Parity-Time Symmetric Optoelectronic Oscillator Based on Intensity-Modulated Sidebands

Abstract

The present invention provides a parity-time symmetric optoelectronic oscillator based on intensity modulation sidebands, comprising: a laser, an electro-optic intensity modulator sequentially connected thereto, a long optical fiber for storing energy, a photodetector, an electrical amplifier, an electrical filter, and an electrical power splitter; the present invention can provide a high-quality microwave signal source, which can stably output a microwave signal with low phase noise and high side-mode suppression ratio in electrical form. In addition, it has the advantages of simple structure, no phase matching and polarization-related problems, and high system stability.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic technology, and particularly relates to an optoelectronic oscillator that realizes parity-time symmetry based on intensity modulation sidebands. Background Art

[0002] Optoelectronic oscillators use optoelectronic feedback loop technology and can generate high-frequency and low-phase-noise microwave signals, which have been widely applied in modern radar technology, aerospace engineering, frequency measurement, fiber radio technology and other fields. In order to obtain microwave signals with ultra-low phase noise, a high-Q optoelectronic feedback loop needs to be constructed in the optoelectronic oscillator. An effective implementation scheme is to use a long optical fiber to construct a long loop. However, the long loop will cause the microwave signals generated by the optoelectronic oscillator to have dense longitudinal mode intervals, and an extremely narrow-band electrical filter is required to generate microwave signals with single longitudinal mode oscillation. High-frequency, extremely narrow-bandwidth and high-Q electrical filters are difficult to implement due to the influence of the electronic bottleneck. Therefore, seeking a new and stable mode selection mechanism has become a difficult point that needs to be overcome by optoelectronic oscillators. In recent years, the parity-time symmetry principle has been used as a novel mode selection mechanism for optoelectronic oscillators. Even if there is a long loop in the optoelectronic oscillator and no extremely narrow-band electrical filter or optical filter is used, by making the optoelectronic oscillator satisfy the symmetry breaking condition, microwave signals with single longitudinal mode oscillation can still be generated. Therefore, it has important research value and application value to transform traditional optoelectronic oscillators using the mode selection mechanism of the new and easy-to-operate parity-time symmetry system in the broken state. The Chinese invention patent "Optoelectronic Oscillator Based on Parity-Time Symmetry Principle" has a structure of a parity-time symmetric optoelectronic oscillator based on a dual-polarization spatial double-loop structure. The Chinese invention patent "An Optoelectronic Oscillator, Optical Fiber System and Integrated Optoelectronic System Based on Parity-Time Symmetry Principle" has a structure of a parity-time symmetric optoelectronic oscillator based on a spatial single-loop structure of a dual-wavelength laser. The optoelectronic oscillators mentioned in the above two patents face complex system interference problems such as double loops or double wavelengths, and polarization-related problems, making the anti-interference ability of their systems weak. In addition, these systems are large in volume and complex in structure. Summary of the Invention

[0003] The main purpose of the present invention is to overcome the disadvantages and deficiencies of the prior art, and propose an optoelectronic oscillator that realizes parity-time symmetry based on intensity modulation sidebands, which has a spatial single-loop structure. An electro-optic intensity modulator is used in the structure and the optical polarization state of the system does not need to be adjusted. It will not face polarization-related problems, has strong anti-interference ability, and can stably generate high-frequency microwave signals with high purity and low phase noise.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A parity-time symmetric optoelectronic oscillator based on intensity modulation sidebands, comprising:

[0006] — A laser for generating a laser with a wavelength of λ. The output end of the laser is a polarization-maintaining fiber, and the polarization direction of the linearly polarized laser output by the laser is consistent with the slow axis of the polarization-maintaining fiber;

[0007] — An electro-optic intensity modulator having an optical signal input port, an optical signal output port, a microwave signal input port, and a voltage input terminal. The optical signal input port of the electro-optic intensity modulator is connected to the polarization-maintaining fiber at the output end of the laser, and the linearly polarized laser output by the laser is coupled into the extraordinary optical axis of the electro-optic intensity modulator through the slow axis of the polarization-maintaining fiber;

[0008] The electro-optic intensity modulator is used to receive an optical carrier and modulate the microwave signal generated by the optoelectronic oscillator onto the optical carrier to generate an optical signal with multi-order modulation sidebands;

[0009] — A voltage source that outputs a stable and adjustable DC bias voltage and is connected to the voltage input port of the electro-optic intensity modulator;

[0010] — A long fiber, whose input end is connected to the optical signal output port of the electro-optic intensity modulator;

[0011] — A photodetector, whose input end is connected to the output end of the long fiber;

[0012] — An electrical amplifier for enabling the gain of the optoelectronic signal in the loop to be greater than the loss to start oscillation. Its input end is connected to the output end of the photodetector;

[0013] — An electrical filter for controlling the frequency range of the generated microwave signal. Its input end is connected to the output end of the electrical amplifier;

[0014] — An electrical power splitter having an input port and two microwave signal output ports. The input port of the electrical power splitter is connected to the output end of the electrical filter, and one of the microwave signal output ports of the electrical power splitter is connected to the microwave signal input port of the electro-optic intensity modulator.

[0015] Furthermore, the laser is a semiconductor laser with polarization-maintaining output, a DFB laser with polarization-maintaining output, or a fiber laser with polarization-maintaining output. A non-polarization-maintaining fiber and a polarization controller can be used instead of the polarization-maintaining fiber.

[0016] Furthermore, the long fiber is used to store energy, and the length of the long fiber is selected between 10 and 20000 m according to the requirements of phase noise for different applications.

[0017] Furthermore, the electrical amplifier, electrical filter, and electrical power splitter are not limited to the current connection positions and can be connected at any position and in any order between the output end of the photodetector and the microwave signal input port of the electro-optic intensity modulator.

[0018] Furthermore, for the single-mode microwave signal emitted by the optoelectronic oscillator, its side-mode suppression ratio is greater than 20 dB.

[0019] Furthermore, the electro-optic intensity modulator is an electro-optic intensity modulator with a bandwidth of 5 - 100 GHz that can generate first-order sidebands and second-order sidebands, or other types of electro-optic intensity modulators that achieve the same effect;

[0020] The electrical filter is an electrical filter with a center frequency of 1 - 100 GHz, or other types of electrical filters that achieve the same effect as the electrical filter with a frequency of 1 - 100 GHz.

[0021] Furthermore, the voltage source is an adjustable voltage source that can output -10V - 10V, or other types of voltage sources that can output a certain voltage.

[0022] Furthermore, the photodetector is a photodetector with a bandwidth of 1 - 100 GHz, or other types of photodetectors that achieve the same effect as the photodetector with a bandwidth of 1 - 100 GHz;

[0023] The electrical amplifier is an electrical amplifier with a bandwidth of 40 kHz - 100 GHz and a gain coefficient sufficient to enable the oscillator to oscillate, or other types of electrical amplifiers that achieve the same effect;

[0024] The electrical power splitter is an electrical power splitter with a bandwidth of 1 - 100 GHz, or other types of electrical power splitters that achieve the same effect as the electrical power splitter with a bandwidth of 1 - 100 GHz.

[0025] Furthermore, the multi-order sideband modulated optical signal realizes the gain and loss matching in the parity-time symmetric system by adjusting the voltage of the voltage source and the power of the laser output by the laser. An adjustable optical attenuator can be used to replace the power of the tuned laser, and an adjustable optical filter can be used to replace the voltage of the adjusted voltage source.

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

[0027] 1. The present invention realizes a parity-time symmetric optoelectronic oscillator based on intensity modulation of multi-order sidebands, does not rely on a high-performance extremely narrow bandwidth electrical filter, and adjusts the energy ratio and overall energy level of the carrier, first-order sidebands, and second-order sidebands output by the electro-optic intensity modulator by adjusting the bias voltage of the voltage source and the power of the laser output, so as to realize the adjustment of the gain and loss intensity of the optoelectronic loop, and realize the symmetry-breaking state for mode selection. The adjustment method is simple and easy to implement.

[0028] 2. The present invention does not require the use of multiple lasers and electro-optical modulators with complex structures, nor does it require the adjustment of the optical polarization state of the system. It only uses one laser and an electro-optical intensity modulator with a simple structure, which enables the system to avoid polarization-related problems. In addition, the system structure is significantly simplified, which greatly improves the stability of the system and enables the stable generation of microwave signals with low phase noise and high side-mode suppression ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic structural diagram of the present invention;

[0030] Figure 2 is a spectrogram of generating the first-order sideband and the second-order sideband by electro-optical intensity modulation;

[0031] Figure 3 is a spectrogram of realizing the output of a single-longitudinal-mode microwave signal with a high side-mode suppression ratio after satisfying the parity-time symmetry breaking condition;

[0032] Figure 4 is the phase noise spectrum of the microwave signal output by the optoelectronic oscillator.

[0033] Explanation of the reference numerals in the drawings: 1 - laser; 2 - electro-optical intensity modulator; 3 - voltage source; 4 - long optical fiber; 5 - photodetector; 6 - electrical amplifier; 7 - electrical filter; 8 - electrical power splitter. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The present invention will be further described in detail below in conjunction with the embodiments and the drawings, but the embodiments of the present invention are not limited thereto.

[0035] Embodiment

[0036] As Figure 1 shown, the present invention, an optoelectronic oscillator based on the parity-time symmetry realized by the sidebands of intensity modulation, includes:

[0037] - A laser 1 for generating a laser with a wavelength of λ. The output end of the laser is a polarization-maintaining optical fiber, and the polarization direction of the linearly polarized laser output by the laser is consistent with the slow axis of the polarization-maintaining optical fiber;

[0038] - An electro-optical intensity modulator 2, which has an optical signal input port A, an optical signal output port B, a microwave signal input port C, and a voltage input port; the optical signal input port A of the electro-optical intensity modulator is connected to the polarization-maintaining optical fiber at the output end of the laser, and the linearly polarized laser output by the laser is coupled into the extraordinary optical axis of the electro-optical intensity modulator through the slow axis of the polarization-maintaining optical fiber;

[0039] The electro-optic intensity modulator is used to receive an optical carrier and modulate the microwave signal generated by the optoelectronic oscillator onto the optical carrier to generate an optical signal with multiple-order modulation sidebands;

[0040] — A voltage source 3, the voltage control module outputs a stable and adjustable DC bias voltage, which is connected to the voltage input port of the electro-optic intensity modulator;

[0041] — A long optical fiber 4, its input end is connected to the optical signal output port B of the electro-optic intensity modulator;

[0042] — A photodetector 5, its input end is connected to the output end of the long optical fiber;

[0043] — An electrical amplifier 6, used to make the gain of the optoelectronic signal in the loop greater than the loss to start oscillation, its input end is connected to the output end of the photodetector;

[0044] — An electrical filter 7, used to control the frequency range of the generated microwave signal, its input end is connected to the output end of the electrical amplifier;

[0045] — An electrical power splitter 8, which has an input port D, two microwave signal output ports E and F. The input port D of the electrical power splitter is connected to the output end of the electrical filter, and one microwave signal output port E of the electrical power splitter is connected to the microwave signal input port C of the electro-optic intensity modulator.

[0046] The single-mode microwave signal emitted by the optoelectronic oscillator has a side-mode suppression ratio greater than 20 dB.

[0047] The electrical amplifier, the electrical filter, and the electrical power splitter are not limited to the current connection positions and can be connected at any position between the output end of the photodetector and the microwave signal input port of the electro-optic intensity modulator.

[0048] The laser is a polarization-maintaining output semiconductor laser or a polarization-maintaining output DFB laser or a polarization-maintaining output fiber laser.

[0049] The voltage source is an adjustable voltage source that can output -10V to 10V or other types of voltage sources that can output a certain voltage between -10V and +10V, including but not limited to bias controllers, arbitrary waveform generators, DC voltage adapters, etc.

[0050] The long optical fiber is used to store energy. The length of the long optical fiber is selected between 10 and 20,000 meters according to the requirements of phase noise for different applications. In this embodiment, the length of the long optical fiber is 10.1 kilometers; the long optical fiber is not limited to using G652 ordinary single-mode optical fiber and can use other types of optical fibers to achieve the same effect as G652 ordinary single-mode optical fiber.

[0051] The photodetector is a photodetector with a bandwidth of 1 to 100 GHz or other types of photodetectors that can achieve the same effect as a 1 to 100 GHz photodetector. In this embodiment, a 50 GHz bandwidth photodetector is used.

[0052] The bandwidth of the electrical amplifier 6 is 40 kHz to 100 GHz, and its gain coefficient is sufficient to cause the oscillator to oscillate, or other types of electrical amplifiers that can achieve the same effect as an electrical power amplifier with a bandwidth of 40 kHz to 100 GHz; in this embodiment, an electrical amplifier with a gain coefficient of 26 dB is used.

[0053] The electro-optic intensity modulator is an electro-optic intensity modulator with a bandwidth of 5 to 100 GHz or other types of electro-optic intensity modulators that can achieve the same effect as an electro-optic intensity modulator with a bandwidth of 5 to 100 GHz; the electro-optic intensity modulator is a modulator that can generate first-order sidebands and second-order sidebands or other types of modulators that can generate first-order and second-order sidebands.

[0054] The center frequency of the electrical filter 7 is 1 to 100 GHz, or other types of electrical filters that can achieve the same effect as an electrical filter with a frequency of 1 to 100 GHz; in this embodiment, a band-pass filter with a center frequency of 10 GHz and a bandwidth of 20 MHz is used.

[0055] The electrical power splitter 8 is an electrical power splitter with a bandwidth of 1 to 100 GHz or other types of electrical power splitters that can achieve the same effect as an electrical power splitter with a bandwidth of 1 to 100 GHz. In this embodiment, an electrical power splitter with an operating bandwidth of 4 to 40 GHz is used.

[0056] The optoelectronic oscillator described in this embodiment realizes the regulation of the relative intensity relationship of the energies of each order of optical sidebands by adjusting the bias voltage of the intensity modulator, and realizes the overall energy control of each order of optical sidebands by controlling the power of the laser, so as to make it meet the condition that the gain and loss in the parity-time symmetric system are equal. The principle of realizing the high side-mode suppression ratio mode output is based on the parity-time symmetric mechanism, rather than relying solely on an electrical filter to achieve single-longitudinal mode output. In the embodiment, an optical fiber with a length of up to 16 km can be used, and the adjacent mode frequency interval is only 13 kHz, which is much smaller than the 3 dB bandwidth of the electrical filter.

[0057] In this embodiment, as Figure 2As shown; the electro-optic intensity modulator can output an optical signal with positive and negative first-order sidebands and positive and negative second-order sidebands. The photoelectric conversion of the positive and negative first-order sidebands and the optical carrier within the loop, as well as the photoelectric conversion of the first-order sidebands and the second-order sidebands within the loop, are equivalent to two photoelectric loops. By adjusting the bias voltage of the voltage source and the power of the laser output by the laser, the energy ratio and overall power between each sideband are controlled, so as to realize the adjustment of the gain and loss intensity of these two photoelectric loops. If the gain and loss of these two photoelectric loops do not satisfy that the gain of one loop is equal to the loss of the other loop and the symmetry-breaking state is not achieved, a multi-longitudinal-mode output microwave signal will be obtained; if the bias voltage of the voltage source and the power of the laser output by the laser are adjusted to make the gain and loss of these two photoelectric loops satisfy that the gain of one loop is equal to the loss of the other loop and the symmetry-breaking state is achieved, a single-longitudinal-mode microwave signal output similar to Figure 3 as shown will be obtained. The side-mode suppression ratio exceeds 20 dB. At this time, the spectrogram of the electro-optic intensity modulation optical signal is as Figure 2 shown. When the fiber length is 10.1 km, the phase noise at a frequency offset of 10 kHz is as low as -142 dBc / Hz, as Figure 4 shown.

[0058] In this embodiment, actually only retaining the carrier, +1st-order sideband and +2nd-order sideband, or only retaining the carrier, -1st-order sideband and -2nd-order sideband can also achieve the symmetry-breaking state and generate a single-longitudinal-mode output microwave signal, which has the same effect as the electro-optic intensity modulation optical signal retaining the carrier, two 1st-order sidebands and two 2nd-order sidebands.

[0059] It should also be noted that in this specification, terms such as "including", "comprising" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or also includes elements inherent to this process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, method, article or device including the said element.

[0060] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A parity-time symmetric optoelectronic oscillator based on intensity modulation sidebands, characterized in that, Comprising: A laser for generating laser light with a wavelength of λ. The output end of the laser is a polarization-maintaining optical fiber, and the polarization direction of the linearly polarized laser output by the laser is aligned with the slow axis of the polarization-maintaining optical fiber; An electro-optic intensity modulator having an optical signal input port, an optical signal output port, a microwave signal input port, and a voltage input port; The optical signal input port of the electro-optic intensity modulator is connected to the polarization-maintaining optical fiber at the output end of the laser, and the linearly polarized laser output by the laser is coupled into the extraordinary optical axis of the electro-optic intensity modulator through the slow axis of the polarization-maintaining optical fiber; An electro-optic intensity modulator for receiving an optical carrier and modulating the microwave signal generated by the optoelectronic oscillator onto the optical carrier to generate an optical signal with multiple-order modulation sidebands; A voltage source that outputs a stable and adjustable DC bias voltage and is connected to the voltage input port of the electro-optic intensity modulator; A long optical fiber whose input end is connected to the optical signal output port of the electro-optic intensity modulator; A photodetector whose input end is connected to the output end of the long optical fiber; An electrical amplifier for enabling the gain of the optoelectronic signal in the loop to be greater than the loss to start oscillation, and its input end is connected to the output end of the photodetector; An electrical filter for controlling the frequency range of the generated microwave signal, and its input end is connected to the output end of the electrical amplifier; An electrical power splitter having an input port and two microwave signal output ports. The input port of the electrical power splitter is connected to the output end of the electrical filter. One microwave signal output port of the electrical power splitter is connected to the microwave signal input port of the electro-optic intensity modulator, and the other microwave signal output port of the electrical power splitter outputs the generated microwave signal outside the oscillator.

2. The optoelectronic oscillator according to claim 1, characterized in that, The laser is a semiconductor laser with polarization-maintaining output, a DFB laser with polarization-maintaining output, or a fiber laser with polarization-maintaining output. A non-polarization-maintaining optical fiber and a polarization controller are used instead of the polarization-maintaining optical fiber.

3. The optoelectronic oscillator according to claim 1, wherein The long optical fiber is used to store energy, and the length of the long optical fiber is selected between 10 and 20,000 m according to the requirements of phase noise for different applications.

4. The optoelectronic oscillator according to claim 1, wherein, The electrical amplifier, the electrical filter, and the electrical power splitter are not limited to the current connection positions and can be connected at any position and in any order between the output end of the photodetector and the microwave signal input port of the electro-optic intensity modulator.

5. The optoelectronic oscillator according to claim 1, characterized in that The single-mode microwave signal emitted by the optoelectronic oscillator has a side-mode suppression ratio greater than 20 dB.

6. The optoelectronic oscillator according to claim 1, characterized in that, The electro-optic intensity modulator is an electro-optic intensity modulator with a bandwidth of 5 - 100 GHz and capable of generating first-order and second-order sidebands; The electrical filter is an electrical filter with a center frequency of 1 - 100 GHz.

7. The optoelectronic oscillator according to claim 1, wherein The voltage source is an adjustable voltage source that can output -10V - 10V.

8. The optoelectronic oscillator according to claim 1, characterized in that, The photodetector is a photodetector with a bandwidth of 1 - 100 GHz; The electrical amplifier is an electrical amplifier with a bandwidth of 40 kHz - 100 GHz and a gain coefficient sufficient to enable the oscillator to oscillate; The electrical power splitter is an electrical power splitting functional device with a bandwidth of 1 - 100 GHz.

9. The optoelectronic oscillator according to claim 1, wherein The multi-order sideband modulated optical signal realizes the gain and loss matching in the parity-time symmetric system by adjusting the voltage of the voltage source and the power of the laser output by the laser. An adjustable optical attenuator is used instead of tuning the power of the laser, and an adjustable optical filter is used instead of adjusting the voltage of the voltage source.

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