Dual-clad bismuth-doped fiber-based cladding-pumped broadband optical amplifier, method and system

Abstract

The present application relates to a kind of based on double-clad bismuth-doped fiber cladding pumped broadband optical amplifier, method and system, amplifier is by pump signal combiner It is coupled to the core of double-clad bismuth-doped fiber to be amplified signal light, multi-mode pump light is coupled to inner cladding, so that multi-mode pump light propagates in inner cladding and is absorbed by bismuth-related active center, and the stimulated emission amplification of signal light to be amplified is carried out;After amplification, signal light is sequentially output through cladding pump light stripper, mode field adapter and output optical isolator.Output.The present application utilizes multi-mode semiconductor laser to carry out cladding pumping, reduces the dependence on high-brightness single-mode pump source, and improves the transmission and operation stability of all-fiber link by residual pump stripping, mode field transition and feedback suppression;Single-stage, single-pass structure is adopted, and the optical path is simple, and broadband amplification of E-band or S-band continuous single-mode signal light and noise-like pulse signal light can be compatible.

Description

Technical Field

[0001] This invention relates to the fields of optical fiber amplification and optical communication technology, and in particular to a cladding-pumped broadband optical amplifier, method and system based on double-clad bismuth-doped optical fiber. Background Technology

[0002] With the continuous development of cloud computing, artificial intelligence, data center interconnection, and machine-to-machine communication services, optical fiber communication systems are placing higher demands on transmission capacity and available spectral bandwidth. Existing commercial optical fiber communication systems primarily use erbium-doped fiber amplifiers to amplify C-band and L-band signals, while the low-loss transmission window of standard silica fiber also includes the O-band, E-band, and S-band. To further improve the available bandwidth of a single fiber without replacing the existing optical fiber transmission infrastructure, it is necessary to amplify optical signals in bands that are difficult for traditional erbium-doped fiber amplifiers to effectively cover.

[0003] Existing non-C-band and L-band optical amplification technologies mainly include Raman fiber amplification, semiconductor optical amplification, fiber parametric amplification, and doped fiber amplification. Raman fiber amplifiers can change the operating band by adjusting the pump wavelength, but typically require high pump power and have complex system structures and noise control. Semiconductor optical amplifiers are small in size, but are limited by output power, noise, polarization sensitivity, and nonlinear distortion. Fiber parametric amplifiers can achieve a wide operating bandwidth, but require high-power pumping and strict phase-matching conditions. In contrast, bismuth-related active centers have broad near-infrared absorption and emission bands. Using bismuth-doped fiber as the gain medium, a corresponding optical gain can be formed near 1.4 micrometers, providing a gain basis for amplifying E-band and S-band signal light.

[0004] Based on the pump light injection method, bismuth-doped fiber amplifiers are mainly divided into core-pumped and cladding-pumped types. Core-pumped amplifiers typically use a wavelength division multiplexer to couple the single-mode pump light and the signal light to be amplified to the core of the bismuth-doped fiber, resulting in a high spatial overlap between the pump light, the signal light to be amplified, and the bismuth-related active centers. This pumping method can achieve high pump utilization, but it usually relies on a high-brightness single-mode pump source of a specific wavelength. The output power, cost, and supply conditions of the pump source limit the amplifier's power expansion. Some broadband amplification schemes also require multiple pump sources of different wavelengths, or the use of bidirectional pumping, two-way amplification, and multi-stage amplification structures, increasing the number of optical components and the complexity of the optical path.

[0005] Cladding-pumped amplifiers employ double-clad bismuth-doped fiber. Multimode pump light is coupled to the inner cladding of the fiber, while the signal light to be amplified is coupled to the fiber core. The multimode pump light propagates within the inner cladding and passes through the core multiple times, gradually being absorbed by bismuth-related active centers within the core, thus amplifying the signal light propagating in the core through stimulated emission. This pumping method allows the use of high-power multimode semiconductor lasers. However, because the effective area of ​​the inner cladding is larger than the core area, the average spatial overlap between the multimode pump light and the bismuth-related active centers in the core is low. Therefore, the gain fiber length needs to be appropriately set based on the cladding absorption characteristics of the double-clad bismuth-doped fiber. If the gain fiber length is too short, some multimode pump light may not be fully absorbed; if the gain fiber length is too long, it may increase transmission loss, nonlinear effects, and the difficulty of fiber coiling and packaging.

[0006] During cladding-pumped amplification, residual cladding pump light may remain unabsorbed at the output end of the double-clad bismuth-doped fiber. If this residual pump light continues to enter the subsequent output optical path, it may cause pump light leakage and localized heat accumulation at the output end. Simultaneously, the double-clad gain fiber and the ordinary single-mode output fiber may differ in core diameter, mode field diameter, numerical aperture, and cladding structure. Direct connection can easily lead to mode field mismatch and additional transmission loss, and may excite higher-order modes. End-face reflections or external feedback at the output end may also return to the gain fiber along the optical path, interfering with the amplification process. Therefore, the cladding-pumped amplification link also needs to simultaneously address issues such as residual cladding pump light stripping, mode field transition between the double-clad fiber and the single-mode output fiber, and isolation of reflected light at the output end. Existing solutions do not adequately consider the connection sequence and coordinated configuration of these devices, making it difficult to simultaneously manage residual pump, transmission mode transition, and unidirectional output light transmission within the same continuous all-fiber amplification link.

[0007] Furthermore, the structural and material differences between double-clad and single-mode fibers increase the difficulty of heterogeneous fiber fusion splicing. Improper fusion parameter settings can easily lead to increased insertion loss, reduced mechanical strength, or leakage of cladding pump light into the splice area. Longer double-clad bismuth-doped fibers may also experience localized bending, fiber crossing, axial stretching, and stress concentration during coiling and encapsulation. Heat generated during amplification may also accumulate in the gain fiber, cladding pump light stripper, and splice area, thus affecting the long-term operational status of the all-fiber amplification link.

[0008] Most existing bismuth-doped fiber amplifiers are designed for small-signal gain, noise figure, or communication transmission performance of continuous weak signals. For noise-like pulse signals composed of multiple random ultrashort subpulses, their autocorrelation curves typically include narrow coherence spikes and a wide substrate carrying these spikes. When propagating in gain fiber, they are simultaneously affected by factors such as gain spectrum, gain saturation, self-phase modulation, dispersion, and amplified spontaneous emission. Current technology lacks a relatively simple single-stage, single-path all-fiber amplification link that can amplify continuous single-mode signals in the E-band or S-band, and also perform stimulated emission amplification and nonlinear spectral broadening for noise-like pulse signals. Summary of the Invention

[0009] Therefore, the technical problem to be solved by the present invention is to overcome the problem that the existing bismuth-doped fiber amplifier relies on a high-brightness single-mode pump source of a specific wavelength, which makes it difficult to simultaneously achieve effective absorption of multimode cladding pump light, removal of residual cladding pump light, mode field transition between double-clad fiber and single-mode fiber, and isolation of output reflected light. Furthermore, it is difficult to use a single-stage, single-pass all-fiber amplification link to amplify continuous single-mode signal light and noise-like pulse signal light.

[0010] To address the aforementioned technical problems, this invention provides a cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber, comprising: The system includes an amplified signal input terminal, a multimode semiconductor laser, a pump signal combiner, a standard double-clad fiber, a double-clad bismuth-doped fiber, a clad pump light stripper, a mode field adapter, an output optical isolator, and an amplified signal output terminal. The pump signal combiner includes a combiner pump input, a combiner signal input, and a combiner common output. The multimode semiconductor laser is connected to the combiner pump input, the amplified signal input is connected to the combiner signal input, and the combiner common output is connected to the input of the double-clad bismuth-doped fiber. The pump signal combiner, the double-clad bismuth-doped fiber, the clad pump light stripper, and the mode field adapter are arranged sequentially along the optical signal transmission direction. Adjacent pairs are connected by a standard double-clad fiber. The output end of the mode field adapter is connected to the input end of the output optical isolator, and the output end of the output optical isolator is connected to the amplified signal output end, forming an all-fiber amplification link. The double-clad bismuth-doped fiber includes a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The common output end of the combiner is optically coupled to the core and the inner cladding respectively through a standard double-clad fiber. The pump signal combiner is used to couple the signal light to be amplified input through the signal input end of the combiner to the core and to couple the multimode pump light output from the multimode semiconductor laser to the inner cladding. The wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped optical fiber.

[0011] In one embodiment of the present invention, the all-fiber amplification link is a single-stage, single-pass all-fiber amplification link, wherein the signal light to be amplified passes through the double-clad bismuth-doped fiber once in the all-fiber amplification link; The fiber core is a single-mode fiber core used to transmit the signal light to be amplified in a single-mode manner; The signal light to be amplified is either a continuous single-mode signal light or a noise-like pulse signal light, and the center wavelength of the continuous single-mode signal light and the noise-like pulse signal light is located in the E-band or S-band.

[0012] In one embodiment of the present invention, the cladding pump light stripper is connected to the mode field adapter via a double-clad output optical fiber, and the mode field adapter is connected to the output optical isolator via a single-mode output optical fiber. The mode field adapter includes a double-clad fiber end connected to the standard double-clad fiber and a single-mode fiber end connected to the single-mode output fiber. The forward transmission direction of the output optical isolator is consistent with the direction in which the signal light to be amplified is transmitted from the double-clad bismuth-doped optical fiber to the amplified signal output end.

[0013] In one embodiment of the present invention, the double-clad bismuth-doped optical fiber and the standard double-clad optical fiber are connected by arc fusion splicing to form a fusion splice area, and the insertion loss of a single fusion splice point between the double-clad bismuth-doped optical fiber and the standard double-clad optical fiber is not higher than 0.05 dB. The fusion splice area is covered with a low-refractive-index adhesive, which is in contact with the outer cladding of the optical fiber at the fusion splice area. The refractive index of the low-refractive-index adhesive after curing is 1.31 to 1.35.

[0014] In one embodiment of the present invention, the double-clad bismuth-doped optical fiber has a cladding absorption coefficient within the cladding absorption band; The double-clad bismuth-doped fiber is continuously disposed between the common output end of the combiner and the cladding pump light stripper, and the length of the double-clad bismuth-doped fiber is determined according to the cladding absorption coefficient.

[0015] In one embodiment of the present invention, the multimode semiconductor laser is a multimode pumped semiconductor laser with a center wavelength of 808 nanometers and a maximum pump power of 36 watts. The pump signal combiner is a (2+1)×1 type pump signal combiner, which includes two combiner pump input terminals, one combiner signal input terminal, and one combiner common output terminal.

[0016] In one embodiment of the present invention, the invention further includes an optical fiber disk and a heat sink, wherein the optical fiber disk is disposed on the heat sink, and the double-clad bismuth-doped optical fiber is wound around the optical fiber disk in turn, wherein the winding diameter of the double-clad bismuth-doped optical fiber is not less than the minimum allowable bending diameter of the double-clad bismuth-doped optical fiber, and adjacent optical fiber loops are arranged sequentially in the optical fiber disk.

[0017] Based on the same inventive concept, this invention also provides a cladding-pumped broadband optical amplification method based on double-clad bismuth-doped fiber, implemented using the aforementioned cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber, comprising: The signal light to be amplified is input to the combiner signal input terminal of the pump signal combiner via the amplification signal input terminal. The multimode pump light output from the multimode semiconductor laser is input to the combiner pump input terminal of the pump signal combiner. The wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped fiber. The signal light to be amplified and the multimode pump light are output to a standard double-clad fiber through the pump signal combiner. After transmission through the standard double-clad fiber, the signal light to be amplified is coupled to the core of the double-clad bismuth-doped fiber, and the multimode pump light is coupled to the inner cladding of the double-clad bismuth-doped fiber. The signal light to be amplified propagates in a single-mode manner in the fiber core, while the multimode pump light propagates in the inner cladding and passes through the fiber core multiple times. The multimode pump light is absorbed by the bismuth-related active centers in the fiber core, and the signal light to be amplified is amplified by stimulated emission to obtain the amplified signal light. The amplified signal light is transmitted sequentially through a standard double-clad optical fiber to the clad pump light stripper and the mode field adapter, and then output through the amplified signal output terminal after passing through the output optical isolator. The signal light to be amplified passes through the double-clad bismuth-doped fiber once in the all-fiber amplification link.

[0018] In one embodiment of the present invention, the signal light to be amplified is a noise-like pulse signal light with a center wavelength located in the 1.4 micrometer band. The noise-like pulse signal light is composed of multiple random ultrashort subpulses. The autocorrelation curve of the noise-like pulse signal light includes a narrow coherent peak and a wide substrate carrying the narrow coherent peak. The noise-like pulse signal light passes through the double-clad bismuth-doped fiber once in the fiber core, and undergoes stimulated emission amplification and nonlinear spectral broadening under the cladding pumping of the multimode pump light.

[0019] The present invention also provides a broadband optical amplification system, including the cladding-pumped broadband optical amplifier based on double-clad bismuth-doped optical fiber.

[0020] The technical solution of the present invention has the following advantages compared with the prior art: The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber described in this invention couples multimode pump light to the inner cladding of the double-clad bismuth-doped fiber through a pump signal combiner, and couples the signal light to be amplified to the fiber core. Stimulated emission amplification of the signal light is achieved using multimode pump light with wavelengths within the cladding absorption band, reducing dependence on high-brightness single-mode pump sources at specific wavelengths. Determining the length of the double-clad bismuth-doped fiber based on the cladding absorption characteristics improves the utilization of the cladding pump light. Furthermore, by sequentially setting cladding bands on the output side of the double-clad bismuth-doped fiber... The layer pump light stripper, mode field adapter, and output optical isolator collaboratively manage residual cladding pump light, output mode field, and reflection feedback, and simplify the optical path structure by employing a single-stage, single-path all-fiber amplification link. Simultaneously, low-loss fusion splicing, low-refractive-index adhesive encapsulation, and fiber coiling heat dissipation structure improve the transmission and operational stability of long-span double-clad bismuth-doped fibers, enabling the same all-fiber amplification link to be compatible with continuous single-mode signal light in the E-band or S-band and noise-like pulse signal light, and to perform stimulated emission amplification and nonlinear spectral broadening on noise-like pulse signal light. Attached Figure Description

[0021] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0022] Figure 1 This is a schematic diagram of the structure of the cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber in an embodiment of the present invention; Figure 2 This is a graph showing the test results of continuous single-mode signal optical amplification performance and thermal stability in an embodiment of the present invention; Figure 3 This is a graph showing the characteristic test results of a noise-like pulse seed signal in an embodiment of the present invention; Figure 4 This is a diagram showing the characteristic test results of amplified noise-like pulse signal light in an embodiment of the present invention; Figure 5 This is a graph showing the test results of the output power and pulse energy of the noise-like pulse signal light as a function of the amplifier pump power in an embodiment of the present invention. Figure 6This is a schematic flowchart of the cladding-pumped broadband optical amplification method based on double-clad bismuth-doped optical fiber in an embodiment of the present invention.

[0023] Explanation of reference numerals in the accompanying drawings: 1. Amplified signal input terminal; 2. Multimode semiconductor laser; 3. Pump signal combiner; 4. Double-clad bismuth-doped fiber; 5. Cladding pump light stripper; 6. Mode field adapter; 7. Output optical isolator; 8. Amplified signal output terminal; 9. Standard double-clad fiber. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0025] Example 1: like Figure 1 As shown, the present invention provides a cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber, including an amplified signal input terminal 1, a multimode semiconductor laser 2, a pump signal combiner 3, a double-clad bismuth-doped fiber 4, a cladding pump light stripper 5, a mode field adapter 6, an output optical isolator 7, an amplified signal output terminal 8, and a standard double-clad fiber 9. The pump signal combiner 3 includes a combiner pump input terminal, a combiner signal input terminal, and a combiner common output terminal. The multimode semiconductor laser 2 is connected to the combiner pump input terminal, the amplified signal input terminal 1 is connected to the combiner signal input terminal, and the combiner common output terminal is connected to the input terminal of the double-clad bismuth-doped fiber 4. The pump signal combiner 3, the double-clad bismuth-doped fiber 4, the clad pump light stripper 5, and the mode field adapter 6 are arranged sequentially along the optical signal transmission direction. Adjacent pairs are connected by a standard double-clad fiber 9. The output end of the mode field adapter 6 is connected to the input end of the output optical isolator 7, and the output end of the output optical isolator 7 is connected to the amplified signal output end 8, forming an all-fiber amplification link. The double-clad bismuth-doped fiber 4 includes a fiber core, an inner cladding surrounding the fiber core, and an outer cladding surrounding the inner cladding. The common output end of the combiner is optically coupled to the fiber core and the inner cladding respectively through a standard double-clad fiber 9. The pump signal combiner 3 is used to couple the signal light to be amplified input through the signal input end of the combiner to the fiber core, and to couple the multimode pump light output by the multimode semiconductor laser 2 to the inner cladding. The wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped optical fiber 4.

[0026] This invention uses a pump signal combiner 3 to couple the signal light to be amplified to the core of a double-clad bismuth-doped fiber 4 and to couple the multimode pump light output from a multimode semiconductor laser 2 to the inner cladding. A standard double-clad fiber 9 is set between the pump signal combiner 3 and the double-clad bismuth-doped fiber 4, between the double-clad bismuth-doped fiber 4 and the cladding pump light stripper 5, and between the cladding pump light stripper 5 and the mode field adapter 6. The cladding pump light stripper 5, the mode field adapter 6, and the output optical isolator 7 are sequentially set on the output side of the double-clad bismuth-doped fiber 4 to form an all-fiber amplification link. With the above structure, multimode pump light with wavelengths within the cladding absorption band of the double-clad bismuth-doped fiber 4 can be used to provide pump energy for the bismuth-related active centers in the fiber core, achieving stimulated emission amplification of the signal light to be amplified, reducing dependence on a specific wavelength high-brightness single-mode pump source. At the same time, the residual cladding pump light in the amplification link, the mode field transition between the standard double-clad fiber 9 and the subsequent single-mode optical path, and the output reflection feedback are continuously processed through the cladding pump light stripper 5, the mode field adapter 6, and the output optical isolator 7. Thus, the synergy between multimode cladding pumping and single-mode signal light transmission is achieved in a relatively simple all-fiber link, which is beneficial to improving the integration, output stability, and applicability of the amplification link to amplify wide-band signal light.

[0027] Specifically, the amplified signal input terminal 1 receives the signal light to be amplified and sends it along the signal input optical path to the combiner signal input terminal of the pump signal combiner 3. The multimode semiconductor laser 2 serves as the cladding pump source, generating multimode pump light, which enters the combiner pump input terminal of the pump signal combiner 3 from its output terminal. Before entering the pump signal combiner 3, the signal light to be amplified and the multimode pump light propagate along independent input paths and are received by the combiner signal input terminal and the combiner pump input terminal, respectively. By setting the signal input channel and the pump input channel separately, the signal light to be amplified and the multimode pump light can enter the pump signal combiner 3 along different input paths, providing an optical path basis for subsequent coupling to the core and inner cladding of the double-clad bismuth-doped fiber 4, respectively.

[0028] Pump signal combiner 3 is used to transmit the two input beams via standard double-clad fiber 9 and couple them to double-clad bismuth-doped fiber 4. The signal light to be amplified, input through the signal input end of the combiner, is transmitted through the common output end of the combiner via standard double-clad fiber 9 and then coupled to the core of double-clad bismuth-doped fiber 4. The multimode pump light, input through the pump input end of the combiner, is transmitted through the common output end of the combiner via standard double-clad fiber 9 and then coupled to the inner cladding of double-clad bismuth-doped fiber 4. Therefore, after the signal light to be amplified and the multimode pump light are transmitted through standard double-clad fiber 9 to double-clad bismuth-doped fiber 4, they enter optical regions adapted to their respective transmission modes, thus forming a spatially separated transmission structure where the signal light to be amplified propagates along the fiber core and the multimode pump light propagates along the inner cladding. Using this coupling method, the multimode pump light output from the multimode semiconductor laser 2 can be directly used for cladding pumping, while the signal light to be amplified continues to propagate along the fiber core.

[0029] In the double-clad bismuth-doped fiber 4, the core serves as the gain propagation region for the signal light to be amplified, and contains bismuth-related active centers. The inner cladding, located around the core, provides the propagation region for the multimode pump light. The outer cladding surrounds the inner cladding, forming an optical boundary that restricts the propagation of the multimode pump light within the inner cladding. The multimode pump light, after being transmitted from the common output of the combiner through the standard double-clad fiber 9, enters the inner cladding and propagates along the length of the double-clad bismuth-doped fiber 4, passing through the core multiple times during propagation. Because the wavelength of the multimode pump light lies within the cladding absorption band of the double-clad bismuth-doped fiber 4, it is gradually absorbed by the bismuth-related active centers in the core as it propagates and passes through the core, providing these centers with the pump energy needed to generate optical gain.

[0030] Meanwhile, the signal light to be amplified propagates along the core of the double-clad bismuth-doped fiber 4. After absorbing the multimode pump light, the bismuth-correlated active centers in the core generate stimulated emission gain on the signal light propagating along the core, thereby transferring the energy provided by the multimode pump light to the signal light to be amplified, resulting in the amplified signal light. The multimode pump light and the signal light to be amplified utilize the inner cladding and the core as transmission regions, respectively, and achieve energy coupling through the bismuth-correlated active centers in the core. This allows multimode cladding pumping and single-mode signal transmission to proceed synergistically within the same double-clad bismuth-doped fiber 4, which is beneficial for utilizing higher-power multimode pump light while maintaining single-mode signal transmission.

[0031] Furthermore, the all-fiber amplification link adopts a single-stage, single-pass structure, where the signal light to be amplified completes amplification in a single pass through the double-clad bismuth-doped fiber 4. The core of the double-clad bismuth-doped fiber 4 is a single-mode core, allowing the signal light to propagate in single-mode within the core. This all-fiber amplification link completes the gain process in a single pass through the double-clad bismuth-doped fiber 4, reducing the need for additional optical components and connection nodes required for two-way propagation or multi-stage amplification. Pump input, signal gain, and output management are all completed within a continuous all-fiber link, simplifying the optical path configuration and reducing additional losses and feedback paths that may be introduced by multiple transmissions.

[0032] The signal light to be amplified can be either continuous single-mode signal light or noise-like pulse signal light, with the center wavelengths of both located in the E-band or S-band. Both types of signals can pass through the amplified signal input terminal 1 and the pump signal combiner 3, and then be transmitted through the standard double-clad fiber 9 into the double-clad bismuth-doped fiber 4, where they propagate and are amplified along the same fiber core gain path. Therefore, it can be applied to input signals with different time characteristics while maintaining the basic optical path structure of the amplifier. For continuous single-mode signal light, power amplification can be achieved using the stimulated emission gain in the double-clad bismuth-doped fiber 4; for noise-like pulse signal light, while obtaining stimulated emission gain, corresponding nonlinear spectral broadening can also be generated in the double-clad bismuth-doped fiber 4, making the same all-fiber amplification link compatible with the amplification of both continuous single-mode signal light and noise-like pulse signal light.

[0033] Specifically, the operating wavelength of the multimode semiconductor laser 2 is selected based on the absorption spectrum of the double-clad bismuth-doped fiber 4. Preferably, the multimode semiconductor laser 2 is a multimode pumped semiconductor laser with a center wavelength of 808 nm; more preferably, the maximum pump power of the multimode pumped semiconductor laser is 36 watts. The pump signal combiner 3 is preferably a (2+1)×1 type pump signal combiner, including two combiner pump inputs, one combiner signal input, and one combiner common output. With the above configuration, the multimode pump light output from the multimode semiconductor laser 2 can be transmitted through the combiner pump input and the combiner common output via the standard double-clad fiber 9 into the inner cladding of the double-clad bismuth-doped fiber 4, while the signal light to be amplified is transmitted through the combiner signal input and the combiner common output via the standard double-clad fiber 9 into the fiber core. The 808 nm multimode semiconductor laser is a preferred pump configuration that is compatible with the absorption characteristics of the double-clad bismuth-doped fiber 4 used in this embodiment. It can directly utilize the pump power provided by the multimode semiconductor laser 2 without the need for an additional independent wavelength conversion structure to convert the multimode pump light into single-mode pump light before cladding pumping.

[0034] Furthermore, the double-clad bismuth-doped fiber 4 has a corresponding cladding absorption coefficient within the cladding absorption band and is positioned in the gain optical path between the pump signal combiner 3 and the cladding pump light stripper 5. The input and output ends of the double-clad bismuth-doped fiber 4 are connected to the standard double-clad fiber 9, respectively. The length of the double-clad bismuth-doped fiber 4 is determined based on the cladding absorption coefficient. Under cladding pumping conditions, the effective transmission area of ​​the multimode pump light in the inner cladding is larger than that in the fiber core area, and the average spatial overlap between the multimode pump light and the fiber core is relatively low. Therefore, it is necessary to increase the number of times the multimode pump light passes through the fiber core by increasing the propagation length, so that the multimode pump light can be gradually absorbed by the bismuth-related active centers during propagation.

[0035] By setting the length of the double-clad bismuth-doped fiber 4 according to the cladding absorption coefficient, the gain activation length can be matched with the actual pump absorption capability. When the length of the double-clad bismuth-doped fiber 4 is too short, the multimode pump light may reach the output end of the gain fiber before it has been fully absorbed; while simply increasing the fiber length will correspondingly increase the optical signal propagation distance and the requirements for coiling and encapsulation. Therefore, determining the gain activation length based on the cladding absorption characteristics is beneficial for achieving a proper match between the pump light absorption degree and long-span fiber transmission.

[0036] Preferably, within the target wavelength range of 700 nm to 808 nm, the maximum small-signal cladding absorption coefficient of the double-clad bismuth-doped fiber 4 is measured to be between 0.03 dB / m and 0.057 dB / m. Based on the above cladding absorption characteristics and the spatial overlap between the cladding mode and the fiber core, this embodiment preferably uses a double-clad bismuth-doped fiber 4 with a length of approximately 200 meters. This allows the multimode pump light to pass through the fiber core multiple times in the relatively long inner cladding propagation path and be gradually absorbed by the bismuth-related active centers. When the cladding absorption characteristics of the double-clad bismuth-doped fiber 4 change, its length can be adjusted accordingly to adapt the gain length to the actual absorption of the multimode pump light.

[0037] Along the propagation direction of the amplified signal light, the output end of the double-clad bismuth-doped fiber 4 is connected to the cladding pump light stripper 5 via a standard double-clad fiber 9. Although the multimode pump light is gradually absorbed by the bismuth-related active centers in the fiber core during propagation within the inner cladding, residual cladding pump light may still remain at the output end of the double-clad bismuth-doped fiber 4. The amplified signal light and the residual multimode pump light are transmitted to the cladding pump light stripper 5 via the standard double-clad fiber 9. If the residual cladding pump light continues to enter subsequent output optical paths, it may increase pump light leakage in subsequent optical paths and potentially generate additional heat at relevant connection points. Therefore, the cladding pump light stripper 5 is positioned after the double-clad bismuth-doped fiber 4 to remove the residual multimode pump light still propagating in the inner cladding from the signal output link, ensuring that the optical path after this location primarily transmits the amplified signal light that has already achieved its gain. By placing the residual cladding pump light processing after the gain fiber and before the mode field adaptation, the possibility of residual multimode pump light continuing to enter the subsequent single-mode output optical path can be reduced.

[0038] Furthermore, the cladding pump light stripper 5 is connected to the mode field adapter 6 via a standard double-clad fiber 9, and the mode field adapter 6 is connected to the output optical isolator 7 via a single-mode output fiber. The mode field adapter 6 includes a double-clad fiber end and a single-mode fiber end, wherein the double-clad fiber end is connected to the standard double-clad fiber 9, and the single-mode fiber end is connected to the single-mode output fiber. Since the standard double-clad fiber 9 and the single-mode output fiber may differ in terms of mode field diameter, numerical aperture, and cladding structure, direct connection may result in a certain mode field mismatch. The mode field adapter 6 is positioned between the two types of fibers, allowing the fundamental mode signal transmitted by the standard double-clad fiber 9 to enter the single-mode output fiber after passing through a continuous mode field transition, thereby reducing the additional loss caused by mode field mismatch and reducing the possibility of higher-order modes being excited.

[0039] The mode field adapter 6 can also isolate the back-propagating cladding stray light, allowing the output signal after processing with the residual cladding pump light to further complete the transmission mode transition. The cladding pump light stripper 5 and the mode field adapter 6 are arranged sequentially, ensuring that the residual cladding pump processing and the mode transition from the standard double-clad fiber 9 to the single-mode output fiber proceed continuously according to the transmission direction of the amplified signal light. This helps to reduce the impact of the residual pump light and the fiber mode field difference on the subsequent output optical path.

[0040] The amplified signal light output from mode field adapter 6 enters output optical isolator 7 along the single-mode output fiber. The forward transmission direction of output optical isolator 7 is consistent with the direction in which the signal light to be amplified is transmitted from double-clad bismuth-doped fiber 4 to the amplified signal output terminal 8, enabling the amplified signal light to pass through output optical isolator 7 in the forward direction and be output through amplified signal output terminal 8. End-face reflected light or other feedback light generated by external optical paths and propagating in the opposite direction is restricted by output optical isolator 7, thereby reducing the possibility of reverse light re-entering double-clad bismuth-doped fiber 4. By setting output optical isolator 7 after residual pump stripping and mode field adaptation are completed, the impact of output end reflection feedback on the gain process and output state can be further reduced.

[0041] From the perspective of the complete propagation path of the signal light to be amplified, the signal light to be amplified is input from the amplification signal input terminal 1, enters the pump signal combiner 3 through the combiner signal input terminal, and is output from the common output terminal of the combiner. It is then transmitted through the standard double-clad fiber 9 to the double-clad bismuth-doped fiber 4 and coupled to its core. It propagates in the core and obtains stimulated emission gain, forming the amplified signal light. After being output from the double-clad bismuth-doped fiber 4, the amplified signal light enters the cladding pump light stripper 5 through the standard double-clad fiber 9, and then enters the mode field adapter 6 through the standard double-clad fiber 9. After mode field adaptation, it enters the single-mode output fiber and is output sequentially through the output optical isolator 7 and the amplified signal output terminal 8.

[0042] From the complete propagation path of the multimode pump light, the multimode semiconductor laser 2 generates multimode pump light, which enters the pump signal combiner 3 through the pump input end of the combiner. After being output from the common output end of the combiner, it is transmitted through the standard double-clad fiber 9 and coupled to the inner cladding of the double-clad bismuth-doped fiber 4, and propagates along the inner cladding towards the output direction of the double-clad bismuth-doped fiber 4. During propagation, the multimode pump light passes through the fiber core multiple times. Part of the pump energy is absorbed by the bismuth-related active centers in the fiber core and used to generate stimulated emission gain for the signal light to be amplified. The unabsorbed residual multimode pump light continues to propagate to the output end of the double-clad bismuth-doped fiber 4, and after being output from the common output end of the combiner, it is transmitted through the standard double-clad fiber 9 to the cladding pump light stripper 5 for stripping. The above structure separates the signal light transmission path from the multimode pump light transmission path within the double-clad bismuth-doped fiber 4. At the same time, it establishes a connection between pump energy and signal gain through bismuth-related active centers, thereby enabling multimode clad pumping, single-mode signal transmission, and stimulated emission amplification to be completed in the same gain fiber.

[0043] Furthermore, to reduce the additional loss caused by heterogeneous fiber connections in the all-fiber amplification link, both ends of the double-clad bismuth-doped fiber 4 are connected to the standard double-clad fiber 9 by arc fusion splicing to form a fusion splice area. Since the double-clad bismuth-doped fiber 4 and the standard double-clad fiber 9 differ in core composition, cladding structure, material composition, and thermal properties, fusion splicing parameters can be calibrated for the actual fiber combination being connected. Specifically, the corresponding fiber to be fused can be selected, one end connected to a test light source, and the other end connected to an optical power meter to measure the change in optical power before and after fusion splicing in real time. Based on the initial parameters of the fusion splicer, the discharge intensity and discharge duration are adjusted, and the insertion loss, appearance, and mechanical stability of the fusion point are tested under different parameter conditions. The corresponding fusion splicing parameters are determined based on the test results.

[0044] Preferably, the insertion loss of a single fusion splice point in the fusion splice region is controlled within 0.05 dB. By adjusting the fusion splicing parameters to account for the structural and material differences between the double-clad bismuth-doped fiber 4 and the standard double-clad fiber 9, the additional loss caused by the fusion splice position to the optical signal transmission is reduced while forming a stable mechanical connection. This helps maintain the transmission efficiency in the continuous all-fiber amplification link and reduces local energy deposition caused by fusion splicing loss.

[0045] Furthermore, a low-refractive-index adhesive is applied to the outside of the fusion splice area, and this adhesive contacts the outer cladding of the optical fiber at the fusion splice area. When the double-clad bismuth-doped fiber 4 is fused with the standard double-clad fiber 9, changes in the local cladding structure of the fusion splice area may affect the original transmission boundary of the multimode pump light in the inner cladding. Therefore, a low-refractive-index optical boundary is formed outside the exposed cladding area using a low-refractive-index adhesive to maintain the total internal reflection transmission condition of the multimode pump light in the corresponding area and reduce the outward scattering of the pump light.

[0046] Preferably, the refractive index of the cured low-refractive-index adhesive is 1.31 to 1.35; further, its refractive index after curing is approximately 1.33. By combining low-loss arc welding and low-refractive-index adhesive encapsulation, the cladding pump light transmission boundary at the welding position can be appropriately treated while connecting the double-clad bismuth-doped fiber 4 to the standard double-clad fiber 9. This helps to reduce pump light leakage from the welding area and the local heat accumulation caused by pump light leakage.

[0047] Since the double-clad bismuth-doped fiber 4 can employ a relatively long gain length, the amplifier also includes a fiber optic disk and a heat sink to facilitate the arrangement and heat dissipation of the long gain fiber. The fiber optic disk is mounted on the heat sink, and the double-clad bismuth-doped fiber 4 is wound around the fiber optic disk one turn at a time. During the winding process, the winding diameter of the double-clad bismuth-doped fiber 4 is ensured to be no less than its minimum allowable bending diameter, and adjacent fiber loops are arranged sequentially within the fiber optic disk to reduce fiber crossing, mutual compression, and localized stress concentration.

[0048] During the winding process, the winding tension can be adjusted according to the actual bending state of the double-clad bismuth-doped fiber 4 to avoid additional transmission loss caused by excessive local bending, excessive axial stretching, or scratches on the coating layer. For long double-clad bismuth-doped fibers 4, by arranging them turn by turn and controlling the winding diameter, the gain fiber can be arranged within a limited installation space, and a larger contact range can be formed between the fiber and the heat dissipation structure, which is beneficial for heat transfer during the operation of long gain fibers.

[0049] Preferably, in this embodiment, the approximately 200-meter-long double-clad bismuth-doped fiber 4 has a coiling diameter of approximately 130 millimeters and is mounted on a heat sink for passive heat dissipation. During operation, when passive heat dissipation cannot release the heat generated by the relevant devices in a timely manner, a conventional fan can be used for forced air cooling, allowing air to flow through the double-clad bismuth-doped fiber 4, the cladding pump light stripper 5, and the relevant fusion splice areas to promote heat transfer to the external environment.

[0050] By combining fiber optic trays, heat sinks, and forced air cooling configured according to the operating status, the impact of continuous heat accumulation in long gain fiber sections, residual pump processing locations, and fusion splice areas on the amplifier's operating status is reduced.

[0051] Based on the above structure, this amplifier can be used to perform cladding-pumped broadband optical amplification of the signal light to be amplified. The signal light to be amplified is input to the combiner signal input terminal of the pump signal combiner 3 via the amplification signal input terminal 1. The multimode pump light generated by the multimode semiconductor laser 2 is input to the pump signal combiner 3 via the pump input terminal of the combiner, wherein the wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped fiber 4. The signal light to be amplified and the multimode pump light output from the pump signal combiner 3 are transmitted through the standard double-clad fiber 9 and then coupled to the core and inner cladding of the double-clad bismuth-doped fiber 4, respectively. The signal light to be amplified propagates in single-mode in the core, while the multimode pump light propagates in the inner cladding and passes through the core multiple times. The bismuth-related active centers in the core gradually absorb the multimode pump light, generating stimulated emission gain in the signal light to be amplified, thus obtaining the amplified signal light. The amplified signal light is transmitted through the standard double-clad fiber 9 to the cladding pump light stripper 5. After processing by the residual cladding pump light, it enters the mode field adapter 6 through the standard double-clad fiber 9 again, and then passes through the output optical isolator 7 before being output through the amplified signal output terminal 8. In the above process, the signal light to be amplified passes through the double-clad bismuth-doped fiber 4 only once in the all-fiber amplification link.

[0052] Furthermore, in the aforementioned cladding-pumped broadband optical amplification process, the signal light to be amplified and the multimode pump light propagate along the fiber core and inner cladding, respectively, and establish an energy transfer relationship between them through the bismuth-related active centers in the core of the double-clad bismuth-doped fiber 4. The multimode pump light propagates along the length of the double-clad bismuth-doped fiber 4 in the inner cladding and passes through the fiber core multiple times during propagation, causing the bismuth-related active centers in the fiber core to gradually absorb the multimode pump light; the signal light to be amplified propagates in a single-mode manner in the fiber core and completes stimulated emission amplification under the gain provided by the bismuth-related active centers.

[0053] After passing through the double-clad bismuth-doped fiber 4, the amplified signal light enters the cladding pump light stripper 5 via the standard double-clad fiber 9. After processing by residual cladding pump light, it enters the mode field adapter 6 via the standard double-clad fiber 9 again, and then enters the output optical isolator 7 via the single-mode output fiber. The cladding pump light stripper 5 is used to process the residual multimode pump light still present in the inner cladding after passing through the double-clad bismuth-doped fiber 4, reducing the amount of residual multimode pump light entering the subsequent single-mode output optical path. The mode field adapter 6 forms a mode field transition between the standard double-clad fiber 9 and the single-mode output fiber to reduce the mode field mismatch between different fibers and reduce the impact of high-order mode excitation and reverse cladding stray light on the output optical path. The output optical isolator 7 transmits the amplified signal light in the forward direction according to the output direction to reduce the possibility of reflected light and external feedback light entering the double-clad bismuth-doped fiber 4 in the reverse direction. This allows the residual cladding pump processing, mode field transition, and reverse feedback suppression to be completed sequentially in the same continuous optical path.

[0054] Furthermore, the signal light to be amplified can be either a continuous single-mode signal light or a noise-like pulse signal light. When the signal light to be amplified is a continuous single-mode signal light, it propagates once along the core of the double-clad bismuth-doped fiber 4 and obtains stimulated emission gain under the cladding pump energy provided by the multimode pump light. When the signal light to be amplified is a noise-like pulse signal light, it also propagates once along the fiber core, obtaining stimulated emission gain while also undergoing nonlinear spectral broadening in the longer double-clad bismuth-doped fiber 4. Both continuous single-mode and noise-like pulse signal lights can use signals with center wavelengths in the E-band or S-band, enabling the same single-stage, single-path all-fiber amplification link to be applied to signal lights with different time characteristics without the need to reconfigure independent gain links for the two types of signals.

[0055] This invention, in its embodiment, forms a cladding pump gain link using a multimode semiconductor laser 2, a pump signal combiner 3, and a double-clad bismuth-doped fiber 4. A standard double-clad fiber 9 enables an all-fiber connection between the pump signal combiner 3, the double-clad bismuth-doped fiber 4, the cladding pump light stripper 5, and the mode field adapter 6. By combining a gain length adapted to the cladding absorption characteristics, low-loss fusion splicing and low-refractive-index adhesive encapsulation between the double-clad bismuth-doped fiber 4 and the standard double-clad fiber 9, coiled heat dissipation of the long double-clad bismuth-doped fiber 4, and an output management link consisting of the cladding pump light stripper 5, the mode field adapter 6, and the output optical isolator 7, pump input, signal amplification, residual pump processing, mode field transition, output isolation, and heat dissipation of the long gain fiber encapsulation can all be completed within a continuous all-fiber structure. The entire amplification link adopts a single-stage, single-pass structure, enabling cladding pumping using multimode pump light while maintaining single-mode signal transmission, and is compatible with amplification of continuous single-mode signal light and noise-like pulse signal light.

[0056] Example 2: Based on the cladding-pumped broadband optical amplifier described in Example 1, this embodiment performs amplification tests on continuous single-mode signal light and noise-like pulse signal light to verify the amplifier's amplification performance for different types of signal light near 1.4 micrometers, and detects the output spectrum, output power, pulse time characteristics, radio frequency characteristics, and thermal stability during the amplification process.

[0057] During testing, a near-infrared spectrometer was preferred for measuring the input and output spectra, obtaining the center wavelength and spectral bandwidth; a 20 GHz bandwidth oscilloscope was used to detect the pulse time-domain sequence, and the repetition frequency was determined based on the time interval between adjacent pulses; a 40 GHz bandwidth spectrum analyzer was used to detect the radio frequency spectrum, and the difference between the signal strength at the fundamental frequency and the intensity of adjacent noise was used to characterize the radio frequency signal-to-noise ratio; an autocorrelation analyzer was used to detect the time structure of noise-like pulses; and a power meter was used to measure the average output power. The near-infrared spectrometer used was a Yokogawa AQ6370D, the oscilloscope used was a LeCroy Wavemaster 820Zi-B, the spectrum analyzer used was a Rohde FSV40, the autocorrelation analyzer used was an APE pulseCheck 50NX, and the power meter used was a Solvay PM100D; the measurement position of the ultrafast laser output used an angled physical contact type fiber optic connector.

[0058] In the continuous single-mode signal amplification test, a continuous single-mode laser with a wavelength around 1.4 micrometers is used as the seed signal. Preferably, a single-mode semiconductor laser with a center wavelength of approximately 1454 nanometers is used as the continuous laser seed oscillator. The generated continuous single-mode signal light is input into the all-fiber amplification link via the amplification signal input terminal, while a multimode pump light is input to the pump signal combiner from a multimode semiconductor laser. The continuous single-mode signal light and the multimode pump light are combined by the pump signal combiner and transmitted through a standard double-clad fiber, and coupled to the core and inner cladding of the double-clad bismuth-doped fiber, respectively. The continuous single-mode signal light passes through the double-clad bismuth-doped fiber once in the core and gains stimulated emission gain under the cladding pumping action of the multimode pump light. After amplification, the signal light enters the cladding pump light stripper through the standard double-clad fiber to process the residual cladding pump light, and then enters the mode field adapter through the standard double-clad fiber, and is output through the output optical isolator.

[0059] like Figure 2 As shown, the spectra of the amplified continuous single-mode signal were detected at 25℃ and 70℃. Under both temperature conditions, the center wavelength of the amplified signal remained at approximately 1454 nm, and the main peak shape, spectral bandwidth, and adjacent broadband background remained largely consistent. No significant center wavelength drift, spectral distortion, or additional parasitic peaks were observed. No significant residual pump peak was observed near 808 nm, indicating that after absorption by the double-clad bismuth-doped fiber and processing by the cladding pump light stripper, the residual multimode pump light did not significantly enter the subsequent output optical path. Under both temperature conditions, the signal main peak maintained an optical signal-to-noise ratio of over 40 dB relative to the adjacent amplified spontaneous emission background. No significant enhancement of amplified spontaneous emission or parasitic oscillations was observed at 70℃. Therefore, it can be seen that within the above test temperature range, the amplified continuous single-mode signal can maintain a relatively consistent center wavelength and main peak spectral morphology.

[0060] Furthermore, under conditions of 50 mW input power for the continuous single-mode signal light, room temperature, and no forced air cooling, the output power of the detection amplifier, as well as the temperature variations of the cladding pump stripper, the double-clad bismuth-doped fiber, and the first splice point between the double-clad bismuth-doped fiber and the standard double-clad fiber, were measured with increasing pump power. When the amplifier pump power reached 35 W, the output power of the continuous single-mode signal light increased from 50 mW to 164 mW, approximately 3.28 times the input power; simultaneously, the highest temperatures of the cladding pump stripper, the double-clad bismuth-doped fiber, and the aforementioned splice point did not exceed 78°C. The test results indicate that, under the corresponding pumping conditions, the multimode pump light can provide effective stimulated emission gain for the continuous single-mode signal light, and the correlation gain, residual pump treatment, and splice temperature can be maintained within the corresponding test ranges.

[0061] Furthermore, the amplifier was subjected to long-term operation tests at room temperature, with forced air cooling provided by a regular fan. Before forced air cooling was activated, the amplifier operated continuously for approximately 1.5 hours, during which the output power remained stable at around 164 milliwatts, with a maximum power fluctuation of no more than 0.5 milliwatts. After forced air cooling was activated, the previously accumulated heat was gradually released, and the output power changed somewhat during the establishment of a new thermal equilibrium, stabilizing at approximately 157 milliwatts after reaching this new thermal equilibrium. After forced air cooling was deactivated, the output power gradually returned to its state before forced air cooling was activated. Therefore, it can be concluded that the change in output power caused by changes in heat dissipation conditions is reversible; the amplifier can re-establish a relatively stable output state under both passive cooling and forced air cooling conditions.

[0062] After completing the continuous single-mode signal amplification test, the noise-like pulse signal light was further amplified and tested. The noise-like pulse signal light consists of multiple random ultrashort sub-pulses, and its autocorrelation curve includes a narrow coherent spike and a wide substrate supporting that narrow coherent spike. The multiple random ultrashort sub-pulses are distributed within a relatively wide overall time envelope, giving the noise-like pulse light both a short internal characteristic time scale and a wide overall time scale. Therefore, it can be characterized by a two-scale autocorrelation structure of a narrow coherent spike superimposed on a wide substrate.

[0063] Preferably, a 1.4-micron ultrafast mode-locked bismuth-doped fiber laser is used to generate a noise-like pulse seed signal. The seed signal is subjected to spectral, autocorrelation, time-domain pulse sequence, and radio frequency spectrum tests to determine the noise-like pulse state before the input amplifier. Figure 3 As shown, the center wavelength of the noise-like pulse seed signal is 1465.44 nm, the 3 dB bandwidth is 9.18 nm, and the output spectrum is generally smooth with no obvious sidebands observed. Within the 50 picosecond measurement range, the autocorrelation curve includes a narrow coherence peak of approximately 284 femtoseconds and a wide picosecond-scale floor, exhibiting a clear dual-scale time structure.

[0064] Within a 5-microsecond sampling range, the temporal pulse sequence of the noise-like pulse seed signal exhibits an equally spaced distribution, with an adjacent pulse interval of approximately 0.255 microseconds, corresponding to a repetition frequency of approximately 3.916 MHz, and no obvious sidelobe pulses were observed. Under test conditions of a resolution bandwidth of 500 Hz and a video bandwidth of 1 Hz, the fundamental frequency signal-to-noise ratio of the seed signal is approximately 64.95 dB. Combined with the relatively smooth broad spectrum, the dual-scale autocorrelation structure of narrow coherence spikes superimposed on a broad substrate, the stable pulse sequence, and the corresponding RF spectral characteristics, it can be determined that this seed signal operates in a noise-like pulse state.

[0065] During noise-like pulse amplification, the noise-like pulse signal light is input to the pump signal combiner via amplification signal input terminal 1. The noise-like pulse signal light and the multimode pump light output from the multimode semiconductor laser are combined by the pump signal combiner and transmitted through a standard double-clad fiber, and coupled to the core and inner cladding of the double-clad bismuth-doped fiber, respectively. The noise-like pulse signal light passes through the double-clad bismuth-doped fiber once in the core and undergoes stimulated emission amplification under the cladding pump energy provided by the multimode pump light. Since the noise-like pulse signal light consists of multiple random ultrashort subpulses, it is accompanied by nonlinear effects during propagation within the relatively long double-clad bismuth-doped fiber, causing nonlinear spectral broadening during the amplification process. The amplified noise-like pulse signal light enters the cladding pump light stripper through the standard double-clad fiber to process the residual cladding pump light, and then enters the mode field adapter through the standard double-clad fiber, and is output sequentially through the output optical isolator and the amplified signal output terminal.

[0066] Preferably, the pump power of the noise-like pulse seed source is fixed at 151.7 milliwatts, at which point the seed source output power is 13.59 milliwatts and the pulse energy is 3.47 nanojoules. Under the above seed condition, the amplifier's pump power is increased to 36 watts. For example... Figure 4 As shown, the amplified noise-like pulse signal light has a center wavelength of 1469.23 nm, a 3 dB bandwidth of 15.55 nm, a 10 dB spectral width that increases from 14.38 nm to 41.83 nm, and a 20 dB spectral width that increases from 19.63 nm to 183.28 nm, forming a spectral gain covering the range of 1400 nm to 1700 nm under the aforementioned pumping conditions. Compared to the seed signal, the center wavelength of the amplified output spectrum exhibits a certain redshift, and the 3 dB bandwidth increases by approximately 69%, indicating that the noise-like pulse signal light produces a relatively significant nonlinear spectral broadening while gaining power.

[0067] Autocorrelation tests were performed on the amplified noise-like pulse signal light. It was observed that the autocorrelation curve still exhibited a two-scale structure with a narrow coherent peak superimposed on a wide substrate. Under optimal conditions, the width of the narrow coherent peak decreased from approximately 284 femtoseconds before amplification to approximately 211 femtoseconds. Both the narrow coherent peak and the wide substrate underwent a certain degree of contraction, indicating that the characteristic timescale of the random ultrashort subpulses and their distribution within the overall time envelope changed during amplification. However, the two-scale time structure characterizing the noise-like pulse state still remained.

[0068] Furthermore, the amplified noise-like pulse signal still exhibits an equally spaced pulse sequence within a 5-microsecond sampling range, with the interval between adjacent pulses remaining at approximately 0.255 microseconds, corresponding to a repetition frequency essentially consistent with the seed signal. While the amplified pulse intensity fluctuates somewhat relative to the seed signal, the overall pulse sequence remains continuous, and no obvious sidelobe pulses were observed. This indicates that after stimulated emission amplification and nonlinear spectral broadening, the noise-like pulse signal retains its original repetition frequency and the dual-scale time structure of narrow coherent spikes superimposed on a wide substrate.

[0069] Under RF test conditions with a resolution bandwidth of 500 Hz and a video bandwidth of 1 Hz, the fundamental frequency signal-to-noise ratio (SNR) of the amplified noise-like pulse signal is approximately 59.52 dB, a decrease of about 5.43 dB compared to the original fundamental frequency SNR of approximately 64.95 dB. Simultaneously, the noise floor in the RF spectrum is slightly raised. Despite the decrease in RF SNR after amplification, the amplified pulse sequence maintains the same repetition frequency as the seed signal and retains the characteristic two-scale autocorrelation structure of the noise-like pulse. This indicates that the amplification process can maintain its basic pulse operating state while increasing the output power of the noise-like pulse and broadening the spectrum.

[0070] like Figure 5 As shown, the changes in amplifier output power and pulse energy with amplifier pump power were further examined. With increasing pump power, the output power and pulse energy of the noise-like pulse signal light increased accordingly. Preferably, when the amplifier pump power reached 36 watts, the output power of the noise-like pulse signal light reached 100.25 milliwatts, representing an increase of approximately 7.4 times in both output power and pulse energy, and approximately 10 times in peak power, compared to the seed source output power of 13.59 milliwatts. Figures 3 to 5 The output spectrum, autocorrelation curve, time-domain pulse sequence, RF spectrum, and output power test results shown demonstrate that this single-stage, single-path all-fiber amplification link can amplify noise-like pulse signal light through stimulated emission and generate a wide range of nonlinear spectral broadening during amplification, while maintaining the repetition frequency, continuous pulse sequence, and basic dual-scale time structure of the noise-like pulse.

[0071] The tests conducted on continuous single-mode signal light and noise-like pulse signal light demonstrate that the cladding-pumped broadband optical amplifier used in this embodiment can amplify two signals with different time characteristics near 1.4 micrometers without altering the main amplification link. Specifically, for continuous single-mode signal light, it maintains a relatively stable spectrum and output state under different test temperatures and heat dissipation conditions. For noise-like pulse signal light, it produces significant nonlinear spectral broadening while increasing output power and pulse energy, maintaining its repetition frequency and the dual-scale time structure of narrow coherence spikes superimposed on a wide substrate. These test results validate the amplifier structure and amplification process in Embodiment 1 from aspects such as continuous signal amplification, pulse signal amplification, output spectrum, time characteristics, RF characteristics, and thermal state.

[0072] Example 3: like Figure 6 As shown, in this embodiment of the invention, a cladding-pumped broadband optical amplification method based on double-clad bismuth-doped fiber is provided. This method is implemented using the cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber described in Embodiment 1, and includes: The signal light to be amplified is input to the combiner signal input terminal of the pump signal combiner via the amplification signal input terminal. The multimode pump light output from the multimode semiconductor laser is input to the combiner pump input terminal of the pump signal combiner. The wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped fiber. The signal light to be amplified and the multimode pump light are output to a standard double-clad fiber through the pump signal combiner. After transmission through the standard double-clad fiber, the signal light to be amplified is coupled to the core of the double-clad bismuth-doped fiber, and the multimode pump light is coupled to the inner cladding of the double-clad bismuth-doped fiber. The signal light to be amplified propagates in a single-mode manner in the fiber core, while the multimode pump light propagates in the inner cladding and passes through the fiber core multiple times. The multimode pump light is absorbed by the bismuth-related active centers in the fiber core, and the signal light to be amplified is amplified by stimulated emission to obtain the amplified signal light. The amplified signal light is transmitted sequentially through a standard double-clad optical fiber to the clad pump light stripper and the mode field adapter, and then output through the amplified signal output terminal after passing through the output optical isolator. The signal light to be amplified passes through the double-clad bismuth-doped fiber once in the all-fiber amplification link.

[0073] Specifically, in the aforementioned cladding-pumped broadband optical amplification process, the signal light to be amplified and the multimode pump light output from the pump signal combiner are transmitted via a standard double-clad fiber to a double-clad bismuth-doped fiber, and coupled to the fiber core and inner cladding respectively. The signal light to be amplified propagates along the fiber core in a single-mode manner, while the multimode pump light propagates along the inner cladding and passes through the fiber core multiple times during propagation. The bismuth-related active centers in the fiber core absorb the multimode pump light and generate stimulated emission gain in the signal light to be amplified. After amplification, the signal light is output from the double-clad bismuth-doped fiber, then transmitted via a standard double-clad fiber to a cladding pump light stripper for residual cladding pump light processing, then transmitted via a standard double-clad fiber to a mode field adapter to complete mode field transition, and finally output after passing through an output optical isolator. In the above process, the signal light to be amplified passes through the double-clad bismuth-doped fiber only once, completing signal amplification in a single-stage, single-pass manner.

[0074] Furthermore, when the signal light to be amplified is a noise-like pulse signal light with a center wavelength in the 1.4 micrometer band, the noise-like pulse signal light is composed of multiple random ultrashort subpulses, and its autocorrelation curve includes a narrow coherent spike and a wide substrate carrying the narrow coherent spike, so as to form a dual-scale time structure of the noise-like pulse signal light.

[0075] Noise-like pulse signal light is transmitted through a standard double-clad fiber and coupled to the core of a double-clad bismuth-doped fiber. It then passes through the double-clad bismuth-doped fiber once in the core and undergoes stimulated emission amplification under the cladding pumping action of multimode pump light, resulting in nonlinear spectral broadening during propagation. In the single-stage, single-pass all-fiber amplification process, the noise-like pulse signal light is amplified in power and broadband spectral output is achieved.

[0076] Example 4: This embodiment provides a broadband optical amplification system, including the cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber as described in Embodiment 1. The specific structure, connection relationship and operation mode of the cladding-pumped broadband optical amplifier are as described in Embodiment 1, and will not be repeated here to avoid redundancy.

[0077] The broadband optical amplification system receives the signal light to be amplified and amplifies and outputs the signal light through the aforementioned cladding-pumped broadband optical amplifier. The signal light to be amplified can be continuous single-mode signal light or noise-like pulse signal light. Continuous single-mode signal light undergoes power amplification through the cladding-pumped broadband optical amplifier, while noise-like pulse signal light undergoes stimulated emission amplification through the cladding-pumped broadband optical amplifier, resulting in nonlinear spectral broadening. The broadband optical amplification system can amplify signal light with different time characteristics using the same cladding-pumped broadband optical amplifier, making the system suitable for broadband amplification of E-band or S-band signal light.

[0078] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber, characterized in that, include: The system includes an amplified signal input terminal, a multimode semiconductor laser, a pump signal combiner, a standard double-clad fiber, a double-clad bismuth-doped fiber, a clad pump light stripper, a mode field adapter, an output optical isolator, and an amplified signal output terminal. The pump signal combiner includes a combiner pump input, a combiner signal input, and a combiner common output. The multimode semiconductor laser is connected to the combiner pump input, the amplified signal input is connected to the combiner signal input, and the combiner common output is connected to the input of the double-clad bismuth-doped fiber. The pump signal combiner, the double-clad bismuth-doped fiber, the clad pump light stripper, and the mode field adapter are arranged sequentially along the optical signal transmission direction. Adjacent pairs are connected by a standard double-clad fiber. The output end of the mode field adapter is connected to the input end of the output optical isolator, and the output end of the output optical isolator is connected to the amplified signal output end, forming an all-fiber amplification link. The double-clad bismuth-doped fiber includes a core, an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding. The common output end of the combiner is optically coupled to the core and the inner cladding respectively through a standard double-clad fiber. The pump signal combiner is used to couple the signal light to be amplified input through the signal input end of the combiner to the core and to couple the multimode pump light output from the multimode semiconductor laser to the inner cladding. The wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped optical fiber.

2. The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber according to claim 1, characterized in that: The all-fiber amplification link is a single-stage, single-pass all-fiber amplification link, in which the signal light to be amplified passes through the double-clad bismuth-doped fiber once in the all-fiber amplification link. The fiber core is a single-mode fiber core used to transmit the signal light to be amplified in a single-mode manner; The signal light to be amplified is either a continuous single-mode signal light or a noise-like pulse signal light, and the center wavelength of the continuous single-mode signal light and the noise-like pulse signal light is located in the E-band or S-band.

3. The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber according to claim 1, characterized in that: The cladding pump light stripper is connected to the mode field adapter via a double-clad output optical fiber, and the mode field adapter is connected to the output optical isolator via a single-mode output optical fiber. The mode field adapter includes a double-clad fiber end connected to the standard double-clad fiber and a single-mode fiber end connected to the single-mode output fiber. The forward transmission direction of the output optical isolator is consistent with the direction in which the signal light to be amplified is transmitted from the double-clad bismuth-doped optical fiber to the amplified signal output end.

4. The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber according to claim 1, characterized in that: The double-clad bismuth-doped fiber and the standard double-clad fiber are connected by arc fusion splicing to form a fusion splice area. The insertion loss of a single fusion splice point between the double-clad bismuth-doped fiber and the standard double-clad fiber is not higher than 0.05 dB. The fusion splice area is covered with a low-refractive-index adhesive, which is in contact with the outer cladding of the optical fiber at the fusion splice area. The refractive index of the low-refractive-index adhesive after curing is 1.31 to 1.

35.

5. The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber according to claim 1, characterized in that: The double-clad bismuth-doped optical fiber has a cladding absorption coefficient within the cladding absorption band. The double-clad bismuth-doped fiber is continuously disposed between the common output end of the combiner and the cladding pump light stripper, and the length of the double-clad bismuth-doped fiber is determined according to the cladding absorption coefficient.

6. The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber according to claim 1, characterized in that: The multimode semiconductor laser is a multimode pumped semiconductor laser with a center wavelength of 808 nanometers and a maximum pump power of 36 watts. The pump signal combiner is a (2+1)×1 type pump signal combiner, which includes two combiner pump input terminals, one combiner signal input terminal, and one combiner common output terminal.

7. The cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber according to claim 1, characterized in that: It also includes an optical fiber disk and a heat sink. The optical fiber disk is disposed on the heat sink. The double-clad bismuth-doped optical fiber is wound around the optical fiber disk in turn. The winding diameter of the double-clad bismuth-doped optical fiber is not less than the minimum allowable bending diameter of the double-clad bismuth-doped optical fiber. Adjacent optical fiber loops are arranged sequentially in the optical fiber disk.

8. A cladding-pumped broadband optical amplification method based on double-clad bismuth-doped fiber, implemented using the cladding-pumped broadband optical amplifier based on double-clad bismuth-doped fiber as described in any one of claims 1 to 7, characterized in that... include: The signal light to be amplified is input to the combiner signal input terminal of the pump signal combiner via the amplification signal input terminal. The multimode pump light output from the multimode semiconductor laser is input to the combiner pump input terminal of the pump signal combiner. The wavelength of the multimode pump light is located within the cladding absorption band of the double-clad bismuth-doped fiber. The signal light to be amplified and the multimode pump light are output to a standard double-clad fiber through the pump signal combiner. After transmission through the standard double-clad fiber, the signal light to be amplified is coupled to the core of the double-clad bismuth-doped fiber, and the multimode pump light is coupled to the inner cladding of the double-clad bismuth-doped fiber. The signal light to be amplified propagates in a single-mode manner in the fiber core, while the multimode pump light propagates in the inner cladding and passes through the fiber core multiple times. The multimode pump light is absorbed by the bismuth-related active centers in the fiber core, and the signal light to be amplified is amplified by stimulated emission to obtain the amplified signal light. The amplified signal light is transmitted sequentially through a standard double-clad optical fiber to the clad pump light stripper and the mode field adapter, and then output through the amplified signal output terminal after passing through the output optical isolator. The signal light to be amplified passes through the double-clad bismuth-doped fiber once in the all-fiber amplification link.

9. The cladding-pumped broadband optical amplification method based on double-clad bismuth-doped optical fiber according to claim 8, characterized in that: The signal light to be amplified is a noise-like pulse signal light with a center wavelength located in the 1.4 micrometer band. The noise-like pulse signal light is composed of multiple random ultrashort sub-pulses. The autocorrelation curve of the noise-like pulse signal light includes a narrow coherent peak and a wide substrate carrying the narrow coherent peak. The noise-like pulse signal light passes through the double-clad bismuth-doped fiber once in the fiber core, and undergoes stimulated emission amplification and nonlinear spectral broadening under the cladding pumping of the multimode pump light.

10. A broadband optical amplification system, characterized in that, Includes the cladding-pumped broadband optical amplifier based on double-clad bismuth-doped optical fiber as described in any one of claims 1 to 7.

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