Abstract:
A wavelength tunable optical transmitter is used in a protection system of a WDM network, and its conventional wavelength tuning process has to perform steps of tuning intermediate wavelengths which are used by other optical transmission equipment in the network. Thus the conventional system interferes with normal communications in the other optical transmission equipment. To prevent the interference, wavelength tunable optical transmission equipment is provided with an optical gate for selectively on and off an optical signal output. A controller closes the optical gate while wavelength tunable optical transmission equipment and opens the optical gage to pass the signal through the optical gate once the target wavelength is attained and virtually stabilized.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to optical fiber transmission system technologies based on wavelength division multiplexing (hereinafter called WDM) to send optical signals at different wavelengths on an optical fiber and particularly to wavelength tunable optical transmission equipment and an optical network using this type of transmission equipment.  
           [0002]    An optical source with tunable wavelengths has been described, for example, in U.S. Pat. No. 5,173,909 (Japanese Patent Application Provisional Publication No. 72783/92). It is possible to realize wavelength tunable optical transmission equipment by the use of this type of optical source. This wavelength tunable optical transmission equipment has been attracting attention as auxiliary transmission equipment for WDM networks that have been recently gaining share. The reason is as follows. If conventional wavelength-fixed optical transmission equipment is used in WDM networks, a duplicate number of expensive optical transmitters is needed in order to provide a protection system. On the other hand, if wavelength tunable optical transmission equipment is adopted for the protection system, it is possible to reduce the required number of protection optical transmitters based upon a number of wavelengths available within the wavelength tunable range.  
         SUMMARY OF THE INVENTION  
         [0003]    A problem can arise if the above wavelength tunable optical transmission equipment is used in a WDM network. In a WDM network, when changing the wavelength (e.g. λ 1 ) of certain optical transmission equipment to a different  5  wavelength (e.g. λ 4 ), continuous transition from the current wavelength λ 1  to the new wavelength λ 4  via wavelengths λ 2  and λ 3  would occur in the wavelength tuning process and thus the transmission equipment would pass through steps of intermediate wavelengths λ 2  and λ 3 . However, wavelengths λ 2  and λ 3  are delivered by other optical transmission equipment and used for transmission of other optical data signals. Within the network, there are also pieces of optical receiving equipment that receive optical signals at the wavelengths λ 2 , λ 3 .  
           [0004]    Consequently, these pieces of optical receiving equipment are compelled to receive the optical signals at λ 2  and λ 3  that were generated in the wavelength tuning process by the above wavelength tunable optical transmission equipment. In addition, other optical transmission equipment may simultaneously delivers optical signals at λ 2  and λ 3  wavelengths to the same optical receiving equipment. This prevents the optical receiving equipment from receiving signals properly.  
           [0005]    The object of this invention is to provide optical transmission equipment that solves the above problem as well as an optical network which uses such optical transmission equipment. The object is achieved by wavelength tunable optical transmission equipment with an optical gate which selectively allows an output optical signal to pass. A controller closes the optical gate to block signal output while wavelength tuning is under way in the transmission equipment, and opens it to allow output signals to pass through it once the target wavelength is attained and virtually stabilized.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which:  
         [0007]    [0007]FIG. 1 is a block diagram illustrating a first embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0008]    [0008]FIGS. 2A to  2 D are diagrams illustrating the wavelength tuning process in the embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0009]    [0009]FIG. 3 is a block diagram illustrating a second embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0010]    [0010]FIG. 4 is a block diagram illustrating a third embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0011]    [0011]FIG. 5 is a block diagram illustrating a fourth embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0012]    [0012]FIG. 6 is a block diagram illustrating a fifth embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0013]    [0013]FIG. 7 is a block diagram illustrating a sixth embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0014]    [0014]FIG. 8 is a block diagram illustrating a seventh embodiment of the wavelength tunable optical transmission equipment according to the invention,  
         [0015]    [0015]FIG. 9 is a block diagram illustrating embodiment of the WDM network according to the invention, and  
         [0016]    [0016]FIG. 10 is a block diagram illustrating another embodiment of a WDM network according to the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The preferred embodiments of the present invention are explained below referring to the drawings attached.  
         [0018]    As a preferred embodiment of the optical transmission equipment according to the invention is described next with reference to FIGS. 1 and 2. FIG. 1 is a block diagram for the optical transmission equipment, and FIGS. 2A to  2 D illustrate its wavelength tuning process.  
         [0019]    Optical transmission equipment  100 A as shown in FIG. 1 includes a wavelength tunable optical transmitter  1 , an optical gate  2 , an optical gate drive circuit  3  and a controller  4 . The wavelength tunable optical transmitter  1  can tune wavelength λX of output optical signal. The optical gate  2  receives an optical input signal from the wavelength tunable optical transmitter  1  and blocks or pass the input signal. The optical gate drive circuit  3  drives the optical gate  2 . The controller  4  outputs information about the target wavelength to a wavelength control circuit  8  of the wavelength tunable optical transmitter  1 . The controller  4  receives information about the monitored wavelength from the wavelength monitor circuit  7  in the wavelength tunable optical transmitter  1 . The controller  4  generates a control signal and outputs to the optical gate drive circuit  3 , and based upon the control signal, the optical gate drive circuit  3  selectively makes the above optical gate  2  impassable while wavelength tuning is under way. The optical gate drive circuit  3  also makes the optical gate  2  passable once the target level of wavelength is attained and virtually stabilized.  
         [0020]    The wavelength tunable optical transmitter  1  includes a wavelength tunable optical source  5 , an optical divider  6 , a wavelength monitor circuit  7 , a wavelength control circuit  8  and an optical source drive circuit  9 . The optical divider  6  divides the output from the wavelength tunable optical source  5  into two parts. One of the two parts is monitored as an input by the wavelength monitor circuit  7 . The wavelength control circuit  8  generates a control signal so that the difference between the actual wavelength detected by the wavelength monitor circuit  7  and the target wavelength becomes close to zero. According to the signal from the wavelength control circuit  8 , the optical source drive circuit  9  drives the wavelength tunable optical source  5 . The wavelength monitor circuit  7  is implemented by a commercial spectrum analyzer or wavelength monitor and sends monitoring information to the controller  4 . The wavelength control circuit  8  generates a control signal proportional to the difference between the target wavelength informed of by the controller  4  and the actually detected or monitored wavelength and is embodied by using such a circuit as typically used for negative feedback control. A device such as an optical switch or modulator is as the optical gate  2 .  
         [0021]    According to the above configuration, since the controller  4  not only controls the wavelength in the wavelength tunable optical transmitter  1  but also monitors the actual wavelength, it is possible to realize optical transmission equipment which makes the optical gate  2  selectively impassable during the wavelength tuning process and subsequently makes it passable once the wavelength is virtually stabilized at the target level.  
         [0022]    [0022]FIGS. 2A to  2 D illustrate the operations of the first embodiment. Shown here as an example is the wavelength transition in FIGS.  2 A and FIG. 2D for the wavelength λX to be outputted from the wavelength tunable optical transmission equipment. As the wavelength is changed from the initial wavelength λ 1  to the target wavelength λ 4 , the optical gate status transition as shown in FIG. 2B and the optical signal level transition as shown in FIG. 2C correspondingly change.  
         [0023]    [0023]FIG. 2A shows the wavelengths of an optical signal to be sent to the optical gate in section A in FIG. 1. The wavelength is held at the initial level λ 1  before time t 1 . The initial level λ 1  gradually turns into wavelength λ 2  and then to λ 3 , and finally reaches the target wavelength λ 4  at time t 2 . Thereafter the wavelength remains stably at λ 4 . In parallel with this wavelength transition, the optical gate status as shown in FIG. 2B is controlled so that it is passable before time t 1 , and then it is impassable at time t 1  and stays impassable until time t 2 . After time t 2 , it is passable again. In the above sequence, the optical signal level as shown in FIG. 2C in FIG. 1 during the period from t 1  to t 2  is null or substantially zero in an optical signal B. Therefore, the wavelength as shown FIG. 2D of the optical signal outputted from the optical gate is λ 1  before t 1  and λ 4  after t 2 . In contrast, no optical signal is outputted from the optical gate during the period from t 1  to t 2 . By controlling the optical gate in this way, communication between other optical transmission equipment and the receiving equipment using wavelengths λ 2  or λ 3  will not be disturbed. According to this embodiment, with the above arrangement, the wavelength tunable optical transmission equipment outputs no optical signal during the wavelength tuning process and outputs the tuned optical signal once the tuning is finished without affecting the characteristics of communications at other wavelengths during the tuning.  
         [0024]    As the wavelength tunable optical source  5 , a modulator-integrated optical source is one choice. If this type of optical source is used, an arrangement similar to the above described gate will be needed since there is no feedback to the wavelength control circuit. Alternatively, wavelength tuning is theoretically possible with the closed integrated modulator. The wavelength control circuit  8  is also incorporated in the controller  4  instead of in the wavelength tunable optical transmitter  1 . This alternative arrangement applicable to the following other embodiments which will be described subsequently.  
         [0025]    A second embodiment of the optical transmission equipment according to the present invention is explained below with reference to FIG. 3. The figure is a block diagram of the optical transmission equipment and the same reference numerals are used for the substantially identical components as in FIG. 1. These reference numerals will be also used for the same components in the subsequent embodiments discussed later.  
         [0026]    The difference between the optical transmission equipment  100 B in the second embodiment and the first embodiment  100 A of FIG. 1 is that the optical gate  2  also serves as an optical modulator optical gate in the second embodiment  100 B. Control signals from the controller  4  and data signal are inputted to the optical gate drive circuit  3  so that during normal data transmission, the optical gates selectively turns on or off the light from the wavelength tunable optical source  5  at a constant wavelength, according to a drive signal which depends on the data signal. Thereby the gate  2  permits the optical data signal to be output.  
         [0027]    The optical gate  2  must operate at high velocity so it is preferably a Mach-Zehnder type external optical modulator which uses a semiconductor material or lithium niobate, or an electroabsorptive external optical modulator which uses a semiconductor material or a similar device.  
         [0028]    In the second embodiment, the same wavelength tuning control as illustrated in FIGS. 1 and 2 is also implemented by replacing the control signal for optical gate drive circuit  3  from the data signal with the optical gate control signal from the controller  4 . This also enables the wavelength tunable optical transmission equipment not to affect the characteristics of communications at other wavelengths during the wavelength tuning process.  
         [0029]    Referring to FIG. 4, a third embodiment of the optical transmission equipment according to the invention is next explained. FIG. 4 is a block diagram of the optical transmission equipment, and the optical transmission equipment  100 C in this embodiment is different from the transmission equipment  100 A in the first embodiment as shown in FIG. 1 in that it has an optical modulator  10  between the optical divider  6  and the optical gate  2 . This optical modulator  10  is turned on or off by the optical modulator drive circuit  11  according to the drive signal which depends on the data signal so that optical data signal is selectively outputted.  
         [0030]    Referring to FIG. 5, a forth embodiment of the optical transmission equipment according to the invention will be next explained. FIG. 5 is a block diagram of the optical transmission equipment. The optical transmission equipment  100 D in this embodiment is different from the transmission equipment  100 A in the first embodiment as shown in FIG. 1 in that an optical modulator  10  is connected to the output of the optical gate  2 . This optical modulator  10  is turned on or off by the optical modulator drive circuit  11  according to the drive signal which depends on the data signal so that optical data signal is selectively outputted.  
         [0031]    In both the embodiments as shown in FIGS. 4 and 5, the optical modulator  10  is added to the embodiments as shown in FIG. 1. Thus, while wavelength tuning is under way as shown in FIG. 2, no optical signal can be outputted. After wavelength tuning is finished, an optical signal is outputted so that the wavelength tunable optical transmission equipment does not affect the characteristics of communications with other wavelengths during the wavelength tuning process.  
         [0032]    Referring to FIG. 6, a fifth embodiment of the optical transmission equipment according to the invention will be next explained. FIG. 6 is a block diagram of the optical transmission equipment.  100 E that includes a wavelength tunable optical transmitter  1 , an optical-signal level controller  17  and an optical controller  4 . The wavelength tunable optical transmitter  1  modifies a wavelength λX of an output optical signal. An optical-signal level controller  17  receives the optical signal from the wavelength tunable optical transmitter  1  as an input and determines whether to permit it to pass through or block it. The controller  4  outputs information on the target wavelength to control the wavelengths of the wavelength tunable optical transmitter  1 . The controller  4  also receives information on the monitored wavelength from the wavelength tunable optical transmitter  1 . Based upon the received information, the controller  4  enables the optical-signal level controller  17  to block the optical signal while wavelength tuning is under way. The controller  4  also enables the optical-signal level controller  17  to permit the optical signal to pass through once the target level of wavelength is attained and virtually stabilized. The controller  4  receives information on the monitored optical signal level from the optical-signal level controller  17  and outputs a level control signal or information on the target optical signal level to control the output of the optical signal level controller  17 .  
         [0033]    The structure of the wavelength tunable optical transmitter  1  is the same as shown in FIG. 1. The optical-signal level controller  17  includes a variable optical attenuator  12 , a second optical divider  13 , a variable optical signal level monitor circuit  14 , a variable optical signal level control circuit  15  and a variable optical attenuator drive circuit  16 . The variable optical attenuator  12  attenuates the light from the wavelength tunable optical transmitter  1 . The second optical divider  13  divides the light from the variable optical attenuator  12  into, for example, nine parts, or at a ratio of 9:1. The optical signal level monitor circuit  14  monitors the level of the optical signal from the second optical divider  13 . The optical-signal level control circuit  15  compares the level of the electric signal as a result of conversion in the optical-signal level monitor circuit  14  with the target optical signal level informed of by the controller  4 . Based upon the comparison, the optical signal level control circuit  15  generates a control signal indicative of the difference, and the control signal is used to make the difference zero. The variable optical attenuator drive circuit  16  drives the variable optical attenuator  12  according to the above control signal from the optical signal level control circuit  15 . In one preferred embodiment, the variable optical attenuator  12  is implemented with a polymer-based variable optical attenuator or an erbium-doped fiber (EDF).  
         [0034]    With the above described structures in the preferred embodiment, the controller  4  not only controls the wavelength of the wavelength tunable optical transmitter  1  but also monitors the actual wavelength so that the optical transmission equipment enables the variable optical attenuator  12  selectively to block optical signals during the wavelength tuning process. Upon returning to the state, where the target level of wavelength is attained and virtually stabilized, the optical transmission equipment allows the optical signal to pass through. According to the preferred embodiment, the wavelength tunable optical transmission equipment controls the variable optical attenuator  12  so as to bring the output optical signal to a prescribed level.  
         [0035]    As in the first embodiment as shown in FIG. 1, a modulator-integrated optical source is used as the wavelength tunable optical source  5 . In this case, however, an arrangement similar to the above described structure will be needed since there is no feedback to the wavelength control circuit. However, wavelength tuning is theoretically possible with the closed integrated modulator. Alternatively, the optical-signal level control circuit  15  is included in the controller  4  instead in the optical-signal level controller  17 . This alternative approach is possible for the other embodiments discussed below.  
         [0036]    Referring to FIG. 7, a sixth embodiment of the optical transmission equipment according to the invention will be next explained. FIG. 7 is a block diagram of the optical transmission equipment. The optical transmission equipment  100 F in this embodiment is different from the transmission equipment  100 E as shown in FIG. 6 in that an optical modulator  10  is located between the wavelength tunable optical transmitter  1  and the optical-signal level controller  17 . This optical modulator  10  is turned on or off by the optical modulator drive circuit  11  according to the drive signal which depends on the data signal so that optical data signal is outputted.  
         [0037]    Referring to FIG. 8, a seventh embodiment of the optical transmission equipment according to the invention will be next explained. FIG. 8 is a block diagram of the optical transmission equipment. The optical transmission equipment  100 G in this embodiment is different from the transmission equipment  100 E as shown in FIG. 6 in that an optical modulator  10  is connected to the output of the optical-signal level controller  17 . This optical modulator  10  is selectively turned on or off by the optical modulator drive circuit  11  according to the drive signal which depends on the data signal so that optical data signal is outputted.  
         [0038]    In both the embodiments as shown in FIGS. 7 and 8, the optical modulator  10  is added to the configuration as shown in FIG. 6. In these embodiments, while wavelength tuning is under way as shown in FIG. 2, no optical signal can be outputted. After wavelength tuning is finished, an optical signal can be outputted so that both pieces of the wavelength tunable optical transmission equipment do not affect the characteristics of communications at other wavelengths even during the wavelength tuning process. The wavelength tunable optical transmission equipment also controls the variable optical attenuator  12  so as to bring the output optical signal to a prescribed level.  
         [0039]    Referring to FIG. 9, a preferred embodiment of the WDM network according to this invention is explained. FIG. 9 is a block diagram of the WDM network. This embodiment assumes that four wavelength channels are provided for wavelength multiplexing. When the optical network is working normally, data signals are sent to the respective wavelength fixed optical transmission equipment  19 - 1  through  19 - 4  for the corresponding wavelengths λ 1  through λ 4 . The output optical signal from each of the wavelength fixed optical transmission equipment  19 - 1  through  19 - 4  is wavelength-multiplexed by a wavelength multiplexer  20  and then transmitted through the optical fiber for the optical-signal transmission  22  to a wavelength demultiplexer  23  for wavelength demultiplexing. The demultiplexed separate optical signals at wavelengths are received by respective optical receiving equipment  24 - 1  through  24 - 4 . During this process, outputs from the wavelength tunable optical transmission equipment  100  are blocked.  
         [0040]    However, if a failure occurs in one of the above four pieces of wavelength fixed optical transmission equipment  19 - 1  through  19 - 4 , the wavelength tunable optical transmission equipment  100  changes its status from an optical output blocking status to an optical output enabling status after the output wavelength is changed to λ 4  and stabilized. At the same time, for example, if the transmission equipment  19 - 4  fails to output the λ 4  signal, a selector  25  selects the data signal which was sent to  19 - 4  before the failure prior to outputting it.  
         [0041]    As a result of the above operation, the wavelength tunable optical transmission equipment  100  outputs the same optical signal as the wavelength fixed optical transmission equipment  19 - 4 . The output optical signal from the wavelength tunable optical transmission equipment  100  is combined or multiplexed with other wavelengths λ 1  to λ 3  by an optical combiner  21  before transmitted on the optical fiber  22 . The optical combiner  21  is an optical component that connect optical signals regardless of wavelengths. For instance, an optical coupler is used as an optical combiner in the preferred embodiment.  
         [0042]    A recovery sequence is achieved by the wavelength tunable optical transmission equipment  100  even if a failure occurs in either of the three other pieces of wavelength fixed optical transmission equipment  19 - 1  through  19 - 3  which respectively output wavelengths different from λ 4 . Any of the various types of the wavelength tunable transmission equipment,  100 B,  100 C,  100 D and  100 F with a modulator or either of the modulator-integrated optical sources  100 A and  100 E is used as wavelength tunable optical transmission equipment  100  in a preferred embodiment of the WDM network. The same is said of embodiments discussed below.  
         [0043]    According to this embodiment, by providing one piece of wavelength tunable optical transmission equipment  100  mentioned above as an auxiliary unit in a WDM network using multiple pieces of wavelength fixed optical transmission equipment, the network recovers from the failure even if one of the above wavelength fixed optical transmission equipment fails. This recovery process also works even if two or more pieces of wavelength fixed optical transmission equipment fail at a time, the network resets to normal operation by using additionally available two or more pieces of wavelength tunable optical transmission equipment.  
         [0044]    In this and subsequent embodiments, wavelength tunable optical transmission equipment is installed in place of wavelength fixed optical transmission equipment.  
         [0045]    A further embodiment of the WDM network according to this invention is explained with reference to FIG. 10. FIG. 10 is a block diagram of the WDM network. Composed of nodes  26 - 1 ,  26 - 2  and  26 - 3 , this network uses a system that enables data transmission from the node  26 - 1  to the node  26 - 3  with four wavelengths by combining the two conventional systems of data transmission between the nodes  26 - 1  and  26 - 2  and between the nodes  26 - 2  and  26 - 3  with two wavelengths λ 1  and λ 2 .  
         [0046]    At the node  26 - 1 , two pieces of wavelength fixed optical transmission equipment  19 - 1 - 1  and  19 - 1 - 2  are used to multiplex wavelengths λ 1  and λ 2  for data channels  1  and  2  through an optical combiner  21 - 1 , and the output from the optical combiner  21 - 2  is then transmitted to the node or a transponder  26 - 2 . At the node  26 - 2 , two pieces of fixed wavelength optical transmission equipment  19 - 2 - 1  and  19 - 2 - 2  are used to multiplex wavelengths λ 1  and λ 2  for data channels  3  and  4  through an optical combiner  21 - 2 , and the output from the optical combiner  21 - 2  is then transmitted to the node  26 - 3 . Also, at the node  26 - 2 , the wavelength multiplexed signal that was sent from the node  26 - 1  is demultiplexed or divided into wavelengths λ 1  and λ 2  by wavelength demultiplexer  23 - 1 . The optical signal at wavelength λ 1  and λ 2  is once converted into electric signals through optical receiving equipment  24 - 1 - 1 ,  24 - 1 - 2  and are subsequently converted into optical signals at wavelengths at λ 3  and λ 4  by wavelength tunable optical transmission equipment  100 - 1  and  100 - 2 . The two optical signals at wavelengths λ 3  and λ 4  from channels  1  and  2  are combined with the optical signal at wavelengths λ 1  and λ 2  from channels  3  and  4  through an optical combiner  21 - 2  before being transmitted to node  26 - 3 . At the node  26 - 3 , the wavelength demultiplexer  23 - 2  divides the combined signal into wavelengths λ 1  through λ 4 , which are converted into electric data signals by optical receiving equipment  24 - 1  through  24 - 4 . Signals in channels  1  and  2  have been converted to have wavelengths λ 3  and λ 4  as a result of wavelength conversion at the node  26 - 2 . Advantageously, in this embodiment, transmission between nodes  26 - 1  and  26 - 3  is made possible by wavelength conversion at the node  26 - 2  without modifying the equipment at the node  26 - 1 .  
         [0047]    A device that combines optical transmission equipment and optical receiving equipment with its transmission wavelength stabilized at a prescribed level is called an optical transponder. In general, on the light receiving side, the wavelength demultiplexer works with high accuracy so the optical receiving equipment may have a broad bandwidth for wavelengths including λ 1  through λ 4 . For this reason, the transponder which combines optical receiving equipment and wavelength tunable optical transmission equipment is flexible enough to be able to receive and send signals at any of the wavelengths λ 1  to λ 4 . On the other hand, a conventional transmission system have as many transponders as a number of wavelengths for a transponder failure because the transmission wavelength is fixed for each transponder. By providing a transponder with the wavelength tunable optical transmission equipment according to the current invention as an auxiliary transponder, even if any of the fixed wavelength transponders fails in the network, the failed transponder is replaced by the auxiliary transponder. This decreases the required number of auxiliary transponders.  
         [0048]    According to this invention, an arrangement is made to disallow output optical signals during the wavelength tuning process and allow output of optical signals after the tuning is finished so that wavelength tunable optical transmission equipment for a WDM system does not affect the characteristics of communications at other wavelengths. In addition, a cost-efficient WDM network is implemented using wavelength tunable optical transmission equipment according to the current invention.