Apparatus and method for an optical coupler

Abstract

To provide an optical resonator having a higher quality factor.SOLUTION: An optical coupler 100 comprising a first core 101-1 and a second core 101-2, each of the first core and the second core being covered by a cladding 119, wherein a first spacing between a first surface of the first core and a second surface of the second core is adiabatically tapered to narrow toward a second portion of the first core and the second core; The width of the second portion of the second core tapers to become narrower or wider from the first portion of the second core or after the first portion of the second core towards the third portion of the second core; The width of the third portion of the second core is narrower than the width of the first core, and a second spacing between the first surface along the third portion of the first core and the second surface along the third portion of the second core adiabatically tapers wider away from the first core and the second portion of the second core.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] (Statement regarding federally sponsored research and development) This invention was made with government support. The government has certain rights in this invention. [Background technology]

[0002] Optical couplers are widely used in optical systems. Conventional optical couplers have higher than desirable losses. Such losses result from the emission of optical energy received by the optical coupler into free space and / or from coupling energy into undesired modes. Summary of the Invention

[0003] In some aspects, the technology described herein provides an optical coupler including: a first core including a first portion, a second portion, and a third portion, the second portion of the first core being optically connected between the first portion and the second portion of the first core, the first core having a first surface; and a second core including a first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, the second core having a second surface, each of the first core and the second core being covered by a cladding, each of the first core and the second core having a refractive index higher than the refractive index of the cladding, an optical coupler in which a first spacing between a first surface along the first portion and a second surface along the first portion of the second core adiabatically tapers to narrow toward the first core and the second portion of the second core; a width of the second portion of the second core tapers to narrow or widen from or after the first portion of the second core toward a third portion of the second core; a width of the third portion of the second core is narrower than the width of the first core; and a second spacing between the first surface along the third portion of the first core and the second surface along the third portion of the second core adiabatically tapers to widen away from the first core and the second portion of the second core.

[0004] In some aspects, techniques described herein include a method of reducing insertion loss in an optical coupler including a first core including a first surface and a second core including a second surface, the method comprising receiving an input optical signal consisting of only a transverse electric fundamental mode or a transverse magnetic fundamental mode at a first port of a first portion of the second core, the second core including a first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, a first spacing between the first surface along the first portion of the first core and the second surface along the first portion of the second core adiabatically tapering toward the first core and the second portion of the second core, each of the first cores being surrounded by a cladding; receiving a first core and a second core, each having a refractive index greater than the refractive index of the cladding; coupling a combined optical signal from a second portion of the second core into the second portion of the first core, the combined optical signal comprising at least a portion of the power of the input optical signal; emitting a first output optical signal from a third portion of the second core, the first output optical signal comprising at least another portion of the power of the input optical signal; and emitting a second output optical signal from the third portion of the second core, the second output optical signal comprising at least a portion of the power of the combined optical signal, the second spacing between a first surface along the third portion of the first core and a second surface along the third portion of the second core being adiabatically tapered to be wider away from the first core and the second portion of the second core.

[0005] In some aspects, the techniques described herein provide an optical resonator, comprising: a first optical coupler; a first core including a first portion, a second portion, and a third portion, the second portion of the first core being optically connected between the first portion and the second portion of the first core, the first core having a first surface; and a second core including a first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, the second core having a second surface. each of the first core and the second core is covered by a cladding, and each of the first core and the second core has a refractive index higher than that of the cladding; a first spacing between a first surface along a first portion of the first core and a second surface along a first portion of the second core is adiabatically tapered to narrow toward the first core and the second portion of the second core; and a width of the second portion of the second core narrows or widens from the first portion of the second core or after the first portion of the second core to a third portion of the second core. a first optical coupler, wherein the width of the third portion of the second core is narrower than the width of the first core, and a second spacing between a first surface along the third portion of the first core and a second surface along the third portion of the second core is adiabatically tapered to widen away from the first core and the second portion of the second core; and a second optical coupler, wherein the third core includes a first portion, a second portion, and a third portion, and the second portion of the third core is optically connected between the first portion and the second portion of the third core. a third core having a third surface; and a fourth core including a first portion, a second portion, and a third portion, the second portion of the fourth core being optically connected between the first portion and the second portion of the fourth core, the fourth core having a fourth surface, each of the third core and the fourth core being covered by a cladding, each of the third core and the fourth core having a refractive index higher than the refractive index of the cladding, a third spacing between the third surface along the first portion of the third core and the fourth surface along the first portion of the fourth core beingadiabatically tapering toward the third core and the second portion of the fourth core, the width of the second portion of the fourth core tapering from or after the first portion of the fourth core toward the third portion of the fourth core, the width of the third portion of the fourth core being narrower than the width of the third core, a fourth spacing between a third surface along the third portion of the third core and a fourth surface along the third portion of the fourth core adiabatically tapering away from the third core and the second portion of the fourth core, the first resonator optical waveguide optically connected between a first port of the first portion of the first core and a second port of the third portion of the fourth core, and the second resonator optical waveguide optically connected between the second port of the third portion of the first core and a first port of the first portion of the fourth core. [Brief explanation of the drawings]

[0006] Example embodiments will be described with additional specificity and detail using the accompanying drawings, with the understanding that the drawings depict example embodiments only and therefore should not be considered limiting in scope. [Figure 1] 1 illustrates a plan view of an embodiment of an optical coupler having reduced insertion loss. [Figure 2] 2 illustrates a cross-sectional view of an optical coupler with reduced insertion loss taken along line AA-AA′. [Figure 3] 1 illustrates a plan view of an optical cavity including two optical couplers with reduced insertion loss. [Figure 4] 1 illustrates one embodiment of a method for optically combining optical signals in an optical coupler.

[0007] According to common practice, the various illustrated features are not drawn to scale but rather to emphasize specific features relevant to the exemplary embodiments. Reference characters denote like elements throughout the figures and text. DETAILED DESCRIPTION OF THE INVENTION

[0008] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and structural, mechanical, and electrical changes may be made. Furthermore, the methods presented in the drawings and specification should not be construed as limiting the order in which individual steps may be performed. Therefore, the following detailed description is not to be construed in a limiting sense.

[0009] An embodiment of the present invention is an optical coupler that has a lower insertion loss than conventional optical couplers. Furthermore, such lower insertion loss is over a wider bandwidth compared to conventional optical couplers. Optical couplers with lower insertion loss can be used to create resonators with higher quality factors, thus requiring lower optical pump sources.

[0010] FIG. 1 illustrates a plan view of one embodiment of an optical coupler (or "optical coupler") 100 with reduced insertion loss. The optical coupler 100 includes a first core 101-1 and a second core 101-2. Each of the first core 101-1 and the second core 101-2 is surrounded by a cladding 119. Thus, the refractive index of each core 101-1, 101-2 is greater than the refractive index of the cladding 119. The first core 101-1 surrounded by the cladding 119 forms a first optical waveguide 111-1. The second core 101-2 surrounded by the cladding 119 forms a second optical waveguide 111-2. Optionally, each core may be made of silicon nitride, and the cladding may be made of silicon dioxide.

[0011] For teaching purposes, first core 101-1 is illustrated as being linear and second core 101-2 is illustrated as being substantially linear, although in other embodiments, as described elsewhere herein, first core 101-1 may not be straight, e.g., may be curved, and second core 101-2 may not be substantially straight, e.g., may be curved. Optionally, each of first optical waveguide 111-1 and second optical waveguide 111-2 is a planar optical waveguide formed on optional substrate 102, which may, for example, optionally be a semiconductor or insulator, although each of optical waveguides 111-1, 111-2 may be formed by other types of optical waveguides.

[0012] Each of the first core 101-1 and the second core 101-2 includes a first portion 101-1-1, 101-2-1, a second portion 101-1-2, 101-2-2, and a third portion 101-1-3, 101-2-3. The first portions 101-1-1, 101-2-1 are adjacent to each other over a first length L1. The second portions 101-1-2, 101-2-2 are adjacent to each other over a second length L2. The third portions 101-1-3, 101-2-3 are adjacent to each other over a third length L3. A line AA-AA' bisects the adjacent second portions 101-1-2, 101-2-2.

[0013] Each of the sections 101-1-1, 101-2-1, 101-1-2, 101-2-2, 101-1-3, 101-2-3 has a first port P1-1-1, P1-1-2, P1-1-3, P1-2-1, P1-2-2, P1-2-3 and a second port P2-1-1, P2-1-2, P2-1-3, P2-2-1, P2-2-2, P2-2-3. The second ports P2-1-1, P2-2-1 of the first part 101-1-1, 101-2-1 of the core, i.e., the first core 101-1 or the second core 101-2, are optically connected to the first ports P1-1-2, P1-2-2 of the second part 101-1-2, 101-2-2 of the core. The second ports P2-1-2, P2-2-2 of the second part 101-1-2, 101-2-2 of the core are optically connected to the first ports P1-1-3, P1-2-3 of the third part 101-1-3, 101-2-3 of the core.

[0014] The first port P-1-1-1 of the first portion 101-1-1 of the first core 101-1 is also the first port of the first optical waveguide 111-1. The second port P-2-1-3 of the third portion 101-1-3 of the first core 101-1 is also the second port of the first optical waveguide 111-1. The first port P-1-2-1 of the first portion 101-2-1 of the second core 101-2 is also the first port of the second optical waveguide 111-2. The second port P-2-2-3 of the third portion 101-2-3 of the second core 101-2 is also the second port of the second optical waveguide 111-2.

[0015] The first core 101-1 has a first width W1-1. Optionally, the first width W1-1 is constant along the first core 101-1, for example, in each of the first portion 101-1-1, the second portion 101-1-2, and the third portion 101-1-3 of the first core 101-1.

[0016] The second core 101-2 has a variable width. The first portion 101-2-1 of the second core 101-2 has a second width W2-1. Optionally, the second width W2-1 is constant. The second portion 101-2-2 of the second core 101-2 has a tapered width, e.g., an adiabatic tapered width, that narrows or widens after the first port P1-2-2 of the second portion 101-2-2 of the second core 101-2 in a direction toward the second port P2-2-2 of the second portion 101-2-2 of the second core 101-2. This facilitates coupling a portion of the input optical signal IOS to the second portion 101-1-2 of the first core 101-1. Therefore, the third width W2-2-1 at the first port P1-2-2 of the second portion 101-2-2 of the second core 101-2 is greater than the fourth width W2-2-2 at the second port P2-2-2 of the second portion 101-2-2 of the second core 101-2.

[0017] The third portion 101-2-3 of the second core 101-2 has a fifth width W2-3. The fifth width W2-3 anywhere along the third portion 101-2-3 of the second core 101-2 is smaller (or narrower) than the first width W1 anywhere along the first core 101-1. Optionally, the fifth width W2-3 is constant along the third portion 101-2-3 of the second core 101-2.

[0018] The first core 101-1 has a first surface S1 facing and adjacent to the second surface S2 of the second core 101-2. A first spacing SP1 between the first surface S1 (of the first portion 101-1-1 of the first core 101-1) and the second surface S2 (of the first portion 101-2-1 of the second core) tapers, e.g., adiabatically narrows, after the first port P1-2-1 of the first portion 101-2-1 of the second core 101-2 toward the second port P2-2-1 of the first portion 101-2-1 of the second core 101-2.

[0019] A second spacing SP2 is between the first surface S1 (of the second portion 101-1-2 of the first core 101-1) and the second surface S2 (of the second portion 101-2-2 of the second core 101-2). Optionally, the second spacing SP2 is constant along the second portions 101-1-2, 101-2-2. A third spacing SP3 between the first surface S1 (of the third portion 101-1-3 of the first core 101-1) and the second surface S2 (of the third portion 101-2-3 of the second core 101-2) tapers, e.g., adiabatically widens, after the first port P1-2-3 of the third portion 101-2-3 of the second core 101-2, toward the second port P2-2-3 of the third portion 101-2-3 of the second core 101-2.

[0020] Optionally, the first port of the second optical waveguide 111-2 (or the first port P1-2-1 of the first portion 101-2-1 of the second core 101-2) may be configured to receive the input optical signal IOS, which may have, for example, a single fundamental mode, e.g., a transverse electrical (TE) mode (TE) or a fundamental transverse magnetic (TM) mode (TM). A portion, for example, substantially all, of the input optical signal IOS is incident on the first port P1-2-2 of the second portion 101-2-2 of the second core 101-2. A portion of the optical power of the portion of the input optical signal IOS that is incident on the first port P1-2-2 of the second portion 101-2-2 of the second core 101-2 is coupled to the second portion 101-1-2 of the first core 101-1.

[0021] An optical signal including such coupled optical power is also referred to as a coupled optical signal COS. A portion of the optical power of the portion of the input optical signal IOS that enters the first port P1-2-2 of the second portion 101-2-2 of the second core 101-2 is propagated to the second port P2-2-2 of the second portion 101-2-2 of the second core 101-2. An optical signal that has been propagated, i.e., includes uncoupled optical power, is also referred to as a non-coupled optical signal NCOS.

[0022] The second port of the first optical waveguide 111-1 (or the second port P2-1-3 of the third portion 101-1-3 of the first core 101-1) is configured to emit a first output optical signal OOS1, which is a portion, e.g., substantially all, of the uncoupled optical signal NCOS.

[0023] The combined optical signal COS is emitted from the second port P2-1-2 of the second portion 101-1-2 of the first core 101-1. The combined optical signal COS may have the same fundamental mode (TE0 or TM0) as the input optical signal IOS or a higher order mode.

[0024] When the second width W2-1 and the third width W2-2-1 (thus, the sixth width W2 of the first portion 101-2-1 of the second core 101-2) are each larger than the first width W1, and the fifth width W2-3 and the fourth width W2-2-2 (thus, the sixth width W2 of the first portion 101-2-1 of the second core 101-2) are each smaller than the first width W1, a coupled optical signal COS having the same fundamental mode as the input optical signal IOS is obtained. When the second width W2-1 and the third width W2-2-1 are each smaller than the first width W1, and the fifth width W2-3 and the fourth width W2-2-2 are each smaller than the first width W1, a coupled optical signal COS having a higher-order mode rather than the fundamental mode is obtained as the input optical signal IOS, i.e., a coupled optical signal COS having a higher-order mode. As the second width W2-1 and the third width W2-2 become smaller, the order of the higher-order mode increases.

[0025] The uncoupled optical signal NCOS is emitted from the second port P2-2-2 of the second portion 101-2-2 of the second core 101-2. The uncoupled optical signal NOS has the same fundamental mode (TE0 or TM0) as the input optical signal IOS. The second port of the second optical waveguide 111-2 (or the second port P2-2-3 of the third portion 101-2-3 of the second core 101-2) is configured to emit a first output optical signal OOS1. The second port of the first optical waveguide 111-1 (or the second port P2-1-3 of the third portion 101-1-3 of the first core 101-1) is configured to emit a second output optical signal OOS2.

[0026] The amount of power of the combined optical signal COS, and therefore the first output optical signal OOS1 and the second output optical signal OOS2, depends on the length L2 of the second portions 101-1-2, 101-2-2, the second spacing SP2, and the difference between the third width W2-2-1 and the fourth width W2-2-2. Reducing the second length L2 reduces the power of the combined optical signal COS. Increasing the second spacing SP2 reduces the power of the combined optical signal COS. Reducing the difference between the third width W2-2-1 and the fourth width W2-2-2 reduces the power of the combined optical signal COS. Optionally, the optical power of the first output optical signal OOS1 is about 90 percent of the optical power of the input optical signal IOS, and the power of the second output optical signal OOS2 is about 10 percent of the optical power of the input optical signal IOS. In other words, the optical power of the first output optical signal OOS1 is about nine times the optical power of the second output optical signal OOS1.

[0027] By tapering, for example, adiabatically, (a) narrowing the first spacing SP1 and (b) widening the third spacing SP3 along an axis parallel to the direction of travel of the input optical signal IOS, less optical power of the input optical signal IOS (in the first portions 101-1-1, 101-2-1) and each output optical signal OOS1, OOS2 (in the third portions 101-1-3, 101-2-3) is lost to both free space emission and undesired higher-order modes. Thus, substantially all of the input optical signal IOS received at the first port P1-2-1 is emitted from the second ports P2-1-1, P2-2-1 of the first portions 101-1-1, 101-2-1 of the first core 101-1 and the second core 101-2, respectively. Furthermore, substantially all of the optical signal received from the second port P2-2-2 of the second portion 101-2-2 of the second core 101-2 is emitted as a first output optical signal OOS1 from the second port P2-2-3 of the third portion 101-2-3 of the second core 101-2, and substantially all of the optical signal received from the second port P2-2-2 of the second portion 101-1-2 of the first core 101-1 is emitted as a second output optical signal OOS2 from the second port P2-1-3 of the third portion 101-1-3 of the first core 101-1.

[0028] 2 illustrates a cross-sectional view of optical coupler 200 with reduced insertion loss along line AA-AA'. The illustrated cross-section is of adjacent second portions 201-1-2, 201-2-2. Each first, second, and third portion is formed by a high-index core (or cores) 201-1, 201-2 surrounded by a low-index cladding (or cladding) 219. Optionally, cladding 219 is formed on optional substrate 202.

[0029] 3 illustrates a plan view of an optical resonator 330 including two optical couplers 300, 300′ with reduced insertion loss. Each optical coupler 300, 300′ may be implemented using one or more of the techniques described elsewhere herein.

[0030] The optical resonator 330 includes a first optical coupler 300, a second optical coupler 300′, a first optical waveguide R1, and a second optical waveguide R2. Optionally, the first optical waveguide R1 and the second optical waveguide R2 are planar optical waveguides; however, such first core R1 and second core R2 may be formed from other types of optical waveguides.

[0031] For instructional purposes, the optical resonator 330 is illustrated as an optical ring resonator, where the ring is formed by the first optical waveguide 311-1 of the first optical coupler 300, the second optical waveguide 311-2′ of the second optical coupler 300′, the first resonator optical waveguide R1, and the second resonator optical waveguide R2. The optical resonator 330 can be any other type of optical resonator, such as a racetrack optical resonator or an elliptical optical resonator.

[0032] The first port P1-1-1 of the first optical waveguide 311-1 of the first optical coupler 300 and the second port P2-2-3' of the second optical waveguide 311-2' of the second optical coupler 300' are optically connected by a first resonator optical waveguide R1. The second port P2-1-3 of the first optical waveguide 300-1 of the first optical coupler 311 and the first port P1-2-1' second optical waveguide 311-2' of the second optical coupler 300' are optically connected by a second resonator optical waveguide R2.

[0033] The optical resonator 330 is configured to receive the input optical signal IOS at the first port P1-2-1 of the second optical waveguide 311-2 of the first optical coupler 300. The coupled optical signal COS is configured to be optically coupled from the second optical waveguide 311-2 of the first optical coupler 300 to the first optical waveguide 311-1 of the first optical coupler 300. The coupled optical signal COS is optionally 10 percent or less of the power of the input optical signal IOS. All or most of the power of the input optical signal IOS that is not coupled into the coupled optical signal COS is transmitted from the second port P2-2-3 of the second optical waveguide 311-2 of the first optical coupler 300 as the first output optical signal OOS1.

[0034] A second output optical signal OOS2, which is at least a portion of the combined optical signal COS, is configured to propagate into the second resonator optical waveguide R2. Another input optical signal IOS', which is at least a portion of the second output optical signal OOS2, is configured to be received at the first port P1-2-1' of the second optical waveguide 311-2' of the second optical coupler 300'. The other combined optical signal COS' is configured to be optically coupled from the second optical waveguide 311-2' of the second optical coupler 300' to the first optical waveguide 311-1' of the second optical coupler 300'. The other combined optical signal COS' optionally has a power of 10 percent or less of the other input optical signal IOS'. All or most of the power of the other input optical signal IOS' that is not coupled to the other coupled optical signal COS' is transmitted from the second port P2-2-3' of the second optical waveguide 311-2' of the second optical coupler 300' to the first resonator optical waveguide R1.

[0035] By reducing the insertion loss in each optical coupler 300, 300′, the optical resonator 330 has a higher Q-factor resonator. A higher Q-factor resonator requires less optical power to be injected into the optical resonator 330. Therefore, a cheaper, lower-power light source can be used with the optical resonator 330. Furthermore, by reducing the optical power injected into the optical resonator 330, nonlinear effects that may occur in the resonator may be reduced due to the reduced optical power.

[0036] FIG. 4 illustrates one embodiment of a method 440 for optically coupling optical signals in an optical coupler. To the extent that a method shown in any figure is described herein as being implemented with any of the systems illustrated herein, it should be understood that other embodiments may be implemented in other manners. Optionally, method 440 may be performed by the optical couplers described with respect to FIGS. 1-3. The blocks of the flow diagram are generally arranged in a sequential manner for ease of explanation; however, it should be understood that this arrangement is merely exemplary, and that the operations associated with the method (and the blocks shown in the figures) may occur in a different order (e.g., at least some of the operations associated with the blocks are performed in a parallel and / or event-driven manner). The definitions set forth herein for high-confinement optical waveguides and low-confinement optical waveguides are applicable to method 440.

[0037] In block 442, an input optical signal containing only the fundamental transverse electric mode or the fundamental transverse magnetic mode is received, for example, at a first port of the first portion of the first core. In block 444, a coupled optical signal consisting of either the fundamental (TE or TM) mode is coupled from the second portion of the second core to the second portion of the first core. In block 446, a first output optical signal (consisting only of the fundamental (TE or TM) mode) is transmitted, for example, from a second port of the third portion of the second core, the first output optical signal being at least a portion of the input optical signal. In block 448, a second output optical signal (consisting of either the fundamental mode or a higher-order (TE or TM) mode) is transmitted, for example, from a second port of the third portion of the first core, the second output optical signal being at least a portion of the coupled optical signal.

[0038] As used herein, relative position terms are defined based on a plane parallel to, or in the case of the term "coplanar," the conventional plane or working surface of, a layer, wafer, or substrate, regardless of orientation. As used herein, the terms "horizontal" or "lateral" are defined as a plane parallel to, or in the case of the term "coplanar," the same plane as, a conventional plane or working surface of a layer, wafer, or substrate, regardless of orientation. The term "vertical" refers to a direction perpendicular to the horizontal. Terms such as "on," "side (as in "sidewall")," "higher," "lower," "over," "top," and "under" are defined with respect to a conventional plane or working surface that is the top surface of a layer, wafer, or substrate, regardless of orientation. As used herein, the term "coplanar" is defined as a plane in the same plane as, or in the case of the term "coplanar," the conventional plane or working surface of a layer, wafer, or substrate, regardless of orientation.

[0039] Exemplary Embodiments Example 1 is an optical coupler comprising: a first core including a first portion, a second portion, and a third portion, the second portion of the first core being optically connected between the first portion and the second portion of the first core, the first core having a first surface; and a second core including a first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, the second core having a second surface, each of the first core and the second core being covered by a cladding, each of the first core and the second core having a refractive index higher than the refractive index of the cladding, an optical coupler in which a first spacing between the first surface and the second surface along the first portion of the second core is adiabatically tapered to narrow toward the first core and the second portion of the second core; a width of the second portion of the second core is tapered to narrow or wide from or after the first portion of the second core toward a third portion of the second core; a width of the third portion of the second core is narrower than the width of the first core; and a second spacing between the first surface along the third portion of the first core and the second surface along the third portion of the second core is adiabatically tapered to widen away from the first core and the second portion of the second core.

[0040] Example 2 includes the optical coupler of example 1, in which the first core covered by the cladding and the second core covered by the cladding are each on a substrate.

[0041] Example 3 includes the optical coupler according to example 1 or 2, wherein the widths of the first core, the first portion of the second core, and the third portion of the second core are constant.

[0042] Example 4 includes the optical coupler according to any one of Examples 1 to 3, in which (a) the width of the first portion of the second core and (b) the width at the first port of the second portion of the second core connected to the first portion of the second core are each greater than the width of the first core.

[0043] Example 5 includes the optical coupler of Example 4, in which the widths of the first core, the first portion of the second core, and the third portion of the second core are constant.

[0044] Example 6 includes the optical coupler according to any one of Examples 1 to 5, wherein the width of each of (a) the first portion of the second core and (b) the first port of the second portion of the second core connected to the first portion of the second core is smaller than the width of the first core.

[0045] Example 7 includes the optical coupler of Example 6, wherein the widths of the first core, the first portion of the second core, and the third portion of the second core are constant.

[0046] Example 8 includes the optical coupler according to any one of Examples 1 to 7, wherein the third distance between the first surface of the second portion of the first core and the second surface of the second portion of the second core is constant.

[0047] Example 9 is a method for reducing insertion loss in an optical coupler including a first core including a first surface and a second core including a second surface, the method including receiving an input optical signal consisting of only a transverse electric fundamental mode or a transverse magnetic fundamental mode at a first port of a first portion of the second core, the second core including a first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, a first spacing between the first surface along the first portion of the first core and the second surface along the first portion of the second core adiabatically tapering to narrow toward the first core and the second portion of the second core, each of the first cores being covered by a cladding, each of the cores has a refractive index greater than the refractive index of the cladding; coupling a combined optical signal from the second portion of the second core into the second portion of the first core, the combined optical signal comprising at least a portion of the power of the input optical signal; emitting a first output optical signal from the third portion of the second core, the first output optical signal comprising at least another portion of the power of the input optical signal; and emitting a second output optical signal from the third portion of the first core, the second output optical signal comprising at least a portion of the power of the combined optical signal, the second output optical signal comprising at least a portion of the power of the combined optical signal, the second spacing between the first surface along the third portion of the first core and the second surface along the third portion of the second core adiabatically tapering to become wider away from the first core and the second portion of the second core.

[0048] Example 10 includes the method of example 9, wherein the first core covered by a cladding and the second core covered by a cladding are each on a substrate.

[0049] Example 11 includes the method of example 9 or 10, wherein the width of each of the first core, the first portion of the second core, and the third portion of the second core is constant.

[0050] Example 12 includes the method of any of Examples 9-11, wherein (a) the width of the first portion of the second core and (b) the width at the first port of the second portion of the second core connected to the first portion of the second core are each greater than the width of the first core.

[0051] Example 13 includes the method of any of Examples 9-12, wherein (a) the width of the first portion of the second core and (b) the width at the first port of the second portion of the second core connected to the first portion of the second core are each smaller than the width of the first core.

[0052] Example 14 includes the method of any one of Examples 9-13, wherein the third distance between the first surface of the second portion of the first core and the second surface of the second portion of the second core is constant.

[0053] Example 15 is an optical resonator, a first optical coupler, including: a first core including a first portion, a second portion, and a third portion, the second portion of the first core being optically connected between the first portion and the second portion of the first core, the first core having a first surface; and a second core including the first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, the second core having a second surface. Each of the cores is covered by a cladding, and each of the first core and the second core has a refractive index higher than that of the cladding, a first spacing between a first surface along a first portion of the first core and a second surface along a first portion of the second core is adiabatically tapered to narrow toward the first core and the second portion of the second core, and a width of the second portion of the second core is tapered to narrow or widen from the first portion of the second core or after the first portion of the second core to a third portion of the second core. a first optical coupler and a second optical coupler, the second optical coupler including a third core having a first portion, a second portion, and a third portion, the second portion of the third core being optically connected between the first portion and the second portion of the third core, the third portion of the second core having a width narrower than the width of the first core, and a second spacing between a first surface along the third portion of the first core and a second surface along the third portion of the second core being adiabatically tapered to widen away from the first core and the second portion of the second core; a third core having a surface; and a fourth core including a first portion, a second portion, and the third portion, the second portion of the fourth core being optically connected between the first portion and the second portion of the fourth core, the fourth core having a fourth surface, each of the third core and the fourth core being covered by a cladding, each of the third core and the fourth core having a refractive index higher than the refractive index of the cladding, a third spacing between the third surface along the first portion of the third core and the fourth surface along the first portion of the fourth core beinga second optical coupler, the second optical coupler being adiabatically tapered to narrow toward the third core and the second portion of the fourth core, the width of the second portion of the fourth core tapering from or after the first portion of the fourth core toward the third portion of the fourth core, the width of the third portion of the fourth core being narrower than the width of the third core, and a fourth spacing between the third surface along the third portion of the third core and the fourth surface along the third portion of the fourth core adiabatically tapering to widen away from the third core and the second portion of the fourth core; a first resonator optical waveguide optically connected between a first port of the first portion of the first core and a second port of the third portion of the fourth core; and a second resonator optical waveguide optically connected between the second port of the third portion of the first core and a first port of the first portion of the fourth core.

[0054] Example 16 includes the optical resonator of Example 15, wherein at least one of the widths of the first core, the first portion of the second core, and the third portion of the second core are constant, and the widths of the third core, the first portion of the fourth core, and the third portion of the fourth core are constant.

[0055] Example 17 includes the optical resonator of Example 15 or 16, wherein at least one of (a) the width of the first portion of the second core and (b) the width at the first port of the second portion of the second core connected to the first portion of the second core is greater than the width of the first core, and (a) the width of the first portion of the fourth core and (b) the width at the first port of the second portion of the fourth core connected to the first portion of the fourth core is greater than the width of the third core.

[0056] Example 18 includes the optical resonator according to any one of Examples 15 to 17, wherein at least one of (a) the width of the first portion of the second core and (b) the width at the first port of the second portion of the second core connected to the first portion of the second core is smaller than the width of the first core, and (x) the width of the first portion of the fourth core and (y) the width at the first port of the second portion of the fourth core connected to the first portion of the fourth core is smaller than the width of the third core.

[0057] Example 19 includes the optical resonator of any of Examples 15 to 18, wherein at least one of the following is true: a fifth distance between the first surface of the second portion of the first core and the second surface of the second portion of the second core is constant; and a sixth distance between the first surface of the second portion of the third core and the second surface of the second portion of the fourth core is constant.

[0058] Example 20 includes the optical resonator according to any one of Examples 15 to 19, wherein each of the first, second, third, and fourth cores, the first resonator optical waveguide, and the second resonator optical waveguide is formed by a planar optical waveguide on a substrate.

[0059] While specific embodiments have been illustrated and described herein, those skilled in the art will recognize that any arrangement which is expected to achieve the same purpose may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims

1. An optical coupler, a first core including a first portion, a second portion, and a third portion, the second portion of the first core being optically connected between the first portion and the second portion of the first core, and the first core having a first surface; a second core including a first portion, a second portion, and a third portion, the second portion of the second core being optically connected between the first portion and the second portion of the second core, and the second core having a second surface; each of the first core and the second core is covered with a cladding, and each of the first core and the second core has a refractive index higher than a refractive index of the cladding; a first spacing between the first surface along the first portion of the first core and the second surface along the first portion of the second core adiabatically tapers toward the first core and the second portion of the second core; the width of the second portion of the second core tapers from the first portion of the second core or after the first portion of the second core toward the third portion of the second core, either narrowing or widening; a width of the third portion of the second core is narrower than a width of the first core; an optical coupler, wherein a second spacing between the first surface along the third portion of the first core and the second surface along the third portion of the second core is adiabatically tapered to widen away from the first core and the second portion of the second core.

2. 2. The optical coupler of claim 1, wherein the width of each of the first core, the first portion of the second core, and the third portion of the second core is constant.

3. 2. The optical coupler of claim 1, wherein (a) a width of the first portion of the second core and (b) a width at a first port of the second portion of the second core connected to the first portion of the second core are each greater than the width of the first core.

Images