Soliton optical frequency comb and optical microwave signal generation device and method thereof

Abstract

The application discloses a soliton optical frequency comb and an optical microwave signal generation device, which comprises an optical source unit and an input light control unit, is suitable for obtaining amplified first pump laser, amplified second pump laser and amplified third pump laser; a micro resonant cavity unit comprising a micro resonant cavity, is suitable for generating a soliton optical frequency comb through a nonlinear four-wave mixing parametric oscillation process in the micro resonant cavity by using the amplified first pump laser; is suitable for compensating optical power changes caused by frequency mismatch of the amplified first pump laser by using the amplified second pump laser in the micro resonant cavity; and is suitable for suppressing thermal mechanical vibration noise of the micro resonant cavity based on optical force refrigeration effect by using the amplified third pump laser, so as to suppress phase noise of the soliton optical frequency comb; and a soliton optical frequency comb monitoring unit comprising a microwave signal generation module, the microwave signal generation module is suitable for generating an optical microwave signal by using the soliton optical frequency comb.

Description

Technical Field

[0001] At least one embodiment of the present invention relates to an optical frequency comb applied in the fields of optical information light sources and precision measurement of optical frequencies, optical sensing and coherent optical communication, and particularly to a soliton optical frequency comb based on a micro-resonant cavity and a device and method for generating photogenerated microwave signals. Background Technology

[0002] Optical frequency combs are a novel type of light source with low phase noise, exhibiting an equally spaced coherent comb-like spectrum in the frequency domain and mode-locked optical pulses in the time domain. Traditional methods for generating optical frequency combs are based on femtosecond mode-locked lasers. In recent years, on-chip integrated microcavity optical frequency combs have seen significant development due to their advantages such as high stability, large tooth spacing, small size, and integrability. A microcavity optical frequency comb is formed by a cascaded four-wave mixing coherent spectrum generated by continuous-wave laser pumping a microcavity and producing hyperparametric oscillations through the Kerr nonlinear effect of the microcavity medium. This type of chip-integratable optical frequency comb is also known as a Kerr micro-comb.

[0003] Optical frequency combs exhibit various nonlinear dynamic states, such as ordinary cascaded four-wave mixing optical combs (also known as "Turing mode" optical combs or modulation-unstable MI optical combs), chaotic optical combs, and time-domain dissipative Kerr cavity soliton optical combs (DKS). Among these, the time-domain dissipative Kerr cavity soliton optical comb is a special type of optical frequency comb, which can be further subdivided into single-soliton optical frequency combs, multi-soliton optical frequency combs, and soliton crystal optical frequency combs. In the microcavity, a soliton optical frequency comb exhibits stable propagation of isolated waves in the time domain, maintaining its shape even when propagating in a dispersive medium. Its physical mechanism is a "double balance": on the one hand, the dissipative soliton compensates for the losses in the cavity medium through the parametric gain provided by an external pump; on the other hand, the soliton compensates for the pulse broadening caused by the dispersive medium through "self-focusing" caused by the nonlinear Kerr effect. Under this "double balance" mechanism, the soliton can propagate stably within the cavity, which is the fundamental process for forming a Kerr soliton frequency comb in a microcavity.

[0004] Currently, spontaneous formation of self-organized dissipative Kerr solitons can be observed in optical microcavities with parametric gain under specific pump parameter control. Time-domain dissipative Kerr cavity soliton optical frequency combs provide a mechanism for generating low-phase-noise coherent optical frequency combs with bandwidths exceeding one octave. Due to their wide bandwidth, high coherence, and high repetition rate, DKS optical combs have been widely applied in numerous high-tech fields such as ultra-widebandwidth coherent optical communication, ultrafast distance measurement, optical atomic clocks, precision spectral measurement, optical frequency synthesizers, astronomical spectral calibration for exoplanet exploration, optical coherence tomography, low-phase-noise microwave photonic devices, and photonic radar. Summary of the Invention

[0005] In view of this, the present invention provides a soliton optical frequency comb and a photogenerated microwave signal generation device and method thereof, so as to obtain a microcavity soliton optical frequency comb and a photogenerated microwave signal source with high stability and low phase noise performance.

[0006] This invention provides a soliton optical frequency comb and an optically generated microwave signal generating device, comprising:

[0007] The light source unit includes a first laser, a second laser, and a third laser, which are adapted to generate first pump lasers, second pump lasers, and third pump lasers with different wavelengths respectively;

[0008] The input optical control unit is suitable for amplifying the first pump laser, the second pump laser, and the third pump laser respectively, and combining the amplified second pump laser and the amplified third pump laser into a single beam;

[0009] A microresonant cavity unit, including a microresonant cavity chip, wherein the microresonant cavity chip includes a microresonant cavity, is suitable for generating a soliton optical frequency comb within the microresonant cavity using an amplified first pump laser through a nonlinear four-wave mixing parametric oscillation process; suitable for compensating for optical power changes caused by frequency detuning of the amplified first pump laser within the microresonant cavity using an amplified second pump laser; and suitable for suppressing thermomechanical vibration noise of the microresonant cavity, thereby suppressing phase noise of the soliton optical frequency comb, using an amplified third pump laser based on photomechanical cooling effect; wherein, the wavelength of the first pump laser is relative to the wavelength of the microresonant cavity... The wavelength of the first preselected resonance peak is red-shifted and detuned; the wavelength of the second pump laser is blue-shifted and detuned relative to the wavelength of the second preselected resonance peak of the microresonator; the wavelength of the third pump laser is red-shifted and detuned relative to the wavelength of the third preselected resonance peak of the microresonator; and the amplified optical power of the first pump laser is greater than the nonlinear parametric oscillation threshold of the microresonator, and the amplified optical power of the third pump laser is greater than the optical force oscillation threshold of the microresonator; the first, second, and third preselected resonance peaks are any one of the resonance peaks in the transmission spectrum of the microresonator obtained by scanning using the third laser; and

[0010] The soliton optical frequency comb monitoring unit includes a microwave signal generation module, which is suitable for generating photogenerated microwave signals using the soliton optical frequency comb.

[0011] The present invention also provides a soliton optical frequency comb and a method for generating optically generated microwave signals, which are implemented using the above-mentioned generating apparatus, including:

[0012] The third optical amplifier is set to a low gain state, and the third laser is used to perform wavelength scanning around the micro-resonant cavity chip. The transmission spectrum of the micro-resonant cavity chip and the resonance peaks and corresponding wavelength peak positions in the transmission spectrum are recorded by a spectrometer.

[0013] Set the third optical amplifier to high gain mode;

[0014] The wavelength of the first pump laser output by the first laser is set to redshift and detuned relative to the wavelength of the first preselected resonant peak of the micro-resonant cavity chip using an arbitrary waveform signal generator, so as to excite a soliton optical frequency comb in the micro-resonant cavity of the micro-resonant cavity chip.

[0015] By using an arbitrary waveform signal generator, the wavelength of the second pump laser output by the second laser is set to be blue-shifted and detuned relative to the wavelength of the second pre-selected resonant peak of the micro-resonant cavity chip, so as to compensate for the change in optical power caused by the frequency detuning of the amplified first pump laser within the micro-resonant cavity of the micro-resonant cavity chip.

[0016] By using an arbitrary waveform signal generator, the wavelength of the third pump laser output by the third laser is set to be red-shifted and detuned relative to the wavelength of the third pre-selected resonant peak of the micro-resonant cavity chip, so as to suppress the thermomechanical vibration noise of the micro-resonant cavity based on the photomechanical cooling effect, and thus suppress the phase noise of the soliton optical frequency comb.

[0017] The full spectrum of the soliton optical frequency comb is input to a tunable bandpass filter using a second optical beamsplitter. Two adjacent comb lines filtered from the full spectrum of the soliton optical frequency comb are then input to a second high-frequency photodetector to beat the comb teeth, generating a photogenerated microwave signal. The spectrum of the photogenerated microwave signal is then monitored using an electronic spectrum analyzer.

[0018] The full spectrum of the soliton optical frequency comb is input to the first high-frequency photodetector using the second optical beam splitter. The first high-frequency photodetector performs photoelectric conversion on the full spectrum of the soliton optical frequency comb, and the time-domain waveform of the soliton optical frequency comb is monitored using a high-speed oscilloscope.

[0019] Among them, the first preselected resonance peak, the second preselected resonance peak, and the third preselected resonance peak are any one of the resonance peaks in the transmission spectrum of the micro-resonant cavity obtained by scanning using a third laser; the first preselected resonance peak, the second preselected resonance peak, and the third preselected resonance peak are the same resonance peak or different resonance peaks.

[0020] According to the soliton optical frequency comb and photogenerated microwave signal generation device provided in the above embodiments of the present invention, a soliton optical frequency comb is generated in a micro-resonant cavity by amplified first pump laser through a nonlinear four-wave mixing parametric oscillation process; amplified second pump laser is used in the micro-resonant cavity to compensate for the optical power change caused by the frequency detuning of the amplified first pump laser; and amplified third pump laser is used to suppress the thermomechanical vibration noise of the micro-resonant cavity based on the photomechanical cooling effect, thereby suppressing the phase noise of the soliton optical frequency comb, so as to output a soliton optical frequency comb and photogenerated microwave signal with high stability and low phase noise. Attached Figure Description

[0021] Figure 1This is a schematic diagram of the "soliton step" that appears in the total energy curve of the microcavity soliton optical frequency comb when the pump laser wavelength scanning method is used to excite the soliton optical frequency comb; the inset in the figure is an enlarged view of the region where the "soliton step" appears;

[0022] Figure 2 A schematic diagram illustrating the principle of using laser thermal compensation to generate a stable soliton optical frequency comb;

[0023] Figures 3A to 3D A schematic diagram illustrating the principle of using a pump laser to achieve optical amplification or optical cooling;

[0024] Figure 4 This is a schematic diagram of a soliton optical frequency comb and a photogenerated microwave signal generating device according to an embodiment of the present invention;

[0025] Figure 5 A flowchart illustrating the soliton optical frequency comb and the method for generating photogenerated microwave signals according to an embodiment of the present invention; and

[0026] Figures 6A-6B This is a schematic diagram of a micro-ring core resonator with optical oscillation effect according to an embodiment of the present invention.

[0027] [Explanation of Labels in the Attached Image]

[0028] 10 - Light source unit;

[0029] 11-First laser;

[0030] 12-Second laser;

[0031] 13-Third laser;

[0032] 14-Arbitrary waveform signal generator;

[0033] 20 - Input optical control unit;

[0034] 21- Bundle combiner;

[0035] 22-First polarization controller;

[0036] 23-Second polarization controller;

[0037] 24 - Third polarization controller;

[0038] 25 - First optical amplifier;

[0039] 26 - Second optical amplifier;

[0040] 27 - Third optical amplifier;

[0041] 28 - First optical circulator;

[0042] 29-Second optical circulator;

[0043] 30-Tap-lens fiber optic coupler unit;

[0044] 31-First tapered lens fiber coupler;

[0045] 32-Second tapered lens fiber coupler;

[0046] 40-Micro resonant cavity unit;

[0047] 41-Micro resonant cavity chip;

[0048] 42-Temperature control module;

[0049] 43-Temperature-controlled driver power supply module;

[0050] 50 - Output light control unit;

[0051] 51-Notch filter;

[0052] 52 - First optical beam splitter;

[0053] 53 - Second optical beam splitter;

[0054] 60-Solidon Optical Frequency Comb Monitoring Unit;

[0055] 61-Low-frequency photodetector;

[0056] 62 - First high-frequency photodetector;

[0057] 63 - Second high-frequency photodetector;

[0058] 64-Tunable bandpass filter;

[0059] 65 - High-speed oscilloscope;

[0060] 66-Spectrometer;

[0061] 67-Electronic Spectrum Analyzer. Detailed Implementation

[0062] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. However, this invention can be implemented in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to make the invention thorough and complete, and to fully convey the scope of the invention to those skilled in the art. In the accompanying drawings, for clarity, the dimensions and relative dimensions of layers and regions may be exaggerated, and the same reference numerals denote the same elements throughout.

[0063] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0064] In the practical application of DKS soliton optical combs, obtaining and maintaining single soliton states is a critical requirement fraught with technical challenges. While related technologies disclose the generation of DKS soliton optical combs in some high-Q microcavities, the soliton generation process is inherently random. Although it is possible to generate different numbers of soliton states in the microcavity, the switching between these different soliton states occurs randomly. At pump power exceeding a threshold, by adjusting the frequency detuning of the laser wavelength relative to the microcavity resonant frequency, soliton state optical frequency combs with different numbers of solitons in the microcavity can be obtained. This corresponds to a "soliton step" where the total optical energy or transmitted light energy within the microcavity changes discontinuously with the pump light detuning, such as... Figure 1 As shown, where Figure 1 The horizontal axis represents the scanning time, and the vertical axis represents the total energy of the optical frequency comb.

[0065] However, for microcavities with large thermo-optic coefficients, the soliton state obtained during soliton generation is unstable due to the transient thermal effect caused by the absorption of pump light within the microcavity. In the process of generating solitons using the traditional frequency sweep method, the soliton presence region (i.e., the soliton step width) is usually determined by the step-like microcavity energy transmission spectrum displayed on an oscilloscope. However, during the frequency sweep, when the microcavity comb transitions from a high-energy modulation unstable state to a lower-energy soliton state, the energy within the microcavity drops sharply, causing a significant change in the instantaneous temperature. This results in a significant shift in the resonant wavelength of the microcavity, leading to a sharp increase in its detuning from the pump wavelength. This often causes the soliton step displayed on the oscilloscope to shorten considerably or even disappear entirely. Therefore, thermal effects are a defect that needs to be addressed by the microcavity comb used in the frequency sweep method for soliton generation. The auxiliary laser thermal compensation method is an effective way to reduce the difficulty of soliton modulation by modulating an auxiliary laser to a suitable position near the microcavity resonant wavelength, maintaining a constant energy within the microcavity during the main frequency laser sweep, thereby offsetting the thermal effects of the microcavity. Stable soliton optical combs can be easily obtained by using assisted laser injection into a microcavity to compensate for intracavity energy, such as... Figure 2 As shown. This method uses an auxiliary laser to automatically and synchronously compensate for the energy changes in the cavity caused by pump laser detuning. It can use any slow pump laser frequency scan to generate stable multi-solitons or single solitons and expand the stability range of solitons (i.e., expand the width of the soliton step).

[0066] On the other hand, the phase noise of a microcavity Kerr optical frequency comb is a very important performance indicator for some applications involving precision metrology. The generation of phase noise in a microcavity optical comb is a very complex process, involving nonlinear dynamics at different stages of the comb's evolution and various related physical effects, including linewidth broadening phase noise mechanisms such as self-phase modulation / cross-phase modulation, pump laser phase noise / linewidth, microcavity thermal oscillation noise, microcavity thermo-optic noise, thermoelastic noise, Brownian motion noise, thermal refractive index noise, and ponderomotive noise. Optical frequency comb phase noise affects the performance of some applications that have high requirements for comb coherence. Phase noise manifests as: (1) broadening of the comb tooth linewidth; (2) generation of broad RF beat notes; and (3) damage to the temporal coherence of the comb output pulse. Therefore, realizing a low-phase-noise optical frequency comb is crucial for practical applications. Thermal noise in the microcavity is an important source of phase noise. Therefore, suppressing microcavity thermal noise becomes the top priority for reducing the phase noise of a microcavity optical comb. Typically, microcavity optical comb chips need to be packaged in a sealed metal casing, and the chips require a TEC cooler for constant temperature control. However, this cooling method can only control the temperature of slow, large-amplitude thermal drift, and is powerless against rapid, random, and small-amplitude thermal noise.

[0067] Figure 3A and Figure 3C This diagram illustrates the principle of optical amplification achieved by using blue-shift detuning of a pump laser. Figure 3B and Figure 3D A schematic diagram illustrating the principle of photomechanical cooling achieved by redshift detuning of a pump laser, wherein... Figure 3B The horizontal axis represents frequency, the vertical axis represents energy, and Δ represents frequency detuning.

[0068] Current research on microcavity optomechanical effects has made significant progress. Researchers have proposed and experimentally verified radiation pressure cooling techniques for microcavity thermal noise based on microcavity optomechanical oscillations (OMOs). Using optomechanical cooling, the thermal vibration noise of the microcavity can be cooled to near quantum noise levels. Under the influence of radiation pressure inside the microcavity, the vibrating surface of the microcavity can generate a pair of moving sidebands in the microcavity optomechanical oscillation mode spectrum; that is, a low-frequency (Stokes) sideband and a high-frequency (anti-Stokes) sideband are generated above and below the mechanical oscillation mode, such as... Figure 3C , Figure 3D As shown. The sidebands are generated by Doppler-shifted photons leaving the microcavity, which is due to some intracavity photons colliding with the expanding or contracting walls of the microcavity, resulting in a Doppler shift. Among them, the blue-shifted photons (f+Ω) m This extracts energy from the mechanical motion of the microcavity (equivalent to cooling the microcavity), while redshifted photons (f-Ω)m This adds energy to the mechanical motion of the microcavity (amplifying the microcavity mechanical oscillations). By tuning the pump laser frequency around the optical resonance peak of the microcavity, it is possible to excite the sidebands of these mechanical oscillation modes. When the pump laser is redshifted and detuned, the oscillating microcavity walls reduce scattering to low-frequency mechanical oscillation sideband modes while increasing scattering to high-frequency mechanical oscillation sideband modes. This results in an asymmetric structure of mechanical oscillation modes and produces a net energy transfer from the mechanical oscillation mode to the optical resonance mode. This process can cool the thermal noise oscillation modes of the microcavity through a mechanism called "dynamical backaction," or "photomechanical cooling." Achieving the cooling of the thermal noise mechanical oscillation modes of the microcavity has significant application value, enabling the observation of quantum mechanical effects such as squeezed states and quantum nondestructive measurements. Research indicates that it is possible to use photomechanical cooling technology to cool the microcavity mechanical oscillation modes to their quantum ground state, i.e., a phonon population close to zero.

[0069] The dual-pump excitation method for the stable generation of microcavity Kerr soliton optical frequency combs has been experimentally verified. This method utilizes a main pump laser wavelength scan to generate a soliton optical frequency comb at a redshift relative to the microcavity resonant wavelength, while simultaneously injecting a thermally assisted laser of a different wavelength into the blueshifted detuned position of another resonant peak in the microcavity. This serves as automatic compensation for the abrupt instability of intracavity optical energy caused by the main pump laser generating different numbers of soliton states due to wavelength scanning. However, the dual-pump excitation method is still not ideal in suppressing the phase noise of the microcavity soliton optical comb caused by random thermal noise vibrations within the microcavity. Specifically, the phase noise of the microcavity soliton optical comb generated by the dual-pump excitation method and the optically generated microwave signal source based on the beat frequency of such a microcavity soliton optical comb teeth remains relatively large.

[0070] In view of this, the present invention provides a soliton optical frequency comb and a device and method for generating optical microwave signals that simultaneously have high stability and low phase noise performance.

[0071] Figure 4 This is a schematic diagram of a soliton optical frequency comb and a photogenerated microwave signal generating device according to an embodiment of the present invention.

[0072] According to an exemplary embodiment of the present invention, the present invention provides a soliton optical frequency comb and an apparatus for generating photomicrowave signals, see reference. Figure 4 As shown, it includes: a light source unit 10, an input light control unit 20, a tapered lens fiber optic coupler unit 30, a micro resonant cavity unit 40, an output light control unit 50, and a soliton optical frequency comb monitoring unit 60.

[0073] According to an embodiment of the present invention, the light source unit 10 includes a first laser 11, a second laser 12, and a third laser 13 adapted to generate first pump lasers, second pump lasers, and third pump lasers with different wavelengths respectively. The first laser 11 is a tunable narrow-linewidth main pump laser with a linewidth less than 10 kHz; the second laser 12 is a single-frequency narrow-linewidth auxiliary pump laser; and the third laser 13 is a low-power continuous-wave tunable probe laser with a power less than 10 mW. It should be noted that the thermal effect of the pump laser with a power less than 10 mW injected into the microresonator is negligible, thus avoiding significant spectral distortion of the microresonator.

[0074] According to an embodiment of the present invention, the light source unit 10 further includes an arbitrary waveform signal generator 14, which is suitable for controlling the wavelength of the pump lasers output by the first laser 11, the second laser 12 and the third laser 13 respectively.

[0075] According to an embodiment of the present invention, the input light control unit 20 includes a first polarization controller 22, a second polarization controller 23, and a third polarization controller 24. The first polarization controller 22 is adapted to adjust the polarization state of the first pump laser, the second polarization controller 23 is adapted to adjust the polarization state of the second pump laser, and the third polarization controller 24 is adapted to adjust the polarization state of the third pump laser, so that the polarization states of the first pump laser, the second pump laser, and the third pump laser are matched with the fundamental mode polarization state of the microresonator.

[0076] According to an embodiment of the present invention, the input optical control unit 20 further includes a first optical amplifier 25, a second optical amplifier 26, and a third optical amplifier 27. The first laser 11 is sequentially connected to a first polarization controller 22 and the first optical amplifier 25; the second laser 12 is sequentially connected to a second polarization controller 23 and the second optical amplifier 26; and the third laser 13 is sequentially connected to a third polarization controller 24 and the third optical amplifier 27. The first optical amplifier 25 is used to amplify the first pump laser, the second optical amplifier 26 is used to amplify the second pump laser, and the third optical amplifier 27 is used to amplify the third pump laser. The input optical control unit 20 also includes a beam combiner 21, which is used to combine the amplified second pump laser and the amplified third pump laser to obtain a combined beam.

[0077] According to an embodiment of the present invention, the micro-resonator unit 40 includes a micro-resonator chip 41, which includes an on-chip integrated micro-resonator and an input-output coupled optical waveguide. It is suitable for generating a soliton optical frequency comb within the micro-resonator using an amplified first pump laser through a nonlinear four-wave mixing parametric oscillation process; suitable for compensating for optical power changes caused by frequency detuning of the amplified first pump laser within the micro-resonator using an amplified second pump laser; and suitable for suppressing the thermomechanical vibration noise of the micro-resonator using an amplified third pump laser based on a photomechanical cooling effect, thereby suppressing the phase noise of the soliton optical frequency comb. Specifically, the wavelength of the first pump laser is red-shifted and detuned relative to the wavelength of the first preselected resonance peak of the microresonator; the wavelength of the second pump laser is blue-shifted and detuned relative to the wavelength of the second preselected resonance peak of the microresonator; and the wavelength of the third pump laser is red-shifted and detuned relative to the wavelength of the third preselected resonance peak of the microresonator. Furthermore, the optical power of the amplified first pump laser is greater than the nonlinear parametric oscillation threshold of the microresonator, and the optical power of the amplified third pump laser is greater than the optical force oscillation threshold of the microresonator. The first, second, and third preselected resonance peaks are any one of the resonance peaks in the transmission spectrum of the microresonator obtained by scanning using the third laser 13.

[0078] According to an embodiment of the present invention, the frequency detuning of the third pump laser relative to the third preselected resonant peak is the same as the mechanical oscillation frequency of the micro-resonant cavity, so as to generate optical-mechanical coupling.

[0079] According to an embodiment of the present invention, the micro resonant cavity unit 40 further includes a temperature control module 42 and a temperature control drive power supply module 43. The temperature control module 42 includes a cooler and a heat sink, and is suitable for temperature control of the micro resonant cavity chip 41. The temperature control drive power supply module 43 is suitable for powering the temperature control module 42 and controlling the cooling current.

[0080] According to embodiments of the present invention, the microresonator includes, but is not limited to, one of the following: a microring core resonator, a microring resonator of an air-bridge photonic / phonon hybrid waveguide with an etched and suspended bottom layer, and a spherical microresonator; the microresonator is formed using a transparent material with second- or third-order nonlinear optical polarizability in the soliton optical frequency comb operating band; the transparent material includes silicon-based materials, magnesium fluoride, calcium fluoride, diamond, lithium niobate, III-V compounds, or polymers. III-V compounds include, but are not limited to, aluminum nitride and indium phosphide. Silicon-based materials include, but are not limited to, silicon nitride and silicon dioxide.

[0081] According to an embodiment of the present invention, the input optical control unit 20 further includes a first optical circulator 28 and a second optical circulator 29. It should be noted that the first optical circulator 28 and the second optical circulator 29 are three-port devices, including an input port, a bidirectional port and an output port. Optical signals input from the input port are output from the bidirectional port, optical signals input from the bidirectional port are output from the output port, and optical signals input from the output port are blocked and have no output.

[0082] According to an embodiment of the present invention, the first optical circulator 28 is adapted to input the amplified first pump laser received from the input port into the microresonator through the bidirectional port, and to output the wavelength scanning light signal output by the third pump laser 13 received from the bidirectional port through the microresonator through the output port; the second optical circulator 29 is adapted to input the combined beam of the amplified second pump laser and the amplified third pump laser received from the input port into the microresonator through the bidirectional port, and to output the soliton frequency comb generated by the microresonator received from the bidirectional port through the output port.

[0083] According to an embodiment of the present invention, the tapered lens fiber coupler unit 30 includes a first tapered lens fiber coupler 31 and a second tapered lens fiber coupler 32. The first tapered lens fiber coupler 31 is adapted to couple the amplified first pump laser output from the bidirectional port of the first optical circulator 28 to the microresonator, and to couple the wavelength scanning light signal output from the third laser 13 via the microresonator to the bidirectional port of the first optical circulator 28. The second tapered lens fiber coupler 32 is adapted to couple the combined beam of the amplified second pump laser and the amplified third pump laser output from the bidirectional port of the second optical circulator 29 to the microresonator, and to couple the soliton frequency comb output from the microresonator to the bidirectional port of the second optical circulator 29.

[0084] According to an embodiment of the present invention, the input port of the first optical circulator 28 is connected to the first optical amplifier 25, the bidirectional port of the first optical circulator 28 is connected to the microresonator chip 41 via the first tapered lens fiber coupler 31, and the output port of the first optical circulator 28 is connected to the first optical beam splitter 52. The input port of the second circulator 29 is connected to the beam combiner 21, the bidirectional port of the second optical circulator 29 is connected to the microresonator chip 41 via the second tapered lens fiber coupler 32, and the output port of the second optical circulator 29 is connected to the notch filter 51. The microcavity soliton optical frequency comb output from the microresonator is input to the second optical circulator 29 via the second tapered lens fiber coupler 32 and the bidirectional port of the second optical circulator 29, and output to the notch filter 51 via the output port of the second optical circulator 29.

[0085] According to an embodiment of the present invention, the output optical control unit 50 includes a notch filter 51, which is suitable for filtering out the residual amplified first pump laser transmitted through the micro-resonant cavity, so as to prevent the amplified first pump laser from interfering with the generated soliton optical frequency comb spectrum; and outputs the soliton optical frequency comb generated by the micro-resonant cavity.

[0086] According to an embodiment of the present invention, the notch filter 51 can be an FBG notch filter (fiber Bragg grating notch filter).

[0087] According to an embodiment of the present invention, the output optical control unit 50 further includes a first optical beam splitter 52 and a second optical beam splitter 53, wherein the splitting ratio of the first optical beam splitter 52 is 1:2 and the splitting ratio of the second optical beam splitter 53 is 1:3.

[0088] According to an embodiment of the present invention, the soliton optical frequency comb monitoring unit 60 includes a microwave signal generation module, which is adapted to generate photogenerated microwave signals using the soliton optical frequency comb. The microwave signal generation module includes a tunable bandpass filter 64 and a second high-frequency photodetector 63. The tunable bandpass filter 64 is adapted to filter out two adjacent comb lines from the full spectrum of the soliton optical frequency comb output from the notch filter 51 via the second optical beam splitter 53. The second high-frequency photodetector 63 is adapted to receive the two adjacent comb lines output from the tunable bandpass filter 64 and perform comb beat frequency analysis to generate photogenerated microwave signals.

[0089] According to an embodiment of the present invention, the soliton optical frequency comb monitoring unit 60 further includes a monitoring module, which includes a low-frequency photodetector 61, a first high-frequency photodetector 62, a high-speed oscilloscope 65, a spectrometer 66, and an electronic spectrum analyzer 67.

[0090] According to an embodiment of the present invention, the low-frequency photodetector 61 is adapted to perform photoelectric conversion on the transmission spectrum of the micro-resonant cavity obtained by scanning using the third laser 13, and feed it back to the high-speed oscilloscope 65 to obtain the time-domain waveform of the transmission spectrum of the micro-resonant cavity; the spectrometer 66 is adapted to obtain the transmission spectrum of the micro-resonant cavity to record the resonance peak of the transmission spectrum of the micro-resonant cavity and the wavelength peak position corresponding to the resonance peak.

[0091] According to an embodiment of the present invention, the first high-frequency photodetector 62 is adapted to perform photoelectric conversion on the full spectrum of the soliton optical frequency comb output by the notch filter 51 through the second optical beam splitter 53, and feed it back to the high-speed oscilloscope 65 to observe the time-domain waveform of the soliton optical frequency comb; the spectrometer 66 is adapted to acquire and record the full spectrum of the soliton optical frequency comb output by the notch filter 51.

[0092] According to an embodiment of the present invention, the electronic spectrum analyzer 67 is suitable for monitoring the spectrum of photogenerated microwave signals generated by the microwave signal generation module.

[0093] Figure 5 This is a flowchart of a soliton optical frequency comb and a method for generating photogenerated microwave signals according to an embodiment of the present invention.

[0094] According to an exemplary embodiment of the present invention, the present invention provides a soliton optical frequency comb and a method for generating photogenerated microwave signals, which is implemented using the above-described generation apparatus, with reference to... Figure 5 As shown, it includes steps S01 to S07.

[0095] In step S01, the third optical amplifier 27 is set to a low gain state, the third laser 13 is used to perform wavelength scanning around the micro resonant cavity chip 41, and the spectrometer 66 is used to record the transmission spectrum of the micro resonant cavity chip 41 and the resonance peaks and corresponding wavelength peak positions in the transmission spectrum.

[0096] According to an embodiment of the present invention, the first laser 11 is set to the off state, the second laser 12 is set to the off state, and the third laser 13 is set to the on state. The third optical amplifier 27 is set to a low gain state. The third laser 13 is used to perform wavelength scanning around the resonance peak of the micro-resonant cavity chip 41. The wavelength scanning light signal output by the third laser 13 scanning the micro-resonant cavity chip 41 is input to the bidirectional port of the first optical circulator 28 through the first tapered lens fiber coupler 31, and output to the spectrometer 66 through the output port of the first optical circulator 28 and the first optical beam splitter 52, so as to use the spectrometer 66 to record the resonance spectrum of the micro-resonant cavity chip 41 and the specific wavelength peak position corresponding to the resonance peak.

[0097] According to an embodiment of the present invention, the wavelength scanning optical signal output by the micro resonant cavity chip 41 is input to the bidirectional port of the first optical circulator 28 via the first tapered lens fiber coupler 31, and output to the low-frequency photodetector 61 via the output port of the first optical circulator 28 and the first optical beam splitter 52 for photoelectric conversion, and then fed back to the high-speed oscilloscope 65 for observation.

[0098] In step S02, the third optical amplifier 27 is set to a high-gain state.

[0099] In step S03, the wavelength of the first pump laser output by the first laser 11 is set to redshift and detuned relative to the wavelength of the first preselected resonant peak of the micro-resonant cavity chip 41 using the arbitrary waveform signal generator 14, so as to excite a soliton optical frequency comb in the micro-resonant cavity of the micro-resonant cavity chip 41.

[0100] According to an embodiment of the present invention, by controlling the wavelength of the first pump laser output by the first laser 11 to be scanned from the blue-shifted detuning of the first preselected resonance peak of the micro-resonant cavity chip 41 to the red-shifted detuning and stopping at the red-shifted detuning point, a soliton optical frequency comb is generated in the micro-resonant cavity through a nonlinear four-wave mixing parametric oscillation process.

[0101] In step S04, the wavelength of the second pump laser output by the second laser 12 is set to be blue-shifted and detuned relative to the wavelength of the second pre-selected resonant peak of the micro-resonant cavity chip 41 using the arbitrary waveform signal generator 14, so as to compensate for the change in optical power caused by the frequency detuning of the amplified first pump laser in the micro-resonant cavity of the micro-resonant cavity chip 41.

[0102] According to an embodiment of the present invention, the second pump laser output by the second laser 12 is used to automatically compensate for the temperature drift in the micro-resonant cavity caused by the power change of the first pump laser, so as to improve the applicable range of the wavelength of the first pump laser generated by the soliton in the micro-resonant cavity (i.e., to expand the width of the soliton step) and improve the thermal stability of the soliton optical frequency comb.

[0103] In step S05, the wavelength of the third pump laser output by the third laser 13 is set to redshift and detuned relative to the wavelength of the third preselected resonance peak of the micro-resonant cavity chip 41 using the arbitrary waveform signal generator 14, so as to suppress the thermomechanical vibration noise of the micro-resonant cavity based on the photomechanical cooling effect, thereby suppressing the phase noise of the soliton optical frequency comb.

[0104] It should be noted that the first preselected resonance peak, the second preselected resonance peak, and the third preselected resonance peak are any one of the resonance peaks in the transmission spectrum of the micro-resonant cavity obtained by scanning using the third laser 13; the first preselected resonance peak, the second preselected resonance peak, and the third preselected resonance peak are the same resonance peak or different resonance peaks.

[0105] According to an embodiment of the present invention, a soliton optical frequency comb with high stability and low phase noise and the generation of photogenerated microwave signals are achieved through the triple synergistic excitation effect of the first pump laser output by the first laser 11 with wavelength red-shifted and detuned relative to the first preselected resonance peak, the third pump laser output by the third laser 13 with wavelength red-shifted and detuned relative to the third preselected resonance peak, and the second pump laser output by the second laser 12 with wavelength blue-shifted and detuned relative to the second preselected resonance peak.

[0106] In step S06, the full spectrum of the soliton optical frequency comb is input to the tunable bandpass filter 64 using the second optical beam splitter 53. The two adjacent comb lines filtered out from the full spectrum of the soliton optical frequency comb are input to the second high-frequency photodetector 63 to perform comb frequency beat frequency generation, thereby generating a photogenerated microwave signal. The spectrum of the photogenerated microwave signal is monitored by the electronic spectrum analyzer 67.

[0107] In step S07, the full spectrum of the soliton optical frequency comb is input to the first high-frequency photodetector 62 using the second optical beam splitter 53. The first high-frequency photodetector 62 performs photoelectric conversion on the full spectrum of the soliton optical frequency comb, and the time-domain waveform of the soliton optical frequency comb is monitored using a high-speed oscilloscope 65.

[0108] It should be noted that the spectrometer 66, high-speed oscilloscope 65, and electronic spectrum analyzer 67 are used to monitor the generation of soliton optical frequency combs and their spectral characteristics to determine whether they are in a single-soliton or multi-soliton state, thus deterministically generating soliton optical frequency combs. Among them, the spectrometer 66 is suitable for monitoring the characteristic spectrum of soliton optical frequency combs to determine whether they exhibit hyperbolic secant envelope spectra as shown by single-soliton or multi-soliton states; the high-speed oscilloscope 65 is suitable for monitoring whether soliton optical frequency combs are generated, i.e., determining whether soliton pulse waves are generated; and the electronic spectrum analyzer 67 is suitable for monitoring whether there are power oscillations and noise in the characteristic spectrum.

[0109] According to an embodiment of the present invention, when the high-speed oscilloscope 65 detects the generation of a soliton optical frequency comb, the spectrometer 66 displays the characteristic spectrum of the soliton optical frequency comb, and the electronic spectrum analyzer 67 displays the spectral characteristics without noise features, it is determined that a soliton optical frequency comb has been generated.

[0110] Figures 6A-6B This is a schematic diagram of a micro-ring core resonator with optical oscillation effect according to an embodiment of the present invention.

[0111] According to an embodiment of the present invention, a molten silicon dioxide thin film material on a silicon substrate for a microresonant cavity is prepared by a multi-step photolithography process combined with dry / wet etching. In operation, the external pump light and the optical frequency comb generated inside the cavity are coupled to the outer edge of the microring cavity through a "double-cone" tapered optical fiber, thereby realizing the coupling input and output of the optical frequency comb.

[0112] refer to Figure 6A As shown, when the optical power of the third pump laser does not reach the optical oscillation threshold of the micro-ring core resonator, the optical power of the third pump laser, whose output laser frequency ω is located on the redshift detuning side of the resonant peak frequency ω0 of the micro-ring core resonator, is not strong enough to generate optical oscillation in the micro-ring core resonator.

[0113] refer to Figure 6BAs shown, when the optical power of the third pump laser exceeds the optical oscillation threshold of the micro-ring core resonator, the third pump laser excites a mechanical oscillation mode of n=3 (n is the radial mode index of the mechanical oscillation mode of the micro-ring core resonator) in the micro-ring core resonator. The mechanical oscillation of the micro-ring core resonator with an oscillation frequency of Ω will generate two frequency sidebands output light on both sides of the frequency of the input third pump light: a Stokes output light with a frequency of (ω-Ω) and an anti-stokes output light with a frequency of (ω+Ω). Stokes photons with a frequency of (ω-Ω) amplify the mechanical vibrations within the microcavity, achieving a "transfer of pump light energy to microcavity mechanical oscillation energy," or "optical amplification" effect. Conversely, anti-Stokes photons with a frequency of (ω+Ω) extract energy from the microcavity mechanical oscillations (corresponding to thermal noise motion), achieving a "transfer of microcavity mechanical oscillation energy to pump light energy," or "optical cooling" effect. Both optical amplification and optical cooling effects exist simultaneously. However, because the cavity enhancement factor of the resonant cavity for anti-Stokes photons is greater than that for Stokes photons when the pump light is redshifted and detuned, the overall effect when the pump light is redshifted and detuned is net optical cooling. Therefore, the random mechanical thermal motion of the microcavity can be cooled, reducing the thermal phase noise of the microcavity optical frequency comb.

[0114] According to the soliton optical frequency comb and photogenerated microwave signal generation device provided in the above embodiments of the present invention, a soliton optical frequency comb is generated in a micro-resonant cavity by amplified first pump laser through a nonlinear four-wave mixing parametric oscillation process; amplified second pump laser is used in the micro-resonant cavity to compensate for the optical power change caused by the frequency detuning of the amplified first pump laser; and amplified third pump laser is used to suppress the thermomechanical vibration noise of the micro-resonant cavity based on the photomechanical cooling effect, thereby suppressing the phase noise of the soliton optical frequency comb, so as to output a soliton optical frequency comb and photogenerated microwave signal that have both high stability and low phase noise as a multi-wavelength light source for coherent optical communication.

[0115] The use of ordinal numbers such as "first," "second," "third," etc., in the specification and claims to modify the corresponding elements does not imply that the element has any ordinal number, nor does it represent the order of one element with another element, or the order of manufacturing methods. The use of these ordinal numbers is only to enable a named element to be clearly distinguished from another element with the same name.

[0116] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A soliton optical frequency comb and a device for generating optical microwave signals, characterized in that, include: The light source unit (10) includes a first laser (11), a second laser (12), and a third laser (13) suitable for generating first pump lasers, second pump lasers, and third pump lasers with different wavelengths respectively; The input optical control unit (20) is adapted to amplify the first pump laser, the second pump laser, and the third pump laser respectively, and to combine the amplified second pump laser and the amplified third pump laser into a single beam; The micro-resonant cavity unit (40) includes a micro-resonant cavity chip (41), which includes a micro-resonant cavity and is suitable for generating a soliton optical frequency comb in the micro-resonant cavity by means of an amplified first pump laser through a nonlinear four-wave mixing parametric oscillation process. It is applicable to using the amplified second pump laser to compensate for the optical power change caused by the frequency detuning of the amplified first pump laser in the micro-resonant cavity; and it is applicable to using the amplified third pump laser to suppress the thermomechanical vibration noise of the micro-resonant cavity based on the photomechanical cooling effect, thereby suppressing the phase noise of the soliton optical frequency comb; Wherein, the wavelength of the first pump laser is red-shifted and detuned relative to the wavelength of the first preselected resonant peak of the micro-resonant cavity, the wavelength of the second pump laser is blue-shifted and detuned relative to the wavelength of the second preselected resonant peak of the micro-resonant cavity, and the wavelength of the third pump laser is red-shifted and detuned relative to the wavelength of the third preselected resonant peak of the micro-resonant cavity. Furthermore, the optical power of the amplified first pump laser is greater than the nonlinear parametric oscillation threshold of the micro-resonant cavity, and the optical power of the amplified third pump laser is greater than the optical force oscillation threshold of the micro-resonant cavity. The first preselected resonance peak, the second preselected resonance peak, and the third preselected resonance peak are any one of the resonance peaks in the transmission spectrum of the micro-resonant cavity obtained by scanning using the third laser (13); and The soliton optical frequency comb monitoring unit (60) includes a microwave signal generation module, which is suitable for generating photogenerated microwave signals using the soliton optical frequency comb.

2. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 1, characterized in that, The light source unit (10) further includes an arbitrary waveform signal generator (14), which is suitable for controlling the wavelength of the pump lasers output by the first laser (11), the second laser (12) and the third laser (13), respectively.

3. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 1, characterized in that, The input optical control unit (20) also includes: The system comprises a first polarization controller (22), a second polarization controller (23), and a third polarization controller (24). The first polarization controller (22) is adapted to adjust the polarization state of the first pump laser, the second polarization controller (23) is adapted to adjust the polarization state of the second pump laser, and the third polarization controller (24) is adapted to adjust the polarization state of the third pump laser, so that the polarization states of the first pump laser, the second pump laser, and the third pump laser match the fundamental mode polarization state of the microresonator.

4. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 1, characterized in that, The input optical control unit (20) also includes: The first optical circulator (28) and the second optical circulator (29) are three-port devices, including an input port, a bidirectional port and an output port. The first optical circulator (28) is adapted to input the amplified first pump laser received from the input port into the micro resonant cavity through the bidirectional port, and is adapted to output the wavelength scanning light signal of the third laser (13) received from the bidirectional port through the micro resonant cavity through the output port. The second optical circulator (29) is adapted to input the combined beam of the amplified second pump laser and the amplified third pump laser received from the input port into the micro-resonator through the bidirectional port, and output the soliton optical frequency comb generated by the micro-resonator received from the bidirectional port through the output port.

5. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 4, characterized in that, It also includes a tapered lens fiber coupler unit (30), comprising: The first tapered lens fiber coupler (31) is adapted to couple the amplified first pump laser output from the bidirectional port of the first optical circulator (28) to the micro resonant cavity, and is adapted to couple the wavelength scanning light signal output by the third laser (13) through the micro resonant cavity to the bidirectional port of the first optical circulator (28). The second tapered lens fiber coupler (32) is adapted to couple the combined light output from the bidirectional port of the second optical circulator (29) to the micro resonant cavity, and couple the soliton frequency comb output from the micro resonant cavity to the bidirectional port of the second optical circulator (29).

6. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 1, characterized in that, The micro-resonant cavity unit (40) further includes: The temperature control module (42) and the temperature control drive power supply module (43) are provided. The temperature control module (42) includes a cooler and a heat sink and is suitable for temperature control of the micro resonant cavity chip (41). The temperature control drive power supply module (43) is suitable for power supply and cooling current control of the temperature control module (42).

7. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 1, characterized in that, The micro-resonant cavity includes one of the following: a micro-ring core resonant cavity, an air-bridge photonic / phonon hybrid waveguide micro-ring resonant cavity with an etched and suspended bottom layer of the optical waveguide, or a spherical micro-resonant cavity. The microresonant cavity is formed using a transparent material with second- or third-order nonlinear optical polarizability in the operating band of the soliton optical frequency comb.

8. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 7, characterized in that, The transparent material includes silicon-based materials, magnesium fluoride, calcium fluoride, diamond, lithium niobate, III-V compounds, or polymers.

9. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 1, characterized in that, It also includes an output light control unit (50), The output optical control unit (50) includes a notch filter (51) adapted to filter out the residual amplified first pump laser transmitted through the micro-resonant cavity and output the soliton optical frequency comb generated by the micro-resonant cavity.

10. The soliton optical frequency comb and photogenerated microwave signal generating device according to claim 9, characterized in that, The microwave signal generation module includes: a tunable bandpass filter (64) and a second high-frequency photodetector (63). The tunable bandpass filter (64) is adapted to filter out two adjacent comb lines from the full spectrum of the soliton optical frequency comb output by the notch filter (51). The second high-frequency photodetector (63) is adapted to receive the two adjacent comb lines output by the tunable bandpass filter (64) and perform optical frequency combing to generate the photogenerated microwave signal. The soliton optical frequency comb monitoring unit (60) also includes a monitoring module, which includes a low-frequency photodetector (61), a first high-frequency photodetector (62), a high-speed oscilloscope (65), a spectrometer (66), and an electronic spectrum analyzer (67). The low-frequency photodetector (61) is adapted to perform photoelectric conversion on the transmission spectrum of the micro-resonant cavity obtained by scanning using the third laser (13), and feed it back to the high-speed oscilloscope (65) to obtain the time-domain waveform of the transmission spectrum of the micro-resonant cavity; the spectrometer (66) is adapted to obtain the transmission spectrum of the micro-resonant cavity to record the resonance peak of the micro-resonant cavity and the wavelength peak position corresponding to the resonance peak. The first high-frequency photodetector (62) is suitable for photoelectric conversion of the full spectrum of the soliton optical frequency comb output by the notch filter (51) and feeding it back to the high-speed oscilloscope (65) to observe the time-domain waveform of the soliton optical frequency comb; The spectrometer (66) is suitable for acquiring and recording the full spectrum of the soliton optical frequency comb output by the notch filter (51); The electronic spectrum analyzer (67) is suitable for monitoring the spectrum of the photogenerated microwave signal generated by the microwave signal generation module.

11. A soliton optical frequency comb and a method for generating optically generated microwave signals, characterized in that, Implemented using the generating apparatus as described in any one of claims 1 to 10, comprising: The third optical amplifier (27) is set to a low gain state, and the third laser (13) is used to perform wavelength scanning around the micro resonant cavity chip (41). The transmission spectrum of the micro resonant cavity chip (41) and the resonance peak in the transmission spectrum and the wavelength peak position corresponding to the resonance peak are recorded by the spectrometer (66). Set the third optical amplifier (27) to a high-gain state; The wavelength of the first pump laser output by the first laser (11) is set to redshift and detuned relative to the wavelength of the first preselected resonance peak of the micro-resonant cavity chip (41) using an arbitrary waveform signal generator (14) to excite a soliton optical frequency comb in the micro-resonant cavity of the micro-resonant cavity chip (41). The wavelength of the second pump laser output by the second laser (12) is set to be blue-shifted and detuned relative to the wavelength of the second pre-selected resonance peak of the micro-resonant cavity chip (41) using the arbitrary waveform signal generator (14) in order to compensate for the change in optical power caused by the frequency detuning of the amplified first pump laser in the micro-resonant cavity of the micro-resonant cavity chip (41). The wavelength of the third pump laser output by the third laser (13) is set to redshift and detuned relative to the wavelength of the third preselected resonance peak of the micro-resonant cavity chip (41) using the arbitrary waveform signal generator (14), so as to suppress the thermomechanical vibration noise of the micro-resonant cavity based on the photomechanical cooling effect, thereby suppressing the phase noise of the soliton optical frequency comb. The full spectrum of the soliton optical frequency comb is input to a tunable bandpass filter (64) using a second optical beam splitter (53). Two adjacent comb lines filtered from the full spectrum of the soliton optical frequency comb are then input to a second high-frequency photodetector (63) to perform comb beat frequency generation, thereby generating a photogenerated microwave signal. The spectrum of the photogenerated microwave signal is monitored using an electronic spectrum analyzer (67). The full spectrum of the soliton optical frequency comb is input to the first high-frequency photodetector (62) using the second optical beam splitter (53), and the full spectrum of the soliton optical frequency comb is photoelectrically converted using the first high-frequency photodetector (62). The time-domain waveform of the soliton optical frequency comb is monitored using a high-speed oscilloscope (65). Wherein, the first preselected resonance peak, the second preselected resonance peak and the third preselected resonance peak are any one of the resonance peaks in the transmission spectrum of the micro-resonant cavity obtained by scanning using the third laser (13); the first preselected resonance peak, the second preselected resonance peak and the third preselected resonance peak are the same resonance peak or different resonance peaks.

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