JP2014530373A - Method and structure for coupling light into a waveguide - Google Patents

Abstract

Method and structure for coupling light into a waveguide (501, 502). The method includes providing a nano-sized or sub-micron scattering element in a coupling spot (503) disposed in the plane of the waveguide core (501) of the waveguide; directing light (504) to the coupling spot And exposing the light to scattering by the scattering element and coupling at least a portion of the scattered light to the waveguide core (501). [Selection] Figure 5

Description

  The present invention generally relates to a method and structure for coupling light into a waveguide.

  When light is coupled between the light emitting element or the light receiving element and the optical waveguide, there is usually a problem that the optical coupling efficiency is low. Some of the current approaches for optical coupling are described below.

  A diffraction grating has been used to change the direction of light by diffraction and change the angle of incident light to a desired direction. However, the diffraction grating is limited to a specific light wavelength. In applications using a diffraction grating, the coupling efficiency between the light source and the waveguide is usually low. In order to improve the coupling efficiency, it is necessary to implement high-cost integration technology.

  In addition, the direction of light is changed by 90 degrees by reflection using a 45-degree mirror. However, in applications utilizing such mirrors, the coupling efficiency between the light source and the waveguide is usually low. Furthermore, the angular accuracy, position, and surface flatness of the mirror can affect the coupling efficiency. It will also be appreciated that alignment between mirrors or reflectors is also a problem. In particular, the mirror angle, and thus the mirror alignment, is very important in achieving the desired result. Such problems associated with mirrors are exacerbated when the waveguide dimensions are reduced, for example, when the waveguide thickness is, for example, less than 50 μm.

  A prism coupling method using a high refractive index prism has been used for phase matching between a propagation constant in a waveguide and a propagation constant in incident light. However, prisms are usually expensive and still require some degree of alignment for optical coupling. Evanescent wave methods have also been used that excite propagation modes to promote optical coupling. However, such methods use very thin waveguides, eg less than 50 μm, and are usually not efficient.

  In view of the above, there is a need for an optical coupling structure and method for coupling light into a waveguide that attempts to address at least one of the problems described above.

  According to a first aspect of the present invention, a method for coupling light into a waveguide is provided. The method includes providing a nano-sized or sub-micron scattering element in a coupling spot disposed in a plane of a waveguide core of the waveguide, and directing the light into the coupling spot. And exposing the light to scattering by the scattering element such that at least a portion of the scattered light is coupled into the waveguide core.

  The method may further include reflecting the light or the scattered light to redirect the scattering element at the first boundary of the combined spot.

  The method may further comprise reflecting the reflected light to redirect the scattering element at a second boundary of the combined spot.

  The first boundary and the second boundary may be arranged in respective planes of mutually facing surfaces of the waveguide core.

  The mirror used for reflection at the second boundary may include an opening for directing the light into the coupling spot.

  The first boundary and the second boundary may form an optical cavity configured to match cavity resonance to the wavelength of the light.

  The scattering element may be configured such that dipole or multipolar resonance matches the wavelength of the light.

  The method is to subject the light to scattering at the scattering element such that at least a respective portion of the scattered light is coupled into a respective plurality of waveguide core portions of the waveguide. You may include that.

  The plurality of waveguide core portions may be arranged in a star shape around the coupling spot.

  The method may further comprise using a coupling unit that directs the light into the coupling spot.

  The coupling unit may include a collimating element that collimates the light.

  The coupling unit may include a focusing element that focuses the light in the coupling spot.

  According to a second aspect of the present invention, a structure for coupling light into a waveguide is provided. The structure includes a nano-sized or sub-micron scattering element in a coupling spot disposed in the plane of the waveguide core of the waveguide, the scattering element taking at least a portion of the light by scattering. It is configured to couple into the waveguide core.

  The structure may further include means for reflecting the light or the scattered light to redirect it to the scattering element at a first boundary of the coupling spot.

  The structure may further include means for reflecting the reflected light to redirect it to the scattering element at a second boundary of the coupling spot.

  The first boundary and the second boundary may be arranged in respective planes of mutually facing surfaces of the waveguide core.

  The means for reflecting at the second boundary may include an opening that directs the light into the combined spot.

  The first boundary and the second boundary may form an optical cavity configured to match cavity resonance to the wavelength of the light.

  The scattering element may be configured such that dipole or multipolar resonance matches the wavelength of the light.

  The structure may include a plurality of waveguide core portions of the waveguide, and the structure is configured such that at least each portion of the scattered light is coupled into the respective plurality of waveguide core portions. ing.

  The plurality of waveguide core portions may be arranged in a star shape around the coupling spot.

  The structure may further include a coupling unit that directs the light into the coupling spot.

  The coupling unit may include a collimating element that collimates the light.

  The coupling unit may include a focusing element that focuses the light in the coupling spot.

  The material of the coupling spot may be the same as the material of the waveguide core or may be optically aligned.

  The material of the scattering element may be different from the material of the waveguide core.

  Embodiments of the invention will be more readily understood and readily apparent to those skilled in the art from the following written description, taken in conjunction with the drawings, for illustration purposes only.

FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. 2 illustrates simulated optical coupling efficiency for the structure and method of FIG. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 4 shows simulated optical coupling efficiency for the structure and method of FIG. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. 6 illustrates simulated optical coupling efficiency for the structure and method of FIG. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 shows a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 shows a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 shows a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. 9 shows a graph illustrating an exemplary effective surface area of total internal reflection for the structure of FIG. FIG. 10 shows a graph showing an exemplary effective surface area of total internal reflection for the structure of FIG. 8 expressed for parameters different from FIG. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 6 shows a graph illustrating an exemplary effective surface area of total internal reflection for another structure according to an exemplary embodiment. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 3 is a schematic diagram illustrating a structure and method for coupling light into a waveguide, according to an exemplary embodiment. FIG. 6 is a schematic diagram illustrating a statistical approach to scattering of multiple photons, according to an exemplary embodiment. FIG. 6 is a schematic diagram illustrating a statistical approach to scattering of multiple photons, according to an exemplary embodiment. 4 is a respective flowchart illustrating a method for calculating coupling efficiency according to each exemplary embodiment. 4 is a respective flowchart illustrating a method for calculating coupling efficiency according to each exemplary embodiment. FIG. 6 is a schematic diagram illustrating probability calculations for photon scattering in an optical cavity, according to an example embodiment. A flowchart 1900 illustrating a method for coupling light into a waveguide is shown. FIG. 2 is a schematic diagram illustrating a computer system that implements the exemplary embodiment design calculation method and system.

  The described exemplary embodiment places one or more nano-sized or sub-micron scattering centers, for example with a diameter of less than 1 μm, for example in a local region in the core of the polymer waveguide. The local region containing the scattering center may be referred to as a scattering spot or scattering volume.

  In an exemplary embodiment, when incident light from an external light source passes through the waveguide, the light is scattered by the scattering center, changing the trajectory of the incident light and propagating in the region of the waveguide for optical coupling. It has become.

  Similarly, in different embodiments, light propagating in the waveguide can be coupled out of the waveguide by a scattering spot that includes a scattering center.

  In the exemplary embodiment, the scattering centers are diffused in a polymer matrix whose refractive index is close to the refractive index of the core material of the polymer waveguide. The polymer material forming the polymer matrix may be any material, but preferably meets the criteria described below.

  Diffused scattering centers in the waveguide scatter (ie re-radiate) light depending on a specific configuration, such as a nanoparticle size distribution, concentration gradient, or material configuration. For example, depending on the specific configuration of the nanoparticle scatterer, the scattering may be isotropic or directional (anisotropic).

  Some exemplary embodiments of light scattering that couple light into the waveguide are as follows.

  A first exemplary embodiment (single pass) is shown in FIG. 1, and this first exemplary embodiment includes a waveguide core 101, a waveguide cladding 102, and an optical coupling spot 103. Simulations were performed to observe the relationships among structural parameters such as waveguide core thickness (t), optical coupling spot diameter (d), particle number density (N), and particle radius (r). In this embodiment, the optical coupling spot 103 includes a matrix of the waveguide core 101 material and scattering particles. The refractive indices of waveguide core 101, cladding 102, and particles at an exemplary wavelength of 633 nm are 1.5918, 1.5389, and 2.87, respectively. The range of simulation parameters is listed in Table 1. Details of the coefficients used in the governing equations used are described in detail below and are further listed in Table 4 below. FIG. 2 shows a simulation of optical coupling efficiency when t = 20 μm, d = 1000 μm, and r = 150 nm. Under this condition, a coupling efficiency of about 1% can be achieved.

  Another exemplary embodiment (double path) is shown in FIG. 3, which includes a waveguide core 301, a waveguide cladding 302, an optical coupling spot 303, and a reflective mirror 306. A simulation was performed to observe the relationship among the structural parameters such as the thickness (t) of the waveguide core, the diameter (d) of the optical coupling spot, the particle number density (N), and the particle radius (r). In this embodiment, the optical coupling spot 303 includes a matrix of waveguide core material and scattering particles. The refractive indices of waveguide core 301, cladding 302, and particles at an exemplary wavelength of 633 nm are 1.5918, 1.5389, and 2.87, respectively. The range of simulation parameters is listed in Table 2. Details of the coefficients used in the governing equations are listed in Table 4. FIG. 4 shows a simulation result of the optical coupling efficiency when t = 20 μm, d = 1000 μm, and r = 150 nm. Under this condition, a coupling efficiency of about 3% can be achieved.

  Another exemplary embodiment (multipath) is shown in FIG. 5, which includes a waveguide core 501, a waveguide cladding 502, an optical coupling spot 503, a reflective mirror 506, and an opening 507 having a reflective surface. , And a condensing lens 508. A simulation was performed to observe the relationship among the structural parameters such as the thickness (t) of the waveguide core, the diameter (d) of the optical coupling spot, the particle number density (N), and the particle radius (r). The optical coupling spot in this embodiment includes a matrix of waveguide core material and scattering particles. The refractive indices of the waveguide core 501, cladding 502, and particles at an exemplary wavelength of 633 nm are 1.5918, 1.5389, and 2.87, respectively. The diameter of the opening 507 is fixed to about 250 μm. Table 3 lists the ranges of the distance (D) between the condenser lens 508 and the top surface of the coupling spot 503, the curvature (R) of the condenser lens 508, and all other simulation parameters. Details of the coefficients used in the governing equations are listed in Table 4. FIG. 6 shows a simulation of optical coupling efficiency when t = 20 μm, d = 1000 μm, r = 150 nm, D = 1.62 mm, and R = 1.88 mm. Under this condition, a coupling efficiency of about 4.2% can be achieved. The opening 507 effectively functions as another mirror and reflects light scattered at a certain angle, but does not function as a resonance in this embodiment.

  In another exemplary embodiment shown in FIGS. 7a and 7b (bipolar or multipolar resonance), the light 700 incident on the scatterer within the coupling spot 703 is matched to the bipolar resonance (or multipole) wavelength of the nanoparticle. Selected. This resonance condition depends on the size and material of the nanoparticle scatterer. On the one hand, an incident light 700 selected to match the bipolar resonance (or multipole) wavelength of the nanoparticle and an incident light 708 that is not matched to the bipolar resonance (or multipole) wavelength of the nanoparticle, respectively. As shown by a comparison of the combined light intensity graphs 704, 706 in 7a and 7b, the scattering efficiency of the nanoparticle scatterer can be increased when the wavelength of the input light 700 matches the wavelength of the nanoparticle scatterer. This mechanism can be combined with the embodiments described herein to improve the scattering efficiency for each case.

In another exemplary embodiment (cavity multipass), an optical cavity may be implemented. The nanoparticles preferably resonate at the wavelength of the incident light, and more preferably resonate with the optical cavity. In this case, the scattering efficiency is determined by the interaction between the incident light, the optical resonant cavity and the bipolar nanoparticles as a whole system. The highest scattering efficiency can be achieved if the resonance between the nanoparticles and the optical cavity is “tuned” to a single wavelength. As shown in FIG. 7 c, nanoparticles 720 resonate at a wavelength within the wavelength range of incident light 722. On the other hand, the optical cavity 724 in the coupling spot 733 shown in FIG. 7d can capture light and resonate at one or more wavelengths. In a combination in one embodiment, the input light source generates incident light 725 with a wavelength λ _res that matches the resonant wavelength of the nanoparticles, where λ _res is one of a plurality of cavities 724 at the resonant wavelength of the coupling spot 733. Sometimes, the overall scattering efficiency can preferably be maximized, as shown by the graph 726 coupling the coupled light in FIG. 7e. In this embodiment, the optical cavity 724 is formed by a two-way mirror 728 formed on the waveguide core 730 and a bottom cladding or mirror 731 formed on the substrate 732. In one exemplary embodiment, the mirror 731 is a metal mirror, preferably providing a broadband and highly reflective surface with no appreciable absorption. A fully reflective metal cladding or substrate 732 can result in a lossy waveguide, so the bottom mirror 731 is preferably localized in the region of the cavity 724 in this embodiment. In an alternative embodiment, if light loss is acceptable, the substrate or cladding 732 may be a reflective metal layer and thus also incorporate the functionality of the bottom mirror 731. The bi-directional mirror 728 can be implemented, for example, as a partially reflective Bragg grating in one embodiment.

  Note that in other embodiments, the cavity can be implemented without matching the resonances of the nanoscatterer.

The scattering center in the host medium is preferably significantly different in refractive index from the host polymer. For example, in an exemplary embodiment, the refractive index of the polymer is 1.59, while the refractive index of the scatterer is about 2.87. In an exemplary embodiment, the combination of the size of the nanoscatterer and the difference between its refractive index and the host medium determines the direction and amount of scattered light. Examples include titanium dioxide (TiO ₂ ), silicon dioxide (SiO ₂ ), diamond, silicon, silicon nitride (Si ₃ N ₄ ), zirconium oxide (ZrO ₂ ), zinc oxide (ZnO), aluminum oxide (Al ₂ O ₃ ) inorganic nano-scatterers in the form of nanoparticles; metal nanoparticles such as gold, silver, aluminum, chromium, platinum; or organic scatterers such as polystyrene, polymethyl methacrylate, polycarbonate, polyimide, and the like. In other exemplary embodiments, nano-sized or sub-micron bubbles in the polymer matrix can also serve as scatterers that couple light into the waveguide.

  One advantage of utilizing a scattering-based coupling mechanism in the exemplary embodiment is that the mechanical alignment tolerance between the light source and the coupling spot is significantly reduced. The coupling spot may be several hundred microns to several millimeters in diameter, for example, and may couple light into a waveguide having a thickness on the order of, for example, a few microns. This relaxed alignment tolerance can be an advantage when considering the actual costs associated with aligning the light source with the planar waveguide.

For optical waveguides, the propagation angle is defined by the critical angle of the waveguide configuration. The propagation angle of the waveguide should be larger than the critical angle and can be expressed as:
However, n _clad and n _core are the refractive indexes of the core and the cladding of the optical waveguide. In the case of an asymmetric waveguide, n _clad is the refractive index of the cladding material with the higher refractive index. Thus, in the exemplary embodiment, only light scattered from the scattering spot in the direction defined by the receiving cone of the waveguide is coupled and guided into the waveguide.

For the cavity multipath embodiments described herein, the total power coupled from the cavity into the waveguide varies with different scatterer distributions. This combined power average can be expressed as:

  Where f (z) is the distribution function of the nano scatterers in the optical cavity and is averaged over many distributions of nano scatterers in the optical cavity.

  In the above, the inventors ignore the Purcell factor enhancement in the above analysis. This is because the value of Q / V considered in the exemplary embodiment is very small. Where Q and V are the optical cavity quality factor and mode volume, respectively. For the optical cavity in the exemplary embodiment, the Q / V is on the order of tens to hundreds, which is best limited by losses due to partial mirrors and nanoscatterers, which is very high for a typical optical cavity. Very low. In this case, there is no preferred direction of scattering within the optical cavity, and without additional confinement structures, the scattering of light by the nanoscatterer depends on its individual scattering pattern.

Furthermore, since the nano scatterer is added, the refractive index of the medium in the optical cavity changes, and the change appears as a shift in the resonance wavelength. This shift can be approximated by Maxwell-Garnet effective medium theory. This takes into account that the uniform refractive index of the medium changes due to the presence of nano-inclusions. This change in refractive index can be expressed as:
Where ε _ΝP , ε _med , n _eff , and n _med are the dielectric constant and refractive index of the nano scatterer and the medium in the optical cavity, respectively, and δ is the volume fraction of the nano scatterer in the optical cavity. The shift of the resonant wavelength (frequency) due to this change in the refractive index of the medium can be calculated as:
Accordingly, the frequency (wavelength) of the light 504 should be selected to take into account changes in the resonance conditions of the optical cavity 509 due to the presence of the nanoscatterer.

  Due to the radiation pattern and polarization characteristics of the scattered light, the incident light 504 is preferably s-polarized and the light scattered by the nanoscatterer couples the light into the waveguide 505 oriented in a direction perpendicular to the incident light 504. Therefore, it has a preferable radiation pattern. Further, the light coupled into the waveguide 505 is preferentially p-polarized with respect to the propagation direction of the light in the waveguide due to the polarization characteristics of the scattered light.

  The binding spot in the exemplary embodiment may include nanoparticles made of any material. The size of the nanoparticles has a preferred range defined by the governing equations described in detail below. The surface of the nanoparticles may be chemically modified to uniformly diffuse the particles into the polymer matrix. Milk-like materials such as emulsions may be made by mixing two or more immiscible polymer materials by using phase separation.

For the matrix polymer in which the nanoparticles are placed, any material with a refractive index close to or equal to the waveguide core may be selected. The polymer optical transparency is preferably greater than 80% / cm, although lower optical transparency is possible. UV curable properties are preferred for simple lithography processes or UV molding processes. For high temperature properties, a T _g (glass transition temperature) higher than 150 ° C. is preferred.

  In one embodiment, to increase the dispersity of the nanoparticles in the polymer solution (the polymer is soluble in the solvent), to introduce the monomer component of the polymer matrix used, or to increase the dispersibility of the nanoparticles in the solvent The surface of the nanoparticles is chemically modified, such as by changing hydrophobic / hydrophilic properties.

  The treated nanoparticles are dispersed in the polymer solution via a polymer solvent.

  The polymer solution in which the nanoparticles are dispersed is disposed on a designated region. This region is defined in the exemplary embodiment by the surface of the bottom cladding and the sides of the waveguide core.

  The polymer solution is solidified by a photocuring method, a heat curing method, or a drying method.

  In an exemplary embodiment, the coupling spot diameter is preferably greater than 500 microns for efficient optical coupling. If the diameter is less than 500 microns, the desired performance can be achieved by utilizing an appropriate centering mechanism. The thickness (depth) of the coupling spot is substantially determined by the thickness of the waveguide.

  As described above, a reflective mirror may be disposed on the surface of the waveguide cladding for double path, multipath, and cavity multipath. Furthermore, for multipath, a reflective mirror, such as a metal mirror / opening, may also be placed on the surface of the waveguide core. Further, for cavity multipath, a partially reflecting mirror such as a Bragg grating may be disposed on the surface of the waveguide core.

For the polymer waveguide in the exemplary embodiment, any material may be selected as long as the refractive index of the core material is higher than the cladding material. The optical transparency of the polymer is preferably greater than 80% / cm. UV curable properties are preferred for simple lithography processes or UV molding processes. For high temperature properties, a T _g (glass transition temperature) higher than about 150 ° C. and a low coefficient of thermal expansion (CTE) are preferred.

  In the exemplary embodiment, the incident light is scattered isotropically at the combined spot unless polarized light is used as the incident light. Therefore, in order to effectively couple the scattered light into the waveguide, the waveguide core that receives the scattered light may be arranged radially from the optical coupling spot as the center, such as a “star shape”. And so on.

FIG. 8 shows a top view of an exemplary embodiment of the present invention showing an elliptical focusing structure. As shown in FIG. 8, the combined spot is arranged at the focal point F. Another focal point F ′ is also shown.

The direction of the long axis (x) is the propagation direction of the waveguide. As shown in FIG. 8, the light emitted from one focal point is collected at the other focal point based on the focusing characteristic of the ellipse 800 and the total internal reflection characteristic of the interface of the core 802 / cladding 804. When the incident angle θ _P is larger than the critical angle θ _c = sin ⁻¹ (n ₀ / n ₁ ), the interface of the focusing structure acts as a mirror. The light emitted from the coupling spot at the local point F is reflected by the interface of the coupling structure, and the reflected light then collects at the other local point F ′.

  The effective surface area of total internal reflection can be calculated as:

point
Is represented by coordinates expressed in parameters.

The direction of the tangent of the ellipsoid at is defined as:

The incident angle θ _P is expressed as follows.

point
For total internal reflection condition is satisfied, theta _P satisfies the condition of the following formula.
However,

Therefore, the following equation holds.

On the other hand, θ _guide satisfies the total internal reflection condition.

These two formulas can be rewritten as the following formula using the ratio of the major axis to the minor axis.

As a result, the effective surface area of total internal reflection is defined by equations (14) and (15). FIG. 9 shows the relationship between the parameter t and the left side of these equations when a / b = 4, n ₀ = 1, and n ₁ = 1.5918. As can be seen from FIG. 9, the shaded area divided by the three curves indicates the value of the parameter t that satisfies the above equations (14) and (15). From FIG. 9, when the value of t is about 30 to 165, the expressions (14) and (15) are satisfied. The relationship between θ _F and t is expressed by the following equation.

Replacing the value of t in theta _F, it can be redrawn as shown in FIG. 10 to the graph of FIG. 9. As can be seen from the shaded area divided by the three curves, in this example, the acceptance angle of scattered light from the scattering spot is about 5 degrees to 100 degrees.

  Since the light scattered from the scattering spot is omnidirectional, the light can be scattered into one or more waveguides, thus maximizing the sum of the light captured by the waveguides. However, it appears that the number of waveguides should also be kept to a minimum in order to optimize light utilization and maximize the light coupled into each waveguide. Therefore, it is preferable to balance the maximization of the total sum of light to be coupled with the minimization of the total number of waveguides.

FIG. 11 illustrates an exemplary embodiment that combines two ellipsoidal structures 1100, 1102 to couple light from a single scattered spot 1104 into two waveguide channels 1106, 1108. This exemplary embodiment may be useful when the effective maximum value of θ _F is between 90 and 180 degrees. Similarly, when the effective maximum value of θ _F is between 60 degrees and 90 degrees, as shown in FIG. 12, three elliptical structures 1200, 1202, 1204 are combined to effectively use scattered light. can do.

In order to obtain some value of θ _F satisfying the calculation formula (16) when the difference in refractive index is small, such as Δn = 0.01 (for example, n ₀ = 1.5818, n ₁ = 1.5918). The ratio between the major axis and the minor axis is increased. For example, if the ratio of the major axis to the minor axis is 12, as shown by the shaded area in FIG. 13, if the angle of θ _F is between 3.5 degrees and 10.7 degrees, the calculation formula Satisfies (16). This means that the coupling structure can have a total of 20 degrees for optical coupling (−10 degrees <θ _F <+10 degrees). Therefore, as shown in FIG. 14, a star shape 1400 having an 18-channel waveguide can be an ideal structure for this difference in refractive index.

  For the manufacture of the optical waveguide in the exemplary embodiment, any polymer material may be selected, but preferably the material does not have a large propagation loss at the operating wavelength. Three methods of manufacturing waveguides using polymer materials are described below by way of example and not limitation.

  Photolithography: In this case, the polymer can be directly patterned by exposing selected portions of the polymer to ultraviolet light through a photomask. The exposed area of the polymer will polymerize when exposed to ultraviolet light to define the waveguide structure.

Physical etching: In this method, the waveguide structure is defined by physically implanting suitable ions into selected areas of the polymer in a vacuum vessel. This etching process can be performed in various ways including reactive ion etching (RIE), inductively coupled plasma (ICP) etching, ion beam etching (IBE), electron cyclotron resonance (ECR) etching, and the like. For selective etching, preferably a mask is made on the polymer film to be patterned. This mask may be either a commercially available patterned photoresist or a hard material such as patterned silicon dioxide (SiO ₂ ) or silicon nitride (Si ₃ N ₄ ).

  Molding: In this method, a waveguide pattern mold is first formed, for example using a photolithographic, physical etching, or electron beam lithography process, by filling the mold with a polymer and applying a selected amount of temperature and pressure. A waveguide pattern is formed. The resulting molded structure made of the core polymer defines the waveguide structure. The mold may be a hard mold made of any hard metal such as nickel or chromium, or may be a soft mold that can be made of a commercially available polymer such as PDMS or SU8.

  Another molding method is by using an ultraviolet curable polymer and exposing the polymer through an ultraviolet transmissive mold. In this case, the mask may be made of any polymer or glass structure that is transparent at ultraviolet wavelengths.

  As shown in FIG. 15, the system further includes a light source, preferably a laser diode, for example in the form of a laser diode 1501, a collimating lens 1502, a condensing lens 1503, a diverging ray of a particular diameter in the scattering volume / combining spot 1505. A sized metal opening 1504, a bottom reflecting mirror 1506, an optical waveguide core 1507, and an optical waveguide cladding 1508 may be included. Preferably, the light source 1501 can provide an optical frequency between 400 nm and 2500 nm and covers the visible and near infrared spectra.

  The metal opening 1504 also functions as a mirror that prevents back reflection of the scattered light and further scatters the back reflected light into the scattering medium. In this embodiment, the diameter of the metal opening 1504 is a fixed diameter, and is a fixed ratio with respect to the diameter of the cylindrical scattering volume / coupling spot 1505, for example, about 0.5 or less.

  In order to obtain a light beam having a diameter approximately equal to the diameter of the opening 1504, the condenser lens 1503 is disposed at a certain fixed distance from the opening 1504. This distance is determined by the focal length of the lens 1503, and the diameter of the light beam exceeding the focal length can be analytically determined by Gaussian beam optics.

  Prior to collecting the light from the laser diode 1501, the light is collimated to produce the desired focusing effect. This collimation of the light beam from the laser diode 1501 can be performed by a suitable aspheric collimating lens 1502. The collimating and focusing package may also be a combined unit. The main purpose of this optical unit is to advantageously provide a uniform divergent beam of photons that enter the scattering volume and are scattered with high probability into the waveguide structure.

  For scattering centers, the probability of scattering in a direction perpendicular to the direction of incident light is generally low. By utilizing divergent rays, the incident light can preferably be spread over a large range of angles. This angular spread of incident light is determined by the numerical value of the aperture of the condenser lens 1503. The range of the light source (ray diameter) may also depend on the diameter of the aperture 1504, and the choice may depend on the requirements of the reflective surface 1506 and the illuminance of the scattering volume. In this exemplary embodiment, the reflective aperture 1504 is disposed at a specific distance from the focal length of the condenser lens 1503. This distance can be calculated by fixing the diameter of the aperture 1504 and the parameters of the condenser lens 1503.

  Some portions of the following description have been made explicitly or implicitly in terms of algorithms and functions or symbolic representations of operations on data in computer memory. Such algorithmic descriptions and functions or symbolic representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others. The algorithm is here and generally considered to be a self-consistent sequence of steps leading to the desired result. These steps are those requiring physical manipulation of physical quantities such as electrical, magnetic, or optical signals that can be manipulated in storage, transfer, combination, comparison, and other ways. is there.

  Unless otherwise noted, and as will be apparent from the following, throughout this specification "scan", "calculation", "decision", "replacement", "generation", "initialization", "output", etc. A terminology description is a computer that manipulates data represented as physical quantities in a computer system and converts them into other data that is also represented as physical quantities in a computer system or other information storage, transmission, or display device. It will be understood that it refers to the operation and process of a system or similar electronic device.

  The present specification also discloses an apparatus for performing the operations of the method. Such an apparatus may be specially configured for the required purpose and may include a general purpose computer or other device selectively activated or reconfigured by a computer program stored in the computer. The algorithms and displays presented herein are not inherently related to any particular computer or other device. Various general purpose machines may be used with programs in accordance with the teachings herein. Alternatively, it may be appropriate to build a more specialized device that performs the required method steps. The structure of a conventional general-purpose computer will be apparent from the following description.

  In addition, this specification also implicitly describes a computer program in that it will be apparent to those skilled in the art that the individual steps of the methods described herein can be performed by computer code. Disclose. This computer program is not intended to be limited to any particular programming language and implementation thereof. It will be appreciated that the teachings of the disclosure contained herein may be implemented using various programming languages and their coding. Further, the computer program is not intended to be limited to any particular control flow. There are many other variations of this computer program that can use different control flows without departing from the spirit or scope of the present invention.

  Further, one or more of the steps of the computer program may be performed in parallel rather than sequentially. Such a computer program can be stored on any computer readable medium. The computer readable medium may include a storage device such as a magnetic or optical disk, a memory chip, or other storage device suitable for interfacing with a general purpose computer. Computer-readable media may also include wired media, such as those illustrated in the Internet system, or wireless media, such as those illustrated in the GSM mobile phone system. When the computer program is loaded and executed on such a general purpose computer, it effectively results in an apparatus that implements the steps of the preferred method.

  In order to approximate the preferred conditions or parameters of the scattering spot in the exemplary embodiment, the inventors have identified a governing equation that defines the optical coupling efficiency in the exemplary embodiment.

  To find such a formula, we first consider a statistical approach to photon scattering. As shown in FIG. 16a, the photon scattering process is assumed to be based on a classical interaction between photons and scattering centers governed by a Gaussian stochastic process. In this case, a photon can have multiple scattering events in the scattering volume, the event being determined by the mean free path of the photons in the scattering volume and the optical path length of the photons in the scattering volume.

Next, a scattering volume, that is, a spot embedded in a waveguide structure composed of randomly distributed ^{nanoscatterers} having a number density of n ₀ cm ⁻³ was examined. The scattering volume was assumed to be a cylindrical volume with height L and radius R. If N ₀ photons per second hit the scattering volume in the direction along the axis of the cylindrical volume, the probability that a given photon will be scattered k times in the scattering volume is expressed as [1] Dominated by Poisson distribution.

Here, <s> is the average number of interactions between photons and nanoscatterers for the entire photon, which can be expressed as [2].

Where σ _sca and n _med are the scattering cross section of the _{nanoscatterer} and the refractive index of the medium, respectively. Thus, the probability that there is no scattering event can be simply written as P (0), and the probability that a photon will experience a scattering event at least once can be written as:

Assuming that scattered photons are uniformly distributed in the scattering volume before they scatter out, specify the differential probability per unit volume of photons scattered in the scattering volume and coupled into the waveguide structure Can do. FIG. 16b shows a top view of the scattering spot. In this case, consider a very small volume defined by a cylindrical scattering volume circle with a width dr at a distance r from the scattering volume axis. Then, the differential probability that the photons in this volume element couple into the waveguide, eg, the receiving cone defined by the critical angle, can be written as:

In the above equation, assuming that photons present in the annulus must travel an average distance (R−r) before coupling into the waveguide, σ _d is within the receiving cone of the waveguide structure. It is a differential cross section of the nano scatterer which emitted the scattered light. θ _c is the critical angle of the waveguide and is determined by the refractive index of the waveguide core and the cladding material. Assuming that the above equation is integrated with respect to r and that the integration of the differential cross section does not depend on the position of the nanoscatterer in the scattering volume, the following equation holds:

Together with the previous equation, the total probability that photons will couple into the cavity, and thus the coupling efficiency, is:
The total efficiency thus depends on the length and diameter of the scattering volume, the number density and scattering coefficient of the nanoscatterers, and the waveguide core and cladding materials. The above formula applies to Mie scattering and the same nanoscatterer that isotropically scatters light. The above results show that it can be applied to nano-scatterers with a radius smaller than 50 nm.

FIG. 17a shows a flowchart illustrating steps 1701 to 1705 for calculating the coupling efficiency of an exemplary embodiment in which one scattering event occurs from the parameters of L, R, n _med , N ₀ , σ _sca , and θ _c . FIG. 17b shows another example of steps 1711 to 1716 that calculate the coupling efficiency of another exemplary embodiment in which multiple scattering events occur from the parameters of L, R, n _med , N ₀ , σ _sca , θ _c . A flowchart is shown.

  An empirical approach that extends the scope to double-pass and multi-pass configurations in different embodiments was also evaluated. A non-sequential ray tracing method of ZEMAX ™ (Radiant ZEMAX LLC., USA) using the Mie scattering function (Mie.dll) was used to evaluate the coupling efficiency of the scattering based coupling structure.

An embodiment of the present invention including a waveguide core, waveguide cladding, and optical coupling spot is simulated to determine the waveguide core thickness (t), optical coupling spot diameter (d), particle number density (N And the relationship between structural parameters such as particle diameter (r). The optical coupling spot includes waveguide core material and particles. The mean free path (MFP) is calculated from the particle number density (N) and the scattering cross section (σ _sca ).

Based on the relationship between the structural parameter and the optical coupling efficiency, the inventors have found that the efficiency can be roughly described by a Gaussian function such as

The parameters a, b and c were further analyzed to find the relationship between the structural parameters.

A resonator optical resonator can capture light of a particular wavelength (energy), and the captured light remains in the resonator until it can be dissipated by losses due to absorption or scattering. The light captured in the resonator may be enhanced compared to the incident intensity due to constructive interference of light waves that bounce back and forth within the resonator. The magnitude of this enhancement depends on the way in which light can be lost from the resonator. If a nanoscatterer that has a significant scattering cross section at the resonant wavelength of the optical resonator (or exhibits itself a resonant behavior at the resonant wavelength of the cavity) is in the optical cavity, the increased intensity is Scattering may increase by circulating in the interior. However, such an increase in scattering is only at the resonant wavelength of the optical resonator.

  Further theoretical approaches were also evaluated for optical resonators in which scattering centers exist within the optical cavity structure in the exemplary embodiment. If there is a nanoparticle scatterer in the path of incident light, light will scatter in all directions depending on the optical properties and size of the nanoparticle scatterer. In the case of a waveguide, if the waveguide is oriented in a plane perpendicular to the direction of the incident light, only light scattered in a direction within the receiving cone of the waveguide is guided along the waveguide. That is, it couples into the waveguide. However, if light is confined in a cavity containing nanoparticle scatterers, the light that circulates in the cavity may scatter multiple times, depending on the lifetime of the photons in the cavity. Becomes higher. As a result, the scattered light may increase in the waveguide, and the strength of optical coupling may increase. FIG. 18 shows the scattering of incident light 1800 by nanoparticles 1802 within cavity 1804.

Placing the same nanoscatterer that couples light into the waveguide increases the loss in the optical cavity, thereby changing the round trip loss factor term in the equation for the optical cavity. There are two components of light scattering in the cavity:
Light scattered in the forward direction (θ = 0 degrees, the part that remains in the optical cavity); and light scattered out of the optical cavity.

  Assume that the phase lag between unscattered light and forward scattered light in the cavity is negligible. It is further assumed that independent scattering is followed by scattering from many identical nanoparticles, thereby making the phase difference between the fields scattered by the individual scatterers random.

First, the amplitude round trip loss factor due to the nanoparticle scatterer in the optical cavity is calculated. To approximate this, consider a Fabry-Perot optical cavity of length L _cav and cross-sectional area A _cav . P _f1 is the power incident on the nanoparticle scatterer. Assume that the incident light is in the form of a plane wave whose spatial extent matches the cross-sectional area of the optical cavity. The total power (P _f1 ) in the forward direction on the way to the mirror disappears by an amount determined by the incident intensity (P _f1 / A _cav ) and the scattering cross section of the _{nanoscatterer} .

Assume that all of the scattered light is lost from the optical resonator, except for some of the light scattered in the forward direction (θ = 0 degrees). This remaining power P _f2 = P _b1 is then reflected from the mirror and re-enters the nanoscatterer and the same process is repeated again. Assume that incident light and scattered light can reasonably approximate a plane wave. Thus, there is two chances for light to be scattered by the nanoscatterer in a single round trip. At this time, the power (P _b2 ) returned after one round trip can be expressed as the following equation.
P _b2 = P _{f1 −Light} loss from the cavity into the waveguide = P _{f1 −Light} scattered in the waveguide from the first pass−Light scattered in the waveguide from the second pass after reflection = P _f1 -(Light scattered from the first passage-light scattered forward from the first passage)-(light scattered from the second passage-light scattered forward from the second passage)

The above expression is simplified and expressed as the following expression.

The above equation represents the power scattered for a single nanoscatterer as a function of scattering cross section and cavity cross section. Assuming that a total of N identical scatterers scatter independently of each other, the above equation can be written as:

Therefore, for the amplitude, the above equation can be written as:

Therefore, the amplitude round trip loss factor for an optical cavity in which a nanoscatterer is present can be written as:

If there are both scattering and absorption losses in the optical cavity, the combined amplitude loss factor can be written as:

Therefore, in order to satisfy the critical coupling condition of the optical cavity, the following conditions must be satisfied.

By substituting the equations for a _abs and a _NP , an approximate total number of nanoparticles that satisfy the critical coupling condition for the optical cavity can be calculated. This approximation ignores second order terms in the formula for the amplitude round trip loss factor due to scattering. Therefore, the “critical number” of nanoparticles can be roughly estimated by the following equation.

  This number depends on the scattering properties of the nanoparticles, the amplitude reflection coefficient of the partial mirror, the absorption coefficient of the optical cavity, and the total length of the cavity. This equation is the governing equation for determining the density of the nanoscatterer in the optical cavity necessary to achieve optimal coupling efficiency.

  The enhancement factor of the optical cavity is also determined by the loss factor and varies as a function of the number of nanoparticles. For a given cavity design, the increase factor decreases as the number of nanoscatterers increases. This is because as the number of nanoscatterers increases, the loss due to scattering within the optical resonator also increases because the loss of light from the resonator increases due to scattering. This loss corresponds to a decrease in the lifetime of the photons in the cavity which reduces the intensity increase factor of the optical resonator. The resonance bandwidth indicated by the half-wave height full width value (FWHM) is an index of loss existing in the optical resonator. Therefore, as the number of scatterers in the optical resonator increases, the resonance bandwidth also increases. Therefore, a balance must be struck between the number of nanoscatterers in the optical cavity and the characteristics of the optical cavity. The performance of the coupler based on the increased scattering of the optical cavity is a complex interaction between the optical cavity and the scattering properties of the nanoscatterer. The coupling of scattered light into the waveguide depends on the properties of the scattered light from the optical cavity and the refractive index of the waveguide core and cladding materials.

  FIG. 19 shows a flowchart 1900 illustrating a method for coupling light into a waveguide. In step 1902, nano-sized or sub-micron scattering elements are provided in the coupling spots located in the plane of the waveguide core of the waveguide. In step 1904, light is directed into the combined spot. In step 1906, the light is subjected to scattering by the scattering element such that at least a portion of the scattered light is coupled into the waveguide core.

  Exemplary embodiments of the present invention can increase the efficiency of coupling light into an optical waveguide and reduce alignment tolerances. The combination of materials that form the binding spot can be quite flexible. In one embodiment, a star-shaped waveguide can efficiently use light coupled into the waveguide.

  The exemplary embodiment design calculation method and system may be implemented on a computer system 2000 schematically illustrated in FIG. This may be implemented as software, such as a computer program, executed within computer system 2000 and instructing computer system 2000 to perform the methods of the exemplary embodiments.

  The computer system 2000 includes a computer module 2002, input modules such as a keyboard 2004 and a mouse 2006, and a plurality of output devices such as a display 2008 and a printer 2010.

  The computer module 2002 is connected to the computer network 2012 by a suitable transmission / reception device 2014 and can access, for example, the Internet or other network systems such as a local area network (LAN) or a wide area network (WAN).

  The computer module 2002 in the example includes a processor 2018, a random access memory (RAM) 2020, and a read only memory (ROM) 2022. The computer module 2002 also includes a plurality of input / output (I / O) interfaces, such as an I / O interface 2024 to the display 2008 and an I / O interface 2026 to the keyboard 2004.

  The components of computer module 2002 typically communicate via interconnect bus 2028 in a manner known to those skilled in the relevant art.

  Application programs are typically encoded on a data storage medium such as a CD-ROM or a flash memory carrier, supplied to a user of computer system 2000, and read using a corresponding data storage medium drive of data storage device 2030. The application program is read and controlled by the processor 2018 when executed. The intermediate storage of the program data may be performed using the RAM 2020.

  Those skilled in the art will recognize that the present invention can be modified and / or modified without departing from the spirit or scope of the invention as broadly described, as shown in the specific embodiments. . Therefore, it should be considered that this embodiment is illustrative and not restrictive in all respects.

For example, while embodiments have been described with respect to polymeric waveguides / materials, it will be appreciated that the present invention can be applied to other waveguides / materials such as Si, Ge, SiO ₂ , LiNbO ₃ .

  Also, although embodiments have been described with respect to planar waveguides, it will be appreciated that the present invention can be applied to other waveguides such as strips, fibers, photonic crystal fibers and the like.

Claims (20)

A method for coupling light into a waveguide,
Providing a nano-sized or sub-micron scattering element within a coupling spot, wherein the coupling spot thereby forms a scattering volume that is a local region of the waveguide core of the waveguide;
Directing the light into the combined spot;
Subjecting the light to scattering at the scattering element such that at least a portion of the scattered light is coupled into the waveguide core;
Reflecting the light or the scattered light to redirect the scattering element at the first boundary of the combined spot;
A mirror used for reflection at the second boundary that reflects the reflected light to redirect the scattering element at the second boundary of the coupling spot includes an opening that directs the light into the coupling spot. And coupling the light into the waveguide.   The method of claim 1, wherein the first boundary and the second boundary are disposed in respective planes of mutually opposing surfaces of the waveguide core.   The method of claim 1 or 2, wherein the first boundary and the second boundary form an optical cavity configured to match cavity resonances to the wavelength of the light.   4. A method according to any one of the preceding claims, wherein the scattering element is configured such that a dipole or multipolar resonance is matched to the wavelength of the light.   Subjecting the light to scattering at the scattering element, such that at least a respective portion of the scattered light is coupled into a respective plurality of waveguide core portions of the waveguide; The method of any one of Claims 1-4.   The method according to claim 1, wherein the plurality of waveguide core portions are arranged in a star shape around the coupling spot.   The method according to claim 1, further comprising using a coupling unit that directs the light into the coupling spot.   The method of claim 7, wherein the coupling unit includes a collimating element that collimates the light.   9. A method according to claim 7 or 8, wherein the coupling unit comprises a focusing element that focuses the light into the coupling spot. A structure for coupling light into a waveguide, comprising a nano-sized or sub-micron scattering element within the coupling spot, whereby the coupling spot is thereby a local region of the waveguide core of the waveguide Forming a volume, wherein the scattering element is configured to couple at least a portion of the light into the waveguide core by scattering;
The structure further includes means for reflecting the light or the scattered light to redirect it to the scattering element at a first boundary of the combined spot, and the scattered light is reflected at the second boundary of the combined spot. Further comprising means for reflecting back towards the element;
The means for reflecting at the second boundary includes a mirror having an opening for directing the light into the combined spot;
A structure that couples light into a waveguide.   The structure of claim 10, wherein the first boundary and the second boundary are disposed in respective planes of mutually opposing surfaces of the waveguide core.   12. The structure of claim 10 or 11, wherein the first boundary and the second boundary form an optical cavity configured to match a cavity resonance to the wavelength of the light.   13. A structure according to any one of claims 10 to 12, wherein the scattering element is configured such that dipolar or multipolar resonances are matched to the wavelength of the light.   14. A structure according to any one of claims 10 to 13, comprising a plurality of waveguide core portions of the waveguide, wherein the structure comprises at least each portion of the scattered light being the plurality of the respective plurality. 14. A structure as claimed in any one of claims 10 to 13 configured to couple into a waveguide core portion of the.   15. The structure according to any one of claims 10 to 14, wherein the plurality of waveguide core portions are arranged in a star shape around the coupling spot.   16. A structure according to any one of claims 10 to 15, further comprising a coupling unit for directing the light into the coupling spot.   The structure of claim 16, wherein the coupling unit includes a collimating element that collimates the light.   18. The structure of claim 16 or 17, wherein the coupling unit includes a focusing element that focuses the light into the coupling spot.   19. A structure according to any one of claims 10 to 18, wherein the material of the coupling spot is the same as or optically matched with the material of the waveguide core.   The structure of claim 18, wherein a material of the scattering element is different from a material of the waveguide core.