Abstract:
A single-mode semiconductor diode laser includes a single-mode section, a tapered mode-transformer section coupled to the single-mode section, and a power-supply section coupled to the tapered mode transformer section. The power-supply section has a second width larger than the width of the single-mode section. The tapered mode-transformer section is characterized in that optical energy from said single-mode section adiabatically couples to the power-supply section through the mode-transformer section.

Description:
This application claims priority to provisional application No. 60/176913, filed on Jan. 20, 2000, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor diode lasers. In particular, the present invention relates to high-power single-mode semiconductor diode lasers. 
     BACKGROUND 
     Many technologies employ single-mode high-power lasers. For example, erbium doped fiber amplifiers (EDFAs) use single-mode high-power pump lasers for optimal amplification of optical signals across optical fibers. Unfortunately, producing single-mode laser radiation is not always compatible with emitting laser radiation at high power. One of the reasons for this involves the width of the laser cavity. Single-mode radiation can be produced in a relatively narrow cavity, but the small size of the cavity limits the power output. If the cavity width is increased, thereby increasing the power output, multiple modes are produced. 
     FIG. 1 displays five known types of single-mode laser diodes, all of which exhibit varying degrees of strengths and weaknesses. For example, straight active region  101 , which can be implemented in a ridge configuration or a buried configuration, is inexpensive and reliable. This configuration, however, incurs catastrophic optical damage when used to create 0.98 μm radiation, and it leads to power saturation when used to create 1.48 μm radiation. Thus, the configuration shown in  101  has inherent power limitations. Additionally, this configuration produces a highly divergent elliptical beam. 
     Master oscillator power amplifier (MOPA)  102  has a straight active region with a tapered output. This configuration can produce single-mode radiation at high power. It exhibits problems, however, in that it is complex to fabricate, has relatively low efficiency, and produces a highly-divergent beam with current-dependent astigmatism. 
     Unstable-cavity taper-emitting diode laser  103  exhibits a relatively high continuous-wave power. This configuration, however, has a high vertical and lateral divergence, and produces a highly divergent beam with current-dependent astigmatism. This configuration also exhibits a relatively low efficiency. 
     Diode laser with integrated beam expander  104  produces a low-divergence, non-astigmatic output beam. Unfortunately, the twin-waveguide structure exhibited in the figure is difficult to produce. In addition, this configuration exhibits additional losses and a decrease in output power. 
     Alpha-DFB laser  105  exhibits a very high single-mode power output. This configuration, however, is difficult to fabricate. Additionally, this configuration has a differential efficiency 25-30% less than for conventional laser diodes. Finally, this configuration emits beams with a relatively high beam divergence. 
     In general, those known apparatuses that include a narrow section and a tapered section (e.g., MOPA  102  and unstable cavity  103 ) do not allow for adiabatic coupling from the narrow part of the device to the tapered section. Without adiabatic coupling, additional modes are created. Furthermore, any time the coupling is nonadiabatic, the output beam&#39;s lateral divergence depends only on the width of the narrowest part of the device; the lateral divergence can be as great as that for the straight active region devices,  105 . Finally, these known devices exhibit current-dependent astigmatism. 
     Thus, a laser is needed that is relatively simple to fabricate, and that produces single-mode high-power radiation with a wide active region and a low beam divergence. 
     SUMMARY OF THE INVENTION 
     To alleviate the problems in the known art, an apparatus is provided that produces single-mode high-power laser light. In one embodiment of the present invention, a single-mode semiconductor diode laser comprises a single-mode section with a first width S 0  that satisfies the equation S 0 &lt;λ(8 Δn n) −1/2 , a tapered mode-transformer section coupled to said single-mode section, and a power-supply section coupled to said tapered mode transformer section. The power-supply section in this embodiment has a second width larger than the first width. Additionally, the tapered mode-transformer section is characterized in that optical energy from the single-mode section couples adiabatically to the mode-transformer section and then to the power-supply section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of various known single-mode pumping diode lasers. 
     FIG. 2 is a schematic diagram of a top view of an embodiment of the present invention. 
     FIG. 3 is a graph of taper angle as a function of power-supply section width, for two given coupling efficiencies. 
     FIG. 4 is a graph of mode-transformer-section length as a function of power-supply section width, for two given coupling efficiencies. 
     FIG. 5 is a schematic diagram of an embodiment of the present invention in which cavity length is shown to be optimized as a function of cleavage position. 
     FIG. 6 is a schematic diagram demonstrating that the radiation not confined in the single-mode section can be returned to the waveguide of the single-mode section after reflection. 
     FIG. 7 is a graph of fiber mode coupling coefficient as a function of single-mode section length, according to an embodiment of the present invention. 
     FIG. 8 is a schematic diagram of a top view of an embodiment of the present invention. 
     FIG. 9 is a schematic diagram of a top view of an embodiment of the present invention that includes a beam-expander section. 
     FIG. 10 is a schematic diagram of a top view of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention include single-mode lasers that generate a relatively high power output. In general, embodiments of the present invention include a section that adiabatically couples single-mode laser light to a power-supply section where the laser&#39;s output power is increased above that of laser&#39;s that do have a power-supply section. 
     FIG. 2 is a schematic diagram of a top view of an embodiment of the present invention. In this embodiment, single-mode section  201  is coupled to mode transformer section  202 , and mode-transformer section  202  is coupled to power-supply section  203 . The three sections are designed to have specific characteristics that provide in combination single-mode high-power output light. 
     With regard to single-mode section  201 , single-mode section  201  is a single-mode section of semiconductor laser that is narrow relative to the other parts of the laser. For example, single-mode section  201  can be an InP-based laser section with a lateral width of 1-4 μm that emits single-mode light at 1.48 μm. Alternatively, for example, single-mode section  201  can be a GaAs-based laser section with a lateral width of 1-3 μm, and that emits single-mode light at 0.98 μm. In general, the width S 0  of single-mode section  201  should satisfy the single-mode condition S 0 &lt;λ(8 Δn n 1 ) −1/2 , where Δn is the refractive index step at the lateral active region boundaries, and n 1  is the refractive index in the active region at wavelength λ. 
     Single-mode section  201  has a refractive index n 2  higher than the refractive index n 1  in the adjacent unpumped chip areas. The difference n 2 −n 1 , called Δn, in one embodiment of the present invention should be at least approximately 0.01. 
     Mode transformer section  202  has a length L, and a variable width that is measured by taper angle α, as shown in FIG.  2 . The width of mode-transformer section  202  is narrowest at the point where it couples to single-mode section.  201 , and widest where it couples to power supply section  203 . 
     Mode-transformer section  202  is characterized in that radiation produced in single-mode section  201  is adiabatically coupled into and adiabatically propogates in mode-transformer section  202 . To couple the radiation adiabatically from single-mode section  201  into the power-supply section, taper angle α, in one embodiment of the present invention, can be chosen such that the single-mode radiation from single-mode section  201  is coupled adiabatically into power-supply section  203 . For the purposes of the present invention, the term “adiabatically,” and its grammatically-related forms, Means that no substantial losses occur due to reflection at the boundaries between the sections, or due to any other interactions at the interface, or due to creation of additional modes. Thus, when laser light from single-mode section  201  is coupled into power supply section  203  through mode-transformer section  202 , the near-field distribution of radiation is increased without increasing the number of transverse modes in the laser cavity. 
     Power-supply section  203 , coupled to mode-transformer section  202 , is used to increase the device&#39;s output power. The device&#39;s output power is proportional to the current that can be supplied to the device, and the maximum current is proportional to the total device area shown, for example, in FIG.  2 . Thus, power-supply section  203  increases the power output of the laser by increasing the total active device area. 
     FIGS. 3 and 4 display the relationship between the physical characteristics of embodiments of the present invention for fixed transformation efficiency η t . Transformation efficiency η t  is used as the transformation coefficient for the zero mode, and is defined as the ratio of zero-mode power in the single-mode section to the zero-mode power in the power supply section. To increase the width S p  of power-supply section  203  without degrading the transformation efficiency coefficient η t , length L of the mode-transformer section cannot be merely increased while keeping taper angle α constant. Rather, to support η t  at a fixed level, α should decrease as S p  increases. This is shown in FIGS. 3 and 4. 
     FIG. 3 shows the relationship between taper angle α and power-supply section width S p . The calculations for this figure were performed for η t =99% and η t =99.9%. The values of S 0  and Δn were taken as 2 μm and 0.01, respectively. Note that varying the parameters S 0  and Δn by approximately 1.5 times does not meaningfully effect the results represented in the graphs. These calculations do not depend on the specific material structure of the laser. Rather, the results depend only on the radiation wavelength and the effective refractive indices. The power difference that corresponds to 1−η t  in the graphs results from losses due to the transformation to additional modes that can propagate in the power-supply section, thereby causing wave front distortion. The calculations were performed using the well-known Beam Propagation Method (BPM) in the effective index approximation. As can be seen from FIG. 3, for a given η t , after quickly peaking, taper angle α generally decreases as power-supply section width S p  increases. The calculations that result in FIG. 3 assume a taper with a simple linear shape. If the shape of the taper changes, the results can change as well. 
     FIG. 4 shows the relationship between mode-transformer section length L and power-supply section width S p . Again, for a given η t , the mode-transformer section length L increases superlinearly with increasing power-supply section with S p . 
     For η t =99.9%, the amplitude distortion of the wavefront is less than 10%, and the phase distortion is smaller than 0.1 rad. For η t =99%, on the other hand, the amplitude distortion increases to 30%, and the phase distortion increases to 0.3 Rad. Thus, to emit a substantially nonastigmatic beam from the power-supply section external facet, an η t =99.9% is advantageous. In general, the higher the value of η t , the lower the probability of creating additional modes. It should be appreciated that, to satisfy the conditions of a high η t , the lasers should be relatively long (i.e., longer than 1 mm), and therefore the laser structures used for the device fabrication should have very low internal losses (on the order of &lt;3 cm −1 ). 
     The functional relationships displayed in FIGS. 3 and 4 are independent of the materials used, and are scaled with the wavelengths. Thus, to find the geometry of the mode-transformer section for devices with λ different from λ 0 , the values of L and S p  should be scaled by a factor of λ/λ 0 . The length of the single-mode section and the length of the power-supply section can be varied in the process of bar cleaving, shown in FIG.  5 . 
     The length of the power-supply section depends on the losses in the device&#39;s epilayers. If losses are too high, the laser can be fabricated to limit the length of the power-supply section. The optimum length of the power-supply section can be chosen, in part, based on the value of the laser&#39;s internal losses. To keep the external efficiency high, internal losses should be less than effective output losses that are determined by the laser length, reflectivity of the output facet, and, in the high power regime, by the degree of saturation. Lasers with different power-supply-section lengths cleaved from a single wafer will have the same optical parameters and can be used for loss determination in finding the optimum length. 
     When power is supplied to the mode-transformer section, more than one transverse mode will be created. Under the proper circumstances, however, these additional modes will not couple into the single-mode section, and so will not provide feedback for lasing at these modes. FIG. 6 is a schematic diagram of an embodiment of the present invention that illustrates this phenomenon. In FIG. 6, it can be seen that the first mode (that is, the first nonzero mode) has a nonzero probability of reflecting off the chip facet and reentering the laser. This probability depends on the length of the single-mode section. The longer the section, the smaller the probability this will occur. Thus, the single-mode section can be used as a filter for the nonzero modes; the longer the single-mode section, the higher the probability that nonzero modes will be filtered out. 
     FIG. 7 demonstrates the dependence of the first-non-zero-mode coupling coefficient χ on the length of the single-mode section. The χ values characterize the additional losses that result from producing the first non-zero mode. If χ=0.15, then for the first non-zero mode the mirror losses are 85% higher than that for the zero-mode. After translating this difference in the reflectivity into the difference in the threshold gain for the fundamental and first nonzero mode, for the 2 mm laser cavity, approximately 5 cm −1  has been obtained. This difference should be enough to sustain the single mode regime and prevent the generation of first non-zero mode. Thus, from FIG. 7, it follows that the length of the single-mode section should be, optimally, between 200-400 μm for full elimination of the non-zero modes. 
     The data in FIG. 7 were calculated for the specific parameters of the single mode and taper sections, with the same BPM approach as the other calculations. A length range of the single-mode section of approximately 200 to approximately 400 μm is sufficient for suppressing the first non-zero mode for the lasers that produce 0.98 μm and 1.5 μm radiation. 
     Using the three-section basic design, different configurations of pumping sources for lasers of wavelengths shorter than 1 μm and longer than 1 μm can be applied. The difference in lateral configuration follows from the difference in the mechanisms that limit output power for short and long wavelength lasers. 
     For short-wavelength GaAs-based lasers, output power is typically limited by the output power density at the front mirror facet. Because near-field intensity is much higher at the output mirror of a laser cavity than at the rear high-reflection mirror, performance can, in some embodiments, be improved by using an output side that is broader than the reflection side. FIG. 8 is a schematic diagram of a GaAs-based embodiment of the present invention. In this embodiment, the output radiation has a wavelength of 0.98 μm. Single-mode section  801  is 2 μm wide, and can be backed by a high-reflection coating. The width of power-supply section  803  can range from approximately 3 to approximately 20 μm, depending on length W and angle α. Because output power in this embodiment is limited by catastrophic optical damage, the laser radiation is emitted from the wider power-supply section, which can be lined with an anti-reflective coating. 
     FIG. 9 is another embodiment of a GaAs-based embodiment of the present invention. In this embodiment, a beam expander  904  is coupled to single-mode section  901 . With the beam expander, laser radiation can be emitted from the power-supply section output facet with diminished likelihood of incurring catastrophic optical damage at the high-reflective expander facet. To provide nearly equal optical power density at the reflecting and the emitting facet, the width of the reflective expander facet S er  can be found from the relation: 
     
       
           S   er &gt;2 * S   0ut   *R    0.5   
       
     
     where R is reflectance at the output facet and S out  is the width of the output facet. It is seen from this relation that the width of the reflecting facet is less than the width of the power supply output facet. If S er  is smaller than S 0  the expander section is not required. 
     It should be appreciated by those skilled in the art that emitting the laser radiation from the power-supply section is not limited to GaAs-based embodiments. The discussion above is meant to illustrate an embodiment of the present invention for which catastrophic optical damage is an issue; GaAs-based lasers are discussed merely for ease of explanation. 
     FIG. 10 is an embodiment of the present invention in which single-mode section  1001  includes an output side for emitting the laser light, and power-supply section  1003  includes a reflection side. In one embodiment, the apparatus can be an InP-based laser that emits light with a wavelength of 1.48 μm. InP-based lasers belong to the class of lasers for which output power is not so severely constrained by catastrophic optical damage. Thus, in the embodiment displayed in FIG. 10, single-mode section  1001  can include an output side for emitting the laser radiation from the laser cavity. 
     The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims. For example, although GaAs-based and InP-based lasers are discussed, embodiments of the invention are not limited to these materials. Rather, embodiments of the present invention can be implemented using any materials practicable for making such lasers with the properties as described above.