Single mode laser with large optical mode size

A laser including a grating configured to reduce lasing threshold for a selected vertically confined mode as compared to other vertically confined modes.

BACKGROUND

Field

This application relates generally to single-mode lasers.

Description of the Related Art

Lasers are widely used in telecommunications, sensing, and test and measurement applications. Many high-power lasers are not single-mode while many single-mode lasers do not provide high optical powers.

SUMMARY

High-power single-mode lasers that are capable of providing high optical power and single mode operation can be useful for many applications. Example embodiments described herein have several features, no single one of which is indispensable or solely responsible for their desirable attributes. Without limiting the scope of the claims, some of the advantageous features will now be summarized.

Certain embodiments provide a laser comprising a waveguide configured to support a vertically confined fundamental optical mode and at least one vertically confined higher order optical mode. The laser further comprises an active region at a first position with respect to the waveguide. The laser further comprises a grating at a second position with respect to the waveguide. The first position of the active region and the second position of the grating are configured to reduce a first lasing threshold for the fundamental optical mode and to increase a second lasing threshold for the at least one higher order optical mode.

Certain embodiments provide a method for designing a laser comprising a waveguide, an active region, and a grating. The method comprises providing a position of the active region and a position of the grating. The method further comprises calculating at least a vertically confined first optical mode and at least one vertically confined second optical mode supported by the waveguide for the position of the active region and the position of the grating. The method further comprises adjusting the positions of the active region and the grating such that a first product of an overlap of the first optical mode with the grating and an overlap of the first optical mode with the active region is greater than a second product of an overlap of the at least one second optical mode with the grating and an overlap of the at least one second optical mode with the active region. The method further comprises re-calculating at least the first optical mode and the at least one second optical mode and determining perturbations of at least the first optical mode and the at least one second optical mode resulting from the adjusted positions of the active region and the grating. The method further comprises calculating a difference between the first product and the second product. The method further comprises adjusting, if the difference is less than a threshold value, the positions of the active region and the grating such that the first product is larger than the second product.

These and other features will now be described with reference to the drawings summarized above. The drawings and the associated descriptions are provided to illustrate embodiments and not to limit the scope of the disclosure or claims. Throughout the drawings, reference numbers may be reused to indicate correspondence between referenced elements.

DETAILED DESCRIPTION

Semiconductor lasers are widely used in many applications ranging from telecommunications to sensing, medical applications, and optical pumping (e.g., pumping other laser mediums or amplifier mediums). Semiconductor lasers can comprise a substrate including an optically active layer thereon. In some implementations, the optically active layer can be configured as an epitaxial layer grown over the substrate using semiconductor growth technology. For many applications, it can be advantageous for these semiconductor lasers to output a single longitudinal mode. One method for achieving single longitudinal mode lasing in III-V semiconductor lasers is by incorporation of a Bragg grating, to create either a Distributed Feed Back (DFB) Semiconductor Laser or a Distributed Bragg Reflector (DBR) Laser. Many embodiments of DBR or DFB laser devices can be configured as edge emitting laser devices that are configured to output light from an edge of the laser device. The plane including the edge can be generally oriented along a direction normal to the substrate of the DBR or DFB laser device. The light output from certain embodiments of the edge emitting DBR and DFB laser devices can be coupled to fibers or other passive waveguide circuits. However, the optical mode output from various embodiments of DFB or DBR lasers can be small and/or asymmetrical and may not be matched with the size and/or shape of optical fibers or waveguides of other passive waveguide circuits into which the light from the DBR or DFB laser device is to be coupled. Accordingly, certain embodiments described herein advantageously provide edge emitting laser devices that are configured to output a single spatial mode having a large size that is matched with the size and/or shape of optical fibers or waveguides of other passive waveguide circuits into which the light from these laser devices is to be coupled.

Certain embodiments described herein provide edge emitting semiconductor laser devices that comprise mode converters that are configured to convert the mode of the light output from the edge emitting semiconductor laser devices to larger and/or a more symmetric shape that can be easily coupled into a glass optical fiber, a plastic optical fibers, a polymer waveguide, a doped glass waveguide, a silicon waveguide, and/or a silicon nitride waveguide. In certain such embodiments, the mode converters can advantageously provide a low loss optical connection between the laser and other optical components.

Without relying on any particular theory, the near field optical mode size, in many implementations of semiconductor lasers comprising a III-V material can be small and elliptical. In many implementations, the full width of the elliptical mode along the minor axis (e.g., the distance along the minor axis between positions at which the intensity is 1/e2of the maximum intensity of the elliptical mode) can be approximately less than or equal to 1 micron and the full width of the elliptical mode along the major axis (e.g., the distance along the major axis between positions at which the intensity is 1/e2of the maximum intensity of the elliptical mode) can be less than or equal to 5 microns. The minor axis of the elliptical mode can be oriented parallel to the crystal growth direction. This type of elliptical mode profile that is compressed along a direction parallel to the crystal growth direction may not be compatible with the size and shape of many implementations of optical fibers and/or doped glass waveguides. For example, many implementations of optical fibers and/or doped glass waveguides can comprise a circular core having a diameter of about 9 microns. Many implementations of SiNxwaveguides can be configured to have an elliptical cross-sectional shape such that an optical mode having an elliptical shape mode can be efficiently in-coupled into the waveguide. However, even in such implementations, the small size of the optical mode can make the optical alignment process difficult as even a small misalignment can increase optical losses.

One approach to increase the size of the optical mode includes providing a symmetric buried heterostructure waveguide with a tapered section which allows a large optical mode to be supported by a small, symmetric buried waveguide. The size of the optical mode is adiabatically enlarged over the length of the taper. However, some optical loss can be incurred as the optical mode propagates through the length of the taper over which the optical mode is adiabatically enlarged. This approach is advantageous to increase the size of optical modes that are symmetric but have a size less than or equal to about one micron. Other approaches to increase the size of an optical mode can include waveguide couplers in which the optical mode from the laser is coupled vertically or laterally to an adjacent passive waveguide that can support an optical mode with a large optical size. However, these approaches can introduce absorption and scattering optical losses as the optical mode propagates through the length of the passive waveguide over which the mode transfer occurs.

Another approach to increase the size of the optical mode output from a semiconductor laser includes using large waveguide cores. However, this approach may not be practical for single mode operation because most conventional laser designs on semiconductor materials (e.g., materials from the III-V group) become multimode when the waveguide thickness is increased such that the core of the waveguide is enlarged. A first implementation of a semiconductor laser capable of outputting a large and symmetric optical mode is a slab-coupled optical waveguide laser (SCOWL), which uses a large weak confinement slab waveguide beneath the active region. The size of the near-field optical mode in such structures can be 2 to 3 microns in diameter, or even larger.FIG. 1schematically illustrates an example implementation of a SCOWL comprising a cladding region103(e.g., substrate), a waveguide layer102(e.g., slab waveguide) over the cladding region103, an active region101over and parallel to the waveguide layer102, and a ridge100over the waveguide layer102. The active region101can comprise quantum wells, a bulk material, quantum dots, quantum lines or quantum dashes that provide optical gain to the laser. The active region101can have a higher refractive index than the material of the waveguide layer102and the material of the ridge100. In various implementations, the waveguide layer102can comprise a quaternary material (e.g., a combination of indium phosphide (InP) and some other material). In various implementations, the ridge100can also comprise quantum wells.

In various implementations, the thickness of the waveguide layer102can be between about 0.5 micron and about 20 microns. For example, the thickness of the waveguide layer102can be greater than 0.5 micron and less than or equal to 2 microns, greater than or equal to 1.5 microns and less than or equal to 5 microns, greater than or equal to 4 microns and less than or equal to 8 microns, greater than or equal to 7.5 microns and less than or equal to 10 microns, greater than or equal to 9.0 microns and less than or equal to 15 microns, greater than or equal to 12.5 microns and less than or equal to 20 microns, or any value in any range and/or sub-range defined by these values.

In various implementations, the cladding region103can comprise semiconductor materials such as, for example, InP, AlGaAs, InGaP, or combinations thereof. The waveguide layer102can comprise semiconductor materials such as, for example, InGaAsP, AlInGaAs, AlGaAs or combinations thereof. In implementations in which the cladding region103and the waveguide layer102comprise AlGaAs, the doping concentration of AlGaAs in the waveguide layer102can be different from the doping concentration of AlGaAs in the cladding region103. The ridge100can comprise semiconductor materials such as, for example, InP, AlGaAs having a same doping concentration as the AlGaAs of the cladding region103, InGaP, or combinations thereof.

In various implementations of the SCOWL, as schematically illustrated byFIG. 1, an optional passive layer107can be disposed over exposed portions of the surface of the semiconductor laser (e.g., excluding surface portions that are configured to provide electrical contact to the various layers and/or regions of the SCOWL). A conducting material (e.g., a metal)108can be disposed on surface portions that are configured to provide electrical contact to the various layers and/or regions of the SCOWL. The profile of the fundamental optical mode104of the light output from the SCOWL is shown on the left-side ofFIG. 1. The fundamental mode104is in the waveguide layer102and not localized around the quantum wells of the active region101. As discussed above, it can be difficult to design the waveguide layer102(e.g., having a thickness between 0.5 micron and 20 microns) such that the light output is single mode. Furthermore, it can be difficult to fabricate the thick waveguide layer102to have an index of refraction that is less than about 4% of the refractive index of the cladding region103.

A second implementation of a semiconductor laser capable of outputting a large and symmetric optical mode is a super-large optical cavity (SLOC) laser, an example of which is schematically illustrated inFIG. 2. Like the SCOWL, a SLOC laser comprises a cladding region103(e.g., substrate), a ridge100, and a waveguide layer201between the cladding region103and the ridge100. In the SLOC laser, the active region101that provides the optical gain in the laser is positioned within the waveguide layer201such that the overlap with the active region101of the fundamental mode104asupported by the waveguide layer201is greater than the overlap with the active region101of any of the higher order modes supported by the waveguide layer201(e.g., second order mode104b; third order mode104c). The waveguide layer201in various implementations of the SLOC laser can be configured similar to the waveguide layer102of the SCOWL described above. For example, the waveguide layer201can comprise materials similar to the waveguide layer102and/or can have a thickness in a range similar to the thickness range of the waveguide layer102. The waveguide layers in a SLOC laser or a SCOWL can be configured to output light with circular mode profiles. Accordingly, implementations of the SLOC laser and the SCOWL can advantageously provide optical modes having a size and a shape that are compatible to be in-coupled into optical fibers or other waveguide devices with reduced optical losses as compared to lasers comprising tapered mode converters. Moreover, additional epitaxial growth steps are not required as in buried heterostructure spot-size converters.

Certain embodiments described herein utilize laser designs and/or architectures that comprise a waveguide layer having an enlarged thickness (e.g., similar to the SCOWL and SLOC laser architectures described above) and further comprising a grating (e.g., grating layer; grating structure) in the laser cavity to filter the vertically confined modes of the laser down to fewer vertically confined modes (e.g., to a single vertically confined mode). As used herein, the term “vertically confined mode” has its broadest reasonable meaning, including referring to a mode that is confined in a direction parallel to the growth direction of the semiconductor crystal. Various laser structures described herein can be configured to output light having wavelengths between about 200 nanometers and about 8000 nanometers. Certain embodiments described herein comprise a laser that is grown on a substrate comprising GaAs, InP, silicon, or other crystalline materials. Certain embodiments described herein comprise a cladding region that includes materials such as, for example, InP, AlGaAs, GaAs, AlInGaAs, AlInGaP, InGaAsP, InGaP, InGaAs, InAsP. Similarly, certain embodiments described herein comprise a waveguide layer and an active region that comprise any of the materials described above, as well as others, such as GaN, AlGaN. Certain embodiments described herein comprise a grating layer placed so as to suppress lasing of higher order modes and enhance the lasing of a fundamental mode.

FIG. 3schematically illustrates an example laser in accordance with certain embodiments described herein. The laser comprises a region103including a cladding material103, a layered waveguide structure105, an active region101over the layered waveguide structure105, and a ridge100over the layered waveguide structure105. The layered waveguide structure105comprises a plurality of alternating layers comprising a first material and a second material different from the first material. The plurality of layers comprising the first material can be interleaved with the plurality of layers comprising the second material. In certain embodiments, the first material is the material of the cladding region103and the second material has a refractive index higher than the refractive index of the first material. In certain embodiments, the difference between the refractive index of the second material and the refractive index of the first material is less than about 0.4%. In certain embodiments, the thickness of the individual layers comprising the first material and the second material depends on the refractive index of the first and the second material. For example, the thickness of an individual layer comprising the second material can be between about 0.01 micron and about 0.5 micron. By tailoring the ratio of thickness of the layers comprising the first material and the layers comprising the second material, the layered waveguide structure105can be configured as a weakly confining waveguide. For example, the ratio of the thickness of the layers comprising the first material and the layers comprising the second material can be between about 1:20 and 20:1 to achieve weak confinement in the waveguide. The layered waveguide structure105of certain embodiments advantageously has a refractive index substantially close to the refractive index of the material of the cladding region103, such as, for example less than 4% of the refractive index of the cladding region103. The total thickness of the layered waveguide structure105can be less than about 20 microns in certain embodiments. For example, the total thickness of the layered waveguide structure105can be less than or equal to about 2 microns, less than or equal to about 5 microns, less than or equal to about 10 microns, and/or greater than 0.5 micron. The total thickness of the layered waveguide structure105can have a value in a range/sub-range defined by any of these values. In certain embodiments, the laser ofFIG. 3has an optical confinement low enough to cut-off all higher order modes except the fundamental spatial mode in the X-Y plane parallel to the cross-section shown inFIG. 3.

In certain embodiments, the laser comprises a grating layer106over the active region101in the ridge100, as schematically illustrated byFIG. 3. The grating layer106is configured to reduce the number of lasing longitudinal modes. For example, the grating layer106can be configured to reduce the number of lasing longitudinal modes to a single longitudinal mode. In certain embodiments, as schematically illustrated inFIG. 3, the waveguide has a layered waveguide structure, while in certain other embodiments, the waveguide has a structure similar to that of the waveguide layer102schematically illustrated inFIG. 1.

In certain embodiments, the grating layer106is in the active region101(e.g., within the active quantum well region), while in certain other embodiments, the grating layer106is in the layered waveguide105, as schematically illustrated inFIG. 4. The grating layer106can be positioned to interact strongly with the fundamental mode (or the first order mode) and to interact weakly (or not interact at all) with the second order mode. For example, the grating layer106can be positioned to coincide with the peak of the fundamental mode104aand the null of the second order mode104b. In certain such embodiments, despite the second order mode104bhaving comparable or better overlap with the active region101, only the fundamental mode104alases because the feedback from the grating layer106for the second order mode104b(or the coupling between the grating layer106and the second order mode104b) is much lower as compared to the feedback from the grating layer106for the fundamental mode104a(or the coupling between the grating layer106and the fundamental mode104a). In certain such embodiments, the second order mode104bhas a higher lasing threshold as compared to the fundamental mode104a. For example, in the example laser ofFIG. 4, when the fundamental (e.g., first order) mode104abegins lasing, the carrier threshold is approximately clamped, and additional injection current only serves to enhance the power of the fundamental mode104a. In certain embodiments, as schematically illustrated inFIG. 4, the waveguide has a layered waveguide structure, while in certain other embodiments, the waveguide has a structure similar to that of the waveguide layer102schematically illustrated inFIG. 1.

In certain embodiments, the grating layer106comprises a material having higher or lower refractive index as compared to the material of the waveguide105or the cladding region103. In certain embodiments, as schematically illustrated inFIG. 5, the grating layer106can be etched in one or more layers of the layered waveguide structure105.FIG. 5schematically illustrates a cross-section of the example laser ofFIG. 4in the Y-Z plane with the Z axis oriented along the left to right direction. The ridge etch region100is schematically illustrated inFIG. 5by diagonal line hatching and includes the active region101. In certain embodiments, as schematically illustrated byFIG. 5, a single layer of the layered waveguide105is etched to form the grating106, while in certain other embodiments, multiple layers of the layered waveguide105are etched to form the grating106. This approach of etching the grating106into one or more layers of the layered waveguide105can advantageously allow a single calibration of the example laser during fabrication of both the grating106and the layers of the layered waveguide105.

Growing a thick slab waveguide102comprising a material that is different from the material of the cladding region103over the cladding region103can be difficult and can cause defects in the thick slab waveguide102. In certain embodiments, the layered waveguide105(e.g., comprising relatively thinner layers of the waveguide material alternating with thin layers of the cladding material) is simpler to fabricate than a slab waveguide102. For example, the layered waveguide105can comprise relatively thin layers of a waveguide material comprising quaternary or ternary layers or other layers interleaved with relatively thin layers of InP that are grown on a cladding region103comprising InP. The grating etch can etch or punch through one or more of the non-InP layers, resulting in a very well controlled coupling coefficient where the thickness of the grating106can be controlled only by the thickness of the non-InP layers. Another stack of relatively thin layers of InP interleaved with relatively thin layers of the waveguide material can be further grown over the waveguide layer or layers comprising the grating106.

FIG. 6schematically illustrates an example laser having a SLOC-like laser structure in accordance with certain embodiments described herein. The example laser comprises a layered waveguide105configured to form a weakly confining waveguide and can support two or more guided modes (e.g., by appropriate tailoring of the thickness and refractive index of the various layers of the layered waveguide105). As in a SLOC laser, the example laser ofFIG. 6comprises an active region101comprising one or more quantum wells configured (e.g., positioned within the layered waveguide105) to have a first overlap with the fundamental optical mode104aand a second overlap with a higher order mode (e.g., second order mode104b), the second overlap less than the first overlap. The example laser ofFIG. 6further comprises a grating layer106near the active region101and positioned within the layered waveguide105. In certain embodiments, the grating layer106is configured to provide overlap with the fundamental mode104ato a larger extent as compared to the higher order mode (e.g., the second order mode104b), thereby achieving both higher gain and higher feedback for the fundamental mode104acompared to the higher order mode (e.g., second order mode104b) which will result in a much lower threshold gain for the fundamental mode104acompared to the higher order mode (e.g., second order mode104b). In certain other embodiments, the grating layer106is positioned elsewhere in the example laser structure, because the second order mode104bcan experience virtually no gain and will not lase even if the second order mode104bexperiences a somewhat higher coupling coefficient than does the fundamental mode104a.

In certain embodiments, for lower confinement within the quantum wells and/or placement of the quantum wells closer to one side of the waveguide (e.g., at the top near a p-doped side of the waveguide in a laser grown with n-doping on the bottom), the grating layer106is positioned at or near a null or minimum of the second order mode104b, as schematically illustrated inFIG. 7. In certain embodiments, the second order mode104bcan experience slightly more gain than does the fundamental optical mode104a, and the grating layer106is configured to interact only with the fundamental mode104asuch that only the fundamental mode104alases. The example laser inFIG. 7can have a lower quantum well overlap with the lasing mode and thus can be conducive to higher power distributed feedback (DFB) laser designs. In certain embodiments, hole injection to the quantum well and electron confinement in the quantum well can result in better performance with the quantum well or other gain medium near the p-cladding. In certain such embodiments, the grating layer106selectively lowers the threshold of the fundamental mode104a, despite the placement of the gain nearer to the most concentration of light in the higher order cavity mode (e.g., the second order mode104b). In certain embodiments, the active region101and the grating layer106are configured (e.g., positioned) to select only the second order mode104b. In certain embodiments, the active region101and the grating layer106are configured (e.g., positioned) to select two modes (e.g., fundamental mode104aand the second order mode104b).

In certain embodiments, the grating106in a SLOC-like laser architecture provides an additional parameter to suppress higher order modes to ensure single mode lasing. For example, the active region101(e.g., comprising quantum wells) can be placed such that the fundamental mode104aexperiences higher gain than do other higher order optical modes (e.g., the second order mode104b). Tailoring the placement of the grating layer106can provide an additional mode selection method to preferentially select the fundamental mode104a(or, similarly, to deselect other higher order modes).FIG. 8schematically illustrates an example laser comprising both an active region101and a grating layer106within a large optical cavity that is configured to support multiple modes in accordance with certain embodiments described herein. The example laser ofFIG. 8uses both the placement of the active region101and the placement of the grating layer106to select only the fundamental mode104a. For example, the active region101ofFIG. 8is positioned within the thick waveguide201such that the active region101coincides with a null or a minimum of the second order mode104b, and the grating layer106is positioned within the thick waveguide201such that the grating layer106coincides with a null or a minimum of the third order mode104c. In certain embodiments, the thickness of the waveguide201is between about 0.5 micron to 20 microns (e.g., depending on the wavelength of light that the example laser is designed to emit).

In certain embodiments, the active region101and/or the grating layer106is configured (e.g., positioned) to suppress lasing of one or more modes (e.g., all but the fundamental mode104aor some selected higher order mode). For example, as shown inFIG. 9, the grating layer106can be positioned within the thick waveguide201to coincide with a null or a minimum of the second order mode104bso as to suppress the second order mode104b, and the active region101can be positioned within the thick waveguide201to coincide with a null or a minimum of the third order mode104cso as to suppress the third order mode104c.FIG. 10schematically illustrates another example laser in which the active region101and the grating layer106are both positioned in regions of the thick waveguide201that are equally unfavorable to the second order mode104band the third order mode104c, while being favorable to the fundamental mode104a(e.g., coincide with or overlap with the peak of the fundamental mode104a). In certain embodiments, the thick waveguide201can comprise a slab of a material such as, for example, InGaAsP or AlInAs. In certain embodiments, the waveguide201can be a layered waveguide comprising alternate layers of a first material and a second material, as discussed above in connection withFIGS. 3, 4 and 6.

The method of designing and/or fabricating a laser by positioning the active region101and/or the grating layer106to selectively reduce the number of spatial modes supported by the waveguide is not limited to ridge waveguide architectures (e.g., schematically illustrated inFIGS. 3-10), but are also applicable to a wide variety of waveguide designs and architectures, including but not limited to a buried waveguide architecture.FIG. 11schematically illustrates an example laser comprising a buried waveguide201that is confined vertically (e.g., in a direction along the growth direction) between the cladding region103(e.g., a lower cladding region) and another cladding region1100(e.g., an upper cladding region) and confined laterally by regions112comprising a material having a refractive index substantially equal to that of the material of the cladding region103. In certain embodiments, the cladding region1100comprises the same material as does the cladding region103and/or the cladding region1100comprises a material having a refractive index substantially equal to that of the material of the cladding region103. In certain embodiments, the cladding region1100and the regions112comprise the same material as does the cladding region103. In certain embodiments, the material of the regions112can also be electrically blocking (e.g., similar to existing buried heterostructure waveguide architectures). The thick waveguide201can comprise a bulk material, as discussed above, or can comprise a layered structure (e.g., similar to the layered waveguide105discussed above). In certain embodiments, as schematically illustrated inFIG. 11, the grating layer106and the active region101are configured (e.g., positioned) to inhibit or prevent lasing of the vertically confined second order mode104band the vertically confined third order mode104c(e.g., similar to the example laser schematically illustrated inFIG. 9). In certain embodiments not having a ridge (e.g., broad area structures), the grating layer106and the active region101are configured such that higher order modes in the vertical direction are suppressed.

As described herein, designing and/or fabricating the various example lasers comprises the placement of the grating layer106and the active region101. In certain embodiments in which only two vertical modes that are perpendicular to the direction of the material growth are present, the grating layer106, the active region101, or both can be positioned at or near a center of the waveguide so as to coincide with a null or a minimum of the second order mode104b. In certain other embodiments in which three vertical modes are present, the grating layer106and/or the active region101can be offset from the center of the waveguide so as to coincide with a null or a minimum of two higher order modes (e.g., second order mode104b; third order mode104c). During the design phase, the positions of the grating layer106and the active region101can be calculated using a mathematical model to simulate the modes which are supported by the waveguide, and the positions of the active region101and the grating layer106can be iteratively changed relative to the peaks and nulls of the fundamental mode104aand the higher order modes. Without relying on any particular theory, the position of the active region101can perturb the mode profile significantly as a result of its thickness and relatively high index of refraction. The grating layer106, however, provides a small perturbation to the mode profile, and can be moved within the waveguide without significantly altering the nature of the supported modes. In certain embodiments, the iterative process can be advantageous to improve or optimize the position of the active region101. In certain embodiments, the placement of the grating layer106can be calculated initially and does not change much during the iterative process.

FIG. 12is a flow chart that illustrates an example iterative method for designing a laser in accordance with certain embodiments described herein. The method comprises determining the position of the active region101and the grating layer106to selectively reduce the threshold for lasing of a single vertically-confined mode (e.g., the fundamental mode104a) while suppressing lasing of other vertically-confined modes (e.g., second order mode104b; third order mode104c). For example, the position of the active region101and the grating layer106can be selected to reduce the lasing threshold for the fundamental mode104a. In certain embodiments, the iterative method ofFIG. 12increases or maximizes the combined effect of the coupling coefficient of the grating layer106(also referred to as grating feedback) proportional to Γgrat(e.g., the overlap of an optical mode with the grating layer106) and gain from the active region101proportional to Γqw(e.g., the overlap of the optical mode with the active region101) for the fundamental mode104a. In certain embodiments, Γgratand Γqware calculated for each mode by measuring the portion of the mode that is within the grating layer106or the active region101of the waveguide. If Γqwand/or Γgratcan be kept very low for higher order modes as compared to their values for the fundamental mode104a, the lasing threshold for the fundamental mode104acan be much lower as compared to the lasing threshold for other modes and only the single fundamental mode104ais supported. In certain embodiments, the product Γqw×Γgratfor the fundamental mode104acan be in a range of about 1-20 dB (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB or any value in a range/sub-range defined values between 1 dB and 20 dB) greater than the product Γqw×Γgratfor other higher order modes present in the waveguide to selectively reduce the lasing threshold for the fundamental mode104a.

In an operational block1101, the example method ofFIG. 12comprises calculating the different modes that are supported by an optical waveguide for an initial position of the active region101and the grating layer106. In an operational block1103, the example method further comprises moving the positions of the active region101(e.g., comprising one or more quantum wells, a bulk material, quantum dots, quantum lines or quantum dashes that provide optical gain to the laser) and the grating layer106within the waveguide such that the product Γqw×Γgratfor a desired mode (e.g., fundamental mode104a) is greater than the product Γqw×Γgratfor one, two, three, four or all other modes supported by the waveguide. In an operational block,1105, the example method further comprises calculating the different modes supported by the waveguide again to determine the perturbation of the different modes resulting from a change in the position of the active region101and the grating layer106. In an operational block1107, the example method further comprises calculating a difference between the product Γqw×Γgratfor a desired mode (e.g., fundamental mode104a) determined in block1105and the product Γqw×Γgratfor one, two, three, four or all other modes supported by the waveguide determined in block1105. In an operational block1109, the example method further comprises adjusting, if the difference calculated in block1107is less than a threshold value (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these values), the positions of the active region101and the grating structure106such that the product Γqw×Γgratfor the desired mode (e.g., fundamental mode104a) is larger than the product Γqw×Γgratfor the one, two, three, four or all other modes supported by the waveguide. If the difference calculated in block1107is greater than a threshold value (e.g., 1 dB, 3 dB, 10 dB, 15 dB, 20 dB, or any value in a range/sub-range defined by any of these values), then the positions of the active region101and the grating structure106can be considered to be optimized.

In certain embodiments, as schematically illustrated byFIG. 13, the laser comprises an active region101and a grating layer106that are combined together (e.g., which can be referred to as a gain-coupled laser). For example, the active region101and the grating layer106can be combined together by partially or completely etching away the gain region from certain portions of the active region101. In certain embodiments, as schematically illustrated inFIG. 13, the active region101and the grating layer106are placed together within the thick waveguide201at a position that overlaps with the peak of the fundamental mode104aand thus selectively reduce the lasing threshold for the fundamental mode104aand suppresses other higher order modes.

In certain embodiments, as schematically illustrated byFIG. 14, a high power single mode laser utilizes a higher index portion of cladding within the ridge100. This example laser is like a hybrid between the SLOC and SCOWL architectures, with the active region101below the ridge100, and a region109comprising a material having a refractive index greater than that of the cladding material of the ridge100, as schematically illustrated inFIG. 14. This architecture is different from the architecture of the SCOWL in that the active region101is below the ridge100and is different from the SLOC laser architecture in that the ridge100comprises a high refractive index material as well as cladding material. Certain such embodiments have an advantage over the SCOWL architecture in that the active region101is not etched through by the ridge100, leading to easier processing and better reliability. In certain embodiments, the waveguide105of the laser schematically illustrated inFIG. 14is configured to support only a large single optical mode and to not support any higher order modes. In certain embodiments, the waveguide105of the laser schematically illustrated inFIG. 14supports two, three or more modes, and the placement of the grating layer106and/or the grating layer106is selected (e.g., optimized) to allow lasing of a desired mode. In certain embodiments, the waveguide105has a layered waveguide structure, as schematically illustrated inFIG. 14, while in certain other embodiments, the waveguide105has a slab waveguide structure comprising a material having a refractive index less than the refractive index of the region109. In certain embodiments, the grating layer106is above or below the active region101, or within the ridge100. In certain embodiments, the grating layer106is configured (e.g., positioned) to reduce or minimize overlap with higher order modes while increasing or maximizing overlap with the fundamental mode104a, thereby providing for lasing of the fundamental mode104a.

Certain embodiments described herein can be configured to output optical power greater than or equal to about 10 mW (e.g., greater than or equal to about 20 mW, greater than or equal to about 30 mW, greater than or equal to about 50 mW, greater than or equal to about 75 mW, greater than or equal to about 100 mW, greater than or equal to about 150 mW) and/or less than or equal to 50 W (e.g., less than or equal to 25 W, less than or equal to 10 W, less than or equal to 5 W, less than or equal to 1 W), or any optical power in a range/sub-range defined by these values. Certain embodiments described herein are configured to output a single vertically confined mode. Accordingly, the light output from certain embodiments described herein have a large side mode suppression ratio (SMSR). For example, the SMSR of light output from certain embodiments described herein can be between about 10 dB and about 150 dB (e.g., between about 10 dB and about 20 dB, between about 15 dB and about 30 dB, between about 20 dB and about 40 dB, between about 30 dB and about 60 dB, between about 40 dB and about 80 dB, between about 50 dB and about 100 dB, between about 60 dB and about 120 dB, between about 70 dB and about 140 dB, or any value in any range/sub-range defined by these values.).

Although for various embodiments of lasers discussed herein, the active region and/or the grating layer can be described as being positioned at the peak and/or at the null of the vertically confined mode, it should be appreciated that the active region and/or the grating layer can be positioned near or in proximity to the peak and/or near or in proximity to the null of the vertically confined mode to increase/decrease lasing threshold of the vertically confined mode.

In certain embodiments, a computer system is used for some or all of the calculations described herein. For example, the computer system can comprise hardware (e.g., at least one microprocessor) operative to execute software (e.g., code stored on computer-readable non-transitory memory media). It will be appreciated that one or more portions, or all of the code may be remote from the user and, for example, resident on a network resource, such as a LAN server, Internet server, network storage device, etc. In certain embodiments, the computer system comprises a standard personal computer. The computer system can comprise standard communication components (e.g., keyboard, mouse, trackball, touchpad, toggle switches) for receiving user input (e.g., commands and/or data from a human operator), and can comprise standard communication components (e.g., image display screen, alphanumeric meters, printers) for displaying and/or recording output data, and computer-readable non-transitory memory media (e.g., random-access memory (RAM) integrated circuits; hard-disk drives).

While the foregoing detailed description discloses several embodiments, it should be understood that this disclosure is illustrative only and is not limiting. It should be appreciated that specific configurations and operations in accordance with certain embodiments described herein can differ from the particular example described herein, and that the example apparatus and methods described herein can be used in other contexts. Additionally, components can be added, removed, and/or rearranged. Additionally, processing steps can be added, removed, or reordered. A wide variety of designs and approaches are possible.

Various modifications to the embodiments described herein may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Additionally, a person having ordinary skill in the art will readily appreciate, the terms “upper” and “lower” are sometimes used for ease of describing the figures, and indicate relative positions corresponding to the orientation of the figure on a properly oriented page, and may not reflect the proper orientation of the device as implemented.