Narrow linewidth laser based on microcavity mode interference feedback

Abstract

The invention provides a narrow linewidth laser based on microcavity mode interference feedback, which is characterized in that the output end of a laser generator is connected with the first end of a first coupler, the second end of the first coupler is connected with a feedback unit through a tapering optical fiber, the third end of the first coupler is used as a laser signal output end, and the tapering optical fiber is coupled with an echo wall microcavity; the laser generator couples and transmits generated laser signals to the echo wall microcavity through the first coupler and the coupling front section of the tapering optical fiber, two echo wall microcavity modes are excited, the signals in the two modes interfere at the echo wall microcavity, and electromagnetic induction transparent mode EIT interference signals are generated. The EIT interference signal is transmitted to the feedback unit through the coupling rear section of the tapering optical fiber, and the feedback unit transmits the EIT interference signal back to the laser generator without the isolator so as to perform line width compression on the laser signal. According to the invention, the EIT interference signal is used as a feedback signal for line width compression, the feedback intensity is high and has certainty, and the problem that the Rayleigh scattering feedback signal in a conventional all-pass microcavity is weak in intensity and random is solved.

Description

Technical Field

[0001] The present invention belongs to the field of lasers, and particularly relates to a narrow-linewidth laser based on microcavity mode interference feedback. Background Art

[0002] A microcavity external cavity semiconductor laser (ECDL) directly introduces microcavity feedback outside the main cavity of the laser to achieve linewidth compression. The commonly used resonators include a standing wave cavity based on a Fabry-Perot (FP) cavity and a traveling wave cavity based on a Whispering Gallery Mode Resonator (WGMR). Compared with the standing wave cavity based on the FP cavity, the traveling wave cavity based on the WGMR uses continuous total reflection to confine the optical field, and usually can easily achieve a higher quality factor, showing a wavelength selection characteristic with a narrower bandwidth, and can obtain a better linewidth narrowing effect. According to the coupling structure, the WGMR can be divided into an all-pass type WGMR and an Add-drop type WGMR. The all-pass type WGMR uses backscattered Rayleigh scattering as the feedback signal and has the characteristics of a simple structure. However, both the amplitude and phase of the feedback signal are random, which will affect the consistency of the device and the laser output performance, and a phase controller needs to be added for regulation. Although the Add-drop type structure can generate a feedback signal with a determined intensity, it is necessary to avoid causing multiple longitudinal modes through regulation, and this configuration has a complex structure and a large coupling difficulty, inevitably increasing the insertion loss of the device, resulting in a reduction in the quality factor of this configuration, and further affecting the linewidth compression effect. Summary of the Invention

[0003] The present invention provides a narrow-linewidth laser based on microcavity mode interference feedback to solve the problems in the current narrow-linewidth lasers based on WGMR that either the intensity of the feedback signal for linewidth compression is weak and not deterministic, or the feedback signal has a high intensity and is deterministic, but has a complex structure, large loss, and poor linewidth compression effect.

[0004] According to the first aspect of the embodiments of the present invention, a narrow-linewidth laser based on microcavity mode interference feedback is provided, including a laser generator, a first coupler, a tapered fiber, a feedback unit, and a Whispering Gallery microcavity coupled to the tapered fiber. The output end of the laser generator is connected to the first end of the first coupler. The second end of the first coupler is connected to the feedback unit through the tapered fiber, and the third end is used as a laser signal output end.

[0005] The laser generator couples the generated laser signal to the whispering gallery microcavity through the first coupler and the coupling front section of the tapered fiber, exciting two whispering gallery modes. The signals in the two modes interfere at the whispering gallery microcavity to generate an electromagnetically induced transparency mode (EIT) interference signal. The EIT interference signal is coupled and transmitted to the coupling rear section of the tapered fiber, and then transmitted to the feedback unit through the coupling rear section of the tapered fiber. The feedback unit transmits the EIT interference signal back to the laser generator to compress the linewidth of the laser signal.

[0006] Optionally, the phase detuning between the signals in the two modes is 0, and for laser signals with different central wavelengths, there is a corresponding parameter combination, which includes the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling spacing and coupling position between the tapered fiber and the whispering gallery microcavity.

[0007] Optionally, the feedback unit is a circulator. The second end of the first coupler is connected to the second end of the circulator, and the third end of the circulator is connected to the first end of the circulator through the tapered fiber. The laser generator couples and transmits the generated laser signal to the second end of the circulator through the first coupler. The third end of the circulator couples the laser signal to the whispering gallery microcavity through the coupling front section of the tapered fiber, exciting two whispering gallery modes. The two modes interfere at the whispering gallery microcavity to generate an electromagnetically induced transparency mode (EIT) interference signal. The EIT interference signal is coupled and transmitted to the coupling rear section of the tapered fiber, and then transmitted to the first end of the circulator through the coupling rear section of the tapered fiber. The second end of the circulator transmits the EIT interference signal back to the laser generator through the first coupler to compress the linewidth of the laser signal.

[0008] Optionally, a phase controller is provided between the laser generator and the first coupler. The second end of the circulator transmits the EIT interference signal to the phase controller through the first coupler, and the phase controller adjusts the phase of the EIT interference signal to make the phase of the EIT interference signal match the phase of the laser signal.

[0009] Optionally, it further includes a broadband light source, a second coupler, a third coupler and a first spectral analyzer. The broadband light source is connected to the first end of the second coupler, the second end of the first coupler is connected to the second end of the second coupler, and the third end of the second coupler is connected to the second end of the circulator; the third end of the circulator is connected to the first end of the third coupler through the tapered fiber, the second end of the third coupler is connected to the first end of the circulator, and the third end is connected to the first spectral analyzer;

[0010] For laser signals with different central wavelengths, determine the parameter combinations corresponding to each central wavelength according to the following steps: control the broadband light source to generate a laser signal corresponding to the central wavelength, and control the laser generator to stop working. Adjust at least one of the parameters of the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance between the tapered fiber and the whispering gallery microcavity, and the coupling position. When the EIT interference signal appears in the first spectrometer, the parameter combination at this time corresponds to the central wavelength. Before linewidth compression, according to the corresponding relationship between each central wavelength and the parameter combination, and the central wavelength of the laser signal to be compressed generated by the laser generator, determine the corresponding parameter combination, and adjust at least one of the parameters of the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance between the tapered fiber and the whispering gallery microcavity, and the coupling position, so that the laser signal of the central wavelength will generate an EIT interference signal after passing through the coupled tapered fiber and whispering gallery microcavity.

[0011] Optionally, an isolator is further included. The third end of the first coupler is connected to one end of the isolator, and the other end of the isolator serves as the laser signal output end.

[0012] Optionally, a fourth coupler and a second spectrometer are further included. The third end of the first coupler is connected to the first end of the fourth coupler, the second end of the fourth coupler is connected to the second spectrometer, the third end is connected to one end of the isolator, and the other end of the isolator serves as the laser signal output end.

[0013] Optionally, with the coupling point of the tapered fiber and the whispering gallery microcavity as the boundary, the fiber segment of the tapered fiber on the side of the laser generator is the pre-coupling segment, and the fiber segment on the other side is the post-coupling segment;

[0014] Both the pre-coupling segment and the post-coupling segment are tapers with a gradually decreasing diameter towards the coupling point.

[0015] Optionally, the tapered fiber is drawn from a single-mode fiber.

[0016] Optionally, the EIT interference signal is formed by the interference of signals in the high-quality factor Q mode and the low Q mode. When the phase detuning between the signals in the two modes is less than the linewidth of the low Q mode, the signals in the two modes begin to interact to form a Fano line shape signal; as the phase detuning further decreases, the interaction between the signals in the two modes becomes stronger, and the Fano line shape signal has a greater contrast; when the phase detuning decreases to 0, the Fano line shape signal is converted into an EIT interference signal.

[0017] The beneficial effects of the present invention are:

[0018] 1. In the present invention, by coupling a tapered optical fiber with a whispering gallery microcavity, after the laser signal is coupled from the tapered optical fiber to the whispering gallery microcavity, two whispering gallery modes are excited to generate an EIT interference signal. The EIT interference signal is transmitted back to the laser generator by the feedback unit to compress the linewidth of the laser signal generated by the laser generator. Compared with the all-pass WGMR that uses the backward Rayleigh scattering signal to compress the linewidth, the intensity of the accumulated backward Rayleigh scattering signal is low and has strong randomness. The present invention uses this EIT interference signal as the feedback signal for linewidth compression, which can greatly improve the feedback intensity and avoid the randomness of the feedback signal. Compared with the add-drop multiplexing WGMR, which has a large number of waveguides, large insertion loss, complex device structure and difficult coupling, the present invention only needs to use one tapered optical fiber, has a small number of waveguides, simple structure, and is an all-pass structure with less insertion loss. In addition, the linewidth compression effect of the present invention is also improved.

[0019] 2. In the present invention, by setting a phase controller between the laser generator and the first coupler to adjust the phase of the EIT interference signal, the phase of the EIT interference signal and the laser signal can be matched, thereby further improving the linewidth compression effect.

[0020] 3. The present invention sets a broadband light source to generate laser signals with different central wavelengths, transmits the laser signals with different central wavelengths to the coupled tapered optical fiber and whispering gallery microcavity, and determines whether the parameters such as the diameter of the whispering gallery microcavity, the waist diameter of the tapered optical fiber, the coupling distance between the tapered optical fiber and the whispering gallery microcavity, and the coupling position are adjusted in place by observing whether an EIT interference signal appears in the spectral analyzer. Thus, the parameter combinations corresponding to each different central wavelength can be determined. When compressing the linewidth of laser signals with different central wavelengths, it is convenient to call the parameter combinations and adjust the parameters to ensure the smooth progress of linewidth compression. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic structural diagram of an embodiment of a narrow linewidth laser based on microcavity mode interference feedback of the present invention;

[0022] Figure 2 is a schematic diagram of the coupling relationship between the tapered optical fiber and the whispering gallery microcavity of the present invention;

[0023] Figure 3 is a schematic structural diagram of another embodiment of a narrow linewidth laser based on microcavity mode interference feedback of the present invention;

[0024] Figure 4 is a spectrogram of the EIT interference signal of the present invention;

[0025] Figure 5 is a schematic diagram of the linewidth before and after linewidth compression of the laser signal of the present invention;

[0026] Figure 6 It is a schematic diagram of the side mode suppression ratio before and after the line width compression of the laser signal line of the present invention;

[0027] Figure 7 It is a schematic diagram of the frequency noise before and after the line width compression of the laser signal line of the present invention. Detailed implementation manners

[0028] In order to enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and make the above-mentioned objects, features, and advantages of the embodiments of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0029] In the description of the present invention, unless otherwise specified and limited, it should be noted that the term "connection" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the communication inside two components. It can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific situations.

[0030] See Figure 1 , which is a schematic structural diagram of an embodiment of the narrow line width laser based on microcavity mode interference feedback of the present invention. Combining Figure 2 As shown, the narrow line width laser can include a laser generator 1, a first coupler C1, a tapered fiber 2, a feedback unit, and a whispering gallery microcavity 3 coupled to the tapered fiber 2. The output end of the laser generator 1 is connected to the first end of the first coupler C1. The second end of the first coupler C1 is connected to the feedback unit through the tapered fiber 2, and the third end is used as the laser signal output end. The laser generator 1 couples the generated laser signal to the whispering gallery microcavity 3 through the coupling front section 21 of the first coupler C1 and the tapered fiber 2, exciting two whispering gallery microcavity modes. The signals in the two modes interfere at the whispering gallery microcavity 3 to generate an electromagnetically induced transparency mode EIT interference signal. The EIT interference signal is coupled and transmitted to the coupling rear section 22 of the tapered fiber 2, and then transmitted to the feedback unit through the coupling rear section 22 of the tapered fiber 2. The feedback unit transmits the EIT interference signal back to the laser generator 1 to compress the line width of the laser signal.

[0031] In this embodiment, theoretically speaking, the EIT interference signal is formed by the interference of the signals in the high-quality factor Q mode and the low Q mode. Combining Figure 2 As shown, the laser signal a inCoupled from the coupling front section 21 of the tapered optical fiber to the whispering gallery microcavity 3 by means of near-field coupling to excite two whispering gallery modes a1 and a2, the intrinsic losses generated by the two are γ1 and γ2 respectively, but the sum of γ1 and γ2 is still less than the insertion loss of the add-drop multiplexing type WGMR. According to the coupled mode equation, the output spectrum is related to the phase detuning of the signals in the two modes. Among them, when the phase detuning of the signals in the two modes is greater than the linewidth of the low-Q mode, the signals in the two modes do not interact with each other, and the transmission spectrum a output from the coupling rear section of the tapered optical fiber out appears two separate dips; when the phase detuning of the signals in the two modes is less than the linewidth of the low-Q mode, the signals in the two modes begin to interact with each other to form a Fano line shape signal; as the phase detuning further decreases, the interaction between the signals in the two modes becomes stronger, and the Fano line shape signal has a greater contrast; when the phase detuning decreases to 0, the Fano line shape signal is converted into an EIT interference signal. It can be seen that in order to generate an EIT interference signal, the phase detuning between the signals in the two modes needs to be 0, and for laser signals with different central wavelengths, there are corresponding parameter combinations, and the parameter combination includes the diameter of the whispering gallery microcavity, the waist diameter of the tapered optical fiber, the coupling distance and the coupling position between the tapered optical fiber and the whispering gallery microcavity. When the central wavelength of the laser signal to be compressed changes, at least one parameter in the parameter combination needs to be adjusted. The waist diameter of the tapered optical fiber can be less than 2 μm, for example, 1.56 μm; the whispering gallery microcavity can be prepared into a microsphere cavity by the arc discharge method.

[0032] In addition, the feedback unit can be a circulator OC. The second end of the first coupler C1 is connected to the second end of the circulator OC. The third end of the circulator OC is connected to the first end of the circulator OC through the tapered optical fiber. The laser generator 1 couples and transmits the generated laser signal to the second end of the circulator OC through the first coupler C1. The third end of the circulator OC transmits the laser signal to the whispering gallery microcavity 3 through the coupling front section 21 of the tapered optical fiber 2 to excite two whispering gallery modes. The signals in the two modes interfere at the whispering gallery microcavity 3 to generate an electromagnetically induced transparency mode EIT interference signal. The EIT interference signal is coupled and transmitted to the coupling rear section 22 of the tapered optical fiber 2, and then is transmitted to the first end of the circulator OC through the coupling rear section 22 of the tapered optical fiber 2. The second end of the circulator OC transmits the EIT interference signal back to the laser generator through the first coupler C1 to compress the linewidth of the laser signal. Wherein the tapered optical fiber 2 is bounded by its coupling with the whispering gallery microcavity. The optical fiber section on the side of the laser generator 1 is the coupling front section 21, and the optical fiber section on the other side is the coupling rear section 22; both the coupling front section 21 and the coupling rear section 22 can be tapered with a gradually decreasing diameter towards the coupling; the tapered optical fiber 2 can be drawn from a single-mode optical fiber.

[0033] As can be seen from the above embodiments, in the present invention, by coupling a tapered optical fiber with a whispering gallery microcavity, after the laser signal is coupled and transmitted from the tapered optical fiber to the whispering gallery microcavity, two whispering gallery microcavity modes are excited to generate an EIT interference signal. The EIT interference signal is transmitted back to the laser generator by the feedback unit to compress the linewidth of the laser signal generated by the laser generator. Compared with the all-pass type WGMR that uses the backward Rayleigh scattering signal to compress the linewidth, the intensity of the accumulated backward Rayleigh scattering signal is relatively low and has strong randomness. The present invention uses this EIT interference signal as the feedback signal for linewidth compression, which can greatly improve the feedback intensity and avoid the randomness of the feedback signal. Compared with the add-drop multiplexing type WGMR, which has a large number of waveguides, large insertion loss, complex device structure and difficult coupling, the present invention only needs to use one tapered optical fiber, with fewer waveguides, simple structure, and the present invention is an all-pass type structure with less insertion loss. In addition, the linewidth compression effect of the present invention is also improved.

[0034] See Figure 3 , which is a schematic structural diagram of another embodiment of the narrow-linewidth laser based on microcavity mode interference feedback of the present invention. Figure 3 And Figure 1 The difference between the narrow-linewidth laser based on microcavity mode interference feedback shown is that a phase controller PC can be provided between the laser generator 1 and the first coupler C1. The second end of the circulator OC transmits the EIT interference signal to the phase controller PC through the first coupler C1, and the phase controller PC adjusts the phase of the EIT interference signal so that the phase of the EIT interference signal matches the phase of the laser signal. By setting a phase controller between the laser generator and the first coupler in the present invention to adjust the phase of the EIT interference signal, the phase of the EIT interference signal and the laser signal can be matched, so that the linewidth compression effect can be further improved.

[0035] Figure 3 And Figure 1 The difference between the narrow-linewidth laser based on microcavity mode interference feedback shown is that an isolator ISO can also be included. The third end of the first coupler C1 is connected to one end of the isolator ISO, and the other end of the isolator ISO is used as the laser signal output end, so as to ensure the stable output of the laser signal after linewidth compression.

[0036] Figure 3 And Figure 1The difference of the narrow-linewidth laser based on the microcavity mode interference feedback is that it may further include a broadband light source BBS, a second coupler C2, a third coupler C3, and a first optical spectrum analyzer OSA1. The broadband light source BBS is connected to the first end of the second coupler C2. The second end of the first coupler C1 is connected to the second end of the second coupler C2. The third end of the second coupler C2 is connected to the second end of the circulator OC. The third end of the circulator OC is connected to the first end of the third coupler C3 through the tapered fiber 2. The second end of the third coupler C3 is connected to the first end of the circulator OC, and the third end is connected to the first optical spectrum analyzer OSA1. For laser signals with different central wavelengths, the parameter combinations corresponding to each central wavelength are determined according to the following steps: control the broadband light source to generate a laser signal corresponding to the central wavelength, and control the laser generator 1 to stop working. Adjust at least one of the parameters including the diameter of the whispering gallery microcavity 3, the waist diameter of the tapered fiber 2, the coupling distance between the tapered fiber 2 and the whispering gallery microcavity 3, and the coupling position. When the EIT interference signal appears in the first optical spectrum analyzer OSA1, the parameter combination at this time is corresponding to the central wavelength. Before linewidth compression, according to the corresponding relationship between each central wavelength and the parameter combination, and the central wavelength of the laser signal to be compressed generated by the laser generator, determine the corresponding parameter combination, and adjust at least one of the parameters including the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance between the tapered fiber and the whispering gallery microcavity, and the coupling position, so that the laser signal with the central wavelength will generate an EIT interference signal after passing through the coupled tapered fiber and whispering gallery microcavity. The coupling ratios of the second end and the third end of the first coupler C1 can be 70:30, and the coupling ratios of the second end and the third end of the third coupler C3 can be 90:10.

[0037] The present invention sets a broadband light source to generate laser signals with different central wavelengths, transmits the laser signals with different central wavelengths to the coupled tapered fiber and whispering gallery microcavity, and determines whether the parameters such as the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance between the tapered fiber and the whispering gallery microcavity, and the coupling position are adjusted in place by observing whether the EIT interference signal appears in the optical spectrum analyzer. Thus, the parameter combinations corresponding to each different central wavelength can be determined. When performing linewidth compression on laser signals with different central wavelengths, it is convenient to call the parameter combinations and adjust the parameters to ensure the smooth progress of linewidth compression.

[0038] Figure 3 And Figure 1The narrow-linewidth laser based on microcavity mode interference feedback shown in the figure is different in that it may further include a fourth coupler C4 and a second optical spectrum analyzer OSA2. The third end of the first coupler C1 is connected to the first end of the fourth coupler C4. The second end of the fourth coupler C4 is connected to the second optical spectrum analyzer, and the third end is connected to one end of the isolator ISO. The other end of the isolator ISO serves as the laser signal output end. In the present invention, the second optical spectrum analyzer is provided to monitor the linewidth compression condition of the laser signal. The coupling ratio between the second end and the third end of the fourth coupler C4 may be 10:90.

[0039] The spectrogram of the EIT interference signal of the present invention is as Figure 4 shown. The EIT signal at 1549.8720 nm is selected as the feedback waveform. Figure 5 is the schematic diagram of the linewidth of the laser signal of the present invention before and after linewidth compression; after Lorentz fitting, the linewidth during the free running of the DFB is about 1.18 MHz, and the linewidth after compression is about 4.73 kHz. Figure 6 is the schematic diagram of the side mode suppression ratio of the laser signal of the present invention before and after linewidth compression; the SMSR before and after compression is 35.29 dB and 60.96 dB respectively. Figure 7 is the schematic diagram of the frequency noise of the laser signal of the present invention before and after linewidth compression; the frequency noise during the free running is 5.43×10 4 Hz 2 / Hz, and the corresponding intrinsic linewidth is 170.50 kHz. The frequency noise after compression is 17.03 Hz 2 / Hz, and the corresponding intrinsic linewidth is 53.47 Hz.

[0040] As can be seen from the above embodiments, in the present invention, by coupling the tapered fiber with the whispering gallery microcavity, after the laser signal is coupled from the tapered fiber to the whispering gallery microcavity, two whispering gallery modes are excited to generate an EIT interference signal. The EIT interference signal is transmitted back to the laser generator by the feedback unit to compress the linewidth of the laser signal generated by the laser generator. Compared with the all-pass type WGMR that uses the backward Rayleigh scattering signal to compress the linewidth, the intensity of the accumulated backward Rayleigh scattering signal is low and has strong randomness. The present invention uses the EIT interference signal as the feedback signal for linewidth compression, which can greatly improve the feedback intensity and avoid the randomness of the feedback signal at the same time; compared with the add-drop multiplexing type WGMR, it has a large number of waveguides, large insertion loss, complex device structure and large coupling difficulty. The present invention only needs to use one tapered fiber, has a small number of waveguides, simple structure, and the present invention is an all-pass type structure with less insertion loss; in addition, the linewidth compression effect of the present invention is also improved.

[0041] Other embodiments of the present invention will be readily apparent to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include known common general knowledge or conventional technical means in the technical field not disclosed in the present invention. The specification and examples are only to be considered as exemplary, and the true scope and spirit of the invention are pointed out by the following claims.

[0042] It should be understood that the present invention is not limited to the exact structures already described and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only governed by the appended claims.

Claims

1. A narrow linewidth laser based on microcavity mode interference feedback, characterized in that It includes a laser generator, a first coupler, a tapered fiber, a feedback unit, and a whispering gallery microcavity coupled to the tapered fiber. The output end of the laser generator is connected to the first end of the first coupler. The second end of the first coupler is connected to the feedback unit through the tapered fiber, and the third end serves as the laser signal output end. The laser generator couples the laser signal to the whispering gallery microcavity through the first coupler and the front section of the tapered fiber coupling, exciting two whispering gallery modes. The two modes interfere at the whispering gallery microcavity to generate an electromagnetically induced transparency mode (EIT) interference signal. The EIT interference signal is coupled and transmitted to the rear section of the tapered fiber coupling, and then transmitted to the feedback unit through the rear section of the tapered fiber coupling. The feedback unit transmits the EIT interference signal back to the laser generator to compress the linewidth of the laser signal.

2. The narrow linewidth laser based on microcavity mode interference feedback according to claim 1, wherein The phase detuning between the signals in the two modes is 0, and for laser signals with different central wavelengths, there are corresponding parameter combinations, which include the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance and coupling position between the tapered fiber and the whispering gallery microcavity.

3. The narrow linewidth laser based on microcavity mode interference feedback according to claim 1 or 2, characterized in that, The feedback unit is a circulator. The second end of the first coupler is connected to the second end of the circulator. The third end of the circulator is connected to the first end of the circulator through the tapered fiber. The laser generator couples and transmits the generated laser signal to the second end of the circulator through the first coupler. The third end of the circulator couples the laser signal to the whispering gallery microcavity through the front section of the tapered fiber coupling, exciting two whispering gallery modes. The two modes interfere at the whispering gallery microcavity to generate an electromagnetically induced transparency mode (EIT) interference signal. The EIT interference signal is coupled and transmitted to the rear section of the tapered fiber coupling, and then transmitted to the first end of the circulator through the rear section of the tapered fiber coupling. The second end of the circulator transmits the EIT interference signal back to the laser generator through the first coupler to compress the linewidth of the laser signal.

4. The narrow linewidth laser based on microcavity mode interference feedback according to claim 3, characterized in that, A phase controller is provided between the laser generator and the first coupler. The second end of the circulator transmits the EIT interference signal to the phase controller through the first coupler. The phase controller adjusts the phase of the EIT interference signal to make the phase of the EIT interference signal match the phase of the laser signal.

5. The narrow linewidth laser based on microcavity mode interference feedback according to claim 4, wherein It further includes a broadband light source, a second coupler, a third coupler, and a first spectral analyzer. The broadband light source is connected to the first end of the second coupler. The second end of the first coupler is connected to the second end of the second coupler. The third end of the second coupler is connected to the second end of the circulator. The third end of the circulator is connected to the first end of the third coupler through the tapered fiber. The second end of the third coupler is connected to the first end of the circulator, and the third end is connected to the first spectral analyzer. For laser signals with different central wavelengths, determine the parameter combinations corresponding to each central wavelength according to the following steps: Control the broadband light source to generate a laser signal corresponding to the central wavelength, and control the laser generator to stop working. Adjust at least one of the parameters including the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance between the tapered fiber and the whispering gallery microcavity, and the coupling position. When the EIT interference signal appears in the first spectrum analyzer, the parameter combination at this time corresponds to the central wavelength. Before linewidth compression, according to the correspondence between each central wavelength and the parameter combination, and the central wavelength of the laser signal to be compressed generated by the laser generator, determine the corresponding parameter combination, and adjust at least one of the parameters including the diameter of the whispering gallery microcavity, the waist diameter of the tapered fiber, the coupling distance between the tapered fiber and the whispering gallery microcavity, and the coupling position, so that the laser signal with this central wavelength will generate an EIT interference signal after passing through the coupled tapered fiber and whispering gallery microcavity.

6. The narrow linewidth laser based on microcavity mode interference feedback according to claim 4, wherein It further includes an isolator. The third end of the first coupler is connected to one end of the isolator, and the other end of the isolator serves as the laser signal output end.

7. The narrow linewidth laser based on microcavity mode interference feedback according to claim 6, characterized in that, It further includes a fourth coupler and a second spectrum analyzer. The third end of the first coupler is connected to the first end of the fourth coupler. The second end of the fourth coupler is connected to the second spectrum analyzer, and the third end is connected to one end of the isolator. The other end of the isolator serves as the laser signal output end.

8. The narrow linewidth laser based on microcavity mode interference feedback according to claim 1, wherein Taking the coupling point of the tapered fiber and the whispering gallery microcavity as the boundary, the fiber section of the tapered fiber on the laser generator side is the pre-coupling section, and the fiber section on the other side is the post-coupling section; Both the pre-coupling section and the post-coupling section are tapers with a gradually decreasing diameter towards the coupling point.

9. The narrow linewidth laser based on microcavity mode interference feedback according to claim 1, wherein The tapered fiber is drawn from a single-mode fiber.

10. The narrow linewidth laser based on microcavity mode interference feedback according to claim 1 or 2, characterized in that, The EIT interference signal is formed by the interference of signals in the high-quality factor Q mode and the low-Q mode. When the phase detuning between the signals in the two modes is less than the linewidth of the low-Q mode, the signals in the two modes start to interact with each other to form a Fano line shape signal; as the phase detuning further decreases, the interaction between the signals in the two modes becomes stronger, and the Fano line shape signal has a greater contrast; when the phase detuning decreases to 0, the Fano line shape signal is converted into an EIT interference signal.

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