Pumping polarization-maintaining (PM)raman lasers and amplifiers

Abstract

The present disclosure provides systems and methods for pumping of polarization-maintaining (PM) Raman fiber lasers (RFLs) and Raman fiber amplifiers (RFAs). Briefly described, one embodiment uses a low-noise source as a pump for a PM RFL. Another embodiment uses a low-noise PM semi-random RFL as a pump source for a PM RFA

Description

PUMPING POLARIZATION-MAINTAINING (PM) RAMAN LASERS AND AMPLIFIERSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patent application serial number 63 / 672,027, filed 2024-Jul-16, having the title "low noise pumping of PM Raman lasers and amplifiers," with Andrew T. Grimes as the first-named inventor, which is incorporated herein by reference in its entirety as if expressly set forth herein.BACKGROUNDFIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to Raman lasers and Raman amplifiers and, more particularly, to polarization-maintaining (PM) Raman fiber lasers (RFLs) and PM Raman fiber amplifiers (RFAs).DESCRIPTION OF RELATED ART

[0003] Because RFLs and RFAs are flexible sources of high-power radiation over a broad wavelength range, there are ongoing efforts to improve performance of RFLs and RFAs.SUMMARY

[0004] The present disclosure provides systems and methods for pumping of polarization-maintaining (PM) Raman fiber lasers (RFLs) and Raman fiber amplifiers (RFAs). Briefly described, one embodiment uses a low-noise source as a pump for a PM RFL.Specifically, low-noise is defined herein as being free of longitudinal mode structure. Another embodiment uses a low-noise, PM, semi-random RFL as a pump source for a PM RFA. Other systems, devices, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

[0006] FIG. l is a block diagram showing one embodiment of a polarization-maintaining (PM) laser pump.

[0007] FIG. 2 is a block diagram showing one embodiment of a master oscillator power amplifier (MOPA).

[0008] FIG. 3 is a block diagram showing one embodiment of a Raman fiber laser (RFL).

[0009] FIG. 4 is a block diagram showing one embodiment of a Raman fiber amplifier(RFA).

[0010] FIG. 5 is a graph showing trace signals from a spectrum analyzer, with output power (y-axis) plotted as a function of frequency (x-axis) in one embodiment of the system.

[0011] FIG. 6 is a graph showing output power (y-axis) as a function of pump power (x- axis) for one embodiment of the system.

[0012] FIG. 7 is a graph showing intensity (y-axis) as a function of wavelength (x-axis) for one embodiment of the system.

[0013] FIG. 8 is a graph showing output power (y-axis) as a function of pump power (x- axis) for one embodiment of the system.

[0014] FIG. 9 is a graph showing intensity (y-axis) as a function of wavelength (x-axis) in another embodiment of the system.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] Raman fiber lasers (RFLs) and Raman fiber amplifiers (RFAs) are flexible sources of high-power radiation over a broad wavelength range, especially for applications that require wavelengths that are outside of the traditional rare-earth (RE) ion emission bands. Some applications, such as narrow-linewidth RFAs (pumped by RFLs), provide a flexible approach for high-power narrow-linewidth amplification, but the Raman process increases overall noise. Traditionally, a RFL is pumped by a high-power, RE (e g., Ytterbium (Yb)) doped fiberoscillator, which exhibits relative intensity noise (RIN). Alteranitively, other rare-earth fiber lasers, such as Erbium (Er), ErYb, Neodymium (Nd), or Thulium (Tm) could also be used as a pump source for Raman fiber lasers. The RIN is caused by longitudinal modes that exist in the laser cavity. In cascaded Raman resonators, the RIN causes unwanted scattering to higher-order Stokes lines, thereby limiting the amount of power available at the final desired wavelength. Insofar as Raman gain is polarized, this effect becomes particularly detrimental in polarizationmaintaining (PM) cascaded Raman resonators, which limits the performance of PM RFLs pumped by PM Yb-doped lasers. In applications, such as quantum metrology, the increases in noise can reduce the effectiveness of these types of laser sources.

[0016] To mitigate the problems introduced by noise, this disclosure teaches systems and methods that use low-noise PM sources as pumps for PM semi-random RFLs. Low noise, as defined herein, means being free of longitudinal mode structure. Insofar as longitudinal modes are understandable to those having skill in the art, further explanations of longitudinal modes are omitted from this disclosure. A suitable low-noise, PM pump can be implemented in different ways. An amplified spontaneous emission (ASE) source can be polarized with a polarizer, and then further amplified to make a suitable PM pump source. Alternatively, a narrow linewidth single-frequency laser with only one longitudinal mode could be linewidth broadened using a phase modulator to prevent stimulated Brillouin scattering (SBS), and then amplified to make a suitable low-noise, PM pump source. Random Yb lasers are also free of longitudinal mode structure. PM semi -random RFLs are also known as half-open cavity lasers because there is no fully closed cavity in PM semi -random RFLs. Additionally, this disclosure teaches using PM semi-random RFLs as pump sources for PM RFAs. Generally, a low-RIN pump based on amplified spontaneous emission (ASE) is used as a pump source for a PM semi-random RFL. The PM semi-random RFL is, in turn, used to pump a PM RFA. As shown in greater detail, below, a comparison of a Yb-doped pump source with an ASE-based pump source demonstrates markedly better performance by the ASE-based pump source. Briefly, for purposes of illustration, FIG. 1 and FIG. 2 show two (2) different embodiments of pump sources, each of which is used separately to pump a PM RFL (such as that shown in FIG. 3). That PM RFL is, in turn, used to pump, for example, a PM RFA (such as that shown in FIG. 4). System performance between the systems is shown in various graphs (shown in FIG. 5 through FIG. 9).

[0017] Having provided a broad technical solution to a technical problem, reference isnow made in detail to the description of the embodiments as illustrated in the drawings. While several embodiments are described in connection with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents.

[0018] FIG. 1 is a block diagram showing one embodiment of a PM laser pump 100, which, for example, is a 1090 nanometer (nm) Ytterbium (Yb) laser pump (meaning, a Yb- pumped laser with a center wavelength of 1090nm). While 1090nm wavelength was chosen for this experiment, Yb lasers can be used as a pump for Raman fiber lasers when operated over a wide wavelength range from 976nm to 1190nm. As shown in FIG. 1, the PM laser pump 100 comprises diodes 105, such as, for example, four (4) 915nm pump diodes, which provide pump light 115.

[0019] Pump light 115 is injected into pump combiner (PC) 120, which is coupled into a multi-mode (MM) output fiber 125 which is then optically coupled to a PM high reflector (HR) 130. For purposes of illustration and clarity of understanding, fiber orientation 135 at the PM HR 130 is shown to be vertical. Unused pump ports 110 of the PC 120 are end terminated with a low reflectivity termination such as coreless fiber.

[0020] The PM HR 130 is optically coupled (e.g., spliced) to a PM Yb-doped gain fiber 145, with the fiber orientation 150 at the PM Yb-doped gain fiber 145 being shown to be the same as the fiber orientation 135 at the PM HR 130. For the results shown herein (e.g., FIG. 5 through FIG. 9), the PM Yb-doped gain fiber 145 that was used is a fifteen meter (15m) length of 6 / 125 (or 6 micrometer (pm) core and 125pm cladding) PM Yb-fiber with reflective fiber Bragg gratings (FBGs) written to match passive PM fiber.

[0021] The PM Yb-doped gain fiber 145 is optically coupled to an output coupler (OC) 160. The OC 160 is shown to have an orthogonal orientation 165 (shown as horizontal or ninety degrees (90°) with reference to the orientation 145 of the PM HR 130). The combination of the HR 130, PM Yb fiber 145, and OC 160, along with relative orientations for HR 130 and OC 160, provide a PM laser cavity that produces polarized output when energized by the pump light from the diodes 105. The PM laser pump 100 produces a high-power pump output 170 (e.g., 1090nm high-power PM pump output (as shown from the orientation 175 of the output 170).

[0022] The high-power pump output 170 is used as a pump source in subsequent stages of an optical system (such as that shown in FIG. 3, below).

[0023] By comparison, FIG. 2 is a block diagram showing one embodiment of a master oscillator power amplifier (MOP A) 200 (e.g., 1090nm ASE PM MOP A), which can be used as a pump source (instead of the PM laser pump 100 of FIG. 1). As shown in FIG. 2, the MOPA 200 comprises one or more diodes 205 that provide pump light 21 .

[0024] Unused pump ports 210 of the pump combiner 120 are end terminated with a low reflectivity termination such as coreless fiber. The PC 220 is optically coupled to a gain fiber 230 (e.g., 50m non-PM 6 / 125 Yb-doped gain fiber with an output power of thirty milliwatts (30mW)). The gain fiber 230 is next optically coupled (e.g., spliced) to a single-mode fiber 235 that relays the signal to an input of a fiber polarizer 240.

[0025] The output of the fiber polarizer 240 is optically coupled to a PM fiber 245, which, in turn, is optically coupled to an isolator and band-pass filter (BPF) 255. For the values shown herein (e.g., FIG. 5 through FIG. 9), the isolator-BPF 255 had a center wavelength of 1092nm and a three decibel (3dB) passband of 7nm, thereby producing PM ASE 260 at 1092nm. The PM ASE 260 from the isolator-BPF 255 is amplified at an output amplifier (such as a two- stage amplifier 265) to produce a high-power pump output 275 (e.g., 1090nm high-power PM pump output). The high-power pump output 275 is used as a pump source in subsequent stages of an optical system (such as that shown in FIG. 3).

[0026] Continuing, FIG. 3 shows one example, among others, of such a subsequent stage to either FIG. 1 or FIG. 2. Specifically, FIG. 3 shows a RFL 300 (e.g., 1212nm PM semirandom (half-open cavity) RFL). As shown in FIG. 3, the RFL 300 comprises a high-power pump 305, which produces high-power pump light 310.

[0027] In one embodiment, the PM laser pump 100 of FIG. 1 is used for the high-power pump 305 in FIG. 3 (thus, the high-power pump output 170 of FIG. 1 is the pump light 310 of FIG. 3). In another embodiment, the MOPA 200 of FIG. 2 is used for the high-power pump 305 of FIG. 3 (thus, the high-power pump output 275 of FIG. 2 is the pump light 310 of FIG. 3). Comparisons of the RIN between the PM laser pump 100 and the MOPA 200 are shown in FIG.5 and discussed in greater detail, below.

[0028] The RFL 300 comprises PM FBGs 315, which provide feedback for the PM semirandom RFL 300. For the embodiment of FIG. 3, the PM FBGs 315 are written with high reflectivity at 1148nm and 1212nm.

[0029] The PM signal 320 from feedback from the PM FBGs 315 and the pump light 310are combined at a wavelength division multiplexer (WDM) 325, which provides a combined signal-pump to a PM Raman gain fiber 335. The PM Raman gain fiber 335 produces a polarized output. In one embodiment, the RFL 300 is designed to achieve two (2) Stokes shifts and, thus, the polarized output is an amplified signal at 1212nm.

[0030] For characterization of system performance, the output is coupled into an anti- reflective (AR) coated end cap 340. For measurements shown herein (e.g., FIG. 6 and 7), the polarized output was used to measure the polarization extraction ratio (PER), the spectrum, and the power. The measurements are shown and discussed in greater detail, below, with reference to FIG. 6 and FIG. 7. The PER for the Yb pump sources, RFL, and RFA are all greater than twenty decibels (>20dB).

[0031] The RFL 300 itself is used as a pump source to an RFA, such as that shown in FIG. 4. As shown in FIG. 4, one embodiment of a RFA 400 (e g., 1280nm PM RFA) comprises a PM Raman pump 405. In the embodiment of FIG. 4, the PM Raman pump 405 is the RFL 300 from FIG. 3. The PM Raman pump provides pump light 410 (e.g., 1212nm PM Raman pump light from the PM Raman gain fiber 335 of FIG. 3).

[0032] The RFA 400 also comprises a distributed feedback (DFB) laser 415, which provides a signal 420 to a boosted optical amplifier (BOA) 425, which in turn produces an amplified signal 430 (e.g., 75mW amplified signal). The amplified signal 430 and the pump light 410 are combined at a WDM 435, which produces a combined signal-pump 445 that is optically propagated through a PM fiber 450 which produces a polarized signal 455, which is subsequently propagated through a PM Raman gain fiber 460. For purposes of characterizing the system of FIG. 4, the RFA 400 comprises an AR-coated end cap 465.

[0033] A comparison of performance characteristics between various embodiments is shown in FIG. 5 through FIG. 9.

[0034] FIG. 5 is a graph 500 that shows trace signals from a spectrum analyzer, with output power (y-axis) plotted as a function of frequency (x-axis), comparing RIN measurements from a PM Yb-doped laser (such as the preferred embodiment of a pump laser 100 shown in FIG. 1) with RIN measurements from an ASE PM MOPA (such as the preferred embodiment of a MOPA 200 shown in FIG. 2). Specifically, FIG. 5 shows maximum power for the ASE PM MOPA 510, low power for the ASE PM MOPA 520, maximum power PM Yb-doped laser 530, low power for the PM Yb-doped laser 530, and background signal 550.

[0035] As shown from FIG. 5, the RIN from the PM Yb-doped laser has distinct peaks that originate at the resonant frequency of the cavity length (around six megahertz (6MHz)). The noise for the PM Yb-doped laser increased and broadened as output power increased.

[0036] By comparison, the ASE PM MOPA traces 510, 520 showed no discrete noise peaks. The ASE PM MOPA trace at low power 520 was virtually at the measured background level 550. The ASE PM MOPA at maximum power 510 showed a slight increase in the overall signal level.

[0037] Next, FIG. 6 and FIG. 7 are graphs 600, 700, which show output power (y-axis) as a function of pump power (x-axis) and intensity (y-axis) as a function of wavelength (x-axis), respectively, comparing output of the RFL 300 (FIG. 3) when pumped with PM Yb-doped laser (FIG. 1) and output of the RFL 300 (FIG. 3) when pumped with the ASE PM MOPA (FIG. 2). The graph 600 of measured output power includes traces for ASE pump 610, PM Yb-pump 615 and output power at wavelengths of 1090nm (for ASE pump 620 and for PM-Yb pump 625), 1148nm (for ASE 630 and for PM-Yb 635), 1212nm (for ASE 640 and for PM-Yb 645), and 1280nm (for ASE 650 and for PM-Yb 655). The graph 700 of measured intensity shows traces for both ASE maximum power 710 and PM-Yb maximum power 720.

[0038] For the measurements shown in FIG. 6 and FIG. 7, the PER were all >20dB regardless of the pump source. The PM-Yb pumped system achieved a maximum output power of 58W from a 1090nm pump of 108W, with an in-band spectral purity at 1212nm of only 49.8 percent (%) for the PM-Yb pumped system. High RIN prevents full power transfer to each Stokes component.

[0039] In contrast, the ASE PM MOPA pumped system achieved maximum output power of 87W at 1212nm from 1 1090nm pump power of 125W, with an in-band spectral purity at 1212nm of 99.7%, which is remarkably higher than the spectral purity obtained with the PM- Yb pumped system.

[0040] FIG. 8 and FIG. 9 are graphs 800, 900, which show output power (y-axis) as a function of pump power (x-axis) and intensity (y-axis) as a function of wavelength (x-axis), respectively, comparing the output of the RFA 400 (FIG. 4) when pumped with PM Yb-doped laser (FIG. 1) and output of the RFL 300 (FIG. 3) when pumped with the ASE PM MOPA (FIG. 2). Specifically, for varying pump power at 1090nm, FIG. 8 shows traces measured for output power at different wavelengths, e.g., 1090nm (820), 1148nm (830), 1212nm (840), 1280nm(850), and 1360nm (860). The total power 810 and 3dB linewidth 870 are also shown in FIG. 8.Wavelength-dependent intensity (in dB) for 32W output (910) and DFB seed 920 are shown in FIG. 9.

[0041] The measurements in FIG. 8 and FIG. 9 show that the threshold for 1280nm amplification was 69 Watts (69W) of 1090nm pump and the maximum power achieved was 32W with an in-band percentage of 99.2% at a 1090nm pump power of 94W. For the particular embodiment shown herein, linewidth broadened with increased output power to a final full- width-at-half-maximum (FWHM) value of 0.5dB. Further scaling of the output power appears to be possible with higher power 1212nm pump sources.

[0042] The embodiments in FIG. 1 through FIG. 9 demonstrate systems for pumping PM RFLs and PM RFAs with low noise pump sources, along with measurements comparing one embodiment of a PM-Yb pumped system and one embodiment of an ASE PM MOPA pumped system. As demonstrated from the disclosed embodiments, substantial improvements to PM RFLs and PM RFAs are achievable with the use of low-noise pump sources.

[0043] For components that are described as being optically coupled together, one having ordinary skill in the art will understand that the coupling may be direct (e.g., directly splicing together of optical fibers or other optical components) or indirect (e.g., providing an optical path (either directly or indirectly) between two components). Furthermore, one having ordinary skill in the art will understand that optical components (e.g., pump combiners (PCs), fiber Bragg gratings (FBGs), polarizers, isolators, band-pass filters (BPFs), amplifiers, wavelength division multiplexers (WDMs), distributed feedback (DFB) lasers, boosted optical amplifiers (BOAs), etc.) have their respective inputs (where light enters) and outputs (where light exits), with some components (e.g., WDMs, PCs, etc.) sometimes having multiple inputs and multiple outputs.Insofar as conventional or standard optical components are known to those having skill in the art, inherent features (such as inputs and outputs) are not expressly discussed in detail herein, with the understanding that those having skill in the art have a firm grasp on those inherent features.

[0044] Although exemplary embodiments have been shown and described, it will be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the disclosure as described may be made. All such changes, modifications, and alterations should therefore be seen as within the scope of the disclosure.

Claims

What is claimed is:

1. A system comprising: a low -noise polarization-maintaining (PM) pump source that is free of longitudinal mode structure; a PM semi-random Raman fiber laser (RFL) pumped by the low-noise PM pump source; and a narrow-linewidth polarization-maintaining (PM) Raman fiber amplifier (RFA) pumped by the PM semi-random RFL.

2. A system comprising: an amplified spontaneous emission (ASE) polarization-maintaining (PM) master oscillator power amplifier (MOP A) comprising: a pump diode; a pump combiner (PC) comprising: a PC input optically coupled to the pump diode; and a PC output; a first gain fiber optically coupled the PC output; a polarizer comprising: a polarizer input optically coupled to the first gain fiber; and a polarizer output; a first PM fiber optically coupled to the polarizer output; an isolator and band-pass filter (BPF) comprising: an isolator / BPF input optically coupled to the first PM fiber; and an isolator / BPF output; and an output amplifier comprising: an output amplifier input optically coupled to the isolator / BPF output; and an output amplifier output; a polarization-maintaining (PM) Raman fiber laser (RFL) comprising:PM fiber Bragg gratings (FBGs); a first wavelength division multiplexer (WDM) comprising: a first WDM pump input optically coupled to the output amplifier output; a first WDM output; and a feedback input optically coupled to the first WDM output; and a first PM Raman gain fiber optically coupled to the first WDM output.

3. The system of claim 2, further comprising: a PM Raman fiber amplifier (RFA) comprising: a distributed feedback (DFB) laser; a boosted optical amplifier (BOA) comprising: a BOA input optically coupled to the DFB; and a BOA output; a second WDM comprising: a second WDM pump input optically coupled to the first PM Raman gain fiber; a second WDM output; and a second WDM feedback input optically coupled to the second WDM output; a second PM fiber optically coupled to the second WDM output; and a second PM Raman gain fiber optically coupled to the second PM fiber.

4. A system comprising: a low-noise polarization-maintaining (PM) pump source that is free of longitudinal mode structure; and a polarization-maintaining (PM) semi-random Raman fiber laser (RFL) having a pump input that is optically coupled to an output of the low-noise PM pump source.

5. The system of claim 4, further comprising: a PM Raman fiber amplifier (RFA) optically coupled to an output of the PM semirandom RFL, the PM RFA being pumped by the PM semi-random RFL.

6. The system of claim 4, wherein the low-noise polarization-maintaining (PM) pump source comprises an amplified spontaneous emission (ASE) PM master oscillator power amplifier (MOP A) comprising: a pump diode; a pump combiner (PC) comprising: a PC input optically coupled to the pump diode; and a PC output; a first gain fiber optically coupled the PC output; a polarizer comprising: a polarizer input optically coupled to the first gain fiber; and a polarizer output; a first PM fiber optically coupled to the polarizer output; an isolator and band-pass filter (BPF) comprising: an isolator / BPF input optically coupled to the first PM fiber; and an isolator / BPF output; and an output amplifier comprising: an output amplifier input optically coupled to the isolator / BPF output; and an output amplifier output.

7. The system of claim 6, wherein the pump diode has a center wavelength of 915 nanometers (nm).

8. The system of claim 6, wherein the first gain fiber comprises a ytterbium (Yb) doped gain fiber.

9. The system of claim 8, wherein the Yb-doped gain fiber is a fifty meter (50m) Yb-doped gain fiber.

10. The system of claim 6, wherein the output amplifier comprises a two (2) stage amplifier configured to output pump light at a center wavelength of 1090 nanometers (nm).11 . The system of claim 4, wherein the PM semi-random RFL comprises:PM fiber Bragg gratings (FBGs); a first wavelength division multiplexer (WDM) comprising: a first WDM pump input optically coupled to the output amplifier output; a first WDM output; and a feedback input optically coupled to the first WDM output; and a first PM Raman gain fiber optically coupled to the first WDM output.

13. The system of claim 5, wherein the PM Raman fiber amplifier (RFA) comprises: a distributed feedback (DFB) laser; a boosted optical amplifier (BOA) comprising: a BOA input optically coupled to the DFB; a BOA output; a second WDM comprising: a second WDM pump input optically coupled to the first PM Raman gain fiber; a second WDM output; and a second WDM feedback input optically coupled to the second WDM output; a second PM fiber optically coupled to the second WDM output; and a second PM Raman gain fiber optically coupled to the second PM fiber.

14. The system of claim 13, wherein the DFB laser operates at a center wavelength of 1280 nanometers (nm).

15. The system of claim 13, wherein the second PM fiber has an operating center wavelength of 980 nanometers (nm).

16. The system of claim 13, wherein the second PM Raman gain fiber comprises a ytterbium (Yb) doped gain fiber.

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