Optical transmitter having optical modulator

Abstract: An optical transmitter having a Mach-Zender waveguide optical modulator (48) comprising a signal electrode (12) fed with a driving signal for effecting modulation, a bias electrode (16) for controlling operating point and a bias feedback control circuit. Because the signal electrode and the bias electrode are isolated from each other, a driving circuit (54) and the signal electrode (12) can be connected in a DC setup. This permits stable operating point control and improves waveform characteristics. Also it is disclosed an optical transmitter having a Mach-Zender waveguide optical modulator with a delay optical waveguide (118) directionally coupled to at least one of the Mach-Zender branching optical waveguides and provided with an electrode (124,126) for controlling the directional coupling ratio.

Description: The present invention relates to an optical modulator for use with an optical communication system and comprising an optical interferometer type modulating means having an input port for receiving light from a light source, a first and a second branching waveguide for transmitting the light supplied to said input port after branching the supplied light in two directions, an output port for converging the branched light from said first branching waveguide and said second branching waveguide before outputting the converged light, and a signal electrode arranged to give phase change to light transmitted through said modulating means, and driving means, connected operatively to said signal electrode, for supplying said signal electrode with a driving signal based on an input signal. Such an optical modulator is known from EP-A- 0 444 688. Heretofore, optical transmitters used by optical communication systems have adopted the so-called direct modulation as their operating principle. The method involves modulating the current that flows through laser diodes by use of data signals. A major drawback of direct modulation is the difficulty in implementing long-distance data transmission due to wavelength dispersion. Efforts of recent years to circumvent the disadvantage have led to the development of external modulation. This is the kind of optical modulation that is highly immune to the adverse effects of wavelength dispersion over optical fiber cables. For example, what is known as the LiNbO 3 Mach-Zehnder modulator has drawn attention for its excellent modulation characteristic and high resistance to wavelength dispersion. Optical transmitters based on the external modulation principle are subject to a number of requirements: (a) the operating point of the optical modulator should be controlled to be stable; (b) the optical modulator should be driven with a low voltage; (c) there should be minimum waveform distortion attributable to the capacitor between a driving circuit and the electrodes of the optical modulator as well as to the capacitor between the electrodes and a terminating resistor; and (d) there should be minimum changes in modulation characteristic which are attributable to abrupt changes in the so-called mark rate. It is also required that the presence or absence of chirping (dynamic wavelength fluctuation) in the optical signal output by the optical modulator be optional. For example, where long-distance transmission is effected using a wavelength that approximately matches the zero-dispersion wavelength of the optical fiber and where the dispersion value can be either positive or negative, there should be no chirping. On the other hand, where the polarity of the wavelength dispersion of the optical fiber is predetermined and where optical pulse compression may be effected using a kind of chirping that corresponds to the dispersion polarity, the presence of chirping should be selected. A typical Mach-Zehnder modulator comprises an input port that receives light from a light source, a pair of branching waveguides that transmit the light received through the input port after branching the light in two directions, an output port that converges the branched light streams coming out of the branching waveguides, and electrodes that give phase change to the light transmitted through the branching waveguides. When the light streams from the branching waveguides converge in phase (with a phase difference of 2 nπ , n being an integer), the light output is turned on; when the light streams converge opposite to each other in phase (with a phase difference of (2n + 1)π , n being an integer), the light output is turned off. Thus intensity modulation is performed by varying the voltage given to the electrodes by use of an input signal. Where the voltage fed to the electrodes is varied as per the input signal, it is necessary to compensate for the operating point drift of the optical modulator caused by temperature fluctuation and other ambient conditions. One way to do this is first to supply the electrodes with a bias voltage such as to keep the operating point where optimum and then to superimpose a driving signal onto the bias voltage so that the output light is turned on and off while the operating point is being held in its optimum position. Prior art techniques of the above kind for stabilizing the operating point of the optical modulator are described illustratively in Japanese Patent Laid-Open No. 49-42365 and in Japanese Patent Laid-Open No. 3-251815. Also known to the Applicants is a Mach-Zehnder optical modulator of a symmetrical dual electrode driving type designed to lower the voltage to a signal electrode of the optical modulator and to eliminate chirping in the modulated output light. That is, the branching waveguides have a signal electrode each. These electrodes are fed with driving voltages opposite to each other in phase. This technique is disclosed illustratively in Technical Digest of IOOC' 89, 19D4-2, 1989, "Perfectly Chirpless Low Drive Voltage Ti:LiNbO 3 Mach-Zehnder Modulator with Two Traveling-Wave Electrodes". U.S. Patent Application No. 07/662,412 discloses related techniques of the optical transmitter permitting the selection of the presence of absence of chirping. The driving signal for optical modulation has a repetitive pulse waveform or an AC waveform, while the bias voltage for operating point control comes from a DC source. Thus the circuit for controlling the operating point is connected to the electrode in a DC setup, whereas the driving circuit is connected to the electrode in an AC setup via a capacitor for DC decoupling. Where the electrode of the optical modulator is built as a traveling-wave type, the electrode is connected to the terminating resistor also in an AC setup. If there exists a capacitor between the driving circuit and the electrode and/or between the terminating resistor and the electrode, the low-frequency component of the driving signal is cut off. This can promote distortion of the signal waveform of the modulated light upon abrupt change in the mark rate. If the frequency characteristic of the capacitor is insufficient, the waveform of the driving signal of as high as several Gb/s is distorted by the capacitor. That in turn distorts the waveform of the modulated light. Whereas the symmetrical dual electrode type optical modulator may lower the driving voltage, its application to the optical transmitter with a stabilized operating point is yet to be implemented extensively. It is therefore an object of the present invention to provide an optical transmitter that has stable control on the operating point of the optical modulator and has little waveform distortion against the change in the mark rate by eliminating the capacitor connecting the electrode of the optical modulator to the driving circuit. It is another object of the invention to provide an optical transmitter that eliminates the capacitor connecting the electrode of the optical modulator to the terminating resistor if the electrode is built as a traveling-wave type. It is a further object of the invention to provide an optical transmitter that lowers the driving voltage to the optical modulator. It is yet another object of the invention to provide an optical transmitter that permits easy selection of the presence or absence of chirping in the optical signal output by the optical modulator. According to the invention, there is provided an optical modulator as initially defined, characterised in that: the modulating means also has a bias electrode insulated from a the signal electrode, said optical modulator further comprises operating point control means, connected operatively to said bias electrode, for supplying said bias electrode with a bias voltage controlled in accordance with the light output from a signal converter connected operatively to said output port for control of the operating point of said modulating means, the driving means is connected directly to said signal electrode, and the signal electrode and the bias electrode are arranged to give phase change to the light transmitted through the first branching waveguide and the second branching waveguide. The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. Fig. 1 is a plan view of a Mach-Zehnder optical modulator that may be used to practice the invention; Fig. 2 is a cross-sectional view taken on line A-A of the optical modulator in Fig. 1; Fig. 3 is a cross-sectional view of another optical modulator that may be used to practice the invention; Fig. 4 is a plan view of another optical modulator that may be used to practice the invention; Fig. 5 is a view illustrating the input/output characteristic of a Mach-Zehnder optical modulator; Fig. 6 is a block diagram of an optical transmitter used to implement symmetrical modulation; Fig. 7 is a set of views depicting the waveforms of an optical signal output from the optical transmitter of Fig. 6; Figs. 8 and 9 are views illustrating the waveforms of optical signal outputs effected by the optical transmitter of Fig. 6 when the transmitter develops an operating point drift in the positive and negative directions; Fig. 10 is a view illustrating typical probabilities of occurrence of the space portion, mark portion, and leading and trailing portion of an eye pattern; Fig. 11 is a block diagram of an optical transmitter practiced as a first embodiment of the invention; Figs. 12A through 12E are views of waveforms generated by the optical transmitter of Fig. 11 as it is operating; Fig. 13 is a schematic view depicting key parts of an optical transmitter practiced as a second embodiment of the invention; Fig. 14 is a block diagram of the entire optical transmitter of Fig. 13; Figs. 15A through 15G are views of waveforms generated by the optical transmitter of Fig. 14 as it is operating; Fig. 16 is a block diagram of an optical transmitter practiced as a third embodiment of the invention; Figs. 17A through 17H are views of waveforms generated by the optical transmitter of Fig. 16 as it is operating ; Fig. 18 is a block diagram of an optical transmitter practiced as a fourth embodiment of the invention; and Figs. 19A through 19G are views of waveforms generated by the optical transmitter of Fig. 18 as it is operating. Fig. 1 is a plan view of a Mach-Zehnder optical modulator that may be used to practice the invention, and Fig. 2 is a cross-sectional view taken on line A-A of that optical modulator. The optical modulator comprises a Z-cut type LiNbO 3 substrate into which titanium (Ti) is thermally dispersed to form optical waveguides thereon. The optical waveguides are loaded with electrodes. The optical waveguides formed on the substrate 2 include an input port 4 that receives light from a light source, not shown; a pair of branching waveguides 6 and 8 which transmit the light from the input port 4 after branching the received light in two directions; an output port 10 that converges the branched light transmitted through the branching waveguides 6 and 8. The branching waveguides 6 and 8 are loaded respectively with a signal electrode 12 and a bias electrode 16. Reference numeral 14 indicates a grounding electrode located close to the signal electrode 12, while reference numeral 18 is a grounding electrode furnished close to the bias electrode 16. Although a buffer layer 20 composed primarily of SiO 2 is formed between each electrode and the substrate 2, this layer is omitted from Fig. 1 so as to maintain the visibility of other key components. A driving signal is applied across the signal electrode 12 to the grounding electrode 14, and a bias voltage is fed between the bias electrode 16 and the grounding electrode 18. The signal electrode 12 is built as a traveling-wave type that allows the electric field of the driving signal to travel in the same direction as that of the light through the branching waveguide 6. The driving signal is supplied from an upstream edge 1201 of the signal electrode 12. A downstream edge 1202 of the signal electrode 12 is provided with a terminating resistor, to be described later. Built as a traveling-wave type, the signal electrode 12 permits appreciably higher modulation than electrodes of other types. Because the substrate of this example is a Z-cut type, the branching waveguides 6 and 8 are loaded from directly above with the signal electrode 12 and bias electrode 16, respectively, with the grounding electrodes 14 and 18 located close to the electrodes 12 and 16, respectively. This arrangement permits effective application of the electric field to the branching waveguides 6 and 8. Where the substrate is made of Z-cut LiNbO 3 , the electric field is applied effectively to the branching waveguides in the manner described below. Fig. 3 is a cross-sectional view of another optical modulator that may be used to practice the invention. In this example, the branching waveguides 6 and 8 are loaded from directly above with the signal electrodes 12 and 16, respectively, and a common grounding electrode 22 is formed on the back side of the substrate 2. Forming the signal electrode, bias electrode and grounding electrode on different planes as illustrated still affords necessary phase change to the light transmitted through the branching waveguides. Fig. 4 is a plan view of another optical modulator that may be used to practice the invention. While the typical optical modulators described so far have one of the two branching waveguides loaded with the signal electrode and the other loaded with the bias electrode, this optical modulator has one of the branching waveguides 8 loaded with the signal electrode 12 and bias electrode 16 and has the other branching waveguide 6 furnished with the grounding electrodes 14 and 18 corresponding respectively to the signal electrode 12 and bias electrode 16. Where the signal electrode is insulated from the bias electrode as described, there may be various modes to choose from in which to construct the signal and bias electrodes. If optical waveguides are formed on a crystal plane other than the Z-cut plane of LiNbO 3 , the electrode layout may be varied so as to permit optimally effective application of the electric field. For example, the electrodes may be formed alongside of the branching waveguides. The mutually isolated signal electrode and bias electrode characterize the Mach-Zehnder optical modulator. The advantages of this type of modulator are highlighted illustratively by describing the operating characteristic of an ordinary optical modulator in which the signal and bias electrodes are integrally furnished. One such ordinary optical modulator may be the one in Fig. 4 minus the bias electrode 16 and grounding electrode 18. Fig. 5 illustrates the input/output characteristic of a typical Mach-Zehnder optical modulator. In Fig. 5, reference numeral (1) indicates the characteristic in effect before an operating point drift occurs, and reference numeral (2) points to the characteristic in effect after an operating point drift has occurred. The operating point drift refers to a drift in the direction of voltage increase or decrease along the operation characteristic curve indicating the relationship between output light power and driving voltage. As depicted, the operation characteristic curve of the Mach-Zehnder optical modulator has periodicity with respect to the voltage change. Thus the use of driving voltages V 0 and V 1 for which the output light power is both minimized and maximized permits efficient binary modulation. With the optical signal output by that Mach-Zehnder optical modulator, the driving voltages V 0 and V 1 staying constant upon occurrence of an operating point drift cause waveform distortion and extinction ratio deterioration due to the above-mentioned periodicity. Where the operating point drift is represented by dV, the drift should preferably be compensated by calculations of V 0 + dV and V 1 + dV, where V 0 and V 1 are the driving voltages. One known method for controlling the operating point in compensation for its drift is described illustratively in Japanese Patent Laid-Open No. 49-42365. This method involves driving the optical modulator by superimposing a low-frequency signal onto one of the two logic levels of a driving signal. The operating point is then controlled by use of the phase of the low-frequency signal detected from the optical signal output. However, in the case of asymmetrical modulation involving the superimposing of the low-frequency signal onto one of the two logic levels of the driving signal (here, "modulation" refers not to the modulation for signal transmission but to the modulation based on a low-frequency signal for operating point control), the optimum operating point may not be maintained if the rise time or fall time of the input signal is significantly long (the reason for this will be discussed later in quantitative terms). As disclosed in Japanese Patent Laid-Open No. 3-251815, there is a prior art method of symmetrical modulation for operating point control whereby waveform distortion of the output optical signal and extinction ratio deterioration following the operating point drift are prevented without regard to the input signal. How the symmetrical modulation method is practiced and how it works will be described below in detailed quantitative terms because a study of this method is deemed indispensable for better understanding the advantages of the present invention. Fig. 6 is a block diagram of an optical transmitter used to implement symmetrical modulation. In Fig. 6, a Mach-Zehnder optical modulator 26 having a signal electrode 12 is supplied with light from a light source 24. The output light from the optical modulator 26 is branched in two directions by an optical branching circuit 28. One of the two light streams branched by the optical branching circuit 28 is used as an optical signal output, and the other light stream is converted to an electrical signal by an optical-to-electrical signal converter 30. A low-frequency superimposing circuit 36 superimposes a low-frequency signal from an oscillator 34 onto a driving signal from a driving circuit 32. After the superimposing of the low-frequency signal, the driving signal is fed to the signal electrode 12 via a capacitor C of a bias tee 38. The output terminal of the signal electrode 12 is connected via a capacitor C of a bias tee 40 illustratively to a 50-ohm terminating resistor 42. A phase detecting circuit 44 compares in phase the frequency component of the low-frequency signal in the electrical signal from the optical-to-electrical signal converter 30 with the low-frequency signal from the oscillator 34. The phase detecting circuit 44 then outputs a DC signal whose polarity is determined by the direction of the operating point drift and whose level depends on the magnitude of the drift. A bias control circuit 46 provides feedback control on the bias voltage to the signal electrode 12 in such a way that the DC signal coming from the phase detecting circuit 44 becomes zero. As indicated by (a) in Fig. 7, the waveform of the driving signal to be converted to an optical signal by the Mach-Zehnder optical modulator 26 shows that low-frequency signals are superimposed opposite to each other in phase on the space side and the mark side of the driving signal (i.e., symmetrical modulation). When this driving signal drives the optical modulator 26 having input/output characteristic (operation characteristic curve) indicated by (b) in Fig. 7, the optical modulator 26 yields an output optical signal. As illustrated by (c) in Fig. 7, this output optical signal is a signal that is amplitude-modulated with a signal having a frequency of 2 f 0 (f 0 is the frequency of the low-frequency signal). Where there is no operating point drift, the driving voltages V 0 and V 1 matching the two logic levels of the input signal correspond to the minimum and maximum values located adjacent to one another along the operation characteristic curve. Thus in the output optical signal, the envelope on the space side becomes opposite in phase to the envelope on the mark side, and the frequency of the superimposed component is 2 f 0 . It follows that the frequency component of f 0 is not detected from the output of the optical-to-electrical signal converter 30. But if an operating point drift occurs, the space-side envelope and the mark-side envelope in the output optical signal become in phase with each other, as depicted in Figs. 8 and 9. The mean power of the output optical signal varies at a frequency f 0 depending on the above-described in-phase modulation. The phase of the frequency component f 0 becomes 180 degrees different in accordance with the direction of the operating point drift. Thus from the output of the phase detecting circuit 44 emerges the DC signal whose polarity is determined by the phase difference between the frequency component f 0 and the low-frequency signal from the oscillator 34 and whose level depends on the magnitude of the operating point drift. In accordance with the signal reflecting the operating point drift, the bias control circuit 46 controls the bias voltage in such a manner that the frequency component f 0 is not included in the output optical signal, with the result that the operating point drift is optimally compensated. The operating principle of operating point control based on symmetrical modulation will now be described in quantitative terms. Suppose that the amplitude of the driving signal is given as Vπ (= V 0 - V 1 ) Suppose also that P(V) represents the power of the output optical signal normalized with its peak value, and that V denotes the driving voltage normalized with Vπ. Then the input/output characteristic of the optical modulator 26 is given as P (V) = (1-cos (π (V - Vd)))/2 where, Vd is the operating point drift voltage normalized with Vπ. Suppose that the driving signal is amplitude-modulated with a modulation factor m by use of a low-frequency signal having a frequency of f 0 (= ω 0 /2π). If this modulation is symmetrical modulation, the driving voltages V 0 and V 1 corresponding respectively to the logic levels of 0 and 1 of the input signal are given as V 0 = msin (ω 0 t) V 1 = 1-msin (ω 0 t) If the modulation factor m is sufficiently small, those power levels P 0 and P 1 of the output optical signal which correspond to the two logic levels are given by the following approximate expressions: P 0 = P (V 0 ) ≒ (1-cos (π Vd) - π msin (ω 0 t) sin (π Vd))/2 P 1 = P (V 1 ) ≒ (1 + cos (π Vd) -π msin (ω 0 t) sin (π Vd))/2 In addition, the following approximate expression gives the mean power P 2 of the output optical signal at a rise time and a fall time of the input signal: Fig. 10 illustrates typical probabilities of occurrence of the space portion (P 0 ), mark portion (P 1 ), and leading and trailing portion (P 2 ) of an eye pattern. In Fig. 10, M denotes the mark rate of the input signal, and r represents the constant that defines the relationship between the bit rate f b of the input signal on the one hand, and the rise and fall time (= r(1/f b )) of the input signal on the other. The expression below calculates the power P av of the output optical signal averaged over a time sufficiently shorter than the period of the low-frequency signal (= 1/f 0 ) by use of the probabilities of occurrence in Fig. 10: P av = (r (1-M) 2 + (1-r) (1-M))P 0 + (rM 2 + (1-r) M) P 1 + 2r (1-M) MP 2 ≡ K 0 P 0 +K 1 P 1 +K 2 P 2 where, K 0 , K 1 and K 2 are proportional constants. The following expression calculates the component P of the frequency f 0 of the low-frequency signal contained in the power P av of the output optical signal: P = - {((r (1-M) 2 + (1-r) (1-M)) + (rM 2 +(1-r) M)) (π/2) + 2r (1-M) M (π/2)} × msin (π Vd) sin (ω 0 t) Thus the component P of the frequency f 0 has a phase difference of 180 degrees depending on the direction of the operating point drift (i.e., polarity of Vd). If the frequency component P is multiplied by a reference frequency sin (ω 0 t), a positive or negative DC component may be detected depending on the direction of the operating point drift. That is, maintaining the DC component to zero allows the operating point to be held optimum. From Expression (8), P = 0 if Vd = 0. In this manner, the operating point is controlled optimally without regard to the mark rate M and the rise/fall time r varying with the waveform of the input signal. The operating principle of operating point control based on asymmetrical modulation may be described in the same quantitative terms as with symmetrical modulation. What follows is a description of major differences between the two kinds of modulation, and any repetitive descriptions will be omitted. In the case of asymmetrical modulation, Expressions (2), (4), (6) and (8) are replaced respectively with the following expressions: V 0 =0 P 0 = (1-cos (π Vd))/2 P = -{K 1 (π/2) sin (π Vd) + K 2 (1/2π) (π cos (π Vd) + 2sin (π Vd))} × msin (ω 0 t) = Kmsin (π Vd + ) sin (ω 0 t) Values K and  in Expression (8') are given by the following expressions: K = ((K 2 /2) 2 + (π K 1 /2+K 2 /π) 2 ) 1/2  = tan -1 (π K 2 /(π 2 K 1 +2K 2 )) Thus in the case of asymmetrical modulation, P = 0 only when sin (π Vd +  ) = 0 , as evident from Expression (8'). The stable point is shifted by - /π from the optimum operating point. This can lead to waveform distortion and extinction ratio deterioration depending on the rise time, fall time and mark rate of the input signal. In the optical transmitter of Fig. 6, the driving signal with the low-frequency signal superimposed thereon is supplied via the capacitor C to the signal electrode 12 of the optical modulator 26 for two objectives. One objective is to prevent the DC bias voltage fed to the driving circuit 32 from adversely affecting the stable performance of the transmitter. The other objective is to acquire a symmetrical driving waveform as indicated by (a) in Fig. 7, the waveform being specific to symmetrical modulation. A symmetrical driving waveform is obtained by causing the driving signal with the low-frequency signal simply superimposed thereon to pass through the capacitor C so as to remove the low-frequency component. The removal of the capacitor brought about by the invention is effective in preventing the deterioration of the signal waveform caused by an insufficient frequency characteristic of the capacitor. Another benefit of the absence of the capacitor is the ability derived therefrom to prevent the deterioration of the signal waveform in case the mark rate abruptly changes. More specifically, where operating point control is effected based on symmetrical modulation, the operating point is controlled to be optimum even if the mark rate is other than 1/2. Thus if the change in the mark rate is sufficiently delayed compared with the time constant of an operating point control loop, the signal waveform does not deteriorate. However, the signal waveform does deteriorate in case of a mark rate change that may occur approximately between the time constant of the operating point control loop and the time constant corresponding to the frequency cut off by the capacitor. This is where the removal of the capacity is called for. Fig. 11 is a block diagram of an optical transmitter practiced as the first embodiment of the invention. In Fig. 11, reference numeral 48 is a Mach-Zehnder optical modulator having a signal electrode 12 and a bias electrode 16. This modulator has the same structure as that in Fig. 1. A distributed feedback laser diode 50 serves as the light source. The light from the laser diode 50 is fed to the optical modulator via an optical isolator 52. The light from the optical modulator 48 is branched in two directions by an optical branching circuit 28 composed primarily of an optical coupler arrangement. One of the branched light streams is transmitted as the optical signal output over an optical transmission path, not shown. The other light stream from the optical branching circuit 28 goes to an optical-to-electrical signal converter 30 comprising a photo-diode arrangement for conversion to an electrical signal. After conversion, the electrical signal is amplified by an amplifier 64 before reaching a phase detecting circuit 44. Reference numeral 54 is an amplifier circuit that acts as a driving circuit. The amplifier circuit 54 amplifies the input signal and outputs the result as a driving signal having a predetermined amplitude. Reference numeral 56 is an amplitude modulating circuit that serves as a low-frequency superimposing circuit for superimposing a low-frequency signal onto the driving signal from the amplifier circuit 54. A low-frequency signal from an oscillator 34 is supplied to the amplitude modulating circuit 56 via a variable resistor 58. The frequency of the low-frequency signal is set to be sufficiently lower (e.g., 100 kHz) than the frequency corresponding to the bit rate of the input signal. The variable resistor 58 adjusts the modulation factor of the amplitude modulation based on the low-frequency signal. The phase detecting circuit 44 compares in phase the frequency component of the low-frequency signal in the electrical signal from the optical-to-electrical signal converter 30 with the low-frequency signal from the oscillator 34. The phase detecting circuit 44 then outputs a DC signal whose polarity is determined by the direction of the operating point drift and whose level depends on the magnitude of the drift. The phase detecting circuit 44 includes a synchronous detecting circuit 60 and a low-pass filter 62. Reference numeral 66 is an operational amplifier that serves as a bias control circuit. The operational amplifier 66 feeds a bias voltage to the bias electrode 16 of the optical modulator 48 in such a manner that the supplied DC signal becomes zero. In this first embodiment wherein the driving signal is fed direct to the signal electrode 12, i.e., without passage through a capacitor, a symmetrical driving waveform is not obtained. To implement symmetrical modulation requires superimposing a low-frequency signal onto the bias voltage supplied to the bias electrode 16. Specifically, the low-frequency signal from the oscillator 34 is sent to the bias electrode 16 by way of the variable resistor 68 and a coupling capacitor 70. How the optical transmitter of Fig. 11 works will now be described. Because the advantages of symmetrical modulation have already been discussed in quantitative terms, what follows is a qualitative description of how symmetrical modulation is implemented based on the structure of Fig. 11. Figs. 12A through 12E are views of waveforms generated by the optical transmitter of Fig. 11 when the transmitter is in operation. Fig. 12A shows a waveform of the driving signal as it is fed to the signal electrode 12, with 1's and 0's alternated. Fig. 12B depicts a waveform of the bias voltage as it is supplied to the bias electrode 16. Fig. 12C illustrates an optical phase change  1 of the branching waveguide 6 loaded with the signal electrode 12. Fig. 12D portrays an optical phase change  2 of the branching waveguide 8 loaded with the bias electrode 16. Fig. 12E shows an optical phase difference ( 1 -  2 ) between the light streams from the branching waveguides 6 and 8 as the light streams converge. As illustrated in Fig. 12B, an AC signal in phase with the low-frequency signal and having a predetermined amplitude is superimposed onto the DC bias voltage. The amplitude of this AC signal is determined in such a way that the amplitude of the space-side envelope becomes the same as that of the mark-side envelope in the waveform of the phase difference between the converging light streams from the branching waveguides 6 and 8, as shown in Fig. 12E. The amplitude of the low-frequency signal superimposed onto the driving signal and the amplitude of the AC signal superimposed onto the bias voltage may be adjusted using the variable resistors 58 and 68 in Fig. 11. Where the optical modulator of Fig. 4 is used in which either of the branching waveguides 6 and 8 is loaded with the signal electrode 12 and bias electrode 16, the sum of the optical phase change caused by the signal electrode 12 and the optical phase change brought about by the bias electrode 16 corresponds to the optical output power. Thus the AC signal superimposed onto the bias voltage is set opposite in phase to the low-frequency signal superimposed onto the driving signal. Superimposing an AC signal having a predetermined phase and a predetermined amplitude onto the bias voltage implements symmetrical modulation on the same principle as with the optical transmitter of Fig. 6. That is, the operating point is controlled optimally without regard to the mark rate of the input signal and other parameters. Because the driving signal is fed to the signal electrode 12 without the intervention of a capacitor and because the signal electrode 12 and the terminat-resistor 42 are connected in a DC setup, the signal waveform will not deteriorate even if the mark rate of the input signal changes abruptly. Fig. 13 schematically depicts key parts of an optical transmitter practiced as the second embodiment of the invention. Whereas the first embodiment of Fig. 11 has only one signal electrode 12, the second embodiment comprises signal electrodes 12A and 12B which correspond respectively to the branching waveguides 6 and 8. Bias electrodes 16A and 16B are provided so as to apply bias voltages to the branching waveguides 6 and 8, respectively. Reference numerals 72 and 74 are grounding electrodes; 76 is a driving circuit; 78 is an inverting circuit; 80 and 82 are transistors constituting a differential amplifier; 84 is a current source; 86 is an operational amplifier; and 88 and 90 are resistors. The grounding electrodes 72 and 74 are interconnected, although the connection is not shown in Fig. 13. The driving circuit 76 is connected in a DC setup to the terminals on one end of the signal electrodes 12A and 12B. A terminating resistor 42A is furnished in a DC setup between the signal electrode 12A and the grounding electrode 72, and a terminating resistor 42B is provided also in a DC setup between the signal electrode 12B and the grounding electrode 74. A bias voltage Vb for operating point control is fed to the bias electrode 16A, and a bias voltage inverted by the inverting circuit 78 is applied to the bias electrode 16B. Thus supplying the signal electrodes 12A and 12B with driving signals causes the electric field between signal electrode 12A and grounding electrode 72 to be sent to the branching waveguide 6, and also causes the electric field between signal electrode 12B and grounding electrode 74 to be fed to the branching waveguide 8. Likewise, giving the bias voltages to the bias electrodes 16A and 16B causes the electric field between bias electrode 16A and grounding electrode 72 to be supplied to the branching waveguide 6, and causes the electric field between bias electrode 16B and grounding electrode 74 to be applied to the branching waveguide 8. Because the gates of the transistors 80 and 82 are supplied respectively with an input signal V in and an inverted input signal *V in and because the signal electrodes 12A and 12B are connected in a DC setup to the drains of the transistors 80 and 82 respectively, the signal electrodes 12A and 12B are fed with driving signals that are opposite to each other in phase. With a low-frequency signal F m fed to the current source 84 to which a voltage E is applied, the driving signal coming from the driving circuit 76 and corresponding to the input signal is modulated by the low-frequency signal F m . The light supplied to the input port 4 from a light source, not shown, is modulated in phase in accordance with the driving signals while being transmitted through the branching waveguides 6 and 8. At the output port 10, the two light streams converge in phase or opposite to each other in phase. This reinforces or cancels the intensity of the two light streams, yielding an intensity-modulated light beam. Where the low-frequency signal F m is superimposed onto the driving signals and where the operating point of the optical modulator is kept optimum, the resulting light beam is one which is modulated in amplitude with a frequency twice the frequency fm of the low-frequency signal F m . Thus the modulated light is rid of the component of the low-frequency signal F m . Because an operating point shift of the optical modulator will cause the component of the low-frequency signal F m to be included in the modulated light, the bias voltage Vb is applied so as to minimize the component of the low-frequency signal F m . In this manner, the operating point is kept optimum. Fig. 14 is a block diagram of the entire optical transmitter of Fig. 13. In addition to the structure of Fig. 13, the optical transmitter of Fig. 14 further includes a distributed feedback laser diode 50 that serves as the light source, an optical isolator 52, an optical branching circuit 28, an optical-to-electrical signal converter 30, an amplifier 64, a synchronous detecting circuit 60, a low-pass filter 62, a proportional circuit 92, and an oscillator 34. The proportional circuit 92 contains an operational amplifier 66 and resistors 94 and 96. The optical-to-electrical signal converter 30, amplifier 64, synchronous detecting circuit 60, low-pass filter 62, proportional circuit 92, inverting circuit 78 and oscillator 34 constitute operating point control means. A current source 84 in a driving circuit 76 serves as a low-frequency superimposing circuit. The light from the laser diode 50 is fed to the input port 4 of the optical modulator via the optical isolator 52. The output port 10 of the optical modulator outputs modulated light. Part of the modulated light is branched by the optical branching circuit 28 and is sent to the optical-to-electrical signal converter 30 for conversion to an electrical signal. After conversion, the electrical signal is synchronously detected by the synchronous detecting circuit 60 using a low-frequency signal from the oscillator. The synchronously detected signal is output as a DC signal. The DC signal is supplied to the proportional circuit 92 via the low-pass filter 62. A bias voltage Vb output by the proportional circuit 92 is fed unmodified to a bias electrode 16A. A bias voltage inverted by the inverting circuit 78 is supplied to a bias electrode 16B. Figs. 15A through 15G are views of waveforms generated by the optical transmitter of Fig. 14 when the transmitter is in operation. Fig. 15A shows a waveform of an input signal V in with 1's and 0's alternated; Fig. 15B depicts a waveform of a low-frequency signal F m ; Fig. 15C portrays a waveform of a driving signal fed to one of the signal electrodes 12A; Fig. 15D illustrates a waveform of a driving signal supplied to the other signal electrode 12B; Fig. 15E is a waveform of an output phase  1 of one of the branching waveguides 6; Fig. 15F shows a waveform of an output phase  2 of the other branching waveguide 8; and Fig. 15G sketches a waveform of the phase difference ( 1 -  2 ) between the optical outputs from the branching waveguides 6 and 8. As evident from Figs. 15C and 15D, the driving signal fed to the signal electrode 12A is opposite in phase to the driving signal sent to the signal electrode 12B. As shown in Fig. 15G, the phase difference ( 1 -  2 ) between the optical outputs from the branching waveguides 6 and 8 is in fact the phase difference between zero and π . The center of that phase difference is given as ( 10 -  20 ), where  10 and  20 are reference phases for the optical outputs of the branching waveguides 6 and 8, respectively. When these optical outputs converge on the output port 10, an intensity-modulated light beam is obtained. The second embodiment shown in Fig. 13 or 14 supplies the signal electrodes 12A and 12B with driving signals that are opposite to each other in phase so as to create a predetermined phase difference (zero or π ) between the transmitted light outputs from the branching waveguides 6 and 8. This lowers the driving voltage of the optical modulator and virtually eliminates the chirping thereof. Because the second embodiment of Fig. 14 implements symmetrical modulation with no need for a capacitor as in the case of the first embodiment of Fig. 11, the second embodiment provides stable operating point control against any abrupt change in the mark rate and improves the signal waveform. Fig. 16 is a block diagram of an optical transmitter practiced as the third embodiment of the invention. The third embodiment differs from the second embodiment of Fig. 14 in the following aspects: One of the transistors 82 in the differential amplifier of the driving circuit 76 outputs a driving signal that is attenuated by an attenuator 98 before being fed to the signal electrode 12B. An inverted bias voltage from the inverting circuit 78 is supplied via a resistor 100 to the bias electrode 16B on the same side as the signal electrode 12B. In addition, the bias electrode 16B is supplied through a coupling capacitor 102 with a low-frequency signal from the oscillator 34. Figs. 17A through 17H are views of waveforms generated by the optical transmitter of Fig. 16 when the transmitter is in operation. Fig. 17A shows a waveform of an input signal V in with 1's and 0's alternated; Fig. 17B depicts a waveform of a low-frequency signal F m ; Fig. 17C portrays a waveform of a driving signal fed to the signal electrode 12A; Fig. 17D illustrates a waveform of a driving signal supplied to the signal electrode 12B; Fig. 17E is a waveform of a low-frequency signal fed to the bias electrode 16B; Fig. 17F shows a waveform of the phase  1 of the output light from the branching waveguide 6; Fig. 17G illustrates a waveform of the phase  2 of the output light from the branching waveguide 8; and Fig. 17H sketches a waveform of the phase difference ( 1 -  2 ) between the optical outputs from the branching waveguides 6 and 8. The amplitude of Fig. 17C is set to be greater than that of Fig. 15C so that the sum of the amplitude of Fig. 17C and that of Fig. 17D will equal the sum of the amplitude of Fig. 15C and that of Fig. 15D. As shown in Fig. 17D, the driving signal fed to the signal electrode 12B is attenuated by the attenuator 98. Thus the change in the phase  2 of the output light from the branching waveguide 8 becomes smaller than the change in the phase  1 of the output light from the branching waveguide 6, as indicated in Fig. 17G. With the low-frequency signal supplied to the bias electrode 16B, the phase  2 varies depending on that low-frequency signal. As shown in Fig. 17H, the phase difference ( 1 -  2 ) between the optical outputs from the branching waveguides 6 and 8 falls between zero and π , and the output port 10 outputs an intensity-modulated light beam. As with the second embodiment of Fig. 14, the third embodiment permits stable operating point control against the change of the mark rate, reduces the waveform deterioration, and lowers the driving voltage of the optical modulator. In contrast to the second embodiment of Fig. 14, the third embodiment keeps unbalanced its driving signals that are fed to the signal electrodes 12A and 12B so that chirping is created in the light output by the optical modulator. This cancels the adverse effects of wavelength dispersion over optical fiber cables where the polarity of the dispersion is predetermined. Fig. 18 is a block diagram of an optical transmitter practiced as the fourth embodiment of the invention. As opposed to the second embodiment of Fig. 14, the fourth embodiment is characterized by the provision of mutually independent driving circuits 76A and 76B that feed driving signals to signal electrodes 12A and 12B. The gates of transistors 80A and 82A constituting a differential amplifier in the driving circuit 76A are supplied respectively with an input signal V in and an inverted input signal *V in . The drain of the transistor 80A is connected in a DC setup to the signal electrode 12A. The gates of transistors 80B and 82B constituting a differential amplifier in the driving circuit 76B are fed respectively with the input signal V in and the inverted input signal *V in . The drain of the transistor 80B is connected in a DC setup to the signal electrode 12B. The low-frequency signal F m from the oscillator 34 is applied to two current sources: one current source 84A for the transistors 80A and 82A, the other current source 84B for the transistors 80B and 82B. When the driving circuits 76A and 76B are designed to have the same characteristics, the optical transmitter of Fig. 18 works in the same way as that of Fig. 14. Where the driving circuits 76A and 76B are made to differ from each other in characteristics through the use of such parameters as the maximum current values of the current sources 84A and 84B, chirping is deliberately created. Figs. 19A through 19G are views of waveforms generated by the optical transmitter of Fig. 18 when the transmitter is in operation. Fig. 19A shows a waveform of an input signal V in with 1's and 0's alternated; Fig. 19B depicts a waveform of a low-frequency signal F m ; Fig. 19C portrays a waveform of a driving signal fed to the signal electrode 12A; Fig. 19D illustrates a waveform of a driving signal supplied to the signal electrode 12B; Fig. 19E shows a waveform of the phase  1 of the output light from the branching waveguide 6; Fig. 19F illustrates a waveform of the phase  2 of the output light from the branching waveguide 8; and Fig. 19G sketches a waveform of the phase difference ( 1 -  2 ) between the optical outputs from the branching waveguides 6 and 8. These waveforms are in effect when the driving circuits 76A and 76B differ from each other in characteristics. As with the second embodiment of Fig. 14, the fourth embodiment permits stable operating point control against the abrupt change of the mark rate, improves the waveform deterioration, and lowers the driving voltage of the optical modulator. Because two mutually independent driving circuits are provided, the same transmitter structure is capable of addressing both the provision of chirping and the removal thereof. Thus the optical transmitter in an optical transmission system is readily optimized in characteristics by taking into account the characteristics of the target optical fiber for optical signal transmission. As described, the first through the fourth embodiments of the invention provide an optical transmitter that performs stable modulation based on high-speed input signals of at least several Gb/s. Although the description above contains many specificities, these should not be construed as limiting the scope of the invention as defined by the appended claims but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, with the fourth embodiment of Fig. 18, capabilities to permit the choice of either the presence or the absence of chirping may be added easily to eliminate the adverse effects of wavelength dispersion where the polarity of the wavelength dispersion over optical fiber cables is known beforehand.

Claims: An optical modulator comprising: an optical interferometer type modulating means (48) having an input port for receiving light from a light source (50), a first and a second branching waveguide (6, 8) for transmitting the light supplied to said input port after branching the supplied light in two directions, an output port (10) for converging the branched light from said first branching waveguide (6) and said second branching waveguide (8) before outputting the converged light, and a signal electrode (12) arranged to give phase change to light transmitted through said modulating means (48); and driving means (54, 56), connected operatively to said signal electrode (12), for supplying said signal electrode with a driving signal based on an input signal; characterised in that: the modulating means (48) also has a bias electrode (16) insulated from the signal electrode (12); said optical modulator further comprises operating point control means (44), connected operatively to said bias electrode (16), for supplying said bias electrode (16) with a bias voltage controlled in accordance with the light output from a signal converter (30) connected operatively to said output port for control of the operating point of said modulating means (48); the driving means (54, 56) is connected directly to said signal electrode (12); and the signal electrode (12) and the bias electrode (16) are arranged to give phase change to the light transmitted through the first branching waveguide (6) and the second branching waveguide (8). An optical modulator according to claim 1, wherein said driving means (54, 56) is arranged to supply said signal electrode (12) with said driving signal in such a manner that the light transmitted through said first branching waveguide (6) and said second branching waveguide (8) will be given a phase change which turns on and off the light output from said output port (10) in accordance with the logic level of said input signal; and wherein said operating point control means (44) is arranged to detect the drift of the operating characteristic curve of said modulating means (48) based on the optical intensity level of the light output from said output port (10), and the operating point control means (44) is arranged to supply said bias electrode (16) accordingly with said bias voltage so that the light transmitted through said first branching waveguide (6) and said second branching waveguide (8) will be given a phase change which keeps said operating point positionally constant with respect to said operating characteristic curve. An optical modulator according to claim 2, wherein said operating point control means includes : an oscillator (34) for outputting a low-frequency signal; a phase detecting circuit (44), connected operatively to said optical-to-electrical signal converter and to said oscillator (34), for comparing in phase the frequency component of said low-frequency signal in said electrical signal with said low-frequency signal from said oscillator (34), said phase detecting circuit (44) being further arranged to output a DC signal of which the polarity is determined by the direction of said drift and of which the level depends on the magnitude of said drift; and a bias control circuit (66), connected operatively to said phase detecting circuit (44) and to said bias electrode (16), for providing feedback control on said bias voltage so as to bring said DC signal to zero; and wherein said driving means (54, 56) includes a low-frequency superimposing circuit (56), connected operatively to said oscillator (34), for modulating in amplitude said driving signal using said low-frequency signal and wherein said signal converter (30) is an optical-to-electrical signal converter. An optical modulator according to claim 3, wherein the arrangement is such that, in use, an AC signal, in any one of in- and opposite-phase conditions with respect to said low-frequency signal and having a predetermined amplitude, is arranged to be superimposed onto said bias voltage; and wherein the phase and amplitude of said AC signal are determined so that the space-side and mark-side envelopes of a waveform representing any one of phase difference and phase sum between the converging light streams from said first branching waveguide (6) and said second branching waveguide (8) will be opposite in phase and will have the same amplitude. An optical modulator according to claim 1, wherein said signal electrode (12) has a first end portion and a second end portion and said signal electrode is built as a traveling-wave type such that, in use, the electric field derived from said driving signal travels in the same direction as the transmitted light through said first branching waveguide (6) and said second branching waveguide (8); and said driving means is connected to said first end portion; and said optical modulator further comprises a terminating resistor (42) connected to said second end portion in a DC setup. An optical modulator according to claim 5, wherein said signal electrode (12) is loaded onto any one of said first branching waveguide (6) and said second branching waveguide (8); and wherein said bias electrode (16) is loaded onto the other branching waveguide. An optical modulator according to claim 5, wherein said signal electrode (12) and said bias electrode (16) are loaded onto any one of said first branching waveguide (6) and said second branching waveguide (8). An optical modulator according to claim 5, wherein said signal electrode (12) is constituted by a first signal electrode (12A) and a second signal electrode (12B) which are loaded onto said first branching waveguide (6) and said second branching waveguide (8), respectively; and wherein the arrangement is such that, in use, the driving signal fed to any one of said first signal electrode (12A) and said second signal electrode (12B) is one obtained by inverting the driving signal supplied to the other signal electrode. An optical modulator according to claim 8, wherein said operating point control means includes an oscillator (34) for outputting a low-frequency signal, and a low-frequency superimposing circuit (76) connected operatively to said oscillator for modulating in amplitude said driving signal using said low-frequency signal; and wherein said operating point control means (60, 62, 78, 92) is arranged to control said bias voltage in such a manner that the component of said low-frequency signal contained in the output light from said optical modulating means (48) will be minimized. An optical modulator according to claim 9, wherein said driving means (76) includes a differential amplifier having a first and a second transistor (80, 82) connected respectively to said first signal electrode (12A and said second signal electrode (12B), and a current source (84) for supplying said differential amplifier with a current; and wherein said current source (84) is arranged to be modulated by said low-frequency signal. An optical modulator according to claim 10, wherein one of said first and said second transistors (80, 82) is connected directly to one of said first and said second driving electrodes (12A, 12B) and the other transistor is connected via an attenuator (95) to the other driving electrode; and wherein the arrangement is such that, in use, said bias voltage is supplied to said bias electrode (16) with said low-frequency signal. An optical modulator according to claim 9, wherein said driving means includes a first and a second differential amplifier (80A, 82A, 80B, 82B) which are provided respectively for said first signal electrode (12A) and said second signal electrode (12B); and wherein said first differential amplifier (80A, 82A) and said second differential amplifier (80B, 82B) are provided with current sources (84A, 84B) which are arranged to be modulated by use of said low-frequency signal. An optical modulator according to claim 8, wherein said bias electrode (16) is constituted by a first and a second bias electrode (16A, 16B) which are loaded respectively onto said first branching waveguide (6) and said second branching waveguide (8); and wherein the arrangement is such that, in use, the bias voltage fed to any one of said first bias electrode (16A) and said second bias electrode (16B) is one obtained by inverting the bias voltage supplied to the other bias electrode. An optical modulator according to any preceding claim, wherein said modulating means (48) includes an optical waveguide construction having titanium dispersed in a Z-cut LiNbO 3 substrate. An optical modulator according to claim 1, wherein said light source is a laser diode.

Inventors: HAKOGI HIRONAO, MINAMI TAKATOSHI, NISHIMOTO HIROSHI, HAKOGI, HIRONAO, MINAMI, TAKATOSHI, NISHIMOTO, HIROSHI, HAKOGI, HIRONAO, C/O FUJITSU LIMITED, Minami, Takatoshi, c/o Fujitsu Limited, Nishimoto, Hiroshi, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G02F1
IPC codes: H04B10, G02F1