A module for simultaneously providing carrier envelope offset frequency detection and supercontinuum output

Abstract

The application discloses a module for simultaneously providing carrier envelope offset frequency detection and supercontinuum spectrum output in the field of laser science and technology. The module comprises a mode-locked laser, a fiber array, a thin film lithium niobate chip, a photoelectric detection module and a signal processing circuit. The output end of the mode-locked laser is optically connected with the input end of the thin film lithium niobate chip through the fiber array. The output end of the thin film lithium niobate chip is directly spatially output onto the light-sensitive surface of the photoelectric detection module. The output end of the photoelectric detection module is connected with the signal processing circuit through a signal adapter. The module simultaneously realizes the spectrum octave expansion output of the optical frequency comb and the high signal-to-noise ratio repetition frequency and carrier envelope offset frequency signal detection under the condition of low pulse energy, and provides strong support for developing low-power, small-size, high-stability and fully-locked optical frequency comb light sources.

Description

Technical Field

[0001] The present invention relates to the field of laser science and technology, in particular to a module that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output. Background Art

[0002] The application of optical frequency combs relies on highly stable frequency locking. Currently, this is typically achieved by locking the repetition rate and carrier-envelope offset frequency (CEO) to stabilize the comb teeth. The repetition rate signal of an optical frequency comb can be directly measured using a high-speed photodetector, while the CEO frequency signal originates from the phase deviation between the carrier and its envelope during pulse propagation. Detecting and locking the CEO frequency signal has always been a core and challenging aspect of optical comb development.

[0003] In the past two decades, the f-2f self-reference technology has been commonly used to detect carrier envelope offset frequency signals, and feedback locking has been performed through digital phase-locked loop technology. If the spectrum broadening satisfies the condition of being greater than one octave, the f can be obtained by detecting the beat frequency between the high and low frequencies within the spectrum. ceo This technology requires phase-coherent, cross-octave supercontinuum generation (SCG). The conventional method uses highly nonlinear optical fibers for waveguide broadening and then uses a self-referenced f-2f interferometer for interferometric detection. However, this method has problems such as difficulty in managing fiber dispersion, poor spectrum broadening, and bulky and complex detection equipment.

[0004] In recent years, the medium for generating supercontinuum (SC) has shown a trend of evolving from nonlinear optical fibers to nonlinear integrated optical waveguides. Among the various waveguide materials, lithium niobate has excellent second-order and third-order nonlinearities. SC output greater than one octave can be obtained on a waveguide of only centimeters in length, while the energy of the pump pulse only needs to be in the tens to hundreds of pJ level. Therefore, in highly nonlinear integrated optical waveguides, high peak power can be achieved at the χ (2) The nonlinear effect enables efficient second harmonic generation (SHG), and sufficient excess pump power is generated by the χ (3) The nonlinear effect realizes cross-octave broadening of the spectrum. By simultaneously performing SCG and SHG, and then performing an on-chip f-2f self-reference process, the carrier-envelope offset frequency signal can be directly detected to obtain a completely stable frequency comb. This method is simple and efficient, and is expected to solve the problems of high-precision detection and high signal-to-noise ratio locking of the carrier-envelope offset frequency signal of the optical frequency comb.

[0005] In 2021, Ewelina Obrzud et al. of the Swiss Center for Electron Microscopy combined a thin film lithium niobate (TFLN) waveguide with a silicon photodiode to build a compact repetition frequency and carrier envelope offset frequency detection module. By injecting low-energy pulses (~140pJ) into the waveguide, direct on-chip f-2f interference was performed, and the resulting frequency comb was fully stabilized with a high signal-to-noise ratio of 56dB. However, the SC produced by this method was not filtered, and all the broadband optical signals entered the detector, which could easily cause the detector to saturate. In addition, the SC produced in this scheme was not exported, and an additional SCG device needed to be built for applications such as optical comb spectrum and laser frequency reference. In 2023, Scott A. Diddams of the National Institute of Standards and Technology combined dispersion management and chirp quasi-phase matching on a TFLN waveguide to achieve a spectral coverage range of 330-2400nm with a pulse energy of only 90pJ, and the carrier envelope offset frequency intensity was 38dB. Although the carrier envelope offset frequency was detected, the signal-to-noise ratio was low, and frequency signal detection and SC output could not be achieved simultaneously. These research results show that TFLN is a material platform suitable for optical frequency comb frequency signal detection and SCG applications, and is expected to replace the cumbersome standard f-2f interferometer for optical frequency comb locking, providing support for providing ultra-wideband spectral output and excellent noise performance for optical frequency combs. SUMMARY

[0006] The purpose of the present application is to provide a module that simultaneously provides carrier envelope frequency detection and supercontinuum output, solving the technical problems of complex structure, low signal strength, high cost, and inability to directly output wide spectrum for existing optical frequency comb repetition frequency and carrier envelope offset frequency detection devices. Under low pulse energy conditions, the module simultaneously achieves optical spectrum octave expansion output and high signal-to-noise ratio repetition frequency and carrier envelope offset frequency signal detection, providing strong support for the development of low-power, small-size, high-stability, and fully-locked optical frequency comb light sources.

[0007] To achieve the above purpose, the technical solutions of the present application are as follows:

[0008] The present application provides a module that simultaneously provides carrier envelope frequency detection and supercontinuum output, comprising a mode-locked laser, a fiber array, a thin film lithium niobate chip, a photodetection module, and a signal processing circuit. The output end of the mode-locked laser is optically connected to the input end of the thin film lithium niobate chip through the fiber array, the output end of the thin film lithium niobate chip is directly spatially output to the photosensitive surface of the photodetection module, the output end of the photodetection module is connected to the signal processing circuit through a signal adapter, and the detection and filtered output of the repetition frequency and carrier envelope offset frequency are realized.

[0009] As one aspect of a module that simultaneously provides carrier envelope offset frequency detection and supercontinuum output according to the present invention, the optical fiber array is two polarization-maintaining optical fibers or a tapered lens optical fiber array arranged at equal intervals.

[0010] As one aspect of a module that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output according to the present invention, the optical fiber array includes a fiber core, a sleeve disposed outside the fiber core, and a coating layer disposed on the fiber core.

[0011] As one aspect of a module of the present invention that simultaneously provides carrier envelope frequency deviation detection and supercontinuum spectrum output, the thin-film lithium niobate chip is provided with an optical connection end and a free space output end, the optical connection end is an inverted cone structure, and the coupling loss between the end face of the optical connection end and the optical fiber array is ≤6dB.

[0012] As one aspect of a module of the present invention that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output, the thin-film lithium niobate chip includes a silicon dielectric layer, a silicon dioxide substrate support layer and a TFLN waveguide layer from bottom to top; the cross-section of the TFLN waveguide layer is trapezoidal or rectangular.

[0013] As one aspect of the module of the present invention that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output, the TFLN waveguide layer is provided with an asymmetric optical waveguide coupler, and the asymmetric optical waveguide coupler is a TFLN waveguide with an asymmetric waveguide width design.

[0014] As one aspect of a module of the present invention that simultaneously provides carrier envelope frequency deviation detection and supercontinuum spectrum output, the TFLN waveguide includes a straight waveguide and a curved waveguide; the input end of the straight waveguide is connected to the core of an optical fiber of the optical fiber array, and the input end of the curved waveguide is connected to the core of another optical fiber of the optical fiber array.

[0015] As one aspect of a module of the present invention that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output, the photoelectric detection module uses a Si-PIN detector or a Si-APD detector to detect the repetition frequency of light pulses and the carrier envelope offset frequency; the bandwidth of the photoelectric detection module is between 100 MHz and 4 GHz, and the size of the photosensitive surface of the photoelectric detection module is 100 μm to 5 mm.

[0016] As one aspect of a module that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output according to the present invention, the bandwidth of the signal processing circuit is greater than the repetition frequency of the optical frequency comb pulse.

[0017] By adopting the above technical solution, the present invention has the following advantages:

[0018] The present invention provides a module that simultaneously provides carrier-envelope offset frequency detection and supercontinuum spectrum output. By structurally designing a TFLN chip waveguide, the present invention solves the technical problems of the complex structure, low detection signal intensity, high cost, and inability to directly apply wide-spectrum output to existing optical frequency comb repetition frequency and carrier-envelope offset frequency detection devices for optical frequency combs. The present invention achieves supercontinuum spectrum generation with a spectral range exceeding an octave at low pulse energy, simultaneously performs second harmonic generation and supercontinuum spectrum generation on a single chip, realizes high-precision detection and high signal-to-noise ratio locking of carrier-envelope offset frequency signals, and simultaneously provides ultra-wideband mid-infrared supercontinuum spectrum output, simplifying system links and locking difficulty. The present invention has the advantages of integration, small size, light weight, high reliability, and high stability, and can provide strong support for the development of low-power, small-size, highly stable, and fully locked optical frequency comb light sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A block diagram of a module that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output according to the present invention;

[0020] Figure 2 An embodiment of the module for simultaneously providing carrier envelope offset frequency detection and supercontinuum spectrum output according to the present invention;

[0021] Figure 3 Schematic diagram of the cross-sectional structure of the TFLN waveguide layer in an embodiment of the present invention;

[0022] Figure 4 This is a schematic structural diagram of a thin-film lithium niobate chip according to an embodiment of the present invention;

[0023] Figure 5 is a repetition frequency spectrum diagram in an embodiment of the invention;

[0024] Figure 6 1 is a diagram of a carrier envelope offset frequency spectrum in an embodiment of the present invention;

[0025] Figure 7 This is a spectrum diagram of the ultra-wideband mid-infrared supercontinuum in an embodiment of the present invention. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is described in detail below with reference to the accompanying drawings. It should be noted that, in this document, relational terms such as first and second, etc., are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include," "comprise," or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or apparatus.

[0027] A module that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output can be used to simultaneously provide repetition frequency and carrier envelope offset frequency detection for a femtosecond optical frequency comb with a central wavelength of 1530-1600nm, and realize ultra-wideband infrared supercontinuum spectrum output. The module for generating optical frequency comb repetition frequency, carrier envelope offset frequency detection and supercontinuum spectrum output includes a mode-locked laser 1, an optical fiber array 2, a thin film lithium niobate chip 3 (TFLN chip), a photoelectric detection module 4 and a signal processing circuit 5. Figure 1 、 Figure 2 As shown in the figure, the output of the mode-locked laser 1 is optically connected to the input of the thin-film lithium niobate chip 3 via an optical fiber array 2. The optical fiber array 2 provides femtosecond optical frequency comb pulse input and ultra-wideband infrared supercontinuum output. The output of the thin-film lithium niobate chip 3 is directly spatially output to the photosensitive surface of the photodetection module 4. The output of the photodetection module 4 is connected to the signal processing circuit 5 via a signal adapter cable to detect and filter the repetition frequency and carrier envelope offset frequency.

[0028] In addition, the size of the module that provides carrier envelope offset frequency detection and supercontinuum spectrum output is within 50mm*30mm*5mm. The optical fiber array 2 is an array of two polarization-maintaining optical fibers PM2000 or tapered lens optical fibers arranged at equal intervals. The optical fiber spacing between the two polarization-maintaining optical fibers is 125-130μm; the optical mode field size of the polarization-maintaining optical fibers is 1-10μm; the micro lens at the front end of the tapered lens optical fiber has the functions of focusing and collimating, and the output light spot mode field size is 1-10μm. The optical fiber array is specifically as follows Figure 2 As shown, it includes a fiber core 23, a sleeve 21 arranged outside the fiber core 23, and a coating layer 22 arranged on the fiber core.

[0029] The thin film lithium niobate chip (TFLN chip) 3 has a length of 1 to 30 mm and a width of 1 to 30 mm, and includes a silicon dielectric layer, a silicon dioxide substrate support layer and a TFLN waveguide layer from bottom to top. The silicon dioxide substrate support layer is used to protect the TFLN waveguide layer; the cross section of the TFLN waveguide layer is a trapezoidal or rectangular shape. Figure 3As shown, an optical waveguide is etched onto the TFLN waveguide layer. The waveguide width w ranges from 500 to 3000 nm, the etch depth h ranges from 200 to 400 nm, and the waveguide tilt angle θ ranges from 45° to 90°. Leveraging the excellent linear electro-optical effect of lithium niobate and the high refractive index contrast of the TFLN platform, both SCG and SHG are performed simultaneously. The TFLN waveguide layer is equipped with an asymmetric optical waveguide coupler. These asymmetric waveguide couplers are TFLN waveguides with asymmetric waveguide widths, enabling wavelength division processing of high- and low-frequency supercontinuum energy. The wavelength range of the through-waveguide light is 700 to 900 nm, enabling direct spatial output of optical signals within this band and cross-coupling of optical signals within the 900 to 2500 nm band.

[0030] The thin film lithium niobate chip 3 is provided with an optical connection end and a free space output end. The optical connection end is an inverted cone structure. The coupling loss between the end face of the optical connection end and the optical fiber array is ≤6dB.

[0031] The TFLN waveguide includes a straight waveguide and a curved waveguide. The input end of the straight waveguide is connected to the core 23 of one optical fiber in the optical fiber array 2, while the input end of the curved waveguide is connected to the core 23 of another optical fiber in the optical fiber array 2. The straight waveguide of the TFLN chip can generate a supercontinuum output of 700 to 2500 nm under pulse pumping with a pulse energy of less than 100 pJ and a pulse width of 100 fs.

[0032] The photoelectric detection module 4 uses a Si-PIN detector or Si-APD detector to detect the repetition frequency and carrier envelope offset frequency of the light pulse; the bandwidth of the photoelectric detection module is between 100MHz and 4GHz, and the photosensitive surface size of the photoelectric detection module 4 is 100μm to 5mm.

[0033] The signal processing circuit 5 is used to amplify and filter the carrier envelope offset frequency and repetition frequency signals. The bandwidth of the signal processing circuit 5 is greater than the repetition frequency of the optical frequency comb pulse.

[0034] The working principle of a module that provides both carrier envelope offset frequency detection and supercontinuum spectrum output is as follows:

[0035] The output end of the mode-locked laser 1 is connected to the input end of the thin-film lithium niobate chip 3 via an optical fiber array. The core 23 of one optical fiber in the optical fiber array 2 is connected to the input end 31 of the straight waveguide of the thin-film lithium niobate chip 3. The core 23 of another optical fiber in the optical fiber array 2 is connected at one end to the input end 32 of the curved waveguide of the thin-film lithium niobate chip 3, and at the other end to an optical instrument such as a spectrum analyzer for analyzing the received supercontinuum optical frequency components. An asymmetric optical waveguide coupler is designed on the TFLN chip 3. Light is spatially coupled into the waveguide from the input end 31 of the straight waveguide. A portion of light is directly output spatially from the output end 34 through the straight waveguide, while the other portion is coupled into the curved waveguide at the coupling connection 33 of the asymmetric optical waveguide coupler and spatially output from the other end 32 of the curved waveguide. Figure 4 The straight waveguide output end 34 of the TFLN chip 3 directly outputs and is aligned and connected to the photodetector 4 through spatial coupling. The output end of the photodetector 4 is connected to the signal processing circuit 5 through a signal adapter cable to achieve frequency division, filtering, amplification and locking of the repetition frequency and carrier envelope offset frequency signals.

[0036] The repetition frequency spectrum diagram of the present invention is as follows Figure 5 As shown, the repetition frequency detected by the photoelectric detector and the signal processing circuit is 125MHz, and the signal-to-noise ratio is greater than 70dB. Figure 6 As shown, the repetition frequency range detected by the photodetector and signal processing circuit is 0-125MHz, and the signal-to-noise ratio is greater than 35dB. Figure 7 As shown, the spectrum broadening range is 900~2500nm.

[0037] Finally, it should be pointed out that although the present invention has been described with reference to the current specific embodiments, ordinary technicians in this technical field should realize that the above embodiments are only used to illustrate the present invention and are not used to limit the present invention. Various equivalent changes or substitutions can be made without departing from the concept of the present invention. Therefore, as long as the changes and modifications to the above embodiments are within the scope of the essential spirit of the present invention, they will fall within the scope of the claims of the present invention.

Claims

1. A module that simultaneously provides carrier envelope offset frequency detection and supercontinuum spectrum output, characterized in that: The system comprises a mode-locked laser, an optical fiber array, a thin-film lithium niobate chip, a photoelectric detection module, and a signal processing circuit; the output end of the mode-locked laser is optically connected to the input end of the thin-film lithium niobate chip via the optical fiber array, the output end of the thin-film lithium niobate chip is directly spatially output to the photosensitive surface of the photoelectric detection module, and the output end of the photoelectric detection module is connected to the signal processing circuit via a signal adapter cable to realize detection and filtering output of repetition frequency and carrier envelope offset frequency; Among them, an asymmetric optical waveguide coupler is designed on the thin-film lithium niobate chip. Light is spatially coupled into the waveguide from the input end of the straight waveguide, and part of it is directly output from the output end space through the straight waveguide. The other part is coupled into the curved waveguide at the coupling connection of the asymmetric optical waveguide coupler and output from the other end space of the curved waveguide. The straight waveguide output end of the thin-film lithium niobate chip is directly output and is aligned and connected to the photoelectric detection module through spatial coupling.

2. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 1, characterized in that: The optical fiber array is two polarization-maintaining optical fibers or a tapered lens optical fiber array arranged at equal intervals.

3. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 2, characterized in that: The optical fiber array includes a fiber core, a sleeve arranged outside the fiber core, and a coating layer arranged on the fiber core.

4. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 3, characterized in that: The thin-film lithium niobate chip is provided with an optical connection end and a free-space output end. The optical connection end is an inverted cone structure. The coupling loss between the end face of the optical connection end and the optical fiber array is ≤6 dB.

5. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 4, characterized in that: The thin film lithium niobate chip comprises a silicon dielectric layer, a silicon dioxide substrate support layer and a TFLN waveguide layer from bottom to top; the cross section of the TFLN waveguide layer is trapezoidal or rectangular.

6. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 5, characterized in that: The TFLN waveguide layer is provided with an asymmetric optical waveguide coupler, and the asymmetric optical waveguide coupler is a TFLN waveguide with an asymmetric waveguide width design.

7. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 6, characterized in that: The TFLN waveguide includes a straight waveguide and a bent waveguide; the input end of the straight waveguide is connected to the core of one optical fiber of the optical fiber array, and the input end of the bent waveguide is connected to the core of another optical fiber of the optical fiber array.

8. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 5, characterized in that: The photoelectric detection module uses a Si-PIN detector or a Si-APD detector to detect the repetition frequency and carrier envelope offset frequency of the light pulse; the bandwidth of the photoelectric detection module is between 100 MHz and 4 GHz, and the size of the photosensitive surface of the photoelectric detection module is between 100 μm and 5 mm.

9. The module for simultaneously providing carrier envelope frequency deviation detection and supercontinuum spectrum output according to claim 1, characterized in that: The bandwidth of the signal processing circuit is greater than the repetition frequency of the optical frequency comb pulse.

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