An ultra-narrow linewidth ring fiber laser based on a random distributed feedback structure

Abstract

An ultra-narrow linewidth ring fiber laser based on a randomly distributed feedback structure relates to the technical field of narrow linewidth fiber lasers. A wavelength division multiplexer, erbium-doped fiber, randomly distributed fiber gratings, an isolator, a polarization controller, a circulator, and a coupler are sequentially connected end to end to form a ring cavity. The wavelength division multiplexer uses a 1-to-2 wavelength division multiplexer. The pump source is connected to the remaining input end of the wavelength division multiplexer, one end of the enhanced scattering fiber is connected to the circulator, and a light reflector is connected to the other end of the enhanced scattering fiber. The coupler uses a 1×2 coupler, and the remaining output end serves as the output end of the ring cavity. By arranging randomly distributed fiber gratings and enhanced scattering fibers to form a randomly distributed feedback structure, and connecting a light reflector to the end of the enhanced scattering fiber, the feedback gain is increased, and deep compression and stable narrow linewidth output of a single-frequency fiber laser can be achieved, thereby improving the utilization rate of the pump light and lowering the threshold.

Description

Technical Field

[0001] The present invention relates to the technical field of narrow linewidth fiber lasers, in particular to an ultra-narrow linewidth ring fiber laser based on a random distributed feedback structure. Background Art

[0002] The output of a single-frequency laser source with an ultra-narrow linewidth is of great significance for applications such as optical sensing, lidar, gravitational wave detection, and the next generation of coherent optical communications. Single-frequency fiber lasers with ultra-narrow linewidth have extremely low phase noise and an extremely long coherence length. However, the linewidth of conventional fiber lasers is typically in the kilohertz range and above, which is increasingly difficult to meet the detection distance requirements in applications such as remote sensing and ranging, affecting the receiver sensitivity and detection accuracy requirements in coherent detection systems. Therefore, there is an urgent need for a design solution for an ultra-narrow linewidth laser with a linewidth in the hundreds of hertz range that can operate stably with high gain characteristics.

[0003] Traditional narrowband filtering and short cavity methods can only achieve single-frequency operation of the laser but cannot further narrow the laser linewidth. With the continuous development of optical devices, the method based on external cavity optical feedback has become the most promising linewidth compression mechanism, which includes single external cavity optical feedback, resonant external cavity optical feedback and randomly distributed external cavity feedback. The narrowing methods of single external cavity optical feedback and resonant external cavity optical feedback are relatively complex in structure, which will increase the loss of the laser and affect the stability. Randomly distributed external cavity feedback mainly utilizes the random distribution characteristics of randomly distributed fiber grating structures and enhanced Rayleigh scattering fibers to achieve deep compression of the linewidth while ensuring the single longitudinal mode operation of the laser. It can achieve stable narrow-linewidth laser output without the need for additional filters, and has excellent performance in terms of spectral bandwidth and stability. However, the spectrum of randomly distributed external cavity feedback is random and will lead to the problem of reduced laser output power, requiring precise control of device parameters.

[0004] Therefore, taking into account the precise control of linewidth narrowing and high-gain laser output by the random distributed feedback structure, an effective method for realizing high-power ultra-narrow linewidth single-frequency fiber laser is proposed, which has important scientific significance and practical value. Summary of the Invention

[0005] To address the shortcomings of the background technology, the present invention provides an ultra-narrow linewidth ring fiber laser based on a randomly distributed feedback structure. The randomly distributed feedback structure is composed of randomly distributed fiber gratings and enhanced scattering fibers, and a light reflector is connected to the end of the enhanced scattering fiber to improve the feedback gain. This can achieve deep compression and stable narrow linewidth output of single-frequency fiber lasers, improve the utilization rate of pump light, and lower the threshold.

[0006] To achieve the above objectives, the present invention adopts the following technical solutions: an ultra-narrow linewidth ring fiber laser based on a randomly distributed feedback structure, comprising a pump source, a wavelength division multiplexer, an erbium-doped fiber, a randomly distributed fiber Bragg grating, an isolator, a polarization controller, a circulator, an enhanced scattering fiber, a light reflector, and a coupler;

[0007] The wavelength division multiplexer, the erbium-doped optical fiber, the randomly distributed optical fiber Bragg grating, the isolator, the polarization controller, the circulator and the coupler 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 remaining input end of the wavelength division multiplexer, one end of the enhanced scattering optical fiber is connected to the circulator, and the optical reflector is connected to the other end of the enhanced scattering optical fiber. The coupler adopts a 1×2 coupler and the remaining output end serves as the output end of the ring cavity.

[0008] Furthermore, the randomly distributed fiber Bragg grating has a central wavelength of 1550 nm and a reflectivity of 3% to 5%, and serves as a transmission filter of the ring cavity to further select / filter the light in the ring cavity and the light fed back by the enhanced scattering fiber.

[0009] Furthermore, the enhanced scattering optical fiber uses an optical fiber with a backscattering coefficient higher than that of SMF-28e as a reflection filter.

[0010] Furthermore, the enhanced scattering optical fiber is selected from 50m UHNA3 or 110m tapered optical fiber.

[0011] Furthermore, the coupler is divided into two output ends, 90% and 10%, the 90% output end of the coupler is connected to the wavelength division multiplexer, and the 10% output end of the coupler serves as the output end of the ring cavity.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] 1. The present invention can pre-filter the laser by arranging randomly distributed fiber Bragg gratings in a ring cavity. Together with the enhanced scattering fiber outside the ring cavity, a randomly distributed feedback structure is formed. The seed light, which is initially frequency-selected by the randomly distributed fiber Bragg grating, then returns to the ring cavity through the enhanced scattering fiber with Rayleigh gain for multiple oscillation amplification, thereby achieving deep compression of single-frequency fiber laser.

[0014] 2. The present invention uses a polarization controller to finely adjust polarization rotation, which can accurately control the two random structures of randomly distributed fiber Bragg gratings and enhanced scattering fiber to achieve selective resonant tunneling effect, forming a sharp narrow linewidth peak in lasing and realizing stable narrow linewidth output;

[0015] 3. The present invention improves the gain value per unit length of erbium-doped optical fiber, thereby achieving effective amplification of laser light at a shorter length. At the same time, a light reflector is connected to the end of the enhanced scattering optical fiber, so that the seed light amplifies the stimulated Rayleigh scattered light again and returns to the ring cavity, which can further improve the utilization rate of the pump light and lower the threshold. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic structural diagram of the laser of the present invention;

[0017] Figure 2 This is a schematic diagram of the principle of longitudinal mode suppression of the enhanced scattering optical fiber in the laser of the present invention;

[0018] Figure 3 It is the Rayleigh gain spectrum of the seed light extracted by the enhanced scattering optical fiber in the laser of the present invention.

[0019] In the figure: 1. Pump source; 2. Wavelength division multiplexer; 3. Erbium-doped fiber; 4. Randomly distributed fiber Bragg gratings; 5. Isolator; 6. Polarization controller; 7. Circulator; 8. Enhanced scattering fiber; 9. Optical reflector; 10. Coupler. DETAILED DESCRIPTION

[0020] 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.

[0021] like Figures 1 to 3 As shown, an ultra-narrow linewidth ring fiber laser based on a randomly distributed feedback structure includes a pump source 1, a wavelength division multiplexer 2, an erbium-doped fiber 3, a randomly distributed fiber grating 4, an isolator 5, a polarization controller 6, a circulator 7, an enhanced scattering fiber 8, a light reflector 9 and a coupler 10.

[0022] Combine Figure 1As shown, the wavelength division multiplexer 2 adopts a 1-to-2 wavelength division multiplexer, the circulator 7 is provided with 3 ports, and the coupler 10 adopts a 1×2 coupler. The output end of the wavelength division multiplexer 2 is connected to the input end of the erbium-doped fiber 3, the length and doping concentration of the erbium-doped fiber 3 are selected as needed, the output end of the erbium-doped fiber 3 is connected to the input end of the randomly distributed fiber Bragg grating 4, the output end of the randomly distributed fiber Bragg grating 4 is connected to the input end of the isolator 5, the output end of the isolator 5 is connected to the input end of the polarization controller 6, the output end of the polarization controller 6 is connected to the first port of the circulator 7, the third port of the circulator 7 is connected to the input end of the coupler 10, and the coupler 10 has two output ends, 90% and 10%. The 90% output end of the coupler 10 is connected to the second input end of the wavelength division multiplexer 2. The wavelength division multiplexer 2, the erbium-doped fiber 3, the randomly distributed fiber grating 4, the isolator 5, the polarization controller 6, the circulator 7 and the coupler 10 are connected in sequence and end to end to form a ring cavity. The 10% output end of the coupler 10 serves as the output end of the ring cavity. In addition, the output end of the pump source 1 is connected to the first input end of the wavelength division multiplexer 2, one end of the enhanced scattering fiber 8 is connected to the second port of the circulator 7, and the light reflector 9 is connected to the other end of the enhanced scattering fiber 8.

[0023] Among them, the laser emitted by the pump source 1 is input into the erbium-doped fiber 3 through the wavelength division multiplexer 2, so that the number of particles in the erbium-doped fiber 3 is reversed to stimulate stimulated radiation to generate broadband seed light, which is then incident on the randomly distributed fiber Bragg grating 4. The transmission and reflection of the grating end face inscribed in the randomly distributed fiber Bragg grating 4 perform preliminary frequency selection on the seed light. The seed light after coarse frequency selection passes through the isolator 5 and the polarization controller 6 and enters the enhanced scattering fiber 8 from the second port of the circulator 7. The seed light in the ring cavity enters the enhanced scattering fiber 8 to generate randomly distributed feedback light. The isolator 5 ensures unidirectional transmission of the laser in the ring cavity. The randomly distributed fiber Bragg grating 4 and the enhanced scattering fiber 8 form a randomly distributed feedback structure for laser mode selection and The linewidth is narrowed, and the polarization controller 6 is used to adjust the matching of the reflection spectrum of the randomly distributed fiber grating 4 and the gain spectrum of the enhanced scattering fiber 8, providing a selective resonance tunnel effect to achieve a stable filtering effect. The backward Rayleigh scattered light fed back after passing through the enhanced scattering fiber 8 enters the ring cavity through the third port of the circulator 7 to narrow the linewidth of the seed light. The remaining seed light is reflected by the light reflector 9 at the end of the enhanced scattering fiber 8 and enters the enhanced scattering fiber 8 again to amplify the randomly distributed feedback light generated, thereby improving the utilization rate of the seed light. The seed light that resonates multiple times in the ring cavity outputs a stable narrow-linewidth laser through the coupler 10. At the same time, the coupler 10 completes the linewidth adjustment and power amplification of the laser.

[0024] The randomly distributed fiber Bragg gratings 4 are randomly spaced at regular intervals, with a central wavelength of 1550 nm and a low reflectivity (3% to 5%). They serve as the transmission filter for the ring cavity, further selecting / filtering the light within the ring cavity and reflected from the enhanced scattering fiber 8. For example, 2000 uniform sub-gratings randomly written at 1-2.5 μm intervals on a 5 cm optical fiber have a reflectivity of 3%. Alternatively, a randomly spaced grating string of 7-10 gratings randomly spaced at the end of a single-mode optical fiber has a central wavelength of 1549.8 nm and a reflectivity of 5%.

[0025] The enhanced scattering fiber 8 is different from ordinary single-mode optical fiber. It is an artificial waveguide structure or inhomogeneous optical fiber with a high Rayleigh scattering coefficient. As a reflection filter, an optical fiber with a back Rayleigh scattering coefficient higher than SMF-28e should be used, such as a 50m UHNA3 or a 110m tapered optical fiber.

[0026] Combine Figure 2 As shown, a large number of scattering particles are randomly distributed in the enhanced scattering fiber 8. These scattering particles can be regarded as a series of randomly distributed feedback planes. When the initial laser signal enters the randomly distributed feedback structure, it is reflected by a large number of randomly distributed feedback planes to form a feedback signal, which interacts with the injected initial signal. Due to the narrow gain spectrum characteristics of Rayleigh scattering, the feedback signal can filter the initial signal. After multiple round trips, the side mode laser of the initial signal is suppressed, and the power spectrum of the laser output becomes purer and purer, thereby achieving compression of the laser linewidth.

[0027] Combine Figure 3 As shown, it can be seen from the Rayleigh gain spectrum of the enhanced scattering fiber 8 that the gain bandwidth of the stimulated Rayleigh scattering excited by the seed light is around 24 kHz, which is much smaller than the linewidth of the ordinary single-frequency seed light. The backward Rayleigh scattered light fed back after passing through the enhanced scattering fiber 8 enters the ring cavity through the third port of the circulator 7, which can narrow the linewidth of the seed light.

[0028] The following is an experimental method for outputting laser light according to the present invention: a 980nm pump source 1 is used to enter the ring cavity through a 980 / 1550nm wavelength division multiplexer 2, and its particle number is reversed by an erbium-doped fiber 3 to generate stimulated radiation. The stimulated radiation light is incident on a randomly distributed fiber grating 4, which is inscribed by a femtosecond laser direct writing method and serves as a reflection filter. Reflection and transmission occur in the randomly distributed fiber grating 4 to realize the frequency selection process of the laser. The coarsely selected seed light enters the first port of the circulator 7, and is input into the enhanced scattering fiber 8 through the second port of the circulator 7. Stimulated Rayleigh scattering is stimulated in the enhanced scattering fiber 8 to form a randomly distributed feedback filter with a narrowband gain spectrum for the seed light. The light is further filtered, and the remaining seed light is reflected by the optical reflector 9 at the end of the enhanced scattering fiber 8 and enters the enhanced scattering fiber 8 again to amplify the randomly distributed feedback light generated. The light is incident through the second port of the circulator 7 and input into the ring cavity through the third port of the circulator 7. The peak of the reflection spectrum is adjusted by the polarization controller 6 in the ring cavity to achieve matching of the reflection spectrum of the randomly distributed fiber grating 4 and the gain spectrum of the enhanced scattering fiber 8, providing a selective resonant tunnel effect and achieving a stable filtering effect. Under the structure of the ring cavity, the stimulated radiated seed light undergoes random feedback and the feedback oscillation is continuously enhanced in the ring cavity for multiple cycles. Finally, the coupler 10 outputs a stable single-frequency laser with an ultra-narrow linewidth.

[0029] 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.

[0030] 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. An ultra-narrow linewidth ring fiber laser based on a random distributed feedback structure, characterized by: The invention comprises a pump source (1), a wavelength division multiplexer (2), an erbium-doped optical fiber (3), a randomly distributed optical fiber grating (4), an isolator (5), a polarization controller (6), a circulator (7), an enhanced scattering optical fiber (8), an optical reflector (9), and a coupler (10); The wavelength division multiplexer (2), the erbium-doped optical fiber (3), the randomly distributed optical fiber Bragg grating (4), the isolator (5), the polarization controller (6), the circulator (7) and the coupler (10) 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), one end of the enhanced scattering optical fiber (8) is connected to the circulator (7), the optical reflector (9) is connected to the other end of the enhanced scattering optical fiber (8), and the coupler (10) adopts a 1×2 coupler and the remaining output end serves as the output end of the ring cavity; The randomly distributed fiber Bragg grating (4) and the enhanced scattering fiber (8) form a randomly distributed feedback structure; the randomly distributed fiber Bragg grating (4) has a central wavelength of 1550nm and a reflectivity of 3% to 5%, and acts as a transmission filter of the ring cavity to further select / filter the light in the ring cavity and the light fed back by the enhanced scattering fiber (8); the enhanced scattering fiber (8) uses an optical fiber with a back Rayleigh scattering coefficient higher than SMF-28e as a reflection filter; the laser emitted by the pump source (1) is input to the erbium-doped fiber (3) through a wavelength division multiplexer (2) to generate broadband seed light, and the randomly distributed fiber Bragg grating ( 4) The transmission and reflection of the grating end face are used to select the frequency of the seed light. The seed light after frequency selection enters the enhanced scattering fiber (8) to generate randomly distributed feedback light. The backward Rayleigh scattered light fed back after passing through the enhanced scattering fiber (8) enters the ring cavity to narrow the line width of the seed light. The remaining seed light is reflected by the optical reflector (9) and enters the enhanced scattering fiber (8) again to amplify the generated randomly distributed feedback light. The polarization rotation is adjusted by the polarization controller (6) to control the randomly distributed feedback structure to achieve a selective resonant tunneling effect. The seed light that resonates multiple times in the ring cavity is output through the coupler (10).

2. The ultra-narrow linewidth ring fiber laser based on a random distributed feedback structure according to claim 1, characterized in that: The enhanced scattering optical fiber (8) is selected from 50m UHNA3 or 110m tapered optical fiber.

3. The ultra-narrow linewidth ring fiber laser based on a random distributed feedback structure according to claim 1, characterized in that: The coupler (10) is divided into two output ends, 90% and 10%, the 90% output end of the coupler (10) is connected to the wavelength division multiplexer (2), and the 10% output end of the coupler (10) serves as the output end of the ring cavity.

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