CN109725446B - Microwave photon transverse filter based on single-wavelength and multimode fiber delay line module - Google Patents
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Abstract
The invention provides a microwave photon transverse filter based on a single-wavelength and multi-mode optical fiber delay line module, and belongs to the field of microwave photonics. The core part of the filter is an optical delay line module based on a single optical wavelength and a single multimode fiber, and the optical delay line module combines the technical advantages of a coherent system and a non-coherent system. The optical delay line module is constructed by adopting a single optical wavelength in a coherent system and a multimode fiber in an incoherent system, the single optical wavelength reduces the cost and simplifies the system, the multimode fiber not only provides time delay between modes generated by modal dispersion, but also eliminates the optical coherence phenomenon by the provided spatial wavelength division multiplexing delay line structure, and has high resolution and flexible frequency response. The incidence angle of the light beam entering the multimode optical fiber can be adjusted by relying on the mechanical slit, and the tunable characteristic of the filter tap is realized. The filtering experiment of the filter shows that the wave-limiting suppression ratio of the invention reaches more than 30dB and the stability is good.
Description
Technical Field
The invention relates to the technical field of microwave photonic filters, in particular to a microwave photonic transverse filter based on a single-wavelength and multi-mode optical fiber delay line module.
Background
Microwave photonics combines microwave and photonics, and thus integrates the advantages of microwave and photonics, transparently transforming between radio frequency waves and optical fibers, microwave providing a low cost mobile wireless connection, and optical fibers providing a low loss broadband connection, and being immune to electromagnetic interference.
The microwave photon filter is an optical subsystem which processes microwave signals by an optical method and realizes a filtering function. As a key technology in microwave photonics, a microwave photonic filter is mainly used for processing radio frequency signals instead of a traditional method, namely, the radio frequency signals are used for directly modulating optical carriers and are directly processed in an optical domain, and the microwave photonic filter has the characteristics of large bandwidth, low loss, electromagnetic interference resistance, tunability, flexibility, reconfigurability and the like. In recent years, with the increasing demand of people for broadband communication capacity, microwave photonic filters are becoming hot spots for researches of scholars at home and abroad, and have important applications in the fields of radio over fiber communication systems, radar navigation, electronic countermeasure, deep space exploration and the like, as well as in photonic beam control array antennas and communication signal processing.
Microwave photonic filters fall into two categories, incoherent and coherent. The incoherent system is based on a time delay line structure, a discrete digital filter technology is adopted, a plurality of taps are obtained by a mode of cutting a wide-spectrum light source or a laser array by frequency spectrum, time delay between adjacent taps is obtained by modulating signals through linear dispersion media with different lengths, the filter is stable in performance and insensitive to environmental change, and the frequency spectrum response presents a multi-pass band; the coherent system is realized by using a single light source and based on a phase modulation to intensity modulation (PM-IM) conversion technology, the coherent system essentially belongs to spectrum mapping, the response of a spectrometer is in a single passband, namely, an optical filter is mapped to a microwave filter, and the property of the microwave filter is completely dependent on the optical filter, so that the development of a high-performance optical filter is very important.
The optical delay line module is a key part of the microwave photonic transversal filter, wherein the two-tap optical delay line module is the simplest. Since optical interference is very sensitive to environmental changes, in order to avoid optical interference effects, most existing microwave photonic filters adopt an incoherent delay line structure system based on finite-impulse-response (FIR-finite-impulse-response). Multiple filter taps are typically provided with a laser array containing multiple optical wavelengths, and the time delay between the filter taps can be obtained from modal dispersion between the different wavelengths, avoiding the concern of optical signal polarization instability. However, the classical design has the disadvantage of high cost and complexity, and only limited filtering functions can be implemented, limiting the characteristics and application range of the filter. Low cost solutions often use a single broadband incoherent light source to achieve modal dispersion between multiple wavelengths based on spectral slices and thus obtain the time delay between different modes, but this solution comes at the cost of a low signal-to-noise ratio due to the introduction of a noise-like light source.
On the other hand, the photon filter may also be implemented with a coherent regime. The advantage of using a single wavelength light source is a reduced cost and at the same time a simplified construction. However, since the microwave filter response is directly dependent on the optical and spectral filter responses involved, the operation of the spectral filter response with sub-GHz resolution is challenging, and thus, coherent microwave photonic filters are still limited by limited frequency resolution and low stability.
Disclosure of Invention
The invention aims to provide a microwave photon transverse filter based on a single-wavelength and multimode fiber delay line module aiming at the technical defects of the prior art so as to solve the technical problems of lower resolution and poorer stability of the conventional microwave photon filter.
Another technical problem to be solved by the present invention is how to improve the filtering performance of a microwave photonic filter and achieve tunability.
In order to achieve the technical purpose, the invention adopts the following technical scheme:
the microwave photon transverse filter based on the single-wavelength and multimode fiber delay line module comprises a single-wavelength light source, a polarization controller, a Mach-Zehnder modulator, a radio-frequency signal generator, a laser power amplifier, a first collimating lens, a mechanical slit, a planoconvex lens, a multimode fiber, a second collimating lens, a beam splitter, an infrared imaging device, a focusing lens and a space photoelectric detector, wherein the single-wavelength light source is connected to the polarization controller through the fiber, the polarization controller is connected to the Mach-Zehnder modulator through the fiber, the Mach-Zehnder modulator is simultaneously connected with the radio-frequency signal generator through a cable, the output end of the Mach-Zehnder modulator is connected to the laser power amplifier through the fiber, and the output end of the laser power amplifier emits a beam;
a first collimating lens and a mechanical slit are sequentially arranged on the path of the light beam, and the emergent end of the mechanical slit is provided with a plurality of parallel light beams; a plano-convex lens is arranged on the path of the parallel light beams, and the emergent end of the plano-convex lens is a converged light beam; the path of the converged light beam is provided with a multimode optical fiber, a second collimating lens and a beam splitter are sequentially arranged on the path of the light beam output by the multimode optical fiber, the beam splitter is provided with two emergent ends, an infrared imaging device is arranged on one emergent path of the beam splitter, a focusing lens is arranged on the other emergent path of the beam splitter, the emergent end of the focusing lens is a focused light beam, and a space photoelectric detector is arranged on the path of the focused light beam.
Preferably, the wavelength of the optical carrier output by the single-wavelength light source is 1556.4 nm.
Preferably, the polarization controller is a three-ring polarization controller.
Preferably, the radio frequency signal generated by the radio frequency signal generator is loaded to the optical carrier wave through the mach-zehnder modulator, and the optical carrier wave is subjected to radio frequency modulation with the frequency of 1 GH.
Preferably, the laser power amplifier is an erbium-doped fiber amplifier.
Preferably, the multimode optical fiber has a length of 1 m, a core diameter of 105 μm, and a numerical aperture of 0.37.
Preferably, the spatial photodetector has an effective area of 50 × 50 μm2Free space photodetectors.
Preferably, the system further comprises an oscilloscope and a spectrometer, wherein the oscilloscope detects the radio frequency signal collected by the space photoelectric detector, and the spectrometer measures the frequency response of the filter.
Preferably, the oscilloscope is a real-time digital oscilloscope; more preferably, the oscilloscope is a real-time digital oscilloscope with a product model of TektronixDPO72304 DX.
In the above technical solution, the single-wavelength light source may also be referred to as a single-wavelength output laser light source, and is configured to provide a single-wavelength optical carrier; the Mach-Zehnder modulator is used for loading radio-frequency signals to an optical carrier; the radio frequency generator is used for generating a radio frequency signal and loading the radio frequency signal to an optical carrier through the Mach-Zehnder modulator; the laser power amplifier is used for amplifying the power of the optical carrier after the radio frequency modulation; the first collimating lens is used for collimating and expanding light beams output by the optical fibers; the mechanical slit is also called as a micro-mechanical control slit device and is used for dividing the intensity of an incident light beam to generate several parallel light beams with extremely narrow sizes; the plano-convex lens is used for converging the parallel light beams; the second collimating lens is used for collimating the light beam output by the multimode optical fiber; the beam splitter is used for splitting an incident beam into two beams with different propagation directions; the infrared imaging device is an infrared imaging device with computer analysis software and is used for acquiring light intensity distribution of different taps, namely a light spot mode; the focusing lens is used for converging the parallel light.
The working principle of the invention is as follows: the whole filter system is divided into a delay line front part, a delay line part and a delay line rear part.
Before a filter delay line, a single-wavelength laser light source emits laser light, the output single wavelength is 1556.4nm, when the laser light passes through a polarization controller and a Mach-Zehnder modulator (MZM), a radio frequency signal is loaded on an optical carrier, the single-wavelength optical carrier is subjected to Radio Frequency (RF) modulation with the frequency of 1GH, and power amplification is carried out through a laser power amplifier.
And the first collimating lens collimates and expands the power amplified laser beam, and the laser beam is divided into two parallel narrow beams after passing through a space mask plate with a mechanical slit, the incident angles of the emergent directions of the beams relative to the optical axis of the multimode fiber are 0 degree and 10 degrees respectively, namely, the two excited modes are a fundamental mode and a certain-order high-order mode respectively. The multimode fiber is focused on the incident end face of the multimode fiber after passing through the plano-convex lens, the multimode fiber is incident at different incident angles, the length of the multimode fiber is 1 meter, the diameter of the fiber core is 105 mu m, different spatial modes in the fiber core are excited after the multimode fiber is coupled into the fiber core to form a filter tap, and due to the fact that the different modes have different optical paths in the transmission process in the fiber, group velocity delay is formed between adjacent taps when the multimode fiber is emergent. It is worth to say that it has been proved by the conclusion that, in the optical delay line module based on the coherent system, the modal dispersion in the large-core multimode fiber is more effective than the chromatic dispersion in the common single-mode fiber in terms of generating the time delay, and in addition, for a large-core step-index multimode fiber, because of the large numerical aperture, a large number of spatial modes can be generated, and the spatial mode order in the fiber and the generated group delay have a linear relationship. In conclusion, the optical delay line module design based on the coherent system single optical wavelength and the incoherent system multimode fiber can obtain large time delay. If the incident beam is further narrowed, the time delay between excited modes in the fiber will be further increased or increased. Also, for microwave photonic filters, shortening the length of the multimode fiber will result in a larger free spectral range, while reducing the time delay between modes.
After the delay line, the light beam output by the multimode fiber passes through a second collimating lens and an optical beam splitter, one part of the light beam enters an infrared light camera for measuring the intensity of the light beam, the other part of the light beam enters a space Photoelectric Detector (PD) after being converged by a focusing lens, and the effective area is 50 x 50 mu m2And then connecting with an oscilloscope to detect radio frequency signals or connecting with a frequency spectrograph to measure the effect of the filter.
The invention provides a microwave photon transverse filter based on a single-wavelength and multi-mode optical fiber delay line module. The technical scheme combines the advantages of a coherent system and an incoherent system, adopts a single optical wavelength in the coherent system and a multimode fiber in the incoherent system to construct an optical delay line module, reduces the cost by the single optical wavelength and simplifies the system, and the multimode fiber not only provides time delay between modes generated by modal dispersion, but also eliminates the optical coherence phenomenon by the provided spatial wavelength division multiplexing delay line structure, thereby having high resolution and flexible frequency response.
The core part of the microwave photonic transversal filter is an optical delay line module. In the present invention, optical delay is achieved by modal dispersion occurring within a single multimode fiber (MMF). The specific mechanism is that a single-wavelength light beam is divided into narrow light beams with different incidence angles by intensity to form each tap of a filter, the light beams enter a fiber core of the multimode optical fiber after being focused, then different spatial modes in the fiber core of the multimode optical fiber are excited, and time delay is generated between the modes due to the fact that the different modes experience different optical paths in the transmission process in the optical fiber. In this design, optical interference between different modes in the fiber is eliminated based on spatial demultiplexing occurring in the multimode fiber, and further, since the time delay between taps of the filter is obtained by large modal dispersion in the multimode fiber, the time delay between adjacent taps of the filter can be easily adjusted by changing the angle of incidence of the multiple beams.
In summary, the novel adjustable coherent optical delay line module provided by the invention is a device based on a single optical wavelength and a single multimode fiber, has the advantages of simple structure, low cost and easy reconstruction, and has great application potential in the field of actual microwave photon signal processing.
The invention provides a microwave photon transverse filter based on a single-wavelength and multi-mode optical fiber delay line module, and belongs to the field of microwave photonics. The core part of the filter is an optical delay line module based on a single optical wavelength and a single multimode fiber, and the optical delay line module combines the technical advantages of a coherent system and a non-coherent system. The optical delay line module is constructed by adopting a single optical wavelength in a coherent system and a multimode fiber in an incoherent system, the single optical wavelength reduces the cost and simplifies the system, the multimode fiber not only provides time delay between modes generated by modal dispersion, but also eliminates the optical coherence phenomenon by the provided spatial wavelength division multiplexing delay line structure, and has high resolution and flexible frequency response. The incidence angle of the light beam entering the multimode optical fiber can be adjusted by relying on the mechanical slit, and the tunable characteristic of the filter tap is realized. The filtering experiment of the filter shows that the wave-limiting suppression ratio of the invention reaches more than 30dB and the stability is good.
Drawings
FIG. 1 is a view showing the structure of an apparatus according to the present invention as a whole;
FIG. 2 is a graph of the pattern intensity profile obtained by an infrared imaging device in accordance with an embodiment of the present invention;
FIG. 3 is a diagram of RF signals received by a PD in accordance with an embodiment of the present invention;
FIG. 4 is an eye diagram of two modes output simultaneously in an embodiment of the present invention;
FIG. 5 is a graph of a frequency response in an embodiment of the present invention;
fig. 6 is a diagram of radio frequency signals received by the tunable characteristic verification experiment part PD according to the embodiment of the present invention;
FIG. 7 is a frequency response graph of the tunable property verification experiment section in accordance with an embodiment of the present invention;
in the figure:
1. a single wavelength light source 2, a polarization controller 3, a Mach-Zehnder modulator 4, a radio frequency signal generator
5. Laser power amplifier 6, first collimating lens 7, mechanical slit 8, plano-convex lens
9. Multimode optical fiber 10, second collimating lens 11, beam splitter 12 and infrared imaging device
13. Focusing lens 14, space photoelectric detector
Detailed Description
Hereinafter, specific embodiments of the present invention will be described in detail. Well-known structures or functions may not be described in detail in the following embodiments in order to avoid unnecessarily obscuring the details. Approximating language, as used herein in the following examples, may be applied to represent quantitative representations that could permissibly vary in number without resulting in a change in the basic function. Unless defined otherwise, technical and scientific terms used in the following examples have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
A microwave photon transverse filter based on a single-wavelength and multi-mode fiber delay line module is shown in figure 1 and comprises a single-wavelength light source 1, a polarization controller 2, a Mach-Zehnder modulator 3, a radio-frequency signal generator 4, a laser power amplifier 5, a first collimating lens 6, a mechanical slit 7, a plano-convex lens 8, a multi-mode fiber 9, a second collimating lens 10, a beam splitter 11, an infrared imaging device 12, a focusing lens 13 and a spatial photoelectric detector 14, wherein a single wavelength light source 1 is connected to a polarization controller 2 through an optical fiber, the polarization controller 2 is connected to a Mach-Zehnder modulator 3 through an optical fiber, the Mach-Zehnder modulator 3 is connected with the radio frequency signal generator 4 through a cable at the same time, the output end of the Mach-Zehnder modulator 3 is connected to the laser power amplifier 5 through an optical fiber, and the output end of the laser power amplifier 5 emits light beams;
a first collimating lens 6 and a mechanical slit 7 are sequentially arranged on the path of the light beam, and the emergent end of the mechanical slit 7 is provided with a plurality of parallel light beams; a plano-convex lens 8 is arranged on the paths of the parallel light beams, and the emergent end of the plano-convex lens 8 is a converged light beam; the path of the converged light beam is provided with a multimode fiber 9, a second collimating lens 10 and a beam splitter 11 are sequentially arranged on the path of the light beam output by the multimode fiber 9, the beam splitter 11 is provided with two emergent ends, one emergent path of the beam splitter 11 is provided with an infrared imaging device 12, the other emergent path of the beam splitter 11 is provided with a focusing lens 13, the emergent end of the focusing lens 13 is a focused light beam, and the path of the focused light beam is provided with a space photoelectric detector 14.
Wherein, the wavelength of the optical carrier output by the single-wavelength light source 1 is 1556.4 nm. The polarization controller 2 is a three-ring polarization controller. The radio frequency signal generated by the radio frequency signal generator 4 is applied to an optical carrier via the mach-zehnder modulator 3, and the optical carrier is subjected to radio frequency modulation with the frequency of 1 GH. The laser power amplifier 5 is an erbium-doped fiber amplifier. The length of the multimode optical fiber 9 is 1 meter, the diameter of the fiber core is 105 μm, and the numerical aperture is 0.37. The spatial photodetector 14 has an effective area of 50 x 50 μm2Free space photodetectors. Further comprising an oscilloscope for detecting the radio frequency signal collected by the space photodetector 14 and a spectrometer for measuring the frequency response of the filter. The oscilloscope is a real-time digital oscilloscope.
The filtering effect of the present invention is verified experimentally as follows.
The beam intensity profile image measured by the infrared camera confirms this design, the first two parts of fig. 2 show the speckle pattern when only one beam is active, while when two beams are simultaneously incident on the multimode fiber, the fiber simultaneously emits two speckle patterns as shown in the third part of fig. 2, fully demonstrating that space division multiplexing has been successfully achieved, it is worth emphasizing that there is no optical interference between the two spatial patterns despite the use of a strongly coherent single wavelength light source. The spatial demultiplexing is also maintained during the photodetection process, otherwise, optical interference is generated near the receiving end face of the photodetector after the two modes are focused. In order to confirm that no optical interference occurs between the two modes in the space photoelectric detection process, a real-time digital oscilloscope (Tektronix DPO72304DX) is used for measuring radio frequency signals (RF) received by a PD in the experiment and MATLAB is used for plotting the experimental data.
The first two parts of fig. 3 show the 1GH rf signal recovered when only the fundamental and high order modes are operating, respectively, and in both cases the rf signal is well-obtained. It is worth mentioning that, since the length of the multimode fiber is 1 meter, the time delay of the two modes in the fiber is small, and therefore, the time delay between the two received radio frequency signals is not very obvious. When the two modes are operated simultaneously, the RF signal detected by the space PD is as shown in the third part of fig. 3, and the result shows that the resulting RF signal is stronger than when the single mode is operated and no optical interference phenomenon occurs. The 1GH radio frequency signal has slight distortion, which also confirms the existence of a time delay, which is relatively small but has high stability. To further confirm that the optical coherence phenomenon in the single-wavelength optical delay line module has been eliminated, we measured the eye patterns of the two modes at the same time output with an oscilloscope and compared them with the results measured by a single-mode fiber Mach-Zehnder interferometer (MZI), as shown in fig. 4, the optical coherence phenomenon occurring inside the single-mode fiber system was successfully avoided in our multimode fiber module.
The above experimental results fully confirm that the designed optical delay line module avoids optical interference and has high stability, and based on this, the filter experiment is performed. The output of the spatial PD was connected to a spectrometer to obtain a frequency response curve from 0Hz to 5GHz, as shown in fig. 5. It can be seen that the filter system successfully realizes band-pass filtering, and the rejection ratio of the band-limited frequency band reaches 30 dB.
In order to verify the tunable characteristic of the optical delay line of the core part in the filter system, the incidence angle of incident light is adjusted through a mechanical slit in an experiment, an oscilloscope measures an RP modulation signal as shown in fig. 6, and the experimental result of the filter is shown in fig. 7. The result proves that the designed optical delay line module based on the coherent system single-wavelength light source and the incoherent system multimode fiber can successfully realize the time delay of different taps so as to realize the microwave photon transverse filter, has tunability, can very conveniently adjust the orders of the space modes of the different taps through the mechanical slit, and has high stability.
The embodiments of the present invention have been described in detail, but the description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention. Any modification, equivalent replacement, and improvement made within the scope of the application of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A microwave photon transverse filter based on a single-wavelength and multimode fiber delay line module is characterized by comprising a single-wavelength light source (1), a polarization controller (2), a Mach-Zehnder modulator (3), a radio-frequency signal generator (4), a laser power amplifier (5), a first collimating lens (6), a mechanical slit (7), a planoconvex lens (8), a multimode fiber (9), a second collimating lens (10), a beam splitter (11), an infrared imaging device (12), a focusing lens (13) and a space photoelectric detector (14), wherein the single-wavelength light source (1) is connected to the polarization controller (2) through an optical fiber, the polarization controller (2) is connected to the Mach-Zehnder modulator (3) through an optical fiber, and the Mach-Zehnder modulator (3) is connected with the radio-frequency signal generator (4) through a cable at the same time, the output end of the Mach-Zehnder modulator (3) is connected to a laser power amplifier (5) through an optical fiber, and the output end of the laser power amplifier (5) emits light beams;
a first collimating lens (6) and a mechanical slit (7) are sequentially arranged on the path of the light beam, and the emergent end of the mechanical slit (7) is provided with a plurality of parallel light beams; a plano-convex lens (8) is arranged on the paths of the parallel light beams, and the emergent end of the plano-convex lens (8) is a converged light beam; a multimode fiber (9) is arranged on a path of the converged light beam, a second collimating lens (10) and a beam splitter (11) are sequentially arranged on a light beam path output by the multimode fiber (9), the beam splitter (11) is provided with two emergent ends, an infrared imaging device (12) is arranged on one emergent path of the beam splitter (11), a focusing lens (13) is arranged on the other emergent path of the beam splitter (11), the emergent end of the focusing lens (13) is a focused light beam, and a space photoelectric detector (14) is arranged on a path of the focused light beam;
wherein optical interference between different modes in the fiber is excluded based on spatial demultiplexing that occurs in the multimode fiber.
2. The microwave photonic transversal filter based on single wavelength and multimode fiber delay line module according to claim 1, characterized in that the optical carrier wavelength outputted by the single wavelength light source (1) is 1556.4 nm.
3. The single wavelength and multimode fiber delay line module based microwave photonic transversal filter according to claim 1, characterized in that the polarization controller (2) is a three-ring polarization controller.
4. The microwave photonic transversal filter based on single wavelength and multimode fiber delay line module according to claim 1, characterized in that the radiofrequency signal generated by the radiofrequency signal generator (4) is applied to the optical carrier via the mach-zehnder modulator (3), which is subjected to radiofrequency modulation with a frequency of 1 GH.
5. The microwave photonic transversal filter based on single wavelength and multimode fiber delay line modules according to claim 1, characterized in that said laser power amplifier (5) is an erbium doped fiber amplifier.
6. The microwave photonic transversal filter based on single wavelength and multimode fiber delay line module according to claim 1, characterized in that the multimode fiber (9) has a length of 1 meter, a core diameter of 105 μm and a numerical aperture of 0.37.
7. The single wavelength and multimode fiber delay line module based microwave photonic transversal filter according to claim 1, characterized in that the spatial photodetector (14) is a free space photodetector with an active area of 50 x 50 μm 2.
8. The single wavelength and multimode fiber delay line module based microwave photonic transversal filter according to claim 1, further comprising an oscilloscope and a spectrometer, wherein said oscilloscope detects radio frequency signals collected by the spatial photodetector (14), and said spectrometer measures the frequency response of said filter.
9. The single wavelength and multimode fiber delay line module based microwave photonic transversal filter of claim 8, characterized in that said oscilloscope is a real time digital oscilloscope.
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| JP2009204943A (en) * | 2008-02-28 | 2009-09-10 | Active Inc | Sleeve for optical fiber, and core pin |
| CN104303084A (en) * | 2012-03-05 | 2015-01-21 | 阿尔卡特朗讯 | Multimode optical fiber, mode delay adjuster for fiber systems, and methods to use such fibers, adjusters, and systems |
| CN102636694A (en) * | 2012-05-11 | 2012-08-15 | 厦门大学 | Single-response microwave photonic filter-based frequency measurement device and measurement method |
| CN103856266A (en) * | 2014-03-13 | 2014-06-11 | 北京交通大学 | Optical correlator based on modal dispersion |
| CN107528638A (en) * | 2017-07-27 | 2017-12-29 | 西安电子科技大学 | Wide-band microwave Direction-of-arrival method based on microwave photon filtering |
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