Photonic integrated circuit

Abstract

A photonic integrated circuit and a method for manufacturing is disclosed. The photonic integrated circuit comprises a semiconductor gain medium configured to generate laser radiation that is spatially separated from an output coupling region. The photonic integrated circuit further comprises a semiconductor optical waveguide that optically couples the semiconductor gain medium and output coupling region and defines a device plane of the photonic integrated circuit. The output coupling region comprises a semiconductor grating which redirects the laser radiation out of the device plane to form an output field of the photonic integrated circuit. The characteristics of the semiconductor grating structure can be used to manipulate the output field properties. Furthermore, the above photonic integrated circuit exhibits reduced back-reflected light toward the semiconductor gain medium thus reducing the need for further optical isolating components to be incorporated within the photonic integrated circuit.

Description

[0001] Photonic Integrated Circuit

[0002] The present invention relates to the field of photonic integrated circuits. Specifically, the present invention relates to photonic integrated circuits comprising a semiconductor laser system and methods for manufacturing the same.

[0003] A photonic integrated circuit (PIC) is a chip that integrates two or more photonic components onto a single semiconductor chip. A PIC can manipulate and control light and enables compact, efficient, and cost-effective solutions for various applications including optical communication, sensing, and signal processing.

[0004] The typical photonic components forming the PIC usually include a light source and a waveguide. Other components found on the PIC may include optical modulators, detectors, power splitters, filters, and more. The waveguide is responsible for guiding and directing light to enable communication between different components on the chip.

[0005] Semiconductor laser devices commonly function as the light sources of the PIC. Semiconductor laser devices are those based on a 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.

[0006] 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 feedback 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).

[0007] 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 feedback is produced by the employment of a grating structure.

[0008] While significant advancements have been made to enhance the performance and efficiency of photonic integrated circuits it is desirable to provide further improvements and alternative devices for the large range of commercial applications. In particular, improved output power efficiency, beam quality, beam control, increased wavelength range, single mode operation and multi-wavelength production are all desirable features. Such features require further development for both existing and potential future applications of photonic integrated circuits.

[0009] Summary of Invention

[0010] It is therefore an object of an embodiment of the present invention to provide an alternative photonic integrated circuit to those known in the art.

[0011] It is a further object of embodiment of the present invention to provide a photonic integrated circuit that obviates or mitigates one or more drawbacks or disadvantages of the prior art.

[0012] 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 photonic integrated circuit comprising: a semiconductor gain medium configured to generate laser radiation; a semiconductor optical waveguide defining a device plane of the photonic integrated circuit and optically coupled to the semiconductor gain medium; an output coupling region of the semiconductor optical waveguide spatially separated from the semiconductor gain medium; wherein the output coupling region comprises a semiconductor grating structure configured to redirect laser radiation optically coupled within the semiconductor optical waveguide out of the device plane to form an output field of photonic integrated circuit.

[0013] Laser radiation optically coupled within the semiconductor optical waveguide interacts with the semiconductor grating structure which redirects the light out of the photonic integrated circuit due to the specific arrangement of elements within the semiconductor grating structure. In particular, the properties of the semiconductor grating structure may be used to select the precise output wavelength of the output field, in addition to shaping and guiding the incoming light.

[0014] The structure of the above-described photonic integrated circuit provides several advantages. These include the ability to select the output power level of the output field, determining the precise wavelength of the output field, and control over the beam quality and emission direction. Additionally, the semiconductor grating structure helps reduce the amount of light reflected back towards the semiconductor gain medium. This reduction in the level of back reflected light reduces the need for further optical isolating components to be incorporated within the photonic integrated circuit.

[0015] Significantly the components of the photonic integrated circuit can all be processed from the same semiconductor material, the choice of which is dependent on the desired wavelength of the first output field. For example, the semiconductor material may comprise Indium Phosphide (InP), Gallium Arsenide (GaAs), Gallium Nitride (GaN) or Gallium Antimonide (GaSb). This allows for the photonic integrated circuit to be produced as an integrated structure based on a single semiconductor material. Preferably the semiconductor grating structure comprises a first layer of periodic elements forming an even order linear grating. Most preferably the first layer of periodic elements forms a second order linear grating.

[0016] Most preferably, the semiconductor grating structure further comprises a second layer of periodic elements located on top of the first layer of periodic elements, the second layer of periodic elements forming an odd order linear grating. Most preferably the second layer of periodic elements forms a first order linear grating.

[0017] Preferably a refractive index of the first layer of periodic elements (n2) is greater than or equal to a refractive index ( ) of the semiconductor optical waveguide. Most preferably a refractive index of the second layer of periodic elements (n3) is greater than the refractive index (n2) of the first layer of periodic elements.

[0018] The semiconductor grating structure is preferably arranged on the semiconductor optical waveguide, or partially or fully embedded in the semiconductor optical waveguide.

[0019] Preferably the photonic integrated circuit comprises a semiconductor laser in which the semiconductor gain medium is located. Preferably the semiconductor laser comprises an edge emitting laser.

[0020] Preferably, the semiconductor optical waveguide comprises a first tapered section wherein the first tapered section reduces in width in a direction from the semiconductor gain medium towards the output coupling region.

[0021] Preferably, the semiconductor optical waveguide comprises a second tapered section wherein the second tapered section increases in width in a direction from the semiconductor gain medium towards the output coupling region.

[0022] Preferably the semiconductor gain medium comprises a gain tapered section wherein the gain tapered section reduces in width in a direction from the semiconductor gain medium towards the output coupling region.

[0023] Most preferably the gain tapered section is located between first tapered section and the output coupling region. Optionally, the photonic integrated circuit comprises a spacer semiconductor layer located between semiconductor gain medium and the semiconductor optical waveguide. Most preferably the spacer semiconductor layer comprises a tapered section that reduces in width in a direction from the semiconductor gain medium towards the output coupling region.

[0024] The tapered sections of the semiconductor optical waveguide, the semiconductor gain medium and the spacer semiconductor layer allow for the light to efficiently couple into the optical waveguide and determines the shape and size of the laser radiation within the waveguide.

[0025] Optionally, the semiconductor optical waveguide comprises a notch located between the semiconductor gain medium and the output coupling region.

[0026] The notch acts to reduce the amount of back reflected light that is directed towards the semiconductor gain medium. This reduction in back reflected light reduces the need for optical isolating components to be incorporated within the photonic integrated circuit.

[0027] Preferably, the width of the notch is in the range of 10nm to 500nm. Alternatively, the width of the notch is in the range of 50nm to 150nm. Preferably the notch is rectangular in shape. The notch may comprise a tapered cross sectional profile.

[0028] Preferably the notch extends fully through the semiconductor optical waveguide. Optionally the notch extends partially through the semiconductor optical waveguide. Preferably the notch comprises an air gap in the semiconductor optical waveguide. Optionally the notch is filled with a semiconductor or dielectric material.

[0029] Optionally the semiconductor waveguide comprises an angled face on one end of the semiconductor optical waveguide. The reduction in back reflected light within the photonic integrated circuit is further improved by providing this angled face on the semiconductor optical waveguide. Most preferably the angled face is arranged on the opposite end of the semiconductor waveguide to the output coupling region. Preferably the first output field is parallel to a normal of an output surface of the photonic integrated circuit. Alternatively, the first output field is non parallel to the normal of the output surface.

[0030] Preferably the photonic integrated circuit further comprises an output coupler.

[0031] Preferably the output coupling region comprises an array of semiconductor grating structures each of which generates an output field from the photonic integrated circuit.

[0032] Most preferably the elements of the array of semiconductor grating structures are arranged to be coplanar. The elements of the array of semiconductor grating structures may be coplanar with a plane that is parallel to the device plane.

[0033] Preferably the array of semiconductor grating structures is a regular array, wherein a semiconductor grating structure is located at each site of the array. Alternatively, the array of semiconductor grating structures is an irregular array. The irregular array may comprise a regular array where there is no semiconductor grating structure located at one or more of the array sites.

[0034] Preferably, the photonic integrated circuit generates two or more output fields. The two or more output fields may be at different wavelengths or the same wavelength.

[0035] Preferably the two or more output fields are directed into separate output couplers. Optionally, the two or more output fields are directed into the same output coupler.

[0036] Optionally one or more wavelength-selective reflectors may be placed around one or more sides of the semiconductor optical waveguide. The presence of the one or more reflectors act to reduce the effects of undesired light leakage from the photonic integrated circuit.

[0037] According to a second aspect of the present invention there is provided a method of manufacturing a photonic integrated circuit the method comprising: providing a semiconductor gain medium configured to generate laser radiation; providing a semiconductor optical waveguide defining a device plane of the photonic integrated circuit and optically coupled to the semiconductor gain medium; providing an output coupling region of the semiconductor optical waveguide that is spatially separated from the semiconductor gain medium, providing a semiconductor grating structure within the output coupling region; and configuring the semiconductor grating structure to redirect laser radiation optically coupled within the semiconductor optical waveguide out of the device plane to form an output field of photonic integrated circuit.

[0038] Preferably providing a semiconductor grating structure within the output coupling region comprises providing a first layer of periodic elements forming an even order linear grating. Most preferably the first layer of periodic elements forms a second order linear grating.

[0039] Most preferably, providing a semiconductor grating structure within the output coupling region further comprises providing a second layer of periodic elements located on top of the first layer of periodic elements, the second layer of periodic elements forming an odd order linear grating. Most preferably the second layer of periodic elements forms a first order linear grating.

[0040] Preferably a refractive index of the first layer of periodic elements (n2) is greater than or equal to a refractive index ( ) of the semiconductor optical waveguide. Most preferably a refractive index of the second layer of periodic elements (n3) is greater than the refractive index (n2) of the first layer of periodic elements.

[0041] Embodiments of the second aspect of the present invention may comprise features to implement the preferred or optional features of the first aspect of the present invention.

[0042] According to a third aspect of the present invention there is provided a semiconductor grating structure comprising a first layer of periodic elements forming an even order grating and a second layer of periodic elements, located on the first layer of periodic elements, the second layer of periodic elements forming an odd order grating.

[0043] Most preferably the first layer of periodic elements forms a second order grating.

[0044] Most preferably the second layer of periodic elements form a first order grating. Optionally, the semiconductor grating structure further comprises a semiconductor substrate on which first layer of periodic elements is formed.

[0045] Preferably a refractive index of the first layer of periodic elements (n2) is greater than or equal to a refractive index ( ) of the semiconductor substrate. Most preferably a refractive index of the second layer of periodic elements (n3) is greater than the refractive index (n2) of the first layer of periodic elements.

[0046] Embodiments of the third aspect of the present invention may comprise features to implement the preferred or optional features of the first or second aspects of the present invention.

[0047] Brief Description of Drawings

[0048] There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which:

[0049] Figure 1 presents (a) a cross sectional view and (b) a top view of a photonic integrated circuit (PIC) in accordance with an embodiment of the present invention;

[0050] Figures 2 (a) to (d) present theoretical modelling of the position and shape of the generated laser radiation at the different regions (A, B, C and D) of the photonic integrated circuit (PIC) of Figure 1 ;

[0051] Figures 3 (a) to (c) present cross-sectional views of semiconductor grating structures in accordance with an embodiment of the present invention;

[0052] Figure 4 presents theoretical modelling of the light emitted from the photonic integrated circuit as a function of thickness of the waveguide layer, for a semiconductor grating structure of the type shown in Figure 3 (a);

[0053] Figure 5 presents theoretical modelling of the light emitted from the photonic integrated circuit as a function of the number of periods of the semiconductor grating structure, for a semiconductor grating structure of the type shown in Figure 3 (c); Figure 6 presents (a) a cross sectional view and (b) a top view of a photonic integrated circuit (PIC) in accordance with an alternative embodiment of the present invention;

[0054] Figure 7 presents (a) a cross sectional view and (b) a top view of a photonic integrated circuit (PIC) in accordance with an alternative embodiment of the present invention;

[0055] Figure 8 presents (a) a cross sectional view and (b) a top view of a photonic integrated circuit (PIC) in accordance with an alternative embodiment of the present invention;

[0056] Figure 9 presents (a) a cross sectional view and (b) a top view of a photonic integrated circuit (PIC) in accordance with an alternative embodiment of the present invention;

[0057] Figure 10 presents a cross sectional view of a photonic integrated circuit (PIC) in accordance with an alternative embodiment of the present invention; and

[0058] Figure 11 presents a top view of a photonic integrated circuit (PIC) in accordance with an alternative embodiment of the present invention.

[0059] 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.

[0060] Detailed Description

[0061] Figure 1 (a) and (b) present a cross sectional view and a top view, respectively, of a photonic integrated circuit (PIC), as generally depicted by reference numeral 1 , in accordance with an embodiment of the present invention. Axes are included for reference, where the width, length, and height of the photonic integrated circuit 1 are along the x, y and z axis, respectively.

[0062] The photonic integrated circuit 1 can be seen to comprise a semiconductor gain medium 2, located on a spacer semiconductor layer 3. The semiconductor gain medium 2 generates the laser radiation of the PIC 1 and defines a laser region 4. The PIC 1 also comprises an output coupling region 5, where the output coupling region 5 is spatially separated from the laser region 4 by a semiconductor optical waveguide 6. The semiconductor optical waveguide 6 acts to optically couple the semiconductor gain medium 2 and the output coupling region 5. In the presently described embodiment, the semiconductor optical waveguide 6 extends from the laser region 4 into the output coupling region 5 and defines a device plane of the photonic integrated circuit 1 (i.e. the x-y plane of Figure 1). As will be appreciated by a person skilled in the art, the entire photonic integrated circuit 1 may be located on a substrate layer, not explicitly shown in Figure 1 .

[0063] The semiconductor gain medium 2 of the PIC 1 may be provided by a semiconductor laser, for example an edge emitting laser. The edge emitting laser may be a Fabry Perot laser (FP), a distributed-feedback laser (DFB laser) or distributed Bragg reflector laser (DBR laser). Within each type of edge emitting laser various semiconductor layers are carefully engineered to create a functional laser device. These layers serve specific purposes in achieving optical gain and the controlled emission of laser light.

[0064] The spacer semiconductor layer 3 depicted in Figure 1 comprises a semiconductor material such as Indium Phosphide (InP). Each layer that is formed on the spacer semiconductor layer 3 is then generally made from different combinations of InP or other materials like Indium Gallium Arsenide (InGaAs) or Indium Gallium Phosphide (InGaP). Such materials generally result in a typical output wavelength of 1 .3 pm. This output wavelength is commonly used in fibre-optic communication systems for long-distance data transmission. However, it is known in the art to produce wavelengths in the range of 1 pm to 2.1 pm from Indium Phosphide (InP) based devices.

[0065] In the embodiment presented in Figure 1 , an Indium Phosphide (InP) layer provides the semiconductor optical waveguide 6 of the photonic integrated circuit 1 . The semiconductor optical waveguide 6 can be seen to comprise two distinct adiabatic tapered sections, where the first and second adiabatic tapered sections are denoted as 7a and 7b, respectively. The adiabatic tapered sections 7a and 7b efficiently couple light between different regions of the PIC 1 . They provide for a gradual change in the width and geometry of the semiconductor optical waveguide 6 within the device plane to match the mode profile of light between the different regions of the PIC 1 . The adiabatic tapered sections 7a and 7b are essentially employed as spot size converters, where gradually adjusting the semiconductor optical waveguide 6 dimensions allows for the light to be effectively channelled with minimal dispersion and reduced optical loss. The first adiabatic tapered section 7a provides a gradual transition of the width of the semiconductor optical waveguide 6 and the spacer semiconductor layer 3 within the region of the PIC 1 comprising the semiconductor gain medium 2 i.e., at the output of the laser region 4. In particular, the first adiabatic tapered section 7a results in the semiconductor optical waveguide 6 and the spacer semiconductor layer 3 becoming narrower as they move away from the laser region 4, following the direction of the y-axis, as illustrated in Figure 1 . The first adiabatic tapered section 7a aids in efficiently coupling generated laser radiation from the semiconductor gain medium 2 into the semiconductor optical waveguide 6, as the light travels from the laser region 4 towards the output coupling region 5. The first adiabatic tapered section 7a of the semiconductor optical waveguide 6 and the spacer semiconductor layer 3 facilitates efficient mode conversion during optical coupling into the semiconductor optical waveguide 6. Moreover, it reduces optical losses by gradually matching the optical mode to the required size of the semiconductor optical waveguide 6 i.e. the size at the end of the adiabatic taper section 7a.

[0066] In the region between the first 7a and second 7b adiabatic tapered sections the size of the semiconductor optical waveguide 6 does not change. Between the first 7a and second 7b adiabatic tapered sections the semiconductor optical waveguide 6 is passive, i.e. it does not actively manipulate the light passing through it. This part of the semiconductor optical waveguide 6 acts to guide the light to the output coupling region 5.

[0067] The second adiabatic tapered section 7b of the semiconductor optical waveguide 6 then gradually increases the width along the direction of the y-axis, as the semiconductor optical waveguide 6 enters the output coupling region 5. Therefore, the second adiabatic tapered section 7b of the semiconductor optical waveguide 6 ensures that the laser radiation is efficiently coupled into the output coupling region 5 of the PIC 1 .

[0068] It is noted that the semiconductor gain medium 2 also comprises an adiabatic tapered section 8. The adiabatic tapered section 8 of the semiconductor gain medium 2 provides a gradual change in the size or shape of the optical mode for the generated laser radiation. In Figure 1 , the adiabatic tapered section 8 acts to reduce the spot size of the laser radiation. The adiabatic taper section 8 matches the mode of incoming light to the mode of the semiconductor optical waveguide 6, to further enhance the coupling efficiency. In the present embodiment, the adiabatic tapered section 8 of the semiconductor gain medium 2 follows directly after the first adiabatic tapered section 7a of the semiconductor optical waveguide 6 and the spacer semiconductor layer 3.

[0069] Figures 2(a) through (d) present theoretical modelling depicting the position and shape of the generated laser radiation at various locations within the photonic integrated circuit (PIC) 1 of Figure 1 . More specifically, it shows the position and shape of the generated laser radiation at the locations A, B, C and D respectively,

[0070] In Figure 2(a), the laser radiation is depicted within the laser region 4 of the PIC 1 (i.e. location A of Figure 1 ). The laser radiation is generated within the semiconductor gain medium 2 in the laser region 4. Therefore, in Figure 2(a) the laser radiation is primarily within the semiconductor gain medium 2.

[0071] However, as the laser radiation emerges from the laser region 4, it passes through the first adiabatic tapered section 7a of the semiconductor optical waveguide 6 and the spacer semiconductor layer 3. Figure 2(b) shows the position of the laser radiation directly after exiting the first adiabatic tapered section 7a, corresponding to location B of Figure 1 . The profile of the light is elongated along the vertical direction (i.e. along the z axis of Figure 1 ) indicating the initiation of coupling into the semiconductor optical waveguide 6.

[0072] Figure 2(c) depicts location C of Figure 1 , showing the position of the laser radiation directly after travelling through the adiabatic tapered section 8 of the semiconductor gain medium 2. This adiabatic tapered section 8 acts to efficiently couple the generated laser radiation into the semiconductor optical waveguide 6. This is clear from Figure 2(c) where the light is fully coupled into the semiconductor optical waveguide 6 leaving only a negligible amount of laser radiation within the semiconductor gain medium 2 of the PIC 1 .

[0073] Lastly, Figure 2(d) shows the shape and position of the light within the semiconductor optical waveguide 6, before proceeding into the second adiabatic tapered section 7b of the semiconductor optical waveguide 6 and to the output coupling region 5 of the PIC 1 . More specifically, it illustrates the light within the passive area of the semiconductor optical waveguide 6 at location D of Figure 1 , where the light remains confined within the semiconductor optical waveguide 6. As seen in Figure 1 , the output coupling region 5 comprises a semiconductor grating structure 9. The semiconductor grating structure 9 is a patterned structure consisting of repeating features or elements which may be arranged in a regular or irregular grid across one, two or three dimensions. Such semiconductor grating structures 9 are employed to manipulate the behaviour of light or other electromagnetic waves based on their interactions with the periodic structure. The properties of the semiconductor grating structure 9 depend on the spacing, size, shape, and orientation of the repeating elements.

[0074] In the presently described embodiment, the semiconductor grating structure 9 comprises a one-dimensional grating structure extending along the length of the device (i.e. along the y-axis) the function of which is to redirect light optically coupled within the semiconductor optical waveguide 6 out of the device plane (the x-y plane), to form an output field 10 of the photonic integrated circuit 1 .

[0075] As can be seen from Figure 1 , the photonic integrated circuit 1 may further comprise a reflector 11 located within the output coupling region 5 at the far side of the semiconductor optical waveguide 6. The function of the reflector 11 is to retain light optically coupled into the semiconductor optical waveguide 6 within the device plane (the x-y plane) of the photonic integrated circuit 1 . In the presently described embodiment, the reflector 11 comprises a distributed Bragg reflector (DBR). The DBR 11 is configured to reflect light that is incident upon it, and in particular, in the device plane light travelling out of the semiconductor optical waveguide 6. The DBR 11 is specifically designed to reflect light at a selected wavelength i.e., the wavelength of the output field 10 The DBR 11 may comprise alternative layers of semiconductor materials. In the present embodiment, the layers of the DBR 11 are again based on InP and may comprise InP, InGaAsP or InGaP.

[0076] During operation of the PIC 1 , the laser radiation generated within the semiconductor gain medium 2 in the laser region 4 is optically coupled into the semiconductor optical waveguide 6. The adiabatic tapered sections 7a and 8 of the semiconductor optical waveguide 6, the spacer semiconductor layer 3 and the semiconductor gain medium 2 ensure efficient coupling of the light into the semiconductor optical waveguide 6. The semiconductor optical waveguide 6 then guides and confines the light and ensures that the laser radiation travels along the desired path towards the output coupling region 5. The second adiabatic tapered section 7b of the semiconductor optical waveguide 6 then gradually increases the width of the light coupled within the semiconductor optical waveguide 6 to a desired spot size for entering the output coupling region 5. Within the output coupling region 5 the semiconductor grating structure 9 acts to diffract the incoming laser radiation, that travels from the semiconductor gain medium 2, towards the output surface 12, generating the output field l OError! Bookmark not defined..

[0077] The specific wavelength of the output field 10 is determined by the combined properties of the semiconductor materials from which the semiconductor gain medium 2 and the semiconductor grating structure 9 are produced.

[0078] Figures 3(a) to (c) present cross-sectional views of semiconductor grating structures 9, 13, 14 that may be employed within the PIC 1 of Figures 1 .

[0079] The semiconductor grating structure 9 of Figure 3(a) comprises a first layer of periodic elements 15 located on the semiconductor optical waveguide 6. The first layer of periodic elements 15 and the semiconductor optical waveguide 6 are made from the same material and thus have the same refractive index ( ). In the presently described embodiment the first layer of periodic elements 15 forms a second order linear grating having a refractive index of 3.32. In Figure 3(a), the first layer of periodic elements 15 are fabricated directly on the surface of the semiconductor optical waveguide 6. Therefore, when light propagates through the semiconductor optical waveguide 6, a portion of it interacts with the semiconductor grating structure 9 and the output field 10 is diffracted out of the semiconductor optical waveguide 6.

[0080] However, some of the light interacts with the semiconductor grating structure 9 to create a second emitted field 16 that is diffracted in the opposite direction from the desired output path. The second emitted field 16 is produced as the first layer of periodic elements 15 of the semiconductor grating 9 produces both upward and downward components at diffraction angles relative to the direction of propagation within the semiconductor optical waveguide 6. The second emitted field 16 represents an additional loss mechanism from the PIC 1 , as it does not contribute to the desired output field 10 and leads to a reduction in the overall efficiency of the PIC 1 .

[0081] It is noted that the first layer of periodic elements 15 is located on the semiconductor optical waveguide 6, rather than completely located within the semiconductor optical waveguide 6. The first layer of periodic elements 15 may however be located partially, or fully, within the semiconductor optical waveguide 6, provided that there remains a volume of the semiconductor optical waveguide 6 located along the area (x and z directions) of the first layer of periodic elements 15 such that light propagates into the first layer of periodic elements 15 along a substantive part of its length (y direction).

[0082] Figure 3(b) illustrates an alternative semiconductor grating structure 13 again comprising a first layer of periodic elements 15b located on the optical waveguide 6. This embodiment differs from the semiconductor grating structure 9 presented in Figure 3(a) in that first layer of periodic elements 15b is made from a different material to that of the optical waveguide 6. In the presently described embodiment the first layer of periodic elements 15b forms a second order linear grating having a refractive index (n2= 3.4) that is greater than the refractive index of the semiconductor optical waveguide 6. Light propagating within the semiconductor optical waveguide 6 again interacts with the a first layer of periodic elements 15b, thus generating the output field 10. Additionally, some light is again emitted as a second emitted field 16 as a result of the interaction with the first layer of periodic elements 15b.

[0083] Figure 3(c) illustrates a further alternative semiconductor grating structure 14. The semiconductor grating structure 14 again comprising a first layer of periodic elements 15b located on the semiconductor optical waveguide 6. This embodiment differs from the semiconductor grating structure 13 presented in Figure 3(b) in that it further comprises a second layer of periodic elements 17 located on a first layer of periodic elements 15b. The second layer of periodic elements 17 is made from a different semiconductor material to that of the semiconductor optical waveguide 6 and the first layer of periodic elements 15b. In the presently described embodiment, the second layer of periodic elements 17 forms a first order linear grating having a refractive index (n3= 3.47) that is greater than the refractive index of the semiconductor optical waveguide 6 and n2the refractive index of the first layer of periodic elements 15b. The introduction of the second layer of periodic elements 17 acts to modify the optical properties of the semiconductor grating structure 14, allowing for more precise control over the output coupling process. It is noted that that the periodic elements of the second layer 17 are approximately half the width of the first layer of periodic elements 15b.

[0084] Employing the semiconductor grating structure 14 of Figure 3(c) within a PIC is found to significantly enhance the overall efficiency when compared with the semiconductor grating structure 9 of Figure 3(a) and the semiconductor grating structure 13 of Figure 3(b). In comparison to the semiconductor grating structures 9 and 13, semiconductor grating structure 14 is found to significantly reduce the portion of light within the second emitted field 16. Consequently, there is a significant increase in the amount of light diffracted out of the semiconductor grating structure 14 within the output field 10.

[0085] Figures 4 and 5 provide insights into the behaviour of the diffracted output light from a photonic integrated circuit (PIC) using the different semiconductor grating structures 9, 13 and 14 of Figure 3(a) to 3(c), respectively. More specifically, Figures 4 and 5 illustrate the distribution of output light diffracted out of the optical waveguide 6 into the output field 10 (Ti) and into the second emitted field 16 (T2).

[0086] In particular, Figure 4 presents theoretical modelling of the light emitted from a PIC as a function of thickness (tw) of the semiconductor optical waveguide 6 for the semiconductor grating structure 9 of Figure 3(a). As shown in Figure 4, the portion of the diffracted light within the output field 10 (Ti) and the second emitted field 16 (T2) is approximately equal for all thicknesses larger than 0.1 microns. Furthermore, the transmission in both the output field 10 (Ti) and the second emitted field 16 (T2) exhibits a similar pattern, rapidly decreasing as the thickness of the semiconductor optical waveguide 6 increases.

[0087] Figure 5 presents theoretical modelling of the light transmission as a function of the number of periods of the three layer semiconductor grating structure 14 shown in Figure 3(c). The dots and squares represent a first modelling method, while the solid lines depict an alternative second modelling approach. It is evident from Figure 5 that both modelling methods yield similar results.

[0088] For Figure 5, the thickness of the semiconductor optical waveguide 6 is fixed at 0.3 microns. The three-layer semiconductor grating structure 14 clearly demonstrates different transmission values for the output field 10 (Ti) and the second emitted field 16 (T2). Specifically, the output field 10 (Ti) comprises a higher amount of the diffracted light from the semiconductor grating structure 14 compared to the second emitted field 16 (T2). Moreover, increasing the number of periods of the semiconductor grating structure 14 results in a significant increase in the amount of light diffracted into the output field 10 (Ti). However, the amount of light directed into the second emitted field 16 (T2) experiences a slower rate of increase and generally stabilises for period values exceeding 200. For instance, when the number of periods is 600, roughly 50 per cent of the light is coupled into the output field 10 (Ti), whereas the amount coupled into the second emitted field 16 (T2) remains at less than 10 per cent.

[0089] Therefore, from both Figures 4 and 5, it is evident that implementing the three-layer semiconductor grating structure 14 within the PIC enhances the overall performance and efficiency, as significantly more power can be directed into the output field 10 (Ti).

[0090] It will be appreciated by the skilled reader that the materials, and hence the refractive indices, of the semiconductor optical waveguide 6 (m), the first layer of periodic elements 15 (n2) and the second layer of periodic elements 17 (n3) may vary from those detailed above. It is however preferable for the operation of the three-layer grating structure 14 for these refractive indices to satisfy the following inequality, namely m < n2< n3.

[0091] A further important factor for the operation of the three-layer semiconductor grating structure 14 is that the first layer of periodic elements 15 forms an even order grating, and preferably is a second order grating, while the second layer of periodic elements 17 forms an odd order grating, and preferably a first order grating.

[0092] The process of creating the described photonic integrated circuit 1 with a semiconductor optical waveguide 6 and a semiconductor grating structure 9, 13 and 14 involves using known growth and etching processes to selectively deposit different semiconductor layers onto a substrate, forming a single, Indium Phosphide (InP) based integrated structure. The use of masking techniques allows for controlled deposition of materials, creating distinct regions for the photonic integrated circuit 1 (e.g. the output coupling region 5 and the laser region 4).

[0093] The design of the semiconductor grating structures 9, 13 and 14 within the photonic integrated circuit 1 provides a means for selecting predetermined power levels, precise wavelengths and enhanced beam quality and control of the output field 10.

[0094] An additional advantage of the PIC 1 is that there is a reduction in the light levels back reflected towards the semiconductor gain medium 2. For example, the semiconductor grating structures 9, 13 and 14 may be designed to couple around 10% of the output power from semiconductor gain medium 2 into the output field 10. The output field 10 may then be coupled into an optical fibre, or other Silicon Photonics (SiP) system. Such optical systems are known to cause back reflections towards the photonic integrated circuit 1 .

[0095] However, as will be appreciated by the skilled reader, the semiconductor grating structures 9, 13 and 14 will result in less than 10% of the back reflected light being redirected towards the semiconductor gain medium 2. This reduction in the level of back reflected light reduces the need for further optical isolating components to be incorporated within the photonic integrated circuit 1 .

[0096] It will be appreciated by the skilled reader that alternative semiconductor materials to Indium Phosphide (InP) may be employed to produce the photonic integrated circuit 1 of Figure 1 . These alternative semiconductor materials include Gallium Arsenide (GaAs), Gallium Nitride (GaN) or Gallium Antimonide (GaSb), which allow for output wavelengths from the ultra-violet range to the mid-infrared range of the electromagnetic spectrum to be produced within an integrated structure based on a single semiconductor material.

[0097] A photonic integrated circuit 18 in accordance with an alternative embodiment of the present invention is presented in Figures 6(a) and (b). While the photonic integrated circuit 18 has a similar structure to the PIC 1 of Figure 1 , Figure 6 introduces an angled face 19 situated at one end along the length of the semiconductor optical waveguide 6 (i.e. along the y-axis).

[0098] In particular, the angled face 19 is provided at the edge of the semiconductor optical waveguide 6 within the laser region 4 of the PIC 18. The angled face 19 extends along the width of the semiconductor optical waveguide 6 (i.e. along the x-axis). The angled face 19 is defined by an angle 6 ranging from 7 to 11 degrees, measured with respect to the x axis of the PIC 18.

[0099] The angled face 19 acts to further improve the operation of the photonic integrated circuit 18. In particular, the angled face 19 greatly reduces the likelihood of any back reflections travelling back into the semiconductor gain medium 2 and interfering with the generation of light within the semiconductor gain medium 2. Therefore, the angled face 19 further reduces any potential challenges associated with unwanted back reflections.

[0100] A photonic integrated circuit 20 in accordance with an alternative embodiment of the present invention is presented in Figures 7(a) and (b). In Figure 7 an angled face 21 is again present at one end along the length of the semiconductor optical waveguide 6. However, in Figure 7, the angled face 21 extends along the height of the semiconductor optical waveguide 6. The angled face 21 is defined by an angle 9 ranging from 7 to 11 degrees, measured with respect to the z axis of the PIC 20.

[0101] Again, the angled face 21 acts to further improve the operation of the photonic integrated circuit 20 by greatly reducing the likelihood of any back reflections travelling back into the semiconductor gain medium 2 and interfering with the generation of light within the semiconductor gain medium 2.

[0102] As will be appreciated by a person skilled in the art, the angled faces 19 and 21 of the embodiments of Figures 6 and 7 may be combined within a single PIC.

[0103] A photonic integrated circuit 22 in accordance with an alternative embodiment of the present invention is presented in Figures 8(a) and (b). A notch 23 is provided within the semiconductor optical waveguide 6. The notch 23 is located between the semiconductor gain medium 2 and the output coupling region 5. In particular, the notch 23 is located within the region of the semiconductor optical waveguide 6 comprising the second tapered section 7b. However, as will be appreciated by a reader skilled in the art, the notch 23 may be provided in any other suitable location. For example, the notch 23 could instead be located between the first 7a and second 7b tapered sections of the semiconductor optical waveguide 6.

[0104] In the presently described embodiment, the notch 23 is rectangular in shape, but as will be appreciated by the skilled reader, the notch 23 could alternatively be any other suitable shape and or comprise a tapered cross section profile.

[0105] The notch 23 acts to improve the operation of the photonic integrated circuit 22 by again reducing the likelihood of any back reflections travelling from the output coupling region 5, from reaching the semiconductor gain medium 2 located within the laser region 4 of the PIC 22.

[0106] In Figure 8, the width of the notch 23 is in the range of 50 nm to 500 nm, and it extends partially through the height of the semiconductor optical waveguide 6 (along the x axis). However, the notch 23 could alternatively extend fully through the semiconductor optical waveguide 6. The notch 23 of Figure 8 is an air-filled gap, however, as will be appreciated by the skilled reader, the notch 23 could alternatively be backfilled with a semiconductor or dielectric material.

[0107] As will be appreciated the skilled reader, any combination of the embodiments described with reference to Figures 6, 7 and 8 can be used within a single PIC.

[0108] A photonic integrated circuit 24 in accordance with an alternative embodiment of the present invention is presented in Figures 9(a) and (b). While the photonic integrated circuit 24 has a similar structure to the PIC 1 of Figure 1 , Figure 9 introduces a semiconductor grating structure 25 having a chirped period. In particular, the period of the grating structure 25 decreases along the length of the semiconductor optical waveguide 6 (i.e. along the y-axis).

[0109] The chirped semiconductor grating structure 25 provides a means for controlling the shape of the output field 10. In particular, the chirped grating structure 25 provides for an output field 10 that exhibits a more Gaussian profile.

[0110] A photonic integrated circuit (PIC) 26 in accordance with an alternative embodiment of the present invention is presented in Figure 10. Within the photonic integrated circuit 26 of Figure 10 the output coupling region 5 comprises a first semiconductor grating structure 9a and a second semiconductor grating structure 9b. The semiconductor grating structures 9a and 9b are spatially separated along the length of the semiconductor optical waveguide 6, i.e. along the y-axis of the photonic integrated circuit 26.

[0111] When the semiconductor gain medium 2 generates laser radiation within the laser region 4, the generated laser radiation is optically coupled to the output coupling laser region 5 via the semiconductor optical waveguide 6. When the generated radiation enters the output coupling laser region 5 it interacts with both the first 9a and second 9b semiconductor grating structures, creating a first 10a and a second 10b output field from the output surface 12.

[0112] The particular properties of the first 9a and second 9b semiconductor grating structures may be chosen to provide output fields 10a and 10b which have the same wavelength. This allows for the output fields 10a and 10b to be either combined to provide higher output power, or to be used separately, as required. Alternatively, the properties of the first 9a and second 9b semiconductor grating structures may be selected to provide different wavelengths (e.g. 1 ,310 nm and 1 ,315 nm) for the first 10a and second 10b output fields, providing a photonic integrated circuit 26 with multi-wavelength production. In general, the maximum difference between the produced wavelengths for the first 10a and second 10b output fields typically falls within a range of approximately 60 nm to 100 nm.

[0113] As will be appreciated by the skilled reader, further semiconductor grating structures 9 may be arranged along length of the photonic integrated circuit 26 within the output coupling region 5. This allows for further output fields 10 with specific wavelengths, as determined by each associated semiconductor grating structure 9, to be generated by the photonic integrated circuit 26.

[0114] A top view of a photonic integrated circuit (PIC) 27 in accordance with an alternative embodiment of the present invention is presented in Figure 11 . The photonic integrated circuit 27 again comprises an integrated device based on Indium Phosphide (InP) having a semiconductor gain medium 2, a semiconductor optical waveguide 6 and an output coupling region 5.

[0115] However, as can be seen from Figure 11 , a three by three array of semiconductor grating structures 9a to 9i is arranged within the output coupling region 5. Each of these semiconductor grating structures 9a to 9i can create a separate output field 10 with a wavelength determined by the specific properties of each of the distinct semiconductor grating structures 9a to 9i.

[0116] Similar to the previous embodiment presented in Figure 10, the semiconductor grating structures 9a to 9i can each provide an identical wavelength, different wavelengths or a combination thereof may be provided by some or all of the semiconductor grating structures 9a to 9i. Note that a smaller or larger array of semiconductor grating structures 9 could also be arranged within the photonic integrated circuit 27 to provide less or more output fields as required for a particular application.

[0117] It will be appreciated by the skilled reader that the array of semiconductor grating structures 9a to 9i within the photonic integrated circuit 27 can be arranged in any desired geometry. The array of semiconductor grating structures 9a to 9i in Figure 11 comprises a regular two- dimensional array. However, an alternative arrangement, such as an irregular array of semiconductor grating structures 9 e.g. where one or more of the array sites do not comprise a semiconductor grating structure could instead be provided. In summary, the present invention provides an alternative photonic integrated circuit to those known in the art. The semiconductor materials used within the photonic integrated circuit are specifically chosen to provide the desired output wavelength for the output field 10 of the device. In particular, the different layers of the photonic integrated circuit may be fabricated using one type of semiconductor material to create a highly integrated structure.

[0118] The output coupling region 5 is spatially separated from the semiconductor gain medium 2 of the PIC. A semiconductor grating structure 9, 13, 14 or 25 as depicted in Figures 3(a) to (c) and Figure 9 is located within the output coupling region 5 to generate the output field 10 of the PIC. The properties of the semiconductor grating structure 9, 13, 14 or 25 are purposely selected to provide a particular output wavelength. The choice of the materials used within the PIC, coupled with the properties of the semiconductor grating structure 9, 13, 14 or 25 determine the output power and wavelength of the output field. In addition, the properties of the semiconductor grating structure 9, 13, 14 or 25 provide a means to manipulate the output beam characteristics, beam quality and to control the direction of emission.

[0119] An additional advantage of the above-described PIC is that the structure of the device allows for reduced back reflections to the semiconductor gain medium 2, thus reducing the need for additional optical isolating components.

[0120] A photonic integrated circuit and a method for manufacturing is disclosed. The photonic integrated circuit comprises a semiconductor gain medium configured to generate laser radiation that is spatially separated from an output coupling region. The photonic integrated circuit further comprises a semiconductor optical waveguide that optically couples the semiconductor gain medium and output coupling region and defines a device plane of the photonic integrated circuit. The output coupling region comprises a semiconductor grating which redirects the laser radiation out of the device plane to form an output field of the photonic integrated circuit. The characteristics of the semiconductor grating structure can be used to manipulate the output field properties. Furthermore, the above photonic integrated circuit exhibits reduced back-reflected light toward the semiconductor gain medium thus reducing the need for further optical isolating components to be incorporated within the photonic integrated circuit. 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.

[0121] 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.

[0122] 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 photonic integrated circuit comprising: a semiconductor gain medium configured to generate laser radiation; a semiconductor optical waveguide defining a device plane of the photonic integrated circuit and optically coupled to the semiconductor gain medium; an output coupling region of the semiconductor optical waveguide spatially separated from the semiconductor gain medium; wherein the output coupling region comprises a semiconductor grating structure configured to redirect laser radiation optically coupled within the semiconductor optical waveguide out of the device plane to form an output field of photonic integrated circuit.

2. A photonic integrated circuit as claimed in claim 1 wherein the semiconductor grating structure comprises a first layer of periodic elements forming an even order linear grating.

3. A photonic integrated circuit as claimed in claim 2 wherein the first layer of periodic elements forms a second order linear grating.

4. A photonic integrated circuit as claimed in either of claims 2 or 3 wherein the semiconductor grating structure further comprises a second layer of periodic elements located on top of the first layer of periodic elements, the second layer of periodic elements forming an odd order linear grating.

5. A photonic integrated circuit as claimed in claim 4 wherein the second layer of periodic elements forms a first order linear grating.

6. A photonic integrated circuit as claimed in any of claims 2 to 5 wherein a refractive index of the first layer of periodic elements (n2) is greater than or equal to a refractive index ( ) of the semiconductor optical waveguide.

7. A photonic integrated circuit as claimed in any of claims 4 to 6 wherein a refractive index of the second layer of periodic elements (n3) is greater than a refractive index (n2) of the first layer of periodic elements.

8. A photonic integrated circuit as claimed in any of the preceding claims wherein the semiconductor optical waveguide comprises a first tapered section, the first tapered section reducing in width in a direction from the semiconductor gain medium towards the output coupling region.

9. A photonic integrated circuit as claimed in claim 8 wherein the semiconductor optical waveguide comprises a second tapered section, the second tapered section increasing in width in a direction from the semiconductor gain medium towards the output coupling region.

10. A photonic integrated circuit as claimed in any of the preceding claims wherein the semiconductor gain medium comprises a gain tapered section wherein the gain tapered section reduces in width in a direction from the semiconductor gain medium towards the output coupling region.

11. A photonic integrated circuit as claimed in claim 10 wherein the gain tapered section is located between first tapered section and the output coupling region.

12. A photonic integrated circuit as claimed in any of the preceding claims wherein the photonic integrated circuit comprises a spacer semiconductor layer located between semiconductor gain medium and the semiconductor optical waveguide, the spacer semiconductor layer comprising a tapered section that reduces in width in a direction from the semiconductor gain medium towards the output coupling region.

13. A photonic integrated circuit as claimed in any of the preceding claims wherein the semiconductor optical waveguide comprises a notch located between the semiconductor gain medium and the output coupling region.

14. A photonic integrated circuit as claimed in claim 13 wherein a width of the notch is in the range of 10nm to 500nm or in the range of 50nm to 150nm.

15. A photonic integrated circuit as claimed in any of the preceding claims wherein the semiconductor waveguide comprises an angled face on one end of the semiconductor optical waveguide.

16. A photonic integrated circuit as claimed in any of the preceding claims wherein the output coupling region comprises an array of semiconductor grating structures each of which generates an output field from the photonic integrated circuit.

17. A photonic integrated circuit as claimed in claim 16 wherein the array of semiconductor grating structures is a regular array and a semiconductor grating structure is located at each site of the array.

18. A photonic integrated circuit as claimed in claim 16 wherein the array of semiconductor grating structures is an irregular array.

19. A photonic integrated circuit as claimed in any of claims 16 to 18 wherein the photonic integrated circuit generates two or more of the output fields, the two or more output fields having a different wavelength or the same wavelength.

20. A photonic integrated circuit as claimed in any of the preceding claims wherein one or more wavelength-selective reflectors are located around one or more sides of the semiconductor optical waveguide.21 . A method of manufacturing a photonic integrated circuit the method comprising: providing a semiconductor gain medium configured to generate laser radiation; providing a semiconductor optical waveguide defining a device plane of the photonic integrated circuit and optically coupled to the semiconductor gain medium; providing an output coupling region of the semiconductor optical waveguide that is spatially separated from the semiconductor gain medium, providing a semiconductor grating structure within the output coupling region; and configuring the semiconductor grating structure to redirect laser radiation optically coupled within the semiconductor optical waveguide out of the device plane to form an output field of photonic integrated circuit.

22. A method of manufacturing a photonic integrated circuit as claimed in claim 21 wherein providing a semiconductor grating structure within the output coupling region comprises providing a first layer of periodic elements forming an even order linear grating.

23. A method of manufacturing a photonic integrated circuit as claimed in claim 22 wherein the first layer of periodic elements forms a second order linear grating.

24. A method of manufacturing a photonic integrated circuit as claimed in either of claims 22 or 23 wherein providing a semiconductor grating structure within the output coupling region further comprises providing a second layer of periodic elements located on top of the first layer of periodic elements, the second layer of periodic elements forming an odd order linear grating.

25. A method of manufacturing a photonic integrated circuit as claimed in claim 24 wherein the second layer of periodic elements forms a first order linear grating.

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