Abstract:
The present invention is in general related to efficient operation of an external cavity laser and more particularly to a system and method of operating an external cavity laser utilizing one or more controlled linewidth gain elements. Specifically, the linewidth of an gain elements is broadened so that Brillouin scattering mechanisms in a gain medium are eliminated or reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO A MICROFICHE APPENDIX 
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     BACKGROUND OF THE INVENTION 
     Raman amplifiers have been developed to amplify optical signals. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz. 
     When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is conserved as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain. 
     The probability that a Raman scattering event will occur is dependent on the intensity of the light as well as the wavelength separation between the two photons. The interaction between two optical waves due to SRS is governed by the following set of coupled equations:                 I   S            z       =         g   R          I   S          I   P       -       α   S          I   S                          I   P            z       =         -       λ   S       λ   P              g   R          I   S          I   P       -       α   P          I   P                                
     where I s  is the intensity of the signal light (longer wavelength), I p  is the intensity of the pump light (shorter wavelength), g R  is the Raman gain coefficient, λ s  is the signal wavelength, λ p  is the pump wavelength, and α s  and α p  are the fiber attenuation coefficients at the signal and pump wavelengths respectively. The Raman gain coefficient, g R , is dependent on the wavelength difference (λ s -λ p ) as is well known in the art. 
     As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have only come into existence over the past few years. These advances have renewed interest in Raman amplifiers. 
     FIG. 4 depicts a prior art arrangement of optical system  40  which includes a Raman amplifier. Optical system  40  includes optical signal source which generates an optical signal to be detected by detector  44 . For example, telecommunication providers utilize wavelengths within the C Band (1530 to 1565 nm) and L Band (1570 to 1610 nm) to provide channels to carry information optically. Additionally, it is anticipated telecommunication providers may also begin to utilize wavelengths in the S Band (1430 to 1530 nm) and the XL Band (1615 to 1660 nm). Accordingly, the optical signal may comprise one or more wavelengths within these bands. Detector  44  is disposed at some appreciable distance from optical signal source  42 . Raman source  41  provides a Raman pump. Raman source  41  provides the Raman pump to multiplexer  43 . Multiplexer  43  causes the Raman pump to enter optical fiber  45  which also carries the optical signal generated by optical signal source  42 . Due to SRS, the optical signal experiences Raman gain at the desired wavelength(s) in fiber  45 . 
     External cavity diode lasers (ECDL&#39;s) are most typically used to narrow the linewidth of the laser. In this context, linewidth refers to or measures the width of the spectral content of the output of a laser diode. By utilizing an external cavity, the linewidth of a laser can be reduced by many orders of magnitude. An example of an external cavity laser is provided in U.S. Pat. No. 5,319,668. 
     The reduction in linewidth of an ECDL can result in an accompanying process that is referred to as Brillouin scattering. Brillouin scattering is analogous to Raman scattering. The primary differences are that, in lieu of an optical phonon, an acoustic phonon is generated, the Stokes shift in silica fibers is 10 GHz instead of 13 THz, and the Brillouin gain coefficient is about 2 orders of magnitude larger. It will be appreciated that if a typical ECDL is used as a Raman pump source, Brillouin scattering will backscatter the pump light in competition with stimulated Raman scattering. Specifically, this backscattering prevents the pump light from propagating down the length of the fiber to stimulate the Raman process. Accordingly, typical ECDL&#39;s are not suitable for Raman amplifier pump source applications. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method for operating an external cavity laser to obtain a linewidth controlled output. In some embodiments, the system and method modify an incoherently beam combined (IBC) laser to achieve the desired linewidth control. In some embodiments, laser diodes of an IBC laser are modified to cause the laser diodes to operate in a coherence collapse regime (a non-linear region of laser operation defined by feedback effects) by selecting etalon surface reflectivity of the diodes relative to feedback received from the external cavity. By operating the diodes in the coherence collapse regime, the laser diodes are caused to have significantly broadened linewidth due to non-linear effects. In other embodiments, the linewidth of the laser diodes is broadened by utilizing a phase modulator. The relatively broad linewidth of embodiments of the present invention, in turn, can be used to adapt an external cavity device such as an IBC laser for use as a Raman pump. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 depicts an exemplary incoherently beam combined (IBC) laser according to embodiments of the present invention; 
     FIG. 2 depicts an exemplary emitter with feedback from an external cavity according to embodiments of the present invention; and 
     FIG. 3 is a block diagram depicting an IBC laser coupled to a phase modulator according to embodiments of the present invention; and 
     FIG. 4 is a block diagram of an exemplary optical system including a Ramen amplifier according to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Brillouin gain in a material is given by the following equation:            g   B          (     v   B     )       =       2      π                   n   2          p   12   2         c                   λ   L   2          p   O          v   A        Δ                   v   B                                
     where n is the index of refraction, p 12  is the longitudinal elasto-optic coefficient, c is the speed of light, λ L  is the wavelength of the laser light, ν A  is the frequency of the acoustic wave, and Δν B  is the full width at half maximum of the Brillouin gain profile (which is typically of order 40 MHz). 
     It will be appreciated that linewidths, Δν L , of typical ECDL&#39;s are of order from several hundred kHz to several MHz. When the linewidth of a laser is increased to a value above the Brillouin gain width, Δν B , the Brillouin gain, g B , is reduced to:            g   ~     B     =         Δ                   v   B           Δ                   v   B       +     Δ                   v   L                  g   B          (     v   B     )                                
     where {tilde over (g)} B  is the new Brillouin gain and Δν L  is the linewidth of an emitter of the ECDL. 
     The power threshold at which Brillouin scattering has a significant adverse impact is dependent on detailed material parameters of the optical fiber. Typically, the power threshold is of the order of a few milliwatts. To eliminate or reduce the impact of Brillouin scattering to suitable levels, it is advantageous to increase the linewidth of an ECDL above 100 MHz. By increasing the linewidth of an ECDL, the Brillouin gain will be relatively reduced as represented in the above equations. The associated backscattering will also be reduced. Accordingly, this allows an ECDL to be utilized as a Raman pump to generate Raman gain, because the output beam of the ECDL is thereby allowed to propagate through an optical medium (e.g., a silica fiber) to stimulate Raman scattering. 
     FIG. 1 depicts exemplary incoherently beam combined (IBC) laser  10  which is adapted according to embodiments of the present invention. Specifically, IBC laser  10  possesses a plurality of gain elements or emitters  12 - 1  through  12 -N which are caused to operate in the coherence collapse regime. By operating in the coherence collapse regime, the linewidth of emitters  12 - 1  through  12 -N are broadened to reduce or eliminate the negative effects of Brillouin scattering in an optical medium. 
     Although emitters  12 - 1  through  12 -N can be implemented as discrete devices, emitters  12 - 1  through  12 -N are advantageously implemented on an integrated device through a variety of techniques including photo-lithographic techniques. Emitters  12 - 1  through  12 -N may comprise any number of semiconductor materials such as GaAIAs, GaAs, InGaAs, InGaAsP, AlGaInAs, and/or the like, which are capable of lasing at particular wavelengths. It shall be appreciated that embodiments of the present invention are not limited to any particular implementation of emitters  12 - 1  through  12 -N. Any number of designs may be utilized including, but not limited to, edge emitters, vertical cavity surface emitting lasers (VCSELs), and grating surface emitting lasers. Accordingly, the term “emitter” is intended to mean any gain material or element capable of lasing in response to feedback. 
     Emitters  12 - 1  through  12 -N respectively include output facets  13 - 1  through  13 -N. Output facets  13 - 1  through  13 -N provide a partially reflective surface at the emitting surface of emitters  12 - 1  through  12 -N. Output facets  13 - 1  through  13 -N can be implemented utilizing dielectric material or films. Additionally, a substantially fully reflective surface (not shown) is placed on the opposite side of emitters  12 - 1  through  12 -N. 
     The external cavity of IBC laser  10  includes micro-optic  14 , collimating optic  15 , diffraction grating  16 , and optical fiber  11 . The light emitted from emitters  12 - 1  through  12 -N is partially collimated by micro-optic  14  which may be implemented as an array of micro-lenses utilizing photo-lithographic techniques or as a cylindrical lens. The partially collimated light is then further collimated by collimating optic  15  (e.g., a lens) such that the chief rays of the beams from individual emitters  12 - 1  through  12 -N intersect or spatially overlap on diffraction grating  16 . The beams are then diffracted on the first order through fiber coupling lens  17 , thereby multiplexing the beams. Fiber coupling lens  17  couples the multiplexed beams into optical fiber  11  via fiber facet  18 . Intra-fiber partial reflector  19  provides feedback to emitters  12 - 1  through  12 -N, thereby controlling their emission wavelengths. 
     It will be appreciated that the geometry of the external cavity of IBC laser  10  defines the resonant wavelengths of emitters  12 - 1  through  12 -N. The center wavelength (λ i ) of the wavelengths fed back to the i th  emitter  12 -i is given by the equation: λ i =A[sin(α i )+sin(β)], where A is the spacing between adjacent rulings on diffraction grating  16 , α i  is the angle of incidence of the light from the i th  emitter on diffraction grating  16 , and β is the output angle which is common to all emitters  12 - 1  through  12 -N. Since the feedback to each emitter  12 -i varies according to its position on the array, a relatively broad spectrum of output light can be generated by IBC laser  10 . Additionally, the ability to combine the output incoherently from a number of emitters  12 - 1  through  12 -N allows IBC laser  10  to achieve a relatively high output power. 
     Although IBC laser  10  utilizes diffraction grating  16  to provide feedback to emitters  12 - 1  through  12 -N and to multiplex their output beams, other feedback and multiplexing elements may be substituted. For example, transmission diffraction gratings, prisms, holograms, and other multiplexing elements including arrayed waveguide gratings (AWGs) and Mach-Zehnder interferometers may be utilized in lieu of reflective diffraction grating  16 . The particular dispersive element used in a given application will influence the geometry of the external cavity. 
     To illustrate how emitters  12 - 1  through  12 -N can be operated in the coherence collapse regime, reference is now made to FIG.  2 . FIG. 2 depicts exemplary emitter  12 - 1  in greater detail. Emitter  12 - 1  is shown to be generating light beam  21  via lasing. Light beam  21  propagates forward and, therefore, a portion of light beam  21  is reflected by facet  13 - 1 . Facet  13 - 1  causes a portion of the power associated with light beam  21  to be reflected backward as light beam  23 . The ratio of the power associated with light beam  21  and the power associated with light beam  23  defines the reflectivity of facet  13 - 1 . The portion of light beam  21  that is not reflected exits emitter  12 - 1  and traverses the external cavity of IBC laser  10  as described above in connection with FIG.  1 . In particular, it is pertinent to note that a portion of the non-reflected light is reflected by intra-fiber reflector  19  and returns to emitter  12 - 1  as feedback light beam  22 . The ratio of the power associated with light beam  23  and the power associated with light beam  22  shall be referred to as the feedback ratio. 
     According to embodiments of the present invention, the feedback ratio is selectively designed such that emitters  12 - 1  through  12 -N possess a linewidth that is significantly above the width of the Brillouin gain profile (Δν B ). Advantageously, the feedback ratio is designed to be within the range of −5 to −40 dB, such that emitters  12 - 1  through  12 -N will operate in a largely non-linear manner. Due to the non-linear operation, the linewidth of emitters  12 - 1  through  12 -N is relatively increased, consequently avoiding appreciable stimulation of Brillouin scattering. 
     It will be appreciated that the feedback ratio is largely controlled by the reflectivity of facets  13 - 1  through  13 -N and of intra-fiber reflector  19 . Some loss occurs within the cavity of IBC laser  10 . For example, a certain amount of loss occurs due to coupling to the fiber and due to diffraction efficiency of diffraction grating  16 . Of course, such loss associated with intra-cavity effects should be considered. However, these effects may be primarily addressed by compensating for such loss through the selection of the reflectivities of facets  13 - 1  through  13 -N and of intra-fiber reflector  19  to achieve the desired feedback ratio. 
     FIG. 3 is a block diagram depicting another embodiment of the present invention in which IBC laser  30  is coupled to optical phase modulator  31 . IBC laser  30  can be implemented in the same manner as IBC laser  10 . However, IBC laser  30  does not necessarily operate its emitters  12  in the coherence collapse regime, since linewidth broadening is provided by optical phase modulator  31 . In this embodiment, optical phase modulator  31  is placed outside of the external cavity of IBC laser  30 . Accordingly, optical phase modulator  31  receives the output beam from IBC laser  30  via optical fiber  11 . 
     Optical phase modulator  31  is a device that is well known in the art and is commercially available. Optical phase modulator  31  allows an optical signal to be phase modulated in response to a control signal. A number of physical mechanisms can be utilized to implement optical phase modulator  31 . For example, an electro-optical approach may be utilized. In an electro-optical phase modulator, a changing electrical signal (also known as a modulating or control signal) is applied between a pair of electrodes mounted on opposite faces of a crystal to create electric field stresses within the crystal. The output of IBC laser  30  propagates through the crystal in a direction perpendicular to the electric field between the electrodes, such that the intermittent interaction between the modulating electric field and the optical field modulates the optical light beam. 
     To produce linewidth broadening, RF frequency generator  32  can be coupled to optical phase modulator  31  to provide the modulating signal. RF frequency generator  32  provides a modulating signal at a frequency significantly above the width of the Brillouin gain profile (Δν B ). In some embodiments, RF frequency generator  32  provides a modulating signal that possesses a frequency that is significantly greater than 40 MHz. By providing the RF frequency signal to optical phase modulator  31 , the linewidth of IBC laser  30  may be sufficiently broadened to eliminate or reduce Brillouin scattering in a gain medium. 
     Both of the techniques described above with respect to FIGS. 1,  2 , and  3  are operable to increase the linewidth of ECDL&#39;s. By increasing the linewidth of ECDL&#39;s, the ECDL&#39;s are suitable for Raman pump applications. For example, the configurations depicted in FIGS. 1 and 3 are suitable to replace Raman source  41  of optical system  40  depicted in FIG.  4 . By possessing broadened linewidth, the output beams of ECDL&#39;s will not experience appreciable Brillouin backscattering in the gain medium. Thus, the output beams will be allowed to generate the desired Raman gain. 
     In other embodiments, IBC laser  10  and IBC laser  30  can be utilized as the excitation source for another laser or laser amplifier. IBC laser  10  and IBC laser  30  can provide their output beam to excite a gain medium that is doped with appropriate materials. For example, IBC laser  10  and IBC laser  30  can excite an optical fiber doped with any of the following materials: Ce, Pr, Nd, Er, Tm, Ho, and Yb. In addition, multiple doping materials can be utilized. In particular, it is advantageous to dope a gain medium with both Yb and Er. It shall be appreciated that the use of IBC laser  10  and IBC laser  30  as the excitation source is advantageous for these types of applications. Specifically, IBC laser  10  and IBC laser  30  are capable of providing a relatively high output power to excite the particular gain medium, because IBC laser  10  and IBC laser  30  are operable to combine the output beams from a plurality of emitters. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.