Abstract:
In order to reduce the possibility of a laser operating in multiple transverse modes at high power, the laser is provided with laterally asymmetric losses that discriminate against modes higher than the fundamental mode. One approach to doing this is form an asymmetric ridge waveguide in the laser, that allows the light of the higher order modes to leak out of the waveguide.

Description:
This claims benefit of 60/409,730 filed Sep. 11, 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed generally to a semiconductor device, and more particularly to a semiconductor device with an asymmetric ridge waveguide. 
     BACKGROUND 
     Optical fiber communications have been a commercial reality now for over twenty years. Research and development efforts still continue to explore new laser sources, new forms of optical amplifiers, novel materials for low-loss optical fibers, and the like. At present, long-haul fiber optic communication systems employ small core (on the order of a few microns) single-mode fibers used in conjunction with laser diodes also operating in their single-mode regime, typically the lowest order transverse mode. This configuration yields low intermodal dispersion and allows for what is commonly called dense wavelength multiplexing (DWDM). 
     Today, low-loss single mode fibers and laser diodes that operate near optimum wavelength for low loss propagation are readily available. Even so, in-line optical amplifiers (all-optical repeaters) are necessary approximately every 400 kilometers (≅250 miles) or so for long haul applications. One such in-line optical booster is the erbium-doped fiber amplifier. This device is capable of simultaneously amplifying multiple wavelengths (multiple optical channels) by optically pumping from a secondary pump laser operating at a shorter wavelength. Notwithstanding these advances, research is still underway to increase the distance between optical repeaters. One approach is to increase the optical energy from the pump laser coupled into the fiber. Practical challenges arise from this approach. First, laser diodes driven at high input currents may transition from single-mode to multimode operation, indicative of the so-called “kink effect” in the light output versus injection current characteristic. Although the laser may have more total output optical energy in the multi-mode state, there are at least two negative drawbacks to this straightforward approach. First, when lasing in the multi-mode regime, the optical energy is less confined near the optical axis of the laser and more divergent as the beam exits the laser end-face. Given this, the coupling efficiency for focusing the laser output into a fiber is reduced. Secondly, the multi-mode operation of the laser may increase the intermodal dispersion in the fiber so as to decrease the available bandwidth for multichannel operation. 
     Given the above, there is a need for a technique to operate laser diodes at increased optical output levels while constraining the laser to continue to operate in the single-mode regime. 
     SUMMARY OF THE INVENTION 
     An approach followed in the present invention to overcome the problem of the “kink effect” is to provide a laser having laterally asymmetric losses that discriminate against modes higher than the fundamental mode. 
     One embodiment of the invention is directed to a semiconductor optical device that has a substrate and first and second semiconductor side regions formed over the substrate. The height of the first side region above the substrate is different from a height of the second side region above the substrate. A semiconductor ridge is disposed between the first and second side regions and over the substrate forming an optical waveguide within the device. The height of the semiconductor ridge above the substrate is greater than the heights of the first and second side regions above the substrate. 
     Another embodiment of the invention is directed to an optical communications system that has an optical transmitter, a fiber optic link coupled to receive optical signals from the optical transmitter, and an optical receiver coupled to the fiber optic link to receive the optical signals. A laser is coupled to inject light into the fiber optic link. The laser includes a substrate having first and second semiconductor side regions formed over the substrate. The height of the first side region above the substrate is different from the height of the second side region above the substrate. A semiconductor ridge is formed between the first and second side regions and over the substrate to introduce an optical waveguide within the device. The height of the semiconductor ridge above the substrate is greater than the heights of the first and second side regions above the substrate. 
     Another embodiment of the invention is directed to a semiconductor laser that has a substrate having an upper surface. A lateral direction is defined parallel to the upper substrate surface. One or more superstrate layers are provided on the substrate. An optical waveguide is disposed over the substrate to guide light passing between ends of the substrate and defining a fundamental optical mode. First and second sides of the optical waveguide provide optical confinement in the lateral direction. The optical confinement for the fundamental optical mode provided on the first side of the optical waveguide is different from the optical confinement provided on the second side of the optical waveguide. 
     Another embodiment of the invention is directed to an optical communications system that has an optical transmitter, a fiber optic link coupled to receive optical signals from the optical transmitter, and an optical receiver coupled to the fiber optic link to receive the optical signals. A laser is coupled to inject light into the fiber optic link. The laser includes a substrate and one or more superstrate layers provided on the substrate. An optical waveguide is disposed over the substrate to guide light passing between ends of the substrate. The waveguide defines a fundamental optical mode. First and second sides of the optical waveguide providing optical confinement in the lateral direction. The optical confinement for the fundamental optical mode provided on the first side of the optical waveguide is different from the optical confinement provided on the second side of the optical waveguide. 
     Another embodiment of the invention is directed to a method of forming a semiconductor device having an asymmetric ridge. The method includes removing material on a first side of a ridge region to a first depth, and removing material on a second side of the ridge region to a second depth different from the first depth, so as to produce the asymmetric ridge in the ridge region. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates an optical communications system that includes a laser according to an embodiment of the present invention; 
         FIG. 2  is a block schematic diagram illustrating a frequency selected laser unit according to the present invention; 
         FIG. 3  illustrates a semiconductor laser with an asymmetric ridge waveguide according to the present invention. 
         FIG. 4  illustrates a first step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device. 
         FIG. 5  illustrates a second step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device. 
         FIG. 6  illustrates a third step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device. 
         FIG. 7  illustrates a fourth step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device. 
         FIG. 8  illustrates a fifth step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device. 
         FIG. 9  illustrates a sixth step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device. 
       While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     
    
    
     DETAILED DESCRIPTION 
     In general, the present invention is directed to a semiconductor device that has an asymmetric ridge waveguide. In particular, the material on one side of the ridge is at a different height from the material on the other side of the ridge. A laser formed with an asymmetric waveguide has improved wave-guiding properties in that there is less lateral confinement for higher order modes above the fundamental mode. As a result, the threshold for a higher order mode is higher than in a laser having an equivalent symmetrical ridge, and so the power level at which the output from the laser changes from being single transverse mode to including a higher order transverse mode is increased. Accordingly, the “kink power”, the power at which the kink occurs in the L-I curve of the laser, is higher, and the laser may be used to operate in a single transverse mode at higher power levels. One of the advantages of higher power operation with a single transverse mode is in pumping fiber devices, such as fiber amplifiers or fiber lasers. The occurrence of the kink is usually accompanied with a reduction in the coupling efficiency into the fiber because the higher order mode is less focusable than the fundamental transverse mode. The increase in the kink power, therefore, permits increased amounts of light to be coupled into the fiber device with high efficiency. 
     A laser having an asymmetric waveguide according to the present invention may be employed in a DWDM communications system  100 , schematically illustrated in  FIG. 1 . The system  100  includes a dense wavelength multiplexed (DWDM) transmitter unit  102  that includes a number of lasers  104   a–   104   n  operating at different wavelengths, λ 1 –λn. In addition, one or more spare lasers  105  may operate as a substitute if any of the lasers  104   a–   104   n  fail. The lasers  104   a–   104   n  and  105  may each include modulators for modulating information onto the respective output light beams. The outputs from the lasers  104   a–   104   n ,  105  may be combined in a DWDM combiner arrangement  106  and launched as a DWDM signal into an optical fiber communications link  108  that is coupled to a DWDM receiver  110 . 
     The transmitter  102  may also include a pump laser unit  112  for transmitting pump light into the fiber link  108 . For example, the pump laser unit  112  may produce pump light having a wavelength, λp, that pumps a fiber amplifier. In one embodiment, the pump laser unit  112  generates light at a wavelength that provides Raman gain for the DWDM signal. The pump light and the DWDM signal may both be injected in to the fiber link  108  from the transmitter  102 , with the result that the DWDM signal is amplified in the initial portion of the fiber link leaving the transmitter. 
     The fiber link  108  may include one or more fiber amplifier units  114  to amplify the DWDM signal as it propagates to the DWDM receiver  110 . An amplifier unit  114  typically includes a length of fiber that provides gain to the DWDM signal and one or more pump lasers in a pump laser unit  116 . The gain fiber may be, for example a rare earth-doped fiber, such as an erbium-doped fiber amplifier (EDFA), or may be a Raman fiber amplifier (RFA). The pump lasers in the pump laser unit  116  provide pump light at a wavelength appropriate for pumping the fiber amplifier. Where the fiber amplifier is an EDFA, the pump light may have a wavelength of around 980 nm. Where the fiber amplifier is a RFA, the pump light may have a wavelength different from the wavelength of the DWDM signal by an amount equal to the Raman Stokes shift. 
     Other elements, such as isolators, switches, add/drop multiplexers and the like may also be disposed along the fiber link  108 . The DWDM receiver  110  demultiplexes the received DWDM signal in a demultiplexer  118  and directs signals at different wavelengths λ 1 –λn to respective channel detectors  120   a–   120   n.    
     A block schematic diagram showing various elements of a pump laser unit  200  that uses a laser according to an embodiment of the present invention is illustrated in  FIG. 2 . A laser  202  generates an output light beam  204  that is coupled to a fiber  206 , typically using a lens system  208  having one or more lenses. The fiber  206  may include a fiber Bragg grating (FBG)  210  that provides feedback to the laser  202  at the selected pump wavelength, λp. This helps to maintain the output wavelength of the laser  202  at the desired pump wavelength, λp. The output wavelength of the laser may be selected using another technique. In some embodiments, a grating such as a distributed Bragg grating may be integrated with the laser  202 , or a FBG or other frequency selective element may be provided at the rear end  212  of the laser  202  to provide frequency selective feedback, thus stabilizing the wavelength of the light  204  output from the laser. 
     The laser  202  may be any suitable type of semiconductor laser that produces an output at a desired wavelength. For example, to produce output at around 980 nm for pumping an EDFA, the laser  202  may use an InGaAs quantum well active region grown over a GaAs substrate. To produce an output in the range 1400 nm –1500 nm for providing Raman gain, the laser  202  may use an InGaAsP quantum well active region formed over an InP substrate. 
     A monitor  214 , for example a tap monitor, may be used to monitor the output power from the laser  202 . The monitor may be positioned to measure a portion of the laser output, as illustrated, or may be positioned at the back of the laser  202  to measure the power leaking out of the rear end  212 . A controller  216  includes the power supply  218  for providing power to the laser  202 . The power supply  218  may be coupled to receive a power signal from the tap monitor  214 , and may regulate the amount of current directed to the laser so as to maintain a constant output power. 
     The laser  202  is typically mounted on a submount  220  that is thermally coupled to a cooling unit  222  that removes excess heat from the laser  202 . The cooling unit  222  may be an active cooler, such as a thermoelectric cooler. Where the cooling unit  222  is an active cooler, the controller  216  may include a cooler control unit  224  coupled to control operation of the cooling unit  222 . 
       FIG. 3  illustrates an end-on view of a semiconductor laser diode  300  with an asymmetric ridge waveguide  302 . The asymmetric ridge waveguide  302  is shown with a left-hand side of the ridge  302 L, having a different height from that of the right-hand side of the ridge  302 R. Bonding pads  304  are typically situated on either side of the asymmetric ridge waveguide  302  for making electrical contact to the laser diode  300 . The laser diode  300  is formed from a number of superstrate layers  306  formed over a substrate  308 . The bottom surface of the substrate  308  typically has a metallization layer  312  for soldering to submount assemblies. The superstrate layers  306  include an active layer  310 , which may be a quantum well active layer. The superstrate layers  306  may also include layers that, in tandem with the asymmetric ridge waveguide  302 , provide optical confinement of laser light within the laser diode  300  in the vertical and/or horizontal direction. 
     The right-hand side of the ridge  302 R may be further in depth into the superstrate  306  and may therefore be closer in proximity to the active region  310  than the left-hand side of the ridge  302 L. The slopes of the ridge surfaces  302 L and  302 R may be the same, or the slope of the right-hand side ridge  302 R may be different from the slope of the left-hand ridge  302 L. In this case, the right-hand side ridge  302 R may be sufficiently close to the active region  310  so as to generate tight lateral confinement/waveguiding. In contrast, the left-hand side ridge  302 L may be sufficiently far removed from the active region  310  so that the waveguide provides weak lateral confinement of higher order modes, while still efficiently guiding the fundamental mode. The net result of this asymmetric configuration, with stronger confinement on one side of the waveguide relative to the other side, may result in the waveguide being less efficient in guiding higher order modes than the fundamental lowest order mode of the waveguide, which leads to an increase in the threshold condition for lasing action for the higher order modes. This may allow the laser  300  to be operated at higher single-mode power levels before the so-called “kink effect,” indicative of the higher order modes reaching threshold, becomes apparent. 
     Similar results may be obtained if the ridge geometry were reversed, for example, the left-hand ridge  302 L may be sufficiently close to the active region  310  to provide tight lateral confinement and the right-hand ridge  302 R may be sufficiently far removed to yield weak lateral confinement of the higher order modes. 
     Asymmetric ridge waveguides may be fabricated by a technique which includes two (or more) optical masks and multiple masking steps for the left and right-hand ridge regions. Fabrication of asymmetric ridge waveguides utilizing this technique may require difficult mask-to-mask alignment accuracies on the order of a micron or less. 
     Another approach to fabricating an asymmetric waveguide laser that employs a single mask and a single masking step is described with reference to  FIGS. 4–9 .  FIGS. 4–9  schematically illustrate a device at different stages of fabricating an asymmetric ridge waveguide according to one particular embodiment of a manufacturing method.  FIG. 4  illustrates the first step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device  400 . The semiconductor device  400  includes a semiconductor substrate  402  with a superstrate layer  404  fabricated, for example, by an epitaxial process such as molecular beam epitaxial (MBE) growth or metal organic chemical vapor deposition (MOCVD). A photoresist layer  406  is deposited on the upper surface of the semiconductor device  400 . A three region optical mask having of a left-hand region  408 L, a middle region  408 M, and a right-hand region  408 R may be placed over the photoresist layer  406 . The three regions of the optical mask  408 L,  408 M, and  408 R have different optical transmission efficiencies. To form an asymmetric ridge with a greater depth on the right-hand side of the ridge, the right-hand side of the optical mask  408 R may have a transmission efficiency near 100%, whereas the left-hand side of the mask  408 L may have a transmission efficiency less than 100%. The middle region of the mask  408 M typically is optically opaque at the exposure wavelength, with nearly 0% transmission. 
     The top surface of the semiconductor device  400  may be illuminated with equal intensities of light  410  incident upon all three regions of the optical mask  408 L,  408 M, and  408 R. Sufficient light  410  may be delivered through the mask such that the photoresist under the right-hand side of the mask  408 R may be fully exposed while the photoresist under the left-hand region  408 L may be only partially exposed. Under these circumstances, the photoresist under the middle region  408 M may be effectively unexposed. After delivery of the illumination light  410 , the optical mask may be removed. 
       FIG. 5  illustrates the results of the second step in the manufacturing process. In the second step, a developer solution may be applied to the top surface of the semiconductor device  500 , which totally removes the photoresist on the top right-hand side of the device  500 . The left-hand region of the photoresist  506 L is partially removed since it was only partially exposed. The middle portion  506 M of the photoresist remains unexposed, and so its thickness is effectively unchanged from its original thickness. 
       FIG. 6  illustrates the results of the third step in the manufacturing process. A chemical etchant has been applied to the top surface of the device  600  and etching has occurred to form the beginnings of the right-hand side ridge  608 R. At this stage, the remaining left-hand  506 L and mid-region  506 M photoresist has protected the remainder of the top surface from chemical etching. 
       FIG. 7  illustrates the results of the fourth step in the manufacturing process. In this step, additional UV light has been delivered to the top surface of the device  700  to fully expose the left-hand region of the photoresist (not shown) and partially expose the mid-region  506 M of the photoresist. The top surface is then treated with developer to completely remove the left-hand portion of the photoresist but leave behind a partial thickness of the mid-region photoresist  506 M. 
       FIG. 8  illustrates the results of the fifth step in the manufacturing process. In this step a chemical etchant has been applied to the top surface of the device  800  and etching has occurred to form the final geometry of the left-hand side region  808 L and right-hand side region  808 R of the asymmetric ridge. 
       FIG. 9  illustrates the results of the sixth and final step in the manufacturing process of forming an asymmetric ridge waveguide on a semiconductor device  900 . In this step the remaining photoresist on the middle region is removed by applying a chemical agent such as acetone. The semiconductor device  900  may now undergo additional processing steps to fabricate, for example, bonding pads, bottom-surface metallization and the like. 
     An advantage of this approach is that a single masking step is required, thus simplifying the manufacturing process of the asymmetric ridge by avoiding the need to align a second mask with a first exposed portion of the semiconductor device. 
     As noted above, the present invention is applicable to semiconductor devices and is believed to be particularly useful for semiconductor devices with as asymmetric waveguide. The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.