Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
  • G02F1/035—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect in an optical waveguide structure
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
  • G02B26/007—Optical devices or arrangements for the control of light using movable or deformable optical elements the movable or deformable optical element controlling the colour, i.e. a spectral characteristic, of the light
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/18—Diffraction gratings
  • G02B5/1861—Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
  • H01S3/107—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using electro-optic devices, e.g. exhibiting Pockels or Kerr effect
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/0607—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying physical parameters other than the potential of the electrodes, e.g. by an electric or magnetic field, mechanical deformation, pressure, light, temperature
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/14—External cavity lasers
  • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/0147—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on thermo-optic effects
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
  • G02F2201/30—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 grating
  • G02F2201/307—Reflective grating, i.e. Bragg grating

Definitions

• the present invention relates to a tunable optical reflector chip and an optical laser assembly comprising said optical reflector chip for spectral tuning of the lasing wavelength of the optical radiation emitted by the optical laser assembly.
• T unable lasers which have a high average power while still exhibiting low noise are essential components in a wide range of applications, such as for instance high-performance coherent communications systems, frequency synthesis, spectroscopy, distributed sensing systems, and light detection and ranging (LiDAR).
• LiDAR light detection and ranging
• current commercial solid-state lasers and fiber lasers cannot compete with semiconductor lasers in terms of size, weight, and power (SWaP) or price.
• SWaP size, weight, and power
• the external waveguide serves as a Bragg-reflector, i.e. as a reflector which reflects a spectrally narrow portion of the optical radiation emitted by the gain chip, and is used as one of the laser's end mirrors.
• a Bragg-reflector i.e. as a reflector which reflects a spectrally narrow portion of the optical radiation emitted by the gain chip, and is used as one of the laser's end mirrors.
• US 20210305781 A1 discloses a laser including a semiconductor gain chip and an external cavity, wherein a first end of the gain chip has a high reflectivity facet forming a first end of the laser cavity and a second end of the gain chip has an low reflectivity facet, and wherein a second part of the external cavity includes a Bragg grating which forms the second end of the laser cavity.
• Heaters positioned close to the waveguide are proposed to vary the temperature and hence the refractive index of the waveguide to tune the Bragg grating and therefore tune the wavelength of the laser.
• tuning via thermal effect is generally slow and thus limits the speed at which the wavelength of the laser can be swept.
• An optical reflector chip for tuning a lasing wavelength of an optical laser assembly comprising: a waveguide arrangement configured to guide optical radiation, the waveguide arrangement comprising a Bragg grating section, the Bragg grating section comprising a periodic structure exhibiting a periodically varying refractive index to reflect the optical radiation within a wavelength reflection band defined by the periodic structure, and a modulation device, the modulation device comprising a modulation element, at least a first electrode and a second electrode, wherein the first electrode and the second electrode are configured to actuate the modulation element upon the application of a voltage between the first electrode and second electrode, wherein the modulation element is arranged such that actuating the modulation element by applying the voltage between the first electrode and the second electrode tunes the periodic structure of the Bragg grating section, whereby a shift of the wavelength reflection band is induced.
• peripherally varying refractive index refers to a refractive index varying in space, i.e. a spatially varying refractive index profile experienced by the optical radiation along a propagation direction of the optical radiation within the Bragg grating section.
• optical wavelength and “optical frequency” are to be considered interchangeable, as an optical wavelength may always be converted into an optical frequency and vice versa by taking into account the properties of the medium in which the optical radiation is propagating.
• the optical reflector chip has a footprint which is smaller than 1 square centimeter (cm 2 ), in particular, the optical reflector chip may have a chip width of 5 millimeters (mm) and a chip length of 10 millimeters (mm).
• the modulation element, the first electrode, the second electrode and the waveguide arrangement are preferably monolithically integrated in the optical reflector chip.
• “monolithically integrated” means that said components are manufactured on a common substrate, preferably in layers arranged on top of each other, such that said component are fixedly, in particular non-removably, arranged with respect to each other within the optical reflector chip.
• the common substrate may in particular be made of silicon and/or sapphire.
• the modulation element can comprise or consist of at least one piezo actuator element in the form of at least one piezo actuator layer or of at least one Pockels actuator element in the form of a Pockels actuator layer
• the optical reflector chip can comprise at least one embedding layer into which the Bragg grating section of the waveguide arrangement is embedded as well as at least one substrate layer.
• the embedding layer When seen along a vertical direction of the optical reflector chip, the embedding layer can be arranged on top of the substrate layer, and the piezo actuator layer (or the Pockels actuator layer) can be arranged on top of the embedding layer.
• the substrate layer can be seen as a common substrate or common substrate layer that is common to the embedding layer and the piezo actuator layer (or the Pockels actuator layer), and to further layers of the optical reflector chip, if present.
• the first electrode and the second electrode may be arranged at least partially on and/or in one of these layers.
• the first electrode may be arranged between the embedding layer and the piezo actuator layer (or the Pockels actuator layer), and the second electrode may be attached to the piezo actuator layer (or the Pockels actuator layer).
• the substrate layer, the embedding layer, the piezo actuator layer (or the Pockels actuator layer), the first electrode and the second electrode are monolithically integrated in the optical reflector chip or, in other words, can be seen as constituting a single-piece element.
• the optical reflector chip may further comprise a micro-heater element for thermally tuning the periodic structure of the Bragg grating section.
• the micro-heater element may be formed at least in part by the first electrode and/or the second electrode.
• the periodic structure of the Bragg grating section comprises a main waveguide and a multitude of perturbation elements, the perturbation elements being arranged equidistantly from each other along the main waveguide, i.e. spaced by a fixed spacing A in propagation direction of along the main waveguide, and preferably arranged at a fixed lateral distance of the main waveguide, the perturbation elements causing the periodically varying refractive index.
• lateral distance refers to a distance measured perpendicularly to the propagation direction of the optical radiation within the main waveguide.
• the perturbation elements may be posts, in particular cylindrical posts having a diameter between 260 nanometers (nm) and 350 nanometers (nm), arranged at a fixed lateral distance of the main waveguide.
• the perturbation elements may be crenellations and/or corrugations formed in the main waveguide.
• the lateral distance at which the perturbation elements are arranged form the main waveguide influences the periodically varying refractive index profile experienced by the optical radiation propagating in the main waveguide: a larger portion of the optical radiation leaks into the perturbation elements when the perturbation elements are closer to the main waveguide than if the perturbation elements are arranged further away from the main waveguide, thus a stronger refractive index contrast is created between subsections of the main waveguide in propagation direction which are flanked by perturbation elements and subsections of the main waveguide that are not flanked by perturbation elements when the perturbation elements are closer to the main waveguide.
• a stronger refractive index contrast also intrinsically leads to a higher reflectivity of the Bragg grating section.
• the refractive index contrast may be adjusted by adjusting a depth of the crenellations and/or corrugations.
• the lateral distance is preferably between 350 nanometers (nm) and 500 nanometers (nm).
• the wavelength reflection band is determined by the spacing A of the perturbation elements in propagation direction.
• the main waveguide may be straight or may have an arbitrary shape, in particular a spiral shape.
• the main waveguide may have a length in propagation direction of the optical radiation which is between 1 centimeters (cm) and 50 centimeters (cm), preferably between 5 centimeters (cm) and 8 centimeters (cm), preferably while still fitting within a footprint of 1 square centimeter or less.
• the main waveguide may have a height in a vertical direction perpendicular to the propagation direction of the optical radiation of 50 nanometers (nm) - 1 micrometers (pm), preferably 200 nanometers (nm).
• the main waveguide may have a width in a horizontal direction perpendicular to the propagation direction and the vertical direction of 2.5 micrometers (pm) to 5 micrometers (pm).
• the Bragg grating section may be divided into sub-grating sections which may be arranged to form a superstructure grating in order to further increase the single-mode stability and further enhance the tunability.
• the main waveguide and perturbation elements are made of the same material.
• the main waveguide and/or the perturbation elements comprise or consist of silicon nitride (Sisl ⁇ ) and/or silicon (Si). In other embodiments, the main waveguide and/or the perturbation elements may comprise or consist of lithium niobate LiNbOs and/or lithium tantalite (LiTaCh).
• the waveguide arrangement may comprise at least one pair of input waveguide sections, each pair of input sections being connected to a combining section, the combining section preferably being a multi-mode interference coupler, wherein the combining section is configured to coherently combine optical radiation entering the combining section via the pair of input waveguide sections, and wherein the combining section has an output for the combined optical radiation that is optically connected to the Bragg grating section.
• the output of the combining section may be directly optically connected to the Bragg grating section, or via additional combining stages, in particular, the waveguide arrangement may comprise an arbitrary number of pairs of input waveguide sections and may comprise at least two stages of combining section or more, which may be arranged in a cascade-like manner to combine the optical radiation into a single high-power output connected to the Bragg grating section.
• the optical reflector chip may comprise microheaters which may be arranged along some or all of the input sections, in particular adjacent to or on top of the input sections.
• the optical reflector chip may comprise an embedding layer, and the Bragg grating section may be embedded in the embedding layer.
• the embedding layer comprises or consists of silicon dioxide.
• the optical reflector chip may further comprise a substrate layer, preferably comprising or consisting of silicon and/or sapphire, and the embedding layer may be arranged on top of the substrate layer with respect to a vertical direction of the optical reflector chip. In such an arrangement, the embedding layer may act as an insulation layer preventing leakage of the optical radiation into the substrate layer.
• the modulation element may comprise or consist of a piezo actuator element exhibiting the piezoelectric effect, wherein actuating the piezo actuator element by voltage application causes a photoelastic change and/or a geometrical change of the periodic structure of the Bragg grating section.
• photoelastic change in the present context refers to a microscopic change within the material itself, i.e. a change of the material's refractive index due to strain.
• geometrical change in the present context refers to a change of geometrical dimensions of the Bragg grating section, in particular for instance a change in the spacing between the perturbation elements in propagation direction, which, as described above, has an impact on the central wavelength of the wavelength reflection band of the optical reflector chip.
• the piezo actuator element preferably comprises or consists of aluminium nitride and/or lead zirconate titanate and/or Sc-doped aluminium nitride.
• Aluminium nitride has the advantage of providing a low non-linearity, while lead zirconate titanate provides a higher modulation depth for a constant voltage than aluminium nitride, but exhibits a higher non- linearity than aluminium nitride.
• the Bragg grating section may define a horizontal propagation plane, wherein the piezo actuator element extends at least partially as a piezo actuator layer being parallel to the horizontal propagation plane, wherein the piezo actuator layer is arranged at a vertical distance from the horizontal propagation plane, the vertical distance being preferably between 2 micrometers (pm) and 5 micrometers (pm), more preferably between 3 micrometers (pm) and 4 micrometers (pm), and wherein the photoelastic change and/or the geometrical change of the periodic structure of the Bragg grating section is caused by strain being transmitted through the embedding layer.
• the first electrode may be arranged between the embedding layer and the piezo actuator layer, in particular attached to the embedding layer and attached to a first side of the piezo actuator layer.
• At least part of the second electrode may be attached to a second side of the piezo actuator layer, the second side being opposite to the first side of the piezo actuator layer.
• the piezo actuator layer may thus be sandwiched in the vertical direction between the first electrode and the second electrode.
• the material of first electrode needs to be preferably made of a metal that is sufficiently hard to prevent the mechanical distortion of the piezo actuator layer from getting entirely absorbed by the first electrode.
• first electrode and/or the second electrode may comprise or consist of molybdenum and/or aluminium and/or gold.
• the first electrode may consist of molybdenum
• the second electrode may consist of aluminium to ease fabrication.
• the first electrode and the second electrode each may comprise a connection patch being attached to the embedding layer, wherein the connection patches are preferably arranged in a common plane to facilitate wire-boding.
• the piezo actuator element in particular the piezo actuator layer may be arranged vertically above the Bragg grating section so that the periodic structure of the Bragg grating section is entirely covered by the piezo actuator element or piezo actuator layer when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane.
• the piezo actuator element in particular the piezo actuator layer, comprises two parts being spatially separate from each other and extending parallel to each other on either side of the Bragg grating section so that the periodic structure of the Bragg grating section is not covered by either part of the piezo actuator element, in particular the piezo actuator layer, when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane.
• the two parts may work as push- pull actuators.
• the piezo actuator layer may form a drum, i.e. it may have a substantially round shape covering the spiral when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane.
• the optical reflector chip may comprise a mechanical mode suppression means configured to attenuate one or more mechanical modes of oscillation of the optical reflector chip, i.e. mechanical resonances, caused by an AC operation of the piezo actuator element.
• a mechanical mode suppression means configured to attenuate one or more mechanical modes of oscillation of the optical reflector chip, i.e. mechanical resonances, caused by an AC operation of the piezo actuator element. Examples of such mechanical mode suppression means are disclosed in WO 2022188990 A1 , which is hereby incorporated by reference.
• the mechanical mode suppression means may include at least one dummy piezo actuator arranged on the embedding layer horizontally displaced with respect to the piezo actuator element.
• the mechanical mode suppression means may include an apodization of the common substrate layer, wherein the substrate layer has at least two edges which are non-parallel to each other, wherein particularly the substrate layer has no parallel edges and/or is polygon-shaped and/or has edges with different lengths.
• the substrate layer may be shaped as an irregular polygon to have non-parallel sides.
• the piezo actuator element in particular the piezo actuator layer, comprises two parts being spatially separate from each other and extending parallel to each other on either side of the Bragg grating section, the two parts may be driven with opposite phases to suppress the mechanical resonances.
• the modulation element may comprise or consist of a Pockels actuator element exhibiting the Pockels effect, wherein actuating the Pockels actuator element by voltage application causes an electro-optic change of the periodic structure of the Bragg grating section, the Pockels actuator element preferably comprising or consisting of lithium niobate and/or lithium tantalate and/or barium titanate.
• the Pockels actuator element may extend at least partially as a Pockels actuator layer being parallel to the horizontal propagation plane.
• the Pockels actuator layer is arranged such that a portion of the optical radiation propagating in the Bragg grating section leaks into the Pockels actuator layer, the Pockels actuator layer being in particular arranged vertically above the Bragg grating section so that the periodic structure of the Bragg grating section is at least partially covered by the Pockels actuator layer when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane.
• a first side of the Pockels actuator layer may be attached to the embedding layer.
• the first electrode and the second electrode may be arranged adjacent to each other on a second side of the Pockels actuator layer, the second side being opposite to the first side of the Pockels actuator layer.
• the first electrode and the second electrode may be arranged such that they do not cover the Bragg grating section when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane.
• the first electrode and/or the second electrode may comprise or consist of tungsten and/or niobium.
• An optical laser assembly comprising: a gain chip comprising at least one gain element being configured to provide optical gain within a spectral gain bandwidth when being pumped; an optical reflector chip as described above being coupled, preferably butt-coupled, to the at least one gain element, so as to reflect optical radiation being emitted from the at least one gain element back to the at least one gain element, wherein the wavelength reflection band is narrower than the spectral gain bandwidth of the at least one gain element and wherein the wavelength reflection band lies at least partially within the spectral gain bandwidth, such that tuning the periodic structure of the Bragg grating section by applying a voltage between the first electrode and the second electrode induces a shift of the wavelength reflection band that induces a spectral tuning of a lasing wavelength emitted by the optical laser assembly when the at least one gain element is being pumped.
• the gain element is a semiconductor, preferably a 11 l-V semiconductor, and is preferably pumped via electrical pumping, i.e. by applying a pump current to the gain element using an external current source.
• the lasing wavelength may be additionally tuned by changing the pump current applied to the gain element.
• a first end mirror is preferably arranged at a first end of each of the at least one the gain elements and the Bragg grating section may act a common second end mirror, such that each of the at least one gain elements may form an optical resonator in combination with the Bragg grating section.
• the first end mirror may be formed by an optical coating arranged on the first end of each of at least one the gain elements.
• the waveguide arrangement of the optical reflector chip may comprise two Bragg grating sections, the two Bragg grating section being arranged at either end of the gain element, thus forming both the first end mirror and the second end mirror of the resonator.
• either one or both of the Bragg grating sections may be modulated.
• the lasing wavelength has a linewidth which intrinsically depends on a length of the resonator: the longer the resonator, the narrower the linewidth. However, longer resonators may lead to higher propagation losses.
• the at least one gain element may be configured a gain waveguide.
• two or more gain waveguides maybe arranged in parallel within the gain chip to enhance an optical average power of the optical radiation emitted by the optical lasing assembly.
• the gain element and the optical reflector chip are integrated onto a common chip platform.
• Different integration techniques may be used to fabricate the optical lasing assembly, in particular a heterogeneous integration approach or a P-side up hybrid integration approach or a P-side down hybrid integration approach or flip-chip integration approach.
• a method of tuning a lasing wavelength of the optical laser assembly described above comprising the steps of: pumping the at least one gain element in order to provide optical gain at the lasing wavelength within the spectral gain bandwidth, and reflecting the optical radiation at the lasing wavelength being emitted from the at least one gain element back to the at least gain element by the optical reflector chip, wherein the method further comprises the step of tuning the periodic structure of the Bragg grating section by applying a voltage between the first electrode and the second electrode, whereby a spectral tuning of the lasing wavelength emitted by the optical laser assembly is induced.
• the method may comprise using a tunable voltage source, such as a function generator, to apply the voltage between the first electrode and the second electrode.
• a tunable voltage source such as a function generator
• Fig. 1 show a schematic top view of an optical laser assembly comprising an optical reflector chip according to a first embodiment and a gain chip;
• Fig. 2 shows another schematic top view of an optical laser assembly comprising an optical reflector chip according to a first embodiment and a gain chip;
• Fig. 3 shows a schematic sectional view along the sectional plane A'-A- of figure 2;
• Fig. 4 shows a sectional view of a finite-element simulation of a modulation device of the optical reflector chip
• Fig. 5 shows a diagram depicting a frequency shift of a lasing wavelength emitted by the optical laser assembly as a function of a cladding thickness
• Fig. 6 shows a diagram depicting a propagation loss of optical radiation emitted by the optical laser assembly as a function of a cladding thickness
• Fig. 7 shows a diagram depicting a frequency shift of a lasing wavelength emitted by the optical laser assembly as a function of a width of a piezo actuator layer of the modulation device of the optical reflector chip;
• Fig. 8 shows a schematic top view of an optical laser assembly comprising an optical reflector chip according to a second embodiment and a gain chip;
• Fig. 9 shows a schematic sectional view along the sectional plane B'-B' of figure 8.
• Fig. 10 shows a schematic top view of an optical laser assembly comprising an optical reflector chip according to a third embodiment and a gain chip;
• Fig. 11 shows a schematic sectional view along the sectional plane C'-C of figure 10;
• Fig. 12 shows an enlarged view of the excerpt E marked in figure 11;
• Fig. 13 shows a schematic sectional view of an optical reflector chip according to a fourth embodiment
• Fig. 14 shows a schematic sectional view of an optical reflector chip according to a fifth embodiment
• Fig. 15 shows a schematic top view of an optical laser assembly comprising an optical reflector chip and a gain chip, wherein the gain chip comprises several gain waveguides;
• Fig. 16 shows another schematic top view of an optical laser assembly comprising an optical reflector chip and a gain chip, wherein the gain chip comprises several gain waveguides;
• Fig. 17 shows a schematic top view of an optical laser assembly according to an embodiment of the heterogeneous integration approach
• Fig. 18 shows a sectional view along the sectional plane D'-D' marked in figure 17;
• Fig. 19 shows a schematic top view of a heterogeneously integrated optical laser assembly
• Fig. 20 shows a schematic top view of an optical laser assembly according to an embodiment of P-side up hybrid integration approach
• Fig. 21 shows a sectional view along the sectional plane F'-F' marked in figure 20
• Fig. 22 shows a schematic top view of an optical laser assembly according to an embodiment of P-side down hybrid integration approach
• Fig. 23 shows a sectional view along the sectional plane G-G' marked in figure 22;
• Fig. 24 shows a schematic top view of an optical laser assembly according to an embodiment of P-side down hybrid integration approach
• Fig. 25 shows a sectional view along the sectional plane H'-H' marked in figure 24;
• Fig. 26 shows a Bragg grating section that is wound in a spiral
• Fig. 27 shows an enlarged view of the excerpt marked in Fig. 26.
• FIGs. 1-3 schematically show an optical laser assembly 1 comprising an optical reflector chip 10 according to a first embodiment of the present invention (not to scale) and a gain chip 20 comprising a gain element 21 that is configured as a gain waveguide, the gain waveguide 21 being configured to provide optical gain within a spectral gain bandwidth AAgam when being pumped, for instance by providing an electrical current to the gain element using an external current source (not shown).
• Figs. 1 and 2 show schematic top views of the optical laser assembly 1 , i.e. as viewed along a vertical z-direction, while Fig. 3 shows a schematic sectional view along the sectional plane A'-A' marked in Fig. 2.
• the optical reflector chip 10 comprises a waveguide arrangement 11 which is configured to guide optical radiation emitted by the gain element 21.
• the waveguide arrangement 11 comprises a Bragg grating section 111 , the Bragg grating section 111 in the embodiment shown here comprising a main waveguide 112 and a multitude of perturbation elements 113 in the form of cylindrical posts, which are equidistantly arranged on either side of the main waveguide 112 in a horizontal propagation plane (x-y-plane) as shown in Fig. 1.
• the perturbation elements 113 have a different refractive index than the medium which surrounds them, which causes the optical radiation to experience a refractive index which periodically varies in propagation direction, i.e. in y-direction in the case of a straight main waveguide as shown in this example.
• the perturbation elements 113 have the same refractive index than the main waveguide 112.
• a first end mirror M1 is arranged at a first end the gain waveguide 21 , the first end mirror M1 being for instance provided by a high-reflection coating at first end of the gain waveguide 21 .
• the Bragg grating section 111 then acts a second end mirror, such that the gain waveguide 21 with the first end mirror M1 forms an optical resonator in combination with the Bragg grating section 111 , whereby the periodicity of the varying refractive index defines a wavelength reflection band AA B ragg of this second end mirror.
• the wavelength reflection band AA B ragg is narrower than the spectral gain bandwidth AAgam of the gain element, and the wavelength reflection band AAcragg lies at least partially within the spectral gain bandwidth AAgam, such that tuning the periodic structure of the Bragg grating section induces a spectral tuning of a lasing wavelength AL emitted by the optical laser assembly when the gain element 21 is being pumped.
• the lasing wavelength AL lies within the spectral gain bandwidth AAgam and corresponds to the wavelength within the spectral gain bandwidth AAgam which experience the strongest effective gain, the effective gain being the difference between the optical gain created by pumping the gain element and the optical losses experienced by each wavelength within the spectral gain bandwidth AA gajn .
• the optical losses comprise primarily the output losses caused by an output portion of the optical radiation that exits the optical reflector chip at an output 116 of the Bragg section 111 as indicated by a dotted arrow in Figs. 1 and 2.
• the optical losses may further comprise propagation losses that occur at an interface between the gain waveguide 21 and the waveguide arrangement on the optical reflector chip 10 and/or within the waveguide arrangement, in particular within the Bragg grating section 111 itself.
• the waveguide arrangement 11 is embedded in an embedding layer 40, preferably made of silicon dioxide.
• the waveguide arrangement 11 of this first embodiment is preferably made of silicon nitride, which has a higher refractive index than silicon dioxide.
• the embedding layer 40 extends in the x-y-plane and preferably has a thickness of 10 micrometers (pm) to 15 micrometers (pm) along the z-direction.
• the embedding layer 40 is arranged on a substrate layer 50.
• the substrate layer 50 is preferably made of silicon and preferably a thickness in z-direction of 250 micrometers (pm) to 925 micrometers (pm), preferably 250-525 micrometers (pm).
• the main waveguide 112 and the perturbation elements 113 comprise or consist of silicon nitride.
• the output 116 of the Bragg section 111 comprises or consists of silicon nitride.
• the optical reflector chip 10 further comprises a modulation device 3 (omitted in Fig. 1 to show the Bragg grating section 111 , but visible in Figs. 2 and 3), the modulation device 3 in the embodiment shown in Figs. 1-3 comprising a piezo actuator element 30 and a first electrode 31 and second electrode 32, wherein the first electrode 31 and the second electrode 32 are configured to actuate the piezo actuator element 30 upon the application of a voltage between the first electrode 31 and second electrode 32.
• a modulation device 3 (omitted in Fig. 1 to show the Bragg grating section 111 , but visible in Figs. 2 and 3)
• the modulation device 3 in the embodiment shown in Figs. 1-3 comprising a piezo actuator element 30 and a first electrode 31 and second electrode 32, wherein the first electrode 31 and the second electrode 32 are configured to actuate the piezo actuator element 30 upon the application of a voltage between the first electrode 31 and second electrode 32.
• the piezo actuator element 30 extends as a piezo actuator layer being parallel to the horizontal propagation plane and is arranged vertically above the Bragg grating section 111 so that the periodic structure of the Bragg grating section 111 is entirely covered by the piezo actuator layer 30 when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane, i.e. along the z-direction.
• the piezo actuator layer has a width that is 10 to 15-times larger than a width of the main waveguide 112 in x-direction.
• the piezo actuator layer 30 is sandwiched in z- direction between the first electrode 31 and the second electrode 32, the first electrode 31 being directly attached to the embedding layer 40 and extending along the entire length of the piezo actuator layer 30 in y-direction and along a major part of the width of the piezo actuator layer 30 in x-direction.
• the first electrode 31 comprises a connection patch 310, to which a wire may be bonded or a voltage probe may be applied.
• a major portion of the second electrode 32 extends on top of the piezo actuator layer 30, covering the entire length of the piezo layer 30 in y-direction and a major part of width of the piezo actuator layer 30 in x-direction.
• a minor portion of the second electrode 32 is attached to the embedding layer 40, forming a connection patch 320 of the second electrode 32, to which a wire 321 may be bonded, as shown in Fig. 3, or a voltage probe may be applied.
• Having both the connection patch 310 of the first electrode 31 and the connection patch 320 of the second electrode 32 on a common plane with respect to the z-direction facilitates automatized wire bonding of the optical reflector chip, since the height in z- direction does not need to be adjusted during the wire bonding process.
• the piezo actuator element comprises or consists of aluminium nitride and/or lead zirconate titanate and/or Sc-doped aluminium nitride.
• the first electrode 31 and the second electrode 32 comprise or consist of molybdenum and/or aluminium.
• the waveguide arrangement 11 further comprises a coupling section 114, which extends from an input facet 15 of the optical reflector chip 10 to the main waveguide 112.
• the input facet 15 of the optical reflector chip is butt-coupled to an output facet 22 of the gain chip 20 such that optical radiation emitted by the gain element 21 , when the latter is being pumped, is coupled into the coupling section 114 of the waveguide arrangement 11.
• the gain waveguide 21 comprises an output section 211 which extends at a an output angle a with respect to a surface normal of the output facet 22, the output angle being non-zero as shown in Figs. 1 and 2, preferably 6°, in order to prevent back reflections at the output facet.
• the coupling section 114 is arranged at an input angle p with respect to a surface normal of the input facet 15 of the optical reflector chip, wherein the input angle is determined using Snell's law of refraction by taking into account the output angle a of the gain waveguide 21 and the refractive index of the gain waveguide 21 and the coupling section 114, respectively.
• Fig. 4 shows a sectional view (to scale) of a finite-element simulation of the piezo actuator layer 30, the main waveguide 112 and the perturbation posts 113 arranged as in the embodiment shown in Figs. 1-3, the multitude of arrows representing strain originating from the voltage-actuated piezo actuator layer 30, wherein the length of the arrows depict the magnitude of the strain and the direction of the arrows depict the direction.
• the piezo actuator layer has a height in z-direction of 1 micrometer (pm) and a width in x-direction of 25 micrometers (pm), while the width of the main waveguide 112 in x-direction is 2.5 micrometers (pm) and its height in z-direction is 0.2 micrometers (pm), and while the perturbation posts have a diameter of 0.35 micrometers (pm).
• a cladding thickness D of 3 micrometers (pm) is used in this example, the cladding thickness D being defined as a thickness of the embedding layer 40 in z-direction between a top surface of the main waveguide and a bottom surface of the piezo actuator layer 30, i.e. as a vertical distance in z-direction at which the piezo actuator layer is arranged from the horizontal propagation plane, as shown in Fig. 4.
• applying a voltage to the piezo actuator layer 30 causes the latter to expand/contract, thereby causing photoelastic stress on the Bragg grating section, which in turn affects the periodically varying refractive index experienced by the optical radiation propagating in the Bragg grating section 111.
• the expansion/contraction of the piezo actuator layer 30 in y-direction i.e. perpendicular to the sectional plane shown in Fig. 4, may furthermore cause a geometrical change of a gap distance in y-direction between the perturbation posts, which also impacts the periodically varying refractive index experienced by the optical radiation propagating in the Bragg grating section 111.
• the first electrode 31 arranged between the embedding layer 40 and the piezo actuator layer 30 is ideally much thinner in z-direction than the piezo actuator layer, preferably between 100 nanometers (nm) and 150 nanometers (nm), and preferably made of a metal that is sufficiently hard to prevent the mechanical distortion of the piezo actuator layer 30 from getting entirely absorbed by the first electrode 31.
• suitable metals are in particular molybdenum and aluminium.
• Fig. 5 shows a frequency shift Av of the lasing wavelength around a center lasing wavelength of 1550 nanometers (nm), i.e. the optical lasing frequency, emitted by the optical laser assembly as a function of the cladding thickness D, for a voltage of 1 Volt being applied between the first electrode 31 and the second electrode 32.
• increasing the cladding thickness D reduces the frequency shift Av.
• cladding thickness D may help to reduce the propagation losses, as shown in Fig. 6, which are caused by a portion of the optical radiation leaking beyond the embedding layer 40 in z-direction, e.g. into the first electrode.
• the dots represent data points for a main waveguide and perturbation posts being made of silicon nitride and having height in z-direction of 200 nanometers (nm)
• the stars represent data points for a main waveguide and perturbation posts being made of silicon nitride and having height in z-direction of 100 nanometers (nm).
• higher losses may be expected for a thinner main waveguide, which is due to the optical radiation (at a fixed lasing wavelength) being less strongly confined in a thinner waveguide than in a thicker waveguide.
• Fig. 7 shows the frequency shift Av of the lasing wavelength around a center lasing wavelength of 1550 nanometers (nm) as a function of the width of the piezo actuator layer 30 in x-direction for a voltage of 1 Volt being applied between the first electrode 31 and the second electrode 32.
• the main waveguide and perturbation posts are made of silicon nitride and have a height in z-direction of 200 nanometers (nm).
• the main waveguide has a width in x-direction of 2.5 micrometers (pm)
• the perturbation posts have a diameter of 0.35 micrometers (pm)
• the gap between the posts in y-direction amounts to 0.992 micrometers (pm)
• the posts are symmetrically arranged on either side of the main waveguide with a lateral gap in x-direction between the main waveguide and the posts of 0.4 micrometers (pm).
• Figs. 8 and 9 schematically show an optical laser assembly 1 comprising an optical reflector chip 10 according to a second embodiment of the present invention (not to scale)
• the gain chip 20 is the same as the one shown in described in Figs. 1-3.
• Fig. 8 shows a schematic top view of the optical laser assembly 1 according to the second embodiment, i.e. as viewed along a vertical z-direction
• Fig. 9 shows a schematic sectional view along the sectional plane B'-B' marked in Fig. 8.
• the second embodiment of the optical reflector chip 10 only differs from the first embodiment in terms of how the modulation element is configured, all other features are the same as described in the paragraph above regarding the embodiment shown in Figs. 1-3.
• the piezo actuator layer 30 has two parts, a first piezo actuator layer part 301 and a second piezo actuator layer part 302, being spatially separate from each and extending parallel to each other on either side of the Bragg grating section so that the periodic structure of the Bragg grating section is not covered by either part 301 ,302 of the piezo actuator layer when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane, i.e. along the z-direction.
• Each of the first and second piezo actuator layer part 301 ,302 is sandwiched between a first electrode 31 and a second electrode 32, the first and second electrodes being configured as described in the first embodiment shown in Figs. 1-3.
• connection patches 310,320 of the first and second electrodes 31 ,32 associated with the first and second piezo actuator layer part 301 ,302, respectively, are arranged so as to extend away from the Bragg grating section in order not cover the Bragg grating section to avoid unnecessary propagation losses.
• Figs. 10-12 schematically show an optical laser assembly 1 comprising an optical reflector chip 10 according to a third embodiment of the present invention (not to scale) and a gain chip 20 comprising a gain element 21 that is configured as a gain waveguide.
• Fig. 10 shows schematic top view of the optical laser assembly 1 according to the third embodiment, i.e. as viewed along a vertical z-direction, while Fig. 11 shows a schematic sectional view along the sectional plane C'-C marked in Fig. 10.
• the optical reflector chip comprises a waveguide arrangement with a Bragg grating section 111 preferably made of silicon nitride and embedded in an embedding layer 40 preferably made of silicon dioxide, analogously to the first and second embodiments described above.
• the embedding layer is arranged on a substrate layer 50 preferably made of silicon or lithium niobate or sapphire.
• the main waveguide 112 and the perturbation elements 113 comprise or consist of silicon nitride.
• the modulation device 3 comprises a Pockels actuator element 30' exhibiting the Pockels effect, wherein actuating the Pockels actuator element 30' by voltage application causes an electro-optic refractive index modulation of the periodic structure of the Bragg grating section 111 , thereby changing the optical frequency of reflected radiation.
• the Pockels actuator element extends as a Pockels actuator layer 30' being parallel to the horizontal propagation plane defined by the main waveguide 112 and the perturbation posts 113.
• a first side of the Pockels actuator layer 30' is directly attached to the embedding layer 40.
• the modulation device 3 also comprises a first electrode 31 and a second electrode 32, however, in this third embodiment, the first electrode 31 and the second electrode 32 are arranged adjacent to each other on a second side of the Pockels actuator layer 30', the second side being opposite to the first side of the Pockels actuator layer 30'. Moreover, the first electrode 31 and the second electrode 32 comprise or consist of gold.
• the Pockels actuator element 30' comprises or consists of lithium niobate or lithium tantalate.
• the pockels actuator element 30’ may be embedded in an embedding element 33 preferably in the form of a second embedding layer and/or consisting of silicon dioxide. However, it should be noted that such an embedding element 33 is not always needed and could likewise be absent.
• Fig. 12 shows an enlarged view of the excerpt E marked in Fig. 11.
• a schematic illustration of a hybrid transverse-electric (TE) electric field amplitude spatial distribution of the optical radiation R propagating in the Bragg grating section is shown.
• the Pockels actuator layer 30' is arranged such that a portion of the optical radiation propagating in the Bragg grating section 111 leaks into the Pockels actuator layer 30', where it may experience modulation due to the Pockels effect when the Pockels actuator layer 30' is actuated via the electrodes 31 ,32.
• the Pockels actuator layer 30' is arranged vertically above the Bragg grating section in z-direction so that the periodic structure of the Bragg grating section 111 is covered by the Pockels actuator layer when viewed in a top view along a viewing direction that is perpendicular to the horizontal propagation plane, i.e. along the z-direction.
• the Pockels actuator layer 30' has a thickness in z-direction of 10 micrometers or less, preferably 1 micrometer or less, particularly 200-700 nanometers.
• the first electrode 31 and the second electrode 32 are therefore preferably arranged at an offset Ax in x-direction from the outer lateral edges of the Bragg grating section 111 , in particular from the lateral outer edges of the perturbation elements 113 as shown in Fig. 11 , the offset Ax being preferably larger than 500 nanometers (nm), in particular between 500 nanometers (nm) and 5000 nanometers (nm).
• Fig. 13 shows a schematic sectional view of an optical reflector 10 according to a fourth embodiment.
• the Bragg grating section is made of a material exhibiting the Pockels effect.
• the Bragg grating section comprise a main waveguide 112 with a trapezoidal cross-section and perturbation elements 113 which are equidistantly arranged on either side of the main waveguide 112 in a horizontal propagation plane (x-y-plane), wherein both the perturbation elements 113 and the main waveguide 112 are made of the same material.
• the main waveguide 112 and the perturbation elements 113 are made of lithium niobate or lithium tantalate.
• the main waveguide 112 may also exhibit crenellations to form the periodic structure exhibiting a periodically varying refractive index.
• the Bragg grating section is not embedded in the embedding layer 40, but is arranged on top of the Pockels actuator layer 30' described in the third embodiment.
• the Bragg grating section may have been obtained by etching of an initially uniform layer exhibiting the Pockels effect, i.e. the Bragg grating section may be formed in one piece with the Pockels actuator layer 30'.
• Both the main waveguide 112 and the perturbation elements in the form of a periodic structure 113 are formed by etching of the material constituting the Pockels actuator layer 30'. In the embodiment shown in Fig.
• the first and second electrodes 31 ,32 are arranged on the Pockels actuator layer 30', which extends as along the embedding layer 40 with respect to a horizontal plane perpendicular to a vertical direction z.
• the Pockels actuator element or layer 30' comprises or consists of lithium niobate or lithium tantalate.
• the embedding layer comprises or consists of silicon dioxide, and the substrate layer 50 comprises or consists of silicon or sapphire.
• the Pockels actuator layer 30’ and the main waveguide 112 and the perturbation elements in the form of the periodic structure 113 may be embedded in an embedding element 33 preferably in the form of a second embedding layer and/or consisting of silicon dioxide.
• an embedding element 33 is not always needed and could likewise be absent.
• Fig. 14 shows a schematic sectional view of an optical reflector 10 according to a fifth embodiment, which differs from the fourth embodiment in that the Bragg grating section 111 is directly arranged on the embedding layer 40 without the Pockels actuator layer 30' shown in Fig 13., i.e. in that the Bragg grating section itself forms the modulation element of the modulation device 3.
• the Bragg grating section may comprise perturbation elements and/or crenellations to form the periodic structure exhibiting a periodically varying refractive index.
• the first and second electrodes 31 ,32 are connected directly on the embedding layer 40 and act as capacitors, i.e.
• the first electrode 31 and the second electrode 32 comprise or consist of gold.
• the embedding layer 40 in the fourth and fifth embodiments preferably consists of silicon dioxide. Furthermore, both the fourth and fifth embodiment further comprises a substrate layer 50, the embedding layer being arranged on the substrate layer 50 with respect to a vertical direction z, the substrate layer consisting here preferably of silicon or lithium niobate.
• the first electrode 31 and the second electrode 32 are arranged at a distance d from one another and extend parallel to each other on either side of the Bragg grating section in y-direction, i.e. perpendicularly to the sectional planes shown in Figs. 13 and 14.
• the first electrode 31 and/or the second electrode 32 may also act as a microheater, i.e. a heating current may be applied to further modulate the Bragg grating section 111 and thus shift the lasing wavelength of the optical laser assembly 1.
• the waveguide arrangement 11 of optical reflector chip 10 may comprise at least one pair of input waveguide sections 110, each pair of input sections 110 being connected to a combining section, the combining section preferably being a multi-mode interference coupler.
• 15 and 16 show an example of an optical reflector chip 10 with two pairs of input waveguide sections 110 and three combining sections, wherein two of the three combining sections form "first-stage" combining sections 1151 , which are configured to coherently combine optical radiation entering the combining section via the pairs of input waveguide sections 110, and wherein the third combining section 1152 forms a "second-stage” combining section, which is configured to coherently combine the optical radiation exiting the "first-stage” combining sections 1151.
• the "second- stage” combining section 1152 has an output for the combined optical radiation that is optically connected to the Bragg grating section 111. It is of course conceivable to increase the number of input waveguide sections 110, as well as the number of combining sections to provide further combining "stages" in a cascade-like manner.
• the gain chip 20 may comprise several gain waveguides 21 , in this particular example four gain waveguides 21 , which are arranged in parallel within the gain chip 20.
• the embodiment shown in Figs. 15 and 16 corresponds to a parallelization of the working principle of the invention described above in the description of Figs. 1-3 and enables a scaling of the average power of the output portion of the optical radiation emitted by the optical laser assembly 1.
• Fig. 16 shows the optical laser assembly with the first electrode 31 and the second electrode 32, the electrodes 31 ,32 being omitted in Fig. 15 to show the Bragg grating section 111.
• each gain waveguide 21 is associated with one of the input waveguide sections 110 on the optical reflector chip 10.
• each gain waveguide 21 comprises an output section 211 which is optically connected to one of the input waveguide sections 110, in particular to a coupling section 114 of the respective input waveguide section 110, the output sections 211 of the gain waveguides 21 and the coupling sections 114 being preferably angled as described above in the description of Fig.1-3.
• a first end mirror M1 is arranged at a first end of each of the at least one the gain waveguides 21 , the Bragg grating section 111 acting here as a common second end mirror, such that each of the gain waveguides 21 and the end mirror M1 forms an optical resonator in combination with the Bragg grating section 111.
• Figs. 17-25 shows different approaches illustrating how the gain chip 20 with the optical reflector 10 may be integrated within the optical laser assembly 1 for embodiments of the optical reflector chip 10 that comprise a piezo actuator layer 30, in particular for the first and second embodiment of the optical reflector chip 10 as described above and shown in Figs.1-4 and Figs. 8-9, respectively.
• a piezo actuator layer 30 vertically sandwiched in z-direction between a first electrode 31 and a second electrode 32 is used, similarly to the embodiment shown in Figs. 1-3.
• Figs. 17-19 illustrate a so-called heterogeneous integration approach.
• Fig. 17 shows a schematic top view, i.e. as viewed along the vertical z-direction, of the optical laser assembly 1 according to an embodiment of the heterogeneous integration approach, while
• Fig. 18 shows a sectional view along the sectional plane D'-D' marked in Fig. 17.
• the waveguide arrangement of the optical reflector chip 10 comprises two Bragg grating sections 111 , i.e. one Bragg grating section 111 arranged at either end of the gain element 21 , each Bragg grating section 11 thus forming an end mirror.
• Fig. 17 shows a schematic top view, i.e. as viewed along the vertical z-direction, of the optical laser assembly 1 according to an embodiment of the heterogeneous integration approach
• Fig. 18 shows a sectional view along the sectional plane D'-D' marked in Fig. 17.
• the waveguide arrangement of the optical reflector chip 10 comprises two
• the modulation device 3 is arranged such that only one of the Bragg grating sections 111 is modulated, however, other embodiments are conceivable in which both of the Bragg grating sections 111 may be modulated.
• Fig. 19 a different embodiment of a heterogeneously integrated optical laser assembly 1 is shown, wherein the end mirror M1 is provided by a backside loop mirror.
• an interposer layer 60 preferably made of silicon, is fabricated in z-direction on top of the embedding layer 40 in which the waveguide arrangement 11 , preferably made of silicon nitride, is embedded, the embedding layer 40 preferably being made of silicon dioxide.
• the embedding layer 40 is arranged on top of a substrate layer, which is preferably made of silicon.
• a gain chip preferably a semiconductor gain chip, in particular a lll-V chip, without mirrors on its facets, which is bonded in z-direction on top of the interposer layer 60 and a gain waveguide 21 is etched into the gain chip.
• the gain waveguide 21 may be fabricated prior to bonding and transferred onto the interposer layer 60 using a PMDS stamp.
• the interposer layer 60 facilitates an adiabatic and evanescent coupling of the optical radiation from the gain waveguide 21 into the waveguide arrangement 11 and vice versa, the coupling occurring in z-direction as indicated by the double-arrow in Fig. 18.
• Figs. 20 and 21 illustrate a so-called P-side up hybrid integration approach.
• Fig. 20 shows a schematic top view, i.e. as viewed along the vertical z-direction, of the optical laser assembly 1 according to an embodiment of P-side up hybrid integration approach, while
• Fig. 21 shows a sectional view along the sectional plane F'-F' marked in Fig. 20.
• a gain chip 20, preferably a semiconductor gain chip 20, in particular a lll-V chip is bonded face up, i.e. with the active gain waveguide 21 being arranged at the top of the gain chip 20 in z-direction, to a ceramic submount 70 for heat dissipation.
• the submount 70 with the gain chip 20 and the optical reflector chip 10 are each attached, preferably using glue 90, to a common submount 80, preferably made of glass.
• An active alignment of the gain chip 20 with respect to the optical reflector chip 10 may be carried out by maximizing the output portion of the optical radiation exiting the optical reflector chip 10 in a pick-and-place process.
• the output facet 22 of the gain chip 20 may be glued to the input facet 15 of the optical reflector chip 10.
• the glue 90 between the output facet 22 of the gain chip 20 and the input facet 15 of the optical reflector chip 10 may be omitted, as it may represent a potential point of failure.
• Figs. 22 and 23 illustrate a so-called P-side down hybrid integration approach.
• Fig. 22 shows a schematic top view, i.e. as viewed along the vertical z-direction, of the optical laser assembly 1 according to an embodiment of P-side down hybrid integration approach, while Fig. 23 shows a sectional view along the sectional plane G'-G' marked in Fig. 22.
• the P- side down hybrid integration approach is identical to the P-side up hybrid integration approach described above, except that the gain chip 20 is bonded face down to the ceramic submount 70, i.e. with the active gain waveguide 21 being arranged at the bottom of the gain chip 20 in z-direction.
• the gain waveguide 21 is thus closer to the heat submount 70 in the P-side down hybrid integration approach than in the P-side up hybrid integration approach, which leads to a better heat dissipation in the P-side down hybrid integration approach.
• the P-side down hybrid integration approach is thus favorable for optical laser assemblies configured to emit optical radiation at high average powers, in particular for optical laser assemblies comprising several parallel gain waveguides 21 as shown in Figs. 15 and 16.
• Figs. 24 and 25 illustrate a flip-chip integration approach.
• Fig. 24 shows a schematic top view, i.e. as viewed along the vertical z-direction, of the optical laser assembly 1 according to an embodiment of P-side down hybrid integration approach
• Fig. 25 shows a sectional view along the sectional plane H'-H' marked in Fig. 24.
• the substrate layer 50 preferably made of silicon
• the optical reflector chip 10 extends in the x-y-plane from underneath the embedding layer 40 and a gain chip 20, preferably a semiconductor gain chip 20, in particular a lll-V chip, is bonded directly to the substrate layer 50 of the optical reflector chip 10 using solder bumps 91.
• Electrical contacts may be provided on the substrate layer 50 (not shown in Fig. 25) via which an electrical pump current may be provided to the gain chip 20, to form an electric connection to a bottom side of the gain chip which faces the substrate layer 50.
• the substrate layer 50 may be glued onto a common submount 80, preferably made of glass.
• Figs. 26 shows a main waveguide 112 which is wound in a spiral, wherein the spiral has a first section with windings spiralling towards a center of the spiral, and a second section with windings which are spiralling back outwards away from the center and which are arranged inbetween the windings of the first section.
• Fig. 27 shows an enlarged view of the excerpt F marked in Fig. 26.
• perturbation elements 113 are arranged on either side of the first section of the spiral to form a Bragg grating section 111.
• Such a spiral configuration allows for long waveguides to be fitted into an optical reflector chip which has a small footprint, thus enabling a narrow linewidth of the lasing wavelength of the optical laser assembly, while keeping the optical laser assembly very compact.
• This type of spiral shape and variations thereof may be used in any of the embodiments discussed above instead of a straight Bragg grating sections.

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