Abstract:
In a stabilized laser system, an output of a desired wavelength is generated. Each of a plurality of n lasers, which, while emitting light and having a preselected portion thereof fed back thereto, causes the fed back portion to be amplified and shifted in wavelength in a first direction which is spaced apart from the center wavelength of the feedback signal. A feedback stabilization arrangement is coupled to output ports of the plurality of n lasers for generating a feedback signal having a wavelength spectrum peaking at a wavelength shifted in an opposite direction to the first direction generated by the lasers in response to the feedback signal so as to provide an output signal at the output of the stabilized laser system having a wavelength spectrum that peaks essentially at the desired wavelength. A reflector is located at a predetermined signal round-trip time delay distance from the feedback stabilization arrangement. The reflector receives the output signal from the feedback stabilization arrangement and passes a first portion thereof therethrough, and reflects a remaining second portion back to the feedback stabilization arrangement as a secondary feedback signal that contributes to each of the plurality of n laser sources being set in a stable coherence collapse mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Ser. No. 10/776,808, which is entitled “High Efficiency Single And Multiple Wavelength Stabilized Laser Systems” (Optovia 6), and U.S. Ser. No. 10/776,810, which is entitled “Single And Multiple Wavelength Reflection And Transmission Filter Arrangements” (Optovia 7), having a common assignee and some common inventors with the present invention, and being filed concurrently with the present invention. 
     FIELD OF THE INVENTION 
     The present invention relates to method and apparatus for providing a stable multimode spectrum for a stabilized laser system based on the use of a transmission filter while reducing excess loss and thereby increasing the laser system&#39;s efficiency. 
     BACKGROUND OF THE INVENTION 
     Pump lasers are generally found in the form of Fabry-Perot (FP) cavity lasers whose multimode spectrums are broadband and extremely sensitive to temperature and laser drive current and, therefore, requires stabilization in most applications such as optical amplifiers. Different methods have been proposed in the past to stabilize a single laser or a system of multiple lasers. 
     In a first method, a pump laser is coupled at its output to a reflection filter, such as Fiber Bragg Gratings (FBG), which selectively reflects part of the laser spectrum back towards the laser and, therefore, stabilizes the laser spectrum and power. This first method has been extensively used to stabilize a single laser. Some multi-wavelength applications have also used the first method to stabilize multiple lasers using individual FBGs followed by a wavelength division multiplexer (WDM) to combine the stabilized laser signals. 
     Referring now to  FIG. 1 , there is shown a schematic of a prior art arrangement  10  for stabilizing a single laser source  11  using a combination of a transmission filter  12  having a wavelength spectral response of F(w) and a reflector  13  that are shown within a dashed line rectangle illustrating the second method indicated hereinabove. In the arrangement  10 , an output/input facet  11   a  of the laser source  11  is coupled to a first input/output port  12   a  of the transmission filter  12  via a path A. A second output/input port  12   b  of the transmission filter  12  is coupled to a first input/output port  13   a  of the reflector  13  via a path B. A second port  13   b  of the reflector  13  provides an output signal from the stabilized laser system arrangement  10  via a path C. 
     The transmission filter  12  sets the wavelength based on its spectral response F(w), and the reflector  13  sets the amount of signal reflection provided back through the transmission filter  12  to the laser source  11 . When a portion of the signal filtered by the transmission filter  12  is reflected by the reflector  13 , it is again filtered by the transmission filter  12  with the spectral response F(w) to provide a feedback signal to the output facet  11   a  of the laser source  11 . In response to the feedback signal, it is found that the laser source  11  produces a wavelength shift δw in a first direction, and generates an output signal that now peaks at a center wavelength that is shifted by an amount δw from the peak wavelength of transmission filter F(w) and is no longer at the desired wavelength as is shown in  FIG. 2 , resulting in excess loss and system inefficiency. 
     Referring now to  FIG. 2 , there is shown a graph of wavelength (w) on the X-axis versus Intensity (dB) on the Y-axis for exemplary curves  16  and  17  representing the forward filter spectral response and the feedback filter spectral response, respectively, and an exemplary output signal center wavelength represented by the line  18 . In operation, the output signal (not shown in  FIG. 2 ) generated by the laser source  11  is filtered once by the transmission filter  12  with a spectral response curve F(w) as is shown by curve  16  to provide an output signal from the system  10  where the power peaks at a center wavelength represented by the line  18 . For the system  10  of  FIG. 1 , the forward filter spectral response for the transmission filter  12  is defined as F o (w)=F(w) as the transmission filter spectral response between the output/input facet  11   a  of the laser source  11  and the output port  13   b  from the reflector  13 . 
     When a portion of this filtered signal is reflected by reflector  13 , it is again filtered for the second time by the transmission filter  12  resulting in a narrower signal and returned to the output/input facet  11   a  of the laser source  11 . The feedback filter spectral response for the system  10  is defined as the spectral response between the forward and backward (feedback) signals found at the output/input facet  11   a  of the laser source  11 , F f (w)=F(w) f ·F(w). As a result of the fed back signal, it is found that the laser source  11  produces a wavelength shift and now generates an output signal that now peaks at the center frequency shown by line  18  which is separated by an amount δw from the peak of curve  16  as is shown in  FIG. 2 . This results in an excess loss of power in the output signal of the system  10 . The above description indicates that the laser source  11  produces a red shift (e.g., a first direction) in response to a feedback signal. The occurrence of a red shift (in the first direction) shown in  FIG. 2  is mostly true for semiconductor diodes lasers. However, there are other types of lasers that actually produce a blue shift (in a second opposite direction from a red shift) in response to the reception of a feedback signal that also causes a similar excess loss. 
     The main requirements for pump sources in optical amplification are power efficiency, relative intensity noise (RIN), stimulated Brillouin Scattering (SBS), and spectral stability. It is desirable to provide a stabilized multi-wavelength source that provides for the requirements of power efficiency, stimulated Brillouin Scattering (SBS), and spectral stability. 
     SUMMARY OF THE INVENTION 
     The present invention relates to method and apparatus for providing a stable multimode spectrum for a stabilized laser system based on the use of a transmission filter while reducing excess loss and thereby increasing the laser system&#39;s efficiency. 
     From a first apparatus aspect, the present invention relates to a stabilized laser system comprising a plurality of n lasers, a feedback generating arrangement, and a reflector. Each of the plurality of n lasers, which, while emitting light and having a preselected portion thereof fed back thereto, causes the fed back portion to be amplified and shifted in wavelength in a first direction which is spaced apart from the center wavelength of the feedback signal. The feedback stabilization arrangement is coupled to output ports of the plurality of n lasers for generating a feedback signal having a wavelength spectrum peaking at a wavelength shifted in an opposite direction to the first direction generated by the plurality of n lasers in response to the feedback signal so as to provide an output signal at the output of the stabilized laser system having a wavelength spectrum that peaks essentially at a desired wavelength. The reflector is located at a predetermined signal round-trip time delay distance from the feedback stabilization arrangement for receiving the filtered and multiplexed output signal therefrom, for passing a first portion thereof therethrough, and for reflecting a remaining second portion thereof back to the feedback stabilization arrangement as a secondary feedback signal that contributes to each of the plurality of n laser sources being set in a stable coherence collapse mode. 
     From a second apparatus aspect, the present invention is directed to apparatus comprising a feedback stabilization arrangement, and a reflector. The feedback stabilization arrangement comprises a multiplexer/demultiplexer comprising a plurality of n first input/output ports and a second input/output port. Each first input/output port is adapted to receive an output signal from a corresponding one of a plurality of n laser sources for filtering and multiplexing the received laser source signals using a first spectral response for generating a filtered and multiplexed output signal for transmission from the feedback stabilization arrangement at the second input/output port thereof. The feedback stabilization arrangement further generates a filtered and demultiplexed feedback signal by the multiplexer/demultiplexer that is wavelength shifted by a predetermined amount and direction for transmission back to an output port of each of the corresponding ones of the plurality of n laser sources for stabilizing each of said plurality of laser sources at a desired output center wavelength. The reflector is located at a predetermined signal round-trip time delay distance from the feedback stabilization arrangement for receiving the filtered and multiplexed output signal from the feedback stabilization arrangement, for passing a first portion thereof therethrough, and for reflecting a remaining second portion back to the feedback stabilization arrangement as a secondary feedback signal that contributes to each of the plurality of n laser sources being set in a stable coherence collapse mode. 
     From a third apparatus aspect, the present invention is directed to a feedback stabilization system comprising a plurality of n laser sources, a feedback stabilization arrangement, and a reflector. Each laser source, which, while emitting light and having a preselected portion thereof fed back thereto, causes the fed back portion to be amplified and shifted in wavelength in a first direction that is spaced apart from a center wavelength of the feedback signal. The feedback stabilization arrangement comprises a multiplexer/demultiplexer comprising a plurality of n first input/output ports. Each first input/output port is coupled for receiving an output signal from a corresponding one of the plurality of n laser sources for filtering the received signals using a first spectral response and multiplexing the received signals for generating a filtered and multiplexed output signal at a second input/output port thereof for use as an output signal from the feedback stabilization arrangement. The feedback stabilization arrangement further generates a filtered and demultiplexed feedback signal by the multiplexer/demultiplexer that is wavelength shifted by a predetermined amount and direction for transmission back to an output port of each of the corresponding ones of the plurality of n laser sources for stabilizing each of said plurality of laser sources at a desired output center wavelength. The reflector is located at a predetermined signal round-trip time delay distance from the feedback stabilization arrangement for receiving the multiplexed output signal from the feedback stabilization arrangement, for passing a first portion thereof therethrough, and for reflecting a remaining second portion back to the feedback stabilization arrangement as a secondary feedback signal that contributes to each of the plurality of n laser sources being set in a stable coherence collapse mode. 
     From a method aspect, the present invention is directed to a method of stabilizing a laser system to generate an output signal having a desired wavelength. In the method, output light signals are generated from each of a plurality of n laser sources, which, while emitting light and having a preselected portion thereof fed back thereto, the output signal of each of the plurality of n laser sources is shifted in wavelength in a first direction which is spaced apart from a center wavelength of the feedback signal. A feedback signal is generated in a feedback stabilization arrangement coupled to the plurality of n lasers having a wavelength spectrum peaking at a wavelength shifted in an opposite direction to the first direction generated by the plurality of n lasers in response to the feedback signal so as to provide an output signal at the output of the stabilized laser system having a wavelength spectrum that peaks essentially at the desired wavelength. A portion of the output signal from the stabilized laser system is reflected back into the laser system as a secondary feedback signal that is delay by a predetermined amount for contributing to each of the plurality of n laser sources being set in a stable coherence collapse mode. 
     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  shows a schematic diagram of a prior art stabilized laser system; 
         FIG. 2  shows a graph of forward and feedback filter spectral responses in the prior art laser system of  FIG. 1 ; 
         FIG. 3  shows a schematic of a basic concept of a stabilized multiple laser system in accordance with the present invention; 
         FIG. 4  shows a graph of forward and feedback filter spectral responses as might be found in the multiple laser system of  FIG. 3 ; and 
         FIG. 5  shows a schematic of an exemplary stabilized multiple laser system in accordance with a preferred embodiment of the present invention; 
     
    
    
     The drawings are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to providing a stabilized multi-wavelength source that provides for requirements of power efficiency, stimulated Brillouin Scattering (SBS), and spectral stability. Power efficiency is defined as a ratio of output optical power to input electrical power to drive a laser. Still further, to provide an efficient laser, insertion loss between the laser and the output optical fiber has to be minimized to increase system efficiency. 
     Stimulated Brillouin Scattering (SBS) is a nonlinear optical effect that causes light to scatter in a reverse direction such that .the light does not contribute to the amplification process and causes a system penalty. There is a threshold level of signal power at which this phenomenon occurs that depends on each of the optical waveguide and the signal properties. There are two methods for SBS suppression which involve laser linewidth broadening and multimode operation. 
     Spectral stability is described as a stable single mode or a multimode spectral density over time and current with no mode hopping. Stabilized external cavity lasers can be designed such that the output signal is multimode with a linewidth so large that a coherence length of the laser signal drops significantly. This operating condition is well known as “coherence collapse” that results in a stable (no mode hopping) multimode signal spectrum. In order to achieve coherence collapse operation, it is necessary to consider the total effective reflectivity, the parameters of different components of a feedback signal, and the external cavity length. 
     Referring now to  FIG. 3 , there is shown a schematic of a stabilized multiple laser system  20  illustrating a basic concept of the present invention. The stabilized multiple laser system  20  comprises a plurality of n lasers sources  21   a – 21   n  that generate a plurality of n output signals w 1 –w n , respectively, a feedback stabilization arrangement  22 , a delay line  24 , and a broadband reflector  26 . The feedback stabilization arrangement  22  can comprise many different arrangements which provide a feedback signal back to each of the plurality of n laser sources  21   a – 21   n  that overcomes a red or blue (hereinafter a first direction) shift of each laser  21   a – 21   n  in response to a feedback signal as was described hereinbefore for the prior art system  10  of  FIG. 1 . For example, various multi-wavelength feedback stabilization arrangements that can be used for the multi-wavelength stabilization arrangement  22  are shown and described in the hereinbefore indicated copending application U.S. Ser. No. 10/776,808, entitled “High Efficiency Single And Multiple Wavelength Stabilized Laser Systems” (Optovia 6), and is incorporated by reference herein rather than describing each of those arrangement herein again. 
     Each of the plurality of n laser sources  21   a – 21   n  is coupled at an input/output front facet  21   p  thereof to a separate one of a plurality of n first input/output ports  22   a  of the feedback stabilization arrangement  22  via a separate path A. A second input/output port  22   b  of the feedback stabilization arrangement  22  is coupled to a first input/output port  24   a  of the delay line  24  via a path B. A second input/output port  24   b  of the delay line  24  is coupled to a first port  26   a  of the broadband reflector  26  via a path C. A second port  26   b  of the broadband reflector  26  provides a stabilized output signal from the stabilized multiple laser system  20  via a path D. 
     In operation, the output signal from each of the plurality of n laser sources  21   a – 21   n  is received at a corresponding one of the plurality of n input/output ports  22   a  of the feedback stabilization arrangement  22  via a path A. In the feedback stabilization arrangement  22 , the plurality of n received signals via paths A are each filtered with a predetermined first spectral response and multiplexed to generate an output signal at the second input/output ports  22   b  thereof for transmission via the path C to the first input/output port  24   a  of the delay line  24 . Concurrently, the feedback stabilization arrangement  22  also generates a feedback signal that is filtered and demultiplexed, and a corresponding one of the demultiplexed signals is returned to each of the laser sources  21   a – 21   n  that is shifted in a second direction to counteract a red or blue (first direction) shift in response to a feedback signal to a laser. Such red or blue shift counteracting feedback signal is generated in the feedback stabilization arrangement  22  in a manner shown and described for the various multi-wavelength feedback stabilization arrangements in the hereinbefore indicated copending application U.S. Ser. No. 10/776,808, entitled “High Efficiency Single And Multiple Wavelength Stabilized Laser Systems” (Optovia 6). 
     The output signal from the feedback stabilization arrangement  22  via path B is delayed by a predetermined amount of time in the delay line  24  and then partially reflected by the broadband reflector  26  and returned via the delay line  24  and the feedback transmission arrangement  22  to each of the lasers  21   a – 21   n . The remainder of the signal that is not reflected in the broadband reflector  24  is transmitted as the output signal of the system  20  via path D. The desired optical path length of the second feedback signal caused by the broadband reflector  26  is represented by the delay line  24 . The main function of placing the delay line  24  and the broadband reflector  26  at predetermined round-trip delay time distance from the output of the feedback stabilization arrangement  22  is to provide a small secondary feedback signal that, upon reaching the lasers  21   a – 21   n , causes each of the plurality of n laser sources  21   a – 21   n  to enter the coherence collapse mode. Therefore, the system  20  has to be designed and built such that a minimum required reflection in a desired polarization reaches the front facet  21   p  of each of the laser sources  21   a – 21   n . However, for a stable operation of the stabilized multi-wavelength laser system  20 , the entire optical path wherein the feedback signal of the feedback stabilization arrangement  22  is traveling must preserve the polarization state (polarization maintaining, PM) so that the feedback signal therefrom is not subjected to a fluctuation in polarization state due to signal and environmental conditions. 
     The secondary feedback signal from the broadband reflector  26  can comprises multiple components obtained from the signal round trip through a cavity constructed between the reflector  26  and a loop or another reflector inside the feedback stabilization arrangement  22  (as will be better understood from the description of  FIG. 5  hereinafter). These components enter the front facet  21   p  of each of the plurality of n laser sources  21   a – 21   n . The feedback stabilization arrangement  22  by itself generates a feedback signal that primarily compensates for a shift in a first direction (e.g., red shift) normally occurring in each laser in response to a reflected feedback signal and thereby avoids excess loss. The delay line  24  and broadband reflector  26  are positioned at predetermined locations after the output port  22   b  of the feedback stabilization arrangement  22  to provide a secondary feedback signal through the feedback stabilization arrangement  22  that provides the conditions for each of the laser sources  21   a – 21   n  to enter into the “coherence collapse” operating mode, and promotes a stable multimode output signal spectrum. 
     Referring now to  FIG. 4 , there is shown a graph of wavelength (w) on the X-axis versus intensity (dB) on the Y-axis for illustrating exemplary forward filter spectral response curves  30   a – 30   n  (only curves  30   a  and  30   n  are shown), and dashed line feedback filter spectral response curves  32   a – 32   n  (only curves  32   a  and  32   n  are shown) as might be found in the multi-wavelength blue shifted feedback stabilization arrangement  22  of  FIG. 3 . The feedback stabilization arrangement  22  is designed such that, for a laser source  21   j  channel, the center wavelength of a feedback filter spectral response for a jth wavelength, F f   j (w), (not shown) is shifted toward a shorter wavelength (blue shift) by an amount δw j  to compensate for a red shift of the laser center wavelength of laser source  21   j  (not shown). The blue shift design of the feedback stabilizing arrangement  22  moves the center wavelength  34   j  of the stabilized laser source  21   j  close to the center wavelength of the forward filter spectral response, F o   j (w),  30   j  as a result of the received blue shifted feedback signal. The same operation occurs for each of the other laser sources  21   a – 21   n  as is shown for the laser sources  21   a  and  21   n  by the curves  30   a  and  32   a , and  30   n  and  32   n , respectively, and the respective center wavelength lines  34   a  and  34   n . The blue shift for the feedback signal for each of the laser sources  21   a – 21   n  minimizes the excess loss shown in  FIG. 2  for the laser source  11  associated with the conventional stabilized laser system  10  of  FIG. 1  using a transmission filter  12 . 
     Referring now to  FIG. 5 , there is shown a schematic of an exemplary stabilized multiple laser system  40  in accordance with a preferred embodiment of the present invention. The exemplary stabilized multiple laser system  40  comprises a plurality of n laser sources  41   a – 41   n , a plurality of n optional polarization rotators  42   a – 42   n , a feedback stabilization arrangement  43  (shown within a dashed line area), a delay line  44 , and a reflector  45 . The feedback stabilization arrangement  43  comprises a first multiplexer/demultiplexer  46 , a first power splitter  47 , a second power splitter  48 , and a second multiplexer/demultiplexer arrangement  49 , (shown within a dashed line rectangle). The second multiplexer/demultiplexer arrangement  49  comprises a first multiplexer/demultiplexer  49   a , and a second multiplexer/demultiplexer  49   b  in a back-to-back relationship interconnected by a plurality of paths  49   c . Hereinafter, the term multiplexer/demultiplexer will be referred to as “Mux/Demux”. 
     A front facet  41   p  of each of the plurality of n laser sources  41   a – 41   n  is coupled to a first input/output port  42   p  of a corresponding one of the plurality of n optional polarization rotators  42   a – 42   n  when present by a separate path A. A second input/output port  42   q  of each of the plurality of n optional polarization rotators  42   a – 42   n  is coupled to a corresponding one of a plurality of n first input/output ports  46   a  of the first Mux/Demux  46  via a separate path B. A second input/output port  46   b  of the Mux/Demux  46  is coupled to a first input/output port  47   a  of the first power splitter  47  via a path C, and a third input/output port  46   b  of the Mux/Demux  46  is coupled to a third input/output port  47   c  of the first power splitter  47  via a path G. 
     A second input/output port  47   b  of the first power splitter  47  provides an output signal from the feedback stabilization arrangement  43  via a path D to a first input/output port  44   a  of the delay line  44 . A second input/output port  44   b  of the delay line  44  is coupled to a first port  45   a  of the reflector  45  via a path E. A second port  45   b  of the reflector  45  provides the output signal from the overall exemplary stabilized multiple laser system  40  via a path F. 
     A fourth input/output port  47   d  of the first power splitter  47  is coupled to a first input/output port  48   a  of the second power splitter  48  via a path H. A second input/output port  48   b  of the second power splitter  48  is coupled to a first input/output port  49   d  of the Mux/Demux arrangement  49  via a path J. A third input/output port  48   c  of the second power splitter  48  is coupled to a second input/output port  49   e  of the Mux/Demux arrangement  49  via a path K. 
     In operation, each of the plurality of n laser sources  41   a – 41   n  generates an output signal that is transmitted via a path A to the first input/output port  42   p  of a corresponding one of the plurality of n optional polarization rotators  42   a – 42   n  when present. The optional polarization rotators  42   a – 42   n  are included between the laser sources  41   a – 41   n , respectively, and the respective first input/output ports  46   a  of the first Mux/Demux  46  in case the laser output state of polarization (TE to TM, or vice versa) is to be rotated. The polarized output signals from the optional polarization rotators  42   a – 42   n  are transmitted to corresponding ones of the first input/output ports  46   a  of the first Mux/Demux  46  via separate paths B. 
     In the feedback stabilization arrangement  43 , the plurality of n output signals received at the first input/output ports  46   a  of the first Mux/Demux  46  and a first portion of the received signals is filtered and multiplexed using a spectral response f 1   j (w), and the resultant filtered and multiplexed signal is directed to the second input/output port  46   b  thereof. A second portion of the received signals is multiplexed and filtered using a spectral response f 3   j (w) and directed to the third input/output port  46   c  thereof. The multiplexed signal from the second input/output port  46   b  of the first Mux/Demux  46  is transmitted via path C and received at the first input/output port  47   a  of the first power splitter  47 . The second portion of the filtered and multiplexed signal is transmitted via path G to the third input/output port  47   c  of the second power splitter  47 . 
     In the first power splitter  47  the multiplexed signal received at the first input/output port  47   a , and any signal received at the fourth input/output port  47   d , is split into first and second portions thereof. The first portion of the signals from the first input/output port  47   a  and the fourth input/output port  47   d  is directed to the second input/output port  47   b  of the first power splitter  47  and becomes the output signal from the feedback stabilization arrangement  43  via path D. The second portion of the signals from the first input/output port  47   a  and the fourth input/output port  47   d  is directed as a feedback signal to the third input/output port  46   c  of the first Mux/Demux  46 . The output signal from the second input/output port  47   b  of the first power splitter  47  is transmitted through the delay line  44  to the reflector  45  via paths D and E. In the reflector  45 , the signal is partially reflected back through the delay line  44  to the second input/output port  47   b  from the first power splitter  47 , and the remaining signal is provided as the output signal from the exemplary stabilized multiple laser system  40 . 
     The multiplexed signals received at each of the second input/output port  47   b  and the third input/output-port  47   c  of the first power splitter  47  are split into first and second portions. Each of the first portions from the second and third input/output ports  47   b  and  47   c  of the first power splitter  47  is directed as a feedback signal to the first input/output port  47   a . Each of the second portions from the second and third input/output ports  47   b  and  47   c  of the first power splitter  47  is directed to the fourth input/output port  47   d.    
     The combined second portions of the signals is directed to the fourth input/output port  47   d  of the first power splitter are transmitted to a first input/output port  48   a  of the second power splitter  48 . The received signal at the first input/output port  48   a  is split into first and second portions in the second power splitter  48  and directed to the second and third input/output ports  48   b  and  48   c , respectively, for transmission to the respective first and second input/output ports  49   d  and  49   e  of the second Mux/Demux arrangement  49 . 
     In the Mux/Demux arrangement  49 , the signal received at the first input/output port  49   d  is demultiplexed in the first Mux/Demux  49   a  and then multiplexed in second Mux/Demux  49   b  while being filtered with the spectral response for an exemplary jth wavelength of f 2   j (w) in the Mux/Demux arrangement  49 , and appears as an output signal at the second input/output port  49   e  thereof. The signal received at the second input/output port  49   e  is demultiplexed in the second Mux/Demux  49   b  and then multiplexed in first Mux/Demux  49   a  while being filtered with the spectral response f 2   j (w) for the exemplary jth wavelength of the Mux/Demux arrangement  49 , and appears as an output signal at the first input/output port  49   d  thereof. Each of the output signals from the first and second input/output ports  49   d  and  49   e , respectively, are transmitted to the respective second and third input/output ports  48   b  and  48   c  of the second power splitter  48  where they are combined and fed back to the fourth input/output port  47   d  of the first power splitter  47 . The combined signal received at the fourth input/output port  47   d  of the first power splitter  47  is split into first and second portions. These first and second portions are directed to the second and third input/output ports  47   b  and  47   c , respectively, to add a component to (a) the respective output signal being sent via path D to the output of the stabilized multiple laser system  40 , and (b) to the feedback signal being sent via path G to the third input/output port  46   c  of the first Mux/Demux  46 . 
     The resultant feedback signals from the first power splitter  47  received at the second and third input/output ports  46   b  and  46   c , respectively, of the first Mux/Demux  46  are demultiplexed and filtered with the spectral responses f 1   j (w) and f 3   j (w), respectively. The demultiplexed and filtered signals are directed to corresponding ones of the plurality of n first input/output ports  46   a . The center wavelength of the spectral response f 3   j (w) between the plurality of n first input/output ports  46   a  and the third input/output port  46   c  is shifted towards a shorter wavelength relative to a center wavelength of the spectral response f 1   j (w) between the plurality of n first input/output ports  46   a  and the second input/output port  46   b . Each of the plurality of n demultiplexed signals from the first Mux/Demux  46  appearing at corresponding ones of the plurality of first input/output ports  46   a  is transmitted through the corresponding ones of the plurality of n optional polarization rotators  42   a – 42   n  and to the front facet  41   p  of the associated one of the plurality of n laser sources  41   a – 41   n.    
     The feedback signal comprises four main components. A first main component is obtained by a signal propagating the optical paths of A→B→C→B→A where it is filtered by the spectral responses f 1   j (w) and f 3   j (w) in sequence. A second main component is obtained by a signal propagating the optical paths of A→B→G→C→B→A where it is filtered by the spectral responses f 3   j (w) and f 1   j (w) in sequence. A third main component is obtained by a signal propagating the optical paths of A→B→G→H→J→K→H→G→B→A. A portion of the reflected feedback signal received by the first power splitter  47  via path H adds a component to the output signal transmitted from the input/output ports  47   b  thereof. A fourth main component is obtained by a signal propagating the optical paths A→B→C→D→E→D→C→B→A. There is a cavity formed between the reflector  45  and the combination of the second power splitter  48  and the Mux/Demux arrangement  49  including the optical paths of E→D→H→J→K→H→D→E. A signal round trip in this cavity adds two components to the feedback signal to the plurality of n laser sources  41   a – 41   n , (one component from the reflector  45 , and a second component fed back via path G). Still further, one component is also added to the output signal transmitted via path D. 
     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 present invention.