CN104767585A - A transceiver for an optical network unit of a TWDM-PON system - Google Patents

Abstract

The invention provides an optical network unit transceiver for a TWDM-PON (Time and Wavelength Division Multiplexed Passive Optical Network) system. The transceiver comprises an optical coupler, a comb filter coupled with the optical coupler, a first optical circulator coupled with the comb filter, an downlink receiver coupled with the first optical circulator, a second optical circulator coupled with the optical coupler and the first optical circulator respectively, and an optical source configured to modulate uplink data into uplink optical signals and output the uplink optical signals.

Description

A transceiver for an optical network unit of a TWDM-PON system

technical field

The present invention relates to a passive optical network (PON), and more particularly, to a transceiver for an optical network unit of a TWDM-PON system.

Background technique

Time and Wavelength Division Multiplexed Passive Optical Network (TWDM-PON) has recently been selected by the Full Service Access Network (FSAN) and the International Telecommunication Union Standardization Sector (ITU-T) Q2 group as the The main architecture of the Next Generation Passive Optical Network (NGPON2). Compared with other candidate solutions such as OFDM-PON and pure WDM-PON, TWDM-PON technology is more interesting due to its increased system capacity, flexible bandwidth allocation, higher efficiency. TWDM-PON is also compatible with existing optical distribution network (ODN) and can be easily migrated from XGPON.

In TWDM-PON, multiple wavelength pairs are stacked together to increase capacity. For example, 40Gb/s TWDM-PON includes four pairs of wavelengths. Each wavelength provides a downstream rate of 10Gb/s and an upstream rate of 2.5Gb/s. Therefore, the four wavelength pairs of the TWDM-PON system can provide the speed of downlink 40Gb/s and uplink 10Gb/s. In a single wavelength channel, TWDM-PON still uses XG-PON downlink multiplexing, uplink access technology, time granularity, etc.

technical problem:

In TWDM-PON, multiple technical problems need to be solved, which are briefly described below:

1) Take 40Gb/s TWDM-PON as an example. Since there are four wavelength pairs in the system, the transmitter of each optical network unit (ONU) needs to be tuned to one of the four upstream wavelengths, and the receiver of the ONU needs to be tuned to one of the four downstream wavelengths. Therefore, the adjustable ONU transmitter and receiver are key technologies in TWDM-PON. However, many conventional tunable ONU transmitters (eg, DFB, DBR lasers) and tunable receivers (eg, thin-film filters) are quite expensive. Furthermore, the ONU transmitter and receiver are two separate components. If both components need to be tuned, the ONU cost of TWDM-PON will increase greatly.

2) In TWDM-PON, due to the limitation of the output power of the transmitter and the sensitivity of the receiver, the power budget of the downlink and uplink is quite limited. This in turn will limit the maximum splitting ratio and thus the number of ONUs as well as the transmission distance. As required by NGPON2G.989.1, the TWDM-PON system should be able to support at least a splitting ratio of 1:64 and a transmission range of 40km. Therefore, it is necessary to increase the splitting ratio and transmission range in the future TWDM-PON system.

3) Another key problem in the TWDM-PON system is the rogue characteristic of the ONU wavelength. For example, even if an ONU correctly receives one of the four downstream wavelengths, the transmitter can still work on a wrong upstream wavelength channel, which affects all ONUs working in that wavelength channel. And, since the emission wavelength of the ONU is located in the wrong channel, at the optical line terminal (OLT) side, the upstream signal transmitted by the ONU will be output from the wrong arrayed waveguide grating (AWG) port, or when its wavelength is shifted from the AWG's It is even blocked by the AWG when passing the band channel.

In the prior art, for the tunable transmitter and receiver at the ONU side, the traditional solution uses an expensive DFB or DBR as the tunable transmitter, and a thin-film filter as the tunable receiver. These are two separate components, so the cost of the entire adjustable ONU is quite high.

In addition, in the prior art, in order to increase the power budget of the link and thus increase the splitting ratio and transmission range, an auxiliary optical amplifier is required at the OLT end or remote node to amplify the power of the downlink signal, thereby improving the splitting ratio accordingly /transmission range. However, this would introduce high nonlinear crosstalk or make the ODN an active system.

Contents of the invention

Therefore, the main purpose of the present invention is to provide a low-cost adjustable ONU transceiver.

According to a first aspect of the present invention, there is provided a transceiver for an optical network unit of a TWDM-PON system, the transceiver comprising: an optical coupler; a comb filter coupled to the optical coupler; A first optical circulator, which is coupled to the comb filter; a downlink receiver, which is coupled to the first optical circulator; a second optical circulator, which is respectively coupled to the optical coupler and the first Optical circulator coupling; and a light source configured to modulate uplink data into an uplink optical signal and output the uplink optical signal; wherein, when performing downlink optical signal transmission, the comb filter receives via the optical coupler The downlink optical signal and a target downlink optical signal are filtered out, and the target downlink optical signal is output to the downlink receiver via the first optical circulator; when the uplink optical signal is transmitted, the uplink optical signal The signal is transmitted to the comb filter through the second optical circulator and the first optical circulator, and the comb filter receives the downlink optical signal and filters out the target uplink optical signal to the optical coupler. an optical signal, the optical coupler splits the target uplink optical signal into a first uplink optical signal and a second uplink optical signal, wherein the first uplink optical signal is transmitted to an optical line terminal, and the second The uplink optical signal is fed back to the light source via the second optical circulator, so that the light source adjusts the uplink optical signal based on the second uplink optical signal.

According to a second aspect of the present invention, an optical network unit including the above-mentioned transceiver is provided.

Different from existing schemes with independent tunable ONU transmitter and receiver, in the present invention, the tunable receiver and transmitter share the same optical element (such as a comb filter) to realize a colorless optical network unit , so the cost of the adjustable ONU is greatly reduced.

By using comb filters, the architecture of the proposed tunable ONU transceiver is able to implement downstream wavelength selection and upstream wavelength generation simultaneously. The wavelength tuning of downlink and uplink optical signals can be flexibly realized by adjusting the transmission peak (spectral response) of the shared comb filter.

In the present invention, the upstream wavelength is automatically a function of the downstream wavelength. Once the downstream wavelength is selected, the corresponding upstream wavelength will also be automatically generated, thus simply solving the rogue characteristic of the ONU wavelength.

In addition, in the present invention, in order to realize downlink wavelength selection and uplink wavelength tuning, it is not necessary to scan transmission peaks over a large range to cover all wavelength channels. Alternatively, only a small amount of transmission peak shift is required for the tunable optical filter, which will enable fast wavelength tuning of the ONU transceiver.

Preferably, a conventional low-cost Fabry-Perot laser diode (FP-LD) or a reflective semiconductor amplifier (RSOA) is used as a light source to form a feedback loop to generate an uplink signal with tunable wavelength.

Preferably, bidirectional amplifiers are used in the invention, which are also shared by downlink and downlink to compensate for signal loss and increase link length. Therefore, the transmission range and the number of ONUs (or splitting ratios) can be increased accordingly.

In short, compared to existing solutions, the present invention uses a single tunable comb filter for simultaneous generation of upstream wavelengths and selection of downstream wavelengths. In addition, the present invention solves the rogue characteristic of the ONU wavelength, uses low-cost FP-LD or RSOA, and simultaneously improves the transmission range under the use of FP-LD and RSOA. Furthermore, in the present invention, rapid wavelength tuning can be performed over a wide range. Further, the present invention can be adapted to future high-capacity TWDM-PON systems.

Various aspects of the present invention will be clarified through the description of specific embodiments below.

Description of drawings

The present invention will be better understood and other objects, details, features and advantages of the present invention will become more apparent through the following description of specific embodiments of the present invention given with reference to the following drawings. In the attached picture:

Fig. 1 shows a schematic diagram of an adjustable ONU transceiver according to an embodiment of the present invention;

Fig. 2 shows the schematic diagram that the adjustable ONU transceiver according to an embodiment of the present invention carries out downlink wavelength selection;

FIG. 3 shows a schematic diagram of an adjustable ONU transceiver performing upstream wavelength generation according to an embodiment of the present invention;

Fig. 4 shows a schematic diagram of the operation of a comb filter with a larger free spectral range according to an embodiment of the present invention;

Figure 5 shows a schematic diagram of the operation of a comb filter with a smaller free spectral range according to another embodiment of the present invention; and

FIG. 6 shows a schematic diagram of wavelength selection by a comb filter with a small free spectral range according to another embodiment of the present invention.

In the figures, the same or similar reference numerals designate the same or corresponding parts or features throughout the different views.

Detailed ways

Fig. 1 shows a schematic diagram of an adjustable ONU transceiver according to an embodiment of the present invention. This adjustable ONU transceiver 1 includes an optical coupler 10, a bidirectional amplifier 11 (such as SOA), a comb filter 12, two optical circulators (the first optical circulator 13 and the second optical circulator 14), a light source 15 And downlink receiver 16. Preferably, FP-LD or RSOA can be used as the light source. In one embodiment of the invention, the optocouplers are arranged 80:20. It should be understood that in practice, the above ratios can be changed to other values (eg 90:10 etc.) depending on the parameters in the construction.

In this scheme, the tunable ONU transmitter and receiver are interrelated by sharing the same bidirectional amplifier 11 and comb filter 12, and thus greatly saves the cost of the ONU. The uplink wavelength is generated through a feedback loop, which includes an optical coupler 10 , a bidirectional amplifier 11 , a comb filter 12 , a first optical circulator 13 and a second optical circulator 14 and a light source 15 . During downlink transmission, the downlink optical signal is first amplified by the bidirectional amplifier 11 to increase the link transmission range and then selected and filtered out by the comb filter 12 to filter out the target downlink optical signal. The selected target downlink optical signal is then output to the downlink receiver 16 via the first optical circulator 13 . Here, downlink wavelength selection and uplink wavelength tuning can be realized simultaneously by tuning a single comb filter 12 .

The implementation method of the proposed tuning ONU transceiver will be described in detail below.

Fig. 2 shows a schematic diagram of downlink wavelength selection performed by an adjustable ONU transceiver according to an embodiment of the present invention. The transmission direction of the downlink optical signal is marked by a dotted line in FIG. 2 . As shown in FIG. 2 , when performing downlink optical signal transmission, downlink optical signals with, for example, four downlink wavelengths from an optical line terminal are first injected into 80% of the ports of the optical coupler 10, and then transmitted through the optical coupler 10 A bidirectional amplifier 11 is introduced. Thus, the power loss caused by optical splitting during transmission and at remote nodes can be compensated to a certain extent. Therefore, a longer transmission range and more ONUs can be supported. Of course, in some alternative embodiments, the bidirectional amplifier 11 can also be omitted. By tuning the transmission peak of the comb filter 12 to align it with the target downlink wavelength, the target downlink optical signal can be selected and input to the first port a of the first optical circulator 13 . Thereafter, the target downlink optical signal will be output from the second port b of the first optical circulator 13, and will be transmitted to the downlink receiver 16 for detection of the target downlink optical signal.

Fig. 3 shows a schematic diagram of generation of uplink wavelength by an adjustable ONU transceiver according to an embodiment of the present invention. The transmission direction of the uplink optical signal is marked by a dotted line in FIG. 3 . As shown in Fig. 3, in the uplink transmission direction, the comb filter 12 is utilized to generate the target uplink wavelength, and therefore downlink wavelength filtering and uplink wavelength generation will share the same tunable filter to realize TWDM-PON low-cost ONU transceiver. According to NGPON2PMD requirements, downlink and uplink wavelengths are allocated in different bands. The ONU transceiver according to the invention can fully fulfill this requirement because the comb filter has a periodic spectral response in a wide wavelength region (eg C-band and L-band). As shown in FIG. 3 , the target uplink wavelength is generated in the feedback loop including optical coupler 10 , bidirectional amplifier 11 , comb filter 12 , first optical circulator 13 , second optical circulator 14 and light source 15 . In order to illustrate the process of generating the uplink wavelength, an FP-LD is used as a light source in one embodiment of the present invention.

First, the multi-longitudinal-mode uplink optical signal is generated by the FP-LD, and injected into the feedback loop via the second optical circulator 14 in the counterclockwise direction. The signal is transmitted to the third port c of the first optical circulator 13 and output from the first port a of the first optical circulator 13 . Therefore, the uplink optical signal of multiple longitudinal modes generated by the FP-LD is passed into the feedback loop through two optical circulators.

Thereafter, the comb filter 12 is used to filter and select the target uplink optical signal from the multi-longitudinal mode uplink optical signal. The selected target uplink optical signal (single-mode uplink optical signal) is transmitted to the bidirectional amplifier 11 to compensate the loss in the feedback loop. The amplified target uplink optical signal is then transmitted to the optical coupler 10 . Here, the optical coupler 10 splits the amplified target uplink optical signal into a first uplink optical signal and a second uplink optical signal. Here, the power of the first uplink optical signal accounts for the power of the target uplink optical signal is greater than the power of the second uplink optical signal accounts for the power of the target uplink optical signal. As mentioned above, the ratio of optocouplers is set at 80:20 here. That is, the first uplink optical signal (accounting for 80% of the power of the target uplink optical signal) is used as an uplink optical signal to be finally transmitted to the optical line terminal, and is output from the feedback loop to the optical line terminal. And the remaining second uplink optical signal (accounting for 20% of the power of the target uplink optical signal) is fed back to the FP-LD via the second optical circulator 14 . By feeding the target single-mode uplink optical signal back to the FP-LD, the output wavelength of the FP-LD will be locked to the selected target uplink wavelength while other modes will be suppressed. Thereafter, the target upstream wavelength will be output again from the FP-LD, and then enter the feedback loop again.

Preferably, depending on the required accuracy, the above feedback process will be implemented until the spectrum of the uplink optical signal stabilizes to the spectrum of the target uplink optical signal.

Therefore, in summary, the feedback part (that is, the 20% part in the figure) in the selected single-mode uplink optical signal will be transmitted through the following optical path: FP-LD15→second optical circulator 14→first Optical circulator 13→comb filter 12→bidirectional amplifier 11→optical coupler 10→second optical circulator 14→FP-LD15. The above process continues in the feedback loop until a stable single-mode uplink optical signal (that is, the target uplink optical signal) is generated. In addition, data modulation can be loaded to FP-LD or RSOA. The front end of the optocoupler 10 can also use an additional FP-LD to support high-speed data modulation up to a rate of 10Gb/s.

In the present invention, once the downstream wavelength is selected by the comb filter, the corresponding upstream wavelength will also be automatically selected, and thus the ONU wavelength rogue characteristic can be effectively removed. In practice, downstream and upstream wavelengths can be assigned to different bands (eg, C-band and L-band). And this will affect the design of the comb filter. Two implementations of the comb filter are given below. The different wavebands are schematically indicated as waveband A and waveband B in FIGS. 4 and 5 .

FIG. 4 shows a schematic diagram of the operation of a comb filter with a large free spectral range according to an embodiment of the present invention. As shown in Figure 4, the free spectral range of the comb filter is large enough to cover both upstream and downstream bands. This can be achieved by using thin film filters. By tuning the transmission spectrum of the comb filter, the transmission peak is aligned with the target downstream wavelength (for example, λ _1d ), and at this time, one of the multi-longitudinal-mode upstream optical signals of the FP-LD and the comb filter The other transmission peaks are aligned to simultaneously generate corresponding target downstream wavelengths (eg, λ _1d ) and target upstream wavelengths (eg, λ _1u ). Wavelength tuning can be achieved by continuously tuning the transmission peaks of the comb filter over four upstream/downstream wavelength ranges.

FIG. 5 shows a schematic diagram of the operation of a comb filter with a small free spectral range according to another embodiment of the present invention. In the case of comb filters with a small free spectral range, there are multiple transmission peaks in the upstream and downstream bands. At this time, it is necessary to configure the free spectral range to be different from the wavelength interval between the uplink/downlink wavelength channels, so that only one downlink wavelength and one uplink wavelength are filtered out, while other wavelengths are not aligned with the transmission peaks. For example, if the interval of each wavelength channel in the downlink optical signal is 100 GHz, the free spectral range of the comb filter can be selected as 105 GHz, for example. This will ensure single mode at the upstream wavelength according to the Vernier effect. In this case, only a small wavelength tuning is required to tune on the downstream and upstream bands (for example, for a downstream wavelength channel spacing of 100 GHz, only a tuning range of less than 100 GHz is required).

FIG. 5 shows a schematic diagram of the operation of a comb filter with a small free spectral range according to another embodiment of the present invention. As shown in FIG. 5 , λ _2d and λ _2u are selected here as the target downlink and target uplink wavelengths by slightly shifting the spectral response of the comb filter. Thus, this enables fast wavelength tuning and reduces ONU latency during tuning.

The present disclosure provides the following advantages:

1. A shared comb filter is selected for downlink wavelength filtering and uplink wavelength generation, which can solve ONU wavelength rogue characteristics and reduce ONU cost.

2. Good wavelength tuning ability. The wavelengths of the downlink optical signal and the generated uplink single-mode optical signal can be continuously tuned to cover all wavelengths required by TWDM-PON. Flexible wavelength tuning can be achieved simply by tuning the comb filter.

3. High-speed wavelength tuning capability in the entire wavelength range in TWDM-PON. The latency of the ONU during wavelength tuning is reduced.

4. The load capacity of the link is increased and thus the reachable range of the downlink optical signal and the uplink optical signal is increased. Therefore, the splitting ratio and the number of ONUs are improved.

5. Use a lower cost FP-LD or RSOA as a light source and make it operate as a single longitudinal mode laser system.

6. Simplifies the control of TC and reduces the number of message interactions between OLT and ONU during wavelength configuration.

7. Good portability. Easily extended to support multiple wavelength channels (eg greater than 4) to increase system capacity.

The above description of the present disclosure is provided to enable any person of ordinary skill in the art to make or use the present invention. Various modifications to the present disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other modifications without departing from the spirit and scope of the invention. Thus, the invention is not to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (9)

1. A transceiver for an optical network unit of a TWDM-PON system, the transceiver comprising: optocoupler; a comb filter coupled to the optical coupler; a first optical circulator coupled to the comb filter; a downlink receiver coupled to the first optical circulator; a second optical circulator coupled to the optical coupler and the first optical circulator, respectively; and a light source configured to modulate uplink data into an uplink optical signal and output the uplink optical signal; Wherein, when performing downlink optical signal transmission, the comb filter receives the downlink optical signal through the optical coupler and filters out the target downlink optical signal, and the target downlink optical signal passes through the first optical circulator is output to the downlink receiver; When the uplink optical signal is transmitted, the uplink optical signal is transmitted to the comb filter via the second optical circulator and the first optical circulator, and the comb filter receives the downlink optical signal and filter the target uplink optical signal to the optical coupler, and the optical coupler splits the target uplink optical signal into a first uplink optical signal and a second uplink optical signal, wherein the first The uplink optical signal is transmitted to the optical line terminal, and the second uplink optical signal is fed back to the light source through the second optical circulator, so that the light source adjusts the uplink optical signal based on the second uplink optical signal Signal. 2. The transceiver according to claim 1, wherein a bidirectional amplifier is coupled between the optical coupler and the comb filter, and it is used to amplify the optical signal when transmitting the downlink optical signal. The downlink optical signal is used for amplifying the target uplink optical signal when transmitting the uplink optical signal. 3. The transceiver according to claim 1, wherein the power of the first uplink optical signal accounts for the power of the target uplink optical signal is greater than the power of the second uplink optical signal accounts for the target uplink optical signal. The power of the signal. 4. The transceiver according to claim 3, wherein the power of the first uplink optical signal accounts for 80% of the power of the target uplink optical signal, and the power of the second uplink optical signal accounts for 80% of the power of the target uplink optical signal. 20% of the power of the target uplink optical signal. 5. The transceiver according to claim 1, wherein the light source is configured to adjust the uplink optical signal based on the second uplink optical signal until the spectrum of the uplink optical signal stabilizes to the The spectrum of the target uplink optical signal. 6. The transceiver according to claim 1, wherein the light source comprises a Fabry-Perot laser diode and a reflective semiconductor amplifier. 7. The transceiver according to claim 1, wherein the free spectral range of the comb filter is configured to cover uplink and downlink bands. 8. The transceiver according to claim 1, wherein the free spectral range of the comb filter is configured to be different from the wavelength interval between uplink/downlink wavelength channels. 9. An optical network unit comprising the transceiver according to any one of claims 1-7.