JP5141472B2 - Optical delay element - Google Patents

Description

  The present invention relates to an optical delay element.

As the optical delay element, those disclosed in Non-Patent Documents 1 and 2 below are known. The optical delay element disclosed in Non-Patent Document 1 has a plurality of resonators connected in series. The optical delay element disclosed in Non-Patent Document 2 includes a linear waveguide and a plurality of resonators. In the optical delay element disclosed in Non-Patent Document 2, each of the plurality of resonators is optically coupled to a linear waveguide at a part thereof.
Ammon Yariv and three others, "Coupled-resonator optical wavegauide: a proposal andanalysis", OPTICS LETTERS, June 1, 1999, Vol. 24, No. 11, p711-p713 John E. Heebner and 1 other, "Slow light, induced dispersion, enhanced nonlinearity, and optical solitons in a resonator-array waveguide", PHYSICAL REVIEW E, 2002, Vol. 65, 036619-1-036619-4

  Both of the optical delay elements disclosed in Non-Patent Documents 1 and 2 utilize a resonance phenomenon in a resonator in order to delay light. That is, the optical delay elements disclosed in these documents can delay light by reducing the group velocity of light only near the resonance frequency. Therefore, the operating wavelength band of the optical delay elements disclosed in these documents is narrow.

  An object of the present invention is to provide an optical delay element having a wide operating wavelength band.

The optical delay element of the present invention includes an input port, an output port, an optical waveguide, a ring resonator, a first coupling portion, and a second coupling portion. The input port is a port through which light is input. The output port is a port for outputting light. The optical waveguide extends from the input port to the output port. The ring resonator has a waveguide extending in a ring shape. The first coupling portion and the second coupling portion couple light between the two portions of the ring resonator and the optical waveguide. The first coupling portion is provided closer to the input port than the second coupling portion. Phase change amount φ _U of the optical waveguide path from the first coupling section to the second coupling section, and the waveguide path of the ring resonator between the first coupling section and the second coupling section The phase change amount φ _H satisfies 0.8 ≦ φ _U / φ _H ≦ 1.2. The light intensity coupling efficiency κ ² in each of the first coupling unit and the second coupling unit is such that the parameter θ having a relationship of κ = sin (θ) with respect to the amplitude coupling coefficient κ is 0.2 ≦ θ / It is set so as to satisfy π ≦ 0.3.

This optical delay element can reduce the group velocity of light in a wide wavelength band. In addition, according to the present optical delay element, the wavelength dependence of the group velocity of light decreases as φ _U / φ _H approaches 1.0 and as θ / π approaches 0.25.

The above-mentioned conditions regarding φ _U / φ _H can be realized by bending the path of the ring resonator a plurality of times. Further, the condition regarding φ _U / φ _H can be realized by setting the refractive index and / or cross-sectional area of the waveguide of the ring resonator and the refractive index and / or cross-sectional area of the optical waveguide to be different from each other. is there.

Another optical delay element of the present invention includes an input port, an output port, an optical waveguide, a plurality of ring resonators, a plurality of first coupling portions, and a plurality of second coupling portions. . The input port is a port through which light is input. The output port is a port for outputting light. The optical waveguide extends from the input port to the output port. Each of the plurality of ring resonators has a ring-shaped waveguide. The plurality of first coupling portions and the plurality of second coupling portions couple light between two portions of the plurality of ring resonators and the optical waveguide. Each of the plurality of first coupling portions is provided closer to the input port than the second coupling portion provided for the same ring resonator among the plurality of second coupling portions. Among the plurality of first coupling units and the plurality of second coupling units, the first coupling unit and the second coupling unit provided for the same ring resonator are connected to the first coupling unit from the first coupling unit. The phase change amount φ _U of the optical waveguide path between the two coupling portions and the phase change amount of the waveguide path of the ring resonator between the first coupling portion and the second coupling portion the phi _H, satisfies _{0.8 ≦ φ U / φ H ≦} 1.2. The light intensity coupling efficiency κ ² in each of the plurality of first coupling portions and the plurality of second coupling portions is such that the parameter θ having a relationship of κ = sin (θ) with respect to the amplitude coupling coefficient κ is 0.2 ≦ It is set so as to satisfy θ / π ≦ 0.3.

  This optical delay element can also reduce the group velocity of light in a wide wavelength band. Further, this optical delay element can obtain an arbitrary amount of delay by appropriately setting the number of ring resonators and the size of the ring resonator.

  As described above, according to the present invention, an optical delay element having a wide operating wavelength band is provided.

  Before describing preferred embodiments of the present invention, conditions to be satisfied by the optical delay element of the present invention will be described below.

  As described above, the optical delay element of the present invention includes an optical waveguide (hereinafter referred to as “reduction waveguide”) extending from the input port to the output port. The optical delay element of the present invention includes one or more ring resonators having a ring-shaped waveguide. Furthermore, the optical delay element of the present invention includes a first coupling portion and a second coupling portion for optically coupling the two portions of the ring resonator and the optical waveguide.

In the optical delay element of the present invention, the phase change amount φ _U of the path of the reduction waveguide between the first coupling portion and the second coupling portion, and the interval between the first coupling portion and the second coupling portion. The phase change amount φ _H of the path of the ring resonator satisfies the following expression (1). Preferably, φ _U / φ _H is 1.0. That is, it is preferable that φ _U / φ _H is closer to 1.0.
0.8 ≦ φ _U / φ _H ≦ 1.2 (1)

Here, the phase variation amount phi _U and the phase variation amount phi _H can be defined by the following equation (2).
φ _i = β _i L _i (i = H, U) (2)
In Equation (2), β _H is the propagation constant of the ring resonator, and β _U is the propagation constant of the reduction waveguide. L _H is the path length of the waveguide of the ring resonator from the first coupling part to the second coupling part, and L _U is the reduction guide from the first coupling part to the second coupling part. This is the path length of the waveguide.

As is clear from Equation (2), φ _U / φ _{H is set} to 1.0 by adjusting at least one of the propagation constant β _H , the propagation constant β _U , the path length L _H , and the path length L _U. It is possible to approach. For example, in order to make φ _U / φ _H close to 1.0, the following three measures can be considered.
(I) A plurality of bends are introduced into the above-described path of the waveguide of the ring resonator (L _i adjustment).
(Ii) The refractive index and / or cross-sectional area of the reduction waveguide and the refractive index and / or cross-sectional area of the waveguide of the ring resonator are made different (adjustment of β _i )
(Iii) Make the path length of the path of the optical waveguide equal to the path length of the path of the ring resonator (adjustment of L _i )

In the optical delay element of the present invention, the light intensity coupling efficiency κ ² in each of the first coupling portion and the second coupling portion has a relationship of κ = sin (θ) with respect to the amplitude coupling coefficient κ. The parameter θ is set so as to satisfy the following expression (3) regardless of the wavelength.
0.2 ≦ θ / π ≦ 0.3 (3)
Preferably, φ _U / φ _H is 0.25 regardless of the wavelength. That is, the light intensity coupling efficiency κ ² of each of the first coupling portion and the second coupling portion is preferably 50% regardless of the wavelength.

In the embodiment described below, φ _U / φ _{H is set} to 1.0 by any one of the measures (i) to (iii) described above. In these embodiments, θ / π is set to 0.25 regardless of the wavelength.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

  FIG. 1 is a plan view schematically showing an optical delay element according to an embodiment. The optical delay element 10 illustrated in FIG. 1 includes an input port 12, an output port 14, a reduction waveguide 16, a ring resonator 18, a first coupling unit 20, and a second coupling unit 22. The optical delay element 10 can be constituted by, for example, a planar waveguide or an optical fiber. Hereinafter, the description will be continued assuming that the optical delay element 10 is constituted by a planar waveguide.

  The input port 12 is a port through which light is input. The output port 14 is a port from which light is output. The optical waveguide 16 extends from the input port 12 to the output port 14. Part of the light input to the input port 12 is guided through the optical waveguide 16 and output from the output port 14.

  The ring resonator 18 has a ring-shaped waveguide. The ring resonator 18 is a resonator that resonates light having a wavelength determined by the optical path length. The circumference of the ring resonator 18 is a length that is an integral multiple of the half wavelength of the light that resonates in the ring resonator 18.

In this example, the optical waveguide 16 and the ring resonator 18 can be configured by a planar waveguide in which a core material is embedded with a cladding material. For example, SiON can be used as the core material. Further, SiO ₂ can be used as the cladding material. Moreover, the width | variety and width | variety of a core can be 0.8 micrometer and 1.6 micrometers, respectively.

  In the present optical delay element 10, light is coupled between the two portions of the ring resonator 18 and the optical waveguide 16 in the first coupling unit 20 and the second coupling unit 22. The first coupling unit 20 is located closer to the input port 12 than the second coupling unit 22.

The first coupling unit 20 and the second coupling unit 22 satisfy θ / π = 0.25 regardless of the wavelength. That is, the strength coupling efficiency κ ² of each of the first coupling unit 20 and the second coupling unit 22 is 50%. A wavelength-independent directional coupler can be used for the first coupling unit 20 and the second coupling unit 22.

  The wavelength-independent directional coupler includes an average distance between the optical waveguide 16 and the waveguide of the ring resonator 18, respective cross-sectional areas of the optical waveguide 16 and the waveguide of the ring resonator 18, and the optical waveguide 16. And by adjusting the curvature of the waveguide of the ring resonator 18. For details on how to design a wavelength-independent directional coupler, see GeorgeT. Paloczi, two others, "Wavelength-Insensitive Nonadiabatic Mode Evolution Couplers", IEEE Photonics Technology Letters, February 2004, Vol.16, No.2p515-517. Please refer to.

Further, in the present optical delay element 10, the above measure (i) is adopted so that φ _U / φ _H becomes 1.0. That is, in the optical delay element 10, a plurality of bends are introduced into the path 18 a of the ring resonator 18 between the first coupling unit 20 and the second coupling unit 22. The propagation constant β _U of the optical waveguide 16 and the propagation constant β _{H of the} waveguide of the ring resonator 18 are the same. Here, φ _U is the phase change amount of the path 16 a of the optical waveguide 16 between the first coupling unit 20 and the second coupling unit 22, and φ _H is the second coupling level from the first coupling unit 20 to the second coupling unit 20. The phase change amount of the path 18a of the waveguide of the ring resonator 18 between the first and second coupling portions 22.

The characteristics of the optical delay element 10 having the above-described configuration will be described with reference to FIGS. The characteristics shown in FIGS. 2 to 4 are obtained by calculation using the effective refractive index method. In this calculation, the following conditions were adopted for the waveguide of the ring resonator 18 and the reduction waveguide 16.
Core width: 1.6 μm
Core height: 0.8μm
Refractive index of core: 2.8
Refractive index of clad: 1.45
Circumference length of ring resonator: 100 μm

First, FIG. 2 will be described. FIG. 2 shows the relative phase change due to the wavelength of light input to the optical delay element and the passage of the optical delay element when the intensity coupling efficiency in the first coupling unit and the second coupling unit is changed as a parameter. It is a graph which shows the relationship with quantity (phi). Parameters theta / [pi shown in FIG. 2, an amount corresponding to the intensity coupling efficiency kappa ^2. In FIG. 2, the vertical axis represents φ / π, and the horizontal axis represents the value obtained by dividing the amount δ corresponding to the deviation from the resonance frequency of the ring resonator by π. Here, δ can be expressed by the following equation (4).
δ = (ω−ω _r ) n _eff L _c / c ₀ (4)
In equation (4), ω is the angular frequency of light, ω _r is the resonant angular frequency, n _eff is the effective refractive index of the waveguide constituting the ring resonator, and L _c is the frequency of the ring resonator. in length, c ₀ is the speed of light in vacuum.

  In FIG. 2, the characteristic when θ / π = 0 is the characteristic of an element obtained by removing the ring resonator from the optical delay element 10. Further, the slope of the line indicating each characteristic in FIG. 2 is inversely proportional to the group velocity of light. Therefore, as apparent from comparing the characteristics in the case of θ / π = 0 shown in FIG. 2 with the characteristics in the case of θ / π = 0.25, the optical delay element 10 has θ / π = 0.25. By satisfying, it is possible to reduce the group velocity of light to 1/3. Further, as is apparent from the characteristic when θ / π = 0.25, in the optical delay element 10, the group velocity of light is reduced to 1/3 without depending on the wavelength.

  As shown in FIG. 2, when θ / π = 0.1 or θ / π = 0.4, the slope of the line indicating the characteristics in these cases is δ / π = ± 0. In 5 and δ / π = ± 1.5, the slope is the same as the slope indicating the characteristics when there is no ring resonator. That is, when θ / π = 0.1 or θ / π = 0.4, there is a wavelength band where the group velocity cannot be reduced. On the other hand, as shown in FIG. 2, when θ / π is 0.2 or more and 0.3 or less, the group velocity is reduced in the entire wavelength band. Therefore, the optical delay element 10 can reduce the group velocity in the entire wavelength band when the condition of 0.2 ≦ θ / π ≦ 0.3 is satisfied.

Next, FIG. 3 will be described. FIG. 3 shows the wavelength of light input to the optical delay element and the passage of the optical delay element when θ / π corresponding to the intensity coupling efficiency is 0.25 and is changed with φ _U / φ _H as a parameter. 6 is a graph showing a relationship with relative phase change amount φ due to. 3, as in FIG. 2, the vertical axis represents φ / π and the horizontal axis represents δ / π.

As shown in FIG. 3, the optical delay element 10 can reduce the group velocity of light to 1/3 without depending on the wavelength by satisfying φ _U / φ _H = 1.0. As is clear from FIG. 3, when φ _U / φ _H is 0.2 or 1.8, there is a wavelength band in which the group velocity does not decrease. On the other hand, when φ _U / φ _H is 0.8 or more and 1.2 or less, the group velocity decreases in the entire wavelength band. Therefore, the optical delay element 10 can reduce the group velocity in the entire wavelength band by satisfying the condition of 0.8 ≦ φ _U / φ _H ≦ 1.2.

  Next, FIG. 4 will be described. FIG. 4 is a diagram showing chromatic dispersion for each of the optical delay element 10 and an element obtained by removing the ring resonator from the optical delay element 10. In FIG. 4, the horizontal axis represents the wavelength of the input light, and the vertical axis represents the dispersion (the value obtained by dividing the speed of light of each wavelength by the wavelength).

  As shown in FIG. 4, the dispersion of the optical delay element 10 (see the characteristic in the case of having a ring resonator) indicates that the group speed of the optical delay element 10 is 1/3 of the group speed of an element without the ring resonator. Corresponding to becoming, it has tripled. Therefore, it can be confirmed from FIG. 4 that in the present optical delay element 10, there is no added chromatic dispersion other than the increase in dispersion accompanying the decrease in group velocity.

  Hereinafter, an optical delay element according to another embodiment of the present invention will be described. FIG. 5 is a plan view schematically showing an optical delay element according to an embodiment. Since the optical delay element 10A shown in FIG. 5 has substantially the same elements as the optical delay element 10 shown in FIG. 1, the difference between the optical delay element 10A and the optical delay element 10 will be described below.

In the optical delay element 10A, the above-described measure (iii) is adopted in order to satisfy φ _U / φ _H = 1.0. Specifically, in the optical delay element 10 </ b> A, the path 16 a of the optical waveguide 16 between the first coupling unit 20 and the second coupling unit 22 and the first coupling unit 20 to the second coupling unit 22. The waveguide path 18a of the ring resonator 18 between them extends substantially linearly. Thereby, the path length of the path 16a and the path length of the path 18a are substantially the same. The propagation constant β _U of the optical waveguide 16 and the propagation constant β _{H of the} waveguide of the ring resonator 18 are the same.

Similarly to the optical delay element 10, the optical delay element 10A can reduce the group velocity of light in a wide wavelength band by satisfying the expressions (1) and (3). Further, the optical delay element 10A has φ _U / φ _H = 1.0, and the strength coupling efficiency κ ² = 50% (θ / π = 0.25) in the first coupling unit 20 and the second coupling unit 22. ), The group velocity of light can be reduced to 1/3 regardless of the wavelength in a wide wavelength band.

Note that the optical delay element of the present invention can also be configured by the above-mentioned measure (ii). That is, the refractive index and / or cross-sectional area of the reduction waveguide and the refractive index and / or cross-sectional area of the waveguide of the ring resonator are made different (by adjusting β _i ), thereby satisfying the formula (1), , Φ _U / φ _H = 1.0 can be satisfied.

  Hereinafter, an optical delay element according to still another embodiment of the present invention will be described. FIG. 6 is a plan view schematically showing an optical delay element according to an embodiment. As shown in FIG. 6, the optical delay element 10B includes an input port 12, an output port 14, a reduction waveguide 16, a plurality of ring resonators 18, a plurality of first coupling portions 20, and a plurality of second couplings. A portion 22 is provided.

  In the optical delay element 10 </ b> B, one ring resonator 18, a first coupling unit 20 and a second coupling unit 22 associated with the ring resonator 18, and the first coupling unit 20 to the second coupling unit 22. The optical waveguide 16 up to this is a unit having the same configuration as the optical delay element 10. The optical delay element 10B has a form in which a plurality of such units are connected in series.

  This will be specifically described below. In the optical delay element 10B, the plurality of ring resonators 18 are optically coupled to the optical waveguide 16 in two portions by the plurality of first coupling portions 20 and the plurality of second coupling portions 22, respectively. Yes. As with the first coupling unit 20 and the second coupling unit 22 of the optical delay element 10, wavelength-independent directional couplers are used for the plurality of first coupling units 20 and the plurality of second coupling units 22. Can be used. That is, the plurality of first coupling portions 20 and the plurality of second coupling portions 22 satisfy θ / π = 0.25 regardless of the wavelength. In addition, the some 1st coupling | bond part 20 and the some 2nd coupling | bond part 22 should just satisfy | fill Formula (3) irrespective of a wavelength.

Further, in the optical delay element 10B, the phase change amount φ _U of the path 16a of the optical waveguide 16 between the first coupling unit 20 and the second coupling unit 22 provided for the same ring resonator 18; The phase change amount φ _H of the waveguide path 18 a of the ring resonator 18 between the first coupling unit 20 and the second coupling unit 22 satisfies φ _U / φ _H = 1.0. Yes. In addition, (phi) _U / (phi) _H should just satisfy | fill Formula (1).

In order to satisfy φ _U / φ _H = 1.0, in the optical delay element 10B, as in the optical delay element 10, the path 16a and the path 18a are configured based on the measure (i). Note that the route 16a and the route 18a may be configured by any one of the measures (i) to (iii).

According to the optical delay element 10B, similarly to the optical delay elements 10 and 10A described above, it is possible to reduce the group velocity of light in a wide wavelength band. Further, by satisfying φ _U / φ _H = 1.0 and θ / π = 0.25, it is possible to reduce the group velocity of light to 1/3 regardless of the wavelength. Further, according to the optical delay element 10B, it is possible to realize an arbitrary delay amount by adjusting the size of each of the plurality of units described above.

  Next, application examples of the optical delay element of the present invention will be described. FIG. 7 is a diagram schematically illustrating an application example of the optical delay element according to the embodiment. FIG. 7 shows an example in which the optical delay element 10B is applied as a part of a Mach-Zehnder interferometer that can be used for a wavelength filter or the like.

  A Mach-Zehnder interferometer 30 shown in FIG. 7 includes an optical delay element 10 </ b> B and an optical waveguide 32. The optical delay element 10B is the same as the optical delay element 10 shown in FIG. 6, and has the three units described above. In the Mach-Zehnder interferometer 30, a part of the light input from the input end of the optical waveguide 32 is coupled to the optical delay element 10B, and the light delayed by passing through the optical delay element 10B is transmitted in the optical waveguide 32. The optical waveguide 32 can be coupled with the guided light and output from the output end of the optical waveguide 32.

  In a Mach-Zehnder interferometer that does not use the optical delay element 10B, it is necessary to use an optical waveguide having a long path instead of the optical delay element 10B. On the other hand, since the Mach-Zehnder interferometer 30 uses the optical delay element 10B instead of the optical waveguide having a long path, the size can be reduced. Thus, the optical delay element of the present invention can be used for various optical components, and the optical components can be miniaturized.

It is a top view which shows roughly the optical delay element which concerns on one Embodiment. The relationship between the wavelength of light input to the optical delay element and the relative amount of phase change due to the passage of the optical delay element when the intensity coupling efficiency in the first coupling unit and the second coupling unit is changed as a parameter It is a graph which shows. in the case of the φ _{U /} φ _H is varied as a parameter, a graph showing the relationship between the relative phase variation phi by passage of the wavelength and the optical delay elements of the light input to the optical delay element. It is a figure which shows the wavelength dispersion about each of the optical delay element of one Embodiment, and the element which remove | eliminated the ring resonator from the said optical delay element. It is a top view which shows roughly the optical delay element which concerns on one Embodiment. It is a top view which shows roughly the optical delay element which concerns on one Embodiment. It is a figure which shows schematically the application example of the optical delay element which concerns on one Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10, 10A, 10B ... Optical delay element, 12 ... Input port, 14 ... Output port, 16 ... Optical waveguide (reduction waveguide), 16a ... Path | route (Optical light from 1st coupling part to 2nd coupling part) (Path of waveguide 16), 18 ... ring resonator, 18a ... path (path of waveguide of ring resonator between first coupling portion and second coupling portion), 20 ... first coupling portion, 22 ... 2nd coupling | bond part, 30 ... Mach-Zehnder interferometer, 32 ... Optical waveguide.

Claims (5)

An input port for input of light;
An output port from which light is output;
An optical waveguide extending from the input port to the output port;
A ring resonator having a waveguide extending in a ring shape;
A first coupling portion and a second coupling portion for coupling light between two portions of the ring resonator and the optical waveguide;
With
The first coupling portion is provided closer to the input port than the second coupling portion,
The phase change amount φ _U of the path of the optical waveguide between the first coupling portion and the second coupling portion, and the ring resonance between the first coupling portion and the second coupling portion. The phase change amount φ _H of the waveguide path of the filter satisfies 0.8 ≦ φ _U / φ _H ≦ 1.2,
The light intensity coupling efficiency κ ² in each of the first coupling unit and the second coupling unit is such that the parameter θ having a relationship of κ = sin (θ) with respect to the amplitude coupling coefficient κ is 0.2 ≦ θ. Is set to satisfy /π≦0.3,
Optical delay element.   The optical delay element according to claim 1, wherein the path of the ring resonator is bent a plurality of times.   The optical delay element according to claim 1, wherein a refractive index of the waveguide of the ring resonator is different from a refractive index of the optical waveguide.   The optical delay element according to claim 1, wherein a cross-sectional area of the waveguide of the ring resonator is different from a cross-sectional area of the optical waveguide. An input port for input of light;
An output port from which light is output;
An optical waveguide extending from the input port to the output port;
A plurality of ring resonators having waveguides extending in a ring;
A plurality of first coupling portions and a plurality of second coupling portions for coupling light between two portions of each of the plurality of ring resonators and the optical waveguide;
With
Each of the plurality of first coupling portions is provided on the input port side from a second coupling portion provided for the same ring resonator among the plurality of second coupling portions,
Among the plurality of first coupling portions and the plurality of second coupling portions, the first coupling portion and the second coupling portion provided for the same ring resonator are separated from the first coupling portion. The phase change amount φ _U of the path of the optical waveguide between the second coupling portion and the path of the waveguide of the ring resonator between the first coupling portion and the second coupling portion. The phase change amount φ _H of the above satisfies 0.8 ≦ φ _U / φ _H ≦ 1.2,
The light intensity coupling efficiency κ ² in each of the plurality of first coupling portions and the plurality of second coupling portions is such that the parameter θ having a relationship of κ = sin (θ) with respect to the amplitude coupling coefficient κ is 0. Set to satisfy 2 ≦ θ / π ≦ 0.3,
Optical delay element.

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