Multi-core fiber ring resonance nested single longitudinal mode narrow linewidth fiber laser and linewidth measurement method thereof

Abstract

The invention discloses a multi-core fiber ring resonance nested single longitudinal mode narrow linewidth fiber laser and a linewidth measurement method thereof. The laser comprises a pump light 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 resonant 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. According to the optical fiber laser, through the four-core three-ring nested coupling type optical fiber resonance ring and the optical fiber laser structure in which the temperature-control stable Butterworth filter is connected, single longitudinal mode, narrow linewidth and power-enhanced output of the optical fiber laser can be realized at the same time, and the structural design is simplified.

Description

Technical Field

[0001] The present invention belongs to the technical field of single longitudinal mode fiber lasers, and specifically relates to a multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser and a linewidth measurement method thereof. The laser is particularly suitable for a large-scale seabed sensing BOTDA monitoring system. Background Art

[0002] Single longitudinal mode narrow linewidth fiber lasers have the characteristics of narrow linewidth, high coherence, and low noise, and can be used in applications such as submarine communications, marine environmental monitoring, and submarine structure health monitoring. In recent years, in order to ensure marine safety and efficient resource development. With the continuous improvement of related application performance, large-scale submarine sensing BOTDA monitoring systems require single longitudinal mode narrow linewidth lasers with higher power output, better frequency selection characteristics, and simpler design structures.

[0003] Typical methods for realizing the above functions include using high-gain active optical fiber, increasing the effective mode field area of ​​optical fiber, using composite dual-ring cavity and saturated absorber, single external cavity feedback laser configuration, etc. The use of high-gain active optical fiber will be limited by the power increase caused by stimulated Brillouin scattering effect; increasing the effective mode field area of ​​optical fiber will reduce the output laser beam quality; using composite dual-ring cavity and saturated absorber can effectively suppress multiple longitudinal modes and achieve single longitudinal mode output, but the remaining longitudinal modes need to be further suppressed by saturated absorber, which increases the complexity of the system; using single external cavity feedback laser configuration will make it difficult to accurately match the phase between the external cavity and the main cavity, which is easy to cause laser phase jump. Summary of the invention

[0004] In view of the problems of low output power, poor frequency selection characteristics and complex structural design of the fiber laser used in the large-scale submarine sensing BOTDA monitoring system, the present invention discloses a multi-core fiber ring resonant nested single longitudinal mode narrow linewidth fiber laser and a linewidth measurement method thereof. Through the fiber laser structure of four-core three-ring nested coupled fiber resonant ring and a temperature-controlled stable Butterworth filter connected inside the ring, the single longitudinal mode, narrow linewidth and power enhanced output of the fiber laser can be achieved simultaneously and the structural design can be simplified.

[0005] In the first aspect, the present invention discloses a multi-core fiber ring resonant nested single longitudinal mode narrow linewidth fiber laser, the laser comprising a pump light 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 resonant ring, a second single-mode bare fiber, a polarization controller, a first single-mode fiber coupler, an optical fiber isolator, a spectrometer and an optical power meter; the four core output ends of the first four-core bare fiber correspond to the four core input ends of the second four-core bare fiber in pairs, respectively, and are connected by using a fused taper technology; the three core input ends of the first four-core bare fiber, including the b end, the c end, and the d end, correspond to the three core output ends of the second four-core bare fiber, including the e end, the g end, and the h end, respectively, and are coupled by single-mode fiber fusion splicing to form a four-core three-ring nested coupled fiber. The fiber resonant ring is a combination of a fiber resonant ring and a fiber core input end at the b end of the first four-core bare fiber and a fiber core output end at the e end of the second four-core bare fiber. The fiber core input end at the d end of the first four-core bare fiber and a fiber core output end at the g end of the second four-core bare fiber are fused and coupled by single-mode fiber to form a second fiber resonant ring. The fiber core input end at the c end of the first four-core bare fiber and a fiber core output end at the h end of the second four-core bare fiber are fused and coupled by single-mode fiber to form a third fiber resonant ring. The fiber core input end at the a end of the first four-core bare fiber is used as the core input end of the four-core three-ring nested coupled fiber resonant ring, and the fiber core output end at the f end of the second four-core bare fiber is used as the core output end 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 combiner, the output end of the combiner is fused and connected to the core input end of the four-core three-ring nested coupled fiber resonant ring via the erbium-doped fiber, the Butterworth filter, and the output end of the first single-mode bare fiber in sequence, the core output end of the four-core three-ring nested coupled fiber resonant ring is fused and 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 the polarization controller, the first output end of the first single-mode fiber coupler is connected to the second input end of the combiner via the fiber isolator, and 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 light source enters the erbium-doped optical fiber through the combiner to generate energy level transition, and then enters the four-core three-ring nested coupled optical fiber resonant ring through the first single-mode bare optical fiber after frequency selection filtering to reduce the number of longitudinal modes, and outputs a single longitudinal mode narrow linewidth laser with enhanced output power; the single longitudinal mode narrow linewidth laser is then output to the first single-mode optical fiber coupler for light splitting after the polarization state is adjusted by the polarization controller, and the split first light beam is transmitted to the combiner through the optical isolator to complete the optical fiber loop, and the second light beam is output to the spectrometer and the 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 optical fiber resonant ring is expressed as:

[0009] mλ=2n eff L i

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

[0011] As a preferred example, the total resonant frequency FSR of the four-core three-ring nested coupled fiber resonator ring is e It is expressed as:

[0012] FSR e =k1×FSR1=k2×FSR2=k3×FSR3

[0013] Where k1, k2, k3 are integers; FSR i is the frequency difference between adjacent resonant modes of the i-th resonant ring, 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 optical fiber resonant ring, if each FSR i The total resonant frequency FSR of the four-core three-ring nested coupled fiber resonator ring is periodically repeated and synchronized with each other. e It is expressed as:

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

[0018] Where LCM is the least common multiple function of FSR1, FSR2, and FSR3. i is the frequency difference between adjacent resonant modes of the i-th resonant ring, 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 loss a2 of the optical fiber are respectively expressed as:

[0020]

[0021] Wherein, γ0 and γ1 are the intensity loss factors of the first four-core bare optical fiber and the second four-core bare optical fiber respectively, and γ2 is the optical fiber fusion loss factor;

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

[0023]

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

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

[0026]

[0027] The output light field E of the c-port of the first four-core bare fiber c The light field E output from the g port of the second four-core bare optical fiber g The relationship between them is expressed as:

[0028]

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

[0030]

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

[0032]

[0033] Wherein, b and β are the transmission loss and propagation constant of the first four-core bare fiber and the second four-core bare fiber, respectively, and a2 is the fusion loss of the fiber; L1, L2, and L3 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 L=L1+L2+L3; a is the coupling loss of the first four-core bare fiber, a1 is the coupling loss a1 of the second four-core bare fiber, a2 is the fusion loss of the fiber, and k1 and k2 are the coupling strength coefficients of the first four-core bare fiber and the second four-core bare fiber, respectively.

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

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

[0036] In a second aspect, the present invention discloses a linewidth measurement method for 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 light beam is used as the light source to be measured and is input into the second single-mode optical fiber coupler simultaneously with the reference light source to generate a beat frequency signal;

[0038] The beat frequency signal output by the second single-mode optical fiber coupler is input to a photodetector and converted into electrical energy, and the converted electrical energy is output to an electric spectrometer, which uses a double-beam heterodyne method to determine the line width of the light source to be measured based on the electric power spectrum 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 to be measured and the reference light source are expressed as follows:

[0040]

[0041] Where E1 is the initial light field intensity of the reference light source, E2 is the initial light field intensity of the light source to be measured, v1 is the center frequency of the reference light source, v2 is the center frequency of the light source to be measured, and v beat is the frequency after the two laser beams beat each other, is the initial phase of the reference light source, is the initial phase of the light source to be measured, is the phase difference between the two laser beams;

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

[0043]

[0044] Where v1 and v2 are the center frequencies of the two lasers, σ1 and σ2 are the standard deviations of the line widths of the two lasers, and f is the frequency of a point in the laser power spectrum density;

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

[0046]

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

[0048]

[0049] The beneficial effects of the present invention are:

[0050] The multi-core fiber ring resonator nested single longitudinal mode narrow line width fiber laser and its line width measurement method of the present invention can effectively realize single longitudinal mode, narrow line width, and power enhanced output of the fiber laser, and can also simplify the laser design structure and reduce production costs. According to tests, the light conversion efficiency of the present invention can reach 40.2%, effectively enhancing the output power, and compressing the line width by 5850 times, thereby realizing single longitudinal mode and narrow line width output. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0053] Figure 3 This is the line width measurement diagram of the 980nm pump light source;

[0054] Figure 4 This is a spectrum 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 resonance nested single longitudinal mode narrow linewidth fiber laser of the present invention. DETAILED DESCRIPTION

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

[0058] Example 1

[0059] 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. Figure 1The laser includes: a pump light source 1, a combiner 2, an erbium-doped optical fiber 3, a Butterworth filter 4, a first single-mode bare optical fiber 5, a first four-core bare optical fiber 6, a second four-core bare optical fiber 7, a four-core three-ring nested coupled optical fiber resonant ring 8, a second single-mode bare optical fiber 9, a polarization controller 10, a first single-mode optical fiber coupler 11, an optical fiber isolator 12, a spectrometer 13, and an optical power meter 14. Among them, the output end of the pump light source 1 is connected to the input end of the combiner 2, the output end of the combiner 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 Butterworth filter 4, the output end of the Butterworth filter 4 is connected to the input end of the first single-mode bare fiber 5, the output end of the first single-mode bare fiber 5 is connected to the a-end core input end of the first four-core bare fiber by fusion splicing, the a, b, c, d-end core output ends of the first four-core bare fiber 6 and the e, f, g, h-end core input ends of the second four-core bare fiber 7 correspond to each other in pairs, and are connected by fusion taper technology, the b, c, d-end core input ends of the first four-core bare fiber 6 and the e, g, h-end core output ends of the second four-core bare fiber 7 correspond to each other in pairs, 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 end of the first four-core bare fiber 6 and the e-end core output end of the second four-core bare fiber 7 are connected by single-mode optical fiber. The first four-core bare optical fiber 6 and the second four-core bare optical fiber 7 are fused together to form a resonance ring i, the d-end core input end of the first four-core bare optical fiber 6 and the g-end core output end of the second four-core bare optical fiber 7 are fused together to form a resonance ring j by using single-mode optical fiber, the c-end core input end of the first four-core bare optical fiber 6 and the h-end core output end of the second four-core bare optical fiber 7 are fused together to form a resonance ring k by using single-mode optical fiber, the f-end core output end of the second four-core bare optical fiber 7 is connected to the input end of the second single-mode bare optical fiber 9 by fusion splicing, the output end of the second single-mode bare optical fiber 9 is connected to the input end of the polarization controller 10, the output end of the polarization controller 10 is connected to the input end of the first single-mode optical fiber coupler 11, the output end of the first single-mode optical fiber coupler 11 is connected to the input end of the optical fiber isolator 12, the output end of the optical fiber isolator 12 is connected to the input end of the combiner 2, the output end of the first single-mode optical fiber coupler 11 is connected to the input end of the spectrometer 13, and the output end of the first single-mode optical fiber coupler 11 is connected to the input end of the optical power meter 14.

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

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

[0062] The laser's operating methods include:

[0063] The pump light emitted by the pump light source 1 enters the erbium-doped optical fiber 3 through the combiner 2, generates energy level transition, and enters the first single-mode bare optical fiber 5 after frequency selection filtering by the Butterworth filter 4 to reduce the number of longitudinal modes. The output end of the first single-mode bare optical fiber 5 is connected to the a-end core input end of the first four-core bare optical fiber 6 by fusion splicing. The a, b, c, and d-end core output ends of the first four-core bare optical fiber 6 correspond to the e, f, g, and h-end core input ends of the second four-core bare optical fiber 7, respectively. The first four-core bare optical fiber 6 is connected by a cone technology, and the b, c, and d end core input ends of the first four-core bare optical fiber 6 correspond to the e, g, and h end core output ends of the second four-core bare optical fiber 7 respectively. After single-mode optical fiber fusion coupling, a four-core three-ring nested coupled optical fiber resonant ring 8 is formed; the b-end core input end of the first four-core bare optical fiber 6 and the e-end core output end of the second four-core bare optical fiber 7 are fusion coupled by single-mode optical fiber to form a resonant ring i, and the d-end core input end of the first four-core bare optical fiber 6 and the e-end core output end of the second four-core bare optical fiber 7 are fusion coupled by single-mode optical fiber to form a resonant ring i. The g-end core output end of the optical fiber 7 is coupled by single-mode fiber fusion splicing to form a resonance ring j, and the c-end core input end of the first four-core bare optical fiber 6 and the h-end core output end of the second four-core bare optical fiber 7 are coupled by single-mode fiber fusion splicing to form a resonance ring k, wherein the length L1 of the fiber resonance ring i is 2m, the length L2 of the fiber resonance ring j is 3m, and the length L3 of the fiber resonance ring k is 4m. Based on the resonance frequency selection principle and Jones matrix theory, a power-enhanced single longitudinal mode narrow linewidth laser is output through a four-core three-ring nested coupled optical fiber resonance ring 8. The f-end of the second four-core bare optical fiber 7 is connected to a second single-mode bare optical fiber 9 by fusion splicing. After passing through the second single-mode bare optical fiber 9, the power-enhanced single longitudinal mode narrow linewidth laser is output to a polarization controller 10, and after adjusting the polarization state through the polarization controller 10, it is output to a first single-mode optical fiber coupler 11, and after being split by 50% by the first single-mode optical fiber coupler 11, it is output to an optical isolator 12, and after preventing the light from propagating in reverse, it enters the combiner 2 to complete the optical fiber loop. The other 50% of the light is output 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 resonance frequency selection principle can be expressed as:

[0065] mλ=2n eff L i

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

[0067] The total resonant frequency FSR of the four-core three-ring nested coupled fiber resonator ring e It is expressed as:

[0068] FSR e=k1×FSR1=k2×FSR2=k3×FSR3

[0069] Wherein, k1, k2, k3 are integers;

[0070] FSR i is the frequency difference between adjacent resonant modes of the i-th resonant ring, 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 resonator ring, if each FSR i The total resonant frequency FSR of the four-core three-ring nested coupled fiber resonator ring is periodically repeated and synchronized with each other. e It is expressed as:

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

[0075] Where LCM is the least common multiple of FSR1, FSR2, and FSR3. e Larger than the 3dB bandwidth of the Butterworth filter.

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

[0077]

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

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

[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 light field E of the b-port of the first four-core bare optical fiber 6 can be obtained by numerical calculation. b The output light field E of the f port of the second four-core bare optical fiber 7 fThe relationship between the c-port output light field E of the first four-core bare optical fiber 6 c The light field E output from the e-port of the second four-core bare optical fiber 7 e The relationship can be expressed as:

[0083]

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

[0085] The output light field E of the c-port of the first four-core bare optical fiber 6 c The light field E output from the g port of the second four-core bare optical fiber 7 g The relationship between them can be expressed as:

[0086]

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

[0088]

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

[0090]

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

[0092] Example 2

[0093] See also Figure 2 This embodiment proposes a linewidth measurement method for a multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser, the linewidth measurement method is implemented by a double-beam heterodyne method, the heterodyne method includes: a reference light source 15, a light source to be measured 16, a second single-mode fiber coupler 17, a photodetector 18, and an electric spectrometer 19; wherein the output ends of the reference light source 15 and the light source to be measured 16 are connected to the input end of the second single-mode fiber coupler 17, the output end of the second single-mode fiber coupler 17 is connected to the input end of the photodetector 18, and the output end of the photodetector 18 is connected to the input end of the electric spectrometer 19.

[0094] In terms of parameter selection, the reference light source 15 of 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 16 to be measured is 50% of the optical signal split by the first single-mode fiber coupler 11; the second single-mode fiber coupler 17 has two inputs and outputs, and the splitting ratio is 50:50; the 3dB bandwidth of the photodetector 18 is 10GHz, and the coupling mode is DC coupling; the bandwidth resolution of the spectrometer 19 is 40MHz, and the measurement range is 70-100GHz.

[0095] The measurement method of this embodiment is implemented by a double-beam heterodyne method, including: a reference light source 15 and a light source to be measured 16 are simultaneously input into a second single-mode optical fiber coupler 17 to generate a beat signal, the beat signal output by the second single-mode optical fiber coupler 17 is output to an electric spectrometer 19 after the light energy is converted into electric energy by a photodetector 18, and the line width of one of the light sources with a larger line width is determined according to the electric power spectrum density of the beat signal.

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

[0097]

[0098] Where E1 is the initial light field intensity of the reference light source, E2 is the initial light field intensity of the light source to be measured, v1 is the center frequency of the reference light source, v2 is the center frequency of the light source to be measured, and v beat is the frequency after the two laser beams beat each other, is the initial phase of the reference light source, is the initial phase of the light source to be measured, is the phase difference between the two laser beams;

[0099] Assume that the power spectrum density S(f) of the two laser beams are both Gaussian, the center frequency is close and their line width is determined. The power spectrum density S1(f) and S2(f) can be expressed as:

[0100]

[0101] Where v1 and v2 are the center frequencies of the two lasers, σ1 and σ2 are the standard deviations of the line widths of the two lasers, and f is the frequency of a point in the laser power spectrum density.

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

[0103]

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

[0105]

[0106] The power spectrum density of the beat frequency signal is mainly determined by the line width of the light source to be measured. The line width of the spectrum line finally measured by the double-beam heterodyne method is the line width of the light source to be measured.

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

[0108] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser, characterized in that: The laser comprises a pump light source, a combiner, an erbium-doped optical fiber, a Butterworth filter, a first single-mode bare optical fiber, a first four-core bare optical fiber, a second four-core bare optical fiber, a four-core three-ring nested coupled optical fiber resonant ring, a second single-mode bare optical fiber, a polarization controller, a first single-mode optical fiber coupler, an optical fiber isolator, a spectrometer and an optical power meter; the four core output ends of the first four-core bare optical fiber correspond to the four core input ends of the second four-core bare optical fiber in pairs, respectively, and are connected by using a fusion taper technology; the three core input ends of the first four-core bare optical fiber, including the b end, the c end and the d end, correspond to the three core output ends of the second four-core bare optical fiber, including the e end, the g end and the h end, respectively, and are coupled by single-mode optical fiber fusion splicing to form a four-core three-ring nested coupled optical fiber resonant ring; wherein, the first four-core bare optical fiber is a first single-mode bare optical fiber, a first single-mode bare optical fiber, a second single-mode bare optical fiber, a polarization controller, a first single-mode optical fiber coupler, an optical fiber isolator, a spectrometer and an optical power meter; the four core output ends of the first four-core bare optical fiber correspond to the four core input ends of the second four-core bare optical fiber in pairs, respectively, and are connected by using a fusion taper technology; the three core input ends of the first four-core bare optical fiber, including the b end, the c end and the d end, correspond to the three core output ends of the second four-core bare optical fiber, including the e end, the g end and the h end, respectively, and are coupled by using single-mode optical fiber fusion splicing to form a four-core three-ring nested coupled optical fiber resonant ring; The b-end core input end of the bare optical fiber and the e-end core output end of the second four-core bare optical fiber are fused and coupled by single-mode optical fiber to form a first optical fiber resonance ring, the d-end core input end of the first four-core bare optical fiber and the g-end core output end of the second four-core bare optical fiber are fused and coupled by single-mode optical fiber to form a second optical fiber resonance ring, the c-end core input end of the first four-core bare optical fiber and the h-end core output end of the second four-core bare optical fiber are fused and coupled by single-mode optical fiber to form a third optical fiber resonance ring, the a-end core input end of the first four-core bare optical fiber is used as the core input end of the four-core three-ring nested coupled optical fiber resonance ring, and the f-end core output end of the second four-core bare optical fiber is used as the core output end of the four-core three-ring nested coupled optical fiber resonance ring; the total resonance frequency of the four-core three-ring nested coupled optical fiber resonance ring 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 combiner, the output end of the combiner is fused and connected to the core input end of the four-core three-ring nested coupled fiber resonant ring via the erbium-doped fiber, the Butterworth filter, and the output end of the first single-mode bare fiber in sequence, the core output end of the four-core three-ring nested coupled fiber resonant ring is fused and 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 the polarization controller, the first output end of the first single-mode fiber coupler is connected to the second input end of the combiner via the fiber isolator, and 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 light source enters the erbium-doped optical fiber through the combiner to generate energy level transition, and then enters the four-core three-ring nested coupled optical fiber resonant ring through the first single-mode bare optical fiber after frequency selection filtering to reduce the number of longitudinal modes, and outputs a single longitudinal mode narrow linewidth laser with enhanced output power; the single longitudinal mode narrow linewidth laser is then output to the first single-mode optical fiber coupler for light splitting after the polarization state is adjusted by the polarization controller, and the split first light beam is transmitted to the combiner through the optical isolator to complete the optical fiber loop, and the second light beam is output to the spectrometer and the optical power meter to measure the spectrum and output power.

2. The multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser according to claim 1, characterized in that: The resonance condition of the four-core three-ring nested coupled optical fiber resonant ring is expressed as: mλ=2n eff L i Where m is the order of the mode, λ is the wavelength of the incident light, and n eff is the effective refractive index of light waves propagating in the resonant ring, L i is the length of the i-th resonant ring, i=1,2,3.

3. The multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser according to claim 1, characterized in that: The total resonant frequency FSR of the four-core three-ring nested coupled fiber resonator ring e It is expressed as: FSR e =k1×FSR1=k2×FSR2=k3×FSR3 Where k1, k2, k3 are integers; FSR i is the frequency difference between adjacent resonant modes of the i-th resonant ring, expressed as: FSR i =w / n eff L i In the formula, c is the speed of light in a vacuum, and i=1,2,3.

4. The multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser according to claim 1, characterized in that: In a multi-ring fiber resonator ring, if each FSR i The total resonant frequency FSR of the four-core three-ring nested coupled fiber resonator ring is periodically repeated and synchronized with each other. e It is expressed as: FSR e =LCM(FSR1、FSR2、FSR3) Where LCM is the least common multiple function of FSR1, FSR2, and FSR3. i is the frequency difference between adjacent resonant modes of the i-th resonant ring, i=1,2,3.

5. The multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser according to claim 1, characterized in that: 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 optical fiber fusion loss a2 are respectively expressed as: Wherein, γ0 and γ1 are the intensity loss factors of the first four-core bare optical fiber and the second four-core bare optical fiber respectively, and γ2 is the optical fiber fusion loss factor; According to the Jones matrix theory, the input optical field E of the c-port of the first four-core bare optical fiber can be obtained: c , the output light field E of the e-port of the first four-core bare optical fiber e , the output light field E of the g port of the first four-core bare fiber g , respectively expressed as: Wherein, k1 and k2 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 narrow linewidth fiber laser according to claim 1, characterized in that: The output optical field E of port b of the first four-core bare optical fiber b The output light field E of the f port of the second four-core bare optical fiber f The relationship between the output light field E of the port C of the first four-core bare fiber c The second four-core bare optical fiber outputs the light field E at port e e The relationships are: The output light field E of the c-port of the first four-core bare fiber c The light field E output from the g port of the second four-core bare optical fiber g The relationship between them is expressed as: The input optical field E of port a of the first four-core bare optical fiber a The optical field E output from the C port of the first four-core bare optical fiber c The relationship between them is expressed as: The input light field E of the four-core three-ring nested coupled fiber resonant ring a With the output light field E f The ratio is expressed as: Wherein, b and β are the transmission loss and propagation constant of the first four-core bare fiber and the second four-core bare fiber, respectively, and a2 is the fusion loss of the fiber; L1, L2, and L3 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 L=L1+L2+L3; a is the coupling loss of the first four-core bare fiber, a1 is the coupling loss a1 of the second four-core bare fiber, a2 is the fusion loss of the fiber, and k1 and k2 are the coupling strength coefficients of the first four-core bare fiber and the second four-core bare fiber, respectively.

7. The multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser according to claim 1, characterized in that: The central wavelength of the pump light source is 980nm, the output power is 100mW, and the 3dB bandwidth is 117GHz; the pump input wavelength of the combiner is 980nm, and the pump working bandwidth is 130nm; the length of the erbium-doped fiber is 5m, and the erbium ion doping concentration is 10 24 m -3 ; The operating band of the Butterworth filter is 1550nm, and the 3dB bandwidth is 1nm; the operating bands of the first single-mode bare optical fiber, the first four-core bare optical fiber, the second four-core bare optical fiber and the second single-mode bare optical fiber are all 1550nm, and the transmission loss is 0.2dB / km; the operating band of the polarization controller is 1550nm, and the insertion loss is 1dB; the operating band of the optical fiber isolator is 1550nm, and the return loss is 40dB; the bandwidth resolution of the spectrometer is 0.05nm; the measurement range of the optical power meter is -50dBm to 26dBm.

8. The multi-core fiber ring resonator nested single longitudinal mode narrow linewidth fiber laser according to claim 1, characterized in that: The splitting ratio of the four-core three-ring nested coupled optical fiber resonant ring is 25:25:25:25; wherein the length L1 of the first optical fiber resonant ring is 2m, the length L2 of the second optical fiber resonant ring is 3m, and the length L3 of the third optical fiber resonant ring is 4m.

9. A method for measuring the line width of a multi-core fiber ring resonator nested single longitudinal mode narrow line width fiber laser according to any one of claims 1 to 8, characterized in that: The line width measurement method comprises the following steps: The second light beam is used as the light source to be measured and is input into the second single-mode optical fiber coupler simultaneously with the reference light source to generate a beat frequency signal; The beat frequency signal output by the second single-mode optical fiber coupler is input to a photodetector and converted into electrical energy, and the converted electrical energy is output to an electric spectrometer, which uses a double-beam heterodyne method to determine the line width of the light source to be measured based on the electric power spectrum density of the beat frequency signal.

10. The line width measurement method of a multi-core fiber ring resonator nested single longitudinal mode narrow line width fiber laser according to claim 9, characterized in that: The field functions E1(t) and E2(t) of the two laser beams emitted by the light source to be measured and the reference light source are expressed as: Where E1 is the initial light field intensity of the reference light source, E2 is the initial light field intensity of the light source to be measured, v1 is the center frequency of the reference light source, v2 is the center frequency of the light source to be measured, and v beat is the frequency after the two laser beams beat each other, is the initial phase of the reference light source, is the initial phase of the light source to be measured, is the phase difference between the two laser beams; The power spectral densities S1(f) and S2(f) of the two laser beams are expressed as: Where v1 and v2 are the center frequencies of the two lasers, σ1 and σ2 are the standard deviations of the line widths of the two lasers, and f is the frequency of a point in the laser power spectrum density; Using the convolution property of the Gaussian function, the power spectral density function S of the beat frequency signal is beat (f) is expressed as: The power spectral density function S of the beat frequency signal beat (f) is rewritten as follows:

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