Patent Publication Number: US-2009225794-A1

Title: High Energy All Fiber Mode Locked Fiber Laser

Description:
The inventor claims priority to provisional patent application No. 61/068,835 filed on Mar. 10, 2008 and provisional patent application No. 61/068,750 filed on Mar. 10, 2008. 
    
    
     BACKGROUND  
     The invention relates generally to the field of using an all fiber ring cavity laser for generating ultra-short high power and high energy laser pulses. 
     SUMMARY  
     In one respect, disclosed is a self-start, seed, mode locked fiber laser comprising: a pump laser; a WDM coupler to couple the pump laser into a Ytterbium doped fiber, where the Ytterbium doped fiber is coupled into a first single mode fiber; a bandpass filter coupled to the first single mode fiber output and to a second single mode fiber; a first in-line polarization controller coupled to the second single mode fiber output and an in-line polarization beam splitter, where the in-line polarization beam splitter comprises a single mode fiber output and a polarization maintaining fiber output, where the polarization maintaining fiber is configured for an output laser pulse; a polarization insensitive isolator coupled to the single mode fiber output of the in-line polarization beam splitter; a second in-line polarization controller coupled to the polarization insensitive isolator and a third single mode fiber, where the third single mode fiber output is coupled into the WDM coupler. 
     In another respect, disclosed is a high energy, ultra-short, mode locked fiber laser system comprising: a seed laser comprising: a pump laser; a WDM coupler to couple the pump laser into a Ytterbium doped fiber, where the Ytterbium doped fiber is coupled into a first single mode fiber; a bandpass filter coupled to the first single mode fiber output and to a second single mode fiber; a first in-line polarization controller coupled to the second single mode fiber output and an in-line polarization beam splitter, where the in-line polarization beam splitter comprises a single mode fiber output and a polarization maintaining fiber output, where the polarization maintaining fiber is configured for an output laser pulse; a polarization insensitive isolator coupled to the single mode fiber output of the in-line polarization beam splitter; a second in-line polarization controller coupled to the polarization insensitive isolator and a third single mode fiber, where the third single mode fiber output is coupled into the WDM coupler where the seed laser is configured to couple the output laser pulses to a preamplifier; a high power amplifier coupled to the output of the preamplifier; and a compressor coupled to the output of the high power amplifier. 
     In yet another respect, disclosed is a method for generating high energy, ultra-short laser pulses, the method comprising: generating electromagnetic radiation from a pump laser; coupling the pump laser electromagnetic radiation to a Ytterbium doped fiber using a WDM coupler; coupling the output from the Ytterbium doped fiber to a first single mode fiber; coupling a bandpass filter to the first single mode fiber output and to a second single mode fiber; coupling a first in-line polarization controller to the second single mode fiber output and an in-line polarization beam splitter comprising a single mode fiber output and a polarization maintaining fiber output configured to emit an output laser pulse; coupling a polarization insensitive isolator to the single mode fiber output of the in-line polarization beam splitter and to a second in-line polarization controller; coupling a third single mode fiber output to the second in-line polarization controller and to the WDM coupler; coupling the output laser pulse to a preamplifier; coupling the preamplifier output to a high power amplifier; and coupling the high power amplifier output to a compressor. 
     Numerous additional embodiments are also possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Other objects and advantages of the invention may become apparent upon reading the detailed description and upon reference to the accompanying drawings. 
         FIG. 1  is a block diagram illustrating a seed all fiber mode locked fiber laser, in accordance with some embodiments. 
         FIG. 2  is a graph showing the repetition rate as a function of fiber length in a ring laser cavity, in accordance with some embodiments. 
         FIG. 3  is a graph showing the chirped pulse width as a function of the fiber cavity length and the bandpass filter bandwidth, in accordance with some embodiments. 
         FIG. 4  is a schematic showing the details of the polarization beam splitter using a polarization cube, in accordance with some embodiments. 
         FIG. 5  is a schematic showing the details of the polarization beam splitter using a birefringence crystal, in accordance with some embodiments. 
         FIG. 6  is a block diagram illustrating a conventional chirped pulse amplification fiber laser system and an all fiber, high energy, ultra-short, mode locked fiber laser system without a pulse picker and fiber stretcher, in accordance with some embodiments. 
         FIG. 7  is a flow diagram illustrating a method to generate ultra-short high power laser pulses, in accordance with some embodiments. 
     
    
    
     While the invention is subject to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and the accompanying detailed description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular embodiments. This disclosure is instead intended to cover all modifications, equivalents, and alternatives falling within the scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION  
     One or more embodiments of the invention are described below. It should be noted that these and any other embodiments are exemplary and are intended to be illustrative of the invention rather than limiting. While the invention is widely applicable to different types of systems, it is impossible to include all of the possible embodiments and contexts of the invention in this disclosure. Upon reading this disclosure, many alternative embodiments of the present invention will be apparent to persons of ordinary skill in the art. 
     In some embodiments, an all fiber mode locked laser can generate mode locked femtosecond and picosecond pulses by utilizing components for polarization and spectral shaping. By varying the fiber cavity length, the repetition rate can vary from 100 MHz to 50 Khz and by also adjusting the position of the output coupler, the pulse width can range from 200 femtoseconds to 1 nanosecond. Such a low repetition rate laser can be used as the seed for a high energy mode locked fiber laser system at 1 micron, thus eliminating the need for a pulse picker, stretcher, and a couple of stages of amplifiers. 
       FIG. 1  is a block diagram illustrating a seed all fiber mode locked fiber laser, in accordance with some embodiments. 
     In some embodiments, a seed mode locked fiber laser  110  comprises a pump laser  115  coupled with a WDM coupler  120 . The WDM coupler  120  couples the pump laser  115  into the gain medium of Ytterbium doped fiber  125 . The output from the Ytterbium doped fiber  125  is coupled into a first single mode fiber  130 . The output of the first single mode fiber  130  is coupled into bandpass filter  135 . The bandpass filter  135  is then coupled to a second single mode fiber  140  that is then coupled to a first in-line polarization controller  145 . The output of the first in-line polarization controller  145  is then coupled with an in-line polarization beam splitter  150  having a single mode fiber output  155  and a polarization maintaining output  160 . The single mode fiber output  155  is then coupled with a polarization insensitive isolator  165 . A second in-line polarization controller  170  is then connected to the polarization insensitive isolator  165 . The output of the second in-line polarization controller  170  is then coupled into a third single mode fiber  175 . The output of the third single mode fiber  175  is then coupled back into the WDM coupler  120 , thus completing the all fiber ring cavity. 
       FIG. 2  is a graph showing the repetition rate as a function of fiber length in a ring laser cavity, in accordance with some embodiments. 
     In some embodiments, the pulse repetition rate can be lowered by using a longer fiber cavity length. A 20 m fiber in the cavity will result in a pulse repetition rate of 10 MHz; whereas a 2 km fiber cavity will result in a pulse repetition rate of 100 kHz. Thus by using a longer fiber in the cavity and lowering the pulse repetition rate in the seed laser, the need for using a pulse picker in the overall laser system is eliminated. 
       FIG. 3  is a graph showing the chirped pulse width as a function of the fiber cavity length and the bandpass filter bandwidth, in accordance with some embodiments. 
     In some embodiments, the need for a stretcher in a high power, high energy ultra-short fiber laser system can be eliminated by using a longer fiber cavity in the seed along with a bandpass filter within the ring cavity of the seed laser.  FIG. 3  shows the dependence of the chirped pulse width as a function of the fiber length in the cavity of the seed laser and the bandwidth of the bandpass filter of the seed laser, assuming a cavity with only the second single mode fiber  140  and without the first single mode fiber  130  and third single mode fiber  175 . The chirped pulse width increases with increasing cavity fiber length and broader bandwidth of bandpass filter. 
       FIG. 4  is a schematic showing the details of the polarization beam splitter using a polarization cube, in accordance with some embodiments. 
     In some embodiments, the polarization beam splitter  410  is of a special design. In conventional polarization beam splitters, the two output fibers consist of polarization maintaining fibers. In the special design for the polarization beam splitter  410 , the collimator  415  from the input fiber hits a polarization splitter cube  420  and then splits the beam into one single mode fiber  425  and a polarization maintaining fiber  430 . The single mode fiber output  425  is coupled back into the ring cavity of the all fiber seed laser. This insures a stable mode locking mechanism. The polarization maintaining output fiber  430  is used as the seed for the high energy ultra-short fiber laser system. 
       FIG. 5  is a schematic showing the details of the polarization beam splitter using a birefringence crystal, in accordance with some embodiments. 
     In some embodiments, polarization beam splitter  510  uses the double refraction of a birefringence crystal  515  to generate an ordinary wave  520  and an extraordinary wave  525  from a single collimated input beam  530 . The split beams are coupled into two output fibers, one single mode fiber  535  and the other polarization maintaining fiber  540 . 
     As in output fibers from the polarization beam splitter using a polarization cube, the single mode fiber  535  is coupled into the ring cavity and the polarization maintaining fiber  540  is used as the polarized laser output from the seed. 
       FIG. 6  is a block diagram illustrating a conventional chirped pulse amplification fiber laser system and an all fiber, high energy, ultra-short, mode locked fiber laser system without a pulse picker and fiber stretcher, in accordance with some embodiments. 
     In some embodiments, the conventional chirped pulse amplification fiber laser system  610  consists of a standard seed laser  615  having narrow pulses with pulse repetition rates of 20 Mz to 200 Mz thus requiring a stretcher  620 , a pulse picker  625 , and an amplifier chain  630 , including a high power amplifier  645 , in order to achieve a high energy, ultra-short, mode locked fiber laser system. By using a low pulse repetition rate seed laser  635 , the stretcher  620 , pulse picker  625 , and a couple amplifier stages of the amplifier chain  630  can be eliminated from the conventional chirped pulse amplification fiber laser system. In both systems, a preamplifier  640 , a high power amplifier  645 , and compressor  650  are still utilized. The preamplifier may include one or two stages of Ytterbium doped fiber amplifiers. Depending on the desired output energy levels, microjoules or millijoules, one or two high power amplifiers may be used. The compressor can either be a grating or fiber type. 
       FIG. 7  is a flow diagram illustrating a method to generate ultra-short high power laser pulses, in accordance with some embodiments. In some embodiments, the method illustrated in  FIG. 7  may be performed by one or more of the devices illustrated in  FIG. 1 ,  FIG. 4 ,  FIG. 5 , and  FIG. 6 . 
     Processing begins with a 980 nm laser pump diode  710  coupled into a WDM  715 . The WDM  715  may either be a 980/1030 or 980/1060 WDM coupler. The WDM  715  is then coupled into a gain medium of Ytterbium doped fiber  720  having a high doping concentration ranging between 10,000 ppm to 2,000,000 ppm. The Ytterbium doped fiber  720  amplifies the pulses circulating in the cavity. A first single mode fiber  725 , such as HI 1060 fiber or SM  25 , is coupled to the output of the Ytterbium doped fiber  720 . After the first single mode fiber  725 , a bandpass filter  730  is used to select the wavelength and stabilize the mode locking of the ring cavity. The bandpass filter  730  has a bandwidth between 1 nm to 20 nm. A second single mode fiber  735 , such as HI 1060 fiber or SM  25 , follows the bandpass filter  730 . Next a first fiber based in-line polarization controller  740  is used to control the polarization of the pulse before entering an in-line polarization beam splitter  745 . When the pulse passes through the polarization beam splitter  745 , only the highest intensity lined up with the splitter will pass and the lower intensity part of the pulse will be filtered. This results in the pulse being shaped and works as a saturable absorber to induce mode locking. The in-line polarization beam splitter  745  may either split the beam using a polarization splitter cube or a birefringence crystal. In both cases, the beam is split into a single mode fiber  750  that is coupled back into the ring of the cavity to insure a stable mode-locking mechanism and a polarization maintaining fiber output  755  to be used as the seed laser in high energy, ultra-short, mode locked fiber laser system. The non-polarization maintaining single mode output fiber  750  from the in-line polarization beam splitter  745  is coupled to a polarization insensitive isolator  760 . The polarization insensitive isolator  760  is used in the ring cavity to insure unidirectional propagation. A second in-line polarization controller  765  follows the isolator  760 . Finally a third single mode fiber  770 , such as HI 1060 fiber or SM  25 , completes the ring cavity by coupling the second in-line polarization controller  765  to the WDM  715 . 
     The just described all fiber seed laser is polarized, self start, and operates at all normal dispersion and without any anomalous dispersion (B″&lt;0) components to achieve stable mode locking pulses with a pulse repetition rate between 50 kHz to 100 MHz. The mode lock mechanism is created by both polarization shaping, due to self phase modulation induced polarization change, and spectral shaping resulting from the bandwidth of the band pass filter. By adjusting the position and lengths of the three single mode fiber segments, the chirped output width can vary from 1 to 30,000 times the dechirped pulse width. The output pulse width can be chirped from 100 fs to 3 ns and the chirped output pulses can be dechirped from 10 fs to 10 ps. The total fiber length in the cavity can range from 1 m to 3000 m. The output spectrum bandwidth of the seed laser ranges from 0.5 nm to 30 nm and has a center lasing wavelength between 1025 nm to 1100 nm. 
     In order to generate ultra-short high power laser pulses, the polarized output from the seed laser  755  is first coupled into a preamplifier  775  consisting of one or two stages of Ytterbium doped fibers. Next, depending on the desired output energy levels of microjoules or millijoules, the preamplifier  775  is coupled to one or two high power amplifiers  780 , respectively. Finally, the pulses are compressed using either a grating or fiber type compressor  785 , resulting in ultra-short high power laser pulses  790 . 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     The benefits and advantages that may be provided by the present invention have been described above with regard to specific embodiments. These benefits and advantages, and any elements or limitations that may cause them to occur or to become more pronounced are not to be construed as critical, required, or essential features of any or all of the claims. As used herein, the terms “comprises,” “comprising,” or any other variations thereof, are intended to be interpreted as non-exclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements, and may include other elements not expressly listed or inherent to the claimed embodiment. 
     While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed within the following claims.