Abstract:
The present invention relates to an optical transmitter that includes an optical modulator configured to modulate an optical signal with a digital data stream, and a heater configured to apply heat to the optical modulator. The optical transmitter also includes an optical receiver configured to receive the modulated optical signal and to convert the modulated optical signal into a received digital data stream. A circuit is configured to compute bit errors in the received digital data stream by comparing the received digital data stream with the digital data stream, and control the heater based on the computed bit errors.

Description:
FIELD OF THE INVENTION 
     The present invention relates, in general, to an optical transmitter that modulates an optical signal based on a digital data stream. A heater is used to apply heat to an optical modulator in the transmitter. 
     BACKGROUND OF THE INVENTION 
     Previously implemented Silicon Photonic optical transmitters include an optical ring resonator modulator that modulates an incoming optical signal. The index of refraction within the ring changes with operating temperature which undesirably shifts the resonance. The index of refraction may also vary as a function of fabrication tolerances (e.g. dimensions of the ring). Some previous systems have implemented an integrated micro-heater that is controlled based on temperature readings from an integrated temperature sensor. In general, the micro-heater applies heat to the ring in an attempt to compensate for undesirable changes in the index of refraction based on the sensed temperature. However, these systems are limited due to the changing characteristics of the temperature sensor due to aging and other effects that degrade the bit error rate (BER) but do not affect the temperature. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an optical transmitter that includes an optical modulator configured to modulate an optical signal with a digital data stream, and a heater configured to apply heat to the optical modulator. The optical transmitter also includes an optical receiver configured to receive the modulated optical signal and to convert the modulated optical signal into a received digital data stream. A circuit is configured to compute bit errors in the received digital data stream by comparing the received digital data stream with the digital data stream, and control the heater based on the computed bit errors. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of an optical transmitter and an optical receiver, according to an embodiment of the present invention. 
         FIG. 2A  is schematic plan view of an optical ring modulator, according to an embodiment of the present invention. 
         FIG. 2B  is a schematic cross-section view of the optical ring modulator shown in  FIG. 2A  along section line  2 - 2 , according to an embodiment of the present invention. 
         FIG. 3A  is a plot of the optical ring modulator frequency response for the thru port, according to an embodiment of the present invention. 
         FIG. 3B  is a plot of the optical ring modulator frequency response for the drop port, according to an embodiment of the present invention. 
         FIG. 4  is a plot of the logic 0 and logic 1 bit error rate (BER) computed by the optical transmitter, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As will be described, the present invention provides a system and method for actively controlling the resonance wavelength of a resonant optical device. In one example, an optical transmitter may include a resonant optical device such as an optical ring resonator having multiple light ports (e.g. a thru port and a drop port). A continuous wave (CW) light signal such as a laser beam may be input to the optical ring resonator where it is digitally modulated when voltage applied to the ring. 
     In one embodiment, the optical transmitter includes an optical receiver for converting the modulated optical signal into an electrical signal. The optical transmitter includes circuitry that may include dedicated analog/digital circuits and/or a processor that performs error detection and outputs a control signal to a heater for heating the optical modulator based on the detected errors. 
     Shown in  FIG. 1  is a block diagram of an optical transmitter  100  that is transmitting an optical signal (e.g. a modulated laser beam) over optical link  116  to receiver  150 . It should be noted that receiver  150  may be on the same chip or may be off chip from optical transmitter  100 . 
     In this embodiment, optical transmitter  100  may include an optical signal (e.g. a laser beam) that is generated by a laser device (not shown) and provides the optical signal to optical modulator  101  where it is modulated (e.g. digitally to transmit logic 1&#39;s and 0&#39;s). In one example, optical modulator  101  may be an optical ring resonator that includes a thru port  103 , drop port  104  and a heater  34  (not shown). 
     Optical transmitter  100  also includes optical receiver  110 , digital logic decision circuit  111 , delay circuit  112 , error detector  113 , error lines  114  and  115 , transmission data line  108 , amplifier  107 , controller  109 , modulation control line  105 , and heater control line  106 . 
     During operation, laser beam  102  is optically modulated (e.g. digitally) by optical modulator  101 . In general, digital data  108  is amplified (at amplifier  107 ) and then a voltage is applied (via control line  105 ) to optical ring modulator  101 . By applying a voltage to the optical ring modulator, the light intensities through ports  103  and  104  may be controlled (e.g. complementary to each other). Thus, as the laser light intensity passing through port  103  is increased, the laser light intensity passing through port  104  is decreased (and vice versa). 
     The modulation may be a result of the index of refraction of the modulator changing as a function of the changing carrier concentration in the device (i.e. the applied voltage to the modulator changes the index of refraction). It is noted that controlling the temperature of the modulator based on bit errors is not limited to a specific modulation method (i.e. the bit error based control method is applicable to a variety of modulation methods that may be used in conjunction with a variety of resonant modulators). 
     In general, the modulated laser beam being transmitted over thru port  103  is transmitted over optical link  116  and received at receiver  150 . The modulated optical signal is then demodulated by optical receiver  117  and a decision is made on whether it is a transmitted logic 1 or a logic 0 at detector  118 . The digital bits are then output over line  119  to another circuit (not shown). It is also contemplated that detector  118  may include error correction to correct transmission errors. 
     As described above, the operating temperature of the optical micro-ring modulator shifts the resonant wavelength. In operation, in order to compensate for this shift, optical transmitter  100  detects transmission errors in the modulated optical signal over drop port  104  and then applies appropriate control voltage to the heater (i.e., to either heat up or cool down modulator  101 ). 
     For example, modulated signal in drop port  104  is converted by receiver  110  into an electrical signal. Circuit  111  then determines if the transmitted signal is logic 1 or a logic 0. Error detection circuit  113  then compares the received digital data stream with a delay compensated version of the original transmission data  108 , to determine if the transmitted bits and received bits are the same. Error detection circuit  113  is then able to determine if a logic 0 error or a logic 1 error has occurred during optical transmission. The logic 1 errors are transmitted to controller  109  via line  114  whereas the logic 0 errors are transmitted over line  115  to controller  109 . Controller  109  then applies a voltage (over control line  106 ) to the optical modulator heater (not shown). 
     Logic 1 errors absent logic 0 errors typically mean that the operating temperature is too cold, whereas a mix of logic 1 and logic 0 errors typically means that the operating temperature is too hot (see  FIG. 4 ). In one example, for every logic 0 error, a negative electrical pulse may be generated and applied to a proportional integral derivative (PID) controller (not shown) included in controller  109 . For every logic 1 error, a positive electrical pulse may be generated and applied to the PID controller. In response to receiving the negative pulses and positive pulses, the PID controller either decreases or increases a voltage applied to the resistive heater (i.e. decreases or increases the heat applied to the ring). 
     Thus, in this example, when a logic 0 error occurs, the PID controller (in response to receiving a negative pulse) decreases the voltage applied to the heater to decrease the temperature of optical resonator  101  (i.e. cool it down). In contrast, when a logic 1 error occurs, the PID controller (in response to receiving a positive pulse) increases the voltage applied to the heater to increase the temperature of optical resonator  101  (i.e. heat it up). 
     In one example, the amplitude of the negative electrical pulses is set to be larger than the amplitude of the positive electrical pulses in order to counteract a scenario where both logic 1 and logic 0 errors occur at similar rates. This example will be further described with respect to  FIG. 4 . 
       FIG. 2A  shows the details of an example optical modulator  101 . Specifically, an optical waveguide ring  14  is supported on a substrate  12  with a pair of optical waveguides  16  evanescently coupled to optical waveguide ring  14  in a location near minimum width  18 . In this embodiment, the optical waveguide ring  14  has a width that is adiabatically increasing to a maximum width  20 . 
     An electrical heater  34  is also located in optical waveguide ring  14  proximate to the location of maximum width  20 . In one embodiment, electrical heater  34  may comprise an impurity doped region when the optical waveguide ring  14  is formed from monocrystalline silicon. The impurity doped region may be a region in the monocrystalline silicon which has been doped with specific doping concentration (e.g. with an impurity such as boron, phosphorus or arsenic). This would make the impurity doped region electrically resistive. When a voltage is applied over opposite ends of electrical heater  34 , the impurity doped region begins to heat ring  14 . In general this changes the index of refraction (i.e. the wavelength) via a thermo-optic effect so that the effective optical path for light  100 ′ circulating around optical waveguide ring  14  is increased (see i.e. this changes the resonant frequency of light  100 ′ in ring  14 ). 
     In general, electrical power may be supplied to heater  34  through a pair of connecting members  22  which act as electrical contacts. Wiring  40 , which connects to contracts  22  may be connected to heater control line  106  that is connected to controller  109  shown in  FIG. 1  (e.g. controller  109  applies the electrical current to the heater through control line  106 ). 
     Thus, laser beam  102  enters waveguide  16  at port  24  and travels to thru port  103 . The laser beam  102  is also optically coupled through the ring modulator to output through drop port  104  (i.e. the ring optically couples to waveguides  16  when a modulating voltage is applied). 
     For example, if a modulating voltage is not applied to ring  14 , then laser beam  100  will pass directly through waveguide  16  and exit port  103  (i.e., logic 1 will be transmitted over the thru port). If a voltage is applied to ring modulator  14 , the intensity of the laser beam will be redirected through the ring and into waveguide  16  and exit through drop port  104  (i.e., the logic 1 signal will be output through the drop port). Thus, in this embodiment, the through port and the drop port  26  have a complementary relationship. 
     As shown in  FIG. 2B , optical ring  14  and waveguide  16  may be supported above substrate  12  on layer  36 . Layer  36  may include the various elements  14 ,  16 ,  22 , and  34  which are formed on a monocrystalline silicon layer of the substrate. Second layer  38  may be deposited over optical ring  14  and optical waveguide  16 . This may be useful for encapsulating elements  14 ,  16 ,  22  and  34  and to provide support for wiring  40 . 
     As described in  FIG. 2A , when voltage is applied to optical ring  14 , laser beam  102  passing through waveguide  16  is either modulated to pass over thru port  103  or drop port  104  (i.e., the intensity of the light is amplitude modulated based on the voltage applied to the ring). This amplitude modulation is shown in  FIG. 3A  for the thru port  103 . 
     It is noted that although an optical ring is described above as the modulator in this example, that the micro-heater may also be incorporated into other micro-resonant devices of different configurations (e.g. a micro-disk modulator). Controlling the temperature of these other micro-resonant devices based on bit errors would be similar to the ring modulator. 
     Curve  302 (A) and curve  304 (A) are the optical frequency responses of the modulator passing through the thru port  103  when 3.5 volts (e.g. a logic 1) and 0 volts (e.g. a logic 0) respectively are applied to the modulator. Curves  302 (A) and  304 (A) may be shifted in frequency from their desired characteristics due to operating temperature and/or fabrication errors in the optical modulator. In this example, with the input laser wavelength at the frequency shift threshold of 0 GHz, a logic 1 and a logic 0 transmission have similar amplitudes  312  and  310  that may be difficult to distinguish at the receiver (i.e. the amplitudes between the different modulated signals are too close to one another). 
     In order to correct this undesired frequency shift of the modulator characteristics, in one embodiment, the heater applies heat to shift the frequency response of the modulator closer to its desired frequency (i.e. a frequency shift of 0 GHz). The shifted frequency response is shown by curves  302 (B) and  304 (B) when 3.5 volts (e.g. a logic 1) and 0 volts (e.g. a logic 0) are applied respectively to the modulator. When the modulator is operating at the proper temperature, the points where the two curves cross the 0 GHz frequency shift threshold  306  and  308  are sufficiently different from one another (i.e., it is easier to distinguish between the intensity of a logic 1 and a logic 0). 
     A similar scenario is shown in  FIG. 3B  for the drop port. Curves  314 (A) and  316 (A) (where 3.5 volts and 0 volts are applied to the modulator respectively), cross the frequency shift threshold of 0 GHz at points  322  and  324  which are hard to distinguish from each other (e.g. logic 0 and logic 1 transmissions have similar light intensities). As described above, this is due to shifts in the resonant wavelength of the modulator caused by operating temperature and/or fabrication tolerances of the modulator. In the example system, once the heater applies the appropriate amount of heat to the modulator based on the bit error rate, the curves shift to become curves  314 (B) and  316 (B) which cross the threshold at points  320  and  318  respectively (e.g. logic 0 and logic 1 transmissions have distinguishable light intensities). 
     In one example, the behavior of logic 0 errors and logic 1 errors are shown in  FIG. 4  with respect to the operating temperature of optical modulator  101 . At temperature shifts less than 0° C., the bit errors are primarily logic 1 errors as shown by graph  402 . However, in this example, between temperature shifts of 0° C. and approximately 4° C., there is a mixture of logic 0 errors and logic 1 errors as shown by the overlapping of curves  402  and  404 . Temperature shifts greater than 4° C. show that the errors are once again primarily logic 1 errors. Thus, in region  406 , the errors are all logic 1 errors, in region  408 , the logic errors are a mixture of logic 1 and 0 errors, and in region  410  the errors are all logic 1 errors. 
     Thus, when controller  109  receives all logic 1 errors (assuming the modulator is operating in region  406 ) a positive amplitude electrical pulse is applied to the PID controller which increases the voltage applied to the heater thereby increasing the temperature of the modulator in an attempt to shift the operating temperature closer to its target value (i.e. obtain a temperature shift close to 0° C.) in order to reduce the errors. 
     If, however, the system is operating in region  408  (i.e., there is a mixture of logic 0 and logic 1 errors), both positive and negative electrical pulses are applied to the PID controller in response to receiving both logic 1 and logic 0 errors. Since there is a mixture of both logic 1 and logic 0 errors, the negative electric pulse that decreases the temperature of the modulator may have a higher amplitude than the positive electrical pulses, otherwise the system may get stuck in region  408 . For example, if both positive and negative pulses have the same unit amplitude, then for every logic 1 error, the temperature would increase and for every logic 0 error, the temperature would decrease, and therefore the temperature would get stuck in a region where the rate of logic 0 and logic 1 errors are similar. 
     By increasing the amplitude of the negative electrical pulse to be higher than the positive electrical pulse (e.g., positive electrical pulse may be 1 unit amplitude, whereas the negative electrical pulse may be 3 unit amplitudes), the PID controller will decrease the voltage applied to the heater for a logic 0 error more significantly than it will increase the voltage applied to the heater for a logic 1 error. Therefore, the system will not get stuck in region  408  where the logic 0 and logic 1 errors are equivalent. 
     It should also be noted that in  FIG. 4 , if all logic 1 errors are detected, the modulator may be operating in either region  406  or  410 . A distinction between these two regions may be made in order to determine whether a negative or a positive electrical pulse should be applied to the heater. This distinction may be made based on the temperature of the modulator (i.e., a temperature-sensing device may be integrated into modulator  101 ) or by knowing that when the device initially starts up, the temperature is colder than it should be, and is initially operating in region  406 . 
     In one example, receiver  110  may have higher noise characteristics than receiver  117  in order to generate errors that are used in the correction algorithm. This can be accomplished by designing a receiver with lower transimpedance than would otherwise be required. Other ways to produce more errors may include measuring error rates at multiple signal to noise ratio thresholds. 
     Optical receiver  117  may be off chip or on chip. Receiver  117  in general, may have a lower noise floor and perform essentially error-free when a modulator wavelength is optimized using the bit error rate corrections as described above. In other embodiments, both may be routed from the same port with a splitter routing part of the signal to each receiver. In another embodiment, bit error correction may be utilized at receiver  117 . 
     In another embodiment, optical modulator  101  may include a cooling device such as a fan (not shown) or a thermoelectric cooler (not shown) that is also controlled by controller  109  to cool down the ring modulator more rapidly. The cooling device may be external to the modulator or may be micro-cooler that is integrated into the modulator structure similar to the heater. 
     In one example, when controller  109  wants to lower the temperature, the amplitude of the electrical signal applied to the heater may be reduced while the amplitude of the electrical signal applied to the cooler may be increased, therefore cooling down the modulator more rapidly. It should also be noted that a heat sink (not shown) may be coupled to the ring modulator through a thermal resistance (not shown) as a passive cooling device which radiates the heat away from the ring modulator more rapidly. 
     It should be noted that the electrical devices (e.g.  111 ,  112 ,  113 ,  107  and  109 ) within optical transmitter  100  may be implemented as dedicated hardware circuits (e.g. analog and/or digital circuits) that may include a field programmable gate array (FPGA) and/or a processor for implementing the method in software. 
     It should also be noted that other types of optical modulators having different geometries and different numbers of ports may also be controlled utilizing the error rate control algorithm described above. These include, without limitation, an electro-absorption modulator (EAM) and a Mach-Zehnder modulator. As described above, if the modulator does not have complementary outputs, a beam splitter may be used on the output port to route the modulated laser beam to the respective optical receivers  110  and  117 , shown in  FIG. 1 . 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.