Abstract:
The present invention discloses a means of the generation of tunable femtosecond pulses from 380 nm to 465 nm near the degenerate point of a 405-nm pumped type-I BBO non-collinearly phase-matched optical parametric amplifier (NOPA). The tunable UV/blue radiation is obtained from sum frequency generation (SFG) between the OPA output and the residual fundamental beam at 810-nm and cascaded second harmonic generation (SHG) of OPA. With a pumping energy of 75 mJ at 405 nm, the optical conversion efficiency from the pump to the tunable SFG is more than 5% and the efficiency of SHG of the OPA is about 2%.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical parametric amplifier (OPA). In particular, the present invention relates to providing the blue light and the near-ultra-violet (380-460 nm) a continuously tunable optical parametric amplifier by using cascaded sum frequency generation of femtosecond non-collinear optical parametric amplifier.  
         [0003]     2. Description of Relative Prior Art  
         [0004]     In the near decade, the applications of the blue light become more important. The most important industry is high intensity storage. This wave length is very important for the application of bio-technology and environment control areas. Moreover, in the application of time-resolved and frequency-resolved studies, real time studies of molecular dynamical and optical spectrum studies, show that the development potential of the blue light and near-ultra-violet radiation. However, the source and the detector of the blue light and the near-ultra-violet light is still not enough. In the near decade, due to the development of non-linear crystal and laser technology is more come to mature, this makes possible of the tunable wave-length optical source of this wave-band. The trend is to generate a higher quality and convenient source and to increase the efficiency.  
         [0005]     Optical parametric amplifier (OPA) is an important means to generate tunable-wavelength. But it is very difficult to generate blue light directly from the OPA. Generally, it needs to go through another non-linear optical process, such as frequency doubling or sum frequency generation, which increases the complexity and cost.  
         [0006]     In the published documents, such as in the articles “ultra fast optical parametric amplifiers”, Giolio cerallo et al., Review of scientific Instruments 74, 1-17(2003), relates to the generation of tunable wave-length optical source by OPA process; In the article “Generation and amplification of ultra-short shaped pulses in the visible by a two-stage non-collinear optical parametric process”, Howe-Siang et al., Opt. Lett. 26, 1812-1814(2001); and “Broadband optical parametric amplification in the near UV-VIS”, Tzankov et al., Opt. Commum., 2003, 107˜(2002); “Broadband amplification of ultraviolet laser pulse”, Osvay et al., Appl. Phys. B: Lasers Opt. B74, S163-2002(2002); emphasized by summing the frequency of the generating long wave-length. This makes the system more complex, and that summing frequency process itself also has time overlap problem, the output is not stable due to mechanical problem. In the article “Extension of tuning range of a femtosecond Ti: sapphire laser amplifier through cascaded second-order nonlinear frequency conversion process”, Petro et al., J. Appl. Phys, 76, 7704-7712(1994); the designed architecture is based on the last stage of photo-frequency mixing or sum frequency generation, which is different from the article “Generation of femtosecond laser pulses tunable from 380 nm to 465 nm via cascaded nonlinear optical mixing in a non-collinear optical parametric amplifier with a type- 1  phase matched BBO crystal”, chao-Kuei lee et al., Opt Express 11,1702-1708(2003), this article makes use of a cascaded non-linear Optical mixing to generate tunable-wavelength of femtosecond non-collinear optical parametric amplifier.  
         [0007]     In the U.S. Pat. No.5,144,629 to basu ; U.S. Pat. No.5,751,472 to jeys et.al., and U.S. Pat. No.5,769,513 to Stamm et.al., are implemented general optical parametric amplifier architecture, and only has signal and idler output.  
         [0008]     The prior art of optical parametric amplifier with blue light out put basically is by using the output of generated long wave-length to generate the necessary short wave-length by sum-frequency, or by using high-order harmonic to generate excited light source of shorter wave-length. The disadvantage is that the wave length of the excited light source generation is not easy because of the transformed efficiency of high-order harmonic is low, group velocity mismatched, short wave-length thin-film deposition is not easy, and high cost. The last stage of optical mixing or sum-frequency architecture makes the system more complex, which is not convenient and limited in application consideration.  
         [0009]     What is needed is an improved tunable wave-length femtosecond non-collinear optical parametric amplifier.  
       OBJECTS OF THE INVENTION  
       [0010]     Therefore, it is an object of the invention to provide a femtosecond non-collinear optical parametric amplification to provide continuously tunable of blue light and green light generating device.  
         [0011]     It is another object of the invention to provide a tunable wave-length femtosecond non-collinear optical amplifier by using cascaded non-linear optical crystal to provide a blue light source with wave band of 308-465 nm, and the wave-length is continuous tunable. It is yet another object of the invention to provide a tunable wave-length femtosecond non-collinear optical amplifier by using cascaded non-linear optical crystal to provide a blue light source with wave band of 308-465 nm, and the wave-length is continuous tunable.  
         [0012]     It is yet another object of the invention to provide a tunable wave-length femtosecond non-collinear optical amplifier by using cascaded non-linear optical crystal, to implement a blue light parametric amplifier by lower cost and more simple method.  
       DISCLOSURE OF THE INVENTION  
       [0013]     The present invention teaches a tunable wave-length femtosecomd non-collinear optical sum-frequency generation, comprising: a frequency doubling device for generating optical parametric amplifier(OPA) excited light transmits to the CaF 2  window to generate a seeder; a rotating table, for capable the non-collinear parametric amplify crystal to rotate around the axis of the crystal; a plurality of lens, silver lens and reflector, to direct the light to a non-collinear optical parametric amplifier crystal; a crystal, for generating tunable wave-length of non-collinear optical parametric amplifier, the axis of the crystal may rotate amplifier, the axis and the wave-length of the cascaded sum frequency generation (SFG) can be tune. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The foregoing and other advantages of the invention will be more fully understood with reference to the description of the best embodiment an I the drawings wherein:  
         [0015]      FIG. 1 ( a ) is a schematic representation of the cascaded sum frequency optical parametric amplifier system  100  in according to one embodiment of the present invention.  
         [0016]      FIG. 1 ( b ) shows the propagation of the seeder for optical parametric generation (OPG).  
         [0017]      FIG. 2 ( a ) is a schematic representation of the parametric amplification process and the optical axis corresponding to the cascaded sum frequency non-collinear optical parametric amplifier of  FIG. 1 .  
         [0018]      FIG. 3  ( a ) is the spectrum of different sum frequency in the tuning range according to one embodiment of the present invention.  
         [0019]      FIG. 3  ( b ) is the spectrum of the double frequency generated by the idler in the architecture. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Referring to  FIG. 1 ,  FIG. 1 ( a ) is a schematic representation of the cascaded sum frequency optical parametric amplifier system  100  in according to one embodiment of the present invention. The cascaded in according to sum frequency optical parametric amplifier of this embodiment includes a frequency doublers apparatus  12  for generating excitation light source  16 , a low frequency seeder generating apparatus  15 (not shown), a non-linear optical crystal  14  for generating tunable wave-length of non-collinear optical parametric amplifier, and some clamping for fixing sample, reflecting mirror, time delayed apparatus, and a rotating table for rotating the crystal (all of them is not shown), such that the crystal  14  may rotate around the axis  22 , providing a crystal axis  22  to change the orientation of the crystal such that change the phase matching condition of the crystal.  
         [0021]     When a Ti: sapphire laser; with output power larger than 1 mj/pulse and wave-length approximately 800 nm of infrared incident light  15  transmits to a 5/95 beam splitter  1 , apart of approximately 5% reflects from the beam splitter go through the reflected light aperture  4 , is reflected 180 degree by the two silver mirror  9 , then focus by the 5 cm lens  5 , generating a seeder light  17  by the window  6  of a CaF 2  with 2 mm thick, the seeder  17  is reflected by the parabolic reflector  7  with focal length of 5 cm, reflected by the silver mirror  9 , and focused by the silver focus mirror with focus length 15 cm, then the seeder  17  is focused and go through a 2 mm thick BBO crystal for optical parametric amplification (OPA). On the other hand, most part of the incident light  15  go through the 5/95 beam splitter 1, then go through a tunable attenuator, i.e. a 30 cm convex lens 2 and a 15 cm concave lens  3 , after adjust the focus length, then reflected 180 degree by two silver mirror  9  on the sliding table  10 , and transmit through a 200 mm thick BBO crystal  12  for doubling the frequency. After doubling the frequency, it is a ˜400 nm blue light  16 . This second harmonic generation (SHG) is used for pumping non-collinear optical parametric amplification (NOPA). This blue light  16  is reflected by the blue light reflected mirror  8  and focused by the silver plating focus mirror  11  with focus length of 15 cm, then focusing on the 2 mm thick BBO crystal  14  for optical parametric amplification (OPA).  
         [0022]      FIG. 1 ( b ) shows the propagation of the seeder for optical parametric generation (OPG), the corresponding generated idler  20  will generate cascaded sum-frequency generation (SFG)  21  with the residue base frequency excitation light  19 . The frequency of the sum frequency excitation light  19 . The frequency of the sum frequency will be tunable changed with the direction change of the crystal axis  22  around the axis  23 . This provides a continuous tunable blue light output of optical amplifier.  
         [0023]     Referring to  FIG. 2 ( a ),  FIG. 2 ( a ) is a schematic representation of the parametric amplification process and the optical axis corresponding to the cascaded sum frequency non-collinear optical parametric amplifier of  FIG. 1 . wherein the definition of the symbols are as follow: θ s  is the angle between the optical axis  22  and the signal, θ i  is the angle between the optical axis  22  and the idler  20 , respectively, α is the angle between the seeder  17  and the exited light  18  (the ultra fluorescent light), δ is the angle between the generated idler  20  and the residue base frequency excitation light  19  for generating white light, δ′ is the angle between the corresponding sum frequency (SFG)  21  and the idler  20 . As shown in  FIG. 2 ( a ), this architecture implements the exciting light  18  and the seeder  17  under a type 1 non-collinear architecture, and the angle α is negative, the idler  20  generated by the OPA will satisfy the condition of phase matching to generate sum frequency of the residue base frequency.  
         [0024]     This embodiment has its corresponding modeling data, when cascaded sum frequency generation (SFG) occur, Energy conservation and phase matching of SFG can be cast into the form  
             {               h   _     ⁢     ω   SFG       =         h   _     ⁢     ω   i       +       h   _     ⁢     ω   800                         h   _     ⁢       k   -&gt;     SFG     (   e   )         =         h   _     ⁢       k   -&gt;     i     (   o   )         +       h   _     ⁢       k   -&gt;     800     (   o   )                           (   1   )             
 
         [0025]     Where ω SFG , ω i , and ω 800  are the frequencies of SFG, idler and base frequency respectively; k sfg , k i , and k 800  are the wave vectors of SFG, idler and base frequency respectively, the upper case of (e) and (o) polarization with respect to the optical axis, which perpendicular to the optical axis is o-ray, otherwise is e-ray. When Eq. 1 are both satisfied, δ′ can be the form  
               δ   ′     =       tan     -   1       ⁡     (           k   -&gt;     800     (   o   )       ⁢   sin   ⁢           ⁢   δ             k   -&gt;     800     (   o   )       ⁢   cos   ⁢           ⁢   δ     +       k   -&gt;     i     (   o   )           )               (   2   )             
 
 The theoretical calculated result is compared to the experiment data of the the above embodiment, please refer to  FIG. 2 ( b ), the vertical axis is the tuning range of SFG and the horizontal axis is the seeding angle α between the seeder and the OPA. The solid line is the calculated theoretical result of the SFG tuning range for different seeding angle α, the solid square is the calculated wave length of optimum phase matching, the solid star is the experimental wave length where the energy transform results in maximum power output in that seeding angle, the open circle and the open cross is the tuning angle for seeding angle of −8.4 degree and −14 degree respectively. The experimental and the theoretical results are in good consistent. The wave length of the maximum power output efficiency is consistent with that of the theoretical optimum phase matching point. The tuning range in different seeding angles is also in the theoretical range, the experiment result and the theoretical value are also consistent. 
 
         [0026]      FIG. 3  ( a ) and ( b )is the spectrum in the tuning range according to one embodiment of the present invention, this embodiment is the result for seeding angle of 8 degree.  FIG. 3  ( a ) is the result of different sum frequency and  FIG. 3  ( b ) is the spectrum of the double frequency generated by the idler in the architecture.  
         [0027]     Although specific embodiments of the invention have been disclosed, the specification and drawings are, accordingly, to be regarded as an illustration rather than a restrictive sense. It will, however, be understood by those having skill in the art that minor changes can be made to the form and details of the specific embodiments disclosed herein, without departing from the spirit and the scope of the invention.