Optical waveguide ring resonator with an intracavity active element

An optical resonator, a photonic system and a method of optical resonance employ optical waveguide segments connected together with total internal reflection (TIR) mirrors to form a closed loop. The optical resonator includes the optical waveguide segments, an intracavity active element coupled to a designated one of the optical waveguide segments, the TIR mirrors and a photo-tunneling input/output (I/O) port. The photo-tunneling I/O port includes one of the TIR mirrors. The method includes propagating and reflecting the optical signal, or a portion thereof, in the optical resonator, transmitting a portion of the optical signal through the I/O port, and influencing the optical signal. The photonic system includes the optical resonator with optical gain and a source of an optical signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to patent application of Wu et al., entitled “Optical Waveguide Ring Resonator with Photo-Tunneling Input/Output Port” Ser. No. 11/829042, filed concurrently herewith, which is incorporated by reference herein in its entirety.

BACKGROUND

1. Technical Field

The invention relates to optical waveguides and photonic circuits. In particular, the invention relates to optical waveguide resonators used in photonic devices and systems.

2. Description of Related Art

An optical ring resonator generally comprises a segment or segments of optical waveguide arranged as a closed loop or ring. The closed loop forms a resonant cavity. Such ring resonators have a variety of important uses in photonic devices and systems. In particular, the optical ring resonator may serve as an active photonic device having one or more active optical components or elements integrated within the resonant cavity of the ring resonator (i.e., an ‘intracavity’ active element(s)). The intracavity active element(s) integrated with the ring resonator may result in ‘cavity enhancement’ of a functionality of the active elements, for example. Such intracavity active elements may be employed to one or more of generate, amplify, and modulate an optical signal of the ring resonator.

The intracavity active elements that are integrated with a ring resonator may be generally classified as either saturable active elements or unsaturable active elements. Saturable active elements include, but are not limited to saturable absorbers, laser and other optical amplifiers. For example, the saturable active element may be a continuous-wave laser amplifier or a directly-modulated laser amplifier that is modulated by an applied electric current. In another example, the intracavity active elements may comprise a laser amplifier in combination with a saturable absorber. The saturable absorber may assist in mode-locking the laser within the resonant cavity (i.e., laser cavity) formed by the ring resonator, for example. An example of an unsaturable active element is an electro-absorption modulator (EAM). Incorporated into the resonant cavity of the ring resonator, the EAM becomes a ‘cavity enhanced’ EAM.

Among the challenges of integrating an intracavity active element into an optical ring resonator is providing sufficient coupling between the intracavity active optical element and a mode or modes of the optical signal propagating in the optical waveguide of the ring resonator. Generally, the coupling must be sufficient to provide for proper and/or efficient operation of the intracavity active optical element. For example, a modulation depth or ratio of an EAM is directly dependent on a percentage of the optical signal that is coupled into the EAM. Sufficient coupling may be difficult to achieve in many cases.

Another challenge of integrating an intracavity active optical element into an optical ring resonator is choosing appropriate materials for the integrated structure. Specifically, an ideal or highly desirable material for realizing the optical waveguide may be less than ideal for or even incompatible with an implementation of the intracavity active element. For example, while silicon may be a relatively attractive material (e.g., low cost) for realizing and implementing the optical waveguide of the optical resonator, silicon is an indirect band gap material and, as such, is not particularly well-suited for constructing active optical elements.

BRIEF SUMMARY

In some embodiments of the present invention, an optical resonator is provided. The optical resonator comprises a plurality of segments of an optical waveguide. The optical resonator further comprises an intracavity active element. The intracavity active element overlies and is optically coupled to a designated segment of the plurality of segments. The optical resonator further comprises a plurality of total internal reflection (TIR) mirrors. A number of TIR mirrors in the plurality of TIR mirrors is equal to a number of segments in the plurality of segments. The optical resonator further comprises a photo-tunneling input/output (I/O) port. The plurality of segments of the optical waveguide is arranged as a closed loop wherein successive pairs of the segments are connected to one another at respective segment ends by successive ones of the TIR mirrors. The photo-tunneling I/O port comprises a designated TIR mirror of the successive TIR mirrors of the plurality of TIR mirrors.

In other embodiments of the present invention, a photonic system is provided. The photonic system comprises an optical source that produces an optical signal and an optical resonator that receives the optical signal. The optical resonator comprises a plurality of segments of an optical waveguide, an intracavity active element, a plurality of total internal reflection (TIR) mirrors, and a photo-tunneling input output (I/O) port. The intracavity active element is optically coupled to a designated segment of the plurality of segments. A number of TIR mirrors in the plurality of TIR mirrors is equal to a number of segments in the plurality of segments. The plurality of segments of the optical waveguide is arranged as a closed loop. The TIR mirrors connect segment ends of respective successive pairs of the segments of the optical waveguide. The photo-tunneling I/O port comprises a designated TIR mirror of the plurality of TIR mirrors.

In other embodiments of the present invention, a method of optical resonance is provided. The method of optical resonance comprises propagating an optical signal along a plurality of segments of an optical waveguide. The segments of the plurality are arranged as a closed loop forming a ring resonator. Successive pairs of the segments are connected to one another at respective segment ends by successive ones of an equivalent plurality of total internal reflection (TIR) mirrors. The method of optical resonance further comprises reflecting a portion of the optical signal at a TIR mirror of the equivalent plurality. The method further comprises influencing the optical signal within an intracavity active element that overlies a designated segment of the plurality of segments. The method further comprises transmitting a portion of the optical signal out of the ring resonator through a photo-tunneling input/output (I/O) port. The photo-tunneling I/O port comprises a designated one of the TIR mirrors of the equivalent plurality.

Certain embodiments of the present invention have other features that are one or both of in addition to and in lieu of the features described hereinabove. These and other features of the invention are detailed below with reference to the following drawings.

DETAILED DESCRIPTION

Embodiments of the present invention employ optical waveguide segments and total internal reflection (TIR) mirrors to realize an optical resonator having low optical loss. In particular, the optical resonator of the present invention is a closed loop, ring-shaped resonant structure that supports a propagating optical signal within the closed loop. In various embodiments of the optical resonator according to the present invention, one or more input/output ports (I/O ports) are provided. The I/O port(s) facilitate one or both of introducing to and extracting from the optical resonator the propagating optical signal or a portion thereof.

Further, the optical resonator provides active optical properties. In particular, an intracavity active element that is integrated with and optically coupled to the optical resonator. Through the optical coupling, the intracavity active element influences (e.g., generates, amplifies or modulates) an optical signal propagating within the optical resonator. The intracavity active element overlies one or more of the optical waveguide segments, according to various embodiments. The intracavity active element is also referred to herein as an active optical element.

The optical resonator with the intracavity active element of the present invention has a relatively compact and space-efficient form factor, according to various embodiments herein. Moreover, the optical resonator with the intracavity active element is readily fabricated in an integrated circuit (IC) form as part of a larger IC circuit or subsystem. For example, the optical resonator with the intracavity active element is well-suited to fabrication on or in a substrate such as, but not limited to, a multilayer semiconductor substrate. Fabrication on or in the substrate facilitates integrating the optical resonator with other photonic components including, but not limited to, one or more of passive photonic components and active photonic components.

In some embodiments, the optical resonator may be fabricated directly in a surface layer (e.g., thin film layer) of a semiconductor substrate. Further, the intracavity active element may be fabricated as a layer affixed to a top surface of the surface layer, according to various embodiments. For example, the optical resonator may be fabricated in a thin film semiconductor layer of a semiconductor-on-insulator (SOI) substrate (e.g., a silicon or polysilicon thin film layer of a silicon-on-insulator substrate). The intracavity active element may be fabricated using another semiconductor layer that is wafer bonded or otherwise affixed to the top surface of the SOI substrate, for example. Through wafer bonding, the wafer-bonded semiconductor layer may comprise a semiconductor material that differs from, and that even has a lattice that is dissimilar to, the semiconductor material of the surface layer of the semiconductor substrate. For example, the semiconductor material of the surface layer may be silicon while the wafer-bonded semiconductor layer may be a III-V compound semiconductor (e.g., GaAs, AlGaAs, InP, InGaAs, or InGaAsP) or even a II-VI compound semiconductor. Silicon and III-V or II-VI compound semiconductors have dissimilar or mismatched lattices.

In various embodiments, the intracavity active element may be either a saturable active element or an unsaturable active element and function as one or more of an optical source/amplifier and an optical modulator. The term ‘optical’ as used in ‘optical’ source/amplifier and ‘optical’ modulator herein generally refer to one or both of a device and a structure that operates directly on an optical signal without prior conversion of the optical signal into an electrical signal. For example, the optical source/amplifier may be a saturable active semiconductor device that directly generates and/or amplifies an optical signal through stimulated emission within the semiconductor device (e.g., a laser without mirrors). Such devices are sometimes referred to as semiconductor optical amplifiers (SOAs). The optical modulator may be an electro-absorption modulator (EAM), for example.

Similarly, other photonic components other than the intracavity active element may be fabricated on or in the same semiconductor substrate, according to some embodiments. Alternatively, the other photonic components may be fabricated in or on the wafer bonded semiconductor layer of the intracavity active element. Such photonic components include, but are not limited to, optical signal transmission structures (e.g., other optical waveguides), optical switches and optical detectors.

According to some embodiments, the optical resonator employs a slab optical waveguide. In some embodiments, the slab optical waveguide comprises a ridge-loaded optical waveguide. According to other embodiments, the slab optical waveguide comprises an inverted or reverse ridge-loaded optical waveguide. In yet other embodiments, other optical waveguides besides a slab optical waveguide is employed. Such ‘non-slab’ type optical waveguides may include, but are not limited to, a strip optical waveguide.

As used herein, ‘slab optical waveguide’ refers to an optical waveguide in which the propagating optical signal is confined to and propagates within a slab or sheet of dielectric material. As such, the slab optical waveguide is a member of the class of dielectric waveguides. The slab optical waveguide is also referred to as simply a ‘slab waveguide’ herein.

In some embodiments, a thickness of the slab waveguide is selected to preferentially sustain a lower order propagating mode of the optical signal. For example, the thickness may be less than a particular thickness such that only a first transverse electric mode (i.e., TE10) can propagate. The particular thickness depends on a refractive index of a material of the slab waveguide as well as specific physical characteristics of the slab waveguide (i.e., optical waveguide type). Design guidelines and equations are readily available for establishing the particular thickness for a given refractive index and optical waveguide type.

FIG. 1Aillustrates a cross sectional view of an exemplary conventional slab optical waveguide known as a ridge-loaded optical waveguide10. The ridge-loaded optical waveguide10is also sometimes referred to as a ‘ridge-loaded waveguide’ or simply a ‘ridge waveguide’. The ridge-loaded waveguide comprises a slab layer12. In some embodiments, the slab layer12may comprise a thin film layer deposited on an underlying layer or an underlying supporting substrate (not illustrated). In such embodiments, the ridge-loaded waveguide10may be termed a ‘thin film’ ridge-loaded waveguide10.

The slab layer12comprises a dielectric material through which an optical signal propagates and is guided within the ridge-loaded waveguide10. In particular, essentially all of the energy of the optical signal is confined to the slab layer12. In general, the slab layer12comprises a dielectric material or a semiconductor material, which behaves essentially as a dielectric material with respect to its use in an optical waveguide. Moreover, the material of the slab layer12is essentially transparent to the optical signal.

For example, the slab layer12may comprise a semiconductor material that is compatible with the optical signal such as, but not limited to, silicon (Si), gallium arsenide (GaAs), and lithium niobate (LiNbO3). Dielectric materials used for the slab layer12may include, but are not limited to, glass (e.g., borosilicate glass) and various polymers (e.g., polycarbonate). Any of a single crystalline, polycrystalline or amorphous layer of the dielectric material or the semiconductor material may be employed, according to various embodiments. The transparency of the slab layer material affects an optical loss of the ridge-loaded waveguide. For example, the less transparent the material, the more loss is experienced by the optical signal.

In some embodiments, the slab layer12is supported by a support layer14. The support layer14physically supports the slab layer12. In some embodiments, the support layer14also facilitates optical confinement in the slab layer12. In some embodiments, the support layer14may comprise a material that differs from the material of the slab layer12. In particular, the support layer14may comprise a material having a refractive index that is different from a refractive index of the slab layer12. For example, the support layer14may be an oxide-based insulator layer (e.g., silicon oxide). In another example, the support layer14is an insulator layer of an SOI substrate. In some embodiments, the different refractive index of the support layer14serves to essentially confine the optical signal to the slab layer12.

The ridge-loaded waveguide10further comprises a ridge16. The ridge16is located on and extends above a top surface of the slab layer12. The ridge16serves to ‘guide’ the optical signal within the slab layer12. In particular, essentially all of the optical energy of the optical signal is concentrated below but adjacent to the ridge16within the slab layer12. For example, the optical signal may be essentially concentrated in a roughly circular region below the ridge16, as illustrated by an exemplary dashed circle inFIG. 1A.

The ridge16may be formed by one or more of an etching process, a selective deposition process, or a printing process, for example. A particular width and height of the ridge16are generally a function of a refractive index of the slab layer12material. Information for determining the width and the height may be readily obtained from conventional design guidelines and using computer design models for ridge-loaded optical waveguides.

FIG. 1Billustrates a cross sectional view of another exemplary conventional slab waveguide known as a reverse ridge-loaded optical waveguide20. The reverse ridge-loaded optical waveguide20is also referred to simply as a ‘reverse ridge-loaded waveguide’ or a ‘reverse ridge waveguide’ herein.

The reverse ridge-loaded waveguide20comprises a slab layer22. The reverse ridge-loaded waveguide20further comprises a support layer24. The support layer24comprises a material having a refractive index that differs from a refractive index of the slab layer22. The slab layer22is essentially similar to the slab layer12of the ridge-loaded waveguide10described above, according to some embodiments. Further, the support layer24may be essentially similar to the support layer14of the ridge-loaded waveguide10described above, according to some embodiments. In some embodiments, the slab layer22comprises a thin film layer deposited on the support layer24, wherein the support layer24is lying on a supporting substrate (not illustrated). In such embodiments, the reverse ridge-loaded waveguide20may be termed a ‘thin film’ reverse ridge-loaded waveguide20.

The reverse ridge-loaded waveguide20further comprises a ridge26. The ridge26extends from an interface between the support layer24and the slab layer22down and into the support layer24. As with the ridge16of the ridge-loaded waveguide10described above, the ridge26of the reverse ridge-loaded waveguide20serves to guide the optical signal within the slab layer22. An exemplary dashed circle above but adjacent to the ridge26illustrates an approximate extent of the optical signal energy associated with the optical signal propagating in the reverse ridge-loaded waveguide20, by way of example.

FIG. 1Cillustrates a cross sectional view of an exemplary conventional strip optical waveguide30. The strip optical waveguide30, or simply ‘strip waveguide’, comprises a strip layer32and a support layer34. The strip optical waveguide30further comprises a strip36formed in or from the strip layer32. In particular, the strip36may be formed in the strip layer32by etching channels38to define the strip36, as illustrated inFIG. 1C. The channels38optically isolate the strip36from the strip layer32. In other embodiments, the strip36comprises the entire strip layer32(not illustrated). For example, the strip layer32may be essentially removed by etching to leave only the strip36during fabrication. As such, channels are not formed.

In contrast to the slab waveguides10,20, the optical energy within the strip waveguide30is essentially confined to the strip36by the presence of sidewalls39of the strip36. A material boundary exists at the sidewalls39between a material of the strip layer32and air or another dielectric material within the channels38. The boundary represents a change in a refractive index across the boundary. The refractive index change causes an optical signal to be tightly bound to the strip36due to total internal reflection therewithin. A dashed circle within the strip36illustrates an approximate extent of the optical energy associated with the optical signal propagating in the strip waveguide30, for example.

The optical resonator according to some embodiments of the present invention may be referred to as a ‘folded cavity’ resonator because mirrors are employed along an optical path within the optical resonator. In particular, mirrors are employed to introduce an abrupt change in a direction of the propagating signal within the optical resonator. In other words, an optical path within the resonator is effectively ‘folded’ by a presence of the mirror. Essentially, the mirrors compensate for weak guiding interaction of the optical waveguides that make up the optical resonator. As such, the mirrors allow the optical resonator to be realized in a more compact and space efficient shape than would be possible otherwise. Total internal reflection mirrors are employed to realize the folded cavity of the optical resonator, according to some embodiments.

A total internal reflection mirror (TIR mirror) is defined as a mirror that reflects or changes a direction of an optical signal using total internal reflection. Total internal reflection is a well-known optical phenomenon. Total internal reflection of an optical signal traveling in a material occurs when the optical signal encounters a material boundary at an angle greater than a critical angle relative to a normal of the boundary. In particular, when the material boundary represents a change in refractive index from a higher refractive index to a lower refractive index, the optical signal will be essentially unable to penetrate the boundary and will be reflected back away from the boundary. The reflection obeys the law of reflection in that a reflection angle equals an angle of incidence on the boundary. An example of a boundary that may provide total internal reflection and thus, be employed as a TIR mirror is a boundary between a dielectric material (e.g., glass or silicon) and air.

The optical resonator according to some embodiments herein employs a photo-tunneling input/output (I/O) port to one or both of introduce optical signals into and extract (i.e., couple) optical signals from the optical resonator. As used herein, a ‘photo-tunneling I/O port’ is a junction between optical waveguide segments that transmit a propagating signal through a photo-tunneling junction. A photo-tunneling junction is a junction that employs a non-propagating or evanescent optical mode to transmit optical energy across the junction.

For example, a gap or channel may be introduced in an optical waveguide that separates the waveguide into two segments. The gap may comprise an air gap or channel that physically separates the two waveguide segments. The gap, having a different (e.g., lower) refractive index than the refractive index of a material of the waveguide segments, effectively causes a reflection of an optical signal incident on the gap. In particular, the presence of the gap interrupts propagation of the optical signal along the optical waveguide. However, if the gap has an extent that is generally less than, and typically much less than, a wavelength of the optical signal, a non-propagating mode or evanescent field of the optical signal may essentially jump or ‘tunnel’ across the gap. The tunneling enables the optical signal to propagate from a first segment of the optical waveguide on one side of the gap to a second segment of the optical waveguide on the other side. Moreover, by controlling a width of the gap or a distance between the two optical waveguide segments, an amount of the optical signal that tunnels across the photo-tunneling junction may be controlled or adjusted.

According to some embodiments of the present invention, the photo-tunneling I/O port comprises a TIR mirror, a gap and a segment of an optical waveguide segment. The gap separates the TIR mirror from an end of the optical waveguide. In other embodiments, the photo-tunneling I/O port comprises a TIR mirror, a gap and another TIR mirror. The gap is generally less than one wavelength of the optical signal incident on the gap. In some embodiments, the gap is approximately one to ten percent (i.e., 1-10%) of the wavelength of the optical signal. Such an arrangement of a TIR mirror, gap and either a segment of optical waveguide or another TIR mirror is also known as a ‘frustrated TIR mirror’.

In some embodiments, the optical resonator or a portion thereof may provide asymmetric propagation of the optical signal. In particular, the asymmetric propagation may be relative to a direction of optical signal propagation around a loop of the optical resonator (e.g., clockwise or counter clockwise). In some of such embodiments, asymmetric propagation is provided by a magneto-optical effect.

For example, the optical waveguide of the optical resonator may be selectively doped with a magnetic material. Selective doping may comprise either doping only a portion of one or more of the optical waveguide segments of the optical resonator, doping one or more optical waveguide segments in their entirety, or doping the entire optical resonator, in various embodiments. In another example the magnetic material providing the magneto-optical effect is confined to a layer or layers associated with the optical resonator. For example, a layer comprising magnetic material may be provided underneath and separated from the optical waveguide by less than about a wavelength such that an optical mode propagating in the waveguide couples (e.g., evanescently) into the magnetic material layer. Using a separate layer(s) for the magnetic material may reduce a detrimental effect of the magnetic material, for example. In various embodiments, the magnetic material layer may be either uniformly distributed across the optical resonator or confined to only a portion thereof. For example, the magnetic material layer may be confined to a single waveguide segment of a rectangular-shaped optical resonator.

An externally applied magnetic field may be employed to bias the magnetic material used to dope the optical resonator, according to some embodiments. Magnetic materials comprising one or more of iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni), cobalt (Co), and various alloys and compounds thereof may be employed. In various embodiments, the use of the magnetic material in conjunction with the optical resonator essentially produces an asymmetric refractive index of the optical waveguide. The asymmetric refractive index essentially provides or facilitates the asymmetric propagation.

For simplicity herein, no distinction is made between a substrate and any layer or structure on the substrate unless such a distinction is necessary for proper understanding. Also for simplicity herein, the term ‘gain’ is defined to include one or both of positive gain (e.g., an increase in signal level) and negative gain (e.g., attenuation). Additionally, all waveguides described herein are optical waveguides so that omission of the term ‘optical’ when referring to a ‘waveguide’ does not change the intended meaning of the embodiment being described. Further, as used herein, the article ‘a’ is intended to have its ordinary meaning in the patent arts, namely ‘one or more’. For example, ‘a segment’ means one or more segments and as such, ‘the segment’ means ‘the segments(s)’ herein. Also, any reference herein to ‘top’, ‘bottom’, ‘upper’, ‘lower’, ‘up’, ‘down’, ‘left’ or ‘right’ is not intended to be a limitation herein. Moreover, examples herein are intended to be illustrative only and are presented for discussion purposes and not by way of limitation. U.S. patent application of Wu et al., entitled, “Optical Waveguide Ring Resonator with Photo-Tunneling Input/Output Port”, filed concurrently herewith, is incorporated by reference herein in its entirety.

FIG. 2illustrates a top view of an optical resonator100according to an embodiment of the present invention. An optical signal102is illustrated, by way of example, propagating in a clockwise direction around a closed path of the optical resonator100. While illustrated as propagating in an exemplary clockwise direction, the optical signal102may propagate equally well in a counter clockwise direction and is within the scope of the embodiments described herein.

The optical resonator100comprises a plurality of segments110of an optical waveguide (i.e., optical waveguide segments110). The optical waveguide segments110are arranged as a closed loop. Each of the optical waveguide segments110guides the optical signal102along a length of the segment110. Specifically, the optical signal102propagates along each of the segments110from an arbitrary starting point and eventually returns to the starting point. As such, when introduced into the optical resonator100, the optical signal102repeatedly retraces or cycles around the closed loop and thus resonates within the optical resonator100. A resonance frequency of the optical resonator100is an inverse of a transit time of the propagating optical signal102around to the closed loop.

In some embodiments, the optical waveguide segments110of the plurality comprise substantially straight or linear segments of an optical waveguide. The substantially linear optical waveguide segments110are arranged as sides of a closed polygon. The polygon is a simple polygon that is either a convex polygon or a concave polygon, according to various embodiments. The optical resonator100embodiment illustrated inFIG. 2comprises four optical waveguide segments110that are arranged as four sides of a rectangle representing the closed loop, by way of example. In other embodiments, the closed loop formed by the optical waveguide segments110may be represented by essentially any closed polygon having more than two sides including, but not limited to, a triangle, a pentagon, and a hexagon.

The optical resonator100further comprises an equivalent plurality of total internal reflection (TIR) mirrors120. The term ‘equivalent plurality’ herein is defined to mean that a number or quantity of TIR mirrors120in the mirror plurality is equivalent to or the same as a number or quantity of optical waveguide segments110in the segment plurality. The TIR mirrors120connect pairs of optical waveguide segments110to one another to form the closed loop. In particular, each TIR mirror120connects together adjacent ends of a respective pair of optical waveguide segments110to successively close the loop. Further, the TIR mirrors120facilitate propagation of the optical signal102within the optical resonator100. When the optical signal102propagating along a particular optical waveguide segment110encounters a TIR mirror120at the end of the particular segment110, the TIR mirror120redirects or reflects the optical signal102such that the optical signal102enters an adjacent or successive optical waveguide segment110.

The optical resonator100further comprises a photo-tunneling input/output (I/O) port130. When functioning as an input port, the photo-tunneling I/O port130admits or introduces the optical signal102, or a portion thereof, into the optical resonator100. When functioning as an output port, the photo-tunneling I/O port130allows the optical signal102, or a portion thereof, to exit from or leave the optical resonator100.

In general, whether the photo-tunneling I/O port130functions as an input port or an output port depends on a direction of propagation of the optical signal102. That is, when the optical signal102is inside the optical resonator100and encounters the photo-tunneling I/O port130, the port130functions as an output port and transmits the optical signal102, or a portion thereof, out of the optical resonator100. Alternatively, the photo-tunneling I/O port130functions as an input port if the optical signal102is incident on the port130from outside the optical resonator100. In either instance, a portion of the optical signal102passes through the photo-tunneling I/O port130by a photo-tunneling effect (e.g., evanescent field coupling across a gap) associated with the port130, as described above. InFIG. 2, the photo-tunneling I/O port130is illustrated as an output port.

FIG. 3illustrates a magnified view of the photo-tunneling I/O port130illustrated inFIG. 2, according to an embodiment of the present invention. The photo-tunneling I/O port130comprises a designated TIR mirror132of the plurality of TIR mirrors120. The photo-tunneling I/O port130further comprises a segment134of an optical waveguide having an end. The designated TIR mirror132is a selected one of the TIR mirrors120adjacent to the end of the segment134of optical waveguide. The optical waveguide segment134is separate and apart from the plurality of optical waveguide segments110of the optical resonator100. The photo-tunneling I/O port130further comprises a gap136that separates the end of the optical waveguide segment134from the designated TIR mirror132. Also illustrated inFIG. 3is a portion102aof the optical signal102that passes through the photo-tunneling I/O port130and into the segment134.

In some embodiments, the gap136is filled with a material. In general, the gap material may comprise either a fluid (e.g., a gas or a liquid) or a solid (e.g., a semiconductor material or a dielectric material), in some embodiments. In other embodiments, the gap136is essentially devoid of a gap material (e.g., a vacuum). In either case, the gap136has a refractive index that differs from and in some embodiments, is lower than a refractive index of respective materials of the designated TIR mirror132and the optical waveguide segment134. For example, the gap material may be a gas (e.g., air) of an environment surrounding the optical resonator100. Air has a refractive index of about one. The respective materials of the designated TIR mirror132and the optical waveguide segment134may have refractive indices of about four, for example. Specifically, the refractive indices of the designated TIR mirror132material and the optical waveguide segment134material is different from the refractive index of the gap material.

In some embodiments, the gap136is formed as a slot or channel that separates the designated TIR mirror132from the optical waveguide segment134. In other embodiments, the gap136is a region filled with a suitable transmissive material having a refractive index that differs from (i.e., is less than that of) a refractive index of the optical waveguide of the designated TIR mirror132and the separate optical waveguide segment134(not illustrated).

Referring again toFIG. 2, the optical resonator100further comprises an intracavity active element140. The intracavity active element140overlies and is optically coupled with a designated segment112of the plurality of segments110of an optical waveguide. In some embodiments (not illustrated), more than one designated segment112is provided. In such embodiments, the intracavity active element140simultaneously overlies and is coupled to each of the designated segments112. In general, a number of designated segments112is less than or equal to the number of optical waveguide segments110in the plurality of segments.

According to various embodiments, the intracavity active element140comprises an active optical structure or layer. As used herein, an ‘active’ optical layer is a layer of material that can directly influence an optical signal within the layer in an active (e.g., non-passive) manner. For example, an active optical layer may influence the optical signal102through stimulated emission within the layer. The active optical layer influence may be controlled by another optical signal (e.g., optical pumping) or an electrical signal (e.g., electrical pumping) applied to the intracavity active element140, for example.

The optical signal102, or a portion thereof, propagating in the designated segment112is coupled into the intracavity active element140. The intracavity active element140influences the coupled portion of the optical signal102. For example, the intracavity active element140may be an optical source140(e.g., a laser amplifier) that one or both of generates the optical signal102that propagates in the designated segment112and adds optical energy to (i.e., amplifies) the optical signal102. The optical source140may influence (i.e., generate and/or amplify) the coupled portion of the optical signal102through stimulated emission within a gain medium or material of the intracavity active element140, for example. Stimulated emission may be facilitated by application of an electrical voltage or current to the intracavity active element140, for example. In another example, the intracavity element140may comprise the optical source and further comprise a saturable absorber. In yet another example, the intracavity active element140may be an electro-absorption modulator (EAM)140. The EAM140influences the coupled portion of the optical signal to modulate (i.e., vary an amplitude of) the optical signal102. The modulation may be controlled by an electrical signal applied to the EAM140, for example.

In some embodiments, another optical port may be employed in addition to the photo-tunneling I/O port130. Referring again toFIG. 2, another optical port is illustrated as a segment150of optical waveguide. The segment150is parallel and proximal to one of the optical waveguide segment110of the optical resonator100, as illustrated. This sort of other optical port is often referred to as a ‘parallel line’ port and may function as either an input port or an output port with respect to the optical resonator100. For example, a portion of an optical signal102bpropagating in the parallel segment150of optical waveguide may be coupled into the optical waveguide segment110. The portion of the optical signal102bcoupled into the optical resonator100may become the optical signal102, for example.

FIG. 4illustrates a perspective view of the optical resonator100ofFIG. 2realized in a surface layer of a substrate200, according to an embodiment of the present invention. In particular, as illustrated inFIG. 4, the substrate200comprises a semiconductor on insulator (SOI) substrate200having a semiconductor layer210situated on top of an oxide-based or similar insulator layer220. The optical resonator100is illustrated within the semiconductor layer210. The semiconductor layer210may be similar to the slab layer12,22described above with respect toFIGS. 1A and 1B, in some embodiments, or the strip layer32illustrated inFIG. 1C, in some embodiments. Similarly, the insulator layer220may be similar to any of the support layers14,24,34described above with respect toFIGS. 1A,1B and1C, in some embodiments.

Four optical waveguide segments110connected by respective ones of four TIR mirrors120form the optical resonator100embodiment illustrated inFIG. 4, by way of example. Further as illustrated, the optical waveguide segments110comprise a reverse ridge-loaded waveguide (e.g., the reverse ridge-loaded waveguide20described above). An approximate extent of a guided portion of an optical signal is illustrated inFIG. 4as a pair of dashed lines on either side of a hypothetical centerline (not illustrated) of the reverse ridge-loaded waveguide segments110.

As illustrated inFIG. 4, the TIR mirrors120are each realized as a hole or cavity212formed in a guiding portion (i.e., semiconductor layer210) of the substrate200. The cavity212creates a TIR wall122(e.g., refractive index difference or discontinuity) across an optical propagation axis (i.e., the center line of the optical waveguide) of a pair of intersecting optical waveguide segments110. The TIR wall122generally extends beyond the extent of the guided portion of the optical signal. In some embodiments, the TIR wall122extends into a ridge portion of the reverse ridge-loaded waveguide.

The photo-tunneling I/O port130is also illustrated inFIG. 4. The photo-tunneling I/O port130is realized by a gap136formed in the semiconductor layer210. The gap136, having a width of less than one wavelength (e.g., 1-10% of a wavelength) of the optical signal, separates the designated TIR mirror132from an optical waveguide segment134that acts as an output path or connection for the optical signal. The optical resonator100may be fabricated in the SOI substrate200using conventional semiconductor fabrication methods, for example.

Further illustrated inFIG. 4is the intracavity active element140. In particular, the intracavity active element140comprises a semiconductor layer240. The semiconductor layer240of the intracavity active element140is affixed to a surface of the semiconductor layer210. For example, the semiconductor layer240may be wafer bonded to the surface of the semiconductor layer210. Moreover, the intracavity active element140is located over a designated segment112of the reverse ridge-loaded waveguide110.

FIG. 5illustrates a cross sectional view through the intracavity active element140of the optical resonator100illustrated inFIG. 4, according to an embodiment of the present invention. In particular,FIG. 5illustrates a cross section through the SOI substrate200that intersects the designated segment112.

In some embodiments, the semiconductor layer240of the intracavity active element140comprises a semiconductor material that is similar to a semiconductor material of the semiconductor layer210. In other embodiments, the semiconductor material of the semiconductor layer240is different from that of the semiconductor layer210. Further, in some embodiments, the semiconductor layer240comprises a semiconductor material having a lattice that is dissimilar or mismatched to that of the semiconductor material of the semiconductor layer210. For example, the semiconductor layer210may comprise silicon while the semiconductor layer240may comprise GaAs. In such embodiments, the optical resonator100may comprise the semiconductor layer240affixed to the surface of the semiconductor layer210by wafer bonding.

In some embodiments, the semiconductor layer240comprises a compound semiconductor. In some embodiments, the compound semiconductor is either a III-V compound semiconductor or a II-VI compound semiconductor. For example, the semiconductor layer240may comprise one or more of GaAs, InP, AlGaAs, InGaAs, InGaAsP, and other direct band gap semiconductor gain materials.

Furthermore, while illustrated by way of example as a single layer, the semiconductor layer240may actually comprise one or more layers, according to various embodiments. In addition, while not illustrated, the semiconductor layer240may further comprise one or more dopants and dopant concentrations as well as an electrical connection to other components or power sources (e.g., electrical contact pad). The dopants and dopant concentrations and the electrical connection are used to realize a particular type or functionality of the electrical gain element140, for example.

For example, the intracavity active element140may comprise a heterostructure diode junction. Such heterostructures may comprises a plurality of variously doped (e.g., n, n+, p, and p+) layers. In another example, the intracavity active element140may comprise a quantum well diode junction often used for solid state (e.g., diode) lasers and optical amplifiers. In yet another example, the intracavity active element140may comprise a plurality of variously doped layers arranged as a separate confinement heterostructure laser structure.

The optical signal that is introduced to propagate in the optical resonator100has one or more propagating modes. Each propagating mode has a physical size or extent. As used here, the extent of the propagating mode (or equivalently the propagating signal) is a bounded region within which a majority of the optical energy associated with the mode is confined during propagation. In some embodiments, an extent of a lowest order propagating mode of the optical signal is greater than a thickness of a guiding portion of the slab optical waveguide of the designated segment112. As a result, a portion of the optical signal extends above a top surface of the slab optical waveguide and into the overlying intracavity active element140. The portion of the optical signal that extends into the intracavity active element140is effectively coupled into the intracavity active element140. Thus, the intracavity active element140is able to influence this portion of the optical signal.

As illustrated inFIG. 5, the approximate extent of the optical signal is depicted as a circular dashed line. The circular dashed line extends into the semiconductor layer240of the intracavity active element140. Further, the optical signal extent is generally located above and extending into a ridge216associated with the reverse ridge-loaded waveguide. As illustrated inFIG. 5, the ridge216comprises the semiconductor material of the semiconductor layer210and extends down into the insulator layer220. As such, a portion of the optical signal is effectively coupled into and propagates within the semiconductor layer240of the intracavity active element140. The portion of the optical signal propagating within the semiconductor layer240is available to be influenced by the intracavity active element140.

FIG. 6illustrates a block diagram of a photonic system300according to an embodiment of the present invention. The photonic system300comprises an optical resonator310. The optical resonator310comprises a plurality of optical waveguide segments312, an equivalent plurality of TIR mirrors314and a photo-tunneling input output (I/O) port316. The optical resonator310further comprises an intracavity active element318. The intracavity active element318is optically coupled to a designated segment312of the plurality of segments312.

The optical waveguide segments312are arranged in a closed loop with TIR mirrors314interconnecting the segments312. The photo-tunneling I/O port316comprises one of the TIR mirrors314of the plurality. In some embodiments, the optical resonator310is essentially similar to the optical resonator100described above.

In some embodiments, the intracavity active element318comprises an optical source/amplifier. In other embodiments, the intracavity active element318comprises an optical modulator (e.g., an EAM). In some embodiments, the intracavity active element318is essentially similar to the intracavity active element140described above with reference to the optical resonator100.

The photonic system300further comprises an optical signal source320that generates an optical signal322. For example, the optical signal source320may comprise a laser or a light emitting diode (LED). The generated optical signal322is coupled into and resonates within the optical resonator310. In some embodiments (not illustrated), the optical signal source320comprises the intracavity active element318.

In some embodiments, the optical signal322is coupled into the optical resonator310via the photo-tunneling I/O port316. For example, the optical signal source320may feed the optical signal322into a segment of an optical waveguide324. In turn, the segment of the optical waveguide324has an end that terminates in the photo-tunneling I/O port316. As such, the I/O port316functions as an input port in this example.

In other embodiments, the optical signal322may be coupled into the optical resonator310via another I/O port. For example, a segment of an optical waveguide (not illustrated) parallel with and in close proximity to one of the optical waveguide segments312of the optical resonator310may act as the I/O port. In some embodiments, either of the photo-tunneling I/O port316, another photo-tunneling I/O port316, or another I/O port (not illustrated) may act as an output port.

For example, the output port may sample a portion of the optical signal322resonating within the optical resonator310. The sampled portion is then coupled to an optical waveguide segment326, for example. As such, the I/O port316functions as an output port in this example. In another example, the sampled portion of the optical signal322is coupled to another component (not illustrated) external to the optical resonator310. In some embodiments, the optical source320may be located within the optical resonator310.

FIG. 7illustrates a flow chart of a method400of optical resonance according to an embodiment of the present invention. The method400of optical resonance comprises propagating410an optical signal along a plurality of optical waveguide segments. For example, the optical waveguide segments are arranged in a closed loop forming a ring resonator. Successive pairs of the optical waveguide segments are connected to one another at respective adjacent segment ends by corresponding ones of an equivalent plurality of TIR mirrors to further define the closed loop of the ring resonator. In some embodiments, the ring resonator is essentially similar to any of the embodiments described above for the optical resonator100.

In some embodiments, the method400of optical resonance further comprises reflecting420the optical signal at a TIR mirror. In some of these embodiments, essentially the entire optical signal is reflected420by the TIR mirror. In particular, the optical signal propagating in a first optical waveguide segment of a successive pair is reflected by the TIR mirror connected to the adjacent ends of the first optical waveguide segment and a second optical waveguide segment of the successive pair. After being reflected420, the optical signal enters and propagates in the second optical waveguide segment of the successive pair. The optical signal ultimately encounters another TIR mirror at an opposite end of the second optical waveguide segment. The other TIR mirror further reflects420the optical signal in a similar fashion into a respective successive optical waveguide segment. By successively propagating410and reflecting420, the optical signal may traverse around or circumnavigate the closed loop of the ring resonator. In other of these embodiments, only a portion of the optical signal is reflected420. A dashed-line box inFIG. 7is employed to indicate that reflecting420is optional, in some embodiments.

The method400of optical resonance further comprises influencing440the optical signal within an intracavity active element that overlies a designated segment of the plurality of optical waveguide segments. Influencing440the optical signal may comprise one or more of amplifying the optical signal, modulating the optical signal, and generating the optical signal, according to various embodiments.

In particular, in some embodiments, influencing440the optical signal comprises coupling a portion of the optical signal from the designated optical waveguide segment into an overlying intracavity active element. In various embodiments, influencing440further comprises one or more of amplifying, modulating, and attenuating the coupled portion of the optical signal within the intracavity active element. Influencing440further comprises subsequently coupling the coupled portion of the optical signal, which is one or more of amplified, modulated, and attenuated, back into the designated optical waveguide segment. In some embodiments, the portion of the optical signal that is influenced440by the intracavity active element is a portion of a propagating mode of the optical signal that extends into the overlying intracavity active element.

In some embodiments, the method400further comprises transmitting430the optical signal through a photo-tunneling input/output (I/O) port. Only a portion of the optical signal is transmitted430through the photo-tunneling I/O port, according to some embodiments. Transmitting430includes within its scope one or both of transmitting out of the ring resonator and transmitting into the ring resonator. In some of these embodiments, the transmitted430optical signal, or the portion thereof, exits out of or leaves the ring resonator. In transmitting430the optical signal out of the ring resonator, the photo-tunneling I/O port is acting as an output port, as defined herein.

The photo-tunneling I/O port comprises a designated one of the TIR mirrors of the ring resonator. The photo-tunneling I/O port further comprises a gap and according to some embodiments, another optical waveguide segment that is not part of the plurality of optical waveguide segments of the ring resonator. The designated TIR mirror of the photo-tunneling I/O port generally does not reflect essentially the entire optical signal due to the aforementioned photo-tunneling effect associated with the photo-tunneling I/O port. Instead, only a portion of the optical signal is reflected420by the designated TIR mirror at the photo-tunneling I/O port, while the rest of the optical signal is transmitted430out of the ring resonator across the gap and into the optional other optical waveguide segment.

In some embodiments, an optical signal from outside of the ring resonator may be transmitted430through the photo-tunneling I/O port and into the ring resonator. Thus, in some embodiments, the photo-tunneling I/O port may act as an input port when the source of the optical signal is outside the ring resonator. In some embodiments, the photo-tunneling I/O port may act either as an input port, an output port, or a combination thereof. In some embodiments, there may be more than one photo-tunneling I/O port, in which case whether or not a particular one of the photo-tunneling I/O ports acts as the output port to transmit430the optical signal out of the ring resonator generally depends on a specific configuration of the ring resonator (e.g., location of a source of the optical signal). A dashed-line box inFIG. 7is employed to indicate that transmitting420is optional, in some embodiments.

FIG. 8Aillustrates a top view of another optical resonator500according to an embodiment of the present invention. Specifically, as illustrated inFIG. 8Athe optical resonator500comprises a segment of optical waveguide510arranged as a circular loop or ring.FIG. 8Billustrates a top view of yet another optical resonator500according to an embodiment of the present invention. As illustrated inFIG. 8B, the segment of optical waveguide510is arranged as an oval or ‘race track’ shaped ring.

In some embodiments, the segment of optical waveguide510is essentially similar to one of the segments of optical waveguide110described above with respect to the optical resonator100except that the segment510is curved to form a loop instead of being a linear segment (side) of a polygon. An optical signal502propagates around the segment of optical waveguide510to resonate within the optical resonator500.

In particular, in some embodiments, the segment of optical waveguide510comprises a segment of reverse ridge-loaded slab optical waveguide. The reverse ridge-loaded slab optical waveguide may comprise a layer of a first semiconductor material adjacent to a surface of a substrate (not illustrated). The reverse ridge-loaded slab optical waveguide segment510may be essentially similar to the reverse ridge-loaded slab optical waveguide20described above. A length of the segment of optical waveguide510establishes a resonant frequency of the optical resonator500.

The optical resonator500further comprises an I/O port530. As illustrated, the I/O port comprises a tangential segment of optical waveguide532separated from the optical waveguide segment510by a gap. In other embodiments (not illustrated), the I/O port530may comprise a photo-tunneling I/O port. An optical signal502bmay be coupled into the optical resonator500to become the optical signal502when the I/O port530is serving as an input port. Alternatively, the I/O port530may act as an output port that couples the optical signal502out of the optical resonator500. In some embodiments, the I/O port530may be essentially similar to either the photo-tunneling I/O port130or the other I/O port (e.g., parallel line port150) described above with respect to the optical resonator100.

The optical resonator500further comprises an intracavity active element540. The intracavity active element540overlies and is optically coupled with a portion512of the segment of optical waveguide510. The intracavity active element540influences the optical signal502passing through the portion512of the optical waveguide segment510. The intracavity active element540comprises an active optical structure or layer. In various embodiments, the intracavity active element540may serve as one or more of an optical modulator, a saturable absorber and an optical source/amplifier to influence the optical signal502. In some embodiments, the intracavity active element540is essentially similar to the intracavity active element140described above with respect to the optical resonator100.

Thus, there have been described embodiments of an optical resonator that employs an intracavity active element, a photonic system employing the optical resonator, and a method of optical resonance employing optical gain. It should be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent the principles of the present invention. Clearly, those skilled in the art can readily devise numerous other arrangements without departing from the scope of the present invention as defined by the following claims.