A single-frequency optical fiber module for suppressing backward Stokes light amplification

Abstract

The present invention provides a single-frequency optical fiber module for suppressing backward Stokes light amplification, comprising a forward pumping structure and a backward pumping structure, wherein the forward pumping structure is used for pre-amplifying a single-frequency seed source, and the backward pumping structure is used for secondary amplification of an output optical signal of the forward pumping structure, wherein the backward pumping structure comprises a second gain optical fiber, the second gain optical fiber comprises a first sub-optical fiber and a second sub-optical fiber, the input end of the second sub-optical fiber is fused with the output end of the first sub-optical fiber; the core diameter of the second sub-optical fiber is greater than the core diameter of the first sub-optical fiber; the present invention plays a role in improving Stokes light transmission loss by arranging gain optical fibers with different core diameters, and effectively suppresses the SBS effect in the single-frequency optical fiber module; meanwhile, the present invention adopts the backward pumping structure to further improve the suppression of the SBS effect.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber lasers, and in particular to a single-frequency optical fiber module for suppressing backward Stokes light amplification. Background Art

[0002] Fiber modules use rare earth ion-doped fiber as gain medium, and have many advantages such as low pump threshold, high coupling efficiency, good beam quality, convenient thermal management, compact structure, and wide tunable range. Among them, single-frequency laser amplification technology has become a hot topic of research at home and abroad. Its spectral line width is very narrow and has good coherence characteristics, which makes it have huge and strong demand in high-precision detection and long-distance detection, such as laser radar, coherent detection, nonlinear frequency conversion and other fields. Improving the output power of single-frequency fiber modules can further expand their application scope and fields, giving them a wider application prospect.

[0003] In order to obtain high output power, the main oscillator power amplifier structure is usually used to amplify the power of the module. However, due to its extremely narrow line width, the power increase is limited by the stimulated Brillouin scattering effect (SBS). Therefore, how to suppress SBS and increase the SBS threshold has become a key issue that needs to be solved urgently.

[0004] Therefore, there is an urgent need for a single-frequency optical fiber module that suppresses backward Stokes light amplification to solve the above technical problems. Summary of the invention

[0005] The object of the present invention is to provide a single-frequency optical fiber module for suppressing backward Stokes light amplification, so as to improve the technical problem of low optical output efficiency of the single-frequency optical fiber module in the prior art due to a small SBS threshold.

[0006] In order to solve the above technical problems, the present invention provides a single-frequency optical fiber module for suppressing backward Stokes light amplification, comprising a forward pumping structure and a backward pumping structure, wherein the forward pumping structure is used to pre-amplify the single-frequency seed source, and the backward pumping structure is used to perform secondary amplification on the output optical signal of the forward pumping structure;

[0007] The backward pumping structure includes a second gain fiber, which includes a first sub-fiber and a second sub-fiber. The input end of the second sub-fiber is fused with the output end of the first sub-fiber. The core diameter of the second sub-fiber is greater than the core diameter of the first sub-fiber.

[0008] Preferably, the forward pumping structure further includes a first isolator and a first wavelength division multiplexer, the input end of the first isolator is connected to the single frequency seed source, and the output end of the first isolator is connected to the first input end of the first wavelength division multiplexer.

[0009] Preferably, the forward pumping structure further includes a forward pumping source and a first gain fiber, the second input end of the first wavelength division multiplexer is connected to the output end of the forward pumping source, and the output end of the first wavelength division multiplexer is fused to the input end of the first gain fiber.

[0010] Preferably, the backward pumping structure further comprises a second isolator, the input end of the second isolator is fused to the output end of the first gain fiber, and the output end of the second isolator is fused to the input end of the first sub-fiber.

[0011] Preferably, the backward pumping structure further includes a second wavelength division multiplexer and a backward pumping source, the input end of the second wavelength division multiplexer is fused with the output end of the second sub-fiber, and the first output end of the second wavelength division multiplexer is connected to the output end of the backward pumping source.

[0012] Preferably, the backward pumping structure further comprises a third isolator, the input end of the third isolator is connected to the second output end of the second wavelength division multiplexer, and the output end of the third isolator is connected to the optical fiber end cap.

[0013] Preferably, the single-frequency seed source is an optical fiber module or a semiconductor module, and the emission wavelength of the single-frequency seed source is located within the first gain fiber and the gain spectrum of the first gain fiber.

[0014] Preferably, the forward pumping structure includes a pumping laser having an emission wavelength of 976 nm, and the backward pumping structure includes a pumping laser having an emission wavelength of 940 nm.

[0015] Preferably, the first gain fiber is a single-mode double-clad erbium-doped fiber, the core diameter of the first gain fiber is 6um, and the cladding diameter is 125um; the first gain fiber is a double-clad erbium-ytterbium co-doped fiber, the core diameter of the first sub-fiber is 6um, and the cladding diameter is 125um; the core diameter of the second sub-fiber is 10um, and the cladding diameter is 125um.

[0016] Preferably, the length of the first gain fiber is 1.8-2.4 m, the length of the first sub-fiber is 0.5-1 m, and the length of the second sub-fiber is 2-3 m.

[0017] The beneficial effects of the present invention are as follows: Different from the prior art, the present invention provides a single-frequency optical fiber module for suppressing backward Stokes light amplification, comprising a forward pumping structure and a backward pumping structure, wherein the forward pumping structure is used to pre-amplify a single-frequency seed source, and the backward pumping structure is used to perform secondary amplification on an output optical signal of the forward pumping structure, wherein the backward pumping structure comprises a second gain optical fiber, the second gain optical fiber comprises a first sub-optical fiber and a second sub-optical fiber, the input end of the second sub-optical fiber is fused with the output end of the first sub-optical fiber; the core diameter of the second sub-optical fiber is larger than the core diameter of the first sub-optical fiber; the backward Stokes light amplification suppressing method provided by the present invention is simple and convenient to operate. The single-frequency fiber module of Stokes light amplification has the effect of improving Stokes light transmission loss by arranging gain fibers with different core diameters in a backward pumping structure near the output end, which can improve the SBS threshold of the single-frequency fiber module and effectively suppress the SBS effect in the single-frequency fiber module; at the same time, the present invention further improves the suppression of the SBS effect by arranging the backward pumping structure, and increases the signal light power injected into the second sub-fiber and suppresses the stimulated spontaneous emission (ASE) of the optical signal of the second sub-fiber during amplification, thereby improving the pump utilization rate and the side mode suppression ratio of the final output, and finally improving the optical output power of the single-frequency fiber module. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the connection of a single-frequency optical fiber module for suppressing backward Stokes light amplification provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] The purpose of the present invention is to address the defects of the prior art and provide a single-frequency optical fiber module 100 that suppresses backward Stokes light amplification, improves the SBS threshold of the single-frequency optical fiber module 100, reduces system complexity, and achieves efficient laser output.

[0021] The technical solution of the present invention is now described in conjunction with specific embodiments.

[0022] See also Figure 1The present invention provides a single-frequency optical fiber module 100 for suppressing backward Stokes light amplification, comprising a forward pumping structure 10 and a backward pumping structure 20, wherein the forward pumping structure 10 comprises a single-frequency seed source 11, a forward pumping source 13 and a first gain optical fiber 15, and the backward pumping structure 20 comprises a second gain optical fiber 22 and a backward pumping source 24; the forward pumping structure 10 is used for pre-amplifying the single-frequency seed source 11, and the backward pumping structure 20 is used for secondary amplification of the output optical signal of the forward pumping structure 10;

[0023] The second gain fiber 22 includes a first sub-fiber 221 and a second sub-fiber 222 . The input end of the second sub-fiber 222 is fused with the output end of the first sub-fiber 221 . The core diameter of the second sub-fiber 222 is greater than that of the first sub-fiber 221 .

[0024] In an embodiment of the present invention, the single-frequency seed source 11 is used to provide a signal source that requires power amplification, and to provide high-quality, high-signal-to-noise ratio continuous or pulsed single-frequency seed light; the single-frequency seed source 11 is a fiber laser or a semiconductor laser, and the emission wavelength of the single-frequency seed source 11 is within the gain spectrum of the first gain fiber 15 and the second gain fiber 22.

[0025] Preferably, the emission wavelength of the single-frequency seed source 11 is 1550 nm (belonging to the L band).

[0026] In the embodiment of the present invention, the forward pumping structure 10 further includes a first isolator 12 and a first wavelength division multiplexer 14 . The input end of the first isolator 12 is connected to the single frequency seed source 11 , and the output end of the first isolator 12 is connected to the first input end of the first wavelength division multiplexer 14 .

[0027] Specifically, the first isolator 12 is a passive optical isolator device that only allows unidirectional light to pass through, and its working principle is based on the non-reciprocity of Faraday rotation. The first isolator 12 is a passive device that allows light to pass through in one direction but prevents it from passing through in the opposite direction. Its function is to limit the direction of light so that light can only be transmitted in one direction. The light reflected by the optical fiber echo can be well isolated by the first isolator 12, thereby improving the efficiency of light wave transmission.

[0028] Furthermore, the first isolator 12 is coupled to the rear end of the single-frequency seed source 11, and its wavelength range covers the range of the single-frequency seed source 11; the first isolator 12 allows the seed signal light to pass through the single-frequency seed source 11 along the direction of optical path propagation, while preventing the backward light of the first wavelength division multiplexer 14 from propagating in the direction of the single-frequency seed source 11, and is used to prevent the backward light generated by the forward pumping structure 10 and the backward pumping structure 20 and the end face reflection from damaging the single-frequency seed source 11, thereby protecting the system components.

[0029] In the embodiment of the present invention, the forward pump source 13 is a pump laser with an emission wavelength of 976 nm. The optical fiber module with the forward pump source 13 structure has a higher output light signal-to-noise ratio but a lower slope efficiency.

[0030] In the embodiment of the present invention, the first input end of the first wavelength division multiplexer 14 is connected to the output end of the first isolator 12, the second input end of the first wavelength division multiplexer 14 is connected to the output end of the forward pump source 13, and the output end of the first wavelength division multiplexer 14 is fused to the input end of the first gain fiber 15; wherein, the internal structure of the first wavelength division multiplexer 14 is generally an all-fiber structure, and the optical fibers are generally combined by direct fusion splicing. This type of structure formed by direct end face fusion coupling and side fusion affinity can be called a pump combiner.

[0031] Specifically, the first wavelength division multiplexer 14 is used to couple the 976 nm pump light and the 1550 nm seed signal light into the first gain optical fiber 15 .

[0032] In the embodiment of the present invention, the first gain fiber 15 is a single-mode double-clad erbium-doped fiber with high light absorption efficiency. The core diameter of the first gain fiber 15 is 6um and the cladding diameter is 125um. The erbium-doped fiber is used as a gain medium. When light is incident on the erbium-doped fiber, the energy carried by the incident light excites the electrons in the medium to a higher energy level due to the absorption of the medium. Through the relaxation phenomenon, the electrons transition from the high energy level to the ground state and release energy, emitting photons, thereby amplifying the luminous power of the seed signal light to provide gain.

[0033] In an embodiment of the present invention, the backward pumping structure 20 is used to perform secondary amplification on the output optical signal of the forward pumping structure 10; wherein, the backward pumping structure 20 also includes a second isolator 21, the input end of the second isolator 21 is fused with the output end of the first gain fiber 15, and the output end of the second isolator 21 is fused with the input end of the first sub-fiber 221.

[0034] Specifically, the structure of the second isolator 21 is the same as that of the first isolator 12. The function of the second isolator 21 is to allow the output optical signal of the forward pumping structure 10 to pass along the optical path propagation direction (first direction), while preventing the backward light of the backward pumping structure 20 from propagating in the direction of the single-frequency seed source 11. It is used to prevent the backward light generated by the backward pumping structure 20 and the end face reflection from damaging the single-frequency seed source 11, thereby protecting system components.

[0035] In the embodiment of the present invention, the input end of the second gain fiber 22 is fused with the output end of the second isolator 21; the second gain fiber 22 includes a first sub-fiber 221 and a second sub-fiber 222, and the input end of the second sub-fiber 222 is fused with the output end of the first sub-fiber 221; the core diameter of the second sub-fiber 222 is greater than the core diameter of the first sub-fiber 221.

[0036] Specifically, the second gain fiber 22 is a double-clad erbium-ytterbium co-doped fiber, the core diameter of the first sub-fiber 221 is 6um, and the cladding diameter is 125um; the core diameter of the second sub-fiber 222 is 10um, and the cladding diameter is 125um;

[0037] The function of the ytterbium ions is to improve the efficiency of the pump light emitted to the pump source 24 after absorption and transfer the energy to the erbium ions to achieve amplification of the seed signal light.

[0038] Furthermore, the principle of suppressing the SBS effect by making the core diameter of the second sub-fiber 222 larger than the core diameter of the first sub-fiber 221 is as follows: the SBS threshold is proportional to the effective mode field area of ​​the optical fiber and inversely proportional to the use length of the optical fiber. Increasing the core diameter of the optical fiber can increase the effective mode field area of ​​the gain optical fiber, which can reduce the power density of the seed signal light, and using an optical fiber with a larger core diameter can improve the absorption efficiency of the pump light, shorten the use length of the optical fiber, and further suppress the SBS effect.

[0039] In an embodiment of the present invention, the backward pump source 24 is a pump laser with an emission wavelength of 940 nm. The optical fiber module having the backward pump source 24 structure can achieve higher slope efficiency and has a certain inhibitory effect on nonlinear effects; wherein, the emission wavelength of the backward pump source 24 is different from the emission wavelength of the forward pump source 13, mainly because the backward pump source 24 is adapted to the second sub-fiber 222 with a higher core diameter.

[0040] In the embodiment of the present invention, the backward pumping structure 20 also includes a second wavelength division multiplexer 23, the input end of the second wavelength division multiplexer 23 is fused with the output end of the second sub-fiber 222, the first output end of the second wavelength division multiplexer 23 is connected to the output end of the backward pumping source 24, and the second output end of the second wavelength division multiplexer 23 is connected to the input end of the third isolator 25.

[0041] Specifically, the structure of the second wavelength division multiplexer 23 is the same as that of the first wavelength division multiplexer 14 , and the second wavelength division multiplexer 23 is used to couple the 940nm pump light and the 1550nm seed signal light in the emission direction opposite to the optical path propagation direction, and output them to the third isolator 25 .

[0042] In the embodiment of the present invention, the input end of the third isolator 25 is connected to the second output end of the second wavelength division multiplexer 23, and the output end of the third isolator 25 is connected to the optical fiber end cap.

[0043] Specifically, the structure of the third isolator 25 is the same as that of the first isolator 12. The function of the third isolator 25 is to allow the output optical signal of the second wavelength division multiplexer 23 to pass along the optical path propagation direction (first direction), while preventing the backward light of the optical fiber end cap from propagating in the direction of the single-frequency seed source 11. It is used to prevent the backward light generated by the optical fiber end cap and the end face reflection from damaging the single-frequency seed source 11, thereby protecting system components.

[0044] In the embodiment of the present invention, the principle of the backward pump source 24 suppressing the SBS effect compared with the forward pump source 13 is as follows: the backward pump source 24 will form a temperature field gradient in the gain fiber with the temperature continuously increasing along the optical path propagation direction (first direction) (while the temperature field gradient of the forward pump source 13 first increases and then decreases along the optical path propagation direction, making it difficult to extend the SBS threshold), which can effectively broaden the SBS gain spectrum and significantly increase the SBS threshold, thereby suppressing the occurrence of SBS.

[0045] The present invention can obtain the best output effect of amplifying the seed signal light by adjusting the length ratio between the first sub-fiber 221 and the second sub-fiber 222, which has a greater advantage in optimizing the best output compared to adjusting the length of a single gain fiber.

[0046] Specifically, the fiber length of the first sub-fiber 221 has a greater impact on the SBS threshold than the fiber length of the second sub-fiber 222. Therefore, in theory, a particularly short first sub-fiber 221 should be used, but using a particularly short first sub-fiber 221 will degrade the spectrum of the amplified seed signal light and increase the noise.

[0047] Specifically, increasing the fiber length of the first sub-fiber 221 will improve the signal-to-noise ratio of the output light, but will reduce the SBS threshold (small-core fiber has a better amplification effect on signal light and has lower noise than large-core fiber, but increasing the fiber length will reduce the SBS threshold).

[0048] Furthermore, the fiber length of the second sub-fiber 222 mainly affects the output optical power / energy. Using a longer length will increase the energy storage in the second sub-fiber 222 and improve the output optical power / energy. However, in the traditional structure, due to the limitation of the SBS threshold, a gain fiber that is too long will not be used, which inhibits the increase in output.

[0049] The SBS threshold of the single-frequency fiber amplifier provided by the present invention can compensate for the adverse effects of the increase in the fiber length of the second sub-fiber 222 by reducing the fiber length of the first sub-fiber 221, and different fiber length ratios can be selected for design according to different index parameters.

[0050] In one embodiment, the length of the first gain fiber 15 is 1.8-2.4 m, the length of the first sub-fiber 221 is 0.5-1 m, and the length of the second sub-fiber 222 is 2-3 m.

[0051] Compared with the prior art, the present invention has the following advantages:

[0052] First, the present invention is beneficial to the energy of backward Stokes light lost in the gain fiber mode with different core diameters, thereby suppressing the SBS effect;

[0053] Second, the present invention cooperates with the backward pumping method to further improve the suppression of the SBS effect, and increases the signal light power injected into the second sub-fiber 222, suppresses the stimulated spontaneous emission of the second sub-fiber 222 during amplification, and improves the pump utilization rate and the side mode suppression ratio of the final output;

[0054] Third, the present invention can obtain the best output effect by adjusting the length ratio between the first sub-fiber 221 and the second sub-fiber 222, which has a greater advantage in optimizing the best output compared to adjusting the length of a single gain fiber.

[0055] In summary, different from the prior art, the present invention provides a single-frequency optical fiber module 100 for suppressing backward Stokes light amplification, including a forward pumping structure 10 and a backward pumping structure 20, the forward pumping structure 10 includes a single-frequency seed source 11, a forward pumping source 13 and a first gain fiber 15, and the backward pumping structure 20 includes a second gain fiber 22 and a backward pumping source 24; the forward pumping structure 10 is used to pre-amplify the single-frequency seed source 11, and the backward pumping structure 20 is used to perform secondary amplification on the output optical signal of the forward pumping structure 10, wherein the second gain fiber 22 includes a first sub-fiber 221 and a second sub-fiber 222, and the input end of the second sub-fiber 222 is fused with the output end of the first sub-fiber 221; the core diameter of the second sub-fiber 222 is larger than that of the first sub-fiber 221. The core diameter of the fiber 221; the single-frequency optical fiber module 100 for suppressing backward Stokes light amplification provided by the present invention plays a role in improving the Stokes light transmission loss by arranging gain fibers with different core diameters in the backward pumping structure 20 near the output end, which can improve the SBS threshold of the single-frequency optical fiber module 100 and effectively suppress the SBS effect in the single-frequency optical fiber module 100; at the same time, the present invention adopts a backward pumping source 24 in the backward pumping structure 20, further improving the suppression of the SBS effect, and increasing the signal light power injected into the second sub-optical fiber 222 and suppressing the stimulated spontaneous emission (ASE) of the optical signal of the second sub-optical fiber 222 during amplification, thereby improving the pump utilization rate and the boundary suppression ratio of the final output, and finally improving the optical output power of the single-frequency optical fiber module 100.

[0056] It should be noted that the above embodiments all belong to the same inventive concept, and the description of each embodiment has its own focus. For matters that are not described in detail in some embodiments, reference may be made to the description in other embodiments.

[0057] The above embodiments only express the implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for those of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A single-frequency optical fiber module for suppressing backward Stokes light amplification, characterized in that: It comprises a forward pumping structure and a backward pumping structure, wherein the forward pumping structure is used to pre-amplify the single-frequency seed source, and the backward pumping structure is used to perform secondary amplification on the output optical signal of the forward pumping structure; The backward pumping structure includes a second gain fiber, the second gain fiber includes a first sub-fiber and a second sub-fiber, the input end of the second sub-fiber is fused with the output end of the first sub-fiber; the core diameter of the second sub-fiber is greater than the core diameter of the first sub-fiber; Wherein, the forward pump junction includes a first gain fiber, which is a single-mode double-clad erbium-doped fiber, with a core diameter of 6um, a cladding diameter of 125um, and a length of 1.8~2.4m; the second gain fiber is a double-clad erbium-ytterbium co-doped fiber, with a core diameter of 6um, a cladding diameter of 125um, and a length of 0.5~1m; the core diameter of the second sub-fiber is 10um, the cladding diameter is 125um, and the length is 2~3m.

2. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 1, characterized in that: The forward pumping structure includes a first isolator and a first wavelength division multiplexer. The input end of the first isolator is connected to the single-frequency seed source, and the output end of the first isolator is connected to the first input end of the first wavelength division multiplexer.

3. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 2, characterized in that: The forward pump structure also includes a forward pump source. The second input end of the first wavelength division multiplexer is connected to the output end of the forward pump source, and the output end of the first wavelength division multiplexer is fused with the input end of the first gain optical fiber.

4. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 3, characterized in that: The backward pumping structure further includes a second isolator, the input end of the second isolator is fused to the output end of the first gain fiber, and the output end of the second isolator is fused to the input end of the first sub-fiber.

5. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 1, characterized in that: The backward pumping structure also includes a second wavelength division multiplexer and a backward pumping source. The input end of the second wavelength division multiplexer is fused with the output end of the second sub-fiber, and the first output end of the second wavelength division multiplexer is connected to the output end of the backward pumping source.

6. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 5, characterized in that: The backward pumping structure further includes a third isolator, an input end of the third isolator is connected to the second output end of the second wavelength division multiplexer, and an output end of the third isolator is connected to an optical fiber end cap.

7. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 3, characterized in that: The single-frequency seed source is an optical fiber module or a semiconductor module, and the emission wavelength of the single-frequency seed source is located within the gain spectra of the first gain fiber and the second gain fiber.

8. The single-frequency optical fiber module for suppressing backward Stokes light amplification according to claim 1, characterized in that: The forward pumping structure includes a pumping laser with an emission wavelength of 976 nm, and the backward pumping structure includes a pumping laser with an emission wavelength of 940 nm.

Images