Abstract:
An optical transmission node including an optical preamplifier to amplify input light and an optical postamplifier to amplify light output from the optical preamplifier, includes the optical postamplifier configured to generate amplified spontaneous emission light without signals input, the optical preamplifier configured to amplify the amplified spontaneous emission light from the optical postamplifier, a loopback switch configured to discouple a path of the light output from the optical preamplifier to the optical postamplifier, and couple a path of the light output from the optical postamplifier to the optical preamplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-143060, filed on Jun. 16, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical transmission apparatus that conducts optical transmission, as well as an optical signal level checking method. 
     BACKGROUND 
     When starting up an optical transmission apparatus and node for the first time, work is carried out to configure individual plugin units and check device operation. 
     For example, in an optical amp, gain is configured by utilizing amplified spontaneous emission (ASE). This involves causing ASE to be produced from an optical amp at a node positioned upstream along the optical fiber line, and then causing the ASE to be input into an optical amp at a node positioned downstream. Subsequently, the gain is configured such that the optical output level from the downstream amp reaches a desired level. 
     Related technology for starting up an apparatus by utilizing ASE is proposed in, for example, Japanese Unexamined Patent Application Publication No. 2004-23437. 
     Besides configuring the gain in the optical amp, another important step when first starting up a system that conducts optical transmission using wavelength-division multiplexing (WDM) is checking whether or not optical signals are being output at normal levels after wavelength separation in the apparatus. However, in the related art, it has been difficult to precisely check the normality of optical signal levels after wavelength separation utilizing ASE. 
       FIG. 11  illustrates a WDM transmission apparatus.  FIG. 11  illustrates the portion of a WDM transmission apparatus  5  configured to receive and separate a WDM optical signal multiplexed with light having a plurality of respectively different wavelengths. The WDM transmission apparatus  5  includes the following plugin units: an optical preamp unit  51 , and a wavelength separation unit  52 . The optical preamp unit  51  includes a preamp  51   a  and a coupler  51   b . Meanwhile, the wavelength separation unit  52  includes a DMUX  52   a , couplers  52   b - 1  to  52   b - n , and photodiodes (PD)  52   c - 1  to  52   c - n.    
     The preamp  51   a  amplifies and outputs a received WDM signal flowing in from upstream along an optical fiber line F. The amplified WDM optical signal is split into two parts by the coupler  51   b , with one part being provided to a post-processor, and the other part being provided to the wavelength separation unit  52 . 
     The DMUX  52   a  separates the received WDM optical signal into n wavelengths. Each of the couplers  52   b - 1  to  52   b - n  then splits the optical signal for one of the wavelengths into two parts, with one part being provided to one of the PDs  52   c - 1  to  52   c - n , and the other part being directed to a tributary and dropped. Each of the PDs  52   c - 1  to  52   c - n  generates an electrical signal by O/E converting the received optical signal for one of the wavelengths. The generated electrical signals are then provided to a predetermined processor. 
     When starting up the WDM transmission apparatus  5  herein for the first time, the configuring and checking work is conducted individually for the respective plugin units (i.e., the optical preamp unit  51  and the wavelength separation unit  52 ). 
     Work performed with respect to the optical preamp unit  51  when starting up the apparatus may involve, for example, causing ASE provided from an upstream node and flowing along the optical fiber line F to be input into the preamp  51   a , and configuring the gain in the preamp  51   a.    
     Meanwhile, work performed with respect to the wavelength separation unit  52  when starting up the apparatus may involve, for example, checking whether or not the optical signals are being output (or dropped) at normal levels after the wavelength separation in the DMUX  52   a . In this case, it can be checked whether or not the optical signals at the respective wavelengths are at normal levels by examining the electrical signal levels after O/E conversion in each of the PDs  52   c - 1  to  52   c - n.    
     In order to precisely check the normality of the optical signal levels after wavelength separation, it is preferable for the optical signal input into the DMUX  52   a  to have an optical power that is close to the power of a WDM optical signal received during normal operation. In other words, it is preferable for the optical signal input into the DMUX  52   a  to have an optical power in the WDM optical signal wavelength band that is nearly identical to the optical power when receiving a WDM optical signal during normal operation. 
     When inputting ASE into the wavelength separation unit  52 , the ASE provided from an upstream node travels along the optical fiber line F, and thus its optical power is extremely low upon arrival. For this reason, in order to make the optical power close to the optical power when receiving a WDM optical signal during normal operation, the low-power ASE is amplified to high-power ASE in the preamp  51   a.    
     However, if the gain is increased to raise the low-power ASE flowing in along the optical fiber line F to an optical power nearly equal to the optical power when receiving a WDM optical signal during normal operation, the shape of the wavelength profile becomes significantly sloped. 
       FIG. 12  illustrates such sloping being produced in the shape of the wavelength profile. In  FIG. 12 , the horizontal axis expresses the optical power (in dBm), while the vertical axis expresses the wavelength (in nm). If a standard erbium-doped fiber amplifier (EDFA) is used as the optical amp, then as the gain of the EDFA is raised, population inversion also rises. It has been established that in such a state with high population inversion, the optical power with respect to the wavelength slope down and to the right in the region corresponding to the WDM optical signal wavelength band, such as near the range from 1530 nm to 1570 nm. (In contrast, if the gain is lowered to create a low population inversion state, then the optical power with respect to the wavelength slope up and to the right.) Meanwhile, it has been established that flatness is obtained when the population inversion is kept to approximately 70%. 
       FIG. 13  illustrates a WDM transmission apparatus.  FIG. 13  illustrates the outputting of ASE produced by a sloped wavelength profile. In the wavelength profile pr 1  at the input stage of the preamp  51   a , the ASE  2  provided from an upstream node has a flat optical power in the WDM optical signal wavelength band (i.e., the optical power is uniform at those wavelengths), but the optical power is also very low overall. 
     Given such ASE  2 , if the gain in the preamp  51   a  is then increased to amplify the ASE  2  to an optical power close to the optical power when receiving a WDM optical signal during normal operation, the wavelength profile of the ASE  2   a  output from the preamp  51   a  loses much of its flatness. In other words, as illustrated in the wavelength profile pr 2 , the resulting ASE is high-power, but the portion that was flat in the wavelength profile pr 1  now slopes down and to the right. 
     If the ASE  2   a  having such a wavelength profile pr 2  is input into the DMUX  52   a , then even if the optical signal levels after wavelength separation are monitored, differences in the optical levels at respective wavelengths are already produced before wavelength separation. For this reason, strict checking of the normality of the optical signal levels after wavelength separation becomes problematic. 
     Meanwhile, it is also conceivable to check the normality of optical signal levels after wavelength separation by forgoing use of the ASE  2  provided from the upstream node, putting the preamp  51   a  into an input-less state, and inputting ASE produced by the preamp  51   a  itself into the DMUX  52   a.    
     In this case, it is still preferable for the ASE input into the DMUX  52   a  to have an optical power close to the optical power when receiving a WDM optical signal during normal operation. Thus, the preamp  51   a  amplifies the self-produced ASE to a predetermined level before output. 
     However, outputting ASE with an optical power close to the incoming level of a WDM optical signal during normal operation from a preamp in an input-less state is dependent upon the amplification performance of the preamp  51   a , and thus imposes restrictions on the manufacturing specifications of the preamp  51   a  itself. This method is not readily applicable to an arbitrary preamp. 
     Even if it were hypothetically possible to increase gain and output ASE that has been amplified to a predetermined level from the preamp  51   a  in an input-less state without imposing restrictions on the manufacturing specifications of the optical preamp unit  51   a  itself, a large gain still be set in the preamp  51   a , and thus the ASE output from the preamp  51   a  have a wavelength profile that has lost some of its flatness, like the above wavelength profile pr 2 . Consequently, it is still difficult to strictly check the normality of optical signal levels after wavelength separation, even with the method of producing ASE from the preamp  51   a  itself. 
     As described above, in a WDM transmission apparatus of the related art, it is difficult to produce ASE having a flat and high-output wavelength profile, and thus it is also difficult to precisely check the normality of optical signal levels after wavelength separation. 
     Being devised in light of such points, one object of the preprovided invention is to provide an optical transmission apparatus and an optical signal level checking method that enable the normal operation of apparatus functions to be checked with high precision by utilizing ASE. 
     SUMMARY 
     An optical transmission node including an optical preamplifier to amplify input light and an optical postamplifier to amplify light output from the optical preamplifier, includes the optical postamplifier configured to generate amplified spontaneous emission light without signals input, the optical preamplifier configured to amplify the amplified spontaneous emission light from the optical postamplifier, a loopback switch configured to discouple a path of the light output from the optical preamplifier to the optical postamplifier, and couple a path of the light output from the optical postamplifier to the optical preamplifier. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 2  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 3  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 4  illustrates an ASE spectrum; 
         FIG. 5  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 6  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 7  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 8  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 9  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 10  illustrates an exemplary configuration of an optical transmission apparatus; 
         FIG. 11  illustrates a WDM transmission apparatus; 
         FIG. 12  illustrates how sloping is produced in the shape of a wavelength profile; and 
         FIG. 13  illustrates a WDM transmission apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the accompanying drawings.  FIG. 1  illustrates an exemplary configuration of an optical transmission apparatus. The optical transmission apparatus  10  transmits optical signals, and is provided with a preamp (i.e., an optical preamplifier)  11 , a postamp (i.e., an optical postamplifier)  12 , a loopback switch  13 , and a controller  14 . 
     The loopback switch  13  includes a switch sw 1  (a first switch), a switch sw 2  (a second switch), and a switch sw 3  (a third switch). The switch sw 1  includes input terminals a 1  and a 2 , as well as an output terminal a 3 . The switch sw 2  includes an input terminal b 1  and an output terminal b 2 . The switch sw 3  includes an input terminal c 1 , as well as output terminals c 2  and c 3 . 
     Herein, the switch sw 1  selects either an optical signal provided from an upstream node via the optical fiber line F 1   a , or the light output from the postamp  12 , and inputs the selection into the preamp  11 . The switch sw 2  selects the light output from the preamp  11 , and either inputs the output light into the postamp  12 , or blocks the light. The switch sw 3  selects the light output from the postamp  12 , and provides the output light to either a downstream node via an optical fiber line F 1   b , or to the preamp  11 . 
     The switches sw 1  to sw 3  are coupled as follows. The input terminal a 1  of the switch sw 1  is coupled to the optical fiber line F 1   a , while the output terminal a 3  of the switch sw 1  is coupled to the input port of the preamp  11 . The input terminal b 1  of the switch sw 2  is coupled to the output port of the preamp  11 , while the output terminal b 2  of the switch sw 2  is coupled to the input port of the postamp  12 . The input terminal c 1  of the switch sw 3  is coupled to the output port of the postamp  12 , while the output terminal c 2  of the switch sw 3  is coupled to the optical fiber line F 1   b . The output terminal c 3  of the switch sw 3  is coupled to the input terminal a 2  of the switch sw 1 . 
     Herein, the preamp  11  amplifies input light. The light output from the preamp  11  is then input into the postamp  12  and amplified. The loopback switch  13  switches the switches sw 1  to sw 3  to perform loopback processing, wherein light output from the postamp  12  is looped back and input into the preamp  11  (the switching operation will be described later). 
     The controller  14  issues loopback processing instructions (i.e., switching instructions) to the loopback switch  13 . Herein, the controller  14  also includes controls for other elements of the optical transmission apparatus  10 , and conducts overall control of the apparatus itself. Furthermore, the user interface functions are also included, whereby the controller  14  couples to a maintenance terminal and performs actions such as issuing notifications regarding the operational status of the apparatus to the maintenance terminal, and configuring settings (such as switching settings) from data provided externally via the maintenance terminal. 
     During loopback processing, the loopback switch  13  sets the switches such that the switch sw 3  provides the light output from the postamp  12  to the preamp  11 , the switch sw 1  inputs the light output from the postamp  12  into the preamp  11 , and the switch sw 2  blocks the light output from the preamp  11  from being input into the postamp  12 . 
     By performing such switching control, ASE  1  (i.e., light by amplified spontaneous emission) from the postamp  12  is produced, and subsequently input into the preamp  11  and amplified. ASE  1   a  is then output from the preamp  11 , the ASE  1   a  having wavelength characteristics (i.e., a wavelength profile) such that the optical power is both high-output and flat (i.e., uniform) for all wavelengths in the wavelength band of optical signals transmitted by the optical transmission apparatus  10 . 
     The case of applying the optical transmission apparatus  10  to an apparatus conducting WDM transmission will now be described.  FIG. 2  illustrates an exemplary configuration of an optical transmission apparatus. The optical transmission apparatus  10 - 1  transmits WDM optical signals, includes optical add-drop multiplexing (OADM) functions, and is provided with a preamp  11 , a postamp  12 , a loopback switch  13 , a controller  14 , a DMUX (i.e., a wavelength demultiplexer)  15 , a MUX (i.e., a wavelength multiplexer)  16 , PDs  17 - 1  to  17 - n , a coupler Cp 1 , and couplers Cp 2 - 1  to Cp 2 - n.    
     The loopback switch  13  includes the switches sw 1  to sw 3  described above. The switches sw 1  to sw 3  are coupled as follows. The input terminal a 1  of the switch sw 1  is coupled to the optical fiber line F 1   a , while the output terminal a 3  of the switch sw 1  is coupled to the input port of the preamp  11 . The input terminal b 1  of the switch sw 2  is coupled to the output port of the MUX  16 , while the output terminal b 2  of the switch sw 2  is coupled to the input port of the postamp  12 . The input terminal c 1  of the switch sw 3  is coupled to the output port of the postamp  12 , while the output terminal c 2  of the switch sw 3  is coupled to the optical fiber line F 1   b . The output terminal c 3  of the switch sw 3  is coupled to the input terminal a 2  of the switch sw 1 . 
     Normal operation will now be described on the basis of  FIG. 2 . During normal operation, switching control is conducted as follows. The switch sw 1  is switched such that the input terminal a 1  is coupled to the output terminal a 3 , and a WDM optical signal flowing in along the optical fiber line F 1   a  is input into the preamp  11 . 
     The switch sw 2  is switched such that the input terminal b 1  is coupled to the output terminal b 2 , and light output from the preamp  11  is input into the postamp  12 . The switch sw 3  is switched such that the input terminal c 1  is coupled to the output terminal c 2 , and light output from the postamp  12  is provided via the optical fiber line F 1   b . The switching control at this point is automatically set on the basis of switching instructions from the controller  14 . 
     A WDM optical signal flowing in from upstream along the optical fiber line F 1   a  is input into the preamp  11 . The preamp  11  amplifies and outputs the WDM optical signal. The coupler Cp 1  splits the amplified WDM optical signal into two parts, with one part being provided to the MUX  16 , and the other part being provided to the DMUX  15 . 
     The DMUX  15  separates the received WDM optical signal into n wavelengths, and outputs the results. Each of the couplers Cp 2 - 1  to Cp 2 - n  then splits the optical signal for one of the wavelengths into two parts, with one part being provided to one of the PDs  17 - 1  to  17 - n , and the other part being directed to a tributary and dropped. Each of the PDs  17 - 1  to  17 - n  generates an electrical signal by O/E converting the received optical signal for one of the wavelengths. The generated electrical signals are then provided to a predetermined processor. 
     The MUX  16  uses wavelength-division multiplexing to multiplex the received WDM optical signal with added optical signals, thereby generating and outputting a new WDM optical signal. The postamp  12  amplifies the WDM optical signal output from the MUX  16 , and provides the result downstream via the optical fiber line F 1   b.    
     Operation during the loopback processing conducted at apparatus startup, for example, will now be described on the basis of  FIG. 3 .  FIG. 3  illustrates an exemplary configuration of the optical transmission apparatus  10 - 1 . During loopback processing, switching control is conducted as follows. The switch sw 1  is switched such that the input terminal a 2  is coupled to the output terminal a 3 , and light output from the postamp  12  is input into the preamp  11 . 
     The switch sw 2  is switched such that the input terminal b 1  and the output terminal b 2  are discoupled, and light output from the preamp  11  is blocked from being input into the postamp  12 . The switch sw 3  is switched such that the input terminal c 1  is coupled to the output terminal c 3 , and light output from the postamp  12  is provided to the preamp  11 . The switching control at this point is automatically set on the basis of switching instructions from the controller  14 . 
     The postamp  12  enters an input-less state, and produces ASE  1  in the WDM optical signal wavelength band. The ASE  1  is looped back and input into the preamp  11 . The preamp  11  amplifies the ASE  1  to an optical power close to the optical power when receiving a WDM optical signal during normal operation, and then outputs the amplified ASE  1   a.    
     The DMUX  15  separates the received ASE  1   a  into n wavelengths, and outputs the results. Each of the couplers Cp 2 - 1  to Cp 2 - n  then splits the optical signal for one of the wavelengths into two parts, with one part being provided to one of the PDs  17 - 1  to  17 - n . Each of the PDs  17 - 1  to  17 - n  generates an electrical signal by O/E converting the received optical signal for one of the wavelengths of the ASE  1   a . The optical signal levels after wavelength separation are then checked from the levels of the electrical signals output from the PDs  17 - 1  to  17 - n.    
     Herein, the ASE  1  produced by the postamp  12  has the wavelength profile Pr 1 , wherein the optical power is low and flat for all wavelengths in the WDM optical signal wavelength band. The ASE  1  is input into the preamp  11  and amplified to produce the ASE  1   a . The ASE  1   a  output from the preamp  11  has the wavelength profile Pr 2 , wherein the optical power is high and flat for all wavelengths in the WDM optical signal wavelength band. 
       FIG. 4  illustrates the spectrum of the ASE  1 . Herein, a standard optical amp (EDFA) is used in the preamp  11  and the postamp  12 . In  FIG. 4 , the horizontal axis expresses the optical power (in dBm), while the vertical axis expresses the wavelength (in nm).  FIG. 4  illustrates the spectrum of the ASE  1  produced by the postamp  12  (i.e., the wavelength profile Pr 1 ). in addition, operational parameters of the postamp  12  are set such that −6.4 dBm ASE is produced without input when the output is set to +0.6 dBm/ch. 
     If the output from the postamp  12  is set such that the primary signal power becomes +0.6 dBm per channel (i.e., per wavelength), then the spectrum of the postamp  12  when −6.4 dBm ASE is produced without input becomes like that illustrated in  FIG. 4 . As illustrated in  FIG. 4 , the resulting wavelength profile is low in optical power, but nearly flat in the WDM optical signal wavelength band. 
     Meanwhile, during normal operation, assume that a WDM optical signal multiplexed from a maximum of 40 channels is input into the preamp  11 . Assume also that the operational parameters of the preamp  11  are set such that the output is +0.6 dBm/ch. 
     The optical power equivalent to a WDM optical signal multiplexed from 40 channels of optical signals thus becomes +16.6 dBm (=+0.6+10 log ((40 channels*1 mW)/1 mW)=+0.6+10 log 40). When causing ASE  1   a  equivalent to 40 channels to be produced from the preamp  11 , the power of the ASE  1   a  thus becomes +16.6 dBm (herein calculated by taking each channel as a 1 mW input). 
     Consequently, in order to produce the optical power of a WDM optical signal multiplexed from 40 channels of optical signals by looping back the ASE  1  into the input of the preamp  11 , the gain in the preamp  11  becomes 23 dB (=16.6−(−6.4)). A value equal to or in the vicinity of 23 dB corresponds to a gain that can be produced by an EDFA while keeping the population inversion to approximately 70%. A flat wavelength profile in the WDM optical signal wavelength band can thus be maintained with such a gain value. Furthermore, such a gain value is a typical gain included in the gain range usually covered by an EDFA, and thus a flat wavelength profile in the WDM optical signal wavelength band can be maintained. 
     The ASE  1   a  output from the preamp  11  is input into the DMUX  15 , and wavelength separation is conducted. Therefore, ASE  1   a  is input into the DMUX  15  having an optical power that is both close to the optical power when receiving a WDM optical signal during normal operation, as well as being flat (i.e., uniform) over all wavelengths in the WDM optical signal wavelength band. Thus, since ASE considered equal to a WDM optical signal during normal operation can be input into the DMUX  15 , it becomes possible to check the normality of the apparatus with high precision. 
     As described earlier with reference to  FIG. 13 , in the case of the related art, ASE  2  provided from an upstream node is input into the preamp. However, since this ASE  2  is transmitted along an optical fiber line, the optical power of the ASE  2  is lower than the optical power of the ASE  1  output from the postamp  12  in the optical transmission apparatus  10 - 1  by approximately 20 dB, for example. 
     Since the optical power of the ASE  2  has such an extremely faint level, using the preamp  11  to amplify the ASE  2  to an optical power equal to the optical power when receiving a WDM optical signal during normal operation involves a gain of over 43 dB (=23 dB+20 dB). 
     If the preamp  11  amplifies and outputs the ASE  2  at a high gain of over 43 dB, then as described earlier with reference to  FIGS. 12 and 13 , the output have the wavelength profile pr 2 , which slopes down and to the right with respect to wavelength. For this reason, such a configuration is unsuitable for use in checking the operation of component units such as the DMUX. 
     As described above, a preamp  11  and a postamp  12  provided in the same apparatus are configured such that ASE  1  produced by the postamp  12  is looped back into the preamp  11  and amplified, thereby producing ASE  1   a  having a high-output and flat wavelength profile. 
     By utilizing such ASE  1   a , it becomes possible to check the operation of individual function units in the apparatus with high precision. For example, faults such as damage to the optical fiber between the preamp  11  and the DMUX  15  or damage to the DMUX  15  itself can be detected with good precision, thereby improving maintenance efficiency. 
     Herein, after conducting loopback processing and once measuring the level of each wavelength in the DMUX  15 , the measured level values are stored as initial values in memory provided in the apparatus. In so doing, during normal operation after starting up the apparatus, it becomes possible to efficiently detect faults by comparing the initial values stored in memory to the wavelength levels during actual operation. 
     An optical transmission apparatus in accordance with another embodiment will now be described.  FIGS. 5 and 6  illustrate exemplary configurations of an optical transmission apparatus. As illustrated in  FIG. 5 , the upstream-to-downstream optical transmission subsystem in the optical transmission apparatus  10 - 2  is provided with a preamp  11  (i.e., an upstream preamplifier), a postamp  12  (i.e., an upstream postamplifier), a DMUX  15  (i.e., an upstream wavelength demultiplexer), a MUX  16  (i.e., an upstream wavelength multiplexer), PDs  17 - 1  to  17 - n , a coupler Cp 1 , and couplers Cp 2 - 1  to Cp 2 - n.    
     As illustrated in  FIG. 6 , the downstream-to-upstream optical transmission subsystem in the optical transmission apparatus  10 - 2  is provided with a preamp  21  (i.e., a downstream preamplifier), a postamp  22  (i.e., a downstream postamplifier), a DMUX  25  (i.e., a downstream wavelength demultiplexer), a MUX  26  (i.e., a downstream wavelength multiplexer), PDs  27 - 1  to  27 - n , a coupler Cp 3 , and couplers Cp 4 - 1  to Cp 4 - n.    
     In addition, a loopback switch  13   a  is provided for both subsystems. The loopback switch  13   a  includes a switch sw 1  (i.e., a first upstream switch), a switch sw 2  (i.e., a second upstream switch), and a switch sw 3  (i.e., a third upstream switch), which are disposed with respect to the upstream-to-downstream optical transmission subsystem. 
     Furthermore, the loopback switch  13   a  also includes a switch sw 1   a  (i.e., a first downstream switch), a switch sw 2   a  (i.e., a second downstream switch), and a switch sw 3   a  (i.e., a third downstream switch), which are disposed with respect to the downstream-to-upstream optical transmission subsystem. A controller  14  is also disposed on a higher level of the apparatus. Since the configuration in  FIG. 5  is fundamentally similar to that illustrated in  FIG. 2 , the following description will focus on  FIG. 6 . 
     The switch sw 1   a  provided in the loopback switch  13   a  includes input terminals d 1  and d 2 , as well as an output terminal d 3 . The switch sw 2   a  includes an input terminal e 1  and an output terminal e 2 . The switch sw 3   a  includes an input terminal f 1 , as well as output terminals f 2  and f 3 . 
     The switches sw 1   a  to sw 3   a  are coupled as follows. The input terminal d 1  of the switch sw 1   a  is coupled to the output terminal c 3  of the switch sw 3 , while the input terminal d 2  of the switch sw 1   a  is coupled to the optical fiber line F 2   a . The output terminal d 3  of the switch sw 1   a  is coupled to the input port of the preamp  21 . 
     The input terminal e 1  of the switch sw 2   a  is coupled to the output port of the MUX  26 , while the output terminal e 2  of the switch sw 2   a  is coupled to the input port of the postamp  22 . The input terminal f 1  of the switch sw 3   a  is coupled to the output port of the postamp  22 , while the output terminal f 2  of the switch sw 3   a  is coupled to input terminal a 2  of the switch sw 1 . The output terminal f 3  of the switch sw 3   a  is coupled to the optical fiber line F 2   b.    
     During loopback processing, the loopback switch  13   a  causes ASE to be produced from the postamp  12 , amplified by the preamp  21 , and then causes ASE to be output from the preamp  21  and the preamp  11  having wavelength characteristics such that the optical power is flat in the WDM optical signal wavelength band. 
     Alternatively, the loopback switch  13   a  causes ASE to be produced from the postamp  22 , amplified by the preamp  11 , and then causes ASE to be output from the preamp  11  and the preamp  21  having wavelength characteristics such that the optical power is flat in the WDM optical signal wavelength band. 
     The controller  14  provides loopback processing instruction to the loopback switch  13   a . The DMUX  25  separates a received WDM optical signal into n wavelengths, and outputs the results. Each of the couplers CP 4 - 1  to Cp 4 - n  then splits the optical signal for one of the wavelengths into two parts, with one part being provided to one of the PDs  27 - 1  to  27 - n , and the other part being directed to a tributary and dropped. Each of the PDs  27 - 1  to  27 - n  generates an electrical signal by O/E converting the received optical signal for one of the wavelengths, and then provides the generated electrical signal to a predetermined processor. The MUX  26  uses wavelength-division multiplexing to multiplex the optical signal provided from the preamp  21  with added optical signals, and then output the multiplexed result. 
     Normal downstream-to-upstream operation will now be described on the basis of  FIG. 6 . (Normal upstream-to-downstream operation has been described with reference to  FIG. 2 , and thus is herein omitted.) During normal operation, switching control is conducted as follows. The switch sw 1   a  is switched such that the input terminal d 2  is coupled to the output terminal d 3 , and a WDM optical signal flowing in along the optical fiber line F 2   a  is input into the preamp  21 . 
     The switch sw 2   a  is switched such that the input terminal e 1  is coupled to the output terminal e 2 , and light output from the preamp  21  is input into the postamp  22 . The switch sw 3   a  is switched such that the input terminal f 1  is coupled to the output terminal f 3 , and light output from the postamp  22  is provided via the optical fiber line F 2   b . The switching control at this point is automatically set on the basis of switching instructions from the controller  14 . 
     A WDM optical signal flowing in from downstream along the optical fiber line F 2   a  is input into the preamp  21 . The preamp  21  amplifies and outputs the WDM optical signal. The coupler Cp 3  splits the amplified WDM optical signal into two parts, with one part being provided to the MUX  26 , and the other part being provided to the DMUX  25 . 
     The DMUX  25  separates the received WDM optical signal into n wavelengths, and outputs the results. Each of the couplers Cp 4 - 1  to Cp 4 - n  then splits the optical signal for one of the wavelengths into two parts, with one part being provided to one of the PDs  27 - 1  to  27 - n , and the other part being directed to a tributary and dropped. Each of the PDs  27 - 1  to  27 - n  generates an electrical signal by O/E converting the received optical signal for one of the wavelengths. 
     The MUX  26  uses wavelength-division multiplexing to multiplex the received WDM optical signal with added optical signals, thereby generating and outputting a new WDM optical signal. The postamp  22  amplifies the WDM optical signal output from the MUX  26 , and provides the result upstream via the optical fiber line F 2   b.    
     Operation during the loopback processing conducted at apparatus startup, for example, will now be described. Herein, there exist two types of loopback processing, one for the case where the ASE originates from the postamp  12 , and one for the case where the ASE originates from postamp  22 . The respective loopback processing for the above cases will be described separately. 
       FIGS. 7 and 8  illustrate exemplary configurations of the optical transmission apparatus  10 - 2 .  FIGS. 7 and 8  illustrate the loopback state for the case where the ASE originates from the postamp  12 . During loopback processing, switching control is conducted as follows. The switch sw 1  is switched such that the input terminal a 2  is coupled to the output terminal a 3 , and light output from the postamp  22  is input into the preamp  11 . The switch sw 2  is switched such that the input terminal b 1  and the output terminal b 2  are discoupled, and light output from the preamp  11  is blocked from being input into the postamp  12 . The switch sw 3  is switched such that the input terminal c 1  is coupled to the output terminal c 3 , and light output from the postamp  12  is provided to the preamp  21 . 
     The switch sw 1   a  is switched such that the input terminal d 1  is coupled to the output terminal d 3 , and light output from the postamp  12  is input into the preamp  21 . The switch sw 2   a  is switched such that the input terminal e 1  is coupled to the output terminal e 2 , and light output from the preamp  21  is input into the postamp  22 . The switch sw 3   a  is switched such that the input terminal f 1  is coupled to the output terminal f 2 , and light output from the postamp  22  is provided to the preamp  11 . The switching control at this point is automatically set on the basis of switching instructions from the controller  14 . 
     In such a switching state, the postamp  12  enters an input-less state, and produces ASE  1  in the WDM optical signal wavelength band. The ASE  1  is looped back and input into the preamp  21 . The preamp  21  amplifies the ASE  1  to an optical power close to the optical power when receiving a WDM optical signal during normal operation, and then outputs the amplified ASE  1   a.    
     The DMUX  25  separates the received ASE  1   a  into n wavelengths, and outputs the results. Each of the couplers Cp 4 - 1  to Cp 4 - n  then splits the ASE  1   a  for one of the wavelengths into two parts, with one part being provided to one of the PDs  27 - 1  to  27 - n . Each of the PDs  27 - 1  to  27 - n  generates an electrical signal by O/E converting the received ASE  1   a  for one of the wavelengths. The optical signal levels after wavelength separation are then checked from the levels of the electrical signals output from the PDs  27 - 1  to  27 - n  (in other words, the wavelength separation function of the DMUX  25  is checked). 
     Herein, the ASE  1  produced by the postamp  12  has the wavelength profile Pr 1 , wherein the optical power is low and flat for all wavelengths in the WDM optical signal wavelength band. The ASE  1  is input into the preamp  21  and amplified to produce the ASE  1   a . The ASE  1   a  output from the preamp  21  has the wavelength profile Pr 2 , wherein the optical power is high and flat for all wavelengths in the WDM optical signal wavelength band. 
     Meanwhile, the ASE  1   a  is input into the preamp  11  via the postamp  22 , and then output from the preamp  11  (herein, the postamp  22  and the preamp  11  allow the ASE  1   a  to pass through, without performing amplification control). 
     The DMUX  15  separates the received ASE  1   a  into n wavelengths, and outputs the results. Each of the couplers Cp 2 - 1  to Cp 2 - n  then splits the ASE  1   a  for one of the wavelengths into two parts, with one part being provided to one of the PDs  17 - 1  to  17 - n . Each of the PDs  17 - 1  to  17 - n  generates an electrical signal by O/E converting the received ASE  1   a  for one of the wavelengths. The optical signal levels after wavelength separation are then checked from the levels of the electrical signals output from the PDs  17 - 1  to  17 - n  (in other words, the wavelength separation function of the DMUX  15  is checked). 
     As described above, the ASE  1  produced herein originates from the postamp  12  disposed in the upstream-to-downstream optical transmission subsystem. Subsequently, the ASE  1  is looped back into the preamp  21  disposed in the downstream-to-upstream optical transmission subsystem, and amplified to produce the ASE  1   a . The ASE  1   a  is then looped back into the preamp  11  disposed in the upstream-to-downstream optical transmission subsystem. 
     The ASE  1   a  has the wavelength profile Pr 2 , which is high and flat in the WDM optical signal wavelength band. For this reason, by utilizing such ASE  1   a , it becomes possible to check the operation of individual function units in the apparatus with high precision, and improve maintenance efficiency. 
     For example, faults such as damage to the optical fiber between the preamp  21  and the DMUX  25  or damage to the DMUX  25  itself can be detected with good precision. In addition, faults such as damage to the optical fiber between the preamp  11  and the DMUX  15  or damage to the DMUX  15  itself can also be detected with good precision. In the foregoing, the postamp  22  and the preamp  11  are described as allowing the ASE  1   a  to pass through, without performing amplification control. However, the ASE  1   a  may be amplified in the postamp  22  or the preamp  11 , as long as such amplification does not significantly impair the flatness of the wavelength profile. 
       FIGS. 9 and 10  illustrate exemplary configurations of the optical transmission apparatus  10 - 2 .  FIGS. 9 and 10  illustrate the loopback state for the case where the ASE originates from the postamp  22 . During loopback processing, switching control is conducted as follows. The switch sw 1  is switched such that the input terminal a 2  is coupled to the output terminal a 3 , and light output from the postamp  22  is input into the preamp  11 . The switch sw 2  is switched such that the input terminal b 1  is coupled to the output terminal b 2 , and light output from the preamp  11  is input into the postamp  12 . The switch sw 3  is switched such that the input terminal c 1  is coupled to the output terminal c 3 , and light output from the postamp  12  is provided to the preamp  21 . 
     The switch sw 1   a  is switched such that the input terminal d 1  is coupled to the output terminal d 3 , and light output from the postamp  12  is input into the preamp  21 . The switch sw 2   a  is switched such that the input terminal e 1  and the output terminal e 2  are discoupled, and light output from the preamp  21  is blocked from being input into the postamp  22 . The switch sw 3   a  is switched such that the input terminal f 1  is coupled to the output terminal f 2 , and light output from the postamp  22  is provided to the preamp  11 . The switching control at this point is automatically set on the basis of switching instructions from the controller  14 . 
     In such a switching state, the postamp  22  enters an input-less state, and produces ASE  1  in the WDM optical signal wavelength band. The ASE  1  is looped back and input into the preamp  11 . The preamp  11  amplifies the ASE  1  to an optical power close to the optical power when receiving a WDM optical signal during normal operation, and then outputs the amplified ASE  1   a.    
     The DMUX  15  separates the received ASE  1   a  into n wavelengths, and outputs the results. Each of the couplers Cp 2 - 1  to Cp 2 - n  then splits the ASE  1   a  for one of the wavelengths into two parts, with one part being provided to one of the PDs  17 - 1  to  17 - n . Each of the PDs  17 - 1  to  17 - n  generates an electrical signal by O/E converting the received ASE  1   a  for one of the wavelengths. The optical signal levels after wavelength separation are then checked from the levels of the electrical signals output from the PDs  17 - 1  to  17 - n  (in other words, the wavelength separation function of the DMUX  15  is checked). 
     Herein, the ASE  1  produced by the postamp  22  has the wavelength profile Pr 1 , wherein the optical power is low and flat for all wavelengths in the WDM optical signal wavelength band. The ASE  1  is input into the preamp  11  and amplified to produce the ASE  1   a . The ASE  1   a  output from the preamp  11  has the wavelength profile Pr 2 , wherein the optical power is high and flat for all wavelengths in the WDM optical signal wavelength band. 
     Meanwhile, the ASE  1   a  is input into the preamp  21  via the postamp  12 , and then output from the preamp  21  (herein, the postamp  12  and the preamp  21  allow the ASE  1   a  to pass through, without performing amplification control). 
     The DMUX  25  separates the received ASE  1   a  into n wavelengths, and outputs the results. Each of the couplers Cp 4 - 1  to Cp 4 - n  then splits the ASE  1   a  for one of the wavelengths into two parts, with one part being provided to one of the PDs  27 - 1  to  27 - n . Each of the PDs  27 - 1  to  27 - n  generates an electrical signal by O/E converting the received ASE  1   a  for one of the wavelengths. The optical signal levels after wavelength separation are then checked from the levels of the electrical signals output from the PDs  27 - 1  to  27 - n  (in other words, the wavelength separation function of the DMUX  25  is checked). 
     As described above, the ASE  1  produced herein originates from the postamp  22  disposed in the downstream-to-upstream optical transmission subsystem. Subsequently, the ASE  1  is looped back into the preamp  11  disposed in the upstream-to-downstream optical transmission subsystem, and amplified to produce the ASE  1   a . The ASE  1   a  is then looped back into the preamp  21  disposed in the downstream-to-upstream optical transmission subsystem. 
     The ASE  1   a  has the wavelength profile Pr 2 , which is high and flat in the WDM optical signal wavelength band. For this reason, by utilizing such ASE  1   a , it becomes possible to check the operation of individual function units in the apparatus with high precision, and improve maintenance efficiency. 
     For example, faults such as damage to the optical fiber between the preamp  11  and the DMUX  15  or damage to the DMUX  15  itself can be detected with good precision. In addition, faults such as damage to the optical fiber between the preamp  21  and the DMUX  25  or damage to the DMUX  25  itself can also be detected with good precision. In the foregoing, the postamp  12  and the preamp  21  are described as allowing the ASE  1   a  to pass through, without performing amplification control. However, the ASE  1   a  may be amplified in the postamp  12  or the preamp  21 , as long as such amplification does not significantly impair the flatness of the wavelength profile. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a illustrating of the superiority and inferiority of the invention. Although the embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.