Triple junction semiconductor laser with optimized far-field profile

Abstract

Laser Radar or LiDAR systems rely on the illumination of the far-field by the laser source to provide an accurate sampling of the object at a given distance. A triple junction diode laser can be designed to provide a single illumination spot in the far field by minimizing the tunnel junction absorption and optimizing the distance between the individual laser diodes. This discloses an optimal design approach for a single lobed far-field in the transverse or fast direction of the triple junction laser diode.

Description

Docket: 0021-0012W01TRIPLE JUNCTION SEMICONDUCTOR LASER WITH OPTIMIZED FAR-FIELD PROFILE RELATED APPLICATIONS[ oooi] This application claims the benefit under 35 U. S. C. § 119(e) of U. S. Provisional Application No. 63 / 735,606, filed on December 18, 2024, which is incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTION

[0002] Semiconductor multiple-junction laser diodes combine two or moreedge-emitting waveguides in a vertical stack separated by tunnel junctions to increase power and brightness within a compact chip footprint. In AlInGaAs / InP material systems operating near 1250-1750 nm, the stacked waveguides may be realized as step-index or graded-index separate-confinement heterostructures (SIN-SCH or GRIN-SCH) with one or more quantum wells per junction.

[0003] V arious techniques have been employed to improve beam quality from multiple-junction devices. Near-field phase manipulation prior to overlap is challenging because the individual waveguide modes expand rapidly; accomplishing selective phase shifts commonly requires imaging optics to reimage and magnify the near field, increasing system size, alignment complexity, and cost. Far-field phase plates can also collapse the multi-lobed pattern but similarly add components and tolerances. On-chip adjustments — such as altering tunnel -junction thickness and absorption, or modestly perturbing waveguide symmetry — can influence beam quality.

[0004] Practical multiple-j nction devices also encounter thermal gradients during operation. Because the p-side is typically bonded to a heat sink, the upper junction runs cooler than lower junction(s). Without compensation, the resulting temperature offsets shift the gain peaks of the respective quantum-well stacks, degrading quality or destabilizing separate-mode operation.SUMMARY OF THE INVENTION

[0005] The disclosure relates to multiple-junction semiconductor light sources, and in particular to AlInGaAs / InP triple-junction devices that deliver a near single-lobed transverse (fast-axis) far-field without external phase-correction optics and with output powers substantially greater than a single-junction laser.Docket: 0021-0012W01

[0006] In one aspect, a multiple-junction laser diode comprises, on an InP substrate, a p-cap, three edge-emitting laser diode structures (each including a p-side waveguide, a multiple-quantum-well active region, and an n-side waveguide), and two tunnel junctions disposed between adjacent laser diode structures. The device emits in the 1250-1750 nm band. Layer compositions are drawn from (AlxGa)In(y)As and InP. Tunnel junctions are heavily doped (>10A18 cm3) and have a combined thickness, per junction, in the range of about 5-300 nanometers (nm).

[0007] The invention uses engineered coupling between the stacked waveguides — set by waveguide thickness / composition, cladding thicknesses, inter-active-region separation, and tunnel -junction thickness / index — to place the device in one of two operating regimes that both yield a substantially single-lobed fast-axis far-field:

[0008] Coherent (supermode) regime. The three waveguides are coupled so that the device oscillates on a vertical-order supermode with nodes approximately at the tunnel junctions. By managing the phase across the composite near field (e.g., by small, controlled asymmetries between p- and n-side waveguides and by selecting tunnel -junction loss), the near-field phase excursion is driven toward 0 or 2JI so that the Fourier-related far-field becomes nearly single-lobed.[ o o o 9 ] Incoherent (decoupled) regime. The coupling between waveguides is intentionally reduced so that each waveguide oscillates on its own lateral mode. The far-fields from the three independent emitters sum to a smooth, single-lobed composite profile with low sidelobes.

[0010] In both regimes the stacked architecture provides substantially higher peak power than a single-junction device; representative measurements show >2.5* increase (e.g., -85 W pulsed for a triple junction vs. ~34 W for a single junction at comparable current).

[0011] In a representative triple-junction embodiment the layer sequence includes: a high-conductivity p-cap (<2 pm GalnAs, >5x10A18 cm-3p-doping); for each laser diode structure, an InP outer cladding, a p-side waveguide of (AlxGa)In(y)As (step or graded index), an MQW active region, and an n-side waveguide of (AlxGa)In(y)As; and InP lower claddings and buffer over an InP substrate. The waveguides can be step-index or GRIN-SCH Quantum-well thickness / composition among the three active regions are selected so that, under the p-side-cooled thermal gradient present in packaged devices, theDocket: 0021-0012W01lasing wavelengths of the three structures align at the operating temperature (e.g., by incrementally adjusting well thickness or Al content from the coolest top structure to the warmest bottom structure).

[0012] Coupling strength between adjacent emitters is controlled by several levers that are chosen in combination according to the desired regime:

[0013] Tunnel junctions. Thin, heavily doped junctions are used. For coherent operation, the junction optical index may be equal to or greater than the adjacent waveguide effective index to encourage evanescent coupling while maintaining acceptable absorption. For incoherent operation, the junction optical index is selected lower than that of the active-region waveguides (e.g., using InGaAsP, AlGalnAs, or InP compositions) and / or the junction is made thin enough that the vertical coupling coefficient |K| falls below a decoupling threshold.

[0014] Waveguide design. In the coherent regime the p- and n-side waveguides can be made nominally symmetric or with small (<— 10%) controlled offsets to set the near-field phase. In the incoherent regime, purposeful asymmetry is introduced: the p-side and / or n-side waveguides of the three structures are made non-identical (different thicknesses and / or compositions), which reduces modal overlap and promotes independent oscillation.

[0015] Cladding and spacing. To increase coupling (coherent regime) claddings may¬ be kept relatively thin; to reduce coupling (incoherent regime) individual InP outer claddings are increased (e.g., >1.5 pm) and the center-to-center separation between active regions is increased (e.g., >3 pm).

[0016] Using these controls, the device can be designed either to (i ) lase on a phase-managed supermode that yields a near single-lobed far-field, or (ii) lase on three separate modes whose summed far-field is single-lobed with reduced sidelobes and good beam uniformity.

[0017] Optional features[ o o ].8 ] The top (p-side) waveguide can include a di stributed Bragg reflector or grating to spectrally lock the lasing emission, and, in the incoherent regime, to frequency-lock multiple independent modes to a common wavelength. A narrow ridge contact (e.g., <5 pm) provides lateral single-mode operation. The structure can also be configured as a semiconductor optical amplifier (SOA), including straight, tilted, or curved ridgeDocket: 0021-0012W01geometries; reflective SOA variants may use a high-reflectivity back facet andlow-reflectivity input facet.

[0019] The disclosed multiple-junction architecture provides a compact source with higher peak power and brightness than a single-junction device while producing a substantially single-lobed fast-axis far-field without external phase plates or imaging optics. The design enables wavelength stabilization on-chip and supports both coherent and incoherent operating modes through structural choices made during epitaxial growth. The devices are well-suited for pulsed, quasi -CW, and CW operation and for deployment in systems such as time-of-flight and FMCW lidar.

[0020] The invention encompasses two complementary families of stacked laser-diode devices that both deliver a substantially single-lobed fast-axis far-field: a coherent (supermode) family and a separate-mode (incoherent) family. In every case the device includes more than one junction (e.g., a triple-junction stack) and each laser-diode structure contains a quantum-well active region with one or more quantum wells. Features described for one family may be combined with features of the other where technically compatible.

[0001] Coherent (supermode) embodiments

[0022] In one set of embodiments the stacked waveguides are evanescently coupled so that the device oscillates on a supermode extending through the junction stack. Spectral control can be provided directly on the chip: a Bragg grating formed in the p-side waveguide narrows and stabilizes the emission linewidth, and in a variant the grating is implemented as a distributed-feedback (DFB) structure in that same p-side waveguide. Lateral single-mode behavior is promoted with a narrow ridge whose contact width is less than 5 pm; the ridge width is measured at the top metal contact over the ridge.

[0023] The supermode profile and its far-field are set by the vertical optics. In one implementation the p-side and n-side waveguides are identical, and the claddings are kept thin and identical to maximize coupling between the junctions. In another, the p-side and n-side waveguides differ only slightly — by less than about ten percent in thickness and / or refractive index — which provides a controlled phase adjustment across the supermode and produces a nearly single-mode far-field. In still another, each of the p-side and n-side waveguides uses a graded composition so that the resulting graded-index profiles are nearly identical from junction to junction.Docket: 0021-0012W01

[0024] The vertical phase can also be managed by tailoring what we call the separation regions — the tunnel -junction region(s), their immediately adjacent claddings, and the intervening spacing between active regions. By adjusting the thickness and refractive index of these separation regions, the net phase shift across the entire stack can be driven to about 0 or 2K, yielding a diffraction-limited single-mode far-field. Devices operating on the coherent supermode are suitable for integration into both time-of-flight lidar and frequency-modulated continuous-wave (FMCW) lidar systems for distance and velocity measurement.

[0025] Separate-mode (incoherent) embodiments

[0026] In another set of embodiments the vertical coupling is intentionally reduced so that each stacked waveguide oscillates on its own mode, and the sum of the three far-fields forms a single-lobed composite profile. Decoupling is engineered structurally. In some versions all three p-side waveguides are made different in thickness; in others all three n-side waveguides are made different; and in still others both p-side and n-side waveguides are all different in thickness. As an alternative or in addition, the compositions of the p-side and / or n-side waveguides are varied between junctions to create different step-index values, or are graded differently to create different graded-index profiles; these asymmetries reduce energy sharing between waveguides and promote separate oscillation whose far-fields sum to a single lobe. Devices operating in this separate-mode regime are likewise well suited for time-of-flight lidar applications.

[0027] Tunnel-junction options

[0028] The tunnel junctions that separate the active regions can be selected and dimensioned to support either regime. For strong decoupling, the tunnel-junction refractive index is chosen to be lower than the refractive index of the quantum-well region, using any material that meets this index relation. Specific realizations include InP tunnel junctions with thickness from 5 nm to 300 nm and p-type or n-type dopant concentration greater than l x IO18cnr3; Al(x)Ga(y)In(l-x-y)As(y) tunnel junctions with x from 0 to 1 and y from 0 to 1, again 5-300 nm thick and >1 * 10'8cm3doped; and Ga(l-x)In(x)As(y)P(l-y) tunnel junctions with x from 0 to 1 and y from 0 to 1 in the 5-300 nm thickness range and with p- or n-type doping >1 > 1018cm3. These examples provide literal support for low-index, thin, heavily doped junctions that suppress supermode propagation when desired.

[0029] SO A and RSOA configurationsDocket: 0021-0012W01

[0030] The same stacked waveguide platform may be configured as an amplifier rather than a laser. In semiconductor optical amplifier (SOA) versions, the ridge may be tilted by an angle less than 10 degrees relative to the cavity axis to mitigate back-reflection. Other SOA geometries include a curved ridge on the low-reflectivity (anti -reflection) facet with facet reflectivity below 0.1 % and a straight ridge on the opposite low-reflectivity facet also with reflectivity below 0.1 %; a double-curved SOA in which both facets employ curved ridges with angles less than 10 degrees and anti -refl ection coatings below 0.1 %; and a U-turn curved SOA in which the input and output are on the same low-reflectivity facet with reflectivity below 0.1 %. In a reflective SOA (RSOA) configuration, the device employs a curved ridge on the low-reflectivity facet (again <0.1 %) and a straight ridge on the opposite facet that is coated for high reflectivity greater than 90 %.

[0031] Together these narrative embodiments track and support the full scope of the claim set: supermode lasers with on-chip spectral locking and lateral single-mode ridges; separate-mode devices with structural decoupling that still yield a single-lobed composite beam; specified low-index, thin, highly doped tunnel-junction materials; and SOA / RSOA geometries with defined facet reflectivities and ridge orientations.

[0032] In one class of embodiments, a multiple-junction semiconductor laser diode device is provided that includes a substrate and a plurality of edge-emitting laser diode structures stacked over the substrate and separated from one another by tunnel -junction regions. Each laser diode structure comprises a p-side waveguide, a multiple-quantum-well (MQW) active region, and an n-side waveguide. In these embodiments, the p-side waveguides of the plurality of laser diode structures have substantially identical thicknesses and compositions, and the n-side waveguides likewise have substantially identical thicknesses and compositions, so that the stacked junctions are vertically symmetric. Each tunnel -junction region is formed with an optical refractive index equal to or greater than an effective refractive index of the active regions and with a thickness sufficient to guide a portion of an optical mode, so that the tunnel junctions act as additional guiding layers rather than purely absorbing separators. As a result, the stacked laser diode structures are optically coupled to one another and the device oscillates on a coherent vertical supermode extending through the plurality of laser diode structures. In representative implementations the plurality consists of three laser diode structures, forming a triple-junction device.Docket: 0021-0012W01

[0033] In certain embodiments, the substrate comprises InP, and each of the p-side waveguides and n-side waveguides comprises (AlxGa)In(y)As with 0.1 < x < 1 and 0 < y < 0.9. Each multiple-quantum-well active region comprises AlGalnAs quantum wells having a thickness less than 11 nm, and the overall layer structure and compositions are chosen so that the device is configured to emit in a wavelength range from about 1250 nm to about 1750 nm. A grating may be formed in at least one of the p-side waveguides, such as a distributed Bragg reflector (DBR) or distributed feedback (DFB) grating, and is configured to lock the vertical supermode to a narrow emission linewidth. In some implementations, the device further comprises a ridge waveguide having a ridge contact width less than 5 um, which provides lateral single-mode operation in combination with the vertically coherent supermode. At least one of the p-side waveguides and the n-side waveguides may have a graded composition to provide a graded-index profile that shapes the mode and facilitates confinement.

[0034] The supermode profile and resulting beam quality can be further engineered by¬ slight, controlled departures from perfect symmetry. In certain embodiments, the p-side waveguides and / or the n-side waveguides of the plurality of laser diode structures differ from one another by less than about 10% in at least one of thickness and refractive index, while the thicknesses and refractive indices of the tunnel -junction regions and cladding layers are selected such that a phase shift across the stack of laser diode structures is approximately an integer multiple of 2π. Under these conditions, the coherent supermode produces a fast-axis far-field that is approximately diffraction-limited and substantially single-lobed, so that the multi-junction device behaves, from the standpoint of beam quality, like a single high-brightness emitter.

[0035] In another class of embodiments, a multiple-junction semiconductor laser diode device is provided in which the same general stacked architecture is used — a substrate, and a plurality of edge-emitting laser diode structures stacked over the substrate and separated by tunnel-junction regions, each laser diode structure again comprising a p-side waveguide, an MQW active region, and an n-side waveguide — but the layer dimensions and compositions are selected to reduce optical coupling between adjacent laser diode structures rather than to form a single supermode. In particular, at least one of the following conditions is satisfied: (i) the p-side waveguides of the plurality of laser diode structures have mutually different thicknesses and / or compositions, (ii) the n-sideDocket: 0021-0012W01waveguides of the plurality ha ve mutually different thicknesses and / or compositions, and (iii) each tunnel -junction region has a thickness selected to reduce optical coupling between adjacent laser diode structures. With these choices of waveguide and tunneljunction dimensions, the stacked structures are sufficiently decoupled that they oscillate on separate laser modes, and the aggregate fast-axis far-field of the device has a single main lobe, for example a Gaussian-like profile formed by the sum of the individual far-fields. In representative versions, the plurality again consists of three laser diode structures, implementing a decoupled triple-junction device.

[0036] Further embodiments sharpen this decoupling. In some implementations, the p-side waveguides of the plurality of laser diode structures have mutually different thicknesses so as to decouple energy sharing between the laser diode structures; in other implementations, the n-side waveguides have mutually different thicknesses for the same purpose. Additionally or alternatively, at least one of the following holds: the p-side waveguides have mutually different compositions and / or graded-index profiles, and the n-side waveguides have mutually different compositions and / or graded-index profiles, so that each junction defines a distinct vertical mode. These variations in thickness and index decouple energy sharing between the laser diode structures and create separate oscillating modes whose far-fields sum to the single main lobe. In some designs, cladding layers between the multipl e-quantum-well active regions are made thicker than about 1.5 pm, and center-to-center separations between the multiple-quantum -well active regions of adjacent laser diode structures are made greater than about 3 pm, to further reduce optical coupling and reinforce the separate-mode behavior.

[0037] The tunnel -junction regions in the decoupled embodiments may also be tailored to suppress supermode formation. In particular, each tunnel -junction region can be formed with an optical refractive index less than an effective refractive index of the active regions, with a thickness in a range from 5 nm to 300 nm, and with a p-type or n-type dopant concentration greater than about 1 * IO’8cm3, so that the junction remains highly conductive while presenting a refractive index barrier to vertical coupling. In specific examples, each tunnel -junction region comprises a semiconductor material selected from InP, Al(x)Ga(y)In(l" X-y)As(y) where 0 < x < 1 and 0 < y < 1, andGa(l-x)In(x)As(y)P(l-y) where 0 < x < 1 and 0 < y < 1, with compositions chosen so that the index relation to the quantum-well region is satisfied. These material and geometryDocket: 0021-0012W01choices provide a flexible design space for thin, low-index, heavily doped tunnel junctions that effectively reduce optical coupling between the stacked emitters.

[0038] The disclosed devices can be integrated into ranging systems. In one embodiment, a ranging system comprises a semiconductor laser transmitter that includes any of the multiple-junction devices described above and optics configured to project the output light of the device toward a target, together with a receiver configured to detect light returned from the target. The receiver processes the returned optical energy to determine at least one of a distance to the target and a velocity of the target, for example using time-of-flight processing or frequency-modulated continuous-wave processing. The high peak power and substantially single-lobed fast-axis far-field of the multiple-junction device make it particularly suitable for such time-of-flight and FMCW lidar applications.

[0039] In addition to laser oscillators, the same stacked-waveguide platform can be configured as a semiconductor optical amplifier. In such embodiments, the multiplejunction device is biased below or near transparency and includes at least one ridge waveguide segment tilted or curved by less than 10 degrees relative to an optical axis to reduce back-reflections. A facet associated with the tilted or curved ridge waveguide segment is provided with an anti -reflection coating having a reflectivity less than 0.1 percent, while an opposite facet optionally has a high-reflection coating having a reflectivity greater than 90 percent, for example to implement a reflective semiconductor optical amplifier confi uration. These amplifier embodiments leverage the same multijunction layer structure and coupling control described above while providing high-gain, low-reflection amplification for external optical signals,

[0040] 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.Docket: 0021-0012W01BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:

[0042] Figure 1 is a schematic cross-section of a triple-junction laser diode device 100, showing a p-cap 101, a first, second, and third laser-diode structure 102, 103, 104 each having an upper waveguide 105, 109, 113, an active region 106, 110, 114, and a lower waveguide 107, 111, 115, with tunnel junctions 108 and 112 between adjacent structures and an n-type region 116.

[0043] Figure 2 illustrates a coherent, tripl e-lobed supermode 200 across the stack of FIG. 1, with anti-nodes 201, 204, 205 in the respective laser-diode structures and nodes 202, 203 located at the tunnel junctions 108, 112; the right-hand plot depicts the corresponding stepped phase profile 206-208 through the stack.

[0044] Figure 3 is a graph 300 of the transverse (fast-axis) far-field produced by the supermode of FIG. 2, showing dominant lobes 301, 302 and additional grating lobes 303, 304, 305.

[0045] Figure 4 presents an exemplary epitaxial layer design 400 for the device of FIG. 1, identifying the p-cap 101; InP claddings 401, 402, 405, 406, 409, 412; waveguides 105, 107, 109, 111, 113, 115; active regions 106, 110, 114; tunnel-junction layers 403, 404 and 407, 408; an InP buffer 413; and the InP substrate 414.

[0046] Figure 5 is a power-versus-current (L-I) plot 500 comparing a triple-junction device curve 501 with a single-junction device curve 502.

[0047] Figure 6 A and 6B show, respectively, the near-field distribution 600 of the first laser-diode structure 102 with a dominant lobe 601 and weakly coupled sidelobes 602, 603, and the corresponding fast-axis far-field 604 exhibiting a nearly single-1 obed profile 605.

[0048] Figure 7 A and 7B show, respectively, the near-field distribution 700 of the second laser-diode structure 103 with central lobe 701 and sidelobes 702, 703, and the corresponding far-field 704 featuring a double-lobed pattern 706, 707 and additional lobes 705, 708.Docket: 0021-0012W01

[0049] Figure 8A and 8B show, respectively, the near-field distribution 800 of the third laser-diode structure 104 with central lobe 801 and sidelobes 802, 803, and the corresponding far-field 804 with a primary lobe 805 and multiple grating lobes 806-811.

[0050] Figure 9 is a graph 900 of the composite far-field obtained by summing the far-fields of FIGS. 6B, 7B, and 8B, showing a strong central lobe 901 with residual features 902-905.

[0051] Figure 10 is a graph 1000 illustrating a single-lobed far-field produced when the stacked laser-diode structures oscillate independently (separate-mode operation).

[0052] Figure 11 presents experimental data demonstrating a single transverse (fast-axis) mode 1101 and a lateral single-mode far-field 1102 for a decoupled (incoherent) embodiment.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0054] As used herein, the term "and / or” includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and / or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and / or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present.

[0055] It will be understood that although terms such as “first' and “second” are used herein to describe various elements, these elements should not be limited by these terms.Docket: 0021-0012W01These terms are only used to distinguish one element from another element. Thus, an element discussed below could be termed a second element, and similarly, a second element may be termed a first element without departing from the teachings of the present invention.

[0056] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0057] AlInGaAs / InP triple junction semiconductor laser diode has three individual laser diode structures grown in a single epi-growth run which are connected by two tunnel junctions (Figure 1) and emitting in wavelength range from 1250nm to 1750nm. The objective of building a device in this manner is to dramatically increase the output from a single device in pulsed or continuous wave (CW) operation. In both CW and pulsed cases, a triple junction laser diode can increase the output power by a factor of 2-3x over a single junction device depending on the lateral confinement and the thermal management system.

[0058] Each laser diode structure can be either a Step Index Separate Confinement Heterostructure (SIN-SCH) or a Graded Index Separate Confinement Heterostructure (GRIN-SCH) with 1, 2 or more Quantum Wells in each laser diode structure to provide gain. The laser diode structures can have modes that oscillate entirely separate from each other (in-coherent) or a supermode (coherent) where the modes evanescently couple energy back and forth.

[0059] The supemiode shown in Figure 2 is determined by the design of each laser diode structure and the thickness and absorption characteristics of the tunnel junctions. The triple junction design shown in Figure 2 has nodes of the mode located at each tunnel junction which is a phase shift in the near-field. The far-field from this supermode is shown in Figure 3 which is the Fourier transform of the near-field amplitude and phase information. This multi-lobed far-field is the typical result for these devices and corresponds to 3rd-vertical order super-mode which is guided by the 3 MQW waveguides. The other modes are guided by two tunnel-junctions and are not lasing due to high tunnel junction’s absorption. Several solutions have been proposed to correct the near-field phaseDocket: 0021-0012W01pattern, including shifting the phase of the lobes in the near field and the far-field with a separate optical element.3Shifting the phases in the near-field is difficult because the beams from each waveguide expand very rapidly making it difficult to selectively phase shift each portion of the near-field mode prior to the fields overlapping. However, a set of imaging optics that reimage the near-field and magnify the image can easily accomplish the phase correction required. But this approach increases the size of the system and increases the cost as well. A second approach of adding a phase shift in the far-field can also accomplish transforming the mode back to a single mode, but again, at an increase in size, complexity and cost to the end product.

[0060] The invention described herein is both a coherent design which provides a near single mode in the far-field and an in-coherent design which provides a near single mode in the far field but with less brightness. This range of performance is achieved by varying the losses in the tunnel junctions (108,112) of Figure 1, the waveguide configuration and the waveguide separations.

[0061] The invention is comprised of an AlInGaAs / InP triple junction emitting in wavelength range from 1250 nm to 1750nm. The triple junction structure is comprised of a p-cap region 101, a first laser diode structure (102), a tunnel junction (108), a second laser diode structure (103), a tunnel junction (112), a third laser diode structure (104) and an n-doped region. The first laser diode structure 101 is comprised of a step index or graded index upper waveguide (105), quantum wells and barriers (106) and a step index or graded Index lower waveguide (107). The second laser diode structure may or may not have the same layers thicknesses and it is comprised of a step index or graded Index upper waveguide (109), an active region comprising quantum wells and barriers (110), and a step index or graded index lower waveguide (111). The third laser diode structure may or may not have the same layers thicknesses and it is comprised of a step index or graded index upper waveguide (113), an active region comprising quantum wells and barriers (114) and a step index or graded index lower waveguide (115). The quantum wells (106,110, 114) may be identical for a short pulse coherent device but for long pulse and CW operation each of the quantum well thicknesses (106,110,114) and compositions must be adjusted to compensate for the thermal profile of the device during operation if the device is operated on a single supermode (200). In the case of an in-coherent system, each quantum wellDocket: 0021-0012W01(106,110,114) may be the same thickness or different thickness depending on the intended operating specifications.

[0062] During operation these devices are bonded to a heat sink on the p-side of the device resulting in the operating temperature of the first laser structure (102) being cooler than the second laser structure (103) which is cooler than the third laser structure (104). Consequently, to maintain coherent operation, the quantum well thicknesses (106,110, 114) must be adjusted during growth to align the operating wavelength of each laser diode structure when the laser diode structure reaches its intended operating temperature. Since the first laser structure (102) operates at the lowest temperature, it defines the nominal thickness for the quantum wells (106, 110, 114). The second laser structure (103) operates at a slightly higher temperature which means the thickness and or the Aluminum composition of the quantum well (110) must be different than the first structure (102) quantum well (106) if the wavelengths are to be aligned at operating temperature. The third laser structure (104) operates at a slightly higher temperature than the second laser structure (103) and its quantum well (114) thickness and / or aluminum composition must be slightly different than the second laser quantum well (106) in order for its lasing wavelength to be aligned with the lasing wavelength of the first two quantum wells (106,110) which are operating at different temperatures. For very short pulses, where the energy deposition is adiabatic and all quantum wells (106,110,114) operate at the same temperatures, the quantum wells (106,110,114) can be identical in thickness.

[0063] The triple junction laser diode stack operates in either a coherent or incoherent mode depending on the strength of the field coupling between each laser diode in the stack. Figure 2 shows a coherent supermode (200) with nodes (202, 203) located in tunnel junctions (108, 112), and anti-nodes (201, 204, 205) in laser diode structures (102,103,104). The nodes represent a weak field coupling between each laser diode structure which results in the supermode (200) depicted.

[0064] The far-field (300) in Figure 3 of the supermode (200) is determined by the spatial distribution of the near field’s amplitude and phase. The far-field (300) pattern consists of grating lobes (303, 301, 304, 302, 305) which are spaced. / d (radians) apart where is the wavelength and d is the center to center spacing of the emitters. The dominate lobes in the far-field (301, 302) are the result of the K phase shifts (206,207,208) between the near field lobes (201,204,205).Docket: 0021-0012W01

[0065] The laser diode structure (400) is compri sed of three laser diode structures (102,103,104) separated by two tunnel junctions (108,112) and grown on an InP substrate (414). The P-cap (101) is GaIn(x)As, it is less than 2pm thick, has a P dopant of (Zinc or carbon) at a level >5e18cm'3which provides a very high electrical conductivity.

[0066] The top laser diode structure (102) is made up of an outer cladding layer (401) of InP, an upper waveguide region 105 that is either a graded index or an index step of (AlxGa)In(y)As (0.1<x<0.9) and (0<y<l), the quantum well region (106), a lower waveguide region (107) that is either a graded index or an index step of (AlxGa)In(y)As (0. l<x<l and 0 <y<0.9) and a lower outer cladding layer (402) of InP. The upper cladding layer of InP is P-doped to a level of >le17cm'3with (Zinc or Carbon) and the thickness of the layer is a multiple of the lasing wavelength (X*WL) of the device with that multiple being 0.1< X<3. The waveguide layer, (105), is doped to >5e16cm' and not exceed <5e18, doping type is Zinc or carbon. The waveguide layer thickness is XI where 0.01pm< Xl<1.5pm using AlInGa / \s material where 0.1<x<l and 0<y<0.9. The waveguide refractive index is a step index or graded index. The quantum well (106) is made up of AlGalnAs with thickness <1 Inm thick. The lower waveguide layer in diode structure (102) is a step index or graded index (AlxGa)In(y)As (0.1<x<l) and (0<y<0.9) layer with N type dopant Silicon (Si) or Tellurium (Te) to a level of >5e16cm“3. The lower waveguide (107) with thickness (Yl) where 0.01< Yl<1.5um, The lower outer cladding layer of laser diode structure (102) is InP with a thickness similar to the upper outer cladding layer (X*lasing wavelength) where 0.1< X<3 doped with N type dopant Si or Te to a level of >le17cm“3. The next two layers are the tunnel junction (108) where the upper layer (403) is InGaAs with thickness >0.005pm and doped to a level of >1 e18cm'3using Si or Te N type dopants. The next tunnel junction layer (404) is also InGaAs that is >0.005 um thick and doped to a level of >le18cm’3using Carbon and Zinc on P type dopants. The refractive index of the tunnel junction region is strictly higher than the refractive index of active region comprising MQW region.

[0067] The second laser diode structure (103) is comprised of an upper outer cladding layer of InP (405), where the thickness is a multiple of the operating wavelength (X*WL) with the multiple (X) being 0.1< X<3 and is doped to the level >le17cm'3with P type (Zinc or Carbon). The upper waveguide layer (109) for structure (103) is (AlxGa)In(y)As (0.1<x<l ) and (0<y<0.9) with a thickness X2 where Xl=n*X2 with n<1.2 and doped withDocket: 0021-0012W01a P dopant to the level of >5e16cnT3, The quantum wells (110) are made up of AlGalnAs and with thickness less thanl 1 nm. The lower waveguide layer (111) in the diode structure (103) is a step index or graded index (AlxGa)In(y)As (0.1 <x<l) and (0<y<0.9) with a thickness Y2 where Yl=n* Y2 and n<l.2 and doped with N type dopant Si or Te to a level of >5e16cm'3. The lower outer cladding layer of the second laser diode structure (103) is InP with a thickness similar to the upper outer cladding layer (X*wavelength) where 0.1< X<3 and doped with N type dopant Si or Te to a level of > le17cm’3. The next two layers are the tunnel junction (112) where the upper layer (407) is InGaAs that is >0.005|im thick and doped to a level of >lei8cnr using Tellurium and Silicon N type dopants. The next tunnel junction layer (408) is also InGaAs that is >0.005mm thick and doped to a level of >1 elscm’3using Carbon and Zinc P type dopants. The refractive index of the tunnel junction region is strictly higher than the refractive index of active region comprising MQW region.

[0068] The third laser diode structure (104) is comprised of an upper outer cladding layer of InP (409), where the thickness is a multiple of the operating wavelength (X*WL) with the multiple (X) being 0.1< X<3 and is doped to the level >le17cm'3with P type (Zinc or Carbon). The upper guiding waveguide layer (113) for structure (104) is (AlxGa)In(y)As (range of 0.1<x< I and 0<y<0.9) with a thickness X3 where Xl=n*X2=n*X3 and n<1.2 and doped with a Zinc or carbon dopant to the level of >5elbcm'3. The quantum wells (114) are made up of AlGalnAs with thickness less than 11 nm. The lower waveguide layer (115) in the diode structure (103) is a step index or graded index (AlxGa)In(y)As (0.1< X<1 and 0<y<0.9) with a thickness Y2 where thickness Yl=n*Y2=n*Y3 and n<1.2 and doped with N type dopant Si or Te to a level of >5elbcm'3. The lower outer cladding layer (412) of the third laser diode structure (103) is InP with a thickness similar to the upper outer cladding layer (X*wavelength) where 0.1< X<3 and doped with N type dopant Si or Te to a level of >le17cm"J. The next layer (413) is an InP buffer layer that is >0.5 um thick and doped with N type dopant Si to a level of >le17cm'3. The final layer (414) is the InP substrate the device is grown on.

[0069] The performance of this device is shown in Figure 5. The power vs. current for both the single junction and the triple junction device are graphed. The single junction (502) reaches a maximum pulsed output power of 34 Watts, while the triple junctionDocket: 0021-0012W01device (501) reaches a maximum pulsed output power of 85Watts. This is a factor of >2.5x the output power of a single junction device.

[0070] This invention is a modification of the base structure shown in Figure 4. A single spatial lobe far-field can be accomplished by making the upper P-waveguides (105,109,113) non-identical by making the thicknesses of the three waveguides (105, 109, 113) different and / or their compositions different. The n-side MQW-WG (107,111,115) are also non-identical by making the thicknesses of the three waveguides (107, 111, 115) different and / or their compositions different. The thickness of upper P-waveguides (105, 11, 115) and lower N- waveguides (107, 111, 115) can have respectively same thicknesses or can have respectively different thicknesses. Also, the separation between the top active region and the middle active region is increased to be > 3 pm and separation between middle active region and the bottom active region is increase to be > 3 pm. The InP outer cladding layers (401,402,405,406,409,412) thicknesses are increased to be > 1.5 um for each cladding layer. The tunnel junctions (108,112,116) are reduced in thickness to less than 0.025 pm for n-InGaAs and to less than 0.025 pm for p-InGaAs. The second option is to change the tunnel junction material to a material with lower refractive index than the active regions refractive indexes. To reduce the refractive index of tunnel junction can be achieved by using n- (AlxGa)In(y)As / p- (AlxGa)In(y)As materials. These asymmetries result in three different laser modes that are weakly coupled to the adjacent laser diode structures (102, 103, 104).

[0071] Figure 6a shows the dominate near field mode for the first laser diode structure (102), the two lobes to the left of the structure (602,603) are weak coupling of power (602,603) into the adjacent laser diode structures (103, 104). Figure 6b shows the resulting far-field which consists of the energy from the two side lobes (603,602) interfering with the energy in (601) resulting in a nearly single lobed far field (605).

[0072] Figure 7a shows the near-field mode for the second laser diode structure (103), the side lobes (702,703) are weakly coupled into the adjacent laser diode structure waveguides (102,104). The resulting far-field (Figure 7b) shows the grating lobes as a result of the three lobes near field. The double lobed far-field (706, 707) is the result of a TC phase shift between the center lobe of the near field (701) and the two side lobes (702,703). The additional lobes (705, 708) in the far-field are a direct result of this phase shift, the underfilled near-field and the near-field spatial frequency (701,702,703). Consequently,Docket: 0021-0012W01the far-field consists of multiple lobes spaced at intervals of A / d (radians) where is the wavelength and d is the separation between the near-field lobes.

[0073] Figure 8a shows the near-field mode for the third laser diode structure (104), the side lobes (802,803) are weakly coupled into the adjacent laser diode structure waveguides (103,102). The resulting far-field (Figure 8b) consists of a primary lobe (805) and multiple grating lobes (806, 807, 808, 809, 810, 811). The innermost grating lobes (806,807) are at A / 2d (801,803), the next outer set (808,809) at A / d spacing (803,802), the final set(810,811) are a mix of the first and second set of grating lobes from (801,802 and 802,802). The key feature to note is the power in these grating lobes reduces the relative magnitude of the center lobe compared to that in Figure 6 (605).

[0074] Figure 9 shows the summation of these far-field from the three oscillating modes. The on-axis intensity receives its power primarily from the two on-axis far-field modes (605,805) which makes the on-axis intensity up to 4x greater than that in Figure 7 (706,707), since two on-axis modes combine, and the two off-axis modes split the power. The features in each of the far-fields are washed out by the summation of all the different modes, although some of the grating lobes can be identified in this far-field (902,903,904,905). While this is not a single coherent mode, there is power shared in all three modes from the p-cap side laser diode structure (102) which means a Bragg Grating on top of this structure (102) will serve to lock the frequency of all three modes to a given wavelength, but the slightly different mode structures will make each one a slightly different frequency. The output power for this device is estimated to be 45Watts at 55amps which is like the power response at current shown in Figure 5. The net result of this multi-mode operation is a more uniform far-field profile.[ o o 75] A second exampl e of an improved far-fi eld profil e is shown in Figure 10 where all three laser structures oscillate independently of each other. In this example the p-waveguides (105, 109, 113) can be similar but must be different from the n-waveguides (107, 111, 115) or they can all be different thicknesses with the same or different graded indexes. The objective is to create an asymmetry to the structure to add losses to the supermodes. The n-waveguides (107,111,115), they can be similar to the p-waveguides (105,109,113) or they can all be different thicknesses with the same or different graded indexes. Another requirement is the tunnel junction refractive index be less than the quantum well region(s) refractive index. Another criteria for decoupling the waveguides isDocket: 0021-0012W01to increase the InP cladding layer (402,405,406,409) such that the mode does not efficiently couple to the adjacent waveguides (107,109,111,113).

[0076] The p-waveguide structures (105,109,113) are composed or comprised of p-(AlxGa)In(y)As where x can vary from 0.1 to 1 and y can vary from 0 to 0.9. The index can be a constant composition in these waveguide structures (105,109,113) or it can be a graded composition to create a graded index, either will be sufficient at defining the mode of the waveguide structure. The thickness of these waveguide structures (105,109,113) can be on the order of 1 um or < 1 pm. The n-waveguide structures (107,111,115) are composed of n-(AlxGa)ln(y)As where x can vary from 0.1 to 1 and y can vary from 0 to 0.9. The index can be a constant composition in these waveguide structures (107,111,115) or it can be a graded composition to create a graded index, either will be sufficient at defining the mode of the waveguide structure.

[0077] The decoupling can be further increased by redesigning the tunnel junctions (108, 112) to be thinner with higher refractive index than quantum well refractive index or to have tunnel junction with different material with a refractive index that is less than the refractive index of the quantum well regions (106, 110, 114). The thickness of this region is between 5nm and 300nm with a doping of >le18cm‘3with the requirement being that it is sufficiently thick to suppress the propagation of a supermode within the laser diode structure. Furthermore, it is necessary to reduce the refractive index of the tunnel junctions (108, 112) by using a different material composition than InGaAs. One material system to use as a tunnel junction is using n-Ga(l-x)In(x)As(y)P(l-y) / p-Ga(l-x)In(x)As(y)P(l-y) where x can vary between 0 to 1 and y can vary from 0 to 1 with the requirement being that the absolution values of x and y result in the refractive index of the tunnel junction be less than the refractive index of the quantum well region (106,110, 114). The tunnel junction thickness is ranging from 5nm to 300 nm and a doping level >5elscm-3of the appropriate p or n type dopant. The thickness is dependent on the design of the overall structure and is determined by first designing the waveguide thicknesses (105, 107, 109, 111,113, 115) and determining the tunnel junction thickness to achieve the decoupling. A second material system to use as a tunnel junction is (AlxGa)In(y)As with its thickness ranging from 5nm to 300 nm and a doping level >le!8cm‘3of the appropriate p or n type dopant. The values of x and y are based on the values it takes to make the refractive index less than that of the quantum well regions (106,110,114). The thickness of the tunnel junction region isDocket: 0021-0012W01dependent on the design of the overall structure and is determined by first designing the waveguide thicknesses (105,107,109,111,113,115) and determining the tunnel junction thickness to achieve the decoupling. A third material system to use as a tunnel junction is InP with its thickness ranging from 5nm to 300 nm and a doping level >le18cm’3of the appropriate p or n type dopant. The thickness of the region is dependent on the design of the overall structure and is determined by first designing the waveguide thicknesses (105, 107, 109, 111,113,115) and determining the tunnel junction thickness to achieve the decoupling. Other options for the tunnel junction include using n-Al(x)Ga(y)ln(l-x-y)As(y) and p-Al(x)Ga(y)Iin(l-x-y)As(y) where x can vary between 0 to 1 and y can vary from 0 to 1 with the requirement being that the absolute values of x and y result in the refractive index of the tunnel junction be less than the refractive index of the quantum well region (106, 110, 114). The thickness of a tunnel junction base on this material system is dependent on the design of the overall structure and is determined by first designing the waveguide thicknesses (105,107,109,111,113,115) and determining the amount of tunnel junction thickness to achieve the decoupling.

[0078] The experimental results for the decoupled waveguide approach (incoherent) is shown in Figure 11. The single transverse (fast axis) mode (1101) indicates success in decoupling the waveguides. The lateral single mode far-field is shown in this Figure (1102) which is accomplished with a simple ridge structure.

[0079] References:1) J. Fricke, J. Wenzel, A. Maasdorf, C. Zink, M. Matalla, R. Unger and A Knigge, “Ridge waveguide lasers with vertically stacked quantum wells and tunnel junctions,” Semiconductor Science Technology, 37 (2022) doi: 10.1088 / 1361-6441 / ac860f.2) H. Wenzel, Jorg Fricke, A. MaaPdorf, N. Ammouri, C. Zink, D. Martin and A. Knigge, “Internally wavelength stabilized 910nm diode laser with epitaxially stacked multiple active regions and tunnel junctions,” Electronic Letters, Feb. 2022, Vol. 58, No. 3, pp.121.3) Z. Chen, J. Kanskar, “High Brightness Coherent Multi -Junction Diode Lasers,” US 10461505B2.Docket: 0021-0012W014) S. Aboujja “AlInGaAs / InGaAs / InP Edge emitting semiconductor laser including multiple monolithic laser diodes” US11152767B1.5) D. Bean “High-Power infrared semiconductor diode light emitting device” US2007 / 0002915A1.

[0080] 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 scope of the invention encompassed by the appended claims.

Claims

Docket: 0021-0012W01CLAIMSWhat is claimed is:

1. A multiple-junction semiconductor laser diode device comprising:a substrate;a plurality of edge-emitting laser diode structures stacked over the substrate and separated from one another by tunnel -junction regions, each laser diode structure comprising a p-side waveguide, a multiple-quantum-well active region, and an n-side waveguide; andwherein the p-side waveguides of the plurality of laser diode structures have substantially identical thicknesses and compositions, the n-side waveguides of the plurality of laser diode structures have substantially identical thicknesses and compositions, and each tunnel -junction region has an optical refractive index equal to or greater than an effective refractive index of the active regions and a thickness sufficient to guide a portion of an optical mode;wherein the laser diode structures are optically coupled to one another and the device oscillates on a coherent vertical supermode extending through all of the laser diode structures.

2. The device of claim 1, wherein the plurality of laser diode structures consists of three laser diode structures.

3. The device of claim 1 or 2, wherein:the substrate comprises InP;each of the p-side waveguides and n-side waveguides comprises (AlxGa)In(y)As with 0.1 ≤ x ≤ 1 and 0 ≤ y ≤ 0.9;each multiple-quantum-well active region comprises AlGalnAs quantum wells having a thickness less than 11 nm; andthe device is configured to emit in a wavelength range from about 1250 nm to about 1750 nm.

4. The device of any of claims 1-3, further comprising a grating formed in at least one of the p-side waveguides, the grating being configured to lock the supermode to a narrow emission linewidth.Docket: 0021-0012W015. The device of any of claims 1-4, wherein the device comprises a ridge waveguide having a ridge contact width less than 5 μm to provide lateral singlemode operation.

6. The device of any of claims 1-5, wherein at least one of the p-side waveguides and the n-side waveguides of the plurality of laser diode structures has a graded composition to provide a graded-index profile.

7. The device of any of claims 1—6, wherein:the p-side waveguides and / or the n-side waveguides of the plurality of laser diode structures differ from one another by less than about 10% in at least one of thickness and refractive index; andthicknesses and refractive indices of the tunnel-junction regions and cladding layers are selected such that a phase shift across the stack of laser diode structures is approximately an integer multiple of 2π;whereby a fast-axis far-field of the device is approximately diffraction-limited and substantially single-lobed.

8. A multiple-junction semiconductor laser diode device comprising:a substrate;a plurality of edge-emitting laser diode structures stacked over the substrate and separated from one another by tunnel-junction regions, each laser diode structure comprising a p-side waveguide, a multiple-quantum-well active region, and an n-side waveguide; andwherein at least one of the following is satisfied:(i) the p-side waveguides of the plurality of laser diode structures have mutually different thicknesses and / or compositions;(ii) the n-side waveguides of the plurality of laser diode structures have mutually different thicknesses and / or compositions; and(iii) each tunnel-junction region has a thickness selected to reduce optical coupling between adjacent laser diode structures;the waveguide and tunnel -junction dimensions being selected to reduce optical coupling between the laser diode structures;Docket: 0021-0012W01wherein the laser diode structures oscillate on separate laser modes and an aggregate fast-axis far-field of the device has a single main lobe.

9. The device of claim 8, wherein the plurality of laser diode structures consists of three laser diode structures.

10. The device of claim 8 or 9, wherein cladding layers between the multiple¬ quantum -well active regions have thicknesses greater than about 1.5 pm to reduce optical coupling between the laser diode structures.

11. The device of any of claims 8-10, wherein center-to-center separations between the multiple-quantum-well active regions of adjacent laser diode structures are greater than about 3 pm to reduce optical coupling between the laser diode structures.

12. The device of any of claims 8-11, wherein at least one of:the p-side waveguides of the plurality of laser diode structures have mutually different compositions and / or graded-index profiles; andthe n-side waveguides of the plurality of laser diode structures have mutually different compositions and / or graded-index profiles;is satisfied, to decouple energy sharing between the laser diode structures and create separate oscillating modes whose far-fields sum to the single main lobe.

13. The device of any of claims 8—12, wherein each tunnel -junction region is formed with an optical refractive index less than an effective refractive index of the active regions.

14. The device of any of claims 8-13, wherein each tunnel-junction region has a thickness in a range from 5 nm to 300 nm, and has a p-type or n-type dopant concentration greater than 1 × 1018cm−3.

15. The device of claim 14, wherein each tunnel -junction region comprises a semiconductor material selected from the group consisting of:InP;Al(x)Ga(y)In(1−x−y)As(y), where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1; andDocket: 0021-0012W01Ga(l -x)In(x)As(y)P(l -y), where 0 < x < 1 and 0 < y < 1.

16. A ranging system comprising:a semiconductor laser transmitter comprising a device according to any of claims 1—15 and optics configured to project output light of the device toward a target; anda receiver configured to detect light returned from the target and to determine at least one of a distance to the target and a velocity of the target using time- of-flight processing or frequency-modulated continuous-wave processing.

17. The device of any of claims 1—15, configured as a semiconductor optical amplifier, wherein at least one ridge waveguide segment is tilted or curved by less than 10 degrees relative to an optical axis and a facet associated with the ridge waveguide segment has an anti-reflection coating providing a reflectivity less than 0.1 percent, and wherein an opposite facet optionally has a high-reflection coating providing a reflectivity greater than 90 percent.

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