WO2007107187A1

WO2007107187A1 - Integrated laser optical source with active and passive sections formed in distinct substrates

Info

Publication number: WO2007107187A1
Application number: PCT/EP2006/060977
Authority: WO
Inventors: Francesco Maria Tassone; Marco Romagnoli
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;200;300;900) comprises an optically active waveguide element (115) having a first end (120) and a second end (135), and an optically-reflecting structure, wherein the optically-reflecting structure includes: an optical power splitter (150;350) having an input port optically coupled to the second end of the optically active waveguide element, and a 10 plurality of N output ports; 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; a plurality of comb-like reflectors (160-1,...160-N;260-1,260-2) is provided, each one optically coupled, at a respective first end thereof, to a respective one of said plurality of output ports. The optically active waveguide element is integrated in a first substrate 15 (110a), and the optically-reflecting structure is integrated in a second substrate (110b) distinct from and optically coupled to the first substrate.

Description

INTEGRATED LASER OPTICAL SOURCE WITH ACTIVE AND PASSIVE SECTIONS FORMED IN

DISTINCT SUBSTRATES § § § § §

5 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 O 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 5 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 O 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. 5 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 multipexers, the fundamental component enabling these future generation 0 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 i now available as commercial products.

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.

In particular, in EP 1 094574 a widely wavelength tunable integrated semiconductor device is disclosed comprising a substrate made of a semiconducting material, a two-sided active section on said substrate, said active section generating radiation by spontaneous emission over a bandwidth around some center frequency and guiding said radiation, said active section having amplification actions, and a plurality of sections on said substrate, all said sections being connected to one side of said active section, at least two of said sections including a waveguide system, defining a reflector.

Summary of the invention

The Applicant has observed that a drawback of the above discussed solution resides in the fact that the whole device, i.e. both the active section and the other sections, is integrated on a common semiconducting substrate.

Due to this, it is difficult, especially from the manufacturing viewpoint, to optimize both the active section and the other device sections, since a common substrate implies that the manufacture of one section influences, or needs to take into account the presence of, the other portions. In particular, additional processing steps are needed for the realization of the active and passive section on the same substrate with respect to the steps needed for the manufacture of the individual sections. Also, the manufacturing process yield, and ultimately the device costs, are adversely affected by the integration of the device active section and the other device sections on a same substrate: some device samples may have to be discarded due to problems in the active device section, whereas the other device sections operate properly, or wee versa.

The Applicant has therefore tackled the problem of how to overcome the above discussed drawbacks. The Applicant has found that it would be desirable to have an integrated laser source suitable to be directly modulated and widely wavelength tuned, having external reflectors exploiting the additive Vernier effect as disclosed for example in EP 1 094 574, wherein however the device sections (e.g. the passive device sections) other than the active device section can be manufactured separately from the active device section, in order to optimize the fabrication technology (and resulting device performances) for both the passive device sections and the active device section, and not to limit the manufacturing process yield.

Thus, according to a first aspect of the present invention, an integrated optical device as set forth in appended claim 1 is 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.

Said optically active waveguide element is integrated in a first substrate, and said optically- reflecting structure is integrated in a second substrate distinct from and optically coupled to said first substrate.

By forming the gain device section and the passive device section in distinct, optically coupled substrates, the two device sections can be both optimized, and a cut on the device costs is made possible, because before associating the relatively expensive active device section to the passive optics, the latter may be preliminarily tested and, if required, discarded, whereas the active device section, if operating properly, can be retained. 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. The Applicant has further observed that in order to overcome a 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 (thereby the use of directly modulated sources is normally restricted to the few applications involving very short distance links), it would be desirable to integrate, in an optical device as set forth above, additional optical processing functions adapted to enhance the reach of the optical 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).

Accordingly, in a preferred embodiment of the present invention, the integrated optical device further includes at least one, possibly several, passive optical processing functional devices.

Advantageously, these additional optical processing functional devices are formed in the same substrate wherein the passive device section is formed. Thanks to this, the additional optical processing functions may be manufactured separately from the active part, and it is thus possible to optimize the fabrication technology so as to optimize performance. This allows integrating several cascaded optical processing elements, including for example filters. For example, functional components having optical responses characterized by sharp features, such as a Full Width Half Maximum (FWHM) as small as few GHz, resulting in effective device lengths of several centimeters, may be integrated (waveguide losses of the order of 1 dB/cm or less are required, a figure that is difficult to be obtained if such processing functions are integrated in the active device section substrate, due to the need of the additional processing steps for the realization of this section).

In a preferred embodiment of the invention, the laser output is taken 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 gain region. In this way, the additional passive optical processing functions may be integrated in the same substrate as the comb-like reflectors on one side of the active device section. The optical coupling of the substrates containing the active and the passive device sections is thus relatively easy. It is noted that there is a synergic effect in the combination of the separation of the substrates in which the active device section and the passive device section are integrated, and the integration of the further passive optical processing functions in the passive section of the device already provided for forming the comb-like reflectors. 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.

However, in alternative invention embodiments, the laser output may be taken from an end of the optical gain region opposite to the end coupled to the comb-like reflectors; in this case, the further passive optical processing functions are provided on the side of the laser cavity opposite to that where the comb-like reflectors are formed.

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 26. The process comprises:

- on a first substrate, 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, 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 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 output ports.

The first substrate and the second substrate are physically distinct from each other, the process further comprising butt-coupling the first and second substrates so that the second end of the gain waveguide element is optically aligned to the input port of the optical power splitter.

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, with active and passive device sections realized in distinct substrates;

Figure 2 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, further including optical processing functions integrated in the substrate of the passive device section; Figure 3 schematically shows in top-plan view an optical device according to an embodiment of the present invention, falling within the general scheme of Figure 2;

Figure 4 is a schematic cross-sectional view of the optical laser source of Figure 3 along the plane denoted IV-IV;

Figure 5 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 3;

Figures 6A, 6B, 6C schematically show alternative reflectors adapted to be used in the optical device of Figures 1, 2 or 3;

Figure 7 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 2; Figure 8 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 2; and

Figure 9 schematically shows in top-plan view, and partly in terms of functional blocks, a general scheme of an optical device according to an alternative embodiment of the present invention, further including optical processing functions integrated in the substrate of the passive device section, but located at the opposite side of the comb-like reflectors.

Detailed description of the preferred embodiments(s)

In the following, several possible invention embodiments will be presented, making reference to the drawings. It is pointed out that identical or similar elements or parts in different drawings will be identified by same reference numerals, even if belonging to different invention embodiments.

With reference to 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 active section 105a and the passive section 105b are integrated in respective, distinct active section (preferably semiconductor) substrate 110a and passive section (preferably semiconductor) substrate 110b, respectively. The two substrates 110a and 110b may for example be mounted on a common supporting submount, not depicted in the drawing, or the substrate 110b may act as the submount for the substrate 110a, or wee versa. The active section 105a includes, formed on the semiconductor substrate 110a, 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 110a, 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 passive section 105b may be realized using the silicon, particularly Silicon-On- lnsulator (SOI) technology. Alternatively, the passive section 105b may be realized using the Nl-V InP technology, as the active section 105a.

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 11Oa; 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 110a, possibly coated with a suitable high-reflectivity coating.

At a second end thereof opposite to the first end, the optical gain structure 115 terminates at a second cleaved facet 130 of the substrate 110a, opposite the first facet 125, 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 135 of the gain structure 115is lower than 40%, preferably lower than 15%, even more preferably lower than 5%.

The substrate 110b is arranged in proximity relationship with, particularly butt-coupled to the active substrate 110a, so as to have a first facet 140 facing the second facet 130 of the substrate 110a. An optically-reflecting structure is integrated on the substrate 110b; the optically-reflecting structure comprising a first optical waveguide segment 145, 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-N.

The first optical waveguide segment 145 has a first, free end terminating at the first facet 140 of the substrate 110b, facing the gain structure 115. An antireflection coating on the first free facet 140 of the optical waveguide segment 145 may be also provided having the same function of the antireflection coating 135 described above.

The "1 x N" optical power splitter 150 has an input port optically coupled to the second end of the first optical waveguide segment 145 and is adapted to (preferably substantially equally) split the optical power received from the optical gain structure 115 into a number N of (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

(preferably substantially equal to 1/Λ0 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%. In case of N equal to 2, this means an acceptable power splitting ratio of about 40/60 instead of 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 Figure 5 for an example). For example, the comb-like optical reflectors160-1,..., 160-W may include tunable Super-Sampled Gratings (SSGs). In particular, the tuning of the comb-like reflectivities may be accomplished by thermo-optic effect, e.g. by way of thin-film heaters, or alternatively by means of carrier injection. Alternatively, tuning may be also provided through the electro-optic effect in the case of IN-V technology. In this case, no injection of free carriers is used, but instead a strong electric field is applied to the waveguide with the use of appropriately placed electrodes. The strong electric field applied to the material forming the waveguide results in a modulation of the band-gap and of the index of refraction, and thus into a tuning of the grating section. The electro-optic effect in silicon is insufficient to provide wide tunability, 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 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 waveguide 145, 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 , ... , 1 QO-N.

Preferably, along one or more of the N optical waveguide segments 155-1,..., 155-W, one or more phase adjustment elements may be provided, Jike 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-W and propagating along the Λ/ optical waveguide segments 155-1,..., 155-ΛMs properly adjusted. The laser light can be outputted from the end of the gain structure 115 where there is the wide-band mirror 120, or alternatively it can be taken from one or more of the comb-like optical reflectors 160-1,..., 160-Af, as will be discussed in greater detail later in connection with the several invention embodiments that will be presented.

The active and passive sections 105a and 105b, being integrated in distinct substrates 110a and 110b, can be manufactured independently from each other. This allows adopting, for each of the active and passive sections, the most suitable technological process, adapted to optimize the respective performances, without having the performances of one device section impaired by the manufacturing steps required to form the other section. Also, the two device sections can be tested autonomously, before being assembled, and this increases the manufacturing process yield and ultimately reduces the device cost.

Passing now to Figure 2, a general scheme of an optical device 200 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 200 is, similarly to the device 100 of Figure 1, 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. In particular, in the device 200 the laser light is taken from the external comb-like reflectors. Preferably, in this case 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%.

The optical device 200, further to the device of Figure 1, includes, integrated in the substrate 110b of the passive section 105b, and located 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 263-1,..., 263-W integrated in the substrate 110b, each one being optically coupled to a respective comb-like optical reflectors 160-1,..., 160-W. The N optical waveguide segments 263-1,..., 263-W converge towards, and are each one optically coupled to a respective input port of an "N x 1" optical power combiner 265, apt to recombine the optical power transmitted by the comb-like optical reflectors 160-1,..., 160-W.

Preferably, along one or more of the 2Λ/ optical waveguide segments 155-1,..., 155-W and

263-1,..., 263-W, one or more phase adjustment elements may be provided, like the phase adjustment element schematically depicted as a broken line block 257 and 267 in the drawing, so to adjust the relative phase of the optical radiations propagating along the 2Λ/ optical waveguide segments 155-1,..., 155-Wand 263-1,..., 263-W.

The optical power combiner 265 couples the received optical power and makes it available at an output port thereof, to which an end of an optical waveguide segment 270 is optically coupled.

It can be appreciated that the power splitter 150, the waveguide 145, the optical waveguide segments 155-1,..., 155-W, the comb-like optical reflectors 160-1,..., 160-W, the N optical waveguide segments 263-1,..., 263-W, the phase adjustment element(s) 257 and 267 and the power combiner 265 form an Λ/-arms Mach-Zehnder Interferometer (MZI) for the transmitted light.

Optically coupled to the optical waveguide segment 270, one or more passive optical processing functional devices are integrated in the substrate 110b, which in the drawing are globally schematized as a block 275. In particular, and merely by way of example, the optical processing functions 275 may include a notch filter, and/or a pass band filter and/or an all pass filter.

Downstream the optical processing functions 275, the optical radiation is coupled into an output waveguide segment 280, integrated in the substrate 110b and terminating, at a free end thereof, at a facet 285 of the substrate 110b. The output light of the optical device 200 is taken from the facet 285 (the substrate facet from which the light is outputted is not limitative for the present invention).

By separating the active and passive sections, forming them in distinct, optically-coupled substrates, it is possible to optimize the optical processing functions 175, integrating in the passive section several cascaded optical elements, including filters.

For example, the additional, integrated optical processing functions may provide for having optical transfer functions with sharp features such as for example 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 would not be easily obtained within the same process producing the active device section, due to the need of the additional processing steps for its manufacturing.. In the following, some examples of optical devices falling within the general scheme of Figure 2 will be presented.

In Figure 3, an optical device 300 according to an embodiment of the present invention is shown schematically, in top-plan view, wherein the integer N is equal to two. Also in the device 300, the laser light is taken from the external comb-like optical reflectors (as shown by the left-to-right arrow of Figure 3).

In the optical device 300 the first optical waveguide segment 145 is connected, at the second end thereof, to a power splitter 350, adapted to substantially equally split the optical power propagating through the first optical waveguide segment 145 and to couple respective optical power fractions into two second optical waveguide segments 355-1, 355-2 (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 115 into the waveguide segment 355-1, half into the waveguide segment 355-2. Preferably, the power splitting is within 40/60, more preferably within 45/55. Any known suitable power splitter or coupler 150 may be used instead of the Y-branch splitter shown in the 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 355-1, 355-2, which are designed to suitably diverge in leaving the power splitter 350, a respective comb-like optical reflector 360-1, 360-2 is formed. In the exemplary embodiment of Figure 3, the comb-like optical reflectors 360-1 and 360-2 include tunable SSGs, in particular realized using SOI technology, whose tuning may be accomplished by thermo-optic effect. To this purpose, heaters 393 and 395, schematically depicted as rectangles in dashed line in the drawing, may be provided on the two comb-like optical reflectors 360-1 and 360-2. The heaters may in particular be formed as thin-film heaters. The two comb-like optical reflectors 360-1 and 360-2 and the power splitter 350 form an additive Vernier-effect optically-reflecting structure; the optical gain structure 115, with the wideband mirror 120 at an end thereof, the power splitter 350, the optical waveguide segments 355-1, 355-2 and the comb-like optical reflectors 360-2, 360-2 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 360-1 , 360-2.

Downstream the two comb-like optical reflectors 360-1, 360-2, in the direction of propagation of the outputted optical radiation (left-to-right direction in the drawing), two third optical waveguide segments 363-1, 363-2 are integrated in the substrate 11Ob, each one being optically coupled to a respective one of the two comb-like optical reflectors 360-1, 360-2. The two third optical waveguide segments 363-1, 363-2 converge towards, and are each one optically coupled to a respective input port of an optical power combiner 365, recombining the optical power transmitted by the comb-like optical reflectors 360-1, 360-2. Preferably, the optical power combiner 265 is structurally substantially identical to the optical power splitter 250.

The power combiner 365 couples the recombined optical power into the optical waveguide segment 170. Moving along the optical waveguide segment 170, one or more passive optical functional devices are integrated in the substrate 110b; in the exemplary embodiment herein considered, a narrow pass band filter 375 is formed, exemplarily implemented by means of a ring resonator. Any suitable resonator may be used instead of the ring resonator 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 375 is coupled into the output waveguide segment 280 which, at a free end thereof, terminates at the second facet 285 of the substrate 110b, where the output light of the optical device 300 is taken. 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 directly taken 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 120, 115, 350, 355-1, 355-2, 360-2, 360-2 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. 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. Other methods 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 11Ob may for example be a silicon-on- insulator substrate (SOI); the waveguides 145, 355-1, 355-2, 363-1, 363-2, 270 and 280 may be formed in correspondence of the top silicon layer. In particular, the waveguides 145, 355-1, 355-2, 363-1, 363-2, 270 and 280 may be buried rectangular waveguides or ridge waveguides, as in the example herein considered and depicted in Figure 4: the passive SOI substrate 110b includes a silicon substrate 405, a silicon dioxide layer 410 and over the silicon dioxide layer 410, a silicon core layer 415. A waveguide is formed by a patterning the silicon core layer, so as to form a slab 420 and a ridge 425. A further silicon dioxide layer 430 covers the structure.

In particular, in a preferred waveguide structure suitable for the waveguides 145, 155- 1 ,...,155-W, which readily couples to the active section 105a as it features a closely matched mode size, the ridge 425 width is approximately 500 nm, the ridge height is approximately 600 nm, the slab 420 thickness is approximately 250 nm, the silicon dioxide layer 410 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 425 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 6A, 6B and 6C.

The upper silicon dioxide cladding layer 430 is adapted to mechanically hold and optically separate the thin film heaters, e.g.393 and 395.

Preferably, one or more additional, e.g. thin-film, heaters are provided. Referring back to Figure 3, 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 390 placed on at least part of the first waveguide segment 145 and the power splitter 350, a second additional heater 392 placed on a portion of the second optical waveguide segment 355-2, a third additional heater 397 placed on at least a portion of the second optical waveguide segment 363-2 downstream the optical reflector 360-2, and a fourth additional heater 399 placed over the filter 375. The first additional heater 390 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 390, a thermo-optic fine tuning of the gain waveguide 115, slightly changing the bias current of the gain section 105a. The second additional heater allows for finely balancing the optical length of the two second optical waveguide segments 355-1, 355-2, so as to optimize the proper function of the reflecting structure 350, 355-1, 355-2, 360-1, 360-2. The third additional heater allows for finely balancing the overall optical length of the paths formed respectively by optical waveguide segments 355-1, 363-1 and 360-1 and by 355-2 363-2 and 360-2, so as to optimize the transmitted power, and thus achieve improved power uniformity (as already pointed out in connection with Figure 2, the two second optical waveguide segments 355-1, 355-2, 363-1, 363-2 and the power splitter 350 and power combiner 365 form a Mach-Zehnder interferometer). The third additional heater 399 allows finely aligning the pass band filter (or, alternatively, the notch 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 different material compositions which include high contents of germanium, 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 diagram of Figure 5 shows exemplary reflectivity (solid curve 505) and transmission (solid curve 510) responses of the optically-reflecting structure (reflectors 360-1 and 360-2 and splitter 350) of Figure 3, 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 5 the dotted line shows the reflectivity response of the SSG having 155 λ_o/2 super periodicity, giving an FSR fore 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 425 width or the height, by about 10 nm. Alternatively, a grating in the slab 420 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 375 implemented by the ring resonator 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 6A, 6B and 6C 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 6A, they are formed by at least a resonator 600 coupled to the respective waveguide segment 255-1 or 255- 2 Wa an optical coupler (in Figure 6A a directional coupler formed by bringing the resonator 600 near to the waveguide by a small gap 602). The structure 605 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 600. Figures 6B and 6C schematically depict exemplary embodiments of the general structure of Figure 6A wherein the low reflectivity, wide- band mirror 605 is formed by a grating 610. For example, a first-order grating 610 realized with 20 periods and giving about 4 % of reflectivity and 92 % of transmission, combined with a ring 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 4, using sections of different widths, differing by about 140 nm. Large coupling ratios between the resonator

600 and the waveguides 355-1, 355-2 may be obtained either by using directional couplers with very small gaps 602, as shown in Figures 6A and 6B, of the order of magnitude of about 100 nm, or, preferably, by using multi-mode couplers 608 (as shown in Figure 6C), which exhibit a coupling ratio which is substantially independent from the wavelength across the C-band and are shorter than directional couplers. The integrated resonating structures of Figures 6A to 6C 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 275 compared to the embodiment of Figure 3. The two further embodiments are schematically depicted in Figures 7 and 8, wherein parts being identical or equivalent to those of the embodiment of

Figure 3 are denoted by the same reference numerals.

In particular, in the embodiment of Figure 7, an optical pass-band filter 705 is realized by cascading two Mach-Zehnder filters 710-1 and 710-2 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 715-1 and 715-2 are preferably provided over the arms of the two Mach- Zehnder filters 710-1 and 710-2: 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 8, similarly to the embodiment of Figure 7, an additional optical processing function 805 including two Mach-Zehnder filters 810-1 and 810-2 is provided, with heaters 815-1 and 815-2 preferably provided over the arms thereof for thermo-optically tuning the arm length. However, the first Mach-Zehnder filter 810-1 is integrated with the structure that in Figure 7 is formed by the optical splitter 350, the tunable optical reflectors 360-1 and 360-2, the waveguide sections 363-1 and 363-2 and the optical power combiner 365, so as to further reduce the overall device size and losses. In particular, In the embodiment of Figure 8 the third optical waveguide segment 363-2 of Figure 7 is modified into a third optical waveguide segment 863-2 having an extra length, and the optical power combiner 365 of Figure 7 is realized with a directional coupler having two output ports. Downstream, the second Mach-Zehnder filter 810-2 is formed as previously described in connection with Figure 7. Heaters 815-1 and 815-2 are preferably provided over the MZI arms for thermo-optically tuning the arm length.

In the further embodiment of Figure 9, a device 900 similar to that depicted in Figure 1 is shown, wherein the laser output light is taken from the end 120 of the optical gain region 115. Preferably, in this case the wide-band mirror 120 has a reflectivity of less than 50%, more preferably less than 40%, even more preferably less than 30% Additional integrated optical processing functional devices 975 (such as optical filters) are integrated in a substrate 110c, which is distinct from the substrate 110a, and may be the same substrate 110b wherein the external comb-like reflectors are integrated (for example, a recessed seat may be formed in the substrate 110b,c for accommodating the substrate 11Oa). A waveguide section 970 is integrated in the substrate 110c, having an end at a free facet 987 of the substrate 110c optically coupled to the end 120 of the gain section 115, and another end optically coupled to the optical processing functions 975. The additional integrated optical processing functions 975 may be structurally and/or functionally equal to the devices shown in Figures 3, 7 and 8. A waveguide section 980 exits from the optical processing functions 975 and terminates at a free facet 989 of the substrate 110c, where the laser light is taken.

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;200;300;900) comprising:

- an optically active waveguide element (115) having a first end (120) and a second end (135); and

- an optical power splitter (150;350) 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 in that said optically active waveguide element is integrated in a first substrate (110a), and said optically- reflecting structure is integrated in a second substrate (110b) distinct from and optically coupled to said first substrate.

2. The integrated optical device of claim 1 , wherein said optically active waveguide element is formed in Nl-V technology.

3. The integrated optical device of claim 1 or 2, wherein the optically-reflecting structure is realized in silicon-on-insulator technology.

4. The integrated optical device of claim 1 or 2, wherein the optically-reflecting structure is realized in a electro-optical material such as low silicon content germanium-silicon material.

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

6. The integrated optical device of claim 3 or 4 or 5, wherein said comb-like reflectors are in a thermo-optical material, and include a respective thermal heater (393,395).

7. The integrated optical device of any one of the preceding claims, further comprising a plurality of optical waveguides (163-1 163-W; 263-1 263-Λ/), said optical waveguides being each optically coupled to a second end of a respective one of said comb-like reflectors, said second end being opposite to the first comb-like reflector end.

8. The integrated optical device of claim 7, further comprising an optical power combiner (265;365) having a plurality of input ports optically coupled to a respective one of said plurality of optical waveguides, and an output port, wherein the optical power combiner is adapted to combine a plurality of optical powers received at the input ports and to make the combined optical power available at the output port.

9. The integrated optical device of claim 8, comprising a further optical waveguide (170;270) optically coupled to the optical power combiner output port.

10. The integrated optical device of any of the preceding claims, further comprising at least one optical processing function (275;705;805;975) optically coupled to at least one of said plurality of optical waveguides (163-1 163-W; 263-1 263-Λ/).

11. The integrated optical device of claim 10 when depending on claim 9, wherein said optical processing function (275;705;805;975) is optically coupled to said further optical waveguide (170).

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

13. The integrated optical device of claim 12, wherein said waveguide grating includes a super-sampled grating.

14. The integrated optical device of any one of claims 1 to 11, wherein at least one of said comb-like reflectors includes a Fabry-Perot resonator.

15. The integrated optical device of any one of claims 11 to 14, wherein said at least one optical processing function includes an optical filter.

16. The integrated optical device of claim 15, wherein said optical filter includes a ring or racetrack resonator.

17. The integrated optical device of claim 15, wherein said optical filter includes at least one Mach-Zehnder filter.

18. The integrated optical device of any one of claims 15 to 17, 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.

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

20. The integrated optical device of any one of claims 7 to 19 when depending on claim 2, wherein said plurality of optical waveguides includes N optical waveguides.

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

22. 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).

23. The integrated optical device of claim 22, wherein said high reflectivity element has a reflectivity higher than approximately 30%, preferably higher than approximately 60%, more preferably higher than approximately 90%.

24. The integrated optical device of claim 22 or 23 wherein the optically active waveguide element (115;215), the high reflectivity element (120) and the optically-reflecting structure form an optical cavity.

25. 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.

26. A process of manufacturing an integrated optical device, comprising:

- on a first substrate (110a), integrating an optically active waveguide element (115), wherein said integrating includes forming the optically active waveguide element with a first end (120), and a second end (135);

- on a second substrate (110b), integrating an optically-reflecting structure including: - an optical power splitter (150;350) 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 output ports; and a plurality of comb-like reflectors (160-1, ...160-W;360-1, 360-2), each one optically coupled, at a respective first end thereof, to a respective one of said output ports, characterized in that the first substrate (110a) and the second substrate (110b) are physically distinct from each other, the process further comprising butt-coupling the first and second substrates so that the second end of the gain waveguide element is optically aligned to the input port of the optical power splitter.

27. The process of claim 26, further comprising: integrating, in the second substrate, at least one optical waveguide (263-1,..., 263- W; 363- 1,363-2) optically coupled to a second end of at least one of said comb-like reflectors, said second end being opposite to the first comb-like reflector end.

28. The process of claim 26 or 27, wherein the optically active waveguide element is realized in Nl-V technology, and the optically-reflecting structure is realized in silicon-on-insulator technology.

29. The process of any one of claims 26 to 28, wherein said first end of the optically active waveguide element is formed highly reflective.