Patent Application
20150153512
The invention relates to a directional coupler.
Optical directional couplers comprising at least one polymer waveguide are known from the prior art. In particular, such a directional coupler could be used as a tuneable wavelength filter as described in DE 1 002 53 07 B4.
It is an objective of the present invention to provide a directional coupler with enhanced tuning capabilities.
According to the invention, a directional coupler is provided, comprising
The first and the second optical waveguide each comprises an input ending providing an input port of the directional coupler and an output ending providing an output port of the directional coupler. Further, the optical waveguides are configured and arranged relative to one another in such a way that an optical wave coupled into one of the two waveguides via its input ending will be transferred to the other waveguide only if the wavelength of the optical wave fed into the waveguide lies in a wavelength range around a (tuneable) centre wavelength. Thus, the directional coupler according to the invention can be operated as a wavelength selective optical filter, wherein the input port of the filter is the input ending of one of the two optical waveguides and the output port of the filter is the output ending of the other optical waveguide. Possible configurations permitting the tuning of the centre wavelength of the filter will be discussed below.
The first and the second optical waveguide are, for example, arranged on a substrate (e.g. a silicon substrate or another semi-conductor or non-semi-conductor substrate) in such a way that they extend in a common plane running essentially parallel to the substrate. In other words, the first and the second optical waveguide are arranged in a lateral and not in a vertical configuration. The lateral configuration may have the advantage over a vertical configuration that both the first and the second optical waveguide can be used for changing the centre wavelength of the directional coupler, i.e. for wavelength tuning.
For example, both the first and the second optical waveguide are embedded in a polymer cladding, and wherein the first optical waveguide comprises a dielectric core and the second optical waveguide comprises a polymer core.
The dielectric core of at least one of the optical waveguides may provide a difference of the effective refractive indices of the two waveguides large enough to enable wavelength tuning by exploiting the well-known principle of grating assisted couplers (as described e.g. in the publication “Directional couplers made of non-identical asymmetrical slabs, Part II: Grating assisted couplers”, J. Lightwave Technol. vol. 5, 268-273 (1987), M. Marcuse, which is incorporated by reference herewith). Furthermore, it may also provide a different thermo-optical coefficient, which may improve the tuning characteristics of the directional coupler. In particular, only one of the optical waveguides comprises a dielectric core embedded in a polymer cladding material or polymer waveguide core material, whereas the other waveguide does not. However, it is also conceivable that both the first and the second optical waveguide comprise a polymer cladding embedding a dielectric core, the cores in that case having different dielectric materials, i.e. different refractive indices.
In particular, the directional coupler according to the invention is an asymmetric directional coupler, i.e. the effective refractive index of the first optical waveguide is different from the effective refractive index of the second optical waveguide (taken at the same temperature of the first and the second optical waveguide). According to an embodiment of the invention, only the first optical waveguide comprises or consists of a dielectric core, wherein the dielectric core is configured in such a way that the effective refractive indices of the first and the second optical waveguides differ by some 0.01-0.03, to indicate an exemplary reasonable range. The refractive index of the dielectric core may be significantly higher than the refractive index of the polymer cladding, e.g. the dielectric core material has a refractive index of about 1.8-about 2 at a wavelength of 1550 nm; in particular, distinctly larger than that of the embedding polymer material with typical values between some 1.35-1.6.
For example, the complete core of one of the optical coupler waveguides is formed by a dielectric material. However, it is also possible that the waveguide core is formed by a polymer core (formed by a polymer material different than the polymer material of the cladding), wherein a dielectric material is embedded in the polymer core (combined polymer/dielectric core, which will be equally termed “dielectric core” in the following). Suitable dielectric materials for the waveguide core are, for example, silicon nitride (SiNx), silicon oxynitride (SiON), tantalum oxide, titanium oxide and/or aluminium oxide. Of course, other materials can be used as dielectric core material, provided that they exhibits low optical loss (in respect to the used optical wavelengths) and are compatible with the polymer cladding.
Further, the dielectric core of one of the optical waveguides of the coupler (different than in, for example, conventional silicon oxide waveguides having a silicon nitride core) may also create a waveguide having a thermo-optical coefficient (TOC) different (e.g. lower) from the TOC of the non-dielectric core waveguide. This is due to the fact that the dielectric core material may have a significantly lower TOC (e.g. about +(1−2)*10−5 K−1) than the polymer cladding (e.g. about −1.1*10−4 K−1). Taking into account and depending on weighed confinement factors of the light intensity in the cladding and the core, the effective TOC of the dielectric core waveguide may be significantly lower than the TOC of a pure polymer waveguide. The lower TOC of the waveguide may improve the tuning characteristics of the directional coupler.
In an asymmetric directional coupler a periodic variation of the effective refractive index of at least one of the optical coupler waveguides (along the length of the respective optical waveguide) may be provided in order to permit the light fed into one of the waveguides to be transferred to the other coupler waveguide. In particular, one of the waveguides is provided with a grating-like variation of the effective refractive index (e.g. by providing a grating-like variation the dimensions of the waveguide core), i.e. the directional coupler is formed as a “grating assisted coupler” (GAC). In particular, only one of the two coupler waveguides comprises a grating. The grating-like variation of the effective refractive may be achieved by providing a waveguide core whose dimensions (the width measured parallel to the substrate and for the thickness measured perpendicular to the substrate) vary periodically along at least a portion of the length of the optical waveguide.
In a GAC a transfer of optical power from one of the coupler waveguides into the other one is possible for a resonant coupling wavelength λ (defining the “centre wavelength” mentioned above of the optical filter provided by the directional coupler), wherein λ is given by:
λ=ΔneffΛ
wherein Δneff is the difference of the effective refractive indices of the first and the second optical waveguide of the directional coupler and Λ is the periodic length of the grating.
According to an embodiment of the invention, only the first optical waveguide has a polymer cladding and a dielectric core, wherein the second optical waveguide has a nondielectric core (e.g. a polymer core), wherein the thickness and/or the width of the dielectric core of the first optical waveguide varies periodically along the length of the first optical waveguide such that the refractive index grating mentioned above is created. For example, assuming a centre wavelength of λ=1550 nm and Δneff=0.02, which can be realized because of the dielectric core of the first optical waveguide, a grating period Λ of 77.5 μm is required, which can be readily realized by conventional lithographic methods.
Different from a symmetric directional coupler the coupling wavelength (the centre wavelength) of an asymmetric coupler may be changed by changing the effective refractive index difference Δneff. The difference Δneff may be changed by changing the effective refractive index of the first and/or the second optical waveguide of the directional coupler, e.g. by changing the temperature of the waveguide(s) (thermo-optic refractive index control) or by applying an electric field across the waveguide(s) if electro-optic polymer material is used in the latter case. Regarding the thermo-optic refractive index control the inducable temperature dependent wavelength change Δλ/ΔT is given by:
Δλ/ΔT=Δ(ΔneffΛ)/ΔT=(TOC1−TOC2)*Λ and
Δλ=(TOC1ΔT1−TOC2ΔT2)*λ/Δneff
wherein TOC1 and TOC2 are the thermo-optic coefficients of the first and the second optical waveguide, respectively, and ΔT1 and ΔT2 is the induced temperature change of the first and the second optical waveguide, respectively.
The directional coupler according to the invention may correspondingly comprise a heating device for heating the first and/or the second optical waveguide. For example, the heating device is configured for heating the first and the second optical waveguide in such a way that the temperature of the first and the second optical waveguide can be altered essentially independently from one another. That is, the two optical waveguides may be sufficiently thermally isolated (e.g. by a common polymer cladding surrounding the core of the first and the second waveguide, favoured by the generally low thermal conductivity of polymer materials) such that the waveguides may assume different temperatures. For example, the heating device comprises at least two separate heating electrodes, wherein at least one heating electrode is assigned to the first optical waveguide and at least one heating electrode is assigned to the second optical waveguide.
For example, the heating device comprises at least one electrode that is embedded in the polymer cladding of the directional coupler (the polymer cladding, for example, enclosing both the first and the second optical waveguide).
Further, the heating device may comprise at least one heating electrode that is arranged between the core of the first or the second optical waveguide and a substrate (on which the optical waveguides are arranged).
Alternatively or in addition, the heating device may comprise at least one heating electrode that is arranged laterally of the first or the second optical waveguide; i.e. one a lateral side of one of the waveguides that faces away from the other waveguide. For example, the laterally arranged heating electrode (which may be formed as a plate) extends transversely with respect to a substrate on which the first and the second optical waveguide are arranged.
It is, of course, also possible that only one heating electrode is provided such that only one waveguide can be heated. Further, it is conceivable that the heating device is configured for simultaneously heating the first and the second optical waveguide, making use of the different TOCs of the waveguides. For example, the heating device comprises a single heating electrode assigned to both the first and the second optical waveguide (having different thermo-optical coefficients). The heaters may be applied to the top of the waveguide structures but also at the bottom or at the sidewalls of etched mesa-like stripes encompassing both of the waveguides.
The invention also relates to an optical add-drop multiplexer (OADM) that comprises a directional coupler as described above. For example, an input port of the first or the second optical waveguide of the directional coupler is connected to different WDM-channels (WDM=wavelength division multiplex), wherein using the directional coupler a wavelength channel can be deselected (drop function) and/or added (add-function) by tuning the device to this particular wavelength. The directional coupler according to the invention may allow a relatively broad tuning range such that optical add-drop-multiplexer for coarse wavelength division multiplex (CWDM) applications may be realized which do not require particularly narrow filter characteristics. For example, an optical add-drop multiplexer for four CWDM-channels having, for example, 20 nm channel spacing may be fabricated.
Also, the directional coupler according to the invention may be used as a wavelength tuneable/wavelength selectable thermo-optical wavelength switch or power divider; in particular, as a 2×2 wavelength switch or power divider. According to this embodiment, light of a predetermined wavelength will be fed into one of the coupler waveguides and if the wavelength matches the coupling wavelength of the coupler, the light will be transferred to the other coupler waveguide (or vice versa). This switch can be operated with low losses, in particular, essentially as a no-loss device (apart from inevitable waveguide or fabrication induced losses).
A complete switching may already be achieved by inducing small temperature changes (e.g. a temperature rise of not more than 10 K) depending on the spectral characteristic of the directional coupler. For example, the centre wavelength (i.e. the maximum of the transmitted spectrum) of the directional coupler is tuned to the input wavelength as set forth above. It is, however, also possible that the directional coupler is tuned in such a way that the input wavelength lies on an edge of the transmitted wavelength range of the coupler such that only a (first) portion of the input power is transferred to the other waveguide, whereas a second portion of the input power remains in the input waveguide. Thus, the directional coupler may be used as an optical power divider having an adjustable split ratio.
According to another embodiment of the invention, a tuneable laser device comprising an intra-cavity filter for selecting an output wavelength of the laser is provided, the intracavity filter comprising a directional coupler as discussed above. Thus, the directional coupler is used as a widely tuneable filter for tuning the laser. For example, an input port (i.e. an ending) of the first or the second optical waveguide of the directional coupler is connected to a laser active component (gain component), wherein an output port of the other optical waveguide is connected to a reflecting element (e.g. a multiple peak reflector) of the laser. The directional coupler is used to select one of the plurality of cavity modes permitted by the reflecting element, wherein a cavity mode wavelength is selected by tuning the centre wavelength of the directional coupler as described above (e.g. by heating at least one of the coupler waveguides).
In particular, the laser device is a hybrid device, wherein at least the gain component is a semiconductor component (i.e. in particular, its laser active part is a semi-conductor structure) and the directional coupler is formed as a polymer component (having at least one polymer waveguide) according to the invention. Accordingly, the gain component and the coupler are arranged on different substrates connected to one another.
Further, the invention also relates to a tuneable laser device comprising a first and a second laser and a directional coupler as described above, wherein light emitted by the first laser is coupled into the first optical waveguide of the directional coupler and light emitted by the second laser is coupled into the second optical waveguide of the directional coupler. In particular, the first and the second lasers are designed as waveguide grating lasers (WGL) containing waveguides with tuneable Bragg gratings.
The two Bragg grating loaded waveguides are, for example, integrated on a common substrate (chip) and hybridly integrated with semi-conductor devices to form a dual WGL structure. The two lasers further are connected to an output port of the laser device via the directional coupler, wherein light emitted by one of the waveguides of the directional coupler is guided towards that output port.
In particular, the two waveguide grating lasers cover different tuning ranges, wherein depending on the desired output wavelength either the first or the second laser is operated. The coupling wavelength of the directional coupler is selected correspondingly such that light of the active laser is guided towards the output port of the laser device. In that example, the directional coupler operates as a switch, which, in particular, has low optical losses and has to only operate in half of the tuning range of the laser device and requires little heating power due to its efficiency.
It is also possible to operate the directional coupler in such a way that only a portion of the optical power generated by the first or the second laser of the laser device is transferred between the optical waveguides of the directional coupler such that a portion of the power will be emitted via a second output port of the directional coupler. This configuration may be used, for example, in optical coherent transceivers for using the same laser device simultaneously as a local oscillator laser and an externally modulated transmitting laser.
According to another aspect, the invention relates to an optical waveguide, in particular for use in a directional coupler as discussed above, the optical waveguide comprising:
The optical waveguide according to the invention may be configured as described above with respect to the first and/or the optical waveguide of directional coupler. For example, the optical waveguide may comprise a pure dielectric core or a polymer core surrounding another core material consisting of a dielectric material.
According to an embodiment, only a section of the waveguide comprises a core with a dielectric material. In particular, the waveguide comprises a first section having a dielectric core and a second section having a non-dielectric (e.g. polymer) core. For example, the optical waveguide comprises a plurality of alternating sections having dielectric and non-dielectric cores.
Further, the transition regions between the different waveguide sections may be (in particular adiabatically) tapered, i.e. the width (measured parallel to the substrate on which the optical waveguide is arranged) and/or the thickness (measured perpendicular to the substrate) of the dielectric core decreases towards the non-dielectric core portion of the waveguide.
In particular, the optical waveguide may comprise a facet region with a dielectric core, wherein other portions of the waveguide may not comprise a dielectric core. The dielectric core in the facet region of the waveguide may increase the waveguide aperture (in particular, in the vertical direction, i.e. perpendicular to the substrate) such that the waveguide mode profile may be better matched to optical components that are to be coupled to the waveguide, for example a laser diode (e.g. in a ridge waveguide configuration) having, e.g., an elliptical cross section. The width and/or the thickness of the dielectric core at the facet may be designed such that the coupling losses are reduced in comparison with a pure polymer waveguide.
Further, the waveguide may comprise a Bragg grating, wherein, in particular, the dielectric waveguide core forms the Bragg grating. Also, the optical waveguide may comprise a phase shifting device, which, for example, comprises at least one heating electrode.
Embodiments of the invention will be explained in more detail hereinafter with reference to the drawings.
a-18f schematically illustrates possible configurations of the laser device shown in
The optical waveguide 1 according to the invention illustrated in
The dimensions (width w1, thickness d1) of the dielectric inner core 12 may compare to the dimensions of dielectric core 12 of
The optical waveguide 1 similar to
Because of the different core sections, the different sections of waveguide 1 comprise different effective refractive indices. For example, the effective refractive index neff,2 of the waveguide section with the dielectric core (left lower sectional view) is larger than the effective refractive neff,1 outside the waveguide section with the dielectric core (right lower sectional view).
Further, the transition region between the dielectric core section and the section outside the dielectric core section may comprise a tapered core region 122, wherein the dielectric core 12 is tapered towards the adjacent section (e.g. a polymer core section), i.e. its width and/or thickness continuously (e.g. linearly or adiabatically) decreases towards the adjacent region to reduce optical transition losses between the sections.
According to
The dielectric core 12 of waveguide 1 may further be tapered (lower drawing of
The overlap with an exemplary elliptical mode profile and the mode profile at facet 13 of waveguide 1 dependent on the width w of the dielectric core layer is depicted in
The first optical waveguide 110 is configured similar to the optical waveguides according to the invention shown in
The directional coupler 10 has a lateral configuration, i.e. the first and the second waveguide 110, 120 are arranged on a substrate (similar to substrate 2 in
Due to the fact that the first optical waveguide 110 comprises a dielectric core, it has a rather large effective refractive index such that the two waveguides 110, 120 have different refractive indices which, in principle, would prevent an optical mode in one of the waveguides from coupling into the other waveguide. However, the effective refractive index of the first optical waveguide 110 varies along the length of the waveguide 110 with a period A, wherein the longitudinal variation of the effective refractive index is generated by providing the dielectric core 112 of the first optical waveguide 110 with a grating-like variation of its width (and/or its thickness), i.e. the directional coupler 10 is a “grating assisted coupler” (GAC). Thus, a transfer (indicated by arrows L in
Because of the dielectric core of the first optical waveguide, also the thermo-optical coefficient of the first optical waveguide 110 is different (e.g. lower) than the thermo-optical coefficient of the second optical waveguide 120. Thus, by changing the temperatures of the first and the second optical waveguide 110, 120, the difference Δneff of their effective refractive indices is altered, thereby changing the resonant (centre) wavelength of the directional coupler 10.
It is possible that both waveguides 110, 120 experience the same temperature change, wherein the tuning wavelength of the coupler 10 would still be altered due to the different thermo-optic coefficients. However, it is also possible that the first and the second optical waveguide have similar thermo-optical coefficients. In that case, the temperature of the first optical waveguide may be changed differently than the temperature of the second optical waveguide, e.g. by using separate heating electrodes as will be explained below. It is also noted that at a certain temperature (e.g. at room temperature) the effective refractive indices of the two optical waveguides may be essentially the same, wherein they are different at another temperature.
In particular, it is possible that only one of the two optical waveguides 110, 120 is heated at a time, wherein, for example, the coupling wavelength of the directional coupler 10 may be shifted towards smaller wavelengths when only the first optical waveguide 110 (comprising the grating and the dielectric core) is heated and towards larger wavelengths when only the second optical waveguide 120 (polymer waveguide without grating) is heated. This is illustrated in
The tuning ranges shown in
In particular, the heating electrodes 51, 52 are configured in such a way that the temperature of the waveguides 110, 120 can be changed essentially independently from one another. Of course, the heating electrodes 51, 52 do not necessarily have to be arranged on top of the waveguides. Rather, as an alternative or in addition, heating electrodes 51′, 52′, 51″, 52″ could be provided that are arranged laterally (e.g. embedded in the cladding 111) and/or below the waveguides 110, 120 (in particular below the waveguide cores) as indicated (dashed lines) in
It is also possible, that a common heating electrode 53 is used assigned to both the first and the second optical waveguide 110, 120 as depicted in
Another embodiment of the invention is illustrated in
According to
An output end of the first optical waveguide 110 is connected (e.g. via integrated waveguides) to an output port 601 of the laser device 60 and the second optical waveguide 120 is connected to a second output port 602 of laser device 60. The lasers 61, 62 have different (e.g. adjacent) tuning ranges, wherein using the directional coupler as a combiner 10 light of both lasers 61, 62 can be directed towards the same output port (the upper output port 601). The centre wavelength of the directional coupler is set depending on which one of the lasers 61, 62 is operated (as already explained above). Further, laser device 60 may be a hybrid integrated device, i.e. in particular the gain sections 65, 66 and the directional coupler are not realized as an integrated device. In particular, the gain sections 65, 66 or the lasers 61, 62 as a whole are integrated on a common chip.
Laser device 70 may also be a hybrid integrated device, wherein some of the components may be semi-conductor (e.g. indium phosphide) based components (such as the gain element 71) and some may be polymer based components (such as the directional coupler 10), i.e., in particular, passive and/or active elements of the components are made by semiconductor layers and polymer layers, respectively.
a to 18f illustrate different embodiments of realising the hybrid integrated laser device 70 of
The different configurations of the laser device 70 may be obtained, for example, by varying the order (i.e. the location along the light path of the laser) of the components of the device. Further, some of the components may be either semi-conductor based or polymer based. For example, the phase section 72 may be either a semi-conductor or a polymer component. The semiconductor (e.g. InP) components are indicated by hatched rectangular areas, whereas the polymer components are symbolized by non-hatched areas.
The laser device configurations shown in
It is noted that elements of different embodiments described above can, of course, also be used in combination. For example, the directional coupler used as an intra-cavity filter according to the embodiment of
A portion of the waveguide 1 may further comprise a (e.g. thermo-optically) tuneable section PS for phase adjustment (phase shifter) as shown in
The Bragg grating 1200 may have a total length of approximately 0.1-1 mm, wherein the width b (
An advantage of this embodiment may be that for fabricating a tuneable waveguide a single polymer material may be necessary, only, such that the choice of suitable polymers is less restricted. Further more, the fabrication of the waveguide may be simplified in comparison with a pure polymer waveguide such that the fabrication may be more cost efficient, in particular if the waveguide core is generated using lithographic lift-off technology.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12179654.4 | Aug 2012 | EP | regional |
| 12191078.0 | Nov 2012 | EP | regional |
This application is a continuation-in-part of international application no. PCT/EP2013/066646, filed Aug. 8, 2013, which claims priority to European patent application nos. 12179654.4, filed Aug. 8, 2012 and 12191078.0, filed Nov. 2, 2012. The foregoing applications are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2013/066646 | Aug 2013 | US |
| Child | 14616555 | US |
