GB2585069A - Vertical Surface Emitting Laser with Improved Polarization Stability - Google Patents

Abstract

A vertical surface emitting laser device has a sub-wavelength grating 62 provided on the surface of the gain structure 112, suppressing one polarisation of the emitted laser improving polarisation stability. The device comprises a gain structure (figure 1, 12) comprising a first mirror structure (figure 1, 18) and an active layer (figure 1, 20); a second mirror structure (figure 1 14) aligned to form a cavity with the first mirror, wherein the gain structure 112 has a surface facing the second mirror structure, and wherein at least one sub-wavelength grating 62 is provided on the said surface and is non-absorbing to the target wavelength of the device; and wherein at least part of the gain structure 112 has an optical absorbency that increase in a direction away from an optical axis of the device.

Description

Vertical Surface Emitting Laser with Improved Polarization Stability

Field of the Invention

This invention relates to vertical emitting lasers and in particular to vertical external-cavity surface-emitting lasers (VECSELs) and vertical cavity surface-emitting lasers (VCSELs).

Background to the Invention

A vertical external-cavity surface-emitting laser (VECSEL), also known as a semiconductor disc laser, is a semiconductor laser that produces laser light propagating perpendicular to the surface of its semiconductor structure, i.e. a vertical emitting laser. Unlike a vertical cavity surface-emitting laser (VCSEL), in which two distributed Bragg reflectors (DBRs) are incorporated into the semiconductor structure to form an optical cavity, in a VECSEL one of the DBRs is located outside the semiconductor gain structure to form an external cavity. The external reflector allows a significantly greater area of the semiconductor structure to participate in generating light in a single mode, resulting in higher output power compared with other types of semiconductor lasers. A vertical cavity surface-emitting laser (VCSEL) is similar except that it does not have an external cavity.

A publication entitled "3-4.5 um continuously tunable single mode VECSEL" by M. Fill et al, Applied 20 Physics B (2012) 109:403-406; DOI 10.10071s00340-012-5096-4, discloses an example of a VECSEL.

It is desirable for the generated laser light to have a single, stable polarization. However, the typical radial-symmetric geometry of a VECSEL device causes the laser light not to have a geometrically defined single-polarization orientation. As a result the polarization of laser light can spontaneously switch or rotate its geometric orientation during operation of the laser device, which is undesirable. A similar problem can arise with VCSEL devices.

It would be desirable to provide an improved VECSEL or VCSEL device, in particular a VECSEL or 30 VCSEL device that emits laser light with improved polarization stability.

Summary of the Invention

The invention provides a vertical surface emitting laser device comprising: a gain structure comprising a first mirror structure and an active layer; and a second mirror structure aligned with said first mirror structure to define an optical cavity between the first and second mirror structures, wherein said gain structure has a surface facing said second mirror structure, and wherein at least one sub-wavelength grating is provided on said surface, said at least one sub-wavelength 40 grating being non-absorbing to light at a target laser emission wavelength of said laser device, and wherein at least part of said gain structure has an optical absorbency that increases in a direction laterally away from an optical axis of the device.

Preferably, said at least one grating is formed in relief from said surface.

Preferably, at least one of said at least one gratings is aligned with, or centred on, an optical axis of said laser device. Optionally, said at least one grating comprises a plurality of gratings provided on said surface. Alternatively, said at least one grating comprises only one grating.

In preferred embodiments, said surface is a surface of the active layer.

In some embodiments, an optically transparent element is provided on said active layer, and said surface is a surface of said optically transparent element.

Preferably, the, or each, grating is formed from material that is non-absorbing to light at a target laser emission wavelength of said laser device.

Typically, the, or each, grating comprises a plurality of spaced apart bars, typically formed from semiconductor material. The bars typically extend across a surface area covered by said grating.

Optionally, the grating has a centrally located portion formed from a solid material, preferably the same material from which said bars are formed.

Optionally, the grating is annular. The grating may have a centrally located portion without said bars 25 or other solid material.

In preferred embodiments, said bars are formed from material having a first refractive index, and wherein the spaces between said bars is formed from material having a refractive index that is lower than said first refractive index.

In preferred embodiments, said bars are formed from material having a first refractive index, and wherein at least part of, and preferably all of, a traversally neighboring region around the grating has a refractive index lower than said first refractive index.

In some embodiments, said second mirror structure is an external mirror structure spaced apart from the gain structure to define an external cavity between said external mirror structure and said gain structure, said external cavity being at least part of said optical cavity.

Typically, said at least part of the gain structure is located beneath said at least one sub-wavelength 40 grating.

In some embodiments, said at least part of the gain structure includes a non-central, preferably annular, region that is laterally spaced apart from the optical axis and has an optical absorbency that is higher than an inner central region.

Typically, said at least part of the gain structure comprises said active layer and/or an electrical contact provided on said gain structure.

From another aspect, the invention provides a gain structure for a vertical surface emitting laser device, said gain structure comprising a first mirror structure and an active layer, wherein at least one sub-wavelength grating is provided on a surface of said gain structure, said sub-wavelength grating being non-absorbing to light at a target laser emission wavelength, and wherein at least part of said gain structure has an optical absorbency that increases in a direction laterally away from an optical axis of the structure.

Advantageously, the sub-wavelength grating has the effect of suppressing one polarization of the emitted laser light causing the device to emit laser light with improved polarization stability.

VECSELs and VCSELs are used in a variety of applications including telecommunications and machining. A particularly common application is as an infrared (especially near infrared and mid-infrared) device in spectroscopy and gas sensing applications. In some embodiments, therefore, VECSEL devices embodying the invention may for example be configured to emit laser light in the near or mid-infrared wavelength range, for example between 3.0 and 5pm, or more generally in the wavelength region above 3 pm, advantageously in a single longitudinal and transversal emission mode. In some embodiments, the single emission-mode is continuously tunable without mode hops.

For example, the emission mode may be tunable over approximately 50-100 nm around the centre wavelength, typically at substantially constant operation temperature. The pulsed output power at room temperature is typically above10 mWp.

Further advantageous aspects of the invention will be apparent to those ordinarily skilled in the art 30 upon review of the following description of specific embodiments and with reference to the accompanying description.

Brief Description of the Drawings

Embodiments of the invention are now described by way of example and with reference to the 35 accompanying drawings in which like numerals are used to denote like parts and in which: Figure 1 is a sectional side view of a vertical external cavity surface emitting laser (VECSEL) device embodying the invention.

Figure 2 is an enlarged view of a gain structure and an external mirror structure, being part of the VECSEL device of Figure 1; Figure 3A is a plan view of a grating structure; Figure 3B is a side sectioned view of the first grating structure provided on a top surface of the gain 5 structure of the VECSEL device of Figure 1; Figure 3C is a plan view of an alternative grating structure suitable for providing on the top surface of the gain structure of the VECSEL device of Figure 1; Figure 4 is an enlarged view of a gain structure and an external mirror structure, being part of an alternative embodiment of a VECSEL device; Figure 5A is a plan view of another alternative grating structure; and Figure 5B is a side sectioned view of the grating structure of Figure 5 provided on a top surface of the gain structure of a VECSEL device comprising the gain structure of Figure 4.

Detailed Description of the Drawings

Referring now to Figure 1 of the drawings there is shown, generally indicated as 10, a laser device, in particular a VECSEL device, embodying the invention. The device 10 comprises a gain structure 12 (which may also be referred to as a laser gain chip or a semiconductor laser chip) and an external mirror structure 14. The mirror structure 14 is external in that it is not integrally formed with the gain structure 12, which is typically formed as a semiconductor chip. The mirror structure 14, which may be formed as a separate chip, is spaced apart from the gain structure 12 to define a gap 16 in between. The gap 16 may comprise free space or one or more spacer layer(s), which may be formed from any suitable material, for example a photoresist material e.g. SU-8 photoresist. In typical embodiments, the length of the gap 16 between the structures 12, 14 is approximately 5 pm to 100 pm, although may be shorter or longer depending on the application. The gain structure 12 and/or the mirror structure 14 may be supported by any suitable support structure, which may for example comprise one or more spacer elements or layer(s) 13 for supporting the mirror structure 14 with respect to the gain structure 12. In alternative embodiments (not illustrated) where the laser device is a VCSEL device rather than a VECSEL device, there is no external cavity between the gain structure 12 and the mirror structure 14, and as such they may be integrally formed.

The gain structure 12 comprises a dielectric mirror (also known as a Bragg mirror), preferably a distributed Bragg reflector (DBR) 18, and an active layer 20 (which may alternatively be referred to as an optical gain layer). The mirror 18 typically comprises multiple layers of semiconductor material as is described in more detail hereinafter. The mirror 18 is typically provided on a suitable substrate 22, for example a single-crystal silicon wafer. The gain structure 12 typically has a total thickness of only a few micrometers (not including the substrate 22). The gain structure 12 may be fabricated by an epitaxial process (e.g. molecular beam epitaxy (MBE) of layers 18, 20 on the substrate 22).

The active layer 20, which may be referred to as the active region of the laser, comprises at least 5 one layer, but typically multiple layers, of semiconductor material(s), typically direct bandgap semiconductor material. The active layer 20 may therefore be referred to as an active gain layer stack, and may be conventional in its configuration. For example III-V semiconductor material may be used although in preferred embodiments the active layer 20 is formed from IV-VI semiconductor materials, for example PbTe and PbSe or ternary alloys such as Pb(Eu/Sn/Sr)(Se/Te) [PbEuSe, PbSnSe, PbSrSe, PbSnTe, PbEuTe, PbSrTe], which have a direct band gap of approximately 0.3 eV at room temperature (RT).

With reference to Figure 2, in preferred embodiments, the active layer 20 comprises at least a middle layer 24 formed from a first semiconductor material having a relatively narrow (e.g. 0.3 eV or around 0.3 eV) bandgap, provided between outer layers 26, 28 of a second semiconductor material having a wider bandgap than the first semiconductor material. Most preferably, the middle layer 24 is configured, e.g. is thin enough, to act as a quantum well (QW) and so the active layer 20 may be described as a QW active layer. In alternative embodiments, the active layer 20 comprises a plurality of layers 24 of the first semiconductor material alternately arranged, or interleaved, with a plurality of layers 26, 28 of the second semiconductor material. Advantageously, each layer 24 of the first semiconductor material provides a QW between its adjacent layers 26, 28 of second semiconductor material. Advantageously, the presence of a plurality of QWs within the active layer 20 is operable to increase the effective gain and reduce threshold power values in-use. Preferably, the active layer 20 comprises 5 to 15 QWs, with particularly preferred embodiments comprising 7 to 13 QWs. In an alternative embodiment (not shown) any one or more of the QW layer(s) may be replaced with a quantum dot (QD) layer.

Referring to Figures 1 and 2, the mirror structure 14 comprises a dielectric mirror 32 (also known as a Bragg mirror), optionally a DBR. The mirror 32 comprises multiple layers 46 of dielectric material, each having a selected thickness and refractive index such that collectively the layers 46 serve as a reflector for light of a relevant design wavelength. In preferred embodiments, the mirror structure 14 includes a substrate 50 on which the mirror 32 is formed. As such, the mirror structure 14 may be formed as a multi-layer chip. The substrate 50 is preferably formed from an optically transparent material (with respect to the pump beam 39). The preferred optically transparent material may have a low refractive index (preferably a refractive index of 2 or below) and may absorb light at least at a wavelength corresponding to the gain layer. A low refractive index is not essential for the substrate 50 in the case of simple DBR, for example as illustrated. For example, silicon may be used to form the substrate 50, and may be doped such that it is light absorbing, but not necessarily only to selectively absorb the emitted light 36. In addition preferred substrate materials are permittive to the wavelength(s) used for optical pumping (preferably allowing at least 80% of the pumping beam 39 to be transmitted through), ideally with absorption close to 0%. In preferred embodiments, the substrate material is selected to absorb light at the design wavelength (preferably at least 30% of the light, typically between 30% to 70% for typical substrate thicknesses). This reduces or avoids back-reflections from the outer surface of the substrate, and reduces fringes in the emitted light 36, while allowing continuous mode-hop free tuning. For example, the substrate 50 may be formed from glass, preferably borosilicate glass such as Pyrex® but it may alternatively be formed from SiO2, BaF2 or any other suitable material. It is noted that SiO2, BaF2 are non-absorbing or substantially non-absorbing.

The provision of the substrate 50 enables the mirror structure 14 to have a relatively large active area, i.e. relatively large active mirror area (for example with an area of the order 10-8 to 10-8 m2, e.g. approximately 200x200 pm, or approximately 500x500 pm or approximately 1x1 mm, or approximately 5x5 mm) without loss of optical properties and with mechanical stability (e.g. with respect to bending radius).

The number of layers 46 forming the mirror 32 may vary depending upon the refractive indexes (and/or other relevant characteristics) of the material(s) used for the layers 46. Typically there are at least ten layers 46.

The gain structure 12 and the mirror structure 14 are spaced apart along an optical axis of the device 10 to define an optical cavity (which may also be referred to as a resonating cavity) between the DBR 18 of the gain structure 12 and the mirror 32 of the mirror structure 14, part of the cavity being provided by the gap 16. As such, the optical cavity may be said to comprise an external cavity. The mirrors 18, 32 together with the optical cavity provide an optical resonator for forming standing light waves (not illustrated) and ultimately the generation of an output laser beam 36. In preferred embodiments, the DBR 18 of the gain structure 12 serves as the output coupler by which the beam 36 is emitted in use.

The gain structure 12 is typically flat. The mirror structure 14 is preferably flat but is optionally curved, e.g. comprising a concave or concave spherical surface.

In preferred embodiments, the VECSEL device 10 is optically pumped, in use, by a light source (not shown), typically comprising one or more laser diodes or other laser light source. The light source is preferably arranged such that its output beam 39, or pump beam, is directed along the optical axis of the VECSEL device 10, the optical axis being substantially perpendicular to the layers that form the gain structure 12 and mirror structure 14. Typically the emitted output beam 39 is substantially aligned with or parallel with the optical axis. In preferred embodiments, the pump beam 39 passes from the light source through the mirror structure 14 to impinge upon the active layer 20. The components of the mirror structure 14 are therefore configured to transmit light at the wavelength of the light source but to reflect light at the wavelength of the generated laser light. A lens (not shown) may be used to shape the pump beam and focus it at the location of active layer 20. It is typically placed between light source and mirror structure 14. In alternative embodiments (not illustrated), the VECSEL may be electrically pumped.

In typical embodiments, the mirror structure 14 is carried by a support structure 40, which may be annular and is connected mechanically to the gain structure 12 (e.g. by one or more spacer blocks or layers 13 or other suitable supporting structure). The support structure 40 preferably comprises a microelectromechanical systems (MEMS) structure, and may be formed from any suitable material, e.g. Si or SiO2. In preferred embodiments, the mirror structure 14 is movable in an axial direction (as indicated by arrow A), which is vertical as viewed in the drawings. To this end, the mirror structure 14 is coupled to the support structure 40 by a membrane 41, which may be co-formed with or otherwise connected to the mirror structure 14 in any conventional manner. The preferred membrane 41 is shaped to define an opening around the optical axis of the device 10 so that the membrane 41 does not interfere with the optical operation of the device 10. The membrane 41 is capable of movement in the axial direction, which movement is imparted to the mirror structure 14. In the illustrated embodiment, the membrane 41 is connected to the support structure 40 by a sub-structure 42, which may be annular and which may be co-formed with, or otherwise connected to, the membrane 41 and support structure 40 in any conventional manner, e.g. using any suitable conventional MEMs techniques in preferred embodiments. In preferred embodiments, the support structure 40, membrane 41 and, when present, the sub-structure 42 are provided as MEMS and are formed using MEMS fabrication techniques.

The membrane 41 is moveable electrically by any convenient conventional actuation means (not shown), e.g. an electrostatic or comb-drive actuation system, which may for example involve providing any suitable conventional means for applying electrical charge to the membrane 41 to cause it to move in the desired manner. The movable membrane 41, and therefore the mirror structure 14, is movable in a direction (in a vertical direction as view in the drawings) towards and away from the gain structure 12 as indicated by arrow A in Figure 1 (which is substantially parallel with the optical axis), thereby decreasing or increasing the length of the gap 16 between the structures 12,14 and correspondingly decreasing or increasing the length of the optical cavity. The membrane 41 may be integrally formed with the mirror structure 14 and the support structure for the mirror structure in any conventional manner, typically involving MEMS and semiconductor device fabrication techniques (deposition, patterning and etching). Alternatively, the membrane 41 and the mirror structure 14 are fabricated separately and joined together, using, for example, a commercial precision pick & place machine, with a suitable conventional adhesive.

In the illustrated embodiment, the support structure 40 is supported on the gain structure 12 by spacer 13, which may be formed from any suitable material, for example a photoresist material e.g. SU-8 photoresist, and may be provided as one or more layers as is convenient. The spacer 13 may be formed on, or fixed to, the gain structure 12 in any conventional manner and is configured, i.e. shaped and dimensioned, to define a gap for receiving the mirror structure 14, the gap providing the external cavity of the device 10, including the gap 16 between the structures 12, 14. Preferably the spacer 13 comprises SU-8 photoresist which is spun on the surface of the gain structure 12 and subsequently structured using standard photo-lithographic techniques. The thickness of the SU-8 photoresist layer is determined by the type of photoresist and the spinner speed. The thickness of the spacer 13 depends on the desired cavity gap 16 and the height of the structures 12, 14, 40 and on the assembly geometry. The mirror structure 14 is coupled to the spacer layer 13, the support structure 40 and the membrane 41. The structure 40 may be fixed to the spacer 13 in any conventional manner, e.g. by bonding or gluing as is convenient. Typically, the integration step to permanently connect the gain structure 12 and the support structure 240 is part of the photolithographic process used to form and define the spacer layer, .e.g. the final baking step.

In alternative embodiments (not shown) the structure 40, which may be made of a single solid material, e.g. aluminium, and the mirror structure 14 are fixed to each other by any convenient conventional means. The structure 40 may be connected to a macroscopic actuating device, typically a piezoelectric actuator (e.g. comprising a piezoelectric crystal), which itself is connected to the structure 12, or to an additional support structure (not shown), which may be made of a single solid material, e.g. aluminium, and is fixed to structure 12 by any convenient conventional means, effectively making the structure 40 and thus the mirror structure 14 moveable with respect to the gain structure 12 as described above.

In use, the relative movement between the gain structure 12 and the mirror structure 14 may be oscillatory with an amplitude of, typically, between 0.5 pm and 3 pm and a frequency that may be in the order of Hz up to kHz, e.g. from 1Hz to 200Hz, depending on the application. Moving the mirror structure 14 relative to the gain structure 12 (and in particular relative to the mirror 18) allows the wavelength of the VECSEL device 10 to be tuned continuously without mode-hop. It will be understood that the amplitude and/or frequency of the movement of the mirror structure 14 may vary from the ranges stated herein depending on the application.

In embodiments where the mirror structure 14 is movable for the purposes of tuning, typical additional requirements for the movable structure(s) 40, 41, as applicable, may include any one or more of the following: vertical movement (i.e. in the direction A shown in Figure 1) of approximately 1 to 5pm, typically approximately 2 pm; ability to carry the mirror structure 14 and to impart oscillatory movement up to the order of kHz, e.g. at approximately 1 kHz; facilitating mechanical stability of the mirror structure 14 (e.g. causing or allowing little or no bending of the external mirror, optionally allowing a small tilt); little or no absorption of the pump light 39; little or no feedback from any back-surface.

Some or all of these design requirements may be achieved by preferred embodiments of the invention in which the movable structure 40 includes or is connected to the membrane 41 in particular a MEMS membrane. One option for reducing or eliminating feedback from a surface (e.g. the top or bottom surface as viewed in Figure 1) of the structure 40 and/or membrane 41 is to provide it with an anti-reflective coating. However, this is difficult to achieve with sufficient quality for mode-hop-free continuous wave tuning on a MEMS membrane, especially while making the thickness of the membrane with the required precision. In preferred embodiments therefore the membrane 41 is shaped to define an aperture 52 through which the mirror structure 14 is exposed. The aperture 52 is aligned in use with the pumping light source so that the pumping beam 39 passes through the aperture 52 and impinges on the mirror structure 14. In preferred embodiments, it is the reverse face of the substrate 50 that is exposed by the aperture 52 and upon which the pumping beam 39 impinges during use. Typically, the aperture 52 is substantially centrally located with respect to the mirror structure. The aperture 52 is preferably centred on the optical axis of the VECSEL device 10. Advantageously, the provision of the aperture 52 means that the membrane 41 serves only as a support for the mirror structure 14 and does not interfere with the optical operation of the VECSEL. In alternative embodiments, the aperture 52 may be replaced by an alternative formation that is highly transmissive to the pump beam 39.

In preferred embodiments, the thickness of the substrate 50 is selected to reduce weight to allow the 15 required oscillatory movement of the mirror structure 14, 114, 214.

The membrane 41 may be formed from any suitable material, typically a semiconductor material, for example Si or SiO2. In preferred embodiments, the membrane 41, 241 is formed from a Silicon on Insulator (S01) substrate, preferably a doped SOI substrate.

Referring now in particular to Figure 2, the preferred gain structure 12 and mirror structure 14 are described in more detail. The substrate 22 may be formed from any suitable material, for example a semiconductor material, e.g. silicon. The DBR 18 is provided on the substrate 22, and the active layer 20 is provided on the DBR 18. The DBR 18 comprises multiple layers 19A, 19B of alternating first and second semiconductor materials with a different refractive index (such that the refractive index of the layers alternates between relatively high and relatively low), preferably type IV-VI semiconductor materials. For example, the layers 19A, 19B may alternately comprise PbEuTe and EuTe. In the illustrated embodiment there are four pairs of layers 19A, 19B, although in alternative embodiments there may be more or fewer pairs. It is noted that there does not have to be an even number of layers in the mirror 18. The layers 19A, 19B may be deposited, grown or otherwise provided on the substrate 22 in any suitable conventional manner, e.g. grown by MBE. The optical thickness of each layer 19A, 19B of the DBR 18 is typically one quarter of the design wavelength, although this may vary depending on the application.

The preferred active layer 20 comprises at least one QW layer 24 of PbSe sandwiched between two layers 26, 28 of PbSrSe. Optionally, the active layer 20 comprises a multiple QW structure comprising a plurality of PbSe QW layers sandwiched between PbSrSe barrier layers and/or PbSrSe outer layers. The barrier and outer layers are typically thicker than the QW layers. For example, a preferred embodiment comprises first and second outer PbSrSe layers (one typically having a thickness of 200 nm and the other typically having a thickness of 25nm) with a central PbSrSe spacer layer (typically having a thickness of 200nm) being arranged between the PbSrSe outer layers. A plurality of PbSe QW layers (typically having a thickness of 5nm or approximately 5 nm) and PbSrSe barrier layers (ideally having a thickness of 15 nm or approximately 15 nm) are provided on either side of the central PbSrSe spacer layer such as to define seven PbSe QWs on one side of the central spacer layer and five PbSe QWs on the other side of the central PbSrSe spacer layer, with it being preferable that the seven PbSe QW layers are sandwiched between the central PbSrSe spacer layer, and the PbSrSe outer layers preferably having a thickness of 200 nm. In alternative embodiments more or fewer QW layers may be sandwiched between the outer layers.

The centre emission wavelength, i.e. the design wavelength, is determined by the thickness of the QW layer(s) and the material composition of the host (i.e. the layers of PbSrSe in this example). In preferred embodiments, the configuration is such that the centre wavelength is between 3.0 pm and 4.5 pm when operated at room temperature. The active layer 20 may for example have an optical thickness approximately equal to a multiple of the half of the centre wavelength. A preferred optical thickness is approximately equal to the centre wavelength. The active layer 20 may be deposited, grown or otherwise provided on the substrate in any suitable conventional manner, e.g. grown by MBE.

To improve the effective gain, it is preferred that the quantum wells in the QW layer are placed where the standing electric field is high. This is normally at the centre and the edges of the active layer 20.

In optically pumped embodiments, the pump beam 39 intensity and therefore the carrier generation rate is the highest near the front surface. Thus, the quantum wells are preferably located at the centre and near the surface of the active layer 20 to distribute the generated carriers evenly among the QW. The configuration depends on the total thickness of the active layer 20.

In some embodiments a conventional anti-reflection or high-reflection optical coating is provided on top of the active layer 20, i.e. between the active layer 20 and the gap 16.

The mirror 32 of the mirror structure 14 comprises at least one pair, but typically a plurality of pairs of layers 46A, 46B, each pair comprising a layer 46A of a first dielectric material and a layer 46B of a second dielectric material, the first and second materials having a different refractive index. For example, the first material may be silicon (Si) and the second material may be silicon oxide (SiO) or silicon dioxide (SiO2). In the illustrated embodiment, the mirror 32 comprises 5 such pairs. In alternative embodiments there may be more or fewer pairs of layers 46A, 46B. It is also noted that there does not have to be an even number of layers in the mirror 32, i.e. there does not have to be a whole number of pairs of layers 46A, 46B. As such the mirror 32 may be said to comprise multiple layers 46 with an alternating refractive index (alternating between relatively high and relatively low). The optical thickness of each layer 46A, 46B of the mirror 32 is typically one quarter of the design wavelength, although this may vary depending on the application.

The gain of the quantum wells is relatively high for the design wavelength and decreases towards shorter and longer wavelengths. Laser emission only occurs when the gain is higher than a lasing threshold. If only one longitudinal mode, as defined by one or more characteristics, e.g. dimensions, of the cavity, meets the threshold criterion, then emission is in one longitudinal mode only. In preferred embodiments, to reach single-mode emission, only one longitudinal mode is supported by the gain structure 12. This may be done by increasing the free spectral range, the distance between two longitudinal modes, with a short cavity of approximately 50 pm in length. The small cavity length promotes single-mode operation, avoiding mode hops. By way of example, in embodiments using active layers 20 based on PbSe QW in PbSrSe host material, single-mode operation near room temperature can be achieved. The maximum operation temperature for laser emission is typically above 50 °C. Alternatively, multiple longitudinal modes may be supported.

Typical design requirements for the mirror structure 14 may include any one or more of the following: to act as a broadband highly reflective mirror, for example with theoretical reflectivity R of approximately 99.9% for the design wavelength, and preferably R of approximately 99.8 % for +1-150 cm-1; to allow greater than 50% transmittance of the pumping light 39, preferably with no or very low absorbance; relatively little back-reflection from any substrate into the cavity to avoid mode-hops or distorted spectral beam shapes.

In alternative embodiments, the gain structure 12 may be replaced with any other laser gain structure that is suitable for vertical emission, e.g. having an active layer comprising an interband cascade laser (ICL) chip or other laser chip. Such laser chips may use any suitable semiconductor material(s), e.g. III-V semiconductors in the case of an ICL chip. The laser chip may be electrically pumped or optically pumped as appropriate. The membrane does not require a central aperture in electrically pumped embodiments. By way of example, "Room-temperature mid-infrared interband cascade vertical-cavity surface-emitting lasers" by W. W. Bewley et al, APPLIED PHYSICS LETTERS 109, 151108 (2016), discloses a vertical-cavity surface emitting laser that includes an ICL chip that is suitable for use with embodiments of the invention.

Figure 4 shows an alternative VECSEL device 110 embodying the invention, in respect of which like numerals indicate like parts and the same or similar description applies as provided in relation to the 30 VECSEL 10 of Figures 1 and 2, as would be apparent to a skilled person, unless otherwise indicated. The VECSEL device 110 has an external mirror structure 114 which may be the same as the mirror structure 14. The gain structure 112 comprises a dielectric mirror 118, preferably a distributed Bragg reflector (DBR), on top of which is provided an active layer 120. The active layer 120 comprises ICL stages, and may be described as an ICL chip. The ICL stages comprise layers of semiconductor material, typically type III-V semiconductor material (e.g. InAs, GaSb and AISb), arranged to act as an ICL. Typically, the ICL chip comprises multiple cascade, or active, stages sandwiched between first and second separate confinement layers (SCLs).

The dielectric mirror 118 may take any suitable conventional form, for example being the same or 40 similar to the mirror 18. The mirror 118 comprises multiple layers 119A, 119B of alternating first and second semiconductor materials with a different refractive index (such that the refractive index of the layers alternates between relatively high and relatively low), preferably type III-V semiconductor materials. For example, the layers 119A, 1198 may alternately comprise GaSb and AIAsSb. In the illustrated embodiment there are four pairs of layers 119A, 1198, although in alternative embodiments there may be more or fewer pairs. It is noted that there does not have to be an even number of layers in the mirror 118. The mirror 118 is provided on a substrate 156, which may be made from any suitable material, e.g. GaSb or InAs. Optionally, a buffer layer 122 is provided between the substrate 156 and the mirror 118. The buffer layer 122, which may be formed from any suitable optically transparent material, reduces strain that may be induced due to lattice mismatch of the substrate 156 and the mirror 118.

The preferred ICL gain structure 112 is electrically pumped and to this end includes a first electrical contact 154 at the top of the structure 112, and a second electrical contact at the bottom of the structure 112. In this example, the electrical contact 154 is provided on top of an electrically insulating layer 152 that is provided on top of the gain structure 112. The substrate 156 may serve as the second electrical contact, or the second electrical contact may be provided beneath the substrate (e.g. provided on the underside of the substrate 156). The first and second contacts 154 may be formed from any suitable electrically conductive material, e.g. metal or conducting semiconductor. In the case where the substrate 156 serves as the second contact, it is doped such that it is electrically conductive. The first contact 154 is annular, defining an aperture 155 that is aligned with the optical axis of the device 110 so that the generated laser light may pass through the contact 154. The first and second contacts 154 may be formed in any conventional manner, e.g. deposited or electroplated as applicable. Optionally, an optically transparent (i.e. optically transparent at the design wavelength) element or contact 158 or is provided to cover the aperture 155. The contact 158 may be made from any suitable material (e.g. Ge or other suitable semiconductor) and may be provided in any convenient manner, e.g. deposited as a layer or film. The optical contact 158 may also serve as an anti-reflective coating.

In use an electrical source (not shown) is connected to the contacts 154, 156 (by any conventional contact means (not shown) to cause current to flow through the gain structure in a direction substantially parallel with the optical axis. The generated laser light 136 may be emitted from the bottom of the gain structure 112, and/or from the top of the mirror structure 114 (shown as 136' in Figure 4). In typical embodiments, light 136, 136' is emitted in both directions. The exiting light intensities depend on the mirror reflectivities, with more light exiting where there is less reflection.

It is desirable for the VECSEL devices 10, 110 to generate single-mode and single-polarization emission. However, such VECSEL devices do not naturally have a geometrically defined single-polarization orientation. As a result the polarization of the emitted light 36, 136 can spontaneously rotate its geometric orientation, which is undesirable.

To address this problem, one or more sub-wavelength grating is provided on the top, or outer, surface of the gain structure 12, 112, i.e. the surface of the gain structure 12, 122 that faces the mirror structure 14, 114. The grating is provided as a surface relief grating, i.e. projecting from the top surface. For example, the, or each, grating may be formed on the outer surface of the contact 158, or on the outer surface of the active layer 120 if the contact 158 is not present. The sub-wavelength grating may optionally be referred to as a sub-wavelength diffraction grating.

Advantageously, the, or each, grating is configured to be non-absorbing of light at the design wavelength(s) (typically the centre wavelength of the emission light 36, 136). This may be achieved by forming the grating(s) from one or more material that is non-absorbing at the design wavelength(s). For example, the grating(s) may be formed from silicon, or silicon oxide, or any suitable semiconductor material (with a sufficiently large band gap). In preferred embodiments, the grating(s) is configured to absorb no light, or substantially none, at the design frequency/wavelength. In preferred embodiments, the grating(s) is non-absorbing in that it absorbs less than 5%, preferably less than 1%, and more preferably less than 0.1%, of light at the design frequency/wavelength.

The surface relief grating projects from the surface of the gain structure in a vertical direction (as viewed in the drawings) that is parallel with the optical axis of the laser, i.e. the axis along which beam 36, 136 is emitted. The grating (in particular the solid bars of the grating) are formed from a material having a first refractive index. At least part of, and preferably all of, the neighboring region that surrounds (at least transversally, i.e. in directions perpendicular to the optical axis, and optionally at the top, i.e. opposite the gain structure surface in the vertical direction) the grating has second refractive index, the second refractive index being lower than the first refractive index. In typical embodiments, the grating is formed from a material having a refractive index of 2 or higher, while the neighboring region may have a refractive index of lower than 2. The spacings between the bars of the grating may be in vacuum, or filled with a gas (e.g. air), a liquid or a solid, and exhibit a refractive index that is lower than the refractive index of the grating bars. The grating spacings and the neighboring region may be integral with one another, e.g. all comprising free space (filled with a gas, e.g. air, or in vacuum) or filled with liquid or a solid, or may be filled with different materials.

More generally, the neighboring region and/or the inter-bar spacings of the grating comprises a vacuum region, or a gas-filled region, or a liquid-filled region, or a solid region, wherein the neighboring region and the inter-bar spacings may be integral with one another or separate, and wherein the neighboring region and the inter-bar spacings have a lower refractive index than the refractive index of the grating. In preferred embodiments, the neighboring region surrounds the grating over substantially the entire vertically extending height of the grating, preferably encircles the grating transversally completely and optionally covers the top of the grating.

It is noted that the refractive index of some materials varies with temperature and/or wavelength and it will be understood that the refractive indices referred to herein are those applicable during operation of the laser device, e.g. when the device is pumped and lasing.

Referring to Figure 3A and 3B, a sub-wavelength, surface relief grating 60 is provided on the top surface of the gain structure 12 (only part of which is shown in Figure 3B). Preferably, the grating 60 is provided directly on the top surface of the gain structure 12. The grating 60 comprises a plurality of spaced-apart parallel bars 62, usually of equal width. The bars 62 may for example be formed from silicon, germanium, silicon oxide or any suitable semiconductor material with a sufficiently large bandgap, or any other material that is non-absorbing of light at the design wavelength. The spaces between the bars 62 may be filled with free space, e.g. air or vacuum, or may be filled with a material having a refractive index lower than the refractive index of the bars 62. The bars 62 are formed in relief from the top surface of the gain structure 12 and as such project from the top surface. In preferred embodiments, the grating 60 is substantially centrally aligned with the optical axis of the device 10.

The period A. of the grating 60 is the periodic distance between adjacent bars 62. As the grating 60 is a sub-wavelength grating, the grating period X is smaller than the design wavelength, the design wavelength being the target wavelength of the laser emission, typically the centre wavelength of the emission light 36. In particular, the grating period is physically and optically shorter than the center wavelength of the output laser beam 36. The duty cycle of the grating 60 is the relative width w of the bars 62 to the width of the spacing between the bars 62. In preferred embodiments the duty cycle is 50%, meaning that the bars 62 and spacings are of equal width. Typically the grating 60 has a duty cycle of 30-70%. It will be understood that there are many combinations of period and duty cycle that may be used to achieve the desired result.

By way of example, in typical embodiments where the emission light 36 has a centre wavelength of 3-5p.m, the grating period X may be 100nm. Wth the preferred duty cycle of 50%, the width w is 50nm, as is width of the inter-bar spacings. By way of example, the height h of the bars 62 may be approximately 10-50nm. As is described in more detail hereinafter, it is desired that the grating 60 is selective between TE (transverse electric) polarized light and TM (transverse magnetic) light, which are disposed perpendicularly to each other. For the TE light, the grating 60 acts as a solid, or full, mesa with reduced optical losses. However, the grating 60 does not affect the TM light and so optical losses are high. For any given embodiment, a range of values for the grating period and duty cycle may be suitable, and conveniently dimensions that facilitate fabrication within acceptable tolerances may be chosen. Typical tolerances may be up to 10%. In an array of gratings, there may be multiple combinations of A and w to account for errors in h. The grating 60 has a width dl. In preferred embodiments, the grating 60 is substantially circular (in cross-section taken along the optical axis) and so the width dl corresponds to the diameter of the grating 60. The grating 60 may take any other cross-sectional shape, e.g. rectangular or polygonal, that covers the cross section of the generated laser beam 36, 136. By way of example the value of dl may be 10-30iam, which corresponds approximately to the spot (cross-sectional) size of the emission beam 36, 136. Typically, the width dl depends on the wavelength of the beam 36, 136 and on the cavity length. It is preferred that the width dl is large enough to fit (all of or most of) the beam of the first transversal mode in it, but small enough to induce losses in the larger second and higher order transversal modes.

Optionally, a plurality of gratings 60 may be provided on top, or outer, surface of the gain structure 5 12, 112, for example arranged in an array spaced apart on the surface. Any one or more of the respective characteristics (e.g. dl, h, 2. and/or the duty cycle) of the gratings 60 may be the same as each other, or may be different from each other (either by design or because of manufacturing tolerances). During fabrication, the pumping beam 39 is aligned with one of the gratings 60 and provision of multiple gratings provides flexibility in this regard. Also, providing gratings 60 with different characteristics increases the option of having a grating 60 with characteristics that are best suited to the device 10, 110.

In the embodiment of Figures 3A and 3B, the bars 62 of the grating 60 extend across the whole surface area covered by the grating 60.

Figure 3C shows an alternative embodiment of the sub-wavelength surface relief grating, generally indicated as 60', which may be the same as the grating 60 except that it has a solid centre portion 64 (i.e. a central portion formed from solid material without spaced apart bars 62), which may be formed from the same material as the bars 62. The width d2 of the portion 64 may depend on the wavelength and the cavity length. It is preferred that d2 is large enough to fit (most of) the beam of the first transversal mode in it, but small still have losses in the larger second and higher order transversal modes. A typical size of d2 is approximately half of dl.

The disc-like grating 60 of Figures 3A to 3C is particularly suited for use with laser devices having 25 active layers formed from IV-VI semiconductor material.

Figure 5A an alternative embodiment of the sub-wavelength surface relief grating, generally indicated as 160, which is annular or ring-like in shape, i.e. the grating has a central portion 166 that does not include the bars 62, and which is preferably not filled with solid material, or at least not the same solid material from which the bars 62 are formed. Preferably the central portion 166 is filled with a gas or a vacuum. The grating 160 may otherwise be the same as or similar to the grating 60 and so the same or similar description applies as would be apparent to a skilled person. However, in some applications the grating 160 is wider than the grating 60. For example the diameter dl may be approximately 60um, while the diameter d3 of the inner aperture 166 defined by the grating 160 may be approximately 404m.

The annular grating 160 is particularly suited for use with electrically pumped laser devices, including ICL based laser devices such as the VECSEL device 110 of Figure 4. Figure 5B shows the grating 160 provided on the top surface of the gain structure 112, in particular on the surface of the optically transparent contact 158. The annular grating 160 of Figures 5A and 5B is particularly suited for use with laser devices having active layers formed from III-V semiconductor material.

Any suitable conventional processing techniques such as lithography and silicon and silicon oxide etching may be used to form the surface relief gratings. The preferred lithography process for forming the surface relied grating involves the following steps: apply photoresist; structure the photoresist using an e-beam writer (or other suitable photoresist structuring tool) according to the desired grating geometry; develop the structured photoresist; deposit silicon on top of the developed photoresist; remove photoresist with silicon, leaving silicon where no photoresist was.

Advantageously, gratings 60, 60', 160 embodying the invention work by exploiting losses, in particular optical or absorption losses, that occur in the active layer 20, 120 and/or as a result of the contacts 154, 158. The losses may for example result from anti-guiding in the active layer, but may alternatively result from any other loss mechanism(s) that may be present, especially those that feature increasing losses radially towards the outside of the device 10, 100. In embodiments where the active layer comprises IV-VI semiconductor material, the losses tend to result from anti-guiding in the active layer. For ICL based active layers, one polarization experiences higher losses from the metal contact 154 and is thus suppressed.

In use, the gain structures 12, 112 have the inherent ability to produce light in first and second different orientations, namely TE polarized light and TM polarized light. The sub-wavelength grating 60, 60', 160 exhibits a respective different effective refractive index for TE and TM polarized light. The alignment of the grating 60, 60', 160 on the gain structure 12, 112 can be arbitrary, as the preferred structure 12, 112 is rotationally symmetric. The polarization is then aligned parallel or perpendicular to the bars depending on configuration.

The interaction between the grating 60, 60', 160 and TE and TM polarized light is therefore different. The grating 60, 60', 160 has a different effect on the optical field of the light of each orientation (or polarization), more particularly on the intensity, or amplitude, of the optical field in a direction perpendicular to the optical axis. In particular, the grating 60, 60', 160 has an anti-guiding, or dispersing, affect on the light of one orientation/polarization (it may be either of the first or second orientations/polarizations depending on the embodiment) such that it has a higher intensity in a non-central region of the device 10, 110 (typically an annular region that is radially or perpendicularly spaced from the optical axis) than the other orientation/polarization. The non-central region of the device 10, 110 is more strongly light-absorbing at the design frequency/wavelength than the central region within it. As a result, the light of the orientation/polarization with higher intensity in the non-central region experiences higher absorption losses and is therefore relatively suppressed. In embodiments where the active layer 20, 120 comprises IV-VI semiconductor material, the non-central region corresponds to the non-pumped region of the active layer, or the region of the active layer that is beyond the grating 60, 60', 160 with respect to the optical axis in the lateral, or radial, direction. The lateral distribution of the pump beam is of a Gaussian shape. Accordingly the pumping level (which corresponds to the gain) decreases laterally with a similar shape, and at some lateral point it is just below threshold for lasing. The emitted laser beam also has a Gaussian shape.

Effectively the overlap of the pump beam and emitted laser beam is better or worse for each polarization.The non-uniformity in absorption may be caused by optical pumping, or other pumping, e.g. electrical pumping. When pumping at relatively high light intensities (typically relative to the laser intensity), electronic states in the semiconductor bands of the active layer materials are filled and the material is much less absorbing for additional light. For ICL based active layers, the non-central region may comprise the metal contact 154. More generally, the material from which the gain structure 12, 112 is formed is such that the optical absorbency of the gain structure 12, 112 at the design frequency increases in a lateral or radial direction away from the optical axis of the device 10, 110, typically in a region of the gain structure 12, 112 that is located below the grating 60, 60', 160, 10 e.g. in the active layer 20, 120 and/or the contact 154 as applicable.

Accordingly, for one orientation of the light (it may be either of the first or second orientations depending on the embodiment), the losses are increased by the presence of the grating 60, 60', 160, whereas for the other orientation of light the losses are reduced. As a result, the polarized light that experiences the increased losses is suppressed and tends to be eliminated such that it does not contribute to the emission beam 36, 136, and so is not lasing. Hence, the other (non-suppressed) polarized light provides the emission beam 36, 136 or lasing mode. For the other (non-suppressed) polarized light the losses are not increased, resulting in no, or only slightly, increased lasing threshold of the overall device 10, 110 in comparison with a standard laser device. Advantageously, for the other (non-suppressed) polarized light, the grating 60 acts as a solid mesa structure (i.e. a solid raised structure in relief from the relevant surface), which helps to confine the light transversally and minimizes leakage.

Accordingly, the selection of polarization of the emission light 36, 136 is stable, and switches 25 between polarizations can be eliminated.

The grating 60' with the solid central region 64 improves, and typically maximizes, the separation of TE and TM polarized light, particularly for the TEMOO lasing mode. Because the solid portion 64 is in the centre of the grating 60', it reduces losses over-proportionally to the first order transversial mode TEMOO. When using the solid centre 64, the lasing threshold of TEMOO is essentially that of a fully solid mesa. When the solid centre 64 is not present, the lasing threshold is higher and the difference with respect to the second order transversal mode is smaller. However, increasing the width of the centre 64 reduces the stability of the polarization. There is therefore a trade-off between the width of the solid centre 64 and no polarization switching.

It is noted that the number of grating bars shown in the drawings is intended to be illustrative only and is not intended to represent the actual number of bars in any given embodiment.

The invention is not limited to the embodiment(s) described herein but can be amended or modified 40 without departing from the scope of the present invention.

Claims (20)

1. CLAIMS1. A vertical surface emitting laser device comprising: a gain structure comprising a first mirror structure and an active layer; and a second mirror structure aligned with said first mirror structure to define an optical cavity between the first and second mirror structures, wherein said gain structure has a surface facing said second mirror structure, and wherein at least one sub-wavelength grating is provided on said surface, said at least one sub-wavelength grating being non-absorbing to light at a target laser emission wavelength of said laser device, and wherein at least part of said gain structure has an optical absorbency that increases in a direction laterally away from an optical axis of the device.
2. 2. The laser device of claim 1, wherein said at least one grating is formed in relief from said surface.
3. 3. The laser device of claim 1 or claim 2, wherein at least one of said at least one gratings is aligned with, or centred on, an optical axis of said laser device.
4. 4. The laser device of any preceding claim, wherein said at least one grating comprises a plurality of gratings provided on said surface.
5. 5. The laser device of any one of claims 1 to 3, wherein said at least one grating comprises only one grating.
6. 6. The laser device of any preceding claim, wherein said surface is a surface of the active layer.
7. 7. The laser device of any of claims 1 to 5, wherein an optically transparent element is provided on said active layer, and said surface is a surface of said optically transparent element.
8. 8. The laser device of any preceding claim, wherein the, or each, grating is formed from material 30 that is non-absorbing to light at a target laser emission wavelength of said laser device.
9. 9. The laser device of any preceding claim, wherein the, or each, grating comprises a plurality of spaced apart bars, typically formed from semiconductor material.
10. 10. The laser device of claim 9, wherein said bars extend across a surface area covered by said grating.
11. 11. The laser device of claim 9, wherein the grating has a centrally located portion formed from a solid material, preferably the same material from which said bars are formed.
12. 12. The laser device of claim 9, wherein the grating is annular.
13. 13. The laser device of claim 12, wherein the grating has a centrally located portion without said bars or other solid material.
14. 14. The laser device of any one of claims 9 to 13, wherein said bars are formed from material having a first refractive index, and wherein the spaces between said bars is formed from material having a refractive index that is lower than said first refractive index.
15. 15. The laser device of any one of claims 9 to 14, wherein said bars are formed from material having 10 a first refractive index, and wherein at least part of, and preferably all of, a traversally neighboring region around the grating has a refractive index lower than said first refractive index.
16. 16. The laser device of any preceding claim, wherein said second mirror structure is an external mirror structure spaced apart from the gain structure to define an external cavity between said 15 external mirror structure and said gain structure, said external cavity being at least part of said optical cavity.
17. 17. The laser device of any preceding claim, wherein said at least part of the gain structure is located beneath said at least one sub-wavelength grating.
18. 18. The laser device of any preceding claim, wherein said at least part of the gain structure includes a non-central, preferably annular, region that is laterally spaced apart from the optical axis and has an optical absorbency that is higher than an inner central region.
19. 19. The laser device of any preceding claim, wherein said at least part of the gain structure comprises said active layer and/or an electrical contact provided on said gain structure.
20. 20. A gain structure for a vertical surface emitting laser device, said gain structure comprising a first mirror structure and an active layer, wherein at least one sub-wavelength grating is provided on a surface of said gain structure, said sub-wavelength grating being non-absorbing to light at a target laser emission wavelength, and wherein at least part of said gain structure has an optical absorbency that increases in a direction laterally away from an optical axis of the structure.