Laser amplification with passive peak power filter

Abstract

A method of generating amplified laser radiation includes generating a forward propagating laser beam in a first waveguide gain stage, amplifying the forward propagating laser beam in a second waveguide gain stage, and directing the forward propagating laser beam from the output waveguide of the first waveguide gain stage to the input waveguide of the second waveguide gain stage via a propagation path through the Kerr medium. The Kerr medium suppresses coupling of high peak power laser radiation between the first waveguide gain stage and the second waveguide gain stage beyond a threshold intensity in the Kerr medium. Self-focusing in the Kerr medium results in a majority of the high peak power laser radiation being outside of at least one of an acceptance aperture and an acceptance angle of a receive waveguide in an output waveguide of the first waveguide gain stage and an input waveguide of the second waveguide gain stage. Each waveguide gain stage may include a gain fiber.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 389,705, filed on July 15, 2022, and U.S. Non-Provisional Application No. 18 / 349,419, filed on July 10, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to high power laser systems having two or more sequential gain stages and to protecting such laser systems from damage caused by power spikes. In particular, the present invention relates to preventing damage to master oscillator fiber amplifier systems and multi-stage fiber amplifiers caused by high power stimulated Brillouin scattered radiation. Background Art

[0004] The gain medium of a fiber laser is an optical fiber. The laser gain is provided by optically active ions such as neodymium (Nd 3+ ), Ytterbium (Yb 3+ ), Thulium (Tm 3+ ) or Erbium 3+ ), these ions are doped into the core of the optical fiber and excited by the pump beam. The type of optically active ions is selected according to the desired output wavelength. For example, neodymium and ytterbium ions provide gain at near-infrared wavelengths, between 1.0 and 1.1 micrometers (μm). Both the pump beam and the generated laser radiation propagate along the length of the optical fiber. The optical fiber prevents the beam from diverging. Therefore, high light intensity can be maintained over long propagation distances through the gain medium, thereby achieving high optical gain in fiber lasers. Similarly, fiber amplifiers use the light confinement of optical fibers doped with optically active ions to amplify laser beams with high gain. Amplifications of 2-3 orders of magnitude can be achieved. In a fiber laser, the gain fiber is part of the laser resonant cavity. The generated laser radiation passes through the fiber multiple times, and a small part of the laser radiation is coupled out of the resonant cavity through the output coupler. In a fiber amplifier, on the other hand, the laser beam to be amplified (the "seed beam") passes through the gain fiber once and is amplified during this single pass.

[0005] In a common architecture for high-power fiber laser systems, a relatively low-power master oscillator provides "seed" laser radiation, which is amplified by a fiber amplifier or a series of fiber amplifiers. Such a master oscillator fiber amplifier (MOFA) system can output output laser radiation with an average power greater than 2 kilowatts (kW) in a single longitudinal mode. The master oscillator is typically a fiber laser or a wavelength-locked laser diode. In a MOFA system, there are typically two or more continuous fiber gain stages, either in the form of a fiber laser followed by at least one fiber amplifier, or in the form of two or more continuous fiber amplifiers, where the first fiber amplifier is seeded by, for example, a laser diode. The system parameters for each fiber gain stage are selected based on the laser power level of that stage. Typically, the fiber size, doping concentration, and pump power for each gain stage are selected to maximize gain while managing nonlinear effects and preventing optical damage to the fiber and other fiber components. In addition, the packaging configuration and materials of the optical fiber and fiber components can be selected based on the expected laser power level.

[0006] Stimulated Brillouin scattering (SBS) is a nonlinear effect that must be considered in the design of most high-power MOFA systems. SBS originates from the third-order magnetic susceptibility of the material, χ (3) Fused silica is the most common material for gain fibers and has a moderate third-order magnetic susceptibility χ (3) ), combined with high optical intensity and long propagation length, can lead to significant SBS. SBS causes back reflections that reduce gain efficiency in the fiber and limit output power. SBS can also permanently damage the MOFA system and render it inoperable.

[0007] Brillouin scattering occurs when coherent laser radiation generates phonons in the fiber core. Photons of the forward-propagating laser radiation can generate phonons and Stokes-shifted backward-propagating photons that (a) have lower energy than the forward-propagating photons due to energy conservation and (b) propagate in opposite directions due to phase matching. The forward-propagating and backward-propagating radiation can interfere, generating a traveling refractive index grating by electrostriction. The contrast of such an index grating depends on the degree of temporal coherence imparted to the backward-propagating radiation and its intensity.

[0008] At low laser intensities, Brillouin scattering occurs only as a spontaneous process and is therefore relatively less of a concern. However, when forward-propagating laser radiation with narrow spectral bandwidth, long-time coherence, and sufficient intensity propagates in a sufficiently long fiber, the phonon density accumulates. This accumulation of phonon density causes the contrast of the interference-induced refractive index grating to grow to a level that results in more Brillouin scattering. At this point, the Brillouin scattering process is stimulated. The resulting SBS radiation experiences nonlinear gain. In sufficiently long fibers, the intensity of the backward-propagating SBS can be comparable to the intensity of the forward-propagating amplified laser radiation, thereby exceeding the intensity of the seed laser radiation by up to 2-3 orders of magnitude.

[0009] If SBS occurs in the fiber of a fiber amplifier, the backward-propagating SBS radiation can permanently damage upstream components, such as the optical fiber, a spliced ​​fiber-to-fiber interface, or another fiber component. The damage mechanism is typically local overheating caused by local absorption of the SBS radiation. Fiber components of the preceding fiber amplifier or fiber laser are typically most susceptible to damage because they are typically designed for lower power levels. Despite the risks associated with SBS, power requirements may require operating a high-power MOFA system at or near the SBS threshold (i.e., the condition where Brillouin scattering is stimulated rather than spontaneous). Typically, the greatest risk of generating SBS radiation exists in the fiber of the last fiber amplifier of a MOFA system.

[0010] Techniques to suppress SBS in optical fibers include increasing the effective mode area of ​​the fiber, chemically modifying the fiber core material, creating a temperature gradient along the fiber, minimizing the length of the fiber by increasing the absorptivity of the pump radiation, and frequency chirping or spectrally broadening the seed laser radiation.

[0011] Optical fiber is just one type of waveguide that can be used as a gain medium. Other suitable types of waveguides include planar waveguides and channel waveguides, where one or two-dimensional waveguides enable relatively high light intensities over relatively long propagation distances. Even laser diodes, especially single-mode laser diodes, rely on waveguides in at least one dimension. SUMMARY OF THE INVENTION

[0013] Many high-power MOFA systems operate at or near the SBS threshold in at least one fiber amplifier (typically the last fiber amplifier). Although commercial high-power MOFA systems are typically designed and operated to control the risk of SBS radiation reaching problematic levels, SBS radiation can occasionally lead to catastrophic failures. In a typical failure scenario, a single high-intensity, backward-propagating SBS pulse is generated in the fiber of a fiber amplifier that is seeded by another fiber amplifier or fiber laser. The fiber that generated the SBS pulse is likely able to withstand the intensity of the SBS pulse. However, the fiber and fiber components of the preceding fiber amplifier or fiber laser are optimized for significantly lower power levels and may therefore be permanently damaged by the backward-propagating SBS pulse. While SBS mitigation techniques exist, these techniques may add excessive complexity, be incompatible with other system considerations, or be insufficient to completely eliminate the risk of catastrophic failure.

[0014] A passive filtering technique for a MOFA system is disclosed herein. The filtering technique exploits the optical Kerr effect to suppress coupling of laser radiation between two consecutive fiber gain stages when a peak power spike occurs. The technique can be used, for example, to suppress coupling of a backward propagating SBS pulse to a preceding gain stage. The filtering can be implemented between two consecutive fiber amplifiers or between a seed laser (e.g., a fiber laser) and a fiber amplifier. The filter itself is completely passive and responds instantly to peak power spikes, resulting in less downtime than MOFA systems that rely on active sensor protection schemes. The filter can be configured as a free-space optical component implemented in a fiber-optic component.

[0015] The present filtering technique is not limited to fiber-based systems. More generally, the filtering technique is applicable to laser systems with two or more continuous laser gain stages in a waveguide gain medium, including planar waveguides, channel waveguides, and laser diodes.

[0016] In one aspect, a method of generating amplified laser radiation includes generating a forward propagating laser beam in a first waveguide gain stage, amplifying the forward propagating laser beam in a second waveguide gain stage, and directing the forward propagating laser beam from an output waveguide of the first waveguide gain stage to an input waveguide of the second waveguide gain stage via a propagation path through a Kerr medium. The Kerr medium suppresses coupling of high peak power laser radiation between the first waveguide gain stage and the second waveguide gain stage that exceeds a threshold intensity in the Kerr medium. The coupling is suppressed by self-focusing of the high peak power laser radiation in the Kerr medium. The self-focusing causes a majority of the high peak power laser radiation to be located outside of at least one of an acceptance aperture and an acceptance angle of a receiving waveguide in the output waveguide of the first waveguide gain stage and the input waveguide of the second waveguide gain stage.

[0017] In another aspect, an amplified laser device includes a first waveguide gain stage for generating a forward propagating laser beam and a second waveguide gain stage for amplifying the forward propagating laser beam. The first waveguide gain stage includes an output waveguide for emitting the forward propagating laser beam from the first waveguide gain stage. The second waveguide gain stage includes an input waveguide for receiving the forward propagating laser beam into the second waveguide gain stage. The amplified laser device also includes a plurality of lenses for coupling the forward propagating laser beam from the output waveguide of the first waveguide gain stage into the input waveguide of the second waveguide gain stage. The lenses include a first lens for collecting the forward propagating laser beam from the output waveguide, and a second lens for coupling the forward propagating laser beam into the input waveguide. In addition, the amplified laser device includes a bulk Kerr medium located in a propagation path of the forward propagating laser beam between the first lens and the second lens, so that coupling between the first waveguide gain stage and the second waveguide gain stage of high peak power laser radiation exceeding an intensity threshold in the bulk Kerr medium is suppressed by Kerr-induced self-focusing in the bulk Kerr medium.

[0018] Any aspects and / or features described above or elsewhere herein may be combined with each other in whole or in part. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention by way of example and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0020] Figure 1 An amplified laser device using a passive peak power filtering approach according to an embodiment is shown.

[0021] Figure 2 The acceptance aperture and acceptance angle of an exemplary optical fiber are shown.

[0022] Figure 3 The acceptance aperture and acceptance angle of an exemplary rectangular waveguide are shown.

[0023] FIG. 4A to FIG. 4C According to the embodiment Figure 1 A configuration of passive peak power filtering between two optical fibers in an apparatus with free-space propagation through a bulk Kerr medium.

[0024] Figure 5 and Figure 6 According to the embodiment Figure 1 Two corresponding configurations of passive peak power filtering between two optical fibers in the device, focusing free-space propagation through a bulk Kerr medium. DETAILED DESCRIPTION OF THE INVENTION

[0026] Referring now to the drawings, wherein like parts are indicated by like numerals, Figure 1 An amplified laser device 100 using a passive peak power filtering method is shown. The device 100 includes two fiber gain stages 110 and 120, and a Kerr medium 130 for passive peak power filtering of laser radiation between the gain stages 110 and 120. The gain stage 110 includes a gain fiber 112 that generates a forward propagating laser beam 180, and an output fiber 116. The laser beam 180 is emitted from the output fiber 116 of the gain stage 110 and propagates along a propagation path 170 to the gain stage 120. The gain stage 120 includes an input fiber 128 and a gain fiber 122. The gain fiber 122 amplifies the laser beam 180 received through the input fiber 128 to generate an amplified forward propagating laser beam 180A. The gain stage 120 can amplify the laser beam 180 by 100 times or more. Each of the output fiber 116 and the input fiber 128 is, for example, a single-mode fiber or a large mode area fiber.

[0027] The wavelength of the laser beam 180 may be in the near infrared, visible or ultraviolet spectral range. The laser beam 180 may be continuous wave (cw) or pulsed, for example with a pulse duration of 100 picoseconds or more.

[0028] The gain stage 110 is a fiber laser or a fiber amplifier. The gain stage 120 is a fiber amplifier. The device 100 may form at least a portion of a MOFA system. The gain stage 110 may include a pump laser 114 that pumps the gain fiber 112, and the gain stage 120 may include a pump laser 124 that pumps the gain fiber 122. Alternatively, the gain stages 110 and 120 are configured to receive a pump laser from a laser source obtained elsewhere. Each of the gain stages 110 and 120 may include additional fiber components, such as a fiber combiner, a fiber splitter, a fiber coupler, and additional optical fibers. For example, the gain stage 110 may include a fiber combiner 115 that couples the pump laser from the pump laser 114 into the gain fiber 112, and the gain stage 120 may include a fiber combiner 125 that couples the pump laser from the pump laser 124 into the gain fiber 122.

[0029] The output fiber 116 may be spliced ​​to the gain fiber 112, coupled to the gain fiber 112 via a fiber optic component, or otherwise fiber-coupled to the gain fiber 112. Alternatively, the output fiber 116 may simply be an end segment of the gain fiber 112. Similarly, the input fiber 128 may be spliced ​​to the gain fiber 122, coupled to the gain fiber 122 via a fiber optic component, or otherwise fiber-coupled to the gain fiber 122, or the input fiber 128 may be an end segment of the gain fiber 122.

[0030] The device 100 also includes a "light coupling element" that couples the laser beam 180 from the output fiber 116 of the gain stage 110 into the input fiber 128 of the gain stage 120. Here, "coupling" light into an optical fiber (or another type of waveguide) refers to coupling light into the core mode of the optical fiber (or waveguide). Therefore, for the purposes of this disclosure, light propagating in the cladding mode of the optical fiber (or waveguide) is not considered to be coupled into the optical fiber (or waveguide). There are many different ways to couple light between the output fiber 116 and the input fiber 128. In certain embodiments, it is preferred in many cases that the output fiber 116 and the input fiber 128 are coupled to each other through one or more optical fiber components, optionally in combination with one or more optical fibers. For example, the output fiber 116 and the input fiber 128 can be coupled to each other through an optical fiber component including a pair of coupling lenses: lens 140 and lens 150. Lens 140 collects the laser beam 180 from the output fiber 116 (or another optical fiber coupled thereto). At the other end of the fiber optic component, lens 150 couples laser beam 180 into input fiber 128 (or another fiber connected thereto). In other embodiments, device 100 utilizes free space propagation along propagation path 170, rather than the form of free space propagation that occurs within the fiber optic component. For example, lenses 140 and 150 may be implemented as true free space components, rather than within the fiber optic component.

[0031] Due to the optical Kerr effect, the Kerr medium 130 has a non-negligible nonlinear refractive index n2. The local instantaneous refractive index in the Kerr medium 130 is n=n0+n2I, where n0 is the linear refractive index and I is the local light intensity. Due to the non-negligible nonlinear refractive index n2, if the laser beam has a non-uniform lateral intensity distribution and the intensity is higher near the central axis (which is usually the case), the laser beam passing through the Kerr medium 130 will induce a so-called Kerr lens therein. The positive Kerr lens is caused by the non-uniform lateral intensity distribution of the laser beam, resulting in the instantaneous refractive index n at the center of the laser beam being higher than the instantaneous refractive index n in the flanks of the beam. The induced Kerr lens has a focusing effect on the laser beam. Since this focusing effect is caused by the laser beam itself, it is a form of "self-focusing". The degree of self-focusing increases with the intensity of the laser beam. The optical Kerr effect responds instantly to the electric field of the laser beam. Therefore, the optical power of the Kerr lens follows the intensity of the laser beam without delay.

[0032] Figure 2 An optical fiber 200 is shown having an end face 210, a core 220, and a cladding 230. Light incident on the end face 210 is coupled into the optical fiber 200 only when the light is within the acceptance aperture 240D and the acceptance angle 240A of the optical fiber 200. Figure 2In the illustrated embodiment, the core 220 has a circular cross-section, the receiving aperture 240D is the diameter of the core 220 , and the receiving angle 240A is cylindrically symmetric about the longitudinal axis 290 of the core 220 .

[0033] Thus, in the context of the apparatus 100, backward propagating laser radiation incident on the output fiber 116 of the gain stage 110 and forward propagating laser radiation incident on the input fiber 128 of the gain stage 120 are coupled into the respective optical fibers only when incident thereon within the acceptance angle and acceptance aperture of the optical fibers. Single-mode optical fibers tend to have relatively small acceptance apertures, and therefore coupling of light into single-mode optical fibers is very sensitive to defocus, pointing errors, and other aberrations. Large mode area optical fibers attempt to reduce this sensitivity. Certain commercially available large mode area optical fibers support only a single mode or a few modes and have larger acceptance apertures than typical single-mode optical fibers. However, this larger acceptance aperture comes at the expense of a smaller acceptance angle in order to limit the number of modes supported. Laser radiation can be incident from free space onto the output fiber 116 or the input fiber 128 through a coupling lens (e.g., lens 140 or lens 150). The efficiency of coupling free-space laser radiation into the output fiber 116 or the input fiber 128 is determined by the beam parameters and propagation direction of the laser radiation.

[0034] During normal operation of the device 100, the laser beam 180 is emitted from the output fiber 116 of the gain stage 110, passes through the Kerr medium 130, and is coupled into the input fiber 128 of the gain stage 120. However, a more intense laser radiation propagating along the propagation path 170 will induce a more powerful Kerr lens and thus experience a stronger self-focusing in the Kerr medium 130. When the self-focusing is strong enough, the effect of the laser radiation on the beam parameters will suppress the coupling of the laser radiation between the gain stages 110 and 120. Therefore, the Kerr medium 130, in cooperation with one or more optical coupling elements of the device 100, acts as a passive peak power filter, which (a) switches on immediately when the laser radiation intensity in the Kerr medium 130 becomes high, and (b) switches off immediately when the laser radiation intensity drops again.

[0035] The dimensions and material composition of the Kerr medium 130 are selected such that self-focusing of the high-peak power laser radiation in the Kerr medium 130 prevents a majority of the high-peak power laser radiation from coupling into the receiving gain stages of the gain stages 110 and 120. As used herein, "high-peak power laser radiation" refers to laser radiation exceeding a certain threshold intensity in the Kerr medium 130, and "nominal operation" refers to operation in the absence of high-peak power laser radiation. Self-focusing of the high-peak power laser radiation in the Kerr medium 130 may reduce the coupling efficiency from nearly 100% to a few percent or even less than one percent. During nominal operation, the laser beam 180 may experience a certain degree of self-focusing in the Kerr medium 130. However, this may be accounted for in the optical design of the device 100 so that during normal operation, the laser beam 180 is coupled into the gain stage 120 with a desired coupling efficiency (e.g., 95% or more). The nonlinear refractive index of Kerr medium 130 can take on a range of values, depending on other aspects of device 100 as well as the power level of laser beam 180 and the high peak power laser radiation to be filtered out.

[0036] Figure 1 A passive filtering scenario is depicted. In this scenario, the gain fiber 122 generates a backward propagating pulse 190 of high peak power laser radiation. The pulse 190 is, for example, a pulse of SBS radiation generated from the laser beam 180 during amplification of the laser beam 180 in the gain fiber 122. The pulse 190 undergoes self-focusing in the Kerr medium 130, which prevents most of the pulse 190 from coupling into the output fiber 116 of the gain stage 110. In one embodiment, the laser beam 180 is a continuous wave beam with an average power of 10 watts (W), the peak power of the pulse 190 is 10 kW, and the self-focusing of the pulse 190 in the Kerr medium 130 suppresses the coupling of the pulse 190 into the output fiber 116 of the gain stage 110 to a coupling efficiency of 1% or less. In this embodiment, the peak power of the small portion of the pulse 190 coupled to the gain stage 110 does not exceed 100 W. The Kerr lens induced by the pulse 190 in the Kerr medium 130 also affects the beam parameters of the laser beam 180. Therefore, coupling of the laser beam 180 into the input fiber 128 of the gain stage 120 is also suppressed while the pulse 190 is present in the Kerr medium 130. However, once the pulse 190 passes through the Kerr medium 130, the laser beam 180 is coupled into the gain stage 120 again and the nominal coupling efficiency is restored.

[0037] In another passive filtering scenario, the gain fiber 112 of the gain stage 110 generates a forward propagating pulse of high peak power laser radiation, i.e., a peak power spike in the laser beam 180. The Kerr medium 130 suppresses the peak power spike from coupling into the input fiber 128 of the gain stage 120, where otherwise amplification of the peak power spike may cause adverse effects. In one embodiment, the laser beam 180 is a continuous wave beam with an average power of 10 W, and SBS causes an undesirable 100 W peak power spike in the forward direction. The self-focusing of the peak power spike in the Kerr medium 130 suppresses the coupling of the laser beam 180 into the input fiber 128 of the gain stage 120 to less than 50%, such as 10% or less.

[0038] In most cases, the peak power of the forward propagating SBS pulses generated in the gain stage 110 cannot exceed the nominal power of the laser beam 180 by as much as the peak power of the backward propagating SBS pulses generated in the gain stage 120, because the backward propagating SBS pulses originate from the laser beam 180 after further amplification in the gain fiber 122. This makes it more challenging to filter out the forward propagating SBS pulses using the Kerr medium 130. Fortunately, any adverse effects caused by the forward propagating SBS pulses generated in the gain stage 110 on the gain stage 120 are generally less detrimental than the adverse effects caused by the backward propagating SBS pulses generated in the gain stage 120 on the gain stage 110. In certain embodiments of the device 100, the dimensions and material composition of the Kerr medium 130 are selected to (a) optimize the transmission of the laser beam 180 during nominal operation and (b) limit the coupling of the backward propagating SBS pulses to the gain stage 110 to achieve lossless power levels. This embodiment of the apparatus 100 may or may not be configured to effectively suppress coupling of forward propagating SBS pulses into the gain stage 120 .

[0039] The device 100 may also include an active protection system based on one or more sensors 160 that monitor one or more corresponding power levels in the device 100. For example, one sensor 160 may monitor the forward propagation power in the propagation path 170 and generate a control signal ( Figure 1 180 ), which turns off the pump laser 124 when the forward propagating power remains below a certain level for a certain amount of time. This active protection prevents the gain stage 120 from being damaged by high-power amplified spontaneous emission pulses that are generated when the gain fiber 122 is not injected by the laser beam 180. Another sensor 160 can monitor the backward propagating power in the propagation path 170 and generate a control signal ( Figure 1130 ), the control signal turns off the pump laser 114 and the pump laser 124 when a pulse 190 with a certain maximum amplitude or minimum duration is detected, thereby preventing the gain stages 110 and 120 from being damaged. However, the response time of the active protection system is at least limited by the transmission and processing speed of the electrical signal in the electronic circuit, whether it is used for shutdown operation or for recovery operation. Therefore, the response time of the active protection system is significantly longer than the response time of the passive filtering mechanism provided by the Kerr medium 130. The instantaneous response of the passive filtering mechanism provided by the Kerr medium 130 can therefore reduce the downtime caused by peak power spikes in the MOFA system.

[0040] The apparatus 100 may include Figure 1 1. The passive peak power filtering implemented by the Kerr medium 130 can be implemented between each pair of consecutive gain stages. However, the peak power spikes propagating back from the last gain stage of the MOFA system are generally more concerning than the peak power spikes generated in earlier gain stages in the MOFA system. Therefore, it may be sufficient and most cost-effective to include the Kerr medium 130 between the last pair of gain stages.

[0041] The device 100 and the passive filtering method based on the Kerr medium 130 can be easily extended to other forms of optical waveguides besides optical fibers. These optical waveguides can provide light confinement in one or two lateral dimensions. Therefore, more generally, each of the gain stages 110 and 120 can be replaced by a corresponding waveguide gain stage, wherein the gain fibers 112 and 122 are replaced by corresponding gain waveguides. The output fiber 116 can be replaced by an output waveguide, and the input fiber 128 can be replaced by an input waveguide. Suitable types of waveguides include planar waveguides and channel waveguides. Planar waveguides have a rectangular cross section. One dimension of the rectangular cross section is relatively small, such as a fraction of a millimeter, and limits one lateral dimension of the light propagating through the waveguide. Channel waveguides also have a rectangular cross section, but both dimensions of the rectangular cross section limit the light propagating through the waveguide. Both planar waveguides and channel waveguides can be (mostly) surrounded by air, or arranged on a substrate having a lower refractive index than the waveguide itself or embedded in a substrate having a lower refractive index than the waveguide itself.

[0042] Figure 3 3 shows a waveguide 300 having a rectangular cross-section that can be implemented in the gain stage 110 and / or 120. The waveguide 300 has a rectangular end face 310. The waveguide 300 can be disposed on a substrate 330, such as Figure 3 As shown, or embedded in the substrate 330 ( Figure 33. The waveguide 300 has an acceptance aperture (i.e., end face 310) and an acceptance angle 340A for light coupling. The acceptance angle 340A may or may not be cylindrically symmetric about the longitudinal axis 390 of the waveguide 300. When the waveguide 300 is implemented in place of the output fiber 116 of the gain stage 110 and in place of the input fiber 128 of the gain stage 120, backward propagating laser radiation incident on the gain stage 110 and forward propagating laser radiation incident on the gain stage 120 are coupled into the corresponding output / input waveguides only when incident thereon within the acceptance angle and acceptance aperture of the waveguide.

[0043] Hereinafter, the device 100 and the associated passive peak power filtering method are discussed in the context of optical fibers. The discussed embodiments can be easily extended to other types of waveguides.

[0044] FIG. 4A to FIG. 4C An arrangement 400 for passive peak power filtering between two optical fibers in device 100 is shown, with free space propagation through a bulk Kerr medium 430. Bulk Kerr medium 430 is an embodiment of Kerr medium 130, and arrangement 400 is suitable for use with device 100. Figure 4A An exemplary propagation of laser beam 180 during nominal operation is shown. Figure 4B An exemplary propagation of a pulse 190 of backward-propagating high-peak power laser radiation is shown. Figure 4C An exemplary forward propagation of laser beam 180 during the presence of backward propagating pulse 190 is shown.

[0045] During nominal operation in configuration 400, as Figure 4A As shown, laser beam 180 is emitted from core 412 of optical fiber 410, collected by lens 440, passes through bulk Kerr medium 430, and is coupled into core 422 of optical fiber 420. Optical fiber 410 may be output optical fiber 116 of gain stage 110 or an optical fiber fiber-coupled to output optical fiber 116. Alternatively, optical fiber 410 may be another optical fiber in the path of laser beam 180 between output optical fiber 116 and bulk Kerr medium 430. Similarly, optical fiber 420 may be input optical fiber 128 of gain stage 120, an optical fiber fiber-coupled to input optical fiber 128, or another optical fiber in the path of laser beam 180 between bulk Kerr medium 430 and input optical fiber 128. Lenses 440 and 450 are embodiments of lenses 140 and 150, respectively.

[0046] The coupling of the laser radiation between the optical fibers 410 and 420 is sensitive to the beam parameters of the laser radiation. In turn, the beam parameters are sensitive to the Kerr lensing effect in the bulk Kerr medium 430. Figure 4BAs shown, the self-focusing of pulse 190 in bulk Kerr medium 430 causes a large portion of pulse 190 to be outside the acceptance aperture and / or acceptance angle of optical fiber 410. In more extreme cases, a portion of pulse 190 may completely miss end face 416. Most likely, a large portion of pulse 190 propagates in the cladding mode of optical fiber 410. Although less intense than propagation in the core mode, this cladding mode portion of pulse 190 may still have adverse effects. Therefore, it may be advantageous to remove this portion of pulse 190 from optical fiber 410. To this end, optical fiber 410 may have a mode stripping feature 418 that leaks out light propagating in the cladding mode. Mode stripping feature 418 may be a surface texture on cladding 414 or a material deposited on the surface of cladding 414. Cladding mode propagation may also be at least partially transmitted through a physical aperture ( Figure 4A to Figure C) to prevent.

[0047] Although pulse 190 exists in bulk Kerr medium 430, laser beam 180 is also focused by the Kerr lens caused by pulse 190. Figure 4C , which shows a Kerr lens suppressing the coupling of the laser beam 180 to the optical fiber 420. The optical fiber 420 may include a mode stripping feature 428 that leaks out light propagating in the cladding mode.

[0048] The bulk Kerr medium 430 can be sized to be compatible with implementations in standard fiber optic component packaging. For example, as a fiber optic component 460 along with lenses 440 and 450. For example, the bulk Kerr medium 430 can be a rod or block having a length 430L less than 20 millimeters (mm) and a maximum transverse dimension 430T less than 5 mm. The fiber optic component 460 interconnects the optical fibers 410 and 420 and includes the lens 440, the bulk Kerr medium 430, and the lens 450. The bulk Kerr medium 430 and the lenses 440 and 450 are aligned within the fiber optic component 460 to ensure proper optical alignment with the optical fibers 410 and 420.

[0049] The optimal value of length 430L depends at least on the material of bulk Kerr medium 430 and its nonlinear refractive index, as well as the nominal power of laser beam 180 and the maximum peak power that can be coupled between gain stages 110 and 120. Bulk Kerr medium 430 may have a length of at least 10 -20 m 2 / W. In one such embodiment, the bulk Kerr medium 430 is made of fused silica and has a length 430L in a range between 10 and 30 mm. The nonlinear refractive index of fused silica is 2.2x10 at a wavelength of approximately 1 micrometer (μm). -20 m 2 / W. A smaller length 430L may be sufficient if the Kerr medium 430 is made of a material with a higher nonlinear refractive index, such as zinc selenide, which has a 430x10 -20 m 2 / W nonlinear refractive index.

[0050] The beam parameters should also be considered when selecting the material and dimensions of the bulk Kerr medium 430. Figure 4A In the illustrated embodiment, laser beam 180 is nominally collimated in bulk Kerr medium 430. At the location where a peak power spike (e.g., pulse 190) is incident on bulk Kerr medium 430, the caustics of the peak power spike are generally similar to the nominal caustics of laser beam 180. Figure 4A In the embodiment, laser beam 180 is nominally collimated between lenses 440 and 450, so Figure 4B The pulse 190 is likely collimated when incident on the bulk Kerr medium 430. Depending on the power level of the peak power spike and the characteristics of the bulk Kerr medium 430, it may be necessary to focus the laser radiation within the bulk Kerr medium 430 to induce the degree of self-focusing required to achieve the desired filtering performance.

[0051] Figure 5 A configuration 500 is shown for passive peak power filtering between two optical fibers in the apparatus 100, wherein focused free space propagation is through a bulk Kerr medium 430. The configuration 500 includes two telescopes 540 and 550 for achieving higher laser beam intensity in the bulk Kerr medium 430, thereby producing a stronger Kerr lensing effect. The configuration 500 is similar to the configuration 400, except that additional lenses 542 and 552 are included. The lens 542 cooperates with the lens 440 to form the telescope 540, which during nominal operation focuses the laser beam 180 to a waist 580W in the bulk Kerr medium 430. In one embodiment, the diameter of the waist 580W is in the range of 10 to 50 μm. The lens 552 cooperates with the lens 450 to form the telescope 550, which during normal operation couples the laser beam 180 into the optical fiber 420. Telescope 550 also focuses the backward propagating laser radiation emitted from optical fiber 420 into bulk Kerr medium 430. Figure 5 As shown, during nominal operation, lens 440 may be configured to collimate laser beam 180 and lens 552 may be configured to re-collimate laser beam 180 .

[0052] Figure 6Another configuration 600 for passive peak power filtering between two optical fibers in device 100 is shown, wherein focused free space propagation is through a bulk Kerr medium. Configuration 600 may be viewed as a variation of configuration 500, wherein bulk Kerr medium 430 and lenses 542 and 552 are replaced by bulk Kerr medium 630 having convex end faces 642 and 652. Convex end face 642 has the same function as lens 542, and convex end face 652 has the same function as lens 552. Configuration 600 integrates the focusing effect of lenses 542 and 552 into the bulk Kerr medium, thereby eliminating two optical elements. In configuration 600, lens 440 may be configured to collimate laser beam 180 during nominal operation, and convex end face 652 may be configured to re-collimate laser beam 180.

[0053] The performance of one embodiment of the configuration 600 was evaluated by modeling. The model considered an embodiment of a bulk Kerr medium 630 made of fused silica, having a length 630L of 17 mm and characterized by a radius of curvature of 2.7 mm for each of the convex end faces 642 and 652. The model assumed that during nominal operation, the laser beam 180 was collimated between the coupling lens 440 and the bulk Kerr medium 630 and between the bulk Kerr medium 630 and the coupling lens 450. The model demonstrated effective suppression of the coupling of laser radiation between the optical fibers 410 and 420 when the peak power was increased from a nominal value of 20 W to 20 kW.

[0054] The present invention has been described above in the form of preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. On the contrary, the present invention is only limited by the claims attached hereto.

Claims

1. A method for producing amplified laser radiation, comprising the following steps: generating a forward propagating laser beam in a first waveguide gain stage; amplifying the forward propagating laser beam in a second waveguide gain stage; as well as The forward propagating laser beam is directed from an output waveguide of the first waveguide gain stage to an input waveguide of the second waveguide gain stage via a propagation path through a Kerr medium, the Kerr medium suppressing coupling of high peak power laser radiation between the first waveguide gain stage and the second waveguide gain stage exceeding a threshold intensity in the Kerr medium, wherein the coupling is suppressed by self-focusing of the high peak power laser radiation in the Kerr medium, the self-focusing causing a majority of the high peak power laser radiation to be located outside of at least one of an acceptance aperture and an acceptance angle of a receiving waveguide in the output waveguide of the first waveguide gain stage and the input waveguide of the second waveguide gain stage.

2. The method of claim 1 , wherein the high peak power laser radiation is back-propagating stimulated Brillouin scattered radiation generated in the second waveguide gain stage, and the self-focusing suppresses coupling of the back-propagating stimulated Brillouin scattered radiation into the output waveguide of the first waveguide gain stage.

3. The method of any one of claims 1-2, wherein the high peak power laser radiation is forward propagating stimulated Brillouin scattering radiation generated in the first waveguide gain stage, and the self-focusing suppresses coupling of the forward propagating stimulated Brillouin scattering radiation into the input waveguide of the second waveguide gain stage.

4. The method according to any one of claims 1 to 3, wherein the propagation path passes through free space between the output waveguide of the first waveguide gain stage and the Kerr medium and between the Kerr medium and the input waveguide of the second waveguide gain stage.

5. The method of claim 4, wherein the directing step comprises focusing the forward propagating laser beam to a waist within the Kerr medium.

6. Amplifying laser device, comprising: a first waveguide gain stage for producing a forward propagating laser beam, comprising an output waveguide for emitting the forward propagating laser beam from the first waveguide gain stage; a second waveguide gain stage for amplifying the forward propagating laser beam, comprising an input waveguide for receiving the forward propagating laser beam into the second waveguide gain stage; a plurality of lenses for coupling the forward propagating laser beam from the output waveguide of the first waveguide gain stage into the input waveguide of the second waveguide gain stage, the lenses comprising: a first lens for collecting the forward propagating laser beam from the output waveguide, and a second lens for coupling the forward propagating laser beam into the input waveguide; as well as A bulk Kerr medium located in the propagation path of the forward propagating laser beam between the first lens and the second lens enables coupling between the first waveguide gain stage and the second waveguide gain stage of high peak power laser radiation exceeding an intensity threshold in the bulk Kerr medium to be suppressed by Kerr-induced self-focusing in the bulk Kerr medium. 7 . The amplified laser device according to claim 6 , wherein the bulk Kerr medium has a first convex end surface and a second convex end surface respectively intersecting the propagation path.

8. The amplified laser device of claim 7, wherein the first lens is configured to collimate the forward propagating laser beam, the first convex end surface is configured to generate a waist of the forward propagating laser beam inside the bulk Kerr medium, and the second convex end surface is configured to re-collimate the forward propagating laser beam.

9. An amplified laser device according to any one of claims 6 to 8, wherein the plurality of lenses are arranged such that the forward propagating laser beam has a waist in the bulk Kerr medium at least in the absence of the high peak power laser radiation.

10. The amplified laser device of claim 9, wherein the plurality of lenses further comprises a third lens and a fourth lens, the third lens cooperating with the first lens to form a telescope between the output waveguide of the first waveguide gain stage and the bulk Kerr medium, and the fourth lens cooperating with the second lens to form a telescope between the bulk Kerr medium and the input waveguide of the second waveguide gain stage.

11. An amplified laser device according to any one of claims 6 to 10, wherein the plurality of lenses and the bulk Kerr medium are implemented in an optical fiber component.

12. An amplified laser device according to any one of claims 6 to 11, wherein each of the output waveguide and the input waveguide is an optical fiber, and wherein each of the first waveguide gain stage and the second waveguide gain stage comprises a gain fiber.

13. A master oscillator fiber amplifier system comprising: a first fiber gain stage for generating a forward propagating laser beam, the forward propagating laser beam being a continuous wave, the first fiber gain stage comprising an output fiber for emitting the forward propagating laser beam from the first fiber gain stage, the output fiber having an acceptance aperture and an acceptance angle relative to coupling backward propagating radiation into the output fiber; a second fiber gain stage for amplifying the forward propagating laser beam, the second fiber gain stage comprising an input fiber for receiving the forward propagating laser beam into the second fiber gain stage; a plurality of lenses for coupling the forward propagating laser beam from the output fiber of the first fiber gain stage into the input fiber of the second fiber gain stage, the lenses comprising: a first lens for collecting the forward propagating laser beam from the output optical fiber, and a second lens for coupling the forward propagating laser beam into the input optical fiber; as well as a bulk Kerr medium for suppressing coupling of backward propagating pulses of high peak power laser radiation generated in the second fiber gain stage into the first fiber gain stage, the bulk Kerr medium being located in a propagation path of the forward propagating laser beam between the first lens and the second lens, so that the backward propagating pulses undergo Kerr-induced self-focusing in the bulk Kerr medium, the self-focusing causing a majority of the backward propagating pulses to be located outside at least one of an acceptance aperture and an acceptance angle of the output fiber of the first fiber gain stage.

14. The master oscillator fiber amplifier system of claim 13, wherein the backward propagating pulses comprise stimulated Brillouin scattering radiation.

15. The master oscillator fiber amplifier system according to any one of claims 13-14, wherein the bulk Kerr medium has a first convex end face and a second convex end face respectively intersecting the propagation path.

16. The master oscillator fiber amplifier system of claim 15, wherein the first lens is configured to collimate the forward propagating laser beam, the first convex end face is configured to create a waist of the forward propagating laser beam inside a bulk Kerr medium, and the second convex end face is configured to re-collimate the forward propagating laser beam.

17. The master oscillator fiber amplifier system of any one of claims 13-16, wherein the plurality of lenses are arranged such that the forward propagating laser beam has a waist in the bulk Kerr medium at least in the absence of high peak power laser radiation.

18. The master oscillator fiber amplifier system of claim 17, wherein the plurality of lenses further comprises a third lens and a fourth lens, the third lens cooperating with the first lens to form a telescope between the output fiber of the first fiber gain stage and the bulk Kerr medium, and the fourth lens cooperating with the second lens to form a telescope between the bulk Kerr medium and the input fiber of the second fiber gain stage.

19. The master oscillator fiber amplifier system of any one of claims 13-18, wherein the plurality of lenses and the bulk Kerr medium are implemented in an optical fiber component.

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