Self-referenced locked optical frequency comb generation system based on thin-film lithium niobate

Abstract

A self-referenced locked optical frequency comb generation system based on thin-film lithium niobate includes: a first pump optical coupler, a first bus waveguide, an active microring, a mode spot converter, an optical amplifier, and a first end face coupler arranged on a rare earth-doped thin-film lithium niobate chip along the propagation direction of the optical signal; a second end face coupler, a wavelength division multiplexer, a dispersion compensation waveguide, a second bus waveguide, a passive microring, and a beam splitter arranged on the thin-film lithium niobate chip along the propagation direction of the optical signal; and a second pump optical coupler, an optical frequency comb output coupler, an optical filter, and a frequency doubling optical output coupler. The present invention realizes the generation of an on-chip self-locked optical frequency comb by using devices such as a mode-locked laser, an optical amplifier, and a high-Q value microring based on thin-film lithium niobate, and has broad application prospects.

Description

Technical Field

[0001] The invention relates to an optical frequency comb, in particular to an optical frequency comb generation system based on self-reference locking of thin-film lithium niobate. Background Art

[0002] Optical frequency combs, referred to as optical frequency combs, are widely used in ultrafast optics, spectroscopy, optical clocks, optical communications and other fields. Traditional optical frequency combs are based on mode-locked lasers, combined with supercontinuum generation and frequency doubling structures to achieve self-reference locking of frequency. In the past decade, optical frequency combs based on high-Q micro-resonators (referred to as microcavity optical frequency combs, microcombs) have received attention. Microcavity optical frequency combs can generate a very wide spectrum through cascaded four-wave mixing in the cavity. However, microcavity optical frequency combs are not strictly speaking optical frequency combs, because they do not achieve self-reference locking of frequency. To achieve self-reference locking, existing reports require the use of two microcavity optical frequency combs with different comb tooth spacing and spectral widths, combined with complex systems such as interlocking, frequency doubling, and auxiliary lasers to achieve self-reference locking (D. Spencer, et al., "An optical-frequency synthesizer using integrated photonics," Nature 557, 81-85 (2018), and Z. Newman, et al., "Architecture for the photonic integration of an optical atomic clock," Optica 6, 680-685 (2019)), and the use of multiple materials also makes photonic integration difficult.

[0003] Therefore, it is necessary to study a self-referenced locked optical frequency comb generation scheme with a simpler system and more convenient photonic integration. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a self-referenced locked optical frequency comb generation system based on thin-film lithium niobate. By adopting devices such as a mode-locked laser, an optical amplifier and a high-Q value microring based on thin-film lithium niobate, the generation of an on-chip self-locked optical frequency comb is achieved, which has broad application prospects.

[0005] The technical solution of the present invention is as follows:

[0006] A self-referenced locked optical frequency comb generation system based on thin-film lithium niobate, which is characterized by:

[0007] A first pump laser, used for providing a first pump light;

[0008] a second pump laser, used for providing a second pump light;

[0009] A first pump optical coupler, a first bus waveguide, an active microring, a mode spot converter, an optical amplifier, and a first end face coupler are arranged on a rare earth-doped lithium niobate chip along the propagation direction of the optical signal; the active microring is an annular single-mode lithium niobate waveguide structure, located on one side of the first bus waveguide, and the coupling of the optical field in the first bus waveguide with the optical field in the active microring is achieved through evanescent waves, and the active microring has an electro-optical phase shifter and a first thermo-optical phase shifter;

[0010] A second end face coupler, a wavelength division multiplexer, a dispersion compensation waveguide, a second bus waveguide, a passive microring and a beam splitter are arranged on the thin film lithium niobate chip along the propagation direction of the optical signal; and a second pump light coupler, an optical frequency comb output coupler, an optical filter and a frequency doubling light output coupler; the passive microring is an annular high-Q value single-mode lithium niobate waveguide structure, located on one side of the second bus waveguide, and realizes the coupling of the optical field in the second bus waveguide with the optical field in the passive microring through the evanescent wave, and a second thermo-optical phase shifter is provided on the passive microring;

[0011] After optical coupling through the first pump light coupler, a portion of the first pump light is coupled into the active microring through the first bus waveguide via evanescent wave coupling to generate gain, and the other portion is coupled into the optical amplifier through the mode spot converter to provide gain for the optical amplifier in a forward pumping manner; the first end face coupler is connected to the second end face coupler, and the two are located at the edges of their respective chips to achieve low-loss optical coupling between the two chips; the signal light sequentially enters the combining port of the wavelength division multiplexer through the first end face coupler and the second end face coupler; the second pump light is optically coupled through the second pump light coupler, and then enters the wavelength division multiplexer through the single-mode waveguide. The pump input port of the multiplexer is divided into two paths through the wavelength division multiplexer, wherein one path of pump light is output from the combining port of the wavelength division multiplexer, and then enters the optical amplifier through the second end face coupler and the first end face coupler in sequence, and provides gain for the optical amplifier in a backward pumping mode; the other path of signal light is output from the signal light port of the wavelength division multiplexer, and then passes through the dispersion compensation waveguide, the second bus waveguide and the beam splitter in sequence, and then is divided into two paths, one path enters the optical frequency comb output coupler through the single-mode waveguide and is output, and then is received by the photodetector, and the other path passes through the optical filter and the frequency-doubled light output coupler in sequence, and then is received by the photodetector;

[0012] A photoelectric detector, used for beating the frequency-doubled light with a signal of a corresponding frequency in the optical frequency comb, and transmitting the beating signal to the controller;

[0013] The controller is used to control the output power of the first pump laser and the second pump laser, and to control the electro-optic phase shifter and the first thermo-optic phase shifter in the active microring, and to control the second thermo-optic phase shifter in the passive microring.

[0014] Preferably, the first pump light coupler is located at the edge of the rare earth-doped lithium niobate chip, and is used to couple the pump light into the rare earth-doped lithium niobate chip.

[0015] Preferably, the electro-optic phase shifter comprises a waveguide section and radio frequency electrodes on both sides thereof. When an electrical signal modulation is applied to the electrodes, a sequence of optical pulses having a repetition frequency equal to the modulation frequency is generated in the active micro-ring through the electro-optic effect.

[0016] Preferably, the first thermo-optic phase shifter and the second thermo-optic phase shifter include a waveguide and a heating resistor on one side thereof. When a voltage is applied to the heating resistor, the refractive index of the waveguide is changed by utilizing the thermo-optic effect, so as to fine-tune the FSR of the active microring or the passive microring.

[0017] Preferably, the dispersion compensation waveguide is a section of single-mode lithium niobate waveguide with a specific width, whose dispersion sign is opposite to the dispersion sign of the optical amplifier waveguide; the length is satisfied, and the total dispersion value of the dispersion compensation waveguide offsets the total dispersion value of the optical amplifier, which is used to compensate for the broadening of the optical pulse caused by the amplification process of the optical amplifier.

[0018] Preferably, the waveguide dispersion of the active microring is anomalous dispersion, and the waveguide dispersion of the passive microring is near-zero dispersion.

[0019] Preferably, the first bus waveguide is a single-mode waveguide, the optical amplifier is a multi-mode waveguide, and the waveguide where the spot mode converter is connected to the first bus waveguide is a single-mode waveguide of the same width, which is then widened into a multi-mode waveguide through adiabatic stretching and connected to the optical amplifier.

[0020] Preferably, one end of the first end face coupler is connected to an optical amplifier, and the waveguide is further widened by adiabatic pulling, so that a wider waveguide width is provided at the edge of the chip; the waveguide widths of the first end face coupler and the second end face coupler at the edges of their respective chips are similar and wider, so that the waveguide coupling between the two chips is insensitive to alignment errors, and the waveguide width of the second end face coupler is adiabatically reduced along the propagation direction of the optical signal, and finally becomes a single-mode waveguide connected to the combining end of the wavelength division multiplexer.

[0021] Preferably, the optical frequency comb output coupler is located at the edge of the thin-film lithium niobate chip, and is used to couple the generated optical frequency comb into free space or output optical fiber.

[0022] Preferably, the central frequency of the transmission spectrum of the optical filter is equal to the doubled frequency of the optical frequency comb signal light, and is used to select the frequency band where the doubled frequency light in the optical frequency comb is located.

[0023] Preferably, the frequency-doubled light output coupler is located at the edge of the thin-film lithium niobate chip, and is used to couple the output light of the optical filter into free space or output optical fiber.

[0024] Preferably, the rare earth-doped lithium niobate chip comprises, from bottom to top, a substrate, a lower cladding and a rare earth-doped lithium niobate waveguide layer, and the first pump optical coupler, the first bus waveguide, the active microring, the mode spot converter, the optical amplifier and the first end face coupler are all prepared on the rare earth-doped lithium niobate waveguide layer by an etching process.

[0025] Preferably, the thin-film lithium niobate chip is composed of a substrate, a lower cladding and a thin-film lithium niobate waveguide layer from bottom to top, and the second end face coupler, wavelength division multiplexer, dispersion compensation waveguide, second bus waveguide, passive microring, beam splitter, second pump light coupler, optical frequency comb output coupler, optical filter and frequency doubling light output coupler are all prepared on the thin-film lithium niobate waveguide layer by etching process.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1) Thin-film lithium niobate materials can provide gain by doping rare earth elements, have ultra-low waveguide loss (less than 5dB / m) and Kerr nonlinearity to achieve efficient spectral broadening, and have second-order nonlinearity to achieve high-speed electro-optical modulation and double frequency. The key optical devices for generating optical frequency combs, such as active microrings, optical amplifiers, dispersion compensation waveguides, passive microrings, optical beam splitters, and optical filters, are all prepared on a thin-film lithium niobate photonic integrated platform. The coupling efficiency between these devices is high and the loss is low, so the efficiency of optical frequency comb generation can be improved.

[0028] 2) Except for the pump laser, optical isolator and photodetector, all optical devices are prepared on a thin-film lithium niobate photonic integration platform with high integration, which helps to improve the stability and reliability of the system.

[0029] 3) Soliton light pulses are generated by electro-optic modulation using an active microring with anomalous dispersion. After amplification, they enter a passive microring with near-zero dispersion to achieve the simultaneous generation of supercontinuum and frequency doubling. This has the advantages of simple structure and high integration, and does not require auxiliary devices such as off-chip frequency doubling and auxiliary lasers. Therefore, it has good practical prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the chip portion of the self-referenced locked optical frequency comb generation system based on thin-film lithium niobate of the present invention.

[0031] Figure 2 It is a complete schematic diagram of the self-referenced locked optical frequency comb generation system based on thin-film lithium niobate of the present invention.

[0032] In the figure: 1-rare earth doped thin film lithium niobate chip, 11-first pump optical coupler, 12-first bus waveguide, 13-active microring resonator, 14-electro-optical phase shifter 14, 15-first thermo-optical phase shifter, 16-mode spot converter, 17-optical amplifier, 18-first end coupler;

[0033] 2-thin-film lithium niobate chip, 21-second end coupler, 22-wavelength division multiplexer, 23-dispersion compensation waveguide, 24-second bus waveguide, 25-passive microring, 26-second thermo-optical phase shifter, 27-beam splitter, 221-second pump light coupler, 271-optical frequency comb output coupler, 272-optical filter, 273-frequency-doubled light output coupler 273;

[0034] 3-first pump laser, 4-first optical isolator, 5-second pump laser, 6-second optical isolator, 7-photodetector, 8-controller. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0036] Please refer to Figure 1 , Figure 1 The present invention is a schematic diagram of a lens-assisted beam scanning device based on thin-film lithium niobate, as shown in the figure, comprising: a rare earth-doped thin-film lithium niobate chip 1 and a thin-film lithium niobate chip 2. On the rare earth-doped thin-film lithium niobate chip, along the propagation direction of the optical signal, there are the first pump optical coupler 11, the first bus waveguide 12, the active microring 13, the mode spot converter 16, the optical amplifier 17, and the first end coupler 18 in sequence; on the thin-film lithium niobate chip 2, along the propagation direction of the optical signal, there are the second end coupler 21, the wavelength division multiplexer 22, the dispersion compensation waveguide 23, the second bus waveguide 24, the passive microring 25, and the beam splitter 27 in sequence; the pump input port of the wavelength division multiplexer 22 is connected to the second pump optical coupler 221 through a single-mode waveguide; the two output ends of the beam splitter 27, one output end is connected to the optical frequency comb output coupler 271 through a single-mode waveguide, and the other output end is connected to the frequency-doubling optical output coupler 273 through an optical filter 272.

[0037] The rare earth-doped lithium niobate chip is composed of a substrate, a lower cladding layer and a rare earth-doped lithium niobate waveguide layer from bottom to top. The first pump optical coupler, the first bus waveguide, the active microring, the mode spot converter, the optical amplifier and the first end face coupler are all prepared on the rare earth-doped lithium niobate waveguide layer by etching and other processes. The substrate is a supporting material, such as silicon or quartz. The lower cladding layer is a material with a lower refractive index than that of the rare earth-doped lithium niobate, such as silicon oxide. The thickness of the lower cladding layer should be much greater than the thickness of the rare earth-doped lithium niobate waveguide layer (more than 5 times) to ensure that the light field in the waveguide layer will not leak into the substrate.

[0038] The thin film lithium niobate chip is composed of a substrate, a lower cladding layer and a non-doped thin film lithium niobate waveguide layer from bottom to top. The second end face coupler, wavelength division multiplexer, dispersion compensation waveguide, second bus waveguide, passive micro-ring, beam splitter, second pump light coupler, optical frequency comb output coupler, optical filter and frequency doubling light output coupler are all prepared on the non-doped thin film lithium niobate waveguide layer by etching and other processes. The substrate is a supporting material, such as silicon or quartz. The lower cladding is a material with a lower refractive index than non-doped thin film lithium niobate, such as silicon oxide. The thickness of the lower cladding should be much greater than the thickness of the non-doped thin film lithium niobate waveguide layer (more than 5 times) to ensure that the light field in the waveguide layer will not leak into the substrate.

[0039] The first pump light coupler 11 is located at the edge of the rare earth-doped lithium niobate chip and is used to couple the pump light into the rare earth-doped lithium niobate chip. The pump light passes through the first bus waveguide, and a part of it enters the active microring through evanescent wave coupling to generate gain, while the other part continues along the first bus waveguide and enters the optical amplifier after passing through the spot mode converter, providing gain for the optical amplifier in a forward pumping manner.

[0040] The first bus waveguide 12 is a single-mode waveguide, operating in a transverse electric (TE) mode or a transverse magnetic (TM) mode.

[0041] The active microring 13 is a ring-shaped single-mode lithium niobate waveguide structure, located on one side of the first bus waveguide, and realizes the coupling of the light field in the first bus waveguide with the light field in the active microring through evanescent waves. A high-speed electro-optic phase shifter 14 is prepared on a part of the waveguide in the active microring, and a thermo-optic phase shifter 15 is prepared on another part of the waveguide.

[0042] The electro-optic phase shifter 14 is a waveguide with high-speed radio frequency electrodes on both sides. When an electrical signal is modulated on the electrodes, the electro-optic effect of lithium niobate will generate periodic disturbances of the refractive index in the microring. When the modulation frequency of the electrical signal is equal to the free spectral range (FSR) of the active microring or an integer multiple of the FSR, and the gain generated by the pump light in the cavity is greater than the loss of the active microring, a sequence of optical pulses with a repetition frequency equal to the modulation frequency will be generated in the active microring. This is an active mode-locked laser structure that can generate picosecond or femtosecond pulses.

[0043] The thermo-optic phase shifter 15 is a device in which a heating resistor is prepared on the top or one side of a waveguide. A voltage is applied to the heating resistor to heat the heating resistor, thereby changing the temperature of the waveguide. The refractive index of the waveguide is changed by the thermo-optic effect of lithium niobate, and the FSR of the active microring is fine-tuned. When the heating resistor is prepared on the top of the waveguide, it is necessary to cover the waveguide with an upper cladding layer with a low refractive index, and then prepare the heating resistor and electrodes on the upper cladding layer. The thickness of the upper cladding layer should ensure that the heating resistor and the electrode will not cause additional loss to the optical signal in the waveguide.

[0044] The mode spot converter 16 is a section of adiabatic widening waveguide, the width of one end of which is connected to the first bus waveguide is the same as the width of the first bus waveguide, and the width of one end of which is connected to the optical amplifier is the same as the waveguide width of the optical amplifier, and the latter is wider than the former. The adiabatic widening waveguide allows the optical signal to remain single-mode when it enters the multi-mode optical amplifier from the single-mode bus waveguide.

[0045] The optical amplifier 17 is a multi-mode rare earth-doped lithium niobate waveguide with a relatively large width and a relatively long length.

[0046] The first end coupler 18 converts the multi-mode waveguide of the optical amplifier into a single-mode waveguide through adiabatic shrinkage, and its output end is located at the edge of the rare earth-doped lithium niobate chip.

[0047] The second end face coupler 21 is a single-mode lithium niobate waveguide, and its spot size at the edge of the thin-film lithium niobate chip should be as similar as possible to the spot size of the end face coupler A at the edge of the rare earth-doped thin-film lithium niobate chip to achieve low-loss optical coupling between the two chips.

[0048] The wavelength division multiplexer 22 is used to couple the pump light and the signal light, and is a three-port device, including a combining port, a signal light port and a pump light port. The combining port is connected to the second end face coupler, and the second pump light enters the pump light port of the wavelength division multiplexer through the second pump light coupler 221 through a section of single-mode waveguide, and the signal light port is connected to the dispersion compensation waveguide 23. The second pump light enters the optical amplifier through the wavelength division multiplexer, the second end face coupler and the first end face coupler, and provides gain for the optical amplifier by backward pumping.

[0049] The dispersion compensation waveguide 23 is a single-mode lithium niobate waveguide with a specific width, whose dispersion sign is opposite to the dispersion sign of the waveguide of the optical amplifier. Its length should be sufficient to ensure that the total dispersion value of the dispersion compensation waveguide just offsets the total dispersion value of the optical amplifier, and is used to compensate for the broadening of the optical pulse caused by the amplification process of the optical amplifier.

[0050] The second bus waveguide 24 is a section of single-mode lithium niobate waveguide.

[0051] The passive microring 25 is a ring-shaped high-Q single-mode lithium niobate waveguide structure, located on one side of the second bus waveguide, and realizes the coupling of the light field in the second bus waveguide with the light field in the passive microring through evanescent waves. A second thermo-optical phase shifter 26 is prepared on a part of the waveguide in the passive microring. The waveguide dispersion in the passive microring is designed to be a near-zero dispersion with a wide spectrum, and its dispersion value within one octave should be as small as possible.

[0052] The second thermo-optic phase shifter 26 is a heating resistor prepared on the top or one side of a waveguide. By applying a voltage to the heating resistor, the heating resistor generates heat, changes the temperature of the waveguide, and uses the thermo-optic effect of lithium niobate to change the refractive index of the waveguide, which is used to fine-tune the FSR of the passive microring so that the FSR of the passive microring is equal to the FSR of the active microring. When the heating resistor is prepared on the waveguide, it is necessary to cover the waveguide with an upper cladding layer with a low refractive index, and then prepare the heating resistor and electrodes on the upper cladding layer. The thickness of the upper cladding layer should ensure that the heating resistor and the electrode will not cause additional loss to the optical signal in the waveguide.

[0053] The beam splitter 27 has an input end and two output ends, the input end is connected to the second bus waveguide, one output end is connected to the optical frequency comb output coupler 271 through a single-mode waveguide, and the other output end is connected to the frequency-doubled light output coupler 273 through an optical filter 272.

[0054] The optical frequency comb output coupler 271 is located at the edge of the thin-film lithium niobate chip and is used to couple the generated optical frequency comb into free space or output optical fiber.

[0055] The central frequency of the transmission spectrum of the optical filter 272 is equal to the double frequency of the optical frequency comb signal light, and is used to select the frequency band where the doubled frequency light in the optical frequency comb is located.

[0056] The frequency-doubled light output coupler 273 is located at the edge of the thin-film lithium niobate chip and is used to couple the output light of the optical filter into free space or output optical fiber.

[0057] The system also includes a first pump laser 3, a first optical isolator 4, a second pump laser 5, a second optical isolator 6, a photodetector 7 and a controller 8, which are arranged outside the chip. The pump light emitted by the first pump laser is coupled into the rare earth-doped thin film lithium niobate chip through the first optical isolator and the first pump optical coupler, and then provides gain for the active microring and the optical amplifier respectively through the first bus waveguide. The pump light emitted by the second pump laser is coupled into the non-doped thin film lithium niobate chip through the second optical isolator and the second pump optical coupler, and then enters the optical amplifier through the wavelength division multiplexer, the second end coupler and the first end coupler to provide gain for it. The light beam output by the frequency-doubled light output coupler enters the photodetector through free space or optical fiber, and the photodetector beats the frequency-doubled light with the signal of the corresponding frequency in the optical frequency comb. The beat signal enters the controller. The controller controls the output power of the first pump laser and the second pump laser, controls the electro-optic phase shifter and the thermo-optic phase shifter in the active microring, and controls the thermo-optic phase shifter in the passive microring.

[0058] The principle and working process of the system are as follows:

[0059] The controller 8 controls the first pump laser 3 and the second pump laser 5 to output pump light to provide gain for the active micro-ring and the optical amplifier.

[0060] The controller 8 applies an electrical signal to the electro-optical phase shifter 14 in the active microring for modulation, and the modulation frequency is equal to an integer multiple of the FSR of the active microring. At this time, a mode-locked optical pulse is generated in the active microring, and the repetition frequency of the optical pulse is equal to the modulation frequency. The optical pulse is coupled into the first bus waveguide through the evanescent wave and amplified in the optical amplifier. The amplified optical pulse enters the dispersion compensation waveguide after passing through the first end coupler, the second end coupler and the wavelength division multiplexer to compensate for its pulse broadening in the optical amplifier. The compressed optical pulse is coupled into the passive microring through the second bus waveguide. The thermo-optical phase shifter of the passive microring is controlled so that the FSR of the passive microring is equal to the FSR of the active microring. At this time, the optical pulse is spectrum-broadened in the passive microring through the four-wave mixing effect with the highest efficiency, that is, an optical frequency comb is generated. Since the waveguide dispersion of the passive microring is designed to be a wide-spectrum near-zero dispersion, the spectrum of the generated optical frequency comb can cover an octave, that is, the spectrum range covers f to 2f, and f is a certain frequency in the spectrum of the optical frequency comb. At the same time, due to the second-order nonlinearity of lithium niobate, a double frequency will be generated in the passive microring, that is, the double frequency 2f of f will be generated. The generated optical frequency comb passes through the beam splitter, and a part of it is directly output by the optical frequency comb output coupler for practical use, and the other part is selected by the optical filter to select the optical signal near 2f0.

[0061] The frequency of the optical frequency comb can be expressed as f m =f ceo +mf R , where m is a positive integer, f ceo is the carrier envelope offset frequency of the frequency comb, f R is the repetition frequency of the light pulse, which is equal to the modulation frequency, and 0≤f ceo <f R . Then when the optical frequency comb contains octaves, it also contains f m and f 2m =f ceo +2mf R . And f m The double frequency is 2f m =2f ceo +2mf R , so the output of the optical filter will also contain f 2m =f ceo +2mf R and 2f m =2f ceo +2mf R When this signal is beat by a photodetector, the carrier-envelope offset frequency f of the optical frequency comb can be extracted. ceo =2f m -f 2mThis beat signal can be compared with the reference clock of the controller, and f can be adjusted by fine-tuning the output power of the first pump laser. ceo , to achieve f ceo On the other hand, by controlling the modulation frequency on the active microring, the repetition frequency f can be achieved. R Thus, a complete self-reference locking of the optical frequency comb is achieved.

[0062] Preferably, the rare earth-doped thin film lithium niobate chip and the undoped thin film lithium niobate chip may be two independent chips, or may be two rare earth-doped and undoped regions on one chip.

[0063] Preferably, the thickness of the rare earth-doped lithium niobate thin film waveguide layer and the undoped lithium niobate thin film waveguide layer is between 100 nm and 3 μm.

[0064] Preferably, the rare earth doped lithium niobate thin film waveguide layer is x-cut.

[0065] Preferably, the rare earth element doped in the rare earth-doped lithium niobate thin film waveguide layer is ytterbium (Yb), erbium (Er) or thulium (Tm), and the corresponding gain wavelengths of the optical signal are around 1 μm, 1.55 μm and 2 μm, respectively.

[0066] Preferably, the output wavelengths of the pump laser A and the pump laser B correspond to the pump wavelength required by the gain wavelength of the rare earth-doped lithium niobate waveguide layer. For example, when the doped rare earth element is erbium, the gain wavelength is around 1.55 μm, and the pump wavelength is 980 nm or 1480 nm.

[0067] Preferably, the electric field generated by the high-speed electrode in the electro-optic phase shifter in the active microring is along the crystal axis direction of the rare earth-doped lithium niobate waveguide layer, that is, the z-axis direction.

[0068] Preferably, the modulation frequency applied by the controller to the electro-optic phase shifter in the active microring is approximately equal to the FSR of the active microring, and the relative deviation between the modulation frequency and the FSR is less than 5%.

[0069] Preferably, the waveguide in the optical amplifier has normal dispersion, so that the optical pulse entering therein can undergo self-similar amplification in the optical amplifier.

[0070] Preferably, the dispersion compensation waveguide has anomalous dispersion to compensate for pulse broadening of optical pulses generated in an optical amplifier.

[0071] Preferably, the relative deviation between the FSR of the passive microring and the active microring is less than 1%.

[0072] Preferably, all the optical devices operate in the same mode, ie, TE mode or TM mode.

[0073] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A self-referenced locked optical frequency comb generation system based on thin-film lithium niobate, characterized in that: include: A first pump laser (3), used for providing a first pump light; A second pump laser (5), used for providing a second pump light; A first pump optical coupler (11), a first bus waveguide (12), an active microring (13), a mode spot converter (16), an optical amplifier (17), and a first end face coupler (18) are arranged on a rare earth-doped lithium niobate thin film chip (1) along the propagation direction of the optical signal; the active microring (13) is an annular single-mode lithium niobate waveguide structure, located on one side of the first bus waveguide (12), and achieves coupling of the optical field in the first bus waveguide (12) with the optical field in the active microring (13) through evanescent waves; and an electro-optical phase shifter (14) and a first thermo-optical phase shifter (15) are provided on the active microring (13); A second end face coupler (21), a wavelength division multiplexer (22), a dispersion compensation waveguide (23), a second bus waveguide (24), a passive microring (25) and a beam splitter (27) are arranged on a thin film lithium niobate chip (2) along the propagation direction of the optical signal; as well as a second pump light coupler (221), an optical frequency comb output coupler (271), an optical filter (272) and a frequency doubling light output coupler (273); the passive microring (25) is an annular high-Q value single-mode lithium niobate waveguide structure, located on one side of the second bus waveguide (25), and realizes coupling of the optical field in the second bus waveguide (25) with the optical field in the passive microring (25) through evanescent waves, and a second thermo-optical phase shifter (26) is provided on the passive microring (25); After optical coupling through the first pump light coupler (11), a portion of the first pump light is coupled through the first bus waveguide (12) into the active microring (13) to generate gain, and the other portion is coupled through the mode spot converter (16) into the optical amplifier (17) to provide gain for the optical amplifier (17) in a forward pumping manner; the first end face coupler (18) is connected to the second end face coupler (21), and the two are located at the edges of their respective chips to achieve low-loss optical coupling between the two chips; the signal light sequentially enters the combining port of the wavelength division multiplexer (22) through the first end face coupler (18) and the second end face coupler (21); after optical coupling through the second pump light coupler (221), the second pump light enters the single-mode waveguide of the wavelength division multiplexer (22) A pump input port is divided into two paths through the wavelength division multiplexer (22), wherein one pump light is output from the combining port of the wavelength division multiplexer (22), and then sequentially passes through the second end face coupler (21) and the first end face coupler (18) to enter the optical amplifier (17), and provides gain for the optical amplifier (17) in a forward pumping mode; another signal light is output from the signal light port of the wavelength division multiplexer (22), and then sequentially passes through the dispersion compensation waveguide (23), the second bus waveguide (24) and the beam splitter (27), and then divided into two paths, one of which enters the optical frequency comb output coupler (271) through a single-mode waveguide and is then received by a photodetector (7), and the other of which passes through the optical filter (272) and the frequency-doubled light output coupler (273) in sequence and is then received by a photodetector (7); A photodetector (7) for beating the frequency-doubled light with a signal of a corresponding frequency in the optical frequency comb, and transmitting the beating signal to the controller (8); A controller (8) is used to control the output power of the first pump laser and the second pump laser, and to control the electro-optic phase shifter and the first thermo-optic phase shifter in the active microring, and to control the second thermo-optic phase shifter in the passive microring.

2. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The first pump light coupler (11) is located at the edge of the rare earth-doped thin film lithium niobate chip (1) and is used to couple pump light into the rare earth-doped thin film lithium niobate chip.

3. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The electro-optic phase shifter (14) comprises a waveguide section and radio frequency electrodes on both sides thereof. When an electrical signal modulation is applied to the electrodes, an optical pulse sequence with a repetition frequency equal to the modulation frequency is generated in the active micro-ring through the electro-optic effect.

4. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The first thermo-optic phase shifter and the second thermo-optic phase shifter include a waveguide and a heating resistor on one side thereof. When a voltage is applied to the heating resistor, the refractive index of the waveguide is changed by the thermo-optic effect, so as to fine-tune the FSR of the active microring or the passive microring.

5. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The dispersion compensation waveguide (23) is a section of single-mode lithium niobate waveguide with a specific width, and its dispersion sign is opposite to the dispersion sign of the optical amplifier (17) waveguide; the length is sufficient, and the total dispersion value of the dispersion compensation waveguide offsets the total dispersion value of the optical amplifier, and is used to compensate for the broadening of optical pulses caused by the amplification process of the optical amplifier.

6. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The waveguide dispersion of the active microring (13) is anomalous dispersion, and the waveguide dispersion of the passive microring (25) is near-zero dispersion.

7. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The first bus waveguide (12) is a single-mode waveguide, the optical amplifier (17) is a multi-mode waveguide, and the waveguide where the mode spot converter (16) is connected to the first bus waveguide (12) is a single-mode waveguide of the same width, which is then widened into a multi-mode waveguide through adiabatic stretching and connected to the optical amplifier (17).

8. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: One end of the first end face coupler (18) is connected to the optical amplifier (17), and the waveguide is further widened by adiabatic pulling, so that the waveguide width is relatively wide at the edge of the chip; the waveguide widths of the first end face coupler (18) and the second end face coupler (21) at the edges of their respective chips are similar and relatively wide, so that the waveguide coupling between the two chips is insensitive to alignment errors; the waveguide width of the second end face coupler (21) is adiabatically reduced along the propagation direction of the optical signal, and finally becomes a single-mode waveguide connected to the combining end of the wavelength division multiplexer (22).

9. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The optical frequency comb output coupler (271) is located at the edge of the thin-film lithium niobate chip (2) and is used to couple the generated optical frequency comb into free space or output optical fiber.

10. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The central frequency of the transmission spectrum of the optical filter (272) is equal to the double frequency of the optical frequency comb signal light, and is used to select the frequency band where the doubled frequency light in the optical frequency comb is located.

11. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to claim 1, characterized in that: The frequency-doubled light output coupler (273) is located at the edge of the thin-film lithium niobate chip (2) and is used to couple the output light of the optical filter into free space or output optical fiber.

12. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to any one of claims 1 to 11, characterized in that: The rare earth-doped lithium niobate chip (1) comprises, from bottom to top, a substrate, a lower cladding layer and a rare earth-doped lithium niobate waveguide layer, and the first pump optical coupler, the first bus waveguide, the active microring, the mode spot converter, the optical amplifier and the first end face coupler are all prepared on the rare earth-doped lithium niobate waveguide layer by means of an etching process.

13. The self-referenced locked optical frequency comb generation system based on thin film lithium niobate according to any one of claims 1 to 11, characterized in that: The thin-film lithium niobate chip (2) comprises, from bottom to top, a substrate, a lower cladding layer and a thin-film lithium niobate waveguide layer; the second end face coupler, a wavelength division multiplexer, a dispersion compensation waveguide, a second bus waveguide, a passive microring, a beam splitter, a second pump light coupler, an optical frequency comb output coupler, an optical filter and a frequency doubling light output coupler are all prepared on the thin-film lithium niobate waveguide layer by an etching process.

Images