Fiber laser system

Abstract

An optical fiber laser system is described, generally having a pump laser generating a pump laser beam and a fiber segment optically coupled to the pump laser, the fiber segment having a laser cavity with a cavity path, a first fiber Bragg grating having a first reflectivity profile, a second filter having a second filter profile, and an optical gain region along the cavity path between the first fiber Bragg grating and the second filter, the first reflectivity profile spectrally detuned from the second filter profile, the first fiber Bragg grating having a first refractive index profile comprising a full-width-at-half-maximum bandwidth of at least 0.2 nm and a Gaussian-like apodization, wherein, upon pumping the optical gain region with the pump laser beam and mode-locking the laser cavity, optical pulses circulate along the cavity path, and an output.

Description

Technical Field

[0001] This improvement generally relates to the field of fiber laser systems, and more specifically to the generation of optical pulses. Background Technology

[0002] Fiber laser systems typically consist of an active laser fiber, a pump laser to pump the active laser fiber, and a laser cavity that supports laser modes when the active laser fiber is pumped by the pump laser. When the laser modes are locked together, they are forced to constructively interfere with each other in a periodic manner, thereby generating one light pulse at a time. While existing fiber laser systems that generate light pulses meet requirements to a certain extent, there is still room for improvement, particularly in limiting the number of free-space optical components, increasing the energy of each light pulse, and / or generating shorter light pulses. Summary of the Invention

[0003] In one aspect, a fiber laser system is described, which generally has a pump laser for generating a pump laser beam. The fiber laser system has an optical fiber segment optically coupled to the pump laser. The fiber segment has a laser cavity with an optical gain region and two fiber Bragg gratings sandwiching the optical gain region. The two fiber Bragg gratings have reflectivity profiles detuned to each other. Therefore, when the optical gain region is pumped with the pump laser beam, and in the mode-locked laser cavity, only light pulses amplified sufficiently to cause spectral broadening while propagating through the optical gain region are reflected back and forth within the laser cavity between the two fiber Bragg gratings. For example, noise that would be reflected by the reflectivity profile of the first fiber Bragg grating but not amplified sufficient to cause spectral broadening will not be reflected by the reflectivity profile of the second fiber Bragg grating, and vice versa. The fiber laser system has an output end optically coupled to the laser cavity for outputting at least a portion of the light pulses reflected within the laser cavity.

[0004] It has been found that, due to the relatively narrow reflectivity profile bandwidth and relatively moderate reflectivity of conventional uniform fiber Bragg gratings, only a portion of each optical pulse can be effectively reflected within the laser cavity. Therefore, it has been found that by providing a first fiber Bragg grating with a first refractive index profile having a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian apodization, optical pulses with greater pulse energy and / or greater compressibility can be obtained. For example, because Gaussian apodization can limit the presence of any sidelobes in the resulting reflectivity profile, this apodization can benefit stability at higher pulse energies, thereby improving the desirability of the resulting optical pulse. In embodiments with a varying grating period (i.e., chirp) in the first refractive index profile, the first reflectivity profile can experience not only a broadening of its bandwidth but also an increase in its maximum reflectivity value, which can reflect a larger portion of the optical pulse present within the laser cavity.

[0005] According to a first aspect of this disclosure, a fiber laser system is provided, comprising: a pump laser that generates a pump laser beam; an optical fiber segment optically coupled to the pump laser, the optical fiber segment having: a laser cavity having a cavity path, a first fiber Bragg grating having a first reflectivity profile, a second filter having a second filter profile, and an optical gain region located along the cavity path between the first fiber Bragg grating and the second filter, the first reflectivity profile being spectrally detuned to the second filter profile, the first fiber Bragg grating having a first refractive index profile including a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian apodization, wherein, when the optical gain region is pumped with the pump laser beam and the laser cavity is mode-locked, the optical pulse circulates along the cavity path; and an output end optically coupled to the laser cavity and outputting at least a portion of the optical pulse.

[0006] Furthermore, according to a first aspect of this disclosure, the first reflectivity profile of the first fiber Bragg grating may, for example, have a maximum reflectivity value of at least 40%, and a full width at half maximum (FWHM) bandwidth of at least 0.5 nm.

[0007] Furthermore, according to a first aspect of this disclosure, the maximum reflectance value of the first reflectance profile may, for example, be at least 50%.

[0008] Furthermore, according to a first aspect of this disclosure, the full width at half maximum (FWHM) bandwidth of the first reflectivity profile can be, for example, between about 4 nm and about 5 nm.

[0009] Furthermore, according to a first aspect of this disclosure, the first refractive index profile may, for example, have a varying grating period, i.e., a chirp.

[0010] Furthermore, according to a first aspect of this disclosure, the grating period of a corresponding refractive index profile in the first and second refractive index profiles can be varied, for example, linearly, thereby providing linear group delay dispersion on a corresponding profile in the first reflectivity profile and the second filter profile.

[0011] Furthermore, according to the first aspect of this disclosure, the linear group delay dispersion can, for example, be ±0.5 ps. 2 Steeper.

[0012] Furthermore, according to a first aspect of this disclosure, the linear group delay dispersion of the varying grating period can be, for example, at least twice as steep as the linear group delay dispersion provided by the optical gain region.

[0013] Furthermore, according to a first aspect of this disclosure, the second filter profile may, for example, have a maximum reflectance or transmittance value that is smaller than the maximum reflectance value of the first reflectance profile, and the output end is optically coupled to the second filter.

[0014] Furthermore, according to a first aspect of this disclosure, the second filter may be, for example, a second fiber Bragg grating, and the profile of the second filter is a second reflectance profile that is spectrally detuned to the first reflectance profile.

[0015] Furthermore, according to a first aspect of this disclosure, the first fiber Bragg grating and the second fiber Bragg grating can, for example, sandwich at least a portion of the optical gain region between them along the cavity path, such that the cavity path is a linear path along which the optical pulse reflects back and forth between the first fiber Bragg grating and the second fiber Bragg grating.

[0016] Furthermore, according to a first aspect of this disclosure, the second fiber Bragg grating may, for example, have a second refractive index profile having a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian-like apodization.

[0017] Furthermore, according to a first aspect of this disclosure, the second reflectivity profile of the second fiber Bragg grating may, for example, have a maximum reflectivity value of at least 40%, and a full width at half maximum (FWHM) bandwidth of at least 0.5 nm.

[0018] Furthermore, according to the first aspect of this disclosure, the maximum reflectance value of the second reflectance profile may, for example, be at least 50%.

[0019] Furthermore, according to a first aspect of this disclosure, the full width at half maximum (FWHM) bandwidth of the second reflectivity profile can be, for example, between about 4 nm and about 5 nm.

[0020] Furthermore, according to a first aspect of this disclosure, the second refractive index profile may, for example, have a varying grating period.

[0021] Furthermore, according to a first aspect of this disclosure, the fiber laser system may, for example, further include a mode-locking device coupled to the fiber segment and facilitating the mode-locking of the laser cavity.

[0022] Furthermore, according to a first aspect of this disclosure, the mode-locked device may, for example, have a broadening element of at least one of a longitudinally broadened first fiber Bragg grating and a second filter, thereby modifying the spectral mistuning between the first reflectivity profile and the second filter profile.

[0023] Furthermore, according to a first aspect of this disclosure, the mode-locking device may, for example, have an optical coupler that is optically coupled to an optical fiber segment and leads to the mode-locking arm.

[0024] Furthermore, according to a first aspect of this disclosure, the optical gain region may be, for example, a laser active erbium-doped region, the pump laser beam has a wavelength of approximately 980 nm, the first reflectivity profile has a center wavelength of approximately 1550 nm, and the second filter profile has a center wavelength of approximately 1565 nm.

[0025] Furthermore, according to a first aspect of this disclosure, the pump laser may be, for example, a first pump laser, and the fiber laser system further includes a second pump laser that is optically coupled to the fiber segment and propagates a second pump laser beam along the optical gain region during the pumping.

[0026] Furthermore, according to a first aspect of this disclosure, the output end may be, for example, a main output end optically coupled to a second filter, and the fiber laser system includes an auxiliary output end optically coupled to a first fiber Bragg grating.

[0027] Furthermore, according to a first aspect of this disclosure, the output optical pulse may, for example, have a similar sub-profile, which, after compression, has a linearly varying instantaneous frequency and a pulse duration of less than 100 fs.

[0028] Furthermore, according to the first aspect of this disclosure, the optical pulse may, for example, have a pulse energy of at least 10 nJ.

[0029] According to a second aspect of this disclosure, a laser system is provided, comprising: a pump laser that generates a pump laser beam; a laser cavity optically coupled to the pump laser, the laser cavity having a cavity path, a first filter having a first filter profile of a Gaussian shape, a second filter having a second filter profile, and an optical gain region located along the cavity path between the first filter and the second filter, the first filter profile and the second filter profile being spectrally detuned to each other, the first filter being dispersive, thereby imparting a dispersive profile on at least a portion of the first filter profile, wherein, when the optical gain region is pumped with the pump laser beam and the laser cavity is mode-locked, the optical pulse circulates along the cavity path; and an output end optically coupled to the laser cavity and outputting at least a portion of the optical pulse.

[0030] Further according to a second aspect of this disclosure, the first filter may be, for example, a fiber Bragg grating having a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian apodization-like refractive index profile.

[0031] Furthermore, according to a second aspect of this disclosure, the refractive index profile may, for example, have a varying grating period.

[0032] Furthermore, according to a second aspect of this disclosure, at least a portion of the laser cavity may be, for example, an optical fiber.

[0033] After reading this disclosure, many further features and combinations thereof regarding the present improvement will become apparent to those skilled in the art. Attached Figure Description

[0034] In the attached diagram,

[0035] Figure 1 This is a schematic diagram of an example of a fiber laser system according to one or more embodiments, the fiber laser system having a pump laser, an optical gain region, and a first fiber Bragg grating and a second fiber Bragg grating sandwiching the optical gain region.

[0036] Figure 1A It is shown according to one or more embodiments Figure 1 A graph of an example of the first reflectivity profile of the first fiber Bragg grating, also showing the group delay varying over at least a portion of the first reflectivity profile;

[0037] Figure 1B It is shown according to one or more embodiments Figure 1 A plot of an example of the second reflectivity profile of the second fiber Bragg grating, also showing constant group delay;

[0038] Figure 1C It is shown according to one or more embodiments Figure 1 A graph of another example of the second reflectivity profile of the second fiber Bragg grating also shows the group delay varying over at least a portion of the second reflectivity profile;

[0039] Figure 2 It is shown according to one or more embodiments Figure 1 A graph of an exemplary refractive index profile of the first fiber Bragg grating, showing chirp and Gaussian-like apodization;

[0040] Figure 3 According to one or more embodiments Figure 1 A schematic diagram of a fiber laser system, showing a mode-locking device including a stretching element of a stretching second fiber Bragg grating;

[0041] Figure 3A It is shown according to one or more embodiments Figure 1 A graph showing the reflectivity profiles of the first and second fiber Bragg gratings in a fiber laser system.

[0042] Figure 4 This is a schematic diagram of another example of a fiber laser system according to one or more embodiments, the fiber laser system having a pump laser, an optical gain region, a first fiber Bragg grating and a second fiber Bragg grating sandwiching the optical gain region, a mode-locked arm, and a pump reflector.

[0043] Figure 4A According to one or more embodiments Figure 4A schematic diagram of the first example of a clamping arm;

[0044] Figure 4B According to one or more embodiments Figure 4 A schematic diagram of a second example of a clamping arm;

[0045] Figure 5 This is a schematic diagram of another example of a fiber laser system according to one or more embodiments, the fiber laser system having two pump lasers, an optical gain region, a first fiber Bragg grating and a second fiber Bragg grating sandwiching the optical gain region, and a mode-locked arm;

[0046] Figure 6 This is a schematic diagram of an example of a fiber laser system according to one or more embodiments, the fiber laser system having a ring laser cavity incorporating a first and a second fiber Bragg grating.

[0047] Figure 7 This is a schematic diagram of another example of a fiber laser system according to one or more embodiments, the fiber laser system having an annular laser cavity incorporating a first fiber Bragg grating and a second dielectric transmission filter;

[0048] Figure 8 This is a schematic diagram of another example of a fiber laser system according to one embodiment, the fiber laser system having a laser cavity having an active fiber laser region sandwiched between a first chirped fiber Bragg grating and a second chirped fiber Bragg grating, and having a first mode-locking device and a second mode-locking device.

[0049] Figure 8A It shows Figure 8 The first reflectivity profile of the first chirped fiber Bragg grating (with a full width at half maximum (FWHM) of 4.7 nm and a maximum reflectivity value of 65%) is shown to be -0.87 ps. 2 The curve of group delay dispersion;

[0050] Figure 8B It shows Figure 8 The second reflectivity profile of the second chirped fiber Bragg grating (with a full width at half maximum (FWHM) of 4.5 nm and a maximum reflectivity value of 41%) is shown to be -0.82 ps. 2 The curve of group delay dispersion;

[0051] Figure 9A It shows the result of Figure 8 The simulated and experimental spectra of the light pulses generated by the fiber laser system are plotted.

[0052] Figure 9B It shows the result of Figure 8The simulated and experimental autocorrelation traces of the optical pulses generated by the fiber laser system are plotted.

[0053] Figure 9C It shows the result of Figure 8 The simulated pulse envelope of the compressed optical pulse generated by the fiber laser system and the curve of the measured pulse envelope;

[0054] Figure 9D It shows the result of Figure 8 A graph comparing the emission spectrum of the optical pulses generated by the fiber laser system with the noise level.

[0055] Figure 10A The graph shows the simulated peak power and pulse energy of an optical pulse after second-order dispersion compensation based on a first-chilled fiber Bragg grating for a second chirped fiber Bragg grating with a maximum reflectivity of 41%.

[0056] Figure 10B The graph shows the simulated peak power and pulse energy of the optical pulse after second-order dispersion compensation based on the first chirped fiber Bragg grating for a second chirped fiber Bragg grating with a maximum reflectivity value of 15%.

[0057] Figure 11A It shows according to Figure 8 The pulse parabolic mismatch parameter M of the length of the active doped region in the fiber of a fiber laser system 2 =∫(II) fit ) 2 dt / ∫I 2 A graph illustrating the exemplary evolution of dt;

[0058] Figure 11B It is shown along Figure 8 The curves of pulse and gain spectrum of the optical pulse at the first longitudinal position of the length of the active doped region of the fiber in the fiber laser system.

[0059] Figure 11C It is a graph showing the pulse and gain spectrum of the optical pulse at a second longitudinal position spaced apart from the first longitudinal position;

[0060] Figure 12A This is a graph illustrating another example of the refractive index profile of a fiber Bragg grating operating at 1552 nm in an erbium-based laser cavity, according to one or more embodiments.

[0061] Figure 12B It is shown according to one or more embodiments Figure 12A A graph of an exemplary reflectance profile of a fiber Bragg grating;

[0062] Figure 13AThe graph illustrates another example of the refractive index profile of a fiber Bragg grating operating at 1050 nm in a ytterbium-based laser cavity, according to one or more embodiments; and

[0063] Figure 13B It is shown according to one or more embodiments Figure 13A A graph of an exemplary reflectance profile of a fiber Bragg grating. Detailed Implementation

[0064] Figure 1 An example of a fiber laser system 100 is shown. As illustrated, the fiber laser system 100 has a pump laser 102 that generates a pump laser beam 104 and an optical fiber segment 106 optically coupled to the pump laser 102.

[0065] The fiber segment 106 has a laser cavity 108, which has a cavity path 109, an optical gain region 110, a first fiber Bragg grating 112 along the cavity path 109, and a second filter 111 spaced apart from the first fiber Bragg grating 112 along the cavity path 109, wherein the optical gain region 110 is located between the first fiber Bragg grating 112 and the second filter 111. The fiber segment 106 also has an output end 116, which is optically coupled to the laser cavity 108 for outputting optical pulses 118 generated within the laser cavity 108.

[0066] In this specific embodiment, the second filter 111 is configured as a second fiber Bragg grating 114. As shown, the first fiber Bragg grating 112 and the second fiber Bragg grating 114 sandwich the optical gain medium 110 in the middle. In this case, the cavity path 109 is a linear path, and the optical pulse 118 reflects back and forth between the first fiber Bragg grating 112 and the second fiber Bragg grating 114 along this linear path.

[0067] Each of the first fiber Bragg grating 112 and the second fiber Bragg grating 114 has a corresponding refractive index profile etched along the corresponding portions 112a, 114a of the fiber segment 106, thereby imparting a corresponding reflectivity profile to the first fiber Bragg grating 112 and the second fiber Bragg grating 114.

[0068] In this specific example, the first fiber Bragg grating 112 has a first reflectivity profile that is detuned to the second reflectivity profile of the second fiber Bragg grating. Figure 1A An example of a first reflectivity profile 122 of a first fiber Bragg grating 112 is shown, in this case centered on a first Bragg wavelength λ1. Figure 1B and Figure 1CAn example of a second reflectivity profile 124 of a second fiber Bragg grating is shown, in which it is centered on a second Bragg wavelength λ2, which is spectrally spaced from the first Bragg wavelength λ1 of the first reflectivity profile 122.

[0069] Understandably, the spectral content of the light pulse reflected by the first reflectivity profile 122 of the first fiber Bragg grating 112 tends to be broadened during its propagation within the laser cavity 108 (see arrow A) to include the second Bragg wavelength λ2 of the second reflectivity profile 124 of the second fiber Bragg grating 114, and vice versa. This allows the light pulse to reflect back and forth between the first fiber Bragg grating 112 and the second fiber Bragg grating 114 when the optical gain region 110 is pumped with the pump laser beam 104 and when the laser cavity 108 is mode-locked. Thus, only light pulses amplified sufficiently to cause spectral broadening when propagating through the optical gain region 110 can oscillate within the laser cavity 108. This structure, based on the presence of two bias filters in a nonlinear laser cavity, can be referred to as a Mamyshev oscillator.

[0070] In this example, the first fiber Bragg grating 112 has a first refractive index profile 126 including a varying grating period 128 (i.e., chirp) and a Gaussian apodization 130. Figure 2 An example of the first refractive index profile 126 is shown. It is understood that the grating period 128 of the first refractive index profile is etched along the fiber segment with a partial variation of the first refractive index profile. The varying grating period produces chirp, thereby enhancing the maximum reflectivity value of the first reflectivity profile and widening its bandwidth. Therefore, in this embodiment, the first fiber Bragg grating is a chirped fiber Bragg grating (CFBG). A Gaussian-like apodization 130 shows the refractive index n varying according to a Gaussian-like profile. A Gaussian-like profile is beneficial for stable optical pulses at higher pulse energies. The term "apodization" refers to a gradual change in refractive index n, which approaches zero at both ends 130a, 130b of the first refractive index profile 126. Apodized fiber Bragg gratings can provide advantages in sidelobe suppression while maintaining the desired reflectivity. The Gaussian-like apodization 130 can be described as any apodization that can achieve a smooth reflectance profile within the main lobe such that the absolute rate of change is less than about 1000% / nm and / or the sidelobe reflectance is less than about -10 dB. In some embodiments, for example, the Gaussian-like apodization 130 may have a slightly asymmetrical shape, which can produce a symmetrical Gaussian reflectance profile. It should be noted that... Figure 2The first refractive index profile 126 shown is merely exemplary, as the varied grating period shown is neither a real value nor proportional. The varied grating period 128 (equivalent to chirp) in the first refractive index profile 126 is only optional. Therefore, in some other embodiments, the first refractive index profile 126 may have a constant or uniform grating period.

[0071] The first fiber Bragg grating is constructed such that the first refractive index profile 126 has a full width at half maximum (FWHM) bandwidth of at least 0.2 nm, which can provide greater pulse energy and / or greater compressibility. In some embodiments, the FWHM bandwidth of the first refractive index profile 126 can advantageously be greater than 0.2 nm. See again Figure 1A Thanks to the first varying grating period and Gaussian apodization of the first refractive index profile, the first reflectivity profile 122 of the first fiber Bragg grating 112 can have a maximum reflectivity value R1 of at least 40% and a full width at half maximum (FWHM) bandwidth Δλ1 of at least 0.5 nm. In some embodiments, the maximum reflectivity value R1 of the first reflectivity profile 122 is at least 50%, preferably at least 55%, and most preferably at least 60%. In some embodiments, the FWHM bandwidth Δλ1 of the first reflectivity profile 122 is at least about 0.2 nm, preferably at least about 2 nm or 3 nm, and most preferably between about 4 nm and about 5 nm.

[0072] Thanks to the maximum reflectivity value R1 of the first reflectivity profile 122 of the first fiber Bragg grating 122 and the full width at half maximum bandwidth Δλ1, the optical pulse 118 output at the output end 116 can have a pulse energy of at least 10 nJ (preferably at least 15 nJ, most preferably at least 20 nJ).

[0073] In some embodiments, such as in a Fabry-Perot laser cavity, the varying grating period of the first fiber Bragg grating 112 can vary linearly over at least a portion of the first reflectivity profile 122. In these embodiments, the first fiber Bragg grating 112 can provide linear group delay dispersion over at least a portion of the first reflectivity profile 122, such as... Figure 1A The dashed line indicates that the linear group delay dispersion can be ±0.5 ps. 2Steeper, examples of which will be described below. In some embodiments, the linear group delay dispersion of the varying grating period of the first fiber Bragg grating 112 is at least twice as steep as the linear group delay dispersion provided by the optical gain region 110. In other embodiments, the varying grating period of the first fiber Bragg grating 112 may also vary in a nonlinear manner. Regardless of how the grating period of the first fiber Bragg grating 112 varies, the first fiber Bragg grating 112 can be used to compensate for dispersion occurring elsewhere within the laser cavity 108. For example, the optical gain region 110 may have normal dispersion, i.e., the optical group velocity increases with wavelength, which causes light pulses propagating through it to experience a situation where their longer wavelengths travel faster than their shorter wavelengths. Anomalous dispersion is the opposite, meaning that the group velocity of the guided optical mode decreases with a gradual increase in wavelength. Rigorous compensation for cavity dispersion on each round trip is not required. However, the group velocity dispersion of the first fiber Bragg grating 112 can be used to compensate for fiber dispersion and to manipulate the phase, intensity, and spectral profile of the filtered pulse. Thus, the following nonlinear amplification can be optimized, which can, for example, improve the compressibility of the output pulse. The first fiber Bragg grating 112 can be used to impart this opposite-sign dispersion to the optical pulse. Thus, in some embodiments, the linear group delay dispersion can be anomalous, while in other embodiments, the linear group delay dispersion can be normal. When opposite-sign dispersion is selected, a higher absolute value of the group delay dispersion can be used for the first fiber Bragg grating 112, which means a lower chirp in the refractive index profile of the first fiber Bragg grating 112, thereby further increasing the maximum reflectivity value R1 of the first reflectivity profile.

[0074] In some embodiments, the second fiber Bragg grating 114 has a second refractive index profile including a constant grating period (i.e., chirp-free), thereby imparting, as Figure 1B The reflectance profile is shown at position 124. In this embodiment, the second reflectance profile 124 of the second fiber Bragg grating 114 may have a small reflectance value R2 and a small full width at half maximum (FWHM) bandwidth Δλ2. In some embodiments, the maximum reflectance value R2 of the second reflectance profile 124 is at least 10%, preferably at least 20%, and most preferably at least 30%. In some embodiments, the FWHM bandwidth Δλ2 of the second reflectance profile 124 is at least about 2 nm, preferably at least about 3 nm, and most preferably between about 4 nm and about 5 nm. In embodiments where the second fiber Bragg grating 114 is chirped, the second fiber Bragg grating 114 may provide zero-group delay dispersion on at least a portion of the second reflectance profile 124, such as... Figure 1B As shown by the dashed line. In any case, in some embodiments, the full width at half maximum (FWHM) bandwidths of the first and second reflectivity profiles are similar to each other.

[0075] Although not mandatory, the second fiber Bragg grating 114 can be chirped in a manner similar to the first fiber Bragg grating 112. For example, the grating period of the second refractive index profile can also vary along the portion of the fiber segment inscribed with the second refractive index profile. This variation in the grating period produces chirping, thereby enhancing the maximum reflectivity value R2 of the second reflectivity profile 124 and widening its bandwidth Δλ2, as... Figure 1C The second reflectivity profile is shown at position 124. The Gaussian-like apodization of the second refractive index profile favors stable light pulses or larger pulse energies. In these embodiments, the second refractive index profile may be similar to the first refractive index profile. In some embodiments, the second reflectivity profile 124 may also have a full width at half maximum (FWHM) bandwidth of at least 0.2 nm.

[0076] See still Figure 1C Due to the variation in the refractive index of the second fiber, the grating period, and the Gaussian apodization, the second reflectivity profile 124 of the second fiber Bragg grating 114 can have a maximum reflectivity value R2 of at least 40% and a full width at half maximum (FWHM) bandwidth Δλ2 of at least 0.5 nm. In some embodiments, the maximum reflectivity value R2 of the second reflectivity profile 124 is at least 50%, preferably at least 55%, and most preferably at least 60%. In some embodiments, the FWHM bandwidth Δλ2 of the second reflectivity profile 124 is at least about 0.2 nm, preferably at least about 2 nm or 3 nm, and most preferably between about 4 nm and about 5 nm.

[0077] In some embodiments, the varying grating period of the second fiber Bragg grating 114 can vary linearly over at least a portion of the second reflectivity profile 124. In these embodiments, the second fiber Bragg grating 114 can provide linear group delay dispersion over at least a portion of the second reflectivity profile 114, such as... Figure 1C The dashed line indicates that the linear group delay dispersion can be ±0.5 ps. 2 Steeper. In some embodiments, the linear group delay dispersion of the varying grating period of the second fiber Bragg grating 114 is at least twice as steep as the linear group delay dispersion provided by the optical gain region 110. In other embodiments, the varying grating period of the second fiber Bragg grating 114 may vary in a non-linear manner. In some embodiments, the linear group delay dispersion may be negative, while in other embodiments, the linear group delay dispersion may be positive.

[0078] In some embodiments, the second filter 111 may not be a fiber Bragg grating. Regardless, the second filter 111 imparts a filter profile that is spectrally detuned to the first reflectivity profile 122 of the first fiber Bragg grating 112, thereby forming a Mamyshev-type oscillator. Therefore, the second filter 111 can be reflective or transmissive, in which case the corresponding second filter profile can be either a reflectivity profile or a transmissivity profile, respectively. In practice, in these embodiments, the second filter 111 can be a dielectric filter, a tunable filter, a loop combining one or more filter units and one or more optical circulators, or any combination thereof. However, in some embodiments, due to the complexity of the corresponding laser cavity, the second filter 111 can conveniently be configured as a fiber Bragg grating.

[0079] like Figure 1 In the example shown, fiber segment 106 includes a first fiber segment 106a having a first fiber Bragg grating 112, a second segment 106b having an optical gain region 110, a third segment 106c having a second fiber Bragg grating 114, and a fourth segment 106d having an output end 116. In this specific example, the first, second, third, and fourth segments are fused together or otherwise optically connected to each other. For example, optical connectors can be used to optically connect these segments to each other. According to this embodiment, fiber segment 106 may have fewer or more than four segments. For example, in some embodiments, the first fiber Bragg grating 112 and the second fiber Bragg grating 114 may be directly inscribed within the optical gain region 110. It should be noted that fiber segment 106 may have fiber segments different from the first fiber Bragg grating 112, the second fiber Bragg grating 114, and the fiber segments of the optical gain region 110. In practice, it is contemplated that fiber segment 106 may include any suitable number of fiber segments, including passive or active fibers. In some embodiments, additional passive fiber segments are used within the laser cavity 108 to control parameters such as the dispersion, nonlinearity, and repetition rate of the resulting fiber laser system.

[0080] This embodiment also shows that the pump laser 102 is configured as a fiber laser diode 132, with its fiber output end 134 optically connected to a first segment 106a of fiber segment 106. More specifically, in this embodiment, the fiber output end 134 of the fiber laser diode 132 is fused or otherwise optically connected to the first end 106a of fiber segment 106. In some other embodiments, the pump laser 102 can be configured as any suitable laser type (including, but not limited to, a fiber laser emitting a pump laser beam).

[0081] Understandably, in this example, the maximum reflectivity value R2 of the second reflectivity profile 114 is less than the maximum reflectivity value R1 of the first reflectivity profile 112. In this case, the output 116 is optically coupled to a second fiber Bragg grating 114, which transmits a greater portion of the optical pulse 118 oscillating within the laser cavity 108 compared to the first fiber Bragg grating 112. In some embodiments, the output optical pulse 118 has a similar sub-profile 136 with a linearly varying instantaneous frequency, thereby allowing a compressed pulse duration of less than 100 fs or less.

[0082] Optical gain region 110 can be achieved by doping with one or more rare earth ions (such as erbium ions). 3+ ), ytterbium ions (Yb 3+ ), thulium ions (Tm 3+ ), holmium ions (Ho) 3+ ), Dysprosium ions (Dy 3+ ), praseodymium ion (Dy 3+ ), neodymium ions (Nd) 3+ This can be any type of laser-active optical fiber (or any combination thereof). The concentration of rare-earth ions can vary depending on the different laser-active doping regions. The laser-active doping regions can have a conventional silica-based matrix with intercalated rare-earth ions. In other cases, the fiber matrix can be a low-phonon-energy glass, such as a glass based on fluorides, chalcogenides, chalcogen halides, or tellurides. For example, in some embodiments, the low-phonon-energy glass is a zirconium fluoride glass whose composition includes ZrF4, such as ZBLAN (ZrF4 / HfF4, BaF2, LaF3, NaF, and AlF3). In some other embodiments, the low-phonon-energy glass is an indium fluoride glass whose composition includes InF3. In alternative embodiments, the low-phonon-energy glass is an aluminum fluoride glass whose composition includes AlF3. In further embodiments, the low-phonon-energy glass is a chalcogenide glass whose composition includes As2S3, As2Se3, AsTe, AsSSe, AsSTe, GaLaS, GeAsS, or GeAsS, etc. Photonic crystal fibers, large mode area (LMA) fibers, and other types of specialized fibers can be used in this fiber laser system. Furthermore, it is important to note that the optical gain region 110 can achieve optical gain through nonlinear effects such as stimulated Raman scattering or any other suitable nonlinear effect or combination thereof. For example, in embodiments where the optical gain region 110 relies on these nonlinear effects, the optical gain region 110 does not need to be doped with rare-earth ions.

[0083] For example, in an embodiment where erbium is doped into the optical gain region 110 using a silica-based matrix, the pump laser beam 104 may have a wavelength of approximately 980 nm. The length of the optical gain region 110 may be at least 5 m, preferably at least 8 m, and most preferably at least 10 m. For example, in this specific embodiment, the optical gain region 110 is 10.8 m long. In these embodiments, the first reflectivity profile 122 may have a first Bragg wavelength λ1 of approximately 1550 nm, while the second reflectivity profile 124 may have a Bragg wavelength λ2 of approximately 1565 nm, thereby leaving a spectral detuning of approximately 15 nm. In some embodiments, the spectral detuning may be less than approximately 15 nm, while in some other embodiments, the spectral detuning may be greater than approximately 15 nm.

[0084] In this specific embodiment, the first fiber Bragg grating 112 is chirped, with a first reflectivity profile having a maximum reflectivity R1 of approximately 65% ​​and a full width at half maximum (FWHM) Δλ1 of approximately 4.7 nm. The second fiber Bragg grating 114 is also chirped, with a second reflectivity profile having a maximum reflectivity R2 of approximately 41% and a FWHM Δλ2 of approximately 4.5 nm. In this embodiment, the fiber segment 106 is entirely made of polarization-maintaining (PM) fiber, which contributes to the stability of optical pulse generation. However, in some other embodiments, the fiber segment 106 may be only partially PM-based.

[0085] As can be understood from the following examples, the fiber laser system described herein may have one or more mode-locking devices, one or more pump lasers, one or more output terminals, one or more optical couplers, one or more polarization isolators, pump reflectors, etc.

[0086] For example, Figure 3 A fiber laser system 100 with an exemplary mode-locking device 140 coupled to an optical fiber segment 106 and facilitating mode-locking of a laser cavity 108 is illustrated. As shown in this example, the mode-locking device 140 has two spaced-apart broadening elements 142 that longitudinally broaden a first fiber Bragg grating 112, thereby modifying the spectral detuning between a first reflectivity profile 122 and a second reflectivity profile 124, which can aid in mode-locking. Figure 3AThis illustrates how a stretching element 142 can be used to bring together or push apart the first reflectivity profile 122 and the second reflectivity profile 124. In some cases, for mode-locking the laser cavity 108, the stretching element 142 is operated such that adjacent tails 146 of the first reflectivity profile 122 and the second reflectivity profile 124 overlap each other by a given amount. Alternatively or additionally, a similar mode-locking device can be used to stretch the second fiber Bragg grating 114. In any case, stretching the fiber Bragg grating tends to cause a redshift in the Bragg wavelength of the corresponding reflectivity profile. Therefore, if the first and second reflectivity profiles are to be brought closer together, the stretching element should preferably be mounted on the fiber Bragg grating having the lower Bragg wavelength. In this case, the first fiber Bragg grating 112 has a first Bragg wavelength λ1 that is smaller than the second wavelength λ2 of the second fiber Bragg grating 114, and therefore the stretching element 142 stretches the first fiber Bragg grating 112. In some other embodiments, the second wavelength λ2 of the second fiber Bragg grating 114 is lower than the first Bragg wavelength λ1 of the first fiber Bragg grating 112, therefore the stretching element 142 is mounted on the second fiber Bragg grating 114 instead of the first fiber Bragg grating 112. In some embodiments, the fiber laser system 100 has a tilted fiber Bragg grating 143 in the cavity path within the laser cavity 108. If necessary, the tilted fiber Bragg grating 143 can provide polarization effects and / or additional filtering within the laser cavity 108.

[0087] Figure 4 Another example of a fiber laser system 200 is shown. As shown, the fiber laser system 200 has a pump 202 and an optical fiber segment 206 optically coupled to the pump 202. The optical fiber segment 206 has a laser cavity 208 having an optical gain region 210 as described above, a first Bragg grating 212 and a second Bragg grating 214, and an output end 216 optically coupled to the optical fiber segment 206. As shown in this specific example, the fiber laser system 200 has a mode-locking device 240 having an optical coupler 246 tapped on the optical fiber segment 206, located upstream of the laser cavity 208 and leading to the mode-locked arm. Figure 4A and Figure 4B Two different examples of this mode-locking device 240 are shown. It is understood that... Figure 4A and 4B The mode-locking device 240 facilitates the mode-locking of the laser cavity 208. In these embodiments, the fiber laser system 200 may have an auxiliary output 248 within the mode-locking device.

[0088] Figure 4AThe mode-locking arm 240' includes a first half-wave plate 270, a first polarizing beam splitter 272, a second half-wave plate 274, a second polarizing beam splitter 276, a rotating mirror 278 movable between an in-path position and an out-of-path position, and a static mirror 280. These components are arranged in series. When the rotating mirror 278 moves to the in-path position, the optical signal output from the auxiliary output terminal 248 propagates through the first half-wave plate 270, the first polarizing beam splitter 272, the second half-wave plate 274, and the second polarizing beam splitter 276, and then returns to the laser cavity via the auxiliary output terminal 248, thereby providing mode-locking feedback to the laser cavity. Once the rotating mirror 278 is moved back to the out-of-path position, the laser cavity can transition to a stable single-pulse mode-locked mode.

[0089] Figure 4B Another example of mode-locking arm 240” is shown. As illustrated, mode-locking arm 240” has a rotating mirror 278, a lens 282, and a saturable absorber mirror 284. When the rotating mirror 278 is moved into the path position, the optical signal output from the auxiliary output 248 will propagate through the lens 282 to the saturable absorber mirror 284, and then return to the laser cavity via the auxiliary output 248, thereby providing mode-locking feedback to the laser cavity. In a similar manner, once the rotating mirror 278 is moved back to the path-out position, the laser cavity can transition to a stable single-pulse mode-locked mode.

[0090] Reference above Figure 3 and Figure 4 The mode-locking devices 140 and 240 described are merely exemplary. Other mode-locking devices may be used, for example. In some embodiments, the mode-locking device is a pump laser modulation device that modulates the pump laser 102 according to predetermined modulation parameters, thereby providing mode-locking feedback to the laser cavity 108 once a steady state is reached. Any other passive or active mode-locking device that a skilled reader deems suitable may be used. The foregoing references may also be omitted. Figure 3 and Figure 4 The mode-locking devices 140 and 240 are used because the fiber laser cavity can automatically lock mode as needed.

[0091] See you again Figure 4 The first fiber Bragg grating 212 and the second fiber Bragg grating 214 sandwich only a portion of the optical gain region 210 in between. For example, the second fiber Bragg grating 214 is inscribed on a portion of the optical gain region 210 of the fiber segment 206. Therefore, in this specific embodiment, the optical gain region 210 extends beyond the second fiber Bragg grating 214 toward the output end 216. In this way, the fiber segment 206 can be free of the optical losses typically associated with the optical connection between the laser cavity 208 and the output end 216. Furthermore, due to this structure, partial amplification of the optical pulse 218 can even occur outside the laser cavity 208.

[0092] This embodiment also illustrates that the pump laser beam 204 passes through the optical gain region 210 and propagates towards the output end 216, then is reflected back towards the optical gain region 210 by a pump reflector 250 downstream of the second fiber Bragg grating 214. In this way, the pump reflector can reflect any remaining portion of the pump laser beam 204 back to the optical gain region 210 for further pumping. For example, the pump reflector 250 may be a fiber Bragg grating. The pump reflector 250 is optional, as it can be omitted in some embodiments.

[0093] In the above embodiments, the fiber laser system 200 is made of a single-clad fiber having a core surrounded by at least one cladding. In this case, the first and second fiber Bragg gratings are inscribed within the core of the single-clad fiber. However, in some other embodiments, another exemplary fiber laser system may be made of a multi-clad fiber having a core surrounded by an inner cladding, which in turn is surrounded by at least one outer cladding. In these later embodiments, since the first and second fiber Bragg gratings can be inscribed within the core of the multi-clad fiber, any pump reflector (e.g., pump reflector 250) can alternatively be a fiber Bragg grating inscribed within the inner cladding of the multi-clad fiber.

[0094] In some embodiments, a fiber polarizer 260 may be envisioned as the laser cavity 208 positioned between the first fiber Bragg grating 212 and the second fiber Bragg grating 214 to fix the polarization state of the optical pulse 218 as needed. In these embodiments, the fiber polarizer 260 may also preferably be positioned directly downstream of the second fiber Bragg grating 214.

[0095] Figure 5 Another example of a fiber laser system 300 is shown. As shown, the fiber laser system 300 has a pump 302 and an optical fiber segment 306 optically coupled to the pump 302. The optical fiber segment 306 has a laser cavity 308 having an optical gain region 310 as described above, a first Bragg grating 312 and a second Bragg grating 314, and an output end 316 optically coupled to the optical fiber segment 306. Figure 4In contrast to the fiber laser system 300, in this example, the pump laser 302 is downstream of the laser cavity 308, and the pump laser beam 304 propagates away from the output end 316 and toward the laser cavity 308. In some embodiments, the fiber laser system 300 may have a second pump laser 302' located upstream of the laser cavity 308, which is optically coupled to the fiber segment 306 via an optical coupler. In this embodiment, the second pump laser 302' propagates the second pump laser beam 304' along the optical gain region 310 to improve the pumping results. In these embodiments, the fiber laser system 300 may be provided with one or more optical isolators, either inside or outside the laser cavity 308, to protect the pump lasers 302, 302'. This is particularly convenient when the pump laser beams 304, 304' have different wavelengths.

[0096] While the aforementioned fiber laser systems 100, 200, and 300 all have a linear cavity path in which the optical pulses reflect back and forth between a first fiber Bragg grating and a second fiber Bragg grating, other embodiments of the fiber laser systems described herein may have a ring cavity around which the optical pulses circulate, combined with Figure 6 and Figure 7 An example of it is described.

[0097] Figure 6 An example of a fiber laser system 400 based on a ring Mamyshev oscillator structure is shown. As illustrated, the fiber laser system 400 has a first pump laser 402a and a fiber segment 406 optically coupled to the first pump laser 402a via a first wavelength division multiplexing (WDM) coupler 446a. The fiber segment 406 has a laser cavity 408 with at least one ring cavity path 409, a first fiber Bragg grating 412 along the ring cavity path 409, a second fiber Bragg grating 414 spaced apart from the first fiber Bragg grating 412 along the ring cavity path 409, and a first optical gain region 410a located between the first fiber Bragg grating 412 and the second fiber Bragg grating 414. An output end 416 is optically coupled to the ring laser cavity 408 for outputting optical pulses 418.

[0098] As shown in the figure, optical circulators 486a and 486b are used to circulate the optical pulses 418 reflected by the corresponding first fiber Bragg grating 412 and second fiber Bragg grating 414 back to different fiber segments, thereby allowing the optical pulses 418 to propagate counterclockwise around the annular cavity path 409. An optical isolator 488 can be provided to block optical pulses propagating in the opposite direction. In this specific embodiment, a second optical gain region 410b is provided along the fiber segment 406 between the first fiber Bragg grating 412 and the second fiber Bragg grating 414. To pump the first optical gain region 410a and the second optical gain region 410b, two additional pump lasers 402b and 402c are optically coupled to the laser cavity 408 using corresponding WDM couplers 446b and 446c. As shown in the figure, the first optical gain region 410a is pumped by two pump laser beams 404 propagating in opposite directions.

[0099] According to the Mamyshev structure, the first fiber Bragg grating 412 and the second fiber Bragg grating 414 have reflectance profiles that are spectrally detuned to each other. In this specific embodiment, the first fiber Bragg grating 412 and the second fiber Bragg grating 414 have refractive index profiles, each with a varying grating period and a Gaussian-like apodization, thereby imparting a satisfactory reflectance profile in terms of maximum reflectance value and full width at half maximum (FWHM) bandwidth. However, as mentioned above, the second fiber Bragg grating 414 is only optional, as any other filter with a satisfactory filter profile can be used.

[0100] Figure 7 Another example of a fiber laser system 500 based on a ring Mamyshev oscillator structure is shown. As illustrated, the fiber laser system 500 has a first pump laser 502a and a fiber segment 506 optically coupled to the first pump laser 502a via a first WDM coupler 546a. The fiber segment 506 has a laser cavity 508 with at least one ring cavity path 509, a first fiber Bragg grating 512 along the ring cavity path 509, a second filter 511 spaced apart from the first fiber Bragg grating 512 along the ring cavity path 509, and a first optical gain region 510a located between the first fiber Bragg grating 512 and the second filter 511. An output terminal 516 is optically coupled to the ring laser cavity 508 via a 90 / 10 optical coupler 546', where an optical pulse 518 is output. In this embodiment, the first fiber Bragg grating 512 is provided with a refractive index profile having a varying grating period and a Gaussian apodization, thereby giving it the reflectivity profile sought herein.

[0101] As shown, the optical circulator 586 is used to circulate the optical pulses 518 reflected by the corresponding first fiber Bragg grating 512 back to the different fiber segments, thereby allowing the optical pulses 518 to propagate counterclockwise around the ring cavity path 509. Conversely, since the second filter 511 is transmissive, an optical circulator is not required to maintain the circulation of the optical pulses 518 along the ring cavity path 509.

[0102] This example also illustrates a fiber laser system 500 having a second pump laser 502b, which is optically coupled to a laser cavity 508 via a second WDM coupler 546b. The laser cavity 508 shown in this example also has a second optical gain region 510b, which can be pumped by at least a third pump laser 502c, which is optically coupled to the second optical gain region 510b via a third WDM coupler 546c. An optical isolator 588 can be provided to block light pulses propagating in opposite directions.

[0103] It should be noted that the fiber laser system 500 and Figure 6 The difference in the fiber laser system 400 is that the second filter 511 is not a fiber Bragg grating providing a reflectivity profile, but a dielectric filter providing a transmittance profile. As can be expected from the Mamyshev structure, the first reflectivity profile of the first fiber Bragg grating 512 is spectrally detuned to the filter profile of the second filter 511, thereby ensuring that only light pulses undergoing spectral broadening can exist in the laser cavity 508, as in any Mamyshev oscillator. In some other embodiments, other examples of the second filter may be employed.

[0104] Example 1 – All-fiber Mamyshev oscillator implemented using chirped fiber Bragg gratings

[0105] Ultrafast fiber lasers are rapidly emerging, promising greater efficiency, compactness, robustness, and simplicity than established large-scale solid-state mode-locked lasers. In the past, fiber laser oscillators, which could be more competitively positioned for high peak power applications, offered the advantage of controlling nonlinearities within their tightly enclosed waveguide media. Recently, fiber laser systems based on a novel nonlinear saturable absorber (SA) (called Mamyshev Oscillators (MOs)) have proven promising compared to existing lasers across a wide range of wavelengths. These fiber laser systems can now deliver megawatt-level peak power while also benefiting from the robustness of PM fibers. Mode-locking in these systems can be supported by self-phase modulation and two detuned spectral filters. To date, high-energy fiber laser systems have employed paired collimators and diffraction gratings to achieve Gaussian filtering. However, it would be beneficial to implement these promising properties in an economical all-fiber form with a reliable self-starting process to begin their widespread use beyond specialized laboratories. The challenge lies in obtaining a reliable, high-efficiency, and preferably tunable, all-fiber filter that is reliable at high power.

[0106] In this example, a linear cavity MO based on two chirped fiber Bragg gratings (CFBGs) with Gaussian apodization is proposed as a spectral filter. Chirp is preferred in both FBGs to achieve high reflectivity for the desired bandwidth. Furthermore, it allows us to control the nonlinearity in the Mamyshev oscillator by utilizing a high-dispersion filter. Here, an experimental MO-based fiber laser system is presented, which, after compression, generates pulses with an energy of 21.3 nJ and a duration of 10⁸ fs. The results are consistent with numerical simulations, allowing us to provide guidelines for CFBG-based designs and investigate the effect of filter dispersion on the nonlinear pulse evolution.

[0107] At first glance, Figure 8The fiber laser system 600 shown likely resembles a standard continuous-wave (CW) fiber laser. It is entirely constructed from PM fiber. A 10.8-meter-long small-core erbium-doped fiber 610 (EDF07-PM SR from OFS) provides gain and normal dispersion at 1550 nm. At each end of the laser cavity 608, a first CFBG 612 and a second CFBG 614 each act as a ~5 nm bandwidth reflective filter. CFBG 612 and CFBG 614 are inlaid in standard passive fiber (SM15 from Fujikura) with a high level of photosensitivity. The two passive segments have a total intracavity length of ~0.60 m. The group delay dispersion (GDD) for a single pass in the laser cavity 608 is estimated to be 0.38 ps. 2 .

[0108] Figure 8A and Figure 8B The measurements of the first reflectance profile 622 and the second reflectance profile 624, along with the corresponding group delay dispersion of CFBG 612 and CFBG 614, are shown. Their linear chirp and apodization were optimized by modeling to achieve a smooth Gaussian reflectance profile with minimal sidelobes, i.e., factors maximizing output pulse energy and mode-locking stability. CFBG 612 and CFBG 614 exhibit sidelobes at -30 dB (LR-CFBG) and -28 dB (HR-CFBG) relative to their respective maximum reflectance values ​​R1 and R2. They were UV-written using a chirped phase mask of 100 nm / cm. In this example, a chirped Gaussian apodization ( ) and high refractive index modulation (~3×10 -3 High reflectivity with a large bandwidth can be achieved through extremely short inscription lengths. However, in some other embodiments, other inscription methods can be employed. As mentioned above, for a given bandwidth and maximum refractive index modulation, chirped FBGs produce higher reflectivity compared to uniform FBGs. Imperfections in the apodization profile can cause deviations from the perfect Gaussian profile and asymmetries in the reflectivity profiles 622, 624. These small deviations also affect the group delay curves 623, 625, which are otherwise perfectly linear over the entire bandwidth Δλ. These artifacts are difficult to avoid because accurate inscription is a considerable challenge for submillimeter CFBGs with high refractive index contrast.

[0109] A 15 nm spectral detuning between the first reflectivity profile 622 and the second reflectivity profile 624 of CFBG 612 and CFBG 614 allows for the presence of spectrally broadened pulses with high peak power in the laser cavity while suppressing any CW oscillations. This fiber laser system behaves like an ideal nonlinear SA with a large modulation depth that favors high-energy pulses. However, in some embodiments, it also prevents self-starting from noise. To address this, two broadening elements 640a spaced 12.5 cm apart are added to maintain and broaden the 1550 nm HR-CFBG 622, causing a redshift in the reflectivity profile of the corresponding grating. Once the filters are close to each other, the modulation depth of the SA decreases, and small noise fluctuations with low peak power can be transmitted through both filters, ultimately leading to the formation of high-energy pulses.

[0110] This behavior was not observed in this example. Instead, in this specific embodiment, the laser cavity tends to emit CW when the spectral filter wings overlap. Therefore, an external start-up arm 640b was added to achieve reliable self-starting for this experiment. Using a saturable absorber mirror (SAM-1550-50-10ps from BATOP GmbH), the external start-up arm 640b utilizes feedback from the auxiliary output 648 to induce a noisy Q-switch at 1565 nm. Once the rotating mirror 670 is lowered, the laser cavity 608 immediately transitions to a stable single-pulse mode-locked mode. At this stage, the stretching element 640a can be used to resonate the HR-CFBG 612 between 1550 nm and 1560 nm without losing mode-lock. Even though this is not necessary for the fiber laser system 600 described herein, it adds a significant degree of freedom. Figures 9A to 9D A complete characterization of the main output pulse 618 and a comparison with simulation results are shown. The simulation considers self-phase modulation (SPM), second-order dispersion (GVD), third-order dispersion (TOD), and spectral gain using the scalar generalized nonlinear Schrödinger equation. Measured reflectance and dispersion distributions are directly used to represent CFBG 612 and CFBG 614. Gain is calculated based on the erbium cross section, and the rate equation is solved in steady state. In the linear configuration, the optical pulse 618 traverses each portion of the optical gain region 610 twice per round trip. Therefore, the calculation of the transition rate at each location along the fiber considers the pulse energy of the optical pulse traveling towards HR-CFBG 612 and the pulse energy of its corresponding reflected optical pulse traveling towards LR-CFBG 614.

[0111] A stable 21.3 nJ pulse sequence at 8.935 MHz was observed in the RF spectrum with an average power of 190 mW. An injected pump power of 850 mW was used to achieve an excellent power conversion efficiency of 22.3%. The pulse energy is limited by the injected pump power. A 11.7 nJ pulse was observed at the secondary output 648. By design, most of the energy was extracted from the pulses co-propagating with the pump laser beam 604. The broad spectrum at the main output can support a 74 fs transform-limited pulse. A 15 cm segment of anomalous GVD passive fiber at the main output 618 of the laser cavity 608 partially handles the pulses. Figure 9A The spectral modulation shown helps to compensate for the spectral aperture left by the LR-CFBG 614. Interestingly, it is noted that a polarizer is not required in the laser cavity 608. The fiber laser system 600 naturally selects a linear polarization state between the two orthogonal modes of the PM fiber, which can be seen by analyzing the main output end 618. Using a polarizer, an extinction ratio of -20 dB was measured. Compared to Cross-Phase Modulation (XPM), SPM has a greater effect on spectral broadening, thus causing this self-polarization effect. Even without considering the group delay difference between the fast and slow axes of the fiber, the vector version of the numerical model shows this dynamic. In the experimental setup, birefringent fiber is essential to prevent any disruption to the polarization state. It also helps to greatly improve robustness to environmental disturbances: touching or bending the fiber has no effect on the mode-locked state or the characteristics of the output pulse. The pulse is dechirped (or compressed) using a grating-pair compressor (600 lines / mm), and Figure 9B The autocorrelation trace is shown. The PICASSO algorithm was used to accurately reconstruct the pulse envelope. The peak power of the compressed pulse was estimated at 95 kW, with an FWHM duration of 108 fs. If the 50% loss in the compressor is taken into account, the actual experimental peak power is 47.5 kW. Furthermore, a significant portion of the pulse energy is lost in the sidelobes distributed over 3 ps. This is due to the nonlinear yet smooth variation of the instantaneous frequency of the output pulse. Potential phase noise from the CFBG is not significant here, as the 4.7 nm bandwidth does not strongly affect the 115 nm spanning spectrum after fully nonlinear broadening. Good agreement with the numerical results validates the measurements and the numerical model. This provides an opportunity to further explore the pulse dynamics within laser cavity 608 and the effects of GDD on CFBG 612 and CFBG 614.

[0112] The effect of the dispersion of HR-CFBG 612 on laser cavity 608 was investigated through simulation over a large GDD range and for two reflectivity values ​​of LR-CFBG 614. For this analysis, the reflectivity parameters of both filters were assumed to be perfectly Gaussian with a bandwidth of 4.5 nm. The passive fiber following LR-CFBG 614 at the main output 618 was not considered. All results were obtained when laser cavity 608 reached steady state. Figures 10A to 10B The pulse energy and its peak power after optimal compression by second-order dispersion compensation based on GDD according to HR-CFBG are shown. Figure 10A Basically the same as the experimental setup, but Figure 10B The effect of lower LR-CFBG reflectance without dispersion is shown. Understandably, as GDD increases, the pulse energy also increases in both normal dispersion morphology and anomalous dispersion regions.

[0113] In steady state, the main pulse and its corresponding reflected pulse share the same local gain along the fiber. Therefore, their nonlinear evolution and amplification processes are coupled to each other through the longitudinal gain distribution. To demonstrate this effect, Figure 10A The study demonstrates that the low LR-CFBG reflectivity without dispersion can significantly improve the main pulse energy by reducing its competition for energy extraction and by truncating a smaller portion of the spectrum. However, filter dispersion does not cause intracavity energy loss, only slightly affecting the pulse energy. This minor perturbation is caused by a modified nonlinear spectral broadening that affects the wavelength-dependent gain along the fiber.

[0114] The laser cavity can support higher dispersion in the negative GDD region. For 41% LR-CFBG, the difference is 0.5 ps. 2 This is similar to single-path GDD. When CFBG dispersion is negative, it must compensate for the positive chirp accumulated along the fiber. This means that for higher anomalous dispersion, the suppression of initial peak power and nonlinear pulse broadening is weaker. For the same reason, laser cavities with lower LR-CFBG reflectivity allow for a smaller maximum filter dispersion.

[0115] The higher peak power after compression indicates a more linear initial chirp and a smoother spectrum, resulting in less energy loss in the sidelobes of the compressed pulse. In this example, the simulation shows +0.55 ps. 2A positive GDD value is optimal for output pulse compressibility. In practice, a CFBG with good performance cannot be achieved at such a low GDD due to limitations in maximum refractive index modulation. More dispersion means that, for the same maximum refractive index modulation and bandwidth, a higher peak reflectivity can be achieved by writing a longer CFBG with less chirp. Therefore, to obtain the necessary bandwidth and reflectivity to maintain high pulse energy, it is necessary to operate at a higher optimal anomalous GDD. Operating the CFBG with a higher negative GDD also has other positive side effects. First, the pulse avoids losses due to coupling with the fiber cladding mode, which would be present if operating at a positive GDD. Second, the simulation shows that higher dispersion can mitigate the effects of filter shape asymmetry inevitably caused by writing defects. In summary, Figure 10A and Figure 10B The results show that nonlinear pulse evolution can be customized without significantly affecting the output pulse energy. A general trend to remember is that an appropriate amount of filter dispersion (positive or negative) is beneficial. To further understand the underlying dynamics, Figures 11A to 11C The simulation results compare the pulse evolution between the worst and best HR-CFBG GDD values ​​within the gain fiber. Solid lines show the evolution of the pulse parabolic mismatch parameters, pulse energy, and pulse bandwidth when a chirped fiber Bragg grating is used. For comparison, dashed lines show the evolution of these parameters when the fiber Bragg grating is not chirped.

[0116] Interestingly, it is noted that the pulse energy increases in a similar manner in both cases. Even with significant control over the nonlinear dynamics, the filter dispersion does not strongly affect the pulse energy, as the steady-state saturation gain distribution remains roughly constant for a fixed pump power and CFBG reflectivity. The evolution of the mismatch parameter M exhibits two distinct nonlinear morphologies. First, the pulse parameters (energy, RMS bandwidth, etc.) follow an exponential growth pattern until a minimum M value is reached. This is typical for a self-similar (SS) morphology that attracts any input pulse to a parabolic shape. Second, the spectrum broadens and eventually reaches the gain bandwidth limit that blocks the nonlinear attractor. The pulse shifts to a simple SPM and gain morphology, where the linear chirp begins to bend as the pulse slowly loses its parabolic shape due to the gain bandwidth effect. This state exhibits behavior similar to the transition state between the self-similar morphology and the gain-managed nonlinearity (GMN) morphology, as the blue portion of the broadened spectrum undergoes some absorption. This manifests as RMS bandwidth saturation of the pulse spectrum towards the fiber end. Pulse broadening before amplification reduces the initial peak power and slows down spectral broadening. As a result, the inflection point between the two morphologies is pushed further along the fiber. By reducing the effective length of the SPM amplification morphology, the output pulse chirp becomes more linear. On the other hand, for initial pulses with higher chirp and duration, a slower rate of convergence to the parabolic shape is expected. This also results in a less smooth spectrum than it is for most of the amplifier length. If this effect becomes too strong, the smooth nonlinear evolution induced by the self-similar morphology becomes impossible. Due to the inability to achieve a parabolic shape, the pulse is disrupted by wave splitting, and the laser cavity becomes unstable. This explains why... Figure 10A and Figure 10B The upper limit of dispersion observed in [the study]. The optimal GDD value gives the best balance between bandwidth effect and weaker self-similar morphology.

[0117] As a summary of this example, an all-fiber Mamyshev oscillator is presented, which emits 21.3 nJ pulses compressible to 108 fs with an efficiency of 22.3% relative to the emitted pump power. This achievement is accomplished by using a submillimeter-gaussian-shaped chirped fiber Bragg grating as a spectral filter at the end of the linear cavity. The chirp of the grating is essential for fabricating a high-reflectivity Gaussian filter with the bandwidth required in an MO. The chirp also provides the opportunity to control the filter dispersion, which has a significant impact on the nonlinear evolution of the pulse in the MO. This example demonstrates that a filter GDD amount comparable to that of a fiber GDD is advantageous for obtaining a larger pulse peak power without affecting the pulse energy. This could be a first step towards more sophisticated pulse shaping schemes for Mamyshev oscillators. Arguably, this is the simplest ultrafast fiber laser structure to date, while still producing competitive results in terms of pulse energy, duration, and peak power. This paves the way for availability and cost-effectiveness, enabling many applications requiring high-energy pulses. This approach also demonstrates the potential to bring the MW power level of ytterbium-based fiber MOs into all-fiber form due to its compatibility with high power requirements.

[0118] In the following example, the refractive index modulation used in the grating writing process is given by the following equation:

[0119] n(z)=n o +max(A)+A(z)cos(z / Λ(z)), (1)

[0120] Where z represents the longitudinal position along the fiber, A is the apodization profile, and n o Λ is the initial refractive index of the optical fiber, and Λ is the grating period.

[0121] Example 2 – Fiber Bragg grating operating at 1552 nm in an erbium-based laser cavity

[0122] Figure 12A An example of a fiber Bragg grating is shown, designed to achieve a Gaussian-like reflectivity profile around 1552 nm for an erbium-based laser cavity. In this specific example, the grating is inscribed with a specific apodization and a linearly chirped phase mask period of 65 nm / cm. The grating being measured is inscribed in a PM980-XP fiber from Nufern. Figure 12B The measured reflectance profile of the fiber Bragg grating is shown. In this specific example, the length of the fiber Bragg grating along the z-axis is approximately 2000 μm, and the apodization profile A is as follows. Figure 12A The Gaussian-like type shown has an initial refractive index n. oThe grating period is given by Λ(z) = 0.362um + z*(32.5nm / cm), where z represents the center of the apodization profile. These specific values ​​are given by way of example only and are not limiting, as they may vary from embodiment to embodiment.

[0123] Example 3 – Fiber Bragg grating operating at 1050 nm in a ytterbium-based laser cavity

[0124] Figure 13A Another example of a fiber Bragg grating is shown. In this specific example, the fiber Bragg grating is designed to present a Gaussian-like reflectivity profile around 1050 nm for a ytterbium-based laser cavity. The grating is inscribed with specific apodization and a uniform grating period, and is inscribed in Liekki Passive-10 / 125DC-PM fiber from nLIGHT. Figure 13B The measured reflectance profile of the fiber Bragg grating is shown. In this specific example, the length of the fiber Bragg grating along the z-axis is approximately 700 μm, and the apodization profile A is as follows. Figure 13A The Gaussian-like type shown has an initial refractive index n. o The value is 1.45201, and the grating period Λ is 0.535 μm. These specific values ​​are given by way of example only and are not limiting, as they may vary from embodiment to embodiment.

[0125] It is understood that the examples described and illustrated above are merely exemplary. For example, in some embodiments, the first fiber Bragg grating may not be a fiber Bragg grating. In these embodiments, the laser system may have a pump laser and a laser cavity optically coupled to the pump laser. In some embodiments, the first fiber Bragg grating may have a uniform grating period. In some embodiments, the laser cavity may be fiber. The laser cavity has a cavity path, a first filter having a first filter profile with a Gaussian-like shape, a second filter having a second filter profile, and an optical gain region located along the cavity path between the first and second filters. As in any Mamyshev oscillator, the first and second filter profiles are spectrally detuned to each other. However, in this embodiment, the first filter is a dispersive filter, thereby imparting a dispersive profile over at least a portion of the first filter profile. Thus, when the optical gain region is pumped using the pump laser and when the laser cavity is mode-locked, optical pulses with the required pulse energy and compressibility can circulate within the laser cavity and then exit the cavity via the output. In these embodiments, the first filter can be any suitable type of dispersion filter, including but not limited to Bragg filters, bulk Bragg filters, fiber Bragg gratings, three-dimensional Bragg filters, etc. This scope is indicated by the appended claims.

Claims

1. A fiber laser system, comprising: A pump laser, which generates a pump laser beam; An optical fiber segment, optically coupled to the pump laser, the optical fiber segment having: A laser cavity having a cavity path, a first fiber Bragg grating having a first reflectivity profile, a second filter having a second filter profile, and an optical gain region located along the cavity path between the first fiber Bragg grating and the second filter, wherein the first reflectivity profile and the second filter profile are spectrally detuned, and the first reflectivity profile and the second filter profile are either spectrally non-overlapping or spectrally overlapping only to the extent that adjacent tails of the first reflectivity profile and the second filter profile overlap with each other, the first fiber Bragg grating having a first refractive index profile including a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian apodization, wherein, when the optical gain region is pumped with the pump laser beam and the laser cavity is mode-locked, the optical pulse circulates along the cavity path; and The output end is optically coupled to the laser cavity and outputs at least a portion of the optical pulse.

2. The fiber laser system of claim 1, wherein the first reflectivity profile of the first fiber Bragg grating has a maximum reflectivity value of at least 40%, and the full width at half maximum (FWHM) bandwidth is at least 0.5 nm.

3. The fiber laser system of claim 2, wherein the maximum reflectivity value of the first reflectivity profile is at least 50%.

4. The fiber laser system of claim 2, wherein the full width at half maximum (FWHM) bandwidth of the first reflectivity profile is between 4 nm and 5 nm.

5. The fiber laser system according to claim 1, wherein the first refractive index profile has a variable grating period.

6. The fiber laser system of claim 5, wherein the varying grating period of the first refractive index profile varies linearly to provide linear group delay dispersion on the first reflectivity profile.

7. The fiber laser system according to claim 6, wherein the linear group delay-dispersion ratio is ±0.5 ps. 2 Steeper.

8. The fiber laser system of claim 6, wherein the linear group delay dispersion of the varying grating period is at least twice as steep as the linear group delay dispersion provided by the optical gain region.

9. The fiber laser system according to claim 1, wherein the second filter profile has a maximum reflectivity or transmittance value that is smaller than the maximum reflectivity value of the first reflectivity profile, and the output end is optically coupled to the second filter.

10. The fiber laser system of claim 1, wherein the second filter is a second fiber Bragg grating, and the profile of the second filter is a second reflectivity profile that is spectrally detuned to the first reflectivity profile.

11. The fiber laser system of claim 10, wherein the first fiber Bragg grating and the second fiber Bragg grating sandwich at least a portion of the optical gain region along the cavity path, such that the cavity path is a linear path, and the optical pulse reflects back and forth between the first fiber Bragg grating and the second fiber Bragg grating along the linear path.

12. The fiber laser system of claim 10, wherein the second fiber Bragg grating has a second refractive index profile having a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian-like apodization.

13. The fiber laser system of claim 11, wherein the second reflectivity profile of the second fiber Bragg grating has a maximum reflectivity value of at least 40%, and the full width at half maximum (FWHM) bandwidth is at least 0.5 nm.

14. The fiber laser system of claim 12, wherein the maximum reflectivity value of the second reflectivity profile is at least 50%.

15. The fiber laser system of claim 12, wherein the full width at half maximum (FWHM) bandwidth of the second reflectivity profile is between 4 nm and 5 nm.

16. The fiber laser system of claim 12, wherein the second refractive index profile has a variable grating period.

17. The fiber laser system of claim 1, further comprising a mode-locking device coupled to the fiber segment and facilitating mode-locking of the laser cavity.

18. The fiber laser system of claim 17, wherein the mode-locking device has a stretching element that longitudinally stretches at least one of the first fiber Bragg grating and the second filter, thereby modifying the spectral mistuning between the first reflectivity profile and the second filter profile.

19. The fiber laser system of claim 18, wherein the mode-locking device has an optical coupler that is optically coupled to the fiber segment and leads to the mode-locking arm.

20. The fiber laser system of claim 1, wherein the optical gain region is an active erbium-doped region, the pump laser beam has a wavelength of 980 nm, the first reflectivity profile has a center wavelength of 1550 nm, and the second filter profile has a center wavelength of 1565 nm.

21. The fiber laser system of claim 1, wherein the pump laser is a first pump laser, and the fiber laser system includes a second pump laser, the second pump laser being optically coupled to the fiber segment and propagating a second pump laser beam along the optical gain region during the pumping.

22. The fiber laser system according to claim 1, wherein the output end is a main output end optically coupled to the second filter, and the fiber laser system includes an auxiliary output end optically coupled to the first fiber Bragg grating.

23. The fiber laser system according to any one of claims 1 to 22, wherein the output optical pulse has a similar sub-profile, the similar sub-profile having a linearly varying instantaneous frequency and a pulse duration of less than 100 fs after compression.

24. The fiber laser system according to any one of claims 1 to 22, wherein the optical pulse has a pulse energy of at least 10 nJ.

25. The fiber laser system according to any one of claims 1 to 22, further comprising a tilted fiber Bragg grating in the cavity path within the laser cavity.

26. A laser system, comprising: A pump laser, which generates a pump laser beam; A laser cavity optically coupled to a pump laser, the laser cavity having a cavity path, a first filter having a first filter profile of a Gaussian shape, a second filter having a second filter profile, and an optical gain region located along the cavity path between the first filter and the second filter, the first filter profile and the second filter profile being spectrally detuned to each other, the first filter profile and the second filter profile being one of the following: spectrally non-overlapping, and spectrally overlapping only to the extent that adjacent tails of the first filter profile and the second filter profile overlap each other, the first filter being dispersive, thereby imparting a dispersive profile on at least a portion of the first filter profile, wherein, when the optical gain region is pumped with the pump laser beam and the laser cavity is mode-locked, the optical pulse circulates along the cavity path; as well as The output end is optically coupled to the laser cavity and outputs at least a portion of the optical pulse.

27. The laser system of claim 26, wherein the first filter is a fiber Bragg grating having a full width at half maximum (FWHM) bandwidth of at least 0.2 nm and a Gaussian apodization-like refractive index profile.

28. The laser system of claim 27, wherein the refractive index profile has a varying grating period.

29. The laser system of claim 26, wherein at least a portion of the laser cavity is made of optical fiber.

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