Abstract:
An apparatus and method for controlling bias in an optical modulator is disclosed. The method is particularly applicable to controlling multi-wavelength modulators and wavelength-tunable transmitters. At a calibration stage, a desired optical performance of the modulator is achieved, and an amplitude of a peak-to-peak variation of the output optical signal at a pre-determined amount of dither is stored in a memory as a reference. At operating stage, a controller of the optical modulator adjusts a bias voltage of the modulator until the measured peak-to-peak optical signal variation matches the reference value stored at the calibration stage. For multi-wavelength modulators and tunable transmitters, the calibration is repeated at each wavelength, and corresponding peak-to-peak optical signal variations are stored in the memory.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 61/307,741 filed Feb. 24, 2010 which is incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to optical modulators and transmitters, and in particular to controlling optical modulators and wavelength-tunable transmitters used in optical communications. 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical modulators are used in optical communication systems to modulate an optical signal with an electrical signal. The electrical signal can have a high frequency, for example a few gigahertz. The modulated optical signal is propagated over large distances in an optical fiber. At the receiver end, the signal is detected using a photodetector, and the electrical signal is restored for further processing or transmission. 
         [0004]    Presently, wavelength-tunable transmitters find an increasing application in optical networking systems capable of providing bandwidth on demand. Wavelength-tunable transmitters are also used as “any-wavelength” backup transmitters in fixed-wavelength transmitter arrays. A tunable transmitter requires a multi-wavelength optical modulator, the optical performance of which does not vary with wavelength and does not change, or changes negligibly, as the tunable transmitter ages. 
         [0005]    Every optical modulator has a transfer curve, which represents a relationship between an amplitude of electrical signal applied and a magnitude of optical modulation obtained at the output of the optical modulator. Performance of many optical modulators depends on a choice of a set point, that is, a point on the transfer curve corresponding to zero modulating electrical signal. The set point can be adjusted by adding a DC signal to the modulating electrical signal, or by segmenting the optical modulator and applying the DC signal to one of its segments. 
         [0006]    The set point of an optical modulator has a tendency to drift with temperature. To reduce thermal drift, a dither voltage is added to the modulating voltage, and a synchronous (lock-in) detection is employed to stabilize the set point. Referring to  FIG. 1 , a typical stabilized modulator system  100  is presented. The system  100  includes a Mach-Zehnder (MZ) optical modulator  102 , a dithering unit  104 , a lock-in detector  106  having a photodetector  107 , a mixer  108  for mixing modulation and dithering signals  112  and  114 , respectively, and a beamsplitter  110 . In operation, an optical signal  116  is applied to the MZ optical modulator  102 . The optical signal  116  is modulated with the modulation signal  112  applied to the MZ optical modulator  102  through the mixer  108 . The dithering unit  104  generates the dithering signal  114 , which is mixed by the mixer  108  into the modulation signal. A small fraction of an output optical signal  118  is directed by the beamsplitter  110  to the photodetector  107 . The lock-in detector  106  generates a DC bias signal  120  based on a synchronously detected component of the output optical signal  118  at the frequency of the dithering signal  114 . 
         [0007]    Various modifications and adaptations of the stabilized modulator system  100  have been disclosed. By way of example, Tipper in U.S. Pat. No. 7,555,226 discloses an automatic bias controller for a MZ optical modulator. The automated bias controller of Tipper uses a microprocessor for both dithering and processing of the modulated optical signal. An optical power detector is used for detecting optical power of light emitted by one of two output arms of the MZ modulator. The detected signal is analyzed, and a bias voltage is adjusted so as to stabilize the set point of the MZ modulator. The other output arm of the MZ modulator is used to output the modulated optical signal. 
         [0008]    Nahapetian et al. in U.S. Pat. No. 7,729,621 disclose a controller of a bias voltage for a MZ modulator, programmed to receive a dither signal, determine a derivative and/or an integral of the dither signal, and control a bias voltage for the MZ modulator based on the derivative and/or the integral of the dither signal. 
         [0009]    Noguchi et al. in U.S. Pat. No. 7,561,810 disclose a bias controller of an optical modulator, wherein a pilot tone is added to the biasing voltage of the optical modulator. A monitor signal is branched into a signal path and a noise path. A notch filter is used in the noise path to suppress the pilot tone. The signals in both paths are synchronously detected, and the synchronously detected noise is subtracted from the synchronously detected signal to improve the signal-to-noise ratio. 
         [0010]    Optical modulator control systems of the prior art are not adapted to control a modulator operating at different wavelengths. Accordingly, it is an object of the present invention to provide a control system, a modulator, and a tunable transmitter usable therewith, that can maintain optimal optical performance at a plurality of wavelengths, over extended periods of time. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with the invention there is provided a method for controlling an optical modulator, comprising:
       (a) applying a first optical signal, a first bias signal, and a first RF modulating signal to the optical modulator and measuring a performance parameter thereof, wherein a magnitude of the first bias signal is selected so as to obtain a pre-determined value of the measured performance parameter;   (b) while applying the first optical signal, the first RF modulating signal, and the first bias signal of the magnitude selected in step (a) to the optical modulator, applying a dither signal to the optical modulator and measuring a target peak-to-peak optical power variation of the first optical signal due to the dither signal application;   (c) storing a magnitude of the first bias signal and the measured target peak-to-peak optical power variation in a memory;   (d) upon completion of steps (a) to (c), applying a second optical signal, a second RF modulating signal, a second bias signal, and the dither signal to the optical modulator, and measuring an operational peak-to-peak optical power variation of the second optical signal due to the dither signal application; and   (e) adjusting the second bias signal, so as to lessen a difference between the operational peak-to-peak optical power variation measured in step (d) and the target peak-to-peak optical power variation stored in step (c), thereby obtaining the pre-determined value of the performance parameter without having to re-measure the performance parameter,
 
wherein steps (a) to (c) are performed during calibration of the optical modulator, and steps (d) and (e) are performed during subsequent operation of the optical modulator.
       
 
         [0017]    In accordance with another aspect of the invention, there is further provided a control unit for controlling a multi-wavelength optical modulator, comprising: 
         [0000]    a dither unit for applying a dither signal to the optical modulator;
 
a measuring unit for measuring a peak-to-peak optical power variation at an output of the optical modulator due to the dither signal application by the dither unit, when the optical signal is applied to an input of the optical modulator;
 
a memory for storing a target peak-to-peak optical power variation measured by the measuring unit for each of a plurality of wavelengths of an optical signal; and
 
a feedback loop controller operationally coupled to the memory, the dither unit, and the measuring unit, for applying a bias signal to the optical modulator, in dependence upon a wavelength of the optical signal applied to the optical modulator input, so as to lessen a difference between the peak-to-peak optical power variation measured by the measuring unit and one of the target peak-to-peak optical power variations stored in the memory, corresponding to the wavelength of the optical signal applied.
 
         [0018]    In accordance with yet another aspect of the invention there is further provided a tunable transmitter comprising: 
         [0000]    a tunable laser source for providing the optical signal;
 
a multi-wavelength optical modulator coupled to the tunable laser source;
 
the abovementioned control unit for controlling the multi-wavelength optical modulator; and
 
a wavelength locker coupled to the control unit, for locking a wavelength of the tunable laser source to any of the plurality of wavelengths;
 
wherein the control unit is adapted to provide a wavelength control signal for tuning the wavelength of the tunable laser source to a first one of the plurality of wavelengths, using the wavelength locker to generate a wavelength error signal proportional to a deviation of the wavelength of the tunable laser source from the first wavelength, and to adjust a bias signal applied to the multi-wavelength optical modulator, so as to lessen a difference between the peak-to-peak optical power variation measured by the measuring unit and a target peak-to-peak optical power variation stored in the memory, corresponding to the first wavelength.
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Exemplary embodiments will now be described in conjunction with the drawings, in which: 
           [0020]      FIG. 1  is a block diagram of a prior-art stabilized modulator system; 
           [0021]      FIG. 2  is a block diagram of a tunable transmitter of the present invention; 
           [0022]      FIG. 3  is a block diagram of an optical modulator control system used in the tunable transmitter of  FIG. 2 ; 
           [0023]      FIG. 4  is a flow chart of an optical modulator calibration method according to the invention; 
           [0024]      FIG. 5  is a flow chart of an optical modulator bias control method according to the invention; 
           [0025]      FIG. 6  is a graph of a dependence of a peak-to-peak optical modulation on a bias voltage applied to an optical modulator used in the tunable transmitter of  FIG. 2 ; 
           [0026]      FIG. 7  is a flow chart of an optical modulator bias control method of the invention, used when a wavelength of the optical signal is changed; 
           [0027]      FIG. 8  is a plan view of an optical subassembly of the tunable transmitter of  FIG. 2 ; 
           [0028]      FIG. 9  is an equivalent electrical circuit of the subassembly of  FIG. 8 ; and 
           [0029]      FIG. 10  is an electrical circuit of a driver for the subassembly of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0031]    Referring to  FIG. 2 , a tunable transmitter  200  includes a tunable laser  202 , a semiconductor optical amplifier (SOA)  204 , a Mach-Zehnder (MZ) modulator  206 , a wavelength locker  208 , and a control unit  210 . The wavelength locker  208  has a Fabry-Perot interferometer or etalon  212 , a beamsplitter  214 , and two photodetectors  216 , preferably photodiodes, for detecting an optical signal reflected from and transmitted through the Fabry-Perot interferometer  212 . In operation, the control unit  210  provides a wavelength control signal  220  for tuning the wavelength of the tunable laser  202  to a wavelength set at a wavelength input  221 , a power control signal  222  for adjusting gain of the semiconductor optical amplifier  204 , and a modulation control signal  224  applied to the MZ modulator  206 , for modulating an input optical beam  218  and controlling bias of the MZ modulator  206 . The modulation control signal  224  includes an RF modulating signal inputted at a Data Input  223 , superimposed with a bias control signal, for example a bias voltage, for adjusting a set point of the MZ modulator  206 . It is to be understood that an optical modulator of a type other than MZ modulator can also be used in place of the MZ modulator  206 . 
         [0032]    The wavelength control signal  220  is generated so as to lessen the wavelength error signal proportional to a deviation of the wavelength of the tunable laser  202  from the wavelength set at the wavelength input  221 . As will be appreciated by those skilled in the art, the reflection characteristic of the Fabry Perot interferometer  212  is selected so as to translate the deviation of the wavelength into a difference of photocurrents of the two photodetectors  216  of the wavelength locker  208 . 
         [0033]    The power control signal  222  is generated so as to set and maintain the optical power of an output beam  219  of the tunable transmitter  200  at a desired level. The measured output power is proportional to a sum of the photocurrents of the photodetectors  216 . The bias of the MZ modulator  206  is controlled based on measurement of output optical power variation, which is derived from a variation of a sum of the photocurrents of the photodetectors  216 . Thus, the photodetectors  216  are shared between the wavelength control, power control, and modulator control circuits of the control unit  210 . Separate photodetectors can also be used, although the shared photodetectors  216  are preferred for cost reduction reasons. 
         [0034]    The control unit  210  and its bias control function will now be described in more detail. Referring to  FIG. 3 , the control unit  210  includes a dither unit  302  for applying a dither signal  316  to the MZ modulator  206 ; a measuring unit  304  for measuring a peak-to-peak output optical power variation due to application of the dither signal  316  by the dither unit  302 ; a memory  306  for storing target peak-to-peak optical power variation values and bias signal values; and a feedback loop controller  308  for applying the bias control signal to the MZ modulator  206 . A mixer  310 , a capacitor  312 , and an inductor  314  are used to combine an AC binary data signal from the Data Input  223  and a DC bias signal, for application to the MZ modulator  206 . A wavelength control circuit  307  is used to provide the wavelength control signal  220  to the tunable laser  202  based on the difference of the photocurrents of the photodetectors  216 , for tuning the tunable laser  202  to a wavelength set at the wavelength selection input  221 . 
         [0035]    In operation, the control unit  210  sets a wavelength of operation of the tunable laser  202  by applying the wavelength control signal  220  to the tunable laser  202  and an output optical power by applying the power control signal  222  to the SOA  204 . Then, the control unit  210  causes the dithering unit  302  to generate the dithering signal  316 , which is applied to the MZ modulator  206  via the mixer  310 . The measuring unit  304  measures peak-to-peak output optical power variation due to the dithering, using the sum of the photocurrents of the photodetectors  216  as noted above. The feedback loop controller  308  adjusts the bias voltage applied to the MZ modulator  206 , so as to lessen a difference between the peak-to-peak optical power variation measured by the measuring unit  304  and a target peak-to-peak optical power variation stored in the memory  306 . The feedback loop controller  308  is preferably a first-order proportional integral-differential (PID) controller. The target peak-to-peak optical variation corresponds to the wavelength selected at the wavelength selection input  221 . Each selectable wavelength has its own target peak-to-peak optical variation value stored in the memory  306 . The target peak-to-peak optical variation values are determined during calibration. 
         [0036]    The process of calibration of the MZ modulator  206  of the tunable transmitter  200  will now be described. Turning to  FIG. 4 , a method  400  of calibration the tunable transmitter  200  starts at a step  402 , in which an optical signal at a wavelength λ 1 , a first bias signal, and a first RF modulating signal are applied to the MZ modulator  206 . The optical signal is preferably generated by the tunable laser  202  and amplified by the SOA  204 , forming the input optical beam  218 . An external optical signal can also be used. After the signals are applied, a performance parameter, such as extinction ratio (ER) or transmit eye crossing point position, is measured. In a step  404 , a magnitude of the first bias signal Vb is adjusted, so as to obtain a pre-determined value of the measured ER. When the desired performance is obtained, then in a step  406 , a dither signal is applied to the MZ modulator  206 , and a peak-to-peak output optical power variation, Ipp, caused by the dither signal application, is measured. The measured peak-to-peak optical power variation Ipp is used as a “target” value during subsequent operation of the device. 
         [0037]    A ratio of amplitudes of the dither signal and the RF modulating signal can be adjusted during calibration, so as to obtain a predetermined value of the measured peak-to-peak optical power variation Ipp. Typically, the dither signal is 2.5% of the RF modulating signal, but the ratios of 2% to 8% can also be used. For example, if the RF modulating signal has an amplitude of 1V and the dither signal is 2.5%, the output modulating signal varies between 0.975V and 1.025V. The frequency of the dithering signal can be between 1 kHz and 20 kHz, preferably 2 kHz to 5 kHz, and most preferably 2.5 kHz. The data rate of the RF modulating signal can range widely, for example from 200 kHz to 40 GHz. A typical data rate is 10 GHz to 12 GHz. 
         [0038]    The measured peak-to-peak output optical power variation Ipp is defined as follows: 
         [0000]        I   pp   ∝P ( V   Rf   +V   D )− P ( V   RF   −V   D )  (1)
 
         [0039]    where P is instantaneous measured optical power, V RF  is RF modulating signal amplitude, and V D  is the dither signal amplitude. 
         [0040]    Preferably, the measured peak-to-peak output optical power variation Ipp is averaged using a finite impulse response filter to reduce noise due to RF modulation. The finite impulse response filter can have a time constant of 0.25 to 0.5 seconds or larger. 
         [0041]    In a step  408 , the value of the bias signal, which can be a bias current or a bias voltage, is stored in the memory  306  along with the target optical power variation Ipp. In the embodiment described, the bias signal is a voltage Vb 0 . A value ΔVb, the use of which will become clear from the following description, is initially set to zero and stored in the memory  306 . In an optional step  410 , the peak-to-peak output optical power variation Ipp is measured as the bias voltage Vb is scanned. The measured dependence Ipp(Vb) can be used during operation of the MZ modulator  206  to set the bias of the MZ modulator  206 . Using the dependence Ipp(Vb) for setting the bias of the MZ modulator  206  will be described in more detail below. Finally, in a step  412 , a wavelength of the optical signal is tuned to a new value, and the steps  402  to  408 , and optionally the step  410 , are repeated. 
         [0042]    Referring now to  FIG. 5 , a method  500  of operating the MZ modulator  206  of the tunable transmitter  200 , calibrated using the method  400  described above, is presented. In a step  502 , the MZ modulator  206  is turned on, or reset. In a step  504 , the previously stored values Vb 0  and ΔVb are recalled form the memory  306 . In a step  506 , a bias voltage Vb is set to Vb 0 +ΔVb and is applied to the MZ modulator  206 . An optical signal at a wavelength λ 1 , an RF modulating signal, and the dither signal of the same amplitude as the amplitude used at calibration  400 , are applied to the MZ modulator  206 . In a step  508 , a peak-to-peak optical power variation Ipp, caused by the dither signal application, is measured. In a step  510 , the measured peak-to-peak optical power variation Ipp is compared to the “optimal” Ipp stored in the memory  306 . If the measured peak-to-peak optical power variation Ipp is different from the “optimal” peak-to-peak optical power variation Ipp, then in a step  512 , the feedback loop controller  308  adjusts the bias voltage Vb so as to lessen a difference between the measured and the “optimal” peak-to-peak optical power variation Ipp. In an optional step  514 , the change of Vb, ΔVb, is stored in the memory  306  for subsequent use. Alternatively or in addition, the adjusted value of Vb can also be stored. In a step  516 , the peak-to-peak optical power variation Ipp is monitored. If a deviation is detected, the process  500  repeats. Thus, the pre-determined value of the performance parameter used in calibration, such as ER, can be achieved without having to re-measure the performance parameter. The performance parameter of interest has been experimentally shown to remain at an optimal value during the lifetime of the optical modulator  200 , without having to re-calibrate the optical transmitter  200 . 
         [0043]    The step  512  of adjusting the bias voltage Vb will now be considered in more detail. Referring to  FIG. 6 , a dependence  600  of a peak-to-peak optical modulation Ipp on the magnitude of the bias voltage Vb applied to the MZ modulator  206  is typically cyclical. The value of peak-to-peak optical power variation Ipp, measured in the step  508 , is labeled in  FIG. 6  as “Ipp_operational”. Since the dependence  600  Ipp(Vb) has been previously measured in the step  410  of the calibration process  400 , the adjustment ΔVb of the bias voltage Vb, required to bring the Ipp value to Ipp_target, can be easily calculated. Advantageously, this allows the adjustment of the bias voltage Vb to be performed much faster. Further, advantageously, the bias voltage Vb and/or the bias voltage adjustment ΔVb, stored in the memory  306  in the step  514 , can be used next time the optical modulator  206  is restarted at the step  502  of the operating process  500 , to speed up the bias adjustment. 
         [0044]    It is to be understood that the methods  400  and  500  can also be used for controlling a single-wavelength optical modulator. In this case, the optical modulator only needs to be calibrated at one wavelength, and only one set of Vb and Ipp needs to be stored in the memory  306 . 
         [0045]    Referring now to  FIG. 7 , a method  700  of operating the MZ modulator  206  of the tunable transmitter  200  is used when a wavelength of the tunable laser  202  is changed from one wavelength to another. It is assumed for certainty that the wavelength is changed from a “previous” wavelength λ 1  to a “new” wavelength λ 2 . It is also assumed that the MZ modulator  206  has been calibrated using the method  400  described above, for both wavelengths λ 1  and λ 2 . In a step  702 , the wavelength of the tunable laser  202  is tuned from λl to λ 2 . In a step  704 , the value Vb 0  corresponding to the new wavelength λ 2  is recalled form the memory  306 . The value ΔVb, corresponding to the previous wavelength λ 1 , is also recalled at this step. In a step  706 , a bias voltage Vb is set to Vb 0 +ΔVb and is applied to the MZ modulator  206 . An RF modulating signal and the dither signal of the same amplitude as the amplitude used at calibration  400 , are applied to the MZ modulator  206 . In a step  708 , a peak-to-peak optical power variation Ipp, caused by the dither signal application, is measured. In a step  710 , the measured peak-to-peak optical power variation Ipp is compared to the “optimal” peak-to-peak optical power variation Ipp for the new wavelength λ 2 , stored in the memory  306  during calibration. If the measured peak-to-peak optical power variation Ipp is different from the “optimal” peak-to-peak optical power variation Ipp, then in a step  712 , the feedback loop controller  308  adjusts the bias voltage Vb so as to lessen a difference between the measured and the “optimal” peak-to-peak optical power variation Ipp. In an optional step  714 , the new value of Vb and/or the change of Vb, ΔVb, is stored in the memory  306  for subsequent use. In a step  716 , the peak-to-peak optical power variation Ipp is monitored. If a deviation is detected, the process  700  repeats. In this way, the pre-determined value of the performance parameter used in calibration, such as ER, can be achieved without having to re-measure the performance parameter or re-calibrate the optical modulator  200 . 
         [0046]    Turning now to  FIG. 8 , an optical subassembly  800  of the tunable transmitter  200  includes a monolithically integrated semiconductor optical transmitter  802  and the wavelength locker  208  mounted on a common thermoelectric cooler (TEC) plate  804 . The semiconductor optical transmitter  802  includes the tunable laser  202 , the SOA  204 , and the planar waveguide MZ modulator  206  monolithically integrated on a common semiconductor substrate. The tunable laser  202  has back and front mirror sections  806 A and  806 B, respectively, a phase section  808 , and a gain section  810 . The semiconductor optical transmitter  802  is described in detail in U.S. Pat. No. 7,633,988 by Fish et al., which is incorporated herein by reference. The MZ modulator  206  has left and right arms  812  and  814  having left and right electrodes  816  and  818 , respectively, for adjusting optical phases of light propagating in the left and right arms  812  and  814 , respectively. 
         [0047]    Referring to  FIG. 9 , an equivalent electrical circuit  900  of the optical subassembly  800  is shown. The monolithically integrated semiconductor optical transmitter  802  (“ILMZ chip” in  FIG. 9 ) is represented by diodes  901 , capacitors  903 , and resistors  905 . In operation, voltages are applied to a back minor electrode  906 A, a phase section electrode  908 , a gain section electrode  910 , a front minor electrode  906 B, a SOA electrode  904 , to generate electrical currents and voltages required for operation of the tunable laser  202  (“SG-DBR laser” in  FIG. 9 ) and the SOA  204 . Modulation and bias voltages are applied to left and right arm electrodes  916  and  918 , respectively, of the MZ modulator  206 . A “DC_GND” electrode  926  is a ground electrode for the tunable laser  202  and SOA  204 . A “RF_GND” electrode  928  is a ground electrode for the MZ modulator  206 . An integrated photodiode  907  is also provided for monitoring output optical power, but not used in the embodiment shown. The TEC  804  has electrodes  924 , and the photodiodes  216  have electrodes  920 . Temperature of the TEC plate  804  is monitored by a thermistor  909  having electrodes  922 . 
         [0048]    The control unit  210  of the tunable transmitter  200  is preferably microcontroller based, with the feedback loop controller  308  implemented in software or firmware. Turning to  FIG. 10 , an electrical circuit  1000  of a driver for driving the optical subassembly  800  is shown. A microcontroller  1002  is suitably programmed to control the MZ modulator  206  according to the calibration method  400  and the operation methods  500  and  700  described above. A modulator driver integrated circuit  1004  coupled to the micro-controller  1002  is used to provide the modulator control signals  224  to the left and right arm electrodes  916  and  918 , respectively, of the MZ modulator  206 , in counter-phase to each other. DC voltage generators  1006  are controlled by the microcontroller to provide the bias voltages Vb to the left and right arm electrodes  916  and  918 , respectively, of the MZ modulator  206 . 
         [0049]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.