A multi-core fiber ring resonant nested single-longitudinal-mode narrow-linewidth fiber laser and a linewidth measurement method thereof

Abstract

This invention discloses a multi-core fiber ring resonator nested single-longitudinal-mode narrow-linewidth fiber laser and its linewidth measurement method. The laser includes a pump source, a combiner, an erbium-doped fiber, a Butterworth filter, a first single-mode bare fiber, a first four-core bare fiber, a second four-core bare fiber, a four-core three-ring nested coupled fiber resonator ring, a second single-mode bare fiber, a polarization controller, a first single-mode fiber coupler, a fiber isolator, a spectrometer, and an optical power meter. This invention, through a fiber laser structure with a four-core three-ring nested coupled fiber resonator ring and a temperature-controlled stable Butterworth filter inside the ring, can simultaneously achieve single-longitudinal-mode, narrow-linewidth, and power-enhanced output while simplifying structural design.

Description

Technical Field

[0001] This invention belongs to the field of single-longitudinal-mode fiber laser technology, specifically relating to a multi-core fiber ring resonator nested single-longitudinal-mode narrow-linewidth fiber laser and its linewidth measurement method. This laser is particularly suitable for large-scale seabed sensing BOTDA monitoring systems. Background Technology

[0002] Single-mode narrow-linewidth fiber lasers (SLLs) are characterized by narrow linewidth, high coherence, and low noise, making them suitable for applications such as seabed communication, marine environmental monitoring, and seabed structure health monitoring. In recent years, to ensure marine safety and efficient resource development, and with the continuous improvement of related application performance, large-scale seabed sensing BOTDA monitoring systems require SLLs with higher power output, better frequency selectivity, and simpler design structures.

[0003] Typical methods for achieving the above functions include using high-gain active fiber, increasing the effective mode area of ​​the fiber, using a composite double-ring cavity and saturable absorber, and a single external cavity feedback laser configuration. Using high-gain active fiber is limited by the power enhancement effect of stimulated Brillouin scattering; increasing the effective mode area of ​​the fiber will reduce the output laser beam quality; using a composite double-ring cavity and saturable absorber can effectively suppress multiple longitudinal modes and achieve single longitudinal mode output, but the remaining longitudinal modes need to be further suppressed using a saturable absorber, which increases the complexity of the system; using a single external cavity feedback laser configuration will make it difficult to accurately match the phase between the external cavity and the main cavity, which can easily cause laser phase jumps. Summary of the Invention

[0004] To address the problems of low output power, poor frequency selectivity, and complex structural design of fiber lasers used in large-scale seabed sensing BOTDA monitoring systems, this invention discloses a multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser and its linewidth measurement method. Through a fiber laser structure consisting of a four-core, three-ring nested coupled fiber resonator ring and a temperature-controlled stable Butterworth filter connected inside the ring, single longitudinal mode, narrow linewidth, and power-enhanced output of the fiber laser can be achieved simultaneously, while simplifying the structural design.

[0005] In a first aspect, the present invention discloses a multi-core fiber ring resonator nested single-longitudinal-mode narrow-linewidth fiber laser. The laser includes a pump source, a combiner, an erbium-doped fiber, a Butterworth filter, a first single-mode bare fiber, a first four-core bare fiber, a second four-core bare fiber, a four-core three-ring nested coupled fiber resonator ring, a second single-mode bare fiber, a polarization controller, a first single-mode fiber coupler, a fiber isolator, a spectrometer, and an optical power meter. The four fiber core outputs of the first four-core bare fiber correspond to the four fiber core inputs of the second four-core bare fiber in pairs, and are connected using fused taper technology. The three fiber core inputs of the first four-core bare fiber (including the b, c, and d ends) correspond to the three fiber core outputs of the second four-core bare fiber (including the e, g, and h ends), and are coupled using single-mode fiber fusion splicing to form a four-core three-ring nested coupled fiber. A four-core, three-ring nested coupled fiber resonant ring is constructed. The first fiber resonant ring is formed by fusion splicing the b-end input of the first four-core bare fiber with the e-end output of the second four-core bare fiber using single-mode fiber. The second fiber resonant ring is formed by fusion splicing the d-end input of the first four-core bare fiber with the g-end output of the second four-core bare fiber using single-mode fiber. The third fiber resonant ring is formed by fusion splicing the c-end input of the first four-core bare fiber with the h-end output of the second four-core bare fiber using single-mode fiber. The a-end input of the first four-core bare fiber serves as the input of the four-core, three-ring nested coupled fiber resonant ring, and the f-end output of the second four-core bare fiber serves as the output of the four-core, three-ring nested coupled fiber resonant ring. The total resonant frequency of the four-core, three-ring nested coupled fiber resonant ring is greater than the 3dB bandwidth of the Butterworth filter.

[0006] The output end of the pump light source is connected to the first input end of the multiplexer. The output end of the multiplexer is sequentially connected to the core input end of the four-core three-ring nested coupled fiber resonator via the output end of the erbium-doped fiber, the Butterworth filter, and the first single-mode bare fiber. The core output end of the four-core three-ring nested coupled fiber resonator is connected to the input end of the second single-mode bare fiber. The output end of the second single-mode bare fiber is connected to the input end of the first single-mode fiber coupler via a polarization controller. The first output end of the first single-mode fiber coupler is connected to the second input end of the multiplexer via a fiber isolator. The second output end of the first single-mode fiber coupler is connected to the spectrometer and the optical power meter, respectively.

[0007] The pump light emitted by the pump source enters the erbium-doped fiber through a combiner, causing energy level transitions. After being filtered by a Butterworth filter to reduce the number of longitudinal modes, it enters a four-core, three-ring nested coupled fiber resonator through the first single-mode bare fiber, outputting a single-longitudinal-mode narrow-linewidth laser with enhanced output power. The single-longitudinal-mode narrow-linewidth laser is then polarized by a polarization controller and output to the first single-mode fiber coupler for beam splitting. The split beam is transmitted to the combiner through an optical isolator to complete the fiber loop, and the second beam is output to a spectrometer and an optical power meter to measure the spectrum and output power.

[0008] As a preferred example, the resonance condition of the four-core, three-ring nested coupled fiber optic resonator is expressed as follows:

[0009] mλ=2n eff L i

[0010] In the formula, m is the mode order, λ is the incident light wavelength, and n eff L is the effective refractive index of light waves propagating in the resonant ring. i It is the length of the i-th resonant ring, where i = 1, 2, 3.

[0011] As a preferred example, the total resonant frequency (FSR) of the four-core, three-ring nested coupled fiber optic resonant ring is... e Represented as:

[0012] FSR e =k1×FSR1=k2×FSR2=k3×FSR3

[0013] In the formula, k1, k2, and k3 are integers; FSR i The frequency difference between adjacent resonant modes of the i-th resonant ring is expressed as:

[0014] FSR i =c / n eff L i

[0015] In the formula, c is the speed of light in a vacuum, and i = 1, 2, 3.

[0016] As a preferred example, in a multi-ring fiber resonant ring, if each FSR i It is a periodically repeating and synchronized four-core, three-ring nested coupled fiber optic resonant ring with a total resonant frequency (FSR). e Represented as:

[0017] FSR e =LCM(FSR1, FSR2, FSR3)

[0018] In the formula, LCM is the least common multiple function of FSR1, FSR2, and FSR3, and FSR... i It is the frequency difference between adjacent resonant modes of the i-th resonant ring, where i = 1, 2, 3.

[0019] As a preferred example, the coupling loss a of the first four-core bare optical fiber, the coupling loss a1 of the second four-core bare optical fiber, and the fusion splicing loss a2 of the optical fiber are respectively expressed as:

[0020]

[0021] In the formula, γ0 and γ1 are the intensity loss factors of the first four-core bare fiber and the second four-core bare fiber, respectively, and γ2 is the fiber fusion splice loss factor.

[0022] According to the Jones matrix theory, the input optical field E at port c of the first four-core bare fiber can be obtained. c The output optical field E of the e-port of the first four-core bare fiber e The output optical field E of the g port of the first four-core bare optical fiber g , respectively represented as:

[0023]

[0024] In the formula, k1 and k2 are the coupling strength coefficients of the first four-core bare fiber and the second four-core bare fiber, respectively.

[0025] As a preferred example, the output optical field E at port b of the first four-core bare optical fiber b The output optical field E of the f port of the second four-core bare fiber f The relationship between the output optical field E at the C port of the first four-core bare fiber. c The output optical field E of the e-port of the second four-core bare fiber e The relationships are as follows:

[0026]

[0027] The output optical field E at port C of the first four-core bare optical fiber c The output optical field E of the g port of the second four-core bare fiber g The relationship between them is represented as follows:

[0028]

[0029] The input optical field E at port a of the first four-core bare optical fiber a The output optical field E of the C port of the first four-core bare fiber c The relationship between them is represented as follows:

[0030]

[0031] The input optical field E of the four-core, three-ring nested coupled fiber resonator ring a With output light field E f The ratio is expressed as:

[0032]

[0033] In the formula, b and β are the transmission loss and propagation constant of the first and second four-core bare optical fibers, respectively, and a2 is the fusion splice loss of the optical fiber; L1, L2, and L3 are the lengths of the first, second, and third fiber resonant rings, respectively, and the total length L = L1 + L2 + L3; a is the coupling loss of the first four-core bare optical fiber, a1 is the coupling loss of the second four-core bare optical fiber, a2 is the fusion splice loss of the optical fiber, and k1 and k2 are the coupling strength coefficients of the first and second four-core bare optical fibers, respectively.

[0034] As a preferred embodiment, the pump light source has a center wavelength of 980nm, an output power of 100mW, and a 3dB bandwidth of 117GHz; the multiplexer has a pump input wavelength of 980nm and a pump operating bandwidth of 130nm; the erbium-doped fiber has a length of 5m and an erbium ion doping concentration of 10. 24 m -3 The Butterworth filter operates at 1550nm with a 3dB bandwidth of 1nm; the first single-mode bare fiber, the first four-core bare fiber, the second four-core bare fiber, and the second single-mode bare fiber all operate at 1550nm with a transmission loss of 0.2dB / km; the polarization controller operates at 1550nm with an insertion loss of 1dB; the fiber optic isolator operates at 1550nm with a return loss of 40dB; the spectrometer has a bandwidth resolution of 0.05nm; and the optical power meter has a measurement range of -50dBm to 26dBm.

[0035] As a preferred example, the splitting ratio of the four-core three-ring nested coupled fiber resonator ring is 25:25:25:25; wherein the length L1 of the first fiber resonator ring is 2m, the length L2 of the second fiber resonator ring is 3m, and the length L3 of the third fiber resonator ring is 4m.

[0036] Secondly, this invention discloses a method for measuring the linewidth of a multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser, the linewidth measurement method comprising the following steps:

[0037] The second beam is used as the light source to be tested, and it is simultaneously input into the second single-mode fiber coupler along with the reference light source to generate a beat frequency signal;

[0038] The beat frequency signal output from the second single-mode fiber coupler is input to a photodetector and converted into electrical energy. The converted electrical energy is then output to an electro-spectrum analyzer, which uses the two-beam heterodyne method to determine the linewidth of the light source under test based on the electric power spectral density of the beat frequency signal.

[0039] As a preferred example, the field functions E1(t) and E2(t) of the two laser beams emitted by the light source under test and the reference light source are expressed as follows:

[0040]

[0041] In the formula, E1 is the initial light field intensity of the reference light source, E2 is the initial light field intensity of the light source under test, v1 is the center frequency of the reference light source, v2 is the center frequency of the light source under test, and v beat The frequency after the two laser beams are beat together. As the initial phase of the reference light source, The initial phase of the light source to be measured. The phase difference between the two laser beams;

[0042] The power spectral density S of the two laser beams t S(f) and S2(f) are represented as:

[0043]

[0044] In the formula, v1 and v2 are the center frequencies of the two lasers, σ1 and σ2 are the standard deviations of the linewidths of the two lasers, and f is the frequency of a point in the laser power spectral density.

[0045] Using the convolution property of Gaussian functions, the power spectral density function S of the beat frequency signal... be2t (f) is represented as:

[0046]

[0047] The power spectral density function S of the beat frequency signal be2t (f) rewritten as:

[0048]

[0049] The beneficial effects of this invention are as follows:

[0050] The multi-core fiber ring resonator nested single-longitudinal-mode narrow-linewidth fiber laser and its linewidth measurement method of the present invention can effectively achieve single-longitudinal-mode, narrow-linewidth, and power-enhanced output of fiber lasers, while also simplifying the laser design structure and reducing production costs. Testing shows that the optical conversion efficiency of the present invention can reach 40.2%, effectively enhancing output power and compressing the linewidth by 5850 times, thereby achieving single-longitudinal-mode, narrow-linewidth output. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the structure of the multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser of the present invention;

[0052] Figure 2 This is a schematic diagram of the linewidth measurement structure of the multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser of the present invention;

[0053] Figure 3 The graph shows the linewidth measurement of the 980nm pump source.

[0054] Figure 4 This is a spectral measurement diagram of the multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser of the present invention;

[0055] Figure 5 This is a power measurement diagram of the multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser of the present invention;

[0056] Figure 6 This is a linewidth measurement diagram of the multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser of the present invention. Detailed Implementation

[0057] The following embodiments are provided to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0058] Example 1

[0059] Figure 1 This is a schematic diagram of the multi-core fiber ring resonator nested single-longitudinal-mode narrow-linewidth fiber laser of the present invention. See also... Figure 1The laser includes: a pump source 1, a combiner 2, an erbium-doped fiber 3, a Butterworth filter 4, a first single-mode bare fiber 5, a first four-core bare fiber 6, a second four-core bare fiber 7, a four-core three-ring nested coupled fiber resonator 8, a second single-mode bare fiber 9, a polarization controller 10, a first single-mode fiber coupler 11, a fiber isolator 12, a spectrometer 13, and an optical power meter 14. In this configuration, the output of pump light source 1 is connected to the input of combiner 2, the output of combiner 2 is connected to the input of erbium-doped fiber 3, the output of erbium-doped fiber 3 is connected to the input of Butterworth filter 4, the output of Butterworth filter 4 is connected to the input of first single-mode bare fiber 5, the output of first single-mode bare fiber 5 is fused to the a-end core input of first four-core bare fiber 6, the a, b, c, and d-end core outputs of first four-core bare fiber 6 are paired with the e, f, g, and h-end core inputs of second four-core bare fiber 7, and connected using fused taper technology, and the b, c, and d-end core inputs of first four-core bare fiber 6 are paired with the e, g, and h-end core outputs of second four-core bare fiber 7, and after single-mode fiber fusion coupling, a four-core three-ring nested coupled fiber resonant ring 8 is formed; the b-end core input of first four-core bare fiber 6 and the e-end core output of second four-core bare fiber 7 are connected using single-mode fiber fusion coupling. A resonant ring i is formed by fiber fusion splicing. The d-end input of the first four-core bare fiber 6 and the g-end output of the second four-core bare fiber 7 are fused together using single-mode fiber to form a resonant ring j. The c-end input of the first four-core bare fiber 6 and the h-end output of the second four-core bare fiber 7 are fused together using single-mode fiber to form a resonant ring k. The f-end output of the second four-core bare fiber 7 is fused together with the input of the second single-mode bare fiber 9. The output of the second single-mode bare fiber 9 is connected to the input of the polarization controller 10. The output of the polarization controller 10 is connected to the input of the first single-mode fiber coupler 11. The output of the first single-mode fiber coupler 11 is connected to the input of the fiber isolator 12. The output of the fiber isolator 12 is connected to the input of the multiplexer 2. The output of the first single-mode fiber coupler 11 is connected to the input of the spectrometer 13. The output of the first single-mode fiber coupler 11 is connected to the input of the optical power meter 14.

[0060] For example, this embodiment proposes a parameter selection scheme for each device:

[0061] Pump light source 1 has a center wavelength of 980nm, an output power of 100mW, and a 3dB bandwidth of 117GHz; the pump input wavelength of combiner 2 is 980nm, and the pump operating bandwidth is 130nm; the erbium-doped fiber 3 has a length of 5m and an erbium ion doping concentration of 10. 24 m -3The Butterworth filter 4 operates at 1550nm with a 3dB bandwidth of 1nm; the first single-mode bare fiber 5 operates at 1550nm with a transmission loss of 0.2dB / km; the first four-core bare fiber 6 operates at 1550nm with a transmission loss of 0.22dB / km; the second four-core bare fiber 7 operates at 1550nm with a transmission loss of 0.22dB / km; the splitting ratio of the four-core three-ring nested coupled fiber resonator 8 is 25:25:25:25, where the length L1 of the fiber resonator i is 2m, and the fiber... The length L2 of the resonant ring j is 3m, and the length L3 of the fiber resonant ring k is 4m; the operating wavelength of the second single-mode bare fiber 9 is 1550nm, and the transmission loss is 0.2dB / km; the operating wavelength of the polarization controller 10 is 1550nm, and the insertion loss is 1dB; the first single-mode fiber coupler 11 has two inputs and outputs, and the splitting ratio is 50:50; the operating wavelength of the fiber isolator 12 is 1550nm, and the return loss is 40dB; the bandwidth 13 of the spectrometer has a resolution of 0.05nm; and the measurement range of the optical power meter 14 is -50 to 26dBm.

[0062] The laser operates as follows:

[0063] Pump light emitted from pump source 1 enters erbium-doped fiber 3 via combiner 2, causing energy level transitions. After frequency selection filtering by Butterworth filter 4 to reduce the number of longitudinal modes, it enters the first single-mode bare fiber 5. The output end of the first single-mode bare fiber 5 is fused to the a-end core input end of the first four-core bare fiber 6. The a, b, c, and d-end core output ends of the first four-core bare fiber 6 correspond to the e, f, g, and h-end core input ends of the second four-core bare fiber 7, respectively, using fusion splicing. The first four-core bare fiber 6 has its b, c, and d ends (input) and the second four-core bare fiber 7 has its e, g, and h ends (output) connected by a cone-shaped technique. These ends correspond to the input ends of the fiber cores in each pair, forming a four-core, three-ring nested coupled fiber resonant ring 8 through single-mode fiber fusion splicing. The b end (input) of the first four-core bare fiber 6 and the e end (output) of the second four-core bare fiber 7 are then coupled using single-mode fiber fusion splicing to form a resonant ring i. The d end (input) of the first four-core bare fiber 6 and the second four-core bare fiber 7 are connected by a cone-shaped technique. The g-end core output of fiber 7 is coupled with single-mode fiber fusion to form a resonant ring j. The c-end core input of the first four-core bare fiber 6 and the h-end core output of the second four-core bare fiber 7 are coupled with single-mode fiber fusion to form a resonant ring k. The length L1 of fiber resonant ring i is 2m, the length L2 of fiber resonant ring j is 3m, and the length L3 of fiber resonant ring k is 4m. Based on the resonant frequency selection principle and Jones matrix theory, a single longitudinal mode narrow linewidth laser with enhanced power is output through the four-core three-ring nested coupling fiber resonant ring 8. The f-end of the second four-core bare fiber 7 is connected with the second single-mode bare fiber 9 through fusion splicing. The single longitudinal mode narrow linewidth laser with enhanced power is output to the polarization controller 10 after passing through the second single-mode bare fiber 9. After the polarization state is adjusted by the polarization controller 10, it is output to the first single-mode fiber coupler 11. After being split by 50% by the first single-mode fiber coupler 11, it is output to the optical isolator 12. After preventing the back propagation of light, it enters the combiner 2 to complete the fiber loop. The remaining 50% of the light output is sent to the spectrometer 13 and the optical power meter 14 to measure the output power and spectrum.

[0064] In this embodiment, the resonance condition in the resonant frequency selection principle can be expressed as:

[0065] mλ=2n eff L i

[0066] In the formula, m is the mode order, λ is the incident light wavelength, and n eff L is the effective refractive index of light waves propagating in the resonant ring. i It is the length of the i-th resonant ring, where i = 1, 2, 3.

[0067] The total resonant frequency (FSR) of the four-core, three-ring nested coupled fiber optic resonant ring is... e Represented as:

[0068] FSR e=k1×FSR1=k2×FSR2=k3×FSR3

[0069] In the formula, k1, k2, and k3 are integers;

[0070] FSR i The frequency difference between adjacent resonant modes of the i-th resonant ring is expressed as:

[0071] FSR i =c / n eff L i

[0072] In the formula, c is the speed of light in a vacuum, and i = 1, 2, 3.

[0073] In a multi-ring fiber resonant ring, if each FSR i It is a periodic repetition and mutual synchronization, the total resonant frequency FSR of the four-core three-ring nested coupled fiber optic resonator ring. e Represented as:

[0074] FSR e =LCM(FSR1, FSR2, FSR3)

[0075] In the formula, LCM is the least common multiple of FSR1, FSR2, and FSR3, and FSR... e It has a bandwidth greater than 3dB of the Butterworth filter.

[0076] The coupling loss a of the first four-core bare fiber 6, the coupling loss a1 of the second four-core bare fiber 7, and the fusion splicing loss a2 of the fiber can be expressed as:

[0077]

[0078] In the formula, γ0 and γ1 are the intensity loss factors of the first four-core bare fiber 6 and the second four-core bare fiber 7, respectively, and γ2 is the fiber fusion splicing loss factor.

[0079] According to the Jones matrix theory, the input optical field at port c of the first four-core bare fiber 6, the output optical field at port e of the first four-core bare fiber 7, and the output optical field at port g of the first four-core bare fiber 6 can be expressed as follows:

[0080]

[0081] In the formula, the coupling strength coefficients of the first four-core bare optical fiber 6 and the second four-core bare optical fiber 7 are k1 and k2, respectively.

[0082] The output optical field E at port b of the first four-core bare optical fiber 6 can be obtained through numerical calculation. b The output optical field E of the f port of the second four-core bare fiber 7 fThe relationship between the output optical field E of the C port of the first four-core bare fiber 6 c The output optical field E of the e-port of the second four-core bare fiber 7 e The relationship can be represented as:

[0083]

[0084] In the formula, b and β are the transmission loss and propagation constant of the four-core optical fiber, respectively, and a2 is the fusion splice loss of the optical fiber.

[0085] The first four-core bare fiber 6 outputs an optical field E at port C. c The output optical field E of the g port of the second four-core bare fiber 7 g The relationship between them can be represented as:

[0086]

[0087] The input optical field E at port a of the first four-core bare fiber 6 a The output optical field E of the c port of the first four-core bare fiber 6 c The relationship between them can be represented as:

[0088]

[0089] The input optical field E of the fiber resonant ring a With output light field E f The ratio can be expressed as:

[0090]

[0091] As the number of coupler rings increases, the ratio of input optical field to output optical field will decrease, output power will increase, and optical conversion efficiency will improve.

[0092] Example 2

[0093] See Figure 2 This embodiment proposes a method for measuring the linewidth of a multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser. The linewidth measurement method is implemented by the two-beam heterodyne method, which includes: a reference light source 15, a light source under test 16, a second single-mode fiber coupler 17, a photodetector 18, and an electro-spectrum analyzer 19. The output terminals of the reference light source 15 and the light source under test 16 are connected to the input terminal of the second single-mode fiber coupler 17, the output terminal of the second single-mode fiber coupler 17 is connected to the input terminal of the photodetector 18, and the output terminal of the photodetector 18 is connected to the input terminal of the electro-spectrum analyzer 19.

[0094] Regarding parameter selection, the reference light source 15 in this embodiment is a single-longitudinal-mode narrow-linewidth fiber laser with a linewidth of 1kHz and an output power of 100mW. The output frequency is 193.42THz. The light source under test 16 is 50% of the optical signal output from the first single-mode fiber coupler 11. The second single-mode fiber coupler 17 has two inputs and outputs with a splitting ratio of 50:50. The photodetector 18 has a 3dB bandwidth of 10GHz and is DC coupled. The spectrometer 19 has a bandwidth resolution of 40MHz and a measurement range of 70-100GHz.

[0095] The measurement method in this embodiment is implemented by the two-beam heterodyne method, including: the reference light source 15 and the light source under test 16 are simultaneously input into the second single-mode fiber coupler 17 to generate a beat frequency signal. The beat frequency signal output by the second single-mode fiber coupler 17 is converted into electrical energy by the photodetector 18 and then output to the electric spectrometer 19. The linewidth of one of the light sources with a larger linewidth is determined according to the electric power spectral density of the beat frequency signal.

[0096] The center frequency of reference light source 15 is v1, and its phase is... The center frequency of the light source under test 16 is v2, and the phase is... Two light beams are coupled to the photosensitive surface of a photodetector for mixing. The field functions E1(t) and E2(t) of the two light beams can be expressed as:

[0097]

[0098] In the formula, E1 is the initial light field intensity of the reference light source, E2 is the initial light field intensity of the light source under test, v1 is the center frequency of the reference light source, v2 is the center frequency of the light source under test, and v beat The frequency after the two laser beams are beat together. As the initial phase of the reference light source, The initial phase of the light source to be measured. The phase difference between the two laser beams;

[0099] Assuming that the power spectral densities S(f) of both laser beams are Gaussian linear, their center frequencies are similar, and their linewidths are fixed, the power spectral densities S1(f) and S2(f) can be expressed as:

[0100]

[0101] In the formula, v1 and v2 are the center frequencies of the two lasers, σ1 and σ2 are the standard deviations of the linewidths of the two lasers, and f is the frequency of a point in the laser power spectral density.

[0102] Using the convolution property of Gaussian functions, the power spectral density function S of the beat frequency signal... be2t (f) can be expressed as:

[0103]

[0104] When σ1 << σ2, σ1 2 The impact on the convolution result is negligible. Therefore, S beat (f) can be expressed as:

[0105]

[0106] The power spectral density of the beat frequency signal is mainly determined by the linewidth of the light source under test. The linewidth of the spectral line measured by the two-beam heterodyne method is the linewidth of the light source under test.

[0107] Figure 3 , Figure 4 , Figure 5 The laser output spectrum has a center wavelength of 1550nm, a signal-to-noise ratio of 110dB, a pump input power of 20dBm, a laser output power of 16.1dBm, an optical conversion efficiency of 40.2%, and enhanced output power. Figure 3 and Figure 6 The medium-pumped light source has an input linewidth of 117 GHz and a laser output linewidth of 20 MHz, with a linewidth compression of 5850 times, achieving single longitudinal mode and narrow linewidth output; the four-core three-ring nested coupled fiber resonator structure can simplify the laser design structure and reduce production costs.

[0108] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A multi-core fiber ring resonator nested single longitudinal mode fiber laser, characterized in that, The laser includes a pump source, a combiner, an erbium-doped fiber, a Butterworth filter, a first single-mode bare fiber, a first four-core bare fiber, a second four-core bare fiber, a four-core three-ring nested coupled fiber resonator, a second single-mode bare fiber, a polarization controller, a first single-mode fiber coupler, a fiber isolator, a spectrometer, and an optical power meter. The four output ends of the first four-core bare fiber correspond to the four input ends of the second four-core bare fiber in pairs, connected using fused taper technology. The three input ends of the first four-core bare fiber (including the b, c, and d ends) correspond to the three output ends of the second four-core bare fiber (including the e, g, and h ends), and are coupled using single-mode fiber fusion splicing to form a four-core three-ring nested coupled fiber resonator. The first four-core... The b-end input of the bare optical fiber and the e-end output of the second four-core bare optical fiber are coupled using single-mode fiber fusion splicing to form a first optical fiber resonant loop. The d-end input of the first four-core bare optical fiber and the g-end output of the second four-core bare optical fiber are coupled using single-mode fiber fusion splicing to form a second optical fiber resonant loop. The c-end input of the first four-core bare optical fiber and the h-end output of the second four-core bare optical fiber are coupled using single-mode fiber fusion splicing to form a third optical fiber resonant loop. The a-end input of the first four-core bare optical fiber serves as the core input of the four-core three-ring nested coupled optical fiber resonant loop, and the f-end output of the second four-core bare optical fiber serves as the core output of the four-core three-ring nested coupled optical fiber resonant loop. The total resonant frequency of the four-core three-ring nested coupled optical fiber resonant loop is greater than the 3dB bandwidth of the Butterworth filter. The output end of the pump light source is connected to the first input end of the multiplexer. The output end of the multiplexer is sequentially connected to the core input end of the four-core three-ring nested coupled fiber resonator via the output end of the erbium-doped fiber, the Butterworth filter, and the first single-mode bare fiber. The core output end of the four-core three-ring nested coupled fiber resonator is connected to the input end of the second single-mode bare fiber. The output end of the second single-mode bare fiber is connected to the input end of the first single-mode fiber coupler via a polarization controller. The first output end of the first single-mode fiber coupler is connected to the second input end of the multiplexer via a fiber isolator. The second output end of the first single-mode fiber coupler is connected to the spectrometer and the optical power meter, respectively. The pump light emitted by the pump source enters the erbium-doped fiber through a combiner, causing energy level transitions. After being filtered by a Butterworth filter to reduce the number of longitudinal modes, it enters a four-core, three-ring nested coupled fiber resonator through the first single-mode bare fiber, outputting a single-longitudinal-mode laser with enhanced output power. The single-longitudinal-mode laser is then polarized by a polarization controller and output to the first single-mode fiber coupler for beam splitting. The first beam is transmitted to the combiner through an optical isolator to complete the fiber loop, and the second beam is output to a spectrometer and an optical power meter to measure the spectrum and output power. The total resonant frequency of the four-core, three-ring nested coupled fiber optic resonant ring Represented as: ； In the formula, It is an integer; It is the first The frequency difference between adjacent resonant modes of a resonant ring is expressed as: ； In the formula, It is the speed at which light travels in a vacuum. .

2. The multi-core fiber ring resonator nested single longitudinal mode fiber laser according to claim 1, characterized in that, The resonance condition of the four-core, three-ring nested coupled fiber optic resonator is expressed as follows: ； In the formula, It is the order of the pattern. It is the wavelength of the incident light. It is the effective refractive index for light waves to propagate in the resonant ring. It is the first The length of each resonant ring, .

3. The multi-core fiber ring resonator nested single longitudinal mode fiber laser according to claim 1, characterized in that, In a multi-ring fiber resonant ring, if each It is a periodic, repetitive, and synchronized four-core, three-ring nested coupled fiber optic resonant ring with a total resonant frequency. Represented as: ； In the formula, yes The least common multiple function, It is the first Frequency difference between adjacent resonant modes of a resonant ring .

4. The multi-core fiber ring resonator nested single longitudinal mode fiber laser according to claim 1, characterized in that, Coupling loss of the first four-core bare optical fiber Coupling loss of the second four-core bare optical fiber Fiber optic splice loss They are represented as follows: ； In the formula, and These are the intensity loss factors of the first four-core bare optical fiber and the second four-core bare optical fiber, respectively. ; According to the Jones matrix theory, the input optical field at port C of the first four-core bare fiber can be obtained. The output optical field of the e-port of the first four-core bare fiber The output optical field of the g port of the first four-core bare optical fiber , respectively represented as: ； In the formula, and These are the coupling strength coefficients of the first four-core bare optical fiber and the second four-core bare optical fiber, respectively. For the output optical field at the f port of the second four-core bare fiber, E a For the input optical field at port a of the first four-core bare fiber, E b E represents the output optical field at port b of the first four-core bare fiber. h The output optical field is the h port of the first four-core bare optical fiber.

5. The multi-core fiber ring resonator nested single longitudinal mode fiber laser according to claim 1, characterized in that, The output optical field of port b of the first four-core bare optical fiber The output optical field of the f port of the second four-core bare fiber The relationship between the C-port output optical field of the first four-core bare fiber and the relationship between the two. The output optical field of the e-port of the second four-core bare fiber The relationships are as follows: The output optical field at port C of the first four-core bare optical fiber The output optical field of the g port of the second four-core bare fiber The relationship between them is represented as follows: The input optical field at port A of the first four-core bare optical fiber The output optical field of the C port of the first four-core bare fiber The relationship between them is represented as follows: The input optical field of the four-core, three-ring nested coupled fiber resonator With output light field The ratio is expressed as: In the formula, , The transmission loss and propagation constant of the first four-core bare optical fiber and the second four-core bare optical fiber are respectively. This refers to the fusion splicing loss of the optical fiber. , , These are the lengths of the first fiber resonant ring, the second fiber resonant ring, and the third fiber resonant ring, respectively, and the total length. ; This represents the coupling loss of the first four-core bare optical fiber. The coupling loss of the second four-core bare fiber , and These are the coupling strength coefficients of the first four-core bare optical fiber and the second four-core bare optical fiber, respectively.

6. The multi-core fiber ring resonator nested single longitudinal mode fiber laser according to claim 1, characterized in that, The pump light source has a center wavelength of 980nm, an output power of 100mW, and a 3dB bandwidth of 117GHz; the pump input wavelength of the combiner is 980nm, and the pump operating bandwidth is 130nm; the erbium-doped fiber has a length of 5m and an erbium ion doping concentration of [missing information]. The Butterworth filter operates at 1550 nm with a 3dB bandwidth of 1 nm; the first single-mode bare fiber, the first four-core bare fiber, the second four-core bare fiber, and the second single-mode bare fiber all operate at 1550 nm with a transmission loss of 0.2 dB / km; the polarization controller operates at 1550 nm with an insertion loss of 1 dB; the fiber optic isolator operates at 1550 nm with a return loss of 40 dB; the spectrometer has a bandwidth resolution of 0.05 nm; and the optical power meter has a measurement range of -50 dBm to 26 dBm.

7. The multi-core fiber ring resonator nested single longitudinal mode fiber laser according to claim 1, characterized in that, The four-core, three-ring nested coupled fiber optic resonator has a splitting ratio of 25:25:25:25; wherein, the length of the first fiber optic resonator is... The length of the second fiber resonant ring is 2m. The length of the third fiber resonant ring is 3m. It is 4m.

8. A method for measuring the linewidth of a multi-core fiber ring resonator nested single-longitudinal-mode fiber laser as described in any one of claims 1-7, characterized in that, The line width measurement method includes the following steps: The second beam is used as the light source to be tested, and it is simultaneously input into the second single-mode fiber coupler along with the reference light source to generate a beat frequency signal; The beat frequency signal output from the second single-mode fiber coupler is input to a photodetector and converted into electrical energy. The converted electrical energy is then output to an electro-spectrum analyzer, which uses the two-beam heterodyne method to determine the linewidth of the light source under test based on the electric power spectral density of the beat frequency signal.

9. The method for measuring the linewidth of a multi-core fiber ring resonator nested single-longitudinal-mode fiber laser according to claim 8, characterized in that, The field functions of the two laser beams emitted by the light source under test and the reference light source Represented as: ； In the formula, The initial light field intensity of the reference light source, The initial light field intensity of the light source to be measured is... The center frequency of the reference light source The center frequency of the light source under test is The frequency after the two laser beams are beat together. As the initial phase of the reference light source, The initial phase of the light source to be measured. The phase difference between the two laser beams; The power spectral density of the two laser beams , Represented as: ； In the formula, These are the center frequencies of the two lasers. These are the standard deviations of the linewidths of the two lasers. The frequency of a point in the laser power spectral density; Using the convolution property of Gaussian functions, the power spectral density function of beat frequency signals... : The power spectral density function of the beat frequency signal Rewritten as:

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