Passive optical network system and optical signal receiving method thereof

A passive optical network system includes: a plurality of optical signal splitter receiving optical signals from a plurality of optical network units (ONUs) to provide a plurality of upstream optical signals having different wavelengths; a hybrid optical filter multiplexing the plurality of upstream optical signals in a wavelength division multiplexing (WDM) scheme; and an optical line terminal (OLT) receiving the multiplexed upstream optical signals in a time division multiplexing (TDM) scheme. Therefore, the network system can be easily expanded when the number of subscribers increases, and the optical loss can be minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2008-0059170, filed on Jun. 23, 2008, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention disclosed herein relates to an optical communication system, and more particularly, to a passive optical network (PON) system and an optical signal receiving method thereof.

Recently, Internet traffic is rapidly increasing due to continuous growth of video-based application services, which require real-time data transmission, and provision of telecommunication/broadcasting convergence services. In order to efficiently cope with the increase of traffic, network operators have continuously increased transmission bandwidths by utilizing Wavelength Division Multiplexing (WDM) technology in inter-metropolitan backbone networks and metro networks.

On the other hand, subscriber networks which distribute traffics transmitted from the backbone network to final subscribers have been used in a state that a typical Very-high-bit-rate Digital Subscriber Line (VDSL) and cable modem based transmission technology and a high-speed Ethernet based technology are mixed. Fundamentally, these technologies have a short network installation area and their transmission bandwidths are extremely limited in stably providing integrated services which are under intensive investigation. To solve these limitations, optical network technologies, which are under intensive development, aim to efficiently provide transmission bandwidth necessary for the integrated services to the final subscribers.

Optical network technologies, which are under intensive investigation, may be classified into a Time Division Multiplexing Passive Optical Network (TDM-PON) technology and a Wavelength Division Multiplexing Passive Optical Network (WDM-PON) technology. In the case of the TDM-PON, an optical line terminal (OLT) and a plurality of optical network units (ONUs) are connected together through a passive optical splitter, and a single transmission wavelength is shared at an optical layer by the plurality of ONUs. In the TDM-PON, a downstream data transmission is achieved by a Time Domain Multiplexing (TDM) scheme, and an upstream data transmission is achieved by a Time Division Multiple Access (TDMA) scheme based on a bandwidth reservation.

On the other hand, the WDM-PON constructs a logical point-to-point configuration by allocating individual transmission wavelengths to ONUs. Since data transmission between the OLT and the ONUs is achieved independently without any time division procedure, high transmission bandwidths can be provided to the subscribers. However, in the case of the WDM-PON, subscriber charge per bandwidth is high due to the expensive transmission system, and thus it is expected that somewhat long time is necessary to reach the practical utilization step. On the contrary, the TDM-PON is considered as next-generation optical network technology because it can efficiently use the same wavelength through the time division and its system price is relatively low. The TDM-PON technology may be classified into Ethernet-PON, G-PON, and B-PON according to the frame format of a transport layer, but their basic concepts of the upstream/downstream transmission control are equal to one another. In the case of the upstream transmission, since the data transmission from a plurality of ONUs and optical network terminals (ONTs) to the OLT, which is the common destination, is achieved by the shared link, an appropriate media access control (MAC) technology is required for preventing data collision. To this end, generally, the ONU and the ONT reserve bandwidth necessary to the next transmission period, based on a total amount of data accumulated in a buffer. After arbitrating such a reservation, the OLT allocates transmission time slots, that is, upstream bandwidths. Therefore, it is possible to maintain high network efficiency and also fairly allocate bandwidths to the ONUs. In this case, since data frames transmitted from the respective ONUs during the time slot period are the point-to-point communication where the primary destination is the OLT, the fairness of the bandwidth allocation can be easily maintained through the control of the time slots.

On the contrary, the downstream data transmission is achieved as follows. That is, all the data frames transmitted from the OLT are split at the optical layer by an optical splitter and broadcast to all the ONUs and ONTs, and the individual ONUs filter only the necessary frames from the received frames at the MAC layer, based on the destination address. In this case, if all the traffics are unicast frames, that is, frames directed to only the single destination, just like the case of the upstream transmission, the OLT can ensure the fairness of the bandwidth allocation by fairly allocating the downstream transmission time slots to the ONUs. However, in the case of the downstream transmission in the TDM-PON, a large amount of multicast traffics always exist due to VoD and SVD services or the like. These multicast traffics are simultaneously shared by a plurality of ONUs through the optical splitting.

FIG. 1is a block diagram illustrating the architecture of a TDM-PON system. Referring toFIG. 1, a plurality of ONUs30through60are connected to one OLT10. The OLT10and the ONUs30through60are connected together through an optical signal splitter20. Each of the ONUs30through60shares optical lines with the OLT10and thus shares the installation cost of the optical lines and the cost of the OLT10. The sharing of the optical lines can reduce the service charges of the ONUs. Therefore, as the number of the ONUs connected to one OLT10increases, the service charge per ONU is reduced. However, if a lot of ONUs are connected, optical loss occurs in the connection nodes. In addition, optical signals having a power higher than a specific level are required for detecting signals at an optical receiver. Thus, a light source having a high power is required for connecting more ONUs. If the number of the ONUs can increase even though the cost of the OLT shared by a plurality of ONUs increases, the cost reduction effect of the optical lines and the ONUs is greater than the increase in the cost of the OLT. Therefore, by increasing the power of the light source applied to the OLT, the service charge per unit ONU can be reduced. However, if the power of the light source applied to the ONU increases, the cost of the ONU increases and the service charge per ONU increases in proportion to the cost of the ONU. Increasing the output power of the ONU is economically inefficient. Accordingly, in the case of the upstream signal, there is a limitation in increasing the optical power. Furthermore, in order to compensate for optical loss occurring at the connection nodes of the ONUs, it is necessary to improve the receiver sensitivity of the optical receiver or compensate for the optical loss.

Therefore, there is an increasing demand for technologies that can reduce optical loss occurring at the optical distribution network and also flexibly cope with the increase of subscribers.

SUMMARY OF THE INVENTION

The present invention provides an OLT, which is capable of receiving more subscribers at one OLT in a TDM-PON, and a structure of an optical filter.

Embodiments of the present invention provide passive optical network systems, including: a plurality of optical signal splitter receiving optical signals from a plurality of optical network units (ONUs) to provide a plurality of upstream optical signals having different wavelengths; a hybrid optical filter multiplexing the plurality of upstream optical signals in a wavelength division multiplexing (WDM) scheme; and an optical line terminal (OLT) receiving the multiplexed upstream optical signals in a time division multiplexing (TDM) scheme.

In other embodiments of the present invention, methods for receiving upstream optical signals from a plurality of optical network units in a passive optical network system, the method including: overlapping and multiplexing a plurality of upstream optical signals having different wavelengths; and receiving, by an optical line terminal, the multiplexed upstream optical signals divided in a time division multiplexing (TDM) scheme.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2is a block diagram illustrating a PON system100according to an embodiment of the present invention. Referring toFIG. 2, the PON system100according to the embodiment of the present invention includes a plurality of optical signal splitters130,140and150connected to an OLT110. As described above, the plurality of optical signal splitters130,140and150are connected to a plurality of ONUs141,142,143,151and152. An optical filter120is provided for connecting the plurality of optical signal splitters130,140and159to the OLT110.

In the PON system100, the OLT110is an element that integrates and monitors all optical signals. The OLT110provides service nodes, such as a broadcasting service node, a video on demand (VOD) service node and an Internet service node, to the ONUs141,142,143,151and152corresponding to a plurality of service subscribers. The OLT110according to the embodiment of the present invention can receive upstream signals λu1, λu2, . . . , λunthrough one optical receiver or a plurality of optical receivers. The receiving method of the upstream signals λu1, λu2, . . . , λunin the OLT110will be described later with reference toFIGS. 3A,3B and3C.

The optical filter120multiplexes the upstream signals λu1, λu2, . . . , λunof different wavelengths, which are transmitted from the respective optical signal splitters130,140and150, and provides them to the OLT120. The optical filter120splits a downstream signal λd, which is provided from the OLT110, to the optical signal splitters130,140and150. According to the embodiment of the present invention, the optical filter120is configured with a hybrid optical filter in which mirrors and thin film type wavelength filters are formed on a single substrate, thereby minimizing optical loss. In addition, by adjusting the positions of the thin film type wavelength filters, the optical filter120minimizes the transmission number of the upstream signals, which are absolute to the optical loss, thereby reducing the optical loss. That is, the upstream signals transmitted to the respective input ports can be set so that they pass through the wavelength filters only one time. A detailed structure of the optical filter120will be described later in detail with reference toFIGS. 4 and 6.

The optical signal splitters130,140and150function to split the optical signals between the OLT110and the ONUs141,142,143,151and152. In order for the transmission of the optical signal, the optical signal splitter130may use a multiplexing scheme, such as Time Division Multiplexing (TDM) or Wavelength Division Multiplexing (WDM).

The ONUs141,142,143,151and152are connected to the OLT110in a tree structure, and the number of the ONUs to be connected to one OLT110is determined according to a branching rate. In order for the optical communication, the ONUs control the conversion of electric signals from the subscriber side into optical signals. The respective ONUs are connected to subscriber terminals through a VDSL modem or Ethernet interface.

The above-described PON system100can reduce the service charge per subscriber corresponding to the respective ONUs141,142,143,151and152. Furthermore, services for the ONUs141,142,143,151and152expanded with the increase of the subscribers can be easily expanded through the optical filter120according to the embodiment of the present invention.

FIGS. 3A,3B and3C are block diagrams illustrating the OLT110ofFIG. 2according to exemplary embodiments of the present invention. The OLT110ofFIG. 3Asplits the upstream signals corresponding to the respective wavelengths in the TDM scheme. The OLT110ofFIG. 3Bis configured to include one optical receiver for each wavelength of the upstream signals. The OLT110ofFIG. 3Cincludes hybrid optical receivers for splitting the respective signals according to the wavelengths in the TDM scheme. Further detailed description will be made below.

Referring toFIG. 3A, the OLT110includes a light source111supplying a downstream optical signal, and an optical receiver112receiving upstream optical signals received through an upstream/downstream signal divider113. The upstream optical signals λu1, λu2, . . . , λunhaving different wavelengths, which are supplied from the ONUs, are input to the single optical receiver112through the single upstream/downstream signal divider113. In this case, the upstream optical signals λu1, λu2, . . . , λunhaving the different wavelengths are detected by the single optical receiver112and time slots are allocated to all the ONUs connected to the OLT110in accordance with the TDM scheme. According to the OLT110ofFIG. 3A, all the ONUs141,142,143,151and152and the optical transmission line should share time and be allocated. Therefore, if the number of subscribers increases and the number of ONUs increases, the time slot that can be allocated to one ONU is reduced. However, the above-described OLT110can be manufactured with a relatively simple structure and can simplify the signal processing of the receiver.

Referring toFIG. 3B, the OLT110according to another embodiment of the present invention includes a light source110supplying a downstream optical signal to ONUs, and a wavelength divider114dividing upstream optical signals received through an upstream/downstream signal divider113into optical signals according to wavelengths. The wavelength divider114divides all the upstream optical signals λu1, λu2, . . . , λunhaving different wavelengths according to the respective wavelengths. In order to receive the respective upstream optical signals divided according to the respective wavelengths, the OLT110includes a plurality of optical receivers1121,1122, . . . ,1124. The upstream optical signals received through the optical receivers1121,1122, . . . ,1124will be supplied to necessary elements. The upstream optical signals λu1, λu2, . . . , λunhaving the different wavelengths are divided according to the wavelengths, and the respective optical receivers receive the divided upstream optical signals having the same wavelengths. Therefore, the upstream signals are combined in a TDM scheme that allocates time slot to the ONUs having the same wavelength. To this end, the OLT110must include the plurality of optical receivers1121,1122, . . .1124, and the wavelength divider114. Thus, compared with the OLT110ofFIG. 3A, the structure of the OLT ofFIG. 3Bis more complicated. However, the OLT110ofFIG. 3Bcan provide a high-speed data rate because the respective upstream signals are used without regard to the allocation of the time slot.

FIG. 3Cschematically illustrates the structure of the OLT110having the advantages of the OLTs ofFIGS. 3A and 3B. Referring toFIG. 3C, the OLT110includes a light source111supplying a downstream optical signal to ONUs, and a wavelength divider115dividing upstream optical signals received through an upstream/downstream signal divider113into optical signals according to wavelengths. The wavelength divider115may divide the upstream optical signals λu1, λu2, . . . , λuninto a plurality of groups according to the wavelengths. In this case, the wavelength divider115divides the upstream optical signals λu1and λu2into one group, and the upstream optical signals λu3, . . . , λun-1into another group. The upstream optical signal λuncorresponding to one wavelength is divided into one group. The upstream optical signals corresponding to the divided groups are received by the optical receivers1121,1124and1125. The upstream optical signals having the wavelengths of λu1and λu2are input to the optical receiver1121, and the upstream optical signals having the wavelengths of λu3, . . . , λun-1are input to the optical receiver1125. Therefore, the upstream optical signals are combined in a TDM scheme that allocates time slot to the ONUs having the wavelengths of λu1and λu2or the wavelengths of λu3, . . . , λun-1. Consequently, the OLT110ofFIG. 3Cuses both the TDM scheme and the WDM scheme, thereby providing the reduced complexity and the improved data rate.

FIG. 4illustrates a detailed structure of the optical filter ofFIG. 2. Referring toFIG. 4, the optical filter120according to the embodiment of the present invention is configured with a hybrid optical filter that overlaps and multiplexes the upstream optical signals λu1, λu2, λu3and λu4and splits the downstream optical signal λdto the respective input ports. Specifically, the optical filter120includes optical signal dividers301,302and303, mirrors304and305, and wavelength filters401,402,403and404. The case of four input ports for the upstream signals will be described below. That is, the upstream optical signals λu1, λu2, λu3and λu4having different wavelengths are input to the optical filter120through four input ports P1, P2, P3and P4.

The optical signal dividers301,302and303and the mirror304receive the downstream optical signal λdtransferred from the OLT110and split it to the respective input ports. The optical signal divider301reflects a part of the downstream optical signal λdto the first input port P1, and transfers a partially transmitted signal to the optical signal divider302. The optical signal divider302reflects a part of the downstream optical signal λd, which is transmitted from the optical signal divider301, to the second input port P2, and transfers a transmitted downstream optical signal to the optical signal divider303. The optical signal divider303reflects a part of the downstream optical signal λd, which is transmitted from the optical signal divider302, to the third input port P3, and transfers a partially transmitted optical signal to the mirror304. The mirror304reflects the downstream optical signal λd, which is transmitted from the optical signal divider303, to the fourth input port P4. The optical signal dividers301,302and303and the mirror304may be manufactured using thin film mirrors. The optical signal dividers301,302and303and the mirror304function as optical signal splitters that split the downstream signal to the respective input ports.

The wavelength filters401,402,403and404and the mirror305are configured to multiplex the upstream optical signals λu1, λu2, λu3and λu4provided to the input ports of the optical filter120. That is, the wavelength filters401,402,403and404and the mirror305are configured to multiplex the upstream optical signals λu1, λu2, λu3and λu4input through different optical paths, and provide the multiplexed upstream signals to the OLT110. First, the upstream optical signals input to the respective input ports are totally reflected by the wavelength filter404. The wavelength filter404has a reflectivity to totally reflect light corresponding to the wavelength ranges of the upstream optical signals λu1, λu2, λu3and λu4, and transmits light corresponding to the wavelength range of the downstream optical signal λd.

The upstream optical signal λu4input to the fourth input port P4is reflected by the wavelength filter404and reflected by the mirror305. The upstream optical signal λu4is transmitted sequentially through the wavelength filters403,402and401and is again reflected to the OLT110by the wavelength filter404. The upstream optical signal λu3input through the third input port P3is totally reflected sequentially through the wavelength filters402and401. Thereafter, the upstream optical signal λu3is transmitted sequentially through the wavelength filters402and401and is again reflected to the OLT110by the wavelength filter404. The upstream optical signal λu2input through the second input port P2is totally reflected sequentially through the wavelength filters404and402. Thereafter, the upstream optical signal λu2is transmitted through the wavelength filter401and is again reflected to the OLT110by the wavelength filter404. The upstream optical signal λu1input through the first input port P1is totally reflected sequentially through the wavelength filters404and401. Thereafter, the upstream optical signal λu1is again reflected to the OLT110by the wavelength filter404.

In order to implement the above-described multiplexing of the upstream optical signals λu1, λu2, λu3and λu4, the wavelength filter401should be manufactured to have a filtering characteristic to totally reflect the wavelength of the upstream optical signal λu1and transmit the wavelengths of the upstream optical signals λu2, λu3and λu4. The wavelength filter402should be manufactured to totally reflect the wavelength of the upstream optical signal λu2and transmit the wavelengths of the upstream optical signals λu3and λu4. The wavelength filter403should be manufactured to totally reflect the wavelength of the upstream optical signal λu3and transmit the wavelength of the upstream optical signal λu4. As described above, the wavelength filter404should have the total reflection characteristic with respect to the wavelengths of the upstream optical signals λu1, λu2, λu3and λu4and have the transmission characteristic with respect to the wavelength of the downstream optical signal λd.

The optical filter120described above with reference toFIG. 4can enables the PON100to efficiently use the output strengths of the upstream and downstream optical signals between the OLT110and the optical signal splitters130,140and150. However, in the typical wavelength filter, the optical loss upon the transmission of the optical signals is about two times greater than the optical loss upon the reflection of the optical signals. Therefore, the arrangement of the wavelength filters and the mirror in the optical filter120can be designed to minimize number of the transmission of the optical signals. The arrangement of the wavelength filters and the mirror in order to minimize the number of the transmission of the optical signals will be described later in detail with reference toFIGS. 5 and 6.

FIG. 5illustrates the hybrid optical filter120according to another embodiment of the present invention. Referring toFIG. 5, the optical filter120according to another embodiment of the present invention can reduce optical loss by minimizing the transmission number of the optical signals (especially, the upstream optical signals). In particular, in this embodiment, a device300may be formed on a single glass substrate by a coating such as a dielectric multi-layer so that the device300has both the mirror characteristic and the wavelength division characteristic. Further detailed description will be made below.

The optical signal dividers301,302and303reflect a part of the downstream optical signal λdand transmits the rest of the downstream optical signal λd, and the mirror304has a total reflection characteristic with respect to the downstream optical signal λd. The wavelength filters401,402,403and404have a transmission characteristic with respect to the downstream optical signal λd. Therefore, the downstream optical signal λdinput from the OLT110and transmitted through the wavelength filter404are split into the respective input ports P1, P2, P3and P4by the optical signal dividers301,302and303and the mirror304.

The wavelength filters401,402,403and404for multiplexing the upstream optical signals λu1, λu2, λu3and λu4provided to the input ports are arranged to have the minimum number of transmission with respect to the respective upstream optical signals λu1, λu2, λu3and λu4. In particular, such a function may be provided through the wavelength filter404formed on the same substrate together with the optical signal divider301. The upstream optical signal λu4input to the fourth input port P4is reflected by the wavelength filters404,403,402and401in sequence, reflected by the wavelength filter404, and transferred to the OLT110. The upstream optical signal λu3input to the third input port P3is reflected by the wavelength filter404, passes through the wavelength filter403, and reflected by the wavelength filter401. The upstream optical signal λu3is reflected by the wavelength filter404and transferred to the OLT110. The upstream optical signal λu2input to the second input port P2passes through the wavelength filter403and is reflected by the wavelength filter402. The upstream optical signal λu2is again reflected by the wavelength filters401and404in sequence, and transferred to the OLT110. The upstream optical signal λ1input to the first input port P1passes through the wavelength filter401, reflected by the wavelength filter404, and transferred to the OLT110.

In order for the reflection and transmission characteristics with respect to the above-described upstream optical signals λu1, λu2, λu3and λu4, the wavelength filter401has the transmission characteristic with respect to only the upstream optical signal λu1and has the reflection characteristic with respect to the upstream optical signals λu2, λu3and λu4The wavelength filter402has the transmission characteristic with respect to only the upstream optical signal λu2and has the reflection characteristic with respect to the upstream optical signals λu3and λu4. The wavelength filter403has the transmission characteristic with respect to only the upstream optical signal λu3and has the reflection characteristic with respect to the upstream optical signal λu4. The wavelength filter404should be manufactured to have the reflection characteristic with respect to the upstream optical signals λu1, λu2, λu3and λu4.

As a result, according to the optical filter120ofFIG. 5, the respective upstream optical signals λu1, λu2and λu3pass through the wavelength filters only one time. The upstream optical signal λu4will be transferred to the OLT110through only the reflection paths with respect to the wavelength filters, without the transmission paths. Hence, the optical loss occurring at the upstream optical signals λu1, λu2, λu3and λu4can be remarkably reduced.

FIG. 6illustrates the hybrid optical filter120according to further embodiment of the present invention. Referring toFIG. 6, the optical filter120according to the further embodiment of the present invention can reduce the optical loss by minimizing the number of the transmission of the optical signals (especially, the upstream optical signals). The embodiment ofFIG. 6can provide the function of multiplexing and dividing the optical signals just like the embodiment ofFIG. 5, without the structure ofFIG. 5configured to simultaneously obtain the mirror characteristic and the wavelength division characteristic.

The optical signal dividers301,302and303reflect a part of the downstream optical signal λd, and transmit the rest of the downstream optical signal λd. The mirror304has the total reflection characteristic with respect to the downstream optical signal λd, and the mirror305has the total reflection characteristic with respect to the upstream optical signal λu4. The wavelength filters401,402,403and404have the transmission characteristics with respect to the downstream optical signal λd. Therefore, the downstream optical signal λd, which is transferred from the OLT110and passes through the wavelength filter404, is split into the input ports P1, P2, P3and P4by the optical signal dividers301,302and303and the mirror304.

The wavelength filters401,402,403and404for multiplexing the upstream optical signals λu1, λu2, λu3and λu4provided to the input ports are arranged to have the minimum number of transmission with respect to the respective upstream optical signals λu1, λu2, λu3and λu4. The wavelength filter401has the transmission characteristic with respect to the upstream optical signal λu1and has the reflection characteristic with respect to the upstream optical signals λu2, λu3and λu4. The wavelength filter402has the transmission characteristic with respect to only the upstream optical signal λu2and has the reflection characteristic with respect to the upstream optical signals λu3and λu4. The wavelength filter403has the transmission characteristic with respect to only the upstream optical signal λu3and the reflection characteristic with respect to the upstream optical signal λu4.

According to the arrangement ofFIG. 6, the respective upstream optical signals λu1, λu2and λu3pass through the wavelength filters only one time. The upstream optical signal λu3will be transferred to the OLT110through only the reflection paths with respect to the wavelength filters, without the transmission paths. Hence, the optical loss occurring at the upstream optical signals λu1, λu2, λu3and λu4can be remarkably reduced.

According to the above-described PON system, the optical loss occurring at the upstream optical signals and the downstream optical signal can be minimized, and the service system can be easily expanded without great increase of cost when the number of subscribers increase.

According to the PON system and the optical signal receiving method thereof, the network expansion can be easily achieved when the number of subscribers increases, and the optical loss can be minimized.