Patent Publication Number: US-2011075686-A1

Title: High energy, all fiber, mode locked fiber laser

Description:
I. BACKGROUND 
     The invention relates generally to the field of using an all fiber ring cavity laser for generating high energy, mode locked femtosecond and picosecond pulses. 
     II. SUMMARY 
     In one respect, disclosed is an all fiber, mode locked fiber laser comprising: a pump laser; and a ring cavity comprising: a pump/signal coupler comprising an input and an output, wherein the pump laser is coupled to the input of the pump/signal coupler; a cladding pumped or double cladding fiber comprising a doped gain medium and an input and an output, wherein the input of the doped fiber is coupled to the output of the pump/signal coupler; a first fiber comprising an input and an output, wherein the input of the first fiber is coupled to the output of the doped fiber; a bandpass filter comprising an input and an output, wherein the input of the bandpass filter is coupled to the output of the first fiber; a second fiber comprising an input and an output, wherein the input of the second fiber is coupled to the output of the bandpass filter; a first in-line polarization controller comprising an input and an output, wherein the input of the first in-line polarization controller is coupled to the output of the second fiber; an in-line polarization beam splitter comprising an input, a polarization maintaining output configured to emit a laser pulse out of the ring cavity, and a non-polarization maintaining fiber output, wherein the input of the in-line polarization beam splitter is coupled to the output of the first in-line polarization controller; a polarization insensitive isolator comprising an input and an output, wherein the input of the polarization insensitive isolator is coupled to the non-polarization maintaining fiber output of the in-line polarization beam splitter; a second in-line polarization controller comprising an input and an output, wherein the input of the second in-line polarization controller is coupled to the output of the polarization insensitive isolator; a third fiber comprising an input and an output, wherein the input of the third fiber is coupled to the output of the second in-line polarization controller and the output of the third fiber is coupled to the input of the pump/signal coupler; and wherein the ring cavity is configured to have no components at anomalous dispersion. 
     In another respect, disclosed is a method for generating mode locked, femtosecond and picosecond laser pulses, the method comprising: generating electromagnetic radiation from a pump laser; and coupling the pump laser electromagnetic radiation to a ring cavity comprising: a pump/signal coupler comprising an input and an output, wherein the pump laser is coupled to the input of the pump/signal coupler; a cladding pumped or double cladding fiber comprising a doped gain medium and an input and an output, wherein the input of the doped fiber is coupled to the output of the pump/signal coupler; a first fiber comprising an input and an output, wherein the input of the first fiber is coupled to the output of the doped fiber; a bandpass filter comprising an input and an output, wherein the input of the bandpass filter is coupled to the output of the first fiber; a second fiber comprising an input and an output, wherein the input of the second fiber is coupled to the output of the bandpass filter; a first in-line polarization controller comprising an input and an output, wherein the input of the first in-line polarization controller is coupled to the output of the second fiber; an in-line polarization beam splitter comprising an input, a polarization maintaining output configured to emit a laser pulse out of the ring cavity, and a non-polarization maintaining fiber output, wherein the input of the in-line polarization beam splitter is coupled to the output of the first in-line polarization controller; a polarization insensitive isolator comprising an input and an output, wherein the input of the polarization insensitive isolator is coupled to the non-polarization maintaining fiber output of the in-line polarization beam splitter; a second in-line polarization controller comprising an input and an output, wherein the input of the second in-line polarization controller is coupled to the output of the polarization insensitive isolator; a third fiber comprising an input and an output, wherein the input of the third fiber is coupled to the output of the second in-line polarization controller and the output of the third fiber is coupled to the input of the pump/signal coupler; and wherein the ring cavity is configured to have no components at anomalous dispersion. 
     Numerous additional embodiments are also possible. 
    
    
     
       III. 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 nonlinear polarization, pulse shaping and spectral shaping, all fiber, mode locked fiber laser at one micron, 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. 
     
    
    
     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. 
     IV. 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 (fs) and picoseconds (ps) pulses by utilizing components for polarization and spectral shaping. By varying the fiber cavity length, the repetition rate can vary from 100 MHz to 10 kHz and by also adjusting the position of the output coupler, the output pulse width can range from 100 femtoseconds to 100 nanoseconds. The output pulse can be compressed from 10 fs to 100 ps. 
       FIG. 1  is a block diagram illustrating a nonlinear polarization, pulse shaping and spectral shaping, all fiber, mode locked fiber laser at one micron, in accordance with some embodiments. 
     In some embodiments, a nonlinear polarization, pulse shaping and spectral shaping, all fiber, mode locked fiber laser, as shown in block  110 , comprises a ring cavity laser comprising a pump laser  115  coupled with a pump/signal coupler  120 . The pump laser  115  may be any high power laser diode emitting electromagnetic radiation between 905 nm to 985 nm. In this embodiment a 976 nm diode laser was selected as the pump laser in matching with the highest absorption peak of Ytterbium (Yb) gain medium. The pump/signal coupler  120  may be a WDM coupler such as either a 980/1060 or a 980/1030 coupler and can be either a fiber bundled combiner or coupler type. The pump/signal coupler  120  couples the laser pulses from the pump laser  115  into the gain medium of a rare Earth doped fiber. Depending on the desired lasing wavelength, dopants such as Ytterbium, Erbium (Er), and Thulium (Tm) and co-dopants such Erbium/Ytterbium may be used as the gain medium in the fiber amplifier component of the ring cavity laser. In this embodiment, a cladding pumped or double cladding Ytterbium doped fiber  125 , having a high doping concentration ranging between 10,000 ppm to 2,000,000 ppm, was used to amplify the laser pulses. For the Ytterbium doped fiber with double cladding, the core can be single mode (SM), large mode area (LMA), and/or multi-mode (MM) fiber. The amplified laser pulses from the Ytterbium doped fiber  125  output are coupled into a first fiber  130 . The first fiber  130  can be single mode fiber, such as HI 1060 fiber or SM 25 fiber, LMA fiber, and/or MM fiber. The output of the first single mode fiber  130  is coupled into a bandpass filter  135  to select the wavelength and stabilize the mode locking of the ring cavity. The bandpass filter  135  has a bandwidth between 1 nm to 100 nm. The bandpass filter  135  is then coupled to a second fiber  140 . The second fiber  140  can be single mode fiber, such as HI 1060 fiber or SM 25 fiber, LMA fiber, and/or MM fiber. The second fiber  140  is then coupled to a first fiber based, 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 non-polarization maintaining fiber output  155  and a polarization maintaining output  160 . When the laser pulse passes through the in-line polarization beam splitter  150 , only the highest intensity that is aligned with the in-line polarization beam splitter  150  will pass and the lower intensity part of the laser pulse will be filtered, thereby shaping the laser pulse and working like a saturable absorber to induce mode locking. The in-line polarization beam splitter  150  may either split the laser beam using a polarization splitter cube or a birefringence crystal. In both cases, the laser beam is split into a non-polarization maintaining fiber  155  that is coupled back into the ring of the cavity to insure a stable mode-locking mechanism and a polarization maintaining fiber output  160 . Both the polarization maintaining fiber output  160  and non-polarization maintaining fiber output  155  can be single mode fiber, such as HI 1060 fiber or SM 25 fiber, LMA fiber, and/or MM fiber. One alternative is to output the laser pulses in free space without the output fiber. The non-polarization maintaining fiber output  155  is then coupled to a polarization insensitive isolator  165 . The polarization insensitive isolator  165  is used in the ring cavity to insure unidirectional propagation. A second fiber based, 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 fiber  175 . The third fiber  175  can be single mode fiber, such as HI 1060 fiber or SM 25 fiber, LMA fiber, and/or MM fiber. The output of the third single mode fiber  175  is then coupled back into the pump/signal coupler  120 , thus completing the all fiber ring cavity. The fibers used in the components of the ring cavity such as the pump coupler  120 , the bandpass filter  135 , the first and second in-line polarization controller  145  and  170 , the polarization insensitive isolator  165 , and the in-line polarization beam splitter  150  can be single mode fiber, such as HI 1060 fiber or SM 25 fiber, LMA fiber, and/or MM fiber. The output power levels from the polarization maintaining fiber output  160  can be changed by adjusting the two in-line polarization controllers  145  and  170  and the in-line polarization beam splitter  150 . 
     By changing the gain medium from Yb fiber to Er doped or Er/Yb co-doped fiber, or Tm doped fiber, the invention can work for other wavelengths in 1550 nm region (Er or Er/Yb) and 1950 nm region (Tm). 
       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, the pulse repetition rate in the laser can be lowered. 
       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, at a given pulse repetition rate, changing the bandpass filter bandwidth can achieve different pulse operation with various pulse bandwidth and pulse width in combination with fiber position.  FIG. 3  shows the dependence of the chirped pulse width as a function of the fiber length in the cavity of the laser and the bandwidth of the bandpass filter of the laser, assuming a cavity with only the second fiber  140  and without the first fiber  130  and third 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 is of a special design. In conventional polarization beam splitters  410 , the collimator  415  from the input fiber comprising SM, LMA, and/or MM fiber  420  hits a polarization splitter cube  425  and then splits the beam into two polarization maintaining output fibers comprising SM, LMA, and/or MM fiber,  430  and  435 . In the special design for the polarization beam splitter  440 , the collimator  445  from the input fiber comprising SM, LMA, and/or MM fiber  450  hits a polarization splitter cube  455  and then splits the beam into a non-polarization maintaining output fiber comprising SM, LMA, and/or MM fiber  460  and a polarization maintaining fiber comprising SM, LMA, and/or MM fiber  465 . The non-polarization maintaining output fiber  460  is coupled back into the ring cavity of the all fiber, mode locked fiber laser. This insures a stable mode locking mechanism. The polarization maintaining output fiber  465  is used as the output from the high energy, all fiber, mode locked fiber laser. 
       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 an input fiber comprising SM, LMA, and/or MM fiber  530  and collimator  535 . The split beams are coupled into two output fibers, one non-polarization maintaining fiber comprising SM, LMA, and/or MM fiber  540 , and the other polarization maintaining fiber comprising SM, LMA, and/or MM fiber  545 . As in the output fibers from the polarization beam splitter using a polarization cube, the non-polarization maintaining fiber  540  is coupled into the ring cavity and the polarization maintaining fiber  545  is used as the output from the high energy, all fiber, mode locked fiber laser. 
     The just described high energy, all fiber, mode locked fiber laser is self start, and operates at all normal dispersion and without any anomalous dispersion (B″&lt;0) fibers or components to achieve stable mode locking pulses having energy between 100 pJ to 1 mJ and a with a pulse repetition rate between 10 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 fiber segments, the chirped output width can vary from 1 to 1,000,000 times the dechirped pulse width. The output pulse width can be chirped from 100 fs to 100 ns and the chirped output pulses can be dechirped from 10 fs to 100 ps. The total fiber length in the cavity can range from 1 m to 20,000 m. The output spectrum bandwidth of the laser ranges from 0.5 nm to 150 nm and has a center lasing wavelength, when using a Ytterbium gain medium, between 1010 nm to 1150 nm. The output from the laser may either be polarized or non-polarized and may either be coupled to a fiber or be free space. 
     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.