On-chip carrier envelope offset frequency detector based on lpwg and method for detecting selfie frequency

Abstract

The application discloses a kind of on-chip carrier envelope offset frequency detector and selfie frequency detection method based on LPWG, the detector includes: ultra-short pulse port, supercontinuum spectrum SCG generation area, long-period waveguide grating LPWG, single-mode straight waveguide, output straight waveguide, coupling straight waveguide, tuning electrode.The present application is highly integrated, only one integrated waveguide chip can complete beat frequency, solve the problem of expensive cost, complex system and the operation difficulty of beat frequency system of existing detection method, benefit from single chip simultaneously generating cross octave supercontinuum spectrum, and double frequency light, no longer need additional frequency doubling crystal, and benefit from the working bandwidth of LPWG itself will limit filtering effect, the present application will no longer need external filtering element, the successful development of the present application will have wide application background in high-precision frequency measurement, optical atomic clock, stable and reliable on-chip optical frequency comb research and so on, has important scientific significance and far-reaching practical significance.

Description

TECHNICAL FIELD

[0001] The application belongs to the fields of integrated optics, nonlinear optics and frequency metrology, and particularly relates to an on-chip carrier envelope offset frequency detector based on an LPWG and a carrier envelope offset frequency detection method. BACKGROUND

[0002] The carrier envelope offset frequency (f CEO ) detection and locking of an optical frequency comb (OFC) is crucial for high-precision frequency measurement, and applications such as frequency metrology, optical clock and spectroscopy rely on the f CEO stability of a mode-locked laser. The OFC can synthesize 10 5 -10 6 harmonic related optical modes from an electrical or optical reference with a fidelity greater than 1 / 10 18 , and more importantly, the OFC realizes the direct conversion of optical frequency to microwave frequency. The OFC can synthesize in a wide spectral range including infrared, visible light and extreme ultraviolet by using the extremely high peak power of ultra-short pulses to excite nonlinear effects. The carrier phase of the pulse is controlled by the offset frequency f CEO , and fine optical frequency tuning is achieved. Therefore, it is necessary to simplify the detection of f CEO in the research and application of the optical frequency comb.

[0003] At present, the f CEO of the optical frequency comb is usually detected by using a method based on f-2f self-reference, which needs to simultaneously rely on the supercontinuum generation (SCG) across an octave in a nonlinear medium (such as a photonic crystal fiber, a nonlinear waveguide, etc.) and the SHG generated by a frequency doubling crystal, and finally the beat frequency is performed after the beams are combined. This design scheme not only increases the complexity of the system structure, but also increases the size and cost of the device. In addition, the method needs additional spatial light alignment and an additional time delay system, and a series of complex spatial light paths reduce the beat frequency efficiency of the system, resulting in a low signal-to-noise ratio of the beat frequency signal and difficulty in locking and application. SUMMARY

[0004] To solve the above technical problems, the application provides an on-chip carrier envelope offset frequency detector based on an LPWG and a carrier envelope offset frequency detection method.

[0005] The technical scheme adopted by the application is as follows: an on-chip carrier envelope offset frequency detector based on an LPWG, comprising: a port 1, a supercontinuum SCG generation 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.

[0006] The port 1 is an ultrashort pulse input end, the supercontinuum spectrum SCG generation area 2 is a parabolic gradually-changing-width waveguide area, 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 application further provides a LPWG-based on-chip carrier envelope offset frequency selfie frequency detection method suitable for the detector, and the specific steps are as follows:

[0008] S1, the incident femtosecond pulse light enters the supercontinuum spectrum SCG generation area 2 through the port 1, and a supercontinuum spectrum SCG across an octave is generated through a nonlinear effect;

[0009] S2, the SCG generated based on the step S1 passes through the long-period waveguide grating LPWG 3, and a part of the SCG, which is mixed with a short-wave dispersion wave and centered on the second harmonic light of the incident femtosecond pulse light, is coupled into the single-mode straight waveguide 4;

[0010] The incident femtosecond pulse light generates a supercontinuum spectrum with the dispersion wave and the second harmonic light effectively overlapping through the long-period waveguide grating LPWG 3; and the remaining part of the SCG passes through the output straight waveguide 5 after passing through the LPWG coupling area.

[0011] S3, based on the output spectrum of a certain bandwidth range centered on the second harmonic light of the incident femtosecond pulse light coupled into the single-mode straight waveguide 4 in the step S2 and the SCG generated in the step S1, the light output from the coupled straight waveguide 6 is directly detected, and the carrier envelope offset frequency f CEO .

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

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

[0014]

[0015] Wherein, z represents the waveguide length of the supercontinuum spectrum SCG generation area 2, A(z,t) represents a slowly-varying pulse envelope, α represents a loss coefficient, β (m) represents the dispersion of each order of the waveguide, γ represents a nonlinear coefficient, A represents a pulse amplitude, |A| 2 represents the optical power, ω0 represents a center frequency, T R represents the first 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 the loss, represents the effect of the high-order dispersion, and iγ|A|2 A represents the effect of self-phase modulation, iγ represents the effect of self-steepening, represents the effect of intrapulse Raman scattering.

[0016] Then the GNLSE equation is solved by using the split-step Fourier method, and the GNLSE equation is simplified, and the expression is as follows:

[0017]

[0018] wherein, represents a linear operator, represents a nonlinear operator, and an approximate result A(z+h, t) of the pulse after passing through a distance h in the optical field is obtained, and the expression is as follows:

[0019]

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

[0021] The second harmonic wave generated by the SCG in the step S1 directly carries out f-2f self-reference beat with the dispersion wave centered at λ / 2 to obtain f CEO of the optical frequency comb, and the expression is as follows:

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

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

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

[0025]

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

[0027]

[0028] When the phase is not matched, the maximum coupling efficiency between the two modes is When the phase matching is satisfied, i.e. Δβ=0, the maximum coupling efficiency 100% can be obtained.

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

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

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

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

[0033] The detector provided by the application comprises 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 application is highly integrated, and only one integrated waveguide chip is needed to complete the beat frequency, thereby solving the problems of high cost, complex system and operation difficulty of the existing detection method. The application benefits from the simultaneous generation of supercontinuum spectrum across an octave and second-harmonic light by a single chip, and does not need an additional frequency-doubling crystal. The application will no longer need an external filtering element, because the working bandwidth of the LPWG itself will limit the filtering effect. The successful development of the application will have a wide application background in high-precision frequency measurement, optical atomic clocks, stable and reliable on-chip optical frequency comb research and other aspects, and has important scientific significance and far-reaching practical significance. BRIEF DESCRIPTION OF DRAWINGS

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

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

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

[0037] The present application will be further described below in combination with the drawings and embodiments.

[0038] As shown in the drawing, Figure 1 a LPWG-based on-chip carrier envelope offset frequency detector of the present application comprises: a port 1, a supercontinuum generation (SCG) 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 (which utilizes the electro-optic effect of lithium niobate to tune the coupling of the LPWG).

[0039] The port 1 is a super-short pulse input end, the SCG region 2 is a waveguide region with a parabolic gradually-changing width, which is convenient for adjusting the dispersive wave to help generate SCG across an octave and a larger second-harmonic frequency; the LPWG 3 is a multimode sidewall type, which couples the part of the mixing of the second-harmonic light and the short-wave dispersive wave into the single-mode straight waveguide 4; and the single-mode straight waveguide 4 is a rectangular waveguide.

[0040] In the preparation process of the detector in the embodiment, thin-film lithium niobate is used for preparation, which is convenient for utilizing the second-order and third-order nonlinear polarizabilities of the material, i.e., the waveguide core layer uses thin-film lithium niobate, the lower cladding layer is silicon dioxide, and the upper cladding layer is air, and the electrode material is gold.

[0041] As shown in the drawing, Figure 2 the simulation result of the coupling region composed of the LPWG 3 and the single-mode straight waveguide 4 in the embodiment is shown. Figure 2 As shown in the drawing, the SCG generated in the front section is coupled into the single-mode straight waveguide 4 by using the coupling characteristics of the LPWG, with a certain bandwidth range of light centered on the resonant wavelength (set as the second-harmonic light), which is determined by the specific grating setting. In an ideal case, the port coupling straight waveguide 6 only detects the light output with a certain bandwidth centered on the second-harmonic light.

[0042] As shown in the drawing, Figure 3 the embodiment also provides a LPWG-based on-chip carrier envelope offset frequency self-frequency detection method suitable for the above detector, and the specific steps are as follows:

[0043] S1, the femtosecond pulse light is incident into the SCG region 2 through the port 1, and a supercontinuum SCG across an octave is generated through nonlinear effects;

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

[0045] Wherein, the incident femtosecond pulse light generates a supercontinuum spectrum with effective overlap of dispersion wave and second harmonic light through the long-period waveguide grating LPWG 3; the remaining part of the SCG after passing through the LPWG coupling area is output through the output straight waveguide 5.

[0046] S3, based on the SCG generated in step S1 and the output spectrum of a certain bandwidth range centered on the second harmonic light of the incident femtosecond pulse light coupled into the single-mode straight waveguide 4 in step S2, the light output from the coupling straight waveguide 6 is directly detected, and by fine-tuning the tuning electrode 7, the carrier envelope offset frequency f 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 after passing through the supercontinuum spectrum SCG generation area 2, it is expressed by the generalized nonlinear Schrödinger equation GNLSE, and the expression is as follows:

[0049]

[0050] Wherein, z represents the waveguide length of the supercontinuum spectrum SCG generation area 2, A(z,t) represents the slowly varying pulse envelope, α represents the loss coefficient, β (m) represents the dispersion of each order of the 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 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 high-order dispersion, iγ|A| 2 A represents the effect of self-phase modulation, iγ represents the effect of self-steep effect, represents the effect of intrapulse Raman scattering.

[0051] Then the GNLSE equation is solved by using the split-step Fourier method, and the GNLSE equation is simplified, and the expression is as follows:

[0052]

[0053] Wherein, represents the linear operator, represents a nonlinear operator, and A(z+h,t) is an approximate result of the pulse after passing through a distance h in the optical field, and the expression is as follows:

[0054]

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

[0056] The second harmonic light generated by the SCG in the step S1 is directly subjected to f-2f self-reference beating with the dispersion wave centered at λ / 2 to obtain f CEO , and the expression is as follows:

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

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

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

[0060]

[0061] wherein, * represents a conjugate, represents a constant related to the LPWG, Δβ represents a phase mismatch, Λ represents a medium perturbation period, κ represents a coupling coefficient, and ε m (x,y) represents the mth Fourier component of the periodic perturbation, and A(0) and B(0) represent the amplitudes of the modes at L=0 respectively. When the light is only input 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:

[0062]

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

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

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

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

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

[0068] In the embodiment, the method of the application simultaneously utilizes the thin film lithium niobate and the strong nonlinear effect of the ultra-short optical pulse to generate the supercontinuum spectrum SCG across an octave, and then utilizes the coupling characteristics of the LPWG to couple the light in a certain bandwidth range centered at the resonant wavelength in the SCG into the adjacent single-mode waveguide, and the output light f CEO signal is detected. When the metal electrode prepared on the waveguide is tuned, the resonant wavelength in the output spectrum is shifted to compensate for the problem that the resonant wavelength deviates from the second harmonic light due to the preparation error of the device.

[0069] In summary, the method of the application solves the problems of the existing detection methods, such as high cost and complex system. Benefiting from the single-chip simultaneously generating the supercontinuum spectrum across an octave and the second harmonic light, the additional frequency doubling crystal is no longer needed, and benefiting from the fact that the working bandwidth of the LPWG itself limits the filtering effect, the application will no longer need an external filtering element. The successful development of the application will have a broad application background in high-precision frequency measurement, optical atomic clocks, stable and reliable on-chip optical frequency comb research, and has important scientific significance and far-reaching practical significance.

[0070] Those skilled in the art will appreciate that the embodiments described herein are presented for the purpose of understanding the principles of the application and should be understood as not limiting the scope of protection of the application to such specific statements and embodiments. The application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application should be included in the scope of protection of the claims of the application.

Claims

1. A method for detecting the frequency of a carrier envelope offset based on a LPWG on-chip, which is suitable for a carrier envelope offset frequency detector based on a LPWG on-chip, comprising: Port (1), supercontinuum generation region (2), long period waveguide grating (3), single-mode straight waveguide (4), output straight waveguide (5), coupling straight waveguide (6), tuning electrode (7); Wherein, the port (1) is an ultrashort pulse input end, the supercontinuum generation region (2) is a parabolic gradually changing width waveguide region, the long period waveguide grating (3) is a multimode sidewall type, and the single-mode straight waveguide (4) is a rectangular waveguide; The specific steps of the method are as follows: S1, the incident femtosecond pulse light enters the supercontinuum generation region (2) through the port (1), and generates a supercontinuum spectrum (SCG) across an octave through nonlinear effects; S2, the SCG generated based on step S1 passes through the long period waveguide grating (3), and the part of the SCG that is mixed with the short wave dispersion wave centered on the second harmonic light of the incident femtosecond pulse light is coupled into the single-mode straight waveguide (4); Wherein, the incident femtosecond pulse light generates a supercontinuum spectrum with effective overlap of dispersion waves and second harmonic light through the long period waveguide grating (3); The remaining part of the SCG after passing through the LPWG coupling region is output through the output straight waveguide (5); S3, based on the SCG generated in step S1 and the output spectrum of a certain bandwidth range centered on the second-harmonic light of the incident femtosecond pulsed light coupled into the single-mode straight waveguide (4) in step S2, the light output from the coupling straight waveguide (6) is directly detected, and by fine tuning the tuning electrode (7), the carrier envelope offset frequency f CEO .

2. The method according to claim 1, wherein the method is based on a LPWG-based on-chip carrier envelope offset frequency self-beat frequency detection. The step S1 is specifically as follows: The incident femtosecond pulse light enters from the waveguide port (1), passes through the supercontinuum generation region (2), and is represented by the generalized nonlinear Schrödinger equation (GNLSE), the expression is as follows: where z denotes the waveguide length of the supercontinuum generation region (2), A(z,t) denotes the slowly varying pulse envelope, a denotes the loss coefficient, β (m) denotes the dispersion of the waveguide, γ denotes the nonlinear coefficient, A denotes the pulse amplitude, |A| 2 denotes the optical power, ω0denotes the central frequency, T R denotes the first moment of the nonlinear response function; the right side of the equation denotes the influence of various linear and nonlinear effects on the evolution of the pulse in the nonlinear medium, denotes the effect of the loss, denotes the effect of the higher-order dispersion, iγ|A| 2 A denotes the effect of the self-phase modulation, denotes the effect of the self-steepening, denotes the effect of the intrapulse Raman scattering; Then the GNLSE equation is solved by using the split-step Fourier method, and the GNLSE equation is simplified, the expression is as follows: where, denotes a linear operator, denotes a nonlinear operator, yielding an approximation of the pulse after a distance h in the optical field A(z + h, t), expressed as follows:

3. The method according to claim 1, wherein the method is based on the LPWG-based on-chip carrier envelope offset frequency self-beat frequency detection. The step S2 is specifically as follows: The second-order frequency light generated in step S1 is directly f-2f self-referenced with the dispersion wave centered at λ / 2 to obtain f of the optical frequency comb CEO The expression is as follows: f CEO = 2 · v N - v 2N = 2 · (Nf rep + f CEO ) - (2N · f rep + f CEO ) wherein v N represents the frequency at the center wavelength l, v 2N represents the frequency at l / 2, N represents the mode number, f rep represents the repetition frequency of the optical frequency comb; The incident pulse light passes through a coupling region with a length of L in the long period waveguide grating (3), and the output amplitudes of the LPWG and the single-mode straight waveguide (4) are A(L) and B(L) respectively, the expression is as follows: wherein, * represents conjugate, represents a constant related to the LPWG, Δβ represents a phase mismatch, κ represents a coupling coefficient, A(0) and B(0) represent the amplitudes of the modes at L=0, respectively; when light is input only from port (1), i.e. B(0)=0, the amplitude expressions of the two output straight waveguides (5) and (6) are as follows: When the phase is not matched, the maximum coupling efficiency between the two modes is When the phase is matched, i.e. Δβ = 0, the maximum coupling efficiency 100% can be obtained. When the coupling length When the wavelength is λ0, Δβ = 1.6|κ|, the full width at half maximum (FWHM) of the filter is Δλ, the smaller the coupling coefficient is, the larger the effective refractive index difference of the modes is, and the narrower the filter bandwidth is. Wherein, λ0=λ / 2 represents the center wavelength satisfying the phase matching.

4. The method according to claim 1, wherein the method is based on the LPWG-based on-chip carrier envelope offset frequency self-beat frequency detection. The step S3 is specifically as follows: Based on step S2, after passing through the waveguide with length z, the supercontinuum spectrum is obtained by solving the GNLSE equation using the split-step Fourier method, and then passing through the coupling region with length L, the light with bandwidth Δλ centered at λ / 2 is filtered out, and the output light from the port-coupled straight waveguide (6) is directly received by the spectrum analyzer to directly measure f CEO .

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