Passive optical network upgrading system based on multi-order polarization shift keying modulation

Abstract

The utility model discloses a passive optical network upgrading system based on multi-order polarization shift keying modulation. The system comprises a central office, a first separator, a plurality of existing optical network units and a plurality of new optical network units. The central office comprises an existing optical line terminal, a new optical line terminal and a wavelength division multiplexer; the existing optical line terminal comprises a first light source and an amplitude shift keying modulation module which are connected; the new optical line terminal comprises a second light source and a multi-order polarization shift keying modulation module which are connected; the amplitude shift keying modulation module and the multi-order polarization shift keying modulation module are respectively connected with the wavelength division multiplexer, the wavelength division multiplexer is connected with the first separator, and the first separator is also respectively connected with a plurality of existing optical network units and a plurality of new optical network units; the new optical network unit comprises an optical filter, a polarization beam splitting unit and a new optical signal receiving unit which are connected in sequence, and the optical filter is also connected with the first separator. Signal crosstalk can be reduced.

Description

Technical Field

[0001] The embodiments of this specification belong to the field of communication technology, and specifically relate to a passive optical network upgrade system based on multi-order polarization shift keying modulation. Background Art

[0002] Passive optical networks have become the ultimate solution for fiber-to-the-home and have been deployed all over the world. To meet the rapidly growing bandwidth demand, the next generation of passive optical networks is expected to provide 50G passive optical networks and 100G passive optical networks.

[0003] The first challenge facing the next generation of passive optical networks is to provide 50Gb / s or 100Gb / s signals using currently commercially available 10G-class transmitters and receivers. The next generation of passive optical networks will not use many wavelengths. For example, 50G Ethernet passive optical networks only apply two wavelengths, so the bit rate of each wavelength should be increased to 25Gb / s or higher. One way to increase the bit rate per wavelength is coherent detection, including orthogonal frequency division multiplexing, orthogonal phase shift keying, and orthogonal amplitude modulation. However, the high cost of coherent detection limits its application in passive optical networks, which are cost-sensitive. Direct detection is the preferred passive optical network application, and high-order modulations that can be directly detected include dual binary modulation, differential quadrature phase shift keying, and pulse amplitude modulation. Four-level pulse amplitude modulation is considered suitable for transmission within 20 kilometers (the transmission range of passive optical networks). However, it is not adopted in 50G passive optical networks because it will cause a large loss of sensitivity due to multiple levels.

[0004] The second challenge is to make the passive optical network develop smoothly, which is very sensitive to cost. The main cost of the passive optical network comes from the construction of the optical distribution network, which can reach 80%. In order to increase revenue and utilization of the passive optical network, the deployed optical distribution network should be reused instead of building a new one. The evolution of the passive optical network should be smooth, and the next generation of passive optical network links are added to the existing fiber infrastructure, sharing the deployed optical distribution network with the existing passive optical network.

[0005] In a smooth PON evolution, new PON links coexist with existing PON links until all existing PON users are upgraded. The signals of the two PONs are usually carried by different wavelengths, so they can be separated by optical filters. Since the new PON link is newly installed, it can be equipped with optical filters or wavelength demultiplexers to filter out the existing PON signals. However, the existing PON links have already been deployed and the new PON was not known at the time of installation, so the existing PON links do not have optical filters or wavelength demultiplexers. Adding filters (or wavelength demultiplexers) will bring about transformation costs. In cases where additional filtering is required, it is better to do it at the optical line terminal, where access may be easier, rather than at the optical network unit, because they are numerous and widely distributed. Since the existing optical network units lack optical filters to protect them from the downstream signals of the new PON, they will also receive the downstream signals of the new PON, resulting in crosstalk.

[0006] Therefore, there is an urgent need for a method that can eliminate or reduce the crosstalk of downstream signals of a new passive optical network to downstream signals of an existing passive optical network during a smooth upgrade of the passive optical network. Utility Model Content

[0007] The embodiments of this specification provide a passive optical network upgrade system based on multi-order polarization shift keying modulation, which can eliminate or reduce the crosstalk of the new passive optical network downstream signal to the existing passive optical network downstream signal during the smooth upgrade process of the passive optical network.

[0008] The embodiment of this specification provides a passive optical network upgrade system based on multi-order polarization shift keying modulation, including: a central office, a first splitter, and a plurality of existing optical network units and a plurality of new optical network units;

[0009] The central office includes an existing optical line terminal, a new optical line terminal, and a wavelength division multiplexer;

[0010] The existing optical line terminal comprises a first light source and an amplitude shift keying modulation module connected to each other;

[0011] The new optical line terminal includes a connected second light source and a multi-order polarization shift keying modulation module;

[0012] The amplitude shift keying modulation module and the multi-order polarization shift keying modulation module are respectively connected to the wavelength division multiplexer, the wavelength division multiplexer is connected to the first separator, and the first separator is also respectively connected to multiple existing optical network units and multiple new optical network units;

[0013] The new optical network unit comprises an optical filter, a polarization beam splitting unit and a new optical signal receiving unit which are connected in sequence, and the optical filter is also connected to the first separator.

[0014] As a preferred solution, the multi-order polarization shift keying modulation module is a fourth-order polarization shift keying modulation module.

[0015] As a preferred solution, the multi-order polarization shift keying modulation module includes a first polarization controller, a first phase modulator, a second polarization controller, and a second phase modulator connected in sequence, the first polarization controller is connected to the second light source, and the second phase modulator is connected to the first separator;

[0016] The multi-order polarization shift keying modulation module also includes a modulation signal generating unit for mapping the new passive optical network signal into a first modulation signal and a second modulation signal with different polarization states for controlling the first phase modulator and the second phase modulator.

[0017] As a preferred solution, the modulation signal generating unit includes a modulation signal generating subunit for mapping the new passive optical network signal into a first modulation signal and a second modulation signal with different polarization states, a first output subunit for outputting the first modulation signal to a first phase modulator, an XOR device for performing XOR processing on the first modulation signal and the second modulation signal to obtain an XOR modulation signal for controlling the second phase modulator, and a second signal output unit for outputting the XOR modulation signal to the second phase modulator;

[0018] The modulation signal generating subunit is connected to the first output subunit and the XOR device respectively. The first output subunit is also connected to the first phase modulator. The XOR device, the second signal output unit and the second phase modulator are connected in sequence.

[0019] As a preferred solution, the new optical network unit further includes a second separator;

[0020] The polarization beam splitting unit includes a first polarization beam splitter and a second polarization beam splitter;

[0021] The new optical signal receiving unit includes a first new optical signal receiver and a second new optical signal receiver;

[0022] One end of the second separator is connected to the optical filter, and the other end of the second separator is connected to the first polarization beam splitter and the second polarization beam splitter respectively;

[0023] The first polarization beam splitter is connected to the first new optical signal receiver, and the second polarization beam splitter is connected to the second new optical signal receiver.

[0024] As a preferred solution, the polarization beam splitting unit only includes a polarization beam splitter, and the new optical signal receiving unit only includes a new optical signal receiver; the optical filter, polarization beam splitter, and new optical signal receiver are connected in sequence; and the new optical network unit is connected to the first separator through a time division multiplexer.

[0025] As a preferred solution, the new optical signal receiver includes a first photodetector, a second photodetector and an electrical subtractor, one end of the first photodetector and the second photodetector are connected to the polarization beam splitter, and the other end of the first photodetector and the second photodetector are connected to the electrical subtractor.

[0026] As a preferred solution, the existing optical network unit includes a connected third photodetector and a low-pass filter.

[0027] As a preferred solution, the amplitude shift keying modulation module adopts a Mach-Zehnder modulator.

[0028] The beneficial effects brought by the technical solutions provided by some embodiments of this specification include at least:

[0029] A new passive optical network upgrade system is proposed, which adopts multi-order polarization shift keying modulation for smooth upgrade of passive optical networks. The new passive optical network downlink signal adopts multi-order polarization shift keying modulation, which eliminates the crosstalk to the coexisting existing passive optical network signals. The two optical signals are transmitted through the existing optical distribution network and then divided into the existing optical network unit and the new optical network unit. In each existing optical network unit, the optical signal is directly detected by a photodetector without any additional equipment or complicated operations. In each new optical network unit, the multi-order polarization shift keying signal is demodulated into several binary amplitude shift keying signals by a polarization beam splitter. Through this new passive optical network upgrade system, smooth upgrade and high-speed transmission are achieved at the same time, and the signal crosstalk problem is avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of this specification, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this specification. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0031] Figure 1 It is a schematic diagram of the overall structure of a passive optical network upgrade system based on multi-order polarization shift keying modulation provided in an embodiment of this specification.

[0032] Figure 2 Schematic diagram of polarization modulation of phase modulator.

[0033] Figure 3Schematic diagram of the process of 4th-order polarization shift keying modulation for new passive optical network signals.

[0034] Figure 4 Schematic diagram of binary signal reception of 4-order polarization shift keying signal.

[0035] Figure 5 Schematic diagram of binary signal reception of 6-order polarization shift keying signal.

[0036] Figure 6 A schematic diagram of the simulation structure of a passive optical network upgrade system based on multi-order polarization shift keying modulation.

[0037] Figure 7 Eye diagrams of a single existing PON signal and an existing PON signal coexisting with a new PON signal.

[0038] Figure 8 Eye diagram of the demodulated polarization shift keying new passive optical network signal for different polarization detuning.

[0039] Fig. 9 This is a bit error rate curve of the existing passive optical network signal.

[0040] Fig.10 This is the bit error rate curve of the new passive optical network signal with polarization shift keying.

[0041] Fig.11 A schematic diagram of the structure of another passive optical network upgrade system based on multi-order polarization shift keying modulation provided in an embodiment of this specification.

[0042] Fig.12 A schematic diagram of the structure of another passive optical network upgrade system based on multi-order polarization shift keying modulation provided in an embodiment of this specification.

[0043] Fig.13 Experimental eye diagrams of alternative solutions for reducing the cost of optical network units.

[0044] Fig.14 Bit error rate curves of alternative solutions to reduce the cost of optical network units.

[0045] In the figure: LD, light source; CWDM, coarse wavelength division multiplexer; OF, optical filter; PBS, polarization beam splitter; PC, polarization controller; PM, phase modulator; ADC, analog-to-digital converter; MZM, Mach-Zehnder modulator; WDM, wavelength division multiplexer; VOA, variable optical attenuator; LPF, low-pass filter. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of this specification will be described clearly and completely below in conjunction with the drawings in the embodiments of this specification.

[0047] The terms "first", "second", "third", etc. in the description and claims of this specification and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally includes steps or units that are not listed, or optionally includes other steps or units inherent to these processes, methods, products or devices.

[0048] The following description provides examples and does not limit the scope, applicability or examples set forth in the claims. Changes may be made to the functions and arrangements of the elements described without departing from the scope of the present specification. Various processes or components may be appropriately omitted, substituted or added to each example. For example, the described method may be performed in an order different from the order described, and various steps may be added, omitted or combined. In addition, features described with respect to some examples may be combined in other examples.

[0049] See also Figure 1 , Figure 1 A schematic diagram of the structure of a passive optical network upgrade system based on multi-order polarization shift keying modulation provided in an embodiment of this specification.

[0050] like Figure 1 As shown, the passive optical network upgrade system includes:

[0051] Central Office, First Splitter (Note: The first splitter is Figure 1 A splitter directly connected to the optical fiber) and multiple existing optical network units and multiple new optical network units;

[0052] The central office includes an existing optical line terminal, a new optical line terminal, a wavelength division multiplexer ( Figure 1 The coarse wavelength division multiplexer is used in the

[0053] The existing optical line terminal includes a first light source connected to the optical line terminal, an amplitude shift keying modulation module (Note: Figure 1 Not shown, see Figure 3 );

[0054] The new optical line terminal includes a connected second light source and a multi-order polarization shift keying modulation module;

[0055] The amplitude shift keying modulation module and the multi-order polarization shift keying modulation module are respectively connected to the wavelength division multiplexer, the wavelength division multiplexer is connected to the first separator, and the first separator is also respectively connected to multiple existing optical network units and multiple new optical network units;

[0056] The new optical network unit comprises an optical filter, a polarization beam splitting unit and a new optical signal receiving unit which are connected in sequence, and the optical filter is also connected to the first separator.

[0057] This embodiment proposes a new modulation method, namely multi-order polarization shift keying modulation, for the smooth upgrade of passive optical networks. The new passive optical network downlink signal adopts multi-order polarization shift keying modulation, which eliminates the crosstalk to the coexisting existing passive optical network signals. The two optical signals are transmitted through the existing optical distribution network and then divided into the existing optical network unit and the new optical network unit. In each existing optical network unit, the optical signal is directly detected by a photodetector without any additional equipment or complicated operations. In each new optical network unit, the multi-order polarization shift keying signal is demodulated by a polarization beam splitter into several binary amplitude shift keying signals. The multi-order polarization shift keying modulation method proposed in this embodiment achieves smooth upgrade and high-speed transmission at the same time, and avoids the problem of signal crosstalk.

[0058] In the embodiments of the present specification, a smooth upgrade of a passive optical network based on polarization shift keying modulation is provided. A schematic diagram of a smooth upgrade of a passive optical network based on polarization shift keying modulation is shown in FIG. Figure 1 As shown, the new passive optical network downstream signal adopts multi-order polarization shift keying modulation. In the central office, a new optical line terminal module is added to map the new passive optical network downstream data into multiple signals, which are then modulated on the optical carrier by polarization shift keying. The two downstream signals have different wavelengths and are combined by a wavelength division multiplexer. The two signals are transmitted to the remote node through optical fiber, and then they are split to the optical network unit. The existing optical network unit receives the two downstream signals. Since the new passive optical network downstream signal using polarization shift keying modulation technology has a constant amplitude, in theory, it will not cause crosstalk to the existing passive optical network downstream signal modulated by amplitude shift keying. The new optical network unit has a filter to filter out the existing passive optical network signal. They only receive the new passive optical network signal. The received new passive optical network signal is modulated by M-order polarization shift keying, divided into several branches, and then demodulated by a polarization beam splitter. The number of branches is M / 2. After passing through the splitter and polarization beam splitter, a multi-order polarization shift keying signal is converted into multiple amplitude shift keying signals. The demodulated amplitude shift keying signal can be received by a traditional binary receiver. Compared with line coding, the proposed multi-order polarization shift keying has no effective bit rate reduction. Its spectral efficiency is higher than that of traditional binary differential phase shift keying or polarization shift keying, and it can achieve 25Gb / s transmission on current 12.5G-class transmitters and receivers.

[0059] The principle of polarization modulation of phase modulator is as follows Figure 2As shown in , multi-order polarization shift keying modulation uses M equally spaced polarization states to carry M-level signals. The polarization state can be modulated by a phase modulator. When the polarization state of the optical carrier is controlled to be a certain degree (θ) away from the modulation axis, the optical carrier will be partially phase modulated. The part parallel to the modulation axis will be phase modulated, and the part perpendicular to the modulation axis will not be modulated. Figure 2 When the signal is 0, there is no modulation (no change in polarization position). When the modulation signal is 1, the optical carrier portion parallel to the modulation axis will be in the opposite phase, so the combined polarization will be in the mirror direction. When there are multiple optical carriers with different polarization states, when the modulation signal is 1, they will all be modulated to the mirror direction.

[0060] Take 4-order polarization shift keying as an example, Figure 3 As shown in Figure 1, four polarization states (named A, B, C, and D) are used to carry 4-level signals. The two original binary data are mapped into 4-level signals. Figure 3 A possible way to generate a 4th-order polarization shift keying signal is shown. Figure 3 As shown in the mapping relationship, the polarization state of the optical carrier is modulated by two phase modulators. The polarization state of the optical carrier is offset by -45° (θ=-45°) from the modulation axis of the first phase modulator through the polarization controller, and the modulation signal is data 1. After modulation, when data 1 is 0 or 1, the polarization state is A or C. Then, the optical carrier is sent to the second phase modulator with a polarization degree of -22.5° by another polarization controller. The results show that the polarization direction of the injected optical carrier deviates from the modulation axis of the second phase modulator by -22.5° (polarization state A) or 67.5° (polarization state C). The modulation signal is data 1 XOR data 2. When data 1 XOR data 2 is 1, the polarization state A / C will change to polarization state B / D. At the receiving end, the 4th-order polarization shift keying signal is split into two polarization beam splitters with axial directions of 112.5° / 22.5° and 67.5° / -22.5° respectively. The upper polarization beam splitter converts polarization states A / B and C / D into different amplitudes, and the lower polarization beam splitter converts polarization states A / D and B / C into different amplitudes. Then, two binary signals corresponding to data 1 and data 2 can be detected.

[0061] exist Figure 3 In , balanced detection is used to enhance the signal quality (i.e., signal detection and reception are performed by two photodetectors and an electrical subtractor). If balanced detection is not applied, it is sufficient to detect only one output of the polarization beam splitter. Figure 3In the example, the modulation signal of the second phase modulator is data 1 XOR data 2, so the detection signal from the next polarization beam splitter is directly data 2. If data 2 is directly used as the modulation signal of the second phase modulator (the mapping relationship changes), the signal detected from the next polarization beam splitter will become data 1 XOR data 2, so decoding is required. Since the optical network unit is more sensitive to cost than the optical line terminal, it is better to encode or decode on the optical line terminal rather than the optical network unit. Therefore, the XOR operation is performed at the sending end to ensure that the receiving end can directly obtain the original data.

[0062] Multi-level reception will increase the complexity of operation and reduce the receiving sensitivity, so the proposed multi-level polarization shift keying signal adopts traditional binary reception. When M / 2 is an even number or an odd number, an M-level polarization shift keying signal can be converted into multiple binary amplitude shift keying signals by a polarization beam splitter, and the axis of the polarization beam splitter bisects two polarization states or aligns with one polarization state. Figure 4 , Figure 5 A fourth-order polarization shift keying signal and a sixth-order polarization shift keying signal are taken as examples for description.

[0063] exist Figure 4 In the figure, the polarization beam splitter axis is located between A and B (C and D), so the angle between A / B and the axis is 22.5°, and the angle between C / D and the axis is 67.5°. The projections of A / B and C / D on the coordinate axes are cos22.5° and cos67.5° respectively. The optical power is proportional to the square of the polarization amplitude, so the four polarization angles are converted into two light amplitudes cos222.5° (~0.85) and cos267.5° (~0.15). The decision level can be set to 0.5 to distinguish between A / B and C / D. The horizontal distance is about 0.7. Another polarization beam splitter with its axis between B and C can distinguish between B / C and A / D.

[0064] exist Figure 5 In the figure, the polarization beam splitter axis is aligned with B, so the projections of B, A / C, D / F, and E on the axis are cos0°, cos30°, cos60°, and cos90° respectively. The six polarization directions are converted into four optical amplitudes of cos20° (1), cos230° (0.75), cos260° (0.25), and cos290° (0). The sixth-order polarization shift keying signal will be converted into a fourth-order amplitude shift keying signal, but it can be received by a traditional binary receiver with a decision level set to 0.5 to distinguish A / B / C and D / E / F. The level distance is 0.5. The other two polarization beam splitters with axes C and D can distinguish B / C / D from A / E / F, C / D / E from A / B / F.

[0065] The proposed smooth evolution based multi-order polarization shift keying modulation is demonstrated and tested by simulation software, e.g. Figure 6 As shown in the figure. In the central office, two laser diodes with 1340nm (wavelength of the next generation 50G passive optical network) and 1490nm (wavelength of the current Gigabit passive optical network) are used as the light sources of the new passive optical network link and the existing passive optical network link respectively. Two different pseudo-random binary sequences are used as the new passive optical network signal and the existing passive optical network signal respectively.

[0066] The new PON signal uses 4th-order polarization shift keying modulation, so it is first mapped into two modulation signals, which will be used to control the phase modulator. The bit rate of the modulation signal is set to 12.5Gb / s, and the bit rates of the existing PON signals are set to 1.25Gb / s, 2.5Gb / s, and 10Gb / s, respectively. The new PON signal is modulated by two phase modulators, each followed by a polarization controller. The polarization angles of the two polarization controllers are -45° and -22.5°, respectively. The modulation index of the phase modulator is set to 180°. The existing PON signal is amplitude shift keyed modulated onto the optical carrier by a Mach-Zehnder modulator. The modulation index is set to 14dB, which is a typical value for a Mach-Zehnder modulator. The optical powers of the two optical signals are adjusted to the same value, then combined by a wavelength division multiplexer) and sent into a 25 km standard single-mode optical fiber. A variable optical attenuator is used after the standard single-mode fiber for bit error rate measurement. After the optical signal is transmitted, it is divided into two branches, which are used for receiving the new passive optical network signal and the existing passive optical network signal. A power meter is set after the distributor to test the received power for bit error rate measurement. In the new passive optical network signal receiving branch, an optical filter is used to filter out the existing passive optical network signal. The polarization shift keying signal is then split into two polarization beam splitters for demodulation. The axis of the polarization beam splitter is set to the corresponding value. In order to test the influence of polarization misalignment, we also test the polarization shift keying signal when the polarization beam splitter axis deviates from the above standard value. The demodulated signal is detected by a photodetector. Balanced detection is applied. In the existing passive optical network signal receiving branch, the two optical signals are directly detected by a photodetector. In order to simulate the limited receiving bandwidth of a traditional receiver, the detected signal is low-pass filtered by an electrical filter. The cut-off bandwidth is set to 75% of the corresponding bit rate.

[0067] The test eye diagram results are as follows: Figure 7 shown. Figure 7 The eye diagrams of a single existing PON signal and an existing PON signal coexisting with a new PON signal at different bit rates are shown. Comparing the two rows, the existing PON signal suffers little crosstalk from the coexisting new PON signal.

[0068] Figure 8 The demodulated PSK PON signals with different polarization misalignments are shown. When the offset reaches 3°, the demodulated PSK signal attenuates to a certain extent.

[0069] To further show the performance of the signal, the bit error rate was measured, such as Fig. 9 As shown. Fig. 9 In the figure, the error rate curves of the existing PON signals of 1.25Gb / s, 2.5Gb / s and 10Gb / s are represented by triangles, squares and spheres respectively. When the existing PON signal is a single signal or coexists with the new PON signal, the error rate curves are solid or dashed lines respectively. The dashed line almost coincides with the solid line, indicating that there is almost no power loss when the signals coexist. The results are consistent with the eye Figure 1 Considering the widespread application of forward error correction coding in passive optical networks, the received power error rate is used as the receiving sensitivity. The receiving sensitivity of 1.25Gb / s, 2.5Gb / s and 10Gb / s traditional passive optical network signals is -33.6dBm, -31.88dBm and -28.5dBm respectively. Assuming that the transmitter output power in the optical line terminal is 3dBm, the power budget is 36.6dB, 34.88dB and 31.5dB respectively. Such a power budget meets the C+ specification of the gigabit passive optical network and the N2 specification of the next generation passive optical network.

[0070] exist Fig.10 In the figure, the bit error rate curves of the polarization shift keying new passive optical network signal are represented by triangles, squares, diamonds, stars and spheres, and the polarization deviations are 0°, 0.5°, 1°, 1.5° and 3° respectively. The received power is tested before the optical filter, so it will suffer the insertion loss of the optical filter, the 3dB splitting loss and the insertion loss of the polarization beam splitter before it is detected. Taking the bit error rate = as the forward error correction limit, when the offset is less than 1.5°, the receiving sensitivity is -22dBm. When the offset is less than 3°, the receiving sensitivity is -21.5dBm. Assuming that the transmitter output power in the optical line terminal is 3dBm, the power budget is 25dB (offset ≤ 1.5°) or 24.5dB (offset = 3°).

[0071] In the simulation, the received optical power is measured before the splitter of the new optical network unit, because the splitter should be considered as a part of demodulation and reception. The power budget should be calculated from the transmitter to the receiver, so the receiving sensitivity should be measured before the split. The 3dB frequency division loss should be considered as the internal loss of the receiver. Balanced detection brings a 3dB gain at the cost of adding a photodetector and an electrical subtractor. If the power is sufficient, balanced detection is not required.

[0072] An alternative approach can avoid the splitter and reduce the cost of each optical network unit, such as Fig.11 As shown. This method can be regarded as polarization shift keying multi-polarization multiplexing. The optical network units are divided into two groups. The optical line terminal can send two downstream signals (data 1 and data 2 in the figure) to two optical network units in different groups at the same time. The new optical network units in the same group share the channel bandwidth through time division multiplexing (Note: This can be achieved through a time division multiplexer). Each optical network unit uses only one polarization beam splitter to convert the 4th order polarization shift keying signal into an amplitude shift keying signal. The axes of the two sets of polarization beam splitters are 112.5° and 67.5° respectively. The required polarization beam splitters and receivers are reduced from two to one, and no division is required. The cost of each optical network unit is greatly reduced, and the power budget is improved by 3dB. The cost of this scheme is that the maximum bit rate of each optical network unit is limited to 12.5Gb / s, which is half of the total downstream bandwidth.

[0073] An important issue of polarization multiplexing technology is the instability of polarization state in fiber transmission. The polarization fluctuation caused by random birefringence of buried optical fiber in the network is generally only 2° to 10°. In passive optical networks, the transmission length is much shorter than that of backbone networks, so the polarization fluctuation is smaller. After testing the performance of the 4th order polarization shift keying signal when the polarization state is not aligned with the polarization beam splitter axis, the test results show that when the polarization misalignment reaches 3°, the power loss is only 0.5dB.

[0074] In the test, the bit error rate = is set as the forward error correction limit, which is a conservative value. The new generation of forward error correction can even increase the forward error correction limit to. For example, the laser diode polarization controller (17280,14592) recommended by the 50G passive optical network can reduce the bit error rate from to. Therefore, the application of the new generation of forward error correction will further improve the receiving sensitivity of the power budget. In the simulation, the symbol rate of the 4th order polarization shift keying signal is 12.5GB / s, so its effective bit rate is 25Gb / s. Therefore, it is able to send and receive 25Gb / s signals per wavelength using the current 12.5G-level transceiver. 50Gb / s downstream signal transmission can further realize the multiplexing of two wavelengths. The spectral efficiency reaches 2b / s / Hz, which is comparable to polarization multiplexing. The proposed 4th order polarization shift keying signal can double the spectral efficiency and realize the transmission of 25Gb / s wavelength on the existing transceiver while maintaining a constant amplitude to reduce the crosstalk of the existing passive optical network signal. If 6th-order polarization shift keying is used, the spectral efficiency will reach 2.5b / s / Hz, exceeding polarization multiplexing. A 10G transceiver can achieve 25Gb / s transmission per wavelength.

[0075] Compared with the evolution of passive optical networks based on line coding, the 4-POPK signal produces almost no crosstalk to the existing passive optical network signals, even if the bit rate of the existing passive optical network signals is as high as 10Gb / s. At the same time, the 4-POPK modulation does not reduce the effective bit rate of the new passive optical network signal, but doubles it.

[0076] Further experimental verification, such as Fig.12 As shown. Set up and Figure 6 Similarly, the existing PON signal is 2.5Gb / s and the new PON signal is 10Gb / s. The new PON signal and the existing PON signal are transmitted at 1550nm and 1530nm respectively. The wavelength division multiplexer is replaced by a coupler. The existing PON signal has no low pass filtering. No balanced detection is performed on the polarization shift keyed new PON signal. Both photodetectors are PIN.

[0077] The eye diagram tested in the experiment is as follows Fig.13 The polarization shift keying signal is a constant amplitude. When it is combined with the amplitude shift keying existing passive optical network signal, it acts like a direct current, which will increase the existing passive optical network signal but will not reduce it. The bit error rate results are shown in Fig.14 The receiving sensitivity (received power bit error rate) of a single existing passive optical network signal, coexisting existing passive optical network signals and new passive optical network signals are -23dBm, -22.8dBm and -16.8dBm respectively, and the power loss when the existing passive optical network signals coexist is 0.2dB.

[0078] Due to poor experimental equipment, the bit rate of the new passive optical network signal is only 10Gb / s, the transmission bit rate is only 20 kilometers, and the receiving sensitivity is much lower than the simulation result. In addition, since the wavelength of the 4th-order polarization shift keying signal is 1550nm, its dispersion is larger than that of the 4th-order polarization shift keying signal carrying 1340nm in the simulation. That is why the receiving sensitivity of the 4th-order polarization shift keying signal is only -16.8dBm. However, the power loss caused by coexistence is still low (0.2dB).

[0079] The experimental results also verify that the crosstalk from the new PON signal modulated by 4th-order polarization shift keying to the existing PON signal is very low. Smooth upgrade based on 4th-order polarization shift keying modulation is feasible. Simulation and experiment verify the effectiveness of the method. In the simulation, 1.25Gb / s, 2.5Gb / s and 10Gb / s existing PON signals were tested respectively, and the new PON signal with or without 4th-order polarization shift keying modulation was tested. The results show that the crosstalk of the existing PON signal is very low, and there is almost no power loss caused by coexistence. A 12.5GB / s 4th-order polarization shift keying signal was tested as a new PON signal with an effective bit rate of 25Gb / s. Assuming the forward error correction limit is bit error rate =, the receiving sensitivity is -22dBm. When the polarization misalignment is less than 3°, the power loss is less than 0.5dB, and the power budget meets the requirements. The experimental results show that the power loss of existing passive optical network signals is still as low as 0.5dB, which verifies the feasibility of evolving to the future 50G passive optical network (25Gb / s per wavelength) based on the proposed 4th-order polarization shift keying modulation.

[0080] To sum up, as a preference, in the present invention, although the spectral efficiency increases with the order M, as M increases, the bit error rate performance will deteriorate, and the mapping between data and M-level signals will become more complicated, greatly increasing the system cost. Therefore, the multi-order polarization shift keying modulation module is preferably adopted as a fourth-order polarization shift keying modulation module.

[0081] Reference Figure 1 , Figure 3 , Figure 6 As shown, the multi-order polarization shift keying modulation module includes a first polarization controller, a first phase modulator, a second polarization controller, and a second phase modulator connected in sequence, the first polarization controller is connected to the second light source, and the second phase modulator is connected to the first separator;

[0082] The multi-order polarization shift keying modulation module also includes a modulation signal generating unit for mapping the new passive optical network signal into a first modulation signal and a second modulation signal with different polarization states for controlling the first phase modulator and the second phase modulator.

[0083] The modulation signal generating unit includes a modulation signal generating subunit for mapping the new passive optical network signal into a first modulation signal and a second modulation signal with different polarization states, a first output subunit for outputting the first modulation signal to the first phase modulator, an XOR device for performing XOR processing on the first modulation signal and the second modulation signal to obtain an XOR modulation signal for controlling the second phase modulator, and a second signal output unit for outputting the XOR modulation signal to the second phase modulator;

[0084] The modulation signal generating subunit is connected to the first output subunit and the XOR device respectively. The first output subunit is also connected to the first phase modulator. The XOR device, the second signal output unit and the second phase modulator are connected in sequence.

[0085] See also Figure 6 As shown, in a passive optical network upgrade system based on multi-order polarization shift keying modulation provided in this specification, the new optical network unit also includes a second separator;

[0086] The polarization beam splitting unit includes a first polarization beam splitter and a second polarization beam splitter;

[0087] The new optical signal receiving unit includes a first new optical signal receiver and a second new optical signal receiver;

[0088] One end of the second separator is connected to the optical filter, and the other end of the second separator is connected to the first polarization beam splitter and the second polarization beam splitter respectively;

[0089] The first polarization beam splitter is connected to the first new optical signal receiver, and the second polarization beam splitter is connected to the second new optical signal receiver.

[0090] See also Fig.11 As shown, in another passive optical network upgrade system based on multi-order polarization shift keying modulation provided in this specification, the polarization beam splitting unit only includes a polarization beam splitter, and the new optical signal receiving unit only includes a new optical signal receiver; the optical filter, the polarization beam splitter, and the new optical signal receiver are connected in sequence; and the new optical network unit is connected to the first separator through a time division multiplexer (Note: the time division multiplexer is not shown in the figure, the time division multiplexing technology is a prior art, and the implementation method of this technology is not described in detail here).

[0091] See also Figure 3 , Figure 6 As shown, the above-mentioned balance detection implementation structure is as follows:

[0092] The new optical signal receiver includes a first photodetector, a second photodetector and an electrical subtractor, one end of the first photodetector and the second photodetector are connected to the polarization beam splitter, and the other end of the first photodetector and the second photodetector are connected to the electrical subtractor.

[0093] Furthermore, the existing optical network unit includes a connected third photodetector and a low-pass filter.

[0094] The embodiments of this specification also provide a passive optical network upgrade method based on multi-order polarization shift keying modulation, which is based on the passive optical network upgrade system based on multi-order polarization shift keying modulation described in any of the above embodiments.

[0095] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the passive optical network upgrade method embodiment, since it is basically similar to the passive optical network upgrade system embodiment, the description is relatively simple, and the relevant parts can be referred to the partial description of the passive optical network upgrade system embodiment.

[0096] The embodiments described above are merely preferred embodiments of this specification and are not intended to limit the scope of this specification. Without departing from the design spirit of this specification, various modifications and improvements made to the technical solutions of this specification by ordinary technicians in this field should fall within the scope of protection determined by the claims of this specification.

Claims

1. A passive optical network upgrade system based on multi-order polarization shift keying modulation, characterized in that: include: A central office, a first splitter, and a plurality of existing optical network units and a plurality of new optical network units; The central office includes an existing optical line terminal, a new optical line terminal, and a wavelength division multiplexer; The existing optical line terminal comprises a first light source and an amplitude shift keying modulation module connected to each other; The new optical line terminal includes a connected second light source and a multi-order polarization shift keying modulation module; The amplitude shift keying modulation module and the multi-order polarization shift keying modulation module are respectively connected to the wavelength division multiplexer, the wavelength division multiplexer is connected to the first separator, and the first separator is also respectively connected to multiple existing optical network units and multiple new optical network units; The new optical network unit comprises an optical filter, a polarization beam splitting unit and a new optical signal receiving unit which are connected in sequence, and the optical filter is also connected to the first separator.

2. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 1 is characterized in that: The multi-order polarization shift keying modulation module is a fourth-order polarization shift keying modulation module.

3. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 2 is characterized in that: The multi-order polarization shift keying modulation module comprises a first polarization controller, a first phase modulator, a second polarization controller, and a second phase modulator which are connected in sequence, wherein the first polarization controller is connected to the second light source, and the second phase modulator is connected to the first separator; The multi-order polarization shift keying modulation module also includes a modulation signal generating unit for mapping the new passive optical network signal into a first modulation signal and a second modulation signal with different polarization states for controlling the first phase modulator and the second phase modulator.

4. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 3 is characterized in that: The modulation signal generating unit includes a modulation signal generating subunit for mapping the new passive optical network signal into a first modulation signal and a second modulation signal with different polarization states, a first output subunit for outputting the first modulation signal to the first phase modulator, an XOR device for performing XOR processing on the first modulation signal and the second modulation signal to obtain an XOR modulation signal for controlling the second phase modulator, and a second signal output unit for outputting the XOR modulation signal to the second phase modulator; The modulation signal generating subunit is connected to the first output subunit and the XOR device respectively. The first output subunit is also connected to the first phase modulator. The XOR device, the second signal output unit and the second phase modulator are connected in sequence.

5. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 4 is characterized in that: The new optical network unit also includes a second separator; The polarization beam splitting unit includes a first polarization beam splitter and a second polarization beam splitter; The new optical signal receiving unit includes a first new optical signal receiver and a second new optical signal receiver; One end of the second separator is connected to the optical filter, and the other end of the second separator is connected to the first polarization beam splitter and the second polarization beam splitter respectively; The first polarization beam splitter is connected to the first new optical signal receiver, and the second polarization beam splitter is connected to the second new optical signal receiver.

6. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 4 is characterized in that: The polarization beam splitting unit only includes a polarization beam splitter, and the new optical signal receiving unit only includes a new optical signal receiver; the optical filter, polarization beam splitter, and new optical signal receiver are connected in sequence; and the new optical network unit is connected to the first separator through a time division multiplexer.

7. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 5 or 6, characterized in that: The new optical signal receiver includes a first photodetector, a second photodetector and an electrical subtractor, one end of the first photodetector and the second photodetector are connected to the polarization beam splitter, and the other end of the first photodetector and the second photodetector are connected to the electrical subtractor.

8. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 7 is characterized in that: The existing optical network unit includes a connected third photodetector and a low-pass filter.

9. The passive optical network upgrade system based on multi-order polarization shift keying modulation according to claim 1, characterized in that: The amplitude shift keying modulation module adopts a Mach-Zehnder modulator.

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