WO2023142992A1 - 一种产生高相位相干度电磁波信号的激光器及方法 - Google Patents

一种产生高相位相干度电磁波信号的激光器及方法 Download PDF

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WO2023142992A1
WO2023142992A1 PCT/CN2023/071222 CN2023071222W WO2023142992A1 WO 2023142992 A1 WO2023142992 A1 WO 2023142992A1 CN 2023071222 W CN2023071222 W CN 2023071222W WO 2023142992 A1 WO2023142992 A1 WO 2023142992A1
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laser
fiber
frequency
line
wavelength
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French (fr)
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施进丹
冯宪
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江苏师范大学
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06729—Peculiar transverse fibre profile
    • H01S3/06733—Fibre having more than one cladding
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1307—Stabilisation of the phase

Definitions

  • the invention belongs to the technical field of lasers, and in particular relates to a laser and a method for generating electromagnetic wave signals with high phase coherence.
  • the millimeter-wave-based 5G technology that has arrived and the terahertz-wave-based 6G technology that has already been researched will enable wireless communication to be several orders of magnitude faster in data speed than the existing technology, while the data transmission delay time will be shortened by several orders of magnitude.
  • any mobile network will eventually need to be incorporated into a 1.5 ⁇ m low-loss optical fiber communication network. Therefore, compared to other technologies, fiber-based millimeter wave and terahertz wave generators are very promising because fiber optic devices can seamlessly Connect the local wireless network with the long-distance fiber optic network.
  • Wireless communication requires millimeter-wave and terahertz wave sources with low phase noise and wide frequency coverage.
  • Optically-carried millimeter wave and terahertz wave signal sources can be generated by laser difference frequency technology, that is, to beat two lasers with a certain frequency difference.
  • Previous dual-wavelength fiber lasers were realized by chirped distributed feedback grating fiber lasers, superimposed dual-wavelength Bragg fiber grating fiber lasers, or dual-polarization fiber lasers, and the phase difference between the generated dual-wavelength lasers is still not small enough , which is not conducive to continue to reduce the 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, it is necessary to make the generated millimeter wave and terahertz wave signals cover a wider frequency Range, multi-wavelength (such as three-wavelength) fiber laser is obviously a better choice, but this puts forward higher requirements for the phase correlation of laser signals of each wavelength, which has not been successfully reported at home and abroad.
  • the object of the present invention is to provide a laser and a method for generating high-phase-coherence electromagnetic wave signals, outputting lasers with multiple wavelengths, high linear polarization, and single-frequency narrow linewidth, and performing a process between laser output signals.
  • the difference frequency technology of two beat frequencies realizes multi-frequency millimeter wave or terahertz wave output with highly stable phase.
  • the process is simple, the manufacturing cost is low, and it has high mechanical stability.
  • the invention provides a method for generating electromagnetic wave signals with high phase coherence, comprising the following steps:
  • the femtosecond laser is focused on the inside of the fiber core through the fiber cladding from directly above the non-polarization maintaining gain fiber, and its focused spot is positioned on the horizontal plane where the central axis of the fiber core is located. Start from one side of the interface of the layer, and move horizontally through the central axis of the fiber core to the other side for line engraving, forming a linear grating surface in the fiber core;
  • n(r) trident-shaped refractive index distribution curve
  • the laser spot moves horizontally and translates for a period along the axis of the fiber core; the next line grating surface is written in a direction 180 degrees opposite to the line engraving direction of the previous grating surface ;
  • a fiber Bragg grating prepared by a single line marking procedure is equivalent to having the same Bragg grating period and three different equivalent refractive indices.
  • the preset length of the FBG and the short active fiber length jointly determine that there is only one narrow linewidth single longitudinal mode in the spectral linewidth of the FBG;
  • the final fiber Bragg grating prepared by line engraving has a large birefringence difference
  • S2 A pair of fiber Bragg gratings written on both ends of the non-polarization-maintaining gain fiber by the femtosecond laser direct writing method and a distributed reflection fiber laser resonator;
  • the pumping light of the pump light source enters the fiber laser resonator composed of fiber optic wavelength division multiplexer, and the laser signal whose laser wavelength is determined by the wavelength of the Bragg fiber grating oscillates repeatedly in the fiber laser oscillation cavity to realize the laser output;
  • the laser output from the fiber laser resonator passes through the fiber wavelength division multiplexer 2 to form a forward output;
  • the laser output from the fiber laser resonator passes through the fiber wavelength division multiplexer 1 to form a reverse output;
  • one of the pair of fiber Bragg gratings obtained by femtosecond laser line marking in step S1 has an asymmetric trident-shaped refractive index distribution curve (n'(r))
  • the above-mentioned distribution consisting of a set of fiber Bragg gratings The reflective fiber laser resonator is equivalent to a fiber laser resonator with three fiber Bragg gratings with different wavelengths, thereby generating laser signals with three different wavelengths;
  • the fiber Bragg grating obtained by femtosecond laser line marking in step S1 has a large birefringence difference in two orthogonal directions
  • the laser signal oscillates multiple times in the fiber laser resonator, and the laser signal in There is mode competition in the two orthogonal polarization directions, resulting in a large polarization extinction ratio, and the output of the laser signal is a very high degree of linear polarization;
  • step S3 Since the three-wavelength laser output achieved in step S3 has high line skewness and single-frequency narrow linewidth, there is a high phase correlation relationship between different wavelength laser signals, and the difference between two beat frequencies is performed between the three-wavelength laser output signals High-frequency technology to realize multi-frequency millimeter wave or terahertz wave output with highly stable phase.
  • the three-wavelength laser output achieved in step S3 has high line skewness and single-frequency narrow linewidth, there is a high phase correlation relationship between different wavelength laser signals, and the two-by-two beat frequency between the three-wavelength laser output signals Difference frequency technology, to achieve a highly stable phase of multi-frequency millimeter wave or terahertz wave output in the frequency domain, there is a fine structure with periodic frequency intervals, sidebands and main peak intensities close together.
  • the writing method of the fiber Bragg grating is:
  • the non-polarization maintaining gain fiber includes the fiber core and the 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 is located in the fiber core.
  • the focused spot is along the radial direction of the fiber core, From the interface between the fiber core and the fiber cladding, make a linear scan across the central axis of the fiber core and write a linear grating surface structure; especially, on the horizontal plane where the central axis of the fiber core is projected, the direction of the laser line marking A small angle is introduced between the radial direction passing through the center of the fiber core; at the same time, especially, a small angle is introduced between the laser line marking direction and the horizontal plane where the central axis of the fiber core is located; after a grating surface is written, the focused spot Translating one Bragg grating period along the axis of the fiber core, and repeating the line scanning of the next linear grating surface; repeating this until the length of the fiber
  • the length of the fiber Bragg grating is between 0.1-10cm; the femtosecond laser line marking length in the fiber core is 10-100% of the core diameter; the laser line marking direction is at the center of the fiber core
  • the small included angle range between the projection on the horizontal plane where the axis is located and the radial direction of the fiber core is between 1 second and 10 degrees; at the same time, the small included angle range between the laser line marking direction and the horizontal plane where the central axis of the fiber core is located is within 1 second Between -10 degrees.
  • a three-wavelength high-degree-of-linear-polarization and single-frequency narrow-linewidth laser output is generated, the three-wavelength laser output signal frequency interval ranges from 0.1GHz to 10THz, and the polarization extinction ratio of the laser signal is >10dB , The 3dB line width of the single-frequency laser signal is less than 5kHz.
  • a highly phase-stable multi-frequency millimeter wave or terahertz wave output is achieved by performing a two-to-two beat frequency difference technology between the three-wavelength laser output signals, and the frequency of the millimeter wave or terahertz wave generated by the beat frequency is Located at 0.1GHz-10THz, its 3dB line width is less than 5kHz.
  • the generated three-frequency millimeter wave or terahertz wave signals each have the fine structure characteristics of a frequency comb, that is, there are heights on both sides centered on its main frequency. Symmetrical, periodically arranged frequency comb teeth, and the intensity of the comb teeth is close to the intensity of the central peak.
  • the intensity ratio of the first sideband on both sides of the main peak of the frequency comb of the millimeter wave or terahertz wave signal to the main peak is in the range of 0.1%-10%, and the period of the frequency comb is in the range of 1kHz-100MHz.
  • Another object of the present invention is to provide a kind of laser that produces high phase correlation degree electromagnetic wave signal, it is characterized in that, comprise fiber laser resonator, described fiber laser resonator is made of non-polarization-maintaining gain fiber and the fiber Bragg grating of two ends one, Composed of two fiber Bragg gratings;
  • It also includes a fiber optic wavelength division multiplexer, the fiber optic wavelength division multiplexer 2 and the fiber optic wavelength division multiplexer 1 are respectively installed on the front and rear sides of the fiber laser resonator;
  • a pumping light source is coupled to one end of the fiber Bragg grating that enters the fiber laser resonator through the optical fiber wavelength division multiplexer; the laser light generated in the pumping light source passes through the optical fiber wavelength division multiplexer The second output is used to obtain the forward output signal of the three-wavelength laser, and the first output of the optical fiber wavelength division multiplexer is used to obtain the reverse output signal of the three-wavelength laser.
  • the grating has the ability to select multiple frequencies, single longitudinal mode, and high polarization degree at the same time;
  • the fiber laser resonator disclosed in the present invention has laser output with multiple wavelengths, high linear polarization, and single-frequency narrow linewidth, which can minimize the phase difference between the three-wavelength laser output signals generated in the same fiber laser resonator, Through the two-to-two beat-frequency difference technology between the three-wavelength laser output signals, the multi-frequency millimeter wave or terahertz wave output with a highly stable phase is realized;
  • the laser disclosed in the present invention can realize an all-fiber structure, has high mechanical stability, and its process is simple; less optical elements are required, the structure of the fiber laser can be greatly simplified, and its manufacturing cost is low, which is conducive to popularization and application.
  • A is a top view schematic diagram of a fiber Bragg grating prepared by the femtosecond laser direct writing method of the present invention
  • B is a cross-sectional schematic diagram of a fiber Bragg grating; Fiber Bragg gratings with small differences in refractive index;
  • Fig. 2 is a schematic structural diagram of a laser prepared by using the operation method of the present invention to generate high-phase-correlation millimeter-wave and terahertz-wave signals;
  • Fig. 3 is the spectrogram that the three-wavelength laser that embodiment 1 produces is embedded in the spectrum analyzer and obtains; Wherein the wavelength interval of three-wavelength laser output signal and adjacent laser signal is 0.028 nanometer;
  • Fig. 4 is the oscilloscope spectrogram that the three-wavelength laser produced in embodiment 1 is placed in the photoelectric probe and the scanning Fabry-Perot interferometer; In the free spectral range of 1.5GHz, the three-wavelength laser output signals are all single Longitudinal mode output;
  • Fig. 5 is an illustration of the degree of polarization of the three-wavelength laser obtained by placing the three-wavelength laser generated in Example 1 into a polarimeter; in the half-hour test time, the degree of polarization of the three-wavelength laser output signal was above 98%, and the root mean square shift Less than 0.5%; at the same time, the root mean square deviation of its laser output power is less than 0.1%;
  • Figure 6 is an example of the frequency domain spectrum obtained after the three-wavelength laser generated in Example 1 is placed in a photoelectric probe and an electronic frequency analyzer for beating; within the designed frequency range, three stable millimeter-wave frequencies can be seen (3.18, 3.22, 6.39GHz) existence;
  • Figure 7 is a partial enlarged example of a millimeter-wave signal at a frequency of 6.39GHz in the frequency domain spectrum obtained after beating in Figure 6; it can be seen that the main millimeter-wave peak centered at 6.3925GHz has a very thin 3dB linewidth ( ⁇ 500Hz); at the same time, because each beat frequency signal has extremely low frequency noise and phase noise, there are a plurality of comb teeth symmetrically distributed on both sides of the main peak, and the comb tooth interval presents a periodic structure, and the comb tooth distance is 100kHz , and the intensity of the sideband comb teeth is strong, taking the first sideband on both sides of the main peak as an example, its intensity is 10% of the intensity of the main peak.
  • Non-polarization maintaining gain fiber 1a, fiber cladding; 1b, fiber core; 2, focusing spot; 3, fiber core central axis; 4, the horizontal plane where the fiber core central axis is located; 5, linear grating surface; 6. The direction of laser line engraving; 7. The radial direction of the fiber core; 8. The 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.
  • fiber Bragg gratings are first written on both ends of the non-polarization-maintaining gain fiber 1 by femtosecond direct writing method to form a distributed Bragg reflection laser resonator, and the length of each fiber Bragg grating is between 0.1-10cm
  • the femtosecond laser focus spot 2 is located in the fiber core 1b, and the line engraving length range in the fiber core is 10-100% of the fiber core diameter; the projection and light of the laser line engraving direction on the horizontal plane where the central axis of the fiber core is located
  • the small angle 8 between the radial directions of the fiber cores ranges from 1 second to 10 degrees, and the small angle 9 between the laser line marking direction and the horizontal plane where the central axis of the fiber core is located ranges from 1 second to 10 degrees;
  • the distributed Bragg reflection laser resonator is pumped by the pump laser to produce three-wavelength high-degree of linear polarization and single-frequency narrow-linewidth laser output.
  • the frequency interval range of the three-wavelength laser output signal is 0.1GHz-10THz, and the laser signal
  • the polarization extinction ratio (the polarization extinction ratio is the output power ratio of the two orthogonal polarization directions of the laser output) > 10dB, and the 3dB line width of the single-frequency laser signal is less than 5kHz;
  • the multi-frequency millimeter wave or terahertz wave output with a highly stable phase is realized.
  • the millimeter wave or terahertz wave is a kind of electromagnetic wave; beat frequency generation
  • the frequency of the millimeter wave or terahertz wave is located at 0.1GHz-10THz, and its 3dB line width is less than 5kHz; on both sides of the main peak of the millimeter wave or terahertz wave generated by the beat frequency, the signal itself due to the beat frequency has high amplitude stability , high frequency stability, and high phase stability, the resulting three-frequency millimeter wave or terahertz wave signals each have the fine structure characteristics of frequency combs, that is, there are highly symmetrical and periodic arrays on both sides of the main frequency.
  • Frequency comb and the intensity of the comb teeth is close to the intensity of the central peak.
  • 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 period ranges from 1kHz-100MHz.
  • the light source used to write a uniform fiber Bragg grating is an 800-nanometer femtosecond laser (pulse width 80 femtoseconds, repetition rate 1 kHz), and the non-polarization-maintaining gain fiber 1 is an erbium-doped non-polarization-maintaining silica glass fiber , the fiber core 1b has a diameter of 4 microns.
  • the non-polarization maintaining gain fiber 1 includes an optical fiber core 1b and an optical fiber cladding 1a;
  • the femtosecond laser is focused on the inside of the fiber core 1b through the fiber cladding 1a from directly above the non-polarization maintaining gain fiber 1, and its focused spot 2 is positioned on the horizontal plane 4 where the central axis of the fiber core is located.
  • the focused spot 2 is from Starting from one side of the interface of the fiber core 1b-fiber cladding 1a, moving horizontally through the central axis 3 of the fiber core 3 to the other side for line engraving, forming a linear grating surface 5 in the fiber core;
  • Fine-tuning the angle between the projection of the laser line marking direction on the horizontal plane where the central axis of the fiber core is located and the radial direction of the fiber core is 8 to 1 minute, and the distribution of refractive index changes induced by femtosecond laser along the line marking direction presents a trident shape Refractive index distribution curve (n(r)); at the same time, fine-tune the small angle 9 between the laser line marking direction and the horizontal plane where the central axis of the fiber core is located, making the small peaks on both sides of the above-mentioned trident refractive index distribution curve
  • the line length is 4 microns, that is, the geometry of the fiber core 1b 100% of the diameter
  • the femtosecond laser focus spot 2 is translated by 1.1 microns along the central axis 3 of the fiber core.
  • the focus spot 2 writes the next line grating surface 5 in a direction 180 degrees opposite to the direction of the first line engraving, so that the line engraving is repeated under the control of the program, and finally the fiber Bragg grating 11 and the fiber Bragg grating 2 are realized.
  • 12 preset lengths (6.6 mm and 11 mm), so that the FBG obtained by one-time femtosecond direct writing line marking is actually equivalent to three sets of Bragg with the same period length and small difference in effective refractive index.
  • Fiber Bragg grating that is, three sets of Bragg fiber gratings with different Bragg wavelengths located in the same fiber laser cavity.
  • the geometric asymmetry formed in the horizontal and vertical directions of the fiber core constitutes the difference in the effective refractive index of the conduction transverse mode in the orthogonal direction.
  • the non-polarization-maintaining gain fiber Bragg grating introduces a strong birefringence difference, so that the grating has the function of selecting the frequency of the output laser and adding the function of selecting the polarization state of the laser, so that the generated laser signal has a very high polarization extinction ratio ;
  • the laser is an erbium-doped 1.59 micron all-fiber three-wavelength, linearly polarized, single-frequency narrow-linewidth fiber laser, which consists of a non-polarization-maintaining gain fiber 1 with a length of 49 mm and a Bragg fiber laser with a length of 6.6 mm.
  • Fiber Bragg grating one 11 and fiber Bragg grating two 12 with a length of 11 millimeters form a fiber laser resonator, and a maximum power of 1 watt and a 976-nanometer semiconductor laser with pigtails are used as a pumping light source 13, through an optical fiber wavelength division multiplexer
  • One 14 is coupled into one end of the fiber Bragg grating 11 of the fiber laser resonator, and the laser light generated in the pump light source 13 is output through the fiber wavelength division multiplexer 2 15 to obtain the forward output signal 16 of the three-wavelength laser, which is passed through the fiber wavelength division multiplexer.
  • Use device one 13a to output and obtain the three-wavelength laser reverse output signal 17 .
  • the difference frequency technology two-two beat frequencies are performed between the three-wavelength laser output signals to achieve a highly phase-stable three-frequency millimeter-wave or terahertz wave signal output, because the generated millimeter-wave or terahertz wave itself has high amplitude stability.
  • the generated three-frequency millimeter wave or terahertz wave signal has the characteristics of a frequency comb, that is, there are highly symmetrical and periodically arranged frequency comb teeth on both sides with its main frequency as the center, and Comb intensity and central peak ratio are close.
  • the realization method of the present invention for generating millimeter wave and terahertz wave signal sources with high phase correlation is low in manufacturing cost, can be seamlessly connected with an optical fiber network, and is conducive to popularization and application.

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Abstract

一种产生高相位相干度电磁波信号的激光器及方法，由非保偏增益光纤（1）和两端通过飞秒激光直写线刻方法刻写的一对布拉格光纤光栅（11、12）构成分布式反射光纤激光谐振腔；光纤光栅刻写中，激光线刻方向（6）在光纤纤芯中心轴线（3）所在水平面（4）上的投影和光纤纤芯径向方向（7）之间引入微小夹角（8），同时在激光线刻方向（6）和光纤纤芯中心轴线（3）所在水平面（4）之间引入微小夹角（9）；激光谐振腔通过泵浦光源（13）的泵浦，产生高线偏振、单频窄线宽、相位高度相关的三波长激光输出信号，最后输出信号之间拍频产生毫米波或太赫兹波输出。能够实现相位高度稳定的多频率毫米波或太赫兹波输出，其工艺简单，制造成本低，具有较高的机械稳定性。

Description

一种产生高相位相干度电磁波信号的激光器及方法 技术领域
本发明属于激光器技术领域，具体涉及一种产生高相位相干度电磁波信号的激光器及方法。
背景技术
已经到来的基于毫米波的5G技术和已经在着手研究的基于太赫兹波的6G技术将使无线通讯在数据速度比现有技术快几个数量级、而数据传输延迟时间则缩短几个数量级。但是，任何移动网络最终需要并入1.5μm低损耗光纤通信网络，因此，相对于其他技术，基于光纤的毫米波和太赫兹波发生器是非常有发展潜力的，这是因为光纤器件可以无缝地将本地无线网络和长途光纤网络进行衔接。
无线通信需要具有低相位噪声和宽频率覆盖范围的毫米波和太赫兹波源。光载毫米波和太赫兹波信号源可以通过激光差频技术产生，即以对两个具有一定频率差的激光器进行拍频。两个具有不同频率的光波可以表示为E i(t)＝E 0i·cos(ω it+Φ i)(i＝1或2)，其中E 0i表示振幅项，ω i表示角频率项，Φ i表示每个波的相位项。通过差频技术产生的位于在毫米波和太赫兹波的分量可以表示为A·cos[(ω 1-ω 2)t+(Φ 1-Φ 2)]给出，其中A是与E 0i相关的振幅。由此可以推导出：只有当两个波的相位高度相关时，即它们之间的相位差ΔΦ(＝Φ 1-Φ 2)尽可能地小时，差频所产生的毫米波和太赫兹波信号才会具有高振幅、频率和相位稳定性。由此理论上可以采用一个双波长、高线偏振度的、单频窄线宽光纤激光器，由于双波长激光是从同一个激光腔内产生的，两者具有较小的相位差，有利于实现具有高强度稳定性、高频率稳定性和高相位稳定性的毫米波和太赫兹波信号。
此前的双波长光纤激光器是通过啁啾分布式反馈光栅结构光纤激光器、叠印双波长布拉格光纤光栅光纤激光器、或双偏振态光纤激光器来实现的，产生的双波长激光之间的相位差依然不够小，不利于继续降低相位噪声以获得高强度稳定性、高频率稳定性和高相位稳定性的毫米波和太赫兹波信号；同时，要使得产生的毫米波和太赫兹波信号覆盖较宽的频率范围，多波长(譬如三波长)光纤激光器明显是更好的选择，但这对各个波长的激光信号的相位相关度提出了更高的要求，这一点目前国内外并无成功的报道。
发明内容
针对上述存在的技术不足，本发明的目的是提供一种产生高相位相干度电磁波信号的激光器及方法，输出具有多波长、高线偏振、单频窄线宽的激光，激光输出信号之间进行两两拍频的差频技术，实现相位高度稳定的多频率毫米波或太赫兹波输出，其工艺简单，制造成本低，具有较高的机械稳定性。
为解决上述技术问题，本发明采用如下技术方案：
本发明提供一种产生高相位相干度电磁波信号的方法，包括以下步骤：
S1：所述布拉格光纤光栅的飞秒激光直写线刻刻写方法为：
在非保偏增益光纤两端通过飞秒激光直写线刻刻写布拉格光纤光栅；
飞秒激光从非保偏增益光纤的正上方向下通过光纤包层被聚焦于光纤纤芯内部，其聚焦光斑被定位于光纤纤芯中心轴线所在水平面上，聚焦光斑从光纤纤芯和光纤包层的界面内的一侧开始、通过光纤纤芯中心轴线向另一侧进行水平移动扫描进行线刻，在纤芯内形成线型光栅面；
特别地，激光线刻方向在光纤纤芯中心轴线所在的水平面上的投影和通过光纤纤芯中心的径向方向之间引入微小夹角，使得在线型光栅面上激光诱导产生的折射率变化分布呈现三叉戟型折射率分布曲线(n(r))；
特别地，同时，在激光线刻方向和光纤纤芯中心轴线所在水平面之间引入微小夹角，使得上述的三叉戟式折射率分布曲线的两侧小峰之间折射率存在微 小差异而呈现微小的不对称性，即最后形成一个非对称三叉戟型折射率分布曲线(n’(r))；
其次，在完成一条线刻光栅面后，激光光斑沿光纤纤芯轴线方向水平移动平移一个周期；采用和上一条栅面的线刻方向反向180度的方向进行下一条线型栅面的刻写；
最终，多次重复线刻，最终刻写出具有预设长度的布拉格光纤光栅；
由于光栅面的折射率分布曲线是一个非对称三叉戟型(n’(r))，由单次线刻程序制备的一个布拉格光纤光栅相当于具有相同布拉格光栅周期、三个不同等效折射率的的三套布拉格光纤光栅；
再次，布拉格光纤光栅的预设长度和较短的有源光纤长度共同决定了光纤光栅光谱线宽内只存在一个窄线宽单纵模；
再次，由于飞秒激光线刻刻写的布拉格光纤光栅在横截面正交两个方向上的几何不对称性，最终的线刻制备的布拉格光纤光栅具有较大的双折射率差；
S2：由非保偏增益光纤两端通过飞秒激光直写线刻方法刻写的一对布拉格光纤光栅和构成分布式反射光纤激光谐振腔；
泵浦光源的泵浦光通过光纤波分复用器一进入由构成的光纤激光谐振腔，激光波长由布拉格光纤光栅波长决定的激光信号在光纤激光振荡腔内多次往返振荡实现激光输出；
从光纤激光谐振腔内的激光输出通过光纤波分复用器二，形成正向输出；
从光纤激光谐振腔内的激光输出通过光纤波分复用器一，形成反向输出；
特别地，由于步骤S1中飞秒激光线刻获得的一对光纤布拉格光栅中的一个具有非对称三叉戟型折射率分布曲线(n’(r))，上述由一套布拉格光纤光栅构成的分布式反射光纤激光谐振腔等效于带有三个波长不同的布拉格光纤光栅的光纤激光谐振腔，从而产生三个不同波长的激光信号；
同时，特别地，由于步骤S1中飞秒激光线刻获得的光纤布拉格光栅在两个正交方向上具有较大的双折射率差，激光信号在光纤激光谐振腔内多次振 荡，激光信号在两个正交偏振方向上存在模式竞争，结果导致极大的偏振消光比，激光信号输出为极高的线偏振度；
最终，由基于由步骤S1制备出的线刻布拉格光纤光栅的、根据步骤S2构成的光纤激光谐振腔，通过全光纤泵浦装置，实现高相位相关、高线偏度、单频窄线宽的三波长激光输出。
S3：由于步骤S3实现的三波长激光输出具有高线偏度、单频窄线宽，不同波长激光信号之间存在高度相位相关关系，通过三波长激光输出信号之间进行两两拍频的差频技术，实现相位高度稳定的多频率毫米波或太赫兹波输出。
特别地，由于步骤S3实现的三波长激光输出具有高线偏度、单频窄线宽，不同波长激光信号之间存在高度相位相关关系，通过三波长激光输出信号之间进行两两拍频的差频技术，实现相位高度稳定的多频率毫米波或太赫兹波输出在频域上存在周期性频率间隔、边带和主峰强度接近的精细结构。
优选地，所述布拉格光纤光栅的刻写方法为：
非保偏增益光纤包括光纤纤芯和光纤包层，飞秒激光从光纤正上方聚焦进入并聚焦于光纤纤芯内的光纤纤芯中心轴线所在水平面上，聚焦光斑沿光纤纤芯径向方向、从光纤纤芯和光纤包层的界面穿越光纤纤芯中心轴线作直线扫描并刻写出线型光栅栅面结构；特别地，在投影于光纤纤芯中心轴线所在的水平面上，激光线刻方向和通过光纤纤芯中心的径向方向之间引入的微小夹角；同时，特别地，激光线刻方向和光纤纤芯中心轴线所在水平面之间引入微小夹角；完成一个栅面刻写后，聚焦光斑沿光纤纤芯轴线方向平移一个布拉格光栅周期，重复进行下一个线型光栅面的线刻扫描；如此重复，直至该布拉格光纤光栅长度达到预设长度。
优选地，所述布拉格光纤光栅的长度在0.1-10cm之间；飞秒激光在光纤纤芯内的线刻长度范围为纤芯直径尺寸的10-100％；激光线刻方向在光纤纤芯中心轴线所在水平面上的投影和光纤纤芯径向方向之间的微小夹角范围在1秒-10度之间；同时，激光线刻方向和光纤纤芯中心轴线所在水平面之间微小夹 角范围在1秒-10度之间。
优选地，在泵浦激光的泵浦之下，产生三波长高线偏振度、单频窄线宽激光输出，三波长激光输出信号频率间隔范围0.1GHz-10THz、激光信号的偏振消光比>10dB、单频激光信号3dB线宽小于5kHz。
优选地，通过所述三波长激光输出信号之间进行两两拍频的差频技术，实现高度相位稳定的多频率毫米波或太赫兹波输出，拍频产生的毫米波或太赫兹波的频率位于0.1GHz-10THz、其3dB线宽小于5kHz。
优选地，在拍频产生的毫米波或太赫兹波的主峰两侧，产生的三频率毫米波或太赫兹波信号各自具有频率梳的精细结构特征，即以其主频率为中心两侧存在高度对称、周期排列的频率梳齿，且梳齿强度和中心峰强度接近。
优选地，所述毫米波或太赫兹波信号频率梳主峰两侧的第一个边带和所述主峰的强度比在0.1％-10％范围，所述频率梳齿周期范围为1kHz-100MHz。
本发明的另一个目的是提供一种产生高相位相关度电磁波信号的激光器，其特征在于，包括光纤激光谐振腔，所述光纤激光谐振腔由非保偏增益光纤及两端的布拉格光纤光栅一、和布拉格光纤光栅二构成；
还包括光纤波分复用器，所述光纤激光谐振腔前后侧分别安装有光纤波分复用器二和安装有光纤波分复用器一；
泵浦光源，所述泵浦光源通过所述光纤波分复用器一耦合进入光纤激光谐振腔的布拉格光纤光栅一一端；所述泵浦光源内产生的激光，通过光纤波分复用器二输出获得三波长激光前向输出信号，通过光纤波分复用器一输出获得三波长激光反向输出信号。
本发明的有益效果在于：
1.通过在传统飞秒激光直写线刻技术上引入两个角度量的作为额外的飞秒直写光纤光栅的空间调控自由度，从而使得在非保偏增益光纤上刻写出的光纤布拉格光栅同时具有多个频率、单纵模、和高偏振度的选择能力；
2.本发明公开的光纤激光谐振腔具有多波长、高线偏振、单频窄线宽的激 光输出，最大限度地降低同一光纤激光谐振腔中产生的三波长激光输出信号之间的相位差，通过三波长激光输出信号之间进行两两拍频的差频技术，实现相位高度稳定的多频率毫米波或太赫兹波输出；
3.本发明公开的激光器可以实现全光纤结构，具有较高的机械稳定性其工艺简单；所需光学元件少，可大幅度简化光纤激光器结构，其制造成本低，有利于推广应用。
附图说明
为了更清楚地说明本发明实施例或现有技术中的技术方案，下面将对实施例或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他的附图。
图1中A为本发明飞秒激光直写线刻方法制备的布拉格光纤光栅的俯视示意图,B为布拉格光纤光栅的横截面示意图；该光栅事实上等效于三套布拉格周期长度相同、而有效折射率则存在微小差异的布拉格光纤光栅；
图2为运用本发明产生高相位相关度毫米波和太赫兹波信号的运行方法制备的激光器结构示意图；
图3为实施例1产生的三波长激光置入光谱分析仪获得的光谱图；其中三波长激光输出信号和相邻激光信号的波长间隔为0.028纳米；
图4为实施例1产生的三波长激光置入光电探头和扫描式法布里-珀罗干涉仪中获得的示波器频谱图；在1.5GHz的自由光谱范围中，三波长激光输出信号均为单纵模输出；
图5为实施例1产生的三波长激光置入偏振计中获得三波长激光的偏振度例图；在半小时测试时间中，三波长激光输出信号的偏振度在98％以上，且方均根偏移小于0.5％；同时其激光输出功率的方均根偏移小于0.1％；
图6为实施例1产生的三波长激光置入光电探头和电子频率分析仪中进行拍频后获得的频域谱例图；在设计的频率范围内，可以看到稳定的三个毫米波频率(3.18、3.22、6.39GHz)的存在；
图7为图6中进行拍频后获得的频域谱中选取在频率6.39GHz的毫米波信号的局部放大例图；可以看到以6.3925GHz为中心的毫米波主峰具有极细的3dB线宽(<500Hz)；同时，由于各拍频信号都具有极低的频率噪声和相位噪声，在主峰两侧对称地分布着多个梳齿，其梳齿间隔呈现周期性结构，梳齿距离为100kHz,且边带梳齿的强度都较强，以主峰两侧第一个边带为例，其强度为主峰强度的10％。
附图标记说明：
1、非保偏增益光纤；1a、光纤包层；1b、光纤纤芯；2、聚焦光斑；3、光纤纤芯中心轴线；4、光纤纤芯中心轴线所在水平面；5、线型光栅面；6、激光线刻方向；7、光纤纤芯径向方向；8、激光线刻方向在光纤纤芯中心轴线所在水平面上的投影和光纤纤芯径向方向之间的微小夹角；9、激光线刻方向和光纤纤芯中心轴线所在水平面之间的微小夹角；10、布拉格光栅周期；11、布拉格光纤光栅一；12、布拉格光纤光栅二；13、泵浦光源；14、光纤波分复用器一；15、光纤波分复用器二；16、三波长激光前向输出信号；17、三波长激光反向输出信号。
具体实施方式
下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅仅是本发明一部分实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本发明保护的范围。
本发明中，首先在非保偏增益光纤1两端通过飞秒直写线刻的方法刻写布拉格光纤光栅，构成分布式布拉格反射激光谐振腔，每个布拉格光纤光栅的长 度在0.1-10cm之间；飞秒激光聚焦光斑2位于光纤纤芯1b内，在纤芯内的线刻长度范围为纤芯直径尺寸的10-100％；激光线刻方向在光纤纤芯中心轴线所在水平面上的投影和光纤纤芯径向方向之间的微小夹角8范围在1秒-10度之间，激光线刻方向和光纤纤芯中心轴线所在水平面之间的微小夹角9范围在1秒-10度之间；
其次,分布式布拉格反射激光谐振腔在泵浦激光的泵浦之下，产生三波长高线偏振度、单频窄线宽激光输出，三波长激光输出信号频率间隔范围0.1GHz-10THz、激光信号的偏振消光比(偏振消光比为激光输出两个正交偏振方向上的输出功率比值)>10dB、单频激光信号3dB线宽小于5kHz；
再次，通过三波长激光输出信号之间进行两两拍频的差频技术，实现相位高度稳定的多频率毫米波或太赫兹波输出,毫米波或太赫兹波属于电磁波的一种；拍频产生的毫米波或太赫兹波的频率位于0.1GHz-10THz、其3dB线宽小于5kHz；在拍频产生的毫米波或太赫兹波的主峰两侧，由于拍频产生的信号本身具有高幅度稳定性、高频率稳定性、和高相位稳定性，由此产生的三频率毫米波或太赫兹波信号各自具有频率梳的精细结构特征，即以其主频率为中心两侧存在高度对称、周期排列的频率梳齿，且梳齿强度和中心峰强度接近，具体的，主峰两侧的第一个边带和主峰的强度比在0.1％-10％范围，频率梳齿周期范围为1kHz-100MHz。
实施例1
一、飞秒激光直写线刻方法制备布拉格光纤光栅结构
如图1所示，用于刻写均匀布拉格光纤光栅的光源为800纳米飞秒激光(脉冲宽度80飞秒，重复频率1千赫兹)，非保偏增益光纤1为掺铒非保偏石英玻璃光纤，光纤纤芯1b直径为4微米。
非保偏增益光纤1包括光纤纤芯1b和光纤包层1a；
飞秒激光从非保偏增益光纤1的正上方向下通过光纤包层1a被聚焦于光纤纤芯1b内部，其聚焦光斑2被定位于光纤纤芯中心轴线所在水平面4上， 聚焦光斑2从光纤纤芯1b-光纤包层1a的界面内的一侧开始、通过光纤纤芯中心轴线3向另一侧进行水平移动扫描进行线刻，在纤芯内形成线型光栅面5；
微调激光线刻方向在光纤纤芯中心轴线所在水平面上的投影和光纤纤芯径向方向之间的微小夹角8为1分，沿线刻方向飞秒激光诱导产生的折射率变化分布呈现三叉戟型折射率分布曲线(n(r))；同时，微调激光线刻方向和光纤纤芯中心轴线所在水平面之间的微小夹角9为1分，使得上述的三叉戟式折射率分布曲线的两侧小峰之间折射率存在微小差异而呈现微小的不对称性，即最后形成一个非对称三叉戟型折射率分布曲线(n’(r))，线刻长度为4微米，即光纤纤芯1b的几何直径的100％；
完成第一条线型光栅面5后，飞秒激光聚焦光斑2沿光纤纤芯中心轴线3平移1.1微米，该距离为设计的飞秒激光直写线刻光纤布拉格光栅周期10，继而飞秒激光聚焦光斑2沿第一条线刻方向的反向180度的方向进行下一个线型光栅面5的刻写，如此在程序控制下反复进行线刻，最终实现布拉格光纤光栅一11和布拉格光纤光栅二12的预设长度(6.6毫米和11毫米)，这样通过一次性飞秒直写线刻获得的布拉格光纤光栅事实上等效于三套布拉格周期长度相同、而有效折射率则存在微小差异的布拉格光纤光栅，即这是位于同一个光纤激光谐振腔内的三套具有不同布拉格波长的布拉格光纤光栅。
其次，由于飞秒直写线刻方法的特点，在光纤纤芯水平和垂直两个正交方向上形成的几何不对称性，构成了传导横模正交方向上的有效折射率的差异，这使得非保偏增益光纤布拉格光栅引入了强双折射率差，使光栅具有对输出激光的频率选择功能之外增加了对激光偏振态的选择功能，这样产生的激光信号具有极高的偏振消光比；
再次，由于较长的布拉格光纤光栅预设长度和较短的有源光纤长度的共同作用，最终光纤光栅光谱线宽内只允许存在一个窄线宽单纵模。
二、产生高相位相关度毫米波和太赫兹波信号激光器
如图2所示，该激光器为掺铒的1.59微米全光纤三波长、线偏振、单频 窄线宽光纤激光器，包括由长度为49毫米的非保偏增益光纤1、长度为6.6毫米的布拉格光纤光栅一11、和长度为11毫米的布拉格光纤光栅二12构成光纤激光谐振腔，一个最大功率为1瓦、带尾纤的976纳米半导体激光器作为泵浦光源13，通过光纤波分复用器一14耦合进入光纤激光谐振腔的布拉格光纤光栅一11一端，泵浦光源13内产生的激光，通过光纤波分复用器二15输出获得三波长激光前向输出信号16，通过光纤波分复用器一13a输出获得三波长激光反向输出信号17。
三、输出信号的检测和分析
将三波长激光前向输出信号16或三波长激光反向输出信号17置入功率计、光谱分析仪、光电检测探头和电子频率分析仪等仪器设备进行功率、光谱、频率、偏振性等性能的检测和分析,检测和分析结果如图3～图7。
数据表明通过以上实行方法，在由非保偏的增益光纤1和其两端一对布拉格光纤光栅构建的一个光纤激光谐振腔中，泵浦产生具有高线偏振度、单频窄线宽的三个波长的激光输出，共享同一激光腔、高线偏振度和单频窄线宽等性能指标说明三个波长的激光信号之间具有极小的相位差，即三波长激光输出信号的相位高度相关。
最后通过差频技术通过三波长激光输出信号之间进行两两拍频，实现高度相位稳定的三频率毫米波或太赫兹波信号输出，由于产生的毫米波或太赫兹波本身具有高幅度稳定性、高频率稳定性、和高相位稳定性，产生的三频率毫米波或太赫兹波信号具有频率梳的特征，即以其主频率为中心两侧存在高度对称、周期排列的频率梳齿，且梳齿强度和中心峰值比例接近。
本发明的产生高相位相关度毫米波和太赫兹波信号源的实现方法制造成本低，可以和光纤网络进行无缝衔接，有利于推广应用。
显然，本领域的技术人员可以对本发明进行各种改动和变型而不脱离本发明的精神和范围。这样，倘若本发明的这些修改和变型属于本发明权利要求及其等同技术的范围之内，则本发明也意图包含这些改动和变型在内。

Claims (7)

  1. 一种产生高相位相干度电磁波信号的方法，其特征在于，
    S1：布拉格光纤光栅的飞秒激光直写线刻刻写方法为：
    飞秒激光从非保偏增益光纤(1)的正上方向下通过光纤包层(1a)被聚焦于光纤纤芯(1b)内部，其聚焦光斑(2)被定位于光纤纤芯中心轴线(3)所在的水平面(4)上，聚焦光斑(2)从纤芯(1a)和包层(1b)的界面内的一侧开始、通过光纤纤芯中心轴线(3)向另一侧进行水平移动扫描进行线刻，在纤芯内形成线型光栅面(5)；
    在所述激光线刻方向(6)在光纤纤芯中心轴线所在水平面(4)上的投影和光纤纤芯径向方向(7)之间引入微小夹角，使得在线型光栅面(5)上激光诱导产生的折射率变化分布呈现三叉戟型折射率分布曲线(n(r))；
    同时，在所述激光线刻方向(6)和所述光纤纤芯中心轴线所在水平面(4)之间引入微小夹角，使得上述的三叉戟式折射率分布曲线的两侧小峰之间折射率存在微小差异而呈现微小的不对称性，即最后形成一个非对称三叉戟型折射率分布曲线(n’(r))；
    在完成一条线型光栅面(5)后，所述聚焦光斑(2)沿光纤纤芯轴线方向水平移动平移一个布拉格光栅周期(10)；采用和上一条栅面的线刻方向反向进行下一条线型光栅面(5)的刻写；
    多次重复线刻，最终刻写出具有预设长度的所述布拉格光纤光栅；
    S2：在非保偏增益光纤(1)两端通过飞秒激光直写线刻刻写所述布拉格光纤光栅，构成分布式光纤激光谐振腔；
    泵浦光源(13)的泵浦光通过光纤波分复用器一(14)进入所述光纤激光谐振腔，激光波长由布拉格光纤光栅波长决定的激光信号在光纤激光振荡腔内多次往返振荡实现激光输出；
    从光纤激光谐振腔内的激光输出通过光纤波分复用器二(15)，形成正向 输出(16)；
    从光纤激光谐振腔内的激光输出通过光纤波分复用器一(14)，形成反向输出(17)；
    最终，由基于由步骤S1制备出的线刻布拉格光纤光栅的、根据步骤S2构成的光纤激光谐振腔，通过全光纤泵浦装置，实现高相位相关、高线偏度、单频窄线宽的三波长激光输出；
    S3：由于步骤S2实现的三波长激光输出具有高线偏度、单频窄线宽，不同波长激光信号之间存在高度相位相关关系，通过三波长激光输出信号之间进行两两拍频的差频技术，实现相位高度稳定的多频率毫米波或太赫兹波输出，并在频域上存在周期性频率间隔、边带和主峰强度接近的精细结构。
  2. 根据权利要求1所述一种产生高相位相干度电磁波信号的方法，其特征在于，所述布拉格光纤光栅的长度在0.1-10cm之间；飞秒激光在光纤纤芯(1b)内的线刻长度范围为纤芯直径尺寸的10-100％；激光线刻方向在光纤纤芯中心轴线所在水平面上的投影和光纤纤芯径向方向之间的微小夹角(8)范围在1秒-10度之间，激光线刻方向和光纤纤芯中心轴线所在水平面之间的微小夹角(9)范围在1秒-10度之间。
  3. 根据权利要求2所述一种产生高相位相干度电磁波信号的方法，其特征在于，在所述泵浦激光(12)的泵浦之下，产生三波长高线偏振度、单频窄线宽激光输出，三波长激光输出信号频率间隔范围0.1GHz-10THz、激光信号的偏振消光比>10dB、单频激光信号3dB线宽小于5kHz。
  4. 根据权利要求3所述一种产生高相位相干度电磁波信号的方法，其特征在于，通过所述三波长激光输出信号之间进行两两拍频的差频技术，实现高度相位稳定的多频率毫米波或太赫兹波输出，拍频产生的毫米波或太赫兹波的频率位于0.1GHz-10THz、其3dB线宽小于5kHz。
  5. 根据权利要求4所述一种产生高相位相干度电磁波信号的方法，其特征在于，在拍频产生的毫米波或太赫兹波的主峰两侧，产生的三频率毫米波或太 赫兹波信号各自具有频率梳的精细结构特征，即以其主频率为中心两侧存在高度对称、周期排列的频率梳齿，且梳齿强度和中心峰强度接近。
  6. 根据权利要求5所述一种产生高相位相干度电磁波信号的方法，其特征在于，所述主峰两侧的第一个边带和所述主峰的强度比在0.1％-10％范围，所述频率梳齿周期范围为1kHz-100MHz。
  7. 一种应用权利要求1所述一种产生高相位相干度电磁波信号的方法的激光器，其特征在于，包括光纤激光谐振腔，所述光纤激光谐振腔由非保偏增益光纤(1)及两端的布拉格光纤光栅一(11)、和布拉格光纤光栅二(12)构成；
    还包括光纤波分复用器，所述光纤激光谐振腔前后侧分别安装有光纤波分复用器二(15)和安装有光纤波分复用器一(14)；
    泵浦光源(13)，所述泵浦光源(13)通过所述光纤波分复用器一(14)耦合进入光纤激光谐振腔的布拉格光纤光栅一(11)一端；所述泵浦光源(13)内产生的激光，通过光纤波分复用器二(15)输出获得三波长激光前向输出信号(16)，通过光纤波分复用器一(14)输出获得三波长激光反向输出信号(17)。
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