GB2582378A - Vertical external cavity surface emitting laser with improved external mirror structure - Google Patents

Abstract

A vertical surface emitting laser (VECSEL) device 10 has a gain structure 12 comprising a first mirror 18 (for example, a Bragg mirror DBR) and an active layer 20 (or optical gain layer); and an external mirror structure 14 spaced apart from the gain structure lo define an external cavity. The external mirror 14 structure may comprise a sub-wavelength high contrast grating (HCG) and a reduced-layer distributed Bragg mirror (DBR) provided together on a flat multi-layer chip. The HCG may have a grating period smaller than a target laser emission wavelength of the laser device. The combined HCG-DBR may facilitate polarization selectivity, which improves the quality of the laser light emitted by the VECSEL.

Description

Vertical External Cavity Surface Emitting Laser with Improved External Mirror Structure

Field of the Invention

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

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.

It would be desirable to provide an improved VECSEL, in particular a VECSEL that emits laser light 20 with improved quality compared to known VECSELs.

Summary of the Invention

The invention provides a vertical surface emitting laser device comprising: a gain structure comprising a first mirror and an active layer; and an external mirror structure spaced apart from the gain structure to define an external cavity, wherein the external mirror structure is aligned with said first mirror to form an optical cavity that comprises said external cavity, and wherein said external mirror structure comprises a sub-wavelength grating and a second mirror 30 provided together on a substrate, said sub-wavelength grating having a grating period that is smaller than a target laser emission wavelength of the laser device.

In preferred embodiments, the sub-wavelength grating is a high contrast grating (HCG).

Typically, said second mirror and said sub-wavelength grating are formed as a multi-layer chip, preferably a flat multi-layer chip.

Preferably, the sub-wavelength grating is a multi-layer structure comprising at least one layer of a dielectric material. Typically, the sub-wavelength grating is formed from multiple layers of any one or 40 more of silicon, silicon oxide or silicon dioxide. Preferably, the sub-wavelength grating comprises a first layer and a second layer, the first layer having a higher refractive index than the second layer.

The first layer is preferably located between the substrate and the second layer. In some embodiments, the first layer is formed from silicon and the second layer is formed from silicon oxide or silicon dioxide.

In preferred embodiments, the second mirror is located between said sub-wavelength grating and said gain structure.

Preferably, at least in an axial direction of the device, said sub-wavelength grating is located between and is in contact with one or more layers of solid material. In preferred embodiments, a first 10 side of said sub-wavelength grating said solid material is provided by said second mirror. Preferably, on a second side of said sub-wavelength grating said solid material is provided by said substrate.

In preferred embodiments, said second mirror comprises multiple layers formed alternately from a first dielectric material and a second dielectric material, the first and second materials having a 15 different refractive index. The first material may be silicon oxide or silicon dioxide. The second material may be silicon.

In preferred embodiments, said second mirror is a dielectric mirror. Advantageously, said second mirror comprises a reduced-layer Bragg mirror or reduced-layer distributed Bragg mirror. Preferably, said second mirror has at least two layers of dielectric material, and at most 11 layers of dielectric material. Said second mirror may provide a reflectivity of up to 95%, preferably between 70% and 90%.

Advantageously, said second mirror and said sub-wavelength grating together provide a reflectivity 25 of more than 95%, preferably between 95% and 99.9%.

In preferred embodiments, said substrate is formed from an optically transparent material, preferably material that is optically transparent to a light beam with which said laser device is optically pumped in use.

Advantageously, said substrate is formed from a material that absorbs light at said target laser emission wavelength, preferably at least 30% of light at said target laser emission wavelength.

Preferably, said substrate is formed from a material having a low refractive index, preferably 2 or 35 lower. Said substrate may be formed from borosilicate glass.

In preferred embodiments, said external mirror structure is movable with respect to said gain structure, and is coupled to at least one movable structure that facilitates movement of said external mirror structure towards and away from said gain structure.

Optionally, said external mirror structure is coupled to a flexible membrane that allows movement of said external mirror structure towards and away from said gain structure. Said membrane may comprise a MEMS membrane.

In typical embodiments, the laser device includes at least one actuator or actuation system for causing said movement of said external mirror structure. Said at least one actuator or actuation system may be configured to impart oscillatory movement to said external mirror structure. Said at least one actuator may comprise a piezoelectric actuating device.

In typical embodiments, said gain structure and said external mirror structure are spaced apart by at least one layer of spacer material. Said at least one spacer layer is conveniently shaped and dimensioned to define said external cavity, and said at least one external mirror structure may be located in said external cavity. Conveniently, said at least one spacer layer supports said membrane.

Typically, said laser device includes means for optically pumping the device along an optical axis.

Advantageously, said membrane is shaped to define an aperture through which the external mirror structure is exposed. Typically, said substrate is exposed by said aperture. Preferably, said aperture is aligned with said optical axis.

In typical embodiments, said active layer is formed from at least one IV-VI semiconductor material.

Preferably, said active layer comprises at least one quantum well layer. In preferred embodiments, the active layer comprises a multiple quantum well structure comprising a plurality of quantum well layers, each being sandwiched between barrier layers and/or outer layers. Preferably, said active layer comprises first and second outer layers, with a central spacer layer arranged between the outer layers, a plurality of quantum well layers and barrier layers disposed either side of the central spacer layer so as to provide seven quantum wells on one side of the central spacer layer and five quantum well layers on the other side of the central spacer layer, the seven quantum well layers being sandwiched between the central spacer layer and the respective outer layer. The quantum well layer(s) may comprise PbSe, and said barrier, outer and central spacer layers comprise PbSrSe.

In preferred embodiments, said sub-wavelength grating and said second mirror together comprise multiple layers of dielectric material having alternately a relatively high refractive index and a 35 relatively low refractive index.

In contrast with known VECSELs that have a full Distributed Bragg Reflector (DBR) as an external resonator, preferred embodiments of the present invention have an external resonating structure comprising a High Contrast Grating (HCG) combined with a reduced, or relatively thin, DBR. The main advantage of using a combined HCG-DBR is polarization selectivity, which improves the quality of the laser light emitted by the VECSEL.

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

Brief Description of the Drawings

Embodiments of the invention are now described by way of example and with reference to the accompanying drawings in which like numerals are used to denote like parts and in respective of 10 which the same or similar descriptions apply unless otherwise indicated and in which: Figure 1 is a schematic perspective view of a vertical external cavity surface emitting laser (VECSEL) device embodying the present invention; Figure 2 is an enlarged view of a gain structure and a mirror structure, being part of a first embodiment of a VECSEL device embodying the invention; and Figure 3 is a sectional side view of a second embodiment of a VECSEL device embodying the invention.

Detailed Description of the Drawings

Referring now to Figure 1 of the drawings there is shown, generally indicated as 10, 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 integral 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 (not shown in Figure 1), which may for example comprise one or more spacer elements or layer(s) for supporting the mirror structure 14 with respect to the gain structure 12.

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 (or 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)) on the substrate 22.

The active layer 20, which may be referred to as the active region of the laser, comprises at least one layer, but typically multiple layers, of semiconductor material(s), particularly 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 RT (room temperature). Preferably, 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.

The mirror structure 14 comprises a reflector which, in preferred embodiments, comprises a sub-wavelength high contrast grating (HCG) 30, and a dielectric mirror 32 (also known as a Bragg mirror). 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 grating 30 is located behind the mirror 32 with respect to the gain structure 12. In preferred embodiments, the grating 30 and the mirror 32 are formed together as a multi-layer chip, preferably a flat chip.

The use of the term sub-wavelength grating should be understood to mean that the distance of the grating period A (i.e. the periodic distance between adjacent grooves or bars of the grating, which is the same as the combined width of a bar and a groove) is smaller than the design wavelength such that the grating behaves as an anisotropic material. The design wavelength is the target wavelength of the laser emission, typically the centre wavelength of the emission light 36. The sub-wavelength grating 30 is preferably a High Contrast Grating (HCG), as is described in more detail hereinafter. As such, the grating 30 has a relatively large contrast in refractive index compared to its surroundings. In preferred embodiments the contrast in refractive index between the grating 30 and its surroundings (which is typically air) is at least 1.5:1, and is typically in the range 1.5:1 to 4:1. The grating 30 is formed from multiple layers of material, typically comprising dielectric material. For example the layers may be formed from any one or more of silicon, silicon oxide or silicon dioxide, or a combination of multiple materials. It is noted that, conventionally a conventional HCG consists of only one layer.

Preferably, the mirror 32 comprises part of a DBR, in particular a DBR that is thinner than a full DBR, i.e. comprising fewer layers 46. The layers 46 may be formed from any conventional material used in DBRs, e.g. silicon, silicon oxide, silicon dioxide or titanium dioxide. Adjacent layers 46 have a different refractive index, e.g. the mirror 32 may comprise alternating layers of a first dielectric material and a second dielectric material, the first and second materials having a different refractive index. A full DBR is a DBR having a reflectively R of approximately 99.9% for the design wavelength. The number of layers of material forming a full DBR may vary depending upon the refractive indexes (and/or other relevant characteristics) of the material(s) used for the layers.

Typically a full DBR comprises at least twelve layers. A thin DBR comprises a DBR having a reflectivity R of up to 95%, typically approximately 70% to 90%, for the design wavelength, the number of layers 46 depending upon the refractive indexes of the used materials. In preferred embodiments, the thin DBR 32 has only six layers 46, but may have more or fewer layers, e.g. between 2 and 8 layers. It will be understood that full and/or thin DBRs may comprise more or fewer layers than described above, and does not need to have an even number of layers. Both a full and thin DBR typically have a transmittance of > 80% for the wavelength used for optical pumping.

The gain structure 12 and the mirror structure 14 are spaced apart along an optical axis of the device 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 reflector 30, 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 mirror 18 and reflector 30, 32 together with the optical cavity provide an optical resonator for forming standing light waves 34 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 DBR 18 of the gain structure 12 is typically flat. The reflector 30, 32 of the mirror structure 14 is preferably flat but is optionally curved, e.g. comprising a spherical surface. Optionally, one or more of the layered structures 14, 32, 18, 20 may be formed to include a light guiding mesa, for example of 35 the type disclosed in W02014/180751.

In preferred embodiments, the VECSEL device 10 is optically pumped, in use, by a light source 38, typically comprising one or more laser diodes or other laser light source, preferably broad area laser diodes. The light source 38 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 38 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 38 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 38 and mirror structure 14. In alternative embodiments (not illustrated), the VECSEL may be electrically pumped.

In some embodiments, the mirror structure 14 is carried by a 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 see Figure 3 -or other suitable supporting structure). The 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 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, 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 2 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.

Referring now to Figure 2, in which like numerals are used to denote like parts and to which the same or similar description applies as provided above in relation to Figure 1 unless otherwise stated, a preferred gain structure 112 and mirror structure 114 are described. The gain structure 112 comprises a substrate 122, which may be formed from any suitable material, for example a semiconductor material, e.g. silicon, on which is provided a DBR 118 on top of which is provided an active layer 120.

The DBR 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 IV-VI semiconductor materials. For example, the layers 119A, 119B may alternately comprise PbEuTe and EuTe. In the illustrated embodiment there are four pairs of layers 119A, 119B, 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 layers 119A, 119B may be deposited, grown or otherwise provided on the substrate in any suitable conventional manner, e.g. grown by MBE. The optical thickness of each layer 119A, 119B of the DBR 118 is typically one quarter of the design wavelength, although this may vary depending on the application.

The preferred active layer 120 comprises at least one QW layer 124 of PbSe sandwiched between two layers 126, 128 of PbSrSe. Optionally, the active layer 120 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) and PbSrSe barrier layers (ideally having a thickness of 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 120 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 120 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 120. 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 120 to distribute the generated carriers evenly among the OW. The configuration depends on the total thickness of the active layer 120.

In some embodiments a mesa structure (not shown), for example of a type disclosed in W02014/180751, or a standard anti-reflection or high-reflection optical coating is provided on top of the active layer 120, i.e. between the active layer 120 and the gap 116.

The sub-wavelength grating 130, which in the preferred embodiment comprises an HCG, may be formed from at least partly from dielectric material. In preferred embodiments the grating 130 is formed from any one or more of silicon, silicon oxide, silicon dioxide, or from a combination of multiple suitable materials. In preferred embodiments, the grating 130 is carried by a substrate 150, preferably formed from an optically transparent material. The preferred optically transparent material has a low refractive index (preferably a refractive index of 2 or below) and absorbs light at least at a wavelength corresponding to the gain layer. 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. This is in contrast to the conventional approach of using ant-reflective coatings to avoid back-reflections. For example, the substrate 150 may be formed from glass, preferably borosilicate glass such as Pyrex® but it may alternatively be formed from 5102, BaF2 or any other suitable material having a low refractive index. The grating 130 may be formed, e.g. deposited or grown or transferred, on the substrate 150 in any conventional manner.

In a preferred embodiment the grating 130 comprises first layer 131A and second layer 131 B carried by the substrate 150, with the first layer 131A being located between the substrate and the second layer 131B. The first layer 131A preferably has a relatively high refractive index and the second layer 131 B has a relatively low refractive index. The first layer 131A preferably comprises Si, the second layer preferably comprising SiO or SiO2. The first grating layer 131A has a relatively high refractive index with respect to its surroundings which, in preferred embodiments, comprises the substrate 150, the second grating layer 131 B and air. The larger the relative refractive index contrast between the first layer 131A and its surroundings the better.

The layers 131A, 131 B may be created using conventional processing techniques such as lithography and silicon and silicon oxide etching to form the grating 130. The grating period is physically and optically shorter than the center wavelength of the output laser beam 36 (i.e. the design wavelength), typically around 1 pm for mid-infrared emission at 3 pm. The typical duty cycle is 50%, resulting in material bars of typical 500 nm width with the respective refractive index, and air/vacuum spacing of approximately the same width and the corresponding refractive index of 1.

The spacings between bars may alternatively be filled with a material of a different refractive index (typically equal to 1) than the bars, as would be understood by a skilled person.

Advantageously, in the vertical, or axial, direction the grating 130 is surrounded by, and is preferably in contact with (or sandwiched between), solid material, in particular the low refractive index substrate 150 and the layers of the mirror 132 (which is a thin, or reduced-layer, DBR in preferred embodiments). The grating 130 acts as wavelength and polarization dependent reflector. The specific configuration of the grating 130 depends on the application and is typically such that for the centre wavelength of the output beam 36 there is high (e.g. 99.9% for TE, or 99°,/o for TM) reflectance of either TE (transverse electric) or TM (transverse magnetic) polarized light, but significantly less reflectance to other polarization.

The mirror 132 of the mirror structure 114 comprises at least one pair, but typically a plurality of pairs of layers 146A, 146B, each pair comprising a layer 146A of a first dielectric material and a layer 146B 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 132 comprises 2.5 such pairs. In alternative embodiments there may be more or fewer pairs of layers 146A, 1468. It is also noted that there does not have to be an even number of layers in the mirror 132, i.e. there does not have to be a whole number of pairs of layers 146A, 146B. As such the mirror 132 may be said to comprise multiple layers 146 with an alternating refractive index (alternating between relatively high and relatively low). In general, the more layers 146 that are present in the mirror 132, the higher its reflectivity (with respect to the light reflected in the external cavity during use). However, the more layers that are present, the more difficult it is to fabricate the mirror 132. Advantageously, using a reduced number of layers 146 in the mirror 132 (i.e. a thin DBR in preferred embodiments) simplifies the fabrication process, while the resulting loss in reflectivity is compensated for by the sub-wavelength grating 130. In preferred embodiments, the configuration is such that the combined reflectivity of the mirror 132 and the grating 130 is equal to (or at least substantially equal to) the reflectivity of a full DBR. Advantageously, the grating 130, particularly when provided in the form of an HCG, provides polarization selection as indicated above.

The optical thickness of each layer 146A, 146B of the mirror 132 is typically one quarter of the design wavelength, although this may vary depending on the application. If required, the mirror 132 may include a phase matching layer (not shown) having a thickness selected to cause the phase of light passing between the mirror 132 and the grating 130 to match.

Conveniently, all layers of the mirror 132 and of the grating 130 are formed together using any suitable conventional fabrication techniques, e.g. the layers being deposited on a substrate in the appropriate sequence followed by a series of etch and bond processes to reach the final structure. Typically, an upside down process is used in which the layers are deposited on a silicon substrate, DBR layers first then HCG layers, following which the HCG may be created using standard silicon etching techniques, the resulting structure then being bonded to a second silicon substrate, the first substrate being completely removed by grinding and etching.

In preferred embodiments, the respective refractive indexes of the layers 131A, 131B of the grating and the layers 146A, 146B of the mirror 132 (i.e. the relevant layers of the external mirror structure 114) and of the substrate 150 alternate between relatively high (H) and relatively low (L) refractive index. Layer 131A has a high refractive index and so in the illustrated example the relative refractive index of the layers of the external mirror structure 114 alternative as follows: 150(L)-131A(H)-131A(L)-146A(H)-146B(L)-146A(H), and so on depending on how many layers there are in the mirror 132.

Referring now to Figure 3, in which like numerals are used to denote like parts and to which the same or similar description applies as provided above in relation to Figures 1 and 2 unless otherwise stated, there is shown an embodiment of the VESCEL device 210 in which the mirror structure 214, which includes the substrate 250, grating 230 and Bragg mirror 232, is carried by a flexible membrane 241 such that it is movable towards and away from the gain structure 212 as indicated by arrow A. Any convenient actuation system, e.g. a piezo-electric, electrostatic or comb-drive actuation system may be coupled to the membrane 241 to impart oscillatory movement to the membrane 241 and so the mirror structure 114 as described above in relation to the other embodiments. The membrane 241 is preferably a MEMS membrane and may be integrally formed with the support structure 240 that supports the mirror structure 214 with respect to the gain structure 212.

In this example, the support structure 240 is supported on the gain structure 212 by a spacer 213, 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 213 may be formed on, or fixed to, the gain structure 212 in any conventional manner and is configured, i.e. shaped and dimensioned, to define a gap for receiving the mirror structure 214, said gap providing the external cavity of the device 210, including the gap 216 between the structures 212, 214. Preferably the spacer 213 comprises SU-8 photoresist which is spun on the surface of the gain structure 212 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 213 depends on the desired cavity gap 216 and the height of the structures 212, 214, 240 and on the assembly geometry. The mirror structure 214 is coupled to the spacer layer 213, the support structure 240 and the membrane 241. The structure 240 may be fixed to the spacer 213 in any conventional manner, e.g. by bonding or gluing as is convenient. Typically, the integration step to permanently connect the gain structure 212 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 preferred embodiments, to reach single-mode emission, only one longitudinal mode is supported by the gain structure 12, 112, 212. 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, 120, 220 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.

To suppress additional transversal modes, the surface of the active layer 20, 120, 220 may be covered with a structured, e.g. formed to include apertures, polyemide or chromium layer (not shown), as described by way of example in international patent application W02014/180751. The apertures may be shaped, dimensioned and located to precisely define the lasing region and increase losses for higher order transversal modes due to their larger beam diameter. As a result, a single TEMOO mode is emitted from the device 10, 210. In some embodiments, continuous tuning is achieved by mounting the mirror structure 14, 114, 214 on a movable piezo-driven movable structure 41, 241. By applying a suitable voltage to the piezo-driver, the cavity length, and thus the emission wavelength changes. This mechanical tuning allows full spectral scans at a repetition rate that is typically in the order of kHz.

Typical design requirements for the mirror structure 14, 114, 214 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 +/- 150 cm-1; to exhibit either transverse-electric (TE) or transverse-magnetic (TM) polarization is significantly greater than the other, e.g. 99.99% vs. 99.9%, or 99.9% vs. 99.8%, and preferably such that no switch between TE and TM modes occurs in the design range; 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.

Some or all of these design requirements may be achieved by preferred embodiments of the invention in which the mirror structure 14, 114, 214 comprises a grating 30, 130, 230, in particular an HCG, preferably formed from silicon or both silicon and silicon dioxide. In order to achieve the required high reflectivity, the grating 30, 130, 230 is stacked together, or otherwise combined, with a Bragg mirror 32, 132, 232. A phase matching layer, which may be provided as an adjunct layer to the HCG, may also be provided.

Advantageously, the combined grating 30, 130, 230 and mirror 32, 132, 232 is attached to and supported by the substrate 50, 150, 250. The substrate 50, 150, 250 is formed from an optically transparent material, in particular a material that is optically transparent at the wavelength of the pumping light 39. Preferably, the material of the substrate 50, 150, 250 is optically absorbent at the wavelength of the generated laser light 34, 36; this eliminates or at least reduces feedback from the outer surface into the cavity. It is further preferred that the material of the substrate 50, 150, 250 has a low refractive index (e.g. 2 or lower than 2). In preferred embodiments, the substrate material is borosilicate. The provision of the substrate 50, 150, 250 enables the mirror structure 14, 114, 214 to have a relatively large active area, i.e. relatively large active mirror area (for example with an area of the order le to 10-6 m2, e.g. approximately 200x200 pm, or approximately 500x500 pm or approximately 1x1 mm) without loss of optical properties and with mechanical stability (e.g. with respect to bending radius).

In embodiments where the mirror structure 14, 114, 214 is movable for the purposes of tuning, typical additional requirements for the movable structure(s) 40, 240, 41, 241, as applicable, may include any one or more of the following: vertical movement (i.e. in the direction A shown in Figures 1 and 2) of approximately 1 to 5pm, typically approximately 2 pm; ability to carry a mirror structure 14, 114, 214, 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, 114, 214 (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, 240 includes or is connected to the membrane 41, 241, 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 3) of the structure 40, 240 and/or membrane 41, 241 is to provide it with an anti-reflective coating. However, this is difficult to achieve on a MEMS membrane, especially while making the thickness of the membrane with the required precision. In preferred embodiments therefore the membrane 41, 241 is shaped to define an aperture 252 through which the mirror structure 14, 114, 214 is exposed. The aperture 252 is aligned in use with the pumping light source 38 so that the pumping beam 39 passes through the aperture 252 and impinges on the mirror structure 14, 114, 214. In preferred embodiments, it is the reverse face 254 of the substrate 250 that is exposed by the aperture 252 and upon which the pumping beam 39 impinges during use. Typically, the aperture 252 is substantially centrally located with respect to the mirror structure. The aperture 252 is preferably centred on the optical axis of the VECSEL device 10, 110, 210. Advantageously, the provision of the aperture 252 means that the membrane 41, 241 serves only as a support for the mirror structure 14, 114, 214 and does not interfere with the optical operation of the VECSEL. In alternative embodiments, the aperture 252 may be replaced by an 10 alternative formation that is highly transmissive to the pump beam 39.

In preferred embodiments, the thickness of the substrate 250 is selected to reduce weight to allow the required oscillatory movement of the mirror structure 14, 114, 214.

The membrane 41, 241 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.

In alternative embodiments (not illustrated), the gain structure 12, 112, 212 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.

VECSELs 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 be configured to emit laser light 36 in the near or mid-infrared wavelength range, for example between 3.0 and 4.5pm, or more generally in the wavelength region above 3 pm, advantageously in a single 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 abovel 0 mWp.

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 (26)

1. CLAIMS: 1. A vertical surface emitting laser device comprising: a gain structure comprising a first mirror and an active layer; and an external mirror structure spaced apart from the gain structure to define an external cavity, wherein the external mirror structure is aligned with said first mirror to form an optical cavity that comprises said external cavity, and wherein said external mirror structure comprises a sub-wavelength grating and a second mirror provided together on a substrate, said sub-wavelength grating having a grating period that is smaller 10 than a target laser emission wavelength of the laser device.
2. 2. The laser device of claim 1, wherein said sub-wavelength grating is a high contrast grating (HCG).
3. 3. The laser device of any preceding claim, wherein said second mirror and said sub-wavelength 15 grating are formed as a multi-layer chip, preferably a flat multi-layer chip.
4. 4. The laser device of any preceding claim, wherein the sub-wavelength grating is a multi-layer structure comprising at least one layer of a dielectric material, the sub-wavelength grating preferably being formed from multiple layers of any one or more of silicon, silicon oxide or silicon dioxide. 20
5. 5. The laser device of any preceding claim, wherein the sub-wavelength grating comprises a first layer and a second layer, the first layer having a higher refractive index than the second layer, and wherein, preferably, the first layer is located between the substrate and the second layer, and wherein, optionally, the first layer is formed from silicon and the second layer is formed from silicon oxide or silicon dioxide.
6. 6. The laser device of any preceding claim, wherein said second mirror is located between said sub-wavelength grating and said gain structure.
7. 7. The laser device of any preceding claim, wherein at least in an axial direction of the device, said sub-wavelength grating is located between and is in contact with one or more layers of solid material, and wherein, preferably, on a first side of said sub-wavelength grating said solid material is provided by said second mirror, and wherein, preferably, on a second side of said sub-wavelength grating said solid material is provided by said substrate.
8. 8. The laser device of any preceding claim, wherein said second mirror comprises multiple layers formed alternately from a first dielectric material and a second dielectric material, the first and second materials having a different refractive index, and wherein, preferably, said first material is silicon oxide or silicon dioxide, and wherein, preferably, said second material is silicon.
9. 9. The laser device of any preceding claim, wherein said second mirror is a dielectric mirror.
10. 10. The laser device of any preceding claim, wherein said second mirror comprises a reduced-layer Bragg mirror or reduced-layer distributed Bragg mirror.
11. 11. The laser device of any preceding claim, wherein said second mirror has at least two layers of dielectric material, and at most 11 layers of dielectric material.
12. 12. The laser device of any preceding claim, wherein said second mirror provides a reflectivity of up to 95%, preferably between 70% and 90%.
13. 13. The laser device of any preceding claim, wherein said second mirror and said sub-wavelength grating together provide a reflectivity of more than 95%, preferably between 95% and 99.9%.
14. 14. The laser device of any preceding claim, wherein said substrate is formed from an optically 15 transparent material, and wherein, preferably, said substrate is formed from a material that is optically transparent to a light beam with which said laser device is optically pumped in use.
15. 15. The laser device of any preceding claim, wherein said substrate is formed from a material that absorbs light at said target laser emission wavelength, preferably at least 30% of light at said target 20 laser emission wavelength.
16. 16. The laser device of any preceding claim, wherein said substrate is formed from a material having a low refractive index, preferably 2 or lower, for example borosilicate glass.
17. 17. The laser device of any preceding claim, wherein said external mirror structure is movable with respect to said gain structure, and is coupled to at least one movable structure that facilitates movement of said external mirror structure towards and away from said gain structure.
18. 18. The laser device of any preceding claim, wherein said external mirror structure is coupled to a 30 flexible membrane that allows movement of said external mirror structure towards and away from said gain structure, and wherein, preferably, said membrane comprises a MEMS membrane.
19. 19. The laser device of claim 17 or 18, further including at least one actuator or actuation system for causing said movement of said external mirror structure, and wherein, preferably, said at least one actuator or actuation system is configured to impart oscillatory movement to said external mirror structure, and wherein, conveniently, said at least one actuator comprises a piezoelectric actuating device.
20. 20. The laser device of any preceding claim, wherein said gain structure and said external mirror 40 structure are spaced apart by at least one layer of spacer material, said at least one spacer layer preferably being shaped and dimensioned to define said external cavity, and wherein, preferably, said at least one external mirror structure is located in said external cavity.
21. 21. The laser device of claim 20, when dependent on claim 18 or 19, wherein said at least one layer 5 of spacer material supports said membrane.
22. 22. The laser device of any preceding claim, further including means for optically pumping the device along an optical axis.
23. 23. The laser device of any one of claims 18 to 22, wherein said membrane is shaped to define an aperture through which the external mirror structure is exposed, and wherein, preferably, said substrate is exposed by said aperture.
24. 24. The laser device of any preceding claim, wherein said active layer is formed from at least one 15 IV-VI semiconductor material.
25. 25. The laser device of any preceding claim, wherein said active layer comprises at least one quantum well layer, and wherein, preferably, the active layer comprises a multiple quantum well structure comprising a plurality of quantum well layers, each being sandwiched between barrier layers and/or outer layers, and wherein, preferably, said active layer comprises first and second outer layers, with a central spacer layer arranged between the outer layers, a plurality of quantum well layers and barrier layers disposed either side of the central spacer layer so as to provide seven quantum wells on one side of the central spacer layer and five quantum well layers on the other side of the central spacer layer, the seven quantum well layers being sandwiched between the central spacer layer and the respective outer layer, and wherein, typically, said quantum well layer(s) comprise PbSe, and said barrier, outer and central spacer layers comprise PbSrSe.
26. 26. The device of any preceding claim, wherein said sub-wavelength grating and said second mirror together comprise multiple layers of dielectric material having alternately a relatively high refractive 30 index and a relatively low refractive index.