On-chip carrier envelope offset frequency detector based on LPWG and self-beat frequency detection method

Abstract

The invention discloses an LPWG-based on-chip carrier envelope offset frequency detector and a self-beat frequency detection method. The detector comprises an ultra-short pulse port, a super-continuum spectrum SCG generation area, a long-period waveguide grating LPWG, a single-mode straight waveguide, an output straight waveguide, a coupling straight waveguide and a tuning electrode. The device is highly integrated, frequency beating can be completed only through one integrated waveguide chip, the problems that an existing detection method is high in cost, a system is complex, and a frequency beating system is difficult to operate are solved, a single chip is benefited to generate a cross-octave super-continuum spectrum and frequency doubling light at the same time, an extra frequency doubling crystal is not needed any more, and the device is simple in structure and convenient to operate. And due to the fact that the working bandwidth of the LPWG can limit the filtering effect, an external filtering element is not needed any more, and the successful research and development of the LPWG has wide application backgrounds in the aspects of high-precision frequency measurement, optical atomic clocks, stable and reliable on-chip optical frequency comb research and development and the like, and has important scientific significance and profound practical significance.

Description

Technical Field

[0001] The present invention belongs to the fields of integrated optics, nonlinear optics and frequency measurement technology, and in particular relates to an on-chip carrier envelope offset frequency detector based on LPWG and a self-beating frequency detection method. Background Art

[0002] The carrier envelope offset frequency (f CEO ) detection and locking are crucial for high-precision frequency measurement. Applications such as frequency metrology, optical clocks, and spectroscopy all rely on the f CEO OFC can be synthesized from electrical or optical references 10 5 -10 6 Harmonic related optical modes with a fidelity greater than 1 / 10 18 More importantly, OFC realizes the direct conversion between optical frequency and microwave frequency. By using the extremely high peak power of ultrashort pulses to stimulate nonlinear effects, OFC can synthesize in a wide range of spectral regions including infrared, visible light and extreme ultraviolet. The carrier phase of the pulse is determined by the offset frequency f CEO control and achieve fine optical frequency tuning. Therefore, how to simplify f CEO The detection of optical frequency combs is very necessary in the research and application of optical frequency combs.

[0003] Currently, the f-2f self-reference method is usually used to detect the f of optical frequency combs. CEO This requires both the generation of supercontinuum (SCG) across octaves in nonlinear media (such as photonic crystal fibers, nonlinear waveguides, etc.) and the generation of SHG by frequency-doubling crystals, and then combining the beams for beat frequency generation. This design not only increases the complexity of the system structure, but also increases the size and cost of the device. In addition, this method requires additional spatial optical alignment and an additional time delay system. The operation of a series of complex spatial optical paths reduces the beat frequency efficiency of the system, resulting in a low signal-to-noise ratio for the beat frequency signal, making it difficult to lock and apply. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes an on-chip carrier envelope offset frequency detector and a self-beating frequency detection method based on LPWG.

[0005] The technical solution adopted by the present invention is: an on-chip carrier envelope offset frequency detector based on LPWG, comprising: a port 1, a supercontinuum SCG generation area 2, a long period waveguide grating LPWG 3, a single-mode straight waveguide 4, an output straight waveguide 5, a coupling straight waveguide 6, and a tuning electrode 7.

[0006] The port 1 is an ultrashort pulse input port, the supercontinuum SCG generation region 2 is a parabolic waveguide region with a gradually varying width, the long period waveguide grating LPWG 3 is a multimode sidewall type, and the single-mode straight waveguide 4 is a rectangular waveguide.

[0007] The present invention also provides an on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG applicable to the above detector, and the specific steps are as follows:

[0008] S1, the incident femtosecond pulse light enters the supercontinuum SCG generation area 2 through port 1, and generates a supercontinuum SCG across octaves through nonlinear effects;

[0009] S2, based on the SCG generated in step S1, the light centered on the frequency doubled light of the incident femtosecond pulse light and the short-wave dispersion wave mixed part in the SCG are coupled into the single-mode straight waveguide 4 through the long-period waveguide grating LPWG 3;

[0010] The incident femtosecond pulse light generates a supercontinuum spectrum in which the dispersion wave and the doubled frequency light effectively overlap through the long period waveguide grating LPWG 3; the SCG outputs the remaining part through the output straight waveguide 5 after passing through the LPWG coupling region.

[0011] S3, based on the SCG generated in step S1 and the output spectrum in a certain bandwidth range centered on the frequency doubled light of the incident femtosecond pulse light coupled to the single-mode straight waveguide 4 in step S2, directly detect the light output from the coupled straight waveguide 6, and directly detect the carrier envelope offset frequency f by fine-tuning the tuning electrode 7 CEO .

[0012] Furthermore, the step S1 is specifically as follows:

[0013] The incident femtosecond pulse light enters from the waveguide port 1 and passes through the supercontinuum SCG generation region 2. It is expressed by the generalized nonlinear Schrödinger equation GNLSE, which is expressed as follows:

[0014]

[0015] Where z represents the waveguide length of the supercontinuum SCG generation region 2, A(z,t) represents the slowly varying pulse envelope, α represents the loss coefficient, and β (m) represents the dispersion of each order of waveguide, γ represents the nonlinear coefficient, A represents the pulse amplitude, |A| 2 represents the optical power, ω0 represents the center frequency, T R represents the first-order moment of the nonlinear response function. The right side of the equation represents the influence of various linear and nonlinear effects on the evolution of the pulse in the nonlinear medium. Represents the effect of loss, represents the effect of higher-order dispersion, iγ|A|2 A represents the effect of self-phase modulation, iγ represents the effect of the self-steepening effect, Represents the effect of intra-pulse Raman scattering.

[0016] Then the step-by-step Fourier method is used to solve the GNLSE equation. The GNLSE equation is abbreviated as follows:

[0017]

[0018] in, represents a linear operator, represents the nonlinear operator, and the approximate result A(z+h,t) after the pulse passes through a distance h in the light field is obtained. The expression is as follows:

[0019]

[0020] Furthermore, the step S2 is specifically as follows:

[0021] In step S1, the doubled frequency light generated by SCG directly performs f-2f self-referenced beat frequency with the dispersion wave centered at λ / 2 to obtain the f of the optical frequency comb. CEO , the expression is as follows:

[0022] f CEO =2·ν N -ν 2N =2·(Nf rep +f CEO )-(2N·f rep +f CEO )

[0023] Among them, ν N represents the frequency at the center wavelength λ, ν 2N represents the frequency at λ / 2, N represents the number of modes, and f rep represents the repetition frequency of the optical frequency comb.

[0024] The incident pulse light passes through a coupling region of a certain length L in the long-period waveguide grating LPWG 3. The output amplitudes of the LPWG and the straight waveguide are A(L) and B(L), respectively, expressed as follows:

[0025]

[0026] in, * represents conjugation, represents the constant related to LPWG, Δβ represents the phase mismatch, Λ represents the medium perturbation period, κ represents the coupling coefficient, and ε m(x, y) represents the mth Fourier component of the periodic perturbation, and A(0) and B(0) represent the amplitude of the mode at L = 0. When light is input only from port 1, that is, B(0) = 0, the amplitude expressions of the output straight waveguide 5 and the coupled straight waveguide 6 at the two output ends are as follows:

[0027]

[0028] When the phases are mismatched, the maximum coupling efficiency between the two modes is When phase matching is satisfied, that is, Δβ=0, the maximum coupling efficiency of 100% can be obtained.

[0029] Wherein, λ0=λ / 2 represents the central wavelength satisfying phase matching, and Λ represents the grating period satisfying phase matching.

[0030] When the coupling length When the wavelength is λ0 and Δβ=1.6|κ|, the full width at half maximum (FWHM) of the filter is Δλ. The smaller the coupling coefficient and the greater the mode effective refractive index difference, the narrower the filter bandwidth.

[0031] Furthermore, the step S3 is specifically as follows:

[0032] Based on step S2, after passing through a waveguide of length z, the GNLSE equation is solved using the step-by-step Fourier method to obtain a supercontinuum spectrum, and then the light with a bandwidth of Δλ centered on λ / 2 is filtered out by passing through a coupling region of length L. The output light from the port-coupled straight waveguide 6 is directly measured using a spectrum analyzer. CEO .

[0033] Beneficial effects of the present invention: The detector of the present invention includes: an ultrashort pulse port, a supercontinuum spectrum SCG generation region, a long period waveguide grating LPWG, a single-mode straight waveguide, an output straight waveguide, a coupled straight waveguide, and a tuning electrode. The present invention is highly integrated, and only one integrated waveguide chip is needed to complete the beat frequency, which solves the problems of expensive cost, complex system and difficult operation of the beat frequency system in the existing detection method. Benefiting from the fact that the single chip simultaneously generates cross-octave supercontinuum spectrum and doubled frequency light, no additional frequency doubling crystal is required, and benefiting from the fact that the working bandwidth of LPWG itself will limit the filtering effect, the present invention will no longer need external filtering elements. The successful development of the present invention will have a broad application background in high-precision frequency measurement, optical atomic clocks, stable and reliable on-chip optical frequency comb development, etc., and has important scientific significance and far-reaching practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a structural diagram of an on-chip carrier envelope offset frequency detector based on LPWG of the present invention.

[0035] Figure 2A schematic diagram of mode coupling and transmission characteristics of the working mechanism provided in an embodiment of the present invention.

[0036] Figure 3 The present invention is a flow chart of an on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0038] like Figure 1 As shown, an on-chip carrier envelope offset frequency detector based on LPWG of the present invention comprises: a port 1, a supercontinuum SCG generating region 2, a long period waveguide grating LPWG 3, a single-mode straight waveguide 4, an output straight waveguide 5, a coupling straight waveguide 6, and a tuning electrode 7 (the coupling condition of the LPWG is tuned by utilizing the electro-optical effect of lithium niobate).

[0039] Among them, the port 1 is the ultrashort pulse input end, the supercontinuum SCG generation area 2 is a waveguide area with a parabolic gradient width, which is convenient for adjusting the dispersion wave and helps to generate SCG across the octave and a larger second harmonic frequency; the long period waveguide grating LPWG 3 is a multi-mode sidewall type, and this area couples the part of the doubled frequency light and the short-wave dispersion wave mixing into the single-mode straight waveguide 4, and the single-mode straight waveguide 4 is a rectangular waveguide.

[0040] In the detector preparation process of this embodiment, thin film lithium niobate is considered to be used for preparation, so as to utilize the second-order and third-order nonlinear polarizability of the material, that is, the optical waveguide core layer is made of thin film lithium niobate, the lower cladding is silicon dioxide, the upper cladding is air, and the electrode material is gold.

[0041] like Figure 2 As shown in FIG. 1 , in this embodiment, the simulation results of the coupling region composed of the long period waveguide grating LPWG 3 and the single mode straight waveguide 4 are shown. Figure 2 As shown, by utilizing the coupling characteristics of LPWG, the light of a certain bandwidth range centered at the resonant wavelength (set as the double frequency light) generated by the SCG in the front stage is coupled into the single-mode straight waveguide 4 through LPWG. Ideally, only the light output of a certain bandwidth centered at the double frequency light is detected at the port coupling straight waveguide 6.

[0042] like Figure 3 As shown, this embodiment also provides an on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG applicable to the above detector, and the specific steps are as follows:

[0043] S1, the incident femtosecond pulse light enters the supercontinuum SCG generation area 2 through port 1, and generates a supercontinuum SCG across octaves through nonlinear effects;

[0044] S2, based on the SCG generated in step S1, the light centered on the frequency doubled light of the incident femtosecond pulse light and the short-wave dispersion wave mixed part in the SCG are coupled into the single-mode straight waveguide 4 through the long-period waveguide grating LPWG 3;

[0045] The incident femtosecond pulse light generates a supercontinuum spectrum in which the dispersion wave and the doubled frequency light effectively overlap through the long period waveguide grating LPWG 3; the SCG outputs the remaining part through the output straight waveguide 5 after passing through the LPWG coupling region.

[0046] S3, based on the SCG generated in step S1 and the output spectrum in a certain bandwidth range centered on the frequency doubled light of the incident femtosecond pulse light coupled to the single-mode straight waveguide 4 in step S2, directly detect the light output from the coupled straight waveguide 6, and directly detect the carrier envelope offset frequency f by fine-tuning the tuning electrode 7 CEO .

[0047] In this embodiment, the step S1 is specifically as follows:

[0048] The incident femtosecond pulse light enters from the waveguide port 1 and passes through the supercontinuum SCG generation region 2. It is expressed by the generalized nonlinear Schrödinger equation GNLSE, which is expressed as follows:

[0049]

[0050] Where z represents the waveguide length of the supercontinuum SCG generation region 2, A(z,t) represents the slowly varying pulse envelope, α represents the loss coefficient, and β (m) represents the dispersion of each order of waveguide, γ represents the nonlinear coefficient, A represents the pulse amplitude, |A| 2 represents the optical power, ω0 represents the center frequency, T R represents the first-order moment of the nonlinear response function. The right side of the equation represents the influence of various linear and nonlinear effects on the evolution of the pulse in the nonlinear medium. Represents the effect of loss, represents the effect of higher-order dispersion, iγ|A| 2 A represents the effect of self-phase modulation, iγ represents the effect of the self-steepening effect, Represents the effect of intra-pulse Raman scattering.

[0051] Then the step-by-step Fourier method is used to solve the GNLSE equation. The GNLSE equation is abbreviated as follows:

[0052]

[0053] in, represents a linear operator, represents the nonlinear operator, and the approximate result A(z+h,t) after the pulse passes through a distance h in the light field is obtained. The expression is as follows:

[0054]

[0055] In this embodiment, step S2 is specifically as follows:

[0056] In step S1, the doubled frequency light generated by SCG directly performs f-2f self-referenced beat frequency with the dispersion wave centered at λ / 2 to obtain the f of the optical frequency comb. CEO , the expression is as follows:

[0057] f CEO =2·ν N -ν 2N =2·(Nf rep +f CEO )-(2N·f rep +f CEO )

[0058] Among them, ν N represents the frequency at the center wavelength λ, ν 2N represents the frequency at λ / 2, N represents the number of modes, and f rep represents the repetition frequency of the optical frequency comb.

[0059] The incident pulse light passes through a coupling region of a certain length L in the long-period waveguide grating LPWG 3. The output amplitudes of the LPWG and the straight waveguide are A(L) and B(L), respectively, expressed as follows:

[0060]

[0061] Among them, * represents conjugation, represents the constant related to LPWG, Δβ represents the phase mismatch, Λ represents the medium perturbation period, κ represents the coupling coefficient, and ε m (x, y) represents the mth Fourier component of the periodic perturbation, and A(0) and B(0) represent the amplitude of the mode at L = 0. When light is input only from port 1, that is, B(0) = 0, the amplitude expressions of the output straight waveguide 5 and the coupled straight waveguide 6 at the two output ends are as follows:

[0062]

[0063] When the phases are mismatched, the maximum coupling efficiency between the two modes is When phase matching is satisfied, that is, Δβ=0, the maximum coupling efficiency of 100% can be obtained.

[0064] Wherein, λ0=λ / 2 represents the central wavelength satisfying phase matching, and Λ represents the grating period satisfying phase matching.

[0065] When the coupling length When the wavelength is λ0 and Δβ=1.6|κ|, the full width at half maximum (FWHM) of the filter is Δλ. The smaller the coupling coefficient and the greater the mode effective refractive index difference, the narrower the filter bandwidth.

[0066] In this embodiment, step S3 is specifically as follows:

[0067] Based on step S2, after passing through a waveguide of length z, the GNLSE equation is solved using the step-by-step Fourier method to obtain a supercontinuum spectrum, and then the light with a bandwidth of Δλ centered on λ / 2 is filtered out by passing through a coupling region of length L. The output light from the port-coupled straight waveguide 6 is directly measured using a spectrum analyzer. CEO .

[0068] In this embodiment, the method of the present invention simultaneously utilizes the strong nonlinear effect of thin-film lithium niobate and ultrashort optical pulses to generate a supercontinuum spectrum SCG across octaves, and then utilizes the coupling characteristics of LPWG to couple the light of a certain bandwidth range centered on the resonant wavelength in the SCG into an adjacent single-mode waveguide through the LPWG, and detects the output light f CEO When the metal electrodes prepared on the tuning waveguide are used, the resonant wavelength in the output spectrum will shift to compensate for the problem of the resonant wavelength deviating from the double frequency light due to the error in device preparation.

[0069] In summary, the method of the present invention solves the problem that the existing detection methods are expensive and the system is complex. Benefiting from the fact that a single chip simultaneously generates a cross-octave supercontinuum spectrum and doubled frequency light, no additional frequency doubling crystals are needed, and benefiting from the fact that the working bandwidth of the LPWG itself will limit the filtering effect, the present invention will no longer require external filtering elements. The successful development of the present invention will have a broad application background in high-precision frequency measurement, optical atomic clocks, and stable and reliable on-chip optical frequency comb development, and has important scientific significance and far-reaching practical significance.

[0070] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. For those skilled in the art, the present invention may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of the claims of the present invention.

Claims

1. An on-chip carrier envelope offset frequency detector based on LPWG, comprising: Port (1), supercontinuum SCG generation region (2), long period waveguide grating LPWG (3), single mode straight waveguide (4), output straight waveguide (5), coupling straight waveguide (6), tuning electrode (7); The port (1) is an ultrashort pulse input port, the supercontinuum SCG generation region (2) is a waveguide region with a parabolic gradient width, the long period waveguide grating LPWG (3) is a multimode sidewall type, and the single-mode straight waveguide (4) is a rectangular waveguide.

2. An on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG applicable to the detector, the specific steps are as follows: S1, the incident femtosecond pulse light enters the supercontinuum SCG generation region (2) through port (1), and generates a supercontinuum SCG across octaves through nonlinear effects; S2, based on the SCG generated in step S1, the light in the SCG centered on the frequency doubled light of the incident femtosecond pulse light and the mixed part of the short-wave dispersion wave are coupled into the single-mode straight waveguide (4) through the long-period waveguide grating LPWG (3); in, The incident femtosecond pulse light passes through the long period waveguide grating LPWG (3) to generate a supercontinuum spectrum in which the dispersion wave and the doubled frequency light effectively overlap; after passing through the LPWG coupling region, the SCG outputs the remaining part through the output straight waveguide (5); S3, based on the SCG generated in step S1 and the output spectrum in a certain bandwidth range centered on the frequency doubled light of the incident femtosecond pulse light coupled to the single-mode straight waveguide (4) in step S2, directly detect the light output from the coupled straight waveguide (6), and by fine-tuning the tuning electrode (7), the carrier envelope offset frequency f can be directly detected. CEO .

3. The on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG according to claim 2, characterized in that: The step S1 is specifically as follows: The incident femtosecond pulse light enters from the waveguide port (1) and passes through the supercontinuum SCG generation region (2). It is expressed by the generalized nonlinear Schrödinger equation GNLSE, which is expressed as follows: Where z represents the waveguide length of the supercontinuum SCG generation region (2), A(z,t) represents the slowly varying pulse envelope, α represents the loss coefficient, and β (m) represents the dispersion of each order of waveguide, γ represents the nonlinear coefficient, A represents the pulse amplitude, |A| 2 represents the optical power, ω0 represents the center frequency, T R represents the first-order moment of the nonlinear response function; the right side of the equation represents the influence of various linear and nonlinear effects on the evolution of the pulse in the nonlinear medium. Represents the effect of loss, represents the effect of higher-order dispersion, iγ|A 2 A represents the effect of self-phase modulation, iγ represents the effect of the self-steepening effect, represents the effect of intra-pulse Raman scattering; Then the step-by-step Fourier method is used to solve the GNLSE equation. The GNLSE equation is abbreviated as follows: in, represents a linear operator, represents the nonlinear operator, and the approximate result A(z+h,t) after the pulse passes through a distance h in the light field is obtained. The expression is as follows:

4. The on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG according to claim 2, characterized in that: The step S2 is specifically as follows: In step S1, the doubled frequency light generated by SCG directly performs f-2f self-referenced beat frequency with the dispersion wave centered at λ / 2 to obtain the f of the optical frequency comb. CEO , the expression is as follows: f CEO =2·ν N -ν 2N =2·(Nf rep +f CEO )-(2N·f rep +f CEO ) Among them, ν N represents the frequency at the center wavelength λ, ν 2N represents the frequency at λ / 2, N represents the number of modes, and f rep represents the repetition frequency of the optical frequency comb; The incident pulse light passes through a coupling region of a certain length L in the long-period waveguide grating LPWG (3). The output amplitudes of the LPWG and the straight waveguide are A(L) and B(L), respectively, expressed as follows: in, * represents conjugation, represents the constant related to LPWG, Δβ represents the phase mismatch, Λ represents the medium perturbation period, κ represents the coupling coefficient, and ε m (x, y) represents the mth Fourier component of the periodic perturbation, A(0) and B(0) represent the amplitude of the mode at L = 0; when light is input only from port (1), that is, B(0) = 0, the amplitude expressions of the two output ends of the straight waveguide (5) and the coupled straight waveguide (6) are as follows: When the phases are mismatched, the maximum coupling efficiency between the two modes is When phase matching is satisfied, i.e., Δβ = 0, the maximum coupling efficiency of 100% can be obtained; Wherein, λ0=λ / 2 represents the central wavelength satisfying phase matching, and Λ represents the grating period satisfying phase matching; When the coupling length When the wavelength is λ0 and Δβ=1.6|κ|, the full width at half maximum (FWHM) of the filter is Δλ. The smaller the coupling coefficient and the greater the mode effective refractive index difference, the narrower the filter bandwidth.

5. The on-chip carrier envelope offset frequency self-beating frequency detection method based on LPWG according to claim 2, characterized in that: The step S3 is specifically as follows: Based on step S2, after passing through a waveguide of length z, the GNLSE equation is solved using the step-by-step Fourier method to obtain a supercontinuum spectrum, and then passing through a coupling region of length L, the light with a bandwidth Δλ centered at λ / 2 is filtered out, and the output light from the port-coupled straight waveguide (6) is received by a spectrum analyzer to directly measure f CEO .

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