Surface emitting laser device

Abstract

A surface emitting laser device and methods for manufacturing and operating is disclosed. The surface emitting laser device comprises an active layer, a semiconductor layer optically coupled to the active layer and a photonic crystal structure arranged within the semiconductor layer. The device further comprises a laser region and a detector region. The laser region comprises a first set of electrical contacts located on opposite sides of the active layer and semiconductor layer and generates an output field of the device. The detector region comprises a second set of electrical contacts located on opposites sides of the active layer and semiconductor layer and is located outside the laser region. The detector region is configured to generate an output current proportional to the output field. The presence of the detector region within the device results in both an improved structure and improved power monitoring techniques when compared with the prior art.

Description

[0001] Surface Emitting Laser Device

[0002] The present invention relates to the field of surface emitting laser devices and methods for manufacturing and operation of these devices. In particular, the present invention relates to a photonic crystal surface emitting laser (PCSEL) device.

[0003] Semiconductor laser devices are solid-state lasers based on semiconductor gain media, where optical amplification is usually achieved through stimulated recombination of charge carriers. Most semiconductor laser devices are laser diodes based on a semiconductor gain medium, which is pumped with an electrical current in a region where n-doped and p- doped semiconductor materials meet. As the photon energy of a laser diode is close to the bandgap energy, compositions with different bandgap energies allow for different emission wavelengths.

[0004] There is a great variety of semiconductor laser devices, spanning wide parameter regions and many different application areas. Fabry Perot (FP) lasers are the original, semiconductor laser device technology. In these devices the laser feedback and emission are both in-plane, and the gain reflection is produced by facet mirrors such that the generated output light comes out of an end of the laser. For this reason, FP lasers are often referred to as edge emitting lasers (EEL). An alternative type of EEL laser is a Distributed Feedback Laser (DFB). DFB lasers also have in-plane feedback and emission, but in these devices the gain reflection is produced by the employment of a grating structure.

[0005] Another type of known semiconductor laser device technology are Vertical Cavity Surface Emitting Lasers (VCSELs). The laser resonator of a VSCEL consists of two distributed Bragg reflector (DBR) mirrors, with an active region arranged between the DBR mirrors. Within a VSCEL, both the laser feedback and emission are both out of plane, where the laser output light emits from a top surface of the laser.

[0006] Photonic crystal surface emitting lasers (PCSELs) are a newer class of semiconductor laser device. PCSELs have been found to have beneficial properties including coherent oscillation, and low divergences of emitted light. PCSELs are also the only semiconductor laser design that employs in-plane feedback and out of plane, surface emission.

[0007] PCSELs can be made from a number of different semiconductor materials. A simplified PCSEL structure, as generally depicted by reference numeral 1 , is presented in Figure 1 . In particular, Figure 1 (a) presents an exploded perspective view of the PCSEL 1 , while Figure 1 (b) presents a cross sectional view of the PCSEL 1 .

[0008] The PCSEL 1 can be seen to comprise a semiconductor substrate 2 upon which the other layers of the PCSEL 1 are formed. The semiconductor substrate 2 usually comprises an n-type semiconductor material but could alternatively comprise a p-type semiconductor material.

[0009] The other layers of the PCSEL 1 , as illustrated in Figure 1 , are the lower cladding layer 3, the active layer 4, the photonic crystal layer 5, and the upper cladding layer 6. As will be appreciated by a person skilled in the art, one or more further intermediate layers may also be arranged within the PCSEL 1 , as required.

[0010] The purpose of the lower 3 and upper 6 cladding layers is to confine the light generated in the active layer 4. The lower 3 and upper 6 cladding layers may comprise first and second graded cladding layers, where the incorporation of graded cladding layers is found to provide for better electrical conduction through the PCSEL 1 and for better optical confinement within the active layer 4.

[0011] The active layer 4 of the PCSEL may contain quantum wells and or quantum dots. For example, it may contain one or more of InGaAs / GaAs quantum wells, InAs / GaAs quantum dots, GaAs / AIGaAs quantum wells, InGaAsP quantum wells and AllnGaAsP quantum wells, although many other active layer designs are known to those in the art.

[0012] The photonic crystal layer 5 comprises a semiconductor material, such as InGaP. However, the photonic crystal layer 5 may comprise an alternative material, such as InGaAsP or any other suitable semiconductor material. The photonic crystal layer 5 is formed by patterning the semiconductor material with periodic regions 7 having a second refractive index which is different from the first refractive index of the semiconductor material, thus forming a periodic lattice structure within the photonic crystal layer 5. The periodic regions 7 having the second refractive index, different from the semiconductor material, may be provided by air gaps or voids. Alternatively, the periodic regions 7 may be filled by overgrowth with a suitable filler material having a second refractive index (e.g., GaAs). The lattice structure of the photonic crystal layer 5 causes Bragg diffraction within the photonic crystal layer 5, which in turn causes light to resonate in the photonic crystal layer 5 at a particular wavelength determined by the periodicity, or lattice constant, of the photonic crystal layer 5.

[0013] Electrical contacts 8a, 8b are located on the external surfaces of the PCSEL 1 . The first electrical contact 8a is located on the output surface 9 of the PCSEL 1 and the second electrical contact 8b is provided adjacent to the lower cladding layer 3.

[0014] As a result of the above-described structure, when an electrical current is provided between the first 8a and second 8b electrical contacts, the PCSEL 1 begins to lase and the output field 10 is emitted from the output surface 9.

[0015] As shown in Figure 1 , the first electrical contact 8a is square shaped, with an aperture 11 , through which output light 10 of the PCSEL 1 is extracted when the PCSEL 1 is lasing. However, as will appreciated by a person skilled in the art, the first electrical contact 8a may be any appropriate shape comprising a central aperture through which the output field 10 can be emitted. For each type of semiconductor laser as described above (EEL, VCSELs and PCSELS), power monitoring of the output field is essential for applications that require precise control of the lasers output power, such as within optical communication systems. Furthermore, power monitoring of the laser device can also help to ensure that the laser is operating safely and efficiently and help to prevent any damage or failure of the device.

[0016] By continuously monitoring a small portion of the output field on a suitable light detector, it is possible to detect any changes in the lasers output power caused by various factors, such as temperature fluctuations, aging of the device, or changes in the input voltage or current. The light detector converts the optical power into an electrical current, which can then be measured by an external circuit to determine the output power. Generally, in order to compensate for any measured output power variations, an appropriate closed loop power control is used to maintain the output power at a fixed level.

[0017] In an EEL, power monitoring of the output light is generally achieved by arranging an external light detector at the back facet of the device. While most of the output power is emitted through the front facet of an EEL, a small portion of the light ultimately escapes through the back facet. The amount of light leakage through the back facet of the EEL is proportional to the amount of light emitted from the front facet. As such, by monitoring the power emitted through the back facet of the EEL any fluctuations in the output power may be compensated for when using a suitable power control loop.

[0018] Unlike an EEL, there is no in-plane feedback or emission within a VCSEL. As such VCSELs employ alternative techniques for power monitoring of the output light. Typically, a portion of the light emitted by the VCSEL through the top surface of the laser is diverted from the main output field and directed to a light detector. As such, this method requires additional external optical elements in order to divert a portion of the output field and to subsequently align the diverted beam onto the light detector.

[0019] As discussed above, the design of a PCSEL 1 permits both in-plane feedback, and out of plane, surface emission (from which a portion of the output can be picked off for monitoring). As such, either of the power monitoring techniques discussed above may be employed for a PCSEL 1 . In particular, and as shown in Figure 1 , by placing a light detector 12 at one edge of the PCSEL 1 , light leakage from a portion of the light travelling in-plane within the photonic crystal layer 5 can be collected by the light detector 12. Any fluctuations in the output power may therefore again be compensated for by using a suitable power control loop 13. Alternatively, a portion of the output light 10 may instead be picked-off from the main output beam for monitoring.

[0020] There are several known disadvantages associated with employing the above techniques for monitoring the output power of a laser device.

[0021] Firstly, careful alignment of the pick-off light and calibration of the power monitoring device 12 are essential for accurate power measurements. However, the alignment of the light directed to the power monitoring device 12 is likely to drift overtime. As a result, realignment of the beam onto the power monitoring device 12 may be required by a user of the laser system on a regular basis. Otherwise, the misaligned beam would result in inaccurate power measurements and an overall decrease in performance of the laser system. This can be undesirable for a variety of applications, in particular for applications where simplicity of use is an important factor.

[0022] Furthermore, the additional optics required for pick-off and alignment can lead to unwanted power losses, reducing the overall output power available from the laser system.

[0023] Another disadvantage of the above-described power monitoring devices 12 is that they can introduce extra noise into the semiconductor laser system. This extra noise can also significantly affect the accuracy of the power measurements.

[0024] For applications where the compactness of the semiconductor laser is essential, the above power monitoring techniques are also not ideal, as the light detector 12 and additional optics also act to increase the overall footprint of the semiconductor laser.

[0025] There is generally a need for an apparatus and / or method which addresses one or more of the problems identified above. Summary of Invention

[0026] It is amongst the aims and objects of the invention to provide an alternative surface emitting laser device which obviates or mitigates one or more drawbacks or disadvantages of the prior art.

[0027] In particular, one aim of an aspect of the invention is to provide a surface emitting laser device, which may be a PCSEL device, that exhibits both an improved structure and improved power monitoring techniques when compared with the prior art.

[0028] According to a first aspect of the present invention there is provided a surface emitting laser device comprising: an active layer; a semiconductor layer optically coupled to the active layer; a first photonic crystal structure, the first photonic crystal structure comprising an array of scattering centres arranged within the semiconductor layer; a first laser region defined by a first set of electrical contacts located on opposite sides of the coupled active layer and semiconductor layer, whereby the first laser region is configured to generate a first output field of the surface emitting laser device; and a first detector region defined by a second set of electrical contacts located on opposites sides of the coupled active layer and semiconductor layer and outside of the first laser region, wherein the first detector region is configured to generate a first output current proportional to the first output field.

[0029] The separate electrical contacts which define the first laser region and the first detector region allow a drive current to be applied to the first laser region while the first detector region outputs a detected current which is proportional, or at least related, to the power output from the first laser region.

[0030] The above surface emitting laser device is thus provided with an integrated detector region by utilising the unique semiconductor structure of the device and in particular the feature that output field is perpendicular to the gain. The presence of the integrated detector region eliminates the need for any external optical elements or light detectors for power monitoring. This results in a surface emitting laser device which is more compact than similar devices known in the art. Furthermore, the integrated detector region improves continuous power monitoring of the surface emitting laser, as constant realignment onto a light detector is no longer necessary, an issue which can commonly arise with external light detectors. Additionally, the use of an integrated detector region increases the accuracy and reduces the overall noise level of the surface emitting laser device, by removing noise elements specifically associated with external light detectors.

[0031] Preferably the first photonic crystal structure is configured to provide an in-plane optical field within the semiconductor layer.

[0032] Preferably the detector region is configured to detect the in-plane optical field generated by the photonic crystal structure.

[0033] Most preferably the first detector region is operated in reverse bias.

[0034] Preferably the first detector region provides feedback to a control loop, wherein the control loop compensates for changes in the detected power of the in-plane optical field to adjust the output power of the first output field. Adjustment of the output power of the first output field may comprise maintaining the output power at a fixed level. A fixed output power is important for a number of commercial applications, and by employing a suitable control loop less input is required from the user of the device to ensure the power remains constant.

[0035] Optionally, the surface emitting laser device further comprises two or more laser regions each characterised by a set of electrical contacts wherein each laser region is configured to generate a separate output field of the surface emitting laser device.

[0036] Preferably the first detector region is configured to detect the in-plane optical field generated within each laser region.

[0037] Preferably the control loop is configured to selectively activate one of the two or more laser regions for generating a single output field.

[0038] Preferably the control loop is configured to automatically switch to selectively activating a different one of the remaining laser regions, when the currently selected laser region no longer produces a required output power. As the output power from each laser region is monitored by the same integrated detector region, the output field from the currently selected laser region can be set at the same fixed output power as the previously selected output field. Each laser region has an associated lifetime and at the end of that lifetime it will no longer be able to produce the required output power. By switching to another laser region, that generates the same output power as the previously selected laser region, the lifetime of the device is extended. Additionally, there is no downtime of the device when switching between different output fields, which can be important for various applications.

[0039] Preferably the surface emitting laser further comprises an output coupler.

[0040] Preferably the two or more laser regions are configured to direct an associated output field into the output coupler.

[0041] Preferably the direction of the output field from one or more of the laser regions is controlled by modifying the photonic crystal structure with a modulation pattern. The photonic crystal structure may be modified by applying a voltage to the photonic crystal structure. By modulating the photonic crystal structure, two output fields (rather than a single output field) are produced from each photonic crystal region.

[0042] Alternatively, the size, shape and or orientation of the scattering centres of the photonic crystal structure are modified to adjust the direction of the output field.

[0043] Alternatively, the surface emitting laser device further comprises beam steering optics configured to adjust the direction of the output field. Optionally, the beam steering optics comprise a microelectromechanical system mirror or a diffraction grating.

[0044] Optionally, the surface emitting laser device further comprises two or more integrated detector regions wherein each integrated detector region is characterised by a set of electrical contacts and each of the two or more integrated detector regions are configured to generate an output current proportional to the one or more output fields.

[0045] Preferably each integrated detector region is configured to detect the in-plane optical field generated within the two or more laser regions. Preferably the active layer comprises a multi quantum layer active layer.

[0046] Preferably the scattering centres comprise voids (commonly referred to in the art as atoms) formed within the semiconductor layer. Most preferably the voids are infilled with a second n-type semiconductor material. Optionally the second n-type semiconductor material comprises Indium Phosphide (InP) and a Tin, Sulphur or Indium dopant material.

[0047] Preferably the surface emitting laser device further comprises a substrate upon which the layers of the surface emitting laser are formed.

[0048] Preferably the surface emitting laser device further comprises a p-type semiconductor layer located between the active layer and the semiconductor substrate, and a n-type semiconductor layer located on the opposite side of the active layer to that of the p-type semiconductor layer.

[0049] Optionally the surface emitting laser device comprises a n-type semiconductor layer located between the active layer and the semiconductor substrate, and a p-type semiconductor layer located on the opposite side of the active layer to that of the n-type semiconductor layer.

[0050] Advantageously, an embodiment of the above-described structure allows for the lifetime of the surface emitting laser device to be extended. A first output field may be monitored by the integrated detector region, and hence the integrated detector region may detect when the power of first output field begins to degrade because of the limited lifetime of the semiconductor structure. As such, the surface emitting laser device can be configured to switch off the first output field when it nears the end of its lifetime, and instead generate a second output field from a second laser region. By using a suitable control loop, the output power from the second output field can be fixed at the same power as the first output field. In this way, a surface emitting laser device with an increased lifetime can be produced.

[0051] Optionally one or more wavelength-selective reflectors may be placed outside the semiconductor layer containing the photonic crystal structures. The presence of the one or more reflectors act to reduce the effects of undesired light leakage from the semiconductor layer.

[0052] Most preferably the electrical contacts associated with the (or each) laser region comprise an aperture which defines an output surface of the surface emitting laser device.

[0053] In one embodiment, the surface emitting laser device comprises two laser regions and a detector region located between them. In another embodiment, the surface emitting laser device comprises an array of laser regions and at least one detector region located between at least two of the laser regions. In such embodiments two or more laser regions can share an integrated detector region. Alternatively, the surface emitting laser device may comprise two or more laser regions and two or more corresponding integrated detector regions.

[0054] According to a second aspect of the invention there is provided a method of manufacturing a surface emitting laser device the method comprising:

[0055] -providing an active layer;

[0056] -providing a semiconductor layer optically coupled to the active layer;

[0057] -providing a first photonic crystal structure, the first photonic crystal structure comprising an array of scattering centres arranged within the semiconductor layer;

[0058] -providing a first laser region defined by a first set of electrical contacts located on opposite sides of the coupled active layer and semiconductor layer, whereby the first laser region is configured to generate a first output field of the surface emitting laser device; and -providing a first detector region defined by a second set of electrical contacts located on opposites sides of the coupled active layer and semiconductor layer and outside of the first laser region,

[0059] -configuring the first detector region to generate a first output current proportional to the first output field.

[0060] Embodiments of the second aspect of the present invention may comprise features to implement the preferred or optional features of the first aspects of the present invention or vice versa. According to a third aspect of the present invention there is provided a method of operating a surface emitting laser device according to the first aspect, the method comprising:

[0061] -providing a voltage across the first set of electrical contacts to supply a drive current to the first laser region and generate an output field; and

[0062] -generating an output current within the first detector region that is proportional to the output field.

[0063] Most preferably the method of operating the surface emitting laser device, further comprises providing a reverse bias voltage across the first detector region via the second set of electrical contacts.

[0064] Preferably the method of operating the surface emitting laser device further comprises adjusting the drive current of the first laser region responsive to the measured (or determined) output power of the output field.

[0065] Preferably the method of operating the surface emitting laser device comprises adjusting the drive current of the first laser region to maintain the output field at a constant power.

[0066] Optionally the method of operating the surface emitting laser device comprises supplying a drive current to two laser regions to generate two output fields of the surface emitting laser device.

[0067] Preferably the method of operating a surface emitting laser device comprises switching off the drive current to the first laser region and supplying drive current to the second laser region responsive to determining failure or loss of function in the first laser region.

[0068] Preferably the method of operating a surface emitting laser device comprises supplying a drive current to two or more laser regions and generating an output current from the first detector region representative of the combined output power of the two or more laser regions.

[0069] Embodiments of the third aspect of the present invention may comprise features to implement the preferred or optional features of the first and second aspects of the present invention or vice versa. Brief of Drawings

[0070] There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which:

[0071] Figure 1 presents (a) an exploded perspective view; and (b) a cross sectional view, of a photonic crystal surface emitting laser as known in the art;

[0072] Figure 2 presents a cross sectional view of a photonic crystal surface emitting laser device in accordance with an embodiment of the present invention;

[0073] Figure 3 presents a cross sectional view of a photonic crystal surface emitting laser device in accordance with an alternative embodiment of the present invention;

[0074] Figure 4 presents a cross sectional view of a photonic crystal surface emitting laser device in accordance with an alternative embodiment of the present invention;

[0075] Figure 5 presents a cross sectional view, of a photonic crystal surface emitting laser device in accordance with an alternative embodiment of the present invention;

[0076] Figure 6 presents a cross sectional view of a photonic crystal surface emitting laser device in accordance with an alternative embodiment of the present invention; and

[0077] Figure 7 presents a photonic crystal surface emitting laser device in accordance with a further alternative embodiment of the present invention.

[0078] Figure 8 presents a photonic crystal surface emitting laser device in accordance with a further alternative embodiment of the present invention.

[0079] In the description which follows, like parts are marked throughout the specification and drawings with the same reference numerals. The drawings are not necessarily to scale, and the proportions of certain parts have been exaggerated to better illustrate details and features of embodiments of the invention. Detailed Description

[0080] A photonic crystal surface emitting laser (PCSEL) device 14 in accordance with an embodiment of the present invention, and its method of production, will now be described with reference to Figure 2. Figure 2 presents a cross sectional view of the PCSEL device 14 in accordance with an embodiment of the present invention.

[0081] The PCSEL device 14 of Figure 2 comprises a similar structure of semiconductor layers when compared to the PCSEL 1 presented within Figures 1 (a) and (b). However, within the PCSEL device 14 a photonic crystal structure 15 is arranged only within a specific region of a semiconductor layer 16. Therefore, in contrast to the photonic crystal layer 5 of the PCSEL 1 depicted in Figures 1 (a) and (b), the scattering centres of the photonic crystal structure 15 do not extend throughout the entire semiconductor layer 16 i.e. there is an absence of scattering centres within at least one region of the semiconductor layer 16.

[0082] The PCSEL device 14 of Figure 2 comprises a first 8a, 8b and a second 17a, 17b set of electrical contacts. The first set of electrical contacts 8a, 8b are arranged on opposite external surfaces of the PCSEL device 14, and more specifically they are arranged to surround a region comprising the photonic crystal structure 15. The location of the first set of electrical contacts 8a, 8b can be said to define a laser region 18 of the PCSEL device 14. The top electrical contact 8a has an aperture 11 through which the output field 10 of the PCSEL device 14 is extracted when the PCSEL device 14 is lasing.

[0083] The second set of electrical contacts 17a, 17b are then provided in a region separate from but adjacent to the laser region 18, again on opposite external surfaces of the PCSEL device 14. This can be said to define a detector region 19 of the PCSEL device 14.

[0084] Due to the structure of the PCSEL device 14, and the placement of the second pair of electrical contacts 17a, 17b, the detector region 19 adjacent to laser region 18 may function as an integrated light detector.

[0085] A light detector is a semiconductor device that converts light into an electrical current, whereby the light detector is made up of a p-type semiconductor and an n-type semiconductor material. When light falls on the p-n junction of the light detector, electronhole pairs are generated which can be collected as a photocurrent, where the current flow is proportional to the incident light intensity. The light detector typically has two terminals, a positive and negative terminal that are used to create an electric field within the diode, allowing it to convert incident light into an electrical current.

[0086] As would be appreciated by a reader skilled in the art, the structure of the PCSEL device 14 requires both an n-doped and p-doped semiconductor layer, which creates the required p-n junction for a light detector. Additionally, as the second set of electrical contacts 17a, 17b of the PCSEL device 14 provide electrical connections for the required terminals of a light detector, the structure of the PCSEL device 14 is suitable for the detector region 19 to operate as an integrated light detector. Preferably, the detector region 19 should be operated in reverse bias, i.e., where the negative terminal is connected to the p-type semiconductor layer, and the positive terminal is connected to the n-type semiconductor material, although it may function in zero bias.

[0087] Light is automatically incident upon (or received by) the detector region 19 when the PCSEL device 14 is in normal operation. More specifically, when a voltage is applied across the laser region 18 of the PCSEL device 14 via the first set of electrical contacts 8a, 8b, the active layer 4 emits light. As already discussed, the photonic crystal structure 15 within the PCSEL device 14 scatters the light linearly, in plane, and orthogonally, out of plane. As such, a small portion of the light travelling in-plane enters the detector region 19 of the semiconductor layer 16, where, in the presently described embodiment, no photonic crystal structure 15 present.

[0088] When the in-plane optical field 20 from the photonic crystal structure 15 reaches the detector region 19, an electrical current is generated as described above, providing a means for determining and thereby monitoring the output power of the output field 10. Furthermore, by employing a suitable power control loop 13, any power fluctuations in the output field 10 can be compensated for and a fixed output power level can be maintained. The output power is proportional to the in-plane optical field and therefore the generated electrical current is proportional (or at least related to) the output power.

[0089] There are numerous advantages to the above described PCSEL device 14 over the prior art. By using an integrated detector region 19 rather than an external light detector 12, there is no need for any additional external optical elements. This reduces any unwanted power loss on such optical elements, so that the maximum power can be provided in the output field 10 of the PCSEL device 14.

[0090] Furthermore, as the detector region 19 is integrated within the PCSEL device 14, the need for continuous realignment of the in-plane light field 20 onto the detector region 19 is eliminated. As such, a user does not need to be concerned about continuously realigning and or calibrating a light detector, leading to a less complex device and operating conditions for the user.

[0091] Additionally, the overall footprint of the PCSEL device 14 is substantially decreased, which can be critical for some applications.

[0092] The PSCEL device 14 of Figure 2 may further comprise one or more wavelength selective reflectors 21 located outside of the semiconductor structure of the PSCEL device 14. The wavelength selective reflectors 21 are designed to reflect the light at a selected wavelength i.e. the wavelength of the output field 10.

[0093] The locations of the one or more wavelength selective reflectors 21 are chosen to reflect the in-plane optical field 20 of the semiconductor layer 16 back into the photonic crystal structure 15. Advantageously, this means that light which leaves the laser region 18 and enters the detector region 19, does not significantly reduce the overall output power of the PSCEL device 14, as the majority of the light incident on the detector region 19 can be reflected back into the laser region 18, where the output field 10 of the PCSEL device 14 is generated.

[0094] It will be appreciated by the skilled reader that the photonic crystal structure 15 may be arranged at any suitable location within the semiconductor layer 16 and indeed may extend across the full length of the semiconductor layer 16. Furthermore, the semiconductor layer 16 itself may alternatively be located in a different position within the layered structure of the PCSEL device 14. As discussed above, in reference to Figure 1 , one or more further layers may also be arranged within the PCSEL device 14, as required.

[0095] For example, the PSCEL device 14 may also comprise a first separate confinement heterostructure (SCH) layer located between the lower cladding layer 3 and the active layer 4 and a second separate confinement heterostructure (SCH) layer located between the upper cladding layer 6 and the active layer 4.

[0096] A PCSEL device 22 in accordance with an alternative embodiment of the present invention is presented in Figure 3. The PCSEL device 22 has a similar structure to that shown in Figure 2, however in Figure 3 both a first 15a and second 15b photonic crystal structure are provided within the semiconductor layer 16 of the PSCEL device 22.

[0097] Furthermore, a third set of electrical contacts 23a, 23b is provided on the external surfaces of the PCSEL device 22 in the region containing the second 15b photonic crystal structure. Both of the electrical contacts 8a and 23a comprise an aperture 11 a, 11b through which the output fields 10a and 10b of the PCSEL device 22 can be extracted. As such, two separate laser regions 18a and 18b, and thus two output fields 10a and 10b can be provided by the PCSEL device 22. The electrical contacts 8, 23 can be said to define first 18a and second 18b laser regions of the PCSEL device 22.

[0098] When a voltage is applied across the PCSEL device 22 via the first set of electrical contacts 8a, 8b, a first in-plane light field 20a from the first photonic crystal structure 15a travels into the adjacent detector region 19. In a similar manner, when a voltage is applied across the PCSEL device 22 via the third set of electrical contacts 23a, 23b a second inplane light field 20b travels from the second photonic crystal structure 15b into the adjacent detector region 19.

[0099] The advantage of the above-described structure is that it allows for the overall lifetime of the PCSEL device 22 to be significantly increased in comparison with the previous PCSEL device 14 of Figure 2. The lifetime of a PCSEL can depend on various factors, such as operating conditions, and manufacturing quality. Usually, at the end of a PCSELs lifetime, the system must be replaced by the user which can be unfavourable in specific applications, as it results in unwanted downtime.

[0100] As intimated, the lifetime of the PCSEL device 22 can be significantly increased by employing the structure of PCSEL device 22. In this case, a voltage is first applied to the PCSEL device 22 via the electrical contacts 8a and 8b, while no voltage is applied to the electrical contacts 23a and 23b. As such, the output field 10a is generated by the PCSEL device 22, and the power of the first in-plane optical field 20a is monitored by the integrated detector region 19 in order to maintain the first output field 10a at a fixed power level.

[0101] When the integrated detector region 19 detects the output field 10a dropping below the fixed power level, or other signs of laser failure are detected, a suitable power control system 24 is employed to switch to the second output 10b. In particular, the voltage applied across electrical contacts 8a and 8b is switched off and a voltage is instead applied across the PCSEL device 22 via the electrical contacts 23a and 23b. As such, the output field 10b is generated by the PCSEL device 22, and the power of the second inplane optical field 20b is monitored by the integrated detector region 19. As the second output field is monitored by the same integrated detector region 19 it can be ensured that the output field 10b provides the same fixed output power as that which was produced by the first output field 10a. In this way, the user can continue to use the laser without interruption. The output from the PCSEL device 22 can remain at a fixed power level, while the overall lifetime of the device is significantly extended.

[0102] Note, that the first 10a and second 10b output fields will be spatially separated, and therefore some realignment of the output beam may be required by the user when switching between each output field. For example, some additional alignment optics may be required to direct the second output field 10b along the same beam path as the first output field 10a.

[0103] Note that in another mode of operation both sets of electrical contacts and hence both laser regions18a and 18b can be supplied with a drive current simultaneously, such that both output fields 10a and 10b are generated simultaneously. In this mode of operation, the detector region 19 generates a current which is representative of the combined output power of the first 18a and second laser regions 18b. This approach can be scaled up such that the integrated detector region 19 can determine the output power of a PSCEL device comprising an array of laser regions 18.

[0104] A PCSEL device 25 in accordance with an alternative embodiment of the present invention is presented in Figure 4.

[0105] In Figure 4, first 15a, second 15b, and third 15c photonic crystal structures are provided within the semiconductor layer 16 of the device. An additional set of electrical contacts 26a and 26b are provided to define a third laser regions 18c, so that the PCSEL device 25 can additionally produce a third output field 10c from the third laser region 18c which contains the third photonic crystal structure 15c.

[0106] A first detector region 19a and a second detector region 19b are also provided within the PCSEL device 25 structure. The first detector region 19a and second detector region 19b each comprise a set electrical contacts 17a, 17b and 27a, 27b respectively, on the external surface of the device. As a result, the in-plane light field 20a, 20b from the first 15a and second 15b photonic crystal structure is incident on (or received by) the first detector region 19a, while the in-plane light field 20c produced by the third photonic crystal structure 15c is incident on (or received by) the second detector region 19b. Optional reflective elements (not shown) may be located between adjacent laser regions 18a, 18b and 18c to confine the in-plane light field to respective adjacent detector regions (i.e., to effectively isolate detector regions19a and 19b from one another).

[0107] The structure of PCSEL device 25 can be used to provide a device with multiple output fields 10a, 10b and 10c, where constant power monitoring on the integrated detector region 19a and 19b can ensure that the combined output power of the output fields 10a, 10b and 10c remains constant. Furthermore, similar to the discussion with reference to Figure 3, the lifetime PCSEL device 25 can be extended by operating the PCSEL device 25 to produce only one output field 10a, 10b or 10c at a time. It is also envisaged that such a device could produce two output fields from two laser regions while the third laser region provides redundancy in case one of the two laser regions fails or falters.

[0108] As will be appreciated by the skilled person, any number of additional laser regions 18 (with corresponding electrical contacts) may be provided within the PCSEL device 25 to produce any number of additional output fields 10. In a similar manner the number of integrated detector regions 19 can also be increased, so that the power of each output field 10 produced by the additional laser regions 18 may be monitored. As intimated above however, a single detector region 19 may monitor the output of several laser regions 18 or an array of laser regions 18.

[0109] A PCSEL device 28 in accordance with an alternative embodiment of the present invention is presented in Figure 5. In a similar manner to Figure 3, a single integrated detector region 19 is provided within the PCSEL device 28, with first 15a and second 15b photonic crystal structures present within the semiconductor layer 16. A first 8 and third 23 set of electrical contacts define corresponding laser regions 18a and 18b on either side of the detector region 19. However, in Figure 5, both the first 10a and second 10b output fields are emitted from the PCSEL device 28 at a significantly different angle than for the previous embodiments, for example to couple the two output fields into a single optical coupler 29.

[0110] There are different approaches to modify the direction of the output field 10 from the PCSEL device 28. One approach requires using beam steering optics external to the PCSEL device 28. For example, a micro-electromechanical system (MEMS) mirror could be placed in the beam path of the PCSEL device 28 to deflect the output beam in any desired direction. However, this approach requires an additional optical element, and hence a larger footprint for the device.

[0111] Another approach to modify the direction of the output field 10 is to change the geometry of the photonic crystal structure 15. This involves changing the size, shape or orientation of the scattering centres of the photonic crystal structure 15, to favour emission in a particular direction. However, for this approach it can be difficult to control or change the direction of the output beam as required.

[0112] A further alternative method shown in Figure 5 involves modifying the first 15a and second 15b photonic crystal structures with a modulation pattern, to modify the direction of the output fields 10a and 10b. By applying a voltage to the first 15a and second 15b photonic crystal structures, it is possible to create a modulation pattern in the refractive index, which can be used to control the direction of the output fields 10a, 10b. This modulation pattern can cause the PCSEL device 28 to emit in two or more directions simultaneously, as depicted in Figure 5. The additional output fields 30a, 30b are shown by the dashed lines in Figure 5, and their presence acts to limit the attainable power in the output fields 10a and 10b, respectively.

[0113] This approach offers an integrated and flexible way to control the direction of the output fields 10a, 10b of the PCSEL device 28. For instance, by modulating the voltage pattern, the output field 10a, 10b can be steered in any required output direction. As shown in Figure 5 the output direction of the output fields 10a and 10b are chosen so that they are both directed into the output coupler 29. In particular, the output angle of the first 10a and second 10b output fields are chosen to achieve maximal coupling into the output coupler 29, which is placed at a central location between the output fields 10a, 10b.

[0114] The lifetime of the PCSEL device 28 can be extended (as for the PCSEL device 22 of Figure 3) by only producing the first output field 10a initially, and using a suitable power control system 24 to change to the second output field 10b when the power of the first output field 10a deteriorates, for example due to aging of the semiconductor structure.

[0115] As both of the in-plane optical fields 20a, 20b generated by the photonic crystal structures 15a, 15b can be monitored by the integrated detector region 19, a constant output power can be continuously maintained for the PCSEL device 28 by using the power control system 24, for example as described above.

[0116] As both the first 10a and second 10b output fields are aligned into the same output coupler 29, no realignment of the PCSEL device 28 is required when switching between each output field 10a, 10b. As a result, the device has an extended lifetime, while requiring less input from a user than for the previous embodiments. As such, the PCSEL device 28 provides greater ease of use and can be advantageous for applications where a high- power output is not critical.

[0117] A PCSEL device 31 in accordance with an alternative embodiment of the present invention is presented in Figure 6. The PCSEL device 31 is similar to that shown in Figure 5, however the output coupler 29 is instead placed directly above the first laser region 18a comprising the first photonic crystal structure 15a. As such, no modulation of the first photonic crystal structure 15a is required in order to couple the first output field 10a into the output coupler 29. However, the second photonic crystal structure 15b is modulated to produce an output field 10b at an angle that allows for maximal alignment into the output coupler 29. Note that an additional output field 30b will be produced, limiting the maximum output power for the output field 10b.

[0118] As for the above described embodiments, the in-plane optical fields 20a, 20b may be monitored by the integrated detector region 19, and hence a constant output power can be maintained for the PCSEL device 31 . As will be appreciated by a reader skilled in the art, the output coupler 29 could alternatively be placed in any suitable location which allows for coupling of the first 10a and second 10b output fields, through modulation of the first 15a and / or second 15b photonic crystal structures. It is also foreseen that the MEMS approach and / or the geometrical approach to beam steering could be employed instead of, or in addition to, the modulation approach in variants of any relevant embodiment.

[0119] A PCSEL device 32 in accordance with an alternative embodiment of the present invention is presented in Figure 7. The PCSEL device 32 comprises four laser regions 18a to 18d defined by electrical contacts 8, 23, 26, 27 that produce four output fields 10a to 10d.

[0120] The in-plane light fields from each of the photonic crystals may be monitored by the integrated detector region 19 located between all four laser regions 18a to 18d. The photonic crystal structures 15a to 15d are arranged and modulated so that each of the four output fields 10a to 10d are coupled into the output coupler 29, though it is foreseen that any beam steering approach may be adopted.

[0121] As a result, the PCSEL device 32 can have a significantly extended lifetime when compared to the lifetimes of the embodiments described in Figure 5 and 6 (and potentially up to four times or more of the lifetime of PCSEL devices comprising a single laser region). Furthermore, as the photonic crystal structures 15a to 15d can be appropriately modulated, no realignment is necessary when switching between output fields 10a to 10d. The in-plane optical field from each photonic crystal structure can be monitored by the integrated detector region 19, and as such a constant power can be maintained for each output field 10a to 10d of the PCSEL device 32.

[0122] A PCSEL device 33 in accordance with a further alternative embodiment of the present invention is presented in Figure 8.

[0123] Again, the PCSEL device 33 comprises laser regions 18a to 18d and produces four output fields 10a to 10d. However, in this case the output coupler 29 is located directly above the laser region 18a within which the first photonic crystal 15a is present. As such no modulation of the first photonic crystal 15a is required. The other photonic crystals 15b to 15d are appropriately modulated to direct the output fields 10c to 10d into the output coupler 29.

[0124] As will be appreciated by a reader skilled in the art, the output coupler 29 can be placed in any suitable location which allows for coupling of each of the four output fields 10a to 10d, through modulation of the photonic crystal structures 15a to 15d. Furthermore, the integrated detector region 19 may also be arranged in any suitable alternative location, provided the in-plane light fields from the photonic crystal structures 15a to 15d can still be incident on the integrated detector region 19.

[0125] It will be also be appreciated by a reader skilled in the art, that further laser regions 18 could be added to the PCSEL device 33 to generate more output fields 10, and hence further extend the lifetime of the PCSEL device 33. Additional integrated detector regions 19 may also be provided within the PCSEL device 33, as required, for monitoring the inplane light generated by the photonic crystal structures 15.

[0126] In summary, the present invention provides an alternative surface emitting laser device to those known in the art, with an improved structure and operating parameters. In particular, the present invention provides a photonic crystal surface emitting laser (PCSEL) device which provides an integrated detector region within the structure of the PCSEL device. The disclosed PCSEL device provides improved power monitoring for easier output power stability. Furthermore, the structure of the disclosed PCSEL device results in a smaller footprint and requires less continuous input and alignment from a user of the device.

[0127] A surface emitting laser device and methods for manufacturing and operating is disclosed. The surface emitting laser device comprises an active layer, a semiconductor layer optically coupled to the active layer and a photonic crystal structure arranged within the semiconductor layer. The device further comprises a laser region and a detector region. The laser region comprises a first set of electrical contacts located on opposite sides of the active layer and semiconductor layer and generates an output field of the device. The detector region comprises a second set of electrical contacts located on opposites sides of the active layer and semiconductor layer and is located outside the laser region. The detector region is configured to generate an output current proportional to the output field. The presence of the detector region within the device results in both an improved structure and improved power monitoring techniques when compared with the prior art. Throughout the specification, unless the context demands otherwise, the term “comprise” or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

[0128] Furthermore, reference to any prior art in the description should not be taken as an indication that the prior art forms part of the common general knowledge.

[0129] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims

Claims1 . A surface emitting laser device comprising: an active layer; a semiconductor layer optically coupled to the active layer; a first photonic crystal structure, the first photonic crystal structure comprising an array of scattering centres arranged within the semiconductor layer; a first laser region defined by a first set of electrical contacts located on opposite sides of the coupled active layer and semiconductor layer, whereby the first laser region is configured to generate a first output field of the surface emitting laser device; and a first detector region defined by a second set of electrical contacts located on opposites sides of the coupled active layer and semiconductor layer and outside of the first laser region, wherein the first detector region is configured to generate a first output current proportional to the first output field.

2. A surface emitting laser device as claimed in claim 1 , wherein the first photonic crystal structure is configured to provide an in-plane optical field within the semiconductor layer.

3. A surface emitting laser device as claimed in claim 2, wherein the first detector region is configured to detect the in-plane optical field generated by the photonic crystal structure.

4. A surface emitting laser device as claimed in claims 2 or 3, wherein the first detector region provides feedback to a control loop, wherein the control loop compensates for changes in the detected power of the in-plane optical field to adjust the output power of the first output field.

5. A surface emitting laser device as claimed in any of the preceding claims, wherein the first detector region is operated in reverse bias.

6. A surface emitting laser device as claimed in any of the preceding claims, wherein the surface emitting laser device further comprises two or more laser regions each characterised by a set of electrical contacts and wherein each of the two or morelaser regions are configured to generate a separate output field of the surface emitting laser device.

7. A surface emitting laser device as claimed in claim 6, wherein the first detector region is configured to detect the in-plane optical field generated within each of the two or more laser regions.

8. A surface emitting laser device as claimed in claims 6 or 7, wherein the first detector region provides feedback to a control loop, wherein the control loop is configured to selectively activate one of the two or more laser regions for generating a single output field.

9. A surface emitting laser device as claimed in claim 8, wherein the control loop is configured to automatically switch to selectively activating a different one of the remaining laser regions, when the currently selected laser region no longer produces a required output power.

10. A surface emitting laser device as claimed in any of claims 6 to 9, wherein the surface emitting laser further comprises an output coupler.11 . A surface emitting laser device as claimed in claim 10, wherein the two or more laser regions are configured to direct an associated output field into the output coupler.

12. A surface emitting laser device as claimed in claims 11 , wherein the direction of the output field from one or more laser regions is controlled by modifying the photonic crystal structure with a modulation pattern.

13. A surface emitting laser device as claimed in claim 11 or 12, wherein the direction of the output field from one or more laser region is controlled by modifying the size, shape and or orientation of the scattering centres of the photonic crystal structure.

14. A surface emitting laser device as claimed in any of claims 11 to 13, wherein the surface emitting laser device further comprises beam steering optics configured to adjust the direction of the output field from one or more laser region.

15. A surface emitting laser device as claimed in any of claims 6 to 14, wherein the surface emitting laser device further comprises two or more integrated detector regions, wherein each integrated detector region is characterised by a set of electrical contacts and each of the two or more integrated detector regions are configured to generate an output current proportional to the one or more output fields.

16. A surface emitting laser device as claimed in claim 15, wherein each of the two or more integrated detector regions are configured to detect the in-plane optical field generated within the two or more laser regions.

17. A surface emitting laser device as claimed in any of the preceding claims wherein wavelength-selective reflectors are placed outside the semiconductor layer containing the photonic crystal structures.

18. A method of manufacturing a surface emitting laser device the method comprising: -providing an active layer;-providing a semiconductor layer optically coupled to the active layer;-providing a first photonic crystal structure, the first photonic crystal structure comprising an array of scattering centres arranged within the semiconductor layer; -providing a first laser region defined by a first set of electrical contacts located on opposite sides of the coupled active layer and semiconductor layer, whereby the first laser region is configured to generate a first output field of the surface emitting laser device;-providing a first detector region defined by a second set of electrical contacts located on opposites sides of the coupled active layer and semiconductor layer and outside of the first laser region, and-configuring the first detector region to generate a first output current proportional to the first output field.

19. A method of operating a surface emitting laser device as claimed in any of claims 1 to 17, the method comprising:-providing a voltage across the first set of electrical contacts to supply a drive current to the first laser region and generate an output field; and-generating an output current within the first detector region that is proportional to the output field.

20. A method of operating a surface emitting laser device as claimed in claim 19, the method further comprising providing a reverse bias voltage across the first detector region via the second set of electrical contacts.21 . A method of operating a surface emitting laser device as claimed in claims 19 or 20, the method further comprising adjusting the drive current of the first laser region responsive to the measured output power of the output field.

22. A method of operating a surface emitting laser device as claimed in claim 21 , the method further comprising adjusting the drive current of the first laser region to maintain the output field at a constant power.

23. A method of operating a surface emitting laser device as claimed in any of claims 19 to 22, the method further comprising supplying a drive current to a first laser region and a second laser region to generate two output fields of the surface emitting laser device.

24. A method of operating a surface emitting laser device as claimed in claim 23, the method further comprising switching off the drive current to the first laser region and supplying drive current to the second laser region responsive to determining failure or loss of function in the first laser region.

25. A method of operating a surface emitting laser device as claimed in any of claims 19 to 24, the method further comprising supplying a drive current to two or more laser regions and generating an output current from the first detector region representative of the combined output power of the two or more laser regions.

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