Methods and devices for evanescently coupling light having different wavelengths to an open dielectric resonator

One feature pertains to an apparatus that includes apparatus that includes an evanescent field coupler having a first surface that evanescently couples light between the evanescent field coupler and an open dielectric resonator. The apparatus also includes a thin film coating covering at least a portion of the first surface of the evanescent field coupler. The thin film coating is specifically designed so that the thin film coating reflects light of a first wavelength.

FIELD

Various aspects of the present disclosure relate to photonics and, more particularly, to methods and apparatuses for equalized evanescent coupling of light having different wavelengths to an open dielectric resonator.

INTRODUCTION

Many opto-electronic devices utilize evanescent field couplers to evanescently couple light into and out of optical resonators including whispering gallery mode resonators (WGMR). For example, an opto-electronic device may include a coherent light source, an evanescent field coupler, and a WGMR. The light source may generate predominately two different wavelengths of light that pass through the coupler and evanescently couple into the WGMR positioned very close to the coupler.

The distance at which the coupler and the WGMR are spaced apart is critical and has a significant impact on the coupling efficiency. Notably, the optimal distance d selected to maximize coupling between the coupler and WGMR is dependent upon the wavelength of the light being coupled. Selecting the distance d between the coupler and WGMR to maximize coupling of light at a first wavelength may not efficiently couple light at a significantly different second wavelength. The effect occurs due to the wavelength dependence of the evanescent field of the light confined in the resonator, so that high efficiency coupling of longer wavelength optical fields prevents high efficiency coupling of shorter wavelength optical fields, and vice versa. Consequently, those wavelengths of light exhibiting higher efficiency coupling than other wavelengths of light may have loaded quality factors and load bandwidth values that vary by orders of magnitude from one another.

There is a need for devices and methods that enable equalization of the coupling efficiency into and out of an open dielectric resonator for different wavelengths of light while keeping the distance between the coupler and resonator fixed. Such devices and methods would allow the resonator to exhibit a loaded quality factor, loaded bandwidth, and loaded finesse values that are relatively close (e.g., less than a factor of 4) for two or more different wavelengths of light.

SUMMARY

One feature provides an apparatus comprising an evanescent field coupler having a first surface configured to evanescently couple light between the evanescent field coupler and an open dielectric resonator, and a thin film coating covering at least a portion of the first surface of the evanescent field coupler and configured to increase reflection of light of a first wavelength. According to one aspect, the thin film coating is configured to increase reflection of light of the first wavelength relative to light of a second wavelength that is different than the first wavelength. According to another aspect, the thin film coating includes a plurality of layers.

According to one aspect, the plurality of layers include a first set of layers composed of a first material and a second set of layers composed of a second material, the first set of layers interleaved with the second set of layers to form an alternating layer structure. According to another aspect, the first material and the second material have different indexes of refraction, and the refractive index of the first material, the refractive index of the second material, and a selected thickness of each layer of the plurality of layers cause constructive interference of light of the first wavelength at the thin film coating to increase reflection of light of the first wavelength when light of the first wavelength is incident upon the first surface of the coupler and/or a surface of the open dielectric resonator at a grazing angle less than 0.1 radians. According to yet another aspect, the plurality of layers each have at least one of a different thickness and/or a different index of refraction.

According to one aspect, the thin film coating is configured to increase reflection of light of the first wavelength when light of the first wavelength is incident upon the first surface of the coupler and/or a surface of the open dielectric resonator at a grazing angle less than 0.1 radians. According to another aspect, the thin film coating operates as a wavelength selective dielectric mirror that reflects more than 90% of the power of the light of the first wavelength. According to yet another aspect, the thin film coating is further configured to decrease the coupling efficiency of light of the first wavelength and boost loaded quality factor Q of light of the first wavelength propagating within the open dielectric resonator. According to another aspect, the evanescent field coupler is a prism or a waveguide.

Another feature provides a method comprising providing an evanescent field coupler having a first surface configured to evanescently couple light out from the evanescent field coupler and into an open dielectric resonator and couple light out from the open dielectric resonator and into the evanescent field coupler, selecting a first wavelength of light, and applying a thin film coating to at least a portion of the first surface of the evanescent field coupler, the thin film coating configured to enhance reflection of light of the first wavelength. According to one aspect, the method further comprises selecting and adjusting one or more thin film coating properties of the thin film coating to enhance reflection of light of the first wavelength. According to another aspect, the thin film coating properties include a number of layers of the thin film coating, a material for each layer of the thin film coating, an index of refraction for each layer of the thin film coating, and a thickness for each layer of the thin film coating.

According to one aspect, the thin film coating is configured to increase reflection of light of the first wavelength relative to light of a second wavelength that is different than the first wavelength. According to another aspect, applying a thin film coating includes forming a plurality of alternating layers over the portion of the first surface of the evanescent field coupler. According to yet another aspect, the plurality of alternating layers are each composed of one of two different materials and each layer of the plurality of layers has a different thickness. According to another aspect, applying a thin film coating includes forming a plurality of layers over the portion of the first surface of the evanescent field coupler, each layer of the plurality of layers having a different index of refraction for a given wavelength of light.

Another feature provides a system comprising a light source configured to generate coherent light having a first wavelength and a second wavelength, the second wavelength substantially different than the first wavelength, an open dielectric resonator, and an evanescent field coupler having a first surface configured to evanescently couple light between the evanescent field coupler and the open dielectric resonator and a second surface through which the evanescent field coupler transmits and receives light to and from the light source, wherein the evanescent field coupler's first surface includes a thin film coating that is configured to decrease light coupling efficiency between the evanescent field coupler and the open dielectric coupler for light of the first wavelength relative to light coupling efficiency of the second wavelength, and the thin film coating further configured to boost loaded quality factor Q for light of the first wavelength within the open dielectric resonator. According to one aspect, the open dielectric resonator is a monolithic whispering gallery mode resonator. According to another aspect, the thin film coating includes a plurality of layers for which thin film coating properties are selected to increase reflectance of light of the first wavelength through the thin film coating relative to light of the second wavelength through the thin film coating.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any implementation or aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects of the disclosure. Likewise, an aspect is an implementation or example. Reference in the specification to “an aspect,” “one aspect,” “some aspects,” “various aspects,” or “other aspects” means that a particular feature, structure, or characteristic described in connection with the aspects is included in at least some aspects, but not necessarily all aspects, of the present techniques. The various appearances of “an aspect,” “one aspect,” or “some aspects” are not necessarily all referring to the same aspects. Elements or aspects from an aspect can be combined with elements or aspects of another aspect.

It is to be noted that, although some aspects have been described in reference to particular implementations, other implementations are possible according to some aspects. Additionally, the arrangement and/or order of circuit elements or other features illustrated in the drawings and/or described herein need not be arranged in the particular way illustrated and described. Many other arrangements are possible according to some aspects.

FIG. 1illustrates a schematic view of a system100for evanescently coupling light into and out of an open dielectric resonator according to one aspect. The system100includes a coherent light source102(e.g., laser), an evanescent field coupler104, and an open dielectric resonator106. In the example shown, the laser102generates light150,160composed of predominately two different wavelengths λ1and λ2(i.e., wavelengths in vacuum) where λ1<λ2. For instance, as one non-limiting, non-exclusive example, the first wavelength λ1of light may be 795 nm and the second wavelength λ2of light may be 1550 nm. The light150,160generated by the laser102passes through the coupler104(e.g., through the coupler's “second surface”) and strikes a bottom surface105(e.g., “first surface”) of the coupler104. The close proximity of the resonator106at a fixed distance d away from the coupler104causes the light's evanescent field at the coupler's bottom surface105to excite propagating (e.g., circulating) light waves152,162in the resonator106having the same wavelengths λ1and λ2. By a similar process, evanescent fields associated with the light152,162circulating within the resonator106causes light170,180to be evanescently coupled out from the resonator106back to the coupler104, which may then travel back to the laser102as shown. In some aspects, a lens assembly (not shown) may be placed between the optical coupler104and the laser102to help direct the light back to the laser102.

The loaded quality factor Q of an open dielectric resonator depends on many factors including the type and shape of the resonator. For example, the loaded quality factor Q of a spherical resonator may be approximated by the formula:

Q≈π2⁢⁢npnr⁢⁢nr2-1np2-nr2⁢l3/2⁢e(4⁢π⁢⁢dλ⁢nr2-1)
where npand nrare the wavelength (λ) dependent refractive indexes of the coupler and the resonator, l is the azimuthal index of the whispering gallery mode, and d is the shortest distance from the coupler's surface to the resonator. It may be observed that for a fixed distance d between the coupler and the resonator, the loaded quality factor (Q) of the resonator decreases as the wavelength λ of light increases. Thus, in the example shown inFIG. 1, since the distance d between the resonator106and the coupler104is fixed, the loaded quality factor Q of the resonator106would ordinarily have significantly different values for the two different wavelengths λ1and λ2of light that are evanescently coupled to it. Specifically, the loaded quality factor Q for the longer wavelength λ2of light would be orders of magnitude less than the loaded quality factor Q for the shorter wavelength λ1of light.

To help equalize the loaded quality factor Q values of the resonator for light having different wavelengths λ1and λ2, the evanescent field coupler104shown inFIG. 1features a thin film coating108on at least a portion of its bottom surface105that is closest to the resonator106where light is evanescently coupled between the coupler104and the resonator106. As described in greater detail below, the thin film coating108, which may be generally planar, is specifically designed to increase reflectivity of the longer λ2wavelength light relative to the shorter wavelength λ1light in order to decrease the coupling efficiency for the longer λ2wavelength light between the coupler104and the resonator106. Accordingly, the thin film coating108acts as an equalizer that controls the coupling efficiency of light having different wavelengths. By reducing the coupling efficiency of the longer λ2wavelength light, the thin film coating108boosts the loaded quality factor Q of the longer λ2wavelength light so that it better matches the loaded quality factor Q of the shorter wavelength λ1light despite the distance d being fixed between the coupler104and the resonator106.

FIG. 2illustrates the system's coupler104having a thin film coating108that is configured to reflect light of a specific wavelength according to one aspect. The laser102generates light250,260composed of predominately two different wavelengths λ1and λ2(i.e., wavelengths in vacuum) where λ1<λ2. For instance, as one non-limiting, non-exclusive example, the first wavelength λ1of light may be 795 nm and the second wavelength λ2of light may be 1550 nm. The light250,260generated by the laser102passes through the coupler104and strikes a bottom surface105of the coupler104having the thin film coating108. In the example shown, the thin film coating is specifically designed with thin film coating properties (e.g., thickness of layers, materials used for the layers, indexes of refraction of the layers, number of layers, etc.) to reflect a significantly larger percentage of the longer λ2wavelength light's energy261than the shorter wavelength λ1light's energy251. For instance, the thin film coating108may reflect about 65% of the first wavelength λ1light's energy but it may reflect more than 99% of the second wavelength λ2light's energy. Thus, only a small fraction of the second wavelength λ2of light262may evanescently couple into and out of the resonator106compared to the first wavelength λ1of light261. Since the thin film coating108effectively makes it more difficult for the second wavelength λ2light propagating within the resonator106from being evanescently coupled out, the loaded quality factor Q for the second wavelength λ2light is boosted and may better match the loaded quality factor Q of the first wavelength)λ1light.

FIG. 3illustrates a schematic representation of a cross-section of the evanescent field coupler104, a thin film coating308, and resonator106according to one aspect. The thin film coating308may operate as a dielectric mirror (e.g., Bragg mirror, quarter-wave mirror, dichroic mirror, etc.) that has been designed so that it reflects substantially more light at a second wavelength λ2(e.g., wavelength in vacuum) than light at a first wavelength λ1(e.g., wavelength in vacuum). As such, according to one non-limiting, non-exclusive example, the thin film coating308may comprise a plurality of alternating layers302a,302b, . . .302kwhere a first set of layers302a,302c, . . .302k, each composed of a first type of material having a first index of refraction (e.g., high-index of refraction n1) and a first thickness (t1), are interleaved with a second set of layers302b,302d, . . .302jeach composed of a second type of material having a second index of refraction (e.g., low-index of refraction n2) and a second thickness (t1).

The alternating layers302a,302b, . . .302khave thicknesses t1, t2and refractive indexes n1, n2that have been specifically selected so that the path-length differences for reflections310a,310b,310cfrom different high-index n1layers are integer multiples of the wavelength λ2for which the thin film coating308is designed to reflect. The reflections from the low-index n2layers may also have exactly half a wavelength λ2in path length difference compared to high-index n1layer reflections, but there is a 180-degree difference in phase shift at a low-to-high index boundary, compared to a high-to-low index boundary, which means that the low-index n2reflections are also in phase. This causes constructive interference of the second wavelength λ2light reflected at the boundary interfaces (e.g., reflected light310a,310b,310c, etc.) of the plurality of alternating layers302a,302b, . . .302k, which results in a very large portion of the second wavelength λ2light reflecting away. Thus, only a very small percentage of the second wavelength λ2light may actually reach the bottom layer302kand be evanescently coupled into the resonator106.

By contrast, the optical path lengths through each of the alternating layers302a,302b, . . .302kfor the first wavelength λ1light is not specifically designed to promote reflection and thus there is significantly less constructive interference of λ1wavelength light at the coupler's bottom surface105. Such λ1wavelength light may therefore pass through the thin film coating308with substantially less reflective power loss and a greater portion of this light may reach the bottom layer302kof the thin film coating308and be evanescently coupled into the resonator106.

Referring toFIGS. 2 and 3, light304,306generated by the laser102and composed of two different wavelengths λ1(e.g., 795 nm in vacuum) and λ2(1550 nm in vacuum) may propagate through the coupler104(e.g., BK7 having refractive index n0=1.51) and be incident on the bottom surface105of the coupler104at an angle θ from the normal to the coupler's bottom surface105. According to one non-limiting example, the thin film coating308may be comprised of a series of alternating layers302a,302b, . . .302kof silicon oxide SiO2and titanium oxide TiO2. The SiO2layers302b,302d, . . .302jmay have a refractive index n1of about 1.44, while the TiO2layers302a,302c, . . .302kmay have a refractive index n2of about 2.45. These specific materials and refractive indexes used are merely exemplary and in aspects of the disclosure of materials and/or different refractive indexes may be used to achieve the same goal of reflecting light at one wavelength while allowing light of another wavelength to pass through.

The alternating series of layers302a,302b, . . .302khave thicknesses t1, t2so that the path-length differences for reflections310a,310b,310cfrom different high-index n1layers are integer multiples of the wavelength λ2for which the thin film coating308is designed to reflect. The reflections from the low-index n2layers also have exactly half a wavelength λ2in path length difference compared to high-index n1layer reflections, but there is a 180-degree difference in phase shift at a low-to-high index boundary, compared to a high-to-low index boundary, which means that the low-index n2reflections are also in phase. This causes constructive interference of the second wavelength λ2light reflected at the boundary interfaces (e.g., reflected light310a,310b,310c, etc.) of the plurality of alternating layers302a,302b, . . .302k, which results in a very large portion of the second wavelength λ2light reflecting away. Thus, only a very small percentage of the second wavelength λ2light actually reaches the bottom layer302kand be evanescently coupled311into the resonator106.

By contrast, the optical path lengths through each of the alternating layers302a,302b, . . .302kfor first wavelength λ1light304is not designed to constructively interfere. This light304may thus pass through the thin film coating308with substantially less power lost to reflection and a greater portion314of such light may reach the bottom layer302kof the thin film coating308and be evanescently coupled316into the resonator106.

In the example illustrated inFIG. 3, a thin film coating308is shown having 11 layers302a,302b, . . .302k. This is merely an example and in practice the thin film coating308may have less layers or more layers. Moreover, the dual alternating SiO2/TiO2layer structure shown is also merely exemplary. In practice, different types of layer materials may be used and more than two different types may be used. Furthermore, the alternating layers302a,302b, . . .302kmay each have different thicknesses and are not limited to alternating thicknesses t1and t2. Generally, many different types of layer materials, layering structures, number of layers, refractive indexes of the layers, etc. may be used to form the thin film coating308. Whatever specific implementation of the thin film coating308is used, the thin film coating308should be specifically designed to cause significantly more constructive reflection of one or more wavelengths of light over other one or more wavelengths of light in order to selectively control the coupling efficiency of different wavelengths of light between the coupler104and the resonator106.

FIG. 4illustrates a close up view of a portion of a cross-section of the thin film coating308shown inFIG. 3according to one aspect. Light306having a wavelength λ2(in vacuum) may be incident on the evanescent field coupler's bottom surface105at an angle θ to the normal of the coupler's bottom surface105. A first portion402of the light's energy is reflected off at the interface boundary between the coupler104and the first thin film coating layer302awhile a portion of the light's energy is transmitted through into the first layer302aaccording to Fresnel's equations.

The portion of the light transmitted through into the first layer302afollows an optical path PB1whose length is given by Snell's law as n1*t1/cos[sin−1((n0/n1)*sin(θ))]. The light continues through the thin film coating308entering into the second thin film coating layer302b. The portion of the light transmitted through into the second layer302bfollows an optical path PB2whose length is given by Snell's law as n2*t2/cos [sin−1((n1/n2)*sin[sin−1((n0/n1)*sin(θ))])].

A portion of the light traveling through the second layer302breflects off of the interface boundary between the second layer302band the third layer302cand travels back through the second layer302balong an optical path PB3whose length is equal to PB2. A portion of the light traveling back through the second layer302balong the optical path PB3enters back into the first layer302aand travels through the first layer302aalong an optical path PB4whose length is equal to PB1. A portion of this light propagating back through the first layer302aexits the first layer302aback into the coupler104and represents a second reflected portion404of the light's306energy.

The optical path difference (OPD) between the first reflected portion402and the second reflected portion404is given by the formula:
OPD=PB1+PB2+PB3+PB4−PA.
In order to facilitate constructive interference of the first and second reflected portions402,404of light, the thin film coating layers302a,302bmay be designed so that the OPD is an integer multiple of the wavelength λ2. For instance, if the OPD is one wavelength λ2in distance then constructive interference between the first and second reflected portions402,404will be realized. Referring toFIGS. 3 and 4, the remaining thin film coating layers302c-302kmay be similarly designed (e.g., thickness, material, index of refraction) so that second wavelength λ2light traveling into these layers similarly results in reflective constructive interference

Similar design considerations may also be used to ensure that reflections from the low-index n2layers (i.e., interface boundaries between first and second layers302a,302b, third and fourth layers302c,302d, fifth and sixth layers302e,302f, etc.) also constructively interfere. To do so, these layers have exactly half a wavelength λ2in path length difference compared to high-index n1layer reflections to account for a 180-degree difference in phase shift at a low-to-high index boundary.

FIG. 5illustrates a schematic representation of a cross-section of a thin film coating508according to one aspect of the disclosure. Similar to the thin film coating308shown and described with respect toFIG. 3, the thin film coating508shown inFIG. 5is composed of a plurality of alternating layers502a,502b, . . .502kthat operate as a dielectric mirror that reflects510substantially more light at a second wavelength λ2506than light at a first wavelength λ1504. However, the layers502a,502b, . . .502kmay have different thicknesses from one another and/or have different indexes of refraction. The specific thickness and index of refraction for each layer502a,502b,502kmay be selected so that the coating508as a whole substantially reflects510longer wavelength light506, such as λ2wavelength light, but allows shorter wavelength light504, such as λ1wavelength light, to pass through the coating508. That is, the collection of layers502a,502b, . . .502kwork together to cause constructive interference of longer wavelength light at the interlayer boundaries and reflect the longer wavelength light.

In some aspects, the same material may be used in alternating layers (e.g., SiO2and TiO2) so that alternating layers have the same index of refraction but each has a different thickness. In other aspects, the thickness of each layer may be the same but the index of refraction for each layer may be different. Computer simulations may be used to determine and select thin film coating properties such as layer thicknesses, layer materials, indexes of refraction of the layers, and number of layers.

Tables 5-1, 5-2, and 5-3 below provide details of one non-limiting, non-exclusive example of a thin film coating that has been specifically designed and validated to reflect longer wavelength light (1550 nm) and transmit shorter wavelength light (795 nm). In the example provided the coating has 19 alternating layers that alternate between SiO2and TiO2.

FIG. 6illustrates a schematic view of the evanescent field coupler104and the open dielectric resonator106according to one aspect. The resonator106may have light602,604of different wavelengths λ1and λ2circulating inside as shown. If the distance d between the coupler104and the resonator106is sufficiently small (e.g., less than a wavelength λ2) then the evanescent fields at the surface605of the resonator106close to the coupler104cause corresponding light waves606,608to be excited at thin film coating608(e.g., starting at the bottom surface607of the thin film coating608) and propagate through the thin film coating608and coupler104. Since the thin film coating608has been specifically designed to reflect second wavelength λ2light, less second wavelength λ2light608is evanescently coupled from the resonator106over to the thin film coating608and coupler104compared with λ1wavelength light606.

Since less λ2wavelength light is evanescently coupled from the resonator106to the coupler104than wavelength λ1light, a greater amount of λ2wavelength light604remains circulating within the resonator106thereby boosting the loaded quality factor Q of the λ2wavelength light. This helps equalize the loaded quality factor Q between the two different wavelengths λ1, λ2of light so that they are more equally matched. In this fashion, the thin film coating608operates symmetrically to reflect λ2wavelength light and transmit λ1wavelength light emanating from the resonator106towards the coupler104in addition to such light traveling from the coupler104to the resonator106.

FIG. 7illustrates another schematic view of the evanescent field coupler104and the open dielectric resonator106according to one aspect. Light702,704circulating within the resonator106may strike the perimeter surface706of the resonator106at a small grazing angle ϕ approximated by ϕ=λ/2nr*R, where R is the radius of the resonator106(e.g., where the resonator is spherical), and nris index of refraction of the resonator106(e.g., MgF2resonator). Thus, for λ1=795 nm, R=0.1 mm nr=1.38, ϕ1is about 2.9×10−3radians and for λ2=1550 nm, R=0.1 mm nr=1.38, ϕ2is about 5.6×10−3radians. The thin film coating708may be designed (e.g., thicknesses of the layers, number of layers, layer materials, refractive indexes of the layers, etc.) to reflect λ2wavelength light coming in at or about the grazing angle ϕ2and transmit λ1wavelength light coming in at or about the grazing angle ϕ1. Conveniently, a thin film coating708designed to reflect λ2wavelength light at a grazing angle ϕ2and transmit λ1wavelength light at a grazing angle ϕ1may still be used with resonators having substantially smaller or larger radial dimensions (e.g., R is one order of magnitude larger or smaller) with negligible effect (e.g., <1%) on the reflection/transmittance frequency spectrum profile of the thin film coating.

The light702,704within the resonator106may be evanescently coupled to the thin film coating708where it generates light710,712that propagates through the thin film coating708. Since the thin film coating708is designed to reflect λ2wavelength light712, significantly less second wavelength λ2light714reaches and propagates through the coupler104(e.g., back toward the laser102(seeFIG. 1)) than λ1wavelength light716. Since less λ2wavelength light704is effectively coupled out of the resonator106, the loaded quality factor Q of the λ2wavelength light is boosted to better match the loaded quality factor Q value of the λ1wavelength light.

The thin film coating708may be designed so that the power P2of the λ2wavelength light714transmitted from the resonator106to the coupler104is less than X % of the power P1of the λ1wavelength light716transmitted from the resonator106to the coupler104. In some as aspects, where X may be any value between 0.1 and 90. For example, in some aspects, the thin film coating708may be designed so that the power P2of the λ2wavelength light714transmitted from the resonator106to the coupler104is less than 10% of the power P1of the λ1wavelength light716transmitted from the resonator106to the coupler104.

FIG. 8illustrates an exemplary graph (e.g., frequency transmittance profile) of the transmittance percentage versus light wavelength (nm) for an exemplary thin film coating108,308,508,608,708that may be applied to a surface of a coupler. In the example shown, the thin film coating108,308,508,608,708has been designed to reflect a large percentage (e.g., 99%) of light at or about 1550 nm while still allowing a significant portion of light (e.g., 35%) of light at or about 795 nm to transmit/pass through when such light is incident upon the thin film coating at a narrow grazing angle (e.g., 10−3radians). The example shown in merely exemplary and the thin film coating108,308,508,608,708may be designed (e.g., materials selected for the layers, layer thicknesses, refractive indexes, etc.) so that it reflects light of one or more different wavelengths and transmits light of one or more different wavelengths.

FIG. 9illustrates an exemplary graph of loaded Q factor versus gap size (i.e., distance d between coupler104and resonator106shown inFIGS. 1, 2, 3, 5, and 6) for light of two different wavelengths (e.g., 795 nm and 1550 nm) when no thin film coating108is applied to the bottom surface105of the coupler104(seeFIG. 1). In the example shown, the 795 nm light has a loaded Q factor that is almost 3 orders of magnitude greater than the loaded Q factor of the 1550 nm light across a range of gap sizes.

FIG. 10illustrates an exemplary graph of loaded Q factor versus gap size for the two wavelengths of light shown inFIG. 9when a thin film coating108,308,508,608,708has been applied to the bottom surface105of the coupler104(see e.g.,FIGS. 1-7). In the example provided, the thin film coating108,308,508,608,708is designed to reflect 1550 nm light and transmit 795 nm light. For instance, the thin film coating108,308,508,608,708may have the transmittance properties shown inFIG. 8. Referring toFIGS. 2, 6, and 10, the thin film coating decreases coupling efficiency of the 1550 nm wavelength light between the coupler104and the resonator106, which increases the loaded Q factor at that wavelength of light. Thus, referring toFIGS. 9 and 10, the loaded Q factor of the 1550 nm wavelength light in a system having the thin film coating (seeFIG. 10) is significantly boosted compared to the loaded Q factor of such light for a system not having the coating (seeFIG. 9). The boosted loaded Q factor of the 1550 nm light better matches the loaded Q factor of the 795 nm light so that the loaded Q factor difference between the two is only off by, for example, a factor of four or less instead of two to three orders of magnitude.

FIG. 11illustrates an exemplary graph of a photodetector's output voltage versus the offset optical frequency of 795 nm wavelength light propagating through the system shown inFIG. 1where a coupler has a thin film coating108applied to its bottom surface105near an open dielectric resonator106. In the example shown, it is assumed that the distance d between the coupler104and the resonator106(seeFIG. 1) is optimized for evanescently coupling 795 nm light and the thin film coating108is designed to reflect 1550 nm light (e.g., exhibits the transmittance profile shown inFIG. 8). The graph inFIG. 11shows that the relative intensity of the 795 nm light is about 0.75 volts from about −400 MHz to +400 MHz centered around the optical frequency of the 795 nm light.

FIG. 12illustrates an exemplary graph of the photodetector's output voltage versus the offset optical frequency of 1550 nm wavelength light propagating through the system shown inFIG. 1for the case where a coupler has a thin film coating108applied to its bottom surface105near an open dielectric resonator106and for the case where no thin film coating108is applied. In the example shown, it is assumed that the distance d between the coupler104and the resonator106(seeFIG. 1) is optimized for evanescently coupling 795 nm light and the thin film coating108when applied is designed to reflect 1550 nm light (e.g., exhibits the transmittance profile shown inFIG. 8). The graph inFIG. 12shows that the relative intensity of the 1550 nm light has a range between 0.22 volts and 0.10 volts from about −18 MHz to +18 MHz centered around the optical frequency of the 1550 nm light for the case where no thin film coating108is applied to the coupler104. By contrast, the relative intensity of the 1550 nm light is boosted and has a range between 0.38 volts and 0.23 volts from about −18 MHz to +18 MHz centered around the optical frequency of the 1550 nm light for the case where the thin film coating108is applied. While the relative intensity of the 1550 nm light with the thin film coating108is still not as strong as the relative intensity of the 795 nm light shown inFIG. 11, its relative intensity is still significantly greater than if no thin film coating108had been applied.

Thus, the thin film coating108allows for loaded quality factor Q equalization for two different wavelengths of light for evanescently coupled resonator systems even though such a system's coupler and resonator are spaced apart a fixed distance d that is optimized for only one wavelength.

FIG. 13illustrates a method1300for manufacturing an apparatus that allows for the wavelength dependent coupling of light into and out of an open dielectric resonator according to one aspect. First, an evanescent field coupler is provided1302having a first surface that is configured to evanescently couple light out from the evanescent field coupler and into an open dielectric resonator and couple light out from the open dielectric resonator and into the evanescent field coupler. Next, a first wavelength of light may be selected1304. Then, a thin film coating is applied to at least a portion of the first surface of the evanescent field coupler, where the thin film coating is configured to enhance reflection of light of the first wavelength.

A coupler having the above described coating may be used in various systems. For example, such a device may be used for efficient lossless retrieval of photons at any wavelength from cavity modes. This may be useful in nonlinear optics systems such as frequency doubling. As another example, such a device may be used to stabilize an open dielectric resonator operating at one wavelength to a reference laser operating at a significantly different wavelength. This is useful for stabilizing lasers and oscillators. As yet another example, such a device may be used to create a wavelength profile of Q factor to achieve operational improvement of an intracavity mode locked laser/frequency comb generator.

The various features of the invention described herein can be implemented in different systems without departing from the invention. It should be noted that the foregoing aspects of the disclosure are merely examples and are not to be construed as limiting the invention. The description of the aspects of the present disclosure is intended to be illustrative, and not to limit the scope of the claims. As such, the present teachings can be readily applied to other types of apparatuses and many alternatives, modifications, and variations will be apparent to those skilled in the art.