A random fiber laser based on cascaded fiber ring mirrors

Abstract

The present invention discloses a random fiber laser based on cascaded fiber loop mirrors, belonging to the field of fiber laser technology, comprising: a fiber Bragg grating, a pump light source, a wavelength division multiplexer, a gain fiber, a cascaded fiber loop mirror, and an isolator; the pump light source and the fiber Bragg grating are respectively connected to the input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is connected to the gain fiber, and the cascaded fiber loop mirror is respectively connected to the gain fiber and the isolator; the cascaded fiber loop mirror includes N cascaded fiber loop mirrors, and the length of the optical fiber connecting adjacent fiber loop mirrors is random, wherein N ≥ 3. The present invention uses cascaded fiber loop mirrors to provide random feedback, and the random feedback characteristics are flexible and controllable, and have the advantages of low cost and simple preparation. At the same time, the wide spectral response characteristics improve the adjustability of the laser.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber lasers, and more particularly, relates to a random optical fiber laser based on cascaded optical fiber ring mirrors. Background Art

[0002] Random fiber lasers, with their simple structure, stable performance, and low cost, are widely used in fiber-optic sensing, fiber-optic communications, and imaging. Currently, random fiber lasers are categorized into two main types based on the method of providing random feedback: utilizing Rayleigh scattering caused by inherent fiber inhomogeneities or creating artificial scattering structures.

[0003] By utilizing the Rayleigh scattering caused by the inhomogeneity of the optical fiber itself, single-mode optical fiber, polarization-maintaining optical fiber, and special optical fiber can be selected according to actual needs. However, this method usually requires a fiber length of kilometers to provide sufficient Rayleigh scattering feedback, making the entire laser large and heavy. In addition, the random feedback characteristics are determined by the optical fiber, and the adjustability is poor.

[0004] Commonly used artificial scattering structures include various random fiber grating arrays, random refractive index modulation planes, solution-filled photonic crystal fibers, etc. The scattering structures prepared in this way have good flexibility, but usually require expensive preparation equipment. Summary of the Invention

[0005] In response to the defects of the existing technology and the need for improvement, the present invention provides a random fiber laser based on a cascaded fiber ring mirror, the purpose of which is to provide a random feedback device that can flexibly control the random feedback characteristics and is simple to prepare and inexpensive.

[0006] To achieve the above object, according to one aspect of the present invention, a random fiber laser based on cascaded fiber ring mirrors is provided, comprising:

[0007] Fiber Bragg gratings, pump light sources, wavelength division multiplexers, gain fibers, cascaded fiber loop mirrors, and isolators;

[0008] The pump light source and the fiber Bragg grating are respectively connected to the input end of the wavelength division multiplexer, the output end of the wavelength division multiplexer is connected to the gain fiber, and the cascaded fiber ring mirror is respectively connected to the gain fiber and the isolator;

[0009] The cascaded fiber optic loop mirror includes N cascaded fiber optic loop mirrors, and the lengths of the optical fibers connecting adjacent fiber optic loop mirrors are random, wherein N≥3.

[0010] Furthermore, the fiber loop mirror includes a fiber coupler, two optical fibers at one end of the fiber coupler are fused together, and two optical fibers at the other end serve as the input end and output end of the fiber loop mirror respectively.

[0011] Furthermore, the optical fiber coupler is a single-mode optical fiber coupler.

[0012] Furthermore, the coupling ratio of the optical fiber coupler of each of the cascaded optical fiber loop mirrors is different.

[0013] Furthermore, the gain fiber is an erbium-doped fiber or an ytterbium-doped fiber.

[0014] Furthermore, one end of the fiber Bragg grating has an inclined cutting edge.

[0015] According to another aspect of the present invention, there is provided a method for preparing a fiber ring mirror used in the random fiber laser according to any one of the first aspects, comprising the following steps:

[0016] S1, cutting the two optical fibers at one end of the optical fiber coupler to the same target length, and fusing the cut two optical fibers together;

[0017] S2. Use the two optical fibers at the other end of the optical fiber coupler as the input end and output end of the optical fiber loop mirror, respectively, and mark the input end and the output end to obtain the optical fiber loop mirror.

[0018] Furthermore, the method further includes: generating the optical fiber lengths between adjacent optical fiber loop mirrors by generating random numbers.

[0019] Furthermore, the cutting length of one end of the optical fiber coupler of each cascaded optical fiber loop mirror is different.

[0020] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0021] (1) The random fiber laser based on cascaded fiber ring mirrors designed by the present invention forms a Fabry-Perot cavity between any two fiber ring mirrors in the multiple cascaded fiber ring mirrors. The fiber lengths between adjacent fiber ring mirrors are random, so that the multiple Fabry-Perot cavities are cascaded and can produce different reflectivities for different wavelengths. The fiber Bragg grating provides feedback at one end of the laser, and the cascaded fiber ring mirrors provide random feedback at the other end of the laser. The random feedback characteristics are determined by the number of fiber ring mirror cascades and the fiber lengths between adjacent fiber ring mirrors. The more cascades there are and the more random the fiber lengths between adjacent fiber ring mirrors, the better the random feedback characteristics of the cascaded fiber ring mirrors. The random feedback characteristics are flexible to control and easy to modify. At the same time, the cascaded fiber ring mirrors have a simple structure, and a single fiber ring mirror is low in price and small in size.

[0022] (2) Each fiber optic ring mirror designed in the present invention is prepared by fusing two optical fibers at one input end of a fiber optic coupler. It does not require expensive and complex preparation equipment, is simple to prepare, has low cost, and can provide sufficient random feedback in a small size and light weight.

[0023] (3) Preferably, the optical fiber coupler is a single-mode optical fiber coupler, which has wider applicability.

[0024] (4) Preferably, when the coupling ratios of the fiber couplers of each of the cascaded fiber loop mirrors are different, the randomness of the feedback characteristics can be improved.

[0025] (5) The cascaded fiber loop mirror designed in the present invention can adapt to different gain fibers, has a wide operating band, and has good wavelength tunability.

[0026] (6) Preferably, the inclined cut at one end of the fiber Bragg grating can avoid Fresnel reflection.

[0027] (7) The present invention also provides a method for preparing a fiber ring mirror used in the random fiber laser. The preparation method is simple and has low cost.

[0028] (8) Preferably, when the cutting lengths of one end of the optical fiber coupler of each cascaded optical fiber loop mirror are different, the randomness of the feedback characteristics can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of a random fiber laser based on cascaded fiber ring mirrors of the present invention.

[0030] Figure 2 Schematic diagram of the cascaded fiber ring mirror of the present invention.

[0031] Figure 3 is the reflection spectrum of the cascaded fiber ring mirror prepared by the present invention;

[0032] FIG4( a ) is an output power curve of the random fiber laser based on the cascaded fiber ring mirror of the present invention.

[0033] FIG4( b ) is the output spectrum of the random fiber laser based on the cascaded fiber ring mirror of the present invention.

[0034] Figure 5 It is the output spectrum of the random fiber laser based on the cascaded fiber ring mirror of the present invention.

[0035] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0036] 11—first fiber ring mirror; 12—optical fiber between adjacent fiber ring mirrors; 13—Nth fiber ring mirror; 31—inclined cut; 32—fiber Bragg grating; 33—pump light source; 34—wavelength division multiplexer; 35—gain fiber; 36—cascaded fiber ring mirrors; 37—isolator. DETAILED DESCRIPTION

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0038] In the present invention, the terms "first", "second", etc. in the present invention and the accompanying drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0039] like Figure 1 As shown, the random fiber laser based on the cascaded fiber ring mirror of the present invention mainly includes: a fiber Bragg grating 32, a pump light source 33, a wavelength division multiplexer 34, a gain fiber 35, a cascaded fiber ring mirror 36 and an isolator 37;

[0040] The pump light source 33 and the fiber Bragg grating 32 are respectively connected to the input end of the wavelength division multiplexer 34 , the output end of the wavelength division multiplexer 34 is connected to the gain fiber 35 , and the cascade fiber loop mirror 36 is respectively connected to the gain fiber 35 and the isolator 37 .

[0041] The pump light source 33 is used to generate pump light, which enters the gain fiber 35 through the wavelength division multiplexer 34 to generate reverse broadband spontaneous emission light; the fiber Bragg grating 32 is used to provide feedback and wavelength selection at one end of the laser, the cascaded fiber ring mirror 36 is used to provide random feedback at the other end of the laser, and the isolator 37 is used to avoid Fresnel reflection and serves as the laser output end.

[0042] like Figure 2 As shown, the cascaded fiber optic loop mirror 36 includes N cascaded fiber optic loop mirrors, adjacent fiber optic loop mirrors are connected by optical fibers, and the lengths of the optical fibers 12 between adjacent fiber optic loop mirrors are random, N≥3. Figure 2 The first fiber optic loop mirror 11 and the Nth fiber optic loop mirror 13 are marked.

[0043] The fiber optic loop mirror includes a fiber optic coupler. Two optical fibers at one end of the fiber optic coupler are fused together. The two optical fibers at the other end of the fiber optic coupler serve as the input and output ends of the fiber optic loop mirror respectively. The output end of each fiber optic loop mirror is connected to the input end of the next fiber optic loop mirror.

[0044] Preferably, the optical fiber coupler is a single-mode optical fiber coupler.

[0045] The preparation method of the cascaded fiber ring mirror comprises the following steps:

[0046] S1. Cut the two optical fibers at one end of the single-mode fiber coupler to the same target length and fuse the cut two optical fibers together.

[0047] S2. Use the two optical fibers at the other end of the single-mode fiber coupler as the input and output ends of the fiber loop mirror, and mark the input and output ends to obtain a single fiber loop mirror;

[0048] S3, prepare N single fiber loop mirrors obtained in S2, connect the output of each fiber loop mirror to the input of the next fiber loop mirror, and generate the fiber length (unit: meter) between adjacent fiber loop mirrors by generating random numbers. In this embodiment, prepare 10 single fiber loop mirrors obtained in S2, and use MATLAB to generate 9 random numbers between 0.3-1: 0.35, 0.31, 0.65, 0.45, 0.86, 0.7, 0.53, 0.66, and 0.59, each of which corresponds to the fiber length between a section of adjacent fiber loop mirrors.

[0049] In S1, in this embodiment, two optical fibers at one end of a single-mode fiber coupler with a coupling ratio of 99:1 are cut to 20 cm, and the two cut optical fibers are fused together. In other embodiments, fiber couplers with other coupling ratios, such as 99.9:0.1, can also be selected. Preferably, the larger the coupling ratio, the more cascaded fiber loop mirrors are selected.

[0050] In other embodiments, the cutting length of the two optical fibers at one end of the single-mode fiber coupler is selected according to actual needs. The shorter the length after cutting, the shorter the length of the cascaded fiber ring mirror finally formed.

[0051] In S3, the parameters of the fiber loop mirrors are the same or different, where the parameters of the fiber loop mirrors include the coupling ratio of the fiber coupler and the cut length of the two optical fibers at one end of the fiber coupler. When the parameters of the fiber loop mirrors are different, the random feedback characteristics provided by the cascaded fiber loop mirrors are better. In this embodiment, to simplify operation and analysis, the parameters of each fiber loop mirror are the same, that is, the coupling ratio of each fiber loop mirror is the same, and the cut length of one end of the fiber coupler is the same.

[0052] In S3, the shorter the fiber length between adjacent fiber loop mirrors, the better the random feedback characteristics provided by the cascaded fiber loop mirrors.

[0053] In the embodiment of the present invention, the total effective length of the prepared cascaded 10 fiber loop mirrors is 8.7 m, and its relatively flat reflection spectrum is as follows: Figure 3 As shown, it has a reflectivity of about -8.3dB, which is enough to provide random feedback at one end of the laser and significantly reduce the lasing threshold.

[0054] One end of the fiber Bragg grating 32 is cut obliquely to form an oblique cut 31 to avoid Fresnel reflection.

[0055] In this embodiment of the present invention, the 976nm pump light generated by the pump light source 33 enters the gain fiber 35 through the wavelength division multiplexer 34 to generate gain; the gain fiber 35 is an erbium-doped fiber with a length of 1.5m. The spontaneous radiation light generated by the gain fiber 35 is wavelength-selected by the fiber Bragg grating 32 and reflected back into the gain fiber 35 for amplification; the cascaded fiber loop mirror 36 provides random feedback at the other end of the laser, and the isolator 37 isolates the reflected light and serves as the laser output end; the length of the laser from the fiber Bragg grating 32 to the left end of the cascaded fiber loop mirror 36 is 6.33m.

[0056] In other embodiments, the gain fiber 35 may also be an ytterbium-doped fiber, and the laser finally outputs laser light in the 1 μm band.

[0057] The output power curve of the random fiber laser designed based on the cascade fiber ring mirror prepared in the embodiment of the present invention is shown in Figure 4(a). The lasing threshold is less than 20mW and the slope efficiency is 1.5%. The output spectrum at 50mW pump power is shown in Figure 4(b). The 3dB linewidth is less than 20pm. Its RF spectrum is as follows Figure 5 As shown, there is no RF peak (9.36-15.79 MHz) corresponding to the cavity length range of 12.66 to 21.36 m, indicating that the lasing laser is a random laser.

[0058] The present invention designs a cascaded fiber loop mirror, wherein a Fabry-Perot cavity is formed between any two of the multiple cascaded fiber loop mirrors, and the optical fiber lengths between adjacent fiber loop mirrors are random, so that the multiple Fabry-Perot cavities are cascaded and can produce different reflectivities for different wavelengths.

[0059] The pump light generated by the pump light source 33 enters the gain fiber 35 through the wavelength division multiplexer 34 to generate reverse broadband spontaneous radiation light. The spontaneous radiation light is selected by the wavelength through the fiber Bragg grating 32 at one end and reflected back into the gain fiber 35 for gain amplification. The amplified radiation light passes through the cascaded fiber loop mirror 36 to produce different reflectivities for different wavelengths. The reflected light returns to the gain fiber 35 for further amplification. The fiber Bragg grating 32 and the cascaded fiber loop mirror 36 simultaneously form a Fabry-Perot cavity, select the wavelength with the maximum gain, and finally form a stable laser output from the isolator 37.

[0060] In the random fiber laser designed in the present invention, the fiber Bragg grating 32 provides feedback at one end of the laser, and the cascaded fiber ring mirror 36 provides random feedback at the other end of the laser. As the power of the pump light source 33 increases, random laser output is gradually formed.

[0061] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A random fiber laser based on cascaded fiber ring mirrors, characterized in that: include: Fiber Bragg grating (32), pump light source (33), wavelength division multiplexer (34), gain fiber (35), cascaded fiber ring mirror (36) and isolator (37); The pump light source (33) and the fiber Bragg grating (32) are respectively connected to the input end of the wavelength division multiplexer (34), the output end of the wavelength division multiplexer (34) is connected to the gain fiber (35), and the cascade fiber ring mirror (36) is respectively connected to the gain fiber (35) and the isolator (37); The cascaded optical fiber loop mirror (36) comprises N cascaded optical fiber loop mirrors, and the lengths of optical fibers connecting adjacent optical fiber loop mirrors are random, wherein N≥3.

2. The random fiber laser according to claim 1, characterized in that The fiber loop mirror comprises a fiber coupler. Two optical fibers at one end of the fiber coupler are fused together, and two optical fibers at the other end serve as the input end and the output end of the fiber loop mirror respectively.

3. The random fiber laser according to claim 2, characterized in that The optical fiber coupler is a single-mode optical fiber coupler.

4. The random fiber laser according to claim 2 or 3, characterized in that The coupling ratio of the optical fiber coupler of each of the cascaded optical fiber loop mirrors is different.

5. The random fiber laser according to claim 1, wherein The gain optical fiber (35) is an erbium-doped optical fiber or an ytterbium-doped optical fiber.

6. The random fiber laser according to claim 1, characterized in that One end of the fiber Bragg grating (32) is provided with an inclined cutting opening (31).

7. A method for preparing a fiber ring mirror used in a random fiber laser according to any one of claims 2 to 4, characterized in that: The steps include: S1, cutting the two optical fibers at one end of the optical fiber coupler to the same target length, and fusing the cut two optical fibers together; S2. Use the two optical fibers at the other end of the optical fiber coupler as the input end and output end of the optical fiber loop mirror, respectively, and mark the input end and the output end to obtain the optical fiber loop mirror.

8. The preparation method according to claim 7, characterized in that Also includes: The optical fiber lengths between adjacent optical fiber loop mirrors are generated by generating random numbers.

9. The preparation method according to claim 7, characterized in that The cutting length of one end of the optical fiber coupler of each of the cascaded optical fiber loop mirrors is different.

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