GB2431288A

GB2431288A - Semiconductor optical Device

Info

Publication number: GB2431288A
Application number: GB0521017A
Authority: GB
Inventors: Jeremy Peter Duck; David James Robbins; Andrew John Ward; Douglas Charles John Reid; Neil David Whitbread
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2005-10-15
Filing date: 2005-10-15
Publication date: 2007-04-18
Also published as: GB0521017D0

Abstract

A semiconductor optical device comprises: (i) an elongate waveguide 24 having a longitudinal axis A-A and an output facet 28; (ii) at least one first doped region adjacent to, or part of, the waveguide; and (iii) a Bragg grating 6,8 comprising a plurality of grating elements. Each grating element of the Bragg grating extends in a direction substantially perpendicular to the longitudinal axis of the waveguide. The output facet of the waveguide intersects the longitudinal axis at a non-perpendicular angle, and wherever there is a first doped region adjacent to, or part of, the waveguide, the waveguide is substantially straight. The device may be a distributed feed back laser and may further comprise a semiconductor amplifier and a Mach-Zehnder modulator.

Description

Semiconductor Optical Device The present invention relates to

semiconductor optical devices, and particularly (although not exclusively) to semiconductor laser devices.

The invention especially relates to semiconductor optical devices comprising two or more component devices, e.g. a laser and a semiconductor optical amplifier, monolithically integrated onto a single semiconductor substrate.

Semiconductor optical devices comprising a semiconductor laser and a semiconductor optical amplifier monolithically integrated on a single semiconductor chip, are well known. For example, United States Patent No. 6,658,035 Bi discloses laser assemblies comprising a tunable laser and an external optical amplifier formed in a single epitaxial structure and sharing a common waveguide. The single epitaxial structure, encompassing the entire assembly comprising the laser and the amplifier, includes an upper p-type semiconductor layer, and a lower n-type semiconductor layer, with the common waveguiding layer situated between these two layers. The optical amplifier is situated adjacent to the output of the laser, and serves to amplify the light output of the laser.

The optical amplifier has an active section of the waveguide, which is straight, and a curved passive section of the waveguide, situated at the output side of the amplifier, which intersects an output facet of the device at an oblique angle. The oblique intersection with the output facet prevent reflections at the output facet from coupling back into the optical amplifier and the laser. The prevention of such back reflections is important, because if they were not prevented they would be amplified as they propagated back through the optical amplifier, and they would pass into the laser where the amplified reflections would then compete with the intended output from the laser, causing uncontrolled output peaks and a poor side-mode suppression ratio (SMSR) for the device.

United States patent application publications US 2003/0210720 Al and US 2004/0208213 Al disclose the use of an optical ballast layer and t an optical superlattice in a semiconductor laser, which result in a high power output beam and a narrow far field of the optical mode, thereby enabling efficient coupling of the optical mode into an optical fibre.

The present invention seeks to provide a semiconductor optical device that provides advantages over the known devices, including the devices disclosed in US 6,658,035 Bi. In particular, the invention seeks to provide a device that is able to produce an enhanced optical power output, while maintaining the prevention (or at least minimisation) of back reflections.

Accordingly, the invention provides a semiconductor optical device, comprising: (i) an elongate waveguide having a longitudinal axis and an output facet; (ii) at least one first doped region adjacent to, or part of, the waveguide; and (iii) a Bragg grating comprising a plurality of grating elements; wherein each grating element of the Bragg grating extends in a direction substantially perpendicular to the longitudinal axis of the waveguide, the output facet of the waveguide intersects the longitudinal axis at a non-perpendicular angle, and wherever there is a said first doped region adjacent to, or part of, the waveguide, the waveguide is substantially straight.

By the "longitudinal axis" of the waveguide is meant an imaginary line running lengthwise along the waveguide substantially along the centre of the waveguide. The longitudinal axis thus denotes the course of the waveguide along its length. In preferred embodiments of the invention, the waveguide is straight (or at least substantially straight) along its entire length (or at least along substantially its entire length). In such embodiments the longitudinal axis of the waveguide will consequently be (substantially) straight along (substantially) its entire length. However, the invention, at least in its broader aspects, does not preclude the possibility of the waveguide being bent (non-straight, e.g. curved) at one or more regions along its length, if those regions do not have a first doped region adjacent to, or part of, the waveguide. It is to be understood that for any embodiment of the invention that includes a region where the waveguide is not straight, the longitudinal axis still follows the course of the waveguide through that region, and thus the longitudinal axis is not straight in that region.

The (or each) first doped region may advantageously comprise an upper cladding layer, e.g. in the form of a p-doped region. Preferably, the waveguide comprises a guiding layer situated below the (or each) first doped region. The Bragg grating, in regions where it is present, may be situated above or below the guiding layer, for example. At least one second doped region, e.g. an n-doped region, preferably is situated below the guiding layer. (Alternatively, if the first doped region comprises an n- doped region, preferably a second doped region comprises a p-doped region. ) Because the first doped region (e.g. an upper cladding p-doped region) may be lossy, the device may include an optical ballast layer and/or an optical superlattice to pull the optical mode away from the first doped layer. However, pulling at least part of the optical mode away from the guiding layer of the waveguide causes the light to be guided less strongly. Consequently, if the waveguide had a bend (e.g. if it was curved), the light might not be sufficiently strongly guided around the bend, and at least some of the light might be lost from the waveguide.

Therefore (in accordance with the invention), the waveguide preferably is substantially straight, at least in any section of the waveguide where there is an adjacent ballast layer and/or optical superlattice arranged to pull at least part of the optical mode away from a first doped region adjacent to the waveguide.

It is to be understood that the waveguide of the device according to the invention preferably is a single-mode waveguide. An optical ballast layer and/or an optical superlattice may additionally or alternatively be useful in order to increase the width of the ridge of a ridge waveguide while preferably keeping the waveguide a single-mode waveguide. It can, for example, be useful to increase the width of the ridge to enable larger electrical currents to be injected into the waveguide, thereby enabling the device to have greater power. However, increasing the width of the ridge can cause the waveguide to become a multi-mode waveguide (which generally is to be avoided); this can be prevented by the provision of an optical ballast layer and/or an optical superlattice, to pull the mode further from the ridge, thereby maintaining the single-mode property of the waveguide.

By the provision of an output facet of the waveguide that is not perpendicular to the longitudinal axis of the waveguide, any back reflections from the output facet are unlikely to propagate back along the waveguide and compete with the intended light output of the device. The invention has the advantage that this is achieved despite the fact that the waveguide is substantially straight wherever there is a first doped region adjacent to, or part of, the waveguide. This is surprising, because known semiconductor devices that include a Bragg grating are unable to achieve this. Instead, known devices, such as the devices disclosed in US 6,658,035 Bi, provide a non-perpendicular output facet by curving the waveguide, and the curved waveguide is adjacent to doped regions of the semiconductor (e.g. the upper p-doped region). The present invention avoids the requirement to curve the waveguide where a first doped region is adjacent to, or part of, the waveguide, and by so doing provides the advantage that a more powerful light output may be guided by the waveguide than would normally be the case if the waveguide were curved.

Consequently, by means of the present invention, the twin goals of an enhanced optical power output and the prevention (or at least minimisation) of back reflections, may be achieved.

As mentioned above, the (or each) Bragg grating of the device comprises a plurality of grating elements, each of which extends in a direction substantially perpendicular to the longitudinal axis of the waveguide. By "grating elements" are meant the lines or stripes (or similar) that make up the grating. The grating elements will normally comprise a plurality of alternating regions of differing refractive indices arranged along, or parallel to, the waveguide (i.e. such that each element is perpendicular to the axis of the waveguide).

The semiconductor optical device preferably further comprises a substrate on which, or in which, the waveguide and the Bragg grating are situated. The substrate advantageously may comprise a semiconductor chip.

The device preferably comprises a semiconductor laser device, e.g. a distributed feedback laser device, but especially a distributed Bragg reflector laser device. Consequently (especially for those embodiments of the invention in which the laser is a distributed Bragg reflector laser device) the semiconductor device preferably further comprises a second Bragg grating. The second Bragg grating advantageously comprises a plurality of grating elements, each grating element extending substantially perpendicular to the longitudinal axis of the waveguide.

The semiconductor optical device may additionally or alternatively comprise a semiconductor optical amplifier (SOA) region of the device and/or a modulator region of the device (e.g. a Mach-Zehnder modulator region) and/or an optical spot-size converter region of the device, for example. Such region or regions of the device preferably are situated on, or in, the substrate, and the waveguide preferably extends through the region(s). Consequently, the semiconductor device preferably comprises a laser device monolithically integrated with one or more other such devices, and with the waveguide being common to both, or all, of the devices.

In some preferred embodiments of the invention, the waveguide extends to an edge of the substrate (e.g. an edge of a semiconductor chip), such that the output facet is formed by the edge of the substrate.

The non-perpendicular angle between the output facet and the longitudinal axis of the waveguide is therefore preferably provided by the longitudinal axis of the waveguide being oriented at a non-perpendicular angle with respect to the edge of the substrate. The edge of the substrate preferably is substantially straight. More preferably, the edge of the substrate may be one of four straight edges of the substrate, defining a substantially rectangular or square shape of the substrate in plan view (e.g. a rectangular or square semiconductor chip). At least part of the length of the waveguide thus preferably is oriented such that its longitudinal axis is not parallel to any of the edges of the substrate, i. e. preferably at least part of the length of the waveguide is oriented at an oblique angle across at least part of the substrate.

Advantageously, the waveguide may be substantially straight along at least a portion of its length extending from its output facet. (In preferred embodiments, there is a first doped region adjacent to, or part of, the waveguide along at least a portion of its length extending from its output facet.) Most preferably, as mentioned above, the waveguide is substantially straight along substantially its entire length. Consequently, in some especially preferred embodiments of the invention, the entire length of the waveguide is substantially straight, and extends at an oblique angle across at least part of the substrate, but preferably from one edge of the substrate to an opposite edge of the substrate. However, in other embodiments of the invention (which presently are less preferred), the waveguide may include at least one bend in a portion of its length where there is no first doped region adjacent to, or part of, the waveguide.

The angle of intersection between the output facet of the waveguide and the longitudinal axis of the waveguide preferably is at least 70 degrees, more preferably at least 75 degrees, especially at least 78 degrees. The intersection angle preferably is no greater than 89 degrees, more preferably no greater than 86 degrees, for example approximately 83 degrees. Generally, the intersection angle should be greater than the angle at which total internal reflection occurs, which the skilled person will be able to determine by trial-and-error and from a knowledge of the refractive index of the semiconductor material.

As mentioned above, the (or each) first doped region may advantageously comprise an upper cladding layer. In many embodiments of the invention, the (or each) first doped region comprises a p-doped region. Preferably, the waveguide comprises a guiding layer situated below the (or each) first doped region. The Bragg grating may be situated above or below the guiding layer, for example. At least one second doped region, e.g. an n- doped region, preferably is situated below the guiding layer. (Alternatively, if the first doped region comprises an n-doped region, preferably a second doped region comprises a p-doped region.) Thus, the device preferably comprises at least one p-i-n diode structure, with either the p-doped region or the n-doped region situated above a guiding layer of the waveguide, and either the n-doped region or the p- doped region (respectively) situated below the guiding layer of the waveguide. Wherever there is such a doped region adjacent to (or part of) the waveguide, the waveguide is substantially straight. Most preferably, wherever there is at least an upper doped region (normally a p-doped region) adjacent to (or part of) the waveguide, the waveguide is substantially straight. In preferred embodiments of the invention, the entire length of the waveguide has a first doped region (e.g. an upper doped region, especially p-doped) adjacent to, or part of, the waveguide, and consequently in such embodiments the entire length of the waveguide is straight.

In order to compensate for any tendency of a first doped region (e.g. an upper p-doped region) to cause absorption of the light propagating along the waveguide, and in order to ensure that the light output of the device has sufficient power, the light may be manipulated by the device in one or more ways. The simplest form of such manipulation may comprise ensuring that the light has sufficiently high power to have the required power when it exits the device, despite any absorption due to the doped region. (This may be achieved by means of a semiconductor optical amplifier of the device, for example.) However, if the waveguide had a bend (e.g. if it was curved), the light might not be sufficiently strongly guided around the bend, and at least some of the light might be lost from the waveguide. Consequently (in accordance with the invention), the waveguide is substantially straight, at least in any section of the waveguide where there is an adjacent first doped region. More preferably, the manipulation of the light may (additionally or alternatively) be more sophisticated than simply boosting its power. Advantageously, the device may include one or more optical ballast layers and/or at least one optical superlattice, arranged to draw (i.e. pull) at least a portion of the optical mode further away from the first doped region. For example, the device may include an optical ballast layer and/or an optical superlattice situated below the guiding layer of the waveguide, on the opposite side of the guiding layer to an upper first doped region (e.g. an upper p-doped region). However, pulling at least part of the optical mode away from the guiding layer of the waveguide causes the light to be guided less strongly. Consequently, if the waveguide had a bend (e.g. if it was curved), the light might not be sufficiently strongly guided around the bend, and at least some of the light might be lost from the waveguide.

Therefore (in accordance with the invention), the waveguide preferably is substantially straight, at least in any section of the waveguide where there is an adjacent ballast layer and/or optical superlattice arranged to pull at least part of the optical mode away from a first doped region adjacent to the waveguide. As explained above, in preferred embodiments of the invention, the entire length of the waveguide has a "first doped region" (e.g. an upper p-doped region) adjacent to it, or part of it. Consequently, in especially preferred embodiments of the invention, the entire length of the waveguide has an optical ballast layer and/or an optical superlattice situated below the guiding layer of the waveguide. In such embodiments of the invention, the entire length of the waveguide is substantially straight.

In preferred embodiments of the invention, the substrate includes at least a first doped region extending throughout substantially the entire area (in plan view) of the substrate. More preferably, substantially the entire area of the substrate comprises a p-i-n doped diode structure, for example comprising an upper p-doped layer (e.g. as a cladding layer), a lower n-doped layer, and an intermediate intrinsic (or at least unintentionally doped) layer. (Alternatively, the upper doped layer may be n-doped, and the lower doped layer may be p-doped.) Above and below the upper and lower doped layers preferably there are electrodes (e.g. metal contact layers), to provide electrical biasing to the p-i-n diode structure. For those embodiments of the invention comprising a laser device, a gain section of the laser preferably is forward-biased in use (i.e. with the positive electrode in contact with the p-doped layer). If a semiconductor optical amplifier is present, that region of the p-i-n diode structure preferably is forward-biased in use. In both cases, light is generated by stimulated emission. The (or each) Bragg grating section of the device may be forward-biased or reverse-biased in use, to produce changes in refractive index in order to tune the reflection spectra of the gratings. For ease of fabrication, for embodiments of the invention that include an optical ballast layer and/or an optical superlattice, these preferably are provided throughout substantially the entire area (in plan view) of the substrate.

In some embodiments of the invention, the width of the waveguide may increase (e.g. it may flare outwardly) as the waveguide approaches the output facet. This may provide multiple advantages, especially if the waveguide forms part of a semiconductor optical amplifier (SOA) in this region. Possible advantages include increased optical gain due to an increased volume of the active layer experienced by the optical mode, and also include possible advantages provided by the concomitant reductions in the density of electrical charge carriers and the density of the optical power.

At least in the broadest aspects of the invention, the waveguide of the semiconductor device may comprise any type of semiconductor waveguide, including, for example, a buried waveguide. In some preferred embodiments, however, the waveguide comprises a ridge waveguide, for example a surface ridge waveguide. As described above, - 10 - the ridge waveguide preferably includes at least one guiding layer, which may be situated below the ridge, in the body of the substrate.

The semiconductor optical device according to the invention may generally be fabricated from any semiconductor materials, but preferred materials include semiconductors formed from elements of Groups III and V of the periodic table of the elements, i.e. so-called 111-V semiconductors, e.g. GaAs and/or InP based semiconductors.

Some preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 shows, schematically and in cross-section, a known distributed Bragg reflector semiconductor laser device suitable for use in the present invention; Figure 2 shows, schematically and in plan view, a semiconductor optical device according to the invention; Figure 3 (views (a) and (b)) shows, schematically and in end views, the semiconductor optical device shown in Figure 2; Figure 4 (views(a) to (c)) shows, schematically and in cross- section, details of the epitaxial structure of three embodiments of optical device according to the invention; and Figure 5 (views (a) and (b) ) shows, schematically and in plan view, two embodiments of semiconductor optical device according to the invention.

Figure 1 shows schematically and in cross-section, a known distributed Bragg reflector semiconductor laser device 18 suitable for use - 11 - in the present invention. The laser device 18 may, for example, be substantially as described in WO 03/012936 (from where Figure 1 is taken). The entire disclosure of WO 03/012936 is incorporated herein by reference.

The laser 18 of Figure 1 is fabricated in a series of epitaxial layers, with a guiding layer 1 which, apart from a gain section 4 of the guiding layer, comprises intrinsic (i.e. not intentionally doped) semiconductor material formed between a lower n-doped layer 2 and an upper p-doped layer layer 3. (Alternatively, the upper layer 3 could be n-doped, and the lower layer 2 could be p-doped.) Thus, the epitaxial structure of the laser 18 comprises a p-i-n diode structure. (There may be other layers in the epitaxial structure, including for example an optical ballast layer and/or an optical superlattice as described below, but for simplicity they are not shown in Figure 1.) The guiding layer 1 has a higher refractive index than do the upper and lower doped layers 3 and 2, so that the optical mode is largely centred on the guiding layer 1. The laser 18 has four principal sections: the gain section 4, a phase change section 5 and front and rear reflecting sections 6 and 7, respectively. The region of the guiding layer 1 in the gain section 4 is an active layer that produces optical gain when an electrical current is passed through the gain section by forward-biasing the pin-diode structure in the gain section. The forward-biasing is achieved by applying a positive electrical potential to the upper p-doped layer 3 via an upper electrode 4a (in the form of a metal contact on the top surface of the p-doped layer 3), and applying a negative electrical potential to a corresponding electrode (not shown) on the bottom surface of the lower n-doped layer. The active layer 1 in the gain section 4 may, for example, contain multiple quantum wells or quantum dots, or it may provide its optical gain properties by means of the bulk properties of the semiconductor material in this section, in a known way.

The rear reflecting section 7 has a distributed Bragg grating reflector 8 formed in the upper p-doped layer 3. Such a reflector produces a "comb" of reflectance peaks at separated wavelengths, and each peak is - 12 - of substantially the same height. The front reflector 6, also in the upper p-doped layer 3, is made up of a series of segments 9 to 16, each segment being a distributed Bragg grating reflector, but each segment reflecting at substantially a single wavelength only. (The rear Bragg grating reflector 8 and/or the front distributed Bragg grating segments 9 to 16, could instead by provided in the lower n-doped layer 2.) Upper electrodes 7a and 9a to 16a are provided in the form of metal contacts on the top surface of the upper p-doped layer 3, and a plurality of corresponding lower electrodes, or a single electrode (not shown), is/are provided in the form of one or more metal contacts on the bottom surface of the lower n-doped layer 2. Each of the wavelengths of the individual peaks of the segments corresponds to a respective peak of the comb of reflectance peaks produced by the front distributed Bragg reflector 8. By means of the upper electrodes 7a and the lower electrode(s), the rear Bragg grating 8 and each of the front Bragg grating segments 9 to 16 may be tuned by applying, and varying, respective forward or reverse electrical biases across the p-i-n diode structure, thereby varying the refractive index in each of those regions. Consequently, the laser 18 may be tuned in a known way (e.g. as described in WO 03/012936).

Figure 2 shows, schematically and in plan view, a semiconductor optical device 20 according to the invention. The device 20 comprises a substrate 22 (e.g. a semiconductor chip), a waveguide 24, a distributed Bragg reflector laser 18, e.g. as illustrated in Figure 1, and a semiconductor optical amplifier (SOA) 26. The laser 18 and the SOA 26 are located on the common waveguide 24, with the SOA situated adjacent to the output end of the laser beyond the front Bragg grating reflector 6.

The two Bragg gratings 6 and 8 of the laser 18 are illustrated schematically as a series of parallel lines representing the grating elements of the gratings.

The waveguide 24 is straight and elongate, has a longitudinal axis A-A, and has an output facet 28 formed by the output end of the waveguide (beyond the front reflector 6 of the laser 18, and the SA 26) - 13 meeting a straight edge 30 of the substrate 22. As shown, the output facet 28 intersects the longitudinal axis A-A of the waveguide at a nonperpendicular angle (e.g. of 83 degrees), because the straight waveguide 24 is oriented on the substrate 22 such that its longitudinal axis A-A is not perpendicular to the edge 30. In particular, the substrate 22 is rectangular in plan view, and the waveguide 24 extends across the substrate at an oblique angle such that it is not parallel to any of the edges (sides) of the substrate. Because the output facet 28 intersects the longitudinal axis A-A of the waveguide at a non-perpendicular angle, any back reflections of light from the facet will not be strongly guided back along the waveguide through the SOA (where they would be amplified and reduce the performance of the device), but instead will tend to escape from the waveguide.

As indicated in Figure 2 by the parallel lines representing the grating elements of the two Bragg gratings 6 and 8, each of the grating elements extends in a direction substantially perpendicular to the longitudinal axis A-A of the waveguide. This has the advantage that the effect of the Bragg gratings 6 and 8 on the light propagating along the waveguide is substantially maximised. (If, instead, the grating elements of the gratings were not substantially perpendicular to the longitudinal axis of the waveguide - e.g. if the grating elements were parallel to the edge 30 of the substrate - then the effect of the gratings would be lessened, or even nonexistent, and consequently the laser 18 would not function efficiently, or at all.) Preferably the grating elements are "written" by means of electron-beam lithography. Alternatively, however, the grating elements could be written holographically for example, by means of two (or more) interfering light beams.

The grating elements preferably comprise a plurality of alternating regions of differing refractive indices arranged along, or parallel to, the waveguide 24 (i.e. such that each element is perpendicular to the axis A-A of the waveguide). As indicated above, the gratings 6 and 8 can be either above or below the guiding layer 1 of the waveguide 24. In an - 14 - arrangement in which there is a p-doped upper cladding layer, if the gratings are below the guiding layer then a higher refractive index grating may be used, in order to pull the optical mode down away from the doped cladding layer, thereby acting as (or part of) an optical ballasting arrangement. Each grating may, for example, be formed as a "surface grating", in which an epitaxial layer is partially etched so that the grating comprises a modulation of the thickness of the layer, or formed as a "floating grating", in which the grating is formed by etching completely through an epitaxial layer and is then over-grown with another material (e.g. the same material as that beneath the grating).

The semiconductor optical amplifier (SOA) 26 of the device 20 illustrated in Figure 2 comprises a gain section of the guiding layer 1 of the waveguide 24 sandwiched between the upper p-doped region 3 and the lowern-doped region 2 of the device. The gain section of the SOA 26 may be the same as, or different to, the gain section 4 of the laser 18, and is an active layer that produces optical gain when an electrical current is passed through the gain section by forward-biasing the pin-diode structure of the SOA. The forward-biasing is achieved by applying a positive electrical potential to the upper p-doped layer 3 via an upper electrode (not shown, but in the form of a metal contact on the top surface of the p-doped layer 3), and grounding a corresponding electrode (also not shown) on the bottom surface of the lower n-doped layer. The active layer of the SOA 26 may, for example, contain multiple quantum wells or quantum dots, or it may provide its optical gain properties by means of the bulk properties of the semiconductor material in this section, in a known way. Additionally, as illustrated, the width of the waveguide 24 at the SOA 26 may increase (e.g. it may flare outwardly) as the waveguide approaches the output facet 28. This may provide multiple advantages, including increased optical gain due to an increased volume of the active layer experienced by the optical mode, and also including possible advantages provided by the concomitant reductions in the density of electrical charge carriers and the density of the optical power.

- 15 - As mentioned above, the common waveguide 24 preferably is a ridge waveguide comprising the guiding layer 1 of relatively high refractive index material within the substrate 22, and a ridge projecting from the upper surface of the substrate, above the guiding layer 1. Figure 3 illustrates the basic epitaxial structure of the substrate 22. View (a) of Figure 3 shows a schematic end view of the device 20 shown in Figure 2, and in particular shows the edge 30 of the substrate 22. The ridge 32 of the waveguide 24 is shown towards the left-hand side of the edge 30, and is shown as being relatively wide due to the flaring of the ridge in the region of the SOA, as described above. View (b) of Figure 3 shows a schematic end view of the device 20 from the opposite edge 34 of the substrate 22, and therefore also shows the ridge 32 towards the left-hand side of the edge 34 (due to the oblique orientation of the waveguide), but shows the ridge as being narrower than at its flared end.

Both views (a) and (b) of Figure 3 show the basic epitaxial structure of the substrate 22 as comprising the n-doped bottom layer 2, on top of which is an optical ballast layer 36 and/or optical superlattice 36, on top of which is a further layer of the n-doped material 2, on top of which is the guiding layer 1, on top of which is the p-doped cladding layer 3, from which the ridge 32 also is formed. The ballast layer 36 (if present) comprises a layer having a higher refractive index than the layers immediately adjacent to it, which thereby attracts a portion of the optical mode and thus pulls part of the optical mode down away from the lossy (absorptive) p-doped cladding layer 3. If (additionally or instead) an optical superlattice 36 (especially a large optical superlattice, LOSL) is present, this comprises a plurality of layers of alternating high and low refractive indices, the layers being arranged one on top of another. The optical superlattice 36 also (or instead) attracts a portion of the optical mode and thus pulls part of the optical mode down away from the lossy (absorptive) p-doped cladding layer 3. As explained above, because the waveguide 24 is substantially straight, the optical mode is still guided by the waveguide despite a weakening of its guiding effect due to part of the optical mode being pulled down from the upper cladding layer 3 by the - 16 - ballast layer and/or optical superlattice, which pulls the optical mode further from the guiding effect of the waveguide ridge. By virtue of the oblique angling of the waveguide across the substrate, a slanted output facet 28 is obtained despite the fact that the waveguide is substantially straight, and consequently the problem of back reflections into the SOA 26 is substantially avoided.

Views (a) to (c) of Figure 4 show, schematically and in cross- section, details of the epitaxial structure of three embodiments of optical device according to the invention, in relation to an optical mode 38 propagating through the device in use. In view (a), the device comprises a p-doped upper cladding layer 3, in which a ridge 32 of the waveguide 24 is formed. Below the upper cladding layer 3 is an intrinsic active layer 1, which constitutes the guiding layer of the waveguide, and consequently most of the optical mode 38 is situated in this layer, as illustrated. Below the intrinsic active layer 1 is an n-doped lower cladding layer 2, and below this is the substrate 22. View (b) of Figure 4 shows an embodiment of the invention that is similar to the embodiment shown in view (a) except that the view (b) embodiment includes an optical ballast layer 36 in the ndoped lower cladding layer. As illustrated, the ballast layer pulls the optical mode 38 towards it to an extent, and therefore the optical mode propagates predominantly in both the intrinsic layer 1 and in the lower cladding layer 2. This means that relatively little of the optical mode 38 propagates through the lossy p-doped upper cladding layer 3 (compared to the embodiment shown in view (a)). View (c) of Figure 4 shows an embodiment of the invention that is similar to the embodiment shown in view (b) except that the view (c) embodiment includes a large optical superlattice 36 in place of the optical ballast layer. The large optical superlattice 36 has substantially the same effect on the optical mode 38 as does the optical ballast layer, as is shown in view (C).

Figure 5 (views (a) and (b)) shows, schematically and in plan view, two embodiments of semiconductor optical device according to the invention. These views are similar to that of Figure 2, but indicate the - 17 - upper electrodes of the gain section 4 and the phase change section 5.

Additionally, the upper electrode of the semiconductor optical amplifier (SOA) 26 is shown. In the embodiment shown in view (a) the end of the waveguide in the region of the SOA is not flared, whereas in the embodiment shown in view (b), it is flared.

As mentioned above, the semiconductor optical device 20 may include a modulator region of the device (e.g. a Mach-Zehnder modulator region) and/or an optical spot-size converter region of the device, for example. Such region or regions of the device preferably are situated on, or in, the substrate 22, and the waveguide 24 preferably extends through the region(s). In a Mach-Zehnder region (not shown) of the device 20, the waveguide 24 splits into two waveguide arms before recombining once more as the single waveguide 24. One or both of the two waveguide arms of the modulator includes an optical phase shifter, and the modulator is able to modulate the amplitude (or power) of the light propagating through it, by applying phase differences to the light propagating along the waveguide arms. If an optical spot-size converter region (not shown) of the device is present, this is situated at or adjacent to the output facet 28, and comprises a region of the device that alters the cross-sectional shape of the optical mode, preferably so that it can be coupled more efficiently into an optical fibre. For example, the spot-size converter may comprise a region of the device in which the waveguide 24 becomes wider and the guiding layer 1 becomes thinner, thereby causing the far field of the optical mode to become more circular, which facilitates coupling into an optical fibre situated adjacent to the output facet 28 of the waveguide.

However, more generally, the spot-size converter may have whatever form is required in order to shape the optical mode in the appropriate way.

Claims

- 18 - Claims 1. A semiconductor optical device, comprising: (i) an

elongate waveguide having a longitudinal axis and an output facet; (ii) at least one first doped region adjacent to, or part of, the waveguide; and (iii) a Bragg grating comprising a plurality of grating elements; wherein each grating element of the Bragg grating extends in a direction substantially perpendicular to the longitudinal axis of the waveguide, the output facet of the waveguide intersects the longitudinal axis at a non-perpendicular angle, and wherever there is a said first doped region adjacent to, or part of, the waveguide, the waveguide is substantially straight.
2. A device according to claim 1, further comprising a substrate on which, or in which, the waveguide and the Bragg grating are situated.
3. A device according to claim 1 or claim 2, which comprises a semiconductor laser device.
4. A device according to claim 3, wherein the laser device is a distributed Bragg reflector laser device.
5. A device according to claim 3, wherein the laser device is a distributed feedback laser device.
6. A device according to claim 2 or any claim dependent thereon, wherein the substrate has an edge, the waveguide extends to the edge such that the output facet is formed by the edge of the substrate, and the nonperpendicular angle between the output facet and the longitudinal axis of the waveguide is provided by the - 19 - longitudinal axis of the waveguide being oriented at a non- perpendicular angle with respect to the edge of the substrate.
7. A device according to claim 6, wherein the edge of the substrate is substantially straight.
8. A device according to claim 7, wherein the edge of the substrate is one of four straight edges thereof defining a substantially rectangular or square shape of the substrate in plan view, and wherein at least part of the length of the waveguide is oriented such that its longitudinal axis is not parallel to any of the edges of the substrate.
9. A device according to any preceding claim, wherein the substrate comprises a semiconductor chip.
10. A device according to any preceding claim, wherein the waveguide is substantially straight along at least a portion of its length extending from its output facet.
11. A device according to any preceding claim, in which the waveguide flares outwardly as the waveguide approaches the output facet.
12. A device according to any preceding claim, wherein there is a said first doped region adjacent to, or part of, the waveguide along at least a portion of its length extending from its output facet.
13. A device according to any preceding claim, wherein the waveguide is substantially straight along substantially its entire length.
14. A device according to any one of claims 1 to 12, wherein the waveguide includes at least one bend in a portion of its length where there is no said first doped region adjacent thereto, or part thereof.

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15. A device according to any preceding claim, wherein the angle of intersection between the output facet of the waveguide and the longitudinal axis of the waveguide is in the range 70 degrees to 89 degrees, preferably in the range 78 degrees to 86 degrees.
16. A device according to any preceding claim, wherein the, or each, said first doped region comprises an upper cladding layer.
17. A device according to any preceding claim, wherein the, or each, said first doped region comprises a p-doped region.
18. A device according to any preceding claim, wherein the waveguide comprises a guiding layer situated below the, or each, said first doped region.
19. A device according to claim 18, wherein the Bragg grating is situated above or below the guiding layer.
20. A device according to claim 18 or claim 19, further comprising at least one second doped region situated below the guiding layer.
21. A device according to claim 20, wherein the, or each, said second doped region comprises an n-doped region.
22. A device according to any one of claims 18 to 21, further comprising an optical ballast layer situated below or above (preferably below) the guiding layer, at least for a portion of the waveguide where there is a said first doped region adjacent to, or part of, the waveguide.
23. A device according to any one of claims 18 to 21, further comprising an optical superlattice situated below or above (preferably below) the guiding layer, at least for a portion of the - 21 - waveguide where there is a said first doped region adjacent to, or part of, the waveguide.
24. A device according to any preceding claim, wherein the waveguide comprises a ridge waveguide.
25. A device according to claim 2 or any claim dependent thereon, further comprising a semiconductor optical amplifier (SOA) region of the device situated on, or in, the substrate, and through which the waveguide extends.
26. A device according to claim 2 or any claim dependent thereon, further comprising a Mach-Zehnder modulator region of the device situated on, or in, the substrate, and through which the waveguide extends.
27. A device according to claim 2 or any claim dependent thereon, further comprising an optical spot-size converter region of the device situated on, or in, the substrate, and through which the waveguide extends.
28. A device according to any preceding claim, further comprising a second Bragg grating comprising a plurality of grating elements, each grating element extending substantially perpendicular to the longitudinal axis of the waveguide.
29. A semiconductor optical device substantially as illustrated in, and/or substantially as described herein with reference to, the accompanying Figures.