Patent Application
20230098658
The invention relates to fiber optical communications, and more particularly to tips that are 3D printed onto the end of a fiber to enhance the coupling of light into and out of the fiber.
Connections between standard single-mode fibers and photonic integrated circuits tend to have relatively high coupling losses due to a difference in mode size and profile between both waveguides. Edge coupling strategies involving specialty fibers are frequently used to obtain the best performance in terms of coupling efficiency, bandwidth and polarization independence.
The integration of many different optical components and functionalities into a compact device or chip has been actively investigated for many years. Research into these so-called Photonic Integrated Circuits (PIC) has led to three main material platforms, namely silicon (Si), silicon nitride (SiN) and indium phosphide (InP). All of these platforms have their intrinsic merits and challenges, and are found to be particularly useful for specific components and applications. The Si-platform can make use of high-yield optimized processing techniques compatible with the complementary metal-oxide-semiconductor (CMOS) manufacturing industry for the fabrication of high-volume and small-footprint chips, whereas SiN is showing better performance for passive components in terms of propagation loss and transparency at visible wavelengths, at the expense of a larger footprint. Finally, the InP-platform allows monolithic integration of active components and hence is the platform-of-choice for optical amplifiers and lasers.
While the many developments in chip-level components bring various fascinating opportunities and applications, one of the limiting factors in the widespread adoption of PICs is their packaging, and in particular their connection to the outside world. As the optical modes in integrated waveguides tend to be significantly smaller than those in standard single-mode fibers, creating a highly efficient, robust and alignment-tolerant fiber-to-chip interface remains a challenge.
The two main coupling strategies that have been conventionally investigated are grating couplers and edge couplers. Grating couplers are not as efficient as edge couplers, and their efficiency is dependent on the wavelength and polarization of the light, which can be restrictive when the PIC is intended to operate over a range of wavelengths and/or polarization is not controlled. The edge-coupling approach, also referred to as “in-plane coupling,” generally offers the best optical performance in terms of coupling efficiency, spectral transmission bandwidth and polarization independence. This approach often relies on an intermediate coupling scheme that transforms and matches the modal field of the chip waveguide to that of the single mode fiber. Various approaches to achieving this in the past include the implementation of on-chip taper features, specialty fibers, or a combination of both.
The most frequently used types of specialty fibers for fiber-to-chip coupling are lensed fibers. These end-shaped fibertips (usually conical) can be micro-polished or laser-ablated out of standard optical fibers to produce a focal spot down to about >1 μm (full width at half maximum). This free-space approach of coupling is subject to Fresnel reflections at the optical interfaces of the glass fiber and the semiconductor chip. Anti-reflection coatings can improve the coupling efficiency, but they also introduce a wavelength dependence, negatively affecting one of the important advantages that the edge coupling approach has over grating coupling. Secondly, the lensed approach is limited in its design freedom on the shape and size of the focused spot, making it hard to exactly match to the chip's modal fields. Third, the additional step of providing the antireflection coating increases the cost of the PIC product.
As an alternative, research has been done on the use of ultra-high numerical aperture (UHNA) fibers to create a true butt-coupled (physical contact) fiber-to-chip connection. This technique permits the use of an index-matching medium to minimize reflections and is more suitable for mutual alignment of an array of fibers in multifiber edge-coupling structures. On the other hand, these UHNA fibers do not come in a large variety, have little freedom in the desired mode-field diameter (MFD), which is usually between 3 and 5 μm, and require thermal core expansion to achieve efficient coupling to standard single mode fibers. The latter process is cumbersome and its repeatability could still greatly be improved.
There remains a need, therefore, to find improved approaches for coupling between optical fibers, particularly single mode optical fibers, and PICs.
One embodiment of the invention is directed to a multifiber optical device that has a multi-fiber aligning element, a first optical fiber terminating at a first fiber end at the multi-fiber aligning element and at least a second optical fiber terminating at a second fiber end at the multi-fiber aligning element. A first tapered optical element is 3D printed on the first fiber end; and a second tapered optical element is 3D printed on the second fiber end. The first tapered optical element and the second tapered optical element have respective coupling ends flush with an end face of the multi-fiber aligning element.
Another embodiment of the invention is directed to a fiber-coupled optical chip device, that includes a first optical fiber having a first end. A first down-tapered optical element is 3D printed on the first end of the first optical fiber. The first down-tapered optical element has a fiber end proximate the first end of the first optical fiber and a coupling end distal from the fiber end. An optical chip has a first waveguide configured for edge coupling. The coupling end of the first down-tapered optical element is aligned with the first waveguide of the optical chip.
Another embodiment of the invention is directed to a fiber optic device that includes an optical fiber having a first end face and a core. A tapered optical element is 3D printed on the first end face of the optical fiber. The tapered optical element has a fiber end and a coupling end, the fiber end of the tapered optical element being aligned with the core of the optical fiber. The fiber end of the tapered optical element has a first cross-sectional shape and the coupling end of the tapered optical element has a second cross-sectional shape different from the first cross-sectional shape.
Another embodiment of the invention is directed to an optical device that includes an optical fiber having an end face. A tapered optical element is 3D printed on the end face of the optical fiber. The tapered optical element has a fiber end proximate the optical fiber and a coupling end distal to the optical fiber. The tapered optical element has an output face at the coupling end, where the output face is configured to alter divergence of light passing through the output face.
Another embodiment of the invention is directed to an optical device that includes an optical fiber having an end face. An optical element is 3D printed on the end face of the optical fiber, the optical element having a fiber end proximate the optical fiber and a coupling end distal to the optical fiber. An optical axis of the optical fiber passes through the optical element. The optical element has a refractive index gradient in a direction perpendicular to the optical axis.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
The approach described herein may be useful for optimizing the fiber side of the fiber-to-chip connection and tailoring the design of the fiber taper to the on-chip waveguide schemes employed in photonic integrated circuit (PIC) devices. The fabrication of free-standing down-taper structures, using two-photon direct laser printing, directly on top of cleaved fiber facets is described. These tapers can find utility for coupling between a fiber and silicon, silicon nitride and indium phosphide PIC chip platforms. The tapered structures may be tailored to provide flexibility in the output modal fields and coupling efficiencies.
An exemplary coupling arrangement 100 is schematically illustrated in
The tapered optical element 102 may be printed on the end facet 110 of the optical fiber 104 using two-photon polymerization (2PP)-based direct laser writing using a dip-in geometry. This fabrication technology allows the creation of 3D structures with design freedom and submicrometer resolution. A taper structure with excellent modal match in terms of shape and size may be fabricated directly on the end face of the optical fiber such that it can be butt-coupled with the photonic chip to minimize reflections. Furthermore, the design flexibility associated with the 2PP, 3D printed, tapered optical element 102 does not put extra constraints on any on-chip coupling structures. The chip 118 may be coupled to other optical fibers using similar tapered optical elements, for example with the fiber 104 providing an input to the chip 118 and the other fiber or fibers being used for an output or outputs from the chip 118 at a different edge of the chip 118. In other embodiments, for example as discussed below, the fiber 104 may be one of a number of optical fibers aligned to the chip 118 in a multi-fiber alignment device.
The 2PP 3D printing technique has been used in the development of a microstructured antireflective coating, phase masks, lenses, and mode-field expansion up-taper structures on optical fiber tips. In the latter, the fabrication method was demonstrated for use in alignment tolerant physical contact expanded beam fiber-to-fiber connections, as described in K. Vanmol et al., “Two-photo direct laser writing of beam expansion tapers on single-mode optical fibers,” Optics and Laser Technology (2019), vo. 112, pp. 292-298, incorporated herein by reference. The 2PP, 3D printing of tapered optical elements on fibers has also been described in International Publication No. WO2019/165205, also incorporated herein by reference.
The present approach may advantageously reduce reflection losses for light passing between the fiber 104 and the chip 118 by, for example, allowing the fiber coupling end 114 to physically contact the edge of the chip 118 at the waveguide 116, or to optically couple to the waveguide 116 via a short path of index matching material. This approach provides a coupling solution for optical chips that does not require re-engineering of the optical chips. Furthermore, connections formed using the tapered optical element can be designed with complete 3D freedom to provide a high degree of matching between the mode-field of the chip 118 and that of the optical fiber 104.
The embodiment of a down-tapered optical element schematically illustrated in
An eigenmode solver in the MODE simulator of the Lumerical Device Multiphysics Simulation Suite, available from Lumerical Inc., Vancouver, Canada, was used to model the modes of the fiber and the waveguides in various chip platforms. All simulations were performed at a wavelength of 1550 nm, which is one of the main wavelengths used in single mode fiber and PIC technology, due to low material absorption in that spectral range and the many telecom-related applications that come with it.
At the coupling end of the down-taper optical element, the polymer core is advantageously mode-matched to the PIC waveguide mode profile. The modal fields of five different types of PICs, across three different material platforms, were modeled.
Two different chips with different input/output waveguide strategies were examined for the silicon platform. The first, shown in
The design was based on the Transverse Electric (TE) polarization state. An analogous methodology may be used to find optimal coupling design for the Transverse Magnetic (TM) polarization states.
An eigenmode analysis and optimization makes it possible to find the optimal taper input and output dimensions for maximal mode overlap with the optical fiber and the optical chips, respectively. The optimal input polymer waveguide diameter at the fiber side is found to be about 15.1 μm, to mode match with the G.652 single mode fiber. At the output side of the tapered optical element, the optimal coupling dimensions depend on the exact PIC waveguide scheme. Such information is often maintained confidential by the manufacturer of the chip.
Many waveguide geometries have strong elliptical mode profiles, such as the profiles for the SiON SSC shown in
The 2PP 3D laser printing process used to manufacture the tapered optical element conveniently allows a wide variety of shapes to be used at the coupling end of the tapered optical element, some of which are schematically illustrated in
In the embodiments discussed with reference to
The Lumerical Mode Solutions software, referenced above, was used to model propagation of light from the fiber through the tapered optical element. A schematic of the model used for these calculations is provided in
A sweep of the taper length permits a (local) maximum in transmission for the fundamental mode to be found. In general, a larger taper length, which corresponds to a more gradual change in cross section, provides better transmission of the fundamental mode, although this is not always the case. It was found that a taper length of 250 μm is quite adiabatic, giving more than 90% fundamental mode transmission for all output diameters. This taper length limit was chosen to prevent the need for stitching of writing fields in the fabrication process.
While linear tapers provide adiabatic operation, it was found that shorter taper lengths could be achieved using nonlinear tapers. The use of a shorter tapered element reduces the chance that it suffers from mechanical damage, which increases propagation loss. Such nonlinear-shaped tapered optical elements can decrease the total taper length, while keeping the radiation loss from mode-field conversion low. To investigate the usefulness of a nonlinear taper in a tapered optical element, the simulations included a nonlinear function for the diameter of the tapered optical element as a function of distance along the elements, D(z), as follows:
D(z)=D2+(D1−D2)·(1−z/L)m (1)
where L is the length of the taper, m is the nonlinear exponent, and D1 and D2 are the taper's input and output diameters respectively.
The profile of different tapers is shown in
The calculation of the evolution of the fundamental mode's transmission as a function of taper length for each of these three taper profiles is illustrated in
Material absorption was not taken into account in the transmission simulations. Cured IP-DIP polymer has an absorption loss of less than 1 dB/cm at 1550 nm. As the length of the tapered optical elements is typically less than one half of one millimeter, material absorption may be sufficiently small as to be neglected.
Reflection of the incoming light at an interface of different refractive index media can impair the coupling efficiency. Even when two waveguides are perfectly butt-coupled, without having any air-gap between them, a mismatch between the effective indices of both waveguides may generate back-reflections. The reflectance, R, for a normally incident light beam passing an interface from medium 1 to medium 2 is given by:
R=(n2−n1)2/(n2+n1)2 (2)
where n1 and n2 respectively represent the refractive or effective indices of the two media or waveguides.
An optimal mode overlap is targeted for every chip platform, and the difference in effective indices between the tapered fiber and the chip is relatively small, leading to an estimated reflectance of <3% per facet in the Si- and SiN-based platforms. In the InP platform, however, which has a relatively high refractive index, the Fresnel reflection is about 15% per facet.
In the investigation of coupling between (sub)micrometer-scale structures, a misalignment tolerance analysis gives many insights into the realistically achievable efficiency of the designed structure in practice. A good understanding of alignment tolerances aids in making decisions in the design phase, in analyzing experimental results, and in defining packaging strategies.
In a butt-coupled (physical contact) connection, lateral misalignments have a higher impact on loss than axial and angular misalignments. The consequences from misalignments are found to be larger with decreasing mode-field diameters. Simulations show that a submicrometer accurate alignment is needed for all chip platforms (Si, SiN and InP) in order to achieve <1 dB misalignment losses. In general, a larger misalignment tolerance may be achieved by making use of wider taper tips, but this may result in lower coupling efficiency.
It may be possible to introduce a small misalignment between the tapered optical element and the fiber core during the taper fabrication process. Nevertheless, the use of high-magnification real-time imaging of the fiber end-facet during the printing process can result in the achievement of submicrometer positioning control.
This disclosure has, so far, only considered an air-clad tapered optical element. However, index matching materials play an important role in photonic chip packaging. Such materials usually come in the form of gels or UV-curable epoxies and may be used to fill the air-gap between the chip and the tapered optical element. Judicious selection of the material may result in reducing unwanted reflections at the optical interfaces, thus reducing transmission losses.
The index matching material may, at the same time as providing index matching, be used as an effective cladding for the down-tapered waveguide, and provide mechanical support to the tapered optical element. The cladding may also prevent contamination of the taper interface that could lead to undesired radiation losses and may possibly also decrease possible scattering due to roughness at the taper's side wall.
A decrease in refractive index difference between the tapered optical element, which is its own waveguide, and the cladding (Δn) reduces the degree of light confinement in the tapered optical element, therefore increasing the size of its mode-field. As such, the minimal mode-field diameter that is possible to obtain increases with decreasing index contrast, as can be seen in
A Photonic Professional GT+ system from Nanoscribe GmbH, Eggenstein-Leopoldshafen, Germany, was used in the dip-in configuration to fabricate various tapered optical elements directly on the end-facet of cleaved G.652 single mode fibers.
The fiber core position is identified by sending red laser light through the fiber and then aligned with the printer axes. The full tapered optical element may be printed in a matter of minutes using the built-in galvo scanner. The processing steps may be carried out without any specific environmental restrictions, e.g. temperature or humidity control, other than processing the undeveloped material in a room with UV-filtered lighting conditions to prevent undesired single-photon polymerization of the resist.
A typical set up 900 for dip-in laser lithography (DiLL), two photon polymerization, 3D printing process is schematically illustrated in
The liquid resist 906 undergoes a multi-photon polymerization reaction at the focal point 912, in most cases a 2-photon reaction, and solidifies. The correct selection of the optical power of the light 908 and the focusing properties of the focusing unit 910 can, therefore, result in the volume element (voxel) where the liquid resist 906 reacts with the light 908 being extremely small, and may be submicron in size. The tapered optical elements discussed below were fabricated using a voxel of around 150 nm×150 nm×500 nm, obtained using a 63×objective with a numerical aperture (NA) of 1.4. The fiber end facet 902 and/or the focal point 912, can be moved in the x-y plane (the y-direction lies out of the plane of the figure) which, together with selective switching of the light 908, results in writing a layer of solidified resist having a desired 2D pattern. The end facet 902 can then be moved in the z-direction by an amount equal to about that of the thickness of the 2D pattern just printed, and another 2D layer can then be printed on top of the previous layer. In this manner, a 3D object can be formed by sequentially printing a number of 2D layers. An advantage of DiLL, in which the liquid resist 906 is polymerized at a point between the fiber 904 and the focusing unit 910 is that the height (z-direction) of the resulting 3D structure is not limited by the focal length of the focusing unit 910.
A system suitable for use in DiLL is the Photonic Professional GT 3D laser lithography system available from Nanoscribe GmbH, Eggenstein-Leopoldshafen, Germany. DiLL is further described in U.S. Pat. No. 9,302,430, incorporated herein by reference. The resist used to fabricate the tapered optical elements is IP-DIP, obtainable from Nanoscribe GmbH. The writing speed and power were according to the manufacturer's specifications.
Good adhesion of the polymer tapered optical element to the fused silica (SiO2) end-facet 902 is an important consideration. For this reason, in some embodiments, a pedestal base layer is first formed on the end facet 902 prior to forming the tapered body, where the base plate has a larger area in contact with the end facet 902 than would the fiber end of the tapered body alone. The base plate layer may be any suitable thickness. In some embodiments, a base plate layer having a thickness of about 6 μm is employed. Simulations show that such a thin base layer has negligible impact on the light transmission.
Additionally, an adhesion promotor may be used to enhance the adhesion of the polymerized material to the surface of the fiber's end facet 902. In some embodiments, the adhesion promotor is a silane, for example, 3-(methacryloyloxy)propyltrimethoxysilane ethanol solution (2%, v/v), which may be applied by dipping the fiber end into the silane solution. This chemical surface treatment may enhance the formation of covalent bonds between the organic polymer and the inorganic fiber silica glass upon laser exposure, effectively connecting both materials. In an experiment to test the ability of the silane to promote adhesion to glass, when an increasing shear force was applied to a tapered optical element that was written without a silane-pretreatment on the glass surface, the tapered optical element broke free of the glass at an applied force of 0.015 N. In contrast, when the experiment was performed on a similar tapered optical element that was written on the glass after a silane-pretreatment, the tapered optical element resiliently withstood an applied shear force of 0.15 N.
Submicrometer fabrication tolerances are obtained with the 2PP direct laser writing system, which is quantitatively verified using an SNE-4500M scanning electron microscope (SEM) (Hirox Europe, Limonest, France), a Contour GT-I white-light interferometer (Bruker Corp, Billerica, Mass., USA), and a VideoCheck UA400 coordinate measurement machine (Werth Messtechnik GmbH, Giessen, Germany), SEM images of the linear and nonlinear down-taper models are shown in
The output modal fields of the tapered optical elements were characterized using a Bobcat SWIR camera from Xenics nv (Leuven, Belgium), accompanied by a Zeiss 100×NA0.9 microscope objective (Carl Zeiss Microscopy GmbH, Jena, Germany) allowing high-magnification and high-resolution images of the modes at telecom wavelengths. An experimentally obtained mode-field image from the end-facet of a linear tapered optical element having a tip diameter of 3 μm is shown in
The point 1108 shows the measured MFD for the thick nonlinear taper having an output diameter of 2 μm. The MFD of the thick nonlinear taper is only slightly larger than that of its linear counterpart, which may be caused by the change in nonlinear taper diameter being more abrupt close to the coupling end of the element, making it more sensitive to the layered fabrication approach inherent to the 3D printing process.
Coupling experiments were carried out using fibers terminated with a tapered optical element having a linear profile, fibers terminated with a tapered optical element having a nonlinear profile and, for comparison with conventional approaches, commercially obtained lensed fibers. The results reported below for the linearly tapered optical element were obtained using an element of around 250 μm long and coupling tip diameters ranging from 1 μm to 7 μm. The linearly tapered optical element had a base like that shown in
The experimental set-up 1200 used in the coupling experiments is schematically illustrated in
A reference measurement, where the chip and functionalized fibers are removed, was made using the experimental set-up 1250, schematically illustrated in
A summary of the minimum coupling losses for each of the PIC platforms is given in Table I below, for the linear tapered optical element and the lensed fiber. For four out of the five PIC platforms the tapered optical element was found to incur lower losses than the lensed fiber. with an improvement in coupling efficiency of respectively 0.97 dB, 0.44 dB, 0.03 dB and 1.43 dB for the SiON, SiN, double-strip SiN, and InP platforms. For the Si-platform, it was found that the linear tapered optical element achieved a slightly lower coupling efficiency (0.59 dB) than for the lensed fiber. It is believed this was due to a small mismatch in mode profile, which is expected to be improved with further development. These observations show that it is beneficial to use an optimized mode-matching design so as to reduce the overall coupling losses in fiber-to-chip connections. An additional advantage of using a tapered optical element over lensed fibers is that a physical contact through butt-coupling can be used, which facilitates a more robust packaging of the PIC. Such approaches are described in more detail below.
In addition, a thick nonlinear tapered optical element (m=1/2) was fabricated on single mode fibers and the per facet coupling loss determined. As discussed above, the nonlinear taper is advantageous in that it may be shorter than its linear equivalent while still transmitting a similar fraction of the optical signal. The nonlinear tapered optical elements were formed with a coupling tip diameter of about 2 μm and a length of about 63 μm. They were tested with the Si and InP platforms, which had been shown to have a minimum coupling loss for a linear tapered optical element having a coupling tip diameter of around 2 μm. The coupling between the Si-chip and the nonlinear tapered optical element was found to produce a per facet coupling loss of −2.78 dB, and −4.27 dB for the InP chip. These results show that the use of a nonlinear taper profile may allow the length of the tapered optical element to be significantly decreased, with only a small penalty, around 0.1 dB, in terms of coupling efficiency.
A coupling comparison was made over a wide range of wavelengths between fibers using a linear tapered optical element and lensed fibers, around 1525 nm-1630 nm, which encompasses the C and L telecommunication bands. For this experiment, the light source 1202 was a Tunics T100S-HP tunable laser obtained from EXFO Inc., Quebec City, Canada. Facet coupling losses were obtained using the SiON platform. The linear tapered optical element had a coupling tip diameter of 6 μm, which was found to be the optimal size for the SiON platform. The results of the comparison are shown in
As the tapered optical element coupling scheme makes use of a true butt-coupled approach, it is advantageous that no damage occurs to either the chip of the tapered optical elements upon physical contact. It has been found that the polymer tapered optical elements are sufficiently bendable, such that the element would bend upon physical contact with the chip without causing any damage (when using translation stages, under practical and realistic circumstances). Unless excessive force is applied, the taper returns to its original straight shape when it is withdrawn from the chip facet, and no degradation in coupling loss is observed.
Tapered optical elements may be used for single fiber connections, for example to a PIC or to another fiber. In fiber-fiber coupling, the tapered optical element may be referred to as an “up-taper” where the cross-sectional area of the element becomes larger for distances further from the base.
There is an increasing need to provide multiple fiber connections in a single device. For example, MPO connectors have become widely used in in fiber-to-fiber connections, and remains a need to provide multiple fiber connections to an optical chip. These connections need to be efficient, i.e. introduce low loss, and be mechanically sturdy to withstand the rigors of deployment in different environments.
Some embodiments of the invention are directed to providing multi-fiber connectivity between a fiber ribbon and another plurality of waveguides by terminating the fibers of the fiber ribbon with tapered optical elements, and housing the fibers in a multi-fiber aligning element. The other plurality of waveguides may comprise, for example, cores of fibers of another ribbon cable, or waveguides in an optical chip. A multi-fiber aligning element is an element that holds multiple optical fibers in a defined, spatial relation. In some embodiments the multi-fiber aligning element may be the ferrule of a multi-fiber coupler, such as an MPO coupler. In other embodiments the multifiber aligning element may be an alignment block provided with alignment grooves. Different embodiments of the invention that provide multi-fiber connectivity are now discussed.
One approach to providing multiple fiber connectivity using fibers mounted in a ferrule, such as an MPO ferrule, and terminated with tapered optical elements is now discussed with reference to
The ribbon cable 1502 is then inserted into a ferrule 1508, for example an MPO ferrule, that is adapted to receive multiple fibers, as is schematically illustrated in
Tapered optical elements 1514 are then fabricated on the ends 1510 of the fibers 1504, for example in a manner as described above, as schematically illustrated in
Next, as is schematically illustrated in
The end surface 1512 of the ferrule 1508 may then be polished, removing any portion of the tapered optical elements 1514 extending beyond the end surface 1514 of the ferrule 1508, along with any excess adhesive 1516′ that extends beyond the end surface 1512. The resulting multi-fiber coupling device 1519, as schematically illustrated in
In some embodiments, the tapered optical elements 1514 within the ferrule 1508 have the same coupling tip dimensions. In other embodiments, one or more of the tapered optical elements 1514 may be larger or smaller than others within the same ferrule 1508.
If the multi-fiber coupling device 1519 is to be used for coupling to an optical chip 1530, as schematically illustrated in
The step of polishing the ends of the tapered optical elements 1514 may remove some material from the tapered optical elements 1514. In some embodiments, the tapered optical elements may be printed with an excess of material at the coupling tip that is provided for polishing off so that, once the polishing process has completed, the tapered optical elements 1514 have the desired length.
Another embodiment of multi-fiber coupling that implements the use of tapered optical elements on the end of single mode fibers is now described with reference to
Fibers 1706 are placed in the alignment grooves 1704, as schematically illustrated in
A cover 1716 may be placed over the exposed claddings 1712 of the fibers 1706, as is schematically illustrated in
The end face 1714 of the alignment block 1702 may then be polished to ensure that it is flat and to remove any portions of the tapered optical elements 1708 that overhang the end face 1714 of the alignment block 1702, as is schematically illustrated in
In some embodiments, the alignment block 1702 may be positioned adjacent the optical chip 1720 so that the tapered optical element 1708 butt-couple to the waveguides 1722 of the optical chip 1720. An exemplary embodiment of such a configuration is schematically illustrated in
Another approach to coupling multiple fibers to an optical chip using tapered optical elements is described with reference to
Tapered optical elements 1708 may then be printed onto the ends of the fibers 1706, in a manner like that discussed hereinabove, as is schematically illustrated in
The alignment block 1702 and the optical chip 1720 may then be brought close together, so that the tapered optical elements 1708 align, and are in contact, with the waveguides 1722 of the optical chip 1720. The gap 1728 between the alignment block 1702 and the chip 1720 may then be filled with an adhesive 1726 and cured so that the alignment block 1702 chip 1720 remain in the same relative position, as shown in
Other embodiments of optical elements may be employed in addition to those discussed above. For example, a tapered optical element may be provided with a curved output surface instead of a flat output surface, so that divergence of the light exiting or entering the element is changed. An embodiment of a tapered optical element 1902 having a curved output surface 1904 is schematically illustrated in
Another embodiment of tapered optical element 1922 having a curved output surface 1924 is schematically illustrated in
Another embodiment of tapered optical element 1942 having a curved output surface 1944 is schematically illustrated in
Some embodiments of printed optical element may include spatial variations in the refractive index. For example, where the printed optical element has a refractive index profile that has a higher refractive index on-axis than off-axis, in other words it has a refractive index gradient in a direction that is perpendicular to its axis, then the printed element may operate as a gradient-index (GRIN) element. One embodiment of a GRIN printed tapered optical element 2002 is schematically illustrated in
The refractive index of the printed material may be varied by altering the writing speed and/or the writing power. This is described, for example, in Zukauskas A. et al., “Tuning the refractive index in 3D direct laser writing lithography: towards GRIN microoptics,” Laser Photonics Rev. (2015) 9 706-712, DOI 10.1002/Ipor.201500170, incorporated herein by reference. A greater refractive index is obtained when the degree of conversion (DC, the percentage of carbon double bonds that are consumed during the polymerization process) is higher, which is obtained by exposing the material to more light. Thus, by increasing the DC in those volume elements (voxels) of the tapered optical element 2002 where it is intended that the refractive index be increased, a desired 3D refractive index profile may be obtained in the tapered optical element 2002.
Another embodiment of a GRIN printed tapered optical element 2022 is schematically illustrated in
Another embodiment of a GRIN printed tapered optical element 2042 is schematically illustrated in
The graded index profile may be present throughout the entirety of the tapered optical element 2042 or, as in the illustrated embodiment, the graded index profile may be restricted to that portion of the tapered optical element 2042 which light from the fiber 2048 will pass through. The dashed lines 2056 indicate the expected divergence of the light from the fiber 2048, and the GRIN material is found between the dashed lines 2056. Since the slower writing speed associated with voxels of relatively high refractive index increases the length of time required to manufacture the tapered optical element 2042, this approach reduces the costs of making the element 2042, as it avoids writing GRIN voxels where the light from the fiber 2048 does not pass through.
The description of the coupling devices is primarily described in terms of optical signals propagating from the fiber to the optical chip. It should be understood that the embodiments of the invention described herein operate for light propagating both from the fiber to the chip and from the chip to the fiber, and there is no intention in the present description to limit the direction in which optical signals propagate through the claimed optical devices, unless otherwise stated.
As noted above, the present invention is applicable to fiber optical communication and data transmission systems. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims.
This application is being filed on Mar. 23, 2021 as a PCT International Patent Application and claims the benefit of U.S. Patent Application Ser. No. 62/993,449, filed on Mar. 23, 2020, and claims the benefit of U.S. Patent Application Ser. No. 63/046,479, filed on Jun. 30, 2020, the disclosures of which are incorporated herein by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/023744 | 3/23/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62993449 | Mar 2020 | US | |
| 63046479 | Jun 2020 | US |
