Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control

Abstract

The present invention discloses a watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating regulation. The oscillator comprises two sections of active optical fiber, which are pumped by two pump sources respectively. The two sections of active optical fiber fully absorb the pump power while still retaining unpumped portions. The standing wave effect in the linear cavity laser generates narrowband refractive index modulation gratings in the unpumped portions of the two sections of active optical fiber. Therefore, the two sections of active optical fiber serve as gain media to provide laser gain and as saturable absorbers to produce a frequency selection effect. The narrowband refractive index modulation gratings in the two sections of active optical fiber form a vernier effect, which enhances frequency selection and ensures that the power output level of the single-frequency fiber oscillator reaches the watt level.

Description

Technical Field

[0001] The present invention relates to the field of single-frequency fiber lasers, and in particular to a watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating regulation. Background Art

[0002] Single-frequency fiber oscillators, due to their excellent properties such as good monochromaticity, long coherence length, and low noise, have become important laser light sources in applications such as scientific research, detection, and communications. The main feature of single-frequency fiber oscillators is single-longitudinal-mode operation, which poses a significant challenge to oscillator design and limits the output power of single-frequency fiber lasers. Currently, the output power of most single-frequency fiber lasers is mostly in the hundreds of milliwatts, with fewer reaching hundreds of milliwatts or watts. The main reason is that as the power level increases, it is difficult to maintain good frequency selection performance.

[0003] At present, the frequency selection methods in oscillators include: ultra-short linear cavity, cascaded sub-cavity, composite linear cavity, saturable absorber, etc. Among them, the ultra-short linear cavity selects the frequency by relying on the shorter cavity length to ensure a larger free spectral range. Although it has a compact structure and excellent performance in suppressing mode hopping, the shorter cavity length also greatly limits the length of the gain medium. In order to obtain higher output power, highly doped active optical fiber is generally used as the gain medium. However, with the increase of laser power, the temperature change of the gain medium causes a slight change in the cavity length, which will seriously weaken the oscillator's ability to select the frequency. Therefore, the frequency selection method based on low-doped fiber oscillators has also been widely studied. The principles of cascaded sub-cavity and composite linear cavity are both based on the vernier effect, thereby increasing the free spectral range of the oscillator. On the one hand, these two methods increase the free spectral range of the oscillator. It increases the complexity of the system, and on the other hand, it requires a high accuracy of the cavity length; the saturable absorber (SA) frequency selection method is generally implemented using an unpumped active optical fiber. The standing wave field formed by the two counter-propagating laser beams in the saturable absorber periodically modulates the refractive index of the optical fiber to obtain a narrow-band refractive index modulation grating, thereby achieving the purpose of frequency selection. It is a reliable single longitudinal mode implementation method, commonly found in ring cavity structures, and can achieve watt-level single-frequency laser output. However, at higher pump powers, the frequency selection effect of the saturable absorber is bleached, so the single longitudinal mode operation of the laser will be destroyed. Using this method, the single longitudinal mode operation can be achieved, and the maximum output power is only on the order of hundreds of milliwatts.

[0004] In summary, the current single-frequency laser frequency selection methods cannot maintain effective frequency selection of the laser at higher output power under high-power operation. In order to obtain higher single-frequency laser output power, it is urgent to develop more effective frequency selection methods to ensure stable single-frequency laser output above watt-level output power. Summary of the Invention

[0005] The present invention provides a Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control. Without adding special components, the present invention improves the frequency selectivity of single-frequency lasers and ensures single longitudinal mode operation under high-power conditions. Details are described below:

[0006] A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control, wherein the oscillator contains two sections of active optical fiber and is pumped by two pump sources respectively.

[0007] While the two sections of active optical fiber absorb the pump power, they still retain unpumped parts;

[0008] The standing wave effect in the linear cavity laser generates a narrow-band refractive index modulated grating in the unpumped parts of the two active optical fibers to produce a frequency selection effect; the narrow-band refractive index modulated grating in the two active optical fibers forms a vernier effect to further enhance the frequency selection effect.

[0009] Furthermore, the oscillator includes four cavity structures.

[0010] In the first solution, the two sections of active optical fiber are:

[0011] The first active fiber is located on one side of the first high-reflection fiber Bragg grating, and the second active fiber is located on one side of the first low-reflection fiber Bragg grating; the first high-reflection fiber Bragg grating and the first low-reflection fiber Bragg grating provide laser feedback; the first active fiber and the second active fiber are reversely pumped by the first pump source and the second pump source, respectively; the pump laser output by the first pump source is coupled and injected into the first active fiber through the first pump coupling device; the pump laser output by the second pump source is coupled and injected into the second active fiber through the second pump coupling device.

[0012] In the second solution, the two sections of active optical fiber are:

[0013] The third active optical fiber and the fourth active optical fiber are forward pumped by the third pump source and the fourth pump source respectively via the third pump coupling device and the fourth pump coupling device.

[0014] In the third solution, the two sections of active optical fiber are:

[0015] The fifth active optical fiber is reversely pumped by a fifth pump source via a fifth pump coupling device, and the sixth active optical fiber is forward pumped by a sixth pump source via a sixth pump coupling device.

[0016] In the fourth solution, the two sections of active optical fiber are:

[0017] The seventh active optical fiber is forward pumped by the seventh pump source via the seventh pump coupling device, and the eighth active optical fiber is reverse pumped by the eighth pump source via the eighth pump coupling device;

[0018] A section of passive optical fiber with matching size is added between the seventh active optical fiber and the eighth active optical fiber. The passive optical fiber is used to isolate the narrow-band refractive index modulation gratings formed in the seventh active optical fiber and the eighth active optical fiber, thereby forming a vernier effect.

[0019] The two sections of active optical fibers are active optical fibers doped with the same or different rare earth ions; the two sections of active optical fibers satisfy both emission cross sections and absorption cross sections at the center wavelengths of the high-reflection fiber grating and the low-reflection fiber grating.

[0020] The active optical fiber is a single-clad optical fiber, a double-clad optical fiber or a triple-clad optical fiber; the central wavelengths of the high-reflection fiber grating and the low-reflection fiber grating are consistent.

[0021] Preferably, the two pump sources are solid-state lasers, fiber lasers, or semiconductor lasers; the two pump sources are single longitudinal mode lasers or multi-longitudinal mode lasers; the two pump sources are single transverse mode lasers or high-order transverse mode lasers.

[0022] The beneficial effects of the technical solution provided by the present invention are:

[0023] 1. Based on this technical means, the frequency selection performance of single-frequency lasers can be improved without adding special devices, ensuring single longitudinal mode operation under high power conditions;

[0024] 2. Based on this technical approach, a feasible single-frequency operation solution is provided for active optical fibers in special bands, that is, for use as SAs without strong absorption in these bands;

[0025] 3. In this solution, the SA has a low absorption cross-section requirement for signal light, which can expand the application band of SA. The single longitudinal mode performance is optimized based on the control of the dynamic refractive index modulation grating. The structure is simple and compact, and the laser longitudinal mode has strong stability.

[0026] 4. This solution can realize high-power single-frequency fiber laser without relying on highly doped active optical fiber, and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The first structural diagram of a Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control;

[0028] Figure 2 The second structural diagram of a Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control;

[0029] Figure 3 The third structural diagram of a Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control;

[0030] Figure 4This is the fourth structural schematic diagram of a Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control.

[0031] In the accompanying drawings, the components represented by the reference numerals are as follows:

[0032] Figure 1 middle:

[0033] 1. The first high-reflection fiber Bragg grating (FBG); 2. The first active optical fiber;

[0034] 3. First pump coupling device; 4. First pump source;

[0035] 5. A second active optical fiber; 6. A second pump coupling device;

[0036] 7. Second pump source; 8. First low-reflection fiber Bragg grating;

[0037] Figure 2 middle:

[0038] 9. Second high-reflection fiber Bragg grating; 10. Third pump source;

[0039] 11. A third pump coupling device; 12. A third active optical fiber;

[0040] 13. A fourth pump source; 14. A fourth pump coupling device;

[0041] 15. Fourth active optical fiber; 16. Second low-reflection fiber Bragg grating;

[0042] Figure 3 middle:

[0043] 17. The third high-reflection fiber Bragg grating; 18. The fifth active optical fiber;

[0044] 19. Fifth pump coupling device; 20. Fifth pump source;

[0045] 21. Sixth pump source; 22. Sixth pump coupling device;

[0046] 23. Sixth active optical fiber; 24. Third low-reflection fiber Bragg grating;

[0047] Figure 4 middle:

[0048] 25. The fourth high-reflection fiber Bragg grating; 26. The seventh pump source;

[0049] 27. Seventh pump coupling device; 28. Seventh active optical fiber;

[0050] 29. Passive optical fiber; 30. Eighth active optical fiber;

[0051] 31. Eighth pump coupling device; 32. Eighth pump source;

[0052] 33. The fourth low-reflection fiber Bragg grating. DETAILED DESCRIPTION

[0053] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0054] 1. Design principle:

[0055] A watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating regulation. The oscillator contains two sections of active fiber, which are pumped by two pump sources respectively. While absorbing the pump power, the two sections of active fiber still retain an unpumped portion. The standing wave effect in the linear cavity laser generates narrowband refractive index modulation gratings in the unpumped portions of the two sections of active fiber. Therefore, the two sections of active fiber act as gain media to provide laser gain on the one hand, and as SA to produce a frequency selection effect on the other hand. The narrowband refractive index modulation gratings in the two sections of active fiber form a vernier effect, enhancing frequency selection to meet the needs of watt-class output power.

[0056] In an embodiment of the present invention, the narrowband refractive index modulated grating formed in the two sections of active optical fiber is the key to achieving single-frequency operation, and the bandwidth of the narrowband refractive index modulated grating is related to the intensity and length of the refractive index modulated grating; therefore, the refractive index modulated grating in the two sections of active optical fiber can be regulated by controlling the power ratio of the two pump sources, thereby achieving optimal frequency selection effect at different power levels and realizing stable single-frequency operation.

[0057] 2. Classification of laser oscillator cavity structures

[0058] According to different pumping directions, there are four types of laser oscillator cavity structures in the embodiments of the present invention:

[0059] 1. The first cavity structure:

[0060] The first high-reflection fiber Bragg grating 1 and the first low-reflection fiber Bragg grating 8 provide laser feedback; the first active fiber 2 is located on one side of the first high-reflection fiber Bragg grating 1, and the second active fiber 5 is located on one side of the first low-reflection fiber Bragg grating 8; the first active fiber 2 and the second active fiber 5 are reversely pumped by the first pump source 4 and the second pump source 7 respectively; the pump laser output by the first pump source 4 is coupled and injected into the first active fiber 2 through the first pump coupling device 3; the pump laser output by the second pump source 7 is coupled and injected into the second active fiber 5 through the second pump coupling device 6.

[0061] 2. The second cavity structure:

[0062] The required components are the same as those of the first cavity structure. In this structure, the third active fiber 12 and the fourth active fiber 15 are forward pumped by the third pump source 10 and the fourth pump source 13 respectively via the third pump coupling device 11 and the fourth pump coupling device 14.

[0063] 3. The third cavity structure:

[0064] The required components are the same as those of the first cavity structure. In this structure, the fifth active fiber 18 is reversely pumped by the fifth pump source 20 via the fifth pump coupling device 19, and the sixth active fiber 23 is forwardly pumped by the sixth pump source 21 via the sixth pump coupling device 22.

[0065] 4. The fourth cavity structure:

[0066] The required components are the same as those for the first cavity-type structure. In this structure, the seventh active fiber 28 is forward pumped by the seventh pump source 26 via the seventh pump coupling device 27, while the eighth active fiber 30 is reverse pumped by the eighth pump source 32 via the eighth pump coupling device 31. A length of dimensionally matched passive fiber 29 is inserted between the seventh active fiber 28 and the eighth active fiber 30. Passive fiber 29 is used to isolate the narrowband refractive index modulated gratings formed in the seventh active fiber 28 and the eighth active fiber 30, thereby creating a vernier effect.

[0067] 3. Optimal device selection

[0068] In the embodiment of the present invention, the two sections of active optical fiber can be active optical fibers doped with the same rare earth ions or active optical fibers doped with different types of rare earth ions. Under the condition of active optical fibers doped with the same rare earth ions, the two sections of active optical fibers can be the same or different. It is only necessary to satisfy both an emission cross section and a certain absorption cross section at the center wavelength of the high-reflection fiber Bragg grating and the low-reflection fiber Bragg grating. The embodiment of the present invention does not impose any restrictions on this.

[0069] The active optical fiber in the embodiment of the present invention can be a single-clad optical fiber or a double-clad optical fiber, and it is only necessary to ensure that the two sections of active optical fiber can provide laser gain and serve as SA.

[0070] In the embodiments of the present invention, the two supporting pump sources can be in the form of solid-state lasers, fiber lasers, semiconductor lasers, etc., as long as they can generate laser light of the wavelength. They can be single longitudinal mode lasers or multi-longitudinal mode lasers, and the embodiments of the present invention do not impose any restrictions on this. They can be single transverse mode lasers or high-order transverse mode lasers, as long as they can be coupled into the fiber laser system and transmit and absorb, and the embodiments of the present invention do not impose any restrictions on this.

[0071] In the embodiment of the present invention, the central wavelengths of the high-reflection fiber Bragg grating and the low-reflection fiber Bragg grating are consistent, and the reflectivity and bandwidth can be any as long as they can achieve laser oscillation, which is not limited in the embodiment of the present invention.

[0072] In the embodiment of the present invention, the pump coupling device may be a wavelength division multiplexer or a pump combiner, and it only needs to couple the pump laser into the resonant cavity to pump the active optical fiber.

[0073] Example 1

[0074] A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control, a typical implementation is as follows Figure 1 shown.

[0075] The oscillator comprises: a first high-reflection fiber Bragg grating 1, a first active fiber 2, a first pump coupling device 3, a first pump source 4, a second active fiber 5, a second pump coupling device 6, a second pump source 7, and a first low-reflection fiber Bragg grating 8 connected in sequence.

[0076] Among them, the first high-reflection fiber Bragg grating 1 has a central wavelength of 2050nm, a reflectivity of >99.9%, and a half-width at half maximum of 0.5nm; the first active fiber 2 and the second active fiber 5 are both thulium-holmium co-doped single-clad fibers, with core and cladding sizes of 8μm and 125μm respectively, an absorption coefficient of 150dB / m at 1570nm, and a length of 6m; the first pump source 4 and the second pump source 7 are both 1570nm erbium-ytterbium co-doped fiber lasers; the first low-reflection fiber Bragg grating 8 has a central wavelength of 2050nm, a reflectivity of 50%, and a half-width at half maximum of 0.09nm; the first pump coupling device 3 and the second pump coupling device 6 are both 1570nm / 2000nm wavelength division multiplexers, which are used to couple lasers in the 1570nm and 2000nm bands.

[0077] Example 2

[0078] A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control, a typical implementation is as follows Figure 2 shown.

[0079] The oscillator comprises: a second high-reflection fiber Bragg grating 9, a third pump source 10, a third pump coupling device 11, a third active fiber 12, a fourth pump source 13, a fourth pump coupling device 14, a fourth active fiber 15, and a second low-reflection fiber Bragg grating 16 connected in sequence.

[0080] The second high-reflection fiber Bragg grating (FBG) 9 has a central wavelength of 1950 nm, a reflectivity of >99.9%, and a half-width (FWHM) of 0.5 nm. The third active fiber 12 is a thulium-doped double-clad fiber with a core / cladding size of 10 / 130 μm and an absorption coefficient of 800 dB / m at 1570 nm. The fourth active fiber 15 is a thulium-doped single-clad fiber with a core / cladding size of 9 / 125 μm and an absorption coefficient of 150 dB / m at 1570 nm. The third pump source 10 and the fourth pump source 13 are both 1570 nm erbium-ytterbium co-doped fiber lasers. The second low-reflection fiber Bragg grating 16 has a central wavelength of 1950 nm, a reflectivity of 70%, and a half-width (FWHM) of 0.05 nm. The third pump coupling device 11 and the fourth pump coupling device 14 are both 1570 nm / 1950 nm wavelength division multiplexers.

[0081] Example 3

[0082] A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control, a typical implementation is as follows Figure 3 shown.

[0083] The oscillator includes: a third high-reflection fiber Bragg grating 17, a fifth active fiber 18, a fifth pump coupling device 19, a fifth pump source 20, a sixth pump source 21, a sixth pump coupling device 22, a sixth active fiber 23, and a third low-reflection fiber Bragg grating 24 connected in sequence.

[0084] The third high-reflectivity fiber Bragg grating (FBG) 17 has a central wavelength of 1064 nm, a reflectivity of >99.9%, and a full width at half maximum of 0.3 nm. The fifth active fiber 18 and the sixth active fiber 23 are both ytterbium-doped single-clad fibers with core / cladding dimensions of 5 / 125 μm and an absorption coefficient of 1500 dB / m at 976 nm. The fifth pump source 20 and the sixth pump source 21 are both 976 nm high-power semiconductor lasers. The third low-reflectivity fiber Bragg grating 24 has a central wavelength of 1064 nm, a reflectivity of 70%, and a full width at half maximum of 0.05 nm. The fifth pump coupling device 19 and the sixth pump coupling device 22 are both 976 nm / 1064 nm wavelength division multiplexers.

[0085] Example 4

[0086] A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control, a typical implementation is as follows Figure 4 shown.

[0087] The oscillator includes: a fourth high-reflection fiber Bragg grating 25, a seventh pump source 26, a seventh pump coupling device 27, a seventh active fiber 28, a passive fiber 29, an eighth active fiber 30, an eighth pump coupling device 31, an eighth pump source 32, and a fourth low-reflection fiber Bragg grating 33 connected in sequence.

[0088] The fourth high-reflectivity fiber Bragg grating (FBG) 25 has a central wavelength of 2800 nm, a reflectivity greater than 99.9%, and a full width at half maximum of 0.3 nm. The seventh active fiber 28 and the eighth active fiber 30 are both erbium-doped ZBLAN fibers with core / cladding dimensions of 15 / 125 μm and an absorption coefficient of 150 dB / m at 976 nm. The seventh pump source 26 and the eighth pump source 32 are both 976 nm high-power semiconductor lasers. The fourth low-reflectivity fiber Bragg grating 33 has a central wavelength of 2800 nm, a reflectivity of 70%, and a full width at half maximum of 0.1 nm. The seventh pump coupling device 27 and the eighth pump coupling device 31 are both 976 nm / 2800 nm wavelength division multiplexers.

[0089] Unless otherwise specified, the embodiments of the present invention do not limit the models of the components. Any component that can perform the above functions may be used.

[0090] Those skilled in the art will understand that the accompanying drawings are only a schematic diagram of a preferred embodiment, and the serial numbers of the embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control, characterized in that: The oscillator contains two sections of active optical fiber and is pumped by two pump sources respectively. While the two sections of active optical fiber absorb the pump power, they still retain unpumped parts; The standing wave effect in the linear cavity laser generates a narrow-band refractive index modulated grating in the unpumped parts of the two active optical fibers to produce a frequency selection effect; the narrow-band refractive index modulated grating in the two active optical fibers forms a vernier effect to further enhance the frequency selection effect.

2. The Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to claim 1, characterized in that: The power ratio of the two pump sources is controlled to regulate the refractive index modulation gratings in the two sections of active optical fiber, achieving the optimal frequency selection effect at different power levels to ensure stable single-frequency operation.

3. The Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to claim 1, characterized in that: The oscillator includes four cavity structures.

4. The Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to claim 3, characterized in that: The two sections of active optical fiber are: The first active fiber is located on one side of the first high-reflection fiber Bragg grating, and the second active fiber is located on one side of the first low-reflection fiber Bragg grating; the first high-reflection fiber Bragg grating and the first low-reflection fiber Bragg grating provide laser feedback; the first active fiber and the second active fiber are reversely pumped by the first pump source and the second pump source, respectively; the pump laser output by the first pump source is coupled and injected into the first active fiber through the first pump coupling device; the pump laser output by the second pump source is coupled and injected into the second active fiber through the second pump coupling device.

5. The Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to claim 3, characterized in that: The two sections of active optical fiber are: The third active optical fiber and the fourth active optical fiber are forward pumped by the third pump source and the fourth pump source respectively via the third pump coupling device and the fourth pump coupling device.

6. The Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to claim 3, characterized in that: The two sections of active optical fiber are: The fifth active optical fiber is reversely pumped by a fifth pump source via a fifth pump coupling device, and the sixth active optical fiber is forward pumped by a sixth pump source via a sixth pump coupling device.

7. The Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to claim 3, characterized in that: The two sections of active optical fiber are: The seventh active optical fiber is forward pumped by the seventh pump source via the seventh pump coupling device, and the eighth active optical fiber is reverse pumped by the eighth pump source via the eighth pump coupling device; A section of passive optical fiber with matching size is added between the seventh active optical fiber and the eighth active optical fiber. The passive optical fiber is used to isolate the narrow-band refractive index modulation gratings formed in the seventh active optical fiber and the eighth active optical fiber, thereby forming a vernier effect.

8. A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to any one of claims 4 to 7, characterized in that: The two sections of active optical fibers are active optical fibers doped with the same or different rare earth ions; the two sections of active optical fibers satisfy the requirement of having effective emission cross sections and absorption cross sections at the center wavelengths of the high-reflection fiber grating and the low-reflection fiber grating.

9. A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to any one of claims 4 to 7, characterized in that: The active optical fiber is a single-clad optical fiber, a double-clad optical fiber or a triple-clad optical fiber; the central wavelengths of the high-reflection fiber grating and the low-reflection fiber grating are consistent.

10. A Watt-class linear cavity single-frequency fiber oscillator based on dynamic refractive index grating control according to any one of claims 4 to 7, characterized in that: The two pump sources are solid-state lasers, fiber lasers, or semiconductor lasers; The two pump sources are single longitudinal mode lasers or multi-longitudinal mode lasers; The two pump sources are single transverse mode lasers or high-order transverse mode lasers.

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