Abstract:
The present invention provides an optoelectronic device, a method of manufacture therefor and an optical communications system including the same. In an exemplary embodiment, the optoelectronic device includes a device body including an active region having a cavity length defined by a back facet and a front facet. The optoelectronic device may further include a diffraction grating optically coupled to the active region, wherein the diffraction grating has a grating length of less than about 25 percent of the cavity length.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation in part of U.S. patent application Ser. No. 09/769,083, filed on Jan. 25, 2001, entitled “OPTICAL COMMUNICATION SYSTEM WITH COPROPAGATING PUMP RADIATION FOR RAMAN AMPLIFICATION” to David Ackerman, et al., which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention is directed, in general, to an optoelectronic device and, more specifically, to an optoelectronic device having a diffraction grating associated therewith, a method of manufacture therefor, and an optical communications system including the same.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optoelectronic devices, such as lasers for use in optical communication systems, have to meet very stringent requirements. More specifically, Distributed Feedback (DFB), Distributed Bragg Reflector (DBR), and Fiber Bragg Grating (FBG) lasers used in Raman applications, have very stringent requirements. Typically, it is desirable for the DFB, DBR, or FBG lasers to have high output power (120 mW-300 mW), wavelength stabilization (within +/−1.5 nm), high Stimulated Brillouin Scattering (SBS), threshold (greater than about 300 mW), and low cost. Additionally, for Raman applications where the pumps propagate in the same direction as the signal (co-propagating), low relative intensity noise (less than about −145 dB/Hz), is also desired.  
           [0004]    Currently, there are limited types of lasers that provide all of the above-listed properties while being compatible with Raman applications. Diffraction gratings have been known to be used to achieve wavelength stabilization. For example, diffraction gratings may be integrated either into a laser structure as in DFB and DBR devices, or in a fiber pigtail in the case of FBG stabilized lasers. A problem exists in that each of the approaches is deficient in at least one of the above-listed properties.  
           [0005]    For example, FBG lasers can meet the power and spectral requirements listed above, however, the long external cavity associated with such devices increases the relative intensity noise to about −125 dB/Hz. This is generally undesirable, because it makes such systems unsuitable for co-propagating Raman applications. Additionally, the attachment of an external diffraction grating also adds to the cost and manufacturing complexity of the pump module.  
           [0006]    Conventional DFB and DBR lasers, on the other hand, have a very narrow linewidth, which typically limits the launch power to less than 10 mW, before SBS effects become significant. This linewidth may be broadened by applying an AC dither to the DC bias, however, this adds considerable complexity to the drive circuitry and, especially in the case of DFB lasers, introduces intensity modulation to the output.  
           [0007]    Accordingly, what is needed in the art is an optoelectronic device that does not experience the drawbacks encountered by the conventional devices listed above. Also needed, is an optoelectronic device that is compatible with Raman applications, that does not experience the drawbacks encountered by the conventional devices listed above.  
         SUMMARY OF THE INVENTION  
         [0008]    To address the above-discussed deficiencies of the prior art, the present invention provides an optoelectronic device, a method of manufacture therefor, and an optical communications system including the same. In an exemplary embodiment, the optoelectronic device includes a device body including an active region having a cavity length defined by a back facet and a front facet. The optoelectronic device may further include a diffraction grating optically coupled to the active region, wherein the diffraction grating has a grating length of less than about 25 percent of the cavity length.  
           [0009]    The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention is best understood from the following detailed description, when read with the accompanying FIGUREs. It is emphasized that in accordance with the standard practice in the optoelectronic industry, various features may not be drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 illustrates a cross-sectional view of an optoelectronic device, which has been constructed in accordance with the principles of the present invention;  
         [0012]    [0012]FIG. 2 illustrates a cross-sectional view of a partially completed optoelectronic device, which is in accordance with the principles of the present invention;  
         [0013]    [0013]FIG. 3 illustrates a cross-sectional view of the partially completed optoelectronic device illustrated in FIG. 2, after formation of a diffraction grating from a grating layer structure;  
         [0014]    [0014]FIG. 4 illustrates a cross-sectional view of the partially completed optoelectronic device shown in FIG. 3, after formation of a spacer layer over the diffraction grating;  
         [0015]    [0015]FIG. 5 illustrates a cross-sectional view of the partially completed optoelectronic device shown in FIG. 4, after formation of a lower confinement layer in accordance with the principles of the present invention;  
         [0016]    [0016]FIG. 6 illustrates a cross-sectional view of the partially completed optoelectronic device illustrated in FIG. 5, after formation of a conventional radiation cavity in accordance with the principles of the present invention;  
         [0017]    [0017]FIG. 7 illustrates a cross-sectional view of the partially completed optoelectronic device shown in FIG. 6, after formation of an upper cladding layer over the active region;  
         [0018]    [0018]FIG. 8 illustrates a graphical representation of a Prior Art laser structure and a graphical representation of the optoelectronic device illustrated in FIG. 1;  
         [0019]    [0019]FIG. 9 illustrates an optical communication system, which may form one environment where an optoelectronic device, similar to the optoelectronic device shown in FIG. 1, may be included; and  
         [0020]    [0020]FIG. 10 illustrates an alternative optical communication system.  
     
    
     DETAILED DESCRIPTION  
       [0021]    Referring initially to FIG. 1, illustrated is a cross-sectional view of an optoelectronic device  100 , which has been constructed in accordance with the principles of the present invention. It should initially be pointed out that the cross-sectional view of the optoelectronic device depicted in FIGS.  1 - 7  is along a length of the optoelectronic device. As such, radiation traversing through the optoelectronic device  100  illustrated in FIG. 1, would typically move from right to left and from left to right across the page, as compared to into the page if a different cross-sectional view were shown.  
         [0022]    The present invention is directed to an optoelectronic device  100  made of any material or compound that may have use in such devices. In the illustrative embodiments described herein, the optoelectronic device  100  is specifically discussed as a group III-V based device, for example an indium phosphide/indium gallium arsenide phosphide (InP/InGaAsP) based device, a gallium arsenide (GaAs) based device, an aluminum gallium arsenide (AlGaAs) based device, or another group III-V based device. Even though the present invention is discussed in the context of a group III-V based device, it should be understood that the present invention is not limited to group III-V compounds and that other compounds located outside groups III-V may be used.  
         [0023]    In the illustrative embodiment shown in FIG. 1, the optoelectronic device  100  includes an optoelectronic substrate  110  having a diffraction grating  120  located thereover. As illustrated, the diffraction grating  120  has a length (L G ). The diffraction grating  120  may further include a kL G  value ranging from about 0.06 to about 1.0, and more preferably from about 0.06 to about 0.35, and even more preferably from about 0.07 to about 0.14, wherein k is a grating coupling constant of the diffraction grating. Located over the diffraction grating  120 , in the embodiment shown in FIG. 1, is a spacer layer  130 . The spacer layer  130  may be specifically tailored to provide a particular reflectivity for the diffraction grating  120 .  
         [0024]    As illustrated, located over the spacer layer  130  may be a lower confinement layer  135 . Additionally, located over the lower confinement layer  135  may be an active region  140 , such as an active region of a Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) laser. In the particular embodiment shown in FIG. 1, the active region  140  comprises a number of quantum well regions, however, any type of radiation cavity  140  is within the scope of the present invention. While it has been shown that the active region  140  is formed over the diffraction grating  120 , it should be noted that the diffraction grating  120  may be formed over the active region  140  without departing from the scope of the present invention. Formed over the active region  140 , in the illustrative embodiment shown in FIG. 1, is an upper confinement layer  145 , an upper cladding layer  150 , and a capping layer  155 . A radiation cavity formed by the active region  140 , upper and lower confinement layers  135 ,  145 , upper cladding layer  150 , and substrate layers  110 ,  120 ,  130 , as shown, has a cavity length (L c ).  
         [0025]    Located on a back facet of the optoelectronic device  100  is a back facet coating  160 . In an exemplary embodiment, the back facet coating  160  is a conventional high reflection (HR) coating. Located on a front facet of the optoelectronic device  100  is a conventional front facet coating  170 . In contrast to the back facet coating  160 , the front facet coating  170  may be an antireflection (AR) coating. It should be noted that in most situations, the front facet is the facet from which the majority of the radiation is emitted. While the present invention has been briefly discussed as having the HR coating on the back facet and the AR coating on the front facet, it should be noted that these may be interchanged. The embodiment of the optoelectronic device  100  illustrated in FIG. 1, further includes a conventional upper contact  180  and a conventional lower contact  190 .  
         [0026]    It has been unexpectedly found that using a diffraction grating having a grating length (L G ) of less than about 25% of the cavity length (L c ), provides superior results over those achieved in prior art devices, as explained below. As such, in the illustrative embodiment shown in FIG. 1, the grating length (L G ) is less than about 25% of the cavity length (L c ). Alternatively, the grating length (L G ) may be less than about 15% of the cavity length (L c ), and more preferably, less than about 9%, or alternatively, less than about 4%.  
         [0027]    The reduced length of the grating (L G ) provides certain benefits that were not provided by the prior art devices. For instance, the reduced length of the grating (L G ) causes a bandwidth of the grating reflectivity to be significantly wider than the cavity mode spacing. This allows multiple cavity modes near the peak reflectivity of the grating to laze simultaneously, and results in a broadened linewidth, which suppresses Stimulated Brillouin Scattering (SBS) effects. In one example, a grating length (L G ) of about 75 μm and a cavity length (L c ) of about 1.3 mm provides a Full Width Half Maximum (FWHM) of the grating reflectivity that exceeds about 4 nm, while the cavity mode spacing is about 0.25 nm. Typically, an RMS spectral width of about 0.2 nm (at about 25 GHz) may be achieved for this device relative to a linewidth of less than about 1 MHz for a standard DFB. Additionally, the present invention provides a reduced side mode suppression ratio (SMSR). In an exemplary embodiment the SMSR is less than about 10 dB.  
         [0028]    It was also unexpectedly found that increasing the number of mode lasing by a small number would dramatically reduce the SBS effects. This is contrary to that understood by those skilled in the art. For example, those skilled in the art would generally understand that the SBS threshold would only increase proportionally to the number of modes, therefore, providing a smaller SBS threshold value of between about 30 mW and about 60 mW. The present invention, contrary to what would be expected by those skilled in the art, may achieve SBS threshold values of greater than about 75 mW, and more importantly, SBS threshold values of greater than about 200 mW. The larger SBS threshold values are obtained in part, from the larger emission spectrum obtainable by the present invention.  
         [0029]    Additionally, wavelength stabilization may be obtained using the diffraction grating, without the cost and complexity of an external Fiber Bragg Grating (FBG). Moreover, relative intensity noise performance of better than about −150 dB/Hz may be obtained, which is significantly higher than that of conventional FBG lasers, and somewhat approaching that of conventional Distributed Feedback (DFB) lasers.  
         [0030]    Other benefits that may be realized by the optoelectronic device  100  include precise control of the output reflectivity. The precise control of the output reflectivity may be obtained by well controlled epitaxial growth of the spacer layer  130  and photolithography of the grating length (L G ). The precise control of the output reflectivity may further provide substantially optimized output power.  
         [0031]    Turning now to FIGS.  2 - 7 , illustrated are cross-sectional views of detailed manufacturing steps illustrating how an exemplary embodiment of an optoelectronic device, similar to the optoelectronic device  100  illustrated in FIG. 1, may be manufactured. FIG. 2 illustrates a cross-sectional view of a partially completed optoelectronic device  200 , which is in accordance with the principles of the present invention. The partially completed optoelectronic device  200  includes an optoelectronic substrate  210 . The optoelectronic substrate  210  may be any layer located in an optoelectronic device, including a layer located at a wafer level or a layer located above or below the wafer level. The optoelectronic substrate  210 , in an exemplary embodiment, is an n-type doped indium phosphide (InP) substrate. The n-type dopant may comprise various elements, however, in an exemplary embodiment the n-type dopant comprises silicon. Other optoelectronic substrates  210 , however, are within the scope of the present invention.  
         [0032]    Formed over the optoelectronic substrate  210  in the particular embodiment illustrated in FIG. 2, is a diffraction grating layer structure  220 . The diffraction grating layer structure  220  may, in an alternative embodiment, comprise multiple layers. For example, in the illustrative embodiment shown in FIG. 2, the diffraction grating layer structure  220  comprises a first grating layer  223  comprising InP, a second grating layer  225  comprising a quaternary material such as InGaAsP, and a third grating layer  228  comprising InP. The optoelectronic substrate  210 , and first, second and third grating layers  223 ,  225 ,  228 , respectively, may be formed using various conventional processes. For example, in one embodiment, they may be formed using a conventional epitaxial process, such as a metalorganic vapor-phase epitaxy, or other similar process.  
         [0033]    Turning now to FIG. 3, illustrated is a cross-sectional view of the partially completed optoelectronic device  200  illustrated in FIG. 2, after formation of a diffraction grating  310  from the grating layer structure  220 . In the illustrative embodiment shown in FIG. 3, the diffraction grating  310  has a grating length (L G ) that is just long enough to control a wavelength of the optoelectronic device  200 . For example, in an exemplary embodiment, the grating length (L G ) is less than about 25% of a cavity length (L c ) In an alternative embodiment, however, the grating length (L G ) is less than about 15% of the cavity length (L c ), and more preferably, and more preferably, less than about 9%, or alternatively, less than about 4%. In as much, one example provides a cavity length (L c ) of greater than about 1.3 mm and a grating length (L G ) ranging from about 50 μm to about 150 μm. While specific ratios and lengths have been given comparing the grating length (L G ) and cavity length (L c ), it should be noted that all ratios and lengths that provide an optimal stabilized mode, are within the scope of the present invention.  
         [0034]    The diffraction grating  310  may be fabricated using various conventional processes. In an exemplary embodiment, however, the diffraction grating  310  is fabricated using a two step photolithographic process. In a first step a selective grating mask is used to expose photoresist over areas of grating layer structure  220  where the grating is not desired. Subsequently, a holographic grating exposure across the entire surface of grating layer structure  220  is performed. When the photoresist is developed, the grating pattern only exists in the areas protected by the selective grating mask in the first step. Thus, when the photoresist is developed and the grating layer structure  220  is etched, the diffraction grating  310  is formed.  
         [0035]    Precise control of the front facet reflectivity may be realized by the aforementioned diffraction grating. In one instance, a thickness of the second grating layer  225  may be optimized to provide a specific diffraction grating  310  depth, and therefore an improved front facet reflectivity. In an another instance, the previously described manufacturing process allows the diffraction grating length (L G ) to be optimized, also providing an improved front facet reflectivity control. In a third instance, the thickness of the spacer layer may be optimized, also providing an improved front facet reflectivity control.  
         [0036]    In an advantageous embodiment of the present invention, the optical period of the grating is varied along the cavity length to obtain more optimum reflectivity spectrum of the grating. This can be accomplished by varying either the physical period grating (e.g., a “chirped” grating) or average effective index of refraction in the grating region of the cavity. For example, a variation of the grating period in the range of about 0.02% to about 0.2% along its length can provide for a substantially “flatter” reflectivity peak for a given reflectivity bandwidth. The same effect can be achieved with a constant physical grating period by varying the lateral dimension of the waveguide, for example changing the mesa width from about 2.4 μm to about 2.7 μm, in the grating region. This embodiment can allow one skilled in the art to separately control the stability of the wavelength and the side mode suppression ratio, preferably to minimize the side mode suppression ratio while maintaining tight control of the lasing wavelength.  
         [0037]    In an advantageous embodiment of the present invention, the diffraction grating  310  is located proximate the front facet. For example, in an exemplary embodiment, the diffraction grating  310  is offset from the front facet by a distance ranging from about 10 μm to about 40 μm. This offset, advantageously allows for errors in a subsequent cleaving process, without substantially reducing the already minimized diffraction grating length (L G ). It should be noted, however, other embodiments exists. For example, the diffraction grating  310  may be located at or near the back facet. In such an embodiment, a low reflection coating could be used over the front facet.  
         [0038]    Turning now to FIG. 4, illustrated is a cross-sectional view of the partially completed optoelectronic device  200  shown in FIG. 3, after formation of a spacer layer  410  over the diffraction grating  310  and in the areas where the grating layer structure  220  has been completely removed. As illustrated, the spacer layer  410  may also be located between individual teeth of the diffraction grating  310 . In the illustrative embodiment shown in FIG. 4, the spacer layer  410  comprises n-type doped InP. It should be understood, however, that the spacer layer  410  is not limited to n-type doped InP, and that other materials, doped or undoped, may be used.  
         [0039]    The spacer layer  410  may be fabricated using various well-known processes. For example, in one embodiment, the spacer layer  410  may be formed using a conventional epitaxial process, such as a metalorganic vapor-phase epitaxy, or other similar process. Additionally, in one advantageous embodiment, the spacer layer  410  may be fabricated to a thickness ranging from about 0.15 μm to about 1 μm. The thickness is generally dependent on a desired strength of a reflectivity associated with the diffraction grating  310 , thus, a wide range of thicknesses are within the scope of the present invention. In one exemplary embodiment, the optoelectronic substrate  210 , the diffraction grating  310 , and the spacer layer  410  form a lower cladding layer for the optoelectronic device  200 .  
         [0040]    Turning to FIG. 5, illustrated is a cross-sectional view of the partially completed optoelectronic device  200  shown in FIG. 4, after formation of a lower confinement layer  510  in accordance with the principles of the present invention. The lower confinement layer  510 , in an exemplary embodiment, is a conventional undoped InGaAsP confinement layer. It should be noted, however, that the lower confinement layer  510  is not limited to an undoped InGaAsP layer, and that other materials, doped or undoped, may be used. For example, in one particular embodiment, the lower confinement layer  510  comprises two different lower confinement layers having varying compositions of InGaAsP.  
         [0041]    The lower confinement layer  510  may be formed using many know fabrication processes. For example, in one embodiment, the lower confinement layer  510  may be formed using a conventional epitaxial process, such as a metalorganic vapor-phase epitaxy, or other similar process.  
         [0042]    Turning now to FIG. 6, shown in a cross-sectional view of the partially completed optoelectronic device  200  illustrated in FIG. 5, after formation of a conventional radiation cavity  610  in accordance with the principles of the present invention. The active region  610 , as previously mentioned during the discussion of FIG. 1, may comprise a number of quantum well regions  623 ,  625 ,  628 . While three quantum well regions  623 ,  635 ,  628  have been illustrated, it should be noted that more or fewer than three quantum well regions are within the scope of the present invention.  
         [0043]    The active region  610  may be formed using a variety of processes. For example, in one embodiment, the active region  610  may be formed using a conventional epitaxial process, such as a metalorganic vapor-phase epitaxy, or other similar process. In an exemplary embodiment of the invention, the active region  610  includes materials chosen from group III-V compounds. The active region  610  is typically intentionally not doped, however, in an alternative embodiment it may be doped as long as a p-n junction placement is taken into consideration.  
         [0044]    Formed over the active region  610  may be an upper confinement layer  630 . The upper confinement layer  630 , in an exemplary embodiment, is a conventional p-type doped InGaAsP confinement layer. It should be noted, however, that the upper confinement layer  630  is not limited to a p-type doped InGaAsP layer, and that other materials, doped or undoped, may be used. For example, in one particular embodiment, the upper confinement layer  630  comprises two different upper confinement layers having varying compositions of InGaAsP.  
         [0045]    Turning to FIG. 7, illustrated is a cross-sectional view of the partially completed optoelectronic device  200  shown in FIG. 6, after formation of an upper cladding layer  710  over the active region  610  and upper confinement layer  630 . The upper cladding layer  710 , in an exemplary embodiment, is a conventional InP cladding layer having a dopant formed therein. The dopant is typically a p-type dopant such as zinc, however, one having skill in the art understands that other dopants, such as cadmium, beryllium or magnesium, may be used in this capacity.  
         [0046]    The upper cladding layer  710  may be formed using a conventional epitaxial process, for example a metalorganic vapor-phase epitaxy, or other similar process. After formation of the upper cladding layer  710 , the capping layer  155 , the back facet coating  160 , the front facet coating  170 , the upper contact  180  and the lower contact  190  (all illustrated in FIG. 1) may all be conventionally formed, resulting in a device similar to the completed optoelectronic device  100  illustrated in FIG. 1.  
         [0047]    In an exemplary embodiment, lateral definition of the optoelectronic device  200  (e.g., direction into the page) may be accomplished prior to completion thereof. In such an embodiment, an initial upper cladding layer would be grown on the active region  610  and upper confinement layer  630 , and then masked and etched. Next, areas outside of the active region  610  would be regrown with a confinement material, such as InP, for optical and electrical confinement thereof. Then, the manufacturing process would continue as described above, by forming the upper cladding layer  710 .  
         [0048]    Alternatively, a ridge waveguide structure could be formed in conjunction with the optoelectronic device  200 . In such an example, and after formation of the capping layer  155 , the optoelectronic device  200  could be etched laterally to provide lateral optical confinement for the active region  610 . An insulation material would then be deposited on the etched regions of the optoelectronic device  200 , providing lateral optical confinement therefor. Then, the upper contact  180  and the lower contact  190  could be formed. While certain embodiments have been illustrated and discussed, other embodiments, many of which have not been discussed, are within the scope of the present invention.  
         [0049]    As previously recited, the completed optoelectronic device  100  may operate in a superior manner to many of the prior art devices. For instance, the bandwidth of the grating reflectivity is significantly wider than the cavity mode spacing. This allows multiple cavity modes near the peak reflectivity of the grating to laze simultaneously, and results in a broadened linewidth. The broadened linewidth, in turn, suppresses Stimulated Brillouin Scattering (SBS) effects, an aspect not present in many of the prior art devices. Additionally, the present invention provides a reduced side mode suppression ratio (SMSR). In an exemplary embodiment the SMSR is less than about  10  dB, as compared to about 30 dB in many prior art devices.  
         [0050]    Turning to FIG. 8, illustrated is a graphical representation  810  of a Prior Art laser structure and a graphical representation  820  of the optoelectronic device  100  illustrated in FIG. 1. As illustrated, both graphical representations  810 ,  820  compare photon density versus axial distance for the various devices. As illustrated in the Prior Art graphical representation  810 , the photon density peaks near the back facet. In contrast, and as depicted in the graphical representation  820 , the photon density peaks near the front facet in the optoelectronic device  100 . Because the photon density peaks near the front facet in the optoelectronic device  100 , a substantially increased output power may be obtained.  
         [0051]    Turning to FIG. 9, illustrated is an optical communication system  900 , which may form one environment where an optoelectronic device  905 , similar to the optoelectronic device  100  shown in FIG. 1, may be included. The optical communication system  900 , in the illustrative embodiment, includes an initial signal  910  entering a source device  920 . The source device  920 , may comprise a number of different devices, however, in an exemplary embodiment the source device  920  comprises an optical signal source, an erbium doped fiber amplifier (EDFA) or a repeater. The source device  920 , receives the initial signal  910 , addresses the signal  910  in whatever fashion desired, and sends the signal  910  across an optical fiber  930  to a receiving device  940 . The receiving device  940  may also comprise a number of different devices, including a receiver, an EDFA or a repeater. The receiving device  940  receives the information from the optical fiber  930 , addresses the information in whatever fashion desired, and sends an ultimate signal  950 .  
         [0052]    As illustrated in FIG. 9, the completed optoelectronic device  905  may be positioned proximate the source device  920 . In such an example, an output signal  908  of the optoelectronic device  905  would co-propagate with the signal  910 , and provide amplification therefor. In an exemplary embodiment, an optical combiner  960  could be used to combine the signal  910  and the output signal  908 .  
         [0053]    Turning briefly to FIG. 10, illustrated is an optical communication system  1000 , having a completed optoelectronic device  1005  located proximate the receiving device  940 . In such an example, an output signal  1008  of the optoelectronic device  1005  would counter-propagate with the signal  910 , and provide amplification therefor. In as much, the optoelectronic devices  905 ,  1005  may act as amplification sources for an already present signal (e.g., Raman amplification), as compared to a signal sources as used in many of the prior art applications.  
         [0054]    The optical communication systems are not limited to the devices previously mentioned. For example, the optical communication systems may further include various other lasers, photodetectors, optical amplifiers, transmitters, and receivers.  
         [0055]    Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.