Abstract:
An optical access communication apparatus and an optical access communication system for the coexistence of two wideband PON systems without using an expensive optical device or module. A low-speed PON and a high-speed PON have a same upstream wavelength, and an OLT receives optical signals by a same optical receiver in the two systems, converts the optical signals into electric signals, amplifies the electric signals, branches the amplified electric signals, and processes the branched signals by clock and data recovery sections of bit rates corresponding to the two PON systems, thereby achieving an optical communication apparatus and an optical communication system for constructing a simple and low-cost triple-play service system of excellent transmission quality.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application JP 2007-246485 filed on Sep. 25, 2007, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The present invention relates to an optical access communication apparatus and an optical access communication system for use in a passive optical network (PON) using time division multiple access (TDMA) and wavelength division multiple access (WDMA) in signal multiplexing techniques for constructing an optical access network. 
     BACKGROUND OF THE INVENTION 
     Services on a communication network have become increasingly diversified, and new services taking advantage of the network have been expanding. A representative example thereof is the integration of broadcasting and communication services such as so-called triple-play service for integrating broadcasting, Internet, and telephone (audio communication) services. To achieve the triple-play service, FTTH (Fiber To The Home) construction with PON is becoming mainstream in an access network. In this PON system, plural subscribers share an optical fiber from a subscriber accommodation office to an optical splitter and facilities in the office, which leads to cost sharing for reductions in initial introduction cost and maintenance cost. In the PON-based FTTH system which is a shared-media network described above, a bandwidth available to a subscriber is roughly equal to a value obtained by dividing the maximum throughput of the system by the number of sharing subscribers. However, due to a low probability of simultaneous access by all subscribers, the subscriber can practically use a greater bandwidth by statistical multiplexing effect. With such a PON-based wideband FTTH system, it is possible to achieve comfortable triple-play service. Current systems include ITU-T G-PON and IEEE GE-PON. Details of G-PON are defined, for example, in ITU-T G.984.1 “Gigabit-capable Passive Optical Networks (GPON): General characteristics”, ITU-T G.984.2 “Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification”, and ITU-T G.984.3 “Gigabit-capable Passive Optical Networks (GPON): Transmission convergence layer specification”, and details of GE-PON are defined in IEEE 802.3ah “CSMA/CD Access Method and Physical Layer Specifications Amendment: Media Access Control Parameters, Physical Layers, and Management Parameters for Subscriber Access Networks.” For example, in the G-PON system, an optical line terminating apparatus (OLT) accommodates up to 64 optical network terminating units (ONUs) through a 2.4-Gbps high-speed optical line. Collision avoidance control is one of the schemes of sharing facilities in the office as described above. Optical signals (upstream signals) are outputted from ONUs to the OLT, with the optical signal powers being mutually superposed by an optical splitter. In order for the OLT to receive plural separate signals, there is performed transmission timing control such that the signals from the ONUs arrive at the OLT at different times without being mutually superposed, that is, collision avoidance control. Currently, standardizing organizations (ITU-T and IEEE) have started to study next-generation PONs subsequent to these current PON systems. For a wider band of the PON system, studies are being conducted on a higher speed of TDMA applied to the current PON, an increase in bit rate, and the like. 
     The wavelength multiplexing transmission technique is applied to the triple-play service in the PON system. A wavelength range of 1550 to 1560 nm is allocated to a video transmission system. In the PON system, 1490 nm band data signal light is allocated to downstream optical signals from the OLT to the ONU, and 1300 nm band data signal light is allocated to upstream optical signals from the ONU to the OLT. It is desirable that a next-generation PON targeted at a communication speed of 10 Gbps share a fiber with the existing GE-PON and G-PON systems for system construction. 
     However, in an optical transmission system having a bit rate of 10 Gbps, a phenomenon called the wavelength dispersion of an optical fiber greatly limits transmission speed and transmission distance. The wavelength dispersion is a phenomenon in which beams having different wavelengths propagate at different speeds in the optical fiber. Since the optical spectrum of an optical signal modulated at high speed includes different wavelength components, these components arrive at a receiver at different times due to the effect of the wavelength dispersion when propagating through the optical fiber. This causes distortion in optical signal waveform after fiber transmission. There is a technique called dispersion compensation to suppress the waveform degradation due to the dispersion. The dispersion compensation is a technique in which an optical element having a wavelength dispersion characteristic inverse to that of an optical fiber used as a transmission line is disposed in an optical transmitter, a receiver, or a repeater so as to cancel the wavelength dispersion characteristic of the optical fiber to prevent the waveform degradation. As this optical element, that is, a dispersion compensator, devices having an inverse dispersion characteristic such as a dispersion compensation fiber and an optical fiber grating have been studied and put to practical use. However, the dispersion compensator is expensive and not practical for the PON system. As a method not using the dispersion compensator, there is a method of using a low-chirp external modulator. The chirp is a minute and dynamic wavelength variation which occurs when an optical carrier emitted from a communication laser is modulated in an optical communication system. The chirp causes group delay according to a wavelength dispersion value of the optical transmission line, thus distorting an optical signal pulse waveform and degrading transmission quality. For a wavelength of 1490 nm or greater used in the PON system, in the case of directly modulating a laser, it is difficult to achieve a transmission distance of 20 km under the influence of the chirp and the dispersion. In this case, it is considered that a method of using EA (Electro-Absorption) modulator with the electroabsorption effect of a semiconductor is favorable. This is because, EA is made of semiconductor material, which facilitates the integration of the external modulator and the laser, thereby making it possible to suppress a cost increase compared to a modulator using optical crystal having electrooptic effect such as LiNbO 3 . Compared to the method of directly modulating a laser, the use of the EA modulator causes a cost increase corresponding to the modulator. However, in the PON system, plural subscribers share facilities in the office for cost sharing; therefore, in the case where the modulator is used in the optical line terminating apparatus, this cost increase is not a fatal problem. 
     SUMMARY OF THE INVENTION 
     However, when such an external modulator is used in an optical network terminating unit, due to inability to share the cost, even a small cost increase becomes a problem. Accordingly, it can be considered that the method of directly modulating a laser without using the external modulator is favorable to a transmitter for transmitting an optical signal (upstream signal) from the ONU to the OLT. However, in the current PON system, the 1300 nm band for enabling 20 km transmission by a direct modulator is already used for upstream signals. Accordingly, for the coexistence of the next-generation PON targeted at a communication speed of 10 Gbps with the current PON system, it is selected whether a different wavelength is used or the same wavelength is used under the collision avoidance control. The use of a different wavelength requires the external modulator as described above, which causes a cost increase. In the case where the same wavelength is used under the collision avoidance control; for signal input to a current PON apparatus and a next-generation PON apparatus, there is a method of branching an optical signal by an optical splitter. However, in this method, each PON apparatus receives half the optical signal power, which causes a problem that a loss budget cannot be ensured, that is, transmission quality cannot be maintained due to degradation in the S/N ratio of the signal beam. Further, there is a method of using a device such as an optical amplifier to compensate the optical signal power, which however causes a problem of increasing system cost due to the introduction of the new device. 
     An optical line terminating apparatus according to the invention includes a fixed wavelength band optical receiver capable of receiving a signal of a wavelength, an electric signal amplifier, an electric signal branch section, and a first clock and data recovery section. The fixed wavelength band optical receiver receives optical signals having a same wavelength and mutually different bit rates from the first and second optical network terminating units and converts the optical signals into electric signals, the electric signal amplifier amplifies the electric signals, the electric signal branch section branches the amplified electric signals into a signal of the first optical network terminating unit and a signal of the second optical network terminating unit and the first clock and data recovery section extracts a first data signal and a first clock signal corresponding to a bit rate of the signal of the first optical network terminating unit from the branched signal of the first optical network terminating unit. 
     By using the optical communication apparatus and system according to the invention, it is possible to construct a simple and low-cost triple-play service system of excellent transmission quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the basic configuration of a PON system according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating the arrangement of wavelengths used in the PON system; 
         FIG. 3  is a diagram illustrating a method for specifying upstream signal transmission timing in the PON system; 
         FIG. 4  is a diagram illustrating upstream signal collision avoidance in the PON system; 
         FIG. 5  is a diagram illustrating a time chart of ranging in the PON system; 
         FIG. 6  is a diagram illustrating the basic configuration of a heterogeneous-PON coexistence system according to an embodiment of the invention; and 
         FIG. 7  is a diagram illustrating the second configuration of the heterogeneous-PON coexistence system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.  FIG. 1  shows the basic configuration of a PON system. In the PON system, an optical line terminating apparatus (OLT) transmitter-receiver  10  is connected to at least one optical network terminating unit (ONU) transmitter-receiver  100  through optical fibers  40  and  41  and an optical splitter  30 . The optical line terminating apparatus transmitter-receiver  10  is composed of a driver amplifier  11 , a modulation-capable light source  12 , a WDM  13 , an optical receiver  21 , a transimpedance amplifier  22 , and a clock and data recovery section  23 . The optical network terminating unit transmitter-receiver  100  is composed of a driver amplifier  106 , a modulation-capable light source  105 , a WDM  104 , an optical receiver  103 , a transimpedance amplifier  102 , and a clock and data recovery section  101 . 
     Signal processing will be described according to signal flows. First, a description will be made of an optical signal (downstream signal) from the OLT to the ONU. A PON-frame-processed signal is processed by a SerDes (SERializer/DESerializer) circuit, and then inputted to the OLT transmitter-receiver  10 . This electric signal is amplified by the driver amplifier  11  to obtain driving power enough for modulation by the modulation-capable light source  12 . The amplified signal allows the modulation-capable light source  12  to output a modulated signal beam. If the bit rate falls within about 2.5 Gbps, the modulation-capable light source  12  can be achieved by the method of directly modulating a laser. In G-PON and GE-PON, a 1.49 μm wavelength band is used for the modulated signal beam, which is passed through the WDM  13  and then transmitted to an optical fiber  40 . This optical signal is passed through the optical fiber  40 , an optical splitter  30 , and an optical fiber  41 , and inputted to the ONU transmitter-receiver  100 . In the ONU transmitter-receiver  100 , after the 1.49 μm wavelength band component is separated by the WDM  104 , the signal beam is inputted to the optical receiver  103 . A photodiode (PD) is used as the optical receiver  103 . More specifically, a PIN-type photodiode based on a PIN junction semiconductor is used, or an avalanche photodiode (APD) is used if sensitivity is required. A minute current change outputted from the photodiode is converted by the transimpedance amplifier  102  into a voltage change, which is amplified and then outputted. From the output signal, a clock signal and a data signal are extracted and outputted by the clock and data recovery section  101 . The signal thus outputted from the ONU transmitter-receiver  100  is processed by a SerDes circuit, and is then PON-frame-processed. 
     Next, a description will be made of an optical signal (upstream signal) from the ONU to the OLT. A PON-frame-processed signal is processed by the SerDes circuit, and then inputted to the ONU transmitter-receiver  100 . This electric signal is amplified by the driver amplifier  106  to obtain driving power enough for modulation by the modulation-capable light source  105 . The amplified signal allows the modulation-capable light source  105  to output a modulated signal beam. If the bit rate falls within about 2.5 Gbps, the modulation-capable light source  105  can be achieved by the method of directly modulating a laser. In G-PON and GE-PON, a 1.3 μm wavelength band is used for the modulated signal beam, which is passed through the WDM  104  and then transmitted to the optical fiber  41 . This optical signal is passed through the optical fiber  41 , the optical splitter  30 , and the optical fiber  40 , and inputted to the OLT transmitter-receiver  10 . In the OLT transmitter-receiver  10 , after the 1.3 μm wavelength band component is separated by the WDM  13 , the signal beam is inputted to the optical receiver  21 . A photodiode (PD) is used as the optical receiver  21 . More specifically, a PIN-type photodiode based on a PIN junction semiconductor is used, or an avalanche photodiode (APD) is used if sensitivity is required. A minute current change outputted from the photodiode is converted by the transimpedance amplifier  22  into a voltage change, which is amplified and then outputted. From the output signal, a clock signal and a data signal are extracted and outputted by the clock and data recovery section  23 . The signal thus outputted from the OLT transmitter-receiver  10  is processed by the SerDes circuit, and is then PON-frame-processed. 
     The arrangement of wavelength bands used in the PON system will be described with reference to  FIG. 2 . In this example, a 1.49 μm wavelength band is used for a downstream signal and a 1.3 μm wavelength band is used for an upstream signal, thereby performing bidirectional signal transmission through one optical fiber. Further, an optical signal of a 1.55 μm wavelength band for video distribution may be multiplexed with a downstream signal. The 1.55 μm wavelength band can also be reserved for system upgrade. These wavelength-multiplexed optical signals are demultiplexed in an office or a user&#39;s home, so that a user can use plural services through the connection of only one optical fiber. 
     In the PON system, all ONUs can receive a downstream signal outputted from the OLT, that is, the PON system has multicast capability. Thus, the OLT performs header writing for each packet or cell of the downstream signal, and each ONU receives only a signal addressed thereto. On the other hand, the technique of collision avoidance is applied to upstream signals, and a description thereof will be made with reference to  FIGS. 3 and 4 .  FIG. 3  shows a method for specifying upstream signal transmission timing in the PON system. In the same manner as in  FIG. 1 , the OLT transmitter-receiver  10  is connected to the ONU transmitter-receivers  100  through the optical fiber  40 , the optical splitter  30 , and the optical fibers  41 . The optical splitter  30  superposes optical signal powers for output. If upstream signals from the plurality of ONU transmitter-receivers  100  are simultaneously inputted to the optical splitter  30 , these signals are mutually superposed and outputted to the OLT. The OLT cannot separate these signals, and therefore cannot receive them correctly. The ONUs need to control transmission timing so that the respective upstream signals arrive at the OLT at different times without being mutually superposed. For this reason, the OLT notifies each ONU of signal transmission permission to specify the transmission timing of each ONU, thereby making it possible to avoid a collision of upstream signals. In  FIG. 3 , gate timing instruction signals  700  instruct the respective ONUs about transmission timing. Further, as shown in  FIG. 4 , upstream signal cells or packets  800  are transmitted from the ONUs according to the timing provided from the OLT, which prevents a collision of upstream signals. In the PON system, the respective transmission distances between the OLT and the ONUs are not the same, and cannot be determined in advance. Accordingly, the OLT measures and stores beforehand the respective transmission times between the OLT and the ONUs, and thereby calculates timing that does not cause a collision of upstream signals from the ONUs and notifies it to the ONUs. The processing for measuring transmission times is referred to as ranging.  FIG. 5  shows this ranging. First, after the OLT sends to an ONU an instruction (measurement signal transmission permission) to transmit a measurement signal after α seconds, the OLT sets a ranging window after α seconds. Then, the ONU transmits a measurement frame α seconds after receiving the instruction. The OLT measures a measurement time from the start of the ranging window to the arrival of the measurement signal from the ONU, and recognizes half the measurement time as a direction transmission time. In the ranging, the OLT inhibits signal transmission by ONUs other than a specific ONU only during a certain time called the ranging window. The OLT transmits and receives the measurement signal to and from the specific ONU in the ranging window, and calculates the transmission time between the OLT and the ONU based on a time of arrival. Since the OLT cannot receive in the ranging window a measurement signal from an ONU having a transmission time greater than the ranging window, the size of the ranging window determines the maximum distance between the OLT and the ONU in the PON system. This distance is referred to as a maximum logical distance, and defined separately from a physical distance determined by the transmission/reception level of an optical signal and a transmission line loss. 
     In the PON system, since a downstream signal is transmitted as one continuous signal obtained by concatenating packets or cells, the ONU receiver is not particularly different from that of a conventional optical transmission system. However, upstream signals received by the OLT become a burst state due to different clock phases and light intensities of individual ONUs; accordingly, the OLT needs a dedicated burst signal receiving circuit. In burst signal reception, it is necessary to eliminate the effect of an immediately preceding received signal. Further, it is necessary to extract signal timing from the overhead of a packet or cell to establish bit synchronization. Further, the PON system is required to effectively utilize bandwidths by best effort. The OLT can detect the traffic of a downstream signal addressed to each ONU from the network of a telecommunications carrier, and therefore can control the bandwidth dynamically by adjusting the size and frequency of a packet or cell addressed to each ONU. On the other hand, in order to dynamically control the bandwidth of an upstream signal, there is required a function in which each ONU notifies a request bandwidth to the OLT and then the OLT allocates bandwidth to each ONU. This function is referred to as DBA (Dynamic Bandwidth Assignment). The DBA function enables not only an improvement in the upstream bandwidth utilization efficiency of the PON system, but also the low-delay transmission of audio and video signals sensitive to delay characteristics. In FIG.  1 , DBA  300  is an instruction section for supporting the DBA function from the OLT. 
     An embodiment of the invention will be described in detail with reference to  FIG. 6 . A heterogeneous-PON coexistence system according to the invention is composed of an OLT transmitter-receiver  10 , at least one first-group optical network terminating unit (ONU) transmitter-receiver  100 , at least one second-group optical network terminating unit (ONU) transmitter-receiver  200 , the optical fibers  40  and  41  and the optical splitter  30  for connecting these apparatuses. The OLT transmitter-receiver  10  is composed of driver amplifiers  11  and  511 , modulation-capable light sources  12  and  512 , WDMs  13  and  14 , an optical receiver  21 , a transimpedance amplifier  22 , an electric signal branch section  31 , clock and data recovery sections  23  and  523 , output signal ports  51  and  52  for upgrade, and a DBA  310 . The first-group ONU transmitter-receiver  100  is composed of a driver amplifier  106 , a modulation-capable light source  105 , a WDM  104 , an optical receiver  103 , a transimpedance amplifier  102 , and a clock and data recovery section  101 . The second-group ONU transmitter-receiver  200  is composed of a driver amplifier  206 , a modulation-capable light source  205 , a WDM  204 , an optical receiver  203 , a transimpedance amplifier  202 , and a clock and data recovery section  201 . Assume that the second-group ONU transmitter-receiver  200  deals with optical signals having a bit rate of 10 Gbps. In order to transmit an optical signal having a wavelength of 1490 nm or greater at this bit rate over a distance of 20 km, it is preferable that the modulation-capable light source  512  be a laser integrated with an EA modulator due to the above reason. 
     Signal processing will be described according to signal flows. First, a description will be made of a downstream signal from the OLT to the first-group ONU. A PON-frame-processed signal is processed by a SerDes circuit, and then inputted to the OLT transmitter-receiver  10 . This electric signal is amplified by the driver amplifier  11 , and the amplified signal allows the modulation-capable light source  12  to output a modulated signal beam. In G-PON and GE-PON, a 1.49 μm wavelength band is used for the modulated signal beam, which is passed through the WDMs  13  and  14  and then transmitted to the optical fiber  40 . This optical signal is passed through the optical fiber  40 , the optical splitter  30 , and the optical fiber  41 , and inputted to the first-group ONU transmitter-receiver  100 . In the ONU transmitter-receiver  100 , after the 1.49 μm wavelength band component is separated by the WDM  104 , the signal beam is inputted to the optical receiver  103 . A PIN-type PD or an APD is used as the optical receiver  103 . The signal outputted from the photodiode is converted, amplified, and outputted by the transimpedance amplifier  102 . From the output signal, a clock signal and a data signal are extracted and outputted by the clock and data recovery section  101 . The signal thus outputted from the first-group ONU transmitter-receiver  100  is processed by a SerDes circuit, and is then PON-frame-processed. 
     Subsequently, a description will be made of a downstream signal from the OLT to the second-group ONU. A PON-frame-processed signal is processed by a SerDes circuit, and then inputted to the OLT transmitter-receiver  10 . This electric signal is amplified by the driver amplifier  511 , and the amplified signal allows the modulation-capable light source  512  to output a modulated signal beam. It is preferable that this signal beam have a wavelength not less than 1570 nm. The signal beam is multiplexed with a 1.49 μm band signal beam outputted from the modulation-capable light source  12  by the WDM  14 , and the multiplexed signal is transmitted to the optical fiber  40 . This optical signal is passed through the optical fiber  40 , the optical splitter  30 , and the optical fiber  41 , and inputted to the second-group ONU transmitter-receiver  200 . In the ONU transmitter-receiver  200 , after the wavelength component not less than 1570 nm is separated by the WDM  204 , the signal beam is inputted to the optical receiver  203 . A PIN-type PD or an APD is used as the optical receiver  203 . The signal outputted from the photodiode is converted, amplified, and outputted by the transimpedance amplifier  202 . From the output signal, a clock signal and a data signal are extracted and outputted by the clock and data recovery section  201 . The signal thus outputted from the second-group ONU transmitter-receiver  200  is processed by a SerDes circuit, and is then PON-frame-processed. 
     Next, a description will be made of an upstream signal from the first-group ONU transmitter-receiver  100  to the OLT. A PON-frame-processed signal is processed by the SerDes circuit, and then inputted to the first-group ONU transmitter-receiver  100 . This electric signal is amplified by the driver amplifier  106 , and modulated by the modulation-capable light source  105 . A 1.3 μm wavelength band is used for the modulated signal beam, which is passed through the WDM  104  and then transmitted to the optical fiber  41 . This optical signal is passed through the optical fiber  41 , the optical splitter  30 , and the optical fiber  40 , and inputted to the OLT transmitter-receiver  10 . In the OLT transmitter-receiver  10 , after the 1.3 μm wavelength band component is separated by the WDMs  14  and  13 , the signal beam is inputted to the optical receiver  21 . A PIN-type PD or an APD is used as the optical receiver  21 . The electric signal outputted from the photodiode is converted, amplified, and outputted by the transimpedance amplifier  22 . From the output signal, a clock signal and a data signal are extracted and outputted by the clock and data recovery section  23 . The signal outputted from the first-group ONU transmitter-receiver  100  through the OLT transmitter-receiver  10  is processed by the SerDes circuit, and is then PON-frame-processed. 
     Subsequently, a description will be made of an upstream signal from the second-group ONU transmitter-receiver  200  to the OLT. A PON-frame-processed signal is processed by the SerDes circuit, and then inputted to the second-group ONU transmitter-receiver  200 . This electric signal is amplified by the driver amplifier  206 , and modulated by the modulation-capable light source  205 . A 1.3 μm wavelength band is used for the modulated signal beam, which is passed through the WDM  204  and then transmitted to the optical fiber  41 . This optical signal is passed through the optical fiber  41 , the optical splitter  30 , and the optical fiber  40 , and inputted to the OLT transmitter-receiver  10 . Even though the modulation-capable light source  205  deals with signals having a bit rate of 10 Gbps, the modulation-capable light source  205  can be achieved by the method of directly modulating a laser due to the use of a wavelength band having a small amount of fiber dispersion. In the OLT transmitter-receiver  10 , after the 1.3 μm wavelength band component is separated by the WDMs  14  and  13 , the signal beam is inputted to the optical receiver  21 . A PIN-type PD or an APD is used as the optical receiver  21 . The electric signal outputted from the photodiode is converted, amplified, and outputted by the transimpedance amplifier  22 . The output signal is branched by the electric signal branch section  31 . From the branched signal, a clock signal and a data signal are extracted and outputted by the clock and data recovery section  523 . The signal outputted from the second-group ONU transmitter-receiver  200  through the OLT transmitter-receiver  10  is processed by the SerDes circuit, and is then PON-frame-processed. It is preferable that the package configuration be divided by a heavy-line frame in  FIG. 6 . With this configuration, in the case of a system without the second-group ONU transmitter-receiver  200 , the PON system can be constructed by having only the functions within the heavy-line frame in  FIG. 6 , that is, an upgrade-function-equipped basic OLT package  1000 . When the second-group ONU transmitter-receiver  200  is needed, the functions outside the heavy-line frame in  FIG. 6  are added through the output signal ports  51  and  52  for upgrade, thereby making it possible to support the second-group PON system as well. Thus, it is possible to reduce initial investment in the heterogeneous-PON coexistence system and provide a mechanism for extracting an upstream signal from the first-group and second-group ONUs. However, since the first-group and second-group ONUs have the same transmission wavelength, the OLT specifies, with the above-described collision avoidance and DBA, transmission timing to receive a signal. With this configuration, in the case where the OLT receives a transmission signal from the first-group and second-group ONUs, there is no factor for increasing a transmission line loss by adding an optical splitter to the existing PON system. Therefore, a large loss budget required in the PON system can be ensured without adding an optical amplifier or the like, thus enabling low-cost and excellent-quality transmission. 
     An embodiment of the invention will be described in greater detail with reference to  FIG. 7 . A heterogeneous-PON coexistence system according to the invention is composed of an OLT transmitter-receiver  10 , at least one first-group optical network terminating unit (ONU) transmitter-receiver  100 , at least one second-group optical network terminating unit (ONU) transmitter-receiver  200 , the optical fibers  40  and  41  and the optical splitter  30  for connecting these apparatuses. The difference between  FIG. 6  and  FIG. 7  is that the receiver of the OLT transmitter-receiver  10  is configured so as to improve its performance. In the OLT transmitter-receiver  10 , compared to the transmitter-receiver shown in  FIG. 6 , the transimpedance amplifier  22  is replaced by a wideband transimpedance amplifier  24 , which amplifies both low-speed signals and high-speed signals. It is difficult for an ordinary transimpedance amplifier to achieve both wideband and gain. Accordingly, a preference for the amplification of signals of 10 Gbps reduces the output of signals of 2.5 Gbps or less. Further, the noise component increases with increase in amplified bandwidth, which degrades the S/N ratios of signals of 2.5 Gbps or less. For this reason, in signal processing by the low-speed side (first-group) PON system, only the necessary bandwidth of a signal branched by the electric signal branch section  31  is passed through a bandpass filter  25  for the elimination of the noise component. In compensation for signal component reduction, the filtered signal is amplified by an electric-signal amplifier  26  in the subsequent stage. Further, a clock signal and a data signal are extracted by a clock and data recovery section  27 , and then PON-frame-processed. In signal processing by the high-speed side (second-group) PON system as well, an electric-signal amplifier  524  in the subsequent stage may be provided to improve the S/N ratio. With this configuration, it is possible to achieve a heterogeneous-PON coexistence system of excellent transmission quality in consideration of part characteristics due to different bit rates. 
     In the above embodiments, the description has been made assuming that the first-group optical network terminating unit is the G-PON system and the second-group optical network terminating unit is the 10G-PON system. However, these units may be a combination of other PON systems. For example, the invention can also be applied to the case where the first-group optical network terminating unit is the GE-PON system and the second-group optical network terminating unit is the 10G-PON system, or the case where the first-group optical network terminating unit is the GE-PON system and the second-group optical network terminating unit is the G-PON system. Since the GE-PON system and the G-PON system have the same downstream wavelength, the WDM  104  is replaced by an optical splitter. 
     As described above, the invention provides an optical access communication apparatus and an optical communication system for constructing a simple and low-cost triple-play service system of excellent transmission quality.