CN114883899A

CN114883899A - Single-frequency fiber laser with tunable line width

Info

Publication number: CN114883899A
Application number: CN202210625700.4A
Authority: CN
Inventors: 高伟清; 郑子其; 周勇; 马晓辉; 张维; 方文坛; 陈小林; 黄松
Original assignee: Hefei University of Technology
Current assignee: Hefei University of Technology
Priority date: 2022-06-02
Filing date: 2022-06-02
Publication date: 2022-08-09
Anticipated expiration: 2042-06-02
Also published as: CN114883899B

Abstract

The invention discloses a single-frequency fiber laser with tunable line width, which combines optical self-injection and stimulated Brillouin effect to realize wide-range tuning of the line width of output laser. In the laser, except for a single-frequency seed source DFB (distributed feedback) and an optical amplification part, the structure of the device can be roughly divided into two parts, wherein one part is a self-injection narrowing line width structure, and the other part is a stimulated Brillouin effect narrowing line width structure. In the process of tuning the line width, firstly, the line width of the single-frequency laser is narrowed by external cavity injection light, the line width narrowing degree is controlled by controlling a tunable attenuator in the external cavity, different line widths obtained after control are output to a stimulated Brillouin structure after passing through, the line widths narrowed in different degrees are obtained by utilizing the strong narrowing line width capability of the stimulated Brillouin effect, and therefore the tunable process of outputting the laser line width from very wide to extremely narrow is completed.

Description

Single-frequency fiber laser with tunable line width

Technical Field

The invention belongs to the technical field of fiber lasers, and particularly relates to a single-frequency fiber laser with a tunable line width.

Background

The single-frequency fiber laser with the ultra-narrow line width plays an important role in atomic clocks, optical precision calculation and coherent communication by virtue of ultra-low phase noise and long coherent length; the single-frequency fiber laser with a wider laser line width and the single-frequency fiber laser with an ultra-wide laser line width play important roles in the fields of single-frequency Q-switching and single-frequency amplification. Therefore, in practical application, the single-frequency fiber laser with tunable line width is valuable.

Tuning of line width involves line width widening and narrowing techniques. For the line width widening technology, the most common mode at present is to perform frequency modulation on the output laser of a single-frequency fiber laser, but the technology introduces electrical noise into the fiber laser, is not an all-fiber structure, and is not beneficial to large-amplitude tuning of the line width. Therefore, compared with the gradual widening of the line width, the method is a more efficient line width tuning mode by utilizing the line width narrowing technology to gradually narrow the single-frequency laser with the wide line width and respectively extracting and outputting the single-frequency laser.

An F-P structure composed of fiber gratings is added into a classical single-frequency structure such as a DBR (distributed Bragg Reflector) or a DFB (distributed Bragg Reflector), so that the line width is narrowed by utilizing a slow light effect, and the method is a practical method. However, the structure of the original laser is changed in the mode, the degree of slow light effect cannot be controlled, and the tuning effect cannot be achieved. Similar line width narrowing methods, including virtual folded cavity technology and unpumped doped fiber filtering technology, cannot achieve tuning effects.

Traditional self-injection utilizes a part of self-output light to serve as external injection light, returns to the main cavity behind the exocoel of high Q value for high frequency noise is suppressed, thereby presses the narrow linewidth, and the structure is very simple and easy, and in suitable exocoel optical power feedback scope, changes the degree that the size of feedback power can control the narrow linewidth, plays harmonious effect. However, this tuning has limitations, and in general, the line width can be narrowed to 4 to 5 times the original line width without controlling the external cavity phase. If a single-frequency laser with the line width of MHz level is used as a seed source, the line width of output laser can not reach the Hz level at all.

The stimulated Brillouin scattering has extremely narrow intrinsic gain spectral line width which is narrower than most filters, so that the stimulated Brillouin scattering has extremely high line width compression and narrowing capability. But it has no tuning capability in itself and, due to the low brillouin gain coefficient, it often requires a sufficiently long gain medium to have sufficient brillouin gain, which results in an increase in cavity length and thus mode hopping.

In summary, the narrow line width technology only has the optical self-injection technology with obvious tuning capability and simple structure, but the tuning range of the self-injection technology has limitations; stimulated brillouin scattering has a high ability to narrow the linewidth, but it has no tuning ability and requires a very long cavity length, which can cause mode hopping.

Disclosure of Invention

The invention aims to provide a single-frequency fiber laser with tunable line width, which belongs to a fiber laser with simple structure, wide tunable line width range and full fiber structure.

In order to solve the technical problems, the invention is realized by the following technical scheme:

the single-frequency fiber laser with tunable line width is characterized in that a first optical pumping source pumps a DFB single-frequency laser through a first wavelength division multiplexer to generate first signal light and second signal light, the DFB single-frequency laser is provided with residual pumping light, and the residual pumping light and the second signal light are separated through a second wavelength division multiplexer;

after being separated from the second wavelength division multiplexer, the second signal light sequentially passes through an outer cavity formed by the delay optical fiber and the optical fiber type tunable attenuator and returns to the inner cavity through the first optical coupler and the optical isolator to complete the self-injection process;

the first signal light is combined with the separated residual pump light in the third wavelength division multiplexer through the optical isolator and the first optical coupler to reach the erbium-doped optical fiber, signal amplification is completed, then the first signal light passes through the second tunable attenuator and enters the second optical circulator to reach the annular cavity, stimulated Brillouin scattering occurs in the annular cavity, first-order Stokes light is generated, and single-frequency light output is generated.

Further, a first optical circulator is installed between the second tunable attenuator and the second optical circulator, and the first optical circulator is connected with a fiber grating.

Further, the second signal light passes through a second optical coupler before reaching the delay optical fiber, and the second optical coupler is connected with an optical power meter.

Further, a third optical coupler is installed between the second tunable attenuator and the second optical circulator, the third optical coupler is connected with the first laser measurement system, the third optical coupler divides the optical signal into parts, one part enters the first laser measurement system, and the other part enters the second optical circulator.

Further, a fourth optical coupler is installed in the ring cavity corresponding to the second optical circulator, and the fourth optical coupler is connected with a second laser measurement system.

Furthermore, a fourth wavelength division multiplexer and a second erbium-doped fiber are installed in the annular cavity corresponding to the second optical circulator, and the fourth wavelength division multiplexer is connected with a second optical pumping source.

Further, the cavity length of the ring cavity corresponding to the second optical circulator is within 10m, so as to ensure that only one longitudinal mode exists in the Brillouin gain range, and the second erbium-doped fiber has a higher doping concentration.

Furthermore, a tunable optical filter is installed in the ring cavity corresponding to the second optical circulator, and the optical filter is located at the rear end of the second erbium-doped fiber.

The principle is as follows:

1. the laser linewidth is first tuned using self-injection. Self-injection is a very simple line width compression mode with obvious effect, and injection power is in the third stage of injection effect by adding a tunable attenuator in an injection loop, so that the line width compression capacity of self-injection can be changed along with the change of injection light power, and the line width compression process has great tuning capacity.

2. Self-injection is combined with a brillouin laser. The output light line width of the brillouin laser is related to the line width of brillouin pumping, and the narrower the line width of brillouin pumping is to some extent, the narrower the output light line width is. The laser output of the self-injection tunable line width is used as the Brillouin pumping of the Brillouin laser, so that the process of Brillouin line width compression has tuning capacity, the range of the self-injection tunable line width is larger, and the tuning range is wider.

3. The gain optical fiber is added in the Brillouin laser as the amplification gain of the signal light and the Stokes light, so that the threshold value of stimulated Brillouin in the cavity is greatly reduced, more importantly, the length required in the cavity is reduced, and single-frequency output is ensured. In the ring cavity, the stimulated Brillouin pumping light is amplified in the erbium-doped fiber by the second optical pumping source firstly, so that the stimulated Brillouin threshold can be reached in a shorter gain medium in the cavity, and after first-order Stokes light is generated, the first-order Stokes light is continuously amplified in a structure formed by the second optical pumping source and the erbium-doped fiber and is output outwards through the 5:5 coupler in the cavity.

In the fiber bragg grating for filtering out the ASE, the reflection bandwidth of the fiber bragg grating needs to cover the signal light waveband, and the narrower the reflection bandwidth is, the more obvious the ASE filtering effect is. The overall cavity length of the ring cavity needs to be controlled below 10m under the condition of ensuring single-frequency characteristics. The narrower the passband of the tunable filter in the cavity is, the better the passband is, the narrower the passband is, the more the gain curve in the cavity can be regarded as a flat curve, and the more effective the suppression of the self-oscillation of the ring cavity is. When the passband of the tunable filter is not narrow enough, the power of the second optical pump source needs to be controlled to suppress the self-oscillation.

The invention has the following beneficial effects:

(1) conventional self-injection techniques, while having tuning capabilities, do not allow for wide-range tuning of line widths. And the single stimulated Brillouin effect has strong line width compression capability but no tuning capability. The self-injection technology in the third stage of the feedback effect is combined with the stimulated Brillouin scattering, and the self-injection tuning capability, the strong narrow linewidth pressing capability of the stimulated Brillouin effect and the characteristic that the narrow linewidth pressing capability of the stimulated Brillouin is sensitive to the linewidth of the Brillouin pump are utilized, so that the laser linewidth can be tuned in a large range.

(2) Conventional stimulated brillouin scattering requires a single mode fiber of several hundred meters in order to obtain sufficient brillouin gain, which tends to make the cavity length too long to cause mode hopping. According to the invention, the linear gain of the erbium-doped fiber is added into the annular cavity, so that the input Brillouin pumping power is amplified, the length requirement of the required annular cavity is reduced, the generated first-order Stokes light can be amplified, the length requirement of the cavity is further reduced, and the whole annular cavity can stably operate under the condition of less than 10 m.

(3) Compared with the space structure of the partial line width tuning technology, the all-fiber tuning device adopts an all-fiber structure, greatly simplifies the system structure, has lower loss and higher efficiency, and has far lower requirements on environment and devices than the partial line width tuning device.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used for describing the embodiments will be briefly introduced below.

FIG. 1 is a schematic view of the structure of the present invention.

Fig. 2 is a schematic diagram of a self-injection tuning DFB laser linewidth apparatus.

Fig. 3 is a schematic diagram of an apparatus for self-injection tuning the linewidth of a DFB laser and amplifying the output laser.

Fig. 4 is a schematic diagram of a general stimulated brillouin laser device.

Fig. 5 is a schematic diagram of a brillouin laser arrangement incorporating erbium doped linear gain.

Fig. 6 is a schematic diagram of an arrangement for incorporating a filter in a brillouin laser incorporating an erbium doped linear gain.

Fig. 7 is a schematic diagram illustrating tuning linewidth range results.

In the drawings, the components represented by the respective reference numerals are listed below: the optical fiber laser system comprises a first optical pumping source 1, a first wavelength division multiplexer 2, a DFB single-frequency laser 3, a second wavelength division multiplexer 4, an optical isolator 5, a first optical coupler 6, an optical fiber type tunable attenuator 7, a delay optical fiber 8, a second optical coupler 9, an optical power meter 10, a third wavelength division multiplexer 11, a first erbium-doped optical fiber 12, a first optical circulator 13, an optical fiber grating 14, a second tunable attenuator 15, a third optical coupler 16, a first laser measurement system 17, a second optical circulator 18, a second optical pumping source 19, a fourth wavelength division multiplexer 20, a second erbium-doped optical fiber 21, a fourth optical coupler 22, a second laser measurement system 23 and an optical filter 24.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.

As shown in fig. 1-2: the single-frequency fiber laser with tunable line width is characterized in that a first optical pumping source 1 pumps a DFB single-frequency laser 3 through a first wavelength division multiplexer 2 to generate first signal light and second signal light, the first signal light and the second signal light are provided with residual pumping light, and the residual pumping light and the second signal light are separated through a second wavelength division multiplexer 4; the first optical pump source 1 is a 980nm optical pump source. The first wavelength division multiplexer 2 is 980/1550 wavelength division multiplexer, the second wavelength division multiplexer 4 is 980/1550 wavelength division multiplexer, the signal output end of DFB single-frequency laser and the pumping end are the same end, the signal light output power of the other end can be ignored.

As shown in fig. 2: a series of devices are added on the basis of fig. 1 to form a self-injection structure, and second signal light is separated from the second wavelength division multiplexer 4, then sequentially passes through an external cavity formed by a delay fiber 8 and a fiber tunable attenuator 7, and returns to the internal cavity through a first optical coupler 6 and an optical isolator 5 to complete a self-injection process; the first optical coupler 6 is a 5:5 optical coupler. The lengths of the fiber-type tunable attenuator 7 and the delay fiber 8 in the external cavity are used for changing the feedback power and the Q value in the external cavity respectively. In the self-injection process, the injection power is changed by using the optical fiber type tunable attenuator 7, so as to change the compression degree of the self-injection on the line width, namely, the whole process of compressing the line width by the self-injection can be tuned by the optical fiber type tunable attenuator 7.

The optical isolator 5 ensures the direction of the signal light in the main cavity, also ensures the power of the external cavity signal light, and ensures the stability of the whole injection process.

As shown in fig. 3-4: the first signal light passes through the optical isolator 5 and the first optical coupler 6 to be combined with the separated residual pump light in the third wavelength division multiplexer 11, reaches the erbium-doped optical fiber 12, completes signal amplification, then passes through the second tunable attenuator 15, enters the second optical circulator 18 to reach the annular cavity, generates stimulated Brillouin scattering in the annular cavity, generates first-order Stokes light and generates single-frequency light output.

The output light property from the injection last is related to the length of the delay fiber 8. Under the condition of ensuring that the final output single-frequency light cannot jump the mode and has a certain side mode suppression ratio, the longer the length of the delay optical fiber 8 is, the stronger the self-injection compressed line width capability is, and the wider the self-injection tuning line width range is.

The output linewidth can be tuned by adjusting the attenuation of the optical fiber type tunable attenuator 7 in the optical path, but in the tuning process, attention should be paid to that the finally output single-frequency light does not jump the mode and has a certain side mode suppression ratio, that is, under the condition that the finally output single-frequency light does not jump the mode and has a certain side mode suppression ratio, the adjustment range of the optical fiber type tunable attenuator 7 can be adjusted in the whole self-injection process, and the tuning range of the linewidth can be adjusted.

The erbium doped fiber 12 for optical amplification should not have a doping concentration too large to prevent the signal light from being too weak, so that the signal light is lost due to reabsorption without optical amplification.

As shown in fig. 3: between the second tunable attenuator 15 and the second optical circulator 18, a first optical circulator 13 is installed, the ports of which are marked with fonts of different colors in the figure, and in the circulator, laser light of 1 port can only exit from 2 ports, and laser light of 2 ports can only exit from 3 ports. The first optical circulator 13 is connected to a fiber grating 14. The first optical circulator 13 and the fiber grating 14 are for removing ase (amplified spin energy emission) generated during amplification. The second wavelength division multiplexer 4 separates the remaining pump, and the separated pump is recombined with the signal light to the first erbium-doped fiber 12 by the third wavelength division multiplexer 11 to amplify the signal light. The doping concentration of the erbium-doped fiber 12 should not be too high at this time, so as to prevent the loss of the signal light due to reabsorption without amplification.

As shown in fig. 3: the second signal light also passes through a second optical coupler 9 before reaching the delay optical fiber 8, and the second optical coupler 9 is connected with an optical power meter 10. The second optical coupler 9 is a 99:1 optical coupler and a power meter 10 is used to detect the power of the injected light.

As shown in fig. 4: a third optical coupler 16 is installed between the second tunable attenuator 15 and the second optical circulator 18, the third optical coupler 16 is connected with a first laser measurement system 17, the third optical coupler 16 divides the optical signal into parts, one part enters the first laser measurement system 17, and the other part enters the second optical circulator 18. The third optical coupler 16 is a 5:5 optical coupler, and can output signal light with a self-injection control line width, and can monitor optical power input to the second optical circulator thereafter. The first laser measurement system 17 is a spectrometer or spectrograph, and is used to measure various characteristics of the brillouin pump light, such as line width, spectrum, longitudinal mode spectrum, and the like. When the brillouin pump light power is very low, the stimulated brillouin effect basically cannot occur because the stimulated brillouin threshold value in the cavity cannot be reached, and thus the application range of the line width tuning device is greatly limited. Therefore, a first measuring system 17 is installed at the front end of the second optical circulator 18 for monitoring whether the power of the signal light satisfies the requirement.

The degree of the self-injection narrowing line width is related to the optical power fed back by the external cavity and the external cavity Q value, and the optical fiber type tunable attenuator 7 and the delay optical fiber 8 in the external cavity are just devices for respectively changing the two parameters. Theoretically, the longer the delay fiber 8 is more beneficial to narrow the line width, however, the longer the delay fiber 8 will also bring about the change of fsr (free Spectral range) of the main cavity, thereby reducing the side mode suppression ratio of the output single-frequency light and possibly even causing mode hopping, so that it is necessary to determine whether the length of the delay fiber 8 is proper by taking the stability of the single-frequency signal output by the 5:5 optical coupler 16 as a reference. In addition, according to the amount of the external cavity to feed back the optical power, the output signal of the main cavity can be divided into five stages, and the feedback quantity increases with the stages: in the first stage, the phase of the external cavity determines the widening or narrowing of the line width of the output light of the main cavity; in the second stage, the phase of the external cavity is controlled, and only the line width widening phenomenon is replaced by rapid mode hopping; in the third stage, the external cavity phase effect is weakened, the output light of the main cavity is stably in a narrowing state but is sensitive to external feedback, and in the stage, the stronger the feedback is, the narrower the narrowing is; a fourth stage, coherence collapse occurs; in the fifth stage, the external cavity phase effect is continuously weakened, and the output light of the main cavity is stably in a narrowing state and is not sensitive to external feedback any more. Due to the high grating reflectivity at two sides of the DFB single-frequency fiber laser, most of the feedback of the DFB single-frequency laser is in the third stage, so that the fiber-type tunable attenuator 7 can adjust the narrowing degree of the line width. However, it should be noted that, at a certain feedback amount, the gain of the external cavity to a certain longitudinal mode of the main cavity may cause a serious decrease in the side-mode suppression ratio, and even a mode jump, which is also required to determine whether the feedback amount is appropriate by the single-frequency stability of the output light of the third optical coupler 16.

As shown in fig. 5: a fourth optical coupler 22 is installed in the annular cavity corresponding to the second optical circulator 18, and a second laser measuring system 23 is connected to the fourth optical coupler 22. The fourth optical coupler 22 is a 5:5 optical coupler. The brillouin gain of single mode fibre is very low and the brillouin fibre laser of figure 4 requires a very long single mode fibre, but the brillouin gain bandwidth is approximately 20MHz, and when the single mode fibre is very long, i.e. the cavity length is very long, the laser light output by the fourth optical coupler 22 will no longer be single frequency light. The second laser measurement system 23 is installed to monitor the intensity of the signal light in the ring cavity to determine whether the bandwidth requirement is met.

As shown in fig. 6: a fourth wavelength division multiplexer 20 and a second erbium-doped fiber 21 are installed in the annular cavity corresponding to the second optical circulator 18, and the fourth wavelength division multiplexer 20 is connected with a second optical pump source 19. The second optical pump source 19 is a 980nm optical pump source and the fourth wavelength division multiplexer 20 is an 980/1550 wavelength division multiplexer. In order to meet the requirement of Brillouin gain bandwidth and solve the problem of low Brillouin gain of single-mode fibers, a 980nm second optical pump source 19, a 980/1550 fourth wavelength division multiplexer 20 and a second erbium-doped fiber 21 are added into the annular cavity, so that both the injected Brillouin signal light and the generated weak first-order Stokes light are amplified, the threshold of stimulated Brillouin is reached by a shorter gain fiber, the required Brillouin pumping power and the cavity length are greatly reduced, and the power of output light is also improved on the premise of ensuring single-frequency output.

The concentration of the second erbium doped fiber 21 used here should be high enough to have sufficient gain to amplify the brillouin pump light and the first order stokes light while ensuring the shortest cavity length.

The cavity length of the ring cavity corresponding to the second optical circulator is within 10m, so that only one longitudinal mode is ensured in the Brillouin gain range, and the second erbium-doped fiber 21 has higher doping concentration. The concentration of the doped fiber needs to meet the following two requirements: 1, under the requirement that the length of the whole cavity needs to be less than 10m, the added doped fiber needs to provide enough gain for the Brillouin pump light and the generated Stokes light; 2, in special cases, when the output laser power of the previous self-injection device is relatively low, the added doped fiber is ensured to be capable of effectively amplifying the weak input brillouin pumping light, but the brillouin pumping light is not amplified and is reabsorbed and consumed. Thereby ensuring that the cavity length is minimized while maintaining a high gain level.

As shown in fig. 6: a tunable optical filter 24 is installed in the ring cavity corresponding to the second optical circulator 18, and the optical filter 24 is located at the rear end of the second erbium-doped fiber 21. By adjusting the optical filter 24, the whole cavity must start to oscillate near the first-order stokes wavelength, and because the first-order stokes wavelength contains extra brillouin gain, the output light must be single-frequency laser due to gain competition.

The pass band width of the optical filter 24 is narrower than or equal to the brillouin frequency shift corresponding to the laser band applicable to the whole device, so that the whole cavity gain within the pass band range can be regarded as the same level, the gain is similar to a certain flat straight line, the gain is still smaller than loss at the moment, and oscillation cannot be started. Therefore, the Brillouin gain at the first-order Stokes light wavelength can be more prominent, and the gain competition can be more obvious.

As shown in fig. 7: the line width of the first stage of the line width is the widest corresponding line width, the line widths of the later stages are sequentially reduced to the line width of the original laser, the line width can be obtained in the output of the third optical coupler 16 when the optical fiber type tunable attenuator 7 is adjusted to be attenuated to be the largest, and the line width of the stage is not tunable;

the second stage of the line width is the line width obtained after the fiber type tunable attenuator 7 is adjusted, the line width can be obtained from the output of the third optical coupler 16 after the tunable attenuator 7 is adjusted, the line width in the stage is tunable, and the tuning range is approximately within the range of the original line width to one fifth of the line width.

The line width in the third stage is the line width after the Brillouin pressure is narrow, which is obtained when the attenuation of the tunable attenuator is maximum, the line width can be obtained in the output of the fourth optical coupler 22 when the attenuation of the optical fiber type tunable attenuator 7 is maximum, and the line width in the stage is not tunable;

the fourth stage is an extremely narrow linewidth, which can be obtained from the output of the fourth optical coupler 22 after the tunable attenuator 7 is adjusted, and the linewidth at this stage is tunable, and if the linewidth of the original single-frequency laser is in the MHz level, the tuning range is approximately in the kHz and Hz levels.

The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, to thereby enable others skilled in the art to best utilize the invention.

Claims

1. The single-frequency fiber laser with tunable line width is characterized in that:

the laser structure adopts an all-fiber structure, a first optical pumping source (1) pumps the DFB single-frequency laser (3) through a first wavelength division multiplexer (2) to generate first signal light and second signal light, the first signal light and the second signal light are provided with residual pumping light, and the residual pumping light and the second signal light are separated through a second wavelength division multiplexer (4);

after being separated from the second wavelength division multiplexer (4), the second signal light sequentially passes through an outer cavity formed by a delay optical fiber (8) and an optical fiber type tunable attenuator (7), and returns to the inner cavity through a first optical coupler (6) and an optical isolator (5) to complete the self-injection process;

the first signal light passes through an optical isolator (5) and a first optical coupler (6) to be combined with the separated residual pump light in a third wavelength division multiplexer (11), reaches a first erbium-doped fiber (12), finishes signal amplification, then passes through a second tunable attenuator (15), enters a second optical circulator (18) to reach an annular cavity, generates stimulated Brillouin scattering in the annular cavity, generates first-order Stokes light and generates single-frequency light output.

2. The line-width tunable single-frequency fiber laser of claim 1, wherein: a first optical circulator (13) is arranged between the second tunable attenuator (15) and the second optical circulator (18), and the first optical circulator (13) is connected with a fiber grating (14).

3. The line-width tunable single-frequency fiber laser of claim 1, wherein: the second signal light also passes through a second optical coupler (9) before reaching the delay optical fiber (8), and the second optical coupler (9) is connected with an optical power meter (10).

4. The line-width tunable single-frequency fiber laser of claim 1, wherein: and a third optical coupler (16) is arranged between the second tunable attenuator (15) and the second optical circulator (18), the third optical coupler (16) is connected with a first laser measurement system (17), the third optical coupler (16) divides an optical signal into parts, one part enters the first laser measurement system (17), and the other part enters the second optical circulator (18).

5. The line-width tunable single-frequency fiber laser of claim 4, wherein: a fourth optical coupler (22) is installed in the annular cavity corresponding to the second optical circulator (18), and the fourth optical coupler (22) is connected with a second laser measuring system (23).

6. The line-width tunable single-frequency fiber laser of claim 5, wherein: and a fourth wavelength division multiplexer (20) and a second erbium-doped fiber (21) are arranged in the annular cavity corresponding to the second optical circulator (18), and the fourth wavelength division multiplexer (20) is connected with a second optical pump source (19).

7. The line-width tunable single-frequency fiber laser of claim 6, wherein: and the cavity length of the annular cavity corresponding to the second optical circulator is within 10m, so that only one longitudinal mode is ensured in the Brillouin gain range, and the second erbium-doped optical fiber (21) has higher doping concentration.

8. The line-width tunable single-frequency fiber laser of claim 6, wherein: a tunable optical filter (24) is arranged in the annular cavity corresponding to the second optical circulator (18), and the optical filter (24) is positioned at the rear end of the second erbium-doped fiber (21).