Abstract:
A Mach-Zehnder optical modulator that can attain a higher transmission speed than the driver signal speed but also attains, by setting multi-value amplitudes, an information transmission volume corresponding to an integer multiple of a single driver while allowing the transmission speed to remain as it is, wherein plural optical waveguide branches and plural Mach-Zehnder modulator portions are used, and by utilizing a propagation delay of a modulation signal for driving each of the modulator portions there is attained a transmission speed proportional to the delay time. This leads to a transmission signal having a frequency higher than the cut-off frequency of, for example, an IC or a transmission line substrate having an electrical characteristic of generating and propagating a modulation signal in such a form as an optical MUX (multiplexer).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a modulator in an optical transmission module and more particularly to a Mach-Zehnder optical modulator.  
           [0002]    Heretofore, as a modulator for an optical transmission module there have been known an electric absorption (EA) type and a Mach-Zehnder interferometer type. The electric absorption type modulator utilizes a mechanism such that if a modulation signal voltage is applied to light propagated through a waveguide, the resulting electric field causes an electric absorption coefficient in a medium to change, thereby intercepting the light. On the other hand, the Mach-Zehnder interferometer type utilizes a mechanism such that when light propagated through a waveguide is branched in two directions and a modulation signal current is flowed through the center of each branch, there occur magnetic fields of opposite phases with respect to grounds provided on opposite sides in a sandwiching relation to the waveguides, so that the phases of light signals propagated through the respective routes become opposite to each other and the phase lead and lag are offset each other when both lights are later combined together. It is the Mach-Zehnder modulator that utilizes this mechanism.  
           [0003]    FIGS.  10  to  12  are schematic diagrams showing a schematic construction of the conventional Mach-Zehnder modulator. In these figures, light guided through an optical waveguide  101  is branched to two optical waveguides  117   a  and  117   b  at an optical waveguide branch point  102  and the light signals propagated respectively through the optical waveguides  117   a  and  117   b  are changed in phase by electric fields developed between an electrode  104  and grounds  105   a,    105   b  in accordance with a modulation signal generated by a modulation signal generator  103 . FIG. 11 shows a case where the modulation signal generator  103  is OFF, while FIG. 12 shows a case where the modulation signal generator  103  is ON.  
           [0004]    With the modulation signal generator  103  OFF as in FIG. 11, the light signals propagated through the branched waveguides are joined at a junction  106  without any change like their branching at the branch point  102 , so that there is no change in the light signal before and after the modulator. On the other hand, when the modulation signal generator  103  is ON as in FIG. 12, refractive indices in the waveguides are changed by electric fields generated respectively between the electrode  104  and the ground  105   a  and between the electrode  104  and the ground  105   b  (Pockels effect), resulting in that the light signals propagated through the waveguides  117   a  and  117   b  become ±90° out of phase with each other and become 180° out of phase at the junction  106 . This can offset the phase lead and lag. Thus, a modulation signal can be imparted to light. In this single Mach-Zehnder modulator, however, the driver&#39;s speed is an upper limit of the transmission speed.  
           [0005]    JP-A-112688/1998 discloses a duobinary signal generating method wherein a semiconductor laser, a light intensity modulator and an optical phase modulator are cascaded and one branched signal is subjected to light intensity modulation, while the other signal is subjected to optical phase modulation.  
         SUMMARY OF THE INVENTION  
         [0006]    Above-described conventional methods are unable to attain a higher transmission speed than a driver signal which drives the modulator. Besides, since it is only whether light passes or is cut off that is available as a working mode, it has been impossible, either, to set multi-value amplitudes. Therefore, for example in such a high-speed signal propagation as exceeds 40 Gbps, a restriction has heretofore been placed by a driver circuit or by a line which is for propagation of the signal to a modulator, independently of characteristics of a laser diode (LD) and an optical fiber.  
           [0007]    In the JP-A-112688/1998, light is phase-modulated and therefore, in case of transmitting light with optical fiber over along distance, it is difficult to reproduce a light signal accurately because the phase changes.  
           [0008]    It is an object of the present invention to solve the above-mentioned problems and provide a Mach-Zehnder optical modulator wherein conventional driver circuit and transmission line left intact, a modulator is used like an optical MUX (multiplexer) and a signal corresponding to an integer multiple of a signal generated by a single driver circuit is used to permit an increase in the amount of information capable of being transmitted during a certain period of time.  
           [0009]    It is another object of the present invention to provide a Mach-Zehnder optical modulator wherein the amount of information capable of being transmitted can be increased by multi-value amplitudes although the transmission speed remains the same.  
           [0010]    For achieving the objects, according to one aspect of the invention, there is provided a Mach-Zehnder optical modulator comprising: a first modulation signal generator for generating a first modulation signal to modulate light signals guided by first and second optical waveguides, the modulation being conducted so that the phases of the light signals become opposite to each other; and a second modulation signal generator for generating a second modulation signal to modulate light signals guided by third and fourth optical waveguides, the modulation being conducted so that the phases of the light signals become opposite to each other; wherein the transmission speed or the amount of information transmitted is increased in accordance with modulation signals provided from the first and second modulation signal generators.  
           [0011]    In the aspect of the invention, there is provided means for generating a phase shift between the first and second modulation signals generated from the first and second modulation signal generators respectively, a phase between light signals modulated respectively in the first and second optical waveguides and in the third and fourth optical waveguides is changed, and thereafter both light signals are combined together and outputted.  
           [0012]    There is also provided means for generating a phase shift between first and second modulation signals generated from the first and second modulation signal generators respectively, and the third optical waveguide is a joined optical waveguide of both first and second optical waveguides.  
           [0013]    Further, the quantity of light passing through the first and second optical waveguides and the quantity of light passing through the third and fourth optical waveguides are made different from each other and the first to fourth optical waveguides are joined together to provide an output.  
           [0014]    According to an another aspect of the invention, there is provided a Mach-Zehnder optical modulator which modulates a light signal propagated through an optical waveguide in accordance with a modulation voltage applied, the optical waveguide having plural, at least two, branches, and which comprises plural, at least two, Mach-Zehnder optical modulator portions for modulating light signals propagated through optical waveguides in accordance with the modulation voltage.  
           [0015]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    A preferred form of the present invention illustrated in the accompanying drawings in which:  
         [0017]    [0017]FIG. 1 is a schematic diagram showing a Mach-Zehnder optical modulator according to a first embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a diagram showing amplitudes and phases of light signals in optical waveguides;  
         [0019]    [0019]FIG. 3 is a schematic diagram showing an example of delaying the phase of a modulation signal in the Mach-Zehnder optical modulator;  
         [0020]    [0020]FIG. 4 is a waveform diagram for explaining a phase lag of a modulation signal caused by a difference in length between signal transmission lines;  
         [0021]    [0021]FIG. 5 is a schematic diagram showing a Mach-Zehnder optical modulator according to a second embodiment of the present invention;  
         [0022]    [0022]FIG. 6 is a diagram showing amplitudes and phases of light signals in optical waveguides shown in FIG. 5;  
         [0023]    [0023]FIG. 7 is a schematic diagram showing a Mach-Zehnder optical modulator according to a third embodiment of the present invention;  
         [0024]    [0024]FIG. 8 is a schematic diagram showing a Mach-Zehnder optical modulator according to a fourth embodiment of the present invention;  
         [0025]    [0025]FIG. 9 is a diagram showing amplitudes and phases of light signals in optical waveguides shown in FIG. 8;  
         [0026]    [0026]FIG. 10 is a schematic diagram showing a schematic construction of a conventional Mach-Zehnder optical modulator;  
         [0027]    [0027]FIG. 11 also illustrates schematically the conventional Mach-Zehnder optical modulator; and  
         [0028]    [0028]FIG. 12 further illustrates schematically the conventional Mach-Zehnder optical modulator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The present invention will be described hereinunder by way of embodiments thereof and with reference to the accompanying drawings.  
         [0030]    A first embodiment of the present invention is now described with reference to FIGS. 1 and 2. FIG. 1 schematically illustrates a Mach-Zehnder optical modulator according to the first embodiment. FIG. 2 illustrates amplitudes and phases of light signals in optical waveguides shown in FIG. 1, in which time t is plotted along the axis of abscissa, while phases −180°, −90°, 0°, 90°, 180° and amplitudes  1 ,  2 ,  3 ,  4  are plotted along the axis of ordinate. The phases are represented by dotted lines and the amplitudes are represented by solid lines.  
         [0031]    A light signal propagated through an optical waveguide  101  in FIG. 1 is a signal having an amplitude of unit  4  and a phase of  0 , i.e., an amplitude  4 , phase  0  signal (# 1 ), as shown in FIG. 2A. At a branch point  102   a,  this signal is divided into two signals, which are conducted to optical waveguides  118   a  and  118   b,  respectively. The signals in the optical waveguides  118   a  and  118   b  are amplitude  2 , phase  0  light signals (# 2 ) and (# 3 ) each having an amplitude of unit  2  and a phase of  0 , as shown in FIG. 2B. These light signals are each divided into two light signals at branch points  102   b  and  102   c  respectively which are provided in the optical waveguides. The light signals branched at the branch point  102   b  are conducted to optical waveguides  119   a  and  119   b,  respectively. And, electric fields are developed between an electrode  104   a  and a ground  105   a  and also between the electrode  104   a  and a ground  105   b  in accordance with a modulation signal generated from a modulation signal generator  103   a.  The phase is changed with the electric fields. The signal in the optical waveguide  119   a  becomes a signal (# 4 ) of amplitude  1  and phase  0  or −0.5 π, as shown in FIG. 2C, while the signal in the optical waveguide  119   b  becomes a signal (# 5 ) of amplitude  1  and phase  0  or 0.5 π (0/0.5 π), as shown in FIG. 2D.  
         [0032]    Likewise, the light signals branched at the branch point  102   c  are conducted to optical waveguides  119   c  and  119   d,  respectively. And, electric fields are developed between an electrode  104   b  and a ground  106   a  and also between the electrode  104   b  and a ground  106   b  in accordance with a modulation signal generated from a modulation signal generator  103   b  which modulation signal has a time lag of ½ clock relative to the modulation signal generated from the modulation signal generator  103   a.  The phase is changed with the electric fields.  
         [0033]    Now, with reference to FIGS. 3 and 4, explained is means for imparting a time lag to the modulation signal provided from the modulation signal generator  103   b  with respect to the modulation signal provided from the modulation signal generator  103   a.    
         [0034]    [0034]FIG. 3 schematically shows an example of delaying the phase of a modulation signal in the Mach-Zehnder optical modulator according to the present invention. As shown in the figure, a modulation signal is fed from a tuned modulation signal generator, in the tuned state, to modulator drive signal terminals  901   a.  and  901   b  and a phase lag can be generated from a difference between lengths L and L′ of which are from electrodes  104   a,    104   b  to signal transmission lines  1005   a,    1005   b.  Where the length L′ of the signal transmission  1005   b  is larger than the length L of the signal transmission line  1005   a,  it is possible to delay the phase of the modulation signal which has passed through the signal transmission line  1005   b.    
         [0035]    [0035]FIG. 4 is a waveform diagram for explaining a phase lag of a modulation signal caused by a difference in length between signal transmission lines. If there is a difference between the length L and L′ of the signal transmission lines  1005   a  and  1005   b  as in FIG. 3, the phase of the signal passing through the longer signal transmission line  1005   b  lags. The reference mark “α” in FIG. 4 represents this phase lag. Thus, by selecting the lengths L and L′ of the signal transmission lines  1005   a  and  1005   b,  the modulation signal generated from the modulation signal generator  103   b  undergoes a phase lag of ½ clock relative to the modulation signal generated from the modulation signal generator  103   a  in FIG. 1. Thus, from a tuned modulation signal  1001  there occur modulation signals  1002   a  and  1002   b  in accordance with the difference between the length L and L′ of the signal transmission lines  1005   a  and  1005   b.  It is seen that a signal having a time lag is generated.  
         [0036]    A description will now be given again with reference to FIG. 2. The light signal in the optical waveguide  119   c  becomes a signal (# 6 ) of amplitude  1  and phase  0  or −0.5 π, as shown in FIG. 2E, while the light signal in the optical waveguide  119   d  becomes a signal (# 7 ) of amplitude land phase  0  or 0.5 π, as shown in FIG. 2F. In this embodiment, the modulation signal from the modulation signal generator  103   b  modulates the light signal in the optical waveguide  119   c  so as to cause a phase change like 0, −0.5 π, 0, −0.5 π, −0.5 π and modulates the light signal in the optical waveguide  119   c  so as to cause a phase change like 0, 0.5 π, 0, −0.5 π, 0.5 π. These phase changes are merely one example. Various other changes can occur depending on the contents of data.  
         [0037]    The light signals in the optical waveguides  119   a  and  119   b  join at a junction  107   a,  while the light signals in the optical waveguides  119   c  and  119   d  join in a junction  107   b.  The joined signals are amplitude  2 , phase  0  signals (# 8 , # 9 ) as shown in FIGS. 2G and H. These signals become signals (# 10 , # 11 ) with a phase difference of ±0.5 π, as shown in FIGS. 2I and J, by electric fields generated between an electrode  109  and a ground  110   a  and also between the electrode  109  and a ground  110   b  in accordance with a signal  108  which is always constant. If both signals have amplitudes when joining at a junction  107   c,  their phase difference becomes ±π and thus offsets. If only one signal has amplitude, it is impossible to make offset even with the phase difference ±π and there remains the amplitude  2 . Therefore, the signal outputted from the optical waveguide  111  becomes an amplitude  2 , phase ±0.5 π signal (# 12 ), as shown in FIG. 2K. This is twice as fast as the modulation signals generated from the modulation signal generators  103   a  and  103   b,  thus permitting the generation of a light signal which exceeds electrical limitations on the modulation signals. Further, by providing Mach-Zehnder modulators in multiple stages, it is made possible to generate an integer-multiple signal. For example, by adding signals # 8  and # 9  it is possible to obtain a light signal having amplitudes  2  and  4 .  
         [0038]    Next, second and third embodiments of the present invention will be described below with reference to FIGS. 5, 6, and  7 .  
         [0039]    [0039]FIG. 5 is a schematic diagram showing a Mach-Zehnder optical modulator according to a second embodiment of the present invention. FIG. 6 illustrates light amplitudes and phases in optical waveguides shown in FIG. 5, in which time t is plotted along the axis of abscissa, while phases −180°, −90°, 0°, 90°, and 180°, as well as amplitudes  1 ,  2 ,  3 , and  4 , are read along the axis of ordinate. The phase and amplitudes are indicated with dotted lines and solid lines, respectively.  
         [0040]    Assuming that a light signal guided by an optical waveguide  601  is a signal (# 1 ) of amplitude  3 , phase  0 , as shown in FIG. 6A, the light signal is divided into two light signals at a branch point  602   a  for branching to two optical waveguides of different sectional areas. For example, if a sectional area of an optical waveguide  611   a  is 1 and that of an optical waveguide  611   b  is 2, the signal in the optical waveguide  611   a  becomes a signal (# 2 ) of amplitude  1 , phase  0 , and the signal in the optical waveguide  611   b  becomes a signal (# 3 ) of amplitude  2 , phase  0 , as shown in FIG. 6B. Further, the signal in the optical waveguide  611   a  is branched at a branch point  602   b  to optical waveguides  612   a  and  612   b  of the same sectional area. Likewise, the signal in the optical waveguide  611   b  is branched at a branch point  602   c  to optical waveguides  613   a  and  613   b  of the same sectional area. The light signals guided by the optical waveguides  612   a  and  612   b  undergo a change in phase by electric fields generated between an electrode  604   a  and a ground  605   a  and also between the electrode  604   a  and a ground  605   b  in accordance with a modulation signal generated from a modulation signal generator  603   a.  The thus phase-changed signals become a signal (# 4 ) of amplitude  0 . 5 , phase  0  or +0.5 π, and a signal (# 5 ) of amplitude  0 . 5 , phase  0  or −0.5 π, respectively, as shown in FIG. 6C. As to which of these phases is to be continued and for what period, they vary depending on the contents of data. FIG. 6 merely shows one example.  
         [0041]    Likewise, the light signals branched at the branch point  602   c  to the optical waveguides  613   a  and  613   b  undergo a change in phase by electric fields generated between an electrode  604   b  and a ground  606   a  and also between the electrode  604   b  and a ground  606   b  in accordance with a modulation signal provided from a modulation signal generator  603   b.  The thus phase-changed signals become a signal (# 6 ) of amplitude  1 , phase  0  or +0.5 π, and a signal (# 7 ) of amplitude  1 , phase  0  or −0.5 π, respectively, as shown in FIG. 6E. The signals (# 4 , # 5 ) and (# 6 , # 7 ) join at junctions  607   a  and  607   b,  respectively, into a signal (# 8 ) of amplitude  1 , phase  0 , as shown in FIG. 6D and a signal (# 9 ) of amplitude  2 , phase  0 , as shown in FIG. 6F. Both signals (# 8 , # 9 ) then join together at a junction  607   c.  At this time, if both signals have respective amplitudes, the amplitudes are combined into 3, while if there is amplitude on only one side, the amplitude becomes 1 or 2 according to the associated signal. If both signals do not have any amplitude, the value of amplitude becomes 0 and the joined signal is conducted to an optical waveguide  608 . Thus, the signal outputted from the optical waveguide  608  becomes a signal (# 10 ) shown in FIG. 6G which has four types of amplitudes  0 ,  1 ,  2 ,  3 . This is a quaternary value having an information volume twice as large as that of the binary value of the modulation signals generated by the modulation signal generators  603   a  and  603   b,  thus permitting generation of a light signal able to transmit information twice as much while maintaining the modulation signal speed. Further, by using various sections of branches and providing Mach-Zehnder modulators in multiple stages, it becomes possible to generate a signal having an integer multiple of information volume.  
         [0042]    In the embodiment illustrated in FIG. 5, waveguides of different sectional areas are provided at the waveguide branch point  602   a  to branch into signals of different quantities of light toward the branch points  602   b  and  602   c.  But there may be adopted such a construction as shown in FIG. 7.  
         [0043]    [0043]FIG. 7 is a schematic diagram of a Mach-Zehnder optical modulator according to a third embodiment of the present invention. In the previous embodiment illustrated in FIG. 5 there is made branching into optical waveguides of different sectional areas at the branch point  602   a,  while in this third embodiment there is made branching at different angles into optical waveguides  703   a  and  703   b,  which branching is made at a branch point  702   a,  thereby making division into signals of different light quantities for the optical waveguides  703   a  and  703   b.    
         [0044]    Now, with reference to FIGS. 8 and 9, a description will be given below about a fourth embodiment of the present invention.  
         [0045]    [0045]FIG. 8 is a schematic diagram of a Mach-Zehnder optical modulator according to a fourth embodiment of the present invention. FIG. 9 illustrates amplitudes and phases of light signals in optical waveguides, in which time t is plotted along the axis of abscissa, while phases −180°, −90°, 0°, 90°, 180°, as well as amplitudes  1 ,  2 ,  3 ,  4 , are read along the axis of ordinate. The phases and amplitudes are represented by dotted lines and solid lines, respectively.  
         [0046]    As shown in FIG. 9A, a signal guided by an optical waveguide  101  in FIG. 8 is a signal (# 1 ) having an amplitude of unit  4  and phase  0 , i.e., an amplitude  4 , phase  0  signal. This signal is divided into two signals at a branch point  102   a  of the optical waveguide, which signals are then guided by optical waveguides  118   a  and  118   b  respectively. As shown in FIG. 2B, the light signals in the optical waveguides  118   a  and  118   b  are light signals (# 2 , # 3 ) having an amplitude of unit  2  and phase  0 , i.e., amplitude  2 , phase  0  signals. The light signal in the optical waveguide  118   a  is divided into two light signals at a branch point  102   b  provided in the optical waveguide  118   a.  The light signals thus divided at the branch point  102   b  is guided by optical waveguides  119   a  and  119   b  respectively. The optical waveguide  119   a  is disposed between an electrode  104   a  and a ground  105   a  and the optical waveguide  119   b  is disposed between the electrode  104   a  and a ground  105   b,  so that electric fields are generated in the optical waveguides  119   a  and  119   b  in accordance with a modulation signal provided from a modulation signal generator  103   a  and the phases are changed with the electric fields. The signal in the optical waveguide  119   a  becomes a signal (# 4 ) of amplitude  1 , phase  0  or −0.5 π, as shown in FIG. 9C, while the signal in the optical waveguide  119   b  becomes a signal (# 5 ) of amplitude  1 , phase  0  or 0.5 π, as shown in FIG. 9D. The signals in both optical waveguides  119   a  and  119   b  are joined at a junction  107   a  into an optical signal (# 8 ) of amplitude  2 , phase  0 , as shown in FIG. 9E, which signal is then conducted to an optical waveguide  120   a.    
         [0047]    The optical waveguide  120   a  is disposed between an electrode  109  and a ground  110   a,  while the optical waveguide  118   b  is disposed between the electrode  109  and a ground  110   b,  and a modulation signal which lags (or leads) ½ clock with respect to the modulation signal generated from the modulation signal generator  103   a  is fed to the electrode  109  and the ground  110   a  or  110   b  from a modulation signal generator  108 . Consequently, as shown in FIG. 9F, the light signal in the optical waveguide  120   a  becomes a light signal (# 10 ) of amplitude  2  or  0 , phase  0  or −0.5 π, which lags ½ clock in comparison with FIG. 9E. Further, as shown in FIG. 9G, the light signal in the optical waveguide  118   b  becomes a light signal (# 11 ) of amplitude  2 , phase  0  or +0.5 π. The light signals in the optical waveguides  120   a  and  118   b  are combined together at a branch point  107   c  into an optical signal (# 12 ) of amplitude  4 ,  2 , or  0 , phase  0  or +0.5 π, as shown in FIG. 9H. Thus, there is obtained a multi-value signal permitting a high-speed transmission in comparison with the modulation signal generated by the driving modulation signal generator.  
         [0048]    According to the present invention, as described above, by combining optical waveguides each having a Mach-Zehnder modulator there can be obtained an optical modulator which can generate a light signal higher in speed and/or larger in information volume than a modulation signal generated by a modulation signal generator.  
         [0049]    In the present invention, as set forth above, there is attained a transmission speed higher by an integer multiple than the driver signal speed.  
         [0050]    Also, by setting multi-value amplitudes there is attained an information transmission volume corresponding to an integer multiple of a single driver while leaving the transmission speed intact.  
         [0051]    The foregoing invention has been described in terms of preferred embodiments. However, those skilled, in the art will recognize that many variations of such embodiments exist. Such variations are intended to be within the scope of the present invention and the appended claims.