 ,  42 , and  43 . When pumped by pump pulse  26 , the amplifier  20  amplifies seed pulse  28  and produces a short, amplified pulse  29 . 
     The amplifier  20  is designed to obtain an output pulse  29  with optimal pulse energy and shortest pulse duration. The general guideline for the design of the laser amplifier  20  is high gain, high loss, and short cavity. 
     The first end mirror  21  and the second end mirror  23  are also dielectric mirrors. The loss of the amplifier cavity is chosen to be high such that the energy depletion time of the amplifier  20  can be short; a short depletion time leads to a short pulse generation. Typically, the first end mirror  21  has a reflectivity of about 98%, which allows effective seeding from the oscillator  10 . The second end mirror  23  has a reflectivity of about 30% to 70% to serve as an output coupler. 
     The gain medium  24  is a same laser crystal as the oscillator  10  such that a single pump pulse can be split to pump both the oscillator  10  and the amplifier  20 . The gain medium  24  has a predetermined length and doping level such that it produces optimal gain at the laser wavelength when pumped by a pump pulse  26 . Cooling to the gain medium  24  is also critical for obtaining constant and stable operation. The selection and cooling of gain medium  24  are known to those skilled in the art. 
     The Q-switch  25  is to obtain high gain operation of the amplifier  20 ; and high gain operation is a key element for short pulse generation. Either AO Q-switch or EO Q-switch can be used for this purpose, while the latter is more preferable. An EO Q-switch can sustain higher gain and switch within a shorter time than an AO Q-switch does. In this application, the Q-switch  25  can be simply synchronized with the pump pulse  30 . The selection and synchronization of a Q-switch  25  is also known to those skilled in the art. 
     The cavity length of the laser amplifier  20  shall be short; and a short cavity length is another key element for short pulse generation. The laser amplifier  20  consists of simply a gain medium  24  and a Q-switch  25  and thus the cavity length can be as short as 5 to 10 cm. 
     Similar to pump laser beam  16  to the gain medium  14  in the oscillator  10 , the pump laser beam  26  to the gain medium  24  in the amplifier  20  shall have a certain incident angle and polarization with respect to the cavity laser beam  28 . In addition, the first end mirror  21  shall have high transmission to the pump laser beam  26 . 
     In the simplified laser oscillator—amplifier system  100  of  FIG. 1 , the coupling optics  27  is a beam splitter to split a good percentage of the pump pulse  30  into the pump pulse  26  for pumping the amplifier  20 . The coupling optics  17  is a turning mirror to direct the split pump pulse  30  into the pump pulse  16  for pumping the oscillator  10 . The coupling optics  41  and  42  are mirrors to direct the seed pulse  18  from the laser oscillator  10  into the laser amplifier  20 . The coupling optics  43  is a lens to control the beam size of the seed pulse  28  to match the mode size of the laser amplifier  20 . Commercially available design software may be used to assist the design of the laser oscillator  10  and the laser amplifier  20  for given parameters. 
     The Q-switch  25  is simply synchronized to the pump pulse  26  with a predetermined delay, which is about the pulse length of the pump pulse  26 . Due to the short pulse length of the amplified pulse  29 , “isolation” between the oscillator  10  and the amplifier  20  can be achieved by simply separating the oscillator  10  from the amplifier  20  with a traveling time longer than the amplified pulse duration. 
     For gain media  14  and  24  being Ti:sapphire laser crystals, the pump pulse  30  can be delivered from a Q-switched, frequency doubled Nd:YAG, Nd:YLF, or Yb:YAG laser. Typical pulse duration from these lasers is around 100 to 200 ns. To generate a deep UV laser beam suitable for photo-refractive surgery, the amplified pulse  29  shall be in the range of 1 mJ to 5 mJ and pulse repetition rate shall be in the range of 200 Hz to 2000 Hz. The pump pulse  30  shall thus have pulse energy of 3 mJ to 15 mJ. The above designed simplified oscillator—amplifier system  100  shall produce amplified pulse  29  with pulse duration of approximately 1 ns to 5 ns and a pulse spectral bandwidth of approximately 0.01 nm to 0.1 nm. 
     In one embodiment, the simplified laser oscillator—amplifier system  100  employs Ti:sapphire laser crystals as gain medium  14  and gain medium  24 . The pump pulse  30  is delivered from a Q-switched, frequency doubled Nd:YLF laser having a pulse duration of about 150 ns, pulse energy of about 8 mJ, and a pulse repetition rate of about 1 kHz. The laser oscillator  10  and the laser amplifier  20  have each a cavity length of about 5 to 10 cm. The pump pulse  30  is split by beam splitter  27  to have about 7 mJ for pump pulse  26  and 1 mJ for pump pulse  16 . The laser oscillator  10  comprises a stack of three quartz plates and produces a seed pulse  18  of about 0.2 mJ with a spectral bandwidth of 0.02 nm and a pulse duration of about 100 ns. The laser amplifier  20  comprises an EO Q-switch  25  and a 50% output coupler  23 . The amplifier  20  is located about 1 meter away from the oscillator  10 . The simplified oscillator-amplifier system  100  is expected to produce an amplified pulse  29  with pulse energy of about 2 mJ, a wavelength around 830 nm, pulse duration shorter than 5 ns, and a spectral bandwidth narrower than 0.05 nm. 
     The amplified pulse  29  is particular useful for fourth harmonics generation to produce deep UV pulse for refractive surgery, as described in U.S. Pat. No. 6,031,854 to Lai. The above figure and description are intended for illustrating the present invention. It is understood that various modification can be made without departing from the scopes of the invention as defined in the appended claims.