Free space Kerr optical frequency comb and generation method thereof

Abstract

The invention discloses a free space Kerr optical frequency comb and a generation method thereof, and relates to the technical field of optical frequency combs. The optical frequency comb comprises a pumping source, an optical isolator, a sampling lens, a focusing lens, a first concave reflecting mirror, a nonlinear medium, a second concave reflecting mirror, a piezoelectric ceramic displacement platform and a frequency locking controller. Pump light emitted by a pumping source sequentially passes through an optical isolator, a sampling lens and a focusing lens and then enters an oscillator composed of a first concave reflector, a nonlinear medium and a second concave reflector. The second concave reflecting mirror is placed on the piezoelectric ceramic displacement platform; the backward reflection Stokes light generated by the oscillator is reflected by the sampling lens and then enters the frequency locking controller, and the frequency locking controller feeds back a signal to the piezoelectric ceramic displacement platform after monitoring the polarization state change of the incident light; and the piezoelectric ceramic displacement platform translates according to the feedback signal, so that the cavity length of the oscillator is changed, and Stokes light frequency stabilization is realized. The invention has the advantages of large output energy, single mode family and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of optical frequency combs, and in particular to a free-space Kerr optical frequency comb and a generation method thereof. Background Art

[0002] An optical frequency comb is a spectrum consisting of a series of evenly spaced frequency components with a coherent, stable phase relationship. It is a means of bridging microwaves to optical frequencies. In recent years, optical frequency combs have attracted increasing attention due to their important applications in emerging fields such as low-noise microwave generation, ultraprecision spectroscopy, precision distance measurement, precision surface measurement, precise wavelength calibration for exoplanet exploration, and coherence control for ultrafast dynamics research. Currently, mainstream optical frequency combs include internal gain optical frequency combs, electro-optically modulated optical frequency combs, and externally driven optical frequency combs.

[0003] The most mature technology currently is the internally gained optical frequency comb, which provides stimulated emission via an inverted population gain medium within the cavity. This technology typically uses a mode-locked laser to generate a comb spectrum, using frequency stabilization techniques to ensure even spacing within the comb spectrum to produce an optical frequency comb. This typically offers advantages such as high flatness and stability. Free-space mode-locked lasers were the earliest method for generating optical frequency combs, but due to the complexity and bulk of the frequency stabilization system design, they were quickly replaced by frequency combs generated by fiber mode-locked lasers. However, the long optical length of the fiber resonator results in extremely small longitudinal mode spacing, limiting its scope of application.

[0004] Optical frequency combs generated using electro-optical modulation are a method for generating large longitudinal mode spacings. Through a series of electro-optic modulations, a single-frequency seed light source is modulated into an optical frequency comb with ultrashort pulses in the time domain and equally spaced teeth in the frequency domain. Because this generation scheme relies on direct modulation, it offers greater freedom in longitudinal mode spacing. However, due to precision issues with the modulation components, the generated optical frequency comb is noisy, requiring additional components for noise reduction.

[0005] Over the past decade, externally driven passive nonlinear optical cavities have emerged as a novel approach for generating ultrashort optical pulses and corresponding broadband frequency combs. These methods typically utilize phase-sensitive Kerr nonlinear interactions within a microcavity / fiber FP cavity to generate the emission spectrum. Their ultra-small size, the absence of locking the longitudinal mode spacing, and their excellent integration have attracted considerable research interest. Furthermore, microcavity optical frequency combs have become commercially available, generating broad-spectrum optical frequency combs spanning eight octaves. Their superior properties stem from frequency matching with the Kerr effect and the strong nonlinear interactions of the high-Q microcavity. However, limitations such as the damage threshold, geometry, dispersion control, and detuning requirements of the microcavity lead to complex mode families, low output energy, and high pump source requirements. Kerr (optical Kerr) optical frequency combs generated in free-space cavities using external driving methods have, to some extent, overcome the challenges of difficult dispersion control and low power, but still face challenges such as complex control and high light source requirements. Summary of the Invention

[0006] In order to overcome the above-mentioned deficiencies of the prior art, the present invention proposes a free-space Kerr optical frequency comb and a method for generating the same.

[0007] According to one aspect of the present invention, a free-space Kerr optical frequency comb is proposed, comprising a pump source 1, an optical isolator 2, a sampling lens 3, a focusing lens 4, a first concave reflector 5, a nonlinear medium 6, a second concave reflector 7, a piezoelectric ceramic displacement platform 9, and a frequency-locked controller 10.

[0008] Among them, the pump light emitted by the pump source 1 passes through the optical isolator 2, the sampling lens 3, and the focusing lens 4 in sequence, and then enters the oscillator composed of the first concave reflector 5, the nonlinear medium 6 and the second concave reflector 7; the second concave reflector 7 is placed on the piezoelectric ceramic displacement platform 9, and the back-reflected Stokes light generated by the oscillator is reflected by the focusing lens 4 and the sampling lens 3 and enters the frequency-locked controller 10. The frequency-locked controller 10 monitors the polarization state change of the incident light and feeds back the signal to the piezoelectric ceramic displacement platform 9; the piezoelectric ceramic displacement platform 9 translates according to the feedback signal, thereby changing the oscillator cavity length and realizing Stokes optical frequency stabilization.

[0009] Furthermore, a long-pass filter 8 is included. The long-pass filter 8 is placed behind the second concave reflecting mirror 7 and is used to filter out residual pump light that is not completely reflected by the second concave reflecting mirror 7 .

[0010] Furthermore, the optical isolator 2 is used for unidirectional passage of linearly polarized light; the optical isolator 2 includes a first half-wave plate 2-1, a first polarizer 2-2, a Faraday rotator 2-3, a second half-wave plate 2-4 and a second polarizer 2-5; wherein the first half-wave plate 2-1 is used to adjust the polarization state of the linearly polarized light so that the linearly polarized light is consistent with the polarization direction selectively transmitted by the first polarizer 2-2; the first polarizer 2-2 is used to transmit linearly polarized light; the Faraday rotator 2-3 is used to generate non-reciprocal optical rotation in a magnetic field based on the magneto-optical effect to achieve unidirectional polarization rotation; the second half-wave plate 2-4 is used to further adjust the polarization state and compensate for the phase delay in the forward light path so that the forward light is consistent with the polarization direction of the second polarizer 2-5 after passing through; the second polarizer 2-5 is used to transmit linearly polarized light.

[0011] Furthermore, the sampling lens 3 is a beam splitter made of a plane optical glass, which partially reflects the pump light and the Raman light.

[0012] Furthermore, the nonlinear medium 6 is a solid medium that can produce normal dispersion of third-order nonlinear effects, and the third-order nonlinear effects include Kerr effect, stimulated Raman scattering and stimulated Brillouin scattering.

[0013] Furthermore, the concave surfaces of the first concave reflecting mirror 5 and the second concave reflecting mirror 7 face the nonlinear medium 6; the first concave reflecting mirror 5 is a plano-concave mirror, the surface of which is coated with a first frequency ω p The antireflection dielectric film of the pump light and the second frequency ω S Stokes light high reflective dielectric film; the second concave reflector 7 is a plano-concave mirror, the surface of which is coated with a first frequency ω p The high reflective dielectric film of the pump light and the second frequency ω S Stokes light is partially transmitted through dielectric films.

[0014] Furthermore, the frequency-locking controller 10 is used to convert the polarized light signal reflected by the sampling lens 3 into an electrical signal, and send it to the piezoelectric ceramic displacement platform 9 after performing a subtraction operation; the frequency-locking controller 10 includes an attenuation plate 10-1, a quarter-wave plate 10-2, a polarization beam splitter 10-3, a first photodetector 10-4, a subtractor 10-5 and a second photodetector 10-6; wherein the attenuation plate 10-1 is used to make the reflected second frequency ω SThe energy of the back-reflected Stokes light is attenuated; the quarter-wave plate 10-2 is used to convert the back-reflected Stokes light after energy attenuation into circularly polarized or elliptically polarized light; the polarization splitter prism 10-3 is used to decompose the circularly polarized or elliptically polarized light into two orthogonal polarized lights; the first photodetector 10-4 is used to convert the optical signal of the parallel polarized light into an electrical signal; the subtractor 10-5 is used to receive the electrical signals of the first photodetector 10-4 and the second photodetector 10-6, calculate the difference between the two signals, and feed it back to the piezoelectric ceramic displacement platform 9; the second photodetector 10-6 is used to convert the optical signal of the vertically polarized light into an electrical signal.

[0015] According to another aspect of the present invention, a method for generating a free-space Kerr optical frequency comb is provided. The method is implemented using the free-space Kerr optical frequency comb described above, and the method comprises:

[0016] Pump source 1 emits the first frequency ω p The pump light passes through the optical isolator 2, the sampling lens 3, and the focusing lens 4 in sequence, and then enters the oscillator composed of the first concave reflector 5, the nonlinear medium 6, and the second concave reflector 7. First, stimulated Raman scattering is generated in the nonlinear medium 6, and the second frequency ω is emitted. S Stokes light; with the increase of pump light power, the second frequency ω in the oscillator cavity S The Stokes light power density also increases accordingly, causing the piezoelectric ceramic displacement platform 9 to fine-tune the oscillator cavity length to stabilize the Stokes light. The first process is that an optical parametric process occurs with the Raman light in the cavity as the center, generating multiple main comb teeth of the Kerr optical frequency comb. The second process is that the optical parametric process continues to occur between every two main comb teeth, thereby generating more sidebands between the two main comb teeth. Thereafter, the first and second processes occur simultaneously, thereby generating an optical frequency comb.

[0017] Furthermore, the oscillator cavity length L is determined according to the ratio of the Kerr effect gain to the stimulated Brillouin scattering gain. The expression of the gain ratio is:

[0018]

[0019] Among them, g K represents the Kerr effect gain; g B represents the stimulated Brillouin scattering gain; g0 represents the center frequency of the stimulated Brillouin scattering gain bandwidth; N×ΔΩ FSR -Ω B represents the center frequency of the resonance mode closest to the stimulated Brillouin scattering gain center, N = [Ω B / ΔΩ FSR ],Ω Brepresents the Brillouin frequency shift, ΔΩ FSR represents the free spectral range, ΔΩ FSR =c / 2L, c represents the speed of light in vacuum; Γ B represents the Brillouin gain bandwidth.

[0020] Furthermore, the transmittance T1 of the Stokes light partially transmitting dielectric film of the second concave reflecting mirror 7 is determined by the pump light power P P and the Raman cavity power P Sintra Determine as follows:

[0021]

[0022] P Sintra =P Sout / T1

[0023] Among them, P Sout is the first-order Raman output power; α represents the loss in the oscillator; l represents the geometric length of the nonlinear medium 6; is the quantum defect of the Raman process, λ P is the pump light wavelength, λ s is the first-order Raman wavelength; is the overlapping area between the pump field and the first-order Stokes field, ω′ P and ω′ S are the waist radii of the pump and Stokes field respectively; g R is the Raman gain coefficient.

[0024] The beneficial technical effects of the present invention are:

[0025] The present invention proposes a free-space Kerr optical frequency comb and its generation method. Using a pulsed or single-frequency continuous laser as a pump source, the optical frequency comb is generated in a hybrid system combining external coherent drive and material gain within a free-space optical cavity. Specifically, under external pump drive, stimulated Raman scattering (SRS) is introduced into the cavity, and the stimulated Raman scattering stimulated within the cavity serves as the true pump for the optical frequency comb. Specifically, the phase-matching properties of the Kerr effect are exploited in free space to generate a Kerr optical frequency comb with the center frequency of the actual pump as its absolute frequency. To overcome the low Q value of a free-space nonlinear oscillator for pump light, a Stokes light with a high Q relative to the nonlinear oscillator is used as the actual pump for the Kerr optical frequency comb. Thanks to the cavity mode matching properties of the laser generated by stimulated gain, tuning the true pump to match the cavity mode is unnecessary, significantly reducing the difficulty of generating the optical frequency comb. Because the SRS frequency shift is constant, a frequency-locked Kerr optical frequency comb can be obtained by simply stabilizing the pump frequency. Furthermore, the theoretical simulation results show that the pure Kerr optical frequency comb (suppressing the Raman optical frequency) can be generated. SThe stimulated Brillouin scattering frequency ω B ).

[0026] Compared with traditional optical frequency combs, the present invention has high energy, small longitudinal mode spacing (finer comb teeth), a single mode family (stable mode), low system complexity, weaker thermo-optical effect and better suppression of other stimulated scattering; compared with existing generation schemes, it has the advantages of large output energy, single mode family, simple generation method, self-matching of optical frequency comb pumps, and no need for tuning, and has great application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily apparent by reading the following detailed description with reference to the accompanying drawings, in which several embodiments of the present invention are shown by way of example and not limitation, in which:

[0028] Figure 1 A schematic diagram of the structure of a free-space Kerr optical frequency comb proposed in an embodiment of the present invention;

[0029] Figure 2 Schematic diagram of the structure of an optical isolator in an embodiment of the present invention;

[0030] Figure 3 Schematic diagram of the structure of the frequency locking controller in an embodiment of the present invention;

[0031] Figure 4 Schematic diagram of a Kerr optical frequency comb generated in an embodiment of the present invention;

[0032] Figure 5 Schematic diagram of the ratio of Kerr gain to Brillouin gain at the cavity mode corresponding to different cavity lengths in an embodiment of the present invention;

[0033] Figure 6 for Figure 5 A partial enlarged view of

[0034] Figure 7 This is an example diagram of the relationship between the pump light power and the peak power of the Stokes light in the cavity in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are provided solely to enable those skilled in the art to better understand and implement the present invention, and are not intended to limit the scope of the present invention in any way. Rather, these embodiments are provided to make this disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0036] The embodiment of the present invention proposes a free-space Kerr optical frequency comb, such as Figure 1 As shown, it includes: a pump source 1, an optical isolator 2, a sampling lens 3, a focusing lens 4, a first concave reflector 5, a nonlinear medium 6, a second concave reflector 7, a long-pass filter 8, a piezoelectric ceramic displacement platform 9 and a frequency-locking controller 10. The pump source 1 emits pump light, which passes through the optical isolator 2, the sampling lens 3, and the focusing lens 4 in sequence and enters an oscillator composed of the first concave reflector 5, the nonlinear medium 6, and the second concave reflector 7. Subsequently, the residual pump light that is not absorbed is filtered out by the long-pass filter 8. The second concave reflector 7 is placed on the piezoelectric ceramic displacement platform 9. The back-reflected Stokes light is reflected by the focusing lens 4 and the sampling lens 3 and enters the frequency-locking controller 10. The frequency-locking controller 10 monitors the polarization state change of the incident light and feeds back the signal to the piezoelectric ceramic displacement platform 9. The piezoelectric ceramic displacement platform 9 moves in real time according to the feedback signal, thereby fine-tuning the distance between the nonlinear medium 6 and the second concave reflector 7, changing the oscillator cavity length, and achieving Stokes optical frequency stabilization.

[0037] Specifically, the pump source 1 emits a first frequency ω p Linearly polarized light with a pump spectrum linewidth of less than 10 GHz continuous, quasi-continuous or pulsed laser.

[0038] like Figure 2 As shown, optical isolator 2 is used to allow unidirectional passage of linearly polarized light, thereby protecting the pump laser. Optical isolator 2 comprises a first half-wave plate 2-1, a first polarizer 2-2, a Faraday rotator 2-3, a second half-wave plate 2-4, and a second polarizer 2-5. By adjusting the first half-wave plate 2-1, the power of the pump light incident on the nonlinear medium 6 can be continuously adjusted without changing the pump light beam quality or spot size. Specifically, the first half-wave plate 2-1 is used to adjust the polarization state of linearly polarized light so that it is consistent with the polarization direction selectively transmitted by the first polarizer 2-2. The first polarizer 2-2 is used to transmit linearly polarized light with the same polarization direction as the forward light. Due to the polarization state change of the returning light by the Faraday rotator 2-3, the second half-wave plate 2-4, and the second polarizer 2-5, the returning light is perpendicular to the polarization direction of the first polarizer 2-2 and cannot return. The Faraday rotator 2-3 is used to generate non-reciprocal optical rotation in a magnetic field based on the magneto-optical effect. The polarization direction of the forward light is rotated 45° clockwise, and the reverse light is also rotated 45°, achieving unidirectional polarization rotation. The second half-wave plate 2-4 is used to further adjust the polarization state and compensate for the phase delay in the forward light path, ensuring that the forward light is consistent with the polarization direction of the second polarizer 2-5 after passing through. The second polarizer 2-5 is used to transmit linearly polarized light with the same polarization direction as the forward light, and has a polarization-selective effect on the returning light.

[0039] The sampling lens 3 is a beam splitter made of a plane optical glass, which is sensitive to the pump light (i.e. the linearly polarized light passing through the optical isolator 2 in one direction) and the second frequency ω S The Raman light is partially reflected.

[0040] The focusing lens 4 is used to focus the first frequency ω p The linearly polarized light is focused on the nonlinear medium 6.

[0041] The nonlinear medium 6 is a solid medium with normal dispersion that can produce third-order nonlinear effects (including Kerr effect, stimulated Raman scattering and stimulated Brillouin scattering), including but not limited to: diamond, fused quartz, lithium niobate, CaF2, MgF2, AlN, GaN, GaP, etc.

[0042] The concave surfaces of the first concave reflecting mirror 5 and the second concave reflecting mirror 7 face the nonlinear medium 6; the first concave reflecting mirror 5 is a plano-concave mirror, the surface of which is coated with a first frequency ω p The antireflection dielectric film of the pump light and the second frequency ω S The second concave reflector 7 is a plano-concave mirror, the surface of which is coated with a high-reflection dielectric film for the first frequency ω p The high reflective dielectric film of the pump light and the second frequency ω S Stokes light is partially transmitted through dielectric films.

[0043] The cutoff frequency that the long-pass filter 8 can pass is the first frequency ω p , which can make the frequency lower than ω p The Stokes light and the generated frequency comb can be output.

[0044] The piezoelectric ceramic displacement platform 9 is used to fine-tune the length of the resonant cavity composed of the first concave reflector 5, the nonlinear medium 6, and the second concave reflector 7, thereby achieving frequency stabilization of Stokes light (actual pumping of the Kerr effect) and generating a frequency comb.

[0045] like Figure 3 As shown, the frequency-locked controller 10 is used to convert the measured polarized light signal into an electrical signal, and then perform a subtraction operation and send it to the piezoelectric ceramic displacement platform 9, so as to achieve fine adjustment of the SBS / SRS / Kerr resonant cavity length by the piezoelectric ceramic displacement platform 9. The frequency-locked controller 10 is composed of an attenuation plate 10-1, a quarter-wave plate 10-2, a polarization beam splitter 10-3, a first photodetector 10-4, a subtractor 10-5, and a second photodetector 10-6. Among them, the attenuation plate 10-1 is used to make the reflected second frequency ω SThe energy of the back-reflected Stokes light is attenuated to prevent damage to the subsequent devices; the quarter-wave plate 10-2 is used to convert the back-reflected Stokes light after attenuation by the attenuation plate 10-1 into circularly polarized or elliptically polarized light; the polarization splitter prism 10-3 is used to decompose the circularly polarized or elliptically polarized light passing through the quarter-wave plate 10-2 into two orthogonal polarized lights (P polarization and S polarization); the first photodetector 10-4 is used to receive the optical signal of the P polarized light and convert it into an electrical signal; the subtractor 10-5 is used to receive the electrical signals of the first photodetector 10-4 and the second photodetector 10-6, calculate the difference between the two signals, and feed it back to the piezoelectric ceramic displacement platform 9; the second photodetector 10-6 is used to receive the optical signal of the S polarized light and convert it into an electrical signal.

[0046] It should be noted that the frequency-locked controller 10 controls the movement of the piezoelectric ceramic displacement platform 9 by only utilizing the physical properties of the device itself and the threshold setting of the physical length of the resonant cavity.

[0047] Another embodiment of the present invention provides a method for generating a free-space Kerr optical frequency comb, which includes: a pump source 1 emits a first frequency ω p After the pump light enters the oscillator composed of the first concave reflector 5, the nonlinear medium 6 and the second concave reflector 7, it first generates stimulated Raman scattering in the nonlinear medium 6 and emits the second frequency ω S Stokes light; with the increase of pump light power, the second frequency ω in the oscillator cavity S The Stokes optical power density also increases, which enables the piezoelectric ceramic displacement platform 9 to fine-tune the oscillator cavity length to stabilize the Stokes light. The first process is an optical parametric process centered on the Raman light in the cavity (this process is dominated by the degenerate four-wave mixing in the Kerr effect, where two photons of the same frequency produce two photons of new frequencies), generating multiple main comb teeth of the Kerr optical frequency comb (such as ω k1 、ω k2 ), the second frequency of Stokes light ω S The frequency relationship between the two parametric lights generated by the Kerr effect satisfies: ω k1 +ω k2 =2ω S ; The second process is every two main comb teeth ω k1 、ω k2 The optical parametric process continues to occur between ω k1 and the second frequency ω S 、ω k2 and the second frequency ω S More sidebands are generated between them; thereafter, the first and second processes occur simultaneously, thus generating an optical frequency comb.

[0048] In this embodiment, ω S =ω p -Δω, Δω is the frequency shift of the Raman medium, which is related to the properties of the nonlinear medium 6 used. The movement range of the piezoelectric ceramic displacement platform 9 is mainly in the area where the stimulated Brillouin scattering is suppressed by the Kerr effect. According to the ratio of the Kerr effect gain to the stimulated Brillouin scattering gain g K / g B The process of determining the tuning cavity length corresponding to the resonance mode that makes the Kerr gain strongest is as follows.

[0049] Assume that the Brillouin (SBS) gain curve is represented by the Lorentizan line where δ′ 2 is the distance between the excitation SBS frequency and the gain bandwidth center frequency g0, which can be expressed as formula (1):

[0050]

[0051] N=[Ω B / ΔΩ FSR ] (1)

[0052] Where, N×ΔΩ FSR -Ω B is the center frequency of the resonant mode closest to the SBS gain center, ΔΩ FSR represents the free spectral range; Ω B represents the Brillouin frequency shift; Γ B represents the Brillouin gain bandwidth; [] indicates rounding down.

[0053] The Kerr effect is a super-parameter gain, and its gain is independent of frequency, so g K / g B Medium K is a constant, g B Dependent on frequency, substitute formula (1) into the Lorentizan linear type g B (δ′), we can get formula (2), that is, g K / g B It can be expressed as:

[0054]

[0055] Considering the SBS frequency in the cavity (given by the Raman light ω S The resonance of the stimulated Brillouin scattering frequency) is distributed with the frequency interval ΔΩ FSR The evolution of the change can be represented by the following expression:

[0056]

[0057] where γ is the full width at half maximum of the Stokes mode.

[0058] The physical cavity length of the oscillator L = n1l1 + n2l2, where n1 is the refractive index of the nonlinear medium 6, l1 is the geometric length of the refractive index of the nonlinear medium 6, n2 is the refractive index of the vacuum, and l2 is the distance from the first concave reflector 5 to the second concave reflector 7 minus l1. L and ΔΩ FSR The relationship is: ΔΩ FSR =c / 2L, c represents the speed of light in vacuum.

[0059] Figure 5 The medium gray solid line shows the ratio of Kerr gain to Brillouin gain at the cavity mode corresponding to different cavity lengths when the physical cavity length is swept from 95mm to 120mm. The dark gray dotted line is g K / g B =1 dividing line; Figure 6 for Figure 5 It can be seen that as the cavity shortens, g K / g B The peak value of is getting higher and higher, indicating that the shorter the cavity is, the better the suppression effect of SBS is. K / g B >1 area.

[0060] For example, the physical cavity length range of 108.02mm to 106.54mm is selected for adjustment. When the physical cavity length is 106.54mm, SBS begins to be significantly suppressed. At this time, g K / g B =1; After reducing the cavity length, the Kerr effect begins to occur. When the cavity length is 107.27 mm, the Kerr gain is the largest relative to the Brillouin gain, g K / g B =27.62, at this time a dense optical frequency comb is generated and SBS is suppressed.

[0061] The optical frequency comb proposed in this invention utilizes stimulated Raman scattering and the Kerr effect in nonlinear optical crystals to achieve free-space Kerr optical frequency comb operation. Therefore, high intracavity Raman power is required. The relationship between the intracavity first-order Raman power and the pump light power is as follows:

[0062]

[0063] P Sintra =P Sout / T1 (5)

[0064] Among them, P P is the pump light power, P Sout is the first-order Raman output power, P Sintrais the first-order Raman cavity power; T1 is the first-order transmittance of the second concave reflector 7; l represents the geometric length of the nonlinear medium 6; α represents the loss in the oscillator; is the quantum defect of the Raman process, λ P is the pump light wavelength (1064nm), λ s is the first-order Raman wavelength; is the overlapping area between the pump field and the first-order Stokes field, ω′ P and ω′ S are the waist radii of the pump and Stokes field respectively; g R is the Raman gain coefficient.

[0065] Therefore, the Stokes optical power inside the nonlinear cavity with different parameters can be estimated, and the threshold of the Kerr effect in the cavity requires a power density J Sintra =0.1GW / cm 2 ,but Therefore, P Sintra And the peak power of pump light P P Substituting into formula (4) and (5), we can get the first-order transmittance T1 of the second concave reflector 7 required to produce the Kerr effect. The relationship between the pump light power and the peak power of the Stokes light in the cavity is as follows: Figure 7 For example, the peak pump power P P is 350W, and the corresponding first-order transmittance of the second concave reflector 7 is T1=0.3%. At this time, the peak power P in the cavity is Sintra Can reach 25kW.

[0066] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division into various aspects does not mean that the features of these aspects cannot be combined to benefit. Such division is only for the convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A free-space Kerr optical frequency comb, characterized in that: It includes a pump source (1), an optical isolator (2), a sampling lens (3), a focusing lens (4), a first concave reflector (5), a nonlinear medium (6), a second concave reflector (7), a piezoelectric ceramic displacement platform (9), and a frequency-locking controller (10); The pump light emitted by the pump source (1) passes through an optical isolator (2), a sampling lens (3), and a focusing lens (4) in sequence, and then enters an oscillator composed of a first concave reflector (5), a nonlinear medium (6), and a second concave reflector (7); the second concave reflector (7) is placed on a piezoelectric ceramic displacement platform (9); the back-reflected Stokes light generated by the oscillator is reflected by the focusing lens (4) and the sampling lens (3) and then enters a frequency-locking controller (10); the frequency-locking controller (10) monitors the polarization state change of the incident light and then feeds back the signal to the piezoelectric ceramic displacement platform (9); the piezoelectric ceramic displacement platform (9) performs translation according to the feedback signal, thereby changing the oscillator cavity length and realizing Stokes optical frequency stabilization.

2. A free-space Kerr optical frequency comb according to claim 1, characterized in that: The invention also comprises a long-pass filter (8), which is placed behind the second concave reflecting mirror (7) and is used to filter out residual pump light that is not completely reflected by the second concave reflecting mirror (7).

3. A free-space Kerr optical frequency comb according to claim 1 or 2, characterized in that: The optical isolator (2) is used for unidirectional passage of linearly polarized light; the optical isolator (2) comprises a first half-wave plate (2-1), a first polarizer (2-2), a Faraday rotator (2-3), a second half-wave plate (2-4) and a second polarizer (2-5); wherein the first half-wave plate (2-1) is used to adjust the polarization state of the linearly polarized light so that the linearly polarized light is consistent with the polarization direction selectively transmitted by the first polarizer (2-2); the first polarizer (2-2) is used to transmit the linearly polarized light; the Faraday rotator (2-3) is used to generate non-reciprocal optical rotation in a magnetic field based on the magneto-optical effect to achieve unidirectional polarization rotation; the second half-wave plate (2-4) is used to further adjust the polarization state and compensate for the phase delay in the forward light path so that the forward light is consistent with the polarization direction of the second polarizer (2-5) after passing through; the second polarizer (2-5) is used to transmit the linearly polarized light.

4. A free-space Kerr optical frequency comb according to claim 1 or 2, characterized in that: The sampling lens (3) is a beam splitter composed of a plane optical glass, which partially reflects the pump light and the Raman light.

5. A free-space Kerr optical frequency comb according to claim 1 or 2, characterized in that: The nonlinear medium (6) is a solid medium capable of generating normal dispersion of third-order nonlinear effects, wherein the third-order nonlinear effects include Kerr effect, stimulated Raman scattering and stimulated Brillouin scattering.

6. A free-space Kerr optical frequency comb according to claim 1 or 2, characterized in that: The concave surfaces of the first concave reflecting mirror (5) and the second concave reflecting mirror (7) face the nonlinear medium (6); the first concave reflecting mirror (5) is a plano-concave mirror, the surface of which is coated with a first frequency ω p The antireflection dielectric film of the pump light and the second frequency ω S Stokes light high reflective dielectric film; the second concave reflector (7) is a plano-concave mirror, the surface of which is coated with a first frequency ω p The high reflective dielectric film of the pump light and the second frequency ω S Stokes light is partially transmitted through dielectric films.

7. A free-space Kerr optical frequency comb according to claim 1 or 2, characterized in that: The frequency-locking controller (10) is used to convert the polarized light signal reflected by the sampling lens (3) into an electrical signal, and send it to the piezoelectric ceramic displacement platform (9) after performing a subtraction operation; the frequency-locking controller (10) comprises an attenuation plate (10-1), a quarter-wave plate (10-2), a polarization beam splitter prism (10-3), a first photodetector (10-4), a subtractor (10-5) and a second photodetector (10-6); wherein the attenuation plate (10-1) is used to make the reflected second frequency ω S The energy of the back-reflected Stokes light is attenuated; a quarter-wave plate (10-2) is used to convert the back-reflected Stokes light after energy attenuation into circularly polarized or elliptically polarized light; a polarization beam splitter (10-3) is used to decompose the circularly polarized or elliptically polarized light into two orthogonal polarized lights; a first photodetector (10-4) is used to convert the optical signal of the parallel polarized light into an electrical signal; a subtractor (10-5) is used to receive the electrical signals of the first photodetector (10-4) and the second photodetector (10-6), calculate the difference between the two signals, and feed it back to the piezoelectric ceramic displacement platform (9); and a second photodetector (10-6) is used to convert the optical signal of the vertically polarized light into an electrical signal.

8. A method for generating a free-space Kerr optical frequency comb, characterized in that: The generation method is implemented using a free-space Kerr optical frequency comb according to any one of claims 1 to 7, and the generation method comprises: The pump source (1) emits a first frequency ω p The pump light passes through the optical isolator (2), the sampling lens (3), and the focusing lens (4) in sequence, and then enters the oscillator composed of the first concave reflector (5), the nonlinear medium (6), and the second concave reflector (7). First, stimulated Raman scattering is generated in the nonlinear medium (6), and the second frequency ω is emitted. S Stokes light; with the increase of pump light power, the second frequency ω in the oscillator cavity S The Stokes light power density also increases accordingly, so that the piezoelectric ceramic displacement platform (9) fine-tunes the oscillator cavity length to stabilize the Stokes light. The first process is that the optical parametric process occurs with the Raman light in the cavity as the center, generating multiple main comb teeth of the Kerr optical frequency comb. The second process is that the optical parametric process continues to occur between every two main comb teeth, thereby generating more sidebands between the two main comb teeth. Thereafter, the first process and the second process occur simultaneously, thereby generating an optical frequency comb.

9. The method for generating a free-space Kerr optical frequency comb according to claim 8, wherein: The oscillator cavity length L is determined according to the ratio of the Kerr effect gain to the stimulated Brillouin scattering gain. The expression of the gain ratio is: Among them, g K represents the Kerr effect gain; g B represents the stimulated Brillouin scattering gain; g0 represents the center frequency of the stimulated Brillouin scattering gain bandwidth; N×ΔΩ FSR -Ω B represents the center frequency of the resonance mode closest to the stimulated Brillouin scattering gain center, N = [Ω B / ΔΩ FSR ],Ω B represents the Brillouin frequency shift, ΔΩ FSR represents the free spectral range, ΔΩ FSR =c / 2L, c represents the speed of light in vacuum; Γ B represents the Brillouin gain bandwidth.

10. The method for generating a free-space Kerr optical frequency comb according to claim 9, wherein: The transmittance T1 of the Stokes light partially transmitting dielectric film of the second concave reflecting mirror (7) is determined by the pump light power P P and the Raman cavity power P Sintra Determine as follows: P Sintra =P Sout / T1 Among them, P Sout is the first-order Raman output power; α represents the loss in the oscillator; l represents the geometric length of the nonlinear medium (6); is the quantum defect of the Raman process, λ P is the pump light wavelength, λ s is the first-order Raman wavelength; is the overlapping area between the pump field and the first-order Stokes field, ω′ P and ω′ S are the waist radii of the pump and Stokes field respectively; g R is the Raman gain coefficient.

Images