Abstract:
An optical modulator uses an optoelectronic phase comparator configured to provide, in the form of an electrical signal, a measure of a phase difference between two optical waves. The phase comparator includes an optical directional coupler having two coupled channels respectively defining two optical inputs for receiving the two optical waves to be compared. Two photodiodes are configured to respectively receive the optical output powers of the two channels of the directional coupler. An electrical circuit is configured to supply, as a measure of the optical phase shift, an electrical signal proportional to the difference between the electrical signals produced by the two photodiodes.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. Pat. No. 9,372,354 which is hereby incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an optical modulator according to the Mach-Zehnder Interferometer principle, and, more particularly, to the stabilization of the operating point of the modulator with respect to variations of various external parameters. 
     BACKGROUND 
       FIG. 1  schematically shows an example of an optical modulator of Mach-Zehnder interferometer type, often called an MZI modulator. An optical wave having a power P 0  arrives through a waveguide to an optical separation unit S 1 , a Y-separator here. The initial wave is separated into two half-power waves, respectively guided in two parallel, upper and lower branches. Each branch comprises, in series, a fast optical phase modulation diode HSPM (“High Speed Phase Modulator”) and a slow optical phase adjusting diode PINPM. 
     Each of the diodes HSPM and PINPM introduces an adjustable phase delay of the optical wave crossing the diode by producing electric charge in the light path. The HSPM diode operates in reverse bias mode; electric charge is pulled from the junction into the optical path by a junction depletion phenomenon, the junction being offset relative to the optical path. The diode PINPM operates in a forward bias mode. It includes a P-I-N junction, the intrinsic region of which is in the light path and receives electrical charge by an injection phenomenon. 
     The diode PINPM reacts slowly to changes in its bias, but it has a wide range of phase delay adjustment—it is used to adjust an optimal quiescent phase delay in the branch. Thus, the diodes PINPM of the two branches (PINPM 1  and PINPM 2 ) receive respective constant bias currents depending on the quiescent phase delays to be introduced in the two branches. 
     The diode HSPM reacts quickly, but has a low phase modulation amplitude—it is used to modulate the phase delay with a digital signal to be transmitted around the quiescent phase delay established by the diode PINPM. Thus, the diodes HSPM of the two branches (HSPM 1  and HSPM 2 ) receive voltage signals that are modulated, based on the digital signal to be transmitted, between 0 and a positive value Vb. The voltage signals applied to the diodes HSPM 1  and HSPM 2  are complementary so as to produce a differential effect in the two branches. 
     The two branches reach an optical junction unit J 1 , here a directional coupler. The optical waves incident on the two channels of coupler J 1  are shifted by 180° at rest, whereby, in the case of a symmetric coupler, the optical power P 1 , P 2  delivered by each channel of the coupler J 1  is 50% of the input power P 0  of the modulator, the absorption losses in the branches being neglected. Diodes PINPM 1  and PINPM 2  are biased by different currents. For example, the diode PINPM 2  is biased by a zero current introducing theoretically a zero phase delay, and the diode PINPM 1  is biased by a current Ib selected to introduce a phase delay of 180°. 
       FIG. 2  is a diagram illustrating the variation of the transmission rate P 1 /P 0  of the modulator, measured at the output of the upper channel of the coupler J 1 , as a function of the phase difference between the waves at the inputs of the coupler J 1 . The transmission rate P 2 /P 0 , not shown, at the output of the lower channel, varies inversely. 
     The initial phase shift of 180° introduced by the diodes PINPM places the operating point of the modulator at the inflection point of a sinusoid, in a region where the linearity is best and the slope is steepest. The diode HSPM 1  causes the phase to vary in a range above 180°, while the diode HSPM 2  causes the phase to vary symmetrically in a range below 180°. By limiting the amplitude of these ranges, the corresponding change in the rate of transmission may be almost linear. For the transmission of digital signals, the linearity is less important, but it may be preferable that the behavior of the modulator remains balanced, which is the case under the conditions of  FIG. 2 . 
     In practice, the bias currents of the diodes PINPM are individually adjusted for obtaining the desired quiescent conditions. However, a drift of the quiescent conditions may be noticeable, particularly as a function of temperature. 
     SUMMARY 
     Thus there is a need for compensating the drift in the quiescent conditions of an MZI modulator. It may also be desirable to avoid individual adjustment of the bias currents of modulators in a production environment. 
     These needs are addressed by an optoelectronic phase comparator configured to provide, in the form of an electrical signal, a measure of a phase difference between two optical waves. The phase comparator comprises an optical directional coupler having two coupled channels respectively defining two optical inputs for receiving the two optical waves to be compared. Two photodiodes are configured to respectively receive the optical output powers of the two channels of the directional coupler. An electrical circuit is configured to supply, as a measure of the optical phase shift, an electrical signal proportional to the difference between the electrical signals produced by the two photodiodes. 
     An MZI optical modulator may thus comprise an optical separation unit for separating an incoming optical wave into a first and a second optical wave; a first and a second optical phase adjusting diode inserted respectively in the paths of the first and second optical waves; and a first and a second optical directional coupler, each having first and second coupled channels, the first channels being inserted respectively in the paths of the first and second optical waves. The MZI optical modulator may also include an optoelectronic phase comparator as mentioned above, having its optical inputs respectively connected to the second channels of the first and second directional couplers; and a circuit for electrically biasing the phase adjusting diodes, connected in a control loop with the optoelectronic phase comparator. 
     According to an embodiment, the optical modulator comprises, between one of the optical inputs of the phase comparator and the second corresponding channel of the first or second directional coupler, an intermediate directional coupler connected such that the phase difference between the two optical waves incident on the inputs of the phase comparator is equal to 180°. 
     According to an embodiment, the optical modulator comprises an optical wave junction unit having two inputs connected respectively to the outputs of the first channels of the first and second directional couplers; and a first and a second optical phase modulation diode respectively inserted in the connections between the junction unit and the first and second directional couplers. 
     According to an embodiment, the separation unit and the junction unit are directional couplers, and the first and second phase adjustment diodes are biased so that they introduce a 90° phase delay one relative to the other. 
     According to an embodiment, the separation unit is a directional coupler, the junction unit is a Y-combiner, and the first and second phase adjustment diodes are biased to apply the same phase delay. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features will become more clearly apparent from the following description of particular embodiments provided for exemplary purposes only and represented in the appended drawings, in which: 
         FIG. 1 , previously described, schematically shows an exemplary Mach-Zehnder interferometer (MZI) modulator as in the prior art. 
         FIG. 2  is a graph illustrating the transmission rate of the modulator of  FIG. 1  as a function of the phase difference between the optical waves of the two branches. 
         FIGS. 3A through 3D  represent different configurations of an MZI modulator combining different types of optical separation and junction units according to the invention. 
         FIG. 4  shows an embodiment of an optoelectronic regulation circuit adapted to a first MZI modulator configuration according to the invention. 
         FIG. 5  shows an alternative embodiment of the optoelectronic regulation circuit adapted to another MZI modulator configuration according to the invention. 
         FIG. 6  shows a detailed example of an optoelectronic regulation circuit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A P-I-N diode of an MZI modulator, even if its bias current is zero, causes a nonzero optical phase delay, a residual phase shift. This residual phase shift depends on the characteristics obtained after manufacture, such as doping and dimensions. If the two PIN diodes of the modulator could be matched, they would provide the same residual phase shift, which would be offset by the differential structure of the modulator. However, the diodes PINPM, which are formed in optical waveguides, are large compared to diodes used for plain electronic functions, and located far apart from each other at the scale of a semiconductor chip. As a result, it is difficult to match these “optical” diodes, and unpredictable offsets that are too great to be neglected are generally observed between the residual phase shifts of the two diodes PINPM. 
     Despite an accurate adjustment of the bias currents, even in modulator configurations where the two diodes PINPM may have the same phase setting, the quiescent conditions drift with temperature. This drift may be explained by the fact that the rate of change of the phase shift as a function of temperature depends on the operating conditions of the diode PINPM. These operating conditions are generally not identical initially for the two diodes, whereby the phase shifts of the two diodes diverge when the temperature varies. 
     The HSPM diodes are also subject to difficulties in matching, but their structure is inherently less sensitive to variability of manufacturing processes. It is noted that the offset between the residual phase shifts of the two HSPM diodes, even after thermal drift, may remain within acceptable limits to be neglected. 
       FIGS. 3A through 3D  show configurations of MZI modulators differing by combinations of separation and junction units requiring different bias conditions for the diodes PINPM. The structures of the two branches of the modulators are unchanged from  FIG. 1 . 
       FIG. 3A  corresponds to the configuration of  FIG. 1 , already described. The separation unit S 1  is a Y-separator and the junction unit J 1  is a symmetric directional coupler. Separator S 1  maintains the phase of the input optical wave on both outputs, whereas the coupler J 1  requires a phase difference of 180° to be in the center of its dynamic range, the desired quiescent condition. Thus, diodes PINPM 1  and PINPM 2  are biased to introduce an initial phase shift of 180° between the inputs of the coupler J 1 . 
     The two waves exiting the coupler J 1  are in phase quadrature, but their phase difference with respect to the input waves is variable depending on the characteristics of the input waves. 
     In  FIG. 3B , the output directional coupler J 1  has been replaced by a Y-combiner J 2 . Such a combiner transmits 50% of the optical power when the waves at its two inputs are in phase quadrature. The two waves arriving at the diodes PINPM being in phase, the diodes PINPM are biased to introduce the desired phase difference of 90° between the two waves. E.g. the diode PINPM 1  is biased to introduce a phase delay of 90° and the diode PINPM 2  receives a zero current, corresponding theoretically to a null phase delay. In practice, the diode PINPM 2  introduces a residual phase delay at zero bias current, which is difficult to predict, for example 1°. In that case, the diode PINPM 1  is biased for introducing a phase delay of 90+1=91°. 
     The configuration of  FIG. 3B  may be preferred to that of  FIG. 3A , because the diode PINPM 1  causes less absorption losses than in  FIG. 3A . Indeed, a greater phase delay is obtained in  FIG. 3A  by injecting more charge in diode PINPM 1 , and absorption losses increase with the number of charges. 
     In  FIG. 3C , the Y-separator S 1  of  FIG. 3A  has been replaced by a symmetrical directional coupler S 2 . The input optical wave is applied to one of the channels of the coupler, for example the lower one. In that case, the wave exiting the upper channel of the coupler is delayed by 90° relative to the wave exiting the lower channel. The output coupler J 1  requiring a phase difference of 180° to operate in the desired quiescent conditions, it is sufficient that the diode PINPM 1  introduce a phase delay of 90° that is added to the delay of 90° introduced by the upper channel of the input coupler S 2 . 
     The absorption losses of the configuration of  FIG. 3C  are similar to those of  FIG. 3B . 
     In  FIG. 3D , the output directional coupler J 1  of  FIG. 3C  has been replaced by a Y-combiner J 2 . The input coupler S 2  directly produces the phase quadrature desired for the quiescent conditions, between the waves input to the combiner J 2 . Thus, the diodes PINPM need not introduce additional phase delay. In this case, the two diodes PINPM may be biased at zero current, in theory. This configuration therefore provides the best performance in terms of absorption losses. 
     Because the two diodes PINPM operate in similar conditions, this configuration also offers the best performance in terms of thermal drift. 
     In practice, the diode PINPM having the highest residual phase delay may be biased at zero current, while the other diode PINPM is biased with a current sufficient to balance the residual phase delay. As it is difficult to know in advance which of the two diodes PINPM has the highest residual phase delay, it is preferred to bias both diodes with non-zero currents, one fixed and one adjustable. 
       FIG. 4  shows an embodiment of an optoelectronic circuit for regulating the quiescent conditions of an MZI modulator. The MZI modulator has a configuration similar to that of  FIG. 3D . With respect to  FIGS. 3A to 3D , the positions of the diodes PINPM have been interchanged with those of the HSPM diodes, so that the diodes PINPM are the first elements in the two branches of the modulator, and are part of a control loop. The HSPM diodes are not included in the loop—as mentioned earlier, the drifts of the HSPM diodes may be neglected. 
     The optoelectronic regulation circuit, whose principles may be applied to various MZI modulator configurations, such as those illustrated in  FIGS. 3A to 3D , measures the optical phase difference between the waves in the two branches of the modulator, and provides the error relative to a desired value in the form of optical power received by photodiodes PD 1 , PD 2 . The electrical signals provided by the photodiodes are exploited to vary the bias currents of the diodes PINPM in a direction tending to reduce the error. 
     The measurement of the phase difference may be achieved using a symmetrical optical directional coupler DC 0  receiving, on its two channels, optical waves derived from the two branches of the modulator. The paths of the derived optical waves are configured so that the phase difference at the input of the coupler DC 0  equals 180° when the phase difference between the derived optical waves corresponds to that desired at the input of the junction unit. Under these conditions, the coupler DC 0  outputs at each of its channels 50% of the total optical power received. If the phase difference is not equal to 180°, one of the channels provides more than 50% of the power, while the other channel provides the complement. The optical waves at the outputs of the two channels of the coupler are provided to two respective photodiodes PD 1 , PD 2 . Thus, the difference between the electrical signals generated by the photodiodes represents the optical phase error. 
     In  FIG. 4 , more specifically, the optical outputs of diodes PINPM 1  and PINPM 2  are provided to the first channels of two respective asymmetric directional couplers DC 1  and DC 2 . The outputs of these first channels are provided to diodes HSPM 1  and HSPM 2  respectively. 
     The couplers DC 1  and DC 2  are asymmetrical in that the optical power received in the first channel is distributed asymmetrically between the outputs of the first and second channels, for example 98% at the output of the first channel, and 2% at the output of the second channel. The fraction of the output power in the second channel is chosen to be detectable by a photodiode in good conditions. 
     The optical waves output by the second channels of the couplers DC 1  and DC 2  have respective phase delays of 180° and 90° relative to the optical wave input to the modulator (each of the couplers DC 1  and DC 2  introduces a phase delay of 90° as the wave passes from the first channel to the second). The phase difference between these waves is thus 90° while the coupler/comparator DC 0  requires 180°. A symmetrical directional coupler DC 3  is provided to add the missing 90° phase delay to the 180° optical wave. The coupler DC 3  receives in its first channel the 180° wave and provides a 270° wave from its second channel to the upper channel of coupler/comparator DC 0 . 
     A directional coupler DC 4  is provided to equalize the optical paths leading to the coupler/comparator DC 0 . Its first channel connects the coupler DC 2  to the lower channel of the coupler DC 0 , without introducing a phase delay. 
     The outputs of the first and second channels of the coupler/comparator DC 0  are respectively sensed by the photodiodes PD 1  and PD 2 . These photodiodes are part of an electrical control circuit  10  configured to adjust the bias currents of the diodes PINPM according to the difference between the sensed optical powers. In this modulator configuration, the bias currents are substantially the same, since the diodes PINPM are designed to introduce the same phase delay (as close to 0° as possible.) 
     The regulator circuit of  FIG. 4  is also usable, as is, in the modulator configuration of  FIG. 3B . 
       FIG. 5  shows an alternative of the optoelectronic regulation circuit, integrated with an MZI modulator of the type of  FIG. 3C . The same elements as in  FIG. 4  are designated by the same references. The diode PINPM 1  is biased here for introducing a phase delay of 90°, so that the second channel of the coupler DC 1  provides an optical wave delayed by 270°. The second channel of the coupler DC 2  still provides a wave delayed by 90°. The phase difference between these waves is 180°, whereby the two waves may be directly applied to the inputs of the coupler/comparator DC 0 . 
     The configuration of  FIG. 5  is simpler than that of  FIG. 4  in that it uses two directional couplers less. 
     The regulator circuit  FIG. 5  is also usable, as is, in the modulator configuration of  FIG. 3A . 
       FIG. 6  shows a detailed example of electronic circuitry of the optoelectronic regulation circuit  10 . The photodiodes PD 1  and PD 2  are reverse biased by two resistors R 1  and R 2 . The cathodes of diodes PD 1  and PD 2  are connected respectively to a non-inverting input and an inverting input of a differential transconductance amplifier  20 . The diodes PINPM 1  and PINPM 2  are forward biased by respective constant current sources Ib 1  and Ib 2 . The anodes of diodes PINPM 1  and PINPM 2  are connected to forward and reverse outputs of the amplifier  20 . 
     The currents Ib 1  and Ib 2  are set by design to the typical values required to introduce the quiescent phase difference corresponding to the modulator configuration (180° for  FIG. 3A , 90° for  FIGS. 3B and 3C , and 0° for  FIG. 3D ). In theory, one of the currents (Ib 2 ) may be zero. In practice, the two currents are non-zero, so that each has a margin of adjustment. Current Ib 2  is selected, for example, to introduce a phase delay of 5°. Then, the current Ib 1  is selected to introduce a phase delay of 185° in  FIG. 3A , 95° in  FIGS. 3B and 3C , and 5° in  FIG. 3D . 
     When the phase difference between the input waves of coupler/comparator DC 0  is 180°, each of the photodiodes PD 1  and PD 2  receives the same optical power, 50% of the power received by the coupler/comparator DC 0 . If the photodiodes are matched, which is easier to achieve than for diodes PINPM, their cathode voltages stand at identical values. Thus, the amplifier  20  sees a zero input voltage and does not influence the bias currents of the diodes PINPM. 
     If the phase difference between the input waves of coupler/comparator DC 0  drops below 180°, it means that the delay introduced by the diode PINPM 1  decreases or the delay introduced by the diode PINPM 2  increases. The optical power received by photodiode PD 1  increases, and the optical power received by the photodiode PD 2  decreases. The cathode voltage of the photodiode PD 1  increases and that of the photodiode PD 2  decreases. The amplifier  20  sees its differential input become positive—it injects a proportional current in the diode PINPM 1  and subtracts a proportional current from the diode PINPM 2 . The diode PINPM 1  increases its phase delay while the diode PINPM 2  decreases its phase delay. 
     A symmetrical behavior is obtained when the phase shift between the waves becomes greater than 180°. 
     An automatic correction is thus obtained for the quiescent phase errors in the modulator. This correction is independent of the nature of the error—the error may be due to a temperature drift, a poor matching between the diodes PINPM, a poor initial choice of the bias currents, or any other cause. The accuracy of the correction depends on the open loop gain of the control loop, which may be easily adjusted by way of amplifier  20 . 
     The accuracy also depends on the parasitic offset referred to the input of the amplifier, caused for instance by a mismatch between the photodiodes or a lack of precision in the coupling coefficient of each of couplers DC 0 , DC 1  and DC 2 . Such an offset may be compensated electrically by techniques known in the field of differential amplifiers.