Abstract:
A fiber grating stabilized pump laser achieves power stability with a relatively low grating reflectivity of 1.0 to 2.3%. Substantial laser diode output facet reflectivity is provided, between 3.0 and 5.5%. When the optical fiber system provides for no polarization control between the grating and the diode laser, the power reflectivity of the grating, is preferably between 1.7 and 2.3%. The power reflectivity of the laser diode output facet is preferably between 3.4 and 4.9%. In contrast, when provisions are made for polarization control between the grating and laser diode, even lower grating reflectivities are possible, between 1.0 and 2.3%.

Description:
RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. application Ser. No. 09/501,809, filed on Feb. 10, 2000, which is a Continuation-in-Part of U.S. application Ser. No. 09/396,174, filed on Sep. 14, 1999 now abandoned, and claims the benefit of the filing date of Provisional Application Ser. No. 60/136,628, filed May 27, 1999, the entire teachings of which are incorporated herein by this reference. 
    
    
     BACKGROUND 
     Laser diodes are used to optically pump optical fiber (gain fiber). The gain fiber can either be regular fiber in a Raman amplification scheme or rare-earth doped fiber, which has been spliced into the optical link to enable amplification of light signals. In common commercial products, 980 nanometer (nm) or 1480 nm diode lasers are used to optically pump erbium-doped fiber amplifiers operating or amplifying typically in a spectral range around 1550 nm. 
     In these diode pump-gain fiber systems, it is important to minimize changes in the amplifier characteristics due to changes in the pump wavelength or power. This is especially true in wavelength division multiplexing (WDM) systems or dense wavelength division multiplexing (DWDM) systems comprising many, spectrally closely-spaced channels. For example, mode hopping in the pump can cause changes in the gain spectrum of the amplifier. These changes result in preferential amplification of channels relative to other channels in the DWDM system. 
     One solution to controlling noise and wavelength shift due to environment temperature or power changes in the pumps uses fiber-grating stabilization. The Bragg grating has the effect of stabilizing the output spectrum from the laser pump or, more specifically, the grating stabilizes the pump against temporal power and frequency fluctuations. Further, in one suggested implementation, the grating is selected, spaced from the laser module, and tuned relative to the laser&#39;s exit facet reflectivity so that the spectrum of the emission is broadened relative to that of a solitary laser. 
     To further stabilize pump lasers, polarization control is many times useful. The light emitted from the output facet of the diode lasers is typically highly polarized. The polarization of the light propagating through regular, non-polarization maintaining fiber, however, can change its orientation due to fiber birefringence, fiber twisting, bending, temperature shifts, and other stresses. Any fluctuation in the polarization of the light returning to the optical device from the grating effectively changes the feedback power ratio, because the laser is insensitive to any reflected light that has a polarization orthogonal to that of the emitted light. For example, if all of the reflected light has its polarization rotated by 90 degrees, the fiber Bragg grating is effectively removed from the system from the laser&#39;s perspective. 
     In applications where polarization control is required between the laser diode and the grating, polarization-maintaining (PM) fiber is used for the fiber pigtail, with the grating being written into the PM fiber. 
     SUMMARY 
     There are many different approaches to selecting facet and fiber grating reflectivities. Generally, the optimization seeks to maximize the pump module output, while minimizing temporal power fluctuations. 
     One approach suggests the use of an antireflected-coated laser diode output or exit facet. This yields a laser diode exit facet that has very low reflectivity, typically on the order of less than 0.5% power reflectivity. The grating reflectivity is then selected to both maintain good temporal power stability while obtaining good power output from the laser diode. 
     The problem with this approach is that, in optical pumping applications, the total power output from the laser diode is typically a critical design criteria. It can result in a fewer number of laser diodes being required to obtain a specified pumping power in the gain fiber. If the grating is used to provide both the power stabilizing feedback, but also the feedback to create laser operation, there is only a single variable—grating reflectivity—to optimize two parameters, total laser power output and laser power stability. In addition, very low reflectivity laser diodes (&lt;0.5%) can be hard to make, evaluate, test and screen during the manufacturing process. 
     As a result, another approach seeks to optimize fiber grating stabilized diode pump lasers by specifying a specific ratio between front facet reflectivity and effective grating reflectivity, taking into account the two-way coupling efficiency between the fiber pigtail and the laser diode. The problem with this approach, however, is that it fails to contemplate real-world manufacturing tolerances and specifications. To achieve acceptable yields, only ranges of front facet reflectivity, grating reflectivity, and coupling efficiency can be specified. Coupling efficiency varies with laser to fiber alignment success and culminating lens placement/alignment focusing. Fiber grating manufactures typically only provide fiber gratings having a reflectivity within a 0.5 to 1% range at reasonable costs. Finally, exit facet reflectivity can vary as much as one percent in laser diode chips manufactured from the same wafer. 
     As a result, according to one aspect, the invention features a fiber-grating stabilized pump module. It comprises a laser diode chip having an output facet and an optical fiber system into which light exiting from the optical facet of the laser diode is coupled. A grating is formed in the optical fiber of the optical fiber system. The grating is used to provide feedback to the laser diode to thereby promote power stability of the pump module. 
     According to the invention, the power reflectivity of the grating and the output facet are specified in absolute terms. Specifically, the power reflectivity of the grating is 1.0% to 2.3%, and the power reflectivity of the output facet is between 3.0 and 5.5%. 
     In specific embodiments, the coupling efficiency between the laser diode and the optical fiber system is between 70 and 82%. 
     When the optical fiber system comprises only regular fiber, with no polarization control or maintenance, a grating power reflectivity of 1.7 to 2.3% is considered to be preferable. Further, the output facet reflectivity is preferably between 3.4 and 4.9 percent. 
     In contrast, when the optical fiber system comprises polarization maintaining fiber, the grating reflectivity is preferably between 1.0 and 2.3%. Again, a power reflectivity of the output facet of 3.4 to 4.9% is preferable. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1 is a block diagram of the fiber-grating stabilized pump laser component according to the present invention; 
     FIG. 2 is a perspective, schematic view of the fiber-grating stabilized pump laser component according to the present invention; 
     FIGS. 3A-3D are schematic, cross-sectional views of a few exemplary PM and non-PM fibers; and 
     FIG. 4 is a plan view showing the relationship between the PM fiber&#39;s axes and micro lens on the end of the fiber pigtail. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 shows a fiber-grating stabilized pump laser component, which has been constructed according to the principles of the present invention. 
     Specifically, an optical source  110 , such as a laser, specifically, pump laser or amplifier, is installed on a submount  112  in the typical implementation. The submount  112  provides mechanical and electrical connections between the laser diode  110  and the module housing  114 . An optical fiber system is used to transmit the light generated by the laser diode  110  to the next device or system  130 . In the preferred implementation, this system is a fiber amplifier system with the light being used to pump the fiber amplifier  130 . For example, the diode pump component  100  can be attached to the fiber amplifier through a WDM coupler to provide the pump light into the fiber amplifier&#39;s core. 
     The optical fiber system comprises a fiber pigtail  118 , which extends through a side wall  120  of the module housing  114 . It is typically rigidly secured to the submount  112  such that the fiber endface  122  is held in proximity to an output facet  116  of the laser diode  110 . 
     The fiber pigtail  118  is preferably constructed from polarization-maintaining fiber. Different types of polarization-maintaining fiber are applicable. For example, panda, elliptical stress bar, elliptical core, and bow tie may be used in addition to other fiber types such as polarization controlling fiber which has only a single transmission axis. 
     FIGS. 3A through 3C are cross-sectional views illustrating a few exemplary types of PM fiber deployable with the present invention, specifically panda, elliptical stress bar, and bow tie, respectively. Each of these PM fiber types is doped to yield a fast and slow axis of light transmission. Light that is coupled into the fiber and polarized along one of these axes is restricted to maintain its polarization with respect to the axes. 
     In other embodiments, the fiber pigtail  118  is constructed from a type of polarization-maintaining fiber that is termed polarization-controlling fiber. This fiber type only propagates light with a single polarization. Moreover, the pigtail is constructed from regular, non-polarization-maintaining fiber in still other embodiments. 
     Returning to FIGS. 1 and 2, the present invention contemplates the various techniques used for ensuring a high collection efficiency for light exiting from the light output facet  116  of the laser diode into the fiber pigtail  118 . Specifically, discrete lenses, butt coupling, and microlenses are various alternatives. Presently, flat top microlenses are used as described in the application entitled “Flat Top, Double-Angled, Wedge-Shaped Fiber Endface”, U.S. Ser. No. 08/965,798, by Jeffrey Korn, Steven D. Conover, Wayne F. Sharfin and Thomas C. Yang, which is incorporated herein by this reference. 
     The advantage of double-angle flat top microlenses is the fact that they are not circularly symmetric, i.e., with respect to the fiber&#39;s axis, to thereby match the elliptical spatial distribution of light from the output facet of the laser diode. In this vein, however, cylindrical fiber lenses or a cylindrical discrete lenses are another alternative. 
     When non-circularly symmetric coupling techniques are used, it is important to align the discrete or formed microlens, for example, with one of either the fast or slow axes of the polarization-maintaining fiber. 
     FIG. 4 illustrates the relationship between the double-angle flat top microlens and the PM fiber&#39;s transmission axes. Specifically, breaklines BL 1 , between polished surface S 1  and cleaved flat top surface S 3 , and breakline BL 2 , between top surface S 3  and polished surface S 2 , are substantially aligned with one of the PM fiber&#39;s axes. Here, the breaklines BL 1  and BL 2  are aligned with the slow axis. It is equally valid to align the breaklines BL 1  and BL 2  with the fast axis, in alternative embodiments. 
     This coupling system ensures a high power coupling efficiency between the laser diode and the optical fiber system and specifically the grating. In present implementations, the efficiency is between 70% and 82%. 
     In the present invention, the accuracy of the alignment is between zero and five degrees relative to either the fast or slow axis. That is, there is less than 5 degrees of angular difference between the breaklines BL 1 , BL 2  and the selected fiber transmission axis. Preferably, however, the alignment is between 0 and 2°. This level of alignment ensures that little light is coupled to travel along the nonfavored axis of transmission, and thus maximizing the polarization-controlling effect of the fiber pigtail  118 . 
     The polarization control maximizes and stabilizes the effect of the fiber grating. Light exiting from the typical diode laser is polarized. As a result, any light that is coupled into the other, non-favored fiber axis, i.e., the axis of the fiber that is not aligned with the polarization of the laser diode, is reflected by the grating, but has little effect on the laser diode since the diode is nonresponsive to light of this polarization. 
     Returning to FIGS. 1 and 2, the polarization-maintaining fiber pigtail  118  is optically coupled to a strand of regular fiber  124  via splice  126 . Regular fiber preferably has a standard circular cross section core, i.e., has no stress bar, or no fast or slow axes, as illustrated in FIG.  3 D. 
     Although the coupling between the polarization-maintaining fiber pigtail  118  and regular, non-polarization-maintaining fiber  124  is shown as a direct fusion splice. The critical feature is the optical coupling between the two fibers. Thus, other techniques for obtaining this coupling can be used such as intervening fiber lengths of a third fiber. 
     In a similar vein, the PM fiber need not directly receive the light from the diode. Instead, the light can be first coupled into a relatively short length of regular fiber for example, and then into PM fiber, which transmits the light over most of the distance to the grating. This is not preferred, however, because of the need for additional splicing. 
     Moreover, in some cases only non-PM is used between the diode and the grating. 
     A grating  128  is written into the regular fiber  124 . These gratings are typically manufactured by UV beam interference. The gratings are usually written to the depth of the core in the fiber. 
     In one implementation, the fiber grating  128  provides reflectivity at about 980 nm, which is within the gain bandwidth of the diode laser  110 . Further, the fiber grating has a bandwidth of 0.4-1 nm, preferably. 
     Moreover, in still another implementation, only PM fiber is used between the diode  110  and the grating  128 , with the grating being written in the PM fiber. This is the simplest design from a component count perspective, but suffers from higher component costs associated with the PM fiber grating and more stringent manufacturing tolerances. 
     Preferably, the grating has a power reflectivity of about 1.0% to 2.3%; and the front facet power reflectivity of the diode laser is 3.0% to 5.5%. Preferably, the front facet reflectivity for the diode has tighter specification. Specifically, the front facet reflectivity is between 3.4 and 4.9%. 
     Preferably, the grating power reflectivity is between 1.0% to 2.3% when the optical fiber pigtail system provides for polarization control between the grating  128  and the laser diode  110 , i.e., when there is polarization-maintaining or polarization-controlling fiber optically interposed between the grating and diode. 
     When the optical fiber pigtail system uses regular fiber without polarization controlling characteristics, i.e., the optical fiber pigtail system does not provide for polarization control between the grating  128  and the laser diode  110 , higher grating reflectivity is typically required to ensure that the diode does not become unlocked on the grating. Specifically, a grating reflectivities in the range of 1.7 to 2.3% are used. 
     In other implementations, also involving erbium-doped fiber amplifiers, the grating  128  is reflective at 1480 nm. In implementations using Raman amplification, the grating has a bandwidth of 0.5-20 nm with a reflectivity peak in the range of 1060-1600 nm. 
     Important to the performance of the inventive systems is the distance (1) between the splice point  126  and the grating  128 , when the splice is present. Generally, this distance should be kept short to minimize the region without polarization control between the laser diode  110  and grating  128 . In the preferred embodiment, the distance (1) is 50-150 mm, but can be a wider range such as 0 mm to 500 mm. 
     In the preferred embodiment, the regular fiber  128  transmits the signal from the optical device  110  to the next device or system  130 . In the preferred implementation, this system is a fiber amplifier system with the light being used to pump the fiber amplifier  130 . For example, the diode pump component  100  can be attached to the fiber amplifier through a WDM coupler to provide the pump light into the fiber amplifier&#39;s core. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.