Abstract:
An automatic bias controller for an optical modulator is provided. The automatic bias controller comprises a driver for providing an electrical data signal to the modulator and a bias voltage source for providing a bias voltage to the modulator. A microprocessor provides a low frequency digital modulation signal, which is converted to an analogue modulation signal by a digital to analogue converter. The analogue modulation signal is applied to the bias voltage source (so as to modulate the bias voltage) or to the driver (so as to modulate the amplitude of the data signal). Intensity detectors for detecting the intensity of light emitted by the modulator are provided, and an analogue to digital converter converts the output of the intensity detectors to a digital intensity signal which is passed to the microprocessor. The digital intensity signal is analysed, and the bias voltage source instructed to adjust the bias voltage on the basis of the analysed signal. The digital modulation signal is preferably a rectangular wave or time multiplexed series of periods in which the bias voltage and data signal amplitude are varied.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an automatic bias controller for an optical modulator for use in optical data communications, and more specifically, although not exclusively, to an automatic bias controller for an electro-optic modulator such as a Mach-Zehnder modulator. The invention also relates to an optical transmitter including a modulator with automatic bias control. 
   BACKGROUND OF THE INVENTION 
   Transmission of data using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is amplitude modulated with a data signal, and propagates to a remote receiver via a system of optical fibres and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electro-optic modulator such as a Mach-Zehnder modulator or optical coupler. 
   Mach-Zehnder electro-optic modulators are well known. An electro-optic modulator based on a Mach-Zehnder interferometer generally includes a monolithic substrate formed of an electro-optic material such as LiNbO 3  or InP. An optical waveguide is formed in the substrate having two arms or branches which extend generally in parallel with each other. The effective index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate. 
   In the absence of an applied electrical bias voltage, an input optical or light beam produced by a laser or the like applied to the waveguide divides between the branches. The optical signals propagating through the branches recombine at the optical output of the waveguide. If the optical path lengths of the branches are equal, or differ by an integral number of wavelengths, then the optical signals recombine in phase with each other, such that their amplitudes are additive and an optical output signal which is essentially similar to the optical input signal appears at the output of the waveguide. 
   If the optical path lengths of the branches differ by a half integral number of wavelengths, the optical signals emerging from the branches are 180° out of phase with each other. The amplitudes of the signals combine subtractively, cancelling each other out, such that a zero output is produced at the optical output. 
   Application of a predetermined electrical bias voltage differential to one branch of the waveguide relative to the other branch causes the complex indices of refraction of the material in the branches to vary differently due to the electro-optic effect, such that the effective optical lengths (and absorption) of the branches vary accordingly. At a bias voltage known in the art as V π , the effective optical lengths have varied to such an extent that the optical signals emerging from the branches are 180° out of phase compared to the situation when no bias voltage is applied. If the path lengths of an unbiased modulator are the same, then at a bias voltage of V π  the optical signals will interfere destructively and cancel each other out, such that a zero output is produced at the optical output. If the path lengths of an unbiased modulator differ by a half integral number of wavelengths, then at a bias voltage of V π  the optical signals will interfere constructively so that the optical output signal is essentially similar to the optical input signal. An electrical data signal, normally in the microwave frequency range, is applied to one or both of the arms. The optical carrier signal exiting the device is thus modulated by the data signal 
   For most optical communication applications, it is desirable to bias the modulator at a voltage V π /2. However, device instabilities and environmental effects, especially temperature variations, cause the operating point to drift over time, and constant readjustment is required to maintain the proper operating point. The bias point must be maintained during operation to achieve maximum dynamic range, since second order harmonic distortion increases rapidly with increasing bias voltage error. 
   The bias point is generally maintained by providing a low frequency (˜400 Hz) sinusoidal pilot tone to the bias voltage or to the data signal. This causes a further low frequency modulation in the output signal, whose phase and amplitude is determined by the distance from the bias point. The output arm of the modulator includes a tap to a photodetector, enabling the output signal to be monitored. The output signal is compared with the pilot tone using a phase sensitive detector, and this enables the use of a feedback loop to maintain the bias point. Examples of modulators employing such a system are described in U.S. Pat. No. 5,003,624 (in which the pilot tone is applied to the bias voltage) and U.S. Pat. No. 5,170,274 (in which the pilot tone is applied to the data signal). 
   A known electro-optic modulator system including an automatic bias adjustment is illustrated in  FIG. 1 , and generally designated  10 . A laser  12  feeds a coherent light beam through an optical fibre  14  into an optical input  16  of a Mach-Zehnder modulator  18 , optical coupler, or other appropriate electro-optic modulator. The light beam propagates through a waveguide having two branches  20  and  22 , which recombine at an optical output  24  of the modulator  18 . An electrical data signal, preferably in the microwave frequency range, is applied to the branches  20 ,  22  via a driver  25  and modulation signal “T” inputs  26 ,  27 . A bias voltage is also applied to the branches  20 ,  22  via the “T” inputs  26 ,  27 . The optical carrier signal constituted by the laser beam is modulated with the data signal, and fed through an optical fibre  28  to a remote receiver (not shown). An amplitude modulation (AM) pilot tone source  29  is applied to the driver of the data signal. 
   A tap  30  is provided in the output optical fibre  28  which leads through an optical fibre pigtail  32  to a photodetector  34 . The output of the photodetector  34  and the pilot tone modulating signal  29  are applied to a phase sensitive detector  36 , which compares the low frequency modulation of the output signal with the pilot tone  29 . Depending on the phase and amplitude of the modulation on the output signal, the bias voltage of the lower arm  20  of the modulator is adjusted to maintain the bias point. 
   This system works well but the tapping of some of the output signal to a photodetector represents optical loss. The overall transmitted power is therefore reduced. Furthermore, the generation of a sinusoidal pilot tone requires hardware which occupies valuable space near the modulator. The feedback loop also requires a phase sensitive detector and DC coupled amplifiers which are expensive and again occupy valuable space. 
   SUMMARY OF THE INVENTION 
   There is therefore a need for an automatic bias controller for an optical modulator which overcomes or at least mitigates the above problems. 
   In accordance with a first aspect of the present invention there is provided an automatic bias controller for an optical modulator, comprising:
         a driver for providing an electrical data signal to the modulator;   bias means for providing a bias voltage to the modulator;   processing means for providing a low frequency digital modulation signal;   a digital to analogue converter for converting the digital modulation signal to an analogue modulation signal and providing the analogue modulation signal to the bias means so as to modulate the bias voltage or to the driver so as to modulate the amplitude of the data signal;   intensity detection means for detecting the intensity of light emitted by the modulator; and   an analogue to digital converter arranged to convert the output of the intensity detection means to a digital intensity signal and provide the digital intensity signal to the processing means;   wherein the processing means is arranged to analyse the digital intensity signal and instruct the bias means to adjust the bias voltage on the basis of the analysed signal.       

   Thus there is no need for a costly and bulky tone generator or phase sensitive detector. The processing means (e.g. a microprocessor) can provide a simple digital signal, and analogue to digital converters and digital to analogue converters are usually present on a modulator board in any event. Thus space utilisation is improved. 
   The digital modulation signal is preferably a rectangular signal, enabling a simple comparison between the amplitude and phase of the digital intensity signal and the amplitude and phase of the digital modulation signal. A finite difference algorithm may be performed on the digital intensity signal so as to determine the approximate first or second derivative of the output power of the modulator with respect to bias voltage. 
   The digital modulation signal preferably comprises a series of discrete time periods, arranged such that during each period an increase or decrease in the amplitude of the data signal or the bias voltage is effected. The signal may be multiplexed so that the effect of the signal on the amplitude of the data signal or bias voltage can be determined from the behaviour of the corresponding period of the digital intensity signal. For example, the signal may change the bias voltage, then return the bias voltage to its original value and change the amplitude of the data signal. 
   The amplitude of the data signal is preferably controlled by the digital modulation signal to produce high amplitude periods of increased amplitude alternating with low amplitude periods of decreased amplitude. An error in the bias of the modulator may then be determined by computing the difference between the integrated intensity of the digital intensity signal during a high amplitude period and the integrated intensity of the digital intensity signal during a low amplitude period. 
   Similarly, the bias voltage is preferably controlled by the digital modulation signal to produce high bias periods of increased bias voltage alternating with low bias periods of low bias voltage. An error in the amplitude of the data signal may then be determined by computing the difference between the integrated intensity of the digital intensity signal during a high bias period and the integrated intensity of the digital intensity signal during a low bias period. 
   Preferably the intensity of light entering the modulator is controlled with a variable optical attenuator. By integrating the digital intensity signal over time, (i.e. determining the sum of the intensity signal over high/low amplitude and high/low bias periods rather than the difference between periods) the error in the power input to the modulator may be determined, and the variable optical attenuator may then be adjusted on the basis of the determined power input error. 
   Preferably the optical modulator is a Mach-Zehnder modulator having two branches. The output of the modulator preferably comprises a sum arm and a difference arm, the sum arm transmitting light having a waveform determined by the sum of the waveforms in the two branches, and the difference arm transmitting light having a waveform determined by the difference between the waveforms in the two branches. The modulated optical light is preferably transmitted in the sum arm. 
   The intensity of light in the difference arm provides a measure of the intensity of the light in the sum arm when integrated over time (i.e. over many bits of the data signal). The frequency of the digital modulation signal (˜10 2  Hz) is much lower than the frequency of the data signal (˜10 9  Hz), so over the course of a single period of the digital modulation signal there are millions of bits transmitted. The detection of the intensity of light emitted by the modulator may therefore be achieved by measuring the intensity of light in the difference arm. This means that there is no need to tap the sum arm carrying the modulated light, and thus no loss in the transmitted light. 
   Preferably the output of the modulator comprises a multimode interferometer which causes the sum waveform to be transmitted in the sum arm and the difference waveform in the difference arm. 
   In accordance with a second aspect of the present invention there is provided a method for controlling the bias of an optical modulator, comprising:
         providing an electrical data signal to the modulator;   providing a bias voltage to the modulator;   providing a low frequency digital modulation signal from a processing means;   converting the digital modulation signal to an analogue modulation signal;   providing the analogue modulation signal to the bias means so as to modulate the bias voltage or to the driver so as to modulate the amplitude of the data signal;   detecting the intensity of light emitted by the modulator;   converting the detected intensity to a digital intensity signal;   providing the digital intensity signal to the processing means;   analysing the digital intensity signal at the processing means; and   adjusting the bias voltage on the basis of the analysed signal.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
       FIG. 1  is a simplified block diagram illustrating a prior art electro-optic modulator having automatic bias adjustment; 
       FIG. 2  is a graph illustrating the transfer function of a Mach-Zehnder modulator, with the optical output plotted as a function of applied bias voltage; 
       FIG. 3  is a simplified block diagram illustrating a modulator including an automatic bias control embodying the present invention; and 
       FIG. 4  is a graph illustrating the linear modulation bias point of a Mach-Zehnder modulator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2  is a graph showing how the optical power output of a Mach-Zehnder modulator depends on the relative bias voltage between the arms. It is clear from the figure that, as the bias voltage is increased, the optical power increases and decreases in a periodic manner. The sine curve shown in  FIG. 2  has a plurality of peaks  54  and a plurality of troughs  56 . Bias points  58  are constituted by positive inflection points in the rising portions of the curve between adjacent troughs  56  and peaks  54 , whereas bias points  60  are similarly constituted by negative inflection points between adjacent peaks  54  and troughs  56 . The bias points  58 ,  60  have voltage values approximately half way between those of the peaks  54  and troughs  56 , and are the points V π /2 referred to above. 
     FIG. 3  is a simplified block diagram of a modulator system  62  in accordance with the present invention. Components similar to those of  FIG. 1  are represented by the same reference numerals. A laser  12  feeds a coherent light beam through a variable optical attenuator (VOA)  15  into an optical input  16  of a Mach-Zehnder modulator  18  having two branches  20 ,  22 . The branches  20 ,  22  recombine at a 2×2 multimode interferometer (MMI)  64  having a “sum” output arm  66  and “difference” output arm  68 . An electrical data signal is applied to the branches  20 ,  22  via a driver  25  and modulation signal “T” inputs  26 ,  27 . A bias voltage is also applied to the branches  20 ,  22  via the “T” inputs  26 ,  27   
   The waveform of light exiting the MMI  64  via the “sum” arm  66  is the sum of the waveforms of light passing through the two arms  20 ,  22  of the modulator  18 . The sum arm  66  forms the output of the modulator, through which the modulated light is transmitted. The waveform of light exiting the MMI  64  via the “difference” arm  68  is the difference of the waveforms of light passing through the two arms  20 ,  22 . When the optical path difference between the two arms  20 ,  22  is an integral number of wavelengths (a phase difference of 2nπ), the light interferes constructively in the sum arm  66  (known as the “ON” state of the modulator), and no light passes through the difference arm  68 . However, when the phase of the arms differs by (2n+1)π where n is an integer, the light interferes destructively in the sum arm  66  (the “OFF” state) so that the intensity in the sum arm is zero, and light exits the modulator through the difference arm  68 . 
   Since the modulator switches between the “ON” and “OFF” states at a very high frequency when data is being transmitted, the integrated light intensity in the difference arm over many bits can be taken to be proportional to the integrated light intensity in the sum arm over the same time when the modulator is biased at V π /2. A photodetector  70  is provided at the output of the difference arm. The output of the photodetector  70  is passed to an analogue to digital converter (ADC)  72  to generate a digital signal, which is received by a microprocessor  74 . This signal can then be used to monitor the output of the modulator without the need to tap light from the sum arm  66 . 
   With a symmetrical modulation characteristic the mean output power (transmitted through the sum arm  66 ) is constant independent of small amplitude variations in the microwave frequency modulation voltage if the device is biased at its point  60  of maximum modulus of 1st derivative, as shown in  FIG. 4 . At this point, the second derivative of the power output with respect to voltage bias should be zero. If biased away from this ideal then the mean output is intensity modulated by the applied signal with its phase reversing as the bias passes through the ideal point. This is therefore suitable for bias error detection schemes. 
   This can be expressed mathematically using a Taylor expansion of the modulation characteristic as: 
             I   ⁡     (   v   )       =       f   ⁡     (   vbias   )       +       vf   ′     ⁡     (   vbias   )       +         v   2       2   !       ⁢       f   ″     ⁡     (   vbias   )         +     higher   ⁢           ⁢   terms             
where I is the current measured by the photodetector  70  and representative of the output signal, v the coupled modulation voltage and f(vbias) the nonlinear transfer function of the modulator.
 
   The microprocessor  74  provides a digital signal which is passed through a digital to analogue converter (DAC) to the driver  25 . This signal causes the modulation depth of the data signal to be increased and decreased at a low rate (˜100−500 Hz) by an amount m. This change in modulation depth is provided as a simple step function resulting in a low frequency square wave. Thus
 
 v= (1± m ) g ( t )
 
where g(t) is the modulation signal. This means that there are two discrete values of I corresponding to the two discrete values of v, and
 
&lt; I   +m   &gt;−&lt;I   31 m &gt;=2 mƒ″&lt;g   2 ( t )&gt;+higher terms
 
where &lt;I +m &gt; is the current from the photodetector  70  when the modulation depth is (1+m) and &lt;I −m &gt; is the current when the modulation depth is (1−m).
 
   This means that the second derivative of the modulation characteristic (and thus the bias error) can be determined by monitoring the amplitude and sign of the ripple in the photodetector current. The digital signal received at the microprocessor from the photodetector  70  via the ADC  72  is either exactly in phase or exactly out of phase with the square wave amplitude modulation applied to the data signal. There is no need for a phase sensitive detector, since the square wave applied to the data signal is generated by the same microprocessor  74 . 
   The microprocessor  74  acts as a digital integrator to determine the sign and magnitude of the second derivative, and produces a digital signal to correct for this. This signal is passed through the DAC  76  to the driver  25  to enable the bias of the arms  20 ,  22  to be corrected. The linear bias can be maintained either at a positive or negative chirp configuration (points  58 ,  60  in  FIG. 2 ) simply by changing the sign in the gain feedback loop. 
   In addition to determining the error in the biasing point, the modulator system  62  can be used to ensure a constant mean output power by the provision of a variable optical attenuator (VOA)  15  located between the laser  12  and modulator  18 . In addition to determining the difference &lt;I +m &gt;−&lt;I −m &gt; between the outputs in the photodetector current, the microprocessor determines the sum &lt;I +m &gt;+&lt;I −m &gt;, which provides a measure of the mean output power of the modulator. In order to keep this at a constant value a compensating signal is emitted from the microprocessor, via the DAC  76 , to the VOA  15 . 
   The system may also be modified to ensure that the amplitude of the modulation is correct, to ensure that the “ON” and “OFF” states (“1” and “0” bits) correspond to constructive and destructive interference in the sum arm  66 —i.e. the peaks  54  and troughs  56  shown in  FIG. 2 . This is achieved by increasing and decreasing the bias voltage slightly by an amount δV using a signal from the microprocessor  74  transmitted via the DAC  76  and the “T” inputs  26 ,  27  to the arms  20 ,  22 . The difference &lt;I +δV &gt;−&lt;I −δV &gt; between the photodetector currents corresponding to an increase and decrease in the bias voltage provides a measure of the modulation depth. 
   Because the signals to the driver  25  and bias arms  20 ,  22  to vary the modulation depth and bias voltage, respectively, are controlled by the microprocessor  74  and passed through the DAC  76 , it is simple to ensure that they are multiplexed so that variations in the photodetector current may easily be assigned to the correct signal. In a suitable scheme the microprocessor applies initial voltages to the modulator arms, the driver gain input and the VOA input. It then applies incremental steps to the gain set and bias of one of the arms in the following sequence: 
   
     
       
             
             
             
             
           
         
             
                 
             
             
                 
                 
                 
               Photodetector 
             
             
               Step number 
               Vbias 
               Vmod 
               Current 
             
             
                 
             
           
           
             
               1 
               Nominal 
               Nominal + m 
               I +m   
             
             
               2 
               Nominal 
               Nominal − m 
               I −m   
             
             
               3 
               Nominal + δV 
               Nominal 
               I +δV   
             
             
               4 
               Nominal − δV 
               Nominal 
               I −δV   
             
             
                 
             
           
        
       
     
   
   As explained above the difference &lt;I +m &gt;−&lt;I −m &gt; is a direct measure of the error in the biasing point and indicates the direction in which the bias voltage should be incremented in order to operate at the point of zero second derivative of the power output-voltage characteristic. The difference &lt;I +δV &gt;−&lt;I −δV &gt; is a direct measure of the error in the modulation swing and indicates the direction the Vmod voltage should be incremented to place the “1”s and “0”s at the peaks and troughs of the power-voltage characteristic. The sum photodetector current I S =&lt;I +m &gt;+&lt;I −m &gt;+&lt;I +δV &gt;+&lt;I −δV &gt; is a direct measure of the output power and indicates the direction the VOA voltage should be incremented to maintain a constant mean output power. 
   The embodiment described above applies to a modulator whose “ON” and “OFF” states (i.e. “1” and “0” bits) correspond to constructive and destructive interference in the sum arm  66 . Referring back to  FIG. 2 , a “1” is transmitted at a peak  54  and a “0” at a trough  56 . The modulator is maintained at a bias point corresponding to a point of inflection  58  or  60  between the peaks and troughs. However, it is possible to operate the modulator so that “1” and “0” bits correspond to constructive interference in the sum arm  66  but with opposite phases—i.e. to adjacent peaks  54 . In such a situation the bias should be maintained at a trough  56  so that the modulation moves the output between peaks  54 . 
   In a further embodiment, it is also possible to deliver a “duobinary” signal, in which the data signal is a three level waveform having “1”, “0” and “−1” levels, in which the adjacent peaks  54  correspond to “1” and “−1” bits and the trough  56  between them corresponds to a “0” bit. In this embodiment the bias should again be maintained at a trough  56 . 
   Since the power output is at a trough rather than a point of inflection at the point at which the bias needs to be maintained, a feedback loop based on the second derivative is not appropriate for these latter two embodiments. To maintain the bias at the trough  56  the bias should be varied by δV as described above. Then &lt;I +δV &gt;−&lt;I −δV &gt; provides a measure of the error in the biasing point. 
   Thus the invention, at least in its preferred embodiments, provides a number of advantages. The use of a “difference” output of the modulator allows optical power to be supplied for detection without reducing the power available for transmission. 
   The use of rectangular modulation (provided by a microprocessor and DAC) on the bias and/or data signals removes the need for tone generating hardware, reducing the number of components required. 
   The use of finite difference algorithms, in firmware running on a microprocessor to compute approximate derivatives of the power-voltage characteristic from the rectangular modulation avoids the need for tone based phase sensitive detectors or analogue multipliers, further reducing the number of components required. 
   The use of time division multiplexing of the rectangular bias and data modulation removes the need for additional hardware or orthogonal signals to separate control loops. 
   The use of a VOA to maintain the mean output power constant compensates for variation in optical input coupling during operation of the modulator. 
   The use of adjustable loop gain polarity achieves chirp configurability without the need for additional hardware. 
   It will be appreciated that variations from the above embodiments may still fall within the scope of the invention. For example, the digital control circuit shown in  FIG. 3  detects light intensity using the difference arm of a four port modulator, but the circuit will work equally well if light is tapped from the output arm of a three port modulator. 
   In addition, the embodiments are described as using a single microprocessor, ADC and DAC. It will be appreciated that more than one of each of these components may be used (for example, to perform different functions) if necessary.