High-power narrow-linewidth fiber laser system with a single-frequency depolarized source

Abstract

The disclosed high-power, narrow-linewidth laser system is configured with a master oscillator power fiber amplifier (MOPFA) architecture, including a single-mode, single-frequency (SMSF) linearly polarized seed and a power fiber amplifier, which outputs a linearly polarized signal at a desired wavelength. The laser system further includes a one-piece fiber depolarizer between the seed and the power fiber amplifier, configured to depolarize the linearly polarized signal generated by the seed. During amplification of the depolarized signal in the fiber amplifier, the occurrence of four-wave mixing nonlinear effects increases, resulting in the fiber amplifier outputting an amplified depolarized signal with an output power exceeding 2 kW and a spectral linewidth matching that of the linearly polarized signal at the input of the one-piece fiber depolarizer.

Description

[Technical Field]

[0001] The present disclosure relates to a high-power fiber laser system of a master oscillator power fiber amplifier (MOPFA) architecture that includes a linearly polarized single-mode (SM) single-frequency (SF) seed and a power fiber amplifier (PA). In particular, the present disclosure relates to a MOPFA with a one-piece fiber depolarizer that scrambles the linearly polarized output of the SF seed to suppress the occurrence of four-wave mixing (FWM) nonlinear effects (NLE) in the PA that substantially outputs at least a 2 kW SM beam with a linewidth that substantially matches the linewidth of light coupled into the input of the PA. [Background technology]

[0002] High-power solid-state / fiber single-transverse-mode (SM) single-frequency (SF) continuous-wave (CW) lasers with high beam quality and narrow linewidth are attracting increasing attention in fundamental scientific research, military, nuclear physics, precision measurement, and many other fields. Yet, SMSF lasers have not been known for the high output power required by burgeoning industrial demands.

[0003] Referring to FIG. 1, the power scalability of SF lasers has been improved with the introduction of laser sources with a master oscillator power amplifier (MOPA) / fiber amplifier (MOPFA) architecture 10. In this architecture, a seed module 12 includes a relatively low-power SNSF signal generator or seed 16 that outputs a signal at a desired wavelength that is further amplified in a module 14 housing one or several amplifiers. Typically, module 14 includes at least one or more preamplifiers 22 and a power fiber amplifier (PA) 24, also known as a booster. Control and stabilization of the SF seed are very simple at low power, while the cavity-free nature of the amplifier allows the seed's spectral purity to be maintained with a slightly broader linewidth at the output of the PA.

[0004] As is well known, for any type of laser, there is a power level that is difficult to exceed without degrading beam quality. Narrow-linewidth MOPA sources support this concept, primarily because narrow-line fiber amplifiers are susceptible to Brillouin scattering (SBS). The SBS threshold can be increased by implementing an electro-optic phase modulator (EOM) 18 (Figure 1), which controllably broadens the linewidth. In addition to SBS, stimulated Raman scattering (SRS) and transverse mode instability (TMI) also contribute to undesirable beam degradation.

[0005] The combination of NLEs mentioned above adversely affects the output power of SM narrow-linewidth MOPFA architectures. This limitation has been alleviated by depolarizing the SF seed, which is generally a linearly polarized source. Depolarization techniques and numerous devices implementing this technique are well known. A fundamental requirement for depolarizing polarized light is a birefringent medium, such as an optical fiber.

[0006] Figure 2, in conjunction with Figure 1, illustrates a Lyot depolarizer 20, well known to those skilled in the art of laser technology, as disclosed in U.S. Provisional Application No. 63 / 441,603, which is incorporated herein by reference in its entirety. The Lyot depolarizer 20 operates by splitting a coupled beam and passing the split beam portions along different optical paths to an output where they are recombined with a differential time delay between the two orthogonal polarization states. A typical Lyot fiber depolarizer includes two sections of birefringent fiber spliced ​​together at a 45° offset to depolarize a randomly oriented linearly polarized SF seed. The downstream output section L2 is twice as long as the upstream input section L1, with a length equal to or greater than the coherence length of the SF seed 16 in Figure 1.

[0007] With all of the above improvements, a continuous-wave (CW) narrow-linewidth fiber laser depolarized source operating in the 1 μm wavelength range and outputting 2-4 kW of light has recently been reported by, among others, the inventors of the subject device. Nevertheless, experiments conducted by the inventors have convincingly demonstrated that, while the output power can indeed reach the 3-4 kW range as a result of depolarization, the linewidth at the output of the MOPFA is substantially broadened compared to that of the seed while still remaining adequate for selected SF applications. For all practical purposes, the output linewidth of the system 10 of FIG. 1 has been measured to be effectively twice as wide as that of the seed. This may not meet the requirements of some industrial applications, including, for example, beam combining technologies.

[0008] Beam combining techniques such as spectral beam combining (SBC) require good beam quality of each individual beam. As is known, each beam diverges as it propagates, resulting in an increase in spot diameter at the focal plane and, as a result, M 2 This leads to a degradation of beam quality, often expressed by a factor of 1. The closer each of the combineable beams is to a diffraction-limited beam, the smaller the divergence angle of the combineable beams relative to each other, and the higher the beam quality of the combined beam. Therefore, as the linewidth of each individual SF laser increases, the divergence of the individual beams relative to each other increases, leading to a degradation of the quality of the combined beam. In practice, the increased divergence limits the number of channels or laser sources that can be combined together without the combined beam quality becoming too poor.

[0009] The line broadening in the subject system is caused by and has been found to be another well-known NLE-FWM, which is rather insignificant in the MOPFA 10 of Figure 1 at output powers up to 2 kW. At higher powers, at least one of the factors responsible for the occurrence of FWM has been identified as the two-piece Lyot depolarizer 20 of Figure 2. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Provisional Application No. 63 / 441,603 [Patent Document 2] U.S. Provisional Application No. 63470566 [Patent Document 3] U.S. Patent No. 7,848,368 [Patent Document 4] U.S. Patent No. 8,655,121 Summary of the Invention [Problem to be solved by the invention]

[0011] Based on the above, there is a need for a narrow linewidth MOPFA with suppressed FWM at output powers above 2 KW. [Means for solving the problem]

[0012] This need is met by incorporating a one-piece fiber depolarizer into the MOPFA scheme of Figure 1. Like two-piece fiber depolarizers, one-piece depolarizers are used to depolarize the output of the SF seed, leading to an increase in the power threshold in the booster where FWM becomes a problem. Like two-piece depolarizers, the MOPFA scheme of Figure 1 with a one-piece depolarizer in accordance with the present invention is capable of outputting 3-4 kW power signals. In contrast to known techniques, the use of a one-piece depolarizer does not result in substantial linewidth broadening at the booster output.

[0013] Depolarization of the linearly polarized seed output results in additional spectral components. Intuitively, it is easy to recognize that a single piece of depolarizing fiber results in fewer additional spectral components than a two-piece depolarizer. FWM is a well-known nonlinear phenomenon whereby interaction between three spectral components generates a fourth. FWM between different spectral components or longitudinal modes is the dominant line broadening mechanism. Therefore, the number of spectral components obtained in a single-piece depolarizer is fewer than that associated with a two-piece scrambler. Consequently, line broadening in a MOPFA structure with the disclosed one-piece depolarizer is limited compared to the same structure incorporating a standard two-piece scrambler.

[0014] The above and disclosed features are described in detail below. Moreover, it should be understood that both the above information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed methods and apparatus.

[0015] Various aspects of at least one embodiment are described below with reference to the accompanying drawings, which are not intended to be drawn to scale. The drawings are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated into and constitute a part of this specification, but are not intended as a definition of the scope of any particular embodiment. The drawings, together with the remainder of the specification, serve to explain the principles and operation of the described and claimed aspects and embodiments. In the drawings, each identical or nearly identical component illustrated in the various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a known high-power narrow-linewidth fiber laser source having a MOPFA configuration. [Figure 2]FIG. 1 is an optical diagram of a known two-piece fiber Lyot depolarizer. [Figure 3] FIG. 1 is an optical system diagram of the disclosed MOPFA. [Figure 4] 4 is a disclosed one-piece fiber depolarizer for the light source of FIG. 3. [Figure 5] 5A and 5B illustrate the linewidth / output power relationships obtained with the two-piece depolarizer of FIG. 2 and the disclosed one-piece depolarizer of FIG. 4, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0017] The disclosed high-power, narrow-linewidth laser source of a MOPFA architecture incorporating a one-piece depolarizer improves upon known, similarly configured laser sources with standard two-piece scramblers not only by outputting a high-quality beam up to four (4) kW, but also by substantially preserving the linewidth of the SMSF seed signal during its amplification. The one-piece fiber depolarizer increases the FWM threshold because fewer spectral components are obtained during depolarization of the linearly polarized SF seed compared to laser sources with a standard two-piece Lyot scrambler. The smaller the number of spectral components, the higher the FWM threshold.

[0018] 3 illustrates the disclosed narrow-linewidth all-fiber source 25 with a MOPFA architecture comprised of a seed module 30 and an amplifier module 40. The seed module 30 functions as a generator of an SM narrow-line signal at a desired wavelength. The generated signal is directed along a path through the fiber array and coupled to an amplifier module 40, which provides gain to the signal. The amplified signal is then directed through the output fiber 50 with an end cap, collimator, or connector between the output fiber 50 and the laser head (not shown here).

[0019] Focusing specifically on the seed module 30, the seed module 30 houses a linearly polarized SMSF seed 32, such as a pigtailed diode or fiber laser, that generates a low-power signal. By way of example, the seed 32 is a distributed feedback diode that generates approximately 10 mW SMSF output power with a narrow spectral linewidth spanning the 0.1-10 MHz range. As is well known, such narrow-linewidth SMSF signals cannot be amplified to desired kW levels, primarily due to the extremely low threshold for SBS in the power amplifier of the amplifier module 40.

[0020] To increase the SBS threshold, the spectral lines must be broadened, which can be achieved by different techniques implemented by the line broadening system 34. This technique may include time modulation, achieved by a current modulator of the current supplied to the SF seed 32. Alternatively, the broadening system 34 may be represented by a phase modulator coupled to the output of the SF seed 32 and configured to modulate its phase. Regardless of the means selected for spectral broadening in the illustrative diagram 25, the spectral lines at the output of the line broadening system 34 are broadened to approximately 30-40 GHz, and even up to 100 GHz if necessary.

[0021] Regarding the desired power level at the output of amplifier module 40, clearly, "desired" is a very relative term, and what may be considered high power for one industrial application may be significantly inadequate for a different application. Still, the goal of the vast majority of high-power laser applications is to squeeze every last ounce of power possible from any given system without compromising the optical modality. Based on this premise, those skilled in the art of laser technology will readily recognize that, in addition to broadened spectral lines, the SMSF signal from seed 32 should be depolarized, provided the goal is the highest possible power level, which is what light source 25 is intended to achieve.

[0022] Referring to FIG. 4 in addition to FIG. 3, depolarization in the disclosed light source 25 is implemented by a fiber depolarizer 36 configured as a one-piece fiber, such as a panda-type fiber, spliced ​​to the seed 32 at a 45° offset. Dimensioned to have substantially the same length as the second section L2 of the prior art depolarizer 20 of FIG. 2, the one-piece depolarizer 36 introduces fewer additional spectral components than a two-piece depolarizer, which in turn leads to an increased FWM threshold. Once the effects of FWM are attenuated, the spectral lines at the output of the amplifier module 40 remain substantially the same, i.e., 30-40 GHz full width at half maximum (FWHM), for light source 25 output powers reaching the 3-4 kW range. As power continues to increase, the lines are expected to broaden. The combination of the depolarized output of the SF seed 32 along with the broadened linewidth of 30-40 GHz not only enables the signal to reach the high power beam described above as it is directed through one or more preamplifiers 38 and boosters 42, but also provides the output beam with an excellent M of 1.05-1.1. 2 The factors are also provided.

[0023] The amplifier module 40 may further comprise an isolator 44 to prevent high-power back-reflected radiation from being directed in the opposite signal propagation direction. The combination of the photodetector 46 for measuring forward- and backward-propagating radiation is disclosed in more detail in U.S. Provisional Application No. 63,470,566, filed concurrently with the present application and incorporated herein by reference in its entirety. The booster 42 is preferably constructed of a multimode (MM) doped fiber configured with a cylindrical axial cross-section. Preferably, the active fiber has a double-bottleneck axial cross-section with a large-diameter central core region extending into smaller-diameter end regions. Examples of such active fibers are disclosed in U.S. Patent Nos. 7,848,368 and 8,655,121, each of which is incorporated herein by reference in its entirety. The dopant is selected from a group of rare-earth metals to output narrow-linewidth light at the desired wavelength. The pump assembly for exciting the booster 42 includes a unidirectional or bidirectional pumping scheme consisting of multiple MM diodes. The exemplary light source 25 was tested at 1055, 1062, and 1070 nm, which corresponds to the emission range of ytterbium (Yb) ions. The upstream section of the fiber array, including all components of the seed module 30, is based on a fiber selected from PANDA, bowtie, or any other specialty fiber with strong built-in birefringence. The downstream section of the fiber array after the depolarizer does not require a specialty fiber.

[0024] Utilizing a one-piece depolarizer 36 has several advantages over a standard two-piece Lyot polarization scrambler. One of these advantages is illustrated in FIG. 5. In particular, the linewidth 50 in a light source according to the present invention remains virtually unchanged during amplification of the seed signal in the booster 42 up to approximately 4 kW. In contrast, when the light source 25 of FIG. 3 incorporates the standard two-piece Lyot depolarizer of FIG. 2, the linewidth 60 is more than doubled. Yet another advantage relates to cost. The light source 25 was tested with a 100-meter-long standard two-piece Lyot depolarizer. Using a 50-meter-long one-piece depolarizer 36 in the light source 25 of FIG. 3 results in a depolarization degree similar to that observed in the light source 25 with a two-piece depolarizer, but the cost of the specialized fiber required for the depolarizer 36 according to the present invention is effectively half the cost of a two-piece scrambler. Considering that one meter of Panda costs just over $6, the savings can be substantial.

[0025] The features disclosed herein in accordance with the present invention are not limited in their application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. These features can be practiced or implemented in various ways in contemplated other embodiments. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements, and features described in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiment.

[0026] Having thus described several aspects of at least one example, it should be recognized that various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, the examples disclosed herein may be used in other contexts. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the following claims. Accordingly, the foregoing description and drawings are by way of example only. [Explanation of symbols]

[0027] 25 Narrow Linewidth All-Fiber Source 30 Seed Modules 32 SMSF linear polarization seeds 34 Linear Magnification System 36 Fiber Depolarizer, One-Piece Depolarizer 38 Preamp 40 Amplifier Module 42 Booster 44 Isolator 46 Photodetector 50 Output fiber, linewidth

Claims

1. 1. A high power narrow linewidth laser system having a master oscillator power fiber amplifier architecture, comprising: a single-mode single-frequency (SMSF) linearly polarized seed that outputs a linearly polarized signal at a desired wavelength; a one-piece fiber depolarizer spaced downstream from the seed and configured to depolarize the linearly polarized signal; a fiber booster for amplifying the depolarized signal, the fiber booster suppressing the occurrence of four-wave mixing nonlinear effects during amplification in the fiber booster, such that the fiber booster outputs the amplified depolarized signal with an output power greater than 2 kW and a spectral linewidth that matches the spectral linewidth of the linearly polarized signal at the input of the one-piece fiber depolarizer; A high-power narrow-linewidth laser system comprising:

2. 10. The laser system of claim 1, further comprising a linewidth broadening unit coupled between the output of the seed and the input of the one-piece fiber depolarizer, the linewidth broadening unit configured to broaden the spectral linewidth of the linearly polarized signal at the output of the seed to the spectral linewidth of the linearly polarized signal at the input of the one-piece fiber depolarizer.

3. 3. The laser system of claim 2, wherein the linearly polarized signal at the output of the seed has the linewidth in the range of 0.1 MHz to 10 MHz, and the linewidth at the input of the one-piece fiber depolarizer is 30 GHz to 40 GHz at the output power ranging between 2 kW and 4 kW.

4. 3. The laser system of claim 2, wherein the line broadening unit comprises an acousto-optic phase modulator or an electro-optic modulator.

5. The fiber booster a multimode (MM) active fiber doped with one or more rare earth ions and having a cross section with a double bottleneck shape; 2. The laser system of claim 1, comprising a pair of single-mode passive fibers spliced ​​to opposite ends of the active fiber, wherein the MM active fiber supports substantially only a fundamental mode having a mode field diameter (MFD) matching the mode field diameter (MFD) of the SM passive fiber.

6. 6. The laser system of claim 5, wherein the fiber booster is pumped by a pump comprising a plurality of multimode diode lasers arranged in a unidirectional pumping scheme or a bidirectional pumping scheme.

7. 4. The laser system of claim 3, wherein the fiber booster is configured to output the amplified depolarized signal.

8. 3. The laser system of claim 2, wherein the SMSF seed, line expansion unit, and one-piece fiber depolarizer are spliced ​​together to define an upstream region of a system fiber array, the upstream region having a specialty fiber with strong built-in birefringence.

9. 10. The laser system of claim 9, further comprising one or more fiber preamplifiers upstream from said fiber booster, said preamplifiers and fiber booster being housed in an amplifier module.

10. 1. A method for generating a narrow linewidth optical signal in a high power laser system having a master oscillator power fiber amplifier architecture, comprising: generating a single-mode single-frequency (SMSF) linearly polarized signal; broadening the linewidth of the SMSF linearly polarized signal to a narrow linewidth; directing the SM linearly polarized narrow linewidth signal through a one-piece fiber depolarizer, thereby obtaining the narrow linewidth depolarized SM signal; amplifying the depolarized SM signal in a fiber amplifier, wherein the amplified depolarized SM signal is output with an output power ranging from 2 to 4 kW and the narrow linewidth remaining unchanged during the amplification; A method comprising:

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