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.
The present application for patent claims priority to provisional application No. 62/519,029 entitled “Scheme for Efficient Coupling of Light with Different Wavelength to the Same Open Resonator” filed Jun. 13, 2017, the entire disclosure of which is hereby expressly incorporated by reference.
FIELDVarious 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.
INTRODUCTIONMany 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.
SUMMARYOne 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.
In the following description, specific details are given to provide a thorough understanding of the various aspects of the disclosure. However, it will be understood by one of ordinary skill in the art that the aspects may be practiced without these specific details. For example, circuits may be shown in block diagrams in order to avoid obscuring the aspects in unnecessary detail. In other instances, well-known circuits, structures and techniques may not be shown in detail in order not to obscure the aspects of the disclosure.
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.
Not all components, features, structures, characteristics, etc. described and illustrated herein need be included in a particular aspect or aspects. If the specification states a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, for example, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.
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.
In each figure, the elements in some cases may each have a same reference number or a different reference number to suggest that the elements represented could be different and/or similar. However, an element may be flexible enough to have different implementations and work with some or all of the systems shown or described herein. The various elements shown in the figures may be the same or different. Which one is referred to as a first element and which is called a second element is arbitrary.
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:
where np and nr are 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 in
To help equalize the loaded quality factor Q values of the resonator for light having different wavelengths λ1 and λ2, the evanescent field coupler 104 shown in
The alternating layers 302a, 302b, . . . 302k have thicknesses t1, t2 and refractive indexes n1, n2 that have been specifically selected so that the path-length differences for reflections 310a, 310b, 310c from different high-index n1 layers are integer multiples of the wavelength λ2 for which the thin film coating 308 is designed to reflect. The reflections from the low-index n2 layers may also have exactly half a wavelength λ2 in path length difference compared to high-index n1 layer 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 n2 reflections are also in phase. This causes constructive interference of the second wavelength λ2 light reflected at the boundary interfaces (e.g., reflected light 310a, 310b, 310c, etc.) of the plurality of alternating layers 302a, 302b, . . . 302k, which results in a very large portion of the second wavelength λ2 light reflecting away. Thus, only a very small percentage of the second wavelength λ2 light may actually reach the bottom layer 302k and be evanescently coupled into the resonator 106.
By contrast, the optical path lengths through each of the alternating layers 302a, 302b, . . . 302k for the first wavelength λ1 light is not specifically designed to promote reflection and thus there is significantly less constructive interference of λ1 wavelength light at the coupler's bottom surface 105. Such λ1 wavelength light may therefore pass through the thin film coating 308 with substantially less reflective power loss and a greater portion of this light may reach the bottom layer 302k of the thin film coating 308 and be evanescently coupled into the resonator 106.
Referring to
The alternating series of layers 302a, 302b, . . . 302k have thicknesses t1, t2 so that the path-length differences for reflections 310a, 310b, 310c from different high-index n1 layers are integer multiples of the wavelength λ2 for which the thin film coating 308 is designed to reflect. The reflections from the low-index n2 layers also have exactly half a wavelength λ2 in path length difference compared to high-index n1 layer 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 n2 reflections are also in phase. This causes constructive interference of the second wavelength λ2 light reflected at the boundary interfaces (e.g., reflected light 310a, 310b, 310c, etc.) of the plurality of alternating layers 302a, 302b, . . . 302k, which results in a very large portion of the second wavelength λ2 light reflecting away. Thus, only a very small percentage of the second wavelength λ2 light actually reaches the bottom layer 302k and be evanescently coupled 311 into the resonator 106.
By contrast, the optical path lengths through each of the alternating layers 302a, 302b, . . . 302k for first wavelength λ1 light 304 is not designed to constructively interfere. This light 304 may thus pass through the thin film coating 308 with substantially less power lost to reflection and a greater portion 314 of such light may reach the bottom layer 302k of the thin film coating 308 and be evanescently coupled 316 into the resonator 106.
In the example illustrated in
The portion of the light transmitted through into the first layer 302a follows an optical path PB1 whose length is given by Snell's law as n1*t1/cos[sin−1((n0/n1)*sin(θ))]. The light continues through the thin film coating 308 entering into the second thin film coating layer 302b. The portion of the light transmitted through into the second layer 302b follows an optical path PB2 whose 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 layer 302b reflects off of the interface boundary between the second layer 302b and the third layer 302c and travels back through the second layer 302b along an optical path PB3 whose length is equal to PB2. A portion of the light traveling back through the second layer 302b along the optical path PB3 enters back into the first layer 302a and travels through the first layer 302a along an optical path PB4 whose length is equal to PB1. A portion of this light propagating back through the first layer 302a exits the first layer 302a back into the coupler 104 and represents a second reflected portion 404 of the light's 306 energy.
The optical path difference (OPD) between the first reflected portion 402 and the second reflected portion 404 is given by the formula:
OPD=PB1+PB2+PB3+PB4−PA.
In order to facilitate constructive interference of the first and second reflected portions 402, 404 of light, the thin film coating layers 302a, 302b may be designed so that the OPD is an integer multiple of the wavelength λ2. For instance, if the OPD is one wavelength λ2 in distance then constructive interference between the first and second reflected portions 402, 404 will be realized. Referring to
Similar design considerations may also be used to ensure that reflections from the low-index n2 layers (i.e., interface boundaries between first and second layers 302a, 302b, third and fourth layers 302c, 302d, fifth and sixth layers 302e, 302f, etc.) also constructively interfere. To do so, these layers have exactly half a wavelength λ2 in path length difference compared to high-index n1 layer reflections to account for a 180-degree difference in phase shift at a low-to-high index boundary.
In some aspects, the same material may be used in alternating layers (e.g., SiO2 and 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 SiO2 and TiO2.
Since less λ2 wavelength light is evanescently coupled from the resonator 106 to the coupler 104 than wavelength λ1 light, a greater amount of λ2 wavelength light 604 remains circulating within the resonator 106 thereby boosting the loaded quality factor Q of the λ2 wavelength light. This helps equalize the loaded quality factor Q between the two different wavelengths λ1, λ2 of light so that they are more equally matched. In this fashion, the thin film coating 608 operates symmetrically to reflect λ2 wavelength light and transmit λ1 wavelength light emanating from the resonator 106 towards the coupler 104 in addition to such light traveling from the coupler 104 to the resonator 106.
The light 702, 704 within the resonator 106 may be evanescently coupled to the thin film coating 708 where it generates light 710, 712 that propagates through the thin film coating 708. Since the thin film coating 708 is designed to reflect λ2 wavelength light 712, significantly less second wavelength λ2 light 714 reaches and propagates through the coupler 104 (e.g., back toward the laser 102 (see
The thin film coating 708 may be designed so that the power P2 of the λ2 wavelength light 714 transmitted from the resonator 106 to the coupler 104 is less than X % of the power P1 of the λ1 wavelength light 716 transmitted from the resonator 106 to the coupler 104. In some as aspects, where X may be any value between 0.1 and 90. For example, in some aspects, the thin film coating 708 may be designed so that the power P2 of the λ2 wavelength light 714 transmitted from the resonator 106 to the coupler 104 is less than 10% of the power P1 of the λ1 wavelength light 716 transmitted from the resonator 106 to the coupler 104.
Thus, the thin film coating 108 allows 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.
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.
One or more of the components, steps, features, and/or functions illustrated in
Also, it is noted that the aspects of the present disclosure may be described as a process that is depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc.
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.
Claims
1. 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.
2. The apparatus of claim 1, wherein 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.
3. The apparatus of claim 1, wherein the thin film coating includes a plurality of layers.
4. The apparatus of claim 3, wherein 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.
5. The apparatus of claim 4, wherein 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.
6. The apparatus of claim 3, wherein the plurality of layers each have at least one of a different thickness and/or a different index of refraction.
7. The apparatus of claim 1, 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.
8. The apparatus of claim 1, wherein 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.
9. The apparatus of claim 1, wherein 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.
10. The apparatus of claim 1, wherein the evanescent field coupler is a prism or a waveguide.
11. 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.
12. The method of claim 11, further comprising:
- selecting and adjusting one or more thin film coating properties of the thin film coating to enhance reflection of light of the first wavelength.
13. The method of claim 12, wherein 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.
14. The method of claim 11, wherein 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.
15. The method of claim 11, wherein applying a thin film coating includes:
- forming a plurality of alternating layers over the portion of the first surface of the evanescent field coupler.
16. The method of claim 15, wherein 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.
17. The method of claim 11, wherein 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.
18. 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.
19. The system of claim 18, wherein the open dielectric resonator is a monolithic whispering gallery mode resonator.
20. The system of claim 18, wherein 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.
Type: Application
Filed: Jun 4, 2018
Publication Date: Oct 1, 2020
Patent Grant number: 11152681
Inventors: Lute MALEKI (Pasadena, CA), Anatoliy A. Savchenko (Glendale, CA), Danny Eliyahu (Pasadena, CA), Wei Liang (Monrovia, CA), Vladimir S. Ilchenko (Arcadia, CA), Andrey B. Matsko (Pasadena, CA)
Application Number: 15/997,627