Abstract:
Methods and apparatus for monitoring the power level of a multi-wavelength optical signal are provided. Also provided are methods and apparatus for adjusting the power level of selected optical emitters to compensate for the changes in power levels.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the field of optical systems, and more particularly to methods and apparatus for monitoring the power of a multi-wavelength optical signal.  
           [0002]    Various forms of optoelectronic devices have been developed and have found widespread use including, for example, semiconductor lasers, semiconductor photodiodes, semiconductor photo detectors, etc. For some of these applications, an optoelectronic emitter such as a semiconductor laser is coupled to an optoelectronic detector (e.g., photodiode or Resonant Cavity Photo Detector) through a fiber optic link or even free space. This configuration can provide a high-speed communication path, which, for many applications, can be extremely beneficial.  
           [0003]    The increased use of all-optical fiber networks as backbones for global communication systems has been based in large part on the extremely wide optical transmission bandwidth provided by optical fiber. This has led to an increased demand for the practical utilization of the optical fiber bandwidth, which can provide, for example, increase communication system user capacity. In the prevailing manner for exploiting optical fiber bandwidth, wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) techniques are used to enable the simultaneous transmission of multiple independent optical data streams, each at a distinct wavelength, on a single optical fiber, with wavelength-selective WDM and WDD control provided for coupling of the multiple data streams with the optical fiber on a wavelength-specific basis. With this capability, a single optical fiber can be configured to simultaneously transmit several optical data streams, e.g., ten optical data streams, that each might not exceed, say, 10 Gb/s, but that together represent an aggregate optical fiber transmission bandwidth of more than, say, 100 Gb/s.  
           [0004]    In order to increase the aggregate transmission bandwidth of an optical fiber, it is generally preferred that the wavelength spacing of simultaneously transmitted optical data streams, or optical data “channels,” be closely packed to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized.  
           [0005]    In addition, in WDM communications systems as well as in many other applications, it is often desirable to monitor the power of each data channel. The power of each data channel may vary for a variety of reasons including, for example, changing operating conditions such as operating voltage, operating temperature, device degradation, etc. If the power of one or more of the data channels falls outside of a desired range, the reliability of the communications link can significantly degrade. In some systems, it is possible to provide a separate detector for each data channel. However, this is not always possible, and in many cases, can add significant cost to the system.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides methods and apparatus for monitoring the power level of a multi-wavelength optical signal. Also provided are methods and apparatus for adjusting the power level of selected optical emitters to compensate for the changes in power levels.  
           [0007]    In one illustrative embodiment of the present invention, a detector is used to detect two or more wavelengths of light, and to provide an indication of the power level of each wavelength of light in a multi-wavelength optical signal. The detector may include, for example, a first absorbing layer, a second absorbing layer situated below the first absorbing layer, and an intermediate layer situated between the first absorbing layer and the second absorbing layer. In some embodiments, the first absorbing layer and the second absorbing layer are a first conductivity type, and the intermediate layer is a second conductivity type. In this configuration, a first PN junction may be formed between the first absorbing layer and the intermediate layer, and a second PN junction may be formed between the second absorbing layer and the intermediate layer.  
           [0008]    The detector may receive a multi-wavelength optical signal. The multi-wavelength optical signal may be provided by, for example, two or more optoelectronic emitters, such as semiconductor lasers, semiconductor light emitting diodes, etc., each providing a different wavelength of light. The first absorbing layer may absorb a first portion of a first wavelength of light and a second portion of a second wavelength of light. For example, the first absorbing layer may absorb a majority of the first wavelength of light and a minority of the second wavelength of light. The second absorbing layer, which is preferably situated below the first absorbing layer, may absorb a third portion of the first wavelength of light and a fourth portion of the second wavelength of light. For example, the second absorbing layer may absorb a minority of the first wavelength of light and a majority of the second wavelength of light. The relative portions of light absorbed by the first absorbing layer and the second absorbing layer may be controlled by, for example, the materials and/or thickness used for the first absorbing layer and/or second absorbing layer. In a preferred embodiment, the first absorbing layer and the second absorbing layer are adapted to collectively absorb all or substantially all of the first wavelength of light and the second wavelength of light.  
           [0009]    When the power of either the first wavelength of light or the second wavelength of light changes, the relative portions absorbed by the first absorbing layer and the second absorbing layer may also change. For example, if the power level of the first wavelength of light decreases by ten percent, the overall light absorbed by the first absorbing layer may decrease more than the overall light absorbed by the second absorbing layer. In this example, this is because the first absorbing layer absorbs more of the first wavelength of light than the second absorbing layer. Thus, by using a measure of the light absorption in the first absorbing layer and a measure of the light absorption in the second absorbing layer, an indication of the change in the power level of the first wavelength of light and/or the second wavelength of light can be identified.  
           [0010]    In some embodiments, a ratio of the measure of the light absorption in the first absorbing layer and the second absorbing layer is used to identify which wavelength of light experienced a power level change. In some embodiments, a sum of the measure of the light absorption in the first absorbing layer and the second absorbing layer may further be used to identify which wavelength of light experienced a power change, and/or if more than one wavelength of light experienced a power change. While only two wavelengths of light are used in this example, it is contemplated that any number of wavelengths may be used.  
           [0011]    In another illustrative embodiment of the present invention, an optical transmitter may be provided that includes a first and second electrical input signal. A first modulator may modulate the first electrical input signal with a first electrical power monitor signal to produce a first electrical modulated signal. The first electrical modulated signal may be provided to a corresponding optoelectronic emitter to produce a first optical output signal. The first electrical power monitor signal may operate at a frequency that is substantially less than the frequency or data rate of the first electrical input signal so that the first electrical power monitor signal represents an average power output of the corresponding optoelectronic emitter. In some embodiments, the first modulator may “amplitude” modulate the first electrical input signal with the first electrical power monitor signal, with the amplitude of the first electrical power monitor signal substantially less than the amplitude of the first electrical input signal.  
           [0012]    A second modulator may also be provided for modulating the second electrical input signal with a second electrical power monitor signal to produce a second electrical modulated signal. The second electrical modulated signal may be provided to an optoelectronic emitter to produce a second optical output signal. The second electrical power monitor signal may operate at a frequency that is substantially less than the frequency or data rate of the second electrical input signal so that the second electrical power monitor signal represents an average power output of the corresponding optoelectronic emitter. In some embodiments, the second modulator may “amplitude” modulate the second electrical input signal with the second electrical power monitor signal, with the amplitude of the second electrical power monitor signal substantially less than the amplitude of the second electrical input signal.  
           [0013]    An optical combiner may combine the first optical output signal and the second optical output signal into a common optical output signal. A detector may then be used to monitor the common optical output signal, and produce a corresponding electrical detection signal. In one embodiment, the detector is a wide band detector.  
           [0014]    A filter or the like may be used to frequency separate the first power monitor signal and the second power monitor signal from the electrical detection signal, resulting in a first detected power monitor signal and a second detected power monitor signal. The power of the first optoelectronic emitter and the second optoelectronic emitter may then be adjusted based on one or more characteristics of the first detected power monitor signal and the second detected power monitor signal. For example, the power of the first optoelectronic emitter and the second optoelectronic emitter may be adjusted based on the amplitude of the first detected power monitor signal and the amplitude of the second detected power monitor signal. While only two wavelengths are used in this example, it is contemplated that any number of wavelengths may be used.  
           [0015]    Rather than using a broad band detector, it is contemplated that the optical transmitter may include a detector that can help provide an indication of the power level of selected wavelengths of light. For example, if four electrical input signals are provided, two of the electrical input signals may be modulated with a first electrical power monitor signal and the remaining two electrical input signals may be modulated with a second electrical power monitor signal. The four modulated electrical input signals may then be provided to four corresponding optoelectronic emitters to produce four optical output signals. An optical combiner may be used to combine the four optical output signals into a common optical output beam.  
           [0016]    The detector may include a first absorbing layer, a second absorbing layer situated below the first absorbing layer, and an intermediate layer situated between the first absorbing layer and the second absorbing layer. The first absorbing layer may absorb a different proportion of the each of the four optical output signals, and the second absorbing layer may absorb the remaining portion of each of the four optical output signals. When the power of any of the four optical output signals changes, the relative portions absorbed by the first absorbing layer and the second absorbing layer may also change. For example, if the power level of a first wavelength of light decreases by ten percent, the overall light absorbed by the first absorbing layer may decrease more than the overall light absorbed by the second absorbing layer, particularly if the first absorbing layer absorbs more of the first wavelength of light.  
           [0017]    In one illustrative embodiment, a first electrical input signal and a third electrical input signal are modulated with a first electrical power monitor signal to produce a first electrical modulated signal and a third electrical modulated signal. Likewise, a second electrical input signal and a fourth electrical input signal are modulated with a second electrical power monitor signal to produce a second electrical modulated signal and a fourth electrical modulated signal. The first, second, third and fourth electrical modulated signals are provided to corresponding optoelectronic emitters to produce first, second, third and fourth optical output signals.  
           [0018]    A detector having a first absorbing layer and a second absorbing layer receives the first, second, third and fourth optical output signals. The first absorbing layer may absorb a different proportion of the each of the four optical output signals, and the second absorbing layer may absorb substantially the remaining portion of each of the four optical output signals. Using a measure of the light absorption in the first absorbing layer and the second absorbing layer, an indication of change in the power level of the first/fourth optical output signal pair, or the second/third optical output signal pair can be identified.  
           [0019]    A filter or the like can be used to separate out the first power monitor signal from the first optical signal and the third optical signal, and the second power monitor signal from the second optical signal and the fourth optical signal. The power of the first optoelectronic emitter may then be adjusted if it is determined that the first optical signal/fourth optical signal pair had an increase or decrease in power level and said first power monitor signal indicates that the first optical signal or the third optical signal had an increase or decrease in power level. Likewise, the power of the second optoelectronic emitter may be adjusted if it is determined that the second optical signal/third optical signal pair had an increase or decrease in power level and the second power monitor signal indicates that said second optical signal or fourth optical signal had an increase or decrease in power level. The power of the third optoelectronic emitter may be adjusted if it is determined that the second optical signal/third optical signal pair had an increase or decrease in power level and said first power monitor signal indicates that the first optical signal or third optical signal had an increase or decrease in power level. Finally, the power of the fourth optoelectronic emitter may be adjusted if it is determined that the first optical signal/fourth optical signal pair had an increase or decrease in power level and said second power monitor signal indicates that said second optical signal or fourth optical signal had an increase or decrease in power level. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:  
         [0021]    [0021]FIG. 1 is a schematic diagram of an optical transmitter system in accordance with one illustrative embodiment of the present invention;  
         [0022]    [0022]FIG. 2 is a cross-sectional view of an illustrative detector in accordance with the present invention;  
         [0023]    [0023]FIG. 3 is a schematic diagram of the illustrative detector of FIG. 2;  
         [0024]    [0024]FIG. 4 is a cross-sectional view of another illustrative detector in accordance with the present invention;  
         [0025]    [0025]FIG. 5 is a graph showing separate relative spectral responses of the two photodiodes of the detector shown in FIG. 2;  
         [0026]    [0026]FIG. 6 is a graph showing cumulative relative spectral responses of the two photo-diodes of the detector shown in FIG. 2;  
         [0027]    [0027]FIG. 7 is a graph showing a ratio of the output signal of a top diode relative to the output signal of a bottom diode versus output power of an incoming light beam;  
         [0028]    [0028]FIG. 8 is a schematic diagram of another illustrative optical transmitter system in accordance with the present invention; and  
         [0029]    [0029]FIG. 9 is a schematic diagram of an illustrative control block for use with the optical transmitter system of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIG. 1 is a schematic diagram of an optical transmitter system in accordance with one illustrative embodiment of the present invention. The illustrative optical transmitter is generally shown at  8 , and includes four optoelectronic emitters  10 ,  12 ,  14  and  16 . The optoelectronic emitters  10 ,  12 ,  14  and  16  maybe any type of optoelectronic emitter including, for example, a conventional laser, a Vertical Cavity Surface Emitting Laser (VCSEL), a light emitting diode (LED), or any other type of optoelectronic emitter. Each optoelectronic emitter  10 ,  12 ,  14  and  16  preferably receives an electronic input signal  20 ,  22 ,  24  and  26 , and provides a corresponding optical output signal  30 ,  32 ,  34  and  36 , as shown. For WDM and other applications, each optoelectronic emitter  10 ,  12 ,  14  and  16  may produce a different wavelength than the other optoelectronic emitters, if desired.  
         [0031]    An optical combiner  40  may be used to combine the various optical output signals  30 ,  32 ,  34  and  36  into a common optical output signal  42 , as shown. In the illustrative embodiment, a partially transmissive plate  46  is used to direct at least a portion of the common optical output signal  42  to a detector  48 . The detector  48  is used to sample the common optical output signal  42 . In an illustrative embodiment, the detector  48  may include a top detector (D 1 ) and a bottom detector (D 2 ), with each detector absorbing a different proportion of each of the wavelengths of light in the common optical output signal  42 .  
         [0032]    A control block  50  receives the output signal(s) from the detector  48 . In one illustrative embodiment, the controller  50  determines a ratio of the output signals from the top detector (D 1 ) and the bottom detector (D 2 ), and in some cases, an overall magnitude (e.g. sum) of the detector output signals. If one of the optical emitters  10 ,  12 ,  14 , or  16  degrades or otherwise produces a change in it&#39;s output power, the signal ratio (D 1 /D 2 ) and overall magnitude (e.g. D 1 +D 2 ) of the output signals from the detectors D 1  and D 2  may change. By monitoring the signal ratio change, and in some cases the overall magnitude of the detected power, the controller  50  may uniquely identify which of the optical emitters  10 ,  12 ,  14 , or  16  has produced a change in output power. Once identified, the controller  50  may adjust the current and/or voltage that is provided to the identified optoelectronic emitter  10 ,  12 ,  14  or  16  via interface  52  to correct for the detected change in output power.  
         [0033]    [0033]FIG. 2 is a cross-sectional view of an illustrative detector in accordance with the present invention. The illustrative detector is generally shown at  60 , and includes from top to bottom, a high bandgap P-type layer  62 , a high bandgap N-type layer  64 , a low bandgap P-type layer  66 , followed by a substrate  68 . In some embodiments, this forms two back-to-back PN junctions, with the top PN junction  70  forming a top detector (D 1 ) and the bottom PN junction  72  forming a bottom detector (D 2 ). The high bandgap P-type layer  62  may be, for example, Al 10 Ga 90 As that is doped P-type. The high bandgap N-type layer  64  may be, for example, Al 10 Ga 90 As that is doped to be N-type. The low bandgap P-type layer  66  may be, for example, GaAs that is doped to be P-type. The substrate  68  may also be doped P-type. In some cases, one or more buffer or other intervening layers may also be provided, depending on the circumstances. It is also contemplated that the conductivity types of the various layers may be changed or reversed. For example, the detector may include, from top to bottom, a high bandgap N-type layer  62 , a high bandgap P-type layer  64 , a low bandgap N-type layer  66 , followed by an N-type substrate  68 .  
         [0034]    In the illustrative embodiment shown, the Al fraction may be used to tune the bandgap of each layer, as desired. For example, the Al fraction of the high bandgap P-type layer  62  may be set so that only wavelengths shorter than a cut-off wavelength are absorbed while longer wavelengths are passed through. The thickness  76  of the high bandgap P-type layer  62  may be adjusted to provide a desired slope in the spectral response curve at the cut-off wavelength. The Al fraction of the high bandgap N-type layer  64  may be similar to that of the high bandgap P-type layer  62 . The Al fraction of the low bandgap P-type layer  66  may be lower than the Al fraction of the high bandgap P-type layer  62 , so that the cut-off wavelength is higher than the cut-off wavelength of the high bandgap P-type layer  62 . In some embodiments, the cut-off wavelength of the low bandgap P-type layer  66  is higher than the longest expected wavelength in the common optical output signal  42 .  
         [0035]    In another embodiment, the Al fraction of layer  62  may be graded, varying smoothly from a first fraction at the top of  62  to a second fraction at the bottom of  62 . This can also have the effect of reducing the slope of response versus wavelength, discussed below in descriptions of FIGS. 5 and 6.  
         [0036]    Rather than varying the bandgap energy of the various layers, it is contemplated that the detector  609  may include a number of layers that have the same or similar bandgap energy. For example, the detector  60  may have a P-type layer  62 , an N-type layer  64 , and a P-type layer  66 , all of which are made of a single material such as silicon. In this embodiment, the thickness of each layer may be adjusted so that each layer absorbs a different proportion of the various wavelengths expected in the common optical output signal  42 .  
         [0037]    In either case, a top contact  80  may make electrical contact to the P-type layer  62 . The top contact  80  may be applied to the top surface of the P-type layer  62 , as shown. A bottom contact  82  may also be provided to make electrical contact to the P-type layer  66  through the substrate  68 . In the illustrative embodiment, the bottom contact  82  is applied to the bottom surface of the substrate  68 . In some embodiments, an intermediate contact may also be provided for making an electrical contact to the intermediate N-type layer  64 . In the example shown, a heavily N-doped region  86  may be provided to complete the electrical connection between the N-type layer  64  and the intermediate contact  84 . Alternatively, a trench could be cut through the P-type layer to afford contact to layer  64 . FIG. 3 is a schematic diagram of the illustrative detector of FIG. 2.  
         [0038]    [0038]FIG. 4 is a cross-sectional view of another illustrative detector in accordance with the present invention. This embodiment is similar to that shown in FIG. 2, but all of the contacts are situated on the top-side of the detector. A top contact  90  is provided on the P-type layer  62  to make electrical contact to the P-type layer  62 . To make electrical contact to the P-type layer  66 , a trench or mesa is cut through the P-type layer  62 , the N-type layer  64 , and the P-type layer  66  down to the substrate  68 . A contact  92  is then provided on the substrate  68 . When the substrate  68  is doped P-type, an electrical connection is made between the contact  92  and the P-type layer  66  through the substrate  68 . To make electrical contact with the N-type layer  64 , another trench or mesa is cut through the P-type layer  62 , as shown. A contact  94  is then provided on the N-type layer  64  as shown. Because all of the contacts  90 ,  92  and  94  are on the top-side of the detector, the cost associated with packaging the detector may be reduced.  
         [0039]    [0039]FIG. 5 is a graph showing separate relative spectral responses of the two photodiodes (D 1 ) and (D 2 ) of the detector shown in FIG. 2. The relative spectral response of the top photo-diode (D 1 ) is shown by curve  96 , and the relative spectral response of the bottom photo-diode (D 2 ) is shown by curve  98 . As discussed above, the Al fraction in each layer of the detector  60  may be used to tune the bandgap of each layer, and thus the cut-off wavelength of each layer. In the illustrative graph, the Al fraction of the high bandgap P-type layer  62  is set so that only wavelengths shorter than a cut-off wavelength  100  are absorbed while longer wavelengths are passed through. The thickness  76  and/or the grading of the high bandgap P-type layer  62  may be adjusted to provide a desired slope  101  in the spectral response curve  96  at the cut-off wavelength  100 .  
         [0040]    Likewise, the Al fraction of the low bandgap P-type layer  66  may be lower than the Al fraction of the high bandgap P-type layer  62 , so that the cut-off wavelength  102  is higher than the cut-off wavelength  100  of the high bandgap P-type layer  62 . In some embodiments, the cut-off wavelength  102  of the low bandgap P-type layer  66  is higher than the longest expected wavelength in the common optical output signal  42 . Again, the thickness of the low bandgap P-type layer  66  may be adjusted to provide a desired slope  103  in the spectral response curve  98  at the cut-off wavelength  102 .  
         [0041]    [0041]FIG. 6 is a graph showing cumulative relative spectral responses of the two photo-diodes (D 1 ) and (D 2 ) of the detector shown in FIG. 2. The optical output signals of optoelectronic emitters  10 ,  12 ,  14  and  16  of FIG. 1 are shown at  30 ,  32 ,  34  and  36 , each having a different wavelength in this case.  
         [0042]    As can be seen, the top photo-diode (D 1 ) absorbs most of the first optical output signal  30 , which has the shortest wavelength. Because most of the first optical output signal  30  is absorbed by the top photo-diode (D 1 ), only a small fraction of the first optical output signal  30  is transmitted to the bottom photo-diode (D 2 ). In the embodiment shown, the bottom photo-diode (D 2 ) absorbs the remainder of the first optical output signal  30 .  
         [0043]    The top photo-diode (D 1 ) also absorbs a majority of the second optical output signal  32 . Because a majority of the second optical output signal  32  is absorbed by the top photo-diode (D 1 ), only a minority of the second optical output signal  32  is transmitted to the bottom photo-diode (D 2 ). In the embodiment shown, the bottom photo-diode (D 2 ) absorbs the remainder of the second optical output signal  32 .  
         [0044]    The top photo-diode (D 1 ) absorbs a minority of the third optical output signal  34 . Because only a minority of the third optical output signal  34  is absorbed by the top photo-diode (D 1 ), a majority of the third optical output signal  34  is transmitted to the bottom photo-diode (D 2 ). In the embodiment shown, the bottom photo-diode (D 2 ) absorbs the remainder of the third optical output signal  34 .  
         [0045]    Finally, the top photo-diode (D 1 ) absorbs only a small fraction of the fourth optical output signal  36 . Because only a small fraction of the fourth optical output signal  36  is absorbed by the top photo-diode (Dl), most of the fourth optical output signal  36  is transmitted to the bottom photo-diode (D 2 ). In the embodiment shown, the bottom photo-diode (D 2 ) absorbs the remainder of the fourth optical output signal  36 .  
         [0046]    As can be seen, when the power of one of the optoelectronic emitters  10 ,  12 ,  14  and  16  changes, the relative portions absorbed by the first absorbing layer  62  of the top photo-diode (D 1 ) and the second absorbing layer  66  of the bottom photo-diode (D 2 ) may also change. For example, if the power level produced by the optoelectronic emitter  10  decreases by ten percent, the overall light absorbed by the first absorbing layer  62  of the top photo-diode (D 1 ) may decrease more than the overall light absorbed by the second absorbing layer  22  of the bottom photo-diode (D 2 ). In this example, this is because the first absorbing layer  62  of the top photodiode (D 1 ) absorbs more of the first optical output signal than the second absorbing layer  66  of the second photo-diode (D 2 ). By using a measure of the light absorption in the first absorbing layer  62  of the first photo-diode (Dl) and/or a measure of the light absorption in the second absorbing layer  66  of the second photo-diode (D 2 ), an indication of the change in power level produced by one or more of the optoelectronic emitters  10 ,  12 ,  14  and  16  can be identified.  
         [0047]    [0047]FIG. 7 is a graph showing a ratio of the output signal  80  of the top photo-diode (D  1 ) relative to the output signal  82  of the bottom photo-diode (D 2 ) versus output power of an incoming light beam. The graph shown in FIG. 7 assumes, for example, that optoelectronic emitters  10 ,  12 ,  14 , and  16  produce optical output signals  30 ,  32 ,  34  and  36 , having wavelengths λ 1 , λ 2 , λ 3 , and λ 4 , respectively. The graph shown in FIG. 7 also assumes that the spectral response of the top detector (D 1 ) is such that it absorbs 10%, 30%, 70% and 90% of wavelengths λ 1 , λ 2 , λ 3 , and λ 4 , respectively, and that the spectral response of the bottom detector (D2) is such that it absorbs 90%, 70%, 30% and 10% of the wavelengths λ 1 , λ 2 , λ 3 , and λ 4 , respectively. For illustration purposes, FIG. 7 also assumes that all four optoelectronic emitters  10 ,  12 ,  14 , and  16  initially produce an output power of 1.0 unit.  
         [0048]    If one of the optoelectronic emitters  10 ,  12 ,  14 , and  16  begins to degrade or produce a change in output power, the ratio of the output signals  80  to  82  from detectors D 1  and D 2  may also change. In addition, the overall magnitude (e.g. sum) of the output signals  80  and  82  from detectors D 1  and D 2  may change. Referring specifically to FIG. 7, degradation curves  120 ,  122 ,  124  and  126  are shown for optoelectronic emitter  10 ,  12 ,  14 , and  16 , respectively. Thus, if a signal ratio (D 1 /D 2 ) of 1.05 is detected, it can be concluded that optoelectronic emitter  10 , which corresponds to λ 1 , has degraded to about 0.88% of full power. Controller  50  may detect this change and increase the current and/or voltage that is provided to optoelectronic emitter  10  to correct for the detected power degradation.  
         [0049]    In another example, if a signal ratio (D 1 /D 2 ) of 1.025 is detected, either optoelectronic emitter  10  (which corresponds to λ 1 ) has degraded sufficiently to cause the detected output power to fall to about 0.94% of full power, or optoelectronic emitter  12  (which corresponds to λ 2 ) has degraded sufficiently to cause the detected output power to fall to about 0.88% of full power. In this case, an overall magnitude (e.g. sum) of the signals  80  and  82  from detectors D 1  and D 2  can be used to determine which of the optoelectronic emitters has actually degraded. For example, if the overall magnitude (e.g. sum) of the signals  80  and  82  from detectors D 1  and D 2  only degraded by a small amount (e.g. about 1.5%), it can be concluded that optoelectronic emitter  10  (which corresponds to λ 1 ) has degraded. If, on the other hand, the overall magnitude (e.g. sum) of the signals  80  and  82  from detectors D 1  and D 2  has degraded by a larger amount (e.g. about 3%), then it can be concluded that optoelectronic emitter  12  (which corresponds to λ 2 ) has degraded.  
         [0050]    It is contemplated that curves  120 ,  122 ,  124  and  126  may be dependent on operating temperature, time and/or any other parameter of interest, if desired. While a signal ratio is described above, other functions may also be used including, for example, sum and/or difference signals, or any other function, as desired.  
         [0051]    When the power output from all optoelectronic emitters change simultaneously, as might result from a change in operating temperature, voltage, etc., the signal ratio (D 1 /D 2 ) may remain substantially constant. However, the overall magnitude (e.g. sum) of the signals  80  and  82  from detectors D 1  and D 2  may change. In this case, the controller may increase the current and/or voltage that is provided to all optoelectronic emitter  10 ,  12 ,  14 , and  16  to correct for the overall power degradation.  
         [0052]    Another illustrative embodiment of the present invention is shown in FIG. 8. FIG. 8 shows an optical transmitter  130  that includes a first electrical input signal  132 , a second electrical input signal  134 , a third electrical input signal  136  and a fourth electrical input signal  138 . For WDM and other applications, each of the electrical input signals  132 ,  134 ,  136  and  138  may have a different wavelength, such as wavelengths λ 1 , λ 2 , λ 3 , and λ 4 , respectively.  
         [0053]    A first modulator  140  may be provided to modulate the first electrical input signal  132  with a first electrical power monitor signal  142  to produce a first electrical modulated signal  144 . The first electrical modulated signal  144  may be provided to a first optoelectronic emitter  146  to produce a first optical output signal  148 . The first electrical power monitor signal  142  may operate at a frequency that is substantially less than the frequency or data rate of the first electrical input signal  132  so that the first electrical power monitor signal  142  may be used to determine an average power output produced by the first optoelectronic emitter  146 . In some embodiments, the first modulator  140  may amplitude modulate the first electrical input signal  132  with the first electrical power monitor signal  142 , with the amplitude of the first electrical power monitor signal  142  substantially less than the amplitude of the first electrical input signal  132 .  
         [0054]    A second modulator  150  may also be provided to modulate the second electrical input signal  134  with a second electrical power monitor signal  152  to produce a second electrical modulated signal  154 . The second electrical modulated signal  154  may be provided to a second optoelectronic emitter  156  to produce a second optical output signal  158 . The second electrical power monitor signal  152  may operate at a frequency that is substantially less than the frequency or data rate of the second electrical input signal  134  so that the second electrical power monitor signal  152  may be used to determine an average power output produced by the second optoelectronic emitter  156 . In some embodiments, the second modulator  150  may amplitude modulate the second electrical input signal  134  with the second electrical power monitor signal  152 , with the amplitude of the second electrical power monitor signal  152  substantially less than the amplitude of the second electrical input signal  134 .  
         [0055]    A third modulator  160  may also be provided to modulate the third electrical input signal  136  with a third electrical power monitor signal  162  to produce a third electrical modulated signal  164 . The third electrical modulated signal  164  may be provided to a third optoelectronic emitter  166  to produce a third optical output signal  168 . The third electrical power monitor signal  162  may operate at a frequency that is substantially less than the frequency or data rate of the third electrical input signal  136  so that the third electrical power monitor signal  162  may be used to determine an average power output produced by the third optoelectronic emitter  166 . In some embodiments, the third modulator  160  may amplitude modulate the third electrical input signal  136  with the third electrical power monitor signal  162 , with the amplitude of the third electrical power monitor signal  162  substantially less than the amplitude of the third electrical input signal  136 .  
         [0056]    A fourth modulator  170  may also be provided to modulate the fourth electrical input signal  138  with a fourth electrical power monitor signal  172  to produce a fourth electrical modulated signal  174 . The fourth electrical modulated signal  174  may be provided to a fourth optoelectronic emitter  176  to produce a fourth optical output signal  178 . The fourth electrical power monitor signal  172  may operate at a frequency that is substantially less than the frequency or data rate of the fourth electrical input signal  138  so that the fourth electrical power monitor signal  172  may be used to determine an average power output produced by the fourth optoelectronic emitter  176 . In some embodiments, the fourth modulator  170  may amplitude modulate the fourth electrical input signal  138  with the fourth electrical power monitor signal  172 , with the amplitude of the fourth electrical power monitor signal  172  substantially less than the amplitude of the fourth electrical input signal  138 .  
         [0057]    An optical combiner  180  may be used to combine the first optical output signal  148 , the second optical output signal  158 , the third optical output signal  168 , and the fourth optical output signal  178  into a common optical output beam  182 . A partially reflective plate  184  may be used to direct at least part of the common optical output beam  182  to a detector  186 . The detector  186  may produce one or more electrical detection signals  183  that are provided to a controller  190 , as shown.  
         [0058]    In some embodiments, the detector  186  is a wide band detector, and the frequency of the first electrical power monitor signal  142 , the second electrical power monitor signal  152 , the third electrical power monitor signal  162 , and the fourth electrical power monitor signal  172  are different. Once receiving the electrical detection signal(s) from the detector  186 , the controller  190  may frequency separate the first power monitor signal  142 , the second power monitor signal  152 , the third power monitor signal  162  and the fourth power monitor signal  172  from the electrical detection signal provided by the detector  186 . This may result in a first, a second, a third and a fourth detected power monitor signal. Based on selected characteristics of each of the first, second, third and fourth detected power monitor signals, the controller  190  may adjust the current and/or voltage that is applied to the first, second, third and/or fourth optoelectronic emitters  146 ,  156 ,  166  and  176  via interface  192 .  
         [0059]    For example, and in one illustrative embodiment, the controller  190  may adjust the current and/or voltage applied to the first, second, third and/or fourth optoelectronic emitters  146 ,  156 ,  166  and  176  based on the amplitude of the first, second, third and fourth detected power monitor signals. Harmonic distortions of the first, second, third and fourth detected power monitor signals may also be used as a relative threshold determination, if desired. While four wavelengths of light are used in this example, it is contemplated that any number of wavelengths of light may be used.  
         [0060]    [0060]FIG. 9 is a schematic diagram of an illustrative control block  190  for use with the optical transmitter system of FIG. 8. The illustrative control block  190  receives an electrical detection signal  183  from the detector  186 , and provides the electrical detection signal  183  to a lock-in amplifier  200 . The lock-in amplifiers receive four lock-in frequencies  202 ,  204 ,  206  and  208 . Each of the four lock-in frequencies  202 ,  204 ,  206  and  208  may match the frequency of the first, second, third and fourth power monitor signals  142 ,  152 ,  162  and  172 , respectively. Using the four lock-in frequencies  202 ,  204 ,  206  and  208 , the lock-in amplifier  200  frequency separates the first, second, third and fourth detected power monitor signals from the electrical detection signal  183  provided by the detector  186 . From this, the lock-in amplifiers  200  provide control signals  210 ,  212 ,  214  and  216  that are proportional to the power (e.g. amplitude) of the first, second, third and fourth detected power monitor signals, respectively. These control signals may be provided to the first, second, third and fourth optoelectronic emitters  146 ,  156 ,  166  and  176 , respectively, via interface  192 , to control the power of each of the optoelectronic emitters. Rather than using a lock-in amplifier  200 , it is contemplated that one or more filters, including passive filters or the like, may be used to frequency separate the first, second, third and fourth detected power monitor signals from the electrical detection signal  183 .  
         [0061]    Rather than using a broad band detector  186 , it is contemplated that the optical transmitter  130  may include a detector similar to that described above with respect to FIG. 2 to provide an indication of the power level of selected wavelengths of light in the common optical output beam  182 . For example, if four electrical input signals  132 ,  134 ,  136  and  138  are provided, two of the electrical input signals  132  and  136  may be modulated using a first electrical power monitor signal and the remaining two electrical input signals  134  and  138  may be modulated using a second electrical power monitor signal. The first electrical power monitor signal may be at a different frequency than the second electrical power monitor signal. The four modulated electrical input signals may then be provided to the optoelectronic emitters  146 ,  156 ,  166  and  176  to produce four optical output signals. Like above, an optical combiner may be used to combine the four optical output signals into a common optical output beam.  
         [0062]    The detector may include, for example, a first absorbing layer, a second absorbing layer situated below the first absorbing layer, and an intermediate layer positioned between the first absorbing layer and the second absorbing layer. The first absorbing layer may absorb a different proportion of the each of the four optical output signals, and the second absorbing layer may absorb the remaining portion of each of the four optical output signals. When the power of any of the four optical output signals changes, the relative portions absorbed by the first absorbing layer and the second absorbing layer may also change. For example, if the power level of a first wavelength of light produced by the first optoelectronic emitter  146  decreases by ten percent, the overall light absorbed by the first absorbing layer may decrease more than the overall light absorbed by the second absorbing layer, particularly if the first absorbing layer absorbs more of the first wavelength of light.  
         [0063]    After the common optical output signal is detected by the detector, the electrical power monitor signals may be frequency separated from the detected signals by, for example, using lock-in amplifiers, band-pass filters, or any other method as desired. The first electrical power monitor signal, which was modulated and provided to optoelectronic emitters  146  and  166 , may be used to determine if the average power from optoelectronic emitter  146  and/or optoelectronic emitter  166  has changed, and by what amount. Likewise, the second electrical power monitor signal, which was modulated and provided to optoelectronic emitters  156  and  176 , may be used to determine if the average power from optoelectronic emitters  156  and/or  176  has changed, and by what amount. Thus, if one of the optoelectronic emitters experiences a change in output power, the first electrical power monitor signal and the second electrical power monitor signal may be used to identify which optoelectronic emitter pair ( 146 / 166  or  156 / 176 ) includes the optoelectronic emitter that produced the change in output power.  
         [0064]    To identify which optoelectronic emitter in the identified pair actually produced the change in output power, a ratio D 1 /D 2  of the detector output signals may be used. For example, and referring to the FIG. 7, if a signal ratio (D 1 /D 2 ) of 1.025 is detected, either optoelectronic emitter  146  (which corresponds to λ 1 ) has degraded sufficiently to cause the detected output power to fall to about 0.94% of full power, or optoelectronic emitter  156  (which corresponds to λ 2 ) has degraded sufficiently to cause the detected output power to fall to about 0.88% of full power. However, if it is already known from examining the first electrical power monitor signal and the second electrical power monitor signal that the optoelectronic emitter pair  146 / 166  produced the change in output power, it can be concluded that optoelectronic emitter  146  must have degraded. Thus, and as can be seen, a measure of the light absorption in the first absorbing layer and a measure of the light absorption in the second absorbing layer can be used to help provide an indication of the change in the power level of selected ones of the optoelectronic emitters. Once identified, a controller or the like can be used to adjust the voltage and/or current that is provided to the identified optoelectronic emitter.  
         [0065]    Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached.