CN102761373A - High-speed high-capacity passive optical network system and method for realizing coherent reception - Google Patents

Abstract

The invention relates to the technical field of passive optical networks, and provides a high-speed and large-capacity passive optical network system and method for realizing coherent reception. The passive optical network system is composed of an optical line terminal (OLT), a trunk optical fiber, a passive optical coupler, a branch optical fiber and an optical network unit (ONU). The access network system supports signal transmission at a rate of 40Gb/s. The 40Gb/s signal sending unit adopts high-order multi-dimensional modulation to reduce the symbol rate of transmission. The corresponding ONU (OLT) receiving unit adopts coherent reception and electrical domain digital signal processing In order to improve the sensitivity of the receiver and the power budget of the system, a coherent receiving local oscillator laser source and a coherent mechanism of the transmitting laser source are proposed, which effectively reduces the system cost.

Description

A high-speed and large-capacity passive optical network system and method for realizing coherent reception

technical field

The invention relates to the technical field of passive optical networks, in particular to a high-speed and large-capacity passive optical network system and method for realizing coherent reception.

Background technique

With the rapid and continuous development of the Internet, high-bandwidth applications such as high-definition IPTV, digital movies, and telepresence systems continue to emerge, placing higher requirements on the bandwidth of future access networks. Considering the requirement of high-capacity, large-branch and low-cost access network, passive optical network PON (passive optical network) is the best choice for the access network. However, in the face of exponentially increasing bandwidth requirements, the current standardized 10Gbps TDM-PON system will not be sufficient to meet future bandwidth requirements. Therefore, the next-generation passive optical network PON will support 40Gbps or even higher rates.

The current 10Gbps PON system transmission unit adopts the on-off keying (OOK) modulation method, and the receiving unit adopts the direct detection method. If the rate of 10Gbps PON system is directly increased to 40Gbps or even higher, the transmission quality of the system will deteriorate sharply because the optical signal will be affected by both linear effect and nonlinear effect when transmitted in the optical fiber. Linear effects include dispersion, polarization mode dispersion, etc., and nonlinear effects include four-wave mixing (FWM), stimulated Raman scattering (SRS), etc. The chromatic dispersion and polarization mode dispersion in the linear effect will increase quadratically with the increase of the transmission rate, so that the transmission quality will deteriorate sharply. Therefore, if the current 10Gbps PON system rate is directly increased to 40Gbps or even higher, the power budget and dispersion budget of the passive optical network system will be far from meeting the demand due to the intensified impact of dispersion.

At the same time, with the massive increase of high-speed and high-quality data services, the backbone network has basically solved the 40Gbps transmission problem. In the access network, how to realize the modulation and demodulation of 40Gbps high-speed signal at low cost is a key technical problem in the development of 40Gbps next-generation optical access network.

Contents of the invention

(1) Technical problems to be solved

Aiming at the shortcomings of the prior art, the present invention provides a high-speed and large-capacity passive optical network system and method for coherent reception in order to solve the problem that it is difficult to realize the modulation and demodulation of high-speed signals in the optical access network in the prior art, The symbol rate of the system is effectively reduced, the utilization rate of the spectrum is improved, and the sensitivity of the receiver and the power budget of the system are improved.

(2) Technical solutions

For this reason, the present invention specifically adopts following technical scheme to carry out:

First, the present invention provides a high-speed and large-capacity passive optical network system that realizes coherent reception, and the system includes:

An optical line terminal OLT1, a trunk optical fiber 2, a passive optical coupler 3, a branch optical fiber 4, and at least one optical network unit ONU5, the optical line terminal OLT1 is connected to the passive optical coupler through the trunk optical fiber 2 3, the passive optical coupler 3 is connected to the at least one optical network unit ONU5 through the branch optical fiber 4; wherein,

The optical line terminal OLT1 consists of an OLT light source 101, a first polarization beam splitting module 102, at least one multi-ary optical modulator 103 and a polarization coupling module 104 to form an OLT sending unit;

The optical network unit ONU5 is composed of a second polarization beam splitting module 501, an ONU light source 502, at least one optical coherent mixer 503, at least one photoelectric conversion module 504, at least one signal sampling module ADC505 and a digital signal processor DSP506 to form an ONU receiving unit.

Preferably, when performing asymmetric coherent reception, the optical network unit ONU5 is composed of the ONU light source 502 and the electroabsorption EA modulator 507 to form an ONU transmission unit; the optical line terminal OLT1 is composed of a photodiode PD105 and a 3R module OLT receiving unit.

Preferably, when performing symmetrical coherent reception, the ONU sending unit of the optical network unit ONU5 has the same structure as the OLT sending unit, and the OLT receiving unit of the optical line terminal OLT1 has the same structure as the ONU receiving unit , and the ONU sending unit and the ONU receiving unit share the ONU light source 502, and the OLT receiving unit and the OLT sending unit share the OLT light source 101.

Further, the present invention also provides a method for realizing coherent reception in a high-speed and large-capacity passive optical network system, wherein the method includes the steps of:

The laser light generated by the OLT light source 101 is divided into two beams with orthogonal polarization states in the V direction and the H direction by the first polarization beam splitting module 102;

Using two parallel multi-ary optical modulators 103 to generate a single-channel V polarization signal and a single-channel H polarization signal through multi-ary encoding;

Using the polarization coupling module 104 to complete polarization multiplexing to generate a multi-ary modulation signal for polarization multiplexing;

The one-way polarization multiplexed multi-ary modulation signal is transmitted downlink through the trunk fiber 2 through the passive optical coupler 3 and then through the branch fiber 4 to at least one optical network unit ONU5;

The signal received by the ONU is first divided into two optical signals of orthogonal polarization states in the X direction and the Y direction through the second polarization beam splitting module 501;

The ONU light source 502 generates the required local oscillator optical signal for coherent reception;

Using the optical coherent mixer 503 to complete the coherent reception of the two polarization signals in the X and Y directions and the local oscillator signal;

The signal output by the optical coherent mixer 503 undergoes photoelectric conversion through the photoelectric conversion module 504, and performs sampling and quantization processing through the signal sampling module ADC505 to complete the analog-to-digital conversion;

Next, the discrete digital sequence after sampling and quantization is sent to the digital signal processor DSP506 for receiving processing.

Preferably, performing receiving processing operations in the digital signal processor DSP506 includes:

Clock synchronization of digital signals through clock recovery;

Eliminate the influence of dispersion through electrical domain equalization and perform polarization demultiplexing;

Eliminate the phase offset caused by the frequency offset through frequency offset estimation, and eliminate the phase difference between the received signal and the local oscillator optical signal through phase estimation;

After the above several processing links, the original sending signal is restored by the judgment unit.

Preferably, if asymmetric coherent reception is performed, during uplink data transmission, the ONU sending unit loads the uplink data signal onto the electro-absorption EA modulator 507, performs on-off keying OOK modulation and then transmits it; the OLT receiving unit adopts a direct detection way to demodulate the received signal.

Preferably, if symmetric coherent reception is performed, the reverse process of downlink transmission is used to transmit data from the ONU to the OLT during uplink data transmission.

Preferably, the specific manner of multi-ary encoding is M-PSK manner or M-QAM manner.

Preferably, the specific steps of clock synchronization through clock recovery are: using an interpolation filter to adjust the symbol sampling time of the receiver, so that the adjusted receiver sampling clock is synchronized with the transmitted symbol clock; the polarization demultiplexing adopts a butterfly Structured equalization filter implementation.

Preferably, the specific method of multi-ary encoding is M-PSK or M-QAM, and the multi-ary encoding in downlink transmission and uplink transmission may be performed in the same or different manner.

(3) Beneficial effects

The high-speed and large-capacity passive optical network system and method for coherent reception provided by the present invention effectively reduce the symbol rate of the system and improve the spectrum utilization rate by adopting high-order multi-ary (multi-dimensional) modulation. At the same time, by adopting coherent reception and Electronic domain digital signal processing improves the sensitivity of the receiver and the power budget of the system, and effectively reduces the cost of the system by using the mechanism of the same source of the local oscillator light source and the transmitting light source.

Description of drawings

Fig. 1 is the general coherent receiving passive optical network (downlink 40Gbps, uplink 10Gbps) system structural block diagram that the present invention proposes;

Fig. 2 is the digital signal processor (DSP) process flow block diagram of general coherent receiving passive optical network system that the present invention proposes;

Fig. 3 adopts the passive optical network (downlink 40Gbps, uplink 10Gbps) system structural block diagram of PM-DQPSK modulation that the present invention proposes;

Fig. 4 adopts the passive optical network (downlink 40Gbps, uplink 10Gbps) system structural block diagram of PM-8PSK modulation that the present invention proposes;

Fig. 5 is a structural block diagram of the uplink and downlink symmetric PON (downlink 40Gbps, uplink 40Gbps) system proposed by the present invention using PM-DQPSK modulation.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The high-speed and large-capacity passive optical network general system (downlink 40Gbps, uplink 10Gbps) that the present invention proposes is as shown in Figure 1, and this system is by optical line terminal (OLT) 1, trunk optical fiber 2, passive optical coupler 3. The branch optical fiber 4 and the optical network unit (ONU) 5 etc., wherein the transmission unit of the OLT is composed of a light source 101 , a polarization beam splitting module 102 , a multi-ary optical modulator 103 , and a polarization coupling module 104 . The receiving unit of the ONU is composed of a polarization module 501, a light source 502, an optical coherent mixer 503, a photoelectric conversion module 504, a signal sampling module (ADC) 505, a digital signal processor (DSP) 506, and the like.

The working principle of the system is as follows: when the OLT sends a 40Gb/s downlink signal to the ONU, in the OLT sending unit, the laser light generated by the laser is divided into two paths with orthogonal polarization states (denoted as V direction and H direction) through a polarization beam splitter (PBS). )Beam. The transmitter sends a signal with a rate of 20Gbps through multi-ary encoding (DQPSK, 8PSK, 16QAM), and then uses two parallel MZ modulators to generate a single channel V polarization signal of M-PSK/M-QAM. Similarly, another channel of H-polarized signal is modulated and generated by the same method. Then, the polarization multiplexing is completed by using a polarization beam splitter (PBC), and a polarization multiplexed multi-ary modulation signal is generated.

When a polarization multiplexed 40Gb/s downlink signal arrives at the ONU receiving unit, the received signal is first divided into two optical signals of orthogonal polarization states (denoted as X direction and Y direction, different in the V direction and the H direction). Then use the 90° mixer and balanced receiver to complete the coherent reception of the two polarization signals and the local oscillator signal. The local oscillator laser at the ONU end outputs two channels of light through the optical splitter, one is used as the local oscillator optical signal required for coherent reception, and the other is used as the emission light source of the ONU sending unit. The mechanism of the same source of the local oscillator light and the emitted light effectively controls cost of the system. The mixed frequency output signal is photoelectrically converted by a balanced photodetector, and is sampled and quantized by an ADC to complete the analog-to-digital conversion. Next, the discrete digital sequence after sampling and quantization is sent to a digital signal processor (DSP) for processing, and the DSP processing flow is shown in Figure 2.

In the DSP processor, the digital signal is clock-synchronized through clock recovery first. Since the sampling clock of the ADC is independent of the symbol clock at the transmitter, it is necessary to use an interpolation filter to adjust the symbol sampling time of the receiver so that the adjusted receiver The sampling clock is synchronized with the transmitted symbol clock, which means that the sampling rate of the ADC is completely consistent with the symbol rate. Then eliminate the influence of dispersion through electrical domain equalization and perform polarization demultiplexing. Due to the non-ideality of the channel, the signal must be affected by the channel during transmission and cause damage. This influence mainly comes from dispersion. To deal with this impairment, equalization techniques must be used. At the same time, because the system adopts the modulation method of polarization multiplexing, each polarization signal will have a certain degree of deflection during transmission, and cause interference to another polarization signal. In order to offset this interference, a butterfly-shaped equalization filter is used Implement polarization demultiplexing. Then, the phase offset caused by the frequency offset is removed through the frequency offset estimation, and the phase estimation eliminates the phase difference between the received signal and the local oscillator source. After the above several processing links, the original sending signal is restored by the judgment unit.

When the ONU sends an uplink signal to the OLT, the uplink rate is 10Gbps. The uplink signal at the ONU end is modulated by on-off keying (OOK), and the OLT end uses direct detection to receive it. The emission light source of the ONU sending unit is the same source as the local oscillator light of the receiving unit, and the transmitter generates a 10Gbps OOK modulation signal through the electro-absorption EA modulator 507 . At the receiving end of the OLT, the received signal is directly detected by the photodiode 105, and then the signal is demodulated through pre-amplification, limiting, clock recovery and judgment regeneration, wherein the pre-amplification, limiting, clock recovery and regeneration are referred to as 3R, completed by the 3R module 106 .

The coherent receiving high-speed and large-capacity passive optical network system proposed by the present invention is applicable to various PON structures and multiple encoding methods, and can be implemented in PON systems with asymmetric uplink and downlink rates, and can also It is implemented in a PON system with symmetrical uplink and downlink rates, and the signal encoding method is not limited, both M-PSK and M-QAM are acceptable. The present invention will be further described in the following sub-cases.

Example 1

Fig. 3 shows a specific embodiment of the present invention-a structural block diagram of an asymmetric 40Gb/s coherent receiving PM-DQPSK system. Asymmetry means that the downlink rate is 40Gb/s, the uplink rate is 10Gb/s, the modulation method used for the downlink signal is PM-DQPSK, and the traditional OOK modulation method is used for the uplink signal. During downlink data transmission, the OLT sending unit has two channels of 20Gb/s signals, which are respectively used as the original signals of the two polarization states. Each signal is first converted into two channels of 10Gb/s signals through serial-to-parallel conversion, and then through differential precoding, and then two parallel MZ modulators are used to generate a single polarization signal of optical DQPSK. In order to realize the optical DQPSK modulation, the parallel modulation mode is adopted, and the MZ modulator shall adopt the push-pull working mode to avoid chirp (Chirp). Then use a polarization beam splitter (PBC) to multiplex the two polarization signals to form a 10Gbaud PM-DQPSK signal for transmission. In the ONU receiving unit, the received signal is first divided into two optical signals of orthogonal polarization states (referred to as the X direction and the Y direction, which are different from the V direction and the H direction) through the PBS, and the optical coherence solution is performed on the two polarized lights respectively. Tune. Then use the 90° mixer and balanced receiver to complete the coherent reception of the polarized optical signal and the local oscillator optical signal, and the local oscillator optical signal is obtained by the laser at the ONU end through the optical splitter. The signal output by the frequency mixing is photoelectrically converted by a balanced photodetector, and two in-phase and quadrature electrical signals can be obtained. Then the two-way electrical signal is sent to the ADC for sampling and quantization processing, and the analog-to-digital conversion is completed to obtain two discrete digital sequences. Then, the discrete digital sequence after sampling and quantization is sent to the DSP device for processing. In the DSP processor, the digital signal is synchronized through clock recovery, the influence of dispersion is eliminated through electrical domain equalization and polarization demultiplexing is performed, and the frequency difference between the local oscillator light source and the transmitted optical carrier and the influence of phase noise are eliminated through carrier phase estimation. . After the above several processing links, the original sending signal is restored by the judgment unit. During uplink data transmission, the ONU sending unit loads the 10Gb/s signal to the electro-absorption (EA) modulator, performs on-off keying (OOK) modulation and then transmits it. The OLT receiving unit demodulates the received signal by means of direct detection.

Example 2

Fig. 4 shows another specific embodiment of the present invention-a structural block diagram of an asymmetric 40Gb/s coherent receiving PM-8PSK system. This embodiment is the same as the previous embodiment to illustrate that the present invention is generally applicable to various modulation patterns. In this embodiment, we assume that the signal bit rate of downlink data transmission is 42Gb/s. The two channels of 21Gb/s signals in the OLT sending unit are respectively used as the original signals of the two polarization states. Each signal is first encoded by 8PSK to generate an 8PSK complex signal with a symbol rate of 7Gbaud, and then the 8PSK complex signal is converted into two real signals and loaded to two parallel MZ modulators to generate a single polarization signal of optical 8PSK. In order to realize the optical 8PSK modulation, the parallel modulation mode is adopted, and the MZ modulator shall adopt the push-pull working mode to avoid chirp. Then, the polarization beam combiner (PBC) is used to multiplex the two polarization signals to form one signal for transmission. In the ONU receiving unit, the process of demodulating signals is the same as the coherent receiving PM-DQPSK system in Embodiment 1, and the transmission process of uplink data is the same as the coherent receiving PM-DQPSK system in Embodiment 1. I won't go into details here.

Example 3

Fig. 5 has shown another specific embodiment of the present invention-the structural block diagram of symmetrical 40Gb/s coherent reception high-order multi-dimensional modulation system, the rate of uplink and downlink in this embodiment is all 40Gb/s, and the transmitting unit of ONU and OLT all adopts PM-DQPSK is used to modulate the signal to achieve the purpose of reducing the symbol rate. Due to the high-order multi-dimensional modulation, the ONU and OLT receiving units adopt coherent reception to demodulate signals. Among them, the laser source of the sending unit of the OLT and the ONU is the same source as the local oscillator laser source of the receiving unit, thereby effectively controlling the cost of the system. During downlink data transmission, the OLT sending unit has two channels of 20Gb/s signals, which are respectively used as the original signals of the two polarization states. Each 20Gb/s signal is converted into two 10Gb/s signals through serial-to-parallel conversion, and then differentially precoded, and then two parallel MZ modulators are used to generate a 10Gbaud optical DQPSK single-channel polarization signal. After the two polarization signals are generated, the polarization beam splitter (PBC) is used to multiplex the two polarization signals to form one polarization multiplexed PM-DQPSK signal, which is sent to the optical fiber for transmission. In the ONU receiving unit, the received signal is first divided into two optical signals of orthogonal polarization states (marked as X direction and Y direction) by the PBS. Then use the 90° mixer and balanced receiver to complete the coherent reception of the two polarization signals and the local oscillator signal, and the local oscillator signal is obtained by the laser at the ONU end through the optical splitter. The mixed output signal is photoelectrically converted by a balanced photodetector to obtain two in-phase and quadrature electrical signals, and the two electrical signals are sent to the ADC for sampling and quantization processing to complete the analog-to-digital conversion. Then, the discrete digital sequence after sampling and quantization is sent to the DSP device for processing. In the DSP processor, the digital signal is synchronized through clock recovery, the influence of dispersion is eliminated through electrical domain equalization and polarization demultiplexing is performed, and the frequency difference between the local oscillator light source and the transmitted optical carrier and the influence of phase noise are eliminated through carrier phase estimation. . After the above several processing links, the original sending signal is restored by the judgment unit. The uplink data transmission is the inverse process of the downlink transmission, which will not be repeated here.

From the above, it can be seen that the high-speed and large-capacity passive optical network system and method of the present invention realize coherent reception. By adopting high-order multi-dimensional modulation, the symbol rate of the system is effectively reduced, and the spectrum utilization rate is improved. At the same time, by adopting coherent The receiving and electronic domain digital signal processing improves the sensitivity of the receiver and the power budget of the system, and effectively reduces the cost of the system by using the mechanism of the same source of the local oscillator light source and the transmitting light source.

The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Those of ordinary skill in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all Equivalent technical solutions also belong to the category of the present invention, and the protection scope of the present invention should be defined by the claims.

Claims (10)

1. A high-speed high-capacity passive optical network system for implementing coherent reception, the system comprising:

the optical line terminal OLT (1) is connected with the passive optical coupler (3) through the trunk optical fiber (2), and the passive optical coupler (3) is connected with the at least one optical network unit ONU (5) through the branch optical fiber (4); wherein,

the optical line terminal OLT (1) comprises an OLT light source (101), a first polarization beam splitting module (102), at least one multilevel optical modulator (103) and a polarization coupling module (104) to form an OLT sending unit;

the optical network unit ONU (5) comprises a second polarization beam splitting module (501), an ONU light source (502), at least one optical coherent mixer (503), at least one photoelectric conversion module (504), at least one signal sampling module ADC (505) and a digital signal processor DSP (506) to form an ONU receiving unit.

2. The system according to claim 1, characterized in that the optical network unit ONU (5) constitutes an ONU transmission unit by the ONU optical source (502) and an electro-absorption EA modulator (507) when performing asymmetric coherent reception; the optical line terminal OLT (1) comprises an OLT receiving unit consisting of a photodiode PD (105) and a 3R module (106).

3. The system according to claim 1, wherein when performing symmetric coherent reception, the ONU transmission unit and the OLT transmission unit of the optical network unit ONU (5) have the same configuration, the OLT reception unit and the ONU reception unit of the optical line terminal OLT (1) have the same configuration, and the ONU transmission unit and the ONU reception unit share the ONU light source (502), and the OLT reception unit and the OLT transmission unit share the OLT light source (101).

4. A method for realizing coherent reception in a high-speed high-capacity passive optical network system is characterized by comprising the following steps:

laser generated by an OLT light source (101) is divided into two paths of orthogonal polarization state light beams in a V direction and an H direction through a first polarization beam splitting module (102);

generating a single-path V polarization signal and a single-path H polarization signal by using two parallel multi-system optical modulators (103) through multi-system coding;

polarization multiplexing is completed by utilizing a polarization coupling module (104), and a path of polarization multiplexing multilevel modulation signal is generated;

the path of polarization multiplexed multilevel modulation signal is transmitted to at least one optical network unit ONU (5) through a trunk optical fiber (2), a passive optical coupler (3) and a branch optical fiber (4) in a downlink manner;

the signal received by the ONU is firstly divided into two paths of orthogonal polarization state optical signals in the X direction and the Y direction through a second polarization beam splitting module (501);

an ONU light source (502) generates a required local oscillation optical signal for coherent reception;

coherent reception of the X, Y-direction two-path polarization signals and the local oscillator signal is completed by an optical coherent mixer (503);

the signal output by the optical coherent mixer (503) is subjected to photoelectric conversion through a photoelectric conversion module (504), and is subjected to sampling and quantization processing through a signal sampling module ADC (505) to complete analog-to-digital conversion;

the sampled and quantized discrete digital sequence is then fed into a digital signal processor DSP (506) for reception processing.

5. The method of claim 4, wherein performing receive processing operations in the Digital Signal Processor (DSP)506 comprises:

the digital signal carries out clock synchronization through clock recovery;

eliminating the influence of dispersion through electric domain equalization and carrying out polarization demultiplexing;

removing phase offset caused by frequency offset through frequency offset estimation, and eliminating phase difference between a received signal and a local oscillator optical signal through phase estimation;

after the above processing links, the original transmission signal is recovered by the decision of the decision device.

6. The method of claim 4, wherein if the asymmetric coherent reception is performed, when the uplink data is transmitted, the ONU sending unit loads the uplink data signal to the electro-absorption EA modulator (507), performs on-off keying OOK modulation, and then transmits the uplink data signal; the OLT receiving unit demodulates the received signal by adopting a direct detection mode.

7. The method of claim 4, wherein if symmetric coherent reception is performed, the data is transmitted from the ONU to the OLT using a reverse process of downstream transmission during upstream data transmission.

8. The method of claim 4, wherein the multilevel coding is M-PSK or M-QAM.

9. The method of claim 5, wherein the clock synchronization by clock recovery comprises the specific steps of: utilizing an interpolation filter to adjust the symbol sampling time of the receiver, so that the adjusted sampling clock of the receiver is synchronous with the transmitting symbol clock; the polarization demultiplexing is realized by adopting an equalizing filter with a butterfly structure.

10. The method of claim 7, wherein the multilevel coding is specifically in an M-PSK mode or an M-QAM mode, and the multilevel coding in the downlink transmission and the multilevel coding in the uplink transmission may be performed in the same or different modes.

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