Abstract:
A method of optimizing SBS suppression is disclosed for use in an optical communication system including a light guide for transmitting light and means for producing phase modulation of the light using at least first and second tones. In the method, an operational region of SBS suppression is established as a function of the phase modulation of the light such that the operational region identifies combinations of first and second phase modulation levels at which optimum SBS suppression is achieved for the first and second tones. Thereafter, based on the operational region, the first and second phase modulation levels are adjusted such that the system operates with optimum SBS suppression. In one aspect, a contour map and associated method are introduced. The contour map is especially suited for use in optimizing SBS suppression in an optical communication system in which light is phase modulated using at least first and second tones for transmission of the phase modulated light via media which exhibits an SBS threshold. In another aspect, one of the sub-regions identified by the contour map may be selected as the operating point of the system based on certain characteristics of the contour map at and around the location of the selected operating point. In this manner, for example, the stability of the optical communication system may be improved in the event that the selected operating point is subject to drift.

Description:
The present invention is related generally to the phenomenon of Stimulated Brillouin Scattering (hereinafter SBS) in the light guide of an optical communication system and more particularly to a method and associated apparatus for optimizing the SBS performance of such an optical communication system by using at least two phase modulation tones and identifying an advantageous operational region based on the phase modulation provided by the tones. A highly advantageous SBS contour map and its method of use are also disclosed. In addition, a modulated light producing arrangement manufactured in accordance with the teachings of the present invention is disclosed. 
     The phenomenon of SBS has been known in the prior art for a number of years. Essentially, SBS results when a threshold power level is exceeded within a sufficiently narrow frequency band in a fiber optic light guide. The problem of SBS has become significant with the development of lasers such as, for example, Single Longitudinal Mode lasers which readily provide an output that exceeds the SBS threshold (typically about 4 mW in, for example, a 50 kilometer fiber optic cable). Moreover, limitation of optical power to a level as low as 4 mW not only fails to utilize the output power available from state of the art lasers, but limits distance transmission through fiber optic cable by an unacceptable margin. Therefore, suppression of SBS has been contemplated in the prior art. 
     One effective method of overcoming the limitations imposed by SBS has been found to be the use of phase modulation. U.S. Pat. No. 4,560,246 broadly describes this technique. Indeed, the utilization of two modulation tones, specifically 2 and 6 GHz tones, to achieve the desired phase modulation is admitted prior art. In fact, as early as 1975, E. P. Ippen recognized in an article entitled non-linear effects in optical fibers that SBS is a limitation on narrow band transmission capabilities, and low SBS threshold can, in practice, be circumvented by the use of short pulses or broad band sources. However, as will be described below, the present invention recognizes a highly advantageous and heretofore unknown method of improving SBS performance for use with at least two phase modulation sources. The method may utilize a highly advantageous phase modulation contour map for the purpose of establishing certain operating parameters of an optical communication system. In addition, a light producing arrangement is disclosed which provides for implementation of the established operating parameters. 
     SUMMARY OF THE INVENTION 
     As will be described in more detail hereinafter, a method of optimizing SBS suppression is disclosed for use in an optical communication system including a light guide for transmitting light and means for producing phase modulation of the light using at least first and second tones. In the method, an operational region of SBS suppression is established as a function of the phase modulation of the light such that the operational region identifies combinations of first and second phase modulation levels at which optimum SBS suppression is achieved for the first and second tones. Thereafter, based on the operational region, the first and second phase modulation levels are adjusted such that the system operates with optimum SBS suppression. 
     In one aspect of the present invention, a contour map and associated method are introduced. The contour map is especially suited for use in optimizing SBS suppression in an optical communication system in which light is phase modulated using at least first and second tones for transmission of the phase modulated light via media which exhibits an SBS threshold. The contour map includes a first axis along which the phase modulation value produced by the first tone is plotted and a second axis along which the phase modulation value produced by the second tone is plotted. An SBS suppression value is assigned to each point within a region defined by the phase modulation values produced by the first and second tones such that sub-regions of SBS suppression are identifiable with the region. 
     In another aspect of the present invention, one of the sub-regions identified by the foregoing contour map may be selected as the operating point of the system based on certain characteristics of the contour map at and around the location of the selected operating point. In this manner, the stability of the optical communication system may be improved, for example, in the event that the selected operating point is subject to drift. 
     In yet another aspect of the present invention, a light producing arrangement for injecting light into a light guide which exhibits SBS is disclosed. The arrangement includes means for generating light and means for phase modulating the generated light for the purpose of suppressing SBS. The phase modulating means includes at least first and second tone generators, having first and second adjustable output power levels, respectively, which are adjustable in increments of less than 0.5 dBm. 
     In still another aspect of the present invention, a tone generator is provided which includes a feedback loop so as to substantially stabilize the output of the tone generator for use in phase modulating the light output of a laser. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below. 
     FIG. 1 is a block diagram illustrating an optical communication system designed in accordance with the present invention. 
     FIG. 2 is graphic, monochrome representation of a contour map produced in accordance with the method of the present invention for use in establishing a modulation operating point for the optical communication system depicted in FIG.  1 . 
     FIG. 3 is modified version of the contour map of FIG. 2 shown here to illustrate the effects of advancing the phase modulation as compared with FIG.  2 . 
     FIG. 4 is a block diagram illustrating a first embodiment of a two-tone generator for use in the optical communication system of FIG.  1 . 
     FIG. 5 is a block diagram illustrating a second embodiment of a two-tone generator for use in the optical communication system of FIG.  1 . 
     FIG. 6 is a schematic diagram generally illustrating a feedback module for use in the two-tone generator depicted by FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Attention is immediately directed to FIG. 1, which diagrammatically illustrates an optical communication system designed in accordance with the present invention and generally indicated by the reference numeral  10 . System  10  includes a continuous wave (hereinafter CW) laser  12  of any suitable type including but not limited to semiconductor distributed feedback, solid state and fiber lasers. CW laser  12  is controlled by a laser control unit  14  which compensates, as an example, for variations in laser power and ambient temperature. CW laser  12  preferably outputs a single longitudinal mode having a predetermined wavelength onto a fiber optic cable  16  which is, in turn, connected with an optical phase modulator  18 . The latter may employ, for example, a lithium niobate crystal which is not shown for purposes of simplicity. With appropriate electrical stimulation such a lithium niobate crystal is capable of modulating a beam of light which is passing through the crystal. Electrical stimulation is provided to the crystal in phase modulator  18  by a two-tone generator  20 . For the moment, it is sufficient to note that two-tone generator  20  includes a 2 GHz tone generator  20   a  and a 6 GHz tone generator  20   b . Detailed descriptions of several highly advantageous embodiments of generator  20  will be provided at appropriate points hereinafter. 
     Still referring to FIG. 1, the output of optical phase modulator  18  is provided via an optical cable  22  to an RF intensity modulator  24 . At the same time, an input signal is provided to an RF pre-distortion module  26 . In this instance, the input signal is a CATV signal, however, it is to be understood that any suitable input signal may be used including but not limited to analog and digital signals. Pre-distortion module  26  performs linearization functions on the CATV input signal and, thereafter, provides a conditioned CATV input signal to RF intensity modulator  24 . A modulator bias control  28  is also connected with the RF intensity modulator so as to enable amplitude modulation control of the conditioned CATV input signal onto the phase modulated signal which is received from optical phase modulator  18 . Thereafter, a phase and amplitude modulated optical signal is provided by the RF intensity modulator to an optical amplifier  30  which, in turn, drives an optical fiber  32  having a length in the range of approximately 40 kilometers to 65 kilometers. Fiber  32  typically exhibits an SBS threshold of approximately 4 milliwatts. In this regard, it should be appreciated that the present invention contemplates the use of any suitable optical light guide which exhibits SBS, either currently available or to be developed. Fiber  32  is terminated by an optical receiver which is not illustrated for purposes of simplicity. 
     Having provided a general description of the components which make up system  10 , attention is now directed to FIG. 2 in conjunction with FIG.  1 . FIG. 2 illustrates a modulation contour map generally indicated by the reference numeral  100  and developed in accordance with the method of the present invention, which method will be described at an appropriate point below. As will be seen, modulation contour map  100  is highly advantageous with regard to understanding the operation of system  10  in terms of the amount of phase modulation provided at the frequencies of tone generators  20   a  and  20   b.    
     Still referring to FIGS. 1 and 2, modulation contour map  100  includes a horizontal axis  102  along which the phase modulation provided by 2 GHz tone generator  20   a  is plotted in terms of the output power of the generator as measured in dBm (decibels referenced to 1 milliwatt). Similarly, modulation contour map  100  includes a vertical axis  104  along which the phase modulation provided by 6 GHz tone generator  20   b  is plotted in terms of its output power as measured in dBm. It should be appreciated that each tone generator provides a predetermined, known phase shift based on its input power to phase modulator  18 . Therefore, the scale on either axis could just as readily be indicated as phase modulation in radians, as opposed to tone generator output power. Alternatively, both phase modulation and tone generator output power may be indicated along the axes of modulation contour map  100 . 
     Axis  102  and  104  of modulation contour map  100  define a region of operation  106  for system  10 . Region  106  includes a plurality of SBS threshold levels  108  wherein each successive level represents a change in SBS threshold of 0.2 dB. One of ordinary skill in the art will appreciate that the SBS threshold level determines the maximum amount of power which may be transmitted down fiber optic member  32  without encountering the SBS phenomenon. Thus, it is desirable to operate at the highest possible SBS threshold level whereby power transmitted down the member may be maximized. It is noted that contour map  100  is a monochromatic representation of an actual color map which was produced in accordance with the present invention. The use of color in such contour maps is an effective technique for purposes of indicating the relative SBS threshold values of the various levels within the map. For example, the highest SBS threshold values may be indicated in red while the lowest values may be indicated in blue. In this manner, one is able to readily distinguish high values on top of “peaks” from low values within “valleys”. Because the present forum does not afford the luxury of illustrating contour map  100  in a color format, details regarding its “topography” will be provided immediately hereinafter. 
     Referring solely to FIG. 2, the lowest SBS thresholds of map  100  lie within a region  110  which is immediately adjacent the origin of the map. The highest SBS thresholds of the map lie within a region  112  at the upper right corner of the map. The difference in the SBS threshold between regions  110  and  112  is approximately 8 dB or a factor of approximately 6.3. An SBS threshold peak  114  is indicated within region  106 . Peak  114  is approximately 2.2 dB down from high SBS threshold region  112 . It is also of interest to note that a number of relatively broad plateaus are present within region  106  such as, for example, a plateau  116  which is approximately 2 dB down from high SBS threshold region  112 . The significance of these various features within region  106  will become more apparent within the context of the discussions which follow. 
     Having generally described the features of contour map  100  of the present invention, a description will now be provided with regard to the way in which the contour map may be used to select an operating point for system  10 . As mentioned above, several embodiments of tone generator  20  will be described herein. State of the art 2 GHz and 6 GHz tone generators are capable of providing adjustable output power up to approximately 33 and 30 dBm, respectively. Therefore, as can be seen from FIG. 2, the tone generators limit the region in which system  10  may operate to only a portion  118  of region  106  as indicated within dashed lines  120 . Removal of this operational constraint will be considered at an appropriate point below. It should be noted that tone generators  20   a  and  20   b  may be implemented by one having ordinary skill in the art. It is evident from the extent of portion  118  that high SBS region  112  is inaccessible using tone generators  20   a  and  20   b . However, SBS peak  114  lies within portion  118  along with part of plateau  116 . 
     It should be appreciated that prior to the development of the contour map of the present invention, appropriate output powers for tone generators  20   a  and  20   b  of two-tone generator  20  were established empirically. That is, the operating point was selected based on laboratory measurements. However, difficulties were encountered with regard to the sensitivity of systems to these adjustments. One difficulty resided in establishing the operating point using initial adjustments. Another difficulty related to an unacceptable number of instances where systems required re-adjustment at some point in time after the initial adjustments were performed. The reasons for these difficulties remained unknown. Based on new found knowledge, the present invention offers the ability to resolve these problems in a highly effective, yet straight forward manner, as will be seen. Moreover, the present invention contemplates the development of systems having a level of stability which has not previously been thought possible. 
     Referring again to FIG. 2, it has been discovered that the prior art “target” of the output power adjustments for the tone generators was, in fact, previously unknown SBS peak  114 . The latter lies at power outputs of approximately 23.75 dBm for the 2 GHz generator and 28.25 dBm for the 6 GHz tone generator. It can be seen that peak  114  resides at the end of a ridge  122 . More importantly, it can also be seen that below and to the left of peak  114 , the SBS threshold drops off in a very rapid manner. In fact, according to the figure, a drop in power output of 1 dBm from the level of peak  114  for each tone generator places the system at an operating point  124  which is nearly 2 dB in SBS threshold down from peak  114  or a factor of approximately 1.6. Thus, operating at peak  114  as compared with at point  124  results in a 60 percent increase in the amount of power which may be transmitted down a fiber optic light guide. Moreover, prior difficulties encountered in performing adjustment of tone generator power level output are now well understood in light of the present invention, as will be discussed immediately hereinafter. 
     Previously, power output was adjusted, for example, by using 0.5 dB attenuators. However, the FIG. 2 demonstrates that peak  114  is very narrow laterally (less than 0.5 dB in width). Therefore, using a 0.5 dB attenuator one could readily jump across the peak rather than centering the adjustment on the peak. The narrowness of peak  114  also serves to explain the aforementioned problem of systems requiring re-adjustment. In this regard, it should be appreciated that even a slight drift in the output power of a tone generator may result in a relatively large change in SBS threshold when a system is initially adjusted for operation on peak  114 . In view of this new information, tone generators have been developed for use in this application having significantly more stable power outputs which are adjustable in increments of less than 0.5 dB, as will be described at an appropriate point below. Using these new tone generators, adjustment of output power may be performed such that system  10  operates in a stable manner on peak  114 . 
     Still referring to FIG.  2  and in view of the foregoing discussion, one of ordinary skill in the art will appreciate that the operational stability of system  10  is determined based, at least in part, on the topography surrounding the operating point of the system. More specifically, operating points centered within relatively broad features of the map will inherently be more tolerant to drift of the operating point, thus providing a more stable SBS threshold. Therefore, due to the fact that peak  114  is a particularly sharp feature on the map, it is submitted that other operating points may prove to be more advantageous. For example, an operating point  126  within previously mentioned plateau  116  may prove to be advantageous. Operating point  126  is at approximately 30 dBm at 2 GHz and 31 dBm at 6 GHz. In fact, plateau  116  has an SBS threshold which is 0.2 dB above peak  114 . According to contour map  100 , operation at point  126  provides a highly advantageous improvement in the stability of the SBS threshold since a drift in tone generator output power of 0.5 dBm, within a dashed circle  128 , in any direction from point  126  results in no change in the SBS threshold. For this reason alone, it is submitted that the contour map of the present invention is highly advantageous. It should be mentioned that contour maps may readily be developed based upon tone generators which operate at frequencies other than those described herein. In this regard, the present invention remains applicable in future systems which may utilize other tones, for example, due to the need for higher data bandwidth. Moreover, three or more tones may be utilized in conjunction with a software implementation of the method of the present invention for purposes of identifying optimum operating points. It is noted that other concerns may arise as the spectrum of the optical signal is broadened. In fact, such concerns may be relevant even with the use of two tones at high levels of phase modulation. For example, one phenomenon of concern is that of frequency dispersion. The latter should be considered and managed appropriately if spectral broadening is to be employed on long distance fiber runs. 
     Having described the way in which the contour map of the present invention is used, one method of generating the contour map will now be described. Generally, the electric field intensity can be written as: 
     
       
           E=E   0   e   jω     0     t+jφ ,  (1) 
       
     
     where E 0  is the amplitude, ω 0  is the frequency of the optic carrier and φ is the phase. For two tone phase modulation at frequencies Ω 1  and Ω 2 : 
     
       
         φ=β 1  sin Ω 1   t +β 2  sin Ω 2   t,   (2) 
       
     
     where β 1  and β 2  are the modulation indexes at frequencies Ω 1  and Ω 2 , respectively. Using Equations 1 and 2, the electric field can be written as: 
     
       
           E=E   0   e   jω     0     t   e   jβ     1      sin Ω     1     t    e   jβ     2      sin Ω     2     t   (3) 
       
     
     By using Bessel expansions on the terms e jβ     1      sin Ω     1     t  and e jβ     2      sin Ω     2     t  we obtain:              E   =       E   0               j                   ω   0        t              ∑     n   =     -   ∞         +   ∞              ∑     k   =     -   ∞         +   ∞                J   n          (     β   1     )              J   k          (     β   2     )                   j        (       n                   Ω   1       +     k                   Ω   2         )          t                       (4)                                
     where J n (β 1 )and J k (β 2 ) are Bessel functions of order n and k, respectively. If we now choose, for example, Ω 2 =3Ω 1 , Equation 4 can be written as:                E   =       E   0               j                   ω   0        t              ∑     m   =     -   ∞         +   ∞                         [       ∑     k   =     -   ∞         +   ∞                J     m   -     3      k              (     β   1     )              J   k          (     β   2     )           ]                          j                 m                   Ω   1        t               ,           (5)                                
     where n from Equation 4 is chosen to be n=m−k. The normalized optical intensity is defined as:              I   =              E        2              E   0          2       .             (6)                                
     At an offset from carrier frequency ω 0  of mΩ 1 , using Equation 5:                  I   m     =       ∑     k   =     -   ∞         +   ∞                J     m   -     3      k       2          (     β   1     )              J   k   2          (     β   2     )             ,           (7)                                
     where m can take integer values from −∞ to +∞. The SBS suppression is determined by the maximum value of I m  which occurs for one value of m depending on the values of β 1  and β 2 . For example, at small values of β 1 , β 2 &lt;&lt;1, the maximum value of I m  occurs at m=0. As β 1  and β 2  increase, the maximum value of I m  will move from m=0 to m=1 and further move to m=2 and to other, higher values of m. In the present circumstances with n=m−3k, Bessel functions J n (β 1 ) and J k (β 2 ) for n and k greater than 10, have a very small value and, thus, may be ignored. Therefore, I m  is calculated by limiting k and n=m−3k to the range of −10 to +10 since this range has been found to yield sufficient accuracy. Moreover, I m  is symmetric with respect to m around m=0, i.e., I m =I m . Thus, to find the maximum I m , we need only consider m=0 to 10. When the maximum value, I m,MAX  is found, the SBS supression in decibels is given as: 
       S=− 10 log 10    I   m,MAX  [dB]  (8) 
     The numerical process used to produce contour plot  100  includes calculating I m  for m=0 to 10 for given values of β 1  and β 2  using Equation 7 where k and m−3k range from −10 to +10. The values of I m  for m=0 to 10 are then compared to find the maximum value which becomes I m,MAX . The log value is then plotted using Equation 8. 
     Turning again to FIG. 2, irrespective of the limitations imposed by the phase modulation capabilities of system  10 , the optimum operating point for the system would reside at the upper right hand corner of contour map  100 , for example, at an operating point  130 . The latter includes an SBS threshold that is 1.8 dB higher than the threshold at plateau  116  which would result in an increase in power down an optical fiber of approximately 1.5, a level which is 50% more power than that which is available at plateau  116 . Presently, operating point  130  is beyond the modulation capabilities of commercially practical systems. Alternatively, operating point  126  is out of the power range of current 6 GHz tone generators by approximately 0.5 dB, but is, however, within the range of current 2 GHz tone generators. Therefore, it is suggested that, with the development of an appropriate 6 GHz tone generator, operating point  126  may serve as an interim operating point using a modified modulation arrangement prior to operating at point  130 . However, it is contemplated that hardware having modulation capabilities placing region  112  within reach will be implemented, particularly in view of the teachings herein. The effect of the development of such a new modulator arrangement on the modulation contour map of the present invention will be described immediately hereinafter. 
     Turning to FIGS. 2 and 3, a contour map  150  is shown which contemplates the development of a modulator arrangement (not shown) that provides significantly higher modulation levels than state of the art modulator arrangements. The effect of the higher modulation levels would essentially be to shift the features of modulation contour map  100  downward and to the left. As points of reference, plateau  116  and operating point  126  are denoted in modulation contour map  126 . The calculation algorithm was terminated for values in region  112 . Even higher levels of SBS suppression exist beyond this region, but are not shown here for purposes of clarity. 
     Attention is now directed to FIG. 4 which illustrates a first embodiment of two-tone generator  20  including 2 GHz generator  20   a  and 6 GHz generator  20   b . A reference oscillator  170  generates a 10 MHz reference signal which is provided to a 2 GHz phase locked loop (hereinafter PLL) synthesizer  172  and to a 6 GHz PLL synthesizer  174 . The synthesizers generate their designated output frequencies and provide outputs to amplifiers  176  and  178 , as indicated. Amplifiers  176  and  178 , in turn, provide outputs to RF attenuators  180  and  182 . The attenuators are configured for providing attenuation of the outputs of amplifiers  176  and  178  in steps of less than 0.5 dB. Preferably, attenuation steps of approximately 0.1 dB are provided. Attenuators  180  and  182  provide attenuated outputs to an RF signal combiner  184  which, in turn, provides a combined RF output to phase modulator  18  (see FIG.  1 ). While two-tone generator  20  may readily be provided by one having ordinary skill in the art, it is important to mention that design practices should be employed wherever possible which ensure a stable output with regard to temperature changes and output drift over time. 
     Turning to FIG. 5, a second embodiment of a two-tone generator is indicated by the reference numeral  20 ′. Because generator  20 ′ includes certain components used in generator  20 , like reference numbers have been applied wherever possible and the reader is referred to previous descriptions of these components. The 2 GHz and 6 GHz tone generators in this second embodiment are referred to by reference numbers  20   a ′ and  20   b ′. Generator  20 ′ includes reference oscillator  170 , 2 GHz PLL synthesizer  172 , 6 GHz PLL synthesizer  174  and RF combiner  184 . The 2 GHz PLL synthesizer provides an output to an amplifier  200  while the 6 GHz PLL synthesizer provides an output to an amplifier  202 . An output of amplifier  200  is provided to an RF tap  204 . Similarly, an output of amplifier  202  is provided to an RF tap  206 . In accordance with the present invention, feedback modules A and B receive respective portions of the outputs of amplifiers  200  and  202  via the RF taps. Amplifiers  200  an  202  each include a feedback input  208  which is connected with a respective one of the feedback modules. Feedback levels provided to the modules are adjustable using a biasing arrangement which is connected with feedback modules A and B. The biasing arrangement associated with each feedback module includes a resistor R 1  connected at one end with a voltage reference designated as “REF” and connected its other end with an variable resistor R 2 . The latter is, in turn, connected with one of the feedback modules such that varying R 2  results in a variable DC bias provided to the feedback module. Outputs provided by RF taps A and B are combined by combiner  184 . The combined RF signal is then provided to phase modulator  18  (see FIG.  1 ). 
     Still referring to FIG. 5, it should be appreciated that tone generator  20 ′ optimizes the 2 GHz and 6 GHz power levels continuously by tapping off the signal powers after the amplifiers, detecting the amplitude of the signals and comparing them to a fixed pre-determined optimum reference level to generate an error signal that is fed back to the RF amplifiers in order to adjust their gains and, thereby, their output powers. This feedback configuration insures that the 2 GHz and 6 GHz power levels remain substantially constant over temperature and time. Moreover, the output power of each tone generator is variable with a resolution of 0.1 dB or better. It is submitted that the implementation of a tone generator in a feedback configuration is highly advantageous and has not been seen in the present application. A specific implementation of the feedback modules will be described immediately below. 
     Referring to FIG. 6, a feedback module designed in accordance with the present invention is generally indicated by the reference numeral  240 . Feedback module  240  includes an RF amplifier  242 , a first buffer amplifier  244 , a second buffer amplifier  246  and a comparator amplifier  248 . Module  240  further includes an RF detector diode D 1  which produces a voltage proportional to the RF level coupled to D 1  through C 2  as representing a sample of RF output  250  produced by RF amplifier  242 . The voltage produced by D 1  is filtered through inductor L 1  and buffered by second buffer amplifier  246 . An adjustable preset voltage, V SET , is produced using a reference voltage V 2  and a potentiometer R 20 . V SET  is buffered by first buffer amplifier  244 . The buffered voltages of the first and second buffer amplifiers are then compared by comparator amplifier  248  to produce an error voltage V ERR . The latter is then applied to a gain or output level control  252  of amplifier  242 . In this way, the RF power output of amplifier  242  is regulated by the level of V SET . Without such regulation, for example, the output of amplifier  242  is assumed to be prone to drift over excursions of temperature, power supply voltages or with aging. 
     It should be appreciated that the concepts of the present invention, as taught herein, may be applied in a number of different ways by one of ordinary skill in the art. As an example, the teachings of the present invention are equally applicable to any laser implementations which may be developed providing for direct modulation of the laser&#39;s output at the tones/frequencies contemplated herein. Therefore, the present examples and method are considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.