CN109560877B - K-K receiving-based dual-polarization QAM modulation direct detection communication system and method - Google Patents
K-K receiving-based dual-polarization QAM modulation direct detection communication system and method Download PDFInfo
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Abstract
The invention provides a K-K receiving-based dual-polarization QAM modulation direct detection communication system, which comprises a signal receiving end and a signal transmitting end, wherein the signal receiving end is connected with the signal transmitting end through a transmission link, and comprises a laser, a beam splitter, a frequency shifter, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first photoelectric detector PD1, a second photoelectric detector PD2, a first KK conversion unit KK1, a second KK conversion unit KK2, a carrier recovery and data judgment unit and a first circulator C1. The invention also provides a double-polarization QAM modulation direct detection communication method based on K-K reception. The beneficial effects of the invention are as follows: on one hand, the spectrum efficiency is greatly improved; on the other hand, power consumption is greatly reduced, so that the method is suitable for short-range communication.
Description
Technical Field
The invention relates to the field of short-distance optical communication, in particular to a dual-polarization QAM modulation direct detection communication system and method based on K-K reception.
Background
The development of cloud services and applications based on data centers has led to an ever-increasing demand for spectrum for short-range communications. Up to now, data Center (DC) communication traffic has been a major component of global communication traffic, and the information interaction amount of 2020DC is about to reach 15Zbit, which is doubled compared with 2017 according to the prediction of cisco global cloud index. Wherein approximately 77% of the communications are interacted within the data center, including primarily data generation, storage, processing, and verification. The tremendous amount of information interaction has stimulated the need for high-rate, low-cost, low-power consumption short-range communication schemes. Based on the current size and capacity of the data center, the data center interconnection network deployed by Single Mode Fiber (SMF) is mainly distributed in the distance range of 500m to 10km, in this case, the power consumption of the subsequent high-speed DSP processing module is overlarge by using the traditional coherent communication scheme, and the realization cost is overlarge, so that the traditional coherent optical communication scheme is difficult to be directly applied to short-distance communication. However, the coherent detection scheme can perform signal modulation in four dimensions, and meanwhile, the introduction of local oscillation light can greatly improve the sensitivity of the receiver, so that the method is one of effective alternatives for realizing high-speed short-distance communication. Therefore, there is a literature proposal to greatly reduce power consumption by replacing complex DSP (digital signal processing ) modules with reasonable structural designs and optical components. However, on the other hand, coherent reception requires a mixer and four pairs of balanced detectors, which increases the cost and also limits the integration level of the device. Therefore, how to implement dual polarization QAM (Quadrature Amplitude Modulation ) signaling in a direct detection manner in combination with a low complexity DSP is an important research direction.
Based on the market drivers and technical challenges described above, a series of solutions have been proposed by related researchers. In terms of reducing DSP complexity, by operating the system around the 0 dispersion wavelength, the effects of dispersion can be ignored; the same laser is used as the local oscillation light and the signal carrier, and the signal path and the local oscillation light path are subjected to phase matching, so that a carrier recovery module in the DSP can be replaced; and the polarization controller is controlled through feedback to realize the compensation of polarization aliasing. However, in the above scheme, the feedback control amount of polarization aliasing compensation is up to three, and the implementation is complex. In addition, through combining Stokes receiving and a Kramers-Kronig (K-K) algorithm, dual-polarization QAM signal transmission can be realized in a direct detection mode. However, this solution requires a complex algorithm to compensate for polarization aliasing, and is also complex in reception and structure.
Based on this, how to propose a simpler and effective polarization aliasing compensation scheme, combining with Kramers-Kronig reception, only two PDs (photodetectors) are needed to realize dual-polarization QAM signal transmission, which is a technical problem to be solved by those skilled in the art.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a dual-polarization QAM modulation direct detection communication system and method based on K-K reception.
The invention provides a dual-polarization QAM modulation direct detection communication system based on K-K reception, which comprises a signal receiving end and a signal transmitting end, wherein the signal receiving end is connected with the signal transmitting end through a transmission link, the signal receiving end comprises a laser, a beam splitter, a frequency shifter, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first photoelectric detector PD1, a second photoelectric detector PD2, a first KK conversion unit KK1, a second KK conversion unit KK2, a carrier recovery and data decision unit and a first circulator C1, the output end of the laser is connected with the input end of the beam splitter, the local oscillation light output end of the beam splitter is connected with the input end of the frequency shifter, the output end of the frequency shifter is connected with the input end of the first polarization beam splitter PBS1, the X-polarization light output end of the first polarization beam splitter PBS1 is connected with the input end of the first photoelectric detector PD1, the Y-polarization output end of the first polarization beam splitter 1 is connected with the second photoelectric detector PD2, the carrier recovery and data decision unit and the first circulator C1, the output end of the first polarization beam splitter is connected with the second polarization beam splitter C2, the second polarization beam splitter is connected with the second polarizer C2, the second polarization splitter C2 is connected with the second polarizer C2, the second polarizer C2 is connected with the second polarizer C2, the QM2 is connected with the QM2, the X-polarized light output end of the third polarization beam splitter PBS3 is connected with the input end of the first IQ modulator IQM1, the output end of the first IQ modulator IQM1 is connected with the input end of the first Faraday rotary mirror FRM1, the output end of the first Faraday rotary mirror FRM1 is connected with the input end of the polarization beam combiner PBC, the Y-polarized light output end of the third polarization beam splitter PBS3 is connected with the input end of the second IQ modulator IQM2, the output end of the second IQ modulator IQM2 is connected with the input end of the second Faraday rotary mirror FRM2, the output end of the second Faraday rotary mirror FRM2 is connected with the input end of the polarization beam combiner PBC 1, the 3 port of the first annular beam combiner PBC 1 is connected with the input end of the second polarization beam combiner PBC 2, the output end of the second polarization beam splitter PBS2 is connected with the input end of the second polarization beam splitter PBS2, the input end of the second PBS2 is connected with the input end of the second polarization beam splitter KK2, the second PBS2 is connected with the input end of the second PBS2, the PD2 is connected with the input end of the PD1, and the PD 1.
As a further development of the invention, the transmission link is a single-mode optical fiber.
As a further improvement of the present invention, the signal transmitting end further includes a third circulator C3 and a fourth circulator C4, wherein a 1 port of the third circulator C3 is connected to an output end of the second IQ modulator IQM2, a 2 port of the third circulator C3 is connected to the second faraday rotator mirror FRM2, and a 3 port of the third circulator C3 is connected to an input end of the polarization beam combiner PBC; the 1 port of the fourth circulator C4 is connected with the output end of the first IQ modulator IQM1, the 2 port of the fourth circulator C4 is connected with the first faraday rotation mirror FRM2, and the 3 port of the fourth circulator C4 is connected with the input end of the polarization beam combiner PBC.
As a further improvement of the present invention, the third polarization beam splitter PBS3 and the first IQ modulator IQM1 are connected through polarization maintaining fibers, the first IQ modulator IQM1 and the 1 port 1 of the fourth circulator C4 are connected through polarization maintaining fibers, the 2 port of the fourth circulator C4 and the first faraday rotator FRM2 are connected through polarization maintaining fibers, the 3 port of the fourth circulator C4 and the polarization beam combiner PBC are connected through polarization maintaining fibers, the third polarization beam splitter PBS3 and the second IQ modulator IQM2 are connected through polarization maintaining fibers, the second IQ modulator IQM2 and the 1 port 1 of the third circulator C3 are connected through polarization maintaining fibers, the 2 port of the third circulator C3 and the second faraday rotator FRM2 are connected through polarization maintaining fibers, and the 3 port of the third circulator C3 and the polarization beam combiner PBC are connected through polarization maintaining fibers.
As a further improvement of the invention, the beam splitter and the frequency shifter are connected through a polarization maintaining fiber, and the 3 port of the first circulator C1 and the second polarization beam splitter PBS2 are connected through the polarization maintaining fiber.
As a further improvement of the invention, the K-K receiving-based dual-polarization QAM modulation direct detection communication system works in an O wave band, and the wavelength is 1260-1360nm.
The invention also provides a K-K receiving-based dual-polarization QAM modulation direct detection communication method, which is based on any one of the K-K receiving-based dual-polarization QAM modulation direct detection communication system, and comprises the following steps: the light emitted by the laser is divided into signal carrier and local oscillation light by the beam splitter, and the local oscillation light divided by the beam splitter is split by the first polarization beam splitter PBS1 after being subjected to frequency shift by the frequency shifter; the signal carrier wave separated by the beam splitter is input through a 1 port of the first circulator C1, output through a 2 port of the first circulator C1, transmitted through a transmission link to a polarization controller PC of a signal transmitting end, enters the second circulator C2 through the polarization controller PC, is input through a 2 port of the second circulator C2, output through a 3 port to a third polarization beam splitter PBS3, and is separated into X polarized light and Y polarized light by the third polarization beam splitter PBS3, and the third polarization beam splitter 3 is a third polarization beam splitterThe X-polarized light output by the three polarization beam splitters PBS3 is modulated by the first IQ modulator IQM1, reflected by the first Faraday rotating mirror FRM1, enters the polarization beam combiner PBC, Y-polarized light output by the third polarization beam splitters PBS3 is modulated by the second IQ modulator IQM2, reflected by the second Faraday rotating mirror FRM2, enters the polarization beam combiner PBC, returns to the 1 port of the second circulator C2 by the polarization maintaining fiber after being combined by the polarization beam combiner PBC, is output by the 2 port of the second circulator C2, is transmitted to the 2 port of the first circulator C1 by the polarization controller PC and the transmission link, is output by the 3 port of the first circulator C1, is split by the second polarization beam splitter PBS2, and the X-polarized light split by the first polarization beam splitters PBS1 and the second polarization beam splitters 2 is respectively emitted into the first photoelectric detector PD1 for photoelectric conversion to obtain output I X And input to a first KK conversion unit KK1, and output to the carrier recovery and data decision unit through the first KK conversion unit KK1, wherein the Y polarized light split by the first polarizing beam splitter PBS1 and the second polarizing beam splitter PBS2 is respectively injected into the second photodetector PD2 for photoelectric conversion, so as to obtain an output photocurrent I Y And the data is input to a second KK conversion unit KK2, and is output to the carrier recovery and data judgment unit through the second KK conversion unit KK2, and the carrier recovery and data judgment unit performs data judgment after carrier recovery.
As a further improvement of the invention, the beam splitting proportion of the beam splitter is adjustable, the beam splitting proportion ensures that the local oscillator light intensity is 7-9dB greater than the reflected signal light intensity, and the polarization controller PC is used for controlling the equal power of the two polarized light states.
As a further improvement of the invention, after the local oscillation light separated by the beam splitter is shifted by the frequency shifter by Δf, Δf=baud rate/2 is respectively loaded into X, Y polarized signals, and photocurrent I is obtained after detection by the first photoelectric detector PD1 and the second photoelectric detector PD2 X And photocurrent I Y The first KK converting unit KK1 obtains the amplitude and phase information of the X polarized signal, and the second KK converting unit KK2 obtains the amplitude and phase information of the Y polarized signal.
As a further development of the invention, the Kramers-Kronig algorithm performed by the first KK switching unit KK1 and the second KK switching unit KK2 is as follows:
wherein phi is X (t)、φ Y (t) the phase of X, Y polarized light field obtained after loading local oscillation light, p.v. is the integral of the Cauchy principal value, deltaf is the frequency shift of the intrinsic light, E LO-X 、E LO-Y The amplitude of the polarization of the local oscillation X, Y is a fixed constant.
The beneficial effects of the invention are as follows: on one hand, the dual-polarization QAM signal transmission is realized in a direct detection mode, and compared with the traditional amplitude modulation-direct detection scheme, the frequency spectrum efficiency is greatly improved; on the other hand, by reasonable optical fiber link design, the complex DSP module in partial dry receiving, including dispersion compensation and polarization aliasing compensation, can be removed, so that the power consumption is greatly reduced, and the method is suitable for short-distance communication.
Drawings
Fig. 1 is a schematic diagram of a dual-polarization QAM modulated direct-detection communication system based on K-K reception in accordance with the present invention.
Detailed Description
The invention is further described with reference to the following description of the drawings and detailed description.
The solid arrows in fig. 1 are normal connections, and the dashed arrows are polarization maintaining connections.
As shown in fig. 1, the dual-polarization QAM modulation direct detection communication system based on K-K reception includes a signal receiving end 10 and a signal transmitting end 20, where the signal receiving end 10 is connected with the signal transmitting end 20 through a transmission link 30, the signal receiving end 10 includes a laser 11, a beam splitter 12, a frequency shifter 13, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first photodetector PD1, a second photodetector PD2, a first KK conversion unit KK1, a second KK conversion unit KK2, a carrier recovery and data decision unit 14 and a first circulator C1, an output end of the laser 11 is connected with an input end of the beam splitter 12, a local oscillator light output end of the beam splitter 12 is connected with an input end of the frequency shifter 13, an output end of the frequency shifter 13 is connected with an input end of the first polarization beam splitter PBS1, the X-polarized light output end of the first polarization beam splitter PBS1 is connected with the input end of the first photodetector PD1, the Y-polarized light output end of the first polarization beam splitter PBS1 is connected with the input end of the second photodetector PD2, the signal transmitting end 20 comprises a polarization controller PC and a second circulator C2, the third polarization beam splitter PBS3, a first IQ modulator IQM1, a first Faraday rotator FRM1, a second IQ modulator IQM2, a second Faraday rotator FRM2 and a polarization beam combiner PBC, the signal carrier output end of the beam splitter is connected with the 1 port of the first circulator C1, the 2 port of the first circulator C1 is connected with the polarization controller PC through the transmission link 30, the polarization controller PC is connected with the 2 port of the second circulator C2, the 3 port of the second circulator C2 is connected with the input end of the third polarization beam splitter PBS3, the X-polarized light output end of the third polarization beam splitter PBS3 is connected with the input end of the first IQ modulator IQM1, the output end of the first IQ modulator IQM1 is connected with the input end of the first Faraday rotary mirror FRM1, the output end of the first Faraday rotary mirror FRM1 is connected with the input end of the polarization beam combiner PBC, the Y-polarized light output end of the third polarization beam splitter PBS3 is connected with the input end of the second IQ modulator IQM2, the output end of the second IQ modulator IQM2 is connected with the input end of the second Faraday rotary mirror FRM2, the output end of the second Faraday rotary mirror FRM2 is connected with the input end of the polarization beam combiner PBC 1, the 3 port of the first annular beam combiner PBC 1 is connected with the input end of the second polarization beam combiner PBC 2, the output end of the second polarization beam splitter PBS2 is connected with the input end of the second polarization beam splitter PBS2, the second optical detector unit KK2 is connected with the input end of the second optical detector 1, the second optical detector unit KK2 is connected with the input end of the second optical detector unit PDKK 2, and the input end of the second optical detector unit KK2 is connected with the input end of the second optical detector unit PDKK 2, and the PD2 is connected with the input end of the PD 2.
As shown in fig. 1, the first KK converting unit KK1, the second KK converting unit KK2, and the carrier recovery and data decision unit 14 are implemented by using a low-power-consumption, low-complexity Digital Signal Processor (DSP).
As shown in fig. 1, the transmission link 20 is preferably a single mode fiber.
As shown in fig. 1, the signal transmitting end further includes a third circulator C3 and a fourth circulator C4, a 1 port of the third circulator C3 is connected to an output end of the second IQ modulator IQM2, a 2 port of the third circulator C3 is connected to the second faraday rotator FRM2, and a 3 port of the third circulator C3 is connected to an input end of the polarization beam combiner PBC; the 1 port of the fourth circulator C4 is connected with the output end of the first IQ modulator IQM1, the 2 port of the fourth circulator C4 is connected with the first faraday rotation mirror FRM2, and the 3 port of the fourth circulator C4 is connected with the input end of the polarization beam combiner PBC.
As shown in fig. 1, the third polarization beam splitter PBS3 and the first IQ modulator IQM1 are connected through polarization maintaining fibers, the first IQ modulator IQM1 and the 1 port 1 of the fourth circulator C4 are connected through polarization maintaining fibers, the 2 port of the fourth circulator C4 and the first faraday rotator FRM2 are connected through polarization maintaining fibers, the 3 port of the fourth circulator C4 and the polarization beam combiner PBC are connected through polarization maintaining fibers, the third polarization beam splitter PBS3 and the second IQ modulator IQM2 are connected through polarization maintaining fibers, the second IQ modulator IQM2 and the 1 port 1 of the third circulator C3 are connected through polarization maintaining fibers, the 2 port of the third circulator C3 and the second faraday rotator FRM2 are connected through polarization maintaining fibers, and the 3 port of the third circulator C3 and the polarization beam combiner PBC are connected through polarization maintaining fibers.
As shown in fig. 1, the beam splitter 12 and the frequency shifter 13 are connected by a polarization maintaining fiber, and the 3 port of the first circulator C1 and the second polarization beam splitter PBS2 are connected by a polarization maintaining fiber.
As shown in fig. 1, in order to reduce the influence of the dispersion effect, the dual-polarization QAM modulation direct detection communication system based on K-K reception works in the O band, and the wavelength is 1260-1360nm. This band is around the SMF zero dispersion wavelength. Accordingly, the dispersion effect of the fiber is substantially negligible, and dispersion compensation in the digital domain is not required. In fact, in the data center communication links currently in commercial use, the standard operating window of systems with 100GBASE-LR4 as the transmit module is also the O-band, under which conditions the dispersion introduced by the 10km signal transmission is negligible.
As shown in fig. 1, a method for directly detecting and communicating dual-polarization QAM modulation based on K-K reception, based on any one of the above-mentioned dual-polarization QAM modulation and direct-detection communication system based on K-K reception, performs the following procedures: the light emitted by the laser 11 is divided into a signal carrier and a local oscillation light by the beam splitter 12, and the local oscillation light divided by the beam splitter 12 is split by the first polarization beam splitter PBS1 after being shifted in frequency by the frequency shifter 13; the signal carrier wave separated by the beam splitter 12 is input through the 1 port of the first circulator C1, output through the 2 port of the first circulator C1, and transmitted to the polarization controller PC of the signal transmitting end 20 through the transmission link 30, enters the second circulator C2 through the polarization controller PC, is input through the 2 port of the second circulator C2, output through the 3 port to the third polarization beam splitter PBS3, and is separated into the X polarized light and the Y polarized light by the third polarization beam splitter PBS3, and the X polarized light output by the third polarization beam splitter PBS3The polarized light is modulated by a first IQ modulator IQM1, reflected by a first Faraday rotating mirror FRM1, enters a polarization beam combiner PBC, Y polarized light output by a third polarization beam splitter PBS3 is modulated by a second IQ modulator IQM2, reflected by a second Faraday rotating mirror FRM2, enters the polarization beam combiner PBC, returns to a 1 port of the second circulator C2 by a polarization maintaining fiber after the polarization beam combiner PBC combines, is output by a 2 port of the second circulator C2, is transmitted to a 2 port of the first circulator C1 by a polarization controller PC and a transmission link 30, is output by a 3 port of the first circulator C1, is split by a second polarization beam splitter PBS2, and the X polarized light split by the first polarization beam splitter 1 and the second polarization beam splitter PBS2 is respectively injected into the first photoelectric detector PD1 for photoelectric conversion to obtain an output photocurrent I X And input to a first KK conversion unit KK1, and output to the carrier recovery and data decision unit through the first KK conversion unit KK1, wherein the Y polarized light split by the first polarizing beam splitter PBS1 and the second polarizing beam splitter PBS2 is respectively injected into the second photodetector PD2 for photoelectric conversion, so as to obtain an output photocurrent I Y And input to a second KK conversion unit KK2, and output to the carrier recovery and data decision unit 14 via the second KK conversion unit KK2, where the carrier recovery and data decision unit 14 performs data decision after carrier recovery.
The scheme adopts a dual-polarization modulation format, and polarization aliasing is inevitably introduced in the transmission process due to the random birefringence effect, and the invention proposes a link scheme shown in figure 1 to realize the self-compensation of the polarization aliasing. As shown in fig. 1, the laser is located at the receiving end, the transmitted signal carrier is transmitted to the transmitting end after passing through the transmission link, is modulated by the dual-polarization IQ modulator, is reflected by the faraday rotation mirror, and returns to the original transmission link through the polarization maintaining fiber. In a stable fiber optic link, the polarization rotation rate induced by random birefringence is typically between 10-200 krad/s, whereas in a data center internal communication link (500 m-10 km), the time required for a round trip light to travel the same point of the fiber is at mostIn such a short time, it is possible toThe Jones matrix of the fiber optic link is considered unchanged, under conditions described in Yamashita S, hotate K, ito M.polar organization properties of a reflective fiber amplifier employing a circulator and a Faraday rotator mirror [ J]Lightwave Technology Journal of,1996,14 (3): 385-390 have shown that the polarization of light emitted by a point is perpendicular to the polarization of light after returning to the point via a faraday rotator mirror. The design of the link thus achieves self-compensation of polarization aliasing with a small loss of power to the signal carrier. In addition, in short-range communications, nonlinear effects and polarization mode dispersion can be ignored.
As shown in fig. 1, the beam splitter 12 has an adjustable beam splitting ratio, the beam splitting ratio ensures that the local oscillator light intensity is 7-9dB greater than the reflected signal light intensity, and the polarization controller PC is used for controlling the power of the two polarized light states to be equal.
As shown in fig. 1, after the local oscillation light separated by the beam splitter 12 is shifted by Δf by the frequency shifter 13, Δf=baud rate/2 is respectively loaded into X, Y polarized signals, and photocurrent I is obtained after detection by the first photodetector PD1 and the second photodetector PD2 X And photocurrent I Y The first KK converting unit KK1 obtains the amplitude and phase information of the X polarized signal, and the second KK converting unit KK2 obtains the amplitude and phase information of the Y polarized signal.
As shown in fig. 1, the Kramers-Kronig algorithm executed by the first KK converting unit KK1 and the second KK converting unit KK2 is as follows:
wherein phi is X (t)、φ Y (t) the phase of X, Y polarized light field obtained after loading local oscillation light, p.v. is the integral of the Cauchy principal value, deltaf is the frequency shift of the intrinsic light, E LO-X 、E LO-Y The amplitude of the polarization of the local oscillation X, Y is a fixed constant.
The invention provides a dual-polarization QAM modulation direct detection communication system and method based on K-K reception, which are a direct detection communication scheme of dual-polarization QAM modulation with low power consumption and low DSP complexity applied to internal communication of a data center, and reduce power consumption by using reasonable structural design and optical components to replace a complex DSP module. And combining with the Kramers-Kronig algorithm, realizing QAM signal transmission in a direct detection mode.
According to the K-K receiving-based dual-polarization QAM modulation direct detection communication system and method, on one hand, dual-polarization QAM signal transmission is realized in a direct detection mode, and compared with a traditional amplitude modulation-direct detection scheme, the frequency spectrum efficiency is greatly improved. On the other hand, by reasonable optical fiber link design, the complex DSP module in partial dry receiving, including dispersion compensation and polarization aliasing compensation, can be removed, so that the power consumption is greatly reduced, and the method is suitable for short-distance communication.
The foregoing is a further detailed description of the invention in connection with the preferred embodiments, and it is not intended that the invention be limited to the specific embodiments described. It will be apparent to those skilled in the art that several simple deductions or substitutions may be made without departing from the spirit of the invention, and these should be considered to be within the scope of the invention.
Claims (9)
1. A dual-polarization QAM modulation direct detection communication method based on K-K reception is characterized in that: the system comprises a signal receiving end and a signal transmitting end, wherein the signal receiving end is connected with the signal transmitting end through a transmission link, the signal receiving end comprises a laser, a beam splitter, a frequency shifter, a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a first photoelectric detector PD1, a second photoelectric detector PD2, a first KK conversion unit KK1, a second KK conversion unit KK2, a carrier recovery and data decision unit and a first circulator C1, the output end of the laser is connected with the input end of the beam splitter, the local oscillation light output end of the beam splitter is connected with the input end of the frequency shifter, the output end of the frequency shifter is connected with the input end of the first polarization beam splitter PBS1, the X-polarization light output end of the first polarization beam splitter PBS1 is connected with the input end of the first photoelectric detector PD1, the Y-polarization output end of the first polarization beam splitter 1 is connected with the second photoelectric detector PD2, the carrier recovery and data decision unit and the first circulator C1, the output end of the first polarization beam splitter is connected with the second polarization beam splitter C2, the second polarization beam splitter is connected with the second polarization splitter C2, the second polarization output end of the second polarization splitter is connected with the second polarization splitter C2, the second polarization splitter C2 is connected with the second polarization splitter C2, the second polarization detector C2 is connected with the second polarization detector C2, the QM2 is connected with the second polarization end of the second polarization detector C1, the QM is connected with the QM2, the QM2 is connected with the input end of the QM, the output end of the first IQ modulator IQM1 is connected with the input end of the first IQ modulator IQM1, the output end of the first IQ modulator IQM1 is connected with the input end of the first faraday rotator FRM1, the output end of the first faraday rotator FRM1 is connected with the input end of the polarization beam combiner PBC, the Y-polarized light output end of the third polarization beam splitter PBS3 is connected with the input end of the second IQ modulator IQM2, the output end of the second IQ modulator IQM2 is connected with the input end of the second IQ modulator IQM2, the output end of the second faraday rotator FRM2 is connected with the input end of the polarization beam combiner PBC, the 3 port of the first circulator C1 is connected with the input end of the second polarization beam combiner PBC 2, the Y-polarized light output end of the second polarization beam splitter PBS2 is connected with the input end of the second polarization beam splitter KK2, the first polarization beam splitter KK2 is connected with the input end of the second polarization beam splitter KK2, the second polarization beam splitter is connected with the input end of the first polarization beam splitter KK2, the second polarization beam splitter is connected with the input end of the second polarization beam splitter KK2, the first polarization beam splitter is connected with the input end of the second polarization beam splitter input end of the first polarization beam splitter output unit, and the first polarization beam splitter output end is connected with the first polarization beam splitter output light, and the output light beam splitter is output signal from the signal;
the following procedure was performed based on the system:
the light emitted by the laser is divided into signal carrier and local oscillation light by the beam splitter, and the local oscillation light divided by the beam splitter is split by the first polarization beam splitter PBS1 after being subjected to frequency shift by the frequency shifter; the signal carrier wave separated by the beam splitter is input through a 1 port of the first circulator C1, output through a 2 port of the first circulator C1, transmitted through a transmission link to a polarization controller PC of a signal transmitting end, enters the second circulator C2 through the polarization controller PC, is input through a 2 port of the second circulator C2, output through a 3 port to a third polarization beam splitter PBS3, is separated into X polarized light and Y polarized light by the third polarization beam splitter PBS3, the X polarized light output by the third polarization beam splitter PBS3 is modulated by a first IQ modulator IQM1, reflected by a first Faraday rotating mirror FRM1 and enters a polarization beam combiner PBC, Y polarized light output by the third polarization beam splitter PBS3 is modulated by a second IQ modulator IQM2, reflected by a second Faraday rotating mirror FRM2, enters a polarization beam combiner PBC, returns to the 1 port of the second circulator C2 by a polarization maintaining fiber after being combined by the polarization beam combiner PBC, is output by the 2 port of the second circulator C2, is transmitted to the 2 port of the first circulator C1 by the polarization controller PC and a transmission link, is output by the 3 port of the first circulator C1, is split by the second polarization beam splitter PBS2, and X polarized light split by the first polarization beam splitter PBS1 and the second polarization beam splitter PBS2 is respectively transmitted to the first photoelectric detector PD1 for photoelectric conversion to obtain output photocurrent I X And input to a first KK conversion unit KK1, and output to the carrier recovery and data decision unit through the first KK conversion unit KK1, wherein the Y polarized light split by the first polarizing beam splitter PBS1 and the second polarizing beam splitter PBS2 is respectively injected into the second photodetector PD2 for photoelectric conversion, so as to obtain an output photocurrent I Y And the data is input to a second KK conversion unit KK2, and is output to the carrier recovery and data judgment unit through the second KK conversion unit KK2, and the carrier recovery and data judgment unit performs data judgment after carrier recovery.
2. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 1, characterized in that: the transmission link is a single mode fiber.
3. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 1, characterized in that: the signal transmitting end further comprises a third circulator C3 and a fourth circulator C4, a 1 port of the third circulator C3 is connected with the output end of the second IQ modulator IQM2, a 2 port of the third circulator C3 is connected with the second Faraday rotator mirror FRM2, and a 3 port of the third circulator C3 is connected with the input end of the polarization beam combiner PBC; the 1 port of the fourth circulator C4 is connected with the output end of the first IQ modulator IQM1, the 2 port of the fourth circulator C4 is connected with the first faraday rotation mirror FRM2, and the 3 port of the fourth circulator C4 is connected with the input end of the polarization beam combiner PBC.
4. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 3, characterized in that: the third polarization beam splitter PBS3 and the first IQ modulator IQM1 are connected through polarization maintaining fibers, the first IQ modulator IQM1 and the 1 port 1 of the fourth circulator C4 are connected through polarization maintaining fibers, the 2 port of the fourth circulator C4 and the first Faraday rotator FRM2 are connected through polarization maintaining fibers, the 3 port of the fourth circulator C4 and the polarization beam combiner PBC are connected through polarization maintaining fibers, the third polarization beam splitter PBS3 and the second IQ modulator IQM2 are connected through polarization maintaining fibers, the second IQ modulator IQM2 and the 1 port 1 of the third circulator C3 are connected through polarization maintaining fibers, the 2 port of the third circulator C3 and the second Faraday rotator FRM2 are connected through polarization maintaining fibers, and the 3 port of the third circulator C3 and the polarization beam combiner PBC are connected through polarization maintaining fibers.
5. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 1, characterized in that: the beam splitter and the frequency shifter are connected through a polarization maintaining fiber, and the 3 port of the first circulator C1 and the second polarization beam splitter PBS2 are connected through the polarization maintaining fiber.
6. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 1, characterized in that: the K-K receiving-based dual-polarization QAM modulation direct detection communication system works in an O wave band, and the wavelength is 1260-1360nm.
7. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 1, characterized in that: the beam splitting proportion of the beam splitter is adjustable, the beam splitting proportion ensures that the local oscillator light intensity is 7-9dB greater than the reflected signal light intensity, and the polarization controller PC is used for controlling the equal power of the two polarized light states.
8. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 1, characterized in that: after the local oscillation light separated by the beam splitter is subjected to frequency shift by Δf through the frequency shifter, Δf=baud rate/2 is respectively loaded into X, Y polarized signals, and photocurrent I is obtained after detection by a first photoelectric detector PD1 and a second photoelectric detector PD2 X And photocurrent I Y The first KK converting unit KK1 obtains the amplitude and phase information of the X polarized signal, and the second KK converting unit KK2 obtains the amplitude and phase information of the Y polarized signal.
9. The K-K reception-based dual-polarization QAM modulation direct-detection communication method of claim 8, wherein: the Kramers-Kronig algorithm performed by the first KK switching unit KK1 and the second KK switching unit KK2 is as follows:
wherein phi is X (t)、φ Y (t) the phase of X, Y polarized light field obtained after loading local oscillation light, p.v. is the integral of the Cauchy principal value, deltaf is the frequency shift of the intrinsic light, E LO-X 、E LO-Y The amplitude of the polarization of the local oscillation X, Y is a fixed constant.
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