Optimized comb tooth spacing optical frequency comb generation method and device based on synthetic Moire lattice

Abstract

The invention relates to a synthetic Moire lattice-based comb tooth spacing optimization optical frequency comb generation method and device, and the method comprises the steps: coupling two optical fiber resonant rings with different sizes, applying resonant electro-optic phase modulation, and constructing a Moire lattice in a frequency dimension to output an optical frequency comb with the comb tooth spacing being the greatest common divisor of the free spectrum range of the two resonant rings; and the spectral distribution of the output optical frequency comb is controlled by changing the coupling strength. The structure is simple, and under the condition that the sizes of the resonant rings are not changed, a denser optical frequency comb can be generated by using a smaller system structure, and meanwhile, controllable electro-optical optical frequency comb output can be realized by flexibly adjusting the coupling strength between the two annular resonant rings.

Description

Technical Field

[0001] The present invention relates to a technology in the field of optical information processing, in particular to a method and device for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice. Background Art

[0002] The concept of optical frequency comb originated from mode-locked lasers, which are manifested as a series of comb teeth with equally spaced frequencies in the frequency domain. Optical frequency combs can be divided into Kerr optical frequency combs and electro-optical frequency combs according to the generation mechanism. Among them, electro-optical frequency combs use electro-optical modulators to modulate lasers to generate sidebands, thereby generating comb teeth with equally spaced frequencies. In recent years, the progress of integrated chips has greatly promoted the development of electro-optical frequency combs. Electro-optical frequency combs generated by micro-ring resonators with high quality factors have greatly improved the bandwidth, stability, and signal-to-noise ratio of electro-optical frequency combs. The principle is to prepare electrodes for micro-ring resonators and load microwave signals so that the modulation frequency of the microwave signal is equal to the free spectral range of the resonant ring. Then, the frequency of the sidebands generated by the electro-optic modulation is consistent with the resonant frequency of the resonant ring, thereby forming an efficient electro-optical frequency comb output. The comb tooth spacing of the electro-optical frequency comb is determined by the size of the resonant ring. If you want to produce an electro-optical frequency comb with a smaller comb tooth spacing, you need to increase the size of the resonant ring, which contradicts the miniaturization requirements of integrated chip devices. Summary of the invention

[0003] In view of the problems that the optical frequency comb generated by the prior art has a small number of comb teeth, the shape of the comb teeth is easily distorted, and the signal-to-noise ratio is low, the present invention proposes a method and device for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice. The method and device have a simple structure and can achieve denser optical frequency comb generation by using a smaller system structure without changing the size of the resonant ring. At the same time, controllable electro-optical frequency comb output can be achieved by flexibly adjusting the coupling strength between the two annular resonant rings.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a method for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice. By coupling two optical fiber resonant rings of unequal sizes and applying resonant electro-optical phase modulation, a moiré lattice is constructed in the frequency dimension to output an optical frequency comb whose comb 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 is then controlled by changing the coupling intensity.

[0006] The frequency dimension refers to: generating a microwave signal through a signal generator and modulating it through electro-optical phase to match the free spectrum range of the optical fiber resonant ring, analogous to the coupling of lattice points in a lattice in real space, constructing an artificial frequency dimension, each resonant mode is a lattice point of the synthetic lattice, and the synthetic lattice period is the free spectrum range, achieving an electro-optical frequency comb output with a smaller comb tooth spacing.

[0007] The lattice periods are Ω 1 =2πv g / L 1 =N 1 Ω and Ω 2 =2πv g / L 2 =N 2 Ω, where: v g is the group velocity of light propagating in the ring, Ω is the greatest common divisor of the free spectral range of the two fiber resonant rings, L 1 and l 2 are the circumferences of the two fiber resonant rings respectively; when the two fiber resonant rings are coupled, a lattice period of N can be constructed 1 N 2 Synthetic moiré lattice of Ω, Ω 1 :Ω 2 Analogous to the torsion angle of a moiré lattice.

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

[0009] In the synthetic moiré lattice, due to the different free spectrum ranges of the two ring resonators, sum and difference frequency effects occur between the frequency modes. 0 The pump light of is incident on the first fiber resonant ring, and after electro-optic phase modulation, a frequency spacing of ω is generated. 0 ±mΩ 1 When the first set of electro-optical frequency combs is coupled into the second fiber resonant ring and further electro-optically phase modulated, a frequency spacing of ω is generated. 0 ±mΩ 1 ±nΩ 2 The first and second electro-optic frequency combs are cascaded together to form an electro-optic frequency comb output with a frequency spacing of Ω. Technical Effects

[0010] The present invention couples two resonant cavities of different sizes and applies resonant electro-optical phase modulation, so that the mode spacing generated by the frequency mode after the difference frequency and frequency effect is the mode of the greatest common divisor of the free spectral range of the two rings, and these modes can be effectively amplified and transmitted in the resonant cavity. At the same time, the flat band effect of the synthetic moiré lattice constructed in this way can effectively regulate the optical frequency comb. Compared with the prior art, the present invention uses a smaller structure to achieve the generation of an optical frequency comb with denser comb teeth without changing the size of the resonant cavity. By utilizing the enhancement effect of the resonant cavity, a relatively large signal-to-noise ratio and the number of comb teeth can be achieved. The flat band and dispersion characteristics of the synthetic moiré lattice are used to effectively regulate the optical frequency comb. This method is simple in concept and does not require complex external modulation, because it is easy to expand to other systems, such as microcavity systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic diagram of the device of the present invention;

[0012] Figure 2 Schematic diagram of the synthesis of moiré lattice;

[0013] Figure 3 Schematic diagram for generating optical frequency combs with smaller tooth spacing;

[0014] Figure 4 is an energy band diagram at different coupling ratios when the length ratio of the first fiber resonant ring to the second fiber resonant ring is 5:2;

[0015] Figure 5 The output spectrum diagram at different coupling ratios when the length ratio of the first fiber resonant ring to the second fiber resonant ring is 5:2;

[0016] In the figure: 1 pump source, 11 continuously tunable laser; 2 dual-resonance 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 spectrum detection unit, 31 photodetector, 32 oscilloscope. DETAILED DESCRIPTION

[0017] like Figure 1 As shown, this embodiment relates to an optical frequency comb generating device with optimized comb tooth spacing based on a synthetic moiré lattice, comprising: a pump source 1, a dual-resonance cavity coupling unit 2 and a spectrum detection unit 3 connected in sequence, wherein: the pump source 1 is connected to the dual-resonance cavity coupling unit 2 through a 2×2 optical fiber coupler 21, and the two resonant rings are connected through an adjustable optical fiber coupler 210.

[0018] The resonant rings described all include: a semiconductor optical amplifier, a polarization controller, an electro-optical phase modulator and a dense wavelength division multiplexer, wherein: the polarization controllers respectively adjust the polarization of the optical signals in the respective resonant rings and the slow axes of the two electro-optical phase modulators to be consistent; the two semiconductor optical amplifiers respectively amplify the signal light in the respective resonant rings and compensate for the loss of light in the rings, so that the gain in each ring is roughly equal to the loss; the two dense wavelength division multiplexers respectively filter the amplified noise generated in the respective resonant rings to improve the signal-to-noise ratio; the two electro-optical phase modulators respectively perform electro-optical phase modulation on the optical signals in the respective resonant rings to generate an electro-optical frequency comb; and the adjustable optical fiber coupler adjusts the coupling strength between the two resonant rings.

[0019] 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 in the first fiber resonant ring are connected in sequence; 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 in the second fiber resonant ring are connected in sequence.

[0020] 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.

[0021] The pump source uses a continuously tunable laser 11 to provide energy for the generation of the electro-optical frequency comb, and the wavelength is continuously tunable within the range of 1520-1580nm. The central wavelength selected in this embodiment is 1550.92nm;

[0022] In this embodiment, the dense wavelength division multiplexer uses the 33rd channel of the International Telecommunication Union, and the central wavelength is 1550.92nm.

[0023] The spectral detection unit includes: a photodetector 31 and an oscilloscope 32 connected in sequence, wherein: 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, converting the time domain signal into a frequency domain signal for analysis.

[0024] The circumferences of the first fiber resonant ring and the second fiber resonant ring are L 1 =20.4m and L 2 =51m, that is, the length ratio of the two rings is L 1 :L 2 =2:5. The free spectral ranges of the first fiber resonant ring and the second fiber resonant ring are Ω 1 =10MHz and Ω2 =4MHz. The greatest common divisor of the free spectral range of the two annular resonator rings is Ω=2MHz.

[0025] The signal generator 211 applies microwave signals with modulation frequencies of 10 MHz and 4 MHz to the first electro-optic phase modulator 24 and the second electro-optic phase modulator 29, respectively, to construct the following in the dimension of artificially synthesized frequency: Figure 2 The synthetic moiré lattice shown. The period of the synthetic moiré lattice is 20 MHz.

[0026] like Figure 3 As shown, when the center frequency is ω 0 The pump light is incident on the first fiber resonant ring and generates a frequency spacing of ω through the first electro-optic phase modulator 24. 0 After the first set of electro-optical frequency combs is coupled into the second fiber resonant ring, it passes through the second electro-optical phase modulator 29 to generate a frequency spacing of ω 0 The second set of electro-optic frequency combs is ±10m±4n (n is an integer). The cascade of the two sets of electro-optic frequency combs forms an electro-optic frequency comb with a frequency spacing of Ω=2MHz.

[0027] The wavelength is scanned near the central wavelength of the pump source, 1550.92nm, and the coupling ratio of the adjustable fiber coupler between the two rings is adjusted, that is, the coupling strength is changed, and the energy band of the synthetic moiré lattice under different coupling ratios is measured with an oscilloscope. On this basis, the pump source is set to a single wavelength output with a wavelength of 1550.92nm, and the output spectrum of the synthetic moiré lattice under different coupling ratios is measured.

[0028] like Figure 4 and Figure 5As shown in the figure, when the coupling ratio between the first fiber resonator ring and the second fiber resonator ring is 100:0, the energy band of the synthetic 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 when the normalized spectral intensity decreases to -70 dB. When the coupling ratio is 100:0, the maximum range of the electro-optic frequency comb is 530 MHz. When the coupling ratio is 99:1, the energy band of the synthetic moiré lattice is still a completely dispersive band, and the maximum range of the optical frequency comb is about 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 energy band of the synthetic moiré lattice contains both flat bands and dispersive bands, and the maximum range of the optical frequency comb is about 370 MHz, of which the distribution range of the frequency modes with a frequency spacing of 2 MHz is 290 MHz. When the coupling ratio is 12:88, the energy band of the synthetic moiré lattice is completely flat, the frequency spacing of the output electro-optic frequency comb is 2MHz, and the maximum range of the frequency comb is reduced to 80MHz. At this time, the intensity distribution of the spectrum is the most uniform. The spectral distribution under different coupling ratios shows that the use of synthetic moiré lattice can produce electro-optic frequency combs with smaller spectral spacing, and the spectral distribution can be controlled by the coupling ratio.

[0029] Compared with the prior art, the present invention uses a smaller structure to achieve the generation of an optical frequency comb with denser teeth without changing the size of the resonant cavity. By utilizing the enhancement effect of the resonant cavity, a relatively large signal-to-noise ratio and the 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.

[0030] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A method for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice, characterized in that: By coupling two fiber resonant rings of unequal sizes and applying resonant electro-optic phase modulation, a moiré lattice is constructed in the frequency dimension to output an optical frequency comb whose comb tooth spacing is 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.

2. The method for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice according to claim 1, characterized in that: The frequency dimension refers to: generating a microwave signal through a signal generator and modulating it through electro-optical phase to match the free spectrum range of the optical fiber resonant ring, analogous to the coupling of lattice points in a lattice in real space, constructing an artificial frequency dimension, each resonant mode is a lattice point of the synthetic lattice, and the synthetic lattice period is the free spectrum range, achieving an electro-optical frequency comb output with a smaller comb tooth spacing.

3. The method for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice according to claim 2, characterized in that: The lattice periods are respectively Ω1 = 2πv g / L1 = N1Ω and Ω2 = 2πv g / L2 = N2Ω, where: v g is the group velocity of light propagating in the loop, Ω is the greatest common divisor of the free spectral ranges of the two fiber optic resonator loops, and L1 and L2 are the circumferences of the two fiber optic resonator loops respectively; after the two fiber optic resonator loops are coupled, a synthetic Moiré lattice with a lattice period of N1N2Ω can be constructed, and Ω1:Ω2 is analogous to the twist angle of the Moiré lattice.

4. The method for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice according to claim 1, characterized in that: The spectral distribution is adjusted in the following manner: by scanning the wavelength of the pump source, adjusting the coupling ratio of the two optical fiber resonant rings, measuring the energy bands of the synthetic moiré lattice under different coupling ratios, or adjusting the wavelength of the pump source to a single wavelength, measuring the spectral distribution under different coupling ratios.

5. The method for generating an optical frequency comb with optimized comb tooth spacing based on a synthetic moiré lattice according to any one of claims 1 to 4, characterized in that In the synthetic moiré lattice, due to the different free spectrum ranges of the two ring resonators, sum and difference frequency effects occur between the frequency modes. When the pump light with a center frequency of ω0 is incident on the first fiber resonator ring, the first set of electro-optical frequency combs with a frequency spacing of ω0±mΩ1 is generated after electro-optical phase modulation. When the first set of electro-optical frequency combs is coupled into the second fiber resonator ring and further electro-optical phase modulation is performed, the second set of electro-optical frequency combs with a frequency spacing of ω0±mΩ1±nΩ2 is generated, where m and n are integers. The cascade of the first and second sets of electro-optical frequency combs forms an electro-optical frequency comb output with a frequency spacing of Ω.

6. An optical frequency comb generator for optimizing the comb tooth spacing to implement the method described in any one of claims 1 to 5, characterized in that: include: A pump source, a dual-resonance cavity coupling unit and a spectrum detection unit are connected in sequence, wherein the pump source and the dual-resonance cavity coupling unit are connected through a fiber coupler, and the two resonant rings are connected through an adjustable fiber coupler.

7. The optical frequency comb generator with optimized comb tooth spacing according to claim 6, characterized in that: The resonant rings described all include: a semiconductor optical amplifier, a polarization controller, an electro-optical phase modulator and a dense wavelength division multiplexer, wherein: the polarization controllers respectively adjust the polarization of the optical signals in the respective resonant rings and the slow axes of the two electro-optical phase modulators to be consistent; the two semiconductor optical amplifiers respectively amplify the signal light in the respective resonant rings and compensate for the loss of light in the rings, so that the gain in each ring is roughly equal to the loss; the two dense wavelength division multiplexers respectively filter the amplified noise generated in the respective resonant rings to improve the signal-to-noise ratio; the two electro-optical phase modulators respectively perform electro-optical phase modulation on the optical signals in the respective resonant rings to generate electro-optical frequency combs; and the adjustable optical fiber coupler adjusts the coupling strength of the respective resonant rings.

8. The optical frequency comb generator with optimized comb tooth spacing according to claim 7, characterized in that: In the first fiber resonant ring, the fiber coupler, the first semiconductor optical amplifier, the first polarization controller, the adjustable 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 adjustable 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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