CN109981182B - Four-phase reflection type coherent optical communication system - Google Patents

Four-phase reflection type coherent optical communication system Download PDF

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CN109981182B
CN109981182B CN201910166988.1A CN201910166988A CN109981182B CN 109981182 B CN109981182 B CN 109981182B CN 201910166988 A CN201910166988 A CN 201910166988A CN 109981182 B CN109981182 B CN 109981182B
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light
modulator
reflective
phase
light wave
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CN109981182A (en
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陈树强
闫淼
金诗文
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University of Electronic Science and Technology of China
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/614—Coherent receivers comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/615—Arrangements affecting the optical part of the receiver
    • H04B10/6151—Arrangements affecting the optical part of the receiver comprising a polarization controller at the receiver's input stage
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/63—Homodyne, i.e. coherent receivers where the local oscillator is locked in frequency and phase to the carrier signal

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Communication System (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

A four-phase reflective coherent optical communication system belongs to the technical field of optical communication systems. The optical communication system comprises a laser, a reflective optical modulator, a polarization controller, a 3dB coupler, a polarization beam splitter, homodyne coherent reception, a digital signal processing module and an adder. The reflection type coherent light communication system provided by the invention adopts the self-coherent demodulation technology, the signal light and the local oscillator light come from the same light source, the closed loop feedback in the traditional coherent light communication system is not needed to realize the wavelength tracking of the local oscillator laser, and the receiving part is relatively simple.

Description

Four-phase reflection type coherent optical communication system
Technical Field
The invention belongs to the technical field of optical communication systems, and particularly relates to a four-phase reflective coherent optical communication system.
Background
Coherent optical fiber communication systems based on homodyne reception are already the mainstream in the current optical fiber communication backbone network. Because an optical phase-locked loop (OPLL) which makes a system sufficiently stable is difficult to realize, an existing coherent optical communication system generally adopts optical and electrical combined asynchronous reception, a transmitted electrical signal is input into an optical modulator at a transmitting end, a modulated optical signal passes through a gain amplifier, and the optical signal and local oscillator light enter an optical coherent receiver at a receiving end together to complete coherent detection processing. And carrying out judgment and decoding by corresponding digital signal processing technology to complete the modulation and demodulation of the signal.
Disclosure of Invention
The invention aims to provide a four-phase reflective coherent optical communication system aiming at the defects in the background art so as to solve the technical problem of unstable optical phase-locked loop in a coherent optical communication system for homodyne reception.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a four-phase reflective coherent optical communication system comprises a laser, a reflective optical modulator, a polarization controller, a 3dB coupler, a polarization beam splitter, a homodyne coherent receiving module, a digital signal processing module (DSP) and an adder, wherein the laser is used for transmitting a laser beam to the 3dB coupler;
the light wave generated by the laser enters the 3dB coupler through the port 1, is divided into two light waves with equal power through the 3dB coupler, wherein one light wave is reflected back to the 3dB coupler through the reflection end of the port 4, the other light wave is output through the port 3 and is transmitted along the optical fiber link to enter the polarization controller, after passing through the polarization controller, the light wave is converted into a single polarization state from any polarization state, the light wave in the single polarization state enters the reflective light modulator, and the modulation of the light wave in the single polarization state is completed through an electric signal loaded to the reflective light modulator by a signal source; the modulated light wave is input into the 3dB coupler through the port 3 after passing through the polarization controller and the optical fiber, and is coupled with the light wave reflected by the reflection end in the 3dB coupler, the coupled light signal is output through the port 2 and enters the polarization beam splitter, two paths of mutually orthogonal light signals are generated by the polarization beam splitter and are respectively input into two homodyne coherent receivers for demodulation, and the demodulated signals are processed by a digital signal processing module (DSP) and then are subjected to an adder to recover the original signals without distortion.
Further, the structure of the reflective optical modulator is as shown in fig. 2, and includes an MZM modulator formed by two reflective Lithium Niobate (LN) phase modulators, a 3dB Y-branch Waveguide (Waveguide), an 3/8 pi phase retarder (phase responder), and a faraday rotator (R) closely attached to one end of the MZM modulator, the other end of the MZM modulator is connected to the Y-branch Waveguide, the 3/8 pi phase retarder is located on one branch of the Y-branch Waveguide and is connected in series with one of the lithium niobate modulators, and the faraday rotator includes a faraday rotator and a reflecting mirror.
When the reflective optical phase modulator shown in fig. 2 is used, the modulation principle is as follows: assuming that the incident direction of the light wave is a positive direction and the light wave enters from the input port in an o-light form, the light wave firstly passes through the Y-shaped branch waveguide structure, is divided into two light waves with the same power and respectively enters the upper and lower parallel waveguides, wherein one light wave passes through the 3/8 pi phase delayer, the o-light modulation efficiency is about one third of that of the e-light, so that the phase change amount is about 1/8 pi at the moment, and then enters the reflective lithium niobate modulator for positive modulation; after the modulated light wave passes through the Faraday rotator, the polarization state direction of the modulated light wave is changed by 45 degrees, then the light wave reaches the reflector, the light wave generated after reflection of the reflector passes through the Faraday rotator again, the polarization state of the light wave is rotated by 45 degrees, at the moment, compared with the polarization state of incident light, the polarization state of reflected light is changed by 90 degrees, and the reflected light is changed into e light. Because the propagation directions of the reversely transmitted light wave and the modulating signal microwave are consistent, the light wave and the modulating signal microwave are reversely modulated, and the polarization state of the forward transmission light signal and the polarization state of the reverse transmission light signal are vertical to each other in the whole light waveguide, so that the formation of light standing waves is avoided. At this time, the e light passes through the 3/8 pi phase retarder, so that the phase of the light is additionally changed by (1/8 pi +3/8 pi ═ 1/2 pi) in addition to the normal modulation; and the other modulator only realizes normal signal modulation, and the reflected waves of the two modulators are combined and then return to the optical waveguide of the original optical path. In the whole process, the reflection type lithium niobate modulator works in a push-pull mode, the delay of the phase delayer can be adjusted through an external voltage, and the 1/2 pi phase shift is guaranteed. Thus, the phase modulator implements four-phase differential modulation signals of 0, 1/2 pi, 3/2 pi of four-phase modulation (such as DQPSK).
Further, the structure of the reflective optical modulator is as shown in fig. 3, and includes two MZM modulators connected in parallel, a 3dB Y-type branch waveguide, an 3/8 pi phase retarder (phase retarder), and a faraday rotator mirror (R), where the faraday rotator mirror is tightly attached to one end of the two MZM modulators, the other ends of the two MZM modulators are respectively connected to the Y-type branch waveguide, the 3/8 pi phase retarder is connected in series with one of the MZM modulators, and the faraday rotator mirror includes a faraday rotator and a reflecting mirror.
When a reflective optical modulator as shown in fig. 3 is used, the modulation of the signal is the same as that of the reflective four-phase modulator based on a phase modulator as described above, only the intensity modulator is used as the phase, i.e. both intensity modulators are biased at 0, and the amplitude of the drive signal fluctuation is 2VπWherein V isπThe voltage is half-wavelength voltage, so that two BPSK signals can be generated for interference, and four-order phase modulation is realized.
Preferably, a low-resistance filter is added between the two homodyne coherent receiving modules and the DSP to ensure the normal operation of the optical communication system.
Compared with the prior art, the invention has the beneficial effects that:
the four-phase reflection type coherent optical communication system provided by the invention has the advantages that the input and the output are on the same optical fiber, so that the confidentiality of the system is stronger, the application in aspects of confidential communication and the like is facilitated, and meanwhile, the transmitter and the local oscillator in the system are the same laser, so that the phase synchronization of carrier signal light and the local oscillator light is conveniently realized.
Drawings
Fig. 1 is a schematic structural diagram of a four-phase reflective coherent optical communication system according to the present invention;
fig. 2 is a schematic structural diagram of a reflective four-phase optical modulator based on phase modulation in a four-phase reflective coherent optical communication system provided in the present invention;
fig. 3 is a schematic structural diagram of a reflective four-phase optical modulator based on an intensity modulator in a four-phase reflective coherent optical communication system provided in the present invention;
FIG. 4 is a BER comparison plot of an embodiment reflective coherent optical system versus a prior art forward system.
Detailed Description
The technical scheme of the invention is detailed below by combining the accompanying drawings and the embodiment.
As shown in fig. 1, the four-phase reflective coherent optical communication system provided by the present invention includes a laser, a reflective optical modulator, a polarization controller, a 3dB coupler, a polarization beam splitter, a homodyne coherent receiver, a digital signal processing module (DSP), and an adder;
the light wave generated by the laser enters the 3dB coupler through the port 1, and is divided into two light waves with equal power through the 3dB coupler, wherein one light wave is reflected back to the 3dB coupler through the reflection end of the port 4, the other light wave is output through the port 3 and is transmitted along an optical fiber link to enter the polarization controller, and because the lithium niobate modulator is sensitive to polarization, the light must first pass through the Polarization Controller (PC) to realize single-polarization light before entering the modulator, after passing through the polarization controller, the light wave is converted into a single-polarization state from any polarization state, the light wave in the single-polarization state enters the reflective light modulator, and the modulation of the light wave in the single-polarization state is completed through an electric signal loaded to the reflective light modulator by a signal source; the modulated light wave is input into the 3dB coupler through the port 3 after passing through the polarization controller and the optical fiber, and is coupled with the light wave reflected by the reflection end in the 3dB coupler, so that the use of a subsequent 90-degree mixer is reduced, the coupled light signal enters the polarization beam splitter after being output through the port 2, two paths of light signals which are different in polarization state and orthogonal to each other are generated through the polarization beam splitter, the two paths of light signals are respectively input into two homodyne coherent receivers to be demodulated, and the demodulated signal is processed through a digital signal processing module (DSP) and then can be recovered to the original signal without distortion through an adder.
Examples
In the present embodiment, the first and second electrodes are,
a laser: 1550nm DFB Laser Diode, PM Output, up to 30 mW;
a polarization beam splitter: the central wavelength is 1550nm, the bearing power is 300mW, and the light ray type PM is obtained;
a polarization controller: the central wavelength is 1550nm, and the input optical power is-20 dBm to 20 dBm;
faraday rotator mirror: the central wavelength is 1550nm, the rotation angle is 45 degrees, and the optical fiber B is a bare optical fiber with the diameter of 250 mu m;
homodyne coherent reception: the speed is 100Gbps (the high-order phase modulation can generally realize the speed of more than 100G), and the differential amplification gain is 2500V/W.
Tests show that the four-phase reflective coherent optical communication system obtained in the embodiment has a BER curve equivalent to that of the existing forward system, and the maximum transmission distance is about 2800km, which shows that the performance of the two systems is roughly the same; the BER versus curve for the reflective coherent optical system versus the forward system of the example is shown in fig. 4. Because the DSP compensation algorithm of the reflection type system is two less than that of the existing forward system, and the DSP calculation pressure is small, the requirement on the complexity of hardware is relatively low in practical application, and the application in a high-speed real-time system is more facilitated.
In the four-phase reflective coherent optical communication system provided by the invention, no matter a phase modulator or an intensity modulator is adopted, the reflective modulation mode involves twice modulation of a back-and-forth signal, so that a light wave signal of a certain wave band is inevitably weaker and becomes system noise, and therefore, a low-resistance filter can be respectively added between a signal receiving end of the optical communication system, namely two homodyne coherent receiving modules and a DSP (digital signal processor) to ensure the normal work of the system.
The working principle is as follows: according to the calculation formula of the bandwidth of the LN (lithium niobate) traveling wave modulator, the modulation bandwidths of the traveling waves modulated in the positive and negative directions are respectively as follows:
B+＝πL(nm-ne)/1.4c
B-＝πL(nm+ne)/1.4c
where c is the speed of light, L is the length of the modulator modulation segment, nmAnd neThe equivalent refractive index of the microwave signal and the refractive index of the e-light, respectively. Since the incident light is modulated in the reverse direction (first pass through the modulator), the modulation efficiency is low, and the transmission direction of the light is opposite to that of the electrical signal, so that the resulting severe speed mismatch makes the modulation bandwidth narrow, according to the above formula, if B is+>10GHz, L40 cm, no 2.2, then B-About 1GHz, where the signal is weaker. Although the signal is weak, the signal still becomes the noise of the system in the wave band, and at the moment, the output of the signal above 1G can be realized by respectively adding a low-resistance filter between the two homodyne coherent receiving modules and the DSP, so that the normal work of the system is ensured. For higher rates, the bandwidth of the backward modulation will not increase significantly, and the proportion of the bandwidth occupied in the system will decrease significantly.
The invention provides a reflection-type coherent optical communication system, which adopts an autocorrelation demodulation technology, wherein signal light and local oscillator light come from the same light source, the closed-loop feedback in the traditional coherent optical communication system is not needed to realize the wavelength tracking of a local oscillator laser, and a receiving part is relatively simple. In addition, in the modulation mode, intensity or phase modulation can be realized by using an external modulator, and most commonly used modulators based on lithium niobate crystals. Taking four-order phase modulation as an example (a common DQPSK system), one is to connect two MZM modulators in parallel and use an additional 90 ° phase shifter, and the other is to connect a phase modulator and an MZ modulator in series, so as to realize the high-order phase modulation of the system.

Claims (2)

1. A four-phase reflective coherent optical communication system comprises a laser, a reflective optical modulator, a polarization controller, a 3dB coupler, a polarization beam splitter, a homodyne coherent receiving module, a digital signal processing module and an adder, wherein the laser is used for transmitting a laser beam to a light source; the reflective optical modulator comprises an MZM modulator formed by two reflective lithium niobate phase modulators, a 3dB Y-shaped branch waveguide, an 3/8 pi phase delay device and a Faraday rotation mirror, wherein the Faraday rotation mirror is tightly attached to one end of the MZM modulator, the other end of the MZM modulator is connected with the Y-shaped branch waveguide, and the 3/8 pi phase delay device is positioned on one branch of the Y-shaped branch waveguide and is connected with one lithium niobate modulator in series;
the light wave generated by the laser enters the 3dB coupler through the port 1, is divided into two light waves with equal power through the 3dB coupler, wherein one light wave is reflected back to the 3dB coupler through the reflection end of the port 4, the other light wave is output through the port 3 and is transmitted along the optical fiber link to enter the polarization controller, after passing through the polarization controller, the light wave is converted into a single polarization state from any polarization state, the light wave in the single polarization state enters the reflective light modulator, and the modulation of the light wave in the single polarization state is completed through an electric signal loaded to the reflective light modulator by a signal source; the modulated light wave is input into the 3dB coupler through the port 3 after passing through the polarization controller and the optical fiber, and is coupled with the light wave reflected by the reflection end in the 3dB coupler, the coupled light signal is output through the port 2 and enters the polarization beam splitter, two paths of mutually orthogonal light signals are generated by the polarization beam splitter and are respectively input into two homodyne coherent receivers for demodulation, and the demodulated signals are processed by the digital signal processing module and then pass through the adder to recover the original signals without distortion.
2. The four-phase reflective coherent optical communication system according to claim 1, wherein the reflective optical modulator comprises two MZM modulators connected in parallel, a 3dB Y-branch waveguide, an 3/8 pi phase retarder, and a faraday rotator mirror attached to one end of the two MZM modulators, the other end of the two MZM modulators being connected to the Y-branch waveguide, respectively, and a 3/8 pi phase retarder connected in series to one of the MZM modulators.
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CN110336610B (en) * 2019-07-10 2022-04-19 电子科技大学 High-safety reflective coherent optical communication system transmitting end and system
CN110266386B (en) * 2019-07-10 2021-08-10 电子科技大学 Reflective coherent optical communication system combining unidirectional optical signal amplification
CN111901044B (en) * 2019-11-28 2021-11-02 阳光学院 Single-beam coherent optical communication device
CN114690436B (en) * 2022-05-31 2022-08-02 华中科技大学 Light polarization control device and polarization diversity self-coherent system
WO2024113118A1 (en) * 2022-11-28 2024-06-06 华为技术有限公司 Optical modulator, transmission apparatus, communication system, and modulation method

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