Abstract:
A control circuit includes a power controller for adjusting a bias current to a laser diode to change the power output of the laser diode, the change in power having a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode and a monitoring circuit for sensing the bias current to the laser diode and for generating an output signal in response to the sensed bias current. The control circuit further includes a wavelength controller which receives the output signal from the monitoring circuit and in response to the output signal compensates for the wavelength shift such that the laser diode maintains operation at the nominal wavelength.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part and claims the benefit of priority under 35 USC § 120 of U.S. application Ser. No. 09/877,921, filed Jun. 7, 2001. The disclosure of the prior application is considered part of and is incorporated by reference in the disclosure of this application. 
    
    
     BACKGROUND OF THE INVENTION 
     Wavelength division multiplexed systems, in which multiple channels are carried at different wavelengths on the same optical fiber, require adjustable output power to address problems such as optical crosstalk between channels and power balancing of optical signals for optical amplifiers. It is common today to control the output power of a semiconductor laser diode to maintain a constant operational output level, for example, 0 dBm. The constant output power laser diode is used in combination with an optical attenuator to provide the adjustable output power that is needed. The type of optical attenuator can be either fixed or variable attenuation. The fixed attenuation type is neither field adjustable nor remotely controllable. The variable attenuation type is large and expensive and can require additional power sensing circuitry. 
     SUMMARY OF THE INVENTION 
     There is a need for an approach to controlling the output power of laser diodes that is less costly and less bulky than those that require external optical attenuators. There is also a need for a power control mechanism that takes into account the relationship between temperature and wavelength in the operation of laser diodes. 
     An apparatus and method of the present approach provides for electrical control of the laser output power without the need for a costly and bulky optical attenuator. The present approach further provides wavelength control to compensate for the relationship between laser diode operating temperature and wavelength. 
     Accordingly, a control circuit for a laser diode includes a power controller and a wavelength controller. The power controller adjusts a bias current to the laser diode to change the power output of the laser diode. The power change can have a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode. The wavelength controller compensates for the wavelength shift such that the laser diode maintains operation at the nominal wavelength. 
     In an embodiment, the power controller includes a bias current source that provides an adjustable bias current to the laser diode. A power monitor loop includes a backfacet diode for monitoring the laser diode power output to provide a power monitor signal. A power control signal added to the power monitor signal provides a power adjust signal. The bias current source adjusts the bias current responsive to a difference between a power reference voltage input of the bias current source and the power adjust signal. 
     In an embodiment, the wavelength controller includes a temperature control circuit that provides a control current to a thermoelectric element for controlling the temperature operation point of the laser diode. A temperature monitor loop includes a temperature sensor for monitoring the temperature operation point to provide a temperature monitor signal. A wavelength compensation signal added to the temperature monitor signal provides a wavelength control signal. The temperature control circuit adjusts the control current to the thermoelectric element responsive to a difference between a temperature reference signal and the wavelength control signal. 
     The wavelength compensation signal may be proportional to the power control signal. 
     In an alternate embodiment, the wavelength controller includes an etalon element for wavelength compensation. 
     In one aspect of the invention, a control circuit includes a power controller for adjusting a bias current to a laser diode to change the power output of the laser diode, the change in power having a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode and a monitoring circuit for sensing the bias current to the laser diode and for generating an output signal in response to the sensed bias current. The control circuit further includes a wavelength controller which receives the output signal from the monitoring circuit and in response to the output signal compensates for the wavelength shift such that the laser diode maintains operation at the nominal wavelength. 
     Embodiments of this aspect of the invention may include one or more of the following features. The monitoring circuit includes a sensing resistor. The power controller includes a bias current source that provides an adjustable bias current to the laser diode and has a power reference voltage input. The power controller also includes a power monitor loop having a backfacet diode for monitoring the laser diode power output to provide a power monitor signal, and a power control signal added to the power monitor signal to provide a power adjust signal. The bias current, source adjusts the bias current responsive to a difference between the power reference voltage input and the power adjust signal. 
     The wavelength controller includes a temperature control circuit that provides a control current to a thermoelectric element for controlling the temperature operation point of the laser diode and having a temperature reference voltage input and a temperature monitor loop including a temperature sensor for monitoring the temperature operation point to provide a temperature monitor signal a wavelength compensation signal added to the temperature monitor signal to provide a wavelength control signal. The temperature control circuit adjusts the control current to the thermoelectric element responsive to a difference between the temperature reference voltage input and the wavelength control signal. The wavelength compensation signal is proportional to the sensed bias current. 
     The control circuit can further include the laser diode and a modulator for modulating the output of the laser diode. 
     In another aspect of the invention, a method of controlling a laser diode includes the following. A bias current to the laser diode is adjusted to change the power output of the laser diode, the power change having a corresponding wavelength shift effect on the nominal operating wavelength of the laser diode. The level of bias current to the diode is sensed. In response to the sensed level of bias current, compensating for the wavelength shift such that the laser diode maintains operation at the nominal wavelength. 
     Embodiments of this aspect of the invention may include one or more of the following steps. Adjusting the change of power output includes monitoring the laser diode power output to provide a power monitor signal, adding a power control signal to the power monitor signal to provide a power adjust signal, and adjusting the bias current responsive to a difference between a power reference voltage signal and the power adjust signal. 
     Compensating for the wavelength shift includes providing a control current to a thermoelectric element for controlling the temperature operation point of the laser diode, monitoring the temperature operation point to provide a temperature monitor signal, adding a wavelength compensation signal to the temperature monitor signal to provide a wavelength control signal, and adjusting the control current to the thermoelectric element responsive to a difference between a temperature reference signal and the wavelength control signal. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a circuit diagram of a laser transmitter of the prior art. 
     FIG. 2 is a chart that illustrates power control characteristics of the transmitter of FIG.  1 . 
     FIG. 3 is a chart that illustrates temperature control characteristics of the transmitter of FIG.  1 . 
     FIG. 4 is a circuit diagram of laser transmitter. 
     FIG. 5 is a chart illustrating power and wavelength control characteristics of the transmitter of FIG.  4 . 
     FIG. 6 is a circuit diagram of another embodiment of a laser transmitter in accordance with the present system. 
     FIG. 7 is a circuit diagram of still another embodiment of a laser transmitter. 
    
    
     DETAILED DESCRIPTION 
     A typical laser transmitter  10  of the prior art is shown in FIG.  1 . The laser transmitter includes a laser module  18  coupled to a variable optical attenuator (VOA)  30  via an optical fiber  32 . The laser module includes a laser diode  20 , a backfacet diode  22  and a modulator  24 . The laser diode  20  typically provides a continuous wave output at a constant bias level corresponding to a constant power level. A data stream input  11  is coupled through gate  16  to modulator  24  to modulate the continuous wave output of the laser diode  20 . For simplicity the modulator  24  is shown as a diode, though it is understood that it is commonly a Mach-Zhender interferometer or lithium niobate waveguide device. The modulated optical signal is coupled to the optical fiber  32 . 
     The constant power output of the laser diode  20  is controlled using a bias current source and a power monitor loop. The bias current source, which includes operational amplifier  12  and transistor  14 , provides an adjustable bias current IDFB to the laser diode. The power monitor loop includes backfacet diode  22  for monitoring the laser diode power output to provide a power monitor signal that is coupled to the negative input of operational amplifier  12 . The output of operational amplifier  12  is coupled to the negative input through capacitor C 1 . The positive input of operational amplifier  12  has a power reference voltage VREF. The operational amplifier  12  adjusts the bias current IDFB responsive to a difference between the power reference VREF and the power monitor signal. For example, if the power monitor signal is less than the power reference VREF, operational amplifier  12  provides more bias current. 
     To control the operating temperature of the laser transmitter, the laser module  18  includes a thermistor  26  and a thermnal electric cooler (TEC) element  28 . Operational amplifier  34  arid transimpedance bridge  36  provide a control current ITEC to the TEC element  28 . A temperature monitor loop includes thermistor  26  for monitoring the temperature operation point to provide a temperature monitor signal that is coupled to the negative input of operational amplifier  34 . The output of operational amplifier  34  is coupled to the negative input through capacitor C 2 . The positive input of operational amplifier  34  has a temperature reference voltage VTEMP. The operational amplifier  34  adjusts the control current ITEC to the TEC element  28  responsive to a difference between the temperature reference VTEMP and the temperature monitor signal. For example, if the temperature monitor signal is less than VTEMP, the operational amplifier  34  provides more current to the TEC element. 
     Direct electrical control of the power output of a laser diode generally is understood to be problematic, given the relationship between operating temperature and wavelength in such devices. In particular, the relationship depends on output power and the characteristics of individual devices. 
     Referring to FIG. 2, the chart illustrates the effect on operating wavelength when the laser output power is adjusted for the exemplary laser transmitter  10  of FIG.  1 . In particular, by applying a voltage VPOWER through a resistor to negative input  40  of operational amplifier  12 , the laser output power is adjusted. Note that the temperature control portion of the laser transmitter is kept constant, i.e., VTEMP is constant. The slope of the power adjustment curve (right vertical axis) is negative. That is, an increase in voltage VPOWER results in a decrease in laser output power. A corresponding change Δλ in operating wavelength occurs (left vertical axis) such that a decrease in laser power output results in a shorter operating wavelength. 
     As shown, a power change from 3.0 mW to below 1.0 mW results in a wavelength shift of about 2000 picometers. In modern dense wavelength division multiplex (DWDM) systems designed for 100 GHz or tighter channel spacings, the channels are only +/−100 picometers wide around a nominal specified center wavelength. Thus, the change in wavelength operation that occurs with the power adjustment shown in FIG. 2 is too large and is unacceptable for modem telecommunication systems. 
     FIG. 3 is a chart that illustrates the effect on operating wavelength when the temperature reference voltage VTEMP is adjusted for the laser transmitter  10  of FIG. 1 while the output power of the laser transmitter and VREF are kept constant. The slope of the curve in FIG. 3 is negative. That is, an increase in temperature reference voltage VTEMP causes the TEC element to operate at a cooler temperature, which results in a shorter operating wavelength for the laser diode. As shown, a change in VTEMP from 2 to 3 volts results in a wavelength shift of about 2000 picometers. 
     It has been found in the present approach that, by taking into account the wavelength shift due to power adjustment and due to temperature, a power control circuit can be implemented that provides variable laser power output while maintaining operation of the laser diode at a nominal wavelength within an acceptable range. 
     In an embodiment of a laser control circuit  100  in accordance with the present approach shown in FIG. 4, a power control signal VMOD is provided that is added to the power monitor signal through resistor network R 1  and R 2  at the negative input of operational amplifier  12  so that the operational power level can be increased or decreased over the nominal set point provided by reference voltage VREF. In addition, to compensate for the wavelength shift of the laser diode  22 , a scaled version  29  of the power control signal VMOD is provided that is added to the temperature monitor signal  27  through resistor R 4  at the negative input of operational amplifier  34 . Note that the control circuit  100  eliminates the need for a VOA (FIG.  1 ). Thus, a simple but elegant solution is provided to solve the problems noted above. 
     Different laser diode devices can exhibit different temperature and wavelength characteristics. Thus, in the control circuit  100  of FIG. 4, the values for resistors R 1 , R 2 , R 3  and R 4  can be accordingly adjusted to fit the characteristics of each laser diode. 
     As described, the control circuit  100  provides an adjustable output power. FIG. 5 shows the laser output power (right vertical axis) as it varies with the applied adjustment voltage, VMOD. Note that for VMOD of 0V the output power is approximately 2.5 mW. With VMOD of 3V the output power is approximately 1.5 mW. Thus, linear adjustment of output power is provided. 
     FIG. 5 also shows a residual amount of wavelength variation (left vertical axis) for the control circuit of FIG.  4 . Note that for VMOD of 0 V the difference between the intended wavelength and the actual wavelength, given as Δλ, is about 25 picometers. The negative sign indicates that the wavelength is less then the intended wavelength. For VMOD of 4.5 V the difference Δλ is about 0. 
     As noted above, DWDM system today require tight channel spacings. Without the wavelength control feature provided as shown in FIG. 4, the variation of the laser wavelength as the power is adjusted from 2.5 mW to 0 mW (FIG. 5) will be very much larger than the acceptable variation. With the control circuit of FIG. 4, the residual wavelength variation is well within the acceptable variation. 
     Referring to FIG. 6, a second embodiment of a control circuit  200  is shown. In this embodiment, a Fabry□Perot etalon locker device  42  is used to provide the wavelength compensation. The etalon locker  42  receives light emitted from laser diode  20 , and based upon the wavelength of the light received, outputs a signal to add to the negative input of operational amplifier  34  for controlling the wavelength. 
     Other embodiments for providing wavelength compensation when the output power of a laser diode is varied are within the scope of the claims. For example, in the embodiment described above in conjunction with FIG. 4, the value of resistor R 4  was selected to provide the appropriate level of wavelength compensation as the output power of the laser diode is varied. However, in that embodiment, as the laser diode “ages,” the bias current needed to provide a given level of output power will no longer be the same, but will increase. The value of resistor R 4  may no longer be appropriate for providing the proper level of wavelength compensation. 
     However, referring to FIG. 7, a laser diode module  300  provides accurate wavelength compensation even as the characteristics of the laser diode changes. In particular, laser diode module provides for wavelength compensation in response to the change in bias current applied to a laser diode  302 . Laser diode module  300  includes many of the same components as the laser circuit shown in FIG.  4 . For example, the output power of laser diode  302  is controlled using a bias current source and a power monitor loop. The bias current source includes an operational amplifier  304  and a transistor  306 , which together provide an adjustable bias current I DFB  to laser diode  302 . The power monitor loop includes a backfacet diode  308  for monitoring the output power to laser diode  302  and to provide a power monitor signal that is coupled to the negative input of operational amplifier  304 . The output of operational amplifier  304  is coupled to the negative input through capacitor C 1 . The positive input of operational amplifier  304  has a power reference voltage VREF. The operational amplifier  304  adjusts the bias current I DFB  responsive to a difference between the power reference VREF and the power monitor signal. For example, if the power monitor signal is less than the power reference VREF, operational amplifier  304  increases the level of bias current. 
     As was the case in the embodiment of FIG. 4, laser diode module  300  includes a temperature monitor loop for monitoring the temperature operation point to provide a temperature monitor signal that is coupled to the negative input of an operational amplifier  314 . The temperature monitor loop has a thermistor  310  and a thermal electric cooler (TEC) element  312 . Operational amplifier  314  and transimpedance bridge  316  provide a control current ITEC to the TEC element  312 . The output of operational amplifier  314  is coupled to its negative input through a capacitor C 2 . The positive input of operational amplifier  314  has a temperature reference voltage V TEMP . The operational amplifier  314  adjusts the control current I TEC  to the TEC element  312  responsive to a difference between the temperature reference V TEMP  and the temperature monitor signal. For example, if the temperature monitor signal is less than V TEMP , the operational amplifier  314  provides more current to the TEC element. A power control signal V MOD  is added to the power monitor signal through a resistor network R 1  and R 2  at the negative input of operational amplifier  314  so that the operational power level can be increased or decreased over the nominal set point provided by reference voltage V REF . Unlike the embodiment of FIG. 4, however, a scaled version of the power control signal V MOD  is not used to compensate for the wavelength shift. Rather, laser module  300  includes a sensing circuit  320  having, in this embodiment, a sensing resistor  322 . An output signal of the sensing circuit  320  is provided via signal line  316  to the negative terminal of operational amplifier  314 . The output signal from sensing circuit  320  provides a connection between that portion of the laser module associated with automatic power control and that portion of the laser module associate with wavelength compensation. 
     Thus, the embodiment shown in FIG. 7 is particularly advantageous in applications where the laser module is to be used for extended periods of time. Rather than being proportional to the output power of the laser diode, the wavelength compensation is proportional the bias current. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.