A method and apparatus for generating optical frequency combs with optimized tooth spacing based on synthetic moiré lattices.

Abstract

A method and apparatus for generating an optical frequency comb based on a synthetic moiré lattice with optimized tooth spacing is disclosed. This method involves coupling two fiber resonant rings of unequal size and applying resonant electro-optic phase modulation to construct a moiré lattice in the frequency dimension. The output tooth spacing is the greatest common divisor of the free spectral ranges of the two resonant rings. The spectral distribution of the output optical frequency comb can then be controlled by adjusting the coupling strength. This invention features a simple structure and, without changing the resonant ring size, achieves denser optical frequency comb generation using a smaller system structure. Furthermore, controllable electro-optic optical frequency comb output can be achieved by flexibly adjusting the coupling strength between the two resonant rings.

Description

Technical Field

[0001] This invention relates to a technology in the field of optical information processing, specifically a method and apparatus for generating optical frequency combs with optimized comb spacing based on synthetic moiré lattices. Background Technology

[0002] Optical frequency combs, or light frequency combs, originated from the concept of mode-locked lasers. In the frequency domain, they appear as a series of comb teeth with equally spaced frequencies. Optical frequency combs can be classified into Kerr optical frequency combs, electro-optic optical frequency combs, etc., based on their generation mechanism. Electro-optic optical frequency combs utilize an electro-optic modulator to modulate laser light to generate sidebands, thus producing an equally spaced frequency comb tooth distribution. In recent years, advancements in integrated circuits have greatly promoted the development of electro-optic optical frequency combs. Electro-optic optical frequency combs based on micro-ring resonators with high quality factors have significantly improved bandwidth, stability, and signal-to-noise ratio. The principle involves fabricating electrodes on the micro-ring resonator and applying a microwave signal, ensuring the modulation frequency of the microwave signal equals the free spectral range of the resonator. The frequency of the sidebands generated by electro-optic modulation then matches the resonant frequency of the resonator, resulting in a highly efficient electro-optic optical frequency comb output. The tooth spacing of the electro-optic optical frequency comb is determined by the size of the resonator. To produce an electro-optic optical frequency comb with a smaller tooth spacing, the size of the resonator needs to be increased, which contradicts the miniaturization requirements of integrated circuit devices. Summary of the Invention

[0003] This invention addresses the problems of existing optical frequency combs, such as a small number of comb teeth, easy distortion of comb tooth shape, and low signal-to-noise ratio. It proposes an optical frequency comb generation method and device based on a synthetic moiré lattice with optimized comb tooth spacing. The structure is simple, and without changing the size of the resonant ring, a denser optical frequency comb can be generated using a smaller system structure. At the same time, controllable electro-optical frequency comb output can be achieved by flexibly adjusting the coupling strength between the two ring resonant rings.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a method for generating an optical frequency comb with optimized tooth spacing based on a synthetic moiré lattice. By coupling two fiber resonant rings of unequal size and applying resonant electro-optic phase modulation, a moiré lattice is constructed in the frequency dimension to generate an optical frequency comb with the tooth spacing being the greatest common divisor of the free spectral range of the two resonant rings. The spectral distribution of the output optical frequency comb is then controlled by changing the coupling strength.

[0006] The frequency dimension refers to the free spectral range of a matching fiber resonant ring, generated by a signal generator and electro-optically modulated to a microwave signal. Analogous to the coupling of lattice points in a crystal lattice in real space, an artificially synthesized frequency dimension is constructed. Each resonant mode is a lattice point of the synthesized crystal lattice, and the period of the synthesized crystal lattice is the free spectral range, thereby achieving an electro-optical frequency comb output with a smaller comb tooth spacing.

[0007] The lattice periods mentioned are Ω1 = 2πv g / L1=N1Ω and Ω2=2πv g / L2=N2Ω, where: v g Let Ω be the group velocity of light propagating in the ring, Ω be the greatest common divisor of the free spectral range of the two fiber resonant rings, and L1 and l2 be the perimeters of the two fiber resonant rings, respectively. After the two fiber resonant rings are coupled, a synthetic moiré lattice with a lattice period of N1N2Ω can be constructed, where Ω1:Ω2 is analogous to the twist angle of the moiré lattice.

[0008] The spectral distribution is adjusted in the following ways: by scanning the wavelength of the pump source, adjusting the coupling ratio of the two fiber resonant rings, and measuring the energy band of the synthesized moiré lattice under different coupling ratios, or by adjusting the wavelength of the pump source to a single wavelength and measuring the spectral distribution under different coupling ratios.

[0009] In artificially synthesized moiré lattices, the different free spectral ranges of the two ring resonators cause sum-frequency and difference-frequency effects between frequency modes. When pump light with a center frequency of ω0 is incident on the first fiber resonator, it generates a first set of electro-optic frequency combs with a frequency spacing of ω0±mΩ1 after electro-optic phase modulation. When the first set of electro-optic frequency combs is coupled into the second fiber resonator and further electro-optically modulated, it generates a second set of electro-optic frequency combs with a frequency spacing of ω0±mΩ1±nΩ2, where m and n are integers. The cascading of the first and second sets of electro-optic frequency combs forms an electro-optic frequency comb output with a frequency spacing of Ω.

[0010] Technical effect

[0011] This invention, by coupling two resonant cavities of different sizes and applying resonant electro-optic phase modulation, allows the frequency modes, after difference-frequency and frequency-frequency effects, to generate modes with a mode spacing equal to the greatest common divisor of the free spectral ranges of the two rings. These modes can be effectively amplified and transmitted within the resonant cavities. Simultaneously, the flat-band effect of the synthetic moiré lattice constructed in this way can effectively control the optical frequency comb. Compared to existing technologies, this invention achieves a denser optical frequency comb using a smaller structure without changing the resonant cavity size. Utilizing the enhancement effect of the resonant cavity, a relatively high signal-to-noise ratio and a large number of comb teeth can be achieved. The flat-band and dispersion characteristics of the synthetic moiré lattice are used to effectively control the optical frequency comb. This approach is simple in concept, requires no complex external modulation, and is easily extended to other systems, such as microcavity systems. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the device of the present invention;

[0013] Figure 2 This is a schematic diagram of the synthesis of a moiré lattice;

[0014] Figure 3 Schematic diagram of the principle of generating an optical frequency comb with smaller tooth spacing;

[0015] Figure 4 The band structure diagrams are shown for different coupling ratios when the length ratio of the first fiber resonator and the second fiber resonator is 5:2.

[0016] Figure 5 The output spectra of the first and second fiber resonant rings at different coupling ratios when the length ratio of the first fiber resonant ring to the second fiber resonant ring is 5:2.

[0017] In the figure: 1 Pump source, 11 Continuously tunable laser; 2 Dual resonant cavity coupling unit, 21 2×2 fiber coupler (splitting ratio 99:1), 22 First semiconductor optical amplifier, 23 First polarization controller, 24 First electro-optic phase modulator, 25 First dense wavelength division multiplexer, 26 Second polarization controller, 27 Second dense wavelength division multiplexer, 28 Second semiconductor optical amplifier, 29 Second electro-optic phase modulator, 210 Tunable fiber coupler, 211 Signal generator; 3 Spectral detection unit, 31 Photodetector, 32 Oscilloscope. Detailed Implementation

[0018] like Figure 1 As shown, this embodiment relates to an optical frequency comb generator with optimized comb spacing based on a synthetic moiré lattice, comprising: a pump source 1, a dual resonant cavity coupling unit 2, and a spectral detection unit 3 connected in sequence, wherein: the pump source 1 and the dual resonant cavity coupling unit 2 are connected by a 2×2 fiber coupler 21, and the two resonant rings are connected by an adjustable fiber coupler 210.

[0019] Each resonant ring comprises: a semiconductor optical amplifier, a polarization controller, an electro-optic phase modulator, and a dense wavelength division multiplexer, wherein: the polarization controller adjusts the polarization of the optical signal in its respective resonant ring and aligns the slow axes of the two electro-optic phase modulators; the two semiconductor optical amplifiers amplify the signal light in their respective resonant rings and compensate for the light loss in the rings, making the gain in each ring approximately equal to the loss; the two dense wavelength division multiplexers filter the amplified noise generated in their respective resonant rings to improve the signal-to-noise ratio; the two electro-optic phase modulators perform electro-optic phase modulation on the optical signal in their respective resonant rings to generate an electro-optic frequency comb; and an adjustable fiber coupler adjusts the coupling strength between the two resonant rings.

[0020] In this embodiment, the fiber coupler 21, the first semiconductor optical amplifier 22, the first polarization controller 23, the adjustable fiber coupler 210, the first electro-optic phase modulator 24, and the first dense wavelength division multiplexer 25 are connected in sequence in the first fiber resonant ring; the adjustable fiber coupler 210, the second polarization controller 26, the second dense wavelength division multiplexer 27, the second semiconductor optical amplifier 28, and the second electro-optic phase modulator 29 are connected in sequence in the second fiber resonant ring.

[0021] The first electro-optic phase modulator 24 and the second electro-optic phase modulator 29 are respectively connected to the signal generator 211 and receive driving microwave signals.

[0022] The pump source is a continuously tunable laser 11, which provides energy for the generation of the electro-optic frequency comb. The wavelength is continuously tunable in the range of 1520-1580nm. In this embodiment, the center wavelength is selected as 1550.92nm.

[0023] In this embodiment, the dense wavelength division multiplexer uses the ITU-T Channel 33, with a center wavelength of 1550.92nm.

[0024] The spectral detection unit includes a photodetector 31 and an oscilloscope 32 connected in sequence. The photodetector 31 is used to convert the optical signal output by the dual resonant ring coupling system 2 into an electrical signal. The oscilloscope 32 is used to receive the electrical signal converted by the photodetector 31, perform time-domain measurement of the signal, and also has the function of Fourier transform to convert the time-domain signal into a frequency-domain signal for analysis.

[0025] The perimeters of the first and second fiber resonant rings are L1 = 20.4 m and L2 = 51 m, respectively, meaning the length ratio of the two rings is L1:L2 = 2:5. The free spectral ranges of the first and second fiber resonant rings are Ω1 = 10 MHz and Ω2 = 4 MHz, respectively. The greatest common divisor of the free spectral ranges of the two ring resonant rings is Ω = 2 MHz.

[0026] The signal generator 211 applies microwave signals with modulation frequencies of 10MHz and 4MHz to the first electro-optic phase modulator 24 and the second electro-optic phase modulator 29, respectively, to construct, in the artificially synthesized frequency dimension, as shown below. Figure 2 The synthesized moiré lattice shown has a period of 20 MHz.

[0027] like Figure 3 As shown, when pump light with a center frequency of ω0 is incident on the first fiber resonant ring, it generates a first set of electro-optic frequency combs (where m is an integer) with a frequency spacing of ω0 ± 10 mMHz after passing through the first electro-optic phase modulator 24. After the first set of electro-optic frequency combs is coupled into the second fiber resonant ring, it passes through the second electro-optic phase modulator 29, generating a second set of electro-optic frequency combs (where n is an integer) with a frequency spacing of ω0 ± 10 m ± 4n. The cascading of the two sets of electro-optic frequency combs forms an electro-optic frequency comb with a frequency spacing of Ω = 2 MHz.

[0028] Wavelength scanning was performed near the center wavelength of the pump source at 1550.92 nm. Simultaneously, the coupling ratio of the adjustable fiber coupler between the two rings was adjusted, thus changing the coupling strength. The energy band of the synthesized moiré lattice was measured using an oscilloscope under different coupling ratios. Based on this, the pump source was set to single-wavelength output at 1550.92 nm, and the output spectrum of the synthesized moiré lattice was measured under different coupling ratios.

[0029] like Figure 4 and Figure 5 As shown, when the coupling ratio between the first and second fiber resonator rings is 100:0, the band structure of the synthesized moiré lattice is a completely dispersive band, and the frequency spacing of the output electro-optic frequency comb is 10 MHz. The maximum range of the electro-optic frequency comb is defined as the frequency position where the normalized spectral intensity decreases to -70 dB. At a coupling ratio of 100:0, the maximum range of the electro-optic frequency comb is 530 MHz. When the coupling ratio is 99:1, the band structure of the synthesized moiré lattice is still a completely dispersive band, and the maximum range of the frequency comb is approximately 440 MHz. Although the output electro-optic frequency comb contains frequency modes with a frequency spacing of 2 MHz, the distribution range of these frequency modes is only 110 MHz. When the coupling ratio is 68:32, the band structure of the synthesized moiré lattice simultaneously contains both a flat band and a dispersive band, and the maximum range of the frequency comb is approximately 370 MHz, of which the frequency modes with a frequency spacing of 2 MHz have a distribution range of 290 MHz. When the coupling ratio is 12:88, the energy band of the synthesized moiré lattice is completely flat, the frequency spacing of the output electro-optic frequency comb is 2 MHz, and the maximum range of the optical frequency comb is reduced to 80 MHz. At this point, the intensity distribution of the spectrum is the most uniform. The spectral distribution under different coupling ratios shows that electro-optic frequency combs with smaller spectral spacing can be generated using the synthesized moiré lattice, and the spectral distribution can be controlled by the coupling ratio.

[0030] Compared with existing technologies, this invention achieves a denser optical frequency comb using a smaller structure without changing the resonant cavity size. Utilizing the enhancement effect of the resonant cavity, a higher signal-to-noise ratio and a larger number of comb teeth can be achieved. The flat band and dispersion characteristics of the synthesized moiré lattice are used to effectively control the optical frequency comb.

[0031] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A method for generating optical frequency combs with optimized tooth spacing based on synthetic moiré lattices, characterized in that, By coupling two fiber resonant rings of unequal size and applying resonant electro-optic phase modulation, a moiré lattice is constructed in the frequency dimension to form an optical frequency comb with the output comb tooth spacing being the greatest common divisor of the free spectral ranges of the two resonant rings. The spectral distribution of the output optical frequency comb can then be controlled by changing the coupling strength. The frequency dimension refers to the free spectral range of a matching fiber resonant ring, generated by a signal generator and electro-optically modulated to a microwave signal. Analogous to the coupling of lattice points in a crystal lattice in real space, an artificially synthesized frequency dimension is constructed. Each resonant mode is a lattice point of the synthesized crystal lattice, and the period of the synthesized crystal lattice is the free spectral range, thereby achieving an electro-optical frequency comb output with a smaller comb tooth spacing.

2. The method for generating optical frequency combs based on optimized comb spacing using a synthetic moiré lattice according to claim 1, characterized in that, The lattice periods are respectively and ,in: Let the group velocity be the velocity of light propagating in the ring. The greatest common divisor of the free spectral ranges of the two fiber resonant rings and These are the perimeters of the two fiber resonator rings; after the two fiber resonator rings are coupled, a lattice period of can be constructed. Synthetic Mohr lattice, Analogous to the twist angle of a moiré lattice.

3. The method for generating optical frequency combs based on optimized comb spacing using a synthetic moiré lattice according to claim 1, characterized in that, The spectral distribution is adjusted in the following ways: by scanning the wavelength of the pump source, adjusting the coupling ratio of the two fiber resonant rings, and measuring the energy band of the synthesized moiré lattice under different coupling ratios, or by adjusting the wavelength of the pump source to a single wavelength and measuring the spectral distribution under different coupling ratios.

4. The method for generating an optical frequency comb based on an optimized comb spacing using a synthetic moiré lattice according to any one of claims 1-3, characterized in that, in In artificially synthesized moiré lattices, the different free spectral ranges of the two ring resonators cause sum-frequency and difference-frequency effects between frequency modes. When the center frequency is... The pump light is incident on the first fiber resonant ring, and after electro-optic phase modulation, a frequency spacing of [value missing] is generated. The first set of electro-optic frequency combs, when coupled into the second fiber resonant ring and further subjected to electro-optic phase modulation, generates a frequency spacing of... The second group of electro-optic frequency combs, in which and The value is an integer. The cascading of the first and second groups of electro-optic frequency combs forms a frequency spacing of... The output of the electro-optical frequency comb.

5. An optimized comb spacing optical frequency comb generating device for implementing the method of any one of claims 1-4, characterized in that, include: The pump source, the dual resonant cavity coupling unit, and the spectral detection unit are connected in sequence. The pump source and the dual resonant cavity coupling unit are connected by an optical fiber coupler, and the two resonant rings are connected by an adjustable optical fiber coupler.

6. The optical frequency comb generator with optimized tooth spacing according to claim 5, characterized in that, Each resonant ring comprises: a semiconductor optical amplifier, a polarization controller, an electro-optic phase modulator, and a dense wavelength division multiplexer, wherein: the polarization controller adjusts the polarization of the optical signal in its respective resonant ring and aligns the slow axes of the two electro-optic phase modulators; the two semiconductor optical amplifiers amplify the signal light in their respective resonant rings and compensate for the light loss in the rings, making the gain in each ring approximately equal to the loss; the two dense wavelength division multiplexers filter the amplified noise generated in their respective resonant rings to improve the signal-to-noise ratio; the two electro-optic phase modulators perform electro-optic phase modulation on the optical signal in their respective resonant rings to generate an electro-optic frequency comb; and an adjustable fiber coupler adjusts the coupling strength of each resonant ring.

7. The optical frequency comb generator with optimized tooth spacing according to claim 6, characterized in that, In the first fiber resonant ring, the fiber coupler, the first semiconductor optical amplifier, the first polarization controller, the tunable fiber coupler, the first electro-optic phase modulator, and the first dense wavelength division multiplexer are connected in sequence; in the second fiber resonant ring, the tunable fiber coupler, the second polarization controller, the second dense wavelength division multiplexer, the second semiconductor optical amplifier, and the second electro-optic phase modulator are connected in sequence.

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