Method and system for monitoring of optical transmission lines

Disclosed herein is a system including a first optical fiber transmission line, a plurality of second optical fiber transmission lines, a branching section for optically connecting these first and second optical fiber transmission lines, a monitor device optically connected to the first optical fiber transmission line, and a plurality of reflecting sections respectively provided in the vicinity of open ends of the second optical fiber transmission lines. The monitor device outputs monitor light through the first optical fiber transmission line and the branching section to each of the second optical fiber transmission lines. The reflecting sections reflect the monitor light to thereby generate identification optical signals having different patterns for respectively identifying the second optical fiber transmission lines. With this configuration, the monitor device can detect an abnormality in each of the second optical fiber transmission lines according to the presence of absence of the corresponding identification optical signal.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to a method and system for
 monitoring (or supervising) of optical transmission lines, and more
 particularly to a method and system for monitoring of optical transmission
 lines suitable for an optical network system for performing transmission
 between a first terminal device (e.g., a central office) and a plurality
 of second terminal devices (e.g., subscriber terminals).
 2. Description of the Related Art
 Referring to FIG. 1, there is shown a conventional system for monitoring of
 optical transmission lines. This system includes a first optical fiber
 transmission line 2, N second optical fiber transmission lines 4(#1) to
 4(#N) (N is an integer greater than 1), and a branching section 6 for
 optically connecting the first optical fiber transmission line 2 and the
 second optical fiber transmission lines 4(#1) to 4(#N). The branching
 section 6 is provided by an optical star coupler, for example.
 A first terminal device 8 is optically connected to an open end of the
 first optical fiber transmission line 2, i.e., an end of the first optical
 fiber transmission line 2 opposite to the branching section 6. N second
 terminal devices 10(#1) to 10(#N) are optically connected to open ends of
 the second optical fiber transmission lines 4(#1) to 4(#N), respectively.
 For example, the first terminal device 8 is a central office in an optical
 fiber network system, and the second terminal devices 10(#1) to 10(#N) are
 subscriber terminals in the optical fiber network system. Between the
 first terminal device 8 and each of the second terminal devices 10(#1) to
 10(#N), one-way or two-way communication or transmission using an optical
 signal having a wavelength .lambda.1 is carried out.
 N monitor devices (or supervisory devices) 14(#1) to 14(#N) are provided in
 a monitor area (or supervisory area) 12, so as to detect an abnormality
 such as a break in each of the second optical fiber transmission lines
 4(#1) to 4(#N). The monitor devices 14(#1) to 14(#N) are optically
 connected to the second optical fiber transmission lines 4(#1) to 4(#N) by
 WDM (wavelength division multiplexing) couplers 16, respectively. The
 monitor devices 14(#1) to 14(#N) perform OTDR (optical time domain
 reflectometry), for example, using monitor light (or supervisory light)
 having a wavelength .lambda.2 to locate abnormal points in the second
 optical fiber transmission lines 4(#1) to 4(#N), respectively, or to
 measure loss characteristics of the second optical fiber transmission
 lines 4(#1) to 4(#N), respectively.
 In the conventional system shown in FIG. 1, the branching section 6 is
 included in the monitor area 12 where the monitor devices 14(#1) to 14(#N)
 are provided. Accordingly, many optical fibers are present in the monitor
 area 12, so that the wiring of the optical fibers in the monitor area 12
 is complicated. Further, the plural monitor devices 14(#1) to 14(#N)
 respectively corresponding to the second optical fiber transmission lines
 4(#1) to 4(#N) are required.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a method and
 system which can monitor (or supervise) a plurality of optical fiber
 transmission lines in a network by using a single monitor device.
 In accordance with a first aspect of the present invention, there is
 provided a system comprising a first optical fiber transmission line; a
 plurality of second optical fiber transmission lines; a branching section
 for optically connecting the first optical fiber transmission line and the
 plurality of second optical fiber transmission lines; a monitor device (or
 supervisory device) optically connected to the first optical fiber
 transmission line; and a plurality of reflecting sections respectively
 provided in the vicinity of open ends of the plurality of second optical
 fiber transmission lines. The monitor device outputs monitor light (or
 supervisory light) through the first optical fiber transmission line and
 the branching section to each of the second optical fiber transmission
 lines. The reflecting sections reflect the monitor light to thereby
 generate identification optical signals having different patterns for
 respectively identifying the second optical fiber transmission lines.
 With this configuration, each reflecting section generates the
 identification optical signal having the pattern for identifying the
 corresponding second optical fiber transmission line. Accordingly, the
 monitor device can identify an abnormal one of the second optical fiber
 transmission lines according to the presence or absence of the
 corresponding identification optical signal, for example, and can also
 locate an abnormal point in this abnormal second optical fiber
 transmission line identified above by applying OTDR. Thus, the plural
 second optical fiber transmission lines can be monitored by the single
 monitor device according to the present invention.
 In accordance with a second aspect of the present invention, there is
 provided a system comprising a first optical fiber transmission line; a
 plurality of second optical fiber transmission lines; a branching section
 for optically connecting the first optical fiber transmission line and the
 plurality of second optical fiber transmission lines; a monitor device
 optically connected to the first optical fiber transmission line; a first
 terminal device optically connected to an open end of the first optical
 fiber transmission line; and a plurality of second terminal devices
 optically connected to open ends of the plurality of second optical fiber
 transmission lines, respectively. The second terminal devices generate
 identification signals for respectively identifying the second optical
 fiber transmission lines. The monitor device detects an abnormality in
 each of the second optical fiber transmission lines according to the
 corresponding identification signal.
 In accordance with a third aspect of the present invention, there is
 provided a system comprising a first optical fiber transmission line; a
 plurality of second optical fiber transmission lines; a branching section
 for optically connecting the first optical fiber transmission line and the
 plurality of second optical fiber transmission lines; a monitor device
 optically connected to the first optical fiber transmission line; and a
 plurality of external modulation units optically connected to the
 plurality of second optical fiber transmission lines in the vicinity of
 open ends of the second optical fiber transmission lines, respectively.
 The monitor device outputs monitor light through the first optical fiber
 transmission line and the branching section to each of the second optical
 fiber transmission lines. The external modulation units modulate the
 monitor light according to identification signals for respectively
 identifying the second optical fiber transmission lines to thereby
 generate identification optical signals, respectively.
 In accordance with a fourth aspect of the present invention, there is
 provided a system comprising a first optical fiber transmission line; N
 second optical fiber transmission lines (N is an integer greater than 1);
 a branching section for optically connecting the first optical fiber
 transmission line and the plurality of second optical fiber transmission
 lines; a 1.times.N optical switch having a first port and N second ports,
 the second ports being optically connected to the second optical fiber
 transmission lines, respectively, in the vicinity of the branching
 section, the first port being selectively connected to one of the second
 ports; and a monitor device optically connected to the first port of the
 1.times.N optical switch for outputting monitor light to each of the
 second optical fiber transmission lines.
 In accordance with a fifth aspect of the present invention, there is
 provided a method for monitoring a plurality of optical fiber transmission
 lines optically connected together by a branching section, comprising the
 steps of (a) identifying an abnormal one of the optical fiber transmission
 lines; and (b) locating an abnormal point in the abnormal optical fiber
 transmission line identified above.
 The above and other objects, features and advantages of the present
 invention and the manner of realizing them will become more apparent, and
 the invention itself will best be understood from a study of the following
 description and appended claims with reference to the attached drawings
 showing some preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Some preferred embodiments of the present invention will now be described
 in detail with reference to the attached drawings. Throughout the drawings
 substantially the same parts are denoted by the same reference numerals.
 Referring to FIG. 2, there is shown a first preferred embodiment of the
 system according to the present invention. This system includes a first
 optical fiber transmission line 2, N second optical fiber transmission
 lines 4(#1) to 4(#N) (N is an integer greater than 1), and a branching
 section 6 for optically connecting the first optical fiber transmission
 line 2 and the second optical fiber transmission lines 4(#1) to 4(#N). The
 branching section 6 is provided by an optical star coupler, for example.
 A first terminal device 8 is optically connected to an open end of the
 first optical fiber transmission line 2, i.e., an end of the first optical
 fiber transmission line 2 opposite to the branching section 6. N second
 terminal devices 10(#1) to 10(#N) are optically connected to open ends of
 the second optical fiber transmission lines 4(#1) to 4(#N), respectively.
 For example, the first terminal device 8 is a central office in an optical
 fiber network system, and the second terminal devices 10(#1) to 10(#N) are
 subscriber terminals in the optical fiber network system. Between the
 first terminal device 8 and each of the second terminal devices 10(#1) to
 lO(#N), one-way or two-way communication or transmission using an optical
 signal having a wavelength .lambda.1 is carried out.
 In this preferred embodiment, a single monitor device (or supervisory
 device) 20 is optically connected through a WDM coupler 18 to the first
 optical fiber transmission line 2, so as to monitor (or supervise) each of
 the second optical fiber transmission lines 4(#1) to 4(#N). A monitor area
 22 including the monitor device 20 is accordingly provided in the vicinity
 of the first terminal device 8.
 The WDM coupler 18 optically couples the first terminal device 8 and the
 branching section 6 by the wavelength .lambda.1 of the optical signal for
 information transmission between the first terminal device 8 and the
 second terminal devices 10(#1) to 10(#N), and optically couples the
 monitor device 20 and the branching section 6 by a wavelength .lambda.2
 different from the wavelength .lambda.1. Accordingly, the monitor device
 20 can output monitor light (or supervisory light) having the wavelength
 .lambda.2 through the first optical fiber transmission line 2 and the
 branching section 6 to each of the second optical fiber transmission lines
 4(#1) to 4(#N).
 N reflecting sections 24 characteristic of this preferred embodiment are
 respectively provided in the vicinity of the open ends of the second
 optical fiber transmission lines 4(#1) to 4(#N) (to which open ends the
 second terminal devices 10(#1) to 10(#N) are connected). The reflecting
 sections 24 reflect the monitor light supplied from the monitor device 20
 to thereby generate identification optical signals having different
 patterns for respectively identifying the second optical fiber
 transmission lines 4(#1) to 4(#N). Accordingly, each identification
 optical signal has the wavelength .lambda.2.
 FIG. 3 is a block diagram showing a preferred embodiment of each reflecting
 section 24 shown in FIG. 2. Each reflecting section 24 is composed of
 optical connectors 26 and 28 connected to each of the second optical fiber
 transmission lines 4(#1) to 4(#N) on the branching section 6 side and on
 the corresponding second terminal device side, respectively, and a timing
 adjusting optical fiber 30 and a reflector 32 cascaded between the optical
 connectors 26 and 28. Accordingly, the timing adjusting optical fiber 30
 and the reflector 32 are arranged in this order from the branching section
 6 side. The reflector 32 generates the corresponding identification
 optical signal with a given timing. The principle of generation of the
 identification optical signals will now be described.
 FIGS. 4A and 4B are diagrams for illustrating an example of the patterns of
 the identification optical signals. Each reflector 32 provided by an
 optical fiber, for example, has a plurality of positions arranged in its
 longitudinal direction at given intervals, and one or more reflection
 points RP are set at one or more inherent positions selected from the
 above positions. For example, the reflector 32 in the reflecting section
 24 corresponding to the second optical fiber transmission line 4(#1) has a
 plurality of reflection points RP at the first, seventh, and eighth
 positions counted from the branching section 6 side as shown in FIG. 4A,
 and the monitor light supplied to this reflector 32 is reflected at these
 reflection points RP to thereby generate an identification optical signal
 having a pattern determined according to the positions of the reflection
 points RP.
 Further, as shown in FIG. 4B, the reflector 32 in the reflecting section 24
 corresponding to the second optical fiber transmission line 4(#N) has a
 plurality of reflection points RP at the first, second, fourth, fifth,
 seventh, and eighth positions counted from the branching section 6 side,
 thereby generating an identification optical signal having a pattern
 different from the pattern shown in FIG. 4A and inherent to the optical
 fiber transmission line 4(#N).
 Letting n denote the number of positions to which the reflection points RP
 are to be allocated, n is given by the number of digits of N in binary
 notation, for example, because n and N are decimal numbers. In the example
 shown in FIGS. 4A and 4B, n =8, so that N can take a maximum number of
 255. In the case that n =4, N can take a maximum number of 15.
 Each reflection point RP may be provided by a splice point between optical
 fibers, for example. In this case, the reflectance at the splice point is
 large enough to generate the identification optical signal and small
 enough not to largely attenuate the optical signal for information
 transmission, so that the splice point is useful in providing each
 reflecting section in the present invention. The splice point can be
 easily fabricated by a fusion splicing apparatus for optical fiber or
 fibers.
 In this preferred embodiment, the single monitor device 20 is used, and it
 is accordingly preferable that the identification optical signals
 respectively corresponding to the second optical fiber transmission lines
 4(#1) to 4(#N) are not to be returned with the same timing. To this end,
 each reflecting section 24 employs the timing adjusting optical fiber 30
 as shown in FIG. 3. By setting different delay periods "1 delay" in the
 timing adjusting optical fibers 30 for the second optical fiber
 transmission lines 4(#1) to 4(#N), the identification optical signals
 respectively corresponding to the second optical fiber transmission lines
 4(#1) to 4(#N) can be returned to the monitor device 20 with different
 timings.
 In this manner, each reflecting section 24 in this preferred embodiment
 reflects the monitor light to thereby generate an identification optical
 signal having a pattern for identifying the corresponding second optical
 fiber transmission line. Accordingly, the monitor device 20 can detect
 whether or not each second optical fiber transmission line is abnormal
 according to the presence or absence of the corresponding identification
 optical signal, for example. A specific example thereof will now be
 described.
 Referring to FIGS. 5A and 5B, there is shown an example of the results of
 measurement in the monitor device 20 in the normal condition and abnormal
 condition of the second optical fiber transmission lines, respectively.
 The monitor device 20 performs OTDR by using the monitor light. In the case
 that all of the second optical fiber transmission lines 4(#1) to 4(#N) are
 normal, the result of measurement shown in FIG. 5A is obtained so as to
 show the time-series appearance of a waveform due to connector reflection
 concerning the monitor device 20, a waveform due to losses in the
 branching section 6, identification optical signals (#1) to (#N)
 respectively corresponding to the second optical fiber transmission lines
 4(#1) to 4(#N), and a waveform due to connector reflection concerning each
 of the second terminal devices 10(#1) to 10(#N) in this order. In each of
 FIGS. 5A and 5B, the vertical axis (direction) represents optical power
 level, and the horizontal axis (direction) represents time. In the result
 of measurement shown in FIG. 5A, the identification optical signals (#1)
 to (#N) for all the second optical fiber transmission lines 4(#1) to 4(#N)
 are obtained, so that it is determined that all the second optical fiber
 transmission lines 4(#1) to 4(#N) are in the normal condition.
 In the result of measurement shown in FIG. 5B, only the identification
 optical signal (#1) corresponding to the second optical fiber transmission
 line 4(#1) is not detected in the monitor device 20. Accordingly, it is
 determined that the second optical fiber transmission line 4(#1) is in the
 abnormal condition. Furthermore, in the result of measurement shown in
 FIG. 5B, a waveform due to abnormal point reflection between the branching
 section 6 and the reflecting section 24 is obtained. Accordingly, since
 the second optical fiber transmission line 4(#1) has been already
 identified as an abnormal optical fiber transmission line, the abnormal
 point in the second optical fiber transmission line 4(#1) can be located
 according to the waveform due to abnormal point reflection.
 In this manner, an abnormal optical fiber transmission line is first
 identified and an abnormal point in this abnormal optical fiber
 transmission line identified above is next located according to this
 preferred embodiment. Accordingly, it is sufficient to provide a single
 monitor device for performing OTDR, for example.
 Further, since the wavelength .lambda.1 of the optical signal for
 information transmission and the wavelength .lambda.2 of the monitor light
 are different from each other, the monitoring of the optical fiber
 transmission lines can be performed while information transmission is
 being performed.
 In the result of measurement shown in FIG. 5B, the abnormal point in the
 optical fiber transmission line 4(#1) is a break point, for example. In
 OTDR, the tilt in the result of measurement gives a loss characteristic of
 an optical fiber transmission line, so that an abnormality in the loss
 characteristic may be detected according to the present invention.
 FIG. 6 is a block diagram showing a second preferred embodiment of the
 system according to the present invention. In this preferred embodiment,
 the second terminal devices 10(#1) to 10(#N) generate identification
 signals for respectively identifying the second optical fiber transmission
 lines 4(#1) to 4(#N), so as to detect an abnormality in each of the second
 optical fiber transmission lines 4(#1) to 4(#N) without providing the
 reflecting sections 24 shown in FIG. 2. The monitor device 20 detects the
 abnormality in each of the second optical fiber transmission lines 4(#1)
 to 4(#N) according to the corresponding identification signal.
 In the case that information transmission is performed by an optical signal
 having a wavelength .lambda.1 between the first terminal device 8 and the
 second terminal devices 10(#1) to 10(#N), each identification signal can
 be transmitted by the optical signal or in association with the optical
 signal. Accordingly, the wavelength related to each identification signal
 is .lambda.1, and the monitor device 20 is therefore optically connected
 to the first optical fiber transmission line 2 by a usual optical coupler
 34 rather than a WDM coupler. The monitor device 20 can identify any
 abnormal one of the second optical fiber transmission lines 4(#1) to 4(#N)
 according to the corresponding identification signal, and can thereafter
 locate an abnormal point in this abnormal second optical fiber
 transmission line by OTDR using monitor light having a wavelength
 .lambda.2 as similarly to the first preferred embodiment, for example.
 Referring to FIGS. 7A and 7B, there is shown an example of the
 identification signals in the second preferred embodiment shown in FIG. 6.
 As shown in FIG. 7A, an identification signal inherent to one of the
 second optical fiber transmission lines 4(#1) to 4(#N) is preliminarily
 added to the trailing end of a main signal for information transmission.
 This identification signal has a pattern determined according to the
 corresponding second optical fiber transmission line. As shown in FIG. 7B,
 an identification signal having a pattern different from the pattern of
 the identification signal shown in FIG. 7A for identifying another one of
 the second optical fiber transmission lines 4(#1) to 4(#N) is
 preliminarily added to the trailing end of the main signal.
 In this manner, the main signal and each identification signal are
 time-division multiplexed to allow easy transmission of the identification
 signal from each of the second terminal devices 10(#1) to 10(#N) to the
 monitor device 20. As a result, the monitor device 20 can detect an
 abnormality in each of the second optical fiber transmission lines 4(#1)
 to 4(#N) according to the presence or absence of the corresponding
 identification signal as similarly to the first preferred embodiment.
 FIGS. 8A and 8B are diagrams showing another example of the identification
 signals in the second preferred embodiment shown in FIG. 6. As shown in
 FIG. 8A, an identification signal having a frequency f.sub.1, is
 superimposed on a main signal to obtain a transmission signal in the
 second terminal device 10(#1). As shown in FIG. 8B, a different
 identification signal having a frequency f.sub.N is superimposed on the
 main signal to obtain a different transmission signal in the second
 terminal device 10(#N).
 In this manner, an identification signal having a frequency determined
 according to each of the second optical fiber transmission lines is
 preliminarily superimposed on the main signal. The monitor device 20
 extracts the identification signal from the transmission signal to thereby
 detect the presence or absence of the identification signal, thus
 identifying any abnormal one of the second optical fiber transmission
 lines according to the result of detection. Further, by using monitor
 light having a wavelength .lambda.2 different from the wavelength
 .lambda.1 of an optical signal for transmitting the main signal, the
 monitor device 20 can locate an abnormal point in the abnormal second
 optical fiber transmission line identified above as similarly to the first
 preferred embodiment.
 FIG. 9 is a block diagram showing a third preferred embodiment of the
 system according to the present invention. In this preferred embodiment, N
 external modulation units 36 are optically connected to the second optical
 fiber transmission lines 4(#1) to 4(#N) in the vicinity of the open ends
 thereof, respectively. The optical connection between the external
 modulation units 36 and the respective second optical fiber transmission
 lines 4(#1) to 4(#N) is effected by N WDM couplers 38.
 This preferred embodiment is similar to the first preferred embodiment
 shown in FIG. 2 in the point that a monitor device 20 is optically
 connected to the first optical fiber transmission line 2 by a WDM coupler
 18. The monitor device 20 outputs monitor light through the first optical
 fiber transmission line 2 and the branching section 6 to each of the
 second optical fiber transmission lines 4(#1) to 4(#N).
 The external modulation units 36 modulate the supplied monitor light
 according to identification signals for respectively identifying the
 second optical fiber transmission lines 4(#1) to 4(#N) to thereby generate
 identification optical signals. Accordingly, as similarly to the first
 preferred embodiment, the monitor device 20 can identify any abnormal one
 of the second optical fiber transmission lines 4(#1) to 4(#N) according to
 the corresponding identification optical signal and can locate an abnormal
 point in the abnormal optical fiber transmission line identified above by
 performing OTDR, for example.
 Referring to FIG. 10, there is shown a preferred embodiment of each
 external modulation unit 36 shown in FIG. 9. Each external modulation unit
 36 includes an optical coupler 40, photodetector 42, identification signal
 generating section 44, external modulator 46, timing adjusting optical
 fiber 48, and total reflector 50.
 Monitor light having a wavelength .lambda.2 dropped from each second
 optical fiber transmission line by the corresponding WDM coupler 38 is
 divided into two branch beams by the optical coupler 40. One of the two
 branch beams is supplied to the photodetector 42 such as a photodiode, and
 the other branch beam is passed through the external modulator 46, then
 reciprocating the timing adjusting optical fiber 48 by the presence of the
 total reflector 50, and being returned to the external modulator 46. The
 identification signal generating section 44 outputs an identification
 signal according to the timing of photodetection by the photodetector 42,
 and the external modulator 46 modulates the light supplied from the timing
 adjusting optical fiber 48 according to the identification signal supplied
 from the identification signal generating section 44, thereby generating
 an identification optical signal. The identification optical signal is
 supplied through the optical coupler 40 and the WDM coupler 38 in this
 order to the corresponding second optical fiber transmission line.
 Referring to FIGS. 11A and 11B, there is shown an example of the
 identification signals in the third preferred embodiment. For example, the
 external modulation unit 36 corresponding to the second optical fiber
 transmission line 4(#1) generates an identification optical signal having
 a pattern inherent to the optical fiber transmission line 4(#1) as shown
 in FIG. 11A. In this case, the external modulation unit 36 corresponding
 to the optical fiber transmission line 4(#N) as an example of any second
 optical fiber transmission line other than the optical fiber transmission
 line 4(#1) generates an identification optical signal having a pattern as
 shown in FIG. 11B, which pattern is different from the pattern shown in
 FIG. 11A. In this manner, by making the patterns of the identification
 optical signals for all the second optical fiber transmission lines 4(#1)
 to 4(#N) different from each other, the monitor device 20 can easily
 identify any abnormal one of the second optical fiber transmission lines
 4(#1) to 4(#N) in which any abnormality such as a break has occurred,
 according to the presence or absence of the corresponding identification
 optical signal.
 Referring to FIGS. 12A and 12B, there is shown another example of the
 identification signals in the third preferred embodiment. In this example,
 two identification signals having different frequencies are shown. Each
 external modulation unit 36 generates an identification optical signal
 according to the corresponding identification signal, and the monitor
 device 20 can easily identify any abnormal one of the second optical fiber
 transmission lines 4(#1) to 4(#N) according to the frequency of the
 corresponding identification signal.
 FIG. 13 is a block diagram showing another preferred embodiment of each
 external modulation unit 36 shown in FIG. 9. In contrast to the preferred
 embodiment shown in FIG. 10 wherein the monitor light is reciprocated in
 the external modulator 46 to perform modulation of the monitor light
 according to the corresponding identification signal on the return path,
 the preferred embodiment shown in FIG. 13 is characterized in that the
 monitor light is only once passed through the external modulator 46 to
 perform modulation of the monitor light according to the corresponding
 identification signal at this time.
 The monitor light dropped from each second optical fiber transmission line
 by the corresponding WDM coupler 38 is divided into two branch beams by
 the optical coupler 40. One of the two branch beams is supplied to the
 photodetector 42, and the other branch beam is supplied through an optical
 coupler 52 to the external modulator 46. The external modulator 46
 modulates the input branch beam according to the identification signal
 from the identification signal generating section 44 to obtain an
 identification optical signal. The identification optical signal is next
 returned through a timing adjusting optical fiber 48' to the optical
 coupler 52. The optical coupler 52 functions as a directional coupler.
 Accordingly, the identification optical signal returned to the optical
 coupler 52 is supplied through the optical coupler 40 and the WDM coupler
 38 in this order to each second optical fiber transmission line.
 The reason for the provision of the timing adjusting optical fiber 48 or
 48' in each external modulation unit 36 is to prevent all the
 identification optical signals from reaching the monitor device 20 with
 the same timing to allow the use of the single monitor device 20.
 FIG. 14 is a block diagram showing a fourth preferred embodiment of the
 system according to the present invention. In this preferred embodiment, a
 1.times.N optical switch 54 is used to allow monitoring of a plurality of
 optical fiber transmission lines by the use of the single monitor device
 20. The optical switch 54 has a first port 54A and N second ports 54(#1)
 to 54(#N). The first port 54A is selectively connected to one of the
 second ports 54(#1) to 54(#N).
 The second ports 54(#1) to 54(#N) are optically connected to the second
 optical fiber transmission lines 4(#1) to 4(#N) by WDM couplers 56 in the
 vicinity of the branching section 6, respectively. The first port 54A
 functions to receive monitor light having a wavelength .lambda.2 from the
 monitor device 20. Particularly in this preferred embodiment, the first
 port 54A is optically connected to the first optical fiber transmission
 line 2 between the WDM coupler 18 and the branching section 6 by a WDM
 coupler 58, so as to pass the monitor light through the first optical
 fiber transmission line 2.
 FIG. 15 is a diagram for illustrating the principle of operation in the
 fourth preferred embodiment shown in FIG. 14. The monitor light output
 from the monitor device 20 is supplied through the WDM coupler 18, the
 first optical fiber transmission line 2, and the WDM coupler 58 in this
 order to the first port 54A of the optical switch 54, and is then output
 from a selected one of the second ports 54(#1) to 54(#N) (e.g., the second
 port 54(#1) as shown in FIG. 15). Accordingly, in the example shown, the
 second optical fiber transmission line 4(#1) can be monitored by
 performing OTDR, for example, using the monitor light. Furthermore, in the
 case that an abnormal point such as a break point is present in the
 optical fiber transmission line 4(#1), the abnormal point can be located.
 By sequentially switching the second ports 54(#1) to 54(#N) of the optical
 switch 54, the above monitoring operation can be performed on all the
 optical fiber transmission lines 4(#1) to 4(#N).
 The reason why the port 54A of the optical switch 54 is optically connected
 to the first optical fiber transmission line 2 by the WDM coupler 58 is to
 allow monitoring of the first optical fiber transmission line 2 by OTDR or
 the like as similarly to the previous preferred embodiments. Accordingly,
 in the case that the monitoring of the first optical fiber transmission
 line 2 is not required, the monitor device 20 may be optically connected
 directly to the port 54A of the optical switch 54 in this preferred
 embodiment, thereby eliminating the need for the WDM couplers 18 and 58 to
 simplify the configuration of the system.
 As mentioned above, this preferred embodiment employs the 1.times.N optical
 switch 54 to allow the monitoring of the N second optical fiber
 transmission lines 4(#1) to 4(#N) by the single monitor device 20.
 In each of the above preferred embodiments, optical fiber wiring in the
 monitor area 22 where the monitor device 20 is provided is dedicated only
 for the monitor device 20, so that the optical fiber wiring in the monitor
 area 22 can be made simpler than that in the conventional system shown in
 FIG. 1.
 The frequencies of the identification signals described with reference to
 FIGS. 8A and 8B are set higher than the frequency corresponding to the bit
 rate (e.g., 50 Mb/s) of the main signal, for example.
 According to the present invention as described above, it is possible to
 provide a method and system which can monitor a plurality of optical fiber
 transmission lines in a network by using a single monitor device. The
 other effects obtained by the specific preferred embodiments of the
 present invention have been described above, so the description thereof
 will be omitted herein.
 The present invention is not limited to the details of the above described
 preferred embodiments. The scope of the invention is defined by the
 appended claims and all changes and modifications as fall within the
 equivalence of the scope of the claims are therefore to be embraced by the
 invention.