Optical modulator

Abstract: A Mach-Zehnder interferometer type modulator, constructed of a first and second optical waveguide (21, 22), a first and second electrode (33, 34) cooperating with the same, and a driving voltage source, wherein the driving voltage source is constructed of a first and second driving unit (31, 32) which drive independently the first and second electrodes (33, 34) in accordance with data input and wherein the first and second driving units (31, 32) apply first and second driving voltages, individually determined, to the first and second electrodes (33, 34).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator used in a transmission apparatus for transmitting high speed digital signals in an optical communication system using optical fibers as a transmission channel. In recent years, optical communication systems using optical signals as the medium of communication have rapidly been put into use. The optical modulators for producing the optical signals in the optical communication systems are extremely important devices. The present invention refers to such optical modulators, in particular external modulation type optical modulators. 2. Description of the Related Art In the conventional direct modulation type optical modulators, when the modulation speed became large, so-called frequency chirping occurred, resulting in deterioration of the high speed characteristics. Frequency chirping is the shift in the wavelength of the light during the rise and fall of the light signal with each such rise and fall. Frequency chirping causes waveform deterioration in the light signal received in the receiving system through the optical fiber due to optical fiber wavelength dispersion. Therefore, in a high speed optical communication system wherein a high speed light modulation of several Gbps is required, it is not possible to use a direct modulation type optical modulator. One known type of external modulation type optical modulator is a Mach-Zehnder interferometer type modulator, which will be explained later. Such a Mach-Zehnder interferometer type modulator has the smallest spread of the spectrum and therefore is able to avoid the waveform deterioration at the receiver caused by the effects of wavelength dispersion of the fiber. That is, in such a Mach-Zehnder interferometer type modulator, the phases of the light propagating through two optical waveguides are modulated in opposite directions by the same magnitude for modulation free from frequency chirping and it is thus possible to reduce the spectral spread to the spread of the modulation side band, by the Fourier component of the modulation waveform. Note that a known reference relating to an optical modulator of this type is F. Koyama et. al., JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 6, No. 1, January 1988, IEEE, pp. 87 to 93. There are, however, the following problems in the conventional Mach-Zehnder interferometer type modulator. First, while there is a desire to make the modulation driving circuit of the optical modulator by a semiconductor integrated circuit (IC) and thus reduce the size, it is not easy to make the modulation driving circuit by an IC chip. The reason is that the conventional Mach-Zehnder interferometer type modulator requires a high voltage of -5 V to -8 V and it is difficult to manufacture an IC able to withstand such a high voltage. Second, while there is a desire to reduce the offset of the optical modulator as much as possible, it is not easy to reduce the offset. Here, "offset" means the offset in the modulation driving voltage and deviation in various characteristics accompanying temperature fluctuations. Third, it is not possible to use an optical modulator for a long distance optical communication system operating at a high transmission speed of over several Gbps. This is due to the so-called wavelength dispersion in the optical fibers. That is, it has not been possible to meet the demand for constructing a high speed, long distance optical communication system using such general optical fibers producing large wavelength dispersions. SUMMARY OF THE INVENTION Therefore, the present invention has as its object the provision of an optical modulator which is able to satisfy each of the first to third desires mentioned above when so demanded by the designer of the optical communication system. Accordingly, in a first aspect, the present invention provides an optical modulator comprising: a substrate; a Mach-Zehnder interferometer having: an input for incoming light, an output for outgoing light, and first and second optical waveguides formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising at least one pair of separated electrodes associated with the first optical waveguide and having a driving voltage side electrode and a ground electrode connected thereto via a terminating resistance; the arrangement of said first and second optical waveguides and said electrode structure being asymmetric as between the first optical waveguide and the second optical waveguide for modulating the phase of light propagating through the first optical waveguide differently from the phase of light propagating through the second optical waveguide by means of the electrooptic effect, even when the same driving voltage is applied to an optional part of the electrode structure associated with the second optical waveguide as to said driving voltage side electrode of the pair of electrodes associated with the first optical waveguide. In a second aspect, the present invention provides an optical modulator comprising: a substrate; a Mach-Zehnder interferometer having: an input for incoming light, an output for outgoing light, and first and second optical waveguides formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising: a first pair of separated electrodes associated with the first optical waveguide and having a first driving voltage side electrode and a first ground electrode connected thereto via a first terminating resistance, and a second pair of separated electrodes associated with the second optical waveguide and having a second driving voltage side electrode and a second ground electrode connected thereto via a second terminating resistance; the arrangement of said first and second optical waveguides and said electrode structure being symmetrical as between the first optical waveguide and the second optical waveguide; and driving means arranged to provide a first driving voltage to the first driving voltage side electrode and a second driving voltage different from the first driving voltage to the second driving voltage side electrode for modulating the phase of light propagating through the first optical waveguide differently from the phase of light propagating through the second optical waveguide by means of the electrooptic effect. In a third aspect, the present invention provides a method of modulating light comprising: providing an optical modulator comprising: a substrate; a Mach-Zehnder interferometer having: an input for incoming light, an output for outgoing light, and first and second optical waveguides formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising at least one pair of separated electrodes associated with the first optical waveguide and having a driving voltage side electrode and a ground electrode connected thereto via a terminating resistance; the arrangement of said first and second optical waveguides and said electrode structure being asymmetric as between the first optical waveguide and the second optical waveguide; applying light to said input of the Mach-Zehnder interferometer; and providing a driving voltage to the driving voltage side electrode of the pair of electrodes associated with the first optical waveguide, thereby modulating the phase of the applied light propagating through the first optical waveguide differently from the phase of the applied light propagating through the second optical waveguide by means of the electrooptic effect, even when applying the same driving voltage to an optional part of the electrode structure associated with the second optical waveguide. In a fourth aspect, the present invention provides a method of modulating light comprising: providing an optical modulator comprising: a substrate; a Mach-Zehnder interferometer having: an input for incoming light, an output for outgoing light, and first and second optical waveguides formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising: a first pair of separated electrodes associated with the first optical waveguide and having a first driving voltage side electrode and a first ground electrode connected thereto via a first terminating resistance, and a second pair of separated electrodes associated with the second optical waveguide and having a second driving voltage side electrode and a second ground electrode connected thereto via a second terminating resistance; the arrangement of said first and second optical waveguides and said electrode structure being symmetrical as between the first optical waveguide and the second optical waveguide; applying light to said input of the Mach-Zehnder interferometer; and providing a first driving voltage to the first driving voltage side electrode and a second driving voltage different from the first driving voltage to the second driving voltage side electrode, thereby modulating the phase of the applied light propagating through the first optical waveguide differently from the phase of the applied light propagating through the second optical waveguide by means of the electrooptic effect. BRIEF DESCRIPTION OF THE DRAWINGS The above object and features of the present invention will be more apparent from the following description of the preferred embodiments with reference to the accompanying drawings, wherein: FIG. 1 is a view schematically showing a first aspect of an optical modulator; FIG. 2 is a view schematically showing a second aspect of the optical modulator; FIG. 3 is a view showing a first prior art of an external modulator; FIG. 4 is a graph for explaining an optical modulation operation in the first prior art; FIG. 5 is a view of a second prior art of an external modulator; FIG. 6 is a graph for explaining an optical modulation operation in the second prior art; FIG. 7 is a block diagram of the principle and constitution of an optical modulator based on the present invention; FIG. 8 is a view for explaining the principle of the first and later embodiments of the present invention; FIG. 9 is a waveform diagram used for explaining FIG. 8; FIG 10A is a view of the construction of a first embodiment of the present invention; FIG. 10B is a view of an example of the circuit of the first and second driving units; FIG. 11 is a time chart showing the operation of the first embodiment; FIG. 12 is a plane view of a second embodiment of the present invention; FIG. 13 is a plane view of a third embodiment of the present invention; FIG. 14 is a sectional view of a fourth embodiment of the present invention; FIG. 15 is a sectional view of a fifth embodiment of the present invention; FIG. 16 is a sectional view of a sixth embodiment of the present invention; FIG. 17 is a plane view of a seventh embodiment of the present invention; FIG. 18 is a plane view of an eighth embodiment of the present invention; FIG. 19 is a graph of the results of a first calculation for explaining the improvement in the transmission characteristics of optical fibers according to the present invention; and FIG. 20 is a graph of the results of a second calculation for explaining the improvement in the transmission characteristics of optical fibers according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiment of the present invention, the related art and the disadvantages therein will be described with reference to the related figures. Figure 1 is a view of a first aspect of the optical modulator, which belongs to what are known as direct modulation type modulators. In the figure, reference numeral 11 is a light source, for example, a laser diode (LD), which is connected in series to a driving signal source 12. The optical modulator shown here applies direct modulation to the laser diode 11 to obtain modulated light MO. In a direct modulation type optical modulator, when the modulation speed is large, as mentioned earlier, there is the disadvantage of the occurrence of frequency chirping, which forces the use of an external modulation type optical modulator. Figure 2 is a schematic view of a second aspect of the optical modulator, which belongs to what are known as external modulation type modulators. In such external modulation type optical modulators, to eliminate the aforementioned frequency chirping, it is necessary to keep the laser diode constituting the light source 11 from blinking and make it transmit continuous light (direct current light) CO. Reference numeral 14 is a direct current source. The continuous light CO is modulated by the external modulator 13. The external modulator 13 is controlled by the driving voltage DV from the driving signal source 12 and transmits the modulated light MO. Therefore, it is possible for the receiving system to receive light signal with minimized waveform deterioration which is caused by frequency chirping and fiber wavelength dispersion. Figure 3 is a view of a first prior art of an external modulator. This external modulator is a known Mach-Zehnder interferometer type modulator, which is constructed by a first optical waveguide 21 and second optical waveguide 22 formed in a substrate (for example, made of LiNbO 3 ), into the input terminals of which continuous light CO, split into two, is input, electrodes 23-1 and 23-2 formed above the same, the already mentioned driving signal source (signal input source) 12 which applies a driving voltage DV to one end of one electrode 23-1, and a terminating resistor R which is connected to the other end. The other electrode 23-2 is ground. To the input terminal is applied the continuous light CO. From the output side is taken out the modulated light MO. Figure 4 is a graph for explaining the optical modulation operation in the first prior art. In the figure, the characteristic curve at the top left is that of the input driving voltage vs the optical output. If the pulse like driving voltage DV shown at the bottom left of the figure is input in accordance with that characteristic curve, the pulse like modulated light MO shown at the top right is obtained. Note that the characteristic curve is drawn facing left because a driving voltage of a negative voltage, for example, -5 V to -8 V, is assumed. For example, if a driving voltage of -5 V to -8 V is applied, an electrooptic effect is created by the electric field formed between the electrodes 23-1 and 23-2 through the substrate (LiNbO 3 ). Underneath the pair of electrodes 23-1 and 23-2, which form so-called traveling wave electrodes, the continuous light CO propagating through the first and second optical waveguides 21 and 22 deviate in phase 180° from each other. The composite lights at the output ends of the optical waveguides 21 and 22 cancel out each other and become zero ("0"). At this time, the modulated light MO becomes zero. Conversely, when the driving voltage DV is zero volt, the above-mentioned composite lights do not cancel each other out and there is a modulated light MO ("1"). Figure 5 is a view of a second prior art of an external modulator. This is substantially the same as the afore-mentioned first prior art (Fig. 3) except that a 3 dB optical coupler 35 is introduced into the optical output end. According to this second prior art, two differential modulated lights MO and MO are obtained. Figure 6 is a graph for explaining an optical modulation operation in the second prior art. The difference from the above-mentioned first prior art is that the driving voltage DV spans two polarities from -2.5 V to -4 V and +2.5 V to +4 V. The advantage of the second prior art from the first prior art is that a complementary modulated light MO can be obtained. There are three major problems with the first prior art and second prior art. The first relates to the driving voltage DV, mentioned previously. That is, in the first prior art, a high voltage of -5 V to -8 V is required and it is difficult to make the driving signal source 12 with an actual circuit, especially with an integrated circuit (IC). Usually, the higher speed the operation required the withstand voltage of an IC becomes smaller. For example, even a GaAs IC has a withstand voltage of less than 3 V. Also the high voltage swing degrades the signal modulation speed. Going into further detail, if it is attempted to provide a drive with a large voltage amplitude of -5 V to -8 V, first the effects of the stray capacitance will he large and therefore high speed operation will become difficult and, second, the terminating resistor R will be fixed (50 Ω) and therefore the drive will have to be with a large current amplitude, the stray capacitance of the driving signal source 12 will increase, and high speed operation will become even more difficult. The second relates to the offset, also mentioned earlier. Annoying voltage drifts in the modulation characteristics and also the temperature fluctuations may arise because of this offset. The cause of this is the asymmetric, coplanar construction of the electrodes (23-1 and 23-2) in both the first and second prior arts, as clear from Fig. 3 and Fig. 5. Further, in the second prior art, the voltage spans both the positive and negative sides, i.e., -2.5 V to -4 V and +2.5 V to +4 V and also the voltage swing of the drive voltage is same as that of the first prior art, so the construction of the driving signal source 12 becomes difficult. If the construction is attempted to be made easier, provision may be made of an electrode for applying a separate DC voltage or the first and second optical waveguides may be made asymmetric in structure, but whatever the case, the construction becomes complicated, which makes it impractical. The third problem, also mentioned above, was that in a high speed optical communication system, it is desired to reduce the spectral spread of the light source as much as possible and prevent the occurrence of waveform deterioration of the optical pulses caused by the spectral spread and the wavelength dispersion of the optical fibers. Therefore, the external modulation system is advantageous in that it enables reduction of the spectral spread, but even if this system is used, there is a limit to the transmission distance at a transmission speed of over several Gbps due to the spectral spread caused by the modulation side band. Therefore, some measures are required for further improving the transmission characteristics. The modulation system featuring the smallest spectral spread and therefore resistance to the effects of fiber wavelength dispersion uses a Mach-Zehnder interferometer type modulator. According to this system, as mentioned earlier, it is possible to reduce the spectral spread. Therefore, in a conventional optical modulation system using a Mach-Zehnder interferometer type modulator, modulation free from frequency chirping has been performed by modulating the phases of the light propagating through the two optical waveguides of the modulator by the same magnitude in opposite directions. However, at a transmission speed of over several Gbps, even if the frequency chirping is made zero, the deformation of the optical pulses by the spectral spread due to the modulation side band and the wavelength dispersion of optical fibers can no longer be ignored. Therefore, there is the problem that long distance transmission is impossible in a fiber with a large fiber wavelength dispersion at a transmission speed of over several Gbps even if the spectral spread due to the modulation is reduced to the spread of just the modulation side band. In consideration of the above-mentioned problems, the present invention provides an optical modulator which can be driven by low voltage, has a driving signal source which can be made by an integrated circuit (IC), which can suppress the offset of the driving voltage and the effects of temperature fluctuations, and which is suited to higher speed operation. With such an optical modulator, the first and second problems can be resolved. Further, the present invention provides an optical modulator which enables long distance transmission using fibers with large fiber wavelength dispersions even at transmission speeds of over several Gbps. Using this optical modulator, it is possible to resolve the above third problem. Figure 7 is a block diagram of the principle and constitution of an optical modulator based on the present invention. The optical modulator of the present invention basically is constructed of a Mach-Zehnder interferometer type modulator comprised of a first optical waveguide 21 and second optical waveguide 22, a first electrode 33 and second electrode 34 which cooperates with the first optical waveguide 21 and second optical waveguide 22, a driving means which imparts relative changes to the light phases of the first and second optical waveguides 21 and 22 through the first and second electrodes 33 and 34. Here, the driving means is constructed of a first driving unit 31 and second driving unit 32 which independently drive the first electrode 33 and second electrode 34. The first and second driving units 31 and 32 apply a first and second driving voltage DV 1 and DV 2 determined individually to the first and second electrodes 33 and 34. The principle of the operation of the above-mentioned optical modulator will be clear form the explanation of the following embodiments disclosed below. The first to eighth embodiments explained below relate in particular to the aforesaid third problem in the prior art. Figure 8 is a view for explaining the principle of the first and later embodiments of the present invention. Figure 9 is a waveform diagram used for explaining Fig. 8. Figure 8 shows the electric field of light at various portions of the Mach-Zehnder interferometer type modulator. In the figure, E 0 is the amplitude of the electric field of the input light CO, ω 0 is the angular frequency of the electric field of the light, t is the time, and  A and  B are the phases of the light modulated in the first and second optical waveguides 41 and 42. Eout(t) is the electric field of the modulated light (MO). Details are given by the following equation (1). Eout(t) = E 0 /2{Cos(ω 0 t +  A ) + Cos(ω 0 t +  B )} = E 0 /2(X 2 + Y 2 ) 1/2 Cos{ω 0 t - tan -1 (Y/X)} where, X = Cos( A ) + Cos( B ) and Y = Sin( A ) + Sin( B ) As will be understood from the above equation (1), phase modulation of tan -1 (Y/X) is applied to the Eout(t). This becomes frequency chirping as shown below. If ω 0 t - tan -1 (Y/X) is set at , the angular frequency can be expressed as ω(t) = d/dt = ω 0 -d{tan -1 (Y/X)}/dt and the wavelength can be expressed as λ = 2πc/ω(t) (where c is the speed of light). Therefore, the phase modulation of tan -1 (Y/X) causes fluctuation of the wavelength λ, that is, frequency chirping. Here, the phase modulation is performed as follows.  A > 0,  B < 0, ABS( A ) > ABS( B ) ABS( B -  A ) ≒ 0(light output high) ABS( B -  A ) ≒ π(light output low) where, ABS() is the absolute value of . That is ||. The operational waveforms of various portions at this time are shown in Fig. 9. As shown by (f) in Fig. 9, the phase of the output light is delayed at the rising edge of the intensity of the output light and advanced at the falling edge. Corresponding to this, the center wavelength λ 0 moves to the long wavelength side at the rising edge and the short wavelength side at the falling edge, as shown by (g) in Fig. 9. In the past, modulation was performed under the condition of  A = - B . In this case, Eout(t) becomes as shown by equation (2): Eout(t) = E 0 cos()Cos(ω 0 t) where,  =  A = - B In this case, the amplitude of the electric field of the light is just modulated by the modulation of , and there is no fluctuation in wavelength accompanying the modulation. By asymmetrically modulating the phase of the light of the optical waveguides 41 and 42 of the optical modulator, the center wavelength λ 0 of the modulated light is, as shown in (g) of Fig. 9, made to move to the long wavelength side at the rising edge and the short wavelength side at the falling edge. On the other hand, the wavelength dispersion of the optical fiber is large in the case of use of a single mode optical fiber with a 1.3 µm band zero dispersion, in the 1.55 µm band, at which the smallest loss is given. The coefficient of dispersion at this time is a maximum 20 ps/nm/km. The longer the wavelength, the slower the speed of propagation through the optical fiber. Therefore, due to the frequency chirping arising due to the principle of Fig. 8 and Fig.9 the rising edge of the optical pulse is delayed by the fiber dispersion, the falling edge is advanced, and pulse compression occurs. This works to compensate for the spread of the waveform caused by both the modulation side band and the optical fiber wavelength dispersion and acts to lengthen the transmittable optical fiber length. Pulse compression means that the pulse width is made narrower toward the center of the pulse. In a Mach-Zehnder interferometer type modulator, as mentioned earlier, use is made of the electrooptic effect for realizing phase modulation of the light. That is, the index of refraction of a substance having an electrooptic effect is changed by the electric field by the electrode so as to change the phase of the light. In a Mach-Zehnder interferometer type modulator, there are several methods conceivable for asymmetrically modulating each phase of the light propagating through the two optical waveguides. One is the method of modulation by driving voltages differing for the optical waveguides. The second is making the driving voltages the same, but the sectional structures of the electrodes asymmetric so as to make the application of the modulating electric fields to the optical waveguides asymmetric. The third is to change the lengths of the electrodes in the optical waveguides and thus change the lengths of the optical waveguides at which the changes in the index of refraction can be sensed. Figure 10A is a view of the construction of a first embodiment of the present invention. In the first embodiment, the driving voltages DV 1 and DV 2 are applied asymmetrically to the electrodes 33 and 34. In the figure, the optical phase modulation in the first optical waveguide 21 is large, and the optical phase modulation in the second optical waveguide 22 is small. Further, the first electrode 33 is comprised of the first pair of separated electrodes 33-1 and 33-2, while the second electrode 34 is comprised of the second pair of separated electrodes 34-1 and 34-2. These are shown by hatching for easier understanding. By these pairs, a so-called travelling-wave type electrode is constructed. Between the electrodes of each pair, as illustrated, there are connected terminating resistors R. The characteristic impedances of the travelling wave type electrodes are matched. The lengths of the electrodes 33 and 34 are equal. Figure 10B is a view of an example of the circuit of the first and second driving units. In the figure, reference numerals 51a and 51b are transistors for outputting the driving voltages DV 1 and DV 2 . At the bases of these transistors are applied the data input Din. However, at one transistor side, the inverter INV is inserted. Further, constant current sources 53a and 53b are connected to the transistors. It is also possible to make the magnitudes of the currents Ia and Ib different (Ia > Ib or Ia < Ib). Figure 11 is a time chart showing the operation of the first embodiment. DV 1 is the driving waveform for the phase modulation in optical waveguide 21, while DV 2 is the driving waveform for the phase modulation of in optical waveform 22. The polarities are reversed by DV 1 and DV 2 and the amplitude of the driving voltage made larger for DV 1 , whereby the phase modulation is made asymmetric. Looking at the position of the separated electrodes 33-1 and 34-1, the first and second optical waveguides 21 and 22 are both formed in the substrate 55, made of a Z-cut electrooptic effect crystal. Over the first and second optical waveguides 21 and 22 are overlappingly formed the driving voltage side electrodes 33-1 and 34-1 of the first and second pairs of separated electrodes 33 and 34. Alternatively, the first and second optical waveguides 21 and 22 are formed in a substrate consisting of X- or Y-cut electrooptic effect crystal. A part from the first and second optical waveguides 21 and 22 are formed the driving voltage side electrodes of the first and second pairs of separated electrodes. The plan view of this state corresponds to the case of equal lengths of the electrodes 33 and 34 in the later mentioned Fig. 13. Figure 12 is a plan view of a second embodiment of the present invention (hereinafter, only electrodes are shown for brevity). In this embodiment, the first and second pairs of separated electrodes 33-1, 33-2 and 34-1, 34-2 have mutually different lengths. The first and second pairs of separated electrodes receive the first and second driving voltages DV 1 and DV 2 having the same levels. By this, the phase modulations of the light may be made mutually asymmetrical for the optical waveguides 21 and 22. In the second embodiment, the first and second optical waveguides 21 and 22 are both formed in the substrate 55, made of a Z-cut electrooptic effect crystal. Over the first and second optical waveguides 21 and 22 are overlappingly formed the driving voltage side electrodes 33-1 and 34-1 of the first and second pairs of separated electrodes 33 and 34. Figure 13 is a plan view of a third embodiment of the present invention. The first and second optical waveguides 21 and 22 are both formed in the substrate 55, made of an X- or Y-cut electrooptic effect crystal. Away from the first and second optical waveguides 21 and 22 are formed the driving voltage side electrodes 33-1 and 34-1 of the first and second pairs of separated electrodes 33 and 34. Figure 14 is a sectional view of a fourth embodiment of the present invention. The positional relationship, when seen from a sectional view, of the first pair of separated electrodes 33 to the first optical waveguide 21 and the positional relationship, when seen from a sectional view, of the second pair of separated electrodes 34 to the second optical waveguide 22 are asymmetrical. That is, the pair of separated electrodes 34 is shifted slightly to the right in the figure. In this case, the first and second optical waveguides 21 and 22 are both formed in the substrate 55, made of a Z-cut electrooptic effect crystal. Over the first and second optical waveguides 21 and 22 are overlappingly formed the driving voltage side electrodes 33-1 and 34-1 of the first and second pairs of separated electrodes 33 and 34. A Z-cut electrooptic effect crystal is used because the electric fields E 1 traverse the inside of the first and second optical waveguides 21 and 22 in the vertical direction. Figure 15 is a sectional view of a fifth embodiment of the present invention. The first and second optical waveguides 21 and 22 are both formed in the substrate 55, made of an X- or Y-cut electrooptic effect crystal. Away from the first and second optical waveguides 21 and 22 are formed the driving voltage side electrodes 33-1 and 34-1 of the first and second pairs of separated electrodes 33 and 34. An X- or Y-cut electrooptic effect crystal is used because the electric fields E 1 traverse the inside of the first and second optical waveguides 21 and 22 in the horizontal direction. Figure 16 is a sectional view of a sixth embodiment of the present invention. The optical modulator of the sixth embodiment is as follows. The first electrode 33 is constructed of a pair of separated electrodes 33-1 and 33-2 separated along the first optical waveguide 21. The second electrode 34 is formed along the second optical waveguide 22 but away from the same (22) and is grounded. One of the pair of separated electrodes, 33-1, forming the first electrode 33 receives at one end a corresponding driving voltage DV 1 and is connected at the other end to one end of the other of the pair of separated electrodes 33-2 through the terminating resistor R, the other end of the other of the pair of separated electrodes 33-2 being grounded. The separated electrode 33-1 of the driving voltage side, when viewed sectionally, is placed at an asymmetric position with respect to the first and second optical waveguides 21 and 22, and the first and second optical waveguides are formed in the substrate, constituted by an X- or Y-cut electrooptic effect crystal. Figure 17 is a plan view of a seventh embodiment of the present invention. Figure 18 is a plan view of an eighth embodiment of the present invention. These embodiments are constituted so that the phases of the light propagated in the first and second optical waveguides 21 and 22 become asymmetric by making the second driving voltage DV 2 always zero volt or making the second separated electrode 34 substantially not present. In the seventh embodiment, the first and second optical waveguides 21 and 22 are formed in a substrate 55 made of a Z-cut electrooptic effect crystal, and the driving voltage side electrode 33-1 of the first pair of separated electrodes 33 is formed overlapping the first optical waveguide. In the eighth embodiment, the first and second optical waveguides 21 and 22 are formed in a substrate 55 made of an X-cut electrooptic effect crystal, and the driving voltage side electrode 33-1 of the first pair of separated electrodes 33 is formed apart from the first optical waveguide 21. Figure 19 is graph of the results of a first calculation for explaining the improvement in the transmission characteristics of optical fibers according to the present invention; and Fig. 20 is a graph of the results of a second calculation for explaining the improvement in the transmission characteristics of optical fibers according to the present invention. Figure 19 shows the results of calculation of the deterioration in the minimum received light power caused by the wavelength dispersion, i.e., the power penalty. When the allowance for the power penalty caused by optical fiber transmission is 0.5 dB, the allowable wavelength dispersion in the conventional modulation method is 500 to 700 ps/nm, while when the phase modulation ratio  A : B is made 5:1, it is improved to 1500 ps/nm or more. Further, Fig. 20 shows the results of similar calculation by another phase modulation ratio. From this it is learned that the phase modulation ratio should be 2:1 or more. In the above explanation, use was made of the example of the case of operation under conditions where the wavelength dispersion value changes only in the positive region (with the object of reducing the transmission loss). Therefore, the center wavelength at the rising edge of the modulated light is shifted to the long wavelength side and the center wavelength at the falling edge of the modulated light is shifted to the short wavelength side. However, in another system using an optical fibers having other characteristics, there is a cases of operation under the conditions where the wavelength dispersion value is close to zero and also a small transmission loss is given. An optical modulator cooperating with such an optical fiber must operate not only under conditions where the wavelength dispersion is in the positive region, but also under conditions where the wavelength dispersion is in the negative region. This being the case, when the wavelength dispersion is negative, the conditions of the previously mentioned wavelength shift must be set to enable setting opposite to the aforementioned case. That is, the center wavelength at the rising edge of the modulated light is to be shifted to the short wavelength side and the center wavelength at the falling edge of the modulated to be light is to be shifted to the long wavelength side. As explained above, according to the present invention, it is possible to halve the driving voltage compared with the past and to easily make the circuit of the driving signal source by an IC. Further, since use is made of symmetric coupled lines, it is possible to eliminate the offset of the driving voltage in the prior art and the effects of temperature fluctuations and possible to receive light signal in the receiving system without waveform deterioration. Further, as understood from the results of calculations of Fig. 19 and Fig.20, according to the present invention, the optical fiber transmission characteristics are improved over the conventional modulation system, greatly contributing to improvement of the performance of high speed optical communication apparatuses. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: An optical modulator comprising: a substrate (55) ; a Mach-Zehnder interferometer having: an input for incoming light (CO), an output for outgoing light (MO), and first and second optical waveguides (21, 22) formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising at least one pair of separated electrodes (33-1, 33-2) associated with the first optical waveguide (21) and having a driving voltage side electrode (33-1) and a ground electrode (33-2) connected thereto via a terminating resistance (R) ; the arrangement of said first and second optical waveguides and said electrode structure being asymmetric as between the first optical waveguide and the second optical waveguide for modulating the phase of light propagating through the first optical waveguide differently from the phase of light propagating through the second optical waveguide by means of the electrooptic effect. An optical modulator according to claim 1, wherein the sectional structure of the electrodes is asymmetric so as to make the application of modulating electric fields by the electrodes to the first and second optical waveguides asymmetric. An optical modulator according to claim 1 or claim 2, wherein the pair of electrodes associated with the first optical waveguide (21) is a first pair (33-1, 33-2) of said electrodes, the second optical waveguide (22) has a second pair (34-1, 34-2) of such electrodes, the first and second pairs of electrodes have mutually different lengths, thus making the length of the first optical waveguide at which a change in the index of refraction of said first waveguide due to the electrooptic effect can be sensed different from the length of the second optical waveguide at which a change in the index of refraction of said second waveguide due to the electrooptic effect can be sensed. An optical modulator according to claim 3, wherein the substrate (55) is a Z-cut electrooptic effect crystal, the driving voltage side electrode (33-1) of the first pair of electrodes overlaps the first optical waveguide and the driving voltage side electrode (34-1) of the second pair of electrodes overlaps the second optical waveguide. An optical modulator according to claim 3, wherein the substrate (55) is an X- or a Y-cut electrooptic effect crystal and the driving voltage side electrodes (33-1, 34-1) of each of said pair of electrodes are formed away from the first and second optical waveguides, respectively. An optical modulator according to claim 2, wherein the substrate (55) is a Z-cut electrooptic effect crystal, the pair of electrodes associated with the first optical waveguide (21) is a first pair (33-1, 33-2) of said electrodes, the second optical waveguide (22) has a second pair (34-1, 34-2) of such electrodes, the driving voltage side electrode (33-1) of the first pair of electrodes overlaps the first optical waveguide, the driving voltage side electrode (34-1) of the second pair of electrodes overlaps the second optical waveguide, and the positional relationship, when seen from a sectional view, of the first pair of electrodes to the first optical waveguide and the positional relationship of the second pair of electrodes to the second optical waveguide are asymmetric. An optical modulator according to claim 2, wherein the substrate (55) is an X- or a Y-cut electrooptic effect crystal, the pair of electrodes associated with the first optical waveguide (21) is a first pair (33-1, 33-2) of said electrodes, the second optical waveguide (22) has a second pair of such electrodes (34-1, 34-2), the driving voltage side electrodes (33-1, 34-1) of each of said pair of electrodes are formed away from the first and second optical waveguides, respectively, and the gap between the first pair of electrodes is different from the gap between the second pair of electrodes. An optical modulator according to claim 1 or claim 2, wherein the substrate (55) in an X- or a Y- cut electrooptic effect crystal, the ground electrode associated with the first optical waveguide (21) is a first ground electrode (33-2), the second optical waveguide (22) has a second ground electrode (34), the driving voltage side electrode (33-1) is positioned between the first ground electrode (33-2) and the second ground electrode (34) and is placed at an asymmetric position with respect to the first optical waveguide (21) and the second optical waveguide (22). An optical modulator according to claim 2, wherein the substrate (55) is a Z-cut electrooptic effect crystal, and the driving voltage side electrode (33-1) of said pair of electrodes overlaps the first optical waveguide (21). An optical modulator according to claim 2, wherein the substrate (55) is an X-cut electrooptic effect crystal, and the driving voltage side electrode (33-1) of said pair of electrodes is formed away from the first optical waveguide (21). An optical modulator comprising: a substrate (55); a Mach-Zehnder interferometer having: an input for incoming light (CO), an output for outgoing light (MO), 'and first and second optical waveguides (21, 22) formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising: a first pair of separated electrodes (33-1, 33-2) associated with the first optical waveguide (21) and having a first driving voltage side electrode (33-1) and a first ground electrode (33-2) connected thereto via a first terminating resistance (R), and a second pair of separated electrodes (34-1, 34-2) associated with the second optical waveguide (22) and having a second driving voltage side electrode (34-1) and a second ground electrode (34-2) connected thereto via a second terminating resistance (R) ; the arrangement of said first and second optical waveguides and said electrode structure being symmetrical as between the first optical waveguide and the second optical waveguide; and driving means arranged to provide a first driving voltage (DV 1 ) to the first driving voltage side electrode (33-1) and a second driving voltage (DV 2 ) different from the first driving voltage to the second driving voltage side electrode (34-1) for modulating the phase of light propagating through the first optical waveguide differently from the phase of light propagating through the second optical waveguide by means of the electrooptic effect. An optical modulator according to claim 11, wherein the driving means comprises: a first driving unit (31) having a first bipolar transistor (51a) for outputting the first driving voltage (DV1) at a collector thereof in response to a data signal (D in ) input at a base thereof, the emitter of said first transistor being connected to a first constant current source (53a), and a second driving unit (32) having a second bipolar transistor (51b) for outputting the second driving voltage (DV 2 ) at a collector thereof in response to an output from an inverter (INV) input at a base thereof, said inverter being supplied at an input thereof with said data signal (D in ), the emitter of said second transistor being connected to a second constant current source (53b). An optical modulator according to claim 11 or claim 12, wherein the substrate (55) is a Z-cut electrooptic effect crystal, the first driving voltage side electrode (33-1) overlaps the first optical waveguide (21) and the second driving voltage side electrode (34-1) overlaps the second optical waveguide (22). An optical waveguide according to claim 11 or claim 12, wherein the substrate (55) is an X- or a Y-cut electrooptic effect crystal, the first driving voltage side electroode (33-1) is formed apart from the first optical waveguide (21) and the second driving voltage side electrode (34-1) is formed apart from the second optical waveguide (22). A method of modulating light comprising: providing an optical modulator comprising: a substrate (55); a Mach-Zehnder interferometer having: an input for incoming light (CO), an output for outgoing light (MO), and first and second optical waveguides (21, 22) formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising at least one pair of separated electrodes (33-1, 33-2) associated with the first optical waveguide (21) and having a driving voltage side electrode (33-1) and a ground electrode (33-2) connected thereto via a terminating resistance (R) ; the arrangement of said first and second optical waveguides and said electrode structure being asymmetric as between the first optical waveguide and the second optical waveguide; applying light to said of the Mach-Zehnder interferometer; and providing a driving voltage to the driving voltage side electrode (33-1) of the pair of electrodes associated with the first optical waveguide (21), thereby modulating the phase of the applied light propagating through the first optical waveguide differently from the phase of the applied light propagating through the second optical waveguide by means of the electrooptic effect, even when applying the same driving voltage to an optional part of the electrode structure associated with the second optical waveguide (22). A method of modulating light comprising: providing an optical modulator comprising: a substrate (55); a Mach-Zehnder interferometer having: an input for incoming light (CO), an output for outgoing light (MO), and first and second optical waveguides (21, 22) formed in said substrate and coupled between said input and said output; and an electrode structure formed on said substrate and comprising: a first pair of separated electrodes (33-1, 33-2) associated with the first optical waveguide (21) and having a first driving voltage side electrode (33-1) and a first ground electrode (33-2) connected thereto via a first terminating resistance (R), and a second pair of separated electrodes (34-1, 34-2) associated with the second optical waveguide (22) and having a second driving voltage side electrode (34-1) and a second ground electrode (34-2) connected thereto via a second terminating resistance (R) ; the arrangement of said first and second optical waveguides and said electrode structure being symmetrical as between the first optical waveguide and the second optical waveguide; applying light to said input of the Mach-Zehnder interferometer; and providing a first driving voltage (DV 1 ) to the first driving voltage side electrode (33-1) and a second driving voltage (DV 2 ) different from the first driving voltage to the second driving voltage side electrode (34-1), thereby modulating the phase of the applied light propagating through the first optical waveguide differently from the phase of the applied light propagating through the second optical waveguide by means of the electrooptic effect. A method according to claim 16, wherein the second driving voltage (DV 2 ) is zero volts. An optical modulator according to claim 8, wherein the sectional structure of the driving voltage side electrode (33-1) and the first optical waveguide (21) relative to the sectional structure of the driving voltage side electrode (33-1) and the second optical waveguide (22) is asymmetrical.

Inventors: AMEMIYA IZUMI, HAMANO HIROSHI, NAMIKI TAKEFUMI, NISHIMOTO HIROSHI, OKIYAMA TADASHI, SEINO MINORU, YOKOTA IZUMI, AMEMIYA, IZUMI, HAMANO, HIROSHI, NAMIKI, TAKEFUMI, NISHIMOTO, HIROSHI, OKIYAMA, TADASHI, SEINO, MINORU, YOKOTA, IZUMI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G02F1
IPC codes: G02F1, H04B10