Coherent synthesis of bichromatic electro-optic modulated optical frequency comb

Abstract

This invention provides a coherently synthesized dual-color electro-optic modulated optical frequency comb, relating to the field of optical frequency comb technology. It includes an optical module and an electrical module. The optical module comprises a laser emitting unit, an electro-optic modulation unit, and a beat frequency unit. The laser emitting unit includes two continuous-wave lasers and a wavelength division multiplexer (WDM). The WDM combines two lasers with different center wavelengths. The electro-optic modulation unit modulates the combined laser beam to generate two electro-optic modulated optical frequency combs. The beat frequency unit detects the beat frequency signals of the two optical frequency combs. The electrical module includes an electronic control unit, a mixer, and a proportional-integral (PI) controller. The electronic control unit drives the electro-optic modulation unit through a drive unit. The mixer mixes a preset signal and the beat frequency signal. The PI controller adjusts the center wavelength of the second continuous-wave laser to achieve coherent synthesis of the electro-optic modulated optical frequency comb. The aforementioned optical frequency comb has a simple structure, a wide spectral range, and low comb noise.

Description

Technical Field

[0001] This invention relates to the field of optical frequency comb technology, and in particular to a coherently synthesized dual-color electro-optic modulated optical frequency comb. Background Technology

[0002] Optical frequency combs have played a crucial role in promoting scientific research in fields such as terahertz optical frequency comb generation, ultrastable optical microwave generation, time-frequency transfer, frequency conversion, laser weapons, spectroscopy, length measurement, and astronomy.

[0003] Currently, there are three main methods for generating optical frequency combs: mode-locked lasers, micro-ring resonator optical frequency combs, and electro-optic modulation optical frequency combs. The repetition frequency of a mode-locked laser is inversely proportional to the laser cavity length, and its repetition frequency is mostly in the range of MHz to 1 GHz. The physical length of the laser resonator is difficult to shorten further, thus making it difficult to further increase the repetition frequency of mode-locked lasers. Micro-ring resonator optical frequency combs can reach repetition frequencies in the hundreds of GHz or even THz range, but they suffer from large timing jitter and poor stability. Electro-optic modulation optical frequency combs, on the other hand, have a very simple and reliable structure, high comb power, and repetition frequencies in the range of MHz to tens of GHz, with a large range of continuous repetition frequency adjustment. However, the spectral coverage of electro-optic modulation optical frequency combs is limited by the half-wave voltage and driving power of the electro-optic phase modulator, typically less than ten nanometers. Furthermore, the accumulated phase noise of the comb teeth gradually increases with the increase of the comb tooth number, and this accumulated phase noise mainly manifests as high-frequency noise in the comb teeth, which is difficult to suppress using phase-locked loop technology. Simultaneously, the accumulated phase noise limits the generation of broadband, low-noise electro-optic modulation optical frequency combs. Summary of the Invention

[0004] To address the shortcomings of the existing technologies, this invention provides a coherently synthesized dual-color electro-optic modulation optical frequency comb, which solves the problems of narrow spectral range and high comb noise in the existing optical frequency combs.

[0005] This invention provides a coherent synthesized dual-color electro-optic modulated optical frequency comb, comprising an optical module and an electrical module;

[0006] The optical module includes a laser emitting unit, an electro-optic modulation unit, an optical amplification unit, and a beat frequency unit. The laser emitting unit includes a first continuous-wave laser, a second continuous-wave laser, and a first wavelength division multiplexer. The outputs of the first and second continuous-wave lasers are respectively connected to the input of the first wavelength division multiplexer. The first wavelength division multiplexer is used to combine the lasers emitted by the first and second continuous-wave lasers. The output of the first wavelength division multiplexer is connected to the input of the electro-optic modulation unit. The electro-optic modulation unit is used to perform intensity modulation and phase modulation on the combined laser emitted by the laser emitting unit to generate two electro-optic modulation optical frequency combs. The output of the electro-optic modulation unit is connected to the input of the optical amplification unit. The first output of the optical amplification unit outputs a coherently synthesized two-color electro-optic modulation optical frequency comb. The second output of the optical amplification unit is connected to the beat frequency unit. The beat frequency unit is used to detect the beat frequency signals of the two electro-optic modulation optical frequency combs. The center wavelengths of the lasers emitted by the first and second continuous-wave lasers are different.

[0007] The electrical module includes an electronic control unit, a drive unit, a mixer, and a proportional-integral controller. The first output terminal of the electronic control unit is connected to the drive unit and is used to output a first preset signal to drive the electro-optic modulation unit. The first input terminal of the mixer is connected to the output terminal of the beat frequency unit, the second input terminal of the mixer is connected to the second output terminal of the electronic control unit, the output terminal of the mixer is connected to the input terminal of the proportional-integral controller, and the output terminal of the proportional-integral controller is connected to the second continuous wave laser. The mixer is used to mix the second preset signal output by the electronic control unit and the beat frequency signal to generate an error signal. The proportional-integral controller adjusts the center wavelength of the second continuous wave laser based on the error signal to achieve coherent synthesis of the electro-optic modulation optical frequency comb.

[0008] Preferably, the electro-optic modulation unit includes a cascaded electro-optic intensity modulator and a cascaded electro-optic phase modulator; the optical amplification unit includes an optical fiber amplifier and an optical fiber beam splitter.

[0009] The output of the laser emitting unit is connected to the input of the cascaded electro-optic intensity modulator. The output of the cascaded electro-optic intensity modulator is connected to the input of the cascaded electro-optic phase modulator. The output of the cascaded electro-optic phase modulator is connected to the input of the fiber amplifier. The output of the fiber amplifier is connected to the input of the fiber beam splitter. The first output of the fiber beam splitter is connected to the output of the coherent synthesized two-color electro-optic modulation optical frequency comb, and is used to output the coherent synthesized two-color electro-optic modulation optical frequency comb. The second output of the fiber beam splitter is connected to the input of the beat frequency unit.

[0010] Preferably, the driving unit includes an electric amplifier, a phase shifter, and a power divider; the electronic control unit includes a first signal generator, a radio frequency reference, and a second signal generator.

[0011] The output terminal of the first signal generator is connected to the input terminal of the power divider to send a first preset signal to the power divider based on the radio frequency reference;

[0012] The output of the power divider is connected to the cascaded electro-optic intensity modulator in sequence via the phase shifter and the electrical amplifier on a branch.

[0013] The output of the power divider is connected to the cascaded electro-optic phase modulator in another branch via the phase shifter and the electrical amplifier in sequence;

[0014] The power divider is used to split the first preset signal, the phase shifter is used to keep the phase of the first preset signal received by the cascaded electro-optic intensity modulator and the cascaded electro-optic phase modulator consistent, and the electrical amplifier is used to amplify the first preset signal.

[0015] The output of the second signal generator is connected to the second input of the mixer to send a second preset signal to the mixer based on the preset radio frequency reference.

[0016] A low-pass filter is also connected between the output of the mixer and the input of the proportional-integral controller.

[0017] Preferably, the beat frequency unit includes a bandpass filter and a photodetector;

[0018] The second output terminal of the optical amplification unit is connected to the input terminal of the bandpass filter, the output terminal of the bandpass filter is connected to the input terminal of the photodetector, and the input terminal of the photodetector is connected to the first input terminal of the mixer.

[0019] The bandpass filter is used to filter out the spectrum of the overlapping portion of the two electro-optic modulated optical frequency combs, and the photodetector is used to detect the beat frequency signal of the two electro-optic modulated optical frequency combs and output it to the mixer.

[0020] Preferably, the beat frequency unit further includes a high-power fiber amplifier, a pulse compression device, and a highly nonlinear optical device;

[0021] The second output terminal of the optical amplification unit is connected to the input terminal of the high-power fiber amplifier, the output terminal of the high-power fiber amplifier is connected to the input terminal of the pulse compression device, the output terminal of the pulse compression device is connected to the input terminal of the highly nonlinear optical device, and the output terminal of the highly nonlinear optical device is connected to the input terminal of the photodetector.

[0022] Preferably, the pulse compression device includes a second wavelength division multiplexer, a first single-mode fiber, a second single-mode fiber, and a third wavelength division multiplexer;

[0023] The output of the high-power fiber amplifier is connected to the input of the second wavelength division multiplexer. The output of the second wavelength division multiplexer is connected to the inputs of the first single-mode fiber and the second single-mode fiber, respectively. The outputs of the first single-mode fiber and the second single-mode fiber are connected to the input of the third wavelength division multiplexer, and the output of the third wavelength division multiplexer is connected to the input of the highly nonlinear optical device.

[0024] The parameters of the second and third wavelength division multiplexers are the same as those of the first wavelength division multiplexer.

[0025] Preferably, the pulse compression device further includes a third single-mode fiber and a fourth single-mode fiber;

[0026] The output of the high-power fiber amplifier is connected to the input of the second wavelength division multiplexer. The output of the second wavelength division multiplexer is connected to the input of the third single-mode fiber and the first input of the third wavelength division multiplexer. The output of the third single-mode fiber is connected to the second input of the third wavelength division multiplexer. The output of the third wavelength division multiplexer is connected to the input of the fourth single-mode fiber. The output of the fourth single-mode fiber is connected to the input of the highly nonlinear optical device.

[0027] The length of the third single-mode fiber is the length difference between the first single-mode fiber and the second single-mode fiber, and the length of the fourth single-mode fiber is equal to the length of the shorter single-mode fiber between the first single-mode fiber and the second single-mode fiber.

[0028] Preferably, the pulse compression device includes a second wavelength division multiplexer, a first fiber chirped Bragg grating, a first fiber circulator, a second fiber circulator, a second fiber chirped Bragg grating, and a third wavelength division multiplexer;

[0029] The output of the high-power fiber amplifier is connected to the input of the second wavelength division multiplexer. The output of the second wavelength division multiplexer is connected to the first fiber circulator and the second fiber circulator, respectively. The first fiber circulator and the second fiber circulator are also connected to the input of the third wavelength division multiplexer, and the output of the third wavelength division multiplexer is connected to the input of the highly nonlinear optical device.

[0030] The first fiber circulator is connected to the first fiber chirped Bragg grating, and the second fiber circulator is connected to the second fiber chirped Bragg grating.

[0031] Both the first and second fiber chirped Bragg gratings are reflective gratings.

[0032] Preferably, the laser emitting unit is provided with N continuous wave lasers, including the first continuous wave laser and the second continuous wave laser, and the number of proportional-integral controllers is N-1, where N is a positive integer greater than or equal to 3;

[0033] The output terminals of the N continuous wave lasers are respectively connected to the input terminals of the first wavelength division multiplexer, and the N-1 proportional-integral controllers are respectively connected to the N-1 continuous wave lasers other than the first continuous wave laser.

[0034] Preferably, the coherently synthesized dual-color electro-optic modulated optical frequency comb further includes a terahertz detector;

[0035] The terahertz detector is connected to the first output terminal of the optical amplification unit, and the terahertz detector is used to output a terahertz optical frequency comb.

[0036] The coherently synthesized two-color electro-optic modulated optical frequency comb provided by this invention combines two laser beams with different center wavelengths emitted by a first continuous-wave laser and a second continuous-wave laser. Intensity and phase modulation are then applied to obtain two electro-optic modulated optical frequency combs with the same bandwidth but different center wavelengths. The beat frequency signals of the two electro-optic modulated optical frequency combs are detected using a beat frequency unit, and the signals undergo mixing in a mixer. A proportional-integral controller is used to fine-tune the center wavelength of the second continuous-wave laser, thereby achieving coherent synthesis of the two electro-optic modulated optical frequency combs. Finally, a coherently synthesized two-color electro-optic modulated optical frequency comb is generated and output, with a spectral coverage of [missing information]. This coherent synthesized dual-color electro-optic modulation optical frequency comb achieves twice the frequency of a single electro-optic modulation optical frequency comb, overcoming the narrow spectral coverage limitation of traditional electro-optic modulation optical frequency combs. Furthermore, by increasing the number of continuous-wave lasers, the spectral coverage can be further expanded. The coherent synthesized dual-color electro-optic modulation optical frequency comb contains two 0-tooth combs, with the comb number of the remaining combs determined by the closer 0-tooth comb. Therefore, compared to traditional electro-optic modulation optical frequency combs, the introduction of the second 0-tooth comb reduces the comb number of its neighboring combs. Since accumulated phase noise is linearly positively correlated with the absolute value of the comb number, passively reducing the comb number can suppress accumulated phase noise. The aforementioned coherent synthesized dual-color electro-optic modulation optical frequency comb is simple in structure, easy to implement, has a wide spectral range, and low comb noise.

[0037] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings.

[0038] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0039] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0040] Figure 1 A schematic diagram of the connection relationship of the coherent synthesis dual-color electro-optic modulation optical frequency comb is provided in one embodiment of this application;

[0041] Figure 2 A schematic diagram of the structure of a coherently synthesized dual-color electro-optic modulation optical frequency comb is provided in one embodiment of this application;

[0042] Figure 3 A schematic diagram of the structure of the coherent synthesis dual-color electro-optic modulation optical frequency comb provided in another embodiment of this application;

[0043] Figure 4 Three schematic diagrams of the pulse compression device are provided in one embodiment of this application;

[0044] Figure 5 A schematic diagram illustrating the principle of a coherently synthesized dual-color electro-optic modulation optical frequency comb with low noise characteristics in one embodiment provided in this application;

[0045] Figure 6 A schematic diagram showing a comparison of the phase noise measurement results of the comb teeth of a coherently synthesized two-color electro-optic modulated optical frequency comb compared with that of a conventional electro-optic comb in one embodiment provided in this application;

[0046] Figure 7 Experimental results of the output spectrum of a coherently synthesized two-color electro-optic modulated optical frequency comb in one embodiment of this application;

[0047] Figure 8 A schematic diagram illustrating the principle of a coherently synthesized dual-color electro-optic modulation optical frequency comb in one embodiment provided in this application;

[0048] Figure 9 A schematic diagram of the principle and structure of a coherently synthesized two-color electro-optic modulated optical frequency comb applied to the generation of a terahertz optical frequency comb is provided in one embodiment of this application.

[0049] In the picture:

[0050] 1. First continuous-wave laser; 2. Second continuous-wave laser; 3. First wavelength division multiplexer; 4. Cascaded electro-optic intensity modulator; 5. Cascaded electro-optic phase modulator; 6. Fiber amplifier; 7. Fiber beam splitter; 8. Output terminal of coherently synthesized two-color electro-optic modulation optical frequency comb; 9. Bandpass filter; 10. Photodetector; 11. Proportional-integral controller; 12. Low-pass filter; 13. Electrical amplifier; 14. Phase shifter; 15. Power divider; 16. First signal generator; 17. Radio frequency reference; 18. Second signal generator; 19. Mixer 20. High-power fiber amplifier; 21. Pulse compression device; 22. Highly nonlinear optical device; 23. Terahertz detector; 24. Terahertz optical frequency comb; 2101. Second wavelength division multiplexer; 2102. First single-mode fiber; 2103. Second single-mode fiber; 2104. Third wavelength division multiplexer; 2105. Third single-mode fiber; 2106. Fourth single-mode fiber; 2107. First fiber chirped Bragg grating; 2108. First fiber circulator; 2109. Second fiber circulator; 21010. Second fiber chirped Bragg grating;

[0051] CW, continuous wave laser; IM, electro-optic intensity modulator; PM, electro-optic phase modulator; Rb, preset radio frequency reference; PI servo, proportional-integral controller. Detailed Implementation

[0052] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0053] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0054] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0055] This invention provides a coherently synthesized dual-color electro-optic modulated optical frequency comb, such as... Figure 1As shown, the system includes an optical module and an electrical module. The optical module includes a laser emitting unit, an electro-optic modulation unit, an optical amplification unit, and a beat frequency unit. The laser emitting unit includes a first continuous-wave laser 1, a second continuous-wave laser 2, and a first wavelength division multiplexer 3. The outputs of the first and second continuous-wave lasers 1 and 2 are respectively connected to the input of the first wavelength division multiplexer 3. The first wavelength division multiplexer 3 is used to combine the lasers emitted by the first and second continuous-wave lasers 1 and 2. The output of the first wavelength division multiplexer 3 is connected to the input of the electro-optic modulation unit. The electro-optic modulation unit is used to perform intensity and phase modulation on the combined laser beam emitted by the laser emitting unit to generate two electro-optic modulation optical frequency combs. The output of the electro-optic modulation unit is connected to the input of the optical amplification unit. The first output of the optical amplification unit outputs a coherently synthesized two-color electro-optic modulation optical frequency comb. The second output of the optical amplification unit is connected to the beat frequency unit. This module is used to detect the beat frequency signals of two electro-optic modulated optical frequency combs, wherein the center wavelengths of the lasers emitted by the first continuous wave laser 1 and the second continuous wave laser 2 are different. The electrical module includes an electronic control unit, a drive unit, a mixer 19, and a proportional-integral controller 11. The first output terminal of the electronic control unit is connected to the electro-optic modulation unit through the drive unit and is used to output a first preset signal to drive the electro-optic modulation unit. The first input terminal of the mixer 19 is connected to the output terminal of the beat frequency unit, the second input terminal of the mixer 19 is connected to the output terminal of the electronic control unit, the output terminal of the mixer 19 is connected to the input terminal of the proportional-integral controller 11, and the output terminal of the proportional-integral controller 11 is connected to the second continuous wave laser 2. The mixer 19 is used to mix the second preset signal output by the electronic control unit and the beat frequency signal to generate an error signal. The proportional-integral controller 11 adjusts the center wavelength of the second continuous wave laser 2 based on the error signal to realize the coherent synthesis of the electro-optic modulated optical frequency comb.

[0056] The coherently synthesized two-color electro-optic modulated optical frequency comb provided by this invention combines two laser beams with different center wavelengths emitted by a first continuous-wave laser 1 and a second continuous-wave laser 2. Intensity and phase modulation are then applied to obtain two electro-optic modulated optical frequency combs with the same bandwidth but different center wavelengths. The beat frequency signals of the two electro-optic modulated optical frequency combs are detected using a beat frequency unit. The signal undergoes mixing in a mixer 19 and fine-tuning of the center wavelength of the second continuous-wave laser 2 using a proportional-integral controller 11, thereby achieving coherent synthesis of the two electro-optic modulated optical frequency combs. Finally, a coherently synthesized two-color electro-optic modulated optical frequency comb is generated and output, and its spectral coverage is [not specified]. The coverage range can reach twice that of a single electro-optic modulation optical frequency comb, overcoming the narrow spectral coverage limitation of traditional electro-optic modulation optical frequency combs. Furthermore, by increasing the number of continuous-wave lasers, the spectral coverage can be further expanded. The coherently synthesized dual-color electro-optic modulation optical frequency comb contains two 0-tooth combs, with the comb number of the remaining combs determined by the closer 0-tooth comb. Therefore, compared to traditional electro-optic modulation optical frequency combs, the introduction of the second 0-tooth comb reduces the comb number of its neighboring combs. Since accumulated phase noise is linearly positively correlated with the absolute value of the comb number, the method of passively reducing the comb number can suppress accumulated phase noise. The aforementioned coherently synthesized dual-color electro-optic modulation optical frequency comb has a simple structure, is easy to implement, has a wide spectral range, and low comb noise.

[0057] The first embodiment of the present invention is as follows: Figure 2As shown, the coherent synthesized dual-color electro-optic modulated optical frequency comb includes an optical module and an electrical module. The optical module includes a first continuous-wave laser 1, a second continuous-wave laser 2, a first wavelength division multiplexer 3, a cascaded electro-optic intensity modulator 4, a cascaded electro-optic phase modulator 5, an optical fiber amplifier 6, an optical fiber beam splitter 7, an output terminal 8 of the coherent synthesized dual-color electro-optic modulated optical frequency comb, a bandpass filter 9, and a photodetector 10. The electrical module includes a proportional-integral controller 11, a low-pass filter 12, an electrical amplifier 13, a phase shifter 14, a power divider 15, a first signal generator 16, an RF frequency reference 17, a second signal generator 18, and a mixer 19. In this system, the first continuous-wave laser 1 and the second continuous-wave laser 2 emit lasers with different center wavelengths. The first wavelength division multiplexer 3 is used to combine the lasers emitted by the first continuous-wave laser 1 and the second continuous-wave laser 2. The combined laser is then modulated by a cascaded electro-optic intensity modulator 4 and a cascaded electro-optic phase modulator 5. Simultaneously, in the electrical module, the first signal generator 16 outputs a first preset signal, which is split by the power divider 15 and then sequentially passed through a phase shifter 14 and an electrical amplifier 13 to drive the cascaded electro-optic intensity modulator 4 and the cascaded electro-optic phase modulator 5, respectively. The phase shifter 14 is used to ensure the phase synchronization of the first preset signal between the cascaded electro-optic intensity modulator 4 and the cascaded electro-optic phase modulator 5. The first preset signal is the driving signal for the cascaded electro-optic intensity modulator 4 and the cascaded electro-optic phase modulator 5. The electrical amplifier 13 is used to amplify the first preset signal. The specific number of the cascaded electro-optic intensity modulator 4 and the cascaded electro-optic phase modulator 5 can be set according to the user's needs. Based on the time-frequency mapping principle, the spectral shape of an electro-optic modulated optical frequency comb is closely related to its pulse envelope shape. The pulse envelope shape is determined by the drive signal, bias voltage, and number of electro-optic intensity modulators. Therefore, the number of electro-optic intensity modulators and their drive signals and bias voltages can be set according to actual application requirements. Furthermore, the spectral width of the electro-optic modulated optical frequency comb is determined by the electro-optic phase modulator. Generally, the more electro-optic phase modulators, the lower the half-wave voltage, and the higher the RF drive signal power, the more teeth the electro-optic modulated optical frequency comb will have, resulting in a wider spectral width. Therefore, the number of electro-optic phase modulators and their modulation parameters can be set according to the user's actual needs.

[0058] In the optical module, fiber amplifier 6 amplifies small optical signals, and fiber beam splitter 7 splits the optical signals, with one part used for coherent combining of electro-optic modulated optical frequency combs and the other part used as the system output. Since the lasers emitted by the first continuous-wave laser 1 and the second continuous-wave laser 2 have different center wavelengths, after being modulated by cascaded electro-optic intensity modulator 4 and cascaded electro-optic phase modulator 5, they will generate two electro-optic modulated optical frequency combs with different center wavelengths. Because the difference in center wavelength between the two continuous-wave lasers is less than the spectral width of a single electro-optic modulated optical frequency comb, the spectral portions of the two electro-optic modulated optical frequency combs overlap. The overlapping spectrum is filtered out by bandpass filter 9 and then detected by photodetector 10. From the radio frequency domain perspective, the signal detected by photodetector 10 is the beat frequency signal of the two electro-optic modulated optical frequency combs. This beat frequency signal is mixed with the second preset signal output by the second signal generator 18 at mixer 19. The first preset signal emitted by the first signal generator 16... The second preset signal emitted by the signal generator and the second signal generator 18 are both referenced to the preset radio frequency reference 17. The error signal after mixing is input to the proportional-integral controller 11 after passing through the low-pass filter 12. The output terminal of the proportional-integral controller 11 is connected to the current pin of the second continuous wave laser 2 to fine-tune the center wavelength of the second continuous wave laser 2. Then, through this phase-locked loop, the beat frequency signal between the two electro-optic modulated optical frequency combs is locked, indicating that the two electro-optic modulated optical frequency combs have achieved coherent synthesis. The spectral width of the finally output coherently synthesized dual-color electro-optic modulated optical frequency comb can be up to twice that of a single electro-optic modulated optical frequency comb. That is, compared with a single electro-optic modulated optical frequency comb, the coherently synthesized dual-color electro-optic modulated optical frequency comb provided in this application has doubled the spectral width.

[0059] In this embodiment, the spectral width of the coherently synthesized two-color electro-optic modulation optical frequency comb is determined by the half-wave voltage, number, and driving signal power of the electro-optic phase modulators. For example, if three electro-optic phase modulators are cascaded, and each electro-optic phase modulator has a half-wave voltage of 3V, a driving signal power of 30dBm, and a driving signal frequency of 10GHz, then the spectral width of a single electro-optic modulation optical frequency comb is approximately 4.8nm. Accordingly, the maximum spectral width of the coherently synthesized two-color electro-optic modulation optical frequency comb can be 9.6nm.

[0060] To further extend the spectral width, this application provides another coherently synthesized two-color electro-optic modulated optical frequency comb, specifically as follows: Figure 3As shown, the coherent synthesized dual-color electro-optic modulated optical frequency comb comprises an optical module and an electrical module. The optical module includes a first continuous-wave laser 1, a second continuous-wave laser 2, a first wavelength division multiplexer 3, a cascaded electro-optic intensity modulator 4, a cascaded electro-optic phase modulator 5, an optical fiber amplifier 6, an optical fiber beam splitter 7, an output terminal of the coherent synthesized dual-color electro-optic modulated optical frequency comb 8, a bandpass filter 9, a photodetector 10, a high-power optical fiber amplifier 20, a pulse compression device 21, and a highly nonlinear optical device 22. The electrical module includes a proportional-integral controller 11, a low-pass filter 12, an electrical amplifier 13, a phase shifter 14, a power divider 15, a first signal generator 16, an RF frequency reference 17, a second signal generator 18, and a mixer 19.

[0061] This implementation method and Figure 2 The difference in the implementation shown is that a high-power fiber amplifier 206, a pulse compression device 21, and a highly nonlinear optical device 22 are added between the fiber beam splitter 7 and the bandpass filter 9. The high-power fiber amplifier 206 further amplifies the output optical power of the electro-optic modulated optical frequency comb to meet the power requirements of subsequent spectral broadening. The pulse compression device 21 compresses the pulse of the electro-optic modulated optical frequency comb to achieve the Fourier transform-limited pulse width, thereby increasing the pulse peak power. The highly nonlinear optical device 22 utilizes strong nonlinear effects to achieve spectral broadening.

[0062] Generally, when a continuous-wave laser is modulated using a cascaded electro-optic intensity modulator 4 and a cascaded electro-optic phase modulator 5 to generate an electro-optic modulated optical frequency comb, the chirp of the output pulse of the electro-optic modulated optical frequency comb is linear. Therefore, compression of its output pulse can be achieved through simple dispersion management, such as... Figure 4 Three specific implementations of the pulse compression device 21 are given, among which, Figure 4 (a) The pulse compression device 21 includes a second wavelength division multiplexer 2101, a first single-mode fiber 2102, a second single-mode fiber 2103, and a third wavelength division multiplexer 2104. The parameters of the second and third wavelength division multiplexers 2101 and 2104 are the same as those of the first wavelength division multiplexer 3. Their main purpose is to split or combine two electro-optic modulation optical frequency combs with different center wavelengths. The first and second single-mode fibers 2102 and 2103 provide second-order dispersion to compress the output pulses of the two electro-optic modulation optical frequency combs with different center wavelengths. Since single-mode fibers have different group velocity dispersions at different wavelengths, the lengths of the first and second single-mode fibers 2102 and 2103 will be slightly different. Specifically, the length of the single-mode fiber can be obtained through numerical simulation or experimentation. Figure 4 (b) shows the pulse compression device 21 as... Figure 4(a) shows a variation of the device, wherein the length of the third single-mode fiber 2105 is the difference between the lengths of the first single-mode fiber 2102 and the second single-mode fiber 2103, and the length of the fourth single-mode fiber 2106 is equal to the length of the shorter of the first single-mode fiber 2102 and the second single-mode fiber 2103; and Figure 4 Compared to implementation (a), this method saves the length of single-mode fiber, especially when the selected single-mode fiber is a polarization-maintaining fiber. Figure 4 (b) The implementation method can significantly reduce system costs; Figure 4 (c) The pulse compression device 21 includes a second wavelength division multiplexer 2101, a third wavelength division multiplexer 2104, a first fiber chirped Bragg grating 2107, a first fiber circulator 2108, a second fiber circulator 2109, and a second fiber chirped Bragg grating 21010. The parameters of the second wavelength division multiplexer 2101 and the third wavelength division multiplexer 2104 are the same as those of the first wavelength division multiplexer 3. The first fiber chirped Bragg grating 2107 and the second fiber chirped Bragg grating 21010 are reflective gratings, which compress the output pulses of two electro-optic modulated optical frequency combs with different center wavelengths. The specific chirp value of the grating can be obtained through numerical simulation. The first fiber circulator 2108 and the second fiber circulator 2109 serve to connect the chirped Bragg grating and the wavelength division multiplexer. In addition, pulse compression can also be achieved using a programmable optical processor. By replacing the first fiber chirped Bragg grating 2107 and the second fiber chirped Bragg grating 21010 with two programmable optical processors, the output pulses of two electro-optic modulated optical frequency combs with different center wavelengths can be compressed respectively.

[0063] Generally, there are various implementation methods for highly nonlinear optical devices 22, such as highly nonlinear optical fibers, photonic crystal fibers, or optical waveguides. These optical devices can be used individually or in combination as highly nonlinear optical devices 22. The specific implementation method can be determined according to the situation.

[0064] The coherent synthesized dual-color electro-optic modulation optical frequency comb provided by this invention has the characteristics of low noise and wide spectrum, for the following reasons:

[0065] Compared to traditional electro-optic modulated optical frequency combs, its comb teeth are generated by phase modulation of a continuous-wave laser. Let the comb tooth with the same frequency as the continuous-wave laser be comb tooth number 0. The comb teeth located on either side of comb tooth number 0 are numbered +n and -n respectively. Then the frequency of the nth comb tooth is v. n =v c +n×f m , where v c f is the frequency of the continuous wave laser. mis the driving frequency of the cascaded electro-optic phase modulator 5. First, based on the sources of the phase noise of the frequency comb, there are mainly two types: the phase noise of the continuous-wave laser and the phase noise of the driving signal of the electro-optic phase modulator. The phase noise of the continuous-wave laser has the same effect on all teeth of the comb. However, the phase noise of the driving signal of the electro-optic phase modulator gradually amplifies as the tooth order of the comb increases. This part of the noise is called the cumulative phase noise. Therefore, the cumulative phase noise increases linearly as the tooth order of the comb increases. Then, based on the frequency distribution of the phase noise, since the continuous-wave laser has extremely low high-frequency noise and the radio-frequency signal generator has extremely low low-frequency noise, the phase noise of the electro-optically modulated optical frequency comb is dominated by the phase noise of the continuous-wave laser in the low-frequency part, that is, it is easy to be suppressed by the phase-locking technology. The high-frequency part is dominated by the phase noise of the radio-frequency signal generator. It can be seen that the cumulative phase noise mainly acts on the high-frequency part, but limited by the limited locking bandwidth of the phase-locked loop, it is difficult to suppress the cumulative phase noise at high frequencies through the phase-locked loop. Therefore, reducing the cumulative phase noise is one of the research difficulties of the electro-optically modulated optical frequency comb. In the coherent-combined dual-color electro-optically modulated optical frequency comb proposed in the present invention, the reduction of the tooth order is achieved by coherently combining two electro-optically modulated optical frequency combs with different central wavelengths, thereby passively reducing the cumulative phase noise. Therefore, the coherent-combined dual-color electro-optically modulated optical frequency comb provided in this application has the advantages of simple structure, low cost, and flexible adjustment of the repetition frequency, etc.

[0066] Among them, Figure 5 shows the principle that the coherent-combined dual-color electro-optically modulated optical frequency comb of the present invention has the characteristic of low noise. In the traditional electro-optically modulated optical frequency comb, the relationship between the cumulative phase noise and the tooth order is as Figure 5 (a) shown by the "V"-shaped curve. In the present invention, two electro-optically modulated optical frequency combs with slightly different central wavelengths are coherently combined by locking the beat frequency signal between them to form a coherent-combined dual-color electro-optically modulated optical frequency comb. Therefore, this coherent-combined dual-color electro-optically modulated optical frequency comb contains two central wavelengths / frequencies. The two teeth with frequencies the same as those of these two continuous-wave lasers are both recorded as the 0th tooth, as Figure 5 (b) shown. Therefore, the tooth order of the teeth close to the second central frequency is greatly reduced. For example, the tooth with the tooth order of n in Appendix 5(a) has the same frequency as the tooth with the tooth order of 3 in Figure 5 (b). However, since 3 << n, the phase noise of the tooth with the tooth order of 3 in the coherent-combined dual-color electro-optically modulated optical frequency comb is much lower than that of the tooth with the tooth order of n in the traditional electro-optically modulated optical frequency comb. Therefore, the cumulative phase noise of the teeth in the coherent-combined dual-color electro-optically modulated optical frequency comb is as Figure 5 (b) shown by the "W"-shaped curve. Compared with the Figure 5 (a) "V"-shaped curve, the cumulative phase noise is significantly suppressed.

[0067] Appendix Figure 6 Experimental results demonstrating the low-noise advantage of the coherent synthesized two-color electro-optic modulation optical frequency comb provided by this invention are presented. The figure shows the phase noise values ​​of the comb teeth at different wavelengths at a 1MHz offset. The triangles in the figure represent direct experimental measurements, obtained by sequentially beating the comb teeth of different wavelengths with a wavelength-tunable narrow-linewidth continuous-wave laser and measuring the phase noise power spectrum of the beat frequency signal. The two center wavelengths of the coherent synthesized two-color electro-optic modulation optical frequency comb provided by this invention are 1550nm and 1560nm. Experimental results show that the phase noise of the comb teeth is lowest at the two center wavelengths, and gradually increases as the comb teeth move away from the center wavelength. The "W"-shaped curve illustrates the cumulative phase noise of the coherent synthesized two-color electro-optic modulation optical frequency comb. As a comparative experiment, the phase noise of the comb teeth in a conventional electro-optic modulation optical frequency comb was also obtained by measuring the phase noise of the beat frequency signal with the same wavelength-tunable narrow-linewidth continuous-wave laser. The phase noise values ​​at a 1MHz offset are shown in the figure. Figure 6 Another solid line in the middle, with solid dots representing measured data, shows a curve shaped like a "V". Figure 6 The trends of the two curves and Figure 5 The curves show the same trend, which also confirms that the coherent synthesis dual-color electro-optic modulation optical frequency comb proposed in this invention has the advantage of low noise.

[0068] The broadband advantage of the coherently synthesized dual-color electro-optic modulated optical frequency comb of this invention is due to... Figure 7 Display. (Using attached) Figure 3 In the illustrated embodiment, the center wavelengths of the first continuous-wave laser 1 and the second continuous-wave laser 2 are 1535 nm and 1560 nm, respectively. After spectral expansion by the highly nonlinear optical device 22, the spectrum covers 1520 nm to 1580 nm, encompassing the entire C-band of communication and part of the S-band and L-band. Considering that the repetition frequency of the electro-optic modulation optical frequency comb can reach tens of GHz, the power per tooth of this coherently synthesized dual-color electro-optic modulation optical frequency comb is also considerable. Therefore, the coherently synthesized dual-color electro-optic modulation optical frequency comb proposed in this invention has very important potential applications in the field of communication.

[0069] Figure 8This diagram illustrates the principle of coherently synthesized multicolor electro-optic modulated optical frequency combs. By increasing the number of continuous-wave lasers, multiple electro-optic modulated optical frequency combs with different center wavelengths can be obtained. Adjacent electro-optic modulated optical frequency combs are coherently synthesized to ultimately achieve a coherently synthesized multicolor electro-optic modulated optical frequency comb. Compared to a single electro-optic modulated optical frequency comb, this coherently synthesized multicolor electro-optic modulated optical frequency comb achieves a significantly increased spectral coverage, and its comb tooth sequence number is further reduced, resulting in lower phase noise.

[0070] Appendix Figure 9 This invention demonstrates the application of the coherently synthesized dual-color electro-optic modulated optical frequency comb proposed in the generation of terahertz optical frequency comb 24.

[0071] Appendix Figure 9 (a) A schematic diagram illustrating the principle of generating a terahertz optical frequency comb 24 using the coherent synthesis of a two-color electro-optic modulation optical frequency comb according to the present invention, with attached... Figure 9 (b) A schematic diagram of the structure of a terahertz optical frequency comb 24 generated using the coherent synthesis dual-color electro-optic modulation optical frequency comb provided by the present invention, wherein... Figure 2 Or attached Figure 3 The output terminal 8 of the coherent synthesized two-color electro-optic modulated optical frequency comb is directly connected to the terahertz detector 23 to generate a terahertz optical frequency comb 24. Because the coherent synthesized two-color electro-optic modulated optical frequency comb provided by this invention has the advantages of high power per comb tooth, flexible and adjustable center wavelength, and tunable repetition frequency, the generated terahertz optical frequency comb 24 has advantages such as high energy, tunable center wavelength, tunable repetition frequency, and simple structure. In addition to its application in the field of terahertz optical frequency comb 24, the coherent synthesized two-color electro-optic modulated optical frequency comb of this invention can also be applied to astronomical optical combs, absolute distance measurement, air refractive index measurement, fiber optic sensing, and other technical fields.

[0072] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A coherently synthesized dual-color electro-optic modulated optical frequency comb, characterized in that, Includes optical and electrical modules; The optical module includes a laser emitting unit, an electro-optic modulation unit, an optical amplification unit, and a beat frequency unit. The laser emitting unit includes a first continuous-wave laser (1), a second continuous-wave laser (2), and a first wavelength division multiplexer (3). The output terminals of the first continuous-wave laser (1) and the second continuous-wave laser (2) are respectively connected to the input terminal of the first wavelength division multiplexer (3). The first wavelength division multiplexer (3) is used to combine the lasers emitted by the first continuous-wave laser (1) and the second continuous-wave laser (2). The output terminal of the first wavelength division multiplexer (3) is connected to the input terminal of the electro-optic modulation unit. The input end is connected to the electro-optic modulation unit, which is used to perform intensity modulation and phase modulation on the laser beam after beam combining emitted by the laser emitting unit to generate two electro-optic modulation optical frequency combs. The output end of the electro-optic modulation unit is connected to the input end of the optical amplification unit. The first output end of the optical amplification unit outputs a coherently synthesized two-color electro-optic modulation optical frequency comb. The second output end of the optical amplification unit is connected to the beat frequency unit, which is used to detect the beat frequency signals of the two electro-optic modulation optical frequency combs. The center wavelengths of the lasers emitted by the first continuous wave laser (1) and the second continuous wave laser (2) are different. The electrical module includes an electrical control unit, a drive unit, a mixer (19), and a proportional-integral controller (11). The first output terminal of the electrical control unit is connected to the drive unit and is used to output a first preset signal to drive the electro-optic modulation unit. The first input terminal of the mixer (19) is connected to the output terminal of the beat frequency unit, the second input terminal of the mixer (19) is connected to the second output terminal of the electrical control unit, the output terminal of the mixer (19) is connected to the input terminal of the proportional-integral controller (11), and the output terminal of the proportional-integral controller (11) is connected to the second continuous wave laser (2). The mixer (19) is used to mix the second preset signal output by the electrical control unit and the beat frequency signal to generate an error signal. The proportional-integral controller (11) adjusts the center wavelength of the second continuous wave laser (2) based on the error signal to realize the coherent synthesis of the electro-optic modulation optical frequency comb.

2. The coherent synthesized dual-color electro-optic modulated optical frequency comb according to claim 1, characterized in that, The electro-optic modulation unit includes a cascaded electro-optic intensity modulator (4) and a cascaded electro-optic phase modulator (5); the optical amplification unit includes an optical fiber amplifier (6) and an optical fiber beam splitter (7); The output end of the laser emitting unit is connected to the input end of the cascaded electro-optic intensity modulator (4), the output end of the cascaded electro-optic intensity modulator (4) is connected to the input end of the cascaded electro-optic phase modulator (5), the output end of the cascaded electro-optic phase modulator (5) is connected to the input end of the fiber amplifier (6), the output end of the fiber amplifier (6) is connected to the input end of the fiber beam splitter (7), the first output end of the fiber beam splitter (7) is connected to the output end (8) of the coherent synthesized two-color electro-optic modulation optical frequency comb, and is used to output the coherent synthesized two-color electro-optic modulation optical frequency comb. The second output end of the fiber beam splitter (7) is connected to the input end of the beat frequency unit.

3. The coherent synthesized dual-color electro-optic modulation optical frequency comb according to claim 2, characterized in that, The driving unit includes an electric amplifier (13), a phase shifter (14), and a power divider (15); the electronic control unit includes a first signal generator (16), a radio frequency reference (17), and a second signal generator (18); The output of the first signal generator (16) is connected to the input of the power divider (15) to send a first preset signal to the power divider (15) based on the radio frequency reference (17); The output of the power divider (15) is connected to the cascaded electro-optic intensity modulator (4) in sequence through the phase shifter (14) and the electrical amplifier (13) on a branch. The output of the power divider (15) is connected to the cascaded electro-optic phase modulator (5) in sequence through the phase shifter (14) and the electrical amplifier (13) on another branch; The power divider (15) is used to split the first preset signal, the phase shifter (14) is used to keep the phase of the first preset signal received by the cascaded electro-optic intensity modulator (4) and the cascaded electro-optic phase modulator (5) consistent, and the electric amplifier (13) is used to amplify the first preset signal. The output of the second signal generator (18) is connected to the second input of the mixer (19) to send a second preset signal to the mixer (19) based on the radio frequency reference (17); A low-pass filter (12) is also connected between the output of the mixer (19) and the input of the proportional-integral controller (11).

4. The coherent synthesized dual-color electro-optic modulated optical frequency comb according to claim 1, characterized in that, The beat frequency unit includes a bandpass filter (9) and a photodetector (10); The second output terminal of the optical amplification unit is connected to the input terminal of the bandpass filter (9), the output terminal of the bandpass filter (9) is connected to the input terminal of the photodetector (10), and the input terminal of the photodetector (10) is connected to the first input terminal of the mixer (19). The bandpass filter (9) is used to filter out the spectrum of the overlapping portion of the two electro-optic modulated optical frequency combs, and the photodetector (10) is used to detect the beat frequency signal of the two electro-optic modulated optical frequency combs and output it to the mixer (19).

5. The coherent synthesized dual-color electro-optic modulation optical frequency comb according to claim 4, characterized in that, The beat frequency unit also includes a high-power fiber amplifier (20), a pulse compression device (21), and a highly nonlinear optical device (22); The second output terminal of the optical amplification unit is connected to the input terminal of the high-power fiber amplifier (20), the output terminal of the high-power fiber amplifier (20) is connected to the input terminal of the pulse compression device (21), the output terminal of the pulse compression device (21) is connected to the input terminal of the high nonlinear optical device (22), and the output terminal of the high nonlinear optical device (22) is connected to the input terminal of the photodetector (10).

6. The coherent synthesized dual-color electro-optic modulation optical frequency comb according to claim 5, characterized in that, The pulse compression device (21) includes a second wavelength division multiplexer (2101), a first single-mode fiber (2102), a second single-mode fiber (2103), and a third wavelength division multiplexer (2104); The output of the high-power fiber amplifier (20) is connected to the input of the second wavelength division multiplexer (2101). The output of the second wavelength division multiplexer (2101) is connected to the inputs of the first single-mode fiber (2102) and the second single-mode fiber (2103). The outputs of the first single-mode fiber (2102) and the second single-mode fiber (2103) are connected to the input of the third wavelength division multiplexer (2104). The output of the third wavelength division multiplexer (2104) is connected to the input of the highly nonlinear optical device (22). The parameters of the second wavelength division multiplexer (2101) and the third wavelength division multiplexer (2104) are the same as those of the first wavelength division multiplexer (3).

7. The coherent synthesized dual-color electro-optic modulation optical frequency comb according to claim 6, characterized in that, The pulse compression device (21) further includes a third single-mode fiber (2105) and a fourth single-mode fiber (2106); The output of the high-power fiber amplifier (20) is connected to the input of the second wavelength division multiplexer (2101). The output of the second wavelength division multiplexer (2101) is connected to the input of the third single-mode fiber (2105) and the first input of the third wavelength division multiplexer (2104). The output of the third single-mode fiber (2105) is connected to the second input of the third wavelength division multiplexer (2104). The output of the third wavelength division multiplexer (2104) is connected to the input of the fourth single-mode fiber (2106). The output of the fourth single-mode fiber (2106) is connected to the input of the highly nonlinear optical device (22). The length of the third single-mode fiber (2105) is the length difference between the first single-mode fiber (2102) and the second single-mode fiber (2103), and the length of the fourth single-mode fiber (2106) is equal to the length of the shorter single-mode fiber between the first single-mode fiber (2102) and the second single-mode fiber (2103).

8. The coherent synthesized dual-color electro-optic modulation optical frequency comb according to claim 5, characterized in that, The pulse compression device (21) includes a second wavelength division multiplexer (2101), a first fiber chirped Bragg grating (2107), a first fiber circulator (2108), a second fiber circulator (2109), a second fiber chirped Bragg grating (21010), and a third wavelength division multiplexer (2104). The output of the high-power fiber amplifier (20) is connected to the input of the second wavelength division multiplexer (2101). The output of the second wavelength division multiplexer (2101) is connected to the first fiber circulator (2108) and the second fiber circulator (2109) respectively. The first fiber circulator (2108) and the second fiber circulator (2109) are also connected to the input of the third wavelength division multiplexer (2104) respectively. The output of the third wavelength division multiplexer (2104) is connected to the input of the highly nonlinear optical device (22). The first fiber circulator (2108) is connected to the first fiber chirped Bragg grating (2107), and the second fiber circulator (2109) is connected to the second fiber chirped Bragg grating (21010). Both the first fiber chirped Bragg grating (2107) and the second fiber chirped Bragg grating (21010) are reflective gratings.

9. The coherent synthesized dual-color electro-optic modulated optical frequency comb according to claim 1, characterized in that, The laser emitting unit is provided with N continuous wave lasers, including the first continuous wave laser (1) and the second continuous wave laser (2), and the number of proportional-integral controllers (11) is N-1, where N is a positive integer greater than or equal to 3; The output terminals of the N continuous wave lasers are respectively connected to the input terminal of the first wavelength division multiplexer (3), and the N-1 proportional-integral controllers (11) are respectively connected to the N-1 continuous wave lasers other than the first continuous wave laser (1).

10. The coherent synthesized dual-color electro-optic modulated optical frequency comb according to claim 1, characterized in that, The coherent synthesized dual-color electro-optic modulated optical frequency comb also includes a terahertz detector (23); The terahertz detector (23) is connected to the first output terminal of the optical amplification unit, and the terahertz detector (23) is used to output a terahertz optical frequency comb (24).

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