Patent Publication Number: US-2021194210-A1

Title: Precision light source

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of priority to U.S. Provisional Appl. Nos. 62/952,030 filed on Dec. 20, 2019 and 63/022,100 filed on May 8, 2020, each of which is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND 
     Field 
     The present application relates generally to the generation and control of few cycle light pulses. 
     Description of the Related Art 
     Ultra-short pulse lasers (e.g., lasers generating pulses with pulse widths in a range of 100 femtoseconds (fs) to 1 picosecond (ps)) have become firmly established in technology during the last two decades and have found applications in many different areas, ranging from laser machining to precision metrology. Though the forefront of laser technology has moved to sub-100 fs laser pulses and even attosecond laser pulses, any such laser systems have barely been used in the commercial realm because of the complexity and limited robustness of such systems. 
     SUMMARY 
     In certain implementations, a nonlinear fiber laser based chirped pulse amplification system is configured to generate output pulses in the femtosecond pulse width range. The system comprises a seed pulse source configured to produce short optical pulses. The system further comprises a fiber Bragg grating (FBG) pulse stretcher system configured to stretch said pulses, at least one amplifier, at least one FBG compressor configured to compress said pulses, and a bulk dispersive element for further compressing the pulses emerging from the FBG compressor. The FBG stretcher dispersion is configured to optimize the pulse quality of said output pulses at a designated elevated power level, said optimization in pulse quality producing a functional dependence of pulse quality on average pulse power or pulse energy in at least three stages: at low powers, the output pulses have a longer temporal width compared to said designated power level, at medium powers, the output pulses exhibit several side pulses with an intensity higher than any side pulses produced at said designated power level, and at said designated power level, the pulse quality is optimized, as characterized by side pulses with an intensity smaller than observed at medium power levels. 
     In certain implementations, a nonlinear fiber laser based chirped pulse amplification system is configured to generate output pulses in the sub-30 femtosecond pulse width range. The system comprises a seed pulse source configured to produce short optical pulses. The system further comprises a fiber Bragg grating (FBG) pulse stretcher system configured to stretch said pulses, at least one amplifier, at least one FBG compressor configured to compress said pulses, a bulk dispersive element configured to further compress the pulses emerging from the FBG compressor, and at least one optical fiber for further pulse compression of the pulses emerging from the FBG compressor. 
     In certain implementations, an optical source comprises a seed source configured to produce short optical pulses, one or more actuators configured to control the carrier envelope offset frequency of the output of said seed source, and a splitter configured to split the output of said seed source into an amplifier branch and an f-2f branch. The optical source further comprises a frequency shifter in said f-2f branch, an f-2f interferometer in said f-2f branch, and a photodetector configured to detect the f-2f signal from said f-2f interferometer. The optical source further comprising a combiner configured to interfere a portion of light from said f-2f branch with a portion of light from said amplifier branch, a photodetector configured to detect light from said combiner, electronics configured to convert the signals from both said photodetectors into a signal representing the carrier envelope offset frequency at the output of said amplifier branch, and a feedback circuit configured to control the carrier envelope offset frequency at the output of said amplifier branch. 
     In certain implementations, a nonlinear fiber laser based chirped pulse amplification system is configured to generate output pulses in the femtosecond pulse width range. The system comprises a seed pulse source configured to produce short optical pulses, at least one fiber Bragg grating (FBG) pulse stretcher or compressor configured to stretch or compress pulses anywhere within said nonlinear fiber based chirped pulse amplification system, adaptive dispersion control of said at least one FBG, and a gas filled hollow fiber compressor for further compression of said output pulses. 
     In certain implementations, a method produces femtosecond pulses with a nonlinear chirped pulse amplification system seeded with an oscillator. The method comprises temporally stretching said pulses with a FBG, amplifying said pulses, and compressing said pulses to produce compressed output pulses. The FBG is configured to optimize the pulse quality of said output pulses at a designated elevated power level, said optimization in pulse quality producing a functional dependence of pulse quality on average pulse power or pulse energy in at least three stages: at low powers, the output pulses have a longer temporal width compared to said designated power level, at medium powers, the output pulses exhibit several side pulses with an intensity higher than any side pulses produced at said designated power level, and at said designated power level the pulse quality is optimized, as characterized by side pulses with an intensity smaller than observed at medium power levels. In certain such implementations, a peak intensity of the pulses is at a maximum at the designated power level. 
     In certain implementations, a pulse source comprises an oscillator configured to generate laser pulses and at least one fiber Brag grating (FBG) pulse stretcher configured to receive the laser pulses from the oscillator and to temporally stretch the laser pulses, the at least one FBG pulse stretcher configured to be adaptively controlled to provide adjustable dispersion. The pulse source further comprises at least one amplifier configured to receive the temporally stretched laser pulses, at least one FBG pulse compressor configured to receive the laser pulses from the at least one amplifier and to temporally compress the laser pulses, and one or more optical compressor components configured to receive and further compress the compressed laser pulses from the at least one FBG pulse compressor. 
     In certain implementations, a pulse transformer for modifying the amplitude and phase of short optical pulses is provided. The pulse transformer comprises a pulse source and a stretcher comprising at least one fiber Bragg grating (FBG) configured to receive pulses from the pulse source. The stretcher has a first second-order dispersion parameter (D 21 ). The pulse transformer further comprises at least one optical amplifier configured to receive pulses from the at least one FBG. The pulse transformer further comprises a compressor configured to receive pulses from the at least one optical amplifier. The compressor has a second second-order dispersion parameter (−D 22 ), an absolute value of the first second-order dispersion parameter (|D 21 |) and an absolute value of the second second-order dispersion parameter (|−D 22 |) that are substantially equal to one another to within 10%. At least one of the stretcher and the compressor is configured to be adaptively controlled. For example, the compressor can comprise an FBG and one or both of the FBG of the stretcher and the FBG of the compressor can be adaptively controlled. 
     In certain implementations, a pulse transformer for generating short optical pulses with reduced pulse curvature is provided. The pulse transformer comprises a pulse source and a positive dispersion pulse stretcher comprising at least one positive dispersion element. The pulse stretcher has a second-order dispersion parameter (D 21 ). The pulse transformer further comprises a first nonlinear element configured to receive stretched pulses from the positive dispersion pulse stretcher and to subject the stretched pulses to self-phase modulation. The pulse transformer further comprises a negative dispersion pulse compressor comprising at least one negative dispersion element. The pulse compressor has a second-order dispersion parameter (−D 22 ) having an absolute value (|−D 22 |) that is substantially equal to an absolute value (|D 21 |) of the second-order dispersion parameter of the pulse stretcher to within 10%. The pulse transformer further comprises a second nonlinear element configured to receive pulses from the pulse compressor and to subject the received pulses to bandwidth broadening. The pulse transformer further comprises a dispersive element configured to compress pulses received from the second nonlinear element. 
     In certain implementations, a high energy passively mode-locked fiber oscillator is provided. The oscillator comprises a pump source and a cavity comprising a fiber Bragg grating at a first end of the cavity. The fiber Bragg grating has a second-order dispersion component (D 21 ). The oscillator further comprises a plurality of cavity elements within the cavity. The plurality of cavity elements comprises at least one optical fiber comprising at least one gain fiber. The at least one optical fiber has a summed second-order dispersion component (D 22 ), where an absolute value of D 21  (|D 21 |) is greater than 20 times than an absolute value of D 22  (|D 22 |), (|D 21 |&gt;20*|D 22 |). The plurality of cavity elements further comprises a generalized fast saturable absorber. A pulse width of pulses propagating within the oscillator fluctuates by more than a factor of 10 along the cavity. 
     In certain implementations, a high energy passively mode-locked fiber oscillator is provided. The oscillator comprises a pump source and a cavity comprising a fiber Bragg grating at one end of the cavity. The fiber Bragg grating has a second-order dispersion component (D 21 ). The oscillator further comprises a plurality of cavity elements within the cavity. The plurality of cavity elements comprises at least one optical fiber comprising at least one gain fiber. The at least one optical fiber has a summed second-order dispersion component (D 22 ), where an absolute value of D 21  (|D 21 |) is greater than 20 times than an absolute value of D 22  (|D 22 |), (|D 21 |&gt;20*|D 22 |). The plurality of cavity elements further comprises a generalized fast saturable absorber configured to generate optical pulses with an intra-cavity pulse width less than or equal to 1 ps at a position within the cavity. 
     In certain implementations, a nonlinear fiber laser based chirped pulse amplification system is provided. The system comprises a seed pulse source configured to produce optical pulses having pulse widths less than 10 picoseconds. The system further comprises at least one fiber Bragg grating (FBG) pulse stretcher or compressor configured to stretch or compress the optical pulses. The at least one FBG has adaptive dispersion control. The system further comprises a gas filled hollow fiber compressor configured to further compress the optical pulses that are stretched or compressed by the at least one FBG pulse stretcher or compressor. For example, optical pulses outputted from the gas filled hollow fiber compressor can have pulse widths less than or equal to 30 femtoseconds 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an example high power femtosecond pulse source in accordance with certain implementations described herein. 
         FIG. 2  schematically illustrates an example adaptive fiber grating pulse stretcher compatible for use in the example pulse source of  FIG. 1  in accordance with certain implementations described herein. 
         FIG. 3  shows a measured dispersion profile induced for optimum pulse quality at the output of the example pulse source of  FIG. 1 . 
         FIG. 4  shows measurements of the pulse quality as a function of average output power obtained at the output of the example pulse source of  FIG. 1 . 
         FIG. 5  schematically illustrates an example system comprising the example pulse source of  FIG. 1  with an added fiber pulse compressor in accordance with certain implementations described herein. 
         FIG. 6  illustrates a FROG trace and the spectrum of a near single cycle pulse generated at the output of the example system of  FIG. 5 . 
         FIG. 7  shows a mid-IR spectrum obtained by focusing the output of the example system of  FIG. 5  into a nonlinear crystal in accordance with certain implementations described herein. 
         FIG. 8  shows an example system configured to control the carrier envelope frequency at the output of the example system of  FIG. 5  in accordance with certain implementations described herein. 
         FIG. 9  shows an example system configured to control the carrier envelope phase at the output of the example system of  FIG. 5  in accordance with certain implementations described herein. 
         FIG. 10  shows another example system configured to control the carrier envelope phase at the output of the example system of  FIG. 5  in accordance with certain implementations described herein. 
         FIG. 11  schematically illustrates an example high energy few cycle pulse generation system further comprising coherent addition of individual pulses in accordance with certain implementations described herein. 
         FIG. 12  schematically illustrates an example precision comb system with two highly nonlinear waveguides for supercontinuum generation in accordance with certain implementations described herein. 
         FIG. 13  schematically illustrates an example adaptive short pulse generation system comprising a hollow fiber compressor in accordance with certain implementations described herein. 
         FIG. 14  schematically illustrates an example adaptive short pulse transformer system in accordance with certain implementations described herein. 
         FIG. 15  schematically illustrates an example substantially static and near-parabolic short pulse transformer in accordance with certain implementations described herein. 
         FIG. 16  schematically illustrates an example application of an adaptive short pulse parabolic pulse compressor of high energy pulses in accordance with certain implementations described herein. 
         FIG. 17A  schematically illustrates an example near dispersion compensated high energy mode-locked oscillator incorporating a highly dispersive fiber Bragg grating in accordance with certain implementations described herein. 
         FIG. 17B  schematically illustrates another example near dispersion compensated high energy mode-locked oscillator incorporating a highly dispersive fiber Bragg grating in accordance with certain implementations described herein. 
         FIG. 18A  schematically illustrates an example evolution of pulse energy as a function of intra-cavity gain fiber length (in meters) in the example mode-locked oscillator of  FIG. 17A . 
         FIG. 18B  schematically illustrates an example relative pulse peak powers as a function of time of the shortest and longest pulses generated in the example mode-locked oscillator of  FIG. 17A . 
         FIG. 18C  schematically illustrates an example close-up view of the shortest pulse generated in the example mode-locked oscillator of  FIG. 17A . 
         FIG. 18D  schematically illustrates an example pulse spectrum of pulses generated in the mode-locked oscillator of  FIG. 17A . 
     
    
    
     The figures depict various implementations of the present disclosure for purposes of illustration and are not intended to be limiting. Wherever practicable, similar or like reference numbers or reference labels may be used in the figures and may indicate similar or like functionality. 
     DETAILED DESCRIPTION 
     Certain implementations described herein advantageously provide compact and highly robust laser systems than can further technological developments with pulse sources generating pulses with pulse widths that are a few tens of femtoseconds, or pulse widths that are less than or equal to 10 fs. 
     Certain implementations described herein advantageously provide compact few cycle fiber laser sources employing several pulse compression stages. In certain implementations, the effects of gain narrowing in fiber amplifiers are counteracted by implementing nonlinear amplification schemes (e.g., nonlinear chirped pulse amplification; similariton amplification). In certain implementations, particularly high pulse energies are reached by combining such fiber laser sources with pulse compression in gas-filled hollow fibers. 
     Certain implementations described herein advantageously enable high precision carrier phase control via coherence transfer between two amplifiers seeded with a single oscillator. In certain such implementations, single-pass common mode wide-band frequency combs can be constructed that allow for efficient coherence transfer from the infrared (IR) radiation to visible light. Moreover, in certain implementations, pump pulses generated with Yb, Er, Tm, or Ho fiber lasers allow for efficient near single cycle mid-IR pulse generation with an output spanning over more than one octave. 
     Overview 
     Adaptive control of the output of pulsed fiber systems has been known for some time (see, e.g., U.S. Pat. Nos. 7,414,780 and 10,096,962). Such systems are generally designed to compensate the dispersion mismatch in chirped pulse amplification systems, incorporating a dispersive pulse stretcher and a dispersive pulse compressor. The pulse stretcher can, for example, be a fiber grating pulse stretcher and the compressor can be a bulk grating compressor or a volume Bragg grating compressor. Moreover, an adaptive pulse stretcher can be implemented for the compensation of self-phase modulation in such systems. 
     Previous efforts have addressed control of the carrier phase of short pulse fiber lasers without addressing any means for tunable carrier phase generation or applications in precision coherence transfer from the IR to the visible (see, e.g., U.S. Pat. No. 9,036,971). scheme for precision coherence transfer from the IR to the visible was discussed in A. Rolland et al., “Ultra-broadband dual-branch optical frequency comb with 10 −18  instability,” Optica, Vol. 5, 1070 (2018), in which both visible and IR supercontinuum (SC) sources, centered respectively at 780 nm and 1560 nm, were used. However, such systems are difficult to assemble, since they utilize two supercontinua of the sources that are overlapped in time with high precision, and with coherence control. 
     Short Pulse Generation 
     Certain implementations disclosed herein provide a simplified scheme for broadband SC generation based on a high power, femtosecond pulse source.  FIG. 1  schematically illustrates an example high power femtosecond pulse source  10  in accordance with certain implementations described herein. The pulse source  10  can be used as the front end to near single cycle pulse sources, as further described herein (see, e.g.,  FIG. 5 ). As described herein, the pulse source  10  of certain implementations disclosed herein comprises a fiber grating pulse compressor that advantageously facilitates a compact design of the pulse source architecture. The pulse source  10  of certain such implementations can further comprise a fiber grating pulse stretcher to advantageously facilitate further compactness. 
     In certain implementations, the pulse source  10  comprises an oscillator  20  configured to generate short laser pulses (e.g., pulses having pulse widths in a range of 30 to 600 femtoseconds). For example, the oscillator  20  can comprise a mode-locked Er fiber laser configured to generate laser pulses with a wavelength at or near 1.55 microns. Other example oscillators  20  compatible with certain implementations described herein include but are not limited to fiber lasers comprising one or more of the following materials: Nd, Yb, Tm, Ho, and Er/Yb; solid-state laser oscillators; semiconductor laser oscillators. In certain implementations, the oscillator  20  comprises at least one pre-amplifier (not shown) configured to amplify the laser pulses after being emitted from the oscillator  20 . 
     In certain implementations, the pulse source  10  further comprises at least one pulse stretcher  30  configured to receive the laser pulses from the oscillator  20  and to temporally stretch the laser pulses (e.g., to increase the pulse widths of the laser pulses to be in a range of 100 fs to 1000 ps). For example, the at least one pulse stretcher  30  can comprise at least one fiber grating pulse stretcher  30  (e.g., at least one fiber Bragg grating (FBG) pulse stretcher) and configured to receive the laser pulses from the oscillator  20  (e.g., via a circulator  32 ) and to reflect and transmit temporally stretched laser pulses. In certain implementations, the circulator  32  comprises bulk optical components with free-space propagation between the optical components (e.g., to reduce or minimize nonlinear pulse distortions. While the at least one fiber grating pulse stretcher  30  of certain implementations provides a predetermined (e.g., desired) dispersion to generate short output pulses, in certain other implementations, the at least one fiber grating pulse stretcher  30  is configured to be adaptively controlled to provide adjustable dispersion, as described more fully herein, to enable further improvement of pulse quality, either because the correct dispersion profile is not known at design time, or laser conditions such as output power are variable. 
     In certain implementations, the pulse source  10  further comprises at least one amplifier  40  configured to receive the temporally stretched laser pulses (e.g., via the circulator  32 ). For example, as schematically illustrated by  FIG. 1 , the at least one amplifier  40  comprises a preamplifier  40   a  and an amplifier  40   b.    
     In certain implementations, the pulse source  10  further comprises at least one pulse compressor  50  configured to receive the laser pulses from the at least one amplifier  40  and to temporally compress the laser pulses (e.g., to decrease the pulse widths of the laser pulses to be in a range of 50 to 1000 femtoseconds. For example, the at least one pulse compressor  50  can comprise at least one fiber grating pulse compressor  50  (e.g., at least one FBG pulse compressor) configured to receive the laser pulses from the at least one amplifier  40  (e.g., via a circulator  42 ) and to temporally compress the laser pulses. In certain implementations, the circulator  42  comprises bulk optical components with free-space propagation between the optical components (e.g., to reduce or minimize nonlinear pulse distortions. 
     In certain implementations, the pulse source  10  further comprises one or more optical compressor components  60  configured to receive and further compress the compressed laser pulses from the at least one pulse compressor  50  (e.g., prior to the laser pulses being emitted by the pulse source  10 ). For example, as schematically illustrated by  FIG. 1 , the one or more optical compressor components  60  can comprise a length of optical fiber pigtail  60   a  after the fiber grating pulse compressor  50  (e.g., between the fiber grating pulse compressor  50  and the circulator  42 ), the optical fiber pigtail  60   a  comprising a nonlinear optical fiber configured to further temporally compress the laser pulses. In certain implementations, the one or more optical compressor components  60  further comprises an essentially dispersion-free free-space propagation section  60   b  configured to receive the temporally compressed laser pulses from the optical fiber pigtail  60   a  (e.g., via the circulator  42 ). 
     In certain implementations, the one or more optical compressor components  60  further comprises one or more other optical compressor components  60   c,  examples of which include but are not limited to, one or more optical fibers, chirped mirrors, or other optical materials configure to provide a predetermined dispersion resulting in a predetermined (e.g., desired) pulse quality, pulse width, and/or pulse peak power for the laser pulses emitted by the pulse source  10 . 
     In certain implementations, the fiber grating pulse stretcher  30  is configured to be adaptively controlled (e.g., to maximize the pulse quality of the laser pulses emerging from the pulse source  10 ). In certain such implementations, the adaptive control of the fiber grating pulse stretcher  30  induces second-, third-, or higher-order dispersion in the fiber grating pulse compressor  50 . Such adaptively induced second-, third-, or higher-order dispersion can compensate for nonlinear self-phase modulation (SPM) induced pulse distortions in the fiber grating pulse compressor  50  and the subsequent fiber pigtail  60   a,  which (e.g., for SPM values greater than 1 or higher) could otherwise severely limit the pulse quality at the output of the free-space propagation section  60   b  and/or the one or more other optical compressor components  60   c.  In certain implementations, the adaptive control can essentially induce higher-order dispersion of any order that pre-compensates for complex SPM induced pulse distortions. Induced adaptive dispersion changes can affect the propagation through the whole system, so in certain implementations, the control parameters in the fiber grating pulse stretcher  30  can be modified by monitoring the pulse quality (e.g., at the output of the one or more optical components  60 ). In certain implementations, an iterative optimization procedure can be implemented to optimize the adaptive FBG control with regard to the optimized compressed pulse quality. Certain implementations described herein are configured to pre-compensate for the nonlinear dispersion terms in the fiber grating pulse compressor  50  and the fiber pigtail  60   a  (e.g., to reduce or minimize the adaptive changes in the dispersion terms of the fiber grating pulse stretcher  30  to improve or optimize pulse quality). Certain such implementations can completely obviate the need for adaptive control for less involved assembly (e.g., without spatially selective thermal control of the fiber grating stretcher  30 ). In certain implementations, adaptive dispersion control can also be implemented directly in the fiber grating pulse compressor  50  which can be advantageous (e.g., for reducing or minimizing cross coupling between nonlinear pulse propagation in the system and the adaptive control). 
       FIG. 2  schematically illustrates an example adaptive fiber grating pulse stretcher  30  compatible for use in the example pulse source  10  of  FIG. 1  in accordance with certain implementations described herein. The adaptive fiber grating pulse stretcher  30  comprises a FBG  100 , a plurality of actuator elements  110  configured to apply perturbations to corresponding portions of the FBG  100 , and a controller  120  in electrical communication with the plurality of actuator elements  110  (e.g., via a plurality of wires  122  in electrical communication with the controller  120  and the actuator elements  110 ). For example, the FBG  100  can have a length of 5 cm and the plurality of actuator elements  110  can comprise  10  or more individually addressable elements. Example actuator elements include, but are not limited to: temperature actuators (e.g., electrical resistive heaters; thermoelectric devices) configured to apply temperature perturbations (e.g., on the order of 10 degrees Celsius or more), bending actuators (e.g., separate arms mechanically fixed to portions of the FBG  100  and configured to be moved relative to one another), and pressure actuators (e.g., piezoelectric transducers) configured to apply pressure perturbations. Example configurations of the example adaptive fiber grating pulse stretcher of  FIG. 2  are disclosed in U.S. Pat. Nos. 7,414,780 and 10,096,962, each of which is incorporated in its entirety by reference herein. 
     In certain implementations, the controller  120  comprises manual controls (e.g., potentiometers for controlling voltages) and/or computerized controls (e.g., integrated circuit; microcontroller) for convenience and automated functions. For example, computerized controls can be used for compensating for system changes over time, for adjusting to requested changes such as output power, or for purposely adjusting the pulse shape or chirp for specific applications. The appropriate setting of the controller  120  can be determined while monitoring a measurement of the laser (e.g., an autocorrelation signal; the output power after frequency doubling) or can be determined once and then recalled for specific situations. In certain implementations, the controller  120  is configured to apply an algorithm (e.g., either manually or automatically) to determine the appropriate settings. For example, the algorithm can comprise repeatedly maximizing the pulse second harmonic autocorrelation peak value for each actuator element  110 , which can be sufficient to achieve good results. More sophisticated algorithms, such as downhill simplex methods or stochastic parallel gradient descent (SPGD) algorithms, can be used for faster convergence. In more complicated implementations, such as generating specific pulse shapes, more flexible algorithms, such as simulated annealing or genetic algorithms, can improve performance. 
       FIG. 3  shows a measured dispersion profile induced for optimum pulse quality at the output of the example pulse source  10  of  FIG. 1 . In  FIG. 3 , the measured dispersion profile of the FBG of the adaptive fiber grating pulse stretcher  30  generates 50 nJ pulses with a pulse width of around 60 fs, with a pulse repetition rate of 100 MHz and an obtained average power of 5.0 W. Once an optimized dispersion profile in the adaptive fiber grating pulse stretcher  30  is set, a distinct evolution of the pulse quality with an increase in pulse energy (or average power) can be observed.  FIG. 4  shows measurements of the pulse quality as a function of average output power obtained at the output of the example pulse source  10  of  FIG. 1 . The obtained pulse quality is shown in  FIG. 4  as the average power is increased from 0.5 W at the bottom of  FIG. 4  to 5.0 W at the top of  FIG. 4 , with about 0.5 W per step. Temporal and spectral intensities are normalized and vertically shifted for ease of viewing. The left-hand side of  FIG. 4  shows a frequency resolved optical gating (FROG) traces of the output pulses, and the middle of  FIG. 4  shows the temporal profile of the output pulses, and the right-hand side of  FIG. 4  shows the spectral measurements. The increase in average power can, for example, be obtained via an increase in pump power delivered to the at least one amplifier  40  (e.g., the final power amplifier  40   b ) in the pulse source  10 . 
     In certain implementations, as shown in  FIG. 4 , the pulse quality goes through at least three distinct regimes: at low pump power (e.g., less than 1.5 W), the pulse source  10  is substantially linear and provides relatively long pulses without pulse structure. At higher power levels (e.g., in the range of 1.5 W to 3.5 W), the pulse quality deteriorates markedly and is worst at the upper portion of the range (e.g., power levels between 2.5 W and 3.5 W) where the interplay of dispersion from the fiber grating pulse stretcher  30  and SPM in the fiber grating pulse compressor  50  and the fiber pigtail  60   a  generates several side-peaks to the main pulse. For power levels greater than 3.5 W, the pulse quality improves and reaches a maximum (e.g., for an average power of 5.0 W), manifested in a minimization of any side peaks. 
     In certain implementations, as schematically illustrated by  FIG. 5 , high quality, high energy output pulses generated by the example pulse source  10  of  FIG. 1  can be compressed to single cycle pulse widths or near single cycle pulse width by injecting the pulses into a fiber pulse compressor  70  (e.g., a short length of optical fiber). The fiber length of the fiber pulse compressor  70  can range from a few millimeters to a few centimeters. Because of the relatively high pulse energy, the fiber pulse compressor  70  of certain implementations utilizes relatively large core fibers (e.g., with mode-diameters greater than 5 microns; greater than 10 microns; greater than 15 μm). The mode diameters can be larger than those typically used for supercontinuum generation from short pulse fiber lasers, and hence supercontinuum spectra with significantly higher spectral density, compared to standard fiber technology, can be obtained. 
       FIG. 6  illustrates a FROG trace and a spectrum of a near single cycle pulse having a pulse width of 17 fs and a pulse energy of 31 nJ generated at the output of the example single cycle pulse source  10  of  FIG. 5 . The spectrum can extend from 1200 nm to 1800 nm and can reach an octave or more when implementing slightly longer fiber lengths in the example single cycle pulse source  10  of  FIG. 5 . For near single cycle pulse generation, the fiber pulse compressor  70  of certain implementations comprises a combination of two sections of compressor fiber (e.g., which can be spliced together): a first fiber section having near zero dispersion near the center wavelength of the injection pulse (e.g., a dispersion value D zero , where |D|&lt;7.5 ps 2 /km) and a second fiber section having negative dispersion (e.g., a dispersion value D neg &lt;−15 ps 2 /km). In certain implementations, the fiber pulse compressor  70  comprises three or more sections of fiber (e.g., which can all be spliced together) comprising a first fiber section configured to generate only limited spectral broadening, a second fiber section configured to compress the pulses to close to the bandwidth limit while generating large spectral broadening, and a third fiber section configured to generate further spectral broadening. 
     As shown in  FIG. 6 , the short pulses generated by certain implementations described herein exhibit two distinct relatively smooth distal spectral regions, which are arranged around a central spectral region. In certain implementations, these two spectral regions can be mixed in a nonlinear crystal to generate mid-IR radiation via difference frequency mixing (see, e.g., U.S. Pat. No. 8,861,555). The central spectral region can also be involved in the nonlinear mixing process, which can also be referred to as intra-pulse difference frequency generation or optical rectification. 
       FIG. 7  shows a mid-IR spectrum obtained by focusing the output of the example near single cycle pulse source  10  of  FIG. 5  into a nonlinear crystal in accordance with certain implementations described herein. The mid-IR spectrum of  FIG. 7  was generated using a nonlinear crystal based on an optically patterned GaP (OPGaP) and extends from 3.7 microns to 17 microns. In certain other implementations, broader spectral coverage can be obtained by using longer compressor fibers for the fiber pulse compressor  70  than those used in generating the mid-IR spectrum of  FIG. 7 . In certain implementations, the output power in the broad mid-IR spectrum can be greater than 500 μW, greater than 1 mW, or greater than 10 mW. By implementing phase matching, certain implementations described herein can further enhance the spectral density in a predetermined spectral region and can obtain further power increases. The high spectral density achievable with few cycle pulse sources in accordance with certain implementations described herein further allows applications in nonlinear microscopy, such as two photon microscopy and coherent anti-Stokes Raman (CARS) microscopy. For some application, further frequency doubling or frequency tripling of the output spectra can be implemented. 
     Measurement and Stabilization of Carrier Envelope Offset Frequency 
     In certain implementations, the pulse source  10  can be configured with precise control of one of the carrier envelope offset frequency f ceo  and the repetition rate, or both for adaptation to frequency comb applications (e.g., frequency transfer, mid-IR generation, control of few-cycle phenomena). In certain frequency comb implementations, the f ceo  of the laser output is measured by an f-2f interferometer in a separate branch which is configured to have the same f ceo  as the main laser branch going to the application. This type of arrangement can be insufficient for precision applications, particularly with strongly amplified lasers and long amplifier lengths, as typical for fiber laser systems. 
     In certain implementations, the pulse source  10  is configured to measure and stabilize the f ceo  of the beam that is going to the application. For example, f ceo  can be stabilized to a continuous range of values from f ceo =0 to the MHz range, making the pulse source  10  universally useable for essentially any comb application. By using a single arm DFG system (see, e.g., U.S. Pat. No. 8,861,555 which is incorporated in its entirety by reference herein), certain implementations described herein can automatically obtain f ceo =0 at the DFG output of the high power branch. 
     In certain implementations, f ceo  of the beam sent to the application is stabilized by optically comparing a fraction of the application beam to a beam from a separately configured f-2f interferometer branch (e.g., arm). Using additional electronic mixing, certain implementations can generate a radio frequency (RF) signal that is appropriate for stabilizing the f ceo  of the application beam. 
       FIG. 8  shows an example system  200  configured to control the carrier envelope frequency at the output of the example pulse source  10  of  FIG. 1  or  FIG. 5  in accordance with certain implementations described herein. The example system  200  of  FIG. 8  can be configured to stabilize the f ceo  of the laser beam used in the application. Stabilization of f ceo  can involve two branches, a diagnostic branch which has a harmonic f-2f interferometer generating an RF signal, and a branch which has the application beam to be stabilized, part of which is interfered with light from the diagnostic branch to generate a second RF signal. The RF signals can be combined to allow stabilization of f ceo  to a desired frequency. A frequency shifter can be used to enable stabilization of f ceo  to frequencies between zero and higher frequencies. 
     In  FIG. 8 , the oscillator  210  comprises an erbium oscillator  20  at 1560 nm and the other components shown in  FIG. 1  or  FIG. 5 . The fiber pulse compressor  70  shown in  FIG. 5  is optional and is not needed for carrier envelope offset frequency control. The oscillator  210  seeds an f-2f interferometer arm  220  that is preceded by an acousto-optic (AO) frequency shifter  230 , which adds frequency “AO” to the optical comb lines (e.g., from an RF generator  240 ). The f-2f interferometer arm  220  can include an amplifier, spectral broadening (e.g., supercontinuum generation), a nonlinear crystal, and a spectral filter to generate an f-2f radio frequency signal (e.g., a beat note frequency “F2f” signal in the 1100 nm spectral range) which is detected by a photodetector  222 . This signal can have the sum of the f ceo  of the oscillator “f 0 ”, the shifting frequency AO, and the phase drifts “φ 1 ” of the f-2f arm  220 . 
     For example, in certain applications, it can be useful to have a zero or small f ceo  to match changes in the carrier envelope phase to the application. In conventional f-2f interferometry, the frequency comb cannot be stabilized to a small frequency as the RF beat notes will appear near zero or multiples of the repetition rate. In certain implementations, shifting the comb by the frequency shifter before the f-2f interferometer arm  220  allows f ceo  to be near zero while still providing a usable RF signal. In certain implementations, it can be advantageous to include the AO frequency shifter  230  and the f-2f arm  220  in a single module. In certain such implementations, the output after supercontinuum generation can be split into two arms (e.g., an IR arm and a near IR arm), where the AO frequency shifter  230  is only inserted into the near-IR arm and pulses from the IR arm are frequency doubled and interfered with the AO shifted pulses from the near IR arm to generate an f-2f signal which is frequency shifted by the AO modulation frequency (see, e.g., U.S. Pat. No. 8,442,373). Moreover, in certain such implementations, the f-2f arm  220  can also be configured to shift the f 2  frequency (or the 1560 nm output) by the modulation frequency of the AO modulator. 
     In certain implementations, a portion of the output from the oscillator  210  is amplified by the at least one amplifier  250  and is interfered with light from the f-2f interferometer arm  220 , as schematically illustrated by  FIG. 8 . The amplified portion of the output can be near the original 1560 nm range of the oscillator  210 , and is not otherwise used in the f-2f interferometer arm  220 . The dashed lines in  FIG. 8  indicate that these beams can propagate in free space to minimize carrier phase changes. These combined beams can be detected by a photodetector  224  which generates an RF beat note “f 12 ” signal that includes the difference of the f ceo  of the two interferometer arms, f 1 -f 2 , where f 1 =f 0 +AO+φ 1 , and f 2 =f 2 +φ 2 , with φ 2  representing the phase drifts in the f-2f interferometer arm  220 , such that f 12 =AO+φ 1 −φ 2 . 
     In certain implementations, the signal F2f is electronically mixed with a radio frequency “Up” signal (e.g., from the RF generator  240 ) to generate a signal F2f+Up=f 0 +AO+φ 1 +Up. This “F2f+Up” signal can then be mixed with the “f 12 ” signal to get a difference signal with frequency F mix =F2f+Up−f 12 =f 2 +Up. In certain implementations, a feedback circuit  260  is configured to lock the difference signal frequency F mix  to frequency Up+f Set  by controllably adjusting the actuator elements  110  of the adaptive fiber grating pulse stretcher  30  that controls the frequency comb. The feedback circuit  260  stabilizes f 2  to f Set , which can be chosen by the user to obtain the desired f ceo  value for the amplified application beam. 
     As schematically illustrated by  FIG. 8 , the example system  200  utilizes three generated radio frequencies. Two of the frequencies, Up and Up+f Set , are well synchronized, as any drifts between the two affects f 2 . For example, such synchronization can be easily achieved by sourcing both frequencies from the same device, or by synchronizing two RF generators to the same clock source. Locking stability is not very sensitive to frequency AO as AO is cancelled when mixing to generate F mix . 
     In certain implementations, separating f ceo  detection from the application beam (e.g., as in the example system  200  of  FIG. 8 ) advantageously allows the application beam to be fully utilized for the application at hand. For example, the application interferometer arm can be used to generate outputs near 1400 nm and 1560 nm for precision metrology applications, outputs from 1560 nm to 1397 nm for coherence transfer, and outputs near 698 nm (after frequency doubling) and 1560 nm for beating with optical clock references, while the f-2f interferometer arm  220  generates the f-2f signal for stabilizing f ceo  of the application arm without any non-common mode noise. Similarly, broadband output can be generated in the application interferometer arm in the range from 1050 nm to 1400 nm, which can allow coherence transfer between most optical clock wavelengths of interest and an optical clock reference near 1560 nm. 
       FIG. 9  shows an example system  200  configured to control the carrier envelope phase at the output of the example pulse source  10  of  FIG. 5  in accordance with certain implementations described herein. The example system  200  of  FIG. 9  can be configured to achieve a specific carrier envelope phase. As schematically shown in  FIG. 9 , the example system  200  further comprises a carrier-envelope phase (CEP) detector  270  (e.g., a current in a semiconductor; stereo above-threshold-ionization system, spectrally-resolved f-2f interferometer, or directly from the CEP sensitive application (see, e.g., K. Wang et al., “Comb offset frequency measurement using two-photon—three-photon quantum interference control,” CLEO 2017, JTh2A.68.pdf; T. Fordell et al., “High-speed carrier-envelope phase drift detection of amplified laser pulses,” Op. Express, Vol. 19, No. 24, pp. 23652-23657 (2011); E. Shestaev et al., “High-power ytterbium-doped fiber laser delivering few-cycle, carrier-envelope phase-stable 100 μJ pulses at 100 kHz,” Op. Lett. Vol. 45, No. 1, pp. 97-100 (2020))) and a feedback circuit  280  configured to receive a signal from the CEP detector  270 . For example, to maintain a constant CEP, the feedback circuit  280  can be configured to control the frequency Up+f Set , effectively adjusting f Set  in such a way as to maintain the desired CEP. 
       FIG. 10  shows another example system  200  configured to control the carrier envelope phase at the output of the example pulse source  10  of  FIG. 5  in accordance with certain implementations described herein. The example system  200  of  FIG. 10  further comprises at least one CEP actuator  290  (e.g., a rotating transparent plate of material, such as glass, having low chromatic dispersion and/or laser pump intensity control) in the laser output and configured to receive a signal from the CEP feedback circuit  280 . Certain such implementations advantageously allow the CEP stabilization to operate independently of the f 2  stabilization system, which locks f 2  to zero, while using CEP stabilization to compensate for drifting of the CEP. 
     As schematically illustrated by  FIGS. 9 and 10 , the CEP detector  270  of certain implementations receives light being sampled by a beam splitter. In certain such implementations, the CEP detector  270  preferentially measures light with the same CEP as the application sees. If certain other implementations, a calibration can be performed by simultaneously comparing the measured CEP at the CEP detector  270 , and at the application. 
     Other Example Implementations 
     In certain implementations, the pulse source  10  utilizes at least one pulse compressor  50  comprising at least one chirped fiber grating pulse compressor  50  (e.g., chirped FBG compressor) in which pulses are subject to significant levels of SPM (e.g., SPM greater than 1, greater than π, greater than 3π, or larger). The pulse source  10  can use relatively long FBG compressor gratings while still obtaining a high level of pulse quality at the output. In certain implementations, the chirped fiber grating pulse compressor  50  can be configured to stretch input pulses to have a pulse width greater than or equal to 100 ps or a pulse width greater than or equal to 1 ns. With increasing length, the level of SPM in the chirped fiber grating pulse compressor  50  also increases. 
     In certain implementations, the pulse source  10  utilizes compact high power pulse sources based on Tm and Ho fiber amplifiers. In certain such implementations, the fiber grating pulse compressor  50  comprises longer FBG lengths configured to provide efficient pulse compression due to the reduced refractive index modulation that is currently available for FBGs operating at wavelengths greater than 1.7 microns. 
     In certain implementations, the pulse source  10  utilizes coherent combination of pulses or pulse stacking in pulse stackers to increase the pulse energy of the few cycle pulse system (see, e.g., U.S. Pat. Appl. Publ. No. 2019/0190224 which is incorporated in its entirety by reference herein). In certain such implementations with an optimized pulse stacking system, the pulse energy of few cycle pulses can be increased by at least a factor of 10 or more, where sub-10 fs pulses can also be reached. 
       FIG. 11  schematically illustrates an example high energy few cycle pulse generation system  300  further comprising coherent addition of individual pulses in accordance with certain implementations described herein. In certain implementations, the system  300  comprises a coherent combination interferometer  310  configured to receive amplified seed pulses from an amplifier, the coherent combination interferometer  310  configured to multiply each pulse into two trains of temporally separated sub-pulses. Each train of the two trains seeds one of two amplifiers, with each pulse becoming a few cycle, high energy pulse. The coherent combination interferometer  310  is further configured to recombine these two trains of amplified pulses into a single output pulse with pulse energy several times higher than each individually amplified pulse in the train. For example, the system  300  can be configured to yield a scaling of the average power from a single amplifier by the number of amplifiers combined (e.g., two as shown in  FIG. 11 ) and a scaling of the pulse energy from a single amplifier by the total number of sub-pulses generated from each seed pulse (e.g., 8 or 16). 
     In certain implementations, the example few cycle pulse system  300  of  FIG. 11  is configured to generate pulse energies in the range of 100 nJ to 100 μJ per pulse or higher. The carrier wavelength of certain implementations can be at or near 1.55 microns, in a range of 1.8 microns to 2.1 microns, or in a range of 900 nm to 1100 nm, with pulse widths of 10 fs or lower. Certain such implementations are configured for use in machining applications. In some applications, solid state laser sources can also be pulse compressed using nonlinear FBGs in accordance with certain implementations disclosed herein. Certain implementations achieve even higher pulse energies by using nonlinear volume Bragg gratings for compression, where adaptive control of the characteristics of the at least one pulse stretcher  30  can compensate for nonlinear pulse distortions in the volume Bragg gratings. 
     Certain implementations described here are configured to be used for efficient THz generation, as well as for direct electric field sampling. The broadband high power supercontinuum frequency comb spectra generated by the pulse sources  10  of certain implementations can further be amplified in optical parametric amplifiers where output levels for frequency combs in the range of 100 mW to 1 W or higher can be generated in the mid-IR. 
     In certain implementations configured for use for coherence transfer, the pulse source  10  can provide a frequency transfer stability between the visible and the IR (e.g., in a range of 698 nm to 1550 nm) of less than 10 −18  in one second, which can be used for precision coherence transfer for precision optical clocks and is better than the stability achievable with other technologies. Certain implementations disclosed herein can be configured, in conjunction with high harmonic generation in gases or from solids, to provide a relatively simple system for efficient VUV light generation (e.g., wavelengths of 100 nm and shorter). 
     In certain implementations, the system  200  shown in  FIG. 8  is configured for precision frequency metrology and coherence transfer with highly nonlinear waveguides, such as silicon nitride (Si 3 N 4 ), here simplified as SiN. Highly nonlinear waveguides are useful for octave spanning supercontinuum generation with pulse energies of only a few pJ. However, due to the lack of an intrinsic second order nonlinearity in SiN, it can be difficult to design SiN waveguides that allow for frequency doubling, as utilized by an f-2f interferometer. In certain implementations, the system  200  shown in  FIG. 8  can be adapted to precision metrology, where a single oscillator feeds two supercontinuum generating SiN waveguides, the first SiN waveguide used for coherence transfer between spectral regions and the second SiN waveguide used as an f-2f interferometer via inserting a doubling crystal down-stream of the SiN waveguides. Additional microwave components, as discussed with respect to  FIG. 8 , can also be used to ensure minimal frequency noise between the various spectral components. 
       FIG. 12  schematically illustrates an example precision (e.g., low noise) comb system  400  with two highly nonlinear waveguides for supercontinuum generation in accordance with certain implementations described herein. In certain implementations, the comb system  400  comprises an oscillator  410  configured to generate an output that is split in two (e.g., by an optical coupler). After optional amplification (not shown) of the split beams, the two beams are coupled into two highly nonlinear waveguides  420  for supercontinuum generation. A first nonlinear waveguide  420   a  of the two highly nonlinear waveguides  420  is configured to generate supercontinuum output that is subsequently interfered with optical references (not shown) for the generation of beat signals. A second nonlinear waveguide  420   b  of the two highly nonlinear waveguides  420  is configured to be used in conjunction with a frequency doubling crystal  430  (e.g., periodically poled nonlinear lithium niobate (PPLN)) for the generation of an F-2f signal. 
     Certain implementations described herein have other benefits, for example, few cycle pulse and mid-IR pulse generation that are compatible with dual comb generation in a single laser cavity (see, e.g., U.S. Pat. No. 5,479,422). Certain implementations described herein are configured to be further adapted for scanning dual comb systems for mid-IR spectroscopy and other applications (see, e.g., U.S. Pat. No. 8,120,778). Other certain implementations of the few cycle pulse sources as described herein are configured to be used with single oscillator dual comb generation and dual comb scanning. 
     Certain implementations described herein are configured to be used with pulse compression in gas-filled nonlinear hollow waveguides (see, e.g., J. S. Travers et al., “High-energy pulse self-compression and ultraviolet generation through soliton dynamics in hollow capillary fibres,” Nature Photonics, Vol. 13, 547 (2019)). 
       FIG. 13  schematically illustrates an example adaptive short pulse generation system  500  comprising a hollow fiber compressor  510  in accordance with certain implementations described herein. The system  500  comprises a short pulse source  10  (e.g., as schematically illustrated by  FIG. 1 ) and a hollow fiber compressor  510  (e.g., a hollow photonic crystal fiber; a Kagome fiber; a hollow capillary) that is filled with gas (e.g., at least one inert gas; He; Ne; Kr; Ar) at high pressures (e.g., in a range from 2 bar to 100 bar). In certain implementations, pulses from the short pulse source  10  (e.g., with pulse energies in the range of 100 nJ to a few 100 μJ) are transmitted into the hollow fiber compressor  510  (e.g., via a focusing system  520 ) and the hollow fiber compressor  510  is configured to compress the femtosecond input pulses to a few cycles. As schematically illustrated by  FIG. 13 , the output of the hollow fiber compressor  510  is collimated by a collimation system  530  and a first portion of the collimated output is directed to a pulse diagnostic system  540  which generates a feedback signal provided to the adaptive gratings of the pulse source  10  (e.g., for optimizing the pulse quality at the output). In certain implementations, the example system  500  of  FIG. 13  advantageously tolerates (e.g., is not substantially affected by optical damage) much higher pulse energies and peak powers than are tolerated by pulse sources utilizing pulse compression in a solid core fiber (see, e.g.,  FIG. 5 ). 
     Certain implementations described herein utilize Yb fiber based pulse sources (see, e.g., U.S. Pat. Nos. 7,688,499 and 9,553,421, each of which is incorporated by reference herein) and are configured to ensure a high optical efficiency of the system. Certain such pulse sources utilize dispersive pulse stretching and pulse compression elements which can be constructed with adaptive control of the dispersion characteristics. In certain implementations, more than one adaptively controlled FBG can be used in such pulse sources. 
     Pulse Transformation 
     In certain implementations, the dispersion modulation enabled by an adaptively controlled FBG (e.g., as schematically illustrated by  FIGS. 1, 2, 5 and 13 ) can also be used for shaping the amplitude of optical pulses or, more generally, for transforming the amplitude and/or phase of optical pulses. This functionality can be particularly advantageous for controlling the pulse amplitude of pulses entering a nonlinear optical element (e.g., the optical fiber pigtail  60   a  in  FIG. 1 ). Though the dispersive modulation imprinted onto the stretched pulses by adaptive fiber stretcher  30  in  FIG. 1  can be small, after compensation of the dispersion of pulse stretcher  30  with the near-opposite dispersion of the pulse compressor  50 , the dispersive modulation can translate to amplitude modulation. 
       FIG. 14  schematically illustrates an example adaptive short pulse transformer system utilizing this principle in accordance with certain implementations described herein. As schematically illustrated in  FIG. 14 , a pulse transformer  600  can be seeded by a short pulse source  601 , and the pulse transformer  600  can comprise a first dispersive element  602  configured for pulse stretching and adjustable phase modulation which can modulate a first dispersion of the first dispersive element  602 . The first dispersive element  602  of certain implementations comprises an adaptively controlled FBG, as discussed herein with respect to  FIG. 2 , while in certain other implementations, the first dispersive element  602  further comprises additional dispersive components (e.g., dispersive fiber or bulk dispersive optics). The pulse transformer  600  can further comprise at least one optical amplifier  603  which can have a gain of unity or higher. 
     The pulse transformer  600  can further comprise a second dispersive element  604 , the second dispersive element  604  having a second dispersion opposite to the first dispersion of the first dispersive element  602 . For example, the absolute value of the first dispersion and the absolute value of the second dispersion can be substantially equal to one another, with the first dispersion and the second dispersion having opposite signs from one another. For another example, the absolute value of at least the second-order dispersion component D 21  of the first dispersive element  602  (e.g., pulse stretcher) and the absolute value of at least the second-order dispersion component D 22  of the second dispersive element  604  (e.g., pulse compressor) are substantially equal to one another (e.g., |D 21 | substantially equal to |D 22 | to within 50%, to within 10%, or to within 1%), with at least the second-order dispersion component D 21  of the first dispersive element  602  and at least the second-order dispersion component D 22  of the second dispersive element  604  having opposite signs from one another. In certain implementations, the second dispersive element  604  comprises an FBG, while in certain other implementations, the second dispersive element  604  further comprises additional bulk optic or dispersive fiber elements. In certain implementations, a dispersion of the at least one optical amplifier  603  can be considered to be a part of the dispersion of the second dispersive element  604 . In certain implementations, the pulse compression FBG can also be provided with adjustable phase modulation, which can modulate the dispersion of the second dispersive element  604  and optimize the pulse quality after compression. 
     Trace  606  of  FIG. 14  schematically represents the pulse intensity versus time for the short input pulses entering the pulse transformer  600  from the source  601 . Trace  607   a  of the inset plot of  FIG. 14  shows the pulse power as a function of time for an example input (e.g., short) pulse entering the pulse transformer  600  and trace  607   b  of the inset plot of  FIG. 14  shows the pulse power as a function of time for an example output (e.g., long) pulse leaving the pulse transformer  600  at output  605 . Traces  607   a,    607   b  were calculated using the dispersion profile parameters shown in  FIG. 3  and with nearly matched second-order dispersion values of the first dispersive element  602  (e.g., pulse stretcher) and the second dispersive element  604  (e.g., pulse compressor), as shown in  FIG. 1 . In certain implementations, additional dispersive and nonlinear optical elements (e.g., as shown in  FIG. 1 ) are configured to ensure that the final output pulse is obtained with high quality. 
     In certain implementations, for the pulse transformer  600  to provide amplitude and phase shaping of an input pulse or to effectively convert dispersion to amplitude modulation, the absolute value of the second-order dispersion components of all the pulse stretching elements (e.g., elements having positive dispersion; comprising an FBG stretcher) of the pulse transformer  600  is matched (e.g., substantially equal) to within 50% (e.g., within 10%; within 1%) of the absolute value of the second-order dispersion components of all the pulse compressing elements (e.g., elements having negative dispersion; comprising an FBG compressor) of the pulse transformer  600 . In certain such implementations, the pulse transformer  600  is configured to effectively convert dispersion modulation to amplitude modulation or to generally function via phase to amplitude conversion. 
     In certain implementations, optional pulse stretching, amplification, and pulse compression stages can be included down-stream of output  605  for further pulse manipulation. These stages can comprise standard fiber or bulk optic components, as well known in the state of the art and are not further shown here. 
     To add an adaptive functionality to pulse transformer  600 , a small fraction of the light from output  605  can be directed (e.g., via beam splitter  608 ) to a pulse diagnostic element  609 . For example, the pulse diagnostic element  609  can comprise an autocorrelator, a frequency-resolved gating instrument, and/or other devices (e.g., as previously discussed with respect to  FIGS. 1, 2 and 13 ). Appropriate control signals can then be generated based on signals from the pulse diagnostic element  609  and the control signals can be delivered (e.g., via electrical connection  610 ) to the adjustable first dispersion element  602  or the adjustable dispersion element  604 . To enhance the sensitivity of the pulse diagnostic element  609 , certain implementations include an additional pulse compression stage  611  up-stream of the beam splitter  608 , as schematically illustrated by  FIG. 14 . This pulse compression stage  611  can include nonlinear optical elements, dispersive elements, and optical amplifiers. 
     In certain implementations, the pulse transformer  600  is configured to generate pulse shapes that are robust against to pulse break-up during pulse propagation in the presence of nonlinearity (i.e., self-phase modulation) in the pulse compression stage  611 . For example, the pulse transformer  600  can be configured to generate chirped pulses with a near parabolic shape, which, during nonlinear pulse propagation, are particularly robust against pulse break-up, both in positive and negative dispersion fiber (see, e.g., U.S. Pat. Nos. 9,553,421 and 10,096,962). Certain implementations described herein advantageously accomplish pulse transformation in a more compact form factor than do the configurations of U.S. Pat. Nos. 9,553,421 and 10,096,962 and without constraints on fiber lengths, amplification values, or dispersion parameters of the fibers involved. This additional flexibility can be partially facilitated via the use of the adjustable and adaptive pulse transformer  600  of certain implementations described herein. 
     In certain implementations, adaptive control of the pulse shape by the pulse transformer  600  produces pulses with a reduced pulse curvature near the center of the pulse, which can propagate along long fiber lengths while avoiding pulse break-up. As used herein, pulse curvature (PC) can be defined as the time derivative of the pulse slope: PC=d 2 I(t)/dt 2 , where I(t) is the pulse intensity as a function of time t. Generally, any physically possible pulses exhibit a constant PC at the highest pulse intensity, whereas PC typically decreases towards the periphery of the pulses. Pulses with a gaussian or sech 2  pulse profile are well known examples of such pulses. In contrast, for parabolic pulses, PC is constant over most of the extent of the pulse, which can produce a particularly stable pulse in the presence of nonlinear pulse propagation. In general, there is a continuum of different pulse shapes ranging from sech 2  to gaussian to parabolic. For example, quartic solitons (e.g., as recently described by A.F.J. Runge et al., “The pure-quartic soliton laser,” arXiv: 1910.10314, 2019) can also be particularly stable in the presence of nonlinear pulse propagation. For certain implementations described herein, pulses with a reduced pulse curvature can be defined as near parabolic pulses that have a reduced PC compared to a standard Gaussian or sech 2  pulses for the same 1/e pulse width (e.g., temporal separation of the points where the pulse intensity decreases to 1/e compared to the peak intensity). This definition also includes quartic soliton pulses under the more general heading of near-parabolic pulses. Other pulse forms that fall within this definition include pulses that have a time-dependent intensity profile that resembles at least partially Jacobi elliptic functions (e.g., the sn Jacobi elliptic function), which can also have a reduced PC near the center of the pulse. 
     Apart from the generation of near parabolic pulses via the adaptively controlled pulse transformer  600  in accordance with  FIG. 14 , as discussed here, near parabolic pulses can also be generated via nonlinear amplification in positive dispersion fiber amplifiers (see, e.g., U.S. Pat. No. 8,031,396). In certain implementations, a pulse transformer can comprise a negative dispersion fiber that is configured to receive a stretched pulse, where the pulse transformer is configured to generate near parabolic pulses from an input pulse. In certain such implementations, the stretched pulses are then submitted to some nonlinearity or to self-phase modulation (e.g., self-phase modulation values less than 10) in a negative dispersion fiber (e.g., using an amplifier), and are recompressed to close to the bandwidth limit. The final pulse can resemble a near parabolic pulse shape. 
       FIG. 15  schematically illustrates an example substantially static and near-parabolic short pulse transformer  700  (e.g., a pulse transformer generating near-parabolic pulses) in accordance with certain implementations described herein. The pulse transformer  700  comprises a first dispersive element  702  with a positive second-order dispersion component D 21  (e.g., optical pulse stretcher) configured to receive near-bandwidth limited pulses generated by a pulse source  701  and to stretch the received pulses. The pulse transformer  700  further comprises a fiber amplifier  703  configured to subject the stretched pulses to self-phase modulation and a second dispersive element  704  (e.g., pulse compressor) having a second-order dispersion component D 22  substantially opposite to a second-order dispersion component D 21  of the first dispersive element  702 , the second dispersive element  704  configured to compress (e.g., recompress) the stretched pulses. In certain implementations, the amount of self-phase modulation in the fiber amplifier  703  can be small compared to the self-phase modulation induced by an additional negative dispersion fiber (not shown) in optical communication with the fiber amplifier  703 . The pulse transformer  700  is configured to generate near parabolic pulses at the output  705 . In  FIG. 15 , the arrows point to the positions along the pulse transformer  700  at which the respective pulse forms are observable. In certain other implementations, optional pulse stretching, amplification, and pulse compression stages (e.g., fiber or bulk optic components) can also be included down-stream of output  705  for further pulse manipulation. 
     Certain implementations in accordance with  FIG. 15  are nearly static in that certain such implementations do not include adaptive control of the pulse shape (e.g., in contrast to certain implementations in accordance with  FIG. 14 ), or include only very limited adaptive control (e.g., via changing the amplifier or the optical powers of the seed pulses from the pulse source  701 ). Thus, the pulse transformer  700  of  FIG. 15  can be simpler than the pulse transformer  600  of  FIG. 14  or the pulse source  10  of  FIG. 1 . The pulse transformer  700  of certain implementations can also allow for the generation of high energy pulses in the presence of large values of self-phase modulation (e.g., self-phase modulation values greater than or equal to 3). 
     Referring back to  FIG. 14 , in certain implementations, dispersion modulation in an adaptive pulse transformer  600  can also modify or modulate the amplitude of the stretched pulse, which via self-phase modulation in the stretched pulse can allow for compensation of higher-order dispersion terms (e.g., as encountered in a high power chirped pulse amplification system, as discussed in U.S. Pat. Nos. 6,885,683 and 10,096,962). 
     Generally, the pulse transformers as discussed herein with respect to  FIGS. 1, 14, and 15  can be configured to: (i) subject a stretched pulse to self-phase modulation; (ii) implement amplitude shaping of a stretched pulse via dispersion modulation of the corresponding fiber Bragg grating stretcher (e.g., as discussed herein); (iii) implement amplitude shaping via phase to amplitude conversion (e.g., as discussed herein with respect to  FIG. 14 ); and (iv) implement amplitude shaping via subjecting a stretched pulse to self-phase modulation in conjunction with compressing the stretched pulse back to near the transform limit (e.g., as discussed herein with respect to  FIG. 15 ). In certain implementations, these four types of pulse transformation can be utilized at the same time to obtain optimum results. For example, such pulse transformers can optimize the output pulse quality of a high power amplifier system in the presence of self-phase modulation and non-compensated dispersion. In certain implementations, such pulse transformers can rely on both modulation of amplitude and phase of an input pulse. FBG pulse stretchers or FBG pulse compressors are particularly useful elements of such pulse transformers in accordance with certain implementations described herein by facilitating both amplitude and phase shaping of an input pulse (e.g., as discussed herein with respect to  FIG. 14 ). Alternatively, a pulse transformer of certain other implementations can be non-adaptive and can rely on subjecting stretched pulses to self-phase modulation (e.g., as discussed herein with respect to  FIG. 15 ). Certain implementations that utilize pulse manipulation as described herein (e.g., as discussed with respect to  FIGS. 3 and 4 ), sub 200 fs pulses (e.g., sub 100 fs pulses) can be generated from an Er-fiber amplifier system with a pulse energy&gt;30 nJ (e.g., a pulse energy&gt;50 nJ; a pulse energy&gt;100 nJ). In certain implementations, the average power of the Er-fiber amplifier system can be greater than 3 W (e.g., greater than 5 W; greater than 10 W). 
     In certain implementations (e.g., using configurations schematically illustrated by  FIG. 1, 2, 5, 14 , or  15 ), the dispersion mismatch between the at least one pulse stretcher  30  (e.g., first dispersive element  602 ,  702 ) and the at least one pulse compressor  50  (e.g., second dispersive element  604 ,  704 ) is increased to increase the pulse energies that are generated. Certain such implementations can produce a longer and more strongly chirped pulse that enters the optical fiber pigtail  60   a  and thus increases the pulse energy limits for spectral bandwidth broadening in the optical fiber pigtail  60   a.  In certain implementations, the pulses are compressed back to near the bandwidth-limit (e.g., using a prism, grism, or grating compressor), with larger dispersion values than are provided with chirped mirrors. In certain implementations, the first dispersive element  602 ,  702  and/or the second dispersive element  604 ,  704  comprise volume Bragg gratings and/or bulk stretchers and compressors instead of fiber Bragg grating stretchers and compressors. 
     In certain implementations (e.g., using configurations schematically illustrated by  FIG. 1, 2, 5, 14 , or  15 ), to increase the pulse energies that are generated, the optical fiber pigtail  60   a  comprises a large mode area fiber (e.g., having larger mode sizes than standard optical fibers) instead of a standard nonlinear optical fiber. Alternatively, in certain implementations, the optical fiber pigtail  60   a  comprises other fiber types (e.g., fluoride fibers, chalcogenide fibers, gas-filled hollow core fibers). Optical bandwidth broadening and pulse compression in such structures can be advantageously implemented in certain implementations by using chirped near parabolic pulse forms for pulse compression, which can be generated, for example, via the pulse transformers as discussed here. Since near parabolic pulse compression utilizes stretched pulses in the bandwidth-broadening stage and further utilizes a final compression stage to obtain short pulses (e.g., the shortest possible pulses), the peak pulse power during bandwidth broadening and propagation through the fiber in certain implementations is reduced (e.g., allowing for the generation of higher energy pulses in such fibers as compared to the use of soliton compression). In soliton compression, a high peak power pulse is generated within the actual bandwidth broadening fiber. Utilizing parabolic pulse compression in certain implementations can thus overcome optical damage limitations in optical fibers or can reduce the onset of gas ionization in gas-filled hollow core fiber compression, which can otherwise reduce the pulse quality of hollow fiber compressors. 
       FIG. 16  schematically illustrates an example application of an adaptive short pulse near parabolic pulse compressor  800  of high energy pulses in accordance with certain implementations described herein. The pulse compressor  800  of  FIG. 16  is configured to receive a short pulse from the source  801  (e.g., near transform limited). 
     The pulse compressor  800  of  FIG. 16  comprises a pulse transformer  802  (e.g., pulse transformers  600 ,  700 ) configured to transform the optical pulse to a near parabolic pulse shape. The pulse transformer  802  of certain implementations comprises at least one chirped fiber Bragg grating with adaptive dispersion control. As discussed herein with respect to  FIGS. 14 and 15 , the pulse transformer  802  can comprise appropriate pulse stretching and compression stages as well as amplification stages. Adaptive control can be implemented in either of or both the pulse stretching and compression stages. In certain implementation, the pulse transformer  802  can comprise at least one fiber selected from the group consisting of: telecom fiber, large mode area fiber, fluoride, lead glass, telluride, chalcogenide or any soft-glass fiber, waveguides, gas-filled hollow core fiber, as well as a nonlinear optical element inserted into multi-pass cell. In all generality, the components of the group listed above can have positive or negative dispersion. When comprising a positive dispersion nonlinear element, the pulse transformer  802  of certain implementations can provide additional flexibility (e.g., compared to U.S. Pat., Nos. 6,885,683 and 10,096,962). In certain implementations in which the source  801  comprises a solid-state laser pulse source, the pulse transformer  802  can comprise a bulk optical pulse shaper (see, e.g., F. Verluise et al., “Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter: pulse compression and shaping,” Opt. Lett., 25, 575 (2000)) and the pulse compression with parabolic pulses can be extended to pulses with pulse energies beyond 1 mJ. 
     The pulse compressor  800  of  FIG. 16  further comprises a nonlinear bandwidth broadening stage  803  with negative dispersion and configured to receive the near parabolic pulse from the pulse transformer  802 . The bandwidth broadening stage  803  can comprise at least one fiber selected from the group consisting of: negative dispersion telecom fiber, large mode area fiber, fluoride, lead glass, telluride, chalcogenide or any soft-glass fiber, any negative dispersion waveguide, gas-filled hollow core fiber operating in the negative dispersion regime, as well as a nonlinear optical element inserted into multi-pass cell (see, e.g., J. Weitenberg et al., “Multi-pass-cell-based nonlinear pulse compression to 115 fs at 7.5 μJ pulse energy and 300 W average power,” Opt. Lett., 25, 20502 (2017)). In certain implementations, using the near parabolic pulses for pulse compression advantageously reduces the peak power inside the bandwidth broadening stage  803  and/or applies the compression to nonlinear optical materials operating at wavelengths greater than or equal to 1.5 μm, where the majority of high quality optical materials have negative dispersion. 
     The pulse compressor  800  of  FIG. 16  further comprises a dispersive element  804  (e.g., pulse compressor) configured to compress (e.g., recompress) the pulses received from the bandwidth broadening stage  803  (e.g., to close to the bandwidth limit), and to emit the resulting pulses at an output  805 . In certain implementation, the dispersive element  804  can comprise at least one element selected from the group consisting of: a chirped mirror pair; a bulk compressor assembled from bulk gratings, grisms, or prisms; a volume Bragg grating, a piece of bulk optical material. 
     As schematically illustrated in  FIG. 16 , trace  806  represents the short near bandwidth-limited pulse at the input to the pulse compressor  800 , trace  807  represents the near parabolic pulse shape obtained at the output of the pulse transformer  802 , and trace  808  represents the compressed pulses generated at the output  805 . 
     In certain implementations, additional pulse compression stages can be included downstream of outputs  605 ,  705 ,  805  (referring to  FIGS. 14, 15 and 16 ). These additional pulse compression stages can be used to compress the output pulses to the few cycle regime (e.g., as discussed herein with respect to  FIG. 5 ). 
     High Power Oscillators 
     High power fiber amplifier systems (e.g., the example pulse sources  10  schematically illustrated in  FIGS. 1 and 5 ) can have a relatively high component count, since compact mode-locked fiber oscillators (e.g., as used for frequency combs) typically only generate a few hundred pJ of pulse energy. Hence, such high power fiber amplifier systems can utilize several amplifier stages, including several isolators, optical pump couplers, etc. For example, mode-locked fiber oscillators that operate close to zero intra-cavity dispersion that generally exhibit the lowest pulse jitter and the lowest carrier phase noise cannot be operated at pulse energies exceeding about 1 nJ without great experimental difficulty. 
     In contrast, the FBG pulse stretchers and compressors of certain implementations discussed herein are configured to allow operation of dispersion-compensated mode-locked fiber lasers at pulse energies exceeding 1 nJ (e.g., exceeding 100 nJ), offering new opportunities for high power ultra-low noise fiber oscillator systems. In certain implementations, adaptive control of the dispersion properties of these FGBs can further optimize the performance of such oscillator systems. 
       FIG. 17A  schematically illustrates an example near dispersion compensated high energy (e.g., high power) mode-locked (e.g., passively mode-locked) oscillator  900  incorporating a highly dispersive fiber Bragg grating in accordance with certain implementations described herein. The oscillator  900  of  FIG. 17A  comprises a chirped FBG  901  and a fiber gain section  902 . The FBG  901  and the fiber gain section  902  can be spliced together for robust operation and convenience. The FBG  901  can also include adaptive dispersion control (not shown). In certain implementations, the FBG  901  is configured to receive pump light from a pump source (not shown) and the pump light is directed through the FBG  901  for pumping the fiber gain section  902  (e.g., via cladding pumping or core pumping). 
     The oscillator  900  of  FIG. 17A  further comprises at least one first lens  903  configured to collimate light emitted from a free end of the fiber gain section  902 , a bulk dispersive element  905  (e.g., bulk grating pair or other dispersive elements) configured to receive the collimated light  906  from the at least one first lens  903  (the collimated light traversing the bulk dispersive element  905  shown by arrow  907 ), and at least one second lens  904  configured to focus (e.g., refocus) the collimated light  908  after traversing the bulk dispersive element  905 . 
     The oscillator  900  of  FIG. 17A  further comprises a generalized saturable absorber  909  configured to receive the focused light from the at least one second lens  904 , the saturable absorber  909  configured to reflect at least a portion of the received light. After reflection from the saturable absorber  909 , the reflected light passes again through the at least one lens  904 , the bulk dispersive element  905 , the at least one first lens  903 , and the fiber gain section  902 , and is reflected from the FBG  901 . The oscillator  900  is configured to use reflections from the FBG  901  and the saturable absorber  909  to repeatedly pass light forward and backward through the oscillator  900 . During the forward and backward passing of light through the bulk dispersive element  905 , an intra-cavity pulse is subject to the same dispersion. As schematically illustrated by  FIG. 17A , the output  910  can be extracted from the non-reflected portion of the light that is transmitted through the FBG  901 . In certain other implementations, the output  910  can be extracted from other locations in the oscillator  900  (e.g., via appropriate beam splitters). 
     In certain implementations, for a dispersion compensated cavity, the dispersions of the FBG  901  and the bulk dispersive element  905  (e.g., including the dispersions of other cavity components) are matched to one another (e.g., having equal magnitudes to within 10%). For example, in certain implementations, as schematically illustrated by  FIG. 17A , the FBG  901  has a dispersion of +2D 21  and the bulk dispersive element  905  has a single-pass dispersion of −D 22  having a magnitude of about half the dispersion of the FBG  901  and having opposite sign from the dispersion of the FBG  901 . 
     In certain other implementations, instead of the bulk dispersive element  905 , the oscillator  900  comprises two additional FBGs with about half of the opposite dispersion of the FBG  901  and arranged via an optical circulator. In such an arrangement, the light is first reflected from the first additional FBG and then directed to the fast generalized saturable absorber  909 , and after reflection from the second additional FBG is directed back to the intra-cavity gain fiber  902 . In certain such implementations, the circulator is configured to allow for an integrated arrangement of the optics, but the fiber pigtails of the circulator may limit achievable pulse energies in some implementations. In certain other implementations (not shown), the example oscillator  900  of  FIG. 17A  is configured to use circulators in a ring cavity configuration. In certain implementations (not shown), the FBG  901  of  FIG. 17A  can also be replaced with a bulk optic positive dispersion producing element. In certain other implementations (not shown), the locations of the positive and negative dispersion elements of  FIG. 17A  can be switched around. In certain implementations, the grating pair  905  of  FIGS. 17A and 17B  generates a spatial chirp in collimated light beam  908  and the at least one lens  904  as well as saturable absorber  909 . For example, this spatial chirp can be compensated by using an arrangement with two successive grating pairs (not shown). 
     In certain implementations, the dispersion of the FBG  901  and the bulk dispersive element  905  are configured such that the total second-order cavity dispersion is substantially equal to zero (e.g., the total cavity dispersion D 2cavity  is smaller than 10% of the second-order dispersion D 2FBG  of the FBG  901 ). In certain implementations in which the bulk dispersion element  905  has a single-pass dispersion which is about half of the dispersion of the FBG  901 , a short pulse is generated at the location of the saturable absorber  909  and a chirped pulse is coupled out of the FBG  901 . Moreover, during propagation through the intra-cavity fiber  902 , the pulses can be strongly chirped, minimizing any nonlinearity from said fiber. 
     In certain implementations, the saturable absorber  909  comprises a semiconductor saturable absorber or any type of “fast” saturable absorber. The saturable absorber  909  can rely on the optical Kerr effect in a nonlinear optical element. For example, the Kerr effect can induce self-focussing or nonlinear polarization ellipse rotation in a nonlinear element that can be used as a fast saturable absorber  909 . In certain other implementations, the saturable absorber  909  can comprise a short nonlinear fiber pigtail utilizing nonlinear polarization evolution in the nonlinear fiber pigtail (see, e.g., U.S. Pat. No. 5,689,519). In brief, nonlinear polarization evolution in a short length of fiber that is part of a Fabry-Perot cavity can be used as a fast saturable absorber  909  by the addition of appropriate waveplates and optional non-reciprocal optical elements on at least one end of the optical fiber. 
       FIG. 17B  schematically illustrates another example near dispersion compensated high energy mode-locked oscillator  900  incorporating a highly dispersive fiber Bragg grating in which the saturable absorber  909  of  FIG. 17A  is replaced with a short length of fiber  911  in accordance with certain implementations described herein. As schematically illustrated by  FIG. 17B , the at least one second lens  904  is configured to couple the intra-cavity light  908  into the fiber  911 . To utilize nonlinear polarization evolution in the fiber  911  as a fast saturable absorber, certain implementations comprise at least one third lens  912  a reflective mirror  913 , and at least two polarization manipulation elements  914 ,  915  having the fiber  911  therebetween. The at least one third lens  912  is configured to collimate and direct the output of the fiber  911  to the reflective mirror  913 . The at least two polarization manipulation elements  914 ,  915  can comprise optical waveplates, polarizers, optical bandpass filters, and/or non-reciprocal elements such as Faraday rotators configured to optimize the fast saturable absorber action of the fiber  911 , the at least one third lens  912 , the reflective mirror  913 , and the at least two polarization manipulation elements  914 ,  915  and/or to ensure environmental stability. 
     In certain implementations, the oscillator  900  further comprises electro-optic repetition rate and loss modulators configured to facilitate the onset of mode-locking and/or fast modulation of the repetition rate and carrier envelope offset frequency of the mode-locked oscillator  900 . The oscillator  900  of certain implementations can use both negative and positive dispersion gain fiber. Fiber gain media such as Nd, Yb, Er, Er/Yb, Tm, Ho doped fiber or any other rare-earth doped fiber can be used as a gain medium. 
       FIGS. 18A-18D  further explain operation of the example oscillators  900  schematically illustrated by  FIGS. 17A and 17B . For  FIGS. 18A-18D , the dispersion of the FBG  901  is 13 ps 2  and is matched to the double pass dispersion of the bulk dispersive element  905 , which can be a bulk grating pair with groove density of 1200 lines/mm and a separation or around 1.5 cm. To compensate for the large value of third- and fourth-order dispersion in the bulk grating pair, the FBG  901  can be designed with appropriate values of third- and fourth-order dispersion. 
       FIG. 18A  schematically illustrates an example evolution of pulse energy along an Er gain fiber as a function of intra-cavity gain fiber length (in meters) in the example mode-locked oscillator of  FIG. 17A . The dashed curve of  FIG. 18A  shows the pulse energy evolution from the FBG  901  to the intra-cavity bulk dispersion element  905 , following the direction of arrow  1  of  FIG. 17A . The solid curve of  FIG. 18A  shows the pulse energy evolution from the intra-cavity bulk dispersion element  905  to the FBG  901 , following the direction of arrow  2  of  FIG. 17A . For  FIG. 18A , the FBG  901  has a reflectivity of 10% (such that most of the cavity loss is located at the FBG end), the Er gain fiber has a length of 0.5 m and a mode diameter of about 5 μm with positive dispersion.  FIG. 18A  shows that a pulse energy greater than or equal to 100 nJ can be extracted from the cavity schematically illustrated by  FIG. 17A . This energy can be extracted from a nearly dispersion-compensated cavity, which can be a great improvement as compared to the energy extraction from other dispersion-compensated Er oscillators (e.g., having a maximum of about 1 nJ). 
       FIG. 18B  schematically illustrates an example relative pulse peak powers as a function of time of the shortest pulses and longest pulses generated in the example mode-locked oscillator of  FIG. 17A .  FIG. 18B  shows the pulse shape  960  after reflection from the FBG  901  and the pulse shape  970  at the location of the fast saturable absorber  909 . The pulse width after reflection from the FBG  901  is about 100 ps, and the pulse width at the location of the fast saturable absorber  909  is about 200 fs (e.g., the pulse width changes by about a factor of 500 around the cavity). Hence, during propagation through the gain fiber  902  of  FIG. 17A , the pulse width can be greatly increased and self-phase modulation inside the intra-cavity fiber can be minimized, explaining the large achievable pulse energies even for a dispersion compensated cavity. 
     In certain implementations, the oscillator  900  of  FIG. 17A  advantageously provides pulse width changes of about a factor of 10 around the cavity for a FBG dispersion greater than or equal to 1.0 ps 2 . The pulse width changes can be measured by comparing the pulse width emerging from the chirped FBG and the pulse width at the location of the saturable absorber. In certain implementations, the absolute value of the FBG second-order dispersion component |D 2FBG | (e.g., +13 ps 2 ) is at least 20 times larger (e.g., at least 100 times larger) than the absolute value of the total second-order dispersion component |D 2fiber | of the intra-cavity fiber (e.g., a round trip Er fiber dispersion of 0.035 ps 2 , resulting in a ratio of absolute values of about 371) and a pulse width (e.g., full width at half maximum width) extractable from the cavity after traversing down-stream dispersion compensation elements is less than or equal to 1 ps. In contrast, previous systems (e.g., M. E. Fermann et al., “Generation of 10 nJ from a modelocked Er fibre laser,” Electronics Letters, 31, 194 (1994)) generated pulses with pulse widths of 300 fs using an FBG with a second-order dispersion component approximately equal to 3.5 ps 2 , the second-order dispersion components of all the intra-cavity fiber of the oscillator summed to about −0.27 ps 2  (e.g., resulting in a ratio between FBG and fiber dispersion of about 13). 
       FIG. 18C  schematically illustrates an example close-up view of the shortest pulse generated in the example mode-locked oscillator  900  of  FIG. 17A , and  FIG. 18D  schematically illustrates the corresponding example pulse spectrum of pulses generated in the mode-locked oscillator  900  of  FIG. 17A . The solid line of  FIG. 18C  shows the pulse shape  970  at the location of the saturable absorber  909  has a pulse width of about 200 fs. The equivalent bandwidth-limited pulse, shown by the dashed line of  FIG. 18C , is essentially indistinguishable from the generated pulse at the resolution of  FIG. 18C . 
     In certain implementations, by adjusting the dispersion profile of the FBG, pulse stability and achievable pulse energy inside the oscillator can be optimized. For example, the introduction of fourth-order and/or sixth-order dispersion in the FBG can improve pulse stability in the presence of negative dispersion gain fiber (e.g., Er, Er/Yb, Tm, Ho, or Ho/Yb doped fiber amplifiers) or positive dispersion gain fiber (e.g., Yb, Nd and Er fiber amplifiers). The cavity of certain implementations can operate equally well with positive as well as negative dispersion gain media. 
     In certain implementations, the pulse energies extractable from the cavities shown in  FIGS. 17A and 17B  can be in a range of 1 to 2 orders of magnitude higher than the maximum pulse energies extractable from standard dispersion compensated fiber oscillators. With optimized dispersion compensation, the pulse width generated from fiber oscillators as shown in  FIG. 17A or 17B  can be smaller than 100 fs. To generate the shortest possible pulses and to minimize the pump power utilized, it can also be useful to minimize intra-cavity loss as much as possible. The timing jitter from certain implementations described herein can be in a range of 3 to greater than 10 times smaller as compared to the timing jitter of standard oscillators, since quantum-limited timing jitter is inversely proportional to the square root of pulse energy. To minimize timing jitter, it can also be useful to generate the shortest possible pulses and to minimize intra-cavity loss as much as possible. Hence, certain implementations described herein provide high power oscillators that can be very useful as ultra-low noise microwave sources, since the achievable phase noise floor of a microwave source based on the stability of the repetition rate of an optical oscillator is inversely proportional to the mode-locked oscillator pulse energy for a given pulse bandwidth. For example, the shot-noise limited phase noise of a 10 GHz microwave signal generated by a mode-locked Er fiber laser according to certain implementations described herein can be less than or equal to −170 dBc/Hz at a carrier offset frequency of 10 kHz, and in certain implementation, a shot noise limited phase noise less than or equal to −180 dBc/Hz can be reached. To minimize timing jitter, athermal packaging of the FBG or at least some of the intra-cavity fiber can be implemented in certain implementations described herein. Athermal packaging of the intra-cavity fiber can, for example, comprise fiber with reduced or negative thermal coefficient of delay. 
     In addition, the individual cavity modes of the oscillator of certain implementations described herein can have a comparatively high power and therefore a low level of shot noise, as compared to standard fiber frequency combs, which can be useful for ultra-high precision optical frequency transfer with frequency combs, precision optical clocks, and metrology applications. 
     Additional Aspects 
     In a first aspect, a nonlinear fiber laser based chirped pulse amplification system is configured to generate output pulses in the femtosecond pulse width range. The system comprises a seed pulse source configured to produce short optical pulses, a fiber Bragg grating (FBG) pulse stretcher system configured to stretch said pulses, at least one amplifier, at least one FBG compressor configured to compress said pulses, and a bulk dispersive element for further compressing the pulses emerging from the FBG compressor. The FBG stretcher dispersion is configured to optimize the pulse quality of said output pulses at a designated elevated power level, said optimization in pulse quality producing a functional dependence of pulse quality on average pulse power or pulse energy in at least three stages: at low powers, the output pulses have a longer temporal width compared to said designated power level, at medium powers, the output pulses exhibit several side pulses with an intensity higher than any side pulses produced at said designated power level, and at said designated power level, the pulse quality is optimized, as characterized by side pulses with an intensity smaller than observed at medium power levels. 
     In a second aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspect 1, wherein the pulses propagating in said FBG compressor are subject to integrated self-phase modulation phase values greater than 1. 
     In a third aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspect 1, further comprising at least one optical fiber for further pulse compression or spectral broadening. 
     In a fourth aspect, a nonlinear fiber laser based chirped pulse amplification system is configured to generate output pulses in the sub-30 femtosecond pulse width range. The system comprises a seed pulse source configured to produce short optical pulses, a fiber Bragg grating (FBG) pulse stretcher system configured to stretch said pulses, at least one amplifier, at least one FBG compressor configured to compress said pulses, a bulk dispersive element configured to further compress the pulses emerging from the FBG compressor, and at least one optical fiber for further pulse compression of the pulses emerging from the FBG compressor. 
     In a fifth aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspect 4, further configured to produce pulses with pulse widths less than 15 fs. 
     In a sixth aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspect 4, further configured to produce sub-3 cycle pulses. 
     In a seventh aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspects 1 or 4, further configured to generate pulses with a pulse energy greater than 20 nJ. 
     In an eighth aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspects 1 or 4, further configured to generate over half an octave of output light with a pulse energy power greater than 10 nJ. 
     In a ninth aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspect 8, further configured to generate a mid-IR output. 
     In a tenth aspect, a nonlinear fiber laser based chirped pulse amplification system according to aspects 1 or 4, said nonlinear chirped pulse amplification system used as a pump source for an optical parametric amplifier. 
     In an eleventh aspect, a nonlinear fiber laser based chirped pulse amplification system according to any one of aspects 1 to 10, further comprising adaptive control of the dispersion characteristics of the FBG stretcher. 
     In a twelfth aspect, a nonlinear fiber laser based chirped pulse amplification system according to any one of aspects 1 to 11, further comprising adaptive control of the dispersion characteristics of the FBG compressor. 
     In a thirteenth aspect, a nonlinear fiber laser based chirped pulse amplification system according to any one of aspects 1 to 12, comprising Er, Yb, Tm, Ho, Er/Yb or Tm/Yb fibers. 
     In a fourteenth aspect, a nonlinear fiber laser based chirped pulse amplification system according to any one of aspects 1 to 13, said FBG compressor being replaced with a volume Bragg grating compressor. 
     In a fifteenth aspect, a nonlinear fiber laser based chirped pulse amplification system according to any one of aspects 1 to 14, further comprising means for coherent addition of at least two pulses. 
     In a sixteenth aspect, an optical source comprises a seed source producing short optical pulses, one or more actuators for controlling the carrier envelope offset frequency of the output of said seed source, a splitter that splits the output of said seed source into an amplifier branch and an f-2f branch, a frequency shifter in said f-2f branch, an f-2f interferometer in said f-2f branch, a photodetector configured to detect the f-2f signal from said f-2f interferometer, a combiner configured to interfere a portion of light from said f-2f branch with a portion of light from said amplifier branch, a photodetector configured to detect light from said combiner, electronics configured to convert the signals from both said photodetectors into a signal representing the carrier envelope offset frequency at the output of said amplifier branch, and a feedback circuit configured to control the carrier envelope frequency at the output of said amplifier branch. 
     In a seventeenth aspect, an optical source according to aspect 16, further comprising an optical amplifier in said amplifier branch. 
     In an eighteenth aspect, an optical source according to aspect 16 or aspect 17, wherein said electronics includes one or more radio frequency generators, and one or more radio frequency mixers. 
     In a nineteenth aspect, an optical source according to any of aspects 16 to 18, further comprising a carrier envelope phase measurement device and a feedback circuit configured to stabilize the carrier envelope phase at the output of said amplifier branch. 
     In a twentieth aspect, an optical source according to aspect 19, further comprising an actuator configured to control the carrier envelope phase of the output of said amplifier branch. 
     In a twenty-first aspect, an optical source according to aspect 20, wherein said actuator is a transmissive plate. 
     In a twenty-second aspect, an optical source according to any of aspects 16 to 21, wherein said seed source and said amplifier comprise the optical source of any one of aspects 1-15. 
     In a twenty-third aspect, a nonlinear fiber laser based chirped pulse amplification system is configured to generate output pulses in the femtosecond pulse width range. The system comprises a seed pulse source configured to produce short optical pulses, at least one fiber Bragg grating (FBG) pulse stretcher or compressor configured to stretch or compress pulses anywhere within said nonlinear fiber based chirped pulse amplification system, adaptive dispersion control of said at least one FBG, and a gas filled hollow fiber compressor for further compression of said output pulses. 
     In a twenty-fourth aspect, a method produces femtosecond pulses with a nonlinear chirped pulse amplification system seeded with an oscillator. The method comprises temporally stretching said pulses with a FBG, amplifying said pulses, and compressing said pulses to produce compressed output pulses. The FBG is configured to optimize the pulse quality of said output pulses at a designated elevated power level, said optimization in pulse quality producing a functional dependence of pulse quality on average pulse power or pulse energy in at least three stages: at low powers, the output pulses have a longer temporal width compared to said designated power level, at medium powers, the output pulses exhibit several side pulses with an intensity higher than any side pulses produced at said designated power level, and at said designated power level the pulse quality is optimized, as characterized by side pulses with an intensity smaller than observed at medium power levels. 
     In a twenty-fifth aspect, the method of aspect 24, wherein a peak intensity of the pulses is at a maximum at the designated power level. 
     In a twenty-sixth aspect, a pulse source comprises an oscillator configured to generate laser pulses and at least one fiber Bragg grating (FBG) pulse stretcher configured to receive the laser pulses from the oscillator and to temporally stretch the laser pulses, the at least one FBG pulse stretcher configured to be adaptively controlled to provide adjustable dispersion. The pulse source further comprises at least one amplifier configured to receive the temporally stretched laser pulses, at least one FBG pulse compressor configured to receive the laser pulses from the at least one amplifier and to temporally compress the laser pulses, and one or more optical compressor components configured to receive and further compress the compressed laser pulses from the at least one FBG pulse compressor. 
     In a twenty-seventh aspect, the pulse source of aspect 26, wherein the laser pulses have pulse widths in a range of 30 to 600 femtoseconds. 
     In a twenty-eighth aspect, the pulse source of aspect 26 or aspect 27, wherein the oscillator comprises a mode-locked fiber laser comprising one or more of the following materials: Er, Nd, Yb, Tm, Ho, and Er/Yb. 
     In a twenty-ninth aspect, the pulse source of any of aspects 26 to 28, wherein the oscillator comprises at least one pre-amplifier configured to amplify the laser pulses prior to being emitted from the oscillator. 
     In a thirtieth aspect, the pulse source of any of aspects 26 to 29, wherein the at least one pulse stretcher is configured to increase the pulse widths of the laser pulses to be in a range of 100 fs to 1000 ps. 
     In a thirty-first aspect, the pulse source of any of aspects 26 to 30, wherein the at least one amplifier comprises a preamplifier and an amplifier. 
     In a thirty-second aspect, the pulse source of any of aspects 26 to 31, wherein the at least one FBG pulse compressor is configured to decrease the pulse widths of the laser pulses to be in a range of 50 to 1000 femtoseconds. 
     In a thirty-third aspect, the pulse source of any of aspects 26 to 32, wherein the one or more optical compressor components comprises an optical fiber pigtail and/or a dispersive free-space compressor. 
     In a thirty-fourth aspect, the pulse source of any of aspects 26 to 33, wherein the one or more optical compressor components comprises at least one optical fiber, chirped mirror, or other optical materials configure to provide a predetermined pulse quality for the laser pulses emitted by the pulse source. 
     Additional Information 
     Example, non-limiting experimental data are included herein to illustrate results achievable by various implementations of the systems and methods described herein. All ranges of data and all values within such ranges of data that are shown in the figures or described in the specification are expressly included in this disclosure. The example experiments, experimental data, tables, graphs, plots, figures, and processing and/or operating parameters (e.g., values and/or ranges) described herein are intended to be illustrative of operating conditions of the disclosed systems and methods and are not intended to limit the scope of the operating conditions for various implementations of the methods and systems disclosed herein. Additionally, the experiments, experimental data, calculated data, tables, graphs, plots, figures, and other data disclosed herein demonstrate various regimes in which implementations of the disclosed systems and methods may operate effectively to produce one or more desired results. Such operating regimes and desired results are not limited solely to specific values of operating parameters, conditions, or results shown, for example, in a table, graph, plot, or figure, but also include suitable ranges including or spanning these specific values. Accordingly, the values disclosed herein include the range of values between any of the values listed or shown in the tables, graphs, plots, figures, etc. Additionally, the values disclosed herein include the range of values above or below any of the values listed or shown in the tables, graphs, plots, figures, etc. as might be demonstrated by other values listed or shown in the tables, graphs, plots, figures, etc. Also, although the data disclosed herein may establish one or more effective operating ranges and/or one or more desired results for certain implementations, it is to be understood that not every implementation need be operable in each such operating range or need produce each such desired result. Further, other implementations of the disclosed systems and methods may operate in other operating regimes and/or produce other results than shown and described with reference to the example experiments, experimental data, tables, graphs, plots, figures, and other data herein. 
     The invention has been described in several non-limiting implementations. It is to be understood that the implementations are not mutually exclusive, and elements described in connection with one implementation may be combined with, rearranged, or eliminated from, other implementations in suitable ways to accomplish desired design objectives. No single feature or group of features is necessary or required for each implementation. 
     For purposes of summarizing the present invention, certain aspects, advantages and novel features of the present invention are described herein. It is to be understood, however, that not necessarily all such advantages may be achieved in accordance with any particular implementation. Thus, the present invention may be embodied or carried out in a manner that achieves one or more advantages without necessarily achieving other advantages as may be taught or suggested herein. 
     As used herein any reference to “one implementation” or “some implementations” or “an implementation” means that a particular element, feature, structure, or characteristic described in connection with the implementation is included in at least one implementation. The appearances of the phrase “in one implementation” in various places in the specification are not necessarily all referring to the same implementation. Conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain implementations include, while other implementations do not include, certain features, elements and/or steps. In addition, the articles “a” or “an” or “the” as used in this application and the appended claims are to be construed to mean “one or more” or “at least one” unless specified otherwise. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are open-ended terms and intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), or both A and B are true (or present). As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: A, B, or C” is intended to cover: A, B, C, A and B, A and C, B and C, and A, B, and C. Conjunctive language such as the phrase “at least one of X, Y and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be at least one of X, Y or Z. Thus, such conjunctive language is not generally intended to imply that certain implementations require at least one of X, at least one of Y, and at least one of Z to each be present. 
     Thus, while only certain implementations have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention. Further, acronyms are used merely to enhance the readability of the specification and claims. It should be noted that these acronyms are not intended to lessen the generality of the terms used and they should not be construed to restrict the scope of the claims to the implementations described therein.