Abstract:
In an optical grating device, a grating arrangement receives different wavelength output signals from a plurality of radiation sources at input ports thereof, and generates therefrom a multiplexed wavelength output signal at a zero diffraction order output port of the grating arrangement. Additionally, the gating arrangement generates at least one predetermined wavelength output signal at one of a group consisting of a separate predetermined location in an at least one of a symmetric non-zero diffraction order of the grating arrangement, within the grating arrangement itself, and a combination thereof. A separate power tap is coupled to detect the power of a separate one of the at least one predetermined wavelength output signal from the grating arrangement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Ser. No. 10/609,857, now U.S. Pat. No. 6,937,795 which is entitled COMBINATION WAVELENGTH MULTIPLEXER AND WAVELENGTH STABILIZER” (Optovia 4), has a common assignee and two common inventors with the present application and is being filed concurrently with the present application. 
     FIELD OF THE INVENTION 
     The present invention relates to devices as, for example, transmitters and wavelength multiplexers and demultiplexers that utilize gratings (such as diffraction gratings, array waveguide gratings (AWG), etc.) wherein one or more proportional taps are needed for obtaining input and/or output power. 
     BACKGROUND OF THE INVENTION 
     Many prior art devices requiring power taps utilize couplers to tap power off of the input and/or output signal power of the device. More recently, other arrangements have been devised to provide optical power taps. 
     U.S. Pat. No. 5,748,815 (Hamel et al.), issued on May 5, 1998, discloses an optical component adapted to monitor a multi-wavelength link for use as an add-drop multiplexer in optical networks. The optical component includes an input optical fiber for launching a multiplexed signal comprising a plurality of different wavelength signals from a zero diffraction order of a flat blazed grating towards the grating for dispersing the wavelength signals in several non-zero orders of diffraction. A first set of optical fibers is located within a first non-zero diffraction order area (e.g., the 1 order) of the grating where each optical fiber of the first set is located to receive a separate one of the dispersed and demultiplexed wavelength signals. A second set of optical fibers is located within a second higher non-zero diffraction order area (e.g., the 2 order) of the grating. Each optical fiber in this set is located to receive a separate one of the dispersed and demultiplexed wavelength signals and transmit the wavelength signal to a separate one of a plurality of photodetectors for ascertaining the average power level coming out of each of these optical fibers. The Hamel et al. optical component is directed only for receiving a multiplexed wavelength input signal and obtaining therefrom demultiplexed wavelength signals within one diffraction area of a grating while concurrently detecting the power of the received demultiplexed wavelength signals within a second diffraction area of the grating. Wavelength drift can also be determined from the demultiplexed wavelength signals. The optical component does not provide for determining overall multiplexed signal power. 
     U.S. Patent Application Publication No. US 2002/0057875 A1 (Kaneko), published on May 16, 2002, discloses arrangements of an arrayed waveguide grating (AWG), optical transmitter, and optical communication system including a monitoring function for a main signal. The AWG comprises an input slab waveguide, an output slab waveguide, and a channel waveguide array having waveguides of progressively increasing lengths interconnecting the input and output slab waveguides. A first plurality of input waveguides are coupled to the input slab waveguide, each waveguide being used to launch a separate wavelength signal into the input slab waveguide. In passing through the input slab waveguide, the channel waveguide array, and the output slab waveguide, the launched waveguide signals are multiplexed and recovered at a zero diffraction order output in an output optical fiber. First and second mirrors are located to intercept multiplexed waveguide signals appearing in the first diffraction order beams of the output slab waveguide. Each mirror redirects the intercepted multiplexed waveguide signals back through the output slab waveguide, the channel waveguide array, and the input slab waveguide to demultiplex the waveguide signals for a second time. In the input waveguide slab, the waveguide signals are angularly dispersed to appear at separate locations on the input side of the input slab waveguide that do not coincide with the locations of the first plurality of input waveguides. A first and a second set of a plurality of monitoring waveguides are each disposed at separate locations of the input slab waveguide so that each waveguide of the set receives a separate one of the demultiplexed wavelength signals for monitoring purposes. In a second embodiment, a feedback loop is connected to intercept the first order diffraction beams at the output slab waveguide instead of at the first and second mirrors and to feed the multiplexed signals back through the (AWG). This arrangement is somewhat inefficient because the waveguide signals propagate twice through the AWG and introduces losses to each of the wavelength signals for each pass therethrough. 
     It is desirable to provide a more efficient grating based wavelength multiplexer/demultiplexer that provides wavelength power taps after only one pass through a grating device, and additionally, if desired, a total power tap. Since a multiplexed signal tap by definition has more power than the individual wavelengths that form the multiplexed signal, higher grating losses are generally acceptable for tapping the total power. 
     SUMMARY OF THE INVENTION 
     The present invention relates to optical grating based devices where wavelength signals generated by at least two light sources are multiplexed for transmission as an output signal from the system along with power taps for overall signal power and/or individual output wavelength powers that are separated within a grating based device. 
     From a first apparatus aspect, the present invention is an optical grating device comprising a grating arrangement, and at least one power tap. The grating arrangement receives a different wavelength signal from each of a plurality of radiation sources at separate input ports thereof, and generates therefrom an output signal of multiplexed wavelengths at a zero diffraction order output port of the grating arrangement. The grating arrangement further generates at least one predetermined wavelength output signal at one of a group consisting of a separate predetermined location in an at least one of a symmetric non-zero diffraction order of the grating arrangement, within the grating arrangement itself, and a combination thereof. Each power tap is coupled to the grating arrangement to detect the power of a separate one of the at least one predetermined wavelength output signal therefrom. 
     From a second apparatus aspect, the present invention relates to an optical grating device comprising a grating arrangement, and a plurality of power taps. The grating arrangement receives sufficiently separated wavelength signals from a plurality of radiation sources at separate input ports thereof, and generates therefrom a multiplexed wavelength output signal at a zero diffraction order output port of the grating arrangement, and separate ones of each of the sufficiently separated different wavelength output signals from the plurality of radiation sources that are focused at separate predetermined spaced apart locations within both areas of a set of predetermined symmetric non-zero diffraction orders of the grating arrangement. Each of the plurality of power taps is coupled to detect the power of a separate one of the sufficiently separated wavelength output signals from the plurality of radiation sources that are focused at the separate predetermined spaced apart locations within both of the predetermined symmetric 
     From a third apparatus aspect, the present invention relates to an optical grating device comprising a grating arrangement, and first and second power taps. The grating arrangement receives each of densely spaced apart wavelength output signals from a plurality of radiation sources at separate input ports thereof, and generates therefrom a multiplexed wavelength output signal at a zero diffraction order output port of the grating arrangement, and first and second multiplexed wavelength output signals of the densely spaced apart wavelength output signals that are focused at separate predetermined locations within first and second ones of a predetermined symmetric non-zero diffraction order, respectively, of the grating arrangement. The first and second power taps are coupled to detect the power of the first and second multiplexed wavelength output signals focused within the first and second ones of the predetermined symmetric non-zero diffraction order, respectively, of the grating arrangement. 
     From a fourth apparatus aspect, the present invention relates to an optical grating device comprising a grating arrangement, and a plurality of power taps. The grating arrangement receives a different wavelength output signal from each of a plurality of radiation sources at separate input ports thereof, and generates therefrom a first multiplexed wavelength output signal at a zero diffraction order output port of the grating arrangement. The grating arrangement comprises a feedback loop for coupling a second multiplexed wavelength output signal appearing at a non-zero diffraction order output port of the grating arrangement back into the grating arrangement adjacent the zero diffraction order output port so that each of the different wavelength output signals in the second multiplexed output signal are demultiplexed and focus to separate input ports of the grating arrangement that are not coincident with the input ports receiving the wavelength signals from the plurality of radiation sources. Each of the plurality of power taps are coupled to the grating arrangement to detect the power of a separate one of the different wavelength output signals focused at said separate input ports thereof. 
     From a fifth apparatus aspect, the present invention relates to an optical grating device comprising a grating arrangement, and a first power tap. The grating arrangement is an Arrayed Waveguide Grating (AWG) comprising a first Free Propagating Region (FPR), a second FPR comprising an input and an output side thereof, and a grating array, and a first power tap. The first Free Propagating Region (FPR) receives a different wavelength output signal from each of a plurality of radiation sources at separate input ports at an input side thereof. The grating array is formed from a plurality of different length optical waveguides that couple an output side of the first FPR to the input side of the second FPR. The grating array generates both a first multiplexed wavelength output signal from the different wavelength output signals received from the plurality of radiation sources at a zero diffraction order of the second FPR, and a second multiplexed wavelength output signal at a first predetermined waveguide forming the plurality of different length optical waveguides of the grating array. The first power tap is coupled to receive the multiplexed wavelength output signal from said predetermined waveguide forming the plurality of different length optical waveguides of the grating array. 
     From a sixth apparatus aspect, the present invention relates to a combination comprising optical grating means, main output means, and auxiliary output means. The optical grating means includes input means for receiving signal power of different optical wavelengths from each of a plurality of radiation sources and output means for providing one of a group consisting of multiplexed power and individual power of the different optical wavelengths, the output means including a zero-order diffraction area and at least one non-zero order diffraction area. The main output means is coupled to the zero-order diffraction area for abstracting said multiplexed power in an output signal of the optical grating means. The auxiliary output means is coupled to the at least one of non-zero diffraction area for abstracting power of one of the group consisting of the multiplexed power and individual powers of the different optical wavelengths. 
     The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a grating based device including individual wavelength optical power taps in accordance with a first embodiment of the present invention; 
         FIG. 2  is an exemplary enlarged portion of a free propagation region (FPR) and individual wavelength power taps obtained therefrom for the grating based device of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a grating based device including power taps in accordance with a second embodiment of the present invention; 
         FIG. 4  is a graph of wavelength on the X-axis versus transmission (dB) on the Y-axis for exemplary multiplexed wavelength signal outputs that might be found at outputs of a predetermined design of the grating based device of  FIG. 3 ; 
         FIG. 5  is a general schematic diagram of a grating based device in accordance with a third embodiment of the present invention; and 
         FIG. 6  is a general schematic diagram of a grating based device in accordance with a fourth embodiment of the present invention. 
     
    
    
     The drawings are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , there is shown a general schematic diagram of a grating based device  10  including a plurality of radiation sources (shown as, and referred to hereinafter as, lasers  12   a  (LASER  1 ),  12   b  (LASER  2 ), and  12   c  (LASER  3 )), an Arrayed Wavelength Grating (AWG) Multiplexer (MUX  14  (shown within a dashed line area), individual wavelength optical taps  16 , and an optional control device  18  in accordance with a first embodiment of the present invention. The AWG MUX  14  comprises a first Free Propagation Region (FPR)  14   a , a second FPR  14   b , and an optical grating section  14   c  formed from a plurality of predetermined different length waveguides. 
     Each of the plurality of lasers  12   a ,  12   b , and  12   c  generates an output signal with a predetermined wavelength different from that of the other lasers. The output wavelength signals from the lasers  12   a ,  12   b , and  12   c  have their wavelengths widely spaced apart (sufficiently separated) from each other, and are coupled to appropriate separate inputs of the first FPR  14   a . The first and second FPRs  14   a  and  14   b  and the optical grating section  14   c  of the Array Waveguide Grating (AWG)  14  utilize the interference of phase shifted signals to multiplex or demultiplex optical signals as is well known in the art. In the typical case of an AWG multiplexer, multiple input signals from the lasers  12   a ,  12   b , and  12   c  are individually positioned on a waveguide slab of the first FPR  14   a  in such a way that after propagating in turn through the waveguide slab of FPR  14   a , the optical grating section  14   c , and a waveguide slab of the second FPR  14   b , the location of the focus point of the zeroth diffraction order (m=0) of each input wavelength signal coincides with the location chosen for the output waveguide  15 . Most of the power of the input signals from the lasers  12   a ,  12   b , and  12   c  is found in the zeroth diffraction order (m=0), but some power resides in non-zero diffraction orders of m=−1, m−+1, m=−2, m=+2, etc, which power decreases as the diffraction order numbers increase. 
     A property of an array waveguide diffraction grating (AWG) is that spacings (hereinafter designated “a”) of array waveguides of the optical grating section  14   c  and the wavelength (hereinafter designated “w”) of a channel from each of the lasers  12   a ,  12   b , and  12   c  determine the location of the focal point of non-zero diffraction orders at the output of the slab of FPR  14   b . This is shown in typical bulk grating equations, such as that shown in equation 1 [Eq. (l)].
 
( m )( w )= a (sin θ i +sin θ m )  Eq. (1)
 
where m=diffraction order, w=wavelength, a=grating spacing, θi=incident angle, and θm=diffraction angle. Additionally, each non-zero diffraction order experiences an angular dispersion. For bulk gratings, this relationship is given by equation 2 [Eq. (2)].
 
Angular Dispersion= m/a  cos θ m=dθm/dw   Eq. (2)
 
     As described by equations (1) and (2), the higher diffraction orders more efficiently separate the individual wavelengths of the lasers  12   a ,  12   b , and  12   c . However, except for blazed gratings, as the diffraction order increases, the intensity of its output signal decreases. The description hereinafter will be directed particularly to a first symmetric non-zero diffraction order (m=+1, m=−1) when referring to non-zero diffraction orders. However, the present concept extends to the higher diffraction orders as well when used in place of the first symmetric non-zero diffraction order, when wavelength channel resolution is not easily achieved within the first non-zero diffraction order area. Equations 1 and 2 can be found on pages 421 and 415, respectively, of the book entitled “Introduction To Optics” by Frank L. Pedrotti and Leno S. Pedrotti, published by Prentice Hall 1987. 
     For densely spaced apart wavelength channels from the lasers  12   a ,  12   b , and  12   c , the shift in focus point for individual channel wavelengths for low diffraction orders is minor (and is zero for m=0). However, for widely spaced apart (coarsely spaced, sufficiently separate) wavelength channels, the optical grating section  14   c  can be designed so that the non-zero diffraction orders both focus and sufficiently resolve individual wavelength channels so that the focused channel signals can be directed into separate individual output waveguides. As is shown in  FIG. 1 , the individual optical power taps  16  (as shown in greater detail in  FIG. 2 ) of the first embodiment of the present invention are coupled to separate outputs that are focused in an localized area of one or more of the symmetric non-zero diffraction orders of the FPR  14   b  to receive the wavelength channel light signals from separate ones of the lasers  12   a ,  12   b , and  12   c  (as will be described in more detail hereinafter with reference to  FIG. 2 ). Each optical power tap  16  detects the intensity of the received light signal and generates an output signal that is a measure of the power in the wavelength signal from the associated one of the plurality of lasers  12   a ,  12   b , and  12   c . The output signals from the optical power taps  16  can be transmitted to either the optional control device  18  for generating output control signals to each of the lasers  12   a ,  12   b , and  12   c  when a power adjustment is required, or to another suitable termination (not shown) such as an automatic alarm or monitoring system that can be activated for subsequent manual intervention when required. Each power tap  16  typically includes an optical waveguide or a free space optical path and can direct the optical power to devices such as photodetectors and an optical spectrum analyzer. 
     Referring now to  FIG. 2 , there is shown an exemplary enlarged portion of the free propagation region (FPR)  14   b  of  FIG. 1  and its outputs  14   a – 14   g  when lasers  12   a ,  12   b , and  12   c  (shown in  FIG. 1 ) generate widely spaced apart (sufficiently separated) wavelength channels in accordance with the first embodiment of the present invention. The location of output port  14   d  of the FPR  14   b  corresponds to the location of the focus point of the zeroth diffraction order (m=0) of the FPR  14   b  and provides the multiplexed output wavelength channels of the lasers  12   a ,  12   b , and  12   c  as the output port  15  from the grating based device  10  of  FIG. 1 . The first FPR  14   a , second FPR  14   b , and the optical grating section  14   c  (shown in  FIG. 1 ) are designed to resolve the wavelength signal output signal from lasers  12   a ,  12   b , and  12   c  (shown in  FIG. 1 ) at, for example, separate distinct locations within each of the m=−1 and the m=+1 diffraction order areas of the FPR  14   b . Where the channels from the lasers  12   a ,  12   b , and  12   c  cannot be resolved sufficiently in the m=−1 and/or m=+1 diffraction order areas, such signals may be resolved in a higher non-zero diffraction order as, for example, m=−2 and m=+2, m=−3 and m=+3, etc. When a higher diffraction order is used, less resolved power will be found in the signals from the lasers  12   a ,  12   b , and  12   c . It is assumed hereinafter that the wavelength channels from the lasers  12   a ,  12   b , and  12   c  are separately adequately resolvable within both the m=+1 and m=−1 diffraction order areas of the FPR  14   b.    
     As is seen in  FIG. 2 , first optical power taps  16   a  and  16   e  are coupled to output ports  14   a  and  14   e , respectively, within the respective m=−1 and m=+1 diffraction order areas of the FPR  14   b . The output ports  14   a  and  14   e  are positioned at the output side of the FPR  14   b  for receiving the resolved angular dispersion and diffracted wavelength (w 1 ) signal from the first laser  12   a . Second optical power taps  16   b  and  16   f  are coupled to output ports  14   b  and  14   f , respectively, within the respective m=−1 and m=+1 diffraction order areas of the FPR  14   b  for receiving the resolved angular dispersion and diffracted wavelength signal (w 2 ) from the second laser  12   b . Third optical power tap  16   c  and  16   g  are coupled to output port  14   c  and  14   g , respectively, within the respective m=−1 and m=+1 diffraction order areas of the FPR  14   b  for receiving the angular dispersion and diffracted wavelength signal (w 3 ) from the third laser  12   c . Because of symmetry, the power in each of the wavelength signals (w 1 , w 2 , and w 3 ) from the lasers  12   a ,  12   b , and  12   c  obtained at the output of the FPR  14   b  can be measured using just the power taps  16   a ,  16   b , and  16   c , respectively, at the m=−1 diffraction order. Similarly, the power in each of the wavelength signals (w 1 , w 2 , and w 3 ) from the lasers  12   a ,  12   b , and  12   c  obtained at the output of the FPR  14   b  can be measured using just the power taps  16   e ,  16   f , and  16   g , respectively, at the m=+1 diffraction order. Alternatively, the power in each of the wavelength signals (w 1 , w 2 , and w 3 ) from the lasers  12   a ,  12   b , and  12   c  obtained at the output of the FPR  14   b  can be measured using the power taps  16   a  and  16   e ,  16   b  and  16   f , and  16   c  and  16   g , respectively, at both the m=−1 and m=+1 diffraction orders. In either case, the measured power of the separate wavelength signals from the lasers  12   a ,  12   b , and  12   c  can be used as needed. For example, the measured power can be either used directly (e.g., when obtained from one diffraction order m=−1 or m=+1), or further analyzed (e.g., when obtained from either one or both of the symmetric diffraction orders m=−1 and m=+1). 
     Referring now to  FIG. 3 , there is shown a schematic diagram of a grating based device  20  including a plurality of radiation sources (shown as lasers  22   a  (LASER  1 ),  22   b  (LASER  2 ), and  22   c  (LASER  3 )), an Arrayed Wavelength Grating (AWG) Multiplexer/Demultiplexer (MUX/DEMUX)  24  (shown within a dashed line area), and overall optical power taps  26   a  and  26   b  positioned in the m=−1 and m=+1 diffraction order areas, respectively, of the AWG MUX/DEMUX  24  in accordance with a second embodiment of the present invention. The AWG MUX/DEMUX  24  comprises a first Free Propagation Region (FPR)  24   a , a second FPR  24   b , and an optical grating section  24   c  formed from a plurality of predetermined different length waveguides. Each of the plurality of lasers  22   a ,  22   b , and  22   c  is arranged to generate an output signal with a predetermined wavelength different from each of the other lasers where the wavelengths of the lasers  22   a ,  22   b , and  22   c  are relatively densely spaced apart. The output signals from each of the lasers  22   a ,  22   b , and  22   c  are coupled to separate appropriate inputs of the first FPR  24   a.    
     The first and second FPRs  24   a  and  24   b  and optical grating section  24   c  of the Array Waveguide Grating (AWG)  24  utilize the principle of interference of phase shifted signals to multiplex or demultiplex optical signals as is well known in the art. In the case of the AWG multiplexer  24 , multiple input signals from the lasers  22   a ,  22   b , and  22   c  are individually positioned on the first waveguide slab of the first FPR  24   a  in such a way that after propagating through each of the first waveguide slab, the optical grating section  24   c , and the second waveguide slab of the second FPR  24   b , the location of the focus point of the zeroth diffraction order (m=0) of all input wavelength signals coincides with the location of the output waveguide  25 . Most of the power of the multiplexed input signals from the lasers  22   a ,  22   b , and  22   c  is found in the zeroth diffraction order (m=0), but some power of the multiplexed input signals resides in non-zero symmetric diffraction orders of m=−1 and m=+1, m=−2 and m=+2, etc, which power decreases as the diffraction order numbers increase. Because of the relative dense spacing of the wavelengths of the laser  22   a ,  22   b , and  22   c , the individual wavelengths from the lasers  22   a ,  22   b , and  22   c  are not adequately resolved to separate locations in the non-zero symmetric diffraction orders of m=−1 and m=+1 in contrast to the case for the coarsely spaced wavelengths in the grating based device  10  of  FIG. 1 . Depending on the design of the AWG  24  and the wavelength range of the signals involved, the signals from the overall power taps  26   a  and  26   b  have to be further analyzed for reasons described hereinbelow in association with  FIG. 4 . 
     Referring now to  FIG. 4 , there is shown a graph of wavelength (nm) on the X-axis versus transmission power (dB) on the Y-axis for multiplexed wavelength signal outputs depending on exemplary ranges of wavelength outputs of the lasers  22   a ,  22   b , and  22   c  that might be found for a predetermined design of the grating based device  20  of  FIG. 3 . The graph of  FIG. 4  illustrates the effect of angular dispersion and the need for analysis of the power data detected by the overall power taps  26   a  and  26   b  of the grating based device  20  of  FIG. 3 . Each curve  30  within a particular wavelength range (e.g., 1510–1525 nm, or 1520–1535 nm) represent the power in a multiplexed output signal from the lasers  22   a ,  22   b , and  22   c  of  FIG. 3  having a wavelength range that is centered at 1545 nm and obtained at the m=0 diffraction order location (at waveguide  25  of  FIG. 3 ) for an exemplary designed AWG  24 . Each of the curves  31  within a particular wavelength range (e.g., 1510–1525 nm, 1520–1535 nm, etc.) represents the power that would be obtained in the multiplexed signal at the m=+1 diffraction order location (at power tap  26   b  of  FIG. 3 ) when the lasers  22   a ,  22   b , and  22   c  of  FIG. 3  generate their different wavelength output signals which are centered within the indicated range. Each of the curves  32  within a particular wavelength range (e.g., 1510–1525 nm, 1520–1535 nm, etc.) represent the power that would be obtained in the multiplexed signal at the m=−1 diffraction order location (at power tap  26   a  of  FIG. 3 ) when the lasers  22   a ,  22   b , and  22   c  of  FIG. 3  generated their different wavelength output signals within the indicated range. 
     The exemplary data shown in the graph are for the grating based device  20  having an exemplary design to cover predetermined input wavelengths between 1510 nm and 1570 nm. In this design, the center wavelength of 1540 nm within the 1510–1570 nm design range is where the curves  30 ,  31 , and  32  will be found to be centered on each other. When the center wavelength of a range of wavelengths of the lasers  22   a ,  22   b , and  22   c  deviates from the center wavelength 1540 nm, angular dispersion causes the associated curves  31  and  32  are no longer centered on each other, and they become separated by increasing distances as the center wavelength of each wavelength range increases in separation from the 1540 nm wavelength. For example, the curves  31  and  32  within the wavelength range of 1510–1525 nm have their peaks separated by a greater distance that the curves  31  and  32  within the wavelength range of 1520–1535 nm. The occurrence of increasing separations of the curve  31  and  32  is mirrored on the opposite higher side of the center wavelength of 1540 nm as can be seen by the curves  31  and  32  within the wavelength range of 1550–1565 nm. Therefore, if only a single overall power tap (e.g.,  26   a  or  26   b ) were positioned at either the m=−1 or the m=+1 location, as is generally found in the prior art, an accurate reading of the overall power would only be obtained only when the lasers  22   a ,  22   b , and  22   c  generate a wavelength range that is centered at 1540 nm. The prior art does not consider the effect of angular dispersion when the input wavelength range (e.g., from any of lasers  22   a ,  22   b , and  22   c ) deviates from a wavelength range centered at 1540 nm. By using two overall power taps  26   a  and  26   b  located at both the m=−1 and m=+1 diffraction order locations, respectively, and performing a data analysis utilizing both readings, a more accurate overall power reading is obtainable within the design range of the grating based device  20 . The data analysis of concurrent readings from the overall power taps  26   a  and  26   b  can be performed by any suitable data processing method as, for example, averaging, weight averaging, etc. to obtain an overall power reading. The use of overall power taps  26   a  and  26   b  permits a grating based device to be used with any input wavelength range within the grating design, and obtain more accurate overall power readings than would be found by using only overall power tap. 
     Referring now to  FIG. 5 , there is shown a general schematic diagram of a grating based device  40  including a plurality of radiation sources (shown as, and referred to hereinafter as, lasers  42   a  (LASER  1 ),  42   b  (LASER  2 ), and  42   c  (LASER  3 )), an Arrayed Waveguide Grating (AWG) Multiplexer/Demultiplexer (MUX/DEMUX)  44  (shown within a dashed line area) including a feedback loop  47 , and individual wavelength optical taps  46   a ,  46   b , and  46   c  in accordance with a third embodiment of the present invention. The AWG MUX/DEMUX  44  comprises a first Free Propagation Region (FPR)  44   a , a second FPR  44   b , and an optical grating section  44   c  formed from a plurality of predetermined different length waveguides. 
     Each of the plurality of lasers  42   a ,  42   b , and  42   c  are arranged to generate an output signal with a wavelength different from that of the other lasers. The output wavelength signals from the lasers  42   a ,  42   b , and  42   c  are assumed to be relatively densely spaced apart, and are coupled to separate inputs of the first FPR  44   a . The first and second FPRs  44   a  and  44   b  and the optical grating section  44   c  of the Array Waveguide Grating (AWG)  44  utilize the principle of interference of phase shifted signals to multiplex or demultiplex optical signals, as is well known in the art. In the typical case of an AWG multiplexer, multiple input signals from the lasers  42   a ,  42   b , and  42   c  are individually positioned on a first waveguide slab forming the first FPR  44   a  in such a way that after propagating, in turn, through the first waveguide slab, the optical grating section  44   c , and a second waveguide slab forming the second FPR  44   b , the location of the focus point of the zeroth diffraction order (m=0) of all input wavelength signals coincides at the location of the output waveguide  45 . Because of the dense spacing of the wavelengths from the lasers  42   a ,  42   b , and  42   c , the individual wavelengths from the lasers  42   a ,  42   b , and  42   c  are not adequately resolved to separate locations in the non-zero symmetric diffraction orders of m=−1 and m=+1, m=−2 and m=+2, etc. as was in the case of the coarsely spaced wavelengths in the grating based device  10  of  FIG. 1 . 
     To better resolve the distinct wavelength signals from the lasers  42   a ,  42   b , and  42   c  to individual wavelength power taps, a feedback loop  47  is provided that intercepts the power of the multiplexed wavelength signals at, for example, the m=−1 diffraction order area and returns the intercepted signal into the FPR  44   b  as close as possible to the m=0 diffraction order area of the FPR  44   b . Redirecting the intercepted multiplexed wavelength signal back into the grating  44   c  spatially close to the center (m=0 diffraction order area) of the FPR  44   b , it reduces grating and launch angle related losses. Still further, the return signal is spaced apart from the m=0 output position such that the individual power taps  46   a ,  46   b , and  46   c  located at the input side of the FPR  44   a  do not overlap the input ports from the lasers  42   a ,  42   b , and  42   c . If overall power taps are also desired, first and second optional overall power taps  48   a  and  48   b  (shown in dashed lines) can be located to intercept the multiplexed wavelength signal at, for example, the m=−2 and m=+2 diffraction orders and process the obtained readings in a manner similar to that described earlier for the overall power taps  26   a  and  26   b  of  FIG. 3 . 
     Referring now to  FIG. 6 , there is shown a general schematic diagram of a grating based device  50  including a plurality of radiation sources (shown as, and referred to hereinafter as, lasers  52   a  (LASER  1 ),  52   b  (LASER  2 ), and  52   c  (LASER  3 )), and an Arrayed Waveguide Grating (AWG) Multiplexer/Demultiplexer (MUX/DEMUX)  54  (shown within a dashed line area) in accordance with a fourth embodiment of the present invention. The AWG MUX/DEMUX  54  comprises a first Free Propagation Region (FPR)  54   a , a second FPR  54   b , and an optical grating section  54   c  formed from a plurality of predetermined different length waveguides. Each of the plurality of lasers  52   a ,  52   b , and  52   c  are arranged to generate an output signal with a predetermined wavelength different from that of the other lasers. The signals from the lasers  52   a ,  52   b , and  52   c  are coupled to separate inputs of the first FPR  54   a . The first and second FPRs  54   a  and  54   b  and the optical grating section  54   c  of the Arrayed Waveguide Grating (AWG)  54  utilize the principle of interference of phase shifted signals to multiplex or demultiplex optical signals as is well known in the art. In the typical case of an AWG multiplexer, multiple input signals from the lasers  52   a ,  52   b , and  52   c  are individually positioned on a first waveguide slab forming the first FPR  54   a  in such a way that after propagating, in turn, through each of the first waveguide slab, the optical grating section  54   c , and a second waveguide slab forming the second FPR  54   b , the location of the focus point of the zeroth diffraction order (m=0) of all input wavelength signals coincides with the location of the output waveguide  55  for a multiplexed output signal from the device  50 . 
     In the fourth embodiment, optical power is tapped after passing through only a portion (0.5, 1.5, etc.) of the AWG  54 . In particular, some of the waveguides forming the optical grating section  54   c  at the output of the first FPR  54   a  can be used for power taps. In a first example of the fourth embodiment, outer waveguides  54   x  and  54   y , which are first and last waveguides of the optical grating section  54   c , are used to provide separate power taps  56   a  and  56   b  after the waveguide signals from the lasers  52   a ,  52   b , and  52   c  have traveled only once through the first FPR  54   a  or the equivalent of approximately one-half (0.5) the distance through the AWG  54 . The outer waveguides  54   x  and  54   y  of the optical grating section  54   c  are preferentially used because of ease of access thereto, and to avoid cross-talk when crossing over other waveguides of the optical grating section  54   c.    
     Each of the waveguides  54   x  and  54   y  carries various proportions of the different wavelengths from the lasers  52   a ,  52   b , and  52   c , and as such, each of the power taps  56   a  and  56   b  would require some calibration based on the proportion of each wavelength power. The arrangement of tapping power after the wavelength power has only propagated through the first FPR  54   a  allows for lower loss since the power does not have to travel through the second FPR  54   b . By not having to travel through the second FPR  54   b , the power tapped directly after the first FPR  54   a  by power taps  56   a  and  56   b  is broadband and gives a better indication of the total power launched by the lasers  52   a ,  52   b , and  52   c  into the first FPR  54   a  as compared to a total power that could be measured after being emitted from the second FPR  54   b.    
     Where the wavelengths from the lasers  52   a ,  52   b , and  52   c  are widely spaced apart (sufficiently separated), the AWG  54  can be designed not only to obtain the overall power at power taps  56   a  and/or  56   b , but also to focus each of the wavelengths at separate adjacent positions within, for example, the m=−1 (not shown) and/or m=+1 diffraction orders at FPR  54   b  in the manner described hereinbefore for the grating based device  10  of  FIG. 1 . Each of the resolved wavelengths from the lasers  52   a ,  52   b , and  52   c  can then be captured at individual power taps  56   c ,  56   d , and  56   e , respectively. 
     Where the wavelengths from the lasers  52   a ,  52   b , and  52   c  are densely spaced apart so that the individual wavelengths cannot normally be resolved in a non-zero diffraction order of the second FPR  54   b , one of the outer waveguides  54   x  and  54   y  of the optical grating section  54   c  can be looped back to an input of the first FPR  54   a . As is shown in  FIG. 6 , the outer waveguide  54   y  is looped back via a dashed path  57   b  to an input of the first FPR  54   a  that is spaced apart from the inputs used for lasers  52   a ,  52   b , and  52   c , instead of going to overall power tap  56   b  via dashed path  57   a . In essence, the multiplexed wavelength signal propagating in waveguide  54   y  is fed back to an input of the first FPR  54   a  and is demultiplexed in propagating through the first FPR  54   a , the optical grating section  54   c , and the second FPR  54   b . Since the multiplexed signal from waveguide  54   y  was launched at a different input location from that of the lasers  52   a ,  52   b , and  52   c , the individual wavelength signals of the multiplexed signal will be demultiplexed and will appear at a non-zero diffraction order area (e.g., m=+1) of the second FPR  54   b . These demultiplexed individual wavelength signals can then be captured by the individual power taps  56   c ,  56   d , and  56   e.    
     It is to be appreciated and understood that the specific embodiments of the present invention that have been described are merely illustrative of the general principles of the present invention. Various modifications may be made by those skilled in the art that are consistent with the principles of the invention set forth. For example, various types of grating can be used as for example, an Arrayed Waveguide Grating (AWG), a Bulk grating, an Echelle grating, etc.