Field of the Invention The present invention relates to a waveguide structure. In particular, the present invention relates to an improved waveguide structure including a waveguide having a thermally controllable section, and a method for manufacturing it.

Background

Where the term "light" is used, this refers generally to electromagnetic radiation, and not specifically to visible light. Where the term "laser" is used, this refers to a semiconductor laser unless specified otherwise. Thermally tuned semiconductor lasers (e.g. distributed Bragg reflector, DBR, lasers) are being developed to improve linewidth performance, compared to known electronically tuned lasers. Each type of tuning acts by modifying the refractive index of one or more components of the laser such as a reflector, causing that component to select for a different wavelength.

Electronically tuned lasers provide high levels of optical loss, which increases the laser threshold current and degrades the linewidth. Furthermore, because electronic tuning has a very fast response (on the order of nanoseconds), electronics noise is easily coupled to the laser output.

In contrast, thermal tuning does not significantly increase optical loss, so there is negligible degradation of linewidth. Furthermore, since the response of thermal tuning is much slower (on the order of several tens of microseconds), the laser output is decoupled from high frequency noise sources. Heat is applied to the waveguide optical core via a resistive heater stripe running on top or closely parallel to the waveguide ridge. The stripe is electrically isolated from the ridge by a passivation dielectric.

A typical electrically tuned laser has a cross section as shown in Figure 1 A. The laser comprises a p-cladding 101 , a waveguide core layer 102, an n-cladding layer 103 and a substrate 104. The p-cladding 101 is etched to form a waveguide ridge 105, to which is attached electrical means for varying the refractive index (not shown). The region of the waveguide core layer under the waveguide ridge forms the waveguide core.

Figure 1 B shows a "buried heterostructure" laser. The laser comprises a p-cladding 11 1 , a waveguide core layer 112, an n-cladding layer 1 13 and a substrate 1 14. Instead of the waveguide ridge 1 15, the waveguide is formed by a structure 1 15 in the upper cladding layer and waveguide core layer, which is isolated by an isolating region 1 16. The waveguide core of the buried heterostructure laser is formed by the section of the waveguide core layer within the structure 1 15.

Each laser is a planar structure of well heat-sunk materials, designed to extract the heat generated by the diodes. However, this means that when adapting such a laser design for thermal tuning, the power required to cause the necessary temperature shifts is very large (e.g. 1W for a 50-70°C temperature change). To improve the efficiency of thermal tuning, it is desirable to thermally isolate the waveguide from the support structures. However, sections of the laser which are not thermally tuned should be in thermal contact with the support structures so that their temperature can be held constant. An example known structure for achieving this is shown for a ridge waveguide laser in Figures 2A and 2B, where Figure 2A is a cross section view of the structure along the line IIA-IIA in Figure 2B, and Figure 2B is a plan view. The laser has an upper p- cladding layer 201 , a waveguide core 202, and a lower n-cladding layer 203. The upper p-cladding layer is etched to form a waveguide ridge 204. A layer of sacrificial material 205 is located between the lower cladding layer and a substrate 206, and this sacrificial material is etched out by a wet etch process to leave an airgap 208 underneath the section containing the waveguide ridge. Vias 207 are provided in the upper cladding layer, waveguide core, and lower cladding layer to allow the wet etch to reach the sacrificial material. Clearly, if the vias completely surround the waveguide, then it would no longer be supported, so support structures 209 are provided in the vias to connect the waveguide to the rest of the substrate.

However, these support structures cause the waveguide to have uneven thermal characteristics - i.e. parts of the waveguide near a support structure will cool more readily than parts distant from a support structure. This uneven heating affects the uniform control of refractive index along the component and reduces the performance of the laser.

Summary

According to a first aspect, there is provided a waveguide structure including a waveguide having a thermally controllable section. The waveguide structure comprises a plurality of layers. The layers comprise, in order: a substrate, a sacrificial layer, a lower cladding layer, a waveguide core layer, and an upper cladding layer. The lower cladding layer, waveguide core layer, and upper cladding layer form the waveguide, the waveguide has a waveguide core. The waveguide structure has a continuous via passing through the upper cladding layer, waveguide core layer, and lower cladding layer and running parallel to the waveguide ridge along substantially the whole length of the thermally controllable section. The waveguide structure also has a thermally insulating region in the sacrificial layer extending at least from the via to beyond the waveguide ridge along the whole length of the thermally controllable section. The sacrificial layer comprises a sacrificial material outside the thermally insulating region, and a thermally insulating gap or thermally insulating material separating the lower cladding layer and substrate inside the thermally insulating region.

According to a further aspect, there is provided a tuneable laser comprising the waveguide structure of the first aspect.

According to a yet further aspect, there is provided a method of manufacturing a thermally controlled waveguide. A waveguide structure is provided. The waveguide structure comprises, in order: a substrate, a sacrificial layer, a lower cladding layer, a waveguide core layer, and an upper cladding layer. The lower cladding layer, waveguide core layer, and upper cladding layer form the waveguide, the waveguide has a waveguide core. The waveguide structure has a continuous via passing through the upper cladding layer, waveguide core layer, and lower cladding layer and running parallel to the waveguide ridge along the whole length of the thermally controllable section. A wet etch is provided to the sacrificial layer through the via in order to remove material from at least a thermally insulating region in the sacrificial layer extending at least from the via to beyond the waveguide ridge along the whole length of the thermally controllable section, in order to create a gap separating the lower cladding layer and substrate in the thermally insulating region. The wet etch etches material of the sacrificial layer and does not etch materials of the substrate and lower cladding layer. Further embodiments of the invention are set out in claim 2 et seq. Brief Description of the Drawings

Figure 1 shows a cross section of a waveguide structure of a typical electrically tuned laser;

Figures 2A and 2B show a cross section and plan view of a known waveguide structure for a thermally tuned laser;

Figure 3 shows a cross section and plan view of an exemplary waveguide structure for a thermally tuned laser;

Figure 4 shows the stages of the manufacture of the structure of Figure 3;

Figure 5 shows a typical temperature profile of an exemplary waveguide structure during heating;

Figure 6 shows a cross section of an exemplary thermally tuned laser. Detailed Description

An alternative undercut structure is presented below. This structure overcomes the limitations of the prior art as it provides a more even heating profile. Furthermore, the structure is highly tolerant to variations in the manufacturing process and, in certain embodiments, allows more effective grounding of the waveguide structure than is possible with prior undercuts.

An exemplary structure is shown in Figure 3A and 3B, where Figure 3A is a cross section view of the structure along the line IIIA-IIIA in Figure 3B, and Figure 3B is a plan view. Similarly to the prior art structure of Figure 2, this structure has an upper p- cladding 301 and lower n-cladding 303 sandwiching a waveguide core 302. The upper p-cladding is etched to provide a waveguide ridge 304. A sacrificial layer 305 is provided between the lower n-cladding an a substrate 306 Rather than providing a series of vias on either side of the waveguide, as in known structures, a single via 307 is provided on one side of the waveguide. This via does not have any support structures crossing it and extends through the upper cladding layer 301 , waveguide core layer 302 and lower cladding layer 303 as far as the sacrificial layer 305. The via runs parallel to the waveguide ridge along the whole length of the thermally controllable section. Etching fluid is provided into the via to etch the sacrificial layer, which results in the section containing the waveguide ridge overhanging an airgap 308 in a cantilever-like arrangement. Adequate thermal properties can be obtained so long as the sacrificial material is etched at least beyond the waveguide since this will result in the waveguide being thermally insulated from the substrate. Any small over-etch will be uniform along the length of the via and has little effect on the thermal properties of the structure.

Figure 4 shows a manufacturing process for producing the structure of Figure 3. A layered structure 400 is fabricated, comprising, in order, a substrate layer 406, a sacrificial layer 405, a lower cladding layer 403, a waveguide core layer 402, and an upper cladding layer 401 . The upper cladding layer comprises an upper cladding material, e.g. a p-cladding material. The waveguide core layer comprises a waveguide core material. The lower cladding layer comprises a lower cladding material, e.g. an n- cladding material. The sacrificial layer comprises a sacrificial material. The substrate comprises a substrate material.

The layered structure 400 is then etched 4000, 4001 (e.g. using dry etching or a combination of dry etching and wet etching) to create intermediate structure 410. The first etching 4000 etches the upper cladding layer 401 to form a waveguide ridge 404 and an etched upper cladding layer 41 1 . The intermediate structure also has a via 407 which has been etched through waveguide core layer 402, and lower cladding layer 403 to leave etched waveguide core layer 412, and etched lower cladding layer 413. The via passes through the upper cladding layer, waveguide core layer, and lower cladding layer to the sacrificial layer. The upper cladding layer 401 may be etched from the location of the via during etching step 4000 (as shown), or it may be etched together with the waveguide core layer 402 and lower cladding layer 403 during etching step 4001 . If the upper cladding layer is only etched during step 4000, then the side of the etched upper cladding layer 41 1 may not be at the edge of the via (as shown in the figure). The intermediate structure is then etched 4002 by the use of a chemically selective wet etch introduced to the via 407 to create the waveguide structure 420. The wet etch preferentially etches the sacrificial material to form the etched sacrificial layer 415, such that the sacrificial material is removed from a region extending at least from the via 407 to beyond the waveguide ridge 404, leaving an air gap 408 between the lower cladding layer 413 and the substrate 406 in that region. The air gap causes the region to be thermally insulating. It will be noted that the waveguide structure 420 is equivalent to the waveguide structure presented in Figure 3. The etching processes 4001 and 4002 may be performed separately, or an intermediate structure 410 may be created by other means and provided for the wet etching process 4002.

Figure 5 shows a thermal model of the structure shown in Figure 3. Heat is applied to the waveguide ridge 304, and the substrate 306 is held at a constant temperature. In Figure 3, a high density of dots represents a high temperature, and a low density of dots represents a low temperature. Heat flow via the remaining sacrificial layer can be clearly seen. The thermal properties will depend on the width and thickness of the overhang, the choice of sacrificial material, and the thickness of the sacrificial material.

Typical overhang widths are 20 to 50 microns. Typical sacrificial material thickness is 0.25 to 2 microns. Typical thickness of each of the upper cladding, lower cladding and waveguide core layers is 1 -3 microns. The sacrificial material is typically 1 -2 microns beneath the optical core. The thermally insulating region typically extends 10-40 microns beyond the waveguide ridge, for example 30 microns beyond the waveguide ridge. In order to avoid thermal effects at the ends of the overhang, the overhang may extend in the direction of the axis of the waveguide at least 20 microns from critical features of the laser such as gratings, at least 50 microns from such features, or at least 100 microns from such features.

The combination of material used for the sacrificial layer, etching fluid, and cladding layers should be selected such that the etching fluid has a strong preference for etching the sacrificial material over the cladding layers. In the case where the waveguide core is vulnerable to the etching fluid, a passivation dielectric may be applied to the exposed surface of the waveguide core within the via, to prevent etching of the waveguide core.

As an example, the sacrificial material used in the sacrificial layer may include one or more of InGaAs, AllnAs, and AIGalnAs, and the cladding layers may include InP. Possible etching fluids which would etch the sacrificial layer but not significantly etch the cladding layer include:

• H ₃ P0 ₄ -H ₂ 0 ₂

· citric acid- H ₂ 0 ₂

• HNO ₃

• tartaric acid-HN0 ₃

• tartaric acid-H ₂ 0 ₂

The sacrificial material in the sacrificial layer remains in place to the sides of the waveguide structure, and may remain in place in regions of the device other than those which are thermally controllable. This ensures that those regions have thermal contact with the substrate, which helps with temperature control of those regions. Instead of leaving an air gap in the etched region, it may be filled or partially filled with a thermally insulating material, i.e. a material which is more thermally insulating than the sacrificial material.

The sacrificial material used in the sacrificial layer may also be formed as more than one discrete layer, although where this is the case, all of these discrete layers still collectively form the sacrificial material. In one arrangement, the sacrificial material may include a lower layer of AllnAs with a top layer of InGaAs. This particular arrangement has a number of advantages. Better growth morphology for subsequent layers appears to be achievable on InGaAs compared to AllnAs, and processing schemes involving a combination of wet etch and dry etch procedures can advantageously be employed. The combination of materials allows for optimisation of thermal conductivity in the layer remaining under the gain section. The optical absorption due to InGaAs helps to control stray (unguided) light. As an alternative, only InGaAs may be used as the sacrificial layer. Figure 6 shows an exemplary structure with further components. The upper and lower cladding 601 , 603, waveguide core layer 602, waveguide ridge 604, sacrificial layer 605, via 607 and substrate 606 are equivalent to those of the structure in Figure 3. The structure further comprises a passivation dielectric 609, a heater resistor 610, and ground contacts 61 1 , 612. The passivation dielectric 609 shields the waveguide core from the wet etch, and electrically isolates the ridge waveguide from the heater resistor 610. The passivation dielectric may be arranged to perform one or both of these functions. Referring back to Figure 4, the passivation dielectric may be applied to the intermediate structure 410 prior to introduction of the wet etch 4002. The heater resistor 610 provides heating to the ridge waveguide as required for thermal tuning of the laser. The ground contacts 61 1 , 612 cause a reduction in carrier density oscillations in the waveguide core and clamping of the Fermi level, which results in a suppression of shot noise and improved performance particularly in the 10MHz- 100MHz range. Because the p ground contact 61 1 is better connected to the waveguide ridge than is possible in prior art undercut designs, the shot noise of the structure shown is significantly reduced compared to an equivalent structure based on that shown in Figure 2. Other configurations of ground contacts and heaters are possible. For example, there may be a ground contact on the top of the waveguide ridge, with heaters on either side of the ridge, or a ground contact within one or both gaps in the upper cladding layer next to the ridge. The heater(s) may be in contact with the top of the ridge, the sides of the ridge, or both.

Figure 7 shows an alternative exemplary structure. The upper and lower cladding 701 , 703, waveguide core layer 702, waveguide ridge 704, sacrificial layer 705, via 707 and substrate 706 are equivalent to those of the structure in Figure 3. The structure further comprises heater resistors 709, 710, ground contacts 71 1 , 712, and a support ridge 713. The ground contact 71 1 is located on top of the waveguide ridge 704, and the heaters 709, 710 are located equidistant to either side of the ridge. A passivation dielectric 714 is provided to prevent electrical contact between the heaters and the semiconductor. The passivation dielectric 714 has a gap to allow contact between the ground contact 71 1 and the waveguide ridge 704. This arrangement provides improved phase noise relative to the arrangement of Figure 6. The support ridge 713 improves the mechanical strength of the overhang, in a manner analogous to a "C- beam" or "parallel flange channel" as used in mechanical fields. The upper cladding 701 of the support ridge 713 may be thickened to increase the mechanical integrity. The additional features of Figure 6 and Figure 7 may be combined in any suitable arrangement, or with other features mentioned in the disclosure but not presented in the figures. For example, a structure may be provided with the arrangement of ground contacts 61 1 , 612 and the support ridge 713, or with the arrangement of heater resistors 709, 710 and ground contact 712, without a support ridge 713.

A similar thermal isolation structure may be applied to a buried heterostructure laser, as shown in Figure 8. Figure 8 shows a waveguide structure comprising an upper and lower cladding 801 , 803, waveguide core layer 802, waveguide 804, isolating regions 808, sacrificial layer 805, via 807 and substrate 806. The substrate 806, via 807, and sacrificial layer 805 are equivalent to those of Figures 6 and 7. The waveguide 804 and isolating regions 808 are equivalent to the structure 1 15 and isolating regions 1 16of Figure 1 B. Ground contacts 81 1 , 812 are shown in a corresponding arrangement to that of Figure 6, but may also be in an arrangement equivalent to that of Figure 7. Heaters may be applied to any suitable location around the waveguide structure 804, e.g. either side of the waveguide structure as shown for heaters 809, 810. A passivation dielectric is applied to prevent electrical contact between the heaters 814 and the underlying components. In order to provide sufficient thermal isolation, the undercut should extend at least beyond the waveguide core, i.e. beyond the waveguide structure 804.

The waveguide structure disclosed above may be used for any waveguide with a thermally controllable section. For example, in a Distributed Bragg Reflector (DBR) laser, the waveguide structure may be used for the rear DBR section and/or the phase control section to provide improved thermal control of those sections.