Laser based on discrete echo wall microcavity and mode locking method

Abstract

The invention provides a laser based on a discrete echo wall microcavity and a mode locking method, and relates to the technical field of laser. The laser provided by the invention comprises a pumping source, a resonant cavity packaging module and a feedback control module, the resonant cavity packaging module comprises a tapered optical fiber group, a phase modulator and an echo wall mode resonant cavity which are packaged in a shell; the tapering optical fiber group is coupled with the echo wall mode resonant cavity at a fixed distance; and the feedback control module is used for locking the mode of the resonant cavity packaging module. According to the laser and the mode locking method provided by the invention, a corresponding structure is designed based on the discrete echo wall micro-cavity, the precise coupling problem of the outer cavity module and the laser chip is solved, the laser is easy to assemble and deploy by realizing the modular design, and a self-adaptive compensation algorithm is provided to enhance the locking effect, so that the mode locking method has the advantages of high reliability and high reliability. And a mode selection function is provided for a laser user so as to better support research in the fields of communication, calculation and the like.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular, to a laser and a mode locking method based on a discrete whispering-gallery microcavity. Background Art

[0002] In recent years, the method of obtaining narrow-linewidth laser by externally cavity self-injection locking to narrow the linewidth of semiconductor lasers has attracted extensive attention of researchers.

[0003] Among various types of external cavities, the whispering-gallery-mode (WGM) optical resonator has an extremely high quality factor (Q value), and thus can obtain a very good linewidth narrowing effect when applied to self-injection locking. In particular, compared with the integrated WGM cavity, the discrete WGM cavity has the highest recorded quality factor in the world, and has very good linewidth compression effect and mode selection performance. For the discrete WGM cavity, generally, semiconductor laser is coupled into the WGM cavity through evanescent wave, and there are mainly two specific methods: one is to generate evanescent wave through total reflection on the prism interface, and the other is to rely on the optical field leakage in the tapered region of the tapered fiber. Prism coupling requires high-precision micro-assembly technology, which increases the cost of the self-injection locking laser, and strict mode field matching is required when collecting the output light, otherwise power loss will occur. And fiber taper coupling generally results in a longer optical path of the feedback light. Coupled with the fragility of the fiber taper itself, it is more vulnerable to environmental disturbances, resulting in very poor locking stability. Moreover, both the fiber and prism coupling methods are not compatible with the original end-face emission output method of the semiconductor laser chip, and collimation needs to be performed through multiple output and input lenses, which increases the system complexity and is not conducive to integration. Summary of the Invention

[0004] Embodiments of the present invention provide a laser and a mode locking method based on a discrete whispering-gallery microcavity. By designing a corresponding structure based on the discrete whispering-gallery microcavity, the problem of precise coupling between the external cavity module and the laser chip is solved. Through modular design, the laser is easy to assemble and deploy, and the tapered fiber group and the whispering-gallery mode resonator are less affected by external disturbances. Moreover, during the operation of the laser, feedback adjustment can be performed through a phase modulator to ensure the stable output of the laser.

[0005] In a first aspect, an embodiment of the present invention provides a laser based on a discrete whispering-gallery microcavity, including a pump source, a resonator packaging module, and a feedback control module;

[0006] The pump source is used to output a pump beam;

[0007] The input end of the resonator packaging module is coupled to the output end of the pump source;

[0008] The resonant cavity packaging module includes a tapered fiber optic group, a phase modulator, and a whispering gallery mode resonator encapsulated in a housing. The phase modulator is used to adjust the optical path between the pump source and the whispering gallery mode resonator.

[0009] The tapered fiber optic group is coupled to the whispering gallery mode resonator at a fixed distance. The tapered fiber optic group is used to transmit the pump beam. The tapered fiber optic group includes a first end and a second end. The first end is the input end of the resonant cavity packaging module, and the second end is the output end of the resonant cavity packaging module.

[0010] After the pump beam is coupled to the whispering gallery mode resonator through the tapered fiber optic group, the whispering gallery mode resonator is used to return part of the pump beam to the pump source to generate a self-injection locking effect.

[0011] The feedback control module is electrically connected to the pump source and the phase modulator respectively. The feedback control module is used to adjust the optical path between the pump source and the resonant cavity packaging module, the drive current of the pump source, and the operating temperature of the pump source according to the laser power output by the resonant cavity packaging module, so as to lock the laser frequency on the resonant mode of the whispering gallery mode resonator.

[0012] Optionally, the feedback control module includes a beam splitter, a detector, and a control unit.

[0013] The input end of the beam splitter is coupled to the output end of the resonant cavity packaging module. The beam splitter includes a first output end and a second output end. The beam splitter is used to divide the laser beam output by the resonant cavity packaging module into a first signal light and a second signal light. The first output end of the beam splitter is used to output the first signal light, and the second output end of the beam splitter is used to output the second signal light.

[0014] The detector is coupled to the second output end of the beam splitter. The detector is communicatively connected to the control unit. The detector is used to receive the second signal light, and the control unit determines the power of the second signal light according to the signal obtained by the detector.

[0015] The control unit is electrically connected to the pump source and the phase modulator respectively. The control unit is used to adjust the optical path between the pump source and the resonant cavity packaging module, the drive current of the pump source, and the operating temperature of the pump source according to the power, so as to make the resonant cavity packaging module output a laser beam of a preset mode.

[0016] Optionally, the laser further includes an isolator. The input end of the isolator is coupled to the output end of the resonant cavity packaging module. The isolator is used to make the laser beam transmit unidirectionally.

[0017] Optionally, the tapered fiber optic group includes a first tapered fiber optic. The first tapered fiber optic includes a third end and a fourth end. The third end is the input end of the resonant cavity packaging module, and the fourth end is the output end of the resonant cavity packaging module.

[0018] Optionally, the tapered fiber group includes a first tapered fiber and a second tapered fiber that are arranged in parallel on both sides of the whispering gallery mode resonator. Both the first tapered fiber and the second tapered fiber are coupled to the whispering gallery mode resonator. The first tapered fiber includes a third end and a fourth end, and the second tapered fiber includes a fifth end and a sixth end. The third end is the input end of the resonator encapsulation module, and the fifth end is the output end of the resonator encapsulation module.

[0019] Optionally, the phase modulator includes a temperature controller, and the resonator encapsulation module further includes an optical fiber clamp;

[0020] The optical fiber clamp is used to hold the tapered fiber group and serve as a temperature conduction medium between the temperature controller and the tapered fiber group;

[0021] The temperature controller is used to change the temperature of the tapered fiber group through the optical fiber clamp to change the optical path between the pump source and the whispering gallery mode resonator.

[0022] In a second aspect, an embodiment of the present invention provides a mode locking method, which is applicable to the laser provided in any embodiment of the present invention. The mode locking method includes:

[0023] Start the laser and record the output power of the laser at this time;

[0024] Control the phase modulator to change the optical path between the pump source and the whispering gallery mode resonator. Each time the optical path is changed, the working temperature of the pump source is changed from the lower limit of the working temperature change to the upper limit of the working temperature change at a constant speed, and record the waveform of the output power of the laser changing with the working temperature of the pump source under each optical path;

[0025] Adjust the optical path to the optical path when the output power of the laser is the lowest, and adjust the working temperature of the pump source until the output power of the laser is less than the threshold power to achieve mode locking.

[0026] Optionally, until the output power of the laser is less than the threshold power, then, it further includes:

[0027] If the output power of the laser is still greater than or equal to the threshold power after the working temperature of the pump source reaches the upper limit of the working temperature change, then change the adjustment range of the optical path and the working temperature of the pump source, and repeat the mode locking method.

[0028] Optionally, until the output power of the laser is less than the threshold power, then, it further includes:

[0029] Record the output power of the laser every Δt. When the output power of the laser is greater than or equal to the threshold power, through the adaptive compensation algorithm, adjust the phase modulator to change the optical path until the output power of the laser is less than the threshold power.

[0030] Optionally, achieving mode locking includes:

[0031] The linewidth of the output mode of the laser is less than or equal to the threshold linewidth.

[0032] The laser provided by the embodiment of the present invention encapsulates the tapered fiber group and the whispering gallery mode resonator in the resonator packaging module, designs the corresponding structure based on the discrete whispering gallery microcavity, solves the problem of precise coupling between the external cavity module and the laser chip, makes the laser easy to assemble and deploy through modular design, and makes the tapered fiber group and the whispering gallery mode resonator less affected by external disturbances; in addition, by installing a phase modulator in the resonator packaging module, an adjustable phase delay parameter is introduced, so that feedback adjustment can be performed during the operation of the laser to ensure the stable output of the laser.

[0033] It should be understood that the content described in this part is not intended to identify the key or important features of the embodiments of the present invention, nor is it used to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0035] Figure 1 is a schematic structural diagram of a laser provided by an embodiment of the present invention;

[0036] Figure 2 is a schematic structural diagram of another laser provided by an embodiment of the present invention;

[0037] Figure 3 is a schematic structural diagram of yet another laser provided by an embodiment of the present invention;

[0038] Figure 4 is a schematic structural diagram of a resonator packaging module provided by an embodiment of the present invention;

[0039] Figure 5 is a schematic internal structural diagram of a resonator packaging module provided by an embodiment of the present invention;

[0040] Figure 6 is an exploded view of the structure of a resonator packaging module provided by an embodiment of the present invention;

[0041] Figure 7 is an exploded view of the structure of yet another resonator packaging module provided by an embodiment of the present invention;

[0042] Figure 8 It is a schematic diagram of the internal structure of another resonant cavity packaging module provided by an embodiment of the present invention;

[0043] Figure 9 It is a mode-locking method provided by the present invention;

[0044] Figure 10 It is a waveform of the power output by the laser at different optical paths changing with the operating temperature of the pump source;

[0045] Figure 11 It is another mode-locking method provided by the present invention;

[0046] Figure 12 It is a flowchart of the mode searching part in the mode-locking method provided by an embodiment of the present invention;

[0047] Figure 13 It is a flowchart of the stable mode-locking part in the mode-locking method provided by an embodiment of the present invention. Detailed implementation manners

[0048] In order to enable those skilled in the art to better understand this solution, the technical solutions in 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. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0049] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and do not have to be used to describe a specific order or sequence. It should be understood that such used data can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device including a series of steps or units does not have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0050] An embodiment of the present invention provides a laser based on a discrete whispering gallery microcavity, Figure 1 It is a schematic diagram of the structure of a laser provided by an embodiment of the present invention, for reference Figure 1, the laser includes a pump source 100, a resonant cavity packaging module 200, and a feedback control module 300; the pump source 100 is used to output a pump beam; the input end of the resonant cavity packaging module 200 is coupled to the output end of the pump source 100; the resonant cavity packaging module 200 includes a tapered fiber group 201 encapsulated in a housing, a phase modulator ( Figure 1 not shown in the figure) and a whispering gallery mode resonator 202, the phase modulator is used to adjust the optical path between the pump source 100 and the whispering gallery mode resonator 202; the tapered fiber group 201 is coupled to the whispering gallery mode resonator 202 at a fixed distance, the tapered fiber group 201 is used to transmit the pump beam, the tapered fiber group 201 includes a first end and a second end, the first end is the input end of the resonant cavity packaging module 200, and the second end is the output end of the resonant cavity packaging module 200; after the pump beam is coupled to the whispering gallery mode resonator 202 through the tapered fiber group 201, the whispering gallery mode resonator 202 is used to return part of the pump beam to the pump source 100 to generate a self-injection locking effect; the feedback control module 300 is electrically connected to the pump source 100 and the phase modulator respectively, and the feedback control module 300 is used to adjust the optical path between the pump source 100 and the resonant cavity packaging module 200, the drive current of the pump source 100, and the operating temperature of the pump source 100 according to the laser power output by the resonant cavity packaging module 200, so that the laser frequency is locked on the resonant mode of the whispering gallery mode resonator 202.

[0051] Reference Figure 1, the pump source 100 includes but is not limited to a Distributed Feedback Laser (DFB) or a Distributed Bragg Reflector (DBR). The pump source 100 can provide a continuous pump beam. Optionally, the wavelength of the pump beam includes but is not limited to 1550 nm, 1064 nm, 980 nm, 785 nm, or 532 nm. The pump beam is coupled into the whispering gallery mode resonator 202 through the tapered fiber array 201. The whispering gallery mode resonator 202 can select modes for the pump beam. Only the modes that meet the resonance conditions of the whispering gallery mode resonator 202 will resonate in the whispering gallery mode resonator 202, thereby eliminating the modes in the pump beam that do not meet the resonance conditions of the whispering gallery mode resonator 202. The whispering gallery mode resonator 202 includes but is not limited to a disk-shaped, rod-shaped, or ring-shaped cavity made of alkali metal fluoride crystal, sapphire, lithium niobate, lithium tantalate, or fused silica. The whispering gallery mode resonator 202 can backscatter part of the pump beam to the output end face of the pump source 100, thereby generating a self-injection locking effect and shortening the linewidth of the laser finally output by the laser. The stronger the self-injection locking effect, the narrower the linewidth of the laser finally output by the laser. The phase modulator can adjust the optical path between the pump source 100 and the whispering gallery mode resonator 202, thereby changing the phase of the pump beam returning to the output end face of the pump source 100, and further affecting the strength of the self-injection locking effect. Since the tapered fiber array 201, the phase modulator, and the whispering gallery mode resonator 202 are encapsulated in the resonator encapsulation module 200 and their relative positions are fixed, the tapered fiber array 201 and the whispering gallery mode resonator 202 are less affected by external disturbances. Moreover, the phase modulator introduces an adjustable phase delay parameter for providing feedback regulation during the operation of the laser to ensure the stable output of the laser.

[0052] The feedback control module 300 can adjust the drive current and the operating temperature of the pump source 100 to make the frequency of the pump beam close to the resonance frequency of the whispering gallery mode resonator 202, so that more pump beams enter the whispering gallery mode resonator 202 to excite the whispering gallery mode. As a result, more pump beams will be backscattered to the pump source 100, enhancing the self-injection locking effect and narrowing the linewidth of the output laser beam. In addition, the feedback control module 300 can detect the power of the laser beam output by the resonator encapsulation module 200. When the power of the laser beam is low, it indicates that the self-injection locking effect is strong and the linewidth of the laser beam is narrow. When the power of the laser beam increases, it indicates that the self-injection locking effect weakens. At this time, the feedback control module needs to adjust the phase modulator to change the optical path between the pump source 100 and the resonator encapsulation module 200 to enhance the self-injection locking effect and thus narrow the linewidth of the laser beam.

[0053] The laser provided by the embodiment of the present invention encapsulates a tapered fiber group and a whispering gallery mode resonator in a resonator packaging module, designs a corresponding structure based on a discrete whispering gallery microcavity, solves the problem of precise coupling between the external cavity module and the laser chip, makes the laser easy to assemble and deploy through modular design, and reduces the influence of the tapered fiber group and the whispering gallery mode resonator on external disturbances; in addition, by installing a phase modulator in the resonator packaging module, an adjustable phase delay parameter is introduced, so that feedback adjustment can be performed during the operation of the laser to ensure the stable output of the laser.

[0054] Optionally, the tapered fiber group includes polarization-maintaining fibers, which can keep the polarization state of the light beam unchanged and improve stability.

[0055] Figure 2 is a schematic structural diagram of another laser provided by the embodiment of the present invention. Refer to Figure 2 , the feedback control module 300 includes a beam splitter 301, a detector 302 and a control unit 303; the input end of the beam splitter 301 is coupled to the output end of the resonator packaging module 200. The beam splitter 301 includes a first output end and a second output end. The beam splitter 301 is used to divide the laser beam output by the resonator packaging module 200 into a first signal light S1 and a second signal light S2. The first output end of the beam splitter 301 is used to output the first signal light S1, and the second output end of the beam splitter 301 is used to output the second signal light S2; the detector 302 is connected to the second output end of the beam splitter 301, and the detector 302 is communicatively connected to the control unit 303. The detector 302 is used to receive the second signal light S2, and the control unit 303 determines the power of the second signal light S2 according to the signal obtained by the detector 302; the control unit 303 is electrically connected to the pump source 100 and the phase modulator respectively. The control unit 303 is used to adjust the optical path between the pump source 100 and the resonator packaging module 200, the driving current of the pump source 100 and the operating temperature of the pump source 100 according to the power, so that the resonator packaging module 200 outputs a laser beam of a preset mode.

[0056] Refer to Figure 2 , the first signal light S1 is the laser finally output by the laser, and the second signal light S2 is the basis for the feedback adjustment of the control unit 303. Optionally, the power ratio of the first signal light S1 to the second signal light S2 is 99:1. The detector 302 can convert the optical signal into an electrical signal and detect the power of the second signal light S2. The control unit 303 can adjust the optical path between the pump source 100 and the resonator packaging module 200, the driving current of the pump source 100 and the operating temperature of the pump source 100 according to the power of the second signal light S2 according to a preset program, so as to realize mode searching, mode locking and feedback control.

[0057] Optionally, the detector 302 includes on-chip devices. The voltage output by the detector 302 can also be read by connecting an oscilloscope or a data acquisition card after the detector 302, and an electrical signal amplifier and a filter can be added between the detector 302 and the subsequent device.

[0058] Optionally, a semiconductor optical amplifier (SOA) or an erbium-doped fiber amplifier (EDFA) can be used to amplify the first optical signal.

[0059] Optionally, the laser further includes a driving module. The driving module is electrically connected to the feedback adjustment module and the pump source respectively. The driving module can receive instructions from the control unit, provide driving current for the pump source according to the instructions, and change the operating temperature of the pump source. The driving current control accuracy of the driving module is at least ±0.5 mA, and the temperature control accuracy of the driving module is at least ±4 mK. Changing the driving current of the pump source will change the frequency and power of the pump beam. The larger the driving current, the lower the frequency and the higher the power of the pump beam; while changing the operating temperature of the pump source will only change the frequency of the pump laser. The higher the operating temperature, the lower the frequency of the pump beam. In addition, the driving module can also drive the phase modulator to adjust the phase, and its phase adjustment accuracy is at least ±0.05 rad.

[0060] Figure 3 is a schematic structural diagram of another laser provided by an embodiment of the present invention. Refer to Figure 3 , the laser further includes an isolator 400. The input end of the isolator 400 is coupled to the output end of the resonator packaging module 200. The isolator 400 is used to make the laser beam transmit unidirectionally. If the beam in the feedback control module 300 is transmitted reversely into the resonator packaging module 200, it may interfere with the generation of the laser beam or cause damage to the optical devices. The isolator 400 can prevent the beam in the feedback control module 300 from being transmitted reversely into the resonator packaging module 200, make the generation of the laser beam more stable, and avoid damage to the optical devices.

[0061] Figure 4 is a schematic structural diagram of a resonator packaging module provided by an embodiment of the present invention. Figure 5 is a schematic internal structural diagram of a resonator packaging module provided by an embodiment of the present invention. Figure 6 is an exploded view of the structure of a resonator packaging module provided by an embodiment of the present invention. Refer to Figure 4 , Figure 5 and Figure 6, optionally, the tapered fiber group 201 includes a first tapered fiber 2011, the first tapered fiber 2011 includes a third end 2013 and a fourth end 2014, the third end 2013 is the input end of the resonant cavity packaging module 200, and the fourth end 2014 is the output end of the resonant cavity packaging module 200. The resonant cavity packaging module 200 further includes a module substrate 203, a module cover plate 204, a phase modulator 205, an optical fiber fixture 206, and a heat conducting base 207. The module substrate 203 and the module cover plate 204 encapsulate the tapered fiber group 201 and the whispering gallery mode resonant cavity 202 inside the resonant cavity packaging module 200 to prevent the tapered fiber group 201 and the whispering gallery mode resonant cavity 202 from being interfered by the outside world. After the first tapered fiber 2011 is encapsulated into the resonant cavity packaging module 200, the end faces of the third end 2013 and the fourth end 2014 of the first tapered fiber 2011 are flush with the surface of the module substrate 203. Optionally, based on the grinding and polishing process, the end faces of the third end 2013 and the fourth end 2014 of the first tapered fiber 2011 form a 90° or 82° angle with the extending direction of the first tapered fiber 2011 for realizing the direct end face coupling between the pump source and the input end of the packaging module. The optical fiber fixture 206 is used to fix the first tapered fiber 2011, and the heat conducting base 207 is in contact with the whispering gallery mode resonant cavity 202. The heat conducting base 207 can absorb the heat of the whispering gallery mode resonant cavity 202 while supporting the whispering gallery mode resonant cavity 202, thereby cooling the whispering gallery mode resonant cavity 202. Optionally, the material of the heat conducting base 207 is copper.

[0062] Reference Figure 4 , Figure 5 and Figure 6 , optionally, the phase modulator 205 includes a temperature controller 2051, and the resonant cavity packaging module 200 further includes an optical fiber fixture 206; the optical fiber fixture 206 is used to clamp the tapered fiber group 201 and serves as a temperature conduction medium between the temperature controller 2051 and the tapered fiber group 201; the temperature controller 2051 is used to change the temperature of the tapered fiber group 201 to change the optical path between the pump source 100 and the whispering gallery mode resonant cavity 202. The temperature controller 2051 can heat the optical fiber fixture 206, and the optical fiber fixture 206 will transfer the heat to the first tapered fiber 2011, causing thermal expansion and thermal refractive index change phenomena in the first tapered fiber 2011. Thermal expansion will cause a change in the size of the first tapered fiber 2011, and thermal refractive index change will cause a change in the refractive index of the first tapered fiber 2011 for the laser of the same wavelength, thereby changing the optical path of the pump beam in the first tapered fiber 2011, and thus changing the optical path between the pump source 100 and the whispering gallery mode resonant cavity 202.

[0063] Figure 7 is an exploded view of the structure of another resonant cavity packaging module provided by an embodiment of the present invention. Reference Figure 7, the phase modulator 205 includes a piezoelectric controller 2052. The piezoelectric controller 2052 is connected to the module substrate 203. The piezoelectric controller 2052 can generate deformation according to the voltage signal applied thereto, thereby driving the entire resonant cavity packaging module 200 to move, thus changing the optical path between the pump source 100 and the whispering gallery mode resonant cavity 202.

[0064] Figure 8 is a schematic diagram of the internal structure of another resonant cavity packaging module provided by an embodiment of the present invention. Refer to Figure 8 , optionally, the tapered fiber group 201 includes a first tapered fiber 2011 and a second tapered fiber 2012 that are arranged in parallel on both sides of the whispering gallery mode resonant cavity 202. Both the first tapered fiber 2011 and the second tapered fiber 2012 are coupled to the whispering gallery mode resonant cavity 202. The first tapered fiber 2011 includes a third end 2013 and a fourth end 2014, and the second tapered fiber 2012 includes a fifth end 2015 and a sixth end 2016. The third end 2013 is the input end of the resonant cavity packaging module, and the fifth end 2015 is the output end of the resonant cavity packaging module. The pump beam will be coupled into the resonant cavity packaging module 200 from the third end 2013, and then coupled into the whispering gallery mode resonant cavity 202 for mode selection. The mode that meets the resonance condition of the whispering gallery mode resonant cavity 202 will be coupled into the second tapered fiber 2012 and output from the fifth end 2015 of the resonant cavity packaging module 200. The beam in the pump beam that is not coupled into the whispering gallery mode resonant cavity 202 will be transmitted along the first tapered fiber 2011 and finally output from the third end 2013. The resonant cavity packaging module 200 enables the mode that meets the resonance condition of the whispering gallery mode resonant cavity 202 and the beam that is not coupled into the whispering gallery mode resonant cavity 202 to be output from two ports respectively, so that the beam noise output from the output end of the resonant cavity packaging module 200 is lower and has a higher signal-to-noise ratio.

[0065] Based on the same inventive concept, an embodiment of the present invention provides a mode locking method, which is applicable to the laser provided by any embodiment of the present invention. Figure 9 is a mode locking method provided by the present invention. Refer to Figure 9 , the mode locking method includes:

[0066] S101. Start the laser and record the output power of the laser at this time.

[0067] Specifically, at this time, the laser is not in the locked state, and the output power of the laser at this time is denoted as A0.

[0068] S102. Control the phase modulator to change the optical path between the pump source and the whispering gallery mode resonator. Each time the optical path is changed, the operating temperature of the pump source is changed from the lower limit of the operating temperature change to the upper limit of the operating temperature change at a constant speed, and the waveform of the power output by the laser as a function of the operating temperature of the pump source is recorded.

[0069] Specifically, control the phase modulator to change the optical path between the pump source and the whispering gallery mode resonator by a first step length ΔL, so that the optical path changes from L i to L f , where L i is the modulation starting point of the phase modulator, and L f is the modulation end point of the phase modulator. The difference between L i and L f is usually greater than the wavelength of the pump light, that is, the phase change corresponding to the difference between L i and L f exceeds 2π, and ΔL is usually less than |L i -L f | / 10. Each time the optical path is changed, the operating temperature of the pump source is increased from T id to T fd , where T id is the lower limit of the operating temperature change of the pump source, and T fd is the upper limit of the operating temperature change of the pump source. Record the waveform A n (T) of the power output by the laser as a function of the operating temperature of the pump source for each optical path, and calculate the difference d n between the minimum power min[A n (T)] in A n and A0, i.e., d n = A0 - min[A Figure 10 is the waveform of the power output by the laser as a function of the operating temperature of the pump source for different optical paths. Refer to Figure 10 , where the phase modulator includes a temperature controller. The number directly above each waveform is the temperature of the temperature controller in °C. The abscissa of each waveform corresponds to the operating temperature, and the ordinate is the power output by the laser. Since the stronger the self-injection locking effect, the narrower the linewidth of the laser beam output by the laser and the lower the power of the laser beam output by the laser, in order to narrow the linewidth of the laser beam, it is necessary to find the optical path between the pump source and the whispering gallery mode resonator and the operating temperature of the pump source corresponding to the strongest self-injection locking effect, that is, the optical path and the operating temperature corresponding to the lowest power of the laser beam, and also the optical path and the operating temperature corresponding to the maximum d n .

[0070] S103. Adjust the optical path to the optical path when the output power of the laser is the lowest, and adjust the operating temperature of the pump source until the output power of the laser is less than the threshold power to achieve mode locking.

[0071] Specifically, adjust the optical path to d n corresponding to the maximum optical path L, where the optical path L is the optical path corresponding to the strongest self-injection locking effect. Then fix the optical path and increase the operating temperature of the pump source from T id in the second step size ΔT d until the output power of the laser is less than the threshold power A th = A0 - η×max(d n ), where 0 < η < 1, and usually η is 0.8. When the output power of the laser is less than the threshold power A th , it indicates that the output power of the laser is close to the lowest value of the laser beam power found in S102, and the self-injection locking effect is strong. The larger η is, the stronger the self-injection locking effect, and the more difficult it is to find the operating temperature of the pump source corresponding to the output power of the laser being less than the threshold power A th .

[0072] When the output power of the laser is less than the threshold power A th , mode locking is achieved.

[0073] Optionally, achieving mode locking includes: the linewidth of the output mode of the laser is less than or equal to the threshold linewidth. The threshold linewidth is a value used to determine whether the linewidth narrowing effect of the output mode of the laser meets the requirements. If the linewidth of the output mode of the laser is less than or equal to the threshold linewidth, it can be considered that the output mode of the laser is locked to the mode with the best linewidth narrowing effect.

[0074] Optionally, if after the operating temperature of the pump source reaches the upper limit of the operating temperature change T fd , the output power of the laser is still greater than or equal to the threshold power, it may be due to one or more incorrect values selected from L i , L f , ΔL, T id , ΔT d and T fd . One or more of the values of L i , L f , ΔL, T id , ΔT d and T fd should be changed, and then S102 and S103 are repeated. If the output power of the laser is still greater than or equal to the threshold power, L i , L f , ΔL, T id , ΔT d and T fdAfter one or more numerical values in it, repeat S102 and S103 until the output power of the laser is less than the threshold power.

[0075] The mode locking method provided by the embodiments of the present invention finds the optical path and operating temperature corresponding to the strongest self-injection locking effect by adjusting the optical path between the pump source and the whispering gallery mode resonator and the operating temperature of the pump source, thereby further narrowing the line width of the laser beam output by the laser.

[0076] After the laser completes mode locking, external interference or the inherent errors of each device may cause the weakening of the self-injection locking effect. Based on this, another mode locking method provided by the embodiments of the present invention Figure 11 is another mode locking method provided by the present invention Figure 11 The mode locking method shown further illustrates how to perform feedback adjustment to keep the output of the laser stable after the laser completes mode locking on the basis of the above embodiments. Refer to Figure 11 , the mode locking method includes:

[0077] S201. Start the laser and record the output power of the laser at this time.

[0078] S202. Control the phase modulator to change the optical path between the pump source and the whispering gallery mode resonator. Each time the optical path is changed, the operating temperature of the pump source is changed from the lower limit of the operating temperature change to the upper limit of the operating temperature change at a constant speed, and record the waveform of the output power of the laser changing with the operating temperature of the pump source under each optical path.

[0079] S203. Adjust the optical path to the optical path when the output power of the laser is the lowest, and adjust the operating temperature of the pump source until the output power of the laser is less than the threshold power to achieve mode locking.

[0080] S204. Record the output power of the laser every Δt. When the output power of the laser is greater than or equal to the threshold power, adjust the phase modulator to change the optical path through the adaptive compensation algorithm until the output power of the laser is less than the threshold power.

[0081] The adaptive compensation algorithm is an algorithm that adaptively adjusts the phase modulator to change the optical path according to the output mode and power of the laser, so that the output power of the laser is less than the threshold power. Specifically, after achieving mode locking, the laser continuously outputs a laser beam, and it is necessary to record the output power A of the laser every Δt i , A iis the output power of the laser before i×Δt. When the power of the laser beam is greater than or equal to the threshold power, it indicates that the self-injection locking effect weakens. It is necessary to adjust the phase modulator to make the power of the laser beam less than the threshold power, that is, to enhance the self-injection locking effect. By adjusting the phase modulator with the third step size ΔL s directionally changes the optical path. Directionally changing the optical path means increasing or decreasing the optical path, and the time interval for changing the optical path is Δt. If the output power of the laser continues to increase, it indicates that the direction of the optical path change is incorrect, and the optical path should be changed in the reverse direction. If the output power of the laser continues to decrease, it indicates that the direction of the optical path change is correct, and the optical path should be changed in the original direction until the output power of the laser is less than the threshold power.

[0082] If when changing the optical path, the change degree of the output power of the laser before Δt is greater than the change degree of the output power of the laser before 2Δt, then reverse the change of the optical path by adjusting the phase modulator until the output power of the laser is less than the threshold power.

[0083] Specifically, the change degree of the output power of the laser before i×Δt can be expressed as ΔA1, and the change degree of the output power of the laser before 2Δt can be expressed as ΔA2, where ΔA1 = A0 - A1 and ΔA2 = A1 - A2. Exemplarily, if ΔA1>0, it indicates that the output power of the laser is rising. If ΔA1<0, it indicates that the output power of the laser is falling. And the change degree of the output power of the laser before Δt being greater than the change degree of the output power of the laser before 2Δt can be expressed as ΔA1>ΔA2.

[0084] It should be noted that if the output power of the laser increases when changing the optical path, there are two possible reasons. One possible reason is that the direction of the optical path change is incorrect, resulting in the output power of the laser increasing with the change of the optical path. At this time, it is necessary to change the direction of the optical path change so that the output power of the laser decreases. Another possible reason is that the direction of the optical path change is correct, and the change of the optical path is making the output power of the laser decrease, but there are interference factors that weaken the self-injection locking effect of the laser, thereby increasing the output power of the laser, and the rate at which the interference factors increase the output power of the laser is greater than the rate at which the change of the optical path decreases the output power of the laser. Thus, the output power of the laser is manifested as increasing.

[0085] If the increase in the output power of the laser is caused by the incorrect direction of the optical path change, then the increase rate of the output power should be getting faster and faster, that is, the change degree of the output power of the laser before Δt is greater than the change degree of the output power of the laser before 2Δt, that is, ΔA1>ΔA2.

[0086] In summary, when ΔA1 > ΔA2, if ΔA1 > 0 and ΔA2 > 0, it indicates that the increasing rate of the output power of the laser is getting faster. If ΔA1 > 0 and ΔA2 < 0, it indicates that the output power of the laser starts to increase before Δt. If ΔA1 < 0 and ΔA2 < 0, it indicates that the decreasing rate of the output power of the laser is getting slower. All these situations indicate that the changing direction of the optical path is incorrect. At this time, the phase modulator should be adjusted to change the optical path in the reverse direction until the output power of the laser is less than the threshold power.

[0087] Optionally, if the change degree of the output power of the laser before Δt is greater than the change degree of the output power of the laser before 2Δt, first compensate for the change in the changing direction of the optical path before Δt, that is, make the optical path return to the size before 2Δt, and then change the optical path in the direction opposite to the changing direction before Δt. The fact that the change degree of the output power of the laser before Δt is greater than the change degree of the output power of the laser before 2Δt indicates that the changing direction of the optical path is incorrect. Making the optical path return to the size before 2Δt is to eliminate the influence of the incorrect changing direction on the optical path. In this case, the change degree of the output power of the laser before i×Δt can be expressed as The change degree of the output power of the laser before 2Δt can be expressed as where ΔA1 = A0 - A1, ΔA2 = A1 - A2, ΔL1 = L0 - L1, ΔL2 = L1 - L2, and L i is the optical path before i×Δt. Adding ΔL1 and ΔL2 to the expression is to eliminate the influence of making the optical path return to the size before 2Δt when changing the changing direction. In this change, the optical path changes by 2ΔL s .

[0088] If, when changing the optical path, the change degree of the output power of the laser before Δt is less than or equal to the change degree of the output power of the laser before 2Δt, continue to change the optical path in the original direction by adjusting the phase modulator until the output power of the laser is less than the threshold power.

[0089] The degree of change in the output power of the laser before Δt is less than the degree of change in the output power of the laser before 2Δt, which can be expressed as ΔA1 < ΔA2. In the case of ΔA1 < ΔA2, if ΔA1 > 0 and ΔA2 > 0, it indicates that the direction of the change in the optical path is correct, and the change in the optical path is reducing the output power of the laser. However, there are interference factors that weaken the self-injection locking effect of the laser, thereby increasing the output power of the laser. Moreover, the rate at which the interference factors increase the output power of the laser is greater than the rate at which changing the optical path reduces the output power of the laser. Thus, the output power of the laser is manifested as an increase. If ΔA1 < 0 and ΔA2 > 0, it indicates that the output power of the laser became decreasing before Δt. If ΔA1 < 0 and ΔA2 < 0, it indicates that the rate of decrease in the output power of the laser is getting faster and faster. All these situations indicate that the direction of the change in the optical path is correct. At this time, the phase modulator should be adjusted to continue changing the optical path in the original direction until the output power of the laser is less than the threshold power.

[0090] The above mode locking method can be executed by a control unit. The mode locking method provided by the embodiments of the present invention reduces the perturbation of environmental noise to the laser through feedback adjustment, ensures the long-term stable operation of the laser, and solves the problem that the optical fiber is vulnerable to environmental interference.

[0091] Figure 12 It is a flowchart of the mode searching part in the mode locking method provided by the embodiments of the present invention. Figure 13 It is a flowchart of the stable mode locking part in the mode locking method provided by the embodiments of the present invention. Refer to Figure 12 and Figure 13 The phase modulator includes a temperature controller. By adjusting the temperature of the phase modulator, the optical path between the pump source and the whispering gallery mode resonator can be changed. Figure 12 The shown process is to first start the laser, and then adjust the temperature of the phase modulator and the operating temperature of the pump source to make the self-injection locking effect the strongest. Figure 13 The shown process is to judge the correct change direction of the phase modulator and adjust the phase modulator to make the output power of the laser beam less than the threshold power.

[0092] The above specific embodiments do not constitute a limitation to the protection scope of the present invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A laser based on a discrete whispering gallery microcavity, characterized in that: It includes a pump source, a resonant cavity packaging module and a feedback control module; The pump source is used to output a pump light beam; The input end of the resonant cavity packaging module is coupled to the output end of the pump source; The resonant cavity packaging module comprises a tapered optical fiber group, a phase modulator and a whispering gallery mode resonant cavity packaged in a housing, wherein the phase modulator is used to adjust the optical path between the pump source and the whispering gallery mode resonant cavity; The tapered fiber group is coupled to the whispering gallery mode resonant cavity at a fixed distance, the tapered fiber group is used to transmit a pump beam, and the tapered fiber group includes a first end and a second end, the first end is the input end of the resonant cavity packaging module, and the second end is the output end of the resonant cavity packaging module; After the pump beam is coupled to the whispering gallery mode resonant cavity through the tapered fiber group, the whispering gallery mode resonant cavity is used to return part of the pump beam to the pump source to generate a self-injection locking effect; The feedback control module is electrically connected to the pump source and the phase modulator, respectively, and is used to adjust the optical path between the pump source and the resonant cavity packaging module, the driving current of the pump source, and the operating temperature of the pump source according to the laser power output by the resonant cavity packaging module, so as to lock the laser frequency on the resonant mode of the whispering gallery mode resonant cavity.

2. The laser according to claim 1, characterized in that The feedback control module includes a beam splitter, a detector and a control unit; The input end of the beam splitter is coupled to the output end of the resonant cavity packaging module, the beam splitter includes a first output end and a second output end, the beam splitter is used to split the laser beam output by the resonant cavity packaging module into a first signal light and a second signal light, the first output end of the beam splitter is used to output the first signal light, and the second output end of the beam splitter is used to output the second signal light; The detector is coupled to the second output end of the beam splitter, the detector is in communication with the control unit, the detector is used to receive the second signal light, and the control unit determines the power of the second signal light according to the signal obtained by the detector; The control unit is electrically connected to the pump source and the phase modulator respectively, and is used to adjust the optical path between the pump source and the resonant cavity packaging module, the driving current of the pump source and the operating temperature of the pump source according to the power, so that the resonant cavity packaging module outputs a laser beam of a preset mode.

3. The laser according to claim 1, characterized in that An isolator is also included, the input end of the isolator is coupled to the output end of the resonant cavity packaging module, and the isolator is used to make the laser beam transmit unidirectionally.

4. The laser according to claim 1, characterized in that The tapered fiber group includes a first tapered fiber, and the first tapered fiber includes a third end and a fourth end, the third end is the input end of the resonant cavity packaging module, and the fourth end is the output end of the resonant cavity packaging module.

5. The laser according to claim 1, characterized in that The tapered fiber group includes a first tapered fiber and a second tapered fiber which are arranged in parallel on both sides of the whispering gallery mode resonant cavity. The first tapered fiber and the second tapered fiber are both coupled to the whispering gallery mode resonant cavity. The first tapered fiber includes a third end and a fourth end, and the second tapered fiber includes a fifth end and a sixth end. The third end is the input end of the resonant cavity packaging module, and the fifth end is the output end of the resonant cavity packaging module.

6. The laser according to claim 1, characterized in that The phase modulator includes a temperature controller, and the resonant cavity packaging module also includes an optical fiber clamp; The optical fiber clamp is used to clamp the tapered optical fiber group and serve as a temperature conduction medium between the temperature controller and the tapered optical fiber group; The temperature controller is used to change the temperature of the tapered optical fiber group through the optical fiber clamp to change the optical path between the pump source and the whispering gallery mode resonant cavity.

7. A mode locking method, characterized in that: Applicable to the laser according to any one of claims 1 to 6, the mode locking method comprises: Starting the laser and recording the output power of the laser at this time; Controlling the phase modulator to change the optical path between the pump source and the whispering gallery mode resonant cavity, each time the optical path is changed, the operating temperature of the pump source is changed from the lower limit of the operating temperature change to the upper limit of the operating temperature change at a constant speed, and recording the waveform of the power output of the laser at each optical path as the operating temperature of the pump source changes; The optical path is adjusted to the optical path when the output power of the laser is the lowest, and the operating temperature of the pump source is adjusted until the output power of the laser is less than the threshold power, so as to achieve mode locking.

8. The mode locking method according to claim 7, characterized in that: Until the output power of the laser is less than the threshold power, then the method further comprises: If the output power of the laser is still greater than or equal to the threshold power after the operating temperature of the pump source reaches the upper limit of the operating temperature change, the mode locking method is repeatedly executed after changing the adjustment range of the optical path and the operating temperature of the pump source.

9. The mode locking method according to claim 7, characterized in that: Until the output power of the laser is less than the threshold power, then the method further comprises: The output power of the laser is recorded every Δt. When the output power of the laser is greater than or equal to the threshold power, the phase modulator is adjusted to change the optical path through an adaptive compensation algorithm until the output power of the laser is less than the threshold power.

10. The mode locking method according to claim 7, characterized in that: Implement mode locking, including: The line width of the output mode of the laser is less than or equal to the threshold line width.

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