Abstract:
An optical apparatus comprising: a source and a loop. The source generates a pump. The resonating cavity of the source includes: a gain medium; and a tunable filter for selecting a wavelength. The loop comprises: an input coupler; a waveguide; and an output coupler. The input coupler receives the pump and a signal and outputs the pump and the signal into the waveguide In the waveguide, energy in the pump is transferred into energy in the signal while a relative center position of the signal is crossing a center position of the pump in a first direction while both are passing through the waveguide and into the output coupler. The output coupler r outputs a first portion of the signal and a second portion of the signal is fed into the input coupler as the signal, completing the loop.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/184,356, filed Jun. 25, 2015. U.S. Provisional Application No. 62/184,356 is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This disclosure generally relates to fiber based optical parametric oscillators. 
     Description of Related Art 
     Light sources based on optical parametric interaction are interesting since they provide access to laser wavelengths that existing gain materials based on electronic transitions cannot provide. An optical parametric oscillator (OPO) can be realized by exploiting the χ (2)  nonlinear optical response in a wide range of crystals or the χ (3)  nonlinear response in optical fibers. 
     Optical fiber based OPO (FOPO) are particularly attractive owning to their potential in achieving low cost, alignment-free and compact laser systems while still providing very wide tuning range and high power operation. 
     The operation of FOPOs is in essence based on degenerated four-wave-mixing (FWM) wherein two pump photons interact with the fiber to generate a signal photon and an idler photon. The exact frequencies of the signal and idler photons are defined by the phase matching condition which depends on the pump laser wavelength, its peak power as well as the dispersion profile of the optical fiber of the FOPO. There are two common ways to pump an OPO. The first approach is continuous pumping where the pump laser is a continuous wave laser or a laser generating long pulses compared to the OPO cavity round trip time. The second approach is based on synchronous pumping of pulsed pump laser. In the context of the present application, the optical frequency of signal photon may be greater than the optical frequency of the idler photon; or the optical frequency of signal photon may be less than the optical frequency of the idler photon. This has been done to simplify the explanation, a resonating ring cavity may be designed to operate to produce either a signal pulse or an idler pulse, while the description below has been written in terms of a signal pulse, but an embodiment may be made to operate to output either a signal pulse, an idler pulse, or both. 
     Pulsed lasers tend to have a broader output optical spectrum than continuous lasers. One way to narrow the spectral bandwidth of a pulsed laser is with the use of a spectral filter, preferably tunable, that is outside of the laser cavity. In which case, the spectral bandwidth of the pulsed laser is influenced mainly by the spectral shape of the spectral filter. Since, the spectral shape of the pump light is tuned with a wavelength tunable external filter; this makes it difficult for a narrow spectral bandwidth to be achieved. This is because the spectral bandwidth of the pump light is almost same as the spectral bandwidth of the tunable external filter. In which case, the conversion efficiency from pump light to signal light is drastically decreased due to the broad spectral bandwidth of the pump light. As a result, it is difficult to achieve a high peak power of the signal light. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment may be an optical apparatus comprising: an optical source and an optical loop. The optical source may generate a pump pulse with a first optical wavelength. The resonating cavity of the optical source may include: a gain medium; and a wavelength tunable filter for selecting light as the first optical wavelength. The optical loop may comprise: an input optical coupler; an optical waveguide; and an output optical coupler. The input optical coupler may comprise: a first port for receiving the pump pulse; a second port for receiving a signal pulse, wherein the signal pulse has a second optical wavelength different from the first optical wavelength; and a third port for outputting the pump pulse and the signal pulse. The optical waveguide may comprise: a fourth port for receiving the pump pulse and the signal pulse; a fifth port outputting the pump pulse and the signal pulse. Wherein in the optical waveguide energy in the pump pulse may be transferred into energy in the signal pulse while a relative center position of the signal pulse is crossing a center position of the pump pulse in a first direction while both are passing through the waveguide. The output optical coupler may comprise: a sixth port for receiving the pump pulse and the signal pulse; a seventh port for outputting a first portion of the signal pulse; and an eighth port for outputting a second portion of the signal pulse. Wherein, the first portion of the signal pulse from the seventh port of the output coupler may be fed into the second port as the signal pulse, completing the optical loop. 
     In an embodiment, the optical source may further comprise: wherein the resonating cavity may be a linear cavity; a saturable absorber may form a first terminus of the linear cavity; wherein the optical source may be passive mode locked to produce the pump pulse at the first optical wavelength; a dispersion compensator may form a second terminus of the linear cavity; the wavelength tunable filter is between the saturable absorber and the dispersion compensator in the linear cavity; and a doped optical fiber having linear optical gain is also between the saturable absorber and the dispersion compensator in the linear cavity. 
     In an embodiment, within the resonating cavity of the optical source may include a variable delay line. 
     In an embodiment, the saturable absorber may mode lock the optical source. 
     In an embodiment, the following equations may be satisfied: 0.1 nm≦Δλ≦1.0 nm; and D cavity ×L≧1 ps/nm. Wherein Δλ is the bandwidth of the tunable filter at full width half maximum. Wherein D cavity  is the dispersion parameter of the cavity of the optical source. Wherein L is the total cavity length of the optical source. 
     In an embodiment, the dispersion parameter D cavity  of the optical source is kept within the limit of following equation by a chirp in a chirped fiber Bragg grating that acts as a dispersion compensator of the cavity of the optical source: D cavity ×L≧1 ps/nm. 
     In an embodiment, the energy of the pump pulse may be converted to the energy of the signal pulse by a parametric process in the waveguide. 
     In an embodiment, a third idler pulse may be produced by the waveguide. 
     In an embodiment, the optical waveguide may comprise a photonic crystal fiber. 
     In an embodiment, the optical loop may further comprise a second waveguide. The length of the second waveguide may satisfy the following equation: L 3 ≧t/D pulse . Wherein L 3  is the length of the third waveguide. Wherein t is the pulse duration of the pump pulse. D pulse  is the dispersion parameter of the signal pulse in the second waveguide. 
     In an embodiment, the second waveguide may change a chirp in the signal pulse produced by the optical waveguide. 
     In an embodiment, the chirp in the second pulse may be positive. 
     In an embodiment, the chirp in the second pulse may be negative. 
     In an embodiment, the second waveguide may be located between the seventh port of the output coupler and the second port of the input coupler, and the first portion of the signal pulse is fed through the second waveguide. 
     In an embodiment, the optical apparatus, may further comprise: an optical amplifier between the optical source and the first port of the input coupler that amplifies the optical power of the pump pulse. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is an illustration of an embodiment. 
         FIG. 2  is an illustration of an embodiment. 
         FIG. 3  is an illustration of an embodiment. 
         FIG. 4  is an illustration of an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described below with reference to the attached drawings. 
     Example 
     When the optical spectral bandwidth of the pump light is almost same as the bandwidth of the tunable filter, the conversion efficiency from the pump light to the signal light is reduced because of the broad spectral of the pump light.  FIG. 1  is an illustration of such an optical apparatus  100 . 
     The optical apparatus  100  is an illustration of a fiber optic parametric oscillator pumped by a standard pulsed light source  102  which is then filtered by a tunable filter  104 . A light source  102  may be a semiconductor laser, or some other type of a laser that produces pulsed light. The light source  102  may be modulated to form a pulse by modulating the current applied to the gain medium of the light source  102 . The light source  102  has a short temporal width but a relatively wide spectral width. 
     The short pulse from the light source  102  is then optically filtered by the tunable filter  104 . The light source  102  and the tunable filter  104  may be fiber coupled. In which case, a fiber from the light source  102  may be spliced to a fiber from the tunable filter  104 . The splice between the fibers is illustrated as a small x as illustrated in  FIG. 1  and the following figures. The optical fibers are illustrated as lines between the optical components as shown in  FIG. 1  and the following figures. A fiber from the tunable filter  104  may be spliced to a first gain fiber  106   a . The first gain fiber  106   a  provides linear optical gain to an intensity of the pulse exiting the tunable filter  104 . 
     The first gain fiber  106   a  may be spliced to a first input port of a first wavelength division multiplexer (WDM)  108   a . An output port of the first WDM  108   a  may be spliced to a first end of a highly non-linear fiber (HNLF)  110 . A second end of the HNLF  110  may be spliced to an input port of an output coupler  112 . A first output port of the output coupler  112  may be spliced to a second input port of the first WDM  108   a . The optical components  108   a ,  110 , and  112  form an optical loop. The optical loop forms an optical parametric oscillator (OPO). A second output port of the output coupler  112  outputs light from the OPO. 
     The spectral bandwidth of the short pulse pump laser  102  is relatively wide as illustrated by the pulse shown above the short pulse pump laser  102  in  FIG. 1 . The spectral bandwidth of the pump light coming out of the tunable filter  104  is almost the same as the filter bandwidth of the tunable filter  104 . The spectral bandwidth of the tunable filter  104  is illustrated above tunable filter  104  in  FIG. 1 . The spectral bandwidth of the optical pulse exiting the tunable filter  104  is illustrated between the tunable filter  104  and the first gain fiber  106   a  in  FIG. 1 . The peak intensity of the optical pulse is increased by the first gain fiber  106   a , while the spectral bandwidth remains the same as illustrated by the pulse shown next to the first gain fiber  106   a.    
     The HNFL  110  provides non-linear gain, which in the context of an OPO means that a pump light pulse with an optical frequency of ω p  is converted into two light pulses, a signal light pulse with a frequency of ω s  and a idler light pulse with a frequency of ω i . Such that 2ω p =ω s +ω i . The efficiency of this non-linear process is dependent upon the peak pulse intensity at the peak optical frequency. Thus, the spectral broadness of the pump light pulse limits the peak intensity of the pulse produced by the HNLF  110 . When the pump light is converted to the signal light in the HNLF  110 , the conversion efficiency is very low, because the spectral bandwidth of the pump light is relatively broad. This non-linear process is illustrated in  FIG. 1  by showing how the black pulse produces a new grey pulse shown next to the HNLF  110 . The new grey pulse represents the seed and idle pulse. The output coupler then splits these pulses and outputs a portion of the produced light as signal light. Another portion of the produced light is feedback into the WDM to recirculate in the optical loop of the OPO. In the context of the present application, the optical frequency of signal light pulse may be greater than the optical frequency of the idler light pulse; or the optical frequency of signal light pulse may be less than the optical frequency of the idler light pulse. 
     Due to the broadness of the pump pulse, it is difficult to achieve a high peak power of the signal light. 
     Embodiment 1 
     The applicants have found a way to increase the peak power of the signal light exiting the FOPO. This may be done by inserting a wavelength tunable filter into the cavity of the pump laser for the FOPO. Such that the total cavity dispersion and the filter bandwidth satisfy the following equations (1). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           0.1 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           nm 
                         
                         ≤ 
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           λ 
                         
                         ≤ 
                         
                           1.0 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           nm 
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             D 
                             cavity 
                           
                           × 
                           L 
                         
                         ≥ 
                         
                           1 
                           ⁢ 
                           
                             ps 
                             nm 
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             ⅆ 
                             T 
                           
                           
                             ⅆ 
                             λ 
                           
                         
                         = 
                         
                           
                             D 
                             cavity 
                           
                           × 
                           L 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In which: Δλ is the spectral bandwidth of the tunable filter; D cavity  is a dispersion parameter of the cavity of the pump laser; L is the total cavity length of the pump laser; and T is total round trip time of the cavity of the pump laser. Wherein, these equations are relevant over the wavelength range of the pump light. 
     If the limitations listed in equation (1) are used to design the pump laser then the optical spectral bandwidth of the pump light is narrower than the optical bandwidth of the internal tunable filter. So, a narrow spectral bandwidth can be achieved, the conversion efficiency from the pump light to the signal light can be increased. Thus, it is possible to get a high peak power signal light. 
     Embodiment 2 
       FIG. 2  is an illustration showing a schematic of a fiber optical parametric oscillator (FOPO)  200  which is an embodiment. This FOPO includes a tunable laser which pumps the oscillator. The tunable laser portion of the FOPO  200  may include a semiconductor saturable absorber mirror (SESAM)  220  which is used as saturable absorber. An example of SESAM that may be used in an embodiment is the SAM-1040-40-500fs-x sold by Batop GmbH of Jena, Del. The absorbance of the SESAM  220  may be 40%. The modulation depth of the SESAM  220  may be 24%. The relaxation time of the SESAM  220  may be 500 femtoseconds (fs). The saturation fluence of the SESAM  220  may be 120 microjoules per square centimeter (μJ/cm 2 ). 
     The SESAM  220  may be fiber coupled. The SESAM  220  may be spliced to a first port of a second WDM  108   b . A fiber coupled first pump laser  202   a  may be spliced to a second port of the second WDM  108   b . The second WDM  108   b  may combine the pump light from the second port with light from the first port and output it through a third output port of the second WDM  108   b.    
     The third output port of the second WDM  108   b  may be spliced to a first end of a second doped gain fiber  106   b . The second gain fiber  106   b  may be a polarization maintaining Ytterbium doped fiber. The length of the second gain fiber  106   b  may be 0.8 meters (m). 
     The second end of the second gain fiber  106   b  may be spliced to a first port of the wavelength tunable filter  104  which may be used for selecting a center wavelength of a seed for the tunable pump laser. The optical spectral bandwidth of the wavelength tunable filter may be 1 nm. The tuning range of the wavelength tunable filter may be 1020-1060 nm. 
     A second port of the wavelength tunable filter may be spliced to a first end of a chirped fiber Bragg grating (CFBG)  222  which may be used as dispersion compensator. The chromatic dispersion in the chirped fiber Bragg grating CFBG  222  may be +1 to +5 picoseconds per nm (ps/nm) at the center wavelength of the CFBG  222 . The reflectivity of the CFBG  222  may be 50%. The range over which the CFBG  222  has this reflectivity and this dispersion may be 1020-1060 nanometers (nm). The SESAM  220  and the CFBG  222  from a resonating cavity for the tunable laser portion of the FOPO  200 . The chirp in the CFBG  222  can be designed such that the dispersion parameter of the pump laser cavity D cavity  is kept within the limit described by equation (1) listed above. 
     The tuning wavelength range of the tunable laser portion of the FOPO  200  may be around 1020-1060 nm. The average power of the tunable laser portion of the FOPO  200  may be around 1 millwatt (mW). The peak power of the tunable laser portion of the FOPO  200  may be around 4 Watts (W). The repetition rate of the tunable laser portion of the FOPO  200  may be around 50 megahertz (MHz). The pulse duration of the tunable laser portion of the FOPO  200  may be around 5 picoseconds (ps). The optical spectral bandwidth of the tunable laser portion of the FOPO  200  may be around 0.3 nm. 
     The tunable laser portion of the FOPO  200  may be operated as a seed laser that is then amplified by a double clad fiber amplifier. The double clad fiber amplifier may include a polarization maintaining double clad Ytterbium doped as the first gain fiber  106   a . The CFBG  222  may be spliced to one end of the first gain fiber  106   a . The length of the first gain fiber  106   a  may be 1.2 m. After being amplified, the average power of the pump light is around 10 mW. The peak power of the pump light after being amplified may be 40 W. 
     A second end of the first gain fiber  106   a  may be spliced to an input port of the first WDM  108   a . An output port of the first WDM  108   a  may be spliced to a first end of the HNLF  110 . A second end of the HNLF  110  may be spliced to an input port of the coupler  112 . An output port of the coupler  112  may be spliced to a second port of the first WDM  108   a . The optical components  108   a ,  110 , and  112  form a ring type resonating cavity of the FOPO. The HNLF  110  of the FOPO resonating cavity converts the pump light from the gain fiber into signal light which is outputted from a second output port of the coupler  112 . 
     In an alternative embodiment, the FOPO resonating cavity may also include a third gain fiber  106   c  (which is not shown in  FIG. 2  and is shown in  FIG. 3 ). The pump light is further amplified by the third gain fiber  106   c  to produce further amplified pump light with an average power of 1 W. The peak power of the pump pulse exiting the third gain fiber  106   c  may be 4 kilowatts (kW). 
     The HNLF  110  may be a polarization maintaining photonic crystal fiber. The zero dispersion wavelength of the HNLF  110  may be 1050 nm. The 3 rd  order dispersion of the HNLF  110  may be 6.71776 E-41 seconds cubed per meter (s 3 /m). The 4 th  order dispersion of the HNLF  110  may be −9.83483 E-56 seconds to the power of four per meter (s 4 /m). The nonlinear coefficient of the HNLF  110  may be 10 watts per kilometer (W/km). The length of the HNLF  110  may be 30 centimeters (cm). 
     The tunable wavelength range of the signal light exiting the second output port of the coupler  112  of the FOPO  200  may be 750-950 nm. The average power of the signal light exiting the second output port of the coupler  112  of the FOPO  200  may be 150 mW. The peak power of the signal light exiting the second output port of the coupler  112  of the FOPO  200  may be 1 kW. The repetition rate of the signal light exiting the second output port of the coupler  112  of the FOPO  200  may be 50 MHz. The pulse duration of the signal light exiting the second output port of the coupler  112  of the FOPO  200  may be 3 ps. The spectral bandwidth of the signal light exiting the second output port of the coupler  112  of the FOPO  200  may be 15 nm. The HNLF  110  produces both signal and idler light. A filter may be placed within the FOPO  200  to remove idler light from the resonating cavity of the FOPO  200 . In an alternative embodiment, a filter may be placed within the FOPO  200  to remove signal light from the resonating cavity of the FOPO  200 . WDM  108   a  may act as such a filter to remove either signal or idler light from the resonating cavity of the FOPO  200 . 
     Embodiment 3 
       FIG. 3  is an illustration showing a schematic of a FOPO  300  which is an embodiment. The FOPO  300  is substantially similar to FOPO  200 . The FOPO  220  may also include a variable delay line  324  inserted between the second WDM  108   b  and the SESAM  220 . The FOPO  300  may also include a first isolator  326   a  between the CFBG  222  and the third WDM  108   c . A second end of the CFBG  222  may be spliced to an input port of the first isolator  326   a . The output port of the first isolator  326   a  may be spliced to a first input port of a third WDM  108   c . A second pump laser  202   b  may be spliced to an input port of the third WDM  108   c . An output port of the third WDM  108   c  may be spliced to the first end of the first gain fiber  106   a . The FOPO  300  may also include a second isolator  326   b  between the first gain fiber  106   a  and the first WDM  108   a . The second end of the first gain fiber  106   a  may be spliced to a first input port of the second isolator  326   b . The output port of the second isolator  326   b  may be spliced to a first input port of the first WDM  108   a . Fourth WDM  108   d  and the third gain fiber  106   c  may be between the first WDM  108   a  and the HNLF  110 . The output port of the first WDM  108   a  may be spliced to an input port of the fourth WDM  108   d . A third pump laser  202   c  may be spliced to a second input port of the fourth WDM  108   d . An output port of the fourth WDM  108   d  is spliced to a first end of the third gain fiber  106   c . A second end of the third gain fiber  106   c  may be spliced to the first end of the HNLF  110 . 
     The variable delay line  324  may be used to optimize the length of the cavity as the lasing wavelength is changed by the tunable filter  104 . The variable delay line  324  may also work in combination with the chirp in the CFBG  222  such that the dispersion parameter of the pump laser cavity D cavity  is kept within the limit described by equation (1) listed above. The first isolator  326   a  may be used to prevent light from the first gain fiber  106   a  and the FOPO ring cavity from reaching the CFBG  222  and being reflected back by the CFBG  222 . Such reflections may destabilize the FOPO  300 . The second isolator  326   b  may be used to prevent light from the FOPO ring cavity from entering the first gain fiber  106   a , which may destabilize the gain in the first gain fiber  106   a  or steal gain from the pump light provided by the first gain fiber  106   a.    
     Embodiment 4 
       FIG. 4  is an illustration showing a schematic of a FOPO  400  which is an embodiment. The FOPO  400  is substantially similar to the FOPO  300 . The FOPO  400  may include a long fiber  430  between the output coupler  112  and the WDM  108   a . A second output port of the output coupler  112  may be spliced to a first end of the long fiber  430 . A second end of the long fiber  430  may be spliced to the second input port of the first WDM  108   a . In an alternative embodiment, the resonating cavity of the FOPO may also include a variable delay line which is varied as the output wavelength of the FOPO is varied. The length L 3  of the long fiber may be 100 meters (m). The length L 3  of the long fiber may be chosen based on equation (2) listed below. 
     
       
         
           
             
               
                 
                   
                     L 
                     3 
                   
                   ≥ 
                   
                     t 
                     
                       D 
                       pulse 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The time t is the full width half max pulse duration of the pump pulse. The dispersion parameter D pulse  is a dispersion parameter of the signal pulse as defined in equation (3) below. 
     
       
         
           
             
               
                 
                   
                     D 
                     pulse 
                   
                   ≥ 
                   
                     
                       1 
                       
                         L 
                         3 
                       
                     
                     ⁢ 
                     
                       
                         ∂ 
                         
                           Φ 
                           ⁡ 
                           
                             ( 
                             λ 
                             ) 
                           
                         
                       
                       
                         ∂ 
                         λ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The function Φ(λ) is the temporal relative position over a narrow wavelength range centered at the wavelength λ of a peak light intensity relative to the average peak light intensity over the entire signal pulse. Alternatively, instead of the peak light intensity it may be the average or median light intensity. The function Φ(λ) may also represent a difference between a first value and a second value. Wherein, a first value represents a full width half max temporal width as function of the wavelength of the signal pulse before it enters the long fiber  430 . Wherein, a second value represents a full width half max temporal width as a function of the signal pulse after it leaves the long fiber  430 . Thus, Φ(λ) represents the effect of the long fiber  430  on the signal pulse. Equation (3) may be evaluated at the peak wavelength of the signal pulse. This long fiber  430  may work as a wavelength filter. The product of the dispersion parameter D pulse  and the length L 3  may be 10 picoseconds per nanometer (ps/nm). The wavelength bandwidth of the tunable filter  104  may be 0.5 nm. 
     The tunable wavelength range of the signal light produced by the FOPO  400  is 750-950 nm. The average power of the signal light produced by the FOPO  400  is 150 mW. The peak power of the signal light produced by the FOPO  400  is 1 kW. The repetition rate of the signal light produced by the FOPO  400  is 50 MHz. The pulse duration of the signal light produced by the FOPO  400  is 3 ps. The spectral bandwidth of the signal light produced by the FOPO  400  is 0.5 nm. 
     The arrangement of these optical components in this manner having these properties allows the FOPO  400  to efficiently convert the pump light to the signal light. It is thus possible to get a high peak power of the signal light. 
     An embodiment is a narrow spectrum, high power tunable fiber laser in 750-950 nm range with a less than 1 nm spectral bandwidth, approximately 100 mW of average optical power, and approximately 3 nJ of optical pulse energy. An embodiment may exploit the four wave mixing (FWM) effect in a bendable Photonic Crystal Fiber (PCF) as the HNLF. 
     An embodiment may include a fiber (such as fiber  430 ) in the resonating ring of the FOPO placed after HNLF  110  that applies a positive or negative chirp to one or both of the signal or idler pulse that exits the HNLF  110 . The applicants have found that this can result in a narrowing of subsequent pulses which exit the HNLF  100 . This structure provides FOPO  400  that produces a narrowing of the output spectrum of the signal pulse while still keeping the same tuning range of the FOPO  400 . 
     The chromatic dispersion in the resonating ring of the FOPO can lead to temporal broadening of the signal pulse or idler pulse. The chromatic dispersion may cause the pulse duration of the signal pulse or idler pulse to exceed the pulse duration of the pump laser amplified by the double-clad Yb fiber amplifier  106   a . The FOPO resonating ring may be designed so that the round-trip time of the signal pulse and/or the idler pulse is an integer multiple of the repetition rate of the pump pulse. In which case only a part of the signal pulse will overlap with the incoming pump pulse. By applying a chirp to the signal pulse, a partial temporal overlap allows for only some of the spectral components of the signal pulse to interact with the new incoming pump pulse. Consequently, this narrowing of the pulse results in a progressive decrease in the spectral bandwidth of the signal pulse and/or idler pulse. This may be accomplished without the use of any narrow-band spectral filtering device in the resonating cavity of the FOPO. In an embodiment, the chirp of the signal pulse or idler pulse may provide a method of tuning the wavelength of the generated signal and idler pulse. By changing the cavity length with the delay line or changing the repetition rate of the pump pulse, the successive pump pulse overlaps with different spectral components of the signal pulse and/or idler pulse. So, the central wavelengths of the signal and idler can be shifted. 
     As a pump pulse which has high peak power is coupled into the HLNF  110  which converts from pump pulse to signal pulse, self-phase-modulation (SPM) occurs in connection fibers which connect the gain fiber  106   a  of main amplifier and the HNLF  110 . As a result, the spectral bandwidth of pump pulse is broadened, and conversion efficiency from pump pulse to a signal pulse is decreased. Thus, it can be difficult to achieve high peak power of signal pulse. By adding the gain fiber  106   c  into the FOPO ring as illustrated in  FIG. 3  the length of the connection fiber may be reduced. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions.