Abstract:
A problem of a disclosed technique is as follows. The present problem is to control wavelengths of light sources in each individual optical transmitters, based on a plurality of reference wavelengths with high accuracy by a wavelength division multiplexing optical transmitting apparatus. Means for solving the problem is as follows. A reference light source is capable of simultaneously or successively generating a plurality of reference wavelengths different from one another. These reference wavelengths are demultiplexed by an optical multiplexer, which in turn are inputted to optical transmitters respectively. In the optical transmitters, their own light-source wavelengths are respectively controlled so as to coincide with the input reference wavelengths or reach inherent or proper values at which differences in wavelength exist.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wavelength stabilized light source. The present invention relates particularly to a wavelength stabilized optical transmitting apparatus and an optical transmitting apparatus both having a plurality of light sources, wherein each of wavelengths employed in a wavelength division multiplexing optical communication system is controlled to a predetermined value. 
     2. Description of the Related Art 
     With abrupt growth of the recent information communication market, an optical transmitter-receiver apparatus or system capable of transmitting a larger quantity of information has actively been developed. The development of a wavelength division multiplexing optical transmitter-receiver system capable of making a leap increase in the information amount per optical fiber becomes pronounced in particular. However, since a limitation is imposed on a wavelength region suitable for transmission executed with a transmission loss less reduced at each optical fiber. It is necessary to increase a number of optical channels for optical or light signals within the limited wavelength region. It is necessary to narrow the interval between adjacent wavelengths of each individual light signals for the purpose of increasing the number of optical channels. A problem arises in that a semiconductor laser principally used as a light source for a conventional wavelength division multiplexing optical transmitting apparatus or system greatly varies in wavelength according to changes in ambient temperature and drive current. 
     Further, a phenomenon has widely been known wherein even if the ambient temperature of the semiconductor laser is kept constant and the drive current is also held constant, the wavelength varies due to a variation with time such as a change in composition of the semiconductor laser if it is used over a long period. There is a possibility that such a variation in wavelength from each set value will cause interference from other light signals, a substantial degradation in transmission quality or a fall into an impossible-to-transmit state. It is therefore essential that an optical transmitter is provided with some wavelength monitoring mechanism and wavelength tunable mechanism to perform control for correcting a shift or deviation made from each set wavelength with a view toward implementing a high-density wavelength division multiplexing optical transmission system capable of narrowing a wavelength-to-wavelength interval. 
     As the optical transmitting apparatus provided with the wavelength monitoring mechanism and the wavelength tunable mechanism, an optical wavelength division multiplexing transmitting apparatus shown in FIG. 9 is known (see Japanese Patent Application Laid-Open No. Hei 10-209973, for example). Namely, optical or light signals  9  through  12  having wavelengths λ 1 , λ 2 , λ 3  and λ 4  different from one another, which are sent from optical transmitters  1  through  4 , are respectively applied to an optical multiplexer or an optical coupler  300  through optical fibers  5 ,  6 ,  7  and  8 , where they are multiplexed into a wavelength division-multiplexed signal  310 , which in turn is applied to an optical power divider  305  through an optical fiber  304 . The wavelength division-multiplexed signal  310  is divided into two by the optical power divider  305 , one of which is sent to a wavelength deviation detector  302  as a wavelength division-multiplexed signal  311  through an optical fiber  307 , and the other of which is transmitted through an optical fiber  306 . 
     The wavelength deviation detector  302  generates deviations equivalent to differences between values of wavelengths included in the wavelength division-multiplexed signal  311  and the set values of the wavelengths assigned to the optical transmitters in advance, and sends their deviation signals  313  to a wavelength control circuit  301  on a time-sharing basis. The wavelength control circuit  301  and the optical transmitters  1  through  4  are respectively connected to one another through wires  321  and  324 . The wavelength control circuit  301  generates wavelength control signals  331  through  334  for controlling the values of the wavelengths of the present signals to the values of the wavelengths assigned in advance, based on the respective deviation signals inputted to the wavelength control circuit  301 . For example, signals for controlling the temperatures of light sources in the optical transmitters  1  through  4  or signals for controlling drive currents of the light sources are used as the wavelength control signals  331  through  334 . The wavelength control signals  331  through  334  are distributed to the optical transmitters  1  through  4  through the wires  321  through  324  respectively. The optical transmitters  1  through  4  respectively control the wavelengths of their own light sources according to the wavelength control signals  331  through  334  and send out the wavelengths assigned to their own optical transmitters in advance. 
     SUMMARY OF THE INVENTION 
     A mechanism for identifying each individual light signals included in wavelength division-multiplexed light signals and measuring the values of wavelengths of the light signals is essential to the aforementioned prior art. 
     For example, intensity modulation is slightly effected on each of light signals sent out from within each individual optical transmitters, based on low-frequency signals different from an information transmission region. The values of the frequencies of these low-frequency signals are respectively set to different values every optical transmitters, and a wavelength deviation detector performs phase sensitive detection with the frequencies of the respective low-frequency signals, whereby deviations corresponding to the differences between the wavelengths of each individual optical transmitters and the wavelengths assigned to the optical transmitters in advance can be detected. However, a problem arises in that the intensity modulation using such low frequencies is unnecessary for original information transmission and might be one cause of degradation in transmission quality. 
     As a mechanism for identifying other light signals and measuring wavelengths thereof, a system is also known wherein a wavelength deviation detector measures the wavelength of each light signal while successively scanning a band-pass wavelength variable filter or an optical power divider or the like, and measures deviations corresponding to the differences between the measured wavelengths of light signals and the wavelengths assigned to each individual optical transmitters in advance. However, a problem arises in that since no identifying information exists in each light signal itself, it is not possible to correct each wavelength when the wavelength of the optical transmitter  1 , which is to be originally set as the wavelength λ 1 , and the wavelength of the optical transmitter  2 , which it to be originally set as the wavelength λ 2 , are respectively replaced by λ 2  and λ 1  with respect to each other. 
     Since, in either case, all the optical transmitters and the wavelength deviation detector are elements which constitute an electrical control loop, there is a possibility that when one optical transmitter is replaced by another due to it trouble or malfunction, it will interfere with the operation of other normal optical transmitters. 
     Thus, a principal object of the present invention is to implement an optical transmitting apparatus capable of stabilizing wavelengths of each individual optical transmitters without superimposing signals other than information necessary for information transmission, i.e., signals dedicated for wavelength control on light signals respectively. 
     Another object of the present invention is to implement an optical transmitting apparatus capable of achieving the above object, and when a failure or malfunction occurs in any of respective optical transmitters, performing such wavelength control as not to exert an influence on other optical transmitters due to the replacement of the faulty optical transmitter by another. 
     In an optical transmitting apparatus of the present invention to achieve the above objects, control units for receiving other light signals used to control characteristics such as wavelengths, optical intensities, optical phases, etc. of light signals respectively sent from optical transmitters, from the outside the optical transmitters and controlling the respective optical transmitters based on the received signals are respectively provided inside the optical transmitters or provided adjacent to the optical transmitters. Namely, a reference light source (wherein the wavelengths of signals generated therefrom are different from those of the light signals transmitted from the optical transmitters. Further, the wavelengths of the generated light signals are regarded as sufficiently stable.) capable of simultaneously or successively generating a plurality of reference wavelength light signals, means for demultiplexing the respective reference wavelength light signals emitted from the reference light-source and inputting the demultiplexed reference wavelength light signals to the plurality of optical transmitters respectively, and a control unit for controlling light-source wavelengths so as to allow light-source wavelengths of the optical transmitters to coincide with the wavelengths of the reference wavelength light signals, corresponding to the corresponding optical transmitters, of the input respective reference wavelength light signals, or so as to reach inherent or proper values at which differences in wavelength exist, are provided for each optical transmitter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a diagram showing a configuration of an embodiment 1 of an optical transmitting apparatus according to the present invention; 
     FIG. 2 is a configurational diagram for describing a reference light source employed in the embodiment 1 of the present invention; 
     FIG. 3 is a diagram for describing an example of a signal outputted from the reference light source employed in the embodiment 1 of the present invention; 
     FIG. 4 is a diagram illustrating a configuration of an embodiment 2 of the present invention; 
     FIG. 5 is a diagram depicting a configuration of an embodiment 3 of the present invention; 
     FIG. 6 is a diagram showing an example of a configuration of one of optical transmitters  1  through  4  employed in the embodiments 1 through 3; 
     FIG. 7 is a diagram for describing one example of control operations of the optical transmitters  1  through  4  employed in the embodiments 1 through 3; 
     FIG. 8 is a diagram for describing one example of other control operations of the optical transmitters  1  through  4  employed in the embodiments 1 through 3; and 
     FIG. 9 is a diagram showing a configuration of a conventional optical transmitting apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. 
     &lt;Embodiment 1&gt; 
     FIG. 1 is a diagram showing a configuration of one embodiment of an optical transmitting apparatus according to the present invention. The optical transmitting apparatus according to the present embodiment includes a plurality of optical transmitters  1 ,  2 ,  3  and  4  for respectively generating light signals  9 ,  10 ,  11  and  12  having wavelengths λ 1  through λ 4  different from one another, optical fibers  5 ,  6 ,  7  and  8  for respectively transmitting the light signals  9 ,  10 ,  11  and  12 , and an optical multiplexer  17  for combining the light signals  9 ,  10 ,  11  and  12  sent through the optical fibers  5  through  8  into one and converting it into a wavelength division-multiplexed signal  18 . A reference light source  23  is provided outside the optical transmitters  1 ,  2 ,  3  and  4 . Each of the optical transmitters  1 ,  2 ,  3  and  4  has a light source for generating a first light signal, and a control means or unit for controlling at least a wavelength of the characteristics of the first light signal by using the first light signal and a second light signal emitted from the reference light source  23 . 
     In the above configuration, the optical multiplexer  17  comprises a Mach-Zehnder interferometer multiplexer, an arrayed waveguide multiplexer or the like. The output of the optical multiplexer  17  is connected to an optical circulator  22 . The optical circulator  22  sends the wavelength division-multiplexed signal  18  with a low loss as a wavelength division-multiplexed signal  19  and transmits a reference wavelength light signal  20  emitted from the reference light source  23  with a low loss as a reference wavelength light signal  21 . As the optical circulator  22 , may be used an already-existing one which applies a Faraday effect of a magneto-optic crystal, for example. 
     A reference wavelength launched from an outgoing port of the optical multiplexer  17  is demultiplexed every wavelengths according to an optical demultiplexed characteristic reciprocal with a multiplexed characteristic of the optical multiplexer. The demultiplexed reference wavelength light signals  13  through  16  are respectively sent to the optical transmitters  1  through  4  through the optical fibers  5  through  8 . The wavelengths of the demultiplexed reference wavelength light signals  13  through  16  are λref 1  through λref 4  respectively. The respective optical transmitters  1  through  4  cause light-source wavelengths in the optical transmitters to coincide with the wavelengths of the demultiplexed reference wavelength light signals  13  through  16  or perform such wavelength control as to fix the wavelengths with predetermined wavelength&#39;s offsets as viewed from the reference wavelength. The details of this operation will be described later with reference to FIG.  6 . Owing to such wavelength control, the wavelengths of the light signals  9  through  12  sent from the optical transmitters  1  through  4  respectively result in wavelengths based on the reference wavelength emitted from the reference light source  23 . 
     Incidentally, the reference wavelength light signals having the wavelengths λref 1  through λref 4  may be emitted simultaneously from the reference light source  23  or may be sent out in time sequence. 
     FIG. 2 shows one example of a configuration of the reference light source  23  employed in the above-described embodiment. This example is a configuration similar to, for example, ┌Optical comb generator┘ described in ┌Wide-Span Optical Frequency Comb Generator for Accurate Optical Frequency Difference Measurement┘ of the paper IEEE Journal of Quantum Electronics, vol.29, p.2693-p.2701 in 1993. 
     A light signal  202  emitted from a light source  201  such as a semiconductor laser or the like whose wavelength is λref, is launched into a Fabry-Perot resonator comprising half mirrors  203  and  207 . Further, a microwave cavity resonator  204  is placed inside the Fabry-Perot resonator. An optical frequency modulator comprised of an LiNbO 3  crystal is placed inside the microwave cavity resonator  204 . A microwave is applied to the microwave cavity resonator  204  by a microwave oscillator  206 . The microwave cavity resonator  204  is designed so as to resonate at a predetermined microwave frequency (100 GHz in the example of FIG.  2 ). The light signal  202  is subjected to optical frequency modulation even by the LiNbO 3  crystal when repeatedly multiple-reflected within the Fabry-Perot resonator. 
     As a result, the light signal transmitted through the half mirror  207  includes wavelengths spaced away from λref by wavelengths of an integral multiple of 100 GHz (=0.8 nm) to the long-wave and short-wave sides of λref except for the wavelength λref. The light signal transmitted therethrough is sent to the optical fiber  20  by a condenser lens  208 . 
     FIG. 3 shows the relationship between the wavelength and optical intensity of a light signal emitted from the light source  201  employed in the embodiment. Light signals are produced on the long-wave and short-wave sides of λref of the light source  201  at equal intervals with an oscillation frequency of the microwave oscillator. Thus, a plurality of reference wavelengths can be obtained simultaneously by making the oscillation frequency of the microwave oscillator identical to the wavelength interval of the wavelength division-multiplexed signal. Incidentally, the wavelength of the light source  201  is stabilized by an absolute optical frequency stabilized circuit using a resonant spectral line of an atom or molecule. Thus, all the wavelengths shown in FIG. 3 can also be stabilized simultaneously. 
     &lt;Embodiment 2&gt; 
     FIG. 4 is a diagram showing a configuration of another embodiment of an optical transmitting apparatus according to the present invention. Optical transmitters  1 ,  2 ,  3  and  4 , optical fibers  5  through  8 , light signals  9  through  12 , an optical circulator  22  and a reference light source  23  are those similar to those shown in FIG.  1  and are also identical in operation to those shown in FIG.  1 . The present embodiment is one in which the optical multiplexer  17  is replaced by an optical coupler  35  as compared with the first embodiment shown in FIG.  1 . Since the optical coupler  35  does not include wavelength selectivity in multiplex and demultiplex operations, a reference wavelength light signal  21  is equally distributed to the optical fibers  5  through  8  on the input side. 
     Thus, the reference light source  23  superimposes an intrinsic identification signal on each of wavelengths λref 1  through λref 4 . Each of the optical transmitters selects only one of the wavelengths λref 1  through λref 4  to be controlled, based on the intrinsic identification signal. For example, a method of intensity-modulating each individual reference light signals having the wavelengths λref 1  through λref 4 , based on low-frequency waves different in frequency can be used as a method of superimposing the intrinsic identification signal thereon. Each of the optical transmitters  1  through  4  selects the wavelength having the identification signal inherent in its own optical transmitter from the wavelengths λref 1  through λref 4  included in reference wavelength light signals  31  through  34 . Thereafter, the optical transmitters cause wavelengths of their light sources to coincide with one another or perform such wavelength control as to fix the wavelengths with predetermined offsets. Incidentally, the reference wavelength light signals having the wavelengths λref 1  through λref 4  emitted from the reference light source  23  may be sent out four simultaneously or may be sent out every predetermined time intervals and in time sequence. 
     While the four optical transmitters are shown in FIG. 4, no limitation is imposed on the number of optical transmitters. Further, the embodiment shown in FIG. 4 shows an example in which the optical fibers are used when the optical transmitters  1  through  4  and the optical coupler  35 , and the optical coupler  35  and the reference light source  23  are respectively connected to one another. However, substances (including even those in air) capable of transmitting light signals can also be used for their interconnection without being limited to the optical fibers. 
     &lt;Embodiment 3&gt; 
     FIG. 5 is a diagram showing a configuration of a further embodiment of an optical transmitting apparatus according to the present invention. Optical transmitters  1  through  4 , optical fibers  5  through  8 , light signals  9  through  12 , and an optical multiplexer  17  are substantially identical in configuration and operation to those identified by the same reference numerals in FIG.  1 . In the present embodiment, a reference light source  23  is directly connected to the respective optical transmitters  1  through  4  by optical fibers  41  through  44  respectively. The reference light source  23  generates reference wavelength light signals  45  through  48  corresponding to the respective optical transmitters  1  through  4  and transmits them to the optical transmitters  1  through  4  through the optical fibers  41  through  44  respectively. The optical transmitters  1  through  4  perform such waveform control as described in FIG.  6 . 
     FIG. 6 is a diagram showing an example of a configuration of one of the optical transmitters  1  through  4  employed in the embodiments 1, 2 and 3. A light signal  81  emitted from a light source  51  is launched into an optical power divider  52  through an optical fiber  58 . The optical power divider  52  sends most of the light signal launched therein to an optical circulator  53  through an optical fiber  59 . The optical power divider  52  delivers part thereof as a light signal  83  through an optical fiber  60  so as to be sent to an input port b of an optical switch  54 . The light signal transmitted to the optical circulator is sent to one input port of the optical multiplexer  17  with a low loss as a light signal  82 . 
     On the other hand, a reference wavelength light signal  85  sent from the reference light source  23  through the optical circulator  22  and the optical multiplexer  17  is transmitted to an input port a of the optical switch  54  through an optical fiber  61  with a low loss by the optical circulator  53 . An outgoing port c of the optical switch  54  is connected to a tunable optical filter  55 . 
     The tunable optical filter  55  can make use of a tunable optical filter or the like which scans an acousto-optic wavelength tunable filter or a dielectric multilayered filter on a mechanical or temperature basis. A light signal  86  is launched into the tunable optical filter  55  and thereafter enters an optical detector  56  through an optical fiber  63  as a light signal  87 . 
     An electric signal  88  photoelectrically-converted by the optical detector  56  is sent to a control circuit  57 . The control circuit  57  generates a signal  91  for controlling the optical switch  54  and a signal  90  for controlling the tunable optical filter  55 . Further, the control circuit  57  also generates a signal  89  for controlling the wavelength of the light source  51 . A signal for varying an operation temperature of the light source, a signal for controlling an operating current of the light source, etc. can be utilized as the signal  89  for controlling the wavelength of the light source. 
     Incidentally, substances (including even those in air) capable of transmitting light signals can be utilized as optical transmission lines given by the optical fibers in the drawing. 
     The operation of allowing the wavelength of the light source to coincide with the reference wavelength under the configuration shown in FIG. 6 will be explained with reference to FIG.  7 . 
     (1) The control circuit  57  first sends a control signal  91  to the optical switch  54  so as to allow the connection of the input port a and the outgoing port c of the optical switch  54  shown in FIG.  6 . 
     (2) In doing so, a reference wavelength light signal having a wavelength λref is inputted to the tunable optical filter  55 . 
     (3) Here, the control circuit  57  sends a control signal  90  for controlling a transmission wavelength of the tunable optical filter  55  to maximize, for example, a current value of an electric signal  88  which is transmitted through the tunable optical filter  55  and subjected to optical power/current conversion by the optical detector  56 . 
     (4) Next, the control circuit  57  sends a control signal  91  to the optical switch  54  so that the input port b and the output port c of the optical switch are connected to one another. 
     (5) In doing so, a light signal having a wavelength λn, which is emitted from the light source  51 , is inputted to the tunable optical filter  55 . 
     (6) Here, the control circuit  57  sends a control signal  89  for controlling the wavelength of the light source  51  to maximize an electric signal  88 , which is transmitted through the tunable optical filter  55  and photoelectrically-converted by the optical detector  56 . 
     (7) Owing to a series of operations described in the paragraphs (1) through (6), the wavelength λn of the light source  51  coincides with the reference wavelength λref n. 
     The control circuit  57  performs the series of operations at all times or at predetermined intervals to allow the wavelength of the light source to always coincide with the reference wavelength. 
     The operation of allowing the reference wavelength and the wavelength of the light source to coincide with each other has been described in FIG.  7 . However, the reference wavelength and the wavelength of the light source may also be stabilized while a predetermined waveform offset is being held. This case will be described with reference to FIG.  8 . FIG. 8 is basically identical in operation to FIG.  7 . Further, the optical transmitting apparatus is also similar in configuration to that shown in FIG.  6 . The operation shown in FIG. 8 is different from that shown in FIG. 7 in terms of transmission wavelength characteristics of the tunable optical filter  55 . 
     While only one intrinsic wavelength is used as the transmission wavelength of the tunable optical filter  55  in FIG. 7, the transmission wavelength of the tunable optical filter  55 , which is suitable for the operation of FIG. 8, is one having a cyclic or periodic transmission characteristic. As one example, may be mentioned, for example, Fabry-Perot etalon wherein both sides of a cylinder of quartz glass are coated with reflecting films. When Fabry-Perot etalon in which the length of the cylinder of the quartz glass is 50 microns, for example, is fabricated, a transmission wavelength characteristic thereof results in one having transmission peaks every about 40 nm. 
     If Fabry-Perot etalon is now created and controlled so that one of the transmission peaks assumes a 1550 nm bandwidth or region often used for optical transmission, then adjacent transmission wavelength peaks result in a 1510 nm region and 1590 nm region. The operation of the optical transmitting apparatus will now be described using FIG. 8 with the reference wavelength as the 1510 nm region by way of example. 
     In a manner similar to FIG. 7, the transmission wavelength of Fabry-Perot etalon is first controlled in such a manner that the reference wavelength transmitted through the Fabry-Perot etalon is brought to the maximum. As this control method, may be utilized a method of controlling the temperature of the Fabry-Perot etalon, etc. Upon such control, a transmission peak of a 1550 nm region also varies by the same one as the quantity of a variation in the transmission peak of the 1510 nm region. This is because the amount of change or displacement of an interval between the transmission peaks is extremely small as compared with the amount of change or displacement of the transmission wavelength in the characteristic of the Fabry-Perot etalon. 
     Next, the wavelength of the light source is controlled so as to coincide with it corresponding transmission peak placed on the long-wave side, which is spaced away 40 nm from the reference wavelength λref n′. 
     The wavelength of the light source is stabilized by this operation so as to have the reference wavelength and a predetermined wavelength offset (40 nm in the example shown in FIG.  8 ). 
     While the embodiments according to the present invention have been described above, the present invention is not necessarily limited to the embodiments. The respective embodiments respectively show the case in which the optical transmitters are provided four. However, the number of the optical transmitters is not limited. Further, the embodiments respectively show the example in which the optical fibers are used to connect the optical transmitters and the optical multiplexer, and the optical multiplexer and the reference light source to one another. However, substances (including even those in air) capable of transmitting light signals can also be used for their interconnection without limitations being imposed on the optical fibers. 
     Further, no limitations are imposed only on the examples shown in FIGS. 1,  4  and  5  as means for distributing a reference wavelength light signal emitted from a reference light source. For instance, optical circulators are inserted in the course of the optical fibers  5  through  8  and a reference wavelength light signal may be distributed to the optical transmitters through the optical circulators. Namely, no reference is made to means for optically connecting the reference wavelength light signal generated from the reference light source to each optical transmitter. 
     In the present invention as described above, there is no need to superimpose information other than information necessary for information transfer on light signals transmitted from each individual optical transmitters. Therefore, the quality of transmission of each transmitted light signal is improved. Further, each individual optical transmitters and their corresponding external devices are respectively connected to one another by optical transmission lines (optical fibers or the like) alone, and no electrical connections are required for wavelength control. Therefore, even when a single optical transmitter is replaced by another due to its trouble or the like, it does not exert an influence on other normal optical transmitters. Further, since no optical device whose wavelength is fixed, is installed within each optical transmitter, the wavelength can be controlled to free wavelengths within a wavelength tunable region of a tunable laser when the tunable laser is used as a light source. Thus, the use of the present invention allows construction of a wavelength stabilized optical transmitting apparatus in which the quality of transmission is high, high reliability is provided and the degree of freedom of each set wavelength is high, and an optical transmission system including the wavelength stabilized optical transmitting apparatus. 
     While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.