WO2024061682A1

WO2024061682A1 - Edge emitting laser device and method of processing an edge emitting laser device

Info

Publication number: WO2024061682A1
Application number: PCT/EP2023/074924
Authority: WO
Inventors: Jens Ebbecke
Original assignee: Ams Osram International GmbH
Current assignee: Ams Osram International GmbH
Priority date: 2022-09-19
Filing date: 2023-09-11
Publication date: 2024-03-28
Anticipated expiration: 2025-03-19

Abstract

The invention concerns an edge emitting laser device, comprising an n-cornered mesa shaped substrate, whereas n is an even number. An elongated gain region with an active area, in particular a quantum well or a multi-quantum well is located within the n-cornered mesa shaped substrate and the facing with its ends two opposite facets of the n-cornered mesa shaped substrate. One facet of the two opposing facets comprises an exit window for light generated by pumping the active region. A resonator region is located within the n-cornered mesa shaped substrate and surrounds the elongated gain region. The elongated gain region and the resonator region are optically coupled.

Description

EDGE EMITTING LASER DEVICE AND METHOD OF PROCESSING AN EDGE EMITTING

LASER DEVICE

The present invention concerns an edge emitting laser device and a method for processing the same .

BACKGROUND

Semiconductor edge emitting lasers usually have a rather broad emission linewidth that is due to different modes being excited in the laser gain region . While this may be sufficient for some applications , other applications require a more stable and particularly smaller emission linewidth .

There are several options to address this issue . In some instances , careful laser design can improve the situation, although the results do unfortunately not satisfy higher requirements . Another approach is to provide some locking mechanism, which requires some form of resonance , where the emitted light is resonating on a carefully selected mode , thereby reducing the emission linewidth . In an exemplary conventional technique , several optical resonating modes are generated by a laser diode chip . The emitted light is used to pump a resonator, for example a ring resonator with a high-quality factor . The ring resonator works as a feedback and offers a couple of different resonating modes . By careful designing the ring resonator in its dimensions , one of the ring resonator modes is within the linewidth of the emitted laser light . By self-inj ection locking onto this mode , an overall narrow linewidth of the pumped laser mode is realised .

This solution, however, may be cumbersome and requires a relatively large space . It is therefore an obj ect of the present application to present a laser device , in particularly an edge emitting laser device , which provides a stable narrow linewidth emission . SUMMARY OF THE INVENTION

This and other obj ects are addressed by the subj ect matter of the independent claims . Features and further aspects of the proposed principles are outlined in the dependent claims .

The inventor proposes to integrate two different resonating modes into one laser chip , where a first resonator mode is generated in a gain region that can be electrically pumped . A semiconductor laser is suitable for generating such first resonator mode . The second resonator mode is implemented without a gain region but realized in the same substrate as the semiconductor laser . To achieve the implementation in the same substrate , the inventor proposes utilizing impurity free quantum well intermixing , with the intermixed area forming a ring resonator surrounding the semiconductor laser . By designing the ring resonator properly, both resonator modes are weakly coupled to each other which results in an inj ection looking of the gain-mode to the gain-free mode leading to a strong linewidth reduction of the pumped laser mode .

In an aspect , a multi-mode laser diode is proposed that is frequency stabilized by an external ring resonator in the same substrate . The external ring resonator is carefully designed . For example , a hexagonal phosphide-based ring resonator with a length of 1000pm comprises a free spectral range of about 0 . 4 nm, meaning that probably more than a single mode will be self-inj ection locked . Reducing the size of the hexagonal laser diode chip to a length of 200 pm provides a free spectral range of about 2 nm, which is enough for a single frequency operation .

Certain structures for the ring resonator may even allow for more than a single mode to be stabilised, which provides an opportunity for stabilised and adj ustable multi-mode applications .

In an aspect , an edge emitting laser device is proposed, comprising an n-cornered mesa shaped substrate , whereas n is an even number . The parameter n usually is 4 , 6 or 8 . At least one elongated gain region with an active area, in particular a quantum well or a multi-quantum well is located within the n-cornered mesa shaped substrate . Its two ends are facing two opposite facets of the n-cornered mesa shaped substrate and may be substantially parallel to it . One facet of the two opposing facets comprises an exit window for light generated by pumping the active region .

In accordance with the proposed principle , a resonator region is located within the n-cornered mesa shaped substrate and surrounds the elongated gain region . The elongated gain region and the resonator region are optically coupled . Due to the optical coupling, one or more modes of the emitted light by the elongated gain region is enhanced and selected by the resonator region . This will provide a selection of a single frequency with a very low FWHM . The proposed solution can be implemented in a very small size , as the resonator surrounds the actual gain region . In particular, it can be implemented in the same semiconductor, which offers low manufacturing costs and high yields .

The proposed solution can be implemented in different semiconductor material systems , like for example but not limited to GaAs , InAlGaAs , InGaAP, InGaAlP, InP , GaN, InGaN and InGaAlN .

The mesa shaped substrate with its dimension substantially defines the resonator region . In some aspects , the resonator region forms a ring resonator particularly along the facets of the n-cornered mesa shaped substrate . Consequently, the resonating modes of the resonator region are defined by the shape and size of the n-cornered mesa shaped substrate . In some instances , the laser device comprises piezo or heating elements on the resonator region, thereby enabling a tuneable resonator frequency . Consequently, the laser device in accordance with the proposed principle becomes a tuneable laser device that provides a multitude of electable emission lines with small FWHM due to the high quality of the ring resonator .

In some further aspects , the resonator region comprises a quantum well intermixed area or a multi-quantum well intermixed layer . In other words , while the actual elongated gain region comprises a quantum well or a multi quantum well structure that is not intermixed, the surrounding layer is intermixed . This results in an electrical barrier in the bandgap, thereby confining the charge carriers inside the elongated gain region .

In some instances , the resonator region comprises an active layer that is characterized by a first bandgap . Said first bandgap is sufficiently larger than a second bandgap of the active area of the elongated gain region, said first bandgap generated by quantum well intermixing of the active layer . As already mentioned, the higher bandgap generates an electrical barrier confining the charge carriers inj ected into the elongated gain region to said region . In addition, the higher bandgap in the surrounding active layer, that is the ring resonator, prevents bandgap absorption of light generated by pumping the active region of the elongated gain region .

In some aspects , the n-cornered mesa shaped substrate comprises a first doped layer and a second doped layer and an active layer in between, the active layer including the active area of the elongated gain region . Consequently, the resonator as well as the elongated gain region can be processed in the same process flow and on the same substrate . It mainly differentiates in the step of amending or changing the bandgap .

In some aspects , the facets facing the ends of the elongated gain region comprise a mirror coating . The mirror coating is adj usted such that one mirror comprises a slightly higher transmission than the other, thereby defining the exit window . Mirror coating can be applied by an atomic layer deposition or other suitable means . In some instances , the mirrors may include a thin layer of a reflective metal or a DBR structure and the like .

As the substrate is mesa shaped, it is suitable to utilize certain crystallographic planes during the mesa etching process . For example , at least the two opposing facets can extend along the crystallographic m-planes of the cornered mesa shaped substrate . While the suitable planes may be different depending on the respective material systems used, etching along certain crystallographic planes achieves smooth and substantially vertical mesa side walls .

In some other aspects , the elongated gain region is smaller than the distance between the two opposite facets of the n-cornered mesa shaped substrate and is in the range of 100 pm to 300 pm and in particularly smaller than 250pm and more particularly smaller than 200 pm . In other words , the gain is restricted to a certain area within the resonator that spans the distance between the two opposing facets .

For a confined carrier inj ection one may additionally utilize a conductive material on the surface of the n-cornered mesa shaped substrate . The area covered by said conductive material can be restricted to the elongated gain region within the n-cornered mesa shaped substrate . Consequently, charge carriers will only be inj ected into the area directly above the non-intermixed active region of the elongated gain region . This increases the quantum efficiency and reduces recombination of charge carriers outside the elongated gain region .

Some further aspects concern a coupling element that is used to facilitate the coupling between the light emitted by the elongated gain region and the surrounding resonator . For this purpose , the coupling element can be arranged between an end of the elongated gain region and the facet of the n-cornered mesa shaped substrate opposite the facet with the exit window . The coupling element is configured to couple light generated in the elongated gain region into the ring resonator . More particularly, the coupling element may couple a light mode of the elongated region that is in resonance with the modes of the ring resonator . This light mode is thereby greatly enhanced and subsequently outcoupled through the exit window .

In some instances , the coupling element comprises a width that is smaller than a width of the respective resonating mode of the resonator to be coupled to . The coupling element may in some instances affect the coupling of the respective modes . For example , the coupling element comprises in some instances a diffusor created in the n-cornered mesa shaped substrate . The diffusor can be implemented or formed by a groove that is arranged inclined with respect to the adj acent facet . Alternatively, the diffusor may comprise such groove or a vertical mesa etch and the like . The inclination angle can be within 30 ° to 60 ° and particularly between 40 ° and 50 ° and more particularly around 45 ° .

In this regard, it is not necessary that the coupling element redirects all light of the intended mode . Rather it has been found that feedback of only 1 part in 10⁴ is enough to stabilise the emission wavelength of an edge emitter by a ring resonator . The depth of the coupling element can vary but is deep enough to achieve the previously mention weak coupling . In some instances , the depth reached down to the active region of the elongated gain region .

As an alternative embodiment , the coupling element comprises a strain in the crystal structure , in particularly an elongated strain that is inclined with respect to the adj acent facet , particularly with an angle of 30 ° to 60 ° and particularly between 40 ° and 50 ° and more particularly around 45 ° .

In some further instances , the proposed laser device comprises an elongated groove , arranged substantially parallel to the elongated gain region . The groove may comprise a depth at least down to the active area of the elongated gain region . Further, the elongated groove can be distanced from the elongated gain region by 10 pm to 100 pm and particularly by 10 pm to 40 pm . The elongated groove will prevent a resonance perpendicular to the elongated gain region . It can also be used to separate two or more elongated gain regions that are arranged in parallel on the same substrate .

Some further aspects concern a method for processing an edge emitting laser device . The method comprises providing a layer stack having a first doped layer and a second doped layer and an active layer in between . The active layer can comprise one of a pn-heterostructure , a quantum well or a multi quantum well structure . An impurity free quantum well intermixing is generated in portions of the layer stack as to form an elongated non-intermixed region in the active layer . In other words , quantum well intermixing in the active layer is conducted except for an elongated region . Then, a mesa etching of the layer stack is performed such that two opposing facets of the etched layer stack are formed, which are substantially parallel to two ends of the elongated non-intermixed region . A mirror coating on the two opposing facets is provided . Finally, a contact is formed on the surface of the layer stack above the elongated non-intermixed region .

Some of the above-mentioned steps can be interchanged, for example the contact can be formed prior to the mesa etching process . Nevertheless , the proposed method provides an elongated gain region surrounded by a resonator element . In operation of such device , the resonator element forces light emitted by the elongated region to its designed resonance frequency causing a stable emission with a very small FWHM .

In some aspects , the contact provided on the surface of the layer stack comprises a conductive metal . The size of the contact however can be smaller than the area of the elongated gain region below . Consequently, the contact may be smaller and/or thinner than the elongated non-intermixed region beneath .

Some aspects concern the step of mesa etching . The etching process is conducted such as to form a resonator region, particularly along the facets of the mesa etched structure . Said resonator region may be formed as a ring resonator along the facets generated by the mesa etching process . The resonator region comprises an active layer that is characterized by a first bandgap gap larger than a second bandgap gap of the active area of the non-intermixed elongated gain region .

In some aspects , the step of mesa etching comprises etching such that at least the two opposing facets extends along the crystallographic m-planes of the cornered mesa shaped substrate . In some other aspects , the method further comprises providing a recess or a groove that is substantially parallel to the elongated non-intermixed region . The recess may be distanced from the elongated non-intermixed region in the range of 30 pm to 200 pm and particularly less than 150 pm. It can reach down to the active layer and will prevent the excitement of optical modes perpendicular to the elongated non-intermixed region .

In some further aspects , a coupling element can be provided . Said coupling elements is arranged between an end of the elongated gain region and one of the two opposing facets . The coupling element can be configured to redirect a portion of light generated in the elongated gain region substantially parallel to the one of the two opposing facets . The previously mentioned steps of providing a coupling element can be implemented in various ways .

For example , a particular vertical recess can be etched in some instances into at least the top layer . The particular vertical recess is inclined with respect the adj acent facet , particularly with an angle of 30 ° to 60 ° and particularly between 40 ° and 50 ° and more particularly around 45 ° . Alternatively or additionally, a particular vertical recess is etched into at least the top layer, wherein the particular vertical recess comprises a width smaller than a width of a respective resonating mode of the resonator to be coupled to .

Asa further alternative , a strained area can be implemented in a portion in the top layer and/or the active layer , in particularly an elongated strained area that is inclined with respect the adj acent facet , particularly with an angle of 30 ° to 60 ° and particularly between 40 ° and 50 ° and more particularly around 45 ° .

SHORT DESCRIPTION OF THE DRAWINGS

Further aspects and embodiments in accordance with the proposed principle will become apparent in relation to the various embodiments and examples described in detail in connection with the accompanying drawings in which

Figure 1 shows a first embodiment of an edge emitting laser in accordance with some aspects of the proposed principle ;

Figure 2 illustrates a second embodiment of an edge emitting laser in accordance with some aspects of the proposed principle ;

Figures 3A to 3C show some steps of processing an edge emitting laser in accordance with some aspects of the proposed principle ;

Figure 4 illustrates a top view of a wafer structure during the processing of an edge emitting laser in accordance with some aspects of the proposed principle as well as a corresponding bandgap distribution;

Figures 5A to 5E show some further steps of processing an edge emitting laser in accordance with some aspects of the proposed principle ;

Figure 6 illustrates a top view of an edge emitting laser in accordance with some aspects of the proposed principle .

DETAILED DESCRIPTION

The following embodiments and examples disclose various aspects and their combinations according to the proposed principle . The embodiments and examples are not always to scale . Likewise , different elements can be displayed enlarged or reduced in size to emphasize individual aspects . It goes without saying that the individual aspects of the embodiments and examples shown in the figures can be combined with each other without further ado , without this contradicting the principle according to the invention . Some aspects show a regular structure or form . It should be noted that in practice slight differences and deviations from the ideal form may occur without , however, contradicting the inventive idea . In addition, the individual figures and aspects are not necessarily shown in the correct size , nor do the proportions between individual elements have to be essentially correct . Some aspects are highlighted by showing them enlarged . However, terms such as "above" , "over" , "below" , "under" "larger" , "smaller" and the like are correctly represented with regard to the elements in the figures . So it is possible to deduce such relations between the elements based on the figures .

Figure 1 illustrates a first embodiment of an edge emitting laser in accordance with some aspects of the proposed principle . The edge emitting laser comprises a mesa shaped structure with six corners forming a hexagonal shape . The mesa shaped structure 20 includes a layer stack comprising a first doped layer 21 , a second doped layer 22 and an active layer 23 in between . The active layer 23 includes a quantum well structure or a multi-quantum well structure . The two doped layers 21 and 22 comprise different doping types , that is n- doping and p-doping . The structured mesa shaped substrate further comprises on its top surface a contact layer 11 for inj ecting charge carriers into the doped layer 22 and the quantum well structure 23 . Contact 11 thereby forms an elongated contact arranged between two opposing facets of the mesa shaped substrate .

The active layer 23 of the mesa shaped substrate 20 outside the area covered by contact 11 is quantum well intermixed in order to obtain a higher band gap in that area . The portion of active layer 23 below the contact 11 is not intermixed and comprises a bandgap, which is smaller than the corresponding bandgap of the intermixed area of active layer 23 . As a result thereof , the substrate 20 comprises an elongated gain region 10 , which is arranged between two opposing facets . In the present application, the elongated gain region is defined by the area with a smaller bandgap and completely covered by contact 11 , however , said elongated gain region can be longer or slightly thicker than the area of contact 11 on top . This may ensure that charge carriers are inj ected mainly into the elongated gain region and do not diffuse in the surrounding region, irrespectively of the bandgap difference . The two opposing facets 24 and 25 are covered by a respective mirror and reflective layer, whereas the facet 25 is partially more transparent compared to facet 24 , thereby forming an exit window for the edge emitting laser . The elongated gain region generates light , which is reflected by the respective mirror facets 24 and 25 and coupled out through the exit window 25 . Consequently, the reflectivity of facet 24 is higher than that of mirror 25 , or in other words , transmittance of mirror 25 is higher than that of mirror 24 . Usually, absorption for both mirror facets is very low .

In a conventional operation of the device , such edge emitting laser usually provide a relatively large linewidth of the emitted light with several modes included . This is mainly due to the fact that a plurality of modes can get enough gain to be excited in the elongated gain region, thereby broadening the linewidth . Variations in the quantum well intermixing or mirror processing may also broaden the spectrum of the emitted light .

To reduce the linewidth, the inventor proposes a specific shape and size for the mesa shaped structure . In particular, the structure is shaped to generate a resonating element along its facets , in this case a ring resonator having a very high-quality factor . For this purpose , the shaped structure comprises an n-cornered mesa shape in the present exemplary embodiment with six corners . Two opposing facets are substantially parallel to each other, whereas the elongated gain region is arranged between two opposing facets substantially parallel to the facet 24 and 25 .

The mesa shaped structure now comprises a ring mode also called "a whispering gallery mode" that can exist along the facets of the mesa shaped structure with a very high-quality factor . Said resonator mode lies within the light spectrum provided by the elongated gain region . Coupling a portion of the emitted light into the resonator area leads to a resonance , greatly enhancing the respective mode defined by the ring resonator . The feedback forces the elongated gain region to mainly emit on said resonance frequency resulting in an emitted light through the window with a small linewidth . For the purpose of controlling or increasing the optical coupling between the elongated gain region and the ring resonator, a small coupling element in form of a diffusor 30 is arranged between the fully reflective facet 24 and the end of the elongated gain region .

The diffusor 30 is implemented as a small groove etched substantially vertical into or close to the active layer 23 with a tilted angle with regards to facet 24 and the end of the elongated gain region . The inclination is about 45 ° for the present case . The diffusor comprises a width that is smaller than the respective linewidth of the resonator mode .

By proper selection of the respective width as well as the distance between the facet 24 and the end of the elongated gain mode , a specific mode for the ring resonator can be enhanced . Hence , by changing parameters like the dimension of the ring resonator' s size , its refractive index or the size of the diffusor , a specific mode of the resonator is selected, thereby providing an enhancement feedback to the elongated gain region .

It has been shown that the coupling between the ring resonator and the elongated gain region can be small in the range of a few %o or 1 /10⁴ , still enough to provide enough feedback to enhance and drive the selected mode . By changing the diffusor , the interaction between the different modes of the elongated gain region and the ring resonator can be enhanced or reduced, which leads to varying copying between the two optical modes .

For the purpose of coupling the two optical modes , it is required that the elongated gain region is arranged between two substantially parallel opposing facets , causing to have a mesa shaped substrate with an even corner number . Tilted mirror facets or an arrangement of the elongated gain region inclined to the facets may reduce the efficiency . Consequently, the mesa shaped structure could have four, six or even eight corners . The distance between two opposing facets provides the resonating modes and also the distance in frequency between two modes . For large structures in the range of 1000pm, the distance between two modes is about 0 . 4 nm . for smaller sized of the ring resonator in the range of 200 gives a free spectrum range of about 2 nm, which is suitable for a self-inj ection locking to a single frequency operation .

The coupling between the elongated gain region and the ring resonator can be a varied by the shape of the ring resonator, its length, the diffusor element as well as the shape and structure of the elongated gain region . In addition, changing the refractive index of the ring resonator' s material ( that is the area along the facets outside the elongated gain region ) , provides a tuneable resonator allowing the edge emitting laser according to the proposed principles to be tuned .

Figure 2 illustrates such embodiment with a certain degree of tuning . The mesa shaped substrate is formed as a rectangular or quadrature body with vertical mesa edges . A piezo element 85 is arranged along the edges of the mesa shaped substrate to tune the resonator frequency . Alternatively, element 85 can also be a thermal element , like a heating resistor and the like . The mesa shaped structure further comprises two elongated gain regions substantially parallel to each other embedded therein .

The two elongated gain regions indicated by the referral number 10 and 10 ' are spaced apart about 50 pm to 100 pm and are located substantially parallel to each other . They are formed such that their spectrum usually does not overlap, so that they cannot enhance and stipulate themselves . Still , the linewidth of the emitted light overlaps with a resonance frequency defined by the ring resonator surrounding both region 30 and 30 ' .

The multi-quantum well structure 23 between the two differently doped layers 21 and 22 is quantum well intermixed in areas outside of the elongated gain regions 10 and 10 ' . An additional V-shaped groove 40 is located between the two elongated gain regions 10 and 10 ' to prevent crosstalk between the two gain regions . In the present case , the groove 40 does not extend towards the mesa facets separating both portions completely, but this is possible to provide to separate ring resonators on the same substrate .

Two diffusor 30 and 30 ' are also provided in the beam path of the respective elongated gain regions . However, the diffusors 30 and 30 ' are implemented slightly different , thereby enhancing different modes of the ring resonator being formed by the quadrature shape of the mesa shaped structure . As a result thereof , two different modes are enhanced by the ring resonator and fed back to the gain regions . The elongated gain regions 30 and 30 ' will emit light through their respective exit windows 25 at slightly different wavelength . The wavelengths are defined by the different nodes being excited in the elongated gain regions .

In the two present embodiments of Figure 1 and Figure 2 , the quantum well intermixing is provided by an impurity free quantum well intermixing outside of the respective elongated gain regions defined by contact areas 11 on the top surface of the mesa shaped structure . However, in some embodiments , the contact 11 can be shorter or thinner than the non-intermixed area of the active layer 23 below . Hence , the elongated gain region, in which quantum well structure 23 is not intermixed may be longer or broader than the respective dimension of contact 11 provided on top of the mesa shaped substrate 20 . While such arrangement may slightly increase absorption in the portion of the non-intermixed areas in active layer 23 uncovered by contact 11 , it also allows to better confine the charge carriers and inj ect them mainly into the non-intermixed area 23 .

Figures 3A to 3C show the first steps of processing an edge emitting laser in accordance with some aspects of the proposed principle . Figure 3A illustrates a substrate 50 , on which a layer stack is epitaxially deposited . The material of the layer stack may be selected from a plurality of possible material systems , including ternary and quaternary semiconductor material . Hence , edge emitting lasers with laser light output from the infrared spectrum to the ultraviolet spectrum are possible by proper choice of the material system, such systems include but are not limited to GaAs , AlGaAs , GaN, GaP, AIN, A1P, InGaN, InGaP, AlGaN, AlGaP , InGaAlN and InGaAlP .

The substrate 50 may also comprise certain buffer layers for crystal lattice matching , for reducing crystal defects , as sacrificial layer, for later re-bonding and the like . In any case , the layer stack comprises an epitaxially grown first doped layer 21 , an active layer 23 deposited on the first doped layer 21 and a second differently doped layer 22 deposited on active layer 23 . The doping concentration in doped layers 21 is constant , it varies in layer 23 due to quantum well intermixing explained further below . Alternatively, the dopant concentration can also vary in layer 21 to provide a good carrier inj ection into active layer 23 . Active layer 23 comprises a quantum well structure , for example , made of the quaternary system like InGaAlP or InGaAlN with different aluminum contents to provide barrier layers and quantum well layers , respectively .

A mas k layer is deposited in a subsequent step shown in Figure 3B on the top surface of the second doped layer 22 and subsequently structured . The structuring provides a small strip 60 of mask material on the top surface .

In a subsequent step illustrated in Figure 3C, a dopant , for example Zn is deposited into active layer 23 causing quantum well intermixing in portions of active layer 23 not covered by the strip mask layer 60 . For this purpose , Zn as a dopant is first deposited on the top surface of layer 22 at a first temperature Tl . The temperature T1 is selected such that Zn substantially remains on the top surface . In a subsequent processing step , the temperature is increased initiating the actual diffusion process . Zn material will now diffuse into the doped layer 22 ( it may contain the same doping type or even the same dopant ) and subsequently into the quantum well structure of active layer 23 . This two-step process provides a highly controllable steep doping profile and more controlled process step , with steep doping edges clearly separating quantum well intermixed areas from nonquantum well intermixed areas . The result , illustrated in a top view presented in Figure 4 , is an elongated region 10 , in which the band gap in the active layer 23 is smaller with respect to the surrounding portions . The right side of Figure 4 illustrate the bandgap across the distance x in a cut view through the elongated portion 10 . The band gap difference caused by the quantum well intermixing may be in the range of 0 . 05 eV to 0 . 15 eV, which is sufficient to create an energy barrier, substantially confining the charge carrier within the smaller bandgap .

Following now with the next process steps illustrated in Figure 5A to 5E . The top surface of doped layer 22 in Figure 5A will be covered by a second mask 70 , which is subsequently structured to provide an opening 71 , exposing a portion of the top surface of layer 22 . The exposed portions of surface of layer 22 is substantially parallel along the elongated region 62 , in which no intermixing took place . The area of active layer 23 being quantum well intermixed have the referral number 61 ' .

In a subsequent step in Figure 5B, a groove is etched into the exposed portion until the active layer 23 is reached . The depth of the groove depends on the optical characteristics later, but usually reaches or even intersects active layer 23 . The groove will reduce and prevent other optical modes particularly optical modes perpendicular to the elongated region to be excited . The distance between the groove 40 and the elongated gain region 10 with the nonintermixed area 62 is approximately 50 pm to 150 pm but can generally be set to the desired needs .

In addition, a small coupling element in form of the diffusor 45 is etched in front of one end face of the elongated gain region with the non-intermixed area 62 . Similar to groove 40 , diffusor 45 can be vertically etched into the top surface layer . However, diffusor 45 may only scratch the active layer or even end prior to reaching the active layer, j ust to implement a disturbance in said portion in order to redirect a small amount of light into the subsequently formed ring resonator . The resulting structure is presented in a top view in Figure 5C . In area 62 , the active layer 23 is not quantum well intermixed, while the surrounding area 61 ' comprises the quantum well intermixed portion with a higher bandgap . Groove 40 and diffusor 45 are etched into the respective quantum well intermixed area with the vertical mesa particularly for the diffusor 45 used out coupling light generated in the elongated gain region 10 .

Figure 5D and 5E illustrate the next process steps . A hard mas k 80 material is deposited in Figure 5D on the top surface of layer 22 and subsequently structured to provide a recess 81 surrounding the elongated gain region 10 . The recess 81 exposes top surface portions of layer 22 . The edges of the recess 81 follow the crystallographic planes of the material system . Using the above-mentioned quaternary material system, the recess follows the crystallographic m-planes thereof .

In a subsequent step, mesa etching is performed illustrated in Figure 5E by removing the doped layers 21 , 23 and the active layer 23 in between above said exposed areas . The mesa etch process is done by a dry etching or a wet etching process , or a combination of both, respectively . Following the crystallographic planes may provide a benefit here , as a smoother and more vertical facet can be achieved reducing the need for additional cleaning or further etching steps .

The etch is performed such that the resulting mesa structure comprises an even number of corners in order that during mode along the facets as well as an extra etching mode along the elongated gain region can exist simultaneously . Therefore the mask layer 80 in the previous figure and/or the mesa etch should ensure that two opposing facets are substantially parallel to the end portions of the elongated region 62 .

Based on the material system, certain shapes of the mesa structure are preferred . For example , a hexagon mesa structure , illustrated in Figure 6 in its perspective view, may be advantages in case of an InGaAlP or an InGaAlN quaternary material system. This is due to the possible etching along a certain crystallographic planes , in particular the m-planes in the previously mentioned material systems .

As a result , the etching process will produce facets that are substantially parallel to each other and almost vertical . After some post processing, the vertical facets are subsequently cleaned and covered by metallic or otherwise reflective layer 24 and 25 , respectively . The mirror facets 24 and 25 a comprise either metal layer of different reflectivity, a DBR structure or combination of both .

Facet 25 is defined as the exit window and is processed to comprise a slightly higher transmission than the corresponding facet 24 in accordance with the proposed principle . The material for the mirror or the DBR structure for facets 24 and 25 is done in a conventional end of line processing, or in subsequent ALD processes . The diffusor 41 is arranged between the highly reflective mirror facets 24 and the end face of the elongated region as depicted in Figure 6 .

LIST OF REFERENCES edge emitting laser , 10 ' elongated gain region top contact n-cornered mesa shaped substrate doped layer doped layer active layer mirror facet mirror facet , exit window , 30 ' diffusor groove , recess growth substrate structured mas k dopant ' intermixed area non-intermixed area structured mas k hard mas k exposed areas

Claims

CLAIMS Edge emitting laser device , comprising : an n-cornered mesa shaped substrate , whereas n is an even number ; an elongated gain region with an active area , in particular a quantum well or a multi-quantum well located within the n- cornered mesa shaped substrate and the facing with its ends two opposite facets of the n-cornered mesa shaped substrate ; wherein one facet of the two opposing facets comprises an exit window for light generated by pumping the active region; a resonator region located within the n-cornered mesa shaped substrate and surrounding the elongated gain region, wherein the elongated gain region and the resonator region are optically coupled . The laser device according to claim 1 , wherein the resonator region forms a ring resonator particularly along the facets of the n-cornered mesa shaped substrate . The laser device according to any of the preceding claims , wherein the resonator region comprises a quantum well intermixed or a multi-quantum well intermixed layer . The laser device according to any of the preceding claims , wherein the resonator region comprises an active layer that is characterized by a first bandgap gap larger than a second bandgap gap of the active area of the elongated gain region, said first bandgap gap generated by quantum well intermixing of the active layer . The laser device according to any of the preceding claims , wherein the n-cornered mesa shaped substrate comprises a first doped layer and a second doped layer and an active layer in between, the active layer including the active area of the elongated gain region . The laser device according to any of the preceding claims , wherein the facets facing the ends of the elongated gain region comprise a mirror coating . The laser device according to any of the preceding claims , wherein at least the two opposing facets extends along the crystallographic m-planes of the cornered mesa shaped substrate . The laser device according to any of the preceding claims , wherein the elongated gain region is smaller than the distance between the two opposite facets of the n-cornered mesa shaped substrate and is in the range of 100 pm to 300 pm and in particularly smaller than 250 pm . The laser device according to any of the preceding claims , wherein the top surface of the n-cornered mesa shaped substrate with the elongated gain region beneath is at least partially covered by a conductive material , said conductive material configured to inj ect charge carriers into the elongated gain region, said conductive material optionally being shorter and/or thinner than elongated gain region beneath . The laser device according to any of the preceding claims , further comprising a coupling element arranged between an end of the elongated gain region and the facet of the n-cornered mesa shaped substrate opposite the facet with the exit window; said coupling element configured to couple light of generated in the elongated gain region into the ring resonator . The laser device according to claim 10 , wherein the coupling element comprises a diffusor created in the n-cornered mesa shaped substrate , said diffusor generated by or comprising by at least one of : a groove that is arranged inclined with respect the adj acent facet , particularly with an angle of 30 ° to 60 ° and particularly between 40 ° and 50 ° and more particularly around 45 ° , further optionally having a depth down to the active region of the elongated gain region; a strain, in particularly an elongated strain that is inclined with respect the adj acent facet , particularly with an angle of 30 ° to 60 ° and particularly between 40 ° and 50 ° and more particularly around 45 ° . The laser device according to claim 10 or 11 , wherein the coupling element comprises a width that is smaller than a width of the respective resonating mode of the resonator to be coupled to . The laser device according to any of the preceding claims , further comprising an elongated groove , arranged substantially parallel to the elongated gain region and comprising a depth at least down to the active area of the elongated gain region, wherein optionally, the elongated groove is distanced from the elongated gain region by 10 pm to 100 pm and particularly by 10 pm to 40 pm. Method of processing an edge emitting laser device , comprising the steps of : providing a layer stack having a first doped layer and a second doped layer and an active layer in between, said active layer comprising one of a pn-heterostructure , a quantum well or a multi quantum well ; generating a -particularly impurity free- quantum well intermixing in portions of the layer stack as to form an elongated non-intermixed region in the active layer ; mesa etching the layer stack such that two opposing facets of the etched layer stack are substantially parallel to two ends of the elongated non-intermixed region; providing a mirror coating on the two opposing facets ; providing a contact on the surface of the layer stack above the elongated non-intermixed region . The method according to claim 14 , wherein the contact on the surface of the layer stack comprises a conductive metal and optionally is smaller and/or thinner than the elongated nonintermixed region beneath . The method according to any of claims 14 to 15 , wherein the step of mesa etching is conducted as to form a resonator region, particularly along the facets of the mesa etched structure , wherein the resonator region comprises an active layer that is characterized by a first bandgap gap larger than a second bandgap gap of the active area of the non-intermixed elongated gain region . The method according to any of claims 14 to 16 , wherein the step of mesa etching comprises etching such that at least the two opposing facets extends along the crystallographic m-planes of the cornered mesa shaped substrate . The method according to any of claims 14 to 17 , further comprisin : providing a recess substantially parallel to the elongated nonintermixed region, said recess being distanced to the elongated non-intermixed region in the range of 30 pm to 200 pm and particularly less than 150 pm. The method according to any of claims 14 to 18 , further comprising : Providing a coupling element arranged between an end of the elongated gain region and one of the two opposing facets ; said coupling element optionally configured to redirect a portion of light generated in the elongated gain region substantially parallel to the one of the two opposing facets . The method according to any of claim 19 , wherein the step of providing a coupling element comprises at least one of :

Etching a particular vertical recess into at least the top layer, the particular vertical recess inclined with respect the adj acent facet , particularly with an angle of 30 ° to 60 ° and particularly between 40° and 50° and more particularly around 45° ;

Etching a particular vertical recess into at least the top layer, the particular vertical recess comprising a width smaller than a width of a respective resonating mode of the resonator to be coupled to;

Providing a strain, in particularly an elongated strain that is inclined with respect the adjacent facet, particularly with an angle of 30° to 60° and particularly between 40° and 50° and more particularly around 45° .