Waveguide structure of optical integrated circuits

Abstract

A waveguide structure (100) for an optical integrated circuit, comprising: a substrate; an active region (102) including a diode junction, the active region including a light-emitting portion (102a) that emits light in a first direction and a second direction perpendicular to the first direction, and a light-absorbing portion (102b) that absorbs light emitted from the light-emitting portion (102a) in the second direction; a first contact corresponding to the light-emitting portion (102a) and a second contact corresponding to the light-absorbing portion (102b).

Description

[Background technology]

[0001] Photonic integrated circuits (PICs) utilize waveguide structures to guide light between components of the PIC.

[0002] Unguided light can be a source of noise for other components of the PIC.

[0003] It is desirable to reduce the level of noise detectable by the components of a PIC. [Brief description of the drawings]

[0004] [Figure 1a] 1 is a plan view (not to scale) of a waveguide structure, according to an embodiment. [Figure 1b] 1 is a cross-sectional view (not to scale) of a waveguide structure, according to an embodiment. [Diagram 2] FIG. 4 is a flow diagram illustrating a method for fabricating a waveguide structure, according to an embodiment. [Figure 3a] 1 is a plan view (not to scale) of a waveguide structure, according to an embodiment. [Figure 3b] 1 is a cross-sectional view (not to scale) of a waveguide structure, according to an embodiment. [Figure 4] 1 is a plan view (not to scale) of a waveguide structure, according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] The embodiments described herein relate to waveguide structures in photonic integrated circuits (PICs). A PIC may be considered an optical circuit in which two or more optical elements (e.g., elements that use light to perform functions) are optically connected. A PIC may include components that perform functions including, for example, emitting light, detecting light, amplifying light, directing light, reflecting light, diffracting light, mixing light of different wavelengths, separating light of different wavelengths, polarization conversion, optical filtering, power separation, amplitude modulation, and phase modulation. A PIC may be considered an optical analog of an electronic circuit. PICs typically operate in the visible or near infrared regions of the electromagnetic spectrum (e.g., light having wavelengths ranging from about 400 nanometers (nm) to about 1650 nm), although PICs can operate in other regions of the electromagnetic spectrum as well.

[0006] The commercially exploited material platform for PICs is indium phosphide (InP), which allows the integration of optically active and optically inactive functions on the same chip, but alternative materials such as silicon (Si), gallium arsenide (GaAs) or lithium niobate (LiNBO) are also available. 3 Other material platforms such as MOS FETs, MOS FETs, and MOS FETs may also be used as the platform for the PIC. A PIC may contain hundreds of components integrated onto a single chip.

[0007] Figures 1a and 1b are schematic diagrams of a waveguide structure 100 that is a component of a photonic integrated circuit (PIC). Figure 1a is a plan view of the waveguide structure 100, and Figure 1b is a cross-sectional view of the waveguide structure 100 taken along the guide axis of the waveguide structure 100 indicated by line AA'.

[0008] Light guided by a waveguide structure is typically not completely confined by the waveguide structure, and some of the light escapes the waveguide structure and is emitted in a direction different from the light guided by the waveguide. In some applications, such unguided light may be undesirably detected by components of the PIC other than the components of the PIC through which the waveguide is designed to guide the light, or may affect the amplification of light within the waveguide structure itself. Such unguided light may be a source of noise for other components of the PIC. The embodiments described herein reduce such noise.

[0009] In some embodiments, such as the embodiment of Figures 1a and 1b, the waveguide structure 100 is a semiconductor optical amplifier (SOA). In other embodiments, the waveguide structure 100 is an entity other than an SOA. For example, the waveguide structure 100 may be an edge-emitting laser.

[0010] The waveguide structure 100 includes a semiconductor structure formed on a semiconductor substrate on which the various layers forming the waveguide structure 100 are grown. For example, the semiconductor substrate may be an indium phosphide (InP) material, and in an embodiment, a lower cladding layer is deposited on the substrate, and a lower core layer having a higher refractive index than the lower cladding layer is deposited on the lower cladding layer. In an embodiment, an active region 102 (described below) is deposited on the lower core layer, and an upper core layer is deposited on the active region. An upper cladding layer having a lower refractive index than the upper core layer is deposited on the active region to form a waveguide and guide light to the upper and lower core layers to increase interaction between light propagating through the waveguide structure and the active region 102.

[0011] In an embodiment, the upper cladding layer, the lower cladding layer, the upper core layer, the lower core layer, and the active region 102 each include multiple layers of different semiconductor materials having appropriate thicknesses and compositions to perform a desired function.

[0012] To form the waveguide structure 100, the substrate is typically thinned or removed and contacts (eg, metal contacts) are formed on the p-side and n-side, respectively, of the semiconductor structure.

[0013] The active region 102 is a region of the waveguide structure 100 that includes a diode junction (such as a pn junction or a pin junction) that can emit photons by recombination of electrons and holes and / or absorb photons by the creation of electron-hole pairs. In its simplest form, a diode junction includes an interface between an n-doped semiconductor material and a p-doped semiconductor material. However, modern photonic components typically have active regions that include electronic confinement structures (such as quantum wells, quantum wires, or quantum dots) to confine charge carriers to a specific spatial region, thereby improving, for example, the recombination rate and / or the waveguide structure that provides optical confinement (such as confinement of photons by refractive index differences), reducing optical losses, and thus increasing the interaction between photons and charge carriers (e.g., increasing the absorption rate and / or stimulated emission rate).

[0014] In some embodiments, the substrate, lower cladding layer, and lower core layer are doped with dopants to form p-doped semiconductor material, and the upper core layer and upper cladding layer are doped with dopants to form n-doped semiconductor layers, whereby the interfaces between the p-doped and n-doped semiconductor layers at or within the active region 102 form a pn junction. In other embodiments, the substrate, lower cladding layer, and lower core layer are n-doped, and the upper core layer and upper cladding layer are p-doped. In some embodiments, the active region 102 itself is undoped (so-called intrinsic semiconductor) to form a pin junction. The waveguide structure 100 also includes one or more waveguides, which in some embodiments, such as the waveguides in the embodiments shown in Figures 1a and 1b, are channel waveguides. For example, the waveguides are ridge waveguides or rib waveguides. In other embodiments, the waveguides are other forms of waveguides, such as isolated-stripe waveguides.

[0015] A ridge waveguide is an example of a channel waveguide as described above. Typically, a ridge waveguide is rectangular in cross section (e.g., has a flat top surface and two sidewall surfaces, each of which forms an interface with a material having a lower refractive index than the material that makes up the ridge). However, in embodiments, the cross section of the ridge is not perfectly rectangular. For example, the sidewalls are not straight and / or are not perfectly perpendicular to the substrate on which the ridge is formed. In some other embodiments, the cross section of the ridge of a ridge waveguide is not rectangular, but has a different shape, for example a trapezoidal cross section, or in other embodiments has rounded or curved sidewalls (convex or concave).

[0016] As shown in Figures 1a and 1b, the active region 102 includes a light emitting portion 102a and a light absorbing portion 102b. A first waveguide corresponds to the light emitting portion 102a and is hereafter referred to as the driven waveguide 104 (because in normal operation the junction below the waveguide is forward biased). A second waveguide, of the same or similar structure as the driving waveguide 104, corresponds to the light absorbing portion 102b and is hereafter referred to as the absorbing ridge 106. The driving waveguide 104 corresponds to the light emitting portion 102a to the extent that a bias applied to the driving waveguide 104 primarily affects the light emitting portion 102a, and the absorbing ridge 106 corresponds to the light emitting portion 102a to the extent that a bias applied to the driving waveguide 104 primarily affects the light absorbing portion 102b. For example, as shown in Figure lb, the drive waveguide 104 is located (in the z-axis) above the light emitting portion 102a and the absorbing ridge 106 is located (in the z-axis) above the light absorbing portion 102b. In other words, at least in the y-axis, the drive waveguide 104 is aligned with or overlaps the light emitting portion 102a and the absorbing ridge 104b is aligned with or overlaps the light absorbing portion 102b.

[0017] The drive waveguide 104 and the absorbing ridge 106 are each formed, for example, in at least an upper cladding layer of a semiconductor structure, which in some embodiments may include an upper core layer. In some embodiments, the drive waveguide 104 and / or the absorbing ridge 106 comprise the active region 102, as described below.

[0018] Light generated within the drive waveguide 104 is emitted in all directions, including a first direction along the x-axis and a second direction perpendicular to the first direction (e.g., along the y-axis). Light generated within the drive waveguide 104 and emitted in a direction that satisfies the condition for total internal reflection is guided along the drive waveguide 104 (e.g., along the x-axis), while light generated within the drive waveguide 104 and emitted in a direction that does not satisfy the condition for total internal reflection (e.g., light emitted in the second direction) exits the drive waveguide 104.

[0019] Light emitted in the second direction may be absorbed by absorbing ridge 106 to prevent such light from escaping waveguide structure 100 and becoming a noise source for other components of the PIC, for example.

[0020] The drive waveguide 104 and absorbing ridge 106 extend longitudinally in a first direction, represented by the x-axis in Figure 1a. Light is directed in a first direction along the longitudinal axis of the drive waveguide 104 such that the first direction is substantially parallel to the longitudinal axis of the drive waveguide 104 (e.g., parallel to the x-axis, within acceptable performance tolerances).

[0021] The active region 102 is planar and extends along an x-axis and also along a y-axis perpendicular to the x-axis, shown in Figures 1a and 1b as the y-axis. The drive waveguide 104 and absorbing ridge 106 extend from the active region 102 along a z-axis perpendicular to both the x-axis and the y-axis.

[0022] A channel-type waveguide, such as the drive waveguide 104, is a waveguide that guides light (or any other propagating electromagnetic radiation) in two orthogonal dimensions. The guiding mechanism is based on differences in refractive index. In particular, the "core" of the waveguide has a higher refractive index than the material surrounding the core (commonly called the "cladding"), so as to provide an effect known as internal reflection (according to Snell's law). As is well known, light propagating through a medium with a first refractive index and incident on an optical interface with a medium with a second, different refractive index, is at least partially reflected. If the second refractive index is lower than the first, the propagating light is totally internally reflected if the angle of incidence (relative to the normal to the plane of the optical interface) of the propagating light is greater than a critical angle.

[0023] Differences in refractive index may be the result of differing intrinsic material properties (e.g., based on the spacing of atoms of the constituent materials of the components) and / or may be based on external factors such as an externally applied electric field (e.g., quantum induction effect) and / or the concentration of charge carriers in a region of the semiconductor material (e.g., gain guiding phenomenon), which can affect the apparent refractive index experienced by an electromagnetic field propagating through the material.

[0024] In some embodiments, the refractive index difference is provided by etching or otherwise removing semiconductor material on either side of the actuation waveguide 104 to form one or more vertical recesses in the top surface of the semiconductor material on which the waveguide structure 100 is fabricated. For example, as shown in FIG. 1b, a first recess is formed between the actuation waveguide 104 and the absorbing ridge 106 by removing semiconductor material to form a first sidewall 112a on a first side of the absorbing waveguide 106, a second sidewall 112b on a first side (opposite the first sidewall 112a) of the actuation waveguide 104, and a base portion 112c joining the first sidewall 112a and the second sidewall 112b. The base portion 112c is spaced a depth D (for subsequent figures, 1 and / or 2 may be added if there are recesses with different depths relative to the top surface of the semiconductor structure) from the top surface of the semiconductor structure. In some embodiments, the first and second sidewalls 112a, 112b are perpendicular to the substrate and the base portion 112c is parallel to the substrate to form a substantially rectangular recess (e.g., the first and second sidewalls 112a, 112b meet the base portion 112c at 90°±10°). In other embodiments, the first and second sidewalls 112a, 112b may be curved such that they meet the base portion 112c to form a non-rectangular recess (e.g., a "U" shaped recess or a "V" shaped recess).

[0025] Similarly, such recesses may be formed on a second side of the actuation waveguide 104 opposite the first side of the actuation waveguide 104, and on a second side of the absorbing ridge 106 opposite the first side of the absorbing ridge 106, as shown in FIG. 1b.

[0026] As shown in FIG. 1b, the recess(es) are at least partially filled with a layer or block of solid dielectric material 113 deposited after the actuation waveguide 104 and absorbing ridge 106 are formed. For example, the recesses may be at least partially filled with a polyimide material or other dielectric material used to planarize semiconductor structures. For example, the recesses are filled with a material such as benzocyclobutene (BCB), polymethylmethacrylate (PMMA), or SU-8. Thus, once fabricated, the waveguide structure 100 may not include a recess because the recess is filled with a dielectric material, or the recess may have a depth less than D because the recess is partially filled. In other embodiments, the dielectric material filling the recess may be air.

[0027] The drive waveguide 104 is located over the light emitting portion 102a of the active region 102. In contact with the drive waveguide 104 is a first contact 108. The first contact 108 may include, for example, one or more metal layers and one or more doped semiconductor layers selected to form an electrical contact through which charge carriers (e.g., electrons or holes) may be injected into and / or extracted from the light emitting portion 102a through the drive waveguide 104. In particular, in normal operation, the first contact 108 is forward biased (e.g., a positive voltage may be applied to the first contact with respect to a common potential) to provide a population inversion in the light emitting portion 102a, thereby causing the light emitting portion 102a to generate light in the form of photons by a process of recombination of electrons and holes (including amplified spontaneous emission and / or stimulated emission).

[0028] Similarly, the absorbing ridge 106 is located above the light absorbing portion 102b of the active region. Contacting the absorbing ridge 106 is a second contact 110. Like the first contact 108, the second contact 110 may include, for example, one or more metal layers and one or more doped semiconductor layers selected to form an electrical contact through which charge carriers (e.g., electrons or holes) may be injected into and / or extracted from the light absorbing portion 102b through the absorbing ridge 106.

[0029] The first contact 108 and the second contact 110 are formed, for example, on the top surfaces of the drive waveguide 104 and the absorbing ridge 106. An additional contact layer 114 may be deposited to form a ground contact on the underside of the waveguide structure 100. As shown in Figures 1a and 1b, the additional contact layer 114 may provide a common ground for the electrical path between both the first contact 108 and the second contact 110.

[0030] In use, a positive voltage (e.g., a forward bias) is typically applied to the first contact 108 relative to the additional contact 114. The additional contact 114 is typically grounded (e.g., at zero volts), but may alternatively be floating. Such a positive voltage provides the necessary population inversion required for amplification of light received at the light emitting portion 102a. For example, the first contact 108 may be electrically connected (e.g., via a solder joint to the first contact 108) to a first bias circuit arranged to provide a first bias (e.g., a forward bias relative to the additional contact 114) to the first contact 108, and the second contact 110 may be electrically connected (e.g., via a solder joint to the second contact 110) to a second bias circuit arranged to provide a second bias (e.g., a negative or reverse bias relative to the additional contact 114, or a ground bias, or a floating bias) to the second contact 110.

[0031] In use, a received optical signal 116a may be applied to a first end 104a of the drive waveguide 104. As the received optical signal traverses the drive waveguide 104, it propagates along the drive waveguide 104 as an optical field 116, is amplified, and is output at a second end 104b of the drive waveguide 104 as an amplified output signal 116b.

[0032] Photons emitted in the light emitting portion 102a by spontaneous emission may be emitted in any direction. Some of these photons may be directed by the drive waveguide 104 along the longitudinal axis of the drive waveguide 104, contributing to amplified spontaneous emission and / or stimulated emission and amplification of the received input signal 116a to produce an amplified output signal 116b. Other photons may be directed not by the drive waveguide 104 but escaping the drive waveguide 104, and such photons may be considered noise as described above.

[0033] The second contact 110 may be grounded (e.g., at zero volts) or may be floating. By grounding the second contact 110, charge carriers generated at the absorbing ridge 106 in response to incident photons (e.g., emitted in a second direction from the drive waveguide 104) are removed from the absorbing ridge 106, such that these charge carriers do not recombine to generate photons that are re-emitted from the absorbing ridge 106. In some embodiments, a reverse bias with respect to the potential of the additional contact 114 may be applied to the second contact 110, which may improve the absorption efficiency of the absorbing ridge 106 in some embodiments.

[0034] At least one interface between the active region 102 and the surrounding semiconductor material is tilted in the xy plane with respect to light traversing the drive waveguide 104 along the x-axis. This is because there is a refractive index difference between the semiconductor material of the active region 102 and the semiconductor material surrounding the active region 102. Such a refractive index difference creates reflective optical interfaces similar to those described above in connection with the operation of the ridge waveguide 104, and such reflections may be detrimental to the operation of the SOA, but by forming the interface at the angle shown in Figure 1a, those reflections may be directed away from the axis of the ridge waveguide 104 at an angle (e.g., greater than the critical angle) that prevents the reflected light from being guided by the ridge waveguide 104.

[0035] For example, as shown in FIG. 1a, the active region 102 is formed to be trapezoidal in plan view, such that the interfaces between each end of the active region 102 and the surrounding semiconductor material lie on the xy plane for light traversing the drive waveguide 104 along the x-axis.

[0036] In other embodiments, one or more of the interfaces between the active region 102 and the surrounding semiconductor material may be perpendicular to light traversing the drive waveguide along the x-axis. For example, when the waveguide structure 100 is used in a laser device, it may be beneficial for light reflected at an optical interface between the active region 102 and the semiconductor material adjacent to the active region 102 to be perpendicular to the axis of the drive waveguide 104 such that the light is reflected into the drive waveguide 104 and amplified.

[0037] Figure 2 illustrates a method of fabricating a waveguide structure for an integrated optical circuit, such as the waveguide structure 100 described above with reference to Figures 1a and 1b. For example, as described above, the waveguide structure may be formed on a semiconductor substrate as described above, and may include core and cladding layers that confine light in the z-axis.

[0038] In block 202, an active region is deposited. The active region may include a diode junction, including, for example, a pn junction or a pin junction as described above. For example, the substrate, lower cladding layer, and lower core layer are doped with dopants to form a p-doped semiconductor material, and the upper core layer and upper cladding layer are doped with dopants to form an n-doped semiconductor layer, whereby the interface between the p-doped and n-doped semiconductor layers at or within the active region 102 forms a pn junction. In other embodiments, the substrate, lower cladding layer, and lower core layer are n-doped, and the upper core layer and upper cladding layer are p-doped. In some embodiments, the active region 102 itself may be undoped (so-called intrinsic semiconductor) to form a pin junction.

[0039] In block 204, a light emitting portion is defined in the active region. For example, the light emitting portion may correspond to the light emitting portion 102a described above with reference to FIG. 1b and may be defined, for example, by the drive waveguide 104 described above with reference to FIGS. 1a and 1b. For example, the light emitting portion is defined by a portion of the active region 102 and is aligned with or overlaps the drive waveguide 104 located above the light emitting portion 102a as shown in FIG. 1b. When the term defined is used in connection with the method 200, it is used to describe that corresponding feature(s) are formed (e.g., with appropriate masking, by etching or deposition processes) such that they are aligned or overlap each other, for example, at least in the y-axis.

[0040] In block 206, a light absorbing portion is defined in the active region. For example, the light absorbing portion corresponds to light absorbing portion 102b described above with reference to Figure lb and is defined, for example, by absorbing ridge 106 described above with reference to Figures 1a and 1b. For example, the light absorbing portion is defined by a portion of the active region and is aligned with or overlaps absorbing ridge 106 located above light absorbing portion 102b, as shown in Figure lb.

[0041] In block 208, a first contact is defined that corresponds to a light emitting portion (e.g., light emitting portion 102a described above with reference to FIG. 1b). For example, the first contact is an electrical contact formed on or in contact with drive waveguide 104, such as first contact 108 described above with reference to FIGS. 1a and 1b.

[0042] In block 210, a second contact is defined that corresponds to the light absorbing portion (e.g., light absorbing portion 102b described above with reference to FIG. 1b). For example, the second contact is an electrical contact formed on or in contact with the absorbing ridge 106, such as second contact 110 described above with reference to FIGS. 1a and 1b.

[0043] In some embodiments, the first and second contacts are formed in openings in a dielectric material deposited over the waveguides of the light emitting and light absorbing portions. For example, as previously described, there may be a recess between the drive waveguide 104 and the absorbing ridge 106 as shown in FIGS. 1a and 1b. As previously described, the recess may be filled with a dielectric material. In practice, application of the dielectric material may result in excess dielectric material being deposited on the surfaces of the drive waveguide 104 and the absorbing ridge 106 to which it is desirable to make electrical contacts. Thus, in some embodiments, the method 200 includes creating openings in such overlying dielectric material that provide access to the drive waveguide 104 and the absorbing ridge 106. For example, the method 200 includes one or more lithography steps to define portions of the drive waveguide 104 and / or the absorbing ridge 106 where the conductive contacts are to be applied, and a subsequent etching step to remove the dielectric material from the defined portions of the drive waveguide 104 and / or the absorbing ridge 106 where the conductive contacts are to be applied.

[0044] 1a and 1b provide a structure that absorbs light (e.g., absorbs light emitted in a second direction) that is not guided along the drive waveguide 104. Other structures are envisioned that improve absorption of light that is not guided along the drive waveguide 104.

[0045] For example, Figures 3a and 3b are respectively a plan view and a cross-sectional view of a waveguide structure 300 according to another embodiment. The waveguide structure 300 is similar to the waveguide structure shown in Figures 1a and 1b and can be manufactured according to a similar method as described above with reference to Figure 2. Accordingly, features shown in Figures 3a and 3b that appear in Figures 1a and 1b are given the same reference numerals (with the addition of a and / or b if there are more than one of the aforementioned features in this embodiment) and the above descriptions of these features apply, where appropriate. Figure 3a is a plan view of the waveguide structure 300 and Figure 3b is a cross-sectional view of the waveguide structure 300 taken along the guide axis of the waveguide structure 300 indicated by the line BB'.

[0046] 3a and 3b differs from the waveguide structure shown in Figures 1a and 1b in that it includes two absorbing ridges, a first absorbing ridge 106a on one side of the actuation waveguide 104 and a second absorbing ridge 106b on the other side of the actuation waveguide 104. Thus, in the embodiment shown in Figures 3a and 3b, light that is not guided along the actuation waveguide 104 but is emitted in a second direction may be absorbed by the first absorbing ridge 106a if emitted from the actuation waveguide 104 in the negative y-direction and may be absorbed by the second absorbing ridge 106b if emitted in a third direction opposite the second direction of the actuation waveguide 104 in the positive y-direction.

[0047] The waveguide structure 300 shown in Figures 3a and 3b includes two of the recesses described above with reference to Figures 1a and 1b. The two recesses include a dielectric material 113 as described above. In the embodiment shown in Figure 3b, the first recess 302 has a first depth D1 relative to the top surface of the semiconductor structure, and the second recess 304 has a second depth D2 relative to the top surface of the semiconductor structure. In Figure 3, the depths D1 and D2 are equal (at least within acceptable manufacturing tolerances).

[0048] The contacts 110a, 110b corresponding to the first absorbing ridge 106a and the second absorbing ridge 106b may both be grounded, floating, or biased to the same reverse bias, or may be biased to different biases to independently control the absorption of each of the absorbing ridges 106a, 106b.

[0049] Figure 4 is a plan view of a waveguide structure 400 according to another embodiment. The waveguide structure 400 is similar to the waveguide structure shown in Figures 3a and 3b, and may be fabricated according to the method previously described with reference to Figure 2. Accordingly, features shown in Figure 4 that appear in Figures 3a and 3b (and Figures 1a and 1b) have been given the same reference numerals (with the addition of a and / or b, in this embodiment, if there is more than one previously described feature) and the previous descriptions of those features apply, where appropriate.

[0050] 4, the first contact 108 is formed over an opening 402 in a dielectric material 113 deposited over the drive waveguide 104. The opening 402 extends a length along the drive waveguide 104. In other embodiments, the dielectric material 113 may be replaced with a semiconductor material and the first contact 108 may be formed on the semiconductor material.

[0051] The absorbing ridges 106a, 106b include one or more extensions arranged to increase absorption of light not guided by the drive waveguide 104. For example, as shown in FIG. 4, there are extensions 404a-404d extending from the ends of each absorbing ridge 106a, 106b in the y-direction toward the drive waveguide 104. In an embodiment, each such extension 404a-404d extends from each end of each absorbing ridge 106a, 106b at a location along the x-axis. Thus, for example, the first extension 404a extends along the y-axis from a first end of the absorbing ridge 106a, and the second extension 404b extends along the y-axis from a second end of the absorbing ridge 106b. The third extension 404c and the fourth extension 404d similarly extend from the first and second ends of the other absorbing ridge 106b. This improves absorption of light not guided by the drive waveguide 402. In particular, light having a component in the x-direction that may not be absorbed by the absorbing ridges 106a, 106b may be absorbed by the extensions 404, thereby preventing the light from escaping the waveguide structure 400.

[0052] The above embodiments should be understood as illustrative examples of the present invention. For example, in the embodiment described above and shown in the cross-sectional views of Figures 1b and 3b, the recesses do not penetrate into the active region, but in additional embodiments, one or more of the recesses may have a depth D that extends to or beyond the active region 102. This can improve electrical isolation between the drive waveguide 104 and the absorbing ridges 106a, 106b. Furthermore, in the embodiment shown in Figure 3b, the depths Dl, D2 of the recesses 302, 304 are equal, but in other embodiments, the depths Dl, D2 are not equal. In some such embodiments, the depth Dl of the first recess 302 is such that the recess 302 extends through the active region 102, and the depth D2 of the second recess 304 is such that the recess 304 does not extend through the active region 102. In other embodiments, the depth Dl of the first recess 302 and the depth D2 of the second recess 304 are such that both recesses extend through the active region 102.

[0053] It should be understood that any feature described with respect to any one embodiment may be used alone or in combination with other features described, or in combination with one or more features of any other embodiment, or in any combination of any other embodiment. Moreover, equivalents or modifications not described above may also be employed without departing from the scope of the appended claims.

Claims

1. A waveguide structure of an optical integrated circuit, comprising: A substrate; an active region including a diode junction, a light emitting portion that emits light in a first direction and in a second direction perpendicular to the first direction; a light absorbing portion that absorbs light emitted in the second direction from the light emitting portion; the active region comprising: a first contact corresponding to the light emitting portion; a second contact corresponding to the light absorbing portion; and a drive waveguide having a longitudinal axis substantially parallel to the first direction; an absorber structure having a longitudinal axis substantially parallel to the first direction for absorbing light not guided by the drive waveguide; the drive waveguide is separated from the absorber structure by a recess in the waveguide structure; the recess includes a first sidewall provided on a side surface of the driving waveguide, a second sidewall provided on a side surface of the absorber structure, and a base portion connecting the first sidewall and the second sidewall, the absorbent structure includes an extension extending in a third direction from an end of the side of the absorbent structure; The third direction is opposite to the second direction.

2. the first contact is configured for electrical connection of the light emitting portion to a first bias circuit; The waveguide structure of claim 1 , wherein the second contact is configured for electrical connection of the light absorbing portion to a second bias circuit.

3. A waveguide structure as described in claim 1 or 2, wherein the extension portion is configured to absorb light emitted at least partially in the first direction from the light emitting portion.

4. A waveguide structure as described in any one of claims 1 to 3, wherein the extension is a first extension, the end is a first end, and the absorber structure includes a second extension extending in the third direction from a second end of the side of the absorber structure.

5. the light absorbing moiety is a first light absorbing moiety; the absorbent structure is a first absorbent structure, The waveguide structure comprises: a second light absorbing portion that absorbs light emitted in the third direction from the light emitting portion; a third contact corresponding to the second light absorbing portion; and a second absorber structure having a longitudinal axis substantially parallel to the first direction, the second absorber structure for absorbing light not guided by the drive waveguide; 5. The waveguide structure of claim 1 , comprising:

6. The recess is a first recess, the side of the drive waveguide is a first side; the base portion is a first base portion; the drive waveguide is separated from the second absorber structure by a second recess in the waveguide structure; 6. The waveguide structure of claim 5, wherein the second recess includes a third sidewall on a second side of the drive waveguide, a fourth sidewall on a side of the second absorber structure, and a second base portion joining the third sidewall and the fourth sidewall.

7. 7. A waveguide structure according to claim 5 or 6, wherein the first contact is between the second contact and the third contact.

8. 8. The waveguide structure of claim 5, wherein the light emitting portion is located between the first light absorbing portion and the second light absorbing portion.

9. A waveguide structure described in any one of claims 5 to 8, wherein the second absorber structure includes an extension portion extending in the second direction from an end portion of the side of the second absorber structure.

10. The extension portion of the second absorbent structure is a first extension portion of the second absorbent structure; the end of the side surface of the second absorbent structure is a first end of the first absorbent structure; 10. The waveguide structure of claim 9, wherein the second absorber structure includes a second extension extending in the second direction from a second end of the side of the second absorber structure.

11. 5. The waveguide structure according to claim 1, wherein the depth of the recess extends from a top surface of the waveguide structure to a position shallower than a depth at which the active region is located, or extends from a top surface of the waveguide structure to a position deeper than a depth at which the active region is located.

12. 8. The waveguide structure of claim 7, wherein the drive waveguide is separated from the absorber structure by at least one of a semiconductor material, a dielectric material, air, polyimide, benzocyclobutene (BCB), polymethylmethacrylate (PMMA), or SU-8.

13. 12. A waveguide structure according to claim 1, wherein the active region includes an interface with a surrounding semiconductor material that is tilted relative to the first direction.

14. 14. A waveguide structure according to any one of claims 1 to 13, wherein the active region is a region in which electron-hole pairs can recombine to emit photons and / or photons can be absorbed to create electron-hole pairs.

15. 15. A waveguide structure according to any preceding claim, wherein the active region comprises one or more quantum wells.

16. A semiconductor optical amplifier comprising a waveguide structure according to any one of claims 1 to 15.

17. An optical integrated circuit comprising the waveguide structure according to any one of claims 1 to 15 or the semiconductor optical amplifier according to claim 16.

18. a first bias circuit arranged to apply a forward bias to the first contact so as to generate light in the light emitting portion of the active region; a second bias circuit arranged to apply a ground, floating or reverse bias to the second contact so as to extract charge carriers generated in the light absorbing portion of the active region; 20. The optical integrated circuit of claim 17, comprising:

19. 1. A method for manufacturing a waveguide structure of an optical integrated circuit, comprising the steps of: depositing an active area including a diode junction; defining a light emitting portion in the active region, the light emitting portion emitting light in a first direction and a second direction perpendicular to the first direction; defining a light absorbing portion that absorbs light emitted in the second direction from the light emitting portion; depositing an etch mask having a longitudinal axis substantially parallel to the first direction, defining a drive waveguide for guiding light, and defining an absorber structure having a longitudinal axis substantially parallel to the first direction, for absorbing light not guided by the drive waveguide; Etching material to form a recess outside the drive waveguide and the absorber structure; defining a first contact corresponding to the light emitting portion; and defining a second contact corresponding to the light absorbing portion; the drive waveguide is separated from the absorber structure by the recess; the recess includes a first sidewall provided on a side surface of the driving waveguide, a second sidewall provided on a side surface of the absorber structure, and a base portion connecting the first sidewall and the second sidewall, the absorbent structure includes an extension extending in a third direction from an end of the side of the absorbent structure; The method, wherein the third direction is opposite to the second direction.

20. depositing a dielectric material over the drive waveguide and into the recess; forming an opening in the dielectric material over the drive waveguide; defining the first contact in the opening; 20. The method of claim 19, comprising:

21. depositing a semiconductor material over the actuation waveguide and into the recess; defining the first contact on the semiconductor material; 20. The method of claim 19, comprising:

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