Surface emitting laser

Abstract

A surface emitting laser device 13 comprises an active layer configured to generate laser radiation and a photonic crystal layer 5 comprising a periodic arrangement within a semiconductor material and

Description

The present invention relates to the field of surface emitting lasers and methods for manufacturing these devices. In particular, the present invention relates to photonic crystal surface emitting lasers (PCSEL) and methods for manufacturing the same. Semiconductor lasers 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 media, 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. There is a great variety of semiconductor lasers, spanning wide parameter regions and many different application areas. Photonic crystal surface emitting lasers (PCSELs) are one class of semiconductor laser. PCSELs have been found to have beneficial properties including coherent oscillation, and low divergences of emitted light. PCSELs differ from most semiconductor laser designs in that they employ two dimensional in-plane feedback and out of plane, surface emission. Semiconductor lasers may be used as a light source for display devices. In particular, semiconductor lasers, such as PCSELs, offer key benefits for use within display devices as they are compact light sources, enabling seamless integration into display systems. Furthermore, PCSELs are highly energy efficient, i.e., at converting electrical energy into light and have long lifespans which is important for ensuring the longevity of a display device. Other advantages include their durability, fast switching speeds and narrow spectral ranges. In general, PCSELs emit directional and coherent light, which can ensure sharp image quality and colour accuracy and their capability for fast modulation suits high refresh rate displays. PCSELs also provide cost-effective production and can produce light outputs at a wide range of wavelengths dependent on the required application. However, one notable drawback associated with employing PCSEL lasers as light sources within display systems stems from their highly coherent output. Coherent light waves have a fixed phase relationship, meaning they have the same frequency, wavelength, and maintain a constant phase difference. This characteristic leads to a phenomenon termed "speckle", which becomes a discernible detrimental feature within the displayed image. The term speckle refers to a random interference pattern that occurs when highly coherent light waves from a source, such as a laser, undergo constructive and destructive interference due to slight variations in the optical path length they traverse. This phenomenon can be observed when coherent light is scattered or reflected off a rough or optically varying surface and results in a granular, grainy pattern on the surface. A useful parameter for quantifying speckle is the level of noise present in a speckle pattern. This is achieved by calculating the speckle contrast (C), which is defined as the ratio of the standard deviation of intensity fluctuation (a) to the mean value of the intensity (I), as defined by equation (1) below: C = ^ (1) Speckle contrast effectively characterises the depth of spatial modulation of the scattered radiation and shows how strong the intensity fluctuations in the speckle sample are compared to the average intensity. Speckle is generally considered undesirable for specific applications, and particularly within display systems, as it can severely degrade the image quality and clarity by making the image appear grainy or uneven, impacting the overall visual experience. In particular, speckle can reduce the contrast of an image by introducing random variations in brightness. These variations can impact the ability to distinguish details within the image. The granular pattern introduced by speckle noise can make the displayed content appear less sharp and well-defined. The random and fluctuating nature of speckle leads to variations in brightness and contrast across the display, making it difficult to reproduce accurate and consistent images. For a viewer of a display, the constantly changing interference patterns due to speckle can strain the eyes and affect the overall perception of image quality. Summary of Invention It is therefore an object of an embodiment of the present invention to provide an alternative photonic crystal surface emitting laser (PCSEL) to those known in the art. It is a further object of embodiments of the present invention to provide a semiconductor laser system that obviates or mitigates one or more drawbacks or disadvantages of the prior art. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the present invention there is provided a surface emitting laser comprising: an active layer configured to generate laser radiation; a photonic crystal layer comprising an array of scattering centres or atoms arranged within a semiconductor material and optically coupled to the active layer; the photonic crystal layer comprising two or more photonic crystal regions that each direct a separate output field to an output surface; and a coherence perturbator; wherein the coherence perturbator acts to control the relative coherence between the two or more output fields. The coherence perturbator allows for control over the relative coherence between the two or more output fields from each photonic crystal region, and hence acts to mitigate speckle in the combined output. Significantly, a degree of incoherence is introduced between the light generated by the different photonic crystal regions, resulting in uncorrelated output fields which no longer have a fixed phase relationship. Therefore, when the output fields comprising uncorrelated phase relationships are superimposed the overall coherence of the light is reduced. This results in a reduction in the visibility of speckle when the combined light is observed or detected. The speckle is further reduced due to the inherent spacing between the photonic crystal regions. Preferably the surface emitting laser device further comprises a first and a second electrical contact arranged on opposite external surfaces of the surface emitting laser device. Most preferably the second electrical contact comprises an aperture which defines the output surface of the surface emitting laser device. Preferably the coherence perturbator comprises an amplitude modulated drive current applied to the electrical contacts. Preferably the amplitude modulation frequency is at least 50 Hz. Most preferably the coherence perturbator comprises modifying one or more properties of the photonic crystal layer within each photonic crystal region. Preferably the coherence perturbator comprises providing a different lattice spacing for the photonic crystal layer within at least two of the photonic crystal regions. The different lattice spacing results in two or more output fields at different wavelengths. Preferably the wavelength difference between the two or more output fields is in the range 0.01 nm to 100 nm. More preferably the wavelength difference between the two or more output fields is in the range 0.01 nm to 50 nm. The wavelength difference between the two or more output fields may be in the range 0.01 nm to 10 nm. The coherence perturbator may comprise providing a different orientation, different size and or different shape of the scattering centres or atoms within at least two of the photonic crystal regions. Alternatively, the coherence perturbator may comprise a different refractive index contrast between the scattering centres or atoms and the semiconductor material within at least two of the photonic crystal regions and or a different filling fraction for the scattering centres or atoms within at least two of the photonic crystal regions. Alternatively, the coherence perturbator comprises a phase shifting film arranged between the photonic crystal layer and the output surface. Preferably the phase shifting film has a varying thickness. Preferably the thickness of the film varies between at least two of the photonic crystal regions. The varying thickness of the phase shifting film results in two or more output fields comprising a different phase. Alternatively, the coherence perturbator comprises a phase modulator. Preferably the phase modulator comprises modulating the phase of the drive current applied to the electrical contacts. Optionally, the surface emitting laser further comprises two or more electrical contacts arranged on the output surface of the surface emitting laser device. Preferably each of the two or more electrical contacts arranged on the output surface comprise an aperture through which the output fields from each photonic crystal region are emitted. Preferably the two or more output fields have wavelengths that are within visible spectrum i.e., a range of 380 nm to 750 nm. Optionally, the two or more output fields have a wavelength in the range of 600 nm to 700 nm. Alternatively, the two or more output fields have a wavelength in the range of 400 nm to 550 nm. Preferably, the surface emitting laser further comprises a first cladding layer and a second cladding layer located on opposite sides of the active layer. Preferably the active layer comprises a Multi-Quantum Well (MQW) active layer. Preferably the surface emitting laser further comprises a substrate upon which the layers of the surface emitting laser are formed. According to a second aspect of the invention there is provided a method of manufacturing a surface emitting laser the method comprising: providing an active layer configured to generate laser radiation; providing a photonic crystal layer comprising an array of scattering centres or atoms within a semiconductor material and optically coupled to the active layer; providing the photonic crystal layer with two or more photonic crystal regions and configuring each photonic crystal region to direct a separate output field to an output surface; and providing a coherence perturbator; wherein the coherence perturbator is configured to control the relative coherence between the two or more output fields. 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. Brief Description of Drawings There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which: 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; Figure 2 presents (a) a cross sectional view of a photonic crystal surface emitting laser and (b) a top view of the photonic crystal layer of the photonic crystal surface emitting laser, in accordance with an embodiment of the present invention; Figure 3 presents an alternative top view of a photonic crystal layer for the photonic crystal surface emitting laser of Figure 2(a); Figure 4 presents an alternative top view of a photonic crystal layer for the photonic crystal surface emitting laser of Figure 2(a); Figure 5 presents an alternative top view of a photonic crystal layer for the photonic crystal surface emitting laser of Figure 2(a); Figure 6 presents an alternative top view of a photonic crystal layer for the photonic crystal surface emitting laser of Figure 2(a); Figure 7 presents a cross sectional view of a photonic crystal surface emitting laser in accordance with an alternative embodiment of the present invention; Figure 8 presents a cross sectional view of a photonic crystal surface emitting laser in accordance with a further alternative embodiment of the present invention; Figure 9 presents a cross sectional view of a photonic crystal surface emitting laser in accordance with a further alternative embodiment of the present invention; and Figure 10 presents (a) a cross sectional view of a photonic crystal surface emitting laser and (b) a top view of the photonic crystal layer of the photonic crystal surface emitting laser in accordance with a further alternative embodiment of the present invention. 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 A simplified PCSEL structure as is known the art, and 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. 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. The other layers of the PCSEL 1, as illustrated in Figure 1, are a lower cladding layer 3, an active layer 4, a photonic crystal layer 5, and an 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. 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. 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. The photonic crystal layer 5 comprises a semiconductor material, such as Gallium Nitride (GaN) or Gallium Arsenide (GaAs). However, the photonic crystal layer 5 may comprise any suitable alternative semiconductor material. The photonic crystal layer 5 is formed by patterning the semiconductor material with an array of scattering centres or atoms 7 having a second refractive index which is different from the first refractive index of the semiconductor material, thus forming a lattice structure within the photonic crystal layer 5. The lattice structure within the photonic crystal layer 5 is generally a periodic lattice structure. These scattering centres or atoms 7 having the second refractive index, different from the semiconductor material, may be provided by air gaps or voids. Alternatively, the scattering centres or atoms 7 may be filled by overgrowth with a suitable filler material having a second refractive index. 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. Electrical contacts 8 and 9 are located on the external surfaces of the PCSEL 1. The first electrical contact 8 is located on the output surface 10 of the PCSEL 1 and the second electrical contact 9 is provided on the opposite external surface of the PCSEL 1. As a result of the above described structure, when an electrical current is provided between the first 8 and second 9 electrical contacts, the PCSEL 1 begins to lase and the output field 11 is emitted from the output surface 10. As shown in Figure 1, the first electrical contact 8 is square shaped, with an aperture 12, through which output light 11 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 8 may be any appropriate shape provided the output field 11 can be emitted. The PCSEL 1 described with reference to Figure 1 is of a type that would produce speckle if incorporated within a display system. It is known that for M uncorrelated speckle sources, the contrast (C) of the resulting speckle pattern can be reduced in accordance with the following equation: r — 1 in\ c — As a result, a speckle pattern can be reduced by controlling one or more parameters of the M light sources e.g. timing of respective light outputs, spatial separation, frequency, phase and or polarisation. A number of techniques for achieving this with PCSEL light sources will now be described with reference to Figures 2 to 10. Figure 2 presents a cross sectional view of a photonic crystal surface emitting laser, as generally depicted by reference numeral 13, in accordance with an embodiment of the present invention. As shown in Figure 2, the structure of the photonic crystal surface emitting laser 13 is similar to that shown in Figure 1 and comprises a semiconductor substrate 2, a lower cladding layer 3, an active layer 4, a photonic crystal layer 5, an upper cladding layer 6 and a set of electrodes 8, 9. However, in Figure 2 the photonic crystal layer 5 is configured to provide two output fields 11a and 11b from the PCSEL 13. In particular, the photonic crystal layer 5 is designed to provide a first photonic crystal region 14a and a second photonic crystal region 14b that each provide a separate output field 11a and 11b respectively. The first 14a and second 14b photonic crystal regions of the PCSEL 13 each comprise a different lattice spacing of the scattering centres or atoms 7 as shown in Figure 2(b). The scattering centres or atoms 7 may be left as air gaps or voids within the photonic crystal layer 5. Alternatively, the scattering centres or atoms 7 may be filled by overgrowth with a suitable filler material. By changing the spacing or period between the scattering centres or atoms 7 in the crystal structure the position of the photonic bandgap is shifted, resulting in a wavelength shift of the output fields 11. Therefore, by providing a first lattice spacing (a) within the first 14a photonic crystal region, and a second lattice spacing (b), different from the first, within the second photonic crystal region 14b the output fields 11a and 11b will emit light at different wavelengths. The wavelength difference of the output fields 11 a and 11 b that can be achieved by varying properties of the photonic crystal layer 5 (such as the lattice spacing) is in the range of 0.01 nm to 100 nm. The lattice spacing may alternatively be adjusted within the first 14a and second 14b photonic crystal regions to introduce wavelength differences in the range 0.01 nm to 50 nm or in the range 0.01 nm to 10 nm. As such the different lattice spacings (a and b) act as a coherence perturbator, where the different lattice spacing of the first 14a and second 14b photonic crystal regions result in a reduced coherence between the output fields 11a and 11b. The different lattice spacing of the first 14a and second 14b photonic crystal regions allows for control over the relative coherence between the output fields 11 a and 11 b from each photonic crystal region 14a and 14b, and hence acts to mitigate speckle in their combined output. By employing different lattice spacings, a degree of incoherence is introduced between the different output fields 11a and 11b. As discussed above, speckle is essentially the result of interference between coherent light waves. Therefore, when output fields 11a and 11b are superimposed, their reduced coherence results in a reduction of observed speckle patterns. The speckle is further reduced due to the inherent spacing between the first 14a and second 14b photonic crystal regions. The PCSEL 13 of Figure 2, further comprises a power supply 15 which supplies a drive current to the electrical contacts 8 and 9. In continuous wave operation, the PCSEL 13 emits light with constant intensity. However, in order to reduce speckle it is possible to modulate the amplitude of the drive current applied to the electrical contacts 8 and 9 in order to modulate the intensity of the output fields 11 a and 11b. By modulating the laser's intensity the coherence between the output fields 11 a and 11 b is again reduced, hence reducing any speckle visible in the output of the PCSEL 13. The amplitude of the drive current should be modulated fast enough to ensure that the modulation is too rapid for the human eye to perceive as flicker (e.g. >50 Hz). This modulation of the amplitude of the drive current creates a rapid variation in the output intensity making any speckle pattern less noticeable to an observer. There are a number of other specific properties of the photonic crystal layer 5 that may alternatively, or additionally, be employed as a coherence perturbator for the PCSEL 13. By way of example, Figures 3 to 6 show some examples of alternative photonic crystal layers 5 that may be incorporated within the PCSEL 13. In Figure 3 the relative size of the scattering centres or atoms 7 are different in the first 14a and second 14b photonic crystal regions. In particular, the size of the circular shaped holes / atoms in the second 14b photonic crystal region are smaller than the first photonic crystal region 14a. In a similar manner to changing the spacing between the scattering centres or atoms 7, changing the relative size of the scattering centres or atoms 7 while keeping the lattice constant the same will alter the spacing between these elements 7. This change in spacing can affect the photonic band structure and, consequently, the optical properties, including the allowed wavelengths. As such, the size of the scattering centres or atoms 7 may also act as a coherence perturbator for the PCSEL 13. Similar wavelength ranges for the output fields 11 a and 11b, as discussed with reference to Figure 2, may be achieved by changing the relative size of the scattering centres or atoms 7. In Figure 4 the shape of the scattering centres or atoms 7 are different in the first 14a and second 14b photonic crystal regions. The first photonic crystal region 14a comprises circular shaped scattering centres or atoms 7 while the second photonic crystal region 14b comprises square shaped scattering centres or atoms 7. Changing the shape of the scattering centres or atoms 7 within a photonic crystal layer 5 can significantly impact its optical properties, including the output wavelengths generated. Different shapes introduce different symmetries to the periodic lattice. This change in symmetry can result in variations in the photonic band structure and dispersion relations. Furthermore, different shapes can influence the diffraction and scattering of light within the photonic crystal layer 5. This can affect the direction and intensity of output beams 11a and 11 b emerging from the PCSEL 13 at specific wavelengths. As will be appreciated by the skilled reader, in alternative embodiments, the scattering centres or atoms 7 of the first 14a and second 14b photonic crystal regions may comprise different regular or irregular geometric shapes (e.g. circular, oval, diamond, square or chevron shapes) as required. As shown in Figure 5, changing the lattice shape of the photonic crystal layer 5 can also have an impact on its optical properties, including how light interacts with the crystal and the output wavelengths of the output fields 11 a and 11 b. Therefore, different lattice geometries can also be used as a coherence perturbator for the PCSEL 13. Note that any suitable lattice structure may be employed for the first 14a and second 14b photonic crystal regions. For example, the first 14a and second 14b photonic crystal regions may comprise a regular or irregular lattice structure (e.g. triangular, hexagonal or Kagome) having different lattice constants. As shown in Figure 6, the orientation of scattering centres or atoms 7 within the photonic crystal layer 5 can also be adjusted to act as a coherence perturbator of the PCSEL 13. The orientation of the scattering centres or atoms 7 can have a significant effect on the polarisation of the output fields 11a and 11b from the PCSEL 13. The specific photonic crystal orientation influences the polarisation characteristics of the emitted light. By carefully controlling the orientation of the scattering centres or atoms 7 during fabrication of the PCSEL 13, it is possible to manipulate the polarisation characteristics of the emitted light. Employing different polarisations for the output fields 11a and 11b by ensuring the specific orientation of the scattering centres or atoms 7 within photonic crystal differs within the first 14a and second 14b photonic crystal regions can be highly effective in reducing speckle. As previously discussed, speckle arises from the coherent nature of PCSELs, which generally provide an output with a single polarisation. By providing output fields 11a and 11b which are different in polarisation the interference patterns causing speckle are disrupted. Therefore, this effect can be used to provide a more uniform and less speckled image. Other properties that may act, or contribute to a coherence perturbator include the refractive index contrast between the materials within the photonic crystal layer 5 and or filling fraction of the scattering centres or atoms 7 within the first 14a and second 14b photonic crystal regions. For instance, a higher refractive index contrast between the materials within of the photonic crystal layer can lead to wider bandgaps and hence control over specific wavelengths of the output beams 11a and 11b. The filling fraction refers to the proportion of the scattering centres or atoms 7 occupied by one of the materials. Adjusting the filling fraction can therefore also alter the photonic band structure. Any suitable property, or combination of the above-described properties, of the photonic crystal layer 5, as discussed with reference to Figures 2 to 6, may be employed to act as the coherence perturbator. This results in output fields 11a and 11b of the first 14a and second 14b photonic crystal regions which are incoherent with respect to one another. When such output fields 11 a and 11b are combined, they disrupt the coherent nature of the light and reduce the intensity variations that lead to speckle patterns. This results in a smoother and more visually appealing output, which can be particularly advantageous for the use of PCSELs within display systems. Note that each of the above techniques can be either used separately or in combination in order to provide the required coherence perturbation for the output fields 11 a and 11 b of the PCSEL 13. Any other suitable property of the photonic crystal layer 5 can instead be modified in order to change the polarisation and / or wavelength of the output fields 11a and 11b relative to one another. It will be appreciated by the skilled reader that semiconductor materials such as Gallium Arsenide (GaAs) or Gallium Nitride (GaN) may be employed to produce the PCSEL 14 of Figure 1. These semiconductor materials provide output wavelengths within the visible range of the electromagnetic spectrum. Therefore, such a wavelength range is particularly useful for utilizing the PCSEL as a light source within display systems. However, alternative semiconductor materials may instead be employed, as suitable, for the specific application. A PCSEL 16 in accordance with an alternative embodiment of the present invention is presented in Figure 7. The PCSEL 16 is similar to the embodiment of Figure 2, where the selection of the specific properties of photonic crystal layer 5 acts as a coherence perturbator. In particular, the properties of photonic crystal layer 5 are designed to be different within the first 14a and second 14b photonic crystal region to provide output fields 11a and 11b with different wavelengths and / or polarisations. However, the PCSEL 16 of Figure 7 comprises a first 8, second 9 and third 17 electrical contact. The first electrical contact 8 is arranged on the external surface PCSEL 16 adjacent to the semiconductor substrate 2. The second electrical contact 9 is arranged on the output surface 10 within the area of the PCSEL 16 comprising the first photonic crystal region 14a. The third electrical contact 17 is arranged on the on the output surface 10 within the area of the PCSEL 16 comprising the second photonic crystal region 14b. The second 9 and third 17 electrical contacts comprise an aperture 12 through which the first 11a and second 11b output fields are emitted. When an electrical current is provided between the first 8 and second 9 electrical contacts the output field 11 a is directed to the output surface 10 from the first photonic crystal region 14a. When an electrical current is provided between the first 8 and third 17 electrical contacts the output field 11b is directed to the output surface 10 from the second photonic crystal region 14b. Therefore, by providing the second 9 and third 17 electrical contacts, both the first 14a and second 14b photonic crystal regions are independently addressable. Since the first 14a and second 14b photonic crystal regions are independently addressable the timing of their output fields 11 a and 11 b can be controlled in order to reduce the effects of speckle. A PCSEL 18 in accordance with a further alternative embodiment of the present invention is presented in Figure 8. In Figure 8, the photonic crystal layer 5 is again configured to provide two output fields 11 a and 11b from the PCSEL 18. However, unlike for the previous embodiments of Figures 2 and 7, in Figure 8 the photonic crystal layer 5 does not act as the coherence perturbator for the PCSEL 18. Instead, a phase shifting film 19 is arranged within the PCSEL 18 to act as the coherence perturbator. In the presently described embodiment, the phase shifting film 19 is located between the photonic crystal layer 5 and the output surface 10. As will be appreciated by the skilled reader the phase shifting film 19 may be located in any suitable location within the PCSEL 18 provided it interacts with the light directed towards the output surface 10 by the first 14a and second 14b photonic crystal regions. The phase shifting film 19 comprises a varying thickness that changes the phase of light as it passes through it. In particular, the phase shifting film 19 is designed to alter the phase of incident light waves depending on the thickness of the film 19 at different points. The varying thickness of the film 19 may be continuous, as presented in Figure 8, or stepwise between the first 14a and second 14b photonic crystal regions. As such, the phase of output fields 11 a and 11 b emitted from the first 14a and second 14b photonic crystal regions respectively, are different as a result of the phase shifting film 19. Therefore, the phase shifting film 19 can address or mitigate speckle. The phase shifting film 19 is engineered to introduce controlled phase shifts between the output fields 11a and 11b that disrupt the interference patterns responsible for speckle. This results in speckle reduction or modification and improves the quality of the PCSEL 18 output for use in display systems. The specific design of the phase shifting film 19 will depend on the particular wavelength of the output fields 11 a and 11 b, the intended application, and desired reduction in speckle. As will be appreciated by the skilled reader, the coherence perturbator of the PCSEL 18 may comprise the phase shifting film 19 in combination with one or more modifications to the photonic crystal layer 5, as discussed with reference to the PCSEL’s 13, 16 of Figures 2 to 6. Furthermore, as for the embodiment of Figure 7, the PCSEL 18 may alternatively comprise separate electrical contacts arranged on the output surface 10 with the area of the PCSEL 18 containing the first 14a and second 14b photonic crystal regions, so that each region can be independently addressable, thus providing a further alternative for addressing or mitigating speckle. A PCSEL 20 in accordance with an alternative embodiment of the present invention is presented in Figure 9. In Figure 9 the coherence perturbator of the PCSEL 20 is provided by a phase modulator 21. More specifically, the phase modulator 21 is incorporated into PCSEL 20 by applying a phase modulated drive current to the first 8 and second 9 electrical contacts. A phase-modulated drive current can be employed as a technique to help reduce or mitigate speckle patterns. Speckle patterns arise from random phase variations in the laser light. Phase modulation involves intentionally changing the phase of the output fields 11a and 11b in a controlled manner to introduce predictable and controlled phase variations into the light and can effectively disrupt the relative coherence between the output fields 11a and 11b. The effectiveness of phase modulation for speckle reduction depends on various factors, including the modulation frequency and amplitude of the drive current. The specific modulation techniques and parameters can therefore be tailored to the particular application, and its requirements. As will be appreciated by the skilled reader, the coherence perturbator of the PCSEL 20 may alternatively be provided by a combination of the phase modulator 21 and one or more of the coherence perturbators discussed with reference to the PCSEL’s 13, 16 and 18 of Figures 2 to 6 and 8. Furthermore, as for the embodiment of Figure 7, the PCSEL 20 may alternatively comprise separate electrical contacts arranged on the output surface 10 with the area of the PCSEL 20 containing the first 14a and second 14b photonic crystal regions, so that each region can be independently addressable. A PCSEL 22 in accordance with an alternative embodiment of the present invention is presented in Figure 10. In particular, Figure 10 (a) illustrates a cross section of the PCSEL 22 and Figure 10 (b) illustrates a top view of the photonic crystal layer 5 of the PCSEL 22. The structure of the PCSEL 22 comprises a semiconductor substrate 2, a lower cladding layer 3, an active layer 4, a photonic crystal layer 5, an upper cladding layer 6 and a set of electrodes 8, 9. The PCSEL 22 further comprises an array of photonic crystal regions 14a to 14i within the photonic crystal layer 5, where each element of the array provides an individual output field 11 a to 111 respectively. In Figure 10, the coherence perturbator is provided by changing one or more of the properties of the photonic crystal layer 5 within at least two of photonic crystal regions 14a to 14i relative to one another. As will be appreciated by the skilled reader the photonic crystal layer 5 could have different properties within each of photonic crystal regions 14a to 14i. As will be further appreciated by the skilled reader, the coherence perturbator of the PCSEL 22 could instead comprise a phase shifting film 19, a phase modulator 21 or a combination of one or more of an phase shifting film, a phase modulator 21 and modified properties of the photonic crystal layer 5 within one or more photonic crystal regions 14. The array of photonic crystal regions 14a to 14i shown in Figure 10 is a regular three by three array. A smaller or larger array of regions 14 could alternatively be provided as appropriate for the required application. Furthermore, an irregular array could alternatively be provided. The embodiment presented in Figure 7 could also be incorporated into the PCSEL 22. In this case a separate electrical contacts are provided on the output surface 10 within each photonic crystal region 14a to 14i. As such an addressable array of photonic crystal regions 14a to 141 is provided, where it is possible to separately control each output field 11a to 11i. In summary, the present invention provides an alternative surface emitting laser device to those known in the art. In particular, the present invention relates to a PCSEL, where the photonic crystal layer of the PCSEL is split into an array of photonic crystal regions that each generate an output field from an output surface. The PCSEL comprises a coherence perturbator which allows for control over the relative coherence between the two or more output fields and hence acts to mitigate speckle in the combined output of the PCSEL. Reducing the speckle from a PCSEL device is advantageous within specific applications, and most notably for use within display systems, where speckle is highly undesirable as it effects the quality and contrast of the displayed image. Providing a degree of incoherence between the different output fields of the PCSEL results in a reduction in the visibility of speckle on a displayed image of a display system. The speckle is further reduced due to the inherent spacing between the photonic crystal regions of the surface emitting laser device. A surface emitting laser device and a method for manufacturing is disclosed. The surface emitting laser device comprises an active layer configured to generate laser radiation and a photonic crystal layer comprising an array of scattering centres or atoms within a semiconductor material and optically coupled to the active layer. The photonic crystal layer comprises an array of two or more photonic crystal regions that each direct a separate output field to the output surface of the device. The surface emitting laser device further comprises a coherence perturbator which acts to control the relative coherence between the two or more output fields. The coherence perturbator acts to reduce speckle in the output from the surface emitting laser device. Such a reduction in speckle is advantageous for a variety of applications, and in particular when utilising the surface emitting laser device as a light source for display devices. 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. 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. 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

ms3456789101112131415161718192021222324252627282930313233341. A photonic crystal surface emitting laser comprising:an active layer configured to generate laser radiation,a photonic crystal layer comprising an array of scattering centres or atoms arranged within a semiconductor material and optically coupled to the active layer, the photonic crystal layer comprising two or more photonic crystal regions that each direct a separate output field to an output surface, anda coherence perturbator comprising a different filling fraction for the scattering centres or atoms within at least two of the photonic crystal regions, wherein the coherence perturbator acts to reduce the relative coherence between the two or more output fields.

2. A photonic crystal surface emitting laser as claimed in claim 1, wherein the photonic crystal surface emitting laser further comprises a first and a second electrical contact arranged on opposite external surfaces of the surface emitting laser device.

3. A photonic crystal surface emitting laser as claimed in claim 2, wherein the second electrical contact comprises an aperture which defines the output surface of the surface emitting laser device.

4. A photonic crystal surface emitting laser as claimed in claims 2 or 3, wherein the coherence perturbator further comprises an amplitude modulated drive current applied to the first and second electrical contacts.

5. A photonic crystal surface emitting laser as claimed in claim 4, wherein amplitude modulated drive current applied to the first and second electrical contacts comprises an amplitude modulation frequency of at least 50 Hz.

6. A photonic crystal surface emitting laser as claimed in any of the preceding claims, wherein the coherence perturbator further comprises providing a different lattice spacing for the photonic crystal layer within at least two of the photonic crystal regions.

357. A photonic crystal surface emitting laser as claimed in any of the preceding claims wherein the coherence perturbator further comprises providing a different orientation, different size and or different shape of the scattering centres or atoms within at least two of the photonic crystal regions.

8. A photonic crystal surface emitting laser as claimed in any of the preceding claims wherein the coherence perturbator further comprises a different refractive index contrast between the scattering centres or atoms and the semiconductor material within at least two of the photonic crystal regions.

9. A photonic crystal surface emitting laser as claimed in any of the preceding claims wherein the wavelength difference between the two or more output fields is in the range 0.01 nm to 100 nm, or is in the range 0.01 nm to 50 nm, or is in the range 0.01 nm to 10 nm.

10. A photonic crystal surface emitting laser as claimed in any of the preceding claims wherein the coherence perturbator further comprises a phase shifting film arranged between the photonic crystal layer and the output surface.

11. A photonic crystal surface emitting laser as claimed in claim 12 wherein the phase shifting film has a varying thickness.

12. A photonic crystal surface emitting laser as claimed in claim 13 wherein the thickness of the phase shifting film varies between at least two of the photonic crystal regions.

13. A photonic crystal surface emitting laser as claimed in any of the preceding claims wherein the coherence perturbator further comprises a phase modulator.

14. A photonic crystal surface emitting laser as claimed in claim 14 wherein the phase modulator comprises modulating the phase of the drive current applied to the electrical contacts.

15. A photonic crystal surface emitting laser as claimed in any of the preceding claims wherein the photonic crystal surface emitting laser further comprises two or more1 electrical contacts arranged on the output surface of the surface emitting laser2 device.34 16. A photonic crystal surface emitting laser as claimed in claim 16 wherein each of the5 two or more electrical contacts arranged on the output surface comprise an6 aperture through which the output fields from each photonic crystal region are7 emitted.89 17. A photonic crystal surface emitting laser as claimed in any of the preceding claims10 wherein the two or more output fields have wavelengths that are within visible11 spectrum.1213 18. A photonic crystal surface emitting laser as claimed in claim 18 wherein the two or14 more output fields have a wavelength in the range of 600 nm to 700 nm.15M 16 19. A photonic crystal surface emitting laser as claimed in claim 18 wherein the two or17 more output fields have a wavelength in the range of 400 nm to 550 nm.CM is19 20. A photonic crystal surface emitting laser as claimed in any of the preceding claims,20 wherein the photonic crystal surface emitting laser further comprises a first1 21 cladding layer and a second cladding layer located on opposite sides of the active22 layer.2324 21. A photonic crystal surface emitting laser as claimed in any of the preceding claims,25 wherein the active layer comprises a Multi-Quantum Well (MQW) active layer.2627 22. A photonic crystal surface emitting laser as claimed in any of the preceding claims,28 wherein the photonic crystal surface emitting laser further comprises a substrate29 upon which the layers of the surface emitting laser are formed.3031 23. A method of manufacturing a photonic crystal surface emitting laser the method32 comprising:33 providing an active layer configured to generate laser radiation;34 providing a photonic crystal layer comprising an array of scattering centres or35 atoms within a semiconductor material and optically coupled to the active layer;CM1 providing the photonic crystal layer with two or more photonic crystal regions and2 configuring each photonic crystal region to direct a separate output field to an3 output surface; and4 providing a coherence perturbator comprising a different filling fraction for the5 scattering centres or atoms within at least two of the photonic crystal regions;6 wherein the coherence perturbator is configured to reduce the relative coherence7 between the two or more output fields.89

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