An ultra-narrow linewidth, flexible repetition rate adjustable and wideband electro-optical comb generation system and method

Abstract

The application relates to the technical field of electro-optical frequency comb generation, and discloses a super-narrow-line-width, wide-band, flexible-repetition-frequency and adjustable electro-optical comb generation system and method, which comprises a pump laser generation branch, a fiber ring cavity, and a frequency shift injection locking branch.The pump laser generation branch comprises a distributed feedback laser, a first polarization controller and a first circulator; the fiber ring cavity comprises a second circulator, a Brillouin gain fiber, a fiber delay line, a fiber coupler, a second polarization controller, an electro-optical modulator, a dispersion compensation fiber, an optical amplifier, a flat filter and an isolator; and the frequency shift injection locking branch comprises a narrow-bandwidth filter, a polarization controller and a frequency shifter.The electro-optical frequency comb generation device of the application has a novel structure, realizes super-narrow-line-width, wide-band, flexible-repetition-frequency and adjustable electro-optical comb output, solves the limitations of narrow bandwidth and wide comb line width of a traditional electro-optical frequency comb generated based on electro-optical modulation, and does not need complex active optoelectronic feedback to maintain stable operation of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electro-optical frequency comb generation, and particularly relates to a system and method for generating an ultra-narrow linewidth, flexible repetition frequency and wideband electro-optical comb. BACKGROUND

[0002] Optical frequency combs are laser light that is discretely distributed in the frequency spectrum, uniformly spaced and has a coherent stable phase relationship, providing a time and frequency reference bridge between the optical domain and the microwave domain, and has a wide application prospect in the fields of optical atomic clocks, atmospheric science, precision measurement, etc. The characteristics of the comb spectrum required by each application may be quite different, which leads to the diversification of OFCG technology capable of generating various comb spectrum outputs. For example, when used as a stable light-based heterodyne source, comb bandwidth is a key consideration factor. For precision spectroscopy applications, the ability to dynamically adjust the repetition frequency opens up the way for its adaptability. When applied to coherent optical communication systems, the flexibility of simultaneously evaluating the comb linewidth, bandwidth and repetition frequency is required to meet the high coherence requirement, improve transmission efficiency and achieve wavelength phase synchronization. As can be seen, how to generate an optical frequency comb that simultaneously considers ultra-narrow linewidth, large bandwidth, flexible repetition frequency tuning and other characteristics has become a focus problem in the field of optical communication technology, and is expected to open up a new field of scientific exploration and technological innovation.

[0003] At present, there are three methods for generating optical frequency combs, which are based on mode-locked lasers, micro resonators and electro-optical modulation. The method based on mode-locked lasers can generate multi-carrier signals across a wide frequency band, but since the cavity length of the laser determines the spacing of the comb lines, it does not have the flexibility of repetition frequency and the repetition frequency is usually below GHz level. The optical frequency comb generated based on the micro resonator is concerned due to its ultra-high repetition frequency and large bandwidth characteristics, but it has poor spectral flatness, does not have the flexibility of repetition frequency, and the comb line width is limited by the pump laser. The optical frequency comb generated based on the electro-optical modulator has a simple structure and the flexibility of repetition frequency, but the output spectrum comb line width is also limited by the pump laser line width, and the bandwidth is usually narrow. Generally speaking, a single electro-optical modulator is not enough to generate a relatively wide electro-optical frequency comb at a high repetition frequency. Because if a too high radio frequency voltage is provided, the electro-optical modulator may be damaged. Researchers use cascaded and nested multiple modulators to perform spectral broadening. The limitation of this method is that the insertion loss of the modulator is large (especially in the cascaded structure), and multiple phase shifters need to be used simultaneously to modulate the phase of the modulator, which is complex to operate and high in cost.

[0004] At present, the typical device for generating a broadband electro-optical frequency comb is to embed an electro-optical modulator into a fiber cavity. In fact, the comb teeth coherence of these devices is highly dependent on the pump laser, and cannot achieve an ultra-narrow linewidth optical frequency comb output, and needs a complex active optoelectronic feedback to maintain the stable operation of the system. At present, although various methods have been proposed to achieve the spectral broadening of the electro-optical frequency comb with the help of the electro-optical modulator, but in summary, these methods all need a complex active optoelectronic feedback to maintain the stable operation of the system, and the output comb teeth linewidth is limited by the pump laser linewidth, and cannot achieve an ultra-narrow linewidth electro-optical frequency comb to meet the coherence requirement in high-speed coherent optical communication applications. In summary, it is still challenging to generate an electro-optical frequency comb with the characteristics of ultra-narrow linewidth, large bandwidth, and flexible adjustable repetition frequency. SUMMARY

[0005] This section is intended to summarize some aspects of the embodiments of the present application and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of the specification of the present application to avoid obscuring the purpose of this section, the abstract and the title, and such simplifications or omissions cannot be used to limit the scope of the present application.

[0006] In view of the above problems existing in the existing electro-optical frequency comb generation system, the present application is proposed.

[0007] Therefore, an object of the present application is to provide an ultra-narrow linewidth, flexible adjustable repetition frequency and broadband electro-optical comb generation system to achieve an ultra-narrow linewidth, large bandwidth, flexible adjustable repetition frequency electro-optical frequency comb output, which is beneficial to the development of high-speed coherent optical communication, precision spectroscopy and other fields.

[0008] To solve the above technical problems, the present application provides the following technical scheme: an ultra-narrow linewidth, flexible adjustable repetition frequency and broadband electro-optical comb generation system, which comprises a pump laser generation branch, a fiber ring cavity and a frequency shift injection locking branch, wherein the pump laser generation branch comprises a distributed feedback laser, a first polarization controller and a first circulator connected in sequence; the fiber ring cavity comprises a second circulator connected with the first circulator, a Brillouin gain fiber, a fiber delay line connected with the second circulator respectively, and a fiber coupler, a second polarization controller, an electro-optical modulator, a dispersion compensation fiber, an optical amplifier, a flat filter and an isolator connected between the Brillouin gain fiber and the fiber delay line; the frequency shift injection locking branch comprises a narrow bandwidth filter, a polarization controller and a frequency shifter connected in sequence; the narrow bandwidth filter is connected with an output end of the fiber coupler, and the frequency shifter is connected with the first circulator.

[0009] As a preferred scheme of the super-narrow linewidth and wideband electro-optical comb generation system, the first and second circulators each include three ports, which are distinguished as a port, a b port and a c port, and the a port to the b port, the b port to the c port and the c port to the a port are unidirectional.

[0010] As a preferred scheme of the super-narrow linewidth and wideband electro-optical comb generation system, the electro-optical modulator is matched with a radio frequency signal source, and different frequency microwave signals are applied to the electro-optical modulator by the radio frequency signal source.

[0011] As a preferred scheme of the super-narrow linewidth and wideband electro-optical comb generation system, a second coupler is further included, which is connected between the fiber coupler and the second polarization controller.

[0012] Another object of the present application is to provide a super-narrow linewidth and wideband electro-optical comb generation method, which realizes the output of the super-narrow linewidth, wideband, and wideband frequency flexible adjustable electro-optical frequency comb by the super-narrow linewidth and wideband electro-optical comb generation system.

[0013] To solve the above technical problems, the present application provides the following technical scheme: a super-narrow linewidth and wideband electro-optical comb generation method, characterized in that the super-narrow linewidth and wideband electro-optical comb generation system is used, and the method includes the following steps:

[0014] S1: an initial laser is output by a pump laser generation branch, and the initial laser enters a fiber ring cavity, and a first-order Brillouin laser is generated by a Brillouin gain fiber;

[0015] S2: the first-order Brillouin laser is divided into one-way laser and two-way laser by a fiber coupler;

[0016] S3: the one-way laser enters a frequency shift injection locking branch, and enters a distributed feedback laser through a first circulator, so as to lock the wavelength of the distributed feedback laser and transfer the narrow linewidth characteristic;

[0017] S4: the two-way laser enters the fiber ring cavity, and generates sidebands under the modulation of an electro-optical modulator, the sidebands circulate in the fiber ring cavity, and the spectrum is expanded;

[0018] S5: after circulating for several times, the sidebands are adjusted to correspond to the cavity longitudinal membrane position;

[0019] S6: Adjusting the radio frequency and radio power of the radio frequency signal source, and outputting the electrical-optical frequency comb with the target characteristics.

[0020] As a preferred scheme of the method for generating the ultra-narrow linewidth and flexible repetition rate and wideband electro-optical comb, wherein: the initial laser is output by the pump laser, and the initial laser enters the fiber ring cavity, and the first-order Brillouin laser is generated by the Brillouin gain fiber; comprising,

[0021] The initial laser emitted by the distributed feedback laser enters the first circulator after the first polarization controller, and then enters the Brillouin gain fiber from the second circulator, and the stimulated Brillouin scattering effect occurs in the Brillouin gain fiber, and the first-order Brillouin laser is generated in the opposite direction of the initial laser.

[0022] As a preferred scheme of the method for generating the ultra-narrow linewidth and flexible repetition rate and wideband electro-optical comb, wherein: the one-way laser enters the frequency shift injection locking branch, and enters the distributed feedback laser through the first circulator, which is used to lock the wavelength of the distributed feedback laser and transfer the narrow linewidth characteristics; comprising,

[0023] S31: The one-way laser enters the distributed feedback laser through the first circulator and the first polarization controller after the narrow bandwidth filter, the polarization controller and the frequency shifter, and the narrow linewidth characteristics of the Brillouin light are transferred to the pump, so that the linewidth of the secondary laser output by the distributed feedback laser is narrowed; and the relative position between the initial pump light and the cavity longitudinal mode is maintained, and the wavelength of the output secondary laser is locked;

[0024] S32: After the secondary laser is output, it enters the distributed feedback laser again through the Brillouin gain fiber and the frequency shift injection locking branch, so that the linewidth of the laser output by the distributed feedback laser is narrowed again;

[0025] S33: After a plurality of groups of circulation, the laser with the locked wavelength and the narrowed linewidth to the limit relative to the cavity longitudinal mode is output by the distributed feedback laser.

[0026] As a preferred scheme of the method for generating the ultra-narrow linewidth and flexible repetition rate and wideband electro-optical comb, wherein: the two-way laser enters the fiber ring cavity, and the sideband is generated under the modulation of the electro-optical modulator; comprising the following steps:

[0027] S41: Turn on the radio frequency signal source of the electro-optical modulator, and keep the radio frequency input;

[0028] S42: the two-way laser passes through the second polarization controller, the electro-optic modulator, the dispersion compensation fiber, the optical amplifier, the flat filter and the isolator, and then passes through the Brillouin gain fiber, the optical fiber delay line to the optical fiber coupler;

[0029] S43: the two-way laser passes through the electro-optic modulator to generate sidebands;

[0030] S44: the generated sidebands are amplified by the optical amplifier, a part of which is output through the optical fiber coupler, and the remaining part passes through the electro-optic modulator again, and the sidebands generate sub-sidebands again;

[0031] S45: continue to circulate and expand the spectrum.

[0032] As a preferred scheme of the method for generating the ultra-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb according to the application, the adjustment is to correspond the sidebands to the cavity longitudinal mode positions, wherein the sidebands are adjusted by changing the radio frequency of the radio frequency signal source, and the cavity longitudinal mode positions are adjusted by changing the length of the fiber ring cavity.

[0033] As a preferred scheme of the method for generating the ultra-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb according to the application, the radio frequency of the radio frequency signal source is adjusted to be an integer multiple of the cavity longitudinal mode spacing of the fiber ring cavity.

[0034] The application has the following beneficial effects:

[0035] The electro-optical frequency comb generation system according to the application has a novel structure, realizes the output of the ultra-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb, solves the limitations of the narrow bandwidth and large comb tooth linewidth of the traditional electro-optical frequency comb generated based on electro-optical modulation, does not need complex active optoelectronic feedback to maintain stable operation of the system, can realize flexible tuning of the repetition frequency, and can realize fine tuning with the cavity longitudinal mode spacing as the tuning step (MHz level) and wide-range tuning with the integer multiple of the cavity longitudinal mode spacing as the tuning step (GHz level). BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without any creative labor. Among them:

[0037] Figure 1 It is a schematic diagram of the overall structure of the ultra-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb generation system according to the application.

[0038] Figure 2The specific connection structure diagram of the super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb generation system of the application.

[0039] Figure 3 The laser direction diagram of the super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb generation system of the application.

[0040] Figure 4 The structure diagram of the super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb generation system of the application.

[0041] Figure 5 The super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb spectrum diagram formed by the Brillouin laser cavity in the embodiment.

[0042] Figure 6 The frequency noise data diagram of the super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb formed by the Brillouin laser cavity in the embodiment.

[0043] Figure 7 The super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb repetition frequency tuning data in the embodiment Figure 1 .

[0044] Figure 8 The super-narrow linewidth and flexible adjustable repetition frequency and wideband electro-optical comb repetition frequency tuning data in the embodiment Figure 2 . DETAILED DESCRIPTION

[0045] In order to make the above objectives, features and advantages of the application more apparent, the specific embodiments of the application will be described in detail below with reference to the accompanying drawings.

[0046] In the following description, a lot of specific details are set forth in order to provide a thorough understanding of the application, but the application can also be practiced by other different ways than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the application, so the application is not limited to the specific embodiments disclosed below.

[0047] Secondly, the "one embodiment" or "embodiment" referred to herein means that the specific features, structures or characteristics can be included in at least one implementation of the application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an independent or alternative embodiment that excludes other embodiments.

[0048] Thirdly, the application is described in detail in conjunction with the schematic diagram, and in the detailed description of the embodiments of the application, the cross-sectional view of the device structure is partially enlarged without general proportion for the convenience of description, and the schematic diagram is only an example, which should not limit the scope of protection of the application here. In addition, three-dimensional spatial dimensions including length, width and depth should be included in actual manufacture.

[0049] Embodiment 1

[0050] With reference to Figures 1-4 For the first embodiment of the present application, a system for generating an ultra-narrow linewidth and flexible repetition rate and wideband electro-optical comb is provided, which is summarized as three parts, namely a pump laser generating branch 100, a fiber ring cavity 200 and a frequency shift injection locking branch 300; wherein the pump laser generating branch 100 is used for laser generation and output, the fiber ring cavity 200 is an extended loop with a Brillouin cavity, which is used for generating a large bandwidth and flexible repetition rate of the electro-optical frequency comb; and the frequency shift injection locking branch 300 is used for locking the relative position of the laser output wavelength and the cavity longitudinal mode and narrowing the linewidth.

[0051] Specifically, the pump laser generating branch 100 includes a distributed feedback laser 101, a first polarization controller 102 and a first circulator 103 connected in sequence; wherein the distributed feedback laser 101 is a fixed wavelength or tunable single-frequency laser; and the first polarization controller 102 is used for controlling the polarization state of the pump laser of the distributed feedback laser 101 to maximize the Brillouin gain of the Brillouin gain fiber 202.

[0052] The fiber ring cavity 200 includes a second circulator 201 connected with the first circulator 103, and a Brillouin gain fiber 202 and a fiber delay line 203 connected with the second circulator 201 respectively, and an optical coupler 204, a second polarization controller 205, an electro-optical modulator 206, a dispersion compensation fiber 207, an optical amplifier 208, a flat filter 209 and an isolator 210 connected in sequence between the Brillouin gain fiber 202 and the fiber delay line 203.

[0053] Further, the Brillouin gain fiber 202 acts as a gain medium of stimulated Brillouin scattering effect to generate a counter-propagating stimulated Brillouin scattering laser in the fiber ring cavity 200; the length of the fiber delay line 203 can be adjusted, which can be used for length adjustment of the fiber ring cavity 200, thereby fine-tuning the FSR (fiber cavity longitudinal mode spacing) of the fiber ring cavity 200 and reducing the requirement for the frequency step of the radio frequency signal source 206a; the optical coupler 204 is used for splitting a beam of laser into two beams; the second polarization controller 205 is used for controlling the polarization in the Brillouin laser cavity, thereby controlling the output power thereof; the electro-optical modulator 206 is used for modulating the Brillouin light to generate sidebands; the dispersion compensation fiber 207 is used for optimizing the dispersion of the fiber cavity; the optical amplifier 208 is used for compensating the intra-cavity loss of the fiber ring cavity 200, thereby reducing the threshold of the stimulated Brillouin laser; the flat filter 209 is used for filtering to keep the required bandwidth of the spectrum reserved in the ring cavity; and the isolator 210 is used for isolating the counter-propagating second-order Brillouin laser generated after the first-order Brillouin laser passes through the Brillouin gain fiber 202 again.

[0054] The electro-optical modulator 206 is matched with a radio frequency signal source 206a, and a microwave signal of different frequency is applied to the electro-optical modulator 206 by the radio frequency signal source 206a; the radio frequency of the radio frequency signal source 206a is an integer multiple of the longitudinal membrane interval of the fiber ring cavity 200.

[0055] The first circulator 103 and the second circulator 201 each include three ports, which are distinguished as a port, a b port and a c port, and the a port to the b port, the b port to the c port and the c port to the a port are unidirectional conduction. It should be noted that each port of the circulator has input and output functions.

[0056] The frequency shift injection locking branch 300 includes a narrow bandwidth filter 301, a polarization controller 302 and a frequency shifter 303 connected in sequence; the narrow bandwidth filter 301 is connected with an output end of the fiber coupler 204, and the frequency shifter 303 is connected with the first circulator 103.

[0057] The radio frequency of the frequency shifter 303 is the frequency corresponding to the maximum stimulated Brillouin scattering gain.

[0058] The narrow bandwidth filter 301 is used for filtering and retaining the laser of the target width range; the polarization controller 302 is used for maintaining the polarization state of the Brillouin light of the frequency shift injection locking; and the frequency shifter 303 is used for frequency shifting the Brillouin light so as to keep the same frequency as the original initial laser. It should be noted that the radio frequency of the frequency shifter 303 is the frequency corresponding to the maximum stimulated Brillouin scattering gain, and due to the injection locking effect, the signal light is locked to the vicinity of the maximum gain of the Brillouin gain region, and the locked distributed feedback laser 101 generates the Brillouin laser with compressed line width again in the Brillouin laser cavity as a new light source, and the cycle is continuously performed until the line width is compressed to the limit.

[0059] Embodiment 2

[0060] Reference Figure 1 For the second embodiment of the application, which is different from the first embodiment, in the pump laser generating branch 100, the distributed feedback laser 101, the first polarization controller 102 and the first circulator 103 are connected in sequence, and a second frequency shifter Y is connected in the b port line of the first circulator 103, or an optical amplifier can be additionally installed; and in the frequency shift injection locking branch 300, the frequency shifter 303 can be removed correspondingly, and the same technical effect as the scheme in the original embodiment 1 can be achieved; the difference lies in that in the embodiment 1, the Brillouin light with lag is frequency shifted to the same frequency phase as the initial laser; while in the present embodiment, the initial laser is frequency shifted before forming the Brillouin light, and is frequency shifted back to the frequency phase position of the initial laser in the subsequent process.

[0061] The second coupler 211 is connected between the fiber coupler 204 and the second polarization controller 205, and is used for output of the final electro-optical frequency comb.

[0062] The rest of the structure is the same as that of Example 1; here, no longer detailed.

[0063] Example 3

[0064] Reference Figures 1-4 For a third embodiment of the present application, a method for generating an ultra-narrow linewidth and flexible repetition rate and wideband electro-optical comb is provided, which uses the ultra-narrow linewidth and flexible repetition rate and wideband electro-optical comb generation system of Example 1, and includes the following steps:

[0065] S1: output initial laser from the pump laser generation branch 100, and the initial laser enters the fiber ring cavity 200, and generates first-order Brillouin laser through the Brillouin gain fiber 202;

[0066] Specifically, the initial laser emitted by the distributed feedback laser 101 enters the first circulator 103 from the a port of the first circulator 103 after the first polarization controller 102, and is output from the b port of the first circulator 103, and then enters the Brillouin gain fiber 202 from the b port of the second circulator 201. The initial laser undergoes stimulated Brillouin scattering effect in the Brillouin gain fiber 202, and generates first-order Brillouin laser in the opposite direction of the initial laser propagation direction. It should be noted that this is the first-order Brillouin laser formed by part of the initial laser passing through the Brillouin gain fiber 202, while part of the initial laser passing through the Brillouin gain fiber 202 is interrupted under the blocking action of the isolator 210; and the isolator 210 is also used to isolate the reverse second-order Brillouin laser generated by the first-order Brillouin laser after passing through the Brillouin gain fiber 202 again.

[0067] S2: the first-order Brillouin laser is divided into one-way laser and two-way laser through the fiber coupler 204; the one-way laser and the two-way laser are both the same first-order Brillouin laser, and are generated at the same time and in the fiber ring cavity 200 and the frequency shift injection locking branch 300, respectively.

[0068] S3: the one-way laser enters the frequency shift injection locking branch 300, and enters the distributed feedback laser 101 through the first circulator 103, and is used to lock the wavelength of the distributed feedback laser 101 and transfer the narrow linewidth characteristic; wherein,

[0069] S31: one laser light sequentially through the narrow bandwidth filter 301, polarization controller 302 and frequency shifter 303, and then by the first circulator 103 and the first polarization controller 102 into the distributed feedback laser 101, the narrow linewidth characteristics of Brillouin light is transmitted to the pump, so that the linewidth of the secondary laser output by the distributed feedback laser 101 is narrowed; and the relative position between the initial pump light and the cavity longitudinal mode is maintained, and the wavelength of the output secondary laser is locked;

[0070] S32: after the secondary laser output, the laser light again passes through the Brillouin gain fiber 202 and the frequency shift injection locking branch 300 and then enters the distributed feedback laser 101, so that the linewidth of the laser output by the distributed feedback laser 101 is narrowed again;

[0071] S33: after several cycles, the distributed feedback laser 101 can output a laser whose wavelength and linewidth are locked to the limit.

[0072] It should be noted that when one laser light enters the distributed feedback laser 101, the narrow linewidth characteristics of Brillouin laser are transmitted to the pump, and the pump light with narrow linewidth characteristics is output; the pump light with new linewidth repeats the optical path of the initial laser for circulation, that is, it continuously enters the distributed feedback laser 101 through the frequency shift injection locking branch 300, continuously narrows the linewidth of the laser output by the laser, and the linewidth is compressed to the limit.

[0073] Further, while one laser light locks the wavelength and narrows the linewidth of the distributed feedback laser 101 by the frequency shift injection locking branch 300; the second laser light also circulates in the fiber ring cavity 200, but the electro-optic modulator 206 in the fiber ring cavity 200 is not turned on; thus, in the circulation process, the second laser light also separates a part of the laser light into the frequency shift injection locking branch 300 under the action of the fiber coupler 204.

[0074] And then the modulation step of the large bandwidth electro-optic frequency comb is performed.

[0075] S4: the second laser light enters the fiber ring cavity 200 and generates sidebands under the modulation of the electro-optic modulator 206, the sidebands circulate in the fiber ring cavity 200 to expand the spectrum; specifically,

[0076] S41: turn on the radio frequency signal source 206a of the electro-optic modulator 206 and keep the radio frequency input;

[0077] S42: the second laser light sequentially passes through the second polarization controller 205, the electro-optic modulator 206, the dispersion compensation fiber 207, the optical amplifier 208, the flat filter 209 and the isolator 210, and then passes through the Brillouin gain fiber 202, the fiber delay line 203 to the fiber coupler 204;

[0078] S43: After the two-way laser passes through the electro-optical modulator 206, sidebands are generated;

[0079] S44: After the generated sidebands are amplified flatly by the optical amplifier 208, part of them is output through the optical fiber coupler 204, and the remaining part passes through the electro-optical modulator 206 again, and the sidebands generate sub-sidebands again;

[0080] S45: Continue to circulate and expand to form a spectrum.

[0081] It should be noted that the two-way laser is a narrow linewidth laser after being compressed by the frequency shift injection locking branch 300 multiple times after circulation. The sidebands are generated under the modulation of the electro-optical modulator 206 in the optical fiber ring cavity 200, and the frequency interval between the sidebands is the radio frequency input to the electro-optical modulator 206.

[0082] S5: After circulating for several times, adjust the sidebands to correspond to the cavity longitudinal mode position;

[0083] Specifically, since the sidebands generated by the electro-optical modulator 206 are coherent and phase-locked, in order to output more coherent and phase-locked sub-sidebands in the optical fiber ring cavity 200, it is necessary to adjust the length of the optical fiber cavity loop or the radio frequency signal to keep the sidebands on the cavity longitudinal mode. The cavity longitudinal mode position is adjusted by adjusting the length of the optical fiber delay line 203 in the cavity (i.e. changing the length of the optical fiber cavity), and the sidebands are adjusted by changing the radio frequency of the radio frequency signal source 206a. It should be noted that among the two, the radio frequency signal source 206a is mainly adjusted, but the step of adjusting the radio frequency signal source 206a is limited, and the optical fiber delay line 203 is used to assist adjustment, so as to achieve the purpose of making the sidebands correspond to the cavity longitudinal mode position.

[0084] S6: Adjust the radio frequency and radio frequency power of the radio frequency signal source 206a to output an electro-optical frequency comb with target characteristics.

[0085] Specifically, the repetition frequency of the sidebands generated by the Brillouin light after being modulated by the electro-optical modulator 206 is determined by the signal frequency of the radio frequency signal source 206a. Therefore, by adjusting the radio frequency of the radio frequency signal source 206a, the radio frequency is an integer multiple of the cavity longitudinal mode interval of the optical fiber ring cavity 200, that is, the radio frequency is N×FSR, which can realize fine tuning and coarse tuning of the repetition frequency of the electro-optical frequency comb. The smallest can realize fine tuning with the cavity longitudinal mode interval as the tuning step (MHz level), and can also realize wide-range tuning with an integer multiple of the cavity longitudinal interval as the tuning step (GHz level).

[0086] Embodiment 4

[0087] Reference Figures 5-8 Based on the system of Embodiment 1 and the method in Embodiment 3, the following examples are performed:

[0088] The initial pump light is set to 1550.1 nm, and the input RF power of the electro-optical modulator 206 in the fiber ring cavity 200 is set to 18.5 GHz.

[0089] After the system is set up according to the embodiment 1, the distributed feedback laser 101 is turned on, and the initial laser emitted thereby enters the fiber ring cavity 200 through the first circulator 103 and the second circulator 201, and a counter-propagating Brillouin laser is generated in the ring cavity, and then the Brillouin laser is output through the fiber coupler 204. Part of the output light enters the frequency shifter 303, and the RF frequency is set to 9.383 GHz. The narrow-band filter 301 is adjusted, and the side mode suppression ratio reaches 20 dB. The light is returned to the distributed feedback laser 101 through the first circulator 103, and the frequency-shifted light injection locking is completed.

[0090] The loss of the fiber ring cavity 200 is greatly reduced due to the addition of the optical amplifier 208 in the cavity, and the threshold of the Brillouin laser is reduced. The RF input of the electro-optical modulator 206 in the fiber cavity is turned on, and the RF input frequency should be set to an integer multiple of the fiber cavity free spectral range (FSR). The length of the ring cavity is adjusted through the fiber delay line 203 in the cavity, and then the FSR of the fiber ring cavity 200 is fine-tuned, and the requirement for the frequency step of the RF signal source 206a is reduced. The counter-propagating Brillouin laser in the cavity generates sidebands through the electro-optical modulator 206, and the frequency interval between the sidebands is the RF frequency input to the electro-optical modulator 206. The generated sidebands are flatly amplified through the optical amplifier 208 in the cavity, and then pass through the electro-optical modulator 206 again, and the sidebands generate sub-sidebands again, and the process is repeated to continuously expand the spectrum.

[0091] Figure 5 As can be seen in the figure, two different color spectrum diagrams are obtained, and the orange spectrum corresponds to the case where the Brillouin light does not pass through the electro-optical modulator 206 to generate sidebands, and the blue spectrum corresponds to the spectrum obtained after the cavity circulation is continuously expanded. It can be seen that the spectrum without cavity circulation has a relatively narrow bandwidth; specifically, the spectrum width reaches 2.25 THz, and the total comb tooth number reaches 126. At this time, the width of the spectrum is just the bandwidth of the flat filter 209 in the cavity.

[0092] The frequency noise of the comb tooth of the electro-optical frequency comb is measured by using the delay self-heterodyne method. The frequency noise of the comb tooth is as shown in the figure. Figure 6As shown, when the offset frequency is greater than 10 kHz, it can be found that the frequency noise of the comb teeth is obviously raised at high frequencies as the comb teeth are far away from the center, which can be attributed to the relaxation noise. Because the comb teeth are generated by the electro-optical modulator 206 symmetrically expanding around the Brillouin laser frequency, the frequency noise of the comb teeth symmetric to the center wavelength is equal. When the offset frequency is between 10 kHz and 100 kHz, the frequency noise corresponding to different order comb teeth is about 0.015-0.3, which corresponds to the intrinsic linewidth of the electro-optical frequency comb teeth being about 1 Hz.

[0093] Figure 7 and Figure 8 The data diagram for the super-narrow linewidth and large bandwidth electro-optical frequency comb with flexible adjustable repetition frequency represents the frequency interval of different optical frequency comb teeth, and the parameters such as the center wavelength of the distributed feedback laser 101 are consistent with Figure 5 The signal frequency of the radio frequency signal source 206a determines the frequency interval of the generated optical frequency comb teeth, and the frequency interval changes with the microwave frequency applied on the modulator. By changing the frequency interval of N times FSR, the repetition frequency can be finely and coarsely adjusted, so as to realize the output of optical frequency comb with different repetition frequencies of 12.5-18.5 GHz.

[0094] In summary, from the above content, it can be seen that based on the system and method, the bandwidth of the generated flat comb source is 2.25 THz, the intrinsic linewidth of the comb teeth reaches the Hz level, the high flatness output of 125 combs is realized at the repetition frequency of 18.5 GHz, the Brillouin light can be locked on the cavity longitudinal mode without active optoelectronic feedback, and the repetition frequency can be flexibly tuned between 12.5 GHz and 18.5 GHz by simply adjusting the radio frequency, and the lowest tuning accuracy can be as low as a single FSR. It has broad application prospects in modern scientific fields such as precision spectroscopy, coherent fiber communication and sensing.

[0095] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, and all of them should be covered in the scope of the claims of the present application.

Claims

1. A broadband electro-optical comb generation system with ultra-narrow linewidth, flexible and adjustable repetition rate, characterized in that: include, The pump laser generation branch (100) includes a distributed feedback laser (101), a first polarization controller (102) and a first circulator (103) connected in sequence. The fiber optic ring cavity (200) includes a second circulator (201) connected to the first circulator (103) and a Brillouin gain fiber (202) and a fiber delay line (203) respectively connected to the second circulator (201), as well as a fiber coupler (204), a second polarization controller (205), an electro-optic modulator (206), a dispersion compensation fiber (207), an optical amplifier (208), a flattening filter (209), and an isolator (210) connected between the Brillouin gain fiber (202) and the fiber delay line (203). The fiber coupler (204), the second polarization controller (205), the electro-optic modulator (206), the dispersion compensation fiber (207), the optical amplifier (208), the flattening filter (209), and the isolator (210) are located between the Brillouin gain fiber (202) and the fiber delay line (203) and are away from the second circulator (201). The frequency shift injection lock branch (300) includes a narrow bandwidth filter (301), a polarization controller (302) and a frequency shifter (303) connected in sequence; the narrow bandwidth filter (301) is connected to one output of an optical fiber coupler (204) and the frequency shifter (303) is connected to a first circulator (103).

2. The ultra-narrow linewidth, flexible and adjustable repetition rate, and broadband electro-optical comb generation system according to claim 1, characterized in that: The distributed feedback laser (101) is a fixed wavelength or a tunable single-frequency laser; The first circulator (103) and the second circulator (201) each include three ports, namely port a, port b and port c, and the connection from port a to port b, port b to port c and port c to port a is unidirectional. The length of the optical fiber delay line (203) is adjustable; The radio frequency of the frequency shifter (303) is the frequency corresponding to the maximum value of the stimulated Brillouin scattering gain.

3. The ultra-narrow linewidth, flexible and adjustable repetition rate, and broadband electro-optical comb generation system according to claim 2, characterized in that: The electro-optic modulator (206) is matched with a radio frequency signal source (206a), and microwave signals of different frequencies are applied to the electro-optic modulator (206) by the radio frequency signal source (206a).

4. The ultra-narrow linewidth, flexible repetition rate, and broadband electro-optical comb generation system according to claim 3, characterized in that: It also includes a second coupler (211) connected between the fiber coupler (204) and the second polarization controller (205).

5. A method for generating an ultra-narrow linewidth, flexible, and adjustable repetition rate broadband electro-optical comb, characterized in that: The ultra-narrow linewidth, frequency repetition rate, and broadband electro-optic comb generation system as described in any one of claims 1 to 4 includes the following steps: The initial laser is output through the pump laser generation branch (100), and the initial laser enters the fiber ring cavity (200) and generates a first-order Brillouin laser through the Brillouin gain fiber (202). The first-order Brillouin laser is split into one laser and two lasers via an optical fiber coupler (204); One laser beam enters the frequency shift injection locking branch (300) and then enters the distributed feedback laser (101) through the first circulator (103) to lock the wavelength and narrow linewidth characteristics of the distributed feedback laser (101). Two laser beams enter the fiber optic ring cavity (200) and generate sidebands under the modulation of the electro-optic modulator (206). The sidebands circulate in the fiber optic ring cavity (200) and expand to form a spectrum. After several cycles, adjust the position so that the sideband corresponds to the position of the longitudinal membrane of the cavity; Adjust the radio frequency and radio power of the radio frequency signal source (206a) to output an electro-optic frequency comb with target characteristics.

6. The method for generating an ultra-narrow linewidth, flexible, adjustable repetition rate, and broadband electro-optical comb according to claim 5, characterized in that: The pump laser generation branch (100) outputs an initial laser, which enters the fiber ring cavity (200) and generates a first-order Brillouin laser through the Brillouin gain fiber (202). include ， The initial laser emitted by the distributed feedback laser (101) enters the first circulator (103) after passing through the first polarization controller (102). After being output from the first circulator (103), it enters the Brillouin gain fiber (202) through the second circulator (201). The initial laser undergoes stimulated Brillouin scattering in the Brillouin gain fiber (202) to generate a first-order Brillouin laser with the opposite propagation direction to the initial laser.

7. The method for generating an ultra-narrow linewidth, flexible, adjustable repetition rate, and broadband electro-optical comb according to claim 6, characterized in that: One laser beam enters the frequency shift injection locking branch (300) and enters the distributed feedback laser (101) through the first circulator (103) to lock the wavelength of the distributed feedback laser (101) and transmit the narrow linewidth characteristics. include, The laser beam passes sequentially through a narrow-bandwidth filter (301), a polarization controller (302), and a frequency shifter (303), and then enters the distributed feedback laser (101) through the first circulator (103) and the first polarization controller (102). This transmits the narrow linewidth characteristics of the Brillouin light to the pump, making the linewidth of the secondary laser output by the distributed feedback laser (101) narrower; and maintaining the relative position between the initial pump light and the cavity longitudinal mode, locking the wavelength of the output secondary laser. After the secondary laser output, it passes through the Brillouin gain fiber (202) and the frequency shift injection lock branch (300) again before entering the distributed feedback laser (101), which again narrows the laser linewidth output by the distributed feedback laser (101). After several cycles, the distributed feedback laser (101) can be made to lock the wavelength relative to the cavity longitudinal mode and narrow the linewidth to the limit.

8. The method for generating an ultra-narrow linewidth, flexible, adjustable repetition rate, and broadband electro-optical comb according to claim 7, characterized in that: The two laser beams enter the fiber optic ring cavity (200) and generate sidebands under the modulation of the electro-optic modulator (206); including the following steps: Turn on the radio frequency signal source (206a) of the electro-optic modulator (206) and maintain the radio frequency input; The two laser beams pass sequentially through a second polarization controller (205), an electro-optic modulator (206), a dispersion compensation fiber (207), an optical amplifier (208), a flat filter (209), and an isolator (210), and then through a Brillouin gain fiber (202) and a fiber delay line (203) to a fiber coupler (204). The two laser beams generate sidebands after passing through the electro-optic modulator (206); The generated sidebands are flattened by an optical amplifier (208), a portion of which is output through an optical fiber coupler (204), and the remaining portion is passed through an electro-optic modulator (206) again to generate sub-sidebands. Continuous circulation expands the spectrum.

9. The method for generating an ultra-narrow linewidth, flexible, adjustable repetition rate, and broadband electro-optical comb according to claim 8, characterized in that: The adjustment causes the sidebands to correspond to the positions of the longitudinal diaphragm in the cavity, wherein the sidebands are adjusted by changing the radio frequency of the radio frequency signal source (206a), and the longitudinal diaphragm spacing is adjusted by changing the length of the fiber optic ring cavity (200).

10. The method for generating an ultra-narrow linewidth, flexible, and adjustable repetition rate broadband electro-optical comb according to claim 9, characterized in that: The radio frequency frequency of the adjusted radio frequency signal source (206a) is made to be an integer multiple of the longitudinal film spacing of the fiber optic ring cavity (200).

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