CN117317791A

CN117317791A - Heavy-frequency self-stabilization laser, design method thereof and all-fiber optical comb structure

Info

Publication number: CN117317791A
Application number: CN202311263138.6A
Authority: CN
Inventors: 曾和平; 刘婷婷; 闻齐; 郭政儒; 邢帅
Original assignee: East China Normal University; Chongqing Institute of East China Normal University; Beijing Changcheng Institute of Metrology and Measurement AVIC
Current assignee: East China Normal University; Chongqing Institute of East China Normal University; Beijing Changcheng Institute of Metrology and Measurement AVIC
Priority date: 2023-09-27
Filing date: 2023-09-27
Publication date: 2023-12-29

Abstract

The invention discloses a design method of a heavy frequency self-stabilization laser, which is based on an NALM laser, wherein the NALM laser comprises a nonlinear optical fiber loop, two laser pulses are reversely transmitted in the nonlinear optical fiber loop, the two laser pulses can generate interference, the two laser pulses are subjected to staggered sequence interference by changing the propagation delay difference between the two laser pulses, and the staggered sequence pulse peak values of the two laser pulses are mutually aligned, so that the heavy frequency self-stabilization laser outputs mode locking pulses with stable repetition frequency; the invention can eliminate the dependence on an additional phase-locked loop, so that the NALM laser can realize the locking of the repetition frequency by utilizing the mode locking effect of the nonlinear optical fiber loop in the interior, and no additional complex hardware element is required to be introduced, thereby effectively reducing the equipment cost and complexity, improving the locking precision and being beneficial to improving the accuracy and convenience of the laser application.

Description

Heavy-frequency self-stabilization laser, design method thereof and all-fiber optical comb structure

Technical Field

The invention relates to the technical field of optical comb repetition frequency signal locking, in particular to a repetition frequency self-stabilization laser, a design method thereof and an all-fiber optical comb structure.

Background

At the end of the 20 th century, henry Shi Xiao group of Germany Max-plane institute realizes simultaneous locking of carrier envelope phase shift frequency and repetition frequency of the femtosecond laser, a first optical frequency comb (hereinafter referred to as optical comb) is developed, and absolute frequency of transition spectral lines between two hyperfine energy level structures of Cs atoms is accurately measured. Since then, the optical comb is widely applied to numerous front technical fields such as astronomical detection, precise spectrum measurement, optical clock, attosecond science and the like as a measuring tool with high precision, high resolution and high sensitivity, and the accurate exploration of a plurality of physical quantities such as time, frequency and the like by human beings is greatly promoted.

The comb tooth frequency of the optical comb can be expressed as f _n ＝nf _r +f _ceo As can be seen from the optical frequency expression, the precision time-frequency domain control precision of the optical frequency depends on the repetition frequency signal (f _r ) Carrier envelope phase offset frequency (f _ceo ) The locking accuracy of these two free parameters. Of these two parameters, the repetition frequency is due to its 10 ⁶ Is dominant for frequency stability. At present, the locking of the repetition frequency signal of the optical comb mainly starts from two ideas: firstly, f is directly realized by controlling the geometric cavity length of the laser _r Tuning of the signal; second, the effective cavity length of the laser is adjusted by changing the refractive index of the medium in the cavity, thereby realizing f _r Tuning of the signal.

For the first kindThe idea is that in the prior art, a piezoelectric ceramic actuator (PZT) is used to stretch the optical fiber in the resonant cavity, so that tuning the geometrical cavity length of the laser is a common way to realize the laser repetition frequency locking. PZT is an electro-active device, and its amount of expansion is proportional to the magnitude of the external driving voltage. The optical fiber in the resonant cavity is stuck on the surface of the columnar PZT or wound on the side surface of the annular PZT, and the precise tuning of the geometric cavity length of the resonant cavity can be realized by regulating and controlling the external driving voltage of the PZT _r Locking of the signal. The components and the technology of the method are mature, but the method has a plurality of defects, including: an additional electronic feedback servo system needs to be constructed, and the high-voltage driving limitation of the PZT is harsh to the phase-locked loop; the PZT has inherent displacement errors and nonlinear delays, which can increase uncertainty for signal locking; since PZT relies on mechanical tuning, its feedback bandwidth is limited to the kHz order, which cannot compensate for high frequency noise.

For the second concept, by tuning the refractive index of the resonant cavity medium, the method for realizing the repetition frequency locking of the laser emerges in various embodiments, for example:

(1) An electro-optic modulator;

an electro-optic modulator (EOM) is added in the resonant cavity, and the laser repeated frequency signal is tuned and locked through the electro-refractive index change of the electro-optic crystal. The method only needs low-voltage driving, and the feedback bandwidth is up to MHz level, so that the noise in the broadband can be compensated, partial shortages of the piezoelectric ceramic brake are made up, but the method needs a complex phase-locked loop, and the EOM of optical fiber coupling can introduce up to 3dB of insertion loss, thereby generating the problems of increasing the mode locking threshold value and increasing the system power consumption.

(2) Pumping power modulates the nonlinear refractive index;

pumping power modulation nonlinear refractive index is realized by changing medium refractive index _r Another possible way of signal locking. Another section of gain fiber and wavelength division multiplexer are inserted into the resonant cavity, the inversion particle number in the gain fiber is changed by regulating the pumping power, and the refractive index of the fiber medium is modulated, so that the laser f is realized _r Locking of the signal. This method requires the construction of an electronic servo feedback system with its feedback bandThe width is limited by the energy level life of the activated ions, and the feedback bandwidth is usually in the order of hundred Hz, so that high-frequency noise cannot be compensated, and the locking precision of the high-frequency noise cannot be influenced.

In summary, in the prior art, the following technical defects exist in the aspect of locking the repetition frequency signal, that is, the present locking of the repetition frequency signal of the laser and the optical comb depends on the passive locking of an additional phase-locked loop, so that the cost and the complexity of the device are increased, and the feedback bandwidth is limited by the phase-locked element due to the limitation of the phase-locked loop, so that the improvement of the locking precision of the repetition frequency signal is limited, and the difficulty is caused to the accuracy and the convenience of the application of the laser.

Disclosure of Invention

The invention aims to provide a design method of a repetition frequency self-stabilization laser, the repetition frequency self-stabilization laser manufactured by the design method of the repetition frequency self-stabilization laser, and an all-fiber optical comb structure configured by the repetition frequency self-stabilization laser, so that the technical problems that in the prior art, the locking of a laser repetition frequency signal depends on the passive locking of an additional phase-locked loop, the cost and the complexity of the device are increased, and the feedback bandwidth is limited by a phase-locked element due to the limitation of the phase-locked loop, the locking precision of the repetition frequency signal is limited, and the application accuracy and convenience of the laser are further difficult are solved.

In order to solve the technical problems, the invention adopts the following technical scheme:

in a first aspect, the invention discloses a design method of a heavy frequency self-stabilization laser, based on a NALM laser, the NALM laser comprises a nonlinear optical fiber loop, two laser pulses are reversely transmitted in the nonlinear optical fiber loop, the two laser pulses can generate interference, the two laser pulses are subjected to staggered sequence interference by changing propagation delay difference between the two laser pulses, and staggered sequence pulse peaks of the two laser pulses are mutually aligned, so that the heavy frequency self-stabilization laser outputs mode locking pulses with stable repetition frequency.

The design method of the heavy frequency self-stabilization laser disclosed by the invention can eliminate the dependence on an additional phase-locked loop when being applied to the NALM laser, so that the NALM laser can realize the locking of the repetition frequency by utilizing the mode locking effect of the nonlinear absorption fiber loop mirror inside, and the equipment cost and complexity can be effectively reduced without introducing additional complex hardware elements. In addition, as the NALM laser realizes the locking of the repetition frequency and has lower requirement on the feedback bandwidth, the locking precision is improved, the accuracy and convenience of the laser application are improved, and the technical problems in the traditional method are overcome.

The invention discloses a second aspect, an embodiment of a heavy frequency self-stabilization laser, and the design method of the heavy frequency self-stabilization laser comprises a resonant cavity, wherein a nonlinear optical fiber ring is arranged in the resonant cavity and used for locking and modulating an optical signal, the nonlinear optical fiber ring comprises an optical fiber, the optical fiber comprises a first optical fiber coupling end and a second optical fiber coupling end, an adjustable delay module is arranged between the first optical fiber coupling end and the second optical fiber coupling end, and the adjustable delay module is used for changing the propagation delay difference between two paths of laser pulses; the heavy-frequency self-stabilization laser also comprises a linear arm, and the nonlinear optical fiber ring is connected with the linear arm through a central optical beam splitter;

the nonlinear optical fiber ring is sequentially connected with a wavelength division multiplexer, a gain optical fiber and a non-reciprocal linear phase shifter along the anticlockwise direction from the central optical beam splitter, and the wavelength division multiplexer is also connected with a pump laser diode;

the pump laser diode is used for providing a pump light signal and transmitting the pump light signal to the gain optical fiber through the wavelength division multiplexer;

the gain optical fiber is used for amplifying the optical signal, and the optical signal amplified by the gain optical fiber is a mode-locked optical signal;

The wavelength division multiplexer is used for coupling the pump light signal and the mode locking light signal into the same optical fiber;

the non-reciprocal linear phase shifter is used for introducing linear phase shift quantity;

the linear arm comprises an optical fiber integrated device, the optical fiber integrated device is connected with the central optical beam splitter through a polarization-maintaining single-mode fiber, the polarization-maintaining single-mode fiber is used for maintaining the polarization state in an optical fiber, and the optical fiber integrated device is used for reflecting signal light and introducing an output optical signal of the resonant cavity.

The working principle of the heavy frequency self-stabilization laser disclosed by the invention is as follows: the pump laser diode provides pumping energy, the wavelength division multiplexer multiplexes the pumping light signal and the mode locking light signal, the gain optical fiber amplifies the mode locking light signal, the nonreciprocal linear phase shifter introduces linear phase shift, the adjustable delay module can adjust the propagation delay of the signal in the nonlinear optical fiber loop, the working state and the output characteristic of the laser can be controlled by adjusting the delay module, the linear arm realizes feedback and interference effects through the polarization maintaining single-mode fiber and the optical fiber reflector and the optical fiber beam splitter integrated device, and the components act together, so that the heavy frequency self-stabilizing laser can realize locking of the frequency and the phase of the light pulse and generate stable light pulses.

Preferably, the scheme provides an embodiment structure of an adjustable delay module, wherein the adjustable delay module comprises a first optical fiber coupling collimating lens, an input end of the first optical fiber coupling collimating lens is used for inputting optical signals, the input end of the first optical fiber coupling collimating lens is coupled with the first optical fiber coupling end, an output end of the first optical fiber coupling collimating lens is connected with an input end of an optical beam splitter, the optical beam splitter comprises a transmission end and a reflection end, the transmission end is connected with a first end of an optical circulator, and the first end is used for inputting the optical signals;

the optical circulator further comprises a second end, the second end is connected with the output end of a second optical fiber coupling collimating lens, the input end of the second optical fiber coupling collimating lens is coupled with the second optical fiber coupling end, the second end can input optical signals from the output end of the second optical fiber coupling collimating lens and also can output optical signals transmitted from the first end, and the input end of the second optical fiber coupling collimating lens is used for inputting reverse optical signals; the optical circulator further includes a third terminal for outputting an optical signal transmitted by the first terminal or the second terminal; the adjustable time delay module further comprises an electric control displacement platform, the electric control displacement platform is electrically connected with an external power supply, and the electric control displacement platform can realize translation by changing the voltage of the external power supply;

The adjustable delay module is also provided with a fixed reflecting mirror group and a movable reflecting mirror group, wherein the fixed reflecting mirror group comprises a plurality of optical plane reflecting mirrors, the fixed positions of the optical plane reflecting mirrors are unchanged, and the movable reflecting mirror group is arranged on the electric control displacement platform and can translate along with the electric control displacement platform;

the fixed reflector group can reflect the light rays from the light beam splitter to the movable reflector group and reflect the light rays to the third end of the light circulator; the light emitted from the third end of the optical circulator can pass through the movable reflecting mirror group to reflect the light to the fixed reflecting mirror group and emit to the reflecting end of the optical beam splitter.

The working principle of the scheme is as follows: the invention aims at a nonlinear absorption optical fiber annular mirror (Nonlinear Absorbing Loop Mirror, NALM) laser, hereinafter referred to as NALM laser, an adjustable delay module is added in the NALM laser, two paths of transmission paths with different propagation directions and different laser pulses with the same frequency and wavelength are constructed by utilizing the optical circulator, different optical path differences are introduced between the two transmission paths, so as to achieve the purpose of adjustable delay, when the delay delta T introduced by the adjustable delay module meets delta t=T, wherein T is the pulse repetition period, and when the peak values of the staggered sequence pulses are aligned, the NALM laser can accumulate enough phase difference, mode locking pulses can be generated, and the repetition frequency f can be obtained through the structure _r Stable mode locking pulse.

According to the technical scheme disclosed by the invention, the dependence on an additional phase-locked loop can be eliminated, so that the NALM laser can realize the locking of the repetition frequency by utilizing the mode locking effect of the nonlinear absorption optical fiber annular mirror in the NALM laser, no additional complex hardware element is required to be introduced, and the equipment cost and complexity can be effectively reduced; in addition, as the NALM laser realizes the locking of the repetition frequency and has lower requirement on the feedback bandwidth, the locking precision is improved, the accuracy and convenience of the laser application are improved, and the technical problems in the traditional method are overcome.

In order to optimize the reflection light path, a first reflecting mirror is further arranged between the movable reflecting mirror group and the third end, and the emergent direction of the third end is opposite to the incident direction of the first reflecting mirror;

the fixed reflecting mirror group comprises a fourth reflecting mirror, a fifth reflecting mirror and a sixth reflecting mirror, and the movable reflecting mirror group comprises a second reflecting mirror and a third reflecting mirror; the emergent direction of the first reflecting mirror is opposite to the incident direction of the second reflecting mirror, the emergent direction of the second reflecting mirror is opposite to the incident direction of the third reflecting mirror, the emergent direction of the third reflecting mirror is opposite to the incident direction of the fourth reflecting mirror, the emergent direction of the fourth reflecting mirror is opposite to the incident direction of the fifth reflecting mirror, the emergent direction of the fifth reflecting mirror is opposite to the incident direction of the sixth reflecting mirror, and the emergent direction of the sixth reflecting mirror is opposite to the emergent direction of the reflecting end of the light beam splitting mirror.

Preferably, the repetition period of the laser pulse is T, the delay amount of the propagation delay difference between the two paths of laser pulses is T, and the phase difference between the two paths of laser pulses is 2npi, where n is a natural number and n∈ {1,2,3, }.

Preferably, the optical path distances between the two ends of the gain fiber and the central optical beam splitter are unequal, and the gain fiber is asymmetrically arranged in the nonlinear fiber loop.

The invention also discloses another heavy frequency self-stabilization laser, and the design method of the heavy frequency self-stabilization laser comprises a resonant cavity, wherein the nonlinear optical fiber ring is arranged in the resonant cavity and is used for carrying out mode locking and modulation on optical signals, the nonlinear optical fiber ring comprises an optical fiber, and an adjustable delay phase shift module is arranged on the optical fiber and is used for changing the propagation delay difference and the phase difference between two paths of laser pulses;

the heavy-frequency self-stabilization laser also comprises a linear arm, and the nonlinear optical fiber ring is connected with the linear arm through a central optical beam splitter;

the nonlinear optical fiber ring is sequentially connected with a wavelength division multiplexer and a gain optical fiber along the anticlockwise direction from the central optical beam splitter, and the wavelength division multiplexer is also connected with a pump laser diode;

The scheme improves the original nonreciprocal linear phase shifter structure in the NALM laser resonant cavity, can realize coarse adjustment and fine adjustment of propagation delay difference between two laser pulses transmitted in opposite directions in a nonlinear optical fiber loop, and fine adjustment of phase shift amount, does not need to additionally increase an adjustable delay module, and can enable the whole device to be more concise and integrated.

Preferably, the scheme provides an embodiment structure of the adjustable delay phase shift module, and the adjustable delay phase shift module disclosed in the embodiment comprises an input port, an output port, a birefringent crystal, a Faraday rotator, an electro-optical modulation crystal, a polarization beam splitter and a stepping motor; the Faraday rotator is positioned between the birefringent crystal and the electro-optic modulation crystal, the polarization beam splitter is positioned between the electro-optic modulation crystal and the stepping motor, and the stepping motor can translate under the action of an external power supply;

The birefringent crystal is used for dividing light rays from an input port or an output port or light rays from the Faraday rotator into different light rays in two directions; the polarization directions of the incident light from the input port and the output port differ by 90 degrees;

the Faraday rotator is used for rotating the polarization state of light rays from the birefringent crystal or light rays of the electro-optical modulation crystal by a preset angle;

the electro-optical modulation crystal can change the refractive index in the crystal by the influence of an external electric field and is used for changing the propagation phase difference between two paths of laser pulses;

the polarization beam splitter is used for dividing light rays from the photoelectric modulation crystal into two light beams with different polarization directions, a first plane reflecting mirror and a second plane reflecting mirror are respectively arranged in the emergent directions of the two light beams with different polarization directions, the second plane reflecting mirror is arranged on the stepping motor and can move along with the movement of the stepping motor, and the stepping motor and the second plane reflecting mirror are used for changing propagation delay differences between two paths of laser pulses.

The working principle of the scheme is that the adjustable delay phase shift module can be used for oppositely transmitting two laser pulses in a nonlinear optical fiber loop and simultaneously providing an adjustable phase difference and a delay difference, and the birefringent crystal can be used for dividing two light beams with mutually perpendicular polarization directions into different paths by utilizing the anisotropic property of the birefringent crystal so as to realize the input and output of light; the polarization state of incident light is rotated by a Faraday rotator, and the refractive index difference of the fast and slow axes of the crystal is changed by changing the external voltage of the electro-optical modulation crystal, so that the phase difference and the delay difference of micro-dimming are changed; dividing incident light into two light beams with different polarization directions through a polarization beam splitter, so that only the light beams with specific polarization directions can pass through or reflect, splitting the light beams through the polarization beam splitter to form two light paths, wherein one light path propagates to a first plane reflecting mirror, and the other light path propagates to a second plane reflecting mirror, and at the moment, a stepping motor converts electric energy into mechanical rotation and moves to different positions under the input of specific stepping electric pulses; the stepping motor drives the second plane reflector to translate, so that the change of the optical path difference of the reflected light path of the second plane reflector is realized, and further, the propagation delay difference is formed between the optical path difference and the reflected light of the first plane reflector, so that two paths of laser pulses are subjected to staggered sequence interference, and the staggered sequence pulse peak values of the two paths of laser pulses are mutually aligned, so that the mode locking pulse with stable repetition frequency is output by the repetition frequency self-stabilization laser; the stepping motor and the second plane reflecting mirror can coarsely adjust the delay difference between the two beams of light; the laser pulses with the same frequency and wavelength and opposite propagation directions can be precisely adjusted in a large range by the combined adjustment of the electro-optical modulation crystal, the stepping motor and the second plane reflecting mirror.

Preferably, the tunable delay phase shift module can provide a total delay difference Δt for two laser pulses with the same frequency and wavelength and opposite propagation directions as follows:

wherein L is ₁ For the first optical path difference, L ₂ Is the second optical path difference, n ₁ The refractive index of air, c is the light velocity in vacuum, l is the length of the electro-optic modulation crystal, d is the thickness of the electro-optic modulation crystal, V ₀ For electro-optically modulating the DC component of the applied electric field, V _m The alternating current component of the electric field externally applied to the electro-optical modulation crystal, f is the modulation frequency of the alternating electric field externally applied to the electro-optical modulation crystal, and gamma _yy Is a dielectric parameter.

In a fourth aspect, the invention also discloses an all-fiber optical comb structure, which utilizes the heavy-frequency self-stabilization laser, and comprises a heavy-frequency self-stabilization laser, an optical power amplifier, a supercontinuum stretcher, an f-2f self-reference detector and a phase-locked loop;

the adjustable delay module or the adjustable delay phase shift module is arranged in the repetition frequency self-stabilization laser and used for generating mode locking pulse laser;

the optical power amplifier is used for increasing the energy and power of the optical signal;

the supercontinuum stretcher is used for generating a continuous spectrum containing a plurality of frequency components;

the f-2f self-reference detector is used for measuring and stabilizing the frequency of the laser pulse;

The phase-locked loop includes a repetition frequency f _r Signal phase-locked loop and optical comb carrier envelope offset frequency f _ceo A signal phase-locked loop;

the repetition frequency f _r The signal phase-locked loop is used for stabilizing the frequency of the optical pulse sequence generated by the laser;

optical comb carrier envelope offset frequency f _ceo The signal phase-locked loop is used to stabilize the phase relationship between the comb frequency and the comb envelope waveform.

Specifically, the heavy-frequency self-stabilization laser is the heavy-frequency self-stabilization laser with the adjustable delay module.

Specifically, the optical power amplifier, the supercontinuum stretcher, the f-2f self-reference detector and the phase-locked loop are all commonly used devices in the prior art, and the working principle is as follows: the working principle of the optical power amplifier is that the energy and the power of an optical signal are increased by utilizing the stimulated radiation process by injecting the weak optical signal into a device doped with an optical amplifying material (such as an erbium-doped optical fiber); the principle of operation of supercontinuum stretcher is to utilize nonlinear optical effects (e.g., self-phase modulation, transient coulomb effect, etc.) and modulated optical pumping to produce a continuous spectrum comprising multiple frequency components; f-2f self-reference detector can divide the input pulse into two parts, and delay and optical frequency shift in the optical lattice on a reference beam, then by comparing the two frequency components, the absolute frequency and related characteristics of the laser pulse can be derived from them; the phase-locked loop being capable of controlling and adjusting the phase relationship between the input signal and the reference signal, in particular the repetition frequency f _r The signal phase locked loop is capable of phase locking the laser frequency to a reference signal frequency (typically a reference oscillator) such that the two maintain a stable phase relationship. By controlling the length of an optical cavity of the laser or the bias of an amplifier, the synchronization of the laser output frequency and the frequency of a reference signal can be realized, and a repetition frequency signal phase-locked loop is commonly used for mode locking of an ultrafast laser; optical comb carrier envelope offset frequency f _ceo The signal phase-locked loop is capable of controlling the carrier frequency and envelope offset frequency of the optical comb by phase locking so that they are aligned with a reference signalThe (typically stable optical oscillator) maintains a fixed phase relationship.

The invention has the following beneficial effects: the design method of the heavy frequency self-stabilization laser disclosed by the invention can eliminate the dependence on an additional phase-locked loop when being applied to the NALM laser, so that the NALM laser can realize the locking of the repetition frequency by utilizing the mode locking effect of the nonlinear absorption fiber loop mirror inside, and no additional complex hardware element is required to be introduced, thereby effectively reducing the equipment cost and complexity; in addition, as the NALM laser realizes the locking of the repetition frequency and has lower requirement on the feedback bandwidth, the locking precision is improved, the accuracy and convenience of the laser application are improved, and the technical problems in the traditional method are overcome.

Drawings

For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings, in which:

fig. 1 is a schematic diagram of one implementation structure of the repetition frequency self-stabilization laser of the present invention.

Fig. 2 is a schematic diagram of the structure of the adjustable delay module of the present invention.

Fig. 3 is a schematic diagram of pulse missequence interference based on an adjustable delay module according to the present invention.

Fig. 4 is a graph showing the variation of the loss in the resonant cavity of the heavy-frequency self-stabilization laser according to the phase shift of two beams of light.

Fig. 5 is a schematic diagram of the present invention for implementing repetition frequency locking of a repetition frequency self-stabilized laser using an adjustable delay module.

Fig. 6 is a schematic diagram of an all-fiber comb of a NALM laser constructed based on an adjustable delay module of the present invention.

Fig. 7 is a schematic diagram of another implementation structure of the repetition frequency self-stabilization laser of the present invention.

Fig. 8 is a schematic diagram of the structure of the adjustable delay phase shift module of the present invention.

Reference numerals illustrate: 100. an adjustable delay module; 101. a first fiber coupled collimating mirror; 102. a light beam splitter; 103. an optical circulator; 1031. a first end; 1032. A second end; 1033. a third end; 104. a second fiber coupled collimating mirror; 105. an electric control displacement platform; 106. a first mirror; 107. a second mirror; 108. a third mirror; 109. a fourth mirror; 110. a fifth reflecting mirror; 111. a sixth mirror; 200. a heavy frequency self-stabilizing laser; 201. a nonlinear optical fiber loop; 202. a linear arm; 203. a first optical fiber coupling end; 204. a second optical fiber coupling end; 205. a central optical beam splitter; 206. a wavelength division multiplexer; 207. a gain fiber; 208. a pump laser diode; 209. a non-reciprocal linear phase shifter; 210. an optical fiber integrated device; 301. an optical power amplifier; 302. a supercontinuum stretcher; 303. f-2f is from the reference detector; 304. repetition frequency f _r A signal phase-locked loop; 305. optical comb carrier envelope offset frequency f _ceo A signal phase-locked loop; 400. an adjustable delay phase shift module; 401. a birefringent crystal; 402. a Faraday rotator; 403. an electro-optic modulation crystal; 404. a polarizing beam splitter; 405. a stepping motor; 406. a first planar mirror; 407. a second planar mirror; 408. a first optical path difference; 409. a second optical path difference; 410. an input port; 411. an output port.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

It should be noted that: like reference numerals and letters denote like items in the following figures, and thus once an item is defined in one figure, no further definition or explanation thereof is necessary in the following figures. In the description of the present invention, it should be noted that, directions or positional relationships indicated by terms such as "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., are directions or positional relationships based on those shown in the drawings, or are directions or positional relationships conventionally put in use of the inventive product, are merely for convenience of describing the present invention and simplifying the description, and are not indicative or implying that the apparatus or element to be referred to must have a specific direction, be constructed and operated in a specific direction, and thus should not be construed as limiting the present invention. Furthermore, the terms "first," "second," "third," and the like are used merely to distinguish between descriptions and should not be construed as indicating or implying relative importance. Furthermore, the terms "horizontal," "vertical," and the like do not denote a requirement that the component be absolutely horizontal or overhang, but rather may be slightly inclined. As "horizontal" merely means that its direction is more horizontal than "vertical", and does not mean that the structure must be perfectly horizontal, but may be slightly inclined. In the description of the present invention, it should also be noted that, unless explicitly specified and limited otherwise, the terms "disposed," "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.

The invention can be applied to the technical field of NALM lasers, and solves the technical problems that the locking of the laser repetition frequency signals in the prior art depends on the passive locking of an additional phase-locked loop, thereby not only increasing the cost and complexity of the device, but also limiting the feedback bandwidth to a phase-locked element due to the limitation of the phase-locked loop, limiting the improvement of the locking precision of the repeated frequency signals, and further causing difficulty to the accuracy and convenience of the laser application.

The design method of the heavy frequency self-stabilization laser disclosed by the invention can eliminate the dependence on an additional phase-locked loop when being applied to the NALM laser, so that the NALM laser can realize the locking of the repetition frequency by utilizing the mode locking effect of the nonlinear absorption fiber loop mirror inside, and no additional complex hardware element is required to be introduced, thereby effectively reducing the equipment cost and complexity; in addition, as the NALM laser realizes the locking of the repetition frequency and has lower requirement on the feedback bandwidth, the locking precision is improved, the accuracy and convenience of the laser application are improved, and the technical problems in the traditional method are overcome.

In a second aspect, the present invention discloses an embodiment of a heavy frequency self-stabilization laser 200, and the design method of the heavy frequency self-stabilization laser includes a resonant cavity, where a nonlinear optical fiber ring 201 is disposed in the resonant cavity and is used for mode locking and modulating an optical signal, where the nonlinear optical fiber ring 201 includes an optical fiber, where the optical fiber includes a first optical fiber coupling end 203 and a second optical fiber coupling end 204, and an adjustable delay module 100 is disposed between the first optical fiber coupling end 203 and the second optical fiber coupling end 204; the adjustable delay module is used for changing the propagation delay difference between two paths of laser pulses;

The working principle of the repetition frequency self-stabilization laser 200 disclosed by the invention is as follows: the pump laser diode 208 provides pump energy, the wavelength division multiplexer 206 multiplexes the pump optical signal and the mode locking optical signal, the gain optical fiber 207 amplifies the mode locking optical signal, the non-reciprocal linear phase shifter 209 introduces a linear phase shift, the adjustable delay module 100 can adjust the propagation delay of the signal in the nonlinear optical fiber loop 201, the working state and the output characteristic of the laser can be controlled by adjusting the delay module, and the linear arm 202 realizes feedback and interference effects through the polarization maintaining single-mode fiber and the optical fiber reflector and the optical fiber beam splitter integrated device. These components cooperate to enable the repetition rate self-stabilizing laser 200 to achieve frequency and phase locking of the light pulses and to generate stable light pulses.

Specifically, the pump light signal and the mode-locked light signal are both laser signals.

Specifically, each module of the nonlinear optical fiber ring 201 and the linear arm 202 is connected through polarization maintaining optical fibers, and optical devices in each module adopt polarization maintaining devices, so that the environmental immunity of the laser is enhanced by the full polarization maintaining structure.

Specifically, the non-reciprocal linear phase shifter 209 module is located between the gain fiber 207 and the adjustable delay module 100 in the non-linear fiber ring 201, and is configured to introduce a linear phase shift amount for two light beams transmitted in opposite directions in the non-linear fiber ring 201.

Specifically, the optical fiber integrated device 210 includes an optical fiber mirror and an optical fiber beam splitter; the fiber mirror reflects a portion of the output optical signal back to the nonlinear fiber loop 201; the optical fiber beam splitter is used for leading out part of the optical signals as output signals of the resonant cavity.

Preferably, the beam splitting ratio of the optical fiber beam splitter is 80:20, 80% of light returns to the resonant cavity to continuously oscillate, and the remaining 20% of light is output as the output light of the heavy-frequency self-stabilizing laser.

Preferably, the optical path distances between the two ends of the gain fiber 207 and the central optical splitter 205 are not equal, and are asymmetrically arranged in the nonlinear fiber loop 201.

Preferably, the gain fiber 207 is an erbium-doped gain fiber, and erbium ions are doped in the erbium-doped gain fiber, so as to amplify the optical signal, and the optical signal amplified by the erbium-doped gain fiber is a mode-locked optical signal.

Specifically, in the erbium ion doped optical fiber, after the erbium ion is excited to a high energy level, the erbium ion can transition to a stable low energy level through the process of stimulated radiation, and the energy of incident light is absorbed at the same time, and the process realizes the amplification of an optical signal in the optical fiber; in addition, erbium ions have a specific electron level structure in rare earth ions, in which the energy levels of stimulated radiation and absorption are generally distributed around 1550nm, and light at a wavelength of 1550nm can generally match the energy level structure of erbium ions and excite their transitions, so that erbium ions exhibit a good light amplification performance in a range around 1550 nm.

When the pump light excites erbium ions, they can increase the intensity of the optical signal by the amplifying action of the erbium-doped gain fiber. The operating range of erbium ions is typically around 1550nm wavelength. In particular, the gain peaks of erbium doped gain fibers are typically between 1540nm and 1560 nm.

Specifically, the wavelength division multiplexer in this scheme includes two input ports and an output port, can multiplex two optical signals of different wavelength, and specifically, one of them input port is connected with pump laser diode, accepts 980nm pump light signal, and another input port is connected with erbium-doped gain fiber for accept the mode locking optical signal after being amplified by gain fiber, then through wavelength division multiplexer's coupling, couple 980nm pump light signal and 1550nm mode locking optical signal to same optic fibre and export through the output port.

Specifically, the pump laser diode is a laser diode for providing energy to the erbium-doped gain fiber, which is capable of emitting light having a wavelength of 980nm for exciting erbium ions in the erbium-doped fiber.

Preferably, the splitting ratio of the central optical beam splitter 205 is 50:50.

Specifically, the polarization-maintaining single-mode fiber is a single-mode fiber, has good optical fiber transmission characteristics and polarization-maintaining performance, and the polarization maintaining means that the polarization state of an optical signal in the optical fiber can be maintained, thereby being beneficial to improving the stability of the optical signal and reducing the noise of a system.

Preferably, the gain fiber 207 may be selected according to the output wavelength, and ytterbium-doped gain fiber or thulium-doped gain fiber may be selected, where the ytterbium-doped gain fiber is used to generate 1 μm laser light, and the thulium-doped gain fiber is used to generate 2 μm laser light. Specifically, ytterbium ions (Yb ³⁺ ) Ytterbium ions have a wide absorption bandwidth and a high amplification cross section, and the absorption and emission wavelength of ytterbium-doped fibers ranges from 980nm to 1100nm, preferably from 1030nm to 1080 nm. In the wavelength range, the ytterbium-doped optical fiber can effectively absorb the energy of the pumping light source and amplify the signal light in the stimulated radiation process, and the ytterbium-doped optical fiber can realize high-power and high-efficiency optical amplification and laser output.

Thulium-doped optical fiber doped with thulium ions (Tm ³⁺ ) The thulium ion has specific energy level structure and spectral line characteristics, and the absorption and emission wavelength range of the thulium-doped optical fiber is between 1800nm and 2100 nm; the preferred wavelength range is between 1900nm and 2000nm, where the thulium doped fiber is capable of absorbing and amplifying optical signals.

As can be seen from fig. 3, when the tunable delay module is not added in the nonlinear optical fiber loop, two paths of laser pulses with the same frequency and wavelength and opposite propagation directions in the loop experience the same optical path, and the two paths of laser pulses reach the central beam splitter to interfere at the same time. After the adjustable delay module is added, the delay delta t between the two beams of light can be controlled through external regulation and control. The purpose is to shift the peak beat frequency of the two beams of light.

The operation of the repetition frequency self-stabilizing laser 200: in the nonlinear optical fiber loop 201, the gain optical fiber 207 spontaneously radiates a small amount of 1550nm laser, then under the action of pumping light, the stimulated radiation generates 1550nm laser and oscillates in the resonant cavity, and the stable mode locking pulse is finally formed by combining the influence of dispersion and nonlinearity. The mode-locked pulse exiting the linear arm 202 is split by a central beam splitter into two columns of pulses that are transmitted clockwise and counterclockwise. The clockwise transmission pulse firstly passes through a single-mode fiber, the adjustable delay module 100 and the nonreciprocal linear phase shifter 209 module, then is amplified through the gain fiber 207, and the anticlockwise transmission pulse firstly passes through the gain fiber 207, then is amplified through the single-mode fiber, the nonreciprocal linear phase shifter 209 module and the adjustable delay module 100, the asymmetrically placed gain fiber 207 provides nonlinear phase shift for laser pulses with the same frequency and wavelength and opposite in the two paths of propagation directions, the nonreciprocal linear phase shifter 209 module provides linear phase shift quantity for the two beams of light, and then the two beams of light are converged and interfered at the central beam splitter to realize pulse narrowing.

Pulse narrowing refers to a process or technique of shortening the time width of a signal or an optical pulse, wherein a narrow pulse generally has a shorter pulse width and higher peak power.

As a preferred embodiment, referring to fig. 2, the adjustable delay module disclosed in this embodiment includes a first optical fiber coupling collimator 101, an input end of the first optical fiber coupling collimator 101 is used for inputting an optical signal, an input end of the first optical fiber coupling collimator 101 is coupled with the first optical fiber coupling end 203, an output end of the first optical fiber coupling collimator 101 is connected with an input end of an optical beam splitter 102, the optical beam splitter 102 includes a transmitting end and a reflecting end, the transmitting end is connected with a first end 1031 of an optical circulator 103, and the first end 1031 is used for inputting an optical signal;

the optical circulator 103 further includes a second end 1032, where the second end 1032 is connected to an output end of the second optical fiber coupling collimator 104, an input end of the second optical fiber coupling collimator 104 is coupled to the second optical fiber coupling end 204, the second end 1032 is capable of inputting an optical signal from the output end of the second optical fiber coupling collimator 104 and outputting an optical signal transmitted from the first end 1031, and an input end of the second optical fiber coupling collimator 104 is used for inputting a reverse optical signal;

The adjustable delay module 100 further comprises an electric control displacement platform 105, the electric control displacement platform 105 is electrically connected with an external power supply, and the electric control displacement platform 105 can realize translation by changing the voltage of the external power supply;

the adjustable delay module 100 is further provided with a first reflecting mirror 106, a second reflecting mirror 107, a third reflecting mirror 108, a fourth reflecting mirror 109, a fifth reflecting mirror 110 and a sixth reflecting mirror 111 at different positions, wherein the second reflecting mirror 107 and the third reflecting mirror 108 are arranged on the electric control displacement platform 105;

the optical circulator 103 further includes a third end 1033, where the third end 1033 is configured to output an optical signal transmitted by the first end 1031 or the second end 1032, an outgoing direction of the third end 1033 is opposite to an incoming direction of the first mirror 106, an outgoing direction of the first mirror 106 is opposite to an incoming direction of the second mirror 107, an outgoing direction of the second mirror 107 is opposite to an incoming direction of the third mirror 108, an outgoing direction of the third mirror 108 is opposite to an incoming direction of the fourth mirror 109, an outgoing direction of the fourth mirror 109 is opposite to an incoming direction of the fifth mirror 110, an outgoing direction of the fifth mirror 110 is opposite to an incoming direction of the sixth mirror 111, and an outgoing direction of the sixth mirror 111 is opposite to an outgoing direction of the reflecting end of the optical beam splitter 102.

Specifically, the light incident on the sixth mirror 111 can be transmitted to the third end 1033 along the fifth mirror 110, the fourth mirror 109, the third mirror 108, the second mirror 107, and the first mirror 106 in this order.

Specifically, the working principle of the optical circulator is that the optical circulator is an irreversible unidirectional three-port device, light can only be transmitted clockwise in the device, a signal input from a first end can be output from a second end in a low loss manner, and a signal input from the second end can be output from a third end in a low loss manner; in contrast, the signal input from the second terminal will generate a large loss at the first terminal and cannot be output, and the signal input from the third terminal will generate a large loss at the first terminal and the second terminal and cannot be output, so that two transmission paths with different propagation directions of laser pulses with the same frequency and wavelength can be constructed by using the optical circulator.

The working process of the adjustable time delay module is as follows: referring to fig. 2, light input from an input end of an input optical fiber is collimated to a light beam splitter 102 by a first optical fiber coupling collimator 101, and is split into two paths by the light beam splitter 102, namely a reflection path and a transmission path, wherein the transmission path is transmitted to a second end 1032 by a first end 1031 of an optical circulator 103, is collected into the optical fiber by a second optical fiber coupling collimator 104, is emitted from an output end of an output optical fiber, enters a resonant cavity to continue to be transmitted, and the reflection path is transmitted to a third end 1033 of the optical circulator by a first reflector 106 to a sixth reflector 111 to be lost;

Conversely, the light input from the output end of the output optical fiber is transmitted to the second end 1032 of the optical circulator 103 through the second optical fiber coupling collimating mirror 104 for incidence, is transmitted to the third end 1033 for emergence in a unidirectional manner, is split into two beams at the optical beam splitting mirror 102 after being reflected by the first to sixth reflectors 106 to 111 in sequence, is collected into the optical fiber through the first optical fiber coupling collimating mirror 101 for reflection, emerges from the input end of the input optical fiber, enters the resonant cavity for continuous transmission, and the transmitted light beam is lost at the optical beam splitting mirror 102.

The working principle of the scheme is as follows: the scheme aims at a nonlinear absorption optical fiber annular mirror (Nonlinear Absorbing Loop Mirror, NALM) laser, hereinafter referred to as NALM laser, an adjustable delay module is added in the NALM laser, two paths of transmission paths with opposite propagation directions and different laser pulses with the same frequency and wavelength are constructed by using the optical circulator, different optical path differences are introduced between the two transmission paths, so that the purpose of adjustable delay is achieved, when the delay delta T introduced by the adjustable delay module meets delta t=T, wherein T is the pulse repetition period, and the peak values of the staggered sequence pulses are alignedAt this time, the NALM laser can accumulate a sufficient phase difference, and the phase difference between the two laser pulses is 2npi, where n is a natural number and n∈ {1,2, 3. }, at which time mode-locked pulses can be generated, and by this structure, the repetition frequency f can be obtained _r Stable mode locking pulse.

According to the technical scheme disclosed by the scheme, the dependence on an additional phase-locked loop can be eliminated, so that the NALM laser can realize the locking of the repetition frequency by utilizing the mode locking effect of the nonlinear absorption optical fiber annular mirror, an additional complex hardware element is not required to be introduced, and the equipment cost and complexity can be effectively reduced; in addition, as the NALM laser realizes the locking of the repetition frequency and has lower requirement on the feedback bandwidth, the locking precision is improved, the accuracy and convenience of the laser application are improved, and the technical problems in the traditional method are overcome.

The scheme provides an application embodiment of the adjustable delay module, the technical scheme is applied to a common 9-shaped NALM laser structure, and a nonlinear optical fiber ring and a linear arm are arranged in a resonant cavity of the common 9-shaped NALM laser, so that the adjustable delay module is a key component for realizing a mode locking function. The working principle of the 9-shaped NALM laser is to utilize the optical nonlinear effect of nonlinear medium to introduce nonlinear phase modulation in the optical pulse propagation process. As the light pulse passes through the nonlinear medium, the phase of the light changes due to the nonlinear response of the medium, thereby changing the spectral and phase characteristics of the light pulse. The modulator is used for controlling the phase and the amplitude of the feedback signal to realize locking of the frequency and the phase of the optical pulse, and can generate stable optical pulse.

Specifically, the mode locking principle of the 9-shaped NALM laser is as follows:

when the resonant cavity is not provided with the adjustable delay module, the 1 st clockwise pulse interferes with the 1 st anticlockwise pulse, and after the adjustable delay is added, the 1 st anticlockwise transmission laser pulse is delayed in time and can only interfere with the subsequent clockwise transmission laser pulse.

Assuming that the 1 st counter-clockwise transmission pulse interferes with the 2 nd clockwise transmission pulse, thisThe adjustable delay amount satisfies Δt=t, where T is the laser pulse repetition period, which is the laser repetition frequency f _r The relationship is shown in the formula (1):

T＝1/f _r (1)

as can be seen from equation (1), the laser repetition frequency f is adjusted _r After balancing the delay delta t between laser pulses with the same frequency and wavelength and opposite to the two paths of propagation directions, the laser can realize the error sequence interference of two rows of pulses, thereby realizing mode locking.

The accumulated phase shift amount between pulses is composed of a linear phase shift amount and a nonlinear phase shift amount, as shown in the formula (2):

in the formula (2), the amino acid sequence of the compound,for the accumulated phase shift between pulses, +.>For the amount of accumulated linear phase shift between pulses, +.>Is the amount of nonlinear phase shift accumulated between pulses. />

When the adjustable delay module is arranged in the resonant cavity, the external power supply voltage of the electric control displacement platform is changed, so that the electric control displacement platform moves towards the direction far away from or close to the first reflecting mirror and the fourth reflecting mirror, and the optical path between the first reflecting mirror and the second reflecting mirror and the optical path between the reflecting mirror 3 and the fourth reflecting mirror are shortened or extended. By regulating the optical path difference Deltal between two beams of light transmitted in opposite directions, the delay difference Deltat between the two beams of light can be controlled, and the relation between the two can be approximately expressed as the following formula:

Δl＝cΔt/n ₁ (3)

In (3)Deltal is the optical path difference between two light beams transmitted oppositely, deltat is the delay difference between two light beams transmitted oppositely, and n ₁ The refractive index of air, c is the speed of light in vacuum.

After the matching of the delay difference delta T and the pulse period T is regulated, the wrong pair interference of positive and negative transmission pulses in the resonant cavity can be realized, and the purpose of accurate locking of the repetition frequency is achieved.

In addition, the light beam splitter with different beam splitting ratios can introduce different losses to two light beams transmitted in opposite directions, and pulse mode locking depends on nonlinear phase difference accumulation related to the intensity between the two light beams, so that the beam splitting ratio of the light beam splitter is adjusted, and the pulse establishment process in the resonant cavity can be regulated.

Referring to fig. 4, fig. 4 shows the relationship between the cumulative phase shift between laser pulses of the same frequency and wavelength with the losses in the "9" NALM laser opposite to the two propagation directions. When the phase shift amount of the two beams reaches 2 pi, the loss of the resonant cavity is minimum, and the mode locking is easier to realize. When the linear phase shift is added to the two beams of light, the loss curve of the resonant cavity generates corresponding translation transformation. Since the light intensity is different at each position of the laser pulse, and the nonlinear phase shift amount caused by the nonlinear effect is positively correlated with the light intensity, the accumulated phase difference is different at each position of the pulse. For the central position of the pulse, the light intensity is strongest, the accumulated total phase shift difference is more easy to reach 2 pi, so that mode locking is realized, and the edge position of the pulse is lost because of insufficient accumulation of phase difference.

Referring to fig. 5, it can be seen from fig. 5 that the 1 st counterclockwise transmission pulse interferes with the 2 nd clockwise transmission pulse after adding the adjustable delay module. When the central positions of the two pulses are not aligned, the accumulated phase difference of the forward and backward transmission pulses cannot reach 2 pi, so that the laser cannot realize mode locking. Only when the central positions of the two pulses are aligned, the total phase difference of 2 pi can be achieved, and the laser achieves mode locking.

This means that the delay amount Δt introduced by the tunable delay module must be equal to the laser pulse repetition period T, and the laser can be locked when the time tolerance is small, and at this time, the pulse repetition period T tends to be constant, and the laser repeats the frequency signal locking.

In summary, when the technical scheme is applied to the common 9-shaped NALM laser structure, when the delay delta T introduced by the adjustable delay module meets delta t=T, wherein T is the pulse repetition period, and the peak values of the staggered sequence pulses are aligned, the NALM laser can accumulate enough phase difference, the mode locking pulse can be generated, and the repetition frequency f can be obtained through the structure _r Stable mode locking pulse.

For the NALM laser in this embodiment, the optical fiber forms a closed loop similar to the shape of the digital "9", the "9" NALM laser only includes a nonlinear optical fiber loop, the optical signal only propagates back and forth in one loop, the phase adjustment is only performed in one loop, and since the cavity length of the "9" structure is shorter, the optical signal has more back and forth times in one loop, thus having higher repetition frequency, being suitable for application scenarios requiring higher repetition frequency, having the characteristics of narrow spectral bandwidth and high stability, in the "9" NALM laser, the phase difference of the two light beams transmitted oppositely is required to be an even multiple of pi, when the total phase difference of the laser pulses with the same frequency and wavelength with opposite propagation directions reaches the even multiple of pi, the loss of the resonant cavity is minimum, and the laser is easier to lock the mode.

When the 9-shaped NALM laser has no delay module, the 1 st clockwise transmission pulse and the 1 st anticlockwise transmission pulse in the nonlinear optical fiber loop have the same optical path, and are converged and interfered at the central beam splitter, and the self-starting mode locking of the laser is realized under the help of accumulated phase difference. In the interference process, the intensity of the central position of the pulse is highest, so that the nonlinear phase shift quantity accumulated in the nonlinear optical fiber loop is largest, and the total phase shift difference of 2 pi is most easily achieved, therefore, when the central beam splitter interferes, the central position of the pulse is transmitted, and the edge position of the pulse is lost, so that the pulse narrowing is realized.

When the adjustable delay module is applied to the 9-shaped NALM laser, the introduced delay amount is deltat= |t ₁ -t ₂ | a. The invention relates to a method for producing a fibre-reinforced plastic composite. With the intervention of a delay module, the 1 st clockwise transmission pulse and the 1 st anticlockwise transmission pulse exist betweenAt adjustable time differences, they may be staggered in the time domain; by adjusting the balance between the delay difference and the laser cavity length, the 1 st clockwise transmitted pulse can be time-domain synchronized with the 2 nd counterclockwise transmitted pulse and the converging interference is performed at the central beam splitter. It should be noted that only when the 1 st clockwise pulse coincides with the 2 nd counterclockwise pulse, a sufficient phase difference can be accumulated, and the laser can achieve mode locking and locking of the repetition frequency signal, otherwise, stable mode locking pulses cannot be formed.

The method is equivalent to reserving a time window of the femtosecond order for the next pulse on the time scale of nanoseconds or microseconds, and realizing mode locking of a laser and phase locking of a repetition frequency signal on the premise of not needing a peripheral phase-locked loop by reversely transmitting self calibration between two pulses. For NALM laser, there is another kind of "8" shape NALM laser, different from "9" shape NALM laser, "8" shape NALM laser include two non-linear optical fiber rings that cross each other, two non-linear optical fiber rings that cross each other form the closed loop, couple light through the optical fiber coupler between two non-linear optical fiber rings, form the transmission of photon between two loops, when the phase difference between two light of opposite transmission in the non-linear optical fiber ring reaches pi odd multiple, the resonant cavity loss is minimum, the laser is easier to lock the mode; in the 8-shaped NALM laser, an optical signal propagates back and forth between two loops to form a longer cavity length, and phase adjustment is mainly carried out between the two loops through an optical fiber polarization controller to adjust the phase difference, because the round trip times of the optical signal between the two loops are small, the heavy frequency of the 8-shaped NALM laser is relatively low, but the 8-shaped NALM laser is easier to realize high-power output relative to the 9-shaped NALM laser, and has the characteristics of strong capability of generating a locking mode, narrow spectral bandwidth of an output optical signal and high stability.

The heavy frequency self-stabilization laser disclosed by the second aspect of the invention has the following technical effects:

1. the heavy frequency self-stabilizing laser disclosed in the second aspect of the invention is provided with the adjustable delay module disclosed in the invention, so that the nonlinear optical fiber ringThe two beams of light transmitted internally are changed from paired interference to staggered pair interference, so that the staggered sequence among pulses is adjustable. When the peak values of the staggered pair interference pulses meet, the accumulated phase difference between the two pulses is the largest, and the laser realizes mode locking; when the peak values of the two pulses are staggered, the accumulated phase difference cannot meet the mode locking requirement. This means that once the laser is mode locked, the delay time between the two pulses (Δt=t=1/f _r ) I.e. locked, the fr signal of the laser is locked, and the laser achieves mode locking and phase locking. After the repetition frequency self-stabilization laser disclosed by the invention is adopted, a laser with precisely locked repetition frequency can be obtained without adding a phase-locked loop system, the structure is simple, the innovation is strong, and a new feasible scheme is provided for locking the repetition frequency of the laser.

2. The adjustable delay module disclosed by the invention can realize self calibration between opposite transmission pulses, realize locking of repeated frequency signals of the laser and realize fs-magnitude time difference locking without a redundant peripheral phase-locked system when being applied to the NALM laser, and only needs to regulate and control the introduced delay amount to achieve balance between the delay amount and the laser cavity length, so that the peak values of the staggered sequence interference pulses are opposite; the adjustable delay module has simple structure, strong innovation and low cost, does not need complex light path and circuit structure, and provides a new feasible scheme for locking the repetition frequency of the laser.

3. The adjustable delay module disclosed by the invention adopts full polarization maintaining fiber coupling and is used as a fiber coupling modulation device to be welded into the full polarization maintaining laser and the optical frequency comb, so that the interference of external environment can be effectively resisted, and the system stability is enhanced.

In a third aspect, referring to fig. 7, the present invention further discloses another heavy frequency self-stabilization laser 200, and the design method of the heavy frequency self-stabilization laser includes a resonant cavity, the nonlinear optical fiber ring 201 is disposed in the resonant cavity, and is used for performing mode locking and modulation on an optical signal, the nonlinear optical fiber ring 201 includes an optical fiber, and an adjustable delay phase shift module 400 is disposed on the optical fiber, where the adjustable delay phase shift module 400 is used for changing propagation delay difference and phase difference between two paths of laser pulses;

the nonlinear optical fiber ring 201 is sequentially connected with a wavelength division multiplexer 206 and a gain optical fiber 207 along the anticlockwise direction from a central optical beam splitter 205, and the wavelength division multiplexer 206 is also connected with a pump laser diode 208;

the pump laser diode 208 is configured to provide a pump light signal and pass the pump light signal through the wavelength division multiplexer 206 to the gain fiber 207;

The gain fiber 207 is used for amplifying an optical signal, and the optical signal amplified by the gain fiber 207 is a mode-locked optical signal;

the wavelength division multiplexer 206 is configured to couple the pump optical signal and the mode-locked optical signal into the same optical fiber;

the linear arm 202 comprises an optical fiber integration device 210, the optical fiber integration device 210 being connected to the central optical splitter 205 by a polarization-maintaining single-mode fiber for maintaining a polarization state in the optical fiber, the optical fiber integration device 210 being for introducing a feedback signal.

The scheme improves the original nonreciprocal linear phase shifter structure in the NALM laser resonant cavity, can realize coarse adjustment and fine adjustment of propagation delay difference between two laser pulses transmitted in opposite directions in a nonlinear optical fiber loop, and fine adjustment of phase shift amount, does not need to additionally increase an adjustable delay module, has higher integration, and can enable the whole device to be more succinctly integrated.

Preferably, the structure of one embodiment of the adjustable delay phase shift module 400 is given, referring to fig. 8, the adjustable delay phase shift module 400 disclosed in this embodiment includes an input port 410, an output port 411, a birefringent crystal 401, a faraday rotator 402, an electro-optical modulation crystal 403, a polarization beam splitter 404, and a stepper motor 405; the faraday rotator 402 is located between the birefringent crystal 401 and the electro-optical modulating crystal 403, the polarization beam splitter 404 is located between the electro-optical modulating crystal 403 and the stepper motor 405, and the stepper motor 405 can translate under the action of an external power supply;

The birefringent crystal 401 is used to split the light from the input port 410 or the output port 411 or the light from the faraday rotator 402 into different light in two directions; the polarization directions of the incident light from the input port 410 and the output port 411 differ by 90 °;

the faraday rotator 402 is configured to rotate the polarization state of the light from the birefringent crystal 401 or the light from the electro-optical modulating crystal 403 by a preset angle;

the electro-optical modulation crystal 403 can change the refractive index in the crystal by the influence of an external electric field, and is used for changing the propagation phase difference and the delay difference between two paths of laser pulses;

the polarizing beam splitter 404 is configured to split light from the photoelectric modulation crystal into two light beams with different polarization directions, the outgoing directions of the two light beams with different polarization directions are respectively and correspondingly provided with a first plane mirror 406 and a second plane mirror 407, the second plane mirror 407 is mounted on the stepper motor 405 and can move along with the movement of the stepper motor 405, and the stepper motor 405 and the second plane mirror 407 are configured to change propagation delay differences between two paths of laser pulses.

The working principle of the scheme is that the adjustable delay phase shift module 400 can simultaneously provide an adjustable phase difference and a delay difference for the internal phase transmission of two laser pulses in the nonlinear optical fiber ring 201, and the birefringent crystal 401 can divide two light beams with mutually perpendicular polarization directions into different paths by utilizing the anisotropic property of the birefringent crystal, so as to realize the input and output of light; the polarization state of incident light is rotated by the Faraday rotator 402, and the refractive index difference of the fast and slow axes of the crystals is changed by changing the external voltage of the electro-optical modulation crystal 403, so that the phase difference and the delay difference of micro-dimming are changed; the incident light is split into two light beams with different polarization directions through the polarization beam splitter 404, so that only the light beam with a specific polarization direction can pass through or reflect, then the light beam passing through the polarization beam splitter 404 is split to form two light paths, one light path propagates to the first plane mirror 406, and the other light path propagates to the second plane mirror 407, at this time, the stepper motor 405 converts electric energy into mechanical rotation, and the mechanical rotation moves to different positions under the input of specific stepping electric pulses; the stepping motor 405 drives the second plane reflector 407 to translate, so that the change of the optical path difference of the reflected light path of the second plane reflector 407 is realized, and further, the propagation delay difference exists between the optical path difference and the reflected light of the first plane reflector 406, so that two paths of laser pulses are subjected to staggered sequence interference, and the staggered sequence pulse peak values of the two paths of laser pulses are mutually aligned, so that the mode locking pulse with stable repetition frequency output by the repetition frequency self-stabilization laser is realized; the stepper motor 405 and the second planar mirror 407 may coarse tune the delay difference between the two beams; by the combined adjustment of the electro-optical modulation crystal 403, the stepping motor 405 and the second plane mirror 407, the large-scale precise adjustment of the delay difference and the phase difference between the laser pulses with the same frequency and wavelength and opposite propagation directions can be realized.

Specifically, the input port 410 and the output port 411 are coupled with the optical fiber by adopting an optical fiber coupling or direct space coupling mode; the direct spatial coupling employs lens coupling or diffractive coupling.

Specifically, the birefringent crystal (Birefringent Crystal) used in the present invention has a crystal with a non-uniform refractive index, and can divide an incident light into different light rays in two directions by the birefringent effect; in the scheme, the birefringent crystal can be used for splitting and combining polarized orthogonal light, and the included angle of the two beams of refracted light is related to the propagation direction and the polarization state of the light wave; the birefringent crystal adopted in the scheme can adopt one of Wollaston prism, nickel prism and polarization beam splitter, but is not limited to the above-listed birefringent crystals, and other crystals capable of realizing the functions of the birefringent crystal in the scheme are all in a protection range.

Preferably, a Faraday Rotator (Faraday Rotator) is capable of rotating light by both Faraday effect and magneto-optical effect, in this case, the Faraday Rotator is used to rotate the polarization of incident light by 45 °.

Specifically, the first plane mirror and the second plane mirror are all optical plane total reflection mirrors and are used for reflecting light, and the operations of reflection, focusing, light splitting, light path adjustment and the like of the light beams can be realized through the reflection performance of the optical plane mirrors.

Specifically, the electro-optical modulation crystal can utilize the influence of an external electric field to change the refractive index in the crystal, so that the modulation of light passing through the crystal is realized, the electro-optical modulation crystal comprises a Fast Axis (Fast Axis) and a Slow Axis (Slow Axis), the Fast Axis refers to the direction of a crystal main Axis with higher response speed of light refractive index change, and in the direction, the response of the crystal to the external electric field is more rapid, and the refractive index change is more sensitive; in contrast, the slow axis refers to the direction of the other crystal main axis perpendicular to the fast axis, which responds slower to the change of the electric field, and the change of the refractive index is smaller.

In particular, the electro-optic modulation crystal used in the invention can adopt lithium niobate crystal (LiNbO) ₃ ) Yttrium vanadate crystal (YVO) ₄ ) Gallium arsenide crystal (GaAs), lithium tantalate crystal (LiTaO) ₃ ) Potassium dihydrogen phosphate crystal (KH) ₂ PO ₄ ) Ammonium dihydrogen phosphate Crystal (NH) ₄ H ₂ PO ₄ ) One of the crystals, potassium tantalate niobate crystals (KTN), etc., but not limited to the above listed crystal types, other crystals that can perform the function of the electro-optic modulation crystal are within the scope of the present solution.

For electro-optically modulated crystals, the change in refractive index can be described by a refractive index ellipsoid. In the principal axis coordinate, when no external electric field is applied, the refractive index ellipsoid equation is shown as formula (4):

Wherein n is _x ，n _y ，n _z The refractive indexes of the ellipsoids in the three main axis directions are respectively.

When the direction of the applied electric field coincides with the light propagation direction, referred to as longitudinal electro-optic effect, the refractive index of the medium along the light propagation direction changes. When the direction of the applied electric field is perpendicular to the light propagation direction, the ellipsoidal section of the refractive index perpendicular to the light propagation direction is changed from a circle to an ellipse, and after the principal axis transformation, the refractive index is as shown in formula (5):

wherein n is _x ，n _y Refractive index in x-axis and y-axis directions of ellipsoid, gamma _yy As dielectric parameters, E _x Is the x-direction component of the electric field strength.

The working principle of the adjustable delay phase shift module 400 is: referring to fig. 8, a schematic structural diagram of an adjustable delay phase shift module 400 according to the present invention is shown; the laser pulse optical signal entering from the input port 410 enters along the o-axis of the birefringent crystal 401, passes through the Faraday rotator 402, rotates 45 degrees in the polarization state of the pulse, transmits along the fast axis of the electro-optic modulation crystal 403, transmits at the polarization beam splitter, passes through the fast axis of the electro-optic modulation crystal 403 and the Faraday rotator 402 again after undergoing the first optical path difference 408, rotates 90 degrees in the polarization state as a whole, and is accumulated to obtain phase delayAnd output along output port 411;

similarly, the laser pulse optical signal input from the output port 411 is incident along the e-axis of the birefringent crystal 401, passes through the faraday rotator 402, rotates the polarization state by 45 ° and transmits along the slow axis of the electro-optic modulation crystal 403, reflects at the polarization beam splitter, passes through the second optical path difference 409, passes through the slow axis of the electro-optic modulation crystal 403 and the faraday rotator 402 again, rotates the polarization state by 90 ° as a whole, and accumulates to obtain a phase delay And exits through input port 410;

the difference of the refractive index of the fast/slow axes of the electro-optical modulation crystal 403 can be changed by controlling the external driving voltage of the electro-optical modulation crystal, so that the phase difference and the delay difference are finely adjusted for the forward and backward transmission of two paths of laser pulses;

adjusting the position of the stepping motor 405 can change the optical path difference between the two laser pulses, so as to realize coarse adjustment of the delay time difference; the electro-optical modulation crystal 403, the stepping motor 405 and the second plane mirror 407 are jointly regulated, so that the large-range precise regulation and control of the laser pulse phase difference and the delay difference of the same frequency and wavelength with opposite propagation directions can be realized, and a new scheme is provided for the mode locking and the repetition frequency precise locking of the NALM type laser. The invention replaces the fixed wave plate in the non-reciprocal linear phase shifter 209 with the electro-optic modulation crystal 403, thereby realizing the effect of real-time adjustment of the non-reciprocal linear phase difference. The new refractive index ellipsoid can rotate 45 degrees around the Z axis without manual adjustment, so that the optical loss and the angle deviation are reduced. This tunable delay phase shift module 400 is used in NALM laser based lasers where the phase difference between pulses is used for self-starting mode locking of the laser and the time difference of the misclassification delay is used for self-locking of the laser repetition frequency signal.

The invention discloses one embodiment of an adjustable delay phase shift module 400, which is used for elaborating how to provide adjustable delay and linear non-reciprocal phase difference for two beams of light which are transmitted oppositely, wherein the adjustable delay phase shift module 400 comprises a birefringent crystal 401, a Faraday rotator 402, an electro-optical modulation crystal 403, a polarization beam splitter 404 and a stepping motor 405; the birefringent crystal 401 is a Wollaston prism, and the electro-optical modulation crystal 403 is a YVO4 crystal.

The tail fibers of the input port 410 and the output port 411 are polarization maintaining fibers, the polarization states of the polarization maintaining fibers differ by 90 degrees, the incident direction of the input port 410 is along the o-axis of the Wollaston prism, and the incident direction of the output port 411 is along the e-axis of the Wollaston prism; wollaston prism, faraday rotator and YVO ₄ The heights of the optical axis and the optical center of the crystal are kept consistent, and the placement angle of the polarization beam splitter is equal to YVO ₄ The crystals were 45 ° apart in the Z direction.

At YVO administration ₄ After a controllable electric field perpendicular to the light propagation direction is applied to the crystal, the optical axis of the crystal rotates 45 degrees along the Z direction, the fast and slow axes rotate from the original x and y directions to the x ', y' directions, and the rotation angles between the x 'axis and the y' axis and the original x and y axes are 45 degrees respectively. YVO ₄ The length of the crystal is l, the thickness is d, and the fast axis refractive index is n _x’ The refractive index of the slow axis is n _y’ . Horizontal deflection incident from the first endAfter passing through Wollaston prism, the polarized light is rotated by 45 degrees by Faraday rotator along YVO ₄ The fast axis x' of the crystal is incident.

The polarizing beam splitter crystal is horizontally polarized light, and the horizontally polarized light is reflected by the optical plane second plane reflector and then returns to the polarizing beam splitter, and the first optical path difference is reflected back and forth.

Then pass through YVO again ₄ The x' axis of the crystal and the Faraday rotator are rotated by 90 degrees in an accumulated way, the polarization state is changed into the vertical direction, and the phase difference obtained by passing through the fast axis twice isFinally, the light exits along the e-axis of the Wollaston prism and is coupled into the output port 411 to be output.

Similarly, vertically polarized light incident from the second end is incident along the e-axis of the Wollaston prism, its polarization state is rotated 45 ° by the Faraday rotator, along YVO ₄ The slow axis y' of the crystal propagates, and for the polarization beam splitter crystal, the crystal is vertical polarized light, and the vertical polarized light is reflected by the polarization beam splitter and then passes through the first plane reflector, finally returns to the polarization beam splitter, and the second optical path difference is reflected back and forth.

Then the polarization state of the crystal is rotated by 90 degrees in a cumulative way through the y' axis and the Faraday rotator again, the polarization state becomes the horizontal direction, and the phase difference obtained by passing through the slow axis twice is Finally, the light exits along the o-axis of the Wollaston prism and is coupled into the input port 410 for output.

Therefore, after the adjustable delay phase shift module, a non-reciprocal linear phase shift difference is obtained between two opposite transmission beams of light, as shown in formula (6):

wherein,the phases of the laser pulses with the same frequency and wavelength and opposite propagation directions are respectively n _x Is YVO ₄ The fast axis refractive index of the crystal, n _y Is YVO ₄ Slow-axis refractive index of crystal, n _x’ Is YVO ₄ The refractive index of the fast axis of the crystal after the optical axis of the crystal rotates 45 degrees along the Z direction, n _y’ YVO after the optical axis rotates 45 degrees along the Z direction ₄ The slow-axis refractive index of the crystal, l is YVO ₄ The length of the crystal, d is YVO ₄ Thickness of crystal, V ₀ Is YVO ₄ Direct current component of crystal external electric field V _m Is YVO ₄ The alternating component of the externally applied electric field of the crystal, f is YVO ₄ The modulation frequency of the alternating electric field is externally applied to the crystal. Gamma ray _yy Is a dielectric parameter.

The delay difference obtained between the two light beams transmitted in opposite directions is shown in formula (7):

Δt ₁ ＝|L ₁ -L ₂ |·n ₁ /c (7)

wherein L is ₁ For the first optical path difference, L ₂ For the second optical path difference, c is the speed of light in vacuum, the magnitude of the delay difference is on the ns- μs scale, n ₁ Is the refractive index of air.

Meanwhile, the second plane reflecting mirror is adhered to the stepping motor, and the external driving voltage V of the motor is regulated and controlled ₂ The position of the second plane reflector can be changed to regulate and control the delay difference delta t between the two beams of light ₁ 。

In addition, fine tuning YVO ₄ External control voltage V of crystal ₁ A small delay delta t can be introduced into two paths of laser pulses with the same frequency and wavelength and opposite propagation directions ₂ As shown in formula (8):

wherein n is _x’ Is YVO ₄ The refractive index of the fast axis of the crystal after the optical axis of the crystal rotates 45 degrees along the Z direction, n _y’ YVO after the optical axis rotates 45 degrees along the Z direction ₄ Slow axis of crystalRefractive index, l is YVO ₄ Length of crystal, delay difference Δt ₂ The magnitude is on the fs-ps scale, c being the speed of light in vacuum.

Pure YVO ₄ The crystal is positive single-axis crystal, n _x ＝n _y ＝n _o ，n _z ＝n _r Therefore, after simplification, the accumulated phase difference between the two beams is

The accumulated delay difference is

Wherein lambda is ₀ For the incident center wavelength, c is the speed of light in vacuum, V ₀ Is YVO ₄ Direct current component of crystal external electric field V _m Is YVO ₄ The alternating component of the externally applied electric field of the crystal, f is YVO ₄ Modulation frequency of externally-applied alternating electric field of crystal, n _o Refractive index in the o-axis direction, gamma _yy Is a dielectric parameter.

The total delay delta t provided by the tunable delay phase shift module for two laser pulses with the same frequency and wavelength and opposite propagation directions is shown as formula (11):

wherein Δt is ₁ Is the delay difference, delta t, adjusted by the second plane mirror and the stepper motor ₂ Is made of YVO ₄ Delay difference of crystal adjustment, L ₁ For the first optical path difference, L ₂ Is the second optical path difference, n ₁ Is the refractive index of air, c is the speed of light in vacuum, l is YVO ₄ The length of the crystal, d is YVO ₄ Thickness of crystal, V ₀ Is YVO ₄ Direct current component of crystal external electric field V _m Is YVO ₄ Ac component of externally applied electric field of crystalF is YVO ₄ Modulation frequency of externally-applied alternating electric field of crystal, gamma _yy Is a dielectric parameter.

According to the embodiment scheme of the adjustable delay phase shift module, coarse adjustment of the delay time difference can be achieved by adjusting the driving voltage of the stepping motor, and fine adjustment of the delay time difference and the phase shift difference can be achieved by adjusting the external driving voltage of the electro-optic modulation crystal. By the combined adjustment of the two, the error sequence interference of the forward and reverse transmission light can be realized, and meanwhile, the accurate locking of the mode locking and the repetition frequency of the laser can be obtained.

Therefore, the heavy frequency self-stabilization laser disclosed in the third aspect has the following technical effects:

1. the invention discloses a heavy frequency self-stabilization laser in a third aspect, which is provided with an adjustable delay phase shift module, wherein the concept of adjustable delay is added into a non-reciprocal linear phase shifter in an NALM laser, so that two light energy which are oppositely transmitted in a non-linear optical fiber ring is ensured to obtain enough phase accumulation, synchronous adjustment of phase difference and delay difference is realized on the premise of realizing mode locking of the laser, two light error sequences which are oppositely transmitted in the non-linear optical fiber ring are interfered, and mode locking and f are realized _r Locking of the signal.

2. The adjustable delay phase shift module provided by the invention can realize synchronous change of the linear phase shift quantity and the delay quantity between two beams of light which are oppositely transmitted only by adjusting the external driving voltage of the electro-optical modulation crystal and the stepping motor, has high accuracy, strong flexibility, wide adjustment range and simple operation, and can help the laser to self-start and lock the mode at a low threshold value by using the adjustable linear phase shift module in an NALM laser; the adjustable delay quantity can lead the staggered sequence interference among the pulses to realize the locking of the laser repetition frequency signal, has multiple purposes, has high integration level and reduces the complexity of the system.

In a fourth aspect, as shown in fig. 6, the present invention also discloses an all-fiber optical comb structure based on the heavy-frequency self-stabilization laser 200 of fig. 2, which includes a heavy-frequency self-stabilization laser 200, an optical power amplifier 301, a supercontinuum stretcher 302, an f-2f self-reference detector 303, and a phase-locked loop;

the repetition frequency self-stabilization laser 200 comprises an adjustable delay module 100 for generating mode-locked pulse laser;

the optical power amplifier 301 is used for increasing the energy and power of the optical signal;

the supercontinuum stretcher 302 is configured to produce a continuous spectrum comprising a plurality of frequency components;

The f-2f self-reference detector 303 is used to measure and stabilize the frequency of the laser pulses;

the phase-locked loop includes a repetition frequency f _r Signal phase locked loop 304 and optical comb carrier envelope offset frequency f _ceo A signal phase locked loop 305;

the repetition frequency f _r The signal phase-locked loop 304 is used to stabilize the frequency of the optical pulse train generated by the laser;

the optical comb carrier envelope offset frequency fceo signal phase lock loop 305 is used to stabilize the phase relationship between the optical comb frequency and the optical comb envelope waveform.

Specifically, the heavy frequency self-stabilizing laser 200 is the heavy frequency self-stabilizing laser 200 including the adjustable delay module 100 disclosed above.

Specifically, the optical power amplifier 301, the supercontinuum stretcher 302, the f-2f self-reference detector 303 and the phase-locked loop are all commonly used devices in the prior art, and the working principle thereof is as follows:

the optical power amplifier 301 operates on the principle that the energy and power of an optical signal are increased by using stimulated radiation processes by injecting a weak optical signal into a device doped with an optical amplifying material (e.g., erbium-doped fiber);

the principle of operation of the supercontinuum stretcher 302 is to utilize nonlinear optical effects (e.g., self-phase modulation, transient coulomb effects, etc.) and modulated optical pumping to produce a continuous spectrum comprising a plurality of frequency components;

The f-2f self-reference detector 303 can divide the input pulse into two parts and delay and optical shift in the optical lattice on one reference beam, from which the absolute frequency and the correlation properties of the laser pulse can then be derived by comparing the two frequency components;

phase locked loops capable of controlling and regulating input signals and referencesPhase relation between signals, in particular repetition frequency f _r The signal phase locked loop 304 is capable of phase locking the laser frequency to a reference signal frequency (typically a reference oscillator) such that the two maintain a stable phase relationship. By controlling the length of an optical cavity of the laser or the bias of an amplifier, the synchronization of the laser output frequency and the frequency of a reference signal can be realized, and a repetition frequency signal phase-locked loop is commonly used for mode locking of an ultrafast laser; optical comb carrier envelope offset frequency f _ceo The signal phase locked loop 305 is capable of controlling the carrier frequency and envelope offset frequency of the optical comb by phase locking so that they maintain a fixed phase relationship with a reference signal (typically a stable optical oscillator).

In particular, the optical comb has a very uniform frequency spectrum with equal spacing between frequencies, one carrier frequency per frequency pulse envelope of the optical comb, and one envelope offset frequency above the repetition frequency of the optical comb. For the traditional optical fiber comb based on ultrashort pulse generation, two or more additional high-feedback bandwidth phase-locking elements are usually inserted into the resonant cavity, and a high-bandwidth phase-locking servo system is assisted to realize the optical comb f _r And f _ceo The two free parameters are locked simultaneously, so that a complete high-stability optical comb is formed. The added phase-locked element and the electronic servo system not only increase the complexity of the system, but also bring challenges to the aspects of system power consumption, volume, cost and the like.

The all-fiber optical comb structure disclosed by the invention has the following technical effects: in the all-fiber optical comb structure disclosed by the invention, the adjustable delay module or the adjustable delay phase shift module is added in the heavy-frequency self-stabilization laser, the internal mode locking and the phase locking element are combined into one, the system integration level is high, an additional phase locking electronic servo system is not needed, and the optical comb f can be realized only by adjusting the external driving voltage of the adjustable delay module _r Locking of the signal. The negative feedback regulating system based on the electro-optic effect and the fast saturation absorption effect has a high feedback bandwidth of MHz magnitude, and can realize the optical comb f _r Precision locking of signals; meanwhile, the repetition frequency locking pulse impulse obtained from the repetition frequency self-stabilization laserThe light can be used as seed source pulse light of the optical comb, so that the optical comb gets rid of a redundant phase-locked loop electronic servo system, and the miniaturization and integration development of the optical comb are promoted; the all-fiber optical comb structure device disclosed by the invention has the advantages of simple structure, low cost and high integration level, and provides a reliable realization way for miniaturized, integrated and practical optical combs.

It will be understood that the invention has been described in terms of several embodiments, and that various changes and equivalents may be made to these features and embodiments by those skilled in the art without departing from the spirit and scope of the invention. Modifications may be made to the features and embodiments of the invention in light of the teachings of the invention to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. The described embodiments of the invention are some, but not all, embodiments of the invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the invention, as presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all other embodiments falling within the scope of the invention as defined by the appended claims, as interpreted according to the breadth to which they are fairly set forth in the claims.

Claims

1. The design method of the heavy frequency self-stabilization laser is based on the NALM laser and is characterized in that the NALM laser comprises a nonlinear optical fiber loop, two laser pulses are reversely transmitted in the nonlinear optical fiber loop, the two laser pulses can generate interference, the two laser pulses are subjected to staggered sequence interference by changing the propagation delay difference between the two laser pulses, and the staggered sequence pulse peaks of the two laser pulses are mutually aligned, so that the heavy frequency self-stabilization laser outputs mode locking pulses with stable repetition frequency.

2. The design method of the heavy frequency self-stabilization laser according to claim 1 is characterized by comprising a resonant cavity, wherein the nonlinear optical fiber ring is arranged in the resonant cavity and is used for carrying out mode locking and modulation on optical signals, the nonlinear optical fiber ring comprises an optical fiber, the optical fiber comprises a first optical fiber coupling end and a second optical fiber coupling end, an adjustable delay module is arranged between the first optical fiber coupling end and the second optical fiber coupling end, and the adjustable delay module is used for changing propagation delay difference between two paths of laser pulses; the heavy-frequency self-stabilization laser also comprises a linear arm, and the nonlinear optical fiber ring is connected with the linear arm through a central optical beam splitter;

the non-reciprocal linear phase shifter is used for introducing linear phase shift quantity for forward and backward transmission of two beams of light;

the linear arm comprises an optical fiber integrated device, the optical fiber integrated device is connected with the central optical beam splitter through a polarization-maintaining single-mode fiber, the polarization-maintaining single-mode fiber is used for maintaining the polarization state in an optical fiber, and the optical fiber integrated device is used for reflecting signal light and introducing output light of a resonant cavity.

3. The heavy frequency self-stabilization laser of claim 2, wherein the tunable delay module comprises a first optical fiber coupling collimator, an input end of the first optical fiber coupling collimator is used for inputting an optical signal, the input end of the first optical fiber coupling collimator is coupled with the first optical fiber coupling end, an output end of the first optical fiber coupling collimator is connected with an input end of an optical beam splitter, the optical beam splitter comprises a transmission end and a reflection end, the transmission end is connected with a first end of an optical circulator, and the first end is used for inputting the optical signal;

4. The heavy frequency self-stabilizing laser according to claim 3, wherein a first reflecting mirror is further arranged between the movable reflecting mirror group and the third end, and the emergent direction of the third end is opposite to the incident direction of the first reflecting mirror;

5. The heavy frequency self-stabilizing laser according to claim 3 or 4, wherein the repetition period of said laser pulses is T, the delay amount by which said tunable delay module is capable of varying the propagation delay difference between two laser pulses is Δt = T, and the phase difference between two laser pulses is 2nδ, where n is a natural number and n e {1,2, 3.

6. The heavy frequency self-stabilizing laser according to claim 3 or 4, wherein the optical path distances between two ends of said gain fiber and said central optical beam splitter are not equal, and are asymmetrically arranged in said nonlinear fiber loop.

7. The design method of the heavy-frequency self-stabilization laser utilizing the heavy-frequency self-stabilization laser according to claim 1 is characterized by comprising a resonant cavity, wherein the nonlinear optical fiber ring is arranged in the resonant cavity and is used for carrying out mode locking and modulation on an optical signal, the nonlinear optical fiber ring comprises an optical fiber, and an adjustable delay phase shift module is arranged on the optical fiber and is used for changing propagation delay difference and phase difference between two paths of laser pulses;

8. The heavy frequency self-stabilizing laser according to claim 7, wherein said tunable delay phase shift module comprises an input port, an output port, a birefringent crystal, a faraday rotator, an electro-optic modulation crystal, a polarizing beam splitter, and a stepper motor; the Faraday rotator is positioned between the birefringent crystal and the electro-optic modulation crystal, the polarization beam splitter is positioned between the electro-optic modulation crystal and the stepping motor, and the stepping motor can translate under the action of an external power supply;

9. The heavy frequency self-stabilizing laser of claim 8, wherein the tunable delay phase shift module is capable of providing a total delay difference Δt for two laser pulses of the same frequency and wavelength having opposite propagation directions as follows:

10. An all-fiber optical comb structure utilizing the repetition frequency self-stabilization laser of claim 3 or claim 8, characterized by comprising a repetition frequency self-stabilization laser, an optical power amplifier, a supercontinuum stretcher, an f-2f self-reference detector and a phase-locked loop;