A narrow-linewidth fiber laser based on self-injection locking with a randomly distributed feedback loop

Abstract

A narrow-linewidth fiber laser based on self-injection locking using a randomly distributed feedback loop relates to the technical field of fiber lasers. A wavelength division multiplexer, an erbium-doped fiber, an optical bandpass filter, optical coupler 1, optical coupler 2, a polarization controller, and an optical isolator are sequentially connected end-to-end to form a ring cavity. A pump source is connected to the wavelength division multiplexer. A randomly distributed feedback structure is connected to optical coupler 3 to form an optical fiber ring, forming a randomly distributed feedback loop. Ports 1 and 3 of the optical circulator are respectively connected to optical coupler 1 and optical coupler 2 to form a self-injection structure. Port 2 is connected to optical coupler 3, and the remaining output end of optical coupler 2 serves as the output end of the ring cavity. Utilizing a randomly distributed feedback loop can reduce the laser frequency drift of the ring cavity. By increasing the number of round trips of the optical signal and increasing the effective length of the randomly distributed feedback structure, a narrowband gain peak can be ensured, resulting in a stable narrow-linewidth single-frequency laser output. This also improves the efficiency and environmental sensitivity of linewidth narrowing.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber lasers, in particular to a narrow-linewidth optical fiber laser based on self-injection locking of a randomly distributed feedback loop. Background Art

[0002] Narrow-linewidth, single-frequency lasers have significant application value in high-precision metrology, coherent optical communications, fiber-optic sensing, and quantum technology. Narrow-linewidth fiber lasers, with their high coherence, low phase noise, and high stability, have become indispensable laser sources in the field of optical information science.

[0003] Narrow-linewidth single-frequency fiber lasers are a method that achieves single-frequency optical signal operation within a resonant cavity by adding additional linewidth reduction. Among the common linewidth reduction methods, narrowband filtering can greatly limit the gain range of the longitudinal mode within the cavity, making it smaller than the free spectral range of the laser, thereby achieving single-frequency laser output. However, the bandwidth of filtering devices such as narrowband phase-shifted fiber Bragg gratings and high-finesse FP etalons is usually limited by processing precision and can only achieve single longitudinal mode selection, but cannot further narrow the linewidth of single-frequency lasers. Saturable absorbers can use the standing wave effect to form narrowband dynamic gratings to suppress side modes and achieve linewidth reduction, but they often require longer doped fibers, resulting in high insertion loss and reduced laser output performance. High-Q microring resonators facilitate the construction of compact single-frequency fiber lasers, acting as mode selectors within the fiber cavity to achieve narrowband laser output, but they are generally expensive and have low output power.

[0004] In addition, the common self-injection locking technology and the linewidth narrowing method based on backward Rayleigh scattering in optical fiber can effectively further narrow the single-frequency output laser. The self-injection locking technology mainly relies on longer delay optical fiber to increase the photon lifetime in the cavity to narrow the laser linewidth, but too long external optical fiber will inevitably couple more environmental noise, which will reduce the stability of the laser; the continuous backward Rayleigh scattering generated at different positions in the optical fiber is a randomly distributed weak feedback signal. Due to the narrow gain spectrum characteristics of Rayleigh scattering, it can be used to deeply narrow the linewidth of the laser, but the accumulation of Rayleigh scattering signals in ordinary optical fibers requires a long length, which will also cause the laser to mode hop and affect the stable operation of the laser.

[0005] Therefore, there is an urgent need for a single-frequency narrow-linewidth laser realization method that can achieve linewidth depth narrowing on the basis of single-frequency laser operation, while having high stability and good output performance. Summary of the Invention

[0006] In order to solve the defect in the prior art that deep linewidth narrowing requires a long optical fiber length, resulting in poor laser stability, the present invention provides a narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop. The randomly distributed feedback loop is used to reduce the laser frequency drift of the ring cavity. By increasing the number of round trips of the optical signal and increasing the effective length of the randomly distributed feedback structure, a narrowband gain peak can be guaranteed, a stable narrow-linewidth single-frequency laser output can be obtained, and the efficiency and environmental sensitivity of linewidth narrowing can be improved.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions: a narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop, comprising a pump source, a wavelength division multiplexer, an erbium-doped fiber, an optical bandpass filter, a first optical coupler, a second optical coupler, a polarization controller, an optical isolator, an optical circulator, a third optical coupler, and a randomly distributed feedback structure;

[0008] The wavelength division multiplexer, the erbium-doped optical fiber, the optical bandpass filter, the optical coupler 1, the optical coupler 2, the polarization controller, and the optical isolator are connected in sequence and end-to-end to form a ring cavity. The wavelength division multiplexer adopts a 1-to-2 wavelength division multiplexer. The pump source is connected to the surplus input end of the wavelength division multiplexer. The optical coupler 3 adopts a 2×2 coupler. The randomly distributed feedback structure is connected to an input end and an output end of the optical coupler 3 to form an optical fiber ring to form a randomly distributed feedback loop. The surplus output end of the optical coupler 3 is idle. The optical coupler 1 adopts a 1×2 coupler, and the optical coupler 2 adopts a 2×2 coupler. The optical circulator has three ports, the first port and the third port of which are respectively connected to the surplus output end of the optical coupler 1 and the surplus input end of the optical coupler 2 to form a self-injection structure. The second port of the optical circulator is connected to the surplus input end of the optical coupler 3. The surplus output end of the optical coupler 2 serves as the output end of the ring cavity.

[0009] Furthermore, the randomly distributed feedback structure adopts randomly distributed fiber gratings.

[0010] Furthermore, the randomly distributed fiber grating randomly writes 5 to 8 sub-gratings on a 40 to 80 cm single-mode optical fiber. The central wavelength of a single grating is 1550 nm, the reflectivity is 3% to 5%, and the intervals between adjacent sub-gratings are uniformly and randomly distributed between 2 and 5 cm.

[0011] Furthermore, the randomly distributed feedback structure adopts enhanced Rayleigh scattering optical fiber.

[0012] Furthermore, the enhanced Rayleigh scattering optical fiber is a germanium-doped quartz optical fiber with a numerical aperture of 0.35.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. The present invention adopts a randomly distributed feedback structure, which can use a randomly distributed fiber Bragg grating with weak reflectivity or an enhanced Rayleigh scattering fiber to shorten the feedback length and reduce the random mode density. It can increase the scattering density based on deep refractive index modulation, ensure a narrowband gain peak, and achieve linewidth compression while avoiding the problem of unstable laser intensity caused by excessive random mode density, which is conducive to obtaining stable narrow-linewidth single-frequency laser output.

[0015] 2. The present invention can reduce the laser frequency drift of the ring cavity through the randomly distributed feedback loop. The function of the randomly distributed feedback structure is to allow multiple random modes of lasing. When the temperature or sound wave changes, the new resonant mode will be quickly established with low noise. The resonant laser condition will remain unchanged in the existing random mode, reducing frequency drift. At the same time, the randomly distributed feedback loop increases the number of round trips of the optical signal in the randomly distributed feedback structure, which is equivalent to increasing the effective length of the randomly distributed feedback structure, greatly improving the efficiency of linewidth narrowing.

[0016] 3. The present invention adopts a randomly distributed feedback loop combined with a self-injection structure to avoid the problem that the linewidth reduction efficiency of traditional self-injection locking technology depends on a longer external optical fiber, thereby coupling more environmental noise, thereby improving environmental sensitivity and facilitating high-stability operation of the laser. At the same time, the self-injection locking technology can lock the resonant mode of the ring cavity to the narrowband gain peak of the randomly distributed feedback structure, and obtain the maximum gain through the erbium-doped optical fiber in the ring cavity, which can effectively suppress the side mode and ensure the laser output performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic structural diagram of the laser of the present invention;

[0018] Figure 2 It is a schematic diagram of the principle of the random distributed feedback structure in the present invention;

[0019] Figure 3 is a schematic structural diagram of the laser in Example 1;

[0020] Figure 4 It is a structural diagram of the laser in the second embodiment.

[0021] In the figure: 1. Pump source; 2. Wavelength division multiplexer; 3. Erbium-doped fiber; 4. Optical bandpass filter; 5. Optical coupler 1; 6. Optical coupler 2; 7. Polarization controller; 8. Optical isolator; 9. Optical circulator; 10. Optical coupler 3; 11. Randomly distributed feedback structure; 12. Randomly distributed fiber Bragg grating; 13. Enhanced Rayleigh scattering fiber. DETAILED DESCRIPTION

[0022] The technical solutions of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0023] like Figures 1 and 2 As shown, a narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop includes a pump source 1, a wavelength division multiplexer 2, an erbium-doped fiber 3, an optical bandpass filter 4, an optical coupler 1 5, an optical coupler 2 6, a polarization controller 7, an optical isolator 8, an optical circulator 9, an optical coupler 3 10 and a randomly distributed feedback structure 11.

[0024] Combine Figure 1 As shown, the wavelength division multiplexer 2 adopts a 1-to-2 wavelength division multiplexer, the optical coupler 1 5 adopts a 1×2 coupler, the optical coupler 2 6 and the optical coupler 3 10 adopt 2×2 couplers, and the optical circulator 9 has three ports. The output end of the wavelength division multiplexer 2 is connected to the input end of the erbium-doped fiber 3, the output end of the erbium-doped fiber 3 is connected to the input end of the optical bandpass filter 4, the output end of the optical bandpass filter 4 is connected to the input end of the optical coupler 1 5, one output end of the optical coupler 1 5 is connected to an input end of the optical coupler 2 6, one output end of the optical coupler 2 6 is connected to the input end of the polarization controller 7, and the laser output and polarization re-injected into the ring cavity from the third port of the optical circulator 9 are controlled. The output end of the polarization controller 7 is connected to the input end of the optical isolator 8 to ensure unidirectional transmission of light in the ring cavity. The output end of the optical isolator 8 is connected to an input end of the wavelength division multiplexer 2. The ring cavity is formed by the wavelength division multiplexer 2, the erbium-doped fiber 3, the optical bandpass filter 4, the optical coupler 1 5, the optical coupler 2 6, the polarization controller 7 and the optical isolator 8, which are connected in sequence end to end. In addition, the pump source 1 is connected to the other input end of the wavelength division multiplexer 2, the first port and the third port of the optical circulator 9 are respectively connected to the other output end of the optical coupler 1 5 and the other input end of the optical coupler 2 6 to form a self-injection structure, the second port of the optical circulator 9 is connected to an input end of the optical coupler 3 10, and one output end of the optical coupler 3 10 is idle. The self-injection structure is used to re-inject the signal after the ring cavity enters the randomly distributed feedback loop and is filtered into the ring cavity, and the effect of line width depth narrowing is achieved through multiple cycles. The randomly distributed feedback structure 11 is connected to the other input end and the other output end of the optical coupler 3 10 to form an optical fiber ring to form a randomly distributed feedback loop. The other output end of the optical coupler 2 6 serves as the output end of the ring cavity.

[0025] Among them, the laser emitted by the pump source 1 is input to the erbium-doped fiber 3 through the wavelength division multiplexer 2, and the particle number in the pump erbium-doped fiber 3 is reversed to generate stimulated radiation. The stimulated radiation light is incident on the optical bandpass filter 4, and a specific output wavelength is selected from the broadband light source. After preliminary frequency selection, the optical signal enters the optical coupler 1 5, enters the first port of the optical circulator 9 from the other output end of the optical coupler 1 5, and then is transmitted from the second port of the optical circulator 9 into the randomly distributed feedback loop. The roughly selected optical signal is further filtered in the randomly distributed feedback structure 11 and is filtered in the optical coupler 3 10 and the randomly distributed feedback structure 11. The optical signal is filtered multiple times in the optical fiber ring where it is located. The filtered optical signal is output from the randomly distributed feedback loop, transmitted from the second port to the third port of the optical circulator 9, and injected into the ring cavity again through the optical coupler 2 6. After the polarization state of the light is adjusted by the polarization controller 7, it passes through the optical isolator 8 and then enters the wavelength division multiplexer 2 again. The narrowed light is continuously circulated and amplified by the gain of the erbium-doped optical fiber 3, and then the line width is narrowed multiple times through the randomly distributed feedback loop and then self-injected back into the ring cavity, finally achieving stable laser oscillation, and obtaining a narrow linewidth single-frequency laser output from the other output end of the optical coupler 2 6 which is the output end of the ring cavity.

[0026] The following is an experimental method for outputting laser light according to the present invention: a laser beam emitted by a 976nm pump source 1 is passed through a 980nm / 1550nm wavelength division multiplexer 2 into an erbium-doped fiber 3, causing a population inversion in the erbium-doped fiber 3 to stimulate stimulated emission and generate broadband initial signal light. This signal light is longitudinally mode-selected by a broadband optical bandpass filter 4 and continuously circulated and amplified in a ring cavity. When the gain of the erbium-doped fiber 3 is consistent with the loss of the ring cavity, a stable broadband laser output is formed in the ring cavity. Figure 2 As shown in the figure, the black envelope contains multiple laser modes, and the free spectrum range is determined by the cavity length of the ring cavity. The stable broadband laser output enters the randomly distributed feedback loop through the optical coupler 5 and the optical circulator 9, and circulates in it. Due to the reflection filtering characteristics of the randomly distributed feedback structure 11, the reflected light generated after multiple passes through the randomly distributed feedback structure 11 in the fiber ring is transmitted from the second port of the optical circulator 9 to the third port. Its reflection spectrum is Figure 2 The green envelope in the image is a narrow peak with a fixed free spectral range, which further filters the broadband initial signal. The third port of the optical circulator 9 is connected to the input of the 2×2 optical coupler 6, allowing the optical signal, which has been further filtered by the randomly distributed feedback loop, to return to the ring cavity for gain amplification. The narrowband gain peak of the randomly distributed feedback loop forcibly locks the ring resonance peak, suppressing the longitudinal mode compression of the laser linewidth and reducing laser frequency drift.

[0027] The self-injection locking structure of the randomly distributed feedback loop is further elaborated, combined with Figure 2As shown, for the optical coupler 3 10, light propagates from port (3) to port (4) / (2), and from port (4) to port (3) / (1). The optical signal E1 for preliminary longitudinal mode selection in the ring cavity enters the optical fiber ring from port (1) of the optical coupler 3 10 and passes through the randomly distributed feedback structure 11. Ports (3) and (4) of the optical coupler 3 10 are connected to the two ends of the randomly distributed feedback structure 11. The optical signal E1 propagates in the random distributed feedback loop for many times, and the back-reflected light E2 generated by it is reflected by port (1) of the optical coupler 3 10. The right side of the figure shows that the black envelope is the broadband laser envelope output from the ring cavity, B is the corresponding laser gain in the ring cavity, the blue line represents the multiple longitudinal modes in the ring cavity, C is the corresponding cavity mode interval, and A represents the free spectral range of the reflection peak of the randomly distributed feedback loop. It can be seen that when the frequency of the randomly distributed feedback loop reflected back to the ring cavity resonates with the gain bandwidth, the self-injection locking of the randomly distributed feedback loop forces the narrow gain peak of the randomly distributed feedback structure 11 to be locked to the resonance peak of the ring cavity, effectively suppressing the side mode and selecting the narrowband longitudinal mode with the highest gain, effectively narrowing the linewidth of the laser signal. At the same time, the random mode and self-injection locking established by the randomly distributed feedback structure 11 reduce the frequency drift of the laser.

[0028] The randomly distributed feedback structure 11 can be a randomly distributed fiber Bragg grating 12 or an enhanced Rayleigh scattering fiber 13. Two embodiments are given below:

[0029] Example 1

[0030] Combine Figure 3As shown, this embodiment uses a randomly distributed fiber grating 12 as a randomly distributed feedback structure 11. The randomly distributed fiber grating 12 uses a femtosecond laser to randomly write 5 to 8 sub-gratings on a 40 to 80 cm single-mode optical fiber. The central wavelength of a single grating is 1550 nm, the reflectivity is 3% to 5%, and the intervals between adjacent sub-gratings are uniformly and randomly distributed between 2 and 5 cm. The laser emitted by the 976nm pump source 1 enters the ring cavity through the wavelength division multiplexer 2, continuously pumping the erbium-doped fiber 3, causing it to produce a population inversion and generate stimulated emission laser. The initial signal light generated is preliminarily selected in the longitudinal mode by the broadband optical bandpass filter 4 and amplified multiple times in the ring cavity to obtain a stable broadband laser output. The output laser is transmitted through the optical coupler 1 5 and the optical circulator 9 to the randomly distributed feedback loop composed of the randomly distributed fiber grating 12 and the optical coupler 3 10. The broadband signal light is reflected by the random distributed fiber grating 12 multiple times and circulates in the optical fiber ring. After the longitudinal mode is suppressed, it is reflected back to the optical circulator 9 through the optical coupler 3 10. The optical circulator 9 is connected to the optical coupler 2 6 in the ring cavity, so that the optical signal after filtering by the random distributed feedback loop is injected into the ring cavity again for gain amplification, and the narrow reflection peak of the random distributed feedback loop is locked with the resonance peak of the ring cavity to achieve narrow linewidth single-frequency laser output.

[0031] Example 2

[0032] Combine Figure 4As shown, this embodiment uses enhanced Rayleigh scattering fiber 13 as the randomly distributed feedback structure 11. This enhanced Rayleigh scattering fiber 13 uses germanium-doped quartz fiber with a numerical aperture of 0.35. Rayleigh scattering in an optical fiber is a continuous, randomly distributed weak feedback signal with a narrowband gain spectrum of approximately 20 kHz for the seed light signal. Conventional single-mode optical fiber typically requires hundreds of meters or even kilometers of length to accumulate sufficient backscattered Rayleigh signal. However, by using enhanced Rayleigh scattering fiber 13, the fiber length can be shortened to 50 meters, effectively improving the stability of laser operation. The principle of this embodiment is similar to that of the first embodiment. Similarly, laser light emitted by a 976 nm pump source 1 enters the ring cavity through a wavelength division multiplexer 2, continuously pumping the erbium-doped fiber 3, causing it to generate a population inversion and produce stimulated emission laser light. The generated initial signal light undergoes preliminary longitudinal mode selection through a broadband optical bandpass filter 4 and is circulated and amplified multiple times within the ring cavity to obtain stable broadband laser output. The output laser is transmitted through an optical coupler 1 5 and an optical circulator 9 to a randomly distributed feedback loop consisting of an enhanced Rayleigh scattering fiber 13 and an optical coupler 3 10. The broadband signal light is reflected multiple times by the enhanced Rayleigh scattering fiber 13 and circulates in the fiber ring, and the longitudinal mode is suppressed. After that, it is reflected back to the optical circulator 9 through the optical coupler 3 10. The optical circulator 9 is connected to the optical coupler 2 6 within the ring cavity. As a result, the optical signal filtered by the randomly distributed feedback loop is re-injected into the ring cavity for gain amplification, and the narrow reflection peak of the randomly distributed feedback loop is locked with the resonance peak of the ring cavity, achieving narrow linewidth single-frequency laser output.

[0033] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other configurations without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations coming within the meaning and range of equivalents of the claims are intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0034] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A narrow-linewidth fiber laser based on self-injection locking using a randomly distributed feedback loop, characterized by: The invention comprises a pump source (1), a wavelength division multiplexer (2), an erbium-doped optical fiber (3), an optical bandpass filter (4), an optical coupler 1 (5), an optical coupler 2 (6), a polarization controller (7), an optical isolator (8), an optical circulator (9), an optical coupler 3 (10), and a randomly distributed feedback structure (11); The wavelength division multiplexer (2), the erbium-doped optical fiber (3), the optical bandpass filter (4), the optical coupler 1 (5), the optical coupler 2 (6), the polarization controller (7) and the optical isolator (8) are connected in sequence and end to end to form a ring cavity, the wavelength division multiplexer (2) adopts a 1-to-2 wavelength division multiplexer, the pump source (1) is connected to the remaining input end of the wavelength division multiplexer (2), the optical coupler 3 (10) adopts a 2×2 coupler, and the randomly distributed feedback structure (11) is connected to one input end and one output end of the optical coupler 3 (10). The output ends are connected to form an optical fiber ring to form a randomly distributed feedback loop. The remaining output end of the optical coupler three (10) is idle. The optical coupler one (5) adopts a 1×2 coupler, and the optical coupler two (6) adopts a 2×2 coupler. The optical circulator (9) has three ports, the first port and the third port of which are respectively connected to the remaining output end of the optical coupler one (5) and the remaining input end of the optical coupler two (6) to form a self-injection structure, the second port of which is connected to the remaining input end of the optical coupler three (10), and the remaining output end of the optical coupler two (6) serves as the output end of the ring cavity.

2. The narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop according to claim 1, characterized in that: The randomly distributed feedback structure (11) adopts a randomly distributed fiber grating (12).

3. The narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop according to claim 2, characterized in that: The randomly distributed fiber grating (12) randomly writes 5 to 8 sub-gratings on a 40 to 80 cm single-mode optical fiber. The central wavelength of a single grating is 1550 nm, the reflectivity is 3% to 5%, and the intervals between adjacent sub-gratings are uniformly and randomly distributed between 2 and 5 cm.

4. The narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop according to claim 1, characterized in that: The randomly distributed feedback structure (11) adopts enhanced Rayleigh scattering optical fiber (13).

5. The narrow-linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop according to claim 4, characterized in that: The enhanced Rayleigh scattering optical fiber (13) is a germanium-doped quartz optical fiber with a numerical aperture of 0.35.

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