Abstract:
Methods and systems for optical chirped pulse amplification and phase dispersion, the system including an active dispersion controller for receiving an input optical pulse from a modelocked laser and controlling a third and fourth order dispersion property of the input optical pulse to produce an optical output pulses, a stretching re-circulating loop for stretching the optical output pulses in time, an optical amplifier for amplifying the stretched optical output pulses, a compressing re-circulating loop for compressing the amplified stretched optical output pulse to produce a compressed optical output pulse, and a feedback loop for feeding a feedback optical signal to the active dispersion controller.

Description:
This application claims the benefit of priority to U.S. Provisional Application No. 60/900,634 filed on Feb. 9, 2007. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to lasers and, in particular, to methods, systems, apparatus and devices for a system for extreme pulse stretching, amplification and compression for an ultrashort pulse laser. 
     BACKGROUND AND PRIOR ART 
     In amplifiers for ultrashort optical pulses, the occurring optical peak intensities can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or of some other optical element may occur. This can be effectively prevented by employing the method of chirped-pulse amplification. 
     Before passing the amplifier medium, the pulses are chirped and temporally stretched to a much longer duration by means of a strongly dispersive element (the stretcher, e.g. a grating pair or a long fiber). This reduces the peak power to a level where the above mentioned detrimental effects in the gain medium are avoided. After the gain medium, a dispersive compressor is used, i.e., an element with opposite dispersion (typically a grating pair), which removes the chirp and temporally compresses the pulses to a duration similar to the input pulse duration. As the peak power becomes very high at the compressor, the beam diameter on the compressor grating has to be rather large. For the most powerful devices, a beam diameter of the order of one meter is required. 
     The method of chirped pulse amplification has allowed the construction of table-top amplifiers which can generate pulses with millijoule energies and femtosecond durations, leading to peak powers of several terawatts. For the highest peak powers in ultrashort pulses, amplifier systems consisting of several regenerative and/or multipass amplifier stages are used, which are mostly based on titanium-sapphire crystals. Such amplifiers can be used e.g. for high harmonic generation in gas jets. Large-scale facilities even reach peak powers in the petawatt range. 
     When ordinary holographic diffraction gratings are used for the compressor, the four reflections on gratings can easily cause a loss of approximately 50%. In order not to lose half of the power at the end, special transmission gratings, fabricated with electron beam lithography, have been developed with losses of only approximately 3% or even less per reflection (at least for one polarization direction), resulting in much better efficiency of chirped-pulse amplifier systems. Another possibility is to use volume Bragg gratings. A single such grating can be used as the stretcher and compressor. 
     Another approach to reduce the compressor losses is down chirped pulse amplification, where the stretcher uses anomalous dispersion so that the compressor can be a simple glass block with normal dispersion. 
     For ultrabroad optical spectra, as are associated with few-cycle laser pulses, the main challenge of the CPA technique is to obtain a sufficiently precise match of the dispersion of stretcher and compressor despite the large stretching/compressing ratio. This is difficult due to higher-order chromatic dispersion. On the other hand, systems for relatively long (picosecond) pulses require enormous amounts of chromatic dispersion, which are not easily provided. Therefore, CPA systems work best for pulse durations between roughly 20 fs and a few hundred femtoseconds. 
     The concept of chirped pulse amplification is also applied to fiber amplifiers. Due to the inherently high nonlinearity of long fibers, CPA has to be applied already for relatively low pulse energies, and even with strong temporal stretching of the pulses, the achievable pulse energies stay quite limited. However, high average powers of tens of watts or even &gt;100 W can be generated. Fiber-based CPA systems are therefore most suitable for high pulse repetition rates combined with high average powers. The fibers used for such systems should be optimized in various respects; they should have features such as e.g. a high gain per unit length, polarization-maintaining properties (strong birefringence), core-less end caps, etc. 
     Unfortunately, all-fiber solutions are normally not possible, since the temporal compression has to be done with a dispersive compressor with a mode area well above that of a fiber. There is some progress, though, towards air-guiding photonic crystal fiber compressors, which at least allow significantly higher pulse energies than previously considered to be realistic for fibers. 
     SUMMARY OF THE INVENTION 
     A primary objective of the invention is to provide apparatus, methods, systems and devices for an extreme pulse stretching, amplification and compression system for an ultrashort pulse laser. 
     A secondary objective of the invention is to provide apparatus, methods, systems and devices for signal from a system with a mode locked laser output pulse that is stretched in time, amplified and compressed a maximum of two amplification stages. 
     A third objective of the invention is to provide apparatus, methods, systems and devices for an extreme pulse stretching and compression circuit using commercially available fiber-based components. 
     A fourth objective of the invention is to provide apparatus, methods, systems and devices for a compact extreme pulse stretching and compression circuit for an ultrashort pulse laser. 
     A first embodiment provides an optical system for chirped pulse amplification and phase dispersion. The optical system includes an active dispersion controller for receiving an input optical pulse from a mode locked laser and controlling a third and fourth order dispersion property of the input optical pulse to produce an optical output pulses, a stretching re-circulating loop serially connected to the active dispersion controller for stretching the optical output pulses in time, an optical amplifier serially connected to the stretching circuit for amplifying the stretched optical output pulses, a compressing re-circulating loop serially connected to the output of the optical amplifier for compressing the amplified stretched optical output pulse to produce a compressed optical output pulse, and a feedback loop connected between an output of the compression re-circulating loop and the active dispersion controller for feeding a feedback optical signal to the active dispersion controller for adjusting the third and fourth order dispersive property of the active dispersion controller to actively fine tune the optical output pulses. 
     The stretching re-circulating loop includes a switch for coupling the stretching re-circulating loop between the mode locked laser output and the optical amplifier input to inject the input optical pulse into the stretching re-circulating loop, a stretching optical amplifier connected to the first switch having for receiving and amplifying the input optical pulse, a stretching circulator connected with an output of the stretching optical amplifier for receiving an amplified stretched optical pulse, a stretching chirped fiber Bragg grating, and a circulator connected to an output of the stretching optical amplifier and coupled with the stretching chirped fiber Bragg grating for stretching of the output optical pulse. 
     The compressing re-circulating loop includes a switch for coupling the compressing re-circulating loop between the optical amplifier output and the feedback loop to inject the amplified stretched optical signal into the compressing re-circulating loop, the compressing re-circulating loop producing the compressed output pulse, a compression optical amplifier for receiving and amplifying the amplified stretched optical pulse, a compressing circulator connected with an output of the compressing optical amplifier for re-circulating the compressed optical signal within the compressing re-circulating loop, and a compression chirped fiber Bragg grating coupled between the feedback loop and the compressing circulator for outputting a compression feedback pulse. 
     The optical system includes a feedback loop having an intensity auto correlation diagnostic device connected to the second switch for receiving a portion of the compressed output pulse and a proportional/integral/differential controller connected between the output of the auto correlation diagnostic device and the active dispersive controller for generating the feedback optical signal. 
     A second embodiment provides a method for generating an optical output pulse stretched in time, amplified and compressed. A dispersive property of an input optical pulse from a mode locked laser is adjusted, the adjusted optical pulse is stretched in a stretching re-circulating loop, then amplified and the amplified stretched optical pulse is compressed in a compression re-circulating loop to produce a compressed output optical pulse. A portion of the compressed output optical pulse is actively fine tuned and coupled into at least one of the stretching and the compressing re-circulating loops. 
     Further objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments which are illustrated schematically in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic block diagram showing the extreme stretch and compression configuration according to the present invention. 
         FIG. 2  is a schematic block diagram showing the extreme stretch and compression configuration using a single chirped fiber Bragg grating according to an embodiment of the present invention. 
         FIG. 3  is a schematic block diagram showing an alternative circuit diagram with each of the stretching and compressing loop each having a chirped fiber Bragg grating according to another embodiment of the present invention. 
         FIG. 4  is a schematic block diagram of for generating an arbitrary RF signal for phase compensation. 
         FIG. 5  is a schematic block diagram of the stretching loop. 
         FIGS. 6   a  and  6   b  are graphs showing the SPM of the laser pulse through the dispersion device showing the optical spectrum and pulse shape, respectively, with SPM. 
         FIGS. 6   c  and  6   d  are graphs showing the laser pulse through the dispersion device  520  showing the optical spectrum and pulse shape, respectively, without SPM. 
         FIG. 7  is a graph showing the EFDA gain spectrum. 
         FIG. 8   a  is a graph showing the RF spectrum with pulse picking at approximately 5 MHz. 
         FIG. 8   b  is a graph showing the optical pulse train with a pulse picking at approximately 1.25 MHz. 
         FIG. 9   a  is a graph showing the optical spectrum of the pre-stretched laser pulse after the pulse picker. 
         FIG. 9   b  is a graph showing the pulse shape of the pre-stretched laser pulse after the pulse picker. 
         FIG. 10  is a graph showing the switch window with a switching period and the switch pulse width. 
         FIGS. 11   a  and  11   b  are graphs showing the chirped fiber Bragg grating ripple and reflectance window, respectively. 
         FIG. 12  is a graph showing the semiconductor amplifier gain vs. DC current. 
     
    
    
     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 other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
     The following is a list of the reference numbers used in the drawings and the detailed specification to identify components: 
     
       
         
               
               
               
               
             
           
               
                   
               
             
             
               
                 100 
                 element 
                 292 
                 chirped fiber Bragg grating 
               
               
                 110 
                 modelocked laser 
                 295 
                 circulators 
               
               
                 120 
                 active dispersion device 
                 296 
                 chirped fiber Bragg grating 
               
               
                 130 
                 extreme stretching device 
                 300 
                 phase compensation circuit 
               
               
                 140 
                 optical amplifier 
                 310 
                 a sinewave generator 
               
               
                 150 
                 extreme compression stage 
                 320 
                 pulse generator 
               
               
                 170 
                 auto correlation device 
                 330 
                 circulator 
               
               
                 180 
                 PID controller 
                 335 
                 arbitrary waveform 
               
               
                 200 
                 alternative circuit diagram 
                 340 
                 phase modulator 
               
               
                 225 
                 2-x-2 switch 
                 350 
                 phase compensated output 
               
               
                 230 
                 stretching loop 
                 500 
                 stretching circuit 
               
               
                 235 
                 optical amplifier 
                 510 
                 laser source 
               
               
                 240 
                 optical amplifier 
                 520 
                 dispersion device 
               
               
                 245 
                 2-x-2 switch 
                 522 
                 EDFA 5 
               
               
                 250 
                 compressing loop 
                 525 
                 pulse picker 
               
               
                 255 
                 optical amplifier 
                 527 
                 2-by-2 switch 
               
               
                 260 
                 auto correlation 
                 535 
                 semiconductor optical 
               
               
                   
                   
                   
                 amplifier 
               
               
                 280 
                 PID controller 
                 592 
                 chirped fiber Bragg grating 
               
               
                 285 
                 PID controller 
                   
                   
               
               
                   
               
             
          
         
       
     
       FIG. 1  is a schematic block diagram showing the extreme stretch and compression configuration according to the present invention. As shown, optical output pulses are generated from a mode locked laser  110  and the optical output pulses are passed through an active dispersion device  120  that controls the third and fourth order dispersive terms of the laser output pulses. The optical pulse output of the active dispersion control device  120  is passed through and extreme stretching stage  130 , followed by optical amplification  140  and an extreme compression stage  150 . A portion of the amplified and compressed optical pulses  160  are characterized by optical intensity auto correlation techniques  170  and the resulting signal is used as an input signal to a feedback loop comprised of a proportional-integral-differential (PID) controller  180 . The output of the PID controller  180  is used to adjust the third and fourth order dispersive property of the active dispersion controller  120 , to maximize the optical intensity auto correlation measurement. 
       FIG. 2  is a schematic block diagram showing the extreme stretch and compression configuration using a single chirped fiber Bragg grating  190  and two re-circulating loop mirrors. As shown, the stretching loop includes a 2-×-2 lithium biobate switching device  125  and the compressing loops includes a 2-×-2 lithium biobate switching device  145 . The stretching loop  130  and the compressing loop  150  share a single chirped fiber Bragg grating  190  between two re-circulating loop mirrors  195  to realize the extreme stretching and compression. Both the stretching loop  130  and the compressing loop  150  include an optical amplifier  135  and  155 , respectively, within the loop. The salient feature using a single chirped fiber Bragg grating is that nonuniformities in the group delay of the chirped fiber Bragg grating, if they exist, that accumulate in the stretching loop  130  are completely removed in the compression loop  150 . 
     Alternatively, chirped fiber Bragg gratings that have a shorter physical length resulting in smaller group delay slopes have a uniform group delay, and hence separate or individual chirped fiber Bragg gratings are used.  FIG. 3  is a schematic block diagram showing the alternative circuit diagram  200  with each of the stretching loop  230  and the compressing loop  250  each having a chirped fiber Bragg grating  292  and  296 , respectively, as a tuning element. As with the first configuration shown in  FIG. 2 , the alternative configuration shown in  FIG. 3  includes an optical amplifier  235  and  255  within the stretching loop  230  and compression loops  250  and a separate re-circulating mirror  294  and  297  coupling the respective loop with the respective chirped fiber Bragg grating. The salient feature of using two separate chirped fiber Bragg gratings is that the optical isolation between the stretching and compression loops is physically achieved much easier. 
     In the second embodiment shown in  FIG. 3 , independent proportion-integration-differential controllers for each chirped fiber Bragg grating is required, however, owing to the potential to tune the dispersive properties of a chirped fiber Bragg grating by 50% (e.g. +/−500 psec on a 1000 psec total stretch) one has the potential to have complete continuous dispersion control spanning a range from a minimum of −500 psec to well over 10 nsec, where the upper limit is determined by the initial repetition rate of the modelocked laser and the number of round trips each pulse stays within the circulating loop. 
       FIG. 4  is a schematic block diagram of for generating an arbitrary RF signal for phase compensation. The phase compensation circuit  300  includes two input sources, a sinewave generator  310  with pulse delay adjustment and a pulse generator  320  that are fed into a circulator  330  to generate the output signal having an arbitrary waveform. The arbitrary waveform is modulated by a phase modulator  340  for active dispersion control to generate a phase compensated pulse with a phase intensity as shown in  FIG. 4 . The phase compensation circuit  300  is a cost effective method for generating an arbitrary RF signal to compensate for the phase distortions associated with optical amplification. 
     Experiments were conducted to show the effects of the extreme stretching according to the present invention. It will be obvious to those skilled in the art that the compression of the stretched and amplified output signal is achieved in a similar manner.  FIG. 5  is a schematic block diagram of the stretching loop circuit  500 . In this experiment, the laser source  510  in this example is a Calmar laser having a center wavelength of approximately 1552 nm, a spectral bandwidth of approximately 3 nm and a pulse width of approximately 546 fs. The optical output pulse has an output power of approximately 3 mWatt with a repetition rate of approximately 50 nsec.  FIG. 6   a  is a graph showing the optical spectrum of the optical output and  FIG. 6   a  is a graph showing the intensity auto correlation. 
     Referring back to  FIG. 5 , the dispersion control device  520  in the circuit has a dispersion of approximately 200 ps/nm and the output optical signal after dispersion is approximately 2 mWatt with a spectral bandwidth of approximately 10 nm.  FIGS. 6   a  and  6   b  are graphs showing the SPM of the laser pulse through the dispersion device  520  showing the optical spectrum and pulse shape, respectively, with SPM.  FIGS. 6   c  and  6   d  are graphs showing the laser pulse through the dispersion device  520  showing the optical spectrum and pulse shape, respectively, without SPM. As shown in  FIGS. 6   c  and  6   a , the dispersion device output pulse without SPM has an output power of approximately 2.13 mW while the output pulse with SPM is approximately 3.25 mW and the resulting output pulse shape shown in  FIG. 6   d  is much sharper than the pulse with SPM shown in  FIG. 6   b.    
     The output pulse from the dispersion device  520  is fed into the erbium doped fiber amplifiers (EDFA)  522  which has a pump power of approximately 120 mA and produces an output pulse after EDFA of approximately 16 mW at approximately 2.5 MHz. As shown in  FIG. 5 , the EDFA output pulse is fed into an AM pulse picker  525  which has an approximately 27 dB extinction and an optical insertion loss of approximately 4.5 dB. The output power of the optical signal after the pulse picker  525  is in a range of approximately 0.37 mW at 1.25 MHz to approximately 0.75 mW at approximately 2.5 MHz repetition rate.  FIGS. 8   a  and  8   b  are graphs showing the RF spectrum with pulse picking at approximately 5 MHz and optical pulse train with a pulse picking at approximately 1.25 MHz, respectively.  FIGS. 9   a  and  9   b  are graphs showing the optical spectrum and the pulse shape, respectively, of the pre-stretched laser pulse after the pulse picker. 
     Referring back to  FIG. 5 , the output pulse from the pulse picker  125  is fed into a 2×2 optical switch  527  which transfers a portion of the pre-stretched laser pulse into the stretching loop  530  where the laser pulse circulates through the chirped fiber Bragg grating  592  and the semiconductor optical amplifier  535  that is used as a continuous wave source at approximately 0.5 mW. The switching window has a switch period of approximately 200 ns and a switch pulse width of approximately 40 ns as shown in  FIG. 10 . Still referring to  FIG. 5 , the chirped fiber Bragg grating  592  in the stretching loop  530  has a dispersion of approximately 50 ps/nm with an optical insertion loss in a range of approximately +8 dB and approximately −3 dB. The output power of the pulse after the chirped fiber Bragg gratings is approximately 0.06 mW at a repetition rate of approximately 2.5 MHz.  FIGS. 11   a  and  11   b  are graphs showing the chirped fiber Bragg grating ripple and reflectance window, respectively. 
     In the stretching loop  530 , the pulse from the chirped fiber Bragg grating  592  is fed into the semiconductor optical amplifier  535 . The output power of the pulse after amplification is approximately 0.27 mW with a pulse bias of approximately 0.2 mW at approximately 2.5 MHz repetition rate.  FIG. 12  is a graph showing the semiconductor amplifier gain vs. DC current. 
     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.