WO2011068458A1

WO2011068458A1 - Integrated chip comprising a laser and a filter

Info

Publication number: WO2011068458A1
Application number: PCT/SE2010/051325
Authority: WO
Inventors: Christofer Silfvenius; Marcin Swillo
Original assignee: Ekklippan Ab
Priority date: 2009-12-04
Filing date: 2010-12-01
Publication date: 2011-06-09
Also published as: SE0950936A1; SE534300C2

Abstract

Integrated chip (1) for data or telecommunication or optical analysis for concurrent emission over at least one wavelength and receiving over at least one other wavelength, wherein the chip (1) has a basic structure which is identical, from a carrier and upwards over the entire surface of the chip (1), wherein waveguides are formed at the upper surface of the chip by way of the basic structure being etched down so that the protruding waveguides (2; 4; 5) are formed, and wherein the chip (1) comprises monolithically integrated components. The invention is characterized in that the chip (1) comprises a wavelength selective optical coupler (6), a grating based laser (41) attached to the coupler (6), a light detector (51) attached to the coupler (6) and a wavelength specific, grating based filter (52), in that both the laser (41) and the filter (52) are produced from the same monolithic basic structure, whereby the properties of the laser (41) and the filter (52) are altered in corresponding way as function of temperature, and in that the filter (52) is arranged between the laser (41) and the light detector (51) and is also opaque to the wavelengths emitted by the laser (41).

Description

Integrated chip comprising a laser and a filter

The present invention relates to an integrated chip for data or telecommunication or analytic applications wherein the chip can be used both for emitting light from narrow-band light sources such as lasers or broadband light sources such as for example light emitting diodes, and to be able to detect laser light or light from other light sources, for example from light emitting diodes or luminescent light from biological samples or the like.

The invention refers especially to an integrated chip comprising both a laser for emitting light and a light detector for detecting light.

Such integrated chips can in general comprise one or more components, such as for example lasers, photo diodes, diplex- ers, or matrices of such components. Each component can in turn be composed of partial components, such as for example where a diplexer comprises a number of waveguides.

As for data and telecommunication, lasers are often used to emit laser light which is modulated with the information to be emitted, and light detectors, such as photo diodes, to detect received laser light.

In applications where light is both emitted and received, possibly over several different wavelengths, there are combined chips with several different components.

Swedish patent no. 0501217-4 describes such a chip which is monolithically integrated. In contrast to the conventional integrating method which is based on the so called butt-joint coupling, the patent mentioned above describes a monolithic integration without splices, wherein the chip has a basic structure, which is the same from a carrier and up over the entire surface of the chip, without any splices in the wave- guide. The chip comprises a number of waveguides and components, all of which are formed through etching of the basic structure. Such a chip offers low manufacturing cost combined with a high yield. A problem during transmitting and receiving optical signals is the risk of optical overhearing between different components. This is particularly true for the case where a laser and a light detector are arranged in the same integrated chip. The laser is in many cases arranged to emit light for detection in an external device, for example the receiving end of a communication link, which demands high transmitting powers. The light detector can at the same time be arranged to receive light that has been emitted from an external source, wherefore the received light can be weak. It is poss- ible that light from the laser in an integrated chip can be reflected undesiredly in, for example, the chip facet and that the reflected light reaches the integrated photo detector. Other reasons that light from the internal laser does not reach the internal photo detector are scattering effects in gratings or as a result of physical variations in the side walls of the waveguide. In the case wherein a WDM-coupler is used, optical overhearing can occur as a consequence of scattering in the walls or as a result of mode mismatch in the transition between single mode and multi mode waveguides. Imperfections in the grating strength of the laser or a grating of higher order can cause scattering down into the substrate, with reflections back up to the photo detector. The reflection can also appear at the transition to the optical fibre or in the fibre-optic network. In such cases, an overhearing problem arises where the incoming external laser light with low optical power reaches the light detector, competing with reflections of light from the internal laser with high optical power, which creates major difficulties since the information in the incoming light is disturbed.

Besides light from an integrated laser, light from externally arranged lasers, spontaneous emission from the chip itself, etc., can also reach the photo detector and thereby cause similar problems.

The present invention solves the problems described above.

Consequently, the invention refers to an integrated chip for data or telecommunication or optical analysis for concurrent emission over at least one wavelength and receiving over at least one other wavelength, wherein the chip has a basic structure which is identical, from a carrier and upwards over the entire surface of the chip, wherein waveguides are formed at the upper surface of the chip by way of the basic structure being etched down so that the protruding waveguides are formed, and wherein the chip comprises monolithically integrated components, and is characterized in that the chip comprises a wavelength selective optical coupler, a grating based laser attached to the coupler, a light detector attached to the coupler and a wavelength specific, grating based filter, in that both the laser and the filter are produced from the same monolithic basic structure, whereby the properties of the laser and the filter are altered in a corresponding way as a function of temperature, and in that the filter is arranged between the laser and the light detector and also opaque to the wavelengths emitted by the laser. The invention will now be described in more detail, with reference to the exemplifying embodiments of the invention and the accompanying drawings, wherein:

Figure 1 is a perspective view of a known monolithically integrated chip for optical signals;

Figure 2 is a top view of an integrated chip according to a first preferred embodiment of the present invention;

Figure 3 is a perspective view of an integrated chip according to a second preferred embodiment of the present invention; and

Figure 4 is a top view of an integrated chip according to a second embodiment of the present invention.

Figure 1 illustrates schematically a monolithically integrated chip 1, prior to metallization of electrical contacts, with a coupling waveguide 2, an optical coupler 6 and two output waveguides 4, 5. Such a chip is known from the Swedish patent No. 0501217-4.

According to the invention, the chip 1 is intended for data- or telecommunication or optical analysis of at least one, two or more wavelengths.

The chip 1 comprises a waveguide 2 with a first port 3 into or out from which light is intended to be conveyed. The waveguide 2 is expanded, in an expanded part 6, from the first port 3 in a direction towards a second waveguide 4 and a third waveguide 5. The various components of the chip 1 are monolithically integrated. In other words, the chip has a basic structure which is identical from a carrier and upwards over the entire surface of the chip, and the waveguides 2, 4 and 5 are formed at the upper surface of the chip by way of the basic structure having been etched down so that protruding waveguides are formed.

Figure 2 shows an integrated chip similar to the one in figure 1. Corresponding parts have the same reference numbers in all the drawings.

In the second waveguide 4 a laser 41 is arranged to emit light A. The laser 41 is produced from the same monolithic basic structure as the rest of the chip by a grating structure which is produced in the second waveguide 4, which grating structure defines the laser cavity. The laser can be a Distributed FeedBack laser (DFB laser) , a Distributed Bragg Reflector laser (DBR laser) , or any other known type of laser that includes a grating which is produced from the monolithic basic structure. By through etching the waveguide 4, it is also possible to produce facets so that a monolithically integrated Fabryt-Perot laser (FP laser) is formed. The emitted light A is preferably intended to be received by an external receiver, but can also be intended to be received by a light receiver (not shown) in the form of another component which is integrated into the chip.

In the third waveguide 5, there is a light detector 51 arranged to detect incident light D. The light detector 51 can be produced from the same monolithic basic structure as the laser 41, which is preferred since this leads to low manufacturing costs, but it is not required. It can further be any suitable known light detector, such as a conventional photo diode .

According to the invention, the expanded part 6 consists of a wavelength selective, optical coupler, preferably a Wavelength Division Multiplexing (WDM) coupler, arranged to, based on the wavelength of the light incident through the first waveguide 2, direct the light towards either the second 4 or the third 5 waveguide. The WDM coupler is in this case also arranged to direct light with a certain wavelength from the second waveguide 4, and from the third waveguide with a certain different wavelength, respectively, towards the first waveguide 2.

According to a preferred embodiment, the coupler is a Multi- Mode Interferometer (MMI) coupler. According to another preferred embodiment, the coupler is an envanescent coupler.

According to the exemplified embodiment shown in figure 2, the laser 41 is arranged to emit light A across a certain wavelength interval, and the light detector 51 is arranged to detect light D across a certain other wavelength interval, wherein said wavelength intervals are different. Herein, the fact that the wavelength intervals are different should be interpreted to mean that they are essentially not overlapping and that the power maximums with respect to the wavelength for emitted or received light, respectively, are at least separated.

The coupler 6 is accordingly arranged to convey light A, emitted from the laser 41, to and out through the first waveguide 2, and also light D, in another wavelength interval and inciding from the first waveguide 2, to the third waveguide 5 and on to the light detector 51.

However, due to for example undesired facet reflections of the laser light A, for instance at the port 3, reflected light C from the internal laser also incides towards the third waveguide 5. According to the invention, a wavelength specific, grating based filter 52 is arranged between the laser 41 and the light detector 51 in the chip 1. The filter 52 is, in a way which correspondsto the laser 41, produced by way of a grating structure being produced in the material of the third waveguide 5, and is therefore monolithically integrated, together with the laser 41, in the basic structure of the chip 1.

Electrical contacts (not shown) are arranged against the laser 41 and over the filter 52, as well as over the other components, such as the waveguides 2, 4, 5 and the expanded part 6 to which a voltage is to be applied.

The chip 1 is thus arranged, by means of the laser 41, to be able to emit optical signals across a first wavelength interval and at the same time to be able to receive optical signals, by means of the photo detector 51, across a second wavelength interval, said second wavelength interval being different from the first wavelength interval. According to the invention, the filter 52 is arranged to be opaque to the wavelengths emitted by the laser 41. The reflected light C from the laser 41 is thereby prevented from inciding towards and disturbing the operation of the light detector 51. Since the light A, emitted from the laser 41, is much stronger than the light D inciding into the expanded part 6 through the first waveguide 2, the undesiredly reflected laser light C can in many cases be strong enough to disturb the information contained in the incident light D to be detected, towards the third waveguide 5, why the function of the light detector 51 can be severely disturbed. This is often true even though only a small amount C of the reflected light reaches the third waveguide 5. In typical applications, the light from the internal laser 41 is attenuated by about 47 dB before it reaches the light detector 51.

It is further preferred, but not necessary, that the filter 52 also can transmit light D of the wavelengths that the light detector 51 is arranged to detect. Thereby is achieved that the light detector 51 can detect the incident light D to be detected without being disturbed by reflections C from the laser 41. The filter 52 does not need to be able to transmit light D of the wavelengths that the light detector 51 is arranged to detect in the case the light D to be detected incides through a waveguide (not shown) that is not in direct connection to the laser 41. Below is an example of this.

The structure illustrated in figure 2, wherein the second waveguide 4 comprises the laser 41 and the third waveguide 5 comprises both the light detector 51 and the filter 52, so that the filter 52 is arranged between the expanded part 6 and the light detector 51, is a preferred embodiment. However, it is also possible to arrange a monolithically integrated filter 52 also in other configurations to screen off a light detector 51 from stray light C emitted from a monolithically integrated laser 41. A monolithically integrated filter can for example be arranged between a monolithically integrated laser and a light detector arranged to receive light inciding towards the light detector from another direction than from the expanded part 6 and via the third waveguide 5 in figure 2. In this case, the filter can thus be opaque even for light of the wavelengths for which the light detector is sensitive. According to a preferred embodiment, the filter 52 is opaque to the wavelengths occurring in stray light from one or more light sources, such as additional lasers or light diodes, arranged in or outside of the chip 1, and also from stray light occurring by spontaneous emission in the chip material.

Thus, the monolithically integrated filter can comprise a band pass filter or a band exclusion filter or a combination thereof .

The chip 1 can comprise more than one monolithically integrated laser, for instance arranged in each waveguide each in turn being connected to an expanding part. To such an expanding part there could be more than one light detector attached, possibly each one of them arranged in a separate waveguide that is connected to the expanded part. In this case it is preferred that either each one of the connected light detectors is provided with a respective monolithically integrated filter between the light detector and all the connected lasers along the route for undesiredly reflected laser light. Several light detectors can also share the same monolithically integrated filter.

According to a preferred embodiment, the chip is arranged to emit light of several wavelengths, and at the same time be able to detect incident light of several other wavelengths. In this case an expanded part 6 is preferably combined with a plurality of different waveguides, each one with a respective monolithically integrated laser for emitting laser light of different emitting wavelengths, and a plurality of different waveguides, each one with a respective light detector for different receiving wavelengths. Between each respective light detector and the expanded part 6, a respective mono- lithically integrated filter is arranged, which filter is opaque to all the emitting wavelengths but able to transmit the corresponding receiving wavelength of the light detector. Since both the laser 41 and the filter 52 are monolithically integrated, the grating defining the laser cavity and the grating constituting the filter 52 be made in the same material system. Moreover, the temperature for the respective grating based part components will be the same. The tempera- ture dependent material features will thereby change in the corresponding way over time for both grating based part components. For instance, the wavelengths for which the filter is oblique will increase in the case that a temperature change increases the wavelength of the emitted light from the laser. The features of the laser 41 and the filter 51 will therefore shift in a corresponding manner. For instance, the light from the laser will always be able to be blocked by the filter 52, even at variable temperature conditions. In other words, a chip 1 is achieved that can retain its functional characteristics even when the operational temperature is changed.

As is clear from the Swedish patent no. 0501217-4, the material system in a monolithically integrated chip according to the present invention can be designed in different materials. However, the material structure is generally composed by different semi-conductive materials, mainly consisting of As, P, Ga, In and/or Al . There are more than one appropriate ways of manufacturing a monolithically integrated grating to be used as laser cavity or filter in accordance with the present invention. The active material can further consist of a bulk layer, a multiple quantum well structure (MQW) or a combination of quantum dots (QD) . According to a first preferred embodiment, the filter 52 consists of a monolithically integrated grating manufactured as a so called buried grating. Above or below the waveguide a first order grating is dry etched, as a part of the manufacturing process of the monolithic material structure of the chip 1, in the form of a corrugation in the material, in a pattern that is defined by electron beam lithography, optical interference, nano imprinting or the like. The corrugated surface is then covered by a material with a different refractive index than the corrugated material. According to another embodiment, InGaAsP with a band gap of 1200 mm may be etched down to a corrugated periodical surface and covered by InP. Each period gives a difference in refractive index between the materials. Each such grating structure is described in the American patent no. 5,580,740.

According to a second preferred embodiment, the filter is manufactured by arranging a grating structure along with, on each side of, the waveguide, consisting of a periodical grouping of holes, a so called "photonic band gap structure". The grouping of holes can also be arranged on the upper side of the waveguide, or in a combination thereof. According to a preferred variant, the holes are not filled, i.e. they are filled with air. This results in a big difference in refractive index. Suitable such photonic band gap structures are described in the article Mulot, M. , et al . , "Low-loss InP- based photonic-crystal waveguides etched with Ar/Cl2 chemically assisted ion beam etching", Department of Microelectronics and Information Technology, Royal Institute of Technology, published 20 Mars 2003, and also in the American patent application US 10/520,837. According to a third preferred embodiment, the filter 52 consists of a corrugated wall, a side wall and/or the upper side of the third waveguide 5, close to the optical mode. The corrugation can either be made by etching the side wall of the waveguide 5 or another wall such as the upper wall (the upper side in the orientation of the chip 1 as it is shown in figure 1 and 3) of the waveguide 5, or by etching a new waveguide with such a corrugated side wall around the third waveguide 5. Useful etching techniques include RIE (Reactive Ion Etch) and ICP (Inductively Coupled Plasma) . Useful mask materials comprise hard masks (SiN_x or SiO_x) and soft masks of photo resist or BCB (bensocyclobuthen polymer) . Typical process gases are Cl₂, Ar and CH₄ in ¾ . Useful such methods are described in the article Docter, B., et al . , "Deep etching of DBR gratings in InP using Cl₂ based ICP processes", Proceedings Symposium IEEE/LEOS Benelux Chapter, 2006, Eindhoven .

Figures 3 and 4 illustrates perspicuously, in perspective and from above, respectively, a fourth preferred embodiment regarding the design of the monolithically integrated filter 52.

An etched ditch 53 is arranged along the side of the third waveguide 5, so that the optical mode in the waveguide 5 is affected by the inner wall 54 of the ditch 53 facing towards the waveguide and also possibly the bottom of the ditch 53. Such a ditch 53 can advantageously be arranged along both sides of the waveguide 5, which can be seen from figure 4. The ditch wall 54 and/or the bottom of the ditch are corrugated 55, so that a first order filter function is achieved. The corrugation 55 of the ditch wall 54 and/or the bottom of the ditch is also preferably achieved by way of etching, in a known way as such.

The distance from the ditch 53 to the waveguide 5 and the physical geometry of the corrugated wall 54 will determine the functionality of the filter 52 with respect to filtering strength, pass/exclusion band, etc., in a way which is conventional as such.

Furthermore, the present inventors have surprisingly found that the size, in the plane of the chip 1 (the plane illustrated in figures 2 and 4), in the area of the ditch 53 that is etched away and also in the total etched off material volume, will affect the quality of the corrugated side wall 54 or the bottom of the ditch. An area that is too small will make it hard for the process gases to reach the etching front, resulting in a deteriorated physical structure of the corrugated wall. This is of course not desirable, since it affects the characteristics of the filter 52. A too large total etched off material volume will, on the other hand, lead to heavy regrowth during etching, which also deteriorates the structure of the corrugated side wall or the bottom of the ditch.

It has been realised that good etching results can be

achieved relating to the corrugation of the ditch wall 54 if the ditch 53 is at least 0.1 μπ\, preferably at least 0.5 im wide and at most 1000 μιη, preferably at most 250 m wide. The width of the ditch 53 is the dimension of the ditch 53 as can be seen in figure 4 perpendicularly to the longitudinal direction of the third waveguide 5. It has also been realised that good etching results can be achieved in the case where the ditch 53 reaches down to a depth of between 50 and 5000 nm, preferably between 200 and 2000 nm. This means that the cross-sectional area of the ditch 53, perpendicularly to its main area of direction, is preferably between 0.005 and 5000 μπι², rather between 0.1 and 500 μπι².

To achieve a good filtering ability, it is preferred that such a monolithically integrated filter comprises at least about 100 grating periods, wherein each grating period is about between 50 and 300 nm.

The distance between the ditch 53 and the waveguide 5 is preferably between 0 and 10 um.

In accordance with a preferred embodiment, the ditch can be exposed to air, giving a high refractive index difference. It can also be covered by a dielectric such as SiO_x, SiN_x, BCB or the like, giving a lower refractive index difference, but on the other hand a better physical protection of the filter. This choice depends for example on the enclosure used for the chip 1.

According to a preferred embodiment, an optical amplifier (not shown) is arranged between the filter 52 and the optical detector 51. This leads to that good detection can be

achieved even in the case where the incident light D is proportionately weak and moreover possibly further weakened in the filter 52. It is preferred that he amplifier is also designed in the same monolithic basic structure as the filter 52 and the laser 41. According to a further preferred embodiment, the waveguide can be unbiased or biased in the forward or backward direction . Preferred embodiments have been described above. However, it is obvious for a person skilled in the art that many modifications can be made to the described embodiments. The invention shall thus not be limited to the described embodiments, but may be varied within the scope of the enclosed claims.

Claims

1. Integrated chip (1) for data or telecommunication or optical analysis for concurrent emission over at least one wavelength and receiving over at least one other wavelength, wherein the chip (1) has a basic structure which is identical, from a carrier and upwards over the entire surface of the chip (1), wherein waveguides are formed at the upper surface of the chip by way of the basic structure being etched down so that the protruding waveguides (2; ; 5) are formed, and wherein the chip (1) comprises monolithically integrated components, and is c h a r a c t e r i s e d in that the chip (1) comprises a wavelength selective optical coupler (6), a grating based laser (41) attached to the coupler (6), a light detector (51) attached to the coupler (6) and a wavelength specific, grating based filter (52), in that both the laser (41) and the filter (52) are produced from the same monolithic basic structure, whereby the properties of the laser (41) and the filter (52) are altered in a corresponding way as a function of temperature, and in that the filter (52) is arranged between the laser (41) and the light detector (51) and is also opaque to the wavelengths emitted by the laser (41) .

2. Integrated chip (1) according to claim 1, c h a r a c t e r i s e d in that the chip (1) comprises a first waveguide (2) with a first port (3), wherein the first waveguide (2) is expanded from the first port (3) in a direction towards at least one second waveguide (4) and a third waveguide (5), in that the second waveguide (4) comprises the laser (41), in that the third waveguide (5) comprises both the light detector (51) and the filter (52) and in that the fil- ter (52) is arranged between the expanding part, which comprises the coupler (6), and the light detector (51).

3. Integrated chip (1) according to claim 2, c h a r a c t e r i s e d in that the waveguide selective coupler (6) comprises a WDM coupler.

4. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that the chip (1) is arranged to, by means of the laser (41) , be able to emit optical signals of a first waveguide interval and at the same time, by means of the light detector (51), be able to receive optical signals of a second waveguide interval that is different from the first waveguide interval.

5. Integrated chip (1) according to claim 4, c h a r a c t e r i s e d in that the filter (52) is arranged to be able to transfer the wavelengths that the light detector (51) is arranged to detect.

6. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that the filter (52) comprises a buried grating produced by etching a corrugated pattern in the material below or above the waveguide layer, during the manufacture of the monolithically integrated circuit, and thereafter covering the corrugated structure with a material having a different refractive index than the one in which the corrugation was created.

7. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that the filter (52) comprises a photonic band gap structure, comprising holes on each side of the third waveguide (5) .

8. Integrated chip (1) according to claim 7, c h a r a c t e r i s e d in that the holes are filled with air.

9. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that the filter (52) comprises a corrugation in a surface of the third waveguide (5) .

10. Integrated chip (1) according to claim 9, c h a r a c t e r i s e d in that at least one etched ditch (53) is arranged adjacent and alongside the third waveguide (5) on each side of the third waveguide (5) , in that a corrugated side wall (54) and/or a bottom wall is arranged in the respective ditch against the side facing the third waveguide (5), and in that the side wall (54) corrugation constitutes the filter (52) .

11. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that the filter (52) comprises a band pass filter.

12. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that the filter (52) comprises a band exclusion filter.

13. Integrated chip (1) according to any one of the preceding claims, c h a r a c t e r i s e d in that an optical amplifier is arranged between the filter (52) and the optical detector ( 51 ) .

14. Integrated chip (1) according to claim 13, c h a r a c t e r i s e d in that the optical amplifier is made in the same monolithic basic structure as the laser (41) and the filter (52) .