Optical fiber single longitudinal mode laser system with stable frequency

Abstract

The invention discloses an optical fiber single longitudinal mode laser system with stable frequency, and belongs to the technical field of laser. The system is composed of a pump light source, a wavelength division multiplexer, a section of erbium-doped optical fiber, two optical fiber collimating mirrors, two parallel plates, an optical fiber circulator, an optical fiber grating, three optical fiber couplers, an optical fiber isolator, two piezoelectric ceramics, a photoelectric detector, a signal generator and a signal processing circuit. The optical fiber and the optical device form an optical fiber composite ring laser resonant cavity comprising two sub-ring resonant cavities. An erbium-doped fiber is used as a gain medium, two parallel flat plates form a Fabry-Perot etalon, light penetrating through the Fabry-Perot etalon is a row of stripes at equal intervals, and a fiber bragg grating reflects light of one stripe to serve as a gain spectrum of an optical fiber composite ring laser resonant cavity, so that the optical fiber single longitudinal mode laser is formed. The signal generator sends a sinusoidal signal to drive the piezoelectric ceramic to tune the cavity length of the optical fiber composite ring laser resonant cavity; after a signal detected by the photoelectric detector passes through the signal processing circuit, the other piezoelectric ceramic is driven to adjust the cavity length of the optical fiber composite ring laser resonant cavity and compensate the influence of environmental interference on the cavity length of the optical fiber composite ring laser resonant cavity, so that the laser frequency is stabilized at the peak value of a gain spectrum, and the system outputs single longitudinal mode laser with stable frequency.

Description

Technical Field

[0001] The invention relates to the field of laser technology, and in particular to a frequency-stable optical fiber single longitudinal mode laser system. Background Art

[0002] The existing literature close to this technology is as follows:

[0003] [1] Sujuan Feng, Qinghe Mao, Yunyun Tian, ​​Yan Ma, Wencai Li, and Li Wei, Widely Tunable Single Longitudinal Mode Fiber Laser With Cascaded Fiber-RingSecondary Cavity, IEEE Photonics Technology Letters, Vol. 25, No. 4, February 15, 2013, P. 323-326. (IEEE Photonics Technology Letters (IEEE Photonics Technology Letters), Volume 25, Issue 4, P.323-326)

[0004] The technical principle of reference [1] is as follows Figure 1 shown.

[0005] This system is a fiber composite ring resonator laser composed of two cascaded fiber ring sub-cavities (Type I and Type II) and the main fiber ring cavity. The light emitted by the pump light source Pump LD reaches the erbium-doped fiber EDF after passing through the wavelength division multiplexer WDM, and excites the erbium-doped fiber EDF to emit fluorescence in the 1550nm band. After passing through the fiber circulator CIR, this fluorescence reaches the fiber grating FBG, and the light of the wavelength that meets the Bragg condition of the fiber grating FBG is reflected by the fiber grating FBG, passes through the fiber circulator CIR again, and reaches the fiber coupler OC1; the light is divided into two paths by the fiber coupler OC1, one path passes through the fiber isolator ISO1, and the other path passes through the Type I sub-cavity and is divided into two paths by the fiber coupler OC1 again, one path passes through the fiber isolator ISO1, and the other path passes through the Type I sub-cavity, and so on. The light passing through the fiber isolator ISO1 passes through the fiber coupler OC2 and the fiber coupler OC3, and is then split into two paths by the fiber coupler OC3. One path reaches the fiber coupler OC4, and the other path passes through the Type II sub-cavity, and then passes through the fiber coupler OC2 and the fiber coupler OC3 again. The light is then split into two paths by the fiber coupler OC3, and one path reaches the fiber coupler OC4, and the other path passes through the Type II sub-cavity, and this cycle repeats. The light reaching the fiber coupler OC4 is then split into two paths by the fiber coupler OC4, and one path is output after passing through the fiber isolator ISO2, and the other path passes through the wavelength division multiplexer WDM and reaches the erbium-doped fiber EDF. When the light passes through the erbium-doped fiber EDF, the light intensity is amplified, and the light with amplified light intensity is repeatedly transmitted along the above optical path. When the gain is greater than the loss, the laser is output from the other end of the fiber coupler OC4. The lengths of the two fiber ring subcavities Type I and Type II are appropriately selected so that the longitudinal mode spacing of the fiber composite ring resonator cavity is expanded to 0.5 to 1 times the 3-dB bandwidth of the reflection spectrum of the fiber grating FBG, so that the fiber grating FBG reflects only one longitudinal mode, thereby achieving single longitudinal mode resonance and outputting single longitudinal mode laser. By stretching the fiber grating FBG, the reflection spectrum of the fiber grating FBG can be adjusted, thereby outputting a single longitudinal mode laser with tunable wavelength.

[0006] The problems and shortcomings of the above prior art are:

[0007] 1. The lengths of the two sub-cavities and the main cavity are affected by environmental disturbances such as temperature drift and vibration, making it difficult for the output laser to suppress the occurrence of mode hopping.

[0008] 2. The length of the two sub-cavities and the main cavity is affected by environmental interference such as temperature drift and vibration, making it difficult to stabilize the output laser frequency. Summary of the invention

[0009] The present invention uses a 980nm pump light source (S) as a pump light source and an erbium-doped fiber EDF as a gain medium; two fiber pigtails of a fiber coupler are used to form two fiber sub-ring resonant cavities with a fiber main ring cavity, and then the two fiber sub-ring resonant cavities form a fiber composite ring laser resonant cavity. Two parallel plates form a Fabry-Perot etalon, and the light passing through the Fabry-Perot etalon is a row of equally spaced stripes. A fiber grating FBG is used as a wavelength selection element, and the fiber grating FBG reflects the light of a stripe passing through the Fabry-Perot etalon as the gain spectrum of the fiber composite ring laser resonant cavity. Light that meets the resonance condition of the fiber composite ring laser resonant cavity and has a frequency within the gain spectrum resonates, and when the gain of the fiber composite ring laser resonant cavity is greater than the loss, the system outputs a single longitudinal mode laser. The signal generator sends out a sinusoidal signal to drive a piezoelectric ceramic to sinusoidally tune the cavity length of the fiber composite ring laser resonant cavity. The optical signal detected by the photodetector is processed by the signal processing circuit, and then drives another piezoelectric ceramic to adjust the cavity length of the fiber composite ring laser resonant cavity, so as to compensate for the influence of environmental interference such as temperature drift and vibration on the cavity length of the fiber composite ring laser resonant cavity, stabilize the laser frequency at the peak of the gain spectrum, and thus make the system output a single longitudinal mode laser with stable frequency.

[0010] The present invention is achieved through the following technical solutions.

[0011] A frequency-stable optical fiber single longitudinal mode laser system is composed of a 980nm pump light source (S), a wavelength division multiplexer WDM, a section of erbium-doped optical fiber EDF, two optical fiber collimators G1 and G2, two parallel plates P1 and P2, an optical fiber circulator C, an optical fiber grating FBG, three optical fiber couplers N1, N2 and N3, an optical fiber isolator I, two piezoelectric ceramics PZT1 and PZT2, a photodetector PD, a signal generator (B1) and a signal processing circuit (B2).

[0012] The light emitted by the 980nm pump light source (S) is coupled into the fiber laser resonant cavity through the wavelength division multiplexer WDM. When passing through the erbium-doped fiber EDF, it excites the erbium-doped fiber EDF to emit fluorescence in the 1550nm band. After being collimated by the fiber collimator G1, this fluorescence is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2. The light passing through the Fabry-Perot etalon is a row of equally spaced stripes. After being coupled into the fiber collimator G2, this light passes through the fiber circulator C and reaches the fiber grating FBG. The fiber grating FBG reflects the light of one stripe back as the gain spectrum of the fiber composite ring laser resonant cavity, and the light of the remaining stripes is transmitted out of the system after passing through the fiber grating FBG. The light reflected back by the fiber grating FBG passes through the fiber circulator C again, and passes through the fiber coupler N1, and the light is divided into two paths by the fiber coupler N1. One path of light passes through the optical fiber L1, reaches the optical fiber end face A, is reflected back by the optical fiber end face A, passes through the optical fiber coupler N1 again, passes through the optical fiber isolator I, reaches the optical fiber coupler N2, and is divided into two paths by the optical fiber coupler N2. One path of light is output through the optical fiber coupler N3, and the other path of light passes through the wavelength division multiplexer WDM, passes through the erbium-doped optical fiber EDF again, and the light intensity is amplified by the erbium-doped optical fiber EDF. The light with amplified light intensity is collimated by the optical fiber collimator G1 again, and is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2 again. After passing through the Fabry-Perot etalon, the light is coupled into the optical fiber collimator G2 again, passes through the optical fiber circulator C again, reaches the optical fiber grating FBG again, is reflected back by the optical fiber grating FBG again, passes through the optical fiber circulator C again, passes through the optical fiber coupler N1 again, and is divided into two paths by the optical fiber coupler N1. One path of light passes through the optical fiber L1, reaches the optical fiber end face A again, and is reflected back by the optical fiber end face A again. This light path is transmitted back and forth in this way, and this light path forms a fiber sub-ring resonant cavity.

[0013] The other light that passes through the fiber coupler N1 passes through the fiber L2, reaches the fiber end face B, is reflected back by the fiber end face B, passes through the fiber coupler N1 again, passes through the fiber isolator I, reaches the fiber coupler N2, and is split into two paths by the fiber coupler N2. One path of light is output through the fiber coupler N3, and the other path of light passes through the wavelength division multiplexer WDM, passes through the erbium-doped fiber EDF again, and the light intensity is amplified by the erbium-doped fiber EDF. The light with amplified light intensity is collimated by the fiber collimator G1 again, and is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2 again. After passing through the Fabry-Perot etalon, the light is coupled into the fiber collimator G2 again, passes through the fiber circulator C again, reaches the fiber grating FBG again, is reflected back by the fiber grating FBG again, passes through the fiber circulator C again, passes through the fiber coupler N1 again, and is split into two paths by N1 again. One path of light passes through the fiber L2, reaches the fiber end face B again, and is reflected back by the fiber end face B again. This light path is transmitted back and forth in this way, and this light path forms another fiber sub-ring resonant cavity.

[0014] The two fiber sub-ring resonators constitute a fiber composite ring laser resonator, which satisfies the resonance conditions of the two fiber sub-ring resonators at the same time, and the light with a frequency within the gain spectrum of the fiber composite ring laser resonator resonates. When the gain of the fiber composite ring laser resonator is greater than the loss, a single longitudinal mode laser is output from one end of the fiber coupler N2.

[0015] The signal generator (B1) sends a sinusoidal signal to drive the piezoelectric ceramic PZT1, and the piezoelectric ceramic PZT1 drives the fiber collimator G1 to move back and forth along the optical path in a sinusoidal pattern, thereby sinusoidally tuning the cavity length of the fiber composite ring laser resonant cavity. The output single longitudinal mode laser is divided into two paths after passing through the fiber coupler N3, one path is used as the output light, and the other path is detected by the photodetector PD. The output signal of the photodetector PD is processed by the signal processing circuit (B2) to drive the piezoelectric ceramic PZT2, and the piezoelectric ceramic PZT2 drives the fiber collimator G2 to move along the optical path, compensating for the influence of environmental interference such as temperature drift and vibration on the cavity length of the fiber composite ring laser resonant cavity, so that the laser frequency is stabilized at the gain spectrum peak of the fiber composite ring laser resonant cavity, thereby achieving the stability of the laser frequency.

[0016] Furthermore, as a preferred solution, two fiber pigtails of the fiber coupler are used to form two fiber sub-ring resonant cavities with the main ring cavity, and then the two fiber sub-ring resonant cavities form a fiber composite ring laser resonant cavity. The longitudinal mode spacing of the fiber composite ring laser resonant cavity can be adjusted by adjusting the difference in the lengths of the two fiber pigtails of the fiber coupler. When the longitudinal mode spacing of the fiber composite ring laser resonant cavity is equivalent to the gain spectrum width of the laser resonant cavity, a single longitudinal mode resonance is generated in the fiber composite ring laser resonant cavity, and the system outputs a single longitudinal mode laser.

[0017] Furthermore, as a preferred solution, the light of one stripe reflected by the fiber grating and transmitted through the Fabry-Perot etalon is used as the gain spectrum of the fiber composite ring laser resonant cavity. The spectral width of this gain spectrum can be adjusted by adjusting the reflectivity of the two cavity mirrors of the Fabry-Perot etalon and adjusting the cavity length of the Fabry-Perot etalon.

[0018] Furthermore, as a preferred solution, a sinusoidal signal is used to drive a piezoelectric ceramic to perform sinusoidal tuning on the cavity length of the fiber composite ring laser resonant cavity, and the output laser is detected by a photodetector. After the signal detected by the photodetector is processed by a signal processing circuit, another piezoelectric ceramic is driven to adjust the cavity length of the fiber composite ring laser resonant cavity, thereby compensating for the influence of environmental interference such as temperature drift and vibration on the cavity length of the fiber composite ring laser resonant cavity, so that the laser frequency is stabilized at the gain spectrum peak of the fiber composite ring laser resonant cavity, thereby achieving the stabilization of the laser frequency.

[0019] The beneficial effects of the present invention mainly include three:

[0020] 1. The present invention uses two fiber pigtails of the fiber coupler to form two fiber sub-ring resonant cavities with the main ring cavity, and then the two fiber sub-ring resonant cavities form a fiber composite ring laser resonant cavity. The longitudinal mode spacing of the fiber composite ring laser resonant cavity can be adjusted by adjusting the difference in the lengths of the two fiber pigtails of the fiber coupler. When the longitudinal mode spacing of the fiber composite ring laser resonant cavity is equivalent to the gain spectrum width of the fiber composite ring laser resonant cavity, a single longitudinal mode resonance is generated in the fiber composite ring laser resonant cavity, and the system outputs a single longitudinal mode laser.

[0021] 2. The present invention uses the light of a stripe reflected by the fiber grating through the Fabry-Perot etalon as the gain spectrum of the fiber composite ring laser resonant cavity. The spectral width of this gain spectrum can be adjusted by adjusting the reflectivity of the two cavity mirrors of the Fabry-Perot etalon and adjusting the cavity length of the Fabry-Perot etalon.

[0022] 3. The present invention uses a sinusoidal signal to drive a piezoelectric ceramic to perform sinusoidal tuning on the cavity length of the fiber composite ring laser resonant cavity, and the output laser is detected by a photodetector. After the signal detected by the photodetector is processed by a signal processing circuit, another piezoelectric ceramic is driven to adjust the cavity length of the fiber composite ring laser resonant cavity, compensating for the influence of environmental interference such as temperature drift and vibration on the cavity length of the fiber composite ring laser resonant cavity, so that the laser frequency is stabilized at the gain spectrum peak of the fiber composite ring laser resonant cavity, thereby achieving the stabilization of the laser frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the schematic diagram of the prior art document [1];

[0024] Figure 2 It is a schematic diagram of the present invention. DETAILED DESCRIPTION

[0025] The following is combined with Figure 2 The present invention is further described with specific embodiments.

[0026] like Figure 2 As shown, it consists of a 980nm pump light source (S), a wavelength division multiplexer WDM, a section of erbium-doped fiber EDF, two fiber collimators G1 and G2, two parallel plates P1 and P2, a fiber circulator C, a fiber grating FBG, three fiber couplers N1, N2, and N3, a fiber isolator I, two piezoelectric ceramics PZT1 and PZT2, a photodetector PD, a signal generator (B1), and a signal processing circuit (B2).

[0027] This system uses a 980nm pump light source (S) as the pump light source and erbium-doped fiber as the gain medium. The end faces A and B of the two pigtails of the fiber coupler N1 are coated with a high-reflection film in the 1550nm band. The two pigtails of the fiber coupler N1 are used to form two fiber sub-ring resonant cavities with the fiber main ring cavity, and then the two fiber sub-ring resonant cavities form a fiber composite ring laser resonant cavity. By adjusting the difference in the length of the two pigtails of the fiber coupler N1, the longitudinal mode spacing of the fiber composite ring laser resonant cavity can be adjusted. One side of the two parallel plates P1 and P2 are coated with a partial reflection film in the 1550nm band, and the other side is coated with an anti-reflection film in the 1550nm band. The parallel plates P1 and P2 form a Fabry-Perot etalon, and the light passing through the Fabry-Perot etalon is a row of equally spaced fringes. Fiber Bragg grating (FBG) is used as a wavelength selection element, and the light of one stripe reflected through the Fabry-Perot etalon is used as the gain spectrum of the fiber composite ring laser resonant cavity. The light of the other stripes is transmitted out of the system after passing through the fiber Bragg grating (FBG). When the longitudinal mode spacing of the fiber composite ring laser resonant cavity is equivalent to the spectral width of the gain spectrum, single longitudinal mode resonance is achieved. At the same time, the resonance conditions of the two fiber sub-ring resonant cavities are met, and the light with a frequency within the gain spectrum resonates. When the gain of the fiber composite ring laser resonant cavity is greater than the loss, a single longitudinal mode laser is output from one end of the fiber coupler N2. The signal generator (B1) sends a sinusoidal signal with a frequency of f0, driving the piezoelectric ceramic PZT1 to drive the fiber collimator G1 to move back and forth along the optical path in a sinusoidal law, thereby sinusoidally tuning the cavity length of the fiber composite ring laser resonant cavity. The output laser is detected by the photodetector PD after passing through the fiber coupler N3. After the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the piezoelectric ceramic PZT2 is driven to drive the fiber collimator G2 to move along the optical path, thereby compensating for the influence of environmental interference such as temperature drift and vibration on the cavity length of the fiber composite ring laser resonant cavity, so that the laser frequency is stabilized at the gain spectrum peak of the fiber composite ring laser resonant cavity, thereby outputting a single longitudinal mode laser with stable frequency.

[0028] The light emitted by the 980nm pump light source (S) is coupled into the fiber laser resonant cavity through the wavelength division multiplexer WDM, and passes through the erbium-doped fiber EDF to excite the erbium-doped fiber EDF to emit fluorescence in the 1550nm band. After being collimated by the fiber collimator G1, this fluorescence is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2. The light passing through the Fabry-Perot etalon is a row of equally spaced stripes. After this light is coupled into the fiber collimator G2, it passes through the fiber circulator C and reaches the fiber grating FBG. The fiber grating FBG reflects the light of one stripe back as the gain spectrum of the laser resonant cavity, and the light of the remaining stripes is transmitted out of the system after passing through the fiber grating FBG. The light reflected by the fiber grating FBG passes through the fiber circulator C again, passes through the fiber coupler N1, and is divided into two paths by the fiber coupler N1. One path of light passes through the fiber L1, reaches the fiber end face A, is reflected back by the fiber end face A, passes through the fiber coupler N1 again, passes through the fiber isolator I, reaches the fiber coupler N2, and is divided into two paths by the fiber coupler N2. One path of light is output through the fiber coupler N3, and the other path of light passes through the wavelength division multiplexer WDM, passes through the erbium-doped fiber EDF again, and the light intensity is amplified by the erbium-doped fiber EDF. The light with amplified light intensity is collimated by the fiber collimator G1 again, and is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2 again. After the light passes through the Fabry-Perot standard, it is coupled into the fiber collimator G2 again, passes through the fiber circulator C again, reaches the fiber grating FBG again, is reflected back by the fiber grating FBG again, passes through the fiber circulator C again, and passes through the fiber coupler N1 again. The light is split into two paths by the fiber coupler N1. One path of light passes through the fiber L1 again, reaches the fiber end face A again, and is reflected back by the fiber end face A again. This light path is transmitted back and forth in this way. This optical path constitutes a fiber sub-ring resonant cavity.

[0029] The other path of light that passes through the fiber coupler N1 passes through the optical fiber L2, reaches the fiber end face B, is reflected back by the fiber end face B, passes through the fiber coupler N1 again, passes through the optical fiber isolator I, reaches the fiber coupler N2, and is divided into two paths by the fiber coupler N2. One path of light is output through the fiber coupler N3, and the other path of light passes through the wavelength division multiplexer WDM, passes through the erbium-doped fiber EDF again, and the light intensity is amplified by the erbium-doped fiber EDF. The light with amplified light intensity is collimated by the fiber collimator G1 again, and is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2 again. After the light passes through the Fabry-Perot etalon, it is coupled into the fiber collimator G2 again, passes through the fiber circulator C again, reaches the fiber grating FBG again, is reflected back by the fiber grating FBG again, passes through the fiber circulator C again, and passes through the fiber coupler N1 again. The light is split into two paths by the fiber coupler N1. One path passes through the fiber L2 again, reaches the fiber end face B again, and is reflected back by the fiber end face B again. This light path is transmitted back and forth in this way. This optical path constitutes another fiber sub-ring resonant cavity.

[0030] The two fiber sub-ring resonators form a fiber composite ring laser resonator, which satisfies the resonance conditions of the two fiber sub-ring resonators and resonates the light with a frequency within the gain spectrum of the fiber composite ring laser resonator. The longitudinal mode spacing of the fiber composite ring laser resonator satisfies equation (1).

[0031]

[0032] In the above formula, c is the speed of light, n is the refractive index of the optical fiber, and L1 and L2 are the lengths of the two pigtails of the optical fiber coupler N1.

[0033] It can be seen from equation (1) that the longitudinal mode spacing of the fiber composite ring laser resonant cavity can be adjusted by adjusting the value of |L1-L2|. By adjusting the value of |L1-L2|, the longitudinal mode spacing of the fiber composite ring laser resonant cavity is equal to the gain spectrum width of the laser resonant cavity, and a single longitudinal mode resonance is generated in the fiber composite ring laser resonant cavity. When the gain of the fiber composite ring laser resonant cavity is greater than the loss, a single longitudinal mode laser is output from one end of the fiber coupler N2.

[0034] The signal generator (B1) emits a sinusoidal signal with a frequency of f0, driving the piezoelectric ceramic PZT1 to drive the fiber collimator G1 to move back and forth along the optical path in a sinusoidal pattern, and sinusoidally tunes the cavity length of the fiber composite ring laser resonant cavity. After the output single longitudinal mode laser passes through the fiber coupler N3, it is divided into two paths, one as the output light, and the other is detected by the photodetector PD. When the frequency of the signal detected by the photodetector PD is 2f0, that is, the frequency of the signal detected by the photodetector PD is twice the frequency of the sinusoidal signal emitted by the signal generator (B1), the laser frequency is at the peak of the gain spectrum of the fiber composite ring laser resonant cavity. After the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the voltage applied to the piezoelectric ceramic PZT2 is zero, and the cavity length of the fiber composite ring laser resonant cavity remains unchanged. When the signal detected by the photodetector PD is in the same frequency and phase as the sinusoidal signal emitted by the signal generator (B1), the laser frequency is less than the frequency at the peak of the gain spectrum. After the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the voltage applied to the piezoelectric ceramic PZT2 is positive, driving the piezoelectric ceramic PZT2 to drive the optical fiber collimator G2 to move along the optical path, so that the cavity length of the optical fiber composite ring laser resonant cavity is reduced, thereby moving the laser frequency to the peak of the gain spectrum. When the signal detected by the photodetector PD is in the same frequency and phase opposite to the sinusoidal signal emitted by the signal generator (B1), the laser frequency is greater than the frequency at the peak of the gain spectrum. After the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the voltage applied to the piezoelectric ceramic PZT2 is negative, driving the piezoelectric ceramic PZT2 to drive the optical fiber collimator G2 to move along the optical path, so that the cavity length of the optical fiber composite ring laser resonant cavity is increased, thereby moving the laser frequency to the peak of the gain spectrum. By adjusting the cavity length of the fiber composite ring laser resonant cavity through such feedback control, the laser frequency is stabilized at the gain spectrum peak of the fiber composite ring laser resonant cavity, thereby achieving the stability of the laser frequency.

[0035] The above-mentioned specific examples are described in order to illustrate the implementation of the present invention, but other changes and modifications of the present invention are obvious to those skilled in the art. Any modification / change or imitation transformation within the scope of the essence and basic principles of the present invention without disclosure shall fall within the scope of protection of the claims of the present invention.

Claims

1. A frequency-stable fiber single longitudinal mode laser system, characterized in that It is composed of a 980nm pump light source (S), a wavelength division multiplexer WDM, a section of erbium-doped fiber EDF, two fiber collimators G1 and G2, two parallel plates P1 and P2, a fiber circulator C, a fiber grating FBG, three fiber couplers N1, N2 and N3, a fiber isolator I, two piezoelectric ceramics PZT1 and PZT2, a photodetector PD, a signal generator (B1) and a signal processing circuit (B2); the end faces A and B of the two pigtails of the fiber coupler N1 are coated with a high-reflection film of the 1550nm band; one side of the two parallel plates P1 and P2 are coated with a partial reflection film of the 1550nm band, and the other side are coated with an anti-reflection film of the 1550nm band, and the parallel plates P1 and P2 constitute a Fabry-Perot etalon; The light emitted by the 980nm pump light source (S) is coupled into the fiber laser resonant cavity through the wavelength division multiplexer WDM, passes through the erbium-doped fiber EDF, and excites the erbium-doped fiber EDF to emit fluorescence in the 1550nm band; after the fluorescence is collimated by the fiber collimator G1, it is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2, and the light passing through the Fabry-Perot etalon is a row of equally spaced stripes; after the light is coupled into the fiber collimator G2, it passes through the fiber circulator C and reaches the fiber grating FBG; the fiber grating FBG reflects the light of one stripe back as the gain spectrum of the laser resonant cavity, and the light of the remaining stripes is transmitted out of the system after passing through the fiber grating FBG; the light reflected back by the fiber grating FBG passes through the fiber circulator C again, passes through the fiber coupler N1, and the light is divided into two paths by the fiber coupler N1, one path of light passes through the fiber L1, reaches the fiber end face A, is reflected back by the fiber end face A, and passes through the fiber coupler again The optical fiber coupler N1 passes through the optical fiber isolator I and reaches the optical fiber coupler N2, and is divided into two paths by the optical fiber coupler N2; one path of light is output through the optical fiber coupler N3, and the other path of light passes through the wavelength division multiplexer WDM and then passes through the erbium-doped optical fiber EDF again, and the light intensity is amplified by the erbium-doped optical fiber EDF; the light with amplified light intensity is collimated by the optical fiber collimator G1 again, and is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2 again, and after passing through the Fabry-Perot etalon, it is coupled into the optical fiber collimator G2 again, passes through the optical fiber circulator C again, reaches the optical fiber grating FBG again, is reflected back by the optical fiber grating FBG again, and passes through the optical fiber circulator C again; the light passes through the optical fiber coupler N1 again, and is divided into two paths by the optical fiber coupler N1 again, and one path of light passes through the optical fiber L1 again, reaches the optical fiber end face A again, and is reflected back by the optical fiber end face A again; this path of light is transmitted back and forth in this way, and this optical path constitutes a fiber sub-ring resonant cavity; The other light that passes through the fiber coupler N1 passes through the optical fiber L2, reaches the fiber end face B, is reflected back by the fiber end face B, passes through the fiber coupler N1 again, passes through the optical fiber isolator I, reaches the fiber coupler N2, and is divided into two paths by the fiber coupler N2; one light is output through the fiber coupler N3, and the other light passes through the wavelength division multiplexer WDM, and then passes through the erbium-doped fiber EDF again, and the light intensity is amplified by the erbium-doped fiber EDF; the light with amplified light intensity is collimated by the fiber collimator G1 again, and is vertically incident on the Fabry-Perot etalon composed of parallel plates P1 and P2 again, and after passing through the Fabry-Perot etalon, it is coupled into the fiber collimator G2 again, passes through the fiber circulator C again, reaches the fiber grating FBG again, is reflected back by the fiber grating FBG again, and passes through the circulator C again; The light passes through the fiber coupler N1 again and is split into two paths by the fiber coupler N1. One path of light passes through the fiber L2 again, reaches the fiber end face B again, and is reflected back by the fiber end face B again. This light path is transmitted back and forth in this way, and this light path constitutes another fiber sub-ring resonant cavity. Two fiber sub-ring resonant cavities constitute a fiber composite ring laser resonant cavity; the resonance conditions of the two fiber sub-ring resonant cavities are met at the same time, and the light of the frequency within the gain spectrum of the laser resonant cavity resonates; the difference between the lengths of the two pigtails of the fiber coupler N1, |L1-L2|, is adjusted so that the longitudinal mode spacing of the fiber composite ring laser resonant cavity is equal to the gain spectrum width of the fiber composite ring laser resonant cavity, and a single longitudinal mode resonance is generated in the fiber composite ring laser resonant cavity; when the gain of the fiber composite ring laser resonant cavity is greater than the loss, a single longitudinal mode laser is output from one end of the fiber coupler N2; The signal generator (B1) emits a sinusoidal signal with a frequency of f0, driving the piezoelectric ceramic PZT1 to drive the optical fiber collimator G1 to move back and forth along the optical path in a sinusoidal pattern, thereby sinusoidally tuning the cavity length of the optical fiber composite ring laser resonant cavity; the output single longitudinal mode laser is divided into two paths after passing through the optical fiber coupler N3, one path is used as the output light, and the other path is detected by the photodetector PD; when the frequency of the signal detected by the photodetector PD is 2f0, at this time, the laser frequency is at the gain spectrum peak of the optical fiber composite ring laser resonant cavity, and after the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the voltage applied to the piezoelectric ceramic PZT2 is zero, and the cavity length of the optical fiber composite ring laser resonant cavity remains unchanged; When the signal detected by the photodetector PD is of the same frequency and phase as the sinusoidal signal emitted by the signal generator (B1), the laser frequency is less than the frequency at the peak of the gain spectrum. After the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the voltage applied to the piezoelectric ceramic PZT2 is a positive value, and PZT2 drives the optical fiber collimator G2 to move along the optical path direction, so that the cavity length of the optical fiber composite ring laser resonant cavity is reduced, thereby moving the laser frequency to the peak of the gain spectrum. When the signal detected by the photodetector PD is of the same frequency and phase opposite to the sinusoidal signal emitted by the signal generator (B1), the laser frequency is greater than the frequency at the peak of the gain spectrum. After the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the voltage applied to the piezoelectric ceramic PZT2 is a negative value, and PZT2 drives the optical fiber collimator G2 to move along the optical path direction, so that the cavity length of the optical fiber composite ring laser resonant cavity is increased, thereby moving the laser frequency to the peak of the gain spectrum. By adjusting the cavity length of the optical fiber composite ring laser resonant cavity in real time, the laser frequency is stabilized at the peak of the gain spectrum.

2. A frequency-stable fiber single longitudinal mode laser system according to claim 1, characterized in that: The two pigtails of the fiber coupler N1 are used to form two fiber sub-ring resonant cavities with the main ring cavity respectively, and then the two fiber sub-ring resonant cavities form a fiber composite ring laser resonant cavity; the longitudinal mode spacing of the fiber composite ring laser resonant cavity can be adjusted by adjusting the difference in the lengths of the two pigtails of the fiber coupler N1; when the longitudinal mode spacing of the fiber composite ring laser resonant cavity is equivalent to the gain spectrum width of the laser resonant cavity, a single longitudinal mode resonance is generated in the fiber composite ring laser resonant cavity, and the system outputs a single longitudinal mode laser.

3. A frequency-stabilized optical fiber single longitudinal mode laser system according to claim 1, characterized in that: The light of a stripe reflected by the fiber grating FBG and transmitted through the Fabry-Perot etalon is used as the gain spectrum of the laser resonant cavity; the spectral width of this gain spectrum can be adjusted by adjusting the reflectivity of the two cavity mirrors of the Fabry-Perot etalon and adjusting the cavity length of the Fabry-Perot etalon.

4. A frequency-stabilized fiber single longitudinal mode laser system according to claim 1, characterized in that: The piezoelectric ceramic PZT1 is driven by a sinusoidal signal to perform sinusoidal tuning on the cavity length of the fiber composite ring laser resonant cavity, and the output laser is detected by the photodetector PD; after the signal detected by the photodetector PD is processed by the signal processing circuit (B2), the piezoelectric ceramic PZT2 is driven to adjust the cavity length of the fiber composite ring laser resonant cavity, thereby compensating for the influence of environmental interference such as temperature drift and vibration on the cavity length of the fiber composite ring laser resonant cavity, and stabilizing the laser frequency at the gain spectrum peak of the laser resonant cavity.

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