An apparatus comprising an optical filter located on a substrate. The optical filter including an optical splitter configured to receive an input light and an interferometer having two waveguide arms having different optical path-lengths from each other. The waveguide arms configured to receive the input light from the optical splitter. At least a portion of one of the two waveguide arms has a narrower core width than a wider core width of the other waveguide arm. The waveguide arm with the longest waveguide portion having the narrower core width has the longest total physical path-length of the two waveguide arms. At least one of the two waveguide arms having a set of discrete waveguide portions, the discrete waveguide portions of the set being connected by optical switches which are configured to tunably select from a plurality of different physical path-lengths through the discrete waveguide portions of the at least one waveguide arm.

TECHNICAL FIELD

The invention relates, in general, to apparatuses having optical filters and, more specifically, to athermal optical filters in a photonic integrated circuit apparatus, and methods of manufacturing such apparatuses.

BACKGROUND

For certain optical network applications, it is desirable to lock multiple optical wavelengths to particular different channels of a grid, such as the dense wavelength division multiplexing (DWDM) standard grid channels of the International Telecommunication Union (ITU). The transmission of dense multiplexed optical carriers with carrier separations of, e.g., 50 or 25 GHz, should be closely locked to their respective dedicated optical wavelength to avoid inter-channel crosstalk.

Sometimes, wavelength locking is achieved using a reference Fabry-Perot etalon along with complex temperature measurement and feedback adjustment circuitry to achieve thermo-optic tuning. Sometimes, the etalon is made of quartz glass due to its small thermo-optic coefficient and linear thermal expansion coefficient. The construction of such a reference etalon on silicon photonic integrated circuits (PICs) can be problematic, however, due to the large thermo-optic coefficients of silicon which makes the temperature stabilization problematic. The integration of quartz glass Fabry-Perot etalons on a PIC also may not be attractive because of the relatively large dimensions of such etalons.

Attempts to provide laser locking using silicon interferometer or ring filter designs are subject to fabrication variations and may require aggressive active thermo-optic tuning procedures due to the large thermo-optic coefficient of silicon. The use of passive athermal filter designs may provide a means to eliminate or reduce such active stabilization requirements. For instance, certain interferometer filter designs, where the two arms of the interferometer have different waveguide widths, have been shown to exhibit athermal behavior in a particular wavelength range.

SUMMARY

One embodiment is an apparatus comprising an optical filter located on a substrate. The optical filter can include an optical splitter configured to receive an input light and an interferometer having two waveguide arms having different optical path-lengths from each other. The waveguide arms can be configured to receive the input light from the optical splitter. At least a portion of one of the two waveguide arms can have a narrower core width than a wider core width of the other waveguide arm. The waveguide arm with the longest waveguide portion having the narrower core width can have the longest total physical path-length of the two waveguide arms. At least one of the two waveguide arms can have a set of discrete waveguide portions, the discrete waveguide portions of the set being connected by optical switches which are configured to tunably select from a plurality of different physical path-lengths through the discrete waveguide portions of the at least one waveguide arm.

In some such embodiments, a first waveguide arm of the two waveguide arms can be the waveguide arm having the set of discrete waveguide portions and a second waveguide arm of the two waveguide arms can have the narrower core width which also can have the longest waveguide portion having the narrower core width.

In some such embodiments, a first waveguide arm of the two waveguide arms can be the waveguide arm having the set of discrete waveguide portions. The first waveguide arm can have the longest waveguide portion having the narrower core width. A second waveguide arm of the two waveguide arms can have a first portion having the narrower core width and a second portion having the wider core width

In any such embodiments, the set of discrete waveguide portions can include a first pair of the discrete waveguide portions. In some embodiments, the set of discrete waveguide portions can further include multiple different pairs of discrete waveguide portions that are series connected together by different pairs of the optical switches. In some such embodiments, each one of the pairs of discrete waveguide portions of the sets can include a first portion having a same first physical path-length and a second portion having a unique second physical path-length that can be greater than the same first physical path-length and different from the second light path-lengths of other ones of the pairs of the sets.

In any such embodiments, the other of the two waveguide arms can include a second set of discrete waveguide portions, the discrete waveguide portions of the second set connected together by different optical switches which can be configured to tunably select from one of a plurality of different physical path-lengths through the discrete waveguide portions of the other waveguide arm. In some such embodiments, the second set of discrete waveguide portions further includes multiple different pairs of the discrete waveguide portions that are series connected together by different ones of the optical switches.

In any such embodiments, the wider core width can have a width value in a range from about 300 to about 500 nanometers and the narrower core width can have a width value that is in a range from about 0.4 to about 0.6 times a value of the wider core width.

In any such embodiments, the waveguide arm with the longest total physical path-length of the two waveguide arms can include a first fixed extension portion that provides at least half of the total physical path-length difference between the waveguide arm and the other waveguide arm. In some such embodiments, the first fixed extension portion can provide from greater than about 50 percent to about 100 percent of the total physical path-length difference between the waveguide arm with the longest total physical path-length and the other waveguide arm.

In any such embodiments, the two waveguide arms each can include second fixed extension portions that have substantially a same physical path-length and that can contribute at least half total physical path-lengths of the respective arms. In some such embodiments, the same physical path-length of the second fixed extension portions can contribute a percentage value ranging from greater than about 50 percent to about 90 percent of the total physical path-length of the respective arms.

Any such embodiments can further including a light input coupled to the optical filter, the light input and the optical filter located on a photonic integrated circuit substrate and the light input can be configured to deliver the input light having a wavelength in an optical telecommunication band to the optical splitter. In some such embodiments, the light input can includes an optical coupling port or a light source. Some such embodiments can further include discrete electrodes on the photonic integrated circuit substrate, the discrete electrodes coupled to each of the optical switches, wherein each of the electrodes can be configured provide an electrical signal to control a direction of transit of the input light through one of the discrete waveguide portions.

Another embodiment is method comprising fabricating an optical filter. Fabricating the optical filter can include providing a substrate, providing an optical layer on the substrate and patterning the optical layer to form an optical filter. Patterning can include forming an optical splitter configured to receive an input light and forming an interferometer having two waveguide arms with different optical path-lengths, the waveguide arms configured to receive the input light from the optical splitter. At least a portion of one of the two waveguide arms can have a narrower core width than a wider core width of the other waveguide arm. The waveguide arm with the longest waveguide portion having the narrower core width can have the longest total physical path-length of the two waveguide arms. At least one of the two waveguide arms can have a set of discrete waveguide portions, the discrete waveguide portions of the set being connected by optical switches which are configured to tunably select from a plurality of different physical path-lengths through the discrete waveguide portions of the at least one waveguide arm.

In some such embodiments, patterning to form the optical filter can include patterning the other of the two waveguide arms to form a second set of discrete waveguide portions, the discrete waveguide portions of the second set connected together by different optical switches which can be configured to tunably select from one of a plurality of different physical path-lengths through the discrete waveguide portions of the other waveguide arm.

Any such embodiments can further include providing a light input on the substrate, the light input can be configured to deliver the input light having a wavelength in an optical telecommunication band to the optical splitter.

Any such embodiments can further include forming discrete electrodes on the substrate, the discrete electrodes coupled to the optical switches. Each of the electrodes can be configured provide an electrical signal to control a direction of transit of the input light through one of the discrete waveguide portions.

In the Figures and text, similar or like reference symbols indicate elements with similar or the same functions and/or structures.

In the Figures, the relative dimensions of some features may be exaggerated to more clearly illustrate one or more of the structures or features therein.

Herein, various embodiments are described more fully by the Figures and the Detailed Description. Nevertheless, the inventions may be embodied in various forms and are not limited to the embodiments described in the Figures and Detailed Description of Illustrative Embodiments.

DETAILED DESCRIPTION

The description and drawings merely illustrate the principles of the inventions. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the inventions and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the inventions and concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the inventions, as well as specific examples thereof, are intended to encompass equivalents thereof. Additionally, the term, “or,” as used herein, refers to a non-exclusive or, unless otherwise indicated. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

As part of the present disclosure, we have found that the production of certain previous passive thermally stabilized silicon interferometer filter designs can be problematic due to fabrication variations. Fabrication variations can cause the filter's dimensions to differ from a targeted design shape. Consequently, the filter may exhibit athermal behavior at a wavelength or wavelength range that is not at the desired value.

The term athermal behavior as used herein refers to an interferometer filter design such as disclosed herein where for a temperature fluctuation of ±60° K the resonance wavelength of the filter changes by ±10 picometer or less or by ±1 GHz or less away from a selected target wavelength.

While not limiting the scope of the disclosure by theoretical considerations, at least in some embodiments, the wavelength λ0where the athermal behavior occurs is thought to be specified by equation (1):
(dneff(λ0)/dT)ΔL=(dΔneff(λ0)/dT)L(1)
where the ΔL equals the difference in length of the sections of the two interferometer arms having a same effective thermo-optic coefficient (e.g., a same core width in some embodiments), L is the length of the part of the waveguide arm having a different effective thermo-optic coefficient (e.g., a narrower core width in some embodiments), and, there is a corresponding part of the second waveguide arm having same length L and the wider core width, Δneffis the effective refractive index difference of the waveguide portions that have the same wider core width versus the narrower core width. The term neffis the effective index of the mode in the waveguide portions having the wider core width.

In accordance with equation (1), we have found that the wavelength where the athermal behavior occurs can be highly sensitive to the filter's waveguide geometric and material properties (e.g., dimensions, thermo-optic coefficient and/or refractive index), which in turn, can cause variations in Δneff. Slight deviation in the fabrication processes away from the target waveguide dimensions, in particular, variations in the waveguide's width and/or height, can result in athermal behavior occurring at the wavelength that is not at the target wavelength. The sensitivity to the filter's waveguide dimensions were found to be problematic for filters designed to provide a desired constant (e.g., 100 GHz or less variation) free spectral range (FSR) because such filters can require long physical lengths of L (e.g., 8000 to 10000 microns in some embodiments).

For instance, small variations (e.g., from ±5 percent to ±1 percent, or, from ±25 nanometers to ±5 nanometers, for some embodiments) in the core width of one or both waveguide arms of the filter can cause the filter's athermal behavior to occur at a wavelength, or in a wavelength range, whose location relative the ITU grid of channels is unpredictable. Additionally, because of the long lengths of the interferometer waveguide arms, it can be difficult to achieve the desired constant FSR (e.g., 25, 50 or 100 GHz in some embodiments) throughout the entire wavelength range of a target telecommunication band. Consequently, photonic integrated circuits having such athermal filter designs are fabricated in lower yields than desired, e.g., in batch fabrication runs on silicon wafers.

To address this problem, as disclosed herein, we have developed an optical filter design architecture that allows the above-mentioned problems associated with fabrication variations to be mitigated. By providing reconfigurable waveguide portions in at least one, and sometimes both, of the interferometer waveguide arms with different thermo-optic coefficients (e.g. different core widths), fabrication variations can be compensated for by tuning the relative physical path-lengths of light traveling through the waveguide arms by switching among different waveguide portions. Having such tunably adjustable waveguide physical path-lengths allows the mitigation of fabrication variations to be done on a circuit-by-circuit basis thereby increasing the yield of useable circuits per fabrication run.

One embodiment is an apparatus.FIG. 1presents a perspective view of an example embodiment apparatus100of the present disclosure. The apparatus100comprises an optical filter102located on a substrate105. The optical filter102includes an optical splitter107(e.g., a 50:50 power splitter) configured to receive an input light110(e.g., via waveguide112). The optical filter102also includes an interferometer115having two waveguide arms (e.g., a first waveguide arm117and a second waveguide arm120) having different optical path-lengths from each other.

For the purposes of the present disclosure, the waveguide arms117,120are defined as including the entire physical waveguide structures located between the splitter107and the combiner121including any wider or narrower core width portions of the waveguide arms or optical switches as further described herein.

The term, optical path-length, refers to the product of the effective refractive index (neff) of the waveguide material and the physical path-length of the waveguide.

The waveguide arms117,120are configured to receive the input light110from the optical splitter107(e.g., each arm receiving 50 percent power portions of the light when the splitter is a 50:50 splitter). After passing through the arms117,120, the light is recombined at an optical combiner121configured to receive light from the waveguide arms117,120.

Embodiments the optical filter102are described herein with the convention of the input light110entering the filter120via the splitter107and exiting the filter120via the combiner121. Such embodiments, however, also would work equally well for an inverse convention, where the light110enters the filter102via a splitter121(e.g., via waveguide113) and exits via a combiner107.

At least a portion of one of the two waveguide arms has a narrower core width than a wider core width of the other waveguide arm. For instance, as shown inFIG. 1, the first waveguide arm117has a wider core width. For instance,FIG. 2Apresents a cross-sectional view of a portion122of the first waveguide arm117, along view line2A inFIG. 1, having the wider core width210. For instance,FIG. 2Bpresents a cross-sectional view of a portion124of the second waveguide arm120along view line2B. As shown inFIG. 1, the second waveguide arm120can have a first portion123that has the same wider core width, and, a second portion124can have a narrower core width215such as shown inFIG. 2B.

The waveguide arm with the longest waveguide portion having the narrower core width also has the longest total physical path-length of the two waveguide arms. For instance, as illustrated inFIG. 1, the second waveguide arm120of the two waveguide arms has the narrower core width215and the second waveguide arm120also has the longest waveguide portion having the narrower core width (e.g., portion124). Therefore the second waveguide arm120has the longest total physical path-length of the two waveguide arms. For instance, as illustrated inFIG. 3, the second waveguide arm120has a first portion123having the narrower core width215and a second portion124having the wider core width210. But, since substantially the entire length of the first waveguide arm117has narrower core width, this is the arm with the longest waveguide portion having the narrower core width. Therefore in this filter embodiment the first waveguide arm117has the longest total physical path-length of the two waveguide arms.

At least one of the two waveguide arms has a set of discrete waveguide portions, the discrete waveguide portions of the set being connected by optical switches which are configured to tunably select from a plurality of different physical path-lengths through the discrete waveguide portions of the at least one waveguide arm, For instance, as illustrated inFIG. 1 or 3, the first waveguide arm117has a set130of discrete waveguide portions (e.g., at least portions132,134). The discrete waveguide portions132,134of the set130are connected by optical switches (e.g., first and second optical switches140,142). The optical switches140,142are configured to tunably select from a plurality of different light path-lengths through the discrete waveguide portions132,134of the first waveguide arm117.

As illustrated inFIGS. 2A and 2B, some embodiments of the waveguide arms117,120can be surrounded by an air cladding. In other embodiments, as familiar to those skilled in the pertinent arts, the waveguide arms117,120can be surrounded by a material cladding (e.g., a silicon oxide cladding).

In some embodiments, the interferometer115can be or include a Mach Zehnder interferometer (MZI) located on a planar silicon-on-insulator (SOI) substrate105. The arms117,120of the interferometer115and other light-guiding components of the apparatus100can be composed of an upper silicon layer of the SOI substrate105. However, in other embodiments, these light guiding components could be composed of high-index-contrast waveguides such as, but not limited to, bulk silicon, silicon-nitride or InP. Example embodiments of the optical switches include Micro-Electro-Mechanical System or MZI switches. One skilled in the pertinent arts would be familiar with other possible embodiments of the optical switch. One skilled in the art would understand how the optical switch140could be configured with a suitable number of input and output ports (e.g., 1×2, 1×3, 1×4, 2×2, 1×2, 1×3, 1×4 . . . switches) and port switch control electrodes to accomplish the tunable selection of one of the different light paths through one of the discrete waveguide portions132,134.

In some embodiments, such as illustrated inFIG. 4, to facilitate efficient light transfer, the first fixed length waveguide portion123having the wider first core width210can be connected to the second fixed length portion124having the narrower second core width215by a tapered waveguide portion410. In such embodiments, the tapered portion can have a core width that gradually transitions from the wider width of the first length portion123to the narrower width of the second length portion124. Likewise, in some embodiments, the second fixed length portion124of the second waveguide arm120can be connected to the combiner121by second tapered waveguide portion412transiting from the narrower width of the second length portion124to an input port of the combiner121that has a wider width. Similarly, as illustrated inFIG. 5, when the first length portion123of the second waveguide arm120has a narrower core width and the second length portion124has the wider core width, then the tapered waveguide portion410can taper from the narrower width of the first length portion123to the wider width of the second length portion124. Similarly second tapered portions412can connect the waveguide arm117and portion123with the narrower core width to the splitter107and/or combiner121.

As illustrated inFIGS. 1 and 2-4, for some embodiments of the interferometer115, the set130of discrete waveguide portions includes a first pair150of discrete waveguide portions132,134. As illustrated inFIG. 1, the first waveguide portion132can have a shorter physical length (e.g., length L1) as compared to the second waveguide portion (e.g., length L1+ΔL1) of the pair150. For some such embodiments, the first optical switch140can be configured as a 1×2 switch and the second optical switch142can be configured as a 2×1 switch. In such embodiments, the optical switches140,142can be configured to select from the different light path-lengths through the discrete waveguide portion132or the discrete waveguide portion134.

In other embodiments, to provide higher degrees of incremental tunable optical light delays while occupying substantially the same area of the substrate105, the set130can further include multiple ones of the discrete waveguide portions that are all connected in parallel to the same two optical switches140,142. For instance, as illustrated inFIG. 6, the set130can include a third discrete waveguide portion136or third and fourth discrete waveguide portions126,138, to form a trio150or quartet150or higher numbers of discrete waveguide portions. Each of the waveguide portions132,134,136,138. . . can have different physical light path-lengths (e.g., L1, L1+ΔL1, L1+2ΔL1, L1+4ΔL1, . . . , respectively) which are tunably selectable by the first optical switch140(e.g., a 1×3 or 1×4 switch etc. . . . ) and the second optical switch142(e.g., a 3×1 or 4×1 . . . switch etc. . . . ). In such embodiments, the optical switches140,142could be configured to select from the different physical path-lengths through one of the discrete waveguide portions132,134,136,138of the set130.

In still other embodiments, to provide more degrees of incremental tunable optical light delays, as illustrated inFIG. 7, the set of discrete waveguide portions130can further include multiple different pairs150,152,154, . . . of discrete waveguide portions. For example, the interferometer115can include a first pair150of waveguide portions132,134, a second pair152of waveguide portions132b,136, and a third pair154of portions132c,138, that are series connected together by different pairs of the optical switches140,142,146,148, . . . (e.g., switches140,142connecting a first pair150, switches142,146connecting a second pair152and switches146,148connecting a third pair154). In such embodiments, the first pair of optical switches140,142can be configured to select from the different light path-lengths through one of the discrete waveguide portions132,134, the second pair of optical switches142,144can be configured to select from the different light path-lengths through one of the discrete waveguide portions132b,136, and so on.

For instance as further illustrated inFIG. 7, in some embodiments, each one of the pairs150,152,154of discrete waveguide portions of the set130can include a first portion having a same first light path-length (e.g., waveguide portion132,132b,132c, . . . , e.g., each of length L1) and a second portion (e.g., one of waveguide portions134,136or138) having a unique second light path-length (e.g., one of lengths L1+ΔL1, L1+2ΔL1, or L1+4ΔL1, respectively) that is greater than the same path-length (e.g., greater than L1) and different from the second light path-lengths of other ones of the pairs of the set130. For instance, the path-length of the second portion134of the first pair150can be greater than the path-length of the first portion132and also have a different path-length than the path-lengths of either of second portions136or138of the second and third pairs152,154.

In some such embodiments, for the pairs150,152,154the same first waveguide portions132,132b,132c, . . . can have a same physical path length equal to L1 where L1 has a value in a range from about 50 to about 100 microns. In such embodiments, the second unique second waveguide portion134has a physical path-length equal to L1+ΔL1, where ΔL1 has a value in a range from about 25 to about 50 microns and in other embodiments from about 50 to about 100 microns. In some such embodiments, for the second pair152, the unique second waveguide portion136can have a physical path-length equal to L1+2ΔL1, and for the third pair154the unique second waveguide portion138can have a physical path-length equal to L1+4ΔL1.

Based on the present disclosure, one skilled in the pertinent art would understand how embodiments of the apparatus100could have various combinations of the filter designs presented inFIGS. 6 and 7, e.g., to provide the additional degrees of incremental tunable optical light delays, Some embodiments, for instance, can include different pairs of discrete waveguide portions connected in series by different pairs of optical switches where there are more than two pairs of tunable waveguide portions connected by the same pairs of optical switches (e.g., to include additional third and fourth waveguide portions136and138shown inFIG. 6).

In some embodiments, the other of the two waveguide arms includes a second set of discrete waveguide portions, the discrete waveguide portions of the second set connected together by different optical switches which are configured to tunably select from one of a plurality of different physical path-lengths through the discrete waveguide portions of the other waveguide arm. Such embodiments can provide additional degrees of incrementally tunable optical light delays.

For example, as illustrated inFIG. 7as part of the second waveguide arm, in addition to discrete waveguide portions that are part of the first waveguide arm, the second waveguide arm can include other discrete waveguide portions. For instance, the portion124of the second waveguide arm120having the second core width215, also includes a second set730of discrete waveguide portions732,734,736. The discrete waveguide portions of the second set730are connected together by different optical switches740,742,744, . . . analogous to that described for the first set130. The optical switches740,742,744, . . . connected to the second set730are configured to tunably select from one of a plurality of different path-lengths through the discrete waveguide portions (e.g., a light path through one of waveguide portions732,734,736, . . . ) of the second waveguide arm120.

As illustrated inFIG. 7, in some embodiments, analogous to that described for first set130, the second set730can include a pair750of discrete waveguide portions, and in some embodiments, includes different pairs750,752,754, . . . of the discrete waveguide portions (e.g., a first pair750including waveguide portions732and734, a second pair752including waveguide portions732band736, . . . ) that are series connected together by different ones of the optical switches between each of the pairs of discrete waveguide portions (e.g., switches740,742, . . . respectively). The different optical switches740,742, . . . can be configured to tunably select from different physical light path-lengths through the discrete waveguide portions of the second waveguide arm120analogous to that described herein in the context of the first waveguide arm117.

In still other embodiments, analogous to embodiments the first waveguide arm117discussed in the context ofFIG. 6, one of more of the pairs of discrete waveguide portions150,152,154. . . can further include third, fourth or additional discrete waveguide portions connected to the same pair optical switches coupled to the second waveguide arm120to form trios, quartets or higher combinations of such discrete waveguide portions.

In some embodiments of the interferometer115where both the first and second arms117,120include tunable discrete waveguide portions, the discrete waveguide portions of the second waveguide arm can have same physical path-lengths as corresponding ones of the pairs of discrete waveguide portions of the first waveguide arm. Having such identically configured tunable waveguide portions in both arms advantageously reduces design complexity because athermal filter is based on waveguide geometry and if the tunable waveguide portions are present in only one waveguide arm then the thermal behavior of those tunable portions needs to be taken into account.

For example, as illustrated inFIG. 7, for some such embodiments, each one of the different pairs750,752,754of discrete waveguide portions732,734,736, . . . of the second waveguide arm120includes a first same waveguide portion (e.g., waveguide portion732,732b,732c, . . . ) that are equal in physical path-length to each other and equal in physical-path length to the corresponding first same waveguide portions of the first waveguide arm117(e.g., waveguide portions132,132b,132c, . . . ). And, the second waveguide portions (e.g., waveguide portion734,736,738, . . . ) of the pairs750,752,754have unique physical path-lengths within the second arm120but the same physical path-lengths corresponding second waveguide portions of the first waveguide arm117(e.g., same physical light path-lengths as the waveguide portions134,136,138, . . . , respectively).

Such embodiments can facilitate changing the physical light path-lengths through the two waveguide arms, which as disclosed in the context ofFIG. 8A-8C, can surprisingly facilitate providing both fine and coarse wavelength tuning of wavelength where the filter102exhibits athermal behavior.

FIG. 8Apresents exemplary fine tuning of resonance shift curves versus carrier frequency for a temperature change of 10° K for an example apparatus100having a filter design analogous to that depicted inFIG. 7. For the purposes of illustration a reference resonance shift curve (ΔL 0/0) is presented. The reference resonance shift curve corresponds to an arbitrary reference total physical path-length (e.g., about 5000 to 20000 microns in some embodiments) and physical path length difference (e.g., about 500 to 5000 microns in some embodiments) between first and second waveguide arms117,120for an arbitrary combination of physical path-lengths obtained by selecting an arbitrary combination of discrete waveguide portions. A first example fined-tuned curve (ΔL +250/+250) corresponds to a scenario where the total physical path-lengths in the first and second waveguide arms117,120have both been increased by a same amount by tuning to select a combination of discrete waveguide portions to provide a 250 micron longer path-length. A second example fined-tuned curve (ΔL −250/−250) corresponds to a scenario where the total path-lengths in the first and second waveguide arms117,120have both been decreased by a same amount by tuning to select a combination of discrete waveguide portions to provide a 250 micron shorter path-length.

FIG. 8Aillustrates that for some such embodiments, by changing the physical path-lengths of both of the waveguide arms117,120by a same amount, e.g., by tuning to select discrete waveguide portions in both waveguide arms, while keeping difference in the path-length between the arms constant, but still change the relative physical path-length difference between the arms, the resonance frequency at which athermal behavior occurs can be shifted by relatively small incremental amounts (e.g., about ±0.5 THz for a ±250 micron path-length change).

FIG. 8Bpresents exemplary coarse tuning of resonance shift curves versus carrier frequency for a temperature change of 10° K for an example apparatus100having a filter design analogous to that depicted inFIG. 7. The reference resonance shift curve (ΔL 0/0) corresponds to the same total physical path-lengths and physical path-length difference as described in the context ofFIG. 8A. A first example coarse-tuned curve (ΔL 0/−50) corresponds to a scenario where the total physical path-length in the first waveguide arm117having the wider core width has not been changed as compared to the reference, and, the total physical path-length in the second waveguide arm120with the narrower core width has been decreased by tuning to select discrete waveguide portions to provide a 50 micron shorter path-length. A second example coarse-tuned curve (ΔL 0/+50) corresponds to a scenario where again, the total physical path-length in the first waveguide arm117has not been changed as compared to the reference, and, the total physical path-length in the second waveguide arms120has been increased by tuning to select discrete waveguide portions to provide a 50 micron longer path-length. As illustrated, increasing the physical path-length of the narrower core width waveguide arm120red-shifts the athermal wavelength range, while decreasing the path-length blue-shifts the athermal wavelength range.

FIG. 8Cpresents additional exemplary coarse tuning of resonance shift curves versus carrier frequency for a temperature change of 10° K for an example apparatus100having a filter design analogous that depicted inFIG. 7. The reference resonance shift curve (ΔL 0/0) corresponds to the same total physical path-lengths and physical path-length difference as described in the context ofFIG. 8A. A first example coarse-tuned curve (ΔL +50/0) corresponds to a scenario where the total physical path-length for the second waveguide arm120has not been changed as compared to the reference, and, the total physical path-length for the wider core first waveguide arm117has been increased by tuning to select discrete waveguide portions to provide a 50 micron longer path-length. A second example coarse-tuned curve (ΔL −50/0) corresponds to a scenario where again, the total physical path-length in the second waveguide arm117has not been changed as compared to the reference, and, the total physical path-length in the second waveguide arm120has been decreased by tuning to select discrete waveguide portions to provide a 50 micron shorter path-length. As illustrated, increasing the physical path-length of the wider core width waveguide arm blue-shifts the athermal wavelength range, while decreasing the path-length red-shifts the athermal wavelength range.

FIGS. 8B and 8Cillustrates that for some embodiments, by changing the physical path-lengths of one of the waveguide arms117,120, e.g., by tuning to select discrete waveguide portions in one of the waveguide arms, the resonance frequency at which athermal behavior occurs can be shifted by relatively large incremental amounts (e.g., about ±5 THz for a ±50 micron path-length change).

Based on the examples presented inFIGS. 8A-8C, one skilled in the pertinent arts would understand how the selection of waveguide portions to change the total physical light path-length both of waveguide arms117,120and the difference in physical path-length between the arms117,120could provide a brad range of tuning options for adjusting the wavelength at which athermal behavior occurs.

As noted in the context ofFIGS. 1-2B, as part of providing athermal filter behavior, at least a portion of one of the two waveguide arms has a narrower core width than a wider core width of the other waveguide arm. For instance, in some embodiments, the wider core width210can have a width value in a range from about 300 to about 1000 nanometers and the narrower core width215can have a width value that is in a range from about 0.4 to 0.6 times the width value of the wider core width210.

As previously noted, the waveguide arm with the longest waveguide portion having the narrower core width also has the longest total physical path-length of the two waveguide arms. In some such embodiments the waveguide arm with the longest total physical path-length of the two waveguide arms includes a first fixed extension portion. For instance, as illustrated inFIGS. 1, 4, 6 and 7, in some embodiments, the second waveguide arm120having the narrower second core width216has the longest total physical path-length. In some such embodiments, as illustrated, the second waveguide arm120can include a fixed (e.g., non-tunable) first extension portion160to facilitate the second arm120having a longer physical path-length than the first arm117. As also illustrated, in some embodiments, the first extension portion160can be part of the fixed first waveguide portion123that is configured to receive light exiting the optical splitter107and guide the light to the second waveguide portion124having the narrower core width (inFIGS. 1, 4 and 6), and in some embodiments, to a first one of the discrete optical switches740coupled to first ones of the discrete portions of the set750(FIG. 7). Similarly, as illustrated inFIGS. 3 and 5, the first fixed portion122of first waveguide arm117having the narrower core width can further includes the first extension portion160.

In some embodiments, the presence of the first waveguide extension portion160in the first waveguide length portion (e.g., portion123inFIGS. 1, 4, 6 and 7or portion122inFIGS. 3 and 5) accounts for the majority (e.g., provides at least half) of the total physical path-length difference between the first waveguide arm117and the second waveguide arm120.

For instance, consider an embodiment of the interferometer115where the total physical path-length difference between the first waveguide arm117and the second waveguide arm120equals ΔLtotaland the physical path-length of the first waveguide extension portion160equals ΔL0. In some such embodiments, ΔL0 equals a percent value range from greater than about 50 percent to about 100 percent of the value of ΔLtotal.

As a non-limiting example, consider an embodiment of the interferometer115, configured as shown inFIG. 7, where ΔLtotalequals about 1000 microns. In some such embodiments, ΔL0 can range from about 50 microns to 500 microns and in some embodiments from to about 500 microns to 1000 microns. Based upon the present disclosure, one skilled in the pertinent art would understand how the selection of a specific value for ΔL0 would depend upon on the specific difference in the waveguide core widths of the first and the second waveguide arms as well as the specific lengths of the portions of the first and the second waveguide arm that can be selected via the optical switches.

Continuing with the same example, in some such embodiments, the contribution to ΔLtotalfrom the tunably selectable discrete waveguide portions can range from zero (e.g., no difference in the path-length through the discrete waveguide portions of the first and second arms) to about ±350 micron (e.g., the path-lengths through the pairs750,752,754of discrete waveguide portions of the second waveguide arm120each equal L1 and the path-lengths through the pairs150,152,154of discrete waveguide portions of the first waveguide arm117equal L1+ΔL1, L1+2ΔL1 and L1+4ΔL1, respectively, and ΔL1 equals 50 microns). In one such embodiment, a path-length increment, ΔL0, equal to 500 microns would contribute from about 100 percent (e.g., if there was no difference in the path-lengths through the discrete waveguide portions of the first and second arms) to about 59 percent (e.g., for a 350 micron path-length difference through the discrete waveguide portions of the first and second arms) to the value of ΔLtotal.

Providing different total path-lengths for the different light portions traveling through the two waveguide arms117,120can facilitate the filter102having the desired athermal behavior at a particular target wavelength range. In some embodiments, providing the majority of the difference in path-lengths two waveguide arms117,120via the first waveguide extension portion160length ΔL0 can facilitate shifting filter's resonance frequency such that athermal behavior occurs at about the desired wavelength range. Then fine and/or coarse tuning can be performed, such as described in the context ofFIGS. 8A-8C, to compensate for fabrication errors that cause variations away from the specific desired range of wavelength with athermal behavior.

In some embodiments, the two waveguide arms each include second fixed extension portions that have substantially a same physical path-length and that contribute at least half total physical path-lengths of the respective arms. For instance, illustrated inFIGS. 1 and 2-7, first and second waveguide arms117,120can further include another, and in some embodiments, additional, fixed (e.g., non-tunable) second waveguide extension portion170,175. Such additional extension portions170,175can facilitate providing a constant free spectral range (FSR; e.g., about 100, 50 or 25 GHz in some embodiments) for the wavelength range of interest and providing athermal behavior over such a broad wavelength range.

In some such embodiments, the second fixed extension portion (e.g., extension portion175inFIG. 1or extension portion170inFIG. 3) that is part of the waveguide arm with the longest total physical path-length also has narrower core width and the second fixed extension portion (e.g., extension portion170inFIG. 1or extension portion175inFIG. 3) that is part of the other waveguide arm has wider core width.

As illustrated, in some embodiments, the second waveguide extension portions170,175have substantially a same physical path-length as each other (e.g., the same within ±1, ±0.1, ±0.01 or ±0.001 percent in various different embodiments). As illustrated, in some embodiments, the second waveguide extension portions170,175have different core widths.

For instance, as illustrated inFIG. 7, the first waveguide arm117can further include a fixed second waveguide extension portion170(e.g., of length L2) coupling light exiting a last one of the discrete optical switches (e.g., switch146in some embodiments) coupled to a last one of the parallel pairs (e.g., pair154in some embodiments) of discrete waveguide portions of the set130, to the optical combiner121of the interferometer115. The second waveguide arm120can further include a corresponding fixed second waveguide extension portion175coupling the light exiting a last discrete optical switch (e.g., switch746in some embodiments) that is coupled to a last pair of discrete waveguide portions (e.g., pair754in some embodiments) of the second set730of pairs of discrete waveguide portions, to the optical combiner121. The fixed second waveguide extension portion170of the first waveguide arm117and the corresponding second waveguide extension portion175of the second waveguide arm120can have substantially a same light path-length (e.g., a same length L2). In some embodiments, such as shown inFIGS. 1, 4, 6-7, the fixed second waveguide extension portion170of the first waveguide arm117has a wider core width (e.g., similar to first core width210) and the corresponding fixed second waveguide extension portion175of the second waveguide arm120has the narrower second core width (e.g., similar to second core width215). In some embodiments, such as shown inFIGS. 3 and 5, the fixed second waveguide extension portion170of the first waveguide arm117has the narrower core width (e.g., similar to second core width215) and the corresponding fixed second waveguide extension portion175of the second waveguide arm120has the narrower second core width (e.g., similar to first core width210).

In some embodiments, the fixed second waveguide extension portion170and the corresponding fixed second waveguide extension portion175contributes at least half of the total physical path-lengths of first and second waveguide arms117,120, respectively. For instance, in some embodiments, the length of the fixed second waveguide extension portion170equals a percentage value ranging from greater than about 50 percent to about 99 percent of the total physical light path-length through the first waveguide arm117. For instance, in some embodiments, the length of the corresponding fixed second waveguide extension portion175equals a percentage value ranging from about 85 percent to about 95 percent of a total physical light path-length through the second waveguide arms120.

For instance, consider an example embodiment of an apparatus100similar to that disclosed inFIG. 7where the total physical path-length through the first waveguide arm117equals 9500 microns and the total physical path-length through the second waveguide arm120equals about 9000 microns (e.g., ΔLtotalequals about 500 microns). In some such embodiments, when the length of the fixed second waveguide extension portion170and the corresponding fixed second waveguide extension portion175both equal about 5000 microns, then these portions170,175contribute about 52 and 55 percent of the total path-length of the first and second waveguide arms117,120, respectively. In some such embodiments, when the length of the fixed second waveguide extension portion170and the corresponding fixed second waveguide extension portion175both equal about 8500 microns, then these portions170,175contribute about 89 and 94 percent of the total path-length of the first and second waveguide arms117,120, respectively.

In some embodiments, as illustrated inFIG. 1, the waveguide arms117,120can further include variable optical attenuators180,182integrated therein. The variable optical attenuators can be adjusted to balance the optical power in the waveguide arms117,120e.g., to facilitate having a maximum extinction ratio. Any of the embodiments of the filter102(e.g., such as depicted inFIGS. 3-7) can optionally include such variable optical attenuators180,182integrated within the waveguide arms117,120.

FIG. 9presents an exemplary optical photonic circuit apparatus100(e.g., photonic integrated circuit) that includes an example reconfigurable athermal optical filter102analogous to that depicted inFIG. 7.FIG. 10presents a detail view of a portion of the optical photonic circuit apparatus100presented inFIG. 9.

Some embodiments of the optical filter102can include a MZI115configured with 2×2 optical switches. As illustrated inFIG. 9the apparatus100can further include a light input910coupled to the optical filter102(e.g., via waveguide112). The light input910and the optical filter102are located on a photonic integrated circuit substrate105(e.g., a silicon photonic integrated circuit substrate) and the light input910can be configured to deliver the input light110(FIG. 1) having a wavelength in an optical telecommunication band to the optical splitter107of the filter102. In some embodiments, the light input910can include or be an optical coupling port (e.g., a horizontal edge coupling port or a vertical edge coupling port) while in other embodiments the light input can be or include a light source such as a cavity laser, pumped laser, laser diode, or hybrid laser, distributed feedback (DFB) laser. As further illustrated inFIG. 9, some embodiments of the apparatus100can further include a light output912, such as a second an optical coupling port.

One skilled in the pertinent art would understand how the filter102, could be tuned to provide thermal behavior at a target optical telecommunication wavelength band. For example, embodiments of the filter could be tuned to provide athermal behavior at a wavelength within any one of the common optical telecommunication bands, including the Original (e.g., about 1260 to about 1360 nm), Extended (e.g., about 1360 to about 1460 nm), Short (e.g., about 1460 to about 1530 nm), Conventional (C-band, e.g., about 1530 to about 1565 nm), Long (from e.g., 1565 to about 1625 nm) or Ultralong (e.g., about 1625 to about 1675) bands.

As illustrated inFIGS. 9 and 10, embodiments of the apparatus100can further include discrete including discrete electrodes920located on the photonic integrated circuit substrate105(e.g., metal electrodes formed on the substrate105) and coupled to the optical switches140,142. . . . Each of the electrodes920can be configured provide an electrical signal to control a direction of transit of the input light110through one of the discrete waveguide portions132,134, . . . . For instance, one skilled in the pertinent art would understand how to use such electrodes to control a phase shifter (e.g., thermo-optic, liquid crystal, electro-optic effects, magneto-optic phase shifters) in a MZI switch, or move mirrors or lens in a MEMS switch, so as to direct the light through the switch to a selected one of the waveguide portions.

As also illustrated inFIGS. 9 and 10, to facilitate keeping track of the state of each of the optical switches140,142, . . . to allow deterministic optical path selection, the apparatus100can further include discrete optical power monitors930. For instance, the optical power monitors930can be optically connected (e.g., via tap waveguides940) to at least one of the discrete waveguide portions (e.g., portions132,134, . . . , or portions732,734, . . . , inFIG. 7) for each of the different pairs of tunable waveguide portions (e.g., pairs150,152, . . . ) in the first waveguide arm117and the pairs (e.g., pairs750,752, . . . ), in the second waveguide arm120.

Another embodiment of the disclosure is a method of manufacturing an apparatus.FIG. 11presents a flow diagram of an example method1100of manufacturing an apparatus of the disclosure, such as any of the example apparatuses100described in the context ofFIGS. 1-10.

As illustrated inFIG. 11, with continuing reference toFIGS. 1-10throughout, the method1100comprises a step1105of fabricating an optical filter102. Fabricating the filter (step1105) can include a step1110of providing a substrate105and a step1115of providing an optical layer on the substrate. In some embodiments, as part of step1115, the optical layer can be provided as part of a layer on the substrate (e.g., in some embodiments, a silicon-on-insulator substrate have a silicon optical layer220and buried oxide layer230located on a bulk silicon layer240, e.g., as illustrated inFIGS. 2A-2B). In other embodiments, as part of step1115, the optical layer can be deposited on the substrate using procedure familiar to those in the pertinent art.

Fabricating the filter (step1105) can include a step1120of patterning the optical layer220to form the optical filter102. The patterning step1120can form any of the embodiments of the filter's component parts, including the splitter107, the interferometer115including the interferometer's waveguide arms117,120, the discrete waveguide portions132,134. . . ,732,734. . . and the optical switches140,142. . . ,740,742, . . . , or other optical components of the filter102(e.g., the optical combiner121, coupling waveguides112, tap waveguides940) as described in the context ofFIGS. 1-9. Embodiments of the patterning step1120can include conventional lithographic and etching procedures familiar to those skilled in the pertinent art to remove portions of the optical layer220laying outside of the optical splitter107, waveguide arms117,120or other optical components of the apparatus100.

In some embodiments, to reduce the variability in the dimension of the waveguide arms117,120and other optical component formed in the patterning step1120, the thickness250of the optical layer220(FIGS. 2A-2B) is preferably highly uniform. In some embodiments, as non-limiting examples, the waveguide arms117,120can have a uniform thickness250of about 200 nanometers, 300 nanometers or 400 nanometers and in some embodiments the thickness250is the same across the lengths of the waveguide arms and between the arms within ±5 percent, or, in some embodiments, within ±1 percent or less, or, in some embodiments, within ±0.5 percent.

Some embodiments of the method1100further include a step1130of providing a light input910on the substrate105. The light input910can be configured to deliver the input light110having a wavelength in an optical telecommunication band to the optical splitter107. One skilled in the pertinent art would be familiar with procedures for, e.g., patterning the optical layer220(e.g., to form mirrors, lens, gratings etc. . . . ) and/or depositing or growing semiconductor materials on the substrate as part of constructing the light input.

Some embodiments of the method1100can further include a step1140of forming discrete electrodes920on the substrate105, the discrete electrodes920coupled to the optical switches of the optical filter102. Each of the electrodes920can be configured to provide an electrical signal to control a direction of transit of the input light110through one of the discrete waveguide portions132,134. . . . The discrete electrodes920can be configured to read a photocurrent from optical power monitors located inside the switches.

In some embodiments, step1140can include a forming a metal layer (e.g., a metal electrode contact) on the optical layer220, e.g., via physical or vapor deposition, electro-deposition, electroless or similar process familiar to one skilled in the pertinent art. The metal layer can be formed in patterns such as presented inFIGS. 9-10so as to contact the switches140,142, . . . , and, to provide wire landing pad electrodes on the substrate105to facilitate connection to control circuitry located external to the substrate105or at a different location on the same substrate. Additional metal layers can be formed and patterned to serve as electrodes to providing electric currents as part of, e.g., providing thermo-optic control of the switches.

Although the present disclosure has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the scope of the invention.