Distributed feedback laser and method of manufacture thereof

Abstract

A distributed feedback laser is fabricated from a semiconductor heterostructure. A lower cladding and waveguiding structure is formed by depositing alternating first and second layers (12, 14) comprising first and second materials of different compositions and Al x1 ln y1 Ga 1-x1-y1 N and Al x2 ln y2 Ga 1-x2-y2 N, with x1, y1 > 0 and x2 + y2 < 1, the compositions being selected such that the refractive index of the first material is lower than that of the second material. At least one of the first layers (12) is etched to form a periodically corrugated surface and then an overgrowth of the second material is carried out to planarize the corrugated surface with a planarizing layer (17). An in-plane Bragg grating (16) for inducing distributed feedback is thereby formed having an in-plane periodic alternation between the first and second materials. Active region, upper cladding and waveguiding layers (18, 20, 22, 24, 25, 26, 28) are then deposited on top of the overgrown planarizing layer (17).

Description

[0001] EXALOS AG - 1 - P10044WO

[0002] TITLE OF THE INVENTION

[0003] DISTRIBUTED FEEDBACK LASER AND METHOD OF MANUFACTURE THEREOF

[0004] BACKGROUND OF THE INVENTION

[0005] The invention relates distributed feedback lasers and to methods of their manufacture.

[0006] The distributed feedback (DFB) laser is a type of edge-emitting semiconductor laser diode (LD) in which a Bragg grating is arranged in the cavity direction. The cavity is typically a linear cavity of the Fabry-Perot type formed by the cleaved faces of the semiconductor chip. The Bragg grating is usually formed by a periodic modulation of refractive index. Most commonly, the refractive index modulation is provided by an in-plane alternation between first and second semiconductor materials with first and second refractive indices. Refractive index modulation may alternatively be provided by a surface (or top) grating, for example as disclosed in:

[0007] • Emily Trageser, Haojun Zhang, Sonya Palmer, Theodore Morin, Joel Guo, Jiaao Zhang, Evan Geske, Heming Wang, Andreas Boes, Shuji Nakamura, John E. Bowers, and Steven P. DenBaars, "Blue GaN-based DFB laser diode with sub-MHz linewidth," Opt. Express 32, 23372-23380 (2024) https: / / doi.org / 10.1364 / OE.525498

[0008] . JP2008166394A from Sharp or in a lateral grating etched into the sidewalls of a ridge

[0009] • Slight, Thomas J. et al. “InGaN / GaN Distributed Feedback Laser Diodes With Deeply Etched Sidewall Gratings.” IEEE Photonics Technology Letters 28 (2016): 2886-2888 https: / / doi.Org / 10.1109 / LPT.2016.2624500

[0010] Another alternative is to define the grating by gain or loss modulation.

[0011] In the common case of a buried semiconductor layer with in-plane modulation of refractive index, the grating is arranged close enough to the active region layers such that the optical mode, as confined vertically by the waveguiding layers, has a significant overlap with the Bragg grating, which then selects a single mode for lasing among the group of modes that are capable of attaining threshold in the cavity. The wavelength in the waveguide is A / neffwhere neffis the waveguide’s average effective refractive index and A is the emission or lasing wavelength in air. The refractive index modulation has a period along the cavity direction, where N is an integer and defines the grating order selected for the distributed EXALOS AG - 2 - P 10044 WO feedback. The lasing wavelength A of the edge-emitting structure is selected by the grating, so that single longitudinal mode operation is achieved.

[0012] DFB lasers are known for semiconductor heterostructures grown on a c-plane GaN substrate with epitaxial layers made of various compositions of the quaternary alloy materials system AlxInyGa^x^yN . In these DFB lasers, it is known to arrange the Bragg grating below the active region layers of the epitaxial stack.

[0013] An example of the Bragg grating being buried below the active region layers in a DFB laser is disclosed in:

[0014] • US2024332911A1 from Nichia

[0015] Here a DFB laser is disclosed which has a Bragg grating formed from a layer of GaN that is etched to form surface corrugations, i.e. , ribs, extending transverse to the cavity direction. The GaN surface corrugations are then overgrown by InGaN to form a GaN / InGaN grating layer. The GaN / InGaN grating layer is arranged within the lower cladding and waveguiding layers and below the active region layers, the latter being made of a GaN / InGaN multiquantum well (MQW) structure. An upper cladding layer of GaN is then deposited on top of the active region layers to complete the semiconductor heterostructure.

[0016] Another example of a Bragg grating being buried below the active region layers in a DFB laser is disclosed in:

[0017] • Shingo MASUI, Kazutaka TSUKAYAMA, Tomoya YANAMOTO, Tokuya KOZAKI, Shin-ichi NAGAHAMA and Takashi MUKAI “CW Operation of the First-Order AllnGaN 405 nm Distributed Feedback Laser Diodes” Japanese Journal of Applied Physics Vol. 45, No. 46, 2006, pp. L1223-L1225 [DOI: 10.1143 / JJAP.45.L1223],

[0018] Here a GaN substrate is used on which a lower cladding layer is grown. The grating structure is fabricated at the interface between the lower cladding layer and a lower waveguiding layer. The grating is formed by etching surface corrugations into the top of the GaN lower cladding layer followed by regrowth with a different material.

[0019] Another published example of the Bragg grating being buried below the active region layers in a DFB laser is disclosed in:

[0020] • JP2008004662 from Nichia.

[0021] A 15 nm thick GaN buffer layer is grown on a sapphire substrate at low temperature followed by a 2500 nm thick GaN under layer, a 3.5 pm thick contact layer of n: Al0 02Ga0 98N , a EXALOS AG - 3 - P 10044 WO

[0022] 0.15 m thick crack prevention layer of n: In0 06Ga0 94N and then an n-type cladding layer formed by a superlattice made up of 260 repeats of 2.5 nm thick undoped Al0 08Ga0 92N and 2.5 nm thick n: GaN up to a total thickness of 1.3 pm. The superlattice is then etched down to a depth of 100 nm using reactive ion etching (RIE) to form surface corrugations with a period of 80 nm. The surface corrugations are then overgrown by a planarizing layer of GaN before proceeding to deposit the active region layers and upper cladding layer. The grating structure is thus formed of GaN overgrown onto ribs of an Al0 08Ga0 92N / GaN superlattice.

[0023] A general limitation with the above known DFB lasers is that the etching processes that are used to create periodic surface corrugations before planarizing overgrowth have a depth control which is limited to a precision of approximately 20-30 nm. Lack of control over the etch depth is problematic, since variation in the etch depth results in variation in the magnitude of refractive index modulation of the subsequently formed grating, so that the device characteristics will vary from wafer to wafer and from chip to chip.

[0024] EXALOS AG - 4 - P 10044 WO

[0025] SUMMARY OF THE INVENTION

[0026] According to one aspect of the invention there is provided a distributed feedback laser configured to lase with a lasing mode at a lasing wavelength A, the laser comprising a semiconductor heterostructure of: a substrate; a c-plane GaN buffer layer arranged on the substrate; an in-plane Bragg grating structure arranged on the c-plane GaN buffer layer that forms an in-plane Bragg grating of period N ■ A / (2 ■ neff), where N is an integer, A is the lasing wavelength, and neffis the average effective refractive index experienced by the lasing mode, the in-plane Bragg grating being formed by at least one grating layer with a periodic in-plane alternation between first and second materials of respective compositions Al^In^Ga^^^N and A / z2 / y2Cai-x2-y2 / V. where xl, yl > 0 and x2 + y2 < 1, embedded above and below by the second material; a multi-quantum well active region layer structure arranged on the in-plane Bragg grating layer structure and comprising alternating third and fourth layers of third and fourth materials, the third and fourth materials having respective compositions Alx3Iny3Ga1-x3-y3N and Al^In^Ga^^^N that have different values of band gap energy; and an upper cladding layer arranged on the multi-quantum well active region layer structure.

[0027] According to a further aspect of the invention there is provided a distributed feedback laser configured to lase with a lasing mode at a lasing wavelength A, the laser comprising a semiconductor heterostructure of: a substrate; a c-plane GaN substrate or buffer layer; a lower cladding and waveguiding structure arranged on the c-plane GaN substrate or buffer layer, the lower cladding and waveguiding structure comprising alternating first and second layers comprising first and second materials having respective compositions Al^In^Ga^^^N and Al^In^Ga^^-y N such that the refractive index of the first material is lower than the refractive index of the second material; an active region arranged on the lower cladding and waveguiding structure, the active region comprising a multi-quantum well structure of alternating third and fourth layers of third and fourth materials having respective compositions Al^In^Ga^^-^N and Al^Iny Ga -^-y N that have different values of band gap energy; and an upper cladding and waveguiding structure arranged on the active region and comprising an upper cladding layer, EXALOS AG - 5 - P 10044 WO wherein the first and second materials have compositions xl,yl > 0 and %2 + y2 < 1, and wherein at least one of the first layers incorporates a periodic alternation between the first and second materials with a period in one direction of N ■ A / (2 ■ to form an in-plane Bragg grating, where N is an integer, A is the lasing wavelength, and neffis the average effective refractive index experienced by the lasing mode.

[0028] According to another aspect of the invention there is provided a method of fabricating a distributed feedback laser configured to lase with a lasing mode at a lasing wavelength A, the method comprising fabricating a semiconductor heterostructure by: providing a substrate; depositing a buffer layer of c-plane GaN on the substrate; depositing on the c-plane GaN buffer layer alternating first and second layers of first and second materials having respective different compositions Al^Iny^a^^^N and Alx2Iny2Ga1-x2-y2N such that the refractive index of the first material is lower than the refractive index of the second material, wherein the first and second materials have compositions xl, yl > 0 and x2 + y2 < 1; etching at least one of the first layers to form a structured surface having a period in one direction of N ■ A / (2 ■ neff), where N is an integer, A is the lasing wavelength, and neffis the average effective refractive index experienced by the lasing mode; regrowing with the second material to planarize the structured surface and thus form an in-plane Bragg grating of the first material embedded in the second material; depositing alternating third and fourth layers of third and fourth materials having respective compositions AZx3 / ny3Ga1-x3-y3 / V and Alx4Iny4Ga1-x4-y4N that have different values of band gap energy to form a multi-quantum well active region structure; and depositing an upper cladding layer.

[0029] The first and second materials may, for example, be Al^Iny^a^^^N and GaN, more particularly AlxlInylN and GaN.

[0030] The use of Al^Iny^a^^^N for the first material in combination with GaN for the second material allows any one of the GaN layers to be used as an etch stop layer, so that the etch can be implemented with a precise depth control terminating at the bottom of a selected layer of the first material, such as the uppermost first material layer or a deeper one. This may be achieved by etching with an etchant that is sensitive to reaching the second material, so that layers of the second material act as etch stop layers. Stopping the etch at the interface between first and second layers can also be achieved by monitoring the release of chemical species during etching, since, for example, there will be a step change in release of Ga if the first layer is Ga-free, and / or a step change in release of Al and In if the second layer is Al- EXALOS AG - 6 - P 10044 WO and / n-free. This approach is feasible provided that the first and second materials are selected such that there is at least one atomic species not shared between the first and second materials. For example, there may be Ga in the second material but not in the first material, and Al and In in the first material but not in the second material. The release of that species, or those species, are then monitored during etching, so that etching can be stopped on sensing a step change in the amount of that or those species that is being released. Tight process control of the etch depth can thus be achieved. The effective refractive index contrast of the grating can be closely defined. More uniform grating spectral characteristics can be achieved in the manufactured device and there is improved reproducibility of the device performance from wafer to wafer.

[0031] The ability to etch through a defined number of the first layers also allows the magnitude of reflectivity for the in-plane Bragg grating to be selected, since the refractive index contrast of the in-plane Bragg grating increases stepwise with increasing numbers of first layers that are etched through before overgrowth.

[0032] The higher refractive index second layers should have a thickness large enough that etching can be stopped reliably within any selected one of the second layers while also being small enough that parasitic waveguide effects in the device do not arise. Typically, these two constraints are met if the second layers have a thickness of at least 1 nm and no more than 150 nm, more particularly between 10 nm and 150 nm. The lower refractive index first layers, at least those that are etched to form the in-plane Bragg grating, preferably have a thickness of at least 10 nm or at least 15 nm prior to being etched.

[0033] In certain embodiments, the first material is AlxlInylN with xl + yl = 1 and the second material is GaN with x2 = y2 = 0. In particular, in some examples, the first material has an aluminum content selected from the group: a. 0.70 < xl < 0.95 b. 0.77 < xl < 0.87 c. 0.80 < xl < 0.84.

[0034] In certain embodiments, the third and fourth materials are / ny3Ga1-y3W and Iny^Ga^y^N with y3 7= yA and x3 = xA = 0.

[0035] In certain embodiments, the upper cladding layer is GaN.

[0036] Certain embodiments further comprise a lower cladding layer arranged between the buffer layer and the in-plane Bragg grating structure. The lower cladding layer material is example Al^Ga^^N where EXALOS AG - 7 - P 10044 WO

[0037] There are various options for the number m of first layers that are etched through, labelled m, to form the in-plane Bragg grating, including: a. m = 1 b. m > 2 c. m is selected from the group m = 2, 3, 4 and 5 d. m < k where k is an integer representing the total number of first layers optionally in combination with any of the options a to c e. m = k optionally in combination with any of the options a to c f. m < 10 optionally in combination with any of the options a to e.

[0038] In some embodiments each of the m first layers that are etched are etched through completely so that m is an integer, i.e. , m e N. In other embodiments, the uppermost one of the layers of the first material is only partially etched through which is achieved by ceasing etching part way through a first material layer, so that the number m of the first layers that are etched through is a non-integer value. For example, m = 0.5 would correspond to half the uppermost first layer being etched through, and m = 1.7 would correspond to all of the uppermost first layer being etched through and 70% of the next uppermost first layer being etched through.

[0039] It may be convenient when all the first layers, as deposited, are the same thickness. However, in general, the thickness of each of the first layers as deposited may be independently chosen and may differ from each other. The same is the case for the second layers.

[0040] Typically, the substrate, the buffer layer and also the layers of the in-plane Bragg grating structure are n-type and the upper cladding layer is p-type. More particularly, the first and second materials are n-type and the upper cladding layer material is p-type.

[0041] Certain embodiments further comprise a lower waveguiding layer arranged between the multi-quantum well active region structure and the in-plane Bragg grating structure. For example, the lower waveguiding layer may be made of n-. InGaN material.

[0042] Certain embodiments further comprise an upper waveguiding layer arranged between the multi-quantum well active region structure and the upper cladding layer. For example, the upper waveguiding layer may be made of p: InGaN material.

[0043] In some embodiments, the periodic surface structure formed by the etch will be a corrugated surface of ribs. The geometric form of the rib cross-section is not critical, so may be stepfunction like or more rounded, such as a wavy cross-section. In other embodiments, the EXALOS AG - 8 - P 10044 WO periodic surface structure need not have a one-dimensional form, such as ribs, but may have a two-dimensional array form, such as a two-dimensional array of pillars.

[0044] In certain embodiments, the substrate is c-plane GaN. Alternatively, a sapphire substrate may be used in combination with a c-plane GaN buffer layer.

[0045] A DFB laser according to the invention may be implemented as an edge-emitting semiconductor laser diode chip in which a linear laser cavity is formed by a front reflector and a back reflector arranged on front and back chip facets. Alternatively, a DFB laser according to the invention may be implemented with a ring cavity, wherein the in-plane Bragg grating extends around part or all of the ring and outcoupling is provided laterally or vertically, for example with a further Bragg grating of second or higher order arranged on a sidewall or on the top surface of the ring-shaped ridge respectively.

[0046] DFB lasers based on the above-disclosed materials combinations will typically have lasing wavelengths A in the range 360-570 nm.

[0047] A DFB laser according to the invention may be monolithically integrated with one or more further device components sharing a common waveguide in series, such as an optical amplifier (gain section), phase modulator, amplitude modulator, absorber section or passive waveguide section, so that the cavity comprises a DFB section with the in-plane Bragg grating and one or more other device component sections arranged in series. A waveguide splitter section may also be arranged within the cavity to split light emitted from the DFB section into two arms, each of which may include a device component, such as an optical amplification section. First and second DFB lasers according to the invention may be monolithically integrated in a common cavity either serially sharing a waveguide or side-by- side having respective waveguides, wherein the cavity may be linear or a ring. The first and second DFB lasers may have a specific frequency (wavelength) offset to each other, so that when mixed a THz optical wave is generated. The mixing may take place on chip by providing the chip with appropriate passive waveguides in a photonic integrated circuit (PIC) or off chip by external optical components for beam combination.

[0048] The invention in another aspect provides an optical module, for example based on a butterfly package, incorporating a plurality of light sources, at least one of which is a DFB laser embodying the invention. The output from the optical module may be via optical fiber or through a window to provide free-space propagating output beams that are collinear or noncollinear, e.g., parallel to each other with specified lateral or vertical offsets.

[0049] DFB lasers according to embodiments of the invention may find use in a variety of applications where they are integrated as components of a larger system, for example: EXALOS AG - 9 - P 10044 WO

[0050] Quantum applications: For example, a DFB laser may be combined with an atomic clock for timing synchronization and positioning or localization systems, where the emission wavelength of the DFB laser precisely matches the atomic transition levels.

[0051] • Blue-laser autofluorescence of fundus: A DFB laser is incorporated in an optical coherence tomography (OCT) system to perform early diagnosis of retinal disease by fundus imaging.

[0052] • Data communication: The DFB laser is used as a transmitter. The DFB laser is modulated in amplitude and / or phase to imprint data on an optical beam according to a data transmission protocol. The modulated light beam is then output to a transmission medium like air and other gases, water and other liquids, glass waveguides (e.g. lithium niobate) or glass fibers (e.g. silica or silica-based optical fibers) or other optical waveguides in solids.

[0053] • LiDAR (Light Detection And Ranging) systems: A DFB laser can be used as a narrow-linewidth source with on / off modulation, e.g. time of flight (TOF) measurement, or frequency modulation (FM) of a continuous wave (cw) beam (FMcw), thereby to perform high-resolution distance measurements, e.g., in air or water.

[0054] • Terahertz (THz) frequency generation: Two DFB lasers are used in combination that have a specific wavelength or frequency offset to each other and thus generate a THz optical wave when mixed.

[0055] • Gas sensing by absorption spectroscopy: the emission wavelength of the DFB laser is specified to precisely match a specific atomic or molecular transition of a gas species of interest.

[0056] • Flow cytometry: one or more DFB lasers are incorporated in the light source.

[0057] • Raman spectroscopy: one or more DFB lasers are incorporated in the light source.

[0058] • Holographic displays such as so-called holographic optical elements (HOEs): A DFB laser embodying the invention on an Al nyGa^^yN materials basis may be used for the blue light source and / or the green light source in an RGB (red-green-blue) optical module. The red light source laser can be made on another materials’ system basis, such as indium gallium phosphide. EXALOS AG - 10 - P 10044 WO

[0059] • Interferometers: These may be used in sensing applications, for example with optical fiber, solid-state waveguide or free-space designs, and based on Michelson or Mach- Zehnder geometries.

[0060] The invention in another aspect provides an atomic clock comprising at least one DFB laser embodying the invention.

[0061] The invention in another aspect provides an OCT system comprising at least one DFB laser embodying the invention as a light source.

[0062] The invention in another aspect provides a transmitter for a data transmission system comprising at least one DFB laser embodying the invention, wherein the DFB laser is modulated in amplitude and / or phase to imprint data on an optical beam according to a data transmission protocol.

[0063] The invention in another aspect provides a LiDAR system comprising at least one DFB laser embodying the invention as a light source.

[0064] The invention in another aspect provides an absorption spectrometer comprising at least one DFB laser embodying the invention as a light source.

[0065] The invention in another aspect provides a flow cytometer comprising at least one DFB laser embodying the invention as a light source.

[0066] The invention in another aspect provides a Raman spectrometer comprising at least one DFB laser embodying the invention as a light source.

[0067] The invention in another aspect provides a HOE comprising at least one DFB laser embodying the invention as at least part of a light source, wherein the HOE may be waveguide based or reflector based, for example.

[0068] The invention in another aspect provides an interferometer system comprising at least one DFB laser embodying the invention as a light source, wherein the interferometer may be based on a Michelson or Mach-Zehnder configuration, for example. EXALOS AG - 11 - P 10044 WO

[0069] BRIEF DESCRIPTION OF THE DRAWINGS

[0070] This invention will now be further described, by way of example only, with reference to the accompanying drawings.

[0071] Figures 1 to 5 show steps in the fabrication of a semiconductor heterostructure for a DFB laser according to one embodiment of the invention with Figure 5 showing the completed semiconductor heterostructure.

[0072] Figures 6 to 10 shows respective DFB laser semiconductor heterostructures according to first to fifth alternative embodiments.

[0073] Figure 11 is a schematic drawing showing how modulation of the average effective refractive index &neff, of the in-plane Bragg grating depends on the number of first layers m of the lower refractive index material that are etched through during fabrication.

[0074] Figures 12A, 12B and 12C show in plan, section and perspective views a DFB laser chip with the semiconductor heterostructure as shown in Figure 5.

[0075] Figures 13A and 13B show in plan and section view a DFB laser chip which monolithic integrates a DFB laser having the semiconductor heterostructure as shown in Figure 5 with a semiconductor optical amplifier (SOA).

[0076] Figure 13C shows in section view an alternative structure to that of Figure 13B which also monolithic integrates a DFB laser having the semiconductor heterostructure as shown in Figure 5 with a SOA.

[0077] Figure 13D shows in plan view another variant which compared with Figure 13A has a partly curved SOA section.

[0078] Figures 14A and 14B show schematic plan and perspective views of a semiconductor chip in which first and second DFB lasers embodying the invention are monolithically integrated.

[0079] Figure 15 is a schematic plan view of a semiconductor chip embodying the invention with a linear cavity that is realized with multiple inline sections formed under a common ridge, namely first and second DFB sections, as well as phase-tuning and gain sections.

[0080] Figure 16 is a schematic plan view of a semiconductor chip embodying the invention with a single ridge forming a waveguide in which is monolithically integrated a DFB section, a phase and / or amplitude modulator section, and an optical amplification section. EXALOS AG - 12 - P 10044 WO

[0081] Figure 17 is a schematic plan view of a semiconductor chip embodying the invention with a DFB section, a passive waveguide splitter section that splits light emitted from the DFB section into two arms, each of which includes an optical amplification section.

[0082] Figure 18 is a schematic plan view of an optical module with an optical fiber output, the optical module accommodating first, second and third light sources, at least one of which is a DFB laser embodying the invention.

[0083] Figure 19 is a schematic plan view of an optical module with a free-space output through a window, the optical module accommodating first, second, third and fourth light sources, at least one of which is a DFB laser embodying the invention.

[0084] Figure 20 shows schematically the flow cell of a flow cytometer with an excitation source in the form of an optical module as described above with reference to Figure 18 or Figure 19.

[0085] Figure 21 is a schematic drawing of a Raman spectrometer incorporating an optical module with a plurality of laser sources, including at least one laser that is a DFB laser according to an embodiment of the invention.

[0086] Figure 22 is a schematic drawing of a waveguide-based HOE incorporating at least one laser that is a DFB laser according to an embodiment of the invention.

[0087] Figure 23 is a schematic drawing of a reflection-based HOE incorporating at least one laser that is a DFB laser according to an embodiment of the invention.

[0088] Figure 24 is a schematic drawing of an interferometer system based on a Michelson interferometer configuration incorporating a DFB laser according to an embodiment of the invention.

[0089] EXALOS AG - 13 - P 10044 WO

[0090] DETAILED DESCRIPTION

[0091] In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a better understanding of the present disclosure. It will be apparent to one skilled in the art that the present disclosure may be practiced in other embodiments that depart from these specific details.

[0092] Numerical values for limits, ratios, ranges etc. are to be interpreted exactly in this document and in particular when construing the claims and not rounded up or down using significant figures, decimal places or otherwise.

[0093] While specific designs are disclosed in the following as using lenses and / or mirrors as components, it will be understood that lenses and mirrors are in principle optically equivalent and may be substituted for one another. Moreover, a single lens and / or mirror may be substituted with a lens and / or mirror combination.

[0094] Figures 1 to 5 show various stages during the fabrication of a semiconductor heterostructure for a DFB laser embodying the invention. It will be understood by the skilled person that the usual growth process for epitaxial layers ot AlxlnyGa^x_yN is by metal-organic chemical vapor phase epitaxy (MOVPE), although other growth processes such as molecular beam epitaxy (MBE) may be used. Further details on MOVPE growth of heterostructures in the quaternary system AlInGaN in particular in relation to AllnN are to be found in:

[0095] • M. Malinverni, A. Castiglia, M. Rossetti, A. Ferhatovic, D. Martin, M. Duelk, C. Velez; “InAIN cladding implementation in green superluminescent diodes and lasers" Appl. Phys. Lett. 15 May 2023; 122 (20): 201104. https: / / doi.Org / 10.1063 / 5.0151764

[0096] • M. Malinverni et al., "Blue and Green Low Threshold Laser Diodes With InAIN Claddings," in IEEE Photonics Technology Letters, vol. 35, no. 24, pp. 1303-1306, 15 Dec. 15, 2023, https: / / doi.org / 10.1109 / LPT.2023.3321420

[0097] The epitaxial stack of heterostructure layers is grown on a substrate. The substrate is typically c-plane GaN. The GaN substrate may be nominally undoped in which case it is intrinsically n-type or doped with an n-type dopant. Alternatively, a sapphire substrate may be used in which case it is necessary to grow a buffer layer of GaN on the substrate initially. In the following, it is assumed a c-plane GaN substrate is used.

[0098] Figure 1 shows the semiconductor heterostructure after depositing on the substrate 10 one or more first layers 12 of a first material in alternation with one or more second layers 14 of a second material. The first and second materials have respective different compositions Al^Iny^a^^^N and Alx2Iny2Ga1-x2-y2N . The first and second materials are selected to EXALOS AG - 14 - P 10044 WO have different values of refractive index such that the refractive index of the first material is lower than the refractive index of the second material. The first and second materials are selected having regard to their lattice constants so that the amount of any mismatch to the lattice constant of the c-plane GaN substrate (or GaN buffer layer if a sapphire substrate is used) is known and acceptable for materials quality. There are in total k lower index first layers deposited, the illustrated number being k = 5. In the illustrated example, the first material is AlxlInylN with xl + yl = 1 and the second material is GaN with x2 = y2 = 0. Using AlxllnylN for the first material is desirable, since it has a low refractive index and, when used in combination with GaN as the second material, an AlxllnylN composition can be selected that provides a large refractive index difference to GaN. More generally, the first material composition will satisfy xl,yl > 0, i.e. will include aluminum and indium and may or may not include gallium. Further, more generally, the second material composition will satisfy x2 + y2 < 1, i.e., will include gallium and may or may not include aluminum and / or indium. It will be understood that the respective compositions of the first and second materials are also selected having regard to their lattice constants so that the amount of any mismatch to the lattice constant of the c-plane GaN substrate is known and acceptable for materials quality.

[0099] Figure 2 shows the semiconductor heterostructure of Figure 1 on which has been formed by lithographic patterning ribs of resist 15, the ribs being equally spaced apart by the intended period for the grating, which as mentioned above is N ■ 2 / (2 ■ in a cavity direction, where N is an integer defining the grating order selected for inducing distributed feedback, A is the intended, i.e., design, lasing wavelength, and is the average effective refractive index experienced by the lasing mode.

[0100] Figure 3 shows the semiconductor heterostructure after etching and removal of the resist 15. Etching between the resist ribs 15 forms a corrugated ribbed surface extending out of the plane of the drawing with the ribs having the above-mentioned period NA / 2. Etching may be, for example, reactive ion etching (RIE) in particular or inductively coupled plasma (ICP) RIE (ICP-RIE). In this example, the uppermost second layer 14 and the uppermost first layer 12 are etched through completely with the etch terminating at the interface between the uppermost first layer 12 and the underlying second layer 14. The etching may etch an integer number m of the first layers 12, such as 1 , 2, 3, 4 or 5 in the illustrated stack.

[0101] Figure 4 shows the semiconductor heterostructure after planarizing overgrowth by depositing the second material 14 to a depth 17 to fill the surface corrugations and thus bury the ribs and provide a flat top surface for growth of the subsequent epitaxial layers. An inplane Bragg grating 16 with a period in the left-right direction of the drawing is thus formed, since the first layer 12 that has been etched and overgrown with the second material has an in-plane periodic alternation between the first and second materials in a cavity direction. The EXALOS AG - 15 - P 10044 WO in-plane Bragg grating layer 16 is also embedded in the second material by the second layer underneath it and the parts of the overgrown planarizing layer that are above it in the stack. The overgrown thickness of the second material may be substantially thicker than the thickness of the underlying second layers 14 as shown in Figure 1 in order to provide improved waveguiding, i.e., to provide a lower waveguiding layer for the semiconductor heterostructure of the DFB laser. However, the thickness should be limited so that the cavity mode that is intended to lase has sufficient spatial modal overlap with the in-plane Bragg grating 16.

[0102] Figure 5 shows the completed DFB laser semiconductor heterostructure. The semiconductor heterostructure is completed from the situation shown in Figure 4 by growing the active region layers and the upper cladding and waveguiding layers on top of the overgrown planarizing layer 17. An optional lower waveguiding layer 18 (e.g., InGaN) is deposited on the overgrown planarizing layer 17. This is then followed by the layers forming a multiquantum well (MQW) active region structure. Specifically, the MQW active region 20 is formed by depositing alternating third layers of a third material and fourth layers of a fourth material, the third and fourth materials having respective different compositions AZx3 / ny3Ga1-x3-y3W and Alx4Iny4Ga-x4-y4N . The third and fourth materials are selected to have different values of band gap energy so that they form well layers and barrier layers for the MQW structure. For example, the third material is / ny3Ga1 -y3W, i.e. the aluminum content is zero, x3 = 0, and the fourth material is Iny4Ga1-y4N , i.e. the aluminum content is zero, %4 = 0. The band gap difference between the third and fourth materials that creates the quantum wells is provided by selecting different amounts of indium and gallium, i.e., y3 #= y4. It will be understood that the respective compositions of the third and fourth materials are also selected having regard to their lattice constants so that the amount of any mismatch to the lattice constant of the c-plane GaN substrate is known and acceptable for materials quality. Next in the stack, two upper waveguiding layers are deposited. First, an upper inner waveguiding layer 22 of InGaN and second an upper outer waveguiding layer 24 of GaN. There then follows deposition of an electron blocking layer (EBL) 25 of AlGaN. This is followed by an upper cladding layer 26 of AlGaN and a top contact layer 28 of GaN. These particular materials choices are given by way of example and may be varied within the quaternary system Al nyGa^x^yN within the usual constraints such as sufficiently close lattice matching and the need for high quality growth as well as the properties required for each particular material or material combination, such as quantum well interband transition energies, refractive index contrast, conductivity level and conductivity type (p or n).

[0103] Although not expressly mentioned above, the DFB laser heterostructure will follow the usual design approach of a separate confinement heterostructure (SCH) with waveguiding layers EXALOS AG - 16 - P 10044 WO arranged above and below the active region in addition to the cladding layers arranged above and below the active region needed for current injection. As such, as is well known, the SCH heterostructure is electrically analogous to a pn-junction, or pin-junction if one labels the active region as i. The n-type and p-type parts of the stack are marked in the illustrations with the curly brackets labelled p and n to the right of the stack. Conventionally with heterostructures based on a GaN substrate, the lower part of the stack is n-type and the upper part is p-type owing to the lack of availability of high quality GaN substrates that are p- type, and this is also the case in the illustrated embodiments.

[0104] Figures 6 to 10 shows DFB laser semiconductor heterostructures according to first to fifth alternative embodiments. For corresponding features, the same reference numerals and other labelling is used as in Figure 5 for ease of comparison.

[0105] Figure 6 shows a semiconductor heterostructure which differs from that of Figure 5 in that the etching has a positive slope leading to the buried ribs of the first material in the in-plane Bragg grating having a shape that approximates to an isosceles trapezium shape, the buried ribs being thinner in the growth direction (or thicker in the etch direction).

[0106] Figure 7 shows a semiconductor heterostructure which differs from that of Figure 5 in that the etching has a negative slope leading to the buried ribs of the first material in the in-plane Bragg grating having a shape that approximates to an isosceles trapezium shape, the buried ribs being thicker in the growth direction (or thinner in the etch direction) as a result of undercutting below the resist by the etchant.

[0107] Figure 8 shows a semiconductor heterostructure which differs from that of Figure 5 in that etching was performed to etch through two of the as-grown first layers instead of one. After planarizing overgrowth, the in-plane Bragg grating 16 thus comprises two periodically modulated layers of ribs, one on top of the other. In further alternative embodiments, it will be understood that the etch may be through three, four, five or more of the as-grown first layers so that the in-plane Bragg grating 16 comprises three, four, five or more periodically modulated layers of ribs that alternate between the first and second materials.

[0108] Figure 9 shows a semiconductor heterostructure which differs from that of Figure 5 in that, before depositing the first and second layers, there is deposited a buffer layer 11 of GaN on the substrate followed by a lower cladding layer 13 of AlGaN. There then follow the alternating first and second layers 12 and 14 as in the previous embodiments with only two second layers 14 and one first layer 12 being illustrated. After etching and overgrowth this provides an in-plane Bragg grating 16 of a single first layer with periodic alternation between the first and second materials similar to Figure 5. In further variations, it will be understood that EXALOS AG - 17 - P 10044 WO the layer structure on top of the lower cladding layer 13 could also follow any of Figures 6, 7 & 8.

[0109] Figure 10 shows a semiconductor heterostructure produced with a different etch compared with the previous embodiments. Instead of etching completely through one or more of the lower index first layers, the approach is taken of etching part way through the uppermost one of the layers of the first material. The in-plane refractive index modulation for the in-plane Bragg grating 16 is thus provided by the upper etched portion of the partially etched first layer. A variation of this approach would be to completely etch through one or more first layers and then partially etch through the next first layer. For example, with reference to Figure 8, if the uppermost first layer was completely etched through and the next layer half the way through, then the structure would look like the section of Figure 8 but with the lower group of buried ribs being of half the height of the upper group of buried ribs.

[0110] Figure 11 is a schematic drawing showing how modulation of effective refractive index Aneff, of the in-plane Bragg grating depends on the number of layers m of the lower refractive index material that are etched through during fabrication. During fabrication of the semiconductor heterostructure, it is possible to vary the number of first layers that are etched through to provide different amounts of in-plane modulation of the effective refractive index in the cavity direction and thus different magnitudes of effective refractive index modulation of the in-plane Bragg grating. The number m of the lower refractive index first layers of the first material (e.g. AllnN) that are etched through determines the refractive index modulation that is achieved. Assuming that all etched first layers are completely etched through, i.e. , assuming m is an integer, the magnitude of the refractive index modulation will vary in a stepwise manner depending on the value of m. As illustrated schematically, if m = 1 then the modulation in refractive index is Aneff= Anl, whereas if m = 2 then it is Aneff= Anl + An2, if m = 3 then it is Aneff= Anl + An2 + An3, and so forth according to Aneff= Y^ Anj. As illustrated schematically, the increase in refractive index modulation per additional first layer that is etched through diminishes, i.e., Anl > An2 > An3 etc.

[0111] The higher refractive index second layers can be used as etch stop layers to achieve a good control of etch depth and hence also good control of the effective reflectivity of the in-plane Bragg grating. Another approach for controlling etch depth to achieve etching through an integer number of first layers is to monitor the flux of one or more chemical species during the etch process to sense when an interface between as-grown first and second layers of the first and second materials is arrived at, so that etching can be stopped at the desired interface as soon as the second material of the next second layer is encountered, or at a depth still within that second layer, so that the next first layer is not reached by the etch.

[0112] Since the troughs will in any case be overgrown with the second material, it is not deleterious EXALOS AG - 18 - P 10044 WO to if the etch is not stopped until a part of the next second layer (of the second material) has been removed.

[0113] The as-grown second layers of the higher refractive index second material (e.g. GaN) which are to be etched should have a thickness d2which is large enough that etching can be stopped reliably within the desired second layer while also being small enough that parasitic waveguide effects are not significant during DFB laser operation. A typical range of thicknesses of the second layers is between 1 nm and 150 nm. The thicknesses of the second layers may vary from layer to layer. The as-grown first layers of the lower refractive index first material (e.g. AllnN) which are to be etched, each of thickness d1, have thicknesses that are individually and collectively selected to provide a desired effective reflectivity of the in-plane Bragg grating, i.e. , a desired amount of refractive index contrast. The thicknesses of the first layers may vary from layer to layer.

[0114] Figures 12A, 12B and 12C show in plan, section and perspective views an edge-emitting DFB laser chip 30 with the semiconductor heterostructure as shown in Figure 5. Any of the layer structures of Figures 6 to 10 could be similarly envisaged. The semiconductor structure of Figure 5 is diced out of the wafer into a chip form by cleaving to form a pair of parallel end facets. The end facets are then coated to form mirrors. The mirror coatings may be multilayer dielectric mirrors, for example, deposited using a vapor deposition technique. A linear laser cavity is thus formed by a front reflector (i.e. mirror) 32 and a back reflector (i.e. mirror) 34. The front and back reflectors 32 and 34 form the output coupler and high reflector respectively of the linear laser cavity. A ridge 36 is formed on the top surface of the chip 30. In the active region layers 20 under the ridge 36 an optical mode 'M' is formed, as shown schematically with the oval. The optical mode, ‘M’, has some power extending into the grating 35 of the first layer 16 (one rib being shown in Figure 12C) so that the grating 35 is effective at mode selection of the optical mode, ‘M’. The ridge 36 thus serves to create the lateral bounds of a waveguide in the active region layers 20. It will be appreciated that the schematic illustration of Figure 12C shows an “unburied" waveguide design. A buried waveguide design could also be adopted, in which regrowth buries the ridge with infill regions. The infill regions may be undoped semiconductor material, for example. The material of the infill is preferably chosen to have a refractive index less than that of the ridge material, so that waveguiding occurs under the buried ridge. In addition, a dielectric material may be deposited on the overgrown infill material around the ridge to promote guiding of the injection current into the gain medium.

[0115] Figures 13A and 13B show in plan and section view an edge-emitting DFB laser chip 30 which monolithic integrates the DFB laser with a SOA. There is thus a DFB section 38 and an SOA section 40 with the output from the chip 30 taking place from the SOA section 40, EXALOS AG - 19 - P 10044 WO i.e. , to the right in the drawings. The ridge 36 shared by the DFB and SOA sections 38, 40 is straight. Referring to Figure 13B, it can be seen that the uppermost first layer 12 that is etched through to form the grating 35 in the DFB section 38 is not etched through in the SOA section 40 and thereby remains a continuous layer in the SOA section 40.

[0116] Figure 13C shows in section view an alternative structure to that of Figure 12B, in which the uppermost first layer 12 that is etched through to form the grating in the DFB section 38 is fully etched away in the SOA section 40, so that the grating layer in the DFB section 38 aligns with an overgrown layer of the second material in the SOA section 40.

[0117] Figure 13D shows in plan view another variant which compared with Figure 13A has an SOA section 40 that is not straight but rather has a curved portion adjacent the front reflector 32. The layer structure may be as shown in the sectional views of Figure 13B or Figure 13C.

[0118] It will be understood that variations following Figures 6 to 10 are also possible with the monolithic integrations of Figures 12A to 12C and Figures 13A to 13C. Moreover, other monolithic integrations of the DFB laser are also contemplated, such as monolithic integration of the DFB laser with a modulator, a phase shifter or a passive waveguide. These elements may also be combined, including combination with an SOA section. Examples of passive waveguide components that may be monolithically integrated with the DFB laser are splitters, couplers, and wavelength combiners. Examples of a modulator that may be monolithically integrated with the DFB laser are an electro-optical amplitude modulators (EAM) for intensity modulation and an electro-optical phase modulator (PM) for phase modulation. Another monolithic integration that is contemplated is a chip with two or more inline or parallel DFB sections within a common cavity defined by the chip end facets. Moreover, one or more individual section or sections that are combined monolithically with a DFB section may be within the chip, i.e., not directly adjacent an output facet. Moreover, with appropriate etching and overgrowth techniques, as employed to fabricate the DFB grating, it will be understood there is full flexibility between chosing either the first material or the second material for the monolithically integrated sections.

[0119] Figures 14A and 14B show schematic plan and perspective views of a semiconductor chip 30, in which first and second DFB lasers are monolithically integrated. This is achieved by etching first and second ridges 36i and 362 into the chip 30 that extend side-by-side, e.g., in parallel to each other, between the front and back reflectors 32 and 34 to form first and second linear cavities. By way of example, a buried waveguide design is shown where regrowth has been used to bury the ridges with infill regions 29 of undoped semiconductor material with a refractive index less than that of the materials making up the first and second ridges 36i and 362 so that waveguiding occurs under the buried ridges. In addition, a EXALOS AG - 20 - P 10044 WO dielectric material may be deposited on the overgrown infill material around the ridges to promote guiding of the injection current into the first and second gain media. The first and second DFB lasers can be realized with the same epitaxial structure for the light-generating active region. In designs where different emission wavelengths 2n22where #= A2are desired the first and second gratings 35i and 352 have different periods. Namely, the first DFB laser is realized with a grating 35i having a first pitch while the second DFB laser is realized with a grating 352 having a second pitch greater or less than the first pitch. Therefore, the first DFB laser will emit light at a first wavelength different from the light emission from the second DFB laser emitting light at a second wavelength A2. For example, the first DFB laser may emit light at a wavelength of 450.0 nm (corresponding to an optical frequency of 666.205 THz) while the second DFB laser may emit light at a wavelength of 451.0 nm (corresponding to an optical frequency of 664.728 THz). The optical output beams of the first and second DFB lasers could be mixed, for example after beam collimation and optical collinear alignment, to generate an optical beat signal with a frequency of 666.205- 664.728 = 1.477 THz. A Terahertz optical signal will thus be generated with an optical carrier wavelength of ~450nm in the blue spectral range. Alternatively, where a higher power beam is desired the first and second emission wavelengths may be the same = A2. Under the first and second ridges 36i and 362, in the active region layers 20, respective optical mode 'MT and ‘M2’ are formed, as shown schematically with the ovals, the optical modes having some power extending into the grating-forming layers 16 (one rib of each grating being shown in Figure 14B) so that each of the first and second gratings is effective at mode selection of the optical mode at the specified emission wavelength A1;A2. Moreover, if the beams from the different DFB lasers is to be combined this may be done on-chip with a suitable PIC or off-chip with external beam combining components. It is further possible to monolithically integrate three, four or more DFB lasers on a single chip as desired.

[0120] Figure 15 is a schematic plan view of a semiconductor chip 30 with a linear cavity that is realized with multiple inline sections formed under a common ridge 36, the inline sections including a first DFB section 38i with a first grating 35i, illustrated by way of example adjacent the back reflector 34, and a second DFB section 382 with a second grating section 352, illustrated by way of example adjacent the front reflector 32. Third, fourth or higher number DFB sections may be added in variants of the illustrated design. An optional phasetuning section 42 is also shown which has the role of matching the round-trip condition of the light generated. An optional amplification (or gain) section 40, for example an SOA section, is also shown, which has the role of amplifying the light circulating in the cavity to increase the output power. A chip with this multiple grating design can be used for achieving a larger wavelength tuning range than a single grating design and / or for achieving a faster EXALOS AG - 21 - P 10044 WO wavelength tuning speed. The chip may operate as a Vernier-type wavelength-tunable semiconductor laser.

[0121] Figure 16 is a schematic plan view of a semiconductor chip 30 with a single ridge 36 forming a waveguide in which is monolithically integrated a DFB section 38 and other elements. The DFB section 38 comprises a grating 35. From back to front, the DFB section 38 is followed by a phase and / or amplitude modulator section 44, and then an optical amplification section 40, such as an SOA section.

[0122] Figure 17 is a schematic plan view of a semiconductor chip 30 with a DFB section 38 with a grating 35 followed by, towards the front reflector 32, a passive waveguide splitter section 46, illustrated as a 1-to-2 splitter by way of example, which splits light emitted from the DFB section 38 in the direction towards the front reflector 32 into first and second arms. The passive waveguide spliiter section 46 provides a low-loss propagation for the narrowlinewidth light selected by the grating 35 in the DFB section 38. This passive waveguide section 46 can be fabricated by an additional regrowth fabrication step where semiconductor material with a different bandgap, or other suitable materials, are deposited. The first and second arms leading from the splitter 46 are connected in series to respective first and second optical amplification sections 40i and 402, for example SOAs. The first and second optical amplifier sections 40i and 4O2 do not change the emission wavelengths of the respective first and second beams output from the front reflector 32 but create an optical phase difference between them while maintaining their coherence. The first and second output beams can then be coherently combined in amplitude and phase off-chip by suitable optical elements (not shown), such as free-space optical components known for beam combination. On-chip combination with a PIC is also possible. Introducing an optical phase difference between the beams enables the optical output power of the combined beams to be significantly higher and more stable compared to the case of non-coherent beam combination and amplification from two independently functioning DFB lasers. Finally, it will be understood that the ridge structure will have the tuning fork shape evident in Figure 17.

[0123] While in the above detailed description, it has been at least implicitly assumed that the cavity is a linear cavity, in particular with cavity end mirrors formed by cleaved end facets of the chip, other embodiments of the invention are envisaged with a ring cavity. The in-plane Bragg grating would then extend over part or all of the ring. Vertical or lateral outcoupling could be realized with a further Bragg grating of second or higher order.

[0124] One or more of the above-described DFB laser chips may be accommodated in an optical module, optionally with other light sources, such as other semiconductor lasers, which may or may not be DFB lasers, and superluminescent light emitting diodes (SLEDs). The optical EXALOS AG - 22 - P 10044 WO module may be based on a butterfly package. Each DFB laser chip may have different emission wavelengths (e.g., fractions of a nanometer to several nanometer or to several tens or hundreds of nanometers) or the same wavelength. The divergent optical output beams are collimated or partly focused by lenses and then spectrally combined, e.g., by dichroic singleedge or double-edge filters. The free-space beams may be collinearly aligned to form a single combined optical beam that is then coupled out of the optical module either through an optical window or through a common optical waveguide, for example a single-mode, polarization-maintaining, multi-mode fiber, through a photonic crystal fiber or any other suitable optical waveguide device. Alternatively, the beams can be brought together with an intentional horizontal and / or vertical beam offset relative to each other to form a group of closely adjacent parallel beams. Furthermore, the optical module may contain multiple photodiodes to monitor the optical output power of each of the DFB output beams or the output power of the combined optical beam. For an optical module providing red, green and blue light (RGB) as needed for colour display applications, a DFB laser embodying the invention on an Al nyGa^x^yN materials basis may be used for the blue light source and / or the green light source. The red light source can be made on another materials’ system basis, such as indium gallium phosphide.

[0125] Figure 18 is a schematic plan view of a specific example optical module 50. The optical module is based on a butterfly package, shown as a 14-pin butterfly package. The butterfly package has a plurality of terminal pins 55 via which electrical connections may be made to components housed in the package. The butterfly package has a housing 53 that forms an enclosure in which a plurality of DFB laser chips 30 are accommodated as well as associated components. The components are at least for the most part mounted directly or indirectly on a main board 51 , which may also be referred to as a carrier board, substrate, optical breadboard or mounting board. The main board 51 is provided with a temperature sensor 54s arranged on the main board to measure the temperature of the main board. The main board 51 should have good thermal conductivity for heat dissipation, and should be mechanically stiff. Suitable materials choices are ceramic, e.g. AIN or AI2O3, a suitable metal, e.g. copper, aluminum or alloys containing either or both of these metals such as CuW.

[0126] The housing 53 and the enclosure it defines by its internal walls are substantially rectangular in plan view as illustrated aligned with orthogonal axes x and z respectively across and along The optical module 50 has an optical fiber output 60. The optical module 50 is illustrated with y being the axis out of the paper, i.e. the vertical. The optical module 50 has its optical output port 60, 62 arranged at one end of the enclosure in an end wall of the housing 53. The optical output port in this embodiment is realised with optical fiber in the form of an optical fiber ferrule 62, which is attached to an optical fiber 60 that may be single mode or multimode EXALOS AG - 23 - P 10044 WO and may be polarisation maintaining (or not) as desired. The ferrule 62 and optical fiber 60 form a so-called pigtail and serve to couple the combined beam from the different DFB laser chips 30 into the end of the optical fiber and thus out of the optical module 50. The ferrule 62 may also be attached to the main board 51, or may be secured to the housing 53, e.g. to the end wall. It will be appreciated that the module also has a lid (not shown) which may be secured removably or non-removably to the housing by fasteners, such as screws or rivets, and / or adhesive bonding, welding or other fastening or sealing means as desired.

[0127] Specifically, the optical module 50 accommodates first, second and third DFB laser chips 30r, 30g and 30b. The DFB laser chips 30 are arranged in the enclosure to emit respective first, second and third beams having first, second and third emission wavelengths into free space within the enclosure along first, second and third beam paths. The three wavelength ranges are labelled as 'r', 'g' and 'b' to indicate red, green and blue emission wavelengths by way of example, and also to provide intuitive labelling. These could however equally well be three wavelengths in some other part of the visible, near-UV or near-infrared (NIR). The DFB laser chips 30r, 30g and 30b are mounted on respective submounts 56, labelled 56r for the red DFB laser chip 30r, 56g for the green DFB laser chip 30g, and 56b for the blue DFB laser chip 30b. The submounts 56 are in turn mounted on the main board 51. The main board 51 is provided with a temperature sensor 54s. The materials choices for the submounting boards 56 are similar to those as described above for the main board 51. The mode of assembly with populated submounts on a main board is referred to as a chip-on-submount (CoS) in the art. The submounts 56 may also have respective temperature sensors 54r / g / b mounted on them, labelled 54r for the red DFB laser submount 56r, 54g for the green DFB laser submount 56g, and 54b for the blue DFB laser submount 56b. The temperature sensors 54r / g / b allow the temperature local to each DFB laser chip 30r / g / b to be monitored. The temperature sensors 54s, 54r / g / b may have their signals used as control inputs for one or more cooling elements (not shown). For example the mounting board 51 may have attached to its upper or lower surface a thermoelectric cooler, e.g. a Peltier device. The submounts 56r / g / b may also have individual cooling elements (not shown) that can be independently controlled via the respective temperature measurements from sensors 54 / r / g / b.

[0128] The output laser beams from the DFB laser chips 30r / g / b are collimated by respective collimating lenses 58r / g / b. All the DFB laser chips are shown arranged on one side of the enclosure. Alternatively, one or more of the DFB laser chips could be arranged on one side of the enclosure and one or more of the others on the other side of the enclosure so that the some beam propagate initially in the opposite direction to other beams, i.e. in negative-x and positive-x directions respectively. EXALOS AG - 24 - P 10044 WO

[0129] The red laser beam after collimation by lens 58r is deflected through 90 degrees from the positive-x direction into the z-direction by a mirror 59r arranged at 45 degrees to x and z. The z-travelling red beam is incident on the back face of a beam combiner 59r-g which has the function of combining the red beam with the green beam. The beam combiner 59r-g is a planar optical element which is made of a suitable glass or crystal material, such as a dichroic mirror. The beam combiner 59r-g has a front side and a back side. The red beam is incident on the back side of the beam combiner 59r-g at an angle of incidence which causes the beam to refract into the beam combiner 59r-g. The back side is preferably coated with an antireflection coating (ARC) that is effective for the wavelength, angle of incidence and polarisation state of the red beam. The red beam is then routed through the glass or crystal to the front side and is once more refracted as it outputs from the front side. The front side of the beam combiner 59r-g is aranged to receive the green beam propagating in the positive-x direction from the collimating lens 58g at a position on the front surface that is the same as where the red beam passes through the front surface. Moreover, the beam combiner 59r-g is configured and arranged so that the green beam reflected from its front surface propagates in the same direction as the red beam output from the front surface, preferably the z-direction as schematically illustrated. The red and green beams thus emerge from the beam combiner 59r-g as a combined beam propagating in free space within the enclosure in direction z. The beam combiner 59r-g will usually be planar, but if desired it could be slightly curved, but still substantially planar, to provide some focusing or defocusing of one or more of the red and green beams. The combined red and green beam is then combined with the blue beam in a similar way using a further beam combiner labelled 59rg-b. Namely, the blue beam output from the blue DFB laser chip 30b travelling in the positive x-direction is collimated by collimating lens 58b and is incident on the front surface of the beam combiner 59rg-b, and the back surface of the beam combiner 59rg-b receives the combined red-green beam. The red, blue and green beams thus emerge from the beam combiner 59rg-b as a combined beam propagating in free space within the enclosure along an optical path in direction z. The combined beam is focused onto the end face of the optical fiber 60 held in the ferrule 62 by a coupling lens 59.

[0130] Individual power monitors 52r / g / b are provided for each of the DFB laser chips 30r / g / b respectively. Each individual power monitor 52r / g / b is arranged to receive a small fraction of the light power that has been transmitted through the deflecting mirror or beam combiner 59, these being configured to have slightly less than 100% reflection to the laser beam, so that a small power fraction, e.g. 1-3%, of the laser beam passes through the mirror or combiner element 59r, 59r-g, 59rg-b to the respective power monitor 52r, 52g, 52b. Other options for arranging the power monitors are possible. For example, the individual power monitors could be arranged adjacent the back facet of each DFB laser chip in order to measure the light that EXALOS AG - 25 - P 10044 WO

[0131] 'leaks', i.e. is emitted, from the back facet of the chip. Having power monitors for each DFB laser chip may be useful in a number of applications. For example, it may be useful when the output needs to meet particular safety standards, and those safety standards specify different safety limits for different ones of the emission wavelengths. The outputs from the power monitors would then be supplied to a controller that would control the drive currents supplied to each DFB laser chip so that the output power from each DFB laser chip did not exceed an upper limit. An addition power monitor to measure the power of the combined beam may also be provided. This may be implemeneted with a tapping mirror (not shown) arranged in the combined beam, e.g. between the coupling lens 59 and fiber ferrule 62, to tap off a small part of the beam to a power monitor, which may be realised as a photodiode, for example. Alternatively, a tap mirror could be omitted, and the combined beam power monitor could face the point where the combined beam is focused onto the fiber end and monitor power through monitoring back-scattered light from the fiber end.

[0132] Other design options may be incorporated into the optical module. For example, edge filters may be incorporated to filter each of the beams prior to them being combined, so as to filter out wavelengths that are outside the emission wavelength of each DFB laser chip. In the case that the beams are combined in order of decreasing wavelength, with the shortest wavelength being combined last, e.g., with a combination order red then green then blue, then each edge filter will cut-off wavelengths shorter than the emission wavelength of each DFB laser. Alternatively, if the beams are combined in order of increasing wavelength, with the longest wavelength being combined last, e.g. a combination order of blue then green then red, then each edge filter will cut-off wavelengths longer than the wavelength of each DFB laser. Edge filters may be incorporated integrally into the front side of the mirror 59r and the front and / or back sides of the beam combiners 59r-g, 59rg-b as coatings. Alternatively, edge filters may be added as separate components and mounted on the main board 51. Band filters could also be used for filtering out unwanted wavelengths in addition to, or instead of edge filters. Another design option is to use a polarisation filter on the combined beam, e.g. prior to or after the coupling lens 59, to increase the polarisation extinction ratio (PER) of the outputted beam. This may be useful when the module is specified to have a high PER, e.g. at least 20-30 dB, whereas the intrinsic PER of one or more of the DFB lasers may be lower, e.g. only 3-10 dB. It will be appreciated that the components 59, 58, 59 etc. may not be single components as illustrated, but may each consist of two or more components, such as isolators (electrical, thermal and / or vibration), and submounts.

[0133] Moreover, physically separate filters, polarisers, apertures or other optical components (not illustrated) may also be included that are attached to the mounting board 51. EXALOS AG - 26 - P 10044 WO

[0134] Figure 19 is a schematic plan view of an optical module 50 containing four DFB laser chips 30 and having a free-space output through a window 64 arranged in the end wall of the housing 53 to allow the combined beam to be output from the housing in the z-direction. Each of the optical output beams of the DFB laser chips 30 is collimated by individual collimation lenses 58. The optical output beams are then spectrally and spatially combined by dichroic single-edge or double-edge filters 59. However, different to the optical module shown in Figure 18, the collimated beams are not collinearly aligned but are having an intentional horizontal and / or vertical beam offset relative to each other before exiting the optical module through an optical window 64. Furthermore, the optical module may contain beam-shaping optics to convert the elliptical output beams of the DFB laser chips into a more circular beam, for example. Another option is that the optical module contains beam-shaping optical elements to generate a specific beam profile, such as an elliptical beam shape, at a specific distance from the optical module. A still further option is that the optical module contains diffractive optical elements like single-axis or double-axis lenses or rotationally symmetric spheric or aspheric lenses, to generate convergent or divergent beams exiting the optical window. There is no requirement for a lens for focusing or otherwise transforming the combined beam before output from the window 64, so this is omitted from the drawing. However, optionally, a lens in the same position as lens 59 of Figure 18 could be included. Such a lens need not be for creating a focus within the enclosure, but rather more likely would be provided to bring the output light beam to a defined focus some specified distance away from the optical module, or for providing an auxiliary or supplementary collimating function for the combined beam additional to that provided by the individual collimating lenses 58. The design of Figure 19 may be considered to combine the three DFB laser chips, and associated components, of the design of Figure 18 with a fourth DFB laser chip 30 / which is arranged to emit along the enclosure in the z-direction and directly project onto the back face of a beam combiner 59 / r-g via a collimating lens 58 / . In this way, the laser beam output from the DFB laser chip 30 / is combined with the output beams from the three DFB laser chips 30r / g / b to form a group of four beams that are offset relative to each other and propagating on parallel beam paths.

[0135] In the above description of optical modules, it will be understood that the repeated references to red, green and blue wavelengths are specific labels that make the description of the examples convenient to understand. While these colors are technically significant for display and projection applications, it will be understood that they may be generalized to mean first, second and third different emission wavelengths from first, second and third DFB lasers. Moreover, one or more of these emission wavelengths need not be in the visible region, since for example one or more of the emission wavelengths may be in the near infrared, or near ultraviolet. EXALOS AG - 27 - P 10044 WO

[0136] The optical modules as described above may be redesigned to omit or add DFB laser chips, and their associated components, as desired, so that there may be 2, 3,4, 5 or more DFB laser chips contained in the optical module. It will also be understood that each DFB laser chip may output more than one emission wavelength. Moreover, not all the emitters need be DFB lasers. For example, the optical module could combine one or more DFB laser chips with other kinds of emitters such as vertical surface emitting lasers, semiconductor diode lasers not based on a DFB design, or SLEDs. With higher numbers of sources, larger packages may be needed, e.g. butterfly packages with more than 18 pins that have more internal volume.

[0137] Figure 20 shows schematically the flow cell 70 of an example flow cytometer with an excitation source in the form of an optical module as described above with reference to Figure 18 or Figure 19. Flow cytometers are used for analyzing properties of cells that are suspended in a liquid. The liquid is passed through a conduit so that the cells flow past a fluorescence-based sensing device one at a time. The cells can thus be counted and classified according to their spectroscopic properties which can then optionally be used to further direct the flow or for other purposes. For example, the cells can be sorted downstream of the detection device according to their fluorescence properties. This is usually referred to as fluorescence activated cell sorting (FACS). The instrument typically is provided with multiple detectors to collect both forward and side scattered light from the cells. Side scatter (SSC) refers to an orthogonal, or at least transverse, alignment of the optical axes of the excitation and collection optics. Forward scatter (FSC) refers to a transmission mode optical set up. A typical instrument will have one detector arranged for forward scatter collection and several detectors arranged for side scatter collection. Usually, fluorescence data at different emission wavelengths are collected by multiple detectors arranged in side scatter. As well as for cell counting, the instrument is sensitive to a variety of cell properties, such as size, morphology, membrane roughness, granularity of the cytoplasm and many others. Generally, each cell type has a unique combination of measured properties, including fluorescence, and correlation of FSC and SSC signals, which allow the cell type of each cell to be identified. Moreover, healthy and diseased cells of the same type may be distinguishable. An optical module 50 as described above with reference to Figure 18 or Figure 19 is provided as the excitation source. Alternatively, in the simplest arrangement, a single DFB laser according to embodiments of the invention could be used that is not packaged in an optical module. Narrow-line lasers are important in flow cytometry because they emit light with a highly specific wavelength and minimal spectral width, enabling precise excitation of fluorescent dyes and proteins. Additionally, the stability and coherence of narrow-line lasers enhance signal-to-noise ratios, ensuring reliable detection of cellular markers. For this reason, DFB lasers according to embodiments of the present invention are EXALOS AG - 28 - P 10044 WO well suited to flow cytometry. Schematically illustrated is an optical module with three DFB laser sources 30i, 302 and 30a by way of example. It is advantageous to provide multiple DFB lasers, since then a variety of excitation wavelengths of interest can be covered. Emission wavelengths (e.g. emitting at 405 nm, 488 nm and or 638 nm) are chosen to match the absorption spectra of specific fluorophores. The optical module 50 may be configured to emit laser beams that are non-collinear, e.g., parallel to each other, being intentionally spaced apart by a fixed amount along the direction of flow, such that the same cell can be sequentially and independently excited by each laser beam, as schematically illustrated. The optical module 50 may thus accommodate multiple DFB lasers 30 as well as suitable optical components, such as lenses and mirrors, to provide collimated beams, or beams focused on a measurement region, that are evenly spaced apart as described with reference to Figure 19 in particular. The different beams may have different wavelengths or the same wavelength.

[0138] The interaction of the laser light with the cells generates fluorescence and scattered light, which are captured by SSC and FSC detectors 86, 88 which separate and quantify the signals, enabling the identification and analysis of various cell properties such as number, size, and the presence of specific markers. The cells may then subsequently be sorted further downstream in a sorting station 92 based on their type identification as determined by an analyzer 90 which receives the data from the SSC and FSC detectors 86, 88 and processes the data using computing resource and appropriate computer code to identify the cell properties of interest.

[0139] In use, sample is introduced into the flow cell 70 through an inlet tube (not shown). The sample inlet tube is connected to an inner capillary tube which is radially enclosed prior to its termination by a sheath 78. As considered in the flow direction, the sheath 78 reduces in its cross-sectional diameter and the inner capillary tube terminates leaving the sample fluid, containing the cells in suspension, and the sheath fluid flowing along a capillary tube 80. Within the capillary tube 80, the sample cells flow radially confined to the central region of the flow by virtue of laminar flow at the interface between the sample fluid and the sheath fluid. The aim of this sheath arrangement is to allow good optical access to the sample in a flow tube that is sufficiently large in diameter to avoid blockages but sufficiently small so that individual cells are exposed by the laser light as they pass through the measurement region. The optical components for optical excitation and light collection are suitably arranged about the measurement region. The optical module 50 outputs one or more collinear or noncollinear (e.g., parallel) laser beams that are focused on the measurement region, which is a central portion of one section of the capillary tube 80, so as to intersect with the sample. Fluorescence from the sample excited by the laser beams is then collected through an SSC EXALOS AG - 29 - P 10044 WO collection lens 82 and the SSC detector 86 which divides the fluorescence into different wavelength bands. Each wavelength band is directed to a suitable channel of the SSC detector 86. Fluorescence from the sample excited by the laser beam 82 is also collected through an FSC collection lens 84 and directed to a FSC detector 88.

[0140] Figure 21 is a schematic drawing of a Raman spectrometer 100 incorporating an optical module 50 with a plurality of laser sources, including at least one laser that is a DFB laser 30 according to an embodiment of the invention, for example as described above with reference to Figure 18 or Figure 19. Alternatively, a single DFB laser according to an embodiment of the invention could be used that is not packaged in an optical module. The output beam from the optical module 50 is directed onto a sample, causing inelastic scattering of light as the laser photons interact with molecular vibrations or other inelastic scattering centers in the sample S. The scattered light is collected and directed into a spectrometer 102, which separates and detects the shifted wavelengths corresponding to specific vibrational modes of the sample’s molecules or other inelastic scattering centers. In the illustrated set-up a dichroic mirror 106 is used to direct light from the optical module 50 onto the sample via a lens 108 which focuses the laser beam onto the sample. Light back-scattered from the sample (dashed line) is collected by the same lens 108 and then is transmitted through the dichroic mirror 106 to the entrance aperture of the spectrometer 102. The collected data is then supplied by the spectrometer 102 to an analyzer 104. The analyzer 104 processes the data using computing resource and appropriate computer code to provide an analysis of the sample, for example to identify molecular species in the sample. Single-frequency excitation is important for Raman spectroscopy, because it ensures high spectral resolution and minimizes noise from overlapping Raman bands or fluorescence background, enabling precise identification and characterization of the sample’s chemical composition, molecular structure, and interactions. The high spectral purity provided by DFB lasers embodying the present invention makes them well-suited as sources for Raman spectroscopy. In some configurations, Raman spectrometers include lasers of different wavelengths for excitation, allowing flexibility in choosing the optimal laser wavelength to avoid sample fluorescence and ensure the best quality spectral data. In this respect, it can be imagined that multiple lasers as described in the present invention and differing in emission wavelength (different active region) could be combined in a compact package together with lenses and mirrors to provide a collinear beam output from the optical module as explained elsewhere in this document.

[0141] Figure 22 is a schematic drawing of a waveguide-based HOE 110 for a single eye, i.e. , in monocle format for monocular vision, with a light source in the form of an optical module 50 with red, green and blue light sources (not shown), wherein at least one of the blue and green light sources is a DFB laser according to an embodiment of the invention. A housing EXALOS AG - 30 - P 10044 WO

[0142] 116 is integrated midway along a temple (arm) 118 and houses an optical module 50, for example as described above with reference to Figure 18 or Figure 19. A combined RGB light beam 112 is output by the optical module 50 and is directed to a beam scanning element 114 which directs the light beams into a waveguide that brings the beam into a spectacle lens 120 by multiple total internal reflections, the light beams further travelling across and within the lens 120 by further total internal reflections until the field of view (FOV) of the wearer’s eye is reached. A diffraction grating 124 is arranged within the FOV to diffract the light onto the wearer's eye E and thus to directly project into the eye and form the image 122 on the retina.

[0143] Figure 23 is a schematic drawing of a reflection-based holographic optical element for a single eye, i.e., in monocle format for monocular vision, with a light source in the form of a laser module with red, green and blue emitting DFB lasers according to one of the abovedescribed embodiments. A housing 116 is integrated partway along a temple (arm) 118 and houses an optical module 50. A combined RGB light beam 112 is output by the optical module 50 and is directed to a scanning element 114 which projects an image on the inside surface of a lens 120, which include a faceted reflector 126 to reflect the light onto a wearer's eye E and thus to directly project into the eye and form the image 122 on the retina.

[0144] It will be understood that an HOE for two eyes, i.e., binocular vision, can be made by doubling up the illustrated designs of Figure 22 & 23 to arrive at a spectacles format. Moreover, the HOE according to these designs may be adapted for different display formats such as goggles format, helmet visor format, head-up display format (e.g., on a vehicle or aircraft windshield). A DFB laser embodying the invention is a suitable light source for a HOE, such as shown by way of example in Figures 22 & 23, since a DFB laser can be fabricated with an emission wavelength precisely matched to the specified operating wavelength, while also maintaining excellent wavelength stability. This is important in head- worn displays and head-up displays, such as augmented, mixed and virtual reality displays, to achieve optimal performance and minimal chromatic aberration.

[0145] Figure 24 is a schematic drawing of an interferometric sensor 130 based on a Michelson interferometer configuration. A DFB laser 30 according to one of the above-described embodiments is used as a light source to provide coherent light at a single frequency. While the illustrated example is of Michelson interferometer configuration, other interferometer configurations, such as Mach-Zehnder, may be suitable depending on the application.

[0146] Light output from the DFB laser 30 is supplied as input to an optical coupler 134 which splits the input light into one part that is output to a sensing arm 136 and another part that is output to a reference arm 138. It will be understood that the arms 132, 136, 138, 142 may be EXALOS AG - 31 - P 10044 WO implemented in optical fiber with the coupler 134 also being implemented in optical fiber. Alternatively, a photonic integrated circuit (PIC) may be used for at least some of the arms and the coupler. In the reference arm 138, light travels along the reference arm, is reflected by a reflector mirror 140 and travels back along the reference arm 138 in the reverse direction back to the coupler 134. In the sensing arm 136, light travels along the sensing arm 136, is reflected from a target, T, and travels back along the sensing arm 136 in the reverse direction back to the coupler 134 where it interferes with the light returning from the reference arm 138 to cause an optical interference signal to be generated. The optical interference signal is directed from the coupler 134 to an output arm 142 which leads to an optoelectronic detector 144 which converts the optical interference signal into an electrical signal. The interferometer is sensitive to variation in the optical path length in the sensing arm caused by a property of the target compared to the optical path length in the reference arm which is not sensitive to the target, e.g., remains constant or is subject to baseline fluctuations that are in common with the reference arm (e.g., ambient thermal fluctuations). In a distance sensor, it is physical movement of a target located beyond the waveguiding part of the sensing arm (e.g. a section of optical fiber) which is measured. In a strain or temperature sensor, it may be strain or expansion in an end portion of an optical fiber that forms the sensing arm which is measured. The electronic signal from the detector 144 is then supplied via an electronic signal line 146 to a data sampler 148 which samples the optical interference signal. The sampled optical interference signal is then supplied via a data communication line 150 to a signal processor 152 for numerical processing with computing resource. The sampled optical interference data is namely processed to extract meaningful measurement parameter information, such as displacement or position of a moving target or some other parameter such as strain or temperature. While the Michelson interferometer is schematically illustrated as being implement with optical fiber components, it will be understood that a free-space implementation is also possible, in which case the coupler could be implemented as a beam splitter cube.

[0147] It will be clear to one skilled in the art that many improvements and modifications can be made to the foregoing exemplary embodiments without departing from the scope of the present disclosure. EXALOS AG - 32 - P 10044 WO

[0148] REFERENCE NUMERALS

[0149] # Label Example Material

[0150] 10 substrate c-plane GaN

[0151] 11 buffer layer n: GaN

[0152] 12 first layer(s) n: AllnN (unetched)

[0153] 13 lower cladding layer n: AlGaN

[0154] 14 second layers GaN

[0155] 15 lithography resist PMMA

[0156] 16 grating-forming first layer(s) n: AllnN / GaN

[0157] 17 overgrown planarizing layer GaN

[0158] 18 lower waveguiding layer n: InGaN

[0159] 20 MQW active region (wells / barriers) i’. InGaN / n AlInGaN

[0160] 22 upper inner waveguiding layer p-. InGaN

[0161] 24 upper outer waveguiding layer p-. GaN

[0162] 25 electron blocking layer (EBL) p-. AlGaN

[0163] 26 upper cladding layer p-. AlGaN

[0164] 28 top contact layer p-. GaN

[0165] 30 DFB laser chip

[0166] 32 Front reflector I output coupler

[0167] 34 Back reflector I high reflector

[0168] 35 grating

[0169] 36 ridge

[0170] 38 DFB section

[0171] 40 SOA section

[0172] 42 phase-tuning section

[0173] 44 amplitude and / or phase modulator section

[0174] 46 splitter

[0175] 50 optical module

[0176] 51 main board

[0177] 52 power monitor

[0178] 53 housing

[0179] 54 temperature sensor

[0180] 55 terminal pin

[0181] 56 submount

[0182] 58 collimating lens

[0183] 59 mirror / combiner

[0184] 60 optical fiber EXALOS AG - 33 - P10044WO

[0185] 62 optical fiber ferrule

[0186] 64 optical window

[0187] 70 flow cytometer I flow cell thereof

[0188] 78 flow cytometer, flow cell sheath

[0189] 80 flow cytometer, flow cell capillary tube

[0190] 82 flow cytometer, SSC focusing lens

[0191] 84 flow cytometer, FSC focusing lens

[0192] 86 flow cytometer, side scatter (SSC) detector

[0193] 88 flow cytometer, forward scatter (FSC) detector

[0194] 90 flow cytometer, analyzer

[0195] 92 flow cytometer, cell sorting station

[0196] 100 Raman spectrometer system

[0197] 102 Raman spectrometer system, spectrometer

[0198] 104 Raman spectrometer system, analyzer

[0199] 106 Raman spectrometer system, dichroic mirror

[0200] 108 Raman spectrometer system, lens

[0201] 110 holographic optical element (HOE)

[0202] 112 HOE beam

[0203] 114 HOE beam scanning element

[0204] 116 HOE housing for source optics

[0205] 118 HOE spectacle temple (arm)

[0206] 120 HOE spectacle lens

[0207] 122 HOE image

[0208] 124 HOE diffraction grating

[0209] 126 HOE reflector

[0210] 130 Interferometer sensor

[0211] 132 input arm optical fiber

[0212] 134 optical fiber coupler

[0213] 136 sensing arm optical fiber

[0214] 138 reference arm optical fiber

[0215] 140 reference arm mirror

[0216] 142 output arm optical fiber

[0217] 144 detector

[0218] 146 electronic signal line

[0219] 148 data sampler

[0220] 150 data communication line

[0221] 152 signal processor

Claims

EXALOS AG - 34 - P 10044 WOCLAIMS1. A distributed feedback laser configured to lase with a lasing mode at a lasing wavelength A, the laser comprising: a substrate (10); a c-plane GaN buffer layer (11) arranged on the substrate; an in-plane Bragg grating structure arranged on the c-plane GaN buffer layer that forms an in-plane Bragg grating of period N ■ A / (2 ■ neff), where N is an integer, A is the lasing wavelength, and neffis the average effective refractive index experienced by the lasing mode, the in-plane Bragg grating being formed by at least one grating layer (16) with a periodic in-plane alternation between first and second materials of respective compositions Al^In^Ga^^^N and A / z2 / y2Cai-x2-y2 / V. where xl,yl > 0 and x2 + y2 < 1, embedded above and below by the second material; a multi-quantum well active region layer structure (20) arranged on the in-plane Bragg grating layer structure and comprising alternating third and fourth layers of third and fourth materials, the third and fourth materials having respective compositions Alx3Iny3Ga1-x3-y3N and Al^In^Ga^^^N that have different values of band gap energy; and an upper cladding layer (26) arranged on the multi-quantum well active region layer structure.

2. The laser of claim 1 , wherein the first material is AlxlInylN with xl + yl = 1.

3. The laser of claim 2, wherein the second material is GaN with x2 = y2 = 0.

4. The laser of claim 3, wherein the first material has an aluminum content selected from the group: a. 0.70 < xl < 0.95; b. 0.77 < xl < 0.87; andC. 0.80 < xl < 0.84.

5. The laser of any one of the preceding claims, wherein the third and fourth materials are fny3Ga^y3N and Iny4Ga1-y4N with y3 7= yA and x3 = xA = 0.EXALOS AG - 35 - P 10044 WO6. The laser of any one of the preceding claims, wherein the number m of the grating layers (16) satisfies one of the following conditions: a. m = 1 b. m > 2 c. m is selected from the group m = 2, 3, 4 and 5 d. m < k where k is an integer representing the total number of first layers e. m = k f. m < 10.

7. The laser of any one of the preceding claims, further comprising a lower cladding layer (13) arranged between the buffer layer (11) and the in-plane Bragg grating structure, the lower cladding layer material being Alx5Iny5Ga1-x5-y5N where x5 #= xl,x2 and y5 #= yl,y2.

8. The laser of any one of the preceding claims, further comprising a lower waveguiding layer (18) arranged between the multi-quantum well active region structure (20) and the inplane Bragg grating structure, and / or further comprising an upper waveguiding layer (22, 24) arranged between the multi-quantum active region well structure and the upper cladding layer (26).

9. A method of fabricating a distributed feedback laser configured to lase with a lasing mode at a lasing wavelength A, the method comprising: providing a substrate (10); depositing a buffer layer (11) of c-plane GaN on the substrate; depositing on the c-plane GaN buffer layer alternating first and second layers (12, 14) of first and second materials having respective different compositions Al^In^Ga^^^N and A / z2 / y2Cai-x2-y2 / V such that the refractive index of the first material is lower than the refractive index of the second material, wherein the first and second materials have compositions xl,yl > 0 and x2 + y2 < 1; etching at least one of the first layers (12) to form a structured surface having a period in one direction of N ■ A / (2 ■ neff), where N is an integer, A is the lasing wavelength, and neff is the average effective refractive index experienced by the lasing mode; regrowing with the second material to planarize the structured surface and thus form an in-plane Bragg grating of the first material embedded in the second material; depositing alternating third and fourth layers of third and fourth materials having respective compositions Al^In^Ga^^-^N and Alx4Iny4Ga1-x4-y4N that have different values of band gap energy to form a multi-quantum well active region structure (20); and depositing an upper cladding layer (26).EXALOS AG - 36 - P 10044 WO10. The method of claim 9, wherein the number m of the first layers (12) that are etched satisfies one of the following conditions: a. m = 1 b. m > 1 c. m is an integer selected from the group m = 2, 3, 4 and 5 d. m < k where k is an integer representing the total number of first layers e. m = k f. m < 10.

11. The method of claim 10, wherein each of the m first layers (12) has a thickness of at least 10 nm prior to etching.

12. The method of claim 9, 10 or 11 , wherein said etching is performed with an etchant that is sensitive to reaching the first material, so that layers of the first material act as etch stop layers.

13. The method of any one of claims 9 to 12, wherein there is at least one atomic species not shared between the first and second materials, the release of which is monitored during etching, wherein etching is stopped responsive to sensing a step change in the amount of that species that is being released.

14. The method of any one of claims 9 to 13, wherein etching is stopped part way through a first material layer so that the number m of the first layers (12) that are etched is a non-integer value.

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