Method and device for evaluating stimulated Brillouin scattering effect in single-frequency fiber lasers

Abstract

A method and device for evaluating the stimulated Brillouin scattering effect in a single-frequency fiber laser, including obtaining the output power and line width of a single-frequency laser output by the single-frequency fiber laser; calculating the spectral power density based on the output power and line width of the single-frequency laser; obtaining a curve of the spectral power density changing with the output power of the single-frequency laser; and evaluating the stimulated Brillouin scattering effect and threshold in the single-frequency fiber laser based on the curve of the spectral power density changing with the output power of the single-frequency laser. When the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density increases with the increase of the output power of the single-frequency laser; when the output power of the single-frequency fiber laser reaches the stimulated Brillouin scattering effect threshold, the spectral power density decreases with the increase of the output power of the single-frequency laser. The present invention can be applied to the evaluation of the stimulated Brillouin effect threshold in various types of single-frequency fiber laser amplifiers.

Description

Technical Field

[0001] The present invention mainly relates to the field of optical fiber laser technology, in particular to a method and a device for evaluating stimulated Brillouin scattering effect in a single-frequency optical fiber laser. Background Art

[0002] In the field of fiber lasers, if the output laser has only one longitudinal mode (i.e., a single frequency), it is called a single-frequency fiber laser. Single-frequency fiber lasers are considered to be ideal light sources for applications such as high-power beam synthesis, gravitational wave detection, lidar, frequency conversion, and precision spectral detection due to their superior temporal coherence, low noise, and high stability. In high-power single-frequency fiber laser systems, with the increase in output power, the influence of nonlinear effects in the optical fiber becomes more and more significant, especially the stimulated Brillouin scattering (SBS) effect, which will transfer most of the energy of the signal light to the frequency-downshifted pulsed Stokes light after the fiber laser system reaches a certain power, thereby limiting the further increase in the output power of the single-frequency fiber laser system. Therefore, in order to ensure the stable operation of the system, it is necessary to ensure that the system operates at a certain power level, which is called the stimulated Brillouin scattering effect threshold.

[0003] Traditional methods for evaluating the threshold of stimulated Brillouin scattering effect mainly include monitoring the nonlinear growth of reverse return light, monitoring the spectrum of reverse return light, monitoring the time domain pulse of forward output laser, etc. However, these traditional methods have some limitations and shortcomings. For example, when judging the SBS threshold by monitoring the nonlinear growth of reverse return light, the judgment of nonlinear growth depends on the subjective consciousness of researchers and there is no unified standard. Although the output power when the return light power reaches a certain proportion of the output power is defined as the SBS threshold of the system, this proportion value can be arbitrarily selected from 1 / 10000 to 1%, so there is still a lack of unified standards, such as Figure 2 As shown, Figure 2 This is a typical range of values ​​for determining the SBS threshold using the return light power. In conventional theoretical or experimental studies, the ratio range for determining the SBS threshold is usually 1 / 10000 to 1%. In addition, since the backlight induced by the SBS effect is an unstable pulse sequence, the power of the backlight fluctuates after the SBS effect occurs, which will seriously affect the accurate calculation of the power ratio value. When determining the SBS threshold by monitoring the spectrum of the reverse return light, the SBS threshold of the system is usually defined as the output power when the reverse Stokes spectrum is higher than a certain value of the Rayleigh scattering spectrum component, such as Figure 3 As shown, Figure 3This is a typical spectrum diagram for determining the SBS threshold using the echo spectrum. When the SBS effect occurs, due to the unstable characteristics of Stokes light, the intensity of the Stokes spectrum component can be seen to fluctuate in the echo spectrum. This threshold definition method also faces the problem of instability of the backward pulse Stokes light, which causes the intensity of the Stokes spectrum component measured at the same power level to fluctuate, further affecting the judgment of the threshold. When the SBS threshold is defined by monitoring the time domain pulse of the forward output laser, since the Stokes light only propagates backward and is amplified in the front half of the optical fiber, the information of the SBS effect pumping energy will be masked to a certain extent after the power amplification in the second half of the optical fiber. In actual systems, the pulse information after the Stokes light pumping energy can only be detected in the output laser when the SBS effect is very serious, such as Figure 4 As shown, Figure 4 It is a timing diagram that uses the output power time domain fluctuation to determine the SBS threshold. After a strong SBS effect occurs, a large number of pulse spikes will appear in the time domain of the output laser. Therefore, using this method to determine the SBS threshold is very likely to damage the laser amplification system. In addition, the above three threshold determination methods can only simply describe the intensity of the SBS effect, and cannot link the SBS effect with the performance of the laser system.

[0004] Based on the current problems and limitations, a stimulated Brillouin scattering effect threshold judgment method is provided, which is universal in single-frequency fiber laser amplifiers and can be linked to the performance of high-power single-frequency fiber laser systems. It has important practical significance for protecting the safety and stability of the laser and improving the performance of high-power single-frequency fiber lasers. Summary of the invention

[0005] In view of the technical problems existing in the prior art, the present invention proposes a method and a device for evaluating the stimulated Brillouin scattering effect in a single-frequency fiber laser.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] In one aspect, the present invention provides a method for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser, comprising:

[0008] Obtain the output power and line width of the single-frequency laser output by the single-frequency fiber laser;

[0009] Calculate the spectral power density based on the output power and line width of the single-frequency laser;

[0010] Obtain the curve of spectral power density changing with the output power of single-frequency laser;

[0011] The stimulated Brillouin scattering effect and threshold in single-frequency fiber laser are evaluated based on the curve of spectral power density changing with the output power of single-frequency laser.

[0012] Preferably, the spectral power density is calculated based on the output power and line width of the single-frequency laser, and the calculation formula is as follows:

[0013]

[0014] in and represent the output power and line width of single-frequency laser, respectively. Represents the spectral power density.

[0015] Preferably, the stimulated Brillouin scattering effect and the threshold value in the single-frequency fiber laser are evaluated based on the curve of the change of the spectral power density with the output power of the single-frequency laser, including:

[0016] When the output power of a single-frequency fiber laser does not reach the threshold of stimulated Brillouin scattering effect, the spectral power density With the output power of single-frequency laser 1. The rise of

[0017] When the output power of a single-frequency fiber laser reaches the threshold of stimulated Brillouin scattering effect, the spectral power density With the output power of single-frequency laser The spectral power density With the output power of single frequency laser The increase in spectral power density With the output power of single frequency laser The turning point where the value of γ decreases with the increase of γ is the threshold of stimulated Brillouin scattering effect.

[0018] Preferably, when the stimulated Brillouin scattering effect begins to occur in the single-frequency fiber laser, but the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density With the output power of single-frequency laser The rate of ascension slows down.

[0019] In another aspect, the present invention provides a device for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser, comprising:

[0020] A testing device, used to obtain the output power and line width of a single-frequency laser output by a single-frequency fiber laser;

[0021] A calculation unit, used for calculating the spectral power density based on the output power and line width of the single-frequency laser;

[0022] A curve drawing unit, used for obtaining a curve of spectral power density changing with the output power of a single-frequency laser;

[0023] The evaluation unit is used to evaluate the stimulated Brillouin scattering effect and threshold value in the single-frequency fiber laser based on the variation curve of the spectral power density with the output power of the single-frequency laser.

[0024] Furthermore, the testing device includes a single-frequency fiber laser, a high-power reflector, a power meter, a focusing lens, a single-mode fiber, a Mach-Zehnder interferometer and a spectrum analyzer. The single-frequency laser output by the single-frequency fiber laser is incident on the high-power reflector, and most of the power is reflected to the power meter for measuring the output power of the single-frequency laser. The remaining small part of the power is transmitted through the high-power reflector and output to the focusing lens, which is coupled into the single-mode fiber by the focusing lens. The single-mode fiber is connected to the Mach-Zehnder interferometer, and the Mach-Zehnder interferometer outputs a beat frequency spectrum to the spectrum analyzer. The line width of the single-frequency laser is obtained based on the spectrum analyzer.

[0025] Furthermore, the Mach-Zehnder interferometer is composed of an input-end fiber coupler, an acousto-optic modulator, a delay fiber and an output-end fiber coupler.

[0026] Compared with the prior art, the technical effects of the present invention are:

[0027] The present invention is based on the output power and line width of the single-frequency laser output by the single-frequency fiber laser, calculates the spectral power density based on the output power and line width of the single-frequency laser, and evaluates the stimulated Brillouin scattering effect and threshold in the single-frequency fiber laser based on the spectral power density, and has better stability.

[0028] The present invention defines the SBS threshold at the power point with the best overall system performance (i.e., the highest spectral power density), which is the best working power point for applications in fields such as long-distance coherent detection.

[0029] The invention has universality and can be widely applied to the evaluation of stimulated Brillouin effect thresholds in various types of single-frequency fiber laser amplifiers.

[0030] The threshold evaluation method provided by the present invention does not require the single-frequency amplifier to work under a strong nonlinear effect, and can better protect the safety and stable operation of the system.

[0031] In summary, the present invention has important application value in the field of high-power lasers, especially in the field of stimulated Brillouin scattering effect threshold evaluation and overall system performance optimization for high-power single-frequency fiber laser amplifiers. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0033] Figure 1 is a flow chart of a method for evaluating stimulated Brillouin scattering effects in a single-frequency fiber laser in one embodiment;

[0034] Figure 2 This is a typical range diagram for determining the SBS threshold using the return light power;

[0035] Figure 3 This is a typical spectrum diagram for determining the SBS threshold using the return spectrum;

[0036] Figure 4 It is a timing diagram for determining the SBS threshold using the output power time domain fluctuation;

[0037] Figure 5 This is a schematic diagram of the test system for the power and line width of a single-frequency laser;

[0038] Figure 6 This is a trend graph of the output laser line width versus output power in a typical single-frequency fiber laser;

[0039] Figure 7 This is a trend graph showing the spectral power density of the output laser in a typical single-frequency fiber laser as the output power changes. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in 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.

[0041] Reference Figure 1 , one embodiment provides a method for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser, comprising:

[0042] Obtain the output power and linewidth of the single-frequency laser output by the single-frequency fiber laser.

[0043] In a high-power single-frequency fiber laser, the output power and line width of the single-frequency laser are the most important core factors affecting its performance, which are related to the transmission distance and temporal coherence of the laser. Therefore, the method for evaluating the stimulated Brillouin scattering effect in a single-frequency fiber laser described in the present invention uses the above two parameters to define the SBS threshold. The spectral power density is defined and calculated based on the output power and line width of the single-frequency laser, and the spectral power density represents the average laser power intensity within a unit line width range.

[0044] Specifically, the spectral power density is calculated based on the output power and line width of the single-frequency laser. The calculation formula is as follows:

[0045]

[0046] in and represent the output power and line width of single-frequency laser, respectively. Represents the spectral power density.

[0047] Obtain the curve of spectral power density changing with the output power of single-frequency laser;

[0048] The situations in which the stimulated Brillouin scattering effect and threshold in a single-frequency fiber laser are evaluated based on the curve of spectral power density changing with the output power of the single-frequency laser include:

[0049] When the output power of a single-frequency fiber laser does not reach the threshold of stimulated Brillouin scattering effect, the spectral power density With the output power of single-frequency laser 1. The rise of

[0050] When the output power of a single-frequency fiber laser reaches the threshold of stimulated Brillouin scattering effect, the spectral power density With the output power of single-frequency laser The spectral power density With the output power of single frequency laser The increase in spectral power density With the output power of single frequency laser The turning point where the value of γ decreases with the increase of γ is the threshold of stimulated Brillouin scattering effect.

[0051] When stimulated Brillouin scattering effect does not occur in a single-frequency fiber laser, the spectral power density will gradually increase with the increase of the output power of the single-frequency fiber laser.

[0052] When stimulated Brillouin scattering (SBS effect) occurs in a single-frequency fiber laser, the line width of the output laser of the single-frequency fiber laser will gradually be nonlinearly broadened. Figure 6 , Figure 6This is a trend chart of the output laser line width versus output power in a typical single-frequency fiber laser. When the stimulated Brillouin scattering effect (SBS effect) does not occur, the output laser line width is well maintained. After the SBS effect occurs, the output laser line width is nonlinearly broadened. When the stimulated Brillouin scattering effect begins to occur in a single-frequency fiber laser, but the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density With the output power of single-frequency laser The rate of ascension slows down.

[0053] When a strong SBS effect occurs in a single-frequency fiber laser, the output power of the single-frequency fiber laser reaches the threshold of the stimulated Brillouin scattering effect, and the spectral power density begins to decrease as the output power of the single-frequency fiber laser increases. The output power at this spectral power density turning point is defined as the stimulated Brillouin scattering effect threshold of the system. Figure 7 This is a trend chart of the spectral power density of the output laser in a typical single-frequency fiber laser changing with the output power. The spectral power density of the output laser first increases linearly with the output laser, and after the SBS effect appears, the growth rate gradually slows down. As the SBS effect further strengthens, the output power of the single-frequency fiber laser reaches the threshold of the stimulated Brillouin scattering effect, and the spectral power density of the output laser begins to decrease with the growth of the output laser. Figure 7 The turning point of the spectral power density of the output laser of the single-frequency fiber laser is 90 W, that is, the SBS threshold of the single-frequency fiber laser is 90 W.

[0054] In another aspect, the present invention provides a device for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser, comprising:

[0055] A testing device, used to obtain the output power and line width of a single-frequency laser output by a single-frequency fiber laser;

[0056] A calculation unit, used for calculating the spectral power density based on the output power and line width of the single-frequency laser;

[0057] A curve drawing unit, used for obtaining a curve of spectral power density changing with the output power of a single-frequency laser;

[0058] The evaluation unit is used to evaluate the stimulated Brillouin scattering effect and threshold value in the single-frequency fiber laser based on the variation curve of the spectral power density with the output power of the single-frequency laser.

[0059] Reference Figure 5A test device for measuring the output power and line width of a single-frequency laser output by a single-frequency fiber laser is provided, comprising a single-frequency fiber laser 1, a high-power reflector 3, a power meter 4, a focusing lens 5, a single-mode fiber 6, a Mach-Zehnder interferometer and a spectrum analyzer 11. The single-frequency laser 2 output by the single-frequency fiber laser 1 is incident on the high-power reflector 3, most of the power is reflected to the power meter 4, which is used to measure the output power of the single-frequency laser, and the remaining small part of the power is transmitted through the high-power reflector 3 and output to the focusing lens 5, which is coupled into the single-mode fiber 6 by the focusing lens 5. The single-mode fiber 6 is connected to the Mach-Zehnder interferometer, and the Mach-Zehnder interferometer outputs a beat frequency spectrum to the spectrum analyzer 11. The line width of the single-frequency laser is obtained based on the spectrum analyzer 11.

[0060] The Mach-Zehnder interferometer is composed of an input fiber coupler 7, an acousto-optic modulator 8, a delay fiber 9 and an output fiber coupler 10. One end of the input fiber coupler 7 and the output fiber coupler 10 is a beam combining end, and the other end has two beam splitting arms. The single-mode optical fiber 6 is connected to the beam combining end of the input fiber coupler 7, one beam splitting arm of the input fiber coupler 7 is connected to the acousto-optic modulator 8, and the other beam splitting arm is connected to a certain length of delay fiber 9 (such as about 2 km long). The other end of the acousto-optic modulator 8 and the other end of the delay fiber 9 are respectively connected to the two beam splitting arms of the output fiber coupler 10, and the beam combining end of the output fiber coupler 10 is connected to the spectrum analyzer 11. The Mach-Zehnder interferometer forms a beat frequency spectrum signal and injects the beat frequency spectrum signal into the spectrum analyzer for measurement and recording, and the line width of the single-frequency laser is obtained by analysis.

[0061] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0062] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating stimulated Brillouin scattering effects in a single-frequency fiber laser, characterized in that include: Obtain the output power and line width of the single-frequency laser output by the single-frequency fiber laser; Calculate the spectral power density based on the output power and line width of the single-frequency laser; Obtain the curve of spectral power density changing with the output power of single-frequency laser; The stimulated Brillouin scattering effect and threshold in the single-frequency fiber laser are evaluated based on the curve of spectral power density changing with the output power of the single-frequency laser, including: when the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density With the output power of single-frequency laser 1. The rise of When the output power of a single-frequency fiber laser reaches the threshold of stimulated Brillouin scattering effect, the spectral power density With the output power of single-frequency laser The spectral power density With the output power of single frequency laser The increase in spectral power density With the output power of single frequency laser The turning point where the value of γ decreases with the increase of γ is the threshold of stimulated Brillouin scattering effect.

2. The method for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser according to claim 1, characterized in that: The spectral power density is calculated based on the output power and line width of the single-frequency laser. The calculation formula is as follows: in and represent the output power and line width of single-frequency laser, respectively. Represents the spectral power density.

3. The method for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser according to claim 1 or 2, characterized in that: When stimulated Brillouin scattering effect begins to occur in the single-frequency fiber laser, but the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density With the output power of single-frequency laser The rate of ascension slows down.

4. An apparatus for evaluating stimulated Brillouin scattering effects in a single-frequency fiber laser, characterized in that include: A testing device, used to obtain the output power and line width of a single-frequency laser output by a single-frequency fiber laser; A calculation unit, used for calculating the spectral power density based on the output power and line width of the single-frequency laser; A curve drawing unit, used for obtaining a curve of spectral power density changing with the output power of a single-frequency laser; An evaluation unit is used to evaluate the stimulated Brillouin scattering effect and threshold in a single-frequency fiber laser based on a curve of spectral power density changing with the output power of a single-frequency laser, including: when the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density With the output power of single-frequency laser 1. The rise of When the output power of a single-frequency fiber laser reaches the threshold of stimulated Brillouin scattering effect, the spectral power density With the output power of single-frequency laser The spectral power density With the output power of single frequency laser The increase in spectral power density With the output power of single frequency laser The turning point where the value of γ decreases with the increase of γ is the threshold of stimulated Brillouin scattering effect.

5. The device for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser according to claim 4, characterized in that: The test device includes a single-frequency fiber laser, a high-power reflector, a power meter, a focusing lens, a single-mode fiber, a Mach-Zehnder interferometer and a spectrum analyzer. The single-frequency laser output by the single-frequency fiber laser is incident on the high-power reflector. Most of the power is reflected to the power meter for measuring the output power of the single-frequency laser. The remaining small part of the power is transmitted through the high-power reflector and output to the focusing lens, which is coupled into the single-mode fiber by the focusing lens. The single-mode fiber is connected to the Mach-Zehnder interferometer. The Mach-Zehnder interferometer outputs a beat frequency spectrum to the spectrum analyzer. The line width of the single-frequency laser is obtained based on the spectrum analyzer.

6. The device for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser according to claim 5, characterized in that: The Mach-Zehnder interferometer consists of an input fiber coupler, an acousto-optic modulator, a delay fiber and an output fiber coupler.

7. The device for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser according to claim 4, 5 or 6, characterized in that: In the calculation unit, the spectral power density is calculated based on the output power and line width of the single-frequency laser. The calculation formula is as follows: in and represent the output power and line width of single-frequency laser, respectively. Represents the spectral power density.

8. The device for evaluating stimulated Brillouin scattering effect in a single-frequency fiber laser according to claim 7, characterized in that: When stimulated Brillouin scattering effect begins to occur in the single-frequency fiber laser, but the output power of the single-frequency fiber laser does not reach the stimulated Brillouin scattering effect threshold, the spectral power density With the output power of single-frequency laser The rate of ascension slows down.

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