Narrow linewidth fiber laser based on random distribution feedback loop self-injection locking

Abstract

The invention discloses a narrow linewidth fiber laser based on random distribution feedback loop self-injection locking, and relates to the technical field of fiber lasers. The wavelength division multiplexer, the erbium-doped optical fiber, the optical band-pass filter, the optical coupler I, the optical coupler II, the polarization controller and the optical isolator are sequentially connected end to end to form an annular cavity, the pumping source is connected with the wavelength division multiplexer, and the random distribution feedback structure is connected with the optical coupler III to form an optical fiber ring to form a random distribution feedback ring. A first port and a third port of the optical circulator are connected with the first optical coupler and the second optical coupler respectively to form a self-injection structure, a second port of the optical circulator is connected with the third optical coupler, and the remaining output end of the second optical coupler serves as the output end of the annular cavity. The laser frequency drift of the annular cavity can be reduced by utilizing the random distribution feedback ring, the effective length of the random distribution feedback structure is increased by increasing the round-trip times of optical signals, a narrow-band gain peak can be ensured, stable narrow-linewidth single-frequency laser output can be obtained, and the linewidth narrowing efficiency and the environmental sensitivity are improved.

Description

Technical Field

[0001] The 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] Single-frequency lasers with narrow linewidth have very important application value in the fields of high-precision metrology, coherent optical communications, fiber-optic sensing and quantum technology. Narrow linewidth fiber lasers have the advantages of high coherence, low phase noise and high stability, and have become an indispensable laser light source in the field of optical information science.

[0003] Narrow linewidth single-frequency fiber laser is a method of adding additional narrow linewidth on the basis of realizing the operation of single-frequency optical signal in the resonant cavity. Among the common methods of narrowing the linewidth: narrowband filtering can greatly limit the gain range of the longitudinal mode in the cavity, making it smaller than the free spectrum range of the laser, thereby realizing the single-frequency output of the laser. However, the bandwidth of filter devices such as narrowband phase-shifted fiber gratings and high-fineness FP etalons is usually limited by the processing accuracy, and can only achieve single longitudinal mode selection, and cannot further narrow the linewidth of the single-frequency laser; saturable absorbers can use the standing wave effect to form narrowband dynamic gratings to suppress side modes and achieve linewidth narrowing, but they often require longer doped fibers, which leads to high insertion loss and reduces the output performance of the laser; high-Q microring resonators are convenient for constructing compact single-frequency fiber lasers, and as a mode selector in the fiber cavity to achieve narrowband laser output, but they are usually 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 and 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 longer 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 method for realizing single-frequency narrow-linewidth laser that can achieve linewidth depth narrowing based on single-frequency laser operation and has high stability and good output performance. Summary of the invention

[0006] In order to solve the defect in the prior art that a long optical fiber length is required for deep linewidth narrowing, resulting in poor laser stability, the present invention provides a narrow-linewidth optical fiber laser based on self-injection locking of a randomly distributed feedback loop, which utilizes a randomly distributed feedback loop to reduce the laser frequency drift of the ring cavity, and increases the effective length of the randomly distributed feedback structure by increasing the number of round trips of the optical signal, thereby ensuring a narrowband gain peak, obtaining a stable narrow-linewidth single-frequency laser output, and improving the efficiency and environmental sensitivity of linewidth narrowing.

[0007] To achieve the above-mentioned object, the present invention adopts the following technical scheme: 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, an optical coupler 1, an optical coupler 2, a polarization controller, an optical isolator, an optical circulator, an optical coupler 3 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 excess 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 a fiber ring to form a randomly distributed feedback loop. The excess 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 excess output end of the optical coupler 1 and the excess input end of the optical coupler 2 to form a self-injection structure, the second port of which is connected to the excess input end of the optical coupler 3, and the excess output end of the optical coupler 2 serves as the output end of the ring cavity.

[0009] Furthermore, the randomly distributed feedback structure uses 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 adopts germanium-doped quartz optical fiber, and its numerical aperture value is 0.35.

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

[0014] 1. The present invention uses a randomly distributed feedback structure, and can use a randomly distributed fiber 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 to ensure a narrowband gain peak. It can achieve line width compression while avoiding the problem of unstable laser intensity caused by excessive random mode density, which is conducive to obtaining a stable narrow line width single-frequency laser output;

[0015] 2. The present invention can reduce the laser frequency drift of the ring cavity through a randomly distributed feedback loop. The function of the randomly distributed feedback structure is to allow multiple random modes of lasers. When the temperature or sound wave changes, a new resonant mode will be quickly established with low noise, and the resonant laser condition will remain unchanged in the existing random mode, reducing the 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 line width narrowing;

[0016] 3. The present invention adopts a randomly distributed feedback loop combined with a self-injection structure to avoid the problem that the line width reduction efficiency of the 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 resonant mode of the ring cavity can be locked to the narrowband gain peak of the randomly distributed feedback structure through the self-injection locking technology, and the maximum gain can be obtained 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 diagram of the structure 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 diagram of the structure of the laser in Example 1;

[0020] Figure 4 It is a schematic diagram of the structure 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 grating; 13. Enhanced Rayleigh scattering fiber. DETAILED DESCRIPTION

[0022] The technical solution 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 of 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 Figure 1-2 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] Combination 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, 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, and 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 in sequence and connected 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 an 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 obtained 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 optical fiber 3 through the wavelength division multiplexer 2, the particle number in the pump erbium-doped optical 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. The optical signal after preliminary frequency selection 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, and the filtered optical signal is output from the randomly distributed feedback loop, transmitted from the second port of the optical circulator 9 to the third port, 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 enters the wavelength division multiplexer 2 again through the optical isolator 8. The narrowed light is continuously amplified by the gain of the erbium-doped optical fiber 3, and then the line width is narrowed by the randomly distributed feedback loop for multiple times, and then it is self-injected back into the ring cavity, finally achieving stable laser oscillation, and obtaining a single-frequency laser output with a narrow line width 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 lasers of the present invention: a laser emitted by a 976nm pump source 1 enters an erbium-doped optical fiber 3 through a 980nm / 1550nm wavelength division multiplexer 2, causing the number of particles in the erbium-doped optical fiber 3 to be reversed and stimulated to emit stimulated radiation to generate broadband initial signal light, which is selected by a broadband optical bandpass filter 4 for longitudinal mode selection and continuously amplified in a ring cavity. When the gain of the erbium-doped optical 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, it is the black envelope in the figure, which contains multiple laser modes, and the free spectrum range is determined by the cavity length of the ring cavity. The stable output broadband laser enters the random distribution feedback loop through the optical coupler 5 and the optical circulator 9, and circulates in it. Due to the reflection filtering characteristics of the random distribution feedback structure 11, the reflected light generated after passing through the random distribution feedback structure 11 in the optical fiber ring for multiple times is transmitted to the third port through the second port of the optical circulator 9, and its reflection spectrum is Figure 2 The green envelope in is a narrow peak with a fixed free spectral range, which can further filter the broadband initial signal. The third port of the optical circulator 9 is connected to the input of the 2×2 optical coupler 2 6, so that the optical signal further filtered by the random distribution feedback loop returns to the ring cavity for gain amplification, and the ring resonance peak is forcibly locked by the narrowband gain peak of the random distribution feedback loop, so as to suppress the longitudinal mode compression laser line width and reduce the 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 respectively adopt a randomly distributed fiber grating 12 or an enhanced Rayleigh scattering fiber 13, and two embodiments are given as follows:

[0029] Embodiment 1

[0030] Combination 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 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, and continuously pumps the erbium-doped fiber 3 to generate a population inversion to generate stimulated emission laser. The generated initial signal light is preliminarily selected by the broadband optical bandpass filter 4 for longitudinal mode selection and amplified multiple times in the ring cavity to obtain a stable broadband laser output. The output laser is transmitted to the randomly distributed feedback loop composed of the randomly distributed fiber grating 12 and the optical coupler 3 10 through the optical coupler 1 5 and the optical circulator 9. The broadband signal light is reflected by the randomly 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 filtered by the randomly distributed feedback loop is injected into the ring cavity again for gain amplification, and the narrow reflection peak of the randomly distributed feedback loop is locked with the resonance peak of the ring cavity to achieve narrow linewidth single-frequency laser output.

[0031] Embodiment 2

[0032] Combination Figure 4As shown, this embodiment uses enhanced Rayleigh scattering fiber 13 as the randomly distributed feedback structure 11, and the enhanced Rayleigh scattering fiber 13 uses germanium-doped quartz fiber, and its numerical aperture value is 0.35. Rayleigh scattering in the optical fiber is a continuous randomly distributed weak feedback signal, which has a narrowband gain spectrum of about 20kHz for the seed light signal. In ordinary single-mode optical fibers, it usually takes hundreds of meters or even kilometers to accumulate enough backward Rayleigh scattering signals. By using enhanced Rayleigh scattering fiber 13, the optical fiber length can be shortened to 50m, effectively improving the stability of laser operation. The principle of this embodiment is similar to that of the first embodiment. Similarly, the laser emitted by the 976nm pump source 1 enters the ring cavity through the wavelength division multiplexer 2, and continuously pumps the erbium-doped fiber 3 to generate a population inversion to generate stimulated emission laser. The generated initial signal light is preliminarily selected by the broadband optical bandpass filter 4 for longitudinal mode selection and amplified multiple times in the ring cavity to obtain a stable broadband laser output. The output laser is transmitted to the randomly distributed feedback loop composed of the enhanced Rayleigh scattering fiber 13 and the optical coupler 3 10 through the optical coupler 1 5 and the optical circulator 9. The broadband signal light is reflected by the enhanced Rayleigh scattering fiber 13 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 filtered by the randomly distributed feedback loop is injected into the ring cavity again for gain amplification, and the narrow reflection peak of the randomly distributed feedback loop is locked with the resonance peak of the ring cavity to achieve narrow linewidth single-frequency laser output.

[0033] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other forms of assembly without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations within the meaning and range of equivalents of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0034] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description 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 may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A narrow linewidth fiber laser based on self-injection locking of a randomly distributed feedback loop, characterized in that: It includes 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. 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 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 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. A 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 optical fiber grating (12).

3. A 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) uses 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 value of 0.35.

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