WO2007107186A1

WO2007107186A1 - Integrated laser optical source

Info

Publication number: WO2007107186A1
Application number: PCT/EP2006/060975
Authority: WO
Inventors: Francesco Maria Tassone
Original assignee: Pirelli & C. S.P.A.
Priority date: 2006-03-23
Filing date: 2006-03-23
Publication date: 2007-09-27

Abstract

An integrated optical device (100) comprises an optically active waveguide element (115; 215) having a first end (120; 220) and a second end (127; 235), and an optically-reflecting structure. The optically-reflecting structure includes: an optical power splitter (150; 250) having an input port optically coupled to the second end of the optically active waveguide element, and a plurality of N output ports, wherein the optical power splitter is adapted to split an optical power inputted at the input port into a plurality of N optical power fractions each one made available at a respective one of the plurality of output ports; and a plurality of comb-like reflectors (160-1,...160- N; 260-1, 260-2), each one optically coupled, at a respective first end thereof, to a respective one of said plurality of output ports. The device further comprises at least one optical waveguide (163- 1,..., 163-N; 263-1,..., 263-N), optically coupled to a second end of one of the plurality of comb-like reflectors, opposite to the first comb-like reflector end.

Description

INTEGRATED LASER OPTICAL SOURCE § § § § §

Background of the invention

Field of the invention:

The invention generally relates to the field of optical communications, and to devices used in such field. In particular, the invention concerns integrated laser optical sources, specifically of the widely wavelength tunable type, adapted to be directly modulated at high bit rates.

Description of related art

Most optical sources used in high-performance optical transmission systems consist of a Continuous Wave (CW) semiconductor laser, followed by a generally bulky and expensive external modulator which modulates the intensity and/or phase of the laser light. Alternatively, appropriately designed semiconductor lasers allow direct modulation of the laser light intensity and/or emission frequency at high speeds.

Bandwidth requirements are driving a fundamental evolution of transmission networks. Currently, deployed networks are based on Wavelength Division Multiplexing (WDM), where several tens or hundreds of wavelengths are available to carry the traffic and multiply the overall available bandwidth in the network. In particular, one or more specific wavelengths are allocated to a definite physical point-to-point connection, according to the bandwidth usually required. However, the structure of such a network is fairly rigid, making it unable to accommodate for any large fluctuation in the load of specific connections.

Dynamical redirection of different wavelengths along different network links has been proposed as a solution to these problems. Indeed, in this case the bandwidth along a specific link can be dynamically increased or decreased by allocating more or less wavelengths to the link.

Along with specific optical devices able to perform such dynamical redirection function, such as optical add-drop multiplexers, the fundamental component enabling these future generation networks is a tunable optical source, capable of quickly switching its operating wavelength over all operating WDM channels. In the last ten years, several such sources have been developed and are now available as commercial products. i Two different solutions emerged: a first one combining several narrowly tunable standard sources, such as Distributed Feedback Lasers (DFB), the second one combining a single semiconductor gain region with an integrated or external widely tunable mirror, in order to realize a widely tunable laser source. Whereas the first solution readily allows high speed modulation when the single components do, up to 12 independent laser sources may be required in order to cover just part of the third optical communication window, typically the C-band from about 1527 nm to 1570 nm. This large number of active sources relates to several drawbacks: core costs, problems of manufacturing yields, and operative reliability. On the other hand, solutions relying on widely tunable external mirrors, either integrated or separated, have the inherent problem of severely lengthening the optical path in the laser. Since the direct modulation speed of the laser is ultimately related to the total length of the optical cavity thereof, the modulation speed is usually also severely limited in these approaches, so that commercial products are usually operated in CW, or can be modulated at low speeds, typically lower than 2.5 Gbit per second. Appropriate integration of the external tunable mirror in the semiconductor device allows for having a shorter cavity and thus achieving higher modulation speeds. However, in fully integrated laser structures tuning is normally provided by injection of electric charges, which brings along absorption. In practice, tuning is restricted to a range of some tens of nm in order to avoid excessive reduction of the tunable mirror transmission and reflectivity. For this reason, using a single tunable mirror does not allow achieving sufficiently wide tunability.

Use of the Vernier effect has thus been proposed in order to enhance the tuning range. However, the standard multiplicative Vernier configuration, in which the two tunable mirrors close the laser cavity on its two sides, has two main drawbacks. The first is the lengthening of the laser cavity, because the mirror length is added in this configuration. Moreover, the output of the laser occurs through one of the two mirrors, and thus the output needs to be amplified (e.g. by means of an additional, external amplifying section), because the absorption in the mirror makes the output power to depend on the tuning.

In EP 1 094 574 and in J. Wesstrom et al., "Design of a widely tunable modulated grating Y-branch laser using the additive Vernier effect for improved super-mode selection", 2002 IEEE 18th International Semiconductor Laser Conference, Conference Digest (Cat. No.02CH37390), 2002, p 99-100, the widely tunable mirror is based on the additive Vernier effect, instead of the multiplicative one. By adopting this solution, M. lsaksson etal., in "10 Gb/s Direct Modulation of 40 nm Tunable Modulated Grating Y-branch Laser", 2005 Optical Fiber Communications Conference Technical Digest (IEEE Cat. No. 05CH37672), 2005, pt. 2, p 3 pp. Vol. 2, reported that the laser efficiently operates at 10 Gbit per second while retaining full tunability over the C-band. Moreover, the output of the laser cavity occurs through a free facet, since the widely tunable mirror is located on one side of the laser cavity only. This allows avoiding the additional, external amplification section, as the reflectivity of the free facet is wavelength independent, and no absorption has to be compensated. This is an important manufacturing advantage, because the structure becomes less complicated, and moreover the laser is easier to drive and control, being the amplification section absent.

Summary of the invention

The Applicant has observed that the above discussed solution suffers from the typical and basically unavoidable limitation of the semiconductor lasers, which, due to the intrinsic chirp of the carrier frequency, have a reach limited to at most a few tens of km of transmission optical fiber, when directly modulated at very high bit rates, such as 10 Gbit per second or more. As a consequence, the use of directly modulated sources is restricted to the few applications involving very short distance links.

The Applicant has thus observed that it would be desirable to have a widely wavelength tunable integrated laser source suitable to be directly modulated, having external tunable reflectors exploiting the additive Vernier effect as disclosed for example in EP 1 094 574, which further integrates additional optical processing functions adapted to enhance the reach of the source. Such additional optical processing functions may for example be filtering functions as described in El.

Lett. Vol.41, no. 9, pag.543-544 (2005). The Applicant has observed that, starting from the structure proposed in EP 1 094 574, it might in principle be possible to integrate one or more passive optical functions in the same semiconductor laser substrate, but many of the advantages of that solution would be lost.

Indeed, since in the above prior art solution the output of the laser is taken directly from a free facet of the active section laser opposite to the widely tunable mirror, the additional passive section providing the additional optical processing functionalities should be contiguous to the active section. Thus, a passive device section would have to be provided also on the side of the active section opposite to that wherein the passive section comprising the reflectors is formed: however, as mentioned in the foregoing, such a spatial alternation of passive and active device sections is disadvantageous, among others from the manufacturing viewpoint. Moreover, a wide-band reflector would have to be used in substitution of the free facet in order to close the laser cavity, making the structure more complicated. Such an additional reflector would also cause a lengthening of the laser cavity, which as discussed above is also disadvantageous.

The Applicant has tackled the problem of providing a technically and economically effective way to realize an optical source to be used in a high performance network, the source being widely tunable and suitable to be directly modulated and moreover suitable to be appropriately filtered for reach enhancement or for implementation of optical functions required in the device or optical transmission system.

The Applicant has found that, in a laser source of the type disclosed in EP 1 094 574, which has external comb-like reflectors arranged to produce the additive Vernier effect, taking the laser output from an end of at least one of the comb-like reflectors opposite to the end coupled with the optical gain region, instead of from the free facet end of the prior art gain region, may provide a solution to the above problems. In this way, it is for example possible to integrate, in cascade to the reflectory) output end(s), further passive optical processing functional devices, e.g. an optical filter, within the passive section of the device already provided for forming the comb-like reflectors, advantageously using passive integrated optics technologies both for the reflectors and for the further processing functions.

According to an embodiment of the present invention, an integrated optical device as set forth in appended claim 1 is thus provided.

The integrated optical device comprises:

- an optically active waveguide element having a first end and a second end; and - an optically-reflecting structure, wherein the optically-reflecting structure includes:

- an optical power splitter having an input port optically coupled to the second end of the optically active waveguide element, and a plurality of N output ports, wherein the optical power splitter is adapted to split an optical power inputted at the input port into a plurality of N optical power fractions each one made available at a respective one of said plurality of output ports; and

- a plurality of comb-like reflectors, each one optically coupled, at a respective first end thereof, to a respective one of said plurality of output ports.

At least one optical waveguide is further provided, said at least one optical waveguide being optically coupled to a second end of one of said plurality of comb-like reflectors, said second end being opposite to the first comb-like reflector end.

5 The Applicant has further observed that it may be useful to include, in the cascaded passive optics, functional components having optical responses characterized by sharp features, such as a Full Width Half Maximum (FWHM) as small as few GHz, resulting in device effective lengths of several centimeters. This would require waveguide losses in the passive device section of the order of 1 dB/cm or less, a figure that is difficult to be obtained within the same technological O process adopted for realizing the active device section, due to the need of the additional processing steps for the realization of this section.

The Applicant has therefore found that it would be desirable to manufacture the optically passive part separately from the active part, in order to optimize the fabrication technology for the passive section so as to obtain the optimal performance which is required for the realization of 5 several cascaded optical processing elements, including for example filters.

Accordingly, in a preferred but not limitative invention embodiment, the gain device section and the passive device section are integrated in distinct, butt-coupled substrates.

This has the further advantage of allowing a further cut on the costs of the laser source: before associating the relatively expensive active device section to the passive optics, the latter O may be preliminarily tested.

It is pointed out that there is a synergic effect in the combination of emitting light from the comb-like reflectors, which allows the integration of the further passive optical processing functions in the passive section of the device already provided for forming the comb-like reflectors, and the separation of the substrates in which the active device section and the passive device section are 5 integrated. The resulting device shows a passive part which can be manufactured with a technology optimized both for allowing tuning of the reflector without the absorption due to the injection of electric charges, such as for example thermal tuning or electro-optical tuning, and for achieving low-loss optical processing functionalities.

In addition, the optical coupling of the two sections may be accomplished by means of a 0 butt-coupling of the two substrates. Should instead the further passive optical processing functions be provided at the side of the laser cavity opposite to that where the comb-like reflectors are formed, the separation of active and passive device sections would require optically coupling the substrate containing the active section to the substrate(s) containing the passive sections at two sides.

The separation of the active and passive device sections also allows increasing the manufacturing yield. In addition, the distinction of the two substrates allows forming external reflector which may be tuned by an optimal technique which avoids tuning-related changes in the absorption of the reflector, independently by the technology used for the formation of the active section. For example, it is possible to avoid the carrier-injection tuning technique, which results in absorption changes, and rely on the thermo-optic or electro-optic tuning techniques. According to a second aspect of the present invention, a process for manufacturing an integrated optical device is provided, as set forth in appended claim 28.

The process comprises:

- on a first substrate portion, integrating an optically active waveguide element, wherein said integrating includes forming the optically active waveguide element with a first end and a second end;

- on a second substrate portion, integrating an optically-reflecting structure including:

- an optical power splitter having an input port optically coupled to the second end of the optically active waveguide element, and a plurality of N output ports, wherein the optical power splitter is adapted to split an optical power inputted at the input port into a plurality of N optical power fractions each one made available at a respective one of said plurality of N output ports; and a plurality of comb-like reflectors, each one optically coupled, at a respective first end thereof, to a respective one of said plurality of N output ports.

The process further comprises integrating, in the second substrate portion, at least one optical waveguide optically coupled to a second end of one of said plurality of comb-like reflectors, said second end being opposite to the first comb-like reflector end.

According to a third aspect of the present invention, a method of emitting an optical radiation for communication is provided, as set forth in appended claim 32.

The method comprises lasing the optical radiation in an optical cavity having an optically active waveguide section with a first end and a second end, an optical power splitter optically coupled to the second end and having a plurality of N output ports, and a plurality of comb-like reflectors each one optically coupled to a respective one of said plurality of N output ports of the power splitter, wherein said optical radiation is emitted from an output end, opposite to the optical power splitter, of at least one of said plurality of comb-like reflectors.

Brief description of the drawings

The features and advantages of the present invention will be made apparent by the following detailed description of some embodiments thereof, provided merely by way of non- limitative examples, in connection with the annexed drawings, wherein:

Figure 1 schematically shows in top-plan view, and partly in terms of functional blocks, a general scheme of an optical device according to an embodiment of the present invention, including a laser source with external comb-like mirrors arranged in an additive Vernier configuration;

Figure 2 schematically shows in top-plan view an optical device according to an embodiment of the present invention, falling within the general scheme of Figure 1 ;

Figure 3 is a schematic cross-sectional view of the optical laser source of Figure 2 along the plane denoted Ill-Ill;

Figure 4 diagrammatically shows a combined optical reflectivity and transmission of two comb-like mirrors arranged in the additive Vernier configuration, like in the device of Figure 2;

Figures 5A, 5B, 5C schematically show alternative reflectors adapted to be used in the optical device of Figures 1 or 2; Figure 6 schematically shows in top-plan view an optical device according to an alternative embodiment of the present invention, still falling within the general scheme of Figure 1 ; and

Figure 7 schematically shows in top-plan view an optical device according to a still further alternative embodiment of the present invention, still falling within the general scheme of Figure 1.

Detailed description of the preferred embodiments(s)

With reference to the drawings, in Figure 1 a general scheme of an optical device 100 according to an embodiment of the present invention is shown schematically, in top-plan view, and partly in terms of functional blocks.

The optical device 100 is a widely wavelength tunable integrated semiconductor laser source with external reflector, adapted to direct modulation at high bit rates, e.g. at 10 Gbit per second or more. The optical device 100 includes an optically active device section 105a, and an optically passive device section 105b (hereinafter also shortly referred to as active section and passive section, respectively). By "optically active" there is meant able to generate and amplify light by stimulated emission of radiation (laser light); by "optically passive" there is instead meant substantially transparent (i.e., that exhibits a low light absorption) at the operating wavelength(s) of interest, that is comprising a material with an absorption edge having an energy larger than the typical energy of the photons of the optical radiation at the wavelength(s) of interest.

The optical device 100 may be integrated in a (preferably semiconductor) substrate 110. Several methods are known in the art for integrating both active and passive waveguides in a same technology on a common substrate. These methods are based on the adoption of material systems which provide for several compositions which are physically compatible, thus allowing growth on a common substrate, but exhibit different band-gaps and absorption edges, which allows to choose between active and passive behavior of the material. By way of non-limiting example, the Quantum Well Intermixing (QWI) process provides for a convenient way of achieving such composition control while reducing the number of steps (such as mask alignments) required in the realization of the device. Alternatively, different material systems for the active and passive sections of the device can be considered, and appropriate mounting provided for achieving stable optical coupling between the two sections.

The active section 105a includes, formed on the semiconductor substrate 110, a light guiding and amplifying, optical gain structure 115. For example, and without entering into excessive details known to those skilled in the art, the substrate 110, and the optical gain structure 115, are realized in Nl-V technology, and include for example an InP substrate over which InGaAsP active, amplifying layers are grown and appropriately patterned to form, in the region of the gain structure 115, an optical waveguide. The active, amplifying layers provide for spontaneous emission and amplification of the spontaneously emitted light in a wide band of typically over 100 nm, centered around the frequencies of interest for the laser source, such as the C-band described above.

The device 100 also includes a wide-band mirror 120, optically coupled to a first end of the optical gain structure 115; in an embodiment of the present invention, the wide-band mirror 120 may consists in an integrated reflector, like a grating integrated in the substrate 110; in an alternative embodiment of the invention, the wide-band mirror 120 may for example be part of a first cleaved facet 125 (depicted in phantom in Figure 1) of the substrate 110, possibly coated with a suitable high-reflectivity coating. Preferably, the wide-band mirror 120 has a reflectivity of at least 30%, more preferably of at least 60%, even more preferably of at least 90%.

At a second end 127 thereof opposite to the first end, the optical gain structure 115 is optically coupled to an optically-reflecting structure comprising a "1 x N" optical power splitter 150, a plurality of optical waveguide segments 155-1 ,..., 155-W and a plurality of comb-like reflectors 160- 1,...160-W.

The "1 x N" optical power splitter 150 has an input port optically coupled to the second end 127 and is adapted to (preferably substantially equally) split the optical power received at the input port from the optical gain structure 115 into a number N of (preferably substantially equal) optical power fractions, and to make each of the N optical power fractions available at a respective power splitter output port; in particular, N may be any positive integer equal to or greater than 2.

The N optical waveguide segments 155-1,..., 155-W are integrated in the substrate 110, each one optically coupled to a respective output port of the power splitter 150. A fraction substantially equal to MN of the optical power received by the power splitter 150 from the optical gain structure 115 is coupled into each of the N optical waveguide segments 155-1,..., 155-W. The term "substantially" takes into account possible deviations from the preferable equal split of the optical power, such deviations not substantially affecting the proper function of the device 100. For example, the fraction may deviate from the ideal MN value by ±25% of MN. In case of N equal 2, this means an acceptable power splitting ratio of about 40/60 instead of the ideal 50/50. Along at least two, possibly along each of the optical waveguide segments 155-1,..., 155-

N, which are designed to suitably diverge from each other in going far from the power splitter 150, a respective comb-like optical reflector 160-1,..., 160-N is formed.

A comb-like optical reflector is apt to back-reflect incident light with a "comb-like" reflectivity response, which for the purpose of the present description and claims is defined as a spectral response having at least two peaks (see Fig. 4 below for an example). For example, the comb-like optical reflectors 160-1,..., 160-W may include tunable Super-Sampled Gratings (SSGs). In particular, the SSGs may be realized within the Nl-V InP technology where the QWI process is used for realization of the passive device section. The tuning of the SSG may be accomplished by carrier injection. Alternatively, tuning may be also provided through the electro-optic effect. In this case, no injection of free carriers is used, but instead a strong electric field is applied to the waveguide, which results in a modulation of the band-gap and of the index of refraction of the material, and thus into tuning of the grating section.

The two or more comb-like optical reflectors 160-1,..., 160-W and the power splitter 150 form an additive Vernier-effect optically-reflecting structure; overall considered, the optical gain structure 115, with the wide-band mirror 120 at an end thereof, the power splitter 150, the optical waveguide segments 155-1,..., 155-W and the comb-like optical reflectors 160-1,..., 160-W form a laser cavity, an end of which is essentially closed by the wide-band mirror 120, whereas the other end is formed by the comb-like optical reflectors 160-1,..., 160-W.

Preferably, along one or more of the N optical waveguide segments 155-1,..., 155-W, one or more phase adjustment elements may be provided, like the phase adjustment element schematically depicted as a broken line block 157 in the drawing. In this way the relative phase of the optical radiations reflected by the optical reflectors 160-1 160-N and propagating along the

Λ/ optical waveguide segments 155-1,..., 155-Wis properly adjusted.

Downstream the comb-like optical reflectors 160-1,..., 160-W, in a direction of propagation of an optical radiation outputted by the optical device (which in the drawing is the left-to-right direction), up to N (and at least one) optical waveguide segments 163-1 ,..., 163-W are integrated in the substrate 110, each one being optically coupled to a respective comb-like optical reflectors 160- 1,..., 160-W. The N optical waveguide segments 163-1,..., 163-W converge towards, and are each one optically coupled to a respective input port of an "N x 1" optical power combiner 165, apt to recombine the optical power transmitted by the comb-like optical reflectors 160-1,..., 160-W. Preferably, along one or more of the N optical waveguide segments 163-1,..., 163-W, one or more phase adjustment elements may be provided, like the phase adjustment element schematically depicted as a broken line block 167 in the drawing. In this way the relative phase of the optical radiations propagating along the N optical waveguide segments 163-1,..., 163-W is properly adjusted. The optical power combiner 165 couples the received optical power and makes it available at an output port thereof, to which an end of an optical waveguide segment 170 is optically coupled.

It can be appreciated that the power splitter 150, the optical waveguide segments 155-1,...,

155-W, the comb-like optical reflectors 160-1,..., 160-W, the N optical waveguide segments 163-

1,..., 163-W, the phase adjustment element(s) 157 and 167 and the power combiner 165 form an N- arms Mach-Zehnder Interferometer (MZI) for the transmitted light.

Optically coupled to the optical waveguide segment 170, one or more passive optical processing functions are integrated in the substrate 110, which in the drawing are globally schematized as a block 175. In particular, and merely by way of example, the optical processing functions 175 may include a notch filter, and/or a pass band filter and/or an all pass filter.

Downstream the optical processing functions 175, the optical radiation is coupled into an output waveguide segment 180, integrated in the substrate 110 and terminating, at a free end thereof, at a facet 185 of the substrate 110. The output light of the optical device 100 is taken from the facet 185 (the substrate facet from which the light is outputted is not limitative for the present invention).

It is observed that although in the above description the active device section and the passive device section were integrated in a same substrate, the Applicant has observed that further advantages may be achieved by separating the active and passive sections, forming them in distinct, optically-coupled substrates.

For example, since the additional, integrated optical processing functions 175 may provide for optical transfer functions with sharp features such as for example a small Full Width Half Maximum (FWHM) of few GHz or a high roll-off of several dB/GHz, amounting to effective device lengths of several centimeters, waveguide losses in the passive section of the order of 1 dB/cm or less would be required; this figure is not easy to be obtained within the same process producing the active device section, due to the need of the additional processing steps for its manufacturing.

Separating the substrates on which the active section and the passive section of the optical device are integrated allows manufacturing the passive section separately from the active section, so that it is possible to realize passive optical elements with higher performance; thanks to this, several cascaded optical elements, including filters, may be integrated in the passive section.

Forming the active section and the passive section in distinct, separated substrates allows in particular using the best suited technology for each of them; for example, the Nl-V technology already mentioned in the foregoing may still be used for forming the active device section, whereas the passive device section may be realized using the silicon, particularly Silicon-On-lnsulator (SOI) technology; in such a case, the comb-like optical reflecting elements may be tunable Super- Sampled Gratings (SSGs), realized using SOI technology, and their tuning may be accomplished by thermo-optic effect, instead of charge injection, e.g. by way of thin-film heaters. Tunable comb- like optical-reflecting elements realized in this way exhibit a significantly lower absorption compared to their counterparts realized in Nl-V technology and exploiting carrier injection. Moreover, by separating the active section and passive section substrates it is possible to preliminary test the passive optics components of the passive section before assembling the passive section to the more expensive active section.

In the following, some examples of optical devices falling within the general scheme of Figure 1 will be presented. It is pointed out that the examples that will be presented refer to optical devices wherein the active device section and the passive device section are formed in separated substrates, as discussed above; however, it should be understood that nothing prevents from realizing the optical devices that will be presented in monolithic form, with the active section and the passive section formed in a same substrate. In Figure 2, an optical device 200 according to an embodiment of the present invention is shown schematically, in top-plan view, wherein the integer N is equal to two.

The optical device 200 includes an active section 205a, and a passive section 205b.

The active section 205a includes an active section substrate 210a, on which a light guiding and amplifying, optical gain structure 215 is formed. For example, the substrate 210a and the optical gain structure 215 are realized in Nl-V technology, and include for example an InP substrate over which InGaAsP amplifying layers are grown and appropriately patterned to form an optical waveguide. The active amplifying layers provide for spontaneous emission and amplification of this spontaneously emitted light in a wide band of typically over 100 nm, centered around the frequencies of interest for the laser source such as the C-band described above. The active section 205a also includes a wide-band mirror 220, placed at, and closing a first end of a laser cavity; the wide-band mirror may for example be part of a first cleaved facet 225 of the active section substrate 210a, possibly coated with a suitable high-reflectivity coating, and having a reflectivity of at least 30%, preferably of at least 60%, more preferably of at least 90%. In alternative embodiments, the wide-band mirror 220 may not be part of a cleaved facet, being instead formed by means of an integrated reflector like a grating, as described before in conjunction with Figure 1.

At a second cleaved facet 230 of the substrate 210a, opposite the first facet 225, an antireflection coating may be provided in order to avoid formation of nested sub-cavities which degrade uniformity of tuning and laser output power. Typically, the reflectivity of a second end 235 of the gain structure 215 is lower than 40%, preferably lower than 15%, even more preferably lower than 5%. The passive section 205b includes a passive section substrate 210b, arranged in proximity relationship with, particularly butt-coupled to the active substrate 210a, so as to have a first facet

240 facing the second facet 230 of the active substrate 210a. The two substrates 210a and 210b may for example be mounted on a common supporting submount, not depicted in the drawing, or the substrate 210b may act as the submount for the substrate 210a or vice versa.

A first optical waveguide segment 245 is integrated on the passive substrate 210b; the first optical waveguide segment 245 has a first, free end terminating at the first facet 240 of the passive substrate 210b, facing the gain structure 215.

An antireflection coating on the first free facet 240 of the optical waveguide segment 245 may be also provided having the same function of the antireflection coating 235 described above.

The first optical waveguide segment 245 is connected, at a second end thereof, to a power splitter 250, adapted to substantially equally split the optical power propagating through the first optical waveguide segment 245 and to couple respective optical power fractions into a number of distinct branches, in the example including two second optical waveguide segments 255-1, 255-2 (thereby forming a "Y" branch); in the example herein considered, the power splitter 250 is a 3 dB splitter, coupling half of the optical power arriving from the optical gain structure 215 into the waveguide segment 255-1, half into the waveguide segment 255-2. Preferably, the power splitting is within 40/60, more preferably within 45/55. Any known suitable power splitter or coupler 250 may be used instead of the Y-branch splitter shown in figure, such as a directional coupler (comprising N coupled waveguides), a multi-mode interference (MMI) coupler, a MZI-based optical splitter, a mode evolution adiabatic coupler or the like.

Along both of the second optical waveguide segments 255-1, 255-2, which are designed to suitably diverge in leaving the power splitter 250, a respective comb-like optical reflecting element 260-1, 260-2 is formed. In the exemplary embodiment of Figure 2, the comb-like optical reflecting elements 260-1 and 260-2 include tunable SSGs, in particular realized using SOI technology, whose tuning may be accomplished by thermo-optic effect. To this purpose, heaters 293 and 295, schematically depicted as rectangles in dashed line in the drawing, may be provided on the two comb-like optical reflectors 260-1 and 260-2. The heaters may in particular be formed as thin-film heaters. The two comb-like optically-reflecting elements 260-1 and 260-2 and the power splitter 250 form an additive Vernier-effect optically-reflecting structure; the optical gain structure 215, with the wide-band mirror 220 at an end thereof, the power splitter 250, the optical waveguide segments 255-1, 255-2 and the comb-like optical reflectors 260-2, 260-2 form a laser cavity, an end of which is essentially closed by the wide-band mirror 220, whereas the other end is formed by the comb-like optically-reflecting elements 260-1, 260-2. Downstream the two comb-like optically-reflecting elements 260-1 , 260-2, in the direction of propagation of the outputted optical radiation (left-to-right direction in the drawing), two third optical waveguide segments 263-1, 263-2 are integrated in the substrate 210b, each one being optically coupled to a respective one of the two comb-like optically-reflecting elements 260-1, 260-2. The two third optical waveguide segments 263-1, 263-2 converge towards, and are each one optically coupled to a respective input port of an optical power combiner 265, recombining the optical power transmitted by the comb-like optically-reflecting elements 260-1, 260-2. Preferably, the optical power combiner 265 is structurally substantially identical to the optical power splitter 250.

The power combiner 265 couples the recombined optical power into a fourth optical waveguide segment 270. Moving along the fourth optical waveguide segment 270, one or more passive optical functional devices are integrated in the substrate 210b; in the exemplary embodiment herein considered, a narrow pass band filter 275 is formed, exemplarily implemented by means of a ring resonator. Any suitable resonator may be used instead of the ring shown in the figure, such as a racetrack resonator, a standing wave resonator, a linear cavity resonator, a Bragg grating resonator or the like.

An output of the filter 275 is coupled into a fifth, output waveguide segment 280 which, at a free end thereof, terminates at a second facet 285 of the substrate 210b, opposite in the shown example the first facet 240 thereof. The output light of the optical device 200 is taken from the facet 285 (however, the fourth waveguide segment 280 may terminate at any facet of the substrate 210b). For example, the fifth waveguide segment 280 may be coupled to the ring resonator as shown in figure 2. It is observed that in case a notch filter is desired, instead of the pass band filter, it is sufficient to slightly modify the described structure so that the output light is taken directly from the waveguide segment 270 instead of the segment 280.

In order to avoid the formation of a nested cavity external to the laser cavity formed by elements 220, 215, 250, 255-1, 255-2, 260-1, 260-2 as described above, an antireflection coating on the free facet 285 of the optical waveguide segment 280 may be also provided. Optical feedback from this free facet is again detrimental to tuning and laser output power uniformity as described above, and may also result into severe instabilities when very long optical paths are realized in the processing optics 275. Alternatively or in conjunction to this precaution, optical isolation elements which cancel optical feedback into the laser cavity may be also integrated in the optical processing section 275. Other methods well known to those skilled in the art such as curving the output of optical waveguide segment 280 may be used in conjunction to the above methods in order to further decrease the level of optical feedback related to the free facet 285.

From a practical viewpoint, the passive substrate 210b may for example be a silicon-on- insulator (SOI) substrate; the waveguides 245, 255-1, 255-2, 263-1, 263-2, 270 and 280 may be formed in correspondence of the top silicon layer. In particular, the waveguides 245, 255-1 , 255-2, 263-1, 263-2, 270 and 280 may be buried rectangular waveguides or ridge waveguides, as in the example herein considered and depicted in Figure 3: the passive SOI substrate 210b includes a silicon substrate 305, a silicon dioxide layer 310 and over the silicon dioxide layer 310, a silicon core layer. A waveguide is formed by patterning the silicon core layer so as to form a slab 320 and a ridge 325. A further silicon dioxide layer 330 covers the structure. The silicon dioxide layers 310 and 330 form part of the cladding.

In particular, in a preferred waveguide structure suitable for the waveguides 245, 255- 1,...,255-W which readily couples to the active section 205a as it features a closely matched mode size, the ridge 325 width is approximately 500 nm, the ridge height is approximately 600 nm, the slab 320 thickness is approximately 250 nm, the silicon dioxide layer 310 is thicker than approximately 1.5 μm. The effective slab refractive index is approximately 2.92 at 1550 nm of wavelength. The TE (Transverse Electric) effective mode refractive index is approximately 3.10, with FWHM of 0.6 μm in the horizontal and vertical directions. The ridge 325 may for example be realized by depositing amorphous silicon in an etched silica layer of appropriate height. Typical attenuations well below 6 dB/cm can be obtained with this method for a straight waveguide.

The same structure above, when used for a bent waveguide, results in an expected excess loss of 12 dB/cm for 20 μm radius bends, so that very compact bends can be implemented, as described with reference to Figures 5A, 5B and 5C.

The upper silicon dioxide cladding layer 330 is adapted to mechanically hold and optically separate the thin film heaters, e.g.293 and 295.

Preferably, one or more additional, e.g. thin-film heaters are provided. Referring back to Figure 2, in the example herein considered, the additional heaters, depicted again as rectangles in dashed line, may include one or more of the group consisting of a first additional heater 290 placed on at least part of the first waveguide segment 245 and the power splitter 250, a second additional heater 292 placed on at least a portion of the optical waveguide segment 255-1 or 255-2, a third additional heater 297 placed on at least a portion of the second optical waveguide segment 263-1 or 263-2 downstream the optical reflector 260-2, and a fourth additional heater 299 placed over the filter 275. The first additional heater 290 allows finely tuning the lasing mode, so that it is properly, finely aligned to the selected wavelength on the ITU grid; in alternative embodiments of the invention, a similar fine tuning result may be obtained by providing, instead of, or in combination with the first heater 290, a thermo-optic fine tuning of the gain waveguide 215, slightly changing the bias current of the gain section 205a. The second additional heater 292 allows for finely balancing the optical length of the two second optical waveguide segments 255-1, 255-2, so as to optimize the proper function of the reflecting structure 250, 255-1, 255-2, 260-1, 260-2. The third additional heater 297 allows for finely balancing the optical length of the overall paths formed respectively by the optical waveguide segments 255-1, 263-1 and 260-1 and by 255-2, 263-2 and 260-2, so as to optimize the transmitted power, and thus achieve improved power uniformity (as already pointed out in connection with Figure 1, the optical waveguide segments 255-1, 255-2, 263-1 263-2, the reflectors 260-1 and 260-2 and the power splitter 250 and power combiner 265 form a Mach- Zehnder interferometer). The fourth additional heater 299 allows finely aligning the pass band filter (or the notch filter or the all-pass filter) to the selected wavelength of the ITU grid. The second, third and fourth additional heaters allow to relax the fabrication tolerances of the respective optical elements in respect of which they provide fine adjustment of the optical length.

Alternatively to the tuning provided by the strong thermo-optic effect in silicon, tuning by conventional carrier injection may be also considered. This is particularly advantageous for providing fine tuning means. In fact, the control of the temperature to very high precisions may be difficult, whereas an independent control achieved through current injection is not. Moreover, for small tuning, additional waveguide absorption related to free carrier injection becomes negligible. The electro-optic effect in silicon is instead very small, but considerable enhancement can be obtained using material compositions which include low silicon - high germanium content, so as to result in a SiGe material with a band-gap energy which is larger, but not too larger, than the energy of the photons traveling in the waveguide. The term "low silicon content" means a content in silicon less than 20%, preferably less than 15%, more preferably less than 10%.

The diagram of Figure 4 shows exemplary reflectivity (solid curve 405) and transmission (solid curve 410) responses of the optically-reflective structure (reflectors 260-1 and 260-2 and splitter 250 of Fig. 2), as a function of the wavelength λ, obtained with the additive Vernier effect using two SSGs based on first order gratings and having four super-sampled periods of 134 and 155 λ_o/2 length, wherein λ₀ is fixed equal to 1.55 μm. In Figure 4 the dotted line shows the reflectivity response of the SSG having 155 λ_o/2 super periodicity, giving an FSR for the comb reflectivity of 10 nm, while the dash-dotted line shows the reflectivity response of the SSG having 134 λ_o/2 super periodicity, giving an FSR for the comb reflectivity of 11.6 nm. The appropriate FWHM of 2.5 nm is obtained with a square modulation of the first order grating of either the ridge 325 width or the height, by about 10 nm. Alternatively, a grating in the slab 320 can also be used. First order gratings in these materials are formed by means of UV or e-beam lithography. Alternatively, higher-order gratings can also be used, at the expense of a slightly increased loss. The modulation of the super-sampling is sinusoidal, with amplitudes of about 1 %. In a preferred embodiment of the present invention, the optical filter implemented by the ring resonator 275 is a pass band filter, with a 25, 50, 100, 200, or 400 GHz of FSR, and a typical FWHM of 5 to 8 GHz. In another preferred embodiment, a second order filter may be realized using two coupled rings, for the purpose of optimizing the transmission capability of the laser. In still another preferred embodiment, a third order filter may be realized. Figures 5A, 5B and 5C schematically show integrated resonating structures adapted to realize the comb-like reflectivity function of the optical reflectors 160-1,..., 160-W, in alternative to the SSGs, with prescribed FSR and FWHM of the reflectivity response. As shown in Figure 5A, they are formed by at least a resonator 500 coupled to the respective waveguide segment 255-1,.., 255-N, via an optical coupler (in Figure 5A a directional coupler formed by bringing the resonator 500 near to the waveguide by a small gap 502). The structure 505 represents a low reflectivity (from 1% to 10%, preferably from 2% to 8%, more preferably from 3% to 5%), wide-band mirror (at least over the C-band) placed along the resonator 500. Figures 5B and 5C schematically depict exemplary embodiments of the general structure of Figure 5A wherein the low reflectivity, wideband mirror 505 is formed by a grating 510. For example, a first order grating 510 realized with 20 periods and giving about 4% of reflectivity and 92% of transmission, combined with a ring (500) diameter of about 20 μm, and a coupling ratio for the optical coupler of about 80%, results in a comb periodicity (FSR) of about 6 nm and a FWHM of about 1 nm. Such a grating can be obtained for example with the ridge waveguide structure described above in connection with Figure 3, using sections of different widths, differing by about 140 nm. Large coupling ratios between the resonator 500 and the waveguide 255-1,.., 255-N may be obtained either by using directional couplers with very small gaps 502, as shown in Figure 5A and 5B, of the order of magnitude of about 100 nm, or, preferably, by using multi-mode couplers 508 (as shown in Figure 5C), which exhibit a coupling ratio which is substantially independent from the wavelength across the C-band and is shorter than the directional couplers. The integrated resonating structures of Figures 5A-C may be realized by means of SOI technique, using e-beam lithography and, compared to SSGs, are more compact and may have technological advantages in easing the control of thermo-optic or electro-optic effects.

The optical devices according to further embodiments of the invention that will be presented in the following incorporate different and/or additional optical processing functions 175 compared to the embodiment of Figure 2. The two further embodiments are schematically depicted in Figures 6 and 7, wherein parts being identical or equivalent to those of the embodiment of Figure 2 are denoted by the same reference numerals.

In particular, in the embodiment of Figure 6, an optical pass-band filter 605 is realized by cascading two Mach-Zehnder filters 610-1 and 610-2 having appropriate respective difference of arm-lengths. In this way, an FSR equal to the values above indicated (from 25 GHz to 400 GHz) can be obtained. Heaters 615-1 and 615-2 are preferably provided over the arms of the two Mach- Zehnder filters 610-1 and 610-2: thermo-optic tuning of the arm length allows fine tuning of the filter characteristics, and alignment to fine-spaced ITU grids.

In the embodiment of Figure 7, similarly to the embodiment of Figure 6, an additional optical processing function 705 including two Mach-Zehnder filters 710-1 and 710-2 is provided. The first Mach-Zehnder filter 710-1 is integrated with the MZI structure including the tunable optical reflectors, the waveguide sections 263-1 and 263-2 and the optical power combiner 265, so as to further reduce the overall device size and losses. In particular, the third optical waveguide segment 263-2 is modified into a third optical waveguide segment 763-2 having an extra-length and the optical power combiner 265 is realized with a directional coupler having two output ports. Downstream, the second Mach-Zehnder filter 710-2 is formed as previously described with reference to Figure 6. Heaters 715-1 and 715-2 are preferably provided over the MZI arms for thermo-optically tuning the arm length. The present invention has been disclosed by describing some exemplary embodiments thereof, however those skilled in the art, in order to satisfy contingent needs, will readily devise modifications to the described embodiment, as well as alternative embodiments, without for this reason departing from the protection scope defined in the appended claims.

For example, any combination of passive optical components, particularly any combination of interferometers and resonators can be integrated in order to realize the appropriate transfer function.

Claims

1. An integrated optical device (100) comprising:

- an optically active waveguide element (115;215) having a first end (120;220) and a second end (127;235); and

- an optically-reflecting structure, wherein the optically-reflecting structure includes:

- an optical power splitter (150;250) having an input port optically coupled to the second end of the optically active waveguide element, and a plurality of N output ports, wherein the optical power splitter is adapted to split an optical power inputted at the input port into a plurality of N optical power fractions each one made available at a respective one of said plurality of output ports; and

- a plurality of comb-like reflectors (160-1,...160-W;260-1, 260-2), each one optically coupled, at a respective first end thereof, to a respective one of said plurality of output ports, characterized by further comprising at least one optical waveguide (163-1 163- W; 263-1 263-A/), said at least one optical waveguide being optically coupled to a second end of one of said plurality of comb-like reflectors, said second end being opposite to the first comb-like reflector end.

2. The integrated optical device of claim 1 , wherein said at least one optical waveguide comprises a plurality of optical waveguides, each one optically coupled to the second end of a respective one of said plurality of comb-like reflectors.

3. The integrated optical device of claim 2, further comprising an optical power combiner (165) having a plurality of input ports optically coupled to said plurality of optical waveguides, and an output port, wherein the optical power combiner is adapted to combine a plurality of optical powers received from the plurality of waveguides and to make the combined optical power available at the output port.

4. The integrated optical device of any of the preceding claims, further comprising at least one optical processing function (175;275;605;705) optically coupled to said at least one optical waveguide (163-1 163-W; 263-1 263-Λ/).

5. The integrated optical device of claim 4, comprising a further optical waveguide (170;270) optically coupled to the optical power combiner output port, wherein said at least one optical processing function (175;275;605;705) is optically coupled to said further optical waveguide (170;270).

6. The integrated optical device of any one of the preceding claims, wherein at least one of said plurality of comb-like reflectors includes a waveguide grating.

7. The integrated optical device of claim 6, wherein said waveguide grating includes a super-sampled grating.

8. The integrated optical device of any of claims 1 to 5, wherein at least one of said plurality of comb-like reflectors includes a Fabry-Perot resonator.

9. The integrated optical device of any one of claims 6 to 8 when depending on claim 5, wherein said at least one optical processing function includes an optical filter.

10. The integrated optical device of claim 9, wherein said optical filter includes a ring or racetrack resonator.

11. The integrated optical device of claim 9, wherein said optical filter includes at least one Mach-Zehnder filter.

12. The integrated optical device of any one of claims 9 to 11 , wherein said optical filter is a band-pass filter with an FSR of 25, or 50, or 100, or 200, or 400 GHz, and an FWHM of approximately 5 to 8 GHz.

13. The integrated optical device of any one of the preceding claims, wherein the optical device is integrated in a same substrate (110).

14. The integrated optical device of any one of the preceding claims, wherein said optical device is realized in Nl-V technology.

15. The integrated optical device of any one of the preceding claims, wherein at least one of said comb-like reflectors is wavelength tunable.

16. The integrated optical device of claim 15 when depending on claim 14, wherein said at least one comb-like reflector is wavelength tunable by electric carrier injection.

17. The integrated optical device of any one of claims from 1 to 12, wherein said optically active waveguide element is integrated in a first substrate (110a), and said optically-reflecting structure and said at least one optical waveguide are integrated in a second substrate (110b) distinct from, and optically coupled to, said first substrate.

18. The integrated optical device of claim 17, wherein the optically active waveguide element is realized in Nl-V technology, whereas the optically-reflecting structure and said at least one optical waveguide are realized in silicon-on-insulator technology.

19. The integrated optical device of any of the preceding claims, wherein at least one of said comb-like reflectors is in a thermo-optical material, and include a respective heater (293,295).

20. The integrated optical device of claim 17, wherein the optically active waveguide element is realized in Nl-V technology, whereas the optically-reflecting structure and said at least one optical waveguide are realized in a electro-optical material such as low silicon content germanium-silicon material.

21. The integrated optical device of any one of the preceding claims, wherein said plurality of comb-like reflectors includes N comb-like reflectors.

22. The integrated optical device of any one of the claims 2 to 21 when depending on claim 2, wherein said plurality of plurality of optical waveguides includes N optical waveguides.

23. The integrated optical device of any one of the preceding claims, wherein N is equal to two.

24. The integrated optical device of any one of the preceding claims, wherein the first end of the optically active waveguide element is optically coupled to a high reflectivity element (120).

25. The integrated optical device of claim 24, wherein said high reflectivity element has a reflectivity higher than approximately 30%, preferably higher than approximately 60%, more preferably higher than approximately 90%.

26. The integrated optical device of claim 24 or 25 wherein the optically active waveguide element (115;215), the high reflectivity element and the optically-reflecting structure form an optical cavity.

27. The integrated optical device of any one of the preceding claims, wherein the optical power splitter (150;250) is adapted to substantially equally split said optical power inputted at the input port into said plurality of N optical power fractions.

28. A process of manufacturing an integrated optical device, comprising: - on a first substrate portion (110;210a), integrating an optically active waveguide element

(115;215), wherein said integrating includes forming the optically active waveguide element with a first end (120;220) and a second end (127;235);

- on a second substrate portion (110;210b), integrating an optically-reflecting structure including: - an optical power splitter (150;250) having an input port optically coupled to the second end of the optically active waveguide element, and a plurality of N output ports, wherein the optical power splitter is adapted to split an optical power inputted at the input port into a plurality of N optical power fractions each one made available at a respective one of said plurality of N output ports; and a plurality of comb-like reflectors (160-1,...160-W;260-1, 260-2), each one optically coupled, at a respective first end thereof, to a respective one of said plurality of N output ports, characterized by further comprising

- integrating, in the second substrate portion, at least one optical waveguide (163-1, ...163- W;263-1 ,263-2) optically coupled to a second end of one of said plurality of comb-like reflectors, said second end being opposite to the first comb-like reflector end.

29. The process of claim 28, wherein the first substrate portion is part of a first substrate, and the second substrate portion is part of a second substrate, the first and second substrate being physically distinct from each other, the process further comprising:

- butt-coupling the first and second substrates so that the second end of the optically active waveguide element is optically aligned to the input port of the optical power splitter.

30. The process of claim 29, wherein the step of integrating on the first substrate

(110;210a) is realized in Nl-V technology, and the step of integrating on the second substrate (110;210b) is realized in silicon-on-insulator technology.

31. The process of claim 28, 29 or 30, wherein said first end of the optically active waveguide element is formed highly reflective.

32. A method of emitting an optical radiation for communication, comprising:

- lasing the optical radiation in an optical cavity having an optically active waveguide section with a first end and a second end, an optical power splitter optically coupled to the second end and having a plurality of N output ports, and a plurality of comb-like reflectors each one optically coupled to a respective one of said plurality of N output ports of the power splitter, characterized in that said optical radiation is emitted from an output end, opposite to the optical power splitter, of at least one of said plurality of comb-like reflectors.