Laser and method for generating high phase coherence electromagnetic wave signal

Abstract

The present invention discloses a laser and method for generating electromagnetic wave signals with high phase coherence, which is composed of a non-polarization-maintaining gain optical fiber and a pair of Bragg fiber gratings engraved at both ends by a femtosecond laser direct writing line engraving method to form a distributed reflection fiber laser resonant cavity; in the fiber grating engraving, a small angle is introduced between the projection of the laser line engraving direction on the horizontal plane where the central axis of the optical fiber core is located and the radial direction of the optical fiber core, and a small angle is introduced between the laser line engraving direction and the horizontal plane where the central axis of the optical fiber core is located; the laser resonant cavity generates a three-wavelength laser output signal with high linear polarization, single frequency narrow line width and high phase correlation through the pumping of a pump source, and finally the beat frequency between the output signals generates millimeter wave or terahertz wave output. The present invention realizes multi-frequency millimeter wave or terahertz wave output with highly stable phase, and its process is simple, the manufacturing cost is low, and it has high mechanical stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lasers, and particularly relates to a laser and a method for generating an electromagnetic wave signal with high phase coherence degree. Background Art

[0002] The upcoming 5G technology based on millimeter waves and the 6G technology based on terahertz waves that is being studied will enable wireless communication to be several orders of magnitude faster in data speed and several orders of magnitude shorter in data transmission delay time than the existing technology. However, any mobile network ultimately needs to be incorporated into the 1.5μm low-loss optical fiber communication network. Therefore, compared with other technologies, the millimeter wave and terahertz wave generators based on optical fibers have great development potential because optical fiber devices can seamlessly connect local wireless networks and long-distance optical fiber networks.

[0003] Wireless communication requires millimeter wave and terahertz wave sources with low phase noise and wide frequency coverage. The optical carrier millimeter wave and terahertz wave signal sources can be generated by optical heterodyne technology, that is, by beating two lasers with a certain frequency difference. Two light waves with different frequencies can be expressed as E i (t)=E 0i ∙cos(ω i t+Φ i ) (i = 1 or 2), where E 0i represents the amplitude term, ω i represents the angular frequency term, and Φ i represents the phase term of each wave. The component located in the millimeter wave and terahertz wave generated by the heterodyne technology can be expressed as A∙cos[(ω 1 -ω 2 )t+(Φ 1 -Φ 2 )], where A is the amplitude related to E 0i . From this, it can be deduced that: only when the phases of the two waves are highly correlated, that is, when the phase difference ΔΦ (=Φ 1 -Φ 2 ) is as small as possible, the millimeter wave and terahertz wave signals generated by the heterodyne will have high amplitude, frequency, and phase stability. Therefore, theoretically, a dual-wavelength, high linear polarization degree, single-frequency narrow linewidth optical fiber laser can be used. Since the dual-wavelength laser is generated from the same laser cavity, the two have a small phase difference, which is conducive to realizing millimeter wave and terahertz wave signals with high intensity stability, high frequency stability, and high phase stability.

[0004] Previously, dual-wavelength fiber lasers were realized through chirped distributed feedback grating structure fiber lasers, overprinted dual-wavelength Bragg fiber grating fiber lasers, or dual-polarization state fiber lasers. The phase difference between the generated dual-wavelength lasers is still not small enough, which is not conducive to further reducing phase noise to obtain millimeter-wave and terahertz-wave signals with high intensity stability, high frequency stability, and high phase stability. At the same time, to make the generated millimeter-wave and terahertz-wave signals cover a wider frequency range, a multi-wavelength (such as three-wavelength) fiber laser is obviously a better choice, but this places higher requirements on the phase correlation of the laser signals of each wavelength, and there is no successful report on this at home and abroad so far. Summary of the Invention

[0005] Aiming at the above-mentioned existing technical deficiencies, the purpose of the present invention is to provide a laser and method for generating electromagnetic wave signals with high phase coherence, outputting lasers with multi-wavelength, high linear polarization, and single-frequency narrow linewidth, using the difference frequency technology of pairwise beat frequency between laser output signals to achieve the output of multi-frequency millimeter-wave or terahertz-wave with highly stable phase, with simple process, low manufacturing cost, and high mechanical stability.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for generating electromagnetic wave signals with high phase coherence, including the following steps:

[0008] S1: The femtosecond laser direct writing line engraving method of the Bragg fiber grating is as follows:

[0009] Engrave Bragg fiber gratings at both ends of the non-polarization-maintaining gain fiber by femtosecond laser direct writing line engraving;

[0010] The femtosecond laser passes downward from directly above the non-polarization-maintaining gain fiber through the fiber cladding and is focused inside the fiber core. Its focused spot is positioned on the horizontal plane where the central axis of the fiber core is located. The focused spot starts from one side inside the interface between the fiber core and the fiber cladding, passes through the central axis of the fiber core, and moves horizontally to the other side for line engraving, forming a linear grating plane inside the core;

[0011] In particular, a small angle is introduced between the projection of the laser line engraving direction on the horizontal plane where the central axis of the fiber core is located and the radial direction passing through the center of the fiber core, so that the refractive index change distribution induced by the laser on the linear grating plane presents a trident-shaped refractive index distribution curve (n(r));

[0012] Specifically, at the same time, a small angle is introduced between the laser line engraving direction and the horizontal plane where the central axis of the optical fiber core is located, so that there is a small difference in refractive index between the two small peaks on both sides of the above-mentioned trident-shaped refractive index distribution curve, showing a small asymmetry, that is, finally an asymmetric trident-shaped refractive index distribution curve (n'(r)) is formed;

[0013] Secondly, after completing one line-engraved grating surface, the laser spot is horizontally translated along the axis direction of the optical fiber core by one period; the next linear grating surface is engraved in a direction that is 180 degrees opposite to the line engraving direction of the previous grating surface;

[0014] Finally, the line engraving is repeated multiple times, and finally a Bragg fiber grating with a preset length is engraved;

[0015] Since the refractive index distribution curve of the grating surface is an asymmetric trident type (n'(r)), a Bragg fiber grating prepared by a single line engraving process is equivalent to three sets of Bragg fiber gratings with the same Bragg grating period and three different equivalent refractive indices;

[0016] Again, the preset length of the Bragg fiber grating and the shorter length of the active optical fiber jointly determine that there is only one narrow linewidth single longitudinal mode within the spectral linewidth of the fiber grating;

[0017] Again, due to the geometric asymmetry of the Bragg fiber grating engraved by femtosecond laser line engraving in two orthogonal directions of the cross section, the finally engraved Bragg fiber grating has a large birefringence difference;

[0018] S2: A pair of Bragg fiber gratings engraved by the femtosecond laser direct writing line engraving method at both ends of the non-polarization-maintaining gain fiber form a distributed feedback fiber laser resonator;

[0019] The pump light of the pump light source enters the fiber laser resonator formed by the fiber wavelength division multiplexer 1. The laser signal whose laser wavelength is determined by the Bragg fiber grating wavelength oscillates back and forth multiple times in the fiber laser oscillation cavity to achieve laser output;

[0020] The laser output from the fiber laser resonator passes through the fiber wavelength division multiplexer 2 to form a forward output;

[0021] The laser output from the fiber laser resonator passes through the fiber wavelength division multiplexer 1 to form a reverse output;

[0022] Specifically, since one of the pair of fiber Bragg gratings obtained by femtosecond laser line engraving in step S1 has an asymmetric trident-shaped refractive index distribution curve (n'(r)), the distributed feedback fiber laser resonator composed of a set of Bragg fiber gratings is equivalent to a fiber laser resonator with three Bragg fiber gratings with different wavelengths, thereby generating three different wavelength laser signals;

[0023] Meanwhile, particularly, since the fiber Bragg grating obtained by femtosecond laser line writing in step S1 has a large birefringence difference in two orthogonal directions, the laser signal oscillates multiple times in the fiber laser resonator. There is mode competition between the laser signals in two orthogonal polarization directions, resulting in a very large polarization extinction ratio, and the laser signal output has a very high degree of linear polarization;

[0024] Finally, a fiber laser resonator based on the line-written Bragg fiber grating prepared in step S1 and constituted according to step S2, through an all-fiber pumping device, realizes the output of a three-wavelength laser with high phase correlation, high linear polarization degree, and single-frequency narrow linewidth.

[0025] S3: Since the three-wavelength laser output realized in step S3 has a high degree of linear polarization and a single-frequency narrow linewidth, and there is a high phase correlation relationship between different wavelength laser signals, through the heterodyne technique of pairwise beating between the three-wavelength laser output signals, the output of multi-frequency millimeter waves or terahertz waves with highly stable phases is realized.

[0026] Particularly, since the three-wavelength laser output realized in step S3 has a high degree of linear polarization and a single-frequency narrow linewidth, and there is a high phase correlation relationship between different wavelength laser signals, through the heterodyne technique of pairwise beating between the three-wavelength laser output signals, the output of multi-frequency millimeter waves or terahertz waves with highly stable phases has a fine structure with a periodic frequency interval, sidebands, and the main peak intensity being close in the frequency domain.

[0027] Preferably, the writing method of the Bragg fiber grating is as follows:

[0028] The non-polarization-maintaining gain fiber includes a fiber core and a fiber cladding. The femtosecond laser is focused from directly above the fiber and focused on the horizontal plane where the central axis of the fiber core in the fiber core is located. The focused spot makes a linear scan along the radial direction of the fiber core and crosses the central axis of the fiber core from the interface between the fiber core and the fiber cladding to write out a linear grating grating surface structure; particularly, a small angle is introduced between the laser line writing direction and the radial direction passing through the center of the fiber core in the horizontal plane projected on the central axis of the fiber core; at the same time, particularly, a small angle is introduced between the laser line writing direction and the horizontal plane where the central axis of the fiber core is located; after writing one grating surface, the focused spot is translated along the fiber core axis by one Bragg grating period, and the line writing scan of the next linear grating surface is repeated; and so on, until the length of the Bragg fiber grating reaches the preset length.

[0029] Preferably, the length of the Bragg fiber grating is between 0.1 - 10 cm; the line engraving length of the femtosecond laser in the fiber core ranges from 10% to 100% of the core diameter size; the range of the small angle between the projection of the laser line engraving direction on the horizontal plane where the central axis of the fiber core lies and the radial direction of the fiber core is between 1 second and 10 degrees; at the same time, the range of the small angle between the laser line engraving direction and the horizontal plane where the central axis of the fiber core lies is between 1 second and 10 degrees.

[0030] Preferably, under the pumping of the pump laser, a three - wavelength, high - degree of linear polarization, single - frequency, narrow - linewidth laser output is generated. The frequency interval range of the three - wavelength laser output signal is 0.1 GHz - 10 THz, the polarization extinction ratio of the laser signal is > 10 dB, and the 3 - dB linewidth of the single - frequency laser signal is less than 5 kHz.

[0031] Preferably, through the difference - frequency technology of pairwise beat - frequency between the three - wavelength laser output signals, a highly phase - stable multi - frequency millimeter - wave or terahertz - wave output is realized. The frequency of the millimeter - wave or terahertz - wave generated by beat - frequency is between 0.1 GHz and 10 THz, and its 3 - dB linewidth is less than 5 kHz.

[0032] Preferably, on both sides of the main peak of the millimeter - wave or terahertz - wave generated by beat - frequency, the generated three - frequency millimeter - wave or terahertz - wave signals each have the fine - structure characteristics of a frequency comb, that is, there are highly symmetric and periodically arranged frequency comb teeth on both sides centered on its main frequency, and the intensity of the comb teeth is close to the intensity of the central peak.

[0033] Preferably, the intensity ratio between the first sideband on both sides of the main peak of the millimeter - wave or terahertz - wave signal frequency comb and the main peak is in the range of 0.1% - 10%, and the frequency comb tooth period range is 1 kHz - 100 MHz.

[0034] Another object of the present invention is to provide a laser for generating an electromagnetic wave signal with a high phase correlation degree, which is characterized in that it includes a fiber laser resonator, and the fiber laser resonator is composed of a non - polarization - maintaining gain fiber and Bragg fiber gratings one and two at both ends;

[0035] It also includes a fiber wavelength - division multiplexer. Fiber wavelength - division multiplexer two and fiber wavelength - division multiplexer one are respectively installed on the front and back sides of the fiber laser resonator;

[0036] A pump light source, the pump light source is coupled into one end of the Bragg fiber grating of the fiber laser resonator through the fiber wavelength - division multiplexer one; the laser generated in the pump light source outputs a three - wavelength laser forward output signal through the fiber wavelength - division multiplexer two and outputs a three - wavelength laser reverse output signal through the fiber wavelength - division multiplexer one.

[0037] The beneficial effects of the present invention are as follows:

[0038] 1. By introducing two angular measures as additional spatial control degrees of freedom for femtosecond direct-written fiber gratings in the traditional femtosecond laser direct writing line engraving technology, it is possible to write fiber Bragg gratings on non-polarization-maintaining gain fibers with the ability to select multiple frequencies, single longitudinal mode, and high polarization degree simultaneously.

[0039] 2. The fiber laser resonator disclosed in the present invention has laser output with multi-wavelength, high linear polarization, and single-frequency narrow linewidth, minimizing the phase difference between the three-wavelength laser output signals generated in the same fiber laser resonator. Through the difference frequency technology of pairwise beat frequency between the three-wavelength laser output signals, multi-frequency millimeter-wave or terahertz wave output with highly stable phase is achieved.

[0040] 3. The laser disclosed in the present invention can achieve an all-fiber structure, having high mechanical stability and simple process; with few required optical components, it can greatly simplify the structure of the fiber laser, with low manufacturing cost, which is conducive to popularization and application. Description of the Drawings

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0042] Figure 1 In the figure, A is a top view schematic diagram of the Bragg fiber grating prepared by the femtosecond laser direct writing line engraving method of the present invention, and B is a cross-sectional schematic diagram of the Bragg fiber grating; this grating is actually equivalent to three sets of Bragg fiber gratings with the same Bragg period length but slightly different effective refractive indices.

[0043] Figure 2 It is a schematic diagram of the laser structure prepared by the operation method for generating millimeter-wave and terahertz wave signals with high phase correlation degree of the present invention.

[0044] Figure 3 It is a spectrogram obtained by placing the three-wavelength laser generated in Example 1 into a spectral analyzer; the wavelength interval between the three-wavelength laser output signal and the adjacent laser signal is 0.028 nanometers.

[0045] Figure 4 It is an oscilloscope spectrogram obtained by placing the three-wavelength laser generated in Example 1 into a photoelectric probe and a scanning Fabry-Perot interferometer; in the free spectral range of 1.5 GHz, the three-wavelength laser output signals are all single longitudinal mode outputs.

[0046] Figure 5Example diagram of obtaining the degree of polarization of a three-wavelength laser by placing the three-wavelength laser generated in Example 1 into a polarimeter; during the half-hour test time, the degree of polarization of the three-wavelength laser output signal is above 98%, and the root mean square deviation is less than 0.5%; at the same time, the root mean square deviation of its laser output power is less than 0.1%.

[0047] Figure 6 Example diagram of the frequency spectrum obtained after beat frequency by placing the three-wavelength laser generated in Example 1 into an optoelectronic probe and an electronic frequency analyzer; within the designed frequency range, the existence of three stable millimeter-wave frequencies (3.18, 3.22, 6.39 GHz) can be seen.

[0048] Figure 7 For Figure 6 Partial enlarged view of the millimeter-wave signal at the frequency of 6.39 GHz selected from the frequency spectrum obtained after beat frequency; it can be seen that the millimeter-wave main peak centered at 6.3925 GHz has an extremely narrow 3dB line width (<500 Hz); at the same time, due to each beat frequency signal having extremely low frequency noise and phase noise, multiple comb teeth are symmetrically distributed on both sides of the main peak, and the comb tooth interval presents a periodic structure, the comb tooth distance is 100 kHz, and the intensity of the sideband comb teeth is relatively strong. Taking the first sideband on both sides of the main peak as an example, its intensity is 10% of the main peak intensity.

[0049] Description of reference numerals:

[0050] 1. Non-polarization-maintaining gain fiber; 1a. Fiber cladding; 1b. Fiber core; 2. Focused spot; 3. Central axis of the fiber core; 4. Horizontal plane where the central axis of the fiber core is located; 5. Linear grating plane; 6. Laser line engraving direction; 7. Radial direction of the fiber core; 8. Small angle between the projection of the laser line engraving direction on the horizontal plane where the central axis of the fiber core is located and the radial direction of the fiber core; 9. Small angle between the laser line engraving direction and the horizontal plane where the central axis of the fiber core is located; 10. Fiber Bragg grating one; 11. Fiber Bragg grating two; 12. Pumping light source; 13a. Fiber wavelength division multiplexer one; 13b. Fiber wavelength division multiplexer two; 14. Forward output signal of the three-wavelength laser; 15. Reverse output signal of the three-wavelength laser. Detailed implementation manners

[0051] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with 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 of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0052] In the present invention, first, Bragg fiber gratings are inscribed at both ends of a non-polarization-maintaining gain fiber 1 by means of femtosecond direct writing line inscription to form a distributed Bragg reflection laser resonator. The length of each Bragg fiber grating is between 0.1 - 10 cm; the femtosecond laser focusing spot 2 is located within the fiber core 1b, and the line inscription length within the core ranges from 10 - 100% of the core diameter size; the small angle 8 between the projection of the laser line inscription direction on the horizontal plane where the fiber core central axis lies and the fiber core radial direction ranges between 1 second - 10 degrees, and the small angle 9 between the laser line inscription direction and the horizontal plane where the fiber core central axis lies ranges between 1 second - 10 degrees;

[0053] Secondly, under the pumping of the pump laser, the distributed Bragg reflection laser resonator generates a three-wavelength, high degree of linear polarization, single-frequency, narrow linewidth laser output. The frequency interval range of the three-wavelength laser output signal is 0.1 GHz - 10 THz, the polarization extinction ratio of the laser signal (the polarization extinction ratio is the ratio of the output powers in two orthogonal polarization directions of the laser output) > 10 dB, and the 3 dB linewidth of the single-frequency laser signal is less than 5 kHz;

[0054] Thirdly, through the heterodyne technique of pairwise heterodyning between the three-wavelength laser output signals, a multi-frequency millimeter wave or terahertz wave output with highly stable phase is realized. The millimeter wave or terahertz wave belongs to a kind of electromagnetic wave; the frequency of the millimeter wave or terahertz wave generated by heterodyning is located in the range of 0.1 GHz - 10 THz, and its 3 dB linewidth is less than 5 kHz; on both sides of the main peak of the millimeter wave or terahertz wave generated by heterodyning, due to the high amplitude stability, high frequency stability, and high phase stability of the signal itself generated by heterodyning, the resulting three-frequency millimeter wave or terahertz wave signals each have the fine structure characteristics of a frequency comb, that is, there are highly symmetric and periodically arranged frequency comb teeth on both sides centered on its main frequency, and the intensity of the comb teeth is close to the intensity of the central peak. Specifically, the intensity ratio of the first sideband on both sides of the main peak to the main peak is in the range of 0.1% - 10%, and the frequency comb tooth period range is 1 kHz - 100 MHz. Embodiment

[0055] I. Preparation of Bragg fiber grating structure by femtosecond laser direct writing line inscription

[0056] As Figure 1 shown, the light source for inscribing a uniform Bragg fiber grating is an 800-nanometer femtosecond laser (pulse width 80 femtoseconds, repetition frequency 1 kilohertz), the non-polarization-maintaining gain fiber 1 is an erbium-doped non-polarization-maintaining silica glass fiber, and the diameter of the fiber core 1b is 4 micrometers.

[0057] The non-polarization-maintaining gain fiber 1 includes a fiber core 1b and a fiber cladding 1a;

[0058] The femtosecond laser passes downward from directly above the non-polarization-maintaining gain fiber 1 through the fiber cladding 1a and is focused inside the fiber core 1b. Its focused spot 2 is positioned on the horizontal plane 4 where the central axis of the fiber core lies. The focused spot 2 starts from one side within the interface of the fiber core 1b - fiber cladding 1a and horizontally moves and scans through the central axis 3 of the fiber core to the other side for line engraving, forming a linear grating surface 5 inside the core;

[0059] The fine-tuning of the small angle 8 between the projection of the femtosecond laser line engraving direction on the horizontal plane where the central axis of the fiber core lies and the radial direction of the fiber core is 1 minute. The refractive index change distribution induced by the femtosecond laser along the line engraving direction presents a trident-shaped refractive index distribution curve (n(r)); at the same time, the fine-tuning of the small angle 9 between the femtosecond laser line engraving direction and the horizontal plane where the central axis of the fiber core lies is 1 minute, so that there is a small difference in the refractive index between the two small peaks on both sides of the above-mentioned trident-shaped refractive index distribution curve, presenting a small asymmetry, that is, finally an asymmetric trident-shaped refractive index distribution curve (n’(r)) is formed, and the line engraving length is 4 microns, that is, 100% of the geometric diameter of the fiber core 1b;

[0060] After the first linear grating surface 5 is completed, the femtosecond laser focused spot 2 is translated 1.1 microns along the central axis 3 of the fiber core. This distance is the designed period of the femtosecond laser direct writing line engraved fiber Bragg grating. Then, the femtosecond laser focused spot 2 engraves the next linear grating surface 5 along the direction 180 degrees opposite to the first line engraving direction. In this way, line engraving is repeatedly carried out under program control, and finally the preset lengths (6.6 mm and 11 mm) of the fiber Bragg grating one 10 and the fiber Bragg grating two 11 are achieved. In fact, the fiber Bragg grating obtained by one-time femtosecond direct writing line engraving is equivalent to three sets of fiber Bragg gratings with the same Bragg period length but with a small difference in the effective refractive index, that is, these are three sets of fiber Bragg gratings with different Bragg wavelengths located in the same fiber laser resonator cavity.

[0061] Secondly, due to the characteristics of the femtosecond direct writing line engraving method, the geometric asymmetry formed in the horizontal and vertical orthogonal directions of the fiber core constitutes the difference in the effective refractive index in the orthogonal directions of the propagating transverse modes. This makes the non-polarization-maintaining gain fiber Bragg grating introduce a strong birefringence difference, adding a function of selecting the polarization state of the laser in addition to the function of selecting the frequency of the output laser. The laser signal generated in this way has an extremely high polarization extinction ratio;

[0062] Thirdly, due to the combined action of the relatively long preset length of the fiber Bragg grating and the relatively short length of the active fiber, finally only one narrow-linewidth single longitudinal mode is allowed to exist within the spectral linewidth of the fiber grating.

[0063] II. Laser for generating millimeter-wave and terahertz-wave signals with high phase correlation

[0064] As shown Figure 2 in the figure, the laser is an erbium-doped 1.59-μm all-fiber three-wavelength, linearly polarized, single-frequency narrow-linewidth fiber laser, which includes a non-polarization-maintaining gain fiber 1 with a length of 49 mm, a first Bragg fiber grating 10 with a length of 6.6 mm, and a second Bragg fiber grating 11 with a length of 11 mm to form a fiber laser resonator. A 976-nm semiconductor laser with a maximum power of 1 W and a pigtail is used as a pump light source 12, which is coupled into one end of the first Bragg fiber grating 10 of the fiber laser resonator through a first fiber wavelength division multiplexer 13a. The laser generated in the pump light source 12 outputs a three-wavelength laser forward output signal 14 through a second fiber wavelength division multiplexer 13b, and outputs a three-wavelength laser reverse output signal 15 through the first fiber wavelength division multiplexer 13a.

[0065] III. Detection and Analysis of Output Signals

[0066] The three-wavelength laser forward output signal 14 or the three-wavelength laser reverse output signal 15 is placed into instruments and equipment such as a power meter, a spectrum analyzer, a photoelectric detection probe, and an electronic frequency analyzer for detection and analysis of performance such as power, spectrum, frequency, and polarization. The detection and analysis results are as Figures 3 to 7 .

[0067] The data shows that through the above implementation method, in a fiber laser resonator constructed by a non-polarization-maintaining gain fiber 1 and a pair of Bragg fiber gratings at both ends thereof, pumping generates laser outputs of three wavelengths with high linear polarization degree and single-frequency narrow linewidth. Sharing the same laser cavity, performance indicators such as high linear polarization degree and single-frequency narrow linewidth indicate that there is a very small phase difference between the laser signals of the three wavelengths, that is, the phases of the three-wavelength laser output signals are highly correlated.

[0068] Finally, through the difference frequency technology, beat frequency is performed pairwise between the three-wavelength laser output signals to realize the output of highly phase-stable three-frequency millimeter-wave or terahertz-wave signals. Since the generated millimeter-wave or terahertz-wave itself has high amplitude stability, high frequency stability, and high phase stability, the generated three-frequency millimeter-wave or terahertz-wave signals have the characteristics of a frequency comb, that is, there are highly symmetric and periodically arranged frequency comb teeth on both sides centered on its main frequency, and the ratio of the comb tooth intensity to the central peak is close.

[0069] The implementation method of the millimeter-wave and terahertz-wave signal source with high phase correlation degree of the present invention has low manufacturing cost, can be seamlessly connected with the fiber network, and is conducive to popularization and application.

[0070] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A method for generating a high phase coherence electromagnetic wave signal, characterized in that: S1: The femtosecond laser direct writing line inscription method of Bragg fiber grating is: A femtosecond laser is focused from directly above a non-polarization-maintaining gain optical fiber (1) downward through an optical fiber cladding (1a) into an optical fiber core (1b), and a focused light spot (2) is positioned on a horizontal plane (4) where a central axis of the optical fiber core is located. The focused light spot (2) starts from one side of an interface between the optical fiber core (1b) and the optical fiber cladding (1a), passes through the central axis (3) of the optical fiber core, and is horizontally moved and scanned toward the other side to perform line engraving, thereby forming a linear grating surface (5) in the optical fiber core. A small angle is introduced between the projection of the laser line engraving direction (6) on the horizontal plane (4) where the central axis of the optical fiber core is located and the radial direction (7) of the optical fiber core, so that the refractive index change distribution induced by the laser on the linear grating surface (5) presents a trident-shaped refractive index distribution curve (n(r)); At the same time, a small angle is introduced between the laser line engraving direction (6) and the horizontal plane (4) where the central axis of the optical fiber core is located, so that there is a small difference in refractive index between the small peaks on both sides of the above-mentioned trident-shaped refractive index distribution curve, thereby presenting a small asymmetry, that is, finally forming an asymmetric trident-shaped refractive index distribution curve (n'(r)); The small angle (8) between the projection of the laser line engraving direction on the horizontal plane where the central axis of the optical fiber core is located and the radial direction of the optical fiber core is in the range of 1 second to 10 degrees, and the small angle (9) between the laser line engraving direction and the horizontal plane where the central axis of the optical fiber core is located is in the range of 1 second to 10 degrees; After completing a linear grating surface (5), the focusing light spot (2) is horizontally moved and translated by one Bragg grating period along the axis of the optical fiber core; and the next linear grating surface (5) is inscribed in the opposite direction to the line engraving direction of the previous grating surface; Repeating the line engraving for multiple times, and finally engraving the Bragg fiber grating with a preset length; S2: writing the fiber Bragg grating at both ends of the non-polarization-maintaining gain optical fiber (1) by femtosecond laser direct writing line inscription to form a distributed fiber laser resonant cavity; The pump light of the pump light source (12) enters the optical fiber laser resonant cavity through an optical fiber wavelength division multiplexer (13a), and the laser signal whose laser wavelength is determined by the wavelength of the optical fiber Bragg grating oscillates back and forth multiple times in the optical fiber laser oscillation cavity to realize laser output; The laser output from the fiber laser resonant cavity passes through the second fiber wavelength division multiplexer (13b) to form a forward output; The laser output from the fiber laser resonant cavity passes through a fiber wavelength division multiplexer (13a) to form a reverse output; Finally, the fiber laser resonant cavity constructed according to step S2 based on the line-engraved fiber Bragg grating prepared in step S1 realizes three-wavelength laser output with high phase correlation, high linear skewness, and single-frequency narrow linewidth through an all-fiber pumping device; S3: Since the three-wavelength laser output achieved in step S2 has high linear skewness and single-frequency narrow linewidth, there is a high phase correlation between laser signals of different wavelengths. By using the difference frequency technology of pairwise beating between the three-wavelength laser output signals, multi-frequency millimeter wave or terahertz wave output with highly stable phase is achieved, and there is a fine structure with periodic frequency intervals, sidebands and main peak intensities in the frequency domain.

2. A method for generating a high phase coherence electromagnetic wave signal according to claim 1, characterized in that: The length of the Bragg fiber grating is between 0.1 and 10 cm; the length of the line engraved by the femtosecond laser in the optical fiber core (1b) is in the range of 10 to 100% of the diameter of the core.

3. The method for generating a high phase coherence electromagnetic wave signal according to claim 2, characterized in that: Under the pumping of the pump light source (12), a three-wavelength high linear polarization degree, single-frequency narrow linewidth laser output is generated, the three-wavelength laser output signal frequency interval range is 0.1 GHz-10 THz, the polarization extinction ratio of the laser signal is greater than 10 dB, and the single-frequency laser signal 3dB linewidth is less than 5 kHz.

4. The method for generating a high phase coherence electromagnetic wave signal according to claim 3, characterized in that: By using the difference frequency technology of two-by-two beat frequencies between the three-wavelength laser output signals, highly phase-stable multi-frequency millimeter wave or terahertz wave output is achieved. The frequency of the millimeter wave or terahertz wave generated by the beat frequency is between 0.1GHz and 10THz, and its 3dB linewidth is less than 5kHz.

5. The method for generating a high phase coherence electromagnetic wave signal according to claim 4, characterized in that: On both sides of the main peak of the millimeter wave or terahertz wave generated by the beat frequency, the three-frequency millimeter wave or terahertz wave signals generated each have the fine structural characteristics of a frequency comb, that is, there are highly symmetrical and periodically arranged frequency comb teeth on both sides of the main frequency as the center, and the comb tooth intensity is close to the central peak intensity.

6. The method for generating a high phase coherence electromagnetic wave signal according to claim 5, characterized in that: The intensity ratio of the first sidebands on both sides of the main peak to the main peak is in the range of 0.1%-10%, and the frequency comb period range is 1kHz-100MHz.

7. A laser using the method for generating a high phase coherence electromagnetic wave signal according to claim 1, characterized in that: The optical fiber laser resonant cavity comprises a non-polarization-maintaining gain optical fiber (1) and a Bragg fiber grating 1 (10) and a Bragg fiber grating 2 (11) at two ends; It also includes an optical fiber wavelength division multiplexer, wherein the front and rear sides of the optical fiber laser resonant cavity are respectively equipped with an optical fiber wavelength division multiplexer 2 (13b) and an optical fiber wavelength division multiplexer 1 (13a); A pump light source (12), the pump light source (12) is coupled into one end of a Bragg fiber grating (10) of a fiber laser resonant cavity through the fiber wavelength division multiplexer (13a); the laser light generated in the pump light source (12) is output through the fiber wavelength division multiplexer (13b) to obtain a three-wavelength laser forward output signal (14), and is output through the fiber wavelength division multiplexer (13a) to obtain a three-wavelength laser reverse output signal (15).

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