Abstract:
A double-ring optical wavelength multiplex network is disclosed, which includes multiple optical transmission apparatuses that can reduce the unit cost of initially installing a small optical network while providing the flexibility to expand. For multiplexed optical signals arriving at a node, the optical transmission apparatus “drops” selected wavelengths for local delivery and “passes” others for continued transmission on the network. For optical signals originating (“added”) at the node, the optical transmission apparatus wavelength multiplexes the “added” signals with the “passing” signals for transmission on the network. For “added” signals, the optical transmission apparatus blocks “passing” signals of the same wavelength.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an optical transmission apparatus, and an optical wavelength multiplex network therewith, and especially relates to an optical transmission apparatus that includes an optical add/drop function for an optical wavelength multiplex network, and an optical wavelength multiplex network therewith.  
           [0003]    2. Description of the Related Art  
           [0004]    With explosive increases in demand for data communications mainly of Internet traffic, backbone networks are required to be capable of accommodating a great volume of traffic, and providing super-long-distance transmissions. At the same time, services available to users must be highly versatile, which requires a network to be economical, flexible, and reliable.  
           [0005]    At present, with wavelength division multiplex (WDM) transmission technology and optical amplification technology, great traffic-accommodating capacity and super-long-distance transmissions of the network have become available, reducing transmission-line costs. However, compared with progress in expanding the capacity, and in reducing of the cost of a transmission line, node apparatuses (transmission apparatuses) are behind in regard to raising throughput and reducing costs, the node apparatuses being arranged on both ends of the transmission line.  
           [0006]    For efficient operations and construction of a network, improvements of signal-processing capacity and cost reduction of the node apparatus (node) that distributes signals to users are essential. However, when the node apparatuses are configured by opto-electricity conversion and an electrical switching method for increasing the throughput, the size of the node becomes large, and consequently, the cost of the node increases, causing a problem that efficient operations and economical construction of the network cannot be attained.  
           [0007]    Accordingly, in order that the node be capable of increased throughput, available at low cost, and made small in size, optical add/drop multiplexers (OADM), and optical cross connect (OXC) apparatuses for carrying out various processes in units of optical paths in the optical wavelength domain are being developed, replacing large-scale electronic circuits with optical parts (refer to patent reference 1).  
           [0008]    An OADM node is a node that performs optical add/drop, and is widely used by metro networks and access networks. The OADM node intercepts and passes through optical signals, adding and dropping desired wavelengths. In this manner, optical paths are provided between nodes, signal connections are made, and communications between users connected to the OADM node are provided.  
           [0009]    The OADM node is required to provide high performances and flexibility at low cost. Here, the performances that are required of the OADM node include high throughput, long transmission distance, scalability in terms of the number of nodes that can be arranged, and great processing capacity of each node. Further, the flexibility that is required of the OADM node includes a capability of providing an optical path by provisioning, a broadcasting function, and a protection function. These are the requirements to be realized at a low cost.  
           [0010]    The requirements are currently answered by configuring an OADM apparatus using an optical multiplexer/demultiplexer, and an optical switch.  
           [0011]    (Patent Reference 1)  
           [0012]    JP, 11-174499, A.  
           [0013]    Although the above-mentioned configuration is capable of providing optical paths as desired by provisioning, and processing all the wavelengths that are received, there is a problem in that initial unit cost becomes high when only a few wavelengths are required, the initial installation is for only the few required wavelengths, and the number of wavelengths is to be increased at a later stage. In other words, the initial cost per wavelength is high, which poses the problem.  
           [0014]    Further, the average number of wavelengths usually dropped (branched) and added (inserted) by an OADM node is between 20% and 50% of all the wavelengths handled by the OADM node, with the remainder being passed through. In consideration of this fact, it is advantageous to limit the number of wavelengths that are processed (added/dropped) by each node, and to design a specialized node configuration for reducing the node cost.  
           [0015]    Further, although OADM node apparatuses that process a few wavelengths are commercially marketed at present, wavelengths of the transmitting luminous source and the optical filter are fixed at predetermined wavelengths. In this case, although initial cost is reduced, the problem is that the network is not flexible, that is, providing future optical paths by provisioning cannot be carried out.  
         SUMMARY OF THE INVENTION  
         [0016]    It is a general object of the present invention to provide an optical transmission apparatus and an optical wavelength multiplex network therewith that will substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.  
           [0017]    Features and advantages of the present invention are set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by the optical transmission apparatus and the optical wavelength multiplex network therewith particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
           [0018]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an optical transmission apparatus that reduces initial cost, and realizes a flexible network configuration and an optical wavelength multiplex network that employs the optical transmission apparatus.  
           [0019]    In order to solve the above-mentioned problem, the present invention provides means as follows.  
           [0020]    The optical transmission apparatus having an optical add/drop function for an optical wavelength multiplex network according to the present invention includes an optical branching (dropping) coupler for dividing wavelength multiplexed optical signals that are input into a wavelength multiplexed optical signal, which is called a passing signal, and another wavelength multiplexed optical signal, which is called a dropping signal, a filter for extracting an optical signal of a predetermined wavelength from the dropping signal, a laser for generating an optical signal of a wavelength (insertion wavelength) to be inserted by the optical transmission apparatus concerned, a blocking filter for blocking a wavelength contained in the passing signal, the wavelength to be blocked being the same as the insertion wavelength, and an optical coupler for coupling the optical signal of the insertion wavelength and the passing signal that passes through the blocking filter.  
           [0021]    The blocking filter and the optical coupler may be replaced with a filter that is capable of serving the functions of the blocking filter and the optical coupler.  
           [0022]    The filter for extracting an optical signal of a predetermined wavelength from the dropping signal is one of a fixed wavelength filter and a variable wavelength filter.  
           [0023]    The present invention further includes an optical wavelength multiplex network that includes the optical transmission apparatus as described above, and is characterized by a double ring configuration having a HUB.  
           [0024]    The optical transmission apparatus according to the present invention realizes a low initial cost, and further provides a flexible network configuration.  
           [0025]    Further, where a filter that is capable of serving the functions of a blocking filter and an optical coupler is employed, according to the present invention, optical attenuation of the transmitted optical signal in the optical transmission apparatus can be made small, since one device (the filter) serves as the filter for inserting an optical signal to the transmission line, and the blocking filter for preventing the inserted optical signal from propagating more than a round on the transmission-line ring.  
           [0026]    Since the optical wavelength multiplex network of the present invention is configured as a double ring network that has a HUB, the network offers high reliability, providing a protection function when a fault takes place.  
           [0027]    According to the configuration of the present invention, all wavelength multiplexed optical signals are branched by the optical branching coupler, and a desired optical signal is extracted, so that wavelength allocation to each optical transmission apparatus in the network is dispensed with.  
           [0028]    Further, since all the wavelengths of the network are branched, and a desired wavelength is selected, a common wavelength can also be selected by two or more nodes in the network such that an optical signal can be broadcast in the network.  
           [0029]    Further, although the optical branching coupler passes selected wavelengths, the blocking filter blocks an optical signal having the same wavelength as an inserted optical signal, thereby cross talk of the optical signals is prevented from occurring. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a schematic diagram of an example of a network;  
         [0031]    [0031]FIG. 2 is a schematic diagram of a first example of an optical transmission apparatus that has an optical add/drop function;  
         [0032]    [0032]FIG. 3 is a schematic diagram of a second example of an optical transmission apparatus that has the optical add/drop function;  
         [0033]    [0033]FIG. 4 is a schematic diagram of an example of an optical transmission apparatus using AOTF;  
         [0034]    [0034]FIG. 5 is a schematic diagram of an example of a network, where a wavelength group is assigned to each node;  
         [0035]    [0035]FIG. 6 shows how communications are accomplished in the network shown in FIG. 5;  
         [0036]    [0036]FIG. 7 shows how branching and inserting are carried out for each wavelength group;  
         [0037]    [0037]FIG. 8 is a schematic diagram of an example of a HUB that includes an optical add/drop multiplexer and an optical switch;  
         [0038]    [0038]FIG. 9 is a schematic diagram of an example of a HUB that includes an optical filter and an optical group filter;  
         [0039]    [0039]FIG. 10 is a schematic diagram of an example of a HUB that includes an optical add/drop multiplexer and a MEMS; and  
         [0040]    [0040]FIG. 11 is a block diagram for explaining the case where a rejection filter and an optical coupler for adding a wavelength are independently arranged. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    According to the present invention, an optical device that has a variable wavelength function in one of the “drop” section and the “add” section of an OADM node is used. Specifically, a variable wavelength filter is used by the drop unit, and a fixed wavelength laser is used by the add unit; or, alternatively, a fixed wavelength filter is used by the drop unit, and a variable wavelength laser is used by the add unit. If a variable wavelength function is provided to one of the two units of the OADM in this manner, an optical path between desired nodes can be established.  
         [0042]    Embodiments of the present invention are explained with reference to attached drawings.  
         [0043]    The network shown by FIG. 1 includes optical loop circuits  1  and  2  that constitute an optical transmission line, a HUB  12  and a HUB  25 , each of which is a node for relaying traffic between networks, nodes, terminals, and the like, OADMs (Optical Add Drop Multiplexers)  13 ,  14 , and  15 , each of which performs branching (drop) and insertion (add) of optical signals, user terminals  21 ,  22 ,  23 , and  24 , and a second network  26 .  
         [0044]    The HUB  12  performs communications between the HUB  12  itself and the OADMs  13 ,  14 , and  15 , and communications with the HUB  25 . In this manner, communications are made possible between the user terminals  21  through  24  connected to the optical loop circuits  1  and  2 , and between the user terminals  21  through  24  and user terminals (not shown) connected to the second network  26 .  
         [0045]    In the present specification, descriptions are made mainly centering on a network consisting of a double optical loop as shown in FIG. 1. However, the optical transmission apparatus of the present invention is applicable not only to loop-like networks, but also to mesh type networks, and networks where a loop-like network and a mesh-like network are intermingled.  
         [0046]    Further, each of the HUBs has functions of an OADM. Further, the number of HUBs, the number of OADMs, and the number of user terminals indicated in the present specification are only examples.  
         [0047]    The loop-like network that includes the HUB  12  and the OADMs  13 ,  14 , and  15  (optical transmission apparatuses) of FIG. 1 is a network of a double optical loop, wherein the optical loop circuit  1  transmits in the direction of A, and the optical loop circuit  2  transmits in the direction of B, and two-way communications are possible between the optical transmission apparatuses. Further, since the optical loop circuits  1  and  2  are closed-path, each optical transmission apparatus can perform communications on each loop circuit. That is, each optical transmission apparatus can perform independent communications using each of the optical loop circuits  1  and  2 ; and it is also possible that one of the optical loop circuits  1  and  2  is used, while the other is reserved as a stand-by. Further, each optical transmission apparatus may perform independent communications and duplex communications using the two optical loop circuits  1  and  2  under normal operating conditions, and in the case of a fault occurring in one of the optical loop circuits  1  and  2 , or at an optical transmission apparatus, communications are still offered by avoiding the faulty section.  
         [0048]    (First Optical Transmission Apparatus)  
         [0049]    [0049]FIG. 2 shows an example of an optical transmission apparatus that has an optical add/drop function according to the present invention. In the example of FIG. 2, the “drop” unit of the OADM node employs a variable wavelength filter. Namely, the optical transmission apparatus shown by FIG. 2 includes an upward transmission unit  30  prepared on an optical transmission line  10  that goes from West to East, a downward transmission unit  31  prepared on an optical transmission line  11  that goes from East to West, optical splitters  34  and  35  for splitting an optical monitoring signal, a 1×2 coupler  36  for splitting optical signals provided by an optical transmitter  40 , a selector  37  for selecting a signal from one of the two optical transmission lines  10  and  11 , monitors  38  and  39  for monitoring the state of the circuits, an optical transmitter OS  40 , an optical receiver OR  41 , and fixed wavelength lasers  42 ,  43 , and  44  for generating optical signals at insertion wavelengths based on monitoring results. Here, the 1×2 coupler  36  may be replaced with a selector for selecting a signal from one of the optical transmission lines  10  and  11  based on the monitoring results. Note that the optical transmitter  40  employs fixed wavelength lasers.  
         [0050]    The upward transmission unit  30  includes a tee (Drop unit)  32  and an insertion unit (Add unit)  33 . The tee  32  includes an optical branching coupler  321  for dividing an input optical signal into a passing signal that is to be passed through, and a dropping signal that is to be dropped, a supervisory-control-signal extraction filter  322  for extracting a supervisory control signal, a WDM preamplifier  323 , a 1×4 coupler  324  for splitting the dropping signal, and variable wavelength filters  325  through  328  that can change the wavelength to be extracted.  
         [0051]    Further, the insertion unit  33  includes a rejection/add filter  331 , a supervisory-control-signal insertion filter  332  for inserting a supervisory control signal, a WDM amplifier  333  for amplifying a wavelength multiplexed optical signal, a 4×1 coupler  334  for coupling multiple optical signals, and single wave amplifiers  335  through  338 . Here, the downward transmission unit  31  is configured similar to the upward transmission unit  30 .  
         [0052]    In addition, although the number of branched wavelengths and the number of the inserted wavelengths are each set at four in the present embodiment, the numbers are only examples, and are not limited to four.  
         [0053]    The rejection/add filter  331  is for blocking a wavelength contained in the passing signal, which wavelength is the same as an inserted wavelength. This is necessary because the inserted wavelength should not agree with any of the wavelengths that are to be further passed through, and because the inserted wavelength should be prevented from propagating beyond a round on the loop transmission line ring.  
         [0054]    The rejection/add filter  331  is also for inserting (add) an optical signal to the transmission line, in addition to preventing the inserted wavelength from propagating beyond one round on the ring transmission line. Accordingly, attenuation of the optical signals caused by the optical transmission apparatus is minimized.  
         [0055]    Here, if the optical attenuation of the transmitted optical signals in the optical transmission apparatus is not a great concern, a rejection filter  3311  and an optical coupler  3312  for inserting an optical signal to a transmission line may be independently provided as shown by FIG. 11.  
         [0056]    Next, operations of the optical transmission apparatus shown by FIG. 2 are explained. Optical signals from West are input into the upward transmission unit  30 . The optical signals input into the upward transmission unit  30  are supplied to the optical branching coupler  321 , and divided into the passing signal that is to be passed through, and the dropping signal that is to be dropped. The passing signal that is branched by the optical branching coupler  321  is provided to the rejection/add filter  331 , an optical signal having the same wavelength as an insertion wavelength is blocked, an optical signal to be inserted is added, an optical signal for surveillance is added by the supervisory-control-signal insertion filter  332 , and a newly configured optical signal is output from the upward transmission unit  30 .  
         [0057]    The dropping signal is branched by the optical branching coupler  321  of the upward transmission unit  30 . The supervisory-control-signal extraction filter  322  extracts a supervisory-control optical signal (OSC) from the dropping signal. The extracted supervisory-control optical signal contains an optical signal for carrying out surveillance and control of the optical transmission apparatus, the optical transmission line, and so on.  
         [0058]    The dropping signal from which the supervisory-control optical signal is extracted is amplified by the WDM preamplifier  323 , and is split by the 1×4 couplers  324 , and provided to the variable wavelength filters  325  through  328 , each of which extracts an optical signal of a respective predetermined wavelength to be dropped. An optical signal destined for the optical transmission apparatus concerned passes the variable wavelength filter  325 , and is monitored by the monitor  38 . Similarly, the optical signal transmitted by the optical transmission line  11  from East, and destined for the optical transmission apparatus concerned is monitored by the monitor  39 .  
         [0059]    Based on the output of the monitors  38  and  39 , the selector  37  selects one of the optical signals destined for the optical transmission apparatus concerned, and the selected optical signal for the optical transmission apparatus concerned is provided to the optical receiver OR  41 .  
         [0060]    In this example, the optical signals extracted by the variable wavelength filters  325  through  328  are provided to user terminals, and the like.  
         [0061]    Next, an explanation is made as to how an optical signal is inserted by the insertion unit  33 , and transmitted from the upward transmission unit  30  prepared in the optical transmission line  10  that transmits signals from West to East.  
         [0062]    The optical signal from the optical transmitter OS  40  is provided to the 1×2 coupler  36 , then to the single wave amplifier  338 , which amplifies the optical signal, and then to the 4×1 coupler  334 . Optical signals of the fixed wavelength lasers  42 ,  43 , and  44  are modulated by respective signals from the user terminals, and the like, amplified by the single wave amplifiers  335  through  337 , and supplied to the 4×1 coupler  334 .  
         [0063]    The 4×1 coupler  334  bundles the inserted optical signals. The WDM amplifier  333  amplifies the bundled optical signal. The rejection/add filter  331  further bundles the bundled optical signal and the passing signal that is branched by the optical branching coupler  321 . The further bundled optical signal is passed through the supervisory-control-signal insertion filter  332 , and transmitted to the optical transmission line  10  serving communications from West to East. Here, the WDM amplifier  333  is not essential, and can be eliminated.  
         [0064]    In this manner, the optical transmission apparatus having the optical add/drop function for the wavelength multiplex network according to the present invention includes the optical branching coupler  321  for dividing the input wavelength multiplexed optical signals into the passing signal and the dropping signal, the variable wavelength filters  325 - 328  for extracting the optical signals of the predetermined wavelengths from the dropping signal, the fixed wavelength lasers  42 - 44  for generating the optical signals to be inserted by the optical transmission apparatus concerned, the rejection filter  331  for blocking the optical signal of the same wavelength as the insertion wavelength from the passing signal, and the optical coupler  331  for coupling the passing signal and the inserted optical signal that is inserted at the insertion wavelength. In this manner, the optical path connection between desired nodes is established.  
         [0065]    That is, the optical path connection between desired nodes is attained by branching a desired wavelength out of the wavelength signals transmitted.  
         [0066]    Further, the optical attenuation of the transmitted optical signal in the optical transmission apparatus is reduced by using the rejection/add filter  331  that functions as a blocking filter for blocking an optical signal having the same wavelength as the insertion wavelength, and as an optical coupler for coupling the optical signal of insertion wavelength and the passing signal that is branched by the optical branching coupler.  
         [0067]    (Second Optical Transmission Apparatus)  
         [0068]    [0068]FIG. 3 shows another example of the optical transmission apparatus having the optical add/drop function according to the present invention. The present example is characterized by the insertion unit using variable wavelength lasers  62  through  64 . The optical transmission apparatus shown by FIG. 3 includes an upward transmission unit  50  prepared on the optical transmission line  10  from West to East, a downward transmission apparatus  51  prepared on the optical transmission line  11  from East to West, splitters  54  and  55  for splitting the optical monitor signal, an optical transmitter  60 , an optical receiver  61 , a 1×2 coupler  56  for splitting an optical signal from the optical transmitter OS  60 , monitors  58  and  59  for monitoring the state of the circuit, a selector  57  for selecting an optical signal of one of the two optical transmission lines  10  and  11  based on the monitoring result, and the variable wavelength lasers  62 ,  63 , and  64  for generating optical signals to be inserted. Here, the 1×2 coupler  56  may be replaced with a selector for selecting a signal of one of the two optical transmission lines  10  and  11  based on the monitoring result. Note that the optical transmitter OS  60  employs variable wavelength lasers.  
         [0069]    The upward transmission unit  50  includes a tee  52  and an insertion unit  53 . The tee  52  includes an optical branching coupler  521  for dividing an input optical signal into the passing signal that is to be passed through, and the dropping signal that is to be dropped, a supervisory-control-signal extraction filter  522  for extracting a supervisory control signal, a WDM preamplifier  523 , a 1×4 coupler  524  for splitting the dropping signal, and fixed wavelength filters  525  through  528  for extracting predetermined wavelengths.  
         [0070]    The insertion unit  53  includes a rejection/add filter  531 , a supervisory-control-signal insertion filter  532  for inserting a supervisory control signal, a WDM amplifier  533  for amplifying wavelength multiplexed optical signals, a 4×1 coupler  534  for coupling two or more optical signals, and single wave amplifiers  535  through  538 . Here, the WDM amplifier  533  is not essential, and can be eliminated. Further, the downward transmission unit  51  is configured similar to the upward transmission unit  50 .  
         [0071]    Although the number of branched wavelengths and the number of the inserted wavelengths are set at four in the present example, the numbers are not limited to four, but can be set as desired.  
         [0072]    The rejection/add filter  531  is for blocking a wavelength contained in the passing signal, which wavelength is the same as the inserted wavelength. This is necessary because the inserted wavelength should not agree with any of the wavelengths that are to be further passed through, and because the inserted wavelength should be prevented from propagating beyond a round on the loop transmission line ring.  
         [0073]    The rejection/add filter  531  is also for inserting (add) an optical signal to the transmission line, in addition to preventing the inserted wavelength from propagating beyond one round on the ring transmission line. Accordingly, attenuation of the optical signals caused by the optical transmission apparatus is minimized.  
         [0074]    Here, when the optical attenuation of the transmitted optical signal in the optical transmission apparatus is not a great concern, the rejection filter and the optical coupler for inserting optical signals in the transmission line can be independently provided like in the case of the first optical transmission apparatus described above (ref. FIG. 11).  
         [0075]    In the present embodiment, i.e., the second optical transmission apparatus shown by FIG. 3, the fixed wavelength filters  525  through  528 , and the variable wavelength lasers  62 ,  63 , and  64  are used, while the first optical transmission apparatus shown by FIG. 2 uses the variable wavelength filters  325  through  328 , and the fixed wavelength lasers  42 ,  43 , and  44 , respectively. Accordingly, operations of the second optical transmission apparatus shown by FIG. 3 are similar to those of the first optical transmission apparatus of FIG. 2, and explanation thereof is omitted.  
         [0076]    As described above, the second optical transmission apparatus having the optical add/drop function in the wavelength multiplex network includes the optical branching coupler for dividing the input wavelength multiplex optical signal into the passing signal that is to be passed through, and the dropping signal that is to be dropped, the fixed wavelength filter for extracting the optical signals of predetermined wavelengths from the dropping signal, the variable wavelength lasers for generating the optical signals to be inserted by the optical transmission apparatus concerned, the rejection filter for blocking the optical signal of the same wavelength as the insertion wavelength from the passing signal, the optical coupler for coupling the passing signal that is passed through the rejection filter, and the optical signal that is inserted. Accordingly, the optical path connection between desired nodes is established.  
         [0077]    That is, the optical path is established by setting the wavelength of the inserted signal to be in agreement with the wavelength of the branch optical signal filter of a node serving as the counterpart of the communications. In this case, the broadcasting function is restricted.  
         [0078]    Further, the optical attenuation of the transmitted optical signal in the optical transmission apparatus can be minimized by using the rejection/add filter that blocks the optical signal of the same wavelength as the insertion wavelength, and couples the inserted optical signal and the passing signal that is branched by the optical branching coupler.  
         [0079]    According to the embodiments having the configurations shown by FIG. 2 and FIG. 3, a low-cost system can be built by employing low-cost wavelength filters, laser optical signal sources, and rejection filters.  
         [0080]    Further, according to the embodiments having the configurations shown by FIG. 2 and FIG. 3, an optical path can be provided as desired by provisioning.  
         [0081]    Further, according to the embodiment having the configurations shown by FIG. 2 and FIG. 3, the insertion loss of the optical signal that is passed through the node is minimized, and an in-line amplifier can be eliminated, reducing the network cost.  
         [0082]    Further, according to the embodiments shown by FIG. 2 and FIG. 3, the configuration can be changed and upgraded according to the required number of wavelengths, providing a low initial cost when a small number of wavelengths are initially installed.  
         [0083]    Further, according to the configurations shown by FIG. 2 and FIG. 3, even in the case that the number of wavelengths is 32 or smaller, the initial cost is lowered, and, further, the network configuration is flexible.  
         [0084]    Here, various kinds of variable wavelength filters can be used as the variable wavelength filters  325  through  328  of the configuration shown by FIG. 2.  
         [0085]    [0085]FIG. 4 shows an example, wherein an AOTF (Acousto-Optic Tunable Filter)  329  is employed as the variable wavelength filters  325  through  328 .  
         [0086]    The AOTF  329  selects an optical wavelength by an RF signal (electrical signal) being applied, the RF signal corresponding to the wavelength that is to be dropped. For example, when the wavelength multiplex optical signal containing wavelengths λ 1  through λ n  is input into the AOTF  329 , and if the wavelengths λ 1  through λ 4  are to be extracted from the wavelengths λ 1  through λ n , RF signals having frequencies f 1  through f 4  corresponding to the wavelengths λ 1  through λ 4 , respectively, are applied to the AOTF  329 .  
         [0087]    Similarly, as the variable wavelength filters  325  through  328 , a dielectric multilayer filter, a FGB (optical fiber Bragg-diffraction grid) type filter, and a Fabry-Perot type filter can be used.  
         [0088]    (Wavelength Group)  
         [0089]    [0089]FIG. 5 shows an example of the network configuration, wherein a wavelength group (for example, a group of four wavelengths) is assigned to each node. The network shown at (A) of FIG. 5 includes an optical loop circuit  69 , a HUB  70 , and optical transmission apparatuses (nodes)  72  through  77 . Here, two or more wavelength groups may be assigned to each of the optical transmission apparatuses  72  through  77 .  
         [0090]    Each of the HUB  70  and the optical transmission apparatuses  72  through  77  branches (drops) and adds (inserts) optical signals transmitted in the optical loop circuit  69  based on the wavelength group, i.e., in units of wavelength groups, thereby providing communications between the HUB  70  and the optical transmission apparatuses  72  through  77 , and communications between the optical transmission apparatuses  72  through  77 . In this manner, communications are provided between user terminals connected to the HUB  70  and the optical transmission apparatuses  72  through  77 .  
         [0091]    The wavelength groups are defined as shown at (B) of FIG. 5, each wavelength group including four wavelengths in the present example. Group 1 includes the wavelengths λ 1  through λ 4 , Group 2 includes the wavelengths λ 5  through λ 8 , and so on up to Group 7 that includes the wavelengths λ 25  through λ 28 . Although an illustration is omitted, Group 8 including wavelengths λ 29  through λ 32  is assigned to the HUB  70 . Each wavelength has an interval of 100 GHz. In addition, an insertion wavelength can be selected as desired, not limited to the plan of the present example.  
         [0092]    As shown at (A) of FIG. 5, the optical transmission apparatuses  71  through  77  are set up so that the Groups 1 through 7, respectively, are inserted. Further, the HUB  70  can branch and insert all the Groups.  
         [0093]    That is, the wavelengths to be inserted are fixed to each of the optical transmission apparatuses  71  through  77 ; however, the optical transmission apparatuses  71  through  77  can branch (drop) any wavelength as desired.  
         [0094]    Communication modes of the network shown by FIG. 5 are explained using FIG. 6. At (A) of FIG. 6, communications are carried out from the HUB  70  to each of the nodes (the optical transmission apparatuses  71  through  77 ), using four wavelengths per node. Here, the HUB  70  transmits optical signals at the wavelengths assigned to Group 1, Group 2, and up to Group 7, which are received by the nodes  71 ,  72  and up to  77 , respectively.  
         [0095]    At (B) of FIG. 6, communications are carried out to the HUB  70  from each of the nodes  71 ,  72  and up to  77 , each node using four wavelengths. Here, the nodes  71 ,  72  and up to 77 transmit optical signals at the wavelengths assigned to Group 1, Group 2 and up to Group 7, respectively. The HUB  70  receives all the optical signals transmitted by the wavelengths of all the Groups.  
         [0096]    At (C) of FIG. 6, the case is shown where communications between each node and the HUB and between the nodes are carried out. In this example, the HUB  70  communicates with the node  72  by the HUB  70  transmitting optical signals at the wavelengths assigned to Group 8, and receiving optical signals at the wavelengths assigned to Group 2; and the node  72  receiving the optical signals at the wavelengths assigned to Group 8, and transmitting the optical signals at the wavelengths assigned to Group 2. Further, in this example, the node  77  communicates with the node  72  by the node  77  transmitting optical signals at the wavelengths assigned to Group 7, and receiving optical signals at the wavelengths assigned to Group 2; and the node  72  receiving the optical signals at the wavelength assigned to Group 7, and transmitting the optical signals at the wavelength assigned to Group 2.  
         [0097]    At (D) of FIG. 6, the case where broadcasting communications are carried out from the HUB  70  to each node is shown. An optical signal at a wavelength assigned to Group 8 is transmitted from the HUB  70 , and is received by the nodes  71  through  77 .  
         [0098]    At (E) of FIG. 6, the case is shown where broadcasting communications are carried out from a node to all the nodes including the HUB  70 . An optical signal at a wavelength assigned to Group 2 is transmitted to the node  72 , and is received by the HUB  70  and the nodes  71 , and  73  through  77 .  
         [0099]    Branching (drop) and insertion (add) of wavelengths in units of wavelength groups performed by each optical transmission apparatus are explained with reference to FIG. 7.  
         [0100]    An optical transmission unit  80  shown by FIG. 7 includes a tee  81 , insertion units  82  and  83 , and an optical amplifier  84 . The tee  81  includes a coupler  811  for dividing an input signal into two parts, namely a passing signal that is to be passed through, and a dropping signal that is to be dropped, a 1×8 coupler  812  for splitting the dropping signal into eight optical signals, and AOTFs  813  and  814  prepared for every four wavelengths. The insertion unit  82  includes a rejection/add filter  821 , a 4×1 coupler  822  for coupling four input optical signals, and single wave amplifiers  823  through  826  prepared for every wavelength. Similarly, the insertion unit  83  includes a rejection/add filter  832 , a 4×1 coupler  833  for coupling four input optical signals, and single wave amplifiers  834  through  837  prepared for every wavelength. The single wave amplifiers  823  through  826 , and  834  through  837  amplify optical signals from laser diodes  828  through  831  and  839  through  842 , respectively, the optical signals being modulated by signals from user terminals, etc. Further, AOTFs  813  and  814  are controlled such that eight desired wavelengths are extracted.  
         [0101]    In this manner, the wavelength multiplexed optical signal that is input to the optical transmission unit  80  is split by the tee  81  into the dropping signal and the passing signal, optical signals are inserted by the insertion units  82  and  83 , and output from the optical transmission unit  80 . Here, the insertion units  82  and  83  add the optical signals at the wavelengths λ1 through λ4 and the wavelengths λ5 through λ8, respectively, to the passing signal supplied by the tee  81 . For the optical signals at the wavelengths λ1 through λ4 and the wavelengths λ5 through λ8 to be added, the rejection/add filters  821  and  832 , respectively, block optical signals that are contained in the passing signal and have the same wavelength as a wavelength to be inserted.  
         [0102]    Further, the dropping signal that is split by the tee  81  is amplified by the optical amplifier  84 , is further split into eight optical signals by the 1×8 coupler  812 , which eight signals are supplied to the AOTF  813  and  814 . The AOTF  813  and  814  pass desired wavelengths based on control from the outside. Then, the optical signals at desired wavelengths λ i  through λ p  are obtained from the AOTF  813  and  814  that are set at the desired wavelengths to drop. The optical signals are provided to user terminals, and the like.  
         [0103]    Thus, by configuring the optical transmission apparatus such that branching (drop) and inserting (add) are carried out on the wavelength group basis as described above, the rejection filter can be designed to block a certain group of wavelengths, and the following four advantages are also obtained.  
         [0104]    (1) Since a group of wavelengths is assigned to each node, the wavelength range required of the fixed wavelength laser or the fixed wavelength filter of the node is limited by the definition of the wavelength group. Accordingly, requirements for the amplification characteristics of the optical amplifiers arranged in front of the fixed wavelength filter or after the fixed wavelength laser of the node are relaxed, i.e., a low-cost amplifier capable of only narrower-band amplification can be used. Since the optical amplifiers represent a significant portion of the cost of the node, the relaxed frequency bandwidth requirements greatly contribute to reducing the cost.  
         [0105]    (2) Since a group of wavelengths is assigned to each node, wavelength assignment is simplified, and network operations become simple.  
         [0106]    (3) Since the rejection filter handles wavelengths that are grouped, rather than handling discrete wavelengths, the cost thereof can be lowered.  
         [0107]    (4) Since expansion of the system is carried out in increments of wavelength groups, system design becomes simple.  
         [0108]    (Protection)  
         [0109]    Next, protection, which is available when using the double optical loop network as shown in FIG. 1, is explained.  
         [0110]    In the following example, the protection function is provided to the HUB; however, the protection function may be provided to a node other than the HUB.  
         [0111]    [0111]FIG. 8 shows an example of a HUB that employs an optical demultiplexer, an optical multiplexer, and optical switches. The HUB includes supervisory-control-signal extraction filters  100  and  200  for extracting a supervisory control signal, WDM amplifiers  101 ,  113 ,  201 , and  213  for amplifying a wavelength multiplexed optical signal, optical demultiplexers  102  and  202 , monitors  103 ,  104 ,  105 ,  203 ,  204 , and  205  for monitoring wavelengths that are split, 1×2 couplers  106 ,  107 ,  108 ,  206 ,  207 , and  208  for dividing an input wavelength multiplexed optical signal into a passing signal and a dropping signal, 2×1 switches  109 ,  110 ,  111 ,  209 ,  210 , and  211  for inserting optical signals at insertion wavelengths, optical multiplexers  112  and  212 , an optical receiver  116 , a 2×1 switch  115  for selecting a signal from one of the upward and downward transmission lines, an optical transmitter  118 , and a 1×2 coupler  117  for splitting the signal from the optical transmitter  118 .  
         [0112]    The 1×2 coupler  117  splits an optical signal from the optical transmitter  118  into two optical signals. One of the divided optical signals is provided to the upward transmission line (from West to East) via the 2×1 switches  109 ,  110 , and  111 . The other of the divided optical signals is provided to the downward transmission line (from East to West) via the 2×1 switches  209 ,  210 , and  211 .  
         [0113]    The optical signal received from the upward transmission line is provided to the 2×1 switch  115  through the 1×2 couplers  106 ,  107 , and  108 . Further, the optical signal received from the downward transmission line is provided to the 2×1 switch  115  through the 1×2 couplers  206 ,  207 , and  208 . Then, the 2×1 switch  115  selects one of the upward and downward transmission lines for receiving the optical signal, and the selected optical signal is received by the optical receiver  116 .  
         [0114]    In this manner, the 2×1 switch  115  is controlled such that use of a faulty transmission line is avoided based on the monitoring result, and communications are provided using a normal transmission line.  
         [0115]    Here, the 1×2 couplers  106 ,  107 , and  108  and 1×2 couplers  206 ,  207 , and  208  may be replaced with 2×1 switches, and the 2×1 switch  115  may be replaced with a 2×1 coupler.  
         [0116]    In this manner, each optical transmission apparatus can operate using one of the transmission lines as an operating circuit, while the other transmission line serves as the standby circuit. Further, when a circuit or an optical transmission apparatus becomes faulty, the above configuration allows communications to continue by avoiding the faulty circuit or faulty optical transmission apparatus, reducing the influence due to the fault to a minimum. Accordingly, the 1×2 couplers  106 ,  107 , and  108 , 1×2 couplers  206 ,  207 , and  208 , and the 2×1 switch  115  constitute a protection unit.  
         [0117]    [0117]FIG. 9 shows an example of a HUB that includes an optical filter and an optical group filter. The HUB includes an upward transmission unit  128 , a downward transmission unit  129 , monitors  140  and  141  each monitoring a single wavelength, 1×2 couplers  143 ,  147 , and  150 , 2×1 switches  142 ,  146 , and  149 , an optical receiver  148 , and an optical transmitter  144 .  
         [0118]    The upward transmission unit  128  includes a tee  130  and an insertion unit  131 . The tee  130  includes optical group filters  132 ,  133 , and  134  for extracting wavelengths of a predetermined wavelength group. The insertion unit  131  includes optical group filters  135 ,  136 , and  137  for inserting wavelengths of a predetermined wavelength group, and for blocking wavelengths that are the same as the wavelengths of the predetermined wavelength group.  
         [0119]    Further, the optical group filter  134  for extracting wavelengths of a predetermined wavelength group includes a wavelength group filter  1341  for extracting the wavelengths of the predetermined wavelength group, and single wavelength filters  1342  through  1345 , each extracting a single wavelength. Other optical group filters  132  and  133  are configured the same as the optical group filter  134 .  
         [0120]    Further, the optical group filter  137  for inserting wavelengths, and for blocking the same wavelengths as the wavelengths that are inserted includes a wavelength group filter  1371  for inserting the wavelengths, and for blocking the same wavelengths as the wavelengths inserted, and single wavelength filters  1372  through  1375 , each inserting and blocking a single wavelength. Other optical group filters  135  and  136  are configured the same as the optical group filter  137 .  
         [0121]    In this manner, the same functions as the HUB shown by FIG. 8 are obtained.  
         [0122]    [0122]FIG. 10 shows an example of a HUB that includes an optical demultiplexer, an optical multiplexer, and a MEMS (Micro Electro-Mechanical Systems) switch. The HUB includes WDM amplifiers  170 ,  174 ,  270 , and  274  for amplifying wavelength multiplexed optical signals, optical demultiplexers  171  and  271 , MEMS switches  172  and  272 , optical multiplexers  173  and  273 , monitors  175  and  275  for monitoring optical signals in units of wavelengths, 1×2 couplers  176  and  276 , filters  177  and  277  for extracting monitoring signals, a 2×1 switch  180 , an optical transmitter  179 , and an optical receiver  181 .  
         [0123]    In this manner, the same functions as the configuration shown by FIG. 8 are obtained.  
         [0124]    As described above, the present invention provides the optical transmission apparatus, and the optical wavelength multiplex network employing the optical transmission apparatus that reduce the initial cost, and are flexible in network configuration.  
         [0125]    Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.  
         [0126]    The present application is based on Japanese priority application No. 2003-019067 filed on 1, 28, 2003 with the Japanese Patent Office, the entire contents of that are hereby incorporated by reference.