WO2012171557A1

WO2012171557A1 - Integration platform incorporating optical waveguide structures

Info

Publication number: WO2012171557A1
Application number: PCT/EP2011/059912
Authority: WO
Inventors: Marko GALARZA GALARZA; Oscar GARCÍA LÓPEZ
Original assignee: Universidad Pública de Navarra; Fundación Investigación Y Desarrollo En Nanotecnología (Fidena)
Priority date: 2011-06-15
Filing date: 2011-06-15
Publication date: 2012-12-20

Abstract

An integration platform for photonic integrated circuits comprising a first down core and a second upper core surrounded with cladding material, wherein the two cores are optically coupled, the platform comprising, in the direction of the light's propagation, a first rib waveguide etched in the down core, at least a vertical resonator, a second rib waveguide etched in the lower core and a third rib waveguide etched in the upper core and placed over the second waveguide having a constant and smaller width than the second waveguide. With this simple, flexible and cost-effective integration scheme, transference of optical power between active and passive components can be made without the need of any tapered structure such that the footprint size of the PIC can be remarkably reduced.

Description

INTEGRATION PLATFORM INCORPORATING OPTICAL WAVEGUIDE

STRUCTURES

DESCRIPTION

FIELD OF THE INVENTION

This invention is related to the field of optical communications and integrated semiconductor devices. In particular the invention relates to optical waveguide structures.

STATE OF THE ART

The explosive growth of the Internet due to consumer applications like video on demand (VOD) has increased remarkably the necessity of bandwidth from the communication networks. To cope with this feature it is predicted that the capacity of those networks will grow exponentially in the next years. However, the Electronic Integrated Circuits (EIC) are touching its physical integration limit and this technology suffers from a great power consumption. Photonic Integrated Circuits (PIC) appears now like the natural evolution of the former owing to its higher bandwidth and its capacity to save power in the communication networks. Photonic integration is the way to build complex PICs with great processing power, low cost per device, improved reliability, and reduced space and power requirements. Those PICs are well suited to applications in such technologies as instrumentation, signal processing, sensors and the aforementioned communication networks. A PIC employs optical waveguides to implement devices, such us transmitters and receivers, amplifiers, switches and couplers, and other passive and active semiconductor optical devices. The integration of active and passive devices in a single chip provides an effective platform for use in photonic industry.

A particularly versatile PIC platform technology is the integrated twin waveguide (TG) structure presented by Yasuharu Suematsu in 1975 for laser-waveguide integration applications. Suematsu's structure is based in evanescent field coupling of active and passive waveguides combined in a vertical directional coupler geometry. More in detail, Suematsu defined a base wafer with two different cores (high refractive index material surrounded by a lower refractive index material with the purpose of confine the light inside of the former material), the lower core to place the passive waveguides and the upper core to place the active waveguides. As is known, the TG structure requires only a single epitaxial growth step and it has the property to be suitable for manufacturing variety of PICs, each with different layouts and components.

All the components are defined by post-growth patterning, avoiding the need for material re-growth or any post-growth modification of the epitaxial structure. In summary, a universal epitaxial structure can be grown in advance without having detailed information about the integrated optical components which will be made. The original TG structure, however, has an important shortcoming. Optical power is constantly going up and down due to the full resonant mode beating phenomena between the odd and even modes of the structure, and it is not possible to keep the light in any waveguide. Besides, any device made from the TG structure is strongly dependent of its length, resulting in the inability to control critical parameters like the lasing threshold current of a laser or the modal gain in a semiconductor optical amplifier. This structure was disclosed in U.S. Pat. No.4,054,363.

A modified TG structure was presented by Forrest et al. in U.S. Pat. No 5,859,866. The Forrest's structure overcomes the stability problems existing in the TG cavity laser adding an absorption layer (or loss layer) between the upper and lower waveguides. This way, an additional loss to the even mode is introduced in the system and as a result of that, the odd mode becomes dominant in the laser performance. Once more, the modified TG structure is designed to have relatively equal confinement factors for both the even and odd modes in each waveguide layer by constructing active and passive waveguides of equal effective index of refraction. The resulting confinement factors are relatively the same because the even and odd optical modes split nearly equally in the active and passive waveguides. The absorption layer suppresses lasing on the even mode while it has minimal effect on the odd mode. Thus, the modal interaction is largely eliminated, resulting in optical power transfer without affecting performance parameters such as the threshold current, modal gain or coupling efficiency. The drawback of the structure presented in US 5,859,866 is that this technology is ineffective in a device with a travelling-wave optical amplifier (TFA), which is an important component in PICs designed for optical communications systems. The reason for this is that in a modified TG structure (with an additional absorption layer) run as a travelling-wave optical amplifier, the additional absorption in the single pass through the active region is insufficient to remove the even mode.

One more time, Forrest et al. based in Suematsu's Twin Waveguide proposed a new integration platform for PICs. In U.S. Pat. No. 6,819,814 the Asymmetric Twin Guide (ATG) platform is disclosed (see Figure 1 ). This invention provided an structure that significantly reduced the negative effects of modal interference and it can be used to implement both lasers and travelling wave optical amplifiers. To achieve this performance in the ATG structure, the effective index of the passive waveguide (the lower core of the TG structure) is varied such that one mode of the even and odd modes of propagation is primarily confined to the passive waveguide and the other to the active waveguide. As a result, the mode with the larger confinement factor in the active waveguide experiences higher gain and becomes dominant.

Nevertheless, there is a lost of flexibility in this design because the light get trapped in one of the layers. To solve this constraint, rather long adiabatic tapers for the transitions upwards and downwards are needed.

More recently, in the late 2000's, Tolstikhin et al. proposed a more sophisticated version of the Suematsu's TG structure. In Patent WO2008/061356 a vertical wavelength multiplexer is presented (VWM) (see Figure 2). This component is based in a controllable, non resonant adiabatic transition of the optical signals, which propagate in a common passive waveguide into a plurality of other wavelength designated waveguides without any significant interaction with them, such that, all designated waveguides are formed at different guiding layers of the same structure. This structure is monolithically integrated on the same semiconductor substrate in one epitaxial growth step. Each waveguide its defined by an specific band-gap wavelength, being the band-gap wavelength of the common passive waveguide shorter than the band-gap wavelength of the other waveguides. In a direct band-gap semiconductor from groups III and V, its refractive index n depends on the relation between its band-gap wavelength and the optical field wavelength, such that for any given wavelength, the longer is the band-gap wavelength the higher is the refractive index. For this reason, the light will be trapped in the layers with the higher band-gap wavelength. Again, there is a loss of flexibility in the integration platform because of the different effective index of each layer. For that reason, long adiabatic tapers are needed to make the transition between the different layers.

Therefore, there is a need in the art of optical communications to provide a simple, flexible and cost-effective integration scheme where the transference of optical power between active and passive components can be made without the need of any tapered structure such that the footprint size of the PIC can be remarkably reduced. There is a further need in the art to provide an integration platform that ensures stability in the laser and other critical components of the chip without a loss of flexibility in the optical power transference between different active and passive components of a PIC. There is a further need in the art to provide an integration platform that significantly reduces negative effects of modal interference without the need of excessive coupling loss. There is a further need in the art to provide an integration platform where it is possible to design and fabricate active micro-ring and micro-disk components easily coupled to an underlying passive waveguide without the requirements of wafer bonding or regrowth technology.

SUMMARY OF THE INVENTION The invention thus provides an integration platform for photonic integrated circuits comprising a first down core and a second upper core surrounded with cladding material, wherein the two cores are optically coupled, the platform comprising, in the direction of the light's propagation, a first rib waveguide etched in the down core, at least a vertical resonator, a second rib waveguide etched in the lower core and a third rib waveguide etched in the upper core and placed over the second waveguide having a constant and smaller width than the second waveguide. The second and/or third waveguides can be bent in different ways. The two cores can have the same or different refractive index, as long as they are optically coupled. Possible materials for the cores and cladding layers are InGaAsP and InP (also doped). BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings illustrate a preferred embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be embodied. The drawings comprise the following figures:

Figure 1 .- is a representations of a prior art's guide.

Figure 2.- is a representation of a guide also according to the prior art.

Figure 3.- is a representation of a first embodiment of the integration platform of the invention.

Figure 4.- is a representation of the first embodiment in accordance with the invention. Three cross - sections taken in the points A, B and C in the direction of the light's propagation are shown.

Figure 5.- is a function of the power coupled from a waveguide etched in the lower core to a waveguide etched in the upper core when varying the length of the directional coupler.

Figure 6.- is a representation of the second embodiment in accordance with the invention.

Figure 7.- is a representation of a third embodiment in accordance with the invention.

Figure 8.- is a representation of a forth embodiment in accordance with the invention.

Figure 9a- is a function of the 90⁵ bending losses versus radius of curvature for different widths of the third rib waveguide of the fourth embodiment. TE polarization is studied.

Figura 9b.- is a function of the 90⁵ bending losses versus radius of curvature for different widths of the third rib waveguide of the forth embodiment. TM polarization is studied.

Figure 10.- is a representation of a fifth embodiment in accordance with the invention.

Figure 1 1 .- is a representation of a sixth embodiment in accordance with the invention. DESCRIPTION OF THE INVENTION

The integration platform according to the simplest embodiment is shown in Figure 3. It can be fabricated by means of conventional lithography techniques from a single epitaxial wafer having two cores vertically stacked and optically coupled. The thicknesses and refractive index of the two cores are chosen to give very close propagation constants when the two cores are well separated (the cores are phase matched). Under this assumptions, the two cores have enough optical coupling when they are in proximity if they have the same width.

According to this first aspect, the invention provides a new and versatile integration platform for photonic components comprising of two cores of high refractive index surrounded by cladding materials of lower refractive index, where the two cores are perfectly optically coupled. In a particular embodiment, depending of the platform's application, the lower core (see (1 ) in Figure 3) can be made of a passive material and the upper core (2) can be made of an active material. This structure can be monolithically fabricated on a single epitaxial structure without the necessity of epitaxial regrowth and it can be effectively used to implement a variety of optical devices with the only use of standard lithography techniques. To achieve this performance the invention incorporates in the direction of the light's propagation (Z) a first rib waveguide etched in the down core, a vertical resonator, a second rib waveguide etched in the lower core and a third rib waveguide etched in the upper core and placed over the second waveguide having a smaller width than the second waveguide.

The rib waveguide in the lower core can be used to make passive optical devices (see (A) in Figure 3). The one or several vertical directional couplers (see (B) in Figure 3) work in a resonantly manner to transfer optical power between the lower core and the upper core. Finally, in a rib waveguide in the upper core over a wider rib waveguide in the lower core (see (C) in Figure 3), such that the effective refractive index in the upper waveguide is lower than the effective refractive index of the lower waveguide, optical power is propagated by the antisymmetric mode of the built structure. We will call this new integration platform Dual Core Antiguide (DCA). In a particular embodiment, shown in Figure 4, the platform is made of two cores of InGaAsP and cladding material of InP. The thickness and the refractive index of the cores in this example are the same. Typical values in the illustrative example of Figure 4 are shown in table 1 :

TABLE 1

Part Material Refractive Thickness

Index (microns)

Air 1 .0

Insulator Polyimide 1 .7

Cladding layer InP 3.17

Upper core InGaAsP 3.297

0.8

Cladding layer InP 3.17

0.8

Down core InGaAsP 3.297

0.8

Substrate InP 3.17

In this structure, light travels in the fundamental mode trough the rib waveguide in the lower core (see (A) in Figure 4), it goes up through the vertical resonator (see (B) in Figure 4) and is coupled to the second order mode (odd mode) of the structure formed by two waveguides, the upper waveguide being narrower than the bottom waveguide (see (C) in Figure 4 .

In section (C) of Figure 4, the light is nearly confined in the upper waveguide because the coupled system formed by the structure is heavily asymmetric, in other words, the upper rib waveguide and the wider waveguide underneath are enough phase mismatched.

The use of the second order mode of the structure makes it possible to confine the light in the upper waveguide even though the lower waveguide has an effective index higher than the upper waveguide. This way the great limitation of Suematsu's Twin Guide structure can be overcome, as with the invention light is kept in a controllable manner in each waveguide. Section (B) of Figure 4 represents the vertical directional coupler that can be fabricated anywhere in the chip by means of standard lithography techniques. A vertical directional coupler works in a resonant way, in other words, in a vertical directional coupler two super modes (the eigenmodes of the coupled waveguide array) are excited and they exchange power along the propagation axis. This means having a very precise system to exchange power between the different functional levels in the chip. This feature overcomes the performance of the state of the art because, using resonant couplers, the exchange of power between the upper and lower waveguides can be realized in a much compact and efficient manner. Figure 5 is a function of the power coupled from a waveguide etched in the lower core to a waveguide etched in the upper core when varying the length of the directional coupler (see (L) in Figure 3) using a rib width of 1 .4 μηι (see (W) in Figure 3). The two polarizations (TE and TM) have been studied.

An estimation of the modal profiles of the confined light in each cross-section of the particular embodiment can be seen in the three boxes named A, B and C on the left side of figure 4. All the cross-sections are made in the direction perpendicular to the light propagation.

DESCRIPTION OF OTHER EMBODIMENTS OF THE INVENTION

In a second embodiment of the invention, shown in Figure 6, the platform is made of two cores of InGaAsP and InP cladding material. In this realisation the thickness and the refractive index of the cores are different. Typical values in the illustrative example of Figure 6 are shown in table 2:

TABLE 2

Part Material Refractive Thickness

Index (microns)

Air 1 .0

Insulator Polyimide 1 .7

Cladding layer InP 3.2

Upper core InGaAsP 3.35

0.5

Cladding layer InP 3.2 InP

In this structure, light travels in the fundamental mode trough the first rib waveguide in the lower core (see (A) in Figure 6), it goes up through the vertical resonator (see (B) in Figure 6) and is coupled to the second order mode (odd mode) of the structure formed by the second and third waveguides, the third waveguide etched in the upper waveguide being narrower than the second waveguide (etched in the lower waveguide) (see (C) in Figure 6).

The vertical resonator of this embodiment (see (B) in Figure 6) is very efficient because the two cores are phase matched when they have the same width. It may achieve a transfer efficiency of about 0.95 with a coupler length lower than 120 μηι in this particular realization of the invention.

In section (C) of Figure 6, the light is nearly confined in the second waveguide (upper waveguide) because the coupled system formed by the structure is heavily asymmetric, in other words, the upper rib waveguide and the wider waveguide underneath are enough phase mismatched.

An estimation of the modal profiles of the confined light in each cross-section of the particular embodiment can be seen in the three boxes named A, B and C on the left side of Figure 6. All the cross-sections are made in the direction perpendicular to the light propagation. These values for the refractive indexes and thicknesses of the cores and cladding layers can be applied to all embodiments described now on, where one or two of the guides are bent.

In a third embodiment of the invention, shown in Figure 7, the platform is made of two cores of InGaAsP and an InP cladding material. In this realisation the thickness and the refractive index of the cores are equal. Typical values in the illustrative example of Figure 7 are shown in table 3: TABLE 3

Part Material Refractive Thickness

Index (microns)

Air 1 .0

Insulator Polyimide 1 .7

Cladding layer InP 3.17

Upper core InGaAsP 3.297

0.8

Cladding layer InP 3.17

0.8

Down core InGaAsP 3.297

0.8

Substrate InP 3.17

In this particular embodiment, the first waveguide (see (A) in Figure 7) and the vertical coupler (see (B) in Figure 7) are the same than those of the embodiment shown in Figure 4. The new feature of this embodiment is that the second and third waveguides (see (C) in Figure 7) are bent. The two waveguides have the same radius of curvature (R).

In this structure, light travels in the fundamental mode trough the first rib waveguide in the lower core (see (A) in Figure 7), it goes up through the vertical resonator (see (B)) and it is coupled to the second order mode (odd mode) of the structure formed by the second and third waveguides, the third waveguide (upper waveguide) being narrower than the second waveguide (lower waveguide, C).

In order to maximize the coupling between the vertical resonator (see (B) in Figure 7) and the second order mode of the structure formed by the second and third waveguides (see (C) in Figure 7), it is also possible to bend the vertical coupler.

The second order mode of the structure formed by the second and third waveguides has very low bending losses, in the order of the fundamental mode of any standard rib waveguide.

An estimation of the modal profiles of the confined light in each cross-section of this particular embodiment can be seen in the three boxes named A, B and C on the left side of Figure 7. All the cross-sections are made in the direction perpendicular to the light propagation.

In a fourth embodiment of the invention, shown in Figure 8, the platform is made of two cores of InGaAsP and a InP cladding material. In this realisation the thickness and the refractive index of the cores are equal. Typical values in the illustrative example of figure 8 are shown in table 4:

TABLE 4

Part Material Refractive Thickness

Index (microns)

Air 1 .0

Insulator Polyimide 1 .7

Cladding layer InP 3.17

Upper core InGaAsP 3.297

0.8

Cladding layer InP 3.17

0.8

Down core InGaAsP 3.297

0.8

Substrate InP 3.17

In this particular embodiment, the first waveguide (see (A) in Figure 8) and the vertical coupler (see (B) in Figure 8) are the same than those of the embodiment shown in Figure 7. The new feature of this embodiment is that the second waveguide (etched in the lower core) is a slab waveguide (see (C) in Figure 8).

In this structure, light travels in the fundamental mode trough the first rib waveguide in the lower core (see (A) in Figure 8), it goes up through the vertical resonator (see (B) in Figure 8) and it is coupled to the second order mode (odd mode) of the structure formed by the second and third waveguides, been the third waveguide (etched in the upper core) narrower than the second waveguide (etched in the lower core) (see (C) in Figure 8).

The second order mode of the structure formed by the second and third waveguides is a combination of a guided mode and an slab mode and therefore, it has some power leakage towards both lateral directions through the slab waveguide in the lower core. This power leakage can be reduced decreasing the radius of curvature (R) of the third waveguide. Figures 9a and 9b show the leakage loss as a function of the bend radius of the second order mode of the structure formed by the second and third waveguides, for different rib widths (see (W) in box named C in Figure 8) of the third waveguide. The two polarizations (TE and TM) have been studied. We observe anti-resonance peaks in loss due to slab layer (second waveguide) under the rib waveguide (third waveguide). An estimation of the modal profiles of the confined light in each cross-section A, B and C can be seen in the three boxes on the left side of Figure 8. All the cross- sections are made in the direction perpendicular to the light propagation.

In a fifth embodiment of the invention, shown in Figure 10, the platform is made of two cores of InGaAsP and a cladding material of InP. In this realisation the thickness and the refractive index of the cores are equal (but again, embodiments where the indexes are different are possible). Typical values in the illustrative example of figure 10 are shown in table 5:

TABLE 5

Part Material Refractive Thickness

Index (microns)

Air 1 .0

Insulator Polyimide 1 .7

Cladding layer InP 3.17

Upper core InGaAsP 3.297

0.8

Cladding layer InP 3.17

0.8

Down core InGaAsP 3.297

0.8

Substrate InP 3.17

In this particular embodiment, the first waveguide (see (A) in Figure 10) has a width larger than 10 μηι and a bend radius lower than 1000 μηι. Under this assumptions, this waveguide works in the Whispering Gallery Mode regime. In the Whispering Gallery Regime the mode will be fully guided by the outer edge of the bend, so that the location of the inner edge becomes irrelevant.

The vertical coupler (see (B) in figure 10) in this realization of the invention is composed by two waveguides having different bend radius, R1 and R2. The lower guide is a wider rib guide whose outer edge is bent (R2). Thus, the outer edges of these waveguides are in general misaligned. Nevertheless, it is possible by means of lithography standard techniques, to keep this misalignment very small or even zero for several microns. We call this misalignment "g" (see (B) and (C) in Figure 10). If g is lower than a certain value, typically 2 microns, there will be transference of energy between the whispering gallery mode of the lower core and the guided mode of the upper core. In this typology of vertical resonator the transfer efficiency is lower than 10% of the total energy of the mode.

The structure formed by the rib waveguide and the wider waveguide underneath become clear when the misalignment parameter g is greater than a certain value, typically 2 microns (see (C) in Figure 10).

In this structure, light travels in the first gallery mode trough the rib waveguide in the lower core (see (A) in Figure 10), it goes up through the vertical resonator while the misalignment factor, g, is lower than a certain value, typically 2 microns, and it is coupled to the second order mode (odd mode) of the structure formed by the second and third waveguides, been the third waveguide (etched in the upper core) narrower than the second waveguide (etched in the lower core) (see (C) in figure 10).

An estimation of the modal profiles of the confined light in each cross-section of the particular embodiment can be seen in the three boxes named A, B and C on the left side of Figure 10.

In a sixth embodiment of the invention, shown in Figure 1 1 , the platform is made of two cores of InGaAsP and cladding material of InP. In this realisation the thickness and the refractive index of the cores are equal. Typical values in the illustrative example of figure 1 1 are shown in table 6: TABLE 6

Part Material Refractive Thickness

Index (microns)

Air 1 .0

Insulator Polyimide 1 .7

Cladding layer InP 3.17

Upper core InGaAsP 3.297

0.8

Cladding layer InP 3.17

0.8

Down core InGaAsP 3.297

0.8

Substrate InP 3.17

This embodiment is a particular situation of the fifth embodiment of the invention. In this embodiment, the rib waveguide etched in the upper core makes an arc of 360⁵.

In this structure, light travels in the first gallery mode trough the rib waveguide in the lower core (see (A) in Figure 10), it goes up through the vertical resonator when the misalignment factor, g, is lower than a certain value, typically 2 microns and it is coupled to the second order mode (odd mode) of the structure formed by the second and third waveguides, the third waveguide (etched in the upper core) being narrower than the second waveguide (etched in the lower core) (see (C) in figure 10).

An estimation of the modal profiles of the confined light in each cross-section of this last embodiment can be seen in the three boxes named A, B and C on the left side of figure 1 1 .

In case the present invention is applied to active components such as semiconductor lasers or semiconductor amplifiers, every layer (core and cladding) requires an electrical doping for an efficient injection of carriers into the active waveguide. The well-known complementary materials that the operation of the laser needs, such as electrical confinement layers, contact layers, insulator layer and electrodes are not shown. This applies to all embodiments of the invention, irrespective of the curvature/shape of the waveguides.

Claims

1 . - An integration platform for photonic integrated circuits comprising a first down core (1 ) and a second upper core (2) surrounded with cladding material, wherein the two cores are optically coupled, the platform comprising, in the direction of the light's propagation, a first rib waveguide (A) etched in the down core, at least a vertical resonator (B), a second rib waveguide etched in the lower core and a third rib waveguide etched in the upper core (C) and placed over the second waveguide having a constant and smaller width than the second waveguide.

2. An integration platform according to claim 1 wherein the third waveguide (C) is bent.

3. An integration platform according to claim 2 wherein the second waveguide is bent.

4. An integration platform according to claim 2 wherein the second waveguide is a slab waveguide.

5. An integration platform according to claim 2 wherein the first waveguide has a bent external edge.

6. An integration platform according to claim 5 wherein the third waveguide is bent 360⁵ forming a ring.

7. An integration platform according to any of the preceding claims wherein the two cores have the same refractive index.

8. An integration platform according to any of the preceding claims wherein the cores are made of InGaAsP and the cladding layers are made of InP.

9. An integration platform according to any of the preceding claims where the cores and cladding layers are doped.